ENGINEERING LIBRARY 19 00- BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Henrg W. Sage 1891 Ar....A^..ZS^^ S/z^iqMY.. The date shows when this volume Was taken. i -* *-'^~^£Cv^-' '*02 All boolcB not in u§e for instruction or re- ' search are limited %o four weeks to all bor- OCT 18 1909 -Mm ^^H-1l5K7 yp/ZK Periodicals of a gen- eral character should be returned as soon as possible ; when needed beyond two weeks a special request should be made. All student borrow- ers are limited to two weeks, with renewal privileges, when the book is not needed by- others. Books not needed during recess periods should be returned to the library, or arrange- ments made for their return during borrow- er's absence, if wanted. Books needed by more than one person belong on the reserve list. Cornell University Library TN 265.P85 1902 The genesis of ore deposits. 3 1924 004 119 792 Vi -1 IK Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004119792 THE GENESIS OF ORE-DEPOSITS. BY V ^ PROFESSOR FRANZ POSEPNY. Reprinted, together with the Discussion Thereof, from Volumes XXIII. and XXIV. of the Transactions of the American Institute of Mining Engineers. SECOND EDITION. Containing also a Biogkaphical Notice of Professor Posepny, and Nu- merous Additional Papers and Discussions by American and Euro- pean Authors, Reprinted from Volumes xxx. and xxxi. OF the Same Transactions. NEW YORK CITY ■ PUBLISHED BY THE INSTITUTE At the Office of the Seckbtary. 190 2. ^.\U^1.%4- GENERAL TABLE OF CONTENTS. Vll ix 188 188 190 192 195 199 204 Preface to the Second Edition, Preface to the First Edition, . . Biographical Notice of Franz Posepny. By B. W. Eaymond. The Genesis of Ore-Deposits. By PRorEssoR Franz Posepny, . . . 1 Discussion at the Chicago Meeting, August, 1893, including Communications Subsequently Keceived ; AV. P. Blake, Arthur Winslow, ...... T. A. KiCKARD, . Horace V. Wincheil, . ... John A. Church, S. F. Emmons, . . ... G. F. Becker, F. M. F. Cazin, . . . . . 206 Discussion at the Virginia Beach Meeting, February, 1894, including Com- munications Subsequently Received : T. A. ElCKARD, . ... . . 211 E. "W. Eaymond, . . . . 226 H. V. Winchell, . . . . 227 Secretary's Kote, . .... 232 Prof. Posepny, ... , . . . 232 E. W. Eaymond, . 252 F. M. F. Cazin, . . . 269 Joseph Le Conte, . 270 Some Principles Controlling the Deposition of Ores. By C. E. Van Hise, 282 The Secondary Enrichment of Ore-Deposits. By S. F. Emmons, . . 433 The Enrichment of Gold and Silver Veins. By Waiter Harvey Weed, 473 Metasomatic Processes in Fissure- Veins. By Waldemar Lindgren, . 498 Discussion : R. Beck (Ptvpers of Van Hise, Emmons, Weed and Lindgren), . , 613 h. deIjAVS AY (Papers of Emmons and Weed), . . 616 Arthur L. Collins {Papers of Emmons and Weed), . 616 H. Foster Bain {Papers of Van Hise, Emmons and Weed), . 622 Charles E. Keyes ( Papers of Van Hise and Lindgren), . .628 Frank D. Adams (Paper of Lindgren), 634 Problems in the Geology of Ore-Deposits. By Prof. J. H. L. Vogt, . . 636 The E61e of the Igneous Eocks in the Formation of Veins. By J. F. Kemp, 681 Tlie Caliche of Southern Arizona : An Example of Deposition by Vadose Circulation. By William P. Blake, 710 (iii) IV GENERAL TABLE OF CONTENTS. PAGE The Character and Genesis of Certain Contact-Deposits. By Waldemae LWDGBEN, . 716 The Formation of Bonanzas in the Upper Portions of Gold-Veins. By T. A. ElCKARD, 734 Discussion : S. F. Emmons (Papers of Collins, Vocjt, De Launay, etc. ), . . . 756 Some Principles Controlling the Deposition of Ores (Concluding Contribu- tion). By C. K. Van HisE, . . . 763 Appendix, 782 PREFACE TO THE SECOND EDITION. The first edition of this volume, issued in 1895, and con- taining only the famous treatise of Posepny and the discus- sions directly based upon it, was exhausted by an unexpectedly large demand from students, teachers and mining engineers in practice. The Council of the Institute authorized the publi- cation of a new edition, which was at first designed to be a simple reproduction of the former one, with the addition of the Biographical Notice of the distinguished author. But the presentation of certain notable papers on this subject at the Washington meeting in February, 1900, led to the decision that these papers, with the discussions thereof, should be in- cluded in the new edition. In the execution of this plan, it was found impossible to in- clude papers of recognized importance, published prior to 1900, or papers on cognate subjects, such as classifications of ore-de- posits, descriptions of special deposits, etc., published since. The Secretary's ITote, constituting the Appendix to this volume, gives ample proof that the valuable material of these classes con- tained in the Transactions of the Institute far exceeds the capacity of a single book. Indeed, the publication of the present collec- tion is not intended to render unnecessary the consultation of many papers not contained in it, by any one who would gain a comprehensive notion of the science to which it relates. Among such papers, it would be easy to point out not a few, equal in value to those which are here reproduced. With regard to all these, the Secretary can only express his regret that they could not be included, and his pride, nevertheless, that the material thus necessarily omitted is so abundant and so important. R. W. Raymond. December, 1901. (V) PREFACE TO THE FIRST EDITION. The name of Franz Posepny appears in the first volume of the IVansaclions of the Institute as one of its foreign members. At the Boston Meeting of February, 1888, he was elected an honorary member, in recognition of his numerous and valuable contributions to the literature of economic geology, and par- ticularly to the science of ore-deposits, which has borne in Germany, at least, since the days of the brilliant Cotta, the name of Erzlagerstattenlehre. The views of Cotta and his associates, sometimes called, for convenience, "the Freiberg school," dom- inated for a generation the current theories and classifications of mining engineers. This is particularly true of the IFnited States, where the excellent translation of Cotta's text-book by Prof. Frederick Prime, one of his pupils, and one of the orig- inal members of the Institute, was for many years the control- ling, and, indeed, the only easily available authority on this subject in the English language. As a personal friend, diligent student and hearty admirer of Bernhard Cotta, and no less as professional critic of his views, I feel myself bound to say that his theories, as stated more than thirty years ago, are still, to a surprising degree, valid and com- prehensive. There is scarcely a single modern modification of them for which he did not,with intuitive prescience, leave a place. On the other hand, it is a fair criticism of the whole " Freiberg school," that it gave too much prominence and attributed too" much typical importance to fissure-veins of the class represented in the Erzgebirge. Such writers as Groddeck and Grimm have undoubtedly aided to modify this disproportionate emphasis. But it has not ceased to infiuence the conceptions entertained by miners, and even by legislators, as the United States mining law (evidently based on the " true fissure-vein " as a general type) abundantly demonstrates. Posepny had contributed to the subject numerous mono- graphs, throwing much-needed light upon it from the detailed study of Bjjecial mining districts. He had been for many years devoted to this particular branch of geology, and had occupied (vii) VIU PREFACE TO THE FIRST EDITION. for ten years a chair as professor in the Przibram Mining Acad- emy, deahug exclusively with the theory of ore-deposits. "When I urged him to contribute, for the International Meeting of the Institute in 1895, a paper on that subject, I did not venture to expect so generous a response as I received, in the free dedica- tion to the Institute of a treatise comprising a summary of the views and observations of the distinguished author, and cover- ing the whole field of his specialty. Besides its wealth of de- tails, this treatise presents a most interesting and suggestive attempt at a genetic classification — a feature confessedly absent from most earlier systems. The translation of Prof Posepny's work was to me a labor both instructive and delightful; and I take pleasure in ac- knowledging here that my task was greatly lightened in that regard by the marvellous accuracy and beauty of the German manuscript, the whole of which came to me in the exquisite handwriting of Madame Posepna. Her husband was for some months unable to write, by reason of an injury to his hand. Probably he regarded this accident as a misfortune ; but I trust he will not be oftended if I say that his American translator had reason to take, with gratitude, a different view of it.* My translation of the paper itself has received the author's approval; bat the translation of his later communication, which appears in this volume in the course of the discussion, goes to press without final revision on. his part. I can only infer, from his omission to return with corrections the copy sent him sev- eral months ago, that he has not found serious errors in it. The presentation of this paper at the Chicago Meeting of 1893 was the signal for a lively and interesting discussion on the part of American geologists. That discussion has by no means come to an end ; and it is likely that the impulse thus given to a renewed study of this important subject will continue to operate for a long time to come. It was, however, necessary to stop somewhere, in preparing the present volume for the con- venient use of readers ; and the line has been drawn at the end of Vol. XXIV. of the Iransactions of the Institute, so as to in- clude, with a complete analytical index, for ready reference, both the original paper and all the discussions of it contained in Vols. XXIII. and XXIV. R. W. Raymond. * See further remarks in the Biographical Notice of Posepny, in tliis volume. BIOGRAPHICAL N"OTICE OF FRAISTZ POSEPKT. Br R. W. RAYMOND, NEW YORK CITY. (Atlanta Meeting, October, 1895.) On the 27th day of March last, the day on which the Florida sessions of the last meeting of the Institute began in Ocala, occurred the death of one of its most distinguished honorary members, Bergrath Franz Posepny, of Vienna, formerly pro- fessor of the science of ore-deposits in the mining school at Przibram, Bohemia. Prof. Posepny had greatly increased his fame among American mining engineers, besides laying the Institute under special obligations of gratitude, by the elabor- ate, brilliant and suggestive treatise on " The Genesis of Ore- Deposits," which he contributed to the International meeting, held at Chicago, in 1893. This essay, constituting the first complete publication of the substance of his course of lectures at Przibram, enriched and perfected by the matured results of his investigation and reflection down to the date of its com- pletion, was a free gift of almost unprecedented value to the society which had distinguished him by honorary membership. In estimating the generosity of the author, it must be borne in mind that the copyright of such a work, the fruit of years of study and of practice as an instructor, is of no little value to a European professor, and constitutes one of the legitimate re- wards of his (otherwise not highly-paid) labor. Moreover, Prof Posepny performed under peculiar difficul- ties his promise to contribute this treatise. Apart from his failing health, an accidental fall had so crippled his hand that he was for months unable to write ; and the whole of the volu- minous German manuscript had to be dictated to his wife, in whose exquisitely clear and beautiful handwriting it came to me for translation and publication. That interesting labor, willingly performed, was greatly lessened by this circumstance; and I did not hesitate to confess to Prof. Posepny that my per- (ix) X BIOSRAPHICAL NOTICE OF FRANZ POSBPNY. sonal regret for his accident was considerably mitigated by the indirect gain thus occasioned to his translator. I trust that I do not transgress propriety by saying in this place a few words concerning Madame Clotilde Posepna, who accompanied her distinguished husband in his visit to the United States in 1876 (as on so many of his other journeys and expeditions), and with whom so many members of the Institute had the pleasure of becoming acquainted at that time. With the exception, perhaps, of Sir Charles and Lady Lyell, I can recall to mind no other husband and wife so highly accom- plished, so thoroughly united and so mutually complementary in scientific work. In the matter of languages, for instance, I remember hearing one of them say that, drawing a meridian through eastern Europe, they had divided the map between them ; he assuming for his province the tongues east of that line, while she took care of those to the west, including all the European languages and literatures that we commonly regard as required for linguistic accomplishment. German, of course, was common ground to both. The inestimable value of such a colleague to Prof Posepny is indicated abundantly in his treatise, already mentioned, which exhibits, on the one hand, the results of much original investigation in Eastern Europe, and, on the other hand, a wide acquaintance vdth the tech- nical literature of western nations. That treatise aroused so much interest among mining engineers in this country, and gave rise to so much suggestive discussion, that a separate vol- ume, containing the original paper, the criticisms which it elicited, and Prof. Posepny's reply thereto, carefully indexed for convenient consultation, has been issued by the Institute to accommodate instructors and students. It was just as this edition was leaving the press that I received the news of Prof Posepny's death ; and in view of the part which his wife had taken in his service to the Institute and to science, I inserted at the beginning of the book these words,* which I here repeat, not doubting that they will be heartily adopted by every one who shall read them : * This dedication, and tlie frontispiece-portrait of Posepny, have been omitted from the present volume. BIOGRAPHICAL NOTICE OF FRANZ POSEPNY. XI TO MADAME CLOTILDE PO^EPNi:, WIFE, COMRADE AND COLLEAGUE OF THE DISTINGUISHED AND LAMENTED AUTHOR OF THIS TREATISE. THE PRESENT VOLUME IS INSCRIBED IN WITNESS OF GRATITUDE FOR HER CO-OPERATION, AND SYMPATHY WITH HER BEREAVEMENT. In attempting to sketch the career of Franz Posepny, I shall make free use of the appreciative obituary notice written by his friend and colleague, Oberbergrath Hitter C. von Ernst, one of the editors of the Oest. Zeitsch. fur Berg- und Hiittenwesen, and published in that journal April 27, 1895. Born March 30, 1836, at Starkenbach, in Bohemia, Posepny, after preliminary courses in various Bohemian schools, entered, in 1852, the Polytechnic School at Prague, with the special pur- pose of pursuing the natural sciences, for which he had a native inclination. In addition to the prescribed curriculum, he zeal- ously frequented the lectures and practical exercises in botany, mineralogy, geology, palaeontology, chemistry, technology, met- allurgy, etc. In order to utilize in the department of mining his knowledge of geology, he went, in 1857, to the mining school at Przibram, where he was specially interested in the lectures of Director Grimm ou the science of ore-deposits. It was from G-rimm (says his German biographer, ou the author- ity of Posepny's own notes) that he heard for the first time the view that ore-deposits are characteristically confined to decom- posed rocks — a doctrine which guided and influenced him for many years after. After finishing his mining course, he en- tered (1859) the State service, and was first assigned, without pay, to the government bureau at Nagybtinya, and thence (1860), at a salary of less than 50 cents a day, to Ohlalapos- banya, in Transylvania. This region, with its complicated mine-workings and vein-phenomena, was peculiarly interesting and stimulating to an ardent young mining ensfineer and in- vestigator ; but he was condemned to the prosaic drudgery of auditing the old accounts of mines which had been destroyed Xll BIOGRAPHICAL NOTICE OF FRANZ POSEPNT. in the rebellion of 1848, and his superior official discouraged his studies underground, telling him that he had " much more important things to attend to than going down into old mines, which could show him only rubbish and dirt." He was obliged, therefore, to pursue his favorite studies in secret until a more favorable position was assigned to him as the director (at about 60 cents per day !) of certain explorations for lignite in the district of Kovar. Here he distinguished himself by the execution of a topographical and geological map of the district, determining, on palseontological evidence, the age of the coal- deposits. This led to a recognition of his peculiar qualifica- tions for the study of problems in economic geology; and in 1862 he was designated (at the increased salary of 75 cents per day !) to make an investigation of the ore-deposits and almost abandoned mines of Rodna, in Transylvania. This work, in which he at first received assistance from the Geologische Eeichs- anstalt at Vienna, was subsequently somewhat peremptorily and prematurely terminated, and, late in 1865, Posepny was ordered to make a similar study of the gold-mines of Veres- patak. This occupied him until 1869, when he was recalled to Vienna, and directed to examine and report upon the mines of Eaibl, in Carinthia. This work consumed a good deal of time, and the authorities were, perhaps, inconsiderate in their re- peated demands for a hasty completion of it. Posepny was still, after 11 years of service, only an "expectant," without title and with scanty pay ; and in justifiable dissatisfaction with this treatment, he accepted, in 1870, the ofier of an independ- ent position — specially created for him — as economic geologist for Hungary, with a salary and allowances amounting to some- thing like $1000 per annum. This he occupied for two years, executing during that period many investigations of value to the Hungarian mining industry. In 1872 he returned to Eaibl; finished and presented, in 1873, his official report on that district, and then went back to Hungary, to continue his study of the Schemnitz region. But by this time his services were required in a wider field ; and he resigned his position in Hungary to accept that of Vice-Secretary in the Eoyal-Imperial Ministry of Agriculture (including mining) of Austria. In this capacity, from 1873 to 1879, he carried out in Tyrol and Salzburg a series of investigations (published in the first vol- BIOGRAPHICAL NOTICE OF FRANZ POSBPNY. XIU ume of his ArcMv fiir Praktische Geologie), and also made journeys to various countries, including an extended tour in the United States. But he was not satisfied with this achievement of official position and its sphere of usefulness. His conviction of the importance to the mining industry of the scientific study of mineral deposits had been expressed incessantly in publica- tions, urging the introduction of special lectures on this subject in mining schools ; and in 1879, the ministry with -which he was connected succeeding in obtaining from the Emperor authority to establish, at the academies of Leoben and Przi- bram, separate chairs devoted to that department, the pro- fessorship at Przibram, together with the title of Bergrath, was given to Posepny, and occupied by him until, in 1888, he re- tired from public service, receiving in recognition of his merit the order of the Iron Crown. In some respects, his labors at Przibram were the most fruit- ful of his life. Besides discharging the duties of the class- room, which served, no doubt, to consolidate and systematize the knowledge gathered in practice, he added to that knowl- edge by a diligent and minute study of the geology and vein- relations of the extensive and productive Przibram mines. This really great investigation was carried through by Pro- fessor Posepny with wonderful persistency, at great personal expense, and without assistance. Its results are to be pub- lished in the second volume of his Archivfur Praktische Geolo- gie, which was in press at the time of his death. After resigning his professorship and retiring from active service, he established himself, with his inseparable helpmate, in a pleasant cottage home in the suburbs of Vienna, where he •devoted himself more exclusively than ever to his favorite studies, making journeys of observation to Transylvania, Ger- many, Switzerland, the TTral, France, England, Sweden, l^or- way, Italy, Sardinia, and finally, in the spring of 1894, to Greece and the Orient, as far as Jerusalem. His principal attention in these journeys was given to the geology and the mining (present, historical or pre-historical) of the countries he visited. That he could appreciate, however, other sentiments and associations, I have touching proof in a note which he sent me from Jerusalem, enclosing a leaf plucked on the Mount XIV BIOGRAPHICAL NOTICE OF FRANZ POSBPNY. of Olives. It should be mentioned also that, in addition to his main specialty, he was an enthusiastic student, and no mean authority, in anthropology and numismatics. During this closing period of his intensely active life, his in- dustry might fairly be called desperate ; for the increase of a long-standing pulmonary v^^eakness, to which in these latter years a disease of the heart was added, produced in him the abiding conviction, not only that his days were numbered, but that they might at any moment suddenly end. What he ac- complished with failing strength and under such a depressing consciousness, is truly amazing. Yet, in his letters to me, he never alluded to the shadow of such an apprehension; and I did not dream that his magnificent contribution to the Insti- tute was the bequest of a dying man, and the last important work of his life. I take the liberty of translating portions of a private letter from his wife, which, although not intended for publication, are calculated to give, better than words of mine could do, a pathetic and inspiring picture of his heroic devo- tion : " Although for many months I had necessarily foreseen the sad termination of his sufferings, I could not help clinging to occasional momentary gleams of hope ; and the end seemed, after all, awfully sudden. "Only with the utmost exertion did we two succeed in so far completing the proof-reading of the second volume of the Archiv, that nothing will now prevent its early publication. " With the kind assistance promised by his professional colleagues, I may also hope to bring out, in a year or two, a third volume. It is a purpose dear to me to publish all that he left behind. Much will, of course, appear in fragmentary form, but it will at least stimulate thought and discussion. " It is almost incredible how hard he worked, giving himself in later years no rest, because he continually looked for death. Outwardly he appeared so full of life and pleasure in life [so lebensfroh), and seemed to be in perfect health. But I Itnew better ; and he himself used to be annoyed when people spoke of his good . looks, for, as he said, he was always ' only a handsomely turfed grave !' " I am unable to give at this time a complete list — still less a critical account — of the published reports and treatises of Prof. Posepny, between one and two hundred in number. This will be done, I understand, in the introduction to the second vol- ume of his Archiv, now in press. N'evertheless, I may venture to express some general reflections concerning his career and his position in scientific literature. 1. Even from the bare outline of his life which I have given. BIOGRAPHICAL NOTICE OF FRANZ POSEPNY. XV it is evident that he trod no easy path to eminence and fame. For many years he was utilized without being adequately ap- preciated ; ordered from place to place ; scantily paid and arbi- trarily overruled; his far-reaching plans, thwarted by short- sighted officialism, intent upon more immediate practical re- sults. For this the government bureaus are not necessarily to be blamed. Posepny was, heart and soul, not a government official, but the lover and slave of science. And governments do not exist for the promotion of science. The utmost which they can legitimately do in that direction is to assist the prog- ress of science on grounds of political economy ; that is, as an element in the industrial prosperity of the commonwealth, and an incident of the intelligent administration of its resources. European states have gone further in theory than our own Federal government (though few have been so loosely liberal in practice) in the range of application given to this principle. But, under any government, immediate administrative necessi- ties may often take precedence of purely scientific investiga- tions, and the subordinates of a bureau may be commanded to devote themselves to barren routine when they would rather be " exploring the unknown." 2. Moreover, not everybody who burns with ambition to distinguish himself by increasing the sum of permanently valu- able human knowledge should, on that account, be enabled, either by public or by private aid, to pursue his supposed mis- sion at the expense of other people. Some peculiar fitness must first be demonstrated ; and, on the whole, there is per- haps no better test than that of patient and obedient service, even under unwelcome restraint. The man who, like Posepny, in spite of, and in addition to, his routine duties, continues with ardor his scientific investigations, is the best man to be subsequently intrusted with such higher work. 3. But this is not all. The best training, even for a specialty, does not consist in simply encouraging the inclination of genius in one direction. We hear a good deal about education as being ideally, as it is etymologically, the " drawing-out " of what is already in the pupil. This is true enough, if we add that the best work of education is the drawing-out of faculties which the pupil does not know or believe to be in him, and that its least important function is the assistance of those domi- XVI BIOGRAPHICAL NOTICE OF FRANZ POSBPNY. nant powers and purposes which need little help. It is often in the branch for which the schoolboy shows no taste or ca- pacity that he should be most rigorously drilled, not merely for the moral, but also for the mental, discipline thus secured. And there is nothing that contributes more potently to success in the larger school of life than the subjection of young men to work which they do not like, and in the knowledge of which they are, consequently, deficient. I say " consequently," but the consequence may be often the cause. The dormant capacity once developed by practice, many a man ends by liking a work which he understands, who began by disliking it because he did not understand it. 4. In the case of Posepny, I am not at all sure that the dis- appointment and drudgery of his early career were not the best things that could have happened to him. Incidentally, they gave him a much wider experience than he w^ould have obtained by rapid promotion — which might have made of him either a conservative official, calmly contemptuous of youthful ambitions, or a library-theorist, learnedly discoursing of nature at second-hand ; of both of which classes the world has enough already. They are useful in their way; but it would have been a pity to waste Posepny, in order to increase either. The result, in his case, of the irksome discipline of fiery, un- conquerable genius, was to reinforce the knowledge of litera- ture and theory with an extensive and intimate direct knowl- edge of nature, and, above all, to make the chemist and geolo- gist also a practical miner and mining engineer. The latter circumstance adds exceptional and characteristic weight to his scientific generalizations. I may add that, in my judgment, the nature of his early labors not improbably bred or deep- ened in him that sense of the vital importance to science of the minute observation, and purely " objective " description, of single groups of phenomena, which is so prominent in all his writings. In accordance with it, his works are mainly de- tailed accounts and discussions of single mining districts. In other words, he continued to the end the method of investiga- tion which was forced upon him in the beginning by superior authority. The difference between such monographs, pro- duced by the patient labor of months in each locality, and the sketchy results of hasty visits by expert tourists, such as con- BIOGRAPHICAL NOTICE OF FRANZ POSEPNY. XVU stitute much of the literature of this class, requires no com- ment. 5. I have emphasized at some length this feature of Po- sepny's work, because I think it carries an important lesson for American mining engineers and geologists. We are making rapid progress in science ; but we do it in a tumultuous and irregular fashion, accumulating a goodly stock of untrust^ worthy data and of premature theories as we go. Our young investigators are often in a hurry to promulgate generaliza- tions ; and, on the other hand, our practicing mining engineers are often too busy to observe and record facts. The two classes could aid each other more than they do ; and especially those who are confined by their duties to one locality might learn from the example of Posepny that the thorough study of one locality is the most valuable contribution that can be made to general science. On the other hand, the authors of theories may profitably note that Posepny himself, as the result of , wider observation, was obliged to change the views he had ex- pressed, under the influence of preconceived imjjressions, in early years. 6. In my brief preface to the separate edition of "The Genesis of Ore-Deposits," issued by the Institute, I have used the following language, which I here repeat, as an introduction to some further observations upon Posepny's Avork : " The views of Cotta and his associates, sometimes called for convenience 'the Freiberg school,' dominated for a generation the current theories and classifications of mining engineers. This is particularly true of the United States, where the excel- lent translation of Cotta's text-book by Prof Frederick Prime, Jr., one of his pupils, was for many years the controlling, and indeed the only easily available, authority on this subject in the English language. " As a personal friend, diligent student and hearty admirer of Bernhard Cotta, and no less as professional critic of his views, I feel myself bound to say that his theories, as stated more than thirty years ago, are still, to a surprising degree, valid and comprehensive. There is scarcely a single modern modification of them for which he did not, with intuitive pres- cience, leave a place. On the other hand, it is a fair criticism of the whole ' Freiberg school,' that it gave too much promi- XVlll BIOGRAPHICAL NOTICE OF FRANZ POSEPNY. nence and attributed too much typical importance to fissure- veins of the class represented in the Erzgebirge. Such writers as Groddeck and Grimm have undoubtedly aided to modify this disproportionate emphasis. But it has not ceased to in- fluence the conceptions entertained by miners, and even by legislators, as the United States mining la^v (evidently based on the true ' fissure-vein ' as a general type) abundantly demon- strates." Of the two authorities named in the above extract, as aiding to modify the views of the " Freiberg school," Bergrath Dr. Albrecht von Groddeck, whose treatise appeared in 1879, was the director of the Prussian Mining Academy at Clausthal. His treatment of the science of ore-deposits was chiefly char- acterized by the recognition of numerous " types," and the citation of leading examples under each type. Oberbergrath Johann Grimm, whose treatise appeared in 1869, was director of the Austrian Mining Academy at Przibram, in Bohemia; and it was in Grimm's lecture-room, from 1857 to 1859, that Posepny received his first working-theory of the nature and origin of mineral deposits. I must confess that I cannot find in Grimm's book, published ten years later, the sweeping gen- eralization to which, on Posepny's authority, Ritter von Ernst (as quoted by me above) alludes; and I am led to suppose that the veteran instructor had seen cause, before 1869, to modify his views. However that may be, it was as a disciple of Grimm that Posepny began his work ; and it was only after years of patient study of facts in the field, that he promulgated any comprehensive system of his own. 7. That system, his matured statement of which is found in " The Genesis of Ore-Deposits," cannot be said to involve any appeal to newly-discovered causes, or any denial of accepted principles in geology. The same is true of all systems pro- posed since the exclusive agency of plutonic action on the one hand, or of aqueous action on the other, was recognized as untenable. They have all been simple attempts to classify the observed facts for fruitful study, and to estimate the relative importance of the several natural agencies which were univer- sally recognized as factors. For the purpose of classification, the chief distinctive characters have always been : (1) The time- relations of a mineral deposit, as formed simultaneously with BIOGRAPHICAL NOTICE OF FRANZ POSEPNY. XIX the enclosing rock, or as a regular member of a series of rocks, or as a later segregation or intrusion ; (2) its form ; (3) the manner and agencies of its origin ; and (4) its contents. Of these characters, sometimes one and sometimes another has been treated as the primary distinction. Gold-, silver-, lead-, and copper-mines, etc., may have been the leading classes in a system designed for convenient use in practice ; veins, stock- works and impregnations may have been separated as groups of independent significance in another practical system ; orig- inal deposits may have been combined with deposits of subse- quent formation, if both were supposed to have originated through the same processes, etc. For the purposes of science, it will probably be admitted that a genetic classification is to be preferred; and such a classification Posepny proposed. That it was not final or complete he acknowledged, not only ex- pressly' in words, but tacitly by his preliminary division of minerals as " idiogenous " and " xenogenous," and the practical confinement of his genetic classification to the latter. It is, of course, plain that the idiogenous minerals must likewise have had a genesis, and that a complete genetic classification would include them, not as a separate primary group, but as parts of other groups, determined by the conditions and agencies of their origin. Posepny's system, beginning as it does with the rocks already formed, and ignoring their prior genetic history, is, to that extent, an avowed compromise. But it is on that basis to be judged, and not by comparison with something more ambitious and comprehensive, at which the author did not pretend to aim. Mj- views on this subject have been sufii- ciently set forth elsewhere ; and the position of Posepny has been so clearly and fully stated by himself as to render further exposition needless. 8. In fact, the present state of the science of mineral de- posits is such as to render any man's system of classification a matter of subordinate pedagogic importance. The declared purpose of Professor Posepny, in the presentation to the Insti- tute of what he at first entitled " Subjective Yiews of the Ori- gin of Ore-Deposits,"* was to invite criticism and discussion. This purpose was unquestionably realized in a discussion (not * See my remarks, Trans, xxiv., 980, and in the present volume, p. 233. XX BIOGRAPHICAL NOTICE OF FRANZ POSEPNY. yet ended) of great interest and value. And the remarkably stimulating effect of that treatise seems to me typical of the chief permanent effect of the author's whole work, in the field in which he became pre-eminent. I believe it will be the ver- dict of his successors, as of his contemporaries : a. That he furnished an example of unselfish and unqualified devotion to science, which will be an inspiration forever. h. That he contributed to science, in his special department, an immense amount of careful and accurate fundamental work, which can be confidently relied upon as trustworthy material for future study, being guaranteed, not only by his eminence in general science, but also by his familiarity, as a mining en- gineer, with operations and observations underground. c. That, on the basis of his wide observation, coupled with his extensive knowledge of technical literature, he exerted a potent influence in promoting the scientific . study of ore-de- posits and in correcting extreme theories and tendencies which have tended to bias and distort that science. 9. In the last proposition, I have in mind more particularly the controversy which Posepny and his friend, the late Prof. Stelzner, of Freiberg, waged against the lateral-secretion the- ory of Prof. Sandberger. In this debate Posepny no doubt as- sumed to some extent the attitude of a partisan ; and, perhaps, in some respects, his controversial utterances may have gone beyond a judicial impartiality. This has been pointed out more than once by his critics, and particularly in the discussion of his recent treatise in the Transactions of the Institute. As I have elsewhere declared, I think he was right in his general view and argument, and I will here do no more than call at- tention to the circumstance that those of his statements which have been seriously contested by American authorities were mainly based upon the publications of others, not upon his own observation. Many such publications are affected with " sub- jective " opinions ; many of them are unaccompanied with ac- curate drawings ; and many of them lack precision in descrip- tion, and are, therefore, liable to misinterpretation. It is no wonder that in single cases Posepny may have mistaken tlie intended meaning of an author or accepted too hastily an as- sertion too hastily made. But it must be confessed that, as a whole, his survey of the literature of his subject was singularly comprehensive, intelligent and fair. BIOGRAPHICAL NOTICE OF FEANZ POSEPNY. XXI 10. As I have observed already, Posepny's final work vras not offered as the last word of science in that field. We now know, what he knew when he wrote it, that it was his last word — the utterance of one who was about to turn over to others the results of a life-labor still incomplete, and surpassing in fruitful suggestion even its illustrious record of accomplished achievement. The loss of such a man at any time is deplora^ hie; but doubly so when he departs in the prime of years, just prepared for the ripest and richest harvest of all his planting. Posepny's views will still incite and reward discussion ; but we shall sorely miss the ablest of expositors and critics in Posepny himself. It has been my endeavor in the foregoing sketch to preserve the standpoint of disinterested j ustice ; but I cannot deny that, while I have been thus coldly analyzing and estimating the scientific leader, there has been constantly present with me a vision of the splendid presence of my own dear friend. I never saw him but once — at the time of his visit to this country in 1876. But that meeting confirmed the personal attraction already exercised upon me by his works ; and our subsequent intercourse by correspondence made the charm perpetual and indissoluble. A few such friends I may still count in foreign lands, unseen, yet ever present; and it sometimes seems to me that these relations of mind and heart, which defy separation in space, are the best types of the relation which defies death also. At all events, I find that he and I, who could be to- gether, though confined to the Old and the ITew "World respec- tively, are not less mutually near, now that one of us has en- tered the World which lies so close to both. THE GENESIS OF OUEDEPOSITS. By Prop. Franz Posepny, Vienna, Austria. CONTENTS. Introduction. Part I. — General Facts and Theories. 1. Systems of Classification Employed Hitherto, 2. Standpoint and View of the Present Paper, . 3. The Xenogenites in General, 4. The Subterranean Water- Circulation, . A. The Vadose Underground Circulation, Filling of Open Spaces Formed by Vadose Circulation, B. The Deep Underground Circulation, . Ascending Waters Encountered in Mines, . Eelaled Phenomena near the Surface, Mineral Springs at the Surface, Chemical Constitution of Mineral Waters, . Minute Metallic Admixtures in Mineral Waters, . Alterations Produced by Mineral Springs, . Structural Features of the Deposits of Mineral Springs, 5. Origin of Ore-Deposits in the Deep Region, . Manner of Filling of Open Spaces in General, Part II. — Examples oe Classes op Deposits, PAGE 3 10 12 17 18 23 26 28 32 38 40 45 48 52 55 64 72 74 Ore-Deposits in Spaces of Discission, . General Features and Illustrations. a. Ore-Veins in Stratified Rocks, . . . .79 Clausthal, 79 ; Andreasberg, 81. b. Ore-Veins in the Neighborhood of Eruptive Masses, 82 The Erzgebirge, 82 ; Przibram, 83. c. Ore-Veins "Wholly Within Large Eruptive For- mations, ....... 85 Hungary, 85 ; Dacian Gold-field, 86 ; Verespatak, 87 ; Vulkoj, 88 ; Comstock lode, 89. Z THE GENESIS OF ORE-DEPOSITS. PAGE 2. Ore-Deposits in Soluble Socks, ..... 95 Fillings of Spaces of Dissolution and Metasomatic Deposits. Eodna, 95 ; Offenbanya, 98 ; Edzbanya, 98 ; Eaibl, 102 ; North of Eng- land, 104 ; Leadville, Colo., 106 ; Ked Mountain, Colo., 109 ; Utah, 110 ; Nevada, 110 ; Deposits in Structural Plateaux, 115 ; Vall^ an 1 Mine la Motte, Mo., 115, 117; Wisconsin, 117. 3. Metamorphous Deposits, ...... 118 G-eneral Features. a. Ores in Distinctly Stratified Rocks, . . .120 Deposition of Ores from Sea- Water, 121 ; from Fresh Water, 123; the Copper-Schists of Mannsfeld, 123 ; of Bohemia, 124 ; of Tliu- ringia, 124 ; of Westphalia, 125 ; the Copper-Sandstone of Bohe- mia, 126; of St. Avoid, 127; Lead-Deposits of Mechernich, 127; Freihung, 129 ; Silver Eeef, Utah, 130 ; Copper-Deposits of New Mexico and Arizona, 131 ; Boleo, Lower Cal. , 132. b. Metasomatic Deposits in Soluble Rocks, . . 133 Calamine Deposits, 134 ; Laurium, 135 ; Bohneisenerz of Alsace, 136 ; Cumberland, 136 ; Wochein, Carniola, 136. c. Depositsin Crystalline Schistsand Eruptive Rocks, 137 Taberg, Sweden, 138; Cornwall, 139 ; Scandinavia, 140; Ammeberg, 141;PrettauinTyrol, 143 ; Lake Superior, 144; Sudbury, Can., 14o. 4. Hysteromorphous Deposits, ..... 147 a. Chemical efifects, ...... 148 Limonite near Bio Tinto, Spain, .... 148 b. Mechanical effects, . . . . . .152 Verchoviky, Surface- Deposits in situ, 152. Theory of the Sinking of Heavier Constituents, 153 ; Stream-detritus, 154 ; Marine detritus, 156 ; Kackar District, Ural Mountains, 157 ; Platinum-placers, 158 ; Tin-placers, 158. c. Hysteromorphs of Older G-eological Formations, 160 Deadwood, S. Dakota, 160; Australia, 161; South Africa, 162; Bohemia, 163. mTRODUCTIOK All serious investigators of this problem have recognized its complex character, and the difficulty of solving it definitely in the present state of our knowledge. Single and simple occur- rences are at present clearly understood ; but the more com- plicated phenomena give rise to discordant and often totally contradictory views, showing that we are still far from the truth upon this subject. The study of it has been the labor of my life ; yet I must confess that the little I have here and there accomplished bears no proportion to the great range of inquiry. I collect, nevertheless, in this paper, some of the personal views THE GENESIS OF ORE-DEPOSITS. 3 to which I have been led, chiefly in order that they may be subnaitted for consideration and discussion to my American colleagues. Looking upon a single, somewhat complicated ore-deposit, we must confess that a superficial, tourist's examination of it could not give satisfactory results. Yet the literature of this subject refers us to such materials chiefly. Even treatises based upon the profound studies of years do not exhaust the subject; for they are aftected by the existing stage of development of the auxiliary sciences, by the existing degree of exploration and exposure of the deposits described, and by the personal views of their authors. Mining, indeed, constantly furnishes fresh evidences in new openings, but it destroys the old at the same time ; and if these are not preserved for science before it is too late, they are lost forever. The whole mining industry is in its nature transitory ; but the nation, which intrusts to the miner, upon certain con- ditions, the extraction of its mineral wealth, has a right to demand that the knowledge thus gained at the cost of a part of the national resources shall not be lost to science. PAET I. GENERAL FACTS AKD THEORIES. 1. Systems of Classification Employed Hitherto. Studies of individual deposits naturally involve speculations concerning their genesis, and many such monographs contain valuable data, which, for the more thoroughly examined min- ing districts, are so well estabhshed and so comprehensive as to invite a systematic arrangement and a genetic explanation. At first, only the form of the ore-deposit was considered in such classifications ; afterwards the barren surrounding medium was included. From this standpoint, unfortunately still taken by some purely empirical experts, the earth's crust is primarily divided into ore-bearing and barren rocks. It was especially the true veins, at one time the principal ob- jects of mining, which gave rise to speculations and discus- 1 4 THE GENESIS OF ORE-DEPOSITS. sions, having now only a historic interest.* A. Werner was the first to frame a scientific theory. He distinguished between ore-deposits contemporaneous in origin with the enclosing rocks and those of subsequent formation, and proved once for all that veins are fissures filled with ore, thus furnishing the most important characteristic for the recognition of primary and secondary formations. As to the manner in which fissures have been filled, Werner's theory, based upon a comparatively limited field of observation, has, like many of his neptunistic views, failed to maintain itself; and this question remains still without a final answer. Curiously enough, many systematizers reproached Werner for having introduced into his system a genetic principle, which they sought to eliminate, confining themselves to the form of deposit as a guide. Thus Waldenstein {op. cit., p. 5) distin- guished (a) tabular deposits (beds and veins) ; (6) stock-deposits, flat^lying or steeply inclined ; and (c) scattered masses, such as nests and pockets. Even Cotta, otherwise an earnest advocate of geological principles, classified ore-deposits according to their form and kind as beds, veins and masses, adding a new and somewhat indefinite group of " impregnations." J. G-rimmf also followed in the main the old principles of classification; included in his system the eruptive ore-breccias which he had personally ex- amined and the tabular segregations of ore, and pronounced not only ore-beds {Erzlager), but also certain bed-masses {Lager- siocke) to be sedimentary formations. Dr. A. von GroddeckJ followed genetic principles already acquiring predominance. He distinguished : (a) original deposits, and (b) deposits of debris. The former he subdivided into (1) those formed contemporane- ously with the country-rock, and stratified (ore-beds, segregated beds, etc.) or massive ; (2) those formed later (cavity-fillings, veins, cave-deposits, metamorphic deposits). He pronounced ore-beds (Erzlager) to be sedimentary, and included in his sys- * The period 1556 to 1791, that is, from G. Agricola to A. Werner, is an illus- tration. See also Die Besonderen Lagerstatten der Mineralien, by J. Waldauf von "Waldenstein, Vienna, 1824, p. 164, etc. ; Die Lehre von den Erzlagerslatten, by B. von Co ta, 2d ed., Freiberg, 1859, p. 85, and the English translation by F. Prime ; and J. A. Phillips' Treatise on Ore-Deposits, London, 1884, p. 74, etc. t -Die Lagerstatten der nutzbaren Mineralien, Prague, 1869. I Die Lehre von den Lagerstatten der Erze. Ein Zweigder Oeologie, Leipzig, 1879. THE GENESIS OF ORE-DEPOSITS. 5 teni the cave-deposits and metamorphic deposits without describ- ing their occurrence in detail. He declared that his system, like all others, had only the purpose of arranging the material of observation conveniently for comprehensive study, and that the manifold products of nature could not be forced into a system of classification. Groddeck's description of the series of forms of deposits is highly original. He presents a number of types, mainly char- acterized by the varying material of the deposits and its mani- fold combinations and transitions. Evidently there vras before him the ideal of combining in a systematic representation the diflerent standpoints from which the subject was to be viewed. At least, if I correctly understood his personal, oral communi- cation of his views, he hoped to represent one standpoint by abscissfe and the other by ordinates, so that the intersection would determine the type of the deposit. This is true enough ; but it presupposes an exhaustive knowledge from both stand- points, which we unfortunately do not possess. My way of looking at the subject was, as appears from his expressions in a later publication, incomprehensible to him.* It seemed to him a sort of heresy to doubt the contemporaneous deposition of the ore of the Mannsfeld copper-schists with the rock, although I assured him that this doubt need only continue until the chemical and physical possibility of such a deposition should be shown. Groddeck's system comprises, it is true, the metamorphic de- posits, but without special definition or illustrative examples. In answer to a criticism of A. Stelzner'sf on this point, he re- plies that he has included in this class those deposits also which have been formed through alteration of the rock-ma- terial by the process which Stelzner had proposed to call meta- somasis, but that the ore-bearing masses thus originated cannot be regarded as separate deposits, because they are only inci- dental phenomena of the filling of cavities. In other words, * " Bemerkungen zur Classifikation der Erzlagerstiitten," Oesterr. Zeitschr., 2885; Hev. Univ. des Mines, 1886, xix. ; Gornoj Jour., 1886, iii., p. 430. " Un- verstiindlich ist es mir, dass Posepny, der sich so grosse Verdienste um die Kenntnisse der Erzlagerstiitten erworben liat, das Vorkommen sedimentiirer Erze ganz ignorirt," etc. t Cited in Das neue Jahrhuch fiir Mineralogie, ii., 1880, p. 50. 6 THE GENESIS OF ORE-DEPOSITS. he grants but subordinate rank to one of the clearest and most important genetic aids to classification, furnished by the oc- currence of rocks transformed into ore. After conceding that deposits of debris should probably be included among strati- fied deposits, he restricts his system to four chief classes : 1. Stratified or sedimentary deposits ; 2. Massive or eruptive de- posits; 3. Cavity-fillings; 4. Metamorphic and metasomatic deposits. This brings him essentially nearer to my view, which groups the first two classes together, as contemporane- ous with the country-rock in origin, with the reservation, how- ever, that the contemporaneity indicated by the stratigraphy should be verified by other evidence. While the work of J. Grimm comprises all useful deposits, that of Groddeck is confined to ore-deposits, although it would be practicable to classify salt, coal and other beds under his system. In England and America the subject has been variously viewed, considerations of practice being predominant, and stratification being regarded as the specially decisive factor. This conception appears first, so far as I know, in the writings of J. D. "Whitney,* who divides mineral deposits primarily into (1) superficial, (2) stratified and (3) unstratified. The strati- fied deposits are divided into (a) those in which the valuable mineral constitutes the mass of a bed, (6) those in which it is disseminated through sedimentary beds, and (c) those origi- nally deposited from aqueous solution, but since metamor- phosed. The unstratified deposits are again divided as irregu- lar [subdivided into (a) masses of eruptive origin (6) dissemi- nated in eruptive rocks ; (c) stock-work deposits ; {d) contact deposits; (e) fahlbands] and regular [subdivided as (/) segregated veins ; [g) gash-veins ; (A) true or fissure-veins] . We find here an explanation of the term " gash-veins," unfamiliar in Europe. Whitney says {op. cit., p. 225) : " Segregated veins, which are peculiar to the altered crystalline, stratified or metamorphic rocks, are usually parallel with the stratification and not to be de- pended on in depth. Gash-veins may cross the formation at any angle, but are peculiar to the unaltered sedimentary rocks. True veins are aggregations of mineral matter, accompanied by metalliferous ores, within a crevice or fissure, * Report of a Geological Survey of the Mississippi Lead Region, Albany, 1868, p. 224, and The Metallic Wealth of the United States, Philadelphia, 1854, p. 34. THE GENESIS OF ORB-DEPOSITS. 7 ■which had its origin in some deep-seated cause, and which may he presumed to extend for an indefinite distance downwards. ' ' Somewhat different is the classification of R. Pumpelly,* who distinguishes: I. Surface-deposits [(1) residuary, (2) stream-, (3) lake- and bog-deposits]. II. Forms due to the texture of the enclosing rock or to its mineral constitution, or to both [(1) disseminated concentrations, further subdivided as (a) impregnations and (6) fahlbands; (2) aggregated concentra- tions, comprising (a) lenticular, (b) irregular masses or " stocks," (c) reticulated veins or " stock-works," (d) contact-deposits] . III. Forms due chiefly to pre-existing cavities or open fissures [(1) cave-deposits; (2) gash-veins; (3) fissure-veins]. Dr. R. W. Raymond,t who followed, in the main, the classification of Lottner,! distinguished : I. Superficial Deposits [(1) Deposits of debris (placers); (2) surface-formations in place (bog-ore, etc.)]. II. Inclosed deposits [(1) sheet-formed or tabular, divided into («) lodes or veins, and (b) beds and seams; (2) mass-deposits, divided into (a) masses, and (b) impregnations, etc. ; and (3) other irregular deposits, such as [a) pockets distributed in large deposits, {b) isolated segrega- tions, gash-veins, etc.]. Prof J. S. ]Srewberry§ adheres mainly to the classification of J. D. Whitney, with some new matter of his own, the value of which has been justly estimated by Raymond. || An analogous line of thought is followed by J. A. Phillips.^f He declares that a careful study of the origin, structure, and composition of ore-deposits, appears to justify their division into the following groups : 1. Superficial [(a) formed by the mechanical action of waters, (b) resulting from chemical action] ; 2. Stratified [(a) constituting the bulk of metalliferous beds formed by precipitation from aqueous solutions, (b) beds * Not possessing the original work, I quote from the monograph of S. F. Em- mons, Oeolopy and Mining Industry of Leadvilte, Washington, 1886, p. 373. t Report of the Commissioner of Mining Statistics, Washington, 1871, and the re- print, Mines and Mining of the Rocky Mountains, New York, 1871, p. 373. X Bergbaukunde, Berlin, 1878. § "The Origin and Classification of Ore-Deposits,'' School of Mines Quarterly, New York, March, 1880 ; also, Eng. and Min. Journal, New York, vol. xxix., 1880, pp. 421 and 437. II Eng. and Min. Journal, vol. xxx., 1880, p. 1. 11 "A Treatise on Ore-Deposits," London, 1884, p. 3. THE GENESIS OF ORB-DEPOSITS. originally deposited from solution, but subsequently altered by metamorphism, (c) ores disseminated through sedimentary beds in which they have been chemically deposited] ; 3. Unstratified [(a) true veins, (6) segregated veins, (c) gash-veins, (d) impreg- nation, (e) stock-works, (/) fahlbands, (g) contact-deposits, (A) chambers or pockets] . In France, comparatively little has been done in framing such systems, higher importance being attached to the syn- thesis of the minerals, the explanation by experiment of geological processes, and the attempt to confirm by the study of mineral-deposits in other countries the theories thus sup- ported. Observations have been made in many cases, not to furnish material for new conclusions, but to prove the truth of existing theories, as, for instance, Elie de Beaumont's theory of " pentagonal symmetry " in the relation between mineral veins and the courses of mountain ranges, etc. In recent times, the chemical standpoint has become domi- nant with the French school, and in the treatise of De Launay,* which has just appeared, the attempt is, in fact, made to base a system of ore-deposits upon a purely chemical view of the subject. He distinguishes : 1, Gites d'indusions (ores as prim- itive constituents of eruptive rocks) ; 2, Gites filoniens (con- taining ores deposited, no matter how, in pre-existing cavities in the rocks); and, 3, Gttes sedimentaires (where metallic sub- stances have been laid down, either as sediments or as precipi- tates, in marine- or fresh-water basins). In another place I will say something of this view, which, in some respects, cor- responds with my own. It is evident from the foregoing mere enumeration of the names of groups and classes of the several systems that, as a general rule, every new observation, considered important by the observer, has been added to the established traditional conception, which, however, was primarily based upon distinc- tions of form and kind, to which genetic principles, if recog- nized at all, were secondary. I may refer, in illustration, to the class of "pipe-veins," and the exhaustive paper of Dr. Raymondf demolishing it. I myself once thought a new * "Formations des Gites Metalliferes," Encydopedie Scientifique, des Aide- memoires publiee sous la direction de M. LeauH, Paris, 1893. t Trans. A. I. M. E., vi., 1887, p. 393. THE GENESIS OF ORE-DEPOSITS. 9 group to be warranted by conclusive observations, namely, typhonic deposits,* in which the ores occur cementing together ^^ the fragments of a brecciated mass. But I soon became con- vinced by the observation of other occurrences, equally difficult to fit into the existing system, that the whole system must be transformed before it could assimilate, without destruction to itself, the new facts observed in the course of time. But a stable and complete system could only be framed, when all the controlling facts — in other words, all the ore- deposits — were accurately known. This is not likely ever to be the case. E"ew observations are constantly made in mining operations, which, moreover, often obliterate the old ones, so that they cannot be verified and compared. It is, however, absolutely necessary, in a field so complicated as that of ore-deposits, to have some general understanding, some sort of system, comprising what is known. And evi- dently, in framing a system, the characters of form, being the most obvious and the most familiar to the miner, would be naturally emphasized, while genetic characters were left in the background. But this ought not to cheek genetic investiga- tion, or the advancing recognition of real relations. A genetic system must, indeed, involve hypotheses, and may not, for a while, be practically useful; but in time it will, like every other cultivated branch of geology, assume more permanent forms. At the Przibram Mining Academy there was established, in 1879, a new chair of " The Geology of Mineral Deposits," which I occupied for about ten years. As the title indicates, it was neither intended merely for instruction in the usual "sci- ence of mineral deposits," nor as a geological course, appended to the technical course in mining, as might be inferred from a title like " Moniangeologie," or " Mining Geology." The leading subject in view was the genesis of the useful mineral deposits. In the present paper I purpose to give a brief statement of the substance of my lectures, which, apart from a few extracts, have never been published. * "Ueber typhonische Gesteinsmassen," Verh. d. k. k. geol. Ecichsaust., 1871, p. 94. 10 the genesis op ore-deposits. 2. Standpoint and View of the Present Paper. The principal genetic distinction is doubtless between de- posits contemporaneous with the country-rock, and those sub- sequently formed in it. The earth's crust consists of rock-elements, chiefly individu- alized as mineral species. Two or three dozen of them — the rock-forming minerals — constitute by far the larger part of the solid earth as known to us. The remainder, much greater in number and variety, ornament our mineral cabinets, but form an insignificant portion of the rocks. The greater part of this group is made up of the legion of minerals occurring in ore- deposits ; and most of these have undoubtedly had a secondary origin in the rocks — for instance, all the cavity-fillings, which of course could only be deposited after the rocks were formed. The secondary origin of some minerals which do not occur in cavity-fillings is less evident. But they occur sometimes in company with those which clearly have this character ; so that we may consider these numerous minerals, occurring in com- paratively small quantities, as secondary. We have two main groups of mineral aggregates : that of the rocks, and that which we will call comprehensively the min- eral deposits. The minerals of the first group belong to it as native and "original ; those of the second are foreigners to the rocks in which they occur. The two groups may therefore be designed (from 'i8w(;, one's own, and fivo?, strange) as Idiog- enous and JCenogenous respectively. It is not necessary here to consider the various origins of rocks, since we take as our starting-point the rocks already formed. The clearly sedimentary rocks consist of the debris of older formations — idiogenous as well as xenogenous; and we must distinguish in them, besides mechanical sediments, chemical precipitates and organic products. The sediment of a basin is the detritus carried into it from the land and deposited in the form of a flat wide cone. Suc- cessive conical envelopes should therefore strictly be the form of such sedimentary beds, though frequently they present apparently level parallel strata. The deposition of a precip- itate, on the other hand, takes place throughout the liquid in the basin, and its form more completely represents the ideal stratum. In both sediments and precipitates, we flnd some- THE GENESIS OF ORE-DEPOSITS. 11 times, besides organic remains, finely divided organic sub- stances, forming the bituminous portions of the rocks. But the great masses of vegetable matter forming the coal-beds were, according to the most widely held opinion, deposited in swampy bottoms, and are therefore neither sediments nor pre- cipitates. Several coal-beds, one above another, indicate a slow sinking of the basin, and its periodical filling-up with detritus from the rivers to such an extent that vegetation could again take root. A coal-basin with several beds becomes on this view the measure of the sinking which is doubtless the cause of every large basin, but which only becomes strikingly evident when the basin contains coal-seams. The foregoing points are mentioned because they indicate original discordances in stratification among the sedimentary laj^ers themselves, and between these and the precipitates and organic formations. If we find in the midst of these formations ores lying exactly between two strata, this relation is not conclusive proof of their sedimentary or precipitative origin. This must be proved in every given case; for in the present state of our knowledge we cannot understand how the metallic sulphides so characteristic of ore-deposits could be formed in that way. As to the eruptive rocks, we do not know what they once were, as we study them only from the moment of cooling. But we observe at once that iron — a metal widely distributed in ore-deposits and in nature generally, occurs primitive in these rocks, in the form of magnetite, a mineral of striking metallic appearance. This idiogenite of the eruptive rocks can be detected without chemical aid ; but with such aid we find traces of other metals besides iron; and this leads us to surmise that the eruptives have brought a whole series of heavy metals up from the " barysphere " into our " lithosphere," aad that it looks as if the metals of our ore-deposits originally belonged to the bary- sphere. This surmise De Launay regards as already proved. He derives, as it were, a priori, all the heavy metals of our ore-deposits from the eruptive rocks, and erects upon this hypothesis an entire system. 12 the genesis of oke-deposits. 3. The Xenogenites in General. With relation to the xenogenites or mineral deposits, the first question concerns the space which every secondary mineral or mineral-aggregate requires to establish its existence. It must either have found this space waiting for it, or it must have made room by driving out an original mineral. Although we shall chiefly consider cavities formed in rocks after the formation of the rocks themselves, we must not forget that some may have been primitive in the rocks. We know that in substances of the greatest apparent density small cavi- ties or pores must exist, since we can, for instance, by adequate pressure, force quicksilver through them. Moreover, we en- counter in the eruptive rocks larger cavities, suited to receive considerable mineral-aggregates — the so-called blow-holes. These phenomena must certainly be considered, although the cavities of secondary origin will first be the subject of attention. With regard to the filling, I observe, first, that the mineral deposits upon the walls of cavities, from liquids circulating within them, usually have a characteristic structure, for which I propose the name " crustification,"'a8 a companion to " strati- fication." (Single crusts were formerly called mineral shells or scales ; and Groddeck introduced the word " crust," which is comprehensible in most languages.) Most frequently mineral crusts occur concentrically in regular succession, and fill the whole cavity (except the central druse), thus forming a symmetrical crustification. They cover, how- ever, not only the cavity-walls, but the surface of every foreign body in the cavity, thus forming crusted kernels which greatly complicate the phenomenon. We shall see, however, that a geode-cavity serves much better than a fissure-cavity to explain the relations of crustification, and that the crusted kernels will give us no trouble in that regard. Sometimes mineral crusts have undergone a secondary alter- ation (carbonates are replaced with silica, etc.). The crustifi- cation is thus made less distinct, or even obliterated. As a general rule, however, crustification is a characteristic feature of cavityfillincj. The cavities are formed either by mechanical or by chemical forces; and these two classes must be sharply distinguished, in view of the important role of each. The former may be THE GENESIS OF ORE-DEPOSITS. 13 the effect of exterior and foreign forces, or of such as are interior, residing in the rock itself. Formerly I called such spaces (with reference mainly to the accompanying fault-phe- nomena) "Spaces of Dislocation;" hut I believe the term " Spaces of Discission " (from scindere, to tear apart) would be more suitable. The latter class I formerly called " Spaces of Corrosion " (with reference to the effect of the leaching and attacking liquids) ; but I would now substitute the more self- explanatory name " Spaces of Dissolution." Spaces of dissolution naturally occur in soluble rocks, espe- cially limestone, and show, with wonderful clearness, the irregu- lar course often followed by underground waters. At and near the surface, we often find the cavity-formations at the contact of soluble with insoluble rocks; and we may infer that this relation affects also the subterranean circulation. Solution seldom extends to the whole mass of the soluble rock. Usually it affects a part only, in which it forms more or less irregular chains of cavities, sometimes so large that pieces of roof fall in, and thus spaces of discission are locally produced. A cavity filled with secondary mineral, however irregular its form may be, and even though it cuts across the stratification, usually shows a predominant course, which we are thus led to recognize as the channel of circulation of the liquid to which we owe the mineral deposit. As I shall show later, we must assume that the liquid which formed the space of dissolution also performed the filling ; in fact, that both processes were almost contemporaneous. Nev- ertheless, they must not be confounded with the metamorphic processes where the idiogenite is expelled, atom by atom, by the xenogenite ; for the deposits in spaces of dissolution show always a distinct crustification, and hence every single crust, at least, must have found free space waiting for it. Concerning the origin of spaces of discission, bo much has .been written that it cannot even be stated in abstract here. Two groups of these are distinguished. Those of the first group do not extend beyond one rock, and the force which produced them probably has its seat in that rock. In the eruptives, they are usually deemed fissures of contraction ; in limes and dolomites, J. D. Whitney called them gash-veins. The cavities of the second group extend out of one rock into 14 THE GENESIS OF ORE-DEPOSITS. another. The force which produced them resided outside of the formation. Considerable movements of one wall along the other are often evident, whence the common name, " fis- sures of dislocation." In a paper upon this subject*, about twenty years ago, I attempted to show that every fissure, in whatever material, must properly be a fissure of dislocation ; that the tendency to dislocation (namely, an unequal tension in the rock) precedes the formation of fissures ; and that whenever the condition of the rock permits, a dislocation of the fissure-walls can be always traced, even in fissures of contraction. As to the filling of spaces of discission, it must not be sup- posed that they represent throughout their entire length open spaces of uniform width. The original fissure was sometimes closed, wholly or partiallj'^, by the detritus originating in the friction of the walls, or by the movement or " swelling " of the country-rock, or by other causes. Only the places remaining open would permit an active circulation of solutions and a regular deposition from them. At points obstructed there would be no circulation, or a very sluggish one. When high pressure was present, and the rock contained interstices, the liquid doubtless penetrated from the fissure into the rock, im- pregnating it with mineral ; or a soluble rock was attacked, and spaces of dissolution were formed, to be filled in like manner as the fissure itself. This explains the fact that, on the same vein-plane, rich de- posits alternate with poor or barren spots, and that the miner, seeking a bonanza, persistently follows the barren traces of the vein, according to a well-known, fundamental law of pros- pecting. From the genetic standpoint, the richer portions are inter- esting as sometimes occupying more or less regular belts in the vein-plane, called " channels," " shoots," " chimneys," etc. These names evidently designate the main channels through which the mineral solutions passed ; and the occurrence of such forms in most kinds of deposits tends to prove that, not- withstanding other differences, they were all formed in a similar way. * "Geol. Betrachtungen fiber die Gangspalten," Jahrb. d. k. k. Bergakademien, xxii., Vienna, 1874. THE GENESIS OF ORE-DEPOSITS. 15 The primitive rock-cavities (pores and blow-holes) may also be filled with secondary minerals. In the former, there results a finely disseminated mineral substance, constituting such a deposit as Cotta denominated impregnation. Blow-holes are very often filled with minerals of the quartz family (opal, chalcedony, etc.), and we are often able to infer from the structure of such geodes the process by which they were filled. "Where the mineral solutions found no cavity already pre- pared, they must have conquered the necessary place by expel- ling a corresponding part of the original material. When one mineral individual was replaced by another, as in cases of pseudomorphs, the nature of the process can often be inferred from a comparison of the composition of the two ; and the laws thus discovered may frequently be applied to the problems of the origin of mineral aggregates. Many phenomena, however, even in the formation of pseudomorphs, are hard to explain, — the fact, for instance, that in some minerals the change com- mences within the mass and progresses outward, etc. Where the original material was expelled, there must have been first an access for the liquids which began and executed this effect. Such may be furnished by original minute rock- cavities, or by secondary cavities. The original substance of the greater part of the pseudo- morphs known to us was composed of soluble minerals, such as carbonates, sulphates, and chlorides, which also occur as the elements of rocks. Hence it may be inferred that metamor- phous or metasomatic deposits will be especially frequent in soluble rocks like limestone, dolomite, etc., and that we may also expect such deposits to occur frequently in company with those which fill spaces of dissolution. Pseudomorphs show us one substance in the crystal-form of another. This indication is lacking for the recognition of metasomatic deposits; yet sometimes the original rock was characterized by peculiar structure, such as lamination or jointing — as, for instance, the cellular structure of the Rauch- wacke (Cargneule), which is reproduced in the cellular calamine which has replaced it. Moreover, the original rock may have contained fossils, which have been replaced, with the rest, by the new mineral, retaining their form ; for instance, the bivalves and moUusks of the Bleiberg limestone in Carinthia and at 16 THE GENESIS OF ORB-DEPOSITS. Wiesloch in Baden, reproduced in galena and calamine ; the brachiopods of the Silurian iron-ores of central Bohemia, etc. Most important for the study of the process are transitional forms between the earlier and the later material ; for instance, coatings of the latter upon kernels of the former, such as limonite upon siderite or ankerite ; and likewise important is the occurrence of regular pseudomorphs, replacing one element in a heterogeneous rock, like those of cassiterite after feldspar in the granite of Cornwall. After the expulsion, atom by atom, of the original material, the resulting deposit must be massive, showing no crustification. Frequently, however, there are only negative indications of the metamorphosis. It can be seen merely that the deposit is not an original rock ; that it has not been deposited in pre- existing primitive or secondary cavities ; and hence, that it must have been formed by replacement. In general, two kinds of metamorphous deposits may be dis- tinguished. In the first, the new material has replaced the more soluble ingredients of a heterogeneous rock, and the re- sult resembles the description of an imjpregnation, in which the new material occupies the original interstices of the rock. In the second, a part or the whole of a homogeneous rock has suffered metamorphosis, and the deposit will bear a certain resemblance to filled cavities of dissolution. As I have shown above, and will illustrate further on with some examples, we may thus establish certain types of deposits entirely without reference to form. Some of these may coin- cide with groups in earlier systems, but others appear together in one and the same group. This seems" at first not to favor the practical usefulness of the above principles, but, as I have said, we do not yet know enough to frame a final system. That must be the aim of future studies, and it is obvious that our purely genetical factors will be more helpful than the arbi- trary characters based upon the exterior form of deposits. We distinguish, then, Idiogenites, or deposits contemporaneous in origin with the rook, from Xenogenites, the deposits of later origin, including not merely those of ores, but mineral deposits in general ; and to these we may add, in harmony with some older systems, the deposits of debris as a third class, Hysterog- enites, or latest formations. THE GENESIS OE ORE-DEPOSITS. 17 The Xenogenites we divide into such as penetrated pre-existing cavities (filling primitive cavitives, spaces of discission, or spaces of dissolution), and the metamorphic or metasomatic deposits, which made room for themselves by the expulsion of an earlier material. The form of all these deposits is not fixed, but depends upon various geological relations of the country-rock. The men- tion, under former systems, of regular forms of deposit, con- templated rather the ideal of the system itself. In reality, the ore-bodies in "veins" and "beds" are irregular, and form masses for which the most various names exist in all countries. "We must now speak more particularly concerning the method of formation of the different deposits. Probably no one doubts at the present day that they are predominantly the result of humid processes of solution and deposition. But such gener- alities are not enough. The processes alleged must be put upon the basis of actual causes, still operative, and capable of being proposed and discussed in explanation of geological phenomena. It is, therefore, necessary to introduce, at this point, the theo- retical chapter which follows. 4. The Subterranean Water-Circulation. In treating of the genesis of mineral deposits, this depart- ment cannot well be so lightly handled as it is in most text- books of general geology. Prof. A. Daubree, in an authorita- tive discussion of the subject,* ascribes the mineral deposits, among other effects, directly to the liquids circulating under- ground. It is my desire, with the aid of personal observations incidental to my continuous study of such deposits, to present a somewhat closer view than that of Prof. Daubree. Surface phenomena exhibit clearly a constant circulation of liquids, and corresponding phenomena, so far as they are observable underground, indicate the persistence of this con- dition, so that we must infer a subterranean circulation con- nected with that of the surface. We have then to consider, first, the surface-phenomena, so far as they concern our pur- pose, and, second, the underground phenomena. As to the former, we know that it is chiefly the solar energy * Les eaux souterraines a Vepoque actuelle, etc., vols. i. and ii., Paris, 1887 ; and Les eava souterraines aux epoques anciennes, etc., Paris, 1887. 18 THE GENESIS OF OEB-DEPOSITS. which initiates the circulation by lifting above the land the water of the sea, and thereby imparting to it the potential energy which is variously exhibited in its return to the sea. The mechanical effects of flowing water* in erosion, transpor- tation and sedimentation need not occupy us here. As to the chemical efl'ects, we know that the mineral constituents of the rocks, dissolved through this circulation, chiefly find their way in the rivers to the sea. In regions without drainage to the ocean, the dissolved minerals are concentrated by evaporation, which may lead to precipitation. I would remark, however, that in my opinion small proportions of salts are mechanically taken up in the evaporation of searwater,f as careful analyses of rain-water have proved, and that this fact leads to the ex- planation of the salt and salt lakes in regions without drain- age, etc. A. The Vadose Underground Circulation. In connection with the underground phenomena, the ground- water has for us a special interest. As is well-known, a portion of the atmospheric precipitate sinks, through open fissures or through the pores of permeable masses, into the rocks, and fills them up to a certain level. When in a given terrain, by wells or other openings, the ground-water (that is, the water- level, Grundwasserspiegel, nappe d'eau) has been reached at several points, it is found that these points are in a gently in- clined plane, dipping towards the deepest point of the surface of the region, or towards a point where an impermeable rock outcrops. The ground-water is not stagnant, but moves, though with relative slowness, according to the difference in height and the size of the interstitial spaces, down the plane mentioned, and finds its way, in the first instance, directly into the nearest surface-stream, or, in the second instance, forms a spring, which takes indirectly a similar course. Thus stated, free from all complications, the phenomenon exhibits clearly the law of circulation. The atmospheric moisture evidently de- scends; and even the movement of the upper la^'er of the ground-water is only apparently lateral, but really downwards *■ Die Wasserjdlle dea Niagara und ihre geologische Bedeutung, by F. Posepnf Vienna, 1879. f "Zur Genesis der Salzablagerungen, besonders Jener im Amerikauischen Westen," Silz. Ber. der k. k Acad. d. W. im Wien, 1877. THE GENESIS OF ORE-DEPOSITS. 19 and is determined (for equal sectional areas of the rock-inter- stices) by the difference in height between the water-level and the surface-outlet. For that part of the subterranean circulation, bounded by the water-level, and called the vadose or shallow underground cir- culation, the law of a descending movement holds good in all cases, even in those complicated ones which show ascending currents in parts. The total difference in altitude between the water-level and the surface-outlet is always the controlling factor. When these two controlling levels are artificially changed, as often happens in mining, the law still operates. In sinking a shaft through permeable ground, it is of course necessary to lift continuously the ground-water. The water-level thus ac- quires an inclination towards the shaft, which may thus receive not only the flow of the immediate vicinity but even also that of neighboring valley-systems. A shaft imparts to the pre- viously plane water-level a depression, giving it the form of an inverted conoid with parabolic generatrix. An adit produces a prismatic depression in the water-level ; and so on for other excavations. On the other hand, a bore-hole, from which the water is not removed, does not affect the water-level. Atmospheric waters falling upon impermeable rocks at the surface cannot penetrate them, but must join the existing sur- face-circulation. The rocks are usually covered with more or less detritus, in the interstices of which the ground-water can move; and the water-level is in most cases at the boundary between the permeable surface-formation and the impermeable rock below. These relations are complicated by the occurrence of fissures (which the ground-water of course fills), and by the communi- cation of such fissures in depth with permeable formations, which come to the surface somewhere at a lower level, though at great distance. In such cases, as is well known, a siphon- action is set up, and the ground-water of one region may find an outlet far away, even beyond a mountain range. Peculiar conditions are created by the occurrence of rela- tively soluble rocks, such as rock-salt, gypsum, hmestone and dolomite, in which, by the penetration of meteoric waters and the circulation of the ground-water, connected cavities are 2 20 THE GENESIS OF ORE-DEPOSITS. formed, constituting complete channels for the vadose circu- lation. It is often possible to observe directly, not only the formation but also the filling of these cavities, and thus to obtain valuable material for the explanation of the origin of xenogenites out- side the vadose circulation, and not observable in the stages of formation. It is for our purpose a most valuable fact, that the phe- nomena of leaching indicate the path of the circulating liquids through soluble rocks, so that we can study the process in its several stages. The water flowing at the bottom of a cave in limestone is unquestionably ground-water; and it follows that the whole complex group of cavities has been eaten out by it. If in another limestone cave we see no flowing water, the cur- rent must have found some lower outlet ; and the cave repre- sents for us an ancient ground-water channel. The many and various phenomena of the Karst region are well known : the Dolins, Ponors and Katravons — points where a surface-stream sinks into the earth ; vertical openings, at the bottom of which flow subterranean streams ; and caves out of which streams issue — illustrating the whole series of the entrance, the course and the exit of subterranean waters. In 1864, I had opportunity to observe, at MAros TJjvar, in Transylvania, a very instructive illustration of this kind, which is shown in Eig. 1. Here the rock-salt comes to the surface with steep zigzag stratification, and is covered only with detri- tus, to the depth of a few meters.* Mining is carried on in great parallelopiped-shaped chambers, by means first of levels run horizontally from a shaft, and win^zes sunk vertically from these. The workings were at that time 125 meters or 409 feet deep. A great difiiculty in the extraction was the entrance of saturated brine from that side of the mi^ie where the Maros river flowed by. Until the mine had been ;^otected by an adit of semi-circular course in the impermeable rock, surrounding the salt-body, the water annually raised and delivered without utilization into the river contained 84,000 tons of salt, or more than twice the weight of the rock-salt mined. * "Studien aus dem Salinargebiete Siebenbiirgens," by F. Posepny, Ja'irb. der k. k. geol. Beichsanst., 1867, xviii., p. 506-516. THE GENESIS OP ORE-DEPOSITS. 21 Various investigations have proved that the water of the river passes through the overlying detritus to the salt-body, which it penetrates at the boundary of the impermeable rock of the hanging-wall, finding its way through separate channels to appear as saturated brine, at the deepest point of the mine- workings. These channels had most frequently a cylindrical shape, smooth walls, and sometimes so great a diameter that a man could crawl in. There were always several to be seen, of which, of course, only the lowest in position brought the brine. The explanation is simple. The water from the river, reach- ing the salt-body through the detritus cover, acted at the border of the salt, where the principal depressions in the sur- face were located, and the saturated brine thus formed filled all interstices in the adjoining salt-body. By the leaching of such solutions into each deeper level opened in the mine, a line of maximum activity of circulation was gradually formed, which was followed also by solutions not yet saturated, with additional leaching and the final creation of open channels as the result. An example on a large scale of such a channel in rock-salt, created, however, without the aid of mining operations, was recently described by H. Winklehner,* who found among other striking phenomena of lixiviation in the rock-salt of the islands of the Persian Gulf, a horizontal natural channel or adit, on the island of Larak, which he was able to follow for about 1^ kilom. (1 mile). It expanded in places to caverns 12 m. (39 feet) high, without ever extending outside of the salt. In precisely the same way were formed the channels in other less soluble rocks, such as limestone, when, the level of the en- trance being above that of the exit of the ground-water, a line of maximum activity of circulation was established between the two points. This line, and the cavities developed along it, would not, indeed, always have the regular parabolic course, but would be dependent upon various influences of the stratifi- cation, the presence of rocks of unequal solubility, or even an intermixture of impermeable rocks. A mass of the latter, oc- * " Salzvorkommen in Siid-Persien," Oenterr. Z. fib- Berg-und Hiittemvesen, 1892 xl.,p. 581. 22 THE GENESIS OE OEE-DBPOSITS. curring on the line connecting tlie two points named, might cause the channel to bend up and down, or even in places to assume an upward inclination. Figs. 2 and 3 illustrate these conditions. S is the soluble, I the impermeable rock ; a, the entrance-point and z the outlet- point of the ground-water; a b c z, the line along which ap- proximately a channel might be made, if the impermeable rock were not present. In its presence, the dissolving current must take another road, adz, following more or less the contact be- tween S and I, and in Fig. 2, descending to a depith propor- tioned to the relation between the original rock-interstices and the hydrostatic head, while in Fig. 3 it first surmounts the dam formed by the impermeable rock, and then plunges towards the outlet z. We see that in this way various channels may originate at the contact of permeable and impermeable rocks, as indeed we find them often in nature. But when to these factors fissures are added, the conditions are essentially changed, for the circulation follows in preference the open fissures, and, if they pass through soluble rocks, en- larges them by solution. Sometimes the position and the level of the outlet are altered — as, for instance, in the progressive erosion of valleys ; and it may then. easily happen that the new channel, representing the new conditions, will take a totally difiterent direction, crossing the line of the old one. Siphon-action is to be observed in soluble, much more fre- quently than in permeable rocks, as the frequency of intermit- tent springs in limestone indicates. Such springs presuppose the existence of a siphon-like channel, through which the ground-water cannot flow to escape from the lower leg until the water-level has risen to the top of the bend of the siphon. We have seen that the ground-water may traverse deep fis- sures leading to soluble or permeable rocks, and may follow such rocks for considerable distances. When the ground-water warmed in depth, has an opportunity to reach the surface, such as is given in Fig. 6 by the difterence, H, in level, a thermal spring is the result — a so-called acroiherm-, if its water is not highly charged with minerals, and not unlike the ground-water of the place. Artesian wells present an analogous case, also explained hither- THE GENESIS OF ORE-DEPOSITS. 23 to by the principle of hydrostatic pressure (see Fig. 7). The outcrop of the permeable layer has been assumed to be neces- sarily higher than the mouth of the well, in order to account for the rising of the water above the latter level. The cause has been conceived as the operation of communicating pipes, the drill hole being one leg, and the permeable layer the other, and it has been overlooked, that the latter is no open pipe, but a congeries of rock-interstices, in which the water has to over- come a great resistance, and that, perhaps, in level regions no hydrostatic head at all can be demonstrated. Certainly the powerful factor of the higher temperature, and in some cases the gaseous contents, of the ascending water, were omitted from the calculation. It would be a matter of surprise to me, if the purely hydro- static and strictly mathematical views heretofore current on this subject had not led to disappointment. I introduce Fig. 7, the conventional diagram of an artesian well, for the purpose of stimulating further thought on the matter. The Filling of the Open Spaces Formed by the Vadose Circula- tion. — This is very important genetically, since it is a matter subject to current and direct observation, and capable of fur- nishing conclusions applicable to inaccessible subterranean occurrences. We can observe spaces on the bottom of which, frequently, the ground-water which excavated them is still flowing, and which are therefore filled for the most part with air. Liquids carrying various minerals drip into these spaces and leave a part of their contents on the walls ; the cause of deposition being, on the one hand, the evaporation of a part of the liquid, or, on the other hand, such changes as the loss of carbonic acid, precipitating as carbonate the soluble bicarbonate of lime ; the oxidation of soluble ferrous to insoluble ferric oxide ; the re- duction of ferrous sulphate by organic matter to sulphide, etc. The form and structure of these precipitates vary at difierent parts of the walls. On the roof occur the stalactites, and on the floor (if it be not covered with water) the corresponding stalag- mites. The wall-deposits have characteristic forms likewise; so that we can recognize by the appearance of any piece of the deposited mineral the place where it was formed. But from water covering the bottom of the cavity only horizontal 24 THE GENESIS OF ORE-DEPOSITS. deposits can orginate. Sometimes the cavity is contracted, so that its whole cross-section is occupied by the liquid. If it is accessible to observation, we can then see that th« deposits from the circulating liquid cover the walls uniformly. This can be much more clearly observed in artifical conduits, where precipitation occurs. We find, for instance, in the pipes which convey concentrated brine, the walls uniformly covered with a deposit, mostly of gypsum. But if air or gas is admit- ted into the pipes, the deposit occurs only at the bottom. We may thence infer that so long as the circulating liquid fills the whole cavity the attraction of the walls for the precipitated particles is controlling ; but that when gas enters, gravity be- comes predominant and draws these particles to the bottom. In opal and chalcedony geodes we can often see both forms of precipitate : the crust uniformly covering the walls, and the horizontal deposit. Fig. 4 represents a geode of iron-opal, from Dreiwasser, in Hungary, in which, besides the crustifica- tion and horizontal deposit, stalactitic and stalagmitic forms also appear. A thin crust of translucent hyalite covers all parts of the wall, including the floor. The cylindrical stalactites are also of hyalite. Some of them extend to the bottom, and are per- haps joined to stalagmites rising from the crust there. The re- maining space is half filled with a milk-white, opaque, opaline substance, in which occurs a thin layer of translucent hyalite. On the same specimen seA'eral other less regular cavities are visible. All of them were lined with the hyalite crust, and some have also the opaline layers. These layers are parallel in all the cavities ; and it cannot be doubted that they were horizontally deposited. The stalagmites stand at right angles to them, and were unquestionably vertical when formed. The geode certainly occupied, therefore, at the place of formation, the position shown in Fig. 4. -I must resist the temptation to describe the manifold forms of deposit in limestone caves. Fig. 5, an ideal diagram, show- ing a wall-accretion, and stalactites and stalagmites, separate and grown together, is given, not to illustrate the variety of the phenomena, but to indicate their analogy with those of the little geode in the iron-opal of Fig. 4. It is easy to conceive, that, under some circumstances, particularly in old cavities, lying above the water-level and not subject to further enlarge- THE GENESIS OF ORE-DEPOSITS. 25 ment, the formation of stalactites, etc., might ultimately fill the whole space. The floor of caves often shows deposits colored with ferric oxide, the explanation of which is obvious. Sometimes we find in the upper caves traces of sediments also ; and, in one instance I found in an outlet-cave pebbles of very hard rocks, which certainly came from the surface.* The chemical reac- tion of the formation and filling of these caves are so simple as to need no discussion here. Much more various observations, however, can be made in the artificial caves, formed by mine-workings. Here we have conditions analogous to those of the natural caves, but much greater variety, since the most widely different substances come into play. The mine-workings are situated at an artificially depressed water-level, and will show, in general, processes analogous to those observed in limestone caves, particularly the formation of stalactites. From calcareous rocks, from mineral deposits, and from the mine-masonry, crusts, stalac- tites arid sinter are formed, analogous to those which occur in cavities at the natural water-level. Processes of oxidation will here also play the leading part, although reduction may also be effected through the more abundant organic matter in the mine-waters. Thus stalactites of pyrites, evidently reduced from ferrous sulphate by organic matter, are often found in metal-mines. A respectably large number of observations already illustrates the processes which are going on under our eyes in mines, and from which we can draw conclusions as to the destruction and creation of many minerals by circulating under-ground solutions. But we must not forget that these proofs apply only to the conditions of the shallow or vadose cir- culation, and that, for the explanation of the formation of the more ancient deposits, we must look to the rock-regions below the water-level. In order to give at least one American example, I refer to the observation of Raymond, who found in an old Spanish mine, in the Cerillos range of N'ew Mexico, an iron pick-axe, the eye of which was filled with beautifully crystallized galena, ♦ ''Geol. mont. Studie der Erzlagerstiltten von KiSzMnya in S. O. Ungarn," von F. Posepny, Vrtc'. geol. Gesdlsch., 1874, p. 48. 26 THE GENESIS OF ORB-DEPOSITS. evidently a reduction of lead sulphate by the decaying wood of the handle of the pick.* It may be said, in general, that the results of the processes of oxidation, chlorination, and reduction, observed in those re- gions of ore-deposits which lie above water-level, have come to pass under conditions analogous to those just described; so that we are able to adduce extended series of proofs, not only as to formations now going on, but also as to similar formations long since finished. B. The Deep Underground Circidation. Thus far, we have considered only such processes as take place in the region above water-level, and are still, in some cases, open to our observation. As we descend to a deeper re- gion, there is less hope of encountering formative processes still active. When we penetrate by mining into the depths, we artificially depress the water-level, and create conditions unlike those which attended the formation of the deposits. But, if we compare the deposits formed below water-level, under proportionally greater pressure and at higher tempera- ture, with those of the upper region, it appears beyond doubt that the former also must have been produced by deposition from fiuid solutions. When we compare the low solubility of certain ingredients of the deposits with the spaces in which they occar, often in large quantity, it is impossible to assume that they could have been precipitated from solutions existing in these spaces only. We must concede that immense volumes of solutions must have flowed through the spaces — in other words, that the de- posits were precipitated from liquids circulating in these chan- nels. The formation of these cavities has been already discussed, and referred to mechanical and chemical causes. It remains to consider the manner of their filling. We have seen that the uppermost layer of the ground-water has an apparently lateral, but really descending movement; and it is very natural to imagine that this top layer slides, as it were, upon a lower mass, which is apparently stagnant. According to this con- * Trans. A. I. M. E., 1883, xi., p. 120. THE GENESIS OF OKE-DEPOSITS. 27 ception, the deep region would be comparable to a vessel filled with various permeable, impermeable, and soluble materials, over which water is continually passed, so that, from the moment when all the interstices have been once filled, only the uppermost water-layer has any movement. But, with increase of depth, the pressure of the water-column increases, as does the temperature. The warm water certainly tends to rise, if not prevented by interstitial friction, as is, no doubt, generally the case. But where the warmed water finds a half-opened channel communicating with the upper region, it will experience much less friction on the walls, and must evidently ascend. It might thus be conceived that the ground- water descends by capillarity through the rock-interstices over large areas,' in order to mount again through open channels at a few points. This subject was viewed by A. Daubree in a much wider significance, and extended to cover the origin of volcanic phe- nomena.* He propounded the inquiry, whether the enormous quantities of steam which are daily liberated from the deeper region are" continually replaced from the surface, and if so, how ? He pointed out that this water-supply could not take place through open fissures, in which the liquid water de- scended at one time and the steam ascended at another, but he showed that the descent could be effected through the porosity and capillarity of the rocks. Jamin's experiments have taught us the influence of capillarity upon the conditions of the equi- librium established by means of a porous body introduced be- tween two opposing columns. Daubree constructed an appa- ratus in which the temperature in one part of the capillary passage was so high that the liquid must assume the form of steam, and thus escape the operation of the laws governing its infiltration. This apparatus comprised a sandstone slab, with water above and a chamber below, the latter provided with a manometer for measuring the pressure of the steam collected in it. The whole was exposed to a temperature of about 160° C. (320° F.), and steam collected in the chamber of 68 cm. mer- cury-column, indicating about 13 pounds over the atmospheric pressure in the manometer, or a total pressure of about 1.9 at- * Synthetische Sludien zur Experimentalgeoloyie, by A. Daubrfee ; German transla- tiun, by Cr. A. Gurlt, Brunswick, 1880, p. ISO. 28 THE GENESIS OF ORE-DEPOSITS. mosphere. This steam could only come from the water above the sandstone through which, in spite of the pressure, a capil- lary filtration took place. The difference in pressure on the two sides of the stone not only did not drive the liquid back, but permitted it to filter quickly from the colder side (100° C. = 212° F.) to the hotter (160° C. =320° F.), and favored the rapid evaporation and the drying of the hot stone surface" {op. cit, p. 184). "According to these experiments', therefore, water may be found by capillarity, operating in the same direction as gravity, against a strong interior counter-pres- sure, to descend from the shallower and cooler regions to deeper and hotter ones, where, by reason of acquired temperature and tension, it is capable of producing great mechanical and chemical effects'" [op. cit, p. 186). Daubree's experiment confirms our view that the portion of the ground-water lying below water-level is not stagnant, but descends by capillarity, and since it cannot be simply consumed in depth, receives there through a higher temperature a ten- dency to return towards the surface, which tendency is most easily satisfied through open channels. Stated summarily:* The ground-water descends in the deep regions also through the capillaries of the rocks ; at a certain depth it probably moves laterally towards open conduits, and, reaching these, it ascends through them to the surface. The solvent power of the water increases with temperature and pressure, and also with the duration of its underground journeying. Hence, while it is descending, it can dissolve or precipitate only the more soluble substances. But the ascend- ing current in the open conduits is undoubtedly loaded more heavily and with less soluble substances, which, as the con- ditions of their solubility (temperature and pressure) gradually disappear in the ascent, must be deposited in the channels themselves. The open channels, in which the solutions ascend, are not the deductions of theoretical speculation. They really exist, as we can prove by induction from appropriate observations. The Ascending Waters Encountered in Mines. — A number of such phenomena are adduced by H. Miiller.f For instance, in * Ueber die Bemegwngsrichtxmg der unlerirdisch cinidirenden Fliissigkeiten, von F. Posepny. Exlrait du compte rendu de la 3me. session du Congris geologique interna- tional. Berlin, 1885, p. 71. t "Ueber die Beziehungen zwischen Mineralquellen und Erzgiingen." Cotta's Gangstuditn, vol. iii., 1860, p. 261. THE GENESIS OF OEE-DEPOSITS. 29 the Gottes Geschick mine, near Schwarzenbach, in the Erzge- birge, at the depth of 110 m. (360 feet) an acid spring contain- ing COjand H^S emerges from a nickel- and cobaltiferous-silver ore-vein {op. cit, p. 286). At the Wolkenstein Bad, an acid spring comes from the druses of an ore-vein containing a crust of barytes and amethyst. In the Alte Hoffnung Erbstollen mine, near Mitweida, bad air and exhalations of carbonic acid led, in 1835, to an analysis of the ground-water, which proved to be weakly acid. In the Churprinz mine at Freiberg a warm (25° C. = 77° F.) acid spring was struck on the Ludwig Spat vein at the depth of about 160 m. (525 feet). Besides these, Miiller names a number of mineral springs occurring in Bohe- mia and Saxony at the outcrops of mineral veins never opened by mining. In spite of the great reserve which he exhibits, he summarizes his view as follows {op. cit, p. 307): " Mineral veins and mineral spiings are certainly adapted to complement each other in genetic theory. On the one hand, the ore-veins, as extended, indefi- nitely deep fissures, gradually filled, indicate a very profound origin for the min- eral springs, and suggest variations caused by time and circumstances in the amount and mutual reactions of their contents, solid or volatile ; and, on ihe other hand, the present relations of mineral springs explain the mode of ingress and deposit of the constituents filling the veins." Soon after this publication (I think in 1864), a thermal spring of 23° C. (73° F.) was struck at a depth of 633 m. (1748 feet) in the Einigkeit shaft, at Joachimsthal, and in the same mine at two other points similar mineral springs, rising with strong pressure, were exposed. They prevented further in- crease in depth of that part of the mine, and were plugged as far as practicable. The analysis made in 1882 showed that they were acid springs containing considerable silica (33 grammes per ton). In one of them arsenic was also proved to the extent of 22 grammes per ton.* The mineral waters of the Joachimsthal mines are said to come in contact, near the place where they were encountered, with basalt-like rocks (called Wacken), which traverse the ore- veins, and are, therefore, of later origin. In general, most of * since the metric ton of 1000 kilo., or the weight of m.' (1 cubic meter) of water, is a rational unit of weight, I refer all tenors to it, and state them in grammes or milligrammes to avoid decimals. Thus 22 grammes per ton repre- sents 0.022 per thousand, or 0.0022 per cent. 30 THE GENESIS OF ORE-DEPOSITS. the ore-deposits of the Erzgebirge appear to have a decidedly recent origin, but even from this standpoint the mineral springs found in mining are to be regarded as nothing else than the continuation of those ascending liquids which have filled the ore-veins. Mining depresses the water-level, so that mineral waters circulating in the neighborhood are forced to those points in the mine where there is only atmospheric pressure. This " neighborhood " may, indeed, extend to a compara- tively long distance. For instance, the thermal spring at Carls- bad, which is the nearest to Joachimsthal, is 17 kilom. (10.5 miles) away and 38i0 m. (1246 feet) above sea-level, while the spring in the Einigkeit shaft at Joachimsthal was struck at 206 m. (675 feet) above sea^level, that is, 174 m. (571 feet) lower than Carlsbad. The irruption of the thermal waters of Teplitz in Bohemia into the lignite-mine of Dux, 7 kilom. (4 miles) away, which took place first in 1879, and has occurred re- cently since, shows plainly that subterranean communications may thus be established for long distances by mining,* Additional data for the study of these relations are furnished by the miners on the Comstock lode, where, with the advan- cing depth of operations, ascending thermal waters were un- expectedly encountered, the abundance and high temperature of which presented extraordinary obstacles to mining. The great richness of the deposit was the reason that the hope of going deeper was not abandoned, as in Joachimsthal, where the only eftbrt was to dam out the waters from existing work- ings ; but that, on the contrary, the struggle was accepted against the waters themselves and the enormous heat which they caused in the mines. As is well known, the upper workings on the Comstock, be- fore any ascending waters had been encountered, were not specially hot, though warmer (21° to 24° C, or 70° to 75° F.) than other mine-workings in similar positions. Dr. F. Baron V. Eichthofen noticed no abnormal mine-temperature, although he ascribed the Comstock to earlier solfataric action. f * " Einige, die Wassereinbriiolie in die Duxer Kohienbergbaue betreffende, geologische Betrachtungen," von F. Posepny. Oesterr. Zeitsch. f. Berg-u. Hiiltenw., 1888, xxxvi., pp. 39-54. t The Comstock Lode, Its Character and Probable Mode of Continuance in Depth, San Francisco, 1866, p. 54. THE GENESIS OF ORE-DEPOSITS. 31 At a later period, upon the cutting through of clay-partings in the rock, the hot-water repeatedly broke into the workings with great force, as, for instance, in the North Ophir mine, when, according to Clarence King,* the workmen had scarcely time to escape. The water is said to have had a temperature of 40° C. (104° F.), and filled the workings immediately to a height of 30 m. (100 feet). In another case the water broke into the 2200-foot level of the Savage mine, and filled the large spaces both of that mine and of the Hale and ISTorcross up to the 1750-foot level, or to a height of 137 m. (450 feet). Gas was continually but not violently evolved ; and although Prof. J. A. Churchf reports it to have been under a pressure of 200 pounds per square inch, he believes that this was not a gaseous, but a hydrostatic pressure. The water which in 1880 flooded the Gold Hill mines came from a bore-hole in the Yellow Jacket shaft, at a depth of 939 m. (3080 feet); had, according to George F. Becker,J a tem- perature of 77° C. (170° F.) ; and was heavily charged with hydrogen sulphide. In the upper levels of the mine, Becker says there is evidence of the presence of carbonic acid, and on the 2700-foot level where the temperature was 66° C. (150° F.) a deposit of sinter was found, consisting mainly of carbonates. Church (p. 206) remarks that it was at first believed that the repeated irruptions of water came from chains of ca\dties exists ing in the rock, but that at the time of his visit the conviction was that they came through shattered and decomposed seams, parallel with the lode, and sometimes of great thickness. Systematic and long-continued temperature-observations in several Comstock mines enabled Becker to represent compre- hensively for different lines the increase of temperature with depth ; and it thus appeared that this increase was greatest in the vicinity of the lode, diminishing with the distance from the lode ; that the vehicle of heat was the water ; and hence that it was through the lode itself that communication with the hot depths took place, and the phenomenon denominated " solfataric action " by Eichthofen was caused. * v. 8. Oeol. E/pl. of the iOtk Parallel, vol. iii. Mining Industry, Washington, 1870, p. 87. t The Comstock Lode, Its Formation and History, New York, 1879, p. 207. X "Geology of the Comstock Lode," etc., U. S. Oeol. Survey, Washington, 1882, pp, 230, 386. 32 THE GENESIS OF ORE-DEPOSITS. The chemical constitution of these intruding waters will be considered further on, after certain phenomena occurring nearer to the surface have received attention. Related Phenomena Near the Surface. — A sort of transition to the corresponding phenomena on the surface itself is illustrated by the mines at Sulphur Bank, Cal., which have furnished some of the most important data contributed by America to the study of the genesis of ore-deposits. This is a once rich, but now (apparently) practically exhausted quicksilver-mine, in the working of which not only thermal waters but gaseous emanations were encountered as obstacles. At the time of my visit in 1876 an open-cut exploitation was in progress, the terraces of which had extended in some places about 5 m. (16 feet) below the natural surface. Sulphur, as well as quicksilver, was won ; but it subsequently appeared that the sulphur-deposit was confined to the uppermost zone, while the quicksilver (or cinnabar) extended in considerable pro- portions to deeper regions. At that time I found sulphur and cinnabar in a decomposed basalt, partly as the filling of irregular fissures, traversing the rock in all directions, partly as impregnations in the rock itself, which had often been reduced to a porous mass. The process of decomposition proceeded unquestionablj' from the fissures, which, moreover, gave forth hot mineral waters and gases. The odor alone was sufiicient proof that the gases contained HjS, to the oxidation of which into SH^O^ the acid reaction of the rock and its moisture was to be ascribed. The miners (mostly Chinese) chiefly followed in extraction the fissures (partly be- cause it was the easiest way to make rapid progress; partly because the richest ores were there concentrated) ; and, as a result, large round blocks, often several meters in diameter, were left standing. These had a distinct shaly structure, but were so loosely held together that a kick would reduce them to ruins. In the interior of the larger, light-gray blocks, was often found a nucleus of solid, dark, undecomposed rock. (Some of these nuclei I have added to the collection of the Przibram Mining Academy.) The cracks were filled chiefly with an opaline mass in which a white, opaque ingredient was variously kneaded, as it were, with a gray to black one, translucent at the edges. The speci- THE GENESIS OF OKE-DEPOSITS. 33 mens taken fell into irregular pieces, bounded by fissures, evidently the result of loss of volume or loss of moisture by the opaline mass. The cinnabar formed either distinct mineral crusts in the crevices or impregnations of the porous neighboring rock. This was true of the sulphur also ; only, the latter appeared, as a rule, in crystalline aggregates upon the cinnabar crusts — • an indication of its later origin. Occasionally the cinnabar was deposited in beautiful crystals on the fissure-walls, but these were generally so loosely attached that it was ditficult to secure a specimen. The pyrites, mostly disseminated in the rock, tended so strongly to decomposition, evidently' by reason of its saturation with sulphuric acid, that specimens containing it soon fell to pieces. These observations sufiice to show that in this case hot mineral-waters ascend through fissures containing ore-crusts and opaline deposits ; and when it is considered that the de- posit of amorphous, hydrated silica is unquestionably the work of the mineral water which decomposed the rock, and, also, that the cinnabar occurs in the interior of the opaline mass, the two phenomena cannot well be separated, and it must be assumed that a metallic sulphide has here been deposited from ah ascending spring. Fig. 10 represents the exposure as sketched in my note-book. Later developments exhibit these relations still more clearly. Le Conte and Becker* found a shaft 50 m. (164 feet) from the basalt, about 92 m. (302 feet) deep in sandstone, from which drifts had been run northward at difterent levels under the outcrops of the deposit. It is to be regretted that their reports are not accompanied with precise descriptions of the mine-workings. In the third level (64 m. = 209 feet below the surface) the drift was 70 m. (230 feet) long, " cutting through the ore-body and reaching only barren rock on the * " The Phenomena o£ Metalliferous Vein-Formation, Now in Progress at Sul- phur Bank," by J. Le Conte and W. B. Rising, Am. Jour. ofSci., xxv., p. 424. "On Mineral Veins, Now in Progress at Steamboat Springs, Compared with the same at Sulphur Bank," by J. Le t'onte, Am. Jour. ofScL, xxv , p. 424. " Geology of the Quicksilver-Deposits of the Pacific Slope," by G. F. Becker, Monograph U. S. Geol. Surv., Washington, 1888, p. 251. 34 THE GENESIS OE ORE-DEPOSITS. other side. The fourth level has been pushed 31 m. (101 feet), and has reached the ore-body." From these data it. is hardly possible to form an idea of the position of the ore-body traversed. The data given concerning the interior structure of the de- posits are, however, important. Sandstones and slates are here broken up by fissures in such a way as often to form a breccia. Whether the fragments belong together, and whether they present the relation which I have denominated typhonic, is not stated ; but it may be inferred from the sketch of an ore-speci- men from this place that the fragments do not belong together, and that their condition has been brought about by more ex- treme dislocations. The subject is highly important for us ; and I have attempted in Fig. 11, although the original is not before me, to represent it according to Le Conte's sketch, so as to place it side by side with other phenomena thoroughly familiar to me. The fragments of slate and sandstone have somewhat rounded edges, and leave varied interspaces, which are filled, partly with a still soft or already indurated paste, containing finely dis- seminated metallic sulphides, partly with cinnabar, for the most part in coherent crusts. A part of the space is usually empty, exhibiting what I call a central druse. Sometimes, it is said, the rock-fragments are cemented together with massive cin- nabar, and kernels of rock crusted with cinnabar occur fre- quently. Hot mineral water and gases carrying H^S force their way through the interstices of the deposit, as was the case observed in the upper zones. The silica deposits are found in all stages of consolidation, from a gelatinous mass to chalcedony and (Leconte, op. cit, p. 29) alternate with layers (crusts) of metallic sulphides (cinnabar and pyrites). Becker examined the whole neighborhood, and extended his studies to similar ore-deposits of the region. He does not consider the basalt of Sulphur Bank, as do G-. Rolland* and Le Conte, to be a lava-stream, but takes it to be an eruptive rock, originating on the spot, which has overfiowed a fresh-water formation of recent age. The bottom proper is a Cretaceous sandstone. The ore-bearing character extends from the basalt (about 16m. =52 feet thick) * "Les Gisements de Mercure de Californie," Anndes des Mities, 1878, p. 26. THE GENESIS OF OKE-DEPOSITS. 35 through the fresh-water layers in to the Cretaceous sandstone. Concerning its relations in the middle layer we have no data, which is unfortunate, since the effects of the acid waters upon this calcareous material must have been considerable, and it is not unlikely that the deposit had in this region a totally differ- ent character. Fresh-water formations adjoining the deposit have preserved to a remarkable degree plant-roots, etc., trans- formed into lime carbonate ; and it would be very instructive to study their forms as metamorphosed by the mineral water. Concerning the chemical constitution of the warm (80° C.= 176° F.) water, I shall speak further. According to Becker's analysis {op. cit., p. 259), it is extraordinarily rich in chlorides, borax and sodium carbonate. The gas liberated from it often proved to be ammoniacal, and consisted in 1000 parts of 898 parts CO2, 2 parts H,S, 79 parts CH, (marsh gas) and 25 parts nitrogen. As to the presence of other metals besides mercury, it is worthy of mention that Dr. Melville found small amounts of gold and copper in the marcasite accompanying the cinnabar, and that G. Becker found in the efflorescence from the mine- workings, besides the substances detected in the mineral water, traces of cobalt and nickel. As will be seen, this deposit furnishes genetic data, concern- ing not only the ores of quicksilver, but also those of other metals. An ascending mineral spring here passes from the deep into the shallow region, and suffers, besides the reduction of pressure and temperature, the oxidation of its H^S, from which result a strong acid and the deposition of sulphur nearest the surface. In depth no sulphur is found, but sulphides of quicksilver and iron, upon or within deposits of silica, both being in dis- tinct alternating mineral crusts. It cannot be doubted that cinnabar and pyrites on the one hand, and silica, on the other, have been precipitated from the solution which still ascends in these channels. At most, it may be doubted whether this pre- cipitation is still going on. Le Conte adduces in support of the probable continuance of the process the occurrence of silica sometimes gelatinous and soft, as if recently precipitated. Becker and Melville tried to obtain direct evidence of the pres- ence of quicksilver dissolved in the ascending mineral water of 36 THE GENESIS OF OUE-DBPOSITS. to-day, but their careful investigations failed to find it. Al- though the water contains ingredients in which quicksilver is soluble, there is no quicksilver dissolved, and it must have been already precipitated by some agent — as they suggest, ammonia. There are among geologists unbelieving Thomases enough, who will believe in the presence of quicksilver in the mineral solution only when it has been actually precipitated for them ; but there are those, on the other hand, who are convinced by the evidence thus far gathered that the sulphide deposits of this locality proceeded from the ascending thermal spring, whether the process of precipitation is still going on or not. Equally weighty data are furnished by Steamboat Springs in Nevada, to which Laur and J. A. Phillips first called attention, and which Le Conte and Becker investigated thoroughly.* In a valley surrounded with eruptive rocks, but underlain chiefly by Archaean rocks, thermal springs may be seen at several points emerging from north-and-south fissures. The action of these springs has covered the ground with a sinter-deposit, predominately of lime carbonates, about 15 m. (49 feet) thick. In this sinter may be traced many fissures, here and there still open, but mostly closed by the deposit of silica on their walls. According to a sketch given by Le Conte, these very clearly crustified deposits extend somewhat above the general level of the surface, forming single mounds or chains of mounds. From some of them hot vapors and gases still issue, chiefly COj containing HjS. In others, such emanations have been so greatly diminished that only by listening can the liberation of vapor in depth be perceived. Some of the fissures are com- pletely filled, and give forth neither mineral water, steam nor gas. In the group, about 200 m. (656 feet) wide and 1 kilom. (0.6 mile) long, which lies nearest to the railway-track, these phe- nomena are most strikingly exhibited. Besides the principal * M. Laur, "Surle gisement et 1' exploitation de I'or en Californie," Annales des Mines, 1863, iii., p. 423. J. A. Phillips, Phil. Mag., 1871, xlii., p. 401. Also ^ Treatise on Ore-Deposits. London, 1884, p. 70. J. Le Conte, "On Mineral Veins now in Progress at Steamboat Springs Com- pared with the Same at Sulphur Bank," Am. Jour. Sci., xxv. , p. 424. G. F. Becker, ' ' Geology of the Quicksilver- Deposits of the Pacific Slope, " 3Iono- graph U. S. Oeol. Survey, Washington, 1888, pp. 331. THE GENESIS OF OKE-DEPOSITS. 37 substances mentioned below in the table, Becker found in this mineral water also small amounts of metallic compounds, as, for instance, HgS, a trace of 'S&.^S, 1.0 gramme per ton of !N'a28bS3, and 8.7 grammes per ton of ISTagAsSj. About IJ kilom. (1 mile) to the west is a group of similar fissures, yielding some steam and COg, but no mineral water. In the mineral crusts of these, however, several metallic sul- phides occur. In 1863, Laur declared that he had seen in them distinct traces of gold. In 1878, one of these fissures was opened by an adit, about 15 m. (49 feet) under the surface, and produced a vein-matter carrying cinnabar, which was mined for a while as quicksilver-ore. The temperature of this mine was not so high as to cause serious trouble to the work- men. G. F. Becker carefully analyzed the filling of several fissures, and found, besides hydrated ferric oxide, considerable quanti- ties of Sb, As, Pb, Cu, Hg sulphides and gold and silver, as well as traces of Zn, Mn, Co and M. Since from 1 to 3.5 kilog. (2.2 to 7.7 lbs.) of the vein-stufl' were employed for each analysis, the results are specially trustworthy, and I give the records of three analyses here, expressing them in grammes per ton (1 ton = 1,000,000 grammes) : Sulphides of antimony and arsenic, Ferric oxide, Sulphide of mercury, Lead, . Copper, Gold, . Silver, I. II. in. 23,000.0 150.0 2,500.0 1.4 2.5 1.0 88.0 21.0 0.3 12.0 0.9 1.0 0.3 0.3 (Considering the gold and silver to be alloyed in the above proportions, we should have bullion 0.750 and 0.769 fine, which is the general grade of the so-called " free gold " of Transyl- vania.) The careful study of the phenomena, particularly by G. F. Becker, leaves no doubt that in this case ascending mineral waters have deposited, besides the various forms of silica (from opal to crystalline quartz), different metallic sulphides, and that the fissure-fillings exhibit a very clear instance of crustifi- cation. It is, indeed, not proved that the process is now going on. But that is not the main point. We may be content to have the proof that it has taken place. 38 THE GENESIS OF ORE-DEPOSITS. Mineral Springs at the Surface. — When we isolate a spring characterized by high temperature, a large quantity of gas or of matter in solution, we notice at once that its level is higher than that of the ground-water. The more thorough the isola- tion or walling-in, the more striking is this phenomenon, so clearly unlike that of the vadose or shallow circulation. Isolation is usually performed by digging as deep as possible, so as to get at the spring below the loose surface-material in an impermeable rock, and then, by building a well-pit, to give it freer ascent. But since the circulation of the ground-water in the loose surface is very lively, the necessary depression of the water-level in such an excavation involves the lifting of large quantities of water. Moreover, the escape of the gas from the mineral spring often hinders the operation ; so that there is, as a rule, little opportunity for thorough investigation. Cases in which accurate observations have been properly re- corded for preservation are very rare. The first good fissure encountered in the bed-rock is deemed to be the channel of the mineral spring, and the well is built over it. Complete isolation from the ground-water is proba- bly seldom practicable. ISTevertheless, the mineral spring, being under higher pressure than the ground-water, Will tend to exclude it from the well. The imperfection of the isolation is shown, however, when we try for any reason to pump out the well. To lower the water-level, say 1 m. (3.28 feet), we have to raise manj^ times the amount of water which the spring itself would normally furnish (even taking into account the de- creased pressure, which affects the fiow in the proportion of the square root of the head). The excess, generally surprisingly great, comes from the ground-water which finds its way into the well. If we allow the mineral water to ascend again quietly in the well, the level rises at first rapidly, then slowly, and finally remains (in the absence of change in the height of the ground- water and in the barometric pressure) stationary at a certain height above the ground-water level. This difference of height represents the ascensional force of the mineral spring. If the spring makes a deposit at its mouth (mostly of lime carbonate, hydrated ferric oxide, and silica) it may thus build a conduit, extending above the ground-water level and the sur- THE GENESIS OF ORE-DEPOSITS. 39 face to the height represented by its ascensional force. Thus, we find conical mounds from the top of which mineral springs flow. This phenomenon is shown in the highest degree by geysers, z.e., thermal springs in which paroxysmal developments of steam and gas occur, often forcing the water to notable heights. Some of the magnificent geysers of the Yellowstone IS'ational Park have built chimney-like conduits of considerable size. Their structure has much similarity to that of stalac- tites ; indeed, we may recognize generally, in the various de- posits of ascending mineral springs (in other words, in the products of the deep circulation), many analogies with the vadose circulation. This circumstance indicates a relation be- tween the phenomena of the two regions which is often entirely ignored or even denied. While, for instance, the geysers have a temperature above boiling-point, some mineral springs rise but little above the mean local temperature of the surface or of the ground-water. This may be especially observed in the acid springs ; yet, these are also ascending springs, and must have been formed in the deep region. "Within the vadose region we have, sometimes, ascending waters, which are, however, mostly to be explained by hydro- static pressure. But, within the deep region, hydrostatic pres- sure can play no part ; and here it is the higher temperature and the presence of gas which cause the ascension of mineral springs. The extreme instances of this kind, such as geysers, steaming springs, mud- volcanoes, petroleum springs, etc., nobody will undertake to explain by hydrostatic pressure, and more moderate results of the same factors can scarcely, with con- sistency, be so explained. It is a striking circumstance that ascending springs occur chiefly in the neighborhood of the later eruptive rocks, such as trachyte, basalt, etc. This is emphatically the case through- out the zone which crosses Europe from west to east, in Prance, Germany, Bohemia, Hungary, and Transylvania. Here the warm springs and the acid springs occur thickly, while north and south of this zone they are only sporadic. Their connec- tion in the zone with the eruptive rocks is evident, and they are often considered as the last echoes of the processes of erup- tion. The sporadic springs, in places where eruptive rocks 40 THE GENESIS OF ORB-DEPOSITS. play no part, must have come through deep fissures of dislo- cation. For example, the line of the fault along which the Alps sank below the Tertiary basin of Vienna is marked by a complete series of thermal springs. This circumstance has another and far-reaching significance. For ore-deposits are similarly distributed. They are most numerous and most closely grouped in the neighborhood of eruptive rocks, especially extended zones of eruptive rocks, as in the American "West, and in Hungary and Transylvania, while among other rocks they are fewer and more scattered. Chemical Constitution of Mineral Waters. — Ascending mineral springs have widely varying composition ; some, like the " aerotherms," representing strictly only warmed ground-water, while others are strongly mineralized, and carry some sub- stances almost to saturation. The material bearing on this sub- ject is too voluminous and heterogeneous to be fully cited and discussed here. I must be content with the exhibit of a few analyses, specially interesting for the present purpose. The following is a list of the localities, etc., represented in the table below : Waters Encountered in Mines. No. Locality. Temperature. °C. "F. 1 Gottesgeschick mine, Sohwarzenberg, . 11. 51.8 2 Einigkeits shaft, Joachimsthal, . . .28.7 83.7 3 The "Sprudel," in Colliery at Briix, Bohemia, 4 Comstoek, Savage, 600-foot level, . . 28. ? 82.4 5 Comstoek, Gould and Curry, 1700-foot level, 48. ? 118.4 6 Comstoek, Gould and Curry, 1800-foot level, 50. ? 122. 7 Comstoek, Hale and Noroross, . . . 70. ? 168. 8 Comstoek, Ophir, 21.1 70. Authority. R. Eichter. J. Seifert. J. Gintl. S. W. Johnson. S. W. Johnson. S. W. Johnson. S. W. Johnson. Attwood. Water in Ore-bearing Fissures. No. Locality. 9 Sulphur Bank, Herman shaft, 10 Sulphur Bank, Parrot shaft, 11 Steamboat Springs, . No. Locality. 12 Sprudel, Carlsbad, 13 Kreuzbrunn, Marienbad, . 14 Wiesenquelle, Franzensbad, 15 Urquelle, Teplitz, Temperature. °C. °F. Authority. 70. ? 158. G. F. Becker. 70. 158. G. F. Becker. 75. 167. G. F. Becker. \al Springs. Temperature. °C. °F. 64. 147.2 12. 53.6 13. 55.4 50. 122. THE GENESIS OF OEE-DEPOSITS. 41 Weak and Strong Mineral Springs. No. Locality. Authority. 16 Ottoquelle, Giesshiibel, Dr. Novak Kratsclimann. 17 Josephsquelle, Bilin (1875), 18 Puits de 1' EdcIos des Celestins, Vichy, 19 Rippoldsau, Josephsquelle (1875), 20 Rippoldsau, Wenzelquelle (1875), 21 Rippoldsau, Leopoldquelle (1875), 22 Kissingen, Pandurquelle (1856), 23 Kissingen, R4koczyquelle (1856), Dr. Ruppert. Bunsen. Bunsen. Bunsen. Bunsen. Liebig. Liebig. 24 Yellowstone, Cleopatra, Mammoth Hot Springs (1888), "1 F. H. Gooch. 25 Yellowstone, Grand Geyser, . . . . i T. E. Whitefield. It is well known that analysts in combining their results do not follow the same rule. One supposes a certain acid to be united with an alkali ; another gives the same acid to an earthy base, etc. "What interests us in the comparison afforded by the table is the substances occurring in large proportions, the car- bonates and sulphates of the alkalies and alkaline earths ; the chlorides, the silica, and the quantity of organic matter (if it were determined by a uniform procedure). I deem it most convenient to take 1 ton of 1000 kilogrammes (representing, for waters not too rich in mineral, the weight of 1 cubic meter), and to express the weights of the salts in grammes, to avoid decimals. In order to show the relations of the salts, one to another, it is well also to represent them on the basis of 1000 parts of the solid matter. For the Comstock waters, the rationally-stated analysis of S. W. Johnson, from the 600-foot level of the Savage mine (C. King, op. cit., p. 87), served me as a guide, according to which I have recalculated the figures (Church, op. cit., p. 204) for other mines and levels. These analyses show the irruptive waters on the Comstock to be poor in dissolved substances. According to the deter- mination of solid residuum by E. S. Bristol (C. King, I. c, p. 88), this would not be the case. He finds the mine-water of the 500-foot level to contain in the Savage north drift 2660 grammes, and in the Yellow Jacket west drift as much as 3271 grammes of solid material in one ton (1000 kilos). But it is a question whether these figures do not refer to ordinary mine- waters, as the term " west drift " seems to indicate. The predominance of sulphates over carbonates is nothing unusual ; but the decided predominance of lime, sulphate or a a &2 a O 00 as OS o as CO (N I> CO i-H o -^ CO CO CO lO lO OJ-^ (N CD : CI -^ uO O lO "^ : o .-( lo CO l-H CO : CO : :o : Cq COrfl OS OS as 00 OS OS as .0 00 1-1 CO 10 CD 00 CO ir- t- Tf :c : lO i-H CO TtH 00 CM : CD OS S5 »o osr^ r^ : t- -^ CD 05 i-H : r- ^ (N ■ 1— 1 1—1 CO l> CO Oi lO f- »o ; lO l> OiOO 10 : (M (M G^ • 1—1 1—1 CO t- rH Tf COOO i-H '^ Oi CO 10 "SI 10-* : CO CD CO : LO ■0 I— ( Tji CO'* I> 00 lO CO lO Oi CO rH CO rH : CO OS CO CO 10 .-1 CD CO CO 10 rH as 10 : CO 10 i lO CO CO CO 00 CO rH C^ 10 CD -* i-H ^ 1—1 00 ir- rH as CO CO CO i-l CO CO rH CO CO rH Tji : rH CO CO 10 OS CO t^ CD CD rjio rH iH lO 00 : t^ CM : 1—i 1— 1 1—i l-H S c3 00 CO 1-1 10 CM rH CO : CO t^ : :o : CO CM 1— ( CD CO t^ (M CM * -(- : r-i rH 10 ; CO OS C<1 . rH CO CO CM lO 00 CO CD 10 ur3 : OS ; I— t CO L^ -i— (M 10 CO CO S §8 10 00 (r- (M 00 10 t-Tfi CMrH CO CD : : •^ : ; CD CO CO CM CO : CO ; 05 as as ■* : : -^ : : CO as 00 CM CD : CM : 00 1-1 ; ; ■^ r-( O) -<* CO : CM : OS CO -* ia : 1—1 ; t-, Oi t- CD 00 (M CD Oi CM CO lO I> co CM 10 Cq 10 lO ^ CO CO rt • ^ : CD OOCM 10 i-l 00 l-( LO rH •71 :co I I 00 ■S- f^- "73 -^ rS -S ^ »3 !S a c3 f-< o w s a oi a O CO CD SO 10 00 ir- cq : ■*« rH CO CM :■* cq CM -«Ii CO 10 CO : as CO cq i- CO cq :co CO i-{ i-H CO r-i ,-^ ^r-i : CO : rH XO rH I> S Tfi ■ rH r-^ CM CO CD rH t> : CO rH rH CO : CD CO l-H CO CM 00 Cq CO CD : CO I-l CO CM CO : Tfi ■'^ I— I : CM CM CM rH :^^rH rH 10 CM CM CM CO rH CO CO •* : i> i—t t* r^ 1— ( rH CO CO 000 CO CO : J> : CO rH rH - 10 CO I> lO 00 rH : rH CO : CO CM 1-i l> CO CO I> 00 CD CM 10 r-i r-i 00 t^ 00 CD CO :cjs ^ r- 10 ; r-l S OiOO CO 00 rfH lO CO 10 1-^ 1—i-rp : r-i 1—i rH -*l OOOS CD CM rH 00 CMO :CD r-i CO -=r t^cO 01 1-- H— r- CD : CD CO — rH : lOrH ^ r^ CM 00 CqOi -K- ^ £- " -^^ : -^t as T-l lO " CO lO 10 c- — >- Q CO r-i : CM t^ CO CD C^ lO CM CO CO TP lO CM OS CM QO l-H rH_.rH ■^ 10 10 ^0-^t^ rH OS : : rH CM CO C^ '^ : : ^ r^ 10 "* CM 00 00 CO CM CD rH tt ' : CM 05 CO rH 00 1> OS CM "* 0; -^ I-- >— f : r- t^ rH CM 1-- CM rt< 1^ rH CM rH r-i OS lO "* CO rH Cq : rH cq : r-i Cq CO CD tT CO CO Tfi :co CD CM J -JS ^- 3 : s "3 § OJ H <1Mfagyb4nya and Felsobdnya, where several domes of trachyte or of andesite, breaking through the late Tertiary "Congerien" strata, are in turn traversed by large veins, which split up near their outcrops, so as to exhibit in vertical cross-section a fan-shaped arrangement. * " Der Bergbau von Schemnitz in Ungarn," Jahrb. d. h. k. geol. B. AmtalL, 1867, p. 403. ob THE GENESIS OP ORE-DEPOSITS. Further east is the Kapnik mining district, containing a series of separate veins; then comes Rota, similar in character; and finally (over the line in Transylvania), the district of Olahldposbdnya, the veins of which are partly in the eruptive rock, partly in the old Tertiary strata which it traverses. Throughout the range, silver-ores predominate, occasionally with a considerable gold-value. In the eastern portion, copper- ores appear. The Dacian Gold-Field. — In southwestern Transylvania, in the Dacian gold-district, all the gold-mines are grouped in con- nection with four separate eruptive zones of recent origin. The main rock of the region is Cretaceous sandstone, with oc- casional exposures of Jurassic and Triassic strata, the latter of which include heavy outflows of melaphyre, and also masses of crystalline rocks. The recent eruptives, comprising por- phyry, diorite, andesite, basalt, etc., occur in a triangle, the base of which is formed by the widest range, the Cietrasian, which strikes WW. and SE., and in which are the mines of ISTagyag, Magura, Fiizesd, Boiza and Ruda. In a second, ap- proximately parallel range, are the mines of Faczebaja and Alm4s ; in a third, those of Yulkoj and Verespatak ; and in a fourth, forming the apex of the triangle, those of Offen- b^nya.* These mines, which are for the most part very ancient (pre- E-oman), I shall treat fully in a monograph now in course of preparation. In the whole Dacian gold-district the predomi- nant deposits are fissure-veins, sometimes represented by mere " knife-blade " seams, continuous for short distances only. In some places, as in the celebrated Verespatak district, other types of deposit are represented, the ores of which, however, also occur in spaces of discission, namely, in eruptive breccias, between the related fragments, in the form which I have else- where called typhonic masses ; but these are ore-bearing only where they are in contact with the ore-veins. The same is true of the conglomerates into which these breccias sometimes pass, and in which the ore takes the place of the interstitial cement, as I have explained in a preceding chapter, and illus- * F. Posepn;^, " AUgem. Bild d. Erzf iihrung im Siebenb. Grolddistrikte, " Jahrb. d. k. k. geol. B. Anstalt., xviii., p. 297. THE GENESIS OP ORB-DEPOSITS. 87 trated in Fig. 18. For further elucidation, I show in Fig. 41 a breccia, and in Fig. 42 a conglomerate. (It should be ob- served that the mutual relation of the fragments of a breccia can be recognized only when they have not suffered much movement after fracture.) In both these specimens, the rock is quartz-porphyry with quartz-crystals of pea-size. In Fig. 41 the interior of the fragments is considerably decomposed, whereas the exterior shows a thin layer, either of undecom- posed rock, or of material subsequently impregnated with silica from the open interstices, and thus made capable of re- sistance. Sometimes the porphyry is found to be traversed by a complex network of fissures, filled (except as to some wider spaces of intersection) with a clastic mass, like sand- stone. The interstices of the conglomerate. Fig. 42 (ex- cept the spaces containing crusts of manganese spar and quartz), are filled with a clastic cement, mostly silicified into hornstone. This sort of ore-filling is comparable in some degree with ore-deposits in soluble rocks, when the filling has passed from the space of discission proper into the rock, after room has been made for it in the latter by dissolution. In the cases be- fore us such room was made by the partial washing away of the (probably clayey) cement of the breccias and conglom- erates. Verespatak. — The gold-district of Yerespatak is situated at the north end of the second eruptive range. The two por- phyry masses of Kirnik and Boi form a center, around which sandstone and porphyry-tufa lie almost horizontally, and in part unconformably, upon folded Cretaceous sandstones below. The whole district is surrounded by a zone of trachytes, an- desites, and their lavas, which once (as may be inferred from the fragments remaining on the porphyry and tufa) overspread the entire district, and have been removed by erosion, laying bare the two older eruptive masses of the porphyry. A funnel-shaped depression seems to have been formed in the folded Cretaceous strata, from the middle of which as- cended the porphyry-outflows, furnishing also the material for the porphyry-tufa, which fills this funnel-shaped basin. The principal gold-bearing rock is the porphyry, yet the tufas and the Cretaceous rocks near the porphyry-outflow carry 88 THE GENESIS OF ORE-DEPOSITS. gold ; whereas, no gold or ore of any kind occurs in the tra- chytic and audesitic lavas which once covered the region. Vulkoj. — At Vulkoj, however, at the southern end of the second eruptive range, almost the opposite is the case. Here the older and deeper quartzose rock carries little ore, while gold abounds in the overlying andesites. Several mines of the Dacian gold-district have encountered in depth the stratified rocks through which the eruptives came, and the result has generally been disastrous to the miner, the ore-veins having either ceased entirely or become pinched to barren fissures. In the first case it would appear that the vein-fissures had been formed by the contraction of the eruptive material. But, in general, it should be said that these phenomena are by no means clearly and reliably reported. The prejudices of the miners play too large a part in their reports. This much is certain, that any fissure, in passing from one rock to another, is likely to exhibit a certain irregularity in both direction and filling, and that a change of this kind should not be allowed to discourage at once all further exploration. In some cases there has been found, below an eruptive rock containing ore-veins, a decomposed breccia of the same, which was quite barren. The great porphyry mass of Kirnik, at Verespatak, has been pierced through and through with ancient and modern workings, like the pores in a sponge. In recent years deep adits have been driven into it to reach fresh ground, but with unsatisfactory results. A short time ago the deepest of these adits encountered in the nucleus of the Kirnik mass, not the ore-bearing porphyry, but decomposed clastic rock and porphyry-breccia, which may be supposed to be the filling of the crater-opening. The Vulkoj mass, which has been almost cut into two halves by very ancient open-workings along its crest, contained a series of IST.-S. veins, the richest of which (the Jeruga) was cut in depth by adits from both sides. On the south side appears a slaty Cretaceous rock, underlying the porphyry, and extending (see Eig. 43) upon the Jeruga plane, with two oflsets, to the deepest adit on the north side, where it strikes the decomposed breccias, in which the very rich ores mined above can no longer be found to continue. As to the continuation of the veins in the slaty rock, the following facts are pertinent. "West of the Vulkoj mass, in the THE GENESIS OF OEB-DBPOSITS. 89 sandstones and slates, there is another gold-field, that of Botesiu, the veins of which are analogous, both in strike and in ore- filling, to those of Vulkoj. Botesiu shows no eruptive rocks; nevertheless, a study of the whole region shows that the forma- tion of its vein-fissures must have been connected with them, and it is even not impossible that they may once have extended as far as this, and may have been removed by subsequent ero- sion. It follows that we must assume the Vulkoj veins to ex- tend below the andesite into the slate, though this has been doubted by some. Fig. 44 shows the situation in an E.-"W. section. In the region of Boitza the eruptive zone (predominantly of quartzose dacites or porphyries) crosses an exposure of Meso- zoic limestones and melaphyrs, and the veins pass directly from the porphyry into the underlying melaphyr. At ISTagyag, Magura, and Fiizesd, in following the gold-veins in depth, masses of Tertiary sandstones and conglomerates are formed, broken through and enveloped by the eruptive rocks. At four places in the Dacian gold-district, namely, Offen- banya, Faczebaja, Fericiel and iN'agyag, telluric ores occur. In the neighborhood of Zalatna there is cinnabar, and at several points near Korosbanya there are copper-ores carrying a little gold. Gold is, however, mainly connected, as has been ob- served, with the four ranges of Tertiary eruptives, and appears chiefly in these rocks, though also in the stratified rocks which they traverse. The occurrence of gold in this case is thus somehow related to the eruptions; but since I have never found it as a primitive or idiogenous constituent of these rocks, I do not believe that it was derived originally from them. There is, therefore, nothing left but to consider the eruptions as the agents of a communication with the deep region, from which at these points the mineral springs ascended. The Dacian gold-district will furnish, upon further exploration, important contributions to the inquiry into the original source of the gold. For in- stance, if the auriferous character of the veins of Vulkoj should be found to continue in the shaly sandstones underlying the andesite, my view would be confirmed. The Comstoek Lode. — The most thoroughly studied Ameri- can vein-phenomena bearing on this question are doubtless 90 THE GENESIS OF OKE-DBPOSITS. those of the Comstock lode. It is not necessary to enter here upon a detailed description. I content myself with a reference to the three large treatises upon the district,* of which Becker especially discusses the genetic question. To appreciate this question, however, some simple illustrations are required ; and these have been compressed into Figs. 68 to 63. As already observed, the general geological conditions of the Comstock lode show a strong analogy to those of the Schem- nitz district. Only occasional bodies of sedimentary rocks are found, while the principal mass of the whole elevated region consists of a great variety of eruptive rocks, principally of the more recent periods. The altitudes of the more important points above sea-level are about as follows : Meters. Feet. Mount Davidson (the highest point of the re- gion) 2420 7941 Outcrop at the Gould and Curry mine (the datum-line for measurements of depth), 1950 6400 The Sutro Tunnel, at different points, 1840 to f 1390 4560 1865 feet below datum-line, ... I 1382 4535 The deepest point in the Belcher and Crown Point shaft, 3414 feet below datum, . . 910 2986 These figures alone indicate the immense extent of the eruptive material. The stratified rocks occur in a considerable continuous body at Grold Hill, in the southern part of the district, while in the northern part only a small body enclosed in eruptive rocks is found in the Sierra Nevada shaft. The several eruptive rocks have been differently defined at different times, according to the changes in petrography and in the methods of investigation pursued. Becker distinguishes : 1. Basalt (B). 2. Later hornblende-andesite (LHA). 3. Augite- andesite (AA). 4. Earlier hornblende-andesite (EHA). 5. Later diabase or black dike (LDb). 6. Earlier diabase (EDb). 7. Quartz-porphyries (QP). 8. Metamorphosed diorites (MDr). 9. Porphyritic diorites (PDr). 10. Granular diorites (GDr). * Clarence King, U. S. Oeol. Explr. of the iOth Parallel, iii., Mining Industry Washington, 1870. J. A. Church, The Comstock Lode : Its Formation and History, New York 1879. G. F. Becker, "Geology of the Comstock Lode," etc. — U. S. Oeol. Survey Monograph, Washington, 1882. THE GENESIS OF ORB-DEPOSITS. 91 11. MetamorpMc rocks (M). 12. Granites (G). This classi- fication is based upon careful microscopic examination.* The two principal veins (the Comstock and the Occidental) strike K-S., and the Comstock has been traced 5 or 7 km. (3 or 4 m.), according as its branches are omitted or included in the measurement. The position and the branching of the veins are shown in the sketch-map, Fig. 58, in which the two most important eruptive rocks, the diorite and the diabase, are emphasized by shading, the others being indicated by letters, as in the above list. The diorite forms the foot-wall from Gold Hill to Virginia City. South of Gold Hill metamorphic slates form the foot-wall, and even extend across in part to the hang- ing-wall side, as does the diorite to the north of Virginia City. Moreover, in one place a dike of diabase — the so-called " black dike," — occurs immediately on the foot-wall. The hanging-wall is principally diabase, at least in depth. In the upper region it is sometimes covered with other erup- tives, most frequently with hornblende-andesite. On the whole (with variations at some places), the Comstock presents wide, gently-dipping masses, predominantly of crushed and decomposed country-rock, and enclosing large flat "horses" of the same. The filling is, as arule, saccharoidal granular quartz (sometimes more compact), in which the ores are very finely disseminated. At some points they have occurred concentrated, forming the bonanzas to which the colossal gold- and silver- production of the district is due. The ores are silver-ores (stephanite, polybasite, argentite), with sometimes galena and zinc-blende. The bullion produced from them contains about half its value, or 6 to V per cent, of its weight, in gold. Some of these bonanzas were in the upper region and came to the surface. Others (like the richest one of all, in the Con- solidated Virginia and California mine) were found in the deep region ; and it is asserted that they were limited on all sides, without connection with other ore-bodies. This would * Messrs. Arnold Hague and J. P. Iddings [Sull. 17, IT. S. Oeol. S., 1885, "On the Development of Crystallization in the Igneous Eocksof Washoe," etc.) have stated as their conclusion that GDr, EDb and AA are identical ; PDr. is EHA ; MDr is LHA ; and LDb is B ; apparent differences being due to con- ditions of cooling. In Bull. No. 6, Cat. Acad, of Sc, 1886, Mr. Becker, after a reinvestigation of the locality, denies this conclusion in toto, so far as the Com- stock rocks are concerned. 92 THE GENESIS OF OEE-DEPOSITS. make them unlike our ore-channels or chimneys, which usually do have interconnection. But I cannot conceive of their forma- tion in any other way than upon the hypothesis that in such places more open spaces existed, through which larger quanti- ties of dilute metallic solutions passed and made deposits. The distribution of the bonanza-areas upon the vein-area is quite irregular ; and it has not been possible hitherto to trace any connection between the bonanzas and the petrographic or structural conditions in their vicinity. In form they are equally without any law, as far as has yet been observed. The bonanzas of the Con. Va. and Cal. consisted of a main body and three lenticular masses higher up, which, taken together, have a flat pitch to the north. The bonanza between Belcher and Yellow Jacket, on the other hand, followed the true dip of the vein; while the bonanza in Justice — a mine on the JSTW.-SE. branch, which dips ISTE. much less steeply than the main lode — shows again a north pitch. This ]SrW.-SE. branch of the Comstock shows a filling dif- ferent in some respects from that of the main lode, and may be considered as a cross-vein, running into the Comstock, or into the black dike which accompanies its foot-wall. (Becker's atlas, ix.) In the Justice mine, namely, the filling is mostly calcite, with little quartz, instead of quartz with very subordinate calcite, as in the main lode. According to Becker (I. c, p. 219) the calcitic filling is characteristic of the whole SE. branch. According to Church {op. eit, 173), compact crusts of calcite alternate in the Justice mine with their quartz crusts. This is the only clear report of crustification anywhere on the Comstock. (I be- lieve, however, that I was able to observe upon a rich speci- men from the Con. Va. bonanza, after polishing, a parallel structure in the mineral aggregate. I received this specimen in 1876 from Mr. Eair, one of the "bonanza kings," as a sort of compensation for the refusal to permit me to enter the then rich mine !) A comparison of the many cross-sections of the Comstock published by King, Church and Becker, and representing, of course, various stages of knowledge of the vein, shows that no normal or average section can be given, because the condition at different points on the strike are so different, and at some THE GENESIS OF ORE-DEPOSITS. 93 places, e.g. the junctions of the branches, developments have not given satisfactorily complete exposures. The sections. Figs. 59 to 63, are given (on a scale too small to show much) merely to illustrate the distribution of the country-rocks. They are reduced from Becker's monograph. In the three northerly sections the foot-wall is granular diorite ; in the two southern (Yellow Jacket and Belcher), and along the SE. branch, it is metamorphic slate. In the southern portion, the so-called black dike (according to Becker, later diabase) appears on the foot-wall, and follows the vein beyond the point where the SE. branch leaves it. The hanging-wall is diabase, except at the northern end, where diorite becomes the hanging-wall as well as the foot-wall. In the upper region, however, earlier diabase is covered by other eruptives. Diabase is the hanging-wall of the SE. branch also ; but in the foot-wall of that branch, besides the metamorphous slates, granular diorite and quartz-porphyry appear. So far as the sources of the eruptive rocks can be inferred, they were all (except that of the diorite) on the hanging-wall side of the vein, as were also the mineral springs which sub- sequently decomposed these rocks. But the ascending thermal waters encountered in these mines were within the vein itself; whence it may be concluded that the ore-bearing solutions came by that road from the deep region, and not, according to the lateral-secretion theory, from the side. In other words, the Comstock ores were not washed from those rocks which have been mined between 1950 and 910 meters (7941 and 2986 feet) above sea-level, but from material lying much deeper. The investigations of G. F. Becker were made at a time when importance was still attached to Sandberger's theory, and the correctness of his method of inquiry was assumed. The mat- ter takes a different aspect when we (quite justifiably) doubt whether the minute metallic admixtures detected by wet or dry analysis were originally in the rock, and acknowledge that they may possibly have entered it afterwards. This is evi- dently the case with the precious metals in the pyrite of the ore-bearing rock. That this pyrite is a secondary impregna- tion can be proved with the microscope, and is admitted by Becker also. In my opinion, any eruptive rock may give rise by metamorphosis to the type which we call, in Hungary, 94 THE GENESIS OF ORE-DEPOSITS. greenstone, greenstone-trachyte, etc., and which F. von Richt- hofen named propylite, because of its frequent occurrence as the country-rock of ore-deposits. Whether the precious metals can he detected in this rock depends wholly upon its impregna^ tion, or that of one of its constituent minerals, with pyrite. But it does not follow that this was the primitive condition. From this standpoint are to he regarded the metallic values reported by Becker, and here reduced, for the sake of better understanding, from cents per ton to grammes per 1000 kilo- grams. A pyrite washed from decomposed diabase, near the face of the north branch of the Sutro tunnel, contained 3 cents silver and 8 cents gold, i.e., 0.72 grm. silver and 0.12 grm. gold, per metric ton. The pyrite from the slates in the Belcher mine carried even 18 c. (4.32 grm.) silver and 20 c. (0.30 grm.) gold. Fresh diabase is said to have contained 4 to 5 c. (0.6 to 0.7 grm.) gold; the diorite of Bullion ravine, only a trace; while the andesite yielded about as much as the diabase. Augite separated by Thoulet's method from the diabase was found to be eight times as rich as a corresponding quantity of the feldspar. Comparative investigations are reported to have shown that the decomposed diabase contains only half as much silver as the fresh — a circumstance which was interpreted in favor of the lateral-secretion theory, on the assumption that the decomposed diabase had given up half its silver to the vein-filling. Since the diorite in the upper portion of Bullion ravine shows only traces of silver, but at the mouth of the ravine, near the vein, contains a considerable amount, Becker considers this indicative rather of an impregnation of the rock proceeding from the vein. Moreover, the andesites and quartz-porphyries also contain small amounts of silver; while the strongly calcareous meta- morphic diorite carries 8 c. (1.92 grms.) per ton, which might be connected with the vein-filling in the Justice mine. Finally, the basalt contains nearly as much silver as the older diabase ; but the basalt cannot be cited as a source, because it comprises the freshest rock in the district, and shows no trace of decom- position in its olivine (Becker, I. c, pp. 223-225). These facts would be favorable to the notion of lateral secretion, if only it could be proved at the same time that the metalliferous char- THE GENESIS OP OEB-DBPOSITS. 95 acter was primitive. But our knowledge does not go so far as that; and the Comstock, like the deep mines of Przibram, ceases, therefore, to be a proof of the lateral-secretion theory. The Comstock differs in many respects from typical ore- veins. It is properly a quartz-vein, in which, at various points, important ore-concentrations have been formed, not showing (except in the Justice mine) any clear crustification, though this may have been present at some time, and may have been ob- literated by metamorphosis of the vein-mass, e.g., through the replacement of calcite by quartz. It is also, in the main, a contact^vein, between a diorite foot^ and a diabase hanging-wall, with steep spurs running upward into the diabase and travers- ing also still more recent eruptives.* Some of these peculiar- ities are represented in other districts. 2. Ore-Deposits in Soluble Rocks. In this group we shall find two genetic types represented : the fillings of spaces of dissolution, and the metasomatic de- posits, the origin of which will be particularly considered, to- gether with some related metamorphic deposits in soluble rocks, which have not yet been sufficiently studied to be classed apart. The expression " soluble rock " is to be understood in its or- dinary sense of solubility in the waters commonly represented on the earth's surface. Acid and caustic waters will attack, more or less, nearly all rocks, though not so as to dissolve them completely, as we see limestone dissolved. I include especially among the soluble rocks, rock-salt, gypsum, limestone, and dolomite. Of the following instances I shall describe most fully those which I have personally studied, giving only the essential outlines of other related occurrences. Bodna. — The ore-deposit of Rodna, in NE. Transylvania, is interesting to me (apart from analogies which it oflfers with Leadville, Colo.), as the first in which I had the opportunity to study the origin of an ore-deposit by replacement. It is situated on the line of two andesite ranges, having a common strike, — the Hungarian Vihorlat Gutine, stretching * This is denied by Hague and Iddings, op. cit., p. 41. — See foot-note on p. 91 of this paper. 96 THE GENESIS OF ORB-DEPOSITS. NW., and the Transylvanian Hargitta range, running SE., — and at the point where this hne cuts through the mass of the Rodna Alps. The predominant rock is mica-slate, with numer- ous intercalations of limestone, and is traversed by many dikes and masses of andesite. Ore-deposits have been found at many points in the district. The most important, situated in the Benyes mountain, was carefully studied by me in 1862, after the ore-bodies in the mine had been worked out. J. Grimm had examined the mine in 1834, and had considered the deposits to be primitive beds at the contact between lime- stone and mica slate, and to have occupied that position be- fore the andesite eruption, by which they had been much shattered. The ores (pyrites, black zinc-blende, and argentiferous galena, slightly auriferous, with quartz and calcite) often occurred, it is true, on the gently-dipping contact-planes; but in certain E. and W. lines they stood steeply, much like veins. In these places the flat deposit, and with it the stratification, had sud- denly turned upward, and it was clear to me that the occurrence represented a peculiar form of fault, namely, a bending of the strata, followed by fracture in the direction of the dislocating force, when the limit of cohesion had been passed. Here and there, in these steep places, the stopes had been carried beyond the contact, and the resulting appearance was as if the steep deposit had been the primary one, and had supplied the ore to the contact. Occasionally eruptive breccias were observed along the steep deposits. At lower levels, in the downward continuation of the fissure of the steep deposit, eruptive rocks and thin breccias occurred ; and these became predominant in the lowest part of the mine. The structure of the ore-beds was mainly massive, and not crustified. In some places, however, druses had been devel- oped, which showed the same paragenetic succession as the mass of the bed, and which contained pseudomorphs of pyrite and galena after calcite. The thickness of the ore-bed was ex- tremely variable, the greater part of the contact-area being scarcely worth working, while at single points colossal masses of ore were found. These circumstances led me to consider the deposits, not as contemporaneous in origin with the rock, but as subsequently formed by the circulation of mineral waters THE GENBSI8 OF OEE-DBPOSITS. 97 along the contact-planes. In other respects I adopted at that time the explanation of J. Q-rimm.* Mining was then active chiefly on the north slope of the Benyes divide; and the sedimentary rocks were cut off towards the south by andesite. I pointed out that on the south slope, beyond the andesite, there were various ancient mines, and rec- ommended that they be explored in depth, by means of an adit. This led to the discovery of several deposits, which gave new life to the industry. After cutting through the andesite, the explorers found steep deposits at the contact of andesite and limestone, and, in the limestone, near its contact with the mica- slate, a flat deposit, which, being above the ground-water level, had been transformed into carbonate of lead. The somewhat complicated conditions are shown in Fig. 70, as far as this can be done in a single section. The deposit at the contact of andesite and limestone indicates at once a genetic connection vnth the eruptive rock, and renders it probable that the ore-beds also are due to the after-effects of the eruption. Even on the north slope there were some reasons for this con- clusion. For instance, at the ore-bodies locally called Thon- strassen, ores occurred in the midst of eruptive breccia, which could not be taken for fragments of the orignal bed. Baron Constantine von Beustf found traces of " ring-ores," indicating a formation in open cavities. In seeking an explanation of all the facts, I was led to give up the view of J. Grimm, J which he, however, still maintained, citing Offenb^nya as another instance in which a pre-existing deposit on the contact between limestone and mica-slate had been shattered by an andesite-eruption. But in that instance, also, I had the opportunity to satisfy myself that the then ac- cessible mine-workings showed no fragments of an earlier ore- deposit, but only ore-formations under the influence of the andesite. G-rimm had had in mind the deposits of Rodna and Offen- * Some results of my studies at Rodna will be found in the Verhandlungen d. k. k. g. R. AnstaU., 1865, pp. 71, 163, 183, and 1870, p. 19. t "Bemerkungen iiber d. Erzvorkommen von Eodna," Verh. d. k. k. geol. B. A., 1869, p. 367. X J. Grimm, " Zur Kenntniss der Erzvorkommen von Kodna, " Fer-A'. d. k. k. geol. B. A., 1869, p. 367 ; and F. Posepn^, "Die Natur der Erzlagerstatten von Rodna," i6id.,1870, p. 19. 98 THE GENESIS OF OKE-DEPOSITS. b4nya when he established, under the first division in his sys- tematic classification,* the second sub-division, " Occurrences of Ores as Fragments of Earlier Deposits, in Breccias," etc. Offenhdnya. — Offenbdnya, in the Transylvania gold district, has various deposits analogous to those of Rodna, and also veins, with telluride ores. We are here interested in its mass- deposits, at the contact of limestone and andesite, one of which is illustrated in Fig. 71. Beneath the limestone widely extending through the district, mining has disclosed a mica-slate (the so-called underground slate) ; and at the contact of the two a flat, pyritous deposit. The whole stratified series is traversed by andesite ; but near its contact with the limestone a steep, rich mass-deposit extends from the surface down to the mica^slate. This deposit is highly crustified, and was evidently formed in a pre-existing space. The flat deposit shows no crustification, and may have been formed by metasomatic replacement of the lime at the contact between the impermeable and the soluble rock. The analogy with the conditions on the south slope of the Benyes mine, at Rodna, is evident, though I do not know whether at Rodna the flat deposit has been followed as yet to its junction with the steep one.f Rezbdnya. — Rezb^nya in SE. Hungary represents different conditions. Here, in an indistinctly stratified Mesozoic lime- stone, occur long spaces filled with ore, descending steeply and irregularly in shape like that of the cavity produced by pour- ing a stream of warm water upon a snow-bank. This extreme case is of great theoretical interest, although such ore-bodies having but one considerable dimension, and that in the most unfavorable direction for mining, mainly downward, are not attractive from a commercial standpoint. I visited Rezbdnya first in 1868, and published some observations concerning it, which may have contributed to induce the Hungarian govern- ment to take up the subject later, and intrust to me a more thorough investigation. I will here mention only some things, * jyU Lagersldtten der nutzbaren MiTiercdien, Prague, 1869, p. 32. t At the time of the visit of G. vom Kath, in 1878 (described by him in the Zeitschr. d. d. geol. Oesellsch., xxx., 1878, p. 556), this ore-body, 28 meters (92 feet) thick, had been developed for a height of 85 meters (280 feet) and a length of 120 meters (394 feet) without reaching its termination. THE GENESIS OF ORB-DEPOSITS. 99 interesting from the genetic standpoint, and refer for details to my published monograph upon the subject.* In the Rezbanya region, lying above clay slates and Permian and Liassic sandstones, appear numerous isolated bodies of limestone, indicated by their fossils to be of various ages, from the Lias to the ITeocomian, seldom distinctly stratified, and, when they are traversed by eruptive rocks, often showing a crystalline structure. The ore-filling is mostly confined to the neighborhood of the ernptives, and sometimes to the contact, where garnet-rock occurs as a well-known product of local metamorphosis. Since my examination, there may have been, in this region, many interesting and scientifically important de- velopments, which are unfortunately unknown to me. On the basis of my old notes only, I shall confine myself to the de- scription of a single district, cut off from commercial commu- nication, that of Valle Sacca. The name is that of the valley, which heads in a high mountain range of Permian and Liassic sandstones, and after a short course ends in a wild limestone canon, leading into the Galbina valley. The sides of Valle Sacca consist chiefly of limestone, which is traversed by a num- ber of eruptive dikes and one larger mass of a syenitic char- acter. Fig. 64 gives a somewhat generalized section of the WW", slope of the valley and district on the line of the so-called fourth adit. At the adit-mouth is cut the syenite mass, which extends also to the opposite slope; and the adjoining portion of the limestone has been metamorphosed to a crystalline mass, while the limestone further SW. is for the most part still com- pact. On the west side, the limestone adjoins sandstone along a E'.-S. line, which doubtless represents a large fault. Approx- imately parallel to it run the greenstone dikes, which, though they seem to be mutually parallel, in reality intersect one an- other at very acute angles, thus constituting a highly elongated net-work. The dikes are not alike. Most of them may be considered aphanitic or dioritic; one, however, is quartz-por- phyry, with dihexahedra of quartz, of pea-size. The principal deposit is the so-called Reichenstein stock, which had been worked, during the period prior to my visit, to a depth of about 400 meters (1300 feet), from its outcrop, * Geologisch-montanistische Sludie der Erzlagentdtten von Bezb&nya, Budapest, 1874. 1 100 THE GENESIS OP ORE-DEPOSITS. 340 meters above the deepest adit, to a level 60 meters below the adit. Fig. 65 shows the form of the ore-channel on the strike. The horizontal section of the body was most frequently circular or elliptical. In some places one dimension strongly predominated, so as to give the appearance of a fissure-filling. At the outcrop, according to the old maps, there was but one channel. Below, this divided into neighboring and mutually connected branches. Several of these might continue parallel and independent for considerable distances. The total sectional area of the channels averaged perhaps 20 to 30 square meters (215 to 322 square feet) ; but at some levels the deposit was only present in traces, whereas at others it had many times its aver- age section. Fig. 66 shows, by the difference between the plumb-line and the arrow, the angle between the true dip and the pitch of the ore-body, oblique to it. The ores were doubtless sulphides originally, but were after- wards oxidized in places. Eich silver-ores predominated, espe- cially argentite, pieces of which weighing several pounds ap- pear to have been no rarity. Besides this mineral there were hessite (telluride of silver), tetrahedrite, redruthite, galena, bis- muthinite, and various pyrites. Taking these together with the oxidized ores, the deposit represented a whole mineral cabinet. The maximum silver-value was reported as 12 to 20 kilos per 1000 (1.2 to 2 per cent.), the gold being 3 grammes to each kilo of silver. The percentage of lead was about twenty times, and that of copper about ten times, as great as of silver. The metric ton (2206 pounds) would yield, at this rate, 24 to 40 per cent, of lead, 12 to 20 per cent, of copper, 12 to 20 kilos (386 to 643 ounces Troy) of silver, and 36 to 60 grammes (1.15 to 1.83 ounces Troy) of gold. The deposit was therefore a bonanza in the American sense. In fact, it yielded from $100 to $150 per ton. Although I could not see this deposit in process of extrac- tion, I was able to conclude positively, from specimens of the ore and from the analogy of similar deposits in the district, that it had been formed by the precipitation of successive crusts. As regarded the origin of the cavity, I was at first iufiuenced in my views by the numerous caves of the region. The mines repeatedly reached caves, into which the mine-water could be THE GENESIS OF ORE-DEPOSITS. 101 discharged without filling them, there being some subterranean outlet. But these caves, as I have explained in Part I., were formed by descending liquids of the vadose circulation ; and to assume a similar origin for the cavities filled by the ore- bodies would be to assume that the latter cavities were formed in a manner directly opposite to that in which they were filled — which is highly improbable. It was not until later, when I had become acquainted with the observations of J. N"oggerath (cited in Part I.) on the thermal springs of Burtscheid, that I recognized that ascending mineral springs are able to cut their own way to the surface, forming the channels which they ultimately fill with ore. The most difiicult feature of all, namely, the nearly cylindrical form of the ore-bodies of Valle Sacca, was thus satisfactorily ex- plained. The channel of the Reichenstein body runs vertically for 400 meters (1312 feet) in limestone between greenstone dikes ; or, in other words, in a zone of lime between two zones of im- permeable rock. The dikes therefore control its direction. It follows downward nearly at the angle of their steepest dip, but with a pitch southward, giving it a " false dip." The sections of the various workings show that the ore-body apparently ended at one side of the dike and recommenced at the other side, as if it had passed through. In that case, porous places in the dike-mass, at the intersection, will have determined the track of the channel. It is significant that the Reichenstein ore-channel passes in depth through the dikes to the SW., towards what is probably a great fault-fissure, and not in the direction of the present drainage. Ifor could the former deep drainage from this channel have been to the NE. along the contact between the limestone and the underly- ing Liassic sandstone (which, in fact, appears at a lower level, where the Yalle Sacca joins the G-albina valley), for the reason that all the barriers of the greenstone dikes, unquestionably extending from the limestone into the sandstone, would have opposed that flow. The stratigraphical conditions thus exclude the possibility that this channel was formed by vadose circula- tion, and render more probable the view that it owes its origin to the ascending waters of the deep circulation, which certainly affected the filling of it. 102 THE GENESIS OP ORB-DEPOSITS. Raibl. — Raibl, in Carinthia, is the best representative of a group of deposits which were at a recent period taken to be genuine beds even by V. M. Lipoid,* then the best authority on the mines of the Alps in general. Here and thei-e, as, for instance, by A. Morlot,t observations were made which threw some doubt on this conception ; but since they did not fit into the prevailing system, they remained disregarded. It was my fortune to establish the truth of the situation. Prof, von Grod- deck kindly characterized my investigation of it as " opening a new path," and adopted the filling of spaces of dissolution as a class in his system (op. ciL, pp. 10, 236, etc.). Such deposits occur in Carinthia, in an E.-W. limestone alpine range, of which Eaibl is the western end; and also somewhat further north, in the zone of Bleiberg, near Villach, chiefiy in a limestone, early denominated for this reason the ore-bearing limestone, and more recently determined as Tri- as sic. The ores occurred mostly in the vicinity of certain inter- calated slates, which seemed always to occupy the same " Eaibl horizon," and thus led to the conclusion that the ore-deposits (naturally believed to be of contemporaneous origin) likewise occupied a fixed horizon. But it soon appeared that the slate at Bleiberg belonged to a somewhat diflferent horizon in the Trias ; and I ventured to assert that the impermeability of the slates, as compared with the solubility of the limestone, had had something to do with the ore-deposition, which was a sec- ondary formation in the rocks. There are found at Eaibl, some distance below the slates, in the limestone which conformably underlies them, what seem indeed at first glance to be beds of ore. They consist chiefly of a coarsely crystalline galena, with pyrites, and a zinc-blende (wurtzite) in very thin crusts, hence called Schalenblende. A closer study, however, of the extremely distinct crustification reveals that it does not represent the stratification, which, on the contrary, it crosses at all angles, being in fact the filling of irregular spaces, traversing the limestone in every direction. Further light is furnished by the seams which here occur. As is generally the case in limestone, these are rarely wide * Jahrb. d. k. k. g. R. Ami., 1862, Verh., p. 292. t Ibid., 1850, i.,p. 266. THE GENESIS OF OEE-DBPOSITS. 103 fissures, but usually mere partings between two polished walls in close contact. Slickensides, etc., identify them at once as results of friction, caused by the forcible rubbing together of walls perhaps originally irregular. The plane of contact with the slates offers a means of determining the extent of the movement along some of these insignificant-looking seams; and it appears that dislocations as great as 40 to 60 meters (131 to 196 feet) have thus taken place. Since the slates possess some flexibility, they were sharply bent in the immediate neighborhood of the fault, a feature which, on account of its theoretical importance, I have illustrated in Pig. 69. In the seams themselves (locally called Blatter or " leaves ") there can be, of course, no deposit of ore ; but such deposition occurs outside of the fissure, when soluble rocks like this lime- stone are traversed. Geode-spaces were thus leached out, and are found filled with distinct mineral crusts, as is shown in Fig. 72, representing the face of a level on the so-called Johanniblatt. It cannot be doubted that the ore-supply came from the seams ; and when we find such seams also in large and rich deposits of similar character, like those on the north slope of the Konigsberg at Eaibl, we must concede to them a similar significance as regards the ore-deposition. To the more important of these seams, J. Waldauf von Waldenstein* and Dr. W. Fuehsf had already called attention. These are the Morgen, Abend, Johann and Josef. The first three meet at an angle of about 30°, and form the boundaries of ore-bodies, extending downwards along the seams with a horizontal length of 40 to 80 meters (131 to 262 feet) and a total thickness (including portions too poor to work) of 10 to 50 meters (33 to 164 feet). Many of the mine-managers be- lieved that there was here a continuous ore-bed which had been faulted into separate bodies by the seams, and numerous exploring levels were undertaken to develop this assumed bed, but all in vain. Nothing was found, except a few more or less independent ore-shoots on one or both sides of the seams, similar to those which have been encountered in recent years at Leadville. * Die besonderm, Lagerstatten d. nutzb. Mineralien, Vienna, 1824, Plate III., Fig. 4. t Seitrdge zur Lehre von den Erzlagerstatten, Vienna, 1846, Plate I., p. 23. 104 THE GENESIS OF ORE-DEPOSITS. The foregoing observations will facilitate a comprehension of Figs 67 and 68, the former showing a section (not strictly in one plane) of the ore-shoots in the government mine, and the latter a similar picture of the Struggl private mine. In the former, separate ore-bodies are observed to the distance of 500 meters (1640 feet) above the bottom of the valley, and in 1870 the continuous ore-shoots extended from 425 meters (1394 feet) above to 150 meters (492 feet) below that level, a total vertical height of 575 meters (1886 feet). It will be seen that the several portions of the slopes descend more or less parallel with the stratification and the lime-slate contact, but with steps or ofisets. The highest portion of the Abendblatt ore-shoot is about 300 meters (984 feet) in the foot- wall of the slate-contact ; at greater depths there are portions 130, 150, 85 and finally 10 meters only (426, 492, 279 and 33 feet) from that plane. It thus appears that the ore-shoots are approaching the con- tact in depth, and will probably follow it below. It is, there- fore, not here the case that a particular layer in the limestone has favored the formation of spaces of dissolution. If that were true, the ore-body, notwithstanding the convergence of the seams southward, should maintain a more or less uniform distance from the contact, which it does not do, either in the section of Fig. 67 or in that of the Struggl mine. Fig. 68, where the opposite occurs, namely, the ore-shoots depart from the contact in depth. I must confess myself unable to explain these variations in the Eaibl ore-shoots with the light afforded by the mine-workings down to 1870. But I am convinced that the explanation will be found by further thorough study. Meanwhile, I can only claim the credit of having placed the inquiry upon what I deem to be the true road, and express my regret that in the twenty years since the publication of my monograph on the Eaibl deposits no further progress seems to have been made in the interpretation of the very numerous analogous ore-deposits. The North of England. — I cannot omit to mention here the region, classic in this respect, of the ]S"orth of England. Lead mining is actively carried on in the carboniferous limestone of Northumberland, Durham, Cumberland and Westmoreland, where the limestone alternates with sandstone and slate, and THE GENESIS OE ORB-DEPOSITS. 105 occasional intercalated eruptives or their tufas. This forma- tion is traversed and faulted by a variety of seams and veins ; and the veins are generally richer where they are in the lime- stone. The thinner and more extensively faulted of the lime- stone strata are entirely severed, so that they appear in dif- ferent horizons on opposite sides of the faulting-fissure. Where they are thicker or less widely thrown by the fault, however, limestone appears on both sides of the latter. It is obvious that an accurate picture of these conditions would furnish valuable data concerning the ore-genesis. The several descriptions of the mines do not specify whether the ore of such veins as become rich in the limestone occurs in the fissures proper or outside of them in spaces of dissolution in the limestone. The latter is clearly the case in the so-called " flats." In certain horizons, where the seams encounter the soluble lime-stratum, the ore-filling departs from the fissure into the geodes of the rock, forming frequently very rich ore- bodies of highly irregular form, but flat, by reason of their following the soluble stratum. The ore-filling continues to a very uncertain distance from the fracture-plane, and is gener- ally accompanied with frequent cavities, the walls of which are covered with crusts of calcite, blende and galena. Empty caverns also occur.* "We cannot but recognize immediately in this description the type as to character and position of the Raibl deposits, the druses of which are here represented by the incrusted cavities. The empty caverns have doubtless been formed by subsequent processes of dissolution. These phenomena occur in the ISTorth of England on a very large scale. Veins are mentioned which have been traced for several miles, and the connected subterranean channels of dis- solution must be also of considerable length. The existence of laterally extensive ore-channels, and hence of an under- ground circulation of mineral waters not formerly suspected is thus revealed, and an entirely new light is thrown upon the so-called " ore-beds." These observations are confirmed in another quarter by de- velopments in "Western North America, where very numerous * See J. A. Phillips, Ore-Deposits, p. 180 ; also, D. C. Davies, Metalliferous Minerals and Mining, London, 1880, p. 216 ; and the works of W. Wallace, T. Sopwith, Westgarth Foster, C. E. De Eance, E. Hunt, etc. 106 THE GENESIS OF ORB-DEPOSITS. ore-deposits are connected with limestone. It is impossible to bring forward here the whole of this material. I must limit myself to certain localities, which have been thoroughly studied and described in publications. Leadville. — I will begin with Leadville, the recent blossom of the mountain-world of Colorado. I am, indeed, not person- ally acquainted with this locality, the importance of which was not recognized until after my visit to the United States ; but my lively interest in it is testified by the article concerning it, which I laboriously compiled in 1879 from the incomplete data then available.* Later, when S. F. Emmons had finished his surveys, but before the publication of his epoch-making work,t I had opportunity to exchange views with him concerning the genetic condition, and to confess that I was unable to share his opinion as to the downward course of the mineralizing solu- tions — an opinion which was opposed to the then prevalent be- lief. The mine-workings have been greatly extended since that time, and Emmons's suggestion has been shown by several mining engineers,! on the basis of thorough studies under- ground, to be untenable ; so that the Leadville deposits appear, as regards the origin of their metallic contents, to form no ex- ception to the history of other similar deposits. I think Em- mons himself must have acknowledged the force of these criticisms, which do not detract in the least from the merit of his accurate investigation of the district. On the west slope of the Mosquito range appears a series of undulating Palaeozoic strata, with heavy layers and intrusive masses of eruptive rocks, and traversed by numerous faults. This formation covers a large area, only a comparatively small portion of which, namely, the vicinity of Leadville, is ore- bearing, — a circumstance which of itself points to a local origin for the ore. As is well-known, the series of rocks has the fol- lowing order downwards : white porphyry, blue limestone, gray porphyry, white limestone, lower quartzite, — which I will de- * "Leadville, die neue Bleistadt in Colorado." — Oesterr. Zeilsck, 1879. t " Geology and Mining Industry of Leadville." — U. S. Oeol. Survey, Monogr. xii., Washington, 18S6. t F. T. Freeland, "The Sulphide-Deposits of South Iron mil"— Trans. A. I. M. E., 1885, liv., 181 ; C. M. Eolker, " The Leadville Ore-Deposits."— JJid., p. 273; A. A. Blow, "The Geology and Ore-Deposits of Iron Hill. "—iiid. , 1889, xviii., 145. THE GENESIS OF ORB-DEPOSITS. 107 note, for brevity, by their initial letters. The ore-deposits occur chiefly at the contact between the first two members of the series, below the WP. and above the BL. In the upper levels they are oxidized and chloridized (doubtless in this, as in other places, through the action of descending ground-water) ; in lower levels they appear in their original form as sulphides. That this was the condition in which they were originally pre- cipitated, Emmons admits ; only their position seems to him to exclude the hypothesis of ascending solutions. He says (op. cit, p. 573) : " The principal water-channel at the time of deposition was evidently the upper contact of the blue limestone with an overlying porphyry ; and from this surface they penetrated downwards into the mass of the limestone. It may be assumed, therefore, that the currents were descending under the influence of gravity, rather than ascending under the influence of heat." But he omits to explain how he conceives it possible that mineral solutions descending by gravity-, and hence certainly having been in contact with the surface-region, could deposit sulphides. Assuming such an explanation to be furnished by reduction through organic substances, the question arises whither such descending currents could go. Here the theory is in cotaflict with our conception of the underground circula- tions. As A. A. Blow has shown, however, a leaching of the WP. cannot by any means have supplied the ore ; for this rock is not at all decomposed, as in that case it must have been. On the other hand, there are found in the intrusive beds and dikes of the lower GP. various indications that this rock had more to do with the ore-deposition. Along these dikes lie the ore- shoots, — in other words, the channels in which ore was de- posited. It was at first tacitly assumed that the ore occupied the whole plane of the contact, although it was known that the richest bodies occupied particular zones in this plane. The im- portance of these ore-shoots was recognized later ; and we may now consider the Leadville occurrence as presenting, not a single contact-deposit or ore-bed, but a complex group of ore- shoots, such as we have observed in other ore-deposits in lime- stone. These ore-shoots lie, in Leadville, at the contact be- tween the soluble and the eruptive rock; while in Eaibl they 108 THE GENESIS OF OEE-DEPOSITS. appear near the contact of two stratified rocks, one soluble and the other impermeable. The physical process forming these ore-shoots was doubtless the same in both cases. The mineral solutions, ascending under pressure, and seeking a path to the surface, followed, as some would say, the line of the least re- sistance ; or, as I would prefer to express it, there was estab- lished in the soluble rock a line of maximum circulation, re- sulting in the dissolving-out of a channel. Such dissolution, however, occurred not only on the contact between WP. and BL., but also at other contacts. Thus L. D. Eicketts (Rolker, I. c, p. 284) gives a section of a mine on Car- bonate Hill, showing a second, deeper ore-horizon between the GP. (dike porphyry) and the underlying limestone. According to Eolker, the BL. of Fryer Hill was relatively thin, and has been replaced with ore and accompanying minerals, all but small remnants of dolomitic sand. These are generally above the ore, i.e., along the upper contact, whereas, according to Em- mons's theory, they should be replaced with ore. The sections given by F. T. Freeland {I. c. Figs. 1 and 6) show two ore-horizons, the thicker of which is below the WP., and the other below an intrusion of GP. ; and Mr. Blow's sections from Iron Hill reveal similar phenomena (see Fig. 73, a section through the McKean shaft). The ore-shoots are, of course, irregular in form ; but a main general direction can be recognized, which is eastward in Fryer Hill, but northeastward in Carbonate and Iron Hill, representing the course of the channel through which the mineral solutions circulated. In the data at hand concerning the structure of the de- posits, nothing is said of a distinct crustification. It is to be remembered, of course, that mining operations hitherto have been largely confined to the upper and decomposed zone, whereas this phenomenon, if ever so fully developed, would show itself clearly only in the undecomposed zone. When we read, however, of great " horses " of country-rock, encountered in the midst of the ore, we must believe that the deposit is due not so much to a metasomatic replacement of the limestone as to the filling of spaces of dissolution ; and hence it should exhibit the characteristic sign of such a filling, namely, crusti- fication. It seems to me that this point has not received the attention it deserves ; and I hope that observations in the un- THE GENESIS OF ORE-DEPOSITS. 109 decomposed ore-zones will give more definite data as to struc- ture. It is difficult to believe that metasomatic processes could produce such pronounced ore-shoots as those described at Lead- ville. Impressed by Emmons's views, and long before the connec- tion of the ore-deposition with the GP. of the dikes had been shown, I wondered, at one time, whether the ore might not have come somehow from the fault-fissures into the contact- channels. But Mr. Emmons pointed out to me that the faults contain only ore which has been dragged in from the pre-ex- isting bodies, the formation of which was complete before the faulting took place. Conditions analogous to those of Leadville are exhibited in most of the ore-deposits in limestone occurring in the Ameri- can West. But, with few exceptions, we have only hasty de- scriptions of them, and sometimes nothing more than business « puff's." Red Mowitain. — A remarkable occurrence has been described in the Red Mountain district, Ouray county, Colorado.* In the midst of the deposits of the San Juan region, which are connected with eruptive rocks, appears a body of Mesozoic strata, carrying, at the contact of a quartzite with the under- lying limestone, a deposit of the sulphides of iron, lead, cop- per, silver, and the products of their decomposition, rich in silver and somewhat auriferous (2110 to 3980 grammes of silver and 3 to 6 grammes of gold per metric ton, or 59 to 111 ounces of silver and 0.08 to 0.17 ounce of gold per ton of 2000 pounds). At certain points the ores extend far down into the limestone, and in the section shown in Eig. 74 the ore follows a fault-fissure through the whole thickness of the limestone into a second quartzite stratum below. The stratified formation is mostly covered with andesite, in which occur ore-bearing veins in fissure-form. In the neighborhood, at Mineral Earm, another contact-de- posit between limestone and quartzite is known, consisting of barite with argentiferous galena and tetrahedrite. Both the above deposits are but briefly described, and perhaps have not been extensively worked. Their conditions of position and the * G. E. Kedzie, " The Bedded Ore-Deposits of Bed Mountain District,'' Trans. A. I. M.E., 1886, xvi., 570. 110 THE GENESIS OF ORE-DEPOSITS. predominance of lead- and silver-ores strangely remind one of Leadville. In the adjacent Territories of ISTew Mexico and Arizona various copper-deposits occur in limestone, and at its contact with eruptive rocks ; as, for instance (according to the outline- description of A. F. "Wendt*), in the Clifton and Bisbee dis- tricts. The sections accompanying Mr. Wendt's paper remind me of some of the deposits described in my monograph, at Eezbanya, at Mednorudjansk, and at Bogoalavsk in the Ural. Fig. 75 is an interesting section from the Clifton district, in Arizona, showing two steep ore-shoots, parallel with the felsite dike, and a flat one, parallel with the bedding. Utah. — With respect to Utah, the paper of 0. J. HoUisterf gives a general survey of the deposits of the Territory, and mentions a number which occur in limestone. Some of those in central Utah I have had the opportunity to see personally, during the period when mining was still confined chiefly to the decomposed upper levels. I refer to the Prince of Wales and the Reed and Benson, in Big Cottonwood ; the Emma and the Flagstafl", in Little Cottonwood ; the Old Telegraph, in West Mountain, and the Hidden Treasure, in Dry Canon district. Palaeozoic strata are here traversed by frequent eruptive dikes, and by two intersecting systems of faults. The ore- deposits, of varying thickness, in the limestone have, as a rule, the form of " chimneys," either lying flat, with the bedding, or standing steeply along the dikes and faults. This gave rise in the beginning (when the nature of the deposits was not under- stood, and the conception of a typical " lode " generally pre- vailed) to a series of disappointments and mistakes in mining, of which the history of the Emma mine furnishes an interest- ing example. Apparently the irregularity and the complica- tions of these deposits came to be better known afterwards. The (sometimes very rich) ores consist chiefly of sulphides of lead and silver, and the products of their decomposition. In some cases {e.g., Hidden Treasure) cuprite occurs, with native copper ; and in the Camp Floyd district cinnabar also is found. Nevada. — In Nevada, adjoining Utah on the west, deposits of this class are likewise abundantly represented. I will men- * "The Copper-Ores of the Southwest," Trans. A. I. M. E., 1886, xv., 25. t "Gold- and Silver-Mining in Utah," Trans., A. I. M. E., 1887, xvi., 3. THE GENESIS OF ORE-DEPOSITS. Ill tion only the two districts which have been most thoroughly studied, namely, White Pine and Eureka. With regard to the former, the work of Arnold Hague (1870)*, demonstrating the peculiar character of the White Pine deposits, led me to seek for European analogues.f I found that, apart from the condition of the ores, which at White Pine are found in the oxidized and chloridized zone, there was an analogy with all the European ore-deposits in limestone, but especially with the conditions at Raibl. Devonian limestones and calcareous slates are overlain at White Pine by Carboniferous clay-slates, sandstones and lime- stones ; and the ores occur only in Devonian limestone and at its contact with the calcareous slates on a JST. and S. anticlinal. The ores and the associated minerals (quartz, calcite, gypsum, fluorspar, barite, rhodonite, rhodochrosite, with the chlorides, bromides, oxides, and carbonates of various metals, especially silver, lead and copper) fill the cavities in the limestone and surround its fragments. The various mines represent different stages in one and the same process. In the Eberhardt, two fissures crossing the anticlinal bound the ore-body (like the Morgmhlatt and the Ahendblatt at Raibl). This consists of a lime-breccia (Kalk- typhon), the fragments of which fit together, and are cemented by ore-bearing quartz seams. The Hidden Treasure mine con- tained the ore in geodes, at the contact of the limestone and slate. In the Aurora, the ore was in bodies stretching IT. and S. In Bromide, Chloride and Pogonip Elats, the ores occurred in geodes and masses included in lime-breccia, in a zone parallel with the bedding. It is Arnold Hague's opinion that the Eberhardt mine probably represents the source of the ore-solutions which impregnated the limestone, wherever cavities existed, up to the level of the overlying calcareous slates, which were impermeable to the solution. The slate-cover having been removed by ero- sion, the ores thus accumulated below it were exposed imme- diately at the surface ; and the surprisingly large product of the district was derived from open cuts and shallow workings. * " Geology of the White Pine District," TJ. S. Oeol. Surv. of the 40th Parallel, vol. iii., Mining Industry, p. 409. t F. Posepny, "Das Erzvorkommen vom White Pine District, u. dessen euro- paische Analogien," Verh. d. k. k. g. E. A., 1872, p. 186. 112 THE GENESIS OF ORE-DEPOSITS. The other leading analogue in Nevada is found in the Eureka district, and was made widely known and practically significant by the law-suit between the Eureka and Richmond companies,* which involved the definition of a deposit not contemplated in the United States mining law. Similar difficulties have arisen under the old European mining codes. Such deposits were known in some districts of Europe, but they were not so widely distributed as the fissure-veins, for the conditions of which the ancient codes were framed. Conflicts were therefore inevita- ble. I will mention only Bleiberg in Carinthia (which presents some degree of analogy with Eureka), where, besides the general mining code, special statutes became necessary, depart- ing from the usual rules with regard to prospecting and the location and the acquisition of claims. The geological conditions of the district have been described in an elaborate monograph by J. S. Curtis, f based on the de- velopments existing in 1882. Further knowledge may have been gained since, but, so far as I know, nothing later has been published. I made a brief visit to Eureka in 1876 ; but as no comprehensive maps of the mine-workings were then available, I could only observe in a general way the analogy with Euro- pean deposits examined by me. According to Arnold Hague,! the series here occurring of Prospect Mt. quartzite. Prospect Mt. limestone. Secret Canon shale, and Hamburg limestone is Cambrian. The ore is con- fined to the limestone first named, and in particular to a por- tion thereof on the 1^. E. slope of Ruby Hill, enclosed between two fault>fissures. The features of the I^W.-SE. ore-bearing zone are too variable to be indicated by a normal cross-section. Fig. 76 shows a generalized and Fig. 77 an actual section, as represented by Curtis. The main fault-fissure separates, in the upper level, the mas- sive limestone in its hanging- from the crushed, ore-bearing limestone in its foot-wall. In the lower levels it shows, in the * E. W. Eaymond, "The Eureka-Eichmond Case," Trans. A. I. M. E., 1877, vi., 371. t " Silver-Lead Deposits of Eureka," U. S. Geol. Surv., Monogr. vii. , Wash- ington, 1884. J "Abstract of Eeport on the Geology of the Eureka District," Third Ann_ Bep. of U. S. Geol. Surv., 1881-1882, Washington, 1883, p. 241. THE aBNESIS OF ORE-DEPOSITS. 113 foo1>wall, quartzite with intercalated "Lower shale," and in the hanging-wall, further down, shale and quartzite. An ideal restoration, above the present saddle of Ruby Hill, of the foot- wall rocks which have been removed by erosion, would bring to light a relative displacement of 150 to 600 meters (492 to 1968 feet), the indications being that the foot-wall has been lifted. This would explain at once the crushing of the lime- stone in the foot-wall, and the creation of a second fault near the contact between the limestone and the underlying quartzite. The ores occur chiefly in the well-known form of chimneys and in individual masses, mostly interconnected by traces of ore, at least at the depth where the two faults come together. In the mines to the SE., about 180 meters (590 feet) from the Eureka-Richmond boundary, the fissures come together at the depth of about 400 meters (1312 feet), the line of their inter- section thus dipping gently 'SW. The ores encountered in the upper zones, above water-level, were, with the exception of a few insignificant remains of sul- phides (mostly argentiferous galena), oxidized ores, such as cerussite and anglesite, chlorides, etc., carrying a considerable amount of silver and a little gold. The present water-level follows approximately the line of intersection of the two faults, but the fact that oxidized ores have been found still deeper in- dicates that the water-level was once lower down. It might consequently be expected that caves formed by the vadose circulation would also occur at considerable depths, especially as the whole wedge of limestone is traversed by ore- shoots, the oxidation of which would, of course, give occasion for cave-formations. The newly-formed caverns would often lie along the ore-channels, and especially in their upper por- tions. (See J. S. Curtis, /. c, p. 100.) Some of the irregularly distributed ore-bodies follow rather the quartzite-limestone contact; others rather the main fissures, with a N"W. dip, like that of the limestone wedge. Of the two largest bodies, which have furnished the chief product of the district, the east ore-body exhibits a steep SE. pitch for nearly 400 meters (1312 feet), and the west ore-body, for nearly an equal distance, a fiat IST.W. pitch. In considering their structure, we must distinguish sharply between their original and their decomposed condition. The 114 THE GENESIS OF ORE-DEPOSITS. latter often hinders a clear recognition of the former. The strata-like deposits of cerussite and other products of decompo- sition mentioned by Curtis {I. c, p. 98) are perhaps, like those in my sketch, Fig. 78, from the Old Telegraph mine, remains of the original crustification, and his statement (p. 104) that " when the ore is not oxidized there are no signs of a banded or concentric structure, and the phenomena observed point en- tirely to substitution of the sulphurets for country-rock," may thus be explained. In like manner his assertion, in the same place, that " the internal structure of the ore-masses in no way resembles those of E,aibl," is so far correct that the original filling is at Raibl extraordinarily distinct, and at Eureka, on the contrary, perhaps, only obscurely traceable. I personally saw in the Eureka mine some small ore-masses which exhibited crustification, if not in a striking degree, yet sujHlciently to be recognized by an impartial observer. Mr. Curtis himself (I. c, p. 98) says that " rounded boulders of limestone as a nucleus " occasionally occur in the ore-mass, and that in a limestone-breccia " small masses of ore sometimes completely fill the spaces between the limestone walls," — ^two phenomena which indicate crustification, and are explained by the hypothesis of a filling of pre-existent spaces. A metasomatic removal of the limestone, such as has taken place in the secondary calamine-deposits of Raibl, cannot well be supposed for the original ore-deposition at Eureka, but may have attended the formation of the secondary, decomposed products. I believe that later mining in deeper zones has developed more clearly the structure of the original Eureka deposits, and that specimens of the ore have shown, after polishing, traces, at least, of crustification. In short, I consider the original Eureka ores to have been deposited in pre-existing spaces by ascending mineral solutions, while their decomposition and the formation of the caverns are the effects of descending surface-waters. I agree with Mr. Curtis that the ore-solutions ascended from the deep region through the " main-fissure " (which has, in the NW., the character of a Blatt at Raibl, and in the SE. part of the district is filled with rhyolite), and that they formed and filled the ore-channels in the soluble, fissured limestone. THE GENESIS OF ORE-DEPOSITS. 115 Missouri and Wisconsin.— We have dealt thus far with ore- deposits in mountain districts, where tilting and folding, as well as the occurrence of eruptives, betray a disturbance of the original relations of stratification. But there are also deposits in limestone in plateau-regions, where the strata show no con- siderable disturbance. Under this head two great districts deserve attention; namely, the lead-regions of Missouri and Wisconsin. Concerning the former, we may refer to a number of more or less detailed descriptions.* We have in this case not a perfect plateau, since here and there domes of the underlying Archsean come to the surface, as especially in the continuation of the Ozark mountains; but the predominant character is nevertheless that of a structural plateau. The ore-deposits, chiefly confined to the Silurian limestone, are in part primary xenogenous and in part hyster- omorphous (debris) deposits; the latter, as is well known, consist of the detritus from the weathering and erosion of the outcrops of the former. In the former we find all the phe- nomena encountered in the deposits of mountain regions. One of these is peculiarly developed, namely, the gently inclined cavities or ore-channels, shown in the Valle and Bish mines of Jefierson and St. Francis counties, concerning which J. R. Gage has given some (unfortunately not very clear) notes and sketches. In the Valle mines, a shaft 49.9 meters (164 feet) deep, situ- ated 33.5 meters (110 feet) above the valley-bottom, encoun- tered at three different depths, respectively of 44.5, 46.3 and 49.9 meters (146, 151 and 164 feet), flat-lying ore-channels, 1 to 2 meters (3 to 6 feet) wide, which, winding in different direc- tions, produce networks, connected at the intersecting points by chimneys from one level to the other. The cross-section of these channels in the horizontal limestone or dolomite con- tracts sometimes to a few square centimeters, or enlarges to * J. R. Gage, "Lead-Mines of S. E. Missouri," Geol. Sun. of Mo., 1873-4, p. 603, and Tram. A 1. M. E., iii., 116. G. C. Broadhead, "TheS. E. Mo. Lead-Districts," Ibid., p. 100. A. Selimidt and A. Leonhard, "The Lead- and Zinc-Begion of S. W. Mo.," Oeol. Surv. of Mo., 1873-4, p. 384. A. Schmidt, "The Lead-Region of Central Missouri," Ibid., p. 503. 116 THE GENESIS OF ORE-DEPOSITS. several square meters, with a height of 3 to 4 meters (10 to 12 feet). The original metallic filling was galena, pyrite and zinc- blende, but is already oxidized to cerussite, anglesite, smith- sonite and calamine, which are accompanied with barite and a red clay. We are specially interested in the original structure of this filling ; but this is not easily detected in the mere dia- grams at hand. Figs. 32 to 35 reproduce four of Mr. Gage's sections, the first three being Pigs. 17, 18 and 19 of his paper in these Transactions, and the fourth. Fig. 72 of his article in the report of the Missouri survey. They indicate for both the' metamor- phosed and the original mineral crusts a prevailing horizontal position, so that we might conclude that the deposits took place in cavities, the upper portions of which were filled with gas only. A very peculiar formation is the red clay which in some instances covers the walls of the caverns and surrounds on all sides the central filling. The data at hand afford no clue to its origin. Mr. Gage's description of Fig. 35 (/. c, p. 618) is as follows : "Fig. [72] represents the occurrence of these minerals. The solid limestone contains a fissure, entirely filled with minerals and gangue. The minerals are completely enveloped by the red clay. Above are two thin folds of silicate of zinc, separated from each other and from the limestone by the red clay. The folds of the zinc-ore are sometimes perfectly solid, being from one to six inches thick, and consisting of alternate layers of the same material in very compact folds ; again, the mass of zinc-ore is from one to six inches in thickness, but, instead of being dense, consists of a thin crust, with a cavity, whose interior walls are lined with beautiful, brilliant crystals of the silicate and occasionally the carbon- ate of zinc. More rarely, crystals of galena are in the cavities, but in this case are invariably covered with a thin coating of the silicate ; and not infrequently portions of the cavities are partially filled with red clay, highly impregnated with oxide of iron, and having the appearance of a highly decomposed brown hematite. Occasionally heavy spar (barytes) lies in a dense mass in close contact with the zinc-ore ; but more frequently it is associated with the galena. Often, but not invariably, immediately below the folds of zinc-ore, occur irregular masses of the zinc-ore in the crystallized form, as pseudomorphs of galena," etc. : . All.the,^doubts which arise concerning the mode of this for- mation would probably be solved by a series of objective pictures of it ; and it is to be hoped that an occurrence so interesting theoretically will be accurately recorded before it is too late. The deposits occurring near the " islands " of granite and THE GENESIS OF ORE-DEPOSITS. 117 porphyry have special interest. "While the Silurian limestones of the surrounding country, farther from these islands, present chiefly only lead- and zinc-ores, other metals, such as copper, cobalt, and nickel, occur as the Archpean foundation-rocks are approached ; and this circumstance is, to my mind, an indica- tion that the source of the lead-deposits also is to be sought in depth. Mine la Motte. — As an example, I may cite the district of Mine la Motte, to which I once made a brief visit. The rock here is usually the same, namely, a Cambrian dolomite, con- taining, however, sandy portions and a clayey stratum charac- terized by numerous fossils [Lingula). The ore occurs pre- dominantly as an impregnation in the rock, more concentrated in a given zone. The so-called sandstone does not here, as in other instances, cut off the impregnation ; it is, in fact, only a sandy limestone and dolomite, and its carbonates can be re- placed by ore as well as those of adjoining strata. I thought that I noticed in the open workings called the Jack and the Seed-tick diggings a very remarkable phenomenon; namely, the ore-impregnation in the almost horizontal stratified rock was conformable not to the bedding but to planes crossing it at a very acute angle (about 10°). A pretty long terrace was exposed; and the impregnation-planes cut pretty regularly through the sandy dolomite also. This appearance indicates plainly a later formation of the ore, independent of the depo- sition of the rock-strata ; and one is almost involuntarily forced to believe that it was the former ground-water surface which formed the cavities to be impregnated. But it was, and is, in- conceivable to me how these cavities could be tilled with sul- phides ; and I can only urge that occurrences of this kind should be subjected to a more thorough study than it has been in my power to give to them. Wisconsin. — In Wisconsin, and in parts of Iowa and Illinois, there is an extensive true plateau, the calcareous members of which contain many and various deposits of lead- and zinc- ores. An excellent monograph concerning them, by my es- teemed friend, Prof. J. D. Whitney,* is at hand. The author * Report of a Geological Survey of the Upper 3Iississippi Lead-Region, Albany, 1862. 118 THE GENESIS OF OEB-DBPOSITS. seeks to show that the mineral solutions depositing these ores came from above, not from below. He appeals to the circum- stance that of the two stratified formations, the upper and the lower Magnesian limestone (underlain by an upper and a lower sandstone, respectively), the ores occur chiefly in the upper, and only seldom, and in small quantity, in the lower ; while the two sandstones (the lower of which is assigned to the Potsdam) do not reveal any traces of ore, as they should do if the solu- tions had come from below. I confess that this conclusion is not obvious to me. There may have been a passage through these sandstones at a distant point, not yet exposed ; and the mineral solutions may have found or created spaces in the soluble rock. The argument that the ores must have come from above because it has not been possible to discover, in the Wisconsin region, fault-fissures and eruptive dikes, such as have brought up similar ores in the north of England and other places, seems to me likewise inconclusive. And as little can I accept the explanation of an occurrence near Dubuque, discovered by T. Lavins and described by "Whitney (op. cit., p. 291 and Fig. on p. 392), which I reproduce in Fig. 79. The fragments of galena, crusted with cerussite, which hang from the roof of a natural cavern, are taken as a proof that the solutions which deposited them must have come from above. But a continuar tion of this cavern is indicated in the bottom, filled with clay, mixed with scattered pieces of galena. In my opinion, this was doubtless originally the filling of a vertical fissure, which was enlarged by the ground-water, as indicated by the dotted line. The symmetrical crusts, as I suppose, of that filling were in part broken up, and fell into the clay accumulating in the space below ; while the upper part of the filling remained at- tached to the rock of the roof. 3. Metamoephous Deposits. Metamorphism has been most truly defined by A. de Lap- parent as the sum of the chemical changes undergone by the sedimentary rocks after their deposition. General or regional metamorphism, affecting the rocks over wide areas, is distin- guished from local or contact-metamorphism, caused in certain groups of strata by eruptive intrusions. In studying the oc- THE GENESIS OF ORE-DEPOSITS. 119 currence of useful minerals, we occupy rather the local stand- point, and start with an assumed original condition of the rock, though its really original character may not always be demon- strable — understanding thereby, for our purpose, a so-called typical condition, usually shown at most places where the rock occurs. "We distinguish the replacement of some constituents of a compound rock, for which the term " impregnation " is more appropriate, from the replacement of the whole homogeneous mass by metasomasis. But since every rock undoubtedly con- tains small primitive cavities, it is difficult, and sometimes impossible, to decide whether a new, xenogenous substance has not been deposited in such pores; and a case of this kind would fall under our notion of impregnation. The new sub- stance may indeed have found entrance through the pores, if the mineral solutions were under sufficient pressure to over- come the friction of their walls, at least in the line of least re- sistance ; and these solutions, thus introduced, may attack and replace one or another element of the rock. The entrance of such solutions will be greatly facilitated by the Assuring of the rock, whether by internal or external forces. "We find in con- nection with ore veins, and also with the thinnest mere seams, an impregnation of the country-rock, which Cotta has called subordinate or dependent (unselhstdndige) impregnation. The particles of certain substances possess a peculiar mutual attraction. In the sandstone of Fontainebleau occur aggre- gates of calcite crystals, which have come together in spite of the separating medium of sandstone ; and in a similar way, as we have seen, another substance of strong crystallizing power, namely, galenite, forms, in the pipe-ores and script-ores of Eaibl, crystalline masses, in spite of the intervening diaphragm of a foreign medium. In like manner are formed the so-called concretions, the cal- careous and marly masses {Losskindlem) in the Loess, and the Marleker of the ancient Scandinavian beaches. For the for- mation of the former, occasion was given by decaying plant- roots ; for that of the latter, by various animal remains, mussels, fishes, etc. In ]S"orway, they have preserved a complete fauna of the Griacial and post-Glacial epochs. Similarly, we find in some spherosiderite concretions of 120 THE GENESIS OF ORB-DEPOSITS. the Saarbriicken coal-basin the remains of fishes. A discerni- ble nucleus is not always found in such concretions ; sometimes no cause for this peculiar formation can be discovered. The concretions occurring in stratified rocks are usually lenticular, comprising portions of several similar strata. Even spherical forms, resembling pisolites, occur. If we imagine, for instance, spherosiderite concretions formed closely side by side in one stratum, we shall have a regular bed of clay-ironstone. Leaving out of view the agency of fissures, or contacts with intruded rocks, impregnations fol- lowing certain strata maj' be formed, constituting a second kind of ore-beds. A third kind may result from the more or less complete replacement of the original rock, especially when the latter is a soluble precipitate, like gypsum or limestone. In thick limestone formations the ore-beds occur at the contact with insoluble rocks, as at Rodna. In all these cases the deposits have the form of a bed, but the ores rarely cover the whole contact-surface, occupying, on the contrary, only certain zones of it. In other words, in these as in other deposits, ore-shoots occur. Much more complicated relations result when the mineral solutions ascend along structural fissures and rock-contacts ; and in order to a comprehensive description of this suite of phenomena, it will be well to consider first the simpler condi- tions obtaining in soluble rocks, and afterwards the more com- plex occurrence of such deposits in crystalline and eruptive rocks. We will, therefore, review the metamorphous deposits as they occur in [a) distinctly stratified rocks ; (h) soluble pre- cipitates ; and (c) crystalline schists and eruptive rocks. a. Metamorphous Ore-Deposits in Distinctly Stratified Bocks. We find in unquestionable sediments not only metallic oxides and salts, but also sulphides, in the form of ore-beds which, by reason of this stratigraphical relation, have been held to be of contemporaneous origin, that is, idiogenous. As a consequence, it has been necessary to assume that they were precipitated in a sea-basin, in which, before and after their precipitation, only barren sediments were deposited. These metals must, there- fore, have been dissolved in the water of the basin, and that in very large quantity, as indicated by the frequently great thick- THE GENESIS OF ORE-DEPOSITS. 121 ness of the ore-beds. But for such an assumption we have no present analogy. The Deposition of Ores from Sea- Water. — In this particular, however, we have to do rather with suggestions than with demonstrations of fact. So far as sea-water is concerned, traces of metals have been found in the water itself, in the ashes of marine plants, and in the solid constituents of marine animals, for instance, corals by Malagutti, Bibra, and Forch- hammer.* Traces of silver, iron and manganese were de- tected in the water, and lead, zinc, cobalt and nickel in the marine organisms; and since there are in sear-water small amounts of hydrogen sulphide, Bischof considers the deposi- tion of metallic sulphides from the sea to have been possible. He observes (op. cit., p. 432) that the occurrence of metallic sulphides in sedimentary rocks, such as that of copper and sil- ver sulphides in Kupferschiefer, or that of lead sulphide in JBuntsandstdn, may be thus explained ; and even indulges (p. 836) in the following teleological conclusion : "Since it cannot be doubted that the rivers flowing into the ocean bring with them metallic salts, though in very dilute solution, it seems a wise arrangement that in the hydrogen sulphide of sea-water a precipitant is presented to throw down the smallest minima, and thus to prevent the gradual accumulation of substances so injuriotis to animal life." Of the various metals dissolved in sea-water, iron is least injurious to animal life. Indeed, animal life assists, in the so- called lake-ores, the segregation of this metal. Moreover, the precipitation of ferrous and ferric oxides from concentrated solutions is probable, so that a precipitation of iron-ores di- rectly from sea-water seems to be established as a possible ori- gin for some iron-ore beds. But the conveyance of metallic salts by rivers to the ocean and the formation of hydrogen sulphide in sea-water are un- questionably continuous ; and the precipitation of metallic sulphides must, therefore, have taken place uniformly in all sediments and precipitates of the, ocean ; whereas, we find the ore-beds in fact only in certain strata. If these are to be thus explained, we must assume that the ocean was at certain periods much more strongly impregnated with metallic salts — a scarcely * G. Bischof, Chem. u. Phys. Geologic, vol. i., Bonn, 1843, pp. 445-447. 122 THE GENESIS OF ORE-DEPOSITS. tenable hypothesis as applied to the mighty deep, — or we must suppose with Carnall, as H. Hoefer has recently done,* a sub- sequent re-deposition of the primitive metallic salts, contained in minute quantities in the sea-deposits — in other words, their solution and re-precipitation at certain horizons. Hoefer cites the lead- and zinc-deposits of Upper Silesia and other districts, which occur in marine Triassic limestones. He assumes the maintenance of uniform horizons by these deposits to be demon- strated, but points out that some of these horizons were already ore-bearing when first formed. In short, a number of investigators have adopted the hy- pothesis of an original ore-deposition from the ocean, without giving any other reason than the observed relations of stratifi- cation. Yet, in a considerable experience with ore-deposits in marine limestones, I have never been able to find genuine ore- beds among them, but always only ores of subsequent intro- duction ; so that I feel warranted in believing that such ore- beds proper do not exist. As to the primitive ore contained in marine sediments and precipitates, innumerable chemical analyses, especially of lime- stone, have failed to show the metallic traces which, according to the above hypothesis, should be present. For this reason, as I have already observed, even Sandberger did not venture to derive the metals from the limestone, preferring, for in- stance, at Eaibl, to look to the overlying slates. The maintenance of certain ore-bearing horizons was set up by A. von Groddeck, to render more plausible the notion of a direct deposition from the ocean ; but I do not believe it pos- sible to prove such an identity of horizon for different ore- deposits. Similar ores and stratigraphical conditions are not confined to the Trias. On the Rhine, in England and in America they occur at much lower horizons in the Palseozoic rocks. Even in Carinthia the ore-bearing limestones of the richest deposits do not occupy the same horizon. That of the Eaibl slate is very different from that of the Bleiberg slate (carrying Ammonites aon), and the deposits in these localities are by no means beds, but, as I have shown, channels in the lime- stone, filled with ore. * " Die Entstehung der Blei-, Zinc- u. Eisenlagerst. in Obersehlesien."— Oesf err. Zeitsch. /. Berg. u. H -wesen, 1893, xli., p. 82. THE GENESIS OF ORE-DEPOSITS. 123 Ore-Deposition in Fresh Water. — The demonstration of direct ore-deposition in fresh-water strata encounters the same diffi- culties, though it may be supported by the same chemical speculations. Here the hypothesis is favored by the analogy of the lakes of regions without drainage to the sea, in which the salts brought in by rivers are necessarily concentrated by evaporation. But since organic life is restricted in these salt lakes to a few animal species, the analogy can have but a limited application. Moreover, it would be necessary to suppose cata^ clysmic changes, like the interposition of a period of no drain- age in the midst of an epoch of fresh-water sedimentation. Without the assumption of such cataclysms, I do not believe that the Mannsfeld Kuj^ferschiefer, in which the organic (fish) remains can be traced continuously from foot- to hanging-wall, could be explained in this way. It deserves mention, that some of the earlier geologists, like Freiesleben, accepted the some- times contorted attitudes of the Palceoniscus in the Kupferschiefer as a proof of contemporaneous ore-depositions, and alleged that these fishes had been thrown into violent contortions by the copper-solution, in which condition they died and were buried in the sediment. The naivete of this diagnosis (which, never- theless, some modern writers have not hesitated to repeat) is evident. Contorted fish-remains occur in other formations out- side of the Kupferschiefer, and clearly show the advanced state of decomposition in which the bodies reached the sediments. The Kupferschiefer of Mannsfeld. — The Mannsfeld Kupfer- schiefer, as is well known, is a thin bed of bituminous slate, lying between the Permian sandstone below, and the marine member of the same formation, the Zechstein, above, and con- taining sulphides of copper, silver, lead, zinc, antimony, mer- cury, nickel and cobalt. The copper amounts to 20 to 30 kilo- grams (44 to 66 pounds), and the silver to 125 to 160 grammes (4 to 5 ounces, Troy) per metric ton of 2204 pounds. In pol- ished sections, the ore can be seen in thin leaves lying be- tween laminse of slate, and often accompanied by gypsum. But the same ores occur in scattered bunches in the sandstone below, and small bodies of redruthite are found in the lime- stone above.* This circumstance alone, that ore occurs also * See Groddeck's Erzlagerslatten, | 58, and Cotta's Manual, § 50. 124 THE GENESIS OF ORE-DEPOSITS. in the marine limestone, above the fresh-water Kupferschiefer, is unfavorable to the contemporaneous origin of ore and rock. Kupferschiefer in Thuringia and Bohemia. — The same bitu- minous slate occurs in the Thuringian forest on the south slope of the Hartz, and in other points a considerable distance away. It must therefore have been deposited in a large basin. But it is a question, whether it anywhere carries ore and deserves the name of Kupferschiefer. In ]SrE. Bohemia, the same Permian slate, with almost the same fossils, is widely distributed, but without the marine member which covers it in Germany. The Permian of Bo- hemia carries copper-ores in many places ; and in one locality, namely, at Hermannseifen, these ores occur in the bitumin- ous slate, which might properly here be called Kupferschiefer. I had opportunity in 1858 to examine the mines. The richness in metal was not unsatisfactory ; but there was much complaint of the numerous faults which seriously enhanced the difficulty of mining. Precisely the same difficulty exists at Mannsfeld and in the Thuringian forest, as Cotta {op. cit., § 50) reports in part as follows : "The fault-fissures themselves are, however, rarely ore-bearing, yet often seem nevertheless to have influenced the ore-bearing character of the strata traversed by them. This influence is shown in the increase or diminution of the propor- tions of ore, not only in the immediate neighborhood, but sometimes also for a considerable distance, even as far as the next master-fault. It is shown also in the transfer of the metallic contents from one stratum to another. ' ' This and other observations concerning the influence of the faults upon the ore-distribution bear decidedly against the con- temporaneity of the ore-deposits, and in favor of a later intro- duction of ore through the fault-fissures. But this conclusion becomes much clearer upon a consider- ation of the remaining occurrences. Thus, according to Cotta (op. cit., § 39), the Kupferschiefer at the edge of the Thuringian forest is not so rich in ore as on the southern border of the Hartz. More important than the copper-slate itself are the fault-fissures which traverse the whole group of strata, but only carry ore in certain zones in which they intersect certain strata — the Kupferschiefer among them. " Strange to say," observes Cotta, " near Camsdorf it is almost exclusively where THE GENESIS OP ORB-DEPOSITS. 125 the Kupferschiefer has suffered such disturbances that it is rich enough to repay mining." In speaking of Eiegelsdorf he says, " The cobalt-ores have in some cases made their way from the veins into the country-rock." Westphalia. — At Stadtberg {pp. cit., p. 76), in Westphaha, there are even several copper-bearing strata, and these are cut by copper-bearing veins. At Bieber, veins traverse the whole group of strata into the underlying mica^slate, and " the irreg- ularly distributed ore occurs, strange to say, chiefly interleaved in the mica-slate, and not, as in the Hartz and the Thuringian forest, in the horizon of the Kupferschiefer ; while, on the other hand, the impregnations from the veins have penetrated chiefly the bituminous marly slate." In consideration of the expressions partly quoted verbatim above, it is difficult to see how there can be any doubt of the secondary nature of the ore-deposits in the Kupferschiefer throughout. Yet Groddeck* has reproved me for coming to this conclusion. He says himselff expressly (evidently having in mind the typical Mannsfeld occurrence) : "The ores were laid down contemporaneously with the slime-deposit, the bitu- minous marly slate as the ore-matrix." .... " It is entirely impossible that the ores could have entered the bed somehow from the fissures, at a later period, after the covering of the marly slate with more recent rocks. If we assume that the ore-solutions were introduced through the fissure faulting the bed, it remains in- conceivable why the filling of metallic sulphides, through a field of many square miles, should be uniformly and exclusively confined to the stratum of marly slate, about J meter (19.5 inches) thick, and should not also occur more or less near the fissures in the strata above and below, there being in these no lack of carbonates and bituminous constituents, available as precipitants of the solutions — the Stink- sehiefer, for instance, lying not far above the Kupferschiefer, being rich in such substances. ' ' Groddeck here overlooked the principle, elsewhere urged by him, that a single link in a whole chain of phenomena should not be exclusively considered. He contemplated only the special development at Mannsfeld ; assumed, moreover, sim- ilar developments for many square miles, which show in fact many variations, and did not take into account the circum- stance that when the Kupferschiefer is not cut by fault-fissures, it is also not valuable for mining. Finally, he was unac- * "Bemerk. zur Classification d. Lagerstatten," S. u. H. Ztg., 1885. f Erzlagerstdtienlehre, 5 142. 126 THE GENESIS OF ORE-DEPOSITS. quainted with the theoretically important occurrence of the Kupferschiefer in Bohemia. The contemporaneous origin of the ore and rock at Mannsfeld was with him, so to speak, a dogma, as may be perceived in some of his expressions (op. cit., p. 302) : "The local ore-bearing character of the foot- and hanging- walls of the Kupfer- schiefer-heA is no proof to the contrary, for it is always confined to the immediate neighborhood of the bed. " (?) "Into the sea, rich in fishes and plants, from which the marly slate was de- posited, flowed abundant metallic solutions, which killed the organisms and were themselves reduced by the products of decay." (?) The first of these propositions becomes logical if it is simply reversed in sense ; and the bold hypothesis of the second indi- cates a doubt which the author is seeking in this way to set at rest. His statement (p. 302) : "It is not to be doubted that metallic sulphides may be formed at the earth's surface, under ordinary pressure and temperature, beneath a water-covering which excludes the air," is quite correct ; but when he adds : "And there is therefore nothing to prevent the belief that sulphuretted ores could be precipitated at the same time with the deposition of sedimentary rocks," it is necessary to add, "provided the metallic salts were present in the sea^basin." This is, indeed, the center of gravity of the whole question ; and, as I have shown, the proposition presents an improba- bility. Various other peculiarities of individual ore-occurrences are cited in favor of the theory of contemporaneous origin; but all of them, when impartially weighed, are equally consistent with a different genetic explanation, and fail to be as signifi- cant as the Mannsfeld type for the theory in question. The Copper-Sandstones of Bohemia. — In Bohemia and on the west slope of the Urals, the copper-ores of the Permian strata occupy by no means a continuous horizon, but occur as im- pregnations in different beds, beside, above, or below one an- other. There are here, as in the German Kupferschiefer mines, fault-fissures which may have served as ore-conduits ; and in these regions the notion of a primary sedimentary origin of the ores has not been so often suggested. At some places in Bo- THE GENESIS OF ORE-DEPOSITS. 127 hernia, as, for instance, at Starkenbach, melaphyres appear above the ore-beds. In almost all these, as in many of the German deposits, the copper sulphides, especially redruthite, occur in the neighbor- hood of plant-remains ; and oxidized copper-ores predominate, as a rule, in the ore-beds in sandstone. Not only Permian, but also Triassic and still more recent sandstones, exhibit analogous deposits, containing lead, silver, and antimony, as well as copper. At Boleo, in Lower Cali- fornia, such an ore-deposit is known in Tertiary strata. The range of illustrations, therefore, is an extensive one. I can mention but a few. St. Avoid. — Concerning the copper-ores in the Triassic sand- stone of St. Avoid and Wallerfangen, Groddeck gives (p. 90) a brief description, based on an article by C. Simon.* The sporadic ores are most abundant in the vicinity of fault-fis- sures ; but only single strata are rich, while other porous layers near by are barren of ore. The ores extend in zones, inde- pendent of the course of the fissures, which they often even cross at right-angles. These two features are said to prove the contemporaneous origin of the ore and rock, " since the en- richment of a zone where it is cut by the fissures can be simply explained by the leaching-out of ores in higher strata, and their re-deposition in or near the fissure." I must confess that this explanation is not satisfactory to me. Figs. 80 and 81 illus- trate the situation. At Bleiberg, in St. Avoid, concretions of galena, of pea-size, occur in the sandstone ; and below the same layer considerable masses of solid galena are encountered. The Lead-Deposit of Mechernich, near Commern.-\ — This de- posit has a special interest in this connection, since it consists of sandstone of considerable thickness, somewhat porous, and impregnated with small concretions of galena (Knoten), which have often been considered as contemporaneous in deposition * Berg. u. H. Ztg. , 1866, p. 412. t Baur, " Das Vorkommen von Bleierzen am Bleiberge bei Commern," Esch- weiler Pumpe, 1859. F. W. Huperts, Der Bergbau u, Huttenbetneh des Meahemieher Sergw. akt. Ve- reins, Koln, 1883. Ellsworth Daggett, "The Lead and Silver Works of the Mechernich Mining Company," E. and M. J., xxiii. 128 THE GENESIS OF ORE-DEPOSITS. with the rock. The district, situated on the north edge of the Eifel Mountains, embraces a zone about 7 kilometers (4J m.) long, through Call, Keldenick, Mechernich and Strempt. Al- ready in the Roman period, at the Tanz Mountain, near Kel- denick, mining was done upon galena veins in the Devonian limestone, which is overlain by the sandstone and conglomerate of the variegated sandstone formation. The conglomerate cov- ering the sandstone has the name of Wackendeckel, and some- times carries ore, the cement between its pebbles being trav- ersed by galena and oxidized products, especially cerussite, which were formerly mined. It is at present the sandstone, impregnated with galena con- cretions (Knoien) to the extent of 5 to 30 kg. (0.5 to 3 per cent.) of lead, and 1 to 6 grammes (0.03 to 0.18 oz. Troy) silver per metric ton of 2204 pounds, which is the principal basis of an extensive mining industry. The thickness of this Knotensandstein, the number of its in- tercalated conglomerate layers, and the richness in ore of each stratum vary greatly, as do also the number, direction and man- ner of throw of the fault-fissures by which it is traversed. Fig. 82, representing the stratigraphy SW. of the boundary of the mining grant at Meinerzhagen, shows the irregularity of the displacements. Within the grant, the several Knoten-lajers are united into a single bed, about 22 meters (72 feet) thick, sep- arated by a conglomerate layer from the Devonian rocks below, and overlain by another conglomerate, the so-called Wacken- deckel, above which is the barren red sandstone. In general terms, there lies here upon an impermeable floor a pervious group composed of sandstones and conglomerates, overlain by argillaceous red sandstone and loam. The Knoten, never larger than peas, exhibit, when prepared in thin sections and mounted in Canada balsam, crystalline ag- gregates of galena, in which the crystal-faces are turned out- wards, away from the center ; that is, they are by no means composed of spherical masses, as they seem to the naked eye to be, when examined as they come from the crumbly rock. Their distribution in the sandstone generally follows the bed- ding ; but in the neighborhood of the cross-faults I observed an accumulation of Knoten in zones parallel to these steep fis- sures. Moreover, I found occasionally in the fissures them- THE GENESIS OF ORE-DEPOSITS. 129 selves threads of galena and pyrite ; and hence I do not doubt that the ore-deposition here was secondary, and proceeded from the fissures. To gain a clear view of this question, it is neces- sary to include the ore-occurrence in the conglomerates, where, as already observed, it impregnates the material cementing the pebbles, and also the nearest ore-occurrence in the Devonian limestone, where it appears in fissure-veins. In my opinion, the loose, pervious sandstone, enclosed be- tween less permeable strata, and cut by many fault-fissures, was impregnated by ascending springs, which employed it as a path in their circulation ; but it cannot be determined what constituted the centers around which the galena concretions are formed. May it have been minute particles of feldspar, such as are still occasionally visible; or was it organic sub- stances, which have now entirely disappeared ? Freihung. — Perhaps additional hints may be furnished by the mines of Freihung in the Bavarian Upper Palatinate, which Cotta considers analogous to those of Mechernich. Here galena and cerussite impregnate the Keuper sandstone, the steep dip of which they share. At the ITuremberg Expo- sition of 1882, maps, ore- and rock-specimens from the mines of the Bavarian Lead-Mining Co. were exhibited. Fig. 83 is a section through the Vesuvius mine. I was struck with nu- merous specimens of tree-stems changed to galena ; and; coming subsequently into possession of such a specimen, I had a polished section prepared from it. The pieces of these stems exhibited are about 20 centimeters (8 inches) long, and elhptical in sections, say 5 to 7 by 10 to 15 centimeters (2 to 3 by 4 to 6 inches). The fiber and the annual rings could be recognized on the surfaces of fracture, but were extremely plain in the polished section. Indeed, they were indicated by the cleavage of the specimens. I have thin slivers, 2 to 4 mm. (0.08 to 0.16 inch) in diameter and several centimeters long, representing the fibers of the original wood. The former bark is replaced by a zone of first pyrite, and then quartz grains cemented with pyrite. I do not know that the determi- nation of the species of the wood has been attempted, but I think it should be approximately practicable. Fig. 84 is a diagram of the section of such a stem altered to galena. Certainly we have here another instance showing that the 130 THE GENESIS OF ORE-DEPOSITS. organic substance attracted metallic solutions and reduced them to sulphides, and this under conditions similar to those of Mechernich. The latter occurrence may, therefore, be most simply explained by the hypothesis of an organic substance, distributed through the rock, which reduced the circulating mineral solutions and occasioned the formation of the concre- tions (Knoten). Silver Reef. — Accustomed as we are to find silver associated with lead-ores, we are surprised by the occurrence, in the Silver Reef district of Utah, in probably Triassic sandstones, of silver accompanied by copper. So far as can be gathered from the various descriptions at hand,* there occur here two beds (the outcrops of which are called " reefs ") which carry silver, either exclusively or with a little copper — the former usually as a chloride, but sometimes native ; and the latter in the ordinary oxidized ores. It may be reasonably inferred that the deposit has been thus far exposed in its upper, chloridized and oxi- dized zones ; and that in depth it would be found to contain sulphide-ores. "Whether such depth has been reached by the miners I do not know. The beds consist of red and gray argillaceous sandstones and arenaceous clay-slates, between the laminae and in the cross- joints of which the ores occur, being the more concentrated, the more highly fissured the condition of the rock. Although traces of silver are found throughout the bed, the pay-ore is confined to separate chimneys or channels, which descend on the true dip, or pitch obliquely to it. The richest bodies are said (Rolker, I. c, p. 25) to be most frequently foiind above a certain thin, very clayey, sandstone stratum. Very often, but not always, the silver-ore is accompanied by carbonized vege- tation, such as trunks and stems of trees, and reed-like plant- remains, which are covered and impregnated with horn-silver. The copper- and silver-ores, while occurring to a certain degree in association, seem to exclude one another, and are seldom found in actual mixture. * " The Silver Beef District, Southern Utah " (by E. P. Kothwell or Thomas Couch?), Eng. and M. Jmir., xxix., pp. 25, 45, 59, 79, 351. C. M. Eolker, "The Silver-Sandstone District of Utah," Tram. A. I. M. E., ix., 21. J. S. Newberry, "Eeport of the Stormont Silver Mining Co.," E. and, M. J., XXX., p. 269. THE GENESIS OF ORE-DEPOSITS. 131 The same sandstone which here carries ore is said to be rep- resented in the plateau cut by the Colorado river ; but there the strata are horizontal and undisturbed, whereas in the ore- district they dip rather steeply, are much disturbed, and are in many places covered with eruptive rocks, including basalt. This neighborhood to eruptives renders it probable that here, as in so many other places in "Western America, the ores have been introduced by the mineral springs which usually follow erup- tive activity. Rothwell, Couch, and Rolker are of this opinion ; whereas, Newberry is inclined to suppose a contemporaneous origin of ores and rock. The principal arguments for his view are, the alleged great area of silver-bearing Triassic strata in that region ; and the circumstance that the richest bedded and lenticular ore-bodies are enclosed in almost impermeable slate- clays, which would not have permitted a subsequent entrance of the mineral solutions. ]!>I'either of these statements disproves the secondary origin of the ores. They could have been de- posited in any given way on a large scale, as well as a small one ; and that the almost impermeable slate-clays did not prevent the entrance of solutions is proved by the subsequent alteration of the original filling to chlorides and oxides.* Moreover, the deposits are not regular strata, but chimneys and channels in parts of strata, and this character, which they possess in common with so many other deposits, should be de- cisive in favor of their secondary origin — a conclusion which, in my opinion, is always reached when observations are not con- fined to single localities, but extended over whole series of anal- ogous phenomena. Copper-Deposits of New Mexico and Arizona. — Traces of sim- ilar ore-distribution in sandstones seem to be not infrequent in the American West. Thus F. M. F. Cazinf says of the copper- ores of the probably Triassic sandstones of the l^acimiento mountains in IST. W. New Mexico, which J. S. Newberry had described in 1860 : ' ' The ore occurs nearly exclusively as tlie petrefaction of the leaves, stones, limbs and trunks of palms. Frequently the ore is coated with a film of jet or coal. It * Compare F. M. F. Cazin, "The Origin of Copper- and Silver-Ores in Triassic Sandrock," E. and M. J., xxx., p. 381. t "New Mexico vs. Lake Superior as a Copper- Producer. " — E. and M. J., XXX., pp. 87, 108. 9 132 THE GENESIS OF ORE-DEPOSITS. is always easily separated from the rock. The ore is predominantly erubesite, copper-glance and melaconite, and it appears to be distributed all over the mass- ive stratum, but is more densely collected on seams and cleavages, in some instances forming a single layer of petrified parts of palm-wood." This occurrence, which is analogous to those in Bohemia and in the province of Perm, was declared to possess great economic importance. Its later developments are not known to me. W. P. Blake* has described an analogous occurrence in the sandstones and conglomerates overlying the granites in Copper Basin, Yavapai county, Arizona, where the copper-ores are found unconnected with any organic substances. In the under- lying granite, however, there are fissures filled with copper-ores. He thinks it probable that copper sulphides circulating in the highly permeable sandstone were precipitated as carbonate by carbonate of soda, while the resulting sulphate of soda escaped in solution, to be concentrated by evaporation, forming deposits of thenardite, which is common in Arizona. Lower California. — At Boleo, opposite G-uaymas, on the pen- insula of Lower California, E. Fuchsf has described a remark- able deposit of copper-ores in Tertiary sandstones, conglomer- ates and tufas, which must be mentioned under this head. The east slope of the (mostly eruptive) mountain range extending through the peninsula is a plateau, gently descending towards the Grulf of California, and cut by precipitous canons. It is formed of strata containing characteristic Miocene fossils. Tufas decidedly predominate, and the series contains three or four copper-bearing beds, covering a large area, and out-crop- ping in many places in the canons. These lie immediately upon conglomerates of pebbles of eruptive rock (difl'erent and characteristic for each horizon) and are overlain by clayey tufas. The whole is traversed by several fissures, of which the largest and most important is a fault-fissure, occurring at the western border of the district and striking about parallel with the sea- shore. In the ore-beds above the ground-water level, disseminated oxidized ores prevail, such as black oxide of copper, and the * " The Copper-Deposits of Copper Basin, Arizona, and their Origin." — Trans. A. I. M. E., xvii., 479. t "Note sur les Gisements de Cuivre du Boleo." — Assoc. Franfaise pour I'Avan- eemeni des Sdenees, 1885. THE GENESIS OF ORE-DEPOSITS. 133 protoxide, with atacamite (CuCl + 3 CuO + SH^O), azurite, malachite and chrysocoUa, with crednerite (2 Mnfi^, 3 CuO). In the second ore-bed (counting downwards) there are peculiar globular concretions, like oolites, of copper oxide and carbon- ate, sometimes several centimeters in diameter, which are lo- cally called boleos, whence the name of the district. Though greatly interested in this type of ore, I have never succeeded in getting specimens, and am unable to form from the hasty description of Fuchs a clear conception as to its genesis. The third bed lies in part below the ground-water level, and contains, in addition to the foregoing minerals, the copper sul- phides chalcosine (CujS) and covelline (CuS). The ore-beds are composed of tufa (the slime, according to Fuchs, of volcanic eruptions), in which ores in disseminated spots and veinlets (" sous forme de mouche ou de vdnules ") as well as globular concretions, are irregularly distributed, with a vis- ible tendency to concentrate towards the bottom of the bed, where they form a compact ore-layer, 15 to 25 centimeters (6 to 10 inches) thick. With regard to genetic questions, we must bear in mind that the fossils found in these strata indicate an open though not very deep sea ; it is, therefore, impossible to assume that iron-, manganese- and copper-ores were dissolved in it, and were pre- cipitated from it at the same time with the rock. A periodical metallic precipitation, three or four times repeated, in an open marine basin, is out of the question; and we are forced in this case, even more strongly than elsewhere, to assume a secondary origin for the ores. The data necessary for its explanation are still wanting, but can undoubtedly be secured by the further advance of mining work. E. Fuchs contented himself with pointing out the after-effects of eruptive processes, and did not enter upon the genetic question. Certainly the conglomerates underlying the ore-bed must have played an important part, representing, very likely, the channels through which the min- eral solutions ascended, to be reduced, probably by the presence of organic matter, in the tufas above. b. Metasomatic Deposits in Soluble Rocks. A metasomatic replacement of the original rock material was clearly proved long ago for some instances — e.g., calamine-de- 134 THE GENESIS OF OKE-DBPOSITS. posits — while in other cases, where proof has not been obtained, analogies in the observed circumstances speak for such an origin. Parts of such deposits, it is true, may be fillings of spaces of dissolution, rendered unrecognizable, as such, by the absence of clearly defined crustification in the ore-precipitates. We must accustom ourselves to the fact that for many deposits, not yet closely enough studied, it is impossible to determine positively the mode of genesis, and we must often choose provisionally, of the two modes just named, the one which appears to represent better the given data. Calamine-Deposits. — The calamine-deposits of Raibl in Car- inthia, Wiesloch in Baden, Vieille Montague, with its vicinity, in Belgium and Grermany, and other places, furnish, in the fossils of the limestone which have been transformed into cala- mine, the clearest proofs of a metasomatic replacement of the carbonate of lime by carbonates and silicates of zinc. More- over, the structure and form of the ore-deposits is characteristic of this origin, these being mostly bodies of irregular outline, with portions projecting far into the country-rock. Often the progress of the replacement can be traced. Thus, at Raibl (Fig. 85), in places where the process has started from seams, the gradual advance from the seam into the rock may be ob- served ; the outermost portions being relatively the most recent, and lying against a peculiarly uneven, rough surface of lime- stone. Sometimes features of the original rock-structure are repeated in calamine, as, for instance, the cellular structure of the so- called Hauchwacke (the eargneule of the Swiss geologists), which consists of a skeleton of thin, smooth lime-partitions, from among which the limestone has been in part dissolved away, or left only in separate decomposed splinters. This is evidently the result of a very complex metamorphosis, which Groddeck has observed also in the quicksilver-deposit of Avala in Servia. The cell-walls, which represent the fillings of cracks in a shat- tered limestone, have been subsequently changed to calamine, and covered with botryoidal clusters of that mineral (Fig. 86). Calamine is frequently formed by atmospheric agencies above the ground-water level, and is a frequent accompaniment of lead- and zinc-deposits in limestone. Space does not permit the description here of the manifold THE GENESIS OF ORB-DEPOSITS. 135 deposits in Belgium, Rhenish Prussia, Westphalia, Upper Silesia, Sardinia, Algiers, etc., which are, moreover, not known to me by personal observation. The text-books of Cotta, Grod- deck and Phillips give some account of them, and refer to sources of more detailed information. Laurium.— It is only in recent periods that the features of the extensive mining region of Laurium in Greece,* worked two thousand years ago, have been described. Although various kinds of deposits are represented, most of them belong under the present head. In the Camaresa district, a series of nearly horizontal, non- fossiliferous limestones and crystalline schists is cut by a num- ber of eruptive dikes, and suddenly assumes on the JSTE. a steep dip, probably indicating a considerable dislocation. The whole group is traversed by a number of ore-veins, which, in the schists, are often rich enough to pay for mining. But the main mass of the ores lies on the contact between limestone and schist, and extends into the former in separate bodies or shoots. At the so-called second and third contacts, the bodies have a prevailing funnel-shape and a vertical position. Fig. 87, an illustration from Huot, shows the apexes of the funnels to point on one contact upward, and on the other downward — but, in either case, into the limestone, according as it overlies or under- lies the schist. The first form may be explained by the pres- sure of the ascending solutions. The second, as shown in this figure, is perhaps somewhat ideally sketched ; at least the sec- tions of this third contact given by Cordelia show ore-bodies following the contact-plane itself. According to Pig 88 (also from Huot) the ore-bodies are funnel-shaped in E". to S. section, but from E. to W. have a flat westward pitch, which is hard to explain unless it repre- sents some kinds of cleavage parallel to the dislocation already mentioned. Below the second contact, which carries chiefly lead, there are (at the Jean Baptiste shaft, for instance, accord- ing to Cordelia) great masses of calamine, the secondary origin of which from zinc-blende is doubtful, since it would involve the assumption that the ground-water zone had extended to * A. Cordelia, La Qrice sous le Rapport Oeologique et Miniraloffique, Paris, 1878 ; and Le Laurium, Marseilles, 1869. A. Huot, Rapport sur les Mines du Sumium, 1880, and Minudres de la Soeiete des Ing. Civ., 1876-78. 136 THE GENESIS OP ORE-DEPOSITS. this depth. As to the present subterranean water-level, I find in the descriptions at hand only the statement that the region generally is very dry, and that the ancients, who mined to the depth of 120 meters (394 feet), had no water to hoist. With regard to the structure of the galena-deposits, I may say that I saw in the exhibit of the Cie. Francaise des Mines de Laurium, at the Paris Exposition of 1867, masses of galena, blende and pyrite showing distinct stratification, but did not learn from which deposit they came. Which of the various eruptive rocks of the district (eurite, porphyry, diabase, serpentine, trachyte) gave occasion for the ascending springs which brought up the ore, cannot as yet be determined. The minerals accompanying the products of decomposition in such deposits, particularly of calamine, are naturally often limonite and other ores of iron. In many countries these play an independent part, being often formed by the metasomasis of limestone, as proved by the irregular masses of the deposits and the contained fossils transformed into ore. Alsace. — An instance is furnished by the so-called Bohneisen- erze of Alsace and adjacent regions which have been described and correctly explained by Daubree.* At Liebfrauenberg, irregular, lean beds of this character, composed principally of limonite, but scarcely workable with profit, lie on both sides of an anticlinal of Vosges sandstone, and are covered with alluvium. In one place, however, near Goersdorf, an undecomposed body of pyrite and mispickel occurs instead of limonite. Cumberland. — In Cumberland, limonite-deposits occur on the contacts of the Carboniferous Mountain limestone, both with the overlying millstone grit and with the underlying Silurian schists. They are connected with faul1>fissures, on both sides of which they appear as is shown in Fig. 89, taken from a paper by Mr. J. D. Kendall.f Carniola. — The Alps offer some remarkable examples of £oh- neisenerze. These occur, according to A. von Morlot,| in the * Les Eaux Souterraines awx Epoques Anciennes, p. 79. t Trans. N. of E. Inst, of M. E., 1878, vol. xxviii., pi. xxviii. % "Geol. Verhaltn. von Ober-Krain," Jahrh. d. k. k. Oeol. B. A., i., 1850, p. 407. THE GENESIS OF ORE-DEPOSITS. 137 region of "Woohein in Carniola (known for its iron-ores and bauxite) in the dolomite and limestone mountains only, and either in the form of beds under the dolomite detritus, or in clay, in the caverns of the dolomite. Fig. 90 is a section show- ing the latter form. In this case the flatter-lying cavern was partly filled with lime-detritus and clay up to its connection with a higher vertical cavern, while the latter was filled with Bohnerze enclosed in loam, and had been mined, according to Morlot, to the considerable depth of 250 meters (820 feet). Here and there a nucleus of pyrite is found in the iron-ore. The beds and mass-deposits of bauxite associated with limonite sometimes show also the " bean-structure." c. Deposits in Crystalline Schists and Eruptive Bocks. Withoiit entering here upon a discussion of the subject of regional metamorphosis, I may remark that, as a general rule, the older a rock is, the more changes it will be found to have undergone ; yet that these changes do not advance in all places uniformly. Many Cretaceous and Tertiary formations of the Alps present a highly metamorphosed, and therefore ancient, appearance ; while many Silurian formations — as, for instance, that which surrounds St. Petersburg — have been so little altered that the fossil shells which they contain still have the mother- of-pearl luster. Some regions, in a word, have been more strongly attacked than others, through causes which we will not here pause to consider ; and when we follow the stratified groups downward, we come upon the various crystalline schists, often traversed by eruptives, and showing no longer any trace of the clastic sediments, which have been wholly transformed to crys- talline masses. We cannot hope to find petrified organisms in these masses ; but the occurrence of disorganized organic ma- terial in the form of anthracite and graphite proves that at the time the rocks were formed, organic life must have been repre- sented in the sediments. Many indications, available in the distinctly sedimentary rocks as guides in the determination of the relative age of their ore- deposits, are here wanting. The bedding becomes more and more obscure, and is sometimes no longer distinguishable from the cleavage. Many of the ore-deposits in these rocks have also become in whole or in part crystalline, adjusting themselves 138 THE GENESIS OF ORB-DEPOSITS. to the prevailing stratification or cleavage, so that most of them present a bed-like structure and form. Whoever believes in the possibility of a contemporaneous formation of the ores with the rocks will not trouble himself here with genetic speculation, but will see in these deposits simply " ore-beds," according to the old classification. Taberg, Sweden. — The circumstance that magnetite is a con- stituent of many eruptive rocks has inclined many geologists to regard masses of magnetite in the neighborhood of such rocks as immediately belonging to them. This theory originated in connection with the Taberg deposit, in Smaland, Sweden, and was propagated by F. L. Haussmann,* W. Kissinger,! and A. Daubree;J and Taberg has been regarded ever since as an ex- ample of the primitive existence of magnetite-deposits, those of Kackanar, Visokaya, Grora, and Blagodat being classed with it. The question arises, where the line is to be drawn between an eruptive rock containing magnetite and a magnetite deposit. An eruptive rock, like that of Samokov, in the Pils moun- tains in Bulgaria, from the weathered detritus of which mag- netite is obtained by ore-dressing, is not properly an ore-deposit ; but, on the other hand, that of Taberg, where the ore is not only finely disseminated in large amount, but also occurs in separate, solid veins, may fairly be so called. According to A. Sjogren, § the rock consists of olivine, magnetite, and a little plagioclase, with mica and apatite as accessories. In other words, it is an already metamorphosed rock. Considering that at several places in Scandinavia magnetite occurs in the crystal- line schists also, it seems unlikely that the magnetite of Taberg belongs to the primitive rock. This is confirmed by the obser- vation of Th. Kjerulf, that all the ore-deposits of ISTorway follow the courses of eruptive rocks. Taberg will scarcely prove to be an exception, and may, therefore, be regarded as a secondary or xenogenous ore-deposit. Before proceeding further I must mention the action of the * Beise durch SkandiTwimen, G-ottingen, 1811-18, i. , p. 165. f Versuch einer minercdog. Oeographie von Schweden (Woehler's translation), 1826, p. 205. J Scandinaviens Erzlagerstdtten (edited by G. Leonhard), Stuttgart, 1846, p. 25. § Neues Jahrb.f. Mineralogie, 1876, p. 434. THE GENESIS OF ORE-DEPOSITS. 139 mineral solutions upon the country-rock of some veins, which might be also classed as impregnation. In this respect tin- deposits are most interesting, because they carry ore, not only in the space of discission, i.e., the vein-fissure, but to a large extent in the neighboring country-rock also. If the veins occur in granite, this is changed for a certain width into greisen, i.e., it is robbed of its feldspar, which is even, in some cases, replaced by cassiterite and associated minerals. Thus are formed the beautiful pseudomorphs of cassiterite after feldspar, which adorn many mineral collections. (See Fig. 91.) Figs. 91-93 are taken from C. Le ISTeve Foster.* Fig. 91 represents the alteration of the granitic country-rock to greisen on both sides of a fissure, which is here filled with symmetrical quartz-crusts, to the central druse or comb. Often such fissures occur close together ; and since each has its own zone of grei- sen, the result is a Stockwerk, constituting a metamorphosis of the granite, and formed by these fissures. Cornwall. — In the slate or killas of the Cornish miners, there is often a disturbance of the bedding in the neighborhood of the fissure (Fig. 92), such as is observed in connection with fault- fissures elsewhere; but in this case the eapel, or adjacent portion of the slate, is altered chemically also, being impregnated with quartz and traversed by streaks of ore. The fissure itself is filled with quartz, cassiterite, chlorite, pyrite, and fragments of the capel. When several fissures come together, the result is somewhat complicated, but can be reduced to the simple case just described. Still more interesting is the tin-deposit of East Wheal Lovell, described by the same authority. f At the side of a narrow quartz vein the ores occur in the granite, from which they are not separated by any definite boundary, so that the ore-body is an almost vertical shoot, confined to the neighborhood of the fissure, yet lying in the country-rock. It is clear that a mineral water of high solvent power must have ascended under great pressure, in order to bring about such effects in a rock ordi- narily regarded as insoluble. Fig. 93 shows the situation of * "Eemarks on some Tin-Lodes in the St. Agnes District," Trans. Roy. Oeol. Soc. of Cornwall, 1877, ix., pi. iii. t C. Le Neve Foster, " Eemarks upon tlie Tin-Deposits of East Wheal Lovell," Trans. Boy. Oeol. Soc. of Cornwall, 1876, Ix., pi. ii. 140 THE GENESIS OF ORE-DEPOSITS. one of these ore-shoots in granite, at the East Wheal Lovell mine. The ore-deposits in metamorphous and eruptive rocks occur especially in the great crystalline northern areas, in Scandi- navia, Canada, and the northeastern United States. Scandinavia-. — In Scandinavia, the science of ore-deposits, like that of petrography, has had a comparatively independent de- velopment. Although these countries have been often visited by foreign observers, few analogies with European deposits have been noted — chiefly, no doubt, because of the peculiar character of the occurrences examined, but also partly because of the differing standpoints and views of native observers. In recent times a difference of interpretation has developed itself between the ITorwegian and the Swedish geologists ; and the former, since Kjerulf, have approached more nearly the Conti- nental view. As already remarked, Kjerulf traces all the ore-deposits of l^orway to the filling of spaces of discission, and particularly of a peculiar space, produced by the sliding of the rock along a bedding-plane, and locally called a Lineal. "With respect to the ore-filling, he points out that the occur- rence of the ore-deposits must always be studied on the large scale, and that this method shows the ore-deposits to occupy certain lines, characterized by the presence of eruptive rocks.* The ores appear chiefly in the crystalline schists, but also in traces along the contact, and sometimes in the eruptive rocks themselves. In the flrst case, the different sulphides, mostly accompanied with quartz, lie parallel with the bedding or cleavage of the rock, and thus look like beds ; but their second- ary origin is indicated by the slickensides, the branching of the deposits and other signs. Sometimes it is made evident by the course of the ore-masses, cutting across the bedding or cleavage. In the museum at Christiania there are many large specimens of the ore, some of which, having been polished, show this structure plainly. Pictures of some of them have also been published by Kjerulf.f * Die Geologic des sudl. u. mittl. Norwegen (authorized German edition, by Dr. A. Gurlt), Bonn, 1880, pp. 81, 284, 293. t "Pragstufer med Braeciestruktur fra Muggruben og Stovarts," Magazinfor Natunridens, Kobern, xxvii., B., p. 335. THE GENESIS OF OKE-DEPOSITS. 141 In this connection, the primitive ore-bearing character of the Fahlbander (so often cited by geologists as primary ore-beds, which enrich the veins by which they are crossed) is entirely denied [l. c, p. 323). It has been proved that the ores of the Modum fahlbands are connected with malakolite and the au- gite rock which intrudes in " lineal " form between the steep strata of quartz-schists. Figs. 94 and 95 are intended to show the appearance of these deposits, formerly deemed to be beds. The former represents a specimen from the Kongens mine at Roras, and the latter a part of the specimen illustrated by Kjerulf, from the Mug mine at Trondhjem. In the former, the subsequent entrance of the ore is at once recognized. The latter appears as if the crystallization of the minerals had taken place after the ore-impregnation. Of course, the political boundary does not divide the nature of the ore-deposits of the Scandinavian kingdoms. Those of Sweden are often the continuations of the Norwegian. The crystalline rocks are here peculiarly developed, and have also been peculiarly named by the Swedish petrographers. In the Swedish granulite, for instance, one would scarcely recognize its Continental namesake. These rocks are not in general so coarsely crystalline that their constituent minerals can be dis- tinguished with the naked eye. The so-called eurites are still finely crystalline, and the halleflinta is almost amorphous, con- sisting only of the ground-mass of the massive rocks. The beds and mass-deposits of the crystalline rocks are often, like many of the Norwegian deposits, associated with talcose and chloritic slates. Sometimes limestone is also present, as at Falun, Tuna- berg, etc., where the ores lie on the limestone contacts. The ores of some of the deposits suffer in depth a remarkable change. Thus the mass of copper pyrites at Falun has diminished in depth ; but on the other hand, gold-bearing quartz-veins appear in the midst of the pyritic body, and have yielded in recent years considerable amounts of gold. Ammeberg. — I will cite as an example one of the most inter- esting deposits, namely, the zinc-blende mine of Ammeberg,* belonging to the Vieille Montague Company, which I have per- sonally examined. ^ " o ' * A. Sjogren, " Undersokning of den omgrifande Bergarten on Ammebergs Gruf- vor." Oeol. Foreningens i Stockholm Forlmndl, 1880, v. 142 THE GENESIS OF ORB-DEPOSITS. In a winding line, chiefly E.-W., and about 3J kilom. (2 m.) in length, occur steeply-dipping beds of zinc-blende in granu- lite, or gneiss resembling granulite. At certain points they show very beautiful, close folds. At first glance they seem to be genuine intercalated beds of the same age as the rock. The ores, however, do not continue along the whole line, but form separate lenses, up to 15 meters (49 feet) thick, which show a distinct stratification, consisting in layers of fine-grained to amorphous material resembling hdlleflinta, alternating with the coarser granulite. Fig. 96 is a polished specimen, which ex- hibits clearly the secondary ore-invasion. The original bedding is here indicated by a series of light and dark dense hdlleflinta layers ; and these are broken through by masses of coarsely crystalline rock and of ore. The entrance of the ore into the coarsely crystalline layers seems to have been attended by an enlargement of their volume, which resulted in their breaking through the dense layers. The same explanation is required for some parts of the bed, in which, between the plane surfaces of two fine-grained barren strata, ore occurs in highly folded and contorted layers. This folding is due by no means to an exterior mechanical energy, but to interior chemical forces. Some of the blende layers carry a considerable admixture of galena, as, for instance, the two ore-layers shown in Fig. 97, separated by a fine-grained, yellow to brown, barren stratum of eurite. The whole mass is traversed by fine fissures perpen- dicular to the bedding, which are filled with leaf-silver, looking like tin-foil. A replacement with ore of the original rock-con- stituents is here beyond question. It is supposed that the blende has taken the place of the mica of the granulite. But the whole country-rock also is metamor- phous. At the open cut of the Godegard II. mine-working I found in the midst of the schists what I took to be limestone, but I subsequently lost on my journey the specimens intended for more careful examination. But petrographers have prob- ably long since determined this point. This Ammeberg deposit, then, although so distinctly bedded, is by no means of primitive origin ; and still less can such an origin be supposed for the others, which occur as lenses of the greatest variety of filling, enclosed in the crystalline schists. THE GENESIS OE ORE-DEPOSITS. 143 If mica may be replaced with zinc-blende, magnetite, etc., such a change will, of course, be confined to certain portions of the rock, immediately within range of its cause ; and these portions, as distinguished from the rest of the country-rock, are to be considered mineral deposits. Some of the ore-deposits of the Alps have a certain simi- larity to those of Scandinavia; for instance, Prettau, in the Ahrn valley, in Tyrol ; Brennthal, near Miihlbach, in Salzburg ; and Schneeberg, near Sterzing, in Tyrol. Prettau in Tyrol. — There is here a very ancient copper-mining industry, which was overwhelmed in 1878 by a great disaster, and will not soon recover ; namely, the settlement at the smeH> ing-works was buried by an avalanche so deep in debris that it has been necessary to sink shafts nearly 20 meters (65 feet) deep and mine out the stock of manufactured copper and other ob- jects of value. The crystalline schists, which here strike E. and W., and dip steeply S., contain impregnations of copper and iron pyrites, very short horizontally, but considerably prolonged on the dip. The deposit has been opened to a vertical depth of 500 meters (1640 feet), representing 600 meters (1968 feet), so that the horizontal projection, or distance between the top and bottom, is only 350 meters (1148 feet). Figs. 56 and 57 are a vertical section and plan. Figs. 54 and 55 are sketches from the roof and side of the Ottilie gallery, where the chlorite-slate and pyrites present highly complicated forms, somewhat like the structure I have observed in the Transylvania rock-salt. It may be explained, in my opinion, either by an interior increase of volume or by a distortion of the chlorite-slate in the steep west> ward-pitching line indicated by the ore-deposit. It is extremely difficult to. form a correct conception of this deposit. I was able to study some of the lower levels only. It is remarkable that the pyrites-mine of Brennthal, near Miihlbach, shows an entirely similar structure and form of ore- bodies, and almost the same westward pitch upon the E.-W. plane of the stratification. It looks as if dynamic movements connected with the mountain had played a leading part in thus determining the same pitch for the ore-bodies of deposits on opposite sides of the Central Alps. "Where the ore-body begins to grow poor, and the pyrites ap- 144 THE GENESIS OF ORB-DEPOSITS. pear disseminated in grains and crystals through the chlorite, the secondary character of the impregnation is clearly recog- nizable. The space for the massive ore-body was probably pre- pared by mechanical forces. That a metamorphosis was the cause is not likely, because the original minerals of the strati- fied group could scarcely have assumed such abnormal form and dimensions. The older rocks occupy in America large areas ; and there also many ore-deposits occur and are worked which, although somewhat unlike those of Scandinavia, belong to a similar type. I do not intend to describe here the numerous and well- known ore-deposits of the Eastern and JSTorthern States ; but I cannot avoid brief mention of some peculiar types. Lake Superior. — The copper-district of Lake Superior oflJers a number of very interesting occurrences, some of which, though developed by extensive mining, and often described at considerable length, have not yet been satisfactorily explained. It is remarkable that copper and silver occur here almost ex- clusively native ; but it is very generally admitted that this is not the usual primitive form of copper. Sulphides seem to occur but seldom, and they receive, as a rule, no attention. I saw once, at Lac-la-Belle, an old working upon pyrite, chal- cosite and galena, which was said to have carried some native copper in its upper levels. But Foster and Whitney do not mention it.* The native copper of this district occurs notoriously in both veins and beds, in a stratified group lying between the Hu- ronian and the Cambrian, and traversed by numerous fiows of eruptive rocks. f We are here concerned with the beds. The ore in the Calumet and Hecla mine is a conglomerate of por- phyry pebbles; another, in the Copper Falls mine,. is a dark lavarflow, the so-called " ash-bed." The latter is impregnated with copper on both sides of the Owl Creek vein, which trav- * Report on the Geology and Topography of a Portion of the Lake Superior Sand- District, i., Washington, 1850, p. 139. t M. E. Wadsworth, "Notes on the Geology of the Inland Copper-Dist. of L. Superior," Bull, of Mus. of Gamp. Zool., Harvard College, Cambridge, vii., 1880. B. Pumpelly, "The Paragenesis and Derivation of Copper and its Associates on Lake Superior," Am. Jour. Sci., 1872, iii. E. Duer Irving, "The Copper-Bearing Eock of L. Superior," U. S. Oeol. Sur., 3d Ann. Sep., Washington, 1883. THE GENESIS OF ORB-DEPOSITS. 145 erses it (Fig. 98) ; while in the Calumet and Hecla conglomer- ate copper sometimes constitutes the cementing material. In both masses the spaces now filled with copper were un- questionably once filled with other substances ; and the present conditions are the result of whole series of complicated re- placements. R. Pumpelly, who originally believed in a contemporaneous origin of the copper and the enclosing rock, became subse- quently convinced that the copper had replaced especially epi- dote and chlorite, and that certain phases of metasomatic pro- cesses were here represented. The eruptive rocks have usually been strongly attacked — for instance, the pebbles of the con- glomerate, the rocks on Isle Royale, etc. Some portions, on the other hand, e.g., the Ash-bed, have been little attacked. The former instance (which the latter, it is true, contradicts) was used, long before Sandberger, as proof of a sort of lateral- secretion theory ; and now and then, where the copper-bearing rock was overlain by an eruptive fiow, the theory of descend- ing solutions was also brought into play. Some of the attempted explanations assume, in my opinion correctly, as the cause of the first ore-depositions, the action of hot springs — in which connection it is only to be emphasized that these thermal efifeets occurred long after the intrusion of the eruptive flows between the sedimentary strata, so that the ores were brought, not by or in the eruptives themselves, but by the later springs, from great depths and perhaps from consid- erable distances. This explanation, applicable to all the de- posits, suits also the exceptional case cited by R. D. Irving, namely, the Nonesuch copper-bed in the sandstone of Porcupine Mountain, far from an eruptive outflow. As to the condition in which the ores were first deposited, and the manner in which they became reduced and associated with zeolites, additional data must be sought for the formation of an opinion. Sudbury, Canada. — Quite recently, A. B. von Foullon has published his observations in the Sudbury region, Canada,* expressing certain theoretical conclusions of great interest, * "Ueber einige Nickelerzvorkommen," Jahrh. d. k. h. R. A., xliii., 1892, p. 276. 146 THE GENESIS OF ORE-DEPOSITS. which, however, flatly contradicted my view. They concern the pyritic deposits which occur in Huronian rocks, but at the borders of eruptive outflows of diorite, etc., and were described by T. G. Bonney* and afterward by E. Bell.f The ores are associated with masses of diorite, intercalated conformably in the stratified rocks. The ore-bodies have the form of " stock- works," and consist of an irregular mixture of rock and metal- lic sulphides (?). In the ore, which contains gold, platinum, tin, lead, silver, zinc and iron, occur also feldspar, quartz and apatite. This account, taken from Bell's description, indicates a strong analogy with the Scandinavian deposits. FouUon, who made in this field a series of highly valuable observations, supported by careful chemical analyses, expresses himself finally, concerning the genesis of these deposits, as follows : ' ' The irregular mixture of pyrites and silicates, presenting copper pyrites and magnetic pyrites enclosed in the rock in the most varied quantities and in all con- ceivable forms ; and, furthermore, the circumstance that sometimes the ore occurs disseminated in the diorite, and sometimes the diorite is enclosed in the ore, now the rock, and again the pyrites, being the ground-mass, prove unmistakably their contemporaneous origin. At certain periods of the diorite eruption, the magma was rich in accessory constituents which rendered possible the formation of the metallic sulphides ; and these were segregated during solidification." E. Bell has expressed himself still more plainly. " The ores are not of humid, but of molten origin, as is proved by their occur- rence in the diorite, with which they ascended. The masses of molten diorite must have remained long liquid, so that the metallic sulphides could separate, become concentrated at certain points, and continue with the fragments of diorite. Large quantities of the molten diorite, and the heavy metals, must have retired again. ' ' These surprising statements assume a chemical impossibility, namely, the presence of metallic sulphides in the magma of the molten eruptive rock, after the fashion conceived by H. C. von Leonhard,J on the strength of metallurgical analogies. Shaft-furnaces, operated for a separation of the ingredients of the charge, produce slag, metallic sulphides (matte) and * " Notes on a Part of the Huronian Series in the Neighborhood of Sudbury," Qmrl. Jour., B., iliv., 1888. t " The Nickel- and Copper-Deposits of Sudbury District," Bull. Qeol. Soc. of Am., ii., Kochester, 1891. X Huitenerzeugnisse und andere auf kiin^tlichan Wege gebildete Mineralien ah Stiitz- punkte geologischer Hypothesen, Stuttgart, 1858. THE GENESIS OF ORE-DEPOSITS. 147 reguline metal. But the above hypothesis involves rather a common fusion of all, and a separation in cooling of slag (diorite) and matte (metallic sulphides). These authors should certainly not omit to explain further the principles upon which their ex- planation is based, taking into consideration at the same time the inner structure and other relations of the deposits in ques- tion, such as their conformity with the stratified rocks of the region ; the occurrence of ore-channels, quite similar to those encountered in deposits formed by aqueous circulation, etc. These pyritic deposits contain almost all the heavy metals, including platinum and gold, and it is remarkable that the latter here occurs in quartz, exactly as it does generally, through- out the world. The untenable character of the explanations above quoted must be evident, and this brief mention of them will be suffi- cient. Yet it appears that there are other inquirers into the genesis of ore-deposits who purpose to take a similar stand- point.* 4. Hysteromorphous Deposits. Under this title are included the deposits formed by the chemical and mechanical influences of the surface-region, from the original deposits of which the conditions of origin have been considered above. These formations have been consid- ered and named from various standpoints. Thus the name " deposits of debris " emphasizes the idea of a mechanical crushing or disintegration ; the German term Seife, like the Spanish and American " placer," is based upon the manner in which such deposits are often mined for their metallic contents, and so on. The expression " secondary deposits " satisfies, it is true, the definition given above, but is rendered ambiguous by its frequent use in other meanings connected with the genesis of ores. I feel warranted, therefore, in proposing for this group the more distinctly significant name " hysteromorphous " (later- formed). The influences of the present surface upon deposits found in the deep region are so characteristic as to permit us to draw conclusions concerning the processes of earlier periods, when * For instance, J. H. L. Vogt, of Christiania, " Bildung von Erzlagerstatten dureh DiSerentiationsprozesse im basischeu Eruptionsmagma." — Zeitsch.f. prakt. Geol, 1893, i., p. 4. 10 148 THE GENESIS OF ORE-DEPOSITS. the surface occupied a very different position. Unquestionably, effects similar to those of to-day were produced then also, and we must include in our consideration of the subject the hystero- morphism of former geological periods. a. Chemical Effects. The chemical effects proceeding from the present surface have been already discussed in many respects. They involve not only phenomena on the surface itself, but extend beneath it to the groundwater level, and even below that level, so far as the vadose circulation is traceable. On the surface it is especially the oxidizing effect of the atmosphere, its contained carbonic acid, and the solvent and chloridizing action of atmospheric precipitation, simultaneously aided by the mechanical effects of wind and moving water, which bring about what Justus Roth* has called " simple weathering," to distinguish it from more complicated forms of decomposition. In considering not merely rocks, but outcrops of complex ore-deposits, we encounter what Eoth calls " com- plicated weathering." Decomposition underground, through the action of the same atmospheric constituents of the surface-water, extends, as is well-known, to the groundwater level, where it may manifest itself in a striking way by reason of the frequent occurrence at that level of the alternation of dryness with moisture, which is a factor greatly promoting decomposition. A similar condition is presented, as was pointed out in Part I., by the workings of mines, where the water-level has been artificially lowered, and a zone of depth previously untouched by the vadose circulation is brought within the domain of that agency. Deep and old metal-mines especially exhibit in a striking way the effects of the vadose circulation, and, in addi- tion, a phenomenon but seldom found in places under the in- fluence of the natural water-level, namely, the effect of the mine-waters upon various surface relations and products. Limonite-Deposit near Rio Tinto, Spain. — One of these rare instances is cited by J. A. Phillipsf in his group, " Deposits * Allgem. u. Chem. Oeologie, vol. i. , Berlin, 1879, pp. 69-159. f A Treatise on Ore-Deposits, London, 1884, p. 15. THE GENESIS OF ORE-DEPOSITS. 149 resulting from chemical action." Namely, in the vicinity of the great iron and copper pyrites-deposits of Rio Tinto, in Spain, there occurs a deposit of hydrated ferric oxide, shown by the fossils it contains (which correspond with species still living in the region) to be of recent origin, and undoubtedly produced by the weathering and decomposition of the neigh- boring pyritic deposit. It was deposited in a swamp-like basin with peaty matter, and subsequent erosion has left of it two remnants only, at Mesa de los Pinos and Cerro de las Vacas respectively. Evidently, in this case, the detritus of the pyritic deposit has not been mechanically swept away and collected elsewhere, but a chemical action has taken place, removing material in solution, exactly as in the formation of bog iron- ores. The formation here is certainly earlier than the Roman period, for Roman tombstones have been found, made of this recent iron-ore. Mine-waters contain the solutions of all substances directly or indirectly dissolved by the vadose circulation, and some of these, encountering suitable precipitants, may be thrown down. Thus, ferrous oxide becomes by oxidation hydrated ferric ox- ide ; many metallic sulphates are reduced by organic matter to sulphides; copper-salts may even be thus reduced to metal, etc. These new precipitates will mark the track of the mine- waters. Finally, while the solutions formed by surface-waters, like those of the mine-waters, mostly find their way to the points where the water-level reaches the surface (drainage-points), yet as a part of the groundwater penetrates to greater depths, such solutions may very likely produce, in the deep region itself, impregnations, which must, however, dift'er in character from those produced by the deep circulation proper. The primitive deposit from which such solutions have come will show remaining in it principally substances not easily soluble, together with such as, like precious stones, resist all atmospheric influences. Meteoric waters, carrying oxygen, some carbonic acid, and small quantities of chlorides, will first oxidize whatever is oxidizable, especially the metallic sulphides. On this subject S. H. Emmens* has published a clear statement. * " The Chemistry of Gossan," E. and M. /., 1892, liv., p. 582. 150 THE GENESIS OF ORE-DEPOSITS. with some practical deductions. He distinguishes in the order of liability to decomposition the following degrees : (1) mar- casite, (2) pyrite, (3) pyrrhotite, (4) chalcopyrite, (5) bornite, (6) folgerite, (7) millerite, (8) chalcosite, (9) galena, and (10) zinc-blende. The acid ferric sulphate formed from the first members of this series immediately attacks the latter members. The carbonic acid contained in the circulating waters has a high solvent power, and, among other things, dissolves the carbonate of lime as a bicarbonate, which reacts upon the basic sulphates, producing gypsum and free carbonic acid, and ulti- mately transforming lead sulphate into carbonate (cerussite). Copper oxide and, under some circumstances, native copper, may be formed from copper sulphate, and so on. For the chlorine of the chlorides, lead and silver have the strongest affinity, and these metals will consequently be often found in the upper zone as chlorides. The decomposition above water-level of gold- and silver- bearing deposits facilitates the extraction of these metals. Metallic gold can be extracted by simple processes of me- chanical concentration and amalgamation from oxidized ma^ terial, while gold in undecomposed sulphides, etc., must be roasted, smelted, or chlorinated with more or less cost and difficulty. Silver likewise occurs, as a rule, in this upper de- composed zone in the form of easily amalgamated combinations (free-milling ores), while the refractory ores of deeper zones are much harder to treat. It is doubtless for these reasons that mining enterprises often come into very critical conditions when they reach water-level, and many mines even cease to be profitable. An important part, no doubt, is played by other causes, such as the necessity of hoisting increased quantities of water, the cost of the required machinery, etc. It is remarkable that in western North America the ground- water level lies deeper than is generally the case in Europe. I suppose the reason to be, that the present area of the Interior Basin of IS^orth America, which has no surface-drainage to the ocean, was formerly cut by deep valleys of erosion, which made a deeper escape of the groundwater possible. This suggestion is confirmed by the level valleys of Utah and Nevada, several miles wide and filled with very recent sediments, between com- THE GENESIS OF ORB-DEPOSITS. 151 paratively narrow mountain ranges, which seem to be, so to speak, the tops only of the former ranges. In Europe, the upper zones of the ore-deposits were worked out long ago, at a time when the science of chemistry was in its infancy. But we know from the remnants in these work- ings that chlorides, lead and silver carbonates, and various sul- phates, such as anglesite, occurred in them, though they were not recognized. In Transylvania the decomposed products of the outcrop-zone were called Braunen (" browns "), with evi- dent reference to the brown hydrated ferric oxide. The well- known maxim of the German miners concerning the " iron hat " is very ancient; and the same may be said of the Cornish proverb, " Gossan rides a high horse." Limonite is certainly a characteristic indication of the outcrop of an ore-deposit; and no doubt its reddish-brown color has chiefly suggested the South American miners' names, pacos and colorados. In a few instances the " iron hat " has been actually mined as an iron-ore. As a rule it is the decomposed, porous and honeycombed vein-material of the upper zone, and is colored only with limonite. The part of the ore-deposit above water- level has a characteristic appearance. Quartz and other refrac- tory gangue-minerals are surrounded and impregnated by earthy limonite masses. As a rule the original texture of the deposit has become obsure; and sometimes fragments of the mineral crusts, broken off and crushed through changes of volume, are found chaotically thrown together. Occasionally, however, the original structure may still be traced in the de- composition-products of the several crusts, unaltered nuclei of the ore being discoverable in them. Some substances (espe- cially calamine formed from zinc-blende) display the stalactitic forms characteristic of the vadose region. Original druses as well as recently formed cavities are filled with new material ; and in this way a secondary crustification may occur. I must not forget to mention that there are some observations according to which gold has been precipitated chemically in hysteromorphous deposits. Oscar Lieber,* F. A. Genth and A. R. C. Selwyn expressed the opinion that detrital gold gen- erally, or a portion of it, has been deposited from solutions. * In Cotta's Gangstudien, and in Oeol. Rep. of S. Carolina, 1860. 152 THE GENESIS OP ORB-DEPOSITS. Laur, J. A. Phillips, Wilkinson, Newberry, Daintree,* Skey, Egleston,t etc., have accepted this view as more or less gener- ally applicable. E. CohenJ has undertaken to examine it critic- ally, and is inclined by his own experience in South Africa " to adopt the conclusion reached by Devereux for the Black Hills of Dakota, and to assume that by far the largest part of the detrital gold has been liberated by the mechanical destruction of older deposits and has been mechanically laid down ; while, on the other hand, a precipitation from solutions undoubtedly takes place, but plays a very subordinate part only." My own opinion on the subject is expressed in the above quotation.! Ifo doubt here and there, in the detrital deposits, traces of chemical activity are discoverable ; but they are not sufficient to weaken the evident proofs of the mechanical origin of detrital gold. b. Mechanical Effects. The mechanical effects of moving air and water, of frost and ice, are grouped under the head of erosion, and are treated at length, so far as rocks in general are concerned, in the geologi- cal text-books. We are here concerned especially with effects of this kind produced upon those portions of ore-deposits which are exposed at the surface. We nptice at once that me- chanical, unlike chemical effects, are confined to the surface or a very small distance below it. In general, we must assume that the chemical changes took place first, but that the prog- ress of erosion brings both to our view at the same time. Verchoviky, or Surface-Deposits in Situ. — Not only water and ice (glaciers), but also wind, takes part in erosion. For in- stance, if an ore-deposit, by reason of its greater resistance, crops out above the level of the country, the wind will con- tinually tend to blow away the finer and lighter portions of the detrites formed by chemical processes of weathering ; so that, in the course of time, there must remain of the original out- crop only the heavier portions, so far as these are not carried * See A. G. Lock's Gold, its Occurrence and Extraction, p. 746-800. t " The Formation of Gold Nuggets and Placer-Deposits," Trans. A.I. M. E., rx., 1881, p. 633. % " Ueber die Entstehung des Seifengoldes," Mitth. d. Naturw. Vereins f. New- pommern u. RUgen, xii. , 1887. § See my article, "Zur Genesis der Metallseifen," Oesterr. Zeitsch. /. B. a. H. wesen, 1887, xxxv., p. 325. THE GENESIS OF OEE-DEPOSITS. 153 away by water. la fact, I have observed in the Urals that the gold-diggings of the valley, undoubtedly formed by water, ex- tended up the slopes to points where this could not have been their origin. The gold-bearing weathering-detritus is then called Nagomyje rozsypy and Verchoviky. A similar feature was observed by W. C. Kerr* in the aurif- erous deposits of North Carolina; and I have seen it in the old gold-workings of Bergreichenstein and ISTesvacil, in Bohemia,! where flat mountain ridges are covered with old pits and dumps. It is impossible to consider them as diluvial terraces, for the alluvium passes over, so to say, into the solid gneissic rock, which is traversed by many quartz veins. The gold occurs concentrated in the deepest portion of the weather-de- tritus, that is to say, on the contact with bed-rock, and has penetrated all the open, loosely-filled fissures in the latter. CottaJ speaks also of deposits of debris in place, which occur on high plateaus and mountain slopes, and consist of products of weathering which are not rounded pebbles or sand and slime, accumulated by water-currents. A. G. Lock§ speaks of surtace-deposits being " a result of the disintegration of the rocks in situ" and says : "The gold it contains is quite angular, hackly, or crystalline, and is derived from auriferous quartz reefs or leaders, existing in the immediate vicinity." Similar conditions obtain very significantly in the Kackar district, to be hereinafter more fully described. Theory of the Sinking of Heavier Constituents. — ^But the great agent in the transportation and re-deposition of the metallic portions of original deposits has unquestionably been flowing water ; and this is an equal factor in the removal of the rock- detritus of erosion, which it is constantly striving to carry to the ocean. River-sediments are notoriously unstable. What is deposited this year is carried further down stream in the years next following, and so on until it comes to comparative * "Some Peculiarities in the Occurrence of Gold in North Carolina," Trans. A. I. M. E.,:s., 475. f "Zur Genesis der Metallseifen," Oesterr. Z. f. B. u. H. wesere, 1887, xxxv., p. 325. J Erzlagerstatten, i., Freiberg, 1859, p. 100. g Oold, its Occurrence, etc., London, 1882, p. 828. 154 THE GENESIS OF ORE-DEPOSITS. rest in the sea. The original deposits, furnishing the material thus transported over great distances and areas by water, are well called by the Russians Korennyje mestorozdenyje, or root- like deposits, — ^that is, as it were, the roots of the scattered hys- teromorphous deposits. The attempt has been made to explain the concentration, es- pecially of heavy metals, like gold and platinum, in certain pay- ing layers of the detritus, by a sort of natural concentration process. The circumstance that the richest gold-deposits most frequently lie in the lowest stratum of the detritus, immediately on the bed-rock, yet that several such horizons occur one over the other, is difficult to explain in this way ; for Cotta's assumed separate periods of formation (op. cit., i., p. 102) are scarcely satisfactory, involving as they do either periodic transportation or periodic deposition, neither of which is probable. I believe that I have found in the Ural gold-placers a much more probable explanation, based on the principle that the spe- cifically heavier elements of a loose mass are able, with the aid of water, to work their way down through the lighter portions. At the Przibram concentrating-works it is found that if the pulp is left standing for a considerable period, the galena will accumulate at the bottom. In gold- and platinum-concentrat- ing establishments it may be often observed that these heavy metals find their way into the fioor and woodwork of the mill, from out of which they are from time to time recovered by working up these materials. Why should this happen in arti- ficial operations only, and not also under natural conditions, where the descent of the heavier portions is essentially aided by the percolation of atmospheric waters through the loose cov- ering-material ? This view is supported by the features of all gold-placers, especially those of the detritus of weathering in place, where the agency of running water cannot be adduced, and the accu- mulations of gold at the contact of the loose and the solid ma- terial must be explained by its sinking through the former. Stream-Detritus. — The detrital deposits produced by running water are generally characterized by the predominance of per- meable material, such as sand, gravel, etc. Under this cover- ing mass lies the solid, impermeable " bed-rock " or " rim-rock" of the Americans, the plotik or posva of the Russians ; and in THE GENESIS OF ORE-DEPOSITS. 155 all the ^old-fields of the world the richest pay-deposits are found, as a rule, at the border between the cover and the bed- rock. If the latter is decomposed, fissured, or otherwise loosened, the fine gold will sink into it, making it sometimes rich enough to be mined and concentrated ; and this occurs without regard to the petrographic character of the rock. Thus in the Ural, palfeozoic schists, limestone and eruptive rocks in- differently are charged with gold. This circumstance indicates also the error of the assumption that these bed-rocks originally carried gold. But layers of impermeable material sometimes occur in the cover, as, for instance, lava-beds in Australia and California, or, in general, solid conglomerates and clays. In such cases there is often a concentration of gold on the more solid layer, called in America the " false bottom," and in the Ural loznyj plotik — that is, a material erroneously taken for the bed-rock. There are often in the detrital cover two or more such gold-bearing layers, which are easily explained on the theory above sug- gested. The hypothesis of a natural concentration in running water is embarrassed by the fact that the material of gold- placers shows no arrangement according to size, but consists, as a rule, of elements of all sizes. The movement of the elements of a loose mass has been already pointed out by W. C. Kerr,* who admits the possibility, according to A. G. Lock, of the sinking of the heavier parti- cles, though this is only in a passing remark, and without in- dication of its far-reaching importance. He says : "The superior weight of the precious atoms would cause them to sink through the moist surrounding matters till a hard layer was met with. The occurrence of this process would constantly add to the deposits, the gold always gravitating to the bottom, quickly or slowly, according to circumstances." It seems to me that this idea must have impressed itself upon other impartial observers also ; and I can only wonder that it has not been more frequently expressed. R. Helmhacker has recently communicated some observa^ tions in the Altai region of Siberia, such as the sinking of heavy metallic objects in the loose wash, which confirm the * "The Gold-Gravels of North Carolina," Trans. A. I. M. E., 1880, viii., p. 462. Oold, its Occurrence, etc., London, 1882, p. 916. 156 THE GENESIS OF ORE-DEPOSITS. above views. Among other things, he identified grains of me- tallic lead formed in the gold-placers as shot, scattered in huni> ing, which had sunk into the earth. As is well known, auriferous detritus occurs not only in present but also in ancient river-beds, long since dry; and since, in the latter, the remains of diluvial animals, such as the mammoth, etc., have been found, a distinction has been made between alluvial and diluvial gold-deposits. But discoveries of yet older organic remains have shown that such gold-deposits were formed in still more ancient periods. The old river-beds of California cross the present streams, and the auriferous de- tritus of the former is covered with thick lava-beds — a feature which may be observed in Australia also. During the depo- sition of the gold, therefore, conditions very different from those of the present day must have obtained. In another respect, also, the relation between ancient and modern river-beds is sometimes peculiar. The late channels have been rendered by erosion deeper than the older ones. But on the eastern slope of the Ural this is almost totally re- versed. The diluvial gold-deposits characterized by the remains of the mammoth often lie below the water-level of the present streams, so that the latter must be diverted in order to mine the ancient beds. This condition apparently extends throughout the whole Siberian plain, and may be taken as evidence that the erosive energy of its rivers has decreased since the Diluvial period, their fall having been reduced, either by the accumula- tion of the erosion-detritus or by changes in the relative alti- tude of the Ural range. The eastern slope of the Ural is characterized by numerous lakes and swamps along the tributary streams, and a number of these contain auriferous detritus, which has been mined for gold. Marine Detritus. — ^In some regions, the auriferous detritus, after being repeatedly deposited and again swept away, to be re-deposited further down the valleys, has at last reached the sea. The coast of Oregon, in western ISTorth America, and Vladivostock, in southeastern Siberia, are examples. Here the ebb and flow of the tide operate very nearly on the principles of artificial ore-dressing ; and one would think that a concen- tration of the heavier particles might be thus effected. But it does not appear that such effects have been recognized hitherto. THE GENESIS OF ORE-DEPOSITS. 157 Kackar District, in the Ural. — At the beginning of this sec- tion, in the discussion of features of auriferous erosion detritus, some characteristics of the Ural placers were described. A few additional particulars concerning them may be of interest. The gold-bearing stratum occurs at no definite depth. As a rule, the whole of the barren or poor cover is stripped off and thrown aside, before the auriferous layer, thus laid bare, is sys- tematically attacked. Open cuts [Razregy) in the surface, of greater or less depth, are thus created, and are usually left to be filled up by the rivers. In the district of Kackar, already mentioned, in the Southern Ural, original gold-deposits (" root- deposits ") of gold have been repeatedly found in the bottom of these cuts. They were well-defined quartz-veins, carrying in the upper zone free gold, but at greater depth sulphides and arsenides rich in gold. In the case I have in mind, the original open cut extended for a considerable distance along the strike of the vein ; but the bed-rock (which was at the same time the country-rock of the vein) was much decomposed, so that the difference between detritus and bed-rock was not strikingly evident ; and the placer-working passed only by gradual stages into vein-mining. Hysteromorphous gold-deposits may thus be said, in a gen- eral way, to occur in the following positions : 1. In the simple detritus of weathering, immediately upon the original deposit (root-deposit). 2. Mixed with the sand and gravel of present streams. 3. At certain points, in the river-bottom, into the crevices and fissures of which the gold has sunk. 4. Mixed with the impermeable material of older water- courses, through which the gold could not sink. 5. On the false bottoms or bed-rocks. 6. On the true bed-rock. 7. In the decomposed bed-rock itself In considering the chemical changes of the outcrops of de- posits (including, of course, those which give rise to hystero- morphous derivatives) we have seen that sulphides suffer total decomposition, and that of their constituents only the unoxi- dizable metals, such as gold and platinum, remain unaffected. Silver-ores and native silver, being attacked by the chlorides of the vadose circulation, are consequently not found in hystero- 158 THE GENESIS OF ORE-DEPOSITS. morphotis deposits. But gold occurring in nature is for the most part alloyed with silver. The gold from the veins of Bud- weis, in Bohemia, contains by weight about two parts of silver, and that of Transylvania contains by weight more than three of silver, to ten of gold. "Whenever I have had opportunity to compare the gold of an original or root-deposit with that of its derived placer, I have found the latter to be of greater fineness, that is, to contain less silver. I am strongly inclined to ascribe this phenomenon to the prolonged contact with water contain- ing chlorides. The dull surface of placer-gold and its fre- quently spongy interior structure, as compared with the luster and solidity of " quartz-gold," favor this explanation. Platinum-Placers. — Detrital deposits of platinum have been, until recently, particularly observed in the Ural onlyj from which the main supply of platinum was derived. Additional localities are now reported in the Altai district of Siberia and in Canada and British Columbia. In the Tulameen district, it is said, the hydraulic method of mining has been introduced for platinum. I have been unable to obtain detailed informa- tion concerning the features of these deposits. In the Ural, and particularly in its most productive district, that of Mznyj Tagil, the conditions closely resemble those of gold-deposits. The richest platiniferous layers are on the true bed-rock. Platinum and its associates, palladium, nevjanskite and siserskite, being found to occur occasionally adhering to olivine and chromite, it was inferred that they were derived from the serpentine, which is itself a secondary product from olivine-rocks. More recently, platinum is said to have been found in an olivine-gabbro not yet metamorphosed; but whether the metal is a primary or an exotic constituent, can as yet scarcely be declared with certainty. Formerly no other occurrence of platinum than the native metal was known; but now a platinum-ore has been found in the Sudbury district, Canada, namely, sperrylite, a compound of platinum and arsenic. Since this is certainly xenogenous, the question as to the original sources of platinum-deposits is advanced to a new phase by its discovery. Tin-Placers. — In connection with the occurrence of tin as cassiterite in detrital deposits, the specific gravity (6.97) of this mineral, nearly equalling that of iron, and the great resistance THE GENESIS OF ORE-DEPOSITS. 159 which it oft'ers to natural agents of decomposition, doubtless play the principal part. Of the numerous and various associates of cassiterite in its original deposits, none, except quartz, are equally able to resist decomposition ; and the consequence is, that the detritus, both of weathering and of erosion, from the outcrops of such deposits, contains, besides the products of the decomposition of these other minerals, chiefly quartz and pieces of cassiterite. The latter, by reason of its high specific gravity, will tend to sink through the lighter detritus and be concen- trated near the bed-rock. The stanniferous detrital deposits of Bohemia and Saxony, as well as Cornwall, were long since exhausted ; those of Aus- tralasia, the South Sea islands and South America are still worked. According to the special monograph of Dr. E. Reyer,* the richest layers are in fact found at the bottom of the detritus, immediately on the bed-rock. With regard to the geological age of the detrital tin-deposits, the rule stated for gold generally obtains, namely, they are for the most part diluvial, yet have sometimes been formed in earlier periods. Thus, at Flatten, in Bohemia, a tin-placer, which has been worked under a bedded flow of basalt, and the detrital deposits of Annaberg in Saxony, which underlie the basalt of the Scheibenberg, were doubtless formed in Tertiary times. The original or root-deposits of tin have been hitherto quite generally considered as very old formations, connected with the eruptions of granite and felsite-porphyry. Recently, however, tin has been found in the Mesozoic lime- stones of Campiglio Maritima ; and it has been shown, more- over, that the root^deposits of tin in Mexico and Bolivia occur in trachytes and andesites, erupted during the Cretaceous or Eocene. Dr. A. W. Stelzner has recently published a notice of the latter occurrence,! and promised a more elaborate de- scription. He says (p. 533) : "The part played in geological history by the tin-ore of Bolivia contrasts sharply with that which has been observed in the Erzgebirge of Saxony and Bohemia, and in Brittany, Cornwall, East India, Australia, Tasmania, and the United States of America, and which has hitherto been willingly regarded as the exclusive method of tin-occurrence. The Bolivian tin-ore does not constitute * Zinn, eine geol.-monlan.-historische Monographie, Berlin, 1881, p. 208. t Zeitsch. d. deutsch. geol. Oesellsch., xliv., 1892, p. 531. 160 THE GENESIS OF ORE-DEPOSITS. aureoles surrounding plutonic granite, and characterized by the contemporaneous presence of minerals containing boron and fluorine. On the contrary, it can only be considered as produced, simultaneously with precious silver-ores and sulphides of copper, iron, lead and zinc, by precipitation from mineral springs, which were connected in point of time, and perhaps also as effects, with outflows of Cretaceous or Lower Tertiary volcanic rocks." c. Hysteromorphous Deposits of the Older Geological Formations. Twenty-five years ago, at a time when no deposits of this kind were known, in an article on the continuance of ore-de- posits (especially of gold) in depth,* I prophesied their dis- covery. They have since heen observed in different gold-dis- tricts. I refer to the characteristic secondary deposits in quartz conglomerates, indicated by their stratigraphical positions and their contained fossils to be of considerable geological age. Such occurrences are often called simply cement-beds, as are the conglomerates of cemented gravel iu recent placers ; and it is difficult in cases where, as in Australia, this term is frequent,f to infer the age of the corresponding conglomerates. It is, however, in some cases unquestionable that these cements actually represent old formations — chiefly Palaeozoic — and are therefore hysteromorphic products from still older primitive deposits. Deadwood, South Dakota. — One of the best described occur- rences is that of Deadwood and Blacktail gulches, in the Black Hills of Dakota.J It is a conglomerate bed, passing upwards into sandstone, and belonging, according to the contained fos- sils, to the Potsdam sandstone (Cambrian). It is by no means a river-deposit ; on the contrary, the fossils indicate a shallow marine basin. The series lies very flat upon crystalline schists ; is at most 100 feet (30 meters) thick, and is covered by a layer of porphyry, which has most probably protected it from ero- sion. Fig. 100, a section given by Mr. Devereux {I. c, p. 468), shows how the deposit is exposed and rendered accessible on the sides of Deadwood and Blacktail gulches, which cut through into the underlying schists. The conglomerates of pebbles of quartz, schist, and hema- * Oesterr. Zeitsch. f. B. u. H. wesen, xv. , 1867. t See, for example, Mr. Lock's Oold, etc., already cited. J W. B. Devereux, " The Occurrence of Gold in the Potsdam Formatio n. Black Hills," Trans. A. I. M. E., 1882, x., 465. THE GENESIS OF ORE-DEPOSITS. 161 tite which lie at the base of this Cambrian series carry partly coarse gold, under such circumstances that there can be no doubt of its secondary origin. It came probably from the Home- stake vein near by. The auriferous detritus is about 2 meters (6.6 feet) thick, and the portions next to the underlying rock are the richest ; so we have here the relation of the " true bed- rock." If my theory be correct, that the gold reached this position by sinking through the lighter detritus, it might be said that the gold was deposited not with, but after, the detritus, and consequently that the Cambrian fossils do not prove the Cambrian age of the gold-deposition. Such an objection might perhaps have weight in other cases of the kind, but in this case, the bed being covered by a porphyry overflow, and hence not at all exposed to later deposits, the objection has no force. The Black Hills contain representatives of the three principal types of gold-occurrence, namely, gold-bearing veins and ancient and recent detrital deposits. The paper of Mr. Devereux is also very interesting in other respects — for instance, with re- gard to the explanation of the differing fineness of vein- and detrital-gold, and with regard to the traces of chemical action in the detrital deposits. Australasia. — The data from Australasia concerning this class of deposits are less conclusive. In 1876 Wilkinson observed in the Talhawang district of 'New South Wales that the Tertiary detrital deposits received their gold from Carboniferous con- glomerates. These conglomerates were associated with sand- stones and slates, in which occurred a fossil plant peculiar to the Carboniferous of ISTew South Wales. The gold occurred in pretty coarse, rounded grains, and on one occasion a nugget was found weighing 5 ounces (155 grammes). Similar condi- tions are said to obtain in the Hawkesbury rocks, at the ISTorth Shore, Sydney, at G-ovett's Leap, and in the conglomerates of the Coal-Measures in the southern district. Gold is also re- ported in the Coal-Measures at Peak Downs in Queensland, near Hobart Town in Tasmania, and in New Zealand.* The question, whether these deposits of gold were really made simultaneously with that of the detritus in the Carbon- * Lock's Oolcl, etc., pp. 515, 516. See also E. Daubr^e's "Note on Certain Modes of Occurrence of Gold in Australia," Quart. Jour. Oeol. Soc, 1878, xxxiv., p. 435. 162 THE GENESIS OF ORB-DEPOSITS. iferous period, may be decided by the circumstance that the conglomerates are or are not covered by Carboniferous strata. In the latter case, it is possible that the gold may have sunk into the gravel at a later period. South Africa. — In South Africa, at "Witwatersrand in the Transvaal, ancient detrital deposits have yielded a considerable gold-production. According to E. Cohen,* the "Witwatersrand consists of sandstones (which resemble closely that of Table mountain at the Cape of Good Hope) and dolomites of high age — undoubtedly Palaeozoic. Conglomerates of the same age, intercalated among these strata, occur in the vicinity of Johannes- burg in several nearly parallel outcrops, and are for certain dis- tances tolerably rich in gold. They are composed mostly of quartz pebbles, sometimes with fragments not entirely rounded, which are united by a strong, ferruginous, arkose-like cement. The quartz pebbles are sometimes porous and impregnated with hydrated ferric oxide, thus presenting the peculiar corroded appearance so characteristic of auriferous quartz. The gold occurs chiefly in the cement, immediately next to the pebbles. It is mostly coarse-grained, and sometimes even crystalline. The latter circumstance has raised the question whether the gold has not here been chemically precipitated, and hence, whether these are detritus-deposits at all. My standpoint in this discussion, as I have remarked at the end of the section on chemical effects in the upper region, is like that of E. Cohen. I do not deny the presence of chemical influences in the de- trital deposits, although I have personally not happened upon them. So far as I can judge from the treatises of A. R. Saw- yerf and Charles A. Alford,J and from a specimen of the "Wit- watersrand conglomerate, kindly sent to me by A. H. Haider, of Pietersburg, it is my opinion that the gold was mechanically brought into the conglomerates from still older auriferous quartz-veins occurring in the rocks which form the basis of this Palaeozoic formation ; and since the idea of a later entrance of the gold is excluded by the almost vertical position of the * " Goldfuhr Conglom. in Sudafrika." Mitth. d. naturw. Ver. f. Neupom- mern, etc. t "The "Witwatersrand Gold-field." Tram. N. Staffordsh., Inst. M. and Meek- K, 1839. X Oeological Features of the Transvaal, London, 1891. THE GENESIS OP ORE-DEPOSITS. 163 conglomerate beds near Johannesburg, I suppose the gold to have been deposited at the same time as the detritus. The greater part of the gold, as has been said, occurs in the cement. There are no vein-like deposits whatever in the conglomerate ; and the only chemical changes which could be presumed are confined to the decomposition of pyrites and the segregation of its contained gold. According to a foot-note in Phillips's Treatise on Ore-De- posits (p. 2), gold is washed out of granular conglomerates of the Lower Carboniferous at Besseges, Department du Gard, France. Bohemia. — In the region of Trautenau, in Bohemia, I ob- served at Gabersdorf and Goldenols considerable traces of an- cient placer-mining, partly in the valley-bottom, partly on the slope, which consisted of old Permian and Carboniferous con- glomerates. These remains looked exactly like other gold- workings in Bohemia, and I could only explain their situation by supposing that this was another case of auriferous Palseozoic detritus. The same may be said of another enigmatical gold- occurrence, at Stupna in Bohemia, where in 1593 a gold of un- usual fineness (0.954) for Bohemia was produced, and must have come from a detrital deposit. The ancient miners pene- trated through bedded flows of melaphyre. The waste-dumps are composed of pebbles from Permian conglomerates. It is therefore possible that these mines were operated upon aurif- erous Permian conglomerates.* * F. Posepny , ' ' Ueber einige wenig bekannte, alte Goldbergbaue Bohmens. ' ' Oesterr. Zeitsch. /. Berg. u. H.-wesen, xxxvii., 1889. 11 Description of Figures. Fig. 1. — ^Erosion of a channel in rock-salt, at Maros TJjvdr, Transylvania. I, impermeable rock; S, rock-salt; H, hydro- static head of vadose circulation. Figs. 2 and 3. — Course of vadose circulation, as affected by the nature of the rocks. S, soluble, I, insoluble rock ; H, hydro- static head ; a, entrance ; z, outlet ; ah c z, natural curve of water-circulation, if I did not intervene ; a d z, actual path under or over I. Fig. 4. — G-eode of Eisenopal (jasp-opal), showing the filling of a cavity in which air or gas is present, besides the liquid. Fig. 5. — ^Diagrammatic representation of deposits in a lime- stone cavern. (Deposits white; empty space, black.) Fig. 6. — Division of ground-water by fissures and permeable strata. Fig. 7. — Conventional representation of an artesian well. Fig. 8. — Spring-mounds at Arczo near Korond, in Transyl- vania. Fig. 9. — Upward erosions in building-stone in the walled pit of a spring at Bourbonne-les-Bains. Jtradleij ^^oaliia.^tis'-a.Ji.F: Description of Figures. Fig. 10. — Deposition of cinnabar and opal in basalt at Sul- phur Bank, Cal. Sketch at the surface by F. Posepny. Fig. 11. — Similar deposition at the same mine, in sandstone, at greater depth (J. Le Conte). Fig. 12. — Carlsbad Sprudelsiein. Fig. 13. — Pisolite with pyrite crusts, from Hammam Mesou- tine. Figs. 14, 15 and 16. — Pisolites formed by dripping solutions at Offenb^nya. Fig. 17. — Sphere-ores, a correction of the illustrations of Cotta (Erzl. Lehre, I., 33) and Daubree [Les eaux aux Ipoques aneiennes, p. 64). Fig. 18. — Gold specimen from the Katrontza ore-body, Veres- patak. Fig. 19. — A crusted rock-nucleus, from Kaibl. Fig. 20. — ^Boiler-scale. Figs. 21 and 22. — Fragments of rock and older crusts, sur- rounded by later crusts, from Zellerfeld (J. C. L. Schmidt). Fig. 24. — Gold-aggregates, surrounded by crusts of calcite, rhodonite, siderite and quartz, from the E^kosi Mangan ore- body, Verespatak. Uradiey j J^ates, Bngr'a, N.T, Description of Figures. Figs. 25, 26, 27 and 28. — Sections of stalactites of galena, blende and pyrite, so-called " pipe-ores," with hollow axis, from Raibl. Fig. 29. — Section of rhodonite stalactites, with axis of gold- aggregates, from the Hungarian IsTational Museum. Fig. 30. — View of the same. Fig. 31. — Section of a similar stalactite in the author's pos- session, from the Rakosi Mangan ore-body. Enlarged to twice the natural size. Figs. 32, 38, 34, 35. — Sections of ore-channels in the lime- stone of the Valle mines, Missouri (J. R. Gage). Fig.25 _V',' !''.•; ;■■;,'>» !.,■■:.-■,•, Fig. 29 ■^-'<:^^^^. Calamine ; E7igr'8,Ii.Y. Description of Figukbs. Fig. 36. — Plan showing gold-bearing veinlets, striking E. to W., in granite (berezite) striking N". to S., at Berezov. Fig. 37. — Network of veins and vein-elay-slates in the Claus- thal district. Localities: a, Lautenstbal; b, Bockwiese ; c, Festenberg ; d, Ober-Scbulenberg ; e, "Wildemann ; /, Zeller- feld; g, Clausthal. Fig. 38. — Network of veins and Buschel in the St. Andreas- berg district. Huschel : a b, Nentsing ; a c, Edellent; dfe,Sil- berberg ; f g h, Abendroth Veins : 1 1, Samson (i, Samson shaft); k k, Bergmannstrost. Fig. 39. — Section through the Franz Josef shaft, Przibram, Bohemia. A B, sesr-level, heights above and below which are given in meters on the left. The Roman numerals on the right indicate the vein-levels, a, post-Cambrian slates ; b, Cambrian sandstone ; c e, faulted stratum of adinole ; d d, diorite dikes ; m, Martyr vein; u u, Marie Hilf vein ; v v v, Sefcin vein; w w, West-dipping vein ; s s, Franz Josef shaft. lira-Mo;/ ^ /Vj(<;8, £-^jr's. .V,l', Description of Figures. Pig. 40. — Ideal section through Bohutin, near Przibram. a, Cambrian sandstone; 6, pre-Cambrian schists; c, granite; d, main fault-fissure ; e e, diorite dikes. (Note : At Przibram it- self the pre-Cambrian schists constitute the hanging-wall of the main fault-fissure to the entire depth of the mines — about 1100 meters, or 3600 feet.) Fig. 41. — Diagrammatic representation of the structure of the Verespatak ore-bodies (Volbura), showing the ore replacing the washed-out cement of a breccia, mostly of porphyritic frag- ments. Fig. 42. — The same, only conglomerate instead of breccia. Fig. 43. — Vertical S. to IS", section through the Vulkoj mines, showing the suprar-position of andesite upon the shaly sand- stone, a a, Ifepomuk adit ; b, sandstone ; c, andesite ; d, Cora- bia open-workings ; e, Jeruga adit ; /, Peter-Paul adit ; g, Her- mann adit. Fig. 44. — Vertical E. to W. section through the gold-mines of Botesiu and Vulkoj. A, Botesiu; B, Vulkoj ; a, ISTepomuk adit ; b, sandstone ; c, andesite ; d, Corabia open-workings ; j, Jeruga vein. Fig. 40 Ilradtej/ J^-batos, Engr\ N. 7. Description of Figures. Fig. 45. — Section in fourth level of Peter Stehend vein, Frei- berg, a, decomposed country-rock ; b, quartz, with brown-spar, pyrites, blende, and silver-ores. (G. A. Von "Weissenbach, No. 2 in his work.) Fig. 46. — Section on third level of Adlerfliigel Stehend vein, Freiberg, a, gneiss fragments ; b, older vein-formation ; c, later quartz-vein matter; d, gneiss. (Weissenbach, No. 21 in his work.) Fig. 47. — Section on third level of Gnade Gottes Stehend vein, Freiberg. ("Weissenbach, No. 22.) Fig. 48. — Section on thirteenth level of Adalbert Liegend vein, Przibram. a, galena and calcite ; b (or, more precisely, the irregular mass shown to the right of 6), zinc-blende ; c, sand- stone. (J. Zadrazil, No. 52.) Fig. 49. — Section on thirteenth level of Adalbert master- lode, Przibram. a, siderite; b, calcite; c, quartz; r, metamorphosed diorite ; P D e, porphyritic dio- rite ; G- D r, granular diorite ; M, metamorphic rocks ; Gr, gran- ite. (G. F. Becker.) Fig. 59. — ^Vertical cross-section through Union shaft. (G-. F. Becker.) Fig. 60. — ^Vertical cross-section through C. and C. shaft. (G. F. Becker.) Fig. 61. — ^Vertical cross-section through Yellow Jacket shaft. (G. F. Becker.) Fig. 62. — ^Vertical cross-section through Belcher shaft. (G. F. Becker.) Fig. 63. — Vertical section on line of Sutro tunnel. I, II, HI, and IV, Sutro tunnel shafts ; s s, lines of solfataric action ; v, vein-material. (G. F. Becker.) Fig. 58 Bradley ^ jPoati^a, £nijr's,,N.y. Description of Figxikes. Fig. 64. — ^Vertical E. and W. section through the mines of Yalle Sacca, near Rezbdnya, Hungary, a, sandstone ; b, Juras- sic limestone; c, Liassic limestone ; d d, crystalline limestone; e, syenite ; /, 3d adit; g, 4th adit; h, new Anton adit; i, Juliana ore-body; A, Mariann a ore-body; ?, Anton ore-body ; mm,-par- allel intercalated dike ; n n, Reichenstein ore-body ; o o, dikes. Fig. 65. — Vertical longitudinal section of Reichenstein ore- body, Valle Sacca, Hungary, n n, ore-body ; b, limestone ; d, Ist adit; e, 2d adit; /, 3d adit. Fig. 66. — Diagram showing the S. W. pitch of the Reichen- stein ore-body, the dikes dipping W. x y, course of dikes ; x w, dip of dikes ; x z, pitch of ore-shoot. Fig. 67. — ^Vertical N. and S. cross-section of the Government mine at Raibl, Carinthia. a, Raibl slates ; b, ore-bearing lime- stone. Adits : c, Johann ; d, Frauen ; e, Sebastian ; /, Franz n., rV., VI. and VH. ; positions of levels numbered upwards from Johann adit. Fig. 68. — Vertical N". and S. section through the Struggl mine at Raibl. a. slate ; b, ore-bearing limestone ; /, Franz adit ; g, Einsiedl level. Fig. 69. — ^Faulting of the contact by a "Blatt." a, slate; h, limestone. Fig. 70. — Vertical IST. and S. cross-section through the Benyes mine, Rodna, Transylvania, a, mica slate; b, andesite; c, lime- stone. Adits : d, Amalia ; e, Zap Peter ; /, Anton ; g, N"epo- muk; A, Teresia. Fig. 71. — Section through the "New Lead-Mass," in Mt. Ambree, OfFenbanya, Transylv&nia. a, mica-slate ; b, andesite ; c, limestone. Levels: d, Segen Gottes; e, Gliick auf; /, ore- shoot. UrattUi/ ^ PjatcB Ligra N V Description of riGURBS. Fig. 72. — ^Face of level on the Josephiblatt, at Raibl, where the ores occur in the country-rock. Fig. 73. — Vertical E. and "W". section through the McKean shaft, Iron Hill, Leadville, Colo. W. P., white porphyry; B. L., blue limestone; G-. P., gray porphyry; W. L., white lime- stone; L. Q., lower quartzite; G-., Granite. (A. A. Blow.) Fig. 74. — Sections from the Eed Mountain district, Colo. A., andesite ; P. Q., pink quartzite ; L., limestone ; L. Q., lower quartzite ; a, Batavia shaft ; a b, Jackson tunnel ; c, adit ; oo, ore. {G. E. Kedzie.) Fig. 75. — Section across Longfellow Hill and Chase Creek, Clifton district, Arizona. A, Longfellow Hill ; B, Chase Creek ; a a, felsite ; b, limestone ; c, sandstone ; d, porphyry ; e, upper adit; /, deep adit. (A. F. Wendt.) Fig. 76. — Ideal sections at Eureka, IsTevada. A, Kuby Hill ; a, Prospect Mountain quartzite ; b, crushed limestone ; c, lime- stone ; d, shale ; e, stratified limestone ; /, Secret Canon shale ; g, Hamburg limestone; i, Logan shaft; p, Lawton shaft. (J. S. Curtis.) Fig. 77. — Combined section at Eureka for comparison with Fig. 76. a, b, c, d, e, f, as above ; k, "Windsail shaft ; I, Bell shaft ; m, Eichmond shaft; x y, east ore-body; VII., Richmond 7th level. (J. S. Curtis.) Fig. 78.— Sketch of face of 310-foot level. Old Telegraph mine, Utah, showing texture of the filling (altered to cerussite). a, hanging-wall clay ; b, quartz ; c, quartzite. Fig. 79. — Section from the lead-region of "Wisconsin, in the neighborhood of Dubuque, Iowa. (J. D. Whitney.) J.f Br^JI. r i <.-:, .N.V. Description op Piuuees. Fig. 80. — Plan of ore-deposits at Wallerfangen and St. Avoid, near Saar Louis. (C. Simon.) Fig. 81. — Cross-section of 81. H, hanging-wall; F, foot- wall. Fig. 82. — Cross-section of the Mechernich deposits, showing irregular faulting of the Knoten sandstone beds. Fig. 83. — Cross-section of the Vesuv mine, Freihang, Bavaria. a, Keuper clay ; b, variegated sandstone ; c, ore-beds ; d, engine- shaft. Fig. 84. — Section of a tree-stem, replaced with galenite, from Freihung. Fig. 85. — Calamine veinlets in the limestone at Eaibl; a, limestone. Fig. 86. — Cellular calamine of Eaibl. Fig. 87. — Section through the Laurium district, Greece; a, limestone; h, schist; c, Hilarion shaft. (A. Cordelia.) Fig. 88. — Section through the Laurium district; a, limestone ; h, schist ; d, porphyry dikes. (A. Huot.) Fig. 89. — Limonite-deposit in "West Cumberland; a, millstone grist ; h, mountain limestone ; c, Silurian schist. Hematite in place. (J. D. Kendall.) Fig. 90. — Bohneisenstein-A&^oa\t of Wochein, Carniola; a, limestone ; b, iron-ore. (A. v. Morlot.) Uradiey ^ I^atca,'Enyr'a. N.i^ Description of Figures. Fig. 91. — Tin-vein in Cornwall, with pseudomorphs of cas- siterite after feldspar in the granite country-rock. (0. Le ITeve Foster.) Fig. 92. — Tin-vein in Cornwall, showing " capel " or altered " killas " country-rock. (C. Le iSTeve Foster.) Fig. 93. — Impregnation of the granite with tin-ore at East Wheal Lovell, Cornwall. (C. Le ISTeve Foster.) Fig. 94. — Specimens of ore from the Kongens mine at Roras, ITorway. (Th. Kjerulf) Fig. 95. — Specimen of ore from the Mug mine, Trondhjem, N"orway ; a, pyrrhotite ; b, mica ; c, quartz ; d, chalcopyrite. (Th. Kjerulf) Fig. 96. — Polished section of ore from Ammeberg, Sweden. Fig. 97. — Ditto, showing leaf-silver in fissures in zinc-blende. Fig. 98. — Section through the Copper Falls mine, Lake Su- perior ; a, trap ; h, ash-bed at depth of 80 feet ; c, amygdaloid ; d, sandstone at depth of 420 feet. Fig. 99. — IsTagynyerges vein at Kisbanya, Transylvania. Fig. 100. — Section through Palaeozoic detrital gold-deposit of the Black Hills ; a, porphyry ; 6, schist ; d, Potsdam (old con- tact-lines dotted) ; e, cement-mines ; ff, placers, the one on the left in the drawing being in Deadwood gulch at Central City ; the one on the right, in Blacktail gulch. (Devereux.) Mradicy ^ J'oaUo, £'n/r'u, JS.Y^ 188 THE GENESIS OF ORB-DEPOSITS. Discussion at the Chicago Meeting, August, 1893, Including Communications Subsequently Received. ~W. P. Blake, ShuUsburg, Wis. : I desire to express my ad- miration of Prof. Posepny's memoir, and particularly of the charming manner and spirit of the introduction. With respect to his mention of the ore-deposits of Missouri and Wisconsin, reference may be made to my paper presented at this meeting {Trans., xxii., 621), showing the existence of dislocations and breaks in the bedding, and their apparent close relation with the localization of the ore-deposits as claimed by Dr. James Gr. Percival, and also so claimed by Dr. Jenney in his paper before us {Trans., xxii., 171). I have in my paper given reasons for believing that the zinc- and lead-ores in the strata above the compact Trenton limestones were formed by lateral secretion and concentration from above downwards, sub- stantially as shown by Prof. J. D. Whitney, and not by the ascent of solutions through the fissures, as Prof. Posepny (p. 118) seems inclined to believe. In regard to the contemporaneity of the ore and the rocks, and in favor of a later introduction of ore through fault-fis- sures, Prof Posepny (p. 124) cites the influence of these fis- sures. In my paper, already referred to, I have endeavored to show how faults may have influenced the deposition of ore with- out being themselves channels for the flow of mineral solutions, and how they may have caused the contemporaneous formation of metallic sulphides from sea-water in the body of a forming rock ; the faulting fissure being formed at an early period in the foundation-rocks, and probably continuing to be a plane of break and movement in the deposits of later formation. Arthur Winslow, Jeflerson City, Mo. : The results of exten- sive and long- continued studies, such as are here presented by Prof. Posepny, deserve most careful consideration before one should undertake to criticise the general conclusions or judge of the broader bearings of his work. I shall not attempt anything of the kind. The remarks made by me {Trans., xxii., 634, THE GENESIS OP ORB-DEPOSITS. 189 735) in the discussions of Mr. Emmons's and Dr. Jenney's papers, presented at this meeting, are to a great extent applica- ble here ; but I wish to add a few more words bearing directly upon Prof. Posepny's statements concerning the Missouri and Wisconsin ores. On page 117 he says that while the deposits, away from the granite and porphyry " islands " of southeastern Missouri, con- sist chiefly of lead- and zinc-ores, " other metals, such as cop- per, cobalt and nickel, occur as the Archean foundation-rocks are approached." This circumstance, he says, is " an indica- tion that the source of the lead-deposits also is to be sought in depth." Whatever may be the value of this indication, I do not think the facts as stated hold generally. I judge that Prof. Posepny reasons from his observations at Mine La Motte, where such conditions exist. At other points, however, these changes in composition are not observed as the crystalline rocks are ap- proached. At Doe Run, a mine recently opened, work is pros- ecuted along the old water-worn pre-Cambrian surface of the Archean granite, among the conglomerate boulders them- selves ; and few or no copper-, cobalt- or nickel-ores are found. Again at other localities, in St. Genevieve, Franklin, Craw- ford and other counties, copper-ores occur remote from any granite or porphyry outcrops and well above the basal beds of the Cambrian. With reference to the Wisconsin deposits, our author seems to think the absence of ores, in the great thicknesses of lime- stones and sandstones which underlie the productive horizons, by no means conclusive as an argument against their deep- seated source, and suggests that the solutions may have come up through a passage not yet exposed, and even that fault-fis- sures and eruptive dikes may exist which have not been dis- covered. From the fact that he refers in this connection only to Whitney's report of 1862, I conclude that he has not had access to the later and more exhaustive works of Strong and Chamberlin. Perhaps with the full light conveyed by these reports and accompanying maps. Prof Posepny might have at- tached more importance to the objections raised. For my own part, I do not see how such a passage for the solutions as he suggests could possibly exist without its presence having been revealed and its course traced through the widespread mining 190 THE GENESIS OF ORE-DEPOSITS. and exploring which has been conducted in this region during the past seventy years. ITeither do I yet see how the solutions could traverse the intervening great thicknesses of water-soaked sandstones without becoming diffused, in great part at least. The failure to find such a passage and the absence of the ores in the beds assumed to have been traversed, though evi- dence of a negative character, is so strong that it becomes of almost positive value in support of the theory of lateral segre- gation. T. A: RicKARD, Denver, Colo. : The distinguished author of this paper has referred to the Leadville monograph of Em- mons as " epoch-making." This judgment has been antici- pated, I believe, by most of us. It serves, however, very well to recall the fact that the publication of that particular mono- graph marked the high-tide of the lateral-secretion theory, which owed its importance more to the fact of its acceptance by certain distinguished geologists than to its incomplete dem- onstration by Sandberger. What Prof. Posepny said of the work of our American geol- ogist we can say, with even greater force, of his present con- tribution. His dissection of the theory promulgated by Sand- berger is most effective. The sympathies of the miner are with him in that demolition of the lateral-secretion theory ; for the latter was an explanation which never found much favor underground, with the miner, but had its stronghold in its own particular habitat, the professor's sanctum. Here I would throw out the suggestion to my fellow mining engineers, whose busi- ness is to observe rather than to theorize, that these distin- guished scientists must, after all, look to the men who spend much time underground for the accumulation of evidence whereon to found their hypotheses. If the genesis of ore-de- posits is to be unravelled, more particularly if this study is des- tined to be capable of further practical and economic applica- tion, it must be through the gathering of facts and not the mere building of theories. Prof. Posepny has very properly pointed out that Sandberger's views gained many disciples be- cause they permitted extensive generalizations to be made above ground, and in comfort, but did not so much require a descent underground and the making of observations under conditions THE GENESIS OF OKE-DEPOSITS. 191 of discomfort. Therefore, I would say, let those of us who have the opportunity aid in the elucidation of truth by the col- lection of the facts and observations without which speculations regarding the origin and formation of ore-deposits are worse than vain. Prof Posepny emphasizes the fact of the ascension of min- eral solutions. T venture to suggest that these terms — " ascend- ing," "lateral," and "descending" — may all be applied to mineral solutions at various periods and under various condi- tions. It is the great fact of circulation which covers all. The water which comes up must have first gone down ; its original descent was as necessary to the process of ore-formation as its subsequent ascent. "When and where in its journeying it be- came a solvent and when and where it became a precipitant — that is what the miner wants to know. The ultimate formation of an ore-deposit is dependent more upon conditions favoring precipitation than upon those determining solution.* Prof Posepny points out more than once that the two great factors which increase the solubility of all substances are heat and pressure. We know by observation that these conditions are increasingly obtainable as we go downward. The deep region is one that favors solution, just as the shallow zone, because it is characterized by lessened heat and diminished pressure, favors precipitation. It is this simple fact which helps to ex- plain the ordinary non-persistence of ore in depth. It is this which explains the comparatively late origin of ore-deposits. The general non-persistence of ore in depth is a fact capable of proof; the comparatively late origin of most ore-deposits is a hypothesis which is founded upon observation and confirmed by the consideration that the older geological formations were at some time overlaid by an enormous thickness of later sedi- ments, and therefore existed under conditions favoring solution, and not that precipitation to which ore-deposition is more di- rectly due. One more point I would wish to refer to. Prof. Posepny demonstrates that at Przibram the metal of the ore-deposits could not have come from the eruptive rock in the immediate vicinity of the lodes. This is most interesting. For many years * Reference is intended particularly to the metals. 192 THE GENESIS OF ORE-DEPOSITS. we have been accustomed to references to dikes and other bodies of eruptive rocks as being the source of the precious metals of certain lode-formations. In fact, a " dike " vras almost as necessary as a " true fissure-vein," a good climate, plenty of timber, fine scenery and other factors, which, in a prospectus, are requisite to the making of a good mine. In my Bendigo paper* I have already suggested that the vicinity of eruptive rocks need not necessarily indicate that they were the source of the metals, but that their extrusion afforded the heat which made the underground waters active. I would add that the contraction, due to cooling, following the extrusion of a sheet or a mass of igneous rock may have afforded a line of least resistance or — as Prof. Posepny would put it — " a line of maximum circulation." In closing I would express the indebtedness which we must all feel to Prof. Posepny for so extensive and so valuable a con- tribution. In my own case I would express it as the gratitude of an apprentice to a master. HoEACB V. "WiNCHBLL, Minneapolis, Minn, (communication to the Secretary) : It is an interesting fact that the opinions here so ably advan&ed by Prof. Posepny were partially stated as long ago as the end of the seventeenth century. A few quo- tations from " An Essay Towards a Natural History of the Earth," by John Woodward, will make this plain. I quote from the third edition, published in 1723, the date of the first edition being 1695. "That there is a perpetual and incessant circulation of water in the atmos- phere ; it arising from the globe in the form of vapour, and falling down again in the form of rain, dew, hail and snow. That the quantity of water thus rising and falling is equal ; as much returning back, in rain, etc., to the whole terraqueous globe, as was exhaled from its vapours. That tho' the quantity of water thus rising and falling be certain and constant as to the whole, yet it varies in the several parts of the globe ; by reason that the vapours float in the atmosphere, sailing in clouds from place to place, and are not restored down again in a perpendicular upon the same precise tract of land, or sea, or both together, from which origi- naly they arose, but any other indifferently" (pp. 132, 133). As to the cause of the circulation of waters beneath the sur- face of the earth he speaks as follows : * Tram., xxii., 289. THE GENESIS OF ORE-DEPOSITS. 193 That there is a nearly uniform fire or heat disseminated throughout the hody of the earth, and especially the interior parts of it ; the bottoms of the deeper mines being very sultry and the stone and ores there very sensibly hot, even in winter, and the colder seasons. That 'tis this heat which evaporates and elevates the water of the Abyss, buoying it up indifferently on every side, and towards all parts of the surface of the globe ; pervading not only the fissures and intervalls of the strata, but the very bodyes of the strata themselves, permeating the interstices of the sand, earth or other matter whereof they consist, yea even the most firm and dense marble and sandstone. . . . That this vapour proceeds up directly towards the surface of the globe on all sides, and as near as possible, in right lines, unless impeded and diverted by the interposition of strata of marble, the denser sort of stone, or other like matter, which is so close and compact that it can admit it only in smaller quantity, and this very slowly and leisurely. "That where the vapour is thus intercepted in its passage, and cannot pene- trate the stratum diametriealy, some of it glides along the lower surface of it, per- meating the horizontal intervall which is betwixt the said dense stratum and that which lies underneath it. The rest passes the interstices of the mass of the subja- cent strata, whether they be of laxer stone, or of marie, or the like, with a direc- tion parallel to the site of those strata, 'till it arrives at their perpendicular inter- valls" (pp. 136, 137). Woodward entertained the idea that " the whole terrestrial globe was taken all to pieces and dissolved at the deluge." "That at length all this metallick and mineral matter, both that which con- tinued asunder, and in single corpuscles, and that which was amassed and con- creted into nodules, subsided down to the bottom ; at the same time that did the shells, teeth, and other like bodyes : as also the sand, cole, marie, and other matter whereof the strata of sand -stone, cole, marie, and the rest are for the most part composed ; and so were included in, and lodged amongst, that matter. . . . And the case of metalls and minerals being the same, 'tis for that reason that in some places we now get iron, or vitriol, but no copper or alum : in others we find these, but not those : and in others both these and those, and perhaps many more. . . . Thus we sometimes see whole strata compiled of metallick and mineral nodules : others of pebles, and of flints, without the interposition of other matter. . . . Thus likewise we find strata consisting almost entirely of common salt : others of ochre : and others of several metalls and minerals, tin, lead, vitriol, nitre, and sulphur promiscuously, without any considerable mixture of coarser terrestrial matter." Of the origin of veins he speaks in these Avords : " That the metallick and mineral matter, which is now found in the perpendicu- lar intervalls of the strata, was all of it originaly, and at the time of the deluge, lodged in the bodyes of those strata ; being interspersed or scatter' d in single corpuscles, amongst the sand, or other matter whereof the said strata mainly con- sist. That it was educed thence and transmitted into these intervalls, since that time ; the intervalls themselves not existing till after the strata were formed, and the metallick and mineral matter was actualy lodged in them ; they being only breaches of the strata, and not made till the very conclusion of the catastrophe, the water thereupon immediately withdrawing again from off the earth. 194 THE GENESIS OP ORB-DEPOSITS. "That the water, which ascends up out of the Abyss, on all sides of the globe, towards the surface of the earth, incessantly pervading the pores of the strata, I mean the interstices of the sand or other matter whereof they consist, detaches and bears along with it all such metallick, mineral, and other corpuscles which lye loose in its way, and are withal so small as to be able to pass those interstices ; forcing them along with it into the perpendicular intervalls ; to which it natur- ally directs its course, as finding there a ready exit and discharge, being partly exhaled thence up into the atmosphere, and partly flowing forth upon the surface of the earth, and forming springs and rivers. " That the water which falls upon the surface of the earth in rain, bears also some, the' a lesser, share in this action ; this, soaking into the strata which lye near the surface, straining through the pores of them, and advancing on towards their perpendicular intervalls, bears thither along with it all such moveable matter as occurs in those pores in much the same manner as does the water which arises out of the Ab3'ss with only this difference, that this passes and pervades none but the superficial and uppermost strata, whereas the other permeates also those which lye lower and deeper. ( The vadose and deep underground circulations of Posepny. ) " That therefore the metalls and minerals which are lodged in the perpendicu- lar intervalls of the strata do still grow [to speak in the mineralogists' phrase], or receive additional increase from the corpuscles which are yet daily borne along with the water into them. Nay they have grown in like manner ever since the time of the Deluge, in all such places where those intervalls are not already so filled that they cannot receive any more : or where the stock of metallick and mineral corpuscles, originally lodged in the strata, is not quite exhausted, and all borne thither already. . . . ' ' That the metallick and mineral matter which lyes in the bodyes of the strata does not grow, . . . but on the contrary, hath been diminished and lessened by so much as hath been conveyed into their perpendicular intervalls, and as hath been brought forth upon the surface of the earth by springs, rivers, and ex- halations from the Abyss, since that time. That notwithstanding there have and do still happen, transitions and removes of it, in the solid strata, from one part of the same stratum to another part of it, occasion' d by the motion of the vapour towards the perpendicular intervalls of these : and in the laxer strata, such as sand, clay, and the like, from the lamer ones to those which lye above them, and even to the very surface of the earth " (pp. 208-216). Although the paragraphs quoted lead us to infer that Wood- ward thought veins were filled by the mechanical transportation of matter in small grains, yet there are in other places indica- tions that he also had an idea of their formation by the deposi- tion of minerals from solution. Thus, nearly a century before "Werner and Hutton, were expressed ideas which were the re- sults of long and careful observation and study which, though tinged with the theological and so-called philosophical doctrines of the day, were yet true to nature and of universal application, and which strike us as extremely valuable and original when put in modern logic and phraseology. THE GENESIS OF ORE-DEPOSITS. 195 John A. Church, New York City (communication to the Secretary) : I cannot agree with all the dicta of Prof. Posepny's valuable paper. He says (page 14, and see page 74) that in fissures " only the places remaining open would permit an active circulation of solutions and a regular deposition from them." The idea of deposition in a free space runs through the whole of the paper, and is applied not only to the ore-deposits of the vadose circulation but with equal uniformity to those of the deep circulation. Such ideas seem to me to be incompatible with the crushing pressure which all agree must be found at depths of 10,000 and 15,000 feet. We have in metasomatic replacement an explanation of ore-formation which accords so well with the conditions supposable at great depth that it seems unnecessary to add to it a requirement that is certainly contradicted by those conditions. I believe I was the first in this country to ascribe the forma- tion of an important vein (the Comstock) to metasomatic altera- tion, which I then called " substitution," the term metasomasis being suggested in the same year. The character of the Com- stock ore forbids the supposition of deposition in an open space ; for it is not quartz but a mixture of quartz and fragments of the wall-rock. In the opinion of experienced men more than half of the rich ore mined from the heart of the great ore-bodies was " porphyry," and at least the proportion was great. My conclusion was disputed by Mr. Becker; but one of the surest advances which vein-geology has made in the last fifteen years has been the steady growth of the idea that the thickest ore- bodies may have been formed by the replacement of masses of wall-rock fragments, or by the spread of siliceous replacement from a narrow crevice through the walls. In deep-seated formations this method of deposition is neces- sarily supposed; for there are not only no open spaces there, but the situation is not even what I conceived it to have been in the Comstock. ISTearer the surface there might be partings which, though minute, would be real openings, while at great depth such partings must be so closely appressed as to be no more than mere breaks of continuity. The tendency of opinion in this- country is toward metasoma- sis acting upon masses of crushed rock in crevices which they 13 196 THE GENESIS OF ORE-DEPOSITS. completely fill ; and I find nothing in Prof. Posepny's paper which need cause a retreat from this view. Prof. Posepny appears to place great reliance upon the ap- pearance of the ore and the walls enclosing it, and I suppose it is because deep-seated deposits in limestone have some strong resemblances to those of the upper circulation, that he con- eludes that the former must be laid down in " spaces of disso- lution," like some of the latter. To me these facts point rather to an identity of active agent than to identical circumstances of its action. To make my meaning clearer I will recall some well-known facts and theories. "We know that the limestone rocks, in proportion to their amount, carry more ore-bodies than the _siliceous rocks, though the latter have actually the greater number. The suitability of limestone for the deposit of ores is usually made to depend upon its solubility in water charged with carbonic acid, which is supposed to be derived from the soil by descending waters. It is carried into the interior of the earth and again discharged, for the earth being a closed vessel already full of water into which a new supply is constantly poured, it is clear that as much must be discharged into the atmosphere by springs as the atmosphere supplies by rain. I find fault with the usual view upon this subject, which apparently assumes that the deep waters must be highly charged with COj derived from the sur- face. On the contrary, it seems to me that the discharging water must bring out as much CO2 as it takes in, for neither water nor gas can be lessened in quantity except by the com- paratively small amount that enters into fixed combinations in the rocks. Since the solubility of gas in water is increased by pressure we must suppose that the dissolved CO^ remains with the water that absorbed it throughout the whole range of cir- culation and that there cannot be any discharge of surface CO2 in the interior. Yet we know that large quantities of COj are discharged from the earth as gas not dissolved in water, besides that which is dissolved; and this gaseous discharge must be in excess of the CO, carried in. May we not find the source of this excess in deep-seated metasomatic replacement? The operation of solutions whose composition we do not know can be judged only by their effects. When metasomatic replacement takes place in limestone it is generally assumed THE GENESIS OF ORE-DEPOSITS. 197 that lime carbonate goes into solution, while its place is taken by the ore-substances, that is to say, that the action is molecu- lar substitution and not atomic ; but it is conceivable that the change should begin by an interchange of acidic elements — that SiOj should drive out COj. Subsequent changes might remove the lime silicate by another process of substitution, since it is more soluble than silica; but the point is that COj would be liberated, and though the original ore-solution were free from CO^, it would immediately become charged with that agent and exert the well-known dissolving power of carbonic acid solutions. In this way a solution which would have but feeble power in other rocks may in limestone set up a chain of reactions that would intensify its effects. These considerations lead to interesting conclusions. We have a source of COj in rocks, however deep-seated, and consequently effects may be produced at any depth, which simulate those of surface-waters, though probably without the production of caverns. Since the mode of solution is the same, the appearance of the walls lining an ore-body and the appear- ance of the ore itself may be almost precisely the same as in the vadose region. Limestone contains the elements for self-destruction, since the breaking up of one lime-carbonate molecule may cause the solution of another ; and, as this cannot be said of any other rock, we reach a possible explanation of the comparative fre- quency of ore-bodies in limestone. The dolomites would, of course, present similar reactions. There are two questions which are distinguished, even in the difficult study of veins, by the obscurity which hangs over them. One is the selection of a favored stratum for ore-depo- sition. In some situations the solutions, before reaching the stratum of actual ore-deposition, must have passed several strata suitable for their action, if they had possessed from the beginning the power of solution which they showed ultimately. I beheve this objection has been urged against the lateral-secre- tion theory as applied to Leadville. Ore-solutions exhibit a selective power which is extraordinary in a water fully supplied with dissolving qualities, but quite exphcable in a solution which lacks this power. I suppose it is impossible at present to determine why the rocks now exposed at Leadville were 198 THE GENESIS OF ORE-DEPOSITS. selected for attack by the solutions ; but I think it is compre- hensible why that action, however extensive, should be localized by the development and action of COj in the neighborhood where it began. An obvious consequence of these considerations is that the aqueous circulation of the earth becomes, through the medium of metasomasis, a means for restoring to the atmosphere accu- mulations of carbon that represent the organic life of past times. The second obscure question is logically one which ought to be answered before we discuss the origin of ore at all. It is the secondary alteration of already-formed ore-deposits. I have no doubt that some of the deep-seated deposits which we see are actually a product of the vadose circulation. Formed ten thousand feet below, they have been raised until they are now ten thousand feet above the sea^level, and, during the immense period through which they have been subjected to the surface circulation, they have not only been re-arranged but may have actually lost their ancient origin. Even the rock in which they were deposited may have been removed and the ore transferred to another member of the series. Structural facts may prove deep-seated deposition, but actually the ore-bodies we see are often in whole or in part hysteromorphs. This is especially true in limestone deposits. Though these facts are well-known, they do not exert the controlling influence upon opinion which I think they deserve, probably because of the extreme difhculty of separating the primary from the secondary phenomena. 'No writer that I have seen has given to this subject half the im- portance which a mining engineer must give it. I cannot agree with the author in giving so much importance to crustification, as he describes it. Certainly a banded struc- ture can arise from the replacement of fragments arranged in layers by pressure and friction, as well as in many other ways, and does not prove deposition in a cavity, whether filled by water or air. He has misunderstood me in saying that I foand crusts of quartz alternating with calcite in the Justice mine (Comstock). I said the thick masses of calcite in that mine rested on a thin layer — an inch or two — of quartz ; but this is not crustification in the author's sense. My view of that oc- currence was that an insignificant quartz seam, probably be- THE GENESIS OF ORE-DEPOSITS, 199 longing to the last period of the Comstock, was first produced, and that the calcite was formed by replacement of the wall-rock at a later period. There is not the least evidence of deposition in a cavity. If there is crustification, that appearance does not have the significance which our author gives to it. I have not attempted to particularize the many points in which I find myself in agreement with the author ; and since my remarks have been rather in criticism, I desire to express, in conclusion, my high appreciation of his admirable treatise. S. F. Emmons, "Washington, D. C. (communication to the Secretary) : Prof. Posepny's paper, or treatise, as it rather de- serves to be called, is a most important contribittion to the theory of ore-deposits. His wide personal observation of most of the important mines in so many different parts of the world and his critical acumen as an observer, combined with his long-con- tinued studies of the subject, give to his words an exceptional authority. Whatever might be said, therefore, in praise of his article (and it would take much time to say it all) would hardly add to its value. But the very high quality of his work ren- ders any errors in it exceptionally hurtful, and I shall therefore confine my remarks mainly to what seem to me to be erroneous teachings, and to points in which I difiter with him either wholly or in part. I would first say, however, that to the greater part of the views put forth in this paper I most heartily subscribe, especially to those on underground circulation, and on the great rarity of ore-deposits which have been formed contemporaneously with the enclosing rocks. It is well known that for some years past there has been a very warm discussion between Posepny and Stelzner on the one side, and Sandberger on the other, in regard to the derivation of the material of ore-deposits, the former holding to the as- cension, the latter to the lateral-secretion theory. Without at- tempting to determine the merits of either side of the contro- versy, which it would be unwise to do without examining personally the deposits in question and their geological sur- rounding, one is inclined to believe that the views of either of such able geologists must have scientific value, whether one or the other may be proved to be erroneous in a particular in- stance. I regret, however, to see this controversy brought into 200 THE GENESIS OF ORE-DEPOSITS. what should be a broad and impartial discussion of the facts of nature, and to detect in certain cases what appears to be a ten- dency on the part of Prof Posepny to adopt a rather forced construction of these facts, in order to make them support his views rather than those of Sandberger. The lateral-secretion theory, which Posepny ascribes to Sand- berger, is much narrower than that which I, and I think most American geologists, hold. It confines the derivation of the vein-contents to the wall-rock in immediate contact with the deposit ; whereas, in my view, a derivation from rocks within reasonable proximity, as opposed to a source at unknown depths (" in the barysphere "), would constitute lateral-secre- tion, and ore-bearing currents may in such cases have had an upward, downward or lateral motion, according to differing local conditions of rock-structure. Prof. Posepny himself ad- mits, in his admirable discussion of vadose or shallow and deep underground circulation, the possibilities of such lateral- secretion when he describes the latter (p. 28) in the follow- ing words : ' ' The ground- water descends in the deep regions also through the capillaries of the rocks ; at a certain depth it probably moves laterally towards open con- duits, and reaching these, it ascends through them to the surface." The distinction between the action of surface and that of deep-seated waters is an important one in the study of ore- deposition ; but I do not think that Prof Posepny is justified in assuming, as he does, that only ascending waters are capable of depositing ores. Furthermore, the necessary derivation of metallic minerals by these ascending waters from the " bary- sphere " seems too far-fetched. At what depth the barysphere will be found, meaning thereby the part of the earth's interior where the rocks have a much higher specific gravity than those that come under our observation, is purely a speculative ques- tion ; but as our surface observations extend over a thickness in round numbers of about 100,000 feet of rocks, and show no appreciable difference of specific gravity between the deeper and more shallow rocks, except such as is due rather to differ- ent degrees of density than to heavier mineral constituents, it seems safe to assume that such a barysphere must exist, if at all, at such great depths as to be beyond the reach of any min- THE GENESIS OF ORE-DEPOSITS. 201 eral-bearing waters. If such a zone rich in heavy metals exists in depth, as there is Bome reason to believe, my own view, as expressed in my paper read at this meeting, is that the heavy metals which constitute the ore-deposits were brought up from it into the outer crust of the earth by the various eruptive rocks, and were partially concentrated in certain parts of these eruptive rocks by differentiation during the process of cooling. In this view I agree with Vogt, whom Prof. Posepny men- tions (p. 147) in a somewhat slighting manner. I differ with Vogt, however, in that I consider the greater part of our ore-deposits, all certainly that have come under my limited observation, to be due to further concentration, perhaps many times repeated, both chemical and mechanical; and I am en- tirely at one with Prof Posepny in considering their final con- centration into their present form to be due to the action of circulating waters. Prof. Posepny's belief in the capabilities of an ascending cur- rent of heated waters or thermal springs seems to me, in some instances, as exaggerated and unreasonable as his rendering makes Sandberger's disbelief, in the instances he cites. He quotes a single observation by Noggerath in 1845 on the find- ing of vertical channels in limestone 8 to 35 inches in diameter, near Aachen, which are supposed to have been eaten out by the ascending spring-waters, and from this draws the wide- reaching conclusion that ascending waters may actually force their way up through rock masses without the necessity of pre- existing cracks or channels. Among instances where he uses this explanation to account for the formation of an ore-deposit the most remarkable is that of Laurium (p. 135), where the ore-deposits as shown by the diagrammatic section (Fig. 87) are funnel-shaped bodies extending outward from the contact of flat-lying schists into subjacent and superjacent limestones, that is both upwards and downwards. My own explanation of this section, deduced by observations in limestone-deposits in this country, would be that the ore-bearing currents circulating along the contact-planes had eaten outward into the more solu- ble rock, upwards from the upper contact, and downwards from the lower contact. But Prof. Posepny explains the funnel-shape of the ore-bodies on the upper contact as produced " by the pressure of the ascending solutions." The lower contact he 202 THE GENESIS OF ORB-DEPOSITS. offers no explanation for, but says " it is perhaps somewhat ideally sketched." It is unprofitable, however, to discuss deposits which neither of us have seen ; for nothing is so liable to misconception as the description of ore-deposits one has not seen by a person with whose qualifications and accuracy of observations one is not familiar. This is shown in Prof. Posepny's remarks upon the Leadville deposits, in which he concludes that I must ac- knowledge that my views in regard to the downward course of the ore-bearing solutions were incorrect, because several mining engineers have shown them to be untenable. It does not seem to occur to him that the views of a mining engineer (who is not necessarily a geologist) based upon studies of a single mine or set of mines would be of less value as applied to such theo- retical questions than those of a trained geologist who had made a study of all the geological conditions and mines of a district. Of the three articles quoted by him, that of Mr. Freeland offers no opinion upon the subject in question. Both this and Mr. Rolker's article were written before my mono- graph was published, otherwise Mr. Rolker would have found his objections on these points foreseen and accounted for there (p. 490). In the summer of 1890 I spent nearly two months at Lead- ville studying the recent developments with the special purpose of testing the correctness of my former deductions, and Mr. Blow accompanied me through the workings of ISTorth Iron Hill, with which he is so familiar. While I naturally found many details of geological structure which were not, and could not have been, correctly represented on the underground sec- tions accompanying my report, I found no reason to change my views of the manner of formation of the ore-deposits, and I con- vinced myself (and I think Mr. Blow also) that his objections were based on a misapprehension of certain geological phe- nomena. It were too long to give here all the results of my observations, which I regret that circumstances beyond my control have as yet prevented me from publishing. I will say, however, as bearing upon this point, that in no case did I find any convincing evidence of the action of ascending solutions. The ore-bodies occur in two general forms, either on the ap- proximately horizontal contact-planes of porphyry and lime- THE GENESIS OF ORE-DEPOSITS. 203 stone, or along nearly vertical fissures crossing the limestone beds. In either case, wherever the form of the ore-body was such as to throw any light upon the probable direction of the ore-forming currents, it showed that they must have descended, for they all terminated more or less in a point or wedged out downwards. Before discussing this further, it may be well to repeat my statement given in the monograph (p. 379) which has evidently been overlooked or misapprehended by my critics. I say, with regard to the immediate source of the ores : "1. That they came from above. 2. That they were derived mainly from the neighboring eruptive rocks. " By these statements it is not intended to deny the possibility that the mate- rials may originally have come from great depths, nor to maintain that they were necessarily derived entirely from eruptive rocks at present immediately in con- tact with the deposits." I do not maintain, as many have assumed, that the ore was derived from the white porphyry. I do not pretend to be able to determine what particular body of porphyry it came from. The objection of Mr. Blow that it could not have come from the white porphyry because this is not all decomposed (not " not at all decomposed," as Prof. Posepny puts it), is based upon a mis- apprehension of what constitutes decomposition. If Prof. Posepny will read the description of the eruptive rocks in my chapter on rock-formations, he will see that all the Leadville porphyries are more or less decomposed within this district; when Mr. Cross and I were making our geological studies we had to go several miles away before we could find a specimen of unaltered white porphyry for microscopical study. My contention with regard to the ores of this district, as opposed to the theoretical views of Prof Posepny and those of his school, would not have been essentially affected, however, if it had been shown that the solutions had ascended to reach the locus of the present deposits. The fissures across the lime- stone which gave access to the solutions forming the ore-shoots of l^orth Iron Hill described by Mr. Blow are, as I showed in my monograph, faults with only a few feet of displacement, and can extend to only limited depth ; in some cases their lower limit could be detected. The great faults which extend several thousand feet in depth are not ore-bearing, except in so far as 204 THE GENESIS OF OBE-DEPOSITS. ore has been dragged into them in the movement of their walls, one upon the other. But the extent in depth, even of these great faults, must be extremely limited as compared with the distance of the barysphere. I believe that the eruptive rocks originally brought up the heavy metals from the depths into the general region in which the ores are now found. Some of these eruptives still contain over four per cent, of them, in spite of all the leaching to which they have been exposed. The ore-deposits are concentrations of these materials by deep underground waters, flowing along natural channels, and de- positing along those which admitted a comparatively free flow, as compared with a capillary circulation. Such a flow may have been upward, downward or lateral, according to varying structural conditions. The ascending solutions which Prof. Posepny contemplates, however, could not have formed ore- bodies of the form of those found in Leadville ; and it was for that reason that I laid stress upon the evidence of their proba- ble downward course. As regards the phenomena of " crustification," I may not have been explicit enough in stating its absence. In my origi- nal examination I searched in vain for any evidence of it. In my second examination, almost entirely in bodies of unaltered sulphides, I found overwhelming evidence that the ore was not deposited in pre-existing cavities, but by metasomatic replace- ment of the limestone. In the great bodies of the A. Y., , Minnie and adjoining mines not only could every detail of the granular structure, joints and cleavage of the original lime- stone be detected at times in the sulphide ore, but even the cracks in the top of the ore-body through which the ore-bear- ing solutions had descended were often visible. In abandoned drifts, where the limestone dust had accumulated on the walls, one would have supposed the walls to be all limestone until the breaking otf of a fresh fragment by the hammer showed the metallic gleam beneath. G. F. Becker, "Washington, D. C. (communication to the President from IsTewport, R. I.) : The paper of Prof Posepny is a very valuable contribution to the science of ore-deposits, and deserves a more careful critical discussion than I am able to assist in giving to it at this time, in the absence of facilities THE GENESIS OF ORE-DEPOSITS. 205 for reference to authorities, etc. A few general observations, therefore, must suffice at present to indicate my views. The theory of the substitution of ore for rock is to be ac- cepted only when there is definite evidence of pseudomorphic, molecular replacement. Prof. Posepny is very clear on this point (p. 13), and I have insisted upon it in my memoir on quicksilver-deposits and in a paper on quicksilver about to be distributed. Prof. Posepny appears to me, on the other hand, to lay too much weight upon the structure which he calls " crustification," as indicating exclusively the filling of open cavities and the absence of replacement. Metamorphic pro- cesses are very frequently accompanied by the formation of layers similar to stratification and crustification, and, indeed, from similar causes. Strata are distinguishable only because the circumstances of deposition undergo more or less marked variations, and the banded structure of agate or hematite is also due to variations in conditions of deposition such as the strength of the solutions, or the rapidity of their flow, or tem- porary changes in the composition of the fluid. It appears to me that the banded structure attending metamorphism, as a matter of observation in many cases, is due to entirely similar causes. Thus a mass of iron immersed in a copper-solution will precipitate the copper as a laminated mass, unless great precautions are taken to secure uniformity of temperature, etc. In short, lamination is an ordinary attendant of processes of deposition, whether by replacement or otherwise, whenever they are so slow as to be subject to changes of condition. Hence crustification seems to me an insufficient guide to genetic diagnosis. The indications of replacement which I should rely upon are twofold : crystalline pseudomorphosis and the irregular enlarge- ment of fissures in the replaced mass. Of the latter. Prof Posepny gives a good illustration (Fig. 85). As for pseudo- morphosis, it has a very important bearing on the work of Mr. Emmons and of J. S. Curtis, for it appears to be thoroughly well established that galena forms pseudomorphs after calcite ; and, therefore, the theory of replacement of limestone which they advocate is, to say the least, possible. The studies of these ob- servers at Leadville and Eureka tend to show that replacement has been the chief process ; but so far as I can recall their re- 206 THE GENESIS OF ORB-DEPOSITS. marks they do not assert the entire absence of deposition in pre-existing openings ; so that even if crustification were an in- fallible sign of filling, the detection of crusts (Posepny, p. 114) would not invalidate their position. Another objection to Mr. Emmons's views is expressed by Prof. Posepny in the sentence (p. 109), " It is difficult to believe that metasomatic processes could produce such pronounced ore-shoots as those described at Leadville." I cannot share this view, for replacement, like so- lution, must occur along fissures or channels, and metasomatic ore-bodies will present analogies in form to the open spaces of caves of solution. It seems substantially certain that open cavities in limestones can form only above the permanent water-level of a country, since in such a country the water below this level must be ap- proximately saturated with calcium carbonate. On the other hand, replacement may take place at any depth. Now, in the Great Basin, the Tertiary and Early Quaternary were very wet periods, and if the Eureka limestones have been excavated by surface waters, the excavation and subsequent ore-deposition, according to Prof. Posepny's view, must be crowded into the late Quaternary. The present precipitation of that region would seem insufficient to bring about much cave-formation, while a greater precipitation would raise the water-level. Thus, so far as Eureka goes, the hypothesis of subsequent filling raises distinct, though perhaps not insuperable, difficulties as to the formation of the cavities. The foregoing notes should be reinforced by examples and citations which I cannot now furnish. F. M. F. Cazin, Hoboken, K J. : If I venture to add a few lines to Prof F. Posepny's treatise on the genesis of ore-de- posits, my justification is derived from practical work done and consequent opportunities enjoyed in a region to which the learned author personally has remained a stranger, and of which in existing literature no such account is available as would afford to him the powerful argument in favor of his the- ories really presented by the region itself, to a degree of im- portance beyond any other mentioned by him. The region to which I refer furnishes a demonstration of the xenogenous origin of ore-deposits, heretofore considered as THE GENESIS OF ORE-DEPOSITS. 207 idiogenous, which I may properly call gigantic, and which is equalled nowhere on the face of the earth as far as known. I refer to the region so tersely described by James Douglas {Trans., xix., 694) in these words : "In the Appalachian chain from Vermont to Georgia there are imbedded in the crystalline schists large masses of pyrites, some consisting of ordinary bisulphide of iron but most of them of pyrrhotite, and all carrying more or less copper." There is, on the long stretch from Vermont (Mr. Douglas might have truly said " from Canada and Maine ") to Georgia, no older mine, and none with more important development on the ore-deposits thus described, than that which has been called by State-legislative act " the Vermont Copper Mine." Its his- tory began before the world knew about copper on the shore of Lake Superior. For many years it produced at the rate of 3,500,000 pounds of copper per annum, and, with adequate im- provements, could do so to-day. I have seen its developments on a deposit dipping 24° N.E. to a distance of 2350 feet from the surface, and to a vertical depth below sea^level of several hundred feet, the lateral expansion of stopes ranging between 50 and 350 feet. Having been connected with this mine from early in 1882 to June of 1888, 1 have had opportunity to search for the origin of the ore-body there exploited. Having discovered unmistakable local evidence as to the true nature of such origin, it remained to ascertain the . identity or uniformity of effect from identical causes or other deposits fall- ing under the description above quoted ; and it was not diffi- cult to establish such identity and harmony. At a distance of ten miles in a northerly direction another mine in the same geological position, at Corinth, offered evi- dence leading to the same conclusions, and in a southern direc- tion at a distance of four miles, the Strafford mines, and at a further distance of sixteen miles the Pompanoosuc mine, all similarly situated, demonstrated the same effects under similar causes. And a visit to many other localities within the Hu- ronian Appalachian region could confirm only the conclusions to which the observations in the Vermont mine had been lead- ing me. Except as to dip, topography and shape of workings. Fig. 56 in Prof. Posepny's paper might well serve as the image of the 208 THE GENESIS OP ORE-DEPOSITS. Vermont ore-deposit, represented on a vertical plane along its dip. And Figs. 54 and 55 may well serve as a representation of horizontal and vertical planes, as they are seen inside and outside of the Vermont, Corinth and Strafl'ord mines, where the designs shown in these figures not only occur in dimensions varying from a few fathoms to many hundreds of fathoms, but also in varying material. In the mine, this consists of the sul- phides of iron and copper, and outside and at distant points therefrom, in an admixture of carbon-matter (graphite) in the country-rock. This rock is a micaceous schist, the graphitic part varying in proportion from a mere trace to 100 per cent., becoming marketable plumbago in many localities, though with- out suflicient extent, as a rule, for exploitation. But it is not on the similarity of design between sulphide and carbon admixtures in the rock alone that my conclusions were built, as a description of the mine will farther show. In their lateral expansion the ore-stopes in the Vermont mine present a figure very similar to the one presented in Prof. Po- sepny's Fig. 93, if the longitudinal extent be assumed as 2350 feet, with the lower part broadened. But similar figurations are also presented on a smaller scale, where in quarries the rock is laid bare on one of its dark seams. The roof and floor of the Vermont ore-deposits are virtually impenetrable to water; the mine at 1000 feet vertical depth being dry. But there is uncovered at a distance of a few hun- dred feet from the outcrop an almost vertical cross-fissure or fault (without perceptible faulting), filled with calcareous spar containing sparsely distributed small seams of galena, which cross-fissure allows a few hundred gallons of water a day to percolate into the workings. Some of this water finds its exit through an abandoned adit. Where it reaches the surface, and where its flow is slow, allowing evaporation, it deposits a slime of virgin-white carbonate of lime ; and as it passes down into the valley, it deposits for miles a mixture of carbonates of lime and iron, giving to the creek-beds their peculiar coat- ing of color, as a result of atmospheric reduction of the iron carbonate. The ore of the Vermont in its mineral character has one main peculiarity, which is common to the deposits as described from Canada to Virginia and Georgia, namely, that quantita^ THE GENESIS OF OEE-DEPOSITS. 209 tive analysis shows neither the figures required to constitute the one of its components as ferro-sulphide, nor those required to show it as ferri-sulphide, these figures varying all the way between those applying to FeS and those applying to FeSj. The structure of the ore is the same as that of the graphitic rock, with the same variations in the ore as to contents in sul- phides, as there are in the country-rock as to contents in carbon- matter. That in no case I have met with a nucleus of carbon in a body of sulphides, I have attributed to a full completion of the metamorphosis. Yet another feature is common to the ores of the described deposits. For a long distance on the northern part of these continental deposits, wherever they occur in the Huronian schists, their ores carry nickel in proportions varying from a mere trace in the copper-metal made therefrom to an available percentage in the ore itself. Although much disinclined to draw generalizing conclusions from isolated geognostic phenomena, I claim justification in the case at hand for the following conclusions, because the evidence is such as repeats itself on a large area, and once understood presses itself upon our attention, so as to be no longer ignored : 1. The iron and copper sulphides occurring in the Huronian crystalline schists on the eastern part of the Iforth American continent have locally displaced carbonaceous matter, where faulting of strata aided water-circulation, such water containing sulphate salts of the metals in solution. 2. The metamorphic action of absorbing mineral carbon and of setting free CO2 is continuous to the present day. 3. The product of such action extending below sea-level being observable in lines nearly parallel with the coast-line of an entire continent, and showing equal peculiarities in com- position on the entire line, it is reasonable to assume oceanic action. It is true that the ocean of our period evinces the presence of copper only by its presence in maritime organisms. But when, on the shores of a once existing Triassic sea we find em- bedded in massive but porous sand-rock an entire palm-vegeta- tion, that has turned into copper-glance, as my eyes have seen it (compare p. 131 of Prof. Posepny's paper), then we may well assume the presence of a perceptible quantity of copper in a 210 THE GENESIS OF OEE-DEPOSITS. Triassic sea, though not necessarily sufficient to destroy animal life. It is even a matter of time only for an ocean like the one of our own period to provide Pecillopora and Heteropora corals with their copper, or to be the means of metamorphosis of car- bon-deposits into copper-sulphides in part; the percentage of copper in these deposits being in general not above three per cent, of the deposits as a whole. I find a further justification for stating these facts and the conclusions to which they lead in the circumstance that the learned author, although mentioning the occurrence of graphite in crystalline schists, does not mention that this graphite any- where accounts for the origin of ore-beds. The description of the Sudbury ore-beds deals with a case far more complicated than those considered by me, because there Huronian strata similar to those met at the difit'erent mines in the Appalachians have been disturbed by more recent dioritic eruptions, which subjected the pre-existing ore-beds to a new partial or second metamorphosis, by which the true state of afl^airs is very materially obscured, misleading the describers into the untenable assumptions, so justly controverted by the learned author. Had he been informed of the facts, as I have described them above, the author of this eminently interesting and progressive essay on the genesis of ore-deposits would have been able to knock the last crutch from under the theory of an eruptive origin of the ore-deposits at Sudbury and elsewhere in the crystalline rocks of the Huronian period. I take this opportunity to furnish, on another point, informa- tion for which Prof. Posepny apparently calls (p. 132). A few months only after my report on the ISTacimiento copper- occurrence was published, with the consent of those interested, in the Engineering and Mining Journal, Aug. 7 and 14, 1880, the United States surveyors, who were commissioned by the Sur- veyor-General of New Mexico to survey the twenty-one mining claims described by me, were driven off these claims by a numerous band of jumpers, who had swarmed into those parts as the usual avant-garde, indicating, as stormbirds the storm, the approach of a new railroad line in those remote parts. To reinstate the legitimate owners either brutal force or litigation had to be employed. The ill-success of other copper-enterprises in ISqw Mexico, though quite foreign to all natural conditions, THE GENESIS OF ORE-DEPOSITS. 211 caused them to resort to neither. When, in 1891, I again visited the upper Rio Grande valley, I found on the platform of the railroad-station at Bernalillo, K M., about a car-load of the precise cuprified palm-vegetation formerly described by me, showing that there had survived some activity at ITacimiento ; but, as stated in my first report, profitable operations are possi- ble only on a scale like that on which lead is obtained from a similar sand-rock at Mechernich in Rhenish Prussia. Discussion at the Virginia Beach Meeting, February, 1894, Including Communications Subsequently Received. T. A. RicKARD, Denver, Colorado (communication to the Secretary) : The paper of Professor Posepny was printed so short a time before the Chicago meeting that it could not re- ceive at that meeting the thorough discussion, based upon care- ful study, which its great importance and value deserved. In the remarks which I made on that occasion, I could do little more than express, with others, our thanks to the distinguished author for this admirable treatise on a subject of such general and permanent interest. Further examination of it has con- firmed the opinion that its appearance marks an epoch, par- ticularly in this country, in the study of ore-deposits and their origin, and has led me to feel that our appreciation of it will be best expressed in aiding its purpose and widening its usefulness by the fi-ee contribution of facts and interchange of views which it invites. I have, elsewhere,* expressed some dissatisfaction with the new names introduced in this paper ; and it has seemed to me, also, that the classification of ore-deposits, which it proposes, is unnecessarily complicated. From the stand-point of a mining engineer, we have had, in my judgment, no classification more practical and sensible than that suggested by Dr. Raymond, twenty-five years ago (outlined on page 7 of Professor Posepny's paper). If any modification of it be permissible, I would sug- gest the following : * Eng. and Min. Jour. 14 212 THE GENESIS OF OEB-DEPOSITS. I. Surface-Deposits. A. Due to mechanical agencies. B. Due to chemical agencies. II. Inclosed Deposits. A. Bedded. a. Contemporaneous, in origin, with country-rock. h. Subsequent, in origin, to country-rock. B. Not bedded. a. Due to dislocation. h. Due to impregnation. Surface-deposits have no regular form, and are, therefore, distinguished primarily by their origin. Class A would be typified by gold-bearing placers, and Class B by deposits of bog iron-ores. When we come to inclosed deposits, we find an extreme complexity ; but, we readily recognize that some are conform- able to the bedding of the country-rock, while others are inde- pendent of it. We can further distinguish those which are of contemporaneous origin, such as the coal-beds, from those which were formed after the deposition of the country-rock. To this class belong ore-deposits which have replaced beds of limestone ; and another pretty example is afibrded by the Ben- digo saddle-reefs, which are conformable to the anticlinal curves of the country-rock, but were clearly formed after both the original sedimentation and the subsequent folding. Among the non-bedded deposits there is no limit to diversity of structure and of origin. We recognize, however, that the fissure-veins which cut across the bedding, but retain a definite position due to their formation along lines of original disloca- tion, may be distinguished from the irregular impregnations, due as much to the chemical composition of the country-rock as to its structure. These two types, however, are forever in- termingled. It is seldom, indeed, that an ore-deposit has not some features, however faint, of form and structure dependent upon those of the country-rock, while, on the other hand, it is not often that a fissure-vein is found which does notxexhibit, in places, a lack of definition, due to metamorphic action upon its inclosing walls. In the discussion of the origin of fissures. Prof. Posepny has THE GENESIS OP OEE-DEPOSITS. 213 touched upon a point which has been the subject of frequent debate. I fully believe that dislocation accompanies the forma- tion of a fissure, and that a movement of its walls is often evinced by slickensides and strise. Yet, this has been ques- tioned by one or two members of the Institute who are known to be both accurate and experienced observers. The question at issue is a vital one, if we desire to obtain a clear idea of the mode of formation of mineral veins. It has been denied that the strise and slickensides observed upon the walls of lodes necessarily prove that movement has taken place; but it has never been clearly shown what other agency did form them. Prof. John A. Church has discussed this matter in a most in- teresting way,* and has pointed out that slickensides may be formed, not only by rubbing but also by " deformation, as when a plastic substance like clay is forced through an open- ing," and again by deposition in fine parallel lines. Recently, Prof. Daubree has experimentally proved that gases under high pressure are capable of producing strise upon rock-surfaces. f It is true that a distinction is made between strise and slicken- sides, but I look upon the two as the work of the same agency. In the former case we have coarse rubbing due to large par- ticles, and in the latter, fine polishing due to minute particles. There is no doubt, however, that certain structures are called strise, which are to be ascribed to causes other than those usually supposed to produce strife and slickensides. As I write I have before me a large piece of rock, the surface of which exhibits fine parallel lines, which, at the mine (the Hillside, in Yavapai county, Arizona), were called strise. The rock was part of the casing of a cavity found in the hanging-wall of the lode, which traversed a quartzose talc-schist. Its surface has been covered]: by a series of siliceous coatings, doubtless deposited by the mineral-bearing waters which circulated over it. The precipi- tation took place along certain parallel lines, probably marking the direction of flow of the circulating waters, and the resulting appearance is to be regarded as a pretty example of a variety * Eng. and Min. Jour., April 30, June 11 and 18, 1892. t Bvll. Soc. Oeol. de France, 3 serie, Feb., 1891, t. xix., p. 313. Comp. rend. Acad., t. cxi., seances du 24 Nov. et 4 Dec, 1890. Compt. rend. Acad., t. cxii., s&nce du 19 Jan., 1891. X As shown by viewing the broken edges of specimens. 214 THE GENESIS OF ORE-DEPOSITS. of crustification, but, coming as it does from a lenticular hole, cannot have been due to rubbing caused by faulting. In the accompanying drawing (Fig. 1), reproduced from a sketch made underground, the cavity above referred to is marked A. There are two others, B and F, of the same kind. D is a seam 6 inches thick, of white talcose gouge, lining the foot-wall, and separating it from C, which is the lode itself. Fig. I HILLSIDE MINE, ARIZONA The latter is 15 to 18 inches wide, and consists of quartz, iron- pyrites, zinc-blende, and a little galena, very much intermingled, and carrying gold and silver in almost equal proportions. The lode itself reproduces to a noticeable extent the structure of the country-rock which it has replaced. The cavities in the hang- ing-wall are also surrounded by talc-schist, which is mineral- ized to such a degree as to constitute " low-grade ore." The vein cuts clear through the foliation, nearly horizontal, of the THE GENESIS OF ORE-DEPOSITS. 215 talc-schist, and the alteration of the country-rock, while most marked in C, extends to a varying distance on either side. JSTot infrequently the quartz of a lode has striated markings which are but the negative of those occurring on the wall- rock.* In such cases the quartz is sometimes entirely solid and unbroken, suggesting that it was deposited upon the pre- viously striated surface, and that it has not only replaced the substance but also reproduced the structure of the rock once inclosed by the fissure-walls. On the other hand, one instance may be cited where it seems necessary to suppose that move- ment took place subsequently to the deposition of the quartz. At the 1800-foot level in the Great Extended Hustler's minef at Bendigo, Australia, the quartz lying against the hanging- wall of the reef exhibited a surface as smooth as polished ivory, but distinctly grooved, and also marked with fine, dark lines, parallel to the grooves. The latter had, I believe, an origin similar to that of ordinary strise, while the dark lines were due to the grinding of particles of pyrite observable in the quartz. Though this quartz seemed to the eye as hard as adamant, it would readily crumble away when pressed between the fingers. It had been crushed to the consistency of com- mon table-salt, which, save for the presence of occasional crys- tals of pyrite, and for its highl}' polished surface, it much re- sembled. Objection has been raised to accepting the occurrence of clay, striae and slickensides as necessary evidence of faulting, because they are occasionally absent where movement may be supposed to have taken place. In such instances, it is reasonable to in- fer that they have been destroyed by agencies identical with those to which the lode-formation is due, namely, the replace- ment of comatry-rock, often in a crushed and shattered con- dition, by ore, through the metamorphic action of percolating solutions. There is a fanciful notion current among miners that a smooth wall and a thick gouge are the necessary adjuncts of a productive " true fissure-vein." Experience does not confirm this belief. A defined wall and a soft seam of clay are natu- » Instances of such are to be seen in the gold-quartz veins of California, t See Trans., vol. xx., 512, et seq. 216 THE GENESIS OF OKE-DBPOSITS. rally welcome to the miner, because they facilitate the actual breaking down of the vein-stufF; but they are no more charac- teristic of productive than of barren lodes. The irregularity in the dip of some veins has been cited as disproving the possibility of their formation along lines of faulting. Occasionally mine-workings show that the dip of a vein is reversed; and the formation of the fracture which it occupies cannot be referred to a continuous line of movement, because that would have involved the shearing-off of the oppos- ing angle. But it is not necessary to suppose, nor do facts sug- gest, that lodes are generally formed along continuous or single lines of movement. As Prof Posepny has well shown, it is the study of the circulation of underground waters which afFords the key to much that is perplexing in ore-deposition. In such cases as are here referred to, it is rational to suppose that the mineralizing solutions searched out the easiest way which offered itself. They did not necessarily percolate along a single definite straight line of Assuring, but often deviated from it, whenever it afforded a less ready passage than was offered by other fractures which united with it or crossed it. An instance which occurs to me as I write, is furnished by the Seven-Thirty mine at Silver Plume, Colorado. The lode con- sists of a system of veins carrying rich silver-ore, the most productive of which is that which bears the name of the mine. It rarely has any considerable width ; it is often only a thread traversing the coarsely crystalline granitoid gneiss and porphy- ritic microcline granite of the region. At the third level, about 280 feet from the surface, there is a very marked irregularity in the course of the vein, presenting some interesting features, which the accompanying sketch (Fig. 2) will help to explain. From the shaft eastward for several hundred feet (A to B) the vein carries ore ; but its width is small and irregular. The lode widens rapidly at B, where it also meets with a sudden deviation in its course. At a first glance, this looks very much like a fault, but subsequent examination will correct such a view. The fissure continues in a straight line from K to L, after the ore has swerved to the south. Instead of maintaining its eastward course, the ore is disposed in two cross-veins, CD and EF, nearly at right angles with that course, which unite with a fissure, MQ, similar in character and parallel in strike THE GENESIS OF OKE-DBPOSITS. 217 to that from which they sprung, AL. Both AL and MQ are continuous so far as they have been followed in the mine- workings. The walls are well-marked, even after they cease to enclose ore. The cross-veins CD and EF lack well-defined boundaries. The western branch, CD, is a streak, about 3 inches wide, carrying ore of a tenor of 300 ounces of silver per ton, while the eastern branch, EF, is larger, about 1 foot wide, and carries ore of lower grade, about 100 ounces per ton. The latter is accompanied by much more galena than the former. The distance between the two is 10 feet; their length is 44 feet. The country separating them is not noticeably altered or mineralized. This is not an instance of faulting ; the ore is found in con- nection with a system of fractures AB, CD, EF and MQ, the 3/. O P Q Fig. 2 SEVEN THIRTY MINE, COLORADO. varied structure and arrangement of which modified the circu- lation of mineralizing solutions, and so brought about the ir- regularity in the deposition of the various minerals comprising the ore. The mineralizing waters met with diverse conditions. From A to B the fissure was tight, and its boundaries were distinct, limiting the circulation to a narrow channel; hence a small streak of ore was found. At B the shattering of the country-rock accompanying the formation of the cross-frac- tures, CD and EF, offered facilities for the ready penetration of the solutions and for chemical interchanges. From C to D and from E to F the irregular fracture across the foliation of the country-rock produced irregular but rich streaks of ore. On meeting with the other main line of fissure the solutions again found well-defined boundaries which put a check to the metamorphic replacement of the country-rock, and it was not 218 THK GENESIS OP ORB-DEPOSITS. till the conditions changed (at 0), that a notable width of ore was again deposited. Many supposed faults found in mine-workings are really of this character. There has been a deviation in the course, and a marked diminution or increase in the amount of ore-deposi- tion, because the mineralizing solutions have circulated along those fractures which presented the easiest passage and offered the conditions most favorable to chemical interchanges. Returning to the subject of strise, slickensides and clay-seams, I must say, that while the questioning of accepted theories is wholesome, and the views quoted above deserve respectful con- sideration, it seems to me that observed facts warrant the gen- eral belief that these phenomena have usually been produced by the rubbing of two faces of rock which have undergone movement ; and I do not sympathize with those who consider that the ordinary explanation is far-fetched. We know that the rock-formations of the upper earth have undergone move- ment, for this is proved by all geological investigation. Further, we have every reason to believe that movement among beds of rock of unequal flexibility must cause some to break. Facts confirm such a belief. Again, every break must be coincident with a movement: for a fracture can hardly be said to exist until made evident by movement however slight. At any rate > a fracture unaccompanied by movement would not give the re- lief required by a series of beds exposed to such strain as ne- cessitated a rupture. Such movement must be accompanied by friction, due to the tendency to smooth down the irregularities of the two opposing rock-faces. Where movement has once occurred, a line of less resistance is established, and a repetition of movement is likely. The result is to break small particles from oflt' projecting points and so form a dust which water makes into mud or clay, also to scratch the surfaces in contact, forming strise, and to polish them, forming slickensides. Why therefore deny the probability, even the necessity, of the move- ment of the walls of a fissure, and why endeavor to give to the markings of rocks underground an origin other than the one which would certainly be ascribed to them if they were found on rocks* at the surface ? * No one questions, for instance, that the scratching seen on boulders from a glacial moraine are the result of rubbing due to movement. THE GENESIS OE ORE-DEPOSITS. 219 The pages which Prof. Posepny devotes to an inquiry into the conditions governing the flovsr of underground waters are among the most valuable of his treatise. His explanations will do much to clarify our conceptions of the mode of behavior of underground waters, and will doubtless suggest further inquiry in the same direction. The word " circulation " is the key to the whole matter. There has been a tendency to speak of de- scending, lateral and ascending currents, as though the one ad- jective would cover the manner of movement of all mineral solutions. An ascending flow was supposed to have formed this lode, descending that one, while others again, steering a middle course, have imagined that ore-formations derived their origin from solutions having a lateral flow. In each case a nar- row view of the subject is both unphilosophic and unscientific; it has too often been the obstacle to progress in this branch of geology. One great fact confronts us, and that is circulation. The distinguished author is himself carried away by his prejudices, and in the latter portions of his treatise* allows his ascensionist views to lead him too far and in part to forget the very forcible teaching given in the earher pages. Much will be done to explain the many puzzling and apparently contra- dictory features exhibited by the ore-deposits of different re- gions if we remember that mineral solutions both descend and ascend, that occasionally they may have an approximately lat- eral flow, and that in each instance their circulation is governed by a diversity of ever-changing conditions. Water must first descend in order to afterwards ascend. The known density of-the earth precludes the supposition that its interior contains any reservoirs of water; the sinking of deep wells and bore-holes has indicated that at a comparatively short distance from daylight the temperature is so high that water could not exist as such, but would be dissociated into its con- stituent gases ; while actual mining exploration has shown that in the deepest mines there is less water encountered in depth than in proximity to the surface. These facts all con- firm the every-day observation that underground waters origi- nate from the rain and snow precipitated from the atmos- phere. * As on page 57. 220 THE GENESIS OF ORE-DEPOSITS. We may compare the circulation of water up and down, through the earth's rocky exterior, to that of the ordinary heater in a house. The water circulates because, when hot, it rises through the length of pipe, and, when cool, it falls back to be reheated. Using this analogy to explain ISTature's oper- ations, we have at one end the condensation and precipitation of moisture due to a fall of temperature, while at the other, and deep down in the earth's rocky confines, we have a heat which sends the water back to the surface. In this matter of ore-deposition we are not concerned with the two ends of the circuit. We have no particular interest for the moment in that part of the water-circulation which intervenes between its ele- vation by evaporation from the earth's surface and its return as rain ; nor, on the other hand, can we see what goes on at the other end of the circuit. We can only guess what conditions obtain and what phenomena occur at depths inaccessible to man. All our investigations must -concern themselves with the intermediate stage, that stage which is most particularly marked by the transition from higher to lower temperatures, and, inversely, from increasing to diminishing pressures. It is the nice adjustment of these conditions which, on the one hand, favors precipitation, and, on the other, compels solution. To the miner, therefore, it may appear most important to investi- gate those factors which bring about precipitation, because to them must be ascribed the immediate agency of ore-deposition. It would simplify his ideas if he could speak of an upper zone of precipitation, where the temperature is low and the pressure light, in contradistinction to a lower region of solution, where the heat is great and the pressure intense. Such attempts to separate the locality of the two processes, however, must not be carried too far. Precipitation has no sooner ceased than solu- tion begins. It is the excess of the one over the other which causes the deposition of ore in one place and its removal to an- other. Similarly, in our talk of " primary " and " secondary " deposits of ore, while some such distinction may be necessary for the purpose of explaining differences of immediate origin, we must not fail to recognize that all the ore-deposits within the ken of man are essentially secondary. There has been nothing original since the world was first evolved from chaos. We have to deal with a continuous rearrangement of material. THE GENESIS OF ORE-DEPOSITS. 221 The ore of one place came thither by removal from another. Whether it be present in minute microscopic particles or in blocks as bi^ as a house, is a distinction more economic and commercial than scientific and philosophic. The decomposition of one mineral is required for the composition of another. Ore-deposits are in their nature concentrations, whether by the mechanical accumulation of disintegrated fragments of older deposits or by the local regathering or segregation by chemical agencies of minerals previously widely and minutely dissem- inated, or finally by the addition, bit by bit, through mechan- ical and chemical force, of the matter brought from above or below by circulating waters. The frequent occurrence of thermal springs in the neighbor- hood of later eruptive rocks is very properly emphasized by Prof. Posepny, and is of immediate importance to the student of ore-deposition because the eruptive rocks are in turn found so often in close association with lode-formations. That thermal springs, eruptive rocks and ore-deposits are intimately inter-re- lated in their origin is generally accepted. In this connection I may be permitted to contribute some additional facts. Besides the localities quoted by Prof. Posepny, I would men- tion the Hauraki or Thames gold-field, in the North Island of New Zealand, where a good opportunity is offered for the study of this subject. In the Coromandel peninsula of the North Island there is a gold-bearing belt extending for nearly a hun- dred miles, from Cape Colville to Te Aroha. The prevailing coantry-rock consists of Tertiary eruptives, through which patches of Carboniferous slate occasionally appear. There are thermal springs scattered throughout the region. At the prin- cipal mining center, the Thames, the escape of carbonic acid gas has often caused a temporary cessation of work in the mines. There are soda-water springs in the vicinity of the Thames. At Te Aroha, at one end of the gold-belt, there is a group of celebrated medicinal hot springs. This last locality is connected by a continuous chain of thermal springs with Eotomahana, about 45 miles distant, the famous hot-lake re- gion, the pink and white sinter-terraces of which were known for their beauty throughout the world until Mt. Tarawera broke out in sudden eruption and destroyed them in 1884. Veins of gold-bearing quartz, recent eruptive rocks, thermal 222 THE GENESIS OF ORB-DEPOSITS. springs, dying solfataric action, and active volcanic force, are all intimately associated in this corner of the world. At the Thames, the leading mining town of the island, bodies of gold-ore of unusual richness have been found. In 1871, the Caledonia mine produced 10 tons of gold and paid three million dollars in dividends. In 1878, at the Moanataeri, 5400 pounds of quartz yielded 14,600 ounces of gold. The prevailing country-rock is an andesite breccia, traversed by zones of de- composition, in which the gold-veins occur. At Eotorua, in the hot-lake district already referred to, the plain is in part covered with fragmentary andesite. This material is usually loose and unconsolidated. Near the edges of the fumaroles, which are numerous, it has, however, become cemented, and then very much resembles the country-rock of the mines. The rims of the fumaroles also exhibit products of decomposition, which are similar in character to those observed in the lode-channels at the Thames, and which, because they are soft and granular, have been termed " tufaceous sandstone." Quartz closely re- sembling that of the gold-veins of the mines can also be seen to be deposited around certain of the fumaroles and hot springs referred to above. My examination of the ore-occurrence and vein-structure, though incomplete, led me to conclude that the deposition of the gold and its associated minerals had followed certain lines of altered country-rock which had been exposed to the effects of dying but lingering solfataric agencies.* Another district which affords evidence to help us in study- ing this subject is that of Pontgibaud, in south-central France, among those volcanic peaks of Auvergne which have been ren- dered classic by the work of Poulet Scrope. The silver-lead lodes of this district have been very extensively developed, and their geological structure has more than once received notice at the hands of competent observers. f The country-rock con- sists of gneiss and mica schist, penetrated by dikes of granulite. J * See also "Certain Dissimilar Occurrences o£ Gold-Bearing Quartz," by the writer, in the Proceedings of the Colorado Scientific Society for 1893. t Annales des Mines, M. Gudnyveaii, 1st series, t. vii., p. 162 to 188. MM. Eivot and Zeppenfeld, 4th series, t. xviii., p. 137 to 257, 361 to 446. Also recently M. Lodin, April, 1892, in a paper entitled " Etude sur les gites m^tallifferes de Pontgibaud," also published in the Annales des Mines. X If it were in our West it would be called "porphyry" — a term which has gradually been losing its distinctive meaning through careless use. THE GENESIS OF ORE-DEPOSITS. 223 The lodes are of later date than the dikes, but older than the Pliocene flows of basalt which cover their croppings. The period of their formation is considered to have been between the middle Miocene and the middle Pliocene, very probably contemporaneous with the extension of the acid eruptives of Mont Dore, which took place at the beginning of the middle Pliocene. The lodes generally follow the veins of granulite, and are productive only when so associated. "When the dike- rock in which the lode occurs is most feldspathic, the metallif- erous filling is most valuable. In this region mineral springs are abundant, and the escape of carbonic acid gas has frequently put a temporary stop to under- ground work. This applies particularly to that part of the dis- trict through which the river Sioule flows between the town of Pontgibaud and the mines at Pranal. Often, while fishing along the stream, I have noted places where there is a constant escape of carbonic acid gas from its bed to the surface. At Pranal there appears to be an intimate connection between the lode- fissures and the volcanic vents. One of the mineral veins has been traced to its connection with what appears to be a vent of the extinct volcano of Chalusset. Powerfully carbonated springs exist close to the mines and on the slope of Chalusset. In both of the two districts above cited, the one in iSTew Zea- land and the other in France, note has been made of the escape of considerable quantities of carbonic acid gas. It is scarcely necessary to emphasize the fact that this is a most common and powerful agent in bringing about changes in rocks and minerals. The action of carbonic acid, and of the alkaline carbonates which it forms, have been recognized by all petrographers. To it we owe the salts occurring in ordinary mineral springs ; to it are due the pseudomorphic replacement of feldspar with chlorite*, and the alteration of olivine into serpentine, and of * And the chlorite afterwards gives place to tinstone. This is a subject much studied by Mr. Richard Pearce, at a time when its importance was not so well rec- ognized as now. — See "The Influence of Lodes on Eocks," Proceedings of the Mining Association of Devon and Cornwall, September 8, 1864. Mr. Pearce directs attention to the difEerence between the granite encasing the lode and that found at some distance from it. He makes note of the joints in the granite, and remarks upon the difference in the minerals found in two well-marked systems of joints having contrary directions. He shows that the changes observed in the rook ad- joining the lodes have their origin in the lodes. Emphasizing the melamorphism 224 THE GENESIS OF ORE-DEPOSITS. limestone into dolomite. Even at ordinary temperatures, car- bonated waters extract magnesia from complex silicates. In this way, biotite loses magnesia and iron, becoming converted into muscovite. The subject of the close association of ore-deposits and igne- ous rocks is a most important one to mining engineers. The detailed geological surveys of several of the most productive mining districts of the West, carried out during the past few years, have done much to emphasize the relation which seems to exist between bodies of eruptive rocks and deposits of gold- and silver-ore found close to them. It has become the fashion, especially since the publication of Emmons's masterly mono- graph on the Leadville region, to suppose that the precious metals of the lodes were derived from the leaching of the adja- cent eruptives ; and some mining engineers have gone so far as to consider the neighborhood of dikes necessary to the occur- rence of a productive lode. This latter notion may be classed with the supposition, now slowly passing away, which, not long ago, was so strong, that a " true fissure-vein " was the only permanent depository of the precious metals. In the United States, in Europe, and in most of the Austral- asian mining regions, the close association of dikes, or other forms of intrusive eruptive rocks, with lode-formations is so marked, that it is not surprising to find such rocks considered as necessary adjuncts to the occurrence of valuable ore-deposits. But generalizations are proverbially dangerous ; and, that this is an illustration of the proverb, the following facts may show. The gold-mining region of the province of Otago, in the South Island 'of I^ew Zealand, is confined, for the most part, to a great series of foliated quartzose schists of an age considered Archaean by some,* and Silurian by others.f These rocks have of the granite, he shows that the lodes consist essentially of altered granite, the most important alteration being the replacement of the feldspar by chlorite, by tinstone and by schorl. He discards the idea of an igneous origin of the tin-ore, and declares that aqueous agency alone can satisfactorily account for the changes in the rocks and the formation of the lodes. He expresses the belief that the subject of the metamorphism of the country-rock, if "diligently investigated, must assist in explaining some of the laws which regulate mineral deposits." This was said thirty years ago I * " On the Foliated Rocks of Otago," Professor F. W. Hutton, F.G.S. Trans, of the New Zealand Institute, vol. xxiv., 1891. t " The Gold-Fields of Otago." Trans. A. I. M. K, xxi., 412. THE GENESIS OP ORE-DEPOSITS. 225 an enormous thickness over a large area; the thickness has been estimated at 50,000 feet, while the area is fully 10,000 square miles. This has been a very successful gold-mining re- gion, although the gravel-deposits have, so far, been more pro- ductive than the quartz-veins. The lodes have certain well- marked structural peculiarities, resulting from the foliated arrangement of the country-rock which they traverse. In a previous contribution, incidental reference was made* to the fact of the remarkable absence, in this auriferous area, of erup- tive rocks. It is interesting to recall so marked an exception to what is often held to be a general rule. That the quartzose schists of Otago are simply altered sedi- mentary beds of very early geological age, there is little reason to doubt. The quartz folia are arranged along the lines of original sedimentation, and not along cleavage-planes. It is a case of " stratification-foliation," as distinguished from " cleav- age-foliation, "f The only rock likely to be a metamorphosed eruptive is the chlorite schist of Queenstown.J The mining regions of Otago do not exhibit any of the phenomena of con- tactmetamorphism ; and the changes which have been pro- duced may be ascribed to what we call " regional " metamor- phism, a vague way of describing those alterations which are forever taking place in rocks wherever there is heat and pres- sure, alterations which are, therefore, most evidenced by the oldest rocks, which have necessarily been overlaid by a great thickness of later-deposited formations. § A treatise which covers so wide a field as that of Professor Posepny can, of necessity, devote but scanty attention to some mining regions which, to those who know them, appear to afford important evidence on the subject of ore-deposition. In this regard, it is to be regretted that Professor Posepny does not seem to have had his attention drawn to certain very excel- lent geological reports contained in the blue books of the * Tram., xxi., 413. t Prof. T. G. Bonney uses these terms in the Quarterly Journal of the Geological Society, vol. xlix. , part i. , p. 95. X As pointed out by Prof. Hutton. Op. cit. ? I do not lose sight of the fact that igneous rocks may become schistose by met- amorphism, especially through pressure, as a dolerite becomes a hornblende schist. There is no reason to suppose that such a metamorphism has occurred in these rocks of Otago. 226 THE GENESIS OF ORB-DEPOSITS. mining departments of Victoria, ISTew South Wales, and ISTew Zealand. Australasia has many object-lessons to offer to the student of economic geology, and the Colonial geological sur- veys have published several accurate and most interesting de- scriptions of them.* In concluding this contribution to the discussion of Prof. Posepny's paper, I may be permitted to express again the be- lief that his destructive criticism of the lateral-secretion theory is most opportune, and that his investigations into the flow of underground waters will do much to illuminate our views of the methods of ore-deposition. At the same time, I cannot but hold that his accumulation of facts and observations will show that neither the lateral, nor the ascensionist, nor any other one narrow theory can cover the multitudinous diversity of the ways in which ore-deposits are found to occur. E. W. Raymond, Kew York City : Concerning Mr. Rickard's proposed classification, I beg to say, while recognizing its con- venience for mining engineers, that it cannot be considered as a substitute for that of Prof. Posepny, for the simple but con- clusive reason that it is not genetic. Its fundamental division is based upon the position of the deposits, which should be, in a genetic classification, a subordinate consideration; and the most profound genetic distinction presented by nature, namely, the distinction between contemporaneous and subsequent forma- tion, appears in this scheme as a division of the third degree, affecting only inclosed bedded deposits. If I were inclined to criticize names, as Mr. Rickard has elsewhere done with regard to Prof Posepny, I might point out that the word " contempo- raneous " does not describe coal-beds, which Mr. Rickard men- tions as typical examples of it. Whatever may be said of a coal-bed, it is not contemporaneous in origin with the country- rock above it or below it. But this is a small matter. The point I make is much more important, namely, that the classi- * I would more particularly instance The Geology of the Vegetable Creek Tin- Mining Field, by T. W. Edgworth David, and the recently published Special Report on the Bendigo Oold-Field, by E. J. Dunn, together with the numerous observations made by E. L. Jack, in Queensland ; H. Y. L. Brown, and H. P. Woodward, in South Australia ; G. H. F. Ulrich, and F. W. Hutton, in New Zealand ; Wilkin- son and Liversedge, in New South Wales ; Murray, Sterling, and Howitt, in Vic- toria. THE GENESIS OP ORE-DEPOSITS. 227 fication itself is neither based on genetic distinctions nor on any other logical arrangement. I say this all the more frankly, because, as Mr. Rickard declares in complimentary phrase, he has largely followed the classification given by me in 1869. But that was, as Mr. Kickard's is, merely a convenient miners' arrangement. ISTow that Prof. Posepny comes forward, pro- posing for the purposes of science, not of mining, a truly genetic classification, a critic may fairly demonstrate its logical defects and suggest remedies, or declare remedies to be impossible. In the latter case, his contention would be that a genetic system cannot be constructed, and that the attempt had better be abandoned. But to say that one prefers, as a mining engineer, the handy non-scientific arrangement of ore-deposits hitherto in use, is no criticism at all. It is as if a botanist, considering a natural system in botany, should say that it was diseourag- ingly complicated, and that he preferred the simple and conve- nient arrangement of Linnaeus, by which one could identify a species from the number of petals and stamens and other arbitrary signs. H. Y. WiNCHELL, Minneapolis, Minn. : While heartily agree- ing with the frequently-expressed opinion that Prof. Posepny's paper is a masterly and exceedingly important discussion of ore-deposits, it still appears that there may be room for difier- ences of opinion on some points. Indeed, they necessarily follow from such decided statements on so important and in- teresting a subject. Those of us who live in the Lake Superior region are wont to believe that we have some conception of the meaning of the term " ore-deposits." We can, and frequently do, point with pride to the great value of our production of iron-ore and the fact that we furnish nearly two-thirds of the total product of the United States. It is an industry employing about 30,000 miners and involving capital to the amount of fully $100,000,- 000. But when we come to treatises on iron-ore deposits we are always disappointed. We find that, while speaking gener- ally and theoretically, iron-ore deposits may be mentioned, yet when it comes to critical discussion, and the illustration of theories by examples, they are omitted. We are constrained to protest that " ore-deposit " does not signify merely a vein 15 228 THE GENESIS OF ORE-DEPOSITS. of gold-, silver- or lead-ore or a stoekwork of tin- or zinc-ore, but that hematite and magnetite form ore-deposits of a com- mercially important and genetically highly interesting class. The value of the raw iron-ore produced in this country in 1889 was equal to the value of the gold bullion produced in the same year. And if we take the value of the pig-iron, which more nearly corresponds with bullion in the degree of removal from the raw material, we find it equal to the value of the gold and silver combined. And yet our author dismisses the entire subject in a couple of pages, and of Fuchs's and DeLaunay's 2000 pages only two are devoted to the most important iron- ore district on the globe. It would not be fair to suggest that iron-ores are overlooked because they do not seem to be explainable by the theories adopted for other classes of deposits. If that were the case, all the more need of giving them attention. It is more probable that it is because of the recentness of their development and the comparatively scant literature on the subject in the libraries of our foreign colleagues. That the circulation of waters carrying different chemical reagents is the all-important factor in the genesis of ores, as we find and mine them, is clearly shown by Prof. Posepny, and is accepted by the majority of writers on the subject. But the prominence which is given to ascending waters and the insig- nificant eflects ascribed to descending solutions will not find such ready acquiescence. It seems likely that ascending waters are the more likely to be eftective and to predominate below the ground-water level than in the vadose circulation. But it can be conclusively demonstrated that many of the immense iron-ore lenses of the Lake Superior region owe their present state of concentration, even to the depth of many hundreds of feet, to the action of the descending waters. Aside from the Mesabi range, the proofs lie partly in the following well-known facts : 1. The ore is a product of concentration in situ, whether the original rock or lean ore was an oxide, a silicate, or a carbon- ate, or whether it was oceanically or otherwise precipitated. 2. The ore-bodies have the shape of highly-inclined lenses, and frequently have an unaltered " capping " of jasper partially covering their upper ends. THE GENESIS OF ORE-DEPOSITS. 229 3. When this capping is present, it can be traced downward into the ore through changes which are clearly the result of oxygenated atmospheric waters. 4. The downward course of the waters is further shown by the protecting action of dikes and other impervious barriers, below which the ore is not found. 5. The ore-lenses lie in basins of greenstone-schists or other rocks, and occur at various depths to at least 2000 feet. 6. At the lower edges of some of these lenses are found de- posits of silica, kaolin, etc., which have plainly been removed from the ore-body above in the process of concentration. This is much below the vadose circulation, as the immense pumping engines and the rivers of water which they throw the year round testify ; but it is an instance of the formation of ore-deposits on the largest scale by descending waters. The circumstances are somewhat diflj'erent on the Mesabi range, but the proof is no less clear that the ore has been formed by solutions percolating downward. There the mines lie along the south side of the coutinental divide or water-shed, from which waters flow north to Hudson Bay and south to the Gulf of Mexico. They thus occupy the highest regions of the northern part of the State. Moreover, the shape of the strata, and the presence of a conglomerate beneath them, indicate that there was a shore-line there when the rocks were deposited. These facts, with the comparatively undisturbed condition of the strata, lead us to believe that the conditions have remained during many geological ages as they were originally and as they are now, viz., such that the inevitable direction of water- circulation would be downward and following to a certain ex- tent the gentle dip of the rocks to the south. Although of remarkable magnitude and chemical purity, these deposits are essentially surface-products and are at pres- ent largely above the ground-water level. The processes of replacement by the removal of silica, and of concentration by the addition of sesquioxide of iron, can be seen in progress in a hundred places. The rock which undergoes this change is a gray, reddish or greenish chert (" taconite ") banded with iron- ore. Figs. 1 and 2, taken from specimens from the Mesabi, illustrate the change mentioned, and show the downward course of the ferruginous solutions. 230 THE GENESIS OF OEB-DEPOSITS. Since we have here examples of iron-ore deposits, both above and below the ground-water level, which have been formed by descending waters, the thought naturally arises that the solutions may not have been so universally ascend- ing, in the case of other mineral deposits, as our author would have us believe. Taconite from the Mesabi range changing to iron-ore by solutions moving from left to right, o 6 is a fault line which conducted the descending waters downward and prevented the right half of the specimen from undergoing the ferrification which is seen in the left half. Another idea on which undue stress seems to have been laid is the correctness of the " ascension theory," and the absolute error of that of " lateral secretion." A consideration of these two ideas leaves me with the impression that they are not in reality so diametrically opposite that if one is true the other can have no scintilla of truth in it. In the deep region the cir- culating waters are supposed to be under considerable pressure, from which they escape by flowing in the direction in which THE GENESIS OF ORE-DEPOSITS. 231 they meet the least resistance. Even if the solution were on the whole ascending, still it must often happen that cracks and fissures would be encountered, leading in a lateral direction into some main fissure, full of ascending waters under slightly less pressure than that behind the waters which entered laterally. In that case it is also quite likely that there would be a dif- ferent chemical reaction at or near the junction of these two Another instance of partial alteration of taconite to ore. There was a joint here along a b whence downward moving waters effected a more rapid change for some distance laterally than the solutions percolating toward this joint along the strata from left to right were able to produce in the solid rock. Specimen collected by J. E. Spurr. circulating fluids from that produced by the action of either one of them on the rocks through which it passed. This might result in the precipitation of certain minerals on the walls of the main fissure near the subsidiary fissure, and thus the re- sulting ore-deposit would owe fully as much to lateral secretion as to ascension. And if these lateral joints and cracks (or even more porous rocks) were sufiiciently numerous, the whole 232 THE GENESIS OF ORE-DEPOSITS. vein, when formed, would be due to the combined actions of lateral secretion and ascension. Moreover, it seems almost necessary for the ascensionists to borrow aid from the lateral secretionists, whether they will or no. For the question arises : Where do the ascending solutions come from, anyhow ? Is there an inexhaustible reservoir at the bottom of each vein-fissure which supplies a ceaseless flow of carbonated and mineralized waters carrying precious metals in solution ? Or does the water start from the surface and per- colate downward until it is forced by heat and generated gases to rise again ? If the latter is the true supposition, is it not evident that the fissures which conduct these ascending waters must receive them from all sides through a thousand small crevices and pores, thus making again a combination of both lateral and ascending motions and depositions ? If ascending waters come from a great depth, descending waters must reach to the same great depth, and since the solu- tions cannot traverse the same path in their ascent that they do in their descent there must be a certain amount of lateral mo- tion at the moment when these solutions are the most dense and carry their heaviest burden of dissolved material. And it is evident that, whatever the depth from which the metallic elements come, there is as much chance for one mode of depo- sition as for the other. Secretary's Note. — The remaining contributions to the dis- cussion published in this volume were presented at the Bridge- port Meeting, October, 1894, or issued with the papers of that meeting, having been received before the Florida Meeting of March, 1895. •") Prof. Posepny (communication, translated by the Secretary) : First let me express my warmest thanks to all those who have so favorably judged my paper on the " Genesis of Ore-Deposits," and likewise to those who have taken this occasion to bring forward, whether in support of my views or in opposition to them, various observations and opinions, whereby our knowledge of ore-deposits has been unquestionably increased. It is exceedingly difficult — indeed, almost impossible — ^to make a correct comprehensive statement of a subject, the sep- THE GENESIS OF ORE-DEPOSITS. 233 arate fundamental data of which are scattered throughout the world ; and my treatise must, of course, be considered as merely an attempt in that direction, inspired by the purpose of con- tributing to this theme an element not yet sufficiently recog- nized, namely, the logical application throughout of the genetic principle. As I indicated on p. 9 of this volume, I expected as a result neither a simplification of systems nor a direct ben- efit to practice. My object was, irrespective of such considera- tions, to approach more nearly to the truth. A single observer may be able to establish a few more or less important facts ; but the great mass of the knowledge required he cannot personally possess. In the most favorable case, gov- ernment institutions, established to benefit single nations, or scientific or business associations, may procure accurate knowl- edge of the mineral resources of separate countries, and these may be combined to increase the knowledge of a considerable territorial complex; but the question still remains, whether the developments and natural exposures in a given region are really typical and conclusive as a basis for general scientific deduc- tions. In this respect, an international union of such endeav- ors, devoted to the advancement of this branch of geology, would be a decisive gain. When the United States Geological Survey began the study of the geological relations of ore-deposits, there was ground for hope that a new era in the knowledge of this subject would be thereby inaugurated. In fact, several monographs of inestima- ble value concerning the most importaiat ore-deposits had been published, when, for reasons unknown to me, the whole activ- ity of the survey in this direction was interrupted — an event much to be lamented. Yet a monograph can give only what is revealed by the de- velopments accessible at the time it is written ; and since mining continually makes new exposures, and for the most part oblit- erates the old ones, a complete scientific inquiry should involve provision for the repeated examination of a given mining dis- trict, and for publication, at intervals of, say, five or ten years, of the new knowledge thus acquired. It is scarcely to be doubted that the investigation of the ge- netic relations of a thing is necessary to complete our knowledge of it, and that this inquiry is therefore obligatory as a part of 234 THE GENESIS OF OKE-DEPOSITS. the study of anything which we desire to know exhaustively. Dr. Raymond (discussion at the Virginia Beach Meeting, p. 226) has defended the introduction of this principle into the science of ore-deposits, for which I thank him heartily. Messrs. W. P. Blake and A. "Winslow have controverted my views concerning the original source of the lead- and zinc-de- posits of Missouri and Wisconsin, condemning at the same time the similar views brought forward at the same Chicago meeting in the paper of Dr. "W". P. Jenney. Since I am personally ac- quainted only through a tourist's journej' with the relations of these deposits, which extend over so large a region, and am, moreover, not master of the wide literature of the subject, I must leave the defence of the principles asserted to Dr. Jenney, and will here simply refer to his reply, contained in the present discussion. With regard to Mr. Winslow's observations, I must confess that I am acquainted neither with the mine at Doe Eun nor with , the publications of Messrs. Strong and Chamberlin. But I know that concerning every region where lead- and zinc-ores occur in limestone and dolomite, the two opposite theories as to their origin invariably appear; and that in terranes consisting of structural plateaux, with nearly undisturbed position of strata, the representatives of the view that these ores were deposited simultaneously with the country-rock have the great advantage that the conditions of stratification are in their favor. Besides the paper here in discussion, I have lately devoted to the deposits of lead- and zinc-ores in soluble rocks a special treatise,* in which I have compared the occurrences of such deposits in plateau-regions with the conditions obtaining in mountain regions with already disturbed stratification. This publication originated in an address delivered by me at a miners' congress in Klagenfurt, that is to say, in the center of a mining industry based upon mineral occurrences of this class. In order to counteract a conception based upon local condi- tions, I have placed side by side the various alpine occurrences of Carinthia with those of the plateaux of Upper Silesia and North America, illustrating them, according to my custom, with * "Ueber die Entstehung der Blei-und Zinklagerstatten in aufloslichen Ge- steinen." — Jahrb. d. k.k. Bergakadeinien, 1893. THE GENESIS OF ORE-DEPOSITS. 235 drawings of the typical features. Among others, the occur- rences in Sardinia and in the ITorth of England are discussed, and use is made of recent literature concerning the Upper Sile- sian plateau. In this place, I can only remark that some of these occurrences in the mountain terranes carry evident traces of the subsequent derivation of the ores from below ; and that this fact alone is an argument for the similar origin of the plateau-de- posits, which so closely resemble the former in all other respects. The treatise I have mentioned does not include the observa- tions made by me in the spring of the present year upon the analogous deposits of Laurium in Greece, which are likewise in a structural plateau ; but I can assure the reader that the de- velopments of that region also indicate the derivation of the ores from below. So far as Mine la Motte is concerned, I can attach no great weight to the observations which I made there, upon a hasty journey. ITevertheless, the specimens of ore disseminated in sandy dolomite which I brought away show distinctly upon the surfaces, after polishing, the secondarj^ intrusion of the ore into the country-rock. "With regard to Mr. T. A. Kickard's criticisms, I would ob- serve that formerly the theories of ascension, descension and lateral secretion were generally spoken of without the assign- ment of any cause for the assumed movements of the subterra- neous liquids. I think, however, that I have secured some definiteness of conception by showing the actual descent of the vadose circulation and the ascent of the deep circulation, and by interpolating the lateral movement between the two. This gives reality to the processes formerly conceived abstractly, and makes it possible to discuss them. Mr. Eickard observes that, with reference to the formation of ore, I have laid special emphasis upon ascending mineral so- lutions (p. 191 of this volume). I meant to do this, however, only with regard to the sulphides. These certainly were not produced from the descending solutions, which carry oxygen now, as they unquestionably did in former geological periods also, and which invariably decompose sulphides wherever (as is the case in the vadose zone) they come into contact with them. "With regard to the sense in which I use the terms ascending and descending, I will say something below. 236 THE GENESIS OF ORE-DEPOSITS. Mr. Rickard suggests (foe. dt.) that, since the increase of pres- sure and temperature favors solution, while their decrease favors precipitation, precipitated ores are to be expected rather in the shallow zone ; and that this might explain the circum- stance that (as he believes) ores do not continue in depth. "Without going into the latter question in detail, I would point out that the conceptions of shallow and deep are only relative, and that in my discussion I could only have in mind the con- ditions existing at the time of the formation of the ores, and not at the present time. What was once shallow may now lie very deep, and vice versa. In this respect, the character of the ores is, I think, the decisive' fact. Oxidized ores must have be- come such in a zone then shallow, and original sulphides must have been deposited in a zone then deep, and beyond the reach of oxidizing agencies. For the present, only the extreme of these processes can be clearly recognized ; but it is not impos- sible that future studies in this direction may distinguish the characteristics of the intervening stages of formation, such as the deposits made during lateral movements of the mineral so- lutions. It would certainly be a step backward to allow the estab- lished characters of the two extremes to disappear under the general term " circulation." In ray description of the vadose circulation I have pointed out that, notwithstanding its course at the ground-water level appears to be almost horizontal, and notwithstanding an actual ascent of the liquid may be locally brought about by siphon-action, nevertheless a decided pre- vailing descent can be proved for the vadose currents. The terms " descending," " ascending " and " lateral " are not applied to a portion, but to the whole line of the current ; and to its cause, as both theoretically and empirically determined. I cannot admit that this is " a narrow view of the subject," likely to hinder progress in this branch of geologj' ; on the contrary, I believe it expresses a series of observed facts, calculated to in- crease our knowledge. Mr. Rickard seems to look at every new conception in this department from the sole standpoint of its immediate usefulness in mining, and not to reflect that the scientific investigator has simply to seek the truth, without regard to such considerations. His criticism might have been more favorable in some particu- THE GENESIS OF ORE-DEPOSITS. 237 lars (f'.^., Virginia Beach Discussion, p. 219, with reference to p. 59 of my paper), if I had taken pains, in many cases in which I was speaking of " ore-deposits," to explain that under this phrase, used for brevity, I was referring to deposits carrying metallic sulphides. Mr. H. V. Winchell complained, at the Virginia Beach meeting, that under the head of ore-deposits the deposits of iron-ore are too often either meagerly or not at all considered. This complaint would be well founded as against a report on the mineral resources of a given region, in which the economic importance of the deposits is a controlling element ; but it is scarcely just in its application to a paper like mine, which was intended only to give single instances in illustration of certain genetic theories. The reason that iron-ore deposits generally receive comparatively little attention in genetic discussion is, I think, the simplicity of their conditions, the knowledge of which is to some extent assumed to be familiar, so that authors interest themselves much more in the discussion of the more complicated occurrences, which have rarely, as a rule, been cor- rectly interpreted. I am indebted to Mr. Winchell for making good my omission by adding to my paper his account of iron-ore deposits known to him. Since the deposits he cites consist of oxidized ores only, they may well have been formed by an originally vadose circulation. I must, however, point out that some iron-ore de- posits may be of idiogenous origin. Thus, I consider the oolitic structure of some deposits (e.g., those of hematite in the Silurian of Central Bohemia) as a sign of their original depo- sition in the basin. I have had, however, far too little to do with these deposits to be able to determine more closely the significance of the remains of brachiopods {e.g., orthis shells), which occur, transformed into hematite, together with the oolites. The iron-ore beds of the Silurian basin of Bohemia have a certain analogy with those of the Huronian basin of Michi- gan, especially as regards the length and continuity of their outcrops, and their connection with tufas of the eruptive rocks. In the latter, as is indicated by the beautiful pseudomorphs of chlorite after garnet, considerable metamorphosis must have taken place. 238 THE GENESIS OF ORE-DEPOSITS. Concerning the Mesabi iron-ores, I am indebted to this critic for the illustrations of two specimens which he has published. They, indeed, suggest reflections as to their probable genesis, upon which, however, I do not trust myself to venture at this time. In reply to Mr. Winchell's criticism that, while laying un- necessary emphasis upon the correctness of the ascension- theory, I appear to concede to the theory of lateral secretion not an atom of truth, I beg to observe : 1. That I deem lateral secretion, in the sense in which it is defined by Professor Sandberger, to be possible only in the zone above the ground-water level, and, therefore, in the formation of oxidized ores only, and not for sulphide-ores. 2. That I am, indeed, obliged (as I have shown on page 28) to assume a lateral movement of liquids in the deep zone. But this is a region in which present processes cannot be directly observed, and, therefore, no clues to the conditions of deposi- tion are found. Hence, I was not able to describe such con- ditions in my paper. It is possible that, in the course of time, conditions of deposition may be discovered which can best be explained in this way. I have not yet encountered such a case. The same is true as to regions in which the two extreme branches of the subterranean circulation take on a lateral course. The case supposed by Mr. Winchell, in which a de- posit can be ascribed to ascension and also to lateral secretion, I do not clearly understand, since a physically weaker current is not capable of displacing a stronger one. While the extreme forms of circulation — that is, both the ascending and descend- ing branches — possess a pronounced character, it must be ex- pected that the character of the branches connecting these ex- ' tremes will be less distinct. Mr. John A. Church does not agree with me regarding ore- deposits in open spaces as a very frequent phenomenon, and ex- presses the opinion that open spaces cannot exist at great depths (such as 3 to 5 kilometers). I must remind him that in order to establish the first proposition the most important observa- tions of a great number of observers for more than a century must be disproved. He cannot have failed to notice that ore- deposits of that form which has been relatively most thor- THE GENESIS OF OKE-DEPOSITS. 239 oughly studied, namely, fissure-veins, consist predominantly of separate crusts, often marvellously distinct, covering what wer^ once the walls of the fissure-space. Even if his proposition he confined to deposits of great thickness and extent in depth, which are deemed to have been formed (as, for instance, the Comstock lode, which he has studied) by substitution, replace- ment or metasomasis, he cannot possibly deny the existence of other thick and deep deposits, the structure of the ores of which evidently represents the filling of open spaces. Eor in- stance, some of the Przibram veins, which have been worked to the depth of more than 1100 meters, and the ore of which often exceeds 10 meters in thickness, must certainly be reck- oned as wide and deep; yet the ores from their deepest por- tions do not difler in the least, so far as structure is concerned, from those which occur in the shallower parts. Both regions present fragments of the country-rock of all sizes, surrounded by the vein-material. Moreover, these fragments surrounded by quartz usually predominate in one or the other of the crusts of the vein-filling. Mr. Church seems to allow small value to the observations which it is possible to make upon the ores themselves and the adjoining country-rock. This is equivalent to the rejection of the only means of obtaining data concerning their probable genesis. It is difficult to discuss such an objection, particularly in its bearing upon the phenomenon of crustification, which I consider one of the most important genetic factors, and con- cerning which I will speak further in connection with my reply to other critics. Mr. Church declares the Comstock vein-mass to be the prod- uct of substitution — that is, of metasomatic alteration — and denies entirely that it is a fissure-vein. He says I have mis- understood him in saying (p. 92 of this volume) that he found crusts of quartz, alternating with calcite, in the Justice mine. The passage to which I referred was the following:* "The ore of the Justice is not quartz but calcite, with but an insignificant amount of silica, and it is noteworthy to find these two components of the lode dispersed in that banded arrangement, which is another of the accepted proofs of a true fissure-vein. The quartz is always on the propylite and the cal- * The CmmioA Lode, etc., by John A. Church, New York, 1870, p. 173. 240 THE GENESIS OF ORE-DEPOSITS. cite on the quartz ; but there is no comparison in respect to quantity. The quartz is always insignificant in thickness, never reaching a layer more than an inch or two, so far as noticed, except in the dyke-vein, while the calcite forms masses which are several yards in thickness," etc. "Why is this not what I call crustification ? It is certainly conceivable that the Comstock was formed by the opening of a space of discission at the contact of diorite and diabase, the filling of this space by the deposition of silica and carbonate of lime from solutions, and the repetition of these processes until the deposit had attained its present thickness. There is, for example, in the collection of the University of Vienna, a large slab from the Adalbert vein at Przibram, showing a series of thin galena-veinlets, the crystals of which meet in the axis of each several veinlet, showing that each was separately filled, and hence that the process of opening and filling, regarded with reference to the Adalbert vein as a whole, was repeated many times, until tHe aggregate thickness of about one meter, shown by this slab, had been attained. The Comstock might have been formed likewise by repeated opening and filling, only the several fillings would have to be thicker, and (since the material varied little) the result might be too indistinct to attract the at- tention of the miner. Mr. Church regards the ore-body of the Justice mine as a deposit separate from the Comstock ; but it is, nevertheless, a branch of the Comstock lode, and certainly has an analogous origin.* The occurrence of a crustified portion, which I think the text of Mr. Church's description indicates, possesses, there- fore, significance for both branches of the Comstock. By crustification, however, I do not mean merely a " banded structure." This may indeed originate, as Mr. Church says, in various ways, but crustification cannot; for true crusts are predominantly chemical precipitates, the crystal-aggregates of which present a certain arrangement. For instance, the quartz- crystals usually stand perpendicular to the former cavity-wall, directing their pyramidal surfaces towards the central druse. Incrusted fragments exhibit the same crusts as the cavity-walls, which is, at the same time, an additional proof of the existence of an open space, etc. It is true, that among these chemical * See Becker's Geology of the Comstock Lode, p. 30. THE GENESIS OF ORE-DEPOSITS. 241 precipitates there sometimes occur mechanical sediments, such as frictional detritus, which may be enveloped by one or another of the crust-substances ; but this is by no means a case under Mr. Church's statement (p. 198 of this volume) : "Certainly, a banded structure can arise from tiie replacement of fragments ar- ranged in layers by pressure and friction, as well as in many other ways, and does not prove deposition in a cavity, whether filled by water or air." Pressure and friction can give rise to no arrangement of xenogenites in separate crusts; in other words, no crustified quartz and calcite filling, such as I suppose to exist in the Corn- stock. I possess, for example, besides the ores from the Con- solidated Virginia bonanza, mentioned in my paper (page 92), some quartz specimens from the 1600-foot level of the Belcher mine, in which separate dark ore-bearing zones may be distin- guished, running parallel with each other, even to the repetition of minute undulations. This is, I confess, not such a convin- cing case as that of the specimen shown in Fig. 53 of my paper, which exhibits numerous successive crusts of baryte, fluorite, etc., no thicker than paper; or those of the Raibl specimens, which consist of thousands of thin layers of zinc-blende (whence the name Schalenblende) ; but it indicates, at least, the probabil- ity of a similar origin. It is, of course, not in every ore-deposit that such incontrovertible proofs as those last mentioned are found and preserved for science. Mr. Church points out (pages 196 and 197 of this volume) that metasomatic processes effected in limestones through the expulsion of the carbonic acid by a stronger acid, may also ex- plain the exhalations of carbonic acid so frequent in certain localities. I much prefer, however, to avoid the adoption of such a purely speculative standpoint, and would only suggest that, upon that view, the enormous volume of such exhalations in volcanic regions would require us to conclude that in those regions immense masses of limestone are undergoing the met- asomatic process referred to. As regards, finally, the subsequent alteration of the original ore-deposit, which, according to Mr. Church, partially passes into hysteromorphism, it is undoubtedly true that mineralo- gists, devoted to the study of pseudomorphs, have collected already valuable data in this field ; yet, I think, prolonged in- 242 THE GENESIS OF ORE-DEPOSITS. vestigation will still be required before general deductions can be profitably discussed. Mr. S. F. Emmons, whom I have to thank warmly for his favorable judgment upon several portions of my paper, natu- rally does not concur in the views I have expressed concerning Prof. Sandberger's lateral-secretion theory, to which he was himself at one time more or less committed. He objects, for instance, to my reference to the barysphere. This is a part of my conception of our planet as consisting out- wardly of several successive, and more or less connected, spher- ical envelopes — atmosphere, hydrosphere, biosphere, litho- sphere, and barysphere — of which only the exterior ones are open to our direct observation. In discussing the mutual re- actions of these great geological factors, which we may briefly call aggregate-spheres, it is unavoidably necessary to refer to the barysphere, which is beyond our observation : and, accord- ing to my habit, I have used this term in speaking of the source of the heavy metals. It is true, the term is only a device to avoid questions still unsolved ; but the same may be said con- cerning the phrases " unknown depths," or " unknown sources in depth," which have a similar meaning. It seems to me that Mr. Emmons and others of my critics have not correctly understood my statements concerning the several branches of the underground circulation ; and I there- fore beg permission to make my meaning clearer, even at the cost of a little repetition. For this purpose I will take for illus- tration, not an ideal case, but conditions actually existing, namely, those developed at Przibram, concerning which there exists an abundant literature, shortly to be increased (in the second volume of my Archiv fur praktische Geologie) by a mon- ograph of my own. The Przibram district lies, in round numbers, about 500 me- ters above sea-level, and the mine-workings extend, as is well known, to more than that distance below sea-level. The ground-water level is but a few meters under the surface. The deepest adit drains the mines to about 100 meters; and every- thing below that level is strictly deep workings, from which the water is lifted to the adit-horizon. A comparison of the water raised from different levels shows that the largest quantities come from the upper ones, and that the amounts diminish with THE GENESIS OF ORE-DEPOSITS. 243 increasing depth, so that at about 300 meters below sea-level no water remains to be raised, the ruling rock- and air-tempera- ture of about 23° C. (74° F.) at that depth sufficing to evap- orate the small existing quantity of water. This is certainly a striking proof that the water encountered in mining is of at- mospheric origin. The ore-deposits are steeply-dipping fissure-veins, which are mined by reason of their richness in silver (about 5 per cent., or 50 kilos per metric ton — or say 1458 ounces Troy per ton of 2000 pounds avoirdupois). Even in the neighborhood of the surface the sulphides predominated, but were mixed with a great variety of beautiful minerals, which have made Przibram famous among collectors, and most of Avhich, according to the results of the investigations of F. A. Reuss and others, are of secondary origin. It cannot well be doubted that this altera- tion is due to the oxidizing properties of the liquids coming from the surface. But this variety of minerals is confined at Przibram to the upper zones. Since mining has penetrated to lower levels, its product has been mainly only rich argentifer- ous galena, with accompanying zinc-blende, etc. The diminu- tion in secondary minerals, so far as it can be determined, seems to follow closely the progressive diminution, in depth, of the quantity of surface-waters. Concerning the origin of the secondary alterations, there is (as Mr. Church may be pleased to know) no doubt at Przibram. The only question at issue concerns the explanation of the orig- inal vein-filling, consisting of sulphides. This must have come, of course, from some rock as a source ; and on this point views are at variance. 1. Professor Sandberger at first conceived that this filling came directly from the country-rock (Nebengestein). The tech- nical term Nebengestein is more definite, perhaps, than " country- rock." It means literally the rock alongside, or the country- rock or wall-rock immediately in contact with the deposit. In this sense, it is impossible to conceive of any other process than that of lateral secretion, which could make the Nebengestein the source of the filling ; and I have attempted in my paper to show the improbability of such a lateral secretion of such a filling. 2. Mr. Emmons, in his paper on " The Geological Distribu- 16 244 THE GENESIS OF OB.E-DBPOSITS. tion of the Useful Metals in the United States," read at the Chicago meeting {Trans., xxii., 53), has connected the deriva- tion of the various metals of different deposits with the observed geological conditions of that country, discussing the metals, iron, manganese, nickel, tin, copper, lead and zinc, mercury and gold and silver separately. In his criticism of my views in this field (pages 200 and 201 of this volume), he has taken occasion to express a general statement for all ore-deposition. Accord- ing to his opinion, the metallic constituents were derived by lateral secretion from rocks within "reasonable proximity;" and " ore-bearing currents may in such cases have had an up- ward, downward or lateral motion, according to differing local conditions of rock-structure." This latter expression I would like to amend in accordance with the fact that, while the local conditions of rock-structure indeed influence the movements of liquids, the true causes of the upward, downward and lateral motion, as explained in my discussion of this point, lie outside the particular rock-structure. I would invite Mr. Emmons to take the standpoint sketched on pages 55 and 56 of this volume, in the depths of the Przi- bram mines, and see how he would get along with his assump- tion of lateral movement. And I must repeat that it is not so much. the local direction of the currents as the general char- acter and cause of the flow which should be kept in view. The general phenomenon of descending currents in the Przi- bram mines is clearly subsequent to the formation of the ore- deposits ; and the existence of lateral movements of the vadose circulation which could form these deposits is inconceivable. Let us see, then, whether such movements could occur in depth, in the sense defined by me on page 28 of this volume, and quoted by Mr. Emmons. "We should be forced to assume that the open vein-channels had not extended much deeper than the point (500 to 700 me- ters below sea-level) at which I have invited Mr. Emmons to stand, and also that there was no special upward tendency of the waters filling these channels. A lateral continuous move- ment would be only possible if there was something " in reason- able proximity " which would consume the moving current, or force it back to the surface. To expect this phenomenon in a terrane already traversed by channels reaching to the surface is THE GENESIS OP ORE-DEPOSITS. 245 irrational. In the only conceivable sense, it would merely make the lateral movement an incidental part of a general up- ward circulation. But this favors my view of the ascent of min- eral solutions from greater depths than have yet been reached in mining, i.e., from " unknown depth," as Mr. Emmons ex- presses it, or from the barysphere, as I have expressed it. He also, by the way, assumes the origin of the heavy metals from the barysphere (or " from the depths," as he prefers to say), and goes so far as to intimate that I would make the theory more plausible by allying it with that of Vogt, according to which a process of so-called diiferentiation, during the coohng of the eruptive rocks, has concentrated their metalhc contents in certain regions more or less accessible to our observation. For my part, I must wait until Vogt's ideas have assumed a more solid form ; but I cannot help suspecting that Mr. Em- mons favors them principally because they bring the concen- trated metals in eruptive rocks within the reach of lateral secre- tion, as a forming process for ore-deposits. Mr. Emmons doubts my conclusion, based upon ISTdggerath's observations, that waters rising under pressure are capable of creating a channel for themselves in soluble rocks. In this connection I must refer to the difficulty encountered in explain- ing the cavities containing pipes of ore in soluble rocks. In my monograph on Eesbanya,* published when Noggerath's work was unknown to me, I was forced to assume, as the cause of the formation of the cavity, the downward vadose currents, and as the cause of the filling, on the other hand, the ascend- ing currents of the deep circulation ; in other words, two pro- cesses, representing the extremes of circulation, and succes- sively acting along the same line. Such a dilemma may be presented by any ore-deposit in limestone. Indeed, I became acquainted subsequently with instances indicating that the t-wo processes of cavity-forming and cavity-filling may have been sometimes almost simultaneous, f I was greatly pleased when I learned of itToggerath's observations and deductions, and I * Oeologisch-montanistische Studie der JErzlagerstdtten von Rezb&nya, in S. Ungarn. PuWished by the Hungarian Geol. Soc, Budapest, 1874. t See my paper in Jahrb. der k. k. Bergakad., 1893, p. 18, "Ueber die Ensteli- ung der Blei- und Zinklagerstittten in aufloslichen Gesteinen," especially Fig. 14, pi. iii. 246 THE GENESIS OF OKE-DEPOSITS. took pains at that time to acquaint Mr. Emmons by letter with the consequent change in my own views. The observation, as I convinced myself in 1885, cannot now be verified, for the whole place at Burtscheid is completely built over ; but 'Sog- gerath was a highly conscientious observer, and there can be no doubt of the correctness of his statement of the facts. Moreover, the phenomenon is, a priori, inevitable. If the highly dilute currents of the vadose circulation, descending by gravity, can eat out their own channels in salt or limestone (as is shown at p. 21 and other places in my paper), all the more might such elfects.be expected from waters ascending under pressure and more highly charged with reagents. Fig. 9 of my paper, show- ing the wedge-shaped spaces of corrosion described by Daubree from Bourbonne-les-Bains, with their summits directed upward, gives actual proof of this. My reference to the wedge-like form of certain deposits at Laurium was based on an ideal profile. In the spring of the present year (1894) I personally visited the district, and strove to secure more accurate drawings of the position and form of the deposits. I must confess that I was not able to find any such drawings, and I must therefore submit to the rebuke of Mr. Emmons. So far as I know the literature concerning the Larium district, the only accurate drawings are those of the French company in the treatise of A. Cambresy.* (I take this opportunity to correct a typographical error in the pamphlet edition of my paper. Fig. 87 was taken, not from Cordelia but from Huot.) "With regard to the essential diflference of opinion concerning the Leadville deposits, I may observe that the reason I ventured to discuss that district without having personally studied it is to be found in the magnificent monograph of Mr. Emmons, the interesting conditions which it describes, and its contradiction of current views as to the origin of the Leadville ores. Pass- ing by all corrections and criticisms on points of minor im- portance, I wish only to keep in view this essential difference of opinion, and to inquire what were the convincing reasons which lead Mr. Emmons to assert in this case a descent of the mineral solutions. * " Le Laurium," par A. Cambresy, Rev. Univ. des Miiies, 3 ser., t. vi., 1889. THE GENESIS OF ORB-DEPOSITS. 247 He separates the sources of the metallic substances into "im- mediate " and " ultimate." The latter, by reason of their purely speculative nature, he does not discuss, but devotes himself to the former. "Without being able to doubt that these substances originally came from great depths, and without being willing to assert that they came wholly from the country-rock actually adjoining the deposits, he believes : 1. That they came from above. 2. That they were derived chiefly from neighboring rocks. With regard to the first of these propositions, I can find in his elaborate monograph no tangible proofs whatever, only con- clusions deduced from certain observations. The shape and position of the ore-deposits, whether of those at the contact between porphyry and lime, or those in the limestone, afford no conclusive proof of descending mineral solutions as their source. Indeed, this is disproved by the fact that the deposits were originally sulphides (as they are now shown still to be at greater depths), and such sulphide-deposits cannot be asserted to have been formed by solutions descending from the surface (unless such an application should be made of the case cited on p. 107 of my paper, namely, the reduction to sulphides by means of organic matter). The interior structure of the deposits and of the country-rock, so far as they are described in the publica- tions on the subject, likewise fail to furnish any conclusive proof of this assumption. In his re-examination of the mines in 1890, Mr. Emmons found, even in the original, unaltered sulphide-ores, no crustifi- cation, from which he concludes that in this case there has been no deposition of ore in open spaces, but a metasomatic replace- ment of the limestone. It is to be hoped that investigations on this point will not be wholly abandoned in future. Mr. Em- mons mentions also his recognition of the granular structure, joints and cleavage of the original limestone in the sulphide- ores of the A. Y. and Minnie mines, and speaks of the cracks in the top of the ore-body, " through which the ore-bearing solutions had descended." This is clearly, as stated in this form, an expression of opinion. A detailed and purely objec- tive description, particularly if accompanied with drawings, would be highly valuable, and might constitute the tangible proof, the absence of which I have pointed out. Mr. Emmons 248 THE GENESIS OF ORE-DEPOSITS. gives us ground to hope for further observations in this direc- tion, based upon the latest developments of the mines. For the present, however, it cannot be said that we have any decisive proof from the interior structure of these deposits. The facts described in the literature concerning Leadville may be equally well used in support of the ascension-theory. As I have remarked (page 107 of this volume), the ores were at first conceived to occur at the contact between porphyry and limestone, or confined to the lime ; but afterwards it became clear that not the whole contact-surface as such, but only cer- tain zones of it, could be regarded as the principal centers of the accumulation of ore. These ore-shoots, lying in and near the contact-plane, were the channels of which the mineral solutions availed themselves. A parallel is thus furnished to various other ore-deposits; for instance, the zinc- and lead-deposits of the Alps, the shoots of which are near a contact of soluble with insoluble rock, and pursue the same direction as the stratifica- tion.* For the establishment of this analogy, credit is due to the mining engineers who have published their observations at Leadville, and, as Mr. Emmons implies, f have rendered val- uable assistance in enlarging our knowledge of the facts as de- veloped by mining. The text of Mr. Emmons's great monograph on Leadville shows plainly (p. 572) that, under the impression produced by the first publication of Professor Sandberger, the author deemed the ascension-theory to have been already completely over- thrown. He assumes that the type of a vein, as described by earlier authorities, is a purely ideal conception, and does not exist in nature. To show that these writers had before them, on the contrary, a real condition, I have cited the developments at Przibram. If we substitute, in that case, for the space of discission the spaces occupied by the Leadville deposits, the situation, as concerns the question of the direction of the ore- bearing circulation, is not altered. The flat dip of the ore- shoots and the solubility of the country-rock at Leadville are scarcely decisive as to this question. Nor does the depth thus '■■ See my treatise (1893), already cited, on the "Origin of Lead- and Zinc-De- posits in Soluble Rocks." t Page 202 of this volume. See also " The Mining Work of the U. S. Geol. Survey," Trans., x., 412 et seq. THE GENESIS OF ORE-DEPOSITS. 249 far attained in Leadville mining aftbrd conclasive evidence. In '"^y judgment, therefore, notwithstanding the dift'erences be- tween Przibram and Leadville, the same inference must be drawn in both eases as to the direction of the ore-bearing cir- culation. In other words, Leadville must be declared to be no exception to the general rule that ore-deposits carrying metallic sulphides have been formed by ascending solutions. Whether the metallic contents were derived wholly or pre- dominantly from the eruptive rocks adjacent to the deposits or occurring within a certain distance, is an independent question. Mr. G. F. Becker's criticism (page 204 of this volume), hav- ing been prepared without opportunity for a thorough combina- tion of authorities, is considered as preliminary only. It deals, as does that of Mr. Emmons, in the main, with metasomatic formations, without reference to formations in open spaces, and, contemplating the former exclusively, seems to disparage the emphasis which I have laid upon crustification as a clear proof of the filling of open spaces. According to his view, the recog- nizability of successive deposits is dependent upon incidental local circumstances, but the instances he gives do not appear to me adapted to prove his proposition that crustification may be produced by other causes than that which I have assigned. The banded structure of agates, so far as I have had oppor- tunity to study it, is a genuine crustification. It exhibits in- crusted nuclei, stalactites, and other formations characteristic of deposition in an open space, quite independently of the question whether changes in concentration or rapidity of cir- culation or in the substances contained in the solution were the occasion of precipitation. In like manner the precipitate formed upon a piece of iron immersed in a solution of copper sulphate is a genuine crust, the iron serving as the cause of the precipi- tation ; and the circumstances of such a precipitation in a space filled with solution, though the process take place above ground, present some analogies with underground conditions. A party of mine-thieves once entered by night an old and ex- tensive mine in Transylvania for the purpose of blasting off and carrying away an exposed mass of gold-ore. The shot opened a hole into an old working {coranda, in the Eoumanian lan- guage), and one of the miners crawled through. The immen- sity of the space in which he found himself astonished him 250 THE GENESIS OF ORE-DEPOSITS. greatly, but his exclamations of wonder were cut short by the crowing of a cock, which revealed to him that he stood under the night sky, in a great snTface-coranda or open quarry, which covered the whole area of the mine. Under some circumstances, therefore, it is clear that underground and above-ground are not so very far apart ! A mineral solution standing in a laboratory-beaker, exposed to the air, may practically represent, from our standpoint, a subterranean space, the lower part of which is filled with liquid and the upper part with gas, as I conceived it on p. 24 of my paper. Mr. Becker doubtless means, by the example he cites, to argue that the banded structure may originate also through re- placement of the idiogenites by xenogenites. This may be true, but his instances do not support the hypothesis ; for the pseudo- morphosis of galena after calcite is not a replacement of lime- stone by galena. Moreover, not every " banded structure " is a crustification. Mr. Becker names two sorts of indications of replacement, namely, crystalline pseudomorphism and the irregular enlarge- ment of fissures in the replaced mass. I beg to say that on pp. 15 and 16 of my paper I have mentioned several other signs, such as the retention of the structure of the original mass ; the transformation of fossils into ore ; the occurrence of remaining nuclei of the original rock, etc., and that I also sup- pose a metasomatic process to have taken place when the evi- dence is merely negative, that is, where indications of cavity- formation, in other words, crustification, are absent. But I have found deposits where the indications of both processes occur side by side, as, for instance, at Rodna, in Transylvania. It was at this place that I had the opportunity, thirty years ago, to demonstrate the metasomatic origin of an ore-deposit. Since that time, however, I have never visited the locality, and have received only superficial data concerning further developments. Outside of calamine-deposits, I have not encountered in my later explorations any cases of metasomatic formation; and I have been led to attach ever-increasing importance to the de- posits formed in open spaces, the list of which, as known to me, has been continually growing, while their definite characteristics have become more and more unmistakably clear. Any differ- THE GENESIS OF ORB-DEPOSITS. 251 ence of opinion which has arisen, as a consequence, between my American colleagues and myself, must be left to the judg- ment of investigators who are equally familiar with both classes of ore-deposits. My statement, " It is difficult to believe that metasomatic processes could produce such pronounced ore-shoots as those described at Leadville," must be explained from the standpoint I have taken as to the origin of cavities in a soluble rock. On p. 21 of my paper I have shown that, before the origin of the cavity, the rock-pores or interstices are filled with saturated solutions, and that a line of maximum flow must be subse- quently set up between the point of entrance and some point of minimum resistance, along which line solutions not yet saturated, finding access to the rock, may ultimately dissolve out open channels or cavities. These will then possess a shape extended in one general direction, such as we encounter almost always in ore-deposits in soluble rocks. The Leadville mining engineers have established such, a form for the Leadville de- posits ; and Mr. Becker has also found it in the quicksilver-de- posits studied by him. If I have correctly conceived the for- mation of these ore-shoots, they should show some indications of free cavity-formation, even when they have been produced in part by the replacement of the original rock. Finally, as regards the Eureka deposits, I seem to have been misunderstood. I did not assert that the spaces originally oc- cupied by the Eureka of e-deposits had been formed by surface- waters. I merely said (in accordance with Mr. J. S. Curtis) that this was the case with the caves, which accompany the ores altered and redeposited by the action of surface-waters. Mr. F. M. F. Cazin has called attention to an American ex- ample, furnished by the Vermont copper-mine, in which graph- ite (or organic matter, the remains of which are now repre- sented by graphite), may have reduced the ore-bearing solutions. Mr. Cazin cites the fossil palms converted into copper-glauce, in the Trias of Mexico, as proof that the copper was originally dissolved in the Triassie ocean, though perhaps in too small a proportion to injure animal life. "With regard to that I must observe that these palms probably occur in a fresh-water basin, from which the character of the ocean of the period cannot be inferred; nor, vice versa, can the traces of copper found in 252 THE GENESIS OP ORE-DEPOSITS. corals be adduced as iadicating the probable presence of copper in such a basin. R. W. Raymond, New York City : The labor and pleasure of translating Prof. Posepny's contributions having fallen to me, I have taken special interest in the discussion vrhich they have elicited ; and I venture to believe that an attempt on my part to summarize the results thus far attained may be useful as a help to the further discussion which I trust will ensue, and will not be deemed an arrogant assumption of the position of a judge, which is as far from my intention as it is beyond my capacity. No amount of latitude in such a discussion is reprehensible if it elicits new facts ; for the accumulation of accurate data is really more important than the mere iteration of argument, and a new fact, however remotely collateral in its bearing, may turn out to be of inestimable value. In this connection, however, it should be noted that the fact is valuable in proportion as it is not merely the expression of an opinion. When we are told by some authority that he " found unmistakable evidences " of this or that, we are simply asked to accept his conclusion, which might or might not have been our own upon the same phe- nomena ; and the weight we give to the fact of his opinion as indicative of the real facts behind it, which are what we want, depends upon our confidence in him, not only as an observer, but also as a reasoner. In my j udgment we should be grateful to Prof. Posepny for the emphasis he has laid, not only in this paper but in many preceding publications, upon the supreme importance of what he has called rein objective Darstellungen, a phrase which I have weakened in my translation by rendering it "accurate descriptions," in the fear that the term "objective," used in that sense, would be misleading. In this connection I may remark, that when the admirable paper of Prof Posepny was sent to me, it bore a title which would have been, literally translated, " Subjective Views on the Genesis of Ore-Deposits," the author meaning thereby to indicate modestly that he offered his paper only as an expression of the opinions to which he had been led by his own studies, and not as a statement of the set- tled results of science. I took the liberty of objecting to this title, on the ground that " subjective " views might be construed THE GENESIS OF ORE-DEPOSITS. 253 as opinions simply " evolved from the inner consciousness," without any foundation vphatever in observed facts ; and as a result of this correspondence, Prof. Posepny permitted the use of the simpler title, accompanied with such introductory explanations as would relieve him from the imputation of dog- matism. Accepting, however, his use of " subjective" and "objective" as connoting statements respectively aflfected or unaffected by individual opinion, we cannot but appreciate and share his de- sire for " purely objective " reports of observed facts in the field of his studies. And, since it is extremely difficult to convey an " objective " description in writing, the superiority of a careful drawing (not an " ideal " diagram, though that has its uses, and is often a better vehicle of description than words) is clear. Prof. Posepny has practiced his own doctrine by illustrating his paper with numerous drawings, and, I may add, he has uncon- sciously enforced that doctrine by betraying his own doubts and difficulties in the interpretation of mere verbal and partly " subjective" descriptions, given by other authors. The misunderstandings thus occasioned may be left to settle themselves through miitual explanations, such as have been made, more or less fully, in the course of this discussion. It need only be added here that Prof. Posepny's conscientious and frank declarations as to the limits of his personal observation, and his careful references to all authorities cited, constitute a safeguard against error, a full guide to further investigation and a model for our imitation. But the chief questions of interest to us, I think, are these : "What are the characteristic and valuable contributions made by this paper to the theory of the genesis of ore-deposits ? and, "What are the definite issues on which Prof. Posepny's views differ from those of other observers, as the latter have been rep- resented in this discussion ? Under the first head I think we may regard as pre-eminent the masterly exposition of the subject of underground circula- tion and the distinction established between the vadose and the deep circulation, the former actuated mainly by gravity and conditioned upon the relative position of the surface-outflow, the latter complicated by the effects of capillarity and pressure due to heat. This distinction supersedes the vague terms 254 THE GENESIS OF ORE-DEPOSITS. " ascending " and " descending," thougli the author has em- ployed these terms, in accordance with popular usage, and has thereby incurred some unnecessary criticism. For it is really not of the slightest importance to the general theory of this sub- ject whether a given mineral solution was moving horizontally or up and down when it produced a given precipitate. The only significant question is whether it was on the way up or down ; that is, whether it belonged to the one or to the other branch of the underground circulation. The third view, namely, that such a solution might belong neither to the vadose downward circulation nor to the deep upward circula- tion, but to a " lateral secretion," Prof. Posepny practically de- clares to be inconceivable. As I understand his argument (or rather, perhaps, as I would state my own view, which I think to be in substantial accordance with his), it may be expressed as follows : 1. The aqueous solutions underground must be conceived either (a) as moving on a general downward course, as parts of the vadose circulation, above ground-water level, or (b) as pen- etrating still deeper into the rocks below drainage-level (the barysphere), or (c) as rising from those depths under pressure, overcoming gravity, towards or to the surface; or (^d) as stand- ing (held by capillarity or otherwise) in rocks, whether above or below the drainage-level, and not participating in the circu- lation at all. 2. Concerning the condition (a), which is most open to our observation, we know a great deal. "We know, for instance, from an overwhelming number of observations, that the solu- tions of the vadose circulation are oxidizing, and that (apart from the, probably rare, re-formation of sulphides by the ac- tion of organic matter) they do not precipitate sulphides, but, on the contrary, attack and decompose them. 3. Concerning (6), we know nothing by direct observation, but are forced to believe, and justified (by Daubree's experi- ments, etc.) in believing, that such a movement actually takes place. 4. Concerning (c), we have the evidence derived from hot springs, etc., which has convinced all observers that there is in fact such an ascending circulation, whatever may be their con- clusions as to the depth of its origin or the degree of its agency THE GENESIS OF ORB-DEPOSITS. 255 in forming mineral deposits. The ascension-theory postulates concerning it only that it comes from the depths below drain- age-level, and is not moved merely by siphon-action, ultimately due to gravity. 5. Concerning (d), it may be said that solutions thus held with- out participation in the general circulation, while they may affect internal changes in the rocks they occupy, cannot begin, until they begin to move, a process of redistributing and concentrating by precipitation elsewhere the substances they hold in solution. 6. Moreover, solutions in the condition (^d), though not par- ticipating in the general circulation, must have reached their locus by means of that circulation. They must be conceived as having been a part either of the downward or of the upward branch, or, in other words, as arrested portions of the circular tion. 7. Whenever they begin to move, they must join one or the other branch of the circulation; and the deposits they may make must be the result of the laws of that branch, operating upon the nature of the solutions, this in turn being partly de- pendent upon their original source. 8. There is, therefore, no room for a hypothesis of ore-con- centration and deposit in bodies of considerable size by " secre- tion," independent of circulation, or for a cycle of circulation, complete in itself, yet not participating in the general phenomena described. For continuous currents must come from somewhere and go somewhere ; and neither inflow nor outlet is provided, except by the conditions of the general underground circulation, as described. 9. From this standpoint it is clear that the source of the sub- stances carried in solution by a current must lie somewhere in the path which that current has traversed. If the theory of lateral secretion means no more than the assertion that the mineral solutions which have precipitated ore in a given fissure or space have traversed and leached some rock before entering that space and that this rock adjoined or lay in " reasonable proximity " to the space of deposition, it would mean, as to the first proposition, nothing that anybody denies ; while as to the second proposition, it would be a somewhat vague assertion, requiring definite proof in each case, and not entitled to the dignity of a general theory. 256 THE GENESIS OE ORE-DEPOSITS. 10. But the theory of lateral secretion, however it may have melted away under the fire of criticism, originally claimed more than this. Prof Sandberger says:* "The so-called descension-theory of Werner is purely neptunic, and regards veins as exclusively filled from above downwards by the deposition of ores from liquids, without answering the question, whence these liquids derived their me- tallic contents. The descension-theory remains good to-day for all cases where, in higher-lying rocks, those substances can be with certainty traced, which have col- lected as ore-deposits in cavities and fissures in lower-lying rocks, not originally containing them. If the ores are accumulated in fissures, they possess all the characters of fissure-veins. So far as my knowledge of ore-deposits goes, the filling of fissures by ores which can be clearly proved to have filtered in from above is not very frequent ; but such filling of irregular cavities are common." After mentioning as an excellent instance the lead- and zinc- deposits of Raibl (which Prof. Posepny has discussed with very difi^erent conclusions), and declaring that he is at present con- cerned specially, not with such deposits, but with true fissure- veins. Prof Sandberger proceeds to state as follows the ascen- sion theory, which he says " still counts many adherents," and which he proposes to controvert : "The ascension-tlieory assumes in all cases that the ores occurring in a vein- fissure were derived either not at all, or only in part, from the immediately ad- jacent country-rock {aus dem unmittelbaren Nebengestdn), but, on the contrary, from greater depths, and have been introduced into the fissures either by ascending mineral springs or by sublimation. The substances deposited in the veins should therefore be diSerent from those of the adjacent rock, and should only occur in the latter as lateral impregnations from the fissures." Confining himself to the supposed agency of ascending min- eral springs, the author asserts that such springs would not, and in fact do not, deposit minerals in their channels, and discusses at some length the case of Sulphur Bank in California, which he declares to be the only instance apparently contradicting his view. He argues against the conclusions drawn by others from this instance, and concludes as follows (p. 17) : "If, then, the only region in which it has been deemed possible to assume the filling of vein-pressures by ascending mineral springs as a process now going on, furnishes no trustworthy proofs of this assumption, what remains? In my opinion, only the leaching of the country-rock which bounds the fissures by seepage-waters which have penetrated it, and which deposit the dissolved materials as ores and * Vntersiichungen «5er Erzgdnge, von Fridolin Sandberger. "Wiesbaden, 1882, Erstes Heft, pp. 3, 4. THE GENESIS OF ORB-DEPOSITS. 257 gangue in the fissures of the same (or, in exceptional cases, the nearest neighbor- ing) rock.* This is the so-called lateral-secretion theory in its most prosaic form ; and it is this to which I have been so distinctly led by many years of observation and investigation that I am forced to consider it applicable to most ore-veins." 11. It is clear that this theory contemplates the exclusion of the agency of waters rising from below drainage-level. That there are such waters, is an admitted fact ; and it must be also admitted that they are under pressure great enough to overcome gravity and friction. All fissures accessible to such waters, they must necessarily occupy; and it seems to follow inevitably that all fissures extending below drainage-level must be filled, up to that level at least, with waters either in actual circulation on their way upward, or temporarily arrested and confined. " Seepage " into such spaces is incon- ceivable. 12. On the other hand, currents under pressure would neces- sarily penetrate into the pores and interstices of the rocks bounding their main channels, and the deposit in such rocks of minerals carried from the fissures is more probable a -priori than the deposit, in the fissures, of minerals dissolved from the adjoining rocks. The opposite would be true if the fissures did not contain water, a condition which can only be assumed when there is a lower outlet, that is to say, only in the zone of vadose circulation. 13. The advocates of lateral secretion must state, at least, their conception of the way in which " seepage " can take place from a porous solid holding water into an adjoining space also filled with water, and under high pressure. That practically no interchange between the two will take place, even if the pressures are equal, is shown by the occurrence of fresh-water springs along our coast, separated by a few feet of sand only from the salt waters of the sea. It is often popularly supposed that the sea-water has been deprived of its salt by " filtration " through the sands ; but the real fact is, that the mass of the sea bars the path of a circulation which would carry the spring- water into it, and the spring seeks another way to the surface. * " Nach meiner Ausicht nur Auslaugung des die Spalten begranzenden Nebenge- steins durch Sickerwiisser, welche dasselbe durchdrungen haben, und die gelosten Stoffe als Erze und Gangarten in den Spalten des gleichen oder ausnahmsweise auch in solchen des niichsten Nachbargesteins. " 258 THE GENESIS OP ORE-DEPOSITS. where it emerges perfectly fresh. The intervening sands are doubtless filled with brackish water, but this takes no part in the circulation, and therefore carries no salt into the channel of the spring. If the Atlantic Ocean cannot " seep " salt into a spring of fresh water, how could a rock, not included in the path of a continuous circulation, impregnate any portion of that path by its " seepage ?" 14. Again, it is conceivable that gash-veins, and other spaces M'hoUy within a given rock-mass, may receive concentrations of mineral by " seepage," though even in this case, if the pro- cess is to result in considerable accumulations of mineral, it must be a long-continued one, supported by an inflow and out- flow ; in other words, it must be a part of a general ascend- ing or descending circulation. ' And since the ascending cir- culation involves a pressure from the fissure towards the wall-rock, that is, in the wrong direction for " seepage," it fol- lows that, except in the vadose region, and apart from highly exceptional conditions, the products of the leaching of any given rock-mass are not likely to be found predominantly in adjoining fissures. 15. The theory of lateral secretion, therefore, is essentially confined to the region of the vadose circulation ; and those who would apply it to the origin of deposits containing sulphides must be prepared to maintain that those sulphides have been deposited from solutions moving downwards or laterally, under the influence of gravity, in other words, surface-waters. Prof. Sandberger does not hesitate to accept this alternative, although he does not perceive, apparently, how it confines the sphere of his theory. According to his view, the metals are disseminated in the country-rocks and silicates, and these rocks contain also sulphate of soda, and other soluble alkaline sulphates, as well as chloride of sodium, all of which, he supposes, are converted by organic matter into alkaline sulphides, which transform the metallic silicates into metallic sulphides. 16. But this explanation encounters two serious difficulties. In the first place, it is opposed to the overwhelming evidence that the downward circulation does not characteristically de- posit sulphides, but attacks them ; that it does not character- istically contain alkaline sulphides, but alkaline carbonates and free carbonic acid and oxygen. In the second place, the expla- THE OENESIS OF OKE-DBPOSITS. 259 nation breaks down in the presence of fissures filled with sul- phides, extending far below any present or conceivable past drainage-level. The sulphide ore-deposits in such fissures, at the greatest depths attained by mining, show no structural dif- ferences or other indications of a different origin, as compared with sulphides in the levels above. There is a change at water- level, but it is notoriously a change from oxidation above, to absence of oxidation below, that level. 17. The lateral-secretion theory, therefore, so far as it is true at all, is no more than a subordinate division of the theory of the formation of deposits in open spaces above drainage-level ; and even here it is neither necessary nor plausible, as the ex- planation of deposits which continue downward, and must be referred, as regards their lower portion, to a deep source. Such deposits may have been altered in character, and even in form, in the vadose region; they probably originated in the deep region. 18. On the other hand, the hypothesis of ascending waters as the vehicle of solution and deposition does not exclude the idea of the leaching of any rock traversed by such waters. It indeed assumes such a leaching as having taken place some- where. But, as opposed to the theory of lateral secretion (mod- ified to lateral circulation) it assumes the rock immediately adjoining a vein-fissure (when the fissure continues deeper) to be the least likely, not the most likely, source of the metallic ores. And on this point it appeals to the phenomena of crustification. Nothing is plainer than the evidence afforded by the successive crystalline crusts of an amethyst geode, for instance, that the deposition took place first upon the walls of the cavity, after- ward upon the crust thus formed, and so on toward the cen- tral druse. The very first deposit evidently covered the wall with an impermeable layer ; and the material for all succeeding deposits must have come (as the sections of many geodes show visibly that it did come), through a passage from without the mass of the geode. In like manner, the crustified filling of a fissure-vein cannot well have come from the walls of the vein at the place where the first crust deposited would necessarily close those walls. The crusts have been deposited from a solu- tion between them. The central druse was not first formed, and then pushed out by successive deposits behind it, as the 17 260 THE GENESIS OF ORE-DEPOSITS. bark of a tree is thickened. The solution depositing the crys- tals in successive crusts must therefore have been part of a current ; and its entrance and exit can scarcely be sought, as a rule, in the walls it has crusted. A side-iissure, entering through either wall, is, of course, not impossible or uncommon. But it cannot be assumed to exist without proof. And when such a thing is actually found, its effect upon the vein is so marked as to raise a strong presumption that the normal source of the vein-solutions was not in that direction. 19. Prof. Posepny has laid much emphasis upon crustifica^ tion, as he has delined that term. I think he is right in so doing; and I may remark incidentally that his use of new special terms (which has been objected to by some) is justified, in this case, as in other cases, by the greater precision of thought thereby secured. The disadvantage of a preference for ordinary and familiar words, when such words may have many meanings, is illustrated by the manner in which Prof. Posepny, on the one hand, and his critics, on the other, have been misled by the ambiguity of " banded structure." He in- terprets " banded structure," or equivalent expressions, in some of the authorities he cites, as meaning crustification, and they say that banded structure may arise in several ways, intimat- ing thereby that crustification is not a sure proof of deposition upon cavity-walls. The verbal misconception being corrected, it seems to me that there is no difference between the parties on this head. 20. The assertion that a current is necessary for the deposi- tion of such crustified accumulations ie not to be construed as excluding variations in velocity, or occasional stoppages and in- termissions. The objection of Prof Sandberger, that mineral springs do not, as a fact, deposit solid substances in their chan- nels, seems to be based upon the conception of such springs as ascending with unvaried velocity, as if through pipes of uni- form diameter. Even pipes, as Professor Posepny reminds us, have been known to receive interior incrustations; but the probability of such deposits is much increased when the effects of variations in the nature and size of the channel are taken into account. Mutatis mutandis, the analogy of the deposition of sediments by a running stream is available here. As sands and clays, carried m suspension where the current is most rapid, THE GENESIS OF ORE-DEPOSITS. 261 are dropped where it is checked through widening of the chan- nel, or from other causes, so the deposits of a mineral circula- tion will naturally be greatest where the movement is slowest, or is even temporarily arrested altogether ; and they will be re- duced to a minimum, other things being equal, where the cur- rent is most rapid. The phenomenon of distinct crustification, in fact, requires the hypothesis of a relative quiescence of the menstruum. And instances are not wanting underground in which the widening of the vein-fissures, or the change to a flatter dip, has apparently favored the deposition of ore.* The ascension-theory does not exclude these obvious considerations. All it asserts is, that the portion of solution entering a given space, and depositing therein a precipitate, must thereafter escape and give place to another portion of solution, if the pro- cess is to be repeated ; and that, with regard to deposits of sul- phides, formed below drainage-level, the only escape is ulti- mately upward. But the phenomena of crustification in veins afford, in my judgment, another argument against the theory of lateral secretion. Namely, it is well known that the crusti- fication, even in typical fissure-veins, is not everywhere distinct. If it can be observed, with its characteristic central druse, in one part of a vein, it is held (properly, I think) to be (in the absence of evidence to the contrary) a proof that the similar ores of other parts of the vein have been similarly deposited. The absence of crustification in some places may be explained, on the ascension-theory, by the varying speed of the current, and the varying nature and dip of the walls, as affecting the deposition of adherent crystalline cruets. The chemical or physical causes inducing precipitation may simply produce a suspended precipitate, to be subsequently deposited as a sedi- ment. But if lateral secretion has produced crustification, such as is observed in fissure-veins (as I think, with Professor Po- * On the other hand, increased width of "vein-matter " has often been due to a splitting of the fissure, and the enclosure of fragments of country-rock, which is afterward more or less transformed into gangue, or remains as horses in the vein. Or, such increased width may be (as in the Cornwall tin-mines) the rer suit of a mineralization of the country-rock beyond the limits of the original fissure, producing a mass of altered rock impregnated with ore (the Zinnzmitter of the Germans). In such cases, while the aggregate of mineral deposited is doubtless much greater than it would have been had the solution passed through the narrow fissure only, the richness of the material is reduced by the admix- ture of gangue and rock. 262 THE GENESIS OF OEE-DBPOSITS. sepny, that it has not), then that structure, it seems to me, should be more uniformly distinct in such veins than it is. For the conception of lateral secretion into a fissure excludes the conception of a current under higher pressure, already occupy- ing that fissure; and the local interference of such a current with the quiet process of crystallization is therefore out of the question. 21. The comparatively small amount of mineral matter con- tained in the ascending springs of the deep circulation, origi- nating below drainage-level, is to my mind some indication that they have already deposited somewhere the larger part of the substances they have held in solution. They are never satu- rated solutions. As we find them, they contain what we may suppose to be only remaining traces of the metallic constitu- ents which they may (we may almost say must) have carried at greater depths, temperatures, and pressures. Is not the presence of these minute remainders really an evidence of the larger amounts once present, and therefore of a precipitation en route ? In connection with this question, the probable con- ditions of the deep zone must be borne in mind, such as, not only the increased solvent power of the waters of that zone, but also the probable slowness of their downward progress, which is practically (according to Daubree) a seepage, and which must favor the formation of saturated solutions. 22. In reply to this suggestion, the question may be raised how the deposition of ores, extending almost or quite to the surface, is to be accounted for, if the solutions now encountered below drainage-level are already so nearly exhausted as to be capable of comparatively little further precipitation. "Without forgetting that the most dilute solutions may still give precipi- tates under chemical or physical changes of condition ; and that such precipitates, however insignificant, may attain a consider- able aggregate amount by long-continued repetition, I think the more comprehensive answer to the above question is found in the conclusion to which we are led by the ascension-theory, that deposits carrying metallic sulphides, though they reach the present surface, were formed mainly below the influence of the vadose circulation, and therefore under conditions such as may now obtain at depths beyond our observation. 23. This suggests another point, to which Prof. Posepny has THE GENESIS OF ORE-DEPOSITS. 263 called attention, and which was acutely recognized by Cotta, many years ago,* namely, the fact that speculations upon the relation between the contents of mineral veins and their depth are largely vitiated by the vagueness and uncertainty of the ele- ment of depth, as estimated by comparison with the present surface. In most mining regions there is unquestionable evi- dence of great denudation, which has probably removed from the surface a larger mass than has been penetrated anywhere by mining. It seems impossible, therefore, to argue as to the con- tinuance of ores " in depth," meaning thereby beyond 1000 or 2000 feet from the present surface, when that surface itself may have been 10,000 feet underground at the time the ores were deposited. "We may imagine that the ascending waters in a vein now rich in metallic deposits " from the grass-roots down," once continued their upward course to the former surface, emerging as dilute solutions ; or never reached that surface in- tact, but encountering the vadose circulation, became a part of it ; and, in either case, precipitated less and less metallic matter as they ascended. Conversely, we may reasonably imagine that, if we could retrace the course of a mineral spring coming from the deep zone, it might lead us back to the region where it had deposited the treasure of which it now exhibits, at most, only faint remaining traces. And what we might thus fairly imagine concerning an actual spring might be equally true of the channel of a former spring now closed altogether, or occu- pied only, under changed conditions of altitude and drainage, by the vadose circulation — that is to say, of a fissure-vein, com- paratively barren or lean at the present surface. In other words, the present surface is an arbitrary section, cutting otf the veins. Those which it happens to intersect in their richer portions are naturally the ones which are developed by mining. Those which it shows to be locally barren are naturally not thus developed, unless local experience supports the hope that they will improve in depth. Such a local experience is doubt- less the foundation of the maxim which Cornish miners have carried throughout the world, that " a fissure-vein grows richer in depth," a proposition for which, as a general guide for mining (apart from the effects of surface-waters, which may be * Die Lehre von den Erzlagerstditen, Freiberg, 1858. Part I., p. 129. 264 THE GENESIS OF ORB-DEPOSITS. sometimes impoverishing), is without foundation in experience. For although a comparatively barren fissure may be, and has often been shown to be, the upper part of a vein carrying rich ores below, there is no general law that it must be so; and, moreover, there is no way of determining a ■priori the depth of the barren zone, measured from the present surface. 24. On the other hand, while the varying positions of the present surface prevent generalization as to the relations of ore to " depth," it is unquestionably possible that there may be, in a given fissure, a relation of that kind. The ascension-theory neither asserts nor denies such a supposition. Mr. Eickard's suggestion that the deeper zone must be the region of solution, and a higher zone the region of precipitation, is speculatively reasonable enough ; but it amounts to a proposed subdivision of the barysphere into two regions ; for the deep zone which Prof. Posepny has called the barysphere includes everything below our observation, and it is in that zone that both solution and precipitation are supposed to have taken place to form the deposits of metallic sulphides. In our ignorance of the con- ditions of that unknown region, it is scarcely possible or neces- sary to frame hypotheses concerning them. The practical bearing of Mr. Eickard's suggestion lies in his connection of it with an alleged general phenomenon of the impoverishment of veins in depth, as shown by experience in mining. 25. As to this alleged general phenomenon, I would say first, that even if it were proved, it could hardly be ascribed to the cause suggested by Mr. Eickard, namely, the predominance of solution in lower zones and the confinement of precipitation to higher ones, because the depths reached in mining are not great enough to warrant such a deduction, and also because the in- stances (such as Przibram) of rich ores continuing for great vertical distances, and down to levels among the deepest ever opened by mining, contradict the hypothesis. But it must be confessed that there is much evidence which seems to corroborate Mr. Eickard's statement as to the ex- haustion- of mines in depth. This evidence needs, however, to be carefully collated and critically sifted, before it can be ac- cepted as the indication of a natural law. a. In such an inquiry all cases must be rejected in which oxidized surface-ores have been mined down to water-level. THE GENESIS OF OEE-DEPOSITS. 265 and the mine has been abandoned by reason of treating the re- fractory sulphides. In many such cases the oxidized ores are actually richer {e.g. in gold) by reason of the alteration they have undergone ; but this is not pertinent to the question of original deposition. b. The abandonment of mines by reason merely of the in- creased cost of deep mining must be also set aside as affording no evidence on this subject. c. The fact that in mining a bonanza is traversed, and a rel- atively barren zone occurs below, does not necessarily indicate a relation between barrenness and depth. The occurrence of a bonanza very frequently involves barrenness of the neighbor- ing portions of the vein. That this is the case on a horizontal line is abundantly proved. An instance in point is furnished by the Bullion mine situated on the Comstock lode, between mines which have produced many millions. The expenditure of millions on the Bullion never produced, so far as I know, a ton of profitable ore. Why should not a similar alternation of rich and barren places occur in the vertical line ? The cost of exploration in depth, and particularly in sinking, naturally dis- courages mine-owners ; and the abandonment of an operation under such circumstances really proves nothing. d. In any case of alleged impoverishment of a vein in depth, not only the actual depth below the present surface, but also (so far as it can be estimated) the probable amount of denuda- tion which the surface has undergone, should be taken into account. e. The nature of the ore also may have a distinct bearing upon this inquiry. It is my impression that of the loose and vague evidence thus far accumulated, a large part refers to gold- ores, and particularly to free gold in quartz, as " giving out " in depth. I remember that in my last conversation with the late Joshua E. Clayton, a close and conscientious observer, he told me that he had personally examined numerous quartz- veins, occurring all along the flanks of the Sierra Nevada, and had found in every case that the veins, as exposed in the deep caiions cross-cutting them, hundreds of feet below their out- crops on the mountains, were poorer in gold than at the higher level. This testimony is valuable, and it may be that it indi- cates a general law as to such gold-veins ; but it must be borne 266 THE GENESIS OF OEE-DEPOSITS. in mind that some of the California gold-mines have been worked deeper than any cafions have cut the veins. Yet, on the other hand, many of the deep gold-mines of the State have been ultimately abandoned. 26. Mr. Rickard's suggestion has a practical side of great importance. IS'amely, although, in my judgment, there is no established general law, discouraging the exploration of a vein in depth, so long as the fissure continues well-defined, and especially if it carries any thread of ore, it is undoubtedly the case that mining explorations are too much confined to sinking and drifting, and that there is too little cross-cutting for par- allel fissures and ore-bodies. To some extent this is one of the results of our absurd United States mining law, which lays so much stress upon the "apex" and the "lode;" but the mis- taken practice of neglecting cross-cuts into the country-rock is not confined to mines operated under that law. 27. Another important point in Prof Posepny's paper is his proposition (based on ISToggerath's observations in the main, but not lacking other support) that open spaces of dissolution may be formed by ascending as well as descending currents. Since the process of solution depends upon the character of the liquid agent, this is only saying that some ascending waters may be able to dissolve portions of the rocks they traverse ; and that if such rocks belong to the class represented by limestone, such currents may produce in them caves and channels, comparable to those notoriously produced by the de- scending waters. I confess, this seems to me a reasonable proposition, however meager may be the proofs thus far ad- duced. And I cannot understand, at all events, how opponents of the ascension-theory should object to it; for they do not deny that there are such things as ascending mineral springs, and that these springs hold in solution such substances as car- bonates and free carbonic acid. "What they deny is that these springs deposit anything in their channels. In that case, they must dissolve without redepositing ; and the evidence that they have actually excavated channels underground is afibrded by their constitution. They bring the evidence of their guilt with them. To reply that they are part of the vadose circulation only, and hence, no matter what their local direction, belong to the descending branch, is not permissible ; for springs en- THE GENESIS OF ORE-DEPOSITS. 267 countered at great depths in mining have the composition re- quired to make them active solvents. How can it be doubted that the hot waters of the springs encountered in the Bohe- mian mines (see ISTos. 1, 2 and 3 of Prof. Posepny's table, p. 42), which contain " a notable quantity of free carbonic acid," would, if they traversed limestone, excavate cavernous chan- nels in it ? 28. Moreover, there is reason why a liquid solvent under pressure, occupying a space in a soluble rock, should eat its way upward rather than downward or laterally — namely, be- cause the insoluble portions of the rock, loosened by the action of the solvent, fall away from the roof of the cavity most easily and completely, leaving fresh surfaces open to further attack. Whoever has visited, as I have done, the salt-mines of the Salz- kammergut, in the Austrian Tyrol, where salt is extracted by Sinkwerke* and has observed how the great underground rooms, repeatedly filled with water under pressure, travel upward through the mass of the saliferous rock, as their roofs are attacked and dissolved, while their floors are relatively protected by the fallen insoluble debris, can scarcely doubt the possibility of the formation of spaces of dissolution by ascending waters. One variety of this extraction — viz., the so-called " continuous water- ing," employed in some of the mines — presents a still closer analogy. In that method the water is not introduced period- ically into each Sinlcwerk, to be withdrawn when saturated, and wholly replaced with fresh water for further solution. On the contrary, the flow of water is made continuous, fresh water being admitted at one point while saturated brine is conducted away at another. It is true that the actual flow of the current is downward, the fresh water being admitted above and the brine drawn otF below ; but this is not an essential feature of the process itself. The actual progress of excavation by solu- tion is upward, and the essential condition is the presence of a pressure sufficient to cause the solvent to penetrate the roof. That being secured, the roof is mainly attacked, the side much less, and the bottom scarcely at all. 29. Prof Posepny's " Theory of the Sinking of Heavier Con- * Described in Serlo's Leii/aden zur Bergbaukunde, 4th ed., Berlin, 1884, vol. i., p. 611 et seq. 268 THE GENESIS OF ORE-DEPOSITS. stituents," as applied to the distributionof gold, etc., in placers, is a valuable addition to our knowledge of such deposits. It is highly desirable that our members engaged in placer or hy- draulic mining should give us the results of careful observa- tion upon the conditions presented by the gold-deposits of this country. Few of them have done so thus far, and'the field is full of interesting data not yet put into shape for preserva^ tion. I am inclined to think, for instance, that "the hypothe- sis of a natural concentration in running water," which Prof. Posepny disparages, and for which he proposes to substitute the theory just mentioned, would find some support in the phe- nomena of many American placers, where the gold is concen- trated not only on a false or true bed-rock, but in distinct channels along that plane, so that the placer-miners, for many years, have pursued the tortuous channels of " pay-dirt," leav- ing large areas unworked, which, for some reason or other, did not pay, though they vs^ere equally " in the gulch," and had the bed-rock under them, like the rest. I do not mean to deny the possible agency of such a concentration by gravity in loose sands and gravels as Prof. Posepny has postulated, but I fancy it would be hard to explain the distribution of the gold in many of our American placers except by including among its factors the action of running water. If I am correct in this impression, I may venture to consider the case as one in which Prof. Posepny's heaviest artillery can be turned back upon him ; since his theory of " settling " may be called a sort of dry " seepage " or secretion by gravity, and my view may be considered as the assertion that, here as elsewhere, there is no deposition without circulation. Concerning the diflferences of opinion developed by this dis- cussion, I think it may be said that, upon closer examination, they are not important, except as to the explanations of certain districts and ore-deposits which Prof. Posepny has rather de- duced from the writings of others than based upon observations of his own. With regard to nearly or quite all of these instances, our own experts are not agreed, so that Prof. Posepny has respectable backing for his views, whether they turn out in the end to be correct or not. Certainly he has presented them with a con- spicuous absence of dogmatism, and they have been received THE GENESIS OF OEB-DEPOSITS. 269 on the part of our members, I am happy to say, with the re- spect due to the merits of a veteran authority, and with grati- tude for the generosity which has enriched the Transactions of the Institute with one of the most important contributions to technical science ever made through that medium. F. M. F. Cazin, Hoboken, N'. J. : Bergrath Posepny rejects my assumption of the presence of copper in the Triassic sea, claiming that the evideiace adduced does not hold good, and ob- serving, in support of his view, " that these palms," the cuprified fossils of which are found in the " coarse yellow sandstones and conglomerates overlying the red beds of the Trias," " on the junction of the Trias with the Cretaceous,"* "probably occur in a fresh-water basin." The fossil in question is identical with " Podozamites crassi- folia," describedf as occurring in the State of Sonora.| Palms cover at this date a narrow belt along the northern coast of South America, disappearing in the interior. The location of the ITacimiento copper-belt is one, from which the Cretaceous sea retired last of all on this continent. Its waters at this date are shed into the Gulf of Mexico, with no indication anywhere of a pre-existing barrier. I am acquainted with the English and ISTorth German Wealden formation, having mined in it; but, as J. S. N"ewberry did not, so I did not, find a trace of evi- dence for assuming a sweet-water formation at the ITacimiento copper-deposits. My assumption, therefore, stands on proper ground, unless more than a probability to the contrary be oifered. * J. S. Newberry, Report on the JExpedition of 1859, pages 117 and 118. t Ibid., p. 145. X It is one of the various strange things observed in geological reports, that are the compound work of many — that, although J. S. Newberry prominently and repeatedly refers to the palm-fossils of Nacimiento, his plates show naught under that head, but do show a true image of these "palm-leaves," described as collected in " quite a number " by Mr. Eedmond from " the Triassic strata at Los Broncos, Sonora," a locality not visited by the expedition of which the report is made. I may mention, as an amusing coincidence, that at Prof. Newberry's and at my time there was at Nacimiento a silver-smith, who hailed from Los Broncos, Sonora, and who, whenever in his trade he needed copper, smelted it in a miniature cru- cible on a miniature Mexican forge with accordion-shaped bellows, using as his material for copper the fossil palm-leaves of Nacimiento, of which within easy walk from his door he could pick all he was in need of, and of which he kept on hand " quite a number." 270 THE GENESIS OF ORE-DEPOSITS. If ever J. S. ]S"ewberry's and my own observations as to the geological position and normal character of the deposits in question have been objected to on the ground of actual local observations, I am ignorant of the fact. Joseph Lb Contb, Berkeley, Cal. : All geologists, but espe- cially students of the phenomena of metalliferous veins, are under deep obligation to Bergrath Posepny for the very lucid exposition and abundant illustrations of these phenomena con- tained in his admirable treatise on the " Genesis of Ore-De- posits." Like the previous treatise of Sandberger, though taking an extreme opposite position, it must powerfully revive the interest of students and observers in the purely scientific theory of metalliferous veins. Although read at the Inter- national Engineering Congress of the World's Fair at Chicago, in 1893, it has only very recently fallen under my eye. As I have thought much, and published somewhat on this subject, I beg leave to say a few words in the way of criticism on this masterly work. All, I think, will agree that one of the chief merits of the work consists in the clear distinction which the author draws between what he calls the vadose, or superficial, and the deep circulation of underground water; the water in the one case containing air, and therefore oxidizing; in the other, destitute of air, and therefore non-oxidizing; the one circulation driven by gravity alone, the direction of the current being determined by the place of outflow, the other driven largely by heat re- ceived in the lower parts of its circuit, and the direction of its current being mainly upward. We are all, I think, especially pleased with the significance he finds in, and the importance he attaches to, the oxidizing and non-oxidizing effects of these two circulations respectively. It follows, from this view, that metallic sulphides are not de- posited from the waters of the vadose circulation, unless under the exceptional conditions of the presence of excess of organic matter ; and therefore, that the presence of metals in the form of sulphides is usually a sign of deposit from ascending currents of the deeper non-oxidizing circulation. Most of us, I think, too (aiad I among the number), will agree with him, as against Sandberger, that since great deep fissures THE GENESIS OF ORE-DEPOSITS. 271 are not empty, air-filled spaces, but are necessarily filled with water, deposits in them cannot take place by seepage or oozing, or lateral secretion from the immediate bounding-walls. Also, that the phenomena of crustification or ribbon-structure of vein- contents seem to negative such a mode of filling as is supposed by Sandberger; that this structure does not indicate a filling by oozing and trickling of waters containing soluble matters, down on a free surface, but rather a deposit in successive layers inward from water contained in the fissure. For all this, and very much more which I cannot repeat here, we are under many obligations to Bergrath Posepny. Nevertheless, I cannot but think that he carries his ascension- views much too far ; that in his zeal against the extreme lateral- secretion views of Sandberger, he has gone to the other extreme of ascensionism ; and that a truer view than either may be found in one that shall combine and reconcile these two extremes. The evidence of the extremeness of his views is found, and indeed is embodied, in his use of the term barijsphere. As con- trasted with liihosphere, this term can only mean a region in the interior of the earth, the materials of which are heavier, because more metalliferous, than the superficial lithosphere visible to us. From such a metalliferous barysphere, he thinks, all the metals of ore-deposits (with trifling exceptions) are derived. It is true, that in his reply to objectors, he speaks of his barysphere as only the equivalent of the " unknown depths " of other writers ; but, it must be remarked, that this latter term, while open to the objection of indefiniteness, does not, necessarily, carry with it any implication of a region peculiar in its density and in the abundance of its metallic contents, although it is doubtless often used with this implication. The word barysphere, on the other hand, fixes definitely an idea which has long floated vaguely in the minds of many writers on this subject. It will, therefore, form the central point of my criticism. I. — Is THERE A Barysphere within Eeach of Circulating Water ? It is true, that the earth increases in density from the surface toward the center, and probably to the very center itself This is shown by the fact that the mean density of the earth is more than double that of the superflcial parts. It is true, also, that 272 THE GENESIS OF ORE-DEPOSITS. the increasing density, while certainly due, in part, to conden- sation by increasing pressure, is probably also due, in part, to diflerence of material, and especially to the presence of metals, as sulphides or native, in greater abundance in the interior parts. It is true, therefore, that the deeper parts of the earth are certainly heavier, and probably more metalliferous, than the superficial parts. In a word, it is true that there is a bary- sphere, and probably in the sense used by Posepny, as being more metalliferous. But how deep must we go to find this barysphere ? Let us see. Taking the density of the superficial parts of the earth (or what Posepny would call the lithosphere) at 2.5, and the mean density of the earth as a whole at 5.5 (Posepny accepts these figures), and assuming the simplest rate of increase, viz., a uni- form rate, then an actual density equal to the mean density of 5.5 would be reached at the depth of 1000 miles, and the central density would be 14.5.* This is an increase of 3 in 1000 miles. At the depth of 100 miles, therefore, the increase would be 0.3 and the density only 2.8. Is it at all probable that we ever have circulating water coming up from any such depth as 100 miles ? And yet, 2.8 is only about the densitj' of our more basic erup- tives, and therefore wholly undeserving the name of a bary- sphere. Circulating water may possibly come up from as deep as 10 miles, but at the same rate of increase, the density there is only 2.53 — an increase over the superficial density wholly in- appreciable. Dr. Eaymond, interpreting Posepny, defines the barysphere as all that interior region, the circulating water of which would not come up at all without the aid of heat. Does this mean all but the superficial region traversed by the vadoae or oxidizing circulation ? If so, it cannot be far from the sur- face, and the term barysphere, as applied to it, is surely wholly inappropriate and misleading. But it may be answered that all this reasoning is based on the assumption of a uniform rate of increase of interior density ; while in fact the great mean density of the earth may be ex- plained by the existence of a highly metalliferous shell at no * By mathematical calculation based on the above conditions, an actual density equal to the mean density of 5.5 is reached at depth of \ radius from the surface. Multiplying this gain of 3 by 4 and adding the surface density of 2.5 makes a cen- tral density of 14.5 (3X4 + 2.5 = 14.5). THE GENESIS OF ORB-DEPOSITS. 273 great distance beneath the surface and therefore within easy reach of circulating waters. To this view I make the following objections : 1. All our general reasonings concerning the cause of the great mean density of the earth, whether («) condensation by increasing pressure, or (6) arrangement of materials of a primal fused earth according to their relative specific gravities, would make the increase progressive to the center. In fact it is hard to conceive the conditions under which a dense metalliferous shell a little way beneath the surface could be formed.* 2. We have abundance of materials coming up in eruptions from depths as great as circulating water is ever likely to reach, and yet these materials show no such density and metalliferous- ness as is implied in the term barysphere. But again, it may be objected that I greatly underestimate the depth which may be reached by underground water. This brings up an important but difficult question. Is there any limit to the depth to which meteoric water may penetrate ? If so, what determines the limit and where is it? These are questions which science is probably not yet prepared to answer definitely. I once thought, that since the pressure of a water- column increases uniformly with the depth, while the elastic tension of steam in contact with water increases with increas- ing heat at an increasing rate, so as to develop a logarithmic curve, there must be a depth at which the tension of steam would be equal to the downward pressure, and that at that depth would be found the limit of underground water ; and I expressed this conclusion in my Elements of Geology, page 99. Further reflection has convinced me that the conclusion is un- warranted. Such a limit would undoubtedly be reached if the increase of tension continued to follow the same law indefi- nitely. But it is now known that at a certain temperature, called the critical point, steam has the same density as the water from which it is formed. At this point, therefore, it may * Of the two causes mentioned above, the first would probably produce increase at an increasing rate and put the place of density equal to mean density deeper than \ radius down. The second might give rise to any kind of rate according to the relative amount of the different kinds of metals ; but not improbably to a de- creasing rate and put the place of mean density higher. The combination of these two would make an indeterminable rate ; but something like a uniform rate is as probable as, perhaps more probable than, any other. 274 THE GENESIS OF ORE-DEPOSITS. be regarded as either steam or water indifferently, and under the slightest change of temperature it takes the one form or the other. Beyond this point it is no longer steam in contact with water, but dry steam, which we know follows an entirely dif- ferent law. ISTow the critical point of water is about 700° F. and the tension of steam at this point is about 200 atmospheres. Taking the increase of underground temperature at 1° for 53 feet, or 100° per mile, the temperature of 700° would be found at the depth of seven miles. But the pressure of a water- column there would be about 1100 atmospheres. The tension has not yet even nearly reached the pressure ; and, as the law changes here, it would seem that the tension would never over- take the hydrostatic pressure at all. Therefore, if the under- ground water is limited at all in its downward course, as is probably the case, it must be limited in some other way, prob- ably by increasing compactness of material, under the increas- ing pressure of superincumbent rock, which, by closing up the pores, would inhibit further penetration, or would make it easier for the water to come up again in ascending currents. I think we may reasonably conclude, therefore, that whether there be a limit to underground water or not, it is certain that below a certain moderate depth, say 8 or 10 miles, such water cannot be circulating; for beyond this the compactness of rock tmder superincumbent pressure would be such, that while capil- larity and weight of water-column might still urge further movement, passages sufficiently open to allow currents of circu- lation could not exist. We may assume, then, that the limit of circulating water cannot be more than 10 miles in depth. Below this, water may indeed penetrate by capillarity and by weight of its own column, but such water does not enter into ordinary circular tion, although it may come up in volcanic eruptions and indeed supply the force of such eruptions. Still, below this again, and even to the very center, there may possibly be what Fisher calls constituent water, i.e., original water occluded in the primal fused magma of the earth, still present in the interior and coming up in volcanoes, and (according to him) the cause of their erup- tions. If there be such, it is not circulating water in the ordi- nary sense, and therefore may be left out of account in this dis- cussion. THE GENESIS OP ORE-DEPOSITS. 275 Underground water may be conceived, therefore, as existing in three possible conditions, but more and more doubtfully in the order named : 1. Circulating meteoric water. This of course is certain. It probably extends but a few miles (8 or 10) below the sur- face. 2. Meteoric water, but not circulating. The existence of this is probable. I have been accustomed to call it " volcanic " water, because it is a probable source of the eruptive force of volcanoes. 3. Constituent water, originally occluded in the primal magma of the incandescent fused earth, and still occluded in the materials of the interior. This, Fisher thinks, is still escaping, and in doing so, fuses its way toward the surface, and finally emerges in volcanic eruptions. This, of course, is very doubtful. Of these three, if they all exist, we are concerned, I believe, with the first only. We have assumed 10 miles as the limit of circulating water, and therefore the limit of depth from which metals may be derived. But at that depth, as already shown, there is no " barysphere " in any intelligible sense of that word. For the diflerence in density and in metalliferousness between the rocks there and those at the surface is quite inappreciable. We have, in fact, much material coming up from this very region, and therefore know its density. Our more basic rocks are indeed far denser and more metalliferous than the average of that region, having acquired greater density by differentiation from an average magma representing that region. I believe, therefore, that the greater abundance of metallic ores in solution in ascending waters is the result, not of the greater abundance of metals in their lower courses, but of the greater heat which they take up in that part of their course and the greater pressure to which they have been subjected there. Both heat and pressure greatly increase the solvent power of water upon the feebly soluble metallic sulphides. Thus heavily freighted, the waters lose, in ascending, both heat and pressure, and therefore deposit abundantly in their upward course. In a word, ascending waters are rich in metallic con- tents, not because they have traversed a barysphere, but because 18 276 THE GENESIS OF ORE-DEPOSITS. they have traversed a thermosphere. With equal heat and pres- sure, I am convinced, they would get as much metal from our more basic rocks here at the surface as they now do from the hypothetical barysphere. These ascending waters are non- oxidizing, not because they have never seen the air, i.e., are not meteoric, but because they have exhausted their oxidizing power by previous oxidation of metals, of organic matters, and other bxidable substances in the upper parts of their downward course. n. — Vadosb vs. Deep Cikculation. Again, I think, Posepny draws much too sharp a distinction between his two kinds of circulation ; not indeed as to their oxidizing and non-oxidizing properties, but as to the force of circulation in the two cases respectively. In his anxiety to distinguish them sharply, he speaks as if the forces of circular tion in the two cases were entirely different, being gravity or hydrostatic pressure in the one case and capillarity in the other. ISTow nothing can be more certain than that hydrostatic pressure is the fundamental cause in both cases alike ; although heat, by lightening the ascending column and thus disturbing the hy- drostatic equilibrium, is the immediately determining cause in the latter. As Mr. Rickard, in the discussion, has justly pointed out, the effect of heat in the underground circulation is exactly like its effect in determining circulation in a system of house- warming pipes. Again, Posepny lays much stress on capillarity as an addi- tional force urging forward the circulation. But surely this cannot be so. Capillarity is indeed a powerful force, urging water to where there is none, but an equally powerful force fixing it where it is. So far from assisting, it powerfully im- pedes circulation, and, where it is strong enough, inhibits it altogether. Dry clay is a powerful absorber of water, but, when once wet, it becomes impermeable to circulation. In fact, Posepny sometimes speaks of the deep barysphere circulation, as contrasted with the vadose circulation, in such terms that one is left in serious doubt whether he regards the former as meteoric water at all ; and yet he speaks of it as cir- culating. Sometimes it seems as if he regarded his vadose water alone as meteoric and his barysphere water as some other kind of water coming up from the deep interior of the earth. THE GENESIS OF ORE-DEPOSITS. 277 like, for example, the constituent water of Fisher. Such water, if there be any such, might indeed be conceived as coming up from a metalliferous barysphere, such as he supposes. But this would be escaping water, not circulating water. If he means anything like this, it ought to be distinctly stated, for it changes entirely the ground of discussion, and much that I have said above would be wide of the mark. For my own part, unless we adopt Fisher's view, I believe that we never have any water coming up which has not previously gone down. This is what is meant by circulation, but I cannot think Posepny can mean that his deep circulating water is not meteoric ; and I therefore say nothing more on this head. III. — Leaching of Wall-Rock. Again, although I fully agree with Posepny and his brilliant expositor. Dr. Raymond, that crustification, when it is well de- veloped, indicates deposit from within, by ascending waters al- ready occupying the fissure, and not by laterally incoming water depositing in the act of incoming (in the manner of seepage- water in empty cavities), yet I cannot agree with them in think- ing that the pressure of such ascending water would necessarily or even usually prevent the incoming of lateral currents from the wall-rock. It is doubtless true that the ascending water in the fissure is under higher pressure than precisely similar water on the outside ; for, in addition to the hydrostatic pressure de- termined by the height of the outlet, it is also under hydraulic pressure in proportion to the velocity of the upward current. But the water saturating the wall-rock is also, of course, under heavy hydrostatic pressure. And when we remember the slow- ness of the ascending current (which is a necessary condition for deposit), and therefore the slight excess of the pressure over that measured by the height of its outlet ; and when we re- member further that the ascending water is hot while the wall- water is cooler, and therefore denser, we may well doubt whether the pressure of the ascending or the lateral waters will be the greater, and therefore whether the current will set outward or inward. The pressure of the ascending water is • greater by virtue of its motion, but that of the wall-rock is greater by vir- tue of its greater density. It seems not unreasonable, therefore, to conclude that sometimes and in some places the current 2Y8 THE GENESIS OE dEE-DEPOSITS. would set outward, and sometimes and in some places it would set inward. In many places, doubtless, the wall-rock is not saturated. In such places, of course, the current would set out^ ward by capillarity, as well as by pressure, until saturation is reached. Of course, also, impediments to upward flow, brought about by filling of the fissure by deposit or otherwise, would increase the interior pressure, and would cause an upward ram- ification and outflow in many places at the surface. Although the analogy is by no means perfect, yet, by way of illustration, the ascending fissure-current, with its freight of dis- solved matters and its tributary drainage from the country-walls, may be roughly compared to a main river with its freight of sus- pended materials and its lateral tributaries. In such a stream, the tributaries usually discharge freely into the main river, in- creasing in volume, though perhaps diminishing its percent- age of freight; but sometimes, by the greater pressure of flood- waters, the main stream may back up the tributaries until equi- librium is restored. So in the case before us, the main ascend- ing fissure-stream, with its freight of dissolved matters, usually receives tributaries from the wall-rock, although, by defect of pressure of the latter or increased pressure of the former, the main current may overflow into the wall-rock. Again, in both cases, the percentage of freight is usually greatest in the main stream, and therefore the deposits by diminished velocity and carrying power in the one case and by diminished heat and pressure and solvent power in the other, are heaviest there, al- though sometimes heavy deposits occur also in the back waters. Again, in both cases, while the tributaries increase the volume of the current, they usually diminish the percentage of freight, although sometimes the reverse may be the fact. Finally, as rivers, when obstructed by their own deposits, may reach their final destination by inverse ramification and through many mouths, so ascending fissure-currents, obstructed by their own deposits, may branch upward and reach the surface by many exits. This, however, can be seen only in ascending currents still depositing, as in the cases of Sulphur Bank and Steamboat Springs. In most cases this part of their course has been carried away by erosion. In a word, there seems no reasonable doubt that while usually the main deposits have been brought up from below, yet the THE GENESIS OF ORE-DEPOSITS. 279 tributaries from the country-wall do contribute, and sometimes in an important degree, to the metallic contents of the veins. This seems well-nigh proved in those cases given by Sandberger and Becker, in which analyses, especially selective analyses, find notable quantities of the required metals in the more basic minerals of the country wall-rock. To discredit the obvious inferences from the results of a method so mach in accord with modern science and substitute a roundabout process of second- ary leachings by vadose circulation of primary impregnations derived from a hypothetical barysphere, as Pos«pny does, must be regarded as a return to the speculative methods of early writers. Again, in cases like the lead-ores of Missouri and "Wisconsin, where there is no evidence of disturbance or of igneous agency of any kind, is it not more rational to derive the metals from the wall-rock, though probably from its deeper parts, than from an unknown barysphere ? IV. — A More Comprehensive Theory Needed. In conclusion, I cannot but think that the views brought for- ward in 1883 in my paper on the " Genesis of Metalliferous Veins " (Am. Jour, of Sci., vol. xxvi., p. 1, 1883), although I would perhaps now modify them slightly on some points, still represent well the present condition of science on this subject. Those who have read that paper will remember that it is an attempt based partly on my own investigations of the phe- nomena of metalliferous vein-formation now going on at Sul- phur Bank and at Steamboat Springs, and partly on a general survey of the whole field, to embody a comprehensive and rational theory, avoiding extremes on both hands. In it I devoted considerable space to combating the extreme lateral- secretion views of Sandberger. I did so because, on account of the recent appearance and signal ability of his treatise, it seemed likely to do harm by carrying scientific opinion too far in one direction. If it had been Posepny's treatise instead of Sandberger's, I should have felt equally compelled to combat it, and on the* same ground. Posepny quotes freely from my papers on " Sulphur Bank " and on " Steamboat Springs," but not from that on " Genesis of Metalliferous Veins." "Whether he has seen it, I do not know. There has always been, and still is, a strong tendency to ex- 280 THE GENESIS OF OKE-DEPOSITS. treme views on this subject. On the one hand, ascensionists would derive all metals from a mysterious metalliferous region — a " barysphere," and so strong is their advocacy that even when analysis finds the required metals in notable quantities in the wall-rock, they discredit the obvious inference by sug- gesting a secondary leaching of materials deposited there by primary baryspheric currents. On the other hand, the lateral- secretionists would derive metals not from ascending currents at all, but wholly from direct secretion from the immediate bounding-walls ; and so strong is their advocacy that even when the deposit of metals from hot ascending currents is proved by direct observation, as at Sulphur Bank and at Steam- boat Springs, they seek to throw discredit on the obvious infer- ence in regard to all metalliferous veins, by giving many cases in which hot springs do not deposit any metals. My paper was an earnest attempt to combine what is true in each, and thus to reconcile these extremes by a more comprehensive view, which explains their differences. According to my view, the source of metals is, indeed, on the one hand, by leaching, but not by lateral secretion ; on the other hand, not from a hypothetical barysphere, but from the wall-rock ; though, again, not from all points alike, but mainly from the deepest parts, and even from below the deepest parts, of sensible fissures. As in the case of many other disputes, I . believe both sides are right and both are wrong. Ascension- ists are right in deriving metals mainly by ascending currents from great depths, but wrong in imagining these depths to be an exceptionally metalliferous barysphere. They are wrong also in not allowing subordinate contributions by lateral cur- rents from the wall-rock higher up. The lateral-secretionists, - on the other hand, are right in deriving metals by leaching, from the wall-rock, but wrong in not making the main source the thermosphere. In the uncolored light of a more comprehensive view, many of the difficulties and obscurities of the subject disappear. 1. Ore-deposits, using the term in its widest sense, may take place from many kinds of waters, but especially from alkaline solutions; for these are the natural solvents of metallic sul- phides, and metallic sulphides are usually the original form of such deposits. THE GENESIS OP ORE-DEPOSITS. 281 2. They may take place from waters at any temperature and pressure, but mainly from those at high temperature and under heavy pressure, because, on account of their great solvent power, such waters are heavily freighted with metals. 3. The depositing waters may be moving in any direction — up-coming, horizontally moving or even sometimes down-going, but mainly up-coming, because by losing heat and pressure at every step, such waters are sure to deposit abundantly. 4. Deposits may take place in any kind of water-ways — in open fissures, in incipient fissures, joints, cracks, and even in porous sandstone, but especially in great open fissures, because these are the main highways of ascending waters from the greatest .depths. . 5.' Deposits may be found in many regions and in many kinds of rocks, but mainly in mountain-regions and in meta^ morphic and igneous rocks, because the thermosphere is nearer the surface, and ready access thereto through great fissures is found mostly in thesfe regions and in these rocks. 282 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. Some Principles Controlling the Deposition of Ores.* BY C. R. VAN HISE, MADISON, WIS. {Washington Meeting, February, 1900.) PART I.— GENERAL PRINCIPLES. PAGE Intkodttction, 284 The Theee Zones of the Lithosphebe, 285 Zone of fracture, 286 Zone of flowage, 286 Factors influencing depth at which flowage occurs, . . . 287 Zone of combined fracture and flowage, ...... 288 The Water-Content and Openings in Eocks, 291 Condition of water in the zone of fracture, 291 The openings in rocks, 293 Size and number of openings, ... .... 295 Ore-deposits derived from zone of fracture, 300 The source of underground water, ........ 302 The cause of the flowage of underground water, 302 Belts of underground circulation, 306 Upper belt of underground circulation, ...... 306 Lower belt of underground circulation 307 Capacity of water for work in the lower belt of underground circulation, 308 Movementsof water in the lower beltof underground circulation, 309 The preferential use by water of large channels, .... 315 Physico-Chemical Principles Controlling the Work of Under- ground Waters, 317 Chemical action 318 Underground aqueous solutions, 319 Tlie relations of solution and temperature, .... 320 The relations of solution and pressure, ..... 321 Precipitation, 322 Precipitatioii by change in temperature, 323 Precipitation by change in pressure, 324 Precipitation by reactions between aqueous solutions, . . 324 Precipitation by reactions between liquid solutions and solids, 325 Precipitation by reactions between gases and solutions, and solids 326 The General Geological Work of Underground Waters, . . 326 Division of the zone of fracture into a belt of weathering and a belt of cementation, 327 ^ Published by permission of the Director of the United States Geological Survey. SOME PRINCIPLES CONTROLLING DEPOSITION OP ORES. 283 PAGE Migration of material from the belt of weathering to the belt of cemen- tation, 329 PAET II.— APPLICATION OF PRINCIPLES TO OEE-DEPOSITS. The Pkecipitation of Ores by Ascending Waters 339 Precipitation by decrease of temperature and pressure, .... 340 Precipitation by mingling of solutions, 340 Reactions due to wall-rocks, . 343 General, 345 The compounds deposited by ascending waters 346 Source of the metals, 346 Source of the sulphur of sulphides, 348 Source of the carbonic acid of carbonates, ..... 351 General, 352 The Precipitation of Ores by Ascending and Descending "Waters Combined, 354 The Association of Lead, Zinc and Iron Compounds, .... 357 Facts of occurrence, 358 First concentration 359 Second concentration , 359 Galena, 360 Sphalerite, 362 Marcasite, ... . 362 General, 363 The Association of Copper and Iron Compounds, 364 The Association of Silver and Gold with the Base Metals, . . 368 Silver, 369 Gold, 371 Concentration by Reaction Upon Sulphides Compared with Metal- lurgical Concentration, . . 376 Other Reactions of Descending Waters, 376 Second Concentration Favored by Large Openings of the Belt of Weathering, 378 Depth of the Effect of Descending Waters 380 Illustrations of Secondary Enrichment and Diminution of Rich- ness WITH Depth, 383 General, 390 The Precipitation of Ores by Descending Waters Alone, . . , 393 Special Factors Affecting the Concentration of Ores, . . . 393 Variations in porosity and structure, 393 The complexity of openings, 394 Impervious strata at various depths, 396 Pitching troughs and arches, 405 Pre-existing channels and replacements, 413 Character of the topography, 416 Effect of the vertical element, 416 Effect of the horizontal element, 417 Physical revolutions, • . 419 Genera], 420 Ore-Chutes, 421 The Classification of Ore-Deposits, 427 284 SOME PRINCIPLES CONTROLLINS DEPOSITION OP ORBS. PART I.— GENERAL PRINCIPLES. Introduction. The following paper upon the principles controlling tlie deposition of ores is adapted from a treatise on Metamor- phism, to be published hereafter as a Monograph of the United States Geological Survey. In the present paper the argument can be made only in outhne. The argument is especially frag- mentary in the treatment of the general principles controlling the circulation of underground water. It will be held in this paper that the deposition of most ores is but a special case of the general work of groundwaters, of exceptional interest to man. In order to understand the special problem, it is necessary to have a profound knowledge of the general principles controlling the circulation and work of groundwaters. In the treatise from which this work is adapted I have attempted to treat this subject more fully and broadly than. has. heretofore been done. From this treatise so much is abstracted as seems absolutely necessary in order to understaiid the special application of the work of groundwaters to the genesis of ore-deposits. Where points are not covered with sufficient fullness, I beg the reader to suspend judgment until' he sees the full treatise. In the treatise, as well as in the following paper, I have of course drawn upon the knowledge contained in the writings of all previous workers. No general treatise upon a broad subject is the work of a single man. It is the conjoint product of aU previous workers and its writer. In the following dis- cussion of ore-deposits I am indebted to all who have contrib- uted'ideas to this subject, from the great Bischof to Sandberger and Posepny. I. have tried to give full credit to all by numerous references; but I cannot be sure that I have done full justice in every case. A comparison with the writings of others will show that I am in accord with Prof. Joseph Le Conte upon more points than with any one else.* Ore-deposits maybe divided into three groups, viz.: (A) * See discussion in this "volume, page 270. "On the Genesis of Metalliferous Veins," by Jos. Le Conte: Am. Joum. Sci., 3d Series, vol. xxvi., 1883, p. 1 et seq. SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 285 ores of direct igneous origin, (B) ores which are the direct result of the processes of sedimentation, and (C) ores which are deposited by underground water. Ore-deposits of direct igneous origin are probably of limited extent. Certain very basic igneous rocks have been worked as iron-ores. In Norway are sulphide ores of various metals which Vogt* holds to be a direct segregation from a magma. Emmonsf has also favored the idea of at least a first concentra- tion of the metallic contents of ore by processes of differentia- tion from igneous rocks, more particularly basic ones. In many cases where ore-deposits, and especially sulphides are supposed to be igneous, the question may pertinently be asked as to how far aqueous agencies have worked in connection with the igneous agencies. I suspect, in most cases, that even if a first concentration has been accomplished by magmatic differ- entiation, that a second and more important concentration has been performed by underground waters, and this position I un- derstand Emmons also to hold. Upon the question as to how far some ore-deposits are the direct processes of igneous agen- cies I do not propose here to enter. To a limited extent ores are also the direct result of pro- cesses of sedimentation. As an instance of such ores may be mentioned some placer deposits. Possibly some ores are due to sublimation. However, in so far as ores are of igneous origin, or are the direct result of the processes of sedimentation, or are the results of sublimation, they are excluded from the scope of the present paper. I intend here to consider only the third group of ores, — those produced through the agency of underground waters. My first and fundamental ■premise is that the greater number of ore-deposits are the result of the work of underground water. The Three Zones op the Lithosphere. In another placej I have shown that the outer part of the crust of the earth may be divided into three zones, depending * J. H. L. Vogt : Zeilschr. Jut prakL. Oeol., Jan. and Apr., 1893; Oct., 1894; Apr., Sept., Nov., Dec, 1895. t Trans. Am. Insl. Mm. Eny., vol. xxii., pp. 53-95. "The Mines of Custer County, Colorado," by S. F. Emmons : Vth Ann. Repl. U. S. Geol. Surv., part ii., 1895-96, pp. 470-472. X "Principles of North American Pre-Cambrian Geology," by C. K. Van Hise 16th Ann. Bep. U. S. Oeol. Surv. for 1894-5, pt. i., p. 589 etseq., 1896. 286 SOME PRINCIPLES CONXKOLLING DEPOSITION OP OKES. upon the character of its deformation : an upper zone of frac- ture, a lower zone of rock flowage, and a middle zone of com- bined fracture and flowage. Zone of Fracture. The zone of fracture is that near the surface. In this zone the rocks are not deformed mainly by flowage, but by fracture. They are adjusted to their new positions mainly by rupture and diflerential movements between the separated parts. When rocks are deformed in the zone of fracture the ruptures which occur are those of faulting, jointing, differential movements be- tween the layers (or accommodation), fissility, and brecciation. The so-called folds in the zone of fracture are chiefly the result of numerous parallel joint-fractures across the strata with slight displacements at the joints, giving each block a slightly differ- ent position from the previous one, and thus as a whole mak- ing a fold. For instance, the folds of the rigid rocks in the Alleghenies are not in the main true flexures, but a series of slightly displaced blocks. Zone of Flowage. In the zone of rock-flowage the deformation is by granulation or recrystallization, no openings being produced, or at least none except those of microscopic size.* This conclusion rests upon arguments which cannot here be fully repeated. How- ever, it may be said in passing that the conclusion that a zone of rock-flowage exists at moderate depth is based, first, upon deduction from known physical principles as to the behavior of solid bodies under pressure, and second, upon observation. It is well known that when a rigid body, such as rock, is sub- jected to stress greater than its ultimate strength, it must rup- ture or flow. If a rock be subjected to a stress in a single direction greater than its ultimate strength in that direction, and the rock is not under pressure in other directions, rupture occurs. However, if we suppose that the rock be subjected to stresses greater than the ultimate strength of the rock in all directions, and that the difference in the stresses in different directions is greater than the ultimate strength of the rock * " Principles," cit., p. 594, el seq. SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 287 under the conditions in which it exists, then if openings could be produced by a rupture, they would almost immediately be closed by pressure. In other words, at a certain depth below the surface of the earth, if we could suppose that cracks and crevices are formed by the deformation to which the rocks are subjected, the pressures in all directions being greater than the ultimate strength of the rock, these cracks and crevices would be almost immediately closed. Since this conclusion was reached, Adams has actually de- formed marble under the conditions supposed to exist at mod- erate depth below the earth, with the result that the rock changed its form without rupture and with no perceptible openings or cracks.* Before the above inductive reasoning or Adams' experiment was made, I had become convinced from observation that at moderate depth rocks are deformed with fracture and differen- tial movements between the solid particles [granulation), and by continuous solution and redeposition by underground water (recrystallization).f It was calculated that for all but the very strongest rocks, flowage must begin at a depth not greater than 12,000 meters,! for ^^ this level the weight of the superincum- bent mass is greater than the ultimate strength of the rocks. Factors Influencing Depth at Which Flowage Occurs. — In the case of anticlinal arches a portion of the load may be removed by the supporting limbs, and thus the depth of the level at which the zone of flowage occurs beneath the arch be theoretically somewhat increased. However, it is highly probable that lateral stresses and increased temperature which always accompany rapid deformation, more than compensate for any removal of load. Time is another important factor. It is well known that a stress which in a short time is insufficient to rupture material may, if long continued, result in its deformation by flowage. The geologist has this factor, time, to a larger extent than scien- tists in any other subject, and it is a factor which he has con- * Experiments in the flow of rocks are still being made at McGill University by Frank D. Adams. A preliminary account was presented to the Geol. Soc. Am., Montreal meeting, 1897. This is summarized in " Science," vol. vii., 1898, pp. 82-83. t "Metamorphismof Eocksand Rock Flowage, " by C. K. Van Hise. Bidl.O. S. A., vol. ix., 1898, pp. 295-313, 318-326. J " Principles," ci(., p. 592. 288 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORBS. stantly to keep in mind. How important this factor is may be illustrated by the deformations of rocks as result of very moderate long-continued pressures. In some cases, in ceme- teries, marble slabs have been placed horizontally and suspended at the ends. In the course of a score or more of years such slabs are found to have sagged in the middle a very consider- able amount. If the slabs had at the outset been bent to this extent they would have undoubtedly been ruptured. The change in form is only possible by rock flowage, either through a differential movement of the solid particles with reference to one another or by solution and redeposition, i.e., recrystal- lization, or the two combined. The consideration of time leads me to believe that the limit of 10,000 to 12,000 meters placed as the level at which flowage of the strong rocks must occur is probably too great, and observations upon deformation in the cores of mountain masses which have been deeply denuded con- firm this conclusion. Rocks, even of the strongest kind, have in many instances been deformed by flowage rather than by fracture, when at depths much less than 10,000 meters. Other factors, such as igneous intrusions or orogenic move- ments, increase the heat and pressure acting on the rock, and thus tend to diminish the depth at which flowage occurs. If this reasoning is correct, it follows that all fissures must disappear at some depth, and that the maximum depth is limited by the depth of the zone of fracture for the strongest rocks. Zone of Combined Fracture and Flowage. There is a zone of combined fracture and flowage below the zone of fracture, because rocks have varying strengths, because there is great variation in the rapidity of deformation, in the temperature at which the deformation occurs, in the moisture present, and in various other factors. A weak rock, for instance a shale, may be deformed by flowage at a much less depth than a strong rock, such as a granite. Thus the belt of combined fracture and flowage is of considerable thickness, possibly as thick as 5000 meters. In this zone we have all combinations of the phenomena of fracture in the various ways above men- •tioned, and of flowage by granulation and recrystallization. It is highly probable that the openings of the zone of fracture SOME PRINCIPLES CONTROLLING DEPOSITION OP ORBS. 289 gradually decrease in size as depth increases, until in the zone of flowage the openings are, as already explained, microscopic or non-existent. If a gradation such as indicated exists, it is a necessary corollary that the deformations of the zone of frac- ture must have their equivalents in the deeper seated zone of flowage and flexure. This point I have fully developed in other places.* It is explained that in depth faults are replaced by flexures, and that any deformation of a large mass of a given rock from one form to another by fracturing may be paralleled by similar changes of form in the zone of flowage, the result being there accomplished by granulation of the mineral par- ticles or by recrystallization, or by both. It might be thought that the above general statement is a deduction which cannot be confirmed by observation, but such is not the case. Many rocks which have been deformed in the zone of flowage or in the zone of combined fracture and flow- age, as a consequence of denudation have reached the surface, and one is able to observe all the transition phenomena of de- formation between the zones of fracture and flowage. These I have somewhat fully described in another place, f An excel- lent illustration of the deformation of a rock mainly by flowage, but in a subordinate way by fracture, is the Berlin rhyolite- gneiss, described by Samuel Weidman.| The formation of this rock was mainly that of recrystallization, but many of the mineral particles were granulated. Also many minute joint crevices were formed which were subsequently filled by cemen- tation. It follows from the above reasoning that fissures may dis- appear at different depths below. "Where there are fractures with large displacements, fissures are likely to extend to very considerable depths. In proportion as the displacements are small, the fissures are likely to disappear below at less depths. Furthermore, as has already been explained, certain rocks are deformed by flowage at a much less depth than are other rocks. Therefore, in a region in which there is a great shale » "Principles," cit, p. 676; " Metamorphism," cit., pp. 313-318. t "Principles," ci«., pp. 601^603; "Metamorphism," cit., pp. 312-313. I "A Contribution to the Geology of the Pre-Cambrian Igneous Rocks of the Fox Eiver Valley, Wisconsin," by S. Weidman. Bull. Wis. Gaol, and Nat. Hist. SMn).,No. in.,pt. 2,1898. 290 SOME PRINCIPLES CONTROLLING DEPOSITION OP ORES. or slate formation at a moderate depth, a strong fissure in more brittle rocks at the surface may disappear as it encounters the shale formation, being replaced there by a flexure. I have little doubt that considerable fissures thus disappear at a depth less than 1000 meters. Illustrations of the disappearance of fissures with depth are found at various places. In the gold belt of the Sierra ISTevadas, Lindgren* says it is " an incontestable fact that many small veins close up in depth." Not only may fissures die out below, but fissures may disap- pear above, the fault along the fissures being replaced by a flexure in the overlying stratum, which yields by flovvage. This is beautifully illustrated by the Enterprise mine, of Rico, Col., described by E,ickard,t where faulted fissures in sandstone and limestone disappear above, at the place where shale is encountered, the shale accommodating itself to the frac- tures below by monoclinal flexures. (See Fig. 9, p. 409.) The marked efiect which the character of the country rock may have upon the nature of a fissure is well illustrated in the Cripple Creek district, where, according to Penrose, J the fis- sures in the hard rocks are sharp, clean-cut breaks, while in the soft rocks they are ordinarily a series of very small cracks, • constituting a displacement of a kind which I call a distributive fault. "Well illustrating this are mines which are partly in hard and partly in soft rock. " The vein on which the Buena Vista, Lee, Smuggler, and Victor mines are located occupies a sharp, clean-cut fissure, partly in the massive rock and partly in the hard breccia; but when it passes into the soft, tufaceous breccia on the east slope of Bull Hill the fissure is represented only by faint cracks occupied by no vein of importance. In this case the force which caused the fissure overcame the co- hesion of the harder rock sufficiently to make a clean break, but in the more plastic rock it overcame cohesion only to the * "The Gold-Quartz "Veins of Nevada City and Grass Valley, California," by Waldemar Lindgren, nth Ann. Re.pt. U. S. Oeol. Surv., pt. ii., 1895-96, p. 162. t "The Enterprise Mine, Eico, Col.," by T. A. Bickard, Trans. Am. Inst. Min. Engineers, vol. xxvi., 1897, pp. 906-980. X "Mining Geology of the Cripple Creek District," by E. A. F. Penrose, Jr. 16wall streak is not sensibly afiected by the faulting ; and they reason that it is a plane of recent movement, on which the silver-minerals have been con- centrated by a secondary migration from the main vein. In several important veins in the San Juan region, I ob- served secondary or post-mineral fractures parallel, and more or less coincident, with the plane of the vein, but no detailed studies have yet been made that would determine whether there has been an enrichment of the minerals, or not. In unpublished notes on the once famous, but now aband- oned, Yankee Girl mine, in the Red Mountain district of the same region, I find mention of a phenomenon which then ap- peared unexplainable, but which it now appears might be accounted for on the theory of secondary enrichment. In the upper levels, the ores, down to about 200 feet from the surface, were mainly silver and lead, galena and pyrite being the prin- cipal minerals. Below this zone, the ore is mainly composed of stromeyerite, bornite, chalcocite, with some gray-copper and barite, yielding about 30 per cent, of copper, and little or no » U. S. Oeol. Sur., 18ih Am,. Bep., Pt. iii., p. 834. 29 452 THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. lead. In depths below 600 or 700 feet, the ore in this and adjoining mines is said to have gradually turned into a low grade pyritous ore, which was of too low grade to work at a profit, and caused the mines to he closed down. The ores in the copper-hearing zone had been exceptionally rich, carrying several thousand ounces of silver to the ton in carload lots. The waters in this region, both mine-waters and spring-waters, are unusually acid, the latter forming abundant deposits of limonite, while the former rapidly corrode any metallic iron- materials, such as water pipes, that are exposed to their action. The most definite instances of secondary enrichment of silver sulphides are, however, those described by Weed, in the mines of ITeihart, Mont.* There the veins have suffered later fractur- ing and secondary enrichment of the zone at or below the water- level, with the deposition of silver sulphides " as crusts or crys- tals lining cavities, or as films or thin coatings along fractures of the primary ore." The primary minerals are galena, blende and pyrite ; the secondary sulphides are polybasite, ruby-silver, more rarely a pure transparent blende. Under the microscope galena is seen altering to a spongy polybasite. Polybasite and pyrargyrite are seen as crystalline aggregates and crusts on all other minerals, but in no case coated by other minerals. The immediate products of superficial alteration, the gossan, are largely removed and make tip a zone at most only a few feet thick. Beneath this is an irregular accumulation of sooty black ore, consisting of manganese and silver sulphide. Where the vein is well-defined the secondary antimonial sulphides occur below, at first, in considerable abundance, but deeper down, only in crevices or fissures partly or wholly lining filled fractures, so that they become less and less abundant in going down on the vein. Eastern Examples. — In his description of the lead- and zinc- deposits of the Mississippi,! W. P. Jenney notes interesting in- stances of deposition by descending-solutions below the zone of oxidation. Among minerals of secondary deposition he enumerates " blende, galena, chalcopyrite and greenockite, produced by * U. S. Geol. Sur., 21st Ann. Rep., Pt. iii. "The Geology of the Little Belt Mountains," hy W. H. Weed, p. 421. f Trans., xxii., 199. THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. 453 alteration from the primary ores in the zone of oxidation in the upper part of the ore-bodies, and re-formed as sulphides by the reducing action of organic-matter in the deeper levels." lies* and Robertsonf have also described secondary deposits of zinc-sulphide vs^hich are assumed by them to have been de- posited from a sulphate solution through the agency of sul- phuretted hydrogen. In the Southern Appalachian region are many copper-de- posits which show excellent instances of secondary enrichment through the agency of mineralizing waters. Most of these have recently been examined by Mr. Weed,| and, as detailed descrip- tions of them will be found in his paper on " Types of Copper- Deposits in the Southern United States," presented at this meet- ing, only a brief mention of the salient points need be made here. Best known are the deposits of Ducktown in the extreme southeast corner of Tennessee, which have become classic in the literature of ore-deposits for their black copper-ores which form a narrow band between the gossan and the unaltered py- ritous ore beneath. The gossan-ores are immense masses of porous iron-oxide which are so free from impurities as to be used in large quantities in the blast-furnaces of Tennessee and Virginia. The original ore is an irregular mixture of pyrrho- tite and chalcopyrite, quite massive and free from vertical frac- turing. The comparatively thin zone of black ore has been considered to be a mixture of black oxide and sulphuret of cop- per ; but chemical examination of the specimens brought in by Mr. "Weed and those in the it^ational Museum show only cop- per-glance and no tenorite. The ore is impregnated with cop- jjer sulphate, showing that the process of concentration is still going on, and recently-formed amorphous black sulphuret is found in clefts in the upper surface of the pyrrhotite. When the black ores were first developed many curious speculations were current as to their origin; but "Whitney§ early recognized their true source, as is most evident in his re- ply to Tuomey in 1855, where he ascribes these rich sulphides * Eng. and Min. Jour., vol. xlix., 1890, p. 499. t Am. Jour. Sci., 3d series, vol. xl., p. 160. i Bull. Oeol. Soc. Amer., vol. xi., pp. 179-206. ^ Metallic Wealth of the United States, p. 322. 454 THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. to an enrichment of the original ore through the decomposi- tion of the zone above (now gossan).* Sterry Huntf makes allusion to this, and similar deposits, at Ore Knob, 'N. C, and in Carroll county, Va., as owing " their origin to the reduction, in some imperfectly explained way, of the sulphates formerly generated by oxidation in the upper portion of the lodes," and OleottJ confirms his view as to the Ore Knob deposit. In all these cases, however, the enrichment is assumed to have taken place at or above the water-level, and there is no question of its extending to any considerable dis- tance below it. In the Union Copper Company's mine at Gold Hill, N. C, as reported by Mr. "Weed, the ore-body is fractured by vertical planes which penetrate to some depths below the oxide zone. There are here no such great masses of pyritous ore as at Duck- town, the primary ore being vein-quartz carrying a small amount of chalcopyrite. Owing to the vertical fractures the lower limits of the brown oxidized ore are very irregular, ex- tending in places down to 180 feet or more below the surface and half that distance into solid unaltered quartz-ore. The surface-ores were originally worked for gold, but the increase of copper with depth interfered with amalgamation. The workings now extend to a depth of over 250 feet. In the main part of the vein are rich masses of chalcopyrite in quartz which, when altered, change directly into chalcocite ; the latter is often surrounded by a film of cuprite, which, in turn, changes to fibrous malachite along cracks in the quartz. Where the alteration has proceeded further, specimens are found showing copper-glance coated with crystalline cuprite and native silver ; others show crystalline glance passing directly into native cop- per, which forms a thin felty covering up to a quarter of an inch thick. Nearer the oxide zone, a sooty black mass is often found directly replacing the chalcopyrite. Specimens from the Blue "Wing mine in the Virgilina district on the borders of Virginia and ISTorth Carolina, taken from be- low the oxide zone, show bornite surrounded by a shell of dull glance with iron oxide outside. Other specimens from below * Amer. Jour Sd., 2d series, vol. xx., p. 53. t Trans., ii., 127. J Trans., iii., 392. THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. 455 the water-level show bornite altered to chalcocite, while the iron is concentrated in the form of nests of specular iron in the quartz. These ores extend to the present limits of the work- ings 300 feet below the surface. That they will pass, like those of Gold Hill, into chalcopyrite in depth, cannot yet be pre- dicated with certainty. Foreign Authorities. — Although it was the Austrian geologist, Posepny, already cited, who most strongly emphasized the dis- tinction between the results of the action of surface- and deep- seated waters, European geologists seem to have been, as a rule, slower to recognize this distinction in their practice than American mining geologists, whose views are generally gath- ered from a wider field of observation. The brilliant French writer, DeLaunay,* has been cited by Mr. Weed as one who seems to have conceived the idea that the enrichment of sulphides by descending surface-waters has been an important element in the formation of ore-deposits ; and indeed the title of his most recent theoretical discussion of the question gives ground for this supposition. A careful reading of this and his other papers, however, leads rather to the conclusion that the secondary migrations he con- templatedwere confined to the zone above the groundwater-level, or oxidized zone, in which sulphides have always been known to occur. The novelty of his view would appear to be that they are not necessarily residual masses of sulphides that have not been completely oxidized, but that in many cases they re- sult from an actual transference of material and a re-deposition as sulphides. Thus, in closing his discussion on the origin of copper and ore-deposits, f he says : ' ' What is the origin of the modifications which have been noted in a general way in the upper part of a vein ; modifications undoubtedly connected with near- ness to the surface, and which disappear in depth ? Is it possible that these modi- fications have sometimes been produced contemporaneously with the filling of the vein, or are they always, as is incontestably the case in many instances, the result * "I. On the importance of deposits by magmatic inclusion and segregation in a classification of ore-deposits. "II. On the part played by the phenomena of superficial alteration and the renewal of movement (migrations) in the formation of ore-deposits." — Ann. des Mines, 9th series, xii., 1897, p. 119-228. t Gites Min^raux et M^tallif6res, Paris, 1893, vol. ii., p. 232. 456 THE SECONDARY ENRICHMENT OF ORB-DEPOSITS. of much later action ; especially of the introduction of surface-waters down to a certain level called hydrostatic?" In his later paper, after describing the succession of minerals found in the silver-deposits of Mexico and South America as an upper zone of chlorides and native metals, and below this a bonanza-zone, where the silver and copper " coming in part from the surface " are concentrated by a sort of cementation as rich sulphides, etc. (both, however, being above the water-level), he speaks of the chemical processes that have probably gone on, as follows : " The atmospheric-waters reached the deposit hy descending and filtering down slowly along the plane of the vein to ascend again to the surface only after a cir- cuit that is more or less long and complex. It results that the elements dissolved are not absolutely lost to the vein, but, on the contrary, a great part of them are only slightly displaced from above downward, and are re-precipitated in depth in contact with unaltered sulphides in the form of insoluble sulphides. This sec- ondary phenomenon, which is particularly marked for copper and silver, often brings about a special concentration of these two metals at a certain distance below the surface, so that below the oxides and carbonates that characterize ^he actual outcrop is found a very rich zone, or, as the miners of the New World express it, a bonanza, in which copper forms gray-copper, glance, bornite, etc. ; while silver separates as argentite, brittle-silver and ruby silver." European Deposits. — The great pyrite-deposits of Eio Tinto, Tharsis, etc., in the Huelva provinces of southern Spain, are enormous bodies, analogous in geological conditions to those of Ducktown, but of even greater volume. The gossan, which extends down to a depth of 130 to 150 feet, consists mainly of iron oxide, with 50 to 55 per cent, iron, a little sulphur and arsenic, and only a trace of copper. Betweeii this and the solid pyrite below is a very regularly distributed zone of earthy, porous material, from a few inches to a foot in thickness, carry- ing an average value of about $35.00 per ton in gold and silver. These values it is assumed by Vogt,* who has last written upon the region, have been brought down in solution by ferric sulphate which, in contact with the underlying pyrite, has been reduced to ferrous sulphate, with precipitation of the gold and silver. It is in the pyrite-mass below the " iron hat " that the copper-values are found ; and these gradually decrease with depth from 4 to 5 per cent, in the upper 100 to 200 feet * Zdtschr. Prak. Geol, July, 1899, pp. 249, 250. THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. 457 to 1 J per cent, at the depth of 900 feet — varying somewhat with local conditions. The geologists who have studied the region account for this decrease by assuming that, during the oxidation of the gossan, the original copper-contents were leached down and re-precipitated in recent fractures and clefts in the pyrite mass, thus enriching its upper part. Says Vogt : "In point of fact we find such minerals — copper-glance, bornite and chalco- pyrite, sometimes with galena, zinc-blende and gray-copper, and, as a rule, ac- companied by quartz — very often in clefts and cracks in pyrite, and these min- erals are, without doubt, of younger (secondary) formation. At times these clefts are so large that they can be separately mined ; thus, the old Koman mining was done chiefly on these richer clefts within the poorer mass of pyrite. Most com- monly, however, they are quite small and constitute a strongly branching net- work in the normal pyrite-mass ; and that the copper-content of these secondary minerals is derived from the weathered, superficial mass is shown quite simply in the fact that these veins are most common in the zone immediately under the ' iron hat. ' They extend downward something like a hundred meters or more, and then commences the solid pyrite-mass, little fissured and comparatively bar- ren of copper." He also mentions a decrease of copper-contents with depth in the great vertical pyrite-deposits of Vigsnas, in Iforway, and Fahlun, in Sweden, of which the former has been worked to a depth of 735 meters and the latter to 350 meters. At Monte Catini, in Italy, the rich copper-ore occurs in masses sometimes of several cubic metres in size with an arrangement of concentric zones which grow successively richer in copper towards the periphery. These masses are sometimes isolated; sometimes connected with metalliferous veins. Balls of glance occur sometimes the size of a man's head. The larger masses consist of chalcopyrite at the center, surrounded first, by bornite, and then on the outside by chalco- cite, which sometimes passes into native-copper. These rich masses occur in a zone of recent movement which is easily penetrable by surface-waters. In depth the rich sulphides cease and the ore consists of chalcopyrite and pyrite, the latter be- coming more and more predominant with depth. There are many other European examples which would doubtless show evidence of secondary enrichment, if examined with that question in view. In the great zinc-, lead- and silver- deposits of Laurium, in Greece, for instance, it seems that this process might explain some of the phenomena about which 458 THE SECONDARY ENRICHMENT OP ORB-DEPOSITS. there has been so much difference of opinion among those that have examined them. Deposits in Arid Regions. — De Launay, who has studied the literature of Mexican and South American mines very exten- sively, makes the following general statement as to the average conditions of the silver-ores in those countries from the surface downwards : ' ' Near the surface the silver in the veins is in the native state, with chlorides, bromides, iodides, etc. , associated with oxides of iron, manganese and often of copper ; if the gangue is siliceous, it shows a honeycomb aspect, resulting from the removal of the sulphides which it formerly held ; frequently red and gray clays are associated with it. These ores are the pacos, casmjos, colorados, etc. , of the Spanish-American miners, which are designated by the general term metcdes ccdidos (free milling ores), but whose tenor in silver is often small compared to the rest of the deposit. ' ' Lower down, at about 80 to 150 meters, appears the bonanza-zone of the Mexi- cans, where by a sort of phenomenon of cementation is concentrated the silver, coming in part from the surface (often with the copper, if this is abundant in the deposit). The silver is here in the state of sulphide (AgjS) ; the copper as chal- cocite, gray-copper (itself often argentiferous) and bornite ; iron is wanting or is in the form of oxide ; lead, not abundant, is mostly in the state of carbonate. " Finally, when one passes below the hydrostatic level, which is rarely lower than 400 or 500 meters, one finds the complex vein-filling of sulphides, autimon- ides and arsenides, which in their primitive form extend indefinitely in depth ; that is, one has, in proportions varying with different deposits, galena, more or less argentiferous, iron and copper pyrites, arsenopyrite, blende, etc., with less abundant silver-minerals. ' ' My personal observations in these countries have been very limited, being confined to a few weeks' visit in Peru and the State of Chihuahua, Mexico, respectively. In Peru, I saw a dull-black copper-glance, brought from the famous Cerro de Pasco mining-district, where silver-mining has been extensively carried on for centuries. This ore is said to be found wherever the workings have been carried down below the water-level, and is estimated to constitute a zone of great extent and value beneath all the old workings. It is evidently a concentration by leaching of the small amount of copper that was dissemi- nated through the now oxidized ores above. In Chihuahua, I had no opportunity to examine the silver- deposits below the water-level ; but in the middle levels of the mines, at 200 to 300 feet below the surface, oxidized and un- altered sulphide ore-bodies were not infrequently found side by side, so to speak, on the same level. The former, generally THE SECONDAKT ENRICHMENT OF ORB-DEPOSITS. 459 in a foot-wall streak, constitute the Colorado ores of tlie Mexican miners, and carry rich silver-minerals in a red clayey material that contains rolled fragments of quartz and ore, and is evi- dently on a plane of movement subsequent to the mineralization. The latter, which consist of pyrite, galena and blende in a dense hard matrix, occur in greater bulk than the richer ores, and ap- parently owe their escape from oxidation to their impenetra- bility to surface-waters. The sulphide is so much lower in grade than the Colorado ore, that it was generally left by the early miners, and now constitutes a concentrating-ore. Evi- dently the foot-wall streak, which often opens out into great " bonanzas," was enriched by the leaching of such parts of the other ore as had been thoroughly decomposed. In Australia, the arid climate of which presents similar con- ditions to those found in Arizona, Mexico and South America, we would expect, as in the latter regions, to find abundant evi- dence of secondary alteration of ore-deposits. In point of fact, the descriptions of many of the important mines of that region make mention of phenomena which seem to be most readily explainable on this theory, although, until they have been ex- amined with this end in view, one cannot be sure how far all the evidence will support it. In the famous Broken Hill lode, as described by Jaquet in the memoirs of the Geological Sur- vey of ITew South Wales, certain occurrences are definitely described by him as secondary sulphides.* The primary sul- phide-ore is an intimate mixture of argentiferous galena and zinc-blende, with quartz, garnet and feldspar, and pyrite, chal- copyrite, arsenopyrite, walfenite and fluorite as accessory con- stituents. These ores contain on an average 5 to 36 oz. of silver per ton, 7 to 50 per cent, of lead, and 14 to 30 per cent. of zinc. Mr. Jaquet says : "Sulphide-ore, secondary, occurs as a thin layer, varying in thickness from 3 inches to 6 feet, which coats the ordinary sulphides at all points where dry ore, rich in silver, comes in contact with them. Kesembling soot somewhat in appear- ance, it has been named ' sooty sulphide-ore ' by the miners. It is, without doubt, ordinary sulphide ore altered and enriched by contact with dry ores.'' These enriched ores carry up to 250 oz. silver per ton and 12 * Mem. Otol. Sur., New South Wales, Geology, No. 5, Sydney, 1894, p. 88. See also Zeitachr. Prak. GeoL, vol. v., p. 95. 1897. 460 THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. per cent, of copper. The dry silver-ores are tlie antimonial and arsenical sulphides of silver, polybasite, stromeyerite, dyscrasite, proustite, pyrargyrite, stephanite, etc., which occur in the lower part of the kaolin-ore that forms the bulk of the oxidized zone in the locality described, and carries oxidized minerals of iron, manganese, lead and copper. This, also, is of much lower grade than the enriched sulphide. It is very evident, therefore, that in the process of alteration by surface-agencies the oxidation products — especially the silver- and copper-combinations — have been leached down and redeposited as sulphides, and are most abundant in contact with the original sulphides of the deposit. It does not appear from the descriptions that the enrichment has extended to any considerable distance below the contact- zone of altered with relatively unaltered material. This may be due to the absence of later fractures which would admit the descending solution ; or it may be that enrichments do exist which have not yet been detected through want of systematic search. In the neighboring Broken Hill Consols mine, as described by Mr. George Smith,* there is evidence of still more exten- sive secondary enrichment of sulphides, the silver-minerals being mainly in the rather rare and unusually rich forms of stromeyerite, dyscrasite, fahlerz and antimonial silver chloride. Although occurring in the same country rocks as the main Broken Hill deposit, instead of being, as in that case, in the form known as " saddle reefs," which conform with the schis- tosity and are of great size, they are found only in compara.- tively narrow veins which distinctly cross the schistosity. The vein is mineralized only at the intersection of what is known as cross-veins, and, of the rich silver-minerals mentioned above, some " have not been met with in the lower workings, though each has been found at some distance below the water-level;" while those that are found at greater depth occur there in smaller quantities and " have been found to assume a distinct track and are evidently the continuation of the larger deposits worked in the upper levels." The so-called cross-veins are de- scribed as " a succession of rock-joints formed along a line of weakness and enlarged in places by a process of removal and * Tram., xxvi., 69, 71, 73. THE SBCONDAKY ENRICHMENT OF ORE-DEPOSITS. 461 replacement." The more important one, which is figured (p. 73), falls vertically on the lode which has a shallow dip near the surface, and immediately below it in the main vein is an accumulation of stromeyerite, dyscrasite and more or less oxi- dized material impregnated with silver chloride. The cross- veins carry in some cases blende and galena; in others only pyrite. While Mr. Smith has recourse to secondary concentration for the enrichment of the oxidized zone, it does not seem to have occurred to him that the concentrations of rich mineral below this zone might also have been produced by solutions along the main fissure from the cross-veins. He brings forward a theory of electro-magnetic currents to account for the precipi- tation of the minerals at these intersections. It is always unsafe to theorize on the observations of others ; but in the present case, it seems legitimate to offer the sugges- tion that, even admitting the possibility of electric action as a stimulant to the chemical reactions, it is necessary to predicate previous conditions which will render the reactions possible. In the case of these cross-veins, as in those of the indicators of Ballarat, and the similar fahlbands of Norway, which are all zones of relatively barren pyritiferous materials crossing the ore-bearing veins, it seems, in a general way, that reactions be- tween salts of the metals and their sulphides have been the un- derlying cause of the concentration of the more valuable metals at or near the intersection. There will have been a great num- ber of differing conditions in such districts, or groups of deposits, and it does not seem absolutely necessary that we should have recourse to descending solutions for the salts of the metals in every case ; they may have been contained in ascending solutions. An instance where it seems probable that there has been enrich- ment from ascending salt-solutions is the Geyser mine at Silver Cliff", Colorado,* where, in a narrow vein at 2000 feet in depth, is an unusually rich and very recent-looking body of sulphides of silver, lead, zinc and copper. The ascending waters which issue from the parallel and adjoining fissures are highly charged with carbonic acid, and carry, apparently in the form of car- bonates, small amounts of the metals found in the veins. "W"hai> * U. S. Geol. Sur., 17th Ann. Rep., Part ii., p. 456. 462 THE SECONDARY ENRICHMENT OF ORB-DEPOSITS. ever may have been the original form in which the metallic minerals were brought to this deposit, it seems evident that at the present day they must be coming in as carbonates and be deposited as sulphides in contact with the already existing sul- phides, thus enriching the latter. Summary. From the foregoing geological evidence, which could be doubtless very much enlarged, it appears to be fairly well es- tablished : 1. That descending waters not only cause migrations, or transference and reconcentration, of the alteration products of the original vein-materials in oxidized form, producing in one place an enrichment, and in another possibly an impoverish- ment of the original deposit, but that in their further down- ward course the oxidized forms are frequently reduced and re- deposited as sulphides, thereby producing a sulphide enrich- ment of the original vein-materials. 2. That this secondary enrichment of sulphides is not nec- essarily a reduction in the presence of organic matter, but is frequent where no organic matter can be supposed to be pres- ent; it occurs mainly in contact with the original sulphides of the deposits, and is, presumably, a result of chemical reaction between these sulphides and the materials brought down in solution by the descending waters. 3. That while this re-deposition of sulphides in many eases appears to commence at or near the groundwater-level, it does not appear to have a necessary connection with that level, and may under favorable conditions extend below that level for a distance as yet undetermined, the most important favoring con- ditions appearing to be recent or post-mineral fractures, which have admitted a relatively free and uninterrupted descent of these waters. In endeavoring to trace back the processes by which the results have been brought about, it is important to bear in mind the physical changes that may be assumed to have taken place during the time that has elapsed since a given ore-deposit was originally formed and before it reached the condition in which it is found at the present day. These changes necessa- rily vary with each mining region or district, being in some THE SECONDARY BNKICHMENT OP ORE-DEPOSITS. 463 cases very considerable, in others relatively slight. They may be classed under two general categories : First, the rock shatterings resulting front dynamic force connected with, earth-movements or eruptive action. These have opened channels for the entrance of surface-waters within the rock-mass and thereby extended the areas to Vfhich the chemical actions produced by the latter may have extended. Second, the erosion or denudation to which the region has been subjected, and which has gradually worn down the origi- nal surface to its present configuration. As a result of this wearing down the lower parts of an ore-deposit have been con- tinually approaching the surface, and in no case, probably, is what was originally the superficial portion of an ore-deposit still in existence. The amount of the wearing away is not always determinable, but it may have been very large ; thus, at Leadville, I estimated that in round numbers a thickness of about 10,000 feet of rocks had been worn away in order to bring the ore-bodies at present exposed to the surface. In the Butte district, where there are no stratified rocks, there exists no criterion by which to estimate accurately the amount of denudation, but the readily disintegrable character of the granite country-rock and the faulting to which I have already alluded show that it must have been very considerable. There the ore-deposits occurring along nearly vertical fissures, and the later shattering having produced extraordinarily abundant secondary fissures nearly coincident with the earlier ones, the conditions were unusually favorable for an abundant leaching down of the material taken up in solution by the sur- face-waters. As the surface gradually lowered we may con- ceive that the insoluble materials were carried off mechanically ; of the soluble minerals, however, but a relatively small propor- tion would have been removed by the actual surface run-off. The greater portion would have been carried back to lower levels before they came near enough to the actual surface to be taken up in the run-off". It will aid our conception to divide the veins theoretically into three horizontal zones. The upper- or surface-zone, that which immediately adjoins the present surface, is necessarily the zone of highest and most recent oxidation. Any sulphides found in it will simply be residual masses which, for some 464 THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. reason, the oxidation has not completely penetrated. The changes which have taken place in this zone will have been mainly of removal, rarely of addition, and any enrichment that will have come about in this zone will have been, as a rule, dif- ferential, resulting from the greater proportion of valueless or base metals removed. The second or intermediate-zone may be called the zone of oxide-enrichment. In this, the less soluble or more readily precipitable metals which have been brought down from the zone above, are found as carbonates or oxides, or in some cases as native-metals. The third zone may be called the zone of sulphide-enrich- ment, in which the materials brought down in solution, and not deposited in the zone next above, are deposited as sulphides (also as sulph-arsenides and sulph-antimonides) or in some cases as native-metals in contact with the original sulphides of the deposit. The presence of organic matter would hasten the reduction to sulphide, and might cause the deposition of the latter, under favorable conditions, even at the surface, but it should be con- sidered as an accidental, rather than an essential occurrence. These zones are, as has been said, a theoretical conception ; in practice they are rarely well-defined, and in many cases one or more may be wanting. One will run into the other, and, as denudation progresses, a lower zone is slowly changing to the one next above it ; thus, as time goes on, it will be a constantly richer zone that rises to the surface to be oxidized, and has part of its oxidation products carried back and re-deposited either as oxides or sulphides. Hence, other things being equal, the longer a deposit has been subjected to denudation the greater will be the enrichment below the surface-zone. The rate of denudation may also have influence upon the amount of enrich- ment, for it can be conceived that the surface-rocks may be so readily disintegrable and the rate of erosion under favoring climatic conditions maybe so rapid, that the surface-removal of the oxidized material, both mechanical and chemical, may pro- ceed so much faster than the downward seepage along the plane of the ore-deposit, that little or no enrichment of the in- terior portion of the deposit may have taken place. Such a rapid denudation may be conceived to have taken THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. 465 place on exposed points during the ice-invasion of the glacial period, at which time, moreover, under the low surface-tem- peratures chemical decomposition would have been relatively sluggish. In arid regions, on the other hand, where the great heat would render chemical decomposition more energetic, and where there has been not only no ice-action, but also compara- tively little erosion by water to wear down the surface, we should expect the zone of oxide-enrichment to extend down to great depths, but if the aridity were so great that there was very little water percolating through the rocks in depth, there might be but little sulphide-enrichment. Chemical Processes Involved. — I do not feel prepared to dis- cuss in detail the chemical processes that are involved in the changes which are shown to have taken place by the above quoted observations. They necessarily vary from one deposit to another under the varying mineralogical and physical condi- tions that prevail in each place. Moreover, the chemical re- actions that are suggested by previous investigations should be tested experimentally before any one can state with any degree of confidence what the succession of chemical processes in a given case has been ; for these investigations have generally been conducted with another object in view, or Avith a different conception of the actual conditions in nature. Mr. Weed, in his paper already cited, has given quite full quotations from all the authorities that bear upon this subject, and I shall, therefore, not repeat them, but only give a brief general statement of the main processes that may be supposed to have contributed to the mineralogical conditions that are found in the ore-deposits cited above, laying more stress on the natural occurrences that illustrate the actual changes. The most common sulphide-minerals in original ore-deposits are the iron sulphides, pyrite, marcasite, pyrrhotite, chalco- pyrite and arsenopyrite ; and next to these, galena, zinc-blende and various copper sulphides. While there is a great variety of other metallic compounds in ore-deposits, yet in most deposits the greater bulk is so far formed by one or more of the above minerals that the chemical changes will be largely governed by the reactions to which these appear to be subject. Of these sulphides marcasite is the most readily decomposed, while pyrite, if occurring by itself in pure crystals, often proves very 466 THE SBCONDAET ENRICHMENT OF ORE-DEPOSITS. resistant to alteration. Where there are mixed sulphides, how- ever, the oxidation is observed to proceed more rapidly and all are readily attacked. The actual changes observed by me in a great body of pyrite carrying galena in a limestone country-rock, which had under- gone partial decomposition from the periphery inwards, are as follows :* The original fresh pyrite or marcasite crystals are first disintegrated and slightly pitted on the surface, then changed to melanterite or hydrated ferrous sulphate and the galena becomes anglesite. In the outer or more fully oxidized zone the iron-vitriol has changed in part to yellow basic sul- phate ; in part to limonite with a separation of native sulphur. The theoretical changes that are assumed to take place by the action of waters carrying oxygen or oxidizing agents are : first, an alteration of the iron sulphide to ferrous sulphate with the formation of sulphuretted hydrogen and sulphur which may have oxidized to sulphuric or sulphurous acid. By further oxidation the ferrous sulphate will become, in part at least, ferric sulphate, and this in its turn will react upon the remaining ferrous sulphate, or upon the sulphides, and form more ferrous sulphate or sulphates of the other metals which are present. By this cycle of reactions a supply of both ferric and ferrous sulphates would seem to be provided in the oxidized zone, but the extending downwards of the ferric salts would decrease as the supply of oxygen in the waters became less abundant. It may be assumed that the sulphates of the metals thus formed would be transported for greater or less distances, gener- ally in proportion to their solubility, the iron sulphates being the most soluble ; next, those of copper and zinc ; silver sul- phate is less soluble and also more readily decomposed, while lead sulphate is extremely insoluble. This accords with the facts generally observed in nature. Thus, from the gossan, which is generally a porous siliceous mass stained by the limonite or hematite resulting from the de- composition of part of the iron sulphate, the copper- and zinc- salts may have been more or less completely removed or trans- formed to less soluble carbonates and silicates. Where galena has been present in considerable amount the sulphate (angle- site) is generally found quite near the surface or forming a coat- * Proc. Colo. Sd. Soc, vol. ii., p. 104. THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. 467 ing around residual masses of galena which some think it has protected from oxidation. Where carbonate of lime is present, as in limestone deposits, it is transformed to the carbonate (cerussite) which is more soluble, especially in the presence of an excess of carbonic acid, and may be transported from its original location and concentrated in bonanzas of more or less crystalline mineral. The silver sulphate formed near the surface is generally transformed to the chloride, but is not infrequently reduced to the native state. Gold probably does not form a sulphate, but when combined, as in the form of the telluride, is directly reduced to the metallic state. It is, however, to a certain extent soluble in ferric sulphide, and would in part be transported by this solution until it is precipitated by the reduc- tion of the ferric to the ferrous condition which may occur in contact with the sulphide or with ferrous salts. Under certain conditions ferric sulphate will decompose the metallic sulphides with the formation of ferrous salts and sul- phates of the metals ; possibly also with a solution of part as sulphides. Actual test has shown that it acts with great readi- ness on the iron sulphides, but much more slowly on silver sul- phides. The action of copper sulphide has not been tested, but is probably intermediate between the two. It would naturally be expected that the oxygen included in surface-waters would gradually be eliminated with depth, and Lepsius* has shown by actual experimental tests of waters taken from bore-holes that there is a gradual and fairly uniform de- crease of contained-oxygen in the waters with depth. The oxygen would be more rapidly exhausted in a region of active chemical action, such as an ore-deposit in process of alteration ; hence it may be assumed that in each case there will be a cer- tain depth at which, owing to the absence of free oxygen, the general tendency in the reactions which take place will be re- ducing rather than oxidizing, and when no organic matter is present we must look to the original sulphide minerals to fur- nish the necessary agents for reducing the sulphates to sul- phides again, or to the native state. The most pertinent investigations bearing upon the reactions that would take place are those undertaken by E. F. Anthonf * Ber. d. Beutsch. Ohem. OeseUechft., vol. xviii., p. 2487. 1885. f Jour. f. Prak. Chem., vol. x., No. 6, p. 353. 30 468 THE SECONDARY ENRICHMENT OF ORB-DEPOSITS. in 1837, primarily foi* facilitating analytical work in the labora- tory, which were continued later on the same lines by E. Schiirmann,* for which reason they are generally known as the " Schiirmann " reactions. By these it was established that in the presence of the sulphides of certain of the metals the salts of other metals would be decomposed and the metal pre- cipitated as sulphide, indicating thus that the latter metal pos- sesses a greater affinity for sulphur than the former, and thus the following series of the more common metals was established in the order of their affinity for sulphur: Mercury, silver, copper, bismuth, cadmium, antimony, tin, lead, zinc, nickel, cobalt, iron, arsenic, thallium, and manganese. In other words, a salt of any metal in the series would be decomposed by the sul- phide of any succeeding metal, and the first metal precipitated as sulphide. Thus, from silver- or copper-salts the metal would be precipitated as sulphide by lead, zinc, or iron sulphides. Sulphuretted hydrogen is an important agent for the precipi- tation of metallic sulphides, and this is not infrequently found in mine-waters. It is assumed to be given oft in the deposition of the various forms of pyrite, and qualitative tests in the lab- oratory of the Survey have shown that it is evolved in the treatment of pyritous ores by very dilute sulphuric acid, though less freely with pure pyrite than with mixed sulphides. Ferrous sulphate will also precipitate many of the metals from their solutions in the native state ; possibly in some cases as sulphides with the formation of ferric sulphate, and it is probable that other ferrous combinations will act in a similar manner. Thus, in the mines of Kongsberg, Norway, which are remarkable for the abundance of native-silver below the water- level, Vogtf assumes, for the occurrences of the metal, as it is sometimes found in fine cracks in the country-rock and even in garnets, that it has been precipitated from its solution through the reducing action of ferrous silicates. The reaction of ferrous sulphate on a solution of silver-salt is easily tested in the laboratory. The silver is readily precipi- tated in the metallic state and the solution colored brown by the' ferric sulphate formed. With an excess of ferrous sulphate preseut,.as near the out-crop of ore-deposits, this might account * Liebig's Ann. d. Ohem., vol. 249, 1888, pp, 326-350. t Jour.f. Prak. OeoL, April, 1899, p. 118. THE SECONDARY ENKICHMENT OP ORB-DEPOSITS. 469 for the separation of native-silver from silver-salts, while on the other hand vs'ith an excess of ferric oxide the silver might be carried further down in solution. In the copper-deposits, to which my studies have been more especially directed, one often finds a black sooty alteration product inpyritous ores just at the water-level which it has been the custom to call an " oxysulphuret," but which, so far as tested, has always proved to be amorphous copper-glance. It is evidently a very recent formation and it might be reasoned that it has not yet had time to become crystalline. On the other hand, chalcopyrite is by some considered to be a com- pound of cuprous sulphide (Cu^S) and ferric sulphide (Fe.^SJ as bornite is, in a similar way, considered to consist of cuprous (CujS) and cupric (CuS) sulphides with ferrous sulphide (FeS). In the attack by a solution of ferric sulphide the iron molecule would first be removed, and in the case of chalcopyrite the Cu^S might be left in the amorphous powder above noted. If, however, the attack was continued until copper sulphate was formed, this being precipitated either by sulphuretted hydrogen or in contact with unaltered sulphide, by analogy with results obtained in the laboratory the precipitated sulphide would be a black amorphous powder. Such a precipitate was obtained by A. P. Brown* by the action of powdered pyrite on a neutral solution of copper sulphate under pressure during his investi- gations, by which he proved, as he assumed, that marcasite is largely composed of the ferrous sulphide, and pyrite contains more ferric sulphide. Bischof t states that the amorphous precipitates from solu- tion by sulphuretted hydrogen gradually assume a metallic luster and tend to become crystalline when extremely dilute solu- tions are used and the reagent is passed sufficiently slowly over the precipitates. In other words, under conditions more nearly approaching those that may be assumed to exist in nature. In recent years Dr. C. Doelter has made a series of synthet- ical experiments in which, by treating metallic salts by sulphu- retted hydrogen solutions, he has succeeded in producing in crys- talline form most of the common sulphide minerals: namely, pj*- rite, chalcopyrite, bornite, chalcocite, covellite, galena, bournon- ■•■■■ Proc. Amer. Fhilos. Soc, vol. xxxiii., No. 145, p. 240. 1894. t Lehrbxich d: Ohem. u. Phys- Oeol., Second edition, Bonn, 1866, vol. iii., p. 721. 470 THE SECONDARY ENRICHMENT OH ORE-DEPOSITS. ite, miargyritejamesonite* and pyrrhotite.f The experiments were conducted at moderately elevated temperatures ; generally about 100° C. Such temperatures are used in the laboratory to hasten the chemical action, but it is probable that the same eiFects would be produced at the ordinary temperatures, such as would be found in ore-deposits, if sufficient time could'be allowed. In nature it is probable that changes in temperature may have been an important factor in producing solution or precipitation of minerals, for it has been found that a mineral-salt which was taken up in solution at a given temperature is sometimes rede- posited under a change in temperature. More significant, however, than laboratory experiments, are the synthetical processes of nature, which, as shown by Daubree and other European geologists, have been detected in the many thermal springs, where Roman metals and coins of copper and other metals have lain for centuries subjected to the action of waters containing feeble solutions of mineral sulphates. Among the minerals thus formed crystals of tetrahedrite, chalcocite, bornite, chalcopyrite and others have been recognized. At the Springs of Bourbon l'Archambault,J the succession of mineral coatings around the metallic copper of the coin is the exact re- verse of the series which has been noted above in copper-mines as the normal change in waters by secondary changes ; namely, next the metallic-copper, first, black copper-glance ; then bor- nite ; then chalcopyrite. That the reactions necessary to produce these changes have taken place in nature in a certain regular series is, in itself, fair ground for assuming that under possible variations of condi- tions, the same changes might take place in reversed order ; for it is recognized by modern chemists that the reactions between two substances which produce two other substances are part of a tendency to establish a condition of equilibrium, as it is called, between the substances involved, and that this tendency can be modified by different conditions of concentration, temperature, pressure, etc., so that it is conceivable that the reaction will proceed in one direction under one set of conditions, and in the reverse direction under another. * Zdtsch.f. KrysUxllogr. u. Mineral., vol. xi., p. 40. t Miller, u. Pelrog. Mitth. v. Tschermak, vol. vii., p. 535. i Ccrmptea Bendus, vol. Ixxx., January- June, 1875, p. 1297. THE SECONDARY ENRICHMENT OF ORE-DEPOSITS. 471 I am indebted to Mr. H. N". Stokes of the U. S. Geological Survey for the following concise statement of the present views of chemists on this subject: " It is now generally recognized by physical chemists that no reaction is complete in the sense expressed by chemical equa- tions. Every reaction tends to a condition of equilibrium lying between the two extremes ; sometimes at an appreciable dis- tance from both ; sometimes so close to one that for practical purposes the reaction may be regarded as complete. This con- dition of equilibrium may be varied as follows : " (1) By an increase of the relative concentration of one of the terms of the equation which tends to shift equilibrium to the other side. This may be effected either by relative concentra- tion of one term on the same side, or by removing the reaction products as fast as formed. " (2) By an increase of temperature which, besides increasing the reaction-velocity, shifts the equilibrium more or less, and always toward that side of the equation which contains the most energy. The reaction velocity is the speed at which the system ap- proaches equilibrium. In some cases this is immeasurably great, in others so small as to require ages. It is always more rapid at first, slowing down as the reaction approaches equilibrium. "(3) By increase of pressure which shifts the equilibrium towards that side of the equation which naturally occupies less volume. " (4) By substances foreign to the reaction which may retard or accelerate the reaction velocity, without, however, influenc- ing the final state of equilibrium (catalytic action)." A practical instance of the reversal of the direction of chemical reaction is furnished in Vogt's* description, already cited, of the famous silver-mines of Kongsberg, IS'orway. In these mines silver is in actual bulk the predominating metal. As mined, it is found mainly in the native state and very largely as wire-silver. This wire-silver, Yogt proves quite conclu- sively, is the result of alteration from silver-glance (argentite). He gives drawings of specimens in which the native silver is found growing out of a base of silver-glance, and in some in- stances still retaining small particles of glance on the ends of the wires. On the other hand, some instances are found where the * Op. cit, p. 119. 4*72 THE SBCONDAKT ENRICHMENT OF ORE-DEPOSITS. wire-silver has been later changed back to silver-glance, the laf^ ter retaining the form of the wire-silver. Thus, three processes are shown in the same mine ; the original deposition as glance ; the change from glance to native-silver, and the reversal of this process in the change from native silver back to silver-glance. Conclusions. — Until a much larger number of ore-deposits have been studied with a definite purpose of determining how far they have been subjected to secondary enrichment, it does not seem safe to draw any far-reaching conclusions from the ob- servations and suggestions noted above. It has long been recog- nized that the superficial alteration of ore-deposits has often pro- duced a very considerable modification of the original constitu- tion of the deposit, and its alteration has so frequently been in the nature of an enrichment in the more valuable metals relatively to the original tenor of the ores that it has given rise to the very hasty decision that all ore-deposits necessarily become poorer in depth, which is almost as unjustifiable as the old assumption by the miner, that the nearer he got to the source of his ore in the unknown depths, the richer it would become. The fact that ores under some conditions may be removed and re-deposited as sulphides, even below groundwater-level, opens a wide field of possibility in accounting for the unusually rich bodies of ore that are in some mines found in the middle levels, and have been fruitlessly sought for at greater depth. In many cases these have undoubtedly resulted from a concen- tration of material leached down from the upper portions of the deposit as they have been gradually worn down and carried away by denudation. Especially in the case of large bodies of pyritous ore carrying small proportions of more valuable metals, is a concentration of those metals by downward per- colating solutions to be looked for. It is, however, not yet safe to say that all rich bonanzas in vein deposits have neces- sarily been formed in this way. Although not yet supported by definite evidence, the im- pression is very strong with me that not infrequently the ascending currents have also produced migrations of already formed deposits and local enrichments under favoring conditions. "What these conditions are, and what are the criteria by which they may be distinguished from concentrations by descending waters, it remains for future investigations to determine-. ENRICHMENT OF GOLD AND SILVER VEINS. 473 The Enrichment of Gold and Silver Veins.* BY WALTER HARVEY WEED, BUTTE, MONT. (Washington Meeting, February, 1900.) Introduction. In a previous paper upon the enrichment of mineral veins by later metallic sulphides, f the writer has shown that certain masses of rich ores, such as are found in many mines, either near the water-line or as bonanzas in depth, are of secondary origin, and are due to a leaching of lean ore and the concentra- tion of the material by reaction between the solution and the unaltered ore below. The geological and mineralogical evi- dence is believed to form an adequate basis for a chemical and physical explanation of the phenomenon. In the present paper the writer will give a brief synopsis of this theory, and will apply it more particularly to deposits of the precious metals, laying special emphasis upon the dependence of such enrich- ments upon the presence of iron sulphide (as pyrite, etc.) in the primary ore, and upon structural features which control the circulation of the enriching solutions below the water-level. It is believed that many, though not all, of the bonanzas and pay-shoots of rich sulphide ores, especially those carrying gold and silver, which are encountered in ore-deposits, are of such secondary origin. Apparently it is essential that the occur- rence and structural relations of such ore-masses should be un- derstood, as the success of the mine is often dependent upon the finding and extraction of these ores. A legitimate deduc- tion, too, is that such ore-deposits decrease in value with depth. As my own studies have been mainly in Montana, my illus- trations must be drawn from the ore-deposits of this and ad- jacent States, with such as I have noted in hurried visits else- where. The literature of ore-deposits doubtless affords also * Published hj permission of the Director of the U. S. Geological Survey. f Bulletin Geological Society of America, vol. xi., pp. 179-206, 1900. 474 ENRICHMENT OF GOLD AND SILVER VEINS. many illustrations of secondary deposition besides those quoted by me in this paper. The recognition of secondary enrich- ment as a factor, and the chief one, in the genesis of many rich ore-deposits was forced upon me several years ago by my study of the IsTeihart, Mont., silver-gold veins. Since then it has proved to be of frequent occurrence in many mines, and its study has led me to the theory of secondary enrichment pro- pounded in a previous paper, and especially applied to precious metals in this paper. Statement oe the Problem. The fact that masses of very rich ore often occur near the water-line in many mines, but do not continue in depth, and the occurrence of pay-streaks and bonanzas in deep mine-work- ings, is a matter of history in many regions. The problem is to explain the genesis of such ores. The theory here presented accounts for such ores as enrichments formed from bodies of lean ore of complex composition, which have been lixiviated, the gold, silver and copper being carried downward below the water-level and precipitated as high-grade sulphide ores. The evidence is mainly mineralogical and geological ; but it is in entire accord with chemical tests and reactions, which have been carried out in the laboratory or are too well known to be called into question. Surface-waters are believed to be commonly the means by which the lean ores are leached and the metallic contents car- ried down and redeposited. In some cases, however, concen- tration has probably been effected by new fractures, resulting, in the cases known to the writer, from later volcanic activity and faulting, and serving as channels for upcoming hot waters. As commonly understood, " surface-waters " are those which have so recently left the surface as to still retain constituents common to waters now found at or near the surface (free car- bonic acid, organic acids, chlorides, etc.), and which produce an oxidation. Thus the effect of superficial alteration, as described by most writers, has been the production of carbonates, chlor- ides, oxides, etc. In those instances where surface-agencies have had a reducing effect, it has been commonly ascribed to organic matter, though Penrose cites the formation of native copper by the action of " a ferrous salt on certain copper salts," ENRICHMENT OF GOLD AND SILVER VEINS. 475 as an instance where the primary chemical action is one of partial oxidation, and the reducing action follows, as the effect of one of the partially oxidized compounds on the other. In this sentence lies the pith of the whole subject of enrichment, since when the leaching of lean ore is performed by oxidizing surface-waters the resulting solutions percolate downward as deoxidized waters, carrying soluble salts that are the result of oxidation, and enrichment is the result of a reaction between these substances in solution and the unaltered ore with which they come in contact in their downward course. Above the ground -water level there is a constant movement of the water downward, but below that level the free oxygen has commonly been used up.* The Zones of Weathering, of Enrichment, and of Primary Sulphides. At the outset a sharp distinction must be drawn between the secondary or later enrichment herein described, which occurs in part at the water-level but usually below it, and the enrich- ment due to simple weathering or superficial alteration of the ore. In the latter case the gold or other values remain, while the worthless constituents are in large part removed, thereby greatly increasing the value per ton of the weathered part of the vein. This process, usually known as superficial, is a com- mon and now well-known feature of ore-deposits the world over. In order to describe the difi:erent parts of veins here under discussion, the writer will follow common usage in calling the upper weathered part the zone of weathering. Beneath this lies the zone of enrichment, underlaid in turn by the zone of primary sulphides. The term " zone " is, of course, only used for convenience, since it is well known that weathering extends down along fractures and other channels for circulating waters, sometimes for hundreds of feet into a mass of otherwise un- altered ore. The zone of enrichment is even more irregular, and may, as illustrated in the ideal conditions represented in * It should be noted that the word oxidation is here used in its original re- stricted sense, as it is commonly understood, and not in the extended sense used by chemists to express the converse of reduction, as, for instance, CujS to CuS, where no oxygen is present. 476 ENRICHMENT OF «OLD AND SILVER VEINS. the diagram, Fig. 1, be separated by unaltered ore from a bonanza mass of secondary ore beneath. The occurrence of enrichments between altered and un- altered vein-matter is one that has not escaped the attention of previous vfriters ; but they have all, so far as known to me, limited such enrichment by the ground-water level. Thus, Penrose says :* "As a result of these various changes, certain minerals are sometimes leached from the upper part of ore-deposits which have become porous by alteration and carried down to the less pervious unaltered parts. Here they are precipitated by meeting other solutions or in other ways, and hence the richest bodies of ore in a deposit often occur between the overlying altered part and the underlying un- altered part. This is not always the case, but it is true of some copper, silver, iron and other deposits." This author, it is true, recognized that surface-waters pass below the zone of oxidation and may gradually sink to very great depths below the permanent water-level. But he ex- pressly declares that the results of such circulation do not re- late to superficial alteration,! which he limits to that of deposits that remain in situ.X De Launay, in his very interesting and valuable essay, § has given us a chemical theory, to the support of which he has marshaled all the facts gathered in the prepa- ration of his great monograph on ore-deposits. This author distinctly recognizes a zone of enrichment, but, like Penrose, limits it by the permanent water-level (niveau hydrostatique). Leaching in the Zone of Weathering. In the sulphide enrichment here discussed, the enriched material is in most cases derived by the leaching out of the metals from the portion of the vein lying above ground-water level. This leaching is due to superficial alteration, and leaves the iron as a gossan while the waters carrying the gold, silver, copper and other metals in solution trickle downward through the partially altered ores into cracks and water-courses which penetrate the ore-body below the water-level. The first partof the process is, therefore, the leaching of the lean ores which occurs * Journ. Oeol, vol. ii., 1894, p. 294. t Loc. dt., p. 298. X Loc. eit, p. 302. g " Contribution a L' Etude des Gltes M6tallif6res," Ann. des Mines, 9th series, vol. xii., 1897, pp. 119-227. ENRICHMENT OF GOLD AND SILVER VEINS. m in the superficial alteration of the vein. This has been dis- cussed by many writers, particularly by Penrose, who, however, does not make any attempt to state the chemical reactions involved. These reactions are complex, and the mass results depend upon the laws of physical chemistry ; yet the general Fig. 1. Bonanza Diagram Showing Belative Positions of Zones of Weathering, Enrichment and Unaltered Ore, and of Bonanzas Formed Along Fault. changes involved may be expressed by equations showing end reactions. The chemistry of weathering, concisely expressed, is as fol- lows : In an ore consisting of either one or all of the following sulphides : — pyrite, arsenopyrite, chalcopyrite, blende, galena, tetrahedrite, — the minerals will oxidize according to their rela- tive affinity for oxygen and inversely as their " affinity " for sulphur.* All the sulphides will be attacked simultaneously, * This statement is suflBciently accurate for the purposes of this discussion. As the mineral decomposition is affected by physical structure, as well as chemi- cal, and by relative amounts of each present, it is apparent that there are many qualifying factors. The ' ' relative affinity ' ' of the metals for sulphur is 1 Hg, 2 Ag, 3 Cu, 4 Sb, 5 Sn, 6 Pb, 7 Zn, 8 Ni, 9 Co, 10 Fe, 11 As, 12 Mn. This is the order in which a salt of one metal will be decomposed by any subsequent one in the series and the first metal precipitated as sulphide. (See E. and M. Jour., Oct. 25, 1890, p. 484. See also Jour, of Soc. Chem. Ind., vol. xi., 1892, p. 869.) 478 ENRICHMENT OF GOLD AND SILVER VEINS. but, inasmuch as pyrite consists of 4 parts ferric and 1 part ferrous sulphide,* and parts with a portion of its sulphur very readily, this mineral will be most attacked. This decomposes first to FeS and S. The sulphur usually forms HjSO,; the FeS forms FeSO,. The latter changes to H^SO,,- Fe(0H)3 and Fej (80^)3. The sulphuric acid attacks more iron sulphide and forms more FeSO^ together -ndth H^S — the latter, in the presence of abundant oxygen, forming HjSO^. The FeSOj changes to Fe2(S0 Jj, which attacks the sulphides of copper, lead, zinc, etc., in a reaction which can be most simply expressed as follows : Cu,S + 5Fe2(SO,)3 + AILfi = 2CuS0. + lOFeSO^ + 4H2SO,. The HjSO^ in mine-waters will attack both copper and iron sulphides and form sulphates without the formation of HjS or the liberation of free S ; and the iron sulphate, oxidizing the iron, is precipitated as limonite. The oxygen may come from either air or water. PbS + Fe2(SO,)3 = PbSO, + 2FeS0, + S ; or PbS + 4Fe2(SO,)3 + 4H2O = PbSO^ + 8FeS0, + 4H2SO,. And ZnS + Fe^CSO j, = ZnSO, + 2FeS0, + S, or, more prob- ably, ZnS -I- 4Fe2(SO,)3 + 4B.fi = ZnSO , + SFeSO^ + 4H,S0,. The above equations simply show that ferric sulphate can oxidize the various sulphides to sulphates, and is itself reduced to ferrous sulphate. However, the sulphuric acid formed by the oxidation of pyrite in the upper zone can also attack sul- phides, and the HjS which is formed may be oxidized by the ferric hydrate into sulphuric acid. This method is probably more likely of occurrence, but no one can say that the oxida- tion is exactly according to any set of equations, as many other reactions are possible. The laws of physical chemistry, verified by experiment, show that blende is more easily attacked by oxidizing waters than galena, and the latter mineral decomposes more readily than chalcocite. The general order of attack of the sulphides is there- fore arsenopyrite, pyrite, chalcopyrite (FeS removed, leaving CuS), blende, galena, chalcocite, while tetrahedrite, being a complex substance without definite percentage-composition, has no fixed place. Gold, if present, may be attacked by Fe2(S04)3 in which it is well known to be readily soluble, and silver goes * "The Chemical Composition of Marcasite and Pyrite," by Amos P. Brown, Proc. Am. Phil. Soc, vol. xxxiii., p. 225, 1894. ENRICHMENT OP GOLD AND SILVER VEINS. 479 into solution as sulphate. The lead, which as sulphate is nearly insoluble and remains about its parent^mineral galena, can only migrate when reduced to carbonate (by calcite, etc.), in which condition it is readily carried off by carbonated waters. "Where these are the only reactions, the outcrop is leached of all its metallic matter, and its soluble gangue-minerals are reduced to a porous spongy mass of silica, such as is sometimes seen. Commonly the iron is not all removed, since the ferrous sulphate, which is the most abundant product of the leaching, absorbs oxygen and water and forms limonite, 2Fe2033H20 (or rarely a basic sulphate of iron), forming the iron-stained quartz or limonite gossans, the " iron cap " of so many vein outcrops. This leaching of the ores is therefore seen to depend upon the tendency of the iron salts to form !Fe(0H)3 as an ulti- mate product which is precipitated from the solution; thus renewing the FeSO^, which renews the ability of the solution to attack more pyrite and metallic sulphides. This cycle of change can be tentatively expressed as follows : Ferric sulphate forms by the oxidation of the iron sulphide of the original ore. This salt attacks pyrite and other sulphides, and is itself re- duced to ferrous sulphate. The latter oxidizes to ferric sulphate, which is partly changed to limonite and sulphuric acid, while the remainder begins anew the cycle of change. Ferric sul- phate is the main vehicle by which the sulphides are dissolved. The Fe(0H)3 is in part eliminated as a precipitate, while a part is acted upon by the sulphuric acid with the production of a solution holding Fe2(SO,)3 + FeSO„ these iron sulphates being in the approximate proportion of 3 : 1. The FeSO^ takes up oxygen and forms Fe(0H)3, and the ultimate production is a yellow basic sulphate insoluble in H^SO,. The result of these changes, due to water and abundant oxygen, is the leaching out of all the constituents of the vein in the weathered zone except iron and silica. The solutions seeping downward contain vari- ous metallic sulphates and much sulphuric acid, the amount of the latter being increased by that formed by hydrolysis from the sulphates, since copper sulphate in solution yields sulphuric acid. Precipitation in the Zone of Weathering. ISTot all the material leached out in the zone of weathering migrates below to the zone of enrichment; for the surface- 480 ENRICHMENT OF GOLD AND SILVER VEINS. waters commonly contain carbon dioxide, some chlorides, organic matter, etc., resulting in the formation of carbonates, chlorides, etc., and of the native metals. Thus, copper can be formed from the oxide by reaction with either free sulphuric acid or iron sulphate (both abundant in the lower part of the superficial zone), viz. : Cnfi + HjSO^ ^ Cu + OuSO^ + HjO ; and 3Cu,0 + 6FeS0, = 6Cu + Fe^O, + 2Fe,(SO,)3. The latter reac- tion accounts for the cement-copper associated with iron sesqui- oxide at Ducktown, Tenn., Gold Hill, "N. C, and elsewhere. [fTative silver is also formed in films and crystalline masses by reduction through ferrous sulphate, viz. : Ag^SO^ + 2FeS04 = 2Ag + Fe2(SOi)3. Gold probably sometimes occurs in the native state because it has not been attacked and is simply left behind, though it is also deposited by precipitation from the ferrous sulphate solution. The Zone of Enrichment. The surface-waters which have leached the vein in the zone of weathering seep downward along cracks and crevices, or along trunk-channels, into the primary ore below. The origin and occurrence of such fractures will be mentioned later. They very commonly exist in ore-deposits, and convey waters downward far below the so-called ground-water level. As we have shown, these waters not only carry various metals in solu- tion, chiefly as sulphates, but they are no longer oxidizing, but are of acid reaction. Penetrating the primary ore, they come in contact with the unaltered metallic sulphides. In such masses pyrite and, more rarely, pyrrhotite are very commonly abundant ; and a reaction at once occurs between the iron sul- phide and the metallic salts (mainly sulphates) held in solution, resulting in their decomposition and the precipitation of new sulphides which encrust the walls of the fractures. This, in the case of copper, is shown by the following theoretical equa- tion, which expresses end reactions only, viz. : 4CuSO^+3FeS2 + 4H,0 = 2Cu,S + 3FeS0, + 2H,S0, + 2H,S + S ; or, more simply, copper sulphate and pyrite yield copper sulphide and ferrous sulphate.* This Cu^S would react in turn upon silver sulphate, AgjSO, + Ou^S. = Ag^S + Cu^SO^, while the pyrite , * The apparent anomaly of cupric sulphate and pyrite giving ferrous sulphate js^explaine4 by the chemical composition, of pyrite as 4 parts f-erric sulphide and ENRICHMENT OF GOLD AND SILVER VEINS. 481 itself will decompose the silver as well as other sulphates, owing to the relative affinity of the metals for sulphur. Chlo- ride or carbonate of silver would he similarly decomposed. For the rich antimonial sulphides of silver various reactions are possible, the pyritous ore reducing the minerals from a solution holding antimony and arsenic derived from impure pyrite. This process is probably aided by the free sulphuric acid brought down in the waters and as hydrolization-product of intermedi- ate steps of above reactions; since a dilute solution of sul- phuric acid attacks iron sulphide, forming iron sulphate and sulphuretted hydrogen, the latter of which would form sul- phides of lead or silver, etc., from the solutions.* For lead the presence of carbonates seems necessary, and if the gangue minerals are of this nature, or the walls are lime- stone, the lead carbonate is decomposed, lime goes into solu- tion, and the H2S set free from pyrite at once forms galena, which is deposited. The Solution and Precipitation of Oold. — The alteration of gold-deposits presents features differing very markedly from those accompanying the alteration of copper- or silver-ores. It is commonly assumed that the unaltered ore contains the gold in association with pyrite or quartz. The most frequent altera- tion of this is to a rusty brown mass of sesquioxide of iron, permeating the quartz and holding nugget-threads of free gold. As a result of further alteration by surface-waters, the iron is leached out, and a porous, spongy, white quartz remains, holding the gold. This kind of alteration is a very common feature of ore-deposits throughout the West. In many cases, however, different conditions prevail. Part of the gold, at least, is taken into solution by ferric sulphate, carried downward as the waters 1 part ferrous sulphide, the latter only being herein considered. Moreover, we may have in oxidation zone: Cu^S + 4Fe2(S04)3 + 4HjO = Cu2S04 + 8FeSOj + 4H2S04. Then any sulphide would precipitate CU2S from the cuprous sul- phate, providing tlie sulphide is soluble enough and the sulphate solution is strong enough to liave enough cuprous ions and sulphide ions to exceed the con- stant of solubility. The formation of cuprous sulphate is theoretical, but its existence is indicated by recent experimental work, as yet unpublished, by C. F. Tolman, Jr. * It must be understood that these equations are given in the simplest and most compact form possible. Thus CuSOj in water really holds Cu(^0H)2 and H,SO-. 482 ENRICHMENT OF GOLD AND SILVER VEINS. seep below, and precipitated as native, leaf, wire or scale gold in minute cracks in sulphide ores, or, what is more commonly the case, the gold is deposited with silver in antimonial sul- phides, especially ruby silver (pyrargyrite). This is the form in which it occurred at the Ruby mine, on Lowland creek (near Butte), where the surface of the quartz crystals lining the open spaces between boulders of decomposed rhyolite and coating these boulders is liberally sprinkled with ruby silver. This mine yielded $600,000 in less than a year, of which one-half the value was gold. The ore was a secondary concentration along a clay fault-fissure, and is now' exhausted. The " in- dicators " of Australian ore-deposits afford a most interesting Fig. 2. Alteration of Bomite to Chalcocite and Limonite, Blue Wing Ore, Virgilina District, N. C. Tiie nucleal masses are bornite ; the black borders represent chalcopyrite ; the stippled area is iron ozide. Drawn from nature, twice the natural size. example of the reduction of gold by pyrite. These indicators are thin layers, sometimes but half an inch thick, of pyrite occurring in shale. In many cases the shales are, it is true, carbonaceous, and the organic matter, may assist in the reduc- tion. As shown by Don and by liickard, the quartz veins are barren except where they intersect these pyritous layers. * The experiments of Liversidgef have shown that gold is precipitated from solution more readily by metallic sulphides than by organic matter. * J. B. Don, Trans., xivii., p. 569. T. A. Eickard, E. and M. Jam., 189£\, Ix., p. 561. t Proe. Bmj. Soc. N. S. W., vol. xzvii., 1893, p. 287. See precipitation of gold by pyrite in the experiments of Daintree, quoted by Eickard in " Origin of the Gold-Bearing Quartz of Bendigo Eeefs," Trans., vol. xxii., p. 313. ENRICHMENT OP GOLD AND SILVER VEINS. 483 USsumi. — It seems unnecessary to expand this section further, as the reactions given are sufficient to show that secondary sulphides are formed in depth. It is evident that a majority of the reactions depend upon the presence of iron sulphides, either as pyrite or in some other form. Pyrite is therefore the great precipitant of secondary sulphides. Evidence that Certain Minerals and Ores have the Genesis Stated Above. That chemical reactions similar to those given do take place in nature, and that the resulting precipitates are true minerals, is shown by abundant mineralogical proof. Thus the aurifer- ous copper-ores of Gold Hill, ITorth Carolina, show chalcopyrite Fig. 3. Barite tablets with parasitic galena, leached of lead, leaving polybasite (pearcite) sponge. Galena, blende Pycite, barite Specimen Showing Leached Galena and Eesidual Polybasite ; Florence Mine, Neihart, Montana. altering about its borders to a spongy mass of black copper sulphide ; the iron being largely carried oflE", but in part form- ing hematite nests near by. In other specimens, the copper sulphide has gone into solution, and has been carried a few inches and redeposited in crystalline masses. In a similar manner bornite alters to chalcocite and iron oxide, as shown in specimens from the Blue "Wing mine at Virgilina, Person county, ISTorth Carolina. Fig. 2, drawn from nature, shows this alteration. In other specimens the iron is carried off and fills cavities with specular iron-ore. At ISTeihart, Montana, polybasite and pyrargyrite encrust barite, quartz, galena and pyrite, which are themselves later 31 484 ENRICHMENT OF GOLD AND SILVER VEINS. than and encrust fractured masses of impure galena, blende and pyrite that constitute the original vein-tilling. These crusts are now forming in vugs and water-courses filled by sluggish descending water. ITucleal masses of impure galena are seen in thin section surrounded by a spongy mass of polybasite, just as chalcopyrite is seen surrounded by amorphous copper glance. Fig. 3 is a diagrammatic representation of a portion of the surface of a specimen of ore from the Florence mine. The main mass of the specimen consists of a breccia of pyrite frag- ments, held in a cement of barite and ankerite spar, with scattered grains of galena. The upper surface shows galena, blende and barite, the latter in projecting tablet-shaped crystals upon which there are parasitic masses of impure argentiferous galena. This galena is etched and leached so that, on part of the specimen, the surface shows a crust of about x^ to | inch thickness consisting of a spongy residue, or a skeleton of the galena. This spongy mass consists largely of polybasite left behind as the more soluble lead was leached out. That some of the antimonial sulphide of silver goes into solution, is shown by the presence nearby of minute newly-formed crystals of the latter. The crystalline polybasite occurs nearby coating frac- tures and showing characteristic triangular markings, or as loose aggregates of rough and mossy-surfaced crystals. It is also probably derived from blende, as it occurs very commonly coating that mineral under conditions which seem to preclude precipitation by that mineral. An examination of numerous specimens from the Florence and Big Seven mines shows that polybasite and pyrargyrite are secondary minerals filling cavi- ties and cracks in the original ore. The material gathered from the lowest level of the Florence mines shows polybasite in the form of crystalline tablets upon barite and other minerals, and also as a moss-like mass of open skeleton-texture, which seems to represent arrested deposition. The latter form is believed to come from a place in the vein where mineral-bearing water is now depositing this mineral, together with spar, quartz, and probably galena. Studied under the microscope, the polybasite appears to be an alteration-product of galena, and itself to be mixed with, and to grade into, pyrargyrite, which is in some cases its undoubted alteration-product. It is certain that poly- basite, as the important constituent of many of the ores, is of ENRICHMENT OF GOLD AND SILVER VEINS. 485 secondary origin. It occurs on all other minerals, and is itself not coated or dotted by them. Pig. 4 is a diagram of a speci- men of the common ore of the district consisting of galena and carbonate " spar " with scattered pyrite and chalcopyrite. The specimen is from the wall of a fracture traversing the somewhat friable bands of galena. The surface of the fracture has been coated with a thin drusy covering of quartz upon which there rests massive polybasite whose upper surface shows the typical triangle striations of polybasite. The speci- men is drawn nearly to natural scale. Sphalerite also occurs in well-formed crystals in some of the vugs and is one of the most recently deposited minerals. |. ^ "-" ti-t ^ ' :ondary "triangle" ore; ■T|l n I III llll ' jlll I IjTlX ybasite encrusting frac- '^ I jiJiMwr^^ ^'k ed surface of primary ore, ^ ** -5 ^^ "^ ? _. Tj^ 'i "^^^ °r6i galena and i ^ t^ bonate nninerals: no ^ %v artz, some pyrite and Specimen from the Florence Mine, Neihart, Montana. While polybasite and pyrargyrite are economically the most important of the secondary minerals formed by enrichment- fractures in the ISTeihart ore, yet other minerals, galena, pyrite, blende and quartz, are also formed. An excellent example is seen where quartz veinlets have filled fractures in the primary ore. Fig. 5 shows a piece of the common spar and galena ore in which the ore is fractured and the fissure filled by a veinlet of quartz in whose center pyrite is seen. At the top the fissure is open and the vug is lined with a drusy coating of quartz, on the surface of which occasional larger crystals of polybasite are seen. The druse (c), connected with the quartz veinlet by a fracture following a layer of spar, shows secondary quartz and pyrargyrite. Where such fractures traverse the ore, and its carbonate gangue is at a decided angle to the banding of the deposit, and the crusts or filling are notably difiFerent in composition, there is no doubt of their being of later origin. A veinlet of this kind is illustrated on a natural scale in the diagram, Fig. 6. In this case various secondary minerals were formed. The figure represents a cross-section of a little quartz 486 ENRICHMENT OP GOLD AND SILVER VEINS. " vein " of the Big Seven mine, which constitutes the high- grade ore-streak of the lode. It shows the relative abundance and association of the minerals, but does not represent the spongy texture of the polybasite and its intimate admixture with both galena and pyrites (chalcopyrite), as this growth is too mossy to be represented well, and the mineral is therefore indicated as polybasite alone. The specimen, seen in thin sec- tion, shows ruby silver and polybasite intimately associated and forming irregular, shreddy and ragged patches. 'No posi- tive identification of galena as the nucleus of such masses was made, but the association with galena is such as to indicate a possible change to polybasite. The pyrite is broken and frac- Fia. 5. Druse forming part of secondary quartz veinlet - quartz' encrusted with pyrite and poiybasite. & = Secondary quartz veiniet witii pyrite center, C= Vug In galena and carbonate ore. shows secondary ieaohing and de- position of quartz in cavity. Specimen of Silver- and Lead-Ore from Neihart, Montana. tured, but the grains are always sharply defined, and no genetic relation to the silver sulphides is recognizable. A blende crystal, seen isolated in the central quartz-filling, shows in thin section a crust of polybasite, the latter holding minute inclu- sions of pyrite. The blende, seen in another section of rich ore, is invariably surrounded by a dark crust which is not iron oxide, nor does it appear to be an iron-rich blende. It is not definitely determinable, but resembles galena or a silver sulphide. Posepny has described stalactitic deposits of sulphide, which, as urged by Dr. A. Schmidt,* form excellent proof of the formation of secondary sulphides by a leaching of ore in the zone of weathering and a redeposition of ore in the zone of enrichment. Posepny, it is true, denied such an origin for these deposits because they occurred below water-level. The * Die Zinkerzlagerstatlen von Wiesloch, in Baden, Heidelberg, 1881, p. 94. Posepny, Genesis of Ore Deposits, p. 63 ; and Trails., xxiii., 259. ENRICHMENT OF GOLD AND SILVER VEINS. 487 existence of open spaces below water-level is a phenomenon frequently encountered in ore-bodies exposed by mine-work- ings. I myself have seen such openings a foot or more across at 1000 feet below the water-level at Elkhorn, Montana, and at 200 feet below water-level at ISTeihart. The pipe-ore of Raibl described by Posepny is, I believe, an excellent example of the formation of secondary minerals by descending waters. Posep- ny's explanation that they are due to ascending waters which were denied access to the cavity except through the roof, seems to me to be an hypothesis opposed to both the facts of observation and physical laws. Moreover, as the geology of the mining regions is more carefully studied, it is certain that they have passed through various physiographic changes, with migration of water-level, so that air-filled spaces below what is now the water-level are not only possible but in some cases probable.* Prof Vogt also describesf the recent concentration of gold and silver in a zone beneath the " iron hat." He says that in the Rio Tinto region the " iron hat" is from 35 to 50 meters deep, and consists of iron oxide or hydrated oxide, with from 35 to 50 per cent, of iron, some silver in part as basic sulphate, and a few ten-thousandths per cent, of arsenic, while on the other hand the copper-contents are, as already remarked, en- tirely oxidized and dissolved out. In one mine, N'orth vein No. 2, at Rio Tinto, there occurred between the iron hat and the underlying comparatively fresh pyrite, a layer of earthy, porous material, bearing gold and silver. This earthy ore, though a few decimeters in thickness, may be followed con- tinuously over the entire ore body. This very marked layer follows closely the irregular plane between the iron hat and the underlying pyrite. It everywhere contains an average gold- and silver-contents of from 15 to 30 grammes gold and 1.025 silver, with a value of about 150 marks per ton. In stripping ofit" the " iron hat," this earthy mass is carefully laid to one side, and has thus yielded fully a thousand tons of ore. It is clear that the formation of this gold- and silver-bearing zone is connected with the oxidizing process that formed the iron hat, and that the gold and silver comes from the very small percentage of such metals in the primary ore. * This volume, p. 69; and Trans., xxiii., 260. f Zeitsch. Prak. OeoL, July, 1899. 488 ENRICHMENT OF GOLD AND SILVER VEINS. I>eduetions. From the chemical reactions given, it is evident that enrich- ment is largely dependent upon the presence of marcasite, pyrite or some other form of iron sulphide in the primary ore, since lixiviation depends upon the presence of the iron sul- phates, and precipitation is mainly effected by the unaltered sulphides. As a consequence of this, it follows that ore-bodies lacking in iron pyrites will not show enrichment, thus explain- ing the absence of any such phenomena in the pure silver-lead bodies of the Cceur d'Alene district and elsewhere. In this region, visited by the writer in 1895, the ore-bodies consist of galena with a siderite gangue and are replacemenl^deposits in quartzite and argillaceous schists. The veins are covered by great masses of barren limonite gossan, beneath which the ores are carbonates and sulphates of lead, which extend along frac- tures to a depth of 200-300 feet. The silver values, which carry about 10 ounces of silver to 1 per cent, of lead, do not show any enrichment. This is quite what would be expected, since, although the galena in decomposing would yield up silver as sulphate, there would be no reducing agent at hand to extract it from the waters as it seeped down into the un- altered ore. Also, at Barker, Montana, the ore-bodies show no enrichment, though a common feature of such deposits, viz., the change of galena to pyrite, in depth, would favor enrichment, if the silver-lead bodies were deeply enough weathered. The writer has also examined the Zosel mines in andesite porphyry near Deer Lodge, Montana, and those in similar rocks on Basin Creek, and near EUiston, as well as the Castle Mountain and Elkhorn ores in limestone, and the Bear Paw ores in basalt, all in Montana, as well as the McMakin in liforth Carolina. These deposits all consist essentially of galena without any notable amount of pyrite, and although favorable physical conditions for enrichment occur, no bonanzas or pay- streaks of rich secondary ores are found. The Occurrence of Bonanzas and Pay-Streaks. The location of bonanzas and pay-streaks of secondary sul- phide-ores is dependant upon physical factors. From a con- sideration of the processes described in the preceding pages, it is evident that the localization of enrichments will depend ENRICHMENT OP GOLD AND SILVEE VEINS. 489 wholly upon structural conditions. If the vein consists of a solid unshattered impermeable body, with no fractures by which the solutions can seep down into the underlying original sul- phides, the zone of enrichment will be confined to the vicinity of the water-lines, and, if above water-level, will constitute the ore-bodies described by Penrose, De Launay and others as one of the results of superficial alterations. In fact, many such en- richments do occur at, or just below, the water-level. If the primary ore-body is shattered by cracks, sheeted by later move- ment or traversed by secondary fractures, faults running with Fig. 6. Polybaslte(& Pyrargyrlte) Galena Pyrlto Blende Quartz-fibrous Quartz Secondary Veinlet of Quartz and Eich Sulphide Ore Filling Fracture in Pri- mary Ore ; Big Seven Mine, Neihart, Montana. Pyrite and galena are angular fragments of original ore. or across the vein, such crevices and fractures will be the chan- nels in which the descending solutions will travel, and along which the secondary ores will form deposits in the unaltered ore below. Such secondary fractures may be now filled with quartz or other gangue-minerals holding ore, or they may be barren and open, or they may be marked by a soft mushy mass of clay or attrition breccia. Very often the so-called sjilits, feeders and stringers of a vein, when examined critically, will be found to be secondary fractures and not true ofi'shoots of the vein itself, the latter phenomenon often being the cause of ore-shoots of primary origin. "Where the later fracturing runs parallel to 490 ENRICHMENT OP GOLD AND SILVER VEINS. the vein, as is so often the case at Butte, and in the silver mines of Jefferson county, it may only be revealed by a clay selvage of a rare slickenside surface, though it is more frequently marked by a soft and mushy mass of mud and breccia, in which fragments of the wall-rock ore and gangue, one or all, may be seen. This is seen at the Comet mine, where second- ary ores were abundant along recent fissures filled with clay and a breccia of leached ore and altered wall-rock. An excellent example is afforded by the ore-body of the Australian Broken Hill Consols mine, N"ew South Wales, de- scribed by Smith.* The occurrence of the largest bonanzas yet Fig. 7. Approx soale Australian Broken Hill Consols Mine, New South Wales. (After George Smith, Trans., xxvi., 73.) A, dyscrasite ; B, stromeyerite ; C, decomposed amphibolite, etc. , assaying under 7 oz. per ton ; D, fahlerz ; E, soft gossany material, containing nodules of silver chloride, stromeyerite, etc. , and averaging about 750 ozs. per ton ; F, limonite, practically free from silver ; G, cross-vein ; H, amphibolite. found in the mine were in association with the vertical vein G, shown in the diagram (Fig. 7). This cross-vein has been faulted by the lode, and is really a succession of joints along a line of * Trans., xxvi., 69. BNKICHMBNT OP GOLD AND SILVER VEINS. 491 weakness. Another bonanza occurs in the same mine, 500 feet to the east, under similar conditions. Mr. Smith's statement is that the lode itself is only ore-bear- ing where it makes j unction with cross-veins. The well-known occurrence in Australia of ore-bodies where veins cross " in- dicators," i.e., pyritic bands a few inches wide, is readily under- stood, either as secondary enrichment or primary, since the reducing effect of pyrite upon gold held in solution has been established by Liversidge. The part played by such secondary fractures at Butte can hardly be appreciated by those not familiar with the ground. In descriptions and diagrams of the veins of this district the occurrence of clay and breccia bands and walls has not been Fig. 8. =0'° ^^^ \z\o=°'°'~^l|]||p\s \ ^^^%^ .^ V^Polybasite ^^"'■"'^■Wiiuas S[ Vy Polybaslte Barren - Level X:Sji „f .^ \ Development 1 Diagram to Show Relation of Mollie Gibson and Smuggler Ore-Bodies and Bonan- zas (of Polybasite) to Fault-Fissures. (From Spurr. ) overlooked, though their true significance appears to have es- caped notice since the fractures so often run parallel with and in the vein itself. Emmons first noted the significance of such fractures, and their genetic connection with glance and bornite veins. It is now known that these fractures are extensive laterally and vertically, and the enrichment probably due to them extends in some instances to a depth of 2000 feet below the present surface. Lest this statement prove misleading, it should be qualified by adding that not all such fractures have caused enrichment, and some of the largest fault-fractures are of relatively recent formation, later than the ore-bodies of glance and bornite, etc., which they cross. In his monograph upon the Aspen district, Colorado,* Mr. » J. E. Spurr, Mining District. U. S. Geol. Survey, Monograph No. xxxi., Otology of the Aspen 492 ENRICHMENT OF GOLD AND SILVER VEINS. Spurr describes the occurrence of the famous ore-bodies of the Smuggler and Mollie Gibson mines at Aspen, Colorado. The ore consists of barite and polybasite, with tennantite. Although Mr. Spurr gives no definite statements as to the possible sec- ondary origin of these ores, yet the sketch which he gives* (Fig. 8) and the descriptions all indicate that the original ore was a silver-bearing lead sulphide, with more or less iron and zinc sulphides, formed along inclined faults, and that subsequent to the formation of these ore-bodies, nearly vertical faults dis- placed the ore and formed the two bodies now worked at the mines mentioned. Although in these vertical or nearly verti- cal fissures rich polybasite ore is now found, it does not extend far in either direction from this fault ; and the description of the ores given by Mr. Spurr indicates that it is derived by sec- ondary alteration-processes from the lead and zinc ore-bodies. This is also indicated by the fact that the polybasite is in part altered to native silver at the extreme lower end of the ore- body. In conversation with me, Mr. Spurr has admitted the possi- ble secondary origin of these polybasite bodies, but had no new evidence upon the subject. He says : "This ore was of a rich character, having large amounts of polybasite and native silver. This polybasite body appears to lie in a sort of subordinate shoot, trending south of east and lying at the Gibson fault-plane. This shoot is marked by exceptionally large and rich bodies of a nature not found elsewhere in the mine. It is noteworthy that this rich shoot is practically the lower termination of the ore of the Gibson fault. Most of the ore below this is native silver, which, from the nature of its occurrence, is manifestly a secondary deposit leached from the rich ore above. Some of these secondary deposits are, however, of consider- able size, and empty vugs are often found beautifully and elaborately festooned with delicate wires of silver. Above the polybasite ore, however, the ore appears to be pretty continuous, but the amount of silver becomes less." It will be noticed that he recognizes the secondary nature of the silver, and that the polybasite lies between the native silver and the lead sulphide. In a chapter upon the chemical geology of the region, where he discusses the alteration of the ore-deposits and of the lime- stones, he does not adduce any new facts concerning the forma- tion of the polybasite ore ; but he does say that iron pyrites carrying small amounts of arsenic, lead, copper, zinc, cadmium, * Op. ciL, 183. ENRICHMENT OF GOLD AND SILVER VEINS. 493 cobalt and nickel is found, and that tetrahedrite is also very common. The polybasite is said to be later than the barite. The description given by Leggett* of the Rosario mine, San Juancito, Honduras, C. A., shows that the secondary enrich- ment may account for the peculiar features of the deposit. This fissure-vein splits into two distinct veins in more barren ground and unites into one consolidated vein where ore-bodies occur and where feeders enter from the hanging-wall — a con- dition paralleled in the Drum Lummon lode in Montana. The oxidized ore of upper levels includes frequent streaks of argen- tite and the rich silver sulphides. The lower levels show the unchanged sulphides of iron, copper, lead and zinc. The gangue is quartz, carrying in the ore-bodies occasional clay- streaks, heavily stained with the hydrated oxides of iron and manganese. Other accompanying minerals found less fre- quently are polybasite, embolite, etc. The vein-material is quartz pure and simple. The foot-wall is usually decomposed and broken, and a clay parting often runs a foot or two inside of the wall, necessitating close stulling till the stope can be filled with waste. The conditions noted here are the counter- part of those encountered in the copper-veins of Butte, where later fractures, marked by clay and attrition-breccia, have been the channels for enriching solutions. The Secondary Enrichment of Veins at Ndhart, Montana. Secondary enrichment has played an unusually important part in the development of the ore-deposits of ITeihart. The ores extracted in the earlier workings and those found to-day where new veins are opened, all show silver sulphides deposited by secondary enrichment as crusts or crystals lining cavities, or as films or thin coatings along fractures of the primary ore, or in the oxidized zone as the s£>-called " sooty sulphide " ores that occur with manganese oxides. It is from this zone of enrichment that the high-grade ores, running from 200 to 1000 ounces of silver to the ton, or even higher, were obtained in the early history of the camp. Although such ores gave out in depth and caused many disappointments and failures, their occurrence played a most beneficial part in the development of the veins. * ■l}rans., xvii., 432, 494 ENRICHMENT OF GOLB AND SILVER VEINS. The secondary minerals recognized are chiefly polybasite (really pearcite) and ruby-silver, the former being more abun- dant. There are also bright metallic coatings, presumably ar- gentite, on crystals and along fracture-planes, and rarely in minutely crystalline masses. The superficial alteration of the Ueihart veins is not a marked one, as there are no great zones of carbonates and oxidized ore. Such ores occur only in limited amounts, being most abundant in the Broadwater vein, where 0= Black "clay" M seam: Colored by ground up galena etc. ft = White spar stippled with galena, pyrite and blende. c = Secondary fault-fissure in primary ore. Filled by clay holding ore fragments. d = Shattered "spar" ore, galena etc. e = Secondary fault-fissure traversing vein: The clay filling Is black and holds secondary antimonial sulphides of silver making good ore. jf = Shattered and altered gneiss holding films of rich sulphide ore and vugs of )^"across holding rich silver sulphides. 5' = Gneiss A = Vein of polybasite Face of Broadwater Vein Exposed August, 1897, on Stope Below Third Level. the partially oxidized ores extend down 170 feet below the out- crop, and, in pipes and along drainage fissures, reach even greater depths. Generally, however, there is another zone of alteration below the level of these altered or highly altered ores — the zone of enrichment. This secondary ore also occurs in the cracks of the shattered country-rock, forming the vein- matter where it is associated with secondary quartz (Fig. 9). Very commonly the polybasite occurs in crystalline masses showing no definite crystal outlines. In the open spaces and ENRICHMENT OF GOLD AND SILVER VEINS. 495 vugs of ihe vein, crystallized specimens have been found associ- ated with barite. It is possible, of course, that this may be due to the meeting of surface- and of deep-seated waters. The zones of impoverishment, of enrichment, and of unaltered primary sulphides recognized in the case of the copper veins are clearly present here, though the uppermost is of limited extent, and the zones are not so sharply or definitely separated from one another as they are in copper deposits, owing to the later fissuring of the vein-filling allowing the secondary enrichment to be mixed with the unaltered sulphides. Polybasite is said by Dana to alter to stephanite and pyrite. In the Neihart ores the mineral seems to show an alteration to pyrargyrite and pyrite, and the former, in turn, changes to native silver in the upper zone. An example of the economic necessity of carefully observing secondary fractures and accompanying enrichment is shown by the Eva May mine, on Cataract creek, near Boulder, Montana. In the early history of the mine much high-grade ore was found consisting of pyrite, together with more or less galena, blende and chalcopyrite, the whole impregnated with scattered bunches of rich antimonial sulphides of silver. The vein is a large one and shows thick ore-shoots of pyritic ore, but the bulk of this away from the enriching fracture is too poor to work. Concentrates made from it have, according to analysis in the Survey laboratory, the composition shown under I. in the fol- lowing Table : I. II. Per cent. Per cent. „.,. ., . , t Insoluble, . . 19.81 10.31 Sihca, Alumina, etc. f „ _^ „ ' .„ -c J Part of ore, . . .50 .56 Fe 24.08 23.59 Pb 9.83 23.93 Zn 6.00 5.74 Cu, 4.56 1.29 Bi, 33 .12 m, 03 trace Ca, none none As 2.01 .98 Sb, .55 trace S 32.30 33.48 100.00 100.00 Gold, ounces per ton, .10 .75 SUver, " " " 1.85 7.25 496 ENRICHMENT OF GOLD AND SILVER VEINS. This material, which looks so much like good ore, has caused the mine to shut down. It will be seen that it furnishes an admirable material for alteration and concentration of silver according to the process outlined in the first part of this paper. In point of fact the pay-streaks and pay ore-bodies of this property are of secondary origin, and it is only by confining mining operations to such places that the mine can be made to pay. Another example is the Frohner mine, 10 miles south of Helena, at the head of Clancey creek. The main ore is a mix- ture of galena and pyrite, and occurs in sufficient abundance to warrant working, if it were not too low in grade. Yet the primary ore, where not enriched, will not pay for concentra- tion. The composition of this ore is given in column II. of the table on the preceding page. The ore has been concen- trated until it carries but 10.3 per cent, silica, with iron and lead present in nearly equal proportions. The sample was carefully collected, so as to represent a true average of the con- centrates as shipped. The rich ore of the mine, carrying as high as 200 oz. of silver per ton, was found near secondary frac- tures, and consisted, I am told, of pyrite and galena with films and nests of antimonial sulphides of silver. A considerable list of mines in Montana might be mentioned in which such phenomena have been observed by the writer. Of many others the past history shows rich surface-ores, be- coming rapidly leaner in depth. This is true not only of silver mines, but of gold-silver properties, near Marysville, Montana, in California at the Mojave mines, etc. "Where telluride ores occur, the only enrichment observed has been due to superficial alteration, as has been observed in the Judith mountains, Little Kocky mountains, the Dolcoath mine near Elkhorn, and the Mayflower mine, all in Montana. The Effect of Physiographic and Climatic Changes. Active degradation favors the accumulation of enrichments, while prolonged degradation of a region, resulting from phys- iographic revolutions, may result in successive migrations of material and the accumulation in a relatively shallow zone of the metals derived from many hundreds, and possibly thou- sands, of feet of the vein Forn aw;ay i^ the degradation of the ENRICHMENT OF GOLD AND SILVER VEINS. 497 land. Climatic conditions, rainfall or aridity, warmth and rapid alteration of vein fracture are agents affecting surface- weathering, and hence, also, enrichment. Active degradation of a region, that is, rapid weathering, favors enrichment by the quickness with which it removes the upper already leached part of the vein, so that a larger amount of vein matter is lixiviated in a given time than would result from slower wasting of the land. Such enrichments are favored by high altitudes. Moreover, the mountainous regions are those in which secondary fractures are most apt to be found. Changes of Water-Level. Prolonged degradation is favorable for a similar reason, since time is a factor in enrichment, and changes in elevation, etc., affect the rate and progress of decay of the vein ; while the crustal movements accompanying physiographic changes favor fractures of the earlier deposit, which , give facilities for leaching and spaces for deposition. If a region passes through several cycles of erosion and elevation, it is evident that their result is likely to be a succession of enrichments in which not only the original ore is leached, but the earlier enrichment- deposits migrate downward. At Butte, Montana, the region has passed through several very pronounced changes in eleva- tion since the formation of the veins in tertiary time. In early Tertiary time the present topography was blocked out, and the mountain ranges and deep intervening valleys were carved. This was succeeded by earth-movements by which the streams became clogged or the valleys dammed, forming lakes ; while volcanoes broke out at numerous places and showered ashes and scoria over the region. The valleys were silted up or in part filled by volcanic debris, before crustal movements drained the valleys and altered the divides. More recent movement, possibly still continuing, is marked by faults and a reversing of stream-courses. The old valley at Butte is filled by hun- dreds of feet of debris, and a mountain wall 2500 feet high marks a north and south fault>line. These changes all caused a migration of water-level facilitating the processes of weather- ing and enrichment, and the great bodies of rich copper-ores of the region are believed to be in part due to this cause. 498 METASOMATIC PROCESSES IN HSStlKE-VBINS. Metasomatic Processes in Fissure-Veins.* BY WALDEMAE LINDGREN, WASHINGTON, D. C. (Washington Meeting, February, 1900.) CONTENTS. PAET I. — G-ENERAL Features. PAGE Purpose and Extent of Inquiry ; Definitions (Fissure- Veins, Metamorphism, Metasomatism, Impregnation, Cementation, Weathering); Metasomatism in Connection with Mineral Deposits, Especially Fissure- Veins ; Calcu- lation of Analyses ; Criteria of Metasomatism ; Crystallization of Sec- ondary Minerals in Other Bodies ; Secondary Alteration of Veins ; Structure and Composition of Metasomatic Vein-Rocks and Their Rela- tion to General Metamorphism, 499 PART II. — Minerals Developed by Metasomatic Processes IN Fissure- Veins. Quartz, including Chalcedonite and Opal ; Eutile and Anatase ; Fluorite ; Calcite ; Magnesite and Dolomite ; Siderite ; Muscovite and Sericite ; Biotite ; Chlorite ; Pyroxene and Amphibole ; Garnet ; Epidote ; Ortho- clase ; Albite ; Tourmaline ; Topaz ; Kaolinite ; Zeolites ; Pyrite ; Mar- casite ; Chaleopyrite ; Arsenopyrite ; Pyrrhotite ; Galena ; Zinc-blende and Other Sulphides ; Tellurides ; Native Copper ; Gold and Silver, . 521 Resistant Minerals, 539 PAET III. — The Fissure- Veins Classified According to Metasomatic Processes. 1. Topaz-Cassiterite Veins (Altenberg and Zinnwald, Saxony ; Mt. Bischoff, Tasmania), 540 2. Scapolite-Apatite Veins, 545 3. Tourmalinic Gold-Copper Veins, . 546 4. Biotitic Gold-Copper Veins, 564 5. Propylitic Gold and Silver Veins (General Features; Comstock Lode ; The Veins of Nagy^g, Hungary ; Pachuca, Mexico ; New Zealand ; Alaska ; Silver Cliff, Colorado ; Silver City, Idaho), .... 585 6. Fluoritic Gold-Tellurium Veins (Cripple Creek ; Other Occurrences), . 574 7. Sericitic and Kaolinitic Gold- and Silver-Veins (General Remarks ; The Freiberg Veins ; De Lamar, Idaho ; Summit District, Colorado), . . 578 8. Sericitic and Calcitic Gold-Silver Veins (The Gold-Quartz Veins of Cali- fornia ; Idaho Types ; San Juan, Colorado ; Treadwell Mine, Alaska ; Gold Veins of Ontario, Canada ; Schwarzwald Fissure-Veins), . . 584 * Presented by permission of the Director of the U. S. G-eological Survey. MBTASOMATIC PROCESSES IN FISSURE-VEINS. 499 9. Silicic and Calcitic Cinnabar- Veins, 595 10. Sericitic Copper-Silver Veins, 596 11. Silicic and Dolomitic Silver-Lead Veins (Aspen, Colorado), . . 596 12. Sideritic Silver-Lead Veins (Wood Biver, Idaho ; Coeur d'Alene Moun- tains, Idaho), 599 13. Sericitic Silver-Lead Veins (Clausthal Veins ; The Democrat Vein, Hailey, Idaho), 602 14. Zeolitic Copper- Veins 606 Observed Alteration by Ascending Waters, 608 Conclnsions, 609 PART I. General Features. Purpose and Extent of Inquiry. A study of the changes in rocks contiguous to ore-bearing fissures is essential to a thorough understanding of the genesis of such deposits. I^evertheless, comparatively little work has been done in this direction, though many mining geologists (for instance, v. Groddeck) long ago emphatically declared the neces- sity of such investigations. Ores and structure have been dealt with in detail; but the important changes which adjacent rocks of known composition have suffered are too often briefly dis- missed, or even incorrectly indicated. It is the purpose of this review to collect the scattered data relating to the alteration of rocks near or between fissures ; to indicate the principal active processes; to classify the veins, if possible, according to the different phases of alteration accompanying them ; and, finally, to draw some conclusions from the facts thus grouped. This first attempt to systematize the metasomatic data of fissure- veins is by no means complete : only such parts of the American and foreign literature are represented as were deemed to be of vital importance. The discussion principally involves the changes which the country-rock has undergone, whether they have resulted in the formation of ores or not; and, in the sec- ond place, some space is devoted to such alterations as fissure- veins already formed sometimes sufl'er through certain sec- ondary agencies. I have excluded, however, all references to weathering, or to the decomposition of vein-materials near the surface by waters containing free oxygen. It is often difficult to draw the line between normal fissure-veins and deposits not 32 500 METASOMATIC PROCESSES IN FISSURE-VEINS. to be regarded as such, strictly speaking, but clearly due to the same genetic causes ; and some of the latter class have been included in this discussion. Definitions. Fissure- Veins. — For the present discussion, a fissure-vein may be regarded as a mineral mass, tabular in form, as a whole, though frequently irregular in detail, occupying or accompany- ing a fracture or set of fractures in the enclosing rock; this mineral mass has been formed later than the country-rock and the fracture, either through the filling of open spaces along the latter, or through chemical alteration of the adjoining rock. Such alteration does not ordinarily extend far from the fissure. Only in regions where the vein-forming agencies have acted with unusual intensity, a partial alteration may extend over larger areas. These zones of alteration being genetically con- nected with the veins proper, must necessarily also be consid- ered in this discussion. Metamorphism. — This term, meaning strictly a change of form, was proposed by Lyell in 1831, and has since been em- ployed in a wider sense, so as to cover any change in the com- position or structure of a rock, through whatever agency, and whether with or without gain or loss of substance. Metasomatism. — This name, meaning a " change of body," is given to that variety of metamorphism which involves a change in the chemical composition of rocks, by the addition or sub- traction of substance.* The terms " replacement," " substitu- tion," " alteration," etc., have been employed in discussions of metasomatism with different shades of meaning. A review of the classification of pseudomorphs, which form the origin of our knowledge of metasomatism, will throw light upon the nomenclature of the subject. The occurrence of organic remains, consisting of material of which they were certainly not originally composed, called atten- tion to the remarkable transformations now known as meta- somatic replacements. As instances, we may recall corals changed into quartz, belemnites converted into barite, and shells of bivalves or gasteropods transformed into pyrite, chalcocite, * Dana (Man. of OeoL, 4tli ed., p. 314) proposes for the same process the term " metachemic ;" but this has not found general acceptance. METASOMATIC PROCESSES IN MSSURE-VEINS. 501 sphalerite or specularite. To the same order of phenomena be- longs the silicified wood, in which the organic substance has been removed and replaced with silica so delicately as to pre- serve in minute detail the original organic structure. This re- placement is probably due to the precipitation of silica from so- lution by the acids generated in the decay of organic matter. More rarely, wood and plant-remains may be replaced by py- rite, chalcocite, galenite, cinnabar, barite, limonite, malachite, etc. But it is the study of pseudomorphs, showing one mineral appearing in the crystal-form of another, that has led to a more detailed knowledge of the chemical laws which govern these remarkable changes. Here was conclusive proof that one min- eral, definitely crystallized, had changed into another, some- times totally different, substance. Naumann says of pseudo- morphs : "Their importance cannot be overestimated, because they enable us to study successfully the laws of the processes which are constantly acting in the rocks and constantly changing them ; for the pseudomorphs represent only one special case of the grand process of chemical alteration going on in the mineral kingdom : namely, that in which the form remained in spite of the change. From these we may draw conclusions as to the chemical processes going on in rocks which may change each grain to another mineral. ' ' * Blum, who made the first extensive examination of pseudo- morphs, divided them into: (1) those produced by partial change in the composition of the original mineral (one or more elements being removed, added or introduced by substitution); and (2) those produced by a complete replacement of the orig- inal mineral with another. (This class includes both those produced by chemical replacement and by previous solution and subsequent filling.) ISTaumann, in his well-known text-book of Mineralogy, di- vided the pseudomorphs into : (1) hypostatic pseudomorphs, formed by the mechanical deposition of substance outwards or inwards from the limiting planes, and again subdivided into pseudomorphs by covering, and pseudomorphs by filling ; (2) metasomatic pseudomorphs, formed by the alteration of the substance by means of its molecular replacement with another mineral while the form has been preserved. The metasomatic * Mineralogie, Naumann-Zirkel, 10th ed., Leipzig, p. 112. 502 METASOMATIC PROCESSES IN FISSUKE-VEINS. pseudomorphs are also designated as " alteration " ( TJynwand- lung) pseudomorphs. This, it will be noted, is the first intro- duction of the word metasomatie in technical literature. In nearly all cases, the metasomatie pseudomorphs involve chem- ical action. Naumann further divides the metasomatie pseudomorphs into three classes, in which, respectively, (a) the original and the secondary substance are identical in chemical constituents (" paramorphic " pseudomorphs); or (6) chemical alteration has left one or more elements of the original in the secondary substance ; or (c) the replacement of constituents has been com- plete, as in the substitution of galenite for calcite, or pyrite for quartz, but the process has been, nevertheless, a chemical one, since the removal and deposition have proceeded simul- taneously, molecule for molecule. The second of these classes is again subdivided, according as the change involved simply the loss of original components (as in the formation of argen- tite from pyrargyrite), or the addition of components {e.g., angle- site from galenite), or the exchange of components {e.g., sericite from oligoclase). The conceptions of Blum were introduced into English technical literature by James D. Dana,* who divided pseudo- morphs into those formed : (1) by infiltration (mechanical de- position in a mould already formed) ; (2) by incrustation (me- chanical covering of crystals) ; (3) by replacement, one mineral gradually replacing another, and assuming at the same time its form, without any interchange of elements (the process being in a certain sense chemical, and wholly different from simple deposition) ; (4) by alteration, some of the elements being re- moved or exchanged, or z^ew ones being added ; and (5) by allomorphism, without chemical alteration ; the body changing to one of the same composition but of different crystallographic system (the paramorphic pseudomorphs of ISTaumann). Somewhat extreme views, differing from the above, were advanced by T. Sterry Hunt,t who classed pseudomorphs as : (1) those produced by chemical alteration, meaning by this a partial exchange of constituents {e.g., limonite after siderite); and (2) those produced by substitution or replacement (these * American Journal of Science, vol. 48, 1845, p. 81. t Systematic Mineralogy, New York, 1892, p. 111. METASOMATIC PROCESSES IN FISSUBB-VEINS. 503 terms being evidently regarded as equivalent). The latter lie believed to be produced by deposition in spaces left by the re- moval of some other matter. The form of the original sub- stance is assumed by the material which displaces, or is substi- tuted for it, e.g., quartz after calcite, barite, etc. "While thus admitting partial alterations, Hunt makes a special case of a complete replacement, refusing to consider it as a chemical process, and regarding it always as an instance of separate dis- solving and refilling. To the theory of metasomatism, which maintains that all the chemical elements in a crystal may be removed, and by molecular processes replaced with foreign substances, Sterry Hunt was strongly opposed. Pseudomorphs of the second group proposed by him are, as is well known, of frequent occurrence, and correspond to I^au- mann's hypostatic division or pseudomorphs formed by me- chanical deposition. Spaces of dissolution, subsequently filled, are also common enough in rocks, and may usually be readily identified as such under the microscope. But that molecular replacement, as defined by ^N'aumann and Dana, also exists, and, moreover, is of the highest importance, seems at present beyond doubt. As the essential process of metasomatism applies as well to an irregular grain as to a perfectly developed crystal, we are justified in extending the conception to aggregates of grains of one or several minerals ; in other words, to rocks and mineral, aggregates in general. In this sense C. R. Van Hise* has de- fined metasomatism as " the process of metamorphism by which original minerals are partly or wholly altered into other min- erals, or are replaced by other minerals, or are recrystallized vsdthout chemical changes, or one or all of these together." S. F. Emmons has defined metasomatism as follows :t ' ' By metasomatic exchange is meant an interchange of substance without nec- essarily involving, as does pseudomorphism, the preservation of the original form of the substance replaced, or even of its original volume." A second definition, based on the consideration that practically simultaneous solution and deposition could certainly be proved * "Principles of Pre-Cambrian Geology," \&ih Ann. Sept., U. S. Oeol. Sur., part i., p. 689. f U. S. Geol. Surv., Monogr. XII., p. 565. 504 METASOMATIC PROCESSES IN FISSURE-VEINS. for many cases, where the exact proof of chemical-molecular replacement could not be furnished, is given by Mr. Emmons as follows:* " By metasomatic interchange I understand an interchange of substances, hut not necessarily molecule by molecule, in such a manner as to preserve the original structure, form or volume of the substance replaced." The fundamental difficulty is that the final result does not always indicate the particular pseudomorphic process which has preceded. Mechanical deposition, for instance, may follow so closely after dissolution, that the two processes really appear as one. It may also be said that molecular replacement is diffi- cult to prove, as molecular processes cannot be followed with the microscope ; and this is, in a sense, true. "We may assert, however, that, with the highest magnifying powers, we are able to follow the transformation of quartz, for instance, into seri- cite, or into calcite, or into siderite, without finding the slightest indication of an intermediate stage of open space. The fiber and blades of sericite project into the quartz without the slightest break in the contact; the rhombohedrons of sider- ite develop in quartzite, their crystal faces cutting across the grains without any interstices. Perfect tourmaline prisms de- velop in feldspar grains, and sharp cubes of pyrite in primary granitic quartz. In cases of complete molecular replacement, such as galena after calcite, the replacing mineral was probably present in the solution, partly dissociated or ionized. The solution of a cer- tain quantity of the original mineral caused the separation of a corresponding quantity of the ions of the replacing substance, according to physico-chemical laws. If carried out on these lines, the process is necessarily molecular and chemical. "Where there were two solutions^one dissolving, the other depositing — and where a certain time intervened, the process is a mechan- ical one and should not, I think, be considered metasomatic. In many cases the distinction may be very difficult to draw. In conclusion, metasomatism might be defined as the pro- cess by which a mineral has suffered, through chemical pro- cesses, a partial or complete change in its chemical constitution. Eocks or aggregates of minerals are " metasomatic," if any or * "The Genesis of Certain Ore-Deposits," Tram., xv., 128, 1886. METASOMATIC PROCESSES IN PISSUEE-VEINS. 505 all of the constituent minerals have undergone such changes. This definition excludes the process of paramorphism which, as already emphasized by Naumann, is exceedingly rare. In the use of the term alteration it would perhaps be best to follow Dana and let it mean a partial change of substance in a mineral or rock. Decomposition, it would seem advisable to re- strict to the cases in which a mineral or rock is dissolved into its component parts ; and a principal use for it would be found in the processes of weathering. As has been shown, the words replacement and substitution have been used in very different ways. The majority of recent authors use them both as equivalent to metasomatism. Dana, however, applies replacement to a complete exchange of sub- stance, reserving alteration for a partial loss, gain or interchange of elements ; while Sterry Hunt gives the name of replacement or substitution to mechanical dissolution and the filling of the resultant cavities. The chemist has, however, a distinct definition of substitution as " the replacing of one or more elements or radicals in a compound by other elements or compounds;" and it would probably be best to adhere to this, and discard substitution as a synonym for metasomatism or alteration. Replacement is, in its general meaning, nearly identical with substitution, although it has no such distinct chemical use. It would seem advisable to regard it as a synonym of metasoma- tism, distinguishing, for the sake of convenience, between par- tial and complete replacement. This is contrary to Dana's dis- tinction ; but the word has been used so generally during late years in this wider sense that it seems best to retain this mean- ing for it. Impregnation. — This term has been applied in so many differ- ent ways — ^to primary disseminations ; to minerals formed by replacement ; and to the filling of cavities or interstitial spaces in rocks — that it might well be rejected altogether as a genetic term, and used only in a structural sense, as descriptive of finely divided material disseminated in a diflfering mineral or rock-mass. Cementation. — This term, proposed by Prof C. R. Van Hise,* * " Pre-Cambrian Geology." 16lh, Ann. Report U. S. O. S., part i., p. 684. 506 METASOMATIC PEOCBSSBS IN FISSURB-VBINS. is convenient and expressive for the purpose of indicating fill- ing of interstices in porous or shattered rocks. Cementation assumes importance in proportion to the porosity of the rock, which, in sandstones and tuflts, may reach 10 or 20 per cent. In most intrusive igneous rocks the porosity is so small as to be a negligible quantity. Weathering. — Under this name are included the changes of rocks near the surface in cohesion and composition, due to the decomposing and oxidizing action of percolating waters above the permanent water-level. The tendency of weathering is to destroy the rock as a geological unit. The final results of metasomatic action are a few resistant minerals, such as quartz, kaolin and limonite. The formation of serpentine, chlorite, epidote and (ordinarily) pyrite is not weathering, but is due to more deeply seated causes. The G-erman usage of Yerwitterung, to cover all secondary changes, due to weathering, thermal and other causes, seems highly objectionable, and especially apt to lead to many misconceptions. In view of the difference of usage as to many of the above definitions, it is to be hoped that writers upon this subject will take pains to indicate the sense in which the various terms are employed by them. Metasomatism in Conn£ction with Mineral Deposits, Es-pecially Fissure- Yeins. It was not long before the principles of metasomatic action, learned by the study of pseudomorphs, were applied to larger masses of rocks. This led, perhaps inevitably, to exaggerated notions, such as that of the formation of true granites from sediments and limestone,* etc. ; and this undue extension was followed by a reaction, exemplified in Sterry Hunt's writings. The observation that ores may be found, not only in the clearly defined vein-filling, but also in the rock adjacent to the fissure, is contemporaneous with almost the earliest scientific records of mining. Sandbergerf mentions the occurrence of masses of native silver, found in 1786 in the altered granite of certain Schwarzwald veins, which greatly astonished the old miners. Vogelgesang,J in Cotta's " Gangstudien," describes * G. Bischof, Ckem. Oeol., Bonn, 1866, vol. iii., p. 34. t Erzgange, part ii., p 418. J Vol. ii., Freiberg, 1854, p. 78. METASOMATIC PROCESSES IN EISSURB-VBINS. 507 the dissemination of argentite, native silver, and various sul- phides, in the gneiss adjoining certain veins near Freiberg. But whether or not it contains ore, the rock adjoining a vein is very commonly softened, bleached and altered for some dis- tance away from the fissure. This phenomenon has been ex- plained in two radically different ways : 1. Bischof says:* "As we find ores in veins, proportionate in quantity to the alteration of the country-rock, what other relation can be thought to exist between the two facts than that the abundance of the ore is a result of this alteration ?' ' Sandberger says if "The extent of the alteration on both sides of the vein corresponds with the area from which the products of leaching have been carried to the vein." 2. The opponents of these views say that the narrow zone of alteration, intense next to the fissure and gradually fading away within a short distance from it, most clearly indicates an agency within the fissure, acting with gradually diminishing energy on the adjoining strip of rock. They also point out that Bischof 's premise, i.e., the coincidence of richness of vein and extent of the altered zone, is not true as a universal proposition. And they show, further, that as the whole altered zone has, in many cases, received an addition of the same metals as are contained in the vein which may more than counterbalance its losses of other constituents, Sandberger's conclusion can certainly not have a general application ; and finally, that, in those veins which have no gangue, but in which the ore has accumulated in the rock during the alteration, the incorrectness of that con- clusion is particularly apparent. Veins carrying cassiterite early attracted attention, as being almost always accompanied by ore impregnating the surround- ing country-rock. The metasomatic character of the process was first shown by DaubreeJ and later by Cotta,§ both of whom, in support of their views, call attention to the well- known occurrence of cassiterite as a pseudomorph after feld- spar. Both explain the alteration as due to gradual replace- * Chem. Oeologie, Bonn, 1866, vol. iii., p. 666. t Erzg'dnge, vol. i. , p. 149. t Ann. d. Mines, 1841, xx., pp. 65, 72, 83. J IXe Lehre von den Erdagerstatten, Freiberg, 1859. 508 MBTASOMATIC PROCESSES IN FISSURE-VEINS. ment by the agency of thermal waters. This explanation was substantiated by more recent and detailed investigations ; for instance, by Richard Pearce (1864) and Lelfeve Foster (1877) in regard to Cornwall; and by A. "W. Stelzner (1864) for Geyer, Saxony. The views of Cotta concerning the alteration of the wall- rocks or veins are well expressed in the following paragraph:* ' ' When lodes are accompanied by ore-impregnations, it is to be assumed that generally the solutions from which the materials of the lode were precipitated — they may have been aqueous, igneous-fluid, or gaseous — also penetrated the wall- rock and there caused certain deposits in fine clefts or in the rock itself. In the last case, crystals have made room for themselves by their power of crystalliza- tion ; or an ore took the place of the mineral dissolved ; for example, tin-ore, that of feldspar." This quotation shows plainly the clear conception which Cotta had of the alteration of rocks, as due, not only to filling of pores and cracks, but also to processes of replacement active within the rocks. Although Cotta made no special division of replacement- veins, he was well aware of their occurrence and perfectly able to distinguish them from filled spaces. Describing the gold- veins of Tauern (Austria),t he says that they have not the character of clearly opened and filled fractures, but consist of several parallel tight fissures, between which lies more or less impregnated and altered country-rock. The gold penetrates into the country-rock from the fissure, and the tenor decreases gradually with increasing distance. In 1873 Posepny published his famous examinations of the Raibl deposits.! These are not connected wi*h fissures, but deserve mention, since entirely similar processes are active in fissure-veins. Posepny found that carbonate of zinc had re- placed carbonate of lime "by metamorphic processes." Yon Groddeck, in his well known text-book (1879), includes " metamorphic " deposits in his system, but limits them to re- placements of limestone by zinc-, iron- and manganese-miner- als. At about the same time, Stelzner, in his lectures, intro- duced a corresponding division of " metasomatic deposits," /t * A Treatise on Ore-Deposits. (Prime's Translation.) New York, 1870, p. 90. t -Die Lehre von den Erzlagerstaiten, Freiberg, 1859, part ii., p. 318. t Jahrbuch d. k. k. Oeol. Mekhsanatalt, xiiii. , 1873, p. 317. METASOMATIC PROCESSES IN EISSURE-VEINS. 509 though they were still limited to a relatively small number of the irregular masses in limestone. Raphael Pumpelly was, I believe, the first who applied the principles of metasomatism to ore-deposits in this country, in describing the copper-deposits of Michigan (in part fissure- veins) in vol. i. of the Geological Survey of Michigan (1873), and in his noted paper on the " Metasomatic Development of the Copper-Bearing Rocks of Lake Superior."* The copper, to a great extent, replaces other minerals. In 1879 J. A. Church published a volume on the Comstock mines,t in which he maintained the origin of the quartz by replacement acting from a number of narrow fissures. In 1882 S. F. Emmons first published the results of his ex- aminations of the Leadville silver-lead deposits, in which it was shown that these were entirely formed by metasomatic replace- ment of the limestone by galena and other minerals. A little later, J. S. Curtis published his first results in regard to the silver-lead deposits of Eureka, Nevada, in which he arrived at substantially the same results as Mr. Emmons. Though these deposits, as well as those of Leadville, are not to be regarded as fissure-veins, it was evident that the same process might be considered as active along fissures, provided the waters circu- lating in them had the composition attributed to those of Lead- ville and Eureka. The results obtained were certainly of the greatest interest to students of fissure-veins, and threw a new light on many obscure facts. During the following years, Mr. Emmons, who had visited a great number of mining regions in the "West containing fissure-veins, published several papers,J in which he maintained that, for a great number of the veins formerly considered as containing ore deposited in open spaces, another and much more plausible explanation could be advanced, namely, that, in many cases, the fissures had not been opened to any noteworthy extent, but only so much as to admit the passage of the mineral-bearing waters. The latter had attacked the rock on either side of the fissure, and, by a process of metasomatic replacement, had deposited ores in the place of * Proc. Am. Acad. Arts and ScL, vol. xiii. (new series, vol. v.), 1878, pp. 253-309. t The Comstock Lode. New York, 1879. I " The Genesis of Certain Ore-Deposits," Trans., xv., 125,1886; "Structural Belationsof Ore-Deposits," Tram., xvi., 804, 1887. 510 MBTASOMATIC PROCESSES IN FISSURE-VEINS. the simultaneously dissolved rock-constituents. Applying this process to composite veins, consisting of a number of narrow fissures, and considering that gradual replacement had taken place, extending into the rock on each side of each smaller fissure, Mr. Emmons succeeded in showing how, under certain circumstances, a banded structure such as had ordinarily been attributed to the filling of open spaces could to some extent result from the process of replacement. During the following years the theory of the formation of fissure-veins by replacement rapidly gained ground; and for some time it seemed as if the old view of deposition in open spaces were doomed to complete extinction. Carried away with the importance and interest of the metasomatic theory, many geologists and mining engineers extended its teachings beyond proper bounds, and were prone to speak of any fissure-vein as unquestionably a product of replacement. Attempts were made to show that open fissures could not exist unsupported, at any rate below the most superficial depths ; and facts and proofs were too often neglected for bare assertions that metasomatic replacement had taken place. Posepny, in the discussion of his paper,* protested against this unwarranted extension of a most excellent and well-founded theory, and stated with some force that the experience and observations of a hundred years were not to be thrown away without very careful scrutiny. The pendulum had now swung to its extreme position ; and it was not unnatural that a reaction should follow. It gradually be- came clear on the one hand that open spaces can and do exist down to a depth of many thousand feet,t and that these open cavities may be filled by the action of mineral-bearing water. On the other hand, it is evident that there is ample room for processes of replacement in fissure-veins, which may either af- fect the surrounding country-rock without producing notable amounts of ore, or, on the other hand, may attack it in such a way as to convert it wholly or partially into valuable minerals. Some kind of metasomatic action is usually noticeable in the rock adjoining the fissure. But it is not to be denied that in many cases this alteration is very slight; and in a few veins it may be entirely absent. * This volume, p. 239. t Van Hise and Hoskins. In "Principles of Pre-Cambrian Geology," 16th Ann. jRept, U. S. Oeol. Swrv., part i. MBTASOMATIC PROCESSES IN FISSURE-VEINS. 511 Calculation of Analyses. In order to trace the metasomatic changes by which one mineral has resulted from another, it is necessary to know the composition of each, and the change in volume during the al- teration. "Without the latter the problem is capable of many solutions, any one of which may be possible, though not true. Only when some definite data, such as the constancy of one constituent, are available, can the changes be determined with- out reference to relative volumes. For instance, the percentage- composition and specific gravity of argentite and pyrargyrite are as follows : S Ag Sb Sp.G Per cent. Per cent. Per cent. Argentite (Ag^S), . . 13.0 87.0 7.0 Pyrargyrite (AgsSbSj), . . 18.0 60.0 22 5.8 Pyrargyrite may be altered into argentite ; but analyses alone give no complete clue to the character of the alteration. Sup- posing, however, that we have found that 1000 cub. centim. of pyrargyrite becomes 670 of argentite ; then we may calculate that about 9 kilos of sulphur and 22 of antimony have been removed from 100 kilos of pyrargyrite during the process, while the silver has remained constant. Supposing, again, that we have foundpyrargyrite altered into argentite without change of volume; then from the original 100 kilos, 2.24 of sulphur and 22 of antimony have been lost, and 44.8 of silver added. To exemplify further the many ways in which even simple metasomatic problems can be solved, we may take the well- known change of olivine to serpentine, consisting, as ordinarily considered, in a simple hydration of the original mineral. And, in order to simplify the matter still further, we may substitute for olivine the pure magnesium orthosilicate, occurring as a mineral under the name of forsterite, and assume the resulting serpentine to contain no iron. The formulas show that serpen- tine cannot be derived from olivine or forsterite by means of a simple addition of water. It may, however, be derived from enstatite (which is a magnesium metasilicate) and forsterite, as follows : Mg^ SiO, + Mg SiO, + 2H,0 = H; Mg, Si, 0,. Translated into kilograms, this means that 50.8 kilos of fors- terite -1- 36.2 of enstatite + 13 of water is equal to 100 kilos of 612 MBTASOMATIC PROCESSES IN FISSURE-VEINS. serpentine. This again translated into volumes by aid of tlie specific gravities, means that 15.8 cb. cm. forsterite + 11.7 cb. cm. enstatite + 13 cb. cm. water is equal to 40 cb. cm. serpentine, or that 27.5 cl^. cm. anhydrous silicates are needed to produce 40 cb.cm. serpentine. In other words, the increase of volume during the process of serpentinization amounts to nearly one- half; — ^the specific gravity of forsterite being 3.24, that of en- statite, 3.1 ; and that of serpentine, 2.5. Serpentine may also be obtained by adding silica and water to forsterite. Thus, 3Mg, SiO^ + 4H,0 + SiO, = 2H^ Mg, Sifl,. Calculating in the same manner as above, we find that 131 cb. cm. of forsterite results in 221 of serpentine, which represents an increase in volume of somewhat more than two-thirds. Still another way of derivation is by subtracting MgO and adding water, as shown by the following formula : 2Mg, SiO, + 2Hp= H, Mg3 Si,0, + MgO. This again is equivalent to the formation of 110.4 cb. cm. of serpentine from 86.3 of forsterite, or an increase of volume of only a little more than one-fourth. Many other formulas could be put forward, which would explain the formation of serpentine, each showing a diflference in the relation of volume of the secondary mineral to that of the fresh. Even in this simple case it might be, in any given problem occurring in nature, extremely difficult to decide with confidence which particular formula should be applied. The problem only becomes definite when we positively know the relation of volume of original substance to that of secondary substance. It is perhaps superfluous to add that the mere knowledge of specific gravities does not give this relation of volumes. When we have to consider metasomatic processes affecting rock-masses, aggregates of from two to six or more minerals, the complexity of the problem becomes immensely greater ; for each of these constituent minerals may have suffered different metasomatic changes. Some may have remained unaltered, while others have been completely replaced; and others, again, may have lost, or gained, or exchanged one or more elements. To calculate the sum total of these changes is often an ex- tremely puzzling task. METASOMATIC PROCESSES IN FISSURE-VEINS. 513 The mineral composition of the altered, as well as the fresh, rock may be accurately calculated by methods known to petro- graphy, if its analysis, as well as those of the constituent min- erals, be available. Even where the latter are only approxi- mately known, a fairly accurate calculation may be made. Thus, for instance, from an analysis of an altered granite containing pyrite, calcite, magnesite, siderite, sericite and quartz, the per- centages of these minerals may be obtained with fair accuracy. The chemical changes suffered during any alteration of a rock may be considered by unil^weight of original substance or by unit-volume of the same. The results will be identical if both rocks are non-porous, or if both have the same porosity. If the actual additions and subtractions should happen to bal- ance, then the percentage-analyses offer, by comparison, direct evidence as to the quantitative alteration. If, besides, during the alteration, the porosity of the two rocks remain the same, then the changes measured by unit-weight will be identical with those measured by unit-volume. This exact balancing of gains and losses, however, is, of course, exceedingly rare. If we do not know the relation of volumes between the two rocks, other ways must be sought for, at least, a partial solution of the problem. If we know that one or more new constituents have been added, we may subtract these, recalculate on 100, and then compare the analyses. This method in many cases leads to fairly correct results ; but it must be applied with the understanding that, ordinarily, it will only give approximate results ; and that, if there be many partial additions and sub- tractions, the inaccuracies may be very great, and actual losses and gains may appear reversed. If we know that one constituent has remained constant, with neither gain nor loss (as the silver in the example cited above), then correct results may be obtained by recalculation on this assumption, as has been shown. This mode of calculation has been used by Scheerer, J. Roth, and lately also by Prof. G. P. Merrill, in his book on " Rocks and Rock-Weathering." It is only rarely, however, that we are able to recognize this con- stancy ; for nearly all constituents undergo some change in the alterations of wall-rocks. Even alumina, often considered to be nearly insoluble, shows great changes in some altered rocks. Besides, if we base recalculation on some compound of which 514 MBTASOMATIC PROCESSES IN FISSURE-VEINS. but a small percentage is present, the multiplication of errors may play havoc with the result. Altogether, this mode of ascertaining gains and losses must be applied with the greatest caution. Any given analysis of fresh and altered rock may correspond to several very different mineral compositions. For any given mineral composition, the constancy of one constituent during the change to another (also known) mineral composition, de- termines the change in volume involved (not considering po- rosity). When the change in volume can be directly ascer- tained, we are definitely able to obtain the absolute gains and losses suffered by unit^volume of the rock; and this comparison is ordinarily the one which throws most light on the processes involved. But relations of volume are difficult to obtain with certainty, especially in regard to a rock made up of a number of minerals which have suffered diflferent changes. As a rule, in fissure-veins, the replacing minerals are denser than those replaced ; so that, if the rock remained compact, there would be a decrease in volume. But as there usually are no indica^ tions of compressive stress in the altered rock, the result of this replacement of lighter by heavier minerals will be a po- rosity expressed by a notable difl^erence in the experimentally determined specific gravity of the rock and that calculated from its known mineralogical composition. This may, in some cases at least, justify the assumption that the rock has not changed its volume as a whole ; and if this be true, a direct comparison between equal volumes of fresh and porous altered rock is practicable. Should it appear probable that an actual change of volume has taken place, either by expansion or contraction, it will ordinarily be a difficult matter in each case to ascertain the exact amount of this change, without which knowledge the calculations cannot be carried out. If there is porosity, the changes by unit-weight of original substance may differ greatly from those obtained by unit-volume; hence porosity is a factor which must not be overlooked. One method may indeed indi- cate the very opposite of the other. For instance, by the first way, it may be ascertained that a rock has gained several per cent, of its weight; while the other method may show that an actual loss per unit-volume of original rock has taken place. METASOMATIC PROCESSES IN FISSURE-VEINS. 515 In the considerations outlined above, it is assumed that the rocks to be compared have undergone no change of volume since their removal from the surrounding mass. In regard to the fresh rocks, there is, as a rule, little fear of this. Certain altered rocks, however, easily soften or crumble when exposed to the air, probably indicating that an increase in volume is taking place. An exceedingly slight action of this kind would evidently be sufficient to break up the rock if it were not con- fined. There is, therefore, little reason to fear that such change of volume has taken place, if the specimens of altered rock remain firm and solid. Oriteria of Metasomatism. Considerable space was devoted to this subject in the dis- cussion of Posepny's paper in the Transactions of the Insti- tute,* and it may therefore be passed with brief notice. It is not always easy to be sure whether metasomatic action really has taken place, and in deciding this question the great- est caution must be observed. The mere occurrence of two minerals together by no means proves that one has been de- rived from the other. The chief difficulty is to draw the dis- tinction between molecular processes involving simultaneous dissolution and precipitation, on the one hand, and previous dissolution and subsequent precipitation on the other. The only decisive criterion is that of metasomatic pseudo- morphism, involving the proof (generally to be furnished by microscopic study) as to whether simultaneous dissolution and deposition have actually taken place. The most satisfactory proof is the distinct alteration of well-defined crystals (or, at least, well-defined grains) of the original mineral into the sec- ondary mineral, in such a way that the latter projects into the former in prisms or fibers, having crystalline outlines. Another proof is afforded by sharply defined crystals of the secondary, embedded in the primary mineral, without any break between their surfaces; but in this case it must be clear that the re- placing mineral is really secondary, and was not formed before the primary. Another satisfactory proof is given when, for instance, in a sandstone, the newly formed mineral has in part * This volume, p. 188. 33 516 MBTASOMATIC PROCESSES IN FISSURE-VEINS. a crystalline form, and its surfaces squarely intersect the grains of clastic material which it partly replaces. There are many other available criteria such as the enlarge- ment of fissures in the replaced mass. An instance is shown in Fig. 30, representing a veinlet of quartz formed by filling a small open fissure, and adjoined on one side by galena, which extends most irregularly into the adjoining quartzite. (See also Fig. 28*.) The retention of the structure of the original mass by the secondary replacing minerals is also an excellent criterion, provided it be identified beyond doubt. Thus, for example, certain porphyritic rocks have suffered nearly com- plete silicification, but preserve almost entirely the outlines of phenoerysts and the structure of the ground-mass. The occur- rence of remaining nuclei of unaltered rock is sometimes an available criterion ; but it must be used with caution, and probably has given rise to misinterpretations, on account of its similarity to actual inclusions of country-rock in vein-filling. In cases of replacement by sulphides, the unaltered residual rock may be sharply defined, and may closely simulate inclu- sion. In cases of replacement by calcite or quartz, there is less of this danger, as the action is usually more gradual. If the alteration or replacement proceeds normally from the out- side of a crystal or angular mass of rock, the tendency will be towards rounded residual portions in the interior of the mass, as may often be seen in altered crystals of olivine. This criterion for replacement, suggested by Gr. F. Becker, may under circumstances prove useful. G-enerally, however, the re- placement proceeds very irregularly, owing to the effect of little cracks and fissures. Slight clay-seams may often inter- pose an absolute barrier, so that sharp contacts of replaced and fresh rocks result. The replacement of crystals or angular fragments may occur without changing in the least, even by the rounding of corners, the form of the masses. In conclusion, I would repeat and adopt the statement of Mr. Becker,t that " the theory of the substitution of ore for rock is to be accepted only when there is definite evidence of pseudomorphic, molecular replacement." * A somewhat similar and excellent illustration is given in Eickard's "Vein- Walls," Trans., xxvi., 195, from the Hillside mine, Arizona, f Discussion of Posepny's paper, this volume, p. 204. METASOMATIC PROCESSES IN FISSURE-VEINS. 517' In many fissure-veins, practically all of the economically im- portant ore has been formed by replacement ; and for these deposits the term replacement-veins is especially used. But if we do not confine ourselves to the fluctuating definition of " pay-ore," practically all fissure-veins are, to some extent at least, replacement-veins. Mr. Emmons* has suggested the following criteria for " re- placement-veins " in the narrower sense of the word : (1) absence of symmetrical banding or comb-structure in the vein-material, and of breccias of country-rock, cemented by vein-material ; (2) great irregularity in the width of the ore-bodies, which may reach very great dimensions; (3) general lack of definition between ore-body and wall-rock. Crystallization of Secondary Minerals in Other Bodies. It has been known for a long time that perfect crystals of minerals, such as quartz, for instance, may be formed in soft rocks such as shale, limestone, clay, etc. ; but concerning their exact mode of formation there has been considerable difference of opinion. Probably the prevailing view, some 20 years ago, was that the growing crystal had, by means of its force of crystallization, pushed apart the surrounding mass. This was indeed the opinion of von Groddeck, who declaresf that the formation of a completely developed crystal in a solid, rigid mass is not possible. Apparent exceptions, such as magnetite in chloritic schists, he considers as caused by development, while the rock was soft, under the influence of metamorphic agencies. There is no doubt good foundation for this view; for in magmas and solutions crystals may grow to perfect de- velopment, and if, for instance, a saturated solution of ferrous sulphate is mixed with some neutral flne powder to a soft pulp, extremely clear and sharply developed crystals of this salt will separate out. But it has gradually become apparent that it is not necessary to assume complete permeation and softening of a rock by concentrated solutions, in order to account for secondarily- developed crystals. It is now well known that the secondary development of crystals in solid material is not only a possible * XJ. S. Oeol. Sun., Folio 38, on Butte, Montana. t Die Lehre von den Lagerstatien- der Erze, Leipzig, 1879, p. 68. 518 METASOMATIC PROCESSES IN FISSURE-VEINS. but an exceedingly common phenomenon, and that it may be caused by simple metasomatic replacement of the surrounding material. The mechanical force of crystallization probably co-operates, to some extent, with the chemical agencies of replacement ; and when the surrounding mass is thoroughly softened and saturated by the depositing solutions, the former force may be alone active. The growing crystal may include parts of the surrounding rock, as is seen in Figs. 3 and 4, representing cal- cite in quartz, and in Fig. 27, showing inclusions of sericite in pyrite. This is analogous to, but not identical with, the occur- rence of inclusions of fluid and glass in crystals separating out from solutions or magmas. It is not uncommon to find new crystals of perfect development generated in a grain of another substance, such as quartz or feldspar, without any disturbance of the optical orientation of the older minerals, such as unfail- ingly would occur were the process simply one of mechanical force. 'So doubt the exchange of substance takes place through the medium of a film of water, but this is generally so exceedingly thin that the strongest powers of the micro- scope fail to reveal it. In many cases, however, the new min- eral begins to grow on the planes of small fractures, traversing the original mineral. Fluid inclusions accumulate on this plane ; and the first separation of the new mineral appears as little dots, closely connected with the inclusions. 'So doubt the line between metasomatism and cavities of dissolution sub- sequently filled is a very fine one, and difficult to draw in many cases ; but when intermediate cavities or subsequent fillings cannot be traced with the microscope, the process may be classed as metasomatic ; and in the great majority of cases this interpretation will be correct. Secondary Alteration of Veins. Under any given conditions, minerals tend to assume the forms most stable under those conditions. Since the conditions prevailing during vein-formation are very different from those prevailing afterwards, it may be inferred that the products of the first process might easily be changed. Such is indeed the case. We find many altered rocks which have evidently under- gone more than one change. Especially near the surface, METASOMATIC PROCESSES IN FISSURE-VEINS. 519 under the influence of oxidizing waters, the minerals formed in the rocks along veins are apt to suffer great changes. Exam- ples are frequent, showing that the minerals which filled the open spaces along a vein have been completely dissolved and partly or wholly replaced by others. This is particularly true of fillings of calcite or barite. Many instances are known in which large masses of these minerals have been completely dis- solved and replaced by quartz. Such are the well-known de- posits of Schneeberg in Saxony, and those of the De Lamar mine in Owyhee county, Idaho. Structure and Composition of Meiasomatic Vein-Roeks and their Rdation to General Metam.orphism. The aggregates replacing the original wall-rocks of veins show great variety of structure. It is most common, perhaps, that the structure of the resulting rock is much finer than that of the primary. As examples may be cited silicification, which nearly always results in micro crystalline and cryptocrystalline aggregates, and sericitization, which generally results in a mass of very fine tufted fibers. This is not, however, a general rule ; because certain easily soluble minerals, when replacing others, produce a much coarser aggregate than that of the original rock. Of such character, for instance, are the carbonates. (See Fig. 27.) Fluorite replacing limestone (see Fig. 14) is an- other instance of coarser grain shown by the secondary rock. As a general rule, the resulting minerals have, on the whole, a greater aggregate specific gravity than the original minerals. Muscovite, sericite, fluorite, the different carbonates, pyrite and other sulphides (as well as topaz and tourmaline, so abundantly formed in tin-deposits) are instances. On the other hand, there are exceptions, such as the development of jasperoids and other quartzose rocks from limestone, in which case the resulting material has less specific gravity than the original. A banded struxjjv^re of the altered rock may possibly, as men- tioned above (p. 590), result from replacement by sulphides in a sheared rock, in which the shear-planes are closely spaced ; but this banding is not likely to be as well marked as the crustifi- cation often caused by the gradual filling of open spaces. From these two sorts of banding a third must be differentiated, namely, the typical " ribbon-structure " caused by shearing of 520 MBTASOMATIC PROCESSES IN PISSTJRB-VBINS. the already-formed vein, in connection with which a secondary concentration of gold and sulphides may have taken place on the shear-planes — whence the richness of vein-material often associated with this structure. In no case, thus far, has any law of progressive alteration of the country-rock of a vein been detected, which would enable us to say that the intensity of the process either increases or decreases from the surface down. !N"or has any instance been shown in which the processes of alteration permanently change with increasing depth. This does not exclude the fact that oc- casionally a different subordinate process of alteration may be introduced. It is known, for instance, that certain parts of the rock near the vein may be locally silicified, although the prin- cipal and prevailing process in depth, as well as near the sur- face, is of a totally different character. Thus, silicification and the formation of greisen may occur side by side in cassiterite veins, and silicification and carbonatization in cinnabar veins. The metasomatic processes in wall-rocks of fissure-veins difier generally from those of regional (static and dynamic) metamor- phism. In most cases oxides of iron and manganese such as magnetite, hematite, ilmenite and pyrolusite, are absent as a primary development; and many silicates, exceedingly com- mon in static and dynamic metamorphism are, as a rule, miss- ing in veins. Among these are amphibole, biotite, garnet, cor- dierite, serpentine, ottrelite, and zoisite. Chlorite and epidote are confined to the vicinity of only one or two classes of veins. Albite, exceedingly common in regional metamorphism, is not known as a metasomatic development in veins, though, like orthoclase, it may occur in the filling of open cavities. Mus- covite, calcite, quartz and pyrite are common to both kinds of metamorphism. As compared with the products of contacts metamorphism, we note in metasomatic vein-phenomena a total absence of the pyroxenes, wollastonite, staurolite, cyanite, andalusite, vesuvianite and garnet. Only two classes of veins are characterized by tourmaline, which is a frequently occurring contact-mineral. Again, as compared with the results of ordi- nary hydro-metamorphism, we note in the results of metaso- matic vein-action the scarcity of amphibole as well as of zeo- lites, except in one or two classes of veins, and also the rela^ tively slight importance of chlorite and epidote. METASOMATIC PEOCBSSBS IN FISSURE-VBINS. 521 The degree of hydration in altered vein-rocks is very mod- erate ; and in some eases, as, for instance, in the change of ser- pentine to magnesite, there is a distinct dehydration. Strongly hydrous minerals are not common on fissure-veins. I have emphasized these differences, to show that the meta- somatie processes in veins cannot simply be identified with those that were active in the other phases of metasomatism mentioned. In the majority of cases, the vein-processes have a distinctive character of their own. PAUT n. Minerals Developed by Mbtasomatio Processes in Fisstjre- Veins. Quartz (including Chaleedonite and Opal). — Though silicic acid is weak, and cannot under ordinary circumstances expel even carbonic acid from its compounds, it is easily deposited instead of other minerals, which are dissolved by more active reagents contained in the same waters. Hence the frequency of quartz in the forms of other minerals. It would be erroneous to say, however, that silicification is a very common metasomatic pro- cess, even in veins containing quartz as a filling ; and very rarely is it the exclusive process in any given vein. It is most common in limestone and other easily soluble rocks ; also in such por- ous rocks as sandstones, though here it is usually to be regarded rather as cementation. In rocks rich in silica, such as rhyolite or quartzite, the tendency to silicification (probably by reason of mass-action) is greater than in more basic rocks in the same district. Quartz replacing limestone along fissures is a common oc- currence. The process usually results in a microcrystalline or cryptocrystalline aggregate of interlocking grains, preserving the original structure, as shown in Fig. 1,* which represents a silicified limestone from the Diadem lode, Plumas county, Cal., and shows the remaining outline of a foraminiferal test. The development of the quartz is shown in Figs. 3 and 4,t represent- * After H. W. Turner, Journal of Oeology, vol. vii., No. 4. f The accompanying figures, with some exceptions noted in the list, were drawn by myself under the microscope, with camera lucida. 522 METASOMATIC PROCESSES IN FISSURE-VEINS. ing rocks from Aspen, Colorado.* Small secondary grains or well-developed crystals appear in the limestones and, gradually extending, finally produce an aggregate which, in this case, is somewhat coarser than in the rock from California. Quartz crystals with double terminals may occur in metasomatic rocks, but are foreign to quartz, filling open cavities. Opal and chal- cedonite may occasionally also be present. The resulting fine- grained rocks, often stained brown or red, may, according to Mr. Spurr's proposal, be called jasperoids. . Daubree describes heavy quartz veins, cutting through gran- ite and overlying sedimentary rocks, in the Central Plateau of France.f Besides quartz, these veins carry fluorite, barite, calcite and galena. Agate and jasper in banded form are also frequently present. From the same description,^ it appears that in some places, where these veins traverse lime- stone (Muschelkalk), there has been a very strong silicification of the enclosing rock, as is proved by means of the occur- rence of crinoids in the compact quartz now forming part of the lode. Another locality, also in the Vosges, is men- tioned as showing a large deposit of fine-grained hornstone- like quartz, also containing barite and fluor-spar, and full of little geodes with projecting crystals of quartz. In this sili- ceous rock, silicified shells of avicula and pecten have been found, showing its derivation from the surrounding lime- stone. The chemistry of the process is apparently simple : waters containing carbon dioxide and silica deposit the latter, while simultaneously dissolving a corresponding proportion of calcite. In contrast to the fine-grained structure of jasperoids, quartz deposited in open spaces is usually characterized by coarse grains, the majority of which show partly developed crystal- faces. Crystals developed at both terminals do not appear, though earlier-developed individuals, growing from some de- posit, are surrounded by later-developed grains. Fig. 2, which shows the normal structure of the quartz in the California gold- veins, illustrates this occurrence. * The thin sections from which these figures were made were kindly loaned to me by Mr. J. E. Spurr. t Daubree, Les Eaux Souterraines aux Epoques Anciemnes, p. 124. % hoc. eit, p. 151. METASOMATIC PROCESSES IN FISSURE-VEINS. 523 Quartz may further replace orthoclase, as shown in Fig. 5, with preservation of the crystal-form. The ordinary course of alteration of the latter mineral is to quartz, sericite, or kao- linite, and potassic carbonate. In complete replacement by quartz, the alumina and potassa have been carried away, and the quartz has received a considerable addition. The process may also be explained as a complete replacement, by means of which the orthoclase, as such, has been removed, and quartz has been deposited. In the same manner, soda-lime feldspars may be replaced by quartz, aswell in phenocrysts as in the ground- mass. Even the ferromagnesian silicates may suffer a similar change. A partial replacement of hornblende by quartz and chlorite (Fig. 6) is common. The ground-mass surrounding crystals of quartz in certain rhyolites (Silver City and De Lamar, Idaho) may be replaced by quartz, forming a secondary aureole around the primary crystal. Under favorable and very exceptional circumstances, veinlets containing coarser quartz, simulating comb-quartz in structure, may be formed by replacement. Fig. 7 represents a contact between chloritic basalt and silicified rhyolite, on which a small quartz vein is developing, the crystals replacing the ground- mass of the silicified rhyolite.* Replacement of minerals by chalcedonite and opal instead of by quartz is less common. Near cinnabar veins, in California and elsewhere, serpentine, transformed into opal, with retention of the primary structure, has been observed. Hutile and Anatase. — These minerals are common in meta- somatic vein-rocks, as secondary products after ilmenite, titan- ite, titaniferous magnetite, biotite, etc. Rutile occurs in nearly every altered titaniferous rock ; anatase (octahedrite) has been found in the altered rocks of Freiberg (Stelzner), Nagyag (KoU- beck), Schwarzwald (Sandberger), and Silver Cliff (Cross). Neither titanite nor ilmenite appear to be stable under the in- fluence of vein-forming solutions. In several publicationsf I have assumed that the milky white flocculent mass (leucoxene) which often results in vein-rocks from the alteration of titan- iferous minerals is titanite ; but this assumption now appears to * 20th Ann. JRept. U. S. Oeol. Sun., part iii., p. 186. t lilh Ann. JRept. U. S. Oeol. Surv., part ii. , p. 276, et seq. 17th Ann. Bepi. U. S. Oeol. Surv., part ii., p. 149, etseq. 524 METASOMATIC PROCESSES IN FISSURE-VEINS. be incorrect. The substance is certainly free titanic acid, as shown by the fact that no titanium is extracted by hydrochloric acid, while the naineral is attacked by boiling sulphuric acid. Muorite. — This mineral may replace many others. It has gen- erally a purplish, unevenly distributed color, and shows under all circumstances a strong tendency to crystal-development. Its formation from limestone is illustrated in Fig. 14, which repre- sents the contact of one of the many small nodules of fluorite scattered in a limestone breccia from a mine in the Judith mountains, in Montana.* The sharp angles of the cube will be seen projecting into the limestone; the latter contains many imperfect fossil shells, and some crystals of secondary quartz. While the reaction involved in this process is not clearly es- tablished, it is probably a complete replacement, the more solu- ble calcite being taken up by the waters and the less soluble fluorite simultaneously deposited. Fluorite, together with quartz and pyrite, is further formed as a replacement-product of orthoclase, as shown in Fig. 10, representing a feldspar grain from a breccia in the Independence mine, Cripple Creek, Colo. The replacement of some of the phonolite and fine-grained granite-andesite breccia from Crip- ple Creek has resulted in a large quantity of crystalline fluorite and quartz (Fig. 9). Wherever calcium silicates are present, and the waters contain sodic fluoride, the result will be sodic silicate and calcic fluoride. In this way the mineral may be formed by interchange of constituents, f Alkaline fluorides and calcic fluorides may exist together in the same solution ; but alkaline carbonates decompose fluorite, yielding alkaline fluorides and calcic carbonate ; hence fluorite cannot exist as such in waters containing alkaline carbonates. Calcite. — This mineral and the allied magnesian and ferrous carbonates are exceedingly common in metasomatic vein-rocks, and their occurrence gives testimony of the energetic altering action of carbon dioxide and alkaline carbonates on nearly all silicates. The metasomatic calcite is of fine or coarse grain — the latter especially when replacing easily soluble minerals. It * This section was prepared for Mr. W. H. Weed, wlio kindly allowed me to use it. t Bischof, Chem. Geol., Bonn, 1864, ii., p. 95. MBTASOMATIO PROCESSES IN FISSURE-VEINS. 525 has very little tendency to crystallize, nearly always occurring in irregular grains. Calcite replaces quartz to a greater or less extent, though in rocks containing also silicates like feldspars and hornblende, these minerals are first attacked, and the replacement of the quartz is usually only partial. The quartz is evidently dis- solved by waters containing alkaline carbonates, and a corre- sponding quantity of calcic carbonate, also dissolved in the water, is deposited in its place. Under ordinary pressure and temperature, water does not dissolve quartz ; but increase of either results in solution to some extent. The presence of car- bon dioxide alone does not promote the solubility. No pseudo- morphs of calcite after quartz are known — an evidence of the resistance of the latter mineral to solution. The replacement of quartz by calcite in granitic rocks is shown in Figs. 13 and 15. The calcite, developed along cracks and fissures, spreads and corrodes the original substance. Small masses of sometimes rhombohedral calcite project into the quartz. Rounded and isolated bodies of calcite may also form on inclined fracture-planes; by extension they finally join and form larger masses. Orthoclase is likewise replaced by calcite in many granitic rocks adjoining veins. The process is similar to the replace- ment of quartz ; but the feldspars are much more easily soluble than quartz. Chemically, the process, as already pointed out by Bischof,* may be considered as simply due to the attack of waters containing calcic bicarbonate. The carbon dioxide of the latter alters the orthoclase ; the resulting alkaline carbon- ates and silica are carried away; just in what form the alumina is removed is not certain. In the majority of cases a simulta- neous formation of sericite occurs ; so that the actual loss of AljO, may be very small. Even more easily effected is the re- placement of soda-hme feldspars by calcite ; for here the orig- inal mineral contains one of the constituents of the result. As is well known, andesine, labradorite and anorthite may be partly converted into calcite under the influence of ordinary cold waters containing carbon dioxide. In the same manner, it is common to find pyroxene, amphi- * Chem. GeoL, ii., p. 428. 526 METASOMATIC PROCESSES IN FISSURE-VEINS. bole and biotite partly converted into calcite. In vein-forming processes, these are usually the first minerals to suffer from the attack. The magnesia, alumina and ferrous oxide usually remain in the form of chlorite or other secondary silicates, though some of the magnesia and iron may also form car- bonates. Magnesite and Dolomite. — Small quantities of magnesian and ferrous carbonates nearly always combine with the newly formed calcite, but in many cases are of no special importance. A change of limestone to magnesite is not known as a vein- forming process. Dolomitization commonly occurs, however, in limestones adjoining fissure-veins, as, for instance, described by Spurr* at Aspen, Colorado. At this place, as the dolomit- ization proceeds irregularly from the fissures, the coarse calcite grains are broken up into smaller rhombohedral crystals, of the yellowish tinge characteristic of dolomite. The process is clearly one of metasomatic replacement, carried on by waters containing magnesic bicarbonate, or even chloride. The cor- rectness of this view has been shown by synthetical experi- ments, f Mr. Spurr shows convincingly that ordinary circulating sur- face-waters do not dolomitize the limestone which they traverse. The reagents which produced this dolomitization must have been more potent. Several hot springs in the vicinity of Aspen, Colorado, carry carbonates of lime and magnesia, and also a large amount of sodium chloride and magnesium chloride. These waters, as shown by analyses, have a distinct dolomitiz- ing infiuence on the adjoining limestone. The change is also accompanied by silicification and ferration. Doloraitic carbonates may also partly replace albite, as shown by Mr. H. "W. TurnerJ in the case of a mineralized dike of albite rock from Tuolumne county, Cal. An accompanying al- most pure magnesite may possibly have resulted from the altera^ tion of the adjoining serpentine. Magnesite and dolomitic carbonates are very apt to form from serpentine, as illustrated in the country-rock adjoining the Idaho vein, G-rass Valley, Cal.§ The fine-grained serpentine is trans- * Monograph XXXI., U. S. Geol. Sun., p. 210. t Doelter, Allgemeine Chemische Oeologie, Leipzig, 1900, p. 158. t Journal of Geology, vol. vii., No. 4, p. 393. § W. Lindgren, 17th Ann. Bept. U. S. Geol. Surv., part ii., p. 153. METASOMATIC PROCESSES IN FISSURE-VEINS. 527 formed into a coarse-grained magnesite, mixed with quartz and some residual serpentine (Fig. 26). The composition of the al- tered rock is Per cent. Magnesic carbonate, .. 34.78 Calcic carbonate, 8.22 Quartz, 26.00 Serpentine (with chlorite), 31.00 100.00 The chemical action involves a substitution of CO^ for SiO^ ; the latter being deposited in the rock. Siderite. — This mineral is less common in altered rocks than the other carbonates. At Aspen, Colorado, Spurr mentions it as forming small rhombohedrons in silicified limestone. In the lead-silver veins of Wood River, Idaho, it replaces calcareous shales. In the lead-silver veins of Coeur d'Alene, Idaho, it replaces the clastic quartz of quartzite in the most energetic manner and abundant quantity (Figs. 16 and 17). The siderite has strong tendency to crystal development ; and the rhombo- hedral crystals often cut squarely across the quartz grains which they partly replace (Fig. 18). To explain the chemistry of this process, we must suppose waters exceedingly rich in al- kaline and ferrous carbonates and poor in silica. SiO^ must be dissolved and FeCOj simultaneously deposited. Muscovite and Sericite. — These two names practically signify the same mineral, though sericite is employed for the fine- grained or fibrous and tufted modifications, resulting from the replacement of other minerals. Sericite is probably the most universal and abundant of all minerals forming in altered rocks near fissures. Only a few classes of ore-deposits, namely, those in limestone and those in recent volcanic rocks, involving pro- pylitic alteration, are comparatively free from it. A vast pro- portion of so-called " talc," " clay " and " kaolin " is really sericite. Sericite forms from quartz in many rocks, though this action is less intense than in the case of the silicates. Foils and fibers of the secondary mineral may develop along cracks, or may in- trude, sharply defined, into the quartz, from the outside of the grain. Complete pseudomorphs after quartz are rare. A com- plex chemical action is probably involved, as sericite is practi- 528 METASOMATIC PROCESSES IN FISSURE-VEINS. cally insoluble. A transportation of potash and alumina must be assumed, though in what form the latter oxide was in solu- tion is not clear. The dissolved quartz may be directly com- bined with these two constituents. It is often observed that the replacement of the quartz is most active when, together with the sericite, calcite is formed (Fig. 15). As is well known, sericite forms easily and abundantly from orthoclase and microcline, the foils and fibers developing on cleavage-planes and cracks, until they invade the whole crys- tal. The reaction may be chemically expressed as follows, water containing carbon dioxide being the only reagent neces- sary : 3K Al Si, O3 + H,0 + CO, = KH, Al, (SiO,)^ + XfiO, + 6SiO,. This reaction is accompanied by a considerable reduction of volume, the sericite occupying less than one-half of the original volume of the orthoclase. If SiO, separates as quartz, the ag- gregate volume of the two secondary minerals shows a reduction of 13 per cent, from the volume of the orthoclase. Yery often, however, the quartz is carried away in solution, to be deposited in neighboring open spaces. Calcite is frequently deposited together with sericite in the feldspar (Fig. 12). Though it is usually fine-grained, large foils may sometimes be formed. Fig. 19 shows radial muscovite forming, together with kaolinite from orthoclase, in the orthoclase of granite adjoining a fissure, in which thermal waters at the present time are depositing a vein.* Sericite forms with equal ease from oligoclase, andesine and labradorite, as from orthoclase, and calcite usually 'also accom- panies it. This interesting fact was first described, I believe, by Bischof,t who also furnished the chemical explanation. The potassic carbonate contained in the water changes the sodic silicate into potassic silicate, which unites with the aluminum silicate to sericite. This will result in a progressive elimina- tion of soda and introduction of potash. In the same manner potassic carbonate decomposes calcic silicate, replacing lime with potash. Bischof gives an excellent illustration of this by describing the surface alteration of a knife of the " stone age," originally made from some flinty rock. * Kindly furnished by Mr. W. H. Weed. t Bischof, Ohem. Oeol, i., p. 31, et seq. ; also p. 44. MBTASOMATIC PROCESSES IN FISSTJKE-VBINS. 529 Even pyroxene and amphibole may alter to sericite, as is frequently shown in the metasomatic vein-rocks of California gold-quartz veins. The explanation is on the lines of the re- actions just described. The resulting sericite is often coarsely fibrous. Biotite alters very easily to coarse muscovite, with loss of magnesia and iron, and separation of rutile (Fig. 27). An instance of replacement of andalusite by muscovite is shown in Fig. 20. Few analyses are available, indicating the exact composition of the sericite contained in metasomatic vein-rocks ; but the satisfactory results obtained from the cal- culation of many rock-analyses on the basis of molecular ratio, closely corresponding to the composition given below, leave little room for doubt that the sericite is practically identical in composition with a normal muscovite. Prof. Beck, of Freiberg, has had the kindness to give me an unpublished analysis, made by Dr. H. Schulze, of a white mica, separated by Prof. Stelzner by heavy solutions from the altered country-rock adjoining the Dietrich Stehenden, Morgenstern ErbstoUn, Himmelfahrt mine, Freiberg. This analysis is aa follows : Per cent. SiO„ .... 47.48 TiOj, trace SnOj, 0.02 AlA. 35.16 FeA. 1-92 CaO, 0.48 MgO, . 1-11 KjO, 10.08 NajO, 0.41 HjO, 4.02 100.68 As minerals most closely related to muscovite, we may men- tion zinnwaldite, containing much fluor and lithia, which re- places feldspar in granite near cassiterite-veins ; also mariposite (fuchsite), containing chromium, which, with magnesite, ap- parently replaces serpentine and allied rocks at ]S"evada City, and on a much larger scale at many places along the Mother Lode of California. Biotite. — Exceedingly common in the form of metamorphism, biotite appears but rarely in fissure-veins. Replacing horn- 530 MBTASOMATIC PROCESSES IN FISSURE-VEINS. blende and feldspars, it is found as small scales in veins carrying tourmaline (Meadow Lake, Cal.) ; replacing the same minerals, it appears abundantly in the gold-copper veins of Rossland, B. C. A greenish mica, probably biotite, occurs, replacing qaartz, in small veinlets, associated with quartz, garnet, tour- maline, actinolite and zinc-blende, in the Bunker Hill and Sul- livan mine, Idaho. Prof. Penrose reports secondary biotite forming in the Ocean "Wave mine. Cripple Creek, Col. Under the influence of waters containing carbon dioxide or alkaline carbonates, biotite is not stable. Chlorite. — ^^This mineral, replacing amphibole, pyroxene and biotite, is commonly found in altered vein-rocks, but ordinarily it is only a transition-form, often abnormally rich in iron, which these minerals assume, under the influence of waters slightly charged with carbon-dioxide, before their final conversion into sericite and carbonates. The chlorite has the ordinary fine fibrous character and shows a strong tendency to migrate into adjoining minerals. In the case of biotite, the conversion should normally result in chlorite, ferrous carbonate, potassic carbonate and silica; in that of amphibole or pyroxene, calcic carbonate may form beside chlorite. Pseudomorphs of chlor- ite and quartz after hornblende are, in fact, very common (Fig. 6). The chloritic alteration is most important in the group of the propylitic veins. Possibly, under the influence of strong alkaline carbonates and carbon dioxide, chlorite can- not exist. Gr. F. Becker mentions chlorite as enclosed in vein- quartz from some localities in the Southern Appalachians, and also in gold-quartz veins from Punter's Bay and Admiralty Island, Alaska. I have described a similar occurrence in an abnormal vein, from Crown Point mine, Grass Valley, Cal. But, on the whole, it is not a common mineral in vein-fillings. Pyroxene and Amphibole. — These minerals are, as a rule, for- eign to fissure-veins, and entirely absent from gold- and silver- veins characterized by sericitic alteration. Amphibole has been noted in the filling of certain copper-veins. Some of these are more or less intimately connected with contact-meta^ morphism; others, like the copper-veins of Bossland, B. C, have probably been formed under dynamic-metamorphic con- ditions. It occurs also in small veinlets of abnormal character, containing garnets, in the Bunker Hill and Sullivan lead-silver mine, Idaho. Khodonite, a bisilicate of manganese allied to pyr- METASOMATIC PROCESSES IN PISSURE-VBINS. 531 oxene, does, however, occur in many veins as part of the fill- ing (Butte, Montana ; Real del Monte, Mexico ; Kapnik, Hun- gary ; Broken Hill, Australia). Garnet. — This mineral is very rare in fissure-veins, though common in dynamic and contact-metamorphism. Mr. S. F. Emmons* states that it replaces limestone at Clifton, Arizona, apparently as a part of the phenomena of mineralization. As part of the filling of gold-quartz veins, it is reported by G. F. Beekerf from several localities in the Southern Appalachians. The remarkable occurrences of Broken Hill, N. 8. W., should be mentioned here. From the extensive literature| it is apparent that opinions difier somewhat widely with regard to these interesting deposits. The deposits of the Barrier ranges near Broken Hill are probably fissure-veins, occurring in crystalline schists of various kinds, perhaps chiefly a garnet-gneiss. Broken Hill Proprie- tary lies parallel to the schistosity, and may, according to some, be considered as a saddle-reef. Other veins, such as the Broken Hill Consols, cut the schistosity in strike and dip. The ores are galena, zinc-blende and rich silver-ores. The gangue in the Proprietary mine is chiefly garnet, with quartz, opal and rhodonite. In the Consols and other veins, siderite and calcite also appear, besides quartz and garnet. If veins, as seems most probable, they represent a decidedly novel type. The sulphides, to some extent, replace other minerals. (See under Galena, below.) Epidote. — This mineral, so common in regions of static and dynamic metamorphism, is not abundant in the altered rocks of flssure-veins, or in the filling of, open spaces. When it occurs, it has a deep yellow color, contains much iron and de- velops in irregular grains, or into radial bunches of imperfect crystals. It occurs chiefly in basic rocks containing labradorite and similar soda-lime feldspars, and may form pseudomorphs after orthoclase, plagioclase, hornblende or augite.' In altered vein-rocks, epidote and muscovite rarely occur together. Epi- dote contains much ferric oxide, and can hardly be formed * Unpublished observations. t l&th Ann. JRept. U. S. Oeol. Surv., partiii., p. 276. J J. B. Jaquet, Mem. 5, Geol. Surv. of N. S. Wales, Sydney, 1894 ; George Smith, Trans., xxvi., 69, 1896 ; E. Beck, Zeitschr. f. prakt. Oeol, March, 1899, etc. 34 532 MBTASOMATIC PROCESSES IN FISSURE-VEINS. under strong reducing influences. It does, however, not follow that it must have been formed under oxidizing conditions ; for rocks ordinarily contain much ferric oxide, and pyrite has often been observed embedded in epidote. Epidote is found in the veins of Lake Superior which carry native copper ; in some veins characterized by tourmaline (Fig. 21) ; and, finally, in the metasomatic rocks accompanying the propylitic Tertiary gold- silver veins. Orthoclase. — As a product of thermal alteration, orthoclase does not commonly appear, and has not been recognized until lately. Closer search will probably reveal it in many altered rocks and vein-fillings of the propylitic type. When forming, it has always a strong tendency to crystallize, and in thin sections usually appears with rhombic, sharply de- fined outlines. The crystal form is similar to that of adular, though the basal plane is small or entirely wanting ; the prisms and dome being the only prominent faces. For this variety, occurring in fissure-veins, the revival of the name of valenci- anite is suggested, proposed by Breithaupt for the mineral as occurring in the Valenciana silver-mine, Gruanajuato, Mex. In certain propylitic gold-silver veins (Silver City, Idaho ; La Va^ lenciana, Mex.) valencianite is prominent as part of the filling of open spaces. In the copper-bearing veins of Lake Superior, orthoclase replaces prehnite, and is deposited on datolite, calcite, anal cite and quartz. In connection with the occurrence of adular at St. Gothard, this mineral is found on calcite. At Bergen Hill, IT. J., the Mesozoic diabases are traversed by veins (1 to 4 inches thick) of quartz and orthoclase, associated with various zeolites, galena, chalcopyrite and pyrite. At Cripple Creek, Colo., orthoclase is an important vein-mineral, occurring partly as a coating of cavities of dissolution in granite (Fig. 11), partly in metasomatic development after many minerals in granite- andesite breccia and phonolite. Orthoclase and calcite have • apparently been formed together in certain metasomatic rocks from Cripple Creek, Colo. Orthoclase has been reproduced artificially by the action of potassic silicate on muscovite at 500° C, but it is evident from many occurrences that a much lower temperature is sufficient for its formation in fissure-veins. In the Silver City, Idaho, veins, for instance, the temperature cannot have been much higher than 100° C. during the deposition of the mineral. METASOMATIC PROCESSES IN PISSUKE-VBINS. 533 Albite. — Though known as a vein-filling, together with quartz, at many places, for instance, in many California gold-quartz veins, this mineral has not been observed hitherto replacing other substances. In metasomatic rocks resulting from other meta- morphic processes it is, as is well known, very abundant. Tourmaline. — This very complex silicate of aluminum, magne- sium, ferric iron and sodium contains also about 10 per cent, of boric acid, as well as a little combined water and fluorine. In metasomatic development, it forms irregularly massed crystals, or single crystals impregnating the mother-mineral. Its ten- dency to crystallization is very strongly marked. It replaces orthoclase and plagioclase, as well as quartz, but is not known to be formed from ferromagnesian minerals. As shown in Fig. 21, small almost perfect crystals may develop in the feldspathic substance, without disturbing its optical orientations. Similar development in quartz is illustrated in Fig. 23. In fine-grained clastic rocks like slate, tourmaline may also form. The only occurring variety is black, usually showing dark brown and dirty bluish or greenish colors in thin section. Tourmaline often occurs in large masses of small felted individuals, together with quartz, entirely replacing the original rock. The mineral is confined to cassiterite-veins and to the allied group of the gold-copper-tourmaline veins. It is not usually associated with carbonates; and the occurrence of siderite with tourmaline, described by von Fircks from Tasmania,* is there- fore of special interest. Topaz. — This fluosilicate of aluminum, containing besides, according to the latest investigations, some chemically com- bined water, is confined to the cassiterite-veins. It usually ap- pears abundantly in the altered rock next to these veins, re- placing the feldspar and even the quartz, f as well as the groundmass of porphyritic rocks. The new-formed topaz may appear in irregular grains, but is often partly crystallized, and then appears in radial masses. The formation of topaz from orthoclase is analogous to kaolinization, silica and potassa being set free. But fiuorine is also introduced, which points to another agent than carbon dioxide as active in this reaction. It has been artificially reproduced by the action of hydro- * Zdtach. d. d. geol. Oes., Bd. li., p. 443, 1899. f Loc. cit.,p. 444. 534 METASOMATIC PROCESSES IN FISSURE-VEINS. fluosilicic acid on silica and alumina. Topaz is not very stable. It is easily altered to kaolinite or sericitic minerals. Kaolinite. — The hydrous silicate of aluminum was formerly supposed to occur very extensively in altered vein-rocks ; but it has been shown that the larger part of the minerals consid- ered as kaolinite or as " talc " are really sericite in finely di- vided form. Kaolinite forms from orthoclase, albite or soda- lime feldspars, with liberation of silica, the reaction in the first case being expressed as follows : 6 (K Al Si, O3) -f 6Hp + SCO, = 3 (H, Al, Si, OJ + 3K,C03 -I- 12SiO,. Ferromagnesian silicates, and even quartz, may be converted into kaolinite, as is shown by the altered rock adjoining a re- cent vein near Boulder, Montana. The mineral is nearly always in an extremely fine state of distribution ; the aggre- gates have a very low bi-refracting power. Kaolinite and seri- cite may form together (Fig. 19), as is also shown by the cal- culated composition of many altered rocks. Wherever abun- dant carbonates form, metasomatically, together with sericite, kaolinite seems to be absent. It often occurs on cassiterite- veins (though it is possible that the kaolinite may here be simply a secondary alteration of topaz) ; further, together with sericite, in veins of the pyritie galena^formation of Freiberg ; in some veins of propylitic character, as at Cripple Creek, and in veins where the action of stronger reagents, such as sulphuric acid, seems probable (Summit District, Colo.,* De Lamar, Idahof). Kaolinite is formed most abundantly in the upper, oxidized zones of many ore-deposits. Zeolites. — These hydrated minerals are almost completely ab- sent from fissure-veins. Exceptions are the silver veins of An- dreasberg in the Hartz, and Kongsberg in Norway, where many zeolites occur as vein-filling with quartz and calcite. As meta- somatic minerals, they occur in the Lake Superior copper-veins replacing feldspars and other minerals. Daubree has described zeolites forming in old bricks at Plombi^res, by the action of thermal waters ascending on a vein which does not contain any * R. C. Hills, Proc. Colorado Sci. Soc, vol. i. , pp. 20-36. t W. Lindgren, 20th Ann. Bept. U. -S. Oeol. Surv., part iii., pp. 171, 172. METASOMATIC PROCESSES IN FISSUBE-VEINS. 535 of these minerals. W. H. "Weed finds stilbite with quartz as the filling of a recent vein by ascending hot waters at Boulder, Montana. The absence of the zeolites from veins is some- what difficult to explain, as it is well known that many of them may be formed at very widely differing pressures and tempera^ tures. Very slight modifications of condition may result in the formation of hydrous or anhydrous minerals. Thus, for in- stance, Friedel and Sarasin* found that when a solution of sili- cate of sodium mixed with silicate of aluminum, in proportions required to form albite, was heated to 600° C. in a closed tube, analcite was formed. When excess of the alkaline silicate was used, albite resulted. Pyrite. — Of all the sulphides occurring as metasomatic min- erals pyrite is naturally the most common. In most fissure- veins, it impregnates the adjoining rock in varying amounts, even if the alteration in other respects has not progressed far. The mineral has a remarkable tendency to crystallization when developing in the rock, as contrasted with its often massive texture when occurring as a filling of open spaces. The forms assumed are either cubes or pentagonal dodecahedrons, or a combination of both. Pyrite develops in nearly every one of the ordinary con- stituents of rocks. By preference, it forms in the new aggre- gates of sericite, carbonates and chlorite so common in altered rocks ; but it also occurs in the fresh original minerals of the rocks, as in quartz, feldspar, hornblende and pyroxene. It is also abundant in calcareous shales adjoining veins. It is common to find small, sharp crystals embedded, for instance, in perfectly clear quartz grains, which show no break in their optical orienta- tion around the secondary crystal, proving that the genesis is by purely metasomatic processes, and not, as may be advocated in the case of crystallization in soft aggregates, by the mechanical pressure of the growing crystal. On the other hand, the devel- opment of a larger crystal in quartz or feldspar will often pro- duce a breaking-up of the grains as an optical unity, and the substitution for it of an interlocking aggregate of smaller grains. To what force this is due is not certain ; there are usually no indications of direct pressure from the growing crystal. * Qmptes rendus, Acad, des Sci., Paris, July, 1883, vol. xcvii., p. 291. 536 METASOMATIC PROCESSES IN FISSURE-VEINS. The pyrite crystals are often bordered by a small rim of cal- cite or quartz ; and little bunches of sericitic fibers may adhere to them, when forming in quartz. On the Avhole, the pyrite seems to obtain some, if not all, of its iron from the ferro-mag- nesian minerals, rather than from the magnetite and titanifer- 0U8 ores, which appear to alter to carbonates and rutile. Marcasite. — In metasomatic development, this mineral is rare, though it has been observed accompanied with kaolin, at De Lamar, Idaho, as the result of the hydrothermal alteration of rhyolite.* It is always crystallized, and the individuals com- bine to arborescent forms. Ghalcopyrite. — This mineral is not common in the metasomatic rocks of gold- and silver-veins, but it forms abundantly in cer- tain replacement-veins, such as those of Butte, Mont., and Ross- land, B. C. In such cases, it may replace any of the ordinary rock-forming minerals. It forms in irregular masses, is rarely crystallized, and is frequently accompanied by a narrow lining of chlorite. Arsenopyrite. — Like pyrite, arsenopyrite nearly always forms in crystals ; these show the simple combination of rhombic prisms and striated dome, and may also replace any of the rock- forming minerals. Next to pyrite it is the most common sul- phide in the altered rocks adjoining veins. Pyrrhotite. — This mineral is not abundant either in vein-fill- ing or in metasomatic rocks. Indeed, in many classes of veins it is entirely absent; and if it happens to be present in the rock close to the vein, it may suffer alteration to pyrite under the influence of the vein-forming agencies, f As a product of replacement of feldspar and ferro-magnesian silicates, it occurs in the Rossland veins of British Columbia, associated with chalcopyrite. The conditions governing the formation of pyrrhotite are not fully known. It has been artificially repro- duced, but not under conditions which seem analogous to those of nature. At any rate, the mineral can only be formed under very strongly reducing influences. Galena. — By preference, galena replaces calcite and dolomite. Hence the great abundance of metasomatic galena-deposits in * 20th Ann. Bept. U. S. Oeol. Surv., part iii., p. 169. t V7th Ann. Bept. U. S. Oeol. Surv., part ii., p. 147. MBTASOMATIO PROCESSES IN FISSUEE-VEINS. 537 limestone, calcareous shale and similar rocks. In crystalline igneous or metamorpliic rocks, it is not abundant as a metaso- matic product. But it may replace other minerals, especially quartz. Metasomatic galena scarcely ever appears in crystal- line form, but often forms wiry, extremely irregular masses. Fig. 25 shows its appearance in primary quartz of a quartz-diorite. It only occurs in quartz which is completely filled with fluid in- clusions ; and its growth begins as little knots and particles, dotted over any given plane of fluid inclusions. These dots, of which some are shown in the flgure, finally appear to have united to larger masses. The quartz grain in which the galena occurs is partly broken up into new quartz aggregates. Galena replacing quartz in quartzite from Northern Idaho is shown in Fig. 16. Gradually extending, the galena unites to larger masses, as illustrated in Fig. 17. The replacement of calcite by galena is illustrated by Fig. 29, representing part of a section from the Elkhorn mine, Montana, which Mr. W. H. Weed kindly put at my disposal. The rock, a crj'stalline limestone, apparently free from organic matter, contains small, partly idiomorphic quartz grains, scat- tered among the larger grains, and also many small veinlets of secondary quartz. There is thus at least an incipient silieifica- tion accompanying the formation of the galena. In the little quartz veins and throughout the rock are small, sharply defined pentagonal dodecahedrons of pyrite, nearly always connected with small quartz grains. The galena appears in several smaller grains closely intergrown with pyrite ; also in larger masses surrounded by a narrow rim of pyrite, and by clusters of small secondary grains and crystals of quartz. The galena is not clearly crystallized, but appears in small, solid masses, developing along the cleavage planes of calcite so that rhombo- hedral grains of the latter may be almost surrounded by galena. Concerning the chemical reactions involved in the replace- ment by galena opinions difier. Some think that the slightly soluble sulphate of lead is reduced from solutions by organic matter — and adduce as confirmation the universal occurrence of galena in limestone or other sedimentary rocks, presumably containing organic matter. Another view is that, the sulphide of lead being soluble to a considerable extent in water contain- ing sodic sulphide (Doelter), a simple precipitation from solu- 538 MBTASOMATIC PROCESSES IN FISSURE-VEINS. tion has taken place, dependent on the simultaneous solution of limestone and separation of silica. For most cases I would be inclined to the latter view, since the small quantity of organic matter available, for instance in the Elkhorn limestone or in the Coeur d'Alene quartzite, seems utterly insufficient to reduce such large masses of galena as are found at these localities. Besides, silver-lead veins may occur in the same districts in very diiferent rocks. Thus, for instance, in the "Wood River region, Idaho, they are found not only in the limestone but also in granite, which cer- tainly does not contain organic substances. In this connection should be mentioned the replacement of rhodonite by galena and zinc-blende, described and figured from Broken Hill, Australia, by Prof E. Beck. From the same local- ity Mr. Jaquet described and figured galena replacing ortho- clase ; and a silver mineral, probably argentite, replacing gar- net, and perhaps quartz, in a garnet schist. Zinc-blende and Other Sulphides. — In metasomatic occurrence zinc-blende is extremely similar to galena. It is usually found as irregular grains, replacing limestone, quartzite (Fig. 17), and many minerals in igneous rocks. Much additional material might be quoted regarding the re- placement of rocks by other sulphides ; the literature on the subject should be used, however, with care, since critical studies of the modes of replacement are very few, and some statements are simply based on casual inspection with the naked eye. Many other sulphides are undoubtedly formed by metaso- matic replacement. Enargite, for instance, is noted by Em- mons* as replacement thus present in the altered vein-rock of Butte, Mont. Tellurides. — Tellurides of gold and silver are found at Crip- ple Creek and elsewhere, under circumstances indicating meta- somatic deposition. Native Copper. — This metal replaces many minerals. Ac- cording to Pumpelly,t it replaces feldspar and various zeolites in the Lake Superior amygdaloids; and most of the large masses of copper there found are believed to be metasomatic. » U. S. Oeol. Sum., Folio 38. f Oeol. Sun. Mich., vol. 1., part ii., p. 19, et seq. METASOMATIC PROCESSES IN FISSURE-VEINS. 539 Gold and Silver. — ITative gold and silver are likewise impor- tant results of replacement in many veins. It is well known that masses of these metals are occasionally found in the country-rock away from the fissure ; and these occurrences are probably to be interpreted as metasomatic, though the process has not been followed in its details. Gold is also often contained in the replacing pyrite and other sulphides; and free gold can be obtained by panning from cer- tain kinds of altered vein-rock. But caution should always be observed in stating such observations. I have known instances of supposed replacement where, in fact, all the value was de- rived from the filling of minute fissures and cracks. Resistant Minerals. Among the minerals which yield not at all or only with dif- ficulty to metasomatic influences are apatite, muscovite, zircon and chromite. The resistance of apatite is very remarkable ; for according to R. Miiller* apatite is soluble with comparative ease in water containing carbon-dioxide. PART in. The Fissure- Veins Classified According to Metasomatic Processes. Under this head, I suggest fourteen classes of fissure-veins, each usually characterized by its own distinctive metasomatic process. These fourteen divisions are not offered as a per- manent classification, though most of them are sharply defined. The principle is not unqualifiedly good for a genetic classifica- tion, for the reason that the same waters may cause a different metasomatic development in different rocks. The list, in each title of which the first word indicates the predominant metasomatic mineral or process, is as follows : 1. Topaz-cassiterite veins; 2. Scapolite-apatite veins; 3. Tour- malinic gold-copper veins; 4. Biotitic gold-copper veins; 5. Propylitic gold- and silver-veins; 6. Fluoritic gold-tellurium veins ; 7. Sericitic and kaolinic gold- and silver-veins ; 8. Seri- citic and calcitic gold- and silver-veins ; 9. Silicic and calcitic quicksilver-veins; 10. Sericitic copper-silver veins ; 11. Silicic » Tsch. Min. Mitt., p. 25, 1877. 540 METASOMATIC PROCESSES IN FISSURE-VEINS. and dolomitic silver-lead veins ; 12. Sideritic silver-lead veins ; 13. Sericitic silver-lead veins ; 14. Zeolitic copper- and silver- veins. 1. Topaz- Cassiierite Veins. This sharply defined class is characterized by exceedingly strong metasomatic action, with fluorine as mineralizing agent, resulting in often coarse-grained, altered rocks, containing topaz and sometimes tourmaline, besides an often considerable percentage of cassiterite. The cassiterite-veins are characterized by their occurrence in connection with intrusive igneous rocks, and by the pneumato- lytic change of the country-rock to greisen, a granular rock consisting chiefly of quartz, topaz and white mica, usually con- taining fluor and lithia. Tourmaline and cassiterite are usually accessory constituents of this rock. The feldspar and the brown mica of the original rock are destroyed, and the min- erals mentioned above are added. Topaz often forms pseudo- morphs after quartz; cassiterite, tourmaline and topaz, after feldspar. The alteration of the country-rock varies somewhat in chemical aspects, but is distinguished by strongly marked transportation of substance. In the granular greisen, the new minerals appear as indi- viduals of considerable extent and optical continuity, perhaps indicating that the processes by which it was formed were more active and energetic than those producing the aggregate structure commonly found in altered vein-rocks. The minerals found in the metasomatic wall-rock appear also in the fissures themselves ; hence the same chemical process must have been active in both. Other rocks, such as gneiss, quartz-porphyry and rhyolite, show similar alteration, though tin-deposits do not so commonly occur in them. A notable feature of the cas- siterite-veins is the occurrence of apatite, a mineral generally unknown in fissure-veins. Under ordinary circumstances, in altered vein-rocks, apatite is the last mineral to remain fresh, after all other primary minerals have been destroyed. Vogt's explanation* of the genesis of cassiterite-veins and the alteration of their wall-rocks follows closely the previously expressed views of Elie de Beaumont, Daubree, Le N'eve Fos- * J. H. L. Vogt, Zeitadvr. f. prakt. Oeologie, 1895, p. 145. METASOMATIC PROCESSES IN FISSURE-VEINS. 541 ter and Dalmer. He assumes that they were formed imme- diately after, or even during, the granitic eruptions, and, fur- ther, that the mineral solutions originated by the action of hydrofluoric acid and hydrochloric acid on the magma, still entirely or partly in igneous fusion. By means of these, fluor- ides of silicon, tin, boron and lithium were extracted, as well as phosphoric acid. These solutions took place under pneu- matolytic conditions, that is, the " critical point "* had been passed and the substances were present in a gaseous state in spite of the high pressure. These extracts in gaseous state ascended on the previously formed fissures and strongly at- tacked the adjoining country-rock, changing it to greisen by means of replacement by minerals containing fluorine and other mineralizing agents. Different rocks were, perhaps, not affected exactly in the same way. For instance, the altered product resulting from schists is not quite similar to that re- sulting from granite, this being possibly due to the fact that the schists were not heated to such a degree as was the granite. While the formation of the greisen took place after the con- solidation of the rock, as is evidenced by the fact that fissures could form in it, still it is believed that the temperature must have been very high, and, in fact, that the lower masses of the granite were not yet consolidated. Altenberg and Zinnwald, Saxony. — The tin-deposits of Alten- berg and Zinnwald, in Saxony, have been lately investigated by K. Dalmer. t The cassiterite-deposits of Altenberg consist, as is well known, of a number of ore-fissures which sometimes carry a notable amount of quartz, mica and topaz, as well as cassiterite. Over a considerable area traversed by these vein- lets appears a greisen, locally called zwitier, which Mr. Dal- mer shows to have resulted unquestionably from the metaso- matic alteration of the granite. The process consisted of a re- placement of feldspar, principally orthoclase, by topaz contain- ing lithium and fluorine. The beginning of the process is often * The critical temperature for water is + 365° C. at a pressure of 200 atmos- pheres. For most other substances the critical point is passed below this tem- perature and pressure. It is, perhaps, not needless to state that the passing of the critical temperature does not mean that the substances are dissociated. t Erliiuterungen zur Geologischen Spedodkarte des Konigreichs Sachsen, Section "Al- tenberg-Zinnwald," Leipzig, 1890. 642 METASOMATIC PROCESSES IN FISSUEE-VBINS. visible, small strings of aggregates of mica and topaz pervad ing the feldspar. These strings repeatedly cross each other, and by extension of the mineral individuals complete replace- ment is attained. The greisen consists of: quartz, 50.28 ; topaz, 12.14; mica, 36.80; and cassiterite, 0.43; total, 99.65 per cent. Dr. Dalmer has calculated the composition of the rock from these percentages, and obtained a result which closely agrees with the old analysis of the same rock by Rube, quoted in Cotta's Gangstudien. The following table shows the result, I being the fresh granite and II the altered product : TiOj, SnO„ AljO,, FeO, CuO, MnO, CaO, MgO, KA Na^O + LijO, Fl, . I. II. Per Cent. Per Cent 74.68 70.41 0.71 0.49 0.09 0.49* 12.73 14.86 1.42 3.00 5.09 0.50 0.29 0.09 0.21 0.35 0.09 4.64 3.01 1.54 0.98 3.10 1.17 99.50 100.44 From these two analyses it is clear that the formation of greisen does not by any means involve silicification. Dal- mer concludes that the principal changes consisted in the addi- tion of FeO, Fl, SnOj, and possibly Al^Oj, while K^O, 'Rafi and SiOj have been subtracted. In the absence of further knowl- edge of the relations of volume during the alteration, it is scarcely possible to conclude from the comparison of these analyses alone what the actual changes have been. The de- termination of specific gravities of the rocks would probably help to obtain a clearer insight in regard to this matter. This much is evident, that the mineral aggregate of the granite has changed to a new aggregate of greater density. It will be observed that the second analysis contains no water. This has probably been unintentionally omitted, as it must * As cassiterite, 0.43; in mica, chemically combined, 0.06. MBTASOMATIC PROCESSES IN FISSUKE-VEINS. 543 surely be present, being contained, chemically combined, both in topaz and in the lithium-mica. In view of the fact that in this alteration not only the potas- sium-aluminum silicate, muscovite, but also the pure aluminum fluo-silicate or topaz appears, it is interesting to note that meta- somatic kaolin is undoubtedly present in cassiterite-veins. Pseudomorphs of it after topaz have been frequently observed, and it is often stated that " steinmark " is present in the veins ; this being really only a synonym for kaolin. Daubree long ago called attention to the connection of kaolin with cassiterite- veins. In several places the alteration of the granite and the adjoin- ing porphyry is of a radically different character, and consists in a complete silicification. While this is not the normal process, yet it appears fairly common, and must be taken into considera- tion in attempting to explain the genesis of these veins. Ml Bischoff, Tasmania : The rocks of the tin-deposits of Mt. Bischoff", in Tasmania, have lately been described by "W". von Fircks,* who devotes considerable space to the alteration which they have suffered. The tin-deposits appear in an area of quartzites and clay-slates with dikes of quartz-porphyry. Granite is present some distance from the mines. The de- posits are in part fissure-veins carrying cassiterite, pyrite, arsenopyrite, fluorite, wolframite, tourmaline and siderite. The latter mineral is notable, because not usually present in veins of this character. Another part of the deposits is formed by replacement, chiefly of porphyry dikes. All rocks in the vicinity of the mines are much altered. The schists and slates contain much tourmaline, and are in part changed to typical tourmalin-fels by complete replacement, only a few grains of the original rock remaining. Besides the tourmaline, some siderite also appears, while topaz is present in but small quan- tity. These altered schists contain cassiterite (rare), and also pyrite, arsenopyrite, pyrrhotite, fluorite, calcite, siderite and pyrophyllite as metasomatic products. The tourmalinization begins with the appearance of needles and bunches of crystals of tourmaline traversing fresh quartz grains, as shown in Fig. 23. These tourmaline crystals, by further growth, finally replace the * Zeitschr. d. d. geol. Ges., Bd. li., p. 433, 1899. 544 MBTASOMATIC PROCESSES IN FISSURE-VEINS. quartz altogether. According to the description, the fissure- veins must have been produced partly, at least, by processes of filling. Where they traverse the schists, their walls show im- pregnation of ores and the development of a great quantity of sericite. Sometimes, it should be added, the wall-rocks are completely silicified. Of greatest interest are the quartz-por- phyry dikes and their alteration-products. The principal sec- ondary mineral in these dikes is topaz, while tourmaline is only of subordinate importance. The groundmass of the por- phyry is changed to aggregates of topaz and quartz. The quartz phenocrysts are usually intact, while the feldspars are often completely replaced with cassiterite, pyrite, pyrrhotite, arsenopyrite and fluorite, as shown in Fig. 22. In the final product, the feldspar and mica have disappeared completely, the zircon being, besides quartz, the only mineral which has withstood the metasomatic infiuences. Here again siderite ap- pears occasionally, seemingly of simultaneous formation with the other metasomatic constituents. There is, as shown by the author, a great similarity between the metasomatic action in these deposits and that described from the vicinity of Schneck- enstein, in Saxony, by Mr. M. Schroeder.* 'None of the rocks from Tasmania can well be designated as " greisen " — a name which ought to be reserved for the granu- lar alteration-products of granite consisting of quartz, lithion- mica, topaz and cassiterite. Here again, as at Altenberg, we find occasionally, seemingly as an exception, a change in the metasomatic processes resulting in complete silicification of the wall-rock. Prof. R. Beckf has discussed the tin-ore deposits from Banca and Billiton, in referring to the work of Mr. Verbeek on the same subject. It has been shown by Prof 01. Winkler that many granites and rocks allied to horn/els contain a small amount (from 0.01 to 0.07 per cent.) of oxide of tin. This is not cassit- erite, but appears to be chemically combined with silicates, partially replacing SiOj. Sandberger had, indeed, also shown long ago the presence of tin in certain muscovites from difterent places in Europe. Another very interesting fact shown by Mr. * Erlaeut. a. Oeol. Sp. Karte des Konigr. Sachsen, Leipzig, 1885, Section " Schneokenstein." t Zeitsehr. f. prakt. Oeologie, 1898, p. 121. MBTASOMATIC PROCESSES IN FISSURE-VEINS. 545 Verbeek is a stanniferous siliceous sinter, deposited at a hot spring in Malacca. This sinter contains, according to an analysis by St. Meunier, SiO^, 91.8; Sn02, 0.5; Fe^Oj, 0.2, and HjO, 7.5 per cent. This observation possesses the greatest importance for our knowledge of tin-deposits, as it shows that the metal may be held in solution and deposited at ordinary pressure by thermal waters. Prof. Beck shows the presence of primary cassiterite in some granites from the same locality, and also points out that the veins are practically identical with tin-deposits from other parts of the world, being characterized by an often strong alteration of the adjoining country-rock, converting it into typical greisen. The feldspar is replaced by topaz, cassiterite and muscovite. The typical greisen is illustrated in Fig. 24, copied from Prof. Beck's article. 2. Scapolite- Apatite Veins. This interesting group of veins has lately been described in detail by Prof. Yogt.* It is characterized by some most re- markable features, closely relating it on one hand to the group of the cassiterite-veins, but showing, on the other hand, strong relationships with the pegmatite-veins, formed under pneu- matolytic conditions and exceedingly high temperature and pressure. The structural features of scapolite-apatite veins are not those of fissure-veins, as they chiefly follow contraction- joints (in gabbro). Indeed, this may be said to some extent of cassiterite-veins ; but the latter often also follow strong, well- defined fissures. At any rate, it seems probable that the apatite- veins were formed almost simultaneously with, or very closely following, the solidification of the magma. The characteristic minerals are apatite, phosphates, rutile, pyrrhotite, specularite, enstatite, scapolite, hornblende and malacolite, many of which are absolutely foreign to ordinary fissure-veins. A most characteristic feature is the presence of chlorine (in apatite and scapolite), just as fluorine persistently appears in cassiterite-veins. In the gabbro adjoining the vein- filling, the labradorite is usually altered to scapolite, and the diallage to hoi'nblende. This alteration is explained (loc. cit., p. 456) as a saturation of the country-rock under high pressure * J. H. L. Vogt, Zeitschr.f. prakt. OeoL, 1895, p. 367. 546 METASOMATIC PROCESSES IN FISSURE-VEINS. by the sodium chloride which acted as a minerahzing agent during the formation of the veins. 3. Tourmalinic Gold-Copper Veins. In the type of veins distinguished by a gangue of tourmaline and quartz the country-rock is generally subject to strong metasomatic changes. The vein-forming agents, which con- tained boron and some fluorine, appear to have attacked the adjoining rock strongly, and caused a more or less complete metasomatic conversion into tourmaline. Von Groddeck* has described an occurrence of this kind from Tamaya, Chile, in which veins containing copper-ores cut gabbro and porphyrites. The tourmaline is here not only present in the filling of the fis- sure but is also distributed through the adjoining country-rock. A further contribution to the knowledge of veins carrying tour- maline was given by A. "W. Stelznerf in his description of oc- currences of this kind from Chile. The rocks examined, from the mining district of Las Condes, 90 miles east of Santiago, consist of granite and greenish porphyritic rocks, which the author is inclined to consider as altered andesites. The vein- filling is pyrite, chalcopyrite, quartz and a loose porous mass of tourmaline needles. By a specific-gravity separation of the latter, Stelzner obtained, as a residue, zircon in well-developed crystals, as well as specularite and anatase. The zircon is probably developed under the influence of the vein-forming so- lutions, the anatase and specularite quite certainly so. Again the fact is emphasized that the adjoining country-rock is bleached and filled with pyrite and tourmaline. Dr. E. HussakJ has recently described the auriferous quartz- vein of Passagem, in Minas Geraes, Brazil. This vein, which lies, parallel to the stratification, between mica schist and itabi- rite (hematite-mica^schist), shows a filling of quartz, tourmaline, and arsenopyrite, with smaller quantities of pyrite and pyrrho- tite. The arsenopyrite is strongly auriferous. A very inter- esting feature is the occurrence of zircon and monazite in the ore, formed apparently simultaneously with it. Here, too, the tourmaline is present in the adjoining country-rock. Musco- * Zeitschr. d. d. geol. Ges., 39, 1887, p. 237. t Posthumously published in Zeitschr. f. prakt. Geologie, 1897, p. 41. X Zeitschr. f. prakt. Oeologie, 1898, p. 345. METASOMATIC PROCESSES IX FISSURE-VEINS. ,")47 Fjg. 1. Siliciiied Calcareous Shale with Outline of Forajiiiniferal Test. Diadem lode, Plumas county, Cal. ( After 11. "\V. Turner. ) Crossed nicols. All quartz. Mag- nified 29 diameters. Primary Vein Quartz from Fillim^'. Federal Loan mine, Nevada county, Cal. Crossed nicols. All Quartz. Magnified l^l diameters. 548 MBTASOMATIC PROCESSES IN FISSURE-VEINS. Fig. 3. Incipient vSilicitication of Limestone. Aspen, Colo. Wliite areas represent quartz crystals with small inclusions of limestone. Ma<;'niiied .">(> diameters. Fifi. 4. Silicified Limestone (" .Jasperoid "). Aspen, (.'olo. Crossed nicols. All quartz. Small inclusions of calcite in .some of the grains. i\Iagnitied 30 diameters. METASOMATIC PROCESSES IN FISSURE-VEINS. Fig. 5. 549 Incipient >Silicification of Orthoclase Crystal in Rhyolite. Silver City, Idaho. Crossed nicols. n, Orthoclase ; h, secondary i|nartz ; f\ sericite. Magnified 34 diameters. Fit:. 6. Hornblende Crystal with Partial Chloritization and Silicification ; in Propylitic Andesite. Virginia City, Xevada. (After G. F. Becker. ) "White, quartz and calcite ; grey, chlorite ; dark grey, liornblende. Magnifieil 70 diameters. Fig. 7. ffmmmmmmmmm, W Veinlet of Quartz (b) on Chloritized Basalt (a), Keplacing Silicified Pihyolite (r) Bishop vein, Silver City, Idaho. Magiiiticd 11 di imeters. 550 METASOJIATIC PROCESSES IN FISSURE-VEINS. Fig. 8. Filled Veinlet in Andesite Breccia. Independence mine, Cripple Creek, Colo. q, Quartz ; o, valencianite (orthoclase) ; p, pyrite; /, fluorite ; g, ground-mass of breccia. Fluorite and pyrite partly replacina diameters. !jround-mass. Magnified 11 Fig. 9. Fluorite Ore, Replacing Phonolite. Portland mine, Cripple Creek, Colo, p, Py- rite ; q, quartz, coarser and finer grains ; /' fluorite. ^Magnified 50 diameters. METASOMATIC PROCESSES IN FISSURE-VEINg. Fig. 10. 5.51 Orthoclase Grain (o) in Granite Aiidesite Breccia, Partly Keplaced by Quartz (q); Fluorite (/) ; Pyrite (p); Sericite (s) ; Ground-Mass of Breccia (g). Inde- pendence mine, Cripple Creek, Colo. Magnified 60 diameters. Fig. 11. Filled Space of Dissolution in Granite. " Granite Ore," Independence raine.Crip- Xjle Creek, Colo, o, Orthoclase of granite ; m, biotite, converted into valen- cianite and pyrite ; v, valencianite ( secondary ortlioclase) , showing crustification ; tj, quartz. Magnified 20 diameters. 5 5 -J METASOMATIC PROCESSES IN" FISSURE-VEINS. Fig. 12. Ande.sine Crystal in Granodinrite, Replaced by Sericite and Calcite. Pinetree vein, Ophir, Placer county, Cal. q, <,luartz ; m, luuscovite ; e, calcite ; s, seri- cite. Magnified 80 diameters. Fk;. 13. Quartz Grain in Same Section, Partly Eeplaced by Calcite. q, Quartz ; c, calcite with some sericite replacing from outside ; also, secondary calcite grains form- ing on inclineiiartzite in same section gradually changing to tliis ore. Jlagnified 35 diameters. METASOMATIC PROCESSES IN FISSURE-VEINS. 5'j5 Fig. 18. Quartzite Partly Replaced by Siderite and Pyrite. Helena and Frisco mine, Cceur d' .A.lene, Idaho, q, Quartz grain,s ; .<, .sericite ; s/, siderite with jiartly rhombohedral form ; blatk, pyrite. Magnified 100 diameters. Fir;. If). Ortlioclase Partially Replaced by Mnscoyite and Kanlinite. From f[uartz-monzo- nite ailjoining recent yein, Boulder, Mont. ", Ortlioclase; 7, (juartz ; m, mus- covite ; /.'. kaolinite. JIagnified 22 diameters. 656 MBTASOMATIC PROCESSES IN FISSURE-VEINS. Fig. 20. Part of Andalusite Crystal Altered into Muscovite, Arsenopyrite, etc. Passagem gold-quartz vein, Brazil. (After E. llussak. ) J/, Pyrrhotite ; .1, arsenopyrite; P, pyrite ; (J», quartz ; S, rutile ; J/h, muscovite ; T, tourmaline. Fifi. 21. Keplacement-Veinlet of Tourmaline in Fresli Andesine Grain. Keystone mine, Meadow Lake, Nevada county, Cal. (, Tourmaline ; /, ande ite. Magnified .50 diameters. epidote; METASOMATIC PROCESSES IN FISSURE-VEINS. Fig. 22. ODt Feldspar Crystal Keplaced by Topaz, Quartz, Fluorite and Cassiterite, in ftround- Mass Converted to Partly Radial Aggregates of Topaz. Mount Bischoff, Tas- mania. (After W. von Fircks.) Fir:. 23 Incipiient Tournialinization of Quartzite. Three quartz grains shown. Xeedles single and in bunches, of tourmaline. Mount Biscliofi, Tasmania. (After W. von Fircks. ) 558 METASOMATIC PROCESSES IN FISSURE-VEINS. 'Greisen" from Tin-Deposits of Banca, ^Malay Peninsula. Derired from gran- ite. (After K. Beck.) g, Litliion-mica ; veinlets of fiuorite and quartz traverse the feldspar. The cavities of dissolution apparently result from the removal of both quartz and micro- cline. Some of them are filled with quartz, valencianite and pyrite ; the secondary feldspar often being deposited with orien- tation parallel to that of older grains. A few grains of sec- ondary feldspar show microcline structure, but most of them are simple individuals. These filled spaces, characterized by crustification, are illustrated in Fig. 11. The processes of replacement are remarkably variable. In some of the granite-andesite breccias the result is quartz, valen- cianite, fiuorite and pyrite. Fig. 10 shows how the replace- ment progresses in a grain of orthoclase. Fig. 8 shows a filled veinlet in the same breccia, which by means of a narrow crack connects with the feldspar grains just mentioned. The valen- cianite shows crustification, while the fluorite and pyrite, by cor- roding the walls, indicate partial metasomatic action. Some of the phonolites of the Independence mine are porous siliceous rocks, completely replaced with quartz, valencianite ,crystals, pyrite, and a few grains of a telluride of gold and * Confer: W. Lindgren, "The Gold and Silver Veins of Silver City," 20ih Ann. Bept. U. S. Oeol. Sw-v., part iii., p. 167. MBTASOMATIC PROCESSES IN MSSUKE-VEINS. 577 silver. Other specimens show only incipient alteration, being impregnated with pyrite crystals and containing a few per cent, of sericite ; they contain, besides, spaces of dissolution filled with quartz, fluorite and valencianite. Certain fine-grained granitic breccias from the Annie Lee shoot, in the Portland mine, show a most peculiar alteration, the quartz and ortho- clase being both replaced by calcite, pyrite and secondary or- thoclase. The final result of the alteration of phonolite is, in many cases, a purple fine-grained rock consisting of quartz, fluorite and pyrite ; as usual, the fluorite is crystallized ; and the quartz also shows, to some extent, idiomorphic outlines (Fig. 9). Other Occurrences. — To this class belong also the so-called Potsdam tellurium-ores oi the Black Hills, Dakota, de- scribed by Devereux, Carpenter, F. Clemes Smith, and lately by J. D. Irving.* According to Mr. Irving, the irregular de- posits are connected with fissures, and consist in a replacement ■ of limestone by silica, vdth fluorite, and gold, partly in the form of tellurides. While the age of these deposits is not fully known, they are believed to be genetically connected with phonolitie and tinguaitic dikes of Tertiary age. Mr. W. H. "Weedf has described interesting occurrences of the same type from the Judith mountains in northern Mon- tana. The deposits, though, strictly speaking, not fissure-veins, are still more or less clearly connected with fractures, and are found in the brecciated contacl^zone between limestone and in- trusive masses of acidic porphyry. The principal gangue-min- erals are quartz and purple fiuorite, the ore-minerals being na- tive gold and tellurides of gold and silver. The mode of ore- deposition is certainly a replacement of limestone, the fluorite occurring in more or less sharply deflned patches in the lime- stone breccia. From a specimen which Mr. "Weed kindly selected for me a section was cut, which is partly illustrated in Fig. 14. The invasion of the normal limestone, still carrying organic remains, by the crystallizing fluorite, is well shown, as well as the incipient silicification which accompanied the meta- somatic action. Much secondary silica, replacing limestone, * "A Contribution to the Geology of tlie Northern Black Hills,'' Ann. N. Y. Acad. Sci., vol. xii., No. 9, pp. 297-314. t 18tA Ann. Bept. TJ. 8. Oeol. Surv., part iii., p. 588. 578 METASOMATIC PROCESSES IN FISSURE-VEINS. also occurs in jaspery or cherty form. Small cubes of fluorite, found in the fresh limestone, represent places of incipient fluor- itization. Dikes and sheets of phonolitic rocks are found in the vicinity of the deposits. A deposit occurring in rhyolite porphyry not far from the limestone shows strong alteration of the country-rock. The feldspar is here changed to sericite, the ferro-magnesian minerals are transformed to calcite, and abundant small crystals of pyrite appear in the rock. Fluorite is also present on some of the veins in rhyolite porphyries. Mr. "Weed regards the deposits as possibly of pneumatolytic origin. It is scarcely necessary, however, to assume the fluor- ine to have been present in the form of free hydrofluoric acid (compare pp. 621 and 524). Similar deposits, with purple fluorite and tellurides, are also described by Mr. "Weed from the Little Rocky Mountains, in Montana.* 7. Sericitic and Kaolinitic Gold- and Silver- Veins. General Remarks. — This class has not been studied as much as some of the other subdivisions, but many deposits will prob- ably be found to belong to it. Apparently the pure aluminic silicate cannot be formed when the generating waters contain much carbon dioxide or alkaline carbonates. But it does form under the influence of some waters containing a small amount of these reagents, and also in the presence of sulphuric acid, which, as is well known, rapidly attacks the feldspars. Even under the latter two conditions, some sericite is ordinarily also formed ; and I am not aware of any veins in which kaolinite forms without sericite. The class may be subdivided accord- ing to the absence or presence of siliciflcation. The Freiberg Veins. — As is well known, several very difierent kinds of veins appear in the Freiberg district, practically all, however, being sharply deflned flssure-veins in gneiss, in which the filling of open spaces constitutes the only ore ; extensive zones of alteration are absent. It is a peculiar fact that very little is known of the metasomatic processes afiecting the coun- try-rock at this celebrated locality, no chemical examinations having been made to determine how the various classes of » Journal of Geol, vol. iv., pp. 399-428 (1896). METASOMATIC PROCESSES IN FISSURE-VEINS. 579 veins may differ in their metasomatic products. A letter from Prof. Beck, of Freiberg, informs me that Prof. A. W. Stelzner devoted much time during the last years of his life to the exami- nation of the changes in the country-rock adjoining the veins, but that his manuscript notes have never been published. "W. "Vogelgesang,* in describing the veins south and southeast of Freiberg containing carbonates and rich silver-ores, describes the impregnation of the adjoining rock vpith ores. He men- tions the occasional introduction of argentite and native silver into the gneiss, also, in places, of arsenopyrite, pyrite, chalco- pyrite, zinc-blende and galena. The impregnation with silver- ores appears only in especially rich places along the vein. In one case, however, the impregnated rock appears as an irregu- lar, limited mass, almost entirely independent of the direction of the vein. In another locality, chalcopyrite and bornite were found in the wall-rock, while the filling of the vein contained neither. In the same paper, the author describes the alteration of the so-called normal gneiss of Freiberg near the veins of the Kiesige Bldformation. The mica is the first mineral attacked ; the second is the feldspar. The former acquires a silvery- white color, often connected with a chloritic appearance. The feldspar is changed to a white " kaolin," and the whole rock is bleached white. By extreme alteration, the quartz disappears, and the rock forms a white or yellowish-white talc-like mass. The altered rock is frequently, in fact usually, filled with arsenopyrite. Some of the crystals are large; others micro- scopic ; and their amount may so increase as to form a con- nected, compact mass of arsenopyrite. All these crystals are twins, excellently developed, with perfect faces. This descrip- tion refers especially to the Dietrich Stehender vein.f In the foot-wall, and partly also in the hanging-wall, of this altered mass appears a rock, recognizable as the ordinary grey gneiss of Freiberg. It contains no aresenopyrite but is strongly im- pregnated with galena, arranged in curved streaks parallel to the schistosity, and replacing one constituent of the rock, namely, the feldspar. The altered rocks have been analyzed; but the determination of alkalies being omitted, the analyses have no special value. * Bernard Cotta, Oangsiudien, vol. ii., Freiberg, 1854, p. 78. t The sericite from this vein has been analyzed by H. Schulze ; see page 609. 37 580 METASOMATIC PROCESSES IN FISSURE-VEINS. The rock adjoining the Gotthold-Stehender vein is likewise strongly impregnated ; but here, besides arsenopyrite, galena, chalcopyrite, pyrite and zinc-blende are also found. As early as 1883 and 1884 Stelzner* devoted considerable at- tention to the examination of the soft and altered rock adjoin- ing the veins of Freiberg. These altered rocks are also usu- ally impregnated with small crystals of arsenopyrite, or pyrite, or (more rarely) zinc-blende and galena. In the normal gneiss of Freiberg, which Prof Stelzner subjected to separation by heavy solutions, he found much zircon, as well as a little tour- maline and a large quantity of apatite. All three of these min- erals were also separated from the altered wall-rocks in the crystalline shape which they had had in the fresh rock ; hence none of them had been attacked. He observes further that the quartz, feldspar and biotite of the fresh gneiss is completely or almost completely changed into white potassium mica, form- ing, in small hexagonal or irregular foils, the chief mass of the rock. This secondary mica contains, according to analysis by Dr. Schulze, as much as 0.41 per cent, of TiOj and 0.54 per cent, of SnOz. As newly-formed minerals in the altered rock, appear small crystals of quartz, rutile and anatase. In certain of the examined rocks only rutile was found, in others only ana- tase (octahedrite), but both were present as sharply defined small crystals. These two minerals are considered as resulting from a decomposition of the biotite, which is rich in titanic acid. In analyzing the heaviest part obtained by the Thoulet solution from the altered rock close to one of the veins, it was found that cassiterite was present in notable quantity. Whether this tin-ore resulted from the decomposition of the biotite, or whether, like the arsenopyrite, it had been introduced from the vein-fissure, is not certain. The latter hypothesis is probable; for the black zinc-blende of the same vein (the Carl Stehender) contains small, but numerous, crystals of cassiterite. Examination of the fresh gneisses of Freiberg discloses the presence of pyrrhotite, pyrite, and probably also a little arseno- pyrite. These are presumably primary ; at least the statement of Prof Stelzner is probably to be interpreted in this sense. The first attempt to examine the altered wall-rocks in a sci- * A. W. Stelzner, " Studies of the Gneiss of Freiberg and its Products of Alter- ation." Neues Jahrbuch, 1884, vol. i., p. 271. METASOMATIC PROCESSES IN FISSURE-VEINS. 581 entific manner was made by Prof. Th. Scheerer,* in connec- tion with his classic paper on the gneissic rocks from the vicinity of Freiberg. According to Scheerer, the normal char- acter of the grey gneiss is always more or less changed in the vicinity of mineral veins, the extension of this alteration being, in general, dependent upon the width of the vein. The mica has turned into a light greenish-grey or white talcose mass ; and the whole rock is softened and easily crushed. The width of this alteration ranges from a few inches up to 6 feet. In the porous gneiss, small bodies of pyrite, zinc-blende and galena have been formed; partly as separate crystals, and partly in little veinlets. Prof. Scheerer examined especially a gneiss closely adjoining the Erzengel vein belonging to the Kiesige JBleiformation, in the Himmelfahrt mine. The specimen was taken from one of the deep levels, thus eliminating the action of surface-water. The analysis gave : SiO^, 61.69; TiO^, 0.73; Al,03, 21.74; FeA, 0.43; CaO, 1.07 ; MgO, 1.15; E;0, 2.69; ]Sra,0, 0.30 ; H,0, 3.96 ; fluorite, 1.20 ; pyrite, 4.26 ; chalcopyr- ite, 0.23 ; galena, 0.09; argentite, trace; total, 99.54 per cent. Scheerer recalculated this analysis on the basis of constant alumina and, combining the result with the analyses of the fresh rock, obtained the table on the following page. The assumed constancy of the alumina is probably not quite correct, but offers an easy and fairly accurate way of approximately judging the changes that have taken place. This result is remarkable in many respects. First, we note that a very decided removal of substance has taken place. Counting by equal weights, only 5 parts in 100 have been added (even less, if we consider that the Fe of FeS is derived from Fe203 and FeO in the fresh rock) while no less than 42.45 parts have been carried away. This contrasts strongly with the results obtained from gold-quartz veins in California and Idaho. Of the silica, 26.62 parts or 40 per cent, is removed. The total bases, except almnina, have been diminished to the extent of 81.01 per cent. Both Kfi and 'Na.fi have been removed J tfae former, however, in much smaller proportion than the latter. Lime, magnesia and iron have also been very greatly reduced. A large part of the iron, however, appears to * Zeitschr. d. d. geol. Oes., vol. xiv., p. 87 (1862). 582 METASOMATIC PROCESSES IN FISSURE-VEINS, SiO, TiO, AlA FeA FeO CaO MgO K,0 Na,0 H,0 Fluorite , Pyrite Cnalcopyrite Galena Argentite Normal Grey Gneiss. Per cent. 65.42 1.05 13.68 4.26 2.88 1.44 2.66 6.18 1.38 1.05' trace trace trace •18.80 100.00 Altered Grey Gneiss. Per cent. 38.80 0.46 13.68 0.27 1 0.67 0.73 1.70 0.19 2.49 0.76 2.68 0.15 0.06 trace 3.56 62.64 Additions and Sub- tractions Suffered by the Altered ilock. Per cent. -26.62 -0.59 -3.99 -2.88 -0.77 -1.93 -4.48 -1.19 -1-1.44 -1-0.76 -i-2.68 -1-0.15 -1-0.06 trace -15.24 -42.45 -)-5.09 37.36 100.00 have been transformed into pyrite. There are no carbonates at all, while a little fluorite has formed. Of course, no microscopic diagnosis of Scheerer's specimens is now available. But from Stelzner's later separations and Schulze's analysis it is clear that considerable sericite is pres- ent. An attempt to calculate Scheerer's analysis shows at once that kaolinite is also present. A rough calculation gives the fol- lowing result : Quartz, 40 ; sericite, 32 ; chlorite, 3.20 ; kaolin- ite, 17.70; titanite, 1.90; fluorite, 1.20 ; pyrite, 4.26; chalco- pyrite, 0.23; galena, 0.09; total, 100.58 per cent. In this calculation K^O H-l^ajO are taken as a basis for sericite, according to Schulze's analysis ; MgO is calculated as chlorite ; the remaining HgO is calculated, with proper quantities of AljOs and SiOj, as kaolinite; and there is a small excess of CaO, possibly belonging to apatite. PjOj is not determined in the analysis. But there remains also an excess of 4 per cent, of AI2O3, which is inexplicable on the basis of this assumed mineral composition ; and it is difficult to see how this should be treated. Possibly the determination of HjO is a little too low. At any rate, a considerable amount of kaolinite is surely present. This result is of great interest. The strong leaching of SiOj and bases, as well as the presence of kaolinite together MBTASOMATIC PROCESSES IN PISSTJRE-VBINS. 583 with sericite, points to a process and to water very different from those by which sericite alone is produced. It is not out of place in this connection to call attention to the presence of fluorite and to the close relationship which, as indicated by Dalmer, exists between the cassiterite-veins and those of the Kiesige Bleiformation of the Erzgebirge, a relationship still further emphasized by the occurrence of cassiterite in the Freiberg veins. Scheerer believed that the grey gneiss, by reason of its easily-attacked biotite, had a precipitating influence on the mineral waters. The red gneiss, containing muscovite in gen- eral, carries no large bodies of ore. De Lamar, Idaho. — The quartz veins of De Lamar, Idaho, belong to that class in which the alteration of the country-rock results in sericite, kaolinite and quartz.* These ores, which carry finely-divided gold, together with some rich silver min- erals, are peculiar, because the quartz now constituting the fill- ing is pseudomorphic after a former vein-filling of calcite. The country-rock has undergone two corresponding changes : the first consisting in a normal sericitization ; the second in a silici- fication, probably under the influence of waters containing sulphuric acid. The final result is that the rhyolite is converted to a silicified rock, in which the" structure is plainly preserved. The phenocrysts of quartz have received aureoles of secondary silica ; and the feldspars are fully converted into fine-grained aggregates of quartz. The composition is as follows : Sericite, 15.43; kaolinite, 3.81; quartz, 78.73; pyrite, 0.90, and water (hygroscopic), 0.51; total, 99.38 per cent. Expressed quantitatively, the process appears to consist in the loss of nearly all of the soda, half of the alumina and much of the ferrous and ferric oxide; and finally, in the addition of several per cent, of silica. The pseudomorphic filling con- sists of crossing laminae of quartz, each consisting of a thin plate of microcrystalline silica, coated on both sides with small quartz crystals. The character of the process involved in this change from calcite to quartz is probably as follows : Calcite-fiUing in veins is often characterized by a prevalence of long, slender or spear- shaped grains. The solutions carrying silica penetrate along * W. Lindgren, lOth Ann. Bept. U. S. Oeol. Surv., part iii., p. 178 et seq. 584 MBTASOMATIC PROCESSES IN FISSURE-VEINS. the contact-planes of these grains and deposit their plates of microcrystalline quartz in the place of dissolved calcite ; from these medial plates small quartz crystals grow into and gradu- ally replace the calcite on both sides. The pseudomorphic plates do not, as a rule, follow the cleavage-planes ; hence there may be in some cases considerable difficulty in determining the original mineral replaced by the quartz. The subject is more fully treated in my report, cited above. Summit District, Colorado. — Some remarkable ore-deposits in the Summit district, Eio Grande county, Colo., have been described by E. C. Hills* as masses of quartz resulting from the alteration and silicification of eruptive rocks along contact- planes, especially between rhyolite and trachyte-breccia. The quartz, which contains gold, enargite, pyrite, galena and zinc- blende, gradually merges into rhyolite, varying in thickness from 3 to 30 feet. Thus silicification of the rhyolite is evident, and has resulted in a change of the ground-mass to compact quartz, while the phenocrysts of that mineral remain intact and conspicuous. The sanidine has been removed, and pseu- domorphic quartz has been deposited. The surrounding rock is notably kaolinized, and contains no lime or potash. The alteration probably took place less than 300 ft. below the original surface. The following is suggested in explanation of the chemical processes : Silica is practically insoluble in solutions contain- ing sulphates and chlorides, hydrogen sulphide and free sul- phuric acid. Under these conditions, aluminum silicates are dissolved and sulphates or chlorides of aluminum are formed, with simultaneous separation of silica; and to sulphuric acid as a reagent the writer believes the alteration to be chiefly due. "Whether the metallic minerals were deposited simultaneously with this alteration, or subsequently, is left an open question, though it is believed that their simultaneous deposition would be possible. 8. Sericitie and Calcitic Gold-Silver Veins. This important metasomatic class comprises an enormous number of veins, differing widely in age and in the character * Proc. Colorado Sci. Soc, vol. i., p. 20. MBTASOMATIC PROCESSES IN FISSURE-VEINS. 585 of the country-rock, but usually characterized by quartz-filling, enclosing auriferous and argentiferous sulphides (often also free gold), while the adjoining rock for a short distance on both sides is converted into an aggregate of quartz, sericite and calcite, with some metasomatic sulphides. Extensive altera- tion-zones are not common; and sometimes fresh rock may adjoin the vein. The relative quantity of sericite and carbon- ates may differ considerably, even in the same mine. The Gold-Quartz Veins of California. — These, perhaps the most prominent representatives of the class, have been studied in some detail.* The gold-quartz veins of California are fissure-veins of Cretaceous age, largely filled with quartz along open spaces. A small amount of calcite may also occur. The ore-minerals comprise native gold, present in a free state at all depths reached hitherto, and a small amount of sulphides consisting of pyrite (never marcasite and rarely pyrrhotite), galena, zinc-blende, arsenopyrite and chalcopyrite. Tellurides are often found, but in small quantity only. The veins are practically independent of the character of the country-rock, cutting almost all the sedimentary and igneous rocks of the Sierra ITevada. Adjoining the veins the country-rock, for a variable distance, but seldom exceeding a few feet, is nearly always altered by metasomatic processes. Clay-slates and sili- ceous schists have been apparently least afiected in this way, except that they are generally impregnated with pyrite. More detailed chemical examinations will probably show that even these rocks have been altered more than their appearance would suggest. In the ordinary course of the metasomatic process, augite, hornblende, epidote, biotite and feldspars are first vigorously attacked. The ferromagnesian silicates are often converted into large foils of muscovite. The alteration proceeds along cracks and cleavage-planes, and a finely felted aggregate of sericite and calcite invades the grains until the replacement is complete. Though the ferromagnesian silicates are, for the most part, directly converted into the minerals mentioned, they form also a chlorite, very rich in iron, as an intermediate * W. Lindgren, Bull. Geol. Soc. Am., vol. vi., pp. 221-240 ; also in U. S. Geol. Surv., lith Arm. RepL, pt. ii., pp. 249 to 284, and 11 ih Arm. RepU, pt. ii., pp. 1 to 262. 586 MBTASOMATIO PROCESSES IN FISSURB-VBINS. stage, which ^s converted later into sericite. An interlacing structure of sericite foils, the triangular or polygonal inter- stices of which are filled with calcite (Fig. 12), is often noted. The quartz is also attacked, but with more difficulty ; and in no case is it completely replaced by the attacking sericite-car- bonate aggregate (Fig. 15). Magnetite seems to be converted mostly into ferrous carbonate, and ilmenite to rutile. Sharp cubes of pyrite form in the new aggregates, but also in the fresh primary minerals. The only other sulphide found is arsenopyrite, which, in some mines, appears as sharply defined rhombic crystals. The sulphides sometimes .include fibers of sericite. The result of the process is the conversion of the rock to sericite, carbonates, quartz and pyrite, with retention of the original structure as shown in Fig. 27. The alteration of serpentine has already been referred to. From many analyses the following eight are selected, A and Ai, B andBi, etc., being respectively the unaltered and altered rock from each locality : Table I. — Analyses of Metasomatic Rocks from Gold- Quartz Veins. SiO, TA AlA FeA FeO FeS^ CujS MnO NiO, ZnO CaO SrO BaO MgO K,0 JTa^O LijO HjO below 110° C. . HjO above 110° C... PA SO, CO^ Total Per cent. 65.54 .39 16.52 1.40 2.49 .06 4.88 not det. not det. 2.52 1.95 4.09 .59 .18 100.61 Per cent. 46.13 .67 15.82 .89 2.27 1.61 ".09 trace 10.68 trace trace 2.13 5.30 .17 trace .12 2.42 .10 n'.24 99.64 Per cent. 45.56 1.11 14.15 1.20 9.83 7.86 .10 .25 trace 2.30 trace trace 6.76 1.18 1.57 trace .23 4.84 .14 .03 3.04 100.15 Per cent. 37.01 .85 12.99 .43 3.57 7.99 trace .24 trace 9.78 trace trace 5.49 4.02 .13 trace .13 1.92 .06 .04 15.04 99.69 Per cent. 66.65 .38 16.15 1.52 2.36 .02 .10 4.53 trace .07 1.74 2.65 3.40 trace .18 .72 .10 100.57 Ci. Per cent. 56.25 .25 17.65 .76 2.64 2.87 4.46 .03 1.69 6.01 .30 .30 2.36 .21 '4.'82 100.60 D. Per cent. 51.01 .98 11.89 1.57 6.08 1.73* trace trace 10.36 none none 8.87 .15 4.17 '".24 2.09 .17 99.31 Per cent 45.74 .36 5.29 .13 2.06 .49 .26 23.85 none trace .94 1.29 .11 trace .22 1.07 .07 18.91 100.79 * Probably present as Fe,S8. MBTASOMATIC PROCESSES IN FISSURE-VEINS. 587 A, Fresh granodiorite, Lincoln, Placer Co. Though not adjoining the vein, it indicates closely the actual composition of the fresh wall-rock. W. F. Hille- brand, analyst. — Ai, Altered granodiorite, Plantz vein, Ophir, Placer Co. W. F. Hillebrand, analyst. — B, Araphibolite schist, Mina Rica vein, Ophir, Placer Co. Fairly fresh, but contains pyrite and calcite. W. F. Hillebrand, analyst. — Bi, Completely altered amphibolite schist, Conrad vein, Ophir, Placer Co. W. F. Hillebrand, analyst. — C, Fresh granodiorite, Nevada City, Nevada Co. W. F. Hillebrand, analyst. — C,, Altered granodiorite, Bellefountain mine, Nevada City. George Steiger, analyst. — D, Fresh diabase, Grass Valley. N. H. Stokes, analyst. — Dj, Altered diabase. North Star mine. Grass Valley. W. F. Hille- brand, analyst. From the chemical and microscopical data the following compositions may be calculated (see reports cited). The only diflference from the sources quoted is due to the fact that by later chemical examination the titanium mineral present has been shown to be rutile instead of titanite. Table II. — Mineralogical Composition of the Altered Hocks of Table I. Quartz Sericite (with a little chlorite) Calcite Magnesite Siderite Rhodonite Rutile Pyrite Apatite Total Per cent. 16.00 41.76 17.53 9.67 5.76 .42 .85 7.99 .13 100.11 B,. Per cent. 24.00 46.97 18.87 2.93 3.67 .14 .67 1.61 .22 99.08 Ci. Per cent. 25.00 61.46 7.23 2.70 .58 .25 2.87 .46 100.55 Di. Per cent. 35.00 21.20 42.15 .71 .36 .50 .15 100.07 As it seems probable that the alumina has remained fairly constant in the first three pairs of analyses in Table I., they may be directly compared for an approximate review of the chemical changes affected. A recalculation on the basis of constant alumina seemed scarcely worth while. The silica has been in all cases greatly reduced. Except in A, which is not from the immediate vicinity of the vein of Aj, the titanic acid has decreased. Ferrous and ferric oxide are both reduced — the latter more than the former ; and the whole or a part of this loss reappears as pyrite. Lime shows great increase except in C, where it is constant. Baryta in C shows partial loss. The loss of magnesia is considerable, except in C, 588 METASOMATIC PROCESSES IN FISSURE-VEINS. where it is slight. Potassa is strongly increased throughout; and there is a corresponding loss of soda. Di differs from the rest in an exceptionally high percentage of introduced lime and carbon-dioxide, and a corresponding loss of magnesia. Moreover, the alumina is so low that re- moval of this constituent must be supposed to have taken place. The characteristic features of the process seem to consist in the decrease of silica, magnesia and soda, and increase of lime, potassa and carbon-dioxide — this calcitic altered rock strongly contrasting with the quartz-filled veins. Sufficient data are not available for the accurate determination of change of volume during the process, and of the actual losses and gains. They could probably be determined by analyses and specific gravity determinations of very carefully selected samples of the fresh rocks, and of altered rocks immediately adjacent to them. It seems probable that, in most cases, the added material has more than balanced the losses. Idaho Types. — In the Rocky Mountain region appear other types related to that of California. These gold-quartz veins cut granites, diorites and various porphyries, and, like the California veins, are of pre-Tertiary, probably Cretaceous, age. They carry a strong percentage of sulphurets, but generally only a subordinate amount of free gold, most of the gold being closely held in the sulphides. The filling constitutes the rich ore, but the narrow zone of metasomatic rock may also yield some low-grade ore. In general character, the metasomatic action is similar to that of the California veins, though the de- tails of chemical change may differ. Gralena, zinc-blende and chalcopyrite, and occasionally also free gold, may appear in the altered rocks. The carbonates are less plentiful, and lime is more often subtracted than added. The following analyses il- lustrate the chemical changes in two prominent types. E and E, are the fresh and altered rock from Willow Creek district, Boise county. The narrow quartz-veins carry scarcely any free gold, but much auriferous galena, pyrite, arsenopyrite and zinc-blende. F and Fj are the fresh and altered rock from the Croesus mine, "Wood River district, Blaine county. The nar- row streaks of filling here consist of quartz, siderite, pyrrhotite and chalcopyrite, with a little galena, arsenopyrite and zinc- MBTASOMATIO PROCESSES IN FISSUEE-VBINS. 589 blende. Here again only a fraction of the gold is in free state. The ore contains very little silver.* Table III. — Analyses of Fresh and Altered Eocksfrom Gold- Quartz Veins. -f SiOj TiO^ AlA FeA FeO CoO, NiO MnO CaO SrO BaO MgO K,0 Na,0 HjO below 100° G. HjO above 100° C. PA CO^ S Fe Co, Ni., Pb Cu As 65.23 .66 16.94 1.60 1.91 trace 3.85 ".19 1.31 3.02 3.57 .18 .88 .19 .25 Ei. 66.66 .49 14.26 .67 1.33 trace 3.37 none .95 4.19 none .36 2.16 .17 3.67 .95 .84 F. 57.78 1.01 16.28 1.02 4.92 .02 .15 6.65 .07 .12 4.60 2.22 3.25 .34 .92 .30 .15 .02 Pi. 58.01 1.08 15.72 .64 3.87 none .17 2.15 none trace ? 2.07 4.79 .10 .31 2.71 .31 2.86 1.25 1.52 .12 .86 .05 1.65 Total 99.78 100.07 99.82 100.24 E, Fresh granitic rock immediately adjoining the Silver Wreath quartz vein, Willow Creek, Idaho. Ei, Altered rock, adjoining the same vein. P, Fresh quartz-pyroxene-diorite, adjoining the Crcesus vein, Hailey, Idaho. F^, Altered rock adjoining the same vein. The composition of the altered rocks may be calculated as shown in table on page 670. The appearance of the altered rock Fi from the Croesus mine is shown in Fig. 25. The specific gravity of E is 2.714. From the mineralogical composition given in the report quoted the specific gravity is calculated to 2.720, which is a close agreement, the diiFerence possibly indicating a very slight porosity. | * For full calculations and description of E and Ei see W. Lindgren, 18th Ann. Jtept. U. S- Geol. Surv., part iii., p. 640 ; for F and Fi see W. Lindgren, 20th Ann. Sept. U. S. Oeol. Surv., part iii., p. 211-232. t Analyst, W. F. Hillebrand. X In this calculation the following figures for specific gravity are used : quartz, 2.65; sericite, 2.83; biotite, 3.00; oligoclase, 2.65; orthoclase, 2.56. 590 METASOMATIC PROCESSES IN FISSURE-VEINS. Table IV. — Mineralogical Composition of E^ and F^, in Table III. El. Ti. Quartz 42.00 46.84 4.80 1.96 1.45 .49 1.78 36.18 38.18 11.76 3.11 1.26 2.19 1.08 .72 .58 .15 trace .99 .15 3.58 Sericite Chlorite Magneaite Rutile Pyrite Zinc-blende Chalcopyrite Total 99.32 99.93 The measured specific gravity of Ej is 2.774, indicating that the rock alters to denser minerals. The calculation of the same specific gravity from Table IV. gives 2.796, which shows a decided porosity of the altered rock. Under these circum- stances, no evidence of pressure being noted, it maybe assumed with fair accuracy that no considerable change in volume has taken place ; and by multiplying the percentages of E and E, by 2.714 and 2.774 respectively, and comparing the results, the absolute gains and losses per cubic meter may be obtained (see Table V.). In the same manner the measured specific gravities of F and Fj are compared with the calculated specific gravities.* This shows that similar conditions prevail here, the porosity being greater. By multiplying the percentages of F and Fj by the measured specific gravities, and comparing these data, the ab- solute gains and losses are again obtained. During the alteration of E to E,, 291 kilograms were added and 229 lost per cubic meter ; the net total being a gain of 62 kilos. During the alteration of F to Fj, 416 kilograms were added and 333 lost per cubic meter; the net total being a gain 83 kilos. A perusal of the table will show very similar results in the '■■ 20(A Ann. Bept. U. S. Oeol. Surv., part iii., pp. 211-232. MBTASOMATIC PROCESSES IN FISSTJRE-VEINS. 591 Table V. — Gains and Losses per Cubic Meter of E and F, Table III. SA TiOj AI2O3 FeA FeO Ck)0, NiO MnO CaO SrO BaO MgO Kfi Na^O H,0 above 105° C. PA CO2 S Fe Co, Ni Pb Cu As Total Gain. Kilos. 79 34 36 93 26 23 291 Per cent. 4.5 41.5 150.0 nearly all. all. all. Loss. s o O Kilos. "4 64 24 15 11 "5 9 97 "6 229 Per cent. 22.2 13.9 55.8 28.8 10.5 160.0 25.7 160.0 Gain. Kilos. 48 2 76 53 79 35 44 4 24 2 48 416 Cm 3 i 00 Per cent. 2.9 25.0 120.6 263.8 nearly all. nearly all. all. all. all. all. all. Loss. Kilos. '4 10 27 1 126 2 4 70 89 "0 333 0) ^ Per cent. .9 34.5 19.4 100.0 67.0 100.0 100.0 , 53.9 96.7 two rocks : a moderate addition of silica and a strong gain of potassa; nearly complete loss of soda, baryta and strontia; partial loss of alumina, magnesia and lime, F, however, losing much more lime than E. In Ej the amounts lost of Fe^Og and FeO are nearly completely converted into Fe (in FeSz). In F these losses are less and not sufficient to account for the gain of Fe; consequently iron must have been added. Phosphoric acid is constant, consistently with the fresh state of the apatite. San Juan, Colorado. — In the San Juan region, southwestern Colorado, are vast eruptive masses of andesites and rhyolites, 592 METASOMATIC PROCESSES IN FISSUKE-VBINS. with their accompanying tuffs and breccias. Some of the gold- quartz veins of Tertiary age occurring in these rocks at Tellu- ride have been described by Mr. C. W. Purington.* The ores consist of quartz containing native gold, with pyrite, galena and other sulphides. In some localities silver is also present in con- siderable amount. The principal gangue is quartz, though car- bonates also are occasionally present, and fluorite in consider- able quantities is mentioned from the Tomboy vein. This is worthy of note ; since, in ordinary gold-quartz veins fluorite, if not entirely absent, at least is exceedingly rare. The quartz forms, as a rule, a well-defined filling of open cavities, and the principal ore is of this character, and not altered country-rock. There are, however, more or less wide zones of partial altera- tion alongside the veins. The veins are often accompanied by large amounts of white, soft clay-like material, which probably is extremely altered country-rock. All of this appears to be sericite. Mr. Purington mentions having failed to establish the presence of kaolinite, except in one or two places. The ordi- nary course of alteration in the diorites, andesites and breccias is distinguished by the development of much calcite and seri- cite, and is consequently very similar to the metasomatic pro- cesses characterizing the California quartz-veins. The altered rocks contain small, abundant and sharply defined crystals of pyrite, more rarely of galena. Much of this pyrite fills the spaces formerly occupied by ferro-magnesian silicates. This pyrite is of low value, compared with the massive mineral oc- curring in the vein-filling. The percentage of silica in the al- tered rock is low, and contrasts with the abundant quartz on the veins. Rhyolite adjoining the veins is changed to felted sericite and some carbonates, as well as pyrite. As an exceptional process Mr. Purington mentions a complete silicification of the diorite from the hanging-wall of the Butter- fly vein, in the Terrible mine. This diorite, which consists of labradorite, hornblende and a little biotite, shows a complete replacement of the feldspar by cryptocrystalline silica, wMle the hornblende is replaced by pyrite. There is a. little sericite, but no carbonate present. The cause of tisiig abnormal altera- * 18(A Ann. JRept. U. S. Qe«L 3irv., part iii., pp. 745-846. METASOMATIC PROCESSES IN FISSUUB-VBINS. 593 tion might be in a local occurrence of waters in which the H^S had been oxidized to HjSO^. A partial analysis of the silicified diorite runs as follows : Per cent. SiOj 70.30 AlA,* 20.00 MgO 0.31 CaO 0.27 K.A 1.78 Na^O, 0.64 The large percentage of alumina indicates the presence of kaolinite, so commonly accompanying silicification. Sericite is also present. A further instance of silicification in part, probably, due to cementation, is mentioned in the case of certain sedimentary rocks adjoining the vein. Here again it is accompanied by a development of pyrite ; and the silicification extends to a dis- tance of 15 feet from the vein. The Treadwell Mine, Alaska. — According to G. F. Becker,t the country-rock of the celebrated Treadwell mine consists of a sodium syenite, which is strongly altered and 'traversed by small seams, carrying a value chiefly in gold. The whole mass of seams and country-rock is mined and milled. The process of alteration consists in a change of the albite, which is the pre- dominant mineral, into carbonates and pyrite. Sericite is also present, as well as a little chlorite. Gold-Veins of Ontario, Canada. — Peter McKellarJ describes quartz-veins in granite of Western Ontario at Lake of the "Woods, which are supposed to be of Archean age. The quartz-veins are only from 3 to 4 in. wide containing auriferous sulphides of copper, lead, zinc and bismuth. These narrow veins are adjoined by from 2 to 5 ft. of altered granite, largely consisting of a greenish fine-grained mineral, probably sericite. Some of this altered granite contains gold, and from 0.5 to 3 per cent, of auriferous pyrite, while the above mentioned sulphides rarely appear in it. The principal ore consists of this altered rock. Schwarzwald Fissure- Veins. — Much material of interest relat- ing to the alteration of country-rock is found in the well-known » Including Fe^O,,, TiOj, and P2O5. t ISlh Annual Beport U. S. Oeol. Surv., partiii., p. 64. t Trans., xxix., 104 (1899). 594 METASOMATIC PROCESSES IN FISSURE-VEINS. investigations of Prof. Sandberger.* In his discussions, as may be expected, superficial weathering is not always strictly separated from deep-seated alteration. In the fissure-veins of Schapbach, in the Schwarzwald, biotite of the granitic country- rock yields a chlorite rich in iron and a substance supposed to be pilite. The decomposition (alteration ?) of the gneiss in the same districts yields a mineral which Sandberger calls hy- grophilite, derived from the alteration of oligoclase. It has a specific gravity of 2.70, and is closely allied to muscovite, if not identical with it. The alteration of the schistose gneiss near the veins, involving a formation of hygrophilite from oli- goclase, shows a concentration of K2O and a decrease of Na^O. In one instance a conversion to carbonates was also noted. For a certain distance on both sides of the veins the rock is soft- ened and altered. The extent of this alteration, which is sup- posed to be favorable to the occurrence of rich ore-bodies, cor- responds to the extent over which its principal leaching has taken place. Sandberger thus derives the minerals of his veins directly from the adjoining country-rock. He further says :t "It is of the greatest importance for the understanding of the veins occur- ring in this granite area to examine the alterations which the rock has suffered by means of waters containing carbonic acid, and by means of weathering with free access of atmospheric oxygen." In the silver-veins of Wittich, Schwarzwald, Prof Sandber- ger finds that the alteration of the biotite is accompanied by the separation of TiOj as anatase or brookite. The oligoclase is transformed to a kind of pinitoid which Sandberger calls lepidomorphite, and which may simply be an impure and mi- crocrystalline muscovite. Two analyses are givenj of fresh and altered granite, the latter occurring close to a vein. The composition of the altered rock is almost the same as that of the fresh, except that a little iron, somewhat over one per cent, of lime, and an equal amount of magnesia, have been carried away. The potash remains practically constant, while about one-half per cent, of soda has been lost. The silica has suf- fered an increase of 2.5 per cent, the alumina of 1 per cent. Sandberger remarks with good reason that these slight changes * Uniersuehungen iiber Erzgange, i. and ii. t Op. eit., ii., p. 343. J Op. eil., ii., p. 347. METASOMATIC PROCESSES IN FISSURE-VEINS. 695 could very well have been effected by vfaters containing a little carbon dioxide. Near the vein of "Wittichen the ore-minerals are not confined to the fissure, but are also present to a remarkable extent in the altered rock adjoining it. The gangue is here quartz, barite, calcite and fluorite. The ores consist of native silver and vari- ous cobaltrminerals. The ore-minerals mentioned, accompanied by small crystals of chalcopyrite, occur abundantly in the altered granite. The veins traverse not only the granite but also the overlying Permian formation, although in these hori- zontal rocks they lose their mineral contents almost completely. They are rich only in the altered granite, the oligoclase and mica of which have been completely, and the orthoclase par- tially altered. It is acknowledged that these veins were formed when 1500 feet of rocks rested on the granite, and that conse- quently the temperature and pressure must have been higher than at the surface. In conformity with Prof. Sandberger's well-known views, the sulphides occurring on the veins are ex- plained by reduction of sulphates. The veins of Schapbach may be compared with those de- scribed from the Central Plateau of France by Daubree. The similarity in occurrence, gangue and ores is very striking; only, in the case of the latter we have undoubted proof of their intimate connection with actual ascending springs. 9. Silicic and Calcitic Oinnabar-Veins. The quicksilver-deposits of the Pacific Coast have been de- scribed by G. F. Becker.* The cinnabar occurs chiefly in zones of fracture or in fissure-veins, and is almost always asso- ciated with quartz and chalcedonic aggregates. Opal is very commonly present in the ores, but the sulphide of mercury is very rarely if ever directly imbedded in it. The main deposit of opal preceded that of cinnabar and quartz. Various rocks, such as diabase, diorite and serpentine, are adjacent to the quicksilver-veins. These rocks are nearly always more or less altered and converted into dolomitic car- bonates. Many of them are also silicified, being converted into opal. Serpentine especially is often transformed in this man- * Monograph XIII., U. S. Oeol. Sun. 38 596 METASOMATIC PROCESSES IN FISSURE-VEINS. ner, and all transitions between the fresh rock and the pure opal may be found ; the latter may retain the color and struc- ture of serpentine. Certain glaucophanes from the Lake Quicksilver mine are altered into microcrystalline quartz. Although Mr. Becker recognizes the fact that the country-rock has been altered by carbonization and silicification, he insists that the cinnabar has been exclusively deposited in open spaces, and does not appear as a product of direct replacement of the vsrall-rock. The quicksilver-deposits are of special interest, be- cause their intimate connection with ascending alkaline waters has been proved. These waters contain but little free carbon dioxide, earthy carbonates and earthy sulphates, but consider- able sodic bicarbonate and sodic chloride, and some hydrogen sulphide. 10. Sericitie Copper-Silver Veins. The copper-veins of Butte, Montana, which have been de- scribed by Emmons, "Weed and Tower,* form an excellent illus- tration of this class and, according to Mr. Emmons, are typical replacement>veins. The deposits appear along well-defined fis- sures in granitic rocks ; the principal gangue-mineral is quartz, the primary ores are pyrite, chalcopyrite, zinc-blende and galena. Bornite, chalcocite and covellite are regarded as sul- phides formed later under secondary influences. In the vicinity of the veins the country-rock is impregnated with vein-mate- rial, generally pyrite and quartz. An impregnation of enar- gite has also been observed. Sericite and, later, kaolin have also been developed in the rock. The extent of the altered zone is generally proportional to the size of the ore-bodies, and may extend to a distance of 100 feet from the vein. According to the proportion of copper in such an altered mass it may con- stitute pay-ore or be considered as barren material. 11. Silieic and Dolomitic Silver-Lead Veins. The association of silver-lead deposits with limestone and other calcareous sedimentary rocks is a well-known fact, occur- ring again and again in all parts of the world. Very many of these deposits are not fissure-veins, or connected with such. But even among those genetically related to fissures, the ores * Folio 88, U. S. Oeol Sun. MBTASOMATIC PROCESSES IN EISSURE-VEINS. 697 seldom form well-defined tabular masses, but occur mostly as irregular bodies, while the ducts through which the solutions found access have received but scanty deposits of ore. This is due to the great tendency of galena and zinc-blende, which in these deposits form the principal ore-minerals, to replace the limestone. Beyond doubt such a replacement very often occurs. It was convincingly established by Mr. Emmons in Leadville, Colo., and by Mr. Curtis in Eureka, Nev. In both these cases the demonstration was furnished by the study of struc- tural relations, without the aid of microscopic examination. Indeed, the latter was scarcely possible, since in both these mining districts operations were still in the zone of oxidation, which obscured the relations of primary ore-minerals to the limestone. Since these reports were published, the conditions governing the replacement of the galena have not been greatly elucidated, except in Spurr's study of the Aspen district. The chemical reactions involved were, and are yet, in some doubt ; the principal question being whether the galena was reduced from solution of sulphate of lead or deposited from sulphide so- lution (see p. 617). The microscopic study of the attending phenomena must help to settle this point. The ores are accompanied either by a gangue of jasperoid, cherty rocks chiefly composed of silica, or by different carbon- ates, such as calcite, dolomite or siderite. (Those accompanied by a siderite gangue will be treated as a separate class.) Sericitic minerals are absent. The gangue-minerals mentioned have also very largely replaced the limestone. In the Elkhorn mine, Mont., studied by W. H. "Weed,* bodies of galena appear in a crystalline limestone and are di- rectly connected with a fissure-vein. The beginnings of replace- ment are shown in specks of intergrown galena and pyrite, scat- tered through the rock, and always accompanied by small crystals of secondary quartz. The larger grains of galena are surrounded by a narrow rim of pyrite (see page 617 and Fig. 29). Mr. Emmons describes the fissure-vein of the Queen of the West mine, Ten Mile district,f Colorado. The principal fissure is partly filled with barren calcite, while galena and blende re- * Unpublished notes. f Folio 48, U. S. Oeol. Surv. 598 METASOMATIC PROCESSES IN FISSURE-VEINS. place the country-rock, consisting of sandstone and intercalated sheets of porphyry. The vein is characterized, besides, by a number of parallel fault-planes, from which replacement has taken place. Aspen, Colorado. — Mr. Spurr, in his description of the Aspen district,* with its wonderfully complicated system of faults, has given a valuable description of the metasomatic processes there observed. The Aspen deposits are not, strictly speaking, fis- sure-veins. The ores form irregular bodies of lead- and silver- minerals in limestone ; but these irregular bodies are closely connected, genetically, with faults which yielded a pathway for the ascending waters. The processes consist in dolomitization, ferration, silicification, and lastly, the introduction of metallic sulphides. The ores occur in part as filling of pre-existing cavities, but more generally replace the limestone adjoining the fissures. The dolomitization which proceeds irregularly from the fissures is well shown under the microscope, the coarse cal- cite being broken up into smaller rhombohedral crystals of the yellowish tinge characteristic of dolomite. Silicification usually accompanies dolomitization. In the limestones the process goes On in the following manner. Many tiny quartz-grains first ap- pear scattered through the rock, chiefly along areas of slight shearing or fracture (Fig. 3). Here and there appear long slen- der quartz crystals, entirely surrounded by fresh limestone. As silicification proceeds, the slender crystals multiply, forming a characteristic network, sometimes enclosing small areas of cal- cite which are sprinkled with small, irregular quartz-grains, down to the most minute dimensions. The final result is a rock made up of crystalline quartz-grains of varying size, in which the retiform structure is still apparent (Fig. 4), and which rock resembles a chert or a fine-grained and altered quartzite, and is generally somewhat porous, drusy, and also often colored red or yellow. In structure, appearance and ori- gin, this cherty rock is identical with the jaspers of Lake Supe- rior. Mr. Spurr proposes " jasperoid " as a term for this rock, consisting essentially of cryptocrystalline, chalcedonic or phan- ero-crystalline silica formed by the replacement of other rocks chiefiy limestone. At Aspen this jasperoid forms big reefs along fault-lines. * J. E. Spurr, Monograph XXXI., U. S. Geol. Sun. METASOMATIC PROCESSES IN FISSURE-VEINS. 599 Dolomitization and silicification are always accompanied by a certain amount of ferration. Usually the iron appears in the partly silicified rocks as small rhombohedrons of siderite ; but pyrite is also present, and in many cases the two minerals have been deposited simultaneously. In the final process of mineralization, the altered limestone is always traversed by reticulated fractures. In every case the ores are first introduced along these crevices ; and often this is the only method of mineralization. With greater alteration, metallic minerals penetrated from the fractures into the rock on both sides. The solutions traveled between adjacent crys- tals of calcite or dolomite, and also along the cleavage-planes of these minerals. In this manner a still finer network was formed, which, by spreading and consolidation, resulted in a continuous mass of sulphides. There is no doubt, Mr. Spurr says, that this is an actual process of replacement ; the calcite or dolomite being taken up, molecule by molecule, and replaced by metallic minerals. The sulphides are often accompanied by granular quartz and dolomite, the relations of which show that they have been simultaneously deposited. 12. Sideritic Silver-Lead Veins. In this class, which, like the preceding, ordinarily occurs in sedimentary rocks, not much secondary silica is formed. The principal gangue-mineral is siderite, often accompanied by other carbonates, and nearly always also with some pyrite; in fact the co-existence of pyrite (often also marcasite) and siderite is a notable feature. The other principal ores are galena and zinc-blende. The Eureka, Nev., deposits probably belong to this type. Wood River, Idaho. — Prominent representatives of this class are the Wood river silver-lead veins, near Hailey, Idaho,* which occur chiefly in calcareous carboniferous shale, and are of pre-Miocene age. In the structure of the vein and arrange- ment of the ore-bodies replacement is clearly indicated ; and galena often occurs as scattered grains throughout the shale. But some filling of pre-existing cavities has also taken place. The rocks clearly contain much organic material ; and the theory of deposition by the reduction of lead sulphate is pos- » W. Lindgren, Wth Ann. Bept. U. S. Oeol. Sun., part iii., pp. 190 to 231. 600 METASOMATIC PROCESSES IN FISSURE-VEINS. sibly applicable. But this argument is greatly weakened by the occurrence of veins of the same composition in a neighbor- ing body of granite. Coeur d'Alene Mountains, Idaho. — There exist, perhaps, no better instances of metasomatic fissure-veins than the celebrated silver-lead deposits of the Coeur d'Alene mountains, in Northern Idaho. They are clearly defined fissure-veins cutting fine- grained greenish quartzites and quartzitic slates of doubtful (though probably Algonkian) age. The principal ores are galena and zinc-blende, but there is also much finely distributed pyrite. Chalcopyrite is ordinarily absent. These are prac- tically the only metallic minerals, and recur in all the deposits. The principal gangue-mineral is siderite, accompanied by minor quantities of quartz and barite. Fluorite is absent. The fis- sures along which the ore-bodies appear are well defined, and sometimes continuous for one or more miles. The ore-bodies do not show much clearly defined crustification or other evi- dence of having been deposited in open spaces. The siderite appears always as an undoubted product of replacement, while many veinlets of quartz have in part resulted from the filling of open small fissures. Evidences of gradual transitions from ore to country-rock are abundant, and are especially prominent in the mines carrying low-grade ore, as, 'for instance, in the Helena and Frisco. In the exposures underground, as well as in the specimens and thin sections, the evidence of replacement is complete and positive. The greenish-grey fine-grained quartzite, which constitutes the prevailing country-rock, contains no sulphides when fresh. It is composed of small, rounded, or subangular quartz grains, closely packed — often, indeed, jointing closely, as in a normal quartzite. Usually, however, a little sericite, in bunches of small fibers, is present as cementing material between the grains. This sericite is apparently an autogenetic mineral, formed during the metamorphism of the sandstone to a quartz- ite. Occasionally small foils of it project into the quartz, show- ing a slight incipient sericitization of the latter mineral. There are few other minerals, except a little feldspar in clastic grains, small prisms of tourmaline, and some grains of calcite. Near the veins minute specks of siderite, zinc-blende, pyrite and galena appear in this quartzite; and these scattered grains METASOMATIC PROCESSES IN FISSUEB-VEINS. 601 gradually merge into bodies containing 3 per cent, and more of galena, thus forming a merchantable ore. The thin sections show how the rock near the veins is filled with small grains of branching and irregular form, which consist of siderite, de- veloped by attack first upon the ground-mass and then upon the grains of clastic quartz. This process is well shown in Fig. 16, which is reproduced from a thin section of Helena and Frisco country-rock. Accompanying the siderite are small grains of zinc-blende, cubes of pyrite and irregular wiry masses of galena. All these sulphides appear not only in or near the siderite, but also in the cementing sericite, and in the apparently perfectly fresh quartz grains. At a more advanced stage (Fig. 17) these areas of siderite extend until they join, and thus completely replace the rock. In the specimen from which Fig. 17 was taken, masses of sid- erite are seen to be merging gradually into the fresh quartzite. In the resulting ore lie scattered many small quartz grains, representing remnants of the clastic constituents of the quartz- ite. Occasionally larger masses of zinc-blende appear to form directly in the quartzite by metasomatic replacement of the quartz. The sericite in the quartz then disappears, though once in a while small foils of it may be detected. During the transition stage, seams and narrow veinlets in the altering rock are filled with sericite, apparently segregated there, when driven out from the main mass. In other specimens from the Helena and Frisco mine, the replacing siderite has a strong tendency to idiomorphic development. Imperfect rhombohedral forms are often seen, sometimes cutting straight across the clastic grains (Fig. 18). Certain specimens from the Bunker Hill and Sullivan mine show quartzose greyish masses of irregular out- line, and apparently merging gradually into the normal green- ish quartzite. These quartzose masses consist of very irregular interlocking grains of quartz, not in the least similar to the quartz usually deposited by processes of filling, but having every appearance of resulting from the silicification of the quartzite. This silicified portion contains irregular grains of pyrite, galena and brown zinc-blende, with a very little siderite. The process, as outlined, is remarkable, as involving a meta- somatic replacement of quartz by siderite, pyrite, galena and zinc-blende, and is the only clearly defined occurrence of this kind of which I am aware. -602 METASOMATIC PROOESSES IN FISSURE-VEINS. This description would not be complete without mention of certain interesting veinlets produced by replacement in the Bunker Hill and Sullivan quartzite. Certain specimens from this mine show a dark greyish-green, very fine-grained quartz- ite, traversed by minute veinlets, carrying quartz and sur- rounded by a greenish material. Under the microscope the rock is seen to be a typical fine-grained quartzite or quartzitic sandstone. The grains are separated, not only by fibers of muscovite, but also by a green mica, probably related to bio- tite. The veinlets are clearly formed by replacement along narrow cracks, and contain a mass of green mica in fine dis- tribution, diminishing away from the seam, together with quartz, garnet, brown zinc-blende, and small prisms of tourma- line, and a small quantity of galena. I have mentioned these peculiar products of replacement because they differ so com- pletely from the deposits as described above. Their formation must be sought in some local cause, involving a change in the mineral-bearing solutions, or in the conditions of the deposi- tion. The presence of garnet in these veinlets is especially re- markable, as this mineral rarely occurs in fissure-veins. 13. Sericitic Lead-Silver Veins. The Clausthal Veins. — The alterations produced in the clay slates adjoining the vein-system of Clausthal have been de- scribed by V. Groddeck.* The fissure-veins at Clausthal, which principally carry galena, pyrite and zinc-blende in quartzose gangue, are enclosed in black clay slate belonging to the Culm formation ; and to the eye these slates, when enclosed in the vein or lying close to it, ordinarily present no alteration, except such as may result from mechanical deformation or crushing. By a series of analyses, v. Groddeck has shown that, as a mat^ ter of fact, these wall-rocks have suffered alteration consider- able in degree, although not apparent to the eye. Some aver- age analyses are given in Table VI. Comparing the first two analyses, it is apparent that a large part of the protoxide of iron has been carried away, and that at the same time the magnesia has been considerably reduced. These subtractions result in an apparent increase of the other * " Studien fiber Thonsohiefer, Gangthonschiefer und Sericitschiefer." Jahr\ buck der konigl. prevss. geol. Landesanstalt, 1885, pp. 1 to 53. METASOMATIO PROCESSES IN EISSURE-VBINS. 603 Table VI. — Analyses of Clausthal Rocks. I. II. III. SiOj Per cent. 56.59 23.14 .61 4.87 'i'.sb .35 3.05 .75 4.01 .38 .64 .67 .14 2.56 Per cent. 59.31 23.72 1.13 1.06 'i'.'ii .36 3.91 .80 4.60 .23 .95 .85 '2.'62 Per cent. 79.12 13.93 .44 'ais .64 1.56 not dt. not dt. not dt. not dt. 1.60 ALO, Fe,0, FeO MnO MgO CaO K.,0 Na^O HjO Rutile Carbon Pyrite Apatite Carbonates I. Black normal clay slate of the Culm formation. Average of three analy- ses. — II. Black clay slate adjoining the vein or enclosed in it. Average of seven analyses. These black altered clay slates are always present in or along the veins. — III. Variegated clay slate adjoining the vein. Average of four analyses. These variegated slates are apparently extreme forms of alteration, and are con- spicuous by means of their red or yellow color. It is remarked, however, that this form of alteration is an unusual one, only appearing locally in a few mines. The processes to which these altered rocks have been subjected are considered to have been different in kind from those producing the ordinary black altered slates. constituents. Very notable is the fact that the alkalies remain nearly constant, and that no soda has been subtracted — a most unusual case. The lime, rutile, carbon, pyrite and carbonate have suffered but little change. The amount of alumina is almost identical in the two analyses; and on the assumption that this constituent has remained constant, the two analyses can be directly compared. Comparing the first with the third, a very strong increase in silica and decrease in alumina is noted, accompanied by an almost complete disappearance of the protoxide of iron, mag- nesia and lime, the alkali apparently remaining practically con- stant. It is clear that the alumina has been carried away to a considerable extent, and the process is, on the whole, similar to the alteration which results from the action of the solutions containing free sulphuric acid on aluminous rocks. Sericite and chlorite form part of the fresh rock, and the former is a prominent constituent of the altered rocks. Basing the calcu- lations on the following formulae : 604 MBTASOMATIC PKOCESSES IN FISSURE-VEINS. Sericite, 2H,0 + (K, Na, Ca) + 3 (Fe Al), 0, + 6SiO„ and Chlorite, 4H,0 + 5 (Mg Fe) + Al^ 0, + 3SiO„ and disregarding the small amounts of carbonate, pyrite, etc., the following results are obtained : Fresh clay slate. Vein clay slate. Variegated clay slate. Per cent. Per cent. Per cent. Sericite, . . . .39.24 47.45 34.89 Chlorite 16.54 4.37 Quartz, . . . .35.30 34.40 63.24 These are the averages of the calculations of all the analyses. The character of the alteration is thus clearly seen to consist in a chemical change of the chlorite into sericite, with simultane- ous subtraction of FeO and MgO. The quartz is practically constant. In the case of the variegated clay slates, the change appears to be of a different kind. A comparison of the third column with the first shows that the following reactions have taken place : 1. The chlorite has been completely destroyed ; its bases have been carried away, and its silica has probably been added to the free quartz. 2. The percentage of sericite has been diminished (more, in fact, than the 4 per cent, shown by the comparison of the calculations, since we must consider the amount of the bases carried away). 3. The percentage of quartz has been increased by the introduction of free silica besides that obtained from the alteration of the sericite and the chlo- rite. It must again be emphasized that this process points to the action of a solvent, probably sulphuric acid, capable of car- rying away considerable amounts of alumina. The Democrat Vein, Railey, Idaho. — The Carboniferous strata near Wood river, Idaho, contain masses of intrusive granite, or, as more specifically determined, quartz-monzonite.* This rock is cut by fissure-veins containing galena, sphalerite and tetrahedrite, with siderite and calcite gangue ; the ore being due, partly, to filling of open fissures, partly to replacement. For a few feet on each side of the vein, the granite is altered and contains some pyrite, galena and zinc-blende. The altered rock is of greyish-green color and its texture unmistakably in- dicates its derivation. The biotite of the granite is converted * W. Lindgren, Wth Ann. Bept. U. S. Oeol. Surv., partiii., pp. 206 and 212. MBTASOMATIC PROCESSES IN FISSUEE-VEINS. 605 to large foils of muscovite ; the feldspars are also completely changed to radial tufts and scaly aggregates of sericite, mixed with calcite grains. The quartz grains are in places vigorously attacked by sericitization and carbonatization, in the manner illustrated in Fig. 13. The apatite is completely unaltered, and the titanite is converted to bunches of rutile needles. A little chlorite remains. For complete analyses and calculations, the reader is referred to the report cited. The altered rock con- sists of: Quartz, 55.07; sericite, 31.78; chlorite, 7.21; calcite, 4.39; siderite, 0.05 ; rutile, 0.40; apatite, 0.23; pyrite, 0.19; pyrrhotite, 0.07; zinc-blende, 0.14; water (hygroscopic), 0.37; total, 99.90 per cent. From the determination of specific gravity it is concluded that no change of volume has taken place, but the granite has altered to an aggregate of denser minerals; the result being a rock of considerable porosity. On this basis, namely, the com- parison of equal volumes, the following changes, expressed in kilograms per cubic meter, have taken place : Table "VTI. — Gains and Losses of Country-Bock of the Democrat Vein, Idaho, During Alteration. SiO^ TiOj AlA , FeA FeO MnO CaO SrO BaO MgO K^O Na^O HjO below 105° C. HjO above 105° C, PA CO, S Fe Co, Ni Zn Gain. Per Cubic Meter of Origi- nal Kock. Kilos. 22 3 31 "43" 4 3 108 Percentage of, for Each Con- stituent. Per cent. 42.3 150.0 155.0 nearly all . nearly all. all. all. Per Cubic Meter of Origi- nal Rock. Kilos. 49 3 99 10 6 1 3 18 33 80 5 308 Percentage of, for Each Con- stituent. Per cent. 2.7 2.3 24.7 61.6 8.5 100.0 100.0 56.2 29.0 93.0 35.8 25.0 606 MBTASOMATIO PROCESSES IN FISSURE-VEINS. The result shows a total loss of substance of 200 kilograms per cubic meter. The losses extend over all the bases and the silica ; baryta and strontia being completely removed without the appearance of barite in the vein. The gains chiefly con- sist in water, carbon dioxide, ferrous oxide, sulphur and zinc. Both potash and soda are removed ; the former only partially, the latter almost completely. Calculated without regard to porosity, by comparing equal weights, the result is reached that the rock has received an addition of substance ; but the manner here indicated is doubtless the correct way of regard- ing the process. 14. Zeolitic Copper- Veins. The copper-deposits of Michigan are in part fissure-veins cut- ting across the beds of melaphyre and other basic igneous rocks so common in that district. It is true, however, that the ore- bodies of the large mines are not to be considered as fissure- veins, but rather as beds or strata along which copper has been deposited by a process of replacement. R. Pumpelly* investi- gated the copper-deposits of Michigan and published part of his results in 1873. Further contributions to the same subject are found in his celebrated paper on " The Metasomatic Develop- ment of the Copper-Bearing Rocks of Lake Superior."t In these investigations the theory of metasomatic replacement was applied to American ore-deposits, and in this field Prof. Pum- pelly is clearly the pioneer in this country. The copper- bearing veins contain a number of minerals not ordinarily present in fissure-veins, and are, therefore, of special interest. Among these minerals are the zeolites : laumontite, apophyllite and analcite. There are also present as gangue minerals, preh- nite, datolite, chlorite, delessite, calcite, orthoclase and quartz. The principal ore-mineral is, of course, the native copper. Of sulphides, chalcocite and bornite are sometimes, but very rarely, encountered. According to Pumpelly's description, the veins must be due in part to filling ; but very largely, perhaps predominantly, the ore results from metasomatic replacement. The stages of this alteration Prof. Pumpelly considers to have been : 1. A forma- * Oeol. Sun. of Mich., vol. i., part ii. t Proe. Am. Acad, of Arts and Sciences, vol xiii., 1877-78, p. 253. METASOMATIC PROCESSES IN FISSURE-VEINS. 607 tion of chlorite in the amygdaloid rock; 2. Individualization of non-alkaline silicates, such as laumontite, prehnite and epi- dote ; 3. Deposition of quartz ; 4. Introduction of native cop- per, accompanying which there was a replacement of prehnite by a green earth or delessite, often intimately connected with the copper; 5. Appearance of the alkaline silicates, such as apophyllite, orthoclase and analcite. This occurrence of secondary orthoclase or adularia is of special interest in view of the fact that the same mineral has lately been found to form an important gangue-constituent in certain Tertiary fissure-veins in volcanic rocks of the West. It is considered that the alkaline silicates represent the final stage, namely, the decomposition of the labradorite of the original rock, while the chloritization represented the first stage of alteration, namely, that of ferro-magnesian silicates to chlorite. Prof Pumpelly thinks that copper was originally present as sulphides in the rocks, and that the changes, consist- ing in leaching and re-disposition in veins, have been effected by surface-waters carrying carbonic acid and some atmospheric oxygen. The copper was deposited after the destruction of the ferro-magnesian minerals, and before the deposition of the products from the decomposition of the feldspars. From the state of sulphide, copper was converted to silicate, carbonate and sulphate. These, salts were then reduced to a metallic state. He thinks also that there is a close genetic relation between this metallic copper and the ferric condition of the iron oxide in the associated silicates. The oxidation of the iron was caused by the reduction of the oxide of copper at the expense of the oxygen of the latter. Prof E. D. Irving, in his report on " The Copper-Bearing Rocks of Lake Superior,"* confirms in general the conclusions of Prof. Pumpelly. He considers the veins as very largely replacement-veins not sharply defined from the surrounding rocks, but simply the result of a rock-alteration entirely analo- gous to that which has brought about the deposition of copper and its associated vein-stone minerals within the cupriferous amygdaloids. They are alteration-zones, which traverse instead of follow the bedding. The replacement of wall-rock by cop- * Monograph V., U. S. Geol. Sun., 1883. 608 METASOMATIC PROCESSES IN FISSURE-VEINS. per masses is a common occurrence ; and the paragenesis of the vein-minerals is identical with that of the copper-bearing amygdaloid rocks. Especially remarkable is the series of replacements which, as shown by Pumpelly, has taken place in these veins. Preh- nite is pseudomorphic after plagioclase ; and many amygdaloids are largely prehnitized. This prehnite is again replaced by orthoclase ; and finally, the latter may change into epidote and quartz. Sericite is absent. These copper-bearing veins are clearly very different from the majority of fissure-veins, and have been formed under very different conditions — in fact, probably not by thermal waters. Of other classes, the orthoclase-albite-zeolite veins of the Alps are most closely related ; while a certain slight resemblance also exists to the propylitic veins, emphasized by the chloritic alteration and the presence of orthoclase. The veins of Kongsberg, Norway, and Andreasberg in the Hartz mountains, both of which also carry zeolites, are not sufficiently known in their metasomatic aspects to be discussed here. Observed Alteration by Ascending Waters. Extremely little exact work has been done in this most im- portant line of investigation, namely, to ascertain actual altera- tions by waters of known composition. . In veins we usually have only the altered rock as a known quantity, and must en- deavor to draw conclusions from this as to the character of the waters. Most interesting and well known is Prof. Daubree's discov- ery of the alteration which the old Roman bricks and mortars have suffered at the place where the mineral springs of Plom- bieres break through the granite, ascending on fissures carry- ing fluorite and quartz. The waters are thermal, having a temperature of 70° C, and may be characterized as weak min- eral waters, containing sulphates and chlorides, with a little hydrogen sulphide ; silicates of potash and soda are also present in them. In the bricks employed by the Romans as curbing for the spring, a number of minerals have been depos- ited. Chief among them are the zeolites. Chabazite, meso- type, and apophyllite are the principal minerals formed in the pores and spaces of dissolution in the old bricks. Besides MBTASOMATIO PROCESSES IN FISSUEE-VEINS. 609 these, opal and chalcedony have been deposited ; and, on one piece of mortar, fluorite, scalenohedrons of calcite, and prisms of aragonite were also found. This is extremely interesting, as the fissures on which the spring rises contain much fluorite. The apophyllite also was found to contain a notable percentage of fluorine. An analysis of this altered brick gave the follow- ingresult: SiO„ 19.39 ; Al.O,, 17.33; Fe,03,5.37; CaO, 51.40 ; MgO, 0.75; E:,0, 5.94; 'Sa.fi, 0.33; total, 100.51 per cent. In this analysis the most remarkable fact is the strong preva- lence of potash and the small quantity of soda present. It is scarcely to be assumed that the ordinary bricks of that locality contained the alkalies in this proportion. The porous bricks were evidently specially adapted for the formation of new minerals, and the large percentage of lime in the mortar also facilitated the process. Daubree mentions that pieces of gran- ite enclosed in the same mass show no zeolitization, and refers at the same time to the fact that pyroxene and feldspar show no alteration in the same superheated glass tubes in which glass is completely transformed into zeolites and into silica. This shows in a striking manner the dependence of the altera- tion of the country-rock upon its structure and composition. Mr. W. H. Weed has recently* found a mineral vein in process of formation by a weak thermal water at Boulder, Montana. The vein-filling consists of quartz, calcite and some stilbite, while the adjoining granite is partly altered to sericite and kaolinite ; these two minerals attacking all of the primary constituents (Fig. 19). A little free silica is also mixed with the kaolinite. In some specimens the kaolinite and sericite are subordinate and the feldspar appears partly silicified. Conclusions. Some of the following conclusions, drawn from the data presented in this paper, may seem trite repetitions of already known facts; but it is perhaps well to remember that our knowledge of the genesis of mineral deposits is not built on such firm foundations that it does not need fortification of its position by conclusions from all possible view-points. 1. Almost all fissure-veins are bordered by altered zones of * Communicated to me from an unpublished paper. 610 METASOMATIC PKOCESSBS IN FISSURE-VEINS.. varying extent and intensity of alteration. In the so-called " replacement-veins " this altered and replaced rock contains the valuable ore. 2. The metasomatic processes in different classes of veins show an almost kaleidoscopic variety. In one class of veins, quartz may be converted into calcite, while in a different class calcite may be converted into quartz. The action is usually in- tense, involving a great change in the chemical composition. 3. The hydration connected with the alteration is only very moderate. 4. The most prominent mineral formed by the metasomatic processes is a potassium mica (muscovite, sericite, zinnwaldite and many other related species). The most prominent process is the progressive elimination of soda and concentration of potash, closely connected with the formation of potassium mijsa. 5. The metasomatic processes in fissure-veins differ distinctly in most cases from those involved in ordinary static, dynamic and contact metamorphism, and the two classes of change have not generally taken place under the influence of the same con- ditions and agencies. Greisen is only found near cassiterite- veins. Granite, thoroughly changed to sericite, calcite and pyrite, is never found as a result of any other metamorphism than in fissure-veins, nor are fiuoritized or sideritized rocks so found. The propylitic and biotitic ^Iteration; the chloritic and zeolitic alteration of Lake Superior copper-veins; and the silicification in limestone and other rocks form exceptions, being similar to certain developments of dynamic, static and hydrochemic metamorphism. 6. Ordinarily, the alteration consists in the total or partial loss of certain constituents ; the gain of others ; and- the intro- duction of new compounds and elements, usually carbon diox- ide and sulphur. The net total of the change per unit of weight or volume may be a gain or a loss, perhaps more often the former. If sulphides are abundantly introduced, the result will usually show. a strong gain in mass. 7. The processes observed are such as can only be explained by aqueous agencies. Possible exceptions are the forms of alteration connected with cassiterite, apatite and tourmaline- veins, in which pneumatolytic conditions may have partly obtained. METASOMATIC PROCESSES IN FISSURE-VEINS. 611 8. The intensity of the processes observed indicates that the aqueous solutions acted under moderately high temperature, pressure and concentration. No cold, pure surface-water could produce such results as are ordinarily found. 9. From the fact that the substances introduced, such as sulphur, carbon-dioxide, fluorine, boron and heavy metals, are only known to be contained in noteworthy quantities in ther- mal waters ascending on fissures, it is concluded that these waters were the agencies usually active in the process of altera- tion. 10. Many of the substances found in the filling of the open spaces along the fissure may be lacking in the altered rock, showing that the latter forms a septum not penetrated with equal ease by all constituents of the solution.* 11. The ascending waters are chiefly surface-waters, which, after a circuitous underground route, have found in a fissure an easy path on which to return. During their long downward passage they doubtless dissolve much material from the rocks which they penetrate ; and this solution was facilitated by the gradually increasing heat and pressure with increasing depth. During the ascending period, much of this material is depos- ited. The metasomatic action on the wall-rock results in further exchanges of constituents, some being dissolved and others deposited. For many veins, this genetic theory may be fully sufficient. But for many others, perhaps for the majority of fissure-veins, something seems to be lacking in this explanation. The differ- ence in the metasomatic processes in veins and in other forms of metamorphism must be taken into consideration, as well as the abundance of certain constituents, such as carbon dioxide and hydrogen sulphide, in mineral waters. The presence of these constituents has not been satisfactorily explained, and cannot be, except in certain cases, on the theory of solutions derived from the solid country-rock. I believe that the majority of fissure- veins are genetically connected with bodies of intrusive rocks, even when the actual deposits are contained in the overlying surface lavas. It is well known that the intrusive rocks, such as granite, diorite * The existence of such osmotic conditions was first suggested by G. F. Becker [U. S. Oeol. Surv., Min. Res., 1892, "Quicksilver Ore-Deposits," p. 159). 39 612 METASOMATIC PROCESSES IN FISSTJRB-VEINS. and gabbro, may contain at the time of their intrusion water, carbon dioxide, fluorine, boron and sulphar. Under decreas- ing pressure, these substances have a tendency to leave the cooling magma; and as many of them form, with the heavy metals also contained in the magma, volatile compounds with a low critical temperature, these heavy metals may be carried away from the magma along with the " mineralizing agents " mentioned above. This is the well-known theory which was originated by Elie de Beaumont and Daubree, and developed by other French investigators ; but until recently it has hardly received the attention which it deserves. The results of these emanations is shown in the contact metamor- phism and in the mineral deposits often appearing near the boundaries of intrusive bodies. Where fissures traverse the cooling magmas, and the rocks surrounding them, it is natural that these mineralizing agents carrying their load of heavy metals should ascend, at first under pneumatolytic conditions, above the critical temperature. Reaching the zone of circu- lating atmospheric waters, it is natural that they should mix with these, which probably greatly predominated in quantity. To this combination of agencies, found in the ascending waters of such regions of igneous intrusion, the formation of most metalliferous veins is probably due. This dependence of veins on intrusive bodies is most clearly perceived in certain cassiterite-, apatite- and tourmaline-veins; but from these all sorts of transitions may be found, to veins of more ordinary character. I am by no means prepared to deny that some classes of veins may be due to circulating surface-waters alone ; but I do not believe that the dissolving power of the latter is sufficient to account for all classes, or even for the majority, of fissure- veins. DISCUSSIONS. (Secbetaby's Note.) The following discussion of the papers of Van Hise, Emmons, Lindgren and Weed, read at the Washington meeting, February, 1900, and printed on pp. 282 to 498 of this volume, comprises communications to the Secretary, received at various times before the Richmond meeting, February, 1901, and, for the most part, presented at that meeting. These communications are introduced at this point in the present volume, in order that they may precede the papers of Vogt, Kemp, Lindgren, Eickard, etc., presented at Richmond, and therefore not forming part of the material considered in them. Since many of the contributors have mentioned in one communication several of the papers re- ferred lo, no attempt is here made to divide their remarks and distribute the fragments under separate headings. In each contribution, however, the several topics are indicated by sub- titles. R. Beck,* Freiberg, Saxony : Prof. Van Hise's Paper. — The paper of Prof. Van Hise (p. 282) represents a great step of scientific progress, in that the circulation of underground waters has never before been presented with such clearness, in the light of modern chemical and physical knowledge. The theories of the formation of ore-deposits which follow the author's general survey of the currents and solvent power of atmospheric waters in the earth's crust do not, indeed, seem to be novel, being essentially an amplification of Le Conte's views; but the proofs adduced in their support are in many particulars so original that no one can read without profit this portion of the paper. Entirely new (though largely in agreement with the papers of Emmons and Weed, presented at the same meeting of the Institute) are the sections dealing with the formation of the rich sulphides of the precious metals, and especially the re- generation of normal sulphides, such as galena, etc., in vein- zones immediately beneath the ground-water level. Nevertheless, it appears to me that Prof "Van Hise, in the course of his most instructive exposition of unquestionable, yet still locally limited, phenomena, has been too much biased in favor of the " descensionists." This is indicated by the small importance which he attaches to the intimate genetic re- lation between epigenetic deposits and the plutonic hearths of the earth's interior. * Professor, Royal Saxon Mining Academy. Translated by the Secretary, and translation approved by the author. ( 613 ) 614 THE GENESIS OF ORE-DEPOSITS. In my treatise, just published,* I have proved from many in- stances that the formation of ore-veins is frequently a direct consequence of the plutonic intrusion, particularly of acid magmas. Prof. Van Hise recognizes such a relation only to this extent, that atmospheric waters, in their downward course, may have happened to reach eruptive masses, perhaps long since solidified, though, in common with the enclosing rocks, still warm, and may have extracted the disseminated mineral compounds from these old magmas. But we still hold to the conception of an immediate relation in time also. We hold as a primary principle that the gases and vapors contained in the fused magmas, and escaping as these cooled, must have played, as carriers of metallic compounds upwards from the region of the plutonic hearth, a very active part. Especially does the study of contact-metamorphism {e.g., at Kristiania, Norway, in the Banat, and at Berggiesshiibel in Saxony) strengthen us in this conviction. As concerns the cassiterite-veins, this view has many ad- herents. The direct connection between granite intrusions and the formation of veins carrying tin-ore, I have lately been able to establish still more firmly by showing that at Zinnwald, in Saxony, small cassiterite-veins in the periphery of the granite mass of that district are cut by veins of a fine-grained "vein- granite." The deposition of the tin-ore must therefore have been still in progress at the time of additional intrusions of granite from below. But there is by no means in the Erzgehirge a sharp separation between the veins of cassiterite and those of silver-lead-ores. The latter sometimes contain constituents characteristic of the tin-ore group ; and they are likewise connected with the in- trusive plutonic masses of the Erzgebirge. The latest work of American observers upon gold-veins, es- pecially that of Spurr and of Hussak (Brazil), has shown that, for many gold-quartz-veins also, there must exist a very in- timate genetic and chronological connection with deep plutonic intrusions. Thus, the nature of many gold-quartz-veins is closely allied to that of the pegmatites — those peculiarly-modi- fied derivatives from deep granitic hearths. In this depart- ment also, the purely hydro-chemical theory of Prof. Van * Lehre von den Erzlagersidtlen, Leipzig, 1901. THE GENESIS OF ORE-DEPOSITS. 615 Hise appears to be inadequate. The facts suggest too strongly an active participation of subterraneous plutonic masses, par- ticularly through the expulsion of gases, which may have be- come mixed with ascending waters. The Papers of Emmons and Weed. — The meritorious work of Messrs. Emmons and Weed (pp. 433 and 473), opening as it does a wide field hitherto unknown, or, at least, entirely ne- glected, will certainly call forth a long series of confirma- tory observations. I have not yet found time to ascertain by closer study to what extent our Freiberg district shows sec- ondary sulphide-enrichment by descending solutions. To a limited degree it is certainly present, e.g. in the not infre- quent thin, sometimes dendritic, coating of silver-glance or native silver on cross-fissures in older vein-fillings, or the druses of beautifully crystallized rich silver-ores in geodes. The crystals of stephanite in the interstices of a breccia in the Him- melsfiirst mine, for instance, may fairly be considered as later deposits from descending waters. But it is very doubtful, to say the least, whether our great bonanzas belong in this cate- gory. In our case, the question is exceedingly complicated, because the ground-water level has probably been more than once displaced, upward or downward. Our veins, admittedly formed, for the most part, before the Cretaceous period, may have stood long already, at the time of the great Cenomanian disturbance and erosion, with their upper zones above ground-water level. For only a couple of miles from Freiberg, and about at the same altitude, Cretaceous strata are now found lying upon the gneiss (in that locality deeply decomposed) which, near the town of Freiberg, encloses the veins. Thick masses of Cenomanian Cretaceous sandstones were unquestionably denuded again, in the Freiberg mining district itself, during the Tertiary. The vein-zones which, during the Cretaceous, were crowded deep below the ground-water level, must have been elevated again, therefore, in Tertiary times, above that level. Exact observations and assured conclusions are moreover made difiicult, practically, by the circumstance that, at the present time, only those vein-zones are being mined which lie far below the natural ground-water level. 616 THE GENESIS OF ORB-DEPOSITS. Students at a distance might, perhaps, infer from descriptions of this district that the rich sulphide-ore-bodies found in our " barytic lead-ore formation," at its crossings with the veins of the " pyrite-blende-lead-ore formation," belong in the category of enrichments so well described by Emmons and "Weed. The mineralogical composition of these bonanzas, with its abun- dance of argentite, proustite, pyrargyrite, acanthite, stephanite, polybasite, and native silver, is indeed similar to that of the bonanzas in Montana silver-veins. Yet the veins of our barytic lead-ore system do not at all exhibit the characteristics of " descensive " formations — their abundance in fluorite alone contradicts such a view. The rich ore-bodies at the intersec- tions referred to must be rather explained as simply due to chemical reaction between the masses of normal sulphides (pyrite, galenite, chalcopyrite, arsenopyrite and sphalerite) in the older veins, and the ascending solutions in the fissures of the later barytic lead-ore system. Emmons himself (p. 472) concedes the probability of such reactions in many cases. However great may be our pleasure and praise in connection with these latest victories of science, we must nevertheless be warned not to attach to them too universal a significance. Mr. Lindgren's Paper. — "With regard to Mr. Lindgren's paper, I will frankly say that since the death of Stelzner nothing has appeared in which the methods of microscopic- chemical research have been applied with such splendid suc- cess to the subject of ore-deposits. I agree (with insignificant exceptions) so thoroughly with the conclusions which the author has drawn from his brilliant investigations, that it would be useless for me to offer at this time any detailed criticisms. I can only express my delight that Stelzner's method has found in Mr. Lindgren an adequate American represen,tative, master at the same time of the European literature of the subject. L. DE Launay,* Paris, France : Papers of Emmons and Weed. — The ideas set forth by Mr. Emmons (p. 433) on the secondary en- richment of ore-deposits, and by Mr.Walter Harvey Weed (p. 473) * Prof., Ecole Superieure des Mines. Translated by the Secretary, and transla- tion approved by the author. THE GENESIS OF ORE-DEPOSITS. 617 on the enrichment of mineral veins, agree almost entirely with my own ; and I can only congratulate myself upon finding their observations so completely in accord with those I have had occasion to make, and thank them for the very kind way in Avhich they have been good enough to cite my writings. I have recently twice reiterated my opinion on these subjects : first, in an article in the Revue GenSrale des Sciences* on " The Variations of Metalliferous Veins in Depth;" secondly, in a little text-book on practical geologyf (in the chapters on super- ficial formations and the alterations of outcrops, pp. 50 to 72). I have, therefore, little to add. As I have said in these essays, I attach more and more importance to the phenomena of sec- ondary alteration, which have produced a number of important modifications (whether enrichments or impoverishments) in those portions of metalliferous deposits accessible to exploi- tation ; and I fully adopt the conclusion of Mr. Weed as to the necessity of taking very careful account of these phe- nomena in practical and industrial estimates. I think, like- wise, that in these secondary and comparatively recent reac- tions should be sought the interpretation of many of the phe- nomena of substitution, lateral alteration, or metasomatism, in the form in which they are now observed ; while I continue to admit, with the school of Elie de Beaumont and Daubree (to which Mr. "Waldemar Lindgren brings valuable support), the primary influence of volatile mineralizers. These must have prepared the way by introducing into the enclosing rocks, or simply by depositing in the vein-fissures, elements such as sul- phides, fluorides, chlorides, etc., which subsequently, dissolved anew by the circulation of superficial waters, have rendered to the latter essential aid in the processes of alteration. In this manner have been produced the large altered zones which are seen, for example, around pyritic masses in the south of Spain. This point I have fully elaborated in my " Contribution to the Study of Metalliferous Deposits. "J In order to formulate the study of the phenomena in ques- tion, I have been led to distinguish in a very general way, in the alterations of terranes and of deposits, three zones (from * For 1900; p. 568. t Oeologie Pratique. Published by Armand Colin, Paris, 1900. t A book of 116 pp., Paris, 1877. 618 THE GENESIS OF ORE-DEPOSITS. the surface downward) which correspond rather to those of Mr. "Weed* than to those of Mr. Emmons,! the two first zones of Mr. Emmons (the reality of which I am far from denying) having been comprised in the first of mine ; and I have thus defined them :l 1. First superficial zone of oxidation, subject, in its upper part, to physical disintegration : a zone characterized by the peroxidation of iron, and, in the case of metalliferous deposits, by the presence of native metals, oxides, carbonates or chlorides (Mr. Weed's " zone of weathering "). 2. The far more important zone of cementation, of de-calcifi- cation, and, more generally, of complex chemical reactions (such as the formation of secondary sulphides), liable to show at its base an increase of certain substances, which have been dis- solved in the upper part and carried away by the descending waters (the " zone of enrichment "). 3. The zone of unaltered equilibrium (unchanged sulphides), below the hydrostatic level (the " zone of primary sulphides "). "With regard to the process of this alteration, I believe with Mr. Emmons that we ought not to attribute too absolute a value to what is called the hydrostatic level (" ground-water level "), and I have insisted at different times in my " Geologie pratique " (pp. 52, 152, etc.) on the iiecessary irregularities of this so-called " level," due to the variable structure of the ter- rane, and leaving, for instance, beneath a former hydrostatic surface, a zone in which the circulation of surface-waters rich in oxygen and carbonic acid could still take place. Moreover, it must be noted that, even in the deep zone, the waters could not be absolutely still or incapable of exercising oxidizing chemical reactions, especially if there be great fissures or faults, permitting the introduction and rapid circulation of waters from the surface, such as appear to exist as correlatives, opposed to the ascent of thermal water8,§ and as Mr. Weed * " Enrichment of Mineral Veins by Later Metallic Sulphides," Bvll. Oeol. Soc. of Am., vol. xi., p. 181 (1900) ; "Enrichment of Gold- and Silver-Veins," Tram., XXX., 424 ; this vol., pp. 473-497. t "Secondary Enrichment of Ore-Deposits," Trarus., xxx., 177; this vol., pp. 433^72. X Qeologie pratique, p. 54 ; Reime ginirale des Sciences, 1900, p. 568. ? TraM des Sources ihermo-min^rales (Baudry, Paris, 1899) ; chapters on the origin and outflow of thermal springs, in which I have called attention to the THE GENESIS OF ORE-DEPOSITS. 619 has well pictured in his Fig. 1.* This may explain the ab- normal occurrence of certain alterations and secondary enrich- ments more deeply situated than might have been at first ex- pected. Here is a very interesting fact which Mr. Weed deserves the credit of bringing to light. Perhaps also, besides the descending waters, the ascending waters, heated by their deep circulation, or even by contact with eruptive phenomena, have in certain cases played a part which their high temperature may have augmented, although we may suppose them to have been robbed of oxidizing re- agents by their subterranean circulation. We know, indeed, that Daubree observed at Plombieres, Bourbonue, etc., evident reactions of this kind, produced upon metals by prolonged con- tact with thermal waters of extremely low mineralization ; and certain minerals, especially, which may be considered as sec- ondary in copper-deposits, are produced under these condi- tions : — secondary sulphides such as those studied by Mr. Weed, who has elsewhere mentioned the possibility of this in- tervention of hot volcanic Avaters. Furthermore, as I have long since remarked, when we are confronted with secondary reactions, the persistence of which in depth is surprising, and appears to contradict existing theo- ries, there is reason to inquire whether the surface of the earth was not, by reason of remote tectonic accidents, very difterent at the time when these reactions took place from what it is to- day. I am very happy to see that in their admirable study of the copper-mines of Butte, Messrs. Emmons and Weed have been led not only to adopt a similar hypothesis, but to give it a local geological confirmation. Finally, I beg again to mention an idea which I have never had occasion to state heretofore except incidentally, but which seems to me to deserve more thorough study. Namely, in re- gions of complex fractures, with numerous systems of inter- secting veins, such as those studied so minutely in Saxony, fact that, to constitute a thermal spring, there mast exist, below the hydrostatic level, an active circulation which I have compared to that which might be pio- duced in a pipe-elbow, plunged in a basin of water (op. cit, pp. 23 to 31). The moving waters thus rapidly brought by accident into contact with the lower por- tion of a deposit might exercise upon it an unforeseen metamorphosis. '- Duns., XXX., 428 (p. 477 of this volume). 620 THE GENESIS OF OHE-DEPOSITS. Bohemia, etc., there is doubtless reason to attribute a very im- portant role to secondarjphenomenaof enrichment, as explain- ing the variations in successive fillings, which have usually- been interpreted as primary phenomena, and the cause of which has been sought in a series of internal movements more or less independent, separated by long intervals of time. Per- haps, for example, the occurrence of a late deposit of highly argentiferous mineral, often accompanied with calcite and co- balt, such as has been noted at Freiberg, Przibram, Wittichen, etc., is only the result of a simple secondary concentration. The same may be true of the cobaltiferous fillings with calcite and barite, which have been observed in sundry faults travers- ing the cupriferous schists of Mansfeld ; and I believe that, in a general way, it is the cause of many enrichments noticed at the intersections of veins, at the junctions of cross-courses, etc., such as those described by Smith at Broken Hill and by Spurr in the Aspen district, which, judging from the published de- scriptions, Mr. Weed appears to me to have interpreted very justly. Arthur L. Collins, Telluride, Colo. : Papers of Emmons and Weed. — Mining engineers owe a great deal to the suggestive papers of Messrs. Emmons and "Weed (pp. 433 and 473), which throw much light on numberless facts in connection with ore- deposits, especially those of copper- and silver-ores. In recent papers on this subject, no reference has been made to the remarkable copper-veins of Cornwall, which, only 60 years ago, furnished the major part of the world's copper- supply, but already seem to be almost forgotten. These de- posits were described by a host of capable observers, includ- ing such men as Delabeche, Henwood and Smythe ; and the separate zones of weathering, enrichment and unaltered ore which they exhibited were so strongly marked and so commer- cially important that one wonders that the relations of these zones were not recognized at the time. The zone of weathering (" gossan ") was often of great ex- tent, reaching not only below present water-level, but far below the level of the neighboring sea. Thus, at Fowey Consols, the gossan extended 100 fathoms below the adit-level ; and at Dol- coath " some of the earthy brown ore was found as far down THE GENESIS OF ORE-DEPOSITS. 621 as the 197-fathoni level."* These gossans generally showed traces of copper ; and it was recognized that they proved the former existence of sulphide-ores, from which they had been formed. Immediately below came the great ore-bodies, such as that at Clifibrd Amalgamated, " 16 or 18 ft. wide, of cindery copper pyrites from wall to wall;" or the " 30 or 40 ft. of dredgy cop- per ore in the best parts of Devon Consols."t These gradually gave place to poorer ores, until, one after another, the mines were abandoned. Some 16 years ago, when one of these old-time Cornish bonanzas, the Tresavean mine, was reopened, the hard quartz ore, sparingly sprinkled with pyrite, mispickel and chalcopyrite, which was encountered, seemed to justify fully its former abandonment. The recog- nition of the essentially superficial origin of rich copper sul- phide ore-bodies of this type will be discouraging in many cases. But it only confirms an opinion long held, on other grounds, by mining engineers. It is noteworthy that the reaction mainly relied upon for the removal of copper from the zone of weathering, namely, the decomposition of copper sulphides by ferric sulphate, is (or was, many years ago, when I was familiar with the district) employed at Eio Tinto on a very large scale in the commercial treatment of copper-ore. The liquors from the lixiviation of heap-roasted ore were run over " raw " fine ore — originally (as I recollect) to lessen the consumption of iron in the precipitate ing-tanks, and to secure a cleaner precipitate. But this was found to be also an efiicient method of extracting part of the copper-contents of raw pyrites. And great heaps of mixed " raw fines," and lixiviated roasted ore, aggregating millions of tons, gradually giving up their copper in solution, largely by means of this reaction, became a feature of the Rio Tinto land- scape. The supposed reaction for the re-precipitation of copper in secondary copper-ores, frotn cupric sulphate solutions by pyrites, can hardly take place under these conditions — it would upset the commercial process. The evidence of secondary enrichment in gold- and silver- * T. H. Collins, Journal Bayal Inst, of Cornwall, No. 38. t Sir W. Smythe, Trans. Royal Geol. Soc. Cornwall, vol. xi., part iv. 622 THE GENESIS OF ORE-DEPOSITS. veins is less striking than in copper-deposits. As to gold in particular, we are accustomed to look for far higher values in the oxidized surface-ores than in the sulphides immediately be- neath. This may be due as much to the ease with which gold is precipitated from its solutions as to its original insolubility; for the native gold in oxidized ores often has every appearance of secondary deposition. As to the Smuggler-Union workings (of which I am at present in charge), a personal examination might give Mr. Emmons reason to doubt the suggestion that the richer silver- minerals have been re-concentrated into a more recent foot-wall streak. ISTor does any such streak remain unaffected by the faulting at the Pandora crossing, so far as our workings show. The great changes in the Smuggler-Union vein with depth seem rather to coincide with the changing strata through which it passes. More striking cases might, I think, be found in the Silver Plume district of Clear Creek county, Colo., where the rich silver-minerals of the upper parts of the veins have given place to low-grade galena and ferruginous blende in depth, without any corresponding change in the enclosing rocks or gangue- minerals. H. Foster Bain,* Des Moines, Iowa : Paper of Van Hise. — The zinc- and lead-deposits of the Mississippi valley, which it has been my fortune to study, give particularly good exam- ples of many of the principles of ore-deposition formulated by Prof. Van Hise. Perhaps there is nowhere clearer evidence supporting his fundamental tenet, that ore-bodies are to be re- garded as a result, and as merely one of the phases, of the work of underground waters. The facts regarding the mines of the upper Mississippi valley have been given in some detail for Wisconsin by Prof. Chamberlinf and, more recently, for the mines west of the Mississippi by the Iowa G-eological Survey.J Fortunately, also, the processes of underground water-circula- tion have been studied in some detail in connection with the * Published by permission of the Director of the U. S. Geological Survey. t Oeology of Wisconsin, vol. iv., pp. 367-571. J Leonard, A. G.. vol. vi.. pp. 9-66. Calvin and Bain, vol. x., pp. 480-697. THE GENESIS OP ORB-DEPOSITS. 623 investigation of artesian waters throughout the region.* In Missouri and Arkansas the mines and ore-bodies have been much studied;! but the general problems of the circulation of underground waters have been neglected. During the season just closed the writer has been engaged in a re-study for the United States Geological Survey of the zinc- and lead-deposits of the Ozark region, with special reference to those of the Joplin area; and his full report is now in preparation. It has been interesting to note how fully the statement that the ore- bodies result from the general action of underground waters is here substantiated. Treated merely as parts of a problem of water-circulation, many of the difficulties regarding the ore- deposits vanish. In brief, the Joplin area is one in which flowing wells would occur, if it were not for the numerous deep fractures which, permitting the free outflow of springs, has had the same effect, in causing loss of head, as the placing of wells too close to- gether. The gathering-ground is the central plateau of the Ozarks ; the overlying impervious layer is the Eureka-Kinder- hook shale, which divides the Carboniferous from the Siluro- Cambrian. The aquifers are the porous dolomitic limestones and interbanded sandstones of the Siluro-Cambrian. At an earlier stage much of the water was transmitted down the dip through the Carboniferous limestones under a coal-measure cover. This earlier circulation seems to have been more im- portant in bringing about the recrystallization of the limestone, and probably to some extent its replacement by chert, than in causing actual ore-deposition. The coal-measure cover is now, however, cut through. The present actual difference in head is about 700 ft. "Water now rises in the Carthage well from the Silurian limestones to within a few feet of the surface, and in the Redell deep-rock well, at Joplin, to within 80 ft. of the surface. That the ore-bodies were deposited by waters rising from these deeper limestones is proved by the following facts : (a) The ores are everywhere associated with great quantities * Leverett, F., U. S. Oeol. Surv., Monograph xxxviii., pp. 550-784. Norton, W. H., Iowa Oeol. Sun., vol. vi., pp. 115-428. t See especially A. Winslow, Mo. Oeol. Surv. , vols. vi. and vii. ; and W. P. Jenney, Trans., xxii., 171-225. 624 THE GENESIS OF ORE-DEPOSITS. of dolomite. The lower limestones are dolomitic, while the Carboniferous limestones (the immediate country-rock) are not. It is also true that the Carboniferous limestones show no dolomitization away from the region of the ore-bodies, even though they have clearly been worked over by circulating water. The magnesia was evidently brought in at the same time as the ores. The magnesian limestones of the Siluro- Cambrian, in both the upper and the lower Mississippi regions, are almost everywhere associated with more or. less ore. The Carboniferous non-magnesian limestones are nowhere associated with ore, except in this particular region, where, as has just been pointed out, the circulating waters passed in their course from the one to the other. These conditions of circulation have been stable for a long time. (b) The ore-bodies of the Joplin region stand in relations, usually close, with a system of fractures and faults of sacL extent and character that we cannot but assume that they have broken the underlying Eurekar-Kinderhook shale and allowed the intermingling of the two circulations above and below it. These fracture- and fault^planes have been much obscured by the irregular manner in which the Carboniferous limestone and its contained chert breaks up, and the very considerable solu- tion which has taken place in this limestone. Nevertheless faults occur, of a minimum throw of 80 ft., traceable across the country for a mile and a half, and are not to be confused with the effects of mere settling as a result of surface-solu- tion. ISTeither are faults of 140 ft. throw, accompanied by over- thrust, to be referred to this category, or confounded with the effects of the pre-coal-measure period of erosion. Such faults were present before the concentration of the ore ; and the ore- bodies stand in close relations with them. They served as the main channels for the upward flow of the ore-bearing waters. Once in the Carboniferous limestones, the solutions wandered widely, and deposited ore under many different conditions. In the upper Mississippi region, the fact of general artesian conditions on the flanks of the Wisconsin axis is well recognized. The presence of alternating pervious and impervious beds affords several distinct circulations, one above the other. Dif- ferences in pressure, composition of the waters, etc., indicate, so far at least as the territory west of the Mississippi has been THE GENESIS OF ORE-DEPOSITS. 625 studied, that each circulation is practically distinct. The ores of the region are mainly found in the Galena-Trenton lime- stone, and especially in the upper, doloraitic portion, to which the name Galena is specifically applied. This lies below a heavy bed of practically impervious shale, to which the terms Maquoketa, Cincinnati, and Hudson River are variously applied. There are minor and local beds of shale in the Galena-Trenton, and a thin but persistent bed cuts it oiF from the St. Peter sandstone below. At an earlier period, as Prof. Van Hise shows, the waters in the Galena-Trenton were under hydrostatic pressure; but now erosion has cut deep into and through the ore-bearing strata, and for a long period of time the movement of the waters and the concentration of the ores has been down- ward. There are locally evidences of movement in the oppo- site direction, as at the Kane Brothers' mine near Dubuque; and throughout the district there are certain phenomena which are best explained by the suggestion of an earlier concentration by water under considerable head. The later effects of present conditions, however, are, in most of the Iowa mines, the more striking. Prof. Van Hise has emphasized the importance of the imper- vious layer, not only as directing the general course of under- ground circulation, but often as closely connected Avith the deposition of the ore. In both the regions under discussion examples of this phenomenon are exceedingly common. In the Joplin region the constant association of ore with the small outliers of coal-measure shale, and the common tendency of the ore in soft ground to " make " against a bar, are essentially phenomena of this sort. Decrease of temperature and pressure cannot be invoked to explain the ore-bodies in either region. There are neither hot nor warm springs in either, nor is there any independent evi- dence that any such have ever existed. The reactions involved in the genesis of the ores are all such as take place under present surface-conditions ; and, when the time and quantity of water are taken into account, there is no necessity for other agents. In the Dubuque (Iowa) region, studies of the artesian wells make it clear that the circulation did not extend to a depth sufficient to make pressure quantitatively important. In Missouri, if the waters be limited practically in their lower 626 THE GENESIS OF OKB-DEPOSITS. circulation by ttie crystalline rocks, the same is true. There is no independent evidence that any considerable portion of the circulation extends into the crystallines ; and many facts sug- gest the opposite view. The mingling of solutions has been especially important in producing deposition in both regions. In the Dubuque and Wisconsin regions, this is shown in the fact that the main ore- deposits are found at points where two crevices cross each other. For instance, in the Stewart's Cave mine, near Dubuque, there are two parallel E-W. crevices, about 60 ft. apart. Along the south crevice a great deal of lead has been found; in the north crevice, practically none. In the south crevice, ore is found only at points where there are ci'oss-crevices. In fact, the south crevice always carries ore where there was a chance for the waters from the other crevice to come in. In this case, one crevice evidently carried the solution which contained the mineral, and the other the precipitating agent. Where the two solutions came together, an ore-deposit was formed. In the Joplin region, the same thing was clearly shown in the fact that the great deposits of zinc are found largely in the Carboniferous limestones, which are cut off from the Cambro- Silurian by a series of shale beds. It is only where these have been broken across by faults that ore-deposits are found. The Carboniferous limestone carries large amounts of bitu- minous matter — so much that, in certain of the mines, when the rock is broken it looks like asphaltum-mastic. This bitu- minous matter is widely distributed throughout the Carbonif- erous limestone of that region, and the waters circulating through these beds become highly charged with it, and hence are reducing agents. The sulphate solutions coming from below have been reduced to sulphides in the limestone, making the ore-deposits. The general principle of the reduction of one sulphide by another does not seem to have been very important in the Missouri region, although we do find evidence there of that process. In the Iowa region, however, it has played a very important part. At the Pike's Peak, near Dubuque, it is con- stantly found that in the little cavities in the rock there is a lining of iron sulphide ; that is, the iron sulphide is between the zinc and the rock, and apparently acted as the reducing- THE GENESIS OF ORE-DEPOSITS. 627 agent. The iron sulphide was there first, and the zinc sul- phate, coming later, has been reduced by it to zinc sulphide. Papers of Emmons and Weed. — The general principle of sec- ondary enrichment, discussed by Messrs. Emmons and Weed also (pp. 433 and 473), finds exemplification all through the Jop- lin region. One of the best cases is the Boston-Get-There mine, at Prosperity, just south of Cartersville. At this mine about 15 acres have been mined out underground, for a thickness of from 10 to 20 ft. The mine is in the top of a big body of chert, underlying limestone. Between the layers of original white flint are thin sheets of black secondary chert; and in these bands the ore is found. The beds dip slightly SW., so that the waters falling there, as shown by the drainage of the mines, flow from WK. to SW. Going NE., up the dip, we encounter more galena and less zinc blende ; and often we can find where the little pieces of zinc blende have been dissolved out of the chert matrix by the water, leaving cavities which have the characteristic form of the zinc blende crystals. It follows that blende has been taken away by the underground waters, and carried down the dip. Farther down the dip we find cavities filled with clear, sharp and apparently new crys- tals of zinc blende. We have here an instance of the removal of zinc blende by the oxidizing waters and its redeposition farther down the dip. The general processes of secondary concentration and enrich- ment are also exemplified in the common fact that the great bodies of zinc blende occur at and below the groundwater-level, which, at Joplin, occurs almost at the surface. Until pumping was carried on vigorously, water was found at a depth of about 30 ft. Now the ore is mined at depths of 150 and 200 ft. by pumping out the water. That the general process, so far as the formation of the richer bodies is concerned, has been one of concentration downward, is shown in the fact that at the sur- face, and in the small deposits below the surface, where we have direct evidence of oxidation, we commonly get galena, while at lower depths great bodies of zinc sulphide occur. Of the common sulphides, galena, blende and pyrite, the galena is the last to go into solution in the presence of oxidizing waters; hence, where we have the three together, the general effect 40 628 THE GENESIS OF ORE-DEPOSITS. of the downward-flowing waters is to carry the zinc blende away from the galena, leaving the latter at the surface, and redepositing the zinc in the lower rich bodies of ore. Another important principle, which is well exemplified in the region as a whole, is, that the form and character of the ore- body are controlled by the character of the rock in which the ore occurs. This is largely due to the fact that the fractures in rock are controlled by the character of the rock. Homogene- ous rocks will yield under stress more uniform fractures than heterogeneous rocks. In the Joplin region, the Carboniferous rocks are interbanded limestones and cherts, which break irreg- ularly ; while the Cambro-Silurian limestones are homogeneous, and break with more regular fractures, like the Wisconsin limestones. The result is seen in the very irregular form of the Joplin ore-bodies as contrasted with the usual regular vein-like form of the deposits in the central, and portions of the south- eastern, districts. The ores of the Ozark region in general, away from the southwestern district, were only studied for the light they would throw on the problems of the latter region. They are different in form, because they occur in a different sort of rock; but enough was seen to justify the statement that they are quite as closely dependent upon the general circulation of the underground waters as are the Joplin ores. In all essential particulars they follow the same general principles. The de- tails of the investigation naturally cannot be given here. Dr. Charles E.. Keyes, Des Moines, Iowa: Paper of Van Hise. — ^It is not too much to say that Prof. Van Hise's paper (p. 282) marks a new epoch in the science of ore-deposits. Few who are not expert petrologists, in the most modern sense of the term, can fully appreciate the profound significance of his remarks. His paper shows more conclusively than ever before that, if we are to make great advancement in the study of ore-deposits, it must be largely along geological lines. Geo- logical occurrence, geological structures and geological rela- tionships come in for first consideration. It is a startling statement that ore-bodies are essentially sur- face-deposits, that is, they are mainly confined to the brittle shell of the globe, — that zone near the surface commonly called THE GENESIS OF ORE-DEPOSITS. 629 by the geologist the zone of fracture. Few of us are fully pre- pared to accept this proposition without some reserve. Yet a little reflection will show that it could hardly be otherwise. The phenomena connected with ore-deposition are merely special cases of a more general problem, with which geologists have long had to deal. Specially opportune is Prof Van Hise's discussion of the upper limit of groundwater, concerning the relations of which to the position and character of ore-bodies, it removes at once many obstacles which have long stood in the way of satisfactory ex- planation of apparently anomalous phenomena. Any change of position of this groundwater level necessarily produces im- portant changes in the mineralogical nature of the ores. Yet some orogenic movements are known to take place much more rapidly than the ores are altered by weathering influences ; and consequently we often find a marked discrepancy between the groundwater-line and the local character of the ores that we should expect to find. In the broader field of general rock- alteration we assume the truth of the observation made by Wadsworth that all such changes are from a less stable to a more stable condition. But this does not express the full sig- nificance of the phenomenon. The conditions themselves are continually changing. The process is essentially continuous, yet sometimes in one, and sometimes in directly the opposite, direction, as is the case with the better understood analogous processes in what we call the organic realm. I am not sure that I fully understand the statement of the third premise of the paper, that " by far the major part of the water depositing ores is meteoric." In the absence of the full ex- planation it may be well not to discuss this point. However, this statement very materially broadens our ordinary conception. There is, perhaps, need of a new term here. But, as it stands, and taking into consideration the related " minor part " in all its aspects, the statement, reduced to its lowest logical terms, merely declares that water is water. Although Prof. Van Hise's paper contains but little regard- ing the classification of ore-deposits, it has an important bear- ing upon that subject; and I am particularly interested in this aspect of it, because it is along the same line- that I have been working for some time in a humble way, and it is on practically 630 THE GENESIS OF ORE-DEPOSITS. the same basis that I presented the first outlines of my own classification in my paper, read at the same meeting of the In- stitute.* When Prof. Van Hise, summing up the situation, says that a " complete theory for many ore-deposits must be a descend- ing, lateral-secreting, ascending theory," he certainly states a conclusion from which there is no escape. We can only attain an adequate explanation of ore-deposition by considering all of these currents, sometimes working independently, perhaps, but usually operating in conjunction and practically contempora- neously. In my own work I was confronted by the labyrinthine com- plexity of any classification based directly on the metamorphic processes, as we know them operating upon the rocks. Prof. Van Hise appears to be profoundly impressed in the same manner. My own position is that any classification of ores in order to be useful to the fullest extent must be, first of all, simple; secondly, capable of being readily applied in the field; and finally, useful as a guide to proper exploitation. No matter how refined and well-fitting a scheme we have, if it does not meet these three requirements, it will not be adopted or even be considered by practical men. I need not here repeat the further arguments and explana- tions of these propositions which I have set forth in my paper, above-mentioned ; and I content myself with saying, in conclu- sion, that if further progress is to be made in the study of ore- deposits, it must be along the lines laid down by Prof. Van Hise. Paper of Lindgren. — We are certainly deeply indebted to Mr. Lindgren(p. 498)for so excellent a review of the subject of molec- ular interchanges associated with the production of ore-bodies occupying fissures. The importance of considering the changes of the wall-rocks of ore-veins has certainly never been adequately recognized. Lying, as it does, in no-man's land, between the territory of the miner and the province of the petrographer, the subject has been sadly neglected by both, instead of being made mutually productive. "Origin and Classification of Ore-Deposits," 2Vans., xxx., 323. THE GENESIS OF OEE-DEPOSITS. 631 While there is, no doubt, great need of an agreed technical terminology to express the multifarious conceptions and the various shades of meaning, I very much question the wisdom of even attempting to adapt, at least in its entirety, the petro- graphical nomenclature, already well established, to the recog- nized phases of ore-formation, where processes are not so well understood, and exact terminology must necessarily remain for some time yet indefinite. The meaning commonly ascribed to metasomatism, when applied to ore-deposits, seems somewhat unhappily chosen. "We sometimes get a clearer insight into things by referring to them under older and entirely different names. The title metasomatism as used by Mr. Lindgren is, I take it, almost, if not exactly, co-extensive with the somewhat older term of mineralogical metamorphism. The latter term has been widely used by petrographers generally, and has come to have a special significance in connection with the microscopic study of rock-masses. So far as ore-deposits are concerned, these two terms may be, without serious impropriety, regarded as identical and in- terchangeable. But the fact should not be lost sight of, that besides strictly metasomatic change, there are other grand groups of molecular changes among which may be mentioned, in particular, paramorphic change. The latter, while it may have no immediate connection with ore-deposits, has an ex- tremely interesting mineralogical role, which cannot well be overlooked, and which greatly elucidates some of the broader phases of rock-metamorphism. As generally used by writers on ore-deposits, the term metasomatism does not signify a simple or definite process, or an assemblage of distinct processes. It is merely a vague title given to an indeterminate group of ordinary chemical activities, in which the only essential feature which the idea carries is that each chemical change is definitely located in space. Among ores it has special emphasis, for the reason that chemi- cal substitution takes place with the desired stationary residuum. Emmons succinctly states the vagueness of the problem when he says that interchange of substance is " not necessarily mole- cule by molecule," but " in such manner as to preserve the original structure, form, or volume of the substance replaced." 632 THE GENESIS OF ORE-DEPOSITS. To illustrate more clearly for present purposes, we may fancy a point of limestone bathed by a stream of moving, mineral-laden water. If the limestone substance is gradually carried away we have simple solution ; if from out the stream mineral matter is left upon the limestone, we may have simple precipitation or incrustation ; but if, as the molecules of lime- stone are dissolved, new molecules immediately take their places, we have substitution or replacement. This last, how- ever, is not necessarily metasomatism, as I understand it. To the student of the general metamorphism of rock-masses, metasomatism is a sharply defined chemical process by which, in the solid rock, usually, mineralogical transformation goes on. At least four well-marked phases are readily distinguished. A characteristic molecule may break up into two or more, with little or no addition or substitution of extraneous elements. Or, there may be reactions between adjoining crystals or sub- stances. Or, thirdly, some of the elements entering into the composition of the new minerals may be brought in from a distance. A fourth phase may occur when a foreign substance entirely displaces a component, molecule by molecule. There are still other distinctions that maybe made reference to which is not necessary at this time. In all of these cases, the interchanges are assumed to take place in the rock-mass with no aid from circulatory waters other than those which may move through the ordinary micro- capillary pores of the stone. In the mineralogical metamorphism of a rock-mass in a region undergoing dynamic compression, such as is initiated by mountain-making forces, the so-called circulatory under- ground waters are only of secondary importance. The fissures through which these waters pass are relatively local in influ- ence ; and changes that may take place along their walls may be regarded as affecting only a very small part of the rock-mass itself. As thus understood, it is doubtful whether ore-deposits of any considerable extent are ever really formed through true metasomatic action. The conditions under which chemical change goes on in and immediately about cavities in rocks are so different from those under which the mineralogical changes in the rock itself take place that it appears inadvisable THE GENESIS OF ORE-DEPOSITS. 633 to attempt to extend the definition of a term already well estab- lished in microscopical petrography, and thereby to do away with its usefulness altogether. Mr. Lindgren himself, I think, recognizes the force of this factor when he specifically calls attention to the wholly distinct character of the alteration taking place in the body of the rock-mass (to certain phases of which I have considered the term metasomatism restricted) from that of the change or re- placement occurring in fissures, and says, " the metasomatic processes in wall-rocks of the fissure-veins differ generally from those of regional (static and dynamic) metamorphism." The restricted petrographical idea of metasomatism is, no doubt, very attractive for application to ore-deposits. But the already widely-used term replacement seems to cover more fully and more appropriately the analogous phases, as exhibited by the ores. The main usefulness of the idea of metasomatism, as applied to ore-bodies, is to give rise to a great taxonomic group of de- posits which are formed often where no previous cavities ex- isted, and hence to set these off, geologically and genetically, from all other classes of ore-formations. It is important to note, in this connection, that the period of maximum activity in the mineralogical change of rock-masses does not often coincide with the period of maximum ore- formation. As a rule, the latter is long subsequent to the former, and is the immediate outcome of activities and condi- tions wholly distinct. In its more extended signification, the term metasomatism is not very far from meaning practically the same as chemical change, at least so far as ore-deposits are concerned. In the sense intended by Mr. Lindgren, replacement appears to meet most nearly the requirements imposed by the conditions pre- sented by the ore-deposits. The exact group of chemical pro- cesses involved, and the definite set of conditions existing in each particular case, are not what are first sought in ore-ex- ploitation. The usefulness of the distinction is really inversely proportional to its success in avoiding expression of exact values. In metasomatism proper, as a mode of rock-alteration due to static or dynamic metamorphism, there are recognized a num- 634 THE GENESIS OF ORE-DEPOSITS. ber of distinct phases, the results of varying physical conditions and differences in chemical composition and mineralogical con- stitution. Such are uralization, sericitization, saussuritization, epidotization, etc. The suggestion of analogous alterations due to contact-metamorphism, or in connection with fissure- veins, does not appear to serve a similar useful purpose ; and in the special case of ore-replacement in veins the central idea is completely lost. Topazization, tourmalinization, scapo- litization, fluoritization, and the like, do not, to my mind, pre- sent practical features for the classification of ore-veins, or features which can be made use of in ore-exploitation. Frank D. Adams,* Montreal, Can. : Paper of Lindgren. — Mr. Lindgren's paper (p. 498) is a valuable contribution to the liter- ature of ore-deposits, bringing together as it does a great num- ber of facts concerning the metasomatic changes developed by vein-forming solutions in the rocks which they traverse. It is also of much interest as an attempt to classify mineral veins according to the character of the metasomatic changes which accompanied their development, and especially according to some 'predominant metasomatic mineral, which they contain. This principle, however, as Mr. Lindgren remarks, seems to have serious limitations when adopted for purposes of classifi- cation — one ot these being the fact that the same waters may give rise to different metasomatic minerals in the case of dif- ferent rocks. Furthermore, just as the various magmas with which Mr. Lindgren considers the various kinds of vein-making solutions to be severally connected pass into one another by impercep- tible gradations, so do these solutions also ; and thus, instead of a series of well-defined classes of mineral veins, an almost con- tinuous series will be met with in nature. This difliculty, how- ever, is shared by all systems of petrographical classification, and by most of the other systems proposed for the classification of mineral veins. In the case of the cassiterite-veins (Mr. Lindgren's Class I.), for instance, the predominant metasomatic mineral is said to be topaz ; but in the most extensive deposits of this class Avhich Prof., McGill University. THE GENESIS OF ORE-DEPOSITS. 635 are known — those of Cornwall — the predominant metasomatic mineral would appear rather to be tourmaline. In the apatite-veins (Class IL), scapolite is taken as the pre- dominant metasomatic mineral. This is true of the ^N'orwegian deposits ; but in the Canadian deposits, which are even more extensive, while this mineral is very common, it cannot be con- sidered as predominant. These Canadian deposits, while in many cases at least occurring in association with basic igne- ous rocks, as in lN"orway, are usually found, not in contrac- tion-joints of the intrusive itself, but as veins cutting the lime- stones and associated rocks of the Laurentian, which are penetrated by these intrusives. The apatite, unlike that of IlTorway, is a fluor-apatite, not a chlor-apatite ; and the pre- dominant metasomatic mineral is malacolite. So notably is this the case that the prospectors in the apatite-districts always look for "pyroxene," and regard it as an almost certain indi- cation of phosphate in the vicinity. IText in abundance to the malacolite is, perhaps, mica (phlogopite and biotite), which in some cases is present in such large amount that apatite-mines which were abandoned on account of the fall in price of that mineral in the years 1893-94 have been, by reason of the more recent demand for phlogopite, opened up and worked anew for this latter mineral. While, therefore, the ISTorwegian and the Canadian apatite-occurrences undoubtedly belong to the same class of deposits, the former is characterized by the presence of chlorine minerals, while in the latter this element is largely replaced by fluorine, which is also so commonly found in as- sociation with cassiterite-veins. The chlorine-bearing scapolite thus cannot be considered in all cases as the predominant meta^ somatic mineral required by the definition of Class II. Mr. Lindgren's views concerning the close genetic associa- tion of most mineral veins with igneous masses seem to be abundantly supported by the facts, as also his conclusions with regard to the preponderating influence of pneumatolitic action in the case of the cassiterite- and apatite-veins, as shown by the constant association of chlorine-, fluorine-, boron-, phosphor- ous-, titanium- and lithium-minerals with them. 636 PKOBLBMS IN THE fiEOLOGY OF ORB-DEPOSITS. Problems in the Geology of Ore-Deposits. BT PROP. J. H. L. VOGT, UNIVERSITr OP KRISTIANIA, NORWAY.* (Richmond Meeting, February, 1901.) TABLE OF CONTENTS. PAGE Inteodoction, 636 I. The Original Source op the Heavy Metals of Ore-Deposits, . 637 Distribution of Elementary Substances in tbe Earth's Crust, . . 639 II. The Eelation between Eruptive Processes and the Formation or Ore-Deposits, Especially such as have been Produced by Eruptive After- Actions, 641 Ore-Deposits Formed by Magmatic Segregation 642 Ore-Deposits Formed by Eruptive After- Actions, 613 Cassiterite- Veins and Apatite- Veins, 645 Ore-Deposits of Contact-Metamorphic Origin, .... 648 Pyritic Deposits, . . .651 Veins of Gold, Silver and Lead-Ore, 653 Conclusions, 658 III. The Nature of the Orb-Solutions in Vein-Fissures, and the Metasomatic Alterations along the Ore- Veins, . . . 658 The Association of Vein-Minerals, 658 Deposition of the Vein-Minerals, ... .... 659 Alteration of the Country-Kock, 660 Classification of Metasomatic Alterations, ...... 660 Additional Observations, . 661 Kaolinizatipn, 661 Comparison between Cassiterite- Veins and Lead-Sulphide Veins, . 665 Comparison between Formation of Greisen, etc. , and Propylitiza- tion, etc., 666 Conclusions, 668 IV. Differences of Depth in the Original Positions of Epigenetic Deposits ; and the Secondary Alterations of Deposits, . . 669 Original Differences of Depth, 670 Secondary Alterations of Ore-Deposits, 675 Introduction. In the latter part of ISTovember, 1900, I received through the Secretary of the Institute the papers of Messrs. Van Hise, Em- mons, Lindgren and "Weed, presented at the "Washington meeting of February in that year, with the request (urged also by Mr. Emmons) that I would furnish for the Richmond meet- * Translated by the Secretary, and translation approved by the Author. PROBLEMS IN THE GEOLOGY OF ORE-DEPOglTS. 637 ing a contribution to the discussion of the geology of ore-de- posits, with reference to these four papers. It is a pleasure and an honor to comply with this request. I entertain a high appreciation of the progress of the natural sciences in America during the last half of the century. We Europeans realize that in many departments of these sciences America is taking the lead ; and it is our desire that the Old and the New World may come closer and closer together in scientific union. This consideration has impelled me to the preparation of the present paper, for the deficiencies of which I must be permitted to offer an excuse in the fact that it was necessarily written within the period from December 3 to December 31. In many respects, therefore, it is incomplete, because time was wanting for a more thorough and comprehensive work. I. The Original Source of the Heavy Metals of Ore- Deposits. It is well known that many investigators, even in most re- cent years, have sought to derive the heavy metals of ore-de- posits from the inaccessible interior of the earth. This hypo- thesis was favored by the remarkably high specific gravity (about 5.6) of the whole globe, which was explained by assum- ing that the heavier metals were concentrated in its interior. A further confirmation was sought in the quantity of iron found in meteorites, and also (by spectral analysis) in the sun. The earth's interior was regarded as a liquid molten mass, and the products of volcanic eruption as furnishing samples of this mass, bringing with them, from the richly metalliferous hearth of interior fusion to the upper horizons, or even to the surface, small quantities of metals and metallic compounds. In support of this hypothesis, the beautiful synthetic production, by sub- limation, of cassiterite,* specular iron, etc., performed by Daubree and other French experimenters in the middle of the nineteenth century, and received with universal and significant interest, has often been cited. This hypothesis is seductively simple, but cannot be main- tained. We must accept as now proved, that the interior of the earth cannot be regarded as a liquid molten mass. In the * According to the equation, SnCl^ + 2HjO = SnOj + 4HC1. 638 PROPLEMS IN THE 6E0L0GT OF ORE-DEPOSITS. words of the distinguished Swedish physicist, Svante Ar- rhenius,* ' ' Modern investigations of astronomers and physicists show that the deforma- tions of the earth's mass under the influence of moon and sun (tides of the earth's crust), and the variations of the earth's axis (called precession and mutation) due to the same outside causes, present such a quantitative order as to be irreconcila- ble with the assumption of a liquid interior. ' ' He concludes that the crust of the earth is solid to the depth of about 40 kilometers. At the temperature of about 1200° C, and the pressure of about 10,840 atmospheres, existing at this depth, most of the ordinary minerals are fused, and dissolve the less fusible materials. That is to say, at the depth of about 40 kilom. begins a liquid molten condition, which, however, cannot continue to much greater depth. For at about 300 kilom. the temperature must without doubt exceed the critical temperature of all known substances; and at this point the liquid magma passes gradually to a gaseous magma, subject to extremely high pressure. The viscosity and lack of compressi- bility of this gaseous magma may be greater than those of the liquid magma. "We must give up, therefore, the old conceptions of the earth's interior condition. There is no reason for supposing that the heavy metals of ore-deposits have come from the enormously compressed earth-interior — which, as some physi- cists declare, must be, in consequence of such compression, " as hard as steel." In fact, no connection has ever been shown between ore-deposits and this heavy interior mass. "We are forced, then, to the conclusion that ore-deposits are derived from the crust of the earth — this crust, however, being regarded as not one or two, but 10, 25, or even 50 kilometers thick. Indeed, as will be shown below, a notable number of ore-deposits may be referred to eruptive processes connected, not with the heavy interior, but with the crust, of the earth. Many deposits, as Van Hise has recently shown, are due to the action of ground-water. Moreover, it has been shown within recent decades that many elements, formerly regarded as very rare — often as totally absent — in rocks, are in fact almost invariably present in de- * Zur Phydk des Vulkanismm {Oeol, Foren. Fork.), Stockholm, 1900. PROBLBte IN THE GEOLOGY OP ORE-DEPOSITS. 639 tectable (though, of course, generally minute) quantity. On this point I may mention the investigations of the American chemists, F. W. Clarke and W. F. Hillebrand, and also my own paper on the relative distribution of the elements, etc.,* from which I here introduce a brief passage. Distribution of Elementary Substances in the Earth's Crust. Of the entire earth-crust, — namely, the rocks, sea and at- mosphere, — oxygen constitutes by weight about one-half, and silicon about one-quarter; the proportions of aluminum, iron, calcium, magnesium, sodium and potassium range from 10 down to 1 per cent. ; those of hydrogen, titanium, carbon and chlorine from 1 to 0.1 per cent. ; those of some eight elements, phosphorus, manganese, sulphur, barium, fluorine, nitrogen, pretty certainly also zirconium and chlorine (but probably no others, with the possible exception of strontium), from 0.1 to 0.01 per cent. Between 0.01 and 0.001 per cent, come nickel, strontium (?), lithium, vanadium, bromine, and pretty certainly also beryllium and boron, but probably not tin, cerium and yttrium, or other elements. Between 0.001 and 0.0001 per cent, are cobalt, argon, iodine, rubidium, pretty certainly tin, cerium and yttrium, and possibly also arsenic and lanthanum, but probably no others. In summary, therefore, we have : Terrestrial Distribution of Groups of Elements. Number of Percentage. Elements. 10 to 1, 6 1 " 0.1, 4 0.1 " 0.01, 8 0.01 " 0.001, 7 0.001 " 0.0001, 7 Similar figures are obtained for the intervals 50 to 5, 5 to 0.5, 0.5 to 0.05 per cent., etc., proving that there is a law of quantitative distribution of the 34 most widely occurring ele- ments, according to which some 4 to 8 elements fall within each decimally-reduced interval. From this law we may with some confidence further infer that of the remaining, say, 37 known elements, some would fall within the next following * " TJAer die relative Verbreitung der Elemente, besonders der Schwei-metaMe." Zeittch. f. praU. Geologie, 1898, pp. 235, 314, 377, 413, and 1899, p. 10. 640 PROBLEMS IN THE GEOLOGY OF ORB-DfeoSITS. smaller intervals, for instance, between 0.0001 and 0.00001, or between the latter and 0.000001 per cent. It may here be noted that all the more widely distributed elements (0, Si, Al, Fe, Ca, Mg, Na, K, H, Ti, C, CI, P, etc.) have relatively small atomic weights. The 25 elements having the lowest atomic weights (up to and including iron) constitute at least 99.8 (more probably 99.85 to 99.9) per cent, of the earth's crust, while the remaining (say) 46 elements (among which barium, strontium, nickel, etc., are the most widely distributed) make up a total of 0.1, or at most 0.2 per cent. This is a re- sult, on one hand, of the laws which controlled the formation of the elements themselves, which are probably to be conceived not as original and simple substances, but as compounds ; on the other hand, of those which controlled the formation of the earth-crust from the original fire-mist of Kant and Laplace. The elements of highest atomic weight are, then, on the whole, relatively the rarest in rocks ; but that they do exist therein, though in minute proportions, and doubtless in some rocks as original constituents, may be shown, by way of illustration, for the platinum metals. These metals are found here and there — often together with segregations of chromite — as primary segregations formed by magmatic concentration, in very basic eruptive rocks (peridotite, and, as reported in one locality, highly basic olivin-gabbro) — a fact which clearly indicates their original presence in minute proportion in these rocks. Moreover, in recent years a small pro- portion of platinum-metals has been found (as at Sudbury, Can., and Klefva, Sweden) in the segregated sulphide-ores of gabbro rocks — a fact which requires the supposition that the gabbro magma originally contained them. Some conception of this original tenor of platinum-metals may be formed from the statement that the nickeliferous pyrrhotites of Sudbury contain, according to many analyses, from 25,000 to 90,000 times as much nickel as platinum-metals ; while the original propor- tion of nickel in the gabbro magma may be set down as about 0.05 per cent. Hence, on the (somewhat arbitrary) assumption that the platinum-metals were concentrated from the magma to the same extent as the nickel, the magma contained, roughly, 0.000001 per cent, of these metals. This figure, of course, has no pretension to accuracy ; but we have at least learned that PROBLEMS IN THE GEOLOGY OP OKE-DEPOSITS. 641 even the platinum-metals are among the normal constituents of the basic eruptive rocks. It can be similarly shown that minute quantities of gold and silver belong in eruptive magmas. For further discussion of this subject, and of the relative concentration of certain ele- ments into the acid, and of others into the basic eruptives, I refer to my treatise cited above. n. The Relation between Ekuptive Processes and the Formation op Ore-Deposits, especially Such as HAVE BEEN PRODUCED BY ErUPTIVE After-Actions. In his latest paper, Prof. Van Hise divides ore-deposits into three groups, namely, those of direct igneous origin ; those which are the direct result of sedimentation ; and those which have been deposited by underground water. His first and fundamental premise is that the greater number of ore-deposits are the work of underground water. He asserts, further, that the material for ore-deposits is derived from rocks within the " zone of fracture " ; that by far the greater part of the water depositing ores is meteoric ; and that the flow of underground water is caused chiefly by gravity. According to his view, by far the larger number of ore- deposits are formed by underground water, ore-deposits of direct igneous origin being " probably of limited extent," and the same being true of those which are the direct result of sedi- mentation (some placers, etc.) ; while possibly some are due to sublimation.* In this paper I shall not discuss the sedimentary ore-deposits; but I may remark here that, in my opinion, there has been, of late, a frequent tendency to underestimate in this connection the importance of sedimentation as a formative agent. From Prof. Van Hise's interesting paper, so rich in new theoretical suggestions, I have learned much ; I believe that he has furnished the key to the genesis of numerous ore-de- posits ; yet at the same time, in my opinion, he ascribes to his theory too great a range, and, in particular, attaches too little importance to the direct genetic relation between ore-deposits * "Some Principles," etc., Trans., xxx., 27, passim; this vol., pp. 282-432. 642 PROBLEMS IN THE GEOLOGY OF ORE-BEPOSITS. and eruptive processes. Many of the occurrences classed by him among the effects of underground water are, according to my view, the results of processes intimately cpnnected with erup- tive magmas, especially through eruptive after-actions (sublima- tion, pneumatolysis, pneumato-hydatogenesis, etc.) by which the heavy metals were in great part extracted from such magmas. In order to make my view clear, I will here briefly mention a number of groups of ore-deposits : Ore-Deposits Formed by Magmatie Segregation. Ore-deposits formed by simple magmatie diflerentiation are confessedly infrequent, and therefore relatively subordinate in importance to other classes. Under this head may be named:* (1) The occurrences of titanic iron-ores in basic and interme- diate eruptives, perhaps also of iron-ores in acid eruptives; (2) those of chromite in peridotites and their secondary serpen- tines (and also, according to J. H. Pratt, those of corundum in the peridotites of IST. C.) ; (3) a number of deposits of sulphide- ores, particularly the nickeliferous pyrrhotites occurring in gabbro (at Sudbury, Can., Lancaster Gap, Pa., many places in !N"orway and Sweden, and Varallo, in Piedmont) ; (4) according to some authorities, the auriferous pyrites of Rossland, B. C. ;t (5) according to B. Lotti, the high-grade copper-ores occurring in serpentinized peridotites in Tuscany and Liguria, Northern Italy (for instance, at Monte Catini), and analogous occurrences in other regions; (6) the occurrence of metallic nickel-iron (without economic value) in eruptive rocks ; (7) those of the platinum-metals in highly basic eruptive rocks, J etc., etc. It may be pretty safely assumed that the foregoing list will * See my articles in the Zeit.f. prakt. Geologic during 1893, 1894, 1895 and 1900 (to be continued in 1901). t Other authorities explain the Eossland occurrence differently. See ' ' Bio- titic Gold-Copper Veins," in Mr. Lindgren's paper, p. 564. X Already mentioned on page 640. It maybe added here that, so far as known, all primary platinum deposits were formed by igneous fusion, and that the plat- inum-metals are either wholly wanting, or only exist in minute traces, in deposits from aqueous solution. The latter fact may be due to the small suscep- tibility of these metals, which are, for example, much less soluble in aqua regia than gold. (See Zeitsch. /. prakt. Oeologie, 1898, p. 321.) PROBLEMS IN THE GEOLOaT OF ORE-DEPOSITS. 643 be enlarged by future investigations, though it can never be- come very extensive.* Ore-Deposits Formed hy Eruptive After-Actions. But a different case is presented by deposits connected with the eruptives by pneumatolytic, pneumato-hydatogenetic, and other similar processes, the heavy metals of which, as I con- ceive, were mostly extracted from the eruptive magmas. To explain this proposition, let us first remark that the erup- tive magmas — at least those of deep origin — are admitted to be hydato-pyrogenic — i.e., to contain a more or less notable admixture of water, with other constituents of hydrous or gaseous character. This is shown directly by observations of volcanoes, and indirectly, for example, by inference from the contact-metamorphism along deep rocks, which is conceived as a recrystallization under the influence of interpenetrating steam. Another indication is given by the enclosures of car- bonic acid in quartz. Concerning the chemical and physical action of water upon the magma, I quote the following passage from the work of Arrhenius, already cited : " The water in the magma . . . acts as an acid ( strong as compared with silicic acid), liberating free silicic acid, HjSiOj, and free b ases. By mixture with the unaltered magma, these become acid and basic silicates — the access of water hav- ing rendered the magma more liquid." As is well known, the ionization of water increases rapidly with its temperature. This explains the activity of water at high temperatures. Thus, for example, Barus has shown that water heated above 185° C. attacks the silicates composing soft glass with astonishing rapidity; and an experiment by Lemberg has proved that water at 210° C. slowly dissolves an- hydrous powdered silicates. * Prof. K. Beck, in the first part (which has just appeared — -Berlin, 1901) of his Lehre von den Erzlagerstatten, classes (with some doubt) the tin-ores of Etta Knob, in the Black Hills of Dakota, among magmatic segregations. This seems to me incorrect. The deposit mentioned, carrying cassiterite with apatite, tri- phyline, tantalite, columbite, spodumene, etc., presents, in its mineral paragen- esis as well as in ils geological occurrence, all the distinguishing marks of the tin- deposits formed by pneumatolytic processes — in this case intimately connected with the eruption of the granite-pegmatite. To this point I shall recur later. 41 644 PROBLEMS IN THE GEOLOGY OP ORE-DEPOSITS. Of special interest in the study of pneumatolytic phenomena is the following passage from the same work : "So far as we know, all gases can be mixed with each other in any desired proportions. In the interior gaseous magma of the earth, therefore, there should be no permanent zones of segregation ; but all occurring difierentiations should lead to continuous transitions, and in the first rank of the forces operative in these would be osmotic pressures, of the detailed nature of which, at these high temperatures, little is yet known. "In cooling, however, it is highly probable that this magma — at least, if it contained sufficient water — separates into two layers, after it has assumed a liquid state. The division takes place at a proportionately lower temperature, the smaller the amount of water. When this is very small, the products of the separation appear only as enclosures of water, carbonic acid, etc., at a very ad- vanced stage of cooling, when the mobility is too small to permit the small drops to flow together and form larger masses. On the other hand, when the water- content is considerable, the aqueous gas collects in larger volumes, and in these are concentrated the bodies which, at the existing temperature, are more sol- uble in water than in the silicate-magma. Among these bodies are carbonic acid, hydrogen sulphide, combinations of univalent ions, such as those of chloric, fluoric and boric acid, with the mostly positive ions, like the alkali-metals, and, less frequently, the earthy metals, calcium, strontium and barium. The univa- lent ions have a strongly marked tendency to go to the water, because their com- pounds are dissociated electrolytically with extraordinary force; And among them the foremost must be those which possess a strong tendency to ionization, or, in the older chemical phrase, show strong affinity. Those ions, also, the hydrates of which are highly soluble in water without becoming dissociated, are favored in this process. This group includes, among others, the ions of carbonic and boric acid and hydrogen sulphide. Of course, silicic acid is likewise taken up by water in proportion to its solubility. (The ions of the bivalent metals — iron, zinc, lead, copper and tin — seem also to follow by preference the negative ions named.) In this solution, composed of bodies so various, the positive and negative ions are to be conceived, not as bound to each other in a definite way, but as mutually independent, as in an ordinary solution, such as sea-water. "The cooling and the consequent separation into two layers occur soonest at the surface of contact between the eruptive and the cool adjacent rock ; and it is natural to assume that later aqueous segregations will by preference accumulate with the earlier ones. Other portions are gradually collected as geodes and veins in the interior of the magmatic mass. By reason of the greatly superior mobility of the aqueous solutions, as compared with the magma, these segrega- tions may send out branches in the form of the finest apophyses. The solution in aqueous gas now gradually cools, and one substance after another separates from it. By reason of the great mobility of the solution, and its consequent strong capability of diffusion, the minerals (provided the cooling be not too rapid) are segregated in large crystals, such as characterize a so-called pegmatitic structure. Gradually, also, the constituents which longest retain a gaseous form — such as water and carbonic acid — escape. "According to this view, all the products required for the formation of ' pneu- matolytic minerals ' are simultaneously present in the aqueous solution ; and it is not necessary to imagine that they come in gaseous form from different regions, to meet at the point of segregation." PROBLEMS IN THE GEOLOGY OP ORE-DEPOSITS. 645 After this theoretic explanation,* we may return to our con- sideration of the pneumatolytic or pneumato-hydatogenetic ore- deposits, beginning with those of tin-ore, the genesis of which has been especially studied by French investigators. f Cassiterite- Veins and Apatite- Veins. — As is well known, the eassiterite-veins, of the type found in Cornwall, the Erzgebirge, Banca and Billiton, Tasmania, etc., are, everywhere in the world,J in connection with acid eruptives, namely, granite and (now and then) the veinstones and ejected rocks of the granite family, such as quartz-porphyry, liparite and rhyolite. Partly for this reason, and partly because of the characteristic paragenesis of the cassiterite-veins (presenting many fluoride-, borate- and phosphate-minerals), and the pneumatolytic meta- morphism of the country-rock (forming Oreisen), Elie de Beau- mont and A. Daubree, as is well known, concluded as early as 1840-1850 that these veins were connected with the granitic eruptions, and that in their formation various volatile fluorides, boron-compounds, etc., took part. Daubree was led to a de- tailed theory by his famous synthetic experiments in sublima- tion. § The genetic relations between the cassiterite-veins and the granite-eruptions may be followed a step further. It is first to be emphasized that the cassiterite-veins were formed imme- diately after the eruption — often, indeed, before the complete cooling — of the granite. One proof of this (among others) is the occurrence of the tin-vein-minerals in many veins of peg- matite with the granite. II It has been proved also by K. Cal- mer in a thorough geological study of the deposits of the Erzgebirge. And it follows that, in this class of cases, ordinary underground water cannot have been active. We may assert, further, that the cassiterite-veins are genetically independent of the immediately adjacent country-rock. * An attempt to explain the physics of magmatic differentiation under the in- fluence of water dissolved in the magma will be found in an article which I shall publish in an early number of the Zdlsch. /. prakt. Oeologie for 1901. f The following statement is mostly a r^sum^ of my treatise in the above-named journal (Nos. 4, 9, 11 and 12, of 1895). J The peculiar silver-tin veins in Bolivia, described by A. W. Stelzner, are not here classed with tin-ore veins proper. Concerning contact-deposits of iron-ore carrying cassiterite, something will be said below. § See above, p. 637. || See footnote, p. 643, above. 646 PROBLEMS IN THE GEOLOGY OF ORB-DEPOSITS. The geological features of these veins force us to the view that their material contents were extracted from the not yet fully congealed granite ; and this view is confirmed by their mineralogical and chemical features. "We find in these veins exactly the series of elements characteristic of the granite peg- matite-veins, such as potassium and lithium ; also, tin, tung- sten, uranium, niobium, etc., as well as beryllium (all also occurring with considerable frequency in the pegmatite-veins) ; and, finally, boron and fluorine. Apatite- Veins. — At this point, I will briefly describe the JJTor- wegian and E'orth-Swedish apatite-veins. These veins are analogous to the cassiterite-veins, from which, however, they difier in many very instructive particulars. The tin-veins are connected with granite ; the apatite-veins with gabbro ; and, in the latter case also, it can be shown that the veins were formed soon after the eruption of the rock, and that they cannot be explained by agencies acting upon the already congealed gabbro. In both classes of veins we find a characteristic pneumato- lytic metamorphism of the country-rock. Each class has in abundance a halogen-element : the tin-veins carrying fluorine (with a very little chlorine), and the apatite-veins chlorine (with a very little fluorine),* which occurs (1) in the mineral scapolite (containing about 2.5 per cent. CI), abundantly represented in the metamorphosed zone along the vein-walls ;t and (2) in the mineral chlorapatite. In the tin-veins also, apatite or other phosphates are almost invariably found — sometimes, even, in considerable quantity.^ This is specially noteworthy, because apatite is wholly (or almost wholly) lacking in lead-silver-ore veins, such as those of the Erzgehirge, the Harz, Kongsberg, Schemnitz, the Comstock Lode, etc., and in gold-veins generally. Instead of the stannic acid, SnOj, found in the tin-veins, we flnd in the apatite-veins titanic acid, TiO^, as rutile (which is often so abundant as to be mined), ilmenite, titanite, etc. * The Canadian apatite-veins carry a larger proportion of fluorine than the Norwegian. t In the well-known apatite-deposit at Odegaarden, Norway, there is, on the whole, some 2. 5 or 3 times as much chlorine as phosphoric acid. X For example, the cassiterite-veins at Montebras in France are mined chiefly for the lithium phosphate, amblygonite. PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 647 The potassium- and lithium-minerals of the tin-veins are re- placed in the apatite-veins by minerals of magnesium and cal- cium-sodium. The apatite-veins often contain some pyrites, and also, exceptionally, a little tourmaline — that is, a silicate containing boron. While the characteristic elements of the tin-veins (Si, Sn, K, Li, Be ; also W, Ur, Va, Ta, with F, B, P, etc.) remind us of the composition of the granite, we find in the characteristic elements of the apatite-veins (especially P, Ti, Fe, Mg, Ca, Na, CI, etc.) a close analogy with the composition of the gabbro. We conclude that the material of the apatite-veins was ex- tracted from the gabbro magma in a manner similar to that of the extraction from the granite of the material of the tin-veins. Since the halogens chlorine and fluorine respectively are so richly represented in these two classes of veins, we may con- clude, further, that the magmatic extraction-process is based chiefly upon a reaction, in the pressure of water, of hydro- chloric (or, as the case may be, hydrofluoric) acid, dissolved in the magma. In my work of 1895, cited above, I have attempted to prove that by such an " acid extraction-process," operating in a granite magma, especially the elements K, Li, Be, Sn, W, Ur, N"b, etc., together with B and P, would be carried into the aqueous hy- drofluoric solution; while, on the other hand, the aqueous hydrochloric solution in the gabbro magma would take up especially P, Ti, Fe, Mg, Ca, ISTa, etc. For this view I now flnd a support in the recent account by Arrhenius of the chemico- physical reactions of aqueous magmatic solutions. Pegmatite- Veins. — A similar argument can be made concern- ing the " nephelin-syenitic pegmatite-veins of the southwest border-zone of the augite-syenite region," near Langesund- Brevig, in southern Norway, which have received so masterly an examination from "W. C. Brogger.* We note specially that we encounter here a whole series of rare minerals, containing boric, zirconic, stannic and thoric acid (B^O^, ZrO^, SnO^, ThO^), and also fluorine and chlorine ; and that Brdgger has established the following four phases of the vein-formation : (1) the phase of magmatic solidification ; (2) the principal phase of pneuma^ * Zeitsch. f. Kryst. u. Min., vol. ivi., 1890. 648 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. tolytic solidification ; (3) the phase of the formation of zeolites ; (4) the phase of the fluocarbonates, carbonates, etc. "We note also that these veins are to be considered as contact phe- nomena. Here we learn, on one hand, the action of the aqueous hy- drochloric-hydrofluoric solution in the augite-syenite magma, and, on the other hand, the various stages of the vein-formation, in which the influence of (a) chlorides and fluorides, (6) water, (e) carbonic acid, etc., is operative. From this brief digression outside the field of ore-deposits, strictly so called, we return to consider Ore-Deposits of Contact- Metamorphic Origin. — These we may more briefiy call " contact-deposits," in a limited sense of that term. As examples, we may take the iron-ore deposits of the Kristiania region, bordering the post-Silurian (pretty certainly Devonian) granites ; also those of southern Hungary (at Vasko or Moravitza, Dognaeka, etc., in the Banat), bordering the late Mesozoic or Tertiary banatite eruptives; also those of the island of Elba, near Tertiary eruptives, particularly granite ; and those of Dielette, in the department of Manche, France.* The characteristics of this group of deposits are : The ores (mainly magnetite and specular hematite, yet often also sulphides of copper, lead, zinc, etc.) occur within the meta^ morphosed contact-zone of deep eruptives, especially granite. Frequently they lie almost immediately at the boundary between the eruptive and the country-rock; frequently from 0.5 to 2 kilom. irom that boundary, and sometimes even farther away (horizontally) ; but never outside of the metamorphosed zone. ISTot seldom they are found in fragments of metamorphosed slate or limestone, which have torn loose, and surrounded by the adjacent eruptive. More particularly, the ores occur in limestones, marly slates and ordinary clay-slates, and are accompanied by the usual * I believe that numerous ore-deposits belong in this group of contact-deposits. But it is sufficient here to describe the«e from typical representatives, upon the following authorities : For the Kristiania region, the studies of Th. Kjerulf and raj own earlier ones (with references in the Zeitsch. f. j)rakt. Geologic for 1894, pp. 177, 464, and 1895, p. 154) ; for the Banat (which I have also visited personally, with Prof. F. Beyschlag of Berlin), the work of B. v. Cotta (1864) and Edward Suess {AnUitz der Erde) ; for the Elba deposits, the investigations of B. Lotti; and for the French deposits, a description by Michel-L^vy. PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 649 contact-minerals, garnet, vesuvianite, scapolite, wollastonite, augite, hornblende, mica, etc. — and also (in the clay-slates) by chiastolite, etc. In other words, the phenomena of contact- metamorphosis are the same here as elsewhere, except that the minerals in the immediate vicinity of the ores are developed as very large individuals ; i.e., these ores have occasioned a con- tact-metamorphism of high potency. Sometimes the ore-deposits are traversed by apophyses of the eruptive, such as veins of granite, quartz-porphyry, etc. The Kristiania Deposits. — A study of the Kristiania contact- deposits indicates that the formation of the ores preceded the solidification of the granitic magma. Even when the ores occur in slates immediately adjacent to the granite, or in the small Silurian fragments completely surrounded by granite, they are never found also in the granite itself This is to be simply ex- plained by the supposition that from the still liquid magma the ores were " blown into "* the adjoining rigid rocks. If they had been introduced later, they would have been deposited in the granite also. In the Kristiania field, the contact-ores are found in pre-granitic rocks of all kinds — not only in limestones, pure and impure, and clay-slates, but also in Archean gneiss and pre-granitic porphyry-outflows. Hence this final deposition is independent of the chemical composition of the adjacent rocks. The presence in these deposits of granitic apophyses, already mentioned, is another proof that they were formed be- fore the solidification of the granite. "We conclude, further, that the material of the ores was de- rived, not from the surrounding rocks, but from the eruptive magma. In the first place, their chemical composition (in the Kristiania region, as often elsewhere, showing a predominance, now of iron, now of copper, or, again, of zinc, lead, etc., asso- ciated with some bismuth, arsenic, antimony, etc.) is independ- ent of that of the country-rock. In the second place, we often find the ores in rock-fragments, completely surrounded by gran- ite, f so small that they could not have furnished the requisite amount of ore-material. Contact-metamorphism is usually referred, in accordance with * I adopt this expression (dngeblasen) from my deceased teacher, Th. Kjerulf . t Of 108 old mines and prospecting-pits in the Kristiania district, 16 per cent, are in small Silurian masses, completely enclosed in the granite ; 20 per cent, im- 650 PROBLEMS IN THE GEOLOGY OF ORB-DEPOSITS. all probability, to the action of heated steam escaping from the eruptive magma and pressed into the surrounding rocks, where it produces a re-crystallization, in most cases without notable addition or subtraction of material. Contact ore-deposite form a special class of this metamorphism (involving " ferrization," etc.), and are explained by the presence of metallic compounds in the heated steam. Other Contact-Deposits. — There is thus a close analogy be- tween such contact ore-deposits as those of Kristiania, the Banat, Elba, etc., on one hand, and the tin-ore veins on the other — the latter being exclusively, and the former mainly, connected with granite eruptions. Indeed, there are also numerous interme- diate transitional cases between these two. We may mention as instances the " tourmalinization " within zones of contact- metamorphism, well known in Saxony, and the similar "axin- itization " of contac1>metamorphic zones in the Pyrenees, which A. Lacroix has recently described. In these cases, that is to say, the boro-silicates, so well known in tin-veins, have been conveyed in great abundance into the metamorphosed zone. Fluorspar, tourmaline, axinite, etc., as well as the scapolite (which contains N'a CI), have also been found in our contact-deposits of iron-ore ; while, on the other hand, specular iron is sometimes abundant in cassiterite-veins. Moreover, there are metamorphic contact ore-deposits (char- acterized by garnet, augite, hornblende, etc.) which, besides magnetite, specular hematite, and sulphide-ores of copper, lead and zinc, carry also cassiterite, with its usual accompanying minerals. An instance is Pitkaranta in Finland, where, accord- ing to A. E. Tornebohm, the order of deposition was (1) iron- ore ; (2) tin-ore ; (3) copper-ore. Another instance is furnished by the " bed-impregnations " near granite, at Schwarzenberg in the Erzgebirge, recently described by K. Dalmer,* which carry as ore-minerals magnetite, specular iron, pyrites, galena, zinc- blende, etc., with cassiterite, wolframite, etc., further accom- panied by pyroxene, actinolite, garnet, epidote, wollastonite, vesuvianite, etc., with fluorite, axinite and titanite. Under mediately upon or near the contact between these rocks ; 44 per cent, within the contact-zone, hut farther away from the granite ; and 20 per cent, in Archean rook and pre-granitic porphyry overflows, near the border of the granite. * Zeitsckf.pralct. Geohgie, 1897, p. 265. PROBLEMS IN THE GEOLOGY OP ORE-DEPOSITS. 651 this head belongs perhaps also the peculiar occurrence of cas- siterite and iron-ore in limestone near Campiglia in Tuscany,* 2.5 Idiom, from a tourmaline-bearing granite. Chemical History. — Chemically, however, the processes form- ing such contact-deposits of iron-ore, on one hand, and the cas- siterite-veins on the other, must have been different. As already observed, the material of the latter was derived through magmatic extraction by an aqueous solution of hydrofluoric (and hydro- chloric) acid; but in the ordinary, non-stanniferous contact- deposits of iron-ore the elements characteristic of the cassiterite- veins (8n, "W", U, Li, Be, B, etc.) are almost or wholly wanting, and, as a rule, fluorine is scantily represented. For these cases, therefore, an extraction by hydrofluoric and hydrochloric acid is apparently excluded. On the other hand, we may assume that the magmatic water itself has here played a specially ener- getic part, and has extracted iron from the magma. The de- tailed explanation is still an open question, in connection with which I may recall the theoretical proposition of Arrhenius, already quoted, that the water of the magma " acts, relatively to SiOz, as a strong acid." Pyritic Deposits. — As an appendix to the foregoing contacts deposits, I mention the pyritic deposits, typically represented at Vigsnas, E,6ros, Sulitelma, etc., in Norway; Rio Tinto, Tharsis and San Domingo, in Spain and the adjacent part of Portugal ; Agordo in Lombardy ; SchmoUnitz in 1^. Hungary ; etc. To these I would reckon also Rammelsberg in the Harz. Concerning the genesis of these deposits, opinions notori- ously diflfer. Some observers assert a sedimentary origin, while, in accordance with many others preceding me,t I ascribe the deposits to after-processes, following eruptive intrusions. J These deposits, which almost always have an apparently stratiform character, occur only either in rocks fully altered by dynamic metamorphism or in formations somewhat less power- fully compressed, and, generally, in close relation with erup- * Described by B. Lotti and K. Dalmer, Zeitseh. f. prakt. Geologie, 1894, p. 400. t Th. Kjerulf in Korway, K. A. Liossen in Germany, L. de Launay in France, Gonzalo y Tarin in Spain, etc. J For my own works on tbis question, see Zeitseh. f. prakt. Qeologie, 1894 (Eoros and Eammelsberg) and 1899 (Huelva). I would mention also the studies of F. Klockmann, who defends the sedimentary hypothesis, Id. for 1895, p. 35, and Sitzungsb. d. k. preuss. Akad. d. Wiss., Berlin, 1894, pp. 1173-1181. 652 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. tives. This last feature is highly characteristic of the numer- ous ISTorwegian deposits scattered between 59° 20' and 70° of !N". lat., along the old mountain-range which consists of Cambro- Silurian slates, probably folded in the middle Paleozoic (Devo- nian) age. Their distribution is such, however, that they ap- pear only in those parts of the range where considerable outbreaks of eruptive rocks (gabbro, often accompanied by a granite rich in soda) took place, at about the period of the mountain-folding. Of 28 Norwegian pyrites-deposits, enumerated in my treatise of 1894, 26 were proved to lie very near, or actually within, regions of compressed gabbro. I can now add that in one of the two cases then excepted we have found the eruptive rock near the mine. Since the deposits, moreover, are independent of the age of the slates (mostly phyllite- and mica-slates), their genetic relation to the eruptives is indisputable. Some of them occur on shearing-planes in the compressed gabbro; but the great majority are in the slates surrounding it, most frequently at a distance of from 50 to 500 meters from the eruptive border, and rarely somewhat farther away. We may note, further, that the pyritic deposits themselves (as has been shown by A. W. Stelzner and others) have some- times been compressed — i.e., they were completely formed be- fore the end of the folding of the mountain-chain. Moreover, in many places they are traversed by apophyses of the erup- tives, i.e., they were formed before the solidification of the deeper portions of the eruptive magma. It follows from these considerations that the Norwegian pyritic deposits are to be classed as phenomena of contact- metamorphism connected with the gabbro and its peculiar accompanying granite, and that their bed-like appearance must be explained by the occurrence of the gabbro eruption during the long period of mountain-folding. The ores were thus formed under extremely high pressure, which favored their introduction up and along existing planes of stratifica- tion. The analogy of these cases with those of bhe ordinary con- tact-deposits already described covers also the origin of the ore-material, which we must assume to have been somehow extracted from the eruptive magma. This view is supported PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 653 by (1) their independence of the adjacent slates; (2) their formation immediately after the gabbro eruption ; and (3) the resemblance of their material to that of the nickel-pyrrhotite deposits, considered to be products of magmatic secretion. The chemico-mineralogical difference between the two classes is, that in the magmatically secreted pyrrhotite deposits nickel predominates over copper, while in the pyritic deposits the contrary is the case. Yet in chemical respects there exist complete intermediate transitional occurrences, which I hope to describe at some future day. The detailed explanation of the magmatic extraction form- ing the pyritic deposits is an entirely open question ; but we may conceive it to be the combined action of water with a sulph ur-compoun d. What I have said of the Norwegian pyritic deposits holds good, I believe, in its main features, though with modifica- tions of detail, for the other deposits of this class, mentioned above. Veins of Gold, Silver and Lead-Ore. — These may be divided into three main groups : (1) relatively recent gold and silver, or silver-lead veins ; (2) old silver-lead veins ; (3) old gold- veins. Grold- and Silver- or Silver-Lead-Veiris of Later Age. — As rep- resentatives of this class we may name those of 'Sagyig- Verespatak in Transylvania; Schemnitz-Kremnitz and ISTagy- banya-Kapnik in upper Hungary (all of which lie along the Karpathian range) ; Cripple Creek, and many other Colorado occurrences in the Boulder, San Juan, Silver Cliff, Rosita and other districts ; the Horn Silver and many mines in Beaver county, Utah; the Comstock, Esmeralda, etc., in Nevada; and San Bernardino in California ; the districts of Durango, Fres- nillo, Zacatecas, Gruanajuato, Pachuca, etc., in Mexico; Cerro de Pasco in Peru; Potosi, Huachuca, Oruro, etc., in Bolivia, and many others along the South American Andes; the Coromandel peninsula (Hauraki) in New Zealand ; and, finally, many places in Japan. This list, though far from complete, may serve to give a notion of the wide distribution and the economic value of the deposits of this group. Its general features were first described by F. v. Richthofen, forty years ago. We may also refer here to the work of E. 654 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. Suess,* and to numerous treatises which have appeared in re- cent decades. The younger gold- and silver-veins stand closely related to recent (especially Tertiary, but sometimes late Mesozoic, and occasionally to Quaternary) eruptive rocks, f But they are not confined to any one of the recent eruptives. Many occur in andesites ; others in dacites ; others, again, in quartz- trachytes, rhyolites, etc., and some in phonolites ; so that they are to be considered rather as products of general volcanic activity. In fact, they belong, as a rule, in each district to the latest, or one of the latest, epochs of volcanic activity for that district. Hot springs, solfataras, etc., are frequently found near them. Very often they carry silver and gold in combination (Corn- stock, Schemnitz, S'agybanya-Kapnik, etc.), the gold being sometimes predominant, with little silver (Cripple Creek, Tran- sylvania), and sometimes vice versa (at many places in Mexico, Bolivia, etc.). Galena is in some cases abundant, but often almost or wholly absent (Transylvania, Cripple Creek, Com- stock). Ores of copper and zinc are, as a rule, scanty ; arsenic and antimony pretty common ; and the frequent abundance of arsenical and antimonial sulphides is noteworthy. A special sub-group is formed by the tin-bearing silver-lead- bismuth-ore veins of Bolivia, examined some years ago by A. "W". Stelzner,J which carry cassiterite, and occasionally also the sulphide, stannite, while the accompanying minerals usual in cassiterite-veius are wanting. Cassiterite has been found also in some recent ore-veins in Mexico (and wolframite at Kapnik, Hungary). Tellurium occurs abundantly in some gold-veins (lif agyag ; Cripple Creek and other places in I^orth America — especially in Colorado ; Hauraki, N". Z.),§ but is lacking, wholly or nearly, in most cases. Selenium occurs occasionally. The gangue-minerals are chiefly quartz and carbonate-spars, sometimes heavy spar (barite). Fluorite is usually absent, but * Zukunfl des Goldes, 1877. f What follows is a summary of my views as expressed in the ZeUaeh. f. prakL Geologic, 1898, pp. 416-420, and 1899, pp. 10-12. X Zeitsch. d. d. geol. Oeselkch., Bd. xlix., 51 (1897). Published after Stelzner's death by Bergeat. 5 The large tellurium-gold- veins at Kalgoorlie, W. Aust., probably belong, not to this younger group, but to the older one above mentioned. PROBLEMS IN THE GEOLOGY OF OKB-DEPOSITS. 655 occurs here and there in abundance, e.g., in the celebrated tellu- rium-gold Held of Cripple Creek, Col.,* and in the Judith mountains, Montana. Of the characteristic alteration of the country-rock along the veins to propylite, with sericite, kaolin, carbonate-spars, etc., Mr. Lindgren's recent paper in these Transactions gives a gen- eral account. I shall say more concerning it in the next part of this paper. The Older Lead-Silver Veins, and the Older Gold- Veins. — The lead-silver deposits of Freiberg, Annaberg and Schneeberg in the Mrzgebirge ; Clausthal and Andreasberg in the Harz; Kongsberg in Norway; Przibram in Bohemia, etc., and also the old gold-quartz veins of the Mother lode in Cal., Berezowsk in the Urals, etc., show in numerous instances an undeniable dependence upon eruptive processes and mountain-foldings. But here also it is impossible to establish a universal relation between a given kind of vein and any particular eruptive rock. The silver-ore veins, for instance, occur in connection now with basic, now with acid eruptions. Between these older and the younger veins there are several well-known differences. The presence of both gold and silver, in considerable proportions of each, displayed by many of the more recent veins, has never been observed, so far as I know, in the older ones. Again, the older veins do not exhibit the propylitization of the country-rock, so characteristic of the later ones ; but there is, instead, in many cases, as described by Lindgren, a somewhat similar alteration (carbonatization or sericitization). Moreover, the quantity of sulphides or, generally, of compounds of arsenic, antimony and bismuth (and, in Bolivia, of tin) is, on the whole, not so large in the older as in the later veins. Yet, notwithstanding these and other differences, we must, in studying the question of genesis, emphasize rather the anal- ogies between the two classes. There is, for instance, a signifi- cant similarity in many respects between the late lead-silver- '"' Some American observers have assumed a genetic relation between fluor-spar and tellurium (or the telluride gold-ores). This I cannot accept, in view of the absence of fluor-spar from other gold-tellurium districts. There is no trace of it at Nagyag, and, so far as I know, none, or in any event very little, at Kalgoorlie and Hauraki. 656 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. gold-veins of Schemnitz and the old lead-silver veins of Claus- tlial; between Zacatecas, Pachuca, etc., in Mexico, and the " no- ble " quartz-formation of Freiberg, etc. By reason of these mineralogical arguments, Prof. R. Beck, in his new treatise,* does not separate the older and younger vein-groups, but de- scribes them together in categories determined by their min- eralogical character, such as the pyritous lead-formation, the carbonate-spathic lead-formation, the barytic lead-formation, the precious (silver) quartz-formation, the noble silver-copper for mation, etc. In some cases it is doubtful whether veins should be reck- oned as belonging to the older or the younger group. For in- stance, the deposits of Pontgibaud, in central France, show, on the one hand, the character of the old galena-veins, but lie, on the other hand, not far from the late eruptives of Auvergne, and parallel with the volcanic fissure of that field. As L. de Launay has pointed out, it is quite possible that the older and newer gold-silver-lead-veins have a mutual relation somewhatlikethat of the formerly so-called "old" and "young" eruptives, which are now distinguished as deep or outflowing, their structural differences being ascribed to crystallization at different depths. To this subject I shall recur later. Source of the Ore. — We may now inquire, Whence comes the ore of these veins ? For the older as well as the younger ones, we may declare that a clear genetic connection with eruptive rocks can be estab- lished. In some eruptive districts the latest eruptives of the series exposed are even later than the ore-veins ; hence the for- mation of the latter must have occurred during the eruptive epoch. Partly for this reason, and partly because of the fact that, on the whole, the veins are generally independent of the petro- graphic nature of the country-rock, f I think we are warranted, in this department also, in assuming, as a working-hypothesis, that the ore-material was extracted from a magma. With re- gard to the younger veins especially, we must keep in mind a possible extraction from a laccolitic magma in depth. * Lehre von den Erilagerstdtten, 1901. t In many cases there is a dependence on tlie country-rock, the nature of which has favored ore-deposition — as, for instance, in the fahlbands of Kongsberg. PKOBLBMS IN THE GEOLOGY OF ORE-DEPOSITS. 657 In support of this hypothesis, we may cite the transitional or intermediate occurrences between the cassiterite- and the sil- ver-lead-veins. Thus, in Cornwall, the tin-, the tin-copper- and the galena-veins are so closely related topographically and geo- logically that a common origin must be assumed for them. The same is true of the cassiterite-veins and the various silver-lead ore-formations of the Erzgehirge ; and the peculiar tin-bearing silver-lead veins of Bolivia may be recalled in this connection. These intermediate groups warrant the conclusion that there can have been no absolute essential difference between the genesis of the cassiterite- and that of the silver-lead-veins. If the tin-veins are to be explained by magmatic extraction, the silver-lead veins may not be attributable to the work of under- ground water. We refer, also, to a recent paper by E. Hussak,* describing an auriferous pyritic quartz-bed-vein at Passagem in Brazil, and asserting that this vein is to be considered as an ultra-acid granitic apophyse. Between the ordinary quartz-veins, deposited from aqueous solutions (and at high temperature), and the granitic apophyses, rich in aqueous solution and highly siliceous, there seem to be gradual transitionary types. "We may also recall the fact that ore-veins often continue to a great depth. As will be shown later on, mining is carried on in many places at a depth of not merely 0.75 to 1.25 kilom., but, in fact, as referred to the original surface, 3, 4, 5, perhaps 6 kilom. The minerals in veins and the alterations of country-rocks show, in many cases, that the solutions in the vein-fissures were specially rich in carbonic acid and compounds of sulphur (hy- drogen and alkaline sulphides, sulphates, etc.), and to these is often added an aqueous solution of silicic acid. As factors in magmatic extraction for such cases we would assume, therefore, water, carbonic acid, and compounds of sulphur, and, in gen- eral, not hydrofluoric or hydrochloric acid. Copper- Ore Deposits. — The copper-ore veins in or near eruptive rocks (e.g., Butte, Mont., and Cornwall), and also the quicksilver- deposits, permit the adoption of a similar genetic hypothesis. * ZeitscLf. prakt. OeoL, 1898, p. 345. 658 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. Conclusions. — That the ore-deposits first mentioned above, viz., the titanic iron-ores in gabbro, the chromite-occurrences in peri- dotites, the nickel-pyrrhotite deposits in gabbro, etc., were formed by magmatic extraction, I think I have scientifically proved beyond doubt; and I believe that the magmatic-extraction theory advanced for the cassiterite- and apatite-veins is in its main proposition correct. For the ore-deposits subsequently considered — the contact-deposits, the pyritic deposits, the gold- veins, silver-lead veins, copper-ore veins, etc. — the view^s here oifered become confessedly more and more hypothetical. But they have much in their favor ; and even if, following in partic- ular the French observers, I have here ascribed to magmatic- extraction too great a significance, I believe, nevertheless, that the hypothesis is worthy of thorough scientific discussion. At the same time, I wish to add emphatically that, beyond doubt, numerous ore-deposits may have been formed by the action of underground waters, so comprehensively investigated and described by Van Hise; e.g., many deposits of iron- and manganese-ores ; the veins of nickel silicate (garnierite) ; pretty certainly also the native copper of Lake Superior ; and many other occurrences. The precise tracing of the boundary between eruptive afteK- action and the work of the underground waters is a labor for the future. m. The N"ature of the Orb-Solutions in Vein-Fissures, AND the MeTASOMATIO ALTERATIONS ALONG THE OrE-VeINS. The composition of these solutions may be deduced: (1) from the association of minerals in veins, and their relative order of individualization (Breithaupt's " paragenesis ") ; and (2) from the alteration of the country-rock proceeding from the vein-fissures. The Association of Vein- Minerals. Upon a knowledge of the quantitative relations among the various minerals which crystallized from the same solution we may base a conception of the physico-chemical mass-actions obtaining in the solution. For example, if a vein consists chiefiy of calcite, with a little silver-glance, the silver, as well as the calcium, must have been present originally as AgHCO PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 659 and CaH2(COj2 respectively, in an aqueous solution of carbonic acid, from which it is pretty certain that the silver was precipi- tated by hydrogen sulphide. Such a case is furnished by the deposits of Kongsberg, Xorway.* By parallel investigations of the paragenesis of the veins and the metasomatism of the country-rock, supported by experi- ments in mineral synthesis, the chemical nature of vein-solu- tions can be fairly well determined. The data at our disposal are now so abundant that this question must soon be ripe for final scientific decision. Deposition of the Vein-Minerals. These have generally crystallized under high pressure and somewhat elevated temperature. Under present conditions a depth of 1 kilom. in the earth's crust represents an increase of about 275 atm. in pressure, and 30° C. in temperature. In many ore-veins, as will be shown later, it can be shown that the minerals were formed at a depth below the original surface of 3, 4, or 5 kilom., or perhaps more. If we assume 4 kilom., and conditions like those of the present day, there must have been a pressure of about 1000 atm., and a temperature about 120° C. higher than at the surface. But it must be considered that in the exceedingly numerous deposits connected in some way with eruptive processes, and often, indeed, formed in the later periods of the eruptive activ- ity, the nearness of the igneous rocks must have caused an in- crease of temperature (and also of pressure ?). This is often so great as to exceed for heavy compounds the " critical tem- perature," as shown for a few substances in the following list. H,0, . HjS, CO,, . CO, (about) SO,, Critical Temperatures. Deg. C. Deg. C. . 364 HCI, ... 52 100 AsCla, 31 SiCl,, 140 SnCli, 157 TiCli, 356 230 319 358 We note especially that the critical temperature of water occurs at 364° (or, according to earlier determinations, 375°) — * See Zeitsch.f. prakt. Geologie for April and May, 1899. Concerning the solu- bility of silver carbonate as AgHCOj in water containing free carbonic acid, see a treatise by Chr. A. Miinster, cited by P. Krusch, Id., 1896, p. 103. 42 660 PROBLEMS IN THE SEOLOGY OF ORE-DEPOSITS. a temperature which certainly must have been exceeded, by magmatic solutions at the moment of leaving the magma. In their upward course — especially determined, perhaps, by their lower specific gravity — the solutions cool ; and (partly by vir- tue of this cooling, and partly through the encountering of various mutually reacting substances) the minerals are succes- sively precipitated. Alteration of tJie Country-Mock. The scientific study of this phenomenon of vein-walls was begun in the '40s by Elie de Beaumont, A. Daubree and others; yet Waldemar Lindgren's recent admirable paper* gives us for the first time a systematic scientific summary of the transfor- mations which it involves. Mr. Lindgren's classification of veins according to metasomatic processes I here condense for convenient reference. (1) Topaz-cassiterite ; (2) scapolitic apatite ; (3) tourmalinic gold-copper ; (4) bi- otitic gold-copper; (5) propylitic guld and silver; (6) fluoritic gold- tellurium ; (7) sericitic and kaolinic gold and silver ; (8) sericitic and calcitic gold and silver; (9) silicic and calcitic quicksilver ; (10) sericitic copper-silver; (11) silicic and dolomitic silver-lead ; (12) sideritic silver-lead ; (13) sericitic silver-lead ;(14) zeo- litic copper and silver. Having busied myself somewhat with this class of problems, I will take the liberty to include here my attempt at a classifi- cation of the metasomatic processes caused by ore-solutions.f Classification of Metasomatic Alterations. 1. Alterations forming greisen, mica^rock, cassiterite-rock, tourmaline-rock, topaz-rock, etc.' 2. Scapolitization. 3. Propylitization (with chloritization, etc.). 4. Kaolinization. 6. Sericitization. 6. Carbonatization (with dolomitization, etc.). * " Metasomatic Processes," etc., Trans., xxx., 578 ; this vol., pp. 498-612. t Taken from the manuscript of a halt-finished paper which I began to write a year or two ago. But I may refer also to my article in the Zeiisch. f. prakl. Geologie, Nos. 4, 11 and 12 of 1895, and No. 12 of 1898. The term " carbonatization," which was new to me, I took, a few years ago, from W. Lindgren's paper, "Characteristic Features of California Gold-Quartz Veins" (Bidl. Oeol. Soc. of Am., 1895, vol. vi., p. 221) ; and I now add, still following Lindgien, the term dolomitization (as a process occurring along ore-veins). PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 661 7. Silicification. 8. Zeolitization. 9. Intense contacl^metamorphism. I would mention also the formation of alum-stone ; quartz- alunite rocks; quartz-diaspore rocks, etc.;* and also the forma- tion of bauxites, etc. But I do not know that these changes have been anywhere observed in genetic relation with ore-veins. Between Lindgren's classification and this one previously written, though not published, by myself, there is, on the whole, a striking resemblance. My final heading, " Intense contact- metamorphism," is not included by Lindgren ; but I believe that it plays an important part in connection, not, indeed, with the vein-fissures wliich he discusses, but with the contact-de- posits described in a previous part of this paper. Additional Observations. To Lindgren's thorough treatise I venture to add, by way of complement and confirmation, a few isolated and fragmen- tary observations. Kaolinization. — As is well-known, A. Daubrfeef called atten- tion to the fact that man}' of the principal kaolin-deposits in Cornwall, central France and the Erzgehirge accompany the tin-ore deposits of those regions. This may, perhaps, suggest the idea that the formation of kaolin, like that of cassiterite, must be explained in some way by the action of fiuorides. Long ago, however, Forchhammer (1835) and BischoflE" (1855), followed by many more recent authorities, ascribed kaolinizar tion to the attack of water carrying carbonic acid; J and this must be the correct view, for the-following reasons : (1) kaolin- ization is in many cases a surface-process, afifected by the weak carbonic-acid solutions of surface-waters; (2) at somewhat greater depths, the feldspars of the rocks are often converted by similarly weak carbonic-acid waters into kaolin, sericite, etc., as well as calcite. Other instances may be given, in which the action of fiuorides is excluded. * See W. Cross, "Geology of Silver CliS, etc., Colo.," and S. F. Emmons, " The Mines of Custer Co., Colo ," llth Ann. Rep. U. S. Oeol. Sur., 1896. t Andes SyTiihetiques de Geologie Expirimentale, 1879 ; and also in his earlier works. t For the literature of the subject, see under " Kaolin," F. Zirkel's Lehrb. d. Petrographie, 1894, vol. iii. 662 PKOBLBMS IN THE GEOLOGY OF ORE-DEPOSITS. It is notoriously orthoclase which is most frequently con- verted into kaolin. Forchhammer states the reaction as follows (in the old notation, which I retain) : Orthoclase, Subtracted, . Added, . AlA, Kaolin, . ALO„ K,0, Kfi, 6SiOj 4SiO, 2H2O 2Si02, 2H2O Other feldspars and silicates of alumina, such as hornblende, augite, beryl, topaz, etc., are known to be occasionally converted into kaolin; but the study of the primary kaolin occurrences of granite shows that the jjotash-feldspar is much more easily or rapidly kaolinized than the silicates of magnesia and iron (magnesia-mica, etc.), while quartz is attacked but very weakly. A few years ago, near Josingfjord, at Ekersund-Soggendal, 4 or 5 kilom. from the ilmenite-deposit of Blaafjeld-Stor- gangen, in southern Norway, a kaolin-deposit in labradorite- rock was discovered, of such importance that it is now worked commercially. The kaolin was formed in situ from the labrador- ite, in which it occurs in pre-glacial fissures. The various stages of the alteration are illustrated by the following table of analyses (hitherto unpublished), some of which, unfortunately, are not complete : Analyses of Labradorite and the Products of its Kaolinization. Labra- dorite. I Labradorite, Partly' Kaolin- ized. Massive Kaolin, More or Less Pure. SiO, Per cent 54.5 AI.O3 27.0 FeoOa 2 5 faO 9.0 1.0 5.0 1.0 MgO H,0 Total 100.0 Per Per ' cent. cent, i 60.03 49.1fi 28.60 29.60 ; 1.62 1.88 i 4.21 3.47 1 2.95 1.67 1 labontlVe™: 1 i-oo ;;lnS. 11.90 I is.e.") II. I I. Per cent. 48.61 29.45 3.40 0.68 0.49 I unde- [- term- ] ined, 16.38 II. Normal Compo- sition of Kao- lin. Per cent. 48.06 ■38.57 unde- Per i cent. 47.83 34.53 i 1.70 0.48 0.59 term- 1 unde- i ined. Merm- J ) ined, 12.95 I 13.76 Per cent. 47.72 37.40 1.59 0.23 0.11 0.76 0.44 kl Per cent 46.85 37.56 1.00 trace, trace. ") unde >-term. j ined. 14.44 Per cent. 46.50 I 39.56 0.00 0.(0 0.00 O.O'I 0.00 100.31 (99.41) I (99.01) (99.68) i.89) 99.91 (99.85) I 100.00 I iSOTE. — Labradorite- rock consists overwhelmingly of labrador-feldspar (contain- ing about 56. 2.5 per cent. SiO.^ ) with a couple of per. cent, of ilinenite and hyper- sthene, somewhat more richly concentrated here and there in spots. Hence the relatively high percentage of MgO in Analysis I. of labradorite, and of Fej03 in I. of kaolin. PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 663 Taken together with the macroscopic and microscopic study of the transitional stages, these analyses show: (1) that ilmen- ite and hypersthene resisted attack better than the labrador- feldspar ; and (2) that from the latter its alkali-silicates were extracted. The larger the amount of ^a.fi, K 0, CaO and MgO (with some SiO^ and Fe^Oj) removed, the smaller becomes the specific gravity. That of the labradorite unaltered is 2.727; of the slightly kaolinized, 2.666; of impure kaolin, still showing the feldspathic structure (Analysis IV. of kaolin, with 47.72 SiOj), 2.254; of almost pure kaolin, 2.193 and 2.192; and of the purest kaolin of the district, 2.178. These determinations hold for the porous masses, inclading the pores : pure non-porous kaolin has a specific gravity of 2.6. That in this case kaolinization has resulted from the action of carbonic acid waters, follows from the fact that occasionally, though rarely, calcite occurs with the kaolin, while most of the NajO, KjO) CaO and MgO (as soluble carbonates), as well as the dissolved SiO^, have 'been entirely removed. If I correctly understand Lindgren, he seems, on pp. 614 and 664 of his paper, to intimate that stronger agents, such as sul- phuric acid, may have operated or co-operated to form kaolin. In my judgment, it is not necessary to assume such stronger agents, especially in view of the well-known kaolinization, by ordinary weathering, of the feldspars of rocks. Moreover, sul- phuric and sulphurous acid appear to produce transformations of a diflferent sort (such as alunite, etc). As already remarked, kaolin occurs in some cassiterite-veins (as well as in their metasomatized wall-rocks), and also in cer- tain districts of sulphide-ores. Examples are found at l^Tagyag, Puda and other places in Transylvania, where propylitization has been occasionally accompanied, in a subordinate degree, by kaolinization. According to the descriptions of Bela von Inkey and Semper, this kaolinization took place along the veins, and was independent of recent weathering by surface-agencies.* ' See E. V. Inkey' s Nagy&g u. Seine Lagerstatten, Budapest, 1885 ; and Bergas- sessor Semper' s Beitrdge zur Kenntniss der Ooldlagerstalten des Siebenbiirgischen Erz- gebirges, in the Abh. d. preuss. geol. LandesaTistalt, 1900, p. 23. According to Kolebeck's analysis (given in the Oest. Z. f. B.-u. Hiittenw., 1888, and referred to by Lindgren), the so-called kaolin of Nagyag is in reality mostly sericite or sericite 664 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. This seems to be true also of the kaolinization of the Cripple Creek district,* where occasionally not only granite but also phonolite and andesitic breccia have been transformed to kaolin. That this is kaolin, and not sericite, Hillebrand's analyses prove. Again, at Schemnitz, Hungary, according to some accounts, propylitization has been accompanied, here and there, by kaolinization. For additional examples, I refer to Lindgren's paper, under " Sericitic and Kaolinic Gold and Silver Veins," and also to the article " Kaolin " in C. Hintze's Handbuch der Mineralogie. On the whole, kaolinization along ore-veins is rather scanty. "We note, then, as the result of many abservations, that the formation of greisen (bordering cassiterite-veins), and also pro- pylitization and serieitization, and probably silicification, are accompanied here and there by kaolinization, which, on the other hand, seems to be wholly absent in cases of carbonatization (along ore-veins), or, as Lindgren says (p. 614) : " Wherever abundant carbonates form metasomatically, to- gether with sericite, kaolinite seems to be absent." Calcite is also, as a rule, wholly absent from the primary kaolin-deposits, formed in situ from granite, gneiss, etc. Even in the kaolin-deposit of Ekersund-Soggendal there is scarcely any lime, though the original labradorite-rock carried consid- erable calcite. Both kaolinization and carbonatization (or the latter with serieitization) result from the attacks of carbonic acid water, but with this important difference, that in the former, lime, magnesia, potash and soda are almost or quite removed, leav- ing the silicate of alumina; whereas in the latter, calcite, and generally also the potash-a\u.m.h\a silicate, sericite, are deposited or precipitated. This difference is due, pretty certainly, to quantitative variations in the constituents of the attacking solution. Thus, we learn from the weathering of granite, etc., that very weak carbonic-acid water can remove lime, magnesia, alkalies, etc., and produce kaolin; and, on the other hand, it mixed with kaolin. The masses produced by kaolinic transformation, described by Semper as rich in kaolin, calcite and pyrite, probably contain considerable sericite. , * Messrs. Cross and Penrose, in 16th Arm. Rep. U. S. Oeol. Sur., Part ii., 1-209. PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 665 may be assumed that water rich in dissolved alkaline and earthy carbonates favors carbonatization and accompanying sericitiza- tion. Comparison- Between Cassiterite-Veins and Lead-Sulphide Veins* — The elder French school drew an absolutely sharp line between the tin-deposits and the sulphide-bearing veins. Some went so far as to divide all ore-veins into two classes : (1) the Jilons stannifhes, products of fumaroles in granites; and (2) the jilons sulfures dites plomhifhes, deposited by thermal springs, and supposed by some observers to be always connected with basic rocks. It is true that the typical cassiterite-veins (Cornwall, Saxony, etc.) are, so far as known, connected with acid eruptions exclusively ; but the converse proposition, that sulphide-veins are connected with basic rocks exclusively, does not fit the facts. As a single instance, instar omnium, we may mention the im- mense copper-silver-ore deposits in theButte granite, in Montana. The division into jilons stannifhres and plombiflres is quite fit- tingf — only there are, here as elsewhere in nature, no sharp boundaries, but, on the contrary, gradual transitionary forms. Among such transitions we may mention the frequent occur- rence in cassiterite-veins of arsenopyrite and other sulphide ores; the tin-copper-ore veins in Cornwall; the connection, in the Erzgehirge and in Cornwall, between lead-silver veins and cassiterite-veins ; also the cassiterite-bearing lead-silver veins of Bolivia, and veins in Tellemarken, ISTorway, which I have briefly characterized as " cassiterite-veins carrying copper-ore instead of cassiterite." Again, we may point out that tourmaline and other boro-silicates (axinite, datolite, etc.) have often been ob- served, even in abundance, in veins carrying sulphide copper-ores or gold. The general treatises of A. v. GroddeckJ and A. W. Stelzner,§ and a series of other publications (some of which Lind- gren cites under "Tourmalinic G-old-Copper Veins "), are author- ities for this statement. Yet, so far as I am aware, galena-silver- ore veins carrying tourmaline in abundance are not known. '' The groups here indicated under these titles are those named by Daubr^e, in his litudes Syntheliques, etc. (1879), les jilons stannifires and les jilons sulfures dites plnmbifires. t Deposits of iron- and manganese-ore are not inchided in this classification. X Zeitschr. d. d. geol. Gessellch,, xxxix., 78, 237 (1887). ^ Zeitschr. f. prakt. OeoL, 1897, p. 41. 666 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. As a characteristic miner alogical difference between the two classes of veins under consideration (^stannifires emd plombifires) we may point out that topaz, so characteristic of the former, has never been observed, either in the ordinary sulphide-ore veins (Freiberg, Clausthal, etc.), or in the tourmaline-bearing veins, whether with sulphide copper-ores or with gold. In the ordinary sulphide-ore veins, moreover, apatite and other pri- mary phosphates are wanting, as is also the lithium-mica, so characteristic of cassiterite-veins. Comparison Between the Formation of Greisen, etc., and Propyl- itization, etc. — Turning now to the metasomatism of the vein- walls, we find that topazization and the formation of topaz- greisen are confined exclusively to cassiterite-veins. On the other hand, we never encounter, along these veins, propylitiza^ tion, serieitization and carbonatization, which belong to the veins of sulphide-ore or gold. Kaolinization, on the contrary, takes place (although subor- dinately) here and there alongside of veins of either kind. The same is true of silicification, as illustrating which I may mention the formation of " quartz-rock " alongside of cassiterite- veins; also the silicification (Verkieselung) of the walk of some later gold-veins (as in a part of the Verespatak district^ in Transylvania) and of some quicksilver-veins. Again, here and there along the cassiterite-veins as well as the sulphide-veins, we find the country altered to mica-rocks, which, although not for the two classes mineralogically identi- cal, present so many analogies that they must have been formed under pretty similar conditions. The mica-rock along the cassiterite-veins is petrographically allied to greisen (and tin- or topaz-bearing greisen), topaz-rock, etc., and consists chiefly (or wholly ?) of lithium-mica. On the other hand, at Tele- marken, Norway, a biotite-granite is likewise altered to mica- rock, along certain quartz-veins carrying chalcopyrite or bor- nite ; but here the mica, which appears often in large crystals, contains no lithium, being a potash-mica (muscovite). In the same locality, subordinate fissures in the vein-material are often lined with mica crystals (1—2 cm. in diameter), exactly as are the similar fissures in the well-known cassiterite-deposits of Zinnwald, in the Erzgehirge. PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 667 As in the ordinary alteration to mica-rock and topaz-greisen, so likewise in propylitization with chloritization and sericitiza- tion, it is, as a rule, the iron-magnesium-silicates (mica, augite, hornblende) which first (before the feldspars) sutler alteration — in other words, offer the smallest resistance to the attacking solutions. On the other hand, mica, augite and hornblende are more resistant than the feldspars to the processes of altera- tion which form kaolin. Concerning the alteration of the later eruptions (andesite, dacite, trachyte, rhyolite, etc., sometimes also basalt) to propy- lite, I would refer in particular to the well-known monograph of Gr. F. Becker on the Comstock Lode (1882), and the investi- gations of B. V. Inkey, Dolter, Judd, Koch, Szabo and many others.* In propylitization, as is well known, the iron-magnesium silicates (augite, hornblende, mica, etc.) are converted chiefly into chlorite, with sericite, actinolite, epidote, serpentine, iron oxides, spathic carbonates, etc. The feldspars lose their luster ; their cleavage is impaired; and they are impregnated with products of decomposition, particularly chlorite, epidote, cal- cite, etc. Moreover, the re-formation of pyrite is very charac- teristic; and, as a rule, the further propylitization has pro- gressed, the larger the quantity of pyrite. Becker has shown thai this pyrite has been derived from the iron-magnesium sili- cates and the iron oxides (magnetite, ilmenite, specular hema- tite) of the original rock, through the action of solutions con- taining alkaline sulphides or hydrogen sulphide.f Rosenbusch describes propylitization as " a process of solfata- ric and thermal^ alteration." Nearly related to it are chloritiza- tion and sericitization. It is confined to later ore-veins, con- nected with extensive rocks, and is absent in the corresponding veins of earlier origin. Possibly the reason of this difference * See chapters on propylite in Eosenbuseh's Mierosc. Physiogr. d. mass. Oesieine (1896), ii., pp. 913-917 ; Zirkel's Lehrb. d. Petrographie (1894), ii., pp. 584-595; and on " Propylitic Gold- and Silver- Veins " inLindgren'spaper, this vol., p. 565. t Here we are reminded of the chemical nature of the solutions of the recent quicksilver-deposits at Steamboat Springs, Sulphur Bank, etc. (investigated by Becker and others), where the quicksilver is found combined with a sodium sul- phide, HgS,wjSra.,S. X Secbetaby's Note. — I understand this terra, as used in German, to mean the action of heated aqueous solutions. — R. W. E. 668 PROBLEMS IN THE GEOLOGY OP OKE-DEPOSITS. maybe that the " old " veins were formed at much greater depths, and hence under much liigher pressure, whereby the escape of solutions (and especially the dissolved gases, H.,S, e^c.) into the country-rock was hindered. I shall presently return to this point. A priori, it is natural to conceive the metasomatism along veins has been generally accompanied by a considerable change (now addition, now subtraction) of material. This does indeed occur in some instances, especially in topazization, tourmalini- zation (with axinitization) and kaolinization ; but in many other metasomatic alterations the change of material is rela- tivel}' insignificant. This is the case in scapolitization ; in intense contact-metamorphosis ; in many alterations resulting in greisen and mica^rock; and also in propylitization with chloritization and sericitizatiou. How small, in the latter processes, are the chemical differences between the original and the altered rock, I have learned with astonishment from the analyses collected by Lindgren. Conclusions. — In conclusion, I will attempt to give a summary of the agencies operative in processes of alteration : 1. Topazization, the formation of topaz-greisen,.tourmalini- zation, axinitization, etc., are chiefly due to the action of fluo- rides — in the two latter cases, of boro-fluorides. 2. Scapolitization is due to re-crystallization under high pressure, with access of a chloride (particularly sodium chlo- ride) solution.* 3. Propylitization is a solfataric and thermal alteration, ef- fected by attacks of hydrogen sulphide or alkaline sulphides, and often also of carbonic acid. 4. Kaolinization, sericitization and carbonatization are pro- duced by the action of waters carrying carbonic acid, or car- bonates of alkalies and earths, in variable proportions. (In kaolinization, the waters carry so much carbonic acid that the alkaline and earthy carbonates are nearly or wholly removed, together with the dissolved silica. In sericitization and car- bonatization, on the contrary, there is a deposit of potassium silicate or calcium carbonate.) 5. Silicification results from percolation bj- a solution of silicic acid. * See, on this subject, Zeitsch. f. prakt. Geol., 1895, pp. 447, 455. PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 669 6. Zeolitization is also produced by silicic acid, but under different conditions (probably, as a rule, by a solution contain- ing silicates of sodium, potassium, calcium and aluminum). 7. Intense contact-metamorphism involves a recrystallization under high pressure, with penetration by heated aqueous vapor, and is, per se, accompanied by a comparatively subordinate change of material. Sometimes, however, it occurs in connec- tion with ferrification, silicilication, tourmalinization or axiniti- zation, etc. 8. The formation of alum-stone or alunite is chiefly effected by the penetration into the rock of a solution of sulphuric or sulphurous acid. Frequently several of the above agencies operate in combina- tion, rendering the results more complicated. r\^. Differences of Depth in the Original Positions of Epi- GENETic Deposits ; and the Secondary Alterations OF Deposits. The attention of both miners and geologists was long ago drawn to these theoretically interesting and economically im- portant problems ; yet only in recent years have they received thorough and comprehensive treatment. The valuable contri- butions made to the Transactions of this Institute by Don, Em- mons, Eickard, Posepny, Van Hise, Weed and others, are fa- miliar to its members, as well as the work of R. A. F. Penrose and his associates of the U. S. Geological Survey, and other American observers Much may be learned from the recent treatise of our celebrated professional colleague, Prof L. de Launay.* These two phenomena — namely, the original differences of depth connected with the formation of ore-deposits, and the secondary alterations of such deposits, occurring often, perhaps even millions of years later — are in many cases, as genetic fac- tors, very widely separated ; yet it may often be difficult to decide what is to be referred to the primary and what to the secondary process. Partly for this reason, and partly because, as Van * ' ' Les variations des filons m^tallif 6res en prof ondeur ' ' {Bev. Oen. des Sci. Pures et Appliquees, xi., 1900 ; discussed by P. Krusch in Zeitsch. f. prakt. Oeol, Oct., 1900). See also De Launay's Contribution A V etude dex gites metatlifh-es in Ann. d. Mines, 9 S^rie, vol. xii., p. 119 (1897). 670 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. Hise's last paper shows, the two factors go, in many localities, hand in hand, I think they may be, with advantage, discussed together. Original Differences of Depth. In considering the original differences of depth, it must be kept in mind, as De Launay has pointed out in the treatises just cited, that the present surface is, in general, very far below the surface existing at the time of the ore-formation. The geolog- ical investigations of recent decades have shown that the work of denudation (or abrasion or erosion) must be measured on a larger scale than was formerly suspected. In the Archean and Algonkian mountain chains (now often removed by this agency down to their base-level), and also in the Paleozoic ranges (showing, as a rule, the effects of extremely energetic denudation, as, for example, the Ural and the ISTorwegian mountains), the difference between original and present; levels is to be generally reckoned, not in such units as 0.1, 0.25, 0.33 or 0.5 kilom., but rather on the scale of 2, 3, 4 or 5 kilom., or even more. Even in the Mesozoic and Tertiary, many denudations of astounding depth have been recognized.* In many epigenetic ore-deposits of Archean-Algonkian or Paleozoic origin {e.g., Kongsberg, Cornwall, Przibram, the Keweenaw peninsula at Lake Superior) mining has been carried * As instances of great denudation, the following may be named : On the E. side of the Kristiania fiord, in Xorway, this process has removed, (1) a series of Devonian conglomerates and porphyry overflows, with Silurian and Cambrian rocks, of an aggregate thickness (according to W. C. Brogger, JVy(. Mtuj. f. Naturv., vol. xxxviii., for 1900) of 2500 meters ; (2) also a large part of the Archean surface — first, during the long pe iod preceding the Cambrian, and again after the removal of the Cambrian, Silurian and Devonian strata. This thickness must also be measured in thousands of meters ; sp that we have here at least 4000, perhaps 5000, 6000 or even more meters of thickness removed. The fiords of the "SV. coast of Norway are often 1.5, sometimes 2 to 2.25 kilom. more deeply eroded than the adjoining high plateau ; and the latter frequently consists of deep eruptives, without any remains of the extensive overflows — show- ing that on the plateau a very extensive denudation, probably to be measured in kilometere, ha,s taken jilace. In the Aspen silver-district, Colo., ■"> kilom. of strata (according to Spurri have been removed by erosion from a range of Tertiary origin. (I quote from Krusch's review of De Launay, ZeitscLf. prakt. Geol., 1900, p. 317. ) In California, according to Lindgren, denudation has extended to a depth of 500 to 1500 or more meters. So far as I know, this denudation has taken place since the beginning of the Cretaceous period. Numerous other similar instances could be easily adduced. PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. 671 to depths of 0.75, 1 to 1.26, and 1.25 to 1.5 kilometers. Tak- ing the depth roughly as 1 kilom., and assuming that in some districts the present surface has been denuded 3, and in others 4, kilom. below the surface at the time of the ore-formation, we may say that mining has reached a depth of 3 to 4 or 4 to 5 kilom. below the original surface. These figures are, of course, somewhat arbitrary ; but mod- ern investigations of the extent of denudation justify us in say- ing that they are not too high for some districts belonging to the ancient geological periods above named. It may be observed, also, that in many deposits of deep and geologically old origin, the deepest portions of the mines have shown no change in the nature of the fissure-formation. Occa- sionally, as at Przibram, Bohemia, and Dolcoath, Cornwall, the richest ore-bodies have been encountered in the deepest mine-workings. "We conclude, then, that, under favorable circumstances, the ore-veins may continue at least to a depth, below the original surface, of 3, 4, 5 or more kilometers. In opposition to this view. Prof. Beck declares* that he has come to the opinion "That ore- veins, and mineral veins generally, can by no means extend to great depths, geologically speaking. . . . Even if we could assume the exist- ence, at a depth between 4000 and GOOD meters, of fissures filled with water, it would be inconceivable that, at that depth, mineral deposits could be made from solutions." I believe, notwithstanding, that future determinations of the extent of denudation, together with the mining of many de- posits to the depth of 1.25, 1.5, or even, perhaps, 2 kilom. below the present surface, will prove that Prof. Beck's conclusion is not correct. It may also be remarked, in passing, that mineral deposits may be made from solutions at above the critical temperature (364° C.) of water — for instance, the deposits of cassiterite, wolframite, apatite, topaz, tourmaline, and even pyrites, in many granite-pegmatite veins. In his latest treatise, which is rich in new conceptions, De * Lehre von den Erzlagerstatten, 190], p. 139. 672 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. Launay compares ore-deposits occurring relatively near the surface, in less denuded regions, with those deep below the surface in strongly denuded regions. As instances of the former, he takes the quicksilver-deposits, which occur chiefly in recent rocks, near volcanic eruptives, while from older ranges, partly destroyed by erosion, they have disappeared, with other debris. As instances of the latter class, he takes the pyritic deposits (Roros, Huelva, Schmollnitz, etc.), which have been found in old mountain-chains or in districts of re- gional metamorphism, and are to be explained as of deep-seated origin. He also mentions very briefly the lead-silver veins. Induced by his description, I have already suggested in this paper the hypothesis that the differences between the later gold-silver-lead veins (Nagyag, Comstock, Potosi, etc.) and the old gold and silver-lead veins (Kongsberg, Erzgebirge, Harz, Przibram, etc.) maybe explained by their formation at different depths. The relative or total absence from the older veins of the propylitization which is so characteristic of the later ones may, perhaps, be due to the fact that hydrogen sulphide (or sodium sulphide), which was a very important factor in this process, could not, under the great pressure due to great depth, make its way from the solutions in the fissures into the country-rock. The later silver-lead veins are, on the whole, richer in silver than the older ones. This may be connected with the fact, inferred on physico-chemical grounds by Van Hise, that at great depth lead sulphide separates in larger proportion than silver-sulphide or sulpho-salts. According to this view, the precious srlver-veins (carrying relatively little galena and zinc-blende) of recent eruptive ranges become, at a very great depth, richer in galena and zinc-blende. This seems to be sometimes the case. (In- stances are given further on.) This hypothetical view is not contradicted by the fact that many of the older silver-lead veins, as at Andreasberg and Kongsberg, are highly "precious" — i.e., relatively poor in galena and zinc-blende; for this character may be due to the small proportion of lead and zinc in the original vein-solutions. In view of the range below the original surface through which mining is carried on, beginning, not at that surface, but alread}' thousands of meters below it, we may easily see that, in many PROBLEMS IN THE GEOLOGY OF OEE-DEPOSITS. 673 districts, the directly observable differences in original depth have little significance. For instance, at Kongsberg there is no difference in the character of the veins from the present surface to 0.5-0.75 kilom. below it. In other districts, however, very important differences of original depth have been established. For instance, these dif- ferences were very distinct in many Cornish mines, where the veins carried : (1) at the uppermost level (in the tin-bearing gossan) tin-stone and a little copper-ore (the latter as the result of a secondary process, the original sulphides having been mostly leached out of the gossan) ; (2) their copper-ore, with some tin-stone (in the Dolcoath mine, to the depth of 0.3 to 0.33 kilom. below the present surface); (8) still deeper, first, a zone of mixed tin-stone and copper-ore, and under that almost exclusively tin-stone. The veins traverse, in depth, chiefly granite; at higher levels, slates. But zones 2 and 3 are not confined to either rock. In this case, then, the tin- stone was originally deposited at a greater depth than the copper-ore. In many silver-lead-zinc veins there is an increase in the proportion of zinc-blende with depth. The Clausthal veins, and many in Mexico (Pachuca, Zacatecas, etc.) are instances. In the latter, very important differences in the depths of orig- inal deposition are often observed. (1) Ifear the surface are very rich silver-ores (the so-called colorados, containing cerus- site with chloride, bromide and iodide of silver, and native silver), the richness of which is the result of secondary pro- cesses.* -(2) Below these, after an intermediate zone of tran- sition, appear for the first time the so-called negros ores — galena, silver-glance, silver sulpho-salts, etc. ; and (8) in the deep work- ings, say 0.5 kilom. below the present surface, the so-called /Meffo-ores,t carrying much zinc-blende and galena, with a scanty admixture of true silver-ores. It is possible that the Tertiary Mexican veins have in depth a character resembling that of the older rather than that of the younger group described in a pre- vious part of this paper. * According to the Mexican geologists and miners at the Paris Exposition of 1900 these ores extend, as a rule, very little below the ground-water level. + That is, "fire "-ores, or, in other words, smelting-ores. The surface-ores are treated by amalgamation. 674 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. "With regard to the increase of zinc-blende in depth, which has been observed in many places, I have already observed that Van Hise, in his last paper, concludes upon theoretical grounds that from an ascending solution containing zinc and lead, the zinc sulphide would be deposited lower down than the lead sulphide. In many veins carrying copper and iron sulphides, the richest copper-ores are found at the higher levels. As Emmons and "Weed have shown for the Butte district, this is in numerous cases the result of a secondary process. In sundry localities, however, the influence of original differences of depth may be recognized. This is the case, for instance, at Vigsnas in JTorway, where the ore, a fine-grained mixture of chalcopyrite with pyrite, occurs in several (about seven) nearly vertical " stocks." In the upper levels the pyritic mixture carried easily 3 to 4 per cent, of copper; at the depth of 735 meters the thickness of the mass was, on the whole, tolerably well maintained ; but the copper-content had sunk to about 1 per cent., or a trifle more. A corresponding phenomenon is not presented, however, by the flat-lying pyritic masses or " lineals " at Roros, which dip respectively 9°, 9° and 15°, and have been worked in these dips to distances of 1080, 1350 and about 2000 meters. In the pyritic deposit at Huelva.(at Rio Tinto, Tharsis, etc.) the secondary concentration in the " zone of enrichment," immediately below the " iron hat," plays a very important part; but there appears to be, besides, a primary distribution according to which the copper diminishes as depth increases.* According to many American reports, there are also in the United States and in Chile many known instances of the depo- sition, from an ascending ore-solution, of pyrite and chalcopy- rite, in which the former was, to a considerable extent, de- posited deeper than the latter. Jfot only in the sulphide-ore deposits, but also in those of iron and manganese oxides, primary differences of depth are recognized. Thus at Romaneche in the Department of Saone et Loire, France, the ore-deposits, occurring in granite, consist of psilomelane (named romanechite by Lacroix, on account of its constant considerable percentage of baryta) and specular * See Zeitachr. f. prakt. GeoL, No. 7, 1899. PROBLEMS IN THE GEOLOGY OF ORB-DEPOSITS. 675 hematite, with quartz and heavy spar, a very little fluor-spar, and traces of calcite. The mine is, with one exception, the largest producer of manganese-ore in France. For our present purpose, the interesting feature is the change of proportion, at different levels, between psilomelane and hematite. Above, the psilomelane predominates; going down, the proportion of hematite increases with considerable regularity. During a visit which I made in the summer of 1900, together with my friend and colleague. Prof. L. de Launay, Mr. L. Chamussy, the director of the mine, called our attention to the fact that this relative increase of iron-ore in depth is found in many manganese-deposits. His explanatiofi was, that the solution containing manganese and iron compounds came from below, and the ores were precipitated mainly through oxidation by the oxygen of the air contained in surface-waters ; that iron thus oxidizes more easily (i.e., sooner) than manganese,* and there- fore, on the whole, the larger proportion of iron-ore would be deposited lower than the manganese. This seems to me quite plausible, t Secondary Alterations of Ore-Deposits. Concerning the secondary alterations more or less directly connected with surface agencies, I would observe, first, that such phenomena have very little importance in the Norwegian and Swedish deposits, which are generally found in very solid rocks, such as gabbro, gneiss, granulite, mica-slate, phyllite, etc. The oceurrrences of magnetite, specular hematite and ilmenite show, as a rule, no trace whatever of a zone of weather- ing — except that here and there apatite has been, to a slight extent, leached out. The dense, massive magnetite resists -:s That from a solution containing protoxides of iron and manganese {e.g., in carbonic-acid water) iron is precipitated by oxidation before manganese has long been known. The literature of the subject is given in my work " Salten ogBanen " (1890-91), in which a geological application of this order of precipitation was attempted. See also Zeitschr. f. prakt. Oeol., 1894, p. 33, and 1895, p. 39; also "The Chemical Belation of Iron and Manganese in Sedimentary Rocks," by R. A. F. Penrose, Jour, of Oeol, 1893, vol. i., p. 350. t I regret that this contribution must be prepared for publication in such haste that I have not time to obtain by correspondence further details concerning this primary difference of distribution in the manganese-deposits. Nevertheless, I venture to give herd the above theoretical explanation. 43 676 PROBLEMS IN THE GEOLOGY OP ORE-DEPOSITS. even denudation, so that, for instance, the extraordinarily large deposit at Kirunawara-Luossawara, in northern Sweden, forms a real " iron mountain," rising about 100 meters above the surrounding rocks. The same is true of the Taberg, a moun- tain of titanomagnetite-olivinite, in southern Sweden. Even the pyritic deposits, like Roros, Sulitelma, Vigsnas, etc., and the nickel-pyrrhotite deposits, like Erteli, 5 to 10 meters in thickness, show a zone of weathering seldom more than one or two meters deep. At Fahlun, where the pyritic mass was very wide, the " iron hat " was probably deeper. This insignificance of the secondary alterations, even in the pyritic deposits, is probably due to two chief causes : (1 ) that the surface was polished clean by the Quaternary ice-sheet; and (2) that the solidity of the country-rocks has permitted very little circulation of water.* In sharp contrast stand the thick pyritic deposits of Rio Tinto, etc., in the Huelva district, where the "iron hat" ex- tends to 35-50 meters. Here the Quaternary ice-period was lacking, and the country-rocks were much more porous than in the corresponding Scandinavian formations. Concerning these secondary alterations, I would refer to an earlier work of my own, which is mentioned, among others, in the papers of Messrs. Emmons and "Weed. Especially noteworthy here is the re- formation of rich sulphides in the " zone of enrichment," and the very characteristic re-formation in Mass II., at Rio Tinto, of a narrow zone, rich in gold and silver, on the boundary be- tween the " iron hat " and the underlying pyritic mass. Concerning the chemistry of the secondary alteration of ore- deposits I can add little, in this hasty review of the subject, to the excellent discussions of Don, Emmons, De Launay, Pen- rose, Van Hise, Weed and others, f Especially interesting are * I know several deep mines in Norway, in which the lowest pump-station is only about 250 meters from the surface. In one of them, water for use in drilling below that level has to be carried down. t I will only introduce some observations upon the solvent effect of the ferric salts, FejfSOjJa and PeClj, upon sulphide ores. To test this point, I made in November, 1896, the following experiment : Samples of 6 grammes each of pulverized chalcocite, bornite, chalcopyrite, pyrrhotile and pyrite were separately treated in Erlenmeyer jars, 100 cub. cm. of neutral aqueous solution containing 30 grammes of FeClj being poured upon ea h sample, after which they were allowed to stand quietly at the ordinary PROBLEMS IN THE GEOLOGY OP ORB-DEPOSITS. 677 the proofs furnished of late years from l!forth America that secondary alteration often extends far below the ground-water level and the re-formation of sulphides in the zone of enrich- ment, investigated especially by Emmons and "Weed. I would here refer to the collection of specimens from the gold-district of West Australia which was exhibited last year at Paris under the direction of Mr. A. G. Holroyd. That the gold of many localities had been first dissolved, most probably in Fe2(SO^)3, and afterwards precipitated, could be clearly seen in a whole series of specimens. 1. From the zone of weathering in many veins were shown small and exquisitely beautiful crystals of gold, sitting upon house-temperature of about 14° C. After a few weeks the chalcocite was almost entirely dissolved, and the bornite had been very strongly attacked. On the other hand, at the end of nine months the chalcopyrite was affected but slightly, the pyrrhotite a little more, and the pyrite not at all. At the present time, after the lapse of 4 years and 1 month, the chalcocite and bornite have long been completely dissolved ; the pyrrhotite is almost all dis- solved ; the chalcopyrite has been somewhat further affected (by far not so much as the pyrrhotite), and the pyrite has been attacked, though very slightly. From the first four, and probably also to a small extent from the pyrite, sulphur has separated. The filtrates from the chalcocite and bornite showed with BaClj a, weak trace of H2SO4 ; that of the chalcopyrite a somewhat stronger trace ; and that of the pyrite a trace stronger still, yet, after all, amounting to little. The formula is : CujS + 4FeCl3 = 2CuClj + 4FeCl2 + S, or Cu^S + 2Fe2(S04)3 = 2CUSO4+ 4FeS04 -fS. Weed gives the formula thus : Cu.,S + 5Fe2(S04;3 + 4H2O = 2CuS04 + lOFeSO^ -f 4H2SO4. In the reactions with CujS and CuS, however, the sulphur does not appear to be oxidized to sulphuric acid, though this occurs in subordinate degree in the reactions with FeS and FeS2. The above experiments were made, as stated, at ordinary house-temperatures. At higher temperatures the process is very much more rapid. I was present in 1893 at an experiment in the Siemens-Halske metallurgical testing-laboratory at Berlin, when pulverized unroasted pyrites from Rio Tinto, containing 3 per cent, of copper and nearly 50 per cent, of sulphur, was stirred in a weakly-acid solution of ferric sulphate (50 grammes of iron to the liter), at 80°-90° C. After 6 hours, the percentage of copper had been reduced to 0.4. Zinc-blende is also attacked, though not as strongly as chalcopyrite. These reactions are metallurgically utilized in the Siemens-Halske electrolysis of copper-ores, and in the present leaching of pyrites at San Domingo, Tharsis, etc., in the Huelva district. Metallic silver also is very rapidly attacked by Fe2(S04)3. Gold will be considered below. Pyrite is one of the commonest minerals in sulphide-deposits ; its weathering yields Fe2(S04)3, which plays an exceedingly important part in the secondary alteration of ore-deposits, as I have shown in earlier publications. 678 PROBLEMS IN THE GEOLOGY OP ORE-DEPOSITS. cobalt-manganese-ore (asbolite), which is unquestionably a sec- ondary mineral, yet older than the gold which has been pre- cipitated upon it. 2. In many samples from gravels or placers, gold could be seen in small breaks in iron-ocher, limonite, etc. 3. Grold appeared also in stalactites, or " drip-stones," con- sisting chiefly of iron-ocher and calcite. In this case the gold was unquestionably in a ferric solution. 4. Again, gold from various localities was seen as a very thin tarnish, " breathed," as it were, upon the pebbles of the placer-conglomerates. 5. Several tree-roots were exhibited, upon which gold was sitting.* Here the gold had been reduced or precipitated from solution by organic substances. 6. Finally, gold was to be seen, in several cases, in fine cracks in the dried clay of the placers, into which it had percolated while dissolved, to be precipitated as a thin coating upon the clay. I am aware that series of similar instances have been de- scribed already from America, Australia and South Africa; but I have dwelt upon these new exhibits from "West Australia because they plainly show that the solubility of gold may play a quantitatively important part.f The same collection showed beautifully the weathering of the telluric gold-veins of Kalgoorlie. The mines, as is well known, carry in depth (down to 1150 feet, in the year 1900) very rich gold-tellurides (calaverite, sylvanite, kalgoorlite, pet- zite), sometimes in masses of extraordinary weight (50-100 kilog).t In the neighborhood of these, the ordinary phe- nomena of flake-, shee1> and wire-gold are often found, the native gold being sometimes intergrown with the telluride mass, and sometimes independent of it. In the highly oxi- * The label read : "Great Boulder Main Eeef. Boot of tree, found at 70-ft. level. Two pieces of wood, with gold-deposition. (Very rare. ) W. A." t The platinum metals, on the contrary, are to be regarded as practically insoluble by the chemical reagents encountered in nature. See note on p. 131. X I will not enter here upon the discussion of so many years' standing con- cerning the "mechanical" to. the "chemical" origin of gold-nuggets in placers. (Notwithstanding the solubility of gold, I adhere to the " mechanical " explana- tion.) But I may say, in passing, that in "West Australia the masses of gold- tellurium found in the veins are as large as the placer-nuggets of other regions. PROBLEMS IN THE GEOLOGY OF GEE-DEPOSITS. 679 dized upper vein-zones, the gold-tellurides have been entirely decomposed, metallic gold and derivative compounds of tellu- rium being formed,* and this metallic gold, appropriately called " sponge-gold," " mustard-gold," etc., could be easily dis- tinguished by its peculiar structure from the native gold occur- ring in depth. This is an indication that the deep native gold is not a secondary formation from gold-telluride, but a primary metallic precipitate. Secondary alteration thus helps us to decide a question which has been discussed for many years, especially in Austria-Hungary, where each of the views just stated has been held by many observers, f It is well known that in numerous ore-deposits, all over the world, unusually rich ore-bodies have been formed by second- ary processes more or less directly connected with the surface. "We need mention only Pachuca and Zacatecas, in Mexico; Pasco, in Peru; Potosi and Oruro, in Bolivia; Chanarcillo,f in Chile; Broken Hill, in Australia; Mednorudjansk, in the Ural, etc. Our knowledge of the secondary formation of very rich bonanzas is now specially enlarged by the investigations of Emmons and "Weed on secondary sulphide-enrichments below the ground-water level, as at Butte, Montana. Since in the development of science it has been so often seen that new ideas or impulses are liable to be overestimated, I will here add that there are innumerable rich " shoots," " chimneys," " edle Sdulen," ^^ Adelsvorschiibe," "bonanzas," etc., which have nothing to do with secondary processes, being of exclusively primary character, and dependent upon the laws which gov- erned the original ore-deposition. I may cite as examples Kongsberg, Andreasberg, Schemnitz, the rich shoots in the Transylvanian gold-veins, etc. And my study of the literature * The same is known to be true of Cripple Creek. f On other grounds, I have formerly expressed my adherence to the latter view — namely, the primary character of the ordinary native gold of the deep zones. See Zeitschr. f. prakt. OeoL, 1898, p. 418 ; 1899, pp. 179-180. X See F. A. Moesta, Ueber das Vorkommen der Chlor-, JBrom- und Jodverbindungen, u. s. w., besonders in Chili (1870). He points out that at the outcrop of the silver- veins of Chanarcillo, etc., the relative proportions of chlorine, bromine and iodine to one another are about the same as in sea-water, to the percolation of which he attributes the formation of these haloids. The explanation given by R. A. F. Penrose {Jour, of Oeol, vol. ii. (1894), p. 34, for the presence of silver-haloids in the arid regions, which connect them with neighboring salt lakes and marshes, seems to me more acceptable. 680 PROBLEMS IN THE GEOLOGY OF ORE-DEPOSITS. of the Oomstock lode has given me the impression that its famous bonanzas were of primary, not secondary, origin. The question, "What is of primary and what of secondary nature ? will doubtless long remain an interesting and often dif- ficult problem for discussion. Postscript. The foregoing contribution is in many respects much less complete and more fragmentary than I would have it. If, with some hesitation on that account, I have decided notwithstand- ing to send it to the Institute, it is in the hope that its defects of form will be judged in the light of the fact stated in the in- troduction, that the manuscript was begun on the 3d and fin- ished on the 31st of December. I close this work of mine on the last day of the nineteenth century, with a miner's hail, " Gliick Auf!" to my numerous American colleagues, unknown to me personally, yet well known through their scientific labors, and held in high esteem. Undoubtedly the new century will fill up many defects and solve innumerable riddles and doubts in the science of ore- deposits. THE IGNEOUS EOCKS IN THE FORMATION OF VEINS. 681 The Role of the Igneous Rocks in the Formation of Veins. BY J. F. KEMP, NEW YORK CITY. (Riohmond Meeting, February, 1901.) CONTENTS. PAGE Introduction, 681 I. The Competence of the Igneous Eocks to Supply the Materi- als OF Veins, 683 The Demonstrated Presence of the Metals in the Igneous Rocks, . . 683 The Presence of the Metals in the Sedimentaryand Metamorphic Eocks, 684 Conclusions, 686 The Abundance in Igneous Eocks of Vapors or Dissociated Gases which will Yield Water on Emission and Cooling, ..... 687 The Sequence of Eruptions, 689 The Sequence of Vein-Formations, . 691 Contact-Metamorphism, 692 Pegmatites, . . 693 Frequency of Pneumatolitic Minerals in Veins,. .... 694 Surface-Flows of Igneous Eock Unfavorable to Vein- Formation, . . 695 II. The Geoundwatee, 695 The Common Conception of the Groundwater, ..... 695 Experience in Deep Mines and Wells, 696 Artesian Basins, 702 Hot Springs 703 The Irregular Distribution of the Groundwater near the Surface, . . 706 III. The Distkibution of Mining Disteicts, 707 Eesume, ... 708 Introduction. The saying that " of all the known regions of the universe, the most unsafe to reason about is that which is under our feet,"* might well be the motto of the present paper, in view of the writer's profound appreciation of the difficulties and un- certainties of the subject. In such a field, the temptation is very strong to announce a probable proposition, and then to defend it with a loyalty insensibly graduating into partisanship. Conscious of this danger, the writer has endeavored to main- tain an impartial and candid form of statement, though others may feel that he has not been wholly successful. The subject, as here considered, falls naturally into two di- * Fisher's Physics of the Earth's Crust, p. 89. 682 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. visions. In the first, the competence of igneous magmas to supply both the contents of veins and the solutions which are the common carriers of the minerals is set forth. In the second, the phenomena and the more or less current conceptions of the groundwater are taken up. This paper is limited to " veins," as the term is ordinarily understood. It practically excludes the common deposits of those metals which appear in appreciable percentages in F. W. Clarke's latest estimated composition of the earth,* namely, Al, 8.16; Fe, 4.64; Ti, 0.41; Mn, 0.07; Cr, 0.01; M, 0.01. Of these, iron and manganese are admittedly favorable subjects for circulating meteoric waters, which are conceded to be of themselves eiffective in the outer 1000 to 2000 ft. of the thick- ness of the earth's crust. It is one thing, for example, that de- posits of iron-ore in the Lake Superior region should result from the rearrangements of iron and silica in a rock which con- tains 15 to 25 per cent, of the former, and quite a diiferent thing for the less common metals, and above all the precious metals, to be concentrated in veins from what we have reason to believe is a condition of excessively sparse dissemination in compact rocks. Experience gained with the former conditions should not be unduly influential in the study of the latter. Of the commoner metals cited above, iron (with titanium), chromium, nickel and perhaps aluminum are at times abun- dant enough in the original minerals of igneous rocks to con- stitute ores. It may be interesting and valuable as an aid in establishing a correct perspective to note the relative proportions of the or- dinary metals in the product of the United States for 1898, the -latest year for which statistics have been furnished by the U. S. Geological Survey. Keducing the weights to grammes, and taking the weight of the gold-product as unity, the ratios are found to be as follows: Iron, 120,950; copper, 2487; lead, 2098 ; zinc, 1090 ; aluminum, 26 ; silver, 22 ; quicksilver, 11. This calculation would be more significant if it covered the product of the world ; but the necessary data are not available. It affords, however, within limits, a certain conception of the relative abundance of the several metals. * Svlktin of the U. S. Oeological Survey, No. 168, p. 15. THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 683 I. The Competence of the Igneous Kocks to Supply the Materials of Veins. The Demonstrated Presence of the Metals in the Igneous Rocks. "Within recent years many assaj-s of rocks have been made, in order to throw some light on the source of the metals in ores. In selecting the samples for analysis, certain precautions are essential. Fresh rock must be taken ; and the possible im- pregnation with small amounts of infiltrated metals must be avoided, or else the significance of the results will be vitiated. The amounts to be measured are excessively small, and their determination taxes the resources of the chemist to the utmost. For example, one ounce to the ton means ^ of 1 per cent. ; and in some dry assays even fractions of a grain — there being 480 grains to the ounce — are determined. Reagents (particu- larly the litharge used in dry assays for gold and silver) must be pure to the last degree. All these precautions have been observed, however, in a large number of cases; and a very considerable amount of trustworthy data has been accumulated, going to show that the common metals are certainly present in igneous rocks, and that one or another of them is contained in nearly all the commoner igneous types (acid, intermediate and basic). Some metals seem to favor one rock and some another — a feature which has been treated at some length by De Launay* and Vogt,t and more briefly summarized by the writer. J It has been shown also that the ferro-magnesian silicates are richer in the metals than is the rock as a whole, and that they are probable sources of the metals. The metals appear in them either as bases or as metallic inclusions. It was the original purpose of the writer to tabulate these results ; but the mass of data was found to be too large to be practicably handled in this way, and therefore only the above general statements are made. Many references, however, are given in the work last cited. We must bear in mind that the results of these assays and * L. De Launay, Annates des Mines, August, 1897. Reprint, p 45. t J. H. L. Vogt, Zeitsch. fur prakt. Oeohgie, vi., 225 (1898). J J. F. Kemp, Ore-Depodls of the United States and Canada. Third edition (1900), p. 35. 684 THE IGNEOUS KOCKS IN THE FORMATION OF VEINS. analyses have, to a large extent, but general interest and ap- plication. Only from the point of view of a lateral-secretionist of the Sandberger type does it follow that the ores in a vein have been derived from the wall-rocks which are accessible for assay. In instances like Butte, Mont., in which two sets of veins, of greatly contrasted mineral contents, are found in the same country-rock, other sources must be assumed for at least one series of them, no matter what are the theoretical predilec- tions of the observer. ISTevertheless, it is a fact of the greatest importance that the presence of the metals -in igneous rocks has been established. l!fot all igneous rocks have yielded such results on assay. The general experience has been that when samples of several varieties have been collected in a given dis- trict, some have proved barren ; and it must be admitted that some negative results have been obtained. As a rule, however, they are decidedly fewer than the positive results. It is like- wise true that not all igneous districts contain veins of ore. Great areas of surface-flows, such as the basalt plains of Idaho, Oregon and Washington, are notably barren, probably for rea- sons that will be subsequently advanced. The elements of the minerals which form the common varie- ties of gangue are found in all the igneous rocks, and in the sedimentary rocks as well. Quartz is much the commonest of the gangue-minerals, and silica is universally present in the rocks. Calcite and fluorite may derive their calcium from an equally wide range of rocks and minerals. Barium and stron- tium are " understudies " of calcium, and available iron for siderite is present on every hand. Where rock, in a stage of greater or less alteration, forms the gangue, it has no special significance in this connection ; and gangue-minerals other than those cited are relatively uncommon and unimportant. The Presence of the Metals in the Sedimentary and Metamorphic Bocks. Wherever the metals are found in the sedimentary or meta- morphic rocks, it is logically necessary to refer them to original sources in the igneous rocks, from which they have been derived either by solution or abrasion.* In the former case, * Ore-Deposits of the United States and Canada. Third edition, p. 32. THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 685 the processes of introduction are essentially those to be sub- sequently discussed ; in the latter, except in the case of placers (which are negligible, in this connection, on account of their small amount), the distribution of the metals is extremely sparse. If we begin with a rock which contains but hun- dredths or thousandths of 1 per cent., and imagine it broken up by the processes of erosion, its minerals subject to solution and dispersion, and to commingling with foreign matter, — or, if they are heavy, to concentration in placers, — the resulting sediment is a less favorable source of supply for migrating solu- tions than was the original igneous rock. The assays and analyses which have been made confirm this general state- ment, but they are hardly as abundant, taking the world over, as are those which have been prepared of the igneous rocks. An exception is the really remarkable work by J. E.. Don* in Australia. In making assays and analyses of sedimentary and meta^ morphic rocks, it is important to observe the same precautions as were outlined for the igneous rocks ; and, in interpreting them, secondary impregnation must be guarded against. The following brief summary of the results of several workers will give an idea of the evidence in hand. Dr. Don has made and tabulated a vast number of analyses of the wall-rocks, chiefly sedimentary, of the Australian gold-veins. He was able to determine the presence of gold in fractions of a grain per ton of rock in a large number ; but his tests indicated that only those rocks which also contained pyrites gave any returns for gold.f There is, therefore, the presumption that the gold and pyrites were introduced as an impregnation ; pyrites not being a mineral favorable to sedimentation. Mr. Winslow,| in con- nection with his most valuable investigations of the lead- and zinc-deposits of Missouri, engaged Mr. J. D. Robertson to pre- pare a series of analyses of the rocks of Missouri, both sedi- mentary and igneous, for lead and zinc. The samples were taken in, near and remote from mines, and in not a few cases * Trails., xxvii., 564. t It may be again remarked that a grain is j|~y of an ounce, and that these values were therefore thousandths and tens of thousandths of an ounce per ton. If one thirty-thousandth of this remote decimal is then calculated, the values in true decimals will Le given. They are almost inconceivably small. % Arthur Winslow. Oeol. Sun. of Mo., vol. vii., p. 479. 686 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. amounts were found equal to several thousandths of one per cent. When, however, we compare the analyses of the sedi- ments with those of the igneous rocks, we find that the latter, as a rule, are by one place of decimals richer than the former, and to that extent are, generally speaking, more favorable sources of the metals. These results justify the statement made above that erosion and sedimentation tend to disperse the original metallic contents of the igneous rocks, and to place them in conditions less favorable for concentration by solution. With regard to the metamorphic rocks especially, it may be said that increasing experience and more accurate knowledge have tended to prove the presence among them of crushed and sheared igneous types, whose foliation is of mechanical origin. Considered as favorable sources of the metals, the same re- marks would apply to them as those already made regarding the unaltered igneous rocks. A good illustration is the gold- belt of the Southern States, which is now recognized to em- brace amid its schistose types a very large proportion which are of this original character. Conclusions. — Sedimentary rocks are far less favorable sources of the metals than are igneous ; but the statement must not he interpreted as a law, though preponderating experience justifies it. Omitting the metals excluded in the opening paragraphs, sedimentary districts not associated with igneous rocks are, as a matter of experience, pre-eminently barren. The lead- and zinc-deposits of the Mississippi valley are almost the only important exceptions which can be suggested, and of these it is fair to say that increasing observation gives some ground for connecting them with dislocations, certainly in southwest Missouri, and to a less degree, perhaps, in southeast Mis- souri, and for favoring the views which have been especially advocated in recent years by W. P. Jenney. The ores are, however, confessedly hard problems. Concentration from the neighboring wall-rock has been upheld in the case of the veins of the Tipper Mississippi, more especially within a year past, by C. E. Yan Hise.* Although there is no known occurrence of igneous intrusions in the two regions cited, or in the gash- * "Some Principles Controlling the Deposition of Ores," Trans., xxx. , 103 (p. 77 of pamphlet edition) ; this vol., pp. 282-432. THE IGNEOUS KOCKS IN THE FORMATION OF VEINS. 687 vein district of Wisconsin, yet it is true that peridotites have been discovered with the lean veins of western Kentucky,* and rocks of this type have elsewhere been found in regions where no eruptives were suspected or anticipated on the basis of the local geology.f In the larger lead- and zinc-districts, however, there is no reason, based on observation, for thinking that such rocks are present ; and in the present state of our knowledge, these districts must be considered as exceptions to the general rule. The Abundance in Igneous Rocks of Vapors or Dissociated Gases which will Yield Water on Emission and Cooling. The ordinary analyses of cold samples of igneous rocks are of little if any value as an indication of the vapors and gases which were present in the hot, fased magma. The observer must turn to active volcanoes and streams of molten lava for his evidence ; and from these we may judge of the composition of intruded masses of rock which never reach the surface. Practically all students of volcanic phenomena are agreed that steam and its dissociated representatives in the molten rock are the chief, if not the only cause of eruption. The paper by Prof. J. H. L. Vogt, presented at this meeting, J discusses at some length the condition of water in the fused rock. All observers are agreed that the first eruption at any volcanic vent must be caused by the steam Avhich is brought up with the lava from the depths of the earth ; but there is a very gen- eral disposition to refer the subsequent outbreaks to meteoric or oceanic waters, which percolate through the rocks near the vent, and which in some way become involved in the molten rock. When the pressure produced by them becomes sufficient, an eruption occurs. It is very generally admitted to be inconceivable that water from any outside source should be able to follow cavities larger than capillary size through solid rock, heated nearly to fusion, to and into molten rock at a temperature of over 2000° F. Any water entering even the outer and moderately heated solid rock would be evaporated and driven outward. It is necessary * J. S. Diller. Mica-peridotite from Kentucky. Amer. Jour. Sci., Oct., 1892, 286. t For example, at Syracuse, Manheim and Ithaca, N. Y. I " Problems in the Geology of Ore-Deposits," Trans. , xxxi. , 125 ; this vol., pp. 636-680. 688 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. therefore to fall back on the capillary conduits, through which to introduce into the magma the accessions of water. In order to prove the possibility of this introduction, recourse is had to Daubree's famous experiment, which has, however, been shown by Osmond Fisher* to have no bearing on the case in point. Daubreef took a slab of sandstone, two cm. (about 0.8 in.) thick, and cemented it between an air-chamber below, and a chamber above which could be filled with water. The tem- perature of the lower chamber was raised until the air-pressure was about two atmospheres. The water from the upper cham- ber was drawn down by capillary attraction, even against this pressure of two atmospheres, and moistened the under side of the slab. It is evident from this that capillary attraction can draw water downward against a pressure ; but, as Fisher acutely remarks, the capillary force was effective because it operated toward a free air space. In fact, it is only under these condi- tions that the difference in surface-tension, which is the real cause of capillary movement, appears between the air and water on the one side and the water and walls on the other. The experiment gives no ground for thinking that water would move through the heated walls confining a reservoir of molten rock and become involved in the latter. There is also some uncertainty about the efficiency of capillary force in rocks which are under great pressure. As I learn from my colleague. Professor R. S. Woodward, no assumptions of its efficiency are based on experimental data. Again, active volcanoes are known, such as Cotopaxi in Ecuador, which are nearly 20,000 feet above sea-level. They must draw on reservoirs below tide, and yet even at tide-level cavities in the rocks, through which water might reach the magma, will have become impossible by reason of the pressure. It would therefore seem necessary to believe that the ejected steam and other vapors of lavas have been brought up with them from the depths ; but it is only fair to say that many think otherwise, although apparently without careful analysis of the problem. Of the abundance of the vapors there can be no question. They often exceed in volume the lava itself The question of their origin only aflfects in a minor way the argu- * Physics of the Earth's Omst, pp. 91, 92. f Geologie exphimentale, p. 236. THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 689 ment to be subsequently made regarding the cause of move- ment of the groundwater. Beyond question, intruded sheets and laccolites are provided with gases similar in all respects to surface-flows ; but, in the nature of the case, the gases are yielded much more gradually, and through longer periods of time. They undoubtedly continue to appear until the rock is nearly as cold as the boiling-point of water at the depth at which they stand. It must be ad- mitted that the hot vapors and waters yielded by an intrusion under these circumstances are extremely vigorous chemical re- agents* and are incomparably superior to the ordinary ground- water, even when the latter exists in any serious amount below 1500 to 2000 ft. It is also important to remark that the pres- ence of even a very small dike in any region is proof of the existence of a relatively very large reservoir of igneous rock, at some point beneath the surface, and at unknown but not great depth. The Sequence of Eruptions. One of the most interesting features of eruptive districts is the sequence of the eruptive rocks. One kind of rock has followed another until, in some instances, a considerable list can be made up. All will recall von Richthofen's observations on the Pacific coast in the late sixties, which led him to infer that eruptions habitually begin with rocks of medium acidity, pass then through a series with increasing silica up to rhyolite, and terminate with basalts. Increasing observation has shown many exceptions to this simple rule ; but of the general fact that molten rocks are jpoured out one after another from what would appear to be a common reservoir, there is no question; and students of the subject have been more and more disposed to explain them by a breaking up of some original parent magma of intermediate composition into the several diverse products. This succession of eruptions holds good in many localities of extensive vein-formation. At Butte, Montana, for example, a basic granite was followed by an acid granite, and both by quartz-porphyry, with which latter the introduction of the ores seems to have had some connection. After the ores had been * Regarding this point, a very valuable paper is that of A. C. Lane, Bulleiin Oeol. Soc. Amer., v., 259. 690 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. deposited, a great outbreak of rhyolite took place, with no attendant vein-formation. At present the quartz-porphyry is by far the least extensive of them all, and forms but a few- minor dikes; yet it is quite possible that it may represent some greater intruded mass, far below, from which the ores have come ; and for the very reason that it is visible in small amount it may be the most important of all the rocks in connection with the genesis of the ores. Again, for example, at Douglass Island, Alaska, albite-diorite (sodium-syenite) and gabbro have been identified by G. F. Becker in the order of their outbreak through slates; but it was only just before or along with the intrusion of a small dike of analcite-basalt that the ore entered. On the Comstock, we find a considerable variety of eruptives in sequence. There is a decided difference of interpretation between Mr. Becker, on the one side, and Messrs. Hague and Iddings on the other; but if the latter are correct in considering Mr. Becker's " later diabase " as the same as the " basalt," which is the youngest eruptive, then it was after the intrusion of the " black dike " of diabase or basalt which is met in depth, that the ores came in along a line of faulting. At Mercur, Utah, a great stratum of carboniferous limestone was penetrated by a sheet of quartz- porphyr}', which itself forked into two thin prolongations. Immediately beneath the lower fork of the sheet are silver ores, after the deposition of which an interval ensued. Later on, gold-ores were deposited beneath the upper fork, having been introduced, as is thought by J. E. Spurr, through the in- fiuence of a laccolite, assumed to exist in depth. Dikes may not at first be evident in mines — as was the case in the Ontario at Park City, Utah. In the early work the vein apparently filled a fissure in quartzite, but in depth a dike was met, which formed one of the walls. Over and over succeeding eruptions have taken place, and then at some stage (usually after a minor intrusion, so far as the exposures give the observer an indication) the ores were introduced, and one may not be able to say whether they came ill with or just after it. It is thus evident that some eruptive rocks are unfavorable in themselves, or unfavorably situated in their present positions, for vein-formation, and that one may appear later whose advent is a signal for the ores to enter. THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 691 The Sequence of Yein- Formations. There are also cases of successive and coutrasted vein-for- mation. More than fifteen years ago R. C. Hills recognized three sets of veins in the San Juan region of Colorado, each with different ores; and the recent work on the Telluride quadrangle of the U. S. Geological Survey has shown in detail many of the structural relations.* There are in this district four sets of fissures, but only one carries the ores — a remark- able state of things if the ores are due to the universal circu- lation of the groundwater. In one instance, the Smuggler vein is faulted by the Pandora, a later vein which does not carry ores sufficiently rich to be mined profitably. The district of Freiberg, Saxony, is a very complex case. If we include with it some of the veins of the ErzgeUrge that lie at a moderate distance, the following groups may be distin- guished: Die Zinnerzgdnge; die kiesige Blderzgange ; die edle J3lei- erzgange ; and die edle Quarzformation. All these are recognized as genetically connected with the great eruptions of granite and porphj-ry in Carboniferous-Permian times. There are, in addition, three other varieties of veins which have usually been considered as later, and even middle Tertiary, viz., die Kobalt- silbererzgdnge ; die barytische Bleierzgdnge; and die Eisenmanganerz- formation. They have been referred to later eruptions of igneous rocks. K. Dalmer, however, developsf some proofs that the first and third date back before the late Cretaceous, and even into the period of the older series. But the point of interest here is the connection with eruptive rocks, which is emphasized by nearly all observers. It is often assumed in such cases that new series of fractures have tapped new sources of ores ; but the hypothesis is not to be ignored that new intrusions may have been responsible for the change of solutions — and experience thus far gained gives the latter at least equal claims with the former. Indeed, new series of fractures can only go down through practically the same rocks as older ones, unless new material is brought in by igneous intrusion ; and hence the second hypothesis, in the absence of proof to the contrary, would seem to have prepon- derant claims over the first. * See C. W. Purington, 18ft Ann. JRept. U. S. Oeol. Sun., part iii., p. 745. f Zeitsch. fiir prakt. Geobgie, Jan. , 1896, p. 1. 44 692 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. Coniact-Meiamorphism. The observed facts of contact-metamorphism and the conclu- sions which have been dravpn from them have an important bearing on this question. It is well known that some intruded igneous rocks have exercised a profound influence on the wall- rocks through which they have come, while again other intru- sions have produced little or no effect. The results depend very largely on the nature of the walls, earthy limestones and argillaceous strata being the most favorable, and quartzose sandstones the least so. Of the igneous rocks, all kinds, in one place or another, have produced notable results, but the acidic and intermediate are the most efficient. Abundance of dis- solved vapors seems to be the essential thing for profound effects, as relatively dry fusion is unfavorable. The intruded igneous rock should also stand in contact with the walls for long periods and at a depth reasonably great below the surface. All these points are very much the same as those which have already been stated regarding the igneous rocks as producers of veins. A divergence of views exists as to the amount of material actually contributed to the metamorphosed rock by the igneous agent. Observers on the continent of Europe have considered the amount to be in some instances large, especially of soda; while from facts noted at Westmoreland, England, where a basaltic tuff is penetrated by granite, a limit of one-twentieth of an inch is set by Alfred Harker for the migration of material. The changes produced in contact-metamorphism are in this in- stance almost entirely those of rearrangement. All observers must, however, admit the general introduction of fluorine, boron and steam, because the distinctive contact-minerals are characteristically provided with these elements. They are therefore described as mineralizers, or as being pneumatoliiic in their nature. Tourmaline, fluorite, fluoric micas, chondrodite and topaz are illustrations of the resultant minerals; while biotite, garnet, albite, woUastonite, vesuvianite and a number of other silicates are common associates. If, now, ores are found associated with these minerals and along the contacts with igneous intrusions, and npt extending far back into the wall-rocks, the inference is well-grounded that they have been derived from the eruptive. In the last paper of Professor Vogt, already cited, the cases of tin-ores and iron-ores to which THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 693 these views apply are given at length ; and in the paper of Mr. Lindgren* copper-deposits of similar nature are cited. Fissured wall-rocks which stand immediately above laccoUtes rich in mineralizers would be in the situation most favorable for these changes; but opportunities for observation are restricted be- cause the laccolite is only revealed by their removal. When they do persist, however, and are thick, the existence of veins would suggest the presence of the laccolites. Pegmatites. Pegmatites have furnished for many years a disputed ques- tion. They are beyond doubt connected with great masses of intruded rock, more often with granite than with any other, and are after-births of the eruptive. "Whether they are them- selves to be considered as true eruptives, or whether dissolved vapors have played so large a part in their genesis that they are veins rather than dikes, or whether some belong to one of these types and some to the other, does not immediately affect the question now before us — their connection with eruptive rocks being the important point. Pegmatites usually present the mineralogy of the granites on a very coarse scale, but they have, in addition, more abundant amounts of the pneumatolitic minerals. They may be rich in feldspar and less rich in quartz, or they may be extremely rich in quartz with only subordinate feldspar or other minerals. The writer believes that in some regions of their extensive de- velopment all gradations can be found, from granitic mixtures to veins of pure quartz. The north shore of Long Island Sound is a case in point. Pegmatites are abundantly developed in connection with granites, and all grades are shown up to practically pure quartz. The great quartz-vein at Lantern Hill, Mystic, Conn., is one of the largest quartz-veins known, being apparently 1000 ft. wide across the comb-in-comb struc- ture, which is at times pronounced. I think it belongs in the pegmatite series, and is only a huge development of veins of a smaller size which are abundant around ITarragansett Bay and elsewhere, t Certain parts of the Lantern Hill quartz show * "The Character and Genesis of Certain Contact-Metamorphic Deposits,'' Trans., xxxi., 226 ; also page 716 of this volume, t J. F. Kemp, Bulletin Oeol. Soc. Amer.. x., 372, 1899. 694 THE IGNEOUS ROCKS IN THE TORMATION OP VEINS. the presence of ferruginous minerals and have yielded on assay a few cents of gold per ton. The gold-bearing pegmatite of Passagem, Brazil, described by Hussak,* has been referred to by Prof. Vogt. In the Trias- sic diabase of the Palisades, pegmatite veins richly charged with pyrite are not uncommon. Last summer, the writer spent several days at Copper Mountain, on the Similkameen river, near Princeton, Yale Dist., B. C, and found a great mass of gabbro, shattered along a wide belt. Into the minute fissures bornite had been introduced in some places, and minute veins of pegmatite in others, while in the Copper Cliff and Copper Reef claims, on the banks of the river, a huge pegmatite vein or dike carried here and there large masses of bornite. The bornite impressed the observer as being as much an original mineral in the vein as any of the other components. In view of the above facts, which could indeed be much amplified, the following statements seem to be justified : Peg- matites are a more or less pronounced pneumatolitic result of igneous intrusion. Pegmatites grade insensibly into quartz- veins. Quartz-veins not visibly associated with pegmatites are open to the same interpretation unless there is positive evi- dence to the contrary. On the other hand, pegmatites, although widely developed, are but rarely provided with metallic minerals in notable amounts, and the same is true of the quartz-veins visibly associated with them. But it is also true that many regions of great development of pegmatite-veins are devoid of ore-bearing veins, as, for instance, E"ew England, and it is prob- able that the magmas did not contain the necessary metals for their production. Frequency of Pneumatolitic Minerals in Veins. Some of the common gangue-minerals contain those ele- ments which are justly associated with pneumatolitic processes. Of these, fluorite is the most important ; and while it cannot be always asserted that it implies the neighborhood of eruptive rocks, it yet creates a presumption in favor of their genetic influence. The gold-ores of Cripple Creek, Colo., and the Potsdam ores of the Black Hills, are cases in point. Lindgren * Zeitsch. fiir prakt. Oeologie, October, 1898, p. 345. THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 695 has already emphasized this connection in his extremely valua- ble paper on "Metasomatic Processes in Fissure-Veins";* and therefore it is only cited here in a brief way. Surface-Flows of Igneous JRock Unfavorable to VeinrFormation. The vapors contained in surface-flows of igneous rock pass off" directly into the atmosphere, and therefore do no geologic work of this character. The most that could be expected of them would be small incrustations in the cracks in their upper and first chilled portions, such as the copper-minerals and specular hematite found in the crevices of Vesuvian lavas. The absence of ore-deposits in flows of this character argues nothing against the efficiency of other forms of igneous rocks. 11. The Groundwater. The Common Conception of the Groundwater. The general conception of the groundwater, that has been hitherto held, has involved the existence of a standing body, quite universally' present, and at a fairly definite depth below the surface, which depth is characteristic of the particular dis- trict. The upper surface is thought to be sharply marked and to be revealed by the boundary between the oxidized or en- riched ores and the unaltered sulphides in an ore-body. The supply of water is kept up by the contribution of that portion of the rainfall which neither runs off nor immediately evapo- rates, but which sinks into the ground, feeds wells and springs, and necessitates pumping in mines. Rocks being more or less porous and crossed by faults, joints and cracks, it has been in- ferred that the waters continually migrate downward, partly by capillary attraction, partly through small crevices and partly through large ones, until, meeting the hotter interior zones of the earth, they are forced by the head of the descending cur- rents (that is by gravitative stress), reinforced by the loss of density due to accessions of heat, to rise again to the upper world. During their journeys they move laterally as well as downward, pass through vast masses of rock, relieve them of their mineral and metallic contents, and deposit this dissolved material more especially on their upward journey. The fact * Trans., xxx., 691 ; p. 498 of this volume. 696 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. that we find a great body of standing water not far below the surface in regions of heavy or moderate rainfall would make it necessary, according to this conception, to believe that the rocks are pretty thoroughly saturated with water down to the depths at which the return journey begins; in fact, as Van Hise often expresses it, there exists a sea of the groundwater. Van Hise in particular rejects specifically the igneous rocks as significant contributors either of material or of energy, and expresses, in the premises or propositions which he seeks to establish, his belief that the waters which fill the veins with minerals are meteoric, and that gravity is their motive power. It is fair to add that the conception is a time-honored one, and has found frequent previous expression ; but we owe to Van Hise an exceptionally clear and logical exposition of it. There are, however, grave objections to this conception, and we may justly examine it in the light of the experience which has been gained in very deep mines and wells, as well as on certain general theoretical grounds. Experience in Deep Mines and Wells. Mines exceeding 2000 or 2500 ft. in depth are of extremely modern development. In several important instances of this class, as well as in many mines of smaller depth, it is possible to impound all the water within a short distance, it may be within 500 ft., of the surface. Below this level the workings are dry and, in a few cases, dusty. The copper-mines on Keweenaw Point are most favorable in their geological structure to the downward passage of water. The shafts, several of which are now between 4000 and 5000 ft. deep, cut a series of sheets of trap and amygdaloid that dip 36° to 39°, and include one or two beds of conglomerate. They are fissured, and at times even brecciated.* As shown by the chart opposite p. 167 of the Report just cited, the N'orth Tam- arack Shaft, 'Eo. 3, at a depth of 3818 ft. had cut 73 different trap and amygdaloid layers. It is fair to infer that the new shaft which has recently grounded in the Calumet conglomerate at 4760 ft. must have cut correspondingly more. Yet the deep workings of these copper mines are not only dry, but in some » Owl. Survey of Mich., vol. v., part i., p. 112. THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 697 cases dusty ; and the water is impounded either at the surface or a short distance down the shaft. Such water as trickles down the shafts from the top is occasionally baled out, and water for the drills has to be- specially sent down into the headings. At Calumet, the only water encountered in the deeper work- ings, or indeed below some such depth as 500 ft, is a highly alkaline variety, tapped in insignificant amounts from occa- sional fissures. It has a painful effect upon cuts and is avoided as far as possible by the miners. As a whole, the rocks are free from visible water. Posepny says that the deep workings at Przibram have afforded a similar experience. Below 800 meters there is no water to be raised, because evaporation removes whatever exists there. Much that has been pumped from the levels immedi- ately above 800 meters is doubtless water that has escaped im- pounding nearer the surface, and has followed down the open- ings made by the mine itself. The deepest workings at Przibram mentioned by Posepny are 1110 meters.* Experience gained in the deep Cornish tin-mines, like the Dolcoath, would be important in this connection, but at the time of going to press it is not available. Mr. B. B. Lawrence has mentioned, in some informal remarks at this meeting, the Pelican-Dives mine, near Georgetown, Colorado, which has now attained a depth of over 2000 ft. The water has been allowed to follow the workings down and is raised from the bottom, but no more is pumped now than when the bottom sump was located in the upper levels. The cases cited merely express the general experience of mining engineers, all of whom are aware that, with impounding of the surface-water, increase of depth, especially below 2000 ft., means almost invariably dry workings. Even if the rocks are " dry " in the miner's sense only, and not in the strict scientific sense, if their contributions of water are removed by evaporation so as not to be noted ; or if, in a great artificial excavation, far larger than the vast majority of natural water- ways, no pumping is necessary, the water in the rocks may be neglected as a producer of veins. The deep mines which are known to be wet, such as those of the Comstock lode, are in * Trails., xxiii., 248; xxiv., 971. 698 THE IGNEOUS KOCKS IN THE FORMATION OF VEINS. regions of expiring vulcanism, as will be emphasized a little further on. Many mines, especially collieries, have been driven under bodies of water, and even under the sea, and yet they have been but slightly if at all troubled by water, and sometimes have been absolutely free from it. Tight shales, in a sedimen- tary series, would partly account for this ; nevertheless the general experience is worthy of emphasis. Artesian borings have in a few cases yielded similar testimony. The deep well near "Wheeling, "West Va., which has been made famous by the measurements of Professor "Wm. Hallock on the increase of temperature with descent, reached a depth of 4500 ft. The last water was cased off at 1500 ft., so that for 3000 ft. the hole was dry. In this 3000 ft. the well penetrated shales and some sandstones, both of marine origin. Shales are admittedly the least favorable of rocks for circulating waters, but it is a surprising fact that this great section afforded a dry hole.* The Pittsburgh well is still more remarkable. In February, 1897, the well had reached a depth of 5386 ft. It was cased only to 900 ft., or slightly beyond, and for over 4400 ft. was dry. The well penetrated a section similar to that at Wheeling. On the other hand, the deep wells at Sperenberg, Schlada^ bach and Reibnik, all in Germany, are wet, so far as the pub- lished descriptions of the measurements of temperature inform us, but as they were bored with the diamond-drill, it is believed that they were not cased. I have, however, no definite informa- tion regarding this point. All these facts, except the last, go to show that the outer portion of the globe is much less permeable to water than has often been assumed, and that, in many places at least, the down- ward percolation is a negligible factor. The groundwater which is met at small though variable depths, and which fills abandoned mine-workings, is held there by the tight rocks be- neath it, and is not to be considered the upper part of a mass of water reaching down to 10,000 ft., or any such depth, in the * Wm. Hallock, Pro(.. Amer. Assoc. Adv. ScL, xl., 257, 1891 ; School of Mines Qimrterly, xviii., 148, 1897. The latter gives details of the Pittsburgh well also. The section of the well at Wheeling will be found in West Va. Geological Survey, vol. i., p. 364, 1899. Professor Hallock states that the well was plugged with an oak plug after his measurements ; two years later, when the plug was removed, the hole was full of water, which all believed leaked in at the end of the casing. THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 699 interior. On the contrary, something like 2000 ft. appears to be its limit, and in some regions it ceases at 500 ft. The explanation lies, no doubt, in the plugging of fissures and crevices with attrition or residual clay, and in the feeble- ness or disappearance of capillary attraction with increase of pressure. The efficiency of a very thin seam of clay in keeping back water is well known to all miners who have been engaged in wet ground. A layer a quarter-inch thick is water-tight, and often every precaution, as remarked by Dr. Raymond at the present meeting, is taken by the miners not to break through even this small thickness. As to the efficiency of capillary attraction with increase of pressure, I learn from Prof. E,. S. "Woodward, as I have already said, that our knowledge is very limited, and he at least would hesitate to affirm that it operates. It has been shown, moreover, as I am informed by Dr. A. A. Julien, that when, in testing the absorption of build- ing-stones, pieces are merely soaked in water, the penetration of the water is insignificant ; but if the air in the stone is ex- hausted under an air-pump, or by boiling, or if the block of stone rests on wet felt, then absorption takes place. The extraordinary impenetrability of some rocks is emphat- ically shown by the storage of petroleum and natural gas. Both of these, but more especially the former, are wanderers to a remarkable degree, yet they are confined in the ground under very great pressure and are unable to escape. Edward Orton, St., satisfactorily demonstrated in 1889 that the pressure of the gas in the comparatively shallow wells of Ohio (1000 ft.) was hydrostatic and due to the groundwater. K"early all geolo- gists believed the same agent to be the universal cause of the rock-pressure of natural gas ; but when the deep gas-wells of !N"ew' York were drilled from 2250 to 2600 ft., to the Trenton lime- stone, it was found that some other factor must enter, because the pressure is too great for a hydrostatic cause. Prof. Orton, therefore, and others with him, have abandoned this view.* In some deep mines water has been encountered in uprising springs, and the same is true of not a few shallow shafts ; but I do not think that any springs at less than 1500 or 2000 ft. depth have a bearing on this question. Posepnyf mentions « Bull. Geol. Soc. Amer., ix., 95-99. t l^ans., xxiii., 223. 700 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. one in the Einigkeit shaft, Joachimsthal, Bohemia, that was met at 533 meters (1774 ft.); but in the next paragraph we learn that the uprising waters at Joachimsthal were tapped along the contacts of the veins with basaltic rocks of com- paratively late origin, and therefore in a situation involving expiring vulcanism. I do not cite this and the subsequent cases with a view of necessarily referring the waters to exhala- tions from fused and consolidating or consolidated magmas, but I do mean to use them, along with other considerations, to show the impotence of purely gravitative motive power. The Comstock lode is the most famous case of a deep, wet mine. Church, King and Becker have all discussed the waters in their several monographs. Water was tapped on the 2200- ft. level of the Savage, and rose both in it and in the Hale and l!forcross to the 1750-ft. level; but there it stopped. Mr. Becker says that two kinds of water have been met in the lode. One is pent up in confined bodies. It was the tapping of such a body that let the water into the Savage and Hale and l^orcross, as just observed. In another case, a cross- cut from the Palmer shaft was invaded by a body of water that rose 100 ft.* and had a temperature of 104° F. The other kind of water rises from the depths. No one doubts that the high temperature of these waters is due to expiring vulcanism, and the focus of the heat is placed by Mr. Becker at not less than two, and more probably four miles in depth, f The re- gion is arid, and it is believed that the water must have come from a distance. A source for it in the Sierras, 12 or 14 miles to the west, is tentatively suggested by Mr. Becker ; but there is good reason for thinking some of it, at least, to be a con- ''tribution from the eruptives themselves. Emmons;|; has recorded a very interesting case of an uprising spring in the G-eyser mine at Silver Cliff, Colo. The shaft was sunk so that it cut at 2000 ft. the contact between overlying porous rhyolite tuff and the underlying granite. Water in small amount and charged with carbonic acid, and different in composition from the descending waters, which had ceased far above, bubbled into the workings along small fissures parallel * Fortieth Parallel, Survey, iii., 87. t U. S. Oeol. Sur., Monograph III., p. 264. t nth Ann. Rep. Dir. U. S. Oeol. Survey, part ii., p. 458. THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 701 with the contact. Here, again, the spring is in a region of vul- eanism of rather recent date, geologically speaking, and it is impossible to assert that an abnormal rise in the isogeotherms from this cause is not a factor in the circulation, although the water exhibited only the temperature of the drifts themselves.. It is not my purpose to attempt to show that water does not descend into the earth below 2000 ft., for I believe that it does, although not by any means in the amounts which have some- times been assumed. I wish to make clear that the amount is probably comparatively small; that there are good grounds for believing that it only descends to great depths by relatively large fissures ; and that these are exceptional. To the same degree that the meteoric waters are limited to the relatively large fissures, they are unfavorably situated for the solution of sparsely distributed minerals and metals. I hope to establish, further on, that even if they descend in this way, by a trickle here and a little seepage there, they can never be brought again to the surface, so as to form springs, by gravity and the normal rise of temperature alone. At the same time, I fully recognize that there is ground for a different view, and that a strong case can be made out for the very slow circulation of water at great depths. But even if it be admitted that this is the case ; that the waters become charged with ores; and that they have some tendency to pass upward, by reason of the heat acquired through the normal rise of tem- perature with depth ; it remains true that, in again ascending, they meet descending currents or mingle with relatively station- ary water; and they become dilute and disseminated and com- paratively weak agents, when contrasted with the much supe- rior efficiency which may be locally conferred upon waters by igneous intrusions. While one cannot deny that, by the former type of circulation and in the long course of geological time, something might be accomplished, yet, a fortiori, all the results might have been brought about, and there is abundant reason to think that they were, brought about, by the aid of igneous rocks, as I shall endeavor to show subsequently, by proof ad- ditional to what has already been said. The interrupted passage of the waters, when they do de- scend, has an important bearing upon the hydrostatic head. Whenever, for example, capillary transmission occurs, the pre- 702 THE IGNEOUS BOOKS IN THE FORMATION OF VEINS. viously acquired head is lost, and the emerging water proceeds on its way only under a newly accumulating head. So far, therefore, as capillary transmission may be assumed, ordinary calculations of hydrostatic pressure, based on distances from the surface, are false. In any event, even with the assumption of channels larger than capillaries, we are forced in these calcu- lations to believe in the practically standing body of water, reaching nearly to the surface, to which objections have been already raised. Artesian Basins. The experience which has been gained with artesian wells is the chief foundation of much that has been written upon the circulation of the groundwater ; and yet artesian basins furnish one of the strongest arguments for the storage of water compara- tively near the surface, and against its descent to great depths. Within the limits of an area thus supplied with underground reservoirs, it is obviously impossible for waters to descend be- low the impervious stratum which is the cause of the reservoir, and it would follow that the lower lying rocks would be dry, except so far as they are supplied with waters which have mi- grated in from points on the surface outside the limits of the catchment-area. In many cases this would involve a journey of many miles, possibly more than a hundred. Artesian basins of themselves permit but slight circulation of the imprisoned waters, and are most unfavorable places for the formation of anything like veins. They represent just so much water cut ofi' from active work, like a convict in a peni- tentiary. They may occasionally be tapped off, downward or upward, by faults, just as once in a while a convict escapes, and then the waters may become geologically active. If they are invaded, however, by igneous intrusions with the attendant cracking of the overlying rock, the accession of heat or energy may make them again active agents. The standing waters, and, what is practically the same thing, the waters which rest under such pressure that they do not reach the surface and flow off, are considered to be too inefllcient to be important in the formation of veins. In cases like the Wheeling and Pittsburgh wells, in which from 3000 to 4500 ft. of marine sediments are apparently dry, one cannot but wonder what has become of the sea-water which THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 703 they must have contained when they were deposited. Instead of receiving new supplies, they have apparently been deprived even of the little which they did possess. Undoubtedly the pressure of overlying masses has eifected this result, or else the water has become combined in some chemical way in the rock itself, and has been thus locked up. Mot Springs. The most suggestive of all geological phenomena in connec- tion with the formation of veins are hot springs; and there is ground for believing that they cannot be explained on any other assumption than that of an abnormal local rise in the isogeotherms. As Osmond Fisher has shown,* the isogeotherms cannot conceivably be raised except by igneous intrusions or by the mechanical production of heat along faults, or belts of shattering ; and the latter do not compare in eflfectiveness with the former. If for a moment we analyze the familiar increase of tempera^ ture with descent, a truer conception will be gained. As ordi- narily stated, and as a fair average, it may be assumed that the temperature increases one degree Centigrade for each thirty meters of descent, which would be about one degree Fahren- heit for each 55 ft. In a region whose mean annual tempera- ture is 50° F. or 10° C. (that of New York is about 51° F.), in order to reach a depth at which the temperature is 100° C. we would be obliged to descend 2700 meters, or not far from 10,000 ft. Now that meteoric waters may flow from the sur- face as a hot spring, which has derived its abnormal heat from this deep-seated source, they must descend to a depth which is at least a large fraction of 10,000 ft. and then return. The depth is a larger fraction of the 10,000 ft. than the temperature of the spring would of itself indicate, because the uprising waters have traversed cooler rocks and necessarily have re- ceived accessions of descending colder waters. One other im- portant factor bearing on this question is, moreover, the ir- regular and more or less choked conduits which have already been emphasized. The following argument has been sometimes advanced, and * Physics of the Earth's Crust, pp. 240-241. 704 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. notably by Van Hise,* in supporting the view that hot springs are the result of normal terrestrial circulations, without acces- sions of heat other than those which would be received through the ordinary increase of temperature with depth. It is argued that, as the descending column of cold water is heavier, and the ascending column of heated water is lighter, therefore a hydrostatic head is afforded. Water expands about 4 per cent, between 0° C. (or, more precisely, 4° C.) and 100° C, and, for illustration, the case is imagined of a descending column at 0° and an ascending one at 100°. This assumption, or any similar one, loses practically all its force if we bear in mind the following important considerations : 1. That the descending column becomes gradually heated, so that, even if the conduits formed practically a long U-tube, there would be little difference in head. 2. That the descending column may move in part in a capil- lary way and lose its head. 3. That water under great load or pressure does not expand according to the 4 per cent, rate named. On the contrary, it may be held by the pressure at fixed volume, despite the added heat. If, for example, we roughly assume a column of water, one square inch in cross-section and two feet high (it is really about 2 ft. S^ in.) as equal to a pressure of a pound to the square inch, in 10,000 ft. we would have a pressure of some- thing near 5000 lbs. or over 2 tons to the square inch ; and in the face of this the expansion of water from an added temperar ture of 100° C. practically becorhes a negligible quantity as contributing to hydrostatic head. 4. "We must bear in mind also that the standing body of cold groundwater fills the interstices of all rocks near the surface, except those in very arid regions, and exerts a retarding influ- ence on uprising currents. When these objections are all appreciated, I think we must admit that, except so far as waters are fed from heights into artesian basins and thence tapped again to the surface, per- haps slightly warmed from having gone to comparatively shal- low depths, such a theory of hot springs, or even of warm springs, is impossible. Hot springs can only be developed in * "Some Principles Controlling the Deposition of Ores," Trans., xxx., 48; p. 303 of this volume. THE IGNEOUS KOCKS IN THE FORMATION OF VEINS. 705 the presence of an abnormal rise of the isogeotherms, which rise can only be effectively produced by intruded masses of igneous rock. I vrill even go so far as to say that it is in the highest degree improbable that any waters which have reached depths even approximating 10,000 ft. can ever again reach the surface and yield flowing springs, except through the propul- sion of stores of energy contributed by still heated masses of igneous rock. I regard it as extremely improbable that the water of any natural spring, whose flow is due simply to hydro- static head, has ever reached more than a very limited depth below the point of emergence. These statements are made in the belief that unless underground water ultimately emerges upon the surface, so as to maintain an activity of movement which this condition implies, its efficiency is so slight and its stagnation so pronounced that it is of small probable impori> ance in connection with vein-formation of any magnitude. Professor Sandberger and those who stand with him are the only logical lateral-secretionists. Even in areas showing the structure of an artesian basin, and possessing a theoretical head of hundreds of feet, the water sometimes rises to a given level in a well and then stands below the surface. Abnormally heated waters, such as those of South Dakota, described by N. H. Darton,* can only be accounted for by the presence of eruptives, although Mr. Darton seems loath even to mention igneoas rocks as a possible explanation. Yet they would appear to be the only reasonable one, and in this region there is ground for inferring their existence. In passing from laboratory-experiments in hydraulics to the phenomena of the earth, there is grave danger of error unless one proceeds with great caution. It is much the same difficulty that formerly arose in drawing profiles of country with exag- gerated, vertical scales. The sense of true perspective was lost. Mr. Rickard's illustrative figure of the hot-water circula- tions in a household heating-plant, f likewise cited by Professor Van Hise, would give a very false conception unless used with so much allowance as to be destructive of its force. The open pipes in a house, extending but 50 or 100 feet in altitude, and * Am. Jour, of Sei., March, 1898, p. 161, and especially p. 168. ■f- Trans., xxiv., 950. 706 THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. with an intense source of heat in the cellar, are not comparable to conduits of irregular size, often choked, at times capillary, and with 10,000 feet of gradually warming walls before even a temperature of 100° C. is reached. In brief, therefore, I believe it to be highly improbable that hot springs are ever produced except in regions of expiring vulcanism ; but it is, on the other hand, highly probable that hot springs have been the great producers of veins. The Irregular Distribution of the Groundwater near the Surface. Recent observations of Emmons and Weed have emphasized the fact that the level of the groundwater is not a regular and sharply marked surface, but is, on the contrary, very irregular and subject to much fluctuation. The presence of oxidized or enriched minerals in some places at depths below the ordinary groundwater level has given rise to this inference. It would appear as if waters become charged with metals within the limits of the gossan, and, descending, react on leaner sulphides so as to enrich them, and that they do so even by diffusion through the standing groundwater and below its level. But it also appears as if there were no standing groundwater and no means of preventing quite deep oxidation and enrichment along some belts, which, because of their open character, may allow the waters to go down, turn and rise again as a spring at some lower point; and this, although neighboring ground, impervious in character, may retain the groundwater at a sharply marked and higher definite level. Naturally, in interpreting the phenomena of gossan-minerals apparently carried downward, we must bear in mind the later geological history of the district, because sub- sidence, together with the choking and elevation of surface- drainage, may raise the groundwater above its »ld level, and it may be that some of the minerals regarded as enriched (bornite, chalcocite, etc.) have been deposited by uprising currents. In regions where the rainfall is small, and where the con- tributions to the groundwater are correspondingly slight, its level may be very far down ; or, if the rocks are shattered, standing groundwater may be entirely lacking, and oxidized ores, so far as they can be produced without the aid of much water, may extend to depths indefinitely great. On the other hand, in an arid region galena may actually outcrop. In the THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 707 Geological Museum of the Columbia School of Mines there is a large specimen, about a cubic foot in volume, that was pried out of the cropping of the Half-Moon vein at Pioche, Nev., by- Prof. Geo. "W. Maynard. It is galena and quartz, the former only oxidized on the surface. All of these points are, however, matters of the anatomy or pathology of already-formed veins, and do not touch the fun- damental problems of genesis, to which, in fact, they are re- lated much as are bodily disorders and amputations to embry- ology and growth. III. — The Distribution of Mining Districts. When one considers the country at large (leaving iron-ores out of the question), it is evident that districts favorable to actual mining are very sparsely distributed. Even in regions like the mountainous parts of Colorado and Montana, where we commonly think of mining as being extensively practiced, the productive areas are separated by vast stretches of country without workable and, I think one may say, without notable vein-development. One rides in a train for hours between the camps, and only for minutes in them. Even making due allow- ance for lack of outcrops, for forests and for veins concealed by the wash, the mining districts must be described as limited areas of intense local vein-formation, which alternate with vast areas of barren ground. In the mining districts igneous rocks are present, practically without exception. If we assert that the assumed circulations of meteoric waters, which are thought to be universal in the rocks, and to be due to the ordinary and ever-present increment of temperature with depth, are the causes of vein-formation, we encounter grave difficulties in trying to explain this general absence of veins. Dislocations are everywhere present, and we ought to find veins in a similarly great abundance. On the other hand, if we remember the points made regarding the igne- ous rocks at the outset, we shall have a much more rational ex- planation both of the presence and of the absence of the veins. It must be appreciated by all who are adequately familiar with both the literature and the phenomena, or with either, that ore-bearing veins, especially when of large size, are altogether exceptional and rare occurrences, and their causes are local and 45 708 THE IGNEOUS KOCKS IN THE FORMATION OF VEINS. exceptional in their nature. No one with a correct sense of perspective can possibly be face to face with the huge stopes of ores of comparatively scarce metals, which some of our mines afford, without marveling greatly that they ever happened to be produced in the course of ITature ; and in dealing with the elu- sive but irresistibly attractive problems which their genesis affords, one cannot be too appreciative of the local and excep- tional nature of the causes which have produced them. One may, therefore, in endeavoring to explain vein-phenomena as a minor corollary to an all-embracing theory of metamorphism, based on the normal circulations of the groundwaters, miss the very kernel of the matter and fall into the same error that von Buch and other disciples of Werner committed, in the early part of the nineteenth century, in endeavoring to establish for rocks in general a "universal hypothesis." Resume. The thesis of vein-formation, however presented, is neces- sarily one of greater or less probability, rather than one of demonstration. The following points may be made in favor of igneous rocks. 1. Igneous rocks contain the metals and the elements of the gangue minerals more abundantly than do sedimentary rocks. 2. Igneous rocks are richly provided with vapors which come up with them from great depths. Igneous rocks are enormous reservoirs of energy. 3. Igneous districts, or districts of combined igneous and sedimentary rocks, are almost always the geological formations in which veins occur. 4. The vapors and solutions from intruded igneous rocks are pre-eminently favorable chemical reagents. 5. Observations in deep mines and the data from very deep wells indicate the general absence of free water in the rocks below moderate depths, except in regions of expiring vulcan- ism. This is a grave objection to the conception of universal groundwater. 6. Capillary attraction is largely an ascensive force and of problematic existence with increasing pressure. Artesian res- ervoirs of themselves are unfavorable to extended circulation. There is a strange absence of the original content of water in THE IGNEOUS ROCKS IN THE FORMATION OF VEINS. 709 deep-seated sediments. Standing water in abandoned shafts is strong evidence of the impenetrability of rocks. 7. Hot springs are necessarily connected with an abnormal rise of the isogeotherms, and this can only be explained by in- truded igneous rocks or by faults and shattering. The latter do not compare with the former as an efficient cause. 8. The distribution of the groundwater is far less uniform than has been supposed. The groundwater may entirely fail in arid regions. 9. The distribution of mining districts can only be satisfac- torily explained by the corresponding distribution of igneous rocks, which have been intruded under circumstances favorable to vein-formation. Under any other view veins should be much more common. In conclusion, I cannot forbear reference to the subject of the classification of ore-deposits. In November, 1892, I pub- lished in the School of Mines Quarterly a paper on the " Classi- fication of Ore-Deposits, a Review, and a Proposed Scheme Based on Origin." The same has been subsequently printed, with one or two minor modifications, in the " Ore-Deposits of the United States." After a review of all the known schemes up to that time, and an analysis of their special features, a scheme was developed which sought more consistently than had been done up to that time to bring the ore-deposits under well-recognized geological phenomena. Aside from the ores of igneous origin, and the placers of various kinds, this in- volved a classification of those phenomena which would give rise to cavities, not of themselves necessarily great, but suflicient to furnish a water-way. These are the determining factors in the location of ore-deposits ; they admit of the least possible difference of theoretical views or of interpretation, and they are the common ground upon which observers can best meet in har- mony. They therefore furnish much the best basis of classifica- tion. I do not believe that any other line of attack of this problem furnishes equal advantages. Therefore, while the conceptions of ascending and descending waters cited by Professor Van Hise in closing his essay give new and significant points of view, yet the interpretation of the phenomena in accordance with them is in- evitably destined to raise such well-grounded differences of opinion as to make the scheme impracticable for general use. 710 THE CALICHE OF SOUTHERN ARIZONA. The Caliche of Southern Arizona : An Example of Depo- sition by the Vadose Circulation. BY WILLIAM p. BLAKE, F.G.S., Director Arizona School of Mines, Tucson, Ariz. (Richmond Meeting, February, 1901.) In southern Arizona and in Mexico the word caliche is in general use to denote a calcareous formation of considerable thickness and volume found a few inches, or a few feet, beneath the surface-soil, upon the broad, dry, gravelly plains and mesas. In western South America the same name is applied to the beds of crude soda-niter (Chili saltpeter). While these deposits of South America and of Arizona are totally different in com- position, and have nothing in common, except that both occur in layers in the strata near the surface, it is probable that an ex- planation of the origin of the calcareous beds may equally apply to the accumulation of soda-niter and other deposits of easily soluble minerals. But the name, taken from the Latin, Calx, is more appropriate to the calcareous beds than to those of niter. Caliche has a wide distribution in the arid regions of Arizona and Mexico. It is usually hidden from view by a slight cover- ing of soil ; but it is easily found by digging, and is often re- vealed by a flow of water during heavy rains. It is practically a continuous sheet, from three to fifteen feet thick, of earthy limestone or travertine, through which the smaller plant-roots find their way with difficulty. The presence of this compara- tively impervious layer of cemented earth may account for the absence of trees, or of the larger shrubs, over wide areas. The shrubs which gain a foothold are those whose roots do not ex- tend far downwards, and which do not require much water, such as Larrea Mexicana and the Cactacece. If trees are planted, it is necessary to break up the caliche by blasting, or at least to crack the upper layers. The top of the caliche is more dense and solid than the lower portions. The surface of this top crust, or layer, is comparatively smooth, though undulating, while the lower portions, under the crust, are irregular, cavern- THE CALICHE OF SOUTHERN ARIZONA. 711 ous, earthy and very porous, blending gradually with the ma- terials of the sandy and gravelly beds, from which they are divided by no sharply defined plane of stratification or separa- tion. The caliche invests, surrounds and includes sand-grains, gravel, and more or less earthy material, but seems to have had the power, especially in its upper crust, of extruding the coarse materials of the soil to a great extent. The deposit does not form a regular horizontal bed conform- able with the rude stratification of the gravels and sands, but conforms roughly with the general surface, rising and falling with the undulations of the mesa. There are, in places, repeti- tions of the compact layers, separated by a few inches of the amorphous and more earthy deposit. In cross-fracture, this upper crust of the caliche exhibits dis- tinct, fine lines of successive layers, in thin sheets, along which the rock splits with some ease, while there is a rude columnar fibrous structure transverse to these layers, sometimes in diver- gent lines from below upward.* Close observation detects in some places small perforations, like pin-holes at the top, which enlarge gradually below and penetrate the entire compact crust, becoming lost in the irregular amorphous granular mass. These holes are often occupied by rootlets of plants ; but this is not regarded as evidence of any connection between the deposition of the caliche and plant-life — a cause of deposition to which great importance is attached by some authorities.f The caliche is an example of deposition independently of the influ- ence of organic agencies. In chemical composition the caliche is essentially a lime * ' ' Sorby has shown that in the calcareous deposits from fresh water there is a constant tendency towards the production of calcite crystals with the principal axis perpendicular to the surface of deposit. When that surface is curved, there is a radiation or convergence of the fibre-like crystals, well seen in sections of stalactites and of some calcareous tufas." Cited by Geikie, Text-Book Geology, 3d Edit., p. 150. t Dana, for example, citing from W. H. Weed, says : "Some of the travertine deposits of Gardiners Elver and elsewhere are a result of the growth and secre- tions of conferva-like plants." {Oeology, ith Edit., -p. 133.) Geikie says : "But besides giving rise to new formations by the mere accumulation of their remains, plants do so also both directly and indirectly by originating or precipitating chem- ical solutions," etc. . . . " Some observers have even maintained that this is the normal mode of production of calc-sinter in large masses, like those of Tivoli." (Geology 3d Edit., p. 482.) 712 THE CALICHE OF SOUTHERN ARIZONA. carbonate, but contains some calcium, magnesium and alumi- num silicates, as more fully shown by the result of an analysis made by my assistant, Mr. J. S. Mann, in the laboratory of the Arizona School of Mines : Calcium carbonate (CaCOa), 78.28 Magnesium carbonate (MgCOj), 2.13 Calcium silicate (CaSiOg), 5.57 Aluminum silicate (AljSiOs), 7.37 Ferric oxide (Fe203), 1-88 Moisture (HjO), 1-20 96.43 This caliche, unlike the deposits of travertine formed in the open air, is not sufficiently compact and solid to be useful in construction, as was the travertine of ancient Rome. When calcined, it yields good caustic lime, which, tempered with sand, makes a strong, quick-setting mortar or cement. It is quarried and used for this purpose in some places. Occurring, as it does, in mixture with gravel and sand, it has the appear- ance of an artificial mixture, and as such was once supposed to have been laid down as a foundation by the builders of the Casa Grande in Arizona. On the line of the Phcsnix and Prescott railway, it has been found that railway ties last longer when laid in the caliche then in ordinary soil. Analysis of this caliche showed that it did not differ essentially from the caliche of other places. The great plain or mesa of Tucson affords one of the best examples of the occurrence ofthecafe'cAe. This mesa, which appears like a great plain, is in reality a combination of gentle slopes from the surrounding mountains. The area within which the phenomena of the caliche are shown is probably not less than 400 square miles, and lies between the Santa Cata- lina Mts. on the north, the Rincon and Rillito mountains and the Santa Rita ranges on the east, and the Tucson Mts. on the west. Toward the south and northwest the country is open in the direction of the valley of the Santa Cruz. The Santa Cruz and the Rillito are the visible channels of drain- age ; but there is, in addition, an extensive underground flow of water as widely spread, possibly, as the area mentioned, but probably strongest in volume under and near the river chan- nels draining to the northwest. The general altitude of the THE CALICHE OP SOUTHERN ARIZONA. 713 mesa above the sea is from 2400 ft. at Tucson to 3000 or 3400 ft. about 20 miles eastward. These declining slopes are formed, for the greater part, of the dibris of the surrounding granitic and gneissic mountains — the " wash " or gravel and sand which has been washed out from the canons through ages of erosion. As a rule, these materials are rudely stratified, the coarsest, heavier gravels lying nearest to the mouths of the canons. In a well 90 ft. deep, near the University, on the mesa about five miles from the channel of the Eillito, the fol- lowing beds were passed through, but the strata were not sharply defined : Section of the Mesa, to Water-Level. Feet. Soil, sandy and porous 1 Caliche, ........... 6 Sand and gravel, 12 Argillaceous earth (red), 2 Sand (red) 2 Caliche, soft and amorphous, 2 Sand, hard, 6 Sand and gravel ("cemented"), 3 Sand cemented and aggregated in lumps, 11 Argillaceous earth, red, 3 Argillaceous earth and sand, red, .30 Sand and boulders mixed, 8 Water in sandy bed, 4 Most of the sand and gravel not enveloped in caliche was found well filled with small sparkling crystals of calcspar, which appears to be the cementing substance holding the grains of sand together. "Wherever these gravels have been pierced by wells in the vicinity of Tucson, an abundance of water has been found at a depth of from 80 to 90 ft., or even less, depending upon the altitude of the surface. This water seems to be inexhaustible; at least it is in such quantity, and flows so freely, that the pumping-plant at the University can be run continuously, dis- charging a 6-inch stream, without exhausting the supply in the well. The general composition of this underground water is shown in the annexed table, compiled from the records of the chemical laboratory of the University of Arizona. 714 THE CALICHE OF SOUl^HERN ARIZONA. Analyses of Well- Waters of Tucson and Vicinity — Parts Per 100,000. Two Miles N. of Uni- versity. R. E. Well. Irrigating Tucson City Water. Hoflf's Well. Oracle. Tucson Water Works. Total Soluble Salt 24.5 42.0 26.0 65.0 45.0 39.0 42.8 NaCl 1.4 3.1 2.0 9.0 1.0 3.0 3.0 15.2 1.3 15.5 trace 2.0 3.0 6.5 1.0 9.5 0.5 2.0 4.4 26.8 1.8 22.0 0.0 2.0 4.5 13.5 1.5 16.5 0.5 2.0 3.4 . 10.6 2.0 14.5 0.5 2.0 3.9i 16.92 18!56 1.74 3.10 (Na, K),SO,.. NajCOj (Ca, Mg) CO3. CaSOi SiOa It is probable that, in the course of the underground flow of the water from the higher levels towards the Santa Cruz, there are considerable areas of basin-shaped depressions in the bed- rocks, where water accumulates and is more sluggish in its movement. So, also, there may be ancient channels, determining a more rapid flow than in other places ; in each case there may be a difference in the amounts of solid matters held in solution. There has been much speculation in regard to the origin of the caliche. It has been generally assumed to be a deposition from some ancient lake, or body of water, once covering the area in which it is found. But such a theory is untenable when all the phenomena are considered. The formation is clearly the result of the upward capillary flow of calcareous water, induced by constant and rapid evaporation at the surface in a comparatively rainless region. With a constant supply of phreatic* calcareous water, the second great essential factor in the formation of caliche is the continued desiccating atmosphere — a condition which prevails, with only short and temporary exceptions, throughout the year. The desert and semi-desert regions of Arizona are character- ized meteorologically by the unusual dryness of the air and its capacity for the absorption of moisture, and the maintenance of continued evaporation from the soil, which determines a constant upward movement of the phreatic water. The occa- sional rains in midsummer and midwinter do not penetrate to great depths, but are sufficient to leach out the soil to the depth of a few inches or feet, turning the calcareous solution back * Eaux phrkiJtiques, Daubr^e, Les Eavx Souierraines d I' ipoque acivAle, i., 19. THE CALICHE OE SOUTHERN ARIZONA. 715 and downwards, and producing the denser upper crust, where it meets the upward flow. Such I conceive to be the origin of the caliche. It may be called a subterranean deposit of travertine ; but it is not the result of a flow from springs, or from any source at the surface, or by the lateral movement of water. Unlike ordinary traver- tine, it is the result not of descending but of ascending currents. The ordinary conditions of vadose circulation are reversed. The caliche is a fine example of the formation of extensive calcareous strata in the midst of pre-existent beds, not by meta- somatic processes, but by precipitation from sources below. This explanation may apply equally well to some other subter- ranean deposits in arid regions, where the upward flow is main- tained in excess of any downward percolation. It may apply, pos- sibly, to the origin of soda-niter, of some beds of gypsum, and of some of the metallic sulphides. In fact, the phenomena of depo- sition of ores in mineral veins are here repeated in kind, though not in form, over broad and approximately horizontal areas, so as to make bedded deposits instead of fillings of fissures. Surface-deposits of soluble salts, such as the chlorides, sul- phates and carbonates of the alkalies, are familiar to all resi- dents of arid regions. The " black-alkali " of the Salt Eiver valley in Arizona, so injurious to vegetation, is an example of the concentration, by evaporation at the surface, of solutions of carbonate of soda. The white efflorescences on the soil in the dry season, known to the Mexicans as tequisquita, are familiar examples. These deposits become snow-white in a dry time, and quickly disappear into the soil during a rain-storm. The presence of caliche in the soil over extended areas in the arid regions I regard as good evidence of the existence of sub- terranean water. The possibility of a change of conditions since the deposit of the caliche should, however, be considered. I have elsewhere directed attention to the possible enrich- ment of the upper portions or croppings of mineral veins by the upward flow of solutions formed by the decomposition of the ores above the permanent water-level in arid regions, and, conversely, the impoverishment of the croppings of lodes in regions of abundant precipitation, where downward circulation predominates. The copper-deposits at Ducktown, Tennessee, afford striking illustrations of the latter process. 716 CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. The Character and Genesis of Certain Contact-Deposits* BY WALDEMAR LINDGREN, WASHINGTON, D. C. (Richmond Meeting, February, 1901.) CONTENTS. I. Chabaotbr op the Deposits, 716 : Prinaipal Features, 716 (Form, 717 ; Posi- tion, 717 ; Constituent Minerals, 717 ; Exceptions, 717) ; Literature, 718 ; Oeo- graphic Distribution, 720 ; (California, 720 ; Idaho, 721 ; Arizona, 723 ; British Columbia, 723 ; Northwest Territory, 723 ; Mexico, 724 ; Other Countries, 725). II. Origin op the Deposits, 725 : Oontact-Melamorphism, 726 ; Cause of Contact- Metamorphism, 727 ; Similar Deposits of Different Origin, 730 ; Genetic CMassifi- cation, 730; Relation of Pegmatite-Veins to Ore-Deposits, 732. I. — Character of the Deposits. 1. Frincipal Features. In many schemes of classification and description the term contact-de-posit has been somewhat loosely applied to all accumu- lations of useful minerals (other than those of unquestioned sedimentary origin) which are enclosed between two different rocks. As thus used, the term may include deposits of widely differing origin, and, unless qualified, is not in place in a genetic classification. The present paper deals with a special class of contact-deposits. In many geological provinces, granular igneous rocks, such as granite, diorite and syenite, have broken through and in- vaded sedimentary rocks. The molten magma may in part have reached the surface and there solidified with relative rapidity as a lava. The largest masses of it, however, did not reach the surface, but cooled very slowly at considerable depth under great pressure, and eventually consolidated into a rock of granitic texture. Uplifts and extensive erosion may have followed ; and at the present day, in many places, thousands of feet of material have been removed, bringing to the surface the intrusive granular rocks and their once deep-seated contacts with the sedimentaries which they shattered at the time of in- * Published by permission of the Director of the IT. S. Geological Survey. CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. 717 trusion. Along these contacts, bodies of useful minerals are often found, most commonly where the sedimentary rock is limestone, or, at least, calcareous. All the world over, this group presents certain characteristics, the more essential of which are the following : Form. — The deposits generally follow the contact, but are ex- tremely irregular in detail, and almost always very bunchy. ISTo regular law has been recognized as governing the form of the ore-bodies, which are sometimes lenticular masses. Position. — The minerals generally occur in the limestone or calcareous rock, immediately on the contact, from which they rarely extend more (usually much less) than a hundred feet. Constituent Minerals. — The gangue contains garnet, wollas- tonite, epidote, ilvaite (lievrite), amphibole, pyroxene, zoisite, vesuvianite, quartz and calcite, rarely fluorite and barite. The ore-minerals are specularite, magnetite, bornite, chalcopyrite, pyrite, pyrrhotite, and, more rarely, galena and zincblende. The sulphides may carry some gold and silver, usually more of the latter than of the former, but are rarely rich. Tellurides are unknown. The characteristic feature is the association of the oxides of iron with sulphides, a combination practically un- known in fissure-veins,* and further the presence of various silicates of lime, magnesia, and iron. The deposits are through- out metasomatie, having been formed by the replacement of limestone; and the filling of open spaces is almost entirely absent. On account of the great solubility of the limestone, well-developed crystals of the gangue minerals are very com- mon. Exceptions. — There are some classes of deposits which, though presenting a certain similarity to this type, must be strictly separated from it. Among these are contact-deposits between limestone and igneous rocks which carry as metaso- matie products (besides galena and zincblende) sericite, dolo- mite, siderite and quartz, and which, upon close investigation, are usually found to be related to fissures and faults. Further, certain deposits of iron-ores, associated with limestone and with garnet-pyroxene-amphibole gangue, but without any apparent * Specularite and arsenopyrite are both known from cassiterite-veins, which, however, in origin, stand close to pegmatite- veins and certain con tact- deposits. 718 CHARACTBE AND GENESIS OF CERTAIN CONTACT-DEPOSITS. close relation to intrusive rocks. This kind will be referred to again in this piaper in the discussion of the genesis of the deposits. 2. Literature. Though the contact-deposits here described are not very abundant, and rarely of great economic importance, they could not long escape the notice of mining geologists. In 1865 B. V. Cotta* described the celebrated mines of the Banat, in Austria, and expressed the opinion that all of them were due to the action of intrusive rocks on a probably Mesozoic lime- stone. Regarding some of these deposits, this view has lately been opposed by H. Sjogren,! who, however, admits that others in the same vicinity may stand in causal relation to the intru- sion. To V. Groddeck belongs the credit of having recognized these deposits as a separate class,| which he calls the Kristi- ania type, and characterizes as follows : ' ' Siderite, magnetite, chalcopy rite, bomite, pyrite, galena, zincblende, etc. , accompanied by garnet, amphibole, wollastonite, axiuite, etc., mingled in very different proportions, forming nests and stocks at the contact of eruptive rocks with granular limestone, or often wholly within the latter. These deposits thus belong in the sphere of contact-metamorphism, and may be briefly characterized as 'contact-deposits.' " Among the examples are mentioned the contact-deposits of the vicinity of Kristiania and those in the Urals (Bogoslowsk). Several others are also included which are more doubtful (Pyrenees, Rodna, Rezbanya, Offenbanya, the Banat and Schwarzenberg), and part of which seem to be due to regional metamorphism, or to the action of ascending thermal waters at the contact of lavas and limestone. V. G-roddeck apparently fails to recognize that the presence of intrusive igneous rocks is necessary to develop this type of deposits. Siderite, mentioned in his definition as one of the characteristic minerals, does not occur in the typical examples, and seems to be neither common nor essential. De Launay§ also describes similar deposits, but includes * Erslagerstdtten im Bariat und in Serbien, 1865. t Jahrbuchd. K. K. Oeol. RekhsanstaU, 1886, xxxvi., pp. 607-668. J Die Lehre von den Lagerstatten der Erze, 1879, p. 260. J Traii& des gttes mStallifires, Paris, vol. ii., pp. 245-258. CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. 719 under the heading several other deposits not so clearly belong- ing to the same category, and hardly recognizes the importance of the presence of intrusive rocks. In addition to the well- established examples from v. Groddeck, de Launay adds excel- lent descriptions of the mines of Mednorudjansk and Ekater- inenbourg in the Urals, which leave little doubt that these, also, should be enrolled under the Kristiania type. In Prof. Kemp's classification,* the following division is found : " Contact^Deposits. Igneous rocks always form one wall. Fumaroles (Greisen)." This is evidently to include several different things under one heading. On p. 222, however. Prof. Kemp recognizes the importance of the type outlined in this paper, one example of which is mentioned, namely, that of the Seven Devils district, Idaho. Relating to the same subject we find (p. 69) the following direct utterance : " In the more characteristic ' contact-deposits ' the igneous rock has apparently been a strong promoter of ore-bearing solutions, and has often been the source of the metals themselves. This form of deposit becomes, then, an attendant phe- nomenon of, or even a variety of, contact-metamorphism." Prof. Vogt describes contact metamorphic desposits in sev- eral of his recent papers.f In that of 1894 the contac1>depo8it8 near Kristiania are described on the basis of his own investiga^ tions and of the previous work of Kjerulf.| These deposits, which are small and do not have much economical importance, occur in the majority of cases exactly on, or very close to, the contact of syenitic rocks and Silurian limestone and slates, along which they are found in great numbers and of irregular form. The mineral aggregates sometimes show a banding parallel to the stratification, and are only found in the sedi- mentary rock, not in the syenite. The gangue-minerals are garnet, amphibole, pyroxene, mica, epidote, vesuvianite, scapo- lite, chiastolite, quartz, calcite, also fluorite and axinite. As ore-minerals appear magnetite, hematite, chalcopyrite, galena, zincblende, and, more rarely, minerals containing bismuth, arsenic and antimony. Besides this locality are mentioned * Ore-Deposits of the United States and Canada, J. F. Kemp, 3d ed., p. 58. t J. H. L. Vogt. Die Kieslagerstdtien RSros-SuUtelma und Rammelsberg, Z. /• prakt. Oeol, 1894, p. 177. Zur Glasdficaiion der Erzvorkommen, Z. f. prakt. Oeol, 1895, p. 154. Concentration des Metallgehaltes zu Erzlagerstatten, Z. f. prakt. Geol. , 1898, p. 416. X Udsigt over det sydlige Norges Oeologi, Kristiania, 1879. 720 CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. others from the Pyrenees, Banat, Pitkaranda (Finland) and Queensland. In the treatise of Phillips-Louis* we find in the preliminary- part no mention of this type of contaci^deposits ; and, in the second part, giving detailed descriptions, such " contact-depos- its " as Leadville, Rodna and the Banat are treated together without genetic distinctions. 3. Geographic Distribution. Deposits of this type are fairly common in America, though little attention has been directed to them, probably because of their smaller economic importance. As might be expected, most of them are found in the regions of the Pacific Cordil- leras, where great intrusions have been followed by uplifts and enormous erosion. They are generally found at the contacts of quartz-monzonites, granodiorites, quartz-diorites and diorites with limestone. Besides the gangue-minerals mentioned above, the characteristic ore-minerals are specularite or mag- netite with bornite or chalcopyrite. A smaller group is dis- tinguished by the additional appearance of galena and zinc- blende which, in places, may overshadow the copper minerals in economic importance. California. — The great area of granodiorite in the Sierra Nevada, accompanied by smaller areas of quartz-diorite, breaks through the Paleozoic and Mesozoic sedimentary rocks. Along the contacts thus presented, ore-deposits of the type here de- scribed are rarely met with, perhaps because limestones and calcareous rocks are not very abundant. However, on the area of the Colfax Folio of the TJ. S. G-eol. Survey, about 10 miles north of the railroad station of Emigrant Gap, N"evada co., a mass of probably carboniferous limestone has been greatly contact-metamorphosed and filled with garnets, etc., but no sulphides appear in it. Along the contacts of the smaller in- trusive areas of granodiorite down on the western slope of the Sierra Nevada small copper-deposits are occasionally found. Near Fairplay, Eldorado co., at the contact of granodiorite and limestone in the canon of the Cosumnes river, garnets and epidote occur, and, intergrown vrith these, small masses of bornite and chalcopyrite. * A Treatise on Ore-Deposits, London, 1896. CHARACTER AND GENESIS OP CERTAIN CONTACT-DEPOSITS. 721 In Alpine co., 12 miles due south of the southern end of Lake Tahoe, an area of sedimentary calcareous rocks of uncer- tain (probably Triassic) age, about 1.5 mile long and 0.5 mile wide, occurs embedded in granodiorite, which is the prevailing rock in that vicinity. The locality is in the upper part of Hope valley. At several places along the contacts, mineralization has taken place. The prospects were visited by Mr. H. W. Turner in 1888, and by myself in 1895. At Rodgers' mine* the strata consist of alternating thin beds of quartzite and lime- stone, the latter carrying the principal value. The ore-bearing strata are in places 100 ft. thick. About $100,000 worth of ore is said to have been extracted from this place some three or four decades ago. The ore-minerals consist of pyrrhotite, chalcopyrite and bornite, and contain gold as well as some sil- ver. The principal gangue-mineral is garnet. On the east slope of Stevens Peak, in the same area, a stratum of limestone near the contact is very crystalline, and contains garnets and zincblende. At Barnes' prospect, in the same area, a wedge of limestone, projecting into the surrounding mass of granodiorite, is highly crystalline and filled with garnets, amphibole, and other con- tact-minerals. Of ore-minerals, magnetite and chalcopyrite as well as bornite were found ; these are reported to contain some gold and silver. Idaho. — A number of deposits of the Kristiania type are be- lieved to occur in this State, though their true nature has rarely been recognized. Position and mineral association indi- cate that the lead-zinc deposits at South Mountain, Owyhee co., are true contact-deposits, though when visited by Mr. F. C. Schraderf the developments did not permit any exact study of structural relations. The ores occur on the contact of lime- stone and diorite or granite ; the ore-minerals are argentiferous galena, zincblende, and a little chalcopyrite and magnetite ; the gangue being garnet, quartz, actinolite and the typical contact- mineral ilvaite or lievrite. According to the description of Mr. G-. H. Eldridge,J certain deposits on Sheep Mountain in central Idaho very likely belong to this type. * MS. notes by Mr. H. W. Turner. t W. Lindgren, Silver Oily and DeLamar, %>ih Ann. Bep. U. S. Qeol. Sur., Part III., pp. 187-189. J Uth Ann. Bep. U. S. Oeol. Sur., Part II., p. 258. 722 CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. Most characteristic are the contact-deposits of the Seven Devils, briefly described in a recent report.* In the Seven Devils district, and in the adjacent Snake River canon, copper-deposits are very abundant. There is, in that vicinity, an extensive series of Triassic basic lavas, with inter- calated layers of slate and limestone. There are also diorites, intrusive in these beds. All of these igneous rocks apparently contain copper which was easily concentrated into deposits of various kinds ; some, fissure-veins ; others, zones of impregna- tion ; others, contact-deposits. In the locality of the original discovery in the Seven Devils, the copper occurs in typical contact-deposits. Small masses of limestones are embedded in a later, intrusive diorite ; at the contact, and usually in the limestone, are found irregular bodies and bunches of bornite, chalcoeite, and a little chalcopyrite, containing, say, 10 oz. of silver and a little gold per ton. The limestone at the contact is very crystalline and contains, associated with the ores, abundant garnet, epidote, quartz, calcite and specularite. The copper sulphides, as shown by their intergrowth, were certainly formed at the same time as the gangue-minerals. The epidote, specularite and garnet, as described by Dr. Palache,t present clear evidence of simultaneous crystallization. At the Peacock mine a large body of medium-grade ore of this character was embedded in diorite. ISTo limestone showed here ; but I am informed that a lower tunnel has lately encountered limestone below the croppings. Other claims in which the ore occurs on the contact of limestone and diorite are the White Monument, Alaska, Blue Jacket, Helena and Decorah. Considerable masses of ore have been exposed at some contacts, though the distribu- tion is extremely irregular. In the Blue Jacket, a rich body of bornite and chalcoeite was lately found ; and it is reported that 500 tons of 40-per cent, ore has been shipped from this mine during the past summer. During 1900, the Boston and Seven Devils Copper Co. shipped from the Peacock and other claims 260 tons, containing 23 per cent, of copper, besides 8 oz. of silver and 0.04 oz. of gold per ton. Still another copper deposit in Idaho which appears to be- * W. Lindgren, 20th Ann. Hep. U. S. Oeol. Sur., Part III., p. 249. t Am. Jmr. Sci., 3d Ser., vol. viii., p. 299, Oct., 1899. CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. 723 long to this type is the "White Knob mine, near Houston, in Lost River valley. Mr. W. Darlington, the general manager of the company, has kindly furnished the following informa- tion. The ore occurs as a deposit between granite and lime- stone ; the trend of the contact is IST. and S., the limestone lying to the E. and the granite to the W. On the surface the ore-bearing zone is 1200 ft. in length, and (as a maximum) 400 ft. in width. The minerals are hematite, magnetite, chalcopy- rite, pyrite and a little galena, in a gangue of garnet and coarsely crystalline calcite. A porphyry dike also occurs on the contact, complicating the geological relations. The oxi- dized zone is very deep, water not having been encountered until the depth of 600 ft. was reached in the shaft. Arizona. — It is well known that many and very important copper-deposits occur associated with limestone and igneous rocks in Arizona. The descriptions published seem to indicate that few of them, if any, are contact-deposits of the Kristiania type. In most of them, also, the zone of oxidation is very deep and their original character has been greatly altered. British Columbia. — Recent literature describing the copper- deposits of Vancouver and Texada islands points without doubt to the existence of numerous and important contact-deposits in those localities. Already indicated by Mr. Carlyle,* this is confirmed by Mr. Wm. M. Brewerf. The deposits always occur in or very near the contacts between limestone and gab- bro or diorite. The mineral association is magnetite, chalcopy- rite, hornblende and garnet. In some places the magnetite predominates, almost to the exclusion of the chalcopyrite. Northwest Territory. — Mr. R. H. Stretch has recently de- scribedj interesting deposits on the Upper Yukon, which, to judge from the excellently presented data, are contact-deposits of the Kristiania type. Mr. Stretch, however, it is fair to say, does not consider them as due to contact-metamorphic origin, but as a result of later mineralization. The locality is a few miles west of White Horse Rapids, lat. 60° 40', long. 135°. The prospects are found along a narrow strip at the base of * Report of the Provincial Mineralogist, 1897. f The Copper-Deposits of Vancouver Island. Trans., xxix., 483. Eng. & Min. Jowr., 1900, Apr. 21, May 5, July 14. t Eng. & Min. Jour., Sept. 8, 1900. Notes on the White Horse Copper-Belt. 46 724 CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. a mountain range, consisting chiefly of limestone. This base is a granite plateau which Mr. Stretch thinks underlies lime- stone ; in fact, a few patches of limestone remain on the plateau. The ores occur at the contact of the two rocks, or in seams of varying size in the granite. Two classes of ores are found : (1) large masses of specularite or magnetite, carrying a moder- ate amount of copper ; (2) outcrops of smaller dimensions, in which the ore is bornite with a little chalcopyrite. Many of these prove to be connected with E.-W. seams penetrating the granite, but nowhere show evidence of massive vein-structure. At all the localities, epidote and lime-garnets are present. The bornite contains some gold and silver ; and a little molybdenite is also found. Dikes of granite occasionally cut the limestone. Mexico. — ^From a perusal of recent geological literature of Mexico, it is clear that contact-deposits of the Kristiania type are very abundant there — more so than in other parts of l^orth America. In a review of the gold-deposits of the republic, Mr. Ordonez* says: ' ' Examples of another type of ore-deposits are found in regions where sedi- mentary Mesozoic rocks appear, that is, on the eastern slopes of the Sierra Madre, towards the Gulf of Mexico. These consist of contact-veins between generally Cretaceous limestones and eruptive granitic rocks, nearly always diorite. The lime- stones are metamorphosed at the contact, and the copper minerals containing gold occur irregularly distributed in contact-metamorphic silicates, such as gar- net and epidote. " Such deposits exist at Encarnacion, district of Zimapan, also in the vicinity of San Jos4 del Oro ; further, at San Jos^, Central district, State of Tamauli- pas, as well as at many other places." Aguilera and Ordonez, mentioning several localities in their sketch of the G-eology of Mexico, f write as follows : ' ' In the region of Mazapil, Zacatecas, an extensive formation of Cretaceous limestone is cut by dioritic rocks. Near the contact extend very important de- posits, worked during many years. The contact is marked by a conversion of the limestone to marble. ' ' "Chalcopyrite, always accompanied by grossularite (garnet), and usually by hematite, occurs in Cretaceous limestone, and its appearance is due to the eruption of igneous rocks, as may be seen at San Jos^ in the Sierra San Carlos, in Ta- maulipas, in which copper-minerals, accompanied by magnetite, appear at the contact of the andesitic diorite." * Note sur les gisements d'or du Mexique, Mexico, 1898, p. 233. ■f Bosquejo geoldgico de Mexico, Mexico, 1897, pp. 68, 222. Boktin del Insiituto geol. de Mexico, Nos. 4, 5, 6. CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. 725 A similar deposit from the State of Chiapas is interestingly described by Mr. E. T. McCarty.* Here limestone of unknown age is invaded from below by rocks called trap, syenite or dolerite. At the contacts the limestone is largely converted into wollastonite and garnet, besides a little quartz, chalcedony, calcite and aragonite. This contact-metamorphosed limestone contains, partly scattered through it, partly in more concen- trated but very irregular " ore-channels," auriferous and argen- tiferous bornite, as well as some chalcopyrite, enargite, galena and linnseite. The average ore consists of 90 per cent, of gar- net with 10 per cent, of quartz and chalcedony, carrying from 3 to 4 per cent, of copper and from 6 to 8 oz. silver, and from |6 to $20 in gold, per ton. The gold is in part free and visible. Regarded as a whole, the ores appear in curved planes, which probably follow the outline of the underlying intrusive. The total width of the ore-bearing limestone is about 30 ft., and within this distance are two ore-bearing streaks. Very often the ore lies directly on the contact. Other Countries. — In the foregoing brief notes I have at- tempted to call attention to the occurrences of this type in America only. But short and incomplete descriptions, found here and there in the literature of the subject, make it more than likely that such contact-deposits occur in "West Australia, Queensland, South Africa and China. From the latter country, for instance, F. L. Garrisonf describes lead- and zinc-deposits in contact-metamorphic limestone, near granite. n. — Origin of the Deposits. The deposits of the Kristiania type may be separated into several subdivisions, according to the prevalence of certain metallic minerals. Thus we have iron-deposits, carrying chiefly magnetite and specularite; copper-deposits, characterized by bornite and chalcopyrite; and finally zinc-lead deposits, containing galena and zincblende. These three groups are connected by transitional examples. In all of them the metallic- * "Mining in the Wollastonite Ore-Deposits of the Santa F^ Mine, Chiapas, Mexico," Trans. Inst. Min. and Met, London., vol. iv., pp. 169-189 (1895- 1896). See also H. F. Collins, Id., Feb., 1900; and Mr. Collins's "Note on Cheap Gold Milling in Mexico," in Trans., xxi., 446. f Mining and MetaUurgy, Feb. 15, 1891, p. 107. 726 CHARACTER AND GENESIS OP CERTAIN CONTACT-DEPOSITS. minerals are intergrown with the various gangue-minerals, — garnet, epidote, woUastonite, etc. — in such a manner that they must be considered as having a simultaneous origin. The the- ory of a subsequent introduction of the metallic ores is decid- edly untenable. Since, on the other hand, the garnets and other gangue-minerals stand in unquestionable relation to the contact- metamorphic action, a theory of the origin of these deposits certainly becomes a branch of the study of contact-metamor- phism. 1. Contaet-Metamorphism. The peculiar action of intrusive igneous bodies upon ad- jacent sedimentary rocks is a well-known fact in geology and petrography. The sedimentaries usually suffer a more or less intense metasomatic alteration, termed contact-metamorphism. Surface-eruptions (lavas), ias a rule, exert no such intense action, though a certain baking or partial melting of the immediately adjoining rock may sometimes be recognized. The metamor- phism exerted by intrusive rocks is characterized by a gradu- ally fading alteration of the sediments, sometimes extending over a width of several kilometers. The contact of the altered rocks with the intrusive is usually sharp, a melting of the former being rarely if ever noticed. Slates and shales in the immediate vicinity of the intrusive rock are changed to highly crystalline schists or massive crystalline rocks, containing an- dalusite, feldspar, cordierite, garnets, etc. ; further away, slighter recrystallization results, with development of mica and accumulation of the carbon of the shales in little knots and masses. In general, there is no considerable addition or subtraction of material during the metamorphism. Limestone usually suffers a stronger contact-metamorphism and becomes a coarse-grained marble. Garnet, woUastonite, amphibole, pyroxene, epidote, etc., often well crystallized in large indi- viduals, form in it. In this case there is usually an addition of silica and a loss of carbon dioxide. In many places the contact-zone has received an access of certain minerals con- taining boron and fluorine not contained in the unaltered rocks ; the most common of these are tourmaline and topaz. Oxides and sulphides, such as magnetite, specularite, ilmenite, pyrite and pyrrhotite, are often contained in contactmetamor- CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. 727 pMc slates and schists.* Magnetite, pyrite and pyrrhotite have been observed in limestones (Morbihan, France) ;t and the Devonian limestones at Eothau, in the Vosges, are metamor- phosed for a few hundred feet from the contact, and contain pyroxene, garnet, epidote and a little galena.| Brogger, in his studies of the contact-metamorphic rocks near Kristiania,§ remarks : " Pyrrhotite appears abundantly in the altered rocks, and is certainly a mineral formed during the contact-metamorphism, for it does not occur in the unaltered rocks. It is not easy to say whether an addition of material has reaUy taken place, or the mineral represents a recrystallization of finely distributed pyrite. Strongly in favor of the hypothesis of direct addition is the fact that large accu- mulations of pyrrhotite exist in the contact-metamorphic rocks — so large, indeed, that mining has been attempted in places." . ... "As already indicated by Kjerulf, we must consider the many small ore-deposits occurring along the con- tacts of granite and syenite with Silurian rocks as contact-formations ; and they should really be included in any study of the contact-metamorphism of this region." The same opinion is strongly held by Prof. Vogt. CaiLse of Contact- Metamorphism. — Petrographers in general agree that contaetmetamorphism is due to the heat of the molten magma combined with the action of the water which it contains. It is well known that during and following vol- canic eruptions, water, hydrogen sulphide, sulphur dioxide and carbon dioxide, as well as compounds of chlorine, fluorine and boron, are emitted. While some of these may result from the contact of the lavas with water and other materials, which they encounter at their eruption, it is extremely probable that a large proportion of them is derived from the magmas them- selves. || This opinion is supported by excellent geological authority — for instance, by Prof. T. C. Chamb^rlin, who says :T[ " It is a familiar fact that enormous quantities of gases are ejected from volca- noes. It has been assumed that these have a, surface-origin, and this is true in part; but, on the other hand, there is abundant ground for the belief that another notable part is brought from the interior, and is a real contribution to the earth's atmosphere and hydrosphere." This is confirmed by the well-known fact that deep-seated * F. Zirkel, Lehrbuch der Petrographie, Leipzig, 1894, ii., p. 97. t Loc. cit., p. 113. t Loc. cit, p. 115. I Die SiLurischen Etagen, 2 and 3. Kristiania, 1882, p. 369. II Braun's C'hemische Mineralogie, Leipzig, 1896, pp. 283-287. 1[ J^^V| CLAY ^^ CROSS SEAM OCCURRENCE OF A 'POCKET.' spongy and was intermixed with quartz. The clay which pene- trated the whole mass was partly red and ochreous, and partly a gray gelatinous material. In the quartz, and associated with the gold, there were acicular black crystals of pitch-blende 752 THE FORMATION OF BONANZAS IN GOLD-VEINS. (uraninite), together with uraaium ochre. This association of gold with uranium is uncommon. New Zealand. — Intersections which coincide with enrichments form a notable characteristic of the Hauraki gold-field* in the north island of New Zealand. In this district the occurrence of patches of native gold is an important feature of the regular mining operations. When I was there, in 1891, each stamp- mill had its " specimen-stamp," a single stamp working in a separate mortar, and employed solely for the treatment of speci- men-ore. These rich patches occur at the places where the " reefs " or lodes cross bands of flinty quartz. The latter are known among the miners as " flinties." They vary in thick- ness from a few inches to mere threads of chalcedonic quartz. They are barren in themselves, but have a favorable eft'ect on the gold-veins. The latter are also intersected by cross-veins, producing an enrichment similar to that caused by the " flint- ies." Fig. 3 is a sketch of one of these intersections, as seen by me in the Moanataeri mine. The lode, AB, consists of a series of small seams of quartz, conforming to the struc- tural lines of the enclosing country, which is hornblende-ande- site. The cross-vein, CD, is a band of soft gray decomposed rock, which also carries a number of small quartz-seams, but only near its crossing with the main lode, AB. The line of CD is parallel to a large fault, to be seen elsewhere in the mine-workings. The " leaders," or quartz-seams, in AB are gold-bearing, and exhibit marked enrichment at the intersec- tion with CD. The prevailing formation of this mining district is an ande- site, which is traversed by soft bands of decomposition, called " sandstone " by the miners. The latter, when penetrated by quartz-seams, are favorable to the finding of ore. The gold- occurrence is essentially sporadic and dependent upon local en- richments, such as have been described. The district is sur- rounded by thermal springs, and is near the well-known volcanic region of Tarawera, which was active in 1884. The mine- waters are heavily mineralized and very acid, so that the metal screens used in the mills are quickly corroded. Tellurides and selenides of gold have been detected in the ores ; but the pre- * It is also known as the Thames district. THE FORMATION OF BONANZAS IN GOLD-VEINS. 753 cious metal is usually found native and in coarse particles, whicli are frequently coated with native arsenic. The district is one which, I think, if thoroughly examined, would afford many suggestions regarding ore-deposition.* i'lG. 3. , '^ "> ■■ ■ - I ^ \ " —■ ' V'' \ \ N • ^'^--^mm^^i^^ • ^>"^.v:-^:v;. ^ Q ' ' 2io™B» 4 0.., jf . r. ENRICHMENT AT INTERSECTION MOANATAERI MINE, NEW ZEALAND. Concluding Remarks. It is to be hoped that the recent recognition of the agencies which bring about the formation of enrichments by surface- waters will not cause too violent a swing in the direction of a sweeping advocacy of the general efficiency of descending * The best description which has come under my notice is "Tlie Geology of the Thames Goldfield," by James Park, read before the Auckland Institute, 1894. See also "On the Eocks of the Hauraki Goldfields," by F. W. Button, Proc. Austral. Assn. Adv. Scl, 1888 ; and J. E. Don, "The Genesis of Certain Auriferous Lodes," Trans., xxvii., 584-589. 754 THE FORMATION OF BONANZAS IN GOLD-VEINS. solutions to form ore-bodies. The study of the problems of ore-occurrence has been hindered in the past by such reactions from one extreme view to its opposite. Therefore, in conclud- ing this contribution to the discussion of the results produced by descending surface-waters, I would emphasize the wider agency of ascending solutions in forming the ore-masses amid which such secondary enrichments are occasionally found. It is agreed that the sulphide-ores are primarily deposited from ascending waters ; it is also likely that such a result is repeated. A region once subjected to fracturing, which has permitted the subsequent passage of mineral-bearing solutions, is likely, at a later period, to be subjected to a repetition of these activi- ties. The geological history of many mining regions gives clear evidence of a repeated disturbance of structure. This is indicated by the existence of several systems of fractures crossing each other, the later ones dislocating the earlier. It is probable that each period was marked by mineralization, the character of which may have varied. The banded arrange- ment of the lodes of certain districts, such as Freiberg, Rico and Butte, suggests this. Enrichment may have been caused by mere addition; the introduction of other metals may have changed the average composition of the ore in the lode so that it is now extremely valuable, whereas before it may have had no economic importance ; a silver-ingredient may have been added to the gold-contents, or the addition of copper may have made a deposit doubly valuable by improving its metallurgical character. I hope the present discussion on ore-deposition will prove as inspiring to further investigation as did Posepny's paper of 1893, and that data concerning the possible secondary enrichment of sulphide-ores by the repetition of ascending solutions will be sought for. There is nothing like a working theory to sharpen the observation. Theories do not alter facts, but they often lead us to find new ones. In cordially welcoming the splendid treatise of Professor Van Hise I need make no reservation. When Posepny made clear the essential character of the upper or " vadose " water- circulation, he did us a great service; and when he combated " lateral secretion " he overthrew a very narrow interpretation of ore-formation, which was calculated to hinder seriously our progress toward the understanding of these difiicult problems. THE FORMATION OP BONANZAS IN GOLD- VEINS. 755 But Posepny was carried so far by his controversy with Sand- berger as to over-emphasize the sole agency of ascending cur- rents. At that time, in 1893, I demurred to this extreme view and said, " the word circulation is the key to the whole matter."* By this I meant that the entire underground water-circulation played a part in the formation of ore, and that to swing from one portion of that circulation to another, restricting oneself to the agency of either, would not (so it seemed to me from ex- perience in the mines) solve the problem. It does not appear to me that Professor Van Hise has erred by exaggerating any particular view of the subject. His elucida- tion of the water-circulation as a complete system is based on a broad conception of the whole matter. Of course, in indicating the work done by an agency hitherto largely overlooked, he was compelled to place some emphasis on certain neglected features of the descending portion of the water-circulation, and thus to give it some prominence in his masterly analysis. This makes the consideration of the question of secondary enrichments by surface-waters one of the most valuable parts of his treatise. Regarding this question of secondary enrichment, it is to be pointed out that all ore-deposits are " secondary," the ore as found by the miner being merely the last term of a series of solutions and precipitations through which its substance has passed in a constant shifting due to the underground water- circulation. However, the last stage of the journey is the only one of immediate importance to the miner ; and the determi- nation of the causes which brought it there is, to him, far the most interesting aspect of the general inquiry. That Mr. Em- mons should also have investigated and illuminated the problem is matter of much pleasure to a great many, engaged in mining throughout the West, to whom his geological con- tributions have seemed to possess a practical bearing and value unfortunately not always found in scientific descriptions of geological phenomena. * Trans., xxiv., 950. 48 DISCUSSION". (Presented at the Eiohmond Meeting, February, 1901.) S. F. Emmons, 'Washington, D. C. : Papers of Collins, Vogt, DeLaunay, etc. — Mr. Collins tells us about facts in the veins of Cornwall that suggest secondary sulphide-enrichment is highly interesting; and I am free to confess that I have not studied the literature of that region as fully as I should have done. !N"evertheless, even if it had been as familiar to me as it is to Mr. Collins, I should probably have hesitated to draw theoretical conclusions without having seen the mines myself; for the personal equation and the point of view of the observer play, perhaps, a larger part in the study of ore-deposits than in that of any other natural phenomena. One important pur- pose of my paper, and its publication at the time of the "Wash- ington meeting, was to cair forth remarks from other geologists upon deposits with which they were personally familiar, or to lead them to re-examine such deposits with the idea of second- ary enrichment in mind. Mr. Collins's remarks on Rio Tinto, which he has the advan- tage of personally knowing, are also interesting. With re- gard, however, to his suggestion — advanced as an apparent ar- gument against our theory — that the re-precipitation of copper from cupric sulphate solution by pyrite can hardly take place there, since it would upset the commercial process, I would remark that, while he is undoubtedly right as to the fact, it does not militate against the reduction and re-precipitation of cupric sulphate in veins ; since on the surface, as at Rio Tinto, there is free access of air, and consequently an excess of ferric sul- phate, whereas in depth the ferric sulphate would have been mostly reduced to ferrous sulphate, and (there being no excess of acid to hold it in solution) the small amount of copper in the presence of an excess of iron sulphide would be precipi- tated either as sulphide or as native copper. To Professor Vogt's analogous remarks, that in his experi- ence sulphuric acid is formed only in subordinate amount in the attack of sulphides by ferric sulphate, I would say that Dr. (756) THE GENESIS OF ORE-DEPOSITS. 757 Stokes's experiments, made in the laboratory of the TJ. S. Geo- logical Survey expressly with a view to determining the efteets of the attack of ferric sulphate on various sulphides, have conclusively demonstrated that sulphuric acid is formed in all such attacks in very considerable amount; much more than he had thought possible a -priori. It is highly gratifying that Professor Vogt has been willing to give us so fully his views on the relation between eruptive processes and ore-deposition, a subject of which he has made a most profound study. His views and those of Prof. Van Hise may be considered to express the opposite poles of geologic opinion; the extreme views of the European and American geologists respectively on this subject — though, among the lat- ter, Prof. Kemp leans more to the European side. To me it seems that a distinction may be drawn between the working geologists, to which class most of the Americans belong, and the professors in universities, which ihclude_Jiiost of our Euro- pean confrhes. The former are more apt to work out theories by practical testing in the mines themselves, while the latter are more dependent upon the literature of the subject, and therefore upon the study of phenomena at second-hand, from the . description given by others. Thus, Prof. Vogt instances the copper-mines of Butte and of Cornwall as attributable to magmatic* extraction. In the former case he very likely based his views on my early suggestion (1886) of a genetic connection between ore-deposition. and the rhyolitic eruption of the "Big Butte"; but the more detailed studies which I have mad^^incef have shown that the deposits are earlier than the rhyolitic eruption, and that the observed facts are such as to preclude pneumatolitic action as the source of the ore in its present- condition. - As regards Cornwall, Prof. Vogt's process of reasoning is that, inasmuch as many tin-deposits have proved to be the result of magmatic (pneumatolitic) processes, and as observations in Cornwall, as well as in the Erzgebirge, seem to show " that there can have been no absolutely essential difference between the * I think the use of the term "magmatic " in this connection very unfortunate. I presume he refers to the pneumatolitic method of extracting the metallic min- erals from igneous magmas. t U. & Geol. Surv., Folio 38, 1897. 758 THE GENESIS OP ORE-DEPOSITS. genesis of the cassiterite and that of the silver-lead veins," the latter are to be attributed to magmatic extraction rather than to the work of underground water. From my point of view, the reverse reasoning, namely, that underground water must have had some part in both kinds of deposition, is at least equally admissible, and more closely fits the facts of nature. Both Prof. Vogt and Prof. Beck quote in support of the magmatic theory Hussak's studies of the gold-quartz vein of Passagem in Brazil, which the latter conceives to be an ultrar acid granitic apophyse. But both Mr. Lindgren* and myself, from a careful consideration of the facts presented by Hussak, consider that he has proved it to be a normal fissure-vein, due to the action of underground waters. With regard to the probable pneumatolitic origin of contact- deposits, there is an essential agreement between Professor Vogt and Mr. Lindgren, as shown in the paper presented by the latter at the present meeting. f On the other hand, I have failed to recognize the distinction upon which both Prof. Vogt and Prof. DeLaunay lay so much stress, namely, between older and younger gold-silver veins. There can be no doubt of the great value of such inter- changes of opinion as this discussion has called forth; and it now remains for each of us, in the cases of difterence of views, to put such views to the critical test of further field-studies and see how far the respective theories are applicable to the phe- nomena of nature. It seems to me that the remarks ot Prof. DeLaunay, at the beginning of his contribution to this discussion, may lead to misconception with regard to his views upon what we consider the essential part of the " secondary-enrichment " idea, viz. : that secondary enrichment has undoubtedly, and indeed, in many cases demonstrably taken place below the groundwater- level. For that reason I take this opportunity to quote from his last article in the Revue GSnirale des Sciences, entitled " The Variations of Metalliferous Veins in Depth," in which he ex- presses himself in more definite terms. Under the caption,. "Secondary Changes of Veins in Depth," after describing the * "Metasomatic Processes in Fissure- Veins, " this volume, pp. 498-610. t ' ' The Character and Genesis of Certain Contact-Deposits, ' ' by W. Lindgren, Eichmond Meeting, February, 1901, this volume, pp. 716-733. CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. 759 two zones, above and helow the groundwater-level, and the reac- tions that may go on there, he summarizes as follows : " A body situated in this zone of permanent waters below this hydrostatic surface (which may have a very complicated form) finds itself in the condition of a wooden pile, which, remaining always immersed in water, suffers no change. On the other hand, above the hydrostatic surface (the groundwater-level) there is a perpetual movement of the waters, a bringing in of oxygen and carbonic acid, alternations of humidity and dry- ness, etc. ; it is there only that are produced the secondary re- actions of which there is question here, and by which all the upper parts of metalliferous deposits are thoroughly modified." Paper of Lindgren on '^ Contact-Deposits." — Mr. Lindgren's paper constitutes a very valuable and very practical contribu- tion to the literature of ore-deposits. It has long been my opinion that the usage which prevails among miners, of calling so great a variety of deposits " contact-deposits " is bad, because the term, as thus applied, is illogical and incapable of defini- tion ; and I have advocated its restriction to such deposits as occur along the contact of eruptive and sedimentary rocks. Mr. Lindgren's usage restricts it still further, but has the great advantage that it rests on a distinctly genetic basis. During the past summer I have had opportunities of observing, though not of studying thoroughly, several deposits which, in many respects, fall within his definition, though I should have hesi- tated in some cases to call them contact-deposits. Most of these deposits were seen in the Boundary district of British Columbia, in mines lying on either side of Bound- ary creek, near the town of Greenwood. They constitute the workable ore-bodies of many of the most important mines of the district, such as the B. C, the Knob Hill and Ironsides, the Mother Lode, and others. The ores of these mines are of very low grade, carrying on the average from 2 to 5 per cent, of copper, with a few dollars in gold per ton. They occur, however, in large bodies, and contain much lime, iron and other bases, with little sulphur, so that they can be mined and smelted at an extremely low cost. By reason of the liberal policy which the Canadian Pacific Eailroad has adopted, of building spurs to all the important mines, so as to connect 760 CHARACTER AND GENESIS OF CERTAIN CONTACT-DEPOSITS. them with the smelting-works, it is estimated that the total cost of mining aiid smelting will be not over |5 or |6 per ton. The region in which the mines occur is very well covered, either by a luxuriant forest growth or by glacial drift, often with both, so that outcrops are comparatively rare and the geological structure is correspondingly difficult to decipher. Hence, in my short visit, I was only able to determine certain very broad general outlines. The immediate valley in which the town of Greenwood lies is carved out of a mass of light grey, coarsely crys- talline granitic diorite, the longer axis of which apparently runs IT. and S. with the valley. As one ascends the tributary ravines on either side, E. or W., one passes into a zone of much altered greenish rock, called by the miners " diorite," beyond which are porphyries, forming, in general, the crests of the bounding ridges. At various points within this zone are outcrops of white crystalline limestone ; and it was soon found that the greater part of the so-called " diorite " is simply al- tered limestone, being largely composed of various normal contact-minerals, the most prominent of which, in the few specimens gathered, was actinolite. Very likely some of these altered rocks maybe of eruptive origin; as interbedded tuffs and breccias were observed at the Ironsides mine, and dikes are frequently found crossing the ore-bodies. Such of the porphyries as were examined under the microscope were found to be of the syenitic lamprophyre type. They are distinctly later than the limestone, cutting it in dikes and sending apophyses into it. The general impression derived in going through the country was that they are also later than the diorite ; but no contacts were found which would aflbrd abso- lute proof of their relative age in this respect. Compared with Mr. Lindgren's type of " contact-deposits," the ore-occurrences of this region show the following striking resemblances : 1. The association with typical contact-minerals, such as the amphiboles, garnet, vesuvianite, zoisite, etc., and the evidence that the ore-minerals were of nearly contemporaneous forma- tion. Mr. Lindgren, who has kindly examined for me, under the microscope, thin sections of ore from the Mother Lode, CHARACTER AND GENESIS OP CERTAIN CONTACT-DEPOSITS. 761 States that "they show pretty clearly that a metasomatie replacement has occurred, during which a granular limestone has been converted into amphibolitic rock, and that simultane- ously, or almost simultaneously, magnetite and sulphides have been developed." 2. The association of magnetic oxide of iron, in cdnsiderable amount, and of contemporaneous formation, with sulphides of iron and copper (more particularly the latter). This peculiar association I had never had occasion to observe until last sum- mer. 3. The irregular manner of occurrence of the ore-bodies. l^ot only does the material grade off insensibly in every direc- tion, inwards as well as outwards, from the so-called " ore " into low-grade rock, but there are no fracture-planes or walls enclosing the ore-shoots, or even defining their direction. This constitutes a very serious element of uncertainty in the mining of such deposits. 4. The ore-bodies are cut by eruptive dikes which appar- ently do not disturb or exert any metamorphic influence on the ore, and yet are not at all mineralized themselves ; so that one is puzzled to say whether the dikes are later than the ore, or the ore later than the dikes. In the B. C. mine, for instance, three such dikes lying in a nearly horizontal position, and ag- gregating some 90 ft. in thickness, have been cut in sinking a vertical shaft 250 ft. through the ore-shoot. On the other hand, the definition of a contact-deposit as in- volving a close proximity with an eruptive body cannot be regarded at present as strictly applicable to these ore-bodies. The belts of metamorphosed limestone appear to be from one to two or more miles wide ; and it is not proved, as yet, that there are considerable eruptive bodies in close proximity with the respective ore-shoots. The final settlement of this ques- tion must, however, await a detailed geological survey of the region. Another probable instance of contact-deposits is seen on the west slope of the Grampian hills, opposite the Horn-Silver mine, in Utah. Here a monzonite intrusion has broken through the dolomitic limestone ; and, along the contact, there is a zone from a quarter- to a half-mile wide on the surface (the actual thickness may of course be very much less, dependent 762 CHARACTER AND GENESIS OE CERTAIN CONTACT-DEPOSITS. on the slope of the contact), of a reddish-brown rock, made up largely of garnet, in which, associated with veins of remarka- bly beautiful fibrous white tremolite, are deposits of copper-, lead- and zinc-ores, the following of which has been found by the miners to be a very difficult and discouraging matter. I was unable to enter any of the mines, and therefore cannot speak of the manner of occurrence of the ore further than to say that it presents the peculiar association of magnetite and contact-minerals with sulphides, mentioned above. SOME PRINCIPLES CONTROLLING DEPOSITION OF ORBS. 763 Some Principles Controlling the Deposition of Ores BY C. R. VAN HISE, MADISON, WIS. ' [Concluding Contribution of Prof. "Van Hise to the Discussion of his Paper, and Others on the Same General Subject, presented at the Washington Meeting, February, 1900 (see Trans., xxx., 27, 177, 323, 424, 578) ; also of the contribu- tions of Vogt, De Launay, Beck, Lindgren, Kemp, Rickard, Bain, Keyes, Collins and Adams, presented at the Eichmond Meeting, February, 1901, and printed in the present volume. J In June, 1900, shortly after my paper was publistied in the Transactions, I made a briefer statement* before the "Western Society of Engineers covering the same ground, which, in cer- tain respects, is somewhat of an improvement. For instance, instead of using the terms descending and ascending with refer- ence to the waters resulting in the two concentrations, my modified statement is as follows : " The first concentration of many ore-deposits is the work of a relatively deep water-circulation, while the reooncentration is the result of reactions upon an earlier concentration through the agency of a relatively shallow water-circulation. Commonly the deep water circulation is lacking in free oxygen, and contains re- ducing agents, and the shallow water contains free oxygen. The deep water is therefore a reducing, and the shallow water an oxidizing agent."t Of the papers upon ore-deposits which, in vol. xxx. of the Transactions, follow my own, or which were presented at the Richmond meeting, a considerable number are wholly con- firmatory of the conclusions which I have presented. Among these are the "Washington paper of Emmons upon the Secondary Enrichment of Ore-Depositsf and his discussion at Richmond of other paper8,§ that of Weed upon the Enrichment of Gold and Silver Veins, || the discussion of Emmons', and at Rich- mond of "Weed's paper by Collinsf and Prof. De Launay,f the paper of Lindgren on Metasomatic Processes in Eissure Vein8,Tf that of Rickard upon the Formation of Bonanzas in the Upper Portions of Grold Veins,** and the remarks of Bain upon the Mississippi Valley lead- and zinc-deposits, ft It is therefore un- Jour. of OeoL, vol. viii., 1900, pp. 730-770. t Ibid., p. 765. % This volume, p. 433. ^ See under "Discussions " in this volume. II This volume, p. 473. 1[ This volume, p. 498. ** This volume, p. 734. tt See under "Discussions" in this volume. 764 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. necessary to discuss tEese papers ; but in this connection those of Messrs. Emmons and Weed are of interest, since their main purpose is to emphasize and illustrate one of the principles stated by me, which they have independently worked out and used ; namely, the principle of secondary concentration by de- scending waters, not only in: the belt above the level of ground- water, but in the sulphide belt below that level. This principle, as well as many of the others stated in my paper, I have been presenting to my students for a number of years. Messrs. Emmons and Weed working at Washington, and I at Madison, were wholly unaware that similar work was being done else- where, and that identical conclusions had been reached. If in- dependent investigation by different men leading to the same results be evidence of the truth of a conclusion, the principle of secondary enrichment by descending waters has such con- firmation. A second class of papers, and especially the admirable papers of Vogt upon the. Geology of Ore-Deposits,* of Lindgren upon the Character and Genesis of Certain Contact Deposits,t and a part of the discussion by Prof. Beck, J have apparently been in- terpreted by some as presenting views radically different from mine. Two fundamental points which Lindgren, Vogt and Beck emphasize are that the main source of the metallic ores is the igneous rocks, and that the heat of the igneous rocks has been instrumental in their production. With these posi- tions I not only agree, but definitely advocate the same ideas in my paper, as is shown by the following quotations : " The original source of much of the material for the metalliferous deposits may, indeed, be largely the centrosphere or the lower part of the llthosphere; for from these sources vast masses of volcanic rocks are injected into the zone of fracture or brought to the surface. This is especially true during great periods of vulcanism. Furthermore, it is well known that in regions of volcanic rocks many ore-deposits are found. Also it is believed that all the rocks of the lithosphere were originally igneous, and that from these igneous rocks the sedimentary rocks have been derived by the epigene forces, i.e., the forces working through the agencies of atmosphere and hydrosphere. It follows, therefore, that the metals of ore-deposits, either directly or indirectly, are derived from igneous rocks. However, the ores are directly derived from rocks in the zone of fracture by cir- culating underground waters. The rocks which furnish the metallic compounds may be intruded igneous rocks ; they may be extruded igneous rocks ; they may be the original rocks of the earth's crust; they may be sedimentary rocks de- * This volume, p. 636. f This volume, p. 716. t See under "Discussions" in this volume. SOME PRINCIPLES CONTROLLING DEPOSITION OF ORBS. 765 rived by any of the processes of erosion from primary rocks ; they may be the altered equivalents of any of these classes."* * *'* » '» X * " The nature of the rooks which contribute the .metallic salts has been much discussed. With Sandberger, I have little doubt that the metallic constituents of ores are in large part derived from the igneous rocks which have been intruded or extruded into the lithosphere ; and especially is this true of the basic rocks. Le Conte points out that the undoubted frequent occurrence of workable ore-deposits in regions of vulcanism may be explained by the heat furnished by the igneous rocks, this promoting the work of underground solutions. That the heat fur- nished by the igneous rocks is a very important factor in the production of ore- deposits, I have no doubt. Since it is very difficult to prove that the metallic content of an igneous rock is original, it is impossible, to make any general state- ment as to whether the metallic content or the heat furnished by the igneous rocks is the more important in the production of ore-deposits. It seems to me clear that both are important ; and equally clear, in many cases, that both work together. That is to say, an igneous rock may furnish all or a part of the metal which appears in an ore-deposit, and the lieat of the same igneous rock may greatly assist its concentration by the underground waters. " While the massive igneous rocks are the undoubted source of a large portion of metallic deposits, it is also equally certain that another large part is derived from the sedimentary rocks and the metamorphosed, or partly metamorphosed, igneous and sedimentary rocks. Lastly, it is also certain that many ore-deposits derive their metalliferous content in part from igneous rocks and in part from sedimentary rocks. Probably this is the mdst frequent of all cases. To give any estimate of the relative amounts of metalliferous materials derived from the original igneous rocks and from the secondary rocks is quite impossible."t Prof. Vogt holds that ore-deposits may be formed by mag- matic segregation, but that such " differentiation "-ores are " confessedly infrequent."J With this conclusion I concur in every particular, and in my classification made a place for ores of this kind: "(A) Ores of Igneous Origin."§ Although agreeing that this class exists, I do not concur in the conclu- sion that all of the ores specifically mentioned by Vogt' as be- longing to it are produced by magmatic segregation alone, without modification by the underground water-circulation. Prof. Vogt holds, however, that the ore-deposits formed by so-called eruptive after-actions are much more important than those directly produced by magmatic segregation. In this class of deposits he places cassiterite-veins, apatite-veins, and pegmatite-veins. Only the first of these groups yields a metallic product; and to the metallic ores my paper is con- fined. There is nothing in it which can be interpreted as dis- * " Some Principles," etc.. Trans., xxx., 45^6; this volume, p. 300. t This volume, pp. 346, 347. X This volume, p. 64--'. I This volume, pp. 284, 285, 428. 766 SOME PRINCIPLES CONTEOLLING DEPOSITION OF ORBS. agreeing with these conclusions of Prof. Vogt, since I refer to no tin-deposits whatever as the product of underground water, and have maintained, as will be seen just below, that the peg- matite veins are connected with igneous action. But the main contention of Prof. Vogt, and one of the prin- cipal ones of Dr. Lindgren, is that there is a large class of ore- deposits of contact-metamorphic origin. The existence of this class I have also distinctly recognized. I say : " In another place I have explained that there are gradations between different classes of rocks, and this statement applies equally well to ore-deposits. I even hold that there are gradations between ore-deposits which may be explained wholly by igneous agencies, and those which may be explained wholly by the work of underground water, or by processes of sedimentation."* Also, in my article in the Journal of Geology I say : ' ' I have elsewhere held that there is complete gradation between waters con- taining rock in solution and rock containing water in solution. If there be no sharp separation between water solutions and magma, it is probable that this is also true in reference to ore-deposits of direct igneous origin and those produced by underground water, "f The fact that I clearly recognize this class of deposits is fully appreciated by Dr. Lindgren, who quotes one of the statements above given, and also the following from my paper upon North American pre-Cambrian Geology : "It is thought highly probable that under sufficient pressure and at a high temperature there are all gradations between heated waters containing mineral material in solution and a magma containing water in solution. If this be so, then there will be all stages of gradation between true igneous injection and aqueous cementation, and all the various phases of pegmatization may thus be fully explained."! It therefore appears that, so far as the classes of ore-deposits are concerned, there is no difference of opinion between my- self and Prof. Vogt and Dr. Lindgren. "We all agree that the class of contact-deposits exists ; that the source of the ores of such deposits is largely the igneous rocks ; and that during the concentration of the ores a high temperature prevailed. The difference of opinion occurs in the interpretation of particular cases. Prof. Vogt and Dr. Lindgren, but more especially the * This volume, p. 429. f Jow- of Geol, vol. viii., 1900, p. 768. I ' ' Principles of North American Pre-Cambrian Geology, " by C. E. Van Hise. 16(A Ann. BepL U. S. Geol. Surv., Part I., p. 687 (1895). SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 767 former, hold that many ore-deposits, including sulphides, are more closely allied to the igneous rocks than to water-deposits ; while I hold that the majority of ores, and espebially those in- cluded under ." Other Contact-Deposits " by Vogt,* as shown by their character and relations, are deposited by underground waters. However, I have distinctly recognized that there may be deposits in which it is difficult to say which of these two agencies predominate. For instance, in the Journal of Geology I say: ' ' There may be ore-deposits in which water-action and magmatic differentiation have been so closely associated that one cannot say whether the resultant ore- deposit is mainly a water-deposit or mainly a magmatic deposit, "f But in the vast majority of cases I hold that there is little difficulty in discriminating between veins and dikes — ^the first representing crystallizations from water-solutions ; the second, crystallization from magma. There are few cases where the discrimination with reference to ore-deposits is not easy. While gradations between water-deposited ores and igneous ores are uncommon, gradations between the different classes of ore-de- posits formed by underground water are common. Concerning pneumatolytic action as an auxiliary in the for- mation of ores, as held by Vogt, Lindgren, Beck and Kemp, I do not deny the existence of ores of this class, but simply say that, while ore-deposits produced by this process are theoreti- cally possible, and very likely exist, I do not know of any in- stance in which it has been shown that pneumatolytic action has actually been a dominating factor in the production of a work- able ore-deposit. However, I think it not unlikely that pneu- matolytic action (in the sense of water-gas under very high pressure, above the critical temperature of water) may have ■ helped in the segregation of the metals by transporting them to the main channels of water-circulation. This condition of the water I distinctly recognize| as producible not only by igneous rocks, but also by dynamic action. But discrimina- tion should be made between what may be true and what has been shown to be true. The presence of such so-called con- tact-minerals as tourmaline and fluorite^ holding such elements * This volume, pp. 650, 651, 652, 653. t Jour, of Geol, vol. viii., 1900, p. 768. J This volume, p. 293. 768 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORBS. as boron and fluorine, is not proof that' they and the other min- erals in the veins containing them were not deposited by heated circulating waters. From the proposition that igneous rocks ar.e an important source of the ores, and that the ores are extracted from them by circulating waters, it by.no means necessarily follows that this work" is chiefly done while the rock is a fused liquid mass. After the rocks crystallize and become partly cooled, deforma- tion, and the cooling itself, may produce many fractures in them, thus furnishing channels through which the hot waters course while they are collecting the metals. In this manner is largely explained the difficulty under which Prof. Kemp labors in understanding how circulating waters may work upon hot igneous rocks.* So far as igneous rocks are deep-seated intru- sives, they may retain after crystallization a very large part of the water which they previously held. This is evidenced by the innumerable liquid inclusions in many such rocks. In this connection I may say that, among the papers pre- sented in this discussion, Lindgfen's admirable paper upon metasomatic processes in fissure-veins seems to me; wholly to confirm the view that the deposition of most metallic deposits is effected by underground water. The metasomatic changes in the rocks which Dr. Lindgren describes occur not only in the veins themselves, but in the walls of the veins. Moreover, in many cases the amount of change decreases in^passing^bm' the walls into the veins. During the metasoitfatic ehaiigfesfw&tals were added and subtracted. Lindgren ^ieclares- that, in: -the great majority of these cases, the chief agentSf through which the metasomatic changes were accomplished were eirculatihg waters. He says : ■ = " The processes observed are sucli as can only be explained by aqueous agencies. Possible exceptions are the forms of alteration connected with cassiterite, apatite, and tourmaline veins, in which pneumatolytic conditions may have partly ob- tained, "t He concludes, further,! that the waters were probably hot; that those which originally deposited the sulphide constituents were probably ascending ; but that the ascending waters are chiefly of surface-origin. Therefore, in all these matters, by * This volume, pp. 687, 688, etc. f This volume, p. 610. t This volume, p. 610. SOME PRINCIPLES CONTROLLING DEPOSITION OF ORBS. 769 his exhaustive study of the metasomatic processes in the voins, Dr. Lindgren fully cotifirms rny most fundamental contentions. It is noteworthy that Prof Yogt and Dr. Lindgren, with ad- mirable scientific restraint, notwithstanding the beliefs which they hold, discriminate clearly between the few cases in which they have shown a probability that the ores are the products of igneous action and the far more numerous cases in which the evidence of such origin is very scanty or wanting altogether. Says Prof. Vogt : "That the ore-deposits first mentioned above, viz., the titanic iron-ores in gab- bro, the chroraite-occurrences in peridotites, the nickel-pyrrhotite deposits in gabbro, etc. , were formed by magmatic extraction, I think I have scientifically proved beyond doubt ; and I believe that the magmatic-extraction theory advanced for the cassiterite- and apatite-veins is in its main proposition correct. For the ore-deposits subsequently considered, — the contact-deposits, the pyritic deposits, the gold-veins, silver-lead veins, copper-ore veins, etc., — the views liere offered become confessedly more and more hypothetical. But they have much in their favor; and even if, following in particular the French observers, I have here ascribed to magmatic-extraction too great a significance, I believe, nevertheless, that the hypothesis is worthy of thorough scientific discussion."* Thus Prof Vogt recognizes clearly that the attribution of the larger class of these ores to igneous action is purely hypothet^ ical. He fully appreciates that, of the great majority of ore- deposits, he has wholly failed to show that igneous agencies have separated the ores from the original rocks and placed them in their present positions. This connection must be made be- fore the hypothesis advanced by Prof Vogt can hope for ac- ceptance. Since the majority of the ore-deposits thought by Prof Vogt to be possibly due to " contact after-action " in some other sense than segregation by underground water diifer in no essential particulars as to their character, the minerals they contain, the relations of these minerals to one another, the re- lations of the ores and minerals to the surrounding rocks, the presence of crustification, and other features, from ore-deposits which many authorities, including Prof Vogt, recognize as de- posited by underground water, I shall hold to the old view that they are the results of water-deposition until evidence is pre- sented showing the contrary. To attempt to prove the propo- sition that these ores are deposited by water would require the * This volume, p. 658. 770 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. repetition throughout of the arguments for such an origin which have been presented during the past half-century by nearly all of the famous men who have discussed ore-deposits. If these arguments are not adequate to convince the reader, I cannot, in closing this discussion, present the case more fully, but must defer the matter until the publication of my treatise upon Metamorphism, in which I consider much more fully the circulation and work of underground water, and the character of the deposits produced by that agent. If it be recognized that, in the majority of the cases cited by Vogt and Lindgren, the materials of the ores were trans- ported and deposited in their present positions by underground waters, it makes no difference to me whether such ore-deposits be called contact-deposits, hydro-thermal deposits, dynamo- metamorphic deposits, or regional metamorphic deposits, as proposed by Lindgren.* As I have pointed out,t dynamic action may increase the temperature of the underground waters, and make the condi- tions much more favorable for the deposition of ores. If in the first part of my paper, discussing general principles, I have not made clear my belief in the extreme efficiency of hot water, as compared with that of cold water, in the segregation of ores, I have failed altogether to convey my ideas. I fully recognize the remarkable relative potency of hot water in all classes of alterations of rocks, including the deposition of ores. I em- phasize especially the effect of high temperature, (1) in pro- ducing a deeper circulation,! (2) in producing a more rapid circulation, § and (3) in very greatly increasing the power of water to do chemical work of all kinds. For instance, I say : ' ' But pure water at a higli temperature is a potent solvent. Barus has shown that water at temperatures above 185° C. attacks the silicates composing soft glass with astonishing rapidity. At 180° C. various zeolites can be dissolved in pure water, the material crystallizing out on cooling. Lemberg shows that water at 210° C. slowly dissolved anhydrous powdered silicates. It is therefore apparent that water in the lower part of the zone of fracture is a most potent chemical agent." II With this conclusion the following quotation shows that Prof. Vogt agrees : * This volume, pp. 730, 731. f This volume, 304. t Ibid., 298. § /Kd., 303-306. || Ibid., 308. SOME PRINCIPLES CONTROLLING DEPOSITION OP ORES. 771 As is well known, the ionization of water increases rapidly with its tempera- ture. Tliis explains the activity of water at high temperatures. Thus, for ex- ample, Barus has shown that water heated above 185° C. attacks the silicates composing soft glass with astonishing rapidity ; and an experiment by Lemberg has proved that water at 210° C. slowly dissolves anhydrous powdered silicates."* Further, I strongly make the point that both the speed of solution and the amount of material which may be taken into solution are greatly increased by high temperature ; f and in proof of the efficacy of hot water in the production of ore- deposits, I cite the Cordilleran region of the "Westl: as one in which the temperature of the water is higher than normal, and in which ore-deposits are common. In conclusion as to this portion of the discussion, I would say that, while I think I have given adequate weight to igneous rocks as a source of the ores, and to the resultant hot waters as an agency in their concentration, I have not elaborated that branch of the subject. The reason is, that these ideas are not new, but have been generally accepted for decades by all who have written upon ore-deposits. A complete treatise upon ore- deposits should, of course, give proportional representation to all parts of the subject; but a paper on the subject necessarily covers the new ground most fully ; and if, in addition to this, it puts new material in its proper relations and proportions to old material, that is all that can be fairl}- expected. I have reserved for separate consideration most of the points raised in Prof. Kemp's paper upon " The Role of the Igneous Eocks in the Formation of Yeins,"§ with the arguments and conclusions of which I am not in such general agreement as with those of the other papers named. I shall state the points both of agreement and of difference between us ; and, I need hardly say, with entire personal esteem and respect for Prof. Kemp. But my position is rendered somewhat embarrassing by the circumstance that this contribution closes the discussion, 80 far as it is to be published by the Institute in the special volume now in press. 1. From the frequent occurrence of ore-bodies in regions of vulcanism, it does not follow, as held by Prof. Kemp,|| that the * This volume, p. 643. t Ibid., 320, 321. % Ibid., 304, 305. ? This volume, pp. 681-709. || This volume, pp. 686, 708, 709. 49 772 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. igneous rocks are the sole source, or, in some cases, even an important source of the ores. As pointed out by Prof. Le Conte many years ago, and as shown by me in my paper (Trans., xxx., 91, 92), "the undoubted occurrence of workable ore-deposits in regions of vulcanism may be explained by the heat furnished by the igneous rocks, this promoting the work of the underground solutions." I have already emphasized the enormously increased activity of solutions with rise of tempera- ture. In the neighborhood of igneous rocks the underground solutions are hot, and these hot solutions may, and in many cases, I believe, undoubtedly do, derive a large part of their metallic material from the sedimentary or metamorphosed rocks, although, as indicated in my original paper,* I maintain that probably the ultimate source of all the ores, and very fre- quently the chief or sole immediate source, has been the igneous rocks. 2. While Prof. Kemp would derive the majority of ores from igneous rocks, he declaresf that surface-flows of such rocks are unfavorable to vein-formation. But, to give an instance to the contrary, the Lake Superior copper-deposits were shown by Pumpelly, years ago, to occur in or associated with surface volcanic rocks. I think the true statement is, that in most dis- tricts very recent volcanic flows have not had time enough for the development of contained or connected ore-deposits; or else, they have not been eroded deeply enough to expose such deposits, if they exist. But in the San Juan region of Colorado, where denudation has taken place on a vast scale in very late geological time, great ore-deposits do occur in Tertiary vol- canic rocks ; and it would be rash to say that ore-deposits are not even now forming in the middle and lower parts of the great lava-flows of the inlateaus of the West. Indeed, I think it highly probable that such formations are going on, and that at some future period the resulting ore-deposits will be at the surface. 3. In asserting the existenoe of gradations between pegma- tite and quartz-veins, I am glad to find Prof. Kemp in full accord with me. I pointed out such gradations some years ago, and, as already explained, advanced as the explanation that * This volume, pp. 300, 301. t This volume, p. 695. SOME PRINCIPLES CONTEOLLING DEPOSITION OF ORES; 773 water and rock, at sufficiently high temperatures and pressures, are miseible in all proportions. 4. From Prof. Kemp's statement* that " practically all stu- dents of volcanic phenomena are agreed that steam and its dis- sociated representatives in the molten rock are the chief, if not the only, cause of eruption," I must wholly dissent, holding with Dutton and Gilbertf that, in areas of regional vulcanism (which are those containing the most extensive ore-deposits), gravita- tive stress is the dominant force producing eruption, although it is agreed that steam plays a subordinate part, and an im- portant part in local vulcanism. 5. Perhaps I do not fully appreciate Prof. Kemp's argu- mentsj concerning capillary attraction as connected with the movements of underground water. Prof. Kemp says that the imperviousness of strata is partly due to the " feeble- ness or disappearance of capillary attraction with increase of pressure." On later pages he says : " Whenever, for example, capillary transmission occurs, the previously ac- quired head is lost, and the emerging water proceeds on its way only under a newly accumulating head." Further, he says that " capillary attraction is largely an ascensive force." I am uncertain whether or not Prof. Kemp intends to imply that I have advocated the view that capillarity is an important force which accounts for the circulation of ground- water in the belt of saturation. As a matter of fact, I have not appealed to the force of capillarity in any way what- ever to explain the circulation in this belt. It seems to me that Prof Kemp has wholly failed to recognize the great differ- ence in the nature and forces which control the circulation of water in the belt of weathering, above the level of ground- water, and the belt of cementation, below that level. § Above the level of groundwater the force of capillarity is important in the movement of ground^^ater. This matter I shall discuss fully in my treatise on Metamorphism, but cannot take up * This volume, p. 687. t " Geology of the High Plateaus of Utah," by C. E. Dutton, Bept. of U. S. Geogr. and Geol. Sun. of the Rocky Mt. Region, Washington, 1880, pp. 113-142. "Geology of the Henry Mts.," by G. K. Gilbert ; Id., pp. 66-74. "Earth-Movements," by C. B. Van Hise. Trans. Wis. Acad. Sci. Arts and Letters, vol. xi., 1898, pp. 495-502. t This volume, pp. 699, 701-2, 705, 708. ? See my remarks, this volume, p. 327. 774 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. here, as it is a complicated one. Below the level of ground- water, the size of the openings, as I have fully explained,* is of very great importance in the movement of the underground water, because friction runs up very rapidly with subdivision of the openings ; but how capillarity is a " descensive " or " as- censive " force in this belt, I am at a loss to understand. 6. Prof. Kemp does me an unintentional injustice when he cites me as supporting " the view that hot springs are the result of normal terrestrial circulations, without accessions of heat other than those which would be received through the ordinary increase of temperature with depth. "f I refer the in- crease of temperature of the underground waters to the normal increase of temperature with depth, to vulcanism, and to dynamic aetion.X Regional vulcanism and orogenic movements I men- tion twice as producing high temperature. 7. But I held, and still hold, that difference in temperature of the ascending and descending columns is a cause which works in the promotion of circulation as an adjunct to the main cause, head. Prof. Kemp§ argues against this conclusion in the following way : " ( 3 ) That water under great load or pressure does not expand according to the 4 per cent, rate named (i.e., 4 per cent, for 100° C). On the contrary, it may be held by the pressure at fixed volume, despite the added heat. If, for ex- ample, we roughly assume a column of water 1 sq. in. in cross-section and 2 ft. high (it is really about 2 ft. 3J in.) as equal to a pressure of a pound to the square inch, ill 10,000 ft. we would have a pressure of something near 5000 lbs., or over 2 tons to the square inch ; and in the face of this, the expansion of water from an added temperature of 100° C. practically becomes a negligible quantity as con- tributing to hydrostatic head." This argument seems to me to be unsound for the following reasons : a. Since I emphasize vulcanism and orogenic movements as chief causes of high temperature in underground water, the depth, and therefore the pressure, may be but a small fraction of that assumed by Prof Kemp. h. The only experiments which I have found- upon the com- pressibility of water at high temperature are those of Barus,|| * See my remarks, Trans., xxx., 40-45. f This volume, p. 704. X Trans., xxx., 49. ^ This volume, p. 704. II "The Compressibility of Colloids," by C. Barus ; Am. Journ. of Sei., 4th ser., vol. vi., 1898, pp. 287-289. SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 775 in which he finds that the compression resulting from the pres- sure cited by Kemp is much less than would be necessary to neutralize the expansion due to the heat mentioned. c. Admitting for the moment that the pressure does neutralize the expansion due to heat, since the pressure is nearly the same upon both the ascending and descending columns, and inasmuch as, under the hypothesis, there is a difference in temperature be- tween the two columns, there would be a difference in density, and therefore a cause for flowage. 8. Upon another point connected with the circulation of un- derground water. Prof. Kemp says : " (4) We must bear in mind also that the standing body of cold groundwater fills the interstices of all rocks near the surface, except those in very arid regions, and exerts a retarding influence on uprising currents."* I am entirely at a loss to understand how the coldness of the water prevents circulation due to difli'erence in head and differ- ence in temperature of the two columns, except as to an effect which I have emphasized, f namely, that due to varying viscosity. After the arguments above mentioned, Prof. Kemp says : "I regard it as extremely improbable that the water of any natural spring whose flow is due simply to hydrostatic head, has ever reached more than a very limited depth below the point of emergence."! We have already found that difference in temperature of the descending and ascending columns are excluded by Prof Kemp as an effective cause of deep circulation. The force to which he appeals to explain the deep circulation is that which proceeds from the igneous rocks. He says : "I will even go so far as to say that it is in the highest degree improbable that any waters which have reached depths even approximating 10,000 ft. can ever again reach the surface and yield flowing springs, except through the propulsion of stores of energy contributed by still heated masses of igneous rock."| I, of course, maintain that the heat-energy of the igneous rocks passes, into and thereby expands the water, thus caus- ing a difference in density between the ascending and de- scending columns,|| and thereby promoting circulation. But * This volume, p. 704. f Trans., xxx., 43. J This volume, p. 705. I This volume, p. 705. II Trans., xxx., 47-49. 776 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORBS. Prof. Kemp has just thrown this use of the energy of the igneous rocks out of consideration ; and how this energy acts in producing a circulation, unless it be by heating and thereby expanding the water, he does not explain. 9. Prof. Kemp says that " standing water in abandoned shafts is strong evidence of the impenetrability of rocks."* This seems to me untenable. Such standing water has come in either from the surface, or through the " impenetrable rocks." The latter hypothesis Prof. Kemp rejects. But if the former be true, why does not the water rise with periodic additions ? According to my view, standing water in shafts, exactly as in wells, indicates the upper limit of the belt of saturation. But the standing water maintains its uniform level (in the absence of pumping) by flowage through the rocks, compensating the local additions or subtractions. Certainly the water standing in the wells of the drift-covered regions of ISTorth America does not prove that there is no active underground circulation in the drift. Passing from specific points concerning the circulation of underground water, I, of course, largely dissent from Prof. Kemp's general view upon this subject, and can only refer to the argument already given in my paper.f If the evidence there presented, showing that the main source of the under- ground water depositing the ores is meteoric, and not de- rived from the igneous rocks, as held by Prof. Kemp (but without giving adequate evidence), does not prove the point, it is useless further to discuss the matter here. In my treatise on Metamorphism I shall cover this part of the subject much more exhaustively. While I hold that the main source of the water depositing ores is meteoric, I recognize that another source of such water is the igneous rocks. I say : "Also, through the agency of vulcanism, water occluded in magma is trans- ferred from the zone of rock flowage, or even possibly from the centrosphere, to the zone of rock-fracture. " J Nor am I able to accept Prof. Kemp's statements as to the small amount and local deficiency of groundwater. He says, " In regions when the rainfall is small," . ..." if the rocks * This volume, p. 709. f Ibid., pp. 282-334. J Ibid., p. 302. SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 777 are shattered, standing groundwater maybe entirely lacking."* And again, " The groundwater may entirely fail in arid re- gions."f I know of no region in the United States which justifies these statements. While I have never advocated a universal uniform under- ground circulation, as implied by Prof. Kemp, I have held, and still hold, that the amount of underground water and its cir- culation is much more general than he believes. This problem may be considered, first, from the point of view of the amount of underground water now present ; and second, and more im- portant, the amount of work which has been done by under- ground water. Upon the first point, it is contended by Prof. Kemp that the amount of underground water in the belt of saturation is usu- ally' small ; but in opposition to this view we have the general experience of mining men and of those who by wells seek under- ground water. While there are notable exceptions, it is in general a difficult and expensive process to lower artificially the level of groundwater which is generally found in both humid and arid regions, though at greater depth in the latter. In the lead- and zinc-districts of Missouri, this operation, called by the miners " beating the water," is usually attempted only by a number of companies, acting jointly, and constitutes the most formidable part of mining-work. . In Wisconsin, it is a grave hindrance to mining below water- level. The lowering of the groundwater by, say, 50 ft., is an exceedingly difficult task, involving enormous expenses for pumping. The subsequent holding of the water at a given level is much easier, as Prof. Kemp has noted. But my con- clusions from these facts are that, in the belt of saturation, the openings are large and the quantity of water is great, but the circulation is, in most cases, not too rapid to be controlled by pumps of moderate capacity — although, in some cases, to hold the water at a given level involves the handling of vast quan- tities of underground water. In this connection Prof. Kemp remarksj that the circulation at smaller depths than 1500 or 2000 ft. has no bearing on the question of ore-deposition. While a few ore-deposits have * This volume, p. 706. f This volume, p. 709. J This volume, p. 699. 778 SOME PRINCIPLES CONTROLLING DEPOSITION OP ORES. been profitably worked to a greater depth than this, it is well known that probably more than 90 per cent, of the metallic wealth of the earth yet mined has come from above the 2000-ft. level ; and therefore there is no warrant for the statement that the circulation above this level is not of vital importance in the production of ore-deposits. In this matter of depth, Prof. Kemp asserts,* of the general circulation of underground water, that " something like 2000 ft. appears to be its limit;" but the only evidence which he presents upon this point is that in some cases this is the fact. The local instances cited are not, to my mind, proof of such a law. On the other hand, the evidence which I have presented to the contrary is reinforced by the arguments of Prof. Vogt,t which, combining the facts of observed depth of denudation of veins with the likeness of their deeper parts to those parts nearer the surface plainly produced by underground waters, clearly lead to the conclusion that, in many cases, the under- ground circulation must have been efficient to a depth several times greater than 2000 ft. On the second point, the work of underground water. Prof. Kemp declares that veins are the exceptional, not the general, work of this agency. He says that while veins occur locally in mining districts, there is a " general absence of veins."J If Prof. Kemp means mineral veins, this is of course true ; but if he means literally what he says (and it is only with this mean- ing that any argument can be made as to the circulation of un- derground water), I wholly dissent from the conclusion. In my field-work throughout the United States and considerable areas of Canada I have yet to find a district in which a series of rocks has been in the belt of saturation for a long time geologically in which there are not extensive, metasomatic changes in the rocks, and many veins. For instance, in the Appalachian re- gion, almost innumerable veins, the work of underground water, may be seen from Maine to Alabama; but only very rarely and locally are there important ore-deposits. Therefore the localization of mineral (i. e., ore-bearing) veins gives no in- formation as to the general circulation of underground water. While I repeat that I have never advocated a universal, uni- * This volume, p. (399. f This volume, p. 669 et seq. % This volume, p. 707. SOMB PRINCIPLES CONTHOLLING DEPOSITION OF ORBS. 779 form, vigorous underground circulation, as implied by Prof. Kemp, I have held and still hold that, almost universally, in those places where ore-deposits occur, a vigorous circulation v^as going on during the time the ores were deposited ; and at the only places of which I know where ore-deposits are now form- ing", such a circulation is going on. The fact that some areas in which ore-deposits are now worked do not now show a vigor- ous circulation has no bearing upon the question of the depo- sition of these ores hj underground water. The very process of vigorous circulation by underground water results in the cementation of the openings, as I have fully explained.* In so far as the innumerable openings are filled, and during the pro- cess here and there ore-deposits are formed, just to that extent the openings are closed. When the openings have been filled to such an extent that they become subcapillary, circulation practically ceases for the time. But subsequent earth-move- ments or igneous intrusions may again produce openings in the rocks, and thus a new circulation may be set up. Of course it is. well known that in the deep copper-deposits of the Lake Superior region, and at various other localities, as at Przibrani, there is not now a vigorous underground circulation. I cannot believe that Prof. Kemp therefore dissents from the conclusion of Pumpelly, Irving, and others, that in the Lake Superior re- gion the deposits of copper in the openings of the conglomerates and amygdaloids, extending to a depth of 5000 ft. or more, are the cementation-result of circulating underground waters. Posepny realized full well that when the ore-deposits were formed at Przibram, there was a vigorous circulation of under- ground waters at a depth below 1100 meters. With regard to the formation of the deep deposits of Lake Superior, Przibram, and many other localities in all parts of the world, cited by Posepny, in some of which there is now but a feeble circula- tion, I am but a follower of Sandberger and Posepny and nearly all the eminent geologists who have written upon ore- deposits, in the belief that these ores were put in place by un- derground waters. In whatever respects I may differ from Sandberger or Posepny, there is absolute identity in our funda- mental contention that the great majority of the metallic ores, * This volume, pp. 326-334. 780 SOME PBINCIPLBS CONTROLLING DEPOSITION OP ORES. to the greatest depth penetrated by man, were deposited in the places where they now exist by circulating underground waters. Probably Prof. Kemp does not intend to argue, although his reasoning could be so interpreted, that because there is not at present a vigorous circulation where ore-deposits exist, such ores were not deposited by the circulation of underground water. Wbile it is clear that the underground circulation is much more vigorous and widespread than is believed by Prof. Kemp, and while I first discuss the circulation which would take place in a homogeneous medium,* I follow with what I regard as one of the most essential points of my paper, viz., the elaborate evidence presented of the very unequal and variable character of the underground water-circulation, due to unequal tempera^ ture caused by normal increase, vulcanism and dynamic action ; the preferential use by water of large channels ;t the variation of the rocks in porosity and structure ; the complexity and ir- regular distribution of the openings ; the intersections of frac- tures; the successions of fractures; the impervious strata at various depths; the pitching troughs and arches; etc. J Prof. Kemp speaks § also of the importance of impervious strata in influencing the circulation of groundwater. I have strongly emphasized II the very profound influence of impervious strata upon the deposition of ores, and have explained that the difffer- ence between pervious and impervious strata is that pervious strata have openings of capillary or supercapillary size, while the openings of impervious strata are subcapillary. The localization of ore-deposits, of which Prof. Kemp speaks,Tf I have shown to be due to the fact that each case of the for- mation of a deposit " requires the fortunate combination of many favorable factors working harmoniously together, the absence of any one of which may prevent the concentration of the ore-deposit."** Only here and there have existed the re- markable combination of circumstances necessary to form an oi'e-deposit, and thus once in a million times, or once in ten million times, a vein formed carries a sufficient amount of the valuable metals to be an ore.f f * This volume, pp. 306-315. f -fM., 315-317. J Ibid., 393-427. § This volume, p. 699. || Ibid., 396-416. T This volume, p. 708. ** This volume, p. 421. tt See my article, Jour, of Geol., vol. viii., 1900, p. 753. SOME PRINCIPLES OONTKOLLING DEPOSITION OF ORES. 781 Therefore, notwithstanding the contrary belief of Prof Kemp, I again assert that the deposition of the great majority of ore-deposits — namely, those produced by underground water — is a special case of the general work of underground water, which can only be adequately explained by a profound knowl- edge of the facts and principles controlling the circulation and work of underground water, and a detailed knowledge of the special modifications necessary to explain the localization and relations of the ores. In closing this discussion I must express deep gratification for the kindly and appreciative manner in which my attempt to solve some of the problems of ore-deposition has been re- ceived by the men who have discussed it. Indeed, the papers of Lindgren, Vogt, Beck and Rickard speak of the paper in a more complimentary way than I could have hoped. When I published it, I anticipated that it would be regarded as too theoretical by geologists who are at work in the field upon the fascinating problem of ore-deposition, and especially by the practical men who are in charge of the development of large mining properties. In this respect, however, I have been wholly mistaken ; for the most hearty appreciation which has come to me has been from these two classes of men. In conclusion, I can only say that I find in the various papers following my own so much which confirms my conclusions (and no reason which appears to me to be sound, advanced against any of them) that, after a careful consideration of all that has been said, I find it unnecessary to modify my paper (beyond the changes in the Journal of Geology to which I have referred) either as to statements of facts and conclusions, or as to their proportional importance. APPENDIX. Secretaey's Note. — The amount, variety and value of the contributions made to the science of ore-deposits by the Amer- ican Institute of Mining Engineers, since its organization in 1871, may be inferred from the list, given below, of papers in its Transactions bearing upon that subject. This list does not include the numerous papers on coal, or other non-metallic de- posits, such as those of clay, apatite, bauxite, etc. On the other hand, it comprises, besides many treatises directly trea1> ing of the formation of ore-deposits, and manj' generalizations of great snggestiveness as to parts of that field, a large number of studies of single mines or districts, in which geological and mineralogical facts of importance are incidentally reported, though greater space is often given to methods of mining, etc. As a whole, I venture to say, this list presents a mass of care- ful observations and intelligent deductions which no student of this science can afibrd to disregard. Papers in the Transactions of the American Institute of Mining Engineers, Bearing Directly or Indirectly upon the Science of Ore-Deposits. Title. vol. page The Geographical Distribution of Mining Districts in the United States. By E. W. Kaymond, I. 33 The Origin of Metalliferous Deposits. By T. Sierry Hunt, . I. 413 The Ore Knob Copper- Mine and some Belated Deposits. By T. Sterry Hunt, II. 123 The Formation of Fissures and the Origin of their Mineral Contents. By A. J. Brown, II. 215 The Magnetic Iron-Ores of New Jersey ; their Geographical Distribution and Geological Occurrence. By J. C. Smock, II. 314 The Ores of Iron ; their Geographical Distribution and Eela- tion to the Great Centers of the World's Iron Industries. By Henry Newton, ... ... III. 380 On the Occurrence of the Brown Hematite Deposits of the Great Valley. By Frederick Prime, Jr., HI. 410 The Cornwall Iron-Mine and some Eelated Deposits in Pennsyl- vania. By T. Sterry Hunt, IV. 319 The Southeastern Missouri Lead- District. By G. C. Broad- head, . V. 100 ( 782) SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 783 Title. vol. page A Study of the Specular and Magnetic Iron-Ores of the New Eed Sandstone in York County, Pa. By Persifor Fra- zer, Jr., V. 132 The Nickel-Ores of Orford, Quebec, Canada. By W. E. C. Eustis, VI. 209 The Mesozoio Formation in Virginia. By Oswald J. Hein- rich, VI. 227 The Eureka Lode of Eureka, Eastern Nevada. By W. S. Keyes ... VI. 344 What is a Pipe-Vein ? By R. W. Raymond, .... VI. 393 Missing Ores of Iron. By Persifor Frazer, Jr. , . . . VI. 531 The Ore Deposits of Eureka District, Eastern Nevada. By William P. Blake, VI. 554 The Antimony-Deposits of Arkansas. By Charles E. Wait, . VIII. 42 Silver Islet. By Thomas Macfarlane, VIII. 226 The Gold-Gravels of North Carolina. By W. C. Kerr, . . VIII. 462 The Silver Sandstone District of Utah. By Charles M. Eolker, IX. 21 Some Copper-Deposits of Carroll County, Maryland. By Per- sifor Frazer, . IX. 33 The Whopper Lode, Gunnison County, Colorado. By Persifor Frazer, IX. 249 Auriferous Slate-Deposits of the Southern Mining Region. By P. H. Mell, Jr., IX. 399 The Gold-Bearing Mispickel- Veins of Marmora, Ontario, Canada. By R. P. Rothwell, IX. 409 The Formation of Gold Nuggets and Placer-Deposits. By T. Egleston, . IX. 633 On the Occurrence of Lustrous Coal with Native Silver in a Vein in Porphyry in Ouray County, Colorado. By G. A. Koenig and Moritz Stockder, IX. 650 Note on Black-Band Iron-Ore in West Virginia. By S. P. Sharpless, . . X. 80 The Geology and Veins of Tombstone, Arizona. By William P. Blake, X. 334 The Gold- Fields of the Southern Portion of the Island of San Domingo. By R. P. Rothwell, . .... X. 345 The Mines and Mills of Gilpin County, Colorado. By A. N. Rogers, ... XL 29 On the Peculiar Features of the Bassick Mine. By L. E. Grabill, XL 110 Notes on the Geology and Mineralogy of San Juan County, Col- orado. By Theodore B. Gomstock, XL 165 The Iron-Ores of the Middle James River. By Persifor Frazer, ... XL 201 Coal and Iron in Alabama. By T. Sterry Hunt, ... XL 236 The Iron-Ores of the Valley of Virginia. By A. S. McCreath, XIL 17 The Copper- Deposits of the South Mountain. By C. Hanford Henderson, XII. 85 Geologico-Geographical Distribution of the Iron-Ores of the Eastern United States. By John C. Smock, . . . XII. 130 Some Canadian Iron-Ores. By Frederick P. Dewey, . . XII. 192 784 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORBS. Title. vol. page The Pyrites-Deposits of Louisa County, Virginia. By W. H. Adams, . . Xn. 527 Note on Tantalite and other Minerals, Accompanying the Tin- Ore in the Black Hills. By Charles A. Schaeffer, . . XIII. 231 The Vallecillo Mines, Mexico. By Richard E. Chism, . . XIII. 351 Geology and Mineral Resources of the Eio Grande Region in Texas and Coahuila. By E. J. Schmitz, .... XIII. 388 The Electrical Activity of Ore-Bodies. By Carl Barus, . . XIII. 417 The Iron-Mines of Putnam County, N. Y. By Arthur F. Wendt 1 . . . . XIII. 478 The Iron-Ores of Pictou County, Nova Scotia. By E. Gil- pin, Jr., XIV. -54 The Geology and Mineral Resources of Sequatchee Valley, Ten- nessee. By W. M. Bowron, XIV. 172 The Sulphide Deposit of South Iron Hill, Leadville, Colorado. By Francis T. Freeland, XIV. 181 The "Centennial" and "Lotta" Gold Properties, Coahuila, Mexico. By Persifor Frazer, XIV. 196 Note on an Exhibition of Banded Structure in a Gold-Vein. By Charles M. Eolker, XIV. 265 Notes on Certain Iron-Ore-Deposits in Colorado. By Charles M. Rolker, ... XIV. 266 Notes on the Leadville Ore-Deposits. By Charles M. Rolker. (See correction, p. 948), XIV. 273 Note on the Apatite Region of Canada. By Dr. T. Sterry Hunt, XIV. 495 The Nova Scotia Gold-Mines. By E. Gilpin, Jr., . . . XIV. 674 Geology of the Low-Moor, Virginia, Iron-Ores. By Benjamin Smith Lyman XIV. 801 Iron-Ore Deposits of Southern Utah. By William P. Blake, . XIV. 809 The Cornwall Iron-Ore Mines, Lebanon County, Pennsylvania. By E. V. D'Invilliers, . . XIV. 873 The Copper-Ores of the Southwest. By Arthur F. Wendt, . XV. 25 Notes on the Geology of the Tilly Foster Ore-Body, Putnam County, N. Y. By Ferdinand S. Eu'ttmann, . . . XV. 79 The Genesis of Certain Ore-Deposits. By S. F. Emmons, . XV. 125 The Diamond Mines of South Africa. By Gardiner F. Wil- liams, XV. 392 Sierra Mojada, Mexico. By Richard E. Chism, . . . XV. 542 Indicative Plants. By R. W. Raymond, . . . . XV. 645 Mining Developments on the Northwestern Pacific Coast and their Wider Bearing. By Amos Bowman, .... XV. 707 The Silver-Mines of Calico, California. By Waldemar Lind- gren, ... XV. 717 Gold and Silver Mining in Utah. By O. J. Hollister, . . XVI. 3 The Old Telegraph Mine, Utah. By G. Lavagnino, . . . XVL 25 Notes on the Geology of Butte, Mont. By S. F. Emmons, . XVI. 49 The Association of Minerals in the Gagnon Vein, Butte City, Montana. By Richard Pearce, XVI. 62 The Rainbow Lode, Butte City, Montana. By William P. Blake, XVI. 65 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 785 Title. vol. page The Chapin Iron-Mine, Lake Superior. By Per Larsson, . . XVI. 119 Mode of Deposition of the Iron-Ores of the Menominee Range, Michigan. By John Fulton, XVI. 525 The Bedded Ore-Deposits of Bed Mountain Mining District, Ouray County, Colorado. By G. E. Kedzie, . . . XVI. 570 Notes on the Topography and Geology of the Cerro de Pasco, Peru. By A. D. Hodges, Jr., XVI. 729 Notes on the Topography and Geology of Western North Caro- lina.— The Hiawassee Valley. By Henry E. Col ton, . XVI. 839 Hot Springs Formation in Eed Mountain District, Colorado : A Eeply to the Criticism of Mr. Emmons. By Theodore B. Comstook, XVII. 261 Notes on the Bosario Mine at San Juancito, Honduras, C. A. By Thomas H. Leggett XVII. 432 The Copper-Deposits of Copper Basin, Arizona, and their Origin. By William P. Blake, XVII. 479 Ore-Deposits of the Black Hills of Dakota. By Franklin B. Carpenter, XVII. 570 The Distribution of Phosphorus in the Ludington Mine, Iron Mountain, Michigan : A Study in Isochemic Lines. By David H. Browne, ... XVII. 616 The Ore- Deposits of Red Mountain, Ouray County, Colorado. By T. E. Schwarz, XVIIL 139 The Geology and Ore-Deposits of Iron Hill, Leadville, Colo- rado. By A. A. Blow, . . XVII I. 145 The Geological Relations of the Principal Nova Scotia Min- erals. By E. Gilpin, Jr., XVIII. 198 Notes on the Geology of the DeKaap Uold-Fields in the Trans- vaal. By W. H. Furlonge, XVIII. 334 The Association of Gold with Other Metals in the West. By Richard Pearce, XVIII. 447 Gold-Quartz. By W. M. Courtis, ... . XVIII. 639 The Iron-Ores of the United States. By T. Sterry Hunt, . . XIX. 3 Geological Notes on the Manganese Ore-Deposit of Crimora, Virginia. By Charles E. Hall, XX. 46 The Mount Morgan Mine, Queensland. By T. A. Rickard, . XX. 133 Notes on the Iron-Ores of Danville, Pennsylvania, with a De- scription of the Long- Wall Method of Mining Used in Working Them. By H. H. Stoek, XX. 369 The Bendigo Gold-Field. By T. A. Rickard, .... XX. 463 Zinc-Blende Mines and Mining near Webb City, Mo. By Carl Henrich, XXI. 3 La Gardette : The History of a French Gold-Mine. By T. A. Rickard, .... . • ■ ■ XXI. 79 Association of Apatite with Beds of Magnetite. By William P. Blake, . • . ' 2CXI. 159 A New Tin Mineral in the Black Hills. By Titus Ulke, . . XXI. 240 The Late Discovery of Large Quantities of Magnetic and Non- Magnetic Pyrites in the Croton Magnetic Iron-Mines. By W. H. Hoffman, XXI. 513 The Mesabi Iron-Range. By Horace V. Winchell, . . . XXI. 644 786 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORBS. Title. vol. page The Bendigo Gold-Field (Second Paper) : Ore-Deposits Other than Saddles. By T. A. Rickard, . . . . ■ XXI. 686 The Cause of Faulting. By John A. Church, .... XXI. 782 Notes on the Geology of the Half-Moon Mine, Pioche, Nevada. By Ernest Wiltsee, . . XXI. 867 The Biwabik Mine. By H. V. Winchell and J. T. Jones, . XXI. 951 Geological Distribution of the Useful Metals in the United States. By S. F. Emmons. (See Discussion, p. 732), . XXII. 53 The Lead- and Zinc-Deposits of the Mississippi Valley. By Walter P. Jenney. (See Discussion, p. 621), . . . XXII. 171 On a Remarkable Deposit of Wolfram-Ore in the United States. By Adolf Gurlt, . . . . XXII. 236 The Origin of the Gold-Bearing Quartz of the Bendigo Reefs, Australia. By T. A. Rickard. (See Discussion, p. 738), . XXII. 289 The Bertha Zinc-Mines at Bertha, Va. By William H. Case. (See Discussion, p. 696j, . . . ♦ . . XXII. 511 The Mineral Deposits of Southwest Wisconsin. By William P. Blake, . XXII. 558 The Genesis of Ore-Deposits. By F. Posepny. (See Discus- sion, p. 587), XXIII. 197 The Silver-Mines of Lake Valley, New Mexico. By Ellis Clark, XXIV. 138 The AUotropism of Gold. By Henry Louis, .... XXIV. 182 The Zinc Ore-Deposits of Southwestern New Mexico. By Wil- liam P. Blake, XXIV. 187 Iron-Ores of East Texas. By W. Kennedy. (See Postscript, p. 862), XXIV. 258 The Geological Structure of the Ringwood Iron Mines, New Jersey. By Frank L. Nason, XXIV. 505 Notes on the Structure of the Franklinite- and Zinc-Ore Beds of Sussex County, New Jersey. By William P. Blake, . XXIV. 521 The Ore-Deposits of Butte City. By R. G. Brown, . . . XXIV. 543 The Nickel-Mine at Lancaster Gap, Pennsylvania, and the Pyrrhotite-Deposits at Anthony's Nose, on the Hudson. ByJ. F.Kemp. (See Discussion, p. 883), . . . XXIV. 620 Lead- and Zinc-Deposits of Missouri. By Arthur Winslow. (See Discussion, p. 931), XXIV. 634 The Mines of the Chalanches, France. By T. A. Rickard, . XXIV. 689 Cinnabar in Texas. By William P. Blake, .... XXV. 68 The Tin-Deposits of Durango, Mexico. By Walter Renton In- galls. (See Discussion, p. 997), XXV. 146 The Ducktown Ore-Deposits, and the Treatment of the Duck- town Copper-Ores. By Carl Henrich, XXV. 173 Hysteromorphous Auriferous Deposits of the Tertiary and Cre- taceous Periods in New Zealand. By Henry A. Gordon, . XXV. 292 The Form of Fissure-Walls, as Affected by Sub-Fissuring and by the Flow of Rocks. By William Glenn, . . . XXV. 499 The Gold-Regions of Georgia and Alabama. By William M. Brewer, . XXV. 569 The Geological Structure of the Western Part of the Ver- million Range, Minnesota. By Henry Lloyd Smyth and J. Ralph Finlay, XXV. 595 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 787 Title. vol. page The Present Condition of Gold-Mining in the Southern Appa- lachian States. By H. B. C. Nitze and H. A. J. Wilkens. (See Discussion, p. 1016), XXV. 661 The Ore-Deposits of the Australian Broken Hill Consols Mine, Broken Hill, New South Wales. By George Smith, . . XXVI. 69 Copper-Ores in the Permian of Texas. By W. J. Schmitz. (See Discussion, p. 1051), XXVI. 97 Vein-Walls. By T. A. Eickard. (See Discussion, p. 1053), . XXVI. 193 Gold in Granite and Plutonic Books. By William P. Blake, . XXVI. 290 Sketch of a Portion of the Gunnison Gold-Belt, including the Vulcan and Mammoth Chimney-Mines. By Arthur Lakes, XXVI. 440 The Smuggler-Union Mines, Telluride, Colorado. By J. A. Porter, . . XXVI. 449 Gold in the Guyanas. By Henry G. Granger, . . . XXVI. 516 The Ore-Shoots of Cripple Creek. By Edward Skewes, . . XXVI. 553 Some Mines of Kosita and Silver Cliff, Colorado. By S. F. Emmons, . . . . . ' . . XXVI. 773 The Occurrence and Treatment of Certain Gold-Ores of Park County, Colorado. By B. Sadtler, .... XXVI. 848 The Occurrence of Gold-Ores in the Kainy Kiver District, On- tario, Canada. By William Hamilton Merritt, . . . XXVI. 853 The Enterprise Mine, Eico, Colorado. By T. A. Eickard, . XXVI. 906 The Manganese-Deposits of the Department of Panama, Colom- bia. By Eduardo J. Chibas, . ... XXVII. 63 The Geology of the Magnetites near Port Henry, N. Y., and Especially those of Mineville. By J. F. Kemp, . . XXVII. 146 The Chromite-Deposits on Port au Port Bay, Newfoundland. By George W. Maynard, . . .... XXVII. 283 The Potsdam Gold-Ores of the Black Hills of South Dakota. By Frank Clemei Smith, . XXVII. 404 Some Dike-Features of the Gogebic Iron-Eange. By C. il. Eoss. (Discussion, p. 978), . . . XXVII. .>56 The Genesis of Certain Auriferous Lodes. By John E. Don. (Discussion, p. 993), . .... XXVIL 564 The Origin and Mode of Occurrence of the Lake Superior Cop- per-Deposits. By M. E. Wadsworth, .... XXVIL 669 The Kotchkar Gold-Mines, Ural Mountains, Eussia. By H. B. C. Nitze and C. W. Purington. (Discussion, p. 844), . XXVIIL 24 Mining Districts of Colombia. By Henry G. Granger and Ed- ward B. Treville. (Discussion, p. 803 ; see also p. 591), . XXVIIL 33 Kalgoorlie, Western Australia, and its Surroundings. By George J. Bancroft. (Discussion, p. 808), .... XXVIIL 88 Notes on the Vein-Formation and Mining of Gilpin County, Colo. By Forbes Eickard, . ... XXVIIL 108 The Manganese-Ore Industry of the Caucasus. By Frank Drake. (Postscript, p. 841 ), XXVIIL 191 Emery, Chrome-Ore and Other Minerals in the Villayet of Aidin, Asia Minor. By W. F. A. Thomae, . . XXVIIL 208 Note on Limonite Pseudomorphs from Dutch Guiana. By E. W. Eaymond, . . • . • . XXVIIL 235 The Auriferous Deposits of Siberia. By Een^ de Batz, . . XXVIIL 452 50 788 SOME PEINCIPLBS CONTEOLLIN& DEPOSITION OP ORBS. Title. vol. page The Alluvial Deposits of Western Australia. By T. A. Kickard, XXVIII. 490 Notes on the Mines of the Frontino and Bolivia Company, Colombia, S. A. By Spencer Cragoe. (Discussion, p. 908; see also pp. 33, 803), XXVIII. 591 The Superficial Alteration of Western Australian Ore-Deposits. By Herbert C. Hoover, XXVIH. 758 The Platinum-Deposits of the Tura Biver-System, Ural Moun- tains, Russia. By C. W. Purington, XXIX. 3 The Occurrence, Origin and Chemical Composition of Chromite, with Especial Eeference to the North Carolina Deposits. By J. H. Pratt, XXIX. 17 The Grold-Bearing Veins of Bag Bay, near Lake of the Woods. By Peter McKellar, ... .... XXIX. 104 Notes on the Geology of Sonora, Mexico. By E. T. Dumble, . XXIX. 122 The Eich Patch Iron Tract, Virginia. By H. M. Chance, . XXIX. 210 Modern Gold-Mining in the Darien. Notes on the Reopening of the Espiritu Santo Mine at Cana. By Ernest B. Woakes, XXIX. 249 The Liberty Bell Gold-Mine, Telluride, Colorado. By Arthur Winslow, XXIX. 285 Iron-Ores of the Potsdam Formation in the Valley of Virginia. By Charles Catlett, . XXIX. 308 The Copper-Deposits of Vancouver Island. By William M. Brewer XXIX. 483 The Mines and Mill of the Atacama Mineral Company, Ltd., Taltal, Chili. By Sidney H. Loram XXIX. 488 The Copper Queen Mine, Arizona. By James Douglas. (Dis- cussion, p. 1056), XXIX. 511 The Peculiar Ore-Deposit of the East Murchison United Gold- Mine, Western Australia. By D. P. Mitchell, . . . XXIX. 556 The Manganese-Deposits of Bahia and Minas, Brazil. By John C. Branner, XXIX. 756 Some Principles Controlling the Deposition of Ores. By C. E. Van Hise XXX. 27 The Secondary Enrichment of Ore-Deposits. By S. P. Em- mons, XXX. 177 A Peculiar Clastic Dike near Ouray, Colorado, and its Asso- ciated Deposit of Silver Ore. By F. L. Eansome, . . XXX. 227 Some Notes on the Nome Gold Region of Alaska. By F. C. Schrader and Alfred H. Brooks, XXX. 236 Notes on the Gold-Mines of Zaruma, Ecuador. By J. Ealph Finlay, XXX. 248 Gold-Ores of the Black Hills, South Dakota. By H. M. Chance, .... XXX. 278 Origin and Classification of Ore-Deposits. By Charles E. Keyes, ... XXX. 323 The Clealum Iron-Ores, Washington. By George Otis Smith and Bailey Willis XXX. 356 The Cripple Creek Volcano. By T. A. Eickard, . . . XXX. 367 Geological Eelations of the Iron-Ores in the Cartersville Dis- trict, Georgia. By C. Willard Hayes, . . . XXX. 403 SOME PRINCIPLES CONTROLLING DEPOSITION OF ORES. 789 Title. vol. page The Enrichment of Gold- and Silver- Veins. By Walter Har- vey AVeed, . . XXX. 424 Types of Copper-Deposits in the Southern United States. By Walter Harvey AVeed, . . . . . XXX. 449 The Oil-Bearing Shales of the Coast of Brazil. By Prof. -John C. Branner, .... XXX. 537 Metasomatic Processes in Fissure- Veins. By Waldemar Lind- gren, XXX. 578 Notes on the Occurrence of Platinum in North America. By David T. Day, . ' XXX. 702 The Telluride-Ores of Cripple Creek and Kalgoorlie. By T. A. Bickard, XXX. 708 An Occurrence of Limburgite in the Cripple Creek District. By E. A. Stevens, XXX. 759 The Iron-Mines of Hartville, Wyoming. By H. M. Chance, . XXX. 987 The Indicator Vein, Ballarat, Australia. . By T. A. Eickard, . XXX. 1004 The Geology and Vein-Phenomena of Arizona. By Prof. Theodore B. Comstock, XXX. 1038 INDEX. Note. — The names of actual contributors to this volume are set in small capitals, preceding the references to their contributions. References to citations, whether of actual contributors or others, are preceded by the names of the authors cited in lower- case type. For mere allusions, not giving further details, the page-references are printed in brackets. Adais-s, F. D. ; remarks on Lindgren, 634. Aguilera, J. and Ordonez, E., on mineral veins of Pachuca, Mexico, 570. Alford, C. A., on geology of the Transvaal, 162. Alte Hoffnung ErbstoUen silver-mine, Miteveida, Saxony, character of ground-water at, 29. Alterations; country-rock, 660; metasomatic, classified, 660; of rocks forming greisen, mica-rock, cassiterite-rock, etc., 660 et seq., 668. Altered rocks from pold-quartz veins, analyses of, 587. Aluminum, proportions of, in the earth's crust, 639. Alum-stone, formation of, 661, 669. Ammerberg, Sweden, character of zinc-blende deposits at, 141 et siq. Amphibole in Bunker Hill and Sullivan lead-silver mine, Idaho, 530; in copper-veins of Eossland, British Columbia, 530. Analyses: altered rocks from gold-quartz veins, 587; brick, altered, 609; Caliche, 712; Clausthal (Germany) rocks, 603; Comstoek clays, 568; country-rock near Himmelfahrt mine, Freiberg, Saxony, 529 ; filling of fissure-veins, 37; fresh and altered rocks from gold-quartz veins, 589 ; gas liberated from warm mineral water, 3") ; gneiss near Himmelfahrt mine, Freiberg, Saxony, 581, 582 ; labra- dorite, etc, 662; metasomatic rocks from gold-quartz veins, 586; mineral waters, 42 ; opaline silica, 575 ; propylitic andesite, 566 ; tin-deposits of Alten- berg and Zinnwald, Saxony, 542; waters encountered in mines, 42 ; well-waters, Tucson, Ariz., 714 ; silicified diorite, 593. Andreasberg, Hartz mountains, silver-ore veins of, 81. Anthon, E. F., investigations in sulphide reactions, 467. Apatite- veins of Canada, 635; in Norway and Sweden, 646. Argon, proportions of, in the earth's crust, 639. Arizona: copper-deposits, 110, 131; Bisbee, 447; Clifton-Morenci, 447; Globe, 447: copper-mines: United Verde, 447; Hillside mine, mineral veins of, 213. Arrhenius, Svante, on the physics of vulcanism, 638 ; on chemical and physical action of water upon magma, 643, 644. Arsenic, proportions of, in the earth's crust, 639 ; in mineral waters, 47. Ascending waters : carbonic acid the most important geological factor, 44 ; encoun- tered in mines, 28; heat of, 31 et seq. ; hydrogen sulphide in, 44. Association of copper and iron compounds, 364 ; of lead, zinc and iron compounds, 357 ; of silver and gold with base metals, 368. Australasia, gold-deposits of, 161, 221 et seq. Australia : copper mines : New South Wales ; Broken Hill Consols, 460, 461. Australian Broken Hill Consols silver-mine. New South Wales, 490. Bain, H. Foster : remarks on Van Hise, 622 ; on Emmons and Weed, 627. Banat ; mines of, 718. 792 INDEX. " Banded structure " and " crustification," 204, 240, 260. Bauxites, formation of, 661. Barium, proportions of, in tlie earth's crust, 639. Barus, C, on the action of hot water on soft glass [643], 308. Barysphere: does it exist within reach of circulating waters? 271; the source at unknown depths of ore-deposits, 11, 73, 79, 200, 242, 264, 271. Beaumont, Elie de, theory of "pentagonal symmetry," 8. Beck, Peof. E., on depth of ore-deposits, 671 ; on the tiu-ore deposits from Banca and 5illiton, 543 ; on the tin-ores of Etta Knob, Black Hills, S. Dak., 643 (foot- note) ; Eemarks on Van Hise, 613 ; on Emmons and Weed, 615 ; on Lindgren, 616. Beckee, a. F. : remarks in discussion of the genesis of ore-deposits, 204. Becker, G. F. : analysis of filling of fissure-veins, 37 ; on the Apollo mine, Unga Island, Alaska, 572; on the country-rock of the Treadwell mine, Alaska, 593; on the geology of the Comstock lode, 566 ; on the quicksilver-deposits of the Pacific Coast, 595 ; on underground temperature, 31. Belcher silver-mine, Storey county, Nev., location of bonanza, 93. Bell, E., on ore-deposits of Sudbury, Canada, 146. Beryllium, proportions of, in the earth's crust, 639. Bescheert Gliick silver-mine, Freiberg, Saxony, character of deposit, 78. Big Seven silver-lead mine, Neihart, Mont., 484. Biotite replacing hornblende and feldspars, in tourmaline veins. Meadow Lake, Cal., 530 ; in gold-copper veins of Eossland, British Columbia, 530. Biotite replacing quartz in Bunker Hill and Sullivan mine, Idaho, 530; secondary, in Ocean Wave mine. Cripple Creek, Colo., 530. Bisbee copper-deposits, Cochise county, Ariz. [446]. Eischof, G. : arsenic in mineral waters found by, 47; on kaolinizatiou [661]; on metallic deposits from sea-water, 121. Black Hills of Dakota : observations in, in connection with contact-deposits, 731. Black Jack copper-silver mines, Florida Mountain, Idaho, 573. Blake, W. P. : remarks in discussion of the genesis of ore-deposits, 188. Blake's account of ore-deposits in Copper Basin,. Ariz., 132. Blue Wing copper-mine, Virgilina district, Va. and N. C, 454. Blue Wing iron-mine. Person county, N. C. [483]. . Blum on pseudomorphs, 501. Bobierre, A., amount of salt in rain-water found by, 43. Bohemia: ancient gold-workings in Trautenau region, 163; copper-sandstones of, 126; kupferschiefer of, 124; ore-deposits at Przibram, 56, 61, 76, 83, 2.39 et seq.; temperature of thermal springs, 40; tin-placers at Flatten, 1,59. Bohneisenerz deposits, 136. Bonanzas at Andreasberg, Germany [679] ; at Kongsberg, Norway [679] ; at Schem- nitz, Hungary [679] ; formation of, in the upper nortions of gold-veins, 734 et seq.; in Transylvanian gold-veins [679]. Boron, proportions of, in the earth's crust, 639. Botesiu gold-field, Dacian district, Transylvania, 89. Brick, altered, analysis of, 609. Brogger, W. C, on genesis of pegmatite- veins, 732 ; on pegmatite- veins in Norway, 647. Broken Hill Consols silver-mine. New South Wales, Australia, 460, 461. Bromine, proportions of, in the earth's crust, 639. Brown, E. G., on the Butte, Mont., copper-deposits, 384. Buena Vista gold-mine, Cripple Creek district, Colo., 290. Bunker Hill and Sullivan silver-lead mine, Coeur d'Alene district, Idaho, 601. Calamine-deposits at Eaibl, Carinthia, 134. Calcium, proportions of, in the earth's crust, 6.39. INDEX. 793 Caledonia gold-mine, Thames district, New Zealand, richness of ore, 222. Caliche of Southern Arizona : An Example of Deposition by the Vadose Circulation (Blake), 710. California, character of copper-ores in peninsula of Lower, 132; silver-mine, Storey county, Nov., location of bonanza, 92. Canada, apatite-deposits of, 635 ; ore-deposits of Sudbury, 145, 210 ; platinum-metals at Sudbury, 640. Canada Hill vein. Grass Valley, Cal., 342. Carbon, proportions of, in the earth's crust, 639. Carbonatization of rocks (with dolomitization, etc,), 661 et seg.,668. Carbonic acid an important geological factor in ascending waters, 44. Carinthia: analogy of Bleiberg ore-deposits with those of Eureka district, Nev., 112; calamine-deposits of Eaibl, 134 ; ore-deposits at Eaibl, 61, 69, 102. Carlsbad Sprudel : analysis and temperature of water, 40, 42 ; character of deposit, 53. Carniola, Bohneiseners at Woehein, 137. Cassiterite: synthetic production of, by sublimation [637]; and iron-ore, occurrence of in limestone near Campiglia, Italy [651] ; cassiterite-veins, 614; in Banca and Billitou [645] ; in Cornwall, England [645] ; in Tasmania [645]. Cause of flowage of underground water, 302. Cavities in rocks, character of filling, 15. Cazin, F. M. F. : remarks in discussion of the genesis of ore-deposits, 206, 269 ; on copper-ores of New Mexico, 131. Cephalonia, water-power of the sea-mills of, 304. Cerium, proportions of, in the earth's crust, 639. Chamberlin, T. C, on ore-deposits of Southwestern Wisconsin [301] ; opinion on cause of contact-metamorphism, 727. Chemical constitution of mineral waters, 40. Chlorine, proportions of, in the earth's crust, 639. Chlorite in gold-quartz of Crown Point mine. Grass Valley, Cal., 530. Church, John A. : remarks in discussion of the genesis of ore-deposits, 195. Church's account of the Justice silver-mine, 239 ; views on source of underground waters, 31. Churprinz silver-mine, Freiberg, ,Saxony, ascending mineral waters at, 29. Cinnabar-deposits of Sulphur Bank, Cal., 32 et seq., 66. Circulating waters of surface-origin, 435. Clarke, F. W. and Hillebrand, W. F., on the occurrence of elementary substances in the earth's crust [639]. Classification of fissure-veins according to metasomatic processes, 540 et seq. ; of ore- deposits, 427; proposed modification of, 211 et seq., 226; systems employed hith- erto, 3. Clansthal (Germany) rocks, analyses of, 603; Hartz mountains, ore-deposits, 79. Clays of Comstock lode, analysis of, 568. Clifibrd amalgamated copper-mine, Cornwall, 62. Clifton-Morenci copper-deposits, Graham county, Ariz., 447. Cobalt, proportions of, in the earth's crust, 639. Cohen, E., on Witwatersrand gold-deposits, South Africa, 162. Collins, Aethuk L. : remarks on Emmons and Weed, 620 ; discussion by Emmons, 756 et seq. Colorado : gold- and silver-mines, 290, 342, 396. Colorado : lead-zinc mines : Aspen district : MoUie Gibson, 492 ; Smuggler, 492 ; ore- deposits of Leadville, 106; Bed Mountain district, Ouray county, character of ore-deposits, 109: ailver-lead-mines [351] : silver-mines: Custer county ; Geyser, 461; MoUie Gibson, 450; Pandora, 451; Smuggler-Union, 451; Smuggler, 450; Yankee Girl, 451. Comstock Lode, Storey county, Nev. . geological conditions of, 90; heat of ascending waters at, 31 ; ore-deposition of. 195, 239. 794 INDEX. Concentration by reaction upon salphides compared with metallurgical concentra- tion, 376. Concentration of ores : special factors affecting, 393 ; character of the topography, 416 ; character of the topography — effect of the vertical element, 416 ; character of the topography — effect of the horizontal element, 417; physical revolutions, 419; variations in porosity and structure, 393; variations in porosity, etc., — com- plexity of openings, 394; variations in porosity, etc., — impervious strata at vari- ous depths, 396 ; variations in porosity, etc., — pitching troughs and arches, 405 variations in porosity, etc., — pre-existing channels and replacements, 413. Condition of water in the zone of fracture, 291. Conical mounds built by ascending mineral waters, 39. Consolidated Virginia silver-mine, Storey county, Nev., location of bonanza, 92. Contact-Deposits: Constituent Minerals, 717 ; Form, 717 ; Genetic classification : "con- tact metamorphic " deposits, 730 ; Dynamo-metamorphie and regional-metamor- phic deposits, 731 ; hydrothermal deposits, 730 ; Geographic Distribution : Arizona, 723; British Columbia, 723; California, 720; Idaho, 721, 722 et seq. ; Mexico, 724, 725 ; Northwest Territory, 723 et seq. ; Other Countries, 725 : Literature : 718 et seq. ; Origin of the Deposits, 725, 726; Position, 717. Contact-metamorphism : 691, 726, 727 ; Cause of Contact- Metamorphism, 727, 728, 729; intense, of rocks, 661 el seq., 669; ore-deposits formed by, 648. Copper-deposits: Arizona: Cochise county; Bisbee, 447; Gila county; Globe, 447; Graham county ; Clifton-Morenci, 447. Copper-mines: Arizona: Cochise county; Copper Queen [446]; Yavapai county; United "Verde, 447 ; Colorado: Boulder county; Orphan Boy, 738; New Mexico: Grant county ; Santa Eita, 449 ; Grant county ; Hanover, 450 ; North Carolina : Eowan county; Union Copper Company's, 454 ; North Carolina anil Virginia : Vir- giliua district; Blue Wing, 454. Copper-ore deposits in Butte, Mont. [657] ; in Cornwall, England [657]. Copper-ores: Arizona, 110, 131; Bohemia, 124, 126; Lake Superior district, 144; Lower California, 132 ; Mannsfeld, 124 ; Mexico : Chihuahua, 4.58 ; New Mexico, 110, 131, 210; Prettau, Tyrol, 143; St. Avoid, 127; Sweden, 140; South Ameeica; Peru: Cerro de Pasco mining district, 458: Vermont, 207; West- phalia, 125. Copper Queen copper-mine, Bisbee, Cochise county, Ariz. [446]. Copper-silver mines : Idaho : Florida Mountain ; Black Jack, 573 ; Trade Dollar, 573. Copper-sandstone of Bohemia, 126. Cordilleran region of the Western U. S., 304. Cornwall (England) copper veins of, 620; tin-deposits of, 1.39. Coronado copper-mines, 449. Cotta, B. von : classification of ore-deposits by, 4 ; on ore-deposition, 124 et seq. ; mines of the Banat, in Austria, 718. Cotta, B. von and Swess, Edward, on the iron-ore deposits in the Banat. Hungary [648] (footnote). Country-rock near Himmelfahrt mine, Freiberg, Saxony, analysis, by Dr. H. Sehulze, 529. Critical temperatures of water, etc., 659. Crosby, W. F. and W. O., on the sea-mills of Cephalonia, 304. Cross, Whitman, and Penrose, E. A. F., on mineral deposits of the Cripple Creek dis- trict, Colo., 574. "Crustification," 12, 198, 204, 240, 260, 277. Crystalline schists, ore-deposits in, 137. Cumberland, England, iron-ore deposits, 136. Dacian gold-fields, Transylvania, 86, 97. Daintree, experiment on crystallization of gold, 743. INDEX. 795 Dakota, gold-deposits of Black Hills, 160. Dalmer, K,, on " bed-impregnatious " at Schwarzenberg, 650. Daubree, Prof. A. : on alterations produced by mineral springs, 48 et seg. ; on causes of striae in lode-walls, 213; on tbe kaolin-deposits of Cornwall, Prance and Ger- many, 661; on subterranean water-circulation, 18 ei aeq. De Launay, Pkop. L. C. : remarks on Emmons and Weed, 616, De Launay on chemical theory of ore-deposits, 8, 63 ; on metalliferous deposits, [476], 389, 455, 672 ; views on "secondary enrichment," 758; on the relation of secondary reactions to the ground water-level, 759. Deposition by yadose circulation. Caliche of Southern Arizona an example of, 710 et seq. Deposition of ores, some principles controlling the, 282 ; of oxides below water-level, 438 ; of sulphides, 438. Detrital deposits, 152. Devereiix, W. B., on occurrence of gold in Black Hills, Dak., 160. Devon Consols copper-mine, Cornwall, 621. Discission, spaces of, 13, 74. Dissolution, spaces of, 13, 95. Distribution of elementary substances in the earth's crust, 639. Doelter, Dr. C, synthetic experiments in sulphide reactions, 469. Dolcoath gold-mine, near Elkhorn, Mont. [496]. Douglas, James, on the Butte, Mont., copper-deposits, 383 ; on the Copper Queen mine, Bisbee, Ariz. [446]. Ducktown, Tenn., copper-deposits, 383, 715. Dux coal-mine (lignite), Bohemia, irruption of thermal waters at, 30. East Wheal Lovell tin-mine, Cornwall, Eng., eliaracter of ore-body, 139. Einigkeit silver-mine, Joachimsthal, Saxony, ascending mineral springs at, 30. Elba: iron-ore deposits [648]. Emmons, S. H., on decomposition of metals, 149. Emmons, 8. F. : The Secondary Enrichment of Ore-Deposits, 433 ; remarks in discussion of Collins, Vogt, De Launay, etc., 756 et seq. ; of Lindgren on " Contact-Depos- its," 759 ; of Posepny, 199. Emmons, S. F., on the Butte, Mont., copper-deposits, 384 ; on descending waters, 107; on the flssure-vein of Queen of the West lead-silver mine, Ten Mile district, Colo., 597; on the Leadville, Colo., silver-deposits, 388; on the mines of Custer county, Colo., 351. Emmons, S. F., and Whitman Cross, on propylitic deposits of Silver Cliff and Kosita Hills, Colo., 572. Emmons, S. F., W. H. Weed and Tower, on copper-veins of Butte, Mont., 596. England ; iron-ores of Cumberland, 136; lead-mining in north of, 104. Enrichment of Gold- and Silver-Veins (Weed), 473. Enrichment of gold-veins near the surface: by concentration, 736; by descending waters, 737; by precipitants, 742 ; by solution, 739; by solution and precipita- tion, 744. Enterprise gold- and silver-mine, Eico, Dolores county, Colo., 290, 342, 396. Epigenetic deposits, differences of depth in the original positions of, and secondary alteration of deposits, 669, 671. Erbsenatein, deposits of, at Carlsbad, 53. Eruptions, sequence of, 689. Eruptive after-actions, ore-deposits formed by, 643; cassiterite- veins and apatite- veins, 645, 646; pegmatite-veins, 647. Eruptive processes, relation of, to the formation of ore-deposits, 641. Eruptive rocks: deposits in, 137 ; influence on ore-deposition, 191 ; relation of thermal springs to, 221. 796 INDEX. Erzgebirge, mineral springs in mines of, 30 ; ore-deposits, 82 ; veins of the, 614, 615. Etta Knob, 643 (footnote) ; tin-ore deposits, Blaclt Hills, S. Dak., 643 (footnote). Eureka district, Nev., geological conditions of, 112. Eva May silver-lead mine, Boulder county, Mont., 495 ; analysis of ore, 495. Factors influencing depth at which rock-flowage occurs, 287. Faribault, E. E., on the gold measures of Nova Scotia, 412. Filons stanniferes, 665. Mlons sulfurh dites plombifires, 665. Fissures : filling, analysis of, 37 ; of dislocation, 14 ; ore-deposits in, 74 ; theories of origin of, 212. Fissure-veins, metasomatic processes in, 498 et seq. Florence silver-lead mine, Neihart, Mont., 484. Flowage: factors influencing depth of occurrence, 287; of underground water, cause of, 302 ; of water in capillary openings, Poiseuille's law, 297 ; zone of, 286. Flow of rocks, experiments in, by F. D. Adams, McGill University [287]. Fluorine, proportions of, in the earth's crust, 639. Fluorite in Independence mine. Cripple Creek, Colo., 524. Forchhammer on kaolinization [661]. Formation of Bonansas in the Upper Portions of Gold-Veins (Eickard), 734. Foster, C. Le Neve, on tin-deposits of Cornwall, 139. FouUon, A. B. von, on ore-deposits of Sudbury, Canada, 145, 146. Fowry Consols copper-mine, Cornwall, 620. Fracture ; condition of water in zone of, 291 ; division of zone of, into a belt of weathering and a belt of cementation, 327 ; ore-deposits derived from zone of, 300 ; zone of, 286 ; zone of, material for ore-deposits derived from rocks in, 302. Fracture and flowage, zone of combined, 288. France : Fontainebleau sandstone, 119; iron-ore deposits of Dielette [648]. Freiberg, Saxony, ore-deposits, 78, 615, 616. Freiberg, Bavarian Upper Palatinate, lead-deposits, 129. Fresh and altered rocks from gold-quartz veins, analysis of, 589. Frohner silver-lead mine, near Helena, Mont., 496 ; analysis of ore, 496. Fuchs, E., on copper-ores of lower California, 132. Furman, H. van F., on mines near Mapimi, Durango, Mex. [445]. Gage, J. E., on occurrence of lead- and zinc-ores in Missouri, 115. Galena: replacing calcite in Elkhoru mine, Mont., 537; tree-stems changed to, at Vesuvius lead-mines, Bavaria, 129. Garnet in gold-quartz veins of Broken Hill, New South Wales, 531. Gas, analysis of, liberated from warm mineral water, 35. Geodes, opal and chalcedony, mineral deposits in, 24. Germany : copper ores, 124 et seq. ; lead-deposits of Mecheruich, near Commern, 127; iron-ore deposits of Alsace, 136 ; tin-placers at Annaberg, Saxony, 159. Geyser silver-mine. Silver Cliff', Custer County, Colo., 461. Gilbert, G. K., on the Cordilleran region of the Western U. S., 304. Globe copper-deposits, Gila county, Ariz., 447. Gneiss near Himmelfahrt mine, Freiberg, Saxony, analysis of, 581, 582. Gold: ancient placer-mining in Bohemia, 163; occurrence of, iu Gympie district^ Queensland, 744; in manganese spar, 67; at Eico, Colo., 744. Gold Hill silver-mine, Storey county, Nov., high temperature of water in, 31. Gold in pyrite of Orphan Boy copper-mine, Boulder county, Colo., 738. Gold-mines: Arizona: Pinal county; Mammoth, salt iu water of, 741; California: Calaveras county: Eathgeb, 725, 749, 750; Colorado: Cripple Creek; Moon Anchor, 747, 748 ; Cripple Creek district ; Buena Vista, 290 ; Lee, 290 ; Smuggler, INDEX. 797 290 ; Victor, 290 ; Foreign Countries : Australia : Bright district ; Myrtle, 746,747; Bright district; Shouldn't Wonder, 746; Western Australia : Kalgoorlie; Great Boulder Main Eeef, 742; Kalgoorlie; Great Boulder Proprietary, salt in water of, 740; Kunanalling ; Sugar Loaf, 742; New Zealand: Thames district; Moanataeri, 7o2; Montana: Elkhorn ; Dolcoath [496]; Elkhorn ; Mayflower [496]. (Note. The numerous passing references to gold-mines, occurring in Prof. Posepay's paper, have not been separately indexed.) Gold-ores: deposits of Black Hills, S. Dak., 160; Beresov, Ural Mountains, 76; Hun- gary, 85 ; of Australasia, 161, 221 et seq. ; in the Ural mountains, 76, 153 ; New Zealand : Hauraki or Thames gold-field, 221 ; Otago gold-field, 224 ; South Africa: Witwatersrand, 162 ; Sweden; Falun, 141; Transylvania, Dacian gold-field, 86; Verespatak, Transylvania, 66, 87. Gold- and silver-mines: Colorado: Dolores county; Enterprise, -iUO, ,342, 396. Nevada: see Comstock Lode. Gold and silver in eruptive magmas, 641. Gold and silver veins, enrichment of, 473 et seq. Gold quartz veins, 614. Gold-veins : enrichment of, near the surface by concentration, 736 ; by descending waters, 737, 738; by solution, 739; by solution and precipitation, 744; formation of bonanzas in the upper portions of, et seq., 734. Gottes Geschick silver-mine, Swarzenberg, Saxony, ascending waters at, 29. Great Boulder Main Eeef gold-mine, Kalgoorlie, W. Australia, 742 ; salt in water of, 740. Great Extended Hustlers gold-mine, Victoria, Australia, character of quartz at, 215. Greece, ore-deposits of Laurium, 135, 246. Grimm, J.: classification of ore-deposits by, 4; examination of Transylvania ore-de- posits by, 96. Groddeck, A. v. ; mines of Banat, 718 ; on ore-deposition, 122 et seq. ; on the vein-sys- tem of Clausthal, Germany, 602; system of classification of ore-deposits, 4. Groundwater, 695 ; artesian basins, 701 ; common conception of, 695 ; experience in deep mines and wells, 698 ; hot-springs, 702 ; irregular distribution of, near the surface, 705; level, 436; movements of, 19. Guiterman, F., on the gold-deposits of Battle Mountain, Colo., 740. Gypsum : origin of some beds, 715. Hale and Norcross silver-mine, Comstock Lode, Nev., flooding of,, by water, 31. Hanover copper-mines. Grant county, N. M., 450. Hartz mountains, ore-deposits, 79. Hauraki or Thames gold-field. New Zealand, 221. Helena and Frisco silver-lead mine, Cceur d'Alene district, Idaho, 600. Helmhacker, E., on ore-deposits of Altai region, Siberia, 155. Highland Boy copper-mine, 450. Hills, E. C, on the ore-deposits of Summit district, Eio Grande county, Colo., 584. Hillside gold- and silver-mine, Ariz., mineral veins of, 213. Hintze, C, on kaolin [664]. Holroyd's (A. G.), collection of specimens from gold-district of West Australia, 677 et seq. Hot mineral waters encountered in mines, 31. Hungary; geode of iron opal from Dreiwasser, 24; iron-ore deposits : in the Banat [648] ; iron-ore deposits : Vask6 [648] : ore-deposits of, 85 et seq. : silver-deposits of Eezbanya, 99. Hunt, T. Sterry, on the copper-deposits at Ore-Knob, N. C, and in Carroll county, Va., 454; on pseudomorphs, 502. Hnssak, E., on auriferous pyritic quartz-bed-veiu at Passagem, Brazil [657] ; on the auriferous quartz- vein of Passagem, Minas Geraes, Brazil, 546. 798 INDEX. Hutton, F. W., on the auriferous veins of Hauiaki gold-fields, Thames district, New Zealand, 571. Hydrogen, proportions of, in the earth's crust, 639; sulphide, important part of, In ascending waters, 44. Hysterogenites, 16, 17. Hysteromorphous ore-deposits, 147. Idiogenous mineral deposits, 10. Igneous rocks: competence of, to supply vein-material, 682; r61e of, in the formation of veins, 680. Illustrations of secondary enrichment and diminution of richness with depth, 383. Independence gold-mine, Cripple Creek, Colo., salt in water of, 741. Iodine, proportions of, in the earth's crust, 639. Iron, proportions of, in the earth's crust, 639. Iron-mines: North Carolina: Person county ; Blue Wing [483]. Iron opal, geode of, from Dreiwasser, Hungary, 24. Iron-ore : deposits at Vasko, Hungary [648] ; of Dielette, France [648] ; of Kristi- ania region, Norway [648], 649 ; in the Banat, Hungary [648] ; in the island of Elba [648] ; increase of, in depth in manganese-deposits, 675. Iron-ores : analogy of Bohemian and Michigan, 237 ; deposition of, in Mesabi range, Minn., 229 ; England, Cumberland, 136 ; Germany, Alsace, 136 ; Lake Superior deposits, 228 ; Norway, 138 ; Spain, Eio Tinto (limonite), 148 ; Sweden, Taberg (magnetite), 138; Switzerland, Carniola, 136; Tyrol, 143. Irving, Prof. E. D., on the copper-bearing rocks of Lake Superior, 607. Jacquet, Mr., on secondary sulphides at Broken Hill lode, N. So. Wales, Austra- lia, 459. Jenney, W. P., on the lead- and zinc-deposits of the Mississippi, 452. Joachimsthal mines, Bohemia, mineral waters of, 30. Joplin lead- and zinc-district. Mo., 623. Justice silver-mine, Storey county, Nev., character of ore-deposit, 92, 198, 239. Kackar district, southern Ural, Kussia, ore-deposits of, 157. Kahleuberg, L., and Lincoln, A. T., on solutions of silicates, 319. Kaolin-deposit at Ekersund-Soggendal, Norway, 662. Kaolinite in vein near Boulder, Mont., 534 ; in piopylibic veins at Cripple Creek, Colo., 534 ; in veins at Delamar, Idaho, 534; in veins of Summit district, Colo., 534 ; and sericite in veins of the pyritic galena-formation of Freiberg, Saxony, 534. Kaoliuization of rocks, 660 et seq., 668. Katrontza gold-mine, Verespatak, Transylvania, character of ore at, 66, 67. Kemp, J. F. : The B6le of the Igneous Bocks in the Formation of Veins, 680. Kendall, J. D., on iron-ores of Cumberland, Eng., 136. Kerr, W. C, on North Carolina gold-deposits, 153. Keweenaw Point, Lake Superior region, copper-bearing rocks of, 344. Keyes, Charles E. : remarks on Van Hise, 628; on Lindgren, 630. Kjerulf, Th. : on the iron-ore deposits of the Kristiania region, Norway [648] (foot- note) ; on iron-ore deposits in Norway, 138. Kongsberg silver-mines, Norway, 468. Kupferschiefer of Mannsfeld, Thuringia and Bohemia, 123, 124. Labradorite, etc., analyses of, 662. Lacroix, A., on axinitization of contact-metamorphic zones in the Pyrenees [650]. Lake Superior region : copper-deposits, 144 ; iron-ores of, 227. Lanthanum, proportions of, in the earth's crust, 639. INDEX. 799 Lapparent, A. de, on metamorphism, 118. Lateral-secretion theory of ore-deposition, 57 et seq., 190, 200, 236, 242, 254, 257, 271. Laurium, Greece, ore-deposits, 135, 246. Leaching: of wall-rock, 277 ; in the zone of weathering, 476 et seq. Lead-ores: Freihung, Bavarian Upper Palatinate, 129; Missouri and Wisconsin, 115; Mechernioh, near Commern, Germany, 127; North of England, 105. Leadville, Colo., ore-deposits, 106. Lead-silver mines: Colorado : Ten Mile district ; Qneeu of the West, 597. Lead-zinc mines: Colorado : Aspen district; MoUie Gihson, 492; Smuggler, 492. Le Conte, PEor. Joseph : on genesis of ore-deposits, 270 ; on mineral vein-forma- tion, 33. Lee gold-mine. Cripple Creek district, Colo., 290. Lemberg's experiment in the solution of anhydrous powdered silicates by boiling water [643]. Limestone, silicified. Diadem lode, Plumas county, Cal., 521. LiNDGEEN, Waldemar : Metasomatic Processes in Fissure- Veins, 498 ; The Character and Genesis of Certain Contact-Deposits, 716. Lindgren, W., on contact-deposits, 759 ; on fissure-veins of the Trade Dollar and Black Jack mines, Idaho, 573; on gold-belt of the Sierra Nevada, 290; on gold-quartz veins of California, 585 ; on granitic and dioritic rocks of Meadow Lake, Nevada county, Cal., 562 ; on the Sierra Nevada, 342 ; on silver-lead veins of Wood Eiver, Idaho, 599 ; on quartz-veins of De Lamar, Idaho, 583. Lithium, proportions of, in the earth's crust, 639. Lithosphere ; zone of flowage, 286 ; zone of fracture, 286. Liversidge, experiments on precipitation of gold from solution by metallic sulphides, [482]. Lock, A. G., on gold-deposits, 152. Lotti, B., on the Elba iron-ore deposits [648] (footnote). Lottner, classification of ore-deposits by, 7. McKellar, Peter, on quartz-veins in granite at Lake of the Woods, Western Ontario, 593. Magmatic segregation, etc. : auriferous pyrites of Rossland, B. C, 642 ; chromite in peridotites and their secondary serpentines [642]; copper-ores (high-grade) in serpentinized peridotites [642]; metallic nickal-iron in eruptive rocks [642]; platinum-metals in highly eruptive rocks [642] ; nickeliferous pyrrhotites in gabbro [642] ; ore-deposits formed by, 642 ; deposits formed by: titanic iron-ores in basic and intermediate eruptives [642]. Magnesite in country-rock near Idaho vein. Grass Valley, Cal., 526. Magnesium, proportions of, in the earth's crust, 639. Magnetite-deposit at Kirunawara-Luossawara, Sweden, 676. Major part of ore-depositing water is meteoric, 302. Mammoth gold-mine, Pinal county, Ariz., salt in water of, 741. Manganese, proportions of, in the earth's crust, 639. Manner of filling open spaces in general, 63. Mannsfeld kupferschiefer, 123. Marine detritus, 157. Mdtyas Kiraly gold-mine, Verespatak, Transylvania, character of native gold at, 68, 70. Mayflower gold-mine, near Elkhorn, Mont. [496]. Mesabi range, Minn., iron-ores of, 229. Metals : found in mineral waters, 45 et seq. ; heavy, original source of the, 637; pres- ence of in igneous rocks, 682; presence of in sedimentary and metamorphic rocks, 684. Metamorphous ore-deposits, 118 et seq. 800 INDEX. Metasomasis, first use of term, 6, 195. Metasomatic ore-deposits in soluble rocks, 133, 250. Metasomatic Processes in Fissure- Veins (Lindgken), 498. Michel-Levy on the French iron-ore deposits [')48]. Mine La Motte, Madison county, Mo., lead-deposits at, 117. Mineral coatings on copper at the Springs of Bourbon TArchauibault, France, 470. Mineral deposits : hysteromorphous, 147 ; idiogeuous and xenogeuous, 10 et seq. ; in limestone caves, 24; in geodes, 24; metamorphous, 118. Mineral springs; at the surface, 37; structural features of deposits by, 52. Mineral waters : alterations produced by. 48 ; analyses of, 37, 42 et seq. ; arsenic in, 47; chemical constitution of, 40; minute metallic admixtures in, 45; temperature of, 40. Mining, depth of [657]. Mining districts, distribution of, 706. Missouri : lead-regions of, 115 ; ore-deposits of, 188. Moanataeri gold-mine, Thames district. New Zealand, 752 ; richness of ore, 222. Mollie Gibson lead-zinc mine. Aspen district, Colo., 492. Mollie Gibson silver-mine, Pitkin county, Colo., 450. Montana :■ gold-mines : Elkhorn ; Dolcoath [496] ; Mayflower [496]. Montana : silver-lead mines : Eva May, 495, analysis of ore, 495; Helena ; Frohner, 49fi, analysis of ore, 496 ; Neihart ; Florence, 484 ; Big Seven, 484. Montana: silver-mines: Butte district ; Euby [482], Moon Anchor gold-mine. Cripple Creek, Colo., 747. Miiller, H., on mineral springs, 28. Myrtle gold-mine. Bright district, Australia, 747. Naumann on pseudomorphs, 501. Nevada: character of ore-deposits, 110 et seq. ; Comstock Lode, 30, 89, 195, 239; Steamboat Springs, thermal waters of, 36 et seq. Newberry, Prof. J. S. : on classification of ore-deposits, 7 ; origin of ores, 131. New Mexico : copper-deposits, 110, 131, 210 ; copper-mines ; Hanover, 450 ; Santa Eita, 449. New Zealand : gold-fields of, 221 et seq. ; gold in coal-measures of, 161. Nickel, proportions of, in the earth's crust, 639. Nickel-pyrrhotite deposit at Erteli, Norway [676]. Nitrogen, proportions of, in the earth's crust, 639. Noggerath, J., on alterations produced by mineral waters, 49, 101, 201, 245. North Carolina : copper-mines ; Union Copper Company's, 454 ; iron-mines : Person county; Blue Wing [483]. North Iron Hill, Lake county, Colo., ore-deposits of, 202 et seq. North Ophir silver-mine, Comstock Lode, Nev., high temperature of water in, 31. Norway : iron-ore deposits, 140 ; Kristiania region [648], 649 kaolin-deposits : Eker- sund-Soggendal, 662. Nuggets, origin of, 678 (footnote). Ofienbinya gold-mines, Dacian district, Transylvania, 86, 87. Opaline silica, analysis of, 575. Openings : in rocks, 293; size and number of, 295. Ore-bonanzas, distribution of, 745. Ore-chutes, 421. Ores; some principles controlling the deposition of, 282. Ore-deposits: at Broken Hill, Australia [679] ; at Chanarcillo, Chile [679]; chemical views of French school, 8, 63; classification of, 427; classification hitherto em- ployed, 3 ; Comstock Lode, Storey county, Nev., 195 ; derived from zone of frac- ture, .300; detrital, 154; examples of classes of, 72; formed by chemical and me- INDEX. 801 chaiiical influences of surface region, 148; hysteromorplious, 147; hysteromor- phous, of older geological formation, 160; in crystalline schists and eruptive rocks, 79, 85, 137; in soluble rocks, 95 ; material for, derived from rocks in zone of fracture, 302; Mednorudjansk, Ural Mts., Russia [679]; metamorphous, 118; metasomatic, in soluble rooks, 133; origin of, in deep regions, 55; Ornro, Bolivia [679] ; Pachuca, Mexico [679] ; Pasco, Peru [679] ; Potosi, Bolivia [679] ; problems in geology of, 636 et seq. ; proposed classification of, 7, 211, 226 ; secondary enrich- ment of, 433 et seq. ; tlie result of work of underground water, 285 ; verchovihy, or surface, 152; Zacatecas, Mexico [679]. Ore-deposition; in fresh water, 123; influence of eruptive rocks, 191; John Wood- ward's opinions, two hundred years ago, 192 ; lateral secretion theory, 57 et seq., 190, 200, 236, 242, 254, 257, 271; in open spaces, 64, 195, 239; from sea-water, 121. Ore-shoots, localization of, 746. Orphan Boy copper-mine, Boulder county, Colo., 738. Orthoclase ( valencianite) : in gold-silver veins. La Valenciana, Mexico, 532 ; in Valen- ciana silver-mine, Guanajuato, Mexico, 532 ; in gold-silver veins. Silver City, Idaho, 532. Otago gold-field. New Zealand, geological formation of, 224. Oxygen, proportions of, in the earth's crust, 639. Pandora silver-mine, San Juan county, Colo., 451. Penrose, A. F. E., on enrichments between altered and unaltered vein-matter, 476 ; on the geology of the Cripple Creek district, 290; on the geology of Cripple Creek, Colo., 343; on the superficial alteration of ore-deposits, 389 [433]. Pegmatites, 692. Pegmatite-veins in Norway, 647. Phillips, J. A. : on appearance of gold at Besseges, France, 163 ; classification of ore- deposits by, 7. Plmsphorus, proportions of, in the earth's crust, 639- Physico-chemical principles controlling the work of underground waters, 317. Placer-deposits, 158. Platinum, placer-deposits of, 158. Platinum-metals, original proportions of, in rocks, 640; at Sudbury, Canada, 640; at Klefva, Sweden, 640. Pneumatolytic minerals in veins, 694. Poiseuille's law of the flowage of water in capillary openings, 297. Pontgibaud, silver-lead district, France, formation of lodes, 222. PSsEPN'^, Peof. Fkanz ; TAe Genesis of Ore-Deposits, 1 ; remarks in discussion, 232. Posepuy on pipe-ore of Eaibl, Bohemia, 487; on stalactite deposits of sulphide, 486. Potassium, proportions of, in the earth's crust, 639. Precipitation in the zone of weathering, 479. Preferential use by water of large channels, 315. Prettau in Tyrol, copper-mines of, 143. Problems in the Geology of Ore-Deposits (Vogt), 636. Propylitic andesite, analysis of, 566. P'ropylitization of rocks (with chloritization, etc.), 660 et seq., 668. Przibram, Bohemia,: ore-deposits, 56, 61, 76, 83, 239 et seq. ; Mining Academy, study of geology at, 9. Pseudomorphs, phenomena in formation of, 15. Psilomelane in ore-deposits at Eomeneche, Prance, 674. Pumpelly, E. : classification of ore-deposits by, 7 ; on Lake Superior copper-deposits, 144 ; on the metasomatic development of copper-bearing rocks of Lake Superior, 606; on metasomatism in ore-deposits, 509. Purington, C. W., on the gold-quartz veins of Telluride, Colo., 592; on the mining industries of the Telluride quadrangle, Colo., 417. 802 INDEX. Pyrite-deposits; in Germany [651] ; in Hungary [651]; in Italy [651]. Foreign COUNTEIES: Italy: Monte Catini, 457; Norway, 651, 652; Vigsnas [457] [676]; Fahlun [676] ; Eoros [676] ; Sulitelma [676] ; ^^Spain : Huelva provinces, 456; Eio Tiuto, 456 ; Eio Tinto, Huelva district, 676 ; Tharsis, 456 ; Spain and Portugal [651] : Sweden : Fahlun [457]. Quartz-alunite rocks, formation of, 661. Quartz-diaspoie rooks, formation of, 661. Queensland, gold in coal-measures of, 161. Quicksilver at Sulphur Bank, Cal., 32 et seq. Eaibl, Carinthia, ore-deposits, 61, 69, 102, 134. Eain-water, salt in, 43. Eathgeb gold-mine, Calaveras county, Cal., 750. Eaymond, R. W. : remarks on the genesis of ore-deposits, 226, 252. Eaymond's classification of ore-deposits, 7, 226. Eed Mountain district, Colo., character of ore-deposits, 109. Eeichenstein silver-mine, Valle Sacca, Hungary, description of ore-deposit, 101. Eeyer, Dr. E., on tin placer-deposits, 159. Rezbdnya, Hungary, geology of ore-deposits at, 98. Ehodonite: in Veins at Broken Hill, Australia, 531 ; at Butte, Mont., 531 ; at Kap- nik, Hungary, 531 ; at Eeal del Monte, Mexico, 531. ElCKARD, T. A. . Tlie Formation of Bonanzas in the Upper Portions of Gold Veins, 734; remarks on Poscpny, 190, 211. Eickard, T. A., on the Enterprise mine of Eico, "Colo., 290, 342. Eio Tinto, Spain, copper-mines, 621 ; limonite deposits, 148. Eock-cavities, mineral deposits in, 15. Eock -salt deposits ; at M4ros Ujvdr, Transylvania, 20; of the Persian Gulf, 21. Eodna, Transylvania, ore-deposits, 95. R6le of the Igneous Bocks in the Formation of Veins (Kemp), 680. Eosario silver-mine, San Juancito, Honduras, Central America, 493. Eosenbusch on the genesis of pegmatite-veins, 732. Eubidium, proportions of, in the earth's crust, 639. Euby silver-mine, Butte district, Mont. [482]. Eussia: gold placer-deposits of Ural Mountains, 156; ore-deposits of Kackar district, in the Ural [153], 157. Eutile in apatite-veins, 646. Salt, in rain-water, 43. Salt-mining in Austrian Tyrol, 267. Sandberger, Prof ; on fissure- veins of Schappach, Schwarzwald, 594 ; lateral-secretion theory of, 57 et seq., 190, 200, 236, 242, 254, 257, 271 ; occurrence of native silver in altered granite [506] ; Przibram rock, analysis of samples by, 62 ; on silver- veins of Wittich-Schwarzwald, 594. Sandstones; copper, of Bohemia, 126; at Fontainebleau, France, 119. Savage silver-mine, Comstock Lode, Nov., flood of water in, 31. Santa Eita copper-mine. Grant county, N. M., 449. Sawyer, A. E., on Witwatersrand gold-field. South Africa, 162. Saxony : tin-deposits of Altenberg and Zinnwald, analysis of, 542. Scandinavia, ore-deposits of, 140. Scapolitization of rocks, 660 et seq., 668. Schists: copper, 124 et seq. ; ore-depnsits in crystalline, 137. Schurmann, E., investigations in sulphide reactions, 468. Sea-mills of Cephalouia, water-powii- of, .304. Sea-water: metallic sulphides from, 188 ; traces of metals in, 121. INDEX. 803 Second concentration favored by large openings of the belt of weathering, 378. Secondary Enrichment of Ore-Deposits (Emmons), 433. Secondary enrichment, absence of, iu silver-lead bodies of Cceur d'Alene district, Idaho, 488; changes of water-level, 497 ; chemical processes involved in, 465 et seq. ; effect of physiographic and climatic changes, 496; instances of: Australia: New South Wales; Brolsieu Hill lode, 459; conditions at Butte, Mont., 440; Eastern U. S., 452; Mexico: Chihuahua, 458 ; Peru: Cerro de Pasco mining dis- tricts, 458 ; Spain : Huelva provinces, 456 : Eio Tinto, 456 ; Tharsis, 456 : Western copper-deposits, 445; Western silver-deposits, 450; zone of enrichment, 475 et seq., 480; solution and precipitation of gold, 481 et seq. ; occurrence of bonanzas and pay-streaks, 488 et seq.; of veins at Neihart, Mont., 493 et seq.; zone of weathering, 475 et seq. ; leaching in, 476 et seq. ; precipitation in, 479. Sericitization of rocks, 660 et seq., 668. Seven-Thirty silver-mine. Clear Creek county, Colo., ore-veins of, 216, 217. Shouldn't Wonder gold-mine, Bright district, Australia, 746. Siberia, Altai region, ore-deposits of, 155. Siderite in lead-silver veins of Coeur d'Alene, Idaho, 527; of Wood Eiver, Idaho, 527. Silicates, solutions of, 319. Silicifloation of rocks, 661 et seq,, 668. Silicified diorite, analysis of, 593. Silicon, proportions of, in the earth's crust, 639. Silver-lead-mines: Colorado: Custer county; Geyser (Security-), 351; Montana: Boulder county; Eva May, 495, analysis of ore, 495; Helena; Frohuer, 496, Neihart ; Big Seven, 484 ; Florence, 484. Silver-mines ; Colorado : Custer County; Geyser, 461 ; Ouray county ; Yankee Girl, 748; Pitkin county; MoUie Gibson, 450; Smuggler, 450; San Juan county ; Pan- dora, 451 ; Smuggler-Union, 451; Yankee Girl, 451; Foreign Countries; Ans- trnliu : New Soutli Wales ; Broken Hill Consols, 460, 461; Central America : Hon- duras; Eosario, 493; New South Wales: Australian Broken Hill Consols, 490; Norway: Kongsberg, 468 ; Montana: Butte district ; Euby [482]. Silver Plume district. Clear Creek county, Colo., 622. Silver Eeef mining district, Utah, 130. Silver- and silver-lead ores : iu Colorado, 202, 216 ; Hartz mountains, 80; atLaurium, Greece, 135; Nevada, 112,198, 251; Pontgibaud, France, 222 ; at Eaibl, Carin- thia, 102; at Eezb4uya, Hungary, 98. Sjogren, A. ; on iron-ore deposits in Sweden, 138 ; mines of Banat, 718. Slichter, C. S. : on the motion of underground waters, 309; on openings in rocks, 294. Slickensides: formation of, 213 et seq. ; at Eaibl silver-lead mines, 103 ; in Scandina- vian mines, 140. Smith, George, on secondary sulphides at Broken Hill Consols mine, N. So. Wales, Australia, 460, 461. Smuggler gold-mine. Cripple Creek district, Colo., 290. Smuggler lead-zinc mine. Aspen district, Colo., 492. Smuggler silver-mines, Pitkin county, Colo., 450. Smuggler-Union silver-mine, San Juan county, Colo., 451, 622. Soda-niter: origin of, 715. Sodium, proportions of, in the earth's crust, 639. Solutions of silicates, 319. Some Principles Controlling the Deposition of Ores (Van Hise), 282 ; discussion through- out later pages of this volume. Source of underground water, 302. South Africa, gold-deposits of^ 162. South Dakota : tin-ore deposits : Black Hills, 643 (footnote). Spaces of discission, 14 ; ore-deposits in, 74. 51 804 INDEX. Spaces of dissolution, 14. Spain, iron -ore deposits of, 148. Spurr, J. E. . on the Aspen mining district, Colo., 342, 598 ; on the geology of the Mercur mining district, Utah, 407; on ore-bodies of the Aspen district, Colo., 492 ; Pegmatite- veins in Yukon section, 733. St. Avoid, copper-ores in sandstone of, 127. Steamboat Springs, Washoe county, Nev., thermal waters of, 36 et seq. Stelzner, Prof. A. W. : ou deposits of tin-ore, 159 ; microscopic methods of research [616]; on pyritic deposits, 652 ; on silver-tin veins in Bolivia [645] (footnote); term of " metasomasis " proposed by, 5; on tourmalinic gold-copper veins in Chile, 546. Stokes, H. N., on chemical reactions, 471., Stream detritus, 154. Striae and slickensides as proofs of movement, 213 et seq. Strontium, proportions of, in the earth's crust, 639. Structural features of the deposits of mineral springs, 52 Struggl silver-lead mine, Eaibl, Carinthia, ore-bodies of, 104. Subterranean water-circulation, 18, 219, 253. Subterranean waters, heat of ascending, 31. Sugar Loaf gold-mine, Kananalling, W. Australia, 742. Sulphide enrichment in veins of Freiberg, Saxony, 615. Sulphide ores, effect of ferric salts upon, 676 (footnote). Sulphur Bank quicksilver-mine. Lake county, Cal., thermal waters at, 32 et seq,, 256. Sulphur, proportions of, in the earth's crust, 639. Surface-flows of igneous rock unfavorable to vein-formation, 694. Sweden: Ammeberg, zinc-blende mine at, 141; copper- and gold-ores, 140; pla- tinum-metals at Klefva, 640 ; Taberg, iron-ore deposits of, 138. Switzerland, Carniola, iron-ore deposits of, 136. Taberg, Sweden, iron-ore deposits, 138. Talhawang gold-district, New South Wales, 161. Tasmania, gold in coal-measures of, 161. Tellurium : in quartz-veins at Cripple Creek, Colo. [654] ; at Hauraki, New Zealand [654]; at Nagyag, Hungary [654]. Temperatures, critical, of water, etc., 659. Temperature : of magmas, 659 ; of mineral waters, 40. Thames gold-fleld, New Zealand, 221. Thermal waters encountered in mines, 29, 30. Thermo-dynamic relations between hot water and soft glass, 308. Thuringia, hupferschiefer of, 124. Tin, proportions of, in the earth's crust, 639. Tin-deposits : of Altenberg and Zinnwald, Saxony, analysis of, 542 ; of Etta Knob, S. Dak., 643 (footnote) : of Cornwall, England, 139; placer-deposits, 158. Tin-, copper- and galena- veins in Cornwall, Eug. [657]. Titanomagnetite-olivinite deposit at Taberg, Sweden, 138. Titanic acid in apatite- veins, 646. Titanium, proportions of, in the earth's crust, 639. Toruebohm, A. E., on contact-deposits at Pitkaranta, Finland [650]. Trade Dollar copper-silver mine, Florida Mountain, Idaho, 573. Transylvania: Dacian gold-district, 86 ; mining rock-salt at Miros TJjv^r, 20; Veres- patak gold deposits, 66 et seq,, 87 ; Vulkoj gold deposits, 88. Transvaal, South Africa, Witwatersrand gold-fields, 162. Tresavean copper-mine, Cornwall, 621. Tyrol: copper-mines of Prettau, 143; iron-ore deposits, 143; salt-mining at the Salzkammergut, 267. INDEX. 805 Underground waters : see innumerable passages throughout this volume. United Verde copper-mines, Yavapai county, Ariz., 447. Union Copper Co.'s copper-mine. Gold Hill, Eowan county, N. C, 454. Ural mountains, gold-districts of, 76. Utah: character of ore-deposits, 110; Silver Eeef mining district, 130. Vadose underground circulation, 18; filling of open spaces formed by, 23; probable source of some deep ore-deposits, 197, 198. Vadose vs. deep circulation, 276. Valle lead-mines, Jefferson county. Mo., character of deposits, 71, 115. Vanadium, proportions of, in the earth's crust, 639. Van Hise (C. R.) ; Some Principles Controlling the Deposition of Ores, 282; remarks iu discussion, 731. Vapors or dissociated gases in igneous rocks, 686. Vein-formations, sequence of, 690. Vein-minerals : association of, 658 ; deposition of, 659. Veins, role of the igneous rocks iu the formation of. 680 et seq. Verchoviky, or surface ore-deposits, 152. Verespatak, Transylvania, structure of gold-deposits at, 66, 87. Vermont, copper-deposits of, 207. Vesuvius lead-mine, Bavaria, Germany, tree-stems changed to galena at, 129. Victor gold-mine. Cripple Creek district, Colo., 290. Vogelgesang on dissemination of native silver in gneiss [506]. VoGT, Pkof. J. H. L. : Problems in the Geology of Ore-Deposits, 636. Vogt's account of cassiterite-veins in Telemarken, Norway, 563 ; of concentration of gold and silver beneath " iron hat" in Eio Tinto region, Spain, 487. Von Fircks, W., on the tin-deposits of Mt. Bischoff, Tasmania, 543. Von Inkey, Bela, on the ore-deposits of Nagyag, Hungary, 568. Vulkoj gold-mines, Transylvania, character of ore-deposits of, 86. Waldstein, classification of ore-deposits by, 4. Waters : analysis of, from mineral springs, 37, 42 ; ascending, encountered in mines, 28 ; subterranean circulation of, 18, 219, 253 et seq. Weed, Waltek Haevey: The Enrichment of Gold- and Silver-Veins, i73. Weed's account of the copper-deposits of the Southern Appalachian region, 453 ; of the Elkhorn mine, Mont., .597 ; of the mines of Neihart, Mont., 452. Weidman, S., on igneous rocks of Fox Eiver Valley, Wis. [289]. Well-waters of Tucson, Ariz., analysis of, 714. Werner, A., theory of ore-deposits of, 3. Westphalia ; copper-ores of, 125 ; pisolitic formation at Warstein, 67. Whitney, Prof. J. D. : classification of mineral deposits by, 6 ; on lead- and zinc-ores of Wisconsin, 118. Winohell, HoK.iCE V. : in discussion of the genesis of ore-deposits, 192, 227. Winklehner, H., on rock-salt deposits of the Persian Gulf, 21. WiNSLOW, Akthur: in discussion of the genesis of ore-deposits, 188. Wisconsin, lead- aud zinc-regions of, 115, 117, 189 et seq. Witwatersrand gold-field, Transvaal, South Africa, 162. Woodward, John, " Essay Towards a Natural History of the Earth," 192. Xeuogenites in general, 12. Xenogenous mineral deposits, 10. Yankee Girl silver-mine, Ouray county, Colo., 748. Yankee Girl silver-mine, San Juan county, Colo., 451. Yellow Jacket silver-mine. Storey county, Nov., location of ore, 93. 806 INDEX. Yellowstone National Park ; analysis of water, 42 ; chimney-like conduits built by geysers, 39. Yttrium, proportions of, in earth's crust, 639. Yukon section ; Pegmatite veins in, 733. Zeolitizatiou of rocks, 661 et seq., 669. Zinc-blende, proportion of, increasing with depth of deposit, 673. Zinc-ores : Ammeberg, Sweden, 141 ; Wisconsin, 117, 189 et seq. Zirconium, proportions of, in the earth's crust, 639. Zones of weathering, of enrichment, and of primary sulphides, 475 et seq. Zone of combined fracture and flowage, 288. Zoppe, G., on salt in rain-water, 43.