v',-'.lV'.\i- . U""^ l^O.MPEND, OF Geology \^^';-';*fr;- ►Va'' ?«'•' •'.'•■»,,\/ •'.■:;i i?tatt CoIIcse of ^sricuUure ^t Cornell ©nibersitp Hifirarp Cornell University Library QE 28.L46 A compend of geology. 3 1924 002 979 338 The original of tliis 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/cu31924002979338 A COMPEFD OF GEOLOGY BY JOSEPH LE CONTE PROFESSOR OP GEOLOGY AND NATURAL HISTORY IN THE UNIVERSITY OP CALIPORNIA; author op "elements op GEOLOGY," ETC. NEW YORK . CISrCINNATI • CHICAGO AMERICAN BOOK COMPANY COPTEISHT, 1884, BT D. APPLETOir & CO. COPTEIGHT, 1898, BY AMEEICAN BOOK COMPANl LE CONTE, QEOU W. P. l8 PREFACE. Ik preparing this little work for the schools I have kept constantly in view two ends : 1. I have tried to make a book which shall interest the pupil, and at the same time convey real scientific knowledge. 2. I have tried, as far as possible, to awaken the faculty and cultivate the habit of observation, by directing the attention of the pupil to geological phenomena occurring and geological agencies at work now on every side, and in the most familiar things. By the former I hope to awaken a true scientific appetite ; by the latter, to cultivate the habits necessary to satisfy that appetite. Joseph Le Oonte. Bbbkelbt, Califobnia, September, 1884. PREFACE TO THE REVISED EDITION. Althottgh a work so elementary as this — embodying only the most general principles of geology — does not require so frequent revision as a more advanced work, yet geology is so rapidly advancing a science that even gen- eral statements must, from time to time, be modified. Especially is it necessary that new and better illustrative figures should be used. 3 4 PREFACE TO THE REVISED EDITION. In this revised edition I have not changed the general plan of the work as already explained in the preface of the previous edition, but have only made such modi- fications and additions as seemed necessary to bring it up to the present condition of science. Among these additions, certainly not the least important are the beau- tiful restorations of vertebrate skeletons and photo- graphs of natural objects for which I am indebted to Professors Marsh, Dean, "Williston, and Scott. I wish hereby to thank these gentlemen for their hearty cooper- ation. Acknowledgment is due also to the American Museum of N'atural History, New York, N. Y., for the photograph of the Great Barrier Eeef, shown on page 97. Joseph Lb Contb, Berkeley, California, Jarmwry, 1898. FAQS CONTENTS. Introduction ... 7 Part I. — Dynamical Geology. CHAFTSB I. Atmospheric Agencies 10 II. Aqueous Agencies 17 III. Organic Agencies . . . * 83 IV. Igneous Agencies 131 Part II. — Steuctueal Geology. I. GrBNBBAL Form and Structure of the Earth . . 173 II. Stratified Rocks 179 III. Unstratified or Igneous Rocks 210 IV. Metamorphic Rocks 334 V. Structures Common to All Rocks .... 338 VI. Denudation, or General Erosion 253 PaET III. HiSTOEICAL GeOLOGY. I. General Principles 250 II. Arch.s;an System and Archeozoic Era . . . 368 III. Paleozoic Rocks and Era 267 IV. Mesozoic Era. — Age of Reptiles 334 V. Cenozoic Bra.— Age of Mammals 363 VI. PsYCHOzoic Era.— Age op Man 407 Index . . 417 5-6 GEOLOGY. INTRODUCTION. Definition of G-eology. — Geology is the science which treats of the^as^ conditions of the earth and of its inhab- itants. It is, therefore, a history of the earth. It is closely allied to physical geography, hut differs in this : Physical geography treats only of the present forms of the earth's features ; geology also, and mainly of their grad- ual formation, or evolution from former conditions. It is also closely allied to natural history, but differs in this : Natural history is concerned only with the present forms and distribution of animals and plants, while geology is chiefly concerned ahout previous forms and distribution, and their changes to the present forms and distribution. In a word, geography and natural history are concerned about how things are j geology, about how they became so. Cultivates Habit of Observation. — We have said geology treats of the history of the past conditions of the earth and its inhabitants. The evidences of the past con- ditions are found in its present structure. But, to under- stand this structure, we must observe the manner in which similar structure is formed now under our eyes. Thus, observation of causes now in operation constitutes the only solid foundation of geology. Fortunately, the proc- esses by which structure is now being formed may be ob- served everywhere ; and the structures which have been 8 INTBODUGTION. thus formed in earlier times may be observed in very many places, if we know how to look for them. Thus geology, perhaps more than any other science, cultivates the habit of fleld-observation ; not, indeed, that minute observation required by mineralogy or botany, but that wider observa- tion which gives interest to mountain-travel or even to rambles over the hills in our vicinity. It cultivates also, in an eminent degree, the habit of tracing effects to their causes — ^for the question ever present to the geologist is, " How came it so 1 " Great Divisions of Geology. — We have said that the history of the earth is recorded in its structure, and that structure is understood by study of causes or processes now in operation. We have thus outlined the great di- visions of geology, and the order in which they must be studied. We must study, first of all, causes and processes now in operation about us everywhere, producing struc- ture. This is called dynamical geology. Next, we must study the rocky structure of the earth to as great a depth as we can, and apply the previously acquired principles in its interpretation ; for this structure has been produced by similar processes acting through all previous time. This is called structural geology. Only after this shall we be prepared to take up the history of the changes through which the earth has passed ; for this history is revealed in structure. This is called historical geology. PAET I. DYNAMICAL GEOLOGY. As already said, this part treats of agencies now in operation producing structure. These are best treated under four heads — ^viz., atmospheric, aqueous, organic, and igneous agencies. The same agencies hare operated from the beginning, though probably with different de- grees of activity. Their accumulated efEect, through inconceivable ages, is the present structure of the earth. We observe these operations now, in order to understand the effects of their operation then. CHAPTER I. ATMOSPHERIC AGBITCIES. Origin of Soil. — If we dig into the earth anywhere, at a certain depth, greater in some places than in others, we find rock. How was the earthy soil formed ? Per- haps some imagine that it is an original clothing intended to cover the rocky nakedness of the new-born earth. But the very first lesson to be learned by the study of geology is that all things that we see, even the most enduring — such as hills, mountains, rocks, etc. — have become what they are, usually by a slow process. Now, soils are no exceptions. All soil is formed by a disintegration or rotting down of rocks. Sometimes the soils remain resting on the rocks from which they were formed ; sometimes they are removed to another place, as, 6. g., from hillsides to bottom-lands ; sometimes they are carried by streams to great distances, and deposited as sediments, and again raised as land ; but in all cases they are formed in the same way — viz., by the rotting down of rocks under the slow action of the atmosphere. The active ingredients of the air in this process are oxygen, carbonic acid (carbon dioxide), and water, as vapor or as moisture. Now, rain-water contains in solu- tion both oxygen and carbon dioxide. Therefore, rain- water, wetting the surface and penetrating the cracks of rocks, is the great agent in the formation of soil. Proofs of this Origin of Soils. — The proofs of this mode of formation are clearest in those cases in which 10 ATMOSPHERIC AajJlNOIJES. 11 the soil still rests on the rock from which it was made. Unfortunately this is rare in the northern part of our country, where the soil has been nearly everywhere shifted during a period which we shall hereafter describe as the Drift period. But in the southern part of the United States, on all the hillsides and mountain-tops, the soil has been undisturbed for ages, and the eyidence is com- plete, and may be observed by any one. If, for example, we note carefully the sections made by railroad and well diggings, we shall see at the top perfect soil, perhaps red ; a little deeper it becomes lighter colored and coarser grained ; then it begins to look like rotten rock ; and, finally, by insensible degrees, it passes into sound rock. The evidence is still more complete if, as is often the case, the rock is traversed by a quartz-vein. In such a case we can trace the quartz-vein through the sound rock, and upward through the rotten rock, the imperfect soil, and the perfect soil, to the surface, where it may usually be traced over hill and dale as white fragments lying on the surface. The reason is this : Quartz is a mineral which will not disintegrate under atmospheric agency ; therefore it remains sound, while all the rest of the rock is changed into soil (Fig. 1). Fib. 1.— Section and perspective view (ideal), a, sound rocls ; J, rotten rock ; c, perfect soil ; d, quartz-vein ; d>, same, outcropping on surface ; e, mass Of more resistant rock imbedded in soli. Sometimes a rounded mass of sound rock, e, is seen imbedded in the soil. This is only a harder piece of 12 DYNAMICAL QEOLO&Y. rock, which has resisted disintegration, while the rest has yielded. These are called bowlders of disintegration. It is not always, even in lower latitudes, that we find this gradation between soil and rock. Often perfect soil is found to rest on sound rock, with sharp limit between. In all such cases there has been shifting of the soil. In northern latitudes (•37°-40° northward), as already stated, the soil nearly everywhere rests on sound rock, and often the underlying rock is smooth and polished. We shall explain this hereafter. But even in the Northern States, if one will notice closely, he will see the process of soil- making going on. Kock-fragments, which were once an- gular, become rounded by rotting of the corners. Cliffs, by their crumbling, gather piles of rock-fragments and iearth {talus) at their bases (Fig. 3). The pupil ought to Via. 2.— CllfE, showing talus, t, and bowlders of disintegration, b, b. observe these things habitually, as it is on just such . observation of simple things that true science rests. Depth of Soil — Since soil is constantly carried away by washing of rain, as will be more fully explained in the next chapter, it is evident that there are two opposite processes here to be considered, viz., soil-formation and soil-removal. The depth of the soil will depend on the relation of these two to each other. More definitely, the depth of the soil depends partly upon the kind of rock (for this affects the rate of formation), and partly on the slope (for this affects the. rate of removal). On high ATMOSPHERIC AGENCIES. 13 slopes the rock is hare (Fig. 3, a), not because there is no soil formed, but because it is removed as fast as formed. A—. -..-^ ^ Fio. 3 — a, soand rock ; J, rotten rock ; o, soil formed in place ; cl, sofl shifted from e. On flat lands, near high slopes, the soil is deep (Pig. 3, b), because not only is it formed here in place, but the wash- ings from above are added. Gate of DlsintegTation. — If rocks were solid, so that the agents of decomposition could act only on the sur- face, the rate would be inconceivably slow, but all rocks are affected with joints in several directions, by which the mass is divided into more or less separable blocks, so that a cliff looks something like a wall of regularly piled blocks without cement (Pig. 2). Water, therefore, pene- trates to great depths, attacking the surface of every block. Also, every block is itself affected throughout with capillary fissures, through which water penetrates to every part (quarry-water of stone-cutters). Thus, the rocky crust of the earth is affected by disintegrating agencies to very great depths — though, of course, most rapidly at the surface. Bowlders of Disintegration. — ^All over the Northern States are found scattered rock-masses (bowlders), lying on the surface. If we examine these, we shall usually find that they are entirely different from the country- rock.. They have been brought from a distance — ^how, we shall explain hereafter. "We have nothing to do with 14 DYNAMICAL GEOLOGY. these now. But in the Southern States also, in many- places, are found huge, isolated masses, lying on the sur- face, and even sometimes forming rocking stones (Fig. 4). If we examine these, we find that they are of the same material as the country-rock. They have heen formed in place. In the general disintegration of rock, and forma- tion and removal of soil, these have resisted, because harder than the rest. Nothing is more interesting than thus to trace the configuration of the surface of the country to unequal resistance to atmospheric agencies. Explanation of Rock-Dlslntegratlon. — If we take a piece of old and very hard mortar, and pour on it a little hydrochloric acid, it quickly breaks down into sand, wet with a solution of calcium chloride. The explanation is simple. Mortar consists of grains of sand cemented into a mass by hydrate or carbonate of lime. The acid dis- solves the lime-cement, and the mass falls to powder. Now, mortar is really artificial stone, and nearly all rock is constituted in a similar manner, i. e., consists of particles cemented together. In all rock some parts are soluble in atmospheric water, and some are not. Under the long- continued action of this agent, therefore, the soluble parts are dissolved, and the mass breaks down into a powder, or dust of the insoluble parts, wet with a solu- tion of the soluble parts. The main difference between the experimental and the natural case is, that in one the process is rapid, and in the other extremely slow. Examples. — One or two examples will make this plain : ATMOaFUEKW AdElSGIES. 15 1. Sandstone is a rock made up of grains of sand cemented into a mass, sometimes by lime carbonate, sometimes by silica. Under the slow action of atmospheric water the cement is dissolved, and the rock crumbles into sand, moistened with a solution of lime carbonate, if this be the cement. 2. Granite and gneiss and many other igneous and metamorphic rocks, such as are found on the eastern slope of the Appalachian Chain everywhere, are an aggre- gation of four minerals, viz., quartz, feldspar, mica, and hornblende. In coarse granite these can be easily seen with the naked eye. The bluish glassy specks are quartz ; the opaque white, or rose-color, are feldspar ; the glistening scales are mica ; and the black spots, horn- blende.* The whole rock may be regarded as grains of quartz, mica, and hornblende cemented into a mass by feldspar. N ow, quartz is not at all, and mica very slightly, affected by atmospheric water ; but the feldspar and hornblende are slowly changed into clay, which, in the case of hornblende, is red, from the presence of iron. Thus, the whole rock rots down to a clay soil, usually red, in which are disseminated grains of quartz and scales of mica, the whole moistened with water, contain- ing in solution a little potash derived from the feldspar. This is the commonest of all soils. 3. Slates and shales are clays hardened into rock by some cement such as lime or silica. When the cement is dissolved the rock crumbles into a clay soil. 4. A pure limestone like mar- ble makes no soil because it is all soluble, but most lime- stones are mixed with clay or sand. When the lime is dissolved the result is a limy clay or limy sand. Mechanical Action of Air ; Frosts. — The soil-for- mation, above explained, is a chemical process, but, in cold climates and mountain-regions, atmospheric water acts also mechanically and very powerfully in rock-dis- * These minerals ought to be shown the pupil, both separately and as aggregated in a specimen of coarse granite. 16 BYNAMIGAL QEOLO&Y. integration. Water penetrating the joints, and freezing, expands with such force that the rocks are riven asunder ; and then, penetrating again into the capillary fissures and freezing, these blocks are in their turn broken into smaller fragments, until the whole crumbles to dust. "Wind. — Again, loose earth, sand, and dust, especially in dry climates, are carried by winds, and sometimes accu- mulate in large quantity and form a peculiar soil. Thus, the sands of Sahara are in some places encroaching on the fertile lands of Egypt. Thus, also, sea-sands are often carried inland from shore, and cover up and destroy fertile lands. The sand-hills to the west of San Fran- cisco are made in this way. The phenomena of sand- dunes may be observed in many places along the coasts of nearly all countries. Some geologists think that in the interior of dry countries, like Asia or the western part of our own country, soil of great thickness has been formed by accumulation of clust. CHAPTER II. AQUEOUS AGENCIES. Aqueous and atmospheric agencies are so closely con- nected that many treat them together under the one head of leveling agencies. Water, as atmospheric moisture or as rain, soaking into the earth, is the chief ^agent of soil- making ; but water, falling more abundantly, runs ofE the surface, and is also the chief agent of soil-removal. In the one case it acts as a chemical, in the other as a me- chanical) agent. The agency of water in soil-making we treated under atmospheric, its agency in soil-removal be- longs to aqueous, agencies. The one, acting at all times and in all places, its effects are obscure and inconspicu- ous ; the other, acting occasionally and concentrating its power on particular places, its effects are easily observed and better understood. Nevertheless, the aggregate ef- fects of the one must be equal to those of the other, for the former prepares the way for the latter. Aqueous agencies have little effect upon rocks unless they have been first rotted down to soils. Although the agency of water is mainly mechanical, yet there is a chemical agency of water other than that of soil-making. The agency of water may therefore be divided into mechanical and chemical. The mechanical agency is best treated under the three heads of rivers, ocean, and ice, and each of these again in cutting away, in carrying, and in throwing down again, or in erosion, transportation, and deposit. The chemical agency we shall consider under the two heads of chemical deposits in springs and in lalces': JjK CONTE, GBOL, 2 18 DYNAMICAL QEOLOGT. {[ Rivers, erosion, transportation, deposit. Mechanical.. J Ocean, " " " I Ice, Chemical i Springs, chemical deposits in, \ Lakes, " " Section I. — Eivebs. Atmospheric or meteoric water falls on land as rain. A portion sinks into the earth, and, after a longer or shorter subterranean course and doing its appropriate work of rock-disintegration and soil-making, comes up again to the surface as springs. Another portion runs ofE the surface, cutting and carrying away the soil everywhere. Quickly, however, it gathers into rills and cuts furrows, these rills uniting into streamlets and cutting gullies. The streamlets, uniting with each other, and with water issuing from springs, form mountain-torrents, and cut out great ravines, gorges, and caflons. Finally, the torrents, amerging on the plains from their mountain home, form great rivers, which deposit their freight of gathered earth and rock-fragments in their courses, and finally in the sea or lake into which they empty. Such is a condensed his- tory of the course and work of water from the time it falls as rain -until it reaches the ocean from which it came. All of this we include under river-agency. It may be defined as the work of rain and rivers, or the work of circulating meteoric water. All that follows on this subject will be but an expansion of the condensed statement given above, and much of it rhay be observed by any one who does not commit the mistake of thinking things insignificant because they are common. 1. Erosion of Rain and Rivers. The rain which falls on land-surface may be divided into three parts : One part runs immediately from the surface, producing universal rain-erosion and the muddy AQUEOUS AGENCIES. 19 floods of tlie rivers. Another part sinks into the earth, and, after doing its appointed work of soil-making, re- appears on the surface as springs, and forms the ordinary flow of rivers in dry times. This part joins the surface drainage, and together they concentrate their work along certain lines, and thus produce stream-erosion. A third portion never reappears on the surface, but finds its way, by subterranean passages, to the sea. By the continued action of rain and rivers all lands (except some rainless deserts) are being cut away and carried to the sea. Every one, each in his own vicinity, may see this process going on. The soil of the hillsides is everywhere being washed away by rain, and carried off in the muddy streams. At what average rate is this wash- ing process going on ? This is a question of extreme importance. Average Rate of Erosion. — By observations made on rivers in all parts of the world it has been estimated that all land-surfaces are being cut away at a rate of about one foot in 3,000 to 5,000 years. The Mississippi cuts down its whole drainage-basin on^ foot in 5,000 years, the Granges one foot in 3,000 years. Some rivers cut still more rapidly, but most less rapidly than these. The rate differs in different parts of the same basin. In mountain- regions the rate is at least three times the average given above, and on steeper slopes still greater. On the lower plains the erosion is small, and in many places there is deposit instead of erosion. Making due allowance for all these variations, it is probable that all land-surfaces are being cut down and lowered by rain and river erosion at a rate of one foot in 5,000 years. At this rate, if we take the mean height of lands as 1,300 feet, and there be no antagonistic agency at work raising the land, all lands would be cut down to the sea-level and disappear in 6,000,000 years. This universal cutting away of land-surfaces we have 30 DYNAMICAL GEOLOGY. divided for convenience into Wo parts^ which, however, graduate completely into each other — viz., rain-erosion and stream-erosion : the one is universal, but small and in- conspicuous in any one place; the other is Qonfined to water-channels, but works with concentrated and con- spicuous efEects. The one may he compared to a univer- sal sand-papering, the other to the action of the graver's tool, cutting ever deeper along the same lines. Of the two, the general rain-erosion, though less conspicuous, is probably far the greater in aggregate amount. They co- operate in cutting away the land, and, if unopposed, would finally 'destroy it. Pure water, however, has comparatively little efEect. Its graving-tools are the sand, gravel, peb- bles, and rock-fragments, which it carries along in its course. Conspicuous Examples of Stream-Erosion. — The efEects of erosion are most conspicuously seen in water- falls, ravines, gorges, and cafions ; but also, in less degree, on every hillside, and in every furrow and gully. Waterfalls ; Niagara. — The Niagara Palls and gorge are an instructive example of stream-erosion, because the efEects are easily observed from year to year. Greneral Configuration of the Country. — Lake Erie is situated on a nearly level plateau, several hundred feet above a similar plateau, on which is situated Lake On- tario. The plateaus are separated by an almost perpen- dicular clifE, running east and west, near Lake Ontario. The Niagara Eiver runs out of Lake Erie, and on the Erie plateau, fifteen to eighteen miles, then drops, by a perpendicular fall, into a narrow gorge, with nearly per- pendicular sides, and runs in the gorge for seven miles, and then emerges on the Ontario plateau just before emptying into that lake. Fig. 5 is an ideal section through the middle of the river, and showing these facts. The light lines show the cliffs on the other side of the gorge. Recession of the Falls. — Ever since their discovery. AQUEOUS AGENCIES. 31 200 years ago, the falls have steadily worked their way back toward Lake Erie. The rate of recession has been estimated at one to three feet per annum. The cause is easily perceived. The strata at the falls consist of E.P. E.F. FiQ. 5.— Section of Niagara Falls and gorge. O.P., Ontario plateau ; E.P., Erie platean ; L.O., Lake Ontario ; /, fall ; mS, stratified mud-banks. solid limestone, represented in the figure by the jointed structure, underlaid by softer .shale. The force of the dashing water cuts away the soft shale and undermines the limestone, causing it to project as overhanging rocks, which fall from time to time into the abyss below. Thus the falls work backward, but remain perpendicular. Gorge formed by Recession. — There can be no doubt that the whole gorge has been formed in this way ; that the river once fell over the cliff which runs across its course near Lake Ontario, and then .worked its way back to its present position ; and the work is still going on. The general configuration of the country suggests this origin even to the casual observer, and close examination entirely confirms it. It is a familiar fact that stratified mud-banks are found in spots along the margins of all rivers, evidently formed by deposits from the river. These stratified muds often contain the shells of the mussels which inhabit the river. N"ow, in several spots {mb) along the top of the gorge-cliff, from the falls to Lake Ontario, are found such stratified mud-deposits contain- ing shells. The deposits were evidently made when the river ran at that level. 33 DYNAMICAL GfEOLOffi'. Time. — Several attempts have been made to estimate the time occupied in this process. Mr. Lyell estimated it at 35,000 years.* A large part, if not the wnole of this time, belongs to the present geological epoch, and was probably witnessed by early man. Other Falls. — Many other perpendicular falls have receded in a similar way and given rise to similar gorges. The most remarkable of these are the Falls of St. Anthony. The Mississippi Eiver, at Fort Snelling (mouth of the Minnesota River), is traversed by an escarpment which separates a higher from a lower plateau. The river runs on the upper plateau as far as Minneapolis, then drops, by a nearly perpendicular fall, into a gorge one hundred feet deep, runs in this gorge eight miles, and then emerges on the lower plateau at Port Snelling. Here, again, we have the upper plateau capped by a hard limestone, under- laid by a soft sandstone. Here, also, the wearing away of the underlying sandstone causes the limestone to project in overhanging tables which fall from time to time into the chasm below, and so the fall works backward. There is no doubt that the Mississippi at one time fell over the escarpment at Port Snelling, and has worked its way back to its present position, and that this all took place during the present geological epoch, and while man inhabited the continent. Professor Winchell has estimated that, at its present-rate recession, it would take nofmore than 8,000 years to accomplish the work. Minnehaha River is a tributary running into the Mis- sissippi about six miles below the falls. It therefore, at one time, fell into the gorge. It has now worked itself back about two miles, and forms the beautiful " Minne- haha Falls," made celebrated by their description in Long- fellow's " Hiawatha." The Columbia River, where it breaks through the Cascade Range, has cut a gorge fifty miles long and 1,000 * Later estimates make it about 11,000 years. AQUEOUS ACfENGIES. 33 to SjOOO feet deep. All the tributaries which run into the river at this point have cut deep side gorges, headed by perpendicular falls. Some of the most exquisite falls are here nestled among the hills in these almost inaccessible gorges. The country rock is a very hard but much Jointed lava, underlaid by 'a softer cement-gravel. The falls have eaten out the gravel and undermined the lava, which from time to time tumbles into the chasm as blocks that are carried away by the stream. In this way the falls have worked back about two miles. Yosemite Falls. — Most perpendicular falls have been made by recession, as explained above, but this is not true of all. The Yosemite Palls (of which there are six, vary- ing in height from 400 to 1,600 feet) have not perceptibly receded. This is because the granite is very hard, and the time too short (probably only a few thousand years)', since the valley was filled with ice (page 394). Ravines, Gorges, Canons. — These are found in all countries, especially in mountainous and high-plateau re- gions. They are always or nearly always formed by run- ning water, although in some cases their places are deter- mined by fractures of the earth's crust (page 239). They are gullies on a large scale. In the Appalachian Chain the most striking examples are the Hudson Eiver gorge in New York, the Tallulah gorge in Georgia, and the French Broad gorge in North Carolina. But it is in the western part of the continent that the finest examples are seen. Nowhere in the world are they on a grander scale, more evidently due to water alone, or more recent in origin. As we are studying " causes now in operation,'' they are the most instructive examples to be found anywhere. In California there was, even since middle geological times, an old river-system difEerent from the present. This will be explained more fully hereafter (page 395). These old river-valleys were filled up with river-gravel, and finally obliterated by lava-flows not long before t/ie 34 DYNAMICAL GEOLO&Y. . advent of man. The displaced rivers have since that time cut new channels, far deeper than the old, so that the old lava-covered channels are high up on the present divides (Pig. 6). Thus, in very recent geological times — ^i. e., in Fig. 6. — Section across old and new river beds of California, t", r, new river beds ; 7^, old river bed; gr^ gravels of present rivers; gr\ old river gravels; dotted limy old configuration of surface. the Quaternary and present epochs — water has cut at least 2,000 feet deep in hard slate-rock. We have selected these cases because of the plain evi- dence of recent work, but the whole western slope of the Sierra is trenched with enormous ravines, 3,000 to 6,000 feet deep, although the history of some of them is, longer than those spoken of above. For example, commencing north and going southward, we have the Columbia Eiver, with its gorge 3,000 feet deep in hard lava. The branches of the Feather, Yuba, and American Elvers have cut gorges 2,000 to 3,000 feet deep in hard slate. These have the structure represented by Fig. 6, and have been cut wholly in very recent geological times. The Tuolumne and Merced Elvers have cut gorges 3,000 to 5,000 feet deep, the famous Hetch-hetchy and Yosemite Valleys being in the course of these. King's Eiver CaQon is 7,000 feet deep, in hard granite. Plateau Region. — But the most wonderful gorges or caflons in the world are found in the high-plateau region — i. e., the region between the Colorado and Wahsatch Mountains, and drained by the Colorado Eiver. This region is 6,000 to 8,000 feet high, and consists of nearly AQUEOUS AQENCIJSf! 25 level strata, which have been cut into by the Colorado and its tributaries in such wise that the whole river system of the country runs far below the general level. The Grand Caflon of the Colorado is 300 miles long and 3,000 ^-o 6,000 feet deep, and all its tributaries come in by side cations of almost equal depth (Fig. 7). Fig. 7.— View of Colorado Cailon and its tributaries, with erosion-columns and mesas in the distance. Besides this prodigious stream cutting, the general rain- erosion has been here upon an equally grand scale. Many thousands of feet have been carried away over the whole area of about 100,000 square miles or more. This is shown hj the isolated peaks and tables of level strata scattered about, and still better by the succession of cliffs shown in 26 DYNAMICAL GEOLO&Y. Fig. 156 (page 250), as will be more fully explained here- after. Time. — The time during which the whole of this enor- mous work was done is but a small portion of the geolog- ical history. It commenced in Middle Tertiary (page 383), continued to the present time, and is still going on. Pot holes.— If we examine the bare rocky beds of swift streams in mountain regions, we often find deep holes with vertical walls like small rock wells. In their bottoms we are sure to find gravel and a good many rounded pebbles. These are called pot holes. They are formed thus : Swift streams form whirling eddies, in which sand, gravel, and rock fragments carried by the stream are whirled about in the same spot until they hol- low out these holes, while the fragments themselves are rounded into pebbles in the process. These become signs of old river beds where rivers no longer exist. / 2. Transportation and Distribution of Sediments, ' -> River agency, it will be remembered, is taken up under three heads. We have already taken up one — Erosion. The other two are best taken up together, as Transporta- tion and Distribution of Sediments. Transporting Power of Water. — Every one is fa- miliair with the fact that running water carries along materials of different degrees of fineness, but the rate at which the carrying or lifting power increases with the velocity is almost incredible to those who have not inves- tigated the subject. It is found that the size or weight of the separate particles or fragments movable by running water increases at the enormous rate of the sixth power of the velocity of the current. Thus, if the velocity of a current be doubled, it can carry a stone sixty-four times as great as before ; if it be increased ten times, it can carry a stone 1,000,000 times as great as before. "We can AQVMOtrS AGENCIES. %1 thus easily understand the prodigious power of mountain- torrents when swollen by heavy rains. It follows from the above that, if a stream be carrying all it can, the least checking of its velocity will cause abundant deposit, and the least increase of its velocity will cause it to take up again what it had previously deposited — i. e., it will scour its bed and banks. Sorting Power of Water. — If we take a handful of earth and throw it into a deep basin, and, after allowing it to settle, pour ofE the water and examine the sediment, we shall find that it is neatly sorted, the coarser particles being at the bottom, and above this finer and finer, until a very fine, smooth mud forms the top. The earth will be still better sorted if we throw it into running water. In this case the coarser will drop first, i. e., higher up, and the finer lower and lower, until only the finest will be carried far down the stream. This is especially the case if the velocity decreases as we go down-stream, as is usually the case in natural streams. Thus, pebbles are found in torrent-beds, and fine mud in lower parts of streams. Stratification. — If we examine carefully the mud or sand of a river-bank or lake-margin, we shall always find them stratified, i. e., in layers of slightly different color and grain. This is easily explained by the sorting power of water. If the water be still, as in a lake or pond, then with every rain earth is brought in, and by settling is sorted, the flLnest falling last. Thus the coarse material of one rain falls on the fine of the previous rain, and every rain is marked by a separate layer. In rivers, the same result follows, but the explanation is a little differ- ent. The velocity of the current is changing from day to day on account of the varying supply of water. The stream-lines also are continually shifting from side to side. Thus the velocity at any one point is all the time changing, and therefore the character of the material 2g DYNAMICAL QEOLOQT. deposited is also changing from day to day, and even from hour to hour — ^now coarser, now finer — and a very distinct, though often irregular, stratification is the result. General Law. — We may therefore state it as a general law that all deposits in water, whether still water, as lakes and seas, or running water, as riyers, are stratified, and, conversely, that all stratified materials, wherever we find them, whether near water or high up on the tops of mountains, and in whatsoever condition we find them, whether as sands and muds or as hard stone, if the strati- fication be a true stratification, i. e., the' result of sorted material, have ieen deposited in water. Upon this very simple law nearly the whole of geological reasoning is based. It is important, therefore, that every one should habitually observe the phenomena described above, not only in lakes and rivers but in shower-rills and pools. We are now in position to explain all the phenomena of rivers. 1. Final Effect of Erosion of Bain and Rivers. There is a certain slope of a river-bed, depending on the amount of sediment carried, at which the river neither cuts nor deposits. This is called its base-level of erosion. Every river is seeking this level. If above it, it seeks to reach it by cutting ; if below it, by building up by sedi- mentation. As soon as the river reaches this level, it rests, so far as down-cutting is concerned, but now begins to sweep from side to side, widening its channel. ' Mean- while the side streams and rain-wash continue to cut down the divides. If this continues without interrup- tion, the final result is a gently undulating land-surface with low divides and broad river channels. This is called a Peneplain. It means that the land has remained steady and the rivers have been working on it, a long time. It is old topography. If the land be now elevated by interior forces the rivers increase in velocity and begin AQUEOUS AeENCIES. 29 cutting again and form deep caflons. These deep caflons are therefore characteristic of neiv topography — i. e., of a rising or newly risen land and of rivers far above their base-level and working hard to reach it. If on the other hand the land sinks, the rivers become more sluggish — they cease to cut and begin to build up by deposit. Thus river channels become delicate indicators of the move- ments of the earth's crust. 2. Winding Course of Rivers. The winding course of rivers is the necessary result of the laws of currents. Streams do not find irregular chan- nels to which they are forced to conform, but they make their own channels. If we straighten these channels, they vnll not remain straight. Some point will wear into a hollow. This will throw, the stream to the other side, which will in like manner be worn, and thus the stream begins to meander. Wow, if we examine any winding stream, we shall see that the swiftest current is on the outer part of the curve and the slowest on the inner side, or, in other words, the current is swifter than the aver- age on the outer and slower than the average on the inner side of the bends. In the figure, the arrows show the Pig. 8. line of swiftest current. Now, if the river is carrying all the sediment its average velocity can, it is evident that it will cut on its outer curve, where the velocity is greater than the average, and deposit and make land on the inner side, where the velocity is less than the average. Thus the outer curve is increased by erosion and the 30 DYNAMICAL &EOLO&T. inner curve by deposit, and the winding tends ever to become greater and greater. This is most conspicuous in cases in which rivers run between mud-banks made by their own deposit. In such cases, the curves become greater and greater, until finally two contiguous curves cut into each other, the river straightens itself, and the old bend is thrown out and becomes a lagoon {d, Fig 9). Fig. 9— a, A, c, sacceeeive stages in the winding course of a liver. Many such lagoons exist in all rivers which run through swamp-lands. Fig. 9 shows the process, and Fig. 10 is a portion of the lower Mississippi Eiver showing the result. Fia. 10. — A portion of lower Mieslssippi. 3. Flood-Plains and Their Deposits. Rivers usually rise in hilly or mountainous regions, and flow in the lower course through flat plains. In flood seasons, the velocity being checked by change of slope, the channels are no longer able to contain their waters, which therefore overflow portions of the flat lands on each side. Tlie area liable to overflow is called the flood-plain. In case of great rivers draining interior continental basins, the flood-plains are very large. The flood-plain of the AQUMOtTS AGJENCmS. 31 Nile is the whole land of Egypt, for without the Kile the whole of Egypt would be a desert. Egypt is literally the daughter of Mlus. The flood-plain of the Mississippi extends from the mouth of the Ohio Eiver to the Gulf — its area is 30,000 square miles. Now, since great rivers always rise in mountain-regions, and since the general rain-erosion in such regions is yery great, it is eTident that in flood seasons they gather abun- dant sediment, and, when these muddy waters overflow, the checking of velocity causes abundant deposit all over the flood-plain. With every flood this deposit is renewed, and the stratum becomes thicker. Thus, the level of the flood-plain is built up by sedimentary deposit, without limit. In the Mississippi Eiver the flood-plain deposit is about fifty feet thick. In the Nile it is forty to eighty feet thick. Time. — On the flood-plain of the Nile stand the oldest monuments of civilization in the world. One of these (the statue of Eameses II), supposed to be 3,000 years old, has been covered about the base with sediment nine feet deep. The whole thickness of the Nile sediment at this point is forty feet — ^nine feet in 3,000 years would make forty feet in 13,330 years as the age of the Nile deposit. This is, of course, but a rough estimate. The rate may not have been uniform. But in any case the whole time belongs to the present geological epoch. licvees, Natural and Artificial. — In rivers which regularly flood their plains we always find a sort of em- bankment on either side near the river, higher than the rest of the flood-plain, and consisting of coarser material. This is called the natural levee (Pig. 11, I, I). When the river is at full flood, e, e, the whole flood-plain is covered, but at half flood d, d, d, it is often divided into three streams, viz., the river channel and the back swamp on either side. The cause of the natural levee may be explained thus : The whole flood-plain is covered with water moving 32 DYNAMICAL QEOLO&T. slowly seaward. Through the middle of this compara- tively still water runs the swift current of the river- FiG. 11.— Ideal Bection of a flooding river, u, a, a, a, original bed ; 6, J, flood- plain ; l^ I, natural levees ; c, low water ; d, d, d^ half flood ; «, «, e, full flood. channel. ISTow, on the two sides of this swift current, just where it comes in contact with the stiller water, and is checked by it, there will be a line of abundant and coarser sediment. Artificial Levees. — Natural levees can not restrain the floods of rivers, since they are made by such floods. By deposit, the bed of the river, the natural levees, and the back swamp, all rise together, maintaining their rela- tive level. If, therefore, we desire to restrain the floods FiQ. 12.— Ideal section of a river-bed and plain which was built up naturally for a time and then restrained by artificial levees^ I, I. and reclaim the flood-plain, we must build artificial levees upon the natural ones. This interference modifies greatly the phenomena of deposit. The river continues to build up its bed as before, and would in time again flood as before, if the levees were not built up higher from time to time. The flood-plain, however, no longer receives deposit. Therefore the river-bed being raised by deposit, and the levees by man, the river finally runs on the top of an embankment, which rises ever higher above the sur- AQUEOUS AGENCIES. 33 rounding plain, and the danger from accidental breakage of the levee is ever greater (Fig. 12). It is said that the river Po, from this cause, now runs above the tops of the houses on the plain. 4. Deltas. The flood-plain of a river may be divided into two parts, viz., the river-swamp and the delta. The river- swamp is that part of the flood-plain which was land- surface when the river began to run, and has been raised only a little by deposit. The delta is that part of the flood-plain which has been reclaimed by the river from the empire of the sea. The river has dumped sediment into the sea or lake, until it filled it up and made a certain amount of land. This made land is the delta. For ex- ample, Upper Egypt is the river-swamp ; Lower Egypt, from Cairo seaward, is the Delta. The flood-plain of the Mississippi, from the mouth of the Ohio to about Baton Eouge, is river-swamp ; thence to the G-ulf it is delta. . A delta may be otherwise defined as an area of flat land at the mouth of rivers, usually of more or less triangular shape, over which the river runs by inverse ramification, emptying by many mouths. The point where the river commences to divide is the head of the delta. The area of some deltas is very great. The delta of the Nile is 10,000 square miles, the delta of the Mississippi is 14,000 square miles, and the common delta of the Ganges and the Brahmapootra is 20,000 square miles. The form of the Mississippi delta is very irregular. It runs out into the Gulf as a narrow tongue fifty miles long, and separated from the Gulf only by low, narrow embank- ments, which are continuations of the natural levees (Fig. 13). Deltas are not formed by all rivers, but only by those which empty into tideless, or nearly tideless, waters. Streams running into pools, ponds, lakes, and rivers run- Lb Conte, Geol. 3 34 DYNAMICAL GEOLOGY. ning into landlocked seas, make deltas ; but rivers emp. tying into strongly tidal seas have wide, bay-like mouths or estuaries. The strong tides and waves not only carry away the sediment brought down and prevent land-mak- ing, but cut away and enlarge the mouths of the rivers. Thus, in this country, all the rivers emptying into the Great Lakes or into the Gulf of Mexico (where the tides Fie. 18.— Mississippi delta. are very small) m.ake deltas, while all emptying into the Atlantic or Pacific have estuaries. So, in Europe, all the rivers emptying into the Mediterranean Sea, the Black Sea, the Caspian Sea, the North Sea, and the Baltic Sea, form deltas, while those emptying into the Atlantic have estuaries. The Ganges (Fig. 14) seems to be an excep- tion to this rule ; for it makes a great delta, although the tides in the Bay of Bengal are strong. The cause of this is the prodigious quantity of mud brought to the sea by the Ganges. Two opposite agencies are at work at the AQUEOUS AaENCIES. 35 mouths of rivers, viz., the river bringing sediment and making land, and the sea carrying it away and destroying land. If the former prevails, a delta is formed; if the latter, an estuary. Fia. 14.— Delta of Ganges and Brahmapootra. (Prom De la Beche.) Mode of Formation. — "We are apt to imagine that we can not observe these phenomena except by becoming travelers. On the contrary, we may observe them in every little stream emptying into a pond. In every such case we shall observe a sand-flat over which the stream runs in many rills, that often change their position. ■ Such a sand-flat is a delta. If we watch the process, we shall see that the stream before entering the pond carries sedi- ment, perhaps is muddy. As soon as it strikes the still water, it spreads out in all directions, the velocity is checked, the sediment falls, the bottom is built up to the surface, and the delta commences. The sand or mud is now carried over the delta, and dumped beyond. Thus the delta grows from day to day (Fig. 15, a). The stream. 36 DYNAMICAL GEOLOGY. as it runs oyer the sand-flat, is often choked with its own deposit, and compelled to seek new channels by dividing. In the figure we have represented the case of a torrent, carrying coarse sediment, rushing down a steep slope into a lake or pond. In such a case the sediment falls quickly, the strata will be irregular and highly inclined ; but, in the case of great rivers carrying sediments for long dis- Fia. 15.— Ideal map (a) and section (S), showing tlie formation of a delta. tances, the coarse material is all dropped higher up, and that which reaches the sea is very fine, and therefore sinks slowly. Hence in great deltas the stratification is nearly horizontal. Again, the figure plainly shows that, if this process goes on, the lake or pond will be entirely filled. All mountain-lakes are being rapidly filled in this way, and a little close observation is sufficient to show that all high mountain-regions, like the Sierra or Colorado mountains, are full of marshes and meadows which are extinct lakes. \. lAge of Deltas — All deltas are growing. The rate of •*^owth in some cases has been observed. The delta of the Po has advanced twenty miles into the Adriatic Sea since Roman times, for the town of Adria, then a sea- port, is now twenty miles inland. The delta of the Eh6ne has grown thirteen miles in the Christian era. The Mis- AQUEOUS AGENCIES. 37 sissippi delta is pushing seaward more rapidly than any- other, evidently because it pushes along narrow lines. It is advancing now at the rate of three hundred and thirty feet per annum, or a mile in sixteen years, or six miles per century. The age of deltas can not, however, be got in this way, because the river often changes its mouth, and dumps its freight now here, now there, along the whole water-front of its delta. The age or time is usually estimated by getting the cubic volume of the delta, and dividing this by the annual mud-discharge [T =• m.-d. But without more accurate observations than have yet been undertaken, these estimates are not entitled to much confidence. 5. Estuaries. The wide mouths of certain rivers are called estuaries. We have already explained why some rivers have estuaries and some make deltas. All the rivers running into the Atlantic and Pacific Oceans have estuaries, because the tidal currents are stronger in carrying away than the river in bringing down and depositing sediments. The Bay of Pundy, the Hudson Eiver to near Albany, the Delaware and Chesapeake Bays, Albemarle and Pamlico Sounds, the Bay of San Francisco, and the Lower Columbia River are estuaries. So also are the wide mouths of the Amazon and La Plata Rivers. So also the firths of Scotland and the fiords of Norway. The velocity and therefore erosive power of tides in estuaries is sometimes enormous. The trumpet-shaped mouth of the river takes in a large mass of the tide-wave. As this passes up it is compressed into a narrower channel, and therefore rises higher and rushes with increasing velocity. In the upper part of Bristol Channel the tide rises forty feet ; in the Bay of Pundy, sixty feet ; in Puget Sound, twenty feet. If the water be shallow and the resistance to advance great, the tide rises 38 DYNAMICAL GEOLOGY. into a breaker, which adyances at a rate sometimes twenty miles an hour. It is evident that the erosive or land- destroying power of such currents is enormous. What- ever is thus gathered in its upward course, together with whatever is brought down as sediment by the river, is all carried by the ebb-tide out to sea, and therefore lost to the land. In fact, of all places along the water-front, the mouths of rivers are the most vulnerable to the attacks of the sea. * Deposits at tlie Mouths of Rivers. — The retreating tide carries away to the sea both what is gathered by the advancing tide and what is brought down by the river. Therefore the estuary is scoured out, rather than receives deposit. Yet, in certain sheltered coves — such as repre- sented in Fig. Yt, at a and i — stratified deposits will often be found. These are peculiar. They consist of an alterna- tion of fresh-water, brackish- water, and salt-water deposits, known each by the shells which they contain. The reason is this : In dry seasons these coves are occupied by salt water only, and in flood seasons by fresh water only. Also along the water-front of deltas some parts will be receiving sediments from the river, while other parts, receiving no such sediments, will be inhabited by marine animals. With the shifting of the mouth of the river, these latter may be again covered with river sediments. Therefore, in all deposits made at the mouths of rivers there will be an alternation of fresh- and salt-water de- posits. And, conversely, stratified deposits, consisting of such alternations, wherever found, are judged h% geologists to have been formed at the mouths of ancient rivers. 6. Bars. The formation of bars is an admirable illustration of the laws of sediment-laden currents. Bars are formed at * Estuaries are also often formed by subsidence of the land. They are then the drowned lower courses of rivers. AQUEOUS AGENCIES. 39 the mouths of all rivers by the fan-like spreading of the currents and the consequent checking of the velocity by contact with still water of the sea or lake. They are usu- ally of semicircular or horseshoe-like form, as shown in Figs. 16, 17. In rivers forming deltas (Fig. 16) this is the only bar ; but in rivers forming estuaries (Fig. 17) there are two bars — one at the mouth of the estuary, and the other at its head. The bar at the mouth is formed in the usual way, by the spreading of the current of the / outgoing tide, the consequent I checking of its velocity, and de- L'^'^'f .. posit of its sediment. It has, ^ ^ ^ 't. therefore, the usual semicircular ifiT^ i'/" °'' horseshoe form. There are i *" usually passes or deeper channels ^ through which the tides ebb and Fig. 16. flow. The bar at the head of the estuary or bay is formed by the meeting of two opposing sediment-laden currents, viz., the up-flowing tide and the down-flowing river. The meeting of these at every tide makes still water at that place, and the sediments from Fig. Vr. both are dropped there. In fact, at the head of the estuary there are three associated phenomena, all pro- duced by the meeting of these opposing currents : 1. 40 DYNAMICAL GEOLOGY. The backing up of the river-water by the tides causes it to overflow. There is, therefore, here a more or less ex- tensive marshy or swampy flood-plain. 2. The river here not only forms a lar, but also a more or less extensive flood-plain deposit. 3. The river winds tortuously and in many channels through the soft, marshy soil, forming many marshy isles. These facts are shown in Fig. Yt. In fact, we have here many of the phenomena of a river- delta. The Hudson Eiver, for example, is an estuary, one hundred and twenty miles long.' The tide runs up and meets the river-current, and makes still water about twenty miles below Albany. At this point is the bar. At this point also is an extensive marshy overflow-land, through which the river winds its tortuous course. The same phenomena are seen at the head of the Bay of San Francisco. The river here winds, by many tortuous channels with islands between, through an extensive marsh (^wfo-lands). The bar is also, of course, found here. Kemoval of Bars.^If a bar be scraped away, it will be re-formed by the same agencies which originally formed it. Only constant dredging can improve it. If the river- channel be contracted by dikes so as to increase the ve- locity of the current, it will indeed scour out the bar, but the latter will again form at a new point of equilibrium a little lower down. In rivers forming deltas the bar has been successfully removed, in some cases, by means of jetties extending beyond the mouth of the river into the sea or gulf. The now swifter current scours out the bar, and the sediment is delivered in deep water, where it must deposit a long time before the bar is re-formed. The most remarkable examples of such improvement of bars are at the mouth of the Danube by the Austrian Government and at the mouth of the Mississippi by the United States Government. We have now traced the agency of rain and rivers from AQUEOUS AaENCIES. 41 mountains to sea. In fact, in the phenomena of estuaries and bars, we have already a cooperation of rivers and sea. This brings us very naturally to the next head, viz.. Agency of the Ocean. Section II. — The Ocean. Waves and Tides. The ceaseless beating of waves on an exposed shore can not fail to impress the observer as a powerful erosive agent. Tides assist the waves, not only by creating pow- erful currents in all bays, inlets, and estuaries, as already explained, but also by lifting the sea-level and therefore presenting new points of attack. As in the case of rivers the erosive power is greatly increased by the sand, gravel, and pebbles carried by the current, so in the case of waves the sand, gravel, shingle, and rock-fragments torn from the cliffs are taken up again and hurled back with vio- lence, and become the chief agents of further erosion. Although, however, so incessant and violent in action and so conspicuous in effects, yet, being confined wholly Pig. 18.— View of inclined strata, with faces exposed to waves. to the shore-line, the aggregate effect of wave-erosion is far less than that of the universal erosion of rain and rivers. 43 DYNAMICAL eEOLO&T. Resulting Forms. — It is interesting to trace the forms of coast-lines to their causes. If the country-rock be stratified, and the strata dip toward the sea so as to present their faces to the wareSj then the erosion will be slower and the coast-line comparatively even (Mgs. 18, 20, a). If, on the contrary, the strata be level and the waves act on the edges (Fig. 19), then the cliff will be Fio. 19.— Section view of level strata, a and b, with edges to waves. undermined; overhanging tables will fall from time to time, and the erosion will be rapid. Finally, if the edges of vertical or inclined strata be turned toward the waves (Mg. 30, t), then the coast-line will be deeply dissected. Fie. 20— Map view of inclined strata, dipping northward, as shown \ij axiow. a, faces to waves ; &, edges to sea. i. e., composed of alternate headlands and inlets. In these inlets, the waves, gathering force as they are pressed into narrower channels, beat with prodigious force. Again : since waves and tides act only on the shore-line as high as they can reach on the one hand, and as deep as they can touch bottom and form breakers on the other, it is evident that they act as a horizontal saw, cutting AQUEOUS AGENCIES. 43 down the land a little below the sea-level. Hence, along . a shore-line which has suffered much from beating waves, we are apt to find first a steep, perhaps overhanging cliflE ; then a level, submarine plateau ; and then, as we go far- ther, a sudden falling ofE to deep water. In Fig. 21, the Pio. 21.— Ideal eectioii view of submarine plateau and shore-cliff : I, sea-leyd ; a, b, submarine plateau ; «, present, and 8\ the original, shore-line. dotted line shows the original configuration of land and position of the shore-line. ' Such level plateaus, termi- nated by cliffs, are often found far inland. In some cases, though not in all, they indicate old sea-cliffs. Nearly all shore-lines are receding under the incessant action of waves and tides, but the rate is very different in different places. As rain-erosion is concentrated on certain lines, giving rise to surface inequalities, such as gorges, ravines, caQons, etc., so wave and tide erosion give rise to nearly all the inequalities of coast-line. The general form of continents, and their largest inequalities, are doubtless due to other (i. e., continent-making and mountain-making) causes ; but all the promontories, har- bors, bays, etc., are due to ocean-erosion. As land scen- ery is due mainly to rain and river erosion, so sea-shore scenery is due mainly to sea-erosion. Every projecting promontory will usually be found to consist of hard rock, and every indentation is determined either by the soft- ness of the rock or else by the mouth of a river giving entrance to powerful tidal currents. Examples are found on every coast. In our own coun- try the rocky shores of New England everywhere show the wasting action of waves. Farther south the coast is 44 DYNAMICAL OEOLOGY. wasting in some places and gaining in others ; for, as we shall see hereafter, waves and tides may make as well as destroy land. In Europe examples are more numerous and striking, and have been more carefully studied. The rushing tide through the English Channel and Dover Strait has greatly enlarged and is still enlarging the channel. The eastern coast of England is now being eaten away at the rate of from three to five feet per annum. The church of the Eeculvers, which stands near the mouth of the Thames, and for many centuries has been a landmark for ships entering that port, stood, in the time of Henry VIII, one and a half miles inland, on a high clifE. It is now on the sea-margin, and would have long ago fallen into the sea if it had not been saved by an artificial sea- wall. Many isles in the German Ocean have entirely dis- appeared in this way. Heligoland is fast going, and already almost gone. The western coast of England, Ireland, and Scotland Fig. 22. is wasting less rapidly at present, but only because noth- ing but hard rock is left. The deeply dissected coast- AQUEOUS .A&ENGIES. 45 lines, with high promontories, separated by deep inlets, show the waste they have suffered in previous geological times. As we go north, the evidences of destruction become more and more conspicuous. To the north of Scotland, among the Hebrides, Orkneys, and Faroe Isl- ;inds, are found groups of bare, wave-worn rocks, standing in the midst of the sea, mere skeletons of once fertile islands (Fig. 22). But all these effects, viz., boldness of the headlands, the depth of the inlets, the intricacy of coast-dissection, reach their highest point on the coast of Norway. Any good map of this country (see Fig. 23) shows that the whole coast consists of alternate promontories and inlets. The promontories are rocky head- lands, 2,000 to 3,000 feet high, and the inlets run 50 to 100 miles inland. Such deep inlets, separating high headlands, are called j?orc?s. Closer inspection shows a line of islands off the coast. These are rocky islands 2,000 to-3,000 feet high, and of hard- est granite. These granite isles are probably the axis of the Scandina- vian mountains — in fact, of the Scan- dinavian Peninsula. If so, then it would seem that the whole western slope of these mountains has been swept away, that the sea has already broken through the axis or backbone, and is now gnaw- ing among the ribs on the eastern flank. On nearly all bold and severely beaten coasts we find such off-shore islands, which are the fragments of a former coast-line. The present form of the Norway coast, however, is not wholly due to sea-erosion, but also largely, as we shall show hereafter, to subsidence. Yet, as Norway is per- haps the oldest part of the European continent, we have I^Q. 83.— Map ot Norway coast, showing the dis- sected coast-line and isl- * ands off shore. 46 D YNAMICAL . GEOLO& Y. probably not exaggerated, in what is said above, the ravages it has suffered from its ancient enemy, the sea. Transportation and Deposit. — The lifting power of waves is immense, often taking up rock-fragments of many tons weight and hurling them with violence against the shore-line ; but they usually carry only a very short distance. Jnder certain conditions, however, waves may transport materials for many hundreds of miles. Thus, on account of the trend of the Atlantic coast and the prevalence of north winds, the coast material is cast up on shore and falls ofE a little southward with every wave. Thusy shore-sands creep southward slowly, even to the point of Florida, although the coast-rock of Flor- ida is all limestone. So, also, the shore-sands of Lake Michigan are carried southward by wave-action, and accumulate about Chicago. Though waves are usually destructive rather than con- structive, yet they often add to the land along shore- lines by deposits. Such deposits are very characteristic : 1. They are usually coarse material and thoroughly water- worn— i. e., round-grained sand, gravel, or shingle. 2. The lamination is often highly inclined and irregular. 3. They are often affected with ripple-marks (Fig. 24). Fig. 24.— Kipple-markB. 4. They are often impressed with tracks of animals and with rain-drops. Now, all these marks are found in rocks far inland and high up the slopes of mountains. We can thus AQUEOUS AGENCIES. 47 often recognize the old shore-lines of preyious geological epochs. Oceanic Currents. The earth is covered with two oceans, an atmospheric and an aqueous. The former covers the whole of it, fifty or more miles deep ; the latter covers three quarters of it, three miles deep. On the surface of the one we swim and sail, on the bottom of the other we crawl. Both of these oceans are in constant circulation in every part. The cur- rents in the one are winds, in the other oceanic streams. The cause of circulation and the general directions of the currents are also the same. There is, indeed, some dif- ference of opinion as to the immediate cause of oceanic currents ; but there can be no doubt that, directly or indi- rectly, they, like winds, are caused by difference of tem- perature between the equator and the poles. In both cases, too, there are disturbing causes which complicate the result. In the one case, local variations of tempera- ture and extreme mobility of the medium ; in the other, the existence of unseen submarine banks, and especially of" impassable barriers, the continents. As our sole object is to discuss their geological agency, we shall describe but one of these great oceanic currents as an example. Gulf Stream. — This stream takes its origin in the equa- torial current which, stretching across the Atlantic from the coast of Africa, strikes the wedge-shaped eastern point of South America and divides north and south. The larger northern branch runs along the coast of South America into the Caribbean, and thence through the Straits of Florida into the North Atlantic. After passing the point of Florida it turns north, runs along -the coast of the United States, turns eastward from the coast of Lab- rador, and, after sending a branch toward the Arctic re- gion north of Europe, turns southward to join again the equatorial current on the coast of Africa. The amount of 48 DYNAMICAL GEOLOGY. water carried by this ocean-stream is probably greater than that of all the rivers in the world. It is equivalent to a stream fifty miles wide, one thousand feet deep, and run- ning at a rate of three miles per hour. Its extreme Telocity, where it passes through the Straits of Florida, is four to five miles per hour. Geological Agency. — Oceanic streams run on beds and between banks of still water, and therefore, probably, have no erosive agency, but they are important agents in the transportation and distribution of sediments. All the debris of land-surfaces are brought by rivers to the water- front and dumped there. Tides, in their retreat, may take these seaward a few miles, but these also soon lose their velocity and drop their freight. Were it not for oceanic currents, therefore, the whole debris of land-surfaces would be dropped within thirty to fifty miles of the shore- line. As it is, nine tenths are so dropped. Marginal sea-bottoms are, therefore, the great theaters of sedimenta- tion. Nevertheless, a small portion of finest sediment is carried within reach of oceanic currents, and by them strewed broadcast over portions of deep-sea bottom. We find good examples of this in the course of the Gulf Stream. The sediments of the Amazon may be traced from its mouth seaward for a great distance. It is then taken by oceanic currents and carried northward, and much of it deposited on the coast of Guiana, three hun- dred miles distant, and the remainder into the Caribbean Sea. According to Humboldt, much sediment is carried from the Caribbean into the Gulf of Mexico. The stream receives there also, possibly, contributions from all the Gulf rivers, especially the Mississippi, and may deposit these again- along the coast of Florida and the Bahama Islands. The surface transparency often conspicuous in oceanic currents is no evidence against their carrying sediments ; for there is this difference between rivers and oceanic AQUEOUS AGENCIES. 49 streams : In rivers, besides the general current, there are partial currents, from side to side and up and down, which keep the stream turbid to the surface ; while ocean-streams, running on beds and between.banks of still water, have no such partial currents. There is nothing to prevent sedi- ments settling exactly as in still water. Thus ocean-cur- rents usually carry sediments, if at all, only in their deeper parts. Deep-sea deposits are undoubtedly of great im- portance, but only recently have attracted much attention. Submarine Banks. — These are formed by checking the velocity of sediment-laden currents, whether tidal or oceanic. The checking may be caused by the meeting of two opposing currents, or by the current passing through Fio. 25.— Tides of the German Ocean. a narrow strait into a wide sea. In other words, subma- rine banks are formed under the same conditions as bars ; Lb Cohtb, Geol. 4 60 DYNAMICAL GEOLOGY. and bars at the mouths of rivers are, in fact, one form of submarine bank. The best examples of banks formed by tidal currents are found in the North Sea.. It is seen in the map, Fig. 25, that the tidal Tvave from the Atlantic, striking on the British Isles, divides into two parts, one entering the North Sea through Dover Strait, the other by the Shetland and Faroe Islands. That through Dover Strait runs swiftly through the narrowing channel, gathering much sediment. As soon as it passes Dover Strait it spreads fanlike, its velocity is checked, and it deposits sediment. In the mean time the other branch, coming in from the north, meets the southern branch, and makes still water at some point, as a, and deposits sediment. Again, all the rivers emptying into this sea from the south form bars at their mouths. To these several causes are due the numerous banks which render the navigation of this shallow sea so dangerous. Banks are formed also by oceanic currents. For example, the Gulf Stream, passing through the Straits of Florida, eddies on both sides and forms the Bahama and Florida Banks. Again, the Banks of Newfoundland are at least partly formed by the Arctic current bearing icebergs loaded with debris from Greenland (see pp. 66, 67), meeting the warm Gulf Stream, whereby the bergs are melted and their burden dropped. Liand formed by the Agency of Waves. — We ha.ve spoken of waves only as destroying land, but under suita- ble conditions they also form land. On submarine banks, however produced, islands are formed by waves. When by sedimentary deposit the bank is built up to near the water-surface, so that the waves touch bottom and form breakers, then the bank is ^eaten up above the surface and forms islands, which continue to grow by the same agency. Such islands are always low, narrow, and long in the direction of the coast-line. In this way are formed the small islands which overdot the surface of extensive AQUEOUS A&MNCIES. 51 submarine banks — also the low islands about the mouths of estuaries and harbors, such as Sandy Hook and Coney Island about New York Harbor, and the long sand-spits off the shores of shallow seas, as, for example, the shores of North Carolina (Fig. 26), and nearly the whole southern Atlan- tic coast. The debris brought down by the rivers to the estuaries is carried by the re- treating tide seaward and dropped near shore. On the sea-margin bank thus formed, the waves beat up long, narrow sand - spits, separated only by tidal inlets. This is the condition on the North Carolina coast. These barriers to the retreating tide then cause the estuaries to fill up, until they are separated from the mainland only by narrow tidal channels. Thus are probably formed the sea-islands on the South Carolina coast. Finally, the tidal channels may be filled up, the islands added to the land, and the coast-line transferred seaward. Along nearly all coasts we find a line of small islands. These are of two kinds. The one consists of high, rocky islands, ofE bold coasts, as in Norway, Greenland, etc.; the other of low, sandy islands, off level coasts, as on the southeastern coast of the "United States. Those of one Fig. 26. — Nortli Carolina coast. 52 DYNAMICAL GEOLO&T. kind are formed by land-destroying, of the other kind by land-forming action of waves. The one are the scattered remnants of an old coast-line, the other the beginnings of a 7iew coast-line. Section III. — Ice. Ice may act either as land-ice — glaciers, or as floating ice — icebergs. Glaciers. The action of glaciers can not be observed by every one in his own locality, since they exist only in very high mountains or in high latitudes ; but the subject is a very fascinating one, and some knowledge of it gives additional interest to mountain-travel. The summits of high mountains, especially in cool, moist climates, are not only covered with perpetual snow, but from this snow-cap there extend down the valleys, far below the region of perpetual snow, solid masses of ice, which are in continual, slow motion downward. These valley prolongations of the snow-caps — these moving masses of ice, these ice-streamS — are called glaciers. All mountain-peaks and mountain-ridges are trenched on the sides with radiating or transverse valleys. Now, if we imagine such a peak or ridge to be covered deeply with snow and ice, and if we imagine, further, that ice is a stiffly viscous substance like pitch, so that under the heavy pressure of the thick mass it runs slowly down the slope of the valleys to half-way down the mountain, then we have the condition of things as they exist in the Alps, or in any other glaciated region. In most mountains the valleys are occupied by rivers ; in glacial regions they are occupied in their upper parts by glaciers, and in their lower parts by rivers. As rivers, so glaciers, have their AQUEOUS AGENCIES. 53 tributaries, only the tributaries of glaciers are far less numerous than those of rivers. We have said that glaciers are in continual, slow motion down the valley ; yet in temperate climates they do not reach the sea — they do not reach beyond a certain point, called the lower limit of glaciers. Under constant condi- tions the snout of the glacier remains unmoved at this place, even though the glacier is in constant current- motion. This apparent anomaly may be explained thus : The glacier may be regarded as under the influence of two opposite forces. Gravity urges it by slow motion downward, and, if this acted alone, the glacier would run into the sea. But the ice is constantly melted, more and more, as the glacier presses downward into warmer regions ; if this alone acted, the point of the glacier would retreat to the summit-snow. Now, where these two forces — one tending to lengthen, the other to shorten the glacier — ^balance each other, is found the lower limit of the glacier where the snout rests unmoved. Some- times, after a succession of cool, moist years, or a succes- sion of heavy snowfall years, the melting is less rapid or the motion more rapid, and the snout of the glacier will slowly advance, perhaps invading cultivated fields and overturning houses. Sometimes, on the contrary, from more rapid melting or less rapid motion, the snout will recede, strewing debris in its former bed. But, whether the snout stands still, or moves forward, or moves bach- ward, the matter of the glacier is moving constantly downward. In this respect, glaciers are like rivers in certain dry regions. These rivers rise in the mountains, run a certain distance, but never reach the sea — ^never pass a certain point where the supply is balanced by waste from evaporation. ♦ We have said that glaciers reach far below the line of perpetual snow. In the Alps, for example, the lower limit of glaciers is 5,000 feet below the snow-line. This 54 DYNAMICAL GEOLOGY. AQUEOUS AQENOmS. 55 shows that the mass of ice is so great that, although mov- ing at a rate of only a few feet a day, it may reach a mile Fig. S8.— Zermatt glacier. (After Agaoaiz.) below, and many miles beyond, the snow-line before it is all melted. In high latitudes, where the snow-line comes nearer the sea-level, glaciers not only touch the sea, but run far into the sea, and, breaking off, form icebergs. General Description of a Glacier. — In glacial re- gions, where the summit snow-fields are large, every val- ley is filled for a certain distance with a glacier. In the Alps the glaciers are five to fifteen miles long, one to three wide, and two hundred to six hundred feet thick. In the Himalayas they are twenty to forty miles long. In the United States (exclusive of Alaska) the largest glaciers occur in Washington. White Kiver glacier, on Mount 56 DYNAMICAL aEOLO&T. Bainier, Washington, is ten miles long and five miles wide. On Mount Shasta, California, glaciers are found five miles long. In the Sierra, California, and in the Wind Eiver Mountains, Northwestern Wyoming, small, imperfect gla- ciers still linger in the highest and shadiest valleys, near the summits. Many glaciers are also found in Norway, and especially in Alaska. Biit it is only "in polar regions that glaciers are developed now in such proportions as to give us any adequate idea of their great importance as a geological agent. Greenland is a land-mass of almost con- tinental size, heing 1,200 miles long and 600 wide. It is apparently completely covered with snow and ice, to a depth of 2,000 to 3,000 feet. This whole ice-mantle moves bodily seaward, and divides only at the coast into separate glaciers, running into the sea through fiords, and there forming icebergs. These separate marginal glaciers are a mere fringe to the great interior ice-sheet, and yet many of them are thirty to forty miles long and many miles wide. General Structure. — ^As we go from the summit down a glacial valley, we pass from ordinary snow through granular ice {neve) to the perfect ice of the glacier proper. This glacier-ice, however, is not clear, solid ice, but mainly a white vesicular ice, though traversed in many places by veins of clear, blue, solid ice, which gives the whole a striped or agate-like appearance. Moreover, the glacier is broken by great transverse fissures, which often reach clear to the bottom (crevasses), by many marginal fissures along the sides, and by smaller, even capillary fissures, which give it a more or less grained structure. As the ice is constantly melting by the heat of the sun and air and by contact with the rocky bed, the surface is full of streams. These soon fall into crevasses, and find their way to the bottom and down the glacier-bed to tho valley below. Thus from the snout of every glacier runs a stream. The surface of a glacier is not smooth, as AQUEOUS A&ENGIES. 6? might at first be supposed, but usually very rough. This roughness is due partly to rock-fragments from the crum- bling clifEs on each side, as will be presently explained ; partly to the unequal melting of the ice by the sun, pro- ducing pinnacles and hollows, as erosion produces hills and valleys on land ; and partly to the crevasses. For these reasons, the travel over the surface is often not only difficult but dangerous, especially as the crevasses are often concealed by recently fallen snow. Moraines ; Lateral Moraines. — On each margin of a glacier, near the bounding cliffs, is found a continuous pile of debris, consisting of earth and rock-fragments of all sizes up to many hundred tons weight. The pile may be twenty to thirty feet high, and is itself raised on an ice-ridge formed by the protection of the ice beneath from the melting power of the sun. These two marginal piles of debris are called the lateral moraines. They are formed by the constant fall of rocks and earth from the crumbling clifEs on each side. But as the clifEs are not everywhere so steep that their fragments reach the glacier, if the glacier were motionless the contributions would be in isolated heaps only. But the motion of the glacier converts these separate contributions into a continuous ridge ; and, conversely, the continuity of the moraines is a proof of the motion of the glacier. Medial Moraine. — When two tributary glaciers unite to form a trunk-glacier, the two interior lateral moraines of the tributaries unite, and from the angle between the two tributaries will train off as a continuous ridge of debris along the middle of the trunk-glacier to its point. This is called a medial moraine. It is still more indispu- table proof of the motion of the glacier, since it is obvi- ously impossible for debris to reach the middle of a glacier in any other way. The number of these medial moraines will depend upon the complexity of the glacial system, for there will be one for every tributary (Fig. 29). Even a 58 DYNAMICAL GEOLOGY. rocky island in the middle of a glacier or of a tributary will give rise to a separate train. Thus, complex glaicie^s, with many tributaries, may be covered with these trains. Terminal Moraine. — Eemembering that glaciers are in constant motion and yet never pass beyond a certain point, it is evident that everything which is carried by the glacier must find its resting-place at the foot. Here, then, we find an enormous, irregularly concentric pile of dSbris, the accumulation of ages. This is called the iermitial moraine. It is composed mainly of materials carried on the surface of the glacier (top moraine), but also to some extent of materials pushed out from beneath (ground moraine). The Motion of Glaciers and its Laws. — That gla- ciers are actually in continual motion downward is proved by the constant change of position, in relation to points on the bounding cliffs, of conspicuous bowlders lying on the surface of the glacier. From day to day and from year to year these are carried farther and farther down the valley. With a good theodolite the movement of objects on the surface may be observed from hour to hour. Thus, not only the fact and the rate but the laws of motion have been determined. The rate of motion of Alpine glaciers is one to three feet per day. The average rate of the Mer de G-lace (Fig. 29) is estimated by Forbes as about five hundred feet per annum. The extreme length of the glacier is ten miles. A stone fallen upon its upper part would find its resting-place on the terminal moraine only at the end of one hundred years. Every- thing upon or beneath or within the substance of the glacier is finally deposited there. A striking and sad illustration of this is found in the fact that, in several cases, the mangled remains of adventurous climbers, who have fallen into crevasses and perished, have appeared, after many years, at the foot of the glacier. Laws of Motion. — A glacier moves, not like a AQUEOUS AGENCIES. 59 body, all together, sliding on its bed, but exactly like a stiffly viscous body. In other words, the motion of a il^G. 29. — Jler . «* 6' Fig. er. a a' is the north and south range of species A, and b b' of species B — the height of the" curve the number and vigor of the individuals, and 6 a' the overlap of ranges. 3. But in specific character there is no such gradual pas- sage of one species into another — no evidence of trans- mutation of one species into another, nor of derivation of one species from another. From this point of view spe- cies seem to come in at once in full perfection, remain substantially unchanged throughout their ranges, and pass out at once on the other border, other species taking their place as if by substitution, not transmutation. It is as if each species originated, no matter how, somewhere in the region where we find them, and then spread in all directions as far as physical conditions and struggle with other species would allow. We can best make this plain by illustrations : The sweet-gum or liquidambar-tree extends from the borders of Florida to the banks of the Ohio. It is most abundant and vigorous, indeed, in the middle regions, and dying out on the borders, where it is replaced by other species : but is everywhere the same species, unmistakable by its five-starred leaf, winged bark, spinous burr, and fragrant gum. Again, the Red-wood {Sequoia) ranges from south- ern California to the borders jf Oregon. It may be most 133 DYNAMICAL anOhUCH^. vigorous in the middle region — it may decrease in yigor and number on its borders ; but in all specific charactersj wood, bark, leaf, and burr, it is the same throughout. The study of species, as they now are, would probably not suggest, certainly could not prove, the theory of their origin by derivation or transmutation. 4. Temperature regions shade into each other. But this is so only where no barriers exist. If there be barriers, such as an east and west mountain-chain, or sea, or desert, then on the two sides of the barrier the species will be very distinct and without gradation by overlapping. Thus, north and south of the Sahara, and north and south of the Himalayas, there is a marked and, as it were, a sudden change of species. 'It is, again, as if the species originated each in its own area and spread, but were pre- vented from mingling and overlapping on their borders by the barrier. 5. Again, although there are similar temperature re- gions on tropical mountains and in high latitudes — and these latter are also repeated north and south of the equator — yet the species are always difEerent in the three cases. This is because the torrid zone is a barrier pre- venting migration. It is, again, as if species originated each in its own place, but were prevented from reaching similar temperature regions elsewhere by the existence of impassable barriers. Zoological Temperature Kegions. — Animal species are limited by temperature, like plants, and therefore also exist in temperature zones ; but they can not be arranged in the same simple way, evident even to the popular eye — i. e., great classes corresponding to great zones. It is true that, if we compare extremes, viz., polar with tropi- cal regions, we find a conspicuous contrast determined by temperature, certain great families being characteristic of each — as, for example, among mammals, the great pachyderms, the elephant, rhinoceros, hippopotamus, and ORaANIC AGENCIES. 133 the great cats, lions, tigers, jaguars ; among birds, tou- cans, parrots, trogons, ostriches ; among reptiles, croco- dilians and pythons ; and among corals, the reef -builders, characterizing the tropics ; while the musk-ox, white bear, seals, walrus, ducks and geese, characterize the polar regions — yet we can not make a zonal arrangement of families as easily as we can with plants. But, confin- ing our attention to species or even genera, animal forms are subject to the same laws as those of plants : 1. All animal species are limited in range ; 3. The range of species is more limited than that of genera, and of genera than that of families, etc.; 3. Contiguous ranges grad- uate into each other by overlapping, the species inter- mingling and coexisting on the margins ; 4. Each species reaches a maximum of number and vigor in middle regions and dies out on the borders ; 5. But in specific character they seem to remain substantially the same throughout their range, and do not change or transmute into other species on the borders ; 6. Physical conditions may limit their range, but do not seem to change them into other species, though varieties may be formed in this way ; 7. Here, again, it is as if Species originated, no matter how, in the places where we find them, and have spread in all directions as far as physical conditions and struggle with other species would allow. All that we shall say hereafter will apply equally to animals and plants. Continental Faunas and Floras. — If there were no barriers to the spread of species around the earth on the same zone, there can be no doubt that they would thus spread, and faunas and floras would be arranged in a series of temperature zones from the equator to the poles, containing the same species all around. But the oceans are impassable barriers between the continents, and there- fore the faunas and floras of difEerent continents are sub- . stantially difEerent. It is, again, as if they originated on the continents where we flnd them, and have been pre- 124 DYNAMICAL GEOLOGY. vented from spreading and intermingling by the impassa- ble barrier of the ocean. Even apparent exceptions, when examined, confirm the law, as we now proceed to show. Pig. 68 is a north-polar view of the earth, and 1, 3, 3, 4, 5, are the temperature zones so often referred to. Now. Fig. 68 commencing with Nos. 4 and 5, the species in the Eastern and Western Continents are substantially the same, for the lands of the two continents approach each other in these zones so nearly that they may be considered as one. There is no barrier to the spread of species all around the pole. But when we come to No. 3, and still more to No. 3, the difference of species is almost complete, and many genera are also different — and that, not because the physical conditions are unsuitable ; for European species introduced in our country do so well that they often kill out our own native species. Nearly all useful and nox- ious species have been thus introduced. They were not here when America was discovered only because could not get here. ORGANIC AGENCIES. 135 We said the difference is almost complete. There are, therefore, some exceptions, but these only confirm the principles on which the rule is founded. They are of three kinds : 1. Hardy or widely migrating species. Some hardy species range through ISTo. 3 into No. 4, and these may pass over from continent to continent. Some birds, like the Canada goose and mallard duck, migrate in sum- mer to No. 4, and thence in winter southward in both continents. 2. Introduced species, which have become wild. 3. ^?jBi«e s^ectes, mostly of insects and plants. It is a curious fact that species of plants and insects, isolated on the tops of high mountains near the snow- line, are similar to each other on the two continents, and also similar to Arctic species. This latter fact gives the key to the explanation. The geological epoch imme- diately preceding the present (glaciaV epoch) was charac- terized by extension of Arctic conditions southward even to the shores of the Mediterranean and the Gulf of Mex- ico. At that time, therefore, Arctic species occupied all Europe and the United States. As the cold abated, Arc- tic species mostly went northward to their present home in the Arctic zone. But some followed the receding Arc- tic conditions upward to the tops of mountains, and were left stranded there, both in Europe and this country. In No. 1 the species on the two continents are still more markedly different, the difference extending even to families and in some instances to orders. Thus, for example, among plants, the cactus order is confined to America. Among animals, the great pachyderms, e. g., elephants, rhinoceroses, hippopotamuses, also the camels, horses, and tailless monkeys, are confined to the Old World, while the sloths, the armadillos, the prehensile- tailed monkeys, the whole family of humming-birds (of which there are over four hundred species), and the family of toucans, are confined to the New. South of the equator the continents do not again ap- 136 DYNAMICAL GEOLOGY. proach, and therefore the fauna of Africa and South America remain very different even to Cape Colony and Fuegia. Subdivisions. — Continental faunas and floras are again subdivided in longitude, more or less completely, by bar- riers in the form of north and south mountain-chains. Thus the fauna and flora of the United States are sub- divided by the Eocky Mountain and Appalachian chain into three sub-faunas and floras, an Atlantic slope, a Mississippi basin, and a Pacific slope. The difference between these is strictly in proportion to the impassdble- ness of the harriers. Thus, between the Atlantic slope and the Mississippi basin the difference is very small, because the Appalachian chain is low and may be over- passed ; but the Pacific slope fauna and flora are almost wholly peculiar. Almost the only exceptions are strong- winged birds, like the turtle-dove, the turkey- vulture, the large blue heron, etc. In many cases the species are very similar and yet different. The meadow-lark and the yel- low-hammer are examples. Similarly the Ural Mountains separate a European from a northern Asiatic fauna and flora. These subdivisions are perhaps more marked in case of plants than animals. The spread of plants is pas- sive (dispersal), the spread of higher animals also by migration. Special Cases. — Isolated islands, and in proportion to the degree of their isolation, have peculiar species. We shall mention only a few cases as examples of a general law. Australia is undoubtedly the most striking case of all. The trees of this isolated continent are so different from those of the rest of the world that the whole aspect of field and forest is peculiar and strange. The animals are not only all different in species, but the genera and fami- lies and even many orders are peculiar. Of two hundred species of mammals, nearly all belong to a distinct sub- ORGANIC A&ENCmS. 127 class (non-plaeentals), including kangaroos, opossums, ornithorhynchus, etc., which, with the exception of a few species of opossums, are found only in Australia and the island appendages of that continent. Madagascar is sep- arated by a deep sea froni Africa, and we therefore find the organic forms entirely different from those of the neighboring continent, or of any other part of the world. It is especially characterized by the great number of lemurs. On the Galapagos (a small group of islands off the western coast of South America, but separated by a deep sea) the animals and plants are all peculiar. Eeptiles of strange aspect abound, but no mammals (except, per- haps, one species of mouse) are known. Thus we see that species are limited north and south by temperature, and in every direction by physical bar- riers. If, now, we add peculiar soil and climates (as in Utah, Arizona, etc.), which, of course, control vegetation and, therefore, animal life, it is easy to see that all these limiting causes produce groups of species confined within certain areas, and differing from other groups, sometimes overlapping and sometimes trenchantly separated. Element of Time. — We have said that faunas and floras differ in proportion to the impassableness of the bar- riers between — i. e., the height and breadth of the moun- tain-chains, the extent of deserts, and the width and depth of seas, etc. But there is still another element of the greatest importance, viz., the length of time elapsed since the iarrier was set up. This element of time connects geographical faunas and floras with geological changes, and thus geographical distribution of species becomes the key to the most recent of these changes. If we suppose species to undergo very slow changes, then the longer faunas are separated the greater becomes their difference. The full discussion of this important point requires a knowledge of the general laws of evolution, which we are not yet prepared to take up. 128 DYNAMICAL GEOLOGY. Primary Regions and Provinces. — Taking all the causes into account, the whole land-surface has been divided by Mr. Wallace into six faunal regions — ^viz. : 1. Nearctic, including all Korth America exclusive of Cen- tral America. 2. Neotropic, including Central and South America. 3. Palmarctic, including Europe, Africa north of the Sahara, and Asia north of the Himalayas. 4. Afri- can, including Africa south of the Sahara and Madagascar. 5. Oriental, including Asia south of the Himalayas and all the adjacent islands. 6. Australian, including Aus- tralia, New Zealand, New Guinea, and the South Sea Islands. These primary regions are subdivided into provinces, and these into sub-provinces, according to the principles already explained. "We will illustrate by only one example. The Nearctic region is divided into four provinces : 1. ORGANIC AGENCIES. 139 AUeghanian; 2. Canadian or boreal j 3. Rocky Mountain ; and, 4. CaUfornian. The limits of these are shown in Fig. 69. Marine Faunas. Conditions are far less diverse in the sea than on land, and the limitation of fauna is less marked, but the same laws hold. Temperature Regions in Latitude. — Fauna are here also arranged in zones determined by temperature. In a north and south coast-line, where the temperature changes gradually, the fauna will also change gradually by the substitution of one species for another; but if for any cause there is a more sudden change of oceanic tempera- ture, there will be a correspondingly rapid change of fauna. For example, on our Atlantic coast, the Gulf Stream hugs the southern coast as far as Cape Hatteras (Fig. 69, a), and then turns away and runs at a greater distance. This makes a great change of temperature at this point. Again, the Arctic current, c, coming out of BaflBba's Bay, hugs the coast of New England as far as Cape Cod, i, and then goes down. Thus Arctic condi- tions prevail in coast waters to this point. Thus there are three very different marine faunas along the coast of the United States — ^viz., a Southern, a Middle State, and a Northern ; and these change somewhat suddenly at Capes Hatteras and Cod. Distribution In Longitude. — By far the larger num- ber of marine species inhabit along shore. For these the deep sea is a barrier no less impassable than the land. Therefore, the species inhabiting the two shores of an ocean like the Atlantic are as completely different as those inhabiting along the two coasts of a continent, as America. Special Cases. — There are many species which live in the open sea and form a distinctive Pelagic fauna. Again, there are others which are conditioned by denth Lb Conte, Geol. 9 130 DYNAMICAL aHOLO&Y. and character of bottom. The most remarkable of these are those inhabiting deep-sea bottom, and forming an abyssal fauna. Again, about the shores of isolated islands, as Madagascar and Australia, the marine fauna are as peculiar as the land fauna. Origin of Geographical Diversity. Until recently the most reasonable view seemed to be that species originated where we find them, and spread in all directions as far as they could. According to this .yiew, the difference between faunas ought to be strictly in proportion to the impassableness of the barriers be- tween. This is largely true, but does not account for all the phenomena. There is another element of equal im- portance, viz., the time during which the iarrier has existed. It is probable that faunas and floras are subject to slow, progressive changes, taking different directions in different places. If there be no barriers, spreading by dispersal or migration prevents extreme diversity. But if a barrier be at any time set up by geological changes, then diversity commences, and increases with time. According to this view, the Australian fauna is so peculiar because this continent has been so long isolated from all others. The fauna of islands off the coasts of continents are often very similar to that of the adjacent mainland, because they have only recently been separated. Thus, for example, the fauna and flora of the British Isles differ but very slightly from those of the Continent, because, as we now know, these islands, even since their inhabitation by man, have been in full connection with Europe. The divergence has commenced, but is only varietal, not specific. This subject will be taken up again, and more fully explained in connection with glacial epoch; p. 403. CHAPTER IV. IGNEOUS AGENCIES. All the agencies which we have thus far discussed tend to destroy the great inequalities of the earth-surface by cutting down the land and filling up the seas. They are therefore called leveling agencies. If they alone acted, they would eventually bring all to the sea-level and inau- gurate a universal ocean. These agencies, however, are opposed by igneous or by elevating agencies, which, acting alone, would make the inequalities much greater than we now find them. The actual amount and distribution of land are the result of the state of balance between these tvfi opposite forces. It is well to observe that the leveling forces are derived from the sun, while the elevating forces are derived from the interior of the earth — being in fact connected with interior heat. It becomes necessary, therefore, first of all to discuss this subject. Interior Heat of the Earth. The surface temperature of the earth varies with lati- tude, but the mean is about 60°. At any place the surface temperature varies between night and day (daily variation), and between summer and winter (annual variation). As we go below the surface, both the daily and the annual variation become less and less, and finally disappear. The daily variation disappears in a few feet, but the annual variation continues and disappears in our latitude only at a depth of sixty or more feet. Below 133 DYNAMICAL GEOLOGY. this the temperature is invariable. The upper limit of the region of invariable temperature is called the stratum of invariable temperature. Its depth varies with latitude, being nearest the surface at the equator, and lying deeper as we go poleward. As already said, below this stratum the temperature is invariable, but it increases as we go deeper. This important fact has been proved by observations in mines and artesian wells. It is true everywhere, but the rate of increase varies, being in some places more rapid (1° in thirty feet), in some less rapid (1° in ninety feet). The average may be taken, for convenience, at 1° for every fifty-three feet, or 100° for every mile of depth. The Interior Condition of the Earth. — Now it is easy to see that at this rate the melting temperature for most rocks, say 3,000°, would be reached at a depth of about thirty miles. Hence, many persons have rashly concluded that the earth is essentially an incandescent, liquid mass, covered with a comparatively thin shell of thirty miles. This would correspond, in a ball of two feet diameter, to a shell of less than one tenth inch thick. On this view, volcanoes are supposed to be openings into this general interior liquid. A little reflection, however, suffices to show that this condition of the interior is improbable. It is almost cer- tain, in the first place, that the rate of increase is not uniform, but decreases, and therefore that the temperature of 3,000° would be found only at a much greater depth than thirty miles. In the second place, 3,000° is the fusing point under atmospheric pressure ; but under the enormous pressure of thirty to fifty miles of rock, the fus- ing point would probably be much higher. Taking these two things into account, it seems certain that, if there be a universal interior liquid at all, the solid shell is much thicker than is usually supposed, and even probable that there is no universal interior liquid at all — and that vol- I&NEOUS AGENCIES. 133 canoes are openings into local reservoirs, not into a uni- versal sea of liquid matter. Eecently there has been a tendency among geologists to accept a compromise between these extremes. It is now well known that rocks, under the combined influence of heat and water, fuse at a much lower temperature. This, to distinguish it from true dry fusion, is called hydrothermal fusion. "While the temperature of true fusion is not less than 8,000°, that of hydrothermal fusion is only 600° to 800°. N'ow, water certainly penetrates the earth to great depths. Therefore many think that the general constitution of the earth is that of a solid nucleus and a solid, crust, separated by a sub-crust layer of liquid or semi-liquid matter. There are many geological phe- nomena that are best explained by such a supposition. The interior heat of the earth is the source of all igne- ous agencies. It shows itself on the surface in three principal forms, viz.: 1. Volcanoes; 2. EartJiquaJces ; 3. Gradual oscillations of the crust. Section I. — Volcanoes. Definition. — A volcano may be defined as a conical mountain, with a pit-shaped, cup-shaped, or funnel- shaped opening atop, from which are ejected, from time to time, materials of various kinds, always hot and often fused. They vary in size from inconspicuous mounds to mountains many thousand feet high. Volcanoes may be active or extinct. Those which have not erupted for a century past are supposed to be extinct. Yet, so-called extinct volcanoes sometimes break out again. Until the great eruption which destroyed Hercu- laneum and Pompeii, Vesuvius was supposed to be" an extinct cone. Since that time it has been very active. Again, in some rare cases, volcanic eruptions are constant. Stromboli and Kilauea, for example, are in feeble erup- 134 DYNAMICAL &I10L0QY. tion all the time. But most yolcanic eruptions are peri- odic. The period of intermission may be ten, or twenty, or fifty, or one hundred years. Ifumtoer, Size, and Distribution. — Humboldt enu- merates 235 volcanoes as known to have erupted in the past century. The number now known is doubtless much greater. In size they vary from little mounds (monticles) to Mount Etna, 11,000 feet; Mauna Loa, 14,000 feet; and Aconcagua, 33,000 feet. In this last case the whole height is not due to volcanic eruptions, for the cone stands on a mountain plateau many thousand feet high ; but the others are wholly built up by eruption. The laws of distribution may be briefly stated as follows : 1. Volcanoes are mostly on islands in the midst of the sea, or on the margins of continents bordering the sea. Only a very few have been found at a distance from the sea. The Pacific Ocean is the greatest theatre of volcanic activity. Its surface is dotted over with volcanic islands, and its margin is belted about with a fiery girdle of volcanic vents. 3. Volcanoes occur usually in lines, as if connected with a crust fissure, or else in groups, as if connected with a subterranean lake of fused matter. The most remarkable linear series of volcanoes is that which, commencing in the volcanoes of Fiiegia, con- tinues, as a chain of active vents, along the Andes and Mexican Cordilleras ; then along the Sierra and Cascade, as the recently extinct volcanoes of these chains ; then along the Aleutian Isles and Kamschatka ; then through the Kurile Isles to Japan and the Philip- pines ; then with more uncertainty to New Guinea, New Zealand, the Antarctic Continent, Deception Island, and back again to Fuegia, after completing the circle of the globe. The most remarkable groups are the Javanese group, the Hawaiian group, the Icelandic and the Medi- terranean groups. 3. Volcanoes are found mostly in strata of comparatively recent date, and the retiring of IGNEOUS AQENGIES. 135 the sea seems in many cases to be associated with their gradual extinction. The recently extinct volcanoes on the east side of the Sierra are good examples. Phenomena of an Eruption. — In some cases, as in the Hawaiian Tolcanoes, the floor of the crater, hardened from previous eruption, becomes hot, then melts ; then the melted lava rises higher and higher, until it overflows and runs down the slope in one or more streams. The volcanic forces being thus relieved, the melted lava again sinks gradually to its former level, and hardens into a floor. Thus all proceeds with but little commotion. In other cases, as in the Javanese volcanoes, premonitions of coming violence are observed in the form of subterranean explosions attended with shakings of the earth ; then, with a powerful explosion, the floor of the crater is broken up, and the fragments are shot with violence, high, some- times miles high, in the air ; then cinders and ashes and smoke are ejected in immense volumes ; then streams of. lava are outpoured, perhaps alternating with explosions of gas and vapor, ejecting cinders and ashes. The ascend- ing vapors are condensed, and fall as deluges of rain, which, with ejected ashes, form streams of mud. In all cases, if the mountain be snow-capped, the melting of the snow produces floods, which are often among the most disastrous features of the eruption. Thus there are two extreme types of eruptions, the quiet and the explosive. In the one, the ejecta are mostly lavas; in the other, gases, vapors, ashes, and cinders. The best type of the former are the Hawaiian, of the latter the Javanese volcanoes. But all grades between exist. The Icelandic volcanoes belong more nearly to the former type, the Mediterranean to the latter. Among Mediterranean, Etna approaches more the former, and Vesuvius the latter. Quantity of Matter lyected. — In the great eruption of Tomboro, in the Island of Sumbawa (one of the Jav- 136 DYNAMICAL GEOLOGY. anese group), in 1815, the explosions are said to have been heard in Ceylon, nine hundred miles distant. The quantity of smoke and ashes was so great that, hanging in the air, they produced absolute darkness for many days, and falling, covered the sea over an area of one hundred miles radius. It has been estimated that the ashes ejected were sufiScient to cover the whole of Ger- many two feet deep, and if piled in one place would make a mass three times the bulk of Mont Blanc (Herschel). Of lava-eruptions, perhaps the greatest is that of Eeyk- janes (Skaptar) in 1783. The mass outpoured has been estimated as twenty-one cubic miles (Herschel). These, however, are extreme cases. One of the greatest erup- tions of Kilauea, that of 1840, poured out a lava-stream forty miles long, which, if accumulated in one place, would cover an area of a square mile eight hundred feet deep. The average of lava-flows, however, is far less. ■One of the greatest eruptions of Vesuvius poured out 600,000,000 cubic feet of lava. This would cover a square mile twenty-two feet deep, or would make a stream seven miles long, one mile wide, and three feet thick. Monticles. — In volcanoes of moderate height, eruptions usually come from the top of the cone or principal crater, but in very lofty volcanoes the pressure necessary to raise lava so high fissures the mountain in a radiating manner. These fissures are filled with liquid matter, which, on hardening, form radiating dikes. Eruptions often take place through these fissures, and thus form subordinate craters and cones about the main cone ; these are called monticles. About six hundred such monticles dot the surface of Mount Etna, some of which are seven hundred feet high above the level of the mountain-slope on which they stand. About Mount Shasta (which is a recently, extinct volcano) are found a number of these monticles. Kature of the Materials Erupted. — The materials I&NEOUS A&ENGIES. 13? erupted are — 1. Eock-fragments. %. Lava. 3. Cinders. 4. Sand. 6. Ashes. 6. Smoke. 7. Gas. The rock-frag- ments are formed in explosire eruptions by the breaking up of the hardened floor of the crater, and require no further explanation. Lava.— This term is applied to melted rock, or to the same after it has hardened again. The degree of liquid- ity depends partly on the degree of heat and partly on the hind of fusion. The lava of Kilauea is as liquid as honey. The bursting of bubbles on the surface of this thin, tIs- Fig. 70.— Lava-tnimel, and "spatter-cone" formed by escaping eteam, Kilauea. (Pliotograph by Libbey.) cous liquid draws it out into hair-like threads like spun- glass, which is borne by the winds and accumulated in certain parts of the crater. This is called " Pele's hair." Thin lava like this, when it first issues from the crater, runs with great velocity, twenty to twenty-five miles an hour : but as it cools it becomes stiller, first like tar, then 138 DYNAMICAL GEOLOGY. like pitch, and therefore runs with less and less speed, until it becomes rigid and stops. Being a bad conductor of heat, lava cools and forms a crust on the surface while it is still liquid and flowing within. The liquid finally flowing out, often leaves a hollow tube or gallery. Again, since all lava contains more or less of gas and vapor, the crust is a sort of concreted lava-foam. This vesicular, spongy lava is called scoria. Sometimes, in very stifily viscous lava, the vapor-bubbles run together and form huge blisters, which, by hardening, form caves. Thus, nearly all lava-beds are full of galleries and caves. It was in the galleries and caves which honey-combed the ancient lava-flows of southern Oregon that a handful of Modocs defied so long the power of the United States Army. Again, the liquidity of lava, and its appearance after solidifying, depend much upon the hind of fusion. Lavas are often in a state of Tiydr other mal fusion (page 133), i. e., half fusion, half solution in superheated water. Such a semi-fused mass, on concreting, makes a kind of earthy stone. Sometimes, in fact, the ejecta are little more than hot mud, and concrete into tufa. Cinders, Sand, and Ashes are only different forms of hardened lava. The liquid lava, before ejection, may be so largely mingled with gas and vapors that it is liter- ally a roch-foam. Masses of this rock-foam, ejected with violence into the air, cool and fall as cinders. Often the greater part of the ejections is of this kind, and thus are formed cinder-cones. Sometimes the violence of the explosions is so great as to break up the liquid mass into rock-spray. This falls again as sand or ashes, according to its fineness ; or else the rock-fragments and cinders are thrown up, and, falling again repeatedly, may be triturated into dust or ashes. The finest rock-dust hang- ing in the air is called smoke; the same, fallen to the earth, ashes. Volcanic ashes, wet with water and con- IGNEOUS AQENGIES. 139 solidated either on the spot or after transportation and sorting, is called tufa. Physical Conditions of Lava. — If lava cools very slowly, the minerals of which it is composed separate and crystallize more or less perfectly. This is stony lava. If it cools rapidly, it forms volcanic glass. If the volcanic glass be full of vapor-bubbles, it forms scoria. If volcanic ashes mixed with water solidifies, it makes tufa. Thus there are four physical states in which we find lava, viz., stony, glassy, scoriaceous, and tufaceous. Classification of Hardened Lavas. — Hardened lava consists essentially of two principal minerals, viz., feld- spar and augite.* If the former predominate, it is called feldspathic ; if the latter, augitic lava. These two min- erals are often not detectable except with the microscope, and yet the two kinds of lavas may usually be distin- guished by the eye. The lighter colored and lighter weighted are usually feldspathic ; the darker and heavier, augitic. The feldspathic lavas are said to be acidic j the augitic, basic. Both of these kinds take on the four physical states mentioned above. Feldspathic lava, in the stony condition, is trachyte; in glassy condition, obsidian j in scoriaceous condition, pumice ; in tufa- ceous, the light-colored tufas. Augitic lava, if stony, is basalt ; if glassy, pitchstone; if scoriaceous and tufa- ceous, blach scoricB and tufas. Gases and Vapors. — The gases ejected from vol- canoes are steam, chlorhydric acid, sulphurous acid, sulphhydric acid, and carbonic acid (H,0, HOI, SO^, HjS, COj). The first three are characteristic of true eruptions, the others of feeble, secondary volcanic activ- ity. Of all, steam is by far the most abundant. In vol- canoes of the explosive type the quantity of steam is often enormous. This fact strongly suggests this vapor as the main agent of eruption. Flames are often spoken * The pupil ought to be shown specimens of these minerals. 140 BYNAMIGAL OEOLOQY. of ia eruptions. It is possible that there may be some- times feeble flame from the combustion of H or H,S, but probably the so-called flame is nothing else than the, ruddy reflection of the glowing liquid in the crater upon the smoke and cloud hanging in the air. Formation of Volcanoes and. their Structure. — It is now generally admitted that volcanic cones are built up mainly by their own eruptions. On this view, their origin and mode of growth may be briefly described as follows : 1, The increase of heat (by causes which we lit- Fio. 71.— Section across Hawaii. Z, Mauna Loa ; K, Mauna Kea. tie understand) at the focus of the volcano thins the crust in that point, until it gives way, and the melted matter is outpoured on the surface around the opening. 3. With every eruption the accumulated material rises higher and spreads farther ; and thus a conical mound is formed. The shape of this mound will depend on the kind of mat- ter erupted. If it be very liquid lava, it will spread far, and the cone will be low in proportion to the base, as in the Hawaiian volcanoes (Fig. 71) ; but if the material be cin- ders, these will pile up into a steep cone (Fig. 73). The repeated lay- ers of lava or cinders produce a stratified appearance ; but this must not be confounded with true stratification. 3. With every eruption, the eruptive throes split the sides of the cone with radiating cracks, which, filling with liquid and hardening, form radiating rocky ribs called dikes (Fig. 73), and these bind the lava or cinder Fio. 72.— Section of cinder cone. IGNEOUS AGENCIES. 141 layers into a stronger mass. 4. When the cone grows very high, eruptions will take place through these fis- Fis. 73.— Dikes in Etna. sures, as well as from the top crater, and thus will be formed secondary cones or monticles. 5. If any of these monticles cease to erupt, they will be coTcred up by ejec- tions from the main crater or other secondary craters. All these facts are shown in Fig. 74. 6. From time to Fig. 74.— Ideal section of a volcano, ss, original surface ; mm, monticles ; m'm', extinct monticles ; cr, cr, original stratified crast» 142 DYNAMICAL &EOLO&T. time, at very long intervals, there occur very great erup- tions. If the volcano be of the quiet type, the whole top of the cone is melted, and, after eruption, is ingulfed ; or if of the explosive type, the whole top of the cone is blown into the air, and the mountain is disemboweled. In either case a yawning chasm many miles in extent is left. 7. Within this great crater, by subsequent erup- tions, is built up a smaller cone, and within this again often still smaller cones. Thus" volcanoes often have about their present eruptive cone a great surrounding. rampart. This rampart is the remains of the great crater. In Vesuvius (Fig. 75), Mount Somma, s, is the V Pig. 75. — Section of VeeuviuB. w, Vesuvius cone ; 8^ Mount Somma ; «', other side of Somma overflowed by lava from Vesuvius. remains of such a great crater, the other side of it being broken down, and now covered by flows from the present crater. Crater Lake. — ^An excellent example of this structure is found in Crater Lake. This beautiful lake, with its splendid blue waters, occupies a yawning chasm on the top of an extinct volcano in southern Oregon. The lake itself is about six miles in diameter and 3,000 feet deep (the deepest lake on the American continent), and is sur- rounded by almost perpendicular walls, 1,000 to 2,200 feet high. From the midst of the blue waters, but nearer one side, there rises a beautiful island (Wizard Isle), 800 feet high, which is in fact a cinder cone with a small crater atop. Fig. 76 gives an ideal section of the lake and island, and also, in dotted outline, the supposed form lONEOUS AGENCIES. 143 and height of the original volcano before ingulfment. This former volcano has been named Mt. Mazama. Fig. 7B.— Ideal sectiou of Crater Lake, Mt. Mazama, and Wizard Isle. (Af tfir Diller.) Age of Volcanoes. — Frota the progressive manner in which volcanoes grow, it would seem that we may esti- mate their age. Such estimates, it is true, must be very- rough, yet they are useful in familiarizing the mind with the idea of the great amount of time necessary to account for geological phenomena. For this purpose we will use Etna. There have been, indeed, other volcanic eruptions great enough to build this mountain at once, but the eruptions of Etna itself have been very regular and mod- erate. Etna is 11,000 feet high, and about thirty miles in di- ameter at its base. "We will take its circumference at one hundred miles. Now, a lava-stream of triangular shape, one foot thick, reaching to the base, and one mile wide, would, we believe, be an average eruption. It would cover seven square miles, one foot deep, and would be equal to more than 300,000,000 cubic feet. It would take one hundred such eruptions to raise the whole mountain-surface one foot. Taking one such eruption every year (eruptions of Etna for the last 2,000 years have been but one in twenty-five years), it would take a century to raise the mountain-surface one foot. But there is a gorge cut into the side of this mountain, revealing 3,000 feet of lava-layers. To have built up these 3,000 feet (irould require 300,000 years. That we have been moder- 144 DYNAMICAL GEOLOGY. ate in our estimate is shown by the fact that there are known on the flanks of Etna lava-flows 2,000 years old, which are still not covered by subsequent flows. We are justified, then, in saying that Etna is probably much more than 300,000 years old. But the birth of Etna is a very recent geological event, for it stands, and has been built up, on the latest tertiary formation. M Cause of Volcanic Eruptions. This question is still very obscure. There are two things to be explained, viz.," volcanic force and volcanic heat — the force necessary to raise lava to the lip of the crater, and the heat necessary to melt the lava. (a. ) Force. — If we consider the height of volcanic cones, we shall be better able to appreciate the greatness of the force. In the accompanying table we give the heights of some well-known volcanoes and the pressure in atmospheres (one atmosphere = fifteen pounds per square inch, or one ton per square foot) necessary to raise lava (taking the specific gravity at 3. 8) to the lip of the crater. It is true lava is sometimes foamy, and therefore lighter, but, on the other hand, we have taken the focus of vol- canoes at sea-level, while it is probably much deeper, and have supposed the force only sufficient to raise to the lip of the crater, whereas it often ejects with violence many thousand feet in the air. NAME. Height. Pressure in atmospberes. Vesuvius 3,900 feet 11,000 " 13,800 " 19,660 " 835 Etna 920 MaiUUEi Tjosi 1,150. Cotopaxi 1,638 What, then, is the agent of this great force ? It is believed that it is the elastic force of compressed gases I&NEOUS AGENCIES. 145 and vapors, especially steam. The power of these agents is.w 3II known ; and gas and steam issue in immense quan- tities during eruptions, especially of the explosive type. On this point there is little difference of opinion. (5.) Heat. — But the cause of the heat necessary to fuse ' the rocks is one of the most difficult of all questions con- nected with the physics of the earth. By most geologists it is thought to he connected with the primal heat of the earth, and the supposed universal melted condition of the interior. This view assumes (a) that the earth was once an incandescent, fused mass. This is almost cer- tainly true ; {V) that in cooling it formed a crust, which thickened by additions to its inner face, until it is now about thirty miles thick ; (c) and that this limit between the solid crust and melted interior is the place of the focus of volcanoes. There are many difficulties in the way of acceptance of this view, some of which are given on page 132. All other theories regard the melted matter as local, but, as to the cause of the fusion, there is yet great diver- sity of view. Some attribute it to chemical action ; some to mechanical crushing. It must be remembered in this connection, however, that in some cases, at least, the amount of heat required is not more than 800° F., for in some lavas the fusion is Tiydrothermal, and in all cases the access of water seems necessary to supply the force. Secondary Volcanic Phenomena. There are many phenomena which linger after the true eruptions have ceased. The chief of these are hot springs, carbonated springs, lime-depositing springs, solfataras, fumaroles, mud-volcanoes, and geysers. These all seem to be the result of circulation of water through lavas which still retain their heat, and are therefore properly called secondary volcanic phenomena. The lavas, outpoured by primary or true eruptions, remain hot in their interior for Lb Contb, Gbol. 10 146 DYNAMICAL GEOLOQY. an indefinite time. If waters, percolating through these, come up again after taking up only heat, they form hot springs. If, in addition, they take up CO^, they form carbonated springs. If lime be taken and deposited on the surface, they form lime-depositing springs (p. 73). If the heat be great, so that vapors are given off and con- densed as clouds, they are called f umaroles. If the waters contain H^S, and AlkS, they are called solfataras. If mud is brought up and deposited about the vent, they are mud-volcanoes. Finally, if the springs are periodi- cally and violently eruptive, they are called geysers. The only variety of these springs which need detain us here is Geysbes. Geysers may be defined as periodically eruptive springs. They seem also usually, if not always, to deposit silica. They are found only in Iceland, in New Zealand, and in Yellowstone Park. The so-called California geysers are solfataric fumaroles. Steamboat Springs in Ifevada may possibly be classed with geysers, but their erup- tions are feeble. The phenomena of true geysers are so splendid that a somewhat full account of them is neces- sary. As they were first studied in Iceland, and the cause of their eruption was first understood there, we Will speak of these first. G-eysers of Iceland. — Iceland may be briefly described as a plateau 2,000 feet high, studded with volcanic peaks, with margin sloping gently to the sea. Only the marginal area is to any extent inhabited. The interior is a scene of desolation, where every form of volcanic phenomena exists in the greatest activity — volcanoes, hot springs, boiling springs, fumaroles, solfataras, and geysers. Of these last there are very many in various degrees of activity. The most celebrated of these is the Qr6a\ Geyser. IG-NEOUS AGENCIES. 147 Great Geyser. — This is a low mound, with a basin- shaped depression at top, from the bottom of which de- scends a tube or well to unknown depth, but may be sounded to eighty feet or more. The basin is fifty feet across, and the tube or throat ten feet in diameter at the top but narrowing downward. Both the basin and the throat are lined with silica deposited from the water, and doubtless the mound itself was built up by similar deposits. In the intervals between eruptions the basin is filled to near the brim with water at 180°. Plieuomena of an Eruption. — As the time for the eruption approaches, the first thing observed is a series of explosions in the bottom of the throat like subterranean cannonading ; then bubbles of vapor are seen to rise and burst on the surface ; then the water of the surface bulges up and overflows. Immediately thereafter the whole of the water in the throat and basin is ejected with violence one hundred feet into the air, forming a fountain of daz- zling splendor, followed by the roaring escape of steam. As the water falls back, it is again ejected, and the foun- tain continues to play several minutes until the steam has all escaped and the water partly cooled ; then all is quiet again until another eruption. The interval between erup- tions is irregular. An eruption may be brought on pre- maturely by throwing large stones down the throat of the geyser. Yellowstone Geysers. — But in splendor of eruption the Icelandic geysers are far surpassed by those of Yellow- stone Park. This, like Iceland, is a volcanic region, but, unlike Iceland, primary volcanic phenomena are all ex- tinct. The geyser phenomena here occur in a narrow valley surrounded on all sides with volcanic rocks of great thickness, of comparatively recent origin, and doubtless, therefore, still hot in their interior. In this little valley there are no less than 10,000 vents of all kinds, hot springs, boiling springs, mud-volcanoes, lime-depositing springs. 148 DYNAMICAL GEOLOGY. and geysers. On Gardiner's River the vents are mostly hot carbonated springs, depositing lime ; on Firehok Eiver they are geysers, depositing silica. In Yellowstone Park alone there are in all 3,000 vents, of which sixty-two are eruptive geysers. In the lime carbonate springs the de- posits on hillsides have given rise to a succession of terraces (Fig. 41, page 75), and sometimes the water descending through a succes- sion of pools from terrace to terrace Fio. 77.— Deposits from carbonated springs. gives rise to beautiful stalactitic forms (Fig. tl). In the geysers the hot alkaline waters collect in pools, and deposit the silica first in a gelatinous condition, which afterward concretes into all kinds of fantastic forms (Fig. 78). The deposit immediately about the eruptive IGNEOUS AGENCIES. 149 150 DYNA31IGAL QMOLO&Y. vents buiids up moundlike, hivelike, and chimneylike forms (Fig. 79). The silica-charged waters, trickling ¥ia. 79.— Crater of Castle Geyser, Yellowstone Park. slowly over the mounds, give rise by deposit to patterns of exquisite and delicate beauty, compared by Hayden to embroidered lace-work with edging-fringe and pendent tassels, and studded with pearls. Similar deposits are formed also in New Zealand ; we give an example in Mg. 80. Only a few of the grandest of these geysers can be mentioned : 1. The Grand G-eyser throws up a column of water six feet in diameter to the height of 300 feet, while the steam ascends 1,000 feet or more. The eruption is repeated every thirty-two hours, and lasts twenty minutes. 2. The Giant (Pig. 81) throws a column five feet in diameter 140 feet in the air, and plays continuously for three hours. 3. The Giantess, the greatest of all, throws up a huge IGNEOUS A&ENCIES. 161 column twenty feet in diameter to the height of sixty feet, and through this great mass it shoots up several lesser jets Fig. 80.— Pink terraces, New Zealand. (After Peale.) to the height of 250 feet. It erupts about once in eleven hours, and plays twenty minutes. 4. The Beehive, so called from the shape of its mound, shoots up a splendid column two to three feet in diameter to the height of 319 feet, and plays fifteen minutes (Fig. 82). 5. Old Faithful, so called from the frequency and regu- larity of its eruptions, throws up a column six feet in diameter to the height of 100 to 150 feet, and plays fifteen minutes (Fig. 83). Cause of Geyser Eruption. '■= — This maybe explained, in a very general way, as follows : Experiments show that the heat of the water rapidly increases as we pass down the geyser-throat. There is no doubt, therefore, that in spite of the increasing pressure (which raises the boiling-point) ■" For a complete discussion of this interesting subject, see author's " Elements," pp. 09-304. 152 DYNAMICAL GEaLO&Y. •1! .'*ISfe Fig. 81.— Giant geyser. (After Hayden.) IGNEOUS AGENCIES. 153 FiQ. 83.— Beehive geyser. (From a drawing by Holmes.) 154 DYNAMICAL GEOLO&Y. the boiling-point is reached and a large quantity of steam is formed first, at some point deep below. The water above is immediately ejected, and the fountain continues Fig. -Old Faithful geyser in action. (After Hayden.) to play until all the steam escapes and the water is some- what cooled. Then all is quiet until the water again heats up to the boiling-point. Section II. — Earthquakes. When we consider the suddenness with which earth- quakes occur, the terror they inspire, and the place of their origin, deep in the interior of the earth, and hidden from observation, it is not surprising that we know so IGNEOUS AGENCIES. 155 little about their cause. In fact, until about forty years ago no attempt had been made to study them scientifi- cally. Now, however, it is believed the foundations of a true science of earthquakes (seismology) have been laid, and a true progress has been made. The basis has been laid by Mr. Mallet, and progress has been made possible by the use of self-registering seismometers. Frequency of Earthquakes. — The slow development of earthquake-science is not due to want of material, but, as has already been stated, partly to the difficulty of the subject, and partly to the terror produced — unfitting the mind for scientific observation. The earthquake catalogue of Alexis Perrey records 18,000 in thirty years (1843- 1873), or nearly two a day. When we remember that three-fourths of the earth's surface is covered with the sea, that a large portion of the land-surface is inhabited by uncivilized races, and that even in civilized countries many slight tremors are unrecorded, it will not seem extravagant to say that, probably, there is not an hour of any day in which the earth is not shaking in some portion of its surface. Phenomena of an Earthquake. — In brief, the phe- nomena of an earthquake are : 1. Sounds, sometimes like underground cannonading ; sometimes a hollow rumbling, or clashing, or grinding. 2. Accompanying, or immedi- ately succeeding, comes the movement of the earth, as a slight tremor, or as a violent shaking ; in extreme cases, so violent that the houses of whole cities are shaken down, like card-houses of children, and bodies on the surface are thrown up a hundred feet into the air, as at Eiobamba in 1797. 3. As to direction, the movement may be up and down, or from side to side, or partaking of both, i. e., obliquely, or it may be rotating or twisting, as, for ex- ample, when chimney-tops are twisted about without being upset, or wardrobes and bureaus turned about before upsetting. 4. One thing is always observed and is of 156 DYNAMICJlL gbolo&y. primary importance, viz., that the shake does not occur everywhere at the same time, hut on the contrary appears first at one place and spreads thence in all directions, precisely like a system of waves when a stone is thrown into the water. This point of first appearance is called the " ejaicentrum ," because it is immediately above the origin. The violence of the earthquake is greatest there, and thence decreases precisely like a system of vndening circular waves. "Velocity of Shock and of Transit. — The velocity of the spread from the center or velocity of travel (transit) must be carefully distinguished from the velocity of the earth-movement (shock). There is no close relation be- tween these. We may best illustrate this by water-waves. Suppose we are in a boat on the surface of a bay traversed by long, low swells. As each swell passes under us, we are slowly heaved up and slowly let down again, but the waves are here, there, and away with great velocity. The velocity of oscillation is small, the velocity of transit is great. But if the surface of the bay be agitated by short, high waves, the oscillation or shaking is more rapid, but the transit is comparatively slow. So in earthquakes, the movement may be only a slow heaving up and down, or swinging back and forth, and yet this movement may travel from place to place with great velocity. Now, as in water-waves generated by a stone thrown in still water, so in earthquakes, the velocity and amount of movement (which is equivalent to the wave-height) is greatest at the center (epicentrum), and diminishes as it spreads, but the velocity of the transit or travel is nearly or quite uniform. Now, the velocity of transit has been determined in many earthquakes by noting the time of arrival at difEerent places. It varies with the kind of rock, being greatest in the hardest, and also with the depth of the origin, being greater for very deep earthquakes. In some cases it is only ten miles per minute ; sometimes fifteen, twenty, IGNEOUS AGENCIES. 157 thirty miles per minute, or even much more. Sometimes the spread is equally rapid in all directions, and the spread- ing wave is circular, or nearly so ; sometimes it is more rapid in one direction than another, and the spreading wave is elliptical. A. dp Cause of Earthquakes. — The origin of earthquakes being deep beneath the surface and hidden from obser- vation, their cause is very obscure. Yet their association with .other forms of igneous agency suggests probable causes : 1. Volcanic eruptions, especially of the explosive type, are always accompanied by slight and sometimes by seri- ous earthquakes. This fact suggests the sudden formation of gases or the sudden collapse of vapors as a possible cause. On this view an earthquake would be like the earth-jar produced by a mine-explosion, or by the explo- sion of large quantities of gunpowder or nitro-glycerine. 2. But great earthquakes are oftener associated with bodily movements of extensive areas of the earth-crust. Thus, for example, in 1835, after a severe earthquake on the western coast of South America, it was found that the whole coast-line of Ohili and Patagonia was raised from two to ten feet above sea-level. Again, in 1832, the same phenomenon was observed in the same region after a great earthquake. Again, in 1819, after a severe earthquake which shook the delta of the Indus, a tract of land fifty miles long and sixteen miles wide was raised ten feet, and an adjacent area of 2,000 square miles was sunk, and became a lagoon. In commemoration of the wonderful event, the elevated tract was called TJllaJh bund, or, the mound of God. Again, in 1811, a severe earthquake — perhaps the severest (except the Charleston earthquake of August, 1886) ever felt in the United States— shook the valley of the Mississippi. Coincidently with the shock, large areas of the river-swamp sank bodily, and have ever since been covered with water. In commemoration of the 158 DYNAMICAL GEOLO&Y. event, this area is still called the sunken country. In all these cases, probably, and in the last two certainly, there was a great fissure of the earth-crust, and a slipping of one side on the other. l^ow, these facts suggest another and, we believe, a more probable cause of earthquakes. It is well known that. there are operating within the earth forces elevating or depressing or crushing together portions of the crust. We will discuss the nature of these forces in Part II. Suffice it to say now. that it is in this way that continents are elevated and mountain-ranges are formed. Now, suppose such forces operating to raise or depress large areas of the crust — e. g., the southern end of South America — it is evident that, the interior forces lifting and the stifE crust resisting, there would come a time when the crust would break — i. e., form a great fissure. Such a sudden break would produce an earth-jar which would propagate itself from the fissure as focus in all directions as an earthquake. Or, again, after such a fissure is formed, the two walls may at any time slip on each other and pro- duce an earth-jar. Now, this is not mere speculation. We find such great fissures intersecting the earth in many places ; they break through miles of thickness of rock, and in many cases the two walls are slipped on each other several thousand feet vertically. It is almost certain that earthquakes are produced iy the formation or the slipping of such fissures. In 1873 there was a severe earthquake in Inyo County, California, just at the eastern base of the Sierra. Now, there is on that side of the range a great fissure and a slip of several thousand feet. It is almost certain that the earthquake was produced by a slight readjustment of the position of the walls of this -fissure. Moreover, the thorough investigations very recently of several earthquakes have seemed to establish the fact that they originated in the formation or the readjustment of a fissure. IGNjEOUS agekcies. 159 Nature of Bartliquake-Waves In any case, it is evident that an earthquake is produced by concussion of some kind somewhere in the interior of the earth, usually at a depth of from six to ten miles. The concussion gives rise to a series of elastic earth-waves, spreading in all directions spherically, like sound-waves, until they reach the surface, and then spread in all directions on the surface as a circular wave, as in Pig. 84. The interior 40m' Fig. 84. — Section and perepective of a portion of the earth's crust Bhalsen by an earthquake, showing origin, x ; section of the spherical waves, a', b', e', etc., and perspective of the outcropping surface waves, a, &, c, etc. point of origin (x) is called the focus, orv centrum ; the point of first emergence (a), the epicentrum. It is the passage of a series of these circular waves beneath the feet of the observer at any point (d) that gives rise to the actual observed phenomena ; so that the scientific discussion of earthquake phenomena is little else than the discussion of such earth- waves emerging and spreading on the surface. ^Earthquakes occurring' toeneatli the Sea. — We have thus far spoken of earthquakes occurring beneath the land ; but three fourths of the earth-crust is covered with water, and therefore it is probable that the larger number of earthquakes have their origin beneath the sea-bed. Besides, as we shall see hereafter in treating of mountain- chains, marginal sea-bottoms are particularly liable to movements. When an earthquake occurs beneath the sea-bed, there are some additional phenomena, which must now be discussed. 160 DYNAMICAL GEOLOGY. Suppose, then, a concussion, from any cause, beneath the sea-bed. There would be formed, as before, about the focus, a series of spherical earth-waves, which, by en- largement, would emerge on the surface of the sea-bed as circular surface-waves. These, spreading beneath the sea, would reach the nearest shore, and produce their de- structive effects there. Some time afterward, perhaps a half -hour or more, there comes rolling in on shore a prodigious water wave, or perhaps a series of water waves, thirty to sixty feet high, deluging the whole shore region, and completing the destruction commenced by the earth- wave. The Grreat Sea Wave. — This very destructive accom- paniment of earthquakes occurring beneath offshore sea- beds may be explained as follows : The bed of the sea at the epicentrum is lifted up perhaps several times. This lifts the whole sea water above, so that the surface is raised into a water mound. This mound immediately sinks as much below the sea level as it was before raised above it, and thus gives origin to a circular water wave (or series of such waves) which spreads exactly like any other water wave, growing lower as its spreads, until it breaks on the nearest shore. Out at sea such great low waves would pass under a ship unobserved, heaving it slowly up and letting it down again. But when they approach shore, on account of their great size, often fifty feet high and one hundred to two hundred miles across the base, they rush forward as a tide fifty feet high and devastate the whole coast within their reach. They are, therefore, sometimes called tidal waves, although they have nothing to do with tides. Though originating at the same place, the great sea wave moves much less rapidly than the earth- wave, and therefore reaches the shore later. Examples of the Great Sea Wave. — 1. In 1755 a terrible earthquake destroyed Lisbon, and, it is said, forty thousand people. The focus of this earthquake was laNJiOUS AOENGIES. 161 beneath the sea-bed, perhaps one hundred miles off shore. The arrival of the earth-wave shook down the houses. Then, after a half-hour, when all was quiet, there came great sea waves sixty feet high and completed the destruc- tion of the city. These waves were sixty feet high at Lisbon, thirty feet at Cadiz, eighteen feet at Madeira, and five feet on the coast of Ireland. They were also felt on the coast of Norway and on the West India Islands, after having traversed the breadth of the Atlantic. 3. In 1854 an earthquake shook the coast of Japan. A half -hour afterward a great wave, thirty feet high, came in and swept the town of Simoda clean away. The epi- centrum was probably a hundred miles off shore. The wave, spreading in all directions, was highest on the coast of Japan, because this was near the epicentrum. But in the other direction it was observed at the Benin Islands fifteen feet high, and — after traversing the Pacific and being nearly exhausted — on the California coast, only eight inches high at San Francisco and six inches at San Diego. 3. In August, 1868, a very destructive earthquake shook the coast of Peru, severest about Arica. The epi- centrum was not far off shore, for in five minutes after- ward there came in great sea waves sixty feet high and desolated the whole coast, carrying ships far inland and stranding them high up on the mountain slopes. These great waves were traced southward to Coquimbo and be- yond, northward to San Francisco, Astoria, and Sitka, southwestward to Australia and New Zealand, and west- ward to Hawaii and Japan, thus having traversed the whole breadth of the Pacific. "Were it not for the obstruct- ing continents, there is no doubt that they would have encompassed the earth in their widening circles. In regard to these waves, there are several points worthy of notice : a. Their velocity, though less than that of earth-waves, Le Conte, Geol. 11 162 DYNAMICAL GEOLOGY. is enormously great for water waves. The wave of 1854 traversed the Pacific, from Japan to San Francisco, a dis- tance of 4,500 miles, in about twelve hours, or at a rate of 370 miles an hour. The wave of 1868 ran across the Pacific with even greater speed. The reason of their great velocity is their enormous size. h. The size of the great sea wave is determined by the principle that every wave runs its owti length in the time of one oscillation. If a boat be lying on smooth water, and a series of water waves passes under it, the boat will be moved up and down once while the waves run the length of one wave ; i. e., from trough to trough. Now, the time of oscillation of the great sea waves of 1854 was about thirty-three minutes. If, then, the waves run 370 miles in an hour (60 minutes), how much did they run in 33 minutes— 60 : 33 : : 370 : 303. Therefore, these waves were 203 miles from trough to trough. c. The mean depth of the ocean may be determined by these waves. The principle on which this is done is as follows : Every one has observed that waves coming in from deep water on to a flat, shelving shore, at a certain depth begin to drag bottom, and are impeded thereby ; also, that the larger the wave, the deeper the water in which it begins to drag. Now, in the case of these enor- mous earthquake sea waves, the ocean itself is not deep enough to prevent them from dragging bottom. As they run over the sea their velocity is impeded everywhere, but more or less according to the varying depth of the ocean. Now, the normal or unimpeded velocity of a wave may be accurately calculated, since it varies as the square root of the wave-length (w oc a/L). Therefore, the amount of retardation will give the depth of the ocean over which it passes. The mean depth of the ocean between Japan and San Francisco, as thus determined, is 12,000 feet ; between Arica and Hawaii it is 18,000 feet. I&NEOUS A&ENCIES. 163 Determination of the Epicentrum and Centrum. —By means of seismometers the direction of the earth's motion may be determined. If this be taken in many places, and the lines of direction be protracted, they will be found to meet at some point from which all seem to radiate. This is the center of the circular surface-waves or epicentrum. Or, by accurate clocks in many stations, the time of arrival of the shock may be recorded. If, now, we draw a line through all the places where the time of arrival was the same, we shall have a curve which represents the form of the wave and the center of which, a, is the epicen- trum. Such lines of simul- taneous arrival of shock are called coseismal lines (c s, Fig. 85). The position of the cen- trum or origin is much more difficult to find, but has been approximately found for several earthquakes. The general conclusion thus arrived at is that an earthquake focus (centrum) is usually only six to ten miles in depth, and that the shock is a jar produced by the formation of a great fissure. Connection of Earthquakes with Phases of the Moon, — By careful comparison of the times of occur- rence of thousands of earthquakes, it 'has been shown — 1. That they are a little more frequent when the moon is on the meridian than when on the horizon. 3. Also at new and full moon than at half moons. 3. Also when the moon is nearest the earth than when she is farthest away. Now, these are the times of flood-tide, and of high flood-tides, and of highest flood-tides. Some have imagined that these facts prove the existence in the Fig. 85. 164 DYNAMICAL QEOLO&Y. interior of the earth of a general liquid subject to tides. But the argument is evidently valueless, ^ for any force tending to lift and break up the crust of the earth would be assisted by the gravitation or lifting power of the moon in passing the meridian, and this lifting power would be greatest at the times indicated above. Suppose, then, an interior force, tending to elevate and break the crust, constantly increasing but resisted by the rigidity of the crust : it is evident that, when the two forces are nearly balanced, the lifting force of the passing moon might well determine the moment of fracture. The moon does not produce the earthquake, but only deter- mines the moment of its occurrence — only adds the last feather that breaks the camel's back. Connection with Season and Weather. — By the discussion of the times of occurrence of a large number of earthquakes it is found that they are a little more fre- quent in winter than in summer. N'o cause for this is known. Again : It is a popular belief that the occurrence is usually associated with an oppressive feeling of the atmos- phere, or with storms. These meteorological phenomena are usually attended with a low condition of the barome- ter. Now, a low barometer means diminished pressure of the atmosphere, and this, again, might determine the moment of fracture of the crust. But this, like the attraction of the moon, must be regarded, not as the cause of the earthquake (which undoubtedly lies wholly within the earth itself), but only as sometimes determin- ing the moment of its occurrence. Seotioit III. — Gkadual Oscillations of the Eaeth- Ckcjst. The movements included under this head are on a grand scale, perhaps affecting whole continents, but usu- laNEOVS AGENCIES. 165 ally so slow as to es(3iipu popular observation. But, though so inconspicuous, they are the most important of all forms of igneous agency, since it is by movements such as these that continents and sea-bottoms, mountains and great valleys, have been formed. Volcanoes and earthquakes occur suddenly, fill the mind with terror, and pass away, leaving behind little effect on the config- uration of the earth ; but gradual movements of the crust, acting over large areas, and without ceasing, through inconceivable ages, have produced all the great inequalities of the earth's surface. Thus is it always — the causes producing the ■ most far-reaching efEects are ever those which, acting slowly, but everywhere and at all times, are scarcely recognized except by the thoughtful tnind. But although the effects of this form of igneous agency are so important, yet they are so obscure, and so little has been accomplished by them in the present geological epoch, that little is known of them, and our account must therefore be brief. It is their accumulated effects through all geological times, as shown in the structure and config- uration of the earth, that alone are conspicuous. These «re shall treat of in Part II. In the meantime, however, a few examples of their action now will prepare us for the discussion of these effects. Elevation. — 1. South America. — "We have already mentioned (page 157) that in 1832 and again in 1835, after severe earthquakes, the southwest coast of South America was elevated several feet along a line of many hundreds of miles. It is not probable that very much is accomplished in this paroxysmal way, but the fact is important as showing the connection of earthquakes with bodily elevation of large tracts. Suppose, then, any force beneath tending to elevate the southern end of the South American Continent, but resisted by the stiffness of the crust : if the crust yielded gradually as the force 166 DYliAJIlGAL GEOLOGY. accumulated, only gradual elevation would take place; but if the stifEness was very great, the yielding might take place paroxysmally, by fracture, earthquake, and su4den elevation. The normal process is, gradual eleva- tion by gradual yielding. Earthquakes are but occasional accidents in the slow march of these grand effects. But, besides these sudden elevations, there has been during an immense time a gradual elevation of the whole southern part of the South American Continent out of the sea. The evidence of this is seen in the old beach- marks one above another to the height of 1,300 feet above the sea and extending along shore 2,000 miles on the western and ].100 miles along the eastern coast. More recently, A. Agassiz has found on the same coast dead corals of recent species sticking to the rocks 3,000 feet above sea. Here, then, we have continent-making forces at work on a grand scale. It is not probable that the whole of these efEects was accomplished during the present geological epoch, but they are the more interest- ■ ing on that very account, since we here trace geological causes directly into causes now in operation. 2. Italy. — The most carefully observed example of gradual elevation is that at the Bay of Baise near Naples. Pig. 86 is a map of the Bay of Baiae. From the present shore-line there runs back a flat plain of stratified vol- canic matter sloping gently to the sea, called the Starza; this is terminated by a perpendicular cliff. In the vicin- ity are evidences of volcanic action in the form of vol- canic cones and solfataras of very recent origin. Fig. 87 is a section of the same. ^ Now, there is abundant proof that this coast has slowly sunk and risen again at least twenty feet, and that this has all taken place certainly since Eoman times, and probably since 1200 A. D. The evidence is briefly as follows :' 1. The Starza consists of stratifled material con- taining recent Mediterranean shells. 2. The cliff which I&NEOUS AGENCIES. 167 terminates the Starza is obviously an old shore-cliff. 3. The face of this cliff up to a line twenty feet above Sozzuoli Pig. 86.— Map of Bay of Baiae. sea-level is riddled with holes bored by lithodomi, a spe- cies of marine-boring shell. 4. On the Starza have been found the remains of an ancient Roman temple. When found, only the upper parts of three fine columns were visible, but, by removal of the soil twelve feet deep, a beautiful tessellated • pavement and many broken columns were exposed. The pave- ment and buried por- tions of the columns were smooth and well preserved ; then fol- lowed nine feet riddled with lithodomi, above which it was again smooth. The uppermost borings were on the same level as those on the cliff, and therefore mark the former level of the sea. Inscriptions on the pavement show that the temple was repaired in the third century, and it was then, therefore, above sea-level. The limit of the borings shows that it subsequently sank twenty-one feet, and again rose slowly to the original level, for the floor is now above sea- FiG. 87.— section of map of Bay of Eaise. 168 DYNAMICAL GEOLOGY. level. All this was done so quietly that it was unre- marked by contemporaneous writers. There is good reason to think that the whole took place between a. d. 1200 and 1600. Writers of the six- teenth century say that in 1530 one might stand on the cliff, 5, and fish in the sea ; this, therefore, was during the period of subsidence. N"ow, in 1198 a great earth- quake destroyed Pozzuoli, and in 1535 Monte Nuovo was formed by eruption. It is probable, therefore, that the history of events was briefly this : After the earthquake of 1198, the sinking commenced, and continued until it reached twenty-one feet ; it remained in this condition until the eruption of 1535, when it began to rise again. During the interval of subsidence, sediments, volcanic ashes, etc., filled up the bottom twelve feet deep, and protected the lower part of the columns, and only the part representing clear water was bored. Other evidences of movements up or down are found all along the coasts of the Mediterranean. The ruins of the Temple of the ISTymphs are now in water. The bridge of Caligula is bored several feet above the sea-level, etc. 3. Sweden and Norway.- — The examples thus far given are in volcanic countries, and possibly caused by volcanic forces ; but such movements are by no means always asso- ciated with volcanism ; for example, Scandinavia is re- markably free from volcanism, and yet the whole coast, both on the Atlantic and the Baltic side, has been for a long time, and is still, rising out of the sea. The rate is less in the southern part and increases northward, the average being about two to three feet per century. That this has been going on for a long time is shown by old beach-marks at various levels up to six hundred feet above sea-level, showing an elevation to that extent, and that during the present geological epoch. At the rate of two and a half feet per century, this would require two hun- dred and forty centuries, or twenty-four thousand years. IGNEOUS AGENCIES. 169 This is of course only an approximate estimate, btft we may say with confidence that for thousands of years the whole of Scandinavia, and perhaps much more, has been rising bodily out of the ocean. Subsidence. — 1. Greenland. — The coast of Greenland, for six hundred miles, is now subsiding, but at what rate is not known. The subsidence is proved by the fact that the houses built by the early Norwegian discoverers are now partially submerged. The fact is so well recognizes,d by the Eskimos that they never build near the sea-level. 3. River Deltas. — In all great river deltas and perhaps we might say in all places where abundant sediments are accumulating, the earth-crust subsides as if weighted down with the ever-increasing load. In digging or boring into the delta of the Mississippi, the Ganges, or the Po, the deposit is found to consist of an alternation of river sediments with old forest-grounds, and sometimes peat several feet thick, and occasional layers of limestone. This is represented in Pig. 88, in which « s is the surface. Pio. 88.— Section of river delta, ss, surface ; rs, river-silt ; fg, f oreatgronnd ; I, limestone.' with growing vegetation and accumulated vegetable mold, and perhaps peat. As we go down we pass through river- silt, r s, then an old submerged forest-ground, /^f, with black mold and stumps in place, as they grew, sometimes 170 DYNAMICAL GEOLOGY. with a considerable layer of peat, then more river-silt, with an occasional layer of limestone, and so on, several times repeated. Such old forest-grounds have been found in the Mississippi delta fifty feet below sea-level, and in the Ganges layers of peat fifty feet below sea-level, and fresh-water shells and river-silt near four hundred feet, In the delta of the Po, peaty layers are found four hun- dred feet below sea-level (Lyell). Now, the only way possible to explain these facts is to suppose a slow subsidence on the one hand and the up- building by sedimentation on the other, but not always absolutely at the same rate. When the upbuilding pre- vailed, the area was reclaimed and overgrown with forest. When the subsidence prevailed, the trees were submerged and destroyed, rotted to stumps and buried in sediments. Sometimes the subsidence was so rapid that salt-water conditions prevailed and limestones were formed. Sub- merged forests are found not only in deltas, but also on many coast-lines, and are among the surest signs of sub- mergence. 3. Mid-Pacific Bottom. — But the grandest example of subsidence, still in progress, is undoubtedly that already discussed under coral reefs. As already shown, we have evidence that over an area of 10,000,000 square miles in mid-Pacific there has been, in comparatively recent geo- logical times, a subsidence of 10,000 feet, and that the subsidence is still going on. Surely, in this case, we have changes now in progress which are of the nature of those by which continents and sea-bottoms were formed. 4. River-beds. — Our examples thus far are all from the coast region. The phenomena are plainest there, because we have the sea-level as a term of comparison. But in the interior of continents we have river beds as indicators of movement. We have seen (p. 38) that in a rising country rivers cut deeper, while in a sinking country they build up by deposit. mUMOUS AGENCIES. 171 Cause of Crust Movements. It is evident that the thing actually observed is only changes in the relative level of sea and land. In the inte- rior of continents we have no means of determining such movements^ except by river beds, as just explained. The cause of these slow changes is very obscure and can not be discussed here.* Suffice it to say that the great inequali- ties of the earth's crust, such as continents, ocean basins, and mountain chains, are probably due to the slow cooling, unequal shrinking, and consequent slight deformation of the whole earth, progressive through all geological time. General Beirospecf. We have discussed briefly the agencies now in operation on the earth's surface, producing structure and form under our eyes. We believe that similar agencies have been at work through all time, and left their effects in the structure and surface forms which we actually find. We study the small and insignificant effects now produced in order that we may throw light on those greater effects which, accumulating through all geological times, are now embodied in the earth's structure. We are now in a posi- tion to examine the actual structure and forms of the earth, and to interpret them by the light of the previous discussions. Again : Of the agencies which we have been discussing there are manifestly two groups. Atmospheric, aqueous, and organic agencies constitute the one, and igneous agencies the other. The one group tends to reduce the inequalities of the surface, and, acting alone, would event- ually bring all to sea-level, and are therefore called levet- ing agencies. The other originally caused, and has ever tended to increase, the inequalities of the surface, and, * Per fuller discussion, see the author's " Elements of Geology," p. 131. 173 DYMAMIOAL GEOLOGY. acting alone, would ere this have made them of incredible dimensions, and are therefore called elevating agencies. The state of the contest between these two opposite forces at any time, determined the distribution of land and water, the height of continents and mountains, and depth of seas, at that time. The one group roughhews, the other shapes, the forms of the earth. PART II. STEUCTURAL GEOLOGY. CHAPTER I. GENERAL FORM AND STRUCTURE OP THE EARTH. General Form. The general form of the earth is that of an oblate spheroid flattened a little at the poles. In other words, it is an ellipsoid of revolution about its minor axis. The equatorial diameter is about twenty-six miles greater than the polar diameter. This general form is taken at sea- level, the land-surfaces rising above and the sea-bottoms sinking below. This form is precisely that which a liquid globe would inevitably assume under the influence of ro- tation. It has, therefore, been somewhat hastily concluded that this general form is demonstrative evidence of the early incandescent liquid condition of the earth. It is certain, however, that the earth would have assumed this form by rotation, whether it were originally liquid or solid.* Therefore, while it is almost certain, from other considerations, that the earth was once liquid, and assumed its oblate spheroid form in that condition, yet this gen- eral form alone can not be regarded as proof of that con- dition. General Structure. — We have already stated (page * This subject is more fully explained in the author's " Elements of Geology.'' 173 174: STRUCTURAL GEOLOGY. 133) that the interior temperature of the earth increases 1° for every fifty-three feet in depth, and that at this rate the fusing temperature of rocks would be reached at about thirty miles ; and, finally, that many have thence hastily concluded that the general structure of the earth is that of a globe of fused rock or lava, covered 'with a thin shell thirty miles thick. But we have also shown there the untenableness of this view. There are only two other views possible, and now held. Some hold that the earth is trnly solid throughout, excepting reservoirs of liquid matter forming the foci of volcanoes. Others hold that the earth consists of — 1. A solid nucleus, which forms its greatest part ; 2. A solid crust, comparatively thin ; and, 3. Separating these, a sub-crust layer of liquid or semi- liquid matter, if not laniversal, at least over large areas. There are many geological phenomena which seem to make this last view most probable. Density of the Earth. — The mean density of the earth, taken as a whole, is 5.6. The density of the crust is about 2.5. Therefore the density of the central parts must be very much greater than 5. 6. It is probably not less than 15 to 16. This greater interior density is due partly to a difference of material (the denser settling toward the center, while the earth was still in a fused con- dition), and partly to condensation iy pressure. Crust of the Earth The surface portion of the earth differs in many respects from the interior, and is, therefore, properly called a crust: 1. It is certainly a lighter portion covering a denser interior. 2. It is a cooler portion, covering an incandescent interior. 3. It is, as we shall see hereafter, a stratified portion covering an unstratified interior. 4. It is probably an oxidized por- tion covering an unoxidized or less oxidized interior (for oxidation comes by contact with air and water). 5. It is probably a solid shell covering a liquid or semi-liquid sub- crust layer. It is this idea of a solid shell covering a FOlill AND STRUCTURE OF THE EARTH. 175 liquid which gave origin to the term crust j but the word is now used only to signify the superficial portions of the sarth, subject to human observation, without any impli- cation as to the interior condition. Means of Geological Observation. — As thus defined, the crust is estimated at from ten to twenty miles in thickness. The manner in which we get a knowledge of the earth to that depth, or the means of geological obser- vation, are — 1. By mines and artesian wells. These pene- trate 4,000 or 5,000 feet. 2, Canons and ravines. These give sections of 6,000 or 7,000 feet. 3. Volcanic ejections. Fie. These bring up matter from unknown but certainly still greater depth. But the most common and efEective means of observation is furnished by — 4. Foldings of the crust, and subsequent erosion. In the section (Fig. 89) in which s s is the present surface, we represent one of the com- monest of all geological phenomena. It is seen that from the point a the strata are repeated on the two sides. The dotted lines show how much has been c\A, away, and what depth of ^strata has been exposed to view. In this way, in very many places, the character of the rocks ten or more miles deep is revealed. Our direct observation is absolutely confined to this superficial portion. We can only speculate about what is ■beneath. It would seem, at first sight, that this is an i;0 STRUCTURAL OEOLOQY. insignificant portion of the earth upon which to found a science of the earth. But it must be remembered that on this superficial portion has been enacted, and in its struc- ture has been recorded, the whole history of the earth. General 'Surface Configuration of the Crust. — The crust of the earth is diversified by greater and smaller features. The greater features are due to interior or elevating, the lesser to exterior or leveling agencies. Under the former head come those greatest features, constitut- ing continental surfaces and oceanic bottoms, and those next greatest, viz., mountain-chains and great valleys. Under the latter come all those peaks and ridges, valleys and ravines, which have been produced by subsequent erosion. The mean height above the sea-level of the continents is about 1,300 to 1,300 feet, or less than one fourth mile, and the raean depth of the ocean-bottoms below the same level is about 15,000 or 16,000 feet, or nearly three miles. The ocean-surface being nearly three times as great as the land-surface, it is evident that, if the inequalities of the crust-surface were removed, there is water enough to cover the whole earth more than two miles deep. General Liaws of Continental Form. — There are certain general laws of continental form which have a bearing on the question of the origin of continents, and which, therefore, must be briefly mentioned. 1. Continents consist essentially of Interior Basins, with Coast-Chain Bims. — The interior basins are drained by the great rivers of the world. This typical structure is well shown in America, Iforth and South, in Australia, and in Africa. For example, in^North America we have the great interior basin drained by the Mississippi Eiver, bordered on the Atlantic side by the Appalachian, and on the Pacific side by the great Eocky Mountain sys- tem or American Cordilleras, consisting of many ranges, of which Colorado, Wahsatch, and the Sierra and Coast FORM AND STRUCTURE OF THE EARTH. 177 Eange of California are tlie most notable (Fig. 90, a). South America has the Andes on one coast, the Brazilian mountains on the other, and the great interior basin drained by the Amazon, La Plata, and Orinoco Rivers (Fig. 90, b). Similarly, the great basin of Africa is drained by the Nile, Niger, Congo, and Zambesi Eivers. Basin. Plateau. Plains. East and west section of North American Continent : cr, coast range ; SJ, San Joa. qnin plain ; 5, Sierra ; w^ Wahsatch ; c, Colorado range ; Ap^ Appalachian. Andes. East and west section across South America. Brazil" m" East and west section across Australia. Fig. 90. — Sections across North and South America and Australia. Australia is also a fine example, as shown in Fig. 90, c. Europe and Asia have similar structure, but less perfect. This continent is elongated east and west, and therefore the section must be north and south. 2. The Greater Range faces the Greater Ocean. — . In America, the North American Cordilleras and the Andes face the Pacific, while the Appalachian and the Brazilian mountains face the Atlantic. In Africa and Australia, on the contrary, the east range faces the greater ocean, and is the greater. 3. The greater chains are usually the most complex and crumpled in structure, and give evidence of greatest vol- canic activity in the present or in the past. Lb Conte, Geol. 12 178 STRUCTURAL GEOLOGY. 4. Continents and ocean-bottoms have not, as some imagine, frequently changed places. On the contrary, the places of continents have been indicated and their outlines sketched out from the beginning, and their forms have been gradually developed, though with many oscil- lations, throughout all geological times. The orlgrin. of continents and ocean-bottoins is very obscure, but it is probably in some way connected with the unequal contraction and therefore deformation of the spheroidal form of the earth, by slow cooling from a former incandescent condition. In such an irregular or deformed spheroid, of course, the water would collect in the hollows, and the protuberances would become conti- nents. The origin of mountaiiia we discuss further on. Rocks. Deflnltlon of Kock. — The term rocTc is used in popu- lar language to designate any substance of stony hard- ness. K"ot so in geology. Any substance constituting a portion of the earth's crust, whether it be hard or soft, is called a rock. Wo distinction based on hardness alone is of any value. The saxpe sandy bed may be found in one place hard enough for building-stone, and in an- other soft enough to be spaded. The same clay stratum may sometimes be trac^l from a condition of slaty hard- ness in one place to good brick-earth in another ; the same bed of lime from marble into chalk, and the same volcanic eruption from stony ^ava into a bed of volcanic ashes. Classes of Eocks. — Eocks are divided, according to their structure and origin, into two principal kinds, viz., stratified and unstratified. Stratified rocks are more or less consolidated sediments, and are therefore aqueous in origin and earthy in structure. Unstratified rocks have been more or less fused, and therefore are igneous in origin and either crystalline or glassy in structure. CHAPTEE II. STRATIFIED ROOKS. Sbctiok I. — Theik Structure AifrD Position. Let any one examine the rocks of a quarry of limestone or sandstone, and he will find that the stone lies in regu- lar heds. In some places these beds will lie level (Pig. 91), in other places they may he inclined (Pig. 93). Por example, throughout the valley of the Mississippi they are usually level, while in mountain-regions they are usually inclined. The next most conspicuous structure will probably be the cross-divisions called joints, by which the beds are broken into separ- able blocks. These are found in all rocks, are not char- acteristic of strati- fied rocks, and therefore we say nothing more about them now. On ex- amining a little more closely, the beds will be seen to be subdivided by faint lines similiar to those observed in a section of sedi- ments, and known to be produced by the sorting power 179 . ■ L.^ ■>! . — ^^H s/t Figs. 91, 93.— Sections of horizontal and inclined strata : g, soil ; ss, sandstone ; eh, shale ; Zs, limestone. 180 STRUCTURAL QEOLO&T. of water (page 27). In a word^ the mass exposed on a clifE or in a quarry, or any large section of stratiflad rock, is seen to be divided by parallel planes into thick beds of different kinds of materials, as sandstone, limestone, etc., and each of these, probably, into thinner beds, differing perhaps in grain or color, and finally these again into thin sheets, produced by the sorting of material. N'ow, the larger beds are called strata, the subdivisions of different color or grain, layers, and the lines of sorted materials are lamihcB. These terms are loosely used, but always in the order mentioned, and the word lamina is always used to signify the marks of water-sorting. Now, the structure we have described is called stratification, and such rocks stratified rocks. Extent and Thickness. — Stratified rocks cover at least nine tenths of the land-surface, and even where they do not occur it is only because they have been removed by erosion or else covered by igneous rocks. Since, as we shall see presently, stratified rocks were formed at the bottom of the water, it is evident that there is no portion of the earth which has not been at some time covered by the sea. The extreme thickness of these rocks is proba- bly ten to twenty miles ; the average thickness is certainly several thousand feet. Principal Kinds. — As defined above, stratified rocks fall naturally into three great groups : 1. Arenaceous or sand-rocks ; 3. A rgillaceous or clay-rocks ; and, 3. Cal- careous or lime-rocks. These may be either in a soft or in a stony condition. The sand-rochs, in their soft or incoherent condition, are beds of sand, gravel, and pebbles or shingle. In their coherent or stony condition they are sandstones, grits, and conglomerates. Breccias differ from conglomerates only in having the fragments angular instead of rounded. They consist of rubble, instead of pebbles, cemented together. STRATIFIED ROOKS. 181 The clay-rocks, in their incoherent condition, are beds of clay, briok-earth, mud, and ooze. In their coherent condition they are the same cemented into shales, or, still harder, into slates. Lime-rocks, in an incoherent condition, are lime-muds, such as exist now in coral lagoons, or in the deep sea (glo- bigerina ooze, page 117) ; in a slightly consolidated con- dition they are chalks, and in a stony condition they are limestones, marbles, and travertines. These different kinds may each produce varieties of different color and grain. They also pass by mixture insensibly into each other, and thus form infinite varie- ties. Thus we may have an argillaceous or calcareous sandstone or calcareous shale, etc. All that need further be said on the subject of the origin of stratified rocks is best thrown into a series of propositions, very simple and yet underlying all geologi- cal reasonings : 1. Stratified Bocks are more or less Consolidated Sediments. — This has been thus far assumed. We wish now to direct the pupil to the observation of the evidence : a. Every gradation may be traced between muds, clays, and sands, which we know were deposited in water ; and shales and sandstones, which we find forming the strata of mountains. 5. In many cases we may see the process of hardening going on under our eyes. For example, at the mouths of rivers carrying lime in solution, like the Rhine, the river-silts are consolidated into calcareous shales. On the shores of coral reefs we find coral mud, coral sand, and coral breccia consolidated into peculiar limestones (page 108). c. Close .examination of many rocks, especially sandstones and shales, clearly shows the sorting of material (water-sorting) along the lines of lam- ination, d. As shells and skeletons of animals are now imbedded in muds of rivers, lakes, and seas, so fossils are found in stratified rocks, e. Other marks, which occur 182 STRUCTURAL GEOLOGY. in recent sediments, such as ripple-marks, rain-prints, sun-cracks, foot-prints of animals, etc., are also found in the hardest stratified rocks. In a word, it may be said that every marlc or peculiarity which has been observed in recent sediments has been found also in stratified rocks. We may assume, then, as certain that stratified rocks are sediments formed originally at the bottom of seas, lakes, rivers, etc., and that when ye find them far in the interior of continents and high up the slopes of mountains we have indubitable evidence of great changes of level. Stratified rocks are all deposits in water. Sandstones and shales are the debris of erosion, and are therefore mechanical deposits; and these rocks are often called fragmental rocks, because they are made up of the frag- ments of previous rocks. Limestones, on the other hand, are either organic or chemical deposits. Again, sand- stones, grits, and conglomerates are formed by violent action, and they indicate either rapid currents or exposed shores ; shales indicate quiet seas or bays ; limestones, open seas. We have already seen (page 37 et seq. ) that sediments are transported soils, and (page 10) that soils are disinte- grated rocks. Now, we see that stratified rocks are con- solidated sediments. We have here an example of a per- petually recurring cycle of changes : rocks are decomposed into soils, soils are carried and deposited as sediments, sediments are again consolidated into rocks, to be raised into land-surfaces, and again disintegrated into soils — and so the cycle goes round. The cause of consolidation is sometimes only the pressure of great thickness of sediment ; sometimes the same, aided by gentle heat ; sometimes there is a distinct cementing substance, the most common being lime car- bonate and silica. When there is a cementing snhstance, the process is often rapid, and may be observed ; as, for example, in the formation of coral rock. But in other HTUATllUMdJ KUUKib. ibs cases the process is very slow, and therefore the newer rocks are often, though not always, imperfectly consolidated. 3. Stratified Rocks have been gradually deposited. — By this we mean that they have not heen formed at once, as some of the older geologists imagined, but by the regular operation of causes similar to those now accumu- lating sediments. The slowness was sometimes extreme. For example : a. "We have strata in which the laminae are as thin as paper, and yet each one represents recurring conditions, as ebb and flow of tide, or flood and low water of rivers, i. In some cases we have a shell attached to the inside of another shell (Fig. 93), in such wise that the latter shell must have been dead before the former attached itself. In such cases a half or quarter inch thickness of rock represents the whole life of the second shell, c. We have seen that some limestones are made up of the accumulated remains of successive genera- tions of microscopic .shells (page 115). Every inch thick- ness of such deposit must rep- resent a long period of time. And yet such deposits are often hundreds or even thousands of feet in thickness. These are, however, extreme cases of slow- n ess. As a general rule, coarser materials are deposited more rapidly than finer — e. g., sands than clays and limestone, but all by regular opera- tion of causes ; and therefore, making due allowance for the nature of the materials, thickness is a rough measure ■ of time. 3. Stratified Rocks were originally horizontal at the Bottom of the Water, — This is i\ necessary conse- FiG. 93.— Serpulse on Interior of a shell. J 184 STRUCTURAL GEOLOGY. quence of the manner in which they were formed. There- fore, when we find them in other positions and at other levels, we- conclude that they have come so by subsequent change. We must not imagine, however, that the planes between the strata were ever absolutely horizontal. Strata must not be likened to continuous, even sheets, but rather to extensive cakes, thickest in the middle and thinning on the margins and there interlapping with other strata or cakes (Fig 94). Coarse materials, like sandstones and Fi&. 94.— Diagram showing thinning out of beds : a, sandstones and conglomerates; i, limestones. grits, are more local, and thin out more rapidly, while fine materials, like clays, are often very widely continuous. This thinning out of strata, however, does not interfere seriously with their appearance of evenness at any point of observation. Another more important apparent exception to original horizontality is what is called cross-lamination or false- hedding (Pig. 95). These are liable to be mistaken for K'///////////,,/ ///,//////v:'///////;/////w///M^^^^^ Fio. 95.— Section on MissiBsippi Central Bailroad at Oxford (after HUgard) : oblique lamination. STRATIFIED ROCKS. 185 tilted strata. But it will be observed that it is the lamina, and not the strata, which are inclined. And, moreover, their extreme irregularity is sufficient to distinguish them from true inclined strata. They seem always to be pro- duced by deposit from rapid, shifting, overloaded currents, and are, therefore, common in river-deposits. After explaining these apparent exceptions, we come back with still more confidence to the proposition that stratified rocks were originally soft sediments in a hori- zontal position at the bottom of seas, lakes, etc. But we usually find them noio in an entirely difEerent condition and position. We indeed find them sometimes soft, but more commonly stony ; sometimes, indeed, still horizon- tal, though raised above the sea and in the interior of continents, but more commonly more or less tilted ; some- times, especially in mountain-regions, not only tilted, but folded, crushed, contorted, broken, and dislocated in the most complex manner, so that it is difBcult to make out their natural order. Sometimes the contortion is in the lamincB, so that it can be seen in a hand-specimen (Fig. 96). Sometimes a series of strata are folded together, /•> FiQ. 96.— Crampled laminsE. (After Geikie.) such as may be seen at one view on an exposed cliff (Fig. 97). Sometimes the strata composing the crust of the 186 STRUCTURAL GEOLO&Y. FiQ. 97,— Contorted strata. ^Prom Logan.) earth, several thousand feet thick, are folded all together so that their foldings form great mountain-ridges, and can only be made out by extensive surveys (Fig 98). As Fig. 98. — Section of Appalachian ciiain. might be expected, the strata by such violent movements are usually broken and dislocated, and always, as seen in Eigs. 97 and 99, large portions of their upper parts have Fig. been carried away by erosion, leaving their edges exposed on the surface. Such exposure of strata on the surface is called outcrop. Fie. 100. STRATIFIED ROCKS. m This important subject must be taken up with some detail, and for this purpose it becomes necessary to define some common geological terms. Dip and Strike. — The angle of inclination of strata with the horizon is called the dip. There are always two elements to be considered j viz., direction and amount. Thus a stratum may dip northward 30°. The angle of dip varies from to 90° — i. e., from horizontality to verticality. Sometimes strata are even pushed over beyond the vertical — such are called overturn-dips (Fig. 99). Examples are found in all great mountain-chains, especially in the Alps. When strata dip regularly, their thickness may be easily estimated. For example, in walking from a to b (Fig. 100), we pass over strata whose thickness is b c {= a i . sin i a c). The dip may be accurately determined by means of a clinometer (Fig. 101). Fig. 101.— Clinometer. The direction of strata, or their line of intersection with a horizontal plane, is called the strike. It is always at right angles to the dip. If the dip is so many degrees north or south, the strike will be east and west. If the surface of the ground is level, the strike will be the same as the outcrop, or appearance on the surface, of the strata ; but this is seldom the case. If the strata are plane, the strike will be a straight line. If the strata are folded, the strike may be very sinuous (Fig. 107). In a map view of strata, the dip and strike are represented by the. sign 1, in which the heavy line represents the strike. 188 STMUGTURAL GEOLOGY. and the perpendicular the dip (Fig. 105). The perpen- dicular is made shorter, as the dip is at a higher angle. Anticline and Syncline. — When a series of strata dip in one direction in one place, the same series will usually be found to dip in a contrary 'direction in another place. In other words, strata are usually disturbed by lateral pressure, which throws them into folds, sometimes wide and gentle, like undulations, sometimes closely appressed. Thus strata usually occur in alternate saddles and troughs (Pigs. 103, 103). The saddles are called anticlines, the troughs synclines. An anticlinal axis, then, may be de- FiG. 103. fined as a line on either side of which the strata repeat one another, dipping in opposite directions, away from the axis. A synclinal axis is a line on either side of which the strata repeat each other, dipping in opposite directions, but toward the axis. In Pigs. 103 and 104, a is an anticline, and s a syncline. In anticlines the strata lie in saddles and in synclines in troughs, but the surface configuration of the ground may or may not correspond. Sometimes the ground is comparatively level, though the foldings are strongly marked (Fig. 103). Sometimes the anticlines are ridges, and the synclines valleys (Pig. 103), and sometimes the Fia. 103. STRATIFIED ROCKS. 189 reverse (Fig. 104). In gently iolded strata it is very common to find the configuration reversed on the surface. Fig. 104. i e., synclinal ridges and anticlinal valleys. Examples of these are given on page 248. Folded strata, which are tilted only iy folding, will outcrop on level ground in parallel bands, as in Fig. 105, II " 1 1 III' " " III III " II » .* *. " " — ' " " u 11 11 M '' 1 " ^ H ; " If ir '■ III " " III 1 " Fro. lO.?. which is a map view of Fig. 102. But if the whole be again tilted in a direction at right angles to the folds. A ■■-■;;;f :::;.-'■ Fio. 106.— Section of undulating strata. then the map of outcrop will be sinuous. Fig. 106 is a section of folded strata thus tilted, and Fig. 107 is a map of the same. The section is along the line C D. Exam- ination of the signs of dip will explain the map. 190 SIEUCTURAL GEOLOQT. FiQ. 107.— Plan of undulating strata. "We have spoken of folded strata and the way in which they outcrop ; but in a survey the process is reversed, i.e., it is the outcrop which is observed, and from this we con- struct the section. Now, when we remember the complex folding, then the tilting after folding, then the displace- ment by fractures, and then, worst of all, the covering of the whole deeply with soil, leaving exposed only patches here and there, we can easily see how difficult a problem it often is to construct a section of the stratified rocks of a country. If the strata be exposed on a clifE or a cafion- side, there is little difficulty, but, in the absence of such, the geologist takes advantage of every exposed patch, examines every gulch or stream-bed, every quarry or railroad-cutting, and thus constructs an ideal section. Conformity and Uuconforinity. — We have just seen that the strata composing the country rock of a land- surface are usually tilted and crumpled and always eroded, so that their edges are exposed (see Kgs. 106, 107). But we have also seen (pages 164-170) that in some places land-surfaces are now sinking beneath the sea, and in others sea-bottoms are rising to become land-surfaces. The same is true for all geological epochs. Now, suppose at any time an eroded land-surface sank below sea-level so that sediments were deposited on the eroded edges and filling the erosion-hollows of the strata, and finally the STRATIFIED HOOKS. 191 whole was again raised above sea and exposed to the in- spection of the geologist ; the phenomena which would Fig. 108.— Some cases of unconformity. be observed are represented by Fig. 108, A, B, 0. This is what is called unconformity. More commonly in such cases there is a want of parallelism between the two series of strata, as in Fig. 108, A, B. But this is not necessary. Fig". 108, G, represents unconformity no less than A and B. In the one case the strata were raised into land-surface and at the same time folded and tilted, and then eroded ; in the other case, they were raised and eroded without folding or tilting. Sometimes the second raising is also attended with tilting, in which case both series are tilted, but in different degrees, as in B. Definition.— After this explanation, we are prepared to define. When a series of strata are parallel, as if formed continuously under similar conditions, they are 192 STRUCTURAL GEOLO&Y. said to be conformable. But if two series are discontinu- ous — i. e., separated by an erosion-surface or old land-sur- face, and therefore formed at different times and under different conditions — they are said to be unconformable. In all the figures the strata of the lower series are con- formable throughout, and so are also those of the upper, but the two series are unconformable with each other, the line of unconformity being an old eroded land-surface. Even so simple sections as Pig. 108, one of the com- monest observed, record many interesting events in the history of the earth, viz. : 1. A long period of quiet, during which the first series of strata was deposited. 2. A period of commotion, during which the sea-bottom here was elevated into land, and perhaps tlie strata crum- pled. 3. A long period during .which it remained land- surface and was deeply eroded and the strata-edges exposed. 4. Another period of commotion, during which it sank again and became sea-bottom. 5. Another long period of quiet, during which the second series of strata was de- posited ; and, 6. Still another period of movement, by which the whole was finally raised and became thus sub- ject to the inspection of the geologist. The following diagrams (Pig. 109) represent the man- ner in which the phenomena may be supposed to have occurred. In A, we have thick sediments, 8d, accumu- lated on an off-shore sea-bottom. In B, the same have been elevated into land, and crumpled. In C, they Have been eroded and their edges exposed. In D, they have again subsided beneath the sea, and received sediments, 8d, on their eroded edges. Since geological history is mainly recorded in stratified rocks, and since, while a place is land-surface and being eroded, there can be no strata formed there, it is evident that a line of unconformity always indicates a period of which there is no record at that place, although the record may be found elsewhere. Unconformity, therefore, al- STRATIFIED ItOOKS. 193 ways represeiats « ffap in the record — a lost interval of time — which may be very long, viz., the whole time dur- ing which the erosion was going on. Fis. 109. — ^In all : L, land ; , sea-level ; SA, shore-line ; Sb, sea-bottom ; Sd, sediments. A group of conformable strata usually form a geologi- cal formation, and a line of unconformity usually sepa- rates two different geological formations. The division of the strata into formations, however, is based also on other characters, viz., the contained fossils. The subject will be taken up again under that head. Cleavage Structure. Stratification is an original structure, i. e., impressed at the time of deposit of sediments. Cleavage is a super- induced or subsequent structure, but it so simulates Le Conte, Geol. 13 194 STRUCTURAL aEOLO&Y. stratification that it seems best to take it up here. It is found in many kinds of rocks, but most perfectly in slates, and is therefore often called slaty cleavage. Definition. — Cleavage is easy splitting in certain di- rections. There are many kinds of cleavage due to dif- erent causes. For example, many crystals split perfectly in certain directions. This is called crystalline cleavage, and is due to molecular arrangement. Certain stratified sands split easily into broad flag-stones in the direction of the laminae. This is lamination cleavage, and is due to the arrangement of the grains by the sorting power of water. Again, wood splits easily in the direction of the silver grain. This wood-cleavage is due to the arrange- ment of the wood-cells. Slaty Cleavage. — Now, there is also an easy splitting of rocks in definite directions, which occurs on an im- mense scale, and in certain slates is a very marked struc ture. The direction of cleavage is usually vertical or highly inclined. Whole mountains are thus cleavable from top to bottom, and rocks over thousands of square miles are often made up of such thin sheets. It is by splitting along these lines of easy fracture that roofing- slates, ciphering-slates, and blackboard-slates are made. On casual examination of strata the cleavage-planes are liable to be mistaken for fine lamince, and we are apt to FiQ. 110.— Cleavage-planes cutting througli strata. think that we are examining a beautiful example of highly inclined strata. But a closer examination will usually show the lines of stratification running in an entirely different direction. In Fig. 110, the strong lines STRATIFIED ROCKS. 195 show the strata strongly folded, while the light lines show the cleavage nearly vertical, cutting through these Pig. 111. — Strata, cleavage-planes, and jointe. in parallel planes. In Pig. Ill, three kinds of structure, which should be kept distinct in the mind, are shown. The rectangular block-faces are joints ; the strong lines, s s, slightly inclined to the right, are strata ; while the highly inclined lighter lines are cleavage-planes cutting through both. Cause of Slaty Cleavage. — Slaty cleavage is undoubt- edly caused by a mashing together of the whole rock-mass in a direction at right angles to the cleavage-planes, and an extension in the direction of these planes ; and, since cleavage-planes are usually nearly vertical, it is the result of a mashing together horizontally, and an up-swelling or extension vertically of the whole cleaved mass. Proof. — This is proved (a) in field-observation by the folding of the strata (Fig. 110), and (J) in hand-speci- mens by the crumpling of the finest laminae in the direc- tion indicated above. Fig. 112 represents a block of slate eighteen inches long, in which the lamination-lines are shown crumpled by the pressure. In the position of the block it is evident that the crushing was horizontal. The cleavage-planes, represented by the light lines, are vertical. One cleavage-face, c p, is shown. The same is proved, also (c), by distorted fossils often found in cleaved slates (Fig. 113). By comparing the natural with the 196 STRUCTURAL GEOLOGY. distorted form the direction of pressure is found to be always at right angles to the cleavage-planes, i. e., the Kg. 118.— a block of cleaved slate. (After Jukes.) fossils are shortened in that direction and elongated in the direction of the planes [d'). In many slates, especially Fig. 113.— Cardium hillanmn : A, natural form ; B and C, deformed by pressure. the purple Cumberland slates, much used in roofing, oblong greenish spots are common. If they be closely examined, they will be found to be very thin in the direc- STRATIFIED BOOKS. 1^1 fcion of the thickness ol the slato or at right augles to cleavage. On the cleavage surface the shape is broad, elliptical (Fig. 114, A), while on sec- tion the shape is very flat, B. These spots before mashing were round pellets of clay. They have been mashed into an ellipsoid of three unequal diameters, the longest, a i, in the dip of the cleavage, and therefore nearly vertical ; the next, c d, in the strike of the cleavage, and therefore horizontal ; and the smallest, b ^ e f, at right angles to cleavage. This Pio- ii4.— Flattened proves that the whole mass has been ^"t^sUtdiew. mashed at right angles to cleavage, and extended in the direction of the dip of cleavage. Micro- scopic examination shows that every constituent granule of the original clay is in the slate mashed into a thin scale, so that the original granular structure is changed into a scaly structure, and it is this which determines the easy splitting. Geological Application. — The amount of mashing to- gether horizontally aind extension vertically shown in these different ways is so great that an original cube or sphere in the unsqueezed mass is changed into an oblong, of which the shortest diameter is to the longest as one to three or four, "one to five or six, one to nine or ten, and even sometimes as one to fifteen. The average in well- cleaved slates is one to six. Now, when we remember that thousands of square miles and thousands of feet thickness of rocks are thus affected, it is evident that this slow mashing together horizontally of whole mountain-regions must be an important agent in the elevation of land, and especially in the formation of mountains. We shall speak of this again under the head of mountains. 198 STRUCTURAL GEOLOGY. Concretionary or Nodular Structure. This, also, is a superinduced structure simulating an original structure. As slaty cleavage simulates stratifi- cation, so concretions or nodules simulate and are apt to be mistaken for fossils. In many strata, especially calcareous sandstones and shales, we find rounded masses often of curious shapes, separable from the general mass of the strata, and differ- ing a little from it in hardness aiid composition. These are called concretions, nodules, septaria, etc. They have evidently been separated out of the general mass after the latter was deposited. This is shown by the fact that the planes of stratification often run right through them (Kg. 115). Forms and Sti'ucture. — In. form they are sometimes perfectly spherical, like can- non-balls, and vary in size from that of a marble to many feet or even yards in diame- ter ; sometimes flattened ellipsoidal, and these, when Fig. 115. Fiu. 116.— KodulcB, hum strata. STRATIFIED ROCKS. 199 marked with polygonal cracks, simulate very much a turtle-shell, and are called turtle-stones ; sometimes dumb- bell-shaped, sometimes rings, sometimes all sorts of strange and fantastic shapes (Fig. 116). In structure they are sometimes solid, sometimes hollow, sometimes afEected with interior cracks, sometimes have a concentric shell-structure, and sometimes a radiated structure. These curious shapes so simulate fossils that even ex- perienced geologists may sometimes be in doubt. By common observers they are very often mistaken for fossil nuts, fossil turtles, etc. They are, however, very inter- esting to the geologist, because they often contain a fossil beautifully preserved in the center. How Formed. — They seem to be formed by the slow .aggregation of more soluble or more suspensible matter from a general mass of insoluble matter, an organism Pio. 117.— Chalk-clifEs with flint nodules. often forming the nucleus of aggregation. Thus, if the mass be a calcareous sandstone, the lime will gather in places, forming sandstones containing more lime than the general mass. So calcareous clays form nodules of lime mixed with clay. These are the hydraulic-cement nod- ules. In chalk the disseminated silica seems to gather SOO STRUCTURAL &EOLO&Y. into nodules of pure flint, and leave the chalk a pure carbonate of lime depriTed of its silica. Hence, chalk usually contains flint nodules, scattered or in layers (Fig. 117). We speak of this nodular structure not on account o± its great importance, but because it is apt to strike the observing eye, and very apt, too, to be mistaken for fossils. Fossils : their Origin and Distribution. Every one must have observed that in many places the stratified rocks contain the exact forms of organisms, especially shells, though these seem to have turned to stone. These are called fossils. They are of extreme interest to geologists, because they reveal the nature of the former inhabitants of the earth. Stratified rocks are the consolidated sediments of former seas, bays, lakes, and rivers. Then, as now, shells lived in the ooze of sea- bottoms, or were cast up on beaches ; the leaves and branches of trees and carcasses of land-animals were car- ried down by rivers to lakes and estuaries and buried in mud. These have been preserved, with more or less change, to the present day. A fossil, then, may be defined as any evidence of the former existence of a living thing. Next to lamination, they are the most constant characteristic of sedimentary rocks. Degrees and Kinds of Preservation. — There are various degrees and kinds of preservation of organic forms. In some cases not only form and structure, but even the organic matter of soft parts, is preserved. More commonly, however, only the shells and skeletons of ani- mals are preserved, and of these sometimes both the form and structure, and sometimes only the form. We shall speak of these under three heads : 1. Organic Matter preserved. — This, of course, is rare. The only perfect examples are those of carcasses STRATIFIED ROOKS. 301 preserved in ice. In the frozen cliffs and soils of Siberia, the carcasses of extinct elephants and rhinoceroses have been exhumed by the rivers, in a condition so perfect that dogs and wolves fed on the flesh. In peat-bogs are found the perfect skeletons (still retaining the organic matter of the bones) of extinct animals ; and in some cases even the flesh is preserved, but changed into a fatty substance (adipocere). These are all in comparatively recent strata. But, even in the oldest strata, organic matters of once living beings are preserved, though changed into coal, lignite, petroleum, bitumen, etc. 3. Organic Structure preserved. — This is the type of what is called petrifaction ; it is best illustrated by petrifled wood. In many strata, but especially in the sub-lava gravels of California (page 395) and the tufa beds of California and the Basin region, drift-wood is found completely changed into stone. In these we have not only the form, not only the general structure — i. e., bark, wood, and pith, concentric rings, medullary rays, and woody wedges — but even the minutest microscopic structure of tissue and markings on the walls of cells, perfectly preserved in the stony matter (usually silica) replacing the wood. Mode of Petrifaction. — It must not be imagined that the wood is turned to stone, but is only replaced by stony matter. As each particle of woody matter passes away by decay, a particle of mineral matter is deposited in its place from solution, thus reproducing its structure per fectly. "Wood best illustrates the process, but in a simi- lar manner the minute structure of bones, teeth, corals, shells, etc., are preserved, even though the original mat- ter is all gone. The most common petrifiers are silica and carbonate of lime. 3. Organic Form only preserved. — In many cases the structure is not preserved, but wo find only a mold of the external form, or a cast of the same in stone. This is 203 STRUCTURAL GEOLOGY. best illustrated by the case of shells. The following figure is a diagram showing four difEerent cases, all of which are very common. In the figure the horizontal Fia. 118.— Section of strata containing fossils. lines represent the stony matrix in which the shell is formed, or mud in which the shell was originally buried, and the rertical lines represent the subsequent filling with finer material. Explanation. — In case a, the living or recently dead shell was buried in mud, and afterward the whole organ- ism was dissolved and removed, leaving only the hollow mold where it lay. In case h, we have the same, only the mold has been subsequently filled and a cast made by the deposit of silica or carbonate of lime from solution. If the rock be broken, the cast will often drop out of the mold. In c, the dead, empty shell was buried in mud and filled with the same, and afterward the shell was removed Pio. 119.— 0, Natural form ; 6, Fig. 120.— Trigonia longa, showing cast (a) of the cast of interior and mold of exterior and (J) of the interior of the shell exterior. STRATIFIED SOCKS. 203 by solution, leaving an empty space corresponding to the thickness of the shell. In d, this hollow space was sub- seqiiently filled by deposit of soluble matters from perco- lating waters. Cases c and d are represented by Figs. 119 and 130. Sometimes we have only the mold and cast of a small part of an organism, as, for example, impressions of the leaves of plants, or the footprints of animals walking on the mud when it was soft. These, however, are of great value, because they are very characteristic parts of plants and animals. Finally, there are all grades of completeness of the process of replacement. In bones, shells, and teeth, sometimes only the organic matter is partly or wholly replaced. Sometimes, also, the mineral matter is replaced by other mineral matter. Distribution of Fossil Species. The kind of fossils which we find in the strata at any place will depend on three things : 1. On the hind of rock ; 2. On the country ; and, 3. On the age of the rock. Kind of Rock. — We have already said (page 130) that at the present time different depths and bottoms are fre- quented by different marine species. Some live on sand- bottoms, some on mud-bottoms, and some on deep-sea ooze. The same was true in previous epochs, and there- fore we ought to expect and we do find that, in the same country, and in strata of the same age, sandstones will contain different fossils from -limestones ; the one being shore and the other open-sea deposit. Again, then as now, lake-deposits contained fresh-water animals, and estuary deposits land plants and animals ; and these are of course different from marine species, though they be of the same age and country. The Country. — In rocks of the same age and same 304 STRUCTURAL GEOLOGY. kind, but ii different continents, we shall often find a great differ?" 'ce of species, for we find the same thing true of living species (page 118). But the geographical diversity of f osr-:"! species, as a general fact, is' not so great as that of living species. Commencing with the earliest times, the geographical differences of species have in- creased more and more to the present time. Tlie Age. — The distribution- of fossil species according to the age of the rocks is the main subject of Part III, or Historical Geology ; but some general notions on this subject are necessary as a basis of classification of strati- fied rocks, and must therefore precede that part. Successive Geological Faunas -and Floras. — The ' fossil species found in rocks, even of the ^ame kind zxA. country, will depend largely on the age of the rocks. The whole earth has been inhabited at different times by entirely different species. All the animals and plants in- habiting the earth at one time are called the fauna and flora of that geological time. Thus we have a fauna and flora of Tertiary times, of Jurassic times, of Devonian times, etc. Definition of Formation and Period. — When the strata are conformable, the change from one geological fauna to another is gradual, but a line of unconformity usually abruptly separates two faunas. A formation, therefore, is a series of conformable strata, in which the fossil species are either the same or change very gradu- ally ; and a geological period is the period during which such a formation has been laid down. There are two tests, therefore, of the limits of a geological formation and a geological period, viz., unconformity of the rock- system and great change in the species. Of these the latter is the more valuable. Law of Gradual Approach to the Present. — It is a fundamental and very important fact that in the suc- cessive changes of geological species there is a steady STRATIFIED SOCKS. 305 approach to living forms, first in families, then in genera, and then in species. Species do not begin to be identical with the living species until the Tertiary period, and thence onward we have an increasing percentage, identical with the living. Now, we determine that rocks belong to the same time, all over the earth, by the general similarity of the fossil species. We find difficulty in applying this rule only in the Tertiary, because then the geographical diversity is beginning to be so great as seriously to interfere with the general similarity. But just here we begin to use another principle, viz., the percentage of the fossil species still living in the immediate vicinity. Similar percentage in- dicates the same age — greater percentage less age, and less percentage greater age.* It is on these principles that is based the classification of stratified rocks. SECTioiir II. — Classification of Stratified Eocks. Geology is a history. Stratified rocks are the leaves of an Liitorical book. Evidently, then, the true basis of 'lassification must be rslative age. In classification, the geologist has two objects in view : I. To arrange all the strata, from lowest to highest, in ine order in which they were formed. 2. Then to separate them into groups and sub-groups for convenient treatment — ^i. e., 1. To arrange the leaves in the order in which they were written, so that the story they contain may be read intelligently. 2. To divide and subdivide into chapters and sections, deter- mined by great events in the history. In a word, he must make first a chronology, and then divide into eras, ages, periods, etc. Chronology; Order of Superposition. — It is evi- dent, from the manner in which sediments are formed, * The teacher should consult the larger work, for a complete state- ment. 206 STRUCTURAL aEOLO&Y. that, if they have not been greatly disturbed, their reh- tive position indicates their relative ages, the uppermost being of course the youngest. If, therefore, ive have a natural section of strata (an exposed sea-clifE or caflon- side), either horizontal or regularly inclined, it is easy to make out the relatire ages. But often the rocks are folded and crumpled, and pushed over beyond the verti- cal ; they are broken and slipped, and a large part worn away by erosion ; they are covered with soil and hidden from view ; so that to make an ideal section showing their real relation is one of the hardest of geological problems.. Nevertheless, if this were all, we might still hope for per- fect success. But all the strata are not represented in any one place — usually only a fraction. Thus, in New York, and all the States westward as far as the Plains, only the older portion of the record is found ; while in California we have mostly the later portion. In many places the record is still more fragmentary. The leaves of this book are scattered about — ^here, perhaps, nearly a whole vol- ume ; there, one or two chapters ; and yonder, only a few leaves. The geologist must gather these and t^rrange them according to their paging' ; and then divide anr" subdivide them into volumes, chapters, etc. Therefore, although the order of superposition must, wherever it can be applied, take precedence of every other method, yet it must be supplemented by careful comparison of the rocks in different localities with one another. There are two means of comparison, viz., the character of the roch and the character of the fossils. Comparison by Rock-Cliaracter. — This method is of little value except in contiguous localities. Sandstones of similar character belong to nearly all times, and are forming now. So, also, of clays and limestones. Coal was once considered characteristic of a particular age, but now is known to occur in strata of many ages. Chalk was once supposed to be characteristic of the Cretaceous, STRATIFIED liOG^S. 307 but is now known to be forming at present in deep seas. But since, both now and in former times, the same kind of deposits formed over wide areas, rocks of similar kind (for example, sandstones of similar grain and color), and especially a group of similar rocks, in contiguous locali- ties, are probably of the same age. But in widely sepa- rated localities, as, for example, in different continents, we can not use this method. To conclude that rocks are of the same age, because they are of similar grain, color, or composition, would almost certainly lead us astray. Comparison of Fossils. — This is the most universal and valuable means of comparison of rocks in all parts of the world. If vrejind a general similarity of species, we conclude that the rocks belong to the same age. But we must make due allowance — 1. For difference of conditions of deposit, whether shore-deposit or deep-sea deposit, whether fresh-water or marine. 2. We must also make due allowance for geographical diversity. We must ex- pect, in fossils of rocks in different continents, not abso- lute identity, but only general similarity. We shall find little difficulty in applying this, until we come to the Tertiary. But here we have another principle to help us, viz., the percentage of living invertebrates found in the rock. Vertebrate, and especially mammalian species, may be used in the Tertiary in much the same way as all species in the lower rocks. Construction of Chronology. — By application of these methods, geologists in all countries, working to- gether, have gradually made a nearly complete chronol- ogy. Breaks in one country are filled by strata in an- other. But a really complete chronology can not be expected until the whole surface of the earth has been' studied, and perhaps not even then, for some missing links are probably concealed beneath the sea. Divisions and Subdivisions. — The next task is to divide and subdivide the whole into primary and second- 208 STRUCTURAL GEOLOGY. ary groups — into volumes, chapters, etc., separated by great changes. As already explained (page 304), there Eras. Ages. Pebiods. Epochs. / 5. Psychozoic. 7. Age of Man. Human. Recent. 4. Cenozoic. 6. Age of Mam- mals. ( Quaternary. L Tertiary. Terrace. < Champlain. Glacial. Pliocene. \ Miocene. Eocene. 3. Mesozoic. Secondary rocks. 5. Age of Rep- tiles. Cretaceous. - Jurassic. Triassic. Upper- Carloniferous \ rocks. 1 4. Age of Aero- } gens and Amphibians. J ("Permian. J Carboniferous. Subcarbonifer- ous. 3. Paleozoic. ' Lower- Devonian rocks. 3. Age of Pishes. 'Catskill. Chemung. - Hamilton. , C'orniferous. . Oriskany. Silurian rocks. 2. Age of Inver- tebrates. Cambrian or primordial rocks. ■ Helderberg. Salina. ■ Niagara. Trenton. Canadian. Upper. - Middle. Lower. 1. Archsean or Arohajozoic. 1. Arohiean rocks. ( Huronian. \ Laurentian. STRATIFIED ROCKS. 209 are two modes of determining the limits of the divisions of the rocks, and corresponding divisions of time, viz., by unconformity of the rocks, and by change of the fossils. These two usually occur together, because they are pro- duced by the same cause, viz., change in physical geogra- phy and climate ; but, if there be discordance between the two, then we follow the change in the fossils rather than unconformity of rocks. By means of the most gen- eral unconformity and greatest change in fossil forms, the primary divisions are established ; and then, by less gen- eral unconformity and less important changes in organic forms, these are divided and subdivided. A generalized schedule of the divisions and subdivisions of the rocks and corresponding divisions of time which will be used in this work, is given on the preceding page. Le Coiitb, Gbol. 14 CHAPTER III. UKSTEATIMEB OR IGNEOUS ROCKS. These differ wholly from the stratified rocTcs — 1. By absence of true stratification, i. e., lamination by sorting of material. 2. By absence of fossils. 3. By a crystal- line or else a glassy texture instead of an earthy texture. 4. By mode of occurrence, as explained below. Origin. — All these characteristics are the result of their mode of origin. They have consolidated from a state of fusion or semi-fusion, and poured out from below, instead of deposited as sediments from above. Their original fused condition is shown by their crystalline or glassy texture, by their occurrence injected into fissures, or even tortuous cracks, and by their effects on the stratified rocks with which they come in contact. Mode of Occurrence. — They occur in three main positions : 1. Underlying the stratified rocks and appear- ing on the surface in great masses, especially in mountain- PiQ. 121.— Ideal section of the earth's crust. UNSTRATIFIED OR IGNEOUS ROCKS. 211 ous regions {a, Fig. 121). 2. In vertical sheets intersect- ing the stratified rocks or other igneous rocks, b. 3. In streams or sheets overlying the stratified, or else between the strata, c d. 4. Sometimes as tortuous veins, d d, connected with the great underlying masses. All of these are connected with, and are extensions of, the great underlying masses. Extent. — As thus defined, igneous rocks occupy but a small portion, certainly not more than one tenth, of the land-surface. But beneath the stratified rocks they are supposed to form the great mass of the earth. Classification of Igneous Kocks. — Igneous rocks can not be classified, like sedimentaries, by relative age. They are best classified partly by texture and partly by mode of occurrence. They thus fall into two strongly contrasted groups, viz., plutonics and volcanics, or ffra- nitics and true eruptives. The rocks of the one group are very coarse-grained and wholly crystalline, of the other, finer-grained or even glassy. The one occurs only in great masses, either underlying the stratified rocks, or appearing on the surface over wide areas, especially in the axes of mountain-ranges ; the other, in sheets injected among the strata, or as streams and sheets outpoured on the surface. The granitics have not usually been erupted at all, althoiigh they often form the reservoirs from which eruptions have taken place. It is sometimes convenient to speak of an intermediate group — trappean. If so, then the three kinds correspond to the three positions mentioned above. The granitic (Fig. 131, a) occur ieneath ; the trappean, 5 h, injected among ; the volcanic, cc, outpoured upon, the stratified rocks. I. — The Massive or Q-eanitic GrKOUP. The rocks of this group occur in great masses, not in sheets or streams. They are all very coarse-grained in 312 STRUCTURAL GEOLOGY. texture, and have. a speckled or mottled appearance, be- cause composed of crystals of considerable size, and of difEerent colors, aggregated together. The crystals of which they mainly consist are, quartz, feldspar, mica, and hornblende. In such a coarse, speckled rock, the bluish, glassy, transparent spots are quartz ; the opaque, whitish, or rose or greenish crystals, with striated surface, are feldspar ; the black spots are usually hornblende ; the • mica may be known by its thin, scaly structure, some- times pearly, sometimes black. The whole group is called granitic, because granite is its best type. In popular language, indeed, all these rocks would be called granite, but sci- ence makes a difEerence. If the rock consists of quartz, feld- spar, and mica, or else of these with hornblende, then it is granite proper. If it consists of feldspar and hornblende, or these with quartz, it is called syenite. If it consists of only quartz and feldspar, and the quartz be m bent plates, looking, on section, like Hebrew characters, it is called pegmatite (Fig. 122). The feldspar in all these is potash- feldspar, or ortJioclase. Diorite is a dark, speckled rock of the same composition as syenite, except that the feld- spar is a soda-lime feldspar or plagioclase. Gahbro and diabase are dark-greenish rocks similar to diorite, except that the hornblende is replaced by augite and olivine.* Mode of Occurrence. — The mode of occurrence of these rocks has been already explained. They never occur in overflows. They rarely or never occur in in- truded sheets or dihes. They occur only in great masses, or sometimes in tortuous veins closely connected with the * The teacher must have a small collection of rocks and of min- erals for illustration. Pie. 183. — Graphic granite. UNSTRATIFIED OR laNEOUS BOCKS. 313 great masses, as if forced into cracks by heavy pressure (Pig. 131, d). Their coarsely crystalline texture and their mode of occurrence are well explained by supposing that they have cooled at great depth in large masses, and consequently slowly. "When they appear at the surface, therefore, they have been exposed by extensive erosion. Two Sub-Groups. — All igneous rocks, whether plu- tonic or volcanic, are divisible into two sub-groups, acidic and iasic. In the acidic, quartz and potash-feldspar (orthoclase) predominate ; in the basic, hornblende or augite and soda-lime feldspar (plagioclase) predominate. The rocks of the former group are lighter colored and less dense ; of the latter, are darker and heavier ; but the two sub-groups run insensibly into each other. Among the granitics, granite is the best type of the acidics ; and diorite, and especially gabbro or diabase, of the basics. Intermediate Series. Between the true plutonics and true voleanics there is an intermediate series, called trappean or intrusives. If the plutonics occur in masses beneath, the voleanics in outpoured streams and sheets upon, these occur in sheets intruded among, the strata, especially of the older rocks. They are finer-grained than the plutonics and more crys- talline than voleanics. The reason, apparently, is that they have cooled more rapidly than the former, and less rapidly than the latter. These are also divisible into acidics and basics. Among the acidics ^^ '"^-^^^Tym. '"""'' would come felsite and por- phyry, and, among basics, diorite and diabase, for these occur both massive and intrusive. 214 STRUCTURAL GEOLOGY. Diorite and diabase have already been described. It is only necessary to say that, when occurring intrusive, they are finer-grained than the massive varieties. Felsite is a fine-grained, light-grayish rock, consisting essentially of orthoclase and quartz. Porphyry is a rock consisting of fine-grained feldspathic paste, with disseminated large crystals of feldspar (Fig. 123). But any rock is said to be porphyritic if it consists of fine-grained paste with large crystals of any kind disseminated. II. — YoLCANics, OE True Eeuptives. The rocks of this group are distinguished from those of the other, both by texture and mode of occurrence. By texture they are not only finer-grained (micro-crystal- line), but there is always more or less of uncrystalline or glassy base or cement, showing that the fused mass has cooled too quickly to allow complete crystallization. Often, also, as already explained under volcanoes (page 139), these rocks are in a wholly glassy and even in a scoriaceous and tufaceous condi- tion. The principal rocks of this group are given in the ac- companying table. Trachyte may be taken as a type of the acidics. It is a light-colored rock, with a rough feel (hence the name), consisting essentially of orthoclase with more or less quartz. When the quartz-grains are con- spicuous, it becomes rhyolite. Phonolite is a dense vari- ety, of light-grayish color, which splits into slabs in weathering, and rings under the hammer almost like metal (hence the name). Obsidian and pumice are glassy and scoriaceous varieties of trachyte. VOLCANIC KOCKS. ACTDIO. BASIC. ( Ehyolite. Stony. J. Trachyte. ( Phonolite. Glassy. ] Obsidian. ( Pumice. Basalt. Dolerite. Andesite. Tachylite. Black soorisB. UN STRATIFIED OR IGNEOLS ROCKS. 315 Basalt is the type of the basics. It is a very dark, almost black, heavy rock, scarcely visibly grained to the naked eye, and breaking with conchoidal fracture. It consists of plagioclase with augite, olivine, and magnetite. Dolerite has a similar composition, but more distinctly crystalline texture, and therefore dark-grayish color. TacJiylite is the glassy variety, which, if vesicular, be- comes black scoria. The following table is a condensed statement of the composition of the principal kinds of rocks numbered above. The sign x x indicates crystals. IGNEOUS ROCKS. I^M .s . u > §•1 lo Bhyottte. Vitreous base. X X of Quartz, Orthoclase (sanidin}. Traehyte. Vitreous base. + X X of Orthoclase (sanidin). PTwnolite. VitreouB base. + X X of Sanidin, Nephelin, Andesite^ Vitreous base. -f X X of Pla^oclase, Augite, or Hornblende. Basalt. Vitreous base. + X X of Plagioclase, Augite, Olivine. Quartz-porphyry. Micro X X ground- mass. + X X of Orthoclase, Quartz. Felsite. Micro X X of Orthoclase, Quartz. Dlorite. See below. Diabcuie. See below. Granite. X X of Quartz, Orthoclase, Mica. Syenite. X X of Orthoclase, Hornblende. Siorite. Diabase: X x of X X of Plagioclase, Plagioclase. Hornblende. Augite. Two Modes of Eruption. — There are two modes of eruption. In the one, the fused mass comes up through chimneys, and flows off in streams (or ejected as cinders and ashes) ; in the other, it comes up through great fissures often hundreds of miles long, and spreads as 316 STRUCTURAL &EOLOaT. extensive sheets. In the one the erupted matters ac- cumulate about the vent as a cone ; in the other they form great lava-fields, or else may be forced between the strata and never come to the surface at all. In the one the/orce of ejection is probably the elastic force of vapors, as explained under volcanoes ; in the other the force is more obscure, but probably of the same nature as that vfhich. forms mountains. The two kinds may be called crater-eruptions amdi fissure-eruptions. At present only the former kind seems to exist ; and therefore in Part I, while treating of causes now in operation, we treated only of this mode. But 'in studying erupted materials of aZZ periods, it is plain that by far the larger quantity have come up in the second way. Modes of Occurrence. — Leaving out of view those modes of occurrence already described under volcanoes, viz., chimney-cones with radiating dikes and lava-streams, the principal modes of occurrence of eruptive rocks are : 1. Dikes. 3. Overflow-sheets. 3. Intercalary beds. 1. Dikesl — Dikes are vertical sheets filling great fis- sures in stratified or other igneous rocks. They are the most common of all modes of occurrence of eruptives and intrusives. In all mountain-regions they are found in great numbers. In width they vary from a few feet to hundreds of feet, and may often be traced outcropping over the surface fifty to one hundred miles. But since rocks are usually covered with soil, they are not always visible at once, but must be looked for wherever the rook is exposed, especially in stream-beds. It is evident that fused matter coming to the surface must overfiow, and therefore dikes thus outcropping on the surface are either the exposed roots of former over- flows which have been removed by erosion, or else are the fillings of fissures which never reached the surface at all (Pig. 121, V). In either case, an outcropping dike is the sign of gieat erosion. If, therefore, the dike is harder UNSTRATIFIED OB IGNEOUS ROCKS. 211 than the country-rock through which it breaks, it will stand aboTe the surface and look like a low, ruined wall Fig. 124.— Dikes. (Fig. 124, a). If, on the contrary, the igneous rock yield more easily to erosion than the country-rock, then it may be traced as a shallow, half-filled ditch(Pig. 134, b). Eflfect of Dikes on Stratified Kocks. — On both sides of a dike the bounding walls of stratified rock are always changed by the intense heat of the fused matter. Sandstones are changed into a rock resembling gneiss (page 225), clays are baked into porcelain jaspers, lime- stones are changed into crystalline marbles, coal-seams into anthracite and sometimes into coke. In all cases the fossils, if any, are more or less completely destroyed. These metamorphic changes usually extend only a few feet or yards from the place of contact. 2. Overflows. — This is the next most common form of occurrence. The liquid matter has come up through great . iiiiiiiilill uuuiiiiiii nil rniiriiiiiiiiiiiii"=^^^^ ;illiiiiiiiiiiiHiiiiiiiii=illllllllllllJ/iil>l",V.Vi Fio. 125^-LaTa sheets. fissures, such as are made by crust-movements, and spread on the surface as extensive sheets. Often sheet after sheet 218 STRUCTURAL QEOLOQY. is outpoured, one on another, until masses 3,000 to 3,000 feet thick are piled up (Fig. 125). The extent and thickness of some of these lava-floods are almost incredible. The great lava-flood of the North- west covers the whole of northern California, north- western Nevada, and a great part of Oregon, Washington, and Idaho, and extends far into Montana and British Columbia. Its area is supposed to be 150,000 square miles, and its thickness, where cut through by the Co- lumbia Eiver, is at least 3,000 feet. There are about a dozen extinct volcanoes dotted, at wide intervals, over this vast area. It seems certain that the lava came up through fissures in the Cascade and Blue Mountains, and spread as sheets which covered the whole intervening space. Afterward eruptive activity continued, in a more feeble form as volcanoes, almost to the present time. The great Deccan lava-field, described by the geologists of India, covers an area of 200,000 square miles, and is in places 6,000 feet thick, and there is no evidence of any crater-eruptions at all. These very extensive sheets are usually basalt. In some parts of the Utah and Nevada Basin region, how- ever, rhyolitic and trachytic lavas are found '5',000 feet thick, but these are far less extensive. As a general rule, the basic lavas, like basalt, were very liquid (superf used), and spread out in thin sheets, while the acidic lavas, like . trachyte, have been stiffly viscous (semi-fused), and were squeezed out dome-shaped. i^TiiiiiiTijimiill Tmniiiimimut \BJSairn^_ ^^ jnSaiiLUJJf^^^^tr: I'lG. 120,— Interqalury l)c4a, UNSTRATIFIED OR I&NEOVS ROCKS. 219 3. Intercalary Beds. — Often sheets are found be- tween the strata, sometimes repeated many times. In such cases they may have been poured out on the bed of the sea or lake, and covered with sediment ; or they may have broken through the strata for a certain distance, and then spread between the separated strata (Fig. 126). Both of these cases occur. If the strata both above and below the sheet are changed by heat, then it has been forced between ; but if only the underlying stratum is changed, then it has been outpoured on the bed of the sea or lake, and covered with sediment. Age of Eruptives. — Where two dikes or streams meet, their relative ages may be known. In case of suc- cessive streams, that which covers is of course the later. If one dike intersects another (Fig. 137), the intersecting dike, a,- is the younger. The absolute age, i. e., the geo- FiG. 127. logical period when the eruption took place, can be de- termined only by the age of the associated stratified rocks. If igneous rocks break through, or are outpoured upon. Fio. 128. or forced between layers of stratified rocks, then the igneous rock must be younger ; but if intercalary beds 330 STRUCTURAL GEOLOGY. are the result of outpouring on the bed of the sea, and covering it with sediment, then the igneous and the stratified rocks are contemporaneous. Finally, if dikes outcropping on the surface are coTcred with other strata through which they do not break (Fig. 138), then they are younger than the lower series, a, and older than the upper, i. Some Structures common to Many Eruptives. Columnar Structure. — Many eruptive rocks, espe- cially of the more basic kinds, seem to be wholly made Fia. 129.— Columnar basalt, New Soatt Wales (Dana). up of regular prismatic columns (Pig. 139). This re- markable structure is most common and perfect in basalt, and is therefore often called basaltic structure. The col- umns vary in size from a few inches to several feet in diameter, and in length from a few feet to one hundred feet ; the number of sides from three to seven, more com- Fia. 130.— Basaltic colomns (after Qeikle). UNSTRATIFIED OR IGNEOUS BOCKS. 321 monly five or six. The columns are not usually continu- ous, but short-jointed, like a vertebral column (Pig. 130). Th.Q position of the columns is usually perpendicular to the cooling surface. Thus, in vertical sheets, like dikes, they are horizontal, and an outcropping dike often pre- sents the appearance of a pile of corded wood (Fig. 131). Fig. 131.— Columnar dike, Lakfi Superior. (After Owen.) In overflow-sheets the columns are vertical (Pig. 129), and at the base of a cliif of such rocks are found piles of separated and disjointed columns. The cause of this structure is shrinkage by cooling. Many substances shrink by drying, and break into pris- matic columns. Mud thus forms polygonal prisms by sun-cracks. Wet starch, poured into boxes and drying, breaks into prismatic pencils. In the case of lava, the shrinkage is by cooling, instead of drying, and the prisms are far more regular. Examples of this structure are found in every country, and give rise to many remarkable scenes. In Europe, the 222 STRUCTURAL GEOLOGY. Giant's Causeway in Ireland, and Pingal's Cave' on the Island of StafEa, are good examples. The Giant's Cause- way is a sea-clifl of Golumnar basalt, consisting of many layers, with softer material between, and the whole rest- ing on stratified rock. By the action of the sea and air the separated and disjointed columns are undermined and fall to the base of the cliS. In this country, the Pali- sades of the Hudson River, and Mounts Tom and Hol- yoke in the Connecticut River Valley, are good examples. Fine examples are found also in the trap of Lake Supe- rior (Fig. 132). But the finest in this country are the Fig. 132.— Basaltic colanmB on sedimentary rock, Lake Superior. (After Owen.) basaltic clifEs of Columbia and Des Chutes Rivers in Ore- gon. On the Des Chutes River at least thirty lava-layers may be counted, one above another, each entirely com- posed of vertical columns. Volcanic Conglomerate and Breccia. — If a lava- stream runs down a stream-bed or a shingly beach, it gathers up the pebbles and forms with them a conglome- rate differing from aqueous conglomerate in the fact that the uniting paste is igneous instead of sedimentary. So, also, a lava-stream may gather up rubble and form a volcanic breccia differing in the same way from sedimen- tary breccia. UN8TRATIFIED Oli IGNEOUS ROCKS. 2)23 Amygdaloid. — The upper part of a lava-stream is ve- sicular, or full of air-bubbles. If such a stream be cov- ered by another stream, percolating waters, charged with silica and carbonate of lime gathered from the lava, will fill up the empty spaces with these materials. If the rock be broken or weathered, these amygdules fall out. They look somewhat like pebbles, and the rock (Fig. 133) might be mistaken for conglomerate, but is formed in an entirely Fig. 133.— Amygdaloid. different way. The filling of the cavities takes place slowly, layer within layer, and the layers are often of dif - ferent colors. It is in this way that are formed the most exquisite agate and carnelian nodules. Tufas. — When volcanic materials disintegrate, and are then moved and deposited in water, they form tufas. Sometimes the fragments may be larger and the mass may simulate volcanic breccia. It is, however, an aqueous breccia of volcanic rock. Such are sometimes called vol- canic agglomerates. CHAPTER IV. MBTAMOEPHIC BOOKS. "We have now finished both the stratified and the un- stratified rocks, but there is yet an intermediate series which must be described. These are stratified like the stratified rocks, but crystalline in texture, and usually destitute of fossils, like the igneous rocks. They are supposed to have been formed from sediments like strati- fied rocks, but have been subsequently changed by heat and other agencies. They are therefore called meia- morphic rocks. They may be traced by gradations, on the one hand, into stratified, and, on the other, into igneous rocks. Extent and Thickness. — They cover large areas, es- pecially among the oldest rocks and along axes of great mountain-chains. The whole of Labrador, the larger por- tion of Canada, the whole eastern slope of the Appala- chian, and also the axes of the Colorado and Sierra, con- sist of them. In Canada they are supposed to be 40,000 to 50,000 feet thick and very much crumpled. Meta- morphism is nearly always associated with great thickness and crumpling. Age. — The oldest rocks are all metamorphic. Hence many regard it as a sign of age. But it is probably more correct to say that metamorphism is found in rocks of all ages if only they be very thick and very much crumpled ; but, since great thickness -and complex crumplings are most common in the oldest rocks, so also is metamorphism. 224 METAMORPHIC ROCKS. 325 Kinds. — The adjoming table shows the principal kinds: Gneiss is a rock having much the appearance and mineral composition of granite — i. e., quartz, feldspar, mica, and hornblende — differing only in a bedded structure. In many places, as, for example, on Manhattan Island, gneiss can be traced by in- sensible gradations into granite. Schists, are rocks having a fissile structure through the abundant Gneiss. Mica-schist. Chlorite-schist. Talcose-schist. Hornblende-schist. Clay-slate. Quartzite. Marble. Serpentine. presence of scales of some kind. In mica-schist they are mica ; in the other schists they are chlorite, or talc, or hornblende. Quartzite and Marble are both white, crystalline, or granular rocks, looking like loaf-sugar ; but in the one case the granules are quartz, in the other, lime-carbonate. Serpentine is a greenish rock, having usually a schistose structure and a greasy feel like talc. It contains a nota- ble quantity of magnesia. Orignin of tliese Kinds. — Metamorphie rocks are prob- ably changed sandstones, limestones, and clays, and mixtures of these. The infinite variety which we find is the result partly of the original kind and partly of the degree of change. For example, sandstones and lime- stones are often perfectly pure. Now, a metamorphie pure sandstone is quartzite, and a metamorphie pure lime- stone is marble. But clays are nearly always impure, being mixed with sand and lime and iron and other bases. A moderately pure clay with a little sand by metamor- phosis makes gneiss or mica-schist. If it contains much iron, it makes a hornblende-schist ; if magnesia, talcose- schist or serpentine. Serpentine is, however, often a changed eruptive rock. Le Conte, Geol. 15 226 STBUGTURAL OMOLOGY. Cause of Metamorphism, There are two kinds of metamorphism which must ba distinguished, viz., local or contact metamorphism, and re- gional metamorphism. The former is produced by direct contact with fused matter, as in dikes or intercalary beds (page 317). There can be no doubt as to the cause in this case. It is intense heat. But the eilect of the heat extends but a little way from the plane of contact. In regional metamorphism, on the contrary, the change is uniyersal over hundreds of thousands of square miles and thousands of feet of thickness. In these cases there is no evidence of intense heat in every part ; the heat was prob- ably very moderate. It is of this kind that we now wish to explain the cause. The Agents of regional metamorphism are — 1. Heat ; 2. Water ; 3. Alkali ; 4. Pressure ; 5. Crushing. To produce metamorphism by heat alone, i. e., dry heat, would require a temperature of 2,500° to 3,000°, but in the presence of water a very moderate heat will change rocks. At 400° Fahr. (= 205° C), incipient change commences ; and at 800° Fahr., complete hydrothermal fusion takes place. If any alkaline carbonate be present in the water, these effects occur at still lower temperature. The quan- tity of water necessary is only ten to fifteen per cent. ; in other words, the included water of sediments is amply sufficient. Pressure is necessary, because it is impossible to have even such moderate heat in the presence of water, unless the whole be under pressure. Application — Suppose, then, we have sediments ac- cumulating along g, shore-line, or at the mouth of a river until a thickness of 10,000, 20,000, or 40,000 feet is reached. It is evident that the isogeotherms (interior isotherms) would rise, and the lower portion of the sedi- ments with their included waters would be invaded by the interior heat of the earth (Fig. 134). At the rate of METAMOBPHIC ROCKS. 1i%t 100° increase per mile (page 133), the lower portion of the sediments 30,000 feet thick would be 400° + 60° (mean surface temperature) = 460°, and 40,000 feet of the sediments would be at the bottom 860°. ' Now, we actually Pia. 134.— *J, original sea-bottom ; s'b', sea-bottom after sediments, sd, have accu- mnlated ; , . . . , isogeotberms of 800° and 400° ; — • — • — , same after accamola' Hon of sediments. have strata 20,000 and 40,000 and even more feet thick. The lower portions of such strata must be completely metamorphic. The figure (Fig. 134) shows how the pro- cess takes place. Crushing. — Pressure alone is a condition, but not a cause of heat. But pressure producing motion, or crush- ing, crumbling, is an active cause of heat. Kow, we usually find metamorphism associated with most complex crumpling of strata. The heat must have been increased also by this cause. Even igneous rocks, by pressure, mashing, and shear- ing, may be made to assume the appearance of metamor- phic stratified. Many schists, especially gneisses, are formed in this way. CHAPTER V. STEUCTTJEES COMMON TO ALL EOCKS. "We have now given a brief account of all the difEerent kinds of rocks. But there are still some structures which are found in all kinds of rocks, and which could not be described until these kinds had been defined. These are: 1. Joints; 2. Great fissures; and, 3. Mineral veins. Mountain-chains, as involving all kinds of rocks and all kinds of structure — in fact, as summing up all the prin- ciples of dynamical and structural geology — we must take last of all. Sectiom' I. — Joints akd IPissuees. Joints. We have already alluded to joints in stratified rocks (page 179), but without describing them, because not characteristic of these rocks. All rocks — sedimentary, igneous, and metamorphic — are divided by cracks in dif- ferent directions into separable blocks of various sizes and shapes. These cracks are called joints. In stratified rocks, one of the division-planes is between the strata, and the other two nearly at right angles to this. The shape and size of the blocks differ in different kinds of rocks. For example, in sandstone the blocks are usually very large and roughly prismatic ; in limestone, they are usually very regularly cubic (Fig. 135) ; in shale, oblong rhom- boidal ; in slate, small and sharply rhombic ; in granite, sometimes largo and roughly cubic, sometimes scaling in 228 STRUCTURES COMMON TO ALL ROCKS. 229 concentric shells, producing domes ; in eruptives, of many shapes, rough cubic, ball-like, regular columnar, tilelike. Fig. 135.— Regular jointing of limestone. For this reason a cliff, especially of stratified rock, looks like a wall of titanic masonry without mortar. Cause — These cracks are supposed to have been formed by the shrinkage of the rocks ; in stratified rocks, in con- solidating from sediments ; in igneous and metamorphic rocks, in cooling from a state of fusion or semi-fusion. In stratified rocks they are usually confined to the stra- tum, though some larger joints (master-joints) run through several strata. They are mentioned mainly that the student should not confound them mth other kinds of stmcture. Great Fissures. Joints are probably shrinkage-cracks. Fissures are fractures by crust-movements. Joints are cracks of the individual strata ; fissures are fractures of the earth's crust, extending through many formations, and continu- ing for many miles. Cause. — We shall see hereafter that the earth's crust is subjected to a powerful horizontal pressure, by which 230 STRUCTURAL GEOLO&Y. it is Bometimes mashed togetlier, sometimes thro-ym into arches and hollows. Such bendings of the crust produce enormous fractures parallel to the axis of the bending, and parallel to mountain-ranges, since mountain-ranges are produced in this way. Sometimes there is a system at right angles to the main system, or in the direction of the cross-valleys of mountains. The characteristics; therefore, of great fissures are — 1. Their occurrence in systems, asually parallel to the axis of elevation. 2. Their length, often extending for hundreds of miles. 3. Their depth, sometimes breaking through miles of thickness of rock. When filled at the moment of formation with fused matter from below, they Pig. 136.— Fault in Southwest Virginia : o, Silurian : d, carboniferous. (After Lesley.) form dihes ; and all great dikes and igneous overflows have been through such fissures. But if not filled at once with fused matter, but slowly afterward with mineral matter, they form the gvesA, fissure-veins. Whether they are filled at once with fused matter, or afterward slowly with mineral matter, or remain empty, the walls do not usually remain in their original position, but nearly always slip one on the other up or down. Such a displacement of the crust on the two sides of a fissure is called a fault. We have already treated of dikes ; we shall hereafter take up mineral veins. We must now speak briefly of faults. Faults. As already explained, these are displacements of fissure- walls. They take place on an immense scale. Lesley STRUCTURES COMMON TO ALL ROCKS. 231 mentions a fissure in Pennsyl- vania in -which the vertical dis- placement is 20,000 feet, and may he traced for twenty miles. Kogers describes one in southern Virginia in which the displace- ment is 8,000 feet, and may be traced for eighty miles (Fig. 136). • According to Powell, there is on the north side of the Uintah Mountains a vertical slip of 20,000 feet. All along the east- ern side of the Sierra there is a slip of not less than 15,000 to 20,000 feet; and King thinks the slip on the west side of the Wahsatch is even 40,000 feet. But they are developed on per- haps the grandest scale in the Colorado plateau region. This high plateau is traversed by a system of north and south fis- sures, 100 to 200 miles long, by which the arched' earth-crust is broken into huge blocks, and these have settled to different levels, some 5,000 to 12,000 feet below others, and thus give rise to a wonderful system of north and south cliffs (Fig. 137). If such a slip takes place sud- denly, then at first there must have been a cliff as great as the slip. The same would be true even with gradual slipping, if \]\l ."^ i\ 332 STRUCTURAL OEOLOGY. there were no erosion. But both the slipping and the erosion have probably been going on slowly all the time, and whether there be a cliff or not;, depends on the age of the fracture and the relative rate of slipping and ero- sion. In many of the faults of the plateau and basin region, the clifE still exists (though not as great as the dis- placement), because of the comparative recency of the fractures and dryness of the climate. The great Sierra fault is marked by a steep slope of 8,000 to 10,000 feet to the east, that of the Wahsatch of 8,000 feet to the west. But in the Uintah fault, and in all the faults of the Appalachian region, there is actually no surface-sign of the fault (Fig. 136). "We may stand astride of the fissure. Two Kinds of Faults. — Faults are of two kinds, ac- cording to the direction of the slip. Fissures are nearly always inclined, and therefore have what miners call a foot (lower) wall and a hanging (upper) wall. More com- monly the hanging wall drops down. These are normal faults. But sometimes the hanging wall is pushed up over the foot wall. These are called reverse faults. In normal faults the broken parts are readjusted by gravity (settling) ; in reverse faults the broken parts are crushed together and forced to slide. The upper wall over the Fia. 138.— Section across Yarrow Colliery, showing the law of faults. (After De la Beche.) lower (Fig. 136) is a reverse fault. Fig. 136 shows only normal faults. Law of Slip. — In cases of displacement of strata it becomes often a matter of great importance, not only to STRUCTURES COMMON TO ALL BOCKS. 333 the field geologist but also to the practical miner, to know which side has gone up or down ; for valuable beds of coal or veins of metal are thus displaced, and it is impor- tant to know which way they went. Now, normal faults are far the more common, and therefore a y&tj general though not universal rule in such cases is this : In case of inclined fissures, the foot-wall or lower side has gone up, or the hanging wall or upper side has dropped down. Or, it may be otherwise expressed, thus : " The dip or hade (slope) of the fissure is toward the down-throw." In Fig. 138, which represents an actual section, the rule is followed in every fissure. The exceptions to this rule (Fig. 136) are found only in the strongly folded rocks of mountain-regions. Section IT.— Mineral Veins. Let any one examine rocks, especially metamorphio rocks, in mountain-regions, and he will see that they are marked with seams and scars running in all directions, as if they had been crushed and broken and again mended ; as indeed they were. Now, all such markings and seam- ings, whatever be their nature and origin, are often called by the general name of veins. Thus, beds of coal, or gypsum, or salt, on the one hand, and the fillings of fissures by fused matter, on the other, are sometimes called veins. It is evident that no scientific progress can be made so long as things so different are confounded under the same name. Definition. — Putting aside, then, all beds formed aa sediments at the bottom of water, such as coal, gypsum, etc., and all fillings of fissures by fused matter, such as dikes, etc., veins may be defined as (usually) the fillings of fissures or cracks by slow deposits from solution in per- colating waters, of materials leached from the surround- ing or underlying rocks. Since the deposit takes place 234 STBUGTUBAL GEOLOGY. from solution, the materials of veins are in a purer and more sparry condition than they exist in the rocks. It is evident that, as thus defined, veins must vary greatly in appearance. Sometimes they are fine lines, the fillings of small cracks produced by rock-crushing. Some- times they are the fillings of larger joints. Sometimes they are the fillings of great fissures breaking through the earth-crust. It is these last which are far the most im- portant ; and it is only on these, therefore, that we shall dwell. Fissure-Veins. — As these are the fillings of those great fissures which are formed by crust-movements, they are of great extent. The fillings of such fissures at once with fused matters are called dikes (page 216) ; the fillings by slow deposit of mineral matter slvq fissure-veins. These veins, therefore, like fissures (page 329), of which they are the fillings, are often many miles in extent, many feet in width, and of unknown but certainly many thousand feet in depth. Like fissures, they occur in sys- tems, parallel to each other and to the axis of elevation of the mountain where they occur. Between the vein and the wall-rock on either side there commonly exists a layer of clay called the selvage. It is very characteristic of true fissure-veins, and probably produced by the solvent effect on the wall-rock of water circulating between the vein and the wall. Metalliferous "Veins. — Metals may occur in beds, for example, iron (page 299), or filling cavities of any kind in rocks, as sometimes lead. But they most commonly occur in veins, especially fissure-veins. The further de- scription of fissure-veins is best undertaken, therefore, under this head. Contents. — We must not imagine that metalliferous veins are filled with metals. The fillings of fissures are of two kinds, viz., the vein-stuff, vein rock, gangue, or matrix (as it is variously called), and the metallic ore. By far STlWCTVIiES COMMON TO ALL liOCKiS. 235 VBIN-STUPF. OBX. + + SiO, + CaCOs F'COi, BaCO, BaS04 CaPl + + MS + MCO, MO M the larger portion is usually vein-stuff ; and through this is disseminated the metallic ore in granules, strings, or larger masses (Pig. 140, c), or sometimes in a central sheet (Figs. 139, 141), as if de- posited last of all. The principal hinds of yein-stuffs are silica, carbonates of lime, iron and baryta, sulphate of baryta, and fluoride of cal- cium (fluor-spar). Often, however, many kinds of minerals are aggregated into a veritable vein-rock. The most common of all is silica, in the form of quartz. Next comes lime-carbonate. The metals sometimes occur free (M), as, for example, always gold and platinum, often silver, and sometimes copper and mercury. But more commonly they occur as metallic sulphides (MS), carbonates (MCO3), and oxides (MO). By far the most common form is sulphides. These facts are given in the schedule. The most abundant kinds are marked with a + . Structure. — ^Veins have nearly always a more or less banded structure, as if the materials were deposited in successive layers, on the two sides alike. Sometimes the iah a b a i d -^ =1 W- Fig. 139.-0, central sheet of ore ; 6J, agate ; d, wall-rock. Fio. 140.— aa, agate ; ft, quartz ; c, copper-bearing lode ; d, wall-rock. successive layers are of the same material, but of different colors (Figs. 139, U, 140, aa); sometimes of different ma- 336 STRUCTURAL GEOLOGY. be haic b ar- terial (Fig. 141). Sometimes the bands are beautifully regular and distinct, like agate (Figs. 139, 140) ; some- times on a larger scale, and irregular. Very often we find several corresponding layers of agate on the two sides, and the center filled with combs of quartz- crystals with interlocking teeth (Fig. 140, I). Irreg-ularities. — Small veins, the fillings of small cracks, are extremely irregular, running in all directions, and intersecting each other in every conceivable way. Great fissure-veins are far more regular. But even these are more or less irregular, partly from the irregularity of the original fis- sure and partly from subsequent movements. Perhaps the most important of these is displacements by fissures or other veins, as explained below. Age of Veins. — Often in the same locality we find two or more systems of-veins, formed at different times, crossing each other. In such cases, as in dikes, the fissure or vein which cuts through the other (Fig. 142, a) is of Fio. 141.-0, galena ; 66, bary ta ; cc, fluor-spar ; d, wall. Pig. 142. course the younger. The absolute age, i. e., the geological period in which the fissure was made, can be known only by the age of the strata through which it breaks. Kecovery of Lost Veins. — Suppose I (Fig. 142) is a valuable vein, and we work down until we strike a. The STRUCTURES COMMON TO ALL ROCKS. 237 rein is here lost by slips ; which way shall we go to recover it ? Eemember the rule already given on page 333 : " The slope or dip or ' hade ' of the displachig fissure (here a) is toward the down-throw." This rule is not invariable, but very general. Surface-Clianges. — We must not imagine that metal- liferous veins outcrop on the surface in the form we have described. If they contain no metal, veins may indeed appear unchanged on the surface. Quartz- veins may, for example, be often traced over hillsides by strewed frag- ments of white quartz. But metalliferous veins are usually so greatly changed on the surface that, without much ex- perience, we would not recognize them at all. Precisely as rocks are usually concealed by soil resulting from sur- face-decomposition, so veins are concealed by surface- changes. To the experienced eye these surface-changes become sartskce-signs, and are therefore of the greatest practical importance. These surface-signs are far too complex and various to be explained here. We only mention them to guard the pupil against supposing that it is easy to see what we have described above, and to stimulate him to observe for himself. Origin of Mineral Veins. — This is a difficult and obscure subject, but the following propositions are prob- ably true : 1. Veins have been formed by deposit of min- eral matters from solutions in percolating or subterranean waters. 2. The movement of the subterranean waters may have been in any direction, but mostly up-coming. 3. The waters may have been at any temperature, but mostly hot. 4. The water-ways may have been of any kind, but the openest water-ways — the highways of ascending waters — are topen fissures. 5. The waters have been usually, though perhaps not always, alkaline, i. e., containing alkaline earionaies or alkaline sulphides, or both. 238 STBVCTURAL QEOLOQY. SeCTIOK III. — MotTNTAIlTS : THEIB OeIGIN iND Stkuctuee. Mountains are the glory of the earth — the culminating points of scenic grandeur and beauty. But few perceiTc that they are so only biecause they are also the culmi- nating points of all geological agencies. This is but one illustration of the general truth, that there is an in- dissoluble and necessary connection between truth and beauty, between science and iine art. It is evident, then, that the study of mountains is the key to dynamical and structural geology. The diflQculty which meets us at the threshold of this subject is the loose use of the term mountain. The term is used to express every conspicuous elevation above the general level, whatever be its extent, and in whatever way it may have been formed. Thus an isolated eminence produced by circum-erosion, or a peak formed by volcanic ejection, a ridge between two stream-gorges, a great bulge produced by the folding of the earth's crust, or a series of such foldings parallel to each other in the same general region — are all called by the same name, mountain. Qualifying terms are indeed often used, such as moun- twn.-peak, mountain-rirf'p^e, mountain-raw^e, etc., but these also are used loosely and interchangeably. It is necessary, therefore, first of all, to define our terms. Definitions — A mountain-c/mni, or, better, moun- tain-system, is an assemblage of ranges parallel to each other in the same general region, but usually formed at different times {polygenetic). All the great moimtain- chains of the world are of this nature. For example : The Appalachian system consists of the Blue Eidge, the Alleghany, and the Cumberland ranges. The Eocky Mountain system, or North American Cordilleras, consists of the Colorado range, the Park range, the "Wahsatch^ STRUCTURES C03IM0N TO ALL ROCKS. 239 the Sierra, the Coast ranges, and many others. So the Alps, the Himalayas, and the Andes consist also of sev- eral parallel ranges. A mountain-range is one of these great components, formed at one time — by one earth-effort {monogenetic), though the effort may have continued through a great period of time. The Colorado, the Uintah, the "Wah- satch, the Sierra, and the Coast ranges are good examples. The Blue Eidge and the Alleghany ranges are also good examples. A mountain-ridge is a subdivision, again, of a range, produced usually by erosion, although sometimes also by foldings of strata. Mountain-peaks are serrations of the crest of a range or a ridge, either by erosion or by volcanic ejections. Mountain-systems are separated by great interior conti- nental basins ; mountain-ranges by great valleys j moun- tain ridges and peaks by narrow valleys or gorges. Such is the simplest view of the form of mountains ; but sometimes a mountain-range seems to be composed of an inextricable tangle of ridges running in all directions. Now, it is evident that any scientific discussion of the origin and structure of mountains must be essentially that of the origin and structure of ranges; for, on the one hand, a mountain-system is a mere adding of range to range, and, on the other, ridges and peaks are the result of subsequent sculpturing by rain and rivers. It is of ranges, therefore, that we shall mainly speak. The surface of the earth has now become cool and its mean temperature fixed, and is, therefore, no longer con- tracting J but the interior is certainly still extremely hot, and still cooling and contracting. The effect of such interior contraction is to thrust the exterior crust upon itself horizontally with irresistible force, crushing it to- gether with many complex foldings of strata, and caus- ing it to bulge up in long wrinkles. Such lines of bulging 240 STRUCTURAL GEOLOGY. or wrinkles are mountain-ranges. So much it was neces- sary to say to render what follows intelligible ; but the origin of mountains is best taken up in connection with their structure. Structure and Origin of Mountains. Mountain-ranges are always made up of series of strata of immense thickness thrown into folds, as if they had been crushed together horizontally, and swelled up verti- Fio. 143. cally. To illustrate : Suppose we had a number of layers of wax, or clay, or other plastic substance of different colors laid one atop another, as in Fig. 143, A ; suppose, further, that the middle portions were softened a Tery little by gentle heat below, and the whole was then crushed together horizontally, as represented by the arrows. The middle softer portions would yield and be Fig. 144.— Section across the Uintah. mashed together, thrown into folds and swelled up, as shown in Fig. 143, B. Now, this is exactly the way in STRUCTURES COMMON TO ALL ROCKS. Ul yhich mountain-ranges seem to have been formed. Some- times, though rarely, there is but one great fold (Pig. 144) ; sometimes there are several open folds (Pig. 145) ; FiQ. 146. — Section across the Jora. more commonly, especially in great mountains, there are many closely appressed folds (Pigs. 146 and 147). In the Coast Range (Pig. 146) there are at least five alternate anticlines and synclines ; in the Alps there are, in some places, seven alternate anticlines and synclines. It la Fia. 146. — Section of Coast Range, showing plication by horizontal pressure. evident that in these cases a great breadth of sediments is squeezed horizontally into a small space, and corre- spondingly swelled upward into a range. In the case of the Coast Eange (Fig. 146), every two or two and a half miles of original breadth has been compressed into one Fig. 147. — Appalachian chain. mile. In the case of the Alps, probably every three miles of original breadth has been crushed into one mile, and, of course, correspondingly swelled up. Sometimes the mashing together is even far greater than represented in Le Conte, Geol. 16 242 STRUCTURAL GEOLOGY. these figures. Fig. 148 shows an example in the Alps, taken from Heim. There is another evidence that mountains are formed wholly by horizontal crushing, viz., the phenomenon oi slaty cleavage. We have already seen (page 195) that slaty Fig. 148.— Section across centia] Alps : j, Jurassic ; t, triassic ; s, schist. cleavage always shows a crushing together horizontally, and an extension vertically, of the whole mass. Now, cleavage is always associated with folded strata and with mountain-ranges. Mountains are often spoken of as due to "upheaval." There is no oLjection to the use of this term, if it be re- membered that the upheaval is not usually due to a force acting /ro?72 helow upward, but to a horizontal force crush- ing together and swelling upward by thickening the whole squeezed mass. We have, in Fig. 143, B, given the ideal structure of a mountain-range if there had been no erosion. But, of course, as soon as a mountain begins to rise, rain-water begins to cut it away, and in all mountains the amount cut away is immense, in many far greater than what is left. This fact is represented in the preceding figures (144^148). In all these figures, however, except the last, the range is composed wholly of stratified rock ; but in most great mountain-ranges we have an axis of cry STRUCTURES COMMON TO ALL BOCKS. 243 rock, granitic or metamorphic, flanked on either side with uptilted and folded strata, as in Figs. 149, 150. It Fig. 149. — Ideal section, Bhowing granite axia. was formerly supposed that the igneous rock in fused con- dition has pushed up and broken through the strata and appeared above them. But it is far more probable that stratified rock once covered the whole, as shown by the dotted lines, and that subsequent erosion has exposed the Fig. 150. — Ideal Bcctlon of a monntain-range. granitic or metamorphic rocks along the crest where the erosion was greatest. Furthermore, when we remember that mountains are composed of immensely thick series of strata, and that very thick strata are sure to be meta- morphic in their lower parts (page 326), and, moreover, that granite is often but the last term of metamorphism of rocks, it becomes probable that even such mountains as those represented in Figs. 149, 150, are really com- posed wholly of horizontally mashed and crumpled strata, only that, on account of the great thickness and strong crumplings, these have become completely metamorphic in their lower parts. 244 STRUCTURAL GEOLOGY. Thickness of Mountain Sediments. — We have said that mountains are composed of enormously thick sedi- ments, crushed together horizontally with many crump- lings, and swelled up proportionally. We will now gire examples of such thickness. The Appalachian consists of folded strata (Fig. 14?) which, according to Hall, are not less than 40,000 feet, or nearly eight miles, in thick- ness. The Wahsatch consists of sediments which, ac- cording to King, are 50,000 feet, or nearly ten miles, in thickness. The Coast Eange of California consists of folded cretaceous and tertiary. The cretaceous alone, according to Whitney, are 30,000 feet thick. The tertiary have not been measured, but cannot be less than 10,000 feet. So that at least 30,000 feet, or nearly six miles, thickness of sediments are involved in the folded struc- ture of this range (Fig. 146). The strata of the Alps are not less than 40,000 to 50,000 feet thick. The same is true of all mountains. Now, we must not imagine that this is evidence of the average thickness of strata, but only revealed in moun- tains by erosion, for the very same strata elsewhere are much thinner. For example, the same strata, which are 40,000 feet thick in the Appalachian Range, thin out west- ward until they are only 4,000 feet thick at the Mississippi River. The very same strata, which are 30,000 feet thick, in the Wahsatch, thin out eastward, and are only 1,000 feet thick on the Plains. Thus, then, mountain-ranges, before they were upheaved, were lines of exceptionally thick sediments. This may be regarded as certain. Mountain-Kangpes are Upheaved Marginal Sea- Bottoms. — Where, then, do we find exceptionally thick sediments ? Where, but along marginal sea-bottoms ? We have seen (page 48) that here are accumulated nearly the whole debris of continental erosion. Therefore, mountain-ranges before they were upjieaved, were mar- ginal sea-bottoms on which have accumulated enormously STRUCTURES C03IM0N TO ALL ROCKS. 245 thick sediments. Every one of our great mountain-ranges caii be shown by geological evidence, which we cannot give here, to have occupied this position until the time of their birth.* Different Stages of Mountain-Life. — We have said " until lirth," but it must not be supposed that there was anything sudden about it. The emergence above water we call its iirth, but a mountain continues to grow stead- ily through many ages. Meanwhile, as soon as it is born, erosion commences, and continues with increasing rate as the range grows higher. When the mountain stops grow- ing, erosion begins to destroy it, and finally levels it com- pletely. Thus, in every mountain there is a period of birth, a period of growth, a period of maturity, a period of decay, and a time of death or obliteration. Many of the earliest mountains have been entirely swept away. We know their places only by their folded structure — fossil bones of extinct mountains. Why Yielding occurs along Lines of Thick Sedi- ments. — Perhaps the pupil has already asked himself, " Why does yielding occur only along lines of thick sedi- ments ?" The probable reason is, that great accumu- lations cause the rise of the interior heat of the earth toward the surface, as already explained on page 227. This h«at, in the presence of the water included in the sediments, causes these, as also the earth-crust beneath, to soften or even semi-fuse ; and thus creates a- line of weakness, and therefore of yielding. This is represented in the experiment with the wax, on page 240, by the gen- tle softening of the middle part. Cause of the Lateral Pressure. — If it be further asked, "What is the cause of the lateral pressure ?" we can only say that this is an obscure point, and one much discussed. It is probable, however, that it is due, as already stated (page 239), to the interior contraction of ■ * For evidence, see " Elements of Geology," p. 265, 246 STRUCTURAL OEOLOGY. the earth, by which the crust, following down the shrink- ing nucleus, is thrust upon itself laterally with irresist- ible force. Mountain-ranges are the lines of yielding. Other Associated Plienomeua.-^If we clearly appre- hend the foregoing account of the structure and origin of mountains, other associated phenomena are easily un- derstood : 1. The strong bendings of the strata neces- sarily produce fissures, mainly parallel to the bendings — i.e., to the axis of the range, and to one another. 2. Since these fissures break through many miles of strata, it is natural that igneous matter should come up through them to the surface, and therefore that Tolcanic and especially great J^sswre eruptions should be associated with mountain-ranges, and that where the OTerflows are cut away by erosion we should find dihes. Again, 3. As the mashing goes on steadily, the fissures first formed would be certain to slip, and thus we find great faults -often as- sociated with mountains. Again, 4. The formation of a fissure or the subsequent slipping of a fissure could not fail to produce an earth-jar ; and thus earthquakes are commonest in mountain-regions. Finally, 5. Fissures which did not fill at the moment of formation by igneous injection would certainly fill slowly afterward by perco- lating water depositing minerals, and thus, also, mineral veins are commonest in mountain-regions. Thus we see now the truth of the proposition with which we set out, viz., that mountains are the culminat- ing points — ^the theaters of greatest activity — of all geo- logical agencies ; of aqueous sedimentary agencies in preparation for the mountain ; of igneous agencies in the birth and growth, and of aqueous erosive agencies in sculpturing and final destruction of the mountain. Mountain- Sculpture. In the life-history of a mountain-range, the work of water in sculpturing is no less important than the work STRUCTURES COMMON TO ALL ROCKS. 347 of interior heat information. If the mountain is rough- hewed by the latter, it is shaped and chiseled by the former. The great swell of the crust, which is only seen from a distance, is due to igneous agency ; but all the scenery, which so charms us when we are among moun- tains, is due wholly to .erosion. Moreoyer, there is a peculiar charm in the study of the latter, because it is more easily understood. The cause of mountain-origin is obscure, and the folded structure of mountains is hidden, and can only be unraveled by the skillful geol- ogist ; but the forms of mountain-sculpture may be studied by all, and their study gives great additional charm to mountain-travel. Eesulting Forms. — The forms produced by erosion are infinitely various, depending upon the kind of rock and upon the amount and style o- folding. They are, therefore, of great interest also as revealing interior struc- ture. We can only touch very lightly on a few of the most common and characteristic forms. 1. Horizontal Strata. — These, when sufficiently hard, give rise to talle forms, the top of the table being deter- mined by a hard stratum of some kind, as sandstone, or by a lava-flotv. In the latter case, however,, we have this form, whatever be the position of the underlying strata (see Fig. 6, page 34). Good examples of this form are FiQ. 151. — Table-mountains. seen in Illinois and in Tennessee (Fig. 151), and espe- cially in the mesas of the Plateau region (Fig. 7, page 25). If, on the contrary, the horizonal strata are soft, and yield easily to erosion, they are worn into the most fan- tastic forms — conical, castellated, pinnacled— such as are found in the "Bad Lands" of the AVest, which are pro- 348 STRUCTURAL GEOLOGY. duced by erosion of the dried-up lake-deposits of this region (Fig. 152). 2. Gently Undulating Strata. — These, also, by ero- sion give rise to table-topped mountains ; but, if carefully Fig. 153.— Mauvaises Terres, Bad Lands. (Alter Hayden.) examined, the ridges are seen to be synclinal, and the val- leys anfdclinal. Pine examples of this form are found on the western slopes of the Appalachian chain, where the folds of the strata are dying away in gentle undulations (Fig. 149). The reason of this form is that the hollows Pig. 153.— Section of coal-field of Pennsylvania. (After Lesley.) become hardened by compression, while the original saddles are loosened or even broken by tension, and ero- . sion therefore takes effect mainly on these latter. STRUCTURES COMMON TO ALL ROCKS. 249 3. Highly Inclined Outcropping- Strata. — These give rise to sharp ridges, determined each by the outcrop of a hard stratum, with intervening valleys determined by the outcrop of softer strata (Fig.- 154). This structure is Pig. 154.— Parallel ridges. «& finely displayed on the flanks of Western mountains and the mountains of Tennessee, and especially in the moun- tains of Virginia. Standing on the top of Warm Springs Eidge, t-welve or more mountain--waves may be counted, each crest determined by the outcrop of a hard sandstone. 4. Very G-ently Inclined Outcropping- Strata. — These, in the Plateau region, give rise to a remarkable series of nearly level tables, terminated by cliffs, a hard stratum forming the surface of the table. In Pig. 156, taken from Po-well, the successive tables are fifteen to t-wenty miles wide, and the cliffs 1,500 to 3,000 feet high. The manner in which these are formed is illustrated in the diagram. Fig. 155, in which a, 5, c, d are hard strata. The dotted space shows the general erosion. 5. Highly Metamorphic and Granitic Kocks. — These reveal internal structure much less perfectly than Fig. 155. — Dotted lines sliow material carried away by erosion. unchanged stratified rocks. Usually the inequalities are very irregular, the peaks being determined by harder, and the valleys by softer, spots. In some cases, however, the peculiar forms may be easily explained. Thus, in the 250 STRUCTURAL GEOLOGY. JTia 156 —Bird 6 eye view of the Terrace Cation. (After Powell.) STItUGTURES COMMON TO ALL ROCKS. 251 high Sierra region, a remarkable dome-structure is very characteristic of the scenery. This is determined by a huge concentric structure of the granite, as in Fig. 157. Fig. 157. — Ideal eectiou showing dome-structure. Dotted line above shows original surface. This, howeTer, must not be confounded with arched strata. These great domes are still scaling ofE concentrically. 6. Outbursts of Igneous Rocks through Dikes often give rise to prominent ridges on account of their superior hardness. Examples are found in the trap ridges of the Connecticut Valley and in many other places. 7. The Nature of the Erosive Agent. — The scenic forms of mountains are also largely determined by the na- ture of the erosive agent. Simple water tends, by erosion, to form rounded summits and ridges, and narrow V-shaped gorges. Ice, on the contrary, tends to make pinnacled summits {aiguilles) and comb-like ridges, and broad, meadow-like valleys. CHAPTER VI. DENUDATION, OE GENERAL EROSION. Definition. — Denudation is a term used to designate the aggregate results of all erosive agents. Its correla- tiye is sedimentation. In the preceding pages we have given the efEects of erosion in many individual cases ; but some general idea of the amount which has taken place, under the action of all agents throughout all geological times, and some very general estimate of geological time based thereon, seem important, as a fitting preparation for Part III, or Historical Geology, which deals especially with time. Agents of Erosion. — The possible agents of erosion are — 1. |lain and rivers. 3. Snow and ice. 3. Waves and tides. 4. Oceanic currents. Of erosion by the last, we have no observation. Oceanic currents run on a bed and between banks of still water, and therefore produce no erosion (page 48). We may probably leave them out of account. Waves and tides are very powerful erosive agents, but their action is confined wholly to the shore- line. It has been estimated that, though so conspicuous, their aggregate effect is certainly less than one fifth that of rain and rivers. Snow is but a different form of rain, and glaciers a different form of rivers ; therefore, in so rough an estimate as we are about to make, we may safely base our estimate upon the action of rain and rivers. Our object, then, will be to give some very general idea of the amount of denudation which has taken place in 253 DENUDATION, OR GENERAL EROSION. 253 geological time ; then of the rate of rain and river erosion ; and then a rough estimate of the time necessary to do the work. Modes 'of determining Amount of Denudation. — There are many ways in which geologists determine the amount of denudation. In case of faults, as in Fig. 158, Rq. 158. in which the strong line, a a, represents actual surface, there must have been great erosion to obliterate all sur- face indications of the slip. Now, there are cases of slips 20,000 feet vertical, as in Pennsylvania and on the north side of Uintah, in which surface indications are entirely removed by erosion. Again, in case of isolated erosion- peaks, like Fig. 159, it is evident that the whole interven- rio. 159.— Denudation of red eandstoni., northwest coast of Koss-shire, Scotland. ing country has been carried away. Now, such peaks are often 3,000 to 3,000 feet high. But the most universal means of estimating the amount of erosion is by resto- ration of folded strata. This is shown in Fig. 160, and in many of the preceding figures on mountains. By all these methods it has been estimated by British 254 STRUCTURAL &EOLOGY. geologists that at least 11^000 feet of thickness has been re- moved from the whole mountainous and hilly portions of the British Isles, and by American geologists that 30,000 FiQ. 160. — Section acrosB middle Tennessee. The dotted lines eliow the amount of matter removed. feet have been removed from the Appalachian region. This has all taken place since the Palaeozoic, which is certainly not more than one quarter of the recorded history of the earth. But the finest examples are from the Pla- teau region. Fig. 161 represents a portion of the Uintah Fig. 161.— Uintah Mountains — upper part restored, showing fault ; lower part show- ing the present condition as produced by erosion. (After Powell.) Mountains, the lower portion as it really is, the upper as it would be if restored with its great fault. According to Powell, 25,000 feet have been removed from the whole area represented, and this, too, since the beginning of the Tertiary, which is but a small fraction of geological time. According to Powell and Button, over the whole DENUDATION, OB GENERAL EROSION. 266 Plateau region, an area of not less than 300,000 square miles, an average of 6,000 to 8,000 feet and an extreme of 13,000 feet, has been removed by erosion, and all since the Middle Tertiary. From these examples it is impos- sible to resist the conclusion that the average erosion over all land-surfaces has been at the very least several thousand feet. There is another way of making the estimate of the amount of general erosion. Evidently the correlative and measure of erosion is sedimentation. The debris of ero- sion have been accumulated as stratified rock. Now, the average thickness of strata can not be less than several • thousand feet. Taking it only as 2,000 feet (it is certainly very much more), since the area of ocean is, and probably always was, three times the area of the land, this would require at least 6,000 feet erosion of all land- surfaces. We may therefore say, with the utmost confi- dence, that over all land-surfaces more than 6,000 feet thickness has been removed by erosion. Time. — Now, we have seen (page 19) that the rate of rain and river erosion is about one foot in 5,000 years. At this rate it would take 30,000,000 years to do the work which we actually find has been done. The time was probably much greater. Exceptions may be taken to some points of our calculation, but, we are sure, not to the result. But this, be it more or less, represents only recorded history. Beyond this, again, is the infinite abyss of the unrecorded. PART III. HISTORICAL GEOLOGY. • CHAPTER I. CtBNBEAL PEINCIPLBS. Geologt is essentially a history. But there are two points of view from which history may be studied, viz. : 1. As a chronicle of thrilling events. 2. As the science of the laws of succession, and of the causes of these events. The interest in the one case is dramatic, in the other, scientific. The one addresses itself mainly to the imagi- nation, the other to the reason. It is almost unnecessary to say that geology is a history in which the second ele- ment predominates. It is a history of the evolution of the earth and of its inhabitants. Now, there are certain general laws of evolution in all departments of nature — certain general principles underlying all history. The most important of these we wish to fix in the mind of the pupil by comparing geology with human history : 1. Human history is divided and siibdivided into eras, ages, periods, epochs, etc., determined by great events. These divisions of time are recorded in separate volumes, chapters, sections, etc., according to their im.portance. So, also, the history of the earth is divided into eras, ages, periods, etc., determined by great changes in physical geography, climate, and forms of organisms ; and these 256 GENERAL PniNCIPLES. 357 divisions of time are recorded in separate rock systems, rock series, rook. formations, according to their importance. 3. These divisions of time, in human history, usually graduate, more or less insensibly, into each other. Yet, at certain points, called revolutions, the steps of change are more rapid. So, also, in geological history, the eras, ages, periods, etc., usually graduate into each other. And yet there are certainly here, also, times of reyolution, in which the steps of change are far more rapid. Thus all history, human or ge_ological, consists of periods of com- parative quiet and prosperity, during which the forces of change are gathering strength ; and periods of revolution, when these forces show themselves in conspicuous effects. 3. In human history, what is distinctively called an age, is marked by the dominance of some characteristic social force or principle. Thus, we have had an age of chivalry, and we look forward to an age of reason. So, also, in geology, what is distinctively called an age, is marked by the dominance of some particular class of animals or plants. Thus, we have an age of mollushs, an age of fishes, an age of reptiles, etc., in which these several classes are successively the dominant types. Now, since the divisions graduate into each other, it is to be expected that the characteristic of each age will commence in the preceding age. This we shall call the law of anticipation. 4. In human history each dominant characteristic, of course, arises, culminates, and declines ; but it does not, therefore, perish. It only becomes subordinated to the next coming and higher characteristic, and society thus becomes not only higher and higher, but also more and more complex in its structure. So, also, in geology we shall find that as each dominant class culminates and declines, it does not perish, but only becomes subordi- nated to the incoming and higher dominant class, and thus the whole organic kingdom becomes not only higher and higher, but also more and more complex in its struc- Lb Conte, Gbol. 17 258 HISTORICAL &EOLO&Y. ture as a whole. This is represented in the diagram (Fig. 162), in which A B \s, the course of time, and the rising and declining curved lines the successive culminations of five great dominant classes of animals. Fio. 162, — Diagram illuBtrating the rising, culmination, and decline of BucceBfiive dominant claBses, and tlie increasing complexity of tlie whole. 6. As in human history, while the whole race, or at least Christendom, advances together, and yet there are special difEerences in rate or direction of advance peculiar to each country and constituting its national civilization ; so also in geology, while the whole earth and its inhabi- tants in every part are affected with a common onward movement in evolution, yet there are special differences in rate or direction of evolution, characteristic of each great division of the earth. The most marked example of this is Australia, which is far hehind other continents in the march of evolution. 6. In a written human history, there are two ways in which we may judge of the subdivisions, viz. : 1. By the artificial divisions of the record, i. e., volumes, chapters, etc.; or, 2. By the nature of the most important con- tents. In a well-written history these will correspond with each other. So also in geology there are two modes of separating and determining the limits of the great divisions and subdivisions of earth-history, viz.: 1. By unconformity of the rock record ; or, 2. By the change in the organic contents. These usually correspond, be- cause they are produced hy the same cause. But, if there be a discordance (as there may be locally), then we follow the changes in the organic forms, rather than the unconformity of the rocks. GENERAL PRINCIPLES. 369 DiTisions of Geological History. Eras. — It is on these principles that the whole history of the earth, and the rocks in which it is recorded, has heen divided and subdivided. The primary divisions, or eras, are five in ^ I I s 1 to CO to CON/r£HS 1^^ X ^ NS AC fiOGE L—^, ^^. \~ '^ X 1 fe^ 1 1 i i \ Ctrl 1- r^ .^/s//ts. 1 1 /i/^rA;/fO'/?oz}6 :: 'mOlLUSKS - « ^ i I "H Si o ft ■sis -ft s i o ^ 5 I number, each embodied in a corresponding system of rocks : 1. Archasozoic,* in the ArchEsan or Primary or Laurentian system of rocks ; 3. Paleozoic, f in the Palaeo- * Archa-ozoie = primeval animal life, f Paleozoic = old life. 260 HISTORICAL GEOLO&Y. zoic, or sometimes called transition system of rocks; 3. Mesozoic,* in the Secondary system ; 4. Cen6zoic,f in the Tertiary and Quaternary systems ; and, 5. Psycho- zoic, J in the present system of sediments. These five are separated in the diagram (Fig. 163) hy the heavy lines, and their names are given on the right. How separated. — These primary divisions (unless ve except the last) are separated hy a universal or almost universal unconformity, indicating wide-spread changes in physical geography at these times ; and by sweeping changes in organic forms, involving not only species, but genera, families, and orders. The changes between the last two were not so great either in the rocks or in the organisms, but the introduction of man, and the sweep- ing changes going on now by his agency, are deemed of sufficient importance to make this a primary division. Ages. — The whole history of the earth is divided, on a different principle, into seven ages, characterized each by a dominant class. In some cases these correspond to, and in some are subdivisions of, the eras. These ages, and their corresponding rocks, when they are subdivisions, are separated usually by unconformity, but not so uni- versal ; and by changes of organisms, but not so sweeping. They are — 1. Archaean or Bozoic age, corresponding to the Eozoic era. 2. Age of MoUusks, or age of Inverte- brates, corresponding to the Silurian system of rocks. 3. The age of Pishes, corresponding to the Devonian. 4. The age of Acrogens, or age of Amphibians, corre- sponding to carboniferous strata. 5. The age of Keptiles, corresponding to the Mesozoic era and Secondary rocks. 6. The age of Mammals, corresponding to the Cenozoic era and the Tertiary and Quaternary rocks. 7. The age of Man, corresponding to the Psychozoic era and the present sediments. * Mesozoic = middle life, f Cenozoic = recent life. X Psychozoic = rational life. GENERAL PBINGIPLES. 261 Tapir, Pecoarj, Bison. Llama. EquoM. Mtgatheriwm, Mylodon, Elep/iaii. PliohippuB Beds. Ptio?tippua, Maatodon, Sag, t Miohippus Beds. ^ioftiRptM, JHcerathmiwm, TMnohyua. Oreodon Beds. Edentates, HytenodUm, Syracodon. Brontotheriutn Beds. MeaoidppuB, Menodfua, JElotkeriumi. Diplacodon Beds. S^Happua, Amynodon. Binoceras Beds. Tmoceraa, UintaHieriu/m, JJimnohyus, Orohippug, Helaletet, CoUmocerae. Corj^pbodoii Beds. Miflippua, Monkeys, CamivoreB, Ungulates, TillcHlontB, Rodenta, Serpenta. Lignite Series. Sadrosaurua, Hryptoaaurwi. Pteranodon Beds. Birds with Teeth, Bespttromie, Ichihyomie. Pterodactyls, Plcsiosa>uiB. Dakotah Group. Comanche Group, Atlantosaurus Beds. Dinosaurs, Apatosaurus, Alloaawrua, Nanoaaurua. Turtles. J>ipUiBawru». Connecticut Kiver Beds. liist Mammals (Uatsupials), {Xh-omatherium). Dinosaur Poot-printa, Amphieavirua. Crocodiles (Belodmt). Permian. Firat ReptUcs Coal-Measares. Sub-carboniferous. Fiist known Amphibians (lAbTrinthodonta). Comiferous. Schobarie Grit. First FiBh Faona. Upper Silurian. nt kBOwn Fishes. Lower Silurian. Primordial. Huronian. Laurentian. No Vertebrates known. Fia. 164 — S' "lion of tbe earth's crust, to illustrate vertebrate life in America. •■tblli^Utly mnditird from MarsbJ 262 HISTORICAL GEOLOOY. In the diagram (Fig. 163) the different rock-systems are placed one on top of the other, and the vertical black spaces represent by their breadth the relative dominance of different classes at different times. Periods and Epochs. — The subdivisions of these again into periods and epochs are founded on more local uncon- formities, and especially on less important changes in the species. We have already, on page 304, given a schedule of the most important divisions and subdivisions adopted in this work ; but we shall not treat separately all of these. As in human history, so in geology, the earliest times are little known, and are touched lightly. As we come toward the present, and events thicken, we shall take up subdivisions more and more — first ages, then periods, and, finally, even epochs. We give here also (Pig. 164) a generalized sec- tion of American strata, which will be found useful for reference. It must not be supposed, however, that all these strata occur in any one place. It is an ideal section, in which all the most important American strata occurring in different places are brought together and arranged in the order of time. We are now ready to commence a rapid survey of the history of the earth. But it must be understood that we can commence only where the record commences. Before this is the abyss of the unrecorded, of which we know nothing positive. Before the historic is the prehistoric ; no history can recall its own beginning. CHAPTER II. ARCH^AN SYSTEM AND ARCHEOZOIC ERA. The events recorded in this oldest system of rocks, in this first Tolume of the booh of time, are so few and so imperfectly recorded that their chief interest consists in the fact that they are the first. There is a fascination about the beginning — the mythical period — of all history. The distinctness of this system was for a long time un- recognized. It has now, chiefly by the labors of Ameri- can" geologists, been completely established. In no single instance have these rocks been found to graduate into the Paleozoic. There is absolutely everywhere an uncon- formity between them and every other system. No such complete and universal break occurs anywhere else in the rocky series as occurs here (Fig. 165). It is, therefore. Pio. 165.— Section showing Primordial unconformable on the Archaean : 1, Acchaean or Laorentian ; 2, Primordial or lowest Silurian. (After Logan.) properly called a distinct system and a distinct era — more distinct, in fact, than any other. Here, then, we have the oldest known rocks. Are they, then, absolutely the oldest — the primitive rocks, as some imagine ? By no means. They are stratified rocks, and therefore consolidated sediments, and therefore, also, the 263 264 HISTORICAL OEOLO&Y. debris of still older rocks, of which we know nothing. Thus, we seek in vain for the absolutely oldest, the primi- tive crust. As already said, no history can write its own beginning. Character of these Kocks. — "We can only say, in brief, that they do not differ very conspicuously from metamorphic rocks of other times. They were probably originally sands, clays, and limestones, much like those of other times ; but, in this case, always very highly meta- morphic and stron,gly crumpled (Fig. 166). The sands are thereby changed into quartzites, the clays into schists, Fig. 166.— Contortion of Laurentian strata. (After Logan.) gneisses, and even granites, and the limestones into mar- bles. Along with these, however, are associated two kinds of beds, which are worthy of note, viz., beds of iron-ore and beds of graphite. In Canada the whole series is not less than 40,000 feet thick. The greatest beds of iron-ore known in any strata are found here. The great iron-ore beds of Sweden, of Lake Superior (Fig. 167), of New Jersey, and the Iron Moun- tain of Missouri, are in these rocks. Eecently, in southern Utah, in rock of this age (or possibly later), have been found the greatest iron-de- posits, perhaps, in the world. The strata here stand on edge, and the beds of iron-ore, being very hard, have been left by erosion standing out as black, castellated, inaccessible crags, 300 feet high, 1,000 feet long, and 500 feet thick. In Canada and elsewhere graphite also occurs in immense beds, sometimes pure, sometimes mixed with the rock. Fio. 167. ABCHJEAN SYSTEM AND ARCHEOZOIC ERA. 265 Area. — 1. These rocks cover the whole of Labrador, nearly the whole of Canada (passing into New York in the region of the Adirondacks), then extend northwest probably to the Arctic regions. This, the greatest Ar- chaean area in Iforth America, forms a broad, open V, inclosing in its arms Hudson Bay. 3. The next largest area is a broad space extending from New England to Georgia, including the Blue Eidge and the eastern slope of the Appalachian. 3. The axes of many of the great mountain-ranges, such as the Colorado, Park, and Wah- satch Eanges, and possibly the Sierra Nevada. 4. Some small, isolated spots, one in Texas and one in Missouri. In the map (page 373) these are represented by V- Physical Geography. — These being stratified rocks, it is evident that the whole Archaean area was sea-bottom at that time. Where, then, was the land from which this debris was derived ? Of this we know nothing. Some have thought that it was to the northeastward. We shall see hereafter that the continent developed southward and westward. Amount of Time. — The Archaean rocks are of enor- mous thickness, probably equal to all other subsequent rocks put together. The amount of time represented is, therefore, probably equal to all the rest of recorded his- tory of the earth. And yet how meager the record ! It is the same with the earliest human history. Life. — Did any living thing exist at that time ? This is a very important question, but we can not yet answer it with absolute certainty. There are, however, some good evidences of life : 1. Iron-ore is accumulated now, and therefore probably also in earlier times, only by means of decaying organic matter (page 89), and is, therefore, justly regarded as a sign of life and a measure of its quantity. 3. Graphite is regarded as the highest anthra- citic condition of coal; and coal is a positive sign of organic matter, -and therefore of the previous existence. 366 EISTORIGAL GEOLOGY. • of life. Limestone, as we have seen (pages 114-117), is now, and at previous geological times, usually, though not always, of organic origin. Judging from these signs, it would seem that life was not only present, but in large quantity. Can we say any- thing as to its kind ? Are there any fossils ? Here we must answer still more doubtfully. Some curious forms are found which are supposed to be those of the lowest order of animals (compound Protozoa). These have been called eozoon or dawn-animal, and therefore some have called this first era eozoic. Most, however, do not accept this animal, and prefer the name Azoic (no animal life), or, better still, Archeean or Archseozoic, as carrying no implication. In conclusion, we may say that the existence of the lowest forms of vegetable life is almost certain, and of the lowest forms of animal life probable. CHAPTER in. PALEOZOIC BOOKS AND EBA. Section I. — General Desoeiptiom'. The Lost Interval. — Between the Archaean and Pale- ozoic rocks occurs the greatest and most uniyersal break in the whole stratified series. At this point in time occurred the greatest and most wide-spread changes in physical geography and climate which has CTer occurred in the history of the earth. The justification for this statement is foxmd in the fact that eTerywhere, even in the most distant localities, we find the lowermost Paleo- zoic (Primordial) lying unconformably on the Archaean. No one has yet seen the two series continuous. Now, when we remember that unconformity always means a previous eroded land-surface (page 192), and stratified rock a sea-bottom, we easily perceive how wide-spread the changes of physical geography must have been at this time. Again, when we remember that unconformity also always means a lost interval unrecorded at the place ob- served, and that the unconformity exists at all observed places, we at once see that right here is an unrecovered, probably an irrecoverable, lost interval of time. During the lost interval wide areas of land existed, which were afterward submerged and covered with Paleozoic sedi- ments. As compared with the early Paleozoic, it was evidently a continental period. Corresponding with the great physical changes here, there was also immense advance in life-forms. During 367 268 HISTORICAL GEOLOGY. the Archaeozoic, as we liave already seen, the life, if any, was only of the lowest possible kind. Life-forms had not differentiated into distinct, recognizable species. There was not yet what could justly be called a fauna and flora. Then came the lost interval, represented by the unconformity. Of what took place then we know nothing. Then the record opens again with the Paleozoic, we have already an abundant and diversified fauna and flora. ?;\ en in the lowest Primordial we find all the great de- ptitments of Invertebrates, and nearly all the classes of thase departments, already represented. It certainly looks like a sudden appearance of somewhat highly organ- ized animals, without progenitors. But we must not forget the lost interval. It is probable that during this period of rapid physical changes there were also rapid changes in organic forms. It is for these reasons that the Paleozoic is regarded as opening a new era, and, in fact, the most distinct in the history of the earth. We have explained its distinct- ness from the Archaean below, but we shall find hereafter that it is almost equally distinct from the Mesozoic above. It is separated on both sides by unconformity and by changes in life — a distinct volume with, as it were, blank boards on either side. Bock-System. — There is nothing very noteworthy in the character of the rocks of the Paleozoic. Only this may be said : as compared with the Archaean rocks, they are far less universally thick, metamorphic, and crumpled. In mountain-regions, indeed, they are very thick (40,000 feet in the Appalachian), very metamorphic, and very much folded ; but in level regions they are often much thinner, entirely unchanged, and level-lying. For exam- ple, in passing from the Appalachian westward, we find (he following four kinds of change : 1. In the Appalachian the Paleozoics are 40,000 feet thick ; they thin out west- ward, until at the Mississippi Eiver they are only 4,000 PALEOZOIC ROCKS AND ERA. 369 feet. 3. In the former, sands, grits, and clays predomi- nate ; in the latter, limestones. 3. In the former the rocks are strongly folded ; these folds die out through gentle undulations to level-lying strata in the latter. 4. In the former the rocks are highly metamorphic ; in the latter they are wholly unchanged. Area in the United States. — 1. Eastern Paleozoic Basin. The Paleozoic rocks cover a large continuous area in the very hest part of the United States. This area is bounded on the north by the chain of the Great Lakes ; on the east by the Blue Eidge of the Appalachian chain ; on the south by a line running through mid- Alabama, turning northward to the mouth of the Ohio River ; then south through mid- Arkansas and Indian Territory ; on the west by the Western grassy plains. 2. Besides this great area, there are several considerable areas scattered about in the Plateau region and exposures along flanks of mountains of the Plateau and Basin regions. Pliysical Geography. — The physical geography of the eastern portions of the North American Continent in Paleozoic times can be made out with considerable cer- tainty. In fact, we can in many places trace the Primor- dial shore-line. Immediately in contact with the Canadian Archaean on the north, and the Blue Ridge Archaean on the east, are found patches, or continuous lines of a coarse sandstone, which contain all the marks characteristic of shore-lines, such as worm-tubes, worm-trails, crustacean tracks, ripple-marks, rain-prints, etc. This is the old Primordial beach. At the beginning of Paleozoic times, therefore, the whole Paleozoic basin was covered by a sea which beat against a land-mass to the north (Canadian Archaean area), and a land-mass to the east (Blue Eidge Archaean area). This is called the great interior Paleo- zoic Sea. There was also a large land-mass in the Basin region, and smaller masses, probably islands, in the Colo- rado mountain-region, but the exact limits of these are 370 HISTORICAL &EOLO&Y. not known. The map (Fig. 168) represents the present state of our knowledge on this subject. It is probable, however, that the Eastern land-mass (Blue Eidge Archjean area) was larger than represented, having been subse- ffuently covered by later deposits, and partly, even now, by the Atlantic Ocean. The change in the rocks, in passing westward from the Appalachian region, is completely explained by the posi- tion of the Appalachian region and the subsequent f orma- FiQ. 168.— Map of physical geography of Primordial timeB ; existiijg eeae and lakes, black ; contlDental seas of that time, light shade ; land of that time, white. The white dotted line shows the probable shore-line of 3 at this time. tion of the mountains. This region was then the marginal bottom of the interior sea, receiving abundant and coarse sediments, which became finer and thinner seaward. This thick marginal line then yielded, was strongly folded and highly metamorphosed in the act of mountain- making which took place at the end of the Paleozoic. PALEOZOIO BOCKS AND ERA. 271 Growth of the Continent during Paleozoic Times. — The map (Pig. 168) represents the continent at the beginning of the Paleozoic. But during that era there was a steady growth from this nucleus by addition south- ward and westward, until, at the end, the whole of the Paleozoic areas were reclaimed from the sea, and the continent was nearly, though not exactly, that represented on page 349. It will be seen that the continent was already outlined at the beginning of the era, and was steadily developed toward its present form« We shall hereafter trace this deyelopment to its completion. Subdivisions of the Paleozoic. — The Paleozoic era and strata are divided into three ages, each represented by corresponding rock-systems : 1. The age ofMollushs, or of Invertebrates, represented by the Cambrian and Silurian system ; 2. The age of Fishes, by the Devonian ; 3. The age of Acrogen Plants and Amphibian Animals, by the Carboniferous. These three rock-systems, in many parts of the world, are unconformable with each other ; but in the United States they are usually entirely conformable. Nevertheless, their life-systems (organic forms) are here, as everywhere, quite different. All these subdivisions are well represented in the Pa- leozoic basin of the United States (Fig. 169). In the fol- lowing map of the main divisions of the geological strata of the Eastern United States, the rocks representing these three ages are all shown. It is important to study this map well, for it will be referred to frequently hereafter in connection with more recent strata. Section II. — Lowbe Paleozoic oe Cambeian and SiLUBiAN System. Age of Invektebeates. Bocks ; Name. — These rooks are called Cambrian and Silurian, from the Eoman name for Wales and the Welsh, because they were first studied in Wales, by Sedgwick and 27!; HISTORICAL 0EOLOBY, PALEOZOIC ROCKS AND ERA. 373 Murchison. But they are far more perfectly represented in the United States. Area. — It will be seen, by reference to the map. Pig. 169, that in 'the great Paleozoic basin these rocks form an irregular border to the Canadian and Blue Eidge Archsean areas. These borders were marginal sea-bot- toms at the beginning of the Silurian times, and were elevated and reclaimed during and at the end of that time. There are many other smaller areas in the "West, but these can not be defined. Physical Geography.— We have already given this for the beginning of the age in the map. Fig. 168. For the end of the age, as just stated, we must add the Silu- rian area to the Archsean area. There was also at the end added a large island of Silurian sea-bottom in Ohio and Tennessee (see map. Fig. 169). Subdivisions. — The Lower Paleozoic rocks are sub- divided into — 1. Primordial, or Cambrian ; 2. Lower Silurian ; 3. Upper Silurian ; and these, again subdivided, as shown in the following schedule. We simply give these by name for reference, if necessary, but will treat of the whole Cambrian and Silurian together : {Helderberg period Salina Niagara " 2. Lower Silurian. \ T'eiit°" / Canada " 1. Cambrian, or j Primordial. j Primordial '" Life-System. We have already spoken of the apparent suddenness of the appearance of a somewhat diversified fauna in the Primordial, and accounted for it by the existence of a lost interval. Immediately after the Primordial the fullness of Paleozoic life became really wonderful. These early Le Contb, Geol. 18 374 HISTORICAL GliOLOQY. seas seem to have swarmed with a life as abuudant as any now existing, but wholly difEerent in species, in genera, and even in families, not only from any now living, but from those living in any other geological period. About 20,000 species are described from the Paleozoic, and of these at last one half, i. e., 10,000 species, are from the Silurian ; and of course these are but a very small frac- tion of the number which actually existed. The number being so great, and the forms so unfamiliar to the pupil, it is impossible to do more than mention and figure a few of the most common and striking forms. Plants. The only kind of plants which are found so early are allied to sea-weeds. * ■ As it is very difficult to determine these species from the very imperfect impressions of them left in the rocks, we shall call them by the general name Fia. 170. B'lG- 171. Fios. 170, 171.— Silurian plants: 170. SphenotlialloB anguBtifoliuB. 171. Buthotre- phis graciliB. of Fucoids, i. e., fucus-Vike plants, from their general resemblance to Fucus (tangle or kelp). We give a few * A few small vascular cryptogams, allied to club-mosses, have been recently found in the Silurian. PALEOZOIC ROCKS AND ERA. ;a75 (Figs. 170, 171), to show their general appearance. They belong to the lowest order of plants. Prv^ <3 Animals. \^xy^ These are far more numerous and diversified than the plants. We can mention only such as may be recognized even by the untrained eye. Corals. — These are very abundant, and seem some- times to have formed veritable reefs. There are three very characteristic forms, viz., Cw^-corals {Cyathophyl- loids, Figs. 173, 173), JIonet/comh-coTals {Favositids, Fig. 174), and C%am-corals (Halysitids, Fig. 175). These Fib. 173. Fig. 172. Fies. 172, 173. — Cyathophylloid corals : 172. LonsdaleiafloriformlB. (After Nichol- son.) 173. Strombodes pentagonue. (After Hall.) are all characteristic of the Paleozoic, and the last char- acteristic of the Silurian. Now, any one can recognize these, especially the Honeycortib and Chain corals, and therefore when these are found any one may identify Paleozoic or even Silurian rocks. Hydrozoa. — In still, sheltered bays, with fine mud- bottom, are now found, attached to sticks, logs, or shells, fine, feathery things, which look like finely dissected sea- weed or sea-moss. They are, indeed, gathered by ama- 276 HISTORICAL &EOLOGY. teur collectors and pressed as sea-weeds. If they be ex- amined with a lens, they are seen to be composed of * KV" Fio. 174. Pio. 175. Fiss. 174, 175.— Favositid and halysitid corals : 174. ColomDaria alveolata. (After Hall.) 175. Halyeitescatenulata. (After Hall.) hollow, branching stems, set on one or both sides with hollow cups, each containing an animal which, if kept undisturbed in sea-water, quickly spreads its thread-like tentacles. These are the Hydrozoa of the present day (Pigs. 176, 177, 178). Now, in fine Silurian shales, which Pig. 176. Fie. 177. Fig. 178. Pigs. 176-178.— Living hydrozoa : 176 and 177. Sertularia. 178. Plumularia. were once fine mud, are found impressions of animals probably similar to these. They are called Oraptolites. Whatever they be, they are easily recognized and wholly PALEOZOIC ROCKS AND ERA. 377 cliM'acteristic of Silurian, and any one may identify Silu- rian by means of them (Figs. 179, 180). FiQ. 179. Fig. 380. Figs. 179, 180. — Silurian hydrozoa : 179. BiplograptuB pristis. (After Kicholeon.l 180. Graptolitea Clintonensis. (After Hall.) Echlnoderms ; Crinoids. — At the present time, if we leave out sea-cucumbers {Holo- _ _^ thurians), because, having no shells, they arei not preserved as fossils, Echinoderms are of three orders : 1. EcMnoids, or sea-ur- chins ; 2. Asteroids, or star-fishes ; and, 3. Crinoids. The first two are free-iaoving, the last is stemmed. The first two are now very abundant, the last rare. But in Silurian times it was the reverse. The Echinoids did not exist at all, the Asteroids were rare, but the Crinoids extremely abundant, though, of course, of ^™- isi. - Living crinoia. T m Tn. Pentacrinua caput-mednsae. species and genera wholly ditter- ent from any now existing (Pig. 181). It is well to ob- serve that the crinoid is a lower form than the other two. 378 HISTORICAL GEOLOGY. as is shown by the fact that some free echinoderms have. steins in the early stages of life, and afterward throw them off and become free. Description of a Crinoid. — A crinoid has a pear- shaped body, containing the viscera, set upon a jointed Fig. 18-a. Flo. 183. Fia. 184. Figs. 18S-184.— Silurian crinoids : 183. HeterocrinuB simplez. (After Meek.) 188. PleurocystitiB squamosas. 184. Lepadocrinus Gebhardii. stem, with mouth on the top of the pear, sometimes sur- rounded by many plumose arms (Kg. 182), sometimes with few simple arms (Fig. 183), sometimes with no arms at all (Fig. 184). Range in Time. — We have said that stemmed echino- derms or crinoids continue from earliest times until now (though the species and genera , change repeatedly), but in diminishing numbers. The free echinoderms, on the contrary, have been constantly increasing. If, then, A B (Fig. 185) represent the course of geological time, and the parallelogram the equal abundance of echinoderms throughout, then the shaded portion below the diagonal would, in a general way, represent the constantly decreas- ing stemmed, and the unshaded space above the diagonal PALEOZOIC ROCKS AND ERA. 279 the constantly increasing free forms. But are there any characters by which we may easily recognize those pecu- -PALEOZOIC STEMMED FiQ. 185.— niagram ehowing diBtribuUoii in tune of crmoids. liar to the Silurian ? There are. Crinoids are subdi- vided into three main groups, viz. : 1. Grinids, or plumose-armed crinoids (Fig. 183) ; 3. Blastids (Fig. 243, page 316), or bud-crinoids ; 3. Cystids (Figs. 183, 184), or bladder-crinoids. The crinids are not character- istic of Silurian, nor even of Paleozoic ; the blastids are characteristic of Paleozoic, though not of Silurian ; the cystids are characteristic of Silurian alone. This is rep- resented by subdivisions of the shaded space in Fig. 186, in relation with the subdivision of the Paleozoic. MoUusks ; Brachiopods. — Bivalve shells are divided into two great groups, viz. : 1. Common bivalves {Lamel- librancJis) ; and, 3. Lamp-shells, or BracMopods. At present, the former are extremely abundant, and the lat- ter rare. The reverse was true in Silurian times. The distribution in time of the two kinds may be roughly Fio. 186.— Diagram showing the general distribution, in time, of brachlopods and lamellibranchs. represented by the diagram (Fig. 186). Now, brachlo- pods are very different from, and much lower than, ordi- nary bivalves. Lamellibranchs have a right and left valve — ^right and left gills, etc. ; in brachiopods the 280 EISTORIGAL GEOLOGY. valTes are tipper and lower, or a back-piece and a breast- plate. The .deeper and more projecting valve is the ven- tral. From the point of this valve comes out a fleshy cord, by which it is attached. It is this which gives it the name of lamp-shells, on account of its resemblance to the ancient lamp (Fig. 187). A large portion of the in- terior of the shell is occupied by long, spiral, fringed arms. It is these which give the name of brachiopod (arm-feet), although they are really gills. These are attached to complex, and sometimes spiral, bony pieces. Fig. 188 is a living brachiopod, showing structure. These shells Fig. 187.- chiopod, -Living bra- Side view. Via. 188.— A living bracMopod Terebiatnla flavescens. are so extremely abundant in Paleozoic, especially Silu- rian rocks, that these rocks may often be identified by them. In Figs. 189, 190, we give two of the most common forms. Are there any characters by which Silurian brachiopods can be easily distinguished? Not by the un- trained eye. Yet the square shoul- dered forms, like those figured here, are very characteristic of Paleozoic, though not of Silu- rian. Liamellibranchs and Grasteropods. — The ordinary bivalve-shells {Lamellibranchs), and the univalves or gas- FiQ. 189.— Silurian bracbio- pods; Ortliis Davidsonii. PALEOZOIC BOOKS AND USA. 281 teropods, like conchs, whelks, etc., are also found; but, in order to avoid confusing tlie mind with too many de- Fig. 190. — Silurian brachiopods : Spirifer Cumberlandise — a, ventral valve ; 6, sutnre. tails, we shall pass over these and confine ourselves only to the most striking and characteristic forms. Gephalopods ; Orthoceratlte. — The great class of Cephalopods, including now the squids, cuttle-fishes, and nautilus, were represented, in Silurian times, by a very remarkable family called Orthoceratite (straight-horn). The appropriateness of the name is recognized by the figures on page 283 (Figs. 192, 193). Cephalopods now are, some of them, naked (squid and cuttle-fish), and some shelled (nautilus). When they have a shell, the shell is cham- iered. The animal lives in the outer part, and all the chambers are empty, full of air only, and connected -^ ' with the animal by a mem- branous tube called the si. phon-tube or siphuncle (Fig. 191). Now, at the present time, nearly all cephalopods are naked. Only one genus of the shelled kind remains, viz., the Nautilus. In Silu- rian times, and indeed long after, there were no naked ones. Only the shelled kinds existed. The naked Pig. 191.— Pearly natitiluB (Nautilus pompilius) : a, mantle ; 6, its dor- sal fold ; 0, hood ; o, eye ; t, ten. tacles ; /, tunnel. 282 HISTORICAL GEOLOGY, kinds are the higher. Again, now, and throughout all later geological times, all the shelled cephalopods were coiled, like the nautilus. But Paleozoic, and especially Silurian times, were characterized by the abundance of long, tapering, straight, chambered shells. These are the Orthoceratites. They are entirely characteristic of s ' e Fig. 192. Flo. 193. Fie. 194. Pias. 192-194.— Silurian cephalopods : 192. Orthoceras muMcameratam. (After Hall.) 193. Orthoceras DuBeri. (After Hall.) 194. Eeetoration of orthocerae, the shell being supposed to be divided vertically, and only its upper part being shown— a, arms ; /, muscular tube (" funnel ") by which water is expelled from the mantle-chamber ; c, air-chambers ; s, siphuncle. (After Nicholson.) Paleozoic, most abundant in the Silurian, and easily rec- ognized by any one. "We give figures of a few (Figs. 192-194), and an attempted restoration of the front part of the shell containing the animal. PALEOZOIC MOCKS AND ERA. 283 Orthoceratites were extremely abundant in Silurian times, and, in some cases, reached an enormous size. In the Silurian of the "Western States, specimens have been found which were eight to ten inches in diameter, and over fifteen feet long. They were the most formidable animals of these early seas. They came in with the Pri- mordial, reached their maximum in the Mid-Silurian, but continued through the Paleozoic, and then passed away forever. Although the straight, chambered shells were by far the most abundant, yet the coiled kinds were also found. Crustacea; Trilobites — Passing over the worms, as being of less importance, although their borings, their tracks, their calcareous tubes, and even their teeth, have been found, we come at once to perhaps the most abun- dant and characteristic of all Paleozoic forms — Trilobites. Description. — The shell of these curious creatures was convex above and flat or, more probably, concave below (Fig. 196). It was divided, like most crustaceans, into many movable joints, but several front joints were always consolidated to make a huchler, or head-shield, and usually, but not always, several hind joints were con- solidated to form B,pygidium, or tail-shield. Longitudi- Pi8. 195.— Strnctnre of the eye of trilobites: a, Dalmanla pleuropteryx; 6, eye i magnified; c, eye more highly magnified. (After Hall.) 384 HISTORICAL GEOLOar. nally, the upper surface of the shell was divided by two depressions into three lobes (hence the name). On each side of the head-shield, in position exact- ly as in the hing-crab {Li- mulus), were placed the eyes; and, strange to say, we find the eye, even at this early time, al- ready a com- plex structure well adapted to form an image (Pig. 195). Recently have been discovered jointed legs, I, I, each with two branches, one for crawl- ing and one for swimming; and also slender, many-jointed antennse, a, a. Fig. 196.— EfiBtoration of upper side of calymene. (After m-. -^n-r^-,, nvmn Beecher.) ^^^^ ^^^^ ^™^' taceans of the present day (Fig. 196). They had the habit, which many crustaceans now have, of folding themselves so as to bring head and tail together in front, as shown in Fig. ly9. In the following figures (Figs. 197, 198) we give some examples of Silurian Trilobites. PALEOZOIG ROCKS AND ERA. 385 Trilobites are found in great numbers, of almost infinite variety of form and markings, and of size varying from a fraction of an inch to twenty inches in length (Fig. 197). Ite. 197. Fia. 198. Pio. 199. B^ss. 197-199.— Silurian trilobites' 197. Paradoxides Harlaiii, x J (after Eogers), American. 198. Calymene Blumenbachii. 199. Same in folded condition. r They come in with the earliest Primordial, reach their maximum in Mid-Silurian, but continue through Palae- ozoic, and pass out forever. They are, therefore, entirely characteristic of the Paleozoic, and especially abundant in Silurian. Although belonging to a distinct order, different from any now living, yet they were more nearly 286 HISTORICAL GEOLOGY. allied to the horseshoe crab {Limulus) than anything else. They are so abundant, so well preserved, and so easily rec- ognized, even by the untrained eye, that they are a very valuable means of identifying Paleozoic, and especially Silurian, strata. Anticipations of the Next Age. — The most highly organized and most powerful animals of Silurian times were undoubtedly the Orthoceratites and the Trilohites. The Orthoceratites especially were the tyrants and scaven- gers of those early seas ; yet, in the uppermost Silurian are found a few insects, scorpions, and cockroaches, and a tew fishes similar to forms far more abundant in Devonian. It is better, therefore, to regard these as anticipations. Section III. — Dbvoniak System. The Age of Pishes. Rock-System ; Name. — The name Devonian was given to these rocks because first studied with success in Devon- shire. In Scotland they were called Old Eed Sandstone, by Hugh Miller. In England it is often unconformable on the Silurian, but in the Eastern United States, as al- ready stated, the Paleozoics are conformable throughout. Nevertheless, even in America there is a great change of life-forms at this point of time ; and, moreover, the first introduction here of a new reigning class — viz., fishes, and a new great department of animals — viz., Vertebrata, or backboned animals, is a prodigious advance^ and en- titles this to be considered a distinct age. It is well to note, however, that some anticipations of this great ad- vance are found in the Silurian. Area in the United States. — By examining the map on page 373, it will be seen that in general the Devonian rocks border on the Silurian area on the south and west and extend far south in the middle region. In the Eocky Mountain region there are considerable areas of Devonian, but their limits are too little known to be described. PALMOZUIO ROCKS AND BRA. 387 Physical Geography. — The land during early Devo- nian times was the Archaaan area, increased by the addi- tion of the Cambrian and Silurian area, the Devonian area being then of course sea-bottom. In the middle of the Devonian Sea there was a large island of Silurian rocks occupying what is now mid-Ohio and running down through mid-Tennessee. At the end of the Devonian, the Devonian area was exposed as land and added to that previously existing. Subdivisions. — The American Devonian is subdivided into at least four groups of strata representing four periods, as shown in the schedule. We shall, however, neglect' these subdivisions in our general accouot of the life- system : 4. Chemung period. 3. Hamilton period. 5. Corniferous period. 1. Oriskany. Idfe-System of Devonian. — Plants. In Silurian times, with the exception of a very few small vascular cryptogams allied to club-moss, we found nothing higher than f ucoids. In addition to these, now, for the first time, land-plants become conspicuous. Here, for the first time, we have a true foresi vegetation. The character of the trees of this first forest is a question of the highest interest. The Devonian forests consisted of the highest cryptogams, vascular cryptogams, and the lowest phenogams, Oymnosperms. More explicitly, there were Ferns, Lepidodendrids, and Sigillarids (gigantic club-mosses), and Calamites (gigantic Equisetce) among vascular cryptogams : and Conifers allied to the yews among gymnospermous phenogams. We shall not describe these now, since they are all much more abundantly represented in the Carboniferous. 288 HISTORICAL GEOLOGY. We shall therefore dismiss them for the present with one or two remarks. 1. In Nova Scotia, in direct connection with the plant- beds, have also been found many fossil forest-grounds. These are marked by dark seams with stumps and roots in place just as the trees grew. In some cases, also, thin seams of coal lie upon the forest-grounds. Thus, there- fore, we have here in the Devonian an anticipation not only of coal vegetation, but also of the conditions neces- sary for the formation and preservation of coal. 3. "We have here a somewhat sudden appearance of land-plants, as if they came without progenitors. But we must remember that we have a feeble beginning of land-plants in the Silurian. It seems probable that in the Devonian we had more favorable conditions, and therefore a rapid development of new forms. Animals. If we bear in mind what we said about Silurian ani- mals, it will be necessary here only to note the great changes, i. e., what old forms pass out, what new forms come in, and what advances are made in the progress of life, dwelling only on the great characteristic of the age, viz., \h.e fishes. Fia. 300. fiG. 201. FiQS. 300, 201.— Devonian corals : 200. FavoBltes bemlspheiica. 301. Zapbientis Worthenl. (After Meek.) PALEOZOIC BOCKS AND ERA. 289 Badiates. — Among corals, the characteristic Siluriau chain-corals {Halysitids) are gone, but the other two forms remain, with different species (Figs. 200, 201). The graptolites are gone, as also the Cystidean crinoids, but the blastids or bud-crinoids are now far more abun- dant, though they reach their maximuni only in the Carboniferous (Fig. 242, page 316). Bivalves and Univalves. — Brachiopods still continue in great numbers, of the characteristic Paleozoic, square- shouldered forms (Pig. 203), and both Lamellibranchs Fig. 202. FiQ. 303. Pia. 804. Flo. 205. Figs. 202-205.— Devonian brachiopods . 202. Spirifer perextensna. (After Meek.) Devonian lamellibranclis and gasteropods : 203. Ctenopistba antiqua. (After Meek.) 204. Lucina Ohioensis. (After Meek.) 805. Orthonema Newberry!. (After Meek.) and Gasteropods (univalTes) are now more abundant. It is well to note that fresh-water and land forms are now for the first time introduced. Gephalopods. — The Orthoceratites still continue in Devonian times, though in greatly diminished number ' and size ; but we note here a great advance in the intro- duction of a new form characteristic of this and the Carboniferous, viz., the Ooniatites (angled stones), so called because the suture or junction of the partition with the shell is angled instead of simple (see Fig. 246, page Le GasTE, Geol. 19 290 HISTORICAL GEOLOGY. SIT). It should be remembered that this is the first introduction of a family {Ammonite family) which in Mesozoic times became extremely abundant. The family is characterized by the dorsal position of the siphon-tube and the complexity of the suture. We shall hereafter trace the increasing complexity of the suture. It only begins in the Groniatite. Crustacea. — Trilobites still continue under new forms (Pig. 206), but in greatly diminished number and size. They have passed their prime. Insects. — Insects are now, and at all previous geologi- cal times have been, closely related to land vegetation. fflo. 30T. Fig. 206. Figs. 206, 207.— 206. Devonian trilobite and insect : Dalmania punctata ; Europe. 207. Wing of platephemera antiqua ; Devonian, America. (After Dawson.) The first conspicuous land vegetation is found in the Devonian, and in connection with this vegetation are found also the first known insects (Fig. 207). These first insects were most nearly allied to cockroaches and dragon- flies — in fact, a connecting link between these orders. PALEOZOIC ROCKS AND ERA. 291 In some a chirping organ has been found. This shows that an auditory apparatus was already developed. Although these first known insects are among the lower orders of the class, and also are connecting links between two such lower orders, yet their somewhat perfect devel- opment indicates that we must look for the very first insects still lower, i. e., in the Upper Silurian. They have been recently found there. Fishes. — The introduction of fishes must be regarded as a great step in the progress of life, for it is the begin- aing not only of a new and higher class, but of a new great department and the highest, viz., Vertebrata. They commenced first in the lowest Devonian or perhaps even in the uppermost Silurian, few in numbers, small in size, and of strange, un-fish-like forms, but soon developed in size and numbers until these early seas swarmed with them, and they quickly became the rulers of the age. The previous rulers, Orthoceratites and Trilobites, there- fore diminish in number and size, and thus seeh safety in subordination. As examples of the great size of Devonian fishes, we mention a few. The Onchyodus (claw-tooth) had jaws eighteen inches long, armed with teeth two or more inches long ; the Dinichthys (huge fish) was fifteen to eighteen ■ feet long, three feet thick, and had jaws two feet long, armed with curious blade-like teeth. These are from America. The Asterolepis (star-scale) of Europe is believed to have been twenty feet long, and of propor- tionate dimensions. We must not imagine, however, that these fishes were at all like most common fishes of the present day. Ifeglect- ing some rare and unusual kinds, fishes may be divided into three great orders, viz., 1. Teleosts (complete bone); %. Ganoids (shining) ; and 3. Elasmoiranchs (plate-gills). The Teleosts include all the ordinary fishes : examples of Ganoids are 'found in gar-fish and sturgeon ; of Elasmo- branchs, in sharks, skates, and rays. At the present 293 HISTORICAL &EOLO&Y. time, nine-tenths of all fishes are Teleosts, but in Devo- nian times all the fishes were Ganoids and sharks, espe- cially the former, though differing in species and genera from Ganoids and sharks of the present day. But we must give some figures of these strange Devonian fishes before discussing their affinities any further. ' Description of Some Devonian Fishes. — The Ceph- alaspis (head-shield. Fig. 308) was a small fish, of very Fig. 208. Fig. 209. Figs. 208, 209.— Devonian flshes— Placoderms : 208. Cephalaspis Lyelli. (After Nich- olson.) 209. Pterichthys comutus. (After Nicholson.) curious shape, with mouth beneath the head-shield. The Pterichthys (winged fish. Pig. 309) was so completely in- cased in bony plates that it must have swum mainly by means of its wing-like anterior fins. The mouth was also beneath. The Coccosteus (berry-bone, Pig. 310) was cov- ered with bony plates in front parts, but the tail was usable for locomotion. The Dinichthys (Fig. 311) was a PALEOZOIC ROCKS AND ERA. 293 huge fish, sometimes eighteen to twenty feet long, very abundant in the Devonian of Ohio. Lilce the Coccosteus, the anterior tail was covered with broad, bony plates. FiQ. 210. v_..._^ Fig. 311. Fias. 210, 211.— Devonian fishes — Placoderms : 310. Ooccosteus liecipiens. (After Owen.) 211. Dinichthys. (After Dean.) The Osteolepis (bony scale. Pig. 212) was covered with a complete coat-of-mail of rhomboidal bony scales, like the gar-fish and polypterus (Fig. 217) of the present day. The Diplacanthus (double spine, Eig. 213) is more fish-like in form, but is also covered with rhomboidal bony scales. We draw attention to the shape of its tail. All these are Ganoids. The sharks, on account of their cartilaginous skeleton and imperfect scales, are known chiefly by their bony spines and by their teeth. A restoration of a Devo- nian shark from Ohio is given in Fig. 214. By examination of the figures, it is seen that Devonian Ganoids are, some of them, wholly or partly covered with large, immovable, bony plates (Figs. 308-211) ; others with smaller, rhomboidal, bony scales (Figs. 212, 213). .The former are called Placo-ganoids (plate-ganoids), or 294 HISTORICAL aHOLOQY. Placoderms (plate-skin); the latter, Lepido-ganoids (scale- ganoids). Now/ the Placo-ganoids are characteristic of the Devonian, and the largest Devonian iishes, such as Fis. 212. Fig. 214. Figs. 212-214. — Devonian fishes — Lepido-ganoids ; 212. Osteolepis. (After Nichol- son.) 213. Diplacantlius gracilis. (After NicliolsoD.) Sharks : 214. Cteuacan- thus vetustus, spine reduced. (After Newberry.) the Dinichthys and Aster olepis, belong to this family. The Lepido-ganoids continued after the Devonian, and are still represented by gar-fishes, etc. The sharks of the Devonian belong, all of them, to a family now almost extinct, called Cestracionts {sharp tool, referring to the spine). Affinities of Devonian Fishes. — There are no living representatives of the Placo-ganoids, but there are such of the Lepido-ganoids. We herewith give figures of those modern fishes which are most like the Devonian fishes. PALEOZOIC ROCKS AND ERA. 295 The first is an Australian fresh-water fish, the recently discoTered Ceratodus (horn-tooth) (Pig. 215). The sec- ond, Lepidosiren (scale-siren), is a very curious animal, intermediate between fish and reptile, found in South America and Africa (Fig. 216). The third is the gar- fish, Polypterus (many fins), from the Nile (Fig. 317). .The fourth is the only living representative of cestraciont sharks — the Cestracion of Australian seas (Fig. 218). Bearing' on Evolution. — It is a curious fact that these fishes, which are most nearly allied to Devonian Fig. 215. Fie. 216. Figs. 215, 216. — Nearest living allies of Devonian fishes : 215. CeratodoB Fosterii, x ^. (After Gunther.) 216. Lepidosiren. fishes, are by no means low in the scale, but, on the coutrary, are, in some respects at least, very high. But one thing is very noteworthy, viz., that they all have amphibian characters united with fish characters — they are all connecting links between fish and amphi- Man. For example, it is seen that the vertebral col- umn in these, and still more in their Devonian allies, runs far into, often to the end of, the tail-fin. The tail-fin is vertebrated. The tail vertebrae are finned on the sides. This is universal in Devonian fishes. Again, 296 HISTORICAL GEOLO&Y. it is observed that in many the paired fins are curiously formed — they are a sort of livabs fringed with fin. Now, a large number of Devonian fishes (Fig. 313) have this style of paired fins. In the third place, all the living Pig. sir. Fig. 218. Pigs. 217, 218.— Nearest living allies of Devonian fishes : 217. Polypteras. tracion Phillippi (a living cestraciont from Australia). 218. Ces- Ganoids given above (Mgs. 215, 217) have a more or less perfect lung, and supplement their water-breathing with air-breathing, in the manner of some amphibian reptiles. It is almost certain that the same was true of Devonian Ganoids. And yet, with all these reptilian characters, all Devonian fishes had cartilaginous skeletons like the embryos of Teleosts. We wish now to take advantage of these facts to an- nounce a very general law. The first introduced exam- ples of any family, order, or class, are not typical forms of that family, order, or class, but intermediate forms or connecting links with other families, orders, etc. Prom such intermediate forms or connecting links have been PALEUZOIG ROCKS AND ERA. 297 afterward deTeloped the more typical forms. To illus- trate : The first fishes were not typical fishes, but con- necting links betwean fish and amphibian, and from this intermediate form, as from a trunk, true fishes and amphibians were afterward separated and developed as branches. Such intermediate forms we shall hereafter call generalized forms, and the more typical forms into which they seem to be afterward developed, specialized forms. We shall find many illustrations of this law as we proceed. Apparent Suddenness of the Appearance of Fishes. — At a certain time fishes seem suddenly to appear, as if they came without progenitors. But we must re- member that the very lowest forms of fishes have neither bony skeleton nor scales, and their remains are not likely to be preserved. We may yet find evidences of such far down in the Silurian. Nevertheless, there can be little doubt that conditions were favorable for the development of fishes about the beginning of the Devonian, and there- fore the steps of development were exceptionally rapid at that time. Section IV. — Carboniferous System. Age of Acro- GENS AND Amphibians. Subdivisions. — The Carboniferous age is subdivided into three periods : 1. Sui-carioniferous ; 2. Carionifer- ous proper, or coal-measures j 3. Permian. The first may be regarded as the preparation, the second the culmina- tion, and the third the transition to the Mesozoic. The whole carboniferous strata in Nova Scotia is 16,000 feet thick, in Wales 14,000 feet, in Pennsylvania 9,000 feet. The sub-carboniferous strata are mostly limestones ; those of the coal-measures mostly, thougli not wholly, sands and clays. The sub-carboniferous are marine de- posits, the coal-measures mainly fresh-water deposits. 298 HISTORICAL GEOLO&Y. The fossils of the former are, therefore, marine animals ; those of the latter mainly land-plants, and fresh-water and land animals. In both Europe and America the sub- ■ carboniferous underlies the coal-measures and outcrops around, and thus forms a penumbral margin about the black areas representing coal-fields on geological maps (see Fig. 169). After this brief comparison and contrast, we shall now concentrate our attention on the coal-measures, because all the characteristics of the Carboniferous age culminate there. In speaking of the fauna, however, we shall take the two together. The Permian will be treated as a transition to the Mesozoic. Carioniferous Proper — Rock-system, or Coal-Measures. Name. — The Carboniferous period is but one of three periods of the Carboniferous age. The Carboniferous age is but one of the three ages of the Paleozoic era. The Paleozoic era is but one of the four great eras, exclusive of the present. The Carboniferous period, therefore, is but a small fraction, certainly not more than one twentieth to one thirtieth of the recorded history of the earth. Yet, during this period were accumulated, and in its strata were preserved, and are now found, nine tenths of all the coal used by man. The name carboniferous, for the period, and coal-measures, for the strata, is surely, there- fore, appropriate. Thickness of the Strata. — Although so small a por- tion of the whole strata of the earth, these coal-measures are often, locally, of great thickness. In Nova Scotia the coal-measures, exclusive of the sub-carboniferous, are •14,000 feet thick, in Wales 12,000, in Arkansas 35,000 (Branner), and in West Virginia 5,000. Mode of Occurrence of Coal. — Such being the thick- ness, it is evident that but a small portion is coal. In PALEOZOIC ROOKS AND ERA. 299 Sh Sh ^^^p fact, the coal-measures consist of alternations of sand- stones, shales, and limestones, like other formations ; but interbedded with these are also seams of coal and beds of iron-ore. These five kinds of strata alternate with each other, and are each repeated many times, but without any regular order, as shown in Fig. 219, which is an ideal column from a coal-field. Thus, the strata of a coal-field may be likened to a ream of sheets of five colors, but arranged without order. Only this may be said, that beneath eyery coal-seam there is al- ways a thin layer of clay, called the under- day, and above is usually, but not always, a shale, called the black shale or roof- sJiale. It is a rich coal-measure in which we find one foot of coal for fifty feet of rock. Subsequent Changes. — The strata of coal-measures, like all other strata, were horizontal when first laid down ; but, like other strata also, they have been elevated, and tilted and folded and crumpled and broken and faulted, especially in moun- tain-regions. And in all cases, whether horizontal (Fig. 321) or folded (Fig. 220), they have been largely carried away by erosion, and the strata left outcropping on the surface (Figs. 220, 221), and often in isolated patches. Since coal-seams, like other strata, are broken and faulted, it is very important to remember the law of slip mentioned on page 232. Thickness and Number of Seams. — The thickness of seams varies from a few inches to many yards. The mammoth seam of Pennsylvania is over one hundred feet thick. The best thickness for easy working is about six Fig. 219.— Ideal sec- tion, showing alter- nation of different kinds of strata : Ss^ sandstone ; Sh, sliale ; /, limestone ; i, iron ; and c, coal. ;iO0 HISTORICAL GEOLOGY. to ten feet. The number of seams in a single field may be a hundred or more, and their aggregate thickness may Fig. 820.— Panther Creek and Summit Hill traverse. (After Daddow.) 3^ j:.-.- ■-rnnt---.-.'-K-lr=::» ^-l-, .|iL-----l.l - 1 ; ! Fig. 231.— Illinois coal-field. (After Daddow.) be, in some cases, one hundred to one hundred and fifty feet of solid coal. Coal-Fields of the United States. — In the map on page 373 the coal-fields of the United States belonging to this period are represented in black. It is seen that there are four of these : 1. The AppalacJiian coal-field, probably the richest in the world. In a general way it may be said to cover the western slope of the Appalachian chain from Pennsylvania southward. It covers an area of 60,000 square miles. 3. The cetitral coal-field. This covers nearly the whole of Illinois, the western portion of Indiana, and northwestern Kentucky, and its area is 47,000 square miles. 3. The great Western coal-field. This cov- ers southern Iowa, northwestern Missouri, eastern Kan- sas, the Indian Territory, western Arkansas, and north- ern Texas. Its area is no less than 78,000 square miles. 4. The Michigan coal-field. This occupies an area of PALEOZOIC ROCKS AND ERA. 301 6,700 square miles in the center of the Michigan Peninsula. Besides these, there is a small area of coal of little value in Ehode Island, and a fine coal-field of 18,000 square miles accessible to us in Nova Scotia. Of the 192,300 square miles of coal within the limits of the United States, 120,000 square miles are estimated as workable. It may be said with confidence that there is no country in the world so liberally supplied with this great agent of modem civilization as our own. Appalachian Central.'. Great Western. Michigan Rhode Island. . , Nova Scotia. Total 60,000 47,000 78,000 6,700 500 193,200 18,000 210,200 Origin of Goal and of its Varieties. There can be no doubt that coal is of vegetable origin. All portions of a coal-seam, even the most structureless to the naked eye, when properly prepared, reveal their vegetable structure to the microscope (Figs. 222, 223). Fig. 833.- -Section of anthracite : a, natural size ; b and c, magnified. (After Bailey.) ^^ iV D O Ko. 323.— Vegetable stnicture in coal. (After Dawson.) Varieties of Coal. — Assuming the vegetable origin of coal, how do we account for the varieties ? These varie- ties are of three kinds : 1. Varieties depending on degrees 302 HISTORICAL GEOLOGY. of purity ; 2. On degrees of bituminization ; 3. On the relative proportion of fixed and volatile matter. 1. Varieties depending: on Degrees of Purity. — Goal consists of combustible and incombustible matter, or ash. The combustible matter is organic, the ash min- eral. Now, the relative proportion of these varies in every degree. The purest coal may contain only 1 to 3 per cent, of ash ; but coal may contain 5 to 10 per cent., 20 to 30 per cent., 50 to 60 per cent., and so on to 90, 95, 99 per cent. ash. If a coal contains not more than 5 per cent, ash, it is probably pure, i. e., its ash is wholly due to ash of original vegetable matter ; but if it contains more than 10 per cent., it is certainly impure, the excess being due to mud deposited with the vegetable matter. 2. Varieties depending' on the Degrees of Bitu- minization. — Coal may be pure, and yet imperfectly bituminized. Such are lignites, irown coal, and the like. This depends mainly on age, the oldest coals being most completely changed. 3. Varieties depending' on the Relative Propor- tion of Fixed and Volatile Matter. — In pure and per- fect coal there are still varieties depending on the relative amount of fixed carbon and volatile hydrocarbon, and it is mainly this which produces the varieties of good coal, and determines its various uses. If the coal contains only 5 to 10 per cent, volatile matter, it is called anthracite, which is a hard, lustrous variety, breaking with a con- choidal fracture, and burning with very little blaze,' but great heat. If it contains 15 to 20 per cent, of volatile matter, it is called semi-bituminous, or steam-coal, because of its excellence in rapid formation of steam. It burns with a long blaze, but does not cahe. If it contains 30 to 40 per cent., it is ordinary bituminous caking coal ; if 50 per cent., or more, highly bituminous, fat, or fusing coal. In-this series we might well put graphite, or plum- bago, above anthracite ; for graphite consists of carbon PALJJOZOIG ROGKS AND ERA. 303 without any. volatile matter, and, although it is not called coal, because incombustible, yet it is but the last term in the above series of varieties. Cause of these Varieties — Vegetable matter decay- ing out of contact with air, i. e., beneath water or buried in mud, loses a large portion of its material in the form of gases (CO,, CH„ and H,0). These (00, and CHJ are the gases -which escape in bubbles when we stir the bottom of a stagnant pool in which plants are growing. They are also the gases which are constantly escaping in every coal-mine, and form the deadly choke-damp and the still more dreaded ^^re-ifam^ of the mines. Now, the relative proportion in which these are given off determines most of the above varieties. Anthracite and graphite may be regarded as metamor- phic coals. The reasons for so thinking are mainly the following : 1. Coal is often made locally anthraeitic by a lava-flow or dike. 2. In the same coal-field, wherever the strata are crumpled and metamorphic, as in eastern Pennsylvania, the coals are anthraeitic ; and where the strata are fiat-lying and unchanged, as in Ohio, the coal is bituminous. . , / Plants of the Coal. In no other strata have the remains of plants been found in so great abundance and variety as in the coal- measures. We could expect nothing else when we re- member that a coal-seam is a mass of vegetable matter, and that, on account of their economic value, these seams are continually explored. The remains of plants are found in the form of leaves, flattened stems and branches, and sometimes fruits, in connection with the black roof- sJiale J and as stumps and roots, in connection with the under-day or floor of the seams. Principal Kinds. — The plants belong mainly to four or five great orders, viz., Conifers and probably Cycads, 304 HISTORICAL &EOLOGY. among gymnosperms, and Ferns, Club-mosses, and Equi- setcB, among yascular cryptogams. These orders were anticipated in the Devonian, but culminate here. 1. Conifers and Cycads. — These are found as leafy '^s^«>« Fio. S24.- -Arancarites gracilis, rednced. (After Dawaon.) Fig, .— Cordaltes. (Beetored hj Dawson.) Pio. 326. Fig. sar. Fia. 828. ■yiGS. 236-228.— Fruits of coal-plants, probably conifers : Cardiocarpon. (Aftar Newberry and Dawson.) PALEOZOIC HOCKS AND ERA. 305 branches (Fig. 224), as scattered leaves, like those in the restored tree (Fig. 225), as nut-like fruits ("Pigs. 226-228), near the top of the coal-seams, and sometimes as drift- logs in the sandstones, interstratifled with the coal. The trunks are known to be conifers by the microscopic structure of the wood, the cells of which are marked with circular disks on lon- gitudinal section (Fig. 229), and on cross-section the wood is destitute of pores. !N'ow, what kind of coni- fers have such leaves and fruit as these ? None but the yew family. All of these have plum-like fruit with nut-like seeds, and many of them have broad leaves (Fig. 230). The cordaites (Fig. 225) has been found with trunk sixty to seventy feet Fig. 229. — Longitudinal section of wood of a living conifer, magnified. Fio. 230.— Living broad-leaved yews. Lb Conte, Gbol. 20 306 HISTORICAL GEOLOGY. long, crowned with broad leaves, and with a spike of fruit. It is probably a Cyead, or else a broad-leaved conifer like the ginkgo. 2. Ferns. — These are the most abundant, but not the largest, plants of the coal. About one half of all the known species of coal-plants are ferns. They are often beautifully preserved, large, complex fronds spread out and pressed, as if between the leaves of a botanist's her- barium, with even the microscopic veining of leaflets visible. They are known to be ferns — 1. By their large, complex fonds (Mg. 331). 2. By the peculiar veining of Fis. 231. Fis. 232. Pia. 233. Figs. 231-2.33.— Coal-fems: 231. Megaphyton, a coal-fern restored. (After Dawson.) 232. Oallipteris Sullivanti. (After Lesqaereux.) 233. Pecopteris Strongii. (After Lesquereux.) the leaves, characteristic of ferns (Fig. 233). 3. By the rows of spore-cases on the under surface of the leaves (Pig. 234). 4. In the case of tree-ferns, by ragged, ovoid marks, leaf-scars left by the fallen fronds. We give a few figures of ferns of the American and French coal-measures. The remaining orders, viz., Lycopods (or club-mosses) and Equisetm (horse-tails or scouring-rushes), are still more important, for two reasons : 1. They formed the principal mass of the coal. 2. They were very remarkable examples of generalized types or connecting links, and PALEOZOIC ROCKS AND ERA. 30? Fio, Dactylothe. ca dentata. (After Zeiler.) a, spore case enlarged. possess a high interest on that account. We shall treat of them under three heads, viz., Lepidodendrids, Sigilla- rids, and Calamites. 1. Lepidodendrids. — Every part of these has been found — roots, stems, branches clothed with leaves and tipped with fruit. They may be restored, therefore, with some degree of confi- dence. Imagine, then, a trunk two, three, or even four feet in diameter at its base where it joins the wide-spread- ing roots; marked with regular rhomboidal figures, which are the leaf-scars (Fig. 236) ; branching widely, but not profusely ; the great branches, clothed with scale-like or needle- like leaves, stretching aloft, like uplifted hairy arms, to the height of fifty or sixty feet, and terminating in scaly cones like club-mosses. The most common findings are flattened stems with beautiful rhomboidal markings (Pig. 236), looking much like rhomboidal scales of a ganoid fish ; hence the name Lepido- dendron, or scale-tree. There can be no doubt that the Lepidodendron was a lyco- pod, or club-moss ; but its inter- nal structure, as well as its great size (club-mosses are now but a few inches, or, at most, a few feet high), ally it strongly with conifers. We may regard it, therefore, as a lycopod, with characters connecting it with conifers. Pig. 235.— Eestoration of a Lepl dodendron. (By Dawson.) 308 HISTORICAL GEOLOGY. 2. Sigillarids. — The family name is taken from the type genus, Sigillaria. It includes Sigillaria and Sigil- larialike plants. The name Sigillaria is taken from the Fio. 236.— Lepidodendron Fig. 837. — Sigillarids : Sigil- Fio. 238.— Eestora- modulatum. (After Les- laria reticulata. (After Les- tion of Sigillaria. quereux.) querenx.) (By Dawson.) seallike markings {sigilla, a seal) left on the trnnk by the falling leaves (Fig. 337). They were the largest of all the coal-trees. Root, stem, branches, and leaves have been found. From these it is possible to reconstruct the general appearance of the tree. Imagine, then, a tree four or five feet in diameter at the base> with widely spreading roots ; the trunk regularly fluted like a Corin- thian column, and ornamented with vertical rows of seal- like impressions (leaf-scars), and towering to the height of one hundred to one hundred and fifty feet ; the top branchless, or else with only a few large branches clothed with grasslike or yuccalike leaves. The fruit is not PALEOZOTO BOOKS AMI ERA. 309 The general appearance is given known with certainty. in Fig. 238. 3. Calamites — These are so named from their jointed, reed-like appearance {cala- mus, a reed). They are usu- ally found in the form of flat- tenedj jointed, and striated stems. They may he de- scribed as follows: Imagine a straight, hollow, jointed, tapering stem, one to two feet in diameter, and twenty, thirty, or forty feet high, ter- minating in a compact, cone- like fruit (Fig. 240), the joints striated, but the grooves in- terrupted at the joints by whorls of scale-like leaves, or else whorls of jointed, thread- like branches (Fig. 239) about the joints. From the basal joints come out thread-like roots. Fig. 239 is a restora- tion of its appearance. N"ow, all that we have said applies, word for word, to equisetae, or horse-tails, except the great size. But equi- setse of the present day are small, rushlike or reedlike plants. Moreover, the internal structure of Calamites shows a close relation with gymnosperms, probably coni- fers. Conclusion. — The general conclusion, then, is that all the plants of the Coal, but especially the Lepidodendrids, the Sigillarids, and the Calamites, were remarkable gen- eralized types, connecting classes and orders now widely separated from each other — viz., the higher or vasculal cryptogams with the lowest or gymnospermous pheno' Fia. 839.— Restoration Fig. 240.— Frnlt of a Calamite. (Af- of Calamite. ter Dawson.) (After Heer.) 310 HISTORICAL QEOLO&Y. gams. The two branches of the tree of life, cryptogam and phenogam, so widely separated now, when traced downward, approach and almost meet here in the Coal period. Mode of Accumulation of Goal. There has been much dispute on this subject, and it is still obscure. There are some things, however, which are reasonably certain. We shall give what is most certain, in the form of three propositions : 1. Coal lias been accumulated In the presence of water. — This is indicated (a) by the nature of the plants, which are mostly swamp-plants ; (5) by the inter- stratified sands and clays, which were, of course, deposited in water ; but, more than all (c) by the preservation of the vegetable matter, which would have entirely disinte- grated and passed ofE, as CO, and H,0, unless completely water-soaked. %. Goal has been formed by accumulation of vegetable matter "in place " — ^i. e., where the plants grew — ^by annual decay of generation after generation, as we see now in peat-bogs and peat-swamps ; and not by accumulation hy driftage, as we see in rafts. The evi- dence of this is complete. We shall only mention one fact, which is demonstrative : The under-day of every coal is full of stumps and roots in position as they grew. Every under-clay is an old fossil forest-ground, or rather swamp-ground. Imagine, then, an old coal-swamp, with its clay bot- tom full of dead stumps and roots, with its accumulation many feet deep of pure peat, with its surface covered with late-fallen leaves, broken branches, and prostrate trunks, and the still growing vegetation shading all. Now, imagine this overwhelmed and buried by sediments, subjected to powerful pressure and slow change, and we have all the phenomena of a coal-seam, with its under- PALEOZOIC ROOKS AND ERA. 311 clay full of roots and its roof-shale full of impressions of leaves and flattened branches, etc. 3. Coal has been accumulated at the mouths of rivers, and therefore subject to overflows and deposits of mud by the river, and to occasional incursions by the sea. This is proved by the alternation of river-sand and clay with marine deposits of limestone. It may be diflaoult to put these three propositions to- gether and form a clear picture of the precise manner of accumulation, and therefore there is still a large field for the play of fancy. Estimate of Length of the Coal Period. If the sands and clays of a coal-field have been accu- mulated by river-deposit, then we have a means of making a rough approximate estimate of the time embraced by the Coal period. It is true, agencies may have acted then at a difEerent rate from now, but our estimate will be liberal. For this purpose we take the Nova Scotia coal-field, because the evidence of river-deposit is very strong in every part. It has been estimated that there were not less than 54,000 cubic miles of river sediment in the original field. Now, the Mississippi Eiver at present ac- cumulates one twentieth cubic mile per annum, and would therefore take twenty years to accumulate one cubic mile, and 1,080,000 years to accumulate 54,000 cubic miles. But, as already said (page 298), the Coal period is but a small fraction, certainly not more than one twentieth to one thirtieth, of the recorded history of the earth. Therefore, this recorded history can not be less than twenty to thirty millions of years. It is proba- bly much more. We only give this estimate in order to accustom the mind to the great periods of time with which geology deals. 313 HISTORICAL GECLOGT. Physical Geography and Climate of the Coal Period. Physical Greography. — The Paleozoic era was a time of gradual growth of the continent from the Archsean nucleus by successive additions, first of the Silurian, then of the Devonian, and now of the Carboniferous area. During Carboniferous times the form of the American Continent probably did not differ greatly from that repre- sented on page 349 (Fig. 303) as the Cretaceous conti- nent, except that the areas of coal-measures were not then permanent land, but were in an uncertain state, sometimes swamp-land, sometimes covered with river- sediment, sometimes covered by the sea. Although the continent had greatly grown, still we must imagine it as small and low compared with its present state. The same is probably true of other continents. Climate. — The climate was probably warm, very moist, very uniform, and the air loaded with CO^. The greater warmth and uniformity are shown by the fact that the plants are those of a tropical climate. Tree-ferns, arbo- rescent lycopods, etc., grew then with ultra-tropical lux- uriance, not only in now temperate regions, but in Mel- ville Island and Grinnell Land, 78°-80° north latitude. The prevalence of the great coal-swamps and the charac- ter of the plants are sufP.cient evidence of greater humid- ity. Finally, when we remember that the whole of the coal in the world represents so much carbon taken from the atmosphere, as CO, with return of the oxygen, we shall be convinced that the quantity of CO, in the air was greater and of oxygen less than now. It is probable, therefore, that in early geological times there were more moisture and CO, and less oxygen than, now. . This would make a paradise for plants, especially the lower orders, but would be unsuitable for air-breath- ing animals. There has been throughout all geological PALEOZOIC ROCKS AND ERA. 313 times a gradual purification of the air of its superabun- dant moisture by increase of the size and height of con- tinents, and of its superabundant 00^ by its withdrawal in many ways, but during the Coal period especially by the growth of plants and the preservation of the carbon as coal. In this process not only was the CO, removed, but oxygen restored, and thus was the air prepared for the use of air-breathing, hot-blooded animals, such as birds and mammals, which were accordingly introduced soon afterward. u.- ~> ^ Petroleum and Bitumen. V, "We take up these here, not because they are peculiar to the coal-measures, for such is not the fact, but because they seem to have been formed from organic matter by a process similar to that of coal, and also because some think they are actually formed from coal by distillation. This, however, is not probable. If bituminous coal, or any organic matter, be heated red-hot, out of contact with air, the volatile matters are driven off, broken up, and recombined, and may be col- lected in a great variety of forms of hydrocarbons — some solid, as coal-pitcJi ; some tarry, as coal-tar ; some liquid, as coal-oil; some volatile, as coal-naphtha; and some gaseous, as coal-gas. Now, a somewhat similar series of hydrocarbons is found in the earth and issuing on its sur- face : some solid, as asphalt, Albertite, Orahamite, etc. ; some tarry, as bitumen ; some liquid, as petroleum ; some volatile, as rock-naphtha; some gaseous, as the gas of burning -springs. It is almost certain also that these are of organic origin. Mode of Occutrence. — Petroleum occurs in the strata much as water does, and the two are often associated. Like water, and with water, it is found in porous and fissured strata, such as sandstones and limestones, when these are covered with a stratum of impermeable shale. 314 HISTORICAL CfEOLOGY, Like water, and with water, it often oozes on the surface as hillside springs. With water, it collects in fissures and cavities of all kinds, from which, through artesian wells, it issues, in some cases, in great quantities as fountains. But, unlike water, there is no great, continuous, peren- nial supply ; and also, unlike water, the force by which it spouts is not by hydrostatic pressure alone, but hydro- static pressure transmitted through the elastic force of compressed gas always associated with the oil. As gas, oil, and water are nearly always found together, these arrange themselves in the order of their relative specific gravities ; and therefore in a flowing well the water usu- ally appears only after the gas and oil are exhausted. The flow of oil wells is hot perennial like water, because the oil is not continually re-supplied. The accumulation of ages is now being exhausted with a rapidity propor- tioned to the abundance of the flow. What is true of oil is much more true of gas. A gas well is necessarily short-lived. Age of Petroleum-bearing' Strata. — Petroleum has been found in strata of nearly all ages, but under the two conditions of local abundance of the organic matter from which this substance is formed, and the absence of metamorphism, which always changes it into asphalt. At one time it was supposed to be characteristic of newer rocks, having been found in foreign localities, mostly in Tertiary strata. But in the Eastern United States it is confined to the Paleozoic rocks, while in California it is again found only in the Tertiary. The great petroleum area of the Eastern United States is the Paleozoic basin. In this basin it is found on sev- eral horizons, but always ielow the coal-measures. The most celebrated, viz., the Pennsylvania horizon, is in the Upper Devonian. The Canada horizon is in the lowest Devonian. In West Virginia it is in the sub-Oarbonifer- PALi:OZOIC HOGKS A^D ERA. 315 ous. In OKio it is in the Devonian and in the Silurian, especially the latter. In California it is in the Miocene Tertiary of the Coast Range. Origin of Petroleum. — It is probahle that petroleum was formed by a change of organic matter, somewhat similar to that which makes coal, but from a different kind of organic matter, and under different conditions. Land-plants, in the presence of fresh water, form coal ; while marine plants, and sometimes lower animals, in the presence of salt water, form petroleum, bitumen, etc. It has been observed that petroleum is often found in connection with salt. Origin of Varieties. — But, however formed in the first instance, there is no doubt that the different varieties or physical conditions are formed from each other by the passing away of gaseous hydrocarbon. In this manner light oil changes into heavy oil, and this into bitumen, and finally into asphalt. Thus there are two series de- rived from organic matter, the coal series and the petro- leum series. By successive changes, coal passes from fat-coals to bituminous, then semi-bituminous, then an- thracite, and finally graphite ; petroleum from light oil to heavy oil, then bitumen, asphalt, jet, and possibly diamond. But the origin of diamond is uncertain. Fauna of the Carboniferous Age. As already stated, we shall take up the fauna of the sub-Carboniferous and Carboniferous together ; only let it be remembered that the land and fresh-water animals are from the coal-measures, especially the vertebrates, and the marine animals are mostly from the sub-Car- boniferous. We shall touch only the most prominent points. We have nothing characteristic to add about corals, but only draw attention here to an exceedingly curious 316 HISTORICAL &EOLO&Y. and characteristic form of coral-making Bryozoan, called from its perfect screw-like form, Archimedes (Fig. 241). This abundant and easily recognized form is wholly characteristic of the sub-Carboniferous By studying the diagram (Pig. 243) the main facts regarding Echinoderms may be easily re- membered. As before (page 279), the lower shaded part represents stemmed, and the up- per unshaded, the free forms. The Cystids, it is seen, are confined to the Silurian, the Blastids commence in the Silurian, continue through the Devonian and Carboniferous, and perish ; while the Crinids continue until now. The Asteroids commence in the Lower Silu- rian, the Echinoids in the Carboniferous, and both continue until now — the species, of course, changing. As Blastids are yery abun- dant in the sub-Carboniferous, we give a fig- ure (Fig. 242). Concerning Mollusca, we touch two points : 1. Fresh-water and land shells, which were in- troduced in the Devonian, are more abundant (Figs. 244, 245). 2. The Goniatites, first in- troduced in the Devonian, are also more numerous here (Fig. 246). Concerning Crustacea, also two points : 1. While Trilobites continue under new forms, ready to perish at the end of this period, Lmiuloids, or horseshoe crabs, a higher type, are here introduced (Fig. 247). The transition from Trilobites to Limuloids may be quite perfectly traced. 2. True typi- cal crustaceans of the long-tailed kind (Ma- j.,q. 242._BiaB- crourans), such as shrimps and the like, were ^d : pentre- first introduced here (Fig. 248). ™i^^ ^Afiac Insects, which first appeared in the Devo- Haii.) Fig. 241. — Archimedes laxa. (After Hall.) PALEOZOIC BOCKS AND ERA. 317 nian in connection with land vegetation, as might be ex- pected, are much more abundant and in greater variety ir SILURIAN, aEV^,cAua? PALEOZOICl N E O Z O I C Fig. S43. Fis. 214.— Dawsonella Meekii. Fia. 215.— Anthracopupa OhioenglB. (After Bradley.) (After WMtfleld.) Via. 246. — Carboniferons goniatites : Goniatites crenistria (Bnropean) ; a, side-view ; b, end-view. Flo. 847.— Carboniferous crustacean : Eu- proBps Danse. (After Meek and Worthen.) the coal-measures. There are spiders, scorpions, centipeds, cockroaches, dragon-flies, and beetles (Figs. 249, 250). It is well to observe that the highest orders of insects. 318 HISTORICAL GEOLO&Y. flower-loving, honey-loving, and social, such as flies, but- terflies, bees, and ants {Dipters, Lepidopters, and Hyme- Fig. 248. — Carboniferous crustacean ; Anthrapalsemon gracilis. (After Meek and Worthen.) FlQ. 249. — Carbonif erons insect ; Blatta madera3, wing • casee, (After Lesqnerenx.) nopfers), are not yet found, because there are not yet any fiowering plants. Fishes. — We have little to add here to what has already been said under the Devo- nian. The same kinds of fishes — viz.. Ganoids and Placoids — still pre- vail. Of the Ganoids, however, the Placo- derms passed away with the Devonian, but the Lepido- ganoids continue, and some of them become still more reptilian. We note also an advance in the Placoids, in that an intermediate form (the Hybodonts) between the Cestracionts and true sharks [Squalodonts) here appears. Fig. 350.— Carboniferous insects : Zylobius sigillariae. (After Dawson.) a, anterior ; J, posterior portion, enlarged. PALEOZOIC ROCKS AND ERA. 319 Amphibians — The introduction of amphibians must be regarded as a great step in the progress of life ; for they are the first true land vertebrates and air-breathing vertebrates. Yet we must remember, on the one hand, that amphibians, as their name implies, all of them at some period of their life, some of them permanently, breathe both air and water — both by gills and by lungs ; and on the other hand we must remember that Ganoid fishes also supplement their gill-breathing by lung- breathing. The amphibians are intermediate between fishes and true reptiles. They are represented now by frogs, toads, newts, etc. Now, in the Carboniferous, and long afterward, am- phibians were very different from any of those mentioned as still living. They belonged to a peculiar order now long extinct, called Labyrinthodonts, from the labyrin- thine structure of their teeth (Fig. 252). All the living amphibians are small creatures ; these were often of huge size. All the living kinds have soft, moist skin ; these were partly covered with large, ganoidal plates. The early Ganoids, too, had the same labyrin- thine structure of the teeth, thoughless „ „„ „, , , ., ^ ,,, ° Pio.251.— structure of a ganoid tooth. (After marked (Fig. 351). In Agaseiz.) fact, the transition from the reptilian Ganoids to the ganoidlike amphibians of the coal-measures is so gradual that it is difficult in some cases to say whether some of these are amphibian reptile or ganoid fish (Fig. 353). Amphibians seem to have been very abundant in the coal-measures — some snakelike forms, with very small or no feet, some lizardlike forms, some fishlike forms, and some huge crocodilian forms, but not with croco- dilian afiBnities. These huge forms were, however, more 320 HISTORICAL eEOLOGY. common later^ i. e., in the Triassic. We will describe only two examples : The Arcliegosaurus {primeval saurian) (Fig. 353) was an animal two to three feet long, with head and body much like a ganoid fish, and covered with ganoid plates and scales. It had probably per- manent gills as well as lungs, and its legs were little more than legged fins, such as are found in some ganoids, and wholly unadapted for locomo- tion on land. It was a re- markable connecting link be- tween ganoid fish and laby- rinthodont amphibian. The Dendrerpeton {tree- reptile) was so called because first discovered (by Dawson) in the hollow stump of a sigil- laria tree. It was of lizard-like form, and about two feet long It is a curious fact that these hollow stumps of Fig. 252. — Section of portion of a tootli of a labyrinthodont. Fig. 253.— Archegoeaurus ; A, plates ; B, section of tooth. sigillaria filled with sandstone (Fig. 254) are very rich in fossils, e. g., skeletons of amphibians, remains of in- sects, and shells of land mollusca. The sigillaria tree was very soft and spongy, but was covered with a hard bark. We may easily picture to ourselves the conditions under PALEOZOIC ROCKS AND BRA. 321 which these remains were entombed. Imagine, then, a large sigillaria tree on the borders of a coal-swamp, rotted down to a hollow stump. A flood then carried thither floating insects, shells, and carcasses of amphibians, which lodged in the hollow stump, and were covered up with sand. The hollow stump changed to coal, the sand to ^^^a sandstone, and the animal re- mains to fossils. The Tery earliest amphibian pm. 254.-section ot toUow eigii- known is recognized by its laria stump, mied with sandstone. tracks (Pig. 256). These are f^*^^"— > found in the sub- Carboniferous of Pennsylvania, in a sand- stone marked with ripple-marks. The animal has been called Sauropus primmvus (primeval reptile-foot). It was evidently a large Labyrinthodont. Not only tracks and ripple-marks, but also rain-prints (Fig. 256) and sun- cracks, are common in the coal-measures. Fig. 256.— Slab of sandstone with sun-cracks a, and reptilian footprints 6, from coal-measures of Pennsylvania ; x \. Some General Observations on the Whole Paleo- zoic. — Before leaving this long and diversified era, we must look back and make some general observations. Progressive Change. — During the whole time we may observe a progressive change going on : 1. There was, as we have seen, a steady growth of the continent Le Conte, Geol. 21 322 HISTORICAL &EOLO&Y. from the Archaeaii nucleus. 2. There vas also a pro- gressive change in the constitution of the atmosphere, especially by removal of excess of water and 00,, fitting it for the introduction of higher animals. 3. In connec- tion with these physical changes, there were also progres- sive changes in life-forms.* Appalachian Kevolu- tiou. — Thus we see a slow, steady, progressive change during the era. But now, at the end, there occurred one of those great and rapid changes in physical geogra- phy and climate which mark the end of the eras, and fe.,i a corresponding sweeping change in the forms of life. The Appalachian chain was formed at this time, and is its monument, and therefore, by American geologists, it is fitly called the Appalachian Revolution. The place of the Appalachian chain during the Silurian and Devonian eras. was the marginal sea- bottom of the great interior Paleozoic Sea, receiving sediments until 30,000 feet were accumulated. During the Carboniferous it was sometimes an inland sea-bottom, sometimes a coal-marsh, and sometimes, perhaps, a lake, but always receiving sediment until 10,000 more feet were accumulated. Now, at last, it yielded to the ever-increas- ing lateral pressure, and was folded and crumbled with all its coal-beds, and swelled up into a great mountain- range. It has since been sculptured by erosion into its present forms. We have said that the change of life-forms produced * For a fuller account of this important point, the teacher is re- ferred to the larger work. Fig. 356.— Fossil rain-prints of \he Coal period. PALEOZOIC BOOKS AND ERA. 333 by this revolution was sweeping. When quiet and pros- perous times again commenced, in the Mesozoic, we find an entirely different condition of things. It is almost like a new world. "We must not imagine, however, that the change was absolutely sudden. The steps of change here were only more rapid, and the general unconformity and loss of record which occur here make it seem sudden. Transition to the Mesozoic Era. — Permian Period. We have seen (page 267) that the Paleozoic commenced after a great revolution. IsTow, the Paleozoic was closed also by a similar revolution. We have called this latter the Appalachian Eevolution, because this range was made at that time ; but it was a time of widespread oscillations, and, therefore, of great changes in physical geography and climate, marked by universal unconformity and by sweeping changes in life-forms. ISTow, as already seen on page 193, unconformity always means lost record at that place. Of the lost record between the Archsan and Paleozoic nothing has been certainly found, but of that between the Paleozoic and Mesozoic, certain leaves have been recovered. These are brought together and called the Permian. Some have allied the Permian with the Mesozoic under the name of Dyas. Others have allied it with the Paleozoic. The truth is, it ought to be regarded as a period of transition or of revolution between the two. Life System. — As might be expected the organisms of the Permian are mainly transitional. Paleozoic forms are passing away and Mesozoic forms are coming in. The very first reptiles are introduced here, but they had not yet attained supremacy. \y^/^^ J.' ! CHAPTER rV. MBSOZOIC EEA. AGE OF EBPTILB8. The Paleozoic era was very long and diversified. It consisted of three ages — ^the age of Invertebrates, the age of Fishes, and the age of Acrogens and Amphibians. The Mesozoic era, on the contrary, consists of but one age — ^the age of Reptiles. Never, in the history of the earth, were reptiles so abundant, of such size and variety, or so highly organized, as then. Characteristics of the Age. — The characteristics of this age are the culmination of the class of reptiles, and the class of cephalopod moUusks among animals, and of cycads among plants ; and the first introduction of mam- mals and birds, and, in the last part, of Teleost fishes and Dicotyledonous trees. The most striking characteristic is the culmination of reptiles, and this, therefore, gives it its name. Subdivisions. — The Mesozoic era and Reptilian age is divided into three periods f3. Cretaceous. — viz. : 1. Triassic, on ac- 3. Jurassic. count of its distinct three- 1. Triassic. fold division in Germany, where first well studied. 3. Jurassic, on account of its splendid development in the folded structure of the Jura Mountains. 3. Creta- ceous, on account of the chalk of England and France being one of its members. These three are very distinct periods in Europe, but in America the Trias and Juras are closely connected 334 Mh\St)ZOia ERA.~AGE OF REPTILES. 325 though very distinct from the Cretaceous ; so that, studied in America alone, it would be most natural to divide the whole age into two periods : 1. Jura- Trias ; and, 2. Cretaceous. Again, the Jura-Trias is much poorer in fossils in this country than in Europe ; so that, if we treated of American strata alone, we should give but a very imperfect picture of the times. Therefore, our j^lan will be to give a brief sketch of the Trias, and then of the Juras, taking illus- trations chiefly from foreign sources, and then a sketch of the Jura-Trias in America. The Cretaceous can be fully illustrated from American strata. Seotioit I. — Tbiassic Period. As already stated, the lowest Mesozoic (Triassic) is always, or nearly always, unconformable with the Coal. The line of break may be between the Triassic and the Permian, but more commonly between the Permian and the Coal. But the fossils of the Triassic are always very different from those of the Permian. The break in the life system is always greatest here. "We will neglect the subdivisions, and take up all together. Life System. — Although the revolution which closed the Paleozoic is passed, and comparative quiet again re- stored, yet it took some time for the old fullness of life to recover .itself. Mesozoic life, therefore, is compara- tively poor in the Triassic compared with the Jurassic and Cretaceous. "We will, therefore, touch very briefly on Triassic life. Tlie Cliang-e. — The most striking fact is the sweep- ing change in life-forms. All the old style corals are replaced by new style ; all the armless crinoids (Blastids and Cystids), the square-shouldered brachiopods, the orthoceratites and trilobites, the lepidodendrids, sigil- larids, and calamites — in a word, all that we found most 326 HISTORICAL QEOLO&Y. characteristic of the Paleozoic, are gone. They are re- placed by other and very different forms. Plants. — As the grand characteristic of the Coal pe- riod was the predominance of the vascular Cryptogams, so that of this period is the predominance of the next higher group of plants, viz., Gymnosperms, i. e.. Conifers and Cycads, especially Cycads (see diagram on page 359). Perns and Equisetse, however, still abounded, though of different genera from those of the Coal. But as the pe- culiar flora of the Mesozoic did not culminate until the Jurassic, we shall put off illustrations until that time. Animals. — Although Cystids and Blastids disappear with the Paleozoic, the Crinids are still represented by many beautiful new forms, with plumose arms, which, when expanded, must have presented a truly flower-like appearance, and their fossilized remains are therefore often called stone-lilies. One of these is shown in Fig. FiQ. 257.— Encrinus lilifonnja. Fig. 258.— Ceratites nodosus. 357, and on page 331 we give a similar form in expanded condition. The Goniatites have passed away. The Ammonite MESOZOIC ERA.— AGE OF REPTILES. 327 family is here represented by Ceratites. They are easily recognized, and entirely characteristic of the Triassic. The complexity of the suture is increased, as shown in Fig. 258. V Pis. 259.— Teeth of Triassic fishes ; Hybodus apicalis. (Af- ter Agassiz.) ' Fig. 260.— Mastodonsaurus Jaegeri ; Among fishes we find still only Ganoids and Placoids, but the Ganoids are assuming more and more the form of ordinary fishes (Teleosts), and the teeth of the Placoids are becoming more shark-like (Mg. 259). Fig. 261.— Triassic reptiles (alter Owen)— anomodonts and therodonts ; Dicynodon lacerticeps. 328 EISTORICAL QEOLO&Y. Amphibians. — Labyrinthodonts, introduced in the Coal, continue and culminate here (Fig. 260), and soon become extinct. Keptiles. — Eeptiles were introduced in the Permian but did not become dominant until the Mesozoic. Cer- tain forms, of which we shall speak hereafter, commence here, but culminate in the Jurassic. But there are also some curious transitional forms entirely characteristic of this period. The Anomodonts (lawless-toothed) were beaked like a turtle, and either toothless or else with long tusks only (Fig. 261), but crocodilian in form. The Therodonts (beast-toothed) were so called because their teeth were in three groups, corresponding to inci- sors, canines, and molars of mammals (Fig. 262). Both of these curious families had many char- acters allying them with the lowest mammals, i. e., Monotremes (Ornithorhynchus, Echidna, etc.), now found only in Australia. They have been fitly called, by Cope, Theromorpha (beast-like). These beast-like reptiles seem to have been introduced first in the Permian. Mammals. — If beast-like reptiles are found here, we might naturally expect also the lower forms of beasts themselves. In the uppermost Triassic, both of Europe and America, remains of small marsupial mammals have indeed been found ; but as only a few have been found, and these in the uppermost Triassic, almost passing into the Jurassic, and as similar remains are far more abun- dant in the Jurassic, we shall put off their description until that time. Fig. 262. — Lycosaurus. ME80Z0IC ERA.— AGE OF REPTILES. 339 No Mrds have been found. It may seem strange that mammals should have been introduced before birds ; but we find the explanation of this in the fact that birds are a sub-hranch of the reptilian ^.nch of the vertebrate stem. ^^^yl ^if^ Section' II.*==DTjrassic JfEEioD. Name. — These strata and the period they represent are called Jurassic, because of their splendid develop- ment in the folded structure of the Jura Mountains (Fig. 145, page 241) and their richness in fossils there. Kock-System. — In England the Jurassic has been subdivided into the Lias, the Oolite, and the Wealden; but we shall neglect these, and speak only of the whole together. Coal. — One point worthy of note here is the occurrence of coal. The Jurassic coal-fields are far smaller than those of the Carboniferous, but the mode of occurrence of the coal is much the same. Examples of such coal are the Yorkshire and Brora coal of Great Britain, and some of the coals of India and China ; also the coals of eastern Virginia and North Caro- lina. Of these last we shall speak again. Many Jurassic coals are of excellent quality, though the average is inferior to the coal of the Carboniferous. Plants. — -The. characteristic families of the Jurassic are Ferns, Conifers, and Cycads. Conifers and Cycads, especially Cycads, culminated in this period ; they are found in extreme abundance in connection with the Jurassic coal in the form of leaves, trunks, and roots. Some Jurassic plants and their living allies are shown in Figs. 263-266. Animals. The culmination of the characteristic animals of the Mesozoic, especially reptiles, occurred in this middle 330 BISTORIGAL GEOLO&Y. period. We shall touch very briefly all except the most important characteristic kinds. Fig. 263.— Zamia spiralis, a living cycad of Australia. Pig. 364.— Stem of cycadeoidea megalophylla. Crinoids, beautiful, plumose-armed, and lilylike, are abundant (Pig. 267) ; but so, also, are the free asteroids and echinoids (Fig. 268). The two kinds, stemmed and free, are evenly balanced. Bivalves are, of course, abundant and of characteristic forms, in this as in all geologi- cal times ; but we can only draw special attention to the oyster family (including Os- trea, OrypJiea, Trigonia, etc.), which were first introduced here (Figs. 269-271). Ammonites. — The Ammo- nite family were introduced first in the Devonian as Goniatites. These were replaced in the Tri- assic by Oeratites. The Am- monites proper, the highest type of the family, were intro- duced in the early Mesozoic, culminated here in the Juras- sic, continued through the Cretaceous, and died out at its Fig. 265.— Jurassic plants : Ptero- phyllum comptum (a cycad). ME80Z0IG JURA.— AGE OF REPTILES. 331 end. It is, therefore, characteristic of the Mesozoic. In the Jurassic they were of extreme abundance, and of all Fia. 266. — Jurassic plants — Conifers : Cone of a piiie. Fig. 288.— Clypens PlotU. 7' ■•"!•*%, FlG. 267. — Apiocrinites restored. (After Buckland.) Fig. 269 Fig. 270. lie. 271. Figs. 269-271.— Jurassic lamellibranchs of England : 269. Trigonia clavellata. 270. Ostrea Sowerbyi. 271. Ostrea Marphii 332 HISTORICAL OEOLOOY. sizeSj from half an inch to three feet in diameter. We give some figures of the most characteristic forms (Figs. 372-374). It is interesting to trace the gradual changes in the I'la. 272. Fia. 273. I'm. 274. « FiQS. 272-274.— Jurassic cephalopods— AmmoniteB : 272. Ammonites margaritanus. 273. Ammonites Jason : side-view. 374. Ammonites cordatus : o, side-view ; 6, showing suture. form of the suture in, shelled cephalopods. In the Silu- rian Orthoceratites the sutures were even; in the Devonian and Carboniferous G-oniatites they vi^ere angled ; in the Fig. 276. Figs. 275, 276.-275. Belemnites Owenii. 276. Belemnites unicanaliculatus. Triassic Ceratites they were scalloped ; finally, here in the Ammonites they were frilled in the most complex patterns. Belemnites. — Now, for the first time, we find the highest order of cephalopods, viz., the naked ones, allied to the squids, cuttle-fishes, etc. This order is represented in Jurassic times by a peculiar form, called Belemnites, MESOZOIC EMA.—AGE OF REPTILES. 333 from the curious, dartlike bone (Figs. 375, 276), which is often the only part found. Sometimes the soft parts have been found ; the ink-bag (Fig. 277) has been found so per- fect that good ink has been made of it, and the animal has even been drawn with its own fossil ink. Prom the various parts found it is possible to restore the animal with some confidence. In Kg. 278 we give such a restoration, and in Fig. 279 aliving squid for comparison. Crustaceans and Insects. — There is a steady development. Fig. ar7. Fig- 278. Pio. 279. Figs. 277-379.-277. Fossil ink-bagS of Beleranites. 378. Belemnite restored. 279. A living sqaid. during the Mesozoic, of crustaceans, toward the highest -form, viz., the crabs. This, however, was fairly attained only in the Cretaceous, though a spider-crab has been found in the Jurassic. Insects also are far more numerous and diversified (Figs. 280, 281) than heretofore, although even yet the highest forms, such as ants, bees, and butterflies, are not found. _^ There is little of importance to be noted in regard to Fishes, We therefore pass on to Reptiles. ^34 HISTORICAL GEOLOGY. Pig. 280. — Jurassic in- sects : Blattina for- mosa. (After Heer.) Reptiles. — These are the rulers of the age, and cul- minate in this period. We shall therefore dwell a little more fully on them. During the Jurassic there was a truly extraordinary development of this class, in number, size, variety, and degree of organization. They were rulers in every department of Ifature : rulers in the sea, in place of whales and sharks of to-day J rulers on the land, in place of beasts ; and rulers in the air, in place of birds. We shall take them up under the three heads indicated, viz.: 1. Ma- rine Saurians (Enaliosaurs). 3. Land Saurians (Dinosaurs). 3. Winged Sau- rians (Pterosaurs). The first were swim- ming, the second walking, the third fly- ing, animals. 1. Marine Saurians. — Among these we shall mention only the two most noted, viz.. Ichthyosaurus and Plesiosaurus. The Ichthyosaurus (fish-reptile) (Fig. 382) was a huge monster, thirty to forty feet long, with thick body, short neck, enormous head, eyes twelve to fifteen inches in diameter, and jaws set with hundreds of conical teeth. The limbs were paddles, suitable for swimming, not for walking. The powerful tail was expanded vertically into a fin at its extremity, and the bodies of the vertebrae were biconcave like those of a fish. Perfect skeletons of this animal have been found ; and even the impressions of its intes- tines, and the contents of its stomach, revealing the nature of its last meal, have been preserved. The Plesiosaurus (lizardlike) (Fig. 383) was a slenderer animal, with a very long neck, small head, short tail, long and powerful paddles, and fishlike vertebrae. 3. Dinosaurs, or Laud Saurians. — The hugest of FiQ. 281.— Glaphyrop- tera gracilis. (After Heer.) MESOZOIG ERA.— AGE OF REPTILES 335 reptiles — in fact, the hugest animals which have ever walked the earth — were of this order. They were also the most highly organized of reptiles ; for, if the marine saurians connected this class with fishes, the dinosaurs Via. 283.— JnrasBic reptiles — IchthyosauruB and Plesiosaurus : Ichthyosaurus com- munis, X iJj. connected it with the higher class of birds. Some of the characters connecting them with birds are tlie following: 1. Many of them had long, powerful hind-legs, large hip- bones, and strong sacrum, and very short and small fore-. Fia, 283.— Jurassic reptiles— Ichthyosaurus and Plesiosaurus : Plesiosaurus doli- chodeirus, restored, x ^. legs. These characters show that they walked mainly on their hind-legs, in the manner of birds. 3. Many of them, like some birds, had only three toes on the hind-feet, so that they made tracks which were bird-like. 3. There were peculiarities about their ankle-joints which were still more bird-like. 336 MISTOmOAL GMOLO&Y. Fig. 284. — Iguanodon BemissartenBis. (After MarBh.) The most noted of this order found in Europe are the Iguanodon and the Megalosaurus. The iguanodon (Iguana-tooth), judging from the size of its bones, was probably several times more bulky than the elephant ; and yet a perfect skeleton, recently found in Belgium (Fig. 284), shows that it walked on the hind-legs alone, supporting itself by its massive tail. The neck was long, flexible, and bird-like, and the jaws were beaked in front and set with herbivorous, iguanalike teeth (Pig. 285) behind. The megalosaur (great saurian) was not quite so large, but prob- ably still more formidable, since it was carnivorous. A restoration of a smaller allied form is given in Fig. 286. This also walked mainly on two Fia. 285.— Tooth of an Igua- nodon. MESOZOIC ERA.—AQE OF REPTILES. 337 legs. Still much larger animals of this order have been found in the United States, as we shall see further on. 3. Pterosaurs, or Winged Sauriaus. — These are per- haps the most extraordinary of all known animals. They Fig. 286.— Compsognathas, x A. (Restored by Marsh.) combined the stout body with keeled breastbone, the long, flexible neck and beaklike jaws of a bird, with the long arms and membranous flying- web of a bat and the essen- tial characters of a reptile. In some cases they had a short, aborted tail, like a bird, but in others a long tail, with yertical expansion at the tip, which was used as a rudder in flying (Fig. 287). The pterosaurs varied in size from two or three feet to eighteen or twenty feet from tip to tip of the extended wings. Birds. — We have seen that the reptiles of this time approached birds, but still more remarkably do the earliest birds approach reptiles. There is in Bavaria a peculiar limestone used the world over for lithographic drawings. This lithographic limestone is equally celebrated for its marvelous preservation of fossils. In 1862 the oldest known bird, the ArchcBopteryx, was found there with even Le Contb. Geol. 22 338 HISTORICAL GEOLOaY. the feathers, and the minute structure of the feathers of the wings and tail, preserved. An undoubted bird, yet FiQ. 387. — Bestoration of Bliampliorhynclius phyllorns. (After Marsh.) One Beventh natural size. how different from modern birds ! Instead of the short, aborted tail, bearing feathers radiating almost from one point, as in all modern birds, it had a long reptilian tail with twenty-one joints, and the feathers given off in pairs on the two sides of each joint. Among many other rep- tilian characters are the possession of socketed teeth, and, instead of the hand being wholly consolidated to form the wing, as in moderji birds, the three fingers remain free, and are armed with claws (Fig. 388). Another fine specimen of this wonderful bird was found, in 1873, in the same locality, and is now in the Berlin Museum. In the Jurassic dinosaurs and this Jurassic bird we have excellent examples of what we have called generalized or connecting types. These two branches — reptile and bird — which seem so widely dis- tinct now, when traced backward in time, approach more and more, until we find almost their point of union. Mammals. — We have already stated, on page 338, that a few small marsupial mammals are found in the uppermost Triassic, both of Europe and the United States. These we regarded as anticipations, and therefore put off their MESOZOIG EBA.—AGE OF REPTILES. 339 discussion. This anticipation is fully realized in the Ju- rassic. In England there have been found about eighteen species, and, in the United States, Marsh has found seven- teen species; so that there are now known about thirty-five species of Jurassic and three species of Triassic mammals. 340 HISTORICAL aEOLO&Y. But, as the first birds were not true typical birds, but reptilian birds, so also the earliest mammals were not true typical mammals, but reptilian mammals, or marsupials. The marsupials live now almost wholly in Australia. They include the kangaroos, the opossums, the bandicoots, the wombats, etc. In Jurassic times they apparently inhab- ited all parts of the earth in great numbers. Now, the marsupials differ so greatly from ordinary mammals that they are put into a distinct sub- class. One striking pe- oulia,rity about them is that their young are born in an exceedingly imperfect state, so that they are almost egg-bearing, semi-oviparous. But neither were the Jurassic marsupials typical mar- supials, but rather generalized types connecting with In- sectivora, the lowest of the true mammals. They were all small animals, varying in size from that of a mole to that of a skunk. They were not able to contend for mastery with the great reptiles. The reign of mammals had not yet come. We give here (Figs. 389, 390) a jaw of a Ju- Fia. 289.— Jaw of a JtiraBsic mammal : Amphitharium Fievostii. Fig. 390.— Myrmecobius faeciatus, banded ant-eater of Australia. MESOZOIC ERA.— AGE OF REFTILE8. 341 rassic marsupial, and also a liying marsupial most nearly allied to them. Section III. — Jura-Tkias in America Areas ; Atlantic Border. — All along the eastern slope of the Appalachian chain, from Nova Scotia to South Caro- lina, in the Archaean region of the map on page 272, are found elongated patches of sandstones and shales which belong to this period. One of these is in Nova Scotia and Prince Edward Island ; the next, going south, is the celebrated Connecticut Eiver Valley sandstone ; the next a long, narrow patch commencing in New York, passing through New Jersey, Pennsylvania, Maryland, and ending in northern Virginia ; then two or three patches in eastern Virginia, about Richmond and Piedmont ; and, lastly, some on the Deep Eiver and the Dan Eiver of North Carolina. They all lie in hollows unconformably on the Archaean gneiss, and therefore their age can not be known except by fossils ; but these, though few, seem to indicate that they represent the whole Jura-Trias, although most writers speak of them as Triassic. In all these patches are found remarkable outbursts of igneous rocks, often columnar in structure, which by erosion have formed the so-called trap-ridges. Such are Mounts Tom and Holy- oke, in the Connecticut Valley patch, and the Palisades of the Hudson River in the New Jersey patch. Interior Region. — Red sandstones, poor in fossils, but probably referable to this period, are found in many places in the Plateau and Basin regions. Pacific Slope. — On both sides of the Sierra, rocks of this age, in a metamorphio condition, form the auriferous slates of this region. Life-System. Life, no doubt, abounded, but the conditions were unfavorable for preservation. We can, therefore, take 343 EI8T0RIGAL QEOLO&Y. up only a few of these localities and give, briefly, the findings. 1. Connecticut Kiver Valley. — This celebrated local- ity is classic ground, through the life-long labors of Dr. Hitchcock. The patch is one hundred and fifty miles long and ten to fifteen miles wide, extending from Ifew Haven Bay, on Long Island Sound, through Connecticut and Massachusetts, and mostly on the two sides of the Connecticut Eiver. As the strata dip regularly to the east, their thickness is easily estimated, and seems to be at least 5,000 to 10,000 feet. They consist of red sandstones and shales, and are in some places beautifully fissile. As might be expected from their redness,* they are very poor in fossils proper ; but in certain parts an immense num- ber of tracks of various animals have been found. There are tracks of (a) insects and crustaceans j (5) of reptiles j (c) possibly, but not probably, of birds. {a) Insects and Crustaceans. — Of the insect and crustacean tracks little can be made out with certainty. We give an example (Fig. 291). (5) Reptiles. — The reptilian tracks vary in size, from Fio. 291.— Tracks of insects. (After Hitchcock.) those of a lizard to those of the huge Otozoum, twenty- two inches long with a stride of four feet. In character, some are five-toed, some four-toed, some three-toed ; some walked on four feet, some on only two hind-feet ; some had long, dragging tails (Fig. 393), and some short tails, or none at all (Figs. 393, 394). (c) As already said, some of these reptiles walked on two legs only, and had only three functional toes, and * Organic matter decolorizes sandstones. — See page 89. MESOZOIG ERA.— AGE OF REPTILES. 343 some were short-tailed or tailless. These have been re- garded by some as wingless birds. They were probably C3 Fis. 292. Fig. 293. Fig. 294. Pigs. 292-294.— Eeptile tracks (after Hitchcock) : 293. GigantitheriQm candatam, X ^. 293. AnomiEpus minor, x J ; a, hind-foot ; S, fore-foot. 294. Track of Brontozoum giganteum, x ^. all reptiles. One of these wonderful two-legged reptiles is given in Fig. 395. The general conclusion, then, is that all these tracks were those of Dinosaurs and, possibly, Labyrinthodonts. In Jura-Trias times there seems to have been in this place an estuary, into which the tides ebbed and flowed. At low tide, reptiles of many kinds were in the habit of vralking on the soft, exposed mud in search of food left by the retreating tide. The incoming tide covered the tracks with fine sediment, and preserved them till now, the sediments, meantime, hardening into stone. 2. New Jersey Patch. — In this patch we find the game redness of the sandstone, and therefore the same poverty of fossils. -Of this sandstone have been built all 344 HISTORICAL GEOLO&Y. the brownstone houses of Kew York city. A few bones and teeth of reptiles, however, hare been found, and these FiQ 395.— Anchisauras colurus, x Ai from Connecticut sandstone. (After Marsh.) confirm the conclusions already given. A few tridactyl tracks also have been recently found, similar to those of the Connecticut patch. In Fig. 296 we give a restoration of fish from the New Jersey sandstone. Fio. 296.— Diplurus longicaudatns, x i. (After Dean.) 3. Virginia and North Carolina Patches. — These are very different from the Northern patches. They form the Richmond and Piedmont coal-fields of eastern Vir- ginia (Fig. 297) and the Deep Eiver and Dan River coal- MESOZOIC EBA.~AGE OF BHPTILBS. 345 FiQ. 298.— Jaw of Sromatherimn sylvestre. Fia S97.— Section across Richmond coal-field. (After Daddow.) fields of Nortli Carolina. In connection with the Coalj plants have heen found in considerable abundance. They are those characteristics of the Jura-Trias every- where, viz., ferns, cy- cads, and conifers. In North Carolina the jaw of a small marsupial has been found about the middle of the series (Kg. 398). The coal of these Jura-Trias fields is of good quality, in thick seams, and easily worked. 4. Atlantosaur Beds. — -These we describe separately, not only because they are recent discoveries, but also and chiefly because they belong to an entirely difEerent horizon, viz., the uppermost Jurassic passing into the Cretaceous. In these uppermost Jurassic beds, called Atlantosaur beds, from their most abundant and characteristic genus. Fig. 299.— Brontosaurus excelsls, x ^ir- (Eestored by Marsh.) have recently been found, in Wyoming and Colorado, great numbers of most extraordinary reptiles, the largest yet known, and also a bird and seventeen species of small marsupial mammals. 346 HISTORICAL aEOLOGY. BeptUes. — The extraordinary number of dinosaurian reptiles found here have thrown much light on this order. Some of them were reptile-footed (Sauropoda) (Fig. 299), some bird-footed (Ornithopoda) (Fig. 300), some beast- FlB. 300.— Laoeaarus, x A- (Restored ty Maisli.) FiQ. 801.— StegosauruB nngalatus, x Jd. (Eeetored by Marsh.) MESOZOIC ERA.—A&E OF REPTILES. 347 footed {Theropoda), aud some curious plate-covered rep- tiles {Stegosauria) (Fig 301). The Ornithopoda and some Theropoda walked almost wholly on their hind-legs in the manner of birds. The size of some of these reptiles is almost inconceivable. A thigh-bone of an Atlantosaur^ found by Marsh, was six or seven feet long, and a vertebra of an Amphicoelias, found by Cope, was six feet high to the top of the spinous process. The Atlantosaur has been estimated to have been seventy to eighty feet long ! In the same beds, as already stated, were found the remains of a bird and of seventeen species of marsupials. A figure of one of these is herewith given (Fig. 303). Via. 302.— Eight lower jaw of Diplocynodon victor (after Marsh), outside view— twice natural size. Disturbances whicli closed, the Jura-Trias Pe- riod. — One of the most important changes which oc- curred at the close of this period was the formation of the Sierra Nevada Range. Until that time the Pacific shore-line was east of the Sierra, and the place of this range was a marginal sea bottom receiving sediment. These sediments finally yielded at the close of this period and were folded and swelled up into this great range. Subsequent erosion sculptured it into its present grand forms. Coincidently with this change in the West, there were on the Atlantic lorder outbursts of igneous matter forming the trap ridges. In the interior region there 348 HISTORICAL GEOLOGY. was a downward moTement of the crust over the whole Plains and Plateau region by which isolated inland seas were changed into the great interior Cretaceous sea. The Sierra Nevada Eange is the most conspicuous monu- ment of this period of change, and therefore it may be called the Sierra revolution. Section IV. — Cretaceous Eocks and Peeiod. General Characteristics. — The Cretaceous is in some respects a transition to, and a preparation for, the next era. Mesozoic types, such as the great reptiles, the, am- monites, etc., continue, but Cenozoic types, like dicoty- ledonous trees and teleost fishes, are introduced, and the two kinds of types coexisted side by side. Rock System ; Areas. — 1. In the Atlantic border region, going southward, we find no cretaceans until we reach Long Island. Going south from this, we find a strip running through New Jersey, Delaware, and Maryland, .lying directly against the Archaean ; then small, isolated patches exposed by erosion in North Carolina, South Carolina, and Georgia. It doubtless extends all along the Southern coast, but is mostly covered with later Ter- tiary deposits. 2. In the Gulf border region it forms a broad, crescentic band, commencing in western middle Georgia, passing through middle Alabama, turning north- ward through Mississippi and Tennessee, to near the mouth of the Ohio. It underdips the Tertiary of the Mississippi River region, and reappears on its west side (see map, page 372). 3. It thence passes northward, covering nearly the whole Plains and Plateau region, though largely concealed by the Tertiary. 4. On the Pacific border it is found on the lower foot-hills of the Sierra Nevada in Northern California, and, together with the Tertiary, forming the whole of the Coast Eange. Physical Geography. — From this distribution we can MESOZOIG EBA.—A&E OF REPTILES. 349 make out witli some confidence the condition of the con- tinent in Cretaceous times. 1. North of New York the Atlantic shore-line was farther out than now. It crossed the present shore-line near New York, passed along the inner border of the Cretaceous of New Jersey, Delaware, and Maryland, and southward nearly along the limit of the low countries. 2. The Gulf shore-line went through Fig. 303.— Map of North America in Cretaceous times. middle Alabama, and northward to the mouth of the Ohio, and southward again on the other side of the Mis- sissippi Eiver. 3. Connected with this extended gulf was a great inland sea five to six hundred miles wide, covering the whole Plains and Plateau region (with some islands in the Colorado mountains region), and stretching northward probably even to the Arctic Ocean, and thus dividing the continent into two parts, an Eastern or Appalachian continent and a Western or Basin regies 350 HISTORICAL QEOLO&Y. continent. The place of the Wahsatch Kange was then the western marginal bottom of this interior sea. 4. The Pacific shore-line was then east of the Coast Eanges, and its waves beat against the lowest foot-hills of the Sierra. This is shown in the map. Fig. 303. Character of the Bocks. — In regard to the kind of strata, there are two points worthy of passing mention. 1. Chalk. — The period takes its name from the chalk of England and France, which belongs here. Chalh h a soft, snow-white, very pure lime-carbonate, scattered Pio. 304.— View of Iowa chalk nnder the microBcope. (After Calvin.) through which are nodules of flint. On account of its softness, it is worn into strange, castellated forms. Pure chalk, as described, was until recently, supposed to be confined to England, and France, and middle Europe, but has now been found in the Cretaceous of Texas and the Plains. When examined with the microscope, it seems ME so ZOIC ERA.—AQE OF REPTILES. 351 to be composed wholly of the remains of low organisms, chiefly foraminifera (Pig. 304). The flints are seen to be composed of shells of Diatoms and spicules of sponges. Ifow, as already shown (page 117), this is exactly the composition of deep-sea ooze (globigerina ooze), except that the silica has been separated and collected in nod- ules. It seems probable, therefore, that chalk is a deep- sea ooze of the Cretaceous times. 3. Coal. — Coal is found, again, in the Cretaceous, both in the TJnited States and elsewhere. But as most of our later coal belongs to a transition period between the Cre- taceous and the Tertiary, we shall put off the discussion of these for the present. Life-System ; Plants. So great is the change and the advance in plants at this point, that if we were guided by plants alone, we would say that the Cenozoic era commenced with the Cretaceous. Here the present aspect of field and forest seems to begin, for here were introduced for the first time, and in great numbers, dicotyls, or ordinary hard-wood trees. The sud- denness of their appearance, however, is due, in part at Fig. 305. Fig. 306. Fig. 307. Figs. 305-307.— Cretaceous plants (after Leequereux) : 305. Sassafras araliopsis. Wi. Sails protesefolia. 307. Fagus polyclada. AH reduced. 352 HISTORICAL GEOLOGY. least, to a lost interval between the Jura-Trias and the Cretaceous. Of the four hundred and sixty species of plants found in the Middle Cretaceous of the West, four hundred are dicotyls. Nearly all the genera of common trees are represented, although, of course, the species are extinct. There were then, as now, oaks, maples, willows, sassafras, dogwoods, hickory, beech, poplar, tulip-tree {Liriodendron), walnut, sycamore, sweet-gum {Liquid- amhar), laurels, myrtles, etc. A few of these are given in Figs. 305-307. Animals. Protozoa. — Though these are found in nearly all the strata heretofore described, we have usually neglected Fiu. 308. Pig. 809. FiG. 310. Figs. 308-310.— Foraminifera of chalk, magnified : 308. Flabellina rugosa. 309. Litnola nautiloidcB. 310. Chrysalidma gradata. (After D'OrWgny.) them, because they are inconspicuous. But here in the Cretaceous they are so abundant that they demand attention. Chalk, as already said, is almost wholly made up of for- aminifers (Figs. 308-310), and sponges are also extremely abundant. Of the former, some are identical with living species. Fig. 311. — Echinoids of the Creraceous of Europe : Galerites albogaleruB. MESOZOIC ERA.—AQE OF REPTILES. 353 Fig. 312.— Hippurites Toucasiana, a large individual witli two small ones attached. in gTr^Y^^^-^. S H n 1 ^ je=s=M§fS^»wte^s^ " ■' II 1. \ '< ^ ■V 1 11 I a,i ' 1 ii % ~ w .. r \ /" r ' « ' 1 II " „ 1 •' A / 1 " ■■ "^4 " " u ' II ■ '. Pie. 348.— Ideal section acroBB river-bed in drift-region. a general way the condition of the rivers in all the drift- region. Beneath the present river-bed, r, there is a much 392 HISTORICAL &EOLO&Y, wider and deeper old river-bed, B R, which is filled up often several hundred feet deep with river-silt, h i, and into this the river is now cutting its bed. The great river- bed, R R, was cut out during the epoch of elevation (Gla- cial) and previous periods. They are preglacial river- beds. The filling was done during this epoch of subsi- dence (Champlain). The river since then has again cut down, but not so deeply. All the rivers in the drift- region, therefore, are bordered on each side by a wide area of old silt, usually much above the present flood- level, and therefore forming high bluffs or terraces, sometimes one, sometimes many, on each side. The Cause of the flooded condition was primarily the great water-supply from melting of the ice-sheet. But it is evident that the subsidence of the land would cause the sea to enter the mouths of many rivers, forming great estuaries ; and also, by diminishing the slope of the river- bed, would tend to increase their floods. From this subsided condition the land gradually rose again, by successive stages, to the present condition. These successive stages are marked by a succession of sea-beaches, lake-terraces, and river-terraces, below the highest just described. As the land rose, successive sea- margins were left ; the outlet of the lakes also cut deeper and deeper, and drained the lakes to lower and lower levels. Also, all the rivers cut deeper and deeper into the old Champlain silts, leaving them as bluffs and ter- races high above the present flood-line (Kg. 348). Some- times there is but one great bluff on each side, as in the Mississippi River. Sometimes there are several terraces, one above the other, as in the case of the Connecticut River. It is evident that when Lake Champlain was first out off from the sea by elevation it was a salt lake. It was freshened in the manner explained on page 79. dMNOZOIG EBA.—AGM OF MAMMALS. 393 Quaternary in the Western Part of the Continent. On the Pacific slope the signs of all these movements are clear ; especially are the signs of extensive glaciation magnificent. "We shall again vary our mode of presenta- tion by tracing the condition of things throughout the Quaternary in seas, glaciers, laJees, and rivers. We take seas first, because by this we establish the oscillations. Seas. — A more elevated condition of land than now exists is plainly shown, not only by the boldness of the Western coast and the existence of a line of bold, rocky islands a little way off shore, a recognized sign of a sunken coast, but also by the remarkable fact that remains of the Quaternary mammoth have been found on one of these islands — the Santa Eosa. When this elephant lived, the island was evidently connected with the mainland. A subsequent subsided condition is demonstrated by sea-margins in many places. We shall describe briefly the condition of the sea. At that time the Bay of San Francisco was enormously enlarged ; for its waters covered the whole of the flat lands about the bay, including the Santa Clara, Napa, and Sonoma Valleys, and then, passing through the Straits of Carquinas, spread all over the great interior valley of California (Sacramento and San Joaquin), forming an inland sea fifty miles wide and three hundred miles long. The old beach-marks may be traced in many places. Lake Tulare is a remnaat of this great inland sea. In Oregon the sea went up the Columbia Eiver, and spread over the Willamette Valley, forming a great sound. Prom this subsided condition the land rose again, making successive terraces down to the present level. Grlaciers. — It is still doubtful if the general ice-sheet ' extended on this coast as far south as California, although abundant evidences are found in British Columbia ; but it 394 HISTORICAL &BOLO&Y. is certain that the whole Sierra was at that time covered with perpetual snow, from which ran great glaciers forty to fifty miles long to the valleys below. It is certain that all the valleys and canons which trench the flanks of the Sierra were filled with glaciers of enormous size. Many Fig. 349.— Glaciated surface and scattered bowlders near Lake Tenaya, Cal. (Prom a photograph by J. N. Le Conte.) of these have been traced in the clearest manner by their polished pathways, their scattered bowlders, and their lateral and terminal moraines (Fig. 349). Liakes. — All the lakes of that time, especially in the Basin region, were greatly enlarged. About Lake Mono, terraces rise, one above another, to 700 feet above the present lake-level, and inclosing an immense area. The lake-waters then washed against the foot of the Sierra, and glaciers ran into its waters and produced icebergs. At the same time, the whole lower part of the Utah and Nevada basins Avas filled each with a great lake. That which filled the Utah basin, called Lake Bonneville, was CENOZOIC ERA— AGE OF MAMMALS. 395 100 miles wide and 300 miles long. Tlie traveler on the Union Pacific Eailway can hardly fail to observe the old terraces, rising up to 1,000 feet above the present lake- level. It drained at that time into the Snake and Co- lumbia Elvers, then lost its outlet, and dried away to the remnants — Great Salt Lake, Utah Lake, and Sevier Lake —which we now have. The lake which filled the Nevada basin — LaTce Laliontan — was of nearly equal size, and its dried-away residues are seen in numerous salt and alkaline lakes, such as Pyramid, Winnemucca, Humboldt, Carson, "Walker, etc., which overdot this great area. Rivers. — The old or preglacial river-beds, on the eastern side of the continent, as we have seen (page 391), underlie the present river-beds — i. e., are in the same place, but deeper. In middle California the relation is quite different and peculiar. Here the old river-beds overlook the new — i. e., they are in a different place, and higher. The old river-beds are on the divides between the new. The reason is this : In middle California, at the beginning of the Glacial epoch, the old river-beds had already been filled up, first with gravel, and then, by igneous outbursts, with lava. The rivers were thus dis- placed, and began to cut new beds. But at the same Pig. 350. — Ideal section tlirough two modem river-bedfl and table-mountain divide : r", old river-bed ; r, r, present river-beds ; s, elate ; ffr, new gravel ; I, lava ; gr^f old gravel under the lava. time there was a considerable lifting of the whole moun- tain-region, and consequently the rivers now cut deeper than before (Pig. 350). Thus it has come to pass that the new river-beds occupy the places of the old divides. 396 HISTORICAL QEOLOQY, and the old river-beds are now found on the top of the present divides. Phenomena similar to those discussed are found in Europe and in all other high-latitude regions, both north and south of the equator. Some General Results of Glacial Erosion, 1. Fiords. — If one examines an accurate map of coast- lines, he will see that, in the region affected by Quater- nary oscillations, there is a bold, deeply dissected coast- line. In Norway these deep inlets are called fiords, and therefore this structure, wherever found, is called fiord-structure. We find it strongly marked in Green- land and in Alaska. This structure, in Norway, is partly due to the action of waves (page 45), but also, and mainly, to the submergence of old glacial valleys. In Greenland and Alaska they are still partly occupied by glaciers. 2. Lakes. — Examine your map of North America. See how the whole northern part is dotted over with lakes, while the southern part is almost destitute of them. See also that the lake-area is also the area of the drift. Now, although lakes may be formed in many ways, and exist in all parts of the world, yet undoubtedly the small lakes at least, which are so thickly sprinkled over the drift-region^ have been produced by glacial agency. There are several ways in which glacial lakes were formed : 1. They are sometimes rocTc-lasins, scooped out by glacial erosion. 2. They are often formed by the damming of drainage waters behind old terminal moraines. These two kinds are ^thickly strewed all over high mountain- regions in the pathways of old glaciers. Standing on the crest of the Sierra, fifty may sometimes be counted at one view. 3. In flat regions, as in northern Minnesota and British America, they are simply hollows produced CENOZOIO ERA.— AGE OF MAMMALS. 397 by inequalities of deposit of the Drift when the ice-sheet retreated. Life-System, of the Quaternary. Plants and Invertebrates. — The plants and inver- tebrate animals were mostly identical with those still living. We dismiss these, therefore, with one important remark. Quaternary species are indeed still living ; not, however, in the same place, but much farther north. This indicated that the climate was much colder in the Qua- ternary than now. Mammals. — It is only in mammals that we find a striking difference as compared with the present time. Those of the Quaternary are peculiar, differing conspicu- ously both from the Tertiary and the living species. "We shall take our first examples from Europe, as they have been best studied there. Quaternary Mammals of Europe. — In Europe they are found sometimes in caves, where in great numbers and of all kinds they have become entombed ; sometimes on river-terraces and old sea-leaches, where their floating carcasses have been stranded and buried ; sometimes in peat-iogs, where, venturing in search of food, they have mired and perished ; and sometimes, as in Arctic regions, in frozen soils, where whole carcasses were sealed up, and are now found perfectly preserved. Tlie Mammalian Age culminates here. — As already said, the mammalian age seems to culminate in the Quaternary just before its downfall. For example, in England alone, during this time, there lived a great elephant, the mammoth {Elephas primigenius), much larger than any now living ; two species of the rhinoceros and one of the hippopotamus ; three species of oxen, two of which were of gigantic size ; a wild horse ; several species of deer, among which were the reindeer and the great Irish elk, a magnificent animal, eleven feet high to 398 EISTORIGAL &EOLOQY. the top of its elevated antlers and ten feet between their tips. Of carnivores there were the great cave-bear, larger than the grizzly ; a lion and a tiger as large as the African lion and the Bengal tiger ; a saber-toothed tiger {Machai' rodus), more formidable than either, with its saber-like tusks projecting six to eight inches beyond the gums ; hyenas in great abundance ; besides many smaller species. The remains of man have also been found associated with these extinct animals. Mammoth. — This great animal deserves more special mention. During Quaternary times, three great elephants roamed in herds over Europe, The greatest of these — in fact the greatest of all elephants, and the most numei ous at this time — was the mammoth {Elephas primigenius). The remains of these are found everywhere, but the most perfect in Siberia. Here perfectly fresh carcasses have been exposed by the undermining, by the river, of the frozen bluffs of the river-banks. The one represented here (Fig. 351) is in the Museum of St. Petersburg. The dried skin still remains on the feet and portions of the head. It is known from these carcasses that this elephant was covered with a thick wool, and over this long hair. Unlike living elephants, it was adapted to endure cold. The same was true of the Quaternary rhinoceros, the carcasses of which have also been found preserved in the same way. Quaternary Mammals in America. — Great mam- mals were equally abundant in America. There roamed in herds all over this country one species of the mastodon and two species of the elephant, viz., the Elephas primi- genius, or mammoth, and the Elephas Americanus. There were also three or four species of the horse, some of gigantic size ; several species of oxen, one of them ten feet from tip to tip of their widely spreading horns ; several species of the elk, one of them equal to the great Irish elk, and a great number of gigantic edentates. CUNOZOIG ERA.—AQE OF MAMMALS. 399 ground-sloths, and armadillos. Carnivores were not so abundant as in Europe ; but there were several species of the bear, a lion, and a saber-toothed tiger. , The Great Mastodon. — The most perfect specimens of the mastodon have been found in the peat-bogs, where, venturing in search of food, they have become mired. 400 HISTORICAL &MOLOGY. Fio. 353. — Mastodon AmericanuB. (After Owen.) FiQ. 35"!.— Tooth of Mas- todon Americanus. Fics. 354.— Molar tootli of a Mammotli (Ble- phas primigenius), grinding surface. In Fig. 352 we give one of the most perfect of these. Any one can dis- tinguish the re- mains of the mas- todon from those of the mammoth, if the jaw-teeth be preserved. The difEerence is shown in Figs. 353, 354. It is doubtful which of these two animals was the greater; but either CENOZOIO ERA.— AGE OF MAMMALS. 401 was probably more than twice the bulk of the. greatest liv- ing elephant. Quaternary Mammals in South America. — We shall mention here only the most characteristic. South America now is characterized by sloths, armadillos (eden- tates), and llamas. In Quaternary times it was similarly characterized, but the species were gigantic. Great ground- sloths and cuirassed animals allied to the armadillo, but bigger than an ox, had their homes in South America, but wandered northward into North America as far as Cali- fornia and Pennsylvania. Among the ground-sloths, the best known are the Megatherium (great beast) and the Mylodon. The hugest of these was the Megatherium (Fig. 355). This was as big as a rhinoceros, and had thigh-bones several times the bulk of those of an elephant. The massiveness of the hind-legs, the hip-bones, and the tail, together with the long arms and prodigious hands, seem to indicate that the animal had the power of stand- ing on its hind-legs while it reached up to tear down branches of trees and feed upon them. T^^m^^^^^ s I ^^ ^^^^^^^L- ^y-- "1 Fie. 355.— Megatherium Cuvieri. • Among the cuirassed edentates, the best known is the Glyptodon, the shell of which was at least five feet long ; Lb Conte, Geol. 26 402 HISTORICAL GEOLOGY, (smilodon) necator, x ^. (After Btirmeister.) but other genera have been found much larger^ one as big as a rhinoceros, and another as big as an ox. The saber-toothed tigers were also abundant in South America at this time (Fig. 356). Quaternary Mammals of Australia. — At the present time the mammals of Australia are all marsupials. So was it also in Quaternary times ; but the species were, again, gigantic. The Diprotodon, for example. Fig. 356.— Head of Machairodus was & kangaroo as big as a rhi- noceros. Many other gigantic species are also found. We see, then, that the present distribution of mamma- lian forms was already established in the Quaternary, but everywhere the species were gigantic. Some Important General Questions. 1. Cause of the Cold of the Glacial Epoch. — The intense cold which characterized the Glacial epoch may have been due to terrestrial or to cosmical causes. It seems right that we should, as far as possible, account for it by terrestrial causes, and resort to the other only if these fail. How, northern elevation would probably produce great cold in the northern hemisphere. This, then, is certainly a probable cause. But the effect has seemed so great and widespread that many think this cause insufficient, and have therefore looked abroad for extra-terrestrial or for cosmical causes. Among the many causes of this kind which have been proposed, the only one which has attracted much attention is that brought forward by Mr. Croll, which attributes it to slow changes in the form and position t)f the earth's orbit.* * For a discussion of this subject, see "Elements," p. 575. QENOZOIC JUBA.—Aai; OF MA3IMALS. 403 2. Migrations during the Glacial Epoch and their Effect uu the Greographical Distribution ol" Organisms. — The oscillations of the earth's crust during glacial times produced great changes in Physical Geog- ■raphy, elevation enlarging and subsidence diminishing the area of the continents. In this manner gateways were opened permitting migrations from one conti- nent to another, as for example between l^orth America and Asia through Bering Straits, and between Europe and Africa through the Mediterranean. Again, the great changes of climate from subtropical mildness to extreme arctic rigor, and back again to temperateness, enforced migrations southward and northward, perhaps several times. These migrations, whether permitted or enforced, produced a mixing of difEerent faunas and floras on the same ground ; and the severe competitive struggles among them, together with the great changes of climatic conditions, caused many changes, partly by extinction and partly by modification. After these migrations, minglings, struggles, and consequent modifications, the resulting faunas and floras were again in many cases reisolated in their new homes by subsidence. In these isolated new homes they have undergone slow changes by evolution until the present time. Thus have come about the present geographical faunas spoken of in chap"- ter iii., section 4, of Part I. (P. 118). Now, as the Glacial epoch is a comparatively recent geological event, it is evident that the migrations of that time furnish a key to the present distribution of organisms ; and conversely, the present distribution of organisms is a key to direction of migrations during that time. We give a few striking examples illustrating this very interesting subject, and completing the explanations given in Part I. 1. TUpine Species. — It is a curious fact that alpine species of plants and insects (i. e., species which live on mountains near the snow line) are very similar in all parts 404 HISTORICAL QEOLOOT. of the world (as for example in North America and Europe), although they are so far separated from one another and completely isolated. It must be observed, however, that they are also very similar to Arctic species. The explanation is found in the migrations of the glacial" times. At that time Arctic species were pushed south- ward on both continents — to the shores of the Mediter- ranean in one and of the Gulf of Mexico in the other. On the return of a temperate climate most of them fol- lowed the retreating ice-foot back to their Arctic home ; but some followed arctic conditions upward to the tops of high mountains, and were stranded there in alpine isola- tion till now. It is true they have been slowly changing since then — some in one direction, some in another — in accordance with a universal law in the case of isolated species; but the time has been too short to effect any great changes. 3. South Africa. — Africa, south of Sahara, is inhabited by two very distinct groups oi mammals. The first group consists of small animals of very low organization, such as insectivores, but very different from those found any- where else. These we shall call indigenes. The other group consists of very large and highly organized ani- mals, mostly also peculiar to Africa, but similar in gen- eral character to those found in Eurasia, especially those of Pliocene times. These we shall call invaders. Now, before glacial times, Africa was isolated from the rest of the world and inhabited by the indigenes only. Then came the glacial elevation, opening gateways through the Mediterranean and into Africa, and the glacial cold driving the Pliocene mammals southward into Africa, where they were shut up by the closing of the passages through the Mediterranean and by the formation of the Desert of Sahara. The subsequent struggles between invaders and indigenes, and the effect of a new environ- ment on the invaders, have greatly changed both, but GENOZOIG EBA.—AOE OF MAMMALS. 405 especially the weaker indigenes. Thus have resulted the mammalian fauna of Africa. 3. The Britisli Isles — The fauna and flora of the British Isles are almost identical with those of Europe, but not quite. They differ in two respects, a. There are varietal, though perhaps not specific, differences of form. 5. The number of species is much less than on the continent. This is especially true of Ireland. Thus, of 90 species of European mammals only 40 are found in England and 22 in Ireland. Of 23 European species of reptiles and amphibians only 13 are found in England and 4 in Ireland. The migrations of glacial times com- pletely explain this. Before glacial times Great Britain was a part of Europe and had the same fauna and flora. During the glacial times it was covered with the ice-sheet, and 8.11 life destroyed or driven southward. After the glacial times it was still connected with the continent, and began to be recolonized by migration from Europe. But before the colonization was completed, especially for more distant Ireland, it was again separated by subsidence from the continent, and at the same time Ireland was separated from England. The time since has not been suflQcient to make species, although it has been enough to make incipient species, i. e., geographical varieties. 4. Coast Islands of California.— The flora of Cali- fornia consists of two groups of species, the one charac- teristically Californian, the other more widely diffused. The first is undoubtedly indigenous, the second is prob- ably composed of invaders from the north. Ifow, off the coast of Southern California there is a string of bold, rocky islands about 2,000 feet high and separated from the mainland by a deep channel 50 miles wide. The flora of these islands is very peculiar. Of 300 known species about 50 are wholly peculiar to the islands and not found elsewhere in the world. The remaining 250 are all characteristic California species. Now the explanation. 406 HISTORICAL GEOLOGY. Before and during the early part of the Glacial epoch the islands were a part of the continent. We have already given proof of this on page 393. At that time all was inhabited by the same flora, viz., the indigenous. Before the invasion from the north the islands were separated from the mainland. Then came the northern invasion and consequent struggle between native and invading species — the destruction of some natives and the modifi- cation of others — and the final result was the California flora as we now know it. But the island flora was spared this conflict, and therefore retained more nearly the orig- inal character of both. In the flora of these islands, therefore, we see a near approach to the flora of both mainland and islands before the separation. CHAPTER VI. PSTCHOZOIC ERA. — AGE OF MAN, In all preTiOTis ages there ruled brute force and ferocity. In this age alone Reason appears as ruler. The order of Nature must be adjusted to this keynote. Therefore^ the great ruling mammals of the previous age must become extinct, and the mammalian class must become subordi- nate ; noxious animals and plants must diminish, and useful ones be preserved. Although in length of time this is not to be compared to an era, nor to an age, nor to a period, nor even to an epoch, yet it deserves to be made one of the primary divisions of time, not only on account of the dignity of man, but also, and mainly, because through his agency there is now going on in organic forms a change as sweep- ing as any which has ever taken place. This change has been going on ever since the introduction of man, and is going on now, but will not be complete until civilized man occupies the whole earth. It is interesting to mark some of the steps of this change. The disappearance of the mammoth, the mas- todon, the cave-bear, and the saber-toothed tiger was due, partly at least, to man. These are among the first. Some of the gigantic oxen of Europe (urns) lingered until Roman times. One species {aurochs) still lingers, being preserved by royal edict in the forests of Lithuania. The bison or buffalo of our Western plains is doomed to speedy extinction, unless saved by domestication. In fact, 407 408 HISTORICAL &MOLOGT. nearly all our domesticated animals and useful plants have been thus saved. A remarkable example of recent extinction of the Qua- ternary species is found in the gigantic wingless birds of New Zealand and Mada- gascar. The bones of the Dinornis and the Epiornis are very abundant in these islands. The Dinornis giganteus (Fig. 357) was twelve feet high. The drumstick was a yard long, and as big as the leg- bone of a horse. A perfect egg of the Epiornis has been found, six times as big as the egg of an ostrich. The ex- _ tinction of these birds, although it occurred before the discovery of these islands by civilized man, was so recent that the feet have been found with dried skin upon them, and eggs with the skeletons of chicks within. Now, in this Fig. 357.— Dinornis giganteas, x A- (Fromapho- gradual change frOm tograph of a skeleton in Christcliurcli Museum, , i* y-v i j_ New Zealand.) the Quaternary to the present fauna and flora, when did man first appear upon the scene and become an agent of change ? And what kind of man was this primeval man? These are questions of tran- scendent importance. PSYCnOZOIG EliA.—AQE OF MAN. 409 Antiquity of Man. On this important question, history, archaeology, and geology meet and cooperate ; and it is to the introduc- tion of geological methods that we must attribute the rapid advances in recent times. Archaeologists long ago divided the history of human progress, according to the nature of the implements used, into three ages — a stone age, a tronze age, and an iron age. Again, by closer study, they subdivided the stone age into an older stone {Paleolithic) and a newer stone {Neolithic) age. In the one, the stone implements are chipped ; in the other, polished. Again, under the guid- ance of geology, the Paleolithic has been subdivided into the mammoth age and the reindeer age. In the former, man was contemporaneous with the mammoth, the cave- bear, and other extinct Quaternary animals ; in the latter, the mammoth had nearly disappeared, but the reindeer was abundant over all middle and southern Europe. The flint implements in the former were so rude that they might well be called flint-flakes ; in the latter they were carefully chipped. The former was coincident with the Mid-Quaternary, i. e., Champlain, or -perhaTps Inter glacial; the latter with the second Glacial or the early Post-glacial. 3. Iron age ) 3. Bronze age [■ Psyohozoic. ( Neolithicr-Domestio animals. ) 1. Stone age I I Reindeer — Late Quaternary. ( Paleolithic •< ( Mammoth — Mid-Quaternary. As seen by the schedule above, the Psychozoic era and age of man commences with the Keolithic. Before that time, man existed, indeed, but contended doubtfully for mastery with the great Quaternary animals. Prom that time his victory is assured and his reign begins. 4:10 HISTORICAL &EOLO&Y. Primeval Man in Europe. According to our schedule, man is traced back to the Mid-Quaternary. Some geologists think that there are signs of his existence still earlier, viz., in the Tertiary ; but the evidence is acknowledged to be unsatisfactory. We shall confine ourselves, therefore, to Quaternary man. We shall commence with Europe, as the evidence is more complete, and all the steps represented. Quateruary Mau ; Maminotli Age ; the Kiver- Drift Man. — Some twenty years ago, M. Boucher de Perthes found, in the undisturbed gravels of the upper terraces of the river Somme, the implements of man asso- ciated with the bones of many extinct Quaternary animals, such as the mammoth, the rhinoceros, the hippopotamus, the hyena, the horse, the Irish elk, the cave-lion, etc. The doubts which were at first entertained by the more cautious geologists have been entirely removed by careful examination. We give this as only one example of very many. In all cases the implements are of the rudest kind of flaked flints, like those figured on page 414. The • Cave - Man. — In Quaternary times, man un- doubtedly contested with the hyena, the lion, the saber- toothed tiger, and the cave-bear the right to occupy the caves as homes. The evidence of this is found in the association of his implements, and even his bones, with those of all the extinct carnivores mentioned, under con- ditions which admit of no doubt of their contempo- raneousness. They are sometimes entombed together, and covered with stalagmitic crust, which has never been broken from Quaternary times until rifled by the geolo- gist. We give a single example. The Mentone Man. — In a cave at Mentone, near Nice, has been recently found the almost perfect skeleton of an old man, of more than average height, lying on his side in un easy position, and about him chipped, im- PSYGHOZOIG ERA.— AGE OF MAN. 411 plements and bones of extinct animals, among which were many pierced reindeer's teeth. All of these were perfectly preserved by a stalagmitic crust. We may well imagine that this old hunter, finding his end approaching, retired to his cave-home, laid himself quietly down, with the implements and trophies of successful chase about him, and gave up the ghost. Good Mother Nature then slowly buried his remains, and sealed them up beneath a crust of stalagmite. The Primeval Aquitanlans. — In southwestern France, on the river Viz^re, a branch of the Dordogne, are found many caves which were inhabited by a ttiore peaceful race. They were not only hunters, but also fishers ; for we find, besides stone implements, many im- plements made of bone, among which are rude fishhooks. They also show evidence of some skill in drawing and carving. Among the bone implements found there are many drawings of extinct animals. Pig. 358 represents a rude but very characteristic sketch of a mammoth. Fig. 358.— Drawing of a Mammotli by contemporaneous man. made by contemporaneous man. In these caves we find a gradual transition from the mammoth to the reindeer age. G-eneral Conclusions.— These all belong to the Qua- ternary. In Europe, therefore, man certainly saw the 413 HISTORICAL GEOLOGY. flooded rivers and lakes, and probably the great glaciers. He certainly hunted the great extinct Quaternary animals, the mammoth, the cave-bear, the cave-lion, the great Irish elk, and the reindeer. All the evidence points to an ex- tremely low, savage state, with little or no tribal organi- zation. There is no evidence yet of either domestic ani- mals or of agriculture. Neolithic Man. Kitchen-Middens ; Refuse-Heaps ; Shell-Mounds. — In many parts of Europe, especially in Denmark and Sweden, are foiind mounds, composed wholly of shells and other refuse of tribal gatherings and foastings. The men of that time seem to have had the habit of gathering annually at some place where food was abundant, usually on the seashore, at the mouth of a river. From year to year the refuse of such gatherings accumulated until mounds of great extent were gradually formed. In these mounds are found the bones of men and animals and the implements of men, and from these we may form a good idea of the character and habits of the men. Here, then, we find a gr-eat and somewhat sudden change : 1. There are no longer any extinct Quaternary animals. 3. We find here, for the first time, domestic animals, viz., the dog, the ox, the sheep, etc., and also evidences of agriculture. 3. The implements are no longer only chipped, but are often carefully polished by rubbing. Rude pottery is also found. 4. We have here for the first time the evidence of tribal organization, similar to the savage races of the present day. 5. The conformation of the skull shows a different race from that of the cave and river-drift men. In a word, we have here the appearance in Europe, probably by migration, of a different and higher race. Until this time man in Europe seems to have contended doubtfully with wild animals : now he seems to have established his su- PSYCnOZOIG ERA.— AGE OF MAK 413 prejnacy. The Psycliozoic era and age of man, there- fore, rightly commence here, and all that follows may be claimed by archaeology and history. Nevertheless, we shall give a very brief sketch of further progress. Transition to the Bronze Age. liake-Dwellers. — In 1850 the lakes of Switzerland be- came very low, and a great number of wooden piles were exposed. Interest being excited, the same was found to be true of all the lakes of middle Europe. By dredging, implements of war, of the chase, of husbandry, and orna- ments and trinkets of all kinds were found in great abun- dance. Some of these were polished stone, but most were bronze, and often beautifully finished. Remnants of grain and fruits of several kinds were also found. Prom these findings the houses (Fig. 359), the habits, and the mode Fig 359 —Lake dwellings, restored. (After Mortillet ) of life of this people have been reconstructed, and even a novel embodying their life has been written.* Thus we might continue, by means of remains alone, to trace progress, through Roman graves, Roman roads * " Realmar," by Arthur Helps. 414 HISTORICAL GEOLOGY. and implements, etc., to the graves in our own church- yards and the machinery of our own times. This all be- longs to history. Thus we trace geology into archaeology, and archaeology into history. Primeval Man in America. It must be remembered that the different men we have described in Europe represent different stages of progress there. The progress has not been at the same rate every- where, and therefore the different stages are not necessa- rily contemporaneous. When America was discovered, the native tribes were still in the stone age, and many savages are only in this stage of advance now. The advance was more rapid in Europe, apparently because of the frequent and extensive migrations and conflict of races there. Ifevertheless, the rudest state (Paleolithic age) seems to have been nearly contemporaneous in America and Europe, and probably elsewhere. Quaternary River-Drift Man in America. — There are many examples of rude flint-flakes in the river-gravels of California and in the glacial drift of N"ew Jersey and Ohio. These were, it is believed, the work of a race cor- Fie. 360.—] ■Paleolith found by Abbott in New Jersey, sligMly reduced. (After Wright.) PSYCIIOZOIG ISRA.—AOE OF MAN. 415 respouding to luul eoutempovaneous with the river-drift man of Europe (Fig. 360). Some doubts have been recently thrown on the antiquity of tliese findings. For this reason we will not dwell on Glacial man in America. Jfeolithlc Mail in America. — The Neolithic age is represented here, as in Europe, by refuse-heaps, which were evidently made in the same way as those already described, and have similar contents. They are abun- dant on the seacoasts everywhere, and some of them are probably no older than the discovery of America ; for, as already said, the native tribes were then still in the stone age. Mound-Builders. — The bronze age is probably, though imperfectly, represented by the mound-builders. In many places, especially in the valley of the Mississippi, are found mounds of enormous size, and fortifications and communal houses of somewhat elaborate construc- tion. In connection with these have also been found not only highly polished stone implements, but also imple- ments of hammered copper. The copper-mines of Lake Superior were evidently worked by tliem, as the old work- ings have been found. The mound-builders were prob- ably a different race from the hunter tribes of Indians, and more advanced, although many now think they are the same. Cliflf-Dwellers. — In the dry regions of ISTew Mexico and Arizona the almost perpendicular cliffs bordering the mesas are studded with remains of many-storied com- munal houses of stone. There are small remnants of sev- eral tribes in that region — Pueblos, Moquis, and Zunis — that live now in similar dwellings, on the flat tops of almost inaccessible mesas. One dwelling with many rooms is occupied by a whole community. These also are entirely different from the roving tribes, and by many are connected with the Aztecs on the one hand, and the mound-builders on the other. 416 HISTORICAL &EOLO&Y. It is needless to repeat that these last three heads be- long to the present epoch. Conclusions. 1. We have thus traced man back to the Mid-Quater- nary. It is possible that he may hereafter be traced still further back ; but this seems very improbable, lio mam- malian species now living can be traced further back than the Quaternary. Man belongs to the present mamma- lian fauna^ and probably came in with other mammalian species in the Quaternary. 2. We have not yet been able to find any undoubted transition forms or connecting links between man and the highest animals.* The earliest known man, the river- drift man, though in a low state of civilization, was as thoroughly human as any of us. 3. The amount of time which has elapsed since man first appeared is still doubtful. Some estimate it at more than a hundred thousand years — some only ten thousand. The question should not be regarded as of any impor- tance, except as a question of science. * Such a link is supposed, by many, to have been recently found in Java, and named Pithecanthropus. We wait for more evidence. INDEX. Acrogens (point-growers or apex- growers), age of, 295. African fauna explained, 404. Agencies, geological, 9. leveling and elevating, 131. Ages, 260. Air^ chemical action of, 14. mechanical action of, 15. Albertite ; a form of asphalt, 313. Alkaline lakes, deposits in, 77. Alpine species, 403. Ammonite (horn of ammon stone), 830. Amphibians (living in both [air and water] ), called also batra- chians, 319-328. age of, 297. Amphicoelias (a/mphi, both sides; Jcoilos, hollow ; double con- cave), 347. Amygdaloid, 228. Anchisaurus, 344. Ancylooeras (curved horn), 353. Andesite, 214. Angustifolius (narrow leaf), 274. Anomodont (lawless tooth), 328. Anomoepus (unlike feet), 343- Anoplotherium (unarmed beast), 880. Anticline and syncline defined, 188. Apiocrinus (pear crinoid), 331. Appalachian revolution, 323. Aqueous agencies, 17. Aquitanians, primeval, 411. Archsean (relating to earliest times), 259. rocks, area of, in the United States, 365. rocks, character of, 364. Lb Conte, Geol. .37 Archaean system, 263. times, life of, 265. times, physical geography of, 365. ArohsBozoic (primeval life), 359. Archegosaurus (primordial liz- ard), 320. Archaeopteryx (primordial winged creature), 337. Artesian wells, 70. Asteroid (star-like), 277. Asterolepis (star-scale), 291. Atlantosaur (great lizard) beds, 845. Atmosphere, chemical action of, 14. mechanical action of, 15. Atmospheric agencies, 10. Baculite (stone-stafE), 354 Bad Lands, 348, 366. Banks, Bahama, 50. in North Sea, 50. of Newfoundland, 50. submarine, 49. Bars, how formed, 38. position of, 38. removal of, 40. Basalt, 139, 215. columnar structure of, 230 Base level of erosion, 38. Bed-rock surface, 887. Belemnite (stone dart), 333. Birds, 337. Bitumen and petroleum, 318* Blastids (bud-like), 379. Borax lakes, 77. Botanical regions, 119. Bowlders, 387. of disintegration, 13. 417 418 INDEX. Braehiopod (arm-foot), 279. Breccia, 180. volcanic, 333. British Isles, fauna of, 405. Brontosaur (giant lizard), 345. Brontotherium (giant beast), 378. Brontozoum (giant animal), 343. Bronze age, 409, 413. Bryozoon (moss-animal, called also PolyzoSn; an order of com- pound moUuscoid animals), 316. Buthotrephis (reared in the deep), 374. Butterfly, a fossil, 873. Calamite (stone reed: a family of coal-plants allied to equisetas), 309. California coast isles, 405. Cambrian, 371. Canons, ravines, gorges, 33. examples of, 34. Carboniferous (coal-bearing) age, 398. age, fauna of, 315. age, subdivisions of, 297. Caves, limestone, 71. Cenozoio (pertaining to recent ani- mal life) era, 363. era, characteristics of, 363. era, subdivisions of, 364. Cephalaspis (head-shield), 393. Cephalopod (head-foot : refer- ring to position of limbs), • 381. Ceratite (stone-horn: a family of shelled cephalopods), 337. Ceratodus (horn-tooth), 395. Cestraeion (sharp tool: referring to the spine), 396. Chalk, 350. Champlain epoch, 390. Chemical deposits in lakes, 76. deposits in springs, 72. deposits of iron oxide, 75. deposits of lime carbonate, 78. deposits of silica, 76. deposits of sulphur, 76. Chronology, construction of geo- logical, 307. Cinders, ashes, etc., 135. Cleavage, slaty, 194. Cleavage, structure, 193. Cliff-dwellers, 415. Coal-iields of the United States, 300. of Eastern Virginia and North Carolina, 344. Coal measures, 898. mode of accumulation of, 310. origin of, 301. period, climate of, 313. period, length of, 303. period, physical geography of, 313. plants of the, 303. varieties of, 301. Coccosteus (berry-bone), 293. Colorado CaSon, 35. Columbia River and tributaries, 33. Columnaria alveolata (ceUulat columns), 376. Columnar structure, 330. structure, cause of, 331. Compsognathus (handsome Jaw), 387. Concretionary or nodular struc- ture, 198. Concretions, how formed, 199. Conformity and unconformity, 190. Conglomerate, volcanic, 332. Connecticut Eiver Valley tracks, 342. Continental faunas and floras,133. faunas and floras, subdivisions of, 126. form, general laws of, 176. Continents and sea-bottoms, ori- gin of, 178. mean height of, 176. Coral, compound, mode of growth, 95. conditions of growth, 98. forest, how formed, 96. islands, closed lagoons, 103. islands, how formed, 97. islands, lagoonless, 103. islands of the Paoiflo, 98. polyp, structure of, 98. reef-rock, 97. reef- rock, diflierent kinds of,108. reefs, 97. reefs, atolls, 103. INDEX. 419 Coral reefs, barrier, 101. reefs, fringing, 100. reefs, how formed, 97. reels of Florida, 109. reefs of the Pacific, 99. reefs, theories of barriers and atolls, 103. Corals in Paleozoic rocks, 275. Cordaites (a coal-plant named after Corda), 304. Coryphodon (peak-tooth), 877. Co-seismal lines (lines connecting points which feel a shock at the same moment), 163. Crater lake, 143. Cretaceous period, 348. period, animals of, 353. period, areas of rooks of, 348. period, coal of, 851. , period, physical geography of, 348. period, plants of, 351. Crinoid (lilyUke stone), 277. range in time, 378. Crust of the earth, 174. general configuration of, 176. Cyathophylloid (cup-leaf like), 375. Cycads : plants of cycas, or sago- palm family, 329. Cystid (baglike), 379. Darwin's subsidence theory of atolls, 104. Deltas, how formed, 33. age of, 36. subsidence of, 169. Dendr^rpeton (tree-reptile), 330. Denudation, or general erosion, 353. modes of determining amount of, 353. Deposits, chemical, 73, 183. deep-sea, 117. made by waves, 50. mechanical, 183. organic, 183. Devonian age, 386. age, animals of, 388. age, fishes of, 391. age, life-system" of, 387. age, physical geography of, 287. age, plants of, 386. Devonian age, rocks of, 286. Devonian fishes, sudden appear- ance of, 397. Diatoms {Sidro)ioi, cut in two) : microscopic plants which multiply by dividing in two, 115, 353. shell deposits of, 116. Diabase, 313, 313. Dicotyls, contraction for dicoty- ledons : plants having two seed-leaves, 351. Dicynodon (two canine-toothed), 327 Dikes, 141, 316. effect on stratified rocks, 317. Dinichthys (huge fish), 393. Dinooeras (huge horned animal), 377. Dinosaur (huge lizard), 334. Dinotherium (huge beast), 381. Diorite, 317. Dip and strike defined, 187. Diplacanthus (double spine), 293. Diplocynodon (double canine- teeth), 347. Diprotodon (two front teeth), 403. Disintegration, rate of, 13. Dolerite, 214. Drift, 386. Drift-timber, 88. Dromatherium (running beast}, 345. Dynamical geology, 9. Earth, crust of, 174. crust, cause of inequalities in, 178-340. crust, cause of movement of, 171. crust, gradual oscillation of, 164. density of, 174. general form of, 173. general structure of, 173. general structure of, means of observing, 175. internal heat of, 131. Earthquake, epicentrum of, 156. focus, mode of determining, 163. wave, nature of, 158. wave, velocity of, 156. 430 INDEX. Earthquakes, 154. beneath the sea-bed, 159. cause of, 157. connection of, with phases of the moon, and with the weather, 163. frequency of, 155. great sea- wave of, 160. phenomena of, 155. Echinoderm (spiny skin), 377. Bchinoid (urchin-like), 277, 352. Echinus (hedgehog or urchin) : a sea-urchin, 277. Blasmobranchs, 291. Elevation and subsidence, cause of, 171. of crust, gradual, 165. Eocene (dawn of recency), 364. Eohippus (dawn of earliest horse/, 377. EozoSn (dawn animal), 266. Epicentrum (upon the center), 156. of an earthquake, mode of de- termining, 163. Epochs, 262. Equisetas : horse-tails, scouring- rush, 287, 806, 326. Eras, 259. Erosion, agents of, 252. amount of, 253. average rate of, 19. general, or denudation, 252. general results of glacial, 396. of rain and rivers, 18. Eruptive rocks, true, 214. Estuaries, deposits in, 38. how formed, 37. Evolution, hearing of Devonian fishes on, 295. Palls, Niagara, 20. of St. Anthony, 22. Minnehaha, 22. Yosemite, 23. False bedding, 184. Faults, amount of displacement, 330. kinds of, 232. law of slip, 333. Faunas and floras, continental, 123 defined, 118. marine. 129. Faunas and floras, geographical, explained, 403. Favositid (honeyoomblike stone), ^75 Pelsrte,"213. Ferns, 306. Fiords (Norwegian term for deep inlets between high head- lands), 45. origin of, 396. Pishes, age of, 286. Fissures, great, 239. great characteristics of, 230. Flood-plain, 30. of the Mississippi, 31 . of the Nile, age of, 31. Floras, defined, 118. Florida, reefs and keys of, 109. Plorifornjis (flower-like), 375. Folded strata, 186. Foraminifera (full of holes : pro- tozoan animals with perfo- rated shells), 351. Formation defined, 304. geological, 193. Fossils, 200. degrees of preservation of, 300. Fossil species, distribution of, 303. Frost, action of, in soil-making, 15. Fucoid (resembling tangle), 374. Fucus (tangle or wrack), 274. Pumaroles (smoking vents), 145. Gabbro, 213. Ganoid (shining : referring to the scales), 391. Gasteropod (belly-foot : referring to mode of walking), 380. Gastornis (Gaston's bird), 374. Geographical distribution of spe- cies, 118. diversity of species, origin of, 130. Geological and human history, correspondence of great prin- ciples of, 256. chronologv, construction of, 207. formation, 193. history, divisions of, 359. Geology, definition of, 7. dynamical, 9. INDEX. 431 Geology, great divisions of, 8. historical, 356. structural, 173. Geyser, Great, 147. Great, phenomena of eruption of, 147. Geysers defined, 146. cause of eruption of, 151. of Iceland, 146. of Yellowstone Park, 147. Qigantitherium (gigantic beast), 343. Glacial (icy), 885. cold, cause of, 403. epoch, 386. epoch, explanation of phenom- ena of, 358. erosion, general results of, 396. Glaciers as a geological agent, 60. characteristic signs of, 63. defined, 53. erosion of, 61. evidences of former greater ex- tension of, 63. in the Sierra, 56. lower limit of, 53. motion of, 58. size of, in various regions, 56. structure of, 56. transportation and deposit by, 61, 03. Globigerina(globule-bearing),117. ooze, 117. Glyptodon (sculptured tooth), 400. Goniatite (angled stone : a family of shelled cephalopods with angled sutures), 390, 317. Gorge formed by recession of falls, 31. Gracilis (graceful), 374. Gradual oscillation of the earth- crust, 164. Grahamite : a form of asphalt, 313. Granite, 313. Granitic rocks, composition of, 311. rocks, mode of occurrence, 313. Graphic granite, 313. Graptolites (stone-writing), 376. Ground-water, perpetual, 68. Gulf Stream, geological agency of, 48. Gulf Stream, origin and cause of, 47. Gymnosperm (naked seed : a class of plants including conifers and cycads), 387, 304. Halysites (stone chain) catenulata (like a little chain), 376. Halysitid (stone chain), 375. Hamite (stone hook), 354. Hesperornis (western bird), 358. Hipparion (little horse), 381. Hippurite (horse-tail), 353. Historical geology, 356. History, general principles of, 356. geological, divisions of, 359. Horse, genesis of, 383. Hybodont (hybodus, hump-tooth : a family of shark-like fishes), 318. Hydrothermal fusion (fusion by heat and water), 133. Hydrozoa (water-animals), 375. Hypsilophodon, 355. Ice, agency of, 53. Icebergs as a geological agent, 66. effect of, compared with gla- ciers, 66. how formed, 64. of Greenland, 64. of the Antarctic, 66. Ice-sheet moraine, 388. Ichthyornis (fish-bird), 858. Ichthyosaur (fish-lizard), 834. Ideal section of earth-crust, 361. Igneous agencies, 131. rocks, 310. rooks, characteristics of, 310. rocks, classification of, 311. rocks, extent of, on the surface, 311. rocks, modes of occurrence of, 311, 316. rocks, origin of, 310. rocks, sub-groups of, 313. Iguanodon (iguana-toothed), 336. Indusium (an inner garment* 371. Insects, 390. Intercalary beds, 319. Invertebrates, age of, 871. Iron accumulations, 88. 422 INDEX. Iron accumvilations, mode of formation of, 89. bog-ore, 89. oxide, deposits of, 75. Islands : coast islands, how formed, 51. Joints, 328. Jurassic period, 339. period, animals of, 339. period, coal of, 339. period, plants of, 329. Jura-Trias, disturbances which closed, 347. in America, 341. life-system of, 341. Kitchen-middens, 413, Labyrinthodont (labyrinthine tooth : a family of extinct amphibians), 319. Lake Agassiz, 391. Bonneville, 394. dwellers, 413. Lahontan, 395. margins, 391. Lakes, alkaline, 77. borax, 77. chemical deposits in, 80. crater, 143. glacial origin of, 396. salt, 76. Lamellibranch (plate-gill), 379. Lamination, cross or oblique, 184. Land, mean height of, 176. Laosanrus, 346. Laramie epoch, 860. epoch, coal of, 361. Lava, classification of, 139. kinds of, 137. sheets, extent of, 317. Lepidodendrid, 387, 307. Lepidodendron (scale-tree), 307. Lepidoganoid (scale-ganoid), 393. Lepidosiren (scaly siren : an am- phibious iish), 295. Levees, artificial, 32. natural, 31. Lime accumulations, 91. carbonate, deposits of, 73. sinks, 73. Limestone oaves, how formed, 71. Limestone shell, 114. Limuloids : Limulus family, 316. Limulus ; horseshoe-crab, or king- crab, 284, 316. Lithodomi (lithos, stone, domus, house : a species of shell-fish which burrow in rocks), 167. Lost intervals explained, 367. Lyoopod (wolfs foot : an order of club-mosses), 307. Machairodus (saber-toothed), 381, 403. Mammals, age of, 363. genesis of orders, 383. of the Tertiary period, 374. Mammoth, 398. Man, antiquity of, 409. Neolithic, 413. of the caves, 410. of the river-drift, 410. primeval, in America, 414. primeval, in Europe, 410. Marmites des grants, 63. Mastodon (nipple-toothed), 399. Mastodonsaur (teat-toothed liz- ard), 337. Mauvaises Terres, 348, 366. Megalosaur (great lizard), 336. Megatherium (great beast), 401. Mentone man, 410. Mesohippus (mid-horse), 379. Mesozoic (pertaining to middle animal life) era, 334. era, characteristics of, 324. era, disturbance which closed, 360. era, general observations on, 359. era, subdivisions of, 334. Metamorphic rocks, 224. Metamorphism, cause of, 336. agents of, 326. Migrations during Glacial epoch, 403. Mineral springs, 71. veins, 333. Miocene (less recent), 364. Miohippus (less horse-like), 378. Mississippi delta, 34. Mode of accumulation of coal, 310. Mollusks, 379. INDEX. 433 Mono Lake In Quaternarr period, 394. Monotremes (one vent : the lowest order of mammals, including ornithorhynohus and echid- na), 338. Monticles (little mountains), 136. Moraines, 57. Mosasaur (Meuse lizard), 357. Mound-builders, 415. Mountain life, difEerent stages of, 245. sculpture, 246. sculpture forms of, 247. strata, thickness of, 244. Mountains, defined, 238. structure and origin of, 238. Murray's theory of atolls, 105. Myrmecobius (ant-liver), 840. Nautilus, 281. Neolithic (new stone), 418. Niagara Falls, recession of, 31. gorge, origin of, 21. Nodules, forms of, 198. how formed, 199. Obsidian, 214. Ocean, agency of, 41. mean depth of, 176. why is it salt ? 79. Oceanic currents, 47. Organic agencies, 83. agencies, subdivisions of, 83. Orohippus (mountain-horse), 377. Orthoceratite (straight stone horn), 383. Orthoclase (right cleavage), 313, 315. Osteolepis (bony scale), 394. Otozoum (giant animal), 343. Outcrop, 186. Overflows, 317. Pacificbottom , subsiding area, 106. Paleolithic (old stone), 409. Paleotherium (old beast), 380. Paleozoic (pertaining to old or ancient animal life), 359. era, general observations on, 331. era, progressive changes during, 331. Paleozoic era, subdivisions of, 271. rocks, 368. rooks and era, 367. rocks, area of, in the United States, 369. system, unconformity of, with Archffian, 367. times, growth of the continent in, 371. times, physical geography of, 369. Peat, antiseptic property of, 83. bogs, 84. bogs, rate of growth of, 87. bogs, structure of, 87. composition of, 84. mode of accumulation of, 85. swamps, 84. Pegmatite, 313. Peneplain, 28. Period, geological, defined, 204. Periods and epochs, 263. Permian period, 323. Petroleum (rock-oil), 313. age of strata of, 314. origin of, 315. Phonolite (ringing-stone), 314. Pithecanthropus, 416. Placode rm (plate-skin), 293. Plaeoganoid (plate-ganoid), 293. Plagioclase (oblique cleavage), 212, 215. Plesiosaur (near to a lizard), 334. Pliocene (more recent), 365. Pliohippus (more horse-like). 379. Plumularia (plume-like), 376. Plutonic rocks, composition of, 313. Polypterus (many-finned), 296. Porphyry, 313. Potholes, 36, 63. Primordial beach, 269. Protohippus (first horse), 379. Protozoa (first and lowest living things), 266, 352, Psychozoic (pertaining to rational life), 260. era, 407. Pteranodon (winged-toothless), 356. Pterichthys (winged fish), 293. 424 INDEX. Pterosaur (winged lizard), 337. Pumice, 214. Quaternary period, 385. , period in Eastern North Amer- ica, 386. period in Western North Amer- ica, 393. period, life-system of, 397. period, mammals of, in Amer- ica, 398. period, mammals of, in Aus- tralia, 403. period, mammals of, in Eu- rope, 397. period, mammals of, in South America, 401. period, subdivisions of, 385. Eafts, 88. Eain and rivers, erosive action of, 18. final effect, 38. rate of erosion by, 19. Ramphorhynchus (beak-snout), 338. Range of species, genera, etc., 130. Ravines, gorges, cafions, 23. Reefs and keys of Florida, 109. how formed. 111. Reefs of Pacific, 97. Regions, botanical, 119. definition of, 130. primary, 138. primary, subdivisions of, 138. zoological, 133. Reptiles, age of, 334. Rhyolite, 314. River-beds of California, old, 395. as indicators of crust-move- ments, 38, 170. deltas, subsidence of, 169. drift-man in America, 414. Rivers, deposits at the mouths of, 38. deposits of old, 393. erosive action of, 30. flood-plain deposits of, 30. winding course of, 39. Rock disintegration, rate of, 18. disintegration, explanation of, 14. Rocks, classes of, 178. Rocks, defined, 178. igneous, 310. metamorphio, 334. stratified, cause of consolida- tion of, 183. stratified, classification of, 305. stratified, description of, 179. stratified, extent of, 180. stratified, origin of, 181. stratified, principal kinds of, 180. structures common to all, 328. unstratified, 210. St. Anthony, Palls of, 32. Saline lakes, how formed, 76. lakes, chemical deposits in, 76. Salt lakes, how formed, 77. Sauropus (lizard-foot), 321. Scaphites (stone-boat), 353. Sea-beaches, elevation of, 391. Section of earth-crust, ideal, ^61. Sediments, transportation and distribution of, 36. Sequoia : genus of conifers, in- cluding Redwoods and Big Trees of California, 868. Sertularia (a little garland), 376. Sharks, 391, 318, 854, 373. Shell limestone, how formed, 114. mounds, 413. Sierra Nevada range, formation of, 348. Sigillaria (from sigillum, a seal), 308. SigiUarid,. 387, 808. Silica, deposits of, 76. Silurian animals, 275. rocks, area of, in the United States, 278. system, 271. times, life-system of, 273. times, plants of, 374. times, subdivisions of, 273. times, physical geography of, 373. Slaty cleavage, 194. Snow-line, 53. Soil, depth of, 13. origin of, 10. Solfataras (hot sulphur springs), 145. Sorting power of water, 27. INDEX. 425 Species, geographical distribu- tion of, 118. origin of geographical diversity of, 130. Sphenothallus (wedge frond), 274. Springs, 69. great, 69. mineral, 71. Squalodont (shark • toothed : a family of true sharks), 318. Stalactites and stalagmites, 72. Stegosaur (covered lizard), 346. Strata, crumpling of, 185. folding of, 185. Stratification explained, 27. Stratified rocks, classification of, 205. rocks, divisions and subdivi- sions of, 207. rocks, how relative age of, is determined, 205. Strike and dip defined, 187. Strombodes pentagonus (five- angled strombus-like ani- mal), 275. Structural geology, 178. Sub-carboniferous, 297. Submarine banks, 41. Subsidence of crust, gradual, 169. of Pacific bottom, amount of, 107. of Pacific bottom, time of, 107. of river deltas, 169. Succinifer (amber-bearing), 871. Sulphur, deposits of, 76. Swamp, Great Dismal, 86. Syenite, 212. Syncline and anticline defined, 188. Table-mountains, 247. Tachylite, 214. Taxodium, bald cypress of South- ern swamps, 368. Teleost (complete bone), 291. Terraces, 892. Tertiary period, 364. period, animals of, 370. period, coal of, 366. period, crust-movements during and closing, 884. Tertiary period, lake deposits of, 377. period, life-system of, 368. period, mammals of, 375. period, physical geography of, 366. period, plants of, 368. period, subdivisions of, 864. system, areas of, 865. Theromorpha (beast-like), 328. Tides and waves, agency of, 41. and waves, effect on coast-line, 43. Toxoceras (bow-horn), 354. Trachyte, 139, 214. Transporting power of water, 26. Trappean rooks, 218. Trioeratops (three-horned face), 362. Triassic period, 325. period, animals of, 826. period, life-system of, 335. period, plants of, 336. Trigonia (three-angled), 381. Trilobite (three-lobed stone), 283. Tufa, 139, 223. Turrulite (stone tower), 354. Unconformity, 190. Unstratified rocks, 210. Vegetable accumulations, 84. Veins, age of, 236. contents of, 234. fissure, 234. irregularities of, 386. metalliferous, 234. mineral, 233. origin of, 337. structure of, 335. surface changes of, 837. Volcanic cinders, ashes, etc., 135. dikes, 141. eruptions, cause of, 144. eruptions, phenomena of, 135. gases and vapors, 139. phenomena, secondary, 145. rocks, 214. rocks, age of, 319. rocks, different kinds of, 315. rooks, intercalary beds of, 219. rocks, modes of eruption, 315. rocks, modes of occurrence, 316. 436 INDEX. Volcanic rooks, sub-groups of, 315. Volcanoes, 133. age of, 143. erupted matters of, 136. mode of formation of. 140. number, size, and distribution of, 134. two types of, 135. Water, agencies of, 17. chemical agency of, 67. mechanical agency of, 18. perpetual ground, 68. sorting power of, 27. transporting power of, 26, Water, underground, 67. Tolcanic, 68. Waterfalls, recession of, 20. Waves, land formed by, 50. nature of deposits by, 46. and tides, agency of, 41. transportation and deposit by 46. Wells, artesian, 70. Winds, action of, 16. Yosemite Palls, 23. Zaphrentis (proper name), 288. Zoological regions, 122, 188. OUTLINES OF BOTANY ?I.OO By ROBERT GREENLEAF LEAVITT, A.M., of the Ames Botanical Laboratory. Prepared at the request of the Botanical Department of Harvard University Edition with Gray's Fidd, Forest, and Garden Flora ^i.8o Edition with Gray's Manual of Botany 2.35 THIS book covers the college entrance requirements in botany, providing a course in which a careful selection and a judicious arrangement of matter is combined with great simplicity and definiteness in presentation. ^ The course offers a series of laboratory exercises in the morphology and physiology of phanerogams ; directions for a practical study of typical cryptogams, representing the chief groups from the lowest to the highest ; and a substantial body of information regarding the forms, activities, and re- lationships of plants and supplementing the laboratory studies. ^ The work begins with the study of phanerogams, taking up in the order the seed, bud, root, stem, leaf, flower, and fruit, and closing with a brief but sufRcient treatment of cryptogams. Each of the main topics is introduced by a cha"pter of laboratory work, followed by a descriptive chapter. Morphology is treated from the standpoint of physiology and ecology. A chapter on minute structure includes a discussion of the cell, while another chapter recapitulates and simpliiies the physiological points previously brought out. ^ The limitations of the pupil, and the restrictions of high school laboratories, have been kept constantly' in mind. The treatment is elementary, yet accurate ; and the indicated laboratory work is simple, but so designed as to bring out fundamental and typical truths. The hand lens is assumed to be the chief working instrument, yet provision is made for the use of the compound microscope where it is available. AMERICAN BOOK COMPANY (174) A NEW ASTRONOMY By DAVID TODD, M.A., Ph.D., Professor of Astron- omy and Navigation, and Director of the Observatory, Amherst College. ASTRONOMY is here presented as preeminently a science of observation. More of thinkipg than of memorizing is required in its study, and greater emphasis is laid on the physical than on the mathematical aspects of the science. As in physics and chemistry the fundamental principles are connected with tangible, familiar objects, and the student is shown how he can readily make apparatus to illustrate them. ^ In order to secure the fullest educational value astronomy is regarded, not as a mere sequence of isolated and imperfectly connected facts, but as an inter-related series of philosophic principles. The geometrical concept of the celestial sphere is strongly emphasized; also its relation to astronomical instru- ments. But even more important than geometry is the philo- sophical correlation of geometric systems. Ocean voyages being no longer uncommon, the author has given rudimerital principles of navigation in which astronomy is concerned. ^ The treatment of the planets is not sub-divided according to the planets themselves, as is usual, but according to special elements and features. The law of universal gravitation is unusually full, clear, and illuminating. The marvelous dis- coveries in recent years and the advance in methods of teach- ing are properly recognized, while such interesting subjects as the astronomy of navigation, the observatory and its instruments, and the stars and the cosmogony receive particu- lar attention. ^ The illustrations demand special mention; many of them are so ingeniously devised that they explain at a glance what many pages of description could not make clear. AMERICAN BOOK COMPANY WILKINSON'S PRACTICAL AGRICULTURE By JOHN W. WILKINSON, A. M., Assistant State Superintendent of Public Instruction, Oklahoma ; for- merly Professor of Agriculture in Northwestern Normal School, Alva, Oklahoma. |l.OO A COMPLETE and practical treatise suited for the eighth grade of grammar schools, or for high or normal schools. It gives the pupil a definite technical training, and fits him for farm life in any part of the United States.. ^The vsrork takes up Agriculture, Horticulture, Forestry, Landscape Gardening, Animal Husbandry, Stock Feeding, Roads and Roadbuilding, and Country Life Conveniences. Air, light, water, and soil, the stapl,e farm crops, fertilizers, the improvement of plant varieties, and the enemies of plants, are discussed in a particularly helpful manner. Besides the descriptive' text, each chapter contains laboratory exercises, questions on the text, and references to more exhaustive works. ^ Nearly one-third of the book is devoted to topics which relate to Civic Improvement. This extension of the field is in accord with the tendency in the schools to broaden the course in agriculture into a course in farm citizenship. It corresponds to the incorporation of prevention of disease and public health in a course in physiology. ^ In the preparation of the book the author has kept constantly in mind the needs of the student as well as the facilities at the disposal of the teacher for making the instruction practical and available. No attempt has been made to exhaust the various topics treated, and in every instance abundant lati- tude is given the instructor to show his own individuality in developing and carrying out the ideas suggested by the text in the most helpful manner. AMERICAN BOOK COMPANY ESSENTIALS OF BIOLOGY By GEORGE WILLIAM HUNTER, A. M., Head of Department of Biology, De Witt Clinton High School, New York City. ^ THIS new first-year course treats the subject of biology as a whole, and meets the requirements of the leading colleges and associations of science teachers. Instead of discussing plants, animals, and man as separate forms of living organisms, it treats of life in a comprehensive manner, and particularly in its relations to the progress of humanity. Each main topic is introduced by a problem, which the pupil is to solve by actual laboratory work. The text that follows explains and illustrates the meaning of each problem. The work throughout aims to have a human interest and a practical value, and to provide the simplest and most easily compre- hended method of demonstration. At the end of each chap- ter are lists of references to both elementary and advanced books for collateral reading. SHARPE'S LABORATORY MANUAL IN BIOLOGY $0.75 IN this Manual the 56 important problems of Hunter's Essentials of Biology are solved ; that is, the principles of biology are developed from the laboratory standpoint. It is a teacher's detailed directions put into print. It states the prob- lems, and then tells what materials and apparatus are necessary and how they are to be used, how to avoid mistakes, and how to get at the facts when they are found. Following each prob- lem and its solution is a full list of references to other books. AMERICAN BOOK COMPANY (16^) - - - CHEMISTRIES By F. W. CLARKE, Chief Chemist of the United States Geological Survey, and L. M. DENNIS, Professor of Inorganic and Analytical Chemistry, Cornell University Elementary Chemistry . jSi.io Laboratory Manual . . ;^o.5o THESE two books are designed to form a course in chemistry which is sufficient for the needs of secondary schools. The TEXT-BOOK is divided into two parts, devoted respectively to inorganic and organic chemistry. Diagrams and figures are scattered at intervals throughout the text in illustration and explanation of some particular experi- ment or principle. The appendix contains tables of metric measures with English equivalents. ^ Theory and practice, thought and application, are logically kept together, and each generalization is made to follow the evidence upon which it rests. The application of the science to human affairs, its utility in modern life, is also given its proper place. A reasonable number of experiments are in- cluded for the use of teachers by whom an organized laboratory is unobtainable. Nearly all of these experiments are of the simplest character, and can be performed with home-made apparatus. 4 The LABORATORY MANUAL contains 127 experi- ments, among which are a few of a quantitative character. Full consideration has been given to the entrance requirements of the various colleges. The left hand pages contain the experi- ments, wliile the right hand pages are left blank," to include the notes taken by the student in his work. In order to aid and stimulate the development of the pupil's powers of observa- tion, questions have been introduced under each experiment. The directions for making and handling the apparatus, and for performing the experiments, are simple and clear, and are illustrated by diagrams accurately drawn to scale. AMERICAN BOOK COMPANY C1625 HOADLEY'S NEW PHYSICS By GEORGE A. HOADLEY, C. E., Sc. D., Professor of Physics, Swarthmore College. Elements of Physics (Text-book) ^1.20 Laboratory Handbook 50 THIS text-book is straightforward and concise. It tells only what everyone should know, and it covers all col- lege entrance requirements in physics. The funda- mental principles are presented in a logical order. The topics have been selected with the greatest care. The treat- ment is clear and simple, practical and interesting. The in- timate relation between, everyday life and applied physics is made plainly evident. ^ The problems also are practical; they deal with real events in both time and place, based upon things that have happened, rather than upon imaginary cases. Important physical laws are verified by well-arranged demonstrations. ^ The numerical answers to the problems given at the close of the book make it easy for the student to verify the accuracy of his solutions. Many of the illustrations are entirely new to works on physics or are from the photographs of real appli- cations. ^ The book meets the requirements of the New York and other State Education Departments, and the recommendations of the National Education Association, those of the College Entrance Examination Board, and those of a number of As- sociations of Teachers of Physics. ^ The Laboratory Handbook contains sixty-two experi- ments, selected with care, and eminently practical. The directions are simple and clear, the apparatus required not elaborate. Throughout, the student is trained to profit from his observations, to exercise his ingenuity, and to depend upon himself. AMERICAN BOOK COMPANY (156) aiiJiMiiMitfiiJffl