LIBRARY 
 
 ANNEX 
 
 I 
 
CORNELL 
 
 UNIVERSITY 
 
 LIBRARY 
 
 GIFT OF 
 THE AUTHOR 
 
°ZSM„?im^i?M^ f'*'"^ references to stru 
 
 3 1924 004 078 469 
 
Cornell University 
 Library 
 
 The original of tiiis book is in 
 tine Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924004078469 
 
DYNAMIC GEOLOGY 
 
 A RECITATION TEXT. 
 
 By O. D. von Engeln. 
 
DYNAMIC GEOLOGY 
 
 (With References to Structural Phenomena in their Relation to Processes) 
 
 A RECITATION TEXT 
 
 BY 
 O. D. von Engeln. 
 
CONTENTS. 
 
 CHAPTER PAGES 
 
 INTRODTTCTORY 5-6 
 
 I. THE EARTH 'S CRUST 7-20 
 
 II. WEATHERING 21-31 
 
 III. UNDERGROUND WATER 32-43 
 
 IV. TRANSPORTATION, EROSION AND DEPOSITION 
 
 BY STREAMS 44-56 
 
 V. TRANSPORTATION, EROSION AND DEPOSITION 
 
 BY WINDS 57-64 
 
 VI, TRANSPORTATION, EROSION AND DEPOSITION 
 
 BY GLACIERS 65-80 
 
 VII. GEOLOGIC PROCESSES IN OCEANS AND LAKES 81-95 
 
 VIII. DIASTROPHISM 96-106 
 
 IX. FAULTING AND EARTHQUAKES 107-126 
 
 X. MOUNTAIN UPLIFT 127-139 
 
 XI. VULCANICITY 140-153 
 
 XII. JIETAMORPHISM 154-160 
 
DYNAMIC GEOLOGY 
 
 INTRODUCTORY 
 
 This volume is intended to be used as a recitation text in a course 
 of Dynamic Geology consisting of lectures, laboratory and field ob- 
 servations, and recitation periods. 
 
 The broader aspects of the subject and its discussional phases may 
 well be presented in lectures with numerous illustrations. Actual ex- 
 perience and knowledge of the material of Geology ought to be gained 
 in the laboratory and by field studies. It is, however, difficult to 
 convey by word of mouth, either in the lecture room or field where 
 students need to take notes while listening, the definite concepts and 
 exact relations that comprise a large body of the science. This can 
 be done much better by means of the diagrams and precise phrases 
 of a printed text since these can be gone over again and again until 
 a given point is clearly understood and not involved in misconceptions 
 due to the speaker's language or faulty hearing on part of the stu- 
 dent. It is, therefore, intended that students study this text para- 
 graph by paragraph and diagram by diagram and be prepared, at the 
 recitation periods, to give exact and confident answers to questions 
 on the content of the sections assigned. In this way and only in this 
 way can a sound basic knowledge of the subject be gained. The lec- 
 tures and field experience will then serve to broadly illuminate, point 
 out the wider applications and the limitations of the text definitions. 
 
 This special text on Dynamic Geology is necessary because the 
 regular elementary geology texts on the market include and inter- 
 mingle in their chapters consideration of all the other sub-divisions 
 of Geology, viz.. Mineralogy, Petrography, Stratigraphy, Paleontology, 
 Economic Geology, and Physiography. In the case of Physiography 
 especially this causes confusion and ought to be avoided. 
 
 Further, Dynamic Geology deals primarily with the processes op- 
 erative on and in the earth, and a knowledge of these is essential to 
 a proper and appreciative understanding of the other phases of geo- 
 logy. Since in nature, however, there are no exact boimdaries; and 
 one natural science, or phase of it, must, therefore, overlap others; 
 so in Dynamic Geology also there must be occasional excursions into, 
 and consideration of some of the other branches of the general subject. 
 
It will, however, be possible to remain clear as to the general 
 fields of the different sub-divisions of the subject if they are thought 
 of as follows: 
 Dynamic Geology— A study of the forces and processes operative in 
 
 causing change in the earth's material, stru'cture and form. 
 Structural and Stratigraphic Geology— A study of the arrangement 
 
 and distribution of the larger units of the earth's material. 
 Historic Geology and Paleontology — A study of the time and life re- 
 lations of geological formations as determined primarily by their 
 
 fossil content. 
 Mineralogy and Petrography — A study of the particular substances 
 
 formed by the dynamic processes with regard to their composition, 
 
 properties and distribution. 
 Economic Geology — A study of the occurrence, commerieal availability 
 
 and uses of the economically valuable earth substances. 
 Physiography and Physical Geography — A study of the forms of the 
 
 earth's surface, their origin, history, distribution, and reactions 
 
 with organisms. 
 
 Before beginning upon a study of the Dynamic phase of Geology 
 it is apparent that a general concept of the substances and structures 
 on and in which the processes are operative will be essential. To that 
 extent at least trespass in the field of Structural and Stratigraphic 
 Geology, Mineralogy and Petrography is necessary at the very outset. 
 In considering the results of processes the domains of Economic Geo- 
 logy and Physiography are occasionally entered. Historic Geology 
 and Paleontological concepts give evidence in regard to the length 
 of time some processes have been active. In other respects, however, 
 these phases of geology are much more dependent on the facts of 
 Dynamic Geology than it on them. For this reason Dynamic Geology 
 best serves as an introductory study to the general science of geology. 
 Occasional references are cited in the text. The student has only 
 to consult several of these to appreciate that something more than 
 knowledge of the elementary principles to be gained from a course 
 such as this is needed to elucidate a geological problem in the field. 
 Such consultation will further serve to impress the necessity of a 
 sound and well grounded Imowledge of the fundamentals, and possibly 
 help to incite a desire for further study of geology. 
 
CHAPTER I 
 
 THE EARTH'S CRUST 
 
 THE GREAT CONCENTRIC SPHERES — Conceive for the moment 
 of a traveler approaching the earth from outer space. Imagine him gifted 
 with powers of discernment of such superior degree that he can in- 
 stantly perceive the relationships and composition of material things. 
 Consider then some of the observations he makes on the final stages 
 of his journey and on his arrival. 
 
 On nearing the planet he notes that it is completely enveloped 
 in a gaseous "shell," the Atmosphere, or sphere of air. On penetrat- 
 ing this air sphere he finds it to be a mechanical mixture of the 
 gases Nitrogen (78 plus per cent.), Oxygen (20 plus per cent.), Argon 
 (1 per cent.). Carbon dioxide (.03 per cent.) with Water Vapor, in 
 varying quantities, and several other minor constituents, serving to 
 make up the full 100 per cent. He further notes that this air blanket 
 extends tangibly for some himdreds of miles above the earth's sur- 
 face, and, in an extremely attenuated state, beyond even that distance. 
 Though composed of light gases he finds that its mass, measured in 
 columns of its great vertical extent, suffices to exert a considerable 
 pressure on the earth's surface (14.7 lbs. per sq. in. at sea level) and 
 that because of this pressure the air is forced for considerable dis- 
 tances into the pores and fissures of the solid substances of the earth. 
 Thus it is clear to the traveler that just as the upper portion of the 
 atmosphere merges imperceptibly into the ether of outer space, so also 
 th« lower side lacks a sharply defined bounding limit because of its 
 varyingly great penetration of the surface on which it rests. 
 
 Conceive now, that, before alighting, the traveler remains for 24 
 hours suspended in the Atmosphere, and, imaffected by gravity, sur- 
 veys the surface features of the earth as these turn past below him. 
 He notes that % of the area is made up of connected basins of water, 
 the oceans, and the remaining i/4 oi land, the continents and islands. 
 Of water he notes in addition to the oceans, filaments, rivers, and ex- 
 tensive areas, lakes and ponds, spread over the surface of the land. 
 On finally descending and investigating more closely he further ob- 
 serves that only the surface of the lands are dry ; that practically 
 everywhere the crevices and pores of the solid material, at varying 
 
8 DYNAMIC GEOLOGY 
 
 depths below the dry upper portions, are filled with water. He finds, 
 therefore, that at least a film of water is coextensive with the earth's 
 surface and is aecordiagly justified in referring to the water of the 
 earth collectively as the Hydrosphere, or sphere of water. He pauses 
 to reflect that large quantities of water vapor are present in the air, 
 and that, both as a vapor and a fluid, water penetrates the material 
 of the land. On making a test he finds, also, that the gases of the air 
 and large quantities of the solids of the land are dissolved in the 
 waters of the Hydrosphere. The traveler is impressed by this inter- 
 penetration of the materials of the land, the Atmosphere and the 
 Hydrosphere and concludes to examine more closely into its sig- 
 nificance. 
 
 First, however, the solid material of the earth claims his atten- 
 tion. He notes that its surface, also, is irregular, but that no height 
 projects over six miles into the Atmosphere. On sounding the deepest 
 oceans he finds that everywhere beneath them solid material extends 
 from shore to shore of the continents; and that their greatest depths 
 are not over six miles. The solid material, therefore, though exhibit- 
 ing an elevation difference of over eleven miles in its upper surface 
 is nevertheless a shell extending completely around the earth like the 
 Atmosphere and the Hydrosphere and may, analogously, be quite 
 properly termed the Lithosphere, or sphere of rock. 
 
 Not yet is he done with the spheres. Living as well a^ inanimate 
 materials demand recognition. Organisms' multitudinous in variety 
 fly in the air, throng the waters and teem on and in the land of the 
 earth. Here then is a Biosphere, or sphere of life, not continuous and 
 connected like the others, but making up for this lack of continuity 
 by the ability of the life forms through locomotion, growth and re- 
 newal to occupy successively, in time, all places that come within the 
 (again) irregular and ill defined upper and lower limits of the Bio- 
 sphere. 
 
 THE PROVINCE OF DYNAMIC GEOLOGY. _By this time the celestial 
 traveler has become well aware that the different spheres he has dis- 
 covered are not static either in form or composition ; that changes 
 are constantly taking place in them; moreover, that they react on each 
 other. Thus the interpenetration of their substance that he had be- 
 fore found an interesting phenomenon, because of its universality, he 
 now perceives to have a deep significance with respect to such reactions 
 and the consequent changes. On making inquiries of mankind he 
 learns that the study of life processes in the Biosphere is Biology, of 
 
THE EARTH'S CRUST 9 
 
 the Atmosphere and its changes is termed Meteorology ; that science 
 of the Hydrosphere is Hydrology or Oceanology and of the Lithosphere 
 and its changes Geology. He recognizes clearly that there may he 
 such separate fields of investigation, each identified with its own. 
 sphere, nevertheless perceives at once that the most complex and 
 interesting reactions must arise where air, water, land and life all 
 meet and mingle because of the merging into each other of the various- 
 spheres. He learns, then, that the science of the processes, agencies 
 and effects in this province constitutes primarily the field of Dynamic 
 Geology. To this study he now gives his attention and finds, that 
 while gifted with superhuman faculties during his journey to the 
 earth, these powers have gradually failed him since, and, that, in 
 common with mankind, he must from hence patiently and laboriously 
 follow the clues that lead to imderstanding in this study, subject 
 also to mankind's limitations as to how far these clues may be fol- 
 lowed by direct observation. Thus the identity of the celestial trav- 
 eler merges itself with that of the other students of Dynamic Geology 
 and he may be thought of, from now on, as struggling ^^-ith them on the 
 problems the science presents. 
 
 VERTICAL EXTENT OF THE LITHOSPHERE While the normal 
 
 habitat of man is on the land it is only with its veriest outer rind 
 that he is well acquainted. Unlike the Atmosphere and Hydrosphere 
 that may be sounded to their greatest depths; the latter actually, the 
 former by well founded inference, the Lithosphere is a sealed book 
 to direct observation at only trifling depths below its surface. As 
 has been pointed out in an earlier paragraph there is a difference of 
 about 11 miles in altitude between the highest mountain peak and 
 the greatest ocean depth (Mt. Everest, Himalayas 29,002 ft.. Pacific 
 Ocean, near Philippine Islands, 32,114 ft.) yet never has the highest 
 peak been scaled nor has any sample of the solid rock of the deep 
 ocean bed ever been obtained. "While there is little doubt about the 
 nature of the rock at the mountain top no valid e^ndence exists as 
 to the character of the foimdations on which the oceans rest. Thus 
 only about % the 11 miles of elevation difference of the surface of 
 the Lithosphere is actually open to inspection. If 514 miles seem of 
 some account, consider that at the equator the radius of the earth 
 is about 4000 miles (3963.) Let 1-16 inch equal 51,4 miles, and then 
 draw a line to show on the same scale the radius of the earth. Now, 
 if 1-16 inch be marked off at the end of this line to represent the 
 maximum elevation above sea level, there will be gained some appre- 
 ciation of how little of the earth's radial dimension is visible on the 
 surface. 
 
10 DYNAMIC GEOLOGY 
 
 Moreover, whether exposed at sea level or on mountain tops, such 
 visible outcrops of the material of the Lithosphere are, after all, only 
 eurfaee matter. This may have been once deeply buried but is not 
 now. "What lies below the present surface? To what depths has the 
 Lithosphere been actually penetrated? Answering in terms of a com- 
 parison (and of a magnitude that the mind may readily grasp the 
 proportions) to only the depth of a tiny scratch or of an almost im- 
 perceptible pin prick in the surface of a ball 10 feet in diameter. The 
 profoundest gorge and the deepest mines penetrate the rocks to depths 
 only slightly greater than one mile (Grand Canyon of the Colorado, 
 6000 ft.; Tamarack Shaft, Lake Superior Copper District, 6070 ft.) 
 There is good evidence that material of similar character extends to 
 depths of 6 or more, perhaps even to 30 miles below the surface, but 
 even these are comparatively insignificant depths, and what substances 
 comprise the great interior mass is only a matter of speculation. Some 
 of its properties, e. g. its density are Imown, but of its actual make 
 up. nothing. Such of its properties as are kno^\'n, however, make it 
 apparent that the unknown interior must be verj^ different from the 
 outer shell of rock, therefore, it has been distinctly termed the Cen- 
 trosphere. 
 
 COMPOSITION OF THE LITHOSPHERE — A discussion of the Com- 
 position of the Liithosphere is properly a part of Structural Geology. 
 But as a knowledge of this aspect of Geology may not be presupposed, 
 on part of the student, an elementary consideration of the subject 
 must have a place in this text. 
 
 The Lithosphere, as is indicated by the Greek term that constitutes 
 the first part of the word, is the sphere of rock. Popularly this is 
 known as the Crust of the Earih. Kock may be defined as any con- 
 siderable mass of material brought together by natural means and 
 constituting a unit portion of the Lithosphere. The defmition is nec- 
 essarily vague, for material that must, as a geological unit, be included 
 under the term of rock is highly divergent in character. Thus banks 
 of clay, sand, and gravel are each rock, geologically speaking, as well as 
 granite, sandstone and limestone. 
 
 MANTLE ROCK AND BED ROCK.^This inclusion in the term rock 
 of unconsolidated material (clay, etc.) as well as the consolidated stony 
 materials, which latter, only, are popularly known as such, is better 
 nnderstood when the geological terms Mantle Kock and Bed Bock are 
 comprehended. 
 
 A sheet of loose unconsolidated material is spread over a large 
 proportion of all the land surface of the world. In places it may be 
 
THE EAETH'S CRUST 11 
 
 composed of quite homogeneous material, fairly uniform throughout 
 large masses, as e. g. a bank of clay. Elsewhere it may be a complex 
 mixture of fragments large and small and of quite diverse composition. 
 It may be himdreds of feet deep or only a thin film. This sheet of 
 material is collectively termed the Mantle Rock. The term Begolith 
 has also been used, to some extent, as a synonym of mantle rock. 
 Usually the surface portion of the mantle rock has characteristics 
 that enable it to support plant growth, admixture of organic matter 
 is perhaps the most important factor; this surface portion constitutes 
 the Soil. Below this upper soil layer is found the Subsoil. 
 
 So general is the distribution of the mantle rock that points where 
 it does not occur are given a special name : Bed Eock Outcrop or more 
 simply Outcrop. In other words, at such points the bed rock is ex- 
 posed, or outcrops, at the surface. Bed Rock, then is r-iimply solid 
 rock, or popularly, Stone, occupying the position in which it acquired 
 its existing characteristics. The term bed rock was originally a min- 
 ing expression to designate the bed of solid rock over which the loose 
 material of the mantle rock was spread. This is still the correct ap- 
 plication but beginning students sometimes misinterpret the term in 
 that they get the impression from it that all solid rock occurs in beds 
 or layers. Such is far from being the case as will appear later. 
 
 Bed rock everywhere underlies the surface of the earth and con- 
 stitutes the bulk of the lithosphere. The mantle rock above it may 
 be thick or thin or absent; but however thick the mantle rock may be 
 at any given place, if a deep enough hole could be bored there bed 
 rock would eventually be encountered. Bed rock, therefore, constitutes 
 a continuous shell around the whole earth, with, however, a very 
 irregular upper surface ; and. like the mantle rock, very diverse com- 
 position at different points in both its horizontal and vertical distribu- 
 tion. Detached fragments of bed rock are knowTi as Boulders or 
 Pebbles according to their size. 
 
 CLASSES OF BED ROCK. —The solid rocks may be conveniently 
 treated in three great classes according to their origin and history. 
 
 CLASS I. IGNEOUS ROCKS, THEIR CONSTITUENTS AND STRUCTURES. 
 
 Igneous Rocks are those whose material owes its existing arrange- 
 ment, composition and solidity to the fact that it cooled to its present 
 temperature from a molten state, precisely as molten iron cools to the 
 solid pig iron form. Such rocks are made up of interlocking crystals 
 of one or several minerals. Each Mineral is (from the point of view 
 of this discussion) a deiinite chemical compound of certain elements 
 in exact proportions. An Element is a unit substance from which 
 
12 DYNAMIC GEOLOGY 
 
 it is impossible to separate simpler things by either chemical or me- 
 chanical means. Oxygen, Silicon, and Aluminum, designated by the 
 ^mbols 0, Si and Al respectively, are, in the order given, the most 
 abundant elements in the minerals that make up the igneous rocks, 
 and, in fact, also the most abundant elements in the lithosphere as 
 a whole. 
 
 It is a curious fact, and if the origin of the universe were fully 
 understood, what would probably be a very significant fact, that the 
 three elements named above, plus a very few others, constitute all but 
 a very small percentage of known terrestrial matter. Although some 
 80 elements have been discovered, the preponderance of the few is 
 shown by the following table of percentages of these present in a 10 
 mile thick shell of the outer lithosphere : 
 
 Element. Per Cent Symbol 
 
 Oxygen 
 
 47.17 
 
 
 
 Silicon 
 
 28.00 
 
 Si 
 
 Aluminum 
 
 7.84 
 
 Al 
 
 Iron 
 
 4.44 
 
 Fe 
 
 Calcium 
 
 3.42 
 
 Ca 
 
 Potassium 
 
 2.49 
 
 K 
 
 Sodium 
 
 2.43 
 
 Na 
 
 Magnesium 
 
 2.27 
 
 Mg 
 
 •Total 98.06 
 
 Thus 8 of the 80 and more elements comprise over 98 per cent, of 
 the lithosphere. 
 
 Again, although some 1000 different minerals are now known, 
 many of which occur in igneous rocks, only a comparatively few of 
 them are at all common, of these a still smaller number make up the 
 great bulk of the rock of the earth's crust and these last only are 
 know as the Essential Bock Forming Minerals. Others that are im- 
 portant in that they occur often, but not in great bulk, are Imown as 
 Accessory Minerals. 
 
 It will at once be appreciated that the rock forming minerals that 
 
 ♦Above table from Clarke, Data of Geochemistry, Bulletin 491, United 
 States Geological Survey, Washington, D. C. Every student who pretends to 
 have the slightest interest in Geology should obtain a copy of this vol- 
 ume. It is free on application by letter to "Director, United States Geo- 
 logical Survey, Washington, D. C." Many other interesting things about 
 the material and origin of the earth that can not be Included in this text 
 was discussed in the bulletin. 
 
THE EARTH'S CRUST 13 
 
 make up the bulk of the igneous rocks must be almost whoUy com- 
 posed of the few elements that comprise so much of all terrestrial ma- 
 terial. But with only 8 elements a great number of combinations in 
 various proportions would be mathematically possible. Two factors, 
 however, operate to prevent any such great number occurring. First 
 the elements combine chemically only in certain proportions accord- 
 ing to a property kno\vn as their valence, second, of the compovmds 
 chemically possible, only such combinations as are relatively quite 
 stable persist in bulk. In other words the great mass of essential 
 rock forming substance comprises those few minerals that are most 
 nearly unchangeable with reference to the conditions that prevail in 
 the lithosphere as a whole. This must not be understood as implying 
 that important changes are not taking place. On the contrary very 
 material readjustments are now occurring, and have occurred con- 
 stantly in the geological past, and, as has been indicated, a study of 
 these changes constitutes a large part of Dynamic Geology; never- 
 theless, by and large, a great equilibrium seems to have been estab- 
 lished in the material of the earth's crust and, because of this, only 
 the few minerals, described below as the most important ones in 
 igneous rocks and certain others occurring in the other two great 
 rock classes, require consideration here. 
 
 Curiously enough by far the most abundant mineral. Feldspar, 
 forming nearly 60 per cent, of the material of igneous rocks, is very 
 little known popularly; probably because igneous rocks outcrop only 
 comparatively infrequently in the more densely settled areas of the 
 earth's surface, and because feldspar is especially prone to alteration 
 near the surface, changing, primarily, to clay. 
 
 Chemically there are a series of feldspars, each variety contain- 
 ing 0, Si and Al plus K, Na, and Ca according to the species. Ortho- 
 clase containing K in addition to 0, Si and Al, and making up the 
 bulk of granite rock, is the most common variety. The characters 
 by which feldspar (and other minerals and rocks) may be identified 
 can be satisfactorily studied and understood only with actual speci- 
 mens in hand and are not, therefore, detailed here. 
 
 Quartz, second only to feldspar in abundance, is quite familiar 
 to almost everyone as the chief constituent of sand, and also in its 
 crystal occurrence. Yet it comprises but 12 per cent, of the litho- 
 sphere. Unlike feldspar, it is chemically very simple, being made up 
 of one part of Si and two parts of O, written SiO^ Because of its 
 simple chemical composition and physical constitution quartz is ex- 
 traordinarily resistant to change, therefore persists so commonly in 
 its original form. As in the case of feldspar there are a great num- 
 
14 DYNAMIC GEOLOGY 
 
 ber of varieties of quartz besides the common, transparent, crystal 
 kind; but unlike feldspar again, these differ not in chemical composi- 
 tion but primarily in physical properties, therefore, are more unlike 
 in appearance. Thus, flint, agate, opal, jasper and chalcedony are 
 non-crystalline forms of quartz with an admixture of water. These 
 are not important as rock forming minerals, especially not m igneous 
 rocks. Bose quartz and milky quartz are colored varieties as indicat- 
 ed by their names, the shades being due to minute inclusions of for- 
 eign material. 
 
 The Amphiboles and Pyroxenes are groups of minerals made uj) 
 of Si and with Mg, Fe and sometimes Al instead of the K, Na and 
 Ca of the feldspars. Hornblende and augite the most common var- 
 ieties, respectively, of the two groups, are usually dark colored or 
 black and occur in many igneous rocks in little grains. However, 
 rocks of which either pyroxenes or amphiboles are bulk ingredients 
 are not uncommon, but the amphibole of such varieties is usually due 
 to changes after the original cooling of the molten material. Basalt 
 is a common igneous rock containing, frequently, large quantities of 
 augite pyroxene. Like the feldspars the pyroxenes and amphiboles 
 are prone to alter into other, commonly softer substances, particularly 
 the minerals chlorite and serpentine, which give many igneous rocks a 
 greenish color. 
 
 Mica, though rarely the dominant mineral of any igneous rock 
 species, is, nevertheless, so widely distributed in rocks that it must be 
 considered a prominent component. Chemically it also is a com- 
 pound of Si and (a silicate) with Al and Hydrogen (H) plus K, 
 Mg and Fe commonly. Its quality of splitting (cleaving) into very 
 thin flexible plates makes it readily recognizable. Two varieties are 
 common, muscovite light colored and biotite dark colored. Biotite is 
 readily altered, but muscovite, under natural conditions, is very diffi- 
 cultly changed. Thus muscovite scales are often found in soils, but 
 eventually such scales change to clay. 
 
 Feldspar, quartz, augite, hornblende and mica make up the bulk 
 of most igneous rocks; are, in them, the essential rock forming min- 
 erals. In addition to these the accessory minerals olivine, pyrite and 
 magnetite deserve mention at this point bcause of their frequent oc- 
 currence in igneous rocks. The first is a silicate of Fe and Mg, the 
 second an Fe and sulphur (S) compound, the third an iron oxide or 
 compound of Fe and 0. 
 
 Both the names and the facts given aboilt the minerals above 
 should be systematically studied by the student, committed to memory 
 
THE EARTH'S CRUST 15 
 
 in fact, since these basic materials and fundamental facts of their 
 composition must be understood as a preparation for any worth while 
 study of many of the processes of Dynamic Geology. On the basis 
 of the character of the minerals that compose them, a general dis- 
 tinction is made between siliceous, or acid, and basic igneous rocks. 
 In the acid types silicon is present in large proportions ; in the basic 
 varieties the minerals are such as have larger proportions of the metal 
 elements, e. g. iron and calcium. As a rule igneous rocks that are 
 light colored or white are of the acid variety ; the basic minerals are 
 commonly dark or black. This distinction is very serviceable because 
 it diiferentiates, in general, between the igneous rocks that are rela- 
 tively stable, chemically, under the atmosphere, the acid kind ; and 
 those which decompose much more readily, the basic kind. 
 
 The Structure or physical characteristics and texture, as well as 
 the composition of rocks, are important to a discussion of the changes 
 they undergo when subject to geological processes. In general the 
 texture of igneous rocks, like their mineral composition, is quite 
 simple. As previously defined igneous rocks are made up of inter- 
 locking crj'stals of one or several minerals. This general statement, 
 however, applies fully only to what may be termed the most character- 
 istic group of the igneous rocks, the Grained or Eolo crystalline type. 
 In this type the different mineral particles are each distinctly visible 
 and recognizable to the vmaided eye, and are usually rather uniform 
 in size. This size may vary from that of small shot or peas or even 
 larger; accordingly a particular grained rock may be referred to as 
 fine, medium or coarse grained depending on the average size of the 
 mineral crystals composing it. The slower the rate of cooling of 
 the molten material or magma, as this is termed, the coarser will be 
 the grain of the resulting igneous rock as a general rule. Since very 
 slow cooling takes place only at great depths below the earth's sur- 
 face, a variety of terms suggestive of this fact, are applied to the 
 grained igneous rocks, e. g. Plutonic, Hypogene, Intrusive, Abyssal. 
 These terms refer to the place of origin of the grained igneous rocks ; 
 another term commonly used, Granitoid, like grained, refers to tex- 
 ture ; and is derived from the name of the best known variety of this 
 type of igneous rocks, granite. The average superior hardness of the 
 igneous rock minerals and the completeness of the interlocking of the 
 mineral crystals, give the grained igneous rocks their great physical 
 strength, resistance to crushing and grinding particularly, that makes 
 them so well adapted for use in massive structures and where they are 
 subject to hard wear. 
 
16 DYNAMIC GEOLOGY 
 
 Not all magmas cool slowly at great depths, hence some igneous 
 rocks have mineral crystals so small that they can be only difficultly 
 or not at all, identified by the naked eye. Such rocks are termed 
 Dense or Cryptocrystalline with reference to their texture and 3yp- 
 abyssal referring to their place of origin. Sometimes the molten 
 rock material is chilled so rapidly that no opportunity is given for 
 individual minerals of definite composition to separate out from the 
 magma solution. The resulting igneous rock resembles artificial glass 
 in physical characteristics, except that of transparency, in fact is a na- 
 tural glass and is, therefore, termed Olassy. Since such rocks are 
 generally formed at or near the earth's surface the terms Volcanic, 
 Epigene and Extrusive are often applied to them. Lava extruded 
 from volcanoes cools rapidly, therefore, commonly gives rise to glassy 
 igneous rocks. 
 
 It may happen that conditions, during the comparatively rapid 
 cooling of a magma, may be such as to favor the separation and form- 
 ation of only a single mineral species. In this event large crystals of 
 that mineral develop while the rest of the material remains undifferen- 
 tiated or glassy. An igneous rock in which such single, large crystals 
 or phenocrysts of one kind of mineral occur, while the rest of its 
 material is a glassy or cryptocrystalline groundmass, is said to have a 
 Porphyritic texture. It will readily be appreciated that the por- 
 phyritic igneous rocks are commonly hypabyssal or extrusive in origin. 
 
 Thus it appears that all variations of texture are possible in ig- 
 neous rocks, from coarse granular to glassy, and to mixtures of the 
 types ; yet that it has been found feasible to apply a few distinctive 
 names to characteristic variations ; names that, nevertheless, are broad 
 enough in their sense to cover the whole range of textures. 
 
 CLASS II. STRATIFIED ROCKS, THEIR CONSTITUENTS AND 
 STRUCTURES. 
 
 It has previously been pointed out that, at variable depths below 
 the earth's surface, a continuous mass of bed rock everywhere under- 
 lies the unconsolidated mantle rock. Also, that feldspars comprise 60 
 per cent, of a 10 mile thickness of the lithosphere. Since the oc- 
 currence of feldspars is confined almost exclusively to igneous rocks, 
 it follows that the igneous rocks must comprise a predominating por- 
 tion of the bed rock material. Yet in surface outcrops the second 
 class of bed rocks. Stratified Bock, is most frequently found. Inas- 
 much as igneous rocks comprise the bulk of the bed rock, this frequent 
 outcropping of the stratified rocks indicates that they must form a 
 thin, but widespread, outer shell, or capping, to a more massive core 
 
THE EARTH'S CRUST 17 
 
 of igneous bed rock. In fact, investigation has shown that, at a max- 
 imum, the bulk of the stratified rock would suffice to form not more 
 than a 1/2 mile deep outer shell of a 10 mile thickness of the litho- 
 sphere. At some places the stratified rocks are wholly absent, at 
 others they are much thicker than a 1/0 mile, yet on the average their 
 occurrence is quite widespread and comparatively thin. 
 
 "Why this should be so is readily understood when it is learned 
 that the stratified rocks are made up of fragments of mechanically 
 and chemically disintegrated igneous rock. It may be that these 
 fragments have one or more times previously to their present occurr- 
 ence formed stratified rocks, and been in turn disrupted, yet originally 
 their material was derived from igneous rocks. Because of this, strati- 
 fied rocks are often referred to as Detrifal rocks and since the frag- 
 ments that compose them have most frequently acquired their present 
 arrangement by settling do-wn through water, Sedimentary rocks. 
 
 The principal structural characteristic of the stratified rocks is 
 that they occur in layers, heels or strata, practically synonymous 
 terms that are used to designate their unit members. These strata, 
 or beds, are separated by what are termed stratification planes or 
 iecIcJing planes, which are found where an abrupt transition occurs 
 in the nature of the material of the two beds separated by the plane, 
 or, if the material in the two beds is similar, mark some interruption 
 during their deposit. Such bedding planes are parallel ^Tith the ex- 
 tension of the strata, and should never be confused with the vertical 
 or nearly vertical, joint planes or joints (resemblance to masonry) 
 that are most characteristically developed in the stratified rocks, but 
 occuj also in the other two great rock classes. Neither the bedding 
 nor the joint planes are necessarily ■v'isible crevices in the rock, yet 
 they commonly serve to give entrance for air, water and other solu- 
 tions to the rock; and along their extensions the rock breaks most 
 readily. Since the igneous rocks have a greater total bulk and lack 
 bedding planes, also because joints in them may be more infrequent 
 and less perfectly developed than in the stratified rocks, their struc- 
 ture is sometimes distinctively referred to as massive. 
 
 As has been previously pointed out the ciuartz granules of ig- 
 neous rocks are practically unalterable. On disruption of igneous 
 rocks such quartz particles, mechanically broken and rounded by 
 grinding, also generally worn to a much smaller size than the original 
 crystal, furnish most of the material of the stratified rock called 
 sandstone. If the fragments are quite large, like peas or walnuts in 
 size, the rock is called a conglomerate. Very infrequently the feld- 
 spar of an igneous rock is broken into fragments and reconsolidated 
 
18 DYNAMIC GEOLOGY 
 
 into a stratified rock. This also is a sandstone, but is distinctively- 
 termed an arkose sandstone. Much more commonly the feldspar is 
 altered into other substances by chemical changes, the bulk of its mass 
 then becomes clay, and when this clay is laid down in beds and consoli- 
 dated the stratified rock resulting is termed sJiale. The calcium that 
 enters into the composition of many igneous minerals generally goes 
 into water solution, as a soluble bicarbonate, when they crumble, and 
 later, often by the aid of organisms, is deposited in the form of the 
 relatively insoluble calcium carbonate. Accumulation and consoli- 
 dation of this calcium carbonate in layers forms the stratified rock, 
 limestone. 
 
 Conglomerate, sandstone, shale and limestone are the principal 
 stratified rocks. Strata or beds of them may be arranged in any 
 order above one another. As a rule their particles are securely bound 
 together or consolidated, but those most recently formed may be quite 
 incoherent. Such incoherent stratified rocks grade into the mantle 
 rock, however, their imiformity of composition and layering, as well 
 as concepts of origin, serve to prevent confusion in the application 
 of terms. 
 
 When consolidated, the stratified rocks owe their splidity of co- 
 herence partly to compaction by pressure but in a much greater 
 measure to the binding action of certain cements. These cements com- 
 prise most frequently calcium and iron compounds and quartz. 
 Quartz, though nearly unalterable chemically, is quite soluble in 
 certain water solutions, the calcium and iron compovmds much more 
 readily so. Consequently these substances are often filtered, in solu- 
 tion, through the pores between the solid particles of the stratified 
 rock, and, under certain conditions, precipitated there. Such a pre- 
 cipitate attaching itself to adjacent particles of solid material serves 
 to bind these together. 
 
 CLASS III. METAMORPHIC ROCKS, THEIR CONSTITUENTS 
 AND STRUCTURE. 
 
 The term metamorphic signifies change of form. The Metamor- 
 phic Rods, therefore, are changed rocks. Geologically the term meta- 
 morphic involves not only change of form bult also change of struc- 
 ture as well. Originally a metamorphic rock may have been an 
 igneous or a stratified rock, but the metamorphic rocks most readily 
 identifiable as such were, in the greater number of instances, strati- 
 fied rocks before the change. Change in rock form and structure may 
 be brought about by a wide variety of processes, as, for instance, the 
 consolidation of the stratified rocks by the precipitation of cementing 
 
THE EAETH'S CRUST 19 
 
 material indicated above, but. as applied to a rock class, the term meta- 
 morphic is used to designate such rocks as have been changed bv 
 processes involving heat and pressure primarily. In some cases the 
 material of a rock remaius the same that it was previously to meta- 
 morphism. only a change in form has taken place. Thus the com- 
 monly irregular and unoriented particles of a limestone, amorphous 
 CAlcimn carbonate, are changed to the crystalline state ia the meta- 
 morphic rock, marllt. In shales new minerals develop, and the material 
 is rearranged as well, by metamorphism. and the rock becomes a 
 slat(. In sandstones the quartz particles partly melt and fuse together 
 forming a quarzite. 
 
 In general metamorphic rocks have crystal structure developed. 
 new minerals formed, material rearranged and are denser and more 
 compact than the originals from which they were derived. Commonly. 
 also, a structural rearrangement giving rise to banding, schiitosity. or 
 slaty ch'ava-ge takes place. These phenomena, in certain respects, re- 
 semble bedding in stratified rocks but are of an altogether different 
 nature. In general they are due to the segregation along certain 
 parallel planes or lines, through the rock, of particular minerals, on 
 something of the same principle that "birds of a feather flock to- 
 gether." If this segregation is only partial, or if the different parallel 
 streaks of the same mineral are rather widely separated, the struc- 
 ttire is termed banding. Gyi^ei^s ''pronounced "nice"'^ is a typical 
 banded, metamorphic rock. When the mineral layers are closely 
 spaced the term schistosity is applied, and when the mineral grains 
 are very small and very closely spaced the metamorphic rock may 
 show slaty cleavage. The parallel bands of minerals need not extend 
 in straight lines, often they are much crumpled and minutely folded. 
 yet retain their parallel relation. 
 
 Banding, schistosity and slaty cleavage are important from the 
 Standpoint of this intrcxiuctory discussion of the materials of the 
 earth "s crust in that, like the bedding planes of the stratified rocks 
 and the .joint planes conunon to all three classes, these structural phe- 
 nomena of the metamorphic rocks are similarly lines and planes of 
 weakness. Along the lines where the relatively weaker minerals of 
 the banded, etc.. metamorphic rocks occur, breaks develop most 
 readily : some minerals dissolve more readily than others and 
 thus give entrance for agents of change to the body of the rock. It 
 should be noted here. also, that there are fracture planes called faults 
 that may extend for long or short distances, in various directions, 
 across all three classes of rock indiscriminately. "Where these occtir 
 
20 
 
 DYNAMIC GEOLOGY 
 
 they afford a further channel by means of which liqiiids and gases may 
 penetrate into the rock. Moreover rock minerals themselves are not 
 impermeable and seldom are the spaces between the particles of sedi- 
 mentary rock completely filled with cement material. Consequently 
 no part of rock material is secure from contact with exterior sub- 
 stances. This vtdnerability of all rocks has a most important bearing 
 on their existence history, and should be fully appreciated if the suc- 
 ceeding pages are to be clearly understood. 
 
 artnmiwi m» afleK^ 
 
 FIG. 1. Diagrammatic section showing position and attitude of various 
 bed rock classes. ("U. S. G. S.) 
 
 Fig. 1 shows in section the relative positions and structures of 
 the three rock classes discussed. The student should study this dia- 
 gram and be prepared to reproduce its essential features. 
 
 It must be understood that in the above paragraphs no attempt 
 has been made to explain the processes by which the principal three 
 rock classes are formed. In this chapter the essential thing is to learn 
 what material is present in the earth's outer mass, its nature and 
 structure ; in order that the operation of the various processes on 
 this material, the next topic to be taken up, may be made intelligible. 
 
 The student who cares to read up further on rocks will find 
 "Kocks and Rock Minerals" by L. V. Pirsson, the best single book on 
 the subject. A careful perusal of its ably and clearly written pages 
 vnll be repaid by a lasting interest in the fascinating study of the 
 rocks and their substance. 
 
CHAPTER II. 
 
 WEATHERING 
 
 DEFINITION. — "Weathering is an inclusive term used to designate 
 the various agencies and effects of the geological processes that cause 
 bed rock to decay and crumble. If these agencies and processes are 
 of a chemical nature, giving rise to alterations or removal of the ma- 
 terial of the rock, what is called rock decomposition takes place; if 
 only a splitting up into fragments of the rock occurs such mechanical 
 disruption is termed rock disintegration. Usually both rock decom- 
 position and rock disintegration are involved in the breakng up of the 
 bed rock of a particular region, though the one or the other commonly 
 predominates according to circumstances. "Weather is primarily a 
 matter of atmospheric conditions, but the term weathering as used 
 geologically has a much broader significance, as will be seen. 
 
 EXTENT OF WEATHERING. — Probably no geological phenomenon 
 is so widespread in its distribution as weathering. In the Arctic, in 
 the Tropics, on mountain top and in valley, in desert spaces as weU. 
 as in humid regions bed rock is rapidly or slowly undergoing dis- 
 ruption by weathering action. Xot rapidly perhaps, in many places, 
 according to human measure of the progress of a process, yet eommonlj' 
 with suffcient rapiditj'. from the geological viewpoint, to have given 
 rise to changes in both the earth's substance and form of the most 
 sweeping character during geological time. 
 
 CAUSE OF WEATHERING. — All terrestrial phenomena that come 
 within the scope of human observation owe their present state of ex- 
 istence to the fact that forces, directed from sources of energj- both 
 within and without the earth, are now acting upon them and have 
 been acting, in a similar manner, throughout long ages of the past. 
 Geological phenomena are no exception to this generalization and the 
 study of weathering, particularlj^, leads to an adequate realization 
 of the importance of the action of these forces in bringing about and 
 conditioning the changes that are occurring, and have occurred, on the 
 earth. 
 
 Of greatest importance are the radiant energy received by the earth 
 from the sun, manifest to human senses primarily as heat and light; 
 
22 DYNAMIC GEOLOGY 
 
 and the heat energy conducted toward the exterior from the hot central 
 core of the earth. Gravitation, the force that holds the planets in 
 their orbits; and gravity, the force that exerts a pull toward the cen- 
 ter of the earth on all terrestrial matter; also have important func- 
 tions in geological economy. These by no means exhaust the list of 
 forces and sources of energy, e. g. terrestrial magnetism and the en- 
 ergy liberated by radium would need to be considered ; but they are the 
 ones that exert the greatest influence and compel recognition of the 
 fact that the earth's matter is nowhere, and at no time, in a static 
 or stable condition; that, on the contrary, inorganic, as well as or- 
 ganic substance, is continually in a state of flux and change. 
 
 Weathering processes are initiated, primarily, by the heat energy 
 from the sun, and are conducted throtfgh the agencies of air, water 
 and life. 
 
 Radiant heat energy from the sim is absorbed by water and rock 
 and raises their temperature. Rock and water in turn part with a 
 portion of the heat energy they receive and convey it by conduction 
 and convection to the air that is in contact with them. Since the rocks 
 and water bodies receive heat only during the time that the sun is 
 above the horizon ; and since, according to season, different areas of 
 the earth's surface receive more heat than others; and, tiaally, because 
 in some areas the heating effect is more intense, during a given time 
 period, than in others ; it follows that at different places the rocks and 
 water convey varying quantities of heat to the air in contact with 
 them. Thus local differences in the temperature of the atmosphere 
 are set up. 
 
 The air is a mixture of gases. Gases expand in volume, on being 
 heated, although their total mass remains unchanged. A column of 
 warm expanded air, therefore, exerts less pressure on a unit area of 
 the earth's surface than does a column of cooler, denser air on an 
 adjacent, equal area of surface. Unequal heating, consequently, sets 
 up unstable conditions in the atmosphere, and, since gases are extreme- 
 ly mobile, air currents or winds are generated tending to equalize these 
 differences in pressure. Heavy air, from regions of lower temperature, 
 flows in and displaces lighter, heated air, pushing it up to higher al- 
 titudes. Differences in heating that involve areas of world magnitude 
 set up great air currents, the planetary winds; local differences of 
 temperature, winds that move only for short distances. The import- 
 ant fact, from the standpoint of this discussion, is that by means of 
 these air currents temperature differences may be conveyed from one 
 locality to another over great as well as small distances. In this man- 
 
WEATHERING 23 
 
 ner rocks may be heated or cooled by contact with air coming from 
 another locality. In short, as day is succeeded by night, as the sea- 
 sons come and go, as winds blow first from one quarter then another, 
 rocks are subjected to slow and rapid changes of temperature. 
 
 A portion of the water warmed by the sun's heat is converted 
 into water vapor, a gas, which mingles with the other gases of the 
 atmosphere and is, by the winds, conveyed to places remote from the 
 point where it was formed. Under favorable conditions, primarily on 
 cooling of the air, the water vapor changes back to water (or snow 
 or hail) and is precipitated on land surfaces. In the form of water 
 much of this precipitated moisture sinks through the soil and mantle 
 rock and soaks into the pores and crevices of the bed rock. The water, 
 during its passage through the air as rain, dissolves in itself some of 
 the atmospheric gases, notably carbon dioxide and oxygen, and carries 
 these with it into the rock. 
 
 Differences in temperature of water and air make for differences 
 in their chemical activity, or ability to combine with other substances. 
 Water heated to a certain temperature may be able to dissolve a given 
 mineral substance, whereas colder water would affect the same sub- 
 stance relatively little. The same thing is true of the oxygen of the air. 
 
 Finally, plant and animal life both owe existence to the light and 
 heat energy received from the sun. This enables them to grow, and 
 growing, to extract part of their substance and to thrust their roots 
 into the underlying rocks. 
 
 Prom the above paragraphs it will be clear, then, that the heat 
 energy of the sun is everywhere effective through and by the agencies 
 of the air, water and life in giving rise to processes of change that 
 cause rocks to disintegrate and decompose, or, acting in concert with 
 a few other related processes, to weather. 
 
 It is important next to consider individually some of the more 
 important of these weathering processes and note the nature and ef- 
 fectiveness of their action. 
 
 EXFOLIATION.— As the sun's rays, during the day, fall on out- 
 cropping bed rock its surface temperature rises rapidly; for rocks ab- 
 sorb a considerable portion of the heat radiation that comes to them. 
 In common with most other substances rock material expands on heat- 
 ing. But the notable temperature changes in outcropping rock, with 
 consequent marked expansion of the rock material, are largely con- 
 fined to an outer zone of comparatively slight depth; because rock, 
 while a aood absorber, is a noor conductor of heat. Thus the surface 
 
24 DYNAMIC GEOLOGY 
 
 layer of rock, which receives the heat first, and which, because of 
 poor conduction, has its temperature raised much higher than the in- 
 terior portions, expands more than the bulk of the rock; accordingly 
 becomes too large for it, therefore, tends to shell off, or pull away 
 from the general mass. Similarly, at evening, the exterior portions 
 cool off much more rapidly than the interior, consequently contract 
 more rapidly, and there develops again the tendency for them to pull 
 away from the less rapidly shrinking general mass of rock beneath. 
 Thus both on heating up and cooling down marked disruptive stresses 
 are set up in the surface layers of exposed bed rock, in consequence of 
 which these layers tend to peel or shell off from the main mass. This 
 process of rock disintegration under the direct action of the sun's rays 
 is known as exfoliation. 
 
 DIFFERENTIAL EXPANSION AND CONTRACTION OF MINERALS 
 
 Igneous rocks, it has been noted, and some metamorphic rocks as 
 well, are typically made ujp of interlocking granules of crystals of 
 different minerals, of which quite a few may be present in one rock 
 variety. As these minerals are made up of different combinations of 
 elements they differ in physical properties, thus they have different 
 rates of expansion on heating. Again some of the minerals are light, 
 others dai'k colored. Dark substances absorb heat much more rapidly 
 than light colored materials, a phenomenon acutely experienced when 
 black clothes are worn in bright, summer svmshine. Therefore, both 
 because they have inherently different coefficients of expansion, and 
 since one mineral may become hotter than its neighbor because it 
 absorbs more heat ; the component granules of a rock made up of dif- 
 ferent minerals, on heating and cooling, strain and thrust against their 
 neighbors ; often exerting such force that the rock is split and shatter- 
 ed. This process is more effective in coarse than in fine grained rock, 
 in igneous than in stratified rocks, since the more uniform the ma- 
 terial the less opportunity is there for differential expansion and 
 eonti'action within its substance. 
 
 Great and frequent changes of temperature favor rock disinte- 
 gration by exfoliation and differential expansion and contraction of 
 minerals. Such conditions are notably present in desert regions, in 
 tropical latitudes and on mountain summits. In desert areas and 
 on moiuitain tops, moreover, plant cover is commonly absent, expos- 
 ures of bed rock of large area occur; weathering by these particular 
 processes is, therefore, especially active in such localities. 
 
 FROST Changes of temperature effect the disintegration of rock 
 
 on a large scale, through the agency of water, in those latitudes where 
 
WEATHERING 25 
 
 the cold in the winter season falls below the freezing point. On freez- 
 ing water expands about 1-11 to 1-10 of its volume and this expan- 
 sion proceeds with great force (as is evidenced by the frequent burst- 
 ing of iron water pipes in houses) exerting a pressure of perhaps 
 2000 pounds per square inch on the surface by which it is confined- 
 As the downward percolation of rain, in moist regions, commonly keeps 
 the pores and crevices of rock near the surface filled with water, it 
 will readily be appreciated that, when the temperature falls below the 
 freezing point, large and small pieces will be pried and wedged off 
 the parent mass of the rock affected by the freezing temperature.. 
 Probably no rock material is sufficiently strong to withstand stresses 
 as great as freezing water exerts. Joint planes, stratification planes 
 and other pores and crevices in the rock are enlarged and pried open. 
 Then, on a thaw, these enlarged cavities again fill with water, and re- 
 freezing further opens and rends the rock. Obviously, therefore, 
 many freezings and thawings favor disintegration of rock by wedge 
 work of ice, accordingly, this process is most effective in temperate 
 latitudes, in early and late winter, and in moist rather than dry re- 
 gions. In dry regions water is not present in quantity to fill the 
 rock crevices, in tropical latitudes freezing point is not reached and 
 in the Arctic the cold continues below the freezing point for months,, 
 giving no opportunity for the many repetitions of the process which 
 make it so effective in the temperate latitudes. Not only is the solid 
 bed rock affected by the wedge work of ice but also the mantle rock, 
 both soil and subsoil. Soil particles are forced further apart, pebbles 
 in the soil are split ; the soil is thus made more friable and finer ; its 
 condition favoring crop growth, or tilth, accordingly improved. Frost 
 action also does damage, in that fence posts and young crops may 
 be heaved or lifted out of the soil, and sometimes its powerful pressures 
 penetrating under the foundations of buildings and other structures 
 affect their stability. 
 
 GRAVITY — Although not dependent on the sun's heat for its ef- 
 fects, the action of gravity in weathering may best be mentioned here, 
 because it, like the other processes so far described, tend=! to disinte- 
 grate or mechanically disrupt rocks. All material on tJie earth is 
 subject to the pull of gravity which tends to bring the material toward 
 the earth's center. Therefore, all overhanging and unstable massei of 
 rock are constantly subjected to this stress, which, while not frequent- 
 ly of sufficient magnitude to break them off, if only instantly ap- 
 plied, yet, by its unremitting pull, eventually suffices to overcome their 
 
26 DYNAMIC GEOLOGY 
 
 resistance to fracture and thus often bring about the separation of large 
 masses. 
 
 Another disintegrating process, due to the force of gravity, is 
 the mechanical beating of raindrops which, if not effective in causing 
 fractures, nevertheless suffices to break out small particles previously 
 loosened by other agencies. 
 
 OXIDATION. -The processes enumerated above were all mechan- 
 ical in their action, resulting in the splitting of large rock masses into 
 smaller and smaller fragments. Of equal, or even greater importance, 
 in weathering are the chemical processes by which the substance of 
 the rock is altered in composition or in part removed. 
 
 Oxygen is an extremely active element chemically, constantly tend- 
 ing to unite with other elements. Burning is simply the union of 
 oxygen with some other element or compound ; thus iron may burn 
 as well as wood. Union of oxygen with inorganic and organic com- 
 pounds may take place much less violently than by burning, this 
 slower union is called oxidation and is facilitated by the presence of 
 moisture. Many rock minerals on contact with the atmosphere unite 
 with its oxygen to form new compounds, and rain water carries oxy- 
 gen in solution to rock below the surface. Oxidation affects particular- 
 ly those igneous rock minerals containing iron, e. g. mica, olivine, augite 
 and hornblende. In their original state such minerals are ulsually dark 
 in tint or black ; when oxidized they change to yellow and red colored 
 substances. Distinctly brown-red and red colors indicate more com- 
 plete oxidation than yellow, and in this the effect of higher tempera- 
 ture is manifest for red colors predominate in the weathered rocks 
 and soils of low latitudes. 
 
 CARBONATION Oxygen combines with rock minerals not only 
 
 as a simple element but also as a compound with other elements. Thus 
 the carbon dioxide (CO^'' of the atmosphere unites with rock minerals 
 on a vast scale forming various carbonate compounds. Like oxygen, 
 carbon dioxide is carried to rocks below the surface dissolved in rain 
 water, and much of this dissolved supply is furnished by decaying 
 (oxidizing) vegetation in the upper soil layer. Accordingly the rate 
 of carbonation is somewhat dependent on the amount of vegetation 
 present, therefore, takes place most rapidly in humid, tropical regions. 
 
 HYDRATION Even more extensive than carbonation in the scale 
 
 of its reactions is hydration or the union of water with minerals pre- 
 viously anhydrous, or lacking water. Carbonation and hydration 
 nsuaUy proceed in unison, and one of the most important results of 
 
WEATHERING 
 
 27 
 
28 DYNAMIC GEOLOGY 
 
 their combined action is the formation of a large proportion of the 
 clay substance found beyond the glaciated regions. Such clay is 
 chiefly derived from the decomposition of feldspar. Thus potassium 
 feldspar (orthoclase) plus water, plus carbon dioxide yields clay 
 (kaolin) potassium carbonate and quartz. (Chemically formulated this 
 reaction has been expressed as follows: 
 a K Al Sis Og plus 2 H2O plus COj equals H2 AI2 Sij O9 plus K2 CO3 plui 4 Si 02. 
 
 Oxidation, carbonation and hydration, all three bring about the 
 decay of rock, in that the new minerals formed have greater volume 
 than those originally present ; with the result that pressures tending to 
 crumble the rock are developed. Such increase in bulk may amount 
 to 50 per cent or more by hydration alone. Heat, further promoting 
 chemical reactions, is liberated. Usually the new minerals formed 
 are softer than the original ones giving better opportunity for the 
 operation of the mechanical weathering processes in breaking up the 
 rock. These three chemical processes affect igneous and metamorphic 
 rocks primarily. The student will find a more detailed statement of 
 their action in a paper by Buckman, H. 0., entitled "Chemical and 
 Physical Processes Involved in the Formation of Residual Clay." 
 (Trans. Amer. Ceramic Soc. Vol. XIII) which also contains a good 
 bibliography on weathering. 
 
 SOLUTION. — Much of the crumbling of rock is due to the re- 
 moval of part of its substance by the dissolving action of water, the 
 process of solution. While pure water dissolves only a few minerals 
 readily, practically all the water in rocks contains impurities that pro- 
 mote its solvent action. Of these impurities carbon dioxide is prob- 
 ably the most important. With its aid percolating water has in cer- 
 tain regions dissolved so much of the limestone rock underground as 
 to give rise to vast caverns as illustrated in Fig. 2. 
 
 Where the solution action is so complete only the impurities of 
 the limestone remain behind, generally comprising an iron stained 
 clay. In other rocks the solution may remove certain minerals only, 
 leaving the rock honeycombed and crumbly ; or it may dissolve the 
 cement that binds together the quartz particles of a sandstone making 
 this extremely porous, if not altogether incoherent. 
 
 LIFE. — Id general, living organisms act as promoters of weather- 
 ing rather than as active agents. Yet plant, especially tree roots, pen- 
 etrate long distances into the joint and bedding plane crevices of bed 
 rock and, in growing, exert notable pressure in forcing these open. 
 Lichens extract substance directly from the rocks on which they grow, 
 
WEATHERING 29 
 
 and other plants give off organic acids from their roots that act vigor- 
 ously on mineral substances. 
 
 Trees, overturned by the wind, promote weathering in that their 
 roots bring up and expose fresh material to other weathering processes. 
 Similarly earthworms in the course of years turn over tons of ma- 
 terial. In Brazil the burrowing and the burying of leaves by ants is 
 important ; in northern latitudes the woodchuck does his share of work. 
 Nor must the activities of man within historic time be overlooked. In 
 his agricultural and building operations man has exerted a tremendous 
 influence in promoting weathering processes. While vegetation in 
 some waj's acts as a direct weathering agent, in others promotes weath- 
 ering; in general its effect is protective, since one of its principal 
 functions is to hold a blanket of decomposed material in place over 
 the fresh bed rock and thus prevent access to this by the more active 
 disintegrating processes. Man, by removing this vegetation cover over 
 large areas, has done much to hasten the rate of rock destruction. 
 
 RATE OF WEATHERING. — In the previous paragraphs, descriptive 
 of the processes, something has been said of their relative, indi^ddual 
 effectiveness, both as to kind and locality. Since, however, the pro- 
 cesses do not act singlj', it is neccssarj^ to consider the factors that de- 
 termine the comparative rate of their attack when they combine 
 forces. 
 
 Much depends on the character of the rock attacked. Open tex- 
 tured rocks admit the agencies of weathering more readily than dense 
 ones, thus open textured rock material facilitates frost action, solu- 
 tion, oxidation, carbonation, hydration. Rocks with dark minerals 
 suffer more by exfoliation and differential expansion and contraction 
 than rocks in which the minerals are light. Again, rock exposed on 
 steep slopes, or in regions where vegetation cover is absent, are more 
 rapidly and continuou.slj' broken down than those in places where the 
 opposite conditions obtain. On gentle slopes the waste of weathering 
 accumulates, forming a blanket that effectively inhibits the action of 
 the mechanical, disintegrating processes ; and a vegetation cover holds 
 such a blanket of waste in place on quite steep slopes. 
 
 Other conditions being the same the rate of weathering in differ- 
 ent localities depends on the number of processes acting and the vigor 
 of their attack. In desert regions nearly all the chemical processes 
 and frost action are checked; in the Arctic, cold checks the action of 
 the chemical processes, while the ice and snow cover present almost 
 totally stops the mechanical processes. 
 
30 
 
 DYNAMIC GEOLOGY 
 
 Probably motmtain summits are places where bed rock is most 
 rapidly crumbled, by mechanical processes, primarily; while rocks 
 imder moist tropical lands suifer most from chemical weathering. 
 Gravity keeps the steep slopes of mountains free from weathered frag- 
 ments, thus fresh rock is continually exposed; daily temperature 
 ranges on mountain summits are extreme, much moisture is present, 
 exfoliation, differential expansion and contraction and frost action are, 
 therefore, effective and chemical processes are not absent. In moist 
 tropical regions rocks are decayed to depths of 300 feet or more, due 
 to the great chemical activity of the warm percolating water, supplied 
 with an abundance of organic acids and carbon dioxide from the 
 luxuriant vegetal growth. 
 
 Finally, the fact that over most of the surface of the land there 
 is spread a mantle of completely crumbled, weathered rock, the soil 
 and subsoil, indicates that, on the whole, weathering proceeds at a 
 more rapid rate than the transporting processes that carry away wea- 
 thering waste. 
 
 RESULTS OF WEATHERING. — Undoubtedly the most important re- 
 sult of weathering is the formation of residual soil, so called from its 
 origin ; and also referred to as a sedentary soil to indicate its position 
 
 FIG. 3. Sectional diagram showing gradations In transition from re- 
 sidual soil at surface, to fresh, unaltered hed rock below. (After Tarr.) 
 
 above the country rock ( as the bed rock of any given locality is called ^ 
 from which it was derived. Residual soil, as the term indicates cora- 
 prises the residual material resulting from the weathering of bed rock 
 occupying the position in which it was formed; though its bulk 
 may represent only a small part of the rock mass from which it was 
 
WEATHERING 31 
 
 derived, some of the original material having been removed by solu- 
 tion ; while much of the volume of the residual soil may be due to 
 the combined oxygen, carbon dioxide and water of the mineral sub- 
 stance formed by weathering. Commonly residual soil is a clay, and 
 as one penetrates from the surface (where this clay is mingled with 
 organic matter) do'vvnward. a gradual transition is found to occur 
 from pure residual material, in the subsoil, to a mixture of residual 
 soil and partly weathered boulders of bed rock below it and finally to 
 the imaltered country rock from which the residual material was 
 formed and on which it is now seated (sedentary.) In the diagram 
 Fig. 3 this transition is graphically shown in section. Be prepared 
 to reproduce and explain this drawing. 
 
 Another important result of weathering is the furnishing of 
 streams and the wind with material to transport and deposit, and to 
 use as tools in wearing down the rock they flow over and blow against. 
 "Weathering also furnishes the dissolved mineral substances that make 
 stream waters "hard" (carbonates) ; gives rise to mineral springs and 
 causes the "saltness" of the ocean. 
 
 Since some minerals and rocks are attacked more readily than 
 others by the weathering processes, and since rocks may vary consid- 
 erably within small areas, weathering results in both the etching on a 
 small scale, and the development of larger relief forms in the rock 
 formations; in other words, plays an important role in the carving 
 out of the topography of the land; the study of which constitutes a 
 principal part of the domain of Physical Geography. 
 
CHAPTER III. 
 
 UNDERGROUND WATER 
 
 SOURCE AND SIGNIFICANCE OF GROUND WATER Atmospheric 
 
 precipitation, moisture in the form of rain, snow, hail, etc., supplies 
 practically all the so-called "fresh" water of the earth. A small 
 amount is probably furnished from deep seated sources, the hot interior 
 of the earth, but this constitutes only a minor part of the volume of 
 fresh water existant at any one time. Water that has its origin in 
 atmospheric precipitation is termed meteoric water. That which comes 
 from deep seated sources, magmatic or juvenile ivater. Meteoric wa- 
 ters constitute all but a possible very small fraction of the volume of 
 streams, ponds and lakes ; but the geologic functions of magmatic wa- 
 ters are, imder certain conditions, quite significant. 
 
 When rain falls or snow melts a part of the water flows off over 
 the surface to rivulets and larger streams, this constitutes the immed- 
 iate run off. A second part is returned to the air by evaporation as 
 water vapor. A third portion sinks into the soil and rocks. This is 
 the underground, or, more concisely, the groutid water. Some of this 
 ground water shortly reappears at the surface, then it also constitutes 
 a part of the run off, but not the immediate run off. Much of the 
 ground water, however, pursues subterranean courses to the sea, per- 
 forming important geologic functions en route. 
 
 The proportion of the precipitation that sinks into the grouiid 
 varies with a number of factors. Steep slopes, dense rocks, absence 
 of vegetation cover, heavy, sudden precipitation all favor a large im- 
 mediate run off. The opposite conditions favor percolation into the 
 ground. These relations are important from a practical standpoint. 
 The recent high waters of the Seine that resulted in the flooding 
 of Paris were due to exceptionally heavy precipitation at the sources 
 of that river which are located in a region of dense rocks and steep 
 slopes with only a thin soil cover. The disastrous floods in Ohio and 
 elsewhere in the Central States in 1913 were also caiised by a very 
 great precipitation ; falling in this case on a wide extent of country 
 practically all under cultivation. Formerly a large part of this re- 
 gion had a thick forest cover that served as a decided cheek on the im- 
 mediate run off; the streams, consequently, never rose so rapidly nor 
 
UNDERGROUND WATER 33 
 
 attained such high stages. As more and more land is cleared of forest 
 the percentage of immediate run off grows larger. Deforestation of the 
 •steep slopes of the Appalachian region, by lumbering operations, has 
 resulted in an increasing number of floods in the Ohio river, and sub- 
 sequently very low water stages during periods of little rainfall. The 
 detriment of such conditions is twofold ; the floods destroy life and 
 property while the protracted periods of low water seriously impede 
 navigation of the stream. As the vegetation cover is the only important 
 factor afiiecting the rate of immediate run off over which man can 
 exercise practical control, it would seem well worth while to think 
 twice before clearing off forest lands, especially from steep slopes. 
 
 THE WATER TABLE — While the Surface of most land is usually 
 dry, even in humid regions, penetration of the mantle rock or soil for 
 -only a few inches or feet usually reveals the presence of moisture; 
 and, if the excavation be made deep enough, a condition of complete 
 Saturation, or filling of all the mantle and bed rock pores and crevices 
 with water, \^'ill be encountered. The level at which this condition of 
 saturation is found is kno^^Ti as the ground water level or icater table. 
 
 EXTENT AND VOLUME OF GROUND WATER. — That, in humid re- 
 gions, the mantle and bed rock pores and crevices are completely filled 
 with water at no great depth below the surface is proved by the gen- 
 eral possibility of securing water from comparatively shallow wells. 
 The presence of such water supplies is also indicated by the flow of in- 
 numerable springs, and by the continual seepage of water into mine 
 shafts. In arid regions the water table may be far below the surface, 
 and as a rule it lies deeper under a hill top or steep slope than under 
 adjacent level, low lying lands. At the level of stream courses, lakes 
 and the ocean the water table comes to the surface, in fact streams owe 
 their perennial flow in large part to the seepage of ground water from 
 their immediate banks and the bottom of their channels. 
 
 From the above it is apparent that the water table is not a 
 level surface. Even in a flat country a local rain will tend to elevate 
 it in that locality, in fact may raise it to the surface. Under the in- 
 fluence of gravity the ground water, like surface waters, flows from 
 higher to lower levels, but is impeded in its flow by the small dia- 
 meter of the passages between rock particles and the consecpient large 
 amount of friction opposing its movement. Thus while a local rise 
 of the water table due to a heavy rain may disappear before the next 
 downpour occurs, yet the movement of the ground water is too slow 
 for a uniform level of the water table to be established, certainly be- 
 tween humid and arid regions or even below hill and valley. In 
 
34 DYNAMIC GEOLOGY 
 
 general the water table follows the surface slope of the country but 
 is at greater depths below hill slopes and summits than in lowlands, 
 consequently has not such steep slopes as the surface topography. Fig. 4 
 shows the relations of the water table to the surface relief and also 
 the conditions for shallow wells and hillside springs. 
 
 FIG. 4. Cro8S"sectioii diagram showing relation of ttie level of the 
 ■water table to surface topography, and the intersection of the water table 
 with the surface, giving rise to springs and supplies of water to streams. 
 (After Fuller.) 
 
 It was formerly thought that the rocks below the water table had 
 their cavities completely filled with water to as great depths as crev- 
 ices existed. Many mining shafts, however, are dry and dusty below 
 depths of 2500 feet. This indicates that the ground water occupies 
 a comparatively near surface zone in the rocks. Where, however, such 
 generally dry, deep shafts intersect large fissures in the rock, water is 
 often encountered. This indicates that water may follow the larger 
 openings to great depths underground. The water in the smaller pores 
 evidently either escapes to the surface in springs, follows near surface 
 courses to the sea, or is used up in the hydration of minerals before 
 it penetrates to great depths. It has been variously estimated that 
 the amount of ground water in the rocks would form a layer covering 
 the whole surface of the earth from 96 to 3500 feet deep. It will be 
 appreciated that an enormous quantity is involved even in the minimum 
 estimate. 
 
 WELLS AND SPRINGS. — As previously stated shallow wells are 
 supplied by groimd water. The permanence and continuity of 
 their supply is dependent on the question whether they have been sunk 
 beneath the lowest level to which the ground water sinks in times of 
 greatest drouth. Since ground water is in movement it is advisable 
 to investigate, before locating such a well or u.sing the water from an 
 existing well, whether there are sources of polhition that may supply 
 typhoid and other disease germs to the ground water that enters the 
 well. In such examination it is not always safe to infer that the 
 ground water flow will follow the surface topography ; the mantle and 
 
UNDEEGROUND WATER 35 
 
 bed rock structure should also be taken into consideration.* 
 
 Wells sunk deeply into the bed rock draw their supplies from 
 porous formations, such as coarse grained sandstone, or from water 
 channels following the larger structural crevices of the bed rock, such 
 as joint planes and stratification planes, or, in limestone regions, from 
 actual caverns and cisterns that the ground wat^r has dissolved out of 
 the solid layers. If a porous layer has impervious beds overlying it 
 of suflScient thickness to prevent the escape of water under pressure 
 
 FIG. 5. Underground conditions for artesian wells. Upper diagram 
 shows artesian basin; lower diagram, dip in one direction only. A — porous 
 layer, B — impervious layer, W — artesian wells. (After Salisbury.) 
 
 and if, further, the porous layer outcrops in a region of sufficient rain- 
 fall and of some elevation above the surface of the countrj' under which 
 it descends, the essential conditions for artesian or flowing wells are 
 present. Rain water enters the porous layer in the elevated region 
 of the outcrop and percolates through it under the impervious beds to 
 lower elevations. The difference in elevation gives rise to hydraulic 
 pressure, just as would be the case if the porous layer were an open 
 pipe, except that friction lessens the pressure in the case of the rock 
 conduit. If now the impervious layers be pierced by a boring, at the 
 lower elevation, the imprisoned water from the porous layer underlying 
 wiU rise to the surface and may actually spurt a number of feet into 
 the air, indeed, in some such wells sufficient pressure to drive mJls is 
 present. It was formerly thought that artesian wells could only be 
 
 *"onderground Water for Farm Use" by M. L. Fuller, Water Supply 
 Paper 255, is an excellent booklet detailing all the geologic conditions or- 
 dinarily encountered in connection with securing supplies of water from 
 underground sources. It also describes methods and costs of sinking wells, 
 precautions, etc. May be had, free, on letter application to Director, U. S. 
 Geological Survey, Washington, D. C. 
 
36 
 
 DYNAMIC GEOLOGY 
 
 obtained when the porous layer rose on all sides from the point of 
 the well locations, forming a basin. More recently artesian wells have 
 been obtained where the porous stratum simply clips in one direction. 
 In that case, however, the water supply comes only from the area of 
 porous rock above the point where the well taps it. In Fig. 5 are 
 shown in section the requisite conditions for both types of artesian 
 well. 
 
 Where the water table intersects the surface on a hill slope seep- 
 age springs may issue where some larger channel favors the collection 
 of a larger volume of water than can be continuously evaporated. More 
 commonly, however, large volumed, permanent springs occur at or 
 above the outcrop of a comparatively impervious stratum, along whose 
 top side the ground water flows. Or it may be that a dipping, por- 
 ous bed carries the water between comparatively imper^'ious layers and 
 is supplied with an outlet by a joint plane or other fissure as shown 
 in Fig. 6. Such springs are known as fissure springs. 
 
 FIG. 6. Diagram of a fissure spring. (After Fuller.) 
 
 Some springs are supplied with water that has penetrated imder- 
 groimd far below the levc4 of its intake or outlet, following an essen- 
 tially U shaped course. The outlet must, however, be below the source. 
 If the difference in elevation between source and ovitlet be sufficiently 
 great such a spring may issue with considerable artesian pressure, and 
 is then known as an artesian spring. 
 
 ZONE OF WEATHERING AND ZONE OF CEMENTATION. — Near 
 
 the surface, at the depths through which the level of the water laf^le fluc- 
 tuates according to supply by precipitation, and in the zone immed- 
 iately below the lowest level to which the water table sinks, the circula- 
 tion of ground water is comparatively active ; accordingly solution, hy- 
 dration, oxidation and carbonation processes are also active. This zone 
 is, therefore, appropriately termed the Zone of Weathcri)ig. At 
 greater depths the ground water circulation is more sluggish, it is 
 heavily charged with mineral matter dissolved from the rock above 
 and part of its volume and gases may have been used uip in hydration 
 
UNDERGROrXD WATER 37 
 
 and carbonation process or have escaped into the air by evaporation 
 through pores leading to the surface. Consequently conditions at such 
 greater depths are apt to favor deposition of mineral matter already 
 in solution rather than further mechanical or chemical destruction 
 of rock. Such deposit may be brought about by a great variety of fac- 
 tors ; decrease in temperature or pressure, mining of water having dif- 
 ferent sub.stances in solution ^causing precipitation of some com- 
 pounds) are among the important causes. Since such deposition tends 
 to bind or cement rocks more firmly together this lower zone is called 
 the Zonii of Ceraentation. It must not be understood that no solution 
 occurs in the zone of cementation or that no deposition occurs in the 
 zone of weathering. Consolidating xjrocesses are dominant in the lower 
 zone, disintegrating processes in the upper one. 
 
 PHENOMENAL FEATURES OF THE ZONE OF WEATHERING AND 
 CEMENTATION. — Solution in the upper levels and deposition in the 
 lower ones is an important relation in connection with formation of 
 deposits of minerals of economic value. ^Minerals containing useful 
 metallic elements are widely disseminated through the various classes of 
 bed rock. It does not pay. however, as a rule, to extract them when 
 scattered in minute particles through great masses of rock. If. how- 
 ever, such metallic compounds are dissolved from wide areas of country 
 rock in the zone of weathering, carried dovnward and later concen- 
 trated by deposition on the walls of joint planes and other fissures 
 in the rock in the zone of cementation, fissure veins are formed; and 
 such veins are the source of much copper, lead. zinc, silver, gold 
 and other ejre. ]\Iany veins contain only quartz and calcite, 
 economically worthless, except imder unusual conditions. Commonly, 
 also, quartz and calcite are associated in the same vein with valuable 
 metallic compounds, in which case the former are called gangue minerals. 
 A wide vein containing metallic minerals is often termed a lode, while 
 rich fillings of larger ca^nties are called bonanzas. Veins may. how- 
 ever, originate elsewhere than in the zone of cementation and by other 
 processes. 
 
 Economically valuable veins distinctively called ore veins, may 
 themselves be further enriched (called seconelanj enrichment) by solu- 
 tion of their upper parts and consequent carrj'ing downward and rede- 
 posit of the metallic compoimds in lower parts of the vein as shown in 
 the diagram Fig. 7. 
 
38 
 
 DYNAMIC GEOLOGY 
 
 ■"^^^^f; 
 
 - ^ — -' 
 
 Removecl\ 
 
 FIG. 7. Diagram of a copper vein enriched by solution above and de- 
 posit belovsr. (After Blackwelder and Barrows.) 
 
 This process is especially important in the case of veins whose 
 original minerals were deposited from heated ascending waters, and 
 are, accordingly, compounds more subject to alteration by weathering 
 processes than minerals previously concentrated by solution in and 
 deposition from descending waters. In caves, release of pressure es- 
 
UNDERGROUND WATER 39 
 
 cape of a carbon dioxide into the air and evaporation of part of the 
 water results in the formation of stalactites, hanging from the roof, 
 while stalagmites, built up from the floor, are due to drippings. In 
 the stalactites the deposit first forms around the circumference of the 
 original drop as it clings to the roof, and, as the water continues to 
 percolate, successive similar annular rings are deposited below and out- 
 side the original one. Stalactites, therefore, commonly show a eon- 
 centric structure and traces of an orifice through the center of their 
 length. Stalagmites, on the other hand, are made up of a solid mass 
 of successive layers. 
 
 Sometimes the depositing solutions form nodular masses (called 
 concretions) in the bed rock; especially in sedimentary strata. These 
 are due to a flowing and concentration, from all sides, of cementing 
 solutions aroimd an original nucleus ; such as a particle of different 
 character than the mass of the rock. Such foreign substances seem 
 to possess ability to cause precipitation. Concretions are generally 
 harder, denser, and of different composition than the surrounding 
 rock, and, if the latter be soft, may force it aside when growing. Con- 
 cretions may be nearly perfect spheres or extremely irregular lumps, 
 and may vary from a few inches to 10 or more feet in diameter. Con- 
 cretions in limestone give rise to flint, an amorphous form of quartz; 
 in shales, concretions commonly contain calcium carbonate or iron 
 carbonate. Small cavities in rocks lined with inward pointing crystals, 
 due to deposit from ground water solutions, are called geodes. 
 
 Organic remains, such as tree trunks and animal forms, buried 
 under sand or other rock material may, as they decay, have their par- 
 ticles carried away in microscopic bits and as rapidly replaced by min- 
 eral matter deposited from percolating solutions. Thus the minutest 
 structures of the organism are preserved and turned into stone. This 
 process of change is termed petrification. It has been of great im- 
 portance in preserving a record of past life forms, giving rise to many 
 kinds of fossils, as all traces of life found in rocks are called. Petri- 
 fied wood is a familiar example of this kind of change due to ground 
 water action. 
 
 HOT SPRINGS AND GEYSERS. — That the interior of the earth ia 
 hot is indicated by the fact that in all borings and mines the tempera- 
 ture is found to rise one degree in the Fahrenheit scale for every 
 50 to 100 feet penetrated from the surface downward. The rate of 
 rise in temperature varies in different localities, and, in any one bor- 
 ing, varies also according to the depth. However, the general rela- 
 
40 DYNAMIC GEOLOGY 
 
 tion of increasing heat with depth is well established. Consequently 
 meteoric waters that penetrate to great depths before reappearing at 
 the surface are apt to emerge at high temperatures, thus giving rise 
 to thermal or hot springs. As has previously been pointed out, rocks 
 are poor conductors of heat, therefore, where igneous intrusions have 
 penetrated to points near the surface, or hot lava has been buried by- 
 later flows, or again where friction due to faulting of rock or chemical 
 reactions have generated heat, it may not be necessary for the water 
 to penetrate to great depths to become highly heated, because such 
 sources of heat may continue to exist for long periods insulated by 
 a comparatively thin overlying mantle of rock; a greater heat loss be- 
 ing in this case probably brought about by the rising water than 
 by conduction through the rock cover. 
 
 Heated waters have a much greater capacity for holding mineral 
 matter in solution than cold; consequently thermal springs on cooling 
 at the surface, commonly precipitate deposits of materials of the rocks 
 through which the waters have passed. Certain minute plant forms 
 that can exist in the warm pools at the surface facilitate such precipi- 
 tation, and the vari-colored deposits around the hot springs in the 
 Yellowstone National Park are largely due to their activities. Not all 
 the dissolved substance, or all the kinds of substance, are deposited 
 on emergence of the hot waters ; thus much mineral matter in solution 
 is contributed to the surface run off. Of course cold springs as well 
 as hot springs bring up dissolved material. Medicinal springs are 
 either hot or cold springs whose waters contain dissolved mineral 
 matter or gases supposed to have a curative value ; carbonate spring 
 waters are also bottled in large quantities for table use. 
 
 Geysers are simply hot springs that, instead of flowing quietly, 
 intermittently erupt violently by virtue of the expansive energy of 
 steam generated in their tubes. They occur in regions where hot 
 lava is buried, Iceland, New Zealand and the Yellowstou'-^ Nati(jnal 
 Park, along the drainage lines, streams, shores of lakes, or other 
 points in such localities where meteoric waters would normally appear 
 as springs. The surface waters percolate slowly through minute 
 fissures to the hot rock, are there greatly heated, partially vaporized, 
 and eventually find a larger opening leading to the surface, the geyser 
 tube. This geyser tube may have been formed earlier by slow dis- 
 solving action of the hot ascending waters. Once the tube is formed 
 the hot percolating waters from the small fissures will slowly fill it 
 after each eruption. Progressively higher temperatures are required 
 to make water boil as pressure on it increases. Thus water at the 
 
UNDERGROUND WATER 
 
 41 
 
 bottom of a geyser tube, under the pressure of the colmnn of water 
 above, can attain a higher temperature without boiling than can the 
 water at the surface. When, however, sufficient heat has been ac- 
 cumulated to bring the water at the bottom to its boiling point, steam 
 is generated, this expanding, lifts the column of water above, and 
 causes an overflow at the top. Such overflow relieves the pressure on 
 all the water below. Much of this has a temperature above the 
 boiling point at the lower pressure, consequently changes to steam 
 and ejects all the water in the tube with great violence. Then the 
 tube once more slowly fills, the water is again heated and another 
 eruption occurs; the time interval between eruptions varying accord- 
 
 ObaeiNed 
 
 10 
 
 20 
 
 ■30 
 
 40 
 
 SO 
 
 •60 
 
 •70 
 75 Jeet 
 
 FIG. 8. Section of Geyser, Iceland, showing temperature relations. The 
 observed temperatures were obtained between eruptions. (After Campbell.) 
 
 ing to the time it takes the tube to fill with water and be reheated. 
 
 This time varies from a few minutes to many days in the Yellowstone. 
 
 In the diagram Fig. 8 a section of the Icelandic geyser, Geyser 
 
 (origin of the name) is shown. Steam and hot water issue from the 
 
42 DYNAMIC GEOLOGY 
 
 fissure at the right marked by the arrow. The steam gradually heats 
 the water layer at A to its boiling point 123.8 degrees Centigrade. Steam 
 in quantity then forms, lifting the water column so that the layer B 
 is brought to the position C. This causes an overflow at the top, 
 consequent shortening of the column. Thus at the position C, water 
 from B is above the boiling point ; the same conditions hold for water 
 above C ; therefore steam is generated through the whole upper column 
 and an eruption occurs. Geyser tubes are probably, in many cases, 
 much deeper than Geyser and heat may be supplied at their very bot- 
 tom, increasing the completeness and effectiveness of the eruptions.* 
 
 LAND-SLIDES AND SOIL CREEP — By dissolving out great masses 
 of limestone groimd water often weakens cavern roofs so that they fall 
 in leaving natural bridges as shown in Fig. 2. In other ways, also 
 ground water causes mechanical destruction and movement of ma- 
 terial. It may so saturate and thus increase the weight of a mass of 
 rock on a slope, that it becomes unstable and gravity sets it in mo- 
 tion, giving rise to a land-slide. Land-slides may involve tons of ma- 
 terial but, more commonly and persistently, smaller unobserved slips 
 of this nature are occuring on most slopes. The process in bed rock 
 is facilitated when clayey material exists under a more porous rock 
 mass above, for the clay, lubricated by water, forms a veritable gliding 
 plane for the material overlying it. In this manner much rock is 
 brought to lower elevations and broken up in its descent. 
 
 Not only bed rock, but also mantle rock and its top layer of soil, 
 are more slowly moved down hill toward stream coiitses by such ac- 
 tion. This movement is called soil-creep. In the surface layers it is 
 facilitated by another process, particularly effective in regions where 
 heavy rains are followed by hot dry periods. The rains saturate the 
 mantle rock, increase its weight and its mass with a consequent thrust 
 down hill. Loss of this moisture by evaporation causes the soil to 
 contract, capillary forces in the pores pulling the soil particles closer 
 together. As a result the soil masses split apart, giving rise to mud- 
 cracks, also called sun-cracks. On a slope, under the influence of 
 gravity, the tendency is for these cracks to open on the lower more 
 than the upper side, thus moving the blocks down hill. When ex- 
 pansion occurs, due to a succeeding rain, the cracks tend to be closed 
 toward the downhill side. Thus both in opening and closing the ma- 
 jor portion of the movement, slow to be sure, but effective in time, is 
 down hill. 
 
 *A very Interesting, illustrated pamphlet entitled "Geysers" may be 
 had, for 10 cents, from the Supt. of Documents, Washington, D. C. 
 
UNDERGROUND WATER 43 
 
 By such processes, and other agencies as well, weathered particles 
 of bed rock and mantle rock are constantly being moved down to 
 stream courses, where they are taken up by the running water and 
 moved onward toward the sea. Removal of the surface weathered 
 rock means that the zone of weathering progressively extends deeper 
 and deeper into the bed-rock, and that, in consequence of such co- 
 operation between weathering and movement down hill of material, the 
 higher areas of land surface are being reduced to lower elevations. 
 Lowering of the land is furthered also by the processes due to stream 
 flow as will appear in the next chapter. 
 
CHAPTER IV. 
 
 TRANSPORTATION, EROSION AND DEPOSITION 
 BY STREAMS 
 
 ORIGIN AND CHARACTERISTICS OF STREAMS AS GEOLOGICAL 
 AGENTS. — Jiist as rock material in land-slides and by the processes of 
 soil creep moves down hill because of gravity pull, so also do ground 
 water and the surface run off. Water, however, flows more rapidly 
 than rock material because the frictional resistance between fluid par- 
 ticles is so much less than between solids; and water flowing over 
 the surface moves more rapidly than ground water because it is not 
 confined to constricted passages and narrow outlets. Moreover, when 
 surface water is gathered into definite channels it flows in greater or 
 less volume, thus further minimizing the frictional resistance of the 
 surface over which it flows. Such definite courses of surface water 
 are collectively termed streams. 
 
 Some streams flow only after a rain, are in other words supplied 
 only by the immediate run off. Such streams are called wet weather 
 or intermittent streams and are common in regions of light rainfall 
 and on the bare hill slopes of more humid regions, where their chan- 
 nels are referred to as rain gullies. Most streams in humid regions 
 flow continually and are, therefore, distinctively called perennial 
 streams. They owe their unfailing water supply (though this may 
 fluctuate greatly in volume) to the slower run off that emerges from 
 springs and to ground water seepage, occurring at the intersection of 
 the water table with the stream channels, as indicated in Pig. 4. Per- 
 ennial streams are called rivulets, runs, brooks, creeks or rivers accord- 
 ing to their volume. 
 
 TRANSPORTATION BY STREAMS. — The waters of some streams 
 are continually roily from the clay substance they contain — like the 
 Missouri which, on that account, has earned the epithet "The Big 
 Muddy." The other streams are turbid only after heavy rains or in 
 the early spring. But it will be found, if continuous observations are 
 made, that even the clearest stream becomes slightly cloudy at times 
 because of the rock material it is carrying. In other words all streams 
 are transporting weathering fragments (collectively spoken of as sedi- 
 ments or the sediment load) to the sea. 
 
STREAMS 4& 
 
 This sediment is supplied to streams in various ways. Bain wash, 
 flowing as a thin sheet of water over areas between stream channels, 
 conveys the finer fragments to wet weather streams and rivulets. Soil 
 creep also furnishes fine grained substance, and practically continuous- 
 ly. Larger fragments are commonly precipitated by gravity into 
 stream courses from their valley sides and land-slides frequently bring 
 tons of material directly into stream channels. In particular localities, 
 or under exceptional circumstances, the wind may blow much material 
 into the running water, volcanoes may supply exploded fragments of 
 rock, or glaciers may furnish a large part of the sediment load. Final- 
 ly the streams get much of the material they transport by their own 
 activity as will be shown in later paragraphs. Ground water also 
 supplies them with vast quantities of material in solution. 
 
 MANNER OF TRANSPORTATION BY STREAMS. — The material sup- 
 plied to streams in these various ways is transported in different fash- 
 ions according to its nature. Thus the dissolved substance can not 
 be detected by mere inspection because carried in solution, usually col- 
 orless. Much solid material is buoyed up by the water, i. e. carried 
 in suspension. The size of particles that may be carried in this way 
 depends (a) on the density of the rock substance, for solid material 
 loses weight in water in proportion to the cubic measure of the water 
 it displaces; (b) on the surface area of the rock fragment, for just 
 as a sheet of glass sinks much more slowly than does a ball of glass of 
 the same mass, so is a rock fragment of large surface area carried 
 along more readily than one of equal weight with less surface, though 
 both are of the same density. 
 
 Rock fragments too heavy to be buoyed up are rolled along the 
 bottom; larger fragments of rounded material than of those that are 
 angular. 
 
 The transporting power of a stream, especially for the large frag- 
 ments rolled and pushed over the bottom, depends primarily on the 
 stream's velocity, or rate of flow; and this in turn is determined by 
 the slope and the volume. On a perfectly flat surface water would 
 stand still, in a vertical drop (a ivaterfall) its acceleration would be that 
 due to gravity minus atmospheric friction and internal friction. Be- 
 tween the two all gradations in velocity are possible dependent on the 
 value of the vertical component of the slope. The volume-velocity re- 
 lations are more indirect, the velocity varies as the cube root of the 
 volume i. e. of two streams flowing over the same slope one must 
 have eight times the volume of the other in order to flow twice as fast. 
 
46 DYNAMIC GEOLOGY 
 
 Increase of velocity, by either increase of slope or volume, has an 
 extraordinary effect on the transporting power of a stream; the trans- 
 porting power varies directly as the sixth power of the velocity. That 
 is, if the velocity of a stream be doubled its transportiag power is in- 
 creased sixty-four times. It is because of this relation that floods are 
 so violently destructive. Consider the effect ia the case of a smaU 
 stream, normally capable of rolling a pebble weighing i/4 of a pound. 
 During a flood its volume may increase 125 fold, its velocity, therefore, 
 5 times and its transporting power 15625 times. In other words it 
 would then be able to move a mass weighing almost two tons. This 
 explains why the small stream of Six Mile Creek near Ithaca, N. Y. 
 was able, in a flood some years ago, to wrench loose and carry along 
 huge iron water pipes, used for the city water suppy ; and why in the 
 Johnstown Flood in Pennsylvania in 1889, locomotives were lifted u^p 
 and transported considerable distances and elsewhere huge masonry 
 dams have been swept away by floods. 
 
 AMOUNT OF MATERIAL TRANSPORTED BY STREAMS. — Normal- 
 ly streams are not loaded to their full capacity in humid regions. In 
 arid regions, on the other hand, they are often overloaded. Estimates 
 of the quantity of material transported by individual streams have, 
 therefore, little significance. But some idea of the maignitude of stream 
 transportation may be gained by consideration of the material annually 
 carried by some large rivers. Thus the Mississippi annually carries 
 7,500,000,000 cubic feet of material into the Gulf of Mexico (equal to a 
 mass 1 square mile in area and 268 feet high) in suspension and 
 pushed along the bottom. It is further estimated that it carries over 
 one-third as much more in solution. At the rate given above the amount 
 of solid material carried would result in the lowering of the land 
 over the entire drainage basin of the Mississippi system by one foot 
 in about 5000 years. 
 
 DEFINITION OF EROSION PROCESSES, —Weathering implies an 
 essentially passive disruption of rock material ; the bed rock crumbles in 
 place, except as part of its substance is carried away in solution or 
 dislodged by gravity. In none of the weathering processes is the idea 
 of grinding or wearing of rock involved. Grinding or wearing action 
 is geologically known as erosion. By some writers the term erosion is 
 geologically applied in a broader sense to include all reduction of land 
 surfaces to lower levels. It is better practice, however, to use the term 
 erosion only for processes where movement of the agency, i. e. water, 
 winds, ice, is involved ; thus including solution action by streams, though 
 
STREAMS 47 
 
 this could not be included in the stricter definition of erosion as 
 mechanical grinding or abrasion. 
 
 To designate the general reduction of land surfaces the term de- 
 nudation is best used. Denudation may also be described as the sum 
 of the processes of both weathering and erosion. Since the land is 
 made up of both hills and hollows the lowering of the hills may be 
 termed degradation to distinguish such processes from those by which 
 the hollows are filled i. e. those of aggradation. As the combined ef- 
 fect of degradation and aggradation processes tends to bring land 
 surfaces to a common level their joint action is termed gradation. It 
 wiU be appreciated that these various terms in certain senses overlap 
 each other, yet each has a distinctive application and the student 
 should cultivate the nice choice of terms that will lead to his proper 
 use of them. 
 
 VARIETIES OF STREAM EROSION. — The erosive activity of a stream 
 may be variously directed. Thus it may deepen its channel, widen it 
 and lengthen it. Again, under water falls the nature of the process 
 differs from that in long, nearly level reaches. The depth and width 
 of the excavation or valley of the stream as a whole are, therefore, the 
 result of the composite action of various types of stream erosion rather 
 than any one of them. 
 
 VERTICAL EROSION BY STREAMS. — Neither water, wind or ice are 
 capable of performing appreciable grinding action by their own sub- 
 stance. True, running water may sweep loose and carry away by 
 its own mass material of the stream bottom if this be loose and inco- 
 herent. But the main erosive action on the stream bottom by which 
 the channel is deepened is accomplished with the rock fragment tools 
 supplied to the stream by the disintegrating processes of weathering. 
 These fragments are rolled and tumbled over and against the bed rock 
 of the stream channel bottom grinding and wearing it deeper. Inci- 
 dentally large fragments are often broken loose from the bottom by 
 this action and these then are also used as tools. In the process the 
 tool fragments are themselves worn down and rounded giving the 
 typical form to stream pehlles. As the water flows over the stream 
 bed it dissolves a considerable body of rock material, this removal 
 must also be counted as stream erosion. To distinguish between the 
 mechanical wear, or abrasion, and the chemical action, the terms cor- 
 rasion and corrosion are respectively used. The water further pro- 
 motes the excavation of the stream bed by wetting and thus softening 
 the rock. 
 
48 DYNAMIC GEOLOGY 
 
 The rate of downeutting depends on a number of factors. Nat- 
 nrally, the less resistant and less massive the bed rock, and the harder 
 and larger the tools, the more rapid the wear. Over a steep slope 
 the water tends to flow in a straight line, is not readily diverted 
 from side to side by obstacles in its course, consequently, its energy 
 is largely directed to down cutting, a straight narrow channel re- 
 sulting. Again, of two streams with the same total, annual volume, 
 flowing over the same slopes, that one of them alternately subject 
 to floods and low water will accomplish more erosion than the one 
 flowing -with uniform volume the year around. How greatly the 
 energy of a stream is increased during a flood was indicated in a pre- 
 vious paragraph. A stream having an abundance of coarse tools, 
 neither overloaded nor underloaded is most capable of cutting down its 
 bed. The Colorado River is a good example of such a stream, the 
 great depth of the Grand Canyon in Arizona being the result of its 
 down cutting. On the other hand the upper Niagara River, above 
 the falls strikingly illustrates the opposite conditions. It is a large 
 stream but practically unprovided with cutting tools, moreover flows 
 over a plain with low velocity and with its volume spread over a wide 
 area, consequently has done little or no down cutting, in fact appears 
 to flow right on the surface of the land. "What little erosion it has 
 accomplished is largely due to corrosion. 
 
 Under favorable conditions the rate of vertical erosion may be 
 very rapid. A stream carrying great quantities of debris and flowing 
 at high velocity wore through a pavement of granite slabs one yard 
 thick in one year in the Sill tunnel in Austria (Scott) and in Alaska 
 a stream loaded with coarse glacial rock waste cut a channel 6 to 9 feet 
 deep in less than three year's time, thouigh in comparatively little re- 
 sistant bed rock. 
 
 POT HOLE EROSION. — Down cutting is especially accelerated at 
 points where the slope of the stream bed is vertical, at a waterfall, 
 and where some change in velocity gives rise to an eddy in the flow of 
 the water. The plunge of the water over the falls and the eddying 
 flow bring about a rotary motion of the water and this movement is 
 transmitted to the rock tools in transport. Consequently these are 
 swirled round and round on a particular spot and thus tend to grind 
 out circular depressions in the bed rock known as poi holes; below 
 large water falls as plunge pools. This process is illustrated in the 
 sectional diagram of Niagara Falls, Pig. 9. As the pot hole, or plunge 
 pool, develops, the tools grind laterally as well as vertically, conse- 
 quently the bottom becomes wider than the top of the depression 
 
STREAMS 
 
 49 
 
 i. e. develops a typical pot shape. In this manner the base of the falls 
 are undercut, and the support of the capping stratum removed, caus- 
 ing large blocks of it to fall; thus the edge or the crest of the fall 
 recedes upstream, a process referred to as the recession of waterfalls. 
 Such undermining of the capping stratum is also brought about in 
 
 PIG. 9. Sectional diagram of Niagara Falls showing rock tools excavat- 
 ing plunge pool below. Also recession of falls by sapping and undermin- 
 ing of weaker rocks under resistant capping stratum. (After Tarr.) 
 
 part by a combination of weathering processes called sapping — ^the 
 action due to the spray from the falls in dissolving, and, by freezing, 
 prying oif material behind the curtain of water. In the Horse Shoe 
 FaUs of Niagara such recession has been found to average 5.3 feet 
 per year along the center of the crest. Not all falls recede upstreain 
 in position, only those underlain by horizontal or nearly horizontal 
 beds of rock varying in resistance to erosion. Palls due to a vertical 
 
50 
 
 DYNAMIC GEOLOGY 
 
 mass of harder rock extending up into the stream bed have their 
 height gradually lowered by the progressive wearing down of the re- 
 sistant rock from the top. 
 
 LATERAL EROSION. — The current of a stream flowing over a com- 
 paratively gentle slope is readily diverted by some obstacle in its course 
 and thus set to swinging from side to side or meandering. Once start- 
 
 
 (^R 
 
 ^v^^-^ 
 
 ^ "# 
 
 /f : 
 
 FIGS. 10 and 11. Diagrams to show development ot meanders in a 
 stream and their progressive enlargement by lateral cutting. Arrowa ?ho\? 
 course of main current of stream. Earlier stage at left. Lateral erosion 
 occurs ■where the margin of the stream channel is cross lined. (After Davis.) 
 
 ed .such meandering tends to propagate itself down stream, for as the 
 stream impinges against one wall of its valley its current is shunted 
 over to the other, and this may be repeated until checked by a change 
 to a steep slope or a resistant formation at a lower part of the course. 
 At the points where the current is thrown against the banks it later- 
 ally erodes a section of the valley side, the width of this eroded band 
 being measured, in general, by the depth of the stream. In time such 
 lateral cutting results in undermining of the bank, a portion of this 
 caves in and is swept away, and then the process is repeated. Thus the 
 meander curves gradually enlarge laterally and also move progressively 
 down stream as indicated by the letters a, b, c, designating the same 
 meander in the diagrams, Figs. 10 and 11. If a very large mass falls 
 in it may suffice to turn the current directly across to the opposite 
 banks and thus lead to the development of a new set of meanders. By 
 
STREAMS 
 
 51 
 
 such alteration of the position of the crtt bank and the progressive 
 down stream migration of a single meander the whole valley of the 
 stream may be widened. 
 
 Some lateral cutting is done by streams that do not meander to 
 any great extent. If the stream is in a region where ice forms on its 
 surface this ice on breaking up and floating down may appreciably 
 
 FIGS. 12 and 13. Progressive stages in the widening of a river valley 
 In stratified rocks by sapping. The resistant layers are marked by letters. 
 Upper figure is earlier stage. (After Davis.) 
 
 increase the amount of lateral erosion because it floats, yet, being solid 
 is capable of scraping the banks; moreover, such ice often has rock 
 fragments frozen into its mass which further promotes its erosive ef- 
 fectiveness. 
 
,62 DYNAMIC GEOLOGY 
 
 Lateral erosion due to the meandering of a stream is probably the 
 principal process in widening valleys in unconsolidated material. In 
 consolidated rock valley widening is due primarily to sapping; like 
 that which takes place under waterfalls that owe their existence to a 
 resistant capping layer. This is especially true of valleys cut in strat- 
 ified rock of varying resistance. In these the vertical erosion of the 
 stream cuts a gash, limited in width to that of the average width of 
 the flowing water ribbon. Such cutting exposes the edges of the rock 
 on the sides of the gash (or gorge) to weathering action. The under- 
 mining weathering processes (solution, hydration, oxidation carbonation 
 £nd frost action, primarily) collectively spoken of as sapping, attack 
 these exposed edges unequally. The weaker layers are eaten away 
 from beneath the more resistant ones. When this undermining has 
 proceeded so far that an intersection of vertical joint plane fissures 
 in the resistant rock is reached, the block of resistant rock that these 
 cut off from the mass fails of support from below, therefore, breaks 
 off and tiimbles down into the stream. By such action the resistant 
 
 SANDST OMg. . 
 j.SMALt SAnO&TOMt 
 
 qLrMtSTOMt 
 
 FIG. 14. Section of the Grand Canyon of the Colorado, showing -wide 
 upper gorge in stratified rocli and narrow inner gorge in massive granite. 
 (After Gilbert and Brigham.) 
 
 layers preserve vertical faces and flat tops, while the edges of the less 
 durable rock are commonly masked by debris fallen from the resistant 
 strata, and the incoherent substance of their o-wn weathering. The 
 widening of valleys by sitch processes is well illustrated in Figs. 12 
 and 13, which show successive stages. In these diagrams layers mark- 
 ed by letters are resistant and show vertical edges. Where streams 
 are eroding vertically in massive resistant rock, valley widening pro- 
 ceeds only very slowly as pieces are pried off by frost and exfoliation 
 action on the exposed sides. This fact is illustrated in the diagram 
 Fig. 14 by the gorge cut in the granite rock showing conditions in the 
 Grand Canyon of the Colorado. The wide upper part of the Canyon 
 is in stratified rock made up of layers of alternating resistance, the 
 narrow inner gorge is in massive granite. In mountain regions of 
 Steep slopes and igneous rock structure, where the vertical stream ero- 
 sion is relatively very rapid as compared to weathering of the sides, 
 such narrow canyons are of common occurrence. 
 
STREAMS 
 
 53 
 
 HEADWATER EROSION, —If a new land surface, essentially flat, 
 but sloping gently toward the sea be conceived, it will be appreciated 
 that at first the rains will flow off this as a thin sheet of water carry- 
 ing along some fine sediment. At some points where a slightly greater 
 voliune collects or the slope is a little steeper the surface material will 
 eventually be cut through to a less durable material below it. In con- 
 sequence vertical erosion will go on more rapidly along this line, and, 
 as the channel is thus deepened, more and more water from neighbor- 
 ing areas will follow it. Thus a stream valley is begun. At the head 
 of such an incipient depression a slight waterfall will develop where 
 the water tumbles from the more durable upper material to the less 
 resistant substance underneath. Under this waterfall sapping will 
 proceed rapidly causing the crest of the falls to recede. Thus the 
 depression will be lengthened inland. Such lengthening of stream 
 valleys is termed headivater erosion and occurs at the sources of near- 
 ly all river systems. Not only the main trunk stream but also the 
 tributaries that come in at its sides are gnawing back into the adjacent 
 lands, in this way, and so extending the whole system. 
 
 Not all the incipient stream channes are equally favored. Some 
 may have originally less volume, less slope or be flowing over mere 
 durable material than others. The more favored ones can, therefore, 
 
 "^"^^'^ 
 
 FIGS 15 and 16. Illustrating river piracy. On the left is shown the 
 earlier stage On the right the Shenandoah by headwater erosiou has cap- 
 tured and diverted the upper course ot Beaverdam Creek. (After Salisbury.) 
 
54 DYNAMIC GEOLOGY 
 
 lengthen and deepen their channels more rapidly than the others. 
 Consequently, it often happens that tributaries, or the heads of the 
 main streams of the more favored systems, encroach upon the territory 
 of adjacent streams that are still flowing at higher levels, tapping and 
 diverting the waters of their tributaries to the lower level course. This 
 process is called river capture or stream piracy, and by it originally 
 smaU stream systems are consolidated. Such action is well shown 
 by the sketch maps Figs. 15 and 16. 
 
 Less durable material lies on either side of the mountain range. 
 The early Potomac had greater volume and velocity than the Beaver- 
 dam Creek consequently was able to cut throiigh the resistant mountain 
 rock more quickly and deeply; its tributary, the young Shenandoah, 
 flowing on the inner area of less durable rock was, therefore, given 
 greater slope and cutting power than the headwaters of the Beaver- 
 dam Creek and in time acted as a pirate by ieheading and diverting 
 the upper section of the Beaverdam creek. By this event the erosive 
 powers of both the Shenandoah and the Potomac were strengthened 
 while those of the Beaverdam Creek were correspondingly decreased 
 by loss of part of its volume. By the continuation of such action one 
 stream eventually becomes the master or trunk stream of a widely 
 ramifying drainage system. 
 
 DENUDATION OF THE LAND. — As weathering processes Continue to 
 bieak up the surface material of t\\^ lithosphere and as stream valleys 
 are deepened, widened, and lengthened the land is progressively de- 
 nuded, its structure exposed by dissection and its higher elevations 
 degraded. Eventually adjacent valleys are separated only by narrow 
 ridge divides. In time these divides, also, are lowered and a region 
 f f originally great elevation and rough topography may be weathering 
 and stream action be worn down to a very low level. But there is a 
 limit below which the land cannot be degraded by stream erosion. 
 This limit is theoretically the surface level of the ocean into which all 
 stream waters flow. Practically, the inflowing streams can cut chan- 
 nels to the extent of their own water depth below the ccean surface 
 level. This limit to the depth of stream erosion is termed the hase 
 level of erosion. At the sea it is practically sea level but, as even the 
 most sluggish stream must have some slope over which to flow, the base 
 level of erosion rises with distance from the sea. The gently sloping, 
 nearly level plain developed when streams flow long enough to reduce 
 their drainage areas to base level is called a peneplain or hase level 
 plain. Such plains have seldom attained perfection in geological time 
 because so called accidents commonly interfere with the normal prog- 
 
STREAMS 55 
 
 ress of the weathering and stream processes in demiding the land. 
 It must also be borne in mind that processes other than weathering 
 and stream erosion are contributary in reducing lands to base level. 
 
 DEPOSITION BY STREAMS. _if a stream at a given point in its 
 course is earrjnng the maximuum amount of sediment that it can trans- 
 port, any reduction in its velocity or volume, e. g. gentler slope or 
 evaporation of its water, must result in its being overloaded, there- 
 fore, under necessity of depositing part of its sediment. If the reduc- 
 tion in velocity or volume be great enough, even an, on the average, un- 
 derloaded stream may be caused to drop part of its load at certain 
 points. Thus river bars are built as the volume of a flood diminishes, 
 or down stream from where a large rock mass, perhaps precipitated 
 into the stream by weathering of the valley sides, obstructs the flow. 
 Sediment is, therefore, not continuously in transport to the sea but 
 goes by irregular stages. A boillder too large to be borne beyond 
 a certain point remains there until worn down by smaller sediment 
 sweeping around it, or until a flood stage of sufficient height to move 
 it onward is attained. 
 
 When streams suffer a notable change in slope, such as descent 
 from a mountain side to a plain at the base, or lose much of their 
 volume by flowing from humid sources (often supplied by snow melt- 
 ing) to arid regions, a general condition of overloading commonly re- 
 sults. Consequently such streams in their lower courses usually ag- 
 grade their courses. The effect of such general aggradation is to 
 steepen the course at the point where it takes place. Thus there is 
 a tendency to establish a regularly curved slope, from the source to 
 the mouth of a stream, by degrading or eroding on the originally 
 steeper slopes and by building up by deposit or aggrading on the 
 gentler slopes. "When this has been fully accomplished sediment se- 
 cured at the headwaters may be carried to the sea without stop and 
 the stream is said to be at "grade." The combined processes of erosion 
 and deposition, have, in other words brought about a gradation of 
 the land along the stream course. This curve of the stream bed slope 
 will be steeper at the source than near the month because in the upper 
 courses the volume is less and the sediment coarser. 
 
 When a stream enters a body of still water such as a pond, lake 
 or sea its current is completely checked, accordingly its solid sediment 
 load all comes to rest, sinks to the bottom. The coarser particles 
 are dropped very near the mouth, and rather rapidly build a deposit 
 up to the water surface ; then the stream flows over this new surface to 
 the advancing shore line. Thus a stream course is lengthened at its 
 
56 DYNAMIC GEOLOGY 
 
 lower end by deposit at the same time that headwater erosion is push- 
 ing back the sources. A deposit of this kind made in still water at a 
 6tream mouth is called a delta and has a characteristic structure de- 
 termined by the manner of the laying down of its different beds. The 
 fine material is carried furthest out by the slackened current and 
 forms a thin layer parallel to the bottom in advance of the general 
 mass, this is called the bottom set bed. Coarser material is carried to 
 the front of the advancing deposit and then dumped, forming steeply 
 dipping layers called the fore set beds; while the coarsest stuff is spread 
 over the top and gives rise to the capping top set beds. The arrange- 
 ment and position of these beds are shown in the diagram Fig. 17. 
 
 PIG. 17. Longitudinal cross section showing the structure of a delta de- 
 posit. (After Salisbury.) 
 
 In lakes of fresh water the suspended fine clay particles may be 
 buoyed up and carried for quite great distances beyond the shore line 
 before they finally settle to the bottom. In the ocean, while currents 
 are usually stronger than in lakes, transportation of such clayey matter 
 in suspension for long distances beyond continents or islands is pre- 
 vented by a property of salt water that causes such clay particles to 
 flocculate, or gather into coherent groups. In this form they present 
 less surface area per unit of mass, consequently sink more readily. 
 Thus even the most finely divided waste of the laud must be deposited 
 near the sea shore, and, because this is true, land materal has been 
 preserved to the continental masses throughout geological time. 
 
 Since all the solid load of a stream is deposited on entering bodies 
 of still water of extensive area, large lakes act as filters to streams, and 
 their outlets are consequently streams of clear water. This filtering 
 action is exceedingly important in connection with the use of streams 
 as water supplies for cities, navigation routes and for water powers. 
 Why in each case ? For this reason also the outlets of lakes are seldom 
 cut down much by erosion, more commonly their basins are filled up 
 by sediments from inflowing streams. This then is another process 
 in the gradation of the land along stream courses. 
 
CHAPTER V. 
 
 TRANSPORTATION, EROSION AND DEPOSITION 
 
 BY WINDS 
 
 THE WIND AS A GEOLOGICAL AGENT. — Winds act both indirect- 
 ly and directly as geological agents. Indirectly they serve as carriers of 
 moisture from bodies of water, primarily the sea, to the land. This moia- 
 ture precipitated in various forms on the land surface returns to the 
 ' sea under gravity puU and on the journey brings about many geo- 
 logical changes, some of which have been noted. Winds also convey 
 temperature differences, warm air heating rock in a cold region and 
 vice versa, thus promoting or checking weathering processes as the case 
 may be. Violent winds also promote weathering by overturning trees 
 whose torn up roots bring up and expose fresh rock material to sur- 
 face processes. Again, strong winds set waves and long shore currents 
 in motion in oceans and lakes and these are factors of considerable 
 geological importance. 
 
 The direct activities of the winds are more apparent. Thus they 
 serve just as streams do to transport, erode and deposit rock material. 
 Their activities as agents in such processes differ, however, from those 
 of streams in several important respects. In the first place their ac- 
 tion is not directed along certain definite lines as that of streams, 
 flowing along their (within limits) fixed courses, must necessarily be. 
 On the other hand, while winds may blow first in one direction then 
 in another and over a wide stretch of surface at any one time, their 
 effectiveness as geological agents is confined almost exclusively to areas 
 that lack a plant cover. Thus the scope of their action is much less 
 imiversal than that of streams. Moreover, winds are, except when of 
 very exceptional violence, unable to transport rock particles much 
 larger than sand grains nor can these be lifted very high above the 
 land surface over which the wind is moving. Thus the direct geo- 
 logical activities of the wind are rather severely restricted both as to 
 area and characteristic processes. 
 
 TYPE REGIONS OF WIND ACTIVITY. — ^As previously stated the ac- 
 tivity of the wind as a dynamic agent is rather effectually checked by 
 a vegetation cover. But there are localities where this is character- 
 istically absent. The most extensive of sack areas are the deserts and 
 
58 DYNAMIC GEOLOGY 
 
 as these have been calculated to spread over something more than 
 one-fifth the land area of the world it will be appreciated that, while 
 restricted, the activities of the wind are not in any sense confined to 
 inconsiderable expanses. Sandy sea coasts also offer opportunity to 
 processes due to winds and these also, in total area comprise a notable 
 portion of the land. Akin to these are the wide sandy flats adjacent 
 to shallow aggrading streams during periods of low water. Mountain 
 tops of sufficient elevation to project above timber line, and very steep 
 slopes, in general, are also free of vegetation ; thus further enlarge 
 the total area open to effective wind action. 
 
 TRANSPORTATION BY WINDS. — Mountain tops projecting into the 
 upper levels of the atmosphere are subjected to very strong winds since 
 the currents move freely and without interruption over the interven- 
 ing spaces. Consequently the fine fragments broken from the rock 
 masses of the peaks by the mechanical weathering processes that there 
 prevail are almost immediately whisked away by -nnnds and carried 
 perhaps to regions quite remote from their origin. Thus rock dust is 
 found on the inland ice of Greenland hundreds of miles away from 
 known exposures of rock material. Dust from volcanic peaks and 
 "volcanic explosions falls on ships far out at sea. Such persistent re- 
 moval of the fine fragments of weathering from areas of mountain 
 (Summits by wind transportation keeps fresh rock constantly exposed 
 for further weathering. Wind transportation, therefore, is an ex- 
 tremely effective factor in the degradation of mountain elevations. 
 
 On sea coasts and lake shores sand is picked up by the wind and 
 carried long distances inland. This action is especially notable in re- 
 gions where the winds are prevailingly on shore and where no moun- 
 tain barrier or dense vegetation cover offers a check to the wind veloc- 
 ity. Thus on the north and west coasts of Europe where the west 
 vpinds blow from the ocean to the land, which is low-lying and has 
 only a thin vegetation cover, there are wide areas of drifting sand. 
 Those on the peninsula of Jutland in Denmark are especially weU 
 known. The same process has resulted in the building up of many 
 coral islands. The coral reefs around their shores are broken up by 
 the waves, the fragments cast up on the shore and from thence carried 
 inland by the winds. The above sea areas of the Bermuda Islands in 
 the Atlantic are of such origin ; their foundation, however, is a volcanic 
 cone that rises from the ocean floor. 
 
 But it is in arid lands and deserts that wind transportation has its 
 greatest geological importance. In such regions occur wide areas of 
 land without a protecting vegetation cover. Consequently weathering 
 
WINDS 59 
 
 processes due to temperature changes, primarily exfoliation, are con- 
 stantly disintegrating the bed rock, thus furnishing much fine rock 
 substance for wind transport. Over wide areas of desert land the 
 winds are sufficiently strong and constant to keep the surface wholly 
 clear of such fragments, thus many miles of desert are bare stony ex- 
 panses and sandy wastes do not extend over the whole of desert re- 
 gions as many people suppose. Sand and fine dust for transport by 
 desert -winds are also furnished by streams that descend from mountain 
 areas bordering the desert ; since such streams evaporate in the arid 
 country, consequently leave behind their sediment load in dry de- 
 posits excellently adapted for further transportation by the wind. 
 Vast areas of arid and desert lands, moreover, are comparatively level, 
 thus affording free sweep to wind currents. Accordingly arid and 
 desert conditions afford both ample supply and ample opportunity for 
 wind transportation of rock material. 
 
 In such regions great dust and sand storms occur. During their 
 continuance the air may contain as much as 126,000 tons of dust and 
 sand per cubic mile. Not all of this is lifted high. In fact much of it 
 moves along quite near or on the surface. Its progress along the 
 ground has been compared to that of writhing snakes. 
 
 WIND EROSION. — It is by means of the sand fragments which it 
 transports that the wind acts as an erosive agent. The sand grains 
 are hurled against the bed rock projections and wear these away; at 
 the same time reducing their own diameters to the fineness of dust. The 
 process is exactly similar to that employed in the industrial arts under 
 the name of the sand blast. A strong current of air is furnished 
 with sand particles and such a jet sufBces to develop ground glass 
 surfaces, remove paint and clean iron castings of firmly adhering in- 
 crustations. In arid lands the natural sand blast action is similarly 
 effective in grinding, scouring and polishing the bed rock. Character- 
 istically such action is localized near the land surface because the greater 
 quantity and the larger fragments of the material carried by the wind 
 are not lifted very high. Again the winds blow from various direc- 
 tions, are not confined to any one channel as are water streams, there- 
 fore all sides of the base of a rock mass may be attacked. This gives 
 rise to undermining, and eventually complete truncation of rock masses, 
 with resultant collapse or overthrow. The coUapsed fragments or over- 
 thrown masses are in twn attacked by the same process and in time 
 completely destroyed and the fragments carried away. This combined 
 erosion and transportation process explains the prevailing absence of 
 an accumulation of weathering fragments at the foot of steep slopes 
 
60 
 
 DYNAMIC GEOLOGY 
 
 in arid and desert regions. The concentration of the erosion and 
 transportation action of the wind near the surface, with the resultant 
 undermining and truncating effects are well illustrated by the dia- 
 grams Figs. 18 and 19, showing rock forms so developed in the African 
 deserts. Since the sand blast action is very nicely discriminative, 
 etching outcrops of soft rock much more rapidly than ones durable un- 
 der its attack, and because wind currents move in complex eddies, fan- 
 tastic and weird rock forms are common in desert regions, being the 
 
 FIGS. 18 and 19. Undermining and truncation of rock by wind erosion 
 and transportation. (After Walther and Andrussow.) 
 
 results of such very irregular sculpture. 
 
 The degradation of desert regions is almost wholly due to wind 
 erosion, wind transportation and exportation, or deflation as the latter 
 is distinctively termed. Water from occasional cloudbursts (for even 
 the driest desert region gets some rainfall at verj' irregular and per- 
 haps long separated periods) aids some in the gradation of desert 
 lands, the restHting torrents washing material from higher to lower 
 levels. But since no water flows out from desert basins the general 
 reduction of the desert elevations is wholly dependent on deflation re- 
 moval of the denudation material. The base level of erosion in desert 
 regions is, therefore, not determined by the ocean level as is the case 
 
WINDS 
 
 61 
 
 in moist regions. Accordingly desert areas may be reduced to plains 
 topography by their characteristic gradation processes, at elevations 
 high above or even actually below sea level. 
 
 WIND DEPOSITS. — Gravity tends to pull back to the surface the 
 sand and dust lifted by the wind currents. Consequently as the wind 
 dies out the particles in transport settle doA\'n to the surface. Any ob- 
 struction that diminishes the force of the wind has the same effect. 
 In areas of relative topographic depression a body of comparatively 
 stagnant air may lead to the accumulation of material. To this and 
 to wash during the floods from cloudbursts are probably due in large 
 part the sandy tracts that occupy the low-lying sections of deserts. 
 
 It must not be inferred, however, that the material of such ac- 
 cumulations is permanently at rest. The great supply there present 
 especially favors movements along the surface and this, combined with 
 the presence of minor obstructions, gives rise to a peculiar form of 
 deposit, the sand dunes which themselves migrate in characteristic 
 fashion. If the object is unyielding and inpenetrable wind eddies 
 form on both sides of it with resulting accumulations as shown by (a) 
 in Fig. 20. If the obstacle is flexible and perforated the check is ef- 
 fective on the lee side and the dune accumvilates there, (b) ; while 
 
 >;■,%■*'• 1 
 
 h;<?. 
 
 
 
 
 
 FIG. 20. Diagrams to illustrate the effect of varying types of obstaclea 
 In causing deposit of wind borne sand. (After Schulze.) 
 
62 DYNAMIC GEOLOGY 
 
 an unyielding but perforated obstruction gives rise to deposit all about 
 it, (c). These relations are important in connection with the checkmg 
 of moving sands near agricultural lands and should be carefully con- 
 sidered in undertaking such operations. Once accumulated the dune 
 may itself move onward independent of the source of supply and the 
 obstruction that gave it origin. Such movement consists in a pro- 
 gressive migration of the particles up the gentle slope to vrindward 
 and deposit in the calm spot on the leeward side of the moving dune, 
 where a steep slope develops as the particles come to rest under grav- 
 ity action, consequently build up as steeply as this vsdll permit. By 
 such migration the sand particles and the land under them are suc- 
 cessively buried and re-exhumed, as the dune mass moves on. The 
 gentle windward and steep lee slope of a dune, and the progress of its 
 migration are shown in the diagrame of Fig. 21. The large arrow 
 shows the prevailing wind direction, the smaller ones indicate the 
 movement of the individual sand particles. 
 
 _\..O.l^O0t^ 
 
 1 669 
 
 FIG. 21. Diagrams to show migration of dune burying the town of Kun- 
 zen, on the Baltic Sea; and its subsequent exhumation, between 1800-1869. 
 
 The occurrence and migration of sand dunes is not confined to 
 desert regions. As indicated by the locality of Fig. 21 they are quite 
 active on sea coasts. In Massachilsetts whole farms have been lost by 
 sand dune submergence following the unwise cutting of forests that 
 had formerly acted as a check to the inland migration of the sand. 
 Sand dunes border the south-eastern shore of Lake Micigan and cover 
 considerable areas adjacent to many of the larger streams flowing 
 across the Great Plains in the United States. In other parts of the 
 world as well, they develop in localities where similar conditions pre- 
 vail. 
 
WINDS 63 
 
 By the process of deflation considerable quantities of fine dust are 
 actuallj- exported from desert regions. Tims in Northern China beds 
 of great thiclaiess of such material have accunnilated by export from 
 the central Asiatic deserts. Deposits of this kind are called loess, a 
 term that, however, refei-s rather to the character of the material (es- 
 pecially its fineness, which is between that of sand and clay) than to its 
 origin, since this latter in some cases may be other than aeolian. Great 
 quantities of dust, and sand as well, are and have been exported by the 
 winds of the Sahara desert and deposited in that portion of the At- 
 lantic Ocean which lies to the west of its borders. 
 
 Wind deposits made on land may be identified as such in part 
 by the narrow range in the size of the particles composing them, the 
 roimding of such particles by grinding, aftrifio)i. and if from desert 
 areas, by tlu> occurrence of fresh mineral particles in the deposits of 
 kinds that under humid climatic conditions readily decompose. The 
 most characteristic feature of such deposits, however, is the cross 
 bedding of their structure. As the winds bring material first from 
 one direction then from another the beds are deposited in irregularly 
 oriented planes, one set nearly horizontal, the next inclined steeply and 
 the next perhaps at lui intermediate angle between the two. Deposits 
 made by rapidly shifting currents of water are similarly cross bedded, 
 but in this case the particles vary greatly in size and may attain far 
 too great actual diameters to admit the possibility of transportation 
 by the %vind. 
 
 Another type of deposit occurring in arid regions, though not pri- 
 marily of wind accumulation may well be mentioned here. These are 
 the deposits of salts resulting from the total or nearly complete evap- 
 oration of the waters in desert basins or lakes. The compounds pre- 
 viously held in solution by the waters are precipitated when the satur- 
 ation point is reached, thus often giving rise to incrxistations that 
 cover large areas in arid regions. Following one of the torrential rains 
 common to arid regions the water precipitated for a time percolates 
 underground, and dissolves rock substance on its way. Later the high 
 evaporation rate of such regions draws much of this water to the sur- 
 face again by capillarj^ action. On evaporation the dissolved substance 
 is deposited on the surface or in the near siirface layers of the soil, 
 forming deposits of the so-called aJkalies that give so miach trouble in 
 the development by irrigation methods of arid land soils. Among such 
 salts due to evaporation carbonates, sulphates and chlorides of calciiun, 
 sodium and magnesium are perhaps most common. These materials 
 
64 DYNAMIC GEOLOGY 
 
 also are drfted about by the wind and occasionally formed into dunes, 
 particularly the grains of gypsum, calcium sulphate. 
 
 Students who are able to read German will find an excellent ac- 
 count of wind processes and desert phenomena in a book entitled Das 
 Gesetz der Wustenbildung by J. Walther. The chapter entitled The 
 Geographical Cycle in an Arid Climate in ' ' Geographical Essays " by "W. 
 M. Davis will also be found interesting reading in this connection. 
 
CHAPTER VI. 
 
 TRANSPORTATION, EROSION AND DEPOSITION 
 BY GLACIERS 
 
 ORIGIN AND NATURE OF GLACIERS, — Glaciers develop wherever 
 the annual accumulation of snow during the cold season is in excess of the 
 amount dissipated by melting and evaporation during the warm season. 
 The excess of accumulation may be very slight or almost total; if 
 slight only insignificant ice tongues develop, if great the glaciation 
 may be very extensive. Great cold does not necessarily imply the de- 
 velopment of large glaciers, for the precipitation may be slight and 
 the rate of evaporation high, both factors that would tend to reduce 
 glaciation to a minimujn. Thus large areas of arctic lands are without 
 glaciers, while mountain areas in temperate latitudes where 
 precipitation of snowfall is great, though the mean annual tempera- 
 ture is only a few degrees below the freezing point, may have very 
 large glaciers. 
 
 If the snowfall of a region is in excess of the annual loss by 
 melting and evaporation there must necessarily be a yearly remainder 
 carried over into the next eason of snow precipitation. Thus with 
 succeeding years the accumulation grows each year greater by the 
 amount of the remainder for that year. If such progressive accumula- 
 tion were to continue indefinitely without other means of dissipation 
 than melting or evaporation a large proportion of the water of the 
 world would shortly be locked up in the form of snow accumulations 
 in regions where such climatic relations prevailed. But this is, and 
 has not been, the case. Once the snow accumulation becomes suf- 
 ficiently heavy its underlying layers are compacted into ice, and this 
 ice, under gravity pull, moves down hill toward sea level. Such mov- 
 ing ice masses are termed glaciers. 
 
 If the excess of snow accumulation over melting and evapora- 
 tion has occurred in mountain regions in temperate latitudes, move- 
 ment, as glacial ice, brings part of it each j^ear to lower altitudes 
 where the rate of melting and evaporation are sufficiently high to 
 change it into water. Such glaciers, therefore, end where an equi- 
 librium between the rate of advance, and the rate of melting and 
 evaporation, exists. In higher latitudes the glaciation may be so ex- 
 tensive and the average temperature so low that the ice tongues de- 
 scend to sea level without melting. In this case the ice river enters 
 
66 DYNAMIC GEOLOGY 
 
 the sea, where its ice mass is either melted or broken off and floated 
 away in the form of ice bergs. If the ice bergs are floated toward 
 higher latitudes they may become part of the polar ice packs. Usually, 
 however, the ice bergs drift to warmer climes and are melted. The 
 same thing is true of a sufficient portion of the polar ice packs to 
 compensate for the accessions of ice bergs from glaciers they may 
 receive. Thus in every case precipitation of snowfall on the land, 
 though it may accumulate to a certain mass, is eventually returned 
 to the sea from which it was originally derived. 
 
 DEVELOPMENT AND MOVEMENT OF GLACIERS. — SnOW is white 
 
 (translucent) because of air included between the flakes. It was formerly 
 thought that partial melting and refreezing was necessary to eliminate 
 this included air and to change the translucent snow into clear, trans- 
 parent, glacial ice. "While such a process is no doubt operative in the 
 snow fields of many glaciers ; changing the surface snow into a gran- 
 idar crust (like that seen after a thaw and freeze on the snow cover 
 occurring generally during northern winters, and on glaciers specially 
 designated by the term neve) it is probably not essential to glacier 
 formation and may afi'ect only the surface layers of each season's 
 precipitation on the snowfield. Recent experiments (some of them 
 conducted by the late Professor Tarr at Cornell University) have 
 shown that snow may be compacted into glacial ice, and the included 
 air squeezed out, at temperatures far below the melting point, by 
 pressure alone. Small ice crystals are thus formed, some larger, how- 
 ever, than others, and these larger ones grow, as time elapses and the 
 pressure continues, by absorbing into themselves the mass of their 
 smaller neighbors. In this manner the nodular ice crystals of gla- 
 ciers, called glacial granules, develop ; an individual crystal in a large 
 glacier sometimes attaining the size of a man's fist, though more com- 
 monly they are like pigeon eggs, with smaller, yet imconsumed in- 
 terstitial crystals between them. The operation of this process is de- 
 pendent on laws governing the molecular growth of crystals (properly 
 a part of the study of Crystallography) in this case probably facilitat- 
 ed by the fact that the ice of most glaciers is near the freezing point 
 for the pressure under which it occurs and also because precipitation 
 is never pure but contains a certain percentage of so called cyclic salts, 
 principally common salt, sodium chloride, brought from the ocean 
 by the winds. As is well known, progressively lower temperatures 
 (within certain limits) are required to freeze salt solutions according 
 to their density. As such solutions freeze the crystals formed are 
 not mixtures of salt and water but either pure water, ice, or salt 
 
GLACIERS 67 
 
 crystals. Thus, as the glacial crystals grow, an interstitial film of 
 denser salt solution is probably accumulated between them, and this 
 may persist unfrozen throughout the glacial mass because the tempera- 
 tures attained imder the pressure conditions existing in the ice may 
 not fall low enough to cause the complete separation of the water and 
 salt solution ; concentrated by the abstraction of a large proportion of 
 the water into the growing glacial granules. Although of only micro- 
 scopic proportions, this film of residual solution probably plays an 
 important part in glacial economy, making of the glacier, as regards 
 movement, a near fluid mass though to superficial examination it is a 
 solid, brittle substance. Moreover, it has been found that, in certain 
 directions, parts of ice crystals will glide over one another, in plates 
 of submicroscopic thinness, when subjected to pressure. Accordingly, 
 while glaciers in many respects act mechanically like solids their 
 movement is essentially that of flowing like a liquid. It should be 
 stated, however, that there is great variance of opinion among in- 
 vestigators as to tlie nature and cause of glacial flow and that the 
 explanation given above would not, therefore, find acceptance every- 
 where. For a discussion of the various theories the student should 
 consult Chamberlin and Salisbury's Geology, Vol. I., pp. 308-323.. 
 
 FIG. 22. Sectional diagram of a glacier tongue. The white triangles 
 over every section indicate the amount of thickness lost by melting and 
 evaporation before the position o£ the next lower section is attained. It will 
 be noted tnat an ice particle which attains a bottom position at the head 
 of the glacier maintains its depth throughout the flow, and only melts when 
 by melting and evaporation the thickness of the glacier is reduced from the 
 surface downward sufficiently to biing about the exposure of that original 
 depth of ice to the air. (After Hess.) 
 
 By the diagram Fig. 22 and its title explanation it will be made 
 clear that snow falling at the highest points of a snowfield will follow 
 a course through the lowest part of the glacier tongue and will be dis- 
 
68 
 
 DYNAMIC GEOLOGY 
 
 sipated last by melting and evaporation (the combined process is 
 termed ablation) at the lower end. Also, that snow from a given 
 point in the snowfield will follow the same course as that which pre- 
 viously accumulated there, accordingly continuous and unbroken flow 
 lines are developed in the ice. The upper section of a glacier where 
 accumulation is in excess of ablation is called the reservoir, the lower 
 part, where ablation is in excess, the dissipator. 
 
 GEOLOGICAL SIGNIFICANCE OF GLACIERS. — Probably all glaciers in 
 the world to-day occupy land surfaces previously shaped by processes due 
 
 FIG. 23. North America at the time of maximum Pleistocene glacla- 
 tlon. The names indicate the centers from which the ice moved radially 
 outwards. D indicates an area not invaded by the ice. 
 
GLACIERS 
 
 69 
 
 FIG. 24. Area of maximum Pleistocene glaciation of Europe. Lines and 
 arrows indicate direction of glacial flow. (After J. Geike.) 
 
 to wind and water agencies of the nature described ia the foregoing 
 chapters. Therefore, the geological action of glaciers has been in 
 large part to niodifj' the results previously produced by other agencies. 
 Possibly subsequent glacial action may have nearly obliterated traces 
 of the operation of earlier processes, yet these must have had a guid- 
 ing influence on the ice processes. 
 
 If glacial action had been confined throughout the past to the 
 regions of present glaciation it would be of comparatively little geo- 
 logical significance. But this is not the case. The last great geological 
 episode involved the covering of much of northeastern North America 
 and northwestern Europe under ice sheets of great thickness in much 
 the same way that the Antarctic continent is now submerged. At the 
 same time the glaciation of present regions of glaciers was mitch more 
 extensive. Examination of the maps Figs. 23 and 2-1 ■«ill disclose that 
 a large part of the most productive agricultural land of both continents 
 was covered by the ice. The extensiveness of the occupation, coitpled 
 with its geological recency have made the glacial invasion a very 
 potent factor in determining the natitre of the agrieidture. and of iu- 
 dustry and commerce as well, over large parts of the civilized countries 
 of the world. A farmer in the glaciated sections ought not. therefore, 
 consider himself a competent judge of their soils if he does not under- 
 stand glacial processes. Engineers engaged in road bitilding or sim- 
 ilar work requiring knowledge of the material to be encountered may 
 
70 DYNAMIC GEOLOGY 
 
 also save contracting parties much money if they apply a knowledge 
 of such processes when working in glaciated regions. The subject, 
 therefore, is of great practical importance as well as being geologically 
 significant. 
 
 TRANSPORTATION BY GLACIERS. — Glaciers imlike streams Or the 
 wind are not limited as to the size of the particles they can carry. The ice 
 is equally competent to carry on its surface, in its mass or drag along 
 its bottom, rock fragments as big as a church or of clay-powder fine- 
 ness. Again, glacier movement is slow, from a few inches to per- 
 haps fifty feet per day, depending on the volume of the ice stream 
 and the slope over which it is flowing. The velocity, therefore, at 
 which glaciers transport material is notably less than that of either 
 water or wind. As transporting agencies, however, the slower rate of 
 flow of glaciers is more than compensated for by their ability to carry 
 rock masses of almost unlimited size, and the finest dust with equal 
 facility. 
 
 Rock material is furnished an ice stream in various ways. Much 
 is detached from mountain sides by snow avalanches plunging down 
 to the snowfields, and is thus embedded in the mass of the glacier 
 or carried with the compacted snow-ice to its bottom. More is sup- 
 plied by frost and other weathering of the steeper mountain slopes on 
 which snow will not lie, or from slopes below the snoiv line (line above 
 which snow lies the year round) since most glacier tongues descend 
 beyond this. ]Most of the surface load of glaciers is supplied by wea- 
 thering of rock masses projecting above them. In moving over its bed 
 a glacier pries loose and breaks off rock fragments. Since the con- 
 tinental glaciers submerged the highest mountains, in regions back 
 from their immediate margins, much of the material they transported 
 must have been pried off at their bottoms. This does not imply that 
 they carried no material imbedded in their mass, for it must be re- 
 membered that in passing over and between mountain peaks opportun- 
 ity was afforded for securing a load through a considerable thickness 
 of the ice. In order to flow from the Labrador Center southward 
 into Pennsylvania the ice must have been sufficiently thick near the 
 source to afford a downhill slope to its surface for all that distance. 
 This would be ample to cover the highest peaks of the Adirondacks. 
 Yet near the southern margin the ice was so thin that it extended in 
 tongues along the valleys. Accordingly material weathered from such 
 valley sides might accumulate on the surface of the glacier. Moreover, 
 as ablation thinned the glaciers near the margins, material imbedded 
 in the mass while passing over the mountains in greater thickness 
 
GLACIERS 71 
 
 woiild be eventually exposed, and from thence on be carried as surface 
 load. 
 
 Thus while the surface of glaciers of continental magnitude 
 {Continental glaciers) completely submerging the land, may be prac- 
 tically free of rock load, like the inland ice of Greenland to-day; 
 glaciers occupying mountain valleys {valley or Alpine glaciers) and 
 the marginal tongues of the continental gaciers, carry debris on their 
 surface, in their mass and on the bottom. 
 
 The material transported by a glacier is termed moraine. Prom 
 vrhat has been said as to the sources of moraine it will be appreciated 
 that morainic debris is not scattered indiscriminately through the 
 mass of the ice but is concentrated along certain lines and areas. 
 Certain terms have been applied according to the place of the con- 
 centration. Thus the material from weathering of the valley accumu- 
 lates on the surface at the lateral margins of an ice tongue and is, 
 accordingly, called lateral moraine. Where a tributary joins a main 
 ice stream one lateral moraine of each will be merged at the point of 
 junction and will from there on continue as a surface concentration 
 of debris nearer the center of the combined streams, such a moraine 
 is, therefore, called a medial moraine. Material embedded in the ice 
 is termed englacial moraine. That carried along the sole of the glacier 
 is termed ground moraine. The relative positions of these different 
 types of morainic concentration are shown in the sectional diagram 
 of a glacier tongue, Fig. 25. 
 
 FIG. 25. Sectional diagram of valley glacier showing position of various 
 moraines. Lr-lateral moraine, M-medial, &englacial, G-ground. (After Hess.) 
 
 A special term, ablation moraine, is applied to the material melted 
 out and accumulated on the fronts of the wasting ends of glaciers. 
 Such moraine forms an extensive general cover over the surface of such 
 glaciers, especially, as flow out from mountain valleys and spread out 
 in comparatively thin bulbs on lowlands at the mountain base, pied- 
 
72 : DYNAMIC GEOLOGY 
 
 mont bulbs, or, merging with neighboring, similarly outflowing gla- 
 ciers, form ice plateau^, called piedmont glaciers. 
 
 EROSION BY GLACIERS. —The water issuing from the melting end of 
 a glacier represents the average annual run off from its drainage area. 
 Such run off is probably greater in volume than that from a similar 
 valley with stream drainage because there is less opportunity for the 
 precipitation to percolate underground in a glacier occupied vaUey. 
 Therefore, the single fact that streams flowing from the fronts of gla- 
 ciers are uniformly heavily charged with sediment, usually overloaded 
 indeed, in itself sufficiently attests the relatively great effciency of 
 glaciers as denuding agencies during a large part of the year. While 
 much of the sediment load furnished streams outflowing from gla- 
 ciers is supplied by weathering of the glacial valley's sides, a prob- 
 ably equally great part results directly from glacial erosive pro- 
 cesses; because the rate of glacial abrasion, deepening and widening 
 the valley below the ice surface, must be sufficiently great to keep 
 the slopes above the glacier surface steep enough to permit of the 
 continuous precipitation of weathered fragments onto the ice. As 
 very steep sides are a typical characteristic of glacially carved valleys 
 it is difficult to perceive on what grounds it is argued by some writers 
 that the effect of the presence of glaciers is essentially protective com- 
 pared with the amount of denudation accomplished by the co-opera- 
 tion of streams and weathering agencies. 
 
 A consideration of the conditions and processes of glacier erosion 
 affords further evidence of its effectiveness. Existing glaciers attain 
 thicknesses of 1000 to 1500 feet. A thickness of 1000 feet of glacial 
 ice exerts a pressure of over 55000 pounds per square foot on the 
 bottom on which it rests. The bottom ice is furnished with rock 
 fragment tools, in many cases of great size, acquired in part from 
 the avalanches that feed the snowfields immediately at the glacier 
 source. As the glacier moves the ice acts as a handle to these rock 
 tools and, exerting the enormous pressure of its overlying mass, veri- 
 tably rasps the rock of the bottom and sides of the valley over which 
 it passes. In places where the glacier has melted away the deep scor- 
 ings in the bed rock due to such abrasion are strikingly evident and 
 persist for a long time. In fact, the abundant occurrence of such 
 striations and groovings afford some of the best evidence of the former 
 great extension of glaciers over what are now temperate, agricultural 
 lands. By such grinding, both the rock tools used and the bed rock 
 are ground to powder, giving rise to the rock flour, so called, that is 
 the chief grist of the glacial mill. It is this rock flour that makes 
 
GLACIERS 73 
 
 streams outflowing from glaciers so persistently turbid and lias led to 
 their waters being called glacial milk, in the Alps, because the forma- 
 tions eroded there furnish white sediment. 
 
 Grinding is not the only process of glacial erosion. By squeezing 
 into joint plane and other crevices of the bed rock a glacier may 
 actually quarry out great blocks from its valley bottom. This process 
 is called plucking. The volume of rock removed by the plucking 
 process is probably even greater than that ground to powder. For 
 this reason a glacier is able to erode more rapidly in a well jointed, 
 though hard rock, than in a massive soft formation lacking a well 
 developed joint system, for the latter type of rock can only be reduced 
 by grinding. If, however, a rock is so minutely fissured as to merit 
 being called shattered the fragments quarried may be so small and 
 many as to clog the sole of the glacier and thus reduce the plucking 
 process to the lower value of the grinding action. 
 
 At the head of the valley glacier, where the snow and neve field 
 mass pulls away from the valley wall, as movement is initiated, an- 
 other type of quarrying process is active. At this point a great 
 crevasse, known as the bergschrimd, develops each season. As the 
 snow accumulates during the winter season it is frozen and forced into 
 the crevices of the bed rock of the sides of the basin in which it 
 collects. Accordingly, when the bergschrund opens, some blocks of 
 rock are pulled out by the downward moving ice mass. Later in the 
 season more rock is wedged loose as melting water from the surface 
 flows down the cliff face and alternately freezes and thaws on account 
 of the great temperature differences between night and day normal 
 at high altitudes. By such processes the head of a glacier basin is 
 caused to recede as a nearly vertical cliff. Thus glaciers as well as 
 streams lengthen their valleys at the head. 
 
 The mechanical action of a glacier is more like that of a solid than 
 of a fluid, therefore, its mass exerts great force against any obstacle 
 that tends to divert its down grade course from a straight line. Be- 
 cause of this glaciers tend to straighten the winding stream courses they 
 come to occupy by actually thrusting off the ends of projecting spurs. 
 By such action, plus also abrasion and plucking, the ends of valley 
 spurs are commonly truncated by glaciers. 
 
 "While the glacial erosive processes described above refer particular- 
 ly to the action of valley glaciers, several of them apply to continental 
 and piedmont glaciers as well. On account of their great thickneiss 
 grinding and plucking action under the bottoms of the great contin- 
 ental glaciers must have been peculiarly effective. By such action 
 
74 DYNAMIC GEOLOGY 
 
 all the residual soil and much fresh bed rock was swept ofiE from 
 the surface regions invaded by the Pleistocene Continental Ice Sheets 
 of North America and Europe. On the Labrador Peninsula and in 
 Norway and Scotland (all centers of ice dispersal) bare, hardrock 
 surfaces; grooved and scoured by the ice, are continuously exposed 
 over wide areas. Where the pre-glacial relief of regions invaded by 
 the ice was great and the larger valleys were parallel to the direc- 
 tion of ice movement, these valleys were notably overdeepened by ice 
 erosion because the ice currents were guided by them, therefore, moved 
 through them in greater thickness and with greater velocity than 
 over adjacent highlands. Since ice erosion, unlike stream erosion, is 
 not limited to a sea base level, rock basins with bottoms perhaps below 
 sea level were commonly carved out as a result of such differential ice 
 movement. The troughs of the Seneca and Cayuga valleys in New 
 York State are notable examples of such action. In regions where, 
 on the other hand, the preglaeial relief was comparatively low the 
 tendency of the ice was to move over the country as a broad sheet 
 of uniform depth and rate of flow. Under such a sheet ice erosion 
 would be directed most actively toward the reduction of the minor 
 ridges projecting up into the ice, thus the effect wotfld be to plane 
 the country off to an even more uniform level than had previously 
 existed. To such erosive action, plus processes of glacial deposition, 
 the Central States, especially those south of the Great Lakes, in large 
 part owe their present level topography. Summarizing, the effect of 
 the ice advances was in general to increase preglaeial relief where 
 this was great, to further reduce topographic differences of elevation 
 where the existing relief was slight. 
 
 Since glacier erosion depends for its effectiveness largely on the 
 pressure of the overlying ice mass ; it follows that a great ice stream 
 will deepen and widen its valley much more rapidly than will a 
 smaller tributary gacier. Moreover, at the point of junction of a 
 tribittary with a main glacier there is no especial tendency for more 
 rapid erosion on the bottom of tributary valley, because of a steeper 
 slope, as is the case at the junction of stream valleys ; for, as glacier 
 ice is a solid, the mass of the tributary glacier merges with the sur- 
 face section of the main glacier, increasing the volume of bitter, there- 
 fore, its erosive power, without effecting any similar acceleration of 
 erosion in the lower end of the valley of the tributary glacier. Con- 
 sequently the ends of tributary glacial valleys often end at levels high 
 above the level of the bottom of the main, glacially eroded valley, a 
 condition described by the term hanging valley. This relation ap- 
 
GLACIERS 
 
 75 
 
 FIG. 26. Mountain topography developed by weathering and stream ero- 
 sion. (Alter Davis.) 
 
 FIG. 27. Occupation of region, shown in Fig. 26, by glaciers. (After 
 Davis.) 
 
 FIG. 28. Topography resulting from ice occupation and ice erosion of re- 
 gion shown in Fig. 26. (After Davis.) 
 
76 DYNAMIC GEOLOGY 
 
 plies also to places in the region of Pleistocene Continental ice in- 
 vasion where branches from the main ice currents moved through 
 smaller upland valleys and, because, again, of less depth and less 
 active flow, failed to erode these as deeply as the larger valleys whose 
 axis extended in the direction of principal movement. 
 
 The differences in character between glacial erosion and stream 
 erosion are clearly brought out by the diagrams Figs. 26-27-28. In 
 Fig. 26 is shown the normal topography developed by weathering 
 and stream erosion, in Pig. 27 the same area is shown occupied by 
 glaciers and in Fig. 28 is seen the topography resulting from an ice 
 invasion as it would appear immediately after the ice had melted 
 away. The overdeepened and steepsided main valley, the truncated 
 spurs, the hanging tributaries with rock basins in their bottoms and 
 cirques in their heads; are all marks of the various erosive effects of 
 the ice. 
 
 DEPOSITS DUE TO GLACIERS. — At the melting end of a glacier 
 transportation by the ice necessarily ceases. Consequently the large 
 rock blocks it may have been carrying, and much finer material as well, 
 accumulate sometimes in great masses, at the glacier front, giving rise 
 to terminal moraines; the term moraine being used for material de- 
 posited directly by the ice as well as for the material it is trans- 
 porting. In part the terminal moraine material accumulates by slid- 
 ing or rolling down the glacier front as it is melted out, in part it 
 is concentrated by the shoving together of material as the glacier front 
 alternately advances bodily and retreats by melting, and in part such 
 moraines are due to the thin ice wedge at the end of a glacier being 
 ttpturned by previously deposited material, so that the ground moraine 
 supply is built up in a mass of progressively greater height. Often 
 the material of terminal moraines is partly stratified, for trickling 
 streams from the melting ice assort the material to a certain degree 
 before it is deposited in the mass. Especially is this the case if the 
 bulk of the material in transport by the ice is comparatively fine 
 grained. Since the ice transported fragments are not uniformly dis- 
 tributed in the glacial mass some parts of the terminal moraine re- 
 ceive greater accessions of material than others, accordingly grow 
 greater in height and bulk in the same time period than other sec- 
 tions. Thus, due to the various processes and varying rates of de- 
 posit, terminal moraines are extremely complex in both structure 
 and form. 
 
 Terminal moraines of notable size accumulate only when the front 
 of the ice is stationary for a considerable time period, i.e. when an 
 
GLACIEKS 77 
 
 equilibrium exists between the rate of forward movement and the rate 
 of loss by melting. If the ice melts back imiformly and rapidly only 
 a thin layer of material is spread over the area it previously oc- 
 cupied, this layer representing only the rock substance actually in 
 transport by the overlying ice at the time of melting. To such de- 
 posits the term till sheet is applied. Till sheet material is largely 
 derived from the rock tools and rock flour of glacial grinding occurring 
 at the bottom of the glacier. Since such material is subjected to 
 great pressure the till sheet is usually very compact and hard while 
 its structure consists of striated, angular bouiders, coarse grit and 
 fine clay confusedly intermingled. 
 
 In some places, especially along the thin, wasting edges of the 
 continental glaciers, the bottom of the ice was clogged with so much 
 material that not all of it could be carried to the very front. This 
 led to successive layers of till sheet material being plastered, as it 
 were, one on another under the wedge end of the melting ice. Some- 
 times the plastering was unequal in amount and this led to the ac- 
 cumidations of higher masses of such material known as drumlins. 
 On the other hand drumlins may have resulted from the unequal 
 erosion of till sheet layers because of a later thickening and more 
 active advance of the ice over an area of previous deposit. 
 
 Accumulation of successive plastered on layers of till, or of drum- 
 lins, in each case made up of rather fine textured materials, was par- 
 ticularly active under the frontal parts of those sections of the con- 
 tiuental glaciers that had passed over wide areas of low relief under- 
 lain by comparatively soft, easily eroded rock, such as the limestones, 
 sandstones and shales that underlie the Central Plains of the United 
 States in a large part of their glaciated area. In the New England 
 country, and in other regions with similar relations, large boulders 
 are more conspicuous both in and on the surface of the till and this 
 is usually thinner. The reason for this difference is apparent on con- 
 sideration of the course of the ice movement. Coming from the 
 Labrador center of dispersion the ice sheet passed over the Adiron- 
 dacks and various moilntainous areas in the west and north of New 
 England, overtopping them all. The mountain masses, however, pro- 
 jected far up into the ice currents, therefore, offered great opportun- 
 ities for plucking action over both their sides and summits, furnish- 
 ing a considerable thickness of ice with large rock blocks to transport. 
 As these mountains are, on the whole, made up of very durable gran- 
 itic types of rock and the fragments were in many cases carried 
 considerable distances above the bottom of the ice, they were not com- 
 
78 DYNAMIC GEOLOGY 
 
 conly reduced to rock flour during the rest of their journey across the 
 lower lands to the east. Moreover, the lower lands, except for a 
 thin sheet of residual soil that was swept away early, also furnished 
 large blocks oi durable material rather than smaller, softer fragments. 
 As a result the glacial debris cover of New England is exceedingy 
 stony over much of the area, and the nature of the glacial deposits 
 has here much impaired the agricultural value of the land. 
 
 During the melting season a tremendous volume of water must 
 necessarily be liberated at glacier fronts. Such water gathers in 
 channels for some distance back under the edges of the sides and the 
 front of the ice. Thus before its emergenc into the air, commonly 
 from ice caves, a considerable part of the water volume from glacier 
 melting pursues submarginal, subglacial courses where it is confined 
 in closed tunnels or conduits by the ice cover. As the water may be 
 of sufficient volume to completely fill such a tunnel it often flows with 
 great velocity under a hydraulic pressure head from the waters in high- 
 er levels of the tube, just as does water in pipes supplied from a stand 
 pipe. Consequently such streams can transport enormous quantities 
 of sediment of which there is an ample supply from the bottom of the 
 ice as well as from fragments carried on the ice surface near the 
 stream sources. As a stream of this nature approaches the place 
 of emergence its pressure is apt to be lessened because of openings 
 to the surface, its velocity, therefore, checked and its carrying power 
 diminished. As a result part of the load may be deposited along the 
 subglacial course forming a gravel deposit on the bed. As this ac- 
 cumulates the stream melts its way higher and higher into the ice 
 mass above it. "When the ice melts away siich a deposit may, there- 
 fore, be of sufficient thickness to form a distinct ridge marking the 
 serpentine bed of the former subglacial stream. Such a deposit is 
 called an esker. If the velocity check is rather sudden a confused 
 mass of stratified gravels of considerable bulk may be deposited in a 
 limited area, such a deposit is known as kame or kame moraine. 
 
 A large proportion of the sediment load of streams due to gla- 
 cier melting is, however, carried beyond the immediate ice front and 
 since such streams are uniformly overloaded, is progressively deposit- 
 ed in sheets of gravel, sand, and clay as distance from the glacier 
 end increases and the streams become more sluggish. Such deposits 
 are known as outtvash plains and constitute a considerable part of 
 the surface material of the south sloping portions of the glaciated 
 plains region of the central United States. Where the wind picked 
 up the finer fragments of such sediment after they had dried, loess 
 
GLACIERS 
 
 79 
 
 deposits like those adjacent to desert regions accumulated during the 
 glacial period. 
 
 In Fig. 29 are shown several types of glacial deposits as they 
 may occur at the end of an ice tongue. Note that, the glacier end 
 rested in the basin to the right behind the moraine loops. 
 
 The material of glacial deposits must be sharply differentiated 
 from residual material due to weathering, both as to the nature of 
 
 MoRAlMt- L 
 
 FIG. 29. Relative position and structure of deposits left by an ice tongue 
 and its meltiig waters. (After Hobbs.) 
 
 the substance and its relation to the bed rock. The residual material 
 of weathering is the insoluble and altered remnant substance accumu- 
 lating in place over the bed rock from which it was derived by 
 chemical and mechanical disintegration. It merges only gradually 
 into fresh bed rock below. It may appropriately be termed rock rot. 
 Glacial deposits, on the other hand, comprise mechanically ground up 
 fresh bed rock for the most part; they contain all the material of the 
 original rock in an unaltered state except as this may have been 
 weathered after deposit. The fine particles of such deposits are, there- 
 fore, well called rock flour. It is becaufce of their possessing all the 
 original rock substance that glacial soils are considered agriculturally 
 stronger and more lasting than some residual soils. Furthermore, 
 glacial deposits bear no relation to the bed rock that they overlie. 
 This difference is well brought out by comparing Fig. 3 -^vith Fig. 30. 
 
 '1^^ 
 
 'i^:::^:>,cS^£-h.^^~'^^22. 
 
 FIG. 30. Glacial deposits resting directly on fresh bed rock surfaces due 
 to ice srosion. The composition of the glacial deposits is not determined 
 by that of the underlying bed rock. (After Tarr.) 
 
80 DYNAMIC GEOLOGY 
 
 In the latter figure it will be noted that there is a sharp line of 
 transition between bed rock below and glacial mantle rock above and 
 that there is no relation between the bed rock and surface topography. 
 In fact glacial deposits are apt to accumulate in greater masses in 
 valleys than on pre-existing hill tops, for the ice tongues, being 
 thicker, will linger longer in valley bottoms than on upland areas, 
 consequently, a much larger proportion of the material transported 
 by the ice gravitates to the lower lying areas during the waning stages 
 of a glacial occupation. Thus areas overdeepened by earlier ice ero- 
 sion tend also to be more deeply filled during retreat of the ice, if 
 not by direct deposit from the glacier front then by outwash deposits 
 due to the melting water. 
 
CHAPTER VII. 
 
 GEOLOGIC PROCESSES IN OCEANS AND LAKES 
 
 GEOLOGIC FUNCTIONS OF THE OCEAN. — The ultimate destinar 
 of all waste from the land is deposit on the ocean floor. Thus the 
 chief geologic function of the ocean is to provide a receptacle for 
 land derived debris, the result of weathering and the erosion of 
 streams, winds and glaciers. The bottoms of lakes and of other smaller 
 bodies of water also afford resting places for material in transport by 
 these various agencies, but only temporary ones, for eventually lake 
 bottom accumulations are destined to continue their journey to the 
 sea. Yet during the period of their existence the geologic fimctions 
 of and processes in lakes are essentially similar to those of the ocean, 
 though necessarily on a much smaller scale. 
 
 The land waste brought to the ocean or lakes does not accumulate 
 in structureless masses but is assorted by the action of the water and 
 disposed in systematic and orderly fashion over their floors. This is 
 a second important geologic function of oceans and lakes. 
 
 While distinctly passive in comparison with the rate of movement 
 of streams or the winds, currents and other water movement suffi- 
 ciently strong to act effectively as agents of erosion and transportation 
 are, nevertheless, developed in oceans and lakes ; so that bodies of so 
 called "standing water" must be counted with streams, winds and 
 glaciers as denuding agencies. 
 
 CAUSE OF MOVEMENT OF OCEAN AND LAKE WATERS.-— The dif- 
 ferential gravitative attraction of the sun and the moon causes the 
 surface of the ocean to dome up on opposite sides of the earth. These 
 domed up areas pass around the earth as it rotates under them, giv- 
 ing rise to the phenomenon of tides. In the open ocean the height 
 of the tides is slight, only one or two feet, but as the great mass of 
 water involved comes into the shallower depths off shores its voliune 
 is often manifest by a change of level of perhaps 10 to 20 feet and 
 under exceptional conditions, as high as 50 feet. While the rise, called 
 the flow, and falling, called the eib of the tides is usually slow, there- 
 fore incapable of much geologic activity, there are places, especially 
 at the heads of long bays narrowing inland, where the rising tide 
 advances as a steep wave with high speed. This is called the bore, 
 
82 DYNAMIC GEOLOGY 
 
 and where it occurs (notably in the Bay of Pundy region, Nova 
 Scotia) it undoubtedly scours and transports sediment as would a 
 stream current of equal volume and velocity. The ebb of such tides 
 also gives rise to swirling currents of some intensity and tidal races 
 through straits connecting arms of the sea that have high tide at dif- 
 ferent time periods are also often strong currents; but on the whole 
 the geologic activity of tides is comparatively slight. 
 
 At the equator oceanic waters are heated to much higher tem- 
 peratures than in higher latitudes. Such heated water expands, 
 therefore floats above the general water level because it is less dense 
 than the colder water below and to the north and south. Being at a 
 higher level than the surrounding water it tends to flow away from 
 the equator, both to the north and south, while colder water from 
 below pushes in to take its place. Thus a general oceanic circulation 
 is set up, cold water from the poles flowing along the bottom toward 
 the equator, while warm, light water flows north and south from the 
 equator at the surface. Such currents are, however, very sluggish 
 and are, therefore, quite ineffective as agents of scour, transportation 
 or deposition. 
 
 Just as unequal heating at the equator and poles sets the ocean 
 waters in movement so also, but to a more marked degree, is the ocean 
 of gases, the atmosphere, set in motion by differences in the amount of 
 heat it receives, and the planetary winds are developed as a result. 
 As these blow over the ocean waters they start the surface layers of 
 water to drifting because of the friction these oppose to the air move- 
 ment. In the open ocean such wind formed currents are of little im- 
 port geologically because of the great depth of the water, but along 
 shore, where the waters are shallower, movement may be imparted 
 to the whole mass of the water of sufficient velocity to transport much 
 sediment. 
 
 In the mobile and relatively unstable gases of the atmosphere 
 strong local disturbances, due primarily, also, to unequal heating, are 
 set up in the general planetary wind currents; giving rise to storms 
 which are variously designated according to their intensity and place 
 of occurrence as gales, hxirricanes, typhoons, etc. In such disturbances 
 the wiind often follows a spiral course, blowing in toward the center, 
 and incidentally it heaps up the ocean waters of that area. At the 
 center of the storm the water is further heaped up because the air 
 pressure there is lower. Thus a notable difference in level is created 
 and this is swept along over the ocean surface as the whole storm 
 moves with the general air current. If this course be such as to bring 
 it in contact with the land the higher level of the water under the 
 
OCEANS AND LAKES 
 
 83 
 
 8torm results in a veritable wall of water being precipitated on low- 
 lying shores. Of this nature was the storm and accompanying water 
 wave that caused the destruction of Galveston, Texas, in 1900. 
 
 In storms extending over larger areas the strong winds tend to 
 set up a strong undulatory movement of the water particles known 
 as wind waves. Away from shore lines the form of such waves movea 
 in the direction of the wind current, but the individual water particles 
 themselves move only up and down in a slightly elliptical course, re- 
 turning essentially to their starting point after each undulation. This 
 is clearly shoNNTi by the analyses of such movement in the diagrams 
 Figs. 31 and 32. 
 
 A > PlRHCTIOnOF v.SftVC MOVtMtMT 
 
 FIG. 31 and 32. Movement of water in waves approaching a shore line 
 and in the open ocean. (After Tarr and Davis.) 
 
 As such waves approach shore lines and encounter shallow water 
 the disturbance extends to the bottom, the movement of the water par- 
 ticles in the lower part of the wave is impeded, the top part movea 
 faster, the wave front becomes steeper and finally topples over, hurling 
 its water mass against the shore in the form of huge breakers, collect- 
 ively surf. In the open ocean high storm winds generate waves meas- 
 uring 30-50 feet from trough to crest. From the center of disturb- 
 ance such storm waves are propagated outward in successive rings to 
 points remote from their place or origin though diminishing in height 
 and velocitj' according to the distance traveled. Accordingly waves 
 
84 DYNAMIC GEOLOGY 
 
 may be breaking along a short line where calm weather prevails. Such 
 waves are known as rollers or the ground swell. 
 
 If a sharp rap be applied to the bottom of a tin basin containing 
 water, a series of ring waves are started from the low water dome 
 created at the center. Such waves are similar to those generated by 
 an earthquake shock on the ocean bottom. They differ from other 
 types of waves in that the whole depth of water is set in motion. 
 Consequently such waves affect great volumes of water and, when 
 they reach shallow off shore waters with comparatively little diminish- 
 ed energy, the magnitude of the disturbance is expressed by its reso- 
 lution into waves of great heights to compensate for the decreasing 
 depth. Such waves may rush far inland and cause great destruction. 
 They are often erroneously styled tidal waves. Still another type of 
 wave deserves mention. This is the iceberg ivave created by the loosing 
 of ice bergs from the fronts of tidal glaciers. Along coasts where 
 numbers of glaciers come down into the sea such waves may act very 
 effectively in attacking the shores on which they break. 
 
 The geologic importance of wave motion, whatever may be its 
 origin, depends on the fact that on approaching shores the undulatory 
 motion of their water particles is changed to one of translation, i.e., 
 horizontal forward movement of the mass, and the great volumes of 
 water involved are hurled against or on the coast. 
 
 EROSION BY WAVES. — Waves like streams, winds and glaciers 
 require to be fiirnished with rock tools if they are to erode effect- 
 ively. As a wave advances into shallow water sand, pebbles and even 
 large boulders, according to the size of the wave, are picked up from 
 the bottom by the forward rushing water and hurled against the 
 rocks of bold shore lines. The mechanical "snolence of such wave at- 
 tack distinguishes it from the other erosive processes. Into the joint 
 plane, bedding plane and other crevices of the roclvs exposed along 
 shore lines water is forced with pressure amoimtiug to nearly three 
 tons per square foot in some places where it has been measured. In- 
 cidentally the air in the crevices is greatly compressed and expands 
 explosively when the pressure is releasd. By such action rock frag- 
 ments in the zone of wave attack are dislodged, supplying the waves 
 with more tools to use in the battering process. The width of the 
 zone exposed to Vi'ave attack is measured by the height of the greatest 
 wave at high tide. As this zone ordinarily does not extend to the 
 height of the cliff, the tendency is for the waves to undercut cliffs 
 and when such undermining has progressed far enough the overhanging 
 rock mass is rendered imstable and breaks off from the top down 
 
OCEANS AND LAKES 85 
 
 bringing additional material withui reach of the waves to be in turn 
 reduced to fine fragments. 
 
 Wave erosion can not, however, progress indefinitely inland bj5 
 such processes if the land level remains stable, for as the cliff recedes a 
 widening, imder water platform, called the wave cut terrace, forms, 
 over which the waves must race in order to come at the base of the 
 cliff. In the shallow water overlying this terrace the force of the 
 waves is diminished by friction so that eventually they reach the rock 
 shore with only slight effectiveness. Further reduction of the bold- 
 ness and height of the cliff is then mainly the result of weatheriag 
 processes ; much in the same manner that a gorge form, stream valley 
 is widened after active vertical and lateral erosion by the .stream 
 ceases. Thus gently sloping shores with wide reaches of shallow water 
 extending seaward eventually result. 
 
 As indicated above, the rock ma.sses dislodged by direct wave ero- 
 sion and by undermining are not suffered to remain imdisturbed after 
 they are precipitated into the sea. They are pounded in turn by the 
 smaller fragments that the waves can lift. The effectiveness of such 
 action, called the wave mill may be gathered from the fact that small 
 waves may come in and break on the .shore as often as 40 times per 
 minute. If such waves each move rock particles only 6 inches and if 
 these are of only sand like fineness, and only rub against each other, 
 the fact that under such conditions each particle wall travel back and 
 forth over an aggregate distance of 1200 feet in one hour indicates 
 how much reduction wave mill grinding may in time bring about. 
 Furthermore, sea waters, like stream waters, dissolve rocks, and prob- 
 ably with greater rapidity because of their higher content of corrosive 
 compounds. 
 
 Wave action is obviously most effective on bold headlands of ex- 
 posed coasts where large waves break without spending much of their 
 energy in overcoming friction on shallowing bottom.?. Similarly soft 
 rocks, much fissured, yield more readily to wave attack than do hard 
 massive formations. The latter, however, supply more effective bat- 
 tering tools. Under exceptionally favorable conditions .shore lines have 
 been measured to retreat as much as 15 feet in a single year and within 
 historic times small islands have been much reduced in area (Heligo- 
 land, north of the German coast) or even completely destroyed by 
 wave attack. 
 
 TRANSPORTATION IN OCEANS AND LAKES. — The water of the 
 breaking wave returns to the greater depths beyond the immediate 
 shore line as a current, known as the undertow, ben'eath the succeeding 
 
86 DYNAMIC GEOLOGY 
 
 incoming wave, as diagramed in Fig. 31. Fragments ground suffi- 
 ciently fine by the wave mill are picked up and carried outward by 
 this current. Though imdertow currents may be quite strong, there- 
 fore able to carry fairly coarse material, they do not, as a rule, trans- 
 port it far beyond the edge of the wave cut terrance ; for beyond that 
 point the water usually deepens rapidly, thus checking the current 
 which merges in its greater volume. Accordingly a wave built terrace 
 formed by deposit from the undertow may extend seaward from the 
 edge of the wave cut terrace. Some of the finest clay sediment brought 
 by the undertow may be buoyed up and carried beyond this in sus- 
 pension for considerable distances off shore, and perhaps settles event- 
 ually to form a deposit of fine mud covering the whole area of the 
 bottom of the lake, but in the sea even such fine material seldom at- 
 tains a position many miles from the coast. 
 
 Much more important, as to the quantities of material moved, than 
 the transporting action of the undertow, is that due to long shore or 
 littoral currents. On the average waves approach coasts with their 
 fronts at an angle with the shore line much more often than they do 
 with their fronts parallel. This advance at an angle is known as the 
 diagonal approach of waves. In consequence of it breakers sweep pro- 
 gressively along the coast, especially if the shore be low, unbroken and 
 sandy ; the movement of the water being such as to carry material 
 along the coast ; as well as up on it and back seaward -nnth the under- 
 tow. Moreover, the wind blowing obliquely also on the shore, tends to 
 start the waters moving in a current along it. Thus sand particles 
 and even pebbles and boulders may be shifted along the coast, pursu- 
 ing a zigzag course, carried on shore and laterally forward by the 
 breaking wave and littoral current, seaward by the undertow, then on 
 shore and laterally forward again by the next wave and the continu- 
 ing current. 
 
 Immense quantities of material, mostly of sandy texture, are trans- 
 ported by such currents. This material is supplied by streams that 
 discharge into the sea as well as by wave mill grinding. For as 
 great a dsitance seaward as shallow water extends the littoral currents 
 may act effectively, and an individual particle may be carried many 
 miles. The flow of such current* is checked only where a deep bay 
 opens back into the coast at right, or nearly right angles to the direc- 
 tion of movement of the waters or where a headland projects out to 
 sea. In both cases the 'long shore current is caused to encounter 
 deeper waters; merging with those of the bay and being diverted into 
 the open sea by the headland. At such points its load is deposited 
 and thus hars across bays, often obstructing harbors, hooks (Sandy 
 
OCEANS AND LAKES 87 
 
 Hook) and spits are formed. An interesting illustration of such a 
 deposit is Crowbar Point in Lake Cayuga, N. Y., described in detail 
 by Tarr. (Wave-formed Cuspate Forelands, Amer. Geol. Vol. XXI, 
 1898, pp. 351-370.) Verj' extensive littoral current transportation and 
 deposition occurs all along the Atlantic and Gidf of Mexico coasts of 
 the United States, from Cape Cod southward ; and it is the rapid shift- 
 ing of such material, often with each storm, that makes coastwise 
 navigation of these sections so dangerous; vessels often going agroimd 
 where deep channels had previously existed. 
 
 DEPOSITION IN OCEANS AND LAKES.— For a great part of their 
 length the coast lines of the continental masses do not mark the line 
 of abrupt descent to true oceanic depths. Submerged shelves, covered 
 by relatively shallow water, termed the continental shelves, project for 
 considerable distances seaward beyond the actual shore line, in some 
 places attaining a width of several liiuadred miles. In other words the 
 seas overfill their basins and extend over portions of the lower lying 
 parts of the continental plateau. 
 
 These submerged projections of the continental masses are the 
 seat of deposit of most of the land derived waste, called terrigtni)us 
 deposits, and apparently have been throughout geological time. Some 
 land waste, di^st, may be carried far out to sea by the winds, and 
 some large rivers (the Congo 600, the Gauges 1000 miles) project their 
 currents, therefore part of their sediment load, many miles beyond the 
 shore. But the latter is exceptional and the volmue of material car- 
 ried by winds relatively small. Thus the bulk of the debris worn from 
 the land is preserved to the continental masses by being deposited 
 on their submerged shelves. In fact, these may have been partl.v built 
 up by such deposits. A further limitation is put upon any tendency 
 that might otherwise exist for very fine material, clay particles, to be 
 buoyed up and carried far out in suspension by the action of sea water 
 which causes such particles to flocculate, or gather together in larger 
 masses, and by thus reducing their surface areas leads to their rapid 
 settling to the bottom. 
 
 SEDIMENTATION. — The water over the continental shelves, though 
 comparatively shallow, is by no means imiform in depth, for the bot- 
 tom everj'where slopes more or less gently seaward. Consequently the 
 deposits laid down vaiy in texture from the shore line outward at 
 any one place, since currents and movement must always be less rapid 
 and strong in deep water than in shallow, when the same forces are 
 acting on both, because of the greater mass of the former. Due to 
 such variation in rate of movement of water with depth, an assorting 
 
88 DYNAMIC GEOLOGY 
 
 process, of which the variouis phases are collectively termed sedimen- 
 tation, is continually being carried on over the areas of the continental 
 shelves. Essentially it involves the sifting of the finer from the coarser 
 sediment and the deposit of the various textures of material in beds 
 or layers according to their kind. Sedimentation begins at the shore 
 line. A stream discharging into the sea carrying gravel, sand and 
 clay has its current checked first and most at the immediate mouth; 
 consequently is forced to deposit the coarsest part of its load. A little 
 farther out the sand settles down and in still deeper water the clay. 
 The products of wave erosion are similarly sorted. In depths of 50 
 feet or less water may be agitated to the bottom by high waves. In 
 this zone gravel pebbles are shifted about and assorted as to size. At 
 greater depths these merge very imperceptibly into coarse grit and 
 sands, and such deposits in turn grade into fine sands and mud. Long 
 shore and tidal currents aid in bringing about the finer gradations of 
 texture. Thus the material furnished by the streams and wave erosion 
 eventually comes to rest in beds of various textures forming parallel 
 belts along the shore line and merging into each other texturally at 
 their lateral edges. Such gradation is shown in the diagram of sedi- 
 mentary deposits, Pig. 33. 
 
 C/oj/ =Sho/a- 
 
 FIG. i3. Diagram to show ideal variation in character of sediment with 
 distance from the shore line. (After Scott.) 
 
 It does not follow that at a given depth deposits of similar char- 
 acter will always be encountered for the currents may be stronger in 
 one place than another. The same relation applies to distance from 
 the shore line. Again, in some places the streams may bring and the 
 waves be able to secure only fine grained material. Consequently 
 coarse deposits will be lacking. At any one place the conditions may 
 vary from time to time. During periods of flood streams may furnish 
 much coarse material to be assorted and deposited, while in a season of 
 drought only clay-fine material will be supplied. Alterations between 
 
OCEANS AND LAKES 89 
 
 storm and calm, high and low tide aso lead to differences in the char- 
 acter of the sedimentation at any one point. Therefore the nature of 
 the material deposited varies vertically as well as horizontally, beds of 
 sand may be succeeded by elaj-s, and these by sands again, or even 
 coarse grit. Thus water laid deposits come to be stratified or divided 
 vertically into layei-s, the plane of separation between one layer or 
 stratum and the next above or below marking the occasion of the tran- 
 sition from one kind of deposit to the other. Total or nearly com- 
 plete cessation of deposit over any one area may also lead to the 
 development of stratification in deposits of very similar texture, for 
 during the period of no deposit some change in the earlier deposited 
 material may serve to prevent its perfect (in the sense of lUibroken, 
 continuous) joining with that next laid down. 
 
 Stratification is a imiversal characteristic of water deposited ma- 
 terial, though it also occurs in wind formed acciuuulatious. The layers 
 may be massive or quite thin (laminae), they may continue of even 
 thickness over wide areas or pinch out (Jentieular beds) horizontally 
 and give way to others of different texture in short distances; accord- 
 ing as conditions of sedimentation remained uniform or were highly 
 variable during a given time period. If the currents were uniform 
 and regular the beds may be deposited nearl.v horizontally ; if they 
 altered rapidly in coiu-se and velocity the layers may dip at various 
 angles, a structure kno'wn as cross or current iedding. If during a 
 clear water period weak waves move over the surface of a newly de- 
 posited bed, they may leave their impress in the form of ripple marks 
 on its top indicating a pai'tial shifting and molding of the material 
 by their action. Similarly at periods of low water rain prints and 
 tracks of animals may be marked on temporarily exposed beds. A suc- 
 ceeding period of active deposition may result in the preservation of 
 such records beneath many feet of overlying material. 
 
 PRESERVATION OF FOSSILS. — All these characteristics are im- 
 portant to the geologist as evidence to determine the conditions pre- 
 vailing when a given stratum or bed was deposited. Even more im- 
 portant from the standpoint of geological chronology is the entombing 
 and preservation of organic remains or traces of them in the stratified 
 deposits. The ways in which this may come about are miiltitudinous. 
 Most commonly such fossils (as all traces of organic life found in the 
 sediments are called) are the hard parts, (or moulds, casts and re- 
 placements of such parts) of shell fi.sh that live in the finer sediments. 
 In the coarser, near shore sediments remains of such creatures are 
 ground to pieces by the wave mill after they die; in deeper waters 
 they do not exist in such numbers, but in the intermediate depths 
 
90 DYNAMIC GEOLOGY 
 
 conditions favor both their abundant occurrence and preservation of 
 their remains in the sediments after death. But bodies of land ani- 
 mals drowned and carried out to sea by river floods, waterlogged 
 branches and vegetable substance of various kinds, insects and birds 
 that fall into the water, may all occasionally settle down to the bottom 
 and be buried under later deposits. Since many feet in thickness of 
 water laid sedimentary deposits are now exposed on the lands; and 
 since in any one set of such beds, where tmdisturbed, the lowest bed 
 represents the earliest deposit, fossils preserved in sediments furnish 
 geologists with a record, very fragmentary to be sure, of the success- 
 ive life forms that have existed on the earth since remote geological 
 ages. Reversing the procedure, the finding of certain types of fossils 
 makes possible the recognition of strata of similar age in regions remote 
 from each other, a matter of exceeding practical importance in locating 
 oil and gas wells, and in mine working. 
 
 PROCESSES AND DEPOSITS IN OCEANS AND LAKES PRIMARILY 
 DUE TO ORGANISMS. — Certain plants (the mangrove tree and salt 
 marsh vegetation) flourish in sea waters. Others grow in the shallow 
 waters of lake shores. By entangling sediments around their roots 
 and stems and by the accumulation of their own remains such organ- 
 isms tend to extend shore lines outward. 
 
 During certain geological periods an almost world-wide condition 
 of low shore lines seems to have existed around the ocean borders, 
 while streams from the land were sluggish and brought little sediment. 
 At the same time climatic conditions were such as to favor very lux- 
 uriant vegetation growth in the shallow off shore seas, giving rise to 
 marine swamps of extraordinary areal extent. As the fern like trees, 
 that seem to have been the characteristic forms in such swamps, grew, 
 their cast off leaves and stems fell into the water below, and when they 
 reached maturity their stems also sank beneath the clear waters. 
 These relations seem to have continued for a long time as great masses 
 of organic matter were accumulated in this way giving rise to coal 
 beds. Vegetable material decays much less rapidly under water than 
 on land, the gaseous constituents escaping while the carbon content re- 
 mains relatively unaltered. Thus while the organic deposits were 
 rjuch diminished in bulk, coal representing only about 7 per cent of 
 the original thickness of the vegetable substance, they became pro- 
 gressively more carbonaceous in composition. The finding of 20-foot 
 seams of coal, therefore, indicates that favorable conditions for such 
 accumulation continiled for great lengths of time, and the purity of 
 the coal in such seams furthermore shows that the waters must have 
 been free from mechanical sediment during all that period. Other 
 
OCEANS AND LAKES 91 
 
 evidence also points to such origin of the coal beds. Underneath them 
 is usually found a characteristic underclay that evidently represents 
 the soil in which the plants grew, as exactly similar material now oc- 
 curs on the bottom of the Dismal Swamp between North Carolina and 
 Virginia in which, in large measure, are duplicated the conditions that 
 obtained when the coal layers were formed. Such clays are commonly 
 fire clays, capable of resisting high temperatures without melting or 
 crujmbling for they consist largely of siliceous sandy material and 
 aluminum silicate clay substance from which the originally present 
 alkaline and iron compounds have to a great degree been leached out 
 by the organic acids exuded by the plant roots and dissolved in the 
 overlying waters. Again, microscopic examination shows that even 
 the hardest coal is made up of determinable vegetable fibres. 
 
 Similar accumulations of vegetable material, though much less 
 extensive in area, have developed and are now developing in many 
 northern lakes, especially those which are comparatively small and 
 shallow. Aroimd the borders of these a moss. Sphagnum, becomes 
 established and gradually spreads out over the water suirface forming 
 a mat of vegetation that gradually grows thick enough to support 
 the weight of a person. Thus a peat bog is developed. As time passes 
 the moss continues to grow at the top surface while the older stems 
 die on the under surface, separate from the living layer and sink to 
 the bottom. As this discarded material partially decays under water 
 the carbonaceous residue builds up layers of peat, which, like coal, may 
 be used for fuel, but as the percentage of carbonaceous substance is 
 less high, and its density much less peat has a much lower heating 
 value than the same bulk of coal. 
 
 Of a different nature than these vegetable accumulations of car- 
 bonaceous matter are the calcareous deposits that form marl icds in 
 fresh water and calcareous oozes in sea waters. Marl is a soft limy de- 
 posit often fotmd below peat beds and in such fresh water lakes as 
 receive only little wash from the adjacent land. It is made up of the 
 fragments of the calcareous shells of fresh water raollusks (snails and 
 the like) that flourish abundantly on such shores, and from the cal- 
 careous residue of lime secreting plants (particularly chara.) In each 
 case the organisms obtain the calcareous material from the soluble 
 compoimds of that substance leached from the surrounding rock by the 
 waters that supply the lake. 
 
 Similar calcareous deposits are laid down on the ocean floor, chief- 
 ly in the deeper waters beyond the zone of deposit of the finest tex- 
 tured muds of land origin. A genus of Poraminifera, the Globigerina, 
 are animals that live in great multitudes in the surface waters of the 
 
92 DYNAMIC GEOLOGY 
 
 oceah, forming shells from the calcareous salts dissolved in the ocean. 
 When the occupants die their shells rain down on the ocean bottom. 
 Near shore the bulk of the deposits formed by such shells is so slight 
 as compared to the quantity of material derived from the land that 
 their presence is completely masked. At depths on the average be- 
 tween 2400 and 17000 feet Glohigerina ooze is the characteristic de- 
 posit over vast areas of the ocean floor, especially that of the Atlantic. 
 At greater depths in the Pacific and Indian Oceans occur deposits of 
 similar origin, but made of silica instead of calcareous substance, these 
 are Radiolarian oozes. Microscopic plants, known as Diatoms, that 
 flourish in both fresh and salt waters, secrete siliceoiLS cases and the 
 accumulation of these on the bottom has also given rise to considerable 
 deposits in lakes and ponds, and in the sea over a wide area of the 
 Antarctic and in the north Pacific, called Diatomaceoiils oozes. Still 
 other kinds of oozes occur, but these are the more important types. 
 In Fig. 33 is indicated the gradation from the finer terrigenous de- 
 posits to the calcareous accumulations that comprise the bulk of the 
 accumulations on the intermediate depths of the ocean floor. 
 
 In the clear waters of tropical seas the temperature of which does 
 not fall below 68 degrees F. and in depths less than 120 feet, reef 
 building coral polyps give rise to notable masses of calcareous sub- 
 stance. In the adult stage the coral polyp attaches itself to a rock 
 or other fixed base and then proceeds to multiply by partial division, 
 giving rise to innumerable other individual polyps which collectively 
 form a colony. In this aggregate of individuals is secreted a skeleton 
 of carbonate of lime extracted from the sea water. Where great num- 
 bers of such colonies exist close together an immense bulk of coral 
 framework is built up. As the colonies grow up from the sea bottom 
 the waves break up the earlier formed substance, grind up the detached 
 fragments, and currents deposit the fine debris between the branches 
 of the standing and unbroken colonies. Thus solid masses of cal- 
 careous material are built up that, consisting largely of wave ground 
 debris, show little trace of their organic origin. Multitudes of other 
 lime secreting animals flourish in and about the coral colonies and 
 their remains also furnish a considerable quota of the bulk of the 
 coral reef that is eventually built up. 
 
 CHEMICAL DEPOSITS IN OCEANS AND LAKES. — Over by far 
 the greater area of the deep sea bottoms, at depths ranging from about 
 13000 feet to the bottom of the most profoimd abyss yet discovered, is 
 found a deposit distinctively termed the Bed clay. This clay repre- 
 sents the insoluble residilal substance of calcareous and other skeletal 
 
OCEANS AND LAKES 93 
 
 remains of oceanic suface forms of animals and plants, as well as of 
 decomposed volcanic ash and terrigenous dust settling down over deep 
 sea areas. The profounder depths of ocean water are so highly 
 charged with carbon dioxide that all but the most resistant compounds 
 of the substances that settle slowly through them are dissolved during 
 the passage downward. Thus the iron oxide and siliceous clay im- 
 purities of the microscopic Globigerina and similar shells make up a 
 great part of the red clay bulk, the iron oxide giving the characteristic 
 red color. Such deposits must, therefore, accumulate with extreme 
 slo^^■ness, as is indicated also by the fact that on their surface meteor- 
 ites occur in such ninnbers that they are frequently brought up from 
 the comparatively limited areas of the deep sea floor that have been 
 dredged over. Similarly certain forms (notably ear bones of whales) 
 of remains from species of sea animals long since extinct still remain 
 in numbers unburied on the surface of the red clay deposits. 
 
 Some of the dissolved substances in the ocean are undoubtedly 
 chemically precipitated but in the deeper waters such processes ^eem 
 to be of relatively rare occurrence. Where shallow arms of the sea 
 extend into regions of arid climate extensive deposits of ocean salts 
 may, however, be precipitated. Probably the most favorable conditions 
 :or such development is when the connection with the open sea is nar- 
 row and barrier-like letting in ocean water only at exceptionally' high 
 tide periods. In the interim between such tides the water in the 
 basin evaporates, the solution attains saturation point for some com- 
 pounds and these are precipitated to the bottom. Calcium, magnosium 
 and sodium salts comprise the bulk of such deposits. In an earlier geo- 
 logical period such relations must have obtained over wide areas, of 
 what is now the central part of the United States, notably northern 
 New York State ; for beds of pure rock salt, sodium chloride, attain- 
 ing a thickness of over 100 feet are found in the underlying stratified 
 rocks of these regions. 
 
 In the arid land regions of the present day, basins occur that 
 have no outlet, i.e., the run off is not sufficient in amoimt to counter- 
 balance the loss by evaporation. Great Salt Lake, Utah, is a typical 
 instance. In such basins the nm off water from the adjacent slopes 
 collects in the lowest depressions and gradually evaporates, concen- 
 trating the content of dissolved substance the streams and under- 
 ground waters bring. Eventually saturation and chemical precipita- 
 tion occur, complete if the water dries up altogether, partial if some of 
 it persists between rainfalls. Thus salt beds and other chemical de- 
 posits, notably calcium carbonate, are found on continental tracts. 
 
94 DYNAMIC GEOLOGY 
 
 CONSOLIDATION OF SEDIMENTS.— Sedimentary deposits on the 
 surface of the bottom of the ocean and other waters are usually loose 
 and incoherent masses and the same thing is generally true of recently 
 accumulated strata occurring on the land. On the other hand strata 
 of more ancient date are, almost universally, hard, firm rock ; in other 
 words stone. It follows, therefore, that with time, certain processes 
 must bring about the change from loose material to the consolidated 
 stony substance. 
 
 Partly such consolidation is due to pressure compaction resulting 
 from the accumulation and weight of overlying, later deposited layers. 
 By such action much clay material has been changed to fairly coherent 
 shale. Clay, however, is notably impervious to water, especially when 
 puddled, therefore is affected least by the most common consolidating 
 process binding other classes of sediment, that of cementation. The 
 ocean water present between the grains of a sand layer at the sea 
 bottom remains comparatively stationary and prolonged contact may 
 result in partial solution of the surrounding grains. Thus a net work 
 of gelatinous silica may develop between the particles. Under the 
 pressure of the accumulating overlying masses a hardening of this may 
 be brought about due to the pressure itself or a partial squeezing out 
 of the water. Then a film of amorphous interstitial silica remains to 
 bind the sand grains together, forming sandstone. Much more com- 
 monly than in the case of the siliceous sand grains such processes of 
 partial resolution and later hardening are operative in calcareous de- 
 posits because the carbon dioxide of ocean waters is particularly ef- 
 fective in dissolving calcium carbonate substances. Thus limestones 
 develop from coral detritus and other calcareous organic remains ac- 
 cumulated on sea bottoms. A strong solution of calcareous waters 
 may percolate very slowly through sands, with resulting partial pre- 
 cipitation, thus binding siliceous sand grains together with calcareous 
 cement. Iron in solution is precipitated by organic matter in natural 
 waters, thus the organic remains entombed with sediments may bring 
 about the filling of the sedimentary interstices with an iron compound. 
 A great variety of analogous cementing processes are no doubt oper- 
 ative in deeply buried sediments. Heat also may play a part in con- 
 solidating sedimentary beds by partially melting the material and 
 thus bringing about the adherence of adjacent grains by making them 
 pasty or semifluid, as is done with clay when it is burnt into bricks. 
 Such action occurs when very deeply buried sediments come within 
 the influence of the earth's interior heat or are adjacent to regions 
 where magmas are being intruded. It may, therefore, be concluded 
 that a great number of processes are acting to bring abodt the con- 
 
OCEANS AND LAKES 95 
 
 solidation of sediments, and from what has been stated above it will be 
 clear that these are of a character amply efficient to develop the rock 
 forms resulting. 
 
CHAPTER VIII. 
 
 DIASTROPHISM 
 
 SIGNIFICANCE OF THE EXISTENCE OF MARKED TOPOGRAPHIC 
 RELIEF. — In the previous chapters processes destructive in type have 
 been discussed for the main part. Such processes tend to bring about 
 the reduction of land levels to that of the sea surface or even lov^er. 
 It is true that a few constructive, or building up, processes have also 
 been considered but these were shown to be effective only in filling up 
 original depressions, that is, in promoting gradation, or, in other words, 
 lending to level up the land and the adjacent sea margins to a uni- 
 form slope with an average lower elevation than existed earlier. If 
 such processes had continued throughout geological time, or even a 
 small fraction of it, without a compensating factor of actual uplift 
 the lands must long since have been worn down to sea level over the 
 whole globe. In that case vast, unbroken plains would extend from 
 shore to shore of the continents and the seas would everywhere be 
 shallow for long distances out from the coast line. 
 
 As is well known, an almost exactly opposite condition actually 
 exists. On every continent are found high mountains and high plateaus 
 with summit elevations thousands of feet above sea level. Yet, it is 
 found on examination that such moimtain and plateau masses are in 
 large part made up of sedimentary rocks containing fossils of marine 
 organisms. On the other hand, these sedimentary strata, now high 
 above sea level, often exhibit in section consecutive layers, hundreds of 
 feet thick; yet all of shallow water origin; i.e. they are made up layer 
 after layer of conglomerates, sandstones and shales in varying altera- 
 tion. No one of these layers could be deposited in depths of water 
 as great as are measured by the vertical extent of some suich sections. 
 Since each layer must have been deposited in shallow water, it is evident 
 that after, or while, each was being former some change in the level 
 of the sea bottom or the sea surface mvust have occurred, to permit of 
 the deposit of superincumbent layers of similar, shallow water sedi- 
 ments. 
 
 What logical conclusion must be drawn from these facts? Very 
 certainly that change in the relative levels of the land and the sea has 
 taken place within the geological history of the earth, not only once 
 bat a number of times as is shown by other evidence. A change in 
 sea level might be due to a notable decrease or increase in the volume 
 of ocean waters, or by sinking of the sea bed over wide areas. In 
 
DIASTROPHISM 97 
 
 exceptional instances an appreciable decrease in the volume of sea 
 water may have occurred, as for instance when great quantities 
 of water were locked up in the huge ice masses of the Continental 
 Glaciers of the Pleistocene Period. No cause is known that could bring 
 about a temporary increase of ocean volume equivalent in amount to 
 such glacial vdthdrawal. On the other hand, it is altogether probable 
 that the ocean bottom is sinking over wide areas of its extent, and a 
 noted German geologist, Professor Suess ("Antlitz der Brde") argues 
 that this would suffice to explain all observed phenomena. 
 
 But the concensus of opinion among geologists is, that not only 
 has the sea floor been depressed, but also, that portions of the 
 continental masses have been and are being bodily upheaved. Further- 
 more, that the forces of upheaval have been quite imiformly in the 
 ascendency ; in other words the vertical measure of the elevation of the 
 land by forces acting from within the earth has more than compensated 
 for the degrading action of the atmospheric agencies of weathering 
 and erosion. Consequently the lands are diversified by relief features 
 of considerable magnitude, notwithstanding the fact, pointed out in an 
 earlier paragraph, that the higher the elevation the more actively do 
 the wearing down processes act. 
 
 CAUSE OF CRUSTAL MOVEMENT. — Changes in the level of the 
 land or the ocean floor either up or down, and horizontal displacements 
 as well, involving the bodily movement of masses of the lithosphere are 
 collectively designated by the term diastrophism. 
 
 The cause of diastrophic movements must be sought in the physical 
 constitution of the interior earth, the so-called centrosphere ; for the 
 forces that oppose, by uplift of the land, the action of the degrading 
 agencies are distinctively terrestrial, i.e. originate within the earth, 
 ■whereas, degradation is primarily effected by extra-terrestrial energies ; 
 in other words, forces originating without the earth. 
 
 Direct observations of conditions deep within the earth are not 
 possible. Consequently all discussion of processes there operative con- 
 sists chiefly of theory and inference about which much difference of 
 opinion exists among geologists. Fundamentally considered, the ques- 
 tion of the nature of terrestrial forces and processes involves specula- 
 tion concerning the origin of the earth. On this subject two rather 
 dissimilar theories have each a considerable number of adherents at the 
 present time. According to the earlier theory, the Nelular Eypothesis, 
 the earth cooled to its present state from an originally highly heated, 
 vast mass of gas, a neiula. As the gas cooled by radiation of heat 
 into space the nebula contracted and its substance condensed to a 
 
98 DYNAMIC GEOLOGY 
 
 sphere of molten material, and in time this solidified at the surface 
 into solid rocks, i.e. the lithosphere. As the temperature of the rocks 
 in the lithosphere increases with depth (though not uniformly either 
 in rate or in different localities) it is inferred that, below a compara- 
 tively thin, cool and rigid outer shell, the interior still remains in a 
 molten condition. Aside from astronomic difficulties, discussion of 
 which falls without this text, the Nebular Hypothesis is open to the 
 objection that the interior of the earth does not behave physically like 
 a molten mass, in the sense ordinarily understood, but acts like a solid 
 with a rigidity greater than that of steel. Because of such objections 
 another, the Planet esimal Hypothesis, has recently been proposed by T. 
 C. Chamberlin (Chamberlin and Salisbury, "Geology.") Chamber- 
 lin conceives a planet built up by the slow aggregation of compara- 
 tively small, cold, solid or liquid particles (planetesimals) into a unit 
 mass from an original cold, spiral nebula. In time the accumulation 
 of originally cool matter developed heat at the center of the growing 
 mass, due to increasing compression, i.e. gravity pressure. Thus the 
 earth must have had a very heterogeneous composition from the be- 
 ginning, that is, it was made up of a mixture of all kinds of plane- 
 tesimals, and must have grown from an originally small nucleus to its 
 present dimensions. As heat developed, by the compression resulting 
 from the ingathering of more material, the less refractory substances 
 were locally and temporarily fused and then forced outward by stress 
 differences in the body of the earth. Thus transfer of heat from the 
 central to the outer parts of the earth occurred and the materials of 
 the mass were gradually separated into radial shells of different den- 
 sity. This would account for the fact that the mass of the earth as a 
 whole has a specific gravity of 5.6 while the outer shell or lithosphere 
 in those parts accessible to observation has an average specific gravity 
 of only 2.6. According to the Planetesimal Hypothesis the earth need 
 never have been hotter at the interior than it is now, planetesimals 
 (meteorites) are still being gathered in, and the process of differentia- 
 tion of its materials may still be in progress. 
 
 While widely variant views obtain in regard to the physical con- 
 ditions of the interior matter, or centrosphere, it is quite generally 
 considered that the lithosphere is probably divisible into several zones, 
 according to depth. Thus the outermost zone consists of rock of var- 
 ious kinds, but all and everywhere penetrated by fissures and cavities 
 large and small. This zone is, accordingly, termed the Zone of Frac- 
 ture. At greater depths it is conceived that the pressure from the 
 overlying masses becomes so great that the existence of open spaces 
 
DIASTROPHISM 99 
 
 is impossible, since movement of the solid rocks by flowage must tend 
 to close all cavities. At such depths, therefore, occurs an intermediate 
 Zone of Fracture and Flowage and below it a Zone of Flowage. Ac- 
 cording to experimental investigation by F. D. Adams (Journal of 
 Geology, 1912, p. 97) the Zone of Fracture may extend to depths of at 
 least 11 miles, and at normal temperatures a small cavity may per- 
 sist to depths of 17 to 20 miles in strong rocks. It appears, therefore, 
 that (accepting either of these hypotheses of the earth's origin, or 
 neglecting them and arguing simply on the basis of the efEeet of earth 
 pressure due to gravity on known rock substance) a relatively rigid and 
 cold, outer zone of the lithosphere, the Zone of Fracture, overlies a 
 hot, comparatively yielding nucleus of interior matter. This interior 
 core may be potentially fluid, that is, so hot that if the pressure were 
 removed it would be in a molten state. Indeed the temperatures may 
 be so great as to rise far above the critical point of all known elements. 
 In that case the interior substance must be, molecularly, in a gaseous 
 condition. Nevertheless, whether potentially fluid or molecularly a gas, 
 the centrosphere behaves physically like a difficultly viscous solid, aa 
 e.g. sealing wax, in that it transmits rapid or constantly changing 
 pressures but yields like a fluid to slow, long continued pressures in 
 one direction. Such apparently contradictory properties may well be 
 characteristic of the earth's interior substance for human experience 
 affords no data in regard to the behavior of matter under the tre- 
 mendous pressure and high temperatures that very probably prevail at 
 great depths below the earth's surface. 
 
 In any event it has been demonstrated with certainty that the in- 
 
 CONTlNtNT 
 
 FIG. 34. Origin of Continent due to proportionally greater downsinking 
 of adjacent areas. (After Salisbury.) 
 
 terior matter is losing heat by conduction through the rocks of the 
 oilter, cold shell of the lithosphere. On this basis it is inferred that 
 the interior nucleus must be contracting as it loses heat, and that, 
 therefore, the relatively cold and rigid lithosphere must be collapsing 
 on the shrinking core, just as the peel of a drying apple wrinkles over 
 
100 DYNAMIC GEOLOGY 
 
 the pulp that is losing substance by evaporation of its juices. Such 
 general collapse, then, of the outer shell, on the shrinking interior, 
 gives rise to diastrophic movement of the masses of the lithosphere. 
 
 It might be reasonably deduced from this that all such movement 
 is dowTiward, i.e. toward the center ; and that the continents and lesser 
 elevations project above the general level only becauise they have suf- 
 fered relatively less displacement toward the center than have adja- 
 cent areas. Such a conception of the origin of the continents is illus- 
 trated by the diagram Fig. 34 in which the continental mass is repre- 
 sented as projecting above the general surface only because of the 
 downsinking of the adjacent areas. Such is not necessarily the case. 
 While the general movement may be one of downsinking due to con- 
 traction, it is to be remembered that in this event the rigid lithosphere 
 shell, which is not appreciably shrinking, miLst tend to become too large 
 to fit over the central core. Therefore compression should be manifest 
 at the edges of collapsing areas. Accordingly continental segments 
 lying between oceanic areas of depression may be virtually wedged up 
 to higher elevations than they had before movement took place. Fur- 
 thermore, the immediate edges of such a wedged up continental block 
 might be intensely compressed, therefore, locally squeezed up. Again 
 the rise of magma accompanying such squeezing may also be capable 
 of bringing about considerable upthrust of overlying material. Thus 
 while the general movement may be downward, in accordance with 
 a Contractional Hypothesis, acceptance of such a hypothesis does not 
 preclude the conception of corollary, actual uplift of certain areas. 
 
 THEORY OF ISOSTACY. — A particular theorj% that of Isostacy has 
 been formulated to express a rather different conception of the relations 
 between the outer and inner portions of the earth than that set forth 
 in the Contractional Hypothesis. The word isostacy indicates an equal 
 stand. According to this theory the exterior portion of the earth 
 is in a sense floating on a denser interior mass. Because of a deficiency 
 in density cf the materials under them, the continental masses float 
 higher than do the areas that comprise the ocean floor. As material 
 is worn off the mountains their mass is lightened, consequently they 
 are uplifted from below to compensate for such loss of material. Sim- 
 ilarly, the adjacent ocean floor, on which the waste from the land is 
 deposited, is depressed by the load of the accumulating sediments, there- 
 fore sinks progressively; making possible the deposit of hundreds of 
 feet of shallow water sediments in successive layers one above the 
 other. The compensating character of such movement fits in very well 
 with observed phenomena of rising mountains and sinking coasts, 
 
DIASTROPHISM 101 
 
 though isostacy only postulates an approximate equilibrium of level 
 between larger masses and does not, therefore, account for rapid and 
 active uplift and depression, per se. 
 
 EVIDENCES OF CHANGE OF LEVEL.— Whatever may be the na- 
 ture of the interior conditions that predicate them, there is no lack of 
 evidence in regard to the actuality of diastrophic movements of the 
 earth's crust. Particularly is this true of regions adjacent to ocean coast 
 lines. Both by direct observation and by geological proof it may be 
 conclusively demonstrated that the lands are sinking in certain locali- 
 ties and rising at other places. If the sea level were changing evi- 
 dence that opposite movements were occurring at the same time in 
 different localities ought not be present, because the ocean consists of 
 connected basins, and a rise in the level of one of these would be 
 apparent in the others as well. 
 
 Diastrophic movements are in general of two distinct types: (1) 
 those that take place slowly and involve large areas, and are. perhaps, 
 isostatic in character, and (2) those that are sudden, and involve dis- 
 location and fracture of crustal blocks in the Zone of Fracture. 
 
 Direct observational confirmation of the slow type of movement 
 is afforded by measurements that have been carried on for a number 
 of years on the coast of Scandanavia. Permanent marks made on the 
 sea cliffs show that north of Stockholm, Sweden, the coast has risen 
 7 feet in 150 years, iuid in some localities is now rising at the rate of 
 2.5 feet per centnrJ^ South of Stockholm, however, the coast is sink- 
 ing. Along the coast of New Jei-sey stumps of a former forest, foimd 
 rooted in the place where the trees grew, are now met with submerged 
 7 feet below low tide level, and there is observational evidence dem- 
 onstrating that a sinking of the land of about 2 feet per century is 
 still in progress in this region. 
 
 E^^dence, partly historical and partly geological, proves both slow 
 uplift and slow depression of the coast of Italy, near Naples. Several 
 columns of an ancient temple, Jupiter Serapis, now above >ea vM, 
 had their bases buried to a depth of 12 feet in marine sediment. Above 
 this buried part of the columns, and now at a level of about 10 feet 
 above the ground, the marble surface of each of the colunms is honey- 
 combed in a zone 9 feet high by the borings of the marine mollusc 
 Lithodomus ; and many of its shells are actually found in the excava- 
 tions the animal made in the rocks. Inscriptions show that the col- 
 umns were above water as late as 235 A. D. After that the temple 
 must have been submerged to a depth of over 20 feet, prmittiug of the 
 deposit of the sediment and the making of the borings. There is doeu- 
 
102 DYNAMIC GEOLOGY 
 
 mentary evidence to prove that the building was completely re-elevated 
 above sea level by 1538. During the past century it has again been 
 sinking. Thus, by historical data and geologic inference, it is clearly 
 shown that repeated downward and upward movement of the land has 
 taken place in that region. 
 
 The occurrence, in many localities, of well preserved raised beaches, 
 often accompanied by ancient sea caves and sea cliffs, all the result of 
 the former erosive activity of ocean waves, but now lifted as much 
 as 100 feet above sea level (Scotland) constitutes very clear evidence 
 of considerable uplift of the land within comparatively recent geo- 
 logic times. Such wave-cut forms, of even more ancient date, have 
 been discovered far inland (Oklahoma) preserved down to the present 
 time because originally excavated in durable granite rock that has 
 crumbled only very slowly under weathering action since the waves 
 have ceased beating against them. Similarly the submerged valleys 
 of many rivers (Hudson, St. Lawrence, Thames, Congo) continued 
 for miles beyond the coast line on the ocean floor, indicate notable 
 subsidence; for rivers can not erode channels on the deeper sea bottom; 
 hence such areas must have been above sea level when the depressions 
 were formed, though now hundreds of feet below the ocean surface. 
 
 It will be noted that the above instances all relate to sea coast 
 occurrences. Change of level inland, unassociated with present or 
 previous oceans, are more difficult to detect. It has, however, been 
 clearly demonstrated that the land about the basins of the Great Lakes 
 is being tilted upward, relatively toward the northeast, and downward 
 toward the southwest. It has been calculated from actual measure- 
 ments that such tilting is now progressing at such a rate as will suffice 
 to cause the Great Lakes to outflow at Chicago, instead of Niagara, 
 in about 5000 years. That a similar tilting has been in progress for 
 a long time is indicated by the fact that along the shore of Lake 
 Ontario a beach of an earlier higher level of the lake has been tilted 
 from its original horizontal position to such an extent that its eastern 
 end is now 400 feet above lake level, whereas the western end dis- 
 pears beneath the waves. Deformation of this kind by tilting has also 
 notably disturbed the horizontality of the old shore lines of a former 
 much greater expansion of Great Salt Lake in Utah. Thus it appears 
 that inland areas, as well as regions near-adjacent to shore lines, are 
 subject to slow changes in elevation, both at the present time and in 
 the recent geological past. 
 
 The finding of fossils of marine organisms in very ancient, strati- 
 fied rock, still horizontally disposed, but now situated at elevations 
 
DIASTROPHISM 
 
 103 
 
 thonsands of feet above sea level, proves that the slow changes of 
 level (demonstrated above to have occurred within historic times and 
 in the recent geologic past) have persisted, also, through the remote 
 ages of the geologic past. On the whole it appears that the areas of 
 land have increased as a result of such movement. The movement has 
 not, however, been continually up; rather, periodic submergences and 
 emergences of the continental areas have followd one another, as is 
 strikingly made clear by the presence of unco7ifon7iities in the rock 
 deposits, one type of which is diagrammed in Fig. 35. 
 
 From the presence of such imconformity a variety of changes in 
 conditions may be reduced. First the lower rock layers must have 
 been deposited on the sea bottom. This was followed by uplift and 
 
 FIG. 35. Diagram of an unconformity between two masses of stratified 
 rock with layers in different attitudes. (After Scott. J 
 
 disturbance of the horizontal position of the sediments. The uplift 
 was great enough to bring the tilted, lower rock layers above sea level, 
 as is indicated by the fact that an erosion surface (marked by the 
 undulating horizontal line in Fig. 35) was developed on their edges, 
 truncating the .strata. Such denudation under the atmosphere, must 
 have been followed by submergence, for on the old erosion surface an- 
 other series of stratified layers were laid down, discontinuous with the 
 earlier, lower beds, both as to time of deposit, juncture and attitude 
 of the layers, hence unconformable. The gap in the regular, orderly 
 succession of the strata, made evident by the unmatched line of con- 
 
104 DYNAMIC GEOLOGY 
 
 tact of the two sets of beds, comprises the physical unconformity ; the 
 same term is also used to denote the time interval missing in the rock 
 record. After the deposit of the second set of beds, vertical upthrust, 
 preserving their horizontality, must have re-elevated the whole mass to 
 its present occurrence above sea level. Thus such an unconformity in 
 itself affords convincing evidence of a long series of geologic changes, 
 (a) deposit, (b) uplift and tilting, (c) atmospheric gradation, (d) sub- 
 mergence and deposit, (e) re-elevation above sea level. 
 
 A single unconformity may persist over very wide areas, indicating 
 that the successive changes of level of which it gives evidence were 
 general rather than local in character. ThiLS notable unconformities 
 afford one basis for separating geologic time, as recorded in the rocks, 
 into distinct periods; inasmuch as they mark the transition from one 
 set of conditions to another. It should be noted, also, that unconform- 
 ities may occur between beds of stratified rock having essentially the 
 same attitude, as well as between those having conspicuously different 
 dip, though in the former case they are much more difficult to detect. 
 
 Again, unconformities may occur between stratified rocks on the 
 one hand and igneous or metamorphic rocks on the other. Only one 
 submergence followed by uplift is necessary for the development of 
 this type of unconformity. 
 
 All the evidence of change of level set forth above applies par- 
 ticularly to the slow general type of movement. It should be noted, 
 however, that while such changes may, on the whole, be slow and 
 continuous, some of the facts indicate spasmodic rather than regular, 
 uniform movement. Thus the raised sea beaches are often developed 
 in distinct, successive terraces, one above the other, rather than merged 
 into a more or less continuous slope. It follows that there must have 
 been periods when uplift ceased and wave cutting acted continuously 
 at one level ; followed by a period of renewed and rather rapid uplift 
 to a new point of equilibrium permitting of the cutting of a lower 
 terrace at that level. 
 
 It is altogether probable that these more rapid movements result 
 where stresses in the Zone of Fracture of the lithosphere exert such 
 force on the rocks as to overcome their shearing strengths. In con- 
 sequence the outer rocks yield by fracture and dislocation, and read- 
 jastment to a new equilibrium condition of no strain takes place by a 
 shifting of individual crust blocks up, down and horizontally with 
 reference to their neighbors. As compared to the slower movements 
 of upwarp and downwarp which are probably accompanied by a slow 
 bending yield of rocks in the Zone of Flowage ; these more rapid, near- 
 
DIASTROPHISM 105 
 
 surface fracture adjustments are usually much more restricted as to 
 the area included in any one shift ; nevertheless they involve masses 
 sometimes many miles long and wide. It would seem that concomi- 
 tant with the slow, possibly isostatic. yielding of the deeper rocks by 
 warping, the outer zone is locally rent and fissured by similar stresses, 
 because there the stresses are unopposed by the compressional force 
 of overlying masses. 
 
 The best known instance of rapid and marked change of level, 
 due to such dislocation and fracture of the outer crust, occurred in the 
 Takutat Bay Eegion. ^\laska. during the month of September, 1899. 
 At that time crustal blocks along the coast line were upUfted as 
 much as 37 to 40 feet, as was proved by the discovery, at such alti- 
 tudes, of raised beaches with barnacles still attached to the rocks on 
 which they grew below sea level. These were, however, extreme cases; 
 on the average the uplift ranged between 5 and 12 feet, and commonly 
 an elevated block was neighbored by one that had been depressed so 
 much as to submerge forests under the sea. In other words, blocks 
 of the land mass moved as individual units up or do^vn so as to afford 
 relief from the strain to which they had been subject before fracture 
 took place. Undoubtedly similar shifts occurred along the inland 
 boundaries of the blocks. 
 
 Along the east base of the Sierra Nevada mountains a fracture 
 occurred in 1872 with a resultant displacement of 25 feet. Along 
 the same line there is evidence of earlier recurring dislocations, in aU 
 amounting to a total vertical displacement of several miles. In the 
 Yakutat Bay Eegion, also, there is evidence of earlier uplifts similar 
 in character to those that took place in 1899. On such grounds it is 
 inferred that after a break and adjustment the existing stresses con- 
 tinue to act until the accumulated strain suffices to fracture the rock 
 anew, and that in certain regions these conditions repeat themselves 
 again and again, so that in time the sum of the displacements often 
 attains such figures as were noted for the break along the east front 
 of the Sierra Nevada. 
 
 The planes along and on which the fractures through the rocks 
 take place are termed faults or fault planes because the strata or other 
 rock masses below the surface, occurring on either side of the break 
 do not match.. Such a condition is well illustrated in the diagram 
 Fig. 36, both as to surface and underground expression. OrdinariUy 
 the cliff or fault scarp developed at the surface as the result of ver- 
 tical displacement is quickly obliterated by conversion into a weather- 
 ing waste slope, but under favorable conditions, especially arid climate 
 
106 
 
 DYNAMIC GEOLOGY 
 
 weathering and marked alternation of durable and little resistant strata 
 (as illustrated to the left in Fig. 36) it may persist and recede from 
 the fault plane location as a denudation escarpment by sapping action. 
 
 Faults probably do not extend as actual fissures to very great 
 depths below the surface because the break at such depths is commonly 
 quickly closed by cementation due to deposits from percolating ascend- 
 ing or descending solutions. Accordingly, while old fault planes are 
 often seats of mineral and ore deposits, they ordinarily fail to exert 
 a notable influence in determining the direction of surface erosion be- 
 cause such filling makes the fissure lines equally durable, on the aver- 
 age, with the adjacent country rock. Thus the Colorado River, in a 
 notably faulted region, flows as frequently across fault lines as parallel 
 
 FIG. 36. Vertical fault planes extending through horizontally stratified, 
 folded and igneous rocks. Natural appearance of outcrop of faulted strata 
 Indicated along side of stream valley. (After Davis.) 
 
 to them, a relation that is illustrated in Fig. 36. The fault scarps 
 giving rise to steep slopes and the accompanying uplift of the land, 
 however, both bring about renewed active stream erosion in regions 
 where such dislocations occur. 
 
 While the particular features and characteristics of different types 
 of faults are discussed in the next chapter the student should realize 
 that only very inadequate expression can be given to the complex sub- 
 ject of Diastrophism as a whole in the pages of this volume. A more 
 comprehensive treatment will be found in "Structural Geology" by 
 C. K. Leith to which the student should refer while studying this topic. 
 
CHAPTER IX. 
 
 FAULTING AND EARTHQUAKES 
 
 DIVISIONAL PLANES IN ROCK STRUCTURE. — Incidental to the dis- 
 cussion of the previous chapter it has been necessary to point out and 
 define the principal types of divisional planes in rock structure ; name- 
 ly, stratification planes, joint planes and fault planes. In order that 
 the student may comprehend as nearly as existing knowledge will per- 
 mit the nature of the processes by which they develop, the dynamic 
 forces operating to produce them and the phenomena that result from 
 such action, it is necessary that these divisional planes be considered 
 consecutively, partly in review, and particularly with reference to the 
 nomenclature that attaches to the fault planes. Understanding of all 
 these structural division planes is essential to obtaining a proper ap- 
 preciation of the universal discontinuity of rock masses and the origin 
 of such discontinuity. Furthermore a knowledge of the nomenclature 
 that applies especially to fault planes is necessary in order that des- 
 criptions of the changes in conditions implied by their presence may 
 be made simple and definite. Moreover, clear concepts of these fea- 
 tures and terms are of first importance in practical work. The geologic 
 accuracy of field interpretations is often dependent on giving precise 
 expression to the nature of such structural features where observed, 
 especially in connection with mining operations. 
 
 STRATIFICATION PLANES, STRIKE AND DIP. — S t r a t i fi C a tion 
 planes develop in rocks made up of fragmentary material, mechanically, 
 chemically or organically derived from previously existing rock ma- 
 terial deposited from and assorted by the action of water or wind. 
 The simple experiment of shaking up one or more teaspoonfuls of soil, 
 in a bottle of water and allowing it to settle will demonstrate the re- 
 markable competency of water to assort rock material into layers vary- 
 ing in coarseness from the bottom to the top. The particles settle 
 more or less rapidly according to their diameters and relative density 
 even under such restricted artificial conditions, and the plane of tran- 
 sition from a layer of particles of one grade of fineness to the next is 
 usually fairly well defined ; it is a stratification plane. In nature such 
 assorting action is much more complete, as a rule, for the scale of the 
 experiment is larger and the process is continued longer. 
 
 A stratification plane is, therefore, not a fissure between layers, 
 yet it constitutes a divisional plane separating beds of different tex- 
 
108 DYNAMIC GEOLOGY 
 
 tured or kinds of material, or materials of the same kind deposited in 
 successive time periods ; one from the other. Nevertheless a stratifica- 
 tion plane marks a structural weakness of the rock mass that may de- 
 velop into a fissure or afford a readier line of passage for percolating 
 solutions than the bodies of the rock layers above and below it. By 
 reason of either the difference in kind of particles it separates, or the 
 inclusion of perhaps a minute film of organic matter between layers of 
 similar material or because the layer below it partially compacted be- 
 fore the next one is deposited; the junction marked by the stratifica- 
 tion plane is ordinarily deficient in coherence as compared to adjacent 
 particles in the bulk of the layers themselves. 
 
 Sediments laid down in standing water bodies have their layers 
 separated by essentially horizontal stratification planes, each parallel 
 to the next above or below it. Some slight variation from horizontality 
 may result from the slope of the bottom on which the deposit is made. 
 But sediments deposited in rapidly moving currents of water may 
 have stratification planes notably tilted from the horizontal and at 
 various angles with reference to each other. Such a structure is termed 
 current or cross bedding; and is typical also of material deposited 
 from wind currents, for wind directions, as is well known, may shift 
 from minute to minute. Cross bedded wind deposits are characteristic- 
 ally different from those formed by water in that the material com- 
 posing them is of sand grain fineness, whereas cross bedded water de- 
 posits may be very coarse, include gravel and large cobbles and may 
 vary much in coarseness from layer to layer. Such cross bedded 
 layers whether water or wind deposited commonly extend only short 
 distances laterally, becoming thinner and pinching out to wedge like 
 edges, thus forming characteristic lenticular beds which add to the 
 irregularity of such deposits. Nor do beds deposited in quiet waters 
 extend indefinitely, laterally, with uniform thickness. They, also, grad- 
 ually pinch out laterally so that any one layer, dissected out from the 
 mass along its stratification plane surfaces, would present the appear- 
 ance of a very much flattened lozenge. This lenticular form, common 
 to beds of both current and quiet water deposits, is due to the fact that 
 at some point in the areal extension of the bed there is a maximum of 
 supply of the kind of material that composes it. 
 
 The greater mass of the stratified rocks exposed the world over, 
 however, exhibit layers rather uniform in thickness for a considerable 
 extension and separated by parallel stratification planes. But while 
 they are parallel to each other the stratification planes of bedded rocks 
 commonly fail to be disposed parallel to the plane of the horizon. In 
 other words the layers jointly slant downwards at a greater or less 
 
FAULTING AXD EARTHQUAKES 109 
 
 angle with the horizon plane. Such inclination is termed the dip of 
 the rocks and may vary from zero degree, when the stratification planes 
 extend parallel t-o the horizon plane ; to 90 degrees when they are at 
 right angles with it. It ^-ill be evident that if the layers dip ap- 
 preciably their successive edges must outcrop in parallel bands at the 
 surface. The intersection of such a line of outcrop and the horizon 
 plane is called the line of strilce or strike of the beds. The dip is al- 
 ways measured in a line at right angles with the line of strike as 
 shown in Fig. 37, and in terms of its angular departure from the hori- 
 .zon plane. Although a given stratum or series of strata may outcrop 
 
 FIG. 37. Diagram to show the strike and dip of strata. (After Marr.) 
 
 continuously for some distance, their line of strike and angle of dip 
 may varj' from point to point according to the manner in which the 
 uplifting and deformational forces varied in intensity and direction. 
 Thus in a given region the attitude of the roek masses may differ great- 
 ly from place to place and such variation may in itself give a 
 considerable measure of complexity to the rock structure of the district. 
 It should be added that the use of the terms dip and strike is not con- 
 fined to sedimentary strata but is applied also in describing the exten- 
 sion and attitude of igneous and metamorphie rock formations show- 
 ing distinct structural lines. 
 
 JOINTS. — ^All rocks are traversed by a series of cracks and fissures 
 that divide them into more or less regular, cubical or prismatic blocks. 
 The fissures themselves are termed joints, and the divisional planes in 
 the rock that their presence occasions are called joint planes. 
 
 In the case of sedimentary rocks such joint planes are commonly 
 disposed vertically with reference to the stratification planes and the 
 more persistent or master joints Bsnally comprise two sets, one parallel 
 
110 DYNAMIC GEOLOGY 
 
 to the strike of the beds, the strike joints, the other set parallel to the 
 dip, the dip joints. The mutual intersection of these two parallel seta 
 of joints divide the strata into prismatic columns, which the stratifica- 
 tion planes mark off into blocks of varying thickness according to the 
 width of the layers. In addition to the master joints, other oblique 
 and irregtdar joints less constant in direction and often confined to a 
 single bed serve to break up sedimentary strata into smaller fragments. 
 Joint cracks are true fissures; not merely directions of potential 
 weakness as is commonly the case with stratification planes. While 
 the more irregular joints, separating rough rock faces in sedimentary 
 recks, may be due to tensional stresses, the typical clean cut master 
 joints, often so narrow as scarcely to permit of the insertion of a thin 
 knife blade in their fissure, are probably due to shear stresses induced 
 by compression of the rock during uplift. Scarcely more than a vibra- 
 tion seems to be required to develop such breaks, for they are present 
 in exactly horizontal beds; and smooth, cleaved faces of identifiable 
 particles in the rock match perfectly, with no trace of lateral or vertical 
 displacement, on either side of the joint. The formation of such 
 cracks may be simulated by slowly compressing a prism of plate glass 
 between the jaws of a vise. 
 
 It is interesting to note that, commonly, in massive igneoas rocks 
 there occurs a third set of joint fissures that correspond in position 
 with the stratification planes of the sedimentary rocks. Vertical inter- 
 secting sets of joints, similar to those in sedimentary rocks, are also 
 present in the massive igneous rocks. In the case of the igneous rocks 
 it is more probable that the joints are practically all due to tensional 
 stresses set up by shrinking as the magma cooled. Such contraction 
 would be altogether adequate to cause the rock to crack and fissure, 
 the uniformity and fineness of the mass controlling the regularity, 
 shape and size of the blocks resulting. Such magma masses can not 
 cool and contract as a whole because their cohesive strength is not 
 suffciently great. In sheet like masses of lava that cool rather rapid- 
 ly under uniform pressure, a regular system of joints, called columnar 
 joints, is particularly apt to develop giving rise to hexagonal prisms 
 generally termed iasaltic columns. When perfect these hexagons are 
 produced by the intersections of systems of three cracks radiating from 
 equidistant points at angles of 120 degrees. Less expenditure of energy 
 is required to produce hexagons than other geometrical figures and 
 the equidistance of the points bespeaks the homogeneity of the cooling 
 magma. Commonly such hexagonal prisms are cut off horizontally 
 by concave-convex, ball and socket like joints, that seem very artificial, 
 but are due to the vertical contraction of the cooling magma. 
 
FAULTING AND EARTHQUAKES 111 
 
 Summarizing them, the facts to remember about joints are (a) 
 that they are common to all kinds of rocks; (b) are tj'pically, plane- 
 surface, clean cut fissures; (c) due to tensional and shearing stresses; 
 (d) usually occurring in two or three well developed parallel sets 
 termed the master joints that mutually intersect each other; (e) divid- 
 ing the rock masses into blocks of greater or less size according as the 
 joints are widely or closely spaced. They originate by vibration, tor- 
 sion, tension and compression during the uplift and distortion of sedi- 
 mentary rocks, and in igneous rocks primarily because of tensional 
 stresses set up by contraction on cooling. When verj' closely spaced 
 they may cause rock masses to be so fragmentary and discontinuous 
 as to merit the use of the term shattered in describing them. 
 
 NATURE OF FAULTS. —Stratification planes and joint planes de- 
 lineate the lesser structural units of rock masses ; the larger crustal 
 blocks are marked out by fault planes. While some faults are quite 
 minute, there is, irrespective of size, an essential difference between 
 joint planes and fault planes, in that previousy adjacent points on op- 
 posite sides of a fault fissure suffer displacement with reference to each 
 other when the fault occurs; whereas in joint-plane breaks such points 
 remain exactly opposite each other. The term fissure is sometimes tech- 
 nically applied to fractures unaccompanied by relative displacement 
 of the opposite sides, when such fractures are of greater continuity 
 and extension than is common to joint planes. 
 
 Faults develop when differential stresses affecting adjacent rock 
 masses are of such magnitude, or are so exerted, as to induce breaking 
 and movement of the rock masses rather than yield by upwarp or 
 downwarp. When such a break occurs the rock mass on one side tends 
 to move past that on the other in a direction parallel with the frac- 
 ture to such distance as may afford relief from the strains which 
 brought about the break. Fault movement may be up, down, sideways 
 or rotatory, with respect to the relative displacement of previously ad- 
 jacent points on opposite sides of the fault fracture, and the fault 
 surface itself may extend in any direction, be plane or curved. With 
 such a variety of possibility in direction and character of movement it 
 is apparent that the phenomena of faulting as affecting rock structure, 
 and with respect to their cause, are very complex. 
 
 NOMENCLATURE OF FAULTS.— Faults are commonly inclined, and 
 the inclination of the fault at any one point measured downward in 
 degress from a horizontal plane, is, like the inclination of a stratum, 
 termed the fault dip. The inclination of the fault surface from the 
 vertical is called the hade of a fault, and a fault is said to hade to the 
 
112 DYNAMIC GEOLOGY 
 
 side toward which it dips. The angle of the hade is the complement 
 of the angle of the dip. The rock mass above the fault surface is 
 termed the hanging wall of the fault, that below the foot-wall. A 
 fault line is the intersection of the fault surface with the earth's sur- 
 face, and, by analogy, with any artificial surface such as that of the 
 floor of a mine. The fault stnke is the direction of the intersection 
 of the fault surface with a horizontal plane. 
 
 The amoimt of displacement due to faulting, measured in a straight 
 line on the fault surface, between two points formerly adjacent, situat- 
 ed respectively on opposite sides of the fault ; is termed the slip or 
 more precisely the net slip. The throiv is the vertical distance, and 
 the heave the horizontal distance between corresponding lines in the 
 two fracture surfaces ; the first measured in a vertical plane in the 
 direction at right angles to the fault strike, the heave in a horizontal 
 plane at right angles to the fault strike. The perpendicular separation 
 (when used in connection with faults in sedimentary strata, called 
 stratigraphic separation) denotes the distance between formerly cor- 
 responding planes, of a dislocated bed, measured at right angles to 
 those planes. 
 
 The cross-section diagram Fig. 38 illustrates the application of 
 most of the terms defined above. The student should carefully study 
 this diagram and be prepared to explain the various relations. 
 
 CLASSIFICATION OF FAULTS.— With respect to the direction of 
 the fault strike, in its relation to the structural arrangement of the 
 rocks, faults may be classified as strike faults, dip faults, and oblique 
 faidts according as the fault strike is parallel to the strike or dip of 
 the strata; or oblique to the strike of the strata or general structure 
 of the region. Of these, the strike faults are most common and great- 
 est, both as to length and amount of displacement. They may die 
 out in a few yards or run for hundreds of miles, but no fault con- 
 tinues indefinitely. The amount of displacement along any one fault 
 fracture diminishes and finally becomes zero. This implies that the 
 break represented by the fault gradually merges into a warping or 
 bending of the strata at its ends. 
 
 With respect to the direction of movement on the fault surface, 
 faults may be classified as strike-slip faults where the net slip is prac- 
 tically in the direction of the fault strike, dip-slip faults where the 
 net slip is in the line of the fault dip. In both these cases aU straight 
 lines on opposite sides of the faudt that were parallel before the 
 displacement, are parallel afterward. Consequently such faults may 
 
FAULTING AND EARTHQUAKES 
 
 113 
 
 be collectively termed translatory faults. When parallel lines before 
 displacement are not parallel afterwards, that is when one side of the 
 fault has suffered rotation relative to the other side, the fault may 
 be termed a rotatory fault. Since faults die out and displacement 
 is not uniform along them, such variations being permitted by varia- 
 
 PIG. 38. Cross-section diagram of normal strike, dip slip fault; show- 
 ing intersection of fault surface (heavy black line) hading to left against 
 the dip of the strata. Hanging wall to left of the fault surface, foot wall 
 to right. The distance b prime to b, measures the perpendicular separation 
 (here stratigraphic separation) in this case measured along the fault sur- 
 lace because this is drawn perpendicular to the stratification planes. The 
 distance, b prime to c, is the throw; and from b to c, the heave. The angle, 
 b prime, b, c, is the angle of dip; b, b prime, c, the angle of hade. 
 
 If there has been no horizontal movement (strike-slip) the stratigraphic 
 separation in this fault diagram also measures the net slip. (Modified after 
 Scott.) 
 
 tions in the degree of bending deformation possible in the rocks ; there 
 is usually some rotation at different points along translatory faults. 
 In general the movement along an extended fault line varies in direc- 
 tion and amount from point to point. 
 
 Considered in relation to the probable character of the stresses. 
 that produced them, faults may be most simply classified into normal 
 faults, also called gravity faults; and reverse faults, also called thrust 
 
114 
 
 DYNAMIC GEOLOGY 
 
 faults, the former implying a tensional, the latter a compressional 
 stress. 
 
 It shoiild be noted that a rotatory fault, involving simply the tilting 
 of the masses on the opposite sides of an inclined fault surface, may 
 produce phenomena indicating normal faulting at one end of the fault 
 line, seen in vertical section, while at the other end reverse faulting 
 would be shown. Yet the shearing force producing the fault might 
 have been either tensional or compressional, and, whichever it was, was 
 probably the same at both ends. 
 
 In a normal fault the hanging wall has been depressed relatively 
 to the foot wall. On account of such relative or apparent depression, 
 the hanging wall side is often referred to as the downthrow side and 
 the foot wall as the upthrow side; but it must be understood that it is 
 seldom possible to tell which side of a fault has actually moved. A 
 normal fault implies a local extension of the are of the earth's sur- 
 
 FIG. 39. Diagrams a and c show transition from simple fold to normal 
 fault under tensional stress, probably gravity pull. In e, a step fault la 
 shown at the left, a trough fault at the right. Diagrams b and d show, 
 respectively, an overfold passing over to a reverse fault under compressional 
 stress. In f, extreme compression has developed an overthrust fault. (After 
 B. Smith.) 
 
 face, for the beds, measured across the fault strike, occupy greater 
 space after the faulting than before. The manner in which a tensional 
 
FAULTING AND EAKTHQUAKES 115 
 
 stress may develop a normal fault is illustrated by the diagrams a and 
 c in Fig. 39 whieli show a simple fold in the rock passing over into 
 a normal fault. 
 
 A series of parallel normal faults, some distance apart, all hading 
 in the same direction give rise to a step fault; if two parallel, normal 
 faults hade toward each other they form a trough fault and include a 
 wedge-shaped mass of rock between them. A tensional stress may 
 also develop a fault fracture with a dip of 90 degrees, a vertical fault ; 
 and parallel vertical fauts (or nearly vertical trough faults) letting 
 down a rock mass between them are collectively termed a graben fault ; 
 and the rock mass is a grahen. If the mass between such vertical faults 
 is left elevated relatively to the surrounding masses it is termed a 
 horst. 
 
 In reverse faults the hanging wall has been raised relatively to 
 the foot wall, so that the hanging wall is, relatively, the upthrow side, 
 (Diagram d in Fig. 39.) Reverse faults are apparently due to com- 
 pressional stresses, the beds occupy less space than before faulting. 
 If the compressional stresses cause great displacement of the strata 
 an extreme form of reverse fault, termed an overthrust fault (diagram 
 f, Fig. 39) is developed. Overthrust faults with movements of trans- 
 lation amounting to 20 miles have been measured in eastern Tennessee, 
 £.nd in the Scottish Highlands they have been observed to extend for 
 75 miles* 
 
 PHENOMENA OF FAULTS. -Faults are not commonly open cracks, 
 nor do they usually consist of a single clean cut fracture. Sometimes 
 they are made up of slips on closely spaced surfaces, deforming the 
 intervening rock and giving rise to what is termed a shear zoyie in the 
 earth's crust. Movement along fault surfaces is generally recurrent; 
 fault lines once developed apparently mark lines of weakness in the 
 lithosphere structure. Because of extended and recurrent movement, 
 rock in th shear zone, especially in the case of thrust faults, is often 
 broken into angular fragments, called fault breccia, that pack the 
 fault fissure. Where the fracture is cleaner, a powder of pulverized 
 rock called gouge may be formed between the walls of the fau'lt. On 
 the more rigid rock the grinding against each other of the rock masses 
 produces a characteristic grooved and polished appearance on the fault 
 surface, called slickensides. Where the displacement has been consider- 
 able the ends of the strata, especially of soft rocks, next to the fault 
 
 *In general the Report of the Committee on the Nomenclature of Faults 
 (Geol. Soc. 01 Amer.) Bull. Geol. Soc. of America, Vol. 24, pp. 163-186, 1913; 
 has been followed in defining terms, etc., In the discussion above. To this 
 report the student should turn for further details on the subject 
 
116 
 
 DYNAMIC GEOLOGY 
 
 surface may be bent upward or downward, according to the direction 
 of movement; such bending is called drag. It is sometimes possible 
 to determine the actual direction of movement from characteristics of 
 the drag and slickensides. A horse is a mass of rock broken from 
 one wall and caught between the walls of the fault; it must not be 
 confused with a horst. 
 
 Faults are by previous definition necessarily limited in depth to 
 that of the Zone of Fracture ; accordingly die out with depth as ad- 
 justment to strain below is brought about in greater and greater de- 
 gree by rock flow. Nor does a single fault, as indicated by the ex- 
 tension of the fault line on the surface commonly continue unbroken 
 for great distances. Parallel fractures with ends over-lapping extend 
 the displacement in the general direction of its prolongation. Such 
 continuation of faults by parallel successive lines of displacement may 
 
 FIG. 40. Map showing some of the fault lines of the San Francisco, 
 California peninsula. (After Ransome.) 
 
FAULTING AND EAETHQUAKES 117 
 
 give rise to depressions at the surface, called rifts or furrows, often 
 marked by the presence of elongated lakes. A rift of this kind haa 
 been traced for 400 miles from northwest to southeast along the coast 
 of California. Its course is marked by the longest of the lines of 
 dashes in Pig. 40. Other lines of faulting essentially parallel to it, 
 though of lesser extension, are also indicated. The word "normal" 
 applied to that class of faults was uSed to indicate the supposed great 
 predominance of that class over others ; indeed, it has been estimated 
 that 90 per cent, of all known faults are normal. It is, however, 
 probable that reverse or thrust faults are more common in rocks that 
 have been deformed deep below the surface. On later exposure at the 
 surface, by erosion, these thrust faults may have superposed on them, 
 in greater number but lesser displacement, normal faults characteristic 
 of surface conditions. 
 
 CAUSE OF FAULTS. — Whatever the ultimate cause of deformation 
 it seems apparent that the Zone of Fracture of the lithosphere is, 
 aa a whole, a weak or failing structure. In other words the outer 
 shell appears to be collapsing, both generally and locally. Thus the 
 great oceanic segments are sinking with respect to the continental plat- 
 forms and much greater fault displacements seem to occur around the 
 margins of the ocean basins than have been observed on the land areas. 
 After such movement on submerged fault lines differences in depth 
 measuring thousands of feet have been discovered to exist between 
 the snapped off ends of oceanic cables. Suboceanic cliffs, probably 
 fault scarps, 3000 to 5000 feet high are known. On the continents 
 the displacements are apparently of less magnitude (in part possibly 
 due to effacement by erosion) but fault lines are found practically 
 everywhere in the bed rock, and in certain areas may locallj^ exhibit 
 extraordinary development, both as to numbers and amomit of dis- 
 placement. Thus a world wide collapse of the outer crust of the 
 earth is indicated. 
 
 Such general collapse must be ascribed primarily to gravity. 
 Whether, though, it indicates a general radial shrinking of the earth's 
 mass, as postulated by the eontractional hypothesis (due to the con- 
 traction by cooling of a highly heated interior) or a sinking of the 
 heavier and stronger segments of the crust that form the ocean basins 
 (with isostatic compensation by the underflow of lighter rock ma- 
 terials to the areas under the continents) is an open question. 
 
 In either event there must be dominant a tangential contraction 
 of the earth's surface area. In other words distances on the siirface 
 must be shortened in order, either, to permit of a general collapse of 
 
118 DYNAMIC GEOLOGY 
 
 the outer crust toward a shrinking interior, or the radial sinking of 
 the by far larger oceanic segments. In both cases the shell comes 
 to be too large. Thus compression must be the prevailing stress in 
 the rocks of the crust. Notwithstanding the fact that normal faults 
 seem to be of much more frequent occurrence at the surface than 
 reverse faults, the general result of faulting, probably, effects such 
 horizontal shortening. This because a single reverse fault of low dip 
 may accomplish horizontal shortening that would require a large num- 
 ber of normal faults for its compensation. A computation by Leith 
 (op. cit. ante) indicates that in the United States a dip of 36 degrees 
 is the average for reverse faults, while the dip of normal faults aver- 
 age 78 degrees. Displacement of a foot on reverse fault plans means, 
 therefore, nearly a foot, on the average of horizontal shortening ; while 
 the same displacement on normal faults will average only a few inches 
 of horizontal lengthening. It must also be noted here that much hori- 
 zontal shortening is brought about by mountain folding as will appear 
 from later paragraphs. 
 
 It may be that the faulting over local, volcanic, areas is due to 
 the violent migrations of liquid magma, thrusting the bed rock up 
 and aside to permit its escape, thus developing tensional stresses and 
 normal faults. After cessation of the eruptions of magma, faulting 
 may continue by the irregular sinking of the crust into the unsolid 
 depths from which the fluid rock has been ejected. Thus both normal 
 and reverse faults may be developed. Such action seems to have 
 occurred in the Tonopah Mining District, Nevada, as described by 
 Spurr. (Professional Paper No. 42 U. S. Geol. Sur.) It has been 
 shown that the poles of the earth shift .slightly in latitudinal position 
 during the year, and by different amounts from year to year, describing 
 rudely spiral paths, with rather abrupt changes of direction from 
 time to time. Since the migrations of the poles correspond to changes 
 in latitude, a distortional adjustment of the globe must occur when 
 they take place ; and this may involve the movement of material in 
 the mobile Zone of Flowage with resulting displacement of the Zone 
 of Fracture similar to that from movement of magmas in local areas. 
 Moreover a certain correspondence has been observed between the 
 period of observed change of direction of the poles and observed maxi- 
 mum disturbance due to faulting. 
 
 CAUSE OF EARTHQUAKES — Any jar to the rocks of the litho- 
 sphere in the Zone of Fracture, such as the collapse of a cavern roof, 
 the sudden descent of an avalanche or landslide, explosion of volcanic 
 gases or injection of molten magma will occasion a vibration in ad- 
 
FAULTING AND EARTHQUAKES 119 
 
 jacent rock masses, therefore, an earthquake. The more important 
 and violent earth tremors, however, are undoubtedly due to the move- 
 ment of great blocks of the earth 's crust past one another along fault 
 lines. In view of the discussion in the previous paragraphs it will be 
 appreciated that such movement is possible over practically the whole 
 area of the earth's surface, since fault fractures are weU nigh uni- 
 versally distributed through the bed rock ; occurring under the sea as 
 weU as in the continental platforms. All the earthquakes of marked 
 intensity that have been recently studied have been shown to be ac- 
 companied by faulting on an extensive scale. Indeed the relation be- 
 tween the two is even more intimate, for while fault movements initiate 
 the earthquake tremors, the latter in turn develop stresses above the 
 breaking point in rock masses already strained almost to rupture; 
 thus induce further fault movement and in consequence further earth 
 shaking occurs. Accordingly earthquakes are to be considered as be- 
 ing both the result and cause of fault movement. 
 
 Such movements as take place on vertical or nearly vertical frac- 
 ture surfaces seem to be the ones primarily responsible for earthquake 
 shocks. Though the displacements occur on vertical planes, the move- 
 ment is not necessarily limited to up and down shifts of the rock 
 masses. The shift may be in a horizontal direction as well, as in the 
 California Earthquake of 1906 when a lateral displacement of from 
 5 to 20 feet occurred while the vertical displacement rarely exceeded 
 4 feet. Notable lateral displacements of this kind may be explained 
 as due to variations in the compactness of the material on the op- 
 posite sides of the fault, by reason of a greater amount of open space 
 from joint planes on one side. The wedging together of such joint 
 fissures, through excessive compression on one side, would bring about 
 a marked degree of lateral discordance observable at the surface. 
 Nevertheless the heavy earthquake shocks are probably due to vertical 
 displacements, which have been quite extensive in observed instances. 
 Thus at Assam, India, in 1897 vertical displacement along the Chedrang 
 fault was in one place no less than 33 feet. The damage done by 
 the resulting earthquake seems all to have been due to the initial 
 shock from this movement. The vertical displacement of the Mino- 
 Owari earthquake in Japan, 1891, was 18 feet; and similar displace- 
 ments of 20 feet were reported from the Indian earthquake of 1818. 
 A great earthquake in Iceland in 1875 was accompanied by a sinking 
 to a maximum of 65 feet between faults of the depression of Sveinagja. 
 Furthermore, there seems to be a direct relation between the amount 
 of movement and the magnitude of the shocks resulting. 
 
120 DYNAMIC GEOLOGY 
 
 When a notable vertical displacement occurs, the great crust block 
 on one side of the fault is severed from the suddenly slips over 
 its neighbor, only to be as suddenly arrested at the end of the 
 movement. The result is a sharp jolt or shock. Some appreciation of 
 its nature may be gained from the feeling experienced when a rapidly 
 descending passenger elevator comes to a sudden stop, or when one 
 has a dream of falling from a great height. Such a shock, and its as 
 sudden reaction, give rise to a series of elastic waves that are pro- 
 pagated outward through the rock in all directions from the line of 
 faulting. While these waves are of greatest amplitude (intensity) and 
 cause the maxima of destruction in earthquake shocks, they are not 
 the only ones sent out. 
 
 Tearing, grinding and frictional resistance develop on a tremen- 
 dous scale as shearing of the rock takes place. Projections hold until 
 the strain becomes too great, then yield suddenly. Yet the whole dis- 
 placement takes place within a few seconds. Consequently a great 
 multitude of rapid period, high velocity vibrations are developed and 
 Bent out through the rock. The effect is exactly like that produced 
 by the rosined surface of a violin bow in passing over the strings, the 
 alternate catching and release of the bow producing vibrations of the 
 strings that are transmitted to the air giving rise to sound waves. 
 When it is realized that a fault movement may be many miles long, 
 and perhaps several miles deep, some conception of the magnitude of 
 the disturbance (caused by the combined waves due to the actual dis- 
 placement shocks and the frictional vibrations) may be gained. Such 
 waves passing through the rocks will be reflected as they encounter 
 different kinds of material, will meet in various phases and have their 
 amplitude increased or diminished as the case may be; so that event- 
 ually a disturbance of great complexity is propagated to the surface 
 adjacent to the fault. 
 
 Eocks are imperfectly ela.stic substance, the degree of elasticity 
 varying with the rock type. Elasticity may be defined as that physical 
 property of a substance by virtue of which it resists deforming or com- 
 pressing stresses and recovers its original unstrained condition when 
 the stresses cease to act. In the degree then that rocks are perfectly 
 elastic they transmit the vibrations of earthquake waves without loss 
 of energy ; in accordance with their viscosity the energ}' of the waves 
 will be absorbed and transformed to heat energy. Thus the more 
 dense and rigid the rock, the better the transmission of especially the 
 waves of high velocity and small amplitude. 
 
FAULTING AND EARTHQUAKES 121 
 
 The waves themselves are both compressional, vibrating parallel to 
 the direction of wave propagation; and transverse, vibrating at right 
 angles to the direction of transmission of the disturbance. 
 
 EARTHQUAKE RELATIONS AND CLASSIFICATION. — The area 
 within which a given earthquake is felt is usually oval or elliptical 
 and incloses a line or spot where the intensity of the shock is at a 
 maximum and from which the disturbance originates. On account of 
 their early kno-svn frequent occurrence in regions of volcanic activity, 
 earthquakes were formerly considered to be wholly due to volcanic 
 explosions. This idea persists today in the classification of earth- 
 quakes into volcanic earthquakes and tectonic earthquakes. "While 
 the great world shaking earthquakes, macroseisrns, are caused by vibra- 
 tions originating through faulting; there are many lesser earthquakes, 
 microseisms, that have their origin at a rather definite point beneath 
 the earth's crust; a type of disturbance that the explosion of volcanic 
 gases may well create. In general all minor shocks originating at 
 essentially a point, such as those due to avalanches, may be classed as 
 volcanic earthquakes. Tectonic earthquakes are linear in their exten- 
 sion and originate from fault movement. 
 
 Several other terms used in Seismology, as the science of earth- 
 quakes is termed, also originated in the older idea of the volcanic 
 cause of earthquakes. Thus the point at which the earthquake 
 originated within the earth was termed the centrum; while the shortest 
 path by which the disturbance could be transmitted to the surface, the 
 seismic vertical, connected the centrum with the epicenter above it. 
 At the epicenter the disturbance would normally be greatest; points 
 at equal radial distance from it ought to suffer equal disturbance ; the 
 successive concentric lines encircling such a point, therefore, constituted 
 isoseismal lines. The isoseismal lines did not describe a circle becanfle 
 of differences in the rocks in different directions about any one earth- 
 quake centrum. The term epicenter is still used to designate points 
 along a fault line where the dislocations and consequent disturbances 
 are greatest; they are known as halitual epicenters if recurrent dis- 
 turbances occur about them. In like manner isoseismal lines are 
 drawn to indicate points within the seismic area that sniffer equal in- 
 tensity of shock. 
 
 Certain regions suffer more from earthquake disturbance than 
 others, they are said to be places of high seismicity. In a general way 
 there are two earthquake zones or belts, one encircling the globe north 
 of the equator passing through the Mediterranean Sea, the Himalaya 
 mountains, the East and West Indian islands. The other encircles the 
 
122 
 
 DYNAMIC GEOLOGY 
 
 Pacific Ocean basin. These belts correspond in location to those of 
 the regions of recent volcanic activity and mountain uplift. Other 
 regions seem relatively more stable. More specifically regions of uni- 
 form rock structure are less liable to earthquakes than those where 
 rock masses of notably different density come in contact with each 
 other. Regions of steep slope, i.e. the margins of continental plat- 
 forms and mountain chains, are especially liable to stronger earth- 
 quake shocks. By far the greater number of all earthquakes originate 
 beneath the sea. More recently it has been pointed out that macro- 
 seisms, tectonic shocks, are most frequent in regions where the rate of 
 increase of teiii])erature with depth is greatst ; suggesting a connection 
 between these disturbances and the outward flow of interior heat. 
 
 DISTANT EARTHQUAKES AND THEIR INTERPRETATION. — The 
 
 recording of earthquake shocks on seisrnograplis in regions remote from 
 the seat of disturbance has brought out many interesting relations. 
 
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 At, RtGisTEREO on ^tisnooRAPH AT 
 H'Oraw Hall Corhell UnivtRsiTY 
 
 FIG. 41. Upper diagram shows a typical, distant earthquake record, di- 
 vided to illustrate the different sections. (After Omori.) Lower diagram 
 shows the main portion of such a record as actually registered on the eastr 
 west oriented pendulum of the seismograph at Cornell University. 
 
 The seismograph permits of the registration of earth tremors too min- 
 ute to be experienced by human senses. Modem seismographs consist 
 
FAULTING AND EARTflQUAKES 123 
 
 of two horizontal pendulums, attached to separate standards which 
 are in turn joined to the bed rock by massive, solid foundations. One 
 pendulum is free to swing in an east-west direction, the other in a 
 north-south direction. At the end of each pendulum a pencil is poised 
 over a rotating drum, also firmly founded on the bed rock. If motion 
 of the earth occurs, the drum and pendulum standard will move with 
 it, but the, theoretically, free mass of the pendulum bob will remain 
 steady. Accordingly the pencil will inscribe, on siiitable paper at- 
 tached round the drum, a record of the horizontal earth movements 
 resolved into east-west and north-south components. The record is not 
 complete, for the vertical components of movement are not registered. 
 A portion of an actual record of this kind made on the seismograph 
 at Cornell University, Ithaca, N. T., is shown in the lower diagram 
 of Fig. 41. 
 
 In the upper diagram of Fig. 41 is shofsn in ideal form a com- 
 plete record of a distant earthquake as registered on the seismograph. 
 
 OJTMriantf Station 
 
 
 .y^/ 
 
 
 <0 
 
 
 '' 
 
 ;Are<t of "Heavy ShocRs 
 
 FIG. 42. Diagram to show the paths of the several waves found in the 
 record of tae Chemaliha (Turkestan) earthquake of August 22, 1902. (After 
 Hobbs.) 
 
124 DYNAMIC GEOLOGY 
 
 It will be noted that the different sections of the record show marked 
 individual characteristics. On this basis they have been divided into 
 four principal sections, (a) the 1st Preliminary Tremors, (b) the 2nd 
 Preliminary Tremors, (c) the Principal Portion, (d) the End Portion 
 or Tail. In the distant study of an earthquake the region of heavy 
 shocks becomes essentially a point, the focus of the earthquake, from 
 which waves are sent out in all directions, both around and through 
 the earth. The two sets of Preliminary Tremors are due to compres- 
 sional waves traveling through the earth along the shortest chord be- 
 tween the focus and the observing station ; as was rather conclusively 
 demonstrated at the time of the Kingston, Jamaica earthquake of 
 July 14, 1907. This earthquake was recorded on seismographs at 
 Washington, D. C. 1400 miles away, almost directly north. Prac- 
 tically no preliminary tremors were registered in the east-west direc- 
 tion, though a very distinct set were registered in the north and south 
 direction. That is, the Avaves must have been due to alternate ex- 
 pansion and compression of material in a north-south direction. On 
 the other hand, the amplitude of the Main Portion waves in the east- 
 west component was five times as great as in the north-south com- 
 ponent. The main waves, in other words, are transverse waves travel- 
 ing around the circumference of the earth. In Fig. 42 are shown in 
 section the courses of these two kinds of waves. 
 
 The transverse waves traveling around the circumference of the 
 earth are propagated at much lower velocities than the compressional 
 waves following chords, about 2.2 and 6.2 miles per second respectively. 
 This is in accordance with the known differences in densities of the 
 outter lithosphere and the earth's interior portion, about 2.6 and 5.5 
 times that of water, respectively. The rate of propagation of the 
 direct or chord waves indicates that the earth's interior has a rigidity 
 nearly twice that of steel, and their uniform velocity argues for homo- 
 geneity of the material traversed. It may be noted, however, that 
 where such waves reach an observing station antipodal to the focus 
 of the shock, they behave slightly differently than do such waves that 
 do not pass through the center of the earth, indicating that the very 
 center of the earth is in a different physical state than intermediate 
 depths. 
 
 The larger, circumference waves of the Principal Portion are 
 divisible into various phases and these have been interpreted to indicate 
 separate disturbances, (a) from waves that have traveled by the short- 
 est circumferential course to the observing station, (b) from waveis 
 that have traveled around the longer arcual distance and (e) from 
 
FAULTING AND EARTHQUAKES 125 
 
 waves that have made the complete circuit of the globe after once 
 passing the observing station. In the Chemakha, Titrkestan record 
 (see Pig. 42) this third (c) phase of waves were recorded. They were 
 relatively very feeble, and came about 3 hours and 31 minutes after 
 the arrival of the first (a) waves; about the time necessary for such 
 waves to make the complete cirucit of the earth in the lithosphere 
 material. By noting the time of arrival of the first Preliminary Tre- 
 mors at several observing stations it is possible to determine with some 
 accuracy the place of occurrence of a distant tectonic earthquake, and 
 thus locations and records of earthquake shocks in regions remote from 
 civilization are now obtained and preserved. 
 
 Another characteristic of seismographic, earthquake records fur- 
 nishes a curious confirmation of the existence of a relatively light and 
 thin outer Zone of Fracture of the lithosphere and a denser Zone of 
 Flowage below. If an earthquake occurs within an area of 620 miles, 
 approximately, of the observing station, no preliminary tremors are 
 recorded. For distances only a little greater the preliminary tre- 
 mors are distinctly present. The natural explanation is that within 
 the area of 620 miles the preliminary tremors and the principal por- 
 tion waves both travel through the less dense maetrial of the Zone of 
 Fracture, hence at about the same velocity. Thus the record of the 
 chord, or preliminary tremor, waves is superposed on that of the main 
 waves and hence lost. The chord of 620 miles measured upon an arc 
 of the earth's surface subtends an angle of about 9 degrees, and its 
 deepest point is only about 12.5 miles below the surface. The conclu- 
 sion is that the Zone of Flowage must begin at a rather sharp line 
 near this depth of 12.5 miles, so that waves traveling along chords 
 from distances greater than 620 miles pass partly through this lower, 
 denser medium. Thus the Zone of Fracture in proportion to the mass 
 of the earth is relatively thinner than the shell of a hen's egg to its 
 mass. 
 
 PHENOMENA ASSOCIATED WITH EARTHQUAKES. — During an 
 earthquake the mantle rock is commonly thrown into wave like un- 
 dulations which observers have declared to attain a height of two feet 
 at the occurrence of heavy earthquakes. Perhaps one foot would more 
 nearly approximate a maximum. It seems that the waves of small 
 amplitude are transformed into slow undulations of great amplitude 
 as they emerge from the highly elastic rocks into loose, incoherent 
 soil material. Such waves are tremendously destructive especially 
 to buildings not foxmded on bed rock. In San Francisco during the 
 earthquake of 1906 buildings on filled in lands suffered most severely. 
 
126 DYNAMIC GEOLOGY 
 
 The ground actually moves impulsively, so that blorl- of stone em- 
 bedded in the soil are loosened and projected several feet from their 
 original positions. The different blocks of massive stone monuments 
 are rotated in position and hurled from their bases. Accordingly 
 buildings foujided on rock, rigidly built of steel, concrete ; and, in gen- 
 eral, well bonded massive structures; that vibrate most nearly in the 
 period of the waves in solid rock, resist earthquake shocks best. The 
 actual motion of a single particle during an earthquake is extremely 
 complex; observers are often unable to retain their footing and be- 
 come "seasick." Undulations of this kind precipitated excessive quan- 
 tities of snow into the reservoirs of glaciers in the Yakutat Bay Region, 
 Alaska, during the earthquakes of September 1899 ; with the result that 
 a notable sudden advance of those ice tongues occurred in the suc- 
 ceeding years. Quite frequently sounds, commonly audible rumblings, 
 like those due to a passing vehicle, precede, acconjpany or follow 
 earthquake shocks. There are due to the compression;! vibrations be- 
 ing transmitted into the air. Gases are squeezed out oi the soil, drain- 
 age lines are deranged, new springs are devloped and old ones may 
 increase their flow during and as a result of earthquake movement. 
 
 For further detailed discussion of earthquakes the student should 
 see "The Physics of Earthquake Phenomena" by C. 
 "Earthquakes" by W. H. Hobbs. 
 
CHAPTER X. 
 
 MOUNTAIN UPLIFT 
 
 DISTINCTION BETWEEN ELEVATION AND MOUNTAIN. — From 
 
 the popular viewpoint mountains are regions of relativel.y great eleva- 
 tion with respect to their surroundings ; and particularly such as 
 have comparatively snuill areas of summit level. Such a definition 
 recognizes elevation but does not distinguish between relr.tive eleva- 
 tion, because of uplift, and relative elevation because of reduction 
 of the surrounding area. Nor does it distinguish between various 
 types aud processes of uplift. To distinguish, therefore, between 
 wh it may be termed typical moimtains, from the viewpoint of geo- 
 log eal process, and elevations in general, it is necessary that con- 
 sid. ratiori be given to the structural characteristics and other relations 
 of a particular elevated region, 
 
 ORIGIN AND NATURE OF VARIOUS TYPES OF ELEVATED RE- 
 
 GIC.'JS. — ^v mass of durable rock may rise to a considerable elevation 
 abov ■ its surroundings because of the more rapid denudation of less 
 resis.ant formations around it, continued for sufficient length of time 
 to bring the harder material into considerable relief. Stone Mountain 
 in Ge >rgia and Mt. Monadnock in New iHampshire are of this type. 
 Siieh elevated areas have been termed nwiaitains of circumdenudutwn 
 or residual mountains. 
 
 Diastrophic movement, involving the uplift of large areas, may 
 give a certain region considerable elevation above sea level without 
 disturbing the attitude or structure of the geologic units involved. 
 If such a region has a wide unbroken expanse of surface level it is 
 termed a plateau, particularly if the structure consists of imdisturbed 
 horizontally bedded sediments. "When, however, such an elevated re- 
 gion has been exposed to denudation long enough to result in its more 
 or less complete dissection by erosional agencies, it is often referred 
 to as a mountain region, though the term dissected plateau is much 
 more expressive and distinctive. The Catskills and the Alleghanies 
 are dissected plateaus. 
 
 Eruption of lava and rock fragments from a given rent in the 
 earth's erust may be of such amount as to build up a high mass 
 about it, forming a volcanic cone, or volcanic mountain. Such an 
 elevation may also be termed a nwuntain of accumulation. 
 
128 DYNAMIC GEOLOGY 
 
 Similarly fluid magma may be projected upward from the earth a 
 interior with such force and in sufficient amount to dome up the 
 over-lying, cold rock formations to a considerable extent without find- 
 ing an outlet to the surface. Such elevations are termed laccolithic 
 mountains and the Black Hills of South Dakota and the Henry Moun- 
 
 FIG. 43. Sectional diagram of the Black Hills, showing doming up 
 of strata by magmatic intrusion (dotted mass) from below. Dotted lines 
 indicate former extension of domed strata, the surface form being du« 
 to erosional removal of the domed up strata, revealing the intruded mass 
 below. (After Davis.) 
 
 tains of southern Utah may be cited as examples. In Fig. 43 the 
 Black Hills are shown in section. The exposure of the intruded mass 
 at the summits is due to erosion. 
 
 As indicated in the preceding chapter, movement along fault 
 lines, while seldom of great magnitude in a single displacement, is 
 
 FIG. 44. Diagram of a fault-block mountain. Dotted lines indicate 
 original dimension of block, also the plane of faulting extending down 
 toward the central waste-filled trough. (After Davis.) 
 
MOUNTAIN UPLIFT 129 
 
 commonly recurrent on the same break or series of breaks, so that 
 eventually very notable differences in elevation result by upthrow 
 or tilting of great crustal blocks. Such elevations are termed fault- 
 hlock mountains and the Basin Ranges of Utah and Nevada are 
 typically of this origin. In Pig. 44 a block uplift by faultiag is dia- 
 grammed. The fault-block mountains of the Great Basin may owe 
 their relief in part to differential erosion. Overthrust faulting, also, 
 may result in the piling up of rock masses in sufScient measure to 
 form mountainous masses. Such faulting, moreover, plays an im- 
 portant part in the development of mountains formed by deforma- 
 tion rather than fracture of rock masses. 
 
 Rocks under strain from compressional and tensional stresses 
 tend to shear, with development of fault fractures if the stresses are 
 rapidly applied and the rock mass lies wholly within the Zone of 
 Fracture. "When, however, rock masses of similar composition are 
 deeply buried under over-lying formations and then have compression- 
 al stress applied to them, they yield to such pressure by bending 
 and flowage. In this manner undulations or folds are developed in 
 the rock formations, commonly in parallel series like successive incom- 
 ing waves. The elevations resulting from such folding constitute 
 typical mountain uplift, from the geological viewpoint, and folded 
 structures are characteristic of nearly all the great mountain chains 
 of the world; though faulting, especially over-thrust faulting, is 
 usually also present. Such folding involves, primarily, thick masses 
 of stratified rock and generally the folds have considerable linear 
 extension. Typical mountain uplifts may, therefore, be defined as 
 consisting of linear elevations of masses of stratified, sedimentary rock 
 in successive parallel extension, due to folding up of the layers by 
 compressive stresses. The Appalachians, the Himalaya and the Jura 
 Moimtains of Switzerland may be cited as examples of such moun- 
 tain uplift. 
 
 TYPES OF FOLDING AND UNITS OF MOUNTAIN UPLIFT. — While 
 
 folding is usually due to compressive stresses, nionoclinal folds, con- 
 sisting of a single bend connecting relatively horizontal-lying rock 
 masses that are at different levels, may be due to tension. (See dia- 
 gram a. Fig. 39.) Thus a normal fault may at its end grade into a 
 monoclinal fold; the rock yielding by bending, rather than fracture, 
 because the stresses were applied with sufficient slo-miess to permit of 
 flow yield without rupture. 
 
 Under compressive stresses uparching of the strata develops 
 anticlines, and the downbends or troughs between the anticlinal arches. 
 
130 
 
 DYNAMIC GEOLOGY 
 
 form synclines. Usually the anticlines and synclines are combined 
 in a series of undulations, the former making the crests and the latter 
 the troughs of the waves. Thus the left side, or limb, of an anti- 
 cline, forms the right hand limb of the parallel adjacent syncline. It 
 is important that this complementary relation of anticlines and syn- 
 clines be recognized. 
 
 In Fig. 45 is diagrammed a single anticlinal fold. If inverted it 
 presents the synclinal attitude. The line along which the fold is 
 
 FIG. 45. Model of anticlinal fold. S-,S, is the line of strike; D, the 
 line of dip; P, the axis of the fold pitching toward the left. The dotted 
 line across the vertical section of the strata marks the intersection of the 
 plane of the axis of the fold. (After Willis.) 
 
 prolonged is termed the anticlinal, {or synclinal) axis. A fold may 
 extend for only a few feet or inches or may be scores of miles long, 
 but whether long or short the fold eventually dies out. 
 
 Thus the summit or bottom of the arch or trough is never per- 
 fectly level ; is more or less steeply inclined and this inclination is 
 termed the pitch of the fold. The pitch is merely a special case of 
 dip taken along the axis of the fold. Dome folds and basin folds are 
 special cases in which the axis is reduced to zero, the layers dipping 
 downward on all sides from, or toward a center, respectively. The 
 curve of the arch of a fold may be broad and gentle or sharp and 
 angular, all gradations being possible. 
 
 When the strata of the two limbs of a fold dip at the same 
 angle in opposite directions, the plane of the axis of the fold is 
 vertical and bisects the fold into equal parts. Such a fold is said 
 to be upright or symmetrical. "When the plane of the axis is oblique 
 and the opposite limbs of the fold have different dips the fold is 
 termed an inclined or asymmetrical fold. The fold shown in Fig. 
 45 is slightly asymmetrical. Both symmetrical and asymmetrical 
 
MOUNTAIN UPLIFT 131 
 
 folds may be pushed over so that their axes are inclined from the 
 vertical, due to such over-turning of the whole fold; such folds are 
 termed over-hirned or recumbent folds according to the degree in 
 which they are upset. Open folds have their limbs widely separated; 
 
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 PIG 46. Complex folding of the Alps. The dotted lines show the ex- 
 tension and connection of similar strata before removal by erosion. (After 
 Helm.) 
 
132 DYNAMIC GEOLOGY 
 
 in closed folds the limbs are in contact ; isoclinal folds are those which 
 have been so bent back on themselves that the limbs of the flexures 
 are all parallel with each other, or nearly so. Further compression 
 of isoclinal folds produces fan folds, in which the arch of the anticline 
 is broader than the base, and the synclines are broader below than 
 above. The dotted line arch above the words Gotthard Massiv in 
 Pig. 46 shows the form of an anticlinal fan fold while the section 
 Valle Leventina shows the synclinal fan fold form. Some idea of 
 the actual contortion of the strata involved in the development of a 
 synclinal fan fold may be gained by inspection of the section be- 
 tween Schachenth and Windgalle at the left of the diagram. 
 
 When viewed as a whole, a series of roughly parallel folds may 
 present a great compoxmd uparching of the strata. Such a structure 
 is termed an anticlinorium and is illustrated by the group of folds 
 at the right hand end of Pig. 46. When the compound folds, as a 
 group, form a trough, the term synclinorium is applied. The sec- 
 ondary folds which make up either of these general series may in 
 themselves be made up of many subordinate folds. If the forma- 
 tions have suffered only a single bend the fold is said to be simple, 
 when the individual strata have themselves been contorted or plicated 
 into minor folds, some perhaps microscopic in size, the fold is termed 
 composite. These terms apply to folds as seen in cross section. A 
 rock mass may have suffered deformation by folding due to com- 
 pression from more than one direction, in this case the axial liae is 
 folded also; that is, the strata have been cross folded and such folds 
 are said to be complex folds. It is worthy of note here that nearly 
 all rock folds in mountain masses are complex. 
 
 In order to hold itself up in a simple anticlinal arch without 
 suffering minor contortions of its own mass, (under a given weight 
 of over-lying material), a stratum must have a certain degree of 
 strength. Such a stratum is said to be competent under those con- 
 ditions. If, however, the formation is less strong it may not be 
 able to hold itself in an arch imder the same weight, and composite 
 folding develops; the body of the material composing the fold is 
 contorted and plicated, the formation, is, in other words incompetent. 
 
 The difference between competency and incompetency of a given 
 stratutai under varying loads is well illustrated by experiment with 
 sheets of blotting paper lying in a pack on a table. When such a 
 pack, lying free on the table, is compressed sidewise the sheets will 
 arch up into parallel competent folds. It should be noted that under 
 these conditions the different sheets of the stack do not arch in equal 
 
MOUNTAIN UPLIFT 133 
 
 amounts and that differential movement between the sheets is essential 
 to make such a relation possible. If the sheets of blotting paper have 
 appreciable thickness, it will be clear that the upper and lower side 
 of any one sheet will under these conditions remain parallel to each 
 other, but that the curvature of no two sheets is the same. Because 
 of this the curvature of the sheets must die out either above or below 
 a given sheet. As noted above such folds in rocks are calledparallel 
 competent folds, the designation parallel referring to the parallel 
 relation of the upper and lower side of any one bed after folding. 
 It is important to realize that slipping takes place between the beds. 
 Such slipping displacement is essentially faulting, though not or- 
 dinarily recognized as such, because the movement takes place parallel 
 to the bedding. 
 
 If next a considerable weight of books be piled on the stack of 
 sheets and sufficient thrust, (a very much greater force is necessary), 
 be applied to the edges in the same manner as before, the sheets mil 
 not arrch up as when free but will be crumpled into minute, con- 
 torted folds. In this case the different sheets wiU lie parallel to one 
 another, but the individual sheets will be thickened and thinned from 
 point to point ; that is the bedding surfaces will no longer be mutually 
 parallel. Since the curvature is in this instance the same for aU beds 
 the folds are said to be similar, that is alike, and the folds do not die 
 out above or below. 
 
 This distinction between parallel and similar folds is practically 
 important in connection with mining operations, in that it enables 
 the geologist to infer the probable position and relations of ore bodies 
 below the surface from the character of the exposed outcrops of the 
 folded strata ; or even from driU cores secured from rocks buried under 
 thick cover of glacial drift. Thus in describing a mass of folded 
 rocks a variety of terms need to be used to make clear the changes 
 that have occurred. With reference to the relation the two limbs 
 bear to each other, an anticline may be symmetrical or asymmetrical; 
 with reference to the inclination of its axial plane due to upsetting of 
 the whole fold it may be an over-turned or recumbent fold; with 
 reference to the degree of compression the fold itself has suffered, 
 a fold may be open or closed. With reference to the nature of the 
 flexures, a mold may be simple, composite or complex, and to make 
 clear how a set of strata behaved under the deforming force, the 
 folds are designated as parallel or similar. The effect of the de- 
 forming force on a single stratum determines its competency or in- 
 competency under those conditions. 
 
134 DYNAMIC GEOLOGY 
 
 Competent parallel folds are characteristically formed in the 
 Zone of Fracture; similar incompetent folds in the Zone of Flowage 
 where the great weight of the over-lying rock masses makes necessary 
 that folding of the beds take place by interior adjustment of all parts 
 of the mass. By contortion and plication the material of the bed be- 
 comes thickened and strengthened and so enabled to support the 
 load. In the Zone of combined Fracture and Flowage some layers 
 may be competent, others incompetent. Consequently the folds de- 
 vtloped in that zone exhibit a combination of both parallel and similar 
 fold structure. Thus in a region of folding where quartzitc bods al- 
 ternate with slate, the quartzite beds may be competent while the 
 slate beds are incompetent. Therefore, the quartzite beds will tend to 
 develop the parallel type of fold and the necessary slipping will occur 
 on the surfaces of the incompetent folds of the weaker slate beds be- 
 tween them. Massive limestone beds and shales alternating show 
 similar relations. Such differential movement wiU tend to overturn 
 (by dragging them along) the composite folds of the incompetent 
 layer, developing what are known as drag folds. Drag folds by their 
 direction of over-turning furnish an important index to the struc- 
 tural relations of outcrops of the more competent layers. 
 
 LOCALIZATION AND NATURE OF MOUNTAIN UPLIFT — Through- 
 out geologic time certain portions of the earth's crust, now included 
 in the continental areas, have been submerged again and again ; these 
 areas have been called negative elements. On the other hand certain 
 regions have tended to rise and thus remain uncovered by marine sedi- 
 ment; these are termed positive elements of the crust. The Pied- 
 mont region of the Southern States may be considered a portion of 
 such a positive element; the extensive area to the west, over-lain by a 
 heavy cover of sediments, comprising practically the whole of the 
 Mississippi Basin is a negative element. From the areas of the posi- 
 tive elements, the highland, detritus has been worn practically con- 
 tinuously and spread out over the negative areas. Even greater 
 negative and positive elements are represented by the areas of the 
 ocean basins and the continental platforms respectively. 
 
 In addition to the individual folds included in mountain masses 
 the earth's crust as a whole is involved in broad and gentle flexures 
 termed geanticlines and geosynclines. Into the great geosynclinal 
 troughs sediments were deposited in great thicknesses during former 
 geological ages, for such great troughs seem to have been continually 
 sinking as the sediments accumulated. Geosynclines are necessarily 
 
MOUNTAIN UPLIFT 135 
 
 aligned next adjacent to the geanticlinal areas of uplift that furnish 
 the detritus. Thus areas of exceptionally heavy deposition have been 
 uniformly disposed along the linear contact of geanticlinal and geo- 
 synelinal areas. 
 
 These areas of former exceptionally heavy deposit are the ones 
 that were later involved in extensive mountain uplift; implying a 
 reversal of the earlier continuous sinking. Thus mountain systems 
 of folded strata are typically located parallel to the continental bor- 
 ders, more or less near the contact lines of the major positive and nega- 
 tive elements. A certain periodicity of recurrence of such uplift 
 seems to have been acting, for a time of mountain formation on one 
 continent was also a time of mountain formation on other continents; 
 thus mountain upheavals may as a rule be dated back to a certain 
 few of the geological time periods. Yet a given moimtain region has 
 commonly been the scene of successive uplifts. 
 
 Enormous thicknesses of strata are involved in such uplifts. In 
 the Appalachian geosynclinal a thickness of 25,000 feet of sediments 
 were deposited and later up-folded into the Appalachian ridges. In 
 the Alps 50,000 feet thickness of sediments were involved, in the 
 Pacific Coast Ranges 30,000, the "Wahsatch Range 31,000 feet. 
 
 The uplifts were brought about by lateral thrusts acting com- 
 pressively with great force, and very slowly, so that the strata are 
 folded up into great undulations with distinct reduction in the width 
 of the area formerly occupied by the sediments. In the Southern Ap- 
 palachian folding, 100 miles of former surface has been crowded into 
 75, with uplift of approximately one mile. On the basic of these 
 figures it has been estimated that the depth affected by folding in 
 this area is about 3 miles. Sometimes the folds are open and sym- 
 metrical like the Jura Mountains of Switzerland, but more commonly 
 the strata are throwTi into closed, asymmetrical, over-turned and 
 complex folds, accompanied by over-thrust faulting on a grand scale. 
 This is the case in the Appalachians. The more deeply buried strata 
 are intensely contorted and plicated, developing composite similar 
 folds suggesting that they have been subject to conditions prevailing 
 in the Zone of Plowage. In the Alps the compression has been so 
 great as to develop typical fan fold structure. Almost uniformly, 
 also, the compression has been accentuated by the fact that the strata 
 have been thrust up against a rigid, unyielding mass not affected by 
 the folding, as in the case of the Alps ; or by the resistance of the 
 continuation of the sediments in the zone of diminishing action beyond 
 the folded section. 
 
136 DYNAMIC GEOLOGY 
 
 The localization of the first and more important flexure is prob- 
 ably determined by the places of slightly greater departures from 
 horizontality in the original bedding of the sediments, the initial dips 
 as they are called. Such slightly greater inclination diverts the com- 
 pression from its horizontal direction and thus determines the point 
 of flexure. 
 
 CAUSE OF MOUNTAIN UPLIFT. —The enormous tangential thrusts, 
 thus localized in the sediments adjacent to the continental edges, may, 
 perhaps, best be considered as the most general expression of the con- 
 dition of collapse that seems to characterize the lithsophere of the 
 earth. It suggests crowding between the oceanic and continental seg- 
 ments, due to the settling of the larger and denser oceanic areas with 
 resultant lateral compression and, perhaps, up-thrusting of the less 
 dense and smaller continental masses. 
 
 Just why the buckling should occur in the thick masses of sedi- 
 ment is a question that has brought out many theoretical answers. 
 Recently it has been proposed that such localization is due to radio- 
 thermal beating. It has been demonstrated the radium content, there- 
 fore, the radium evolution of heat, is much greater in sedimentary 
 than in igneous rock. Consequently, during the ages required for 
 the filling of a geosynclinal trough, the deposited sediments would 
 accumulate a much greater store of radium heat than would adjacent 
 masses of igneous rock. Since rocks are poor conductors of heat there 
 would be little loss of this energy by dissipation at the surface. Ac- 
 cordingly the greater part would be retained in the deeper and deeper 
 sinking sediments. Eventually this might cause the temperatures to 
 rise to the melting point of the sedimentary material. If then a con- 
 dition of strain, due to general collapse, were present iu the outer 
 fihell of the earth, the readiest point or line of yield would be along 
 such partly fused sediments. Consequently buckling would occur 
 there with the resultant uplift of mountains. 
 
 EARTH RELATIONS. -Various theories have been proposed to ac- 
 count for the development of the general configuration of the earth; 
 particularly the form of the continental masses and the linear exten- 
 edon of moimtains along their borders. Prominent among these is 
 the Tetrahedral Theory, which assumes that since the outer shell is 
 eoUapsing on a shrinking nucleus it, the outer shell, will tend to retain 
 ita original spherical dimensions. Now a sphere is the geometrical 
 solid that contains the greatest mass for a given surface area. On the 
 other hand a tetrahedron contains the least mass for a given surface 
 
MOUNTAIN UPLIFT 137 
 
 area. Therefore, on shrinking of the interior, the outer, non-shrink- 
 mg shell -mil tend to assume a tetrahedral form, the continents and 
 mountain ranges corresponding to the points and edges of the tetra. 
 hedron, the ocean basins to the flat faces. If a tetrahedron be stood 
 on one of its points, this point would represent the Antarctic Con- 
 tinent at the south pole, the flat face on top would coincide with 
 the Arctic Basin, while the continental masses of North America, 
 Europe and Asia would be disposed at the three points formed by 
 intersections of the edges along this top face. This accounts for the 
 dominance of land areas in the northern hemisphere. It is interest- 
 ing to note that the areas corresponding most nearly to the actual 
 corners have been positive elements throughout geologic time. Similar- 
 ly. South America, Africa and Australasia would be aligned with the 
 angular edges of a tetrahedron, as it tapers down to a point, and 
 these continents do narrow toward the south. The mountain ridges 
 along the coast lines would mark the lines of compression brought 
 about by the sinking of the master negative segments, the ocean basins. 
 It must not be considered that the earth has actually a tetrahedral 
 shape, but only that it is tending toward such a shape rather than 
 that of some other solid. 
 
 Another generalization suggests that the great negative elements, 
 the sea areas, should be expected to have polygonal outlines, corres- 
 ponding to their primary importance both as to area and with respect 
 to a general condition of collapse. Then the continental areas (the 
 smaller positive elements, lying between the ocean basins) would nor- 
 mally have fewer angles, that is typically triangular outlines; in 
 contrast with the many sided, controlling oceanic segments. This 
 scheme does not require so exact correspondence to the form of any 
 one solid, yet provides a rational way of thinking of the origin of 
 the general configuration of the earth. 
 
 RELIEF OF MOUNTAINS ^Mountains probably never had much 
 greater elevations than those of the highest ranges of the present. Thi» 
 may be postulated on the basis that their height must be limited hj 
 the ability of their foundations to sustain them. Much greater height 
 would result in outflow of material at their bases in the Zone of 
 Plowage. 
 
 On the other hand the sedimentary strata that comprise typical 
 mountain masses and outcrop on their sides and summits would, if 
 extended to complete the original folds, rise to much greater heights 
 than the existing elevations. The forces that bring about mountain 
 growth, while they involve the exertion of tremendous energy, never- 
 
138 DYNAMIC GEOLOGY 
 
 theless act slowly according to human measure of time. Mountain 
 growth is probably in active progress at present but its action is 
 only perceptible in the infrequent, sudden changes of level that ac- 
 company faulting displacement. Geological evidence of such growth 
 is furnished by the elevation of beach lines along the ocean fronts 
 of the rising masses, as for instance along the Pacific side of the 
 Coast Ranges and the Andes Mountains in North and South America 
 respectively. 
 
 During and after mountain uplift denudational agencies are 
 active. Rivers, glaciers and winds plane down the rising masses. 
 While such erosional agencies have proved incompetent to wear down 
 as fast as uplift proceeded, yet mountain growth is so slow that 
 large rivers have often been able to maintain their courses across 
 the uplift ; thus the Columbia River has cut acrross the broad arch 
 of the Cascade Mountains in northwestern United States. 
 
 While some mountain uplifts are made up of regular open, sym- 
 metrical folds like the Jura of Switzerland, much more commonly 
 their structure exhibits the most complex kind of composite cross 
 folding. In comparatively young mountains the outcropping strata 
 in general show competency under the deforming forces ; they were 
 uplifted in the Zone of Fracture without great weight of over-lying 
 masses. In older mountain regions where denvidation has planed off 
 the superficial masses and exposed the core of the moimtains (strata 
 that have been deformed in the Zone of Fracture and Flowage com- 
 bined, or indeed, in the Zone of Flowage) the rocks are all minutely 
 contorted and plicated. Commonly the innermost core of a range 
 consists of a great mass of igneous rock, suggesting that material from 
 deep within the Zone of Flowage was included in the uplift or that 
 the uparching of the strata afforded opportunity for the rise of igneous 
 rock from below. 
 
 Differential erosion acting on such complicated and varied rock 
 structure produces topographic irregularities of corresponding rugged- 
 ness. Both the structural forms and variation in the nature of the 
 strata are effective in bringing about diversified relief. Thus there is 
 a distinct tendency for anticlines to be worn away more rapidly than 
 synclinal forms. The narrower outcrops of the more resistant strata 
 develop ridges with broad valleys on the softer rocks between. In 
 Fig. 47 these relations are graphically shown. The restored folds 
 show the original mass of the strata involved. The effect of the pitch 
 of the folds is shown by the zigzagging of the ridges. Peneplanation, 
 after the first folding, resulted in the leveling of the whole region 
 
MOUNTAIN UPLIFT 139 
 
 to a featureless plain at an elevation accordant with the tops of the 
 ridges. Later uparching of the whole region, without folding, has 
 resulted in the erosional etching into relief of the resistant strata. 
 The straight course of the trunk stream suggests that it was able to 
 maintain its course against the effect of the second uparching, that the 
 
 appalachian 
 rioge: 
 
 FIG. 47. Block diagram Illustrating the effect of peneplanation and 
 later differential erosion (succeeding a second uplift! in the development 
 of the topography of the folded Appalachian belt of mountains. (After 
 Davis.) 
 
 uplift, therefore, must have been slow. While these deductions are 
 physiographic rather than in the province of dynamic geology, they 
 are necessary here to make clear the complex geologic history of up- 
 lift and relief development that is typical of mountain masses. 
 
 The dynamics of mnimtain formation present one of the most 
 complex problems of geology. The student will find the references 
 given below very helpful in gaining a more complete understanding 
 of the processes of mountain origin and growth in so far as they have 
 been worked out. Each of the writers is an authority on the subject. 
 Willis, Bailey, "Mechanics of Appalachian Structure:" 13th Ann. 
 Rept. U. S. Geol. Survey, Part 2, 1893. 
 Same "A Theory of Continental Structure applied to 
 
 North America:" Bull. Geol. Soc. of Amer., Vol. 
 18, 1907. 
 Leith, C. K., Structural Geology, N. Y. 1913. 
 
CHAPTER XI. 
 
 VULCANICITY 
 
 NATURE OF IGNEOUS ACTION -Under the term vulcanicity may 
 be included all those manifestations of interior energy that are in- 
 volved in the rise of molten rock material to or toward the surface 
 of the earth. Some geologists, however, would confine the term vul- 
 canicity to phenomena associated with the actual extrusion of the 
 molten material, applying the more general phrase, igneous action, to 
 the grouped relations of both extrusive and intrusive processes and their 
 effects. Intrusion, then, refers to the invasion of masses of molten 
 material into solid rock at a more or less great depth below the surface. 
 
 On a somewhat similar basis a distinction may be made in the 
 use of the terms magma and lava, the former referring to the molten 
 rock material as such, the latter to such parts of it as have been ex- 
 truded at the surface. 
 
 While such distinctions in the use of terms are worth while for 
 the sake of clearness, they also serve to emphasize the fact that there 
 is a wide difference in the phenomena attending the escape of molten 
 rock to the surface and its invasion of cold rock at depths below 
 the surface. Thus escape of the molten material to the surface ia 
 characterized by the phenomena of volcanic eruptions and explo- 
 sions, accompanied by reactions with the atmosphere and possibly 
 with meteoric waters. Intrusive processes are in general less violent 
 and involve internal modifications of the magma rather than changes 
 resulting from contact with surface conditions, e.g. relief of pressure, 
 lower temperature and reactions with water and the gases of the air. 
 
 CONSTITUTION OF THE MAGMA AND CHARACTER OF MATERIALS 
 ERUPTED. — It must not be conceived that magma consists simply of 
 a uniform, molten rock-substance. Magmas vary radically in the 
 proportion of different elements entering into their composition. They 
 are not simple, melted mixtures of different mineral compounds, but 
 comprise true solu'tions of one compound in another. Furthermore 
 they commonly include enormous quantities of gaseous material which 
 seems also to be dissolved in the magma as a whole, when this exists 
 below the surface, but to escape, as vapor, when pressure is relieved 
 by approach to the earth's surface. 
 
VULCANICITY 141 
 
 In many cases a large proportion of the material extruded at the 
 surface comprises such gases. Among the more important of such 
 exhalations water vapor (steam), oxygen, hydrogen, nitrogen, hydro- 
 gen sulphide, carbon dioxide, chlorine, hydrochloric acid and silicon 
 fluoride may be mentioned. It has been generally stated that water 
 vapor makes up 99 per cent, of the entire gaseous output. More re- 
 cently Brun, on the basis of extended observations, has declared that 
 neither water vapor nor its dissociated elements hydrogen and oxygen, 
 are given off by magma at high temperatures. The dense, white 
 cloud, that characteristically accompanies a volcanic eruption at high 
 temperatures, and which has usually been ascribed to the condensa- 
 tion of exhaled water vapors into steam, is said by Brun to be made 
 up chiefly of extremely fine, solid anhydrous particles, comprising 
 clilorides of potassium, sodium, ammonium and iron, with admixture 
 of finely divided silica and other substances. These solid salts may 
 condense moisture from the atmosphere and thus form vapor clouds, 
 but the eruption does not contribute water to the atmosphere. If this 
 be true, current conceptions of many of the phenomena of vulcanism 
 must be modified. A serious objection to Brim's contention of the 
 anhydrous condition of magma is the presence of micas in very deep- 
 seated, intrusive rocks. Water is an essential constituent of micas 
 and such deep-seated rocks presumably consolidated below the zone 
 of percolating, meteoric waters. 
 
 The fluid portion of extruded magma constitutes lava. This 
 varies much in composition and physical properties, and on its nature 
 the characteristics of a given eruption, or part of an eruption, seem 
 in large measure to depend. The lava may be hasic, containing large 
 percentages of lime, magnesia and iron, which act as fluxes, and make 
 the lava readily fusible. Acid lavas on the other hand contain high 
 percentages of silicia (forming quartz on cooling) and are much 
 less readily fusible than the basic lavas. The more basic lavas fuse at 
 temperatures near 2250 degrees F. and between these and the more 
 acid lavas, which fuse at 2700 degrees F. (remaining pasty at even 
 3100 degrees F.) there are various gradations. At a given temperature 
 above the melting point a basic lava is, therefore, much more fluid 
 than an acid type; thus notably fluid and mobile lavas are always 
 basic. 
 
 In addition to the gaseous and lava materials, solid substances may 
 be ejected during eruptions. Sublimated sulphur is a common product 
 of this kind, and the anhydrous salts mentioned by Brun are also of 
 this class. Large fragmentary ejecta are, however, much more con- 
 
142 DYNAMIC GEOLOGY 
 
 spicTious. Thus gases may be present in such large quantity, and 
 explode so violently, on release of pressure at the surface, as to blow 
 the magma into minute particles, giving rise to volcanic ash; so 
 named because of its appearance and not because it is actually a 
 combustion product. If such fragments are cindery in texture, they 
 are termed scoriae. Portions of still liquid lava, rotated while being 
 thrown out, take on spheroidal form and are called volcanic bombs. 
 Lapillae are smaller, similarly rounded fragments. Blocks of the 
 rock material through which the igneous matter passes are also com- 
 monly torn loose and thrown out on the surface. 
 
 ORIGIN AND SOURCE OF MAGMA — As in the case of other phe- 
 nomena and processes that are excluded from direct observation, be- 
 cause they occur at some depth below the earth's surface; the origin 
 and source of magma may only be determined by inference. The sub- 
 ject, like that of the causes of deformation, is bound up with the 
 broader question of the origin of the planet. Thus the heat energy 
 present may represent a residual portion of the heat of an originally 
 incandescent nebula; it may have been derived from th collision of 
 meteorites; or it may be due to gravitational compression of an ag- 
 gregation of cold planetesimals ; according to the view of the earth's 
 origin that is accepted. Chamberlin has pointed out (Chamberlin, T. 
 C, Journal of Geol. Vol. 19, 1911, p. 673) that his Planetesimal Hy- 
 pothesis of a generally cold and solid earth, developing heat only 
 locally by compression, or the infall of more planetesimals; has been 
 strengthened by the discovery of radio-activity. In the original hetero- 
 geneous mass of planetesimals, radio-active matter would be scattered 
 quite promiscuously. But wherever such heat generating substance 
 f.ccurred it would tend to favor the liqu'ifaction of neighboring ma- 
 terial already hot, due to compresson, by still further raising the tem- 
 peratures. Stress differences would result in the squeezing of '-iich 
 local meltings toward the surface. The hot matter would flux its way 
 outward, carrying the radio-active substance with it. In this manner 
 radio-active mineral matter would tend to be concentrated in or near 
 the superficial portions of the earth ; which is in accordance with the 
 present known facts of its distribution. It is important to recognize 
 such various possibilities of the origin of the earth's interior heat and 
 the development of magma without attaching too much importance 
 or certainty of existence to any one of them. 
 
 Whatever the origin of the heat, it seems quite certain that the 
 isource of the magma is local rather than a general continuous reservoir 
 beneath a solid crust. As already pointed out, the rigidity of the 
 
VULCANICITY 143 
 
 earth, as indicated by the transmission of compressional earthquake 
 waves through its interior portion, argues against any such possibil- 
 ity. Other considerations point to the same conclusion. While the 
 kinds of lava extruded in a broad general region (e.g. the Andean 
 belt along the west coast of South America) often exhibit a marked 
 uniformity, yet outlets closely adjacent often extrude very dissimilar 
 material. Thus, in the Lipari Islands of the Mediterranean, the lava 
 of the volcano Stromboli is basic, while that of Vuleanois very acid. 
 Again between the summits of the lava columns of the Hawaiian vol- 
 canoes Mauna Loa and Kilauea, some 20 miles apart, there is a dif- 
 ference in level of nearly 10,000 feet. Simple hydrostatic law would 
 negative the supposition that they draw from a common source ; that 
 there is a fluid connection between them. It is not necessary, how- 
 ever, to suppose that the magma reservoirs are all relatively small and 
 of temporary duration. As already pointed out lavas may show 
 community of origin over wide areas. Thus it seems probable that 
 very extensive, intercrustal magma-basins, or trains of such basins, 
 Tonderlie such a tract as the Andean region. 
 
 The actual formation of such magma-basins can best be accounted 
 for on the basis of relief of pressure. It is easy to perceive that in 
 any large, relatively permanent, intercrustal reservoir, the magma must 
 have nearly the same temperature as the rocks that enclose it, and 
 this temperature must be approximately that of fusion; unless it is 
 assumed that the country rock and the magma are of very different 
 composition. In other words, it would be impossible to superheat 
 the magma enclosed in a basin of the same kind of material, just as 
 it woidd be impossible to heat water in a vessel of ice. Under such 
 conditions of near equilibrium of temperature, increase of pressure 
 would tend to cause solidification of part of the magma; release of 
 pressure, melting of more of the enclosing rock. 
 
 Reasons for the relief of pressure must be sought in connection 
 with the differential stresses that accompany, and are the causes of 
 crustal deformation and dislocation. These, as has been indicated, are 
 in general compressive, tending to bring about a tangential shortening 
 of the earth's crust. Local relief of pressure would be afforded by 
 the upfolding of mountain ranges, also along the lines of tensional 
 dislocation where broader areas are involved. 
 
 It may be readily be conceived that the same, purely mechanical 
 causes that bring about relief of pressure are also responsible for the 
 transference of part of the magma to levels higher than the reservoir, 
 and even to its extrusion. By gravitational readjustment, that is, by 
 
144 DYNAMIC GEOLOGY 
 
 the sinMng of crustal block, part of the fluid, therefore lighter magma 
 beneath may be forced toward the surface. However, the immediate 
 cause of eruption is ascribed to the expansive force of the gases pres- 
 ent in the magma. "While this does not apply in some cases it may 
 well be possible that when the magma has penetrated to a point near 
 the surface, the volatile substances will be concentrated in its u^per 
 part and the great relief of pressure lead to an explosive outburst. 
 This subject of the ascensive force of the magma is involved in as 
 great obscurity as is the source of the heat energy. 
 
 As to the source of the gases, especially the water vapor, present 
 in magmas, much difference of opinion also exists. By some geologists 
 it is maintained that these are original with the magma, derived from 
 the same deep-seated sources. By others it is declared that they 
 have their source in oceanic water penetrating to the hot magma 
 and being absorbed by it, or even rendering possible the fusion of the 
 rock by its influence in lowering the melting point of minerals. An- 
 other theory holds that masses of crystalline rock are lowered into 
 the heated portion of the earth and that gases are then evolved from 
 these. Water and various gases are present in all igneous rocks, ap- 
 parently in sufSeient quantity to give rise to the amounts observed 
 in connection with magma extrusions. 
 
 CENTRAL ERUPTIONS AND FISSURE ERUPTIONS. — Extrusion of lava 
 at the surface is, in general, of two types in accordance with the na- 
 ture of the vent. These may be designated central eruptions and 
 fissure eruptions. Central eruptions take place from a localized vent, 
 in general a volcano, the extrusions build up a volcanic cone, the out- 
 bursts are more or less violent, and are characteristic of the present. 
 Furthermore, active and recently active volcanic cones are distributed 
 in general near the ocean and follow the lines of growing mountainj. 
 
 Fissure eruptions take place, not through a single orifice, bnt 
 more or less continuously along an extensive fissure or group of parallel 
 fissures, or at points along them. In Fig. 48 is shown the extrusion 
 of lava material along part of such a fissure, the Laki Fissure, Iceland, 
 that reopened for a distance of 20 miles in 1783. Such outpourings 
 are, in general, a tranquil welling out of molten lava, often of enor- 
 mous volume. Thus in the Laki eruption of 1783, it is estimated that 
 three cubic miles of material were extruded. This formed a vast flood 
 of lava that over-flowed the surrounding country. Such eruptions 
 were much more extensive in the geologic past, and then actually 
 built up great lava plateaus. Thus the Columbia Lava Plateaus of 
 
VULCANICITT 
 
 145 
 
 y^ 
 
 til ^«: 
 
 ^^^ 
 
 
 ^^^ 
 
 
 V'^\ 
 
 ■7/ 
 
 ^■^ . 
 
 
 «# 1^?' ■*,*• ■^"e°™ent Of small cones and craters formed In the eruption 
 Of 1783 along part of the Laki Fissure, Iceland. (After Helland.) 
 
146 DYNAMIC GEOLOGY 
 
 Washington, Oregon and Idaho extend over 200,000 square miles and 
 the successive flows that formed them attain a maximum thickness of 
 over 4000 feet. Similar voluminous outpourings from fissures occur 
 in India and South Africa. It is worthy of note that such extensive 
 fissure eruptions consist commonly of basic (basaltic) lavas which are 
 much less viscous than the acid types, consequently favor the spread 
 of the material over wide areas about the fissure source. In contrast 
 with the distribution of volcanic cones, regions of fissure eruptions 
 are generally inland and are typical in plateau regions rather than 
 along the lines of mountain elevations. 
 
 VARIOUS TYPES OF CENTRAL ERUPTIONS — As characterized by 
 the nature of the activity at their vents, central eruptions may be 
 divided into three classes: (a) the quiet lava erupting, (b) the vio- 
 lent ash-erupting, (c) the explosive class. Of these classes, the 
 Hawaiian volcanoes, the West Indian volcanoes and Krakatoa in the 
 East Indies will serve as typical examples, but practically every in- 
 termediate gradation occurs. 
 
 QUIET LAVA ERUPTIONS ^ — A description of the conditions of the 
 eruptions of the volcano Mauna Loa, on the island of Hawaii, will, in 
 general, characterize volcanic activity of the quiet, lava erupting 
 class. Mauna Loa rises to a height of about 14,000 feet above sea 
 level but the whole cone is probably built up of lava flows from the 
 sea bottom. As the ocean is about 16,000 feet deep around the island 
 the cone is 30,000 feet high in its entirety. Nevertheless, it presents 
 a broad, apparently low elevation with very gentle slopes, because the 
 lava is of the basic, very fluid kind, flowing far before it finally cools 
 and solidifles. At the top is a very large crater, as the opening of a 
 volcanic vent is called. In the case of Hawaiian volcanoes the craters 
 are specifically designated calderas, because of their extreme dimen- 
 sions. The caldera of Mauna Loa is three miles long, two miles wide 
 and a thousand feet deep. When the volcano is not active it is possible 
 to descend into the caldera and walk about on its hot, but hard floor. 
 Before an eruption the crater floor rises and lakes of lava appear. 
 Fountains of lava sometimes rise to heights of several himdred feet 
 from these lakes. Sometimes the lava rises to the rim of the caldera 
 and overflows. More commonly the walls of the caldera are unable 
 to withstand the immense pressure of the lava column Consequent- 
 ly fissures open in its sides, the lava spurts out in gigantic jets, at- 
 taning heights of a thousand feet, and then flows down the side of 
 the cone in great streams, lava flows. As the lava flows out, the lava 
 
YULCANICITY 147 
 
 lake in the center of the caldera subsides and great masses of the 
 floor collapse and sink. Then the lava slowly accumulates for another 
 eruption. These occur on the average once in seven years. Com- 
 paratively little gas emission accompanies a Hawaiian eruption, no 
 explosions are heard, earthquakes are rare and of slight intensity. 
 An eruption may continue for months at a time. Hawaii, the island 
 on which Mauna Loa is situated, is one of a chain of similar volcanic 
 islands 400 miles long, and these may mark a fissure in the sea floor. 
 
 LAVA FLOW — The front of a lava stream advances not by glid- 
 ing over the groimd, but by rolling; the bottom being retarded by 
 friction of the ground and the top moving faster, so that it is con- 
 tinually rolling down on the curved end and forming the bottom. 
 Thus the slaggy materials though mostly formed by cooling at the 
 top of the stream, are rolled beneath it and the whole stream is en- 
 closed in a cindery envelope. Sometimes the flow is checked by the 
 mass of slaggy, scoriaeeous mass until the pent up lava acquires suf- 
 ficient pressure to burst out in a fresh stream. The slaggy material 
 is a non-conductor of heat, thus it greatly retards the cooling of the 
 interior mass Avhich may, accordingly, remain hot for years. One of 
 the lava flows of Maima Loa moved 15 miles in two hours, but or- 
 dinarily the rate of lava flow is much slower. (Scott.) Successive 
 lava flows may develop a rude sort of layering that simulates strati- 
 fied rock, but the different character of the material and the usually 
 quite evident flow structure of the igneous substance, commonly ren- 
 ders these readily distinguishable. 
 
 VIOLENT ASH ERUPTIONS. —The second class of central erup- 
 tions is well typified by the outbursts of Mount Peloe on the Island 
 of Martinique, West Indies, in 1902 and 1903. The crater of Pelee, 
 before the eriiptions, was a half mile in diameter and 2000 feet deep. 
 The crater rim was interrupted on the southwest by a deep gash. On 
 April 23rd of 1902 slight earthquakes were noted and on the 25th 
 a heavy cloud of vapors appeared over the volcano. On May 2nd 
 ejections of ash became heavy and frequent, and on May 5, mud that 
 had accumulated in the basin of the crater broke out and flowed down 
 the valley of the gash. During these early stages explosions like the 
 firing of cannon could be heard 300 miles away and all the cables 
 from Martinique were broken. On May 8 came the climax of the 
 outburst when a hea%'y black cloud swept down through the gash and 
 two minutes later struck the city of St. Pierre, five miles away, on 
 the plain to the southwest. The cloud consisted of hot (1500 degrees 
 
148 DYNAMIC GEOLOGY 
 
 F.) sulphurous, steam vapors and glowing ash. The city was at once 
 destroyed, buildings were thrown down, statues hurled from their 
 pedestals and trees torn up. Only one person, a prisoner in an under- 
 ground dungeon, of the 30,000 inhabitants survived the explosion. A 
 few minutes later a deluge of rain, mud and stones fell, completing^ 
 the destruction. The gases were apparently heavier than air and 
 were further forced downward by the resistance of the air colunm 
 above. Accordingly they swept along the ground instead of rising. 
 
 At later dates in May, June, July, August, September in 1902 and 
 in January, March and September in 1903, there were other outbreaks, 
 perhaps 14 in all, the very last one being scarcely less violent than 
 the one on May 8th, 1902. Around the cone, ashes to a depth of 
 several feet accumulated and lesser amounts fell 100 miles away. 
 Bach eruption was followed by heavy rains which washed the loose 
 ash down the steep slopes forming streams of hot mud, mud-flows. 
 On consolidating such mud forms a firm rock, volcanic tuff. 
 
 No lava streams were poured out in these eruptions but the 
 crater was filled with viscous lava which was later slowly protruded 
 above the rim in the form of a "spine" that attained a height of 
 nearly 1500 feet. This spine rapidly crumbled under the attack of 
 weathering agencies, but its lower continuation may be regarded eis a 
 plug to the volcanic vent, which will permit gases to accumulate until 
 they attain suffcient pressure for another destructive outburst. Pelee 
 had previously erupted in similar fashion in the years 1762 and 1851. 
 
 Other volcanoes, for example, Vesuvius, are intermediate in charac- 
 ter between the very quiet lava-erupting Hawaiian volcanoes and such 
 violent ash and gas-erupters as Pelee. Thus Vesuvius alternates ash 
 Mid lava eruptions or extrudes both in one erujption. 
 
 EXPLOSIVE VOLCANOES. — The eruption in 1883 of the volcanic 
 island Krakatoa, situated in the Straits of Sunda between Java and 
 Sumatra illustrates the most violently explosive class of outburst. Pre- 
 vious to the eruption of 1883 Krakatoa had been shaken by earth- 
 quakes and minor explosions for some years. On the morning of the 
 27th of August there were a series of vioelat explosions which blew 
 away about two thirds of the island, as illustrated in Fig. 49. The 
 Bea is now 1000 feet deep where the center of the island formerly 
 stood. Ashes were distributed over an area of 30,000 square miles. 
 The volume of such ejecta sufficed to fill up stretches of water near the 
 island that before had a depth of over 100 feet. It is estimated that 
 dust was thrown 17 to 23 miles up into the air. The explosions are 
 
VULCANICITY 
 
 149 
 
 said to have been heard in Australia, 2200 miles away. They generated 
 air waves that traveled three times round the earth, their passage 
 being recorded by barometers in aU parts of the world. Great sea 
 waves were formed washing 50 feet high over neighboring islands 
 
 FIG. 49. Cross sectionB of the Island of Krakatoa. A, shows the form 
 of Krakatoa in historical time before the explosion of 1883. The dotted 
 line indicates the growth of small cones within the greater crater. B, shows 
 the outline of the crater, as it is now, by the solid line; the dotted line 
 indicates the outline before the explosion of 1883. The horizontal line is 
 ■ea level. (After Judd.) 
 
 and drowning over 30,000 people. Since then Krakatoa has been 
 quiet. There was physiographic evidence that Krakatoa had suffered 
 in prehistoric times from similar paroxysmal outbursts. 
 
 ACTIVE AND EXTINCT VOLCANOES. —No doubt volcanic activity 
 becomes in time extinct in a given region. Thus anciently there were 
 volcanoes in northern New Jersey and in the Connecticut Valley, but 
 there is little likelihood of these becoming active again in the near 
 geological future. Some regions apparently never exhibited volcanic 
 activity, as for instance the Great Plains in western United States. 
 But many volcanoes considered extinct may be only dormant, as is 
 indicated by the varying time periods separating eruptions of the 
 different classes of central vents. Volcanoes that quietly erupt fluid, 
 basic lavas appear to keep their vents open. In the other types, 
 where the lavas are more viscous and exploding gases form a main 
 part of the eruption, there seems to be a tendency to plug up the 
 orifice after pressure has been relieved. Thus all evidences of activity 
 are cut off in such vents, the volcano appears to be extinct, yet is only 
 accumulating energy to again blow off its cover. 
 
 ■ According to geological records there seem to have been marked 
 periods of igneous action in the geologic past, followed by longer 
 
150 
 
 DYNAMIC GEOLOGY. 
 
 periods of comparative quiet. The present seems to be a transition 
 period in this respect. It would be difficult to say whether it marks 
 the end of a period of greater activity or whether it represents the 
 beginning of a new phase of disturbance. Probably the former aS' 
 sumption is more in accord with known facts; for in the western 
 United States there are very many cones that give evidence of being 
 only very recently extinct. 
 
 INTRUSIVE ACTION. — Magma, as has been noted, commonly risea 
 to the surface in fissure and central vent eruptions. Yet the volume 
 of deep-seated, igneous intrusions probably far exceed that extruded 
 at the surface. Thus the mass of intruded igneous rock (exposed 
 by erosion) far exceeds the bulk of extruded material. Dynamically 
 considered, igneous action consists of the movement of magma from 
 one place to another under fluid pressure. Wl^ether part of the 
 magma moved is forced out at the surface depends on circumstances. 
 In other words, it is self evident that intrusion without extrusion 
 may occur, but not the converse case. 
 
 There is little or no direct evidence of intrusive action actually 
 in progress but it undoubtedly accompanies present-day outbursta, 
 as illustrated in Fig. 50. 
 
 PIG. 50. Cross section of region invaded by igneous action showing 
 various types of extrusive and intrusive rocli formations. (After Tarr.) 
 
 The material that plugs up the throat of a volcano in itself 
 constitutes one type of intrusion called a volcanic neck or volcanic 
 plug. From this there may be various types of off-shoots, thus sills are 
 sheets of material injected between the layers of sedimentary strata. 
 The Palisades of the Hudson are a sill of unusual thickness exposed 
 by erosion. Sills are commonly injected at no great depth below the 
 surface, for the over-lying material must be lifted to an amount equal 
 to the thickness of the intruded sheet. If this weight is too great the 
 
VULCANICITY 151 
 
 magma can more readily make its way by breaking through and 
 across the strata; thus dikes are formed and these are typically ver- 
 tical. Dikes break across earlier igneous as well as sedimentary form- 
 ations and seem to follow fissure lines, probably filling these suddenly, 
 as some dikes are very narrow ribbons. Though such slender dikes 
 generally consist of basic, fluid magmas; a very slight cooling would, 
 nevertheless, make the magma quite viscous. Therefore, such narrow 
 dikes must be conceived as having, in a sense, been shot through the 
 rocks. 
 
 The laccolite is an intrusive body that is related to the sill. Lao- 
 colites are intruded between strata like sills, but have less horizontal 
 extension and dome up the layers above them to the proportions 
 of mountain elevations. Typically they have a flat base and 
 are fed from below by wide dikes. In the Henry Mountains in Utah, 
 where this type of intrusive body was first studied by Gilbert, the lacco- 
 lites are nearly circular in ground plan and vary from one-half to 
 four miles in diameter. The thickness ranges from one-seventh to one- 
 third of the horizontal diameter. Chonoliths are intrusive bodies 
 similar to those previously mentioned in that they intrude between, 
 or force apart the country rock, but they have more irregular shape 
 and may occur in any kind of rock, differing in the latter respect 
 from sills and laccoliths. In Pig. 50 the intrusive mass labeled Boss 
 would be more correctly termed a chonolith. 
 
 Dikes, sills, laccoliths and chonoliths are all fed from below 
 through relatively narrow openings, are, therefore, otherwise entirely 
 enclosed by the invaded country rock. Daly terms them injected 
 bodies and draws a distinction between them and bodies which have 
 no observable floor for the intruded body to rest upon, but communi- 
 cate with the earth's interior by great openings that enlarge down- 
 ward. These he calls subjacent bodies. 
 
 The manner of intrusion of the subjacent bodies presents greater 
 difficulties than the injected bodies. In the former case the ascen- 
 sive force of the magma, whether this be due to stress differences in 
 the over-lying Zone of Fracture or the expansive force of gases, may 
 be invoked; for the country rock is pried apart, lifted up or pushed 
 aside to make room for the intrusion. In the case of the subjacent 
 bodies, on the other hand, the surrounding rock often shows no trace 
 of disruption on large enough scale to accommodate the magma mass. 
 It might be assumed that a subjacent, intruded body fuses its way 
 up but chemical and microscopic examination seems to prove that 
 the igneous material is not affected by the character of the rock it 
 
152 DYNAMIC GEOLOGY. 
 
 breaks through. Therefore, it has been proposed that the STibjacent 
 bodies force their way up by breaking loose great blocks of material 
 from the roof against which they thrust and that these dislodged 
 blocks then sink into the fluid magma without being assimilated. 
 Other geologists insist that a cavity must have been formed by up- 
 arching of strata, or that the subjacent bodies, like the injected 
 bodies, do have a base and that if strata are cut through by the 
 intrusion their prolongations must, nevertheless, exist beneath the ig- 
 neous mass. 
 
 Bosses and batholiths are the chief types of subjacent bodies and 
 the distinction between them is chiefly one of size. Bosses vary in 
 diameter from a few feet to several miles, while batholiths are hundreds 
 or even thousands of miles in extent. In general, the coarse-grained 
 igneous rocks are developed on the slow cooling of boss and batholithic 
 masses; of these granite is the commonest type and this rock forna 
 the core of many great mountain ranges like the Sierra Nevada and 
 the Rockies. 
 
 MINERALIZERS AND PNEUMATOLYTIC ACTION. — In intrusions of 
 the batholithic type it is commonly found that the texture of the 
 rock, that is, the size of the mineral crystals, grows coarser from the 
 circumference to the center of the mass. This implies that rapid 
 cooling around the margins prevented the growth of crystals of large 
 size by making the magma too viscous for the free diffusion of the 
 molecular particles of the different minerals. On the other hand dikes 
 of very coarse minerals commonly project as off-shoots from such 
 masses. In these, and in the body of the magma, the formation of 
 large crystals of particuBar minerals is thought to be facilitated by 
 the presence of the volatile constituents, the same ones that give ex- 
 plosive energy to extrusive eruptions. These are called mineralizers. 
 Their role is to act as fluxes keeping the magma fluid at temperatures at 
 which it would otherwise become viscous. At the later stages of the 
 cooling such parts of these gases as may not have already entered 
 into the composition of minerals may redissolve some of the crystallized 
 material, and form particular types of minerals, of very coarse texture, 
 especially around the margins of the solidified body. Such action is 
 termed pneumatolysis. 
 
 MAGMATic DIFFERENTIATION. — Not only does the magma separ- 
 ate into different minerals on slow cooling (such separation being 
 facilitated by the action of mineralizers) but it also resolves itself into 
 quite different rock types. This more general process, of which the 
 
METAMORPHISM 153 
 
 fractional crystallization described is a part, is termed magmatic differ- 
 entiation. Magmatic differentiation may be due to gravity, the more 
 dense minerals sinking as they form, or it may be that certain mineral 
 substances are transferred by diffusion and convection currents toward 
 different parts of the magma mass, probably due to temperature and 
 solution differences. The exact nature of the processes is quite com- 
 plex and they are not fully understood, but their effects are quite 
 evident in the diversity of rock types, often found to have solidified 
 from one great intrusion. 
 
 The student may consult "The Natural History of Igneous 
 Eocks" by Alfred Harker for a more detailed description of the 
 various processes of igneous action. 
 
CHAPTER XII. 
 
 METAMORPHISM 
 
 MEANING AND APPLICATION OF THE TERM METAMORPHISM.— 
 
 The term met amor phism is used geologically to express change in 
 the character of rock substance due either to chemical or physical 
 processes. In its widest sense metamorphism would, therefore, in- 
 clude decomposition and disintegration of rock in the belt of weather- 
 ing; cementation and all processes of consolidation of rock in the 
 belt of cementation; and the more profound reconstructive changes, 
 of both a mineralogical and physical character, that are the result 
 of tangential compression or of the presence of hot magma. The gen- 
 eral use of the term is, however, practically confined to changes of 
 the kind last mentioned, that is alterations both structural and com- 
 positional that result from either the contact of cold rock material 
 with hot magmas or the compression of rock in the process of moutntaiQ 
 folding. When applied with reference to the other classes of change 
 mentioned, the fact of such extension of usage is generally indicated 
 in the context. 
 
 KINDS OF METAMORPHISM. — As the term is used in the more lim- 
 ited sense defined above, two distinct kinds of metamorphism must, 
 nevertheless, be recognized according to the nature and characteristic 
 effects of the agency that brings about the change. These two kinds 
 may be termed (a) contact or local metam/)rphism and (b) dynamic 
 or regional metamorphism. Contact metamorphism as the term im- 
 plies refers to the change effected in surrounding rocks by igneous 
 magmas. Such changes are commonly of rather narrow extent, there- 
 fore, the term local. Dynamic or regional metamorphism, however, 
 involves the reconstruction of rocks on a large scale, in areas covering 
 in some cases thousands of square miles. Sometimes a rock may be 
 altered by contact and dynamic metamorphic processes acting in con- 
 junction. 
 
 NATURE OF CHANGES DUE TO METAMORPHIC PROCESSES Be- 
 fore considering the action of the processes themselves it will be well 
 to dwell for a moment on the nature of the changes they cause. In 
 the first place it should be recognized that both igneous and sedimen- 
 tary rocks may be metamorphosed and it is not always possible to say 
 
METAMORPHISM 155 
 
 whether the metamorphic rock resulting was originally a sediment or 
 an Igneous rock. It is, however, probably safe to assert that a vast 
 majority of the occurrences of metamorphc rock represent altered 
 sediments. It may, indeed, be the case that metamorphic processes 
 go so far as to actually and completely melt sediments. In that 
 event a magma would be produced that would be indistinguishable 
 from a typically igneous one. While so complete metamorphism has 
 not been demonstrated for any large body of rock, its occurrence 
 would make the cycle of rock transformation complete; starting with 
 igneous rock, due to cooling of magma, changing to sedimentary de- 
 posits by processes of weathering and erosion, and these to meta- 
 morphic rock by compression and back again to igneous rock by melt- 
 ing and recooling. This is the more significant because vast areas of 
 igneous and extremely metamorphosed rocks underlie the oldest 
 recognizable sediments. If these be completely altered sediments the 
 length of time occupied in the development of the earth, by processes 
 even now operative, must be much more extended than has commonly 
 been considered, and also the length of time available for the evolution 
 of life forms. 
 
 The specific changes that individual rocks may undergo on altera- 
 tion by metamorphism are quite numerous and varied. Commonly a 
 metamorphic rock is harder and denser than the one from which it 
 was derived, though to this there are exceptions. Crystallization of 
 material previously aggregated in unrelated particles (amorphous) 
 is common, as is also recrystallization and the formation of entirely 
 new minerals. Development of planes of parting, giving the rock 
 properties of cleavage, fissility, banding and schistosity by develop- 
 ment, orientation and segregation of certain types of minerals is an- 
 other typical result of metamorphic processes. 
 
 CONTACT OR LOCAL METAMORPHISM. _As stated before contact 
 metamorphism results from the action of molten magma in contact 
 with other rock ; whether at the surface or at the borders of intrusions. 
 From what has been said in regard to the nature of lava flow, it will 
 be evident that comparatively little metamorphism results from sur- 
 face outflows of molten rock. The clinkery scoriae form a non-heat- 
 conducting mat between the lava flow and the country rock over which 
 it passes. Soils imder lava flows sometimes show evidence of the heat 
 present, in that they are baked to a red color for a few inches depth ; 
 bituminous coal is changed to natiiral coke by driving off of gases; 
 limestone is changed to quick lime by driving off carbon dioxide ; but 
 in general the metamorphic effects of surface flows are trifling. 
 
156 DYNAMIC GEOLOGY. 
 
 On the other hand the magma injected as dikes, sills and laccoliths, 
 and the larger intrusive masses of boss or batholithic type, exert a 
 more notable metamorphic effect on the country rock. The scale of 
 such alterations is in some degree related to the size of the intrusion, 
 though individual characteristics of the invading masses and the nature 
 of the material they enter are also effective factors in determining 
 the extent and kind of changes brought about. 
 
 Intruded masses retain their heat longer than those extruded, 
 as the latter lose their heat to the air. Consequntly the heat from 
 intrusive bodies acts longer on the bed rock. But the most potent 
 factor in metamorphism by intrusions is the action of the vapors and 
 gases that constitute part of the magma. The over-lying rock material 
 prevents the escape of these into the atmosphere. Thus their effects 
 are strongly concentrated around the borders of the intrusion. Acid 
 magmas are more effective in producing metamorphism than basic 
 ones because they contain a greater abundance of such vapors. Shale 
 and limestone country rock are much more readily and profoundly 
 altered than sandstones made up largely of quartz, because the com- 
 position of the former is more susceptible to change by the mineralizing 
 agents. An intrusive mass cutting across strata affects the country 
 rock to a voider extent than one penetrating between bedding planes, 
 because of the greater number of fissures available to distribute the 
 effects. 
 
 In igneous country rock the effect of contact metamorphism is to 
 cause more or less complex mineralogical changes, mostly the restflt 
 of recombination with and under the action of the mineralizing va- 
 pors. In sedimentary strata changes are more distinctive. Sand- 
 stones are changed to quartzites by the deposition of silica from solu- 
 tion around the original grains of sand. Sometimes, also, complete 
 crystallization is effected. Shales change from an outermost tm- 
 changed zone to a zone of slate with the development of mica, mag- 
 netite and other minerals. Still nearer the contact the slate grades 
 into a mica schist, a rock made up largely of mica flakes containing 
 quartz and feldspar ; while at the very contact a very dense rock SBb- 
 Btance called hornfels may develop. Amorphous limestone changes to 
 crystalline marble, its fossil contents are obliterated and its impurities 
 give rise to such minerals as garnet and graphite, the latter being 
 derived from organic matter. Iron and organic impurities are re- 
 sponsible for the variegation, that is, the diyersity of colon in streaks 
 and bands, that many marbles exhibit; as such impurities are char- 
 acteristically segregated by the metamorphic processes. Ib most such 
 
METAMOKPHISM 157 
 
 reactions water vapor, derived either from the country rock or from 
 the magma itself, is present and must be regarded as one of the most 
 important agencies in facilitating changes. 
 
 ORE DEPOSITS DUE TO CONTACT METAMORPHISM Processes 
 
 of magmatic differentiation operating in intruded rock masses, especial- 
 ly those of large size, tend to bring about the concentration of the 
 more basic minerals in the marginal zones of the mass. Among such 
 basic minerals occur those which contain the economically important 
 metallic elements, iron, copper, silver, lead, etc. Magmatic differentia- 
 tion may go so far as to lead to the segregation of nearly pure masses 
 of metallic oxides, as probably is the case of the magnetite iron ores 
 of the Adirondack region, and the zinc ores of New Jersey. More 
 commonly the basic metallic minerals are complex compounds contain- 
 ing too small a proportion of the metallic element to pay for its 
 separation by artificial means. If, however, the intrusive body, with 
 such margiual concentration of complex basic minerals, cuts through 
 shale or limestone country rock ; conditions for natural reactions lead- 
 ing to the separation and concentration of the metallic elements are 
 quite favorable and a contact ore deposit may result. 
 
 The readily alterable shales and limestones act as fluxes and the 
 mineralizing agents further facilitate reactions that lead to the re- 
 combination of the metallic elements in relatively simple compounds. 
 As such they constitute ore deposits if concentrated in sufficient quan- 
 tity to be economically valuable. Such deposits may fill interstices 
 in the contact zone or they may replace the country rock substance 
 for considerable distances away from the contact margin; thus in 
 Montana bed rock one half mile away from the contact with a granite 
 intrusive has been so affected. Deposits of this latter nature are called 
 replacement deposits. It must not be understood that all contact or 
 replacement deposits due to contact metamorphic processes are ore de- 
 posits, or even metallic minerals. Other economically unimportant 
 minerals are probably far more abundant along contacts than are ore 
 concentrations. 
 
 DYNAMIC OR REGIONAL METAMORPHISM. ^in certain respects 
 dynamic metamorphism is not essentially different from contact meta- 
 morphism. Thus the rock is both cases is usually made harder, denser 
 and is commonly crystallized or recrystaUized. The new minerals 
 that develop from either kind of metamorphism are somewhat similar. 
 Aside from the fact that dynamic metamorphism involves the re- 
 construction of rocks on a large scale, while that of contact meta^ 
 
158 DYNAMIC GEOLOGY. 
 
 morphism involves only local change, there is an essential difference 
 of cause that makes distinction between them necessary. Thus con- 
 tact metamorphism results from the reactions between country rock 
 and a hot magma intrusion ; dynamic metamorphism is the result of 
 great compression of rock masses. 
 
 In the rocks themselves this distinction manifests itself, as an 
 effect, primarily in the development of lines of cleaving and parting, 
 a phenomenon which is only secondarily a characteristic of contact 
 metamorphic rocks; and is, in that case also, probably due to minor 
 compressive effects, acting in conjunction with the other processes. 
 On the other hand, development of cleaving and parting lines in rocks 
 is the most pronounced feature due to dynamic metamorphism. 
 
 According to their nature and the conditions ulnder which they 
 were formed such parting lines may be divided into two classes 
 designated, respectively, fractitre cleavage and flow cleavage. 
 
 Fracture cleavage is a phenomenon of the Zone of Fracture, and 
 consists of incipient fractures, like cracks in a plate that has not yet 
 fallen apart. In consequence of such cleavage rocks show fissility, 
 which may be defined a structure by virtue of which rocks are already 
 separated into parallel laminae in their natural state. Between fissU- 
 ity, joints, fissures and fault fractures the distinction is more one of 
 degree of separation and continuity of break than one of kind. While 
 such physical disruption of rock does not come strictly within the 
 definition of metamorphism, as limited in a preceding paragraph, yet 
 it is a result of compression. On this basis Van Hse has formulated 
 a very significant generalization, viz: that the structures impressed 
 on the stratified rocks after their first formation, folds, faults, joints, 
 cleavage, fissility and schistosity are all due to lateral compression act- 
 ing with different degrees of intensity and at different depths, depth 
 and over-lying load being controlling factors of the first importance. 
 
 At great depths where the over-lying load is enormous, in other 
 words in the Zone of Flowage, flow cleavage develops. A rock flows 
 when it is deformed without evident fracture. The flow cleavage de- 
 veloped as an accompaniment of such deformation may be defined 
 as a capacity to part along parallel surfaces, not necessarily planes. 
 
 Under the great compressive stresses in the Zone of Flowage, ac- 
 companied by great heat and the presence of moisture; original sedi- 
 mentary and igneous rock substance tends to be crystallized or re- 
 crystallized into rocks made up primarily of a certain few minerals, 
 mica, hornblende, quartz and feldspar. Fulrthermore these minerals 
 
METAMORPHISM 159 
 
 tend to arrange themselves so that their two longest diameters are at 
 right angles to the direction of greatest press\ire. Thus the individual 
 grains will also be parallel to each other. This parallel arrangement 
 IS not so much due to a mechanical migration of the mineral substance 
 to such position, though this does take place, as to actual formation 
 growth of the mineral in this position. 
 
 A typical example of such metamorphism is the development of a 
 slate from a shale. In the process the original bedding planes of the 
 shale are obliterated. As the rock is laterally compressed, slaty 
 cleavage, due to the growth of mica grains (comprising perhaps 50 
 per cent, by weight of the slate) develops at right angles, or nearly 
 right angles, to the former bedding planes. When the slate is actually 
 Split along the cleavage planes, the parting is seen to take place along 
 the planes of mineral cleavage of the mica grains. In other words, 
 the mica pates split apart in the fashion familiar to every one and 
 the rock splits with them, because the mica grains are so closely 
 spaced and in such uniform parallel position. 
 
 A more advanced degree of metamorphism, resulting from the 
 complete flow and thickening of the rock, under tangential compres- 
 sion with accompanying contortion and plication, is marked by the 
 development of schistosity or foliation. Schistosity is the arrange- 
 ment of the component mineral grains of the completely crystallized 
 (or recrj'stallized) rock, in rudely parallel planes, or undulating sur- 
 faces, at right angles to the direction of greatest pressure while plica- 
 tion was in progress. Mica schist is a typical variety of schistose rock 
 resulting from metamorphism of slates, or is derived from clayey sand- 
 stone. Hornhlende schist is probably derived from basic igneous 
 rocks directly by dynamic metamorphism. Both these typical schistose 
 rocks are named after the predominating metamorphic mineral that, 
 in each, imparts cleavage to the rock along the planes of foliation; 
 in the same manner that the mica grains develop cleavage planes in 
 slate. 
 
 Similar to schistosity is the banding that is characteristic of the 
 gneisses; a class of rocks derived by dynamic metamorphism from 
 granite for the most part; though some gneisses may have a sedimen- 
 tary origin. Gneisses are relatively poor in the cleavable minerals 
 that characterize the schists, and possess a correspondingly greater 
 proportion of feldspar and quartz. In gneisses the minerals are ar- 
 ranged in parallel bands but are of such nature as to lead to partings 
 between the layers of minerals rather than through the minerals them- 
 
160 DYNAMIC GEOLOGY. 
 
 selves. Gneisses have been known to develop by rock flowage from 
 rocks that under other conditions yielded schists. That is, rock of 
 the same composition developed a predominance of schist minerals in 
 one instance, of gneissic min erals in the other. The presence of a 
 higher percentage of moisture during metamorphism seems to be re- 
 sponsible for the more schistose structure.