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 CROSBY" 
 
/B E R K E L E Y 
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 UNIVERSITY OF 
 V CALIFORNIA 
 
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of Natural ^i 
 
 GUIDES FOR SCIENCE-TEACHING 
 
 No. XII 
 
 COMMON MINERALS AND ROCKS 
 
 BY WILLIAM O. CROSBY 
 
 D. C. HEATH & CO., PUBLISHERS 
 BOSTON NEW YORK CHICAGO 
 
Copyright 
 
 BY THE BOSTON SOCIETY OF NATURAL HISTORY 
 1881 
 
 I D 2 
 
 
INTRODUCTION. 
 
 MINERALS and rocks, or the inorganic portions of 
 the earth, constitute the proper field or subject-matter 
 of the science of Geology. Now the inorganic earth, 
 like an animal or plant, may be and is studied in three 
 quite distinct ways, giving rise to three great divisions 
 of geology, which, as will be seen, correspond closely 
 to the main divisions of Biology. 
 
 First, we may study the forces now operating upon 
 and in the earth the geological agencies such as 
 the ocean and atmosphere, rivers, rain and frosts, 
 earthquakes, volcanoes, hot springs, etc., and observe 
 the various effects which they produce. We are con- 
 cerned here with the dynamics of the earth ; and this 
 is the great division of dynamical geology, correspond- 
 ing to physiology among the biological sciences. 
 
 Or, second, instead of geological causes, we may 
 study more particularly geological effects, observing 
 the different kinds of rocks and of rock-structure pro- 
 duced by the geological agencies, not only at the 
 present time, but also during past ages. This method 
 of study gives us the important division of structural 
 geology, corresponding to anatomy and morphology. 
 
6 INTRODUCTION. 
 
 All phenomena present two distinct and opposite 
 aspects or phases which we call cause and effect; and 
 so in dynamical and structural geology we are really 
 studying the opposite sides of essentially the same 
 classes of phenomena. In the first Division we study 
 the causes now in operation and observe their -effects ; 
 and then, guided by the light of the experience thus 
 gained, we turn to the effects produced in the past and 
 seek to refer them to their causes. 
 
 These two divisions together constitute what is 
 properly known as physiography ; and they are both 
 subordinate to the third great division of geology, 
 historical geology, which corresponds to embry- 
 ology. 
 
 The great object of the geologist is, by studying 
 the geological formations in regular order, from the 
 oldest up to the newest, to work out, in their proper 
 sequence, the events which constitute the earth's 
 history ; and dynamical and structural geology are 
 merely introductory chapters, the alphabet, as it were, 
 which must be learned before we are prepared to read 
 understandingly the grand story of the geological 
 record. 
 
 Our work in this short course will be limited to the 
 first two divisions, i.e., to dynamical and structural 
 geology- We will attempt, first, a general sketch of 
 the forces now concerned in the formation of rocks 
 and rock-structures ; and after that we will study the 
 composition and other characteristics of the common 
 minerals and rocks. 
 
 The scope of this work, and its relations to the whole 
 field of geology, are more clearly indicated by the fol- 
 lowing classification of the geological sciences : 
 
GEOLOGY 
 
 INTRODUCTION. 
 
 'DYNAMICAL GEOLOGY (P^y^l Geology. 
 I Chemical Geology. 
 
 {Mineralogy. 
 I Petrology. 
 .HISTORICAL GEOLOGY. 
 
 Many teachers will desire to fill in some of the details 
 of the outline sketch presented in this Guide, and for 
 this purpose the following works are especially recom- 
 mended : 
 
 ELEMENTS OF GEOLOGY. By Prof. Joseph LeConte. 1882. 
 
 D. Appleton & Co., New York. Nearly 600 pages. 
 MANUAL OF GEOLOGY. By Prof. J. D. Dana. Third edition. 
 
 1880. 800 pages. 
 TEXT-BOOK OF GEOLOGY. By- Prof. A. Geikie. 1882. Mao 
 
 millan & Co. London. Nearly looo pages. 
 
 As a reference-book for mineralogy, the following 
 treatise is unsurpassed : 
 
 TEXT-BOOK OF MINERALOGY. By Edward S. Dana. 1883. 
 John Wiley & Sons, New York. 
 
 And, as an introduction to the study of minerals, 
 and, through these, to the study of rocks, 
 
 FIRST LESSONS IN MINERALS. Science Guide No. XIII. By 
 Mrs. E. H. Richards. 
 
 cannot be too highly recommended. Teachers will 
 find this little primer of 46 pages invaluable with young 
 children, and with all who have had no previous train- 
 ing in chemistry. 
 
 As an admirable continuation of the work begun in 
 these pages, teachers are referred to Professor Shaler's 
 " First Book in Geology." In this our brief sketch of 
 
8 1NTR OD UC TION. 
 
 the geological agencies is amplified and beautifully 
 illustrated; and rarely have the wonderful stories of 
 the river, ocean-beach, glacier, and volcano been told 
 so effectively. In the chapter on the history of life on 
 the globe the main outlines of historical geology are 
 skillfully brought within the comprehension of begin- 
 ners. The directions to teachers are fully in accord 
 with the modern methods and ideas, and are a very 
 valuable feature of the book. 
 
DYNAMICAL GEOLOGY. 
 
 WHEN we think of the ocean with its waves, tides, 
 and currents, of the winds, and of the rain and snow, 
 and the vast net-work of rivers to which they give rise, 
 we realize that the energy or force manifested upon the 
 earth's surface resides chiefly in the air and water 
 in the earth's fluid envelope and not in its solid crust. 
 And it would be an easy matter to show that, with the 
 exception of the tidal waves and currents, which of course 
 are due chiefly to the attraction of the moon, nearly all 
 this energy is merely the transformed heat of the sun. 
 Now the air and water are two great geological agen- 
 cies, and therefore the geological effects which they 
 produce are traceable back to the sun. 
 
 Organic matter is another important geological agent ; 
 but all are familiar with the generalization that connects 
 the energy exhibited by every form of life with the sun ; 
 and, besides, it is scarcely necessary to allude to the ob- 
 vious fact that all animals and plants, so far at least as 
 any display of energy is concerned, are merely differ- 
 entiated portions of the earth's fluid envelope. And so, 
 if space permitted, it might be shown that, with the 
 exception of the tides, nearly every form of force 
 
10 DYNAMICAL GEOLOGY. 
 
 manifested upon the earth's surface has its origin in 
 the sun. 
 
 Of this trio of geological agencies operating upon 
 the earth's surface and vitalized by the sun water, 
 air, and organic matter the water is by far the most 
 important, and so it is common to call these collec- 
 tively the aqueous agencies. Hence we have solar 
 agencies and aqueous agencies as synonymous terms. 
 
 The aqueous agencies include, on one side, air and 
 water, or inorganic agencies ; and, on the other, animals 
 and plants, or organic agencies. 
 
 Let us notice briefly the operation of these, begin- 
 ning with the air and water. 
 
 I. AQUEOUS AGENCIES. 
 i. Air and Water, or Inorganic Agencies. 
 
 CHEMICAL EROSION. Attention is invited first to the 
 specimens numbered i, 2, 3, and 4. No. i is a sound, 
 fresh piece of the rather common rock, diabase ; and 
 those who are acquainted with minerals will recognize 
 that the light-colored grains in the rock are feldspar, 
 and the dark, augite. This specimen came from a 
 depth in the quarry, and has not been exposed to the 
 action of the weather. 
 
 The second specimen differs from the first, appar- 
 ently, as much as possible ; and yet, except in being 
 somewhat finer grained, it was originally of precisely 
 similar composition and appearance. In fact, it is a 
 portion of the same rock, but a weathered portion. In 
 this we can no longer recognize the feldspar and augite 
 
AQUEOUS AGENCIES. II 
 
 as such, but both these minerals are very much changed, 
 while in the place of a strong, hard rock we have an 
 incoherent friable mass, which is, externally at least, 
 easily crushed to powder ; and with the next step in 
 the weathering, as we may readily observe in the 
 natural ledges, the rock is completely disintegrated, 
 forming a loose earth or soil. 
 
 We have two examples of such natural powders in 
 the specimens numbered 3 and 4 ; and by washing 
 these (especially the finer one, No. 4) with water, we 
 can prove that they consist of an impalpable substance 
 which we may call clay, and angular grains which we 
 may call sand. The sand-grains are really portions 
 of the feldspar not yet entirely changed to clay. 
 
 Thus we learn that the result of the exposure of this 
 hard rock to the weather is that it is reduced to the 
 condition of sand and clay. What we mean especially 
 by the weather are moisture and certain constituents 
 of the air, particularly carbon dioxide. 
 
 The action of the weather on the rocks is almost en- 
 tirely chemical. With a very few exceptions, the prin- 
 cipal minerals of which rocks are composed, such as 
 feldspar, hornblende, augite, and mica, are silicates, i.e., 
 consist of silicic acid or silica combined with various 
 bases, especially aluminum, magnesium, iron, calcium, 
 potassium, and sodium. 
 
 Now the silica does not hold all these bases with 
 equal strength; but carbon dioxide, in the presence 
 of moisture, is able to take the sodium, potassium, 
 calcium, and magnesium away from the silica in the 
 form of carbonates, which, being soluble, are carried 
 away by the rain-water. 
 
12 DYNAMICAL GEOLOGY. 
 
 The silicate of aluminum, with more or less iron, 
 takes on water at the same time, and remains behind 
 as a soft, impalpable powder, which is common clay. 
 
 In the case of our diabase, contined exposure to the 
 weather would reduce the whole mass to clay. But 
 other rocks contain grains of quartz, a hard mineral 
 which cannot be decomposed, and it always forms sand. 
 Certain classes of rocks, too, such as the limestones 
 and some iron-ores, are completely dissolved by water 
 holding carbon dioxide in solution, and nothing is left 
 to form soil, except usually a small proportion of insol- 
 uble impurities like sand or clay. 
 
 Let us see next how these agents of decay get at 
 the rocks. Neither water nor air can penetrate the 
 solid rock or mineral to any considerable extent, so 
 that practically the action is limited to surfaces, and 
 whatever multiplies surfaces must favor decomposition. 
 
 First, we have the upper surface of the rock where 
 it is bare, but more especially where it is covered with 
 soil, for there it is always wet. 
 
 All rocks are naturally divided by joints into blocks, 
 which are frequently more or less regular, and often of 
 quite small size. Water and air penetrate into these 
 cracks and decompose the surfaces of the blocks, and 
 thus the field of their operations is enormously ex- 
 tended. These rock-blocks sometimes show very 
 beautifully the progress of the decomposing agents 
 from the outside inward by concentric layers or shells 
 of rotten material, which, in the larger blocks v pften 
 envelop a nucleus of the unaltered rock. 
 
 It is interesting to observe, too, that these concentric 
 lines of decay cut off the angles of the original blocks* 
 
AQUEOUS AGENCIES. 13 
 
 so that the undecomposed nucleus, when it is found, 
 is approximately spherical instead of cuboidal. Both 
 these points are well illustrated by specimen No. 2 ; 
 for although now nearly spherical, it was originally 
 perfectly angular, and has become rounded by the 
 peeling off, in concentric layers, of the decomposed 
 material, and in most cases several of these layers are 
 distinctly visible, like the coats of an onion. But by 
 stripping these off we should discover, in all the larger 
 balls at least, a solid, spheroidal nucleus, while in the 
 smaller balls the decomposition has penetrated to the 
 centre. 
 
 In the rocks also we find many imperfect joints and 
 minute cracks. In cold countries these are extended 
 and widened by the expansive power of freezing water, 
 and thus the surfaces of decomposition become con- 
 stantly greater. 
 
 Nearly all rocks suffer this chemical decomposition 
 when exposed to the weather, but in some the decay 
 goes on much faster than in others. Diabase is one of 
 the rocks which decay most readily ; while granite is, 
 among common rocks, one of those that resist decay 
 most effectually. 
 
 The caverns which are so large and numerous in 
 most limestone countries are a splendid example of 
 the solvent action of meteoric waters, being formed 
 entirely by the dissolving out of the limestone by the 
 water circulating through the joint cracks. The pro- 
 cess must go on with extreme slowness at first, when 
 the joints are narrow, and more rapidly as they are 
 widened and more water is admitted. We get some 
 idea, too, of the magnitude of the results accomplished 
 
14 DYNAMICAL GEOLOGY. 
 
 by these silent and unobtrusive agencies when we re- 
 fleet that almost all the loose earth and soil covering 
 the solid rocks are simply the insoluble residue which 
 carbon dioxide and water cannot remove. In low lati- 
 tudes, where a warm climate accelerates the decay of 
 the rocks, the soil is usually from 50 to 300 feet deep,, 
 
 MECHANICAL EROSION. On the edge of the land. 
 Let us trace next the mechanical action of water and 
 air upon the land. First we will consider the edge of 
 the land, where it is washed by the waves of the sea. 
 Whoever has been on the shore must have noticed 
 that the sand along the water's edge is kept in con- 
 stant motion by the ebb and flow of the surf. 
 
 Where the beach is composed of gravel or shingle 
 the motion is evident to the ear as well as the eye ; 
 and when the surf is strong, the rattling and grinding 
 of the pebbles as they are rolled up and down the 
 beach develops into a roar. 
 
 The constant shifting of the grains of sand, pebbles, 
 and stones is, of course, attended by innumerable col- 
 lisions, which are the cause of the noise. Now it is 
 practically impossible, as we may easily prove by ex- 
 periment, to knock or rub two pieces of stone together, 
 at least so as to produce much noise, without abrading 
 their surfaces ; small particles are detached, and sand 
 and dust are formed. 
 
 That this abrasion actually occurs in the case of the 
 moving sand is most beautifully shown by the sand-- 
 blast. We are to conclude, then, that every time a 
 pebble, large or small, is rolled up or down the beach 
 it becomes smaller, and some sand and dust or clay 
 are formed which are carried off by the water. 
 
AQUEOUS AGENCIES. 15 
 
 But what are the pebbles originally? This question 
 is not difficult. A little observation on the beach shows 
 us that the pebbles are not all equally round and smooth, 
 but many are more or less angular. And we soon see 
 that it is possible to select a series showing all grada- 
 tions between the most perfectly rounded forms and 
 angular fragments of rock that are only slightly abraded 
 on the corners. The three principal members of such a 
 series are shown in specimens 5, 6, and 7 from the 
 beach on Marblehead Neck ; but equally instructive 
 specimens can be obtained at many other points on 
 our coast. It is also observable that the well-rounded 
 pebbles are much smaller on the average than the angu- 
 lar blocks. 
 
 From these facts we draw the legitimate inference 
 that the pebbles were all originally angular, and that 
 the same abrasion which diminishes their size makes 
 them round and smooth. 
 
 A little reflection, too, shows that the rounding of 
 the angular fragments is a natural and necessary result 
 of their mutual collisions ; for the angles are at the same 
 time their weakest and most exposed points, and must 
 wear off faster than the flat or concave surfaces. 
 
 Having traced each pebble back to a larger angular 
 rock-fragment, the question arises, Whence come these 
 angular blocks ? 
 
 Behind our gravel-beach, or at its end, we have 
 usually a cliff of rocks. As we approach this it is dis- 
 tinctly observable that the angular pebbles are more 
 numerous, larger, and more angular; and a little 
 observation shows that these are simply the blocks 
 produced by jointing, and that the cliff is entirely 
 
1 6 DYNAMICAL GEOLOGY. 
 
 composed of them. In other words, our cliff is a mass 
 of natural masonry, which chemical agencies, the frost, 
 and the sea are gradually disintegrating and removing. 
 As soon as the blocks are brought within reach of the 
 surf their mutual collisions make them rounder and 
 smaller ; and small round pebbles, sand, and clay are 
 the final result. 
 
 For a more complete account of the formation of 
 pebbles, teachers are referred to the first or introduc- 
 tory number of this series of guides, by Prof.' Hyatt, 
 "About Pebbles." 
 
 Where the waves can drive the shingle directly 
 against the base of the cliff, this is gradually ground 
 away in the same manner as the loose stones them- 
 selves, sometimes forming a cavern of considerable 
 depth, but always leaving a smooth, hard surface, 
 which is very characteristic, and contrasts strongly 
 with the upper portion of the cliff, which is acted 
 on only by the rain and frost. A good example 
 of such a pebble-carved cliff may be seen behind the 
 beach on the sea-ward side of Marblehead Neck. 
 
 The sea acts within very narrow limits vertically, a 
 few feet or a few yards at most ; but the coast-lines of 
 the globe (including inland lakes and seas) have an 
 aggregate length of more than 150,000 miles. Hence 
 it is easy to see that the amount of solid rock ground 
 to powder in the mill of the ocean-beach annually 
 must be very considerable. 
 
 MECHANICAL EROSION. On the surface of the 
 land. I next ask attention to the mechanical action 
 of water upon the surface of the land. 
 
 It is a familiar fact that after heavy rains the road- 
 
AQUEOUS AGENCIES. 17 
 
 side rills carry along much sanfl and clay (which we 
 know have been produced by the previous action of 
 chemical forces), and also frequently small pebbles 01 
 gravel. It is easy to show that in all important respects 
 the rill differs in size only from brooks and rivers ; and 
 the former afford us fine models of the systems of val- 
 leys worn out during the lapse of ages by rivers. The 
 turbidity of rivers is often very evident, and in shallow 
 streams we can sometimes see the pebbles rolled along 
 by the current. 
 
 Now here, just as on the beach, the collisions of 
 rock-fragments are attended by mutual abrasion, sand 
 and clay are formed, and the fragments become smaller 
 and rounder. Our series of pebbles from the beach 
 might be matched perfectly among the river-gravel. 
 In mountain streams especially we may often observe 
 that pebbles of a particular kind of rock become more 
 numerous, larger, and more angular as we proceed up 
 stream, until we reach the solid ledge from which they 
 were derived, showing the same gradation as the beach 
 pebbles when followed back to the parent cliff. 
 
 The pebbles, however, not only grind each other, but 
 also the solid rocks which form the bed of the streams 
 in many places, and these are gradually worn away. 
 When the rocky bed is uneven and the water is swift, 
 pebbles collect in hollows where eddies are formed, by 
 which they are kept whirling and turning, and the hollow 
 is deepened to a pot-hole, while the pebbles, the river's 
 tools, are worn out at the same time. 
 
 By these observations we learn not only that run- 
 ning water carries away sand and clay already formed, 
 but that it also has great power of grinding down hard 
 
l8 DYNAMICAL GEOLOGY. 
 
 rocks to sand and clay. J Of course the pulverized rock 
 always moves in the same direction as the stream which 
 carries it ; and, in a certain sense, all streams run in 
 one direction, viz., toward the sea. Therefore the con- 
 stant tendency of the rain falling upon the land is to 
 break up the rocks by chemical and mechanical action 
 and transport the debris to the sea. 
 
 Rivers, as we all know, are continually uniting to 
 form larger and larger streams ; and thus the drainage 
 of a wide area sometimes, as in the case of the Missis- 
 sippi Valley, reaches the sea through a single mouth. 
 By careful measurements made at the mouth of the Mis- 
 sissippi it has been shown that the 20,000,000,000,000 
 cubic feet of water discharged into the Gulf of Mexico 
 annually carries with it no less than 7,500,000,000 
 cubic feet of sand, clay, and dissolved mineral matter ; 
 and this, spread over the whole Mississippi basin, would 
 form a layer a little more than -^Vs f a f ot m thick- 
 ness. So that we may conclude that the surface of the 
 continent is being cut down on the average about one 
 foot in five thousand years. 
 
 We can only allude in passing to the very important 
 geological action of water in the solid state, as in gla- 
 ciers and icebergs. The moisture precipitated from 
 the atmosphere, and falling as rain, makes ordinary 
 rivers ; but falling in the form of snow in cold regions, 
 where more snow falls than is melted, the excess ac- 
 cumulates and is gradually compacted to ice, which, 
 like water, yields to the enormous pressure of its own 
 mass and flows toward lower levels. When the ice- 
 river reaches the sea it breaks off in huge blocks, which 
 float away as icebergs. Moving ice, like moving water, 
 
AQUEOUS AGENCIES. 19 
 
 is a powerful agent of erosion ; and the glacial marks 
 or scratches observable upon the ledges everywhere 
 in the Northern States and Canada attest the magni- 
 tude of the ice-action at a comparatively recent period. 
 
 We have already noticed incidentally the powerful 
 disintegrating action of water where it freezes in the 
 joints and pores of the rocks ; and it is probable that 
 it thus facilitates the destruction of the rocks in cold 
 countries nearly as much as the higher temperature 
 and greater rain-fall do in warm countries. 
 
 Our observations up to this point show us that ero- 
 sion, by which we mean the breaking up by chemical 
 and mechanical action of the rocks of the land and the 
 transportation of the debris into the sea, is one great 
 result accomplished by the inorganic aqueous agencies. 
 
 MECHANICAL DEPOSITION. Next let us notice what 
 becomes of all this vast amount of clay, sand, and gravel 
 after it is washed into the ocean. By taking up a glass 
 of turbid water from our roadside rill, and observing 
 that as soon as the water is undisturbed the sand and 
 clay begin to settle, we learn that the solid matter is 
 held in suspension by the motion of the water. But 
 it does not remain in suspension long after being washed 
 into the sea, for otherwise the sea would, in the course 
 of time, become turbid for long distances from shore ; 
 and it is a well-known fact that the sea-water is usually 
 clear and free from sensible turbidity close along shore 
 and even near the mouths of large rivers, while at a 
 distance of only 50 or 100 miles we find the transpar- 
 ency of the central ocean. 
 
 Putting these facts together, we see that the ocean, 
 noth withstanding tl?e ceaseless and often violent undu- 
 
*0 DYNAMICAL GEOLOGY. 
 
 lations of its surface, must be as a whole a vast body 
 of still water ; and to the reflecting mind the almost 
 perfect tranquillity of the ocean is one of its most im- 
 pressive features. For it is in striking contrast, in this 
 respect, with the more mobile aerial ocean above it. 
 
 We have got hold, now, of two facts of great geo- 
 logical importance : ( i ) The debris washed off the land 
 by waves and rivers into the still water of the ocean 
 very soon settles to the bottom ; and (2) it nearly all 
 settles on that part of the ocean-floor near the land. 
 
 And now we have in view the second great office 
 of the inorganic aqueous agencies, deposition, the 
 counterpart or complement of erosion. 
 
 The land is the great theatre of erosion and the sea 
 of deposition ; the rocks which are constantly wasting 
 away on the former are as constantly renewed in the 
 latter. 
 
 We will now observe the process of deposition a little 
 more closely. Each of these two bottles contains the 
 same amount of fine yellow clay, but in one the water 
 is fresh, and in the other it is salt. At the beginning 
 of the lesson, as you may have observed, I brought the 
 clay in both bottles into suspension by violent agita- 
 tion, and since then they have remained undisturbed. 
 The main point is that the salt water has become quite 
 clear, while the fresh water is still distinctly turbid, 
 showing that the salt favors the rapid deposition of the 
 clay. At the second lecture, a week later, these two 
 bottles, yet undisturbed, were exhibited, and the fresh 
 water seen to be still sensibly turbid. The fact is, the 
 clay is not held in suspension wholly by the motion of 
 the water ; but, just as in the case of dust in the atmos- 
 
AQUEOUS AGENCIES. 21 
 
 phere, a small portion of the medium is condensed 
 around or adheres to each solid particle, *>., each 
 clay particle in our experiment has an atmosphere of 
 water which moves with it and buoys it up. Now the 
 effect of the salt is to diminish the adhesion of the 
 water to the particles, i.e., to diminish their atmos- 
 pheres, and consequently their buoyancy. The dimin- 
 ished adhesion of the salt water is well shown by the 
 smaller drops which it forms on a glass rod. 
 
 The geological importance of this principle is very 
 great ; for it is undoubtedly largely to the saltness of 
 the sea that we owe its transparency, and the fact that 
 the fine, clayey sediment from the land, like the coarse, 
 is deposited near the shore. 
 
 This bottle of fresh water contains some fine gravel, 
 coarse sand, fine sand, and clay. By agitating the 
 water, all this material is brought into suspension. 
 Now, suddenly placing the bottle in a state of rest, we 
 observe that the gravel falls to the bottom almost instant- 
 ly, followed quickly by the coarse sand, and very soon 
 afterward by the fine sand ; and then there appears to 
 be a pause, the fine particles of clay all remain in sus- 
 pension ; but finally, when the water is quite motion- 
 less, they begin to settle ; they fall very slowly, however, 
 and the water will not be clear for hours. 
 
 This is a very instructive experiment. We learn- 
 from it : 
 
 . First, that the power of the water to hold particles 
 in suspension is inversely proportional to the size of 
 the particles ; 
 
 Second, that all materials deposited in water are as* 
 sorted according to size ; 
 
22 DYNAMICAL GEOLOGY. 
 
 Third, and this is one of the most important facts in 
 geology, all water-deposited sediments are arranged in 
 horizontal layers, i.e., are stratified. And we have now 
 traced to its conclusion, though very briefly, the process 
 of the formation of one great division of stratified rocks, 
 the mechanically-formed orfragmental rocks. These 
 are so called because the clay, sand, and gravel are, 
 in every instance, fragments of pre-existing rocks ; and 
 because the formation, transportation, and especially 
 the deposition of these fragments, are the work chiefly 
 or entirely of mechanical forces. 
 
 CHEMICAL DEPOSITION. It is a well-known fact that 
 the sea holds in solution vast amounts of common salt 
 as well as many other substances; and analyses of 
 river-waters show that dissolved minerals derived from 
 the chemical decomposition of the rocks of the land are 
 being constantly carried into the sea. 
 
 Portions of the sea which are cut off from the main 
 body, and which are gradually drying up, like the 
 Great Salt Lake, Dead Sea, and Caspian Sea, become 
 saturated solutions of the various dissolved minerals, 
 and these are slowly deposited. This process is very 
 nicely illustrated along our shores in summer, where> 
 during storms, salt-water spray is thrown above the 
 reach of the tides, and, collecting in hollows in the 
 rocks, gradually dries up, leaving behind a crust of salt. 
 
 When the sea lays down matter which it held in sus- 
 pension, we call the process mechanical deposition, and 
 the result is mecham'ca/fy-formed rocks. 
 
 But when it lays down matter which it held in solution, 
 we call the process chemical deposition, and the result 
 is chemicall-iQimzd. rocks. 
 
AQUEOUS AGENCIES. 23 
 
 The principal substances which the sea deposits 
 chemically are common salt, forming beds of rock-salt ; 
 sulphate of calcium, forming beds of gypsum ; carbonate 
 of calcium, forming beds of limestone ; and the double 
 carbonate of calcium and magnesium, forming beds of 
 dolomite. 
 
 Inorganic deposition, like inorganic erosion, is both 
 chemical and mechanical. 
 
 2. Animals and Plants, or Organic Agencies. 
 
 We turn now to the consideration of the organic 
 agencies. And I will merely allude in passing to the 
 vast importance of the fossil organic remains found 
 in the stratified rocks as marks by which to determine 
 the relative ages of the formations. 
 
 As regards the destruction of rocks erosion plants 
 and animals are almost powerless ; but in the role of 
 rock-makers they play a very important part, being very 
 efficient agents of deposition. 
 
 FORMATION OF COALS AND BITUMENS. Specimen 
 No. 8 is an example of peat from the vicinity of Bos- 
 ton ; but just as good specimens may be obtained in 
 thousands of places in this and other States. 
 
 The general physical conditions under which peat is 
 formed are familiar facts. We require simply low, level 
 land, covered with a thin sheet of water and abundant 
 vegetation; in other words, a marsh or swamp. If 
 plants decay on the dry land, the decomposition is 
 complete ; they are burned up by the oxygen of the 
 air to carbon dioxide and water just as surely as if 
 
24 DYNAMICAL GEOLOGY. 
 
 they had been thrown into a furnace, though less 
 rapidly, and nothing is returned to the soil but what 
 had been taken from it by the plants during their 
 growth. But if the plants decay under water, as in 
 a peat-marsh or bog, the decay is incomplete, and 
 most of the carbon of the wood is left behind. Now, 
 if this incomplete combustion of vegetable tissues takes 
 place in a charcoal-pit, where the wood is out of con- 
 tact with air from being covered with earth, we call the 
 carbonaceous product charcoal ; but if under the water 
 of a marsh, in Nature's laboratory, we call the product 
 peat. Peat is simply a natural charcoal ; and, just as in 
 ordinary charcoal, its vegetable origin is always perfectly 
 evident. But when the deposit becomes thicker, and 
 especially when it is buried under thick formations 
 of other rocks, like sand and clay, the great pressure 
 consolidates the peat ; it becomes gradually more min- 
 eralized and shining, shows the vegetable tissues less dis- 
 tinctly, becomes more nearly pure carbon, and we call 
 it in succession lignite, bituminous coal, and anthracite. 
 This is, briefly, the way in which all varieties of coal, 
 as well as the more solid kinds of bitumen, like asphal- 
 tum, are formed. But the lighter forms of bitumen, 
 such as petroleum and naphtha, are derived mainly, 
 if not entirely, from the partial decomposition of animal 
 tissues. These, ic is well known, decay much more 
 readily than vegetable tissues ; and the water of an or- 
 dinary marsh or lake contains sufficient oxygen for 
 their complete and rapid decomposition. In the 
 deeper parts of the ocean, however, the conditions are 
 very different, for recent researches have shown, con- 
 trary to the old idea, that the deep sea holds an abun- 
 
AQUEOUS AGENCIES. 25 
 
 dant fauna. All grades of animal life, from the highest 
 to the lowest, have need of a constant supply of oxy- 
 gen. On the land vegetation is constantly returning to 
 the air the oxygen consumed by animals, but in the 
 abysses of the ocean vegetable life is scarce or wanting ; 
 and hence it must result that over these greater than 
 continental areas countless myriads of animals are living 
 habitually on short rations of oxygen, and in water well 
 charged with carbon dioxide, the product of animal 
 respiration. As a consequence, when these animals die 
 their tissues do not find the oxygen essential for their 
 perfect decomposition, and in the course of time 
 become buried, in a half-decayed state, in the ever- 
 increasing sediments of the ocean-floor. 
 
 It is important to observe that an abundance of or- 
 ganic matter decaying under water is not the only con- 
 dition essential to the formation of beds of coal and 
 bitumen ; for this condition is realized in the luxuriant 
 growth of sea-weeds fringing the coast in every quarter 
 of the globe ; and yet coals and bitumens are rarely of 
 sea-shore origin. These organic products, even under 
 the most favorable circumstances, accumulate with 
 extreme slowness ; far more slowly, as a rule, than the 
 ordinary mechanical sediments, like sand and clay, 
 with which they are mixed, and in which they are often 
 completely lost. Consequently, although the deposi- 
 tion of the carbonized remains of plants and animals is 
 taking place in nearly all seas, lakes, and marshes, it is 
 only in those places where there is little or no mechani- 
 cal sediment that they can predominate so as to build 
 up beds pure enough to be called coal or bitumen. In 
 all other cases we get merely more or less carbonaceous 
 
26 DYNAMICAL GEOLOGY. 
 
 sand or clay. Now these especially favorable locali- 
 ties will manifestly not be often found along the sea- 
 shore, where we have strewn the sand and clay brought 
 down by rivers or washed off the land directly by 
 the ever-active surf; but they must exist in the central 
 portions of the ocean, where there is almost no mechan- 
 ical sediment and yet an abundance of life, and in 
 swamps and marshes, where there is scarcely sufficient 
 water to cover the vegetation, and no waves or currents 
 to wash down the soil from the surrounding hills. 
 
 FORMATION OF IRON-ORES. The iron-ores are 
 another class of rocks which are formed only through 
 the agency of organic matter. Iron is an abundant 
 and wide-spread element in the earth's crust, and, but 
 for the intervention of life, we might say that, while 
 there is iron everywhere, there is not much of it in 
 any one place, since it is originally very thinly diffused. 
 All rocks and soils contain iron, but it is mainly in the 
 form of the peroxide, in which state it is entirely insol- 
 uble, and hence cannot be soaked out of the soil by 
 the rain-water and concentrated by the evaporation of 
 the water at lower levels in ponds and marshes, as a 
 soluble substance like salt would be. If carried off 
 with the sand and clay, by the mechanical action of 
 water, it remains uniformly mixed with them, and there 
 is no tendency to its separation and concentration so 
 as to form a true iron-ore. 
 
 But what water cannot do alone is accomplished 
 very readily when the water is aided by decaying organic 
 matter, which is always hungry for oxygen, being, in 
 the language of the chemist, a powerful reducing agent. 
 The soil, in most places, has a superficial stratum of 
 
AQUEOUS AGENCIES. 27 
 
 vegetable mould or half-decayed vegetation. The rain- 
 water percolates through this and dissolves more or 
 less of the organic matter, which is thus carried down 
 into the sand and clay beneath and brought in contact 
 with the ferric oxide, from which it takes a certain 
 proportion of oxygen, reducing the ferric to the ferrous 
 oxide. At the same time the vegetation is burned up 
 by the oxygen thus obtained, forming carbon dioxide, 
 which immediately combines with the ferrous oxide, 
 forming carbonate of iron, which, being soluble under 
 these conditions, is carried along by the water as it 
 gradually finds its way by subterranean drainage to the 
 bottom of the valley and emerges in a swamp or marsh. 
 Here one of two things will happen : If the marsh 
 contains little or no decaying vegetation, then as soon 
 as the ferrous carbonate brought down from the hills 
 is exposed to the air it is decomposed, the carbon 
 dioxide escapes, and the iron, taking on oxygen from 
 the air, returns to its original ferric condition ; and 
 being then quite insoluble, it is deposited as a loose, 
 porous, earthy mass, commonly known as bog-iron- 
 ore, which becomes gradually more solid and finally 
 even crystalline through the subsequent action of 
 heat and pressure. When first deposited, the ferric 
 oxide is combined with water or hydrated, and is 
 then known as limonite (specimen No. 1 2) ; at a 
 later period the water is expelled, and we call the ore 
 hematite (specimen No. 13) ; and at a still later age 
 it loses part of its oxygen, becomes magnetic and more 
 crystalline, and is then known as magnetite (specimen 
 No. 14). Thus it is seen that the iron-ores, as we 
 pass from bog-limonite to magnetite, form a natural 
 
28 DYNAMICAL GEOLOGY. 
 
 series similar to and parallel with that afforded by the 
 coals as we pass from peat to graphite. 
 
 If the drainage from the hills is into a marsh contain- 
 ing an abundance of decaying vegetation, i.e., if peat 
 is forming there, the ferrous carbonate, in the presence 
 of the more greedy organic matter, will be unable to 
 obtain oxygen from the air ; and as the evaporation of 
 the water goes on, it will sooner or later become sat- 
 urated with this salt, and the latter will be deposited. 
 Here we find an explanation of a fact often observed 
 by geologists, viz., that the carbonate iron-ores are 
 usually associated with beds of coal, 
 
 The formation of the iron-ores, like that of the coals 
 and bitumens, is a slow process ; and the ores, like the 
 coals, etc., will be pure only where there is a complete 
 absence of mechanical sediment, a condition that is 
 realized most nearly in marshes. 
 
 FORMATION OF LIMESTONE, DIATOMACEOUS EARTH, ETC. 
 Marine animals take from the sea- water certain min- 
 eral substances, especially silica and carbonate of 
 calcium, to form their skeletons. Silica is used only 
 by the lowest organisms, such as Radiolaria, Sponges, 
 and the minute unicellular plants, Diatoms. The prin- 
 cipal animals secreting carbonate of calcium are Corals 
 and Mollusks. These hard parts of the organisms 
 remain undissolved after death ; and over portions of 
 the ocean-floor where there is but little of other kinds 
 of sediment they form the main part of the deposits, 
 and in the course of ages build up very extensive for- 
 mations which we call diatomaceous earth or tripolite, 
 if the organisms are siliceous, or limestone if they are 
 calcareous. A very satisfactory account of the for- 
 
AQUEOUS AGENCIES. 29 
 
 mation of limestone on a stupendous scale by the 
 polyps in coral reefs and islands is contained in No. 
 IV. of this series of guides. 
 
 The rocks here considered may be, and, as we have 
 already seen, sometimes are, deposited in a purely 
 chemical way, without the aid of life ; and it is impor- 
 tant to observe that in no case do the organisms make 
 the silica and carbonate of calcium of their skeletons, 
 but they simply appropriate and reduce to the solid state 
 what exists ready made in solution in the sea-water. 
 These minerals, and others, as we know, are produced 
 by the decomposition of the rocks of the land, and are 
 being constantly carried into the sea by rivers ; and, 
 if there were no animals in the sea, these processes 
 would still go on until the sea-water became saturated 
 with these substances, when their precipitation as 
 limestone, etc., would necessarily follow. Hence it is 
 clear that all the animals do is to effect the precipitation 
 of certain minerals somewhat sooner than it would 
 otherwise occur ; so that from a geological standpoint 
 the differences between chemical and organic deposition 
 are not great. 
 
 This section of our subject may be summarized as 
 follows : Animals and plants contribute to the forma- 
 tion of rocks in three distinct ways : 
 
 i. During their growth they deoxidize carbon dioxide 
 and water, and reduce to the solid state in their tissues 
 carbon and the permanent gases oxygen, hydrogen, and 
 nitrogen ; and after death, through the accumulation 
 of the half-decayed tissues in favorable localities, 
 marshes, etc., these elements are added to the solid 
 crust of the earth in the form of coal and bitumen. 
 
3 o DYNAMICAL GEOLOGY. 
 
 2. During the decomposition, i.e., oxidation, of the 
 organic tissues, the iron existing everywhere in the soil 
 is partially deoxidized, and, being thus rendered soluble, 
 is removed by rain-water and concentrated in low places, 
 forming beds of iron-ore. 
 
 3. Through the agency of marine organisms, certain 
 mineral substances are being constantly removed from 
 the sea-water and deposited upon the ocean floor, 
 forming various calcareous and siliceous rocks. 
 
 I now bring our study of the aqueous or superficial 
 agencies to a conclusion by noting once more that the 
 great geological results accomplished by air, water, and 
 organic matter or life are : ( i ) Erosion, or the wearing 
 away of the surface of the land; and (2) Deposition, 
 or the formation from the debris of the eroded land of 
 two great classes of stratified rocks, the mechan- 
 ically formed or fragmental rocks, and the chemically 
 and organically formed rocks. 
 
 II. IGNEOUS AGENCIES. 
 
 We pass next to a very brief consideration of opera- 
 tions that originate below the earth's surface. The 
 records of deep mines and artesian wells show that the 
 temperature of the ground always increases downwards 
 from the surface ; and the much higher temperatures of 
 hot springs and volcanoes show that the heat continues 
 to increase to a great depth, and is not a merely super- 
 ficial phenomenon. The observed rate of increase is 
 not uniform, but it seldom varies far from the average, 
 which is about i Fahr. per 53 feet of vertical descent, 
 or, in round numbers, 100 per mile. This rate, if 
 
IGNEOUS AGENCIES. 31 
 
 continued, would give a very high temperature at points 
 only a few miles below the surface ; and until within a 
 few years the idea was generally accepted by geologists 
 that the increase of temperature is sensibly uniform 
 for an indefinite distance downward ; that in the cen- 
 tral regions of the earth the temperature is far higher 
 than anything we can conceive, and that everywhere 
 below a depth of 20 to 40 miles the temperature is 
 above the fusing-point of all rocks ; and hence that 
 the earth is an incandescent liquid globe covered by 
 a thin shell or crust of cold, solid rock. 
 
 Our limited space will not permit us to enter into a 
 discussion of the condition of the earth's interior, and 
 I will merely point out in a few sentences the posi- 
 tion occupied by geologists at the present time. The 
 reasoning of Thompson has shown that the tempera- 
 ture cannot increase downward at a uniform rate, but 
 at a constantly and rapidly diminishing rate ; and 
 that everywhere below a depth of 300 miles the tem- 
 perature is probably sensibly the same, and nowhere, 
 probably, above 8000 to 10,000 Fahr. 
 
 Unlike water, all rocks contract on solidifying and 
 expand on melting, and consequently the high pres- 
 sures to which they are subjected in the earth's inte- 
 rior 10,000,000 to 20,000,000 pounds per square 
 inch must raise their fusing-points enormously, and 
 the probabilities are that they are solid, in spite of the 
 high temperature. But Thompson and Darwin have 
 shown us farther that the phenomena of the oceanic 
 tides could not be what they are known to be if the 
 earth were any less rigid than a globe of solid steel ; 
 while Hopkins has proved that the astronomical phe- 
 
32 DYNAMICAL GEOLOGY. 
 
 nomena of precession and nutation could not be what 
 they are if the earth's crust were less than 800 or 1000 
 miles thick. Putting these considerations together, 
 geologists are almost universally agreed that, while the 
 earth has an incandescent interior, it is still continu- 
 ously solid from centre to circumference, with the 
 exception of a thin plastic stratum at a depth not 
 exceeding 40 or 50 miles, which forms the seat of vol- 
 canic action. 
 
 The earth is not only a very hot body, but it is rotat- 
 ing through almost absolutely cold space, and there- 
 fore must be a cooling body. But, except at the very 
 beginning of the cooling, the loss of heat has gone on 
 almost entirely from the interior; and since cooling 
 means contraction, the heated interior must be con- 
 stantly tending to shrink away from the cold external 
 crust. 
 
 Of course no actual separation between the crust 
 and interior or nucleus can take place, but there is no 
 doubt that the crust is left unsupported to a certain 
 extent, and it must then behave like an arch with a 
 radius of 4000 miles, and the result is an enormous 
 horizontal or tangential pressure. 
 
 This lateral pressure in the earth's crust is one of the 
 most important and most generally accepted facts in 
 geology, and lies at the bottom of many geological the- 
 ories. According to what seems to me to be the most 
 probable theory of the origin of continents and ocean- 
 basins, they are broad upward and downward bendings 
 or arches into which the crust is thrown by the tan- 
 gential pressure. Finally, the strain becomes great 
 enough to crush the crust along those lines where it is 
 
IGNEOUS AGENCIES. 33 
 
 weakest. When the crust is thus mashed up by hori- 
 zontal pressure, a mountain range is formed, the crust 
 becomes enormously thicker, and a weak place becomes 
 a strong one. 
 
 During the formation of mountains the stratified 
 rocks, which were originally horizontal, are thrown into 
 folds or arches, and tipped up at all possible angles ; 
 they are fractured and faults produced ; and by the 
 immense pressure the structure known as slaty cleavage 
 is developed. In fact, a vast amount and variety of 
 structures are produced during the growth of a moun- 
 tain range. 
 
 These great earth-movements are not always per- 
 fectly smooth and steady, but they are accompanied by 
 slipping or crushing now and then ; and, as a result 
 of the shock thus produced, a swift vibratory movement 
 or jar, which we know as an earthquake, runs through 
 the earth's crust. 
 
 Extensive fissures are also formed, opening down to 
 the regions where the rocks are liquid or plastic, and 
 through these the melted rocks flow up to or toward 
 the surface. That portion which flows out on the 
 surface builds up a volcanic cone, while that which 
 cools and solidifies below the surface, in the fissures, 
 forms dikes. Thus among the igneous or eruptive 
 rocks we have two great classes, the dike rocks and 
 the volcanic rocks. 
 
 It is important to observe that all these subterranean 
 operations the formation of continents, of mountain- 
 ranges with all their attendant phenomena of folds, 
 faults and cleavage, and every form and phase of earth- 
 quake and volcanic activity depend upon or originate 
 
34 DYNAMICAL GEOLOGY. 
 
 in the interior heat of the earth. And over against the 
 superficial or aqueous agencies, originating in the solar 
 heat and producing the stratified or sedimentary rocks, 
 we set the subterranean or igneous agencies originating 
 in the central heat, and producing the unstratified or 
 eruptive rocks. 
 
STRUCTURAL GEOLOGY. 
 
 IN geology, just as in biology, there are two ways of 
 studying structure, the small way and the large way. 
 In the case of an organism, we may select a single part 
 or organ, and, disregarding its external form and rela- 
 tions to other parts, observe its composition and minute 
 structure, the various forms and arrangements of the 
 cells, etc. This is histology, and it is the complement 
 of that larger method of studying structure which is 
 ordinarily understood by anatomy. 
 
 The divisions of structural geology corresponding to 
 histology and anatomy are lithology and petrology. 
 Lithology is an in-door science ; we use the microscope 
 largely, and work with hand specimens or thin sections 
 of the rocks, observing the composition and those 
 small structural features which go under the general 
 name of texture. 
 
 In petrology, on the other hand, we consider the 
 larger kinds of rock-structure, such as stratification, 
 jointing, folds, faults, cleavage, etc. ; and it is essen- 
 tially an out-door science, since to study it to the 
 best advantage we must have, not hand specimens, 
 but ledges, cliffs, rail way- cuttings, gorges, and moun- 
 tains. 
 
36 STRUCTURAL GEOLOGY. 
 
 LITHOLOGY. 
 
 A rock is any mineral, or mixture of minerals, oc- 
 curring in masses of considerable size. This distinction 
 of size is the only one that can be made between rocks 
 and minerals, and that is very indefinite. A rock, 
 whether composed of one mineral or several, is always 
 an aggregate ; and therefore no single crystal or min- 
 eral-grain can properly be called a rock. 
 
 Before proceeding to study particularly the various 
 kinds of rocks, a little more preliminary work should 
 be done. As already intimated, the more important 
 characteristics of rocks may be grouped under two 
 general heads, composition and texture. 
 
 Composition of Rocks. 
 
 Rocks are properly defined as large masses or ag- 
 gregates of mineral matter, consisting in some cases 
 of one and in other cases of several mineral species. 
 Hence it is clear that the composition of rocks is of 
 two kinds : chemical and mineralogical ; for the va- 
 rious chemical elements are first combined to form 
 minerals, and then the minerals are combined to form 
 rocks. 
 
 Of course those minerals and elements which can be 
 described as principal or important rock-constituents 
 must be the common minerals and elements. Now it 
 is a very important and convenient fact that although 
 chemists recognize about sixty-five elementary sub- 
 stances, and these are combined to form nearly one 
 thousand mineral species, yet both the common ele- 
 ments and the common minerals are few in number. 
 
LITHOLOGY. 37 
 
 So that, although it is very desirable and even 
 necessary for the student of lithology to know some- 
 thing of chemistry and mineralogy, it by no means fol- 
 lows that he or she must be master of those sciences. 
 A knowledge of the chemical and physical character- 
 istics of a few common minerals is all that is absolutely 
 essential, though it may be added that an excess of 
 wisdom in these directions is no disadvantage. 
 
 Chemical Composition of Rocks. 
 
 The elementary substances of which rocks are chiefly 
 composed, which make up the main mass of the earth 
 so far as we are acquainted with it, number only 
 fourteen : 
 
 Non-Metallic or Acidic Elements. Oxygen, silicon, 
 carbon, sulphur, chlorine, phosphorus, and fluorine. 
 
 Metallic or Basic Elements. Aluminum, magne- 
 sium, calcium, iron, sodium, potassium, and hydrogen. 
 
 The elements are named in each group in about the 
 order of their relative abundance ; and to give some 
 idea of the enormous differences in this respect it may 
 be stated that two of the elements oxygen and sili- 
 con form more than half of the earth's crust. 
 
 Silicon, calcium, and fluorine, although exceedingly 
 abundant, are also very difficult to obtain in the free or 
 uncombined state, and specimens large enough to ex- 
 hibit to a class would be very expensive. With these 
 exceptions, however, examples of these common rock- 
 forming elements are easily obtained. 
 
 My purpose in calling attention to this point is sim- 
 ply to suggest that the proper way to begin the study 
 ef minerals and rocks with children is to first familiarize 
 
38 STRUCTURAL GEOLOGY. 
 
 them with the elements of which they are composed. 
 The most important thing to be known about any min- 
 eral is its chemical composition ; and when a child is 
 told that a mineral corundum, for example is com- 
 posed of oxygen and aluminum, he should have a dis- 
 tinct conception of the properties of each of those 
 elements, for otherwise corundum is for him a mere 
 compound of names. 
 
 It is very important, too, if the pupil has not already 
 studied chemistry, that he should be led to some com- 
 prehension of the nature of chemical union and of the 
 difference between a chemical compound and a me- 
 chanical mixture. For this purpose a few simple ex- 
 periments (the details of which would be out of place 
 here) with the more common and familiar elements 
 will be sufficient. Mrs. Richard's " First Lessons in 
 Minerals " should be introduced here. 
 
 Mineralogical Composition of Rocks. 
 
 The fourteen elements named above are combined to 
 form about fifty minerals with which the student of 
 geology should be acquainted ; but not more than one- 
 half of these are of the first importance. It is desired 
 to lay especial emphasis upon the importance of a per- 
 fect familiarity with these few common minerals. 
 There is nothing else in the whole range of geology so 
 easily acquired which is at the same time so valuable ; 
 for it is entirely impossible to comprehend the defini- 
 tions of rocks, or to recognize rocks certainly and sci- 
 entifically, unless we are acquainted with their con- 
 stituent minerals. 
 
 With one or two exceptions, these common rock- 
 forming minerals may be easily distinguished by their 
 
LIT HO LOGY. 39 
 
 physical characters alone, so that their certain recogni- 
 tion is a matter of the simplest observation, and entirely 
 within the capacity of young children. Furthermore, 
 being common, specimens of these minerals are very 
 easily obtained, so that there is no reason why teachers 
 should not here adopt the best method and place a 
 specimen of each mineral in the hands of each pupil. 
 Typical examples, large enough to show the character- 
 istics well, ought not to cost, on the average, over two 
 cents apiece. 
 
 A MINERAL is an inorganic body having theoretically 
 a definite chemical composition, and usually a regular 
 geometric form. 
 
 THE PRINCIPAL CHARACTERISTICS OF MINERALS. 
 These may be grouped under the following general 
 heads : 
 
 (i) Composition, (2) Crystalline form, (3) Hardness, 
 (4) Specific gravity, (5) Lustre, (6) Color and Streak. 
 
 i. Composition. This, according to the definition 
 of a mineral, ought to be definite, and expressible by a 
 chemical formula. When it is not so, we usually con- 
 sider that the mineral is partially decomposed, or that 
 we are dealing with a mixture of minerals. It is well 
 to impress upon the mind of the pupil the important 
 fact that the more fundamental properties of the ele- 
 ments, such as specific gravity and lustre, are not lost 
 when they combine, but may be traced in the com- 
 pounds. In other words, the properties of minerals 
 are, in a very large degree, the average of the proper- 
 ties of the elements of which they are composed ; min- 
 erals in which heavy metallic elements predominate 
 being heavy and metallic, and vice versa. 
 
40 STRUCTURAL GEOLOGY. 
 
 To fully appreciate this point it is only necessary 
 to compare a mineral like galenite a common ore 
 of lead, and containing nearly 87 per cent, of that 
 heavy metal; or hematite (specimen 13), containing 
 70 per cent, of another heavy metal, iron with 
 quartz (specimen 15), which is composed in nearly 
 equal parts of oxygen and silicon, two typical non- 
 metallic elements. Many minerals contain water, 
 i.e., are hydrated. Now water, whether we consider 
 the liquid or solid state, is one of the lightest and 
 softest of mineral constituents ; and it is a very impor- 
 tant fact that hydrated minerals are invariably lighter 
 and usually softer than anhydrous species of otherwise 
 similar composition. Other striking illustrations of this 
 principle will be pointed out in the descriptions of the 
 minerals which follow. 
 
 2. Crystalline form. A crystal is bounded by plane 
 surfaces symmetrically arranged with reference to cer- 
 tain imaginary lines passing through its centre and 
 called axes. Crystals of the same species are always 
 constant in the angles between like planes, while simi- 
 lar angles usually vary in different species ; so that each 
 species has its own peculiar form. 
 
 " Besides external symmetry of form, crystallization 
 produces also regularity of internal structure, and often 
 of fracture. This regularity of fracture, or tendency to 
 break or cleave along certain planes, is called cleavage. 
 The surface afforded by cleavage is often smooth and 
 brilliant (see specimens 17, 18, and 21), and is always 
 parallel with some external plane of the crystal. It 
 should be understood that the cleavage lamellae are 
 not in any sense present before they are made to ap- 
 pear by fracture." (Dana.) 
 
LITHOLOGY. 41 
 
 Crystals are arranged in six systems, based upon the 
 number and relations of the axes, as follows : 
 
 Isometric System. Three equal axes crossing at 
 right angles. Example, cube. 
 
 Tetragonal System. Two axes equal, third un- 
 equal, all crossing at right angles. Example, square 
 prism. 
 
 Orthorhombic System. Three unequal axes, but 
 intersections all at right angles. Example, rhombic 
 prism. 
 
 Monoclinic System. Three unequal axes, one 
 intersection oblique. Example, oblique rhombic 
 prism. 
 
 Triclinic System. Three unequal axes, all cross- 
 ing obliquely. Example, oblique rhomboidal prism. 
 
 Hexagonal System. Three equal axes lying in 
 one plane and intersecting at angles of 60, and a 
 fourth axis crossing each of these at right angles and 
 longer or shorter. Example, hexagonal prism. 
 
 By the truncation and bevelment of the angles and 
 edges of these fundamental forms a vast variety of sec- 
 ondary forms are produced. The limits of the guide 
 will not permit us to follow this topic farther ; but it 
 may be added that for the proper elucidation of even 
 the simpler crystalline forms the teacher should 'be 
 provided with a set of wooden crystal models and 
 Dana's " Text-Book of Mineralogy." 
 
 The crystallization of a mineral may be manifested 
 in two ways : first, by the regularity of its internal struc- 
 ture -or molecular arrangement, as shown by cleavage 
 and the polarization of transmitted light ; and, second, 
 by the regularity of external form which follows, undef 
 
42 STRUCTURAL GEOLOGY. 
 
 favorable conditions, as a necessary consequence of 
 symmetry in the arrangement of the molecules. 
 
 When a mineral is entirely devoid of crystalline 
 structure, both externally and internally, it is said to be 
 amorphous. 
 
 Perfect and distinct crystals are the rare exception, 
 most mineral specimens being simply aggregates of im- 
 perfect crystals. In such cases, and when the min- 
 eral is amorphous, the structure of the mass may 
 be: 
 
 Columnar or fibrous. 
 
 Lamellar, foliaceous, or micaceous. 
 
 Granular. When the grains or crystalline par- 
 ticles are invisible to the naked eye the mineral is 
 called impalpable, compact, or massive. 
 
 And the external form of the mass may be : 
 
 Botryoidal, having grape-like surfaces. 
 
 Stalactitic, forming stalactites or pendant columns. 
 
 Amygdaloidal or Concretionary, forming separate 
 globular masses in the enclosing rock. 
 
 Dendritic, branching or arborescent. 
 
 3. Hardness. By the hardness of a mineral we 
 mean the resistance which it offers to abrasion. But 
 hardness is a purely relative term, calcite, for example, 
 being hard compared with talc, but very soft com- 
 pared with quartz. Hence mineralogists have found it 
 necessary to select certain minerals to be used as a 
 standard of comparison for all others, and known as 
 the scale of hardness. These are arranged at nearly 
 equal intervals all the way from the softest mineral to 
 the hardest, as follows : 
 
LITHOLOGY. 43 
 
 Scale of Hardness. 
 
 (1) Talc. . (4) Fluorite. (8) Topaz or Beryl 
 
 (2) Gypsum. (5) Apatite. (9) Corundum. 
 
 (3) Calcite. (6) Orthoclase. (10) Diamond. 
 
 (7) Quartz. 
 
 If a mineral scratches calcite and is scratched by 
 fluorite, we say its hardness is between 3 and 4, per- 
 haps 3.5 ; if it neither scratches nor is scratched by 
 orthoclase, its hardness is 6 ; and so on. There are 
 very few minerals harder than quartz, and hence the 
 first seven members of the scale are sufficient for all 
 ordinary purposes ; and these are all included in the 
 series of specimens accompanying this Guide. 
 
 Although it is desirable to be acquainted with the 
 scale of hardness, and to understand how to use it, still 
 the student will learn, after a little practice, that almost 
 as good results may be obtained much more conven- 
 iently by the use of his thumb-nail and a good knife- 
 blade or file. Talc and gypsum are easily scratched 
 with the nail ; calcite and fluorite yield easily to the 
 knife or file, apatite with more difficulty ; while ortho- 
 clase is near the limit of the hardness of ordinary steel, 
 and quartz is entirely beyond it. 
 
 4. Specific Gravity. The specific gravity of a 
 mineral, by which we mean its weight as compared 
 with the weight of an equal volume of water, is deter- 
 mined by weighing it first in air and then in water, and 
 dividing the weight in air by the difference of the two 
 weights. Minerals exhibit a wide range in specific 
 gravity ; from petroleum, which floats on water, to gold, 
 
44 STRUCTURAL GEOLOGY. 
 
 which is nearly twenty times heavier than water. Al- 
 though this is one of the most important properties of 
 minerals, yet, being more difficult to measure than 
 hardness, it is less valuable as an aid in distinguishing 
 species. One can with practice, however, estimate the 
 density of a mineral pretty closely by lifting it in the 
 hand. 
 
 5 . Lustre. Of all the properties of minerals de- 
 pending on their relations to light the most important 
 is lustre, by which we mean the quality of the light re- 
 flected by a mineral as determined by the character or 
 minute structure of its surface. Two kinds of lustre, 
 the metallic and vitreous, are of especial importance ; 
 in fact all other kinds are merely varieties of these. 
 
 The metallic lustre is the lustre of all true metals, as 
 copper and tin, and characterizes nearly all minerals in 
 which metallic elements predominate. The vitreous 
 lustre is best exemplified in glass, but belongs to most 
 minerals composed chiefly of non- metallic elements. 
 Metallic minerals are always opaque, but vitreous min- 
 erals are often transparent. 
 
 Other kinds of lustre are the adamantine (the lustre 
 of diamond), resinous, pearly, and silky. When a 
 mineral has no lustre, like chalk, it is said to be dull. 
 
 It should be made clear to children that lustre and 
 color are entirely distinct and independent. Thus, 
 iron, copper, gold, silver, and lead are all metallic ; 
 while white or colorless quartz, black tourmaline, green 
 beryl, red garnet, etc., are all vitreous. Generally 
 speaking, any color may occur with any lustre. 
 
 6. Color and Streak. The colors of minerals are 
 of two kinds, essential and non-essential. By the 
 
LITHOLOGY. 45 
 
 essential color in any case we mean the color of 
 the mineral itself in its purest state. The non-essential 
 colors, on the other hand, are chiefly the colors of the 
 impurities contained in the minerals. 
 
 Metallic minerals, which are always opaque, usually 
 have essential colors ; but vitreous minerals, which are 
 always more or less transparent, often have non-essen- 
 tial colors. The explanation is this : In opaque min- 
 erals we can only see the impurities immediately on the 
 surface, and these are, as a rule, not enough to affect 
 its color ; but in diaphanous minerals we look into the 
 specimen and see impurities below the surface, and 
 thus bring into view, in many cases, sufficient impurity 
 so that its color drowns that of the mineral. 
 
 To prove this we have only to take any mineral (ser- 
 pentine is a good example in our series) having a non- 
 essential color, and make it opaque by pulverizing it or 
 abrading its surface, when the non-essential color, the 
 color of the impurity, immediately disappears ; just as 
 water, yellow with suspended clay, becomes white when 
 whipped into foam, and thus made opaque. 
 
 What we understand by the streak of a mineral is its 
 essential color, the color of its powder ; and it is so 
 called because the powder is most readily observed by 
 scratching the surface of the mineral, and thereby pul- 
 verizing a minute portion of it. The streak and hard- 
 ness are thus determined at the same time. The streak 
 of soft minerals is easily determined by rubbing them 
 on any white surface of suitable hardness, as paper, 
 porcelain, or Arkansas stone. 
 
 ESSENTIAL AND ACCESSORY MINERALS. Lithologists, 
 regarding minerals as constituents of rocks, divide them 
 
46 STRUCTURAL GEOLOGY. 
 
 into two great classes : the essential and the accessory 
 The essential constituents of a rock are those mineral* 
 which are essential to the definition of the rock. For 
 example, we cannot properly define granite without 
 naming quartz and orthoclase ; hence these are essen- 
 tial constituents of granite ; and if either of these min- 
 erals were removed from granite it would not be 
 granite any longer, but some other rock. But other 
 minerals, like tourmaline and garnet, may be indiffer- 
 ently present or absent ; it is granite still ; hence they 
 are merely accidental or accessory constituents. They 
 determine the different varieties of granite, while the 
 essential minerals make the species. 
 
 This classification, of course, is not absolute, for in 
 many cases the same mineral forms an essential con- 
 stituent of one rock and an accessory constituent of 
 another. Thus, quartz is essential in granite, but ac- 
 cessory in diorite. 
 
 PRINCIPAL MINERALS CONSTITUTING ROCKS. Having 
 studied in a general way the more important charac- 
 teristics of minerals, brief descriptions of the chief 
 rock-forming species are next in order. We will notice 
 first and principally those minerals occurring chiefly as 
 essential constituents of rocks. 
 
 i . Graphite. Essentially pure carbon, though often 
 mixed with a little iron oxide. Crystallizes in hexago- 
 nal system, but usually foliated, granular, or massive. 
 Hardness, 1-2, being easily scratched with the nail. 
 Sp. gr., 2.1-2.3. Lustre, metallic ; an exception to the 
 rule that acidic elements have non-metallic or vitreous 
 lustres. Streak, black and shining (see pencil-mark on 
 white paper). Color, iron-black. Slippery or greasy 
 
LITHOLOGY. 47 
 
 feel. Every black-lead pencil is a specimen of 
 graphite. Specimen 9. 
 
 The different kinds of mineral coal are, geologically, 
 as we have seen, closely related to graphite, but they 
 are such familiar substances that they need not be de- 
 scribed here. 
 
 2. Halite (common salt). Chloride of sodium: 
 chlorine, 60.7; sodium, 39.3; =100. Isometric sys- 
 tem, usually forming cubes. Hardness, 2.5, a little 
 harder than the nail. Sp. gr., 2.1-2.6. Lustre, vit- 
 reous. Streak and color both white, and hence color 
 is essential. Often transparent. Soluble ; taste, purely 
 saline. In specific gravity and lustre it is a good ex- 
 ample of a mineral in which an acidic element pre- 
 dominates. Specimen n. 
 
 3. Limonite. Hydrous sesquioxide of iron : oxygen, 
 25 ; iron, 60 ; water, 15 ; = 100. Usually amorphous ; 
 occurring in stalactitic and botryoidal forms, having a 
 fibrous structure ; and also concretionary, massive, and 
 earthy (yellow ochre) . Hardness, 5-5.5. Sp. gr., 3.6-4. 
 Lustre, vitreous or silky, inclining to metallic, and some- 
 times dull. Color, various shades of black, brown, and 
 yellow. Streak, ochre-yellow ; hence color partly non- 
 essential. Specimen 12. 
 
 4. Hematite. Sesquioxide of iron: oxygen, 30; 
 iron, 70 ; = 100. Hexagonal system, in distinct crystals, 
 but usually lamellar, granular, or compact, columnar, 
 botryoidal, and stalactitic forms being common. Hard- 
 ness, 5.5-6.5 ; good crystals are harder than steel. Sp. 
 gr., 4.5-5.3. Lustre, metallic, sometimes dull. Color, 
 iron-black, but red when earthy or pulverized (red 
 ochre) . Streak, red, and color, therefore, mainly non 
 
48 STRUCTURAL GEOLOGY. 
 
 essential ; sometimes attracted by the magnet. Speci- 
 men 13. 
 
 Hematite has the same composition as limontte, 
 minus the water ; and by comparing the hardness and 
 specific gravity of these two minerals we see that they 
 are a good illustration of the principle that hydrous 
 minerals are softer and lighter than anhydrous minerals 
 of analogous composition. Limonite and hematite are 
 two great natural coloring agents, and almost all yel- 
 low, brown, and red colors in rocks and soils are due 
 to their presence. 
 
 5. Magnetite. Protoxide and sesquioxide of iron : 
 oxygen, 27.6; iron, 72.4; = 100. Isometric system, 
 usually in octahedrons or dodecahedrons. Most abun- 
 dant variety is coarsely to finely granular, sometimes 
 dendritic. Hardness, 5.5-6.5, same as hematite. Sp. 
 gr., 4.9-5.2. Lustre, metallic. Color and streak, iron- 
 black, and hence color essential. Strongly magnetic ; 
 some specimens have distinct polarity, and are called 
 loadstones. Specimen 14. 
 
 The three iron-oxides just described limonite, 
 hematite, and magnetite are all important ores of 
 iron, and form a well-marked natural series. Thus 
 limonite is never, hematite is usually, and magnetite is 
 always, crystalline. Again, limonite with 60 per cent, 
 of iron is never magnetic, hematite with 70 per cent, 
 is sometimes magnetic, while magnetite with 72.4 per 
 cent, is always magnetic. As the iron increases so 
 does the magnetism. We have here an excellent 
 illustration of the principle that the properties of the 
 elements can be traced in those minerals in which they 
 predominate. Iron is the only strongly magnetic 
 
LIT HO LOGY. 49 
 
 element : magnetite contains more iron than any other 
 mineral, and it is the only strongly magnetic mineral. 
 
 These three iron-ores are easily distinguished from 
 each other by the color of their powders or streak, 
 limonite yellow, hematite red, and magnetite black, 
 and from all other common minerals by their high 
 specific gravity. 
 
 6. Quartz. Oxide of silicon or silica : oxygen, 
 53.33 ; silicon, 46.67 ; = 100. Hexagonal system. The 
 most common form is a hexagonal prism terminated 
 by a hexagonal pyramid. Also coarsely and finely 
 granular to perfectly compact, like flint ; the compact 
 or cryptocrystalline varieties often assuming botryoidal, 
 stalactitic, and concretionary forms. It has no cleav- 
 age, but usually breaks with an irregular, conchoidal 
 fracture like glass. Hardness, 7, being No. 7 of the 
 scale ; scratches glass easily. Sp. gr., 2.5-2.8. Lustre, 
 vitreous. Pure quartz is colorless or white, but by 
 admixture of impurities it may be of almost any color. 
 Streak always white or light colored. Quartz is usually, 
 as in specimen 15, transparent and glassy, but may be 
 translucent or opaque. It is almost absolutely infusible 
 and insoluble. 
 
 The varieties of quartz are very numerous, but they 
 may be arranged in two great groups : 
 
 1. Pheno crystalline or vitreous varieties, including 
 rock-crystal, amethyst, rose quartz, yellow quartz, smoky 
 quartz, milky quartz, ferruginous quartz, etc. 
 
 2. Cryptocrystalline or compact varieties, including 
 chalcedony, carnelian, agate, onyx, jasper, flint, chert, 
 etc. Only three varieties, however, are of any great 
 geological importance ; these are : common glassy 
 quartz (spec. 15), flint (spec. 16), and chert. 
 
50 STRUCTURAL GEOLOGY. 
 
 Quartz is one of the most important constituents 
 of the earth's crust, and it is also the hardest and 
 most durable of all common minerals. We have 
 already observed (p. 12) that it is entirely unaltered 
 by exposure to the weather ; i.e., it cannot be decom- 
 posed ; and, being very hard, the same mechanical 
 wear which, assisted by more or less chemical decom- 
 position, reduces softer minerals to an impalpable 
 powder or clay, must leave the quartz chiefly in the 
 form of sand and gravel. This agrees with our obser- 
 vation that sand (spec. 30) , especially, is usually merely 
 pulverized quartz. 
 
 Opal is a mineral closely allied to quartz, and may 
 be mentioned in this connection. It is of similar com- 
 position, but contains from 5 to 20 per cent, of 
 water, and is decidedly softer and lighter. Hardness, 
 5.5-6.5 ; sp. gr., 1.9-2.3. 
 
 7. Gypsum. Hydrous sulphate of calcium : sulphur 
 trioxide (SO 3 ),46.5 ; lime (CaO), 32.6; water (H 2 O), 
 20.9 ; = 100. Monoclinic system. Often in distinct 
 rhombic crystals ; also foliated, fibrous, and finely gran- 
 ular. Hardness, 1.5-2 ; the hardest varieties being 
 No. 2 of the scale of hardness. Sp. gr., 2.3. Lustre, 
 pearly, vitreous, or dull. Color and streak usually white 
 or gray. The principal varieties of gypsum are (a) sele- 
 nite, which includes all distinctly crystallized or trans- 
 parent gypsum; (b) fibrous gypsum or satin-spar ; 
 (c) alabaster, fine-grained, light-colored, and translu- 
 cent. Gypsum is easily distinguished from all com- 
 mon minerals resembling it by its softness and the 
 fact that it is not affected by acids'. Specimen 1 7, 
 
 8. Calcite. Carbonate of calcium : carbon dioxide 
 
LITHOLOGY. 51 
 
 (C O 2 ), 44 ; lime (CaO), 56 ; = 100. Hexagonal system, 
 usually in rhombohedrons, scalenohedrons, or hexago- 
 nal prisms. Cleavage rhombohedral and highly perfect 
 (specimen 18). Also fibrous and compact to coarsely 
 granular, in stalactitic, concretionary, and other forms. 
 Hardness, 2.5-3.5, usually 3 (see scale of hardness). 
 Sp. gr., 2.5-2.75. Lustre, vitreous. Color and streak 
 usually white. Transparent crystallized calcite is 
 known as Iceland-spar, and is remarkable for its strong 
 double refraction. When finely fibrous it makes a 
 satin-spar similar to gypsum. Geologically speaking, 
 calcite is a mineral of the first importance, being the 
 sole essential constituent of all limestones. It is 
 readily distinguished from allied species by its perfect 
 rhombohedral cleavage ; by its softness, being easily 
 scratched with a knife ; and above all by its lively effer- 
 vescence with acids, for it is the only common mineral 
 effervescing freely with cold dilute acid. To apply this 
 test it is only necessary to touch the specimen with a 
 drop of dilute chlorohydric acid. The effervescence, 
 of course, is due to the escape of the carbon dioxide 
 in a gaseous form. Specimen 18. 
 
 9. Dolomite. Carbonate of calcium and magne- 
 sium : carbonate of calcium (CaCO 3 ), 54.35 ; carbon- 
 ate of magnesium (MgCO 3 ),45.65 ; =100. Hexagonal 
 system, being nearly isomorphous with calcite. Rhom- 
 bohedral cleavage perfect. Hardness, 3.5-4 ; sp. gr., 
 2.8-2.9, being harder and heavier than calcite. Lus- 
 tre, color, and streak same as for calcite, from which it 
 is most easily distinguished by its non -effervescence or 
 only feeble effervescence with cold dilute acid, though 
 effervescing freely with strong or hot acid. Spec. 19. 
 
52 STRUCTURAL GEOLOGY. 
 
 10. Siderite. Carbonate of iron : carbon dioxide 
 (C O 2 ), 37.9 ; protoxide of iron (Fe O), 62.1 ; = 100. 
 Crystallization and cleavage essentially the same as for 
 calcite and dolomite. Hardness, 3.5-4.5, and sp. gr., 
 3.7-3.9. Lustre, vitreous. Color, white, gray, and 
 brown. Streak, white. With acid, siderite behaves like 
 dolomite. It is distinguished from both calcite and 
 dolomite by its high specific gravity, which is easily 
 explained by the fact that it is largely composed of the 
 heavy element, iron. 
 
 With one exception, the fifteen minerals which we 
 have yet to study belong to the class of silicates, which 
 includes more than one-fourth of the known species of 
 minerals, and, omitting quartz and calcite, all of the 
 really important rock-constituents. The silicate min- 
 erals may be very conveniently divided into two great 
 groups, the basic and acidic. This is not a sharp 
 division ; on the contrary, there is a perfectly gradual 
 passage from one group to the other ; and yet this is, 
 for geological purposes at least, a very natural classifi- 
 cation. The dividing line falls in the neighborhood of 
 60 per cent, of silica ; i.e., all species containing this 
 proportion of silica or less are classed as basic, since 
 in them the basic elements predominate ; while those 
 containing more than 60 per cent, of silica are classed 
 as acidic, because their characteristics are determined 
 chiefly by the acid element or silica. The principal 
 bases occurring in the silicates, named in the order of 
 their relative importance, are aluminum, magnesium, 
 calcium, iron, sodium, and potassium ; and of these, 
 magnesium, calcium, iron, and usually sodium, are 
 especially characteristic of basic species. 
 
LITHOLOGY. 53 
 
 Iron is the heaviest base ; but all the bases, except 
 sodium and potassium, are heavier than the acid 
 silica ; consequently basic minerals must be, as a rule, 
 heavier than acidic minerals. And since basic mine- 
 rals contain more iron than acidic, they must be darker 
 colored. In general, we say, dark, heavy silicates are 
 basic, and vice versa. All this is of especial importance 
 because in the rocks nature keeps these two classes 
 separate in a great degree. 
 
 1 1 . Amphibole. Silicate of aluminum, magnesium, 
 calcium, iron, and sodium. The bases occur in very 
 various proportions, forming many varieties ; but the 
 only variety of especial geological interest is horn- 
 blende, the average percentage composition of which 
 is as follows : silica (SiO 2 ), 50; alumina (A1 2 O 3 ), 10 ; 
 magnesia (MgO), 18; lime (CaO), 12; iron oxide 
 (FeO and Fe 2 O 3 ), 8; and soda (Na 2 O), 2; = 100. 
 Monoclinic system : usually in rhombic or six-sided 
 prisms which may be short and thick, but are more 
 often acicular or bladed. Hardness, 5-6 ; sp. gr., 
 2.9-3.4. Lustre, vitreous ; color, black and greenish 
 black ; and streak similar to color, but much paler. 
 Compare with quartz, and observe the strong contrast 
 in color possible with minerals having the same lustre. 
 Specimen 20. 
 
 12. Pyroxene. Like amphibole, this species em- 
 braces many varieties, and these exhibit a wide range 
 in composition ; but of these augite alone is an impor- 
 tant rock-constituent. Hence in lithology we practi- 
 cally substitute for amphibole and pyroxene, horn- 
 blende, and augite respectively. 
 
 Augite is very similar in composition to hornblende, 
 
54 STRUCTURAL GEOLOGY. 
 
 but contains usually more lime and less alumina and 
 alkali. Physically, too, these minerals are almost 
 identical, crystallizing in the same system and in very 
 similar forms, and agreeing in hardness, color, lustre, 
 and streak. Augite is heavier than hornblende, sp. gr., 
 3.2-3.5. A certain prismatic angle, which in augite 
 is 875 f , is i243o' in hornblende. Slender, bladed 
 crystals are more common with hornblende than augite. 
 When examined in thin sections with the polarizer, 
 augite does not afford the phenomenon of dichroism, 
 which is strongly marked in hornblende. However, as 
 these minerals commonly occur in the rocks, in small 
 and imperfect crystals, these distinctions can only be 
 observed in thin sections under the microscope ; so 
 that, as regards the naked eye, they are practically 
 indistinguishable. 
 
 It might appear at first that the distinction of mine- 
 rals so nearly identical is not an important matter ; but 
 nature has decreed otherwise. Augite and hornblende 
 are typical examples of basic minerals ; but augite is, 
 both in its composition and associations, the more 
 basic of the two. In proof of this we need only to 
 know that it very rarely occurs in the same rock with 
 quartz, while hornblende is found very commonly in 
 that association. Quartz in a rock means an excess of 
 acid or silica, and almost necessarily implies the absence 
 of highly basic minerals. In other words, hornblende 
 is often, and augite very rarely, found in connection with 
 acidic minerals ; and it is this difference of association 
 chiefly that makes their distinction essential to the 
 proper recognition of rocks ; while at the same time 
 it affords an easy, though of course not absolutely cer- 
 
LITHOLOGY. 55 
 
 tain, means of determining whether the black constit- 
 uent of any particular rock is hornblende or augite. 
 
 Mica Family. Mica is not the name of a single 
 mineral, but of a whole family of minerals, including 
 some half-dozen species. Only two, however, mus- 
 covite and biotite, are sufficiently abundant to engage 
 our attention. These are complex, basic silicates of 
 aluminum, magnesium, iron, potassium, and sodium. 
 The crystallization of biotite is hexagonal, and of mus- 
 covite monoclinic ; but both occur commonly in flat 
 six-sided forms. Undoubtedly the most important and 
 striking characteristic of the whole mica family is the 
 remarkably perfect cleavage parallel with the basal 
 planes of the crystals, and the wonderful thinness, and 
 above all the elasticity, of the cleavage lamellae. The 
 cleavage contrasts the micas with all other common 
 minerals, and makes their certain identification one of 
 the easiest things in lithology. The micas are soft 
 minerals, the hardness ranging from 2 to 3, and being 
 usually easily scratched with the nail. Sp. gr. varies 
 from 2.7-3.1. Lustre, pearly; and streak, white or 
 uncolored. 
 
 The distinguishing features of muscovite and biotite 
 are as follows : 
 
 13. Muscovite. Contains 47 per cent, of silica, 3 
 per cent, of sesquioxide of iron, and 10 per cent, of 
 alkalies, chiefly potash ; and the characteristic colors 
 are white, gray, and, more rarely, brown and yellow. 
 Non-dichroic. Usually found in association with acidic 
 minerals. The mica used in the arts is muscovite. 
 Specimen 21. 
 
 14. Biotite. Contains only 36 per cent, of silica. 
 
56 STRUCTURAL GEOLOGY. 
 
 20 per cent, of oxide of iron, and 17 per cent, of mag- 
 nesia ; colors, deep black to green. Strongly dichroic. 
 Commonly occurs with other basic minerals. Com- 
 pare color with per cent, of iron. 
 
 These differences are tabulated below : 
 
 Muscovite = Biotite 
 
 Acidic mica. Basic mica. 
 
 Non-ferruginous mica. Ferruginous mica. 
 
 Potash mica. Magnesian mica. 
 
 White mica. Black mica. 
 
 Non-dichroic mica. Dichroic mica. 
 
 Feldspar Family. Like mica, feldspar is the name 
 of a family of minerals ; and these are, geologically, the 
 most important of all minerals. They are, above all 
 others, the minerals of which rocks are made, and their 
 abundance is well expressed in the name, feldspar 
 being simply the German for field-spar, implying that 
 it is the common spar or mineral of the fields. 
 
 Chemically, the feldspars are silicates of aluminum 
 and potassium, sodium or calcium. They crystallize 
 in the monoclinic and triclinic systems ; and all pos- 
 sess easy cleavage in two directions at right angles to 
 each other, or nearly so. The general physical char- 
 acters, including the cleavage, are well exhibited in the 
 common species, orthoclase (specimen 22). 
 
 In hardness the feldspars range from 5 to 7, being 
 usually near 6, and almost always distinctly softer than 
 quartz. Sp. gr. varies from 2.5-2.75 ; lustre, from 
 vitreous to pearly; color, from white and gray to red, 
 brown, green, etc., but usually light. Streak, always 
 white ; rarely transparent. By exposure to the weather, 
 feldspars gradually lose their alkalies and lime, become 
 
LITHOLOGY. 57 
 
 hydrated, and are changed to kaolin or common clay. 
 A similar change takes place with the micas, augite, 
 and hornblende ; but these species, being usually rich in 
 iron, make clays which are much darker colored than 
 those derived from feldspars. The fact that the feld- 
 spars contain little or no iron undoubtedly explains 
 their low specific gravity and light colors, as compared 
 with the other minerals just named. The only com- 
 mon minerals for which the feldspars are liable to be 
 mistaken are quartz and the carbonates. From the 
 latter they are easily distinguished by their supertor 
 hardness and non-effervescence with acids ; and from 
 the former, by possessing distinct cleavage, by being 
 rarely transparent, by being somewhat softer, and by 
 changing to clay on exposure to the weather. 
 
 The feldspars of greatest geological interest are five 
 in number, and may be classified chemically as fol- 
 lows : 
 
 Orthoclase, silicate of aluminum and potassium, or 
 
 potash feldspar. 
 A-lbite, . " " " sodium, or 
 
 soda feldspar. 
 Anorthite, " " " calcium, or 
 
 lime feldspar. 
 Oligoclase, " sodium, and calcium, or 
 
 soda-lime feldspar. 
 Labradorite, " " calcium, and sodium, or 
 
 lime-soda feldspar. 
 
 This appears like a complex arrangement, but it can 
 be simplified. Orthoclase crystallizes in the monocli- 
 nic system, and all the other feldspars in the triclinic 
 system. With the exception of albite, which is a com- 
 
58 STRUCTURAL GEOLOGY. 
 
 paratively rare species, the triclinic feldspars all contain 
 less silica than orthoclase ; i.e., are more basic. This is 
 shown by the subjoined table giving the average com- 
 position of each of the feldspars : 
 
 SiO 2 A1 2 O 3 K 2 O Na 2 O CaO Total. 
 
 Orthoclase, 65 18 17 = 100 
 
 Albite, 68 20 12 = 100 
 
 Oligoclase, 62 24 9 5 100 
 
 Labradorite, 53 30 4 13 = 100 
 
 Anorthite, 43 37 20 = 100 
 
 As we should naturally expect, the triclinic feldspars 
 occur usually with other basic minerals, while the mono- 
 clinic species, orthoclase, is acidic in its associations ; 
 furthermore, the triclinic feldspars are often intimately 
 associated with each other, but are rarely important 
 constituents of rocks containing much orthoclase. In 
 other words, the distinction of orthoclase from the basic 
 or triclinic feldspars is important and comparatively 
 easy, while the distinction of the different basic feld- 
 spars from each other is both unimportant and difficult. 
 Hence, in lithology, we find it best to put all these 
 basic feldspars together, as if they were one species, 
 under the name plagiodase, which refers to the oblique 
 cleavage of all these feldspars, and contrasts with 
 orthoclase, which refers to the right-angled cleavage of 
 that species. 
 
 This statement of the relations of the feldspars is, of 
 course, beyond the comprehension of many children j 
 and yet it should be understood by the teacher who 
 would lead the children to any but the most superficial 
 views. 
 
 15. Orthoclase. This is the common feldspar, and 
 
LITHOLOGY. 59 
 
 the most abundant of all minerals, being the principal 
 constituent of granite, gneiss, and many other impor- 
 tant rocks. The most characteristic colors are white, 
 gray, pinkish, and flesh-red. Specimen 22. 
 
 1 6. Plagioclase. Like orthoclase, these species 
 may be of almost any color ; yet these two great divi- 
 sions of the feldspars are usually contrasted in this 
 respect. Thus, bluish and grayish colors are most 
 common with plagioclase, and white or reddish colors 
 with orthoclase. Specimen 23 is labradorite, and, in 
 every respect, a typical example of plagioclase. On 
 certain faces and cleavage-surfaces of the plagioclase 
 crystals we may often observe a series of straight parallel 
 lines or bands which are often very fine, fifty to a 
 hundred in a single crystal. These striae are due to 
 the mode of twinning, and are of especial importance, 
 since, while they are very characteristic of plagio- 
 clase, they never occur in orthoclase. As stated, these 
 twinning striae in plagioclase are often visible to the 
 naked eye ; and when they are not, they may usually 
 be revealed by examining a thin section under the 
 microscope with polarized light. Plagioclase decays 
 much more rapidly when exposed to the weather than 
 orthoclase. This point becomes perfectly clear when 
 we compare weathered ledges of diabase (or any trap- 
 rock, see specimen 2) and granite; for plagioclase 
 is the principal constituent of the former rock, and 
 orthoclase of the latter. 
 
 Hydrous Silicates. Many silicates contain water, 
 and some of these are of great geological importance. 
 What has been stated on a preceding page concerning 
 the softness and lightness of hydrated minerals is espe- 
 
60 STRUCTURAL GEOLOGY. 
 
 cially applicable here ; for all the geologically important 
 hydrous silicates are distinctly softer and lighter than 
 anhydrous minerals of otherwise similar composition. 
 Furthermore, they usually have an unctuous or slippery 
 feel ; and, with one exception (kaolin) , are of a green 
 or greenish color. 
 
 1 7. Kaolinite (Kaolin). Hydrous silicate of alum- 
 inum : silica (SiO 2 ), 46; alumina (A1 2 O 3 ), 40; and 
 water (H 2 O), 14; = 100. Orthorhombic system, in 
 rhombic or hexagonal scales or plates, but usually earthy 
 or clay-like. Hardness, 1-2.5 > S P- g r v 2.4-2.65. 
 The pure mineral is white ; but it is usually colored 
 by impurities, the principal of which are iron oxides 
 and carbonaceous matter. Kaolin is the most abun- 
 dant of all the hydrous silicates, and it is the basis and 
 often the sole constituent of common clay, a very 
 common mineral, but rarely pure. We have already 
 (p. u) noticed the mode of origin of kaolin or clay. 
 It results from the decomposition of various aluminous 
 silicate minerals, especially the feldspars. Under the 
 combined influence of carbon dioxide and moisture, 
 feldspars give up their potassium, sodium, and calcium, 
 and take on water, and the result is kaolin. This min- 
 eral is believed to be always a decomposition product. 
 Perhaps the best, or at least the most convenient, test 
 for kaolin is the argillaceous odor, the odor of moist- 
 ened clay. Specimen 24. 
 
 1 8. Talc. Hydrous silicate of magnesium: silica 
 (SiO 2 ), 63 (acidic); magnesia (MgO), 32; water 
 (H 2 O), 5 ; = 100. Orthorhombic system, but rarely in 
 distinct crystals. Cleavage in one direction very per- 
 fect the cleavage lamellae are flexible, but not elastic, 
 
LITHOLOGY. 6 1 
 
 as in mica. Hardness, i ; see scale. Sp. gr., 2.55-2.8. 
 Lustre, pearly. Color, apple-green to white ; and streak, 
 white. The feel is very smooth and greasy ; and, in 
 connection with the color and foliation, affords the best 
 means of distinguishing talc from allied minerals. Talc 
 sometimes results from the alteration of augite, horn- 
 blende, and other minerals, but it is not always nor 
 usually an alteration product. 
 
 19. Serpentine. Hydrous silicate of magnesium : 
 silica (Si O 2 ), 44 (basic) ; magnesia (Mg O), 44 ; water 
 (H 2 O), 12 ; =100. Essentially amorphous. Hardness, 
 2.5-4; sp.gr., 2.5-2.65. Lustre, greasy, waxy, or 
 earthy. Color, various shades of green and usually 
 darker than talc, but streak always white. Feel, 
 smooth, sometimes greasy. Distinguished from talc 
 by its hardness, compactness, and darker green. 
 Sometimes results from the alteration of olivine and 
 other magnesian minerals, but usually we are to regard 
 it as an original mineral. Specimen 25. 
 
 20. Chlorite. This is, properly, the name of a 
 group of highly basic minerals of very variable compo- 
 sition, but they are all essentially hydrous silicates of 
 aluminum, magnesium, and iron ; and the average 
 composition of the most abundant species, prochlorite, 
 is as follows : silica (SiO 2 ), 30 ; alumina (A1 2 O 3 ), 18 ; 
 magnesia (MgO), 15 ; protoxide of iron (FeO), 26 ; 
 and water (H. 2 O), n ; =100. The chlorites crystallize 
 in several different systems, but in all there is a highly 
 perfect cleavage in one direction, giving, as in talc, a 
 foliated structure with flexible but inelastic laminae. 
 The cleavage scales, however, are sometimes minute, 
 and the structure massive or granular. Hardness of 
 
62 STRUCTURAL GEOLOGY. 
 
 prochlorite, 1-2 ; between talc and serpentine. Sp.gr., 
 2.78-2.96. All the chlorites have a pearly to vitreous 
 lustre. Color usually some shade of green ; in pro- 
 chlorite a dark or blackish green, darker than serpentine, 
 as that is darker than talc. Streak, a lighter, whitish 
 green. Less unctuous than talc, but more so than ser- 
 pentine. The chlorites are produced very commonly, 
 but not generally, by the alteration of basic anhydrous 
 silicates, like augite and hornblende. Specimen 26. 
 
 2 1 . Hydro-mica. This, too, is properly the name 
 of a group of minerals ; but for geological purposes 
 they may be regarded as one species. Taking a gen- 
 eral view of the composition, these are simply the an- 
 hydrous or ordinary micas, which we have already 
 studied, with from 5 to 10 per cent, of water added. 
 In crystallization and structure they are essentially 
 mica-like. Although not distinctly softer than the 
 common micas, they are lighter, always more unctuous 
 and slippery, and usually of a greenish color. The 
 micaceous structure with elastic laminae serves to dis- 
 tinguish the hydro-micas from other hydrous silicates. 
 
 22. Glauconite* Hydrous silicate of aluminum, 
 iron, and potassium : silica(SiO 2 ),5o ; alumina (A1 2 O 3 ), 
 protoxide of iron (FeO), and potash (K 2 O), together, 
 41 ; and water (H 2 O),9 ; = 100. Amorphous, forming 
 rounded and generally loose grains, which often have 
 a microscopic organic nucleus. It is dull and earthy, 
 like chalk, and always soft, green, and light, but not 
 particularly unctuous. Glauconite is the principal, 
 often the sole, constituent of the rock greensand, which 
 occurs abundantly in the newer geological formations, 
 and is now forming in the deep water of the Gulf of 
 
LITHOLOGY. 63 
 
 Mexico and along our Atlantic sea-board. Speci- 
 men 27. 
 
 This completes our list of minerals occurring chiefly 
 as essential constituents of rocks ; and following are 
 three of the more common and important minerals 
 occurring chiefly as accessory, rarely as essential, rock- 
 constituents. 
 
 23. Chrysolite (Oiivine). Silicate of magnesium 
 and iron: silica (SiO 2 ), 41; magnesia (MgO), 51; 
 protoxide of iron (Fe 2 O 3 ), 8 ; = 100. Orthorhombic 
 system ; but usually in irregular glassy grains. Hard- 
 ness, 6-7. Sp. gr., 3.3-3.5. Lustre, vitreous ; color, 
 usually some shade of green ; and streak, white. 
 Chrysolite sometimes closely resembles quartz, but its 
 green color usually suffices to distinguish it. It is a 
 common constituent of basalt and allied rocks. By 
 absorption of water it is changed into serpentine and 
 talc. See examples in specimen. 
 
 24. Garnet. The composition of this mineral is 
 extremely variable ; but the most important variety is a 
 basic silicate of aluminum and iron : silica (SiO 2 ), 37 ; 
 alumina (A1 2 O 3 ), 20 ; and protoxide of iron (FeO),43 '> 
 = 100. Isometric system, usually in distinct crystals, 
 twelve-sided (dodecahedrons) and twenty-four-sided 
 (trapezohedrons) forms being most common. Hard- 
 ness, 6.5-7.5 ; average as hard as quartz. Sp. gr., 
 3.15-4.3 ; compare with the high percentage of iron. 
 Lustre, vitreous ; colors, various, usually some shade of 
 red or brown ; and streak, white. Some varieties con- 
 tain iron enough to make them magnetic. Garnet is 
 easily distinguished by its form, color, and hardness 
 from all other minerals. It is a common but not an 
 
I 
 
 64 STRUCTURAL GEOLOGY. 
 
 abundant mineral, occurring most frequently in gneiss, 
 mica schist, and other stratified crystalline rocks. See 
 examples in specimen. 
 
 25. Pyrite. Sulphide of iron : sulphur, 53.3 ; iron, 
 46.7; = 100. Isometric system, occurring usually in 
 distinct crystals, the cube and the twelve-sided form 
 known as the pyritohedron being the most common. 
 Hardness, 6-6.5, striking fire with steel. Sp. gr., 4.8- 
 5.2 ; heavy because rich in iron. Lustre, metallic and 
 splendent. Color, pale, brass-yellow, and streak, 
 greenish or brownish. Pyrite is sometimes mistaken 
 for gold, but it is not malleable ; while its color, hard- 
 ness, and specific gravity, combined, easily distinguish 
 it from all common minerals. As an accessory rock- 
 constituent, pyrite occurs usually in isolated cubes or 
 pyritohedrons. Specimen 10. 
 
 Textures of Rocks. 
 
 Texture is a general name for those smaller struc- 
 tural features of rocks which can be studied in hand 
 specimens, and which depend upon Reforms and sizes 
 of the constituent particles of the rocks, and the ways 
 in which these are united. 
 
 By "constituent particles" we mean, not the atoms 
 or molecules of matter composing the rocks, but the 
 pebbles in conglomerate, grains of sand in sandstone, 
 crystals of quartz, feldspar, and mica in granite, etc. 
 The four most important textures are : 
 
 (\)Fragmental texture. The rock is composed of 
 mere irregular, angular, or rounded, but visible, frag- 
 ments. Examples : sand, sandstone, gravel, conglom- 
 erate, etc. Specimens 30, 31, 28, 29. 
 
LITHOLOGY. 65 
 
 (2) Crystalline texture. The constituent particles 
 are chiefly, at least, distinctly crystalline, as shown 
 either by external form, or cleavage, or both. Exam- 
 ples : granite, diabase, gneiss, etc. Specimens 45, i, 
 41. 
 
 (3). Compact texture. The constituent particles 
 are indistinguishable by the naked eye, but become visi- 
 ble under the microscope, appearing as separate crystal- 
 line grains or as irregular fragments. In other words, if, 
 in the case of either the granular or crystalline textures, 
 we conceive the particles to become microscopically 
 small, then we have the compact texture. Examples : 
 clay, slate, many limestones, basalt, etc. Specimens 
 
 34, 35, 39- 
 
 (4 ) Vitreous texture. The texture of glass, in which 
 the constituent particles are absolutely invisible even 
 with the highest powers of the microscope, and may 
 be nothing more than the molecules of the substance, 
 which thus, so far as our powers of observation are 
 concerned, presents a perfectly continuous surface. 
 Examples : obsidian, glassy quartz, and some kinds of 
 coal. Specimens 47, 15. 
 
 These four textures, which, it will be observed, are 
 determined by the forms and sizes of the constituent 
 particles, may be called the primary textures, because 
 every rock must possess one of them. We cannot con- 
 ceive of a rock which is neither fragmental, crystalline, 
 compact, nor vitreous. But in addition to one of the 
 primary textures, a rock may or may not have one or 
 more of what may be called secondary textures. These 
 are determined by the way in which the particles are 
 united, the mode or pattern of the arrangement, etc. 
 
66 STRUCTURAL GEOLOGY. 
 
 Following are definitions of the principal secondary 
 textures : 
 
 1 i ) Laminated texture. This exists where the par- 
 ticles are arranged in thin, parallel layers, which may 
 be marked simply by planes of division, or the alter- 
 nate layers may be composed of particles differing in 
 composition, form, size, or color, etc. Among the 
 laminated textures we thus distinguish : (a) the banded 
 texture, where the layers are contrasted in color, tex- 
 ture, or composition, but cohere, so that there is no 
 cleavage or easy splitting parallel with the stratification ; 
 and (b) the schistose or shaly texture, where such fissility 
 or stratification-cleavage exists. If a fragmental, com- 
 pact, or vitreous rock is fissile, we use the term shaly; 
 but a fissile, crystalline rock is described as schistose. 
 The banded texture may occur with the fragmental, 
 banded sandstones, etc. ; with the crystalline, many 
 gneisses, etc. (specimen 41); with the compact, 
 many slates, limestones, felsites, etc. (specimens 34, 
 42); with the vitreous, banded obsidian, furnace 
 slags, and some coal. The schistose texture may occur 
 with the crystalline, mica schist, etc. (specimen 43) ; 
 and the shaly texture with the compact and fragmental, 
 but rarely with the vitreous. 
 
 (2) Porphyritic texture. We have this texture 
 when separate and distinct crystals of any mineral, but 
 most commonly of feldspar, are enclosed in a relatively 
 fine-grained base or matrix, which may be either crystal- 
 line, compact, or vitreous, but rarely fragmental. Speci- 
 mens 5, 6, 7 are examples of the porphyritic compact 
 texture. 
 
LIT HO LOGY. 67 
 
 (3) Concretionary texture. When one or more 
 constituents of a rock have the form, in whole or in part, 
 not of distinct angular crystals, but of rounded concre- 
 tions, the texture is described as concretionary, the 
 concretions taking the place in this texture of the iso- 
 lated crystals in the porphyritic texture. This texture 
 occurs in connection with all the primary textures, but 
 the most familiar example is oolitic limestone. 
 
 (4) Vesicular texture. A rock has this texture 
 when it contains numerous small cavities or vesicles. 
 These are most commonly produced by the expansion 
 of steam and other vapors when the rock is in a plastic 
 state ; and hence the vesicular texture is found chiefly 
 in volcanic rocks. Except rarely, it is associated only 
 with the compact texture, ordinary stony lavas (spec- 
 imen 49) ; and with the vitreous texture, pumice 
 (specimen 48). 
 
 (5 ) Amygdaloidal texture. In the course of time 
 the vesicles of common lava are often filled with various 
 minerals deposited by infiltrating waters, giving rise to 
 the amygdaloidal texture, from the Latin amygdalum, an 
 almond, in allusion to a common form of the vesicles, or 
 amygdules, as they are called, after being filled. The 
 amygdaloidal texture is thus necessarily preceded by the 
 vesicular, and is limited to the same classes of rocks. 
 Specimen 50. 
 
 Besides the foregoing, there are many minor sec- 
 ondary textures. The rocks known as tufas have what 
 may be called the tufaceous texture. Then we have 
 kinds of texture depending on the strength of the union 
 of the particles, as strong, weak, friable, earthy, etc. 
 
68 STRUCTURAL GEOLOGY. 
 
 Classification of Rocks. 
 
 Having finished our preliminary observations on the 
 characteristics of rocks, we are now about ready to 
 begin a systematic study of the rocks themselves ; but 
 it is needful first to say a few words about the classifi- 
 cation of rocks, since upon this depends not only the 
 order in which we shall take the rocks up, but also the 
 ideas that will be imparted concerning their relations 
 and affinities. The classifications which have been 
 proposed at different times are almost as numerous as 
 She rocks themselves. Some of these are confessedly, 
 and even designedly, artificial, as when we classify 
 stones according to their uses in the arts, etc. But we 
 want something more scientific, a natural classification ; 
 that is, one based upon the natural and permanent 
 characteristics of rocks. Rocks have been classified 
 according to chemical composition, mineralogical com- 
 position, texture, color, density, hardness, etc. ; but 
 these arrangements, taken singly or all combined, are 
 inadequate. 
 
 A natural classification may be defined as a concise 
 and systematic statement of the natural relations exist- 
 ing among the objects classified. Now the most, im- 
 portant relations existing among rocks are those due 
 to their different origins. We must not forget that 
 lithology is a branch of geology, and that geology is 
 first of all a dynamical science. The most important 
 question that can be asked about any rock is, not What 
 is it made of? but How was it made ? What were the 
 general forces or agencies concerned in its formation? 
 Rocks are the material in which the earth's history is 
 
Classification of Rocks, 
 
 Sedimentary or Stratified Rocks. 
 
 MECHANICALLY FORMED. 
 
 
 Unconsolidated. Consolidated. 
 
 Conglomerate 
 group. 
 
 Gravel. Conglomerate. 
 
 A renaceous 
 group. 
 
 Sand. Sandstone. 
 
 A rgillaceous 
 group. 
 
 Clay. Slate. 
 
 CHEMICALLY AND ORGANICALLY FORMED. 
 
 Coal 
 
 group. 
 
 Iron-ore 
 
 group. 
 
 C ale ar tons 
 group. 
 
 
 Me amorphic group (Silicates'), 
 Acidic. Basic. 
 
 85 80 70 60 SO 40 30 
 
 Peat. 
 Lignite. 
 Bit. Coal. 
 
 Anthracite. 
 Graphite. 
 Asphaltum. 
 
 Limonite. 
 Hematite. 
 Magnetite. 
 Siderite. 
 
 Limestone. 
 Dolomite. 
 Gypsum. 
 Rock-salt. 
 Phosphate 
 Rock. 
 
 Feldspathic. 
 
 Gneiss. [ Diorite. 
 
 Syenite. Norite. 
 
 Siliceous 
 group. 
 
 Non-Feldspathic. 
 
 Mica Schist. 
 
 Tripolite. 
 Flint. 
 Siliceous 
 Tufa. 
 Novaculite. 
 
 Hornbl. Schist. Amphiboli e. 
 
 Talc Schist. Chi. Schist. 
 i 
 Greensand Serpentine. 
 
 Eruptive or Unstratified Rocks. 
 
 PLUTONIC. 
 
 Thisp 
 is a blan 
 no erupt 
 which ar 
 minerals 
 
 irt of the clas 
 k, for the re. 
 ive rocks ar 
 e chiefly com 
 jelonging to t 
 a Elements, C 
 Sulphates, or 
 , all eruptive 
 nown, are pi 
 1 of minerals V. 
 ss of Silicates 
 
 sification 
 ison that 
 e known 
 posed of 
 ic classes 
 'hlorides 
 
 | 
 I 
 
 | 
 Granite. Diori e. 
 
 Syenite. D abase. 
 
 VOLCANIC. 
 
 Oxides, 
 ates; i.e 
 far as k 
 composec 
 to the cla 
 
 Carbon- 
 rocks, so 
 incipally 
 elonging 
 
 Feldspathic. 
 
 Rhyol te. Andesite. 
 
 Trachyte. Basalt. 
 Obsidian. Tachylite. 
 
 Petrosilex. Porphyrite. 
 
 Felsite. Melaphyr. 
 
 , ,- ; 
 
LITHOLOGY. 71 
 
 written, and what we want to know first concerning any 
 rock is what it can tell us of the condition of that part 
 of the earth at the time it was made and subsequently. 
 
 The geological agencies, as we have already learned, 
 may be arranged in two great classes : first, the aqueous 
 or superficial agencies originating in the solar heat, and 
 producing the sedimentary or stratified rocks ; and, 
 second, the igneous or subterranean agencies originat- 
 ing in the central or interior heat, and producing the 
 eruptive or unstratified rocks. Hence, we want to 
 know first of any rock whether it is of aqueous or 
 igneous origin. Then, if it is a sedimentary rock, 
 whether it has been formed by the action chiefly of 
 mechanical forces, or of chemical and organic forces. 
 And, if it is an eruptive rock, whether it has cooled and 
 solidified below the earth's surface in a fissure, and is a 
 dike or trappean rock, or has flowed out on the sur- 
 face and cooled in contact with the air, and thus be- 
 come an ordinary lava or volcanic rock. 
 
 Here we have the outlines of our classification, and 
 it will be observed that we have simply reached the 
 conclusion, in a somewhat roundabout manner, that 
 there should always be a general correspondence be- 
 tween the classification of rocks and the classification 
 of the forces that produce them. The general plan 
 of the preceding scheme of the classification must 
 now be clear, and the details will be explained as we 
 go along. 
 
72 STRUCTURAL GEOLOGY. 
 
 Descriptions of Rocks. 
 
 I. Sedimentary or Stratified Rocks. 
 
 i. MECHANICALLY FORMED OR FRAGMENTAL ROCKS. 
 These consist of materials deposited from suspension 
 in water, and the process of their formation is through- 
 out chiefly mechanical. The materials deposited are 
 mere fragments of older rocks ; and, if the fragments 
 are large, we call the newly deposited sediment gravel ; 
 if finer, sand ; and, if impalpably fine, clay. These 
 fragmental rocks cannot be classified chemically, since 
 the same handful of gravel, for instance, may contain 
 pebbles of many different kinds of rocks, anjj thus be 
 of almost any and very variable composition. Such 
 chemical distinctions as can be established are only 
 partial, and the classification, like the origin, must be 
 mechanical. Accordingly, as just shown, we recognize 
 three principal groups based upon the size of the frag- 
 ments j viz. : 
 
 (1) Conglomerate group. 
 
 (2) Arenaceous group. 
 
 (3) Argillaceous group. 
 
 This mode of division is possible and natural, simply 
 because, as we observed in an early experiment, mate- 
 rials arranged by the -mechanical action of water are 
 always assorted according to siz When first depos- 
 ited, the gravel, sand, and clay are, of course, perfectly 
 loose and unconsolidated ; but in the course of time 
 they may, under the influence of pressure, heat, and 
 chemical action, attain almost any degree of consoli- 
 dation, becoming conglomerate, sandstone, and slate, 
 
LITHOLOGY. 73 
 
 respectively. The pressure may be vertical where it is 
 due to the weight of newer deposits, or horizontal 
 where it results from the cooling and shrinking of the 
 earth's interior. The heat may result from mechanical 
 movements, or contact with eruptive rocks ; or it may 
 be due simply to the burial of the sediments, which, it 
 will be seen, must virtually bring them nearer the great 
 source of heat in the earth's interior, on the same 
 principle that the temperature of a man's coat, on a 
 cold day, is raised by putting on an overcoat. The 
 effect of the heat, ordinarily, is simply drying, cooper- 
 ating with the pressure to expel the water from the 
 sediments ; but, if the temperature is high, it may bake 
 or vitrify them, just as in brick-making. Sediments 
 are consolidated by chemical action when mineral 
 substances, especially calcium carbonate, the iron 
 oxides, and silica are deposited between the particles 
 by infiltrating waters, cementing the particles together. 
 This principle is easily demonstrated experimentally 
 by taking some loose sand and wetting it repeatedly 
 with a saturated solution of some soluble mineral, like 
 salt or alum, allowing the water to evaporate each time 
 before making a fresh application. The interstices 
 between the grains are gradually filled up, and the 
 sand soon becomes a firm rock. But the student 
 should clearly understand that, in geology, gravel, 
 sand, and clay are just as truly rocks before their 
 consolidation as after. It is plain then that in each 
 of the principal groups of fragmental rocks we must 
 recognize an unconsolidated division and a consoli- 
 dated division. 
 
 (i) Conglomerate group. The rocks belonging in 
 
74 STRUCTURAL GEOLOGY. 
 
 this group we know before consolidation as grave}, 
 and after consolidation as conglomerate. 
 
 Gravel. The pebbles, as we have already seen, are 
 usually, though not always, well rounded or water- 
 worn ; and they may be of any size from coarse grains 
 of sand to boulders. As a rule, however, the larger 
 pebbles, especially, are of approximately uniform size 
 in the same bed or layer of gravel, with, of course, suf- 
 ficient fine material to fill the interstices. Although the 
 same limited mass of gravel may show the widest possi- 
 ble range in chemical and mineralogical composition, 
 yet hard rocks are evidently more likely than soft rocks 
 to form pebbles ; and hence quartz and quartz-bearing 
 rocks usually predominate in gravels. Specimen 28. 
 
 Conglomerate. Consolidated gravel. Children 
 should be led to an appreciation of this point by a 
 careful comparison of the forms of the pebbles in the 
 gravel and conglomerate. The conglomerate seems 
 to contain a larger proportion of fine material than or- 
 dinary gravel. But this is because the gravel is usually, 
 as with our specimen, taken from the surface of the 
 beach, where, of course, the pebbles are clean and sep- 
 arate ; but if it had remained there to be covered by a 
 subsequently deposited layer, enough fine stuff would 
 have been sifted into the holes to fill them. And in 
 the finished gravel, just as in the conglomerate, the 
 pebbles are usually closely packed, with just sufficient 
 sand and clay, or paste, as the material in which the 
 pebbles are imbedded is called, to fill the interstices. 
 The paste is usually similar in composition to the peb- 
 bles, with this difference : hard materials predominate 
 in the pebbles and soft in the paste. 
 
LITHOLOGY. 75 
 
 Stratified rocks generally show the stratification in 
 parallel lines or bands differing in color, composition, 
 etc. ; but nothing like this can be detected in our 
 specimens of conglomerate ; and the question might 
 be asked, How do we know that this is a stratified 
 rock ? In answer, it can be said that our hand-speci- 
 mens appear unstratified simply because the rock is so 
 coarse ; but when we look at large masses, and espe- 
 cially when we see it in place in the quarry, that 
 parallel arrangement of the material which we call strat- 
 ification is usually very evident ; and we often see pre- 
 cisely the same thing in gravel banks. It is, however, 
 wholly unnecessary that we should see the stratification 
 in order to know certainly that this is a stratified or 
 aqueous rock, because the forms of the pebbles show 
 very plainly that they have been fashioned and de- 
 posited by moving water ; and we have in the smallest 
 specimen proof positive that our conglomerate is a 
 consolidated sea-beach. 
 
 Conglomerate shows the same variations in compo- 
 sition and texture as gravel ; it may be composed of 
 almost any kind of material in pebbles of almost any 
 size. We recognize two principal varieties of con- 
 glomerate based on the forms of the pebbles ; if, as is 
 usual, these are well rounded and water-worn, the rock 
 is true pudding-stone (specimen 29) ; but, if they are 
 angular, or show but little wear, it is called breccia. 
 
 (2) Arenaceous Group. The conglomerate group 
 passes insensibly into the arenaceous group ; for, from 
 the coarsest gravel to the finest sand, the gradation is 
 unbroken, and every sandstone is merely a conglome- 
 rate on a small scale. 
 
76 STRUCTURAL GEOLOGY. 
 
 Sand. Like gravel, sand may be of almost any 
 composition, but as a rule it is quartzose ; quartz, on 
 account of its hardness and the absence of cleavage, 
 being better adapted than any other common mineral 
 to form sand. Where the composition of a sand is not 
 specified, a quartzose sand is always understood. By 
 examining a typical sand with a lens, and noting the 
 glassy appearance of the grains, and then testing their 
 hardness on a piece of glass, which they will scratch as 
 easily as quartz, the pupil is readily convinced that 
 each grain is simply an angular fragment of quartz. 
 Specimen 30. 
 
 Sandstone. Consolidated sand. In proving this, 
 children will notice first the granular or sandy appear- 
 ance of the sandstone ; and then, with the lens, that the 
 grains in the sandstone have the same forms as the 
 sand-grains. The stratification cannot be seen very 
 distinctly in our hand-specimens, but in larger masses 
 it is usually very plain, as may be observed in the 
 blocks used for building, and still better in the quarries. 
 However, even if the stratification were not visible to 
 the eye, we could have no doubt that sandstone is a 
 mechanically formed stratified rock ; because the form 
 of the grains, just as in the conglomerate, tells us that. 
 Many sandstones, too, contain the fossil remains of 
 plants and animals, and these are always regarded as 
 affording positive proof that the rocks containing them 
 belong to the aqueous or stratified series. 
 
 There are many varieties of sandstone depending 
 upon differences in composition, texture, etc., but we 
 have not space to notice them in detail. In sandstone, 
 just as in sand, quartz is the predominant constituent, 
 
LITHOLOGY. 77 
 
 although we sometimes find varieties composed largely 
 or entirely of feldspar, mica, calcite, or other minerals. 
 Specimen 31 is an example of the architectural variety 
 known as freestone, which is merely a fine-grained, 
 light-colored, uniform sandstone, not very hard, and 
 breaking with about equal freedom in all directions. 
 The consolidation of sandstones is due chiefly to chem- 
 ical action. The cementing materials are commonly 
 either : ferruginous (iron oxides), giving red or brown 
 sandstones ; calcareous, forming soft "sandstones, which 
 effervesce with acid if the cement is abundant; or 
 siliceous, making very strong, light-colored sandstones. 
 Ferruginous sandstones are the most valuable for archi- 
 tectural purposes ; for, while not excessively hard, they 
 have a very durable cement. Siliceous sandstones are 
 too hard ; and the calcareous varieties crumble when 
 exposed to the weather because the cement is soluble 
 in water containing carbon dioxide, as all rain-water 
 does. Specimen 32 is a good example of a ferruginous 
 sandstone, and it is coarse enough so that we can see 
 that each grain of quartz is coated with the red oxide 
 of iron. The mica scales visible here and there in this 
 specimen are interesting as showing that the grains are 
 not necessarily all quartz ; and it is important to observe 
 that the mica was not made in the sandstone, but, like 
 the quartz, has come from some older rock. 
 
 Quartzite. This rock is simply an unusually hard 
 sandstone. Now the hardness of any rock depends 
 upon two things : ( i ) the hardness of the individual 
 grains or particles ; and (2) the firmness with which 
 they are united one to another. Therefore, the hard- 
 est sandstones must be those in which grains of quarts 
 
78 STRUCTURAL GEOLOGY. 
 
 are combined with an abundant siliceous cement ; and 
 that is precisely what we have in a typical quartzite, 
 such as specimen 33. Quartzite is distinguished, in 
 the hand-specimen, from ordinary quartz by its granu- 
 lar texture (compare specimens 15 and 33) ; and of 
 course in large masses the stratification is an impor- 
 tant distinguishing feature. 
 
 3. Argillaceous group. Just as the conglomerate 
 group shades off gradually into the arenaceous group, 
 so we find it difficult to draw any sharp line of division 
 between the arenaceous group and the argillaceous, 
 but we pass from the largest pebble to the most minute 
 clay-particle by an insensible gradation. For the sake 
 of convenience, however, we draw the line at the limit 
 of visibility, and say that in the true clay and slate the 
 individual particles are invisible to the naked eye ; in 
 other words, these rocks have a perfectly compact tex- 
 ture, while the two preceding groups are characterized 
 by a granular texture. Although clay, like sand and 
 gravel, may be of almost any composition, yet it usually 
 consists chiefly, often entirely, of the mineral kaolin. 
 The reason for this is easily found. Quartz resists both 
 mechanical and chemical forces, ana is rarely reduced 
 to an impalpable fineness ; but all the other common 
 minerals, such as feldspar, hornblende, mica, and cal- 
 cite, on account of their cleavage and inferior hardness, 
 are easily pulverized ; but it is practically impossible 
 that this should happen without their being broken up 
 chemically at the same time. Decomposition follows 
 disintegration ; and, while calcite is completely dis- 
 solved and carried away, the other minerals are reduced, 
 as we have seen, to impalpable hydrous silicates of 
 
LITHOLOGY. 79 
 
 aluminum, /.<?., to kaolin. Hence, we find that the 
 fragmental rocks are composed principally of two min- 
 erals, quartz and kaolin, the former predominating in 
 the conglomerate and arenaceous groups, and the latter 
 in the argillaceous group. 
 
 Clay. That kaolin is the basis of common clay is 
 proved by the argillaceous odor, which is so character- 
 istic of moist clay. Pure kaolin clay is white and im- 
 palpable, like China clay ; but pure clays are the excep- 
 tion. They often become coarse and gritty by admix- 
 ture with sand, forming loam; and they also usually 
 contain more or less carbonaceous matter, which makes 
 black clays ; or more or less ferrous oxide, which 
 makes blue clays ; or more or less ferric oxide, which 
 makes red, brown, and yellow clays. By mixing these 
 coloring materials in various proportions, almost any 
 tint may be explained. Clays are sometimes calcareous, 
 from the presence of shells and shell-fragments or of 
 pulverized limestone. These usually effervesce with 
 acid, and are commonly known as marl. It is the 
 calcareous material in a pulverulent and easily soluble 
 condition that makes the marls valuable as soils. 
 
 Slate. Consolidated clay. The compact texture 
 and argillaceous odor are usually sufficient to prove 
 this. To get the odor we need simply to breathe upon 
 the specimen, and then smell of it. We find all 
 degrees of induration in clay. It sometimes, as every 
 one knows, becomes very hard by simple drying ; but, 
 this is not slate, and no amount of mere drying will 
 change clay into slate ; for, when moistened with water, 
 the dried clay is easily brought back to the plastic 
 state. To make a good slate, the induration must be 
 
80 STRUCTURAL GEOLOGY. 
 
 the result of pressure, aided probably to some extent 
 by heat. True slate, then, is a permanently indurated 
 clay which will not soak up and become soft when wet. 
 
 Slate is usually easily scratched with a knife, and it 
 is distinguished from limestone by its non-effervescence 
 with acid. As we should expect, it shows precisely the 
 same varieties in color and composition as clay. A 
 good assortment of colors is afforded by the roofing- 
 slates. Specimen 34 is a typical slate, for it not only 
 has a compact texture and argillaceous odor, but it is 
 very distinctly stratified. The stratification is marked 
 by alternating bands of slightly different colors, and is 
 much finer and more regular than we usually observe 
 in sandstone, and of course entirely unlike the stratifi- 
 cation of conglomerate. These differences are char- 
 acteristic. Some slates, however, are so homogeneous 
 that the stratification is scarcely visible in small pieces. 
 Thus the roofing-slates (specimen 35) rarely show the 
 stratification ; for it is an important fact that the thin 
 layers into which this variety splits are entirely inde- 
 pendent of the stratification. This is the structure 
 known as slaty cleavage ; it is not due to the stratifica- 
 tion, but is developed in the slate subsequently to its 
 deposition, by pressure. Some roofing-slates, known 
 as ribbon-slates, show bands of color across the flat sur- 
 faces. These bands are the true bedding, and indicate 
 the absolute want of conformity between this structure 
 and the cleavage. Few rocks are richer in fossils than 
 slate, and these prove that it is a stratified rock. 
 Slate which splits easily into thin layers parallel with 
 the bedding is known as shale. 
 
 Porcelainite. This is clay or slate which has been 
 
LITHOLOGY. 8 1 
 
 baked or partially vitrified by heat so as to have the 
 hardness and texture of porcelain. 
 
 2 . CHEMICALLY AND ORGANICALLY FORMED ROCKS. 
 We have already learned that from a geological point 
 of view the differences between chemical and organic 
 deposition are not great, the process being essentially 
 chemical in each case ; and since the limestones and 
 some other important rocks are deposited in both 
 ways, it is evidently not only unnatural, but frequently 
 impossible, to separate the chemically from the organi- 
 cally formed rocks. Unlike the fragmental rocks, the 
 rocks of this division not only admit, but require, a 
 chemical classification. This is natural because they 
 are of chemical origin ; and it is practicable because, 
 with few exceptions, only one class of minerals is de- 
 posited at the same time in the same place, a very 
 convenient and important fact. Therefore our arrange- 
 ment will be mineralogical, thus : 
 
 (1) Coal Group. 
 
 (2) Iron-ore Group. 
 
 (3) Siliceous Group. 
 
 (4) Calcareous Group. 
 
 (5) Metamorphic Group (Silicates). 
 
 Most of the silicate rocks are mixed, *>., are each 
 composed of several minerals ; but some silicate rocks 
 and all the rocks of the other divisions are simple, each 
 species consisting of a single mineral only. 
 
 ( i ) Coal Group. These are entirely of organic 
 origin, and include two allied series, which are always 
 merely the more or less extensively transformed tissues 
 of plants or animals ; viz. : 
 
82 STRUCTURAL GEOLOGY. 
 
 Coals and Bitumens. At the first lesson we exam- 
 ined a sample of peat (specimen 8), and considered 
 the general conditions of its formation, peat being in 
 every instance simply partially decayed marsh vegeta- 
 tion. It was also stated that, as during the lapse of 
 time the transformation becomes more complete, the 
 peat is changed in succession to lignite, bituminous 
 coal, anthracite, and graphite. The coals, indeed, 
 make a very beautiful and perfect series, whether we 
 consider the composition there being a gradual, pro- 
 gressive change from the composition of ordinary 
 woody fibre in the newest peat to the pure carbon in 
 graphite, or the degree of consolidation and mineral- 
 ization since there is a gradual passage from the light, 
 porous peat, showing distinctly the vegetable forms, to 
 the heavy crystalline graphite, bearing no trace of its 
 vegetable origin. This relation is easily appreciated 
 by a child, if a proper series of specimens is presented. 
 The coals also make a chronological series, graphite 
 and anthracite occurring only in the older formations, 
 and lignite and peat in the newer, while bituminous 
 coal is found in formations of intermediate age. 
 
 Bituminous coal is the typical, the representative 
 coal; and from a good specimen of this variety we 
 may learn two important facts : 
 
 (1) That true coals, no less than peat, are of vege- 
 table origin. To see this we must look at the flat or 
 charcoal surfaces of the coal. These soil the fingers 
 like charcoal, and usually show the vegetable forms 
 distinctly. 
 
 (2) That coals are stratified rocks. These dirty 
 charcoal surfaces always coincide with the stratification, 
 
LITHOLOGY. 83 
 
 being merely the successive layers of vegetation de- 
 posited and pressed together to build up the coal ; and 
 when we look at the edge of the specimen the stratifi- 
 cation shows plainly enough. 
 
 The bitumens form a similar though less perfect 
 series, beginning with the organic tissues, and ending, 
 in the opinion of some of the best chemists and miner- 
 alogists, with diamond. In fact the coals and bitumens 
 form two distinct but parallel series. The coals are 
 exclusively of vegetable origin, while the bitumens are 
 largely of animal origin. The organic tissues in which 
 the two series originate are chemically similar, the 
 animal tissues, which produce the lighter forms of bitu- 
 men, however, containing more hydrogen and less 
 carbon and oxygen than vegetable tissues ; while the 
 final terms, as just shown, are probably chemically 
 identical, being pure carbon, graphite for the coals 
 and diamond for the bitumens ; so that the entire pro- 
 cess of change in each series is essentially carboniza- 
 tion, a gradual elimination of the gaseous elements, 
 oxygen and hydrogen, until pure solid carbon alone 
 remains. 
 
 The principal differences between the coals and 
 bitumens are the following : 
 
 Coals are rich in carbon, with some oxygen and little hydrogen. 
 Bitumens are rich in hydrogen, with some carbon and little or 
 
 no oxygen. 
 
 Coals are entirely insoluble. 
 Bitumens are soluble in ether, benzole, turpentine, etc., and the 
 
 solid forms are soluble in the more fluid, naphtha-like varieties. 
 Coals are never liquid, and cannot be melted or, with trifling 
 
 exceptions, even softened by heat. 
 Many bitumens are naturally liquid, and all become so on the 
 
 application of heat. 
 
4 STRUCTURAL GEOLOGY. 
 
 The coals partake of the characteristics of their chief constituent 
 element, carbon, the most thoroughly solid substance known ; 
 while the bitumens similarly show the influence of hydrogen, 
 the most perfectly fluid substance known. 
 
 The two bitumens of the greatest geological impor- 
 tance are asphaltum or mineral pitch and petroleum ; 
 but these substances are too familiar to require any 
 farther description here. 
 
 (2) Iron-ore Group. These interesting and im- 
 portant stratified rocks include the three principal 
 oxides of iron, limonite, hematite, and magnetite, 
 as well as the carbonate cf iron, siderite ; and the 
 rocks have essentially the same characteristics as the 
 minerals. In economical importance they are second 
 only to the coals ; and the history of their formation 
 through the agency of organic matter is one of the 
 most interesting chapters in chemical geology (see 
 page 26). The three oxides are easily distinguished 
 from each other by the colors of their powders or 
 streaks, and the magnetism of magnetite, and from 
 all other common rocks by their high specific gravity. 
 Magnetite is the richest in iron, and limonite the 
 poorest. As regards the degree of crystallization 
 and order of occurrence in the formations, they form 
 a series parallel with the coal series, thus : 
 
 Limonite, never crystalline, and found in recent formations. 
 Hematite, often crystalline, and found in older formations. 
 Magnetite, always crystalline, and found in oldest formations, 
 
 Siderite effervesces with strong acid ; and this sepa- 
 rates it from all other rocks, except limestone and 
 dolomite ; and from these it is distinguished by its 
 
LITHOLOGY. 85 
 
 high specific gravity. As a mineral, siderite is often 
 light colored ; but as a rock it is always dark, and 
 usually black, from admixture chiefly of carbonaceous 
 matter. In studying dynamical geology, we have 
 learned (page 28) tru; reason for the intimate asso- 
 ciation of siderite with beds of coal, and this accounts 
 equally for the carbon contained in the rock itself. 
 The connection of this rock with the coal-formations 
 adds much to its value as an ore of iron. 
 
 Finally, the iron-ores, at least where of much eco- 
 nomical importance, are truly stratified. This can 
 often t>e seen in hand-specimens ; and is well shown 
 by their relations to other rocks, in quarries and 
 mines ; and in many cases, for limonite and hema- 
 tite, by the fossils which they contain. 
 
 (3) Siliceous Group. These rocks are composed 
 of pure silica in the forms of quartz and opal. When 
 first deposited, whether organically, like tripolite, or 
 chemically, like siliceous tufa, the siliceous rocks are 
 soft and light, and the silica is in the form of opal. 
 Subsequently it changes to quartz, and the rocks as- 
 sume the much harder and denser forms of chert and 
 novaculite, respectively. 
 
 Tripolite or Diatomaceous Earth. This interest- 
 ing rock is soft, light, and looks like clay ; but it is 
 lighter, and the argillaceous odor is faint or wanting. 
 It does not effervesce with acid. Hence, it is neither 
 clay nor chalk. Notwithstanding its softness, it is really 
 composed of a hard substance, viz., silica, in the form 
 known as opal. By rubbing off a little of the dust, and 
 examining it- under the microscope, we easily prove 
 that the silica is mainly or entirely of organic origin ; 
 
86 STRUCTURAL GEOLOGY. 
 
 for the dust is seen, to be simply a mass of more or less 
 fragmentary organic remains, occurring in great variety, 
 and of wonderful beauty and minuteness. There are 
 few rocks so unpromising on the exterior, and yet so 
 beautiful within. We have already learned that these 
 organic bodies are principally Diatom cases, Radiolaria 
 shells, and Sponge spicules. We can form some idea 
 of their minuteness from Ehrenberg's estimate that a 
 single cubic inch of pure tripolite contained no less 
 than 41,000,000,000 organisms. 
 
 The lightness of tripolite (sp. gr., 1-1.5) * s due to 
 the facts that opal is a light mineral (sp. gr., 1.9-2.2), 
 and that many of the shells are hollow. Tripolite is a 
 good example of a soft rock composed of a hard min- 
 eral ; and it owes its value as a polishing material to 
 the fact that it consists of a hard mineral in an exceed- 
 ingly fine state of division. Tripolite, when pure, is 
 snow-white ; but it is rarely pure, being commonly 
 either argillaceous or calcareous. This rock is now 
 forming in thousands of places, in both fresh water 
 and the ocean. 
 
 Flint and Chert. During the course of geological 
 time, beds of tripolite are gradually consolidated, chiefly 
 by percolating waters, which are constantly dissolving 
 and re-depositing the silica ; and, finally, in the place 
 of a soft, earthy rock, we get a hard, flinty one, whicn 
 we call flint if it occurs in the newer, or chert if it 
 occurs in the older, geological formations. Besides 
 forming beds of nearly pure silica, which we call tripo- 
 lite, the microscopic siliceous organisms are diffused 
 more or less abundantly through other rocks, especially 
 chalk and limestone. In such cases the consolidatiois 
 
LITHOLOGY. 87 
 
 of the silica implies its segregation also ; /.<?., the silica 
 dissolved by percolating water is deposited only about 
 certain points in the rock, building up rounded con- 
 cretions or nodules. Thus, a siliceous limestone be- 
 comes, by the segregation of the silica, a pure limestone 
 containing nodules of chert, which are usually arranged 
 in lines parallel with the stratification. Specimen 16 
 is a fragment of a flint-nodule from the chalk-forma- 
 tion of England. 
 
 Siliceous Tufa. Hot water, and especially hot alka- 
 line water, circulating through the earth's crust, is 
 always charged with silica dissolved out of the rocks ; 
 and when such water issues on the surface in a hot 
 spring or geyser, it is cooled by contact with the air, 
 its solvent power is diminished thereby, and a large 
 part of the silica is deposited around the outlet as a 
 snowy-white porous material called siliceous tufa. Sil- 
 ica deposited in this way is, like organic silica, always 
 in the form of opal. Siliceous tufa is distinguished 
 from clay, slate, chalk, and limestone by the same 
 tests as tripolite, and from tripolite itself by the ab- 
 sence of microscopic organisms. 
 
 Novaculite. Through the action of percolating 
 water and pressure, siliceous tufa, like tripolite, be- 
 comes harder and denser and is thus changed to 
 novaculite, which holds the same relation to chemi- 
 cally deposited silica that chert and flint do to organi- 
 cally deposited silica. The white novaculite obtained 
 at the Hot Springs of Arkansas, and commonly known 
 as Arkansas stone, is a typical example of this rock. 
 The rock which, on account of the use to which it is 
 put, is known as buhr- stone, is also an excellent exam- 
 
88 STRUCTURAL GEOLOGY. 
 
 pie of chemically deposited silica. It is usually some- 
 what porous and fossiliferous. 
 
 (4) Calcareous Group. These are the lime-rocks ; 
 including the carbonate of lime, in limestone and dolo- 
 mite, the sulphate of lime, in gypsum, and the phos- 
 phate of lime, in phosphate rock. These rocks are 
 even more closely connected in origin than in compo- 
 sition ; and it is for this reason that rock-salt, which 
 of course contains no lime, is also included in this 
 group. Limestones are formed abundantly in the 
 open sea, through the accumulation of shells and 
 corals; but when portions of the sea become de- 
 tached from the main body and gradually dry up, 
 like the Dead Sea and Great Salt Lake, dolomite, 
 gypsum, and rock-salt are deposited in succession as 
 chemical precipitates. Phosphate rock may be re- 
 garded as a variety of limestone, resulting from the 
 accumulation of the skeletons and excrement of the 
 higher animals. 
 
 Limestone. This is the lithologic or rock form 
 of carbonate of lime or calcite, and one of the 
 most important, interesting, and useful of all rocks. 
 Although so simple in composition, calcite bemg 
 the only essential constituent, limestone embraces 
 many distinct varieties, and is really equivalent to a 
 whole family of rocks. A highly fossiliferous lime- 
 stone, such as specimen 38, is, perhaps, the best 
 variety with which to begin the study of the species. 
 The softness of the fossil shells of which the rock is 
 so largely composed, and the fact that they effervesce 
 readily with dilute acid, proves that they are still car- 
 bonate of lime ; and by applying the acid more care- 
 
LIT HO LOGY. 89 
 
 fully, it can be seen that the compact matrix of the 
 rock also effervesces, consisting of shells more finely 
 broken or comminuted and mixed with more or less 
 clay and other impurity, almost the entire rock being 
 of organic origin. 
 
 On the coast of Florida, and in many other places, we 
 find beautiful examples of shell-limestone now in pro- 
 cess of formation. These are at first very open and 
 porous, because the interstices between the nearly entire 
 shells are not yet filled up with smaller fragments and 
 sand. But when that is done, we shall have a rock 
 similar to the old fossiliferous limestone. Specimen 37. 
 
 The shells and fragments, and the grains of calca- 
 reous sand, are, as a rule, quickly cemented together 
 by the deposition of carbonate of lime between them ; 
 so that limestone is nowhere observed occurring abun- 
 dantly in an unconsolidated form. 
 
 Limestone, as a rule, is not distinctly stratified in 
 hand-specimens, but of course that it is a true sedi- 
 mentary rock is abundantly proved by the fossils; 
 and it goes almost without saying that limestone, be- 
 ing necessarily mainly composed of organic remains, 
 must be to a greater extent than any other rock the 
 great store-house of fossils ; and in no other rock are 
 the fossils so well preserved and perfect as in limestone. 
 
 Nevertheless, there are extensive formations of lime- 
 stone containing no discernible traces of fossils. And 
 some of these non-fossiliferous limestones, too, are of 
 very recent formation. Some of the modern coral- 
 reefs, for example, are composed of limestone which 
 was formed only yesterday, as it were, and which, from 
 its mode of formation, must consist entirely of corals ; 
 
90 STRUCTURAL GEOLOGY. 
 
 and yet it shows no trace of its organic origin, but is 
 perfectly compact, or, possibly, crystalline. This fre- 
 quent obliteration of the organic remains, as well as 
 the perfect consolidation of the rock, is attributed to 
 its solubility. The calcium carbonate is gradually dis- 
 solved by the water, and then deposited in the inter- 
 stices in other parts of the rock. 
 
 Specimen 39 is that variety of limestone known as 
 chalk. It is soft and earthy, resembling both clay and 
 tripolite, but differing from the former in lacking the 
 distinct argillaceous odor, and from both by its lively 
 effervescence with acids. It appears to be entirely des- 
 titute of organic remains, but this is a defect of our vis- 
 ion and not of the rock ; for, like the tripolite, it often 
 appears under the microscope to be little else than a 
 mass of shells. Tripolite is a deposit built up of the 
 siliceous shells of Diatoms and Radiolaria, while chalk 
 is chiefly composed of the similar but calcareous shells 
 of Foraminifera. Our specimen is from the Cretaceous 
 formation of England ; but we have good reason to 
 believe that chalk is now forming on a very extensive 
 scale. There are millions of square miles in the deeper 
 parts of the ocean where the dredge brings up little 
 else but a perfectly impalpable, gray, calcareous slime 
 or ooze. When examined microscopically, this is seen 
 to be composed chiefly of Foraminifera shells, and 
 among these the genus Globigerina predominates ; so 
 that the deposit is frequently called Globigerina ooze. 
 Now this gray, calcareous ooze, when dried and com- 
 pacted by pressure, makes a soft, white rock which 
 can scarcely be distinguished from chalk ; in fact, it is 
 a modern chalk. And there seems no good reason 
 
LITHOLOGY. 91 
 
 to doubt that the deposition of chalk has gone on con- 
 tinuously since Cretaceous time for several millions 
 of years at least. 
 
 Specimen 40 is also a white rock, easily scratched 
 with the knife, and effervescing freely with acid, and 
 therefore a variety of limestone. But its texture is very 
 different from the other varieties we have studied. It 
 has a sparkling surface, which we explain by saying that 
 the rock is crystalline. It is, in fact, a mass of minute 
 crystals of calcite. The crystalline limestones have 
 not always been crystalline, but it is safe to assume 
 that they were originally entirely uncrystalline, and in 
 many cases rich in fossils ; but the fossils have been 
 mainly obliterated by the crystallization. 
 
 Crystallization generally in rocks is an indication of 
 great age, so that we usually say crystalline rocks 
 must be older than uncrystalline rocks of the same 
 composition; and this is mainly true with the lime- 
 stones. When the crystallization is rather fine, as in 
 our specimen, resembling granulated sugar, we have 
 what is commonly called saccharoidal limestone. This 
 is the typical marble. Marble is not a scientific name, 
 and the term is usually applied to any calcareous rock 
 which will take a polish, and sometimes even to rocks 
 which are not calcareous at all. 
 
 In the section on dynamical geology, we learned that 
 the carbonate of calcium or calcite is deposited from 
 the sea-water, and limestones formed, in two ways : first, 
 in a purely chemical way, where the water becomes 
 saturated with calcite ; and, second, organically, where 
 the calcium carbonate is taken from the water by ma- 
 rine organisms to form their shells and skeletons, and 
 
92 STRUCTURAL GEOLOGY. 
 
 the gradual accumulation of these on the ocean-floor 
 builds up a limestone. As before stated, the difference 
 between these two methods of deposition is not so 
 great as it often seems, because we know that the ani- 
 mals never make the carbonate of calcium which they 
 secrete, but it comes into the sea ready made with the 
 drainage from the land. 
 
 The limestones forming at the present time are 
 almost wholly organic ; but the rock known as cal- 
 careous tufa is an exception. This is formed under 
 the same general conditions as siliceous tufa, but much 
 more abundantly, and in cold water as well as warm ; 
 because calcite is far more soluble (especially in water 
 containing carbon dioxide) than opal or quartz. It is 
 deposited, not only around the mouths of springs, but 
 also along the beds of the streams which they form, 
 enveloping stones, roots, grasses, etc., and building up 
 usually a loose, spongy mass having a very character- 
 istic turfaceous texture. 
 
 The principal accessory minerals occurring in lime- 
 stone are : ( i ) kaolin, forming argillaceous or slaty lime- 
 stone, which may be recognized by the argillaceous 
 odor and dark color; (2) quartz, forming siliceous 
 or cherty limestone, known by its hardness or by the 
 nodules of flint or chert; (3) dolomite, forming dolo- 
 mitic or magnesian limestone, which effervesces less 
 freely with acid; and (4) serpentine, forming serpen- 
 tinic limestone, which is sharply distinguished by the 
 green grains of serpentine mingled with the white cal- 
 cite. A concretionary texture is common with lime- 
 stone. If the concretions are small, like mustard-seed, 
 we call the rock oolite ; if larger, like peas, pisolite. 
 
LITHOLOGY. 93 
 
 Dolomite. If for calcite, which is the sole essential 
 constituent of all limestone, we substitute the allied 
 mineral dolomite, we have the rock dolomite. As 
 might be inferred from its composition, dolomite is 
 very closely related to limestone, although there are 
 some important differences. Physically, the two rocks 
 differ about as the two minerals do. Dolomite is 
 harder than limestone, and being also less soluble, it 
 resists the action of the weather more. Dolomite, if 
 pure, effervesces feebly, or not at all, with cold dilute 
 acid. Here, however, we have to recognize the fact 
 that dolomite is rarely pure ; but there exists, in conse- 
 quence of the admixture of calcite, a perfectly gradual 
 passage from pure dolomite to pure limestone, and 
 parallel with this every degree of vigor in the reaction 
 with acid. Hence, it is entirely an arbitrary matter 
 as to where we shall draw the line between dolomitic 
 limestone and calcareous dolomite. Dolomite is a very 
 much less abundant rock than limestone, and, unlike 
 limestone, it rarely contains many fossils, and is never 
 of organic origin ; i.e., there are no organisms which 
 secrete the mineral dolomite to form their hard parts 
 or skeletons. Like gypsum and rock-salt, dolomite 
 is probably nevei deposited in the open ocean, but 
 only in closed basins. Like limestone, it occurs with 
 both the compact and the crystalline textures. 
 
 Gypsum. When pure, this rock (specimen 36) is 
 identical with the mineral gypsum (specimen 17), ex- 
 cept that it is rarely crystalline. It is usually, however, 
 not only perfectly compact, but more or less dark- 
 colored from the admixture of clay and other impuri- 
 ties. Its most notable characteristics are its softness, the 
 
94 STRUCTURAL GEOLOGY. 
 
 absence of the argillaceous odor, except where it con- 
 tains much clayey impurity, and its non-effervescence 
 with acids. The first two usually serve to distinguish 
 it from slate, while the acid test separates it readily 
 from limestone and all other carbonate rocks. The 
 deposition of gypsum is purely chemical, and it occurs 
 under about the same physical conditions as the depo- 
 sition of salt ; i.e., in drying-up portions of the sea. 
 Hence we usually find gypsum associated with beds 
 of rock-salt ; and, since drying-up seas are few in 
 number, and small compared with the whole extent 
 of the ocean, we can easily understand why neither 
 rock-salt nor gypsum are abundant rocks, except in 
 a few localities. 
 
 Rock- Salt. This interesting and useful rock, as we 
 have already learned, is deposited in a purely chemical 
 way, and only in drying-up portions of the sea, like the 
 Dead Sea, Great Salt Lake, etc. In some parts of 
 Europe there are beds of solid rock-salt over a hun- 
 dred feet thick. 
 
 Phosphate Rock. Although not specially abundant 
 or attractive, this rock is of great economic interest 
 and importance on account of its extensive use as a 
 fertilizer. Under the general head of phosphate rock 
 are included : ( i ) the typical guano, which is the 
 consolidated excrement of certain marine birds in- 
 habiting in great numbers small coral islands in the 
 dry or rainless regions of the tropics ; (2) the under- 
 lying coral rock, which is often changed to phosphate 
 rock through the percolation of the rain-water falling 
 on the guano ; (3) accumulations of the bones and 
 coprolites of the higher animals ; (4) phosphatic lime- 
 
LITHOLOGY. '- 95 
 
 Stones from which the carbonate of lime has been 
 Jargely dissolved away, leaving the more insoluble 
 phosphate of lime. 
 
 (5) Metamorphic Group (stratified silicates). 
 All the chemically and organically formed rocks which 
 we have studied up to this point are simple, i.e., 
 they consist each of only one essential mineral ; but 
 most of the rocks in this great group of silicates are 
 mixed, or consist each of several essential minerals. 
 Quartz is the only important constituent of these rocks 
 which is not, strictly speaking, a silicate, but in a cer- 
 tain sense it is also not an exception, since it may 
 always be regarded as an excess of acid in the rock. 
 
 This group of stratified rocks composed of silicate 
 minerals is of exceptional importance, first, on account 
 of the large number of species which it includes, and, 
 second, on account of the vast abundance of some of 
 the species. These are, above all others, the rocks of 
 which the earth's crust is composed. With unimpor- 
 tant exceptions, all the rocks of this group are crystal- 
 line ; and they constitute the principal part of what is 
 generally included under the term metamorphic rocks 
 a general name for all stratified rocks which have been 
 so acted upon by heat, pressure, or chemical forces as 
 to make them crystalline. Although the crystalline 
 limestone, dolomite, iron-ores, etc., show us that meta- 
 morphic rocks are not wanting in the other groups. 
 
 As already explained, the metamorphic or crystalline 
 stratified rocks are usually older than the corresponding 
 uncrystalline rocks ; but a point of greater importance 
 here is this : the development in the silicate rocks of 
 crystalline characters has usually made it impossible to 
 
96 STRUCTURAL GEOLOGY. 
 
 determine the method of their deposition, whether 
 mechanical or chemical. In a few cases, as with the 
 rock greensand, we know that the deposition is chemi- 
 cal; while it is equally certain that such common 
 silicate rocks as gneiss, mica schist, and many others, 
 often result from the crystallization of ordinary me- 
 chanical sediments, like sandstone and conglomerate. 
 We classify all these rocks as of chemical origin, how- 
 ever, without considering the mode of their deposition, 
 because the subsequent crystallization is itself essen- 
 tially a chemical process ; and that justifies us in saying 
 that these rocks are made what they now are chiefly 
 by the action of chemical forces. Whatever they were 
 originally, they have become, through their crystalli- 
 zation, rocks having a definite mineral composition 
 which can be classified chemically. 
 
 Some of the details of the classification of this group, 
 as shown in the table, require explanation. In study- 
 ing the silicate minerals it was stated to be important 
 to recognize two classes the acidic and the basic 
 the dividing line falling in the neighborhood of 60 per 
 cent of silica. This division is important simply be- 
 cause Nature has in a great degree kept the acidic and 
 basic minerals separate in the rocks ; and few things in 
 lithology are more important than the distinction of the 
 silicate rocks in which acidic minerals predominate 
 from those in which basic minerals predominate. The 
 amount of silica which any rock of this group contains 
 is shown at a glance by the chart. The vertical broken 
 lines, with the figures at the top, indicate the propor- 
 tion of silica, which increases from 30 per cent on the 
 right to 85 per cent on the left ; so that the percentage 
 
LTTHOLOGY. 97 
 
 of silica which a rock contains determines its position, 
 the acidic species being on the left, and the basic on 
 the right. As most of these rocks are composed of 
 two or more minerals mixed in very various proportions, 
 there is usually a wide range in the percentage of silica 
 which the same species may contain ; and this is ex- 
 pressed in each case by the length of the dotted line 
 under the name of the rock. Thus, in syenite, the sil- 
 ica ranges from 55 per cent, to 65 per cent. The hori- 
 zontal line in the chart separates the gneisses, containing 
 feldspar as an essential constituent, from the schists, in 
 which feldspar is wanting, except as an accessory con- 
 stituent. We will take up the gneisses first. 
 
 Gneiss. This is the most important of all rocks. 
 It forms not far from one-half of New England, and a 
 very large proportion of the earth's crust The name 
 (pronounced same as nice) is known to have origi- 
 nated among the Saxon miners, but its precise deriva- 
 tion is lost in obscurity. To find out what this very 
 important rock is, we will consult specimen 41. The 
 first glance shows us that it is not, like the rocks we 
 have just been studying, composed of a single mineral, 
 but of several minerals, the most conspicuous of which 
 is the pink feldspar orthoclase. This we recognize 
 as a feldspar : (i) by its hardness, which is a little less 
 than that of quartz, and distinguishes it from calcite, 
 a mineral having the general appearance of feldspar ; 
 (2) by its color, which separates it from hornblende 
 and augite ; and (3) by its cleavage, which distinguishes 
 it easily from quartz. Finally, we know it is orthoclase, 
 and not plagioclase, by its general aspect, and by its 
 association with an abundance of quartz, which is the 
 
98 STRUCTURAL GEOLOGY. 
 
 next most important constituent of the rock. The 
 quartz is less abundant than the orthoclase, and more 
 easily overlooked, yet anyone familiar with the min- 
 eral will not fail to recognize it. It forms small, 
 irregular, glassy grains, entirely devoid of cleavage, and 
 scratching glass easily. On weathered surfaces of the 
 rock the orthoclase becomes soft and chalky, while the 
 quartz remains clear and hard, and then the two min- 
 erals are very easily distinguished. Besides these, 
 there are numerous black, thin, glistening scales, which 
 we can easily prove to be elastic, and recognize as 
 mica. 
 
 In most books on the subject, these three minerals 
 orthoclase, quartz, and mica are set down as the 
 normal or essential constituents of gneiss. But it is 
 now recognized by the best lithologists that we may 
 have true gneiss without any mica; or we may have 
 hornblende in the place of mica. Quartz and ortho- 
 clase are the only essential constituents of gneiss ; and 
 when these alone are present, we have the variety 
 known as binary gneiss. The addition to these essen- 
 tial constituents of mica, gives micaceous gneiss ; and 
 of hornblende, hornblendic gneiss. Of these three 
 principal varieties, the micaceous gneiss is by far the 
 most common and important. The mica may be 
 either the white species, muscovite, or the black spe- 
 cies, biotite ; but it is usually the former. 
 
 Orthoclase is the predominant constituent in all typi- 
 cal gneiss, usually forming at least one-half of the rock. 
 The orthoclase may, however, be replaced to a greater 
 or less extent by albitq, or even by oligoclase. But we 
 frequently see the term gneiss carelessly, or ignorantly, 
 
LITHOLOGY. 99 
 
 applied to rocks which are destitute of feldspar, though 
 having the general aspect of gneiss. 
 
 Augite rarely occurs in gneiss ; and hence, when we 
 observe a gneiss containing a black mineral which we 
 know is either augite or hornblende, it is pretty safe to 
 call it the latter. 
 
 Mica and hornblende, although the principal, are 
 not the only, accessory minerals in gneiss ; but the fol- 
 lowing species are also of common occurrence : garnet, 
 cyanite, tourmaline, fibrolite, epidote, and chlorite. 
 Gneisses, as the table indicates, exhibit a wide range 
 in the proportion of silica which they contain, varying 
 from 60 to 85 per cent. ; and there is a concomitant 
 variation in specific gravity, from about 2.5 in the most 
 acidic to 2.8 in the most basic varieties. 
 
 That gneiss is a true, stratified rock is very clearly 
 shown in specimen 41 ; but, unfortunately, the stratifi- 
 cation is not always so evident as in this case. The 
 mica-scales, it will be observed, lie parallel with the 
 stratification, and assist very materially to make it visi- 
 ble ; and gneisses containing little or no mica, as well 
 as some that are rich in mica, frequently appear almost 
 or quite unstratified. These obscurely stratified varieties 
 are commonly known as granitoid gneiss, having the 
 texture and general aspect of granite. The sediment- 
 ary origin of gneiss is also clearly proved by its inter- 
 stratification with undoubted sedimentary rocks, such 
 as limestone, iron-ores, graphite, quartzite, etc. 
 
 Syenite. This is a much abused term, but, as now 
 employed by the best lithologists, it is the name of a 
 rock having a single essential constituent, viz., ortho- 
 clase. Syenite in its simplest variety contains nothing 
 
100 STRUCTURAL GEOLOGY. 
 
 but orthoclase ; but in addition we usually have either 
 hornblende, forming hornblendic syenite, or mica, 
 forming micaceous syenite. 
 
 Syenite, it will be observed, is equivalent to gneiss 
 with the quartz removed ; but, while gneiss is the most 
 abundant of all rocks, syenite is a comparatively rare 
 rock ; and this is simply another way of saying that 
 nearly all orthoclase is associated with quartz. By 
 admixture of quartz we get a perfectly gradual passage 
 from syenite to gneiss. The orthoclase in syenite is 
 more frequently replaced by plagioclase than it is in 
 gneiss. In syenite, too, hornblende is much more 
 abundant than mica ; although just the opposite is 
 true in gneiss. And, again, in gneiss the mica is prin- 
 cipally muscovite ; but in syenite it is almost exclusively 
 biotite. Augite is a common accessory in the more 
 basic syenite ; but garnet, tourmaline, and the other 
 accessory minerals, occurring so frequently in gneiss, 
 are almost unknown in syenite. The specific gravity 
 of syenite varies from 2.7 to 2.9. 
 
 Diorite. This is a more important rock than sye- 
 nite ; but it is of analogous, though more basic, com- 
 position, containing a single essential constituent, viz., 
 plagioclase. Any of the triclinic feldspars may occur 
 in this rock, but oligoclase is most common. Like 
 syenite, diorite usually contains hornblende, often in 
 large proportion, forming hornblendic diorite, which 
 sometimes passes into rocks composed entirely of 
 hornblende. It also, but less frequently, contains 
 mica, forming micaceous diorite. The mica is usually 
 biotite, rarely muscovite. Mica and hornblende also 
 often occur together in diorite, and the same is true of 
 
LITK&LOC* 101 
 
 syenite and gneiss. Quartz is of common occurrence 
 in the more acidic varieties of diorite, and augite in the 
 more basic. 
 
 This is a good example of a basic rock, for all its 
 normal constituents are basic ; but the percentage of 
 silica varies from 45 in those varieties richest in labra- 
 dorite and augite to 60 or more in those containing 
 more or less quartz and orthoclase. There is a corre- 
 sponding change of color from dark to light, and of 
 specific gravity from 2.7 to 3.1. 
 
 Diorite is not rich in accessory minerals ; besides 
 those already mentioned, the most important are chlo- 
 rite, epidote, pyrite, and magnetite. 
 
 Few rocks are more clearly stratified than diorite, 
 whether we consider the hand-specimen, or its relations 
 to other formations. It is an abundant rock in New 
 England. 
 
 Norite. Like diorite, this is essentially a plagio- 
 clase rock ; but there are, nevertheless, important dif- 
 ferences. The plagioclase in diorite is mainly the 
 more acidic species, like oligoclase ; while in norite 
 the more basic species, such as labradorite and an- 
 orthite, predominate. Hornblende, which we have 
 observed to be an important and rather constant con- 
 stituent of diorite and syenite, is much less abundant 
 in norite ; but its place is taken by augite and the 
 allied minerals, hypersthene and enstatite. Black mica 
 is common in norite ; but white mica, orthoclase, and 
 quartz rarely occur. 
 
 Norite is the most basic of all the feldspathic rocks, 
 as gneiss is the most acidic ; while syenite and diorite 
 stand as connecting links, forming a gradual passage 
 
102 STRUCTURAL GEOLOGY. 
 
 between the two extremes. Thus, in passing from 
 gneiss to norite, we have observed a gradual diminu- 
 tion of the quartz, a gradual change in feldspar from 
 orthoclase to the most basic plagioclase ; at first a 
 gradual increase in hornblende, and then a gradual 
 change from hornblende to augite ; and, finally, a 
 gradual substitution of black mica for white. The 
 amount of silica has decreased over 40 per cent. ; and 
 the specific gravity has increased from 2.5 in the light- 
 est gneiss to at least 3.2 in the heaviest norite. We 
 have also passed from light colored rocks to dark ; 
 and from those resisting atmospheric action to those 
 easily decomposed. 
 
 The most characteristic accessory constituents of 
 norite, besides those already mentioned, are magnetite 
 and chrysolite ; though garnet, serpentine, and pyrite 
 often occur. In texture, this rock varies from compact 
 to very coarsely crystalline. The specimen of labra- 
 dorite (No. 23), from the norite of Labrador, affords 
 some idea of the coarseness of the crystallization in 
 much of this rock. It is not a common rock, except 
 in certain regions, the best known of which in eastern 
 North America are the coast of Labrador, various 
 points in Canada north of the St. Lawrence, and the 
 eastern border of the Adirondack Mountains. In hand- 
 specimens, norite rarely appears stratified ; but in the 
 solid ledges the stratification is often as distinct as 
 could be desired. 
 
 Many lithologists call the rocks here designated 
 norite gabbro, and class them all in the eruptive 
 division as essentially a coarse variety of diabase. 
 
LITHOLCGY. 103 
 
 In a similar manner, diorite and syenite are denied a 
 place in the sedimentary series. But the stratified 
 plagioclase rocks seem to have as strong a claim to 
 recognition as gneiss. 
 
 We turn now to the important and interesting divi- 
 sion of the non-feldspathic rocks or schists, 
 
 Mica Schist. This is, next to gneiss, the most 
 abundant rock in New England. Specimen 43 is a 
 typical example, and from it we can readily learn what 
 mica schist is. A glance suffices to show that it is 
 chiefly composed of mica, but not entirely; for, on 
 carefully examining the edges of the specimen, we can- 
 not fail to see thin layers of hard, glassy quartz inter- 
 woven with the mica. The quartz layers are short and 
 overlapping, and we have here a good illustration of 
 the schistose texture ; this is, in fact, a typical schist. 
 
 Mica schist usually consists, as in this instance, of 
 mica and quartz ; but it may be composed of mica 
 alone ; and sometimes kaolin or clay takes the place 
 of the quartz, forming argillaceous mica schist. The 
 mica in the latter is usually in very fine scales and 
 rather inconspicuous, and the rock often passes into 
 ordinary clay slate. Similarly, when the mica becomes 
 deficient in the quartzose mica schist, a passage into 
 ordinary quartzite is the result. A little feldspar is 
 sometimes present in the rock, which thus passes into 
 micaceous gneiss. Specimen 43 contains several crys- 
 tals of red garnet, giving the variety garnetiferous mica 
 schist. There is no other rock that contains such a 
 large variety of beautiful accessory minerals as mica 
 schist ; and for the mineralogist it is one of the most 
 attractive rocks. Few rocks are more distinctly strati- 
 
104 STRUCTURAL GEOLOGY. 
 
 fied ; and the stratification can usually be observed in 
 hand-specimens. The mica in these rocks may be 
 either muscovite or biotite, or both ; but the former is 
 most common. No rock shows a greater variation in 
 the percentage of silica which it contains than mica 
 schist, as we pass from varieties which are nearly all 
 quartz to those which are nearly all mica. 
 
 Closely related to mica schist is the rock now known 
 as hydromica schist, in which the ordinary anhydrous 
 micas are replaced by hydromica. It is distinguished 
 from mica schist by being somewhat softer, less harsh 
 to the touch, and less lustrous. It is to be regarded 
 usually as an incipient mica schist, which has not yet 
 become anhydrous ; though it may sometimes be just 
 the reverse ; viz. : an old mica schist which has become 
 hydrated through the action of meteoric waters. It 
 contains fewer accessory minerals than mica schist. 
 
 Hornblende Schist. This is a stratified aggregate 
 of hornblende and quartz. The quartz is granular and 
 in thin layers, as in mica schist; but the micaceous 
 structure is wanting, and consequently the rock does 
 not cleave readily in the direction of the bedding. 
 The hornblende is mostly finely crystalline, but some- 
 times occurs in large, bladed crystals. Garnet and 
 some other minerals are of common occurrence in the 
 rock ; but it is not rich in accessories like mica schist. 
 The chief difficulty in recognizing this rock consists in 
 determining whether the white mineral is all quartz or 
 partly feldspar. In the latter case, of course, it be- 
 comes a hornblendic gneiss. 
 
 Amphibolite (Hornblende Bock). This is the name* 
 applied to a rock having hornblende as its sole essen 
 
LITHOLOGY. 105 
 
 tiai constituent. Hornblende schist sometimes passes 
 into amphibolite, through the absence of quartz ; and so 
 does diorite, when the feldspar is deficient or wanting. 
 Specimen 20, though small, is a typical example of 
 this rock. The physical and chemical characteristics 
 are essentially the same as for the mineral hornblende. 
 The texture varies from coarsely to finely crystalline. 
 The crystals are usually short and thick, and lie in all 
 directions in the rock, which is thus very massive, the 
 schistose texture being entirely wanting, and the strati- 
 fication rarely showing in small masses. Biotite is a 
 common accessory in amphibolite, and garnet and 
 magnetite frequently occur. 
 
 By the substitution of augite for hornblende, in the 
 description of amphibolite, we get the much rarer, but 
 otherwise very similar, rock, pyroxenite. 
 
 Talc Schist (Steatite or Soapstone). Although not 
 abundant, this is a useful and familiar rock. The 
 composition is implied in the name ; and by comparing 
 it with the specimen of talc (No. 58) we can readily 
 see that they are essentially identical. Typical talc 
 schist is pure talc ; but the talc is often mixed with 
 more or less quartz or feldspar; and mica, chlorite, 
 hornblende, garnet, and other minerals are of common 
 occurrence. 
 
 This rock embraces two distinct varieties, the mas- 
 sive and the schistose, or foliated. The former is the 
 common soapstone (specimen 71), which is a con- 
 fused mass of crystals lying in all directions, and with 
 no visible stratification in the small mass. In the latter, 
 as in specimen , the talc scales lie in parallel planes, 
 giving the rock a micaceous structure, and causing it 
 
106 STRUCTURAL GEOLOGY. 
 
 to split easily in the direction of the stratification. The 
 cleavage surfaces are often wavy or corrugated ; and 
 the same is true of all schistose rocks. Talc schist is 
 easily distinguished from all other rocks by its light- 
 grayish or greenish color, combined with its extreme 
 softness, and its smooth, slippery feel. 
 
 Chlorite Schist. The one essential constituent of 
 this rock is chlorite, and the mineral specimen (No. 26) 
 answers equally well as an example of the rock. As 
 with talc schist, quartz, feldspar, and hydromica are 
 rarely entirely absent. Besides these, the principal 
 accessories are hornblende, magnetite, garnet, and 
 epidote. This rock also agrees with talc schist in pre- 
 senting two principal varieties, the massive and the 
 schistose. It is easily distinguished -from talc schist by 
 its darker color and streak, which are very character- 
 istic ; while its green color, softness, and unctuous feel 
 separate it from all other rocks. 
 
 This is the most basic of all the silicate rocks ; but, 
 in consequence of containing a large proportion of 
 water, it is not the heaviest. It is, in fact, interesting 
 and important to observe that all these hydrous silicate 
 rocks talc schist, chlorite schist, green sand, and 
 serpentine are distinctly lighter in each case than 
 anhydrous rocks containing the same proportion of 
 silica. They are also notable, as a class, for their soft- 
 ness, smooth feel, and green color. 
 
 Serpentine. As the name implies, this rock is simply 
 the mineral serpentine occurring in large masses, and 
 its characteristics are precisely the same. It is fine- 
 grained, massive, compact, rather soft, but very tough, 
 and varies in color from very dark green to light 
 
LITHOLOGY. 107 
 
 greenish-yellow. The dark colors predominate, and 
 specimen 25 is a typical example. 
 
 Serpentine is often intimately associated with lime- 
 stone and dolomite. The white veins running irreg- 
 ularly through* the variety known as Verd Antique 
 Marble, however, are not calcite, as commonly sup- 
 posed, but magnesite. They do not effervesce freely 
 with cold, dilute acid, for the entire rock is magnesion, 
 and it is probable have been at one time simply cracks 
 along which water holding carbon dioxide has pene- 
 trated, changing the magnesia from a silicate to a 
 carbonate. 
 
 Geologists were, at one time, almost unanimous in 
 the opinion that all serpentine is of eruptive origin ; 
 but now it .is conceded by the great majority to be in 
 some cases a sedimentary rock. It is found interstrati- 
 fied with gneiss, limestone, all the schists, and many 
 other stratified rocks. When occupying the position 
 of an eruptive it is never an original rock ; but has 
 been formed by the alteration, in situ, of some basic 
 anhydrous rock, most commonly olivine basalt. 
 
 Greensand. This rock (specimen 27) consists 
 chiefly of the mineral glauconite, mingled usually with 
 more or less sand, clay, or calcareous matter. It is 
 usually very friable, or in an entirely unconsolidated 
 state. It is most abundant in the newer geological 
 formations, especially the Cretaceous and Tertiary ; and 
 is, perhaps, the only one of the stratified silicate rocks 
 now forming on an extensive scale in the ocean. Its 
 value as a fertilizer, for which purpose it is extensively 
 employed, is due to the potash that it contains. 
 
 Following is a systematic summary of the mineral- 
 
io8 
 
 STRUCTURAL GEOLOGY. 
 
 ogical composition of the rocks of this great division 
 of silicates; and this, combined with the classification 
 on page 70, presents in a condensed form all the more 
 important facts contained in the preceding descriptions. 
 Only the more constant and normal constituents of the 
 species are enumerated in each case : 
 
 Names of Species. 
 
 Constituent Minerals. 
 
 Gneiss .... -I 
 
 Orthoclase and Quartz. 
 Orthoclase, Quartz, and Mica. 
 Orthoclase, Quartz, and Hornblende. 
 
 Syenite . . . . ) 
 
 Orthoclase. 
 Orthoclase and Hornblende. 
 Orthoclase and Mica. 
 
 Diorite ....-! 
 
 Plagioclase (chiefly Oligoclase). 
 Plagioclase and Hornblende. 
 Plagioclase and Mica. 
 
 Norite . . . . -j 
 
 Plagioclase (chiefly Labradorite). 
 Plagioclase and Augite (Diallage). 
 Plagioclase and Mica. 
 
 Mica Schist . . ) 
 
 Mica. 
 Mica and Quartz. 
 Mica and Kaolin. 
 
 Hornblende Schist . 
 
 Hornblende and Quartz. 
 
 Amphibolite . . . 
 Pyroxenite .... 
 
 Hornblende. 
 Pyroxene. 
 
 Talc Schist . . . 
 
 Talc. 
 
 Chlorite Schist . . 
 
 Chlorite. 
 
 Serpentine . . . 
 
 Serpentine. 
 
 Greensand . . . 
 
 Glauconite. 
 
LITHOLOGY. 109 
 
 2. Eruptive or Unstratified Rocks. 
 
 The rocks of this great class are formed by the cool- 
 ing and solidification of materials that have come up 
 from a great depth in the earth's crust in a melted and 
 highly heated condition. When the fissures in the 
 earth's crust reach down to the great reservoirs of 
 liquid rock, and the latter wells up and overflows on 
 the surface, forming a volcano, then we may, as was 
 pointed out on page 33, divide the eruptive mass into 
 two parts : first, that which has actually flowed out on 
 the surface, and cooled and solidified in contact with 
 the air, forming a lava flow ; second, that which has 
 failed to reach the surface, but cooled and solidified 
 in the fissure, forming a dike. 
 
 Lava flows or volcanic rocks and dikes or plutonic 
 rocks are identical in composition j but there is a vast 
 difference in texture, due to the widely different con- 
 ditions under which the rocks have solidified. The 
 dike or fissure rocks solidify under enormous pressure, 
 and this makes them heavy and solid free from pores. 
 They are surrounded on all sides by warm rocks : this 
 causes them to cool very slowly, and allows the various 
 minerals time to crystallize. Other things being equal, 
 the slower the cooling the coarser the crystallization ; 
 and hence, the greater the depth below the surface at 
 which the cooling takes place, the coarser the crystal- 
 lization. 
 
 The volcanic rock, on the other hand, cools under 
 very slight pressure ; and the steam, which exists abun- 
 dantly in nearly all igneous rocks at the time of their 
 eruption, is able to expand, forming innumerable small 
 
HO STRUCTURAL GEOLOGY. 
 
 vesicles or bubbles in the liquid lava ; and these remain 
 when it has become solid. Cooling in contact with the 
 air, the lava cools quickly, and has but little chance 
 for crystallization. Hence, to sum up the matter, we 
 3ay : plutonic rocks are solid and crystalline ; and vol- 
 canic rocks are usually porous or vesicular, and 
 uncrystalline. 
 
 As we descend into the earth's crust, it is perfectly 
 manifest that the volcanic must shade off insensibly 
 into the dike rocks, and we find it impossible to draw 
 any but an arbitrary plane of division between them ; 
 but this is no argument against this classification, for, 
 as already stated, all is gradation in geology, and we 
 experience just the same difficulty in drawing a line 
 between conglomerate and sandstone, or between gneiss 
 and mica schist, as between the dike rocks and volcanic 
 rocks. 
 
 We will now observe to what extent the distinc- 
 tions between these two great classes of eruptives can 
 be traced in the rocks themselves, beginning with the 
 dike rocks. But first it is important to notice the 
 general fact, clearly expressed in the classification, that, 
 with perhaps some trifling exceptions which need not 
 be mentioned here, all eruptive rocks are silicates, and 
 nearly all are feldspathic silicates. They are of definite 
 mineralogical composition, and, like the chemically and 
 organically formed stratified rocks, can be classified 
 chemically. But, although there are eruptives corres- 
 ponding closely in composition to the feldspathic sili- 
 cates, which we have just studied, we find among them 
 little to represent the non-feldspathic silicates, and 
 nothing corresponding in composition to the lime- 
 
LITHOLOGY. ill 
 
 stones, dolomites, gypsum, flint, tripolite, siliceous tufa, 
 iron-ores, bitumens or coals. 
 
 i. PLUTONIC (DIKE) ROCKS. These are also known 
 as the ancient eruptive rocks, and for this reason : It 
 is impossible, of course, for us to observe them except 
 where they occur on or near the earth's surface. But, 
 since they are formed wholly below the surface, and 
 usually at great depths in the earth, it is evident that 
 they can appear on the surface only as the result of 
 enormous erosion ; and erosion is a slow process, de- 
 manding, in these cases, many thousands or millions 
 of years. Therefore, the more ancient dike rocks alone 
 are within our reach ; those of recent formation being 
 still deeply buried in the earth's crust. It follows, as 
 a corollary to this explanation, that the coarseness of 
 the crystallization of any dike rock must be a rough 
 measure of its age and of the amount of erosion which 
 the region has suffered since its eruption. 
 
 As regards composition, the dike rocks present, as 
 already stated, essentially the same combinations of 
 minerals as the feldspathic silicates of the stratified 
 series, but occurring under different physical condi- 
 tions and having a widely different origin. The only 
 important difference in texture between the two classes 
 of rocks is that the sedimentary rocks are stratified 
 and the dike rocks are not ; and when we consider 
 that the dike rocks sometimes present a laminated 
 structure that resembles stratification, while the sedi- 
 mentary rocks frequently appear unstratified, it is easy 
 to understand why, in the absence of any marked dif- 
 ference in composition, geologists have often found it 
 difficult to distinguish the two classes of rocks. We 
 
112 STRUCTURAL GEOLOGY. 
 
 also find here the explanation and the justification of 
 the fact that the names of the dike rocks are in most 
 cases the same as those of the sedimentary rocks of 
 similar composition. 
 
 Granite. Granite (from the Latin granum, a grain) 
 is a crystalline-granular rock, agreeing in composition 
 with gneiss. The essential constituents are quartz and 
 orthoclase ; and when they alone are present we have 
 the variety binary granite. Mica, however (commonly 
 muscovite, sometimes biotite, and frequently both) is 
 usually added to these, forming micaceous granite 
 (specimen 44) ; and often hornblende, forming horn- 
 blendic granite (specimen 45). The orthoclase is 
 sometimes replaced in part by triclinic species, espe- 
 cially albite and oligoclase. Accessory minerals are not 
 so abundant in granite as in gneiss ; but, besides those 
 named, garnet, tourmaline, pyrite, apatite, and chlorite 
 are most common. Orthoclase is always the predomi- 
 nant ingredient ; and, except when there is much horn- 
 blende present, usually determines the color of the 
 granite. Thus, specimens 44 and 45 are gray because 
 they contain gray orthoclase ; while all red granites 
 contain red or pink orthoclase. The quartz has usually 
 been the last of the constituents to crystallize or solidify ; 
 and, having been thus obliged to adapt itself to the con- 
 tours of the orthoclase and mica, it is rarely observed 
 in distinct crystals. 
 
 In texture, the granites vary from perfectly compact 
 varieties, approaching petrosilex, to those which are 
 so coarsely crystalline that single crystals of ortho- 
 clase measure several inches in length. Of course 
 one of the most important things to be observed 
 
LITHOLOGY. 113 
 
 about granite, especially in comparing it with gneiss, is 
 the complete absence of anything like stratification ; 
 that, as before stated, being the only important dis- 
 tinction between the two rocks. Gneiss is the most 
 abundant of all stratified rocks, and granite stands in 
 the same relation to the eruptive series. 
 
 Syenite. This is an instance where stratified and 
 eruptive rocks, agreeing in composition, have the same 
 name. That rocks consisting of orthoclase, of ortho- 
 clase and hornblende, or of orthoclase and mica, /.<?., 
 having the composition of syenite, do occur in both 
 the eruptive and stratified series there can be no doubt. 
 They should, however, have distinct names on account 
 of their unlike origins; and would have but for the 
 practical difficulty in determining, in many cases, 
 whether the rock is stratified or not. The best that 
 we can do now, when we desire to be specific, and 
 have the necessary information, is to say stratified 
 syenite or eruptive syenite, as the case may be. 
 
 Diorite. Here, again, we find identity of names, 
 as well as of composition, between the two great series. 
 Eruptive diorite is an abundant and well known rock, 
 and consists of the same minerals as stratified diorite 
 combined in the same proportions. Diorite includes 
 a large part of the dike rocks commonly known as 
 " trap " and " greenstone." The principal accessories 
 are chlorite, epidote, pyrite, magnetite, apatite, and 
 quartz. The texture varies from perfectly compact or 
 felsitic to coarsely crystalline ; averaging, however, less 
 coarse than syenite and granite. 
 
 Diabase. By referring to the classification it will be 
 seen that diabase occupies the same position among 
 
H4 STRUCTURAL GEOLOGY. 
 
 the dike rocks as norite among the stratified rocks 
 Like norite it consists usually of the more basic varie- 
 ties of plagioclase with or without augite, diallage, 01 
 hypersthene. Augite, or one of its representatives, is 
 usually present, and is often the principal constituent. 
 Specimen i shows a somewhat equal development of 
 the feldspar and augite. The name gabbro is some- 
 times applied to the coarser and more feldspathic dia- 
 bases, and especially to those containing diallage or 
 hypersthene in the place of common augite. In the 
 opinion of some high authorities, however, it is un- 
 necessary to recognize two species here ; and it makes 
 the classification more simple and symmetrical not to 
 do it. The principal accessories in diabase are biotite, 
 chlorite, magnetite, pyrite, calcite, and olivine. Chlo- 
 rite is often an important constituent, giving the rock 
 a greenish aspect ; but here, as well as in diorite, the 
 chlorite is due chiefly or entirely to the alteration of 
 the augite and feldspar ; and the chloritic varieties of 
 diorite and diabase together make up the old species 
 " greenstone." Similarly, the more compact and darker 
 varieties of these two rocks, forming regular, wall-like 
 dikes, are known as " trap." Specimen 46. 
 
 In consequence of their more basic composition, 
 diabase and diorite are usually strongly contrasted with 
 granite and syenite in color and specific gravity, being 
 darker and heavier. The basic rocks, too, decay much 
 more readily than the acidic. 
 
 *- 2. VOLCANIC ROCKS. As regards composition, we 
 shall find nothing new in the volcanic series ; for the 
 rocks of this group present essentially the same com' 
 bination of minerals as the dike rocks. In composi 
 
LITHOLOGY. 115 
 
 tion, the dike and volcanic rocks are identical ; but in 
 texture, as already explained, there is a vast difference. 
 The volcanic rocks differ so widely in texture from both 
 the dike and stratified species, that there is rarely any 
 difficulty in distinguishing them ; and hence they have 
 in every instance distinct names. 
 
 Volcanic rocks are rarely found in this part of the 
 world ; and specimens of most of them are difficult 
 to obtain. For this reason they can only be noticed 
 briefly here, since it is the plan of this Guide to give 
 especial attention only to those portions of the subject 
 which can be illustrated by material within easy reach 
 of teachers. 
 
 Rhyolite. This rock corresponds in composition 
 with granite and gneiss, but is less frequently micaceous. 
 The orthoclase in rhyolite, and generally in volcanic 
 rocks, is the clear, pellucid variety sanidine. It is 
 more difficult to separate from quartz than ordinary 
 orthoclase, the chief distinguishing feature being its 
 cleavage. Plagioclase and hornblende are common, 
 but not abundant, constituents. The mica, when pres- 
 ent, is usually biotite. The texture of rhyolite is often 
 more or less distinctly porphyritic, having a finely crys- 
 talline or granular matrix, with interspersed crystals of 
 sanidine and quartz. The rock has usually a rough, 
 harsh feel ; and while the coarser varieties have the 
 aspect of granite, the finer approach petrosilex; but 
 all are somewhat porous, which is seen in the lower 
 specific gravity of rhyolite as compared with granite 
 and gneiss. 
 
 Trachyte. In texture and general aspect rhyolite 
 and trachyte are nearly identical. Trachyte, however,, 
 
Il6 STRUCTURAL GEOLOGY. 
 
 is darker, contains little or no quartz, and more horn- 
 blende and plagioclase. In fact, it agrees in composi- 
 tion with syenite. This is one of the most important 
 of the volcanic rocks. 
 
 Obsidian. Obsidian is sharply distinguished from 
 all other rocks by its perfect vitreous texture ; it is a 
 true volcanic glass. Its surface (specimen 47) is smooth 
 and glassy, and its fracture eminently conchoidal. To 
 the naked eye, and usually under the microscope, the 
 typical variety is perfectly homogeneous ; chemical 
 analysis, however, shows that it has the composition, 
 commonly of rhyolite, but sometimes of trachyte. 
 Obsidian is, in fact, simply rhyolite or trachyte which, 
 cooling quickly, has not had time to crystallize, but 
 has remained permanently in the amorphous or glassy 
 state. The composition is sometimes partially revealed 
 where a portion of the sanidine comes out in distinct 
 crystals porphyritically interspersed through the glass. 
 The homogeneity of the texture is sometimes disturbed: 
 by numerous minute concentric cracks, forming what 
 is known as perlitic structure and the variety perlite ; 
 by numerous small spherical concretions, forming the 
 spherulitic structure and the variety spherulite ; and 
 also by the banding, which is the result of flowing 
 while in a plastic state, whereby portions of the glass 
 of slightly different colors are drawn out into layers and 
 interlaminated. The bands are rarely continuous for 
 any distance, being usually merely elongated lenticular 
 streaks. The glassy state is generally one of inferior 
 density, and hence we find that obsidian is lighter than 
 the crystalline rocks of the same composition. Obsid- 
 ian is a good illustration of a non-essential color, for its 
 
LITHOLOGY. 117 
 
 capacity and jet-black color are due entirely to impuri- 
 ties. In very thin flakes it is transparent and white. 
 It also forms a white powder when crushed, i.e., it has 
 a white streak. 
 
 Obsidian is often vesicular, from the expansion of 
 the steam and other gases which it contained when 
 liquid. The most thoroughly vesicular varieties are 
 known as pumice (specimen 48). The vesicular tex- 
 ture, by rendering the rock impervious to light, con- 
 ceals the impurities, and thus we get a snow-white 
 pumice from black obsidian. The vesicles are fre- 
 quently elongated, sometimes in a definite direction, 
 though often forming an irregular net-work of glassy 
 fibres. Pumice is often light enough to float on water, 
 and it is transported thousands of miles by the oceanic 
 currents. It is employed in the arts, and good speci- 
 mens can be obtained at almost any drug-store. 
 
 Petrosilex and Felsite. Sharply defined groups 
 are unknown in lithology, but all is gradation ; and 
 between rhyolite and trachyte, which are always more 
 or less distinctly crystalline, and obsidian, which is a 
 true glass and perfectly amorphous, there is no break. 
 It is impossible to draw a sharp line and say, Here 
 the vitreous texture ends and the crystalline begins ; 
 for the transition is not abrupt, but gradual. We 
 recognize, really, in these feldspathic rocks, an inter- 
 mediate state, which is neither crystalline nor colloid, 
 but both; and this lithologists have designated the 
 felsitic texture. Felsitic matter cannot, even with the 
 highest powers of the microscope, be resolved into 
 separate grains or particles ; and it does not exhibit, 
 except perhaps very indistinctly, the phenomenon of 
 
Il8 STRUCTURAL GEOLOGY. 
 
 double refraction. In other words, it is not truly crys 
 talline or stony, and yet it is just as clearly not amor- 
 phous or glassy. 
 
 Feldspathic rocks exhibiting the felsitic texture in 
 whole or in part are known as felsites. Many high 
 authorities hold that true felsites are found only among 
 the eruptive rocks; while others claim that they are 
 in part, or wholly, of sedimentary origin. The writer 
 accepts the former view. The felsites are in part acid 
 lavas which have cooled too slowly to form a true glass, 
 like obsidian, and yet too quickly to become truly crys- 
 talline, like rhyolite and trachyte. But they are also 
 in large part simply devitrified obsidian. Glass is an 
 unstable form of mineral matter; and every species 
 of glass, including obsidian, tends with the lapse of 
 time to become crystalline or stony, the amorphous 
 changing to the felsitic structure. Thus, in many 
 cases or usually, what we now call felsites were origi- 
 nally true glassy obsidian. Being perfectly intimate 
 mixtures of the component minerals, the composition 
 of felsites can usually be determined with certainty 
 only by means of chemical analysis. By this means 
 chiefly, it has been proved that there are felsites agree- 
 ing in composition with both rhyolite and trachyte. 
 There is this general difference in composition, how- 
 ever, between these crystalline rocks and the felsites ; 
 viz. : mica, hornblende, and augite are generally want- 
 ing in the latter. From this it follows that the felsites 
 are, with unimportant exceptions, composed either of 
 quartz and feldspar or of feldspar alone. 
 
 The physical differences between the felsites of un- 
 like composition are not great ; but they are sufficient 
 
LITHOLOGY. 119 
 
 to warrant the division of the felsites into two species : 
 a basic species, to which the term felsites may properly 
 be restricted ; and an acidic species, for which petro- 
 silex is a very appropriate name. According to this 
 arrangement, felsite is composed chiefly of orthoclase, 
 and, as the table shows, agrees in composition with 
 trachyte; while petrosilex consists mainly of ortho- 
 clase and quartz, agreeing in composition with rhyo- 
 lite. We find here nothing new in composition ; but 
 petrosilex and felsite are simply the crystalline rocks 
 which we have already studied, repeated under a dif- 
 ferent texture. 
 
 The typical felsite or petrosilex is composed entirely 
 of felsitic matter, and is perfectly homogeneous, like 
 flint or jasper, which it closely resembles in hardness 
 and other physical characteristics. As a rule, however, 
 the rock is not entirely homogeneous, but there is a 
 manifest tendency in the component minerals, and 
 especially in the feldspar, to separate out, usually in 
 the form of crystals. In the banded variety (specimen 
 42) the rock is built up of thin layers, which are often 
 alternately quartzose and feldspathic. There is not a 
 perfect separation of the minerals ; but that the quartz 
 is chiefly in the dark layers, and the feldspar in the 
 light, is shown by the way in which the layers are 
 affected by the weather. 
 
 One of the most common varieties is where a por- 
 tion, frequently a large portion, of the feldspar comes 
 out in the form of distinct, separate crystals, produc- 
 ing a porphyritic texture. Specimens 5, 6, and 7 are 
 examples of porphyritic felsite ; and after examining 
 these we can no longer doubt that feldspar is an im- 
 
120 STRUCTURAL GEOLOGY. 
 
 portant constituent of the rock. Petrosilex and felsite 
 are more generally porphyritic than any other rocks ; 
 and they are commonly called porphyry. It is better, 
 however, since almost any rock may be porphyritic, 
 and since this texture cannot be correlated with any 
 particular composition, not to use porphyry as a rock- 
 name, but simply as the name of a very important 
 rock-texture. The banded and porphyritic textures 
 are about equally characteristic of petrosilex and felsite. 
 In petrosilex, quartz, as well as feldspar, is sometimes 
 porphyritically developed, forming the variety known 
 as quartz-porphyry. There is no limit to the propor- 
 tion of the quartz and feldspar which may crystallize 
 out in this way, and thus we find a perfectly gradual 
 passage from normal petrosilex or felsite to thoroughly 
 crystalline granite and syenite. 
 
 Andesite. This rock has nearly the texture of rhy- 
 olite and trachyte, but is darker and heavier, and 
 corresponds in composition to diorite, consisting of 
 plagioclase and hornblende, with usually more or less 
 sanidine, quartz, augite, biotite, and magnetite. 
 
 Basalt. The rock bearing this familiar name repre- 
 sents diabase among the dike rocks. It is the most 
 basic of the volcanic rocks, and consists of the more 
 basic varieties of plagioclase, especially labradorite, 
 with augite, magnetite, and titanic iron. Olivine is a 
 very common and characteristic constituent, and the 
 plagioclase is often replaced in part by leucite and 
 nephelite. The basalts are usually black, and of high 
 specific gravity ; and vary in texture from compact to 
 coarsely crystalline. The contraction due to cooling 
 frequently results in the development of a columnar 
 
LIT HO LOGY. 121 
 
 structure of remarkable regularity, the columns being 
 normally hexagonal and standing perpendicularly to 
 the cooling surfaces of the mass. This structure occurs 
 in other eruptive rocks, but is most characteristic of 
 basalt. 
 
 Tachylite. Tachylite is a highly basic volcanic 
 glass, standing in the same relation to basalt and ande- 
 site that obsidian does to trachyte and rhyolite. It is 
 much heavier than obsidian, and is perfectly black and 
 opaque, except in the finest fibres. It is a compara- 
 tively rare rock, for the reason that basalt and andesite 
 crystallize more readily than the acidic rocks on pass- 
 ing from the liquid to the solid state. On the surface 
 of the basic lava, however, where it is in contact with 
 the air, and congeals almost instantly, a film of glass 
 is formed ; but this may not be more than a small frac- 
 tion of an inch in thickness. Like obsidian, tachylite 
 is often vesicular ; but the vesicular basic rocks, as well 
 as the solid, are usually stony. They occur in vast 
 abundance in many volcanic regions, and may be con- 
 sidered the typical lava (specimen 49). 
 
 In the more ancient lavas, the vesicles are frequently 
 filled by various minerals chlorite, epidote, quartz, 
 calcite, etc. deposited by infiltrating waters, and de- 
 rived in most cases from the decomposition of the 
 original constituents of the rock. Thus the vesicular 
 is changed to the amygdaloidal texture, and the lava 
 becomes an amygdaloid (specimen 50). The amyg- 
 daloidal texture is common in the basic lavas and rare 
 in pumice, simply because the former are more readily 
 decomposed and contain a greater variety of bases from 
 which secondary minerals can be formed. 
 
122 STRUCTURAL GEOLOGY, 
 
 Porphyrite and Melaphyr. These two rocks hold 
 essentially the same relation as regards origin and 
 structure to the basic lavas that petrosilex and felsite 
 do to the acidic lavas. Porphyrite agrees in compo- 
 sition with andesite, and melaphyr with basalt. They 
 are usually dark-colored rocks having a compact or 
 felsitic texture. Porphyrite is, as the name implies, 
 -rery commonly porphyritic; while melaphyr is often 
 vesicular or brecciated, exhibiting all the structural 
 features of tachylite and basalt, and being in its older 
 forms very generally amygdaloidal. 
 
 Volcanic Tuff and Agglomerate. Besides the crys- 
 talline, glassy, and felsitic lavas, already described, and 
 due chiefly to the rate of cooling of the liquid rock, 
 we may recognize a fourth class to include the very 
 abundant lavas which, (luring explosive eruptions, are 
 ejected in the solid state, being violently blown out of 
 the crater in the form of dust and fragments. Falling 
 on the slopes of the volcano or over the surrounding 
 country, as in the case of the buried city of Pompeii, 
 the fragmental lavas remain largely unstratified. But 
 when, as frequently happens, they fall into the sea, 
 they are assorted by the waves and currents and 
 arranged in layers after the manner of ordinary sedi- 
 ments, with which they are often more or less mixed. 
 Before they become consolidated the finer fragmental 
 lava, of whatever composition, is called volcanic dust, 
 and the coarser lapilli or volcanic sand; while the 
 consolidated materials are known as tuff and agglom- 
 erate respectively. 
 
SUPPLEMENT TO LITHOLOGY. 
 
 VEIN ROCKS. 
 
 ALL rocks are not embraced in the sedimentary and 
 eruptive divisions, but there is a third grand division, 
 which, although rarely mentioned or recognized in the 
 more comprehensive works on geology, it is deemed best 
 not to leave entirely unnoticed here. These are the vein 
 rocks. They present an immense number of varieties, and 
 yet, taken altogether, form but a small fraction of the 
 earth's crust. They are, however, the great repositories 
 of the precious and other metals, and hence are objects of 
 far greater interest to the miner and practical man than the 
 eruptive rocks, or, in some parts of the world, even than 
 the sedimentary rocks. 
 
 The vein rocks, like the eruptive rocks, occupy fissures 
 in the earth's crust intersecting the stratified formations ; 
 but the fissures filled with vein rocks are called veins, and 
 not dikes. We will first notice the mode of formation of 
 a typical vein, and then examine its contents. Geologists 
 are agreed that water penetrates to a very great depth in 
 the earth's crust. All minerals are more or less soluble 
 in water; and we may consider the water circulating 
 through the rocks, especially at considerable depths, as, 
 in most cases, a saturated solution of the various minerals 
 of which- they are composed. Very slight changes in the 
 conditions will cause saturated solutions to deposit part of 
 their mineral load. The water at great depths has a high 
 temperature, and is subjected to an enormous pressure; 
 
124 STRUCTURAL GEOLOGY. 
 
 and both of these circumstances favor solution. Suppose, 
 now, that these hot subterranean waters enter a fissure in 
 the crust and flow upwards, perhaps issuing on the surface 
 as a warm mineral spring ; as they approach the surface, 
 the temperature and pressure, and consequently their sol- 
 vent power, are diminished ; and a portion of the dis- 
 solved- minerals must be deposited on the walls of the 
 fissure, which thus becomes narrower, and in the course 
 of time is gradually filled up. The vein is then complete ; 
 and the mineral waters are forced to seek a new outlet. 
 
 Veins have the same general forms as dikes, since the 
 fissures are formed in the same way for both ; but the 
 vein is of slow growth, and may require ages for its 
 completion, while the dike is formed in an hour or a day. 
 It is now generally believed that water is an important 
 agent in the formation of eruptive rocks ; since they all 
 contain water at the time of their eruption ; and since it 
 has been demonstrated that, while ordinary rocks require 
 a temperature of 2000 to 3000 for their fusion in the ab- 
 sence of water, they are liquified at temperatures below 
 1000 in the presence of water, In other words, common 
 rocks are very infusible and insoluble bodies, and heat and 
 water acting independently have little effect upon them ; 
 but when fire and water are combined in what is now 
 known as aqueo-igneous fusion, they prove very efficient 
 agents of liquefaction. 
 
 If we adopt these views, then it can be shown that, in 
 origin, veins and dikes differ in degree only, and are not 
 fundamentally unlike ; and the formation and relations of 
 the three great classes of rocks may be summarized as 
 follows : 
 
 The ocean and atmosphere, operating on the earth's sur- 
 face, have worked over and stratified the crust/ until the 
 sedimentary rocks have now an average thickness variously 
 estimated at from ten to thirty or forty miles. This entire 
 thickness of stratified rocks, and a considerable depth of 
 
VEIN ROCKS. 125 
 
 the underlying unstratified crust, must be saturated with 
 water; and all but the more superficial portions of this 
 water-soaked crust must be very hot, the temperature 
 increasing steadily downwards from the surface. Both 
 eruptive and vein rocks originate in this highly heated, 
 hydrated crust. Eruptive rocks are formed when the heat, 
 aided by more or less water, softens the rocks, either 
 stratified or unstratified, by aqueo-igneous fusion, and the 
 plastic materials are forced up through fissures to or to- 
 ward the surface. Vein rocks are formed when the water, 
 aided by more or less heat, dissolves the rocks, either 
 stratified or unstratified, by what may be called igneo- 
 aqueous solution, and subsequently deposits the mineral 
 matter in, i.e., on the walls of, fissures leading up to or 
 toward the surface. In the case of the dike rocks, heat is 
 the chief agent, and water merely an auxiliary ; while with 
 the vein rocks it is just the reverse. But between the two 
 it is probably impossible to draw any sharp line. 
 
 The water circulating through the crust, and saturated 
 with its various mineral constituents, has been called the 
 "juice" of the crust; and veins are formed by the con- 
 centration of this earth-juice in fissures. One of the most 
 important characteristics of the vein rocks, as a class, is 
 the immense variety which they present ; for nearly every 
 known mineral is embraced among their constituents ; and 
 these are combined in all possible ways and proportions, 
 so that the number of combinations is almost endless. 
 The solvent power of the subterranean waters varies for 
 different minerals; and appears often to be greatest for 
 the rarer species. In other words, there is a sort of selec- 
 tive action, whereby many minerals which exist in strati- 
 fied and eruptive rocks, so thinly diffused as to entirely 
 escape the most refined observation, are concentrated in 
 veins in masses of sensible size ; and our lists of known 
 minerals and chemical elements are undoubtedly much 
 longer than they would be if these wonderful storehouses 
 
126 STRUCTURAL GEOLOGY. 
 
 of fine minerals which we call veins had never been 
 explored. As a rule, the minerals in veins form larger 
 and more perfect crystals than we find in either of the 
 other great classes of rocks. This is simply because the 
 conditions are more favorable for crystallization in veins 
 than in dikes or sedimentary strata. In both dike and 
 stratified rocks, the growing crystals are surrounded on all 
 sides by solid or semi-solid matter ; and, being thus ham- 
 pered, it is simply impossible that they should become 
 either large or perfect. In the vein, on the other hand, 
 there are usually no such obstacles to be overcome ; but 
 the crystals, starting from the walls of the fissure, grow 
 toward its centre, their growing ends projecting into a 
 free space, where they have freedom to develop their nor- 
 mal forms and to attain a size limited only, in many cases, 
 by the breadth of the fissure. With, possibly, some rare 
 exceptions, all the large and perfect crystals of quartz, 
 feldspar, mica, beryl, apatite, fluorite, and of minerals 
 generally, which we see in mineralogical cabinets, have 
 originated in veins Those fissures which become the 
 seats of mineral veins are really Nature's laboratories for 
 the production of rare and beautiful mineral specimens ; 
 and hence the vein rocks are the chief resort of the miner- 
 alogist, to whom they are of far greater interest than all 
 the eruptive and stratified rocks combined. 
 
 The leading characteristics, then, of the vein rocks may 
 be summarized as follows: (i) They contain nearly all 
 known minerals, including many rare species and elements 
 which are unknown outside of this class of rocks. (2) 
 These mineral constituents, occurring singly and com- 
 bined, give rise to a number of varieties of rocks so vast 
 as to baffle detailed description. (3) They exceed all 
 other rocks in the coarseness of their crystallization, and 
 in the perfection and beauty of the single crystals which 
 they afford. 
 
PETROLOGY. 127 
 
 PETROLOGY. 
 
 In lithology we investigate the nature of the mate- 
 rials composing the earth's crust the various min- 
 erals and aggregates of minerals, or rocks; while in 
 petrology we consider the forms and modes of arrange- 
 ment of the rock-masses, in other words, the archi- 
 tecture of the earth. 
 
 Petrology is the complement of lithology, and in 
 many respects it is the most fascinating division of 
 geology, since in no other direction in this science are 
 we brought constantly into such intimate relations with 
 the beautiful and sublime in nature. The structures of 
 rocks are the basis of nearly all natural scenery ; for 
 what we call scenery is usually merely the external 
 expression, as developed by the powerful but delicate 
 sculpture of the agents of erosion rain and frost, 
 rivers and glaciers, etc. of the geological structure 
 of the country. And to the practised eye of the geolo- 
 gist, a fine landscape is not simply a pleasantly or 
 grandly diversified surface, but it has depth; for he 
 reads in- the superficial lineaments the structure of the 
 rocks out of which they are carved. 
 
 But, while the magnitude of the phenomena adds 
 greatly to the charm of the study, it also increases the 
 difficulties and taxes the ingenuity of the teacher whose 
 work must be done indoors. According to our ideal 
 method, natural science ought to be taught with nat- 
 ural specimens ; and yet here our main reliance must 
 be upon pictures and diagrams. 
 
 Nature, however, has not been wholly unmindful oi 
 our needs; for she has worked often upon a verf 
 
128 STRUCTURAL GEOLOGY. 
 
 small as well as a very large scale ; many of the grand- 
 est phenomena being repeated in miniature. Thus 
 we observe rock-folds or arches miles in breadth and 
 forming mountain masses, and of all sizes from that 
 down to the minutest wrinkle. So with veins, faults, 
 etc. And the wonderful thing is that these small ex- 
 amples, which may be brought into the class-room, 
 are usually, except in size, exactly like the large. Now 
 the aim of every teacher in this department should be 
 to secure a collection of these natural models. It is 
 not an easy thing to do, except one has plenty of 
 time ; for they can rarely be purchased of dealers, but 
 must usually come as the choicest fruit of repeated 
 excursions to the natural ledges and quarries, the sea- 
 shore and the mountains. But for the difficulty of 
 getting the specimens there is some compensation, 
 since it may be truly said that for the collector speci- 
 mens obtained in this way have an interest, a value, 
 and a power of instruction beyond what they would 
 otherwise possess. 
 
 Classification of Structures. 
 
 The structures of rock divide, at the outset, into two 
 classes : ( i ) the original structures, or those pro- 
 duced at the same time and by the same forces as the 
 rocks themselves, and which are, therefore, peculiar 
 to the class of rocks in which they occur (e.g. stratifi- 
 cation, ripple-marks, fossils, etc.) ; and (2) the sub- 
 sequent structures, or those developed in rocks subse- 
 quently to their formation, and by forces that act more 
 or less uniformly upon all classes of rocks, and which 
 
PETROLOGY. 
 
 129 
 
 are, therefore, in a large degree, common to all kinds 
 of rocks (e.g. folds, faults, joints, etc.). 
 
 The original structures are conveniently and natu- 
 rally classified in accordance with the three great classes 
 of rocks: (i) stratified rocks, (2) eruptive rocks, and 
 (3) vein rocks ; while the subsequent structures, not 
 being peculiar to particular classes of rocks, are prop- 
 erly divided into those produced by (i) the subter- 
 ranean or igneous agencies, and (2) the superficial or 
 aqueous agencies. 
 
 mflm'fflih *,fir $fifr\ I'nfoi fr-Mpt T f gff^qf'V^'-''** 
 
 Fig. i. Section through sediment deposited by rain in a roadside pool; 
 a. surface of roadway; b, layer of small pebbles and coarse sand; 
 c. fine sand passing into d', d. the finest sand and mud. 
 
 Original Structures of Stratified Hocks* 
 
 STRATIFICATION. All rocks formed by strewing 
 materials in water, and their deposition in succes- 
 sive, parallel, horizontal layers, are stratified; and this 
 structure is their stratification. It is the most import- 
 ant of all rock structures ; and there is no kind of 
 structure the origin of which is more fully or certainly 
 
1 30 STRUCTURAL GEOLOGY. 
 
 known. The deposition of sediment in carefully assorted 
 horizontal layers is readily brought within the compre- 
 hension of children by simple experiments with sand 
 and clay in water ; and still better by the examination 
 of the deposits formed in roadside pools during heavy 
 rains (Fig. i), and by digging into beaches and sand- 
 bars, which every child will recognize as formed of 
 materials arranged by water. Great stress should be 
 laid upon the fact that a lake like Erie or Champlain 
 is simply a large pool with several more or less turbid 
 streams flowing into it, while the single stream flowing 
 out is clear, the sediment having evidently been de- 
 posited in the lake; and that every lake is, like the 
 roadside pool, being gradually filled up with sedimen- 
 tary or stratified rocks. But the ocean is a still larger 
 pool, receiving mud and sand from many streams ; and 
 since we know that nothing escapes from the ocean but 
 invisible vapor, it is plain that the mud and sand and 
 all other kinds of sediment carried into the ocean must 
 be deposited on its floor, and chiefly, as we have seen, 
 on that part nearest the land. The consolidation of 
 beaches, bars, and mud-flats is all that is necessary to 
 convert them into stratified formations of conglomerate 
 sandstone and slate. 
 
 Let us notice now, more particularly, the causes of 
 visible stratification. As we can easily prove by an 
 experiment with clay in a bottle of water, if the same 
 kind of material is deposited continuously there will 
 be no visible stratification in the deposit. It will be 
 as truly stratified as any formation, but not visibly so ; 
 because there is nothing in the nature of the material 
 or the way in which it is laid down to bring out distinct 
 
PETROLOGY. 13! 
 
 lines of stratification. Continuous and uniform deposi- 
 tion obtains very frequently in nature, but rarely con- 
 tinues long enough to permit the formation of thick 
 beds or strata. Hence, while the stratification is almost 
 always visible on the large surfaces of sandstone, slate, 
 etc., exposed in quarries and railway cuttings, and 
 may usually be seen in the quarried blocks, it is often 
 not apparent in hand specimens, which may represent 
 a single homogeneous layer. There is one important 
 exception, and that is where the particles, although of 
 the same kind, are flat or elongated. Pebbles of these 
 forms are common on many beaches ; and since they 
 are necessarily arranged horizontally by the action of 
 the water, they willj by their parallelism, make the 
 stratification of the pudding-stone visible. The same 
 result is accomplished still more distinctly by the mica 
 scales, etc., in sandstone and slates, the leaves and 
 flattened stems of vegetation in bituminous coal, and 
 the flat shells in limestone. 
 
 In all other cases, visible stratification implies some 
 change in the conditions; either the deposition was 
 interrupted, or different kinds of material were de- 
 posited at different times. The first cause produces 
 planes of easy splitting, or fissility, especially in fine- 
 grained rocks, like shale. This shaly structure or 
 lamination-cleavage may be due, in some cases, to 
 pressure, but it is commonly understood to mean that 
 each thin layer of clay became partially consolidated 
 before the next one was deposited upon it, so that the 
 two could not perfectly cohere. Parallel planes of 
 easy splitting are, however, by themselves, of little 
 value as indications of stratification, since the lamina- 
 
*3 2 STRUCTURAL GEOLOGY. 
 
 tion-cleavage is not easily distinguished from slaty- 
 cleavage (roofing slate) and parallel jointing, structures 
 developed subsequently to the deposition of sediments 
 and quite independent of the stratification. The sec- 
 ond cause, or variations in the kind of sediment, gives 
 alternating layers differing in color, texture, or compo- 
 sition, as is seen frequently in sandstone, slate, gneiss, 
 etc. ; and of all the indications of stratification these 
 are the most important and reliable. 
 
 Fig. 2. Section showing strata and laminae: a. conglomerate; b. sandstone; 
 c. shale ; d. limestone. 
 
 A layer composed throughout of essentially the same 
 kind of rock, as conglomerate or sandstone, and show- 
 ing no marked planes of division, is usually regarded 
 as one bed or stratum, although it may vary consider- 
 ably in texture or color; while the thinner portions 
 composing the stratum and differing slightly in color, 
 texture, and composition, and the thin sheets into 
 which shaly rocks split, are the lamina or leaves* In 
 
PETROLOGY. 
 
 Fig. 2 the strata are designated by letters, and the fine 
 lines and rows of dots show the constituent laminae, while 
 the whole section may be regarded as a small part of 
 a great geological formation. The geological record 
 is written chiefly in the sedimentary rocks ; and the 
 formations, strata, and laminae may be regarded as the 
 volumes, chapters, and pages in the history of the 
 earth. Now every feature of a rock, lithological or 
 petrological, finds its highest interest in the light which 
 it throws upon the history of the rock, i.e., upon the 
 conditions of its formation. Observe what the section 
 in Fig. 2 teaches concerning the geological history of 
 that locality ; premising that any chapter of geologi- 
 cal history written in the stratified rocks should be 
 read from the bottom upwards, since the lowest strata 
 must have been formed first and the highest last. The 
 lowest stratum exposed is conglomerate, indicating a 
 shingle beach swept by strong currents which carried 
 away the finer material. Upwards, the conglomerate 
 becomes finer and shades off into sandstone, and finally 
 into shale, showing that the water has become gradually 
 deeper and more tranquil, the shore having, in conse- 
 quence of the subsidence, advanced toward the land. 
 The next two strata show that this movement is prob- 
 ably reversed ; at any rate, the currents become stronger 
 again, and the shale passes gradually into sandstone 
 and conglomerate. The beach condition prevails now 
 for a long time, and thick beds of sand and gravel are 
 formed. The sea then deepens again, and we observe 
 a third passage from coarse to fine sediment. This 
 locality is now remote from the shore, the gentle cur- 
 rents bringing only the finest mud, which slowly builds 
 
*34 STRUCTURAL GEOLOGY. 
 
 up the thick bed of shale, in the upper part of which 
 shells are abundant, indicating that the deposition of 
 mechanical sediment has almost ceased, and that the 
 shale is changing to limestone. The purity of the 
 limestone, and the crinoids and other marine organ- 
 isms which it contains, prove that this has now become 
 the deep, clear sea ; and this condition is maintained 
 for a long period, for the limestone is very thick, and 
 this rock is formed with extreme slowness. 
 
 The most important point to be gained here is that 
 every line of stratification and every change in the 
 character of the sediments is due to some change of 
 corresponding magnitude in the conditions under which 
 the rock was formed. The slight and local changes 
 in the conditions occur frequently and mark off the 
 individual laminae and strata, while the more important 
 and wide-spread changes determine the boundaries of 
 the groups of strata and the formations. 
 
 Strata are subject to constant lateral changes in tex- 
 ture and composition, i.e., a bed or formation rarely 
 holds the same lithological characteristics over an ex- 
 tended area. There are some striking exceptions, 
 especially among the finer-grained rocks, like slate, 
 limestone, and coal, which have been deposited under 
 uniform conditions over wide areas. It is the general 
 rule, however, particularly with the coarse-grained 
 rocks, which have been deposited in shallow" water 
 near the land, that the same continuous stratum under- 
 goes great changes in thickness and lithological char- 
 acter when followed horizontally. A stratum of con- 
 glomerate becomes finer grained and gradually changes 
 into sandstone, which shades off imperceptibly into 
 
PETROLOGY. 
 
 '35 
 
 slate, and slate into limestone, etc. Where the stratum 
 is conglomerate, its thickness will usually be much 
 greater and more variable than where it is composed 
 of the finer sediments. The rapidity of these changes 
 in certain cases is well shown by the parallel sections 
 in Fig. 3. These repre- 
 sent precisely the same 
 beds, as the connecting 
 lines indicate, at points 
 only twenty feet apart. 
 
 When we glance at 
 the conditions under 
 which stratified rocks 
 are now being formed, 
 it is plain that all strata 
 must terminate at the 
 margin of the sea in 
 which they were de- 
 posited, and in the mar : 
 ginal portions of that 
 sea, especially, must 
 exhibit frequent and 
 rapid changes in com- 
 position, etc. The sedi- 
 ments forming the sur- 
 face of the sea-bottom 
 
 Fig. 3. Parallel sections showing rapid 
 lateral changes in strata: c. clay; 
 s. sand; ss, sandstone; /. lignite; 
 /. fireclay. 
 
 at the present time may 
 be regarded as belonging to one continuous stratum ; 
 and it is instructive to examine a chart of any part of 
 our coast, such as Massachusetts Bay, on which the 
 nature of the bottom is indicated, for each sounding, 
 and observe the distribution of the different kinds oi 
 
136 STRUCTURAL GEOLOGY. 
 
 sediment. On an irregular coast like this, especially, 
 the gravel, sand, and mud of different colors and tex- 
 tures, and the different kinds of shelly bottom, form a 
 patchwork, the patches being, for the most part, of 
 limited extent and shading off gradually into each 
 other. 
 
 On a more regular coast, like that of New Jersey, 
 the sediments are distributed with corresponding uni- 
 formity, the changes are less frequent and more grad- 
 ual, and we have here a better chance to observe the 
 normal arrangement of the sediments along a line from 
 the shore seawards gravel, sand, mud, and shells. 
 On the beach we find the shingle and coarse pebbles, 
 shading off rapidly into fine pebbles and sand. The 
 zone or belt of sandy bottom may vary in width from 
 a mile or two to twenty miles or more, becoming grad- 
 ually finer and changing into clay or mud, which 
 covers, usually, a much broader zone, sometimes ex- 
 tending into the deeper parts of the sea, but gradually 
 giving way to calcareous sediments. Hence we may 
 say that the finer the sediment the greater the area 
 over which it is spread; but, on the other hand, the 
 coarser the sediment the more rapidly it increases in 
 thickness. In other words, the horizontal extent of a 
 formation deposited in any given period of time is 
 inversely, and the vertical extent or thickness is directly, 
 proportional to the size of the particles. 
 
 Observations made in deep wells and mines, and 
 where, by upturning and erosion, the edges of the 
 strata are exposed on the surface, show that the verti- 
 cal order of the different kinds of sedimentary rocks 
 in the earth's crust is extremely variable. But when 
 
PETROLOGY. 137 
 
 we take a general view of a great formation, it is often 
 apparent that it consists chiefly of coarse-grained rocks 
 in the lower part and fine-grained rocks in the upper 
 part. This is, in general, a necessary consequence of 
 the fact that a great thickness of sediments can only 
 be formed on a subsiding sea-floor. Such a formation 
 must consist chiefly of shore deposits, and be deposited 
 near the shore where the sea is shallow. Hence, 10,000 
 feet of sediments implies nearly that amount of subsi- 
 dence. In consequence, the shore line and the several 
 zones of sediment advance towards the land ; and sand 
 is deposited where gravel was at first, and as the sub- 
 sidence continues, both clay and limestone are finally 
 
 Fig. 4. Overlap and unconformability. 
 
 deposited over the original beach. When the sea- 
 floor rises, the order of the sediments is reversed ; 
 and it will be observed that in consequence of the 
 advance and retreat of the shore-line, the formations 
 grow edgewise to a considerable extent. 
 
 OVERLAP AND INTERPOSITION OF STRATA. Another 
 consequence of the constant oscillation of the shore- 
 line is that successive deposits in the same sea will 
 often cover different and unequal areas. When, in 
 consequence of subsidence, one formation extends 
 beyond and covers the edge of another, as shown in 
 Fig. 4, we have the phenomenon described as overlap. 
 
130 STRUCTURAL GEOLOGY. 
 
 Interposition is similar, being the case where a for- 
 mation (Fig. 5, <r.) does not, in certain directions, 
 cover so wide an area as the strata (b. d.) above 
 and below it, which are thus sometimes found in con- 
 tact, although normally separated by the entire thick- 
 ness of the intermediate and, seemingly, interposed 
 stratum. 
 
 Fig- 5- Interposition of strata. 
 
 UNCONFORMABILITY. We have already seen that 
 the rocks on the land are being constantly worn away 
 by the agents of erosion ; and it is also a matter of 
 common observation that the strata thus exposed are 
 often not horizontal, but highly inclined, having been 
 greatly disturbed and crumpled during their elevation. 
 Now, when such a land-surface subsides to form the 
 sea-bottom, and new strata are spread horizontally 
 over it, they will lie across the upturned and eroded 
 edges of the older rocks, and fill the hollows worn 
 out of the latter, as shown in Fig. 6 ; and the new 
 
PETROLOGY. *39 
 
 formation is then said to rest unconformably upon the 
 older. Two strata or formations are unconformable 
 when the older has suffered erosion (Fig. 6), or both 
 disturbance and erosion (Fig. 4) before the deposition 
 of the newer. 
 
 When strata are conformable, the deposition may 
 be presumed to have been nearly or quite continuous ; 
 but unconformability clearly proves a prolonged inter- 
 ruption of the deposition during which the elevation, 
 erosion, and subsidence of the sea-bottom took place. 
 The section in Fig. 7 shows a second unconformability, 
 
 Fig. 6. Unconformability. 
 
 proving that the sea-bottom has here been lifted three 
 times to form dry land. An unconformability may 
 sometimes be clearly established when the actual con- 
 tact of the two formations cannot be seen, as where 
 the new formation is a conglomerate containing frag- 
 ments of the older. 
 
 IRREGULARITIES OF STRATIFICATION. These are es- 
 pecially noticeable in sandstone and conglomerate, 
 which have been deposited chiefly by strong, local, and 
 variable currents ; the kind and quantity of sediment, 
 of course, varying with the strength and direction of 
 the current. Two kinds of irregularity only may be 
 specially noticed here : (i) contemporaneous erosioi) 
 
140 
 
 STRUCTURAL GEOLOGY. 
 
 and deposit, where, in consequence of a change in the 
 currents, fine material recently deposited is partially 
 swept away and its place taken by coarser sediments j 
 and (2) oblique lamination, or current-bedding, where 
 the strata are horizontal as usual, but the component 
 
 Fig. 7. Double Unconformability : q. quartzite; s. sandstone; d. drift. 
 
 laminse are inclined at various angles. This structure 
 is characteristic of sediments swept along by strong 
 currents, especially when deposited in shallow basins 
 or depressions. 
 
PETROLOGY. 141 
 
 RIPPLE-MARKS. All who have been on a beach or 
 sand-bar must have noticed the lines of wavy ridges 
 and hollows, or ripples, on the surface of the sand. 
 These are sand-waves, produced by water moving over 
 the sand, or by air moving over dry sand, as ordinary 
 waves are formed by air moving over water. Each 
 tide usually effaces the ripple-marks made by its pre- 
 decessor and leaves a new series, to be obliterated 
 by the next tide. But where sediment is constantly 
 accumulating, a rippled surface may be gently over- 
 spread by a new layer, and thus preserved. Other 
 series of ripples may, in like manner, be formed and 
 preserved in overlying layers ; and when the beach 
 becomes a firm sandstone, a section of it will.show the 
 rippled surfaces almost as distinctly as when they were 
 first formed (Fig. 8). Ripple-marks are most perfect 
 in fine sand. They are not formed in gravel, because 
 it is too coarse ; nor in clay, because it is too tenacious. 
 They are usually limited to shallow water; and are 
 always regarded as proving that the rocks in which 
 they occur are shallow-water or beach deposits. They 
 are normally at right angles to the current that pro- 
 duces them, and where this changes with the direction 
 of the wind, cross-ripples and other irregularities are 
 introduced. Ripple- marks are also usually parallel 
 with the beach, and when they are found in the rocks 
 they give us the direction, as well as the position, of 
 the ancient shore-line. 
 
 Again, the friction of the water pushes the sand- 
 grains along, rolling them up on one side of the ripple 
 and letting them fall down on the other. Hence 
 ripples, formed by a current are always moving and are 
 
142 STRUCTURAL GEOLOGY. 
 
 unsymmetrical on the cross-section, presenting a long, 
 gentle slope toward the current, and a short, steep slope 
 away from it, the arrow in the figure indicating the 
 direction of the current, or of the sea in the case of a 
 beach. And we may thus learn from the fossil ripples, 
 in some cases, not only the position and direction of 
 the ancient shore, but also on which side the land lay, 
 and on which side the sea. When the water is in a 
 state of oscillation, without any distinct current, more 
 symmetrical ripples are produced. 
 
 Fig. 8. Ripple-marks in sandstone. 
 
 RILL-MARKS, RAIN-PRINTS, AND SUN-CRACKS. " One 
 of the most fascinating parts of the work of a field- 
 geologist consists in tracing the shores of former seas 
 and lakes, and thus reconstructing the geography of 
 successive geological periods." His conclusions, as we 
 have already seen, are based largely upon the nature 
 of the sediments ; but still more convincing is the evi- 
 dence afforded by those superficial features of the 
 strata, which, like ripple-marks, seem, by themselves, 
 quite insignificant. And among these he lays special 
 emphasis upon those which show that during their 
 deposition strata have at intervals been laid bare to 
 sun and air. 
 
 During ebb tide water which has been left at the 
 
PETROLOGY. 143 
 
 upper edge of the beach runs down across the beach 
 in small rills, which excavate miniature channels ; and 
 when these are preserved in the hard rocks, they prove 
 that the latter are beach deposits, and, like the ripple- 
 marks, show the direction of the old shore. 
 
 If a heavy shower of rain falls on a muddy beach or 
 flat, the sediment deposited by the returning tide may 
 cover, without obliterating, the small but characteristic 
 impressions of the individual drops ; and these mark- 
 ings are frequently found well preserved in the hardest 
 slates and sandstones, testifying unequivocally to the 
 conditions under which the rocks were formed. In 
 some cases tlie rain-prints are found to be ridged up 
 on one side only, in such a manner as to indicate that 
 the drops as they fell were driven aslant by the wind. 
 The prominent side of the marking, therefore, indicates 
 the side towards which the wind blew. 
 
 Muddy sediments, especially in lakes and rivers, 
 are often exposed to the air and sun during periods of 
 drouth, and as they gradually dry up, polygonal cracks 
 are formed. The sediment of the next layer will fill 
 these sun-cracks; and when, as often happens, it is 
 slightly different from the dessicated layer, they may 
 still be traced. Sun-cracks preserved in this way are 
 very characteristic of argillaceous rocks, and, of course, 
 prove that in early times, as at the present day, sedi- 
 ments of this class were exposed by the temporary 
 retreat of the water. The foot-prints or trails of land- 
 animals are often, as in the sandstones and shales of 
 the Connecticut Valley, associated with, and of course 
 strongly corroborate, all these other evidences of shore 
 deposits. From the foot-prints preserved in the rocks 
 
144 STRUCTURAL GEOLOGY. 
 
 we pass naturally to the consideration of the fossil 
 remains of plants and animals found entombed in the 
 strata. 
 
 FOSSILS. Although fossils find their highest interest 
 in the light which they throw upon the succession of 
 life on the globe, they may also be properly regarded 
 as structural features of stratified rocks ; and any one 
 who has seen the dead shells, crabs, fishes, etc., on 
 the beach will readily understand how fossils get into 
 the rocks. It is not our province here to study the 
 structure of the fossils themselves, for that would in- 
 volve us in a course in paleontology, a task belonging 
 to the biologist rather than the geologist ; but we will 
 merely observe the three principal degrees in the 
 preservation of fossils : 
 
 1 . Original composition not completely changed. 
 Extinct elephants have been found frozen in the river- 
 bluffs of Siberia so perfectly preserved that dogs and 
 wolves ate their flesh. The bodies of animals are also 
 found well preserved in peat-bogs. All coal is simply 
 fossil vegetation retaining in a large degree the original 
 composition ; and the same is true of ferns, etc., pre- 
 served as black impressions in the rocks. All bones 
 and shells consist of mineral matter which makes them 
 hard, and animal matter which makes them tough and 
 strong. In very many cases, especially in the newer 
 formations, the animal matter is still partially, and the 
 mineral matter almost wholly, intact. 
 
 2. Original composition completely changed, but form 
 and structure preserved. All kinds of fossils are com- 
 monly called petrifactions, but only those preserved in 
 this second way are truly petrified, i.e., turned to stone. 
 
PETROLOGY. 145 
 
 " Petrified wood is the best illustration, and in a good 
 specimen not only the external form of the wood, not 
 only its general structure bark, wood, radiating silver- 
 grain, and concentric rings of growth are discernible, 
 but even the microscopic cellular structure of the wood, 
 and the exquisite sculpturing of the cell-walls, are per- 
 fectly preserved, so that the kind of wood may often 
 be determined by the microscope with the utmost 
 certainty. Yet not one particle of the organic matter 
 of the wood remains. It has been entirely replaced 
 by mineral matter; usually by some form of silica. 
 The same is true of the shells and bones of animals." 
 LE CONTE. 
 
 3. Original composition and structure both obliter- 
 ated, and form alone preserved. This occurs most 
 commonly with shells, although fossil trees are also 
 often good illustrations. The general result is accom- 
 plished in several ways : (a) The shell after being 
 buried in the sediment may be removed by solution, 
 leaving a mould of its external form, (b) This 'mould 
 may subsequently be filled by the infiltration of finer 
 sediment, forming a cast of the exterior of the shell. 
 (c) The shell, before its solution, may have been filled 
 with mud ; and if the shell itself is then dissolved, we 
 have a cast of its interior in a mould of its exterior. 
 
 TIME REQUIRED FOR THE FORMATION OF STRATIFIED 
 
 ROCKS. Many attempts have been made to deter- 
 mine the time required for the deposition of any given 
 thickness of stratified rocks. Of course, only roughly 
 approximate results can be hoped for in most cases ; 
 but these are at least sufficient to make it certain that 
 geological time ig very long. The average relative rate 
 
146 STRUCTURAL GEOLOGY. 
 
 of growth of different kinds of sediment is, however, 
 less open to doubt, for we have already seen that 
 coarse sediments like gravel and sand accumulate 
 much more rapidly than finer sediments like clay and 
 limestone ; and we are sometimes able to compare 
 these two classes of rocks on a very large scale. 
 
 Thus, during what is known as the Paleozoic era, a 
 sea extended from the Blue Ridge to the Rocky Moun- 
 tains. Along the eastern margin of this sea, where the 
 Alleghany Mountains now stand, sediments chiefly 
 conglomerate and sandstone, with some slate and less 
 limestone accumulated to a thickness of nearly 
 40,000 feet. Toward the west, away from the old 
 shore-line, the coarse sediments gradually die out, and 
 the formations become finer and thinner. In western 
 Ohio and Indiana, slate and limestone predominate ; 
 while in the central part of the ancient sea, in Illi- 
 nois and Missouri, the paleozoic sediments are almost 
 wholly limestones, and have a thickness of only 4,000 
 to 5,000 feet. In other words, while one foot of lime- 
 stone was forming in the Mississippi Valley, eight to 
 ten feet of coarser sediments were deposited in Penn- 
 sylvania. 
 
 The best estimates show that coral-reefs rise i.e., 
 limestones are formed on them at the rate of about 
 one foot in two hundred years. But coral limestones 
 grow much more rapidly than limestones in general. 
 Sandstones sometimes accumulate so rapidly that trees 
 are buried before they have time to decay and fall 
 (Fig. 9). Such a buried forest, like a coal-bed, rep- 
 resents a land surface, and proves a subsidence of the 
 land ; and in some cases, as indicated by the section, 
 
PETROLOGY. 
 
 '47 
 
 repeated oscillations of the crust may be proved in 
 this way. 
 
 The mud deposited by the annual overflow of the 
 Nile is forty feet thick near the ancient city of Mem- 
 phis ; and the pedestal of the statue of Rameses II., 
 believed to have been erected B.C. 1361, is buried to 
 a depth of nine feet, four inches, indicating that 13,500 
 years have elapsed since the Nile began to spread 
 its mud over the sands of the 
 desert. 
 
 But the greatest difficulty in 
 estimating the time required 
 for the formation of any series 
 of strata arises from the fact 
 that we cannot usually even 
 guess at the length of the 
 periods when the deposition 
 has been partially or wholly 
 interrupted. Now and then, 
 however, we find evidence that 
 these periods may be very long. 
 A layer of fossil shells in sandstone or slate proves an 
 interruption of mechanical deposition. Beds of coal, 
 fossil forests, and other indications of land surfaces 
 are still more conclusive. The interposition of strata 
 (Fig. 5) proves a prolonged interruption of deposition 
 over the area not covered by the interposed bed. But 
 the most important of all evidence is that afforded by 
 unconformability ; and the length of the lost interval 
 between the two formations is measured approximately 
 by the erosion of the older. 
 
 Fig. 9. Erect fossil trees. 
 
148 
 
 STRUCTURAL GEOLOGY. 
 
 
 
 Fig. 10. Typical dikes. 
 
 Original Structures of Eruptive Rocks. 
 
 The structures of this class are divisible into those 
 pertaining to the volcanic rocks and those pertaining 
 to the fissure or dike rocks. But since volcanoes are 
 rare in this part of the world, while dikes are well 
 
 developed in many sec- 
 tions of our country, it 
 seems best to give our 
 attention chiefly to the 
 latter. 
 
 The term dike is a 
 general name for all 
 masses of eruptive rocks 
 that have cooled and 
 solidified in fissures or 
 cavities in the earth's 
 crust. But the name is 
 commonly restricted to 
 the more regular, wall- 
 like masses (Fig. 10), 
 those having extremely 
 irregular outlines, like 
 most masses of granite 
 
 (Fig. n), being known simply as eruptive masses. 
 The propriety of this distinction is apparent when we 
 consider the origin of dike as a geological term. It 
 was first used in this sense in southern Scotland, where 
 almost any kind of a wall or barrier is called a dike. 
 The dikes traverse the different stratified formations 
 like gigantic walls, which are often encountered by 
 the coal-miners, and on the surface are frequently left 
 
 Fig. ii. Section of a granite mass. 
 
PETROLOGY. 149 
 
 in relief by the erosion of the softer enclosing rock, 
 so that in the west of Scotland, especially, they are 
 actually made use of for enclosures. In other cases 
 the dike has decayed faster than the enclosing rock, 
 and its position is marked by a ditch-like depression. 
 The narrow, straight, and perpendicular clefts or chasms 
 observed on many coasts are usually due to the re- 
 moval of the wall-like dikes by the action of the waves. 
 Dikes are sometimes mere plates of rock, traceable for 
 a few yards only ; and they range in size from that up 
 to those a hundred feet or more in width, and trace- 
 able for scores of miles across the country, their out- 
 crops forming prominent ridges. The sides of dikes 
 are often as parallel and straight of those of built walls, 
 the resemblance to human workmanship being height- 
 ened by the numerous joints which, intersecting each 
 other along the face of a dike, remind us of well-fitted 
 masonry. 
 
 FORMS OF DIKES. A dike is essentially a casting. 
 Melted rock is forced up from the heated interior into 
 a cavity or crack in the earth's crust, cools and solidi- 
 fies there, and, like a metallic casting, assumes the 
 form of the fissure or mould. In other words, the 
 form of the dike is exactly that of the fissure into 
 which the lava was injected. Now the forms of fis- 
 sures depend partly upon the nature of the force that 
 produces them, but very largely upon the structure 
 and especially the joint-structure of the enclos- 
 ing rocks. Nearly all rocks are traversed by planes 
 of division or cracks called joints, which usually run 
 in several directions, dividing the rock into blocks. 
 And it is probable that dike-fissures are most com' 
 
r5 STRUCTURAL GEOLOGY. 
 
 monly produced, not by breaking the rocks anew, but 
 by widening or opening the pre-existing joint-cracks. 
 Hence the straight and regular jointing of slate, lime- 
 stone and most sedimentary rocks is accompanied by 
 wall-like dikes the typical dikes (Fig. 10) ; while 
 the more irregular jointing of granite and other mas- 
 sive rocks gives rise to sinuous, branching, variable 
 dikes. The general dependence of dikes upon the 
 joint-structure of the rocks is proved by the facts that 
 
 dikes, like joints, are 
 normally vertical or 
 highly inclined, and 
 that they are usually 
 parallel with the prin- 
 cipal systems of joints 
 in the same district. 
 
 Fig. . - Dike with regular brariches. The Wall-like dikes also 
 
 give off branches, but 
 usually in a regular manner, as shown in Fig. 12. 
 
 STRUCTURE OF DIKES. The rock traversed by a 
 dike is called the country or wall rock. Fragments 
 of this are often torn off by the igneous material, and 
 become enclosed in the latter. Such enclosed frag- 
 ments may sometimes form the main part of the dike, 
 which then, since they are necessarily angular, often 
 assumes the aspect of a breccia. This is the only im- 
 portant exception to the rule that dikes are homoge- 
 neous in composition ; /.<?., in the same dike we can 
 usually find from end to end, from side to side, and 
 probably from top to bottom no essential difference 
 in composition. But there is often a marked contrast 
 in texture between different parts of a dike, and espe 
 
PETROLOGY. 151 
 
 dally between the sides and central portion. The 
 liquid rock loses heat most rapidly where it is in con- 
 tact with the cold walls of the fissure, and solidifies 
 before it has time to crystallize, remaining compact 
 and sometimes even glassy; while in the middle of 
 the dike, unless it is very narrow, it cools so slowly as 
 to develop a distinctly crystalline texture. There is 
 no abrupt change in texture, but a gradual passage 
 from the compact border to the coarsely crystalline or 
 porphyritic middle portion. It is obvious that a simi- 
 lar gradation in texture must exist between the top 
 and bottom of a dike. 
 
 CONTACT PHENOMENA. Under this head are grouped 
 the interesting and important phenomena observable 
 along the contact between the dike and wall-rock. 
 These throw light upon the conditions of formation of 
 dikes, and are often depended upon to show whether 
 a rock mass is a dike or not. The student will observe 
 here : 
 
 1. The detailed form of the contact. It may be 
 straight and simple, or exceedingly irregular, the dike 
 penetrating the wall, and enclosing fragments of it, as 
 in Fig. n, which is a typically igneous contact. 
 
 2. The alteration of the wall-rock by heat. This 
 may consist in : (a) coloration, shales and sandstones 
 being reddened in the same way as when clay is burnt 
 for bricks ; (ft) baking and induration, sandstone 
 being converted into quartzite and even jasper ; clay, 
 slate, etc., being not only baked to a flinty hardness, 
 but actually vitrified, as in porcelainite ; and bitumi- 
 nous coal being converted into natural coke or anthra- 
 cite; and (c) crystallization, chalk, and other lime- 
 
152 STRUCTURAL GEOLOGY. 
 
 stones being changed to marble, and crystals of pyrite, 
 calcite, quartz, etc., being developed in slate, sand- 
 stone, and other rocks. 
 
 3. The alteration of the dike-rock by (a) more 
 rapid cooling, and (b) the access of thermal waters. 
 
 The alteration of the wall-rock may extend only a 
 few inches or many yards from the dike, gradually 
 diminishing with the distance ; and the cases are sur- 
 prisingly numerous where there is no perceptible alter- 
 ation ; and, again, the alteration is usually mutual, the 
 dike-rock being altered in texture, color, and compo- 
 sition. 
 
 Fig. 13. Ideal cross-section of Fig. 14. Ideal cross-section of 
 
 a laccolite. a volcano. 
 
 INTRUSIVE BEDS. We commonly think of dikes 
 as cutting across the strata, but they often lie in planes 
 parallel with them ; and the same dike may run across 
 the beds in some parts of its course and between them 
 in others (Fig. 12), or the conformable dike may be 
 simply a lateral branch of a main vertical dike, as 
 shown in the same figure. All dikes or portions of 
 dikes lying conformably between the strata are called 
 intrusive beds or sheets. 
 
 When a dike fails to reach the surface, but spreads 
 out horizontally between the strata, forming a thick 
 dome or oven-shaped intrusive bed, the latter is called 
 
PETROLOGY. 153 
 
 a laccolite (Fig. 13). Laccolites are sometimes of 
 immense volume, containing several cubic miles of 
 rock. Fig. 14 enables us to compare the laccolite 
 with the volcano. 
 
 In the one case a large mound of eruptive material 
 accumulates between the strata, the overlying beds 
 being lifted into a dome ; while in the other case the 
 fissure or vent reaches the surface, and the mound of 
 lava is built up on top of the ground. 
 
 COTEMPORANEOUS BEDS. When the lava emitted 
 by a crater is sufficiently liquid, it spreads out hori- 
 zontally, forming a volcanic sheet or bed. If such an 
 eruption is submarine, or the lava flow is subsequently 
 covered by the sea, sedimentary deposits are formed 
 over it ; and beds of lava which thus come to lie con- 
 formably between sedimentary strata are known as 
 cotemporaneous sheets or beds, because they belong, 
 in order of time, in the position in which we find 
 them, being, like any member of a stratified series, 
 newer than the underlying and older than the over- 
 lying strata. Cotemporaneous lava-flows are some- 
 times repeated again and again in the same district, 
 and thus important formations are built up of alternat- 
 ing igneous and aqueous deposits. Evidently, the 
 student who would read correctly the record of igne- 
 ous activity in the past must be able to distinguish 
 intrusive and cotemporaneous beds. The principal 
 points to be considered in making this distinction 
 are : (i) The intrusive bed is essentially a dike, dense 
 and more or less crystalline in texture, altering, and 
 often enclosing fragments of, both the underlying and 
 overlying strata, and frequently jogging across or pene* 
 
154 STRUCTURAL GEOLOGY. 
 
 trating the sediments. (2) The cotemporaneous bed, 
 on the other hand, being essentially a lava-flow, is 
 much less dense and crystalline, being usually distinctly 
 scoriaceous or amygdaloidal, especially at the borders, 
 and the underlying strata alone showing heat action, 
 or occurring as enclosures in the lava ; for the overly- 
 ing strata are newer than the lava, and often consist 
 largely, at the base, of water- worn fragments of the lava. 
 
 AGES OF DIKES. The ages of dikes may be esti- 
 mated in several ways. They are necessarily newer 
 than any stratified formation which they intersect or 
 of which they enclose fragments ; but any formation 
 crossing the top of a dike must usually be regarded as 
 newer than the dike, especially if it contains water- 
 worn fragments of the dike rock. 
 
 The relative ages of different dikes are determined 
 by their relations to the stratified formations ; and still 
 more easily by their mutual intersections, on the prin- 
 ciple that when two dikes cross each other, the inter- 
 secting must be newer than the intersected dike. It 
 is sometimes possible, in this way, to prove several 
 distinct periods of eruption in the same limited dis- 
 trict. The textures of dikes also often afford reliable 
 indications of their ages ; for, as we have already seen, 
 the upper part of a dike, cooling rapidly and under 
 little pressure, must be less dense and crystalline than 
 the deep-seated portion, which cools slowly and under 
 great pressure. 
 
 Now, the lower, coarsely crystalline part of a dike 
 can usually be exposed on the surface only as the 
 result of enormous erosion ; and erosion is a slow 
 process, requiring vast periods of time. Hence, when 
 
PETROLOGY. 155 
 
 we see a coarse-grained dike outcropping on the sur- 
 face, we are justified in regarding it as very old, for 
 all the fine-grained upper part has been gradually worn 
 away by the action of the rain, frost, etc. Other things 
 being equal, coarse-grained must be older than fine- 
 grained dikes ; and the texture of a dike is at once a 
 measure of its age and of the amount of erosion which 
 the region has suffered since it was formed. 
 
 ERUPTIVE MASSES. In striking contrast with the 
 more or less wall-like dikes are the highly irregular, 
 and even ragged, outlines of the eruptive masses ; and 
 it is worth while to notice the probable cause of this 
 contrast. The true dikes are formed, for the most 
 part, of comparatively fine-grained rocks the typical 
 " traps " ; while the eruptive masses consist chiefly of 
 the coarse-grained or granitic varieties. Now we have 
 just seen that the coarse-grained rocks have been 
 formed at great depths in the earth's crust, while the 
 fine-grained are comparatively superficial. But we 
 have good reason for believing that the joint-struc- 
 ture, upon which the forms of dikes so largely de- 
 pend, is not well developed at great depths, where 
 the rocks are toughened, if not softened, by the high 
 temperature. In other words, trap dikes are formed 
 in the jointed formations, which break regularly ; while 
 the granitic masses are formed where the absence of 
 joint-structure and a high temperature combine to 
 cause extremely irregular rifts and cavities when the 
 crust is broken. 
 
 VOLCANIC PIPES OR NECKS. Every volcano and 
 every lava -flow or volcanic sheet must be connected 
 with the earth's interior by a channel or fissure, which 
 
156 STRUCTURAL GEOLOGY. 
 
 becomes a dike when the lava ceases to flow. But 
 the converse proposition is not true, for it is probable 
 that many dikes did not originally reach the surface, 
 but have been exposed by subsequent denudation. 
 This is conspicuously the case with laccolites and 
 other forms of intrusive sheets. Volcanic sheets or 
 beds have probably often resulted from the overflow 
 of the lava at all points of an extensive fissure or sys- 
 tem of fissures ; but the vent of the true volcano must 
 be more circumscribed, an approximately circular open- 
 ing in the earth's crust, although doubtless originating 
 in a fissure or at the intersection of two or more fis- 
 sures, the lava continuing to flow at the widest part of 
 the wound in the crust long after it has congealed in 
 the narrower parts. Such a tube is known as the neck 
 or pipe of the volcano ; and volcanic necks are a 
 highly interesting class of dikes, since they determine 
 the exact location of many an ancient volcano, where 
 the volcanic pile itself has long since been swept away. 
 Necks and dikes are the downward prolongations or 
 roots of the volcanic cone or sheet, and cannot be 
 exposed on the surface until the volcanic fires have 
 gone out and the agents of erosion have removed the 
 greater part of the ejected materials. 
 
 Hence, equally with the dikes which originally failed 
 to reach the surface, they, wherever open to our obser- 
 vation, testify to extensive erosion and a vast antiquity. 
 
 Original Structures of Vein Hocks. 
 
 Many things called veins are improperly so called, 
 such as dikes of granite and trap, and beds of coal 
 and iron-ore. The smaller, more irregular, branching 
 
PETROLOGY. 157 
 
 dikes, especially, are very commonly called veins, and 
 to distinguish the true veins from these eruptive masses, 
 they are designated as mineral veins or lodes, although 
 the term lode is usually restricted to the metalliferous 
 veins. 
 
 ORIGIN OF VEINS. Various theories of the forma- 
 tion of veins have been proposed, but the most of 
 these are of historic interest merely, for geologists are 
 now well agreed that nearly all true veins have been 
 formed by the deposition of minerals from solution in 
 fissures or cavities in the earth's crust. In many cases, 
 especially where the veins are of limited extent, it 
 seems probable that a part or all of the mineral matter 
 was derived from the immediately enclosing rocks, 
 being dissolved out by percolating water; and these 
 are known as segregation or lateral secretion veins. 
 But it is quite certain that as a general rule the min- 
 eral solutions have come chiefly from below, the deep- 
 seated thermal waters welling up through any channel 
 opened to them, and gradually depositing the dis- 
 solved minerals on the walls of the fissure as the tem- 
 perature and pressure are diminished. This case, 
 however, differs from the first only in deriving the 
 vein-forming minerals from more remote and deeper 
 portions of the enclosing rocks ; and thus we see that 
 vein-formation, whether on a large or a small scale, is 
 always essentially a process of segregation. 
 
 We know that every volcano and every lava flow 
 must be connected below the surface with a dike ; and 
 it is almost equally certain that the waters of mineral 
 springs forming tufaceous mineral deposits on the sur- 
 face, as in the geyser districts, also deposit a portion 
 
158 STRUCTURAL GEOLOGY. 
 
 of the dissolved minerals on the walls of the subter- 
 ranean channels, which are thus being gradually filled 
 up and converted into mineral veins, which will be 
 exposed on the surface when erosion has removed the 
 tufaceous overflow. This connection of vein-forma- 
 tion with the superficial deposits of existing springs 
 has been clearly proved in several important instances 
 in Nevada and California. 
 
 Veins occur chiefly in old, metamorphic, and highly 
 disturbed formations, where there is abundant evidence 
 of the former existence of profound fissures, and in 
 regions similar to those in which thermal springs occur 
 to-day. 
 
 In the supplement to the lithological section the 
 student will find the formation of a typical vein briefly 
 described and contrasted with that of a typical dike ; 
 also a brief account of the lithological peculiarities of 
 vein rocks, and general statements concerning their 
 relative abundance and vast economic importance. 
 
 EXTERNAL CHARACTERISTICS OF VEINS. The typical 
 vein may be described as a fissure of indefinite length 
 and depth, filled with mineral substances deposited 
 from solution. Externally, it is very similar to the 
 typical dike, for the fissures are made in the same 
 way for both. Veins are normally highly inclined to 
 the horizon ; they exhibit in nearly every respect the 
 same general relations to the structure of the country 
 rock as dikes ; and the ages of veins are determined 
 in the same way as the ages of dikes. 
 
 The extensive mining operations to which veins 
 have been subjected in all parts of the world, have 
 made our knowledge of their forms below the surface 
 
PETROLOGY. . 159 
 
 very full and accurate. It has been learned in this 
 way that very often the corresponding portions of the 
 walls of a vein do not coincide in position, but one 
 side is higher or lower than the other, showing that 
 the walls slipped over each other when the fissure was 
 formed or subsequently j and this faulting or displace- 
 ment of the walls appears to be much more common 
 with veins than with dikes, perhaps because the fis- 
 sures remained open much longer. This slipping of 
 the walls is the principal cause of the almost constant 
 changes in the width of veins. For, since the walls 
 are never true planes, and are often highly irregular 
 any unequal movements must bring them nearer to- 
 gether at some points than at others. As a rule, the 
 enormous friction accompanying the faulting, either 
 crushes the wall-rock, or polishes and striates it, pro- 
 ducing the highly characteristic surfaces known as 
 slit, ken-sides. Where the wall is finely pulverized in 
 this way, or is partially decomposed before or after 
 the filling of the fissure, a thin layer of soft, argillaceous 
 material is formed, separating the vein proper from 
 the wall-rock. The miners call this the selvage ; and 
 it is a very characteristic feature of the true fissure 
 veins. 
 
 Fragments of the wall-rock are frequently enclosed 
 in veins, and the latter sometimes branch or divide in 
 such a way as to surround a large mass of the wall, 
 which is known as a "horse." A similar result is 
 accomplished when a fissure is re-opened after being 
 filled, if the new fissure does not coincide exactly with 
 the old. It has been proved that veins have thus been 
 re-opened and filled several times in succession ; and 
 
160 STRUCTURAL GEOLOGY, 
 
 in this way fragments of the older vein material be- 
 come enclosed in the newer. 
 
 Although usually determined in direction by the 
 joint-structure of the country rock, veins are often 
 parallel with the bedding, especially in highly inclined,, 
 schistose formations. Such interbedded veins are com- 
 monly distinctly lenticular in form, occupying rifts in 
 the strata which thin out in all directions and are often 
 very limited in extent. 
 
 Whether conforming with the joint-structure or bed- 
 ding, veins are commonly arranged in systems by their 
 parallelism, those of different systems or directions 
 usually differing in age and composition, and the older 
 veins being generally faulted or displaced when inter- 
 sected by the newer. 
 
 INTERNAL CHARACTERISTICS OF VEINS. Internally, 
 veins and dikes are strongly contrasted and it is upon 
 the internal features, chiefly, as previously explained, 
 that we must depend for their distinction. In met- 
 alliferous veins the minerals containing the metal 
 sought for (the galenite, sphalerite, etc.) are the ore ; 
 while the non- metalliferous minerals (the quartz, feld- 
 spar, calcite, etc.) are called the gangue or vein-stone 
 proper. Although the combinations of minerals in 
 veins are almost endless, yet certain associations of 
 ores with each other and with different gangue min- 
 erals are tolerably constant, and constitute an im- 
 portant subject for the student of metallurgy and 
 mining. 
 
 When a vein is composed of a single mineral, as 
 ouartz, it may rival a dike in its homogeneity. Most 
 important veins, however, are composed of several or a 
 
PETROLOGY. I6l 
 
 large number of minerals, which may be sometimes 
 more or less uniformly mixed with each other, but are 
 usually distributed in the fissure in a very irregular 
 manner. The great granite veins which are worked 
 for mica, feldspar and quartz, are good illustrations, 
 on a large scale, of the structure of veins in which sev- 
 eral minerals have been deposited cotemporaneously. 
 The individual minerals are found to a large extent, 
 in great, irregular masses, with no order observable in 
 their arrangement. 
 
 When a mineral is deposited from solution, it crys- 
 tallizes by preference on a surface of similar composi- 
 tion, thus quartz on quartz, feldspar on feldspar, and 
 so on ; and it seems probable that this selective action 
 of the wall-rock may be a principal cause of the irreg- 
 ular distribution of minerals in veins. It has often 
 been observed in metalliferous veins that the richness 
 varies with the nature of the adjacent country rock. 
 This dependence of the contents of a fissure upon the 
 wall-rock may be due in part to the selective deposi- 
 tion of the minerals, and in part to their derivation 
 from the contiguous portions of the country or wall- 
 rock, as in the so-called segregated veins. Temper- 
 ature and pressure exert an important influence upon 
 chemical precipitation, and it is, therefore, probable 
 that the composition of many veins varies with the 
 depth. 
 
 Frequently, perhaps usually, the minerals of com- 
 posite veins are deposited in succession, instead of co- 
 temporaneously, giving rise to the remarkable banded 
 structure so characteristic of this class of veins. The 
 first mineral deposited in the fissure forms a layer 
 
l62 
 
 STRUCTURAL GEOLOGY. 
 
 covering each wall, and is in turn covered by layers of 
 the second mineral, and that by the third, and so on, 
 until the fissure is filled, or the solution exhausted. 
 The distinguishing features of this structure are shown 
 in Fig. 15, in which w w represents the wall-rock, a a, 
 b b, c c are successive layers of quartz, fluorite and 
 barite, and the central band, d, is galenite. Since the 
 vein grows from the outside inward, the outer layers 
 are the oldest, and the central layers are the newest ; 
 again, the layers are symmetrically arranged, being 
 
 repeated in the reverse 
 order on opposite sides 
 of the middle of the vein ; 
 and, lastly, in layers com- 
 posed of prismatic crys- 
 tals, as quartz (see the 
 figure) ; the crystals are 
 perpendicular to the wall 
 and often project into, 
 and even through, the 
 succeeding layers. Such 
 
 a crystalline layer is called a " comb" and the inter- 
 locking of the layers in this way is described as the 
 comb-structure of the vein. The banding of veins is 
 thus strongly contrasted with stratification, and with 
 the structure in dikes due to the more rapid cooling 
 along the walls. The duplicate layers are often dis- 
 continuous and of unequal thickness, on account of 
 the strong tendency to segregation in the materials. 
 This is clearly shown in Fig. 16, drawn on a reduced 
 scale from a polished section of a lead vein in Cum- 
 berland, England, contained in the Museum of the 
 
 Fig. 15. Ideal section of a vein. 
 
PETROLOGY. 163 
 
 Boston Society of Natural History. In this the gangue 
 minerals are fluorite (/) and barite (^). The central 
 band (fg) is a darker fluorite containing irregular 
 masses of galenite. The banded structure of veins 
 is exactly reproduced in miniature in the banding 
 of agates, geodes, and the amygdules formed in old 
 lavas. Unfilled cavities frequently remain along the 
 middle of the vein. When small, these are known 
 
 Fig. 16. Section of a lead vein, one-fifth natural size. 
 
 as " pockets." They are commonly lined with crys- 
 tals ; and when the latter are minute, the pockets are 
 called druses. In metalliferous veins, the ore is much 
 more abundant in some parts than in others, and these 
 ore-bodies, especially when somewhat definite in out- 
 line, are known in their different forms and in different 
 localities, as courses, slants, shoots, chimneys, and 
 bonanzas of ore. The intersections and junctions of 
 veins are often among the richest parts, as if the meet- 
 ing of dissimilar solutions had determined the precipi- 
 tation of the ore. 
 
1 64 STRUCTURAL GEOLOGY. 
 
 Metalliferous veins, especially, are usually deeply 
 decomposed along the outcrop by the action of at- 
 mospheric agencies. The ore is oxidized, and to a 
 large extent removed by solution, leaving the quartz 
 and other gangue minerals in a porous state, stained 
 by oxides of iron, copper, and other metals, forming 
 the gossan or blossom-rock of the vein. 
 
 PECULIAR TYPES OF VEINS. In calcareous or 
 limestone formations, especially, the joint-cracks and 
 bedding-cracks are often widened through the solu- 
 tion of the rock by infiltrating water, and thus become 
 the channels of a more or less extensive subterranean 
 drainage, by which they are more rapidly enlarged to 
 a system of galleries and chambers, and, in some cases, 
 large limestone caverns. The water dripping into the 
 cavern from the overlying limestone is highly charged 
 with carbonate of lime, which is largely deposited on 
 the ceiling and floor of the cavern, forming stalactitic 
 and stalagmitic deposits. These are masses of mineral 
 matter deposited from solution in cavities in the 
 earth's crust, and are essentially vein-formations. 
 Portions of caverns deserted by the flowing streams by 
 which they were excavated, are often rilled up in this 
 way, being converted into irregular veins of calcite. 
 But calcite is not the only mineral found in these 
 cavern deposits, for barite and fluorite, and various 
 lead and zinc ores, especially the sulphides of these 
 metals galenite and sphalerite have also been 
 leached out of the surrounding limestone and concen- 
 trated in the caverns. The celebrated lead mines of 
 the Mississippi Valley, and some of the richest silver- 
 lead mines of Utah and Nevada are of this character, 
 
PETROLOGY. 165 
 
 The forms of these cavern- deposits vary almost indefi- 
 nitely, and are often highly irregular. The principal 
 types are known as gash-veins, flats and sheets (Fig. 
 17), chambers and pockets. 
 
 Where joints and other cracks have opened slightly 
 in different directions and become filled with infiltrated 
 ores, we have what the German miners call a stock- 
 work, an irregular network of small and interlacing 
 veins. 
 
 An impregnation is an irregular segregation of met- 
 alliferous minerals in the mass of some eruptive or 
 
 Fig. 17. Gash-veins and sheets. 
 
 crystalline rock. Its outlines are not sharply defined, 
 but it shades off gradually into the enclosing rock. 
 
 Fahlbands are similar ill-defined deposits or segre- 
 gations in stratified rocks. An impregnation or vein 
 occurring along the contact between two dissimilar 
 rocks is called a contact deposit. These are usually 
 found between formations of different geological ages, 
 and especially between eruptive and sedimentary 
 rocks. 
 
166 STRUCTURAL GEOLOGY. 
 
 Subsequent Structures produced by Subterranean 
 Agencies. 
 
 The subterranean forces concerned in the formation 
 of rocks are chiefly various manifestations of that 
 enormous tangential pressure developed in the earth's 
 crust, partly by the cooling and shrinkage of its 
 interior, but largely, it is probable, by the diminution 
 of the velocity of the earth's rotation by tidal friction, 
 and the consequent diminution of the oblateness of its 
 form. It is well known that the centrifugal force aris- 
 ing from the earth's rotation is sufficient to change the 
 otherwise spherical form of the earth to an oblate 
 spheroid, with a difference of twenty-six miles between 
 the equatorial and polar diameters. It is also well 
 known that while the earth turns from west to east on 
 its axis, the tidal wave moves around the globe from 
 east to west, thus acting like. a powerful friction-brake 
 to stop the earth's rotation. Our day is consequently 
 lengthening, and the earth's form as gradually approach- 
 ing the perfect sphere. This means a very decided 
 shortening and consequent crumpling of the equatorial 
 circumference, and is equivalent to a marked shrink- 
 age of the earth's interior, so far as the equatorial 
 regions are concerned. 
 
 The most important and direct result of the hori- 
 zontal thrust, whether due to cooling or tidal friction, 
 is the corrugation or wrinkling of the crust ; and the 
 earth-wrinkles are of three orders of magnitude, 
 continents, mountain-ranges, and rock- folds or arches. 
 
 Continents and ocean-basins, although the most 
 important and permanent structural features of the 
 
PETROLOGY. 167 
 
 earth's crust, do not demand further consideration 
 nere, since their forms and relations are adequately 
 described in the better text-books of physical geogra- 
 phy. The forms and distribution of mountain-ranges 
 might be dismissed in the same way ; but, unlike con- 
 tinents, the structure of mountains, upon which their 
 reliefs mainly depend, is quite fully exposed to our 
 observation, and is one of the most important fields of 
 the student of structural geology. Mountains, how- 
 ever, as previously explained, combine nearly all the 
 kinds of structure produced by the subterranean agen- 
 cies, and their consideration, therefore, belongs at the 
 end rather than the beginning of this section. 
 
 INCLINED OR FOLDED STRATA. Normally, strata 
 are horizontal, and dikes and veins are vertical or 
 nearly so. Hence the stratified rocks are more ex- 
 posed to the crumpling action of the tangential pres- 
 sure in the earth's crust than the eruptive and vein 
 rocks j and it is for this reason and partly because the 
 stratified rocks are vastly more abundant than the 
 other kinds, that the effects of the corrugation of the 
 crust are studied chiefly in the former. But it should 
 be understood that folded dikes and veins are not 
 uncommon. 
 
 That the stratified rocks have, in many instances, 
 suffered great disturbance subsequent to their deposi- 
 tion, is very evident ; for, while the strata must have 
 been originally approximately straight and horizontal, 
 they are now often curved, or sharply bent and con- 
 torted, and highly inclined or even vertical. All in- 
 clined beds or strata are portions of great folds or 
 arches. Thus we may feel sure when we see a stratum 
 
168 STRUCTURAL GEOLOGY. 
 
 sloping downward into the ground, that its inclination 
 or dip does not continue at the same angle, but that 
 at some moderate depth it gradually changes and the 
 bed rises to the surface again. Similarly, if we look in 
 the opposite direction and think of the bed as sloping 
 upward we know that the surface of the ground is be- 
 ing constantly lowered by erosion, and consequently that 
 the inclined stratum formerly extended higher than it 
 does now, but not indefinitely higher ; for, in imagina- 
 tion, we see it curving and descending to the level of 
 the present surface again. Hence it forms, at the 
 same time, part of one side of a great concave arch, 
 and of a great convex arch, just as every inclined sur- 
 face on the ground indicates both a hill and a valley. 
 And guided by this principle we can often reconstruct 
 with much probability folds that have been more or 
 less completely swept away by erosion, or that are 
 buried beyond our sight in the earth's crust. 
 
 The arches of the strata are rarely distinctly indi- 
 cated in the topography, but must be studied where 
 the ground has been partly dissected, as in cliffs, 
 gorges, quarries, etc. They are also, as a rule, far 
 more irregular and complex than they are usually con- 
 ceived or represented. The wrinkles of our clothing 
 are often better illustrations of rock- folds than the 
 models and diagrams used for that purpose. This 
 becomes self-evident when we reflect that the earth's 
 crust is exceedingly heterogeneous in composition and 
 structure, and must, therefore, yield unequally to the 
 unequal strains imposed upon it. 
 
 The folds or undulations of the strata may be profit- 
 ably compared with water-waves. In fact, the com- 
 
PETROLOGY. 
 
 169 
 
 parison is so close that they have been not inaptly 
 called rock-waves. Folds, like waves,- unless very 
 large, rarely continue for any great distance, but die 
 out and are replaced by others, giving rise to the en 
 echelon or step-like arrangement. The plan of both 
 a wave and a fold is a more or less elongated ellipse, 
 each stratum in a fold being semi-ellipsoidal or boat- 
 shaped. In other words, a normal fold is an elongated 
 mound of concentric strata, being highest at the cen- 
 
 Fig. 18. Anticlinal and synclinal folds. 
 
 tre, sloping very gradually toward the ends, and much 
 more abruptly toward the sides. 
 
 The imaginary line passing longitudinally through a 
 fold, about which the strata appear to be bent, is the 
 axis ; and the plane lying midway between the two 
 sides of a fold and including the axis is the axial 
 plane: The two principal kinds of folds are the anti- 
 cline (Fig. 1 8, A), where the strata dip away from the. 
 axis; and the syncline (Fig. 18, }, where they dip 
 toward the axis. They are commonly, but not al- 
 ways, correlative, like hill and valley. 
 
 Rock-folds are of all sizes, from almost microscopic 
 
170 STRUCTURAL GEOLOGY. 
 
 wrinkles to great arches miles in length and breadth, 
 and thousands of feet in height. The smaller folds, 
 or such as may be seen in hand specimens and even 
 in considerable blocks of stone, are commonly called 
 contortions, and it is interesting to observe that they 
 are, in nearly everything except size, precisely like the 
 large folds, so that they answer admirably as geological 
 models. Large folds, however, are almost necessarily 
 curves, but contortions are frequently angular (Fig. 
 19). With folds, as with waves, the small undulations 
 are borne upon the large ones ; but the contortions 
 are not uniformly distributed. An inspection of Fig. 
 
 Fig. 19. Contorted strata. 
 
 1 8 shows that when the rocks are folded they must be 
 in a state of tension on the anticlines (A], and in a 
 state of compression in the synclines (B), and the 
 latter is evidently the normal position of the pucker- 
 ings or contortions of the strata, as shown in Fig. 20. 
 Contortions are also most commonly found in thin- 
 bedded, flexible rocks, such as shales and schists. 
 And when we find them in hard, rigid rocks, like 
 gneiss and limestone, it must mean either that the 
 structure was developed with extreme slowness, or 
 that the rock was more flexible then and possibly 
 plastic. 
 
 It is very interesting to notice the relations of anti- 
 
PETROLOGY. 
 
 171 
 
 clinal and synclinal folds to the agents of erosion. At 
 the time the folds are made, the anticlinals, of course, 
 are ridges, and the synclinals, valleys, and this relation 
 
 Fig. 20. Contorted syncline. 
 
 sometimes continues, as shown in Fig. 21 ; but we 
 have seen that the rocks in the trough of the synclinal 
 are compressed and compacted, i.e., made more capa- 
 
 Fig. 21. Section of anticlinal mountains. 
 
 ble of resisting erosion, while those on the crest of the 
 anticlinal are stretched and broken, i.e., made more 
 susceptible of erosion. The consequence is that the 
 anticlinals are usually worn away very much faster 
 
172 STRUCTURAL GEOLOGY. 
 
 than the synclinals ; so much faster that in many cases 
 the topographic features are completely transposed, 
 and in place of anticlinal ridges and synclinal valleys 
 
 Fig. 22. Section of synclinal mountains. 
 
 (Fig. 21) we find synclinal ridges and anticlinal val- 
 leys (Fig. 22). 
 
 Besides the anticlinal and synclinal folds already 
 explained, there are folds that slope in only one direc* 
 
 Fig. 23. Monoclinal fold. 
 
 tion, one-sided or monoclinal folds (Fig. 23). Anti- 
 clinal and synclinal folds are symmetrical 'when the dip 
 or slope of the strata is the same on both sides and 
 the axial plane is vertical. The great majority of 
 
PETROLOGY. 173 
 
 folds, however, are unsymmetrical, the opposite slopes 
 being unequal, and the axial planes inclined to the 
 vertical (Fig. 24, A). This means that the compress- 
 ing or plicating force has been greater from one side 
 than from the other, as indicated by the arrows. It 
 acted with the greatest intensity on the side of the 
 gentler slope, the tendency evidently having been to 
 crowd or tip the fold over in the direction of the steep 
 slope. When the steep slope approaches the vertical, 
 this tendency is almost unresisted, and when it passes 
 ,the vertical, gravitation assists in overturning the fold 
 (Fig. 24, B). Such highly unsymmetrical folds, in- 
 
 Fig. 24. Unsymmetrical and inverted folds. 
 
 eluding all cases where the two sides of the fold slope 
 in the same direction, are described as overturned or 
 inverted, although the latter term is not strictly appli- 
 cable to the entire fold, but only to the 'strata compos- 
 ing the under or lee side of it. Fig. 24, B, shows that 
 these beds are completely inverted, the older, as the 
 figures indicate, lying conformably upon the newer. 
 This inversion is one of the most important features 
 of folded strata, and it has led to many mistakes in 
 determining their order of succession. In the great 
 mountain-chains, especially, it is exhibited on the 
 grandest scale, great groups of strata being folded 
 over and over each other as we might fold carpets. 
 
174 
 
 STRUCTURAL GEOLOGY. 
 
 An inverted stratum is like a flattened S or Z, and may 
 be pierced by a vertical shaft three times, as has 
 actually happened in some coal mines. Folds are 
 open when the sides are not parallel, and closed when 
 they are parallel, the former being represented by a 
 half-open, and the latter by a closed, book. Closed 
 folds are usually inverted, and when the tops have 
 been removed by erosion (Fig. 25), the repetition of 
 the strata may escape detection, and the thickness of 
 the section be, in consequence, greatly overestimated. 
 
 Fig. 25. Series of closed folds. 
 
 Thus, a geologist traversing the section in Fig. 25 
 would see thirty-two strata, all inclined to the left at 
 the same angle, those on the right apparently passing 
 below those on the left, and all forming part of one 
 great fold. The repetition of the strata in reverse 
 order, as indicated by the numbers, and the structure 
 below the surface, show, however, that the section 
 really consists of only four beds involved in a series of 
 four closed folds, the true thickness of the beds in this 
 section being only one-eighth as great as the apparent 
 thickness. 
 
 The most important features to be noted in observ- 
 ing and describing inclined or folded strata are the 
 strike and dip. The strike is the compass bearing or 
 
PETROLOGY. 175 
 
 horizontal direction of the strata. It is the direction 
 of the outcrop of the strata where the ground is level. 
 It may also be denned as the direction of a level line 
 on the surface of a stratum, and is usually parallel with 
 the axis of the fold. 
 
 The dip is the inclination of the beds to the plane 
 of the horizon, and embraces two elements : (a) the 
 direction of the dip, which is always at right angles to 
 the strike, being the line of steepest descent on the 
 surface of the stratum, and (b} the amount of the 
 
 Fig. 26. Dip and strike. 
 
 dip, which is the value of the angle between the line 
 of steepest descent and the horizon. 
 
 In Fig. 26, s t is the direction of the strike, and dp 
 that of the dip. The strike and direction of the dip 
 are determined with the compass, and the amount of 
 the dip with the clinometer, an instrument for measur- 
 ing vertical angles. 
 
 The strike is much less variable than the dip, being 
 often essentially constant over extensive districts ; 
 while the dip, except in very large or closed folds, is 
 constantly changing in direction and amount. 
 
 When the dip and surface breadth of a series of 
 strata have been measured, it is a simple problem in 
 
I7 6 STRUCTURAL GEOLOGY. 
 
 trigonometry to determine the true thickness, and the 
 depth below the surface of any particular stratum at 
 any given distance from its outcrop. When the strata 
 are vertical, the surface breadth or traverse measure 
 is equal to the thickness. ' 
 
 By the outcrop of a stratum or formation we ordi- 
 narily understand its actual exposure on the surface, 
 where it projects through the soil in ledges or quarries. 
 But the term is also more broadly defined to mean the 
 exposure of the stratum as it would appear if the soil 
 were entirely removed. It is instructive to observe 
 the relations of the outcrop to the form of the surface. 
 Its breadth varies with its inclination to the surface, 
 appearing narrow and showing its true thickness where 
 it is perpendicular to the surface, and broadening out 
 rapidly where the surface cuts it obliquely. The out- 
 crops of horizontal strata form level lines or bands 
 along the sides of hills and valleys, essentially contour 
 lines in the topography ; and appear as irregular, sinu- 
 ous bands bordering the streams and valleys in the 
 map- view of the country. The outcrops of vertical 
 strata, dikes, or veins, on the other hand, are repre- 
 sented by straight lines and bands on the map. While 
 the outcrops of inclined strata are deflected to the 
 right or left in crossing ridges and valleys, according 
 to the direction and amount of their inclination. 
 
 A geological map shows the surface distribution of 
 the rocks, t.e., gives in one view the forms and arrange- 
 ment of the outcrops of all the rocks in the district 
 mapped, including the trend or strike of the folded 
 strata. The map may be lithological, each kind of 
 rock, as granite, sandstone, limestone, etc., being rep- 
 
PETROLOGY. 177 
 
 resented by a different color ; or, it may be historical, 
 each color representing one geological formation, /.<?., 
 the rocks formed during one period of geological time, 
 without reference to their lithological character. But 
 in the best maps these two methods are combined. 
 The geological section shows the arrangement of the 
 rocks below the surface, revealing the dip of the strata 
 and supplementing the map, both modes of represen 
 tation, the horizontal and vertical, being required to 
 give a complete idea of the geological structure of a 
 country. For a detailed and satisfactory explanation 
 of the construction and use of geological maps and 
 sections, students are referred to Prof. Geikie's " Out- 
 lines of Field Geology." 
 
 CLEAVAGE STRUCTURE. This important structure 
 is now known to be, like rock-folds, a direct result of 
 the great horizontal pressure in the earth's crust. It 
 is entirely distinct in its nature and origin from crys- 
 talline cleavage, and may properly be called lithologic 
 cleavage. It is also essentially unlike stratification and 
 joint-structure. It agrees with stratification in dividing 
 the rocks into thin parallel layers, but the cleavage 
 planes are normally vertical instead of horizontal. And 
 the cleavage planes differ from joints in running in 
 only one direction, dividing the rock into layers j 
 while joints, as we shall see, traverse the same mass of 
 rock in various directions, dividing it into blocks. 
 
 The principal characteristics of lithologic cleavage 
 are : (i) It is rare, except in fine-grained, soft rocks, 
 having its best development in the slates, roofing slates 
 and school slates affording typical examples. Hence 
 it is commonly known as slaty cleavage. (2) The 
 
78 
 
 STRUCTURAL GEOLOGY. 
 
 cleavage planes are highly inclined or vertical, very 
 constant in dip and strike, and quite independent of 
 stratification. (3) It is usually associated with folded 
 strata, and often with distorted nodules or fossils. The 
 more important of these characteristics are illustrated by 
 
 Fig. 27. This 
 represents a 
 block cf con- 
 torted strata 
 in which the 
 dark layers 
 are slate with 
 very perfect 
 cleavage par- 
 allel to the 
 left - hand 
 shaded side 
 of the block ; 
 while the 
 white layers 
 are sandstone 
 and quite 
 destitute of 
 cle avage. 
 Many expla- 
 nations of 
 
 this interesting structure have been proposed, but that 
 first advanced by Sharpe may be regarded as fully 
 established. He said that slaty cleavage is always 
 due to powerful pressure at right angles to the planes 
 of cleavage. All the characteristics of cleavage noted 
 above are in harmony with this theory. Cleavage is 
 
 Fig. 97. Slaty cleavage in contorted strata. 
 
PETROLOGY. 179 
 
 limited to fine-grained or soft rocks, because these 
 alone can be modified internally by pressure, without 
 rupture. Harder and more rigid rocks may be bent 
 or broken, but they appear insusceptible of minute 
 wrinkling or other change of structure affecting every 
 particle of the mass. Since the cleavage planes are 
 normally vertical, the pressure, according to the theory, 
 must be horizontal. That this horizontal pressure 
 exists and is adequate in direction and amount, is 
 proved by the folds and contortions of the cleaved 
 strata; for, as shown in Fig. 27, the cleavage planes 
 coincide with the strike of the foldings, and are thus 
 perpendicular to the pressure horizontally as well as 
 vertically. The distortion of the fossils in cleaved 
 slates is plainly due to pressure at right angles to the 
 cleavage, for they are compressed or shortened in that 
 direction, and extended or flattened out in the planes 
 of cleavage. Again, Tyndall has shown that the mag- 
 netism of cleaved slate proves that it has been power- 
 fully compressed perpendicularly to the cleavage. And, 
 finally, repeated experiments by Sorby and others have 
 proved that a very perfect cleavage may be developed 
 in clay (unconsolidated slate) by compression, the 
 planes of cleavage being at right angles to the line of 
 pressure. When, however, Sharpe's theory had been 
 thus fully demonstrated, the question as to how pres- 
 sure produces cleavage still remained unanswered. 
 Sorby held that clay contains foreign particles with 
 unequal axes, such as mica-scales, etc., and that these 
 are turned by the pressure so as to lie in parallel 
 planes perpendicular to its line of action, thus produc- 
 ing easy splitting or cleavage in those planes. And 
 
l8o STRUCTURAL GEOLOGY. 
 
 he proved by experiments that a mixture of clay and 
 mica-scales does behave in this way. But Tyndall 
 showed that the cleavage is more perfect just in pro- 
 portion as the clay is free from foreign particles, and 
 in such a perfectly homogeneous substance as beeswax, 
 he developed a more perfect cleavage than is possible 
 in clay. His theory, which is now universally accepted, 
 is, that the clay itself is composed of grains which are 
 flattened by pressure, the granular structure with 
 irregular fracture in all directions, changing to a scaly 
 structure with very easy and plane fracture or splitting 
 in one definite direction. 
 
 Observations on distorted fossils and nodules have 
 shown that when slaty cleavage is developed, the rock 
 is, on the average, reduced in the direction of the 
 pressure to two-fifths of its original extent, and corre- 
 spondingly extended in the vertical direction. Thus, 
 whether rocks yield to the horizontal pressure in the 
 earth's crust, by folding and corrugation, or by the 
 flattening of their constituent particles, they are alike 
 shortened horizontally and extended vertically ; and it 
 is impossible to overestimate the importance of these 
 facts in the formation of mountains. 
 
 FAULTS OR DISPLACEMENTS. We may readily con- 
 ceive that the forces which were adequate to elevate, 
 corrugate, and even crush vast masses of solid rock 
 were also sufficient to crack and break them ; and 
 eince the fractures indicate that the strains have been 
 applied unequally, it will be seen that unequal move- 
 ments of the parts must often result. If this unequal 
 movement takes place, i.e., if the rocks on opposite 
 sides of a fracture of the earth's crust do not move 
 
PETROLOGY. l8l 
 
 together, but slip over each other, a fault is produced. 
 The two sides may move in opposite directions, or in 
 the same direction but unequally, or one side may 
 remain stationary while the other moves up or down. 
 It is simply essential that the movement should be 
 unequal in direction, or amount, or both ; that there 
 should be an actual slip, so that strata that were once 
 continuous no longer correspond in position, but lie 
 at different levels on opposite sides of the fracture. 
 The vertical difference in movement is known as the 
 throw, slip, or displacement of the fault. Fault-fract- 
 
 Fig. 28. Section of a normal fault. Fig. 29. Section of a reversed fault. 
 
 ures rarely approach the horizontal direction, but are 
 usually highly inclined or approximately vertical. 
 When the fault is inclined, as in Fig. 28, the actual 
 slipping in the plane of the fault exceeds the vertical 
 throw, for the movement is then partly horizontal, the 
 beds being pulled apart endwise. The inclination of 
 faults, as of veins and dikes, should be measured from 
 the vertical and called the hade. Faults are some- 
 times hundreds of miles in length ; and the throw 
 may vary from a fraction of an inch to thousands of 
 feet. 
 
 Transverse sections, such as are represented by 
 Fig. 28 and many specimens and models, do not give 
 
l82 STRUCTURAL GEOLOGY. 
 
 the complete plan or idea of a fault ; but this is seen 
 more perfectly in Fig. 30. We learn from this that a 
 typical fault is a fracture along which the strata have 
 sagged or settled down unequally. The most impor- 
 tant point to be observed here is that the strata do 
 not drop bodily, but are merely bent, the throw being 
 greatest at the middle of the fault and gradually dimin- 
 ishing toward the ends. In other words, every simple 
 fault must die out gradually ; for we cannot conceive 
 of a fault as ending abruptly, except where it turns 
 
 Fig. 30. Ideal view of a complete fault. 
 
 upon itself so as to completely enclose a block of the 
 strata, which may drop down bodily ; but the fault is 
 then really endless. A fault may be represented on 
 a map by a line ; if a simple fault, by a single straight 
 line. But faults are often compound, and are repre- 
 sented by branching lines; that is, the earth's crust 
 has been broken irregularly, and the parts adjoining 
 the fracture have sagged or risen unequally. 
 
 The rock above an inclined fault, vein, or dike 
 (Fig. 28) is called the hanging wall, and that below 
 the foot wall. Now inclined faults are divided into 
 two classes, according to the relative movements of 
 
PETROLOGY. 183 
 
 the two walls. Usually, the hanging wall slips down 
 and the foot wall slips up, as in Fig. 28. Faults on 
 this plan are so nearly the universal rule that they are 
 called normal faults. They indicate that the strata 
 were in a state of tension, for their broken ends are 
 pulled apart horizontally, so that a vertical line may 
 cross the plane of a stratum without touching it. 
 
 A few important faults have been observed, however, 
 in which the foot-wall has fallen and the hanging-wall 
 has risen (Fig. 29). These are known as reversed 
 
 Fig. 31. Explanation of normal faults. 
 
 faults ; and they indicate that the strata were in a state 
 of lateral compression, the broken ends of the beds 
 having been pushed horizontally past each other, so 
 that a vertical line or shaft may intersect the same bed 
 twice, as has been actually demonstrated in the case 
 of some beds of coal. 
 
 The usual explanation of normal faults is given in 
 Fig. 31. The inclined fractures of the earth's crust 
 must often be converging, bounding, or enclosing large 
 V-shaped blocks (A, J5) . If now, through any cause, 
 as the folding of the strata, they are brought into a 
 
184 STRUCTURAL GEOLOGY. 
 
 state of tension, so that the fractures are widened, the 
 V-shaped masses, being unsupported, settle down, the 
 fractures bounding them becoming normal faults, as 
 is seen by tracing the bed X through the dislocations. 
 The single fracture below the block A is inclined, and 
 the stretching has been accomplished by slipping along 
 it and faulting the bed Z as well as X, the entire section 
 to the right of this fracture being part of a much larger 
 V-shaped block the right-hand boundary of which is 
 not seen. But the united fracture below the block B 
 being vertical, any horizontal movement must widen it 
 into a fissure, which is kept open by the great wedge 
 above and may become the seat of a dike or mineral 
 vein. The beds below the V may, in this case, escape 
 dislocation, as is seen by tracing the bed Z across the 
 fissure. These pairs of converging normal faults are 
 called trough faults ; and this is the only way in which 
 we can conceive of important faults as terminating at 
 moderate depths below the surface, and not affecting 
 the entire thickness of the earth's crust. 
 
 Important reversed faults are believed to occur 
 chiefly along the axes of overturned anticlines (Fig. 
 24) where the strata have been broken by the unequal 
 strains, and those on the upper side shoved bodily 
 over those on the lower or inverted side. 
 
 An extensive displacement of the strata is sometimes 
 accomplished by short slips along each of a series of 
 parallel fractures, producing a step fault. 
 
 Faults cutting inclined or folded strata are divided 
 into two classes, according as they are approximately 
 parallel with the direction of the dip or of the strike. 
 The first are known as transverse or dip faults, and 
 
PETROLOGY. 185 
 
 the second as longitudinal or strike faults. The chief 
 interest of either class consists in their effect upon the 
 outcrops of the faulted strata, after erosion has removed 
 the escarpment produced by the dislocation. 
 
 Dip faults cause a lateral shift or displacement of 
 the outcrops, as shown in Fig. 32, which represents a 
 plan or map-view of the strata traversed by the fault 
 b b, the down throw being on the right and the up 
 throw on the left. The dip of the strata is indicated 
 by the small arrows and the accompanying figures ; 
 
 Fig. 32. Plan of a dip fault. 
 
 and it will be observed on tracing the outcrop of any 
 stratum, a a, across the fault that it is shifted to the 
 right. If the throw of the fault were reversed, the 
 displacement of the outcrop would be reversed also. 
 Strike faults are of two kinds, according as they incline 
 in the same direction as the strata, or in the contrary 
 direction. The effect of the first kind is to conceal 
 some of the beds, as shown in Fig. 33, in which 
 beds 5 and 6 do not outcrop, but we pass on the sur- 
 face abruptly from 4 to 7. The apparent thickness of 
 the section is thus less than the real thickness. When 
 the fault inclines against the strata, on the other hand 
 
186 STRUCTURAL GEOLOGY. 
 
 (Fig. 34), the outcrops of certain strata are repeated 
 on the surface ; and a number of parallel faults of this 
 kind, a step fault, will, like a series of closed folds 
 (Fig. 25), cause the apparent thickness of the section 
 to greatly exceed the real thickness. Repetition of the 
 strata by faulting is distinguished from repetition by 
 folding by being in the same instead of the reverse 
 order. 
 
 Folds and faults are really closely related. In the 
 former the strata are disturbed and displaced by bend- 
 ing ; in the latter by breaking and slipping ; and the 
 
 4 G 9 _*_ a B 4 
 
 Fig. 33. Strike fault, concealing Fig. 34. Strike fault, repeating 
 
 strata. strata. 
 
 displacement which is accomplished by a fold may 
 gradually change to a fracture and slip. This relation 
 is especially noticeable with monoclinal folds (Fig. 23), 
 in which the tendency to shear or break the beds is 
 oftei) very marked. 
 
 Important faults are rarely simple, well-defined frac- 
 tures ; but, in consequence of the enormous friction, 
 the rocks are usually more or less broken and crushed, 
 sometimes for a breadth of many feet or yards. The 
 fragments of the various beds are then strung along 
 the fault in the direction of the slipping, and this cir- 
 cumstance has been made use of in tracing the con- 
 tinuation of faulted beds of coal. In other cases the 
 
PETROLOGY. 187 
 
 direction of the slip is plainly indicated by the bend- 
 ing of the broken ends of the strata (Fig. 35), and the 
 beds are sometimes turned up at a high angle or even 
 overturned in this way. 
 
 Since faults are not plane, but undulating and often 
 highly irregular, fractures, the walls will not coincide 
 after slipping; and if the rocks are hard enough to 
 resist the enormous pressure, the cavities or fissures 
 produced in this way may remain open. Now faults 
 are continuous fractures of the earth's crust, reaching 
 
 Fig- 35 Section of beds distorted by a fault. 
 
 down to an unknown but very great depth ; and hence 
 they afford the best outlets for the heated subterranean 
 waters ; so that it is common to find an important 
 fault marked on the surface by a line of springs, and 
 these are often thermal. The warm mineral waters on 
 their way to the surface deposit part of the dissolved 
 minerals in the irregular fissures along the fault, which 
 are thus changed to mineral veins. This agrees with 
 the fact that the walls of veins usually show faulting 
 as well as crushed rock, slickensides, and other evi- 
 dences of slipping. 
 
 If the earth's surface were not subject to erosion, 
 
i88 STRUCTURAL GEOLOGY. 
 
 every fault would be marked on the surface by an 
 escarpment equal in height to the throw of the fault ; 
 and, notwithstanding the powerful tendency of erosion 
 to obliterate them, these escarpments are sometimes 
 observed, although of diminished height. Thus, ac- 
 cording to Gilbert, the Zandia Mountains in New 
 Mexico are due to a fault of 1 1,000 feet, leaving an 
 escarpment still 7,000 feet high. But, as a rule, there 
 is no escarpment or marked inequality of the surface, 
 the fault, like the fold, not being distinctly indicated 
 in the topography. In all such cases we must con- 
 clude either that the faults were made a very long 
 time ago, or that they have been formed with extreme 
 slowness, so slowly that erosion has kept pace with the 
 displacement, the escarpments being worn away as 
 fast as formed. These and other considerations make 
 it quite certain that extensive displacements are not 
 produced suddenly, but either grow by a slow, creep- 
 ing motion, or by small slips many times repeated at 
 long intervals of time. 
 
 JOINTS AND JOINT-STRUCTURE. This is the most 
 universal of all rock-structures, since all hard rocks 
 and many imperfectly consolidated kinds, like clay, 
 are jointed. Joints are cracks or planes of division 
 which are usually approximately vertical and traverse 
 the same mass of rock in several different directions. 
 They are distinguished from stratification planes by 
 being rarely horizontal, and from both stratification 
 and cleavage planes by being actual cracks or frac- 
 tures, and by dividing the rock into blocks instead of 
 sheets or layers. The art of quarrying consists in 
 removing these natural blocks ; and most of the broad. 
 
PETROLOGY. 189 
 
 flat surfaces of rock exposed in quarries, are the joint- 
 planes (Fig. 36). Some of the most familiar features 
 of rock-scenery are also due to this structure, cliffs, 
 ravines, etc., being largely determined in form and 
 direction by the principal systems of joints ; and we 
 have already seen that the same is true of veins and 
 dikes. 
 
 Joints are divided by their characteristics and modes 
 >f origin into three classes as follows : 
 
 Fig. 36. Quarry showing two systems of parallel joints. 
 
 i . The parallel and intersecting joints. This is by 
 far the most important class, and has its best develop- 
 ment in stratified rocks, such as sandstone, slate, lime- 
 stone, etc. These joints are straight and continuous 
 cracks which may often be traced for considerable 
 distances on the surface. They usually run in several 
 definite directions, being arranged in sets or systems 
 by their parallelism. Thus in Fig. 36 one set of 
 joints is represented by the broad, flat surfaces in 
 
190 STRUCTURAL GEOLOGY. 
 
 light, and a second set crossing the first nearly at right 
 angles, by the narrower faces in shadow. By the inter- 
 sections of the different sets of joints the rock is 
 divided into angular blocks. 
 
 Although many explanations of this class of joints 
 have been proposed, it has long been the general 
 opinion of geologists that they are due to the contrac- 
 tion of the rocks, i.e., that they are shrinkage cracks. 
 We shall soon see, however, that they lack the most 
 important characters of cracks known to be due to 
 shrinkage ; and the present writer has advanced the 
 view that movements of the earth's crust, and especially 
 the swift, vibratory movements known as earthquakes, 
 are a far more adequate and probable cause. It is 
 well known that earthquakes break the rocks ; and, if 
 space permitted, it could be shown that the earthquake- 
 fractures must possess all the essential features of par- 
 allel and intersecting joints. 
 
 2. Contraction joints or shrinkage cracks. That 
 many cracks in rocks are due to shrinkage, there can 
 be no doubt. The shrinkage may result from the 
 drying of sedimentary rocks ; but more generally from 
 the cooling of eruptive rocks. Every one has noticed 
 in warm weather, the cracks in layers of mud or clay 
 on the shore, or where pools of water have dried up ; 
 and we have already seen that these sun-cracks are 
 often preserved in the hard rocks. They have certain 
 characteristic features by which they may be distin- 
 guished from the joints of the first class. They divide 
 the clay into irregular, angular blocks, which often 
 show a tendency to be hexagonal instead of quad- 
 rangular. The cracks are continually uniting and 
 
PETROLOGY. 191 
 
 dividing, but are not parallel, and rarely cross each 
 other. Sun-cracks never affect more than a few feet 
 in thickness of clay, and are an insignificant structural 
 feature of sedimentary rocks. In eruptive rocks, on the 
 other hand, the contraction joints have a very exten- 
 
 Fig. 37. Columnar dike. 
 
 sive, and, in some cases, a very perfect development, 
 culminating in the prismatic or columnar jointing of 
 the basaltic rocks. This remarkable structure has 
 long excited the interest of geologists, and, although 
 the basalt columns were once regarded as crystals, and 
 later as a species of concretionary structure, it is now 
 generally recognized as the normal result of slow cool- 
 
192 STRUCTURAL GEOLOGY. 
 
 ing in a homogeneous, brittle mass. The columns are 
 normally hexagonal, and perpendicular to the cooling 
 surface, being vertical in horizontal sheets and lava 
 flows, as in the classic examples of the Giant's Cause- 
 way and Fingal's Cave, and horizontal in vertical 
 dikes (Fig. 37). They begin to grow on the cooling 
 surface of the mass and gradually extend toward the 
 centre, so that dikes frequently show two independent 
 sets of columns. 
 
 3. The concentric joints of granitic rocks. In 
 quarries of granite and other massive crystalline rocks, 
 it is often very noticeable that the rock is divided into 
 more or less regular layers by cracks which are ap- 
 proximately parallel with the surface of the ground, 
 some of the granite hills having thus a structure re- 
 sembling that of an onion. The layers are thin near 
 the surface, become thicker and less distinct down- 
 wards, and cannot usually be traced below a depth of 
 fifty or sixty feet. These concentric cracks are of 
 great assistance in quarrying, and are now regarded as 
 due to the expansion of the superficial portions of the 
 granite caused by the heat of the sun. In reference 
 to this view of their origin these may be properly 
 called expansion joints. 
 
 STRUCTURE OF MOUNTAIN-CHAINS. Mountains are 
 primarily of two kinds, volcanic and non- volcanic. 
 The structure of the former belongs properly with the 
 original structures of the volcanic rocks ; but the 
 latter the true mountains owe their internal struc- 
 ture and altitude or relief almost wholly to the crump- 
 ling and mashing together of great zones of the earth's 
 crust, being, as already pointed out, the culminating 
 
PETROLOGY. 193 
 
 points of the plication, cleavage, and faulting of the 
 strata. " A mountain-<:to'# consists of a great plateau 
 or bulge of the earth's surface, often hundreds of miles 
 wide and thousands of miles long. This is usually 
 more or less distinctly divided by great longitudinal 
 valleys into parallel ranges and ridges ; and these, 
 again, are serrated along their crests, or divided into 
 peaks by transverse valleys. In many cases this ideal 
 chain is far from realized, but we have instead, a great 
 bulging of the earth's crust composed on the surface 
 of an inextricable tangle of ridges and valleys of ero- 
 sion, running in all directions. In all cases, however, 
 the erosion has been immense ; for the mountain- 
 chains are the great theatres of erosion as well as of 
 igneous action. As a general fact, all that we see, 
 when we stand on a mountain-chain every peak and 
 valley, every ridge and canon, all that constitutes scen- 
 ery is wholly due to erosion." LE CONTE. 
 
 The structure of mountains thus falls under two 
 heads : ( i ) The internal structure and altitude, which 
 are due to the action of the subterranean agencies. 
 (2) The external forms, the actual relief, which are 
 the product chiefly of the superficial agencies or 
 erosion. The study of mountains has shown that : 
 ( i ) They are composed of very thick sedimentary for- 
 mations. Thus the sedimentary rocks have a thickness 
 of 40,000 feet in the Alleghanies ; of 50,000 feet in the 
 Alps ; and of two to ten miles in all important moun- 
 tain-chains. Such thick deposits of sediments, as we 
 have already seen, must be formed on a subsiding sea- 
 floor, and in many mountain-chains, as in the Alle- 
 ghanies, the great bulk of these sediments are still 
 
194 STRUCTURAL GEOLOGY. 
 
 below the level of the sea. Again, thick sedimentary 
 deposits can only be formed in the shallow, marginal 
 portions of the sea; and when such a belt of thick 
 shore deposits yields to the powerful horizontal thrust, 
 and is crumpled and mashed up, it is greatly shortened 
 in the direction of the pressure and thickened verti- 
 cally, so that its upper surface is lifted high above the 
 level of the sea, and a mountain-chain is formed and 
 added to the edge of the continent. We thus find an 
 explanation of the important fact that on the several 
 continents, but notably on the two Americas, the prin- 
 cipal mountain-ranges are near to and parallel with the 
 coast lines. 
 
 2. The mountain-forming sediments are usually 
 strongly folded and faulted, and exhibit slaty cleavage 
 wherever they are susceptible of that structure ; and 
 the older rocks, especially, in mountains are often 
 highly metamorphosed, and are traversed by numer- 
 ous veins and dikes, the infallible signs of intense 
 igneous activity. 
 
 " In other words, mountain regions have been the 
 great theatres (i) of sedimentation before the moun- 
 tains were formed; (2) of plication and upheaval in 
 the formation of the range ; and (3) of erosion which 
 determined the present outline. Add to these the 
 metamorphism, the faults, veins, dikes, and volcanic 
 outbursts, and it is seen that all geological agencies 
 concentrate there." LE CONTE. 
 
 Since mountain-ranges are great up-swellings or 
 bulgings of trfe strata, their structure is always essen- 
 tially anticlinal ; and they sometimes consist of a sin- 
 gle more or less denuded anticline (Fig. 38), the 
 
PETROLOGY. IQS 
 
 oldest and lowest strata exposed forming the summit 
 of the range. More commonly, however, the single 
 great arch or uplift is modified by a series of longitudi- 
 nal folds, as shown in the section of the Jura Moun- 
 tains (Fig. 21). Still more commonly the folds are 
 closely pressed together, overturned, broken, and al- 
 most inextricably complicated by smaller folds, con- 
 tortions, and slips. 
 
 The strata on the flanks of the mountains are usually 
 less disturbed than those near the axis of the range, 
 and are sometimes seen to rest unconformably against 
 
 Fig. 38. Anticlinal mountain. 
 
 the latter. In this way it is proved that some ranges 
 are formed by successive upheavals. But we have 
 still more conclusive evidence that mountains are 
 formed with extreme slowness in the fact that rivers 
 sometimes cut directly through important ranges. 
 This proves, first, that the river is older than the 
 mountains ; second, that the deepening of its channel 
 has always kept pace with the elevation of the range. 
 
 CONCRETIONS AND CONCRETIONARY STRUCTURE. 
 Folds, cleavage, faults, and joints all the subsequent 
 structures considered up to this point are the prod- 
 uct of mechanical forces. Chemical agencies, although 
 very efficient in altering the composition and texture 
 of rocks, are almost powerless as regards the develop- 
 
iq6 STRUCTURAL GEOLOGY. 
 
 ment of rock-structures ; and the only important 
 structure having a chemical origin is that named 
 above. 
 
 Concretions are formed by the segregation of one 
 or more of the constituents of a rock. But there are 
 three distinct kinds of segregation. If the water per- 
 colating through or pervading a rock, dissolves a cer- 
 tain mineral and afterwards deposits it in cavities or 
 fissures, amygdules, geodes, or veins are the result. If 
 the mineral is deposited about particular points in the 
 mass of the rock, it may form crystals, the rock be- 
 coming porphyritic ; or it may not crystallize, but build 
 up instead the rounded forms called concretions, the 
 texture or structure of the rock becoming concretionary. 
 A great variety of minerals occur in the form of con- 
 cretions, but this mode of occurrence is especially 
 characteristic of certain constituents of rocks, such as 
 calcite, siderite, limonite, hematite, and quartz. Con- 
 cretions may be classified according to the nature of 
 the segregating minerals ; and in each class we may 
 distinguish the pure from the impure concretions. A 
 pure concretion is one entirely composed of the seg- 
 regating mineral. Most nodules of flint and chert, 
 quartz, geodes, concretions of pyrite, and many hollow 
 iron-balls are good illustrations of this class. In all 
 these cases the segregating mineral has been able in 
 some way to remove the other constituents of the rock, 
 and make room for itself. But in other cases it has 
 lacked this power, and has been deposited between 
 and around the grains of sand, clay, etc. ; and the 
 concretions are consequently impure, being composed 
 partly of the segregating mineral, and partly of the 
 
PETROLOGY. 197 
 
 other constituents of the rock. The calcareous con- 
 cretions known as clay- stones are a good example of 
 this class, being simply discs of clay, all the minute 
 interstices of which have been filled with segregated 
 calcite. The solid iron-balls are masses of sand filled 
 in a similar manner with iron oxides. 
 
 Concretions are of all sizes, from those of micro- 
 scopic smallness in some oolitic limestones up to those 
 twenty-five feet or more in diameter in some sand- 
 stones. 
 
 The point of deposition, when a concretion begins to 
 grow, is often determined by some concrete particle, 
 as a grain or crystal of the same or a different mineral, 
 a fragment of a shell, or a bit of vegetation, which thus 
 becomes the nucleus of the concretion. The ideal or 
 typical concretion is spherical ; but the form is influ- 
 enced largely by the structure of the rock. In porous 
 rocks, like sandstone, they are frequently very perfect 
 spheres ; but in impervious rocks, like clay, they are 
 flat or disc-shaped, because the water passes much 
 more freely in the direction of the bedding than across 
 it ; while the concretions in limestones, the nodules of 
 flint and chert, are often remarkable for the irregularity 
 of their forms. In all sedimentary rocks the concre- 
 tions are arranged more or less distinctly in layers 
 parallel with the stratification, which usually passes 
 undisturbed through the impure concretions. Many 
 silicious and ferruginous concretions are hollow, ap- 
 parently in consequence of the contraction of the sub- 
 stance after its segregation ; and the shrinkage due to 
 drying is still further indicated by the cracks in the 
 septaria stones. The hollow, silicious concretions are 
 
198 STRUCTURAL GEOLOGY. 
 
 usually lined with crystals (geodes), while the holiow 
 iron-balls frequently enclose a smaller concretion. 
 Rocks often have a concretionary structure when 
 there are no distinct or separable concretions. And 
 the appearance of a concretionary structure (pseudo- 
 concretions) is often the result of the concentric de- 
 composition of the rocks by weathering, as explained 
 on page 13. 
 
 SUBSEQUENT STRUCTURES PRODUCED BY THE SUPER- 
 FICIAL OR AQUEOUS AGENCIES. The superficial agen- 
 cies, as we have seen in the section on dynamical 
 geology, are, in general terms, water, air, and organic 
 matter. Geologically considered, the results which 
 they accomplish, may be summed up under the two 
 heads of deposition and erosion the formation of 
 new rocks in the sea, and the destruction of old rocks 
 on the land. In the role of rock-makers they produce 
 the very important original structures of the stratified 
 rocks ; while as agents of erosion they develop the 
 most salient of the subsequent structures of the earth's 
 crust the infinitely varied relief of its surface. As a 
 general rule, to which recent volcanoes are one im- 
 portant exception, the original and subterranean struc- 
 tures of rocks are only indirectly, and often very 
 slightly, represented in the topography; for this, as 
 we have seen, is almost wholly the product of erosion. 
 Therefore, what we have chiefly to consider in this sec- 
 tion is to what extent and how erosion is influenced 
 by the pre-existing structures of rocks. 
 
 Horizontal or very slightly undulating strata, espe- 
 cially if the upper beds are harder than those below, 
 give rise by erosion to flat-topped ridges or table- 
 
PETROLOGY. 
 
 199 
 
 mountains (Fig. 39). But if the strata be softer and 
 of more uniform texture, erosion yields rounded hills, 
 often very steep, and sometimes passing into pinnacles, 
 as in the Bad Lands of the west. Broad, open folds, 
 as we have seen, give, normally, synclinal hills and 
 anticlinal valleys (Fig. 22), when the erosion is well 
 advanced. But in more strongly, closely folded rocks 
 
 Fig. 39. Horizontal strata and table-mountains. 
 
 the ridges and valleys are determined chiefly by the 
 outcrops of harder and softer strata, as shown in Fig. 
 40, the symmetry of the reliefs depending upon the 
 dip of the strata. This principle of unequal hardness 
 or durability also determines most of the topographic 
 
 Fig. 40. Ridges due to the outcrops of hard strata. 
 
 features in regions of metamorphic and crystalline 
 rocks, in which the stratification is obscure or wanting. 
 The boldness of the topography, and the relation of 
 depth to width in valleys, depends largely upon the 
 altitude above the sea ; but partly, also, upon the dis- 
 tribution of the rainfall, the drainage channels or val- 
 leys being narrowest and most sharply defined in arid 
 regions traversed by rivers deriving their waters from 
 
200 STRUCTURAL GEOLOGY. 
 
 distant mountains. That these are the conditions 
 most favorable for the formation of canons is proved 
 by the fact that they are fully realized in the great 
 plateau country traversed by the Colorado and its 
 tributaries, a district which leads the world in the mag- 
 nitude and grandeur of its canons. But deep gorges 
 and canons will be formed wherever a considerable 
 altitude, by increasing the erosive power of the streams, 
 enables them to deepen their channels much more 
 rapidly than the general face of the country is lowered 
 by rain and frost. This is the secret of such canons 
 as the Yosemite Valley, and the gorge of the Columbia 
 River, and probably of the fiords which fret the north- 
 west coasts of this continent and Europe. For a full 
 description and illustration of the topographic types 
 developed by the action of water and ice upon the 
 surface of the land, and of the various characteristic 
 forms of marine erosion, teachers are referred to the 
 larger works named in the introduction, especially 
 Le Conte's Elements of Geology, and to the better 
 works on physical geography. We will, in closing this 
 section, merely glance at some of the minor erosion- 
 forms, which are not properly topographic, but may be 
 often illustrated by class-room and museum specimens. 
 Mere weathering, the action of rain and frost, develops 
 very characteristic surfaces upon different classes of 
 rocks, delicately and accurately expressing in relief 
 those slight differences in texture, hardness, and solu- 
 bility, which must exist even in the most homogeneous 
 rocks. Every one recognizes on sight the hard, smooth 
 surfaces of water-worn rocks. They are exemplified 
 in beach and river pebbles, in sea-worn cliffs, and 
 
PETROLOGY. 201 
 
 where rivers flow over the solid ledges. The pot-hole 
 (page 17) is a well-marked and specially interesting 
 rock-form, due to current or river erosion. 
 
 Ice has also left highly characteristic traces upon 
 the rocks in all latitudes covered by the great ice- 
 sheet. These consist chiefly of polished, grooved, and 
 scratched or striated surfaces, the grooves and scratches 
 showing the direction in which the ice moved. 
 
 The organic agencies, as already noted, accomplish 
 very little in the way of erosion, especially in the hard 
 rocks, but the rock-borings made by certain mollusks 
 and echinoderms may be mentioned as one unimpor- 
 tant but characteristic form due to organic erosion. 
 
APPENDIX 
 
 The following collections are especially pre- 
 pared and arranged for use with this text : 
 
 2 1 Opalized wood 
 *22 Gypsum 
 *23 Calcite 
 24 Dolomite 
 
 25 Siderite 
 *26 Hornblende 
 27 Pyroxene 
 *28 Muscovite 
 
 29 Biotite 
 *3<D Orthoclase 
 3i Albite 
 *32 Labradorite 
 *33 Kaolinite 
 
 34 Talc 
 *35 Serpentine 
 36 Chlorite 
 
 37 Glauconite (Green Sand) 
 
 38 Chrysolite 
 
 39 Garnet 
 
 40 Pyrite 
 
 Weathering 
 
 i Diabase 
 
 *2 " weathered 
 *3 " disintegrated 
 4 Felsite : Angular fragment 
 5 " Water rounded 
 pebble 
 
 Formation of Coals 
 
 *6 Peat 
 
 ; Lignite 
 8 Bituminous 
 
 *9 Cannel coal 
 io Anthracite 
 n Native coke 
 
 Rock-forming Minerals 
 
 *i2 Graphite 
 I3 Halite 
 *i4 Limonite 
 *I5 Hematite 
 *i6 Magnetite 
 
 17 " Lodestone 
 
 *i8 Quartz: Glassy 
 
 19 " Flint 
 
 20 " Chert 
 
 Sedimentary and Metamorphic 
 Rocks 
 
 *4i Conglomerate : Breccia 
 *42 " Pudding- 
 
 stone 
 
APPENDIX 
 
 203 
 
 *43 Sand : Quartz 
 
 44 " Magnetite 
 
 *45 Sandstone: Ferruginous 
 
 46 " Calcareous 
 
 47 " Arkose 
 *48 Quartzite 
 
 49 Clay: Boulder 
 5<D " Fire 
 *5i Shale 
 *52 " Carbonaceous 
 
 53 Slate: Roofing 
 
 54 " Flagstone 
 
 55 Porcelainite 
 
 56 Tripolite 
 
 57 Siliceous Tufa 
 
 58 Novaculite 
 59 Asphaltum 
 6o Oil Sand 
 
 *6i Limestone: Fossiliferous 
 *62 Coquina 
 
 #63 " Chalk 
 
 64 " Crystalline 
 
 %5 " Compact 
 
 66 " Hydraulic 
 
 67 Calcareous Tufa 
 
 68 Dolomite 
 
 69 Rock Salt 
 
 70 Phosphate Nodule 
 *7i Gneiss : Granitoid 
 *72 " Micaceous 
 73 " Hornblendic 
 
 74 Norite : Hypersthenite 
 *75 Schist : Mica 
 
 76 " Hornblende 
 
 77 " Talc 
 
 78 " Chlorite 
 
 79 Amphibolite 
 
 80 Soapstone 
 
 8 1 Verd Antique (Serpen- 
 
 tine) 
 
 Igneous Rocks 
 
 *82 Granite : Binary 
 83 " Muscovite 
 *84 " Biotite 
 
 85 " Hornblendic 
 
 86 " Red 
 *87 Syenite 
 
 88 " Elaeolite 
 *8g Diorite 
 *9O Diabase : Trap 
 *9i Rhyolite 
 
 92 Trachyte 
 *93 Obsidian 
 
 94 Pumice 
 95 Petrosilex 
 *96 Andesite 
 *97 Basalt^- 
 
 98 " vesicular Lava 
 *99 Melaphyr: Amygdaloidal 
 ioo Volcanic Tuff 
 
 Collection No. Fi. Entire list of 100 museum size spe- 
 cimens (3%x4X)> numbered, labelled and mounted 
 on blocks or in improved trays, for museum display 
 
 and laboratory work $40.00 
 
 (The same, labelled but unmounted, $30.00) 
 
204 APPENDIX 
 
 Collection No. F2. Same as above, but small museum 
 size, mounted in improved trays (2^x3^) .... $25.00 
 
 Collection No. fj. Same as Fz, but hand size speci- 
 mens (2x2) 12.50 
 
 Collection No. F4. 80 specimens, omitting those 
 marked (), in individual trays (2^x1^) and two 
 cloth-board cases, numbered to correspond with ac- 
 companying printed list (no labels) 5.00 
 
 Collection No. F^. 40 specimens marked (*), mounted 
 as collection F4 2.50 
 
 Collection No. F6. 100 pupils' fragments (ixi), num- 
 bered, in paper bags. (Single collection $1.25.) In 
 lots of 5 or more, each i.oo 
 
 Collection No. Fj. 80 pupils' fragments (like F6). 
 
 (Single $1.00.) In lots of 5 or more, each .... .75 
 
 Collection No. F8. 40 pupils' fragments (like F6). 
 
 (Single 5oc.) In lots of 5 or more, each .40 
 
 Collection No. Fg. 25 museum size specimens, illus- 
 trating structure, faults, stratification, etc. Mounted 
 and labelled 10.00 
 
 For further information or in ordering, address 
 
 WARD'S NATURAL SCIENCE ESTABLISHMENT 
 
 84-102 College Ave., Rochester, N. Y. 
 
14 DAY USE 
 
 RETURN TO DESK FROM WHICH BORROWED 
 EARTH SCIENCES LIBRARY 
 
 This book is due on the last date stamped below, or 
 
 on the date to which renewed. 
 Renewed books are subject to immediate recall. 
 
 LD 21-40m-5,'65 
 
 General Library 
 University of California 
 
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