RARY ' ■ TV OP ^ fHIA A CRUZ ^ SANTA CRUZ Z O < u O CO w > Z Dr. Gift ol John H. 4 Feth ^M. H PC m C z < tn ?« en H 03 > SANTA CRUZ A COMPEND OF GEOLOGY BY JOSEPH LE CONTE PROFESSOR OF GEOLOGY AND NATURAL HISTORY IN THE UNIVERSITY OF CALIFORNIA; AUTHOR OF "ELEMENTS OF GEOLOGY," ETC. I^EW YORK . CnsrCIKNATI . CHICAGO AMERICAN BOOK COMPANY Copyright, 1884, by D. APPLETON & CO. Copyright, 1898, by AMERICAN BOOK COMPANY LE CONTE, GEOL. W. P. 9 PREFACE. \ %^2 \^ preparing this little work for the schools I have kept constantly in view two ends : 1. I have tried to make a book which shall interest the pupil, and at the same time convey real scientific knowledge. 2. I have tried, as far as possible, to awaken the faculty and cultivate the habit of observation, by directing the attention of the pupil to geological phenomena occurring and geological agencies at work noio on every side, and in the most familiar tilings. By the former I hope to awaken a true scientific appetite ; by the latter, to cultivate the habits necessary to satisfy that appetite. Joseph Le Conte. Berkeley, California, September, 1884. PREFACE TO THE REVISED EDITION. Although a work so elementary as this — embodying only the most general principles of geology — does not require so frequent revision as a more advanced work, yet geology is so rapidly advancing a science that even gen- eral statements must, from time to time, be modified. Especially is it necessary that new and better illustrative figures should be used. 3 4 PREFACE TO THE REVISED EDITION, In this revised edition I have not changed the general plan of the work as already explained in the preface of the previous edition, but have only made such modi- fications and additions as seemed necessary to bring it up to the present condition of science. Among these additions, certainly not the least important are the beau- tiful restorations of vertebrate skeletons and photo- graphs of natural objects for which I am indebted to Professors Marsh, Dean, Williston, and Scott. I wish hereby to thank these gentlemen for their hearty cooper- ation. Acknowledgment is due also to the American Museum of Natural History, New York, N. Y., for the photograph of the Great Barrier Reef, shown on page 97. Joseph Le Conte. Berkeley, California, January, 1898. CONTENTS PAGE Introduction 7 Part 1. — Dyn^amical Geology. 3HAPTER I. Atmospheric Agencies 10 II. Aqueous Agencies 17 III. Organic Agencies 83 IV. Igneous Agencies 131 Part II. — Structural Geology. I. General Form and Structure of the Earth . . 173 II. Stratified Rocks 179 III. Unstratified or Igneous Rocks 210 IV. Metamorphic Rocks 224 V. Structures Common to All Rocks .... 228 VI. Denudation, or General Erosion 252 Part III. — Historical Geology. I. General Principles 256 II. Arch^an System and Archeozoic Era . . . 263 III. Paleozoic Rocks and Era 267 IV. Mesozoic Era. — Age of Reptiles 324 V. Cenozoic Era. — Age of Mammals 363 VT. PsYCHOZOic Era.— Age op Man 407 Index 417 6-6 Digitized by the Internet Archive in 2007 with funding from IVIicrosoft Corporation http://www.archive.org/details/compendgeologyOOIecorich GEOLOGY. INTRODUCTION. Definition of Geology. — Greology is the science which tieats of the past conditions of the earth and of its inhab- itants. It is, therefore, a history of the earth. It is closely allied to physical geography, but differs in this : Physical geography treats only of the present forms of the earth^s features ; geology also, and mainly of their grad- ual formation, or evolution from former conditions. It is also closely allied to natural history, but differs in this : Natural history is concerned only with the present forms and distribution of animals and plants, while geology is chiefly concerned about previous forms and distribution, and their changes to the present forms and distribution. In a word, geography and natural history are concerned about how things are ; geology, about how they became so. Cultivates Habit of Observation. — We have said geology treats of the history of the past conditions of the earth and its inhabitants. The evidences of the past con- ditions are found in its present structure. But, to under- stand this structure, we must observe the manner in which similar structure is formed now under our eyes. Thus, observation of causes noiv ifi operation constitutes the only solid foundation of geology. Fortunately, the proc- esses by which structure is now being formed may be ob- served everywhere ; and the structures which have been 8 INTRODUCTION, thus formed in earlier times may be observed in very many places^ if we know how to look for them. Thus geology, perhaps more than any other science, cultivates the habit of field-observation ; not, indeed, that minute observation required by mineralogy or botany, but that wider observa- tion which gives interest to mountain-travel or even to rambles over the hills in our vicinity. It cultivates also, in an eminent degree, the habit of tracing effects to their causes — ^for the question ever present to the geologist is, " How came it so f " Great Divisions of Geology. — We have said that the history of the earth is recorded in its structure, and that structure is understood by study of causes or processes now in operation. We have thus outlined the great di- visions of geology, and the order in which they must be studied. We must study, first of all, causes and processes now in operation about us everywhere, producing struc- ture. This is called dynamical geology. Next, we must study the rocky structure of the earth to as great a depth as we can, and apply the previously acquired principles in its interpretation ; for this structure has been produced by similar processes acting through all previous time. This is called structural geology. Only after this shall we be prepared to take up the history of the changes through which the earth has passed ; for this history is revealed in structure. This is called historical geology. PART I. DYNAMICAL GEOLOGY. As already said, this part treats of agencies now in operation producing structure. These are best treated under four heads — ^viz., atmospheric, aqueous, organic, and igneous agencies. The same agencies have operated fifrom the beginning, though probably with different de- grees of activity. Their accumulated effect, through inconceivable ages, is the present structure of the earth. We observe these operations now, in order to understand the effects of their operation then. CHAPTER I. ATMOSPHERIC AGENCIES. Ori^n of Soil. — If we dig into the earth anywhere, at a certain depth, greater in some places than in others, we find rock. How was the earthy soil formed ? Per- haps some imagine that it is an original clothing intended to cover the rocky nakedness of the new-born earth. But the very first lesson to be learned by the study of geology is that all things that we see, even the most enduring — such as hills, mountains, rocks, etc. — have become what they are, usually by a sloiv process. Now, soils are no exceptions. All soil is formed by a^ disintegration or rotting doivn of rocks.- Sometimes the soils remain resting on the rocks from which they were formed ; sometimes they are removed to another place, as, e. g., from hillsides to bottom-lands ; sometimes they are carried by streams to great distaiices, and deposited as sediments, and again raised as land ; but in all cases they are formed in the same way — viz., by the rotting down of rocks under the slow action of the atmosphere. The active ingredients of the air in this process are oxygen, carbonic acid (carbon dioxide), and water, as vapor or as moisture. Now, rain-water contains in solu- tion both oxygen and carbon dioxide. Therefore, rain- water, wetting the surface and penetrating the cracks of rocks, is the great agent in the formation of soil. Proofs of this Origin of Soils. — The proofs of this mode of formation are clearest in those cases in which ATMOSPHERIC AGENCIES. H the soil still rests on the rock from which it was made. Unfortunately this is rare in the northern part of our country, where the soil has been nearly everywhere shifted during a period which we shall hereafter describe as the Drift period. But in the southern part of the United States, on all the hillsides and mountain-tops, the soil has been undisturbed for ages, and the evidence is com- plete, and may be observed by any one. If, for example, we note carefully the sections made by railroad and well diggings, we shall see at the top 'perfect soil, perhaps red ; a little deeper it becomes lighter colored and coarser grained ; then it begins to look like rotten rock ; and, finally, by insensible degrees, it passes into sound rock. The evidence is still more complete if, as is often the case, the rock is traversed by a quartz-vein. In such a case we can trace the quartz-vein through the sound rock, and upward through the rotten rock, the imperfect soil, and the perfect soil, to the surface, where it may usually be traced over hill and dale as white fragments lying on the surface. The reason is this : Quartz is a mineral which will not disintegrate under atmospheric agency ; therefore it remains sound, while all the rest of the rock is changed into soil (Fig. 1). Fia. 1.— Section and perspective view (ideal), a, sound rocli ; 6, rotten rock ; <7, perfect soil ; c?, quartz-vein ; rf', same, outcropping on surface ; e, mass of more resistant rock imbedded in soil. Sometimes a rounded mass of sound rock, e, is seen imbedded in the soil. This is only a harder piece of 12 DYNAMICAL GEOLOGY. rock, which has resisted disintegration, while the rest has yielded. These are called bowlders of disintegration. It is not always, even in lower latitudes, that we find this gradation between soil and rock. Often perfect soil is found to rest on sound rock, with sharp limit between. In all such cases there has been shifting of the soil. In northern latitudes (37°-40° northward), as already stated, the soil nearly everywhere rests on sound rock, and often the underlying rock is smooth and polished. We shall explain this hereafter. But even in the Northern States, if one will notice closely, he will see the process of soil- making going on. Rock-fragments, which were once an- gular, become rounded by rotting of the corners. Cliffs, by their crumbling, gather piles of rock-fragments and earth {talus) at their bases (Fig. 2). The pupil ought to Fig. 2.— Cliff, showing talus, t, and bowlders of disintegration, 6, b. observe these things habitually, as it is on just such observation of simple things that true science rests. Depth of Soil. — Since soil is constantly carried away by washing of rain, as will be more fully explained in the next chapter, it* is evident that there are two opposite processes here to be considered, viz., soil-formation and soil-removal. The depth of the soil will depend on the relation of these two to each other. More definitely, the depth of the soil depends partly upon the kind of 'rock (for this affects the rate of formation), and partly on the slope (for this affects the rate of removal). On high ATMOSPHERIC AGENCIES. 13 slopes the rock is hare (Fig. 3, a), not because there is no soil formed, but because it is removed as fast as formed. v:';;V>1>v;-,rr,-^'-<^ Fia. 3. — «, sound rock ; 6, rotten rock ; c, soil formed in place ; d, soil shifted from e. On flat lands, near high slopes, the soil is deep (Fig. 3, h), because not only is it formed here in place, but the wash- ings from above are added. Rate of Disiutegration. — If rocks were solid, so that the agents of decomposition could act only on the sur- face, the rate would be inconceivably slow, but all rocks are affected with joints in several directions, by which the mass is divided into more or less separable blocks, so that a cliff looks something like a wall of regularly piled blocks without cement (Fig. 2). Water, therefore, pene- trates to great depths, attacking the surface of every block. Also, every block is itself affected throughout with capillary fissures, through which water penetrates to every part (quarry-water of stone-cutters). Thus, the rocky crust of the earth is affected by disintegrating agencies to very great depths — ^though, of course, most rapidly at the surface. Bowlders of Disintegration. — All over the Northern States are found scattered rock-masses (bowlders), lying on the surface. If we examine these, we shall usually find that they are entirely different from the country- rock. They have been brought from a distance — how, we shall explain hereafter. We have nothing to do with 14 DYNAMICAL GEOLOGY. these now. But in the Southern States also, in many places, are found huge, isolated masses, lying on the sur- face, and even sometimes forming rocking stones (Fig. 4). If we examine these, we find that they are of the same Fig. 4. material as the country-rock. They have been formed in place. In the general disintegration of rock, and forma- tion and removal of soil, these have resisted, because harder than the rest. Nothing is more interesting than thus to trace the configuration of the surface of the country to unequal resistance to atmospheric agencies. Explanation of Kock-Disinte^ation. — If we take a piece of old and very hard mortar, and pour on it a little hydrochloric acid, it quickly breaks down into sand, wet with a solution of calcium chloride. The explanation is simple. Moi:tar consists of grains of sand cemented into a mass by hydrate or carbonate of lime. The acid dis- solves the lime-cement, and the mass falls to powder. Now, mortar is really artificial stone, and nearly all rock is constituted in a similar manner, i. e., consists of particles cemented together. In all rock some parts are soluble in atmospheric water, and some are not. Under the long- continued action of this agent, therefore, the soluble parts are dissolved, and the mass breaks down into a powder, or dust of the insoluble parts, wet with a solu- tion of the soluble parts. The main difference between the experimental and the natural case is, that in one the process is rapid, and in the other extremely slow. Examples. — One or two examples will make this plain : ATMOSPHERIC AGENCIES. 15 1. Sandstone is a rock made up of grains of sand cemented into a mass, sometimes by lime carbonate, sometimes by silica. Under the slow action of atmospheric water the cement is dissolved, and the rock crumbles into sand, moistened with a solution of lime carbonate, if this be the cement. 2. Oranite and gneiss and many other igneous and metamorphic rocks, such as are found on the eastern slope of the Appalachian Chain everywhere, are an aggre- gation of four minerals, viz., quartz, feldspar, mica, and liornhlende. In coarse granite these can be easily seen with the naked eye. The bluish glassy specks are quartz ; the opaque white, or rose-color, are feldspar ; the glistening scales are mica ; and the black spots, horn- blende.* The whole rock may be regarded as grains of quartz, mica, and hornblende cemented into a mass by feldspar. Now, quartz is not at all, and mica very slightly, affected by atmospheric water ; but the feldspar and hornblende are slowly changed into clay, which, in the case of hornblende, is red, from the presence of iron. Thus, the whole rock rots down to a clay soil, usually red, in which are disseminated grains of quartz and scales of mica, the whole moistened with water, contain- ing in solution a little potash derived from the feldspar. This is the commonest of all soils. 3. Slates and shales are clays hardened into rock by some cement such as lime or silica. When the cement is dissolved the rock crumbles into a clay soil. 4. A pure limestone like mar- ble makes no soil because it is all soluble, but most lime- stones are mixed with clay or sand. When the lime is dissolved the result is a limy clay or limy sand. Mechanical Action of Air; Frosts. — The soil-for- mation, above explained, is a chemical process, but, in cold climates and mountain-regions, atmospheric water acts also mechanically and very powerfully in rock-dis- * These minerals ought to be shown the pupil, both separately and as aggregated in a specimen of coarse granite. 16 DYNAMICAL GEOLOGY, integration. Water penetrating the joints, and freezing, expands with such force that the rocks are riven asunder ; and then, penetrating again into the capillary fissures and freezing, these blocks are in their turn broken into smaller fragments, until the whole crumbles to dust. Wind. — Again, loose earth, sand, and dust, especially in dry climates, are carried by winds, and sometimes accu- mulate in large quantity and form a peculiar soil. Thus, the sands of Sahara are in some places encroaching on the fertile lands of Egypt. Thus, also, sea-sands are often carried inland from shore, and cover up and destroy fertile lands. The sand-hills to the west of San Fran- cisco are made in this way. The phenemena of sand- dunes may be observed in many places along the coasts of nearly all countries. Some geologists think that in the interior of dry countries, like Asia or the western part of our own country, soil of great thickness has been formed by accumulation of dust. CHAPTER II. AQUEOUS AGENCIES. Aqueous and atmospheric agencies are so closely con- nected that many treat them together under the one head of levelmg agencies. Water, as atmospheric moisture or as rain, soaking into the earth, is the chief agent of soiU making ; but water, falling more abundantly, runs off the surface, and is also the chief agent of soil-removal. In the one case it acts as a chemical, in the other as a me- chanical, agent. The agency of water in soil-making we treated under atmospheric, its agency in soil-removal be- longs to aqueous, agencies. The one, acting at all times and in all places, its effects are obscure and inconspicu- ous ; the other, acting occasionally and concentrating its power on particular places, its effects are easily observed and better understood. Nevertheless, the aggregate ef- fects of the one must be equal to those of the other, for the former prepares the way for the latter. Aqueous agencies have little effect upon rocks unless they have been first rotted down to soils. Although the agency of water is mainly mechanical, yet there is a chemical agency of water other than that of soil-making. The agency of water may therefore be divided into mechanical and chemical. The mechanical agency is best treated under the three heads of rivers, ocean, and ice, and each of these again in cutting away, in carrying, and in throwing down again, or in erosion, trans'portation, and deposit. The chemical agency we shall consider under the two heads of chemical deposits in springs and in lakes : Le Contb, Geol. 2 18 DYNAMICAL GEOLOGY. i Rivers, erosion, transportation, deposit. C iAIeclianical.. J Ocean, Agency 1 ' ^''' / Chemical j Springs, chemical deposits in, I Lakes, •*' *' Section I. — Rivers. Atmospheric or meteoric water falls on land as rain. A portion sinks into the earth, and, after a longer or shorter subterranean course and doing its appropriate work of rock-disintegration and soil-making, comes up again to the surface as springs. Another portion runs off the surface, cutting and carrying away the soil everywhere. Qui'^kly, however, it gathers into rills and cuts furrows, these rills uniting into streamlets and cutting gullies. The streamlets, uniting with each other, and with water issuing from springs, form mountain-torrents, and cut out great ravines, gorges, and caflons. Finally, the torrents, emerging on the plains from their mountain home, form great rivers, which deposit their freight of gathered earth and rock-fragments in their courses, and finally in the sea or lake into which they empty. Such is a condensed his- tory of the course and work of water from the time it falls as rain until it reaches the ocean from which it came. All of this we include under river-agency. It may be defined as the work of rain and rivers, or the work of circulating meteoric water. All that follows on this subject will be but an expansion of the condensed statement given above, and much of it may be observed by any one who does not commit the mistake of thinking things insignificant because they are common. 1. Erosion of Rain and Rivers. The rain which falls on land-surface may be divided into three parts : One part runs immediately from the surface, producing universal rain-erosion and the muddy 1 ^n AQUEOUS AGENCIES, 19 floods of the rivers. Another part sinks into the earth, and, after doing its appointed work of soil-making, re- appears on the surface as springs, and forms the ordinary flow of rivers in dry times. This part joins the surface drainage, and together they concentrate their work along certain lines, and thus produce stream-erosio7i. A third portion never reappears on the surface, but finds its way, by subterranean passages, to the sea. By the continued action of rain and rivers all lands (except some rainless deserts) are being cut away and carried to the sea. Every one, each in his own vicinity, may see this process going on. The soil of the hillsides is everywhere being washed away by rain, and carried off in the muddy streams. At what average rate is this wash- ing process going on ? This is a question of extreme importance. Averag-e Rate of Erosion. — By observations made on rivers in all parts of the world it has been estimated that all land-surfaces are being cut away at a rate of about one foot in 3,000 to 5,000 years. The Mississippi cuts down its whole drainage-basin one foot in 5,000 years, the Ganges one foot in 2,000 years. Some rivers cut still more rapidly, but most less rapidly than these. The rate differs in different parts of the same basin. In mountain- regions the rate is at least three times the average given above, and on steeper slopes still greater. On the lower plains the erosion is small, and in many places there is deposit instead of erosion. Making due allowance for all these variations, it is probable that all land-surfaces are being cut down and lowered by rain and river erosion at a rate of one foot in 5,000 years. At this rate, if we take the mean height of lands as 1,200 feet, and there be no antagonistic agency at work raising the land, all lands would be cut down to the sea-level and disappear in 6,000,000 years. This universal cutting away of land-surfaces we have 20 DYNAMICAL GEOLOGY, divided for convenience into two parts, which, however, graduate completely into each other — viz., rain-erosion and stream-erosion : the one is universal, hut small and in- conspicuous in any one place ; the other is confined to water-channels, but works with concentrated and con- spicuous effects. The one may he compared to a univer- sal sand-paper iiig, the other to the action of the graver's tool, cutting ever deeper along the same lines. Of the two, the general rain-erosion, though less conspicuous, is probably far the greater in aggregate amount. They co- operate in cutting away the land, and, if unopposed, would finally destroy it. Ptire toater, however, has comparatively little effect. Its graving-tools are the sand, gravel, peb- bles, and rock-fragments, which it carries along in its course. Conspicuous Examples of Stream-Erosion. — The effects of erosion are most conspicuously seen in water- falls, ravines, gorges, and cations ; but also, in less degree, on every hillside, and in every furrow and gully. Waterfalls ; Niagara. — The Niagara Falls and gorge are an instructive example of stream-erosion, because the effects are easily observed from year to year. General Configuration of the Country. — Lake Erie is situated on a nearly level plateau, several hundred feet above a similar plateau, on which is situated Lake On- tario. The plateaus are separated by an almost perpen- dicular cliff, running east and west, near Lake Ontario. The Niagara River runs out of Lake Erie, and on the Erie plateau, fifteen to eighteen miles, then drops, by a perpendicular fall, into a narrow gorge, with nearly per- pendicular sides, and runs in the gorge for seven miles, and then emerges on the Ontario plateau just before emptying into that lake. Fig. 5 is an ideal section through the middle of the river, and showing these facts. The light lines show the cliffs on the, other side of the gorge. Recession of the Falls. — Ever since their discovery. AQUEOUS AGENCIES. 21 200 years ago, the falls have steadily worked their way back toward Lake Erie. The rate of recession has been estimated at one to three feet per annum. The cause is easily perceived. The strata at the falls consist of E.E E.P. /■' ,^^f, — - — rnft ^^'""..j _0^R Fig. 5.— Section of Niaguia Falls and gorge. O.P., Ontario plateau ; E.P., Erie plateau ; L.O., Lake Ontario ; /, fall ; mb, stratified mud-banks. solid limestone, represented in the figure by the jointed structure, underlaid by softer shale. The force of the dashing water cuts away the soft shale and undermines the limestone, causing it to project as overhanging rocks, which fall from time to time into the abyss below. Thus the falls work backward, but remain perpendicular. Gorgre formed by Recession. — There can be no doubt that the whole gorge has been formed in this way ; that the river once fell over the cliif which runs across its course near Lake Ontario, and then worked its way back to its present position ; and the work is still going on. The general configuration of the country suggests this origin even to the casual observer, and close examination entirely confirms it. It is a familiar fact that stratified mud-banks are found in spots along the margins of all rivers, evidently formed by deposits from the river. These stratified muds often contain the shells of the mussels which inhabit the river. Now, in several spots (mb) along the top of the gorge-cliff, from the falls to Lake Ontario, are found such stratified mud-deposits contain- ing shells. The deposits were evidently made when the river ran at that level. f 22 DYNAMICAL GEOLOGY. Time. — Several attempts have been made to estimate the time occupied in this process. Mr. Lyell estimated it at 35,000 years.* A large part, if not the whole of this time, belongs to the present geological epoch, and was probably witnessed by early man. "^ Other Falls. — Many other perpendicular falls have receded in a similar way and given rise to similar gorges. The most remarkable of these are the Falls of St. Anthony, The Mississippi Eiver, at Fort Snelling (mouth of the Minnesota Eiver), is traversed by an escarpment which separates a higher from a lower plateau. The river runs on the upper plateau as far as Minneapolis, then drops, by a nearly perpendicular fall, into a gorge one hundred feet deep, runs in this gorge eight miles, and then emerges on the lower plateau at Fort Snelling. Here, again, we have the upper plateau capped by a hard limestone, under- laid by a soft sandstone. Here, also, the wearing away of the underlying sandstone causes the limestone to project in overhanging tables which fall from time to time into the chaspi below, and so the fall works backward. There is no doubt that the Mississippi at one time fell over the escarpment at Fort Snelling, and has worked its way back to its present position, and that this all took place during the present geological epoch, and while man inhabited the continent. Professor Winchell has estimated that, at its present-rate recession, it would take not more than 8,000 years to accomplish the work. Minnehaha River is a tributary running into the Mis- sissippi about six miles below the falls. It therefore, at one time, fell into the gorge. It has now worked itself back about two miles, and forms the beautiful " Minne- haha Falls, ^^ made celebrated by their description in Long- fellow's ^^ Hiawatha.'' The Columbia River, where it breaks through the Cascade Kange, has cut a gorge fifty miles long and 1,000 * Later estimates make it about 11,000 years. AQUEOUS AGENCIES, 23 to 3,000 feet deep. All the tributaries which run into the river at this point have cut deep side gorges, headed by perpendicular falls. Some of the most exquisite falls are here nestled among the hills in these almost inaccessible gorges. The country rock is a very hard but much jointed lava, underlaid by a softer cement-gravel. The falls have eaten out the gravel and undermined the lava, which from time to time tumbles into the chasm as blocks that are carried away by the stream. In this way the falls have worked back about two miles. Yosemite Falls. — Most perpendicular falls have been made by recession, as explained above, but this is not true of all. The Yosemite Falls (of which there are six, vary- ing in height from 400 to 1,600 feet) have not perceptibly receded. This is because the granite is very hard, and the time too short (probably only a few thousand years), since the valley was filled with ice (page 394). Ravines, Oorg-es, Canons. — These are found in all countries, especially in mountainous and high-plateau re- gions. They are always or nearly always formed by run- ning water, although in some cases their places are deter- mined by fractures of the earth's crust (page 229). They are gullies on a large scale. In the Appalachian Chain the most striking examples are the Hudson River gorge in New York, the Tallulah gorge in Georgia, and the French Broad gorge in North Carolina. But it is in the western part of the continent that the finest examples are seen. Nowhere in the world are they on a grander scale, more evidently due to water alone, or more recent in origin. As we are studying ^'causes now in operation,^' they are the most instructive examples to be found anywhere. In California there was, even since middle geological times, an old river-system different from the present. This will be explained more fully hereafter (page 395). These old river-valleys were filled up with river-gravel, and finally obliterated by lava-flows not long before the 24 DYNAMICAL GEOLOGY, advent of man. The displaced rivers have since that time cut new channels, far deeper than the old, so that the old lava-covered channels are high up on the present divides (Fig. 6). Thus, in very recent geological times — i. e., in ailii Fig. 6. — Section across old and new river beds of California, r, r, new river beds ; r', old river bed; gr, gravels of present rivers; g-r', old river gravels; dotted line, old configuration of surface. the Quaternary and present epochs — water has cut at least 2,000 feet deep in hard slate-rock. We have selected these cases because of the plain evi- dence of recent work, but the whole western slope of the Sierra is trenched with enormous ravines, 3,000 to 6,000 feet deep, although the history of some of them is longer than those spoken of above. For example, commencing north and going southward, we have the Columbia River, with its gorge 3,000 feet deep in hard lava. The branches of the Feather, Yuba, and American Rivers have cut gorges 2,000 to 3,000 feet deep in hard slate. These have the structure represented by Fig. 6, and have been cut wholly in very recent geological times. The Tuolumne and Merced Rivers have cut gorges 3,000 to 5,000 feet deep, i\vQ famous Hetch-hetchy and Yosemite Valleys being in the course of these. King^s River Cafion is 7,000 feet deep, in hard granite. . Plateau Region. — But the most wonderful gorges or cafions in the world are found in the high-plateau region — i. e., the region between the Colorado and Wahsatch Mountains, and drained by the Colorado River. This region is 6,000 to 8,000 feet high, and consists of nearly AQUEOUS AGENCIES. 25 level strata, which have been cut into by the Colorado and its tributaries in such wise that the whole river system of the country runs far below the general level. The Grand Caflon of the Colorado is 300 miles long and 3,000 to 6,000 feet deep, and all its tributaries come in by side caflons of almost equal depth (Fig. 7). Fig. 7.— View of Colorado Canon and its tributaries, with erosion-columns and mesas in the distance. Besides this prodigious stream cutting, the general rain- erosion has been here upon an equally grand scale. Many thousands of feet have been carried away over the whole area of about 100,000 square miles or more. This is shown by the isolated peaks and tables of level strata scattered about, and still better by the succession of cliffs shown in 26 DYNAMICAL GEOLOGY. Fig. 156 (page 250), as will be more fully explained here- after. Time. — The time during which the whole of this enor- mous work was done is but a small portion of the geolog- ical history. It commenced in Middle Tertiary (page 383), continued to the present time, and is still going on. Pot holes. — If we examine the bare rocky beds of swift streams in mountain regions, we often find deep holes with vertical walls like small rock wells. In their bottoms we are sure to find gravel and a good many rounded pebbles. These are called pot lioles. They are formed thus : Swift streams form whirling eddies, in which sand, gravel, and rock fragments carried by the stream are whirled about in the same spot until they hol- low out these holes, while the fragments themselves are rounded into pebbles in the process. These become signs of old river beds where rivers no longer exist. , 2. Transportation and Distributio7i of Sediments. River agency, it will be, remembered, is taken up under three heads. We have already taken up one — Erosion. The other two are best taken up together, as Transporta- tion and Distribution of Sediments. Transporting Power of Water. — Every one is fa- miliar with the fact that running water carries along materials of different degrees of fineness, but the rate at which the carrying or lifting power increases with the velocity is almost incredible to those who have not inves- tigated the subject. It is found that the size or weight of the separate particles or fragments movable by running water increases at the enormous rate of the sixth power of the velocity of the current. Thus, if the velocity of a current be doubled, it can carry a stone sixty-four times as great as before ; if it be increased ten times, it can carry a stone 1,000,000 times as great as before. We can AQUEOUS AGENCIES. 27 thus easily understand the prodigious power of mountain- torrents when swollen by heavy rains. It follows from the above that, if a stream be carrying all it can, the least checking of its velocity will cause abundant deposit, and the least increase of its velocity will cause it to take up again what it had previously deposited — i. e., it will scour its bed and banks. Sorting- Power of Water. — If we take a. handful of earth and throw it into a deep basin, and, after allowing it to settle, pour off the water and examine the sediment, we shall find that it is neatly sorted, the coarser particles being at the bottom, and above this finer and finer, until a very fine, smooth mud forms the top. The earth will be still better sorted if we throw it into run7iincf water. In this case the coarser will drop first, i. e., higher up, and the finer lower and lower, until only the finest will be carried far down the stream. This is especially the case if the velocity decreases as we go down-stream, as is usually the case in natural streams. Thus, pebbles are found in torrent-beds, and fine mud in lower parts of streams. Stratification. — If we examine carefully the mud or sand of a river-bank or lake-margin, we sliall always find them stratified, i. e., in layers of slightly different color and grain. This is easily explained by the sorting power of water. If the water be still, as in a lake or pond, then with every rain earth is brought in, and by settling is sorted, the finest falling last. Thus the coarse material of one rain falls on the fine of the previous rain, and every rain is marked by a separate layer. In rivers, the same result follows, but the explanation is a little differ- ent. The velocity of the current is changing from day to day on account of the varying supply of water. The stream -lines also are continually shifting from side to side. Thus the velocity at any one point is all the time changing, and therefore the character of the material 28 DYNAMICAL GEOLOGY. deposited is also changing from day to day, and even from hour to hour — now coarser, now finer — and a very distinct, though often irregular, stratification is the result. General Law. — We may therefore state it as a general law that all deposits in water, whether still water, as lakes and seas, or running water, as rivers, are stratified, and, conversely, that all stratified 7naterials, wherever we find them, whether near water or high up on the tops of mountains, and in whatsoever condition we find them, whether as sands and muds or as hard stone, if the strati- fication be a true stratification, i. e., the result of sorted material, have been deposited in water. Upon this very simple law nearly the whole of geological reasoning is based. It is important, therefore, that every one should habitually observe the phenomena described above, not only in lakes and rivers but in shower-rills and pools. We are now in position to explain all the phenomena of rivers. 1. Final Effect of Erosion of Rain and Rivers. There is a certain slope of a river-bed, depending on the amount of sediment carried, at which the river neither cuts nor deposits. This is called its base-level of erosion. Every river is seeking this level. If above it, it seeks to reach it by cutting ; if below it, by building up by sedi- mentation. As soon as the river reaches this level, it rests, so far as down-cutting is concerned, but now begins to sweep from side to side, widening its channel. Mean- while the side streams and rain-wash continue to cut down the divides. If this continues without interrun- tion, the final result is a gently undulating land- Surface with low divides and broad river channels. This is called a Peneplain. It means that the land has remained steady and the rivers have been working on it, a long time. It is old topography. If the land he now elevated by interior forces the rivers increase in velocity and begin AQUEOUS AGENCIES, 29 cutting again and form deep caflons. These deep caflons are therefore characteristic of new topography — i. e., of a rising or newly risen land and of rivers far above their base-level and working hard to reach it. If on the other hand the land sinks, the rivers become more sluggish — they cease to cut and begin to build up by deposit. Thus river channels become delicate indicators of the move- ments of the earth^'s crust. 2. Winding Course of Rivers, The winding course of rivers is the necessary result of the laws of currents. Streams do not find irregular chan- nels to which they are forced to conform, but they make their own channels. If we straighten these channels, they will not remain straight. Some point will wear into a hollow. This will throw the stream to the other side, which will in like manner be worn, and thus the stream begins to meander. Now, if we examine any winding stream, we shall see that the swiftest current is on the outer part of the curve and the slowest on the inner side, or, in other words, the current is swifter than the aver- age on the outer and slower than the average on the inner side of the bends. In the figure, the arrows show the Fig. 8. line of swiftest current. Now, if the river is carrying all the sediment its average velocity can, it is evident that it will cut on its outer curve, where the velocity is greater than the average, and deposit and make land on the inner side, Avhere the velocity is less than the average. Thus the outer curve is increased by erosion and the 30 DYNAMICAL GEOLOGY, inner curve by deposit, and the winding tends ever to become greater and greater. This is most conspicuous in cases in which rivers run between mud-banks made by their own deposit. In such cases, ^t^^aan^s be^mne greater and greater, until finally two C(mMguouscurves cut into each other, the river straightens~itself, and the old bend is thrown out and becomes a lagoon (d, Fig 9). Fig. 9— a, 6, c, succeesive stages in the winding course of a river. Many such lagoons exist in all rivers which run through swamp-lands. Fig. 9 shows the process, and Fig. 10 is a portion of the lower Mississippi Eiver showing the result. Fig. 10.— a portion of lower Mississippi, 3. Flood-Plains and Their Deposits, Elvers usually rise in hilly or mountainous regions, and flow in the lower course through flat plains. In flood seasons, the velocity being checked by change of slope, the channels are no longer able to contain their waters, which therefore overflow portions of the flat lands on each side. IVie area liable to overflow is called the flood-plain. In case of great rivers draining interior continental basins, the flood-plains are very large. The flood-plain of the AQUEOUS AGENCIES. 31 Nile is the whole land of Egypt, for without the Nile the whole of Egypt would be a desert. Egypt is literally the daughter of Nilus. The flood-plain of the Mississippi extends from the mouth of the Ohio Eiver to the Gulf — its area is 30,000 square miles. Now, since great rivers always rise in mountain-regions, and since the general rain- erosion in such regions is very great, it is evident that in flood seasons they gather abun- dant sediment, and, when these muddy waters overflow, the checking of velocity causes abundant deposit all over the flood-plain. With every flood this deposit is renewed, and the stratum becomes thicker. Thus, the level of the flood-plain is built up by sedimentary deposit, without limit. In the Mississippi Eiver the flood-plain deposit is about fifty feet thick. In the Nile it is forty to eighty feet thick. Time. — On the flood-plain of the Nile stand the oldest monuments of civilization in the world. One of these (the statue of Rameses II), supposed to be 3,000 years old, has been covered about the base with sediment nine feet deep. The whole thickness of the Nile sediment at this point is forty feet — nine feet in 3,000 years would make forty feet in 13,330 years as the age of the Nile deposit. This is, of course, but a rough estimate. The rate may not have been uniform. But in any case the whole time belongs to the present geological epoch. Levees, Natural and Artificial. — In rivers which regularly flood their plains we always find a sort of em- bankment on either side near the river, higher than the rest of the flood-plain, and consisting of coarser material. This is called the natural levee (Fig. 11, I, I). When the river is at full flood, e, e, the whole flood-plain is covered, but at half flood d, d, d, it is often divided into three streams, viz., the river channel and the back swamp on either side. The cause of the natural levee may be explained thus : The whole flood-plain is covered with water moving 32 DYNAMICAL GEOLOGY. slowly seaward. Through the middle of this compara- tively still water runs the swift current of the river- FiG. 11.— Ideal sectiou of a flooding river, a, «, a, a, original bed ; b, b, flood- plain ; I, I, natural levees ; c, low water ; d, d, d, half flood ; e, e, e, full flood. channel. Now, on the two sides of this swift current, just where it comes in contact with the stiller water, and is checked by it, there will be a line of abundant and coarser sediment. Artificial Levees. — Natural levees can not restrain the floods of rivers, since they are made by such floods. By deposit, the bed of the river, the natural levees, and the back swamp, all rise together, maintaining their rela- tive level. If, therefore, we desire to restrain the floods Fig. 12.— Ideal section of a river-bed and plain which was built up naturally for a time and then restrained by artificial levees, I, I. and reclaim the flood-plain, we must build artiflcial levees upon the natural ones. This interference modifies greatly the phenomena of deposit. The river continues to build up its bed as before, and would in time again flood as before, if the levees were not built up higher from time to time. The flood-plain, however, no longer receives deposit. Therefore the river-bed being raised by deposit, and the levees by man, the river finally runs on the top of an embankment, which rises ever higher above the sur- AQUEOUS AGENCIES. 33 rounding plain, and the danger from accidental breakage of the levee is ever greater (Fig. 12). It is said that the river Po, from this cause, now runs above the tops of the houses on the plain. ^^--^ 4. Deltas. 'The flood-plain of a river may be divided into two parts, viz., the river-swamp and the delta. The river- swamp is that part of the flood-plain which was land- surface when the river began to run, and has been raised only a little by deposit. The delta is that part of the flood-plain which has been reclaimed by the river from the empire of the sea. The river has dumped sediment into the sea or lake, until it filled it up and made a certain amount of land. This made land is the delta. For e, ,E.fi) but sometimes of ferrous carbonate (FeOOg). Mode of Formation. — Iron has a very strong affinity for oxygen, as is shown by the rapid rusting of iron when exposed to the weather. But this is true, not only of metallic iron, but also of ferrous oxide, and of ferrous carbonate. In all cases it runs rapidly into the condition of highest oxidation — y\z., ferric oxide. But, although iron has so strong an affinity for oxygen, yet o, portion of the oxygen of ferric oxide will be taken away from it by the superior affinity of organic matter in a state of decom- position. Thus, ferric oxide (Fe^Og) in contact with decomposing organic matter will be reduced to ferrous oxide (FeO), which then readily unites with carbonic acid (COJ, always present in meteoric waters, and forms ferrous carbonate (Fe003). Ferrous carbonate is feebly soluble in water containing CO^. Now, iron is a very abundant substance, but, on account of its affinity for oxygen, it exists most naturally only in the form of ferric oxide ; in which state, therefore, it is almost universally diffused as a red or yellow coloring- matter of soil and rocks. In this state, though abundant, it is unavailable to man. But organic matter, in a state of decay, is everywhere on the surface of the ground. This is dissolved by rain-water, and sinks into the earth. Therefore, all subterranean water contains organic matter in solution. Such water, percolating through red soils or red rocks, first reduces the iron to ferrous oxide, then to ferrous carbonate, then takes it into solution — i. e., washes it out of the soil or rock, leaving these decolor- ized, then comes to the surface, as springs containing 90 DYNAMICAL GEOLOGY. iron carbonate (chalybeate springs). This is where we took it up (page 75). Then, as was there shown, it again comes in contact with air, gives up CO^ and retakes oxy- gen, and is reconverted into ferric oxide, which, being insoluble, is deposited. The above is a complete explanation of the accumula- tions of ferric oxide. In this case the organic matter is consumed, i. e., changed into 00, and H^O, in doing the work of reduction and solution, and there is nothing to prevent the iron from returning to the condition of ferric oxide. But, if there be an excess of organic matter , as peat, for example, in the place where the deposit occurs, then the iron will he deposited as ferrous carhonate, because it can not exist in the form of ferric oxide in the presence of decaying organic matter. This is a sufficient explanation of deposits of iron-carbonate. Familiar Illustrations. — We have gone so far into this explanation because the effects of water containing COg in leaching out the coloring-matter of soils may be observed on every hand, and thus, therefore, afford an excellent field for cultivating the observing power of the pupil. 1. If a dead stump, with roots ramifying in red soil, be examined, it will often be observed that the soil is bleached immediately about each root. This is because water con- taining organic matter, running down the root, leaches out the red coloring-matter of the soil. 2. In every railroad-cutting, or other excavation in red soil, it will be observed that the walls of every fissure in the soil, through which water from the surface descends, will be bleached for a little distance on each side. 3. Red clays exposed to view by excavations, natural or artificial, are often variegated or marbled with irregu- lar streaks and spots of deeper or lighter color. This is produced by irregular percolations of water containing organic matter. I ORGANIC AGENCmS. 91 4. Even in the most intensely red-clay regions, in wooded places the surface-soil, for a foot or more in depth, is bleached. Water containing organic matter from the surface leaches out and carries down the color- ing iron to the subsoil. 5. The clay of uplands may be yellow or red, but the clay of swamp-lands is always hluish. This is because ferric oxide, which is the red or yellow coloring-matter, can not exist in the presence of organic matter, abundant in swamps, but is reduced to ferrous carbonate and its color destroyed. But, if such blue clay be burned to brick the organic matter is destroyed, the iron is peroxi- dized, and the hrick is red. Section III. — Lime Accumulations. "^ Lime accumulations are made mainly by corals and by shells. We shall take up the subject under these two heads. Coral Reefs and Islands. Although corals do not make reefs in temperate regions, and therefore the process can not be observed by every one, yet, for many reasons, the subject is of peculiar interest, both popular and scientific. Coral reefs are of peculiar popular interest — 1. On account of the strange forms and gorgeous beauty of the animals which inhabit them. 2. On account of the gem-like beauty of the isl- ands which form on them. 3. Because a large area is added to the habitable land-surface by the agency of corals ; and especially, 4. Because the largest continuous body of land thus added is on our own coast, viz., in Florida. 5. Because of the great dangers to navigation, especially on the coast of Florida, resulting from the presence of these reefs. The considerable town of Key West owes its existence wholly to the wrecking business. 92 DYNAMICAL GEOLOGY. There are also peculiar points of scientific interest. To the geologist they are of the extremest interest — 1. As agents producing immense accumulations of limestone. 2. As evidences of crust movements on a magnificent scale. These points will be brought out as we proceed. It is a common idea — an idea which has passed into popular literature, and is difficult to eradicate — that corals and coral reefs, like the hills and galleries of ants, are built slowly by the cooperative labor of millions of little insects. It becomes necessary, therefore, to explain somewhat fully the manner in which a reef is really formed. A Simple Polyp. — Fig. 44 represents an ordinary soft Fig. 44. — Simplified fignro of an actinia. polyp {Actinia — sea-anemone), somewhat simplified, such as may be seen clinging to rocks or piers on our sea-shores almost anywhere. Their structure is diagrammatically shown in section (Fig. 45). As seen by these figures the creature may be compared to a hollow, fleshy cylinder, closed at both ends like a yeast-powder can. The lower may be called the /oo^-disk, the upper the mo2^M-disk. The edge of the mouth-disk is surrounded by hollow ten- tacles, t t, which open into the hollow cylinder. In the center of the mouth-disk is the mouth, m, and below it hangs the stomach, s, reaching about half-way down. At ORGANIC AGENCIES. 93 the lower end of the stomach is the pylorus, which may be opened and shut like a second mouth. Eunning from the outer wall, and converging toward the axis, are many partitions, p p, some of which reach the stomach and hold it steadily in the axis, but below the stomach termi- nate in free, scythe-like edges. These converging par- titions divide the body cavity into a number of trian- PiG. 45.— Ideal section, vertical and horizontal, showing structure : t, tentacles ; s, stomach ; jh V-, partitions. gular apartments, which, however, are in free communi- cation with each other below the stomach. Besides the main partitions spoken of, there are very many smaller ones which do not reach so far as the stomach. The whole structure may be briefly summarized by tracing the course of the food. Food is taken by the tentacles, put into the mouth, and passes into the stomach. After digestion, whatever is refuse is thrown back through the mouth, and the digested food is dropped through the pylorus into the general hall below the stomach, and there mixed with sea-water and circulated through all the apartments. Simple Coral, or Stone Polyp. — Now, a simple coral has a similar structure, except that stony matter (lime 94 DYNAMICAL GEOLOGY. carbonate) is deposited in the lower part as high as about the region of the stomach, as shown in Fig. 46. When the animal seems to disappear, it only withdraws the soft upper parts within the stony lower part. But the stony material is everywhere within the living organic matter and covered. When the living organic matter is taken Fig. 46.— Ideal section of a single living coral. The shaded portion contains carbonate of lime. away, as in dead corals, then we have only the radiated structure of the lower part in stone. This is well shown in Fig. 47, a and l. The corals which form reefs, how- ever, are individually extremely small. Fig. 47. — a, stony part of a single coral ; ft, section of same, showing structure. Compound Coral, or Coralluiii. — Many lower ani- mals, like plants, have the power of reproducing by buds. If the buds separate, they form distinct individuals ; but ORGANIC AGENCIES. 95 if they remain attached, then a compound animal is formed, composed of many individuals, united together precisely as a tree is formed of many buds, each of which is in some sense an individual, and capable of independent life. In the compound coral each bud has its own tenta- cles, mouth, stomach, partitions, and other organs neces- sary for life, and yet all are organically connected, and each feeds for all. There is, therefore, a sort of individ- uality in the aggregate, but a more decided individuality in each bud. The form of the aggregate depends on the mode of budding. If the buds grow into branches, then there is formed a tree-coral (Fig. 48) ; but if the buds do not Fig. 48.— Madrepora, a tree-coral. separate, but remain connected to their ends, and form new buds in the intervening spaces, then they form a head- coral (Fig. 49). There are all gradations between these extremes. Coral-trees are often six to eight feet high, so that one may literally climb among the branches. Coral- heads form hemispherical masses fifteen to twenty feet in diameter. In either case the aggregate consists of hun- dreds of thousands of individuals ; in either case, also, the living organic matter is confined to the superficial portion, one quarter to one half an inch thick. As in 96 DYNAMICAL GEOLOGY. case of Ji tree, so in corals, life passes continually outward antl upward, leaving the middle parts dead, and, in fact, wholly composed of mineral matter (lime carbonate), re- FiG. 49.— Astrea, a head-coral. taining, however, the peculiar structure given it while permeated with living matter. Coral Forests. — Corals, however, reproduce also hy eggs. These are formed within, below the stomach, ex- truded through the mouth, and having, like the eggs of many lower animals, the power of locomotion, swim away and settle to the bottom, where, if conditions are favor- able, they form single corals, which, by budding, soon form coral-trees or coral-heads. In this way a coral forest or grove is formed, and spreads in all directions as far as favoring conditions allow (Fig. 50). Coral Reefs. — But coral forests are not yet coral reefs. These are formed by the growth and decay on the same spot of countless generations of coral forests. Each generation in its death leaves its limestone behind ; and thus the coral ground rises or is built up without limit except by reaching the sea-level. As a peat-bog is formed by the accumulated remains of successive generations of ORGANIC AGENCIES. 97 plants growing and dying on the same spot, so a reef is similarly formed by successive generations of corals. As Fig. 50.— Corals iu the Great Barrier Reef, Australia. peat-ground may rise above the surrounding country, so a coral reef rises far above the surrounding sea-bottom. As peat represents so much carbon taken from the air and added to the ground, so a reef represents so much carbonate of lime taken from the sea-water and added to the sea-bottom. The limestone thus formed by the broken remains of corals cemented together is called the reef-rock. Thus a reef is a submarine bank composed of reef -rock, crowned with the present generation of living corals. Coral Islands. — But even coral reefs are not yet coral islands, since corals can not grow above the sea-level. Coral islands are made by the action of waves. Waves Le Conte, Gkol. 7 98 DYNAMICAL GEOLOGY. will form islands on any kind of submarine bank when the water is shallow enough for the waves to touch and chafe the bottom. When, therefore, the reef rises nearly to the surface, the beating waves will break off coral- trees, coral-heads, and even masses of the reef -rock. Great masses are thus rolled up on the inner side of the reef, and form a nucleus about which other masses gather. Among these larger masses smaller masses are thrown, then finely comminuted coral limestone (coral sand) is sifted among these, and the whole is cemented into a solid rock by carbonate of lime in the sea-water. The island rock, therefore, is a breccia of coral limestone, as shown in Fig. 51. The island thus formed is at first barren Fig. 51.— Ideal section across a coral island ; ^, I, sea-level ; R, living reef ; C.R.R., coral reef-rock. rock ^ but, slowly, seeds are brought by waves and wind ; it becomes covered with vegetation, and inhabited by animals and by man. Thus we have traced the whole process, and find no evidence of purpose or will, much less the admirable vir- tues of perseverance and industry, often attributed to them. It is a pity to spoil a moral ; but truth is the best moral. Conditions of Growth. — Reef-building corals do not grow over the whole sea-bottom, nor in all oceans. They are strictly limited by certain conditions : 1. They will not grow where the mean winter tempera- ture of the ocean is less than 68° Fahr. This condition confines them mostly to the tropics. The most notable ORGANIC AGENCIES. 99 apparent exception to this is in the North Atlantic. On the coast of Florida and the Bahamas reefs occur as far as 28° and on the Bermudas as far as 32° north latitude. But this is because the temperature of ^S° is carried northward by the warm waters of the Gulf Stream. 2. Keef -building corals will not grow at a greater depth than about one hundred feet. This condition confines them to submarine banks, and especially to shore lines. In tropic seas corals build all along the shore, and as far out as the depth will allow. Hence results the usual linear form of reefs. 3. They require, also, clear salt water, and are killed by fresh water and by mud. They will not grow, there- fore, along flat, muddy shores where the waves chafe the bottom and stir up mud. Also, if a reef is formed along a shore-line, there will be breaks in the reef oif the mouths of rivers, the corals being prevented from growing there partly by the freshness of the water, and partly by the mud brought down by the river. 4. Corals grow best where they are beaten by the waves — ^viz., on the outer portion of the reef. Some spe- cies, indeed, love the still water on the inner side of the reef, but the strong, reef -building species thrive under the effect of the dashing waves, and will even build up- ward in the face of waves that would wear away a granite Avail. The corals are broken, indeed, and worn, but growth more than makes up for the wear. This is because the crowded life on the reef, both of corals and of animals of all kinds feeding on the corals, rapidly exhausts the water of its oxygen and replaces it with carbonic acid, and thus renders it unfit to support life. But the chafing and foaming of the breakers dis- charges the CO, to the air and restores the oxygen. It is exactly like the ventilation so necessary for air-breath- ing animals. All these conditions refer only to reef-building species. 100 DYNAMICAL GEOLOGY Some species of corals live at great depths and in high latitudes. Description of Pacific Coral Reefs and Islands. — There are in the Pacific two very distinct kinds of islands — viz., volcanic islands and coral islands. The former are high, bold, rocky, and often of considerable size ; the latter low, wave-formed. We will suppose the preexistence of volcanic islands, and proceed to show how coral reefs and islands are formed about them. Pacific reefs, then, are of three principal kinds — viz., fringing reefs, harrier reefs, and circular reefs or atolls. 1. Fringing Beefs. — These grow along any shore- FiG. 52.— Perspective view of volcanic island and fringing reef. line, but the most common and interesting are those about volcanic islands. Suppose, then, a high volcanic island in the midst of the sea. Around such an island corals will build, limited outward by increasing depth, limited inward Fig. 53.— Ideal section of volcanic island and fringing reef ; s.p., shore platform ; r/j, coral platform. by shore-line and upward by sea-level, thus forming a submarine platform clinging close to the island like a ORGANIC AGENCIES. 101 fringe. The existence and extent of such a reef are revealed by the snow-wliite sheet of breakers which sur- rounds the island like a snowy girdle (Fig. 52). Off the mouths of large rivers breaks in the reef will occur. Fig. 53 is an ideal section showing the coral platform, cp, cp. So much for the agency of corals. The waves now break off fragments from the outer part of the reef and pile them up on the inner part against the land, and thus form a low, level sliore platform, s.p., s.p., above the sea-level. Thus we have first the slope of the volcanic island ; then the shore-platform of coral debris ; then the submarine platform of living corals ; and, finally, the deep water. In this case there is no coral island, but only a coral addition to the volcanic island. 2. Barrier Reefs. — About the volcanic island there Fig 54. -Perspective view of volcanic island and barrier. may be little or no fringing reef, but at a distance of five, ten, or fifteen miles away, in deep water, there rises a line of reef like a great rampart surrounding the island, and, as it were, protecting it from the attacks of the sea. 103 DYNAMICAL GEOLOGY. The i^osition of the reef is shown by a snowy girdle of breakers, withiii which, like a charmed circle, there is calm sea in the wildest storm. Between the reef and the island there is a ship-channel, often twenty or thirty fathoms deep. Through breaks or tidal ways in the reef, ships enter and find good harbor in the channel. If it were not for the action of the waves, this would be all, but the beating waves form little coral islands on the reef, so that, instead of a continuous snowy girdle, it is such a girdle gemmed on the inner edge with a string of green islets. By sounding it is found that the inner slope of the reef is gentle, but the outer slope is very steep, and rapidly passes into abyssal depth. All these facts are shown in the perspective view. Fig. 54, and the section. Fig. 55. 3. Circular Reefs, or Atolls. — These are the most remarkable of all. In this case there is no volcanic island or preexisting land of any kind apparent, as a nucleus Fig. 56.— Perspective view of an atoll. for the growth of corals. The reef seems to have been built up from abyssal depth, in an irregular circular form, inclosing a lagoon of still water in the midst (Fig. 56). The j)osition of the reef, r, r, is shown by a circle of snowy foam inclosing and protecting a harbor of still water. Through breaks in the reef-circle ships may enter and find safe anchorage. The lagoon is ten, twenty, thirty, or even fifty miles in diameter, and thirty or ORGANIC AGENCIES. 103 forty fathoms deep. By sounding it is found that the inner reef-slope is gentle, but the outer very steep, so that at a distance of a mile more than a mile depth has been found (Fig. 57). Thus far, the*' action of corals Fig. 57.— Section of an atoll. alone. Now add the action of waves, and the snowy ring is gemmed on the inner edge with small green islets. All these facts are shown in Figs. 56 and 57. 4. Closed Lagoons and Lagoonless Islands. — In the typical atoll the reef-circle is large, and only dotted Pio. 58.— Map view of closed lagoons and lagoonless islands. with small islets, but in small atolls the land is more con- tinuous (Fig. 58, a), or entirely continuous, but the 104 DYNAMICAL GEOLOGY. lagoon open to the sea on one side (Fig. 58, h), or the lagoon may be entirely closed (Fig. 58, c), or the ring may close in upon itself so as to abolish the lagoon (Fig. 58, d). These are so different from the typical atoll that they may be considered a fourth class. Theory of Barriers and A tolls. Fringing reefs need no theory. Corals, finding the con- dition of suitable depth along the shore, build upward to the sea-level and outward to the depth of one hundred feet, and thus form a coral platform clinging to the orig- inal island. But barriers seem at first sight to form far from land in abyssal depth ; and atolls seem to form in deep sea without any island-nucleus. These facts seem to violate the conditions of coral growth. How are they explained ? The most probable explanation was first given by Mr. Darwin. Darwin's Subsidence Theory. — According to Dar- win, every reef began as a fringe, and would have remained so if the floor of the ocean had remained steady. But, in all the region of barriers and atolls, the ocean-floor has slowly subsided, carrying all the volcanic islands with it downward. Now, if the subsidence had been more rapid than the coral ground could rise by accumulations of debris of successive generations, then the corals would have been carried below the depth of one hundred feet and drowned. But the subsidence was not faster than the coral ground could be built up. Therefore the corals building upward, as it were, for their lives, kept their heads at or near the surface. But the reef, building up nearly at the same place, while the volcanic island grew smaller, it is evident that the latter would be separated more and more from the reef. When the island was down waist-deep, the reef was a barrier ; when down head-under, it became an atoll, the reef representing nearly the outline of the original base of the volcanic island. We said nearly, but not per- ORGANIC AGENCIES. 105 fectly. The corals do not build up perpendicularly, but in a steep slope. The barrier, and much more the atoll, is therefore smaller than the original fringe. If, there- fore, the subsidence continues, the atoll will grow smaller and smaller, the separate islets will close together, join each other, and finally close the lagoon. Then the lagoon will close in upon itself and form the lagoonless island, and, last of all, this also will probably disappear. As corals grow best on the outside of the reef, they will not occupy the channel formed by recession of the volcanic island ; or, if they do, they are soon drowned out by subsidence. The channel, however, in case of barriers, or lagoon in case of atolls, will be partly filled by debris carried into it in both cases from the reef, and in the case of barriers also from the volcanic island. Fig. 59 is an Fig. 59.— Ideal Bection diagram showing the formation of an atoll ; I", I", sear-level when reef was a fringe ; I', I', when it was a baiTier, and I, I, the present sea- level. ideal section embodying all these facts. In this figure, for convenience of illustration, instead of the sea-bottom sinking, the sea-level is represented as rising. Murray's Theory. — The subsidence theory, however, is not now universally accepted. Agassiz first showed, by study of the reefs of Florida in 1851, that barriers are formed without subsidence. Murray, traveling in the same region as Darwin, concluded that both atolls and bar- riers may be formed without subsidence. He supposes that atolls are built up by corals on banks previously 106 DYNAMICAL GEOLOGY. formed by other agencies, the corals growing only on the outer margin of the bank, because they find the best condi- tions of growth there. Barriers, he supposes, are fringes separated from the encircled island by dying out of the corals on the landward side and extension on the seaward side of the reef. It is probable that atolls and barriers are formed in several ways, but under present lights it is not improb- able that many, if not all, atolls are formed as Darwin supposes. We will assume that every atoll marks the place of a drowned volcanic island. Area of Subsidence. — The area of the subsiding sea- floor is 6,000 miles long and 3,000 miles wide. It is prob- ably not less than 12,000,000 square miles, or greater than the whole North American Continent. Area of Laud Lost. — This must not be confounded with the sinking area. The sinking area is the whole sea-floor, over millions of square miles ; the land known to have been lost is only the volcanic islands which once overdotted this area. This is, of course, small in com- parison. Estimated by the circles inclosed by atolls, Dana makes it 50,000 square miles. It is doubtless, how- ever, much more than this. For — 1. This estimate takes no account of barriers, but all the area between a barrier and the shore-line is also lost. Now, along the Australian coast, for 1,100 miles, there is a barrier thirty to forty miles distant. This alone would make 33,000 square miles lost for this one barrier. We may with confidence, therefore, double the estimate. But, 2. Atolls themselves, as already shown, are smaller, and closed lagoons and lagoonless islands very much smaller than original volcanic islands. And, 3. In the middle of the coral region there is a blank area of several million square miles, in which there are no islands of any kind. Many islands probably went down here and left no sign, because they went down too rapidly and the corals were drowned. Putting all OHGANIC AGENCIEIS. 107 these facts together, it seems probable that several hun- dred thousand square miles of volcanic land have been lost. Of this only a small fraction has been recovered by the action of corals and waves. Amount of Vertical Subsidence.— This may be roughly estimated in many ways : 1. Soundings a little way oif barriers have reached 2,000 feet, and off atolls 7,000 feet. 2. The average slope of volcanic islands of the Pacific is about 8°, but, taking it even as low as 5°, a barrier ten miles from shore would indicate a subsidence of 4,500 feet. (D B = A D . tan A.) But barriers are found at much greater distances than ten miles. 3. The average height of volcanic islands of the Pacific in non- V W/MMlimmmmmrrmrnrrrr, _ A 3 Fig. 60.— i', volcanic island ; A, shore line ; />, place of barrier ; J.J3, slope of bottom, S". subsiding areas is 6,000 to 10,000 feet. Now, every atoll represents such an island, entirely submerged, and every closed lagoon the same deeply submerged. But it is very improbable that none of these reached the average of those remaining. Taking all these facts together, it is probable that the extreme subsidence is not less than 10,000 feet. Amount of Time Involved. — It is evident that the rate of sinking can not have been greater than the rate of coral ground-rising ; otherwise the corals would have been drowned. Again, the rate of ground-rising is far less than the rate of coral-prong growth. If the annual growth of all the prongs were taken, ground to powder, and strewed over the area shaded by the coral branches, it would give the annual rising of the ground. It is evident that this would be very small in comparison with the growth of the prongs. In addition to this, it must be remembered that large spaces of a coral reef are bare. Taking all these 108 DYNAMICAL OEOLOOY. things into consideration, it has been estimated that one quarter to one half inch per annum is a large estimate of rate of ground-rising. The subsidence can not be greater and may be much less than this. At this rate a subsi- dence of 10,000 feet would require 250,000 to 500,000 years. The whole of this, however, must not be accred- ited to the present geological epoch. It probably extends back into the Tertiary. Geological Application. There are several points in the preceding discussion which throw important light upon the structure and his- tory of the earth. 1. We have here examples of lime- stone rock, formed by coral agency over millions of square miles, and in places many thousand feet thick. 'For not only is limestone formed on the site of the reefs (reef -rock), but the fine coral debris is carried by waves and currents and strewed over the whole intervening space. We find thus a key to the extensive deposits of limestone formed in previous geological times. 2. The hind of rock formed also deserves attention. The reef-roch is, in some parts, a coral breccia; in other parts it consists of rounded granules, cemented together (oolite). In the deep sea of the intervening spaces, the bottom ooze is 2^ fine coral mud, which, dried, looks much like chalk, and by some has been supposed to be indeed the modern representative of chalk ; but more probably, it hardens into a compact limestone. Now, in limestones of previous geological epochs, we find similar structures ; i. e., extensive fine limestone, with areas of coarse coral breccia or of oolites. We are thus able to determine the position of old coral Beas and the lines of old coral reefs, even though they are now occupied by mountain-ranges, as in the case of the Jura Mountains (Heer). 3. Lastly, we have here ex- amples of movements of the earth's crust on a grand ORGANIC AGENCIES. 109 scale — on a scale commensurate with the formation of continents and ocean-bottoms. The phenomena of coral reefs show a down-sinking of the mid- Pacific bottom of several thousand feet, and over an area of many million square miles. This has been going on through later geo- logical times, and is probably still progressing. Now, so wide-spread a downward movement must have its correla- tive in an upward movement somewhere else. It seems probable that we find it in the upheaval of the western half of the American Continent, both North and South. It is well known that during the whole later Tertiary, even to the present time, the western part of North America, especially the plateau region, has been slowly rising, the extreme rise being nearly 20,000 feet. As it rose, the general erosion became greater and the caflons cut deeper and deeper. So that the down-sinking of the Pacific bot- tom, the upheaval of the plateau region, and the cutting of the wonderful caflons of that region, are probably all connected with each other. --. REEFS AlifD KEYS OF FLORIDA. 'The reefs of Florida deserve separate and special treat- ment, not only because they are on our own coast, but also because they are in some important respects entirely pecu- liar : 1. In the Pacific, barrier-reefs are always the result and the sign of subsidence. In Florida, on tlxe contrary, we have barrier-reefs where there has been no subsidence. 2. In the Pacific, corals do not add to the previously ex- isting land-surface ; on the contrary, they only recover a small fraction of a lost land-surface^ But in Florida there has been apparently no loss, but a constant growth of land- surface under the action of corals, assisted by waves and other agents, as we shall presently explain. Attention has not been hitherto sufficiently drawn to the entire uniqueness of these reefs. 110 DYNAMICAL GEOLOGY. Description of Reefs and Vicinity. — Fig. 61, A, is a map of Florida, its keys, reefs, etc., and Fig. 61, B, is a section of the same along the line N 8. The southern Fig. 61.— Map and section of Peninsula and Keys of Florida. In both, a = coast; a' = Jveys ; a" ~ reef ; e = everglade ; e' .= shoal water ; e" = ship-channel ; G8 = Gulf Stream. coast of Florida, a a, \^ a ridge of limestone, twelve to fifteen feet high, inclosing a swamp called the Everglades, e, only one to two feet above the sea-level, covered with ORGANIC AGENCIES, 111 fresh water, overgrown with vegetation, and overdottcd with higher spots, called hummocks. Going south from the coast, the next thing that attracts attention is a line or string of limestone islands (keys) a' a , stretching in a curve from Cape Florida to the Tortugas, a distance of one hundred and fifty miles. Between these and the southern coast is an extensive shoal, almost a mud-flat, navigable only to small fishing-craft. The width of this shoal is thirty to forty miles. It is overdotted with small, low, mud islands, overgrown with mangrove-trees, and entirely different from the true keys. Outside of the line of keys, and separated from it by a ship-channel, five to six miles wide and three to four fathoms deep, is a con- tinuous line of living reef, a" a". On this, by the action of the waves, a few small islands have commenced to form. Outside of all sweep the deep waters of the Gulf Stream, G S. Formed by Coral Agency. — Now, the whole area thus described is a recent coral formation, and has been added to Florida in recent geological times. The proof of this is complete. First : On the living reef, islands have just commenced to form. Some are yet only a collection of large coral fragments, the nucleus of an island. Some are more compacted by smaller fragments thrown in among the larger. Some are small but perfect islands — i. e., coral, sand, and mud have been thrown upon and completely buried the large masses. But none of these are yet clothed with vegetation, much less inhabited by animals and man. Next come the larger inhabited islands of the line of keys. On cutting into these, the same structure as described above is revealed. Undoubtedly these are a string of wave-formed coral islands, and here was once a line of living reef ; but the corals have long ago died, be- cause cut off from the open sea by the formation of another reef farther out. Next comes the southern coast. Ex- 112 DYNAMICAL GEOLOGY. amination of this reveals the same structure precisely. Here, then, was the place of a still earlier reef. Brief History of the Process. — There was, there- fore, a time when the north shore of the Everglades {d, section. Fig. 61, ^) was the southern shore of Florida. At that time the place of the present southern coast was occupied by a living reef. On this reef coral islands were formed, which gradually coalesced into a continuous line of land, the shoal water between it and the mainland was filled up, and the whole added to the mainland ; the southern coast being transferred to its present position, and the shoal water, with its mangrove islands, changed into the Everglades, with its hummocks. In the mean time, however, i. e., while the present southern coast was still a line of keys, another reef was formed in the place of the present line of keys, and the former have therefore died. This new reef in its turn was converted into a line of keys, which will eventually coalesce into a continuous line of land, the shoal water will be filled up, and form another Everglade, with its hummocks, and the coast- line be transferred to the present line of keys. But already another line is formed, and the previous line is dead ; already the process of key-formation has com- menced. We can not doubt that eventually, but proba- bly only after many thousands of years, the Peninsula of Florida will extend even to the present living reef. Far- ther than this it can not go, for the deep water of the Gulf Stream is close at hand, and forms its impassable boundary. Farther, northward, the extent of the coral formation is less known, but it has been found on the eastern coast as far as St. Augustine. The middle and western part of Florida, as far as the north shore of the Everglades, is probably older. The line d d (Fig. 61, A), therefore, probably marks out the area which has been added to Florida by the agencies described. The area already ORGANIC AOENCIES. 113 added is probably not less than 12,000 to 15,000 square miles, and the area which will be added at least half as much ^ore. / Cooperation of Other Agencies. We have seen that the reefs of Florida are unique. It seems certain, therefore, that they were formed under unique conditions. The things to be accounted for are — 1. The constant growth of land ; and, 2. The formation of barriers where there was no sul)sidence. 1. The constant growth of land southward shows that there was a continual extension southward of the conditions of coral-growth, i. e., of moderate depth. In other words, there must have been a gradual extension southward of the submarine bank, on the edge of which the corals grew. If there had been a preexisting bank, obviously the corals would have grown as only one reef on its outer edge ; the formation of successive reefs, one beyond the other, proves that the shallow bank on which they grew must have extended successively in that direc- tion. Thus much seems certain, but the cause of the exten- sion is more uncertain. It is probable, however, that the bank was formed and extended by sedimentary deposit by the Gulf Stream.* Thus, then, the extension of the Peninsula of Florida in recent times has been the result of the cooperation of several agencies : 1. The Gulf Stream built up from deep- sea bottom a bank, and extended it by the same process. 2. The corals took up the work by forming successive bar- * At one time the sediments were supposed to be mechanical sedi- ments from the Gulf rivers, especially the Mississippi. But now they are believed to be organic sediments, partly brought by the Gulf Stream from other coral banks, e. g., the Yucatan bank, but mainly formed in place by the growth of successive generations of deep-sea shells ; the Gulf Stream bringing only the conditions of heat and food necesary for rapid growth. Le Conte, Geol. 8 114 DYNAMICAL GEOLOGY. rier-roefs farther and farther south as the necessary con- dition of moderate depth extended. 3. The waves then took . up the work and converted the line of reef into a line of keys, and finally a line of land twelve to fifteen feet high. 4. The shoal waters between the successive lines of keys and the mainland was filled up by coral debris carried inward from the reef and keys, and southward from the previously formed land, and the mainland was thus ex- tended to the keys. 2. Barrier-reefs without subsidence may be ac- counted for thus : From the manner in which, by this view, the bases of the coral reefs were formed, viz., by sedimentation, there must always have existed a very soft, shallow sea-bottom. Along such a shore-line ?l fringing reef could not form, because the chafing waves stir up the mud. But at a distance from shore, where the water is a hundred feet deep, and the waves no longer touch the bottom, a line of reef would form, limited on the one side by the muddiness and on the other by the increasing depth of the water. This would be in form a barrier- reef, but wholly different in significance from those of the Pacific. Shell Limestone. Lime is constantly carried to the sea by rivers, and yet is the sea-water not saturated. This is because the lime in sea-water is constantly being drafted upon by animals to form their shells and skeletons. These remain after their death, accumulate as lime-deposits, and harden into limestone. We have already spoken of coral limestone, but other animals besides corals form limestones, and some make other kinds of deposits besides lime. Besides corals, the most important are shell-deposits. We shall treat these under two heads, Molluscous Shells and Micro- scopic Shells. And here we would again invite the per- sonal observation of the pupil. ORGANIC AGENCIES. 115 1. Molluscous Shells. — These inhabit mostly shallow water, and therefore accumulate mostly along shore-lines, and may be observed by all who keep their mental eyes open. Each generation takes lime from sea- water, and. leaves it as shell on the bottom. These, therefore, accu- mulate until deposits of enormous thickness and extent are often formed. Sometimes the accumulated mass may consist of one species, as in oyster-banks ; sometimes of many species. The deposit may be purely shelly, or shell mixed with mud, or mud with a few imbedded shells. Again, on exposed shore-lines the shells will be broken or even comminuted, and on quiet shore-lines, as in bays or harbors, they will be perfect. These accumulations are gradually hardened into limestone (Fig. 62). ilu. U;:i.— Modcru bucU limcistoue. (After Scott.) Application.— Now, all these different kinds of lime- stone or shell rock are found far away from present seas and high up in the mountains. We are thus often able to trace out the shore-lines of previous geological times, and determine not only the species which then lived, but also the conditions under which they lived. 2. Microscopic Shell^. — These are some of vege- 116 DYNAMICAL GEOLOGY. table, some of animal origin ; some fresh water, some ma- rine ; some composed of silica, some of lime carbonate. The two most important kinds are silicious fresh-water deposits of vegetable origin, and lime carbonate, deep-sea deposits of animal origin. Fresh- Water Deposits. — It is well known that still waters swarm with microscopic unicelled plants. Most of Fig. 63.— a, diatoma vulgare : a, side view of frustule ; 6, frustule dividing itself. B, grammatopliora serpentina : a, front and side view ; ft, front and end view of dividing frustule. these have no shells and leave no deposit. But one kind — the diatoms — form shells of silica. Generation after generation of these leave their shells until deposits of great thickness and extent are formed. In any clear and sluggish stream, if we examine with the microscope the slime on the stones at the bottom, we shall find living diatoms. These are carried by freshets into ponds, lakes, or seas into which the streams empty. Usually the mud carried with them is so abundant that they will not be detectable in the deposit thus formed. But in large, deep, clear lakes, like Lake Tahoe, beyond the reach of sedimentary deposit, the deep bottom-ooze is found to be composed wholly of the accumulated shells of diatoms. Also in the hot springs of California and in the pools formed by the accumulation of these waters, diatoms are ORGANIC AGENCIES. 117 very abundant, and deposits of these shells are formed comparatively rapidly. Application. — In many countries, and nowhere more abundantly than in California, is found a soft, white, very light and friable earth, often many feet in thickness and many square miles in extent, which, under the microscope, is seen to consist wholly of shells — some perfect, some broken — of diatoms. It is only by the study of deposits now forming that we may hope to understand the condi- tions under which these remarkable deposits were formed. Deep-Sea Deposits. — Many deep-sea explorations have been recently undertaken by the governments of Europe and the United States. From these we learn that the deep-sea ooze is almost everywhere a fine white mud, which dried looks like chalk, and under the micro- scope is seen to be mainly composed of the carbonate-of- lime shells — some perfect, more broken, most of all comminuted — of Forami- nifera (a low form of ani- mals). The most abun- dant form is Globigerina (Fig. 64), and therefore this ooze is often called glo- Mqerina ooze. Among these t. .a -^ t , ^ fe ^"^ Y\G. 64.— Foraminiferal ooze, x ]3. are scattered silicioUS shells (Agasslz after Murray and Renard.) of diatoms and several other kinds of shells, animal and vegetable. All of these prob- ably live at the surface, and on their death drop to the bot- tom. So that we may imagine a continual drizzle of these shells falling to the bottom. These deposits are certainly of enormous extent, and probably of great thickness. Application. — There is one geological stratum which bears a striking resemblance to this deep-sea ooze, viz., the chalk of England, France, the interior of Europe, and our own western plains. The origin of this very peculiar 11$ DYNAMICAL GEOLOGY. stratum will hereafter be discussed in the light of these facts. Section^ IV.— Geographical Distribution of Species. No one can go to a foreign country, or even a distant part of our own country, as, for example, from the eastern to the western coast, without being struck with the great difference in the native animals and plants. If such a one has been trained to observe, he will see that nearly all the species are entirely different. As a broad, general fact, every country has its own native species, differing more or less conspicuously from those of other countries. The laws of this distribution and its causes have recently attracted much attention, and are a subject of very great interest. We can only give the briefest outline. Faunas and Floras. — AVe shall hereafter frequently use the terms fauna and flora, and must therefore define them. The whole group of animals inhabiting one place is called \t^ fauna, and of plants its flora. Thus, we may speak of the fauna and flora of New York, or Illinois, or Oregon. But science cares nothing for such arbitrary limits — it deals only with natural boundaries. A natural fauna or flora is a natural group of animals or plants in one place, differing conspicuously from other groups in other places, and separated from them by natural bound- aries, geographical or climatic. Among the climatic con- ditions limiting faunas and floras, perhaps the most important is temperature, and we shall therefore speak of this first. Again, plants, being fixed to the soil, are more strictly limited than animals, and we shall there- fore illustrate the laws of distribution first by them. Again, temperature conditions change in elevation above the surface, and in latitude. We take the former first. Botanical Temperature Regions in Elevation. — For this we take a high mountain, near the seashore in ORGANIC AGENCIES, 119 tropical regions, because we find there all the regions (Fig. 65). In going up such a mountain, from sea-level, II, we pass through, — 1. A region of palms, so called because of the abundance of palms and palm-like forms. Fig. 65. such as bananas, tree-ferns, etc. ; 2. A region of hard- wood, or ordinary foliferous trees ; 3. A region in which pines and pine-like trees predominate ; 4. A treeless region, in which are only shrubs, herbs, and grasses ; and 5. A plantless region, or region of perpetual snow. These regions, although we have separated them by lines, of course graduate insensibly into each other. The sec- ond region may often be subdivided into a region of evergreens and a region of deciduous hard-woods. Botanical Temperature Re- gions in Latitude. — Now, since the above regions are determined wholly by temperature, and since a similar decrease of temperature is found in going from the equator to the poles, we ought to expect similar regions in latitude. And such we find. In going from the equator to the poles we find — 1. A region of palms, corresponding to the tropic zone ; 2. A region of hard-wood trees, corre- sponding to the temperate zone ; 3. A region of pines and Fig. 66. 120 DYNAMICAL aEOLO&Y. pine-like trees and birches, corresponding to subarctic and arctic zones ; 4. A circumpolar region of shrubs and grasses j and, 5. Perhaps a plantless or nearly plantless region at the pole of cold. Here, again. No. 2 may be subdivided into a «(;«/• w -temperate region of evergreens, and a coo^temperate region of deciduous trees. Here, again, also the regions graduate insensibly into each other. We have been speaking thus far of sea-level or near sea-level. Of course, if a mountain in any latitude rises to perpetual snow, we will have on its sides all the tem- perature regions, except those south of it. Thus, in ascending the Sierra Nevada we have a temperate region (No. 2) at base, a subarctic region (No. 3) half-way up, and a circumpolar region (No. 4) at the summits. Completer Definition of Regions. — 1. All organic forms will spread in all directions as far as physical con- ditions and the struggle for life with other species will allow. The area over which they thus spread may be called their '' range." Now, the range of one species may be much greater than that of another, because more hardy ; but the range of a species is always more restricted than its genus, for when the species can go no farther, another species of the same genus will continue the genus. For the same reason the range of a family is greater than that of its genera, etc. Thus, for example, in going up the Sierra we find the range of pines extend from 2,000 to 10,000 feet above sea-level, but the genus is represented by a succession of species of much more restricted ranges. We find, first, the nut-ipme {Finus iSabiniana), then the yellow-ipme {P. ponder osa), then the sugar-ipine {P. Lam- bertiatia), then the tamarack-iyme (P. contorta), and, last, the mountain-^uiQ {P. flexilis). Hereafter we shall speak mostly of species. 2. We have said that the several temperature regions graduate insensibly into each other. We will now explain ORGANIC AGENCIES. 121 in what sense this is true. Species, then, come in grad- ually on the borders of their range, reach their highest development in number and vigor about the middle, and pass out gradually in number and vigor on the other border, other species taking their place, and the two ranges overlapping on their borders. Thus, in Fig. 67, ^rrrrmTlM MTmTT^ b of i,' Fig. 67. a a! is the north and south range of species A, and h V of species B — the height of the curve the number and vigor of the individuals, and h a! the overlap of ranges. 3. But in specific character there is no such gradual pas- sage of one species into another — no evidence of trans- mutation of one species into another, nor of derivation of one species from another. From this point of view spe- cies seem to come in at once in full perfection, remain substantially unchanged throughout their ranges, and pass out at once on the other border, other species taking their place as if by substitution, not transmutation. It is as if each species originated, no matter how, somewhere in the region where we find them, and then spread in all directions as far as physical conditions and struggle with other species would allow. We can best make this plain by illustrations : The sweet-gum or liquidambar-tree extends from the borders of Florida to the banks of the Ohio. It is most abundant and vigorous, indeed, in the middle regions, and dying out on the borders, where it is replaced by other species : but is everywhere the same species, unmistakable by its five-starred leaf, winged bark, spinous burr, and fragrant gum. Again, the Red- wood {8equoia) ranges from south- ern California to the borders of Oregon. It may be most 122 DYNAMICAL GEOLOGY. vigorous in the middle region — it may decrease in vigor and number on its borders ; but in all specific characters, wood, bark, leaf, and burr, it is the same throughout. The study of species, as they now are, would probably not suggest, certainly could not prove, the theory of their origin by derivation or transmutation. 4. Temperature regions shade into each other. But this is so only where no barriers exist. If there be barriers, such as an east and west mountain-chain, or sea, or desert, then on the two sides of the barrier the species will be very distinct and without gradation by overlapping. Thus, north and south of the Sahara, and north and south of the Himalayas, there is a marked and, as it were, a sudden change of species. It is, again, as if the species originated each in its own area and spread, but were pre- vented from mingling and overlapping on their borders by the barrier. 5. Again, although there are similar temperature re- gions on tropical mountains and in high latitudes — and these latter are also repeated north and south of the equator — yet the species are always different in the three cases. This is because the torrid zone is a barrier pre- venting migration. It is, again, as if species originated each in its own place, but were prevented from reaching similar temperature regions elsewhere by the existence of impassable barriers. Zoological Temperature Regions. — Animal species are limited by temperature, like plants, and therefore also exist in temperature zones ; but they can not be arranged in the same simple way, evident even to the popular eye — i. e., great classes corresponding to great zones. It is true that, if we compare extremes, viz., polar with tropi- cal regions, we find a conspicuous contrast determined by temperature, certain great families being characteristic of each — ^as, for example, among mammals, the great pachyderms, the elephant, rhinoceros, hippopotamus, and ORGANIC AGENCIES, 123 the great cats, lions, tigers, jaguars ; among birds, tou- cans, parrots, trogons, ostriches ; among reptiles, croco- dilians and pythons ; and among corals, the reef-builders, characterizing the tropics ; while the musk-ox, white bear, seals, walrus, ducks and geese, characterize the polar regions — yet we can not make a zonal arrangement of families as easily as we can with plants. But, confin- ing our attention to species or even genera, animal forms are subject to the same laws as those of plants : 1. All animal species are limited in range ; 2. The range of species is more limited than that of genera, and of genera than that of families, etc.; 3. Contiguous ranges grad- uate into each other by overlapping, the species inter- mingling and coexisting on the margins ; 4. Each species reaches a maximum of number and vigor in middle regions and dies out on the borders ; 5. But in specific character they seem to remain substantially the same throughout their range, and do not change or transmute into other species on the borders ; 6. Physical conditions may limit their range, but do not seem to change them into other species, though varieties may be formed in this way ; 7. Here, again, it is as if species originated, no matter how, in the places where we find them, and have spread in all directions as far as physical conditions and struggle with other species would allow. All that we shall say hereafter will apply equally to animals and plants. Continental Faunas and Floras. — If there were no barriers to the spread of species around the earth on the same zone, there can be no doubt that they would thus spread, and faunas and floras would be arranged in a series of temperature zones from the equator to the poles, containing the same species all around. But the oceans are impassable barriers between the continents, and there- fore the faunas and floras of different continents are sub- stantially different. It is, again, as if they originated on the continents where we find them, and have been pre- 124 DYNAMICAL GEOLOGY. vented from spreading and intermingling by the impassa- ble barrier of the ocean. Even apparent exceptions, when examined, coiilinn the law, as we now proceed to show. Fig. 68 is a north-polar view of the earth, and 1, 2, 3, 4, 6, are the temperature zones so often referred to. Now, Fig. 68 commencing with Nos. 4 and 5, the species in the Eastern and Western Continents are substantially the same, for the lands of the two continents approach each other in these zones so nearly that they may be considered as one. There is no barrier to the spread of species all around the pole. But when we come to No. 3, and still more to No. 2, the difference of species is almost complete, and many genera are also different — and that, not because the physical conditions are unsuitable ; for European species introduced in our country do so well that they often kill out our own native species. Nearly all useful and nox- ious species have been thus introduced. They were not here when America was discovered only because they could not get here. ORGANIC AGENCIES. 125 We said the difference is almost complete. There are, therefore, some exceptions, but these only confirm the principles on which the rule is founded. They are of three kinds : 1. Hardy or widely migrating species. Some hardy species range through No. 3 into No. 4, and these may pass over from continent to continent. Some birds, like the Canada goose and mallard duck, migrate in sum- mer to No. 4, and thence in winter southward in both continents. 2. Introduced species, which have become wild. 3. ^/J0^7^e s^eci'es, mostly of insects and plants. It is a curious fact that species of plants and insects, isolated on the tops of high mountains near the snow- line, are similar to each other on the two continents, and also similar to Arctic species. This latter fact gives the key to the explanation. The geological epoch imme- diately preceding the present (glacial epoch) was charac- terized by extension of Arctic conditions southward even to the shores of the Mediterranean and the G-ulf of Mex- ico. At that time, therefore, Arctic species occupied all Europe and the United States. As the cold abated, Arc- tic species mostly went northward to their present home in the Arctic zone. But some followed the receding Arc- tic conditions upward to the tops of mountains, and were left stranded there, both in Europe and this country. In No. 1 the species on the two continents are still more markedly different, the difference extending even to families and in some instances to orders. Thus, for example, among plants, the cactus order is confined to America. Among animals, the great pachyderms, e. g., elephants, rhinoceroses, hippopotamuses, also the camels, horses, and tailless monkeys, are confined to the Old World, while the sloths, the armadillos, the prehensile- tailed monkeys, the whole family of humming-birds (of which there are over four hundred species), and the family of toucans, are confined to the New. South of the equator the continents do not again ap- 126 DYNAMICAL OEOLOOY. proach, and therefore the fauna of Africa and South America remain very different even to Cape Colony and Fuegia. Subdivisions. — Continental faunas and floras are again subdivided in longitude, more or less completely, by bar- riers in the form of north and south mountain-chains. Thus the fauna and flora of the United States are sub- divided by the Rocky Mountain and Appalachian chain into three sub-faunas and floras, an Atlantic slope, a Mississippi basin, and a Pacific slope. The difference between these is strictlij in proportion to the impassaUe- ness of the harriers. Thus, between the Atlantic slope and the Mississippi basin the difference is very small, because the Appalachian chain is low and may be over- passed ; but the Pacific slope fauna and flora are almost wholly peculiar. Almost the only exceptions are strong- winged birds, like the turtle-dove, the turkey-vulture, the large blue heron, etc. In many cases the species are very similar and yet different. The meadow-lark and the yel- low-hammer are examples. Similarly the Ural Mountains separate a European from a northern Asiatic fauna and flora. These subdivisions are perhaps more marked in case of plants than animals. The spread of plants is pas- sive (dispersal), the spread of higher animals also by migration. Special Cases. — Isolated islands, and in proportion to the degree of their isolation, have peculiar species. We shall mention only a few cases as examples of a general law. Australia is undoubtedly the most striking case of all. The trees of this isolated continent are so different from those of the rest of the world that the whole aspect of field and forest is peculiar and strange. The animals are not only all different in species, but the genera and fami- lies and even many orders are peculiar. Of two hundred species of mammals, nearly all belong to a distinct sub- ORGANIC AGENCIES, 127 class (non-placentals), including kangaroos, opossums, ornithorhynchus, etc., which, with the exception of a few species of opossums, are found only in Australia and the island appendages of that continent. Madagascar is sep- arated by a deep sea from Africa, and we therefore find the organic forms entirely different from those of the neighboring continent, or of any other part of the world. It is especially characterized by the great number of lemurs. On the Galapagos (a small group of islands off the western coast of South America, but separated by a deep sea) the animals and plants are all peculiar. Reptiles of strange aspect abound, but no mammals (except, per- haps, one species of mouse) are known. Thus we see that species are limited north and south by temperature, and in every direction by physical bar- riers. If, now, we add peculiar soil and climates (as in Utah, Arizona, etc.), which, of course, control vegetation and, therefore, animal life, it is easy to see that all these limiting causes produce groups of species confined within certain areas, and differing from other groups, sometimes overlapping and sometimes trenchantly separated. Element of Time. — A¥e have said that faunas and floras differ in proportion to the impassableness of the bar- riers between — i. e., the height and breadth of the moun- tain-chains, the extent of deserts, and the width and depth of seas, etc. But there is still another element of the 'greatest importance, viz., the length of time elapsed since the harrier was set up. This element of time connects geographical faunas and floras with geological changes, and thus geographical distribution of species becomes the key to the most recent of these changes. If we suppose species to undergo very slow changes, then the longer faunas are separated the greater becomes their difference. The full discussion of this important point requires a knowledge of the general laws of evolution, which we are not yet prepared to take up. 128 DYNAMICAL GEOLOGY. Primary Regrions and Provinces. — Taking all the causes into account, the whole land-surface has been divided by Mr. Wallace into six faunal regions — ^viz. : Fig. 69. 1. Nearctic, including all North America exclusive of Cen- tral America. 2. Neotropic, including Central and South America. 3. PalcearctiCy including Europe, Africa north of the Sahara, and Asia north of the Himalayas. 4. Afri- can, including Africa south of the Sahara and Madagascar. 5. Oriental, including Asia south of the Himalayas and all the adjacent islands. 6. Australian, including Aus- tralia, New Zealand, New Guinea, and the South Sea Islands. These primary regions are subdivided into provinces, and these into sub-provinces, according to the principles already explained. We will illustrate by only one example. The Nearctic region is divided into four provinces : 1. ORGANIC AGENCIES. 129 Alleghaniarh ; 2. Canadian or boreal ; 3. Eocky Mountain ; and, 4. CaUfornian. The limits of these are shown in Fig. G9. Marine Faunas. Conditions are far less diverse in the sea than on land, and the limitation of fauna is less marked, but the same laws hold. Temperature Regions in Latitude. — Fauna are here also arranged in zones determined by temperature. In a north and south coast-line, where the temperature changes gradually, the fauna will also change gradually by the substitution of one species for another; but if for any cause there is a more sudden change of oceanic tempera- ture, there will be a correspondingly rapid change of fauna. For example, on our Atlantic coast, the Gulf Stream hugs the southern coast as far as Cape Hatteras (Fig. 69, a), and then turns away and runs at a greater distance. This makes a great change of temperature at this point. Again, the Arctic current, c, coming out of Baffin's Bay, hugs the coast of New England as far as Cape Cod, h, and then goes down. Thus Arctic condi- tions prevail in coast waters to this point. Thus there are three very different marine faunas along the coast of the United States — viz., a Southern, a Middle State, and a Northern ; and these change somewhat suddenly at Capes Hatteras and Cod. Distribution in Longitude. — By far the larger num- ber of marine species inhabit along shore. For these the deep sea is a barrier no less impassable than the land. Therefore, the species inhabiting the two shores of an ocean like the Atlantic are as completely different as those inhabiting along the two coasts of a continent, as America. Special Cases. — There are many species which live in the open sea and form a distinctive Pelagic fauna. Again, there are others which are conditioned by d^Dth Le Contb, Geol. 9 130 DYNAMICAL GEOLOGY. and character of bottom. The most remarkable of these are those inhabiting deep-sea bottom, and forming an abyssal fauna. Again, about the shores of isolated islands, as Madagascar and Australia, the marine fauna are as peculiar as the land fauna. Origin of Geographical Diversity. Until recently the most reasonable view seemed to be that species originated where we find them, and spread in all directions as far as they could. According to this view, the difference between faunas ought to be strictly in proportion to the impassableness of the barriers be- tween. This is largely true, but does not account for all the phenomena. There is another element of equal im- portance, viz., the time during which the harrier has existed. It is probable that faunas and floras are subject to slow, progressive changes, taking different directions in different places. If there be no barriers, spreading by dispersal or migration prevents extreme diversity. But if a barrier be at any time set up by geological changes, then diversity commences, and ificreases with time. According to this view, the Australian fauna is so peculiar because this continent has been so long isolated from all others. The fauna of islands off the coasts of continents are often very similar to that of the adjacent mainland, because they have only recently been separated. Thus, for example, the fauna and flora of the British Isles differ but very slightly from those of the Continent, because, as we now know, these islands, even since their inhabitation by man, have been in full connection with Europe. The divergence has commenced, but is only varietal, not specific. This subject will be taken up again, and more fully explained in connection with glacial epoch, p. 403. CHAPTER IV. IGKEOUS AGENCIES. All the agencies which we have thus far discussed tend to destroy the great inequalities of the earth-surface by cutting down the land and filling up the seas. They are therefore called leveling agencies. If they alone acted, they would eventually bring all to the sea-level and inau- gurate a universal ocean. These agencies, however, are opposed by igneous or by elevating agencies, which, acting alone, would make the inequalities much greater than we now find them. The actual amount and distribution of land are the result of the state of balance between these two opposite forces. It is well to observe that the leveling forces are derived from the sun, while the elevating forces are derived from the interior of the earth — being in fact connected with interior heat. It becomes necessary, therefore, first of all to discuss this subject. Interior Heat of the Earth, The surface temperature of the earth varies with lati- tude, but the mean is about 60°. At any place the surface temperature varies between night and day (daily variation), and between summer and winter (annual variation). As we go below the surface, both the daily and the annual variation become less and less, and finally disappear. The daily variation disappears in a few feet, but the annual variation continues and disappears in our latitude only at a depth of sixty or more feet. Below 131 132 DYNAMICAL GEOLOGY. this the temperature is invariable. The upper limit of the region of invariable temperature is called the stratum of invariable temperature. Its depth varies with latitude, being nearest the surface at the equator, and lying deeper as we go poleward. As already said, below this stratum the temperature is invariable, but it increases as we go deeper. This important fact has been proved by observations in mines and artesian wells. It is true everywhere, but the rate of increase varies, being in some places more rapid (1° in thirty feet), in some less rapid (1° in ninety feet). The average may be taken, for convenience, at 1° for every fifty-three feet, or 100° for every mile of depth. The Interior Condition of the Earth. — Now it is easy to see that at this rate the melting temperature for most rocks, say 3,000°, would be reached at a depth of about thirty miles. Hence, many persons have rashly concluded that the earth is essentially an incandescent, liquid mass, covered with a comparatively thin shell of thirty miles. This would correspond, in a ball of two feet diameter, to a shell of less than one tenth inch thick. On this view, volcanoes are supposed to be openings into this general interior liquid. A little reflection, however, suffices to show that this condition of the interior is improbable. It is almost cer- tain, in the first place, that the rate of increase is not uniform, but decreases, and therefore that the temperature of 3,000° would be found only at a much greater depth than thirty miles. In the second place, 3,000° is the fusing point under atmospheric pressure ; but under the enormous pressure of thirty to fifty miles of rock, the fus- ing point would probably be much higher. Taking these two things into account, it seems certain that, if there be a universal interior liquid at all, the solid shell is much thicker than is usually supposed, and even probable that there is no universal interior liquid at all — and that vol- IGNEOUS AGENCIES. 13a canoes are openings into local reservoirs, not into a uni- versal sea of liquid matter. llecently there has been a tendency among geologists to accept a compromise between these extremes. It is now well known that rocks, under the combined influence of heat ayid water, fuse at a much lower temperature. This, to distinguish it from true dry fusion, is called hydrothermal fusion. While the temperature of true fusion is not less than 3,000°, that of hydrothermal fusion is only 600° to 800°. Now, water certainly penetrates the earth to great depths. Therefore many think that the general constitution of the earth is that of a solid nucleus and a solid crust, separated by a sub-crust layer of liquid or semi-liquid matter. There are many geological phe- nomena that are best explained by such a supposition. The interior heat of the earth is the source of all igne- ous agencies. It shows itself on the surface in three principal forms, viz.: 1. Volcanoes; 2. Earthquakes j 3. Gradual oscillations of the crust. /Section I. — Volcanoes. Definition. — A volcano may be defined as a conical mountain, with a pit-shaped, cup-shaped, or funnel- shaped opening atop, from which are ejected, from time to time, materials of various kinds, always hot and often fused. They vary in size from inconspicuous mounds to mountains many thousand feet high. Volcanoes may be active or extinct. Those which have not erupted for a century past are supposed to be extinct. Yet, so-called extinct volcanoes sometimes break out again. Until the great eruption which destroyed Hercu- laneum and Pompeii, Vesuvius was supposed to be an extinct cone. Since that time it has been very active. Again, in some rare cases, volcanic eruptions are constant. Stromboli and Kilauea, for example, are in feeble erup- 134 DYMAMICAL GEOLOGY, tion all the time. But most volcanic eruptions are peri- odic. The period of intermission may be ten, or twenty, or fifty, or one hundred years. Number, Size, and Distribution. — Humboldt enu- merates 225 volcanoes as known to have erupted in the past century. The number now known is doubtless much greater. In size they vary from little mounds (monticles) to Mount Etna, 11,000 feet; Mauna Loa, 14,000 feet; and Aconcagua, 23,000 feet. In this last case the whole height is not due to volcanic eruptions, for the cone stands on a mountain plateau many thousand feet high ; but the others are wholly built up by eruption. The laws of distribution may be briefly stated as follows : 1. Volcanoes are mostly on islands in the midst of the sea, or on the margins of continents bordering the sea. Only a very few have been found at a distance from the sea. The Pacific Ocean is the greatest theatre of volcanic activity. Its surface is dotted over with volcanic islands, and its margin is belted about with a fiery girdle of volcanic vents. 2. Volcanoes occur usually in lines, as if connected with a crust fissure, or else in groups, as if connected with a subterranean lake of fused matter. The most remarkable linear series of volcanoes is that which, commencing in the volcanoes of Fuegia, con- tinues, as a chain of active vents, along the Andes and Mexican Cordilleras ; then along the Sierra and Cascade, as the recently extinct volcanoes of these chains ; then along the Aleutian Isles and Kamschatka ; then through the Kurile Isles to Japan and the Philip- pines ; then with more uncertainty to New Guinea, New Zealand, the Antarctic Continent, Deception Island, and back again to Fuegia, after completing the circle of the globe. The most remarkable groups are the Javanese group, the Hawaiian group, the Icelandic and the Medi- terranean groups. 3. Volcanoes are found mostly in strata of comparatively recent date, and the retiring of I IGNEOUS AGENCIES. 135 the sea seems in many cases to be ' associated with their gradual extinction. The recently extinct volcanoes on the east side of the Sierra are good examples. Phenomena of an Eruption. — In some cases, as in the Hawaiian volcanoes, the floor of the crater, hardened from previous eruption, becomes hot, then melts ; then the melted lava rises higher and higher, until it overflows and runs down the slope in one or more streams. The volcanic forces being thus relieved, the melted lava again sinks gradually to its former level, and hardens into a floor. Thus all proceeds with but little commotion. In other cases, as in the Javanese volcanoes, premonitions of coming violence are observed in the form of subterranean explosions attended with shakings of the earth ; then, with a powerful explosion, the floor of the crater is broken up, and the fragments are shot with violence, high, some- times miles high, in the air ; then cinders and ashes and smoke are ejected in immense volumes ; then streams of lava are outpoured, perhaps alternating with explosions of gas and vapor y ejecting cinders and ashes. The ascend- ing vapors are condensed, and fall as deluges of rain, which, with ejected ashes, form streams of mud. In all cases, if the mountain be snow-capped, the melting of the snow produces floods, which are often among the most disastrous features of the eruption. Thus there are two extreme types of eruptions, the quiet and the explosive. In the one, the ejecta are mostly lavas; in the other, gases, vapors, ashes, and cinders. The best type of the former are the Hawaiian, of the latter the Javanese volcanoes. But all grades between exist. The Icelandic volcanoes belong more nearly to the former type, the Mediterranean to the latter. Among Mediterranean, Etna approaches more the former, and Vesuvius the latter. Quantity of Matter i;jected. — In the great eruption of Tomboro, in the Island of Sumbawa (one of the Jav- 136 DYNAMICAL GEOLOGY. anese group), in 1815, the explosions are said to have been heard in Ceylon, nine hundred miles distant. The quantity of smoke and ashes was so great that, hanging in the air, they produced absolute darkness for many days, and falling, covered the sea over an area of one hundred miles radius. It has been estimated that the ashes ejected were sufficient to cover the whole of Ger- many two feet deep, and if piled in one place would make a mass three times the bulk of Mont Blanc (Herschel). Of lava-eruptions, perhaps the greatest is that of Reyk- janes (Skaptar) in 1783. The mass outpoured has been estimated as twenty-one cubic miles (Herschel). These, however, are extreme cases. One of the greatest erup- tions of Kilauea, that of 1840, poured out a lava-stream forty miles long, which, if accumulated in one place, would cover an area of a square mile eight hundred feet deep. The average of lava-flows, however, is far less. One of the greatest eruptions of Vesuvius poured out 600,000,000 cubic feet of lava. This would cover a square mile twenty-two feet deep, or would make a stream seven miles long, one mile wide, and three feet thick. Monticles. — In volcanoes of moderate height, eruptions usually come from the top of the cone or principal crater, but in very lofty volcanoes the pressure necessary to raise lava so high fissures the mountain in a radiating manner. These fissures are filled with liquid matter, which, on hardening, form radiating dikes. Eruptions often take place through these fissures, and thus form subordinate craters and cones about the main cone ; these are called monticles. About six hundred such monticles dot the surface of Mount Etna, some of which are seven hundred feet high above the level of the mountain-slope on which they stand. About Mount Shasta (which is a recently extinct volcano) are found a number of these monticles. Nature of the Materials Erupted. — The materials IGNEOUS AGENCIES. 137 erupted are — 1. Rock-fragments. 2. Lava. 3. Cinders. 4. Sand. 5. Ashes. 6. Smoke. 7. Gas. The rock-frag- ments are formed in explosive eruptions by the breaking up of the hardened floor of the crater, and require no further explanation. Lava. — This term is applied to melted rock, or to the same after it has hardened again. The degree of liquid- ity depends partly on the degree of heat and partly on the kind of fusion. The lava of Kilauea is as liquid as honey. The bursting of bubbles on the surface of this thin, vis- FiG. 70. —Lava-tunnel, and "spatter-cone" toinidi i)\ i -( ipiuj; --uaiii, Ivu un i (Photograph by Libbey.) cous liquid draws it out into hair-like threads like spun- glass, which is borne by the winds and accumulated in certain parts of the crater. This is called " Pele^s hair." Thin lava like this, when it first issues from the crater, runs with groat velocity, twenty to twenty-five miles an hour ; but as it cools it becomes stilfer, first like tar, then 138 DYNAMICAL GEOLOGY, like pitch, and therefore runs with less and less speed, until it becomes rigid and stops. Being a bad conductor of heat, lava cools and forms a crust on the surface while it is still liquid and flowing within. The liquid finally flowing out, often leaves a hollow tube or gallery. Again, since all lava contains more or less of gas and vapor, the crust is a sort of concreted lava-foam. This vesicular, spongy lava is called scoria. Sometimes, in very stiffly viscous lava, the vapor-bubbles run together and form huge blisters, which, by hardening, form caves. Thus, nearly all lava-beds are full of galleries and caves. It was in the galleries and caves which honey-combed the ancient lava-flows of southern Oregon that a handful of Modocs defied so long the power of the United States Army. Again, the liquidity of lava, and its appearance after solidifying, depend much upon the hind of fusion. Lavas are often in a state of hydrothermal fusion (page 133), i. e., half fusion, half solution in superheated water. Such a semi-fused mass, on concreting, makes a kind of earthy stone. Sometimes, in fact, the ejecta are little more than hot mud, and concrete into tufa. Cinders, Sand, and Ashes are only different forms of hardened lava. The liquid lava, before ejection, may be so largely mingled with gas and vapors that it is liter- ally a roch-foam. Masses of this rock-foam, ejected with violence into the air, cool and fall as cinders. Often the greater part of the ejections is of this kind, and thus are formed cinder-cones. Sometimes the violence of the explosions is so great as to break up the liquid mass into rock-spray. This falls again as sand or ashes, according to its fineness ; or else the rock-fragments and cinders are thrown up, and, falling again repeatedly, may be triturated into dust or ashes. The finest rock-dust hang- ing in the air is called smoke; the same, fallen to the ^arth, ashes. Volcanic ashes, wet with water and con- IGNEOUS AGENCIES. 139 solidated either on the spot or after transportation and sortings is called tufa. Physical Conditions of Lava. — If lava cools very slowly, the minerals of which it is composed separate and crystallize more or less perfectly. This is stony lava. If it cools rapidly, it forms volcanic glass. If the volcanic glass be full of vapor-bubbles, it forms scoria. If volcanic ashes mixed with water solidifies, it makes tufa. Thus there are four physical states in which we find lava, viz., stony, glassy, scoriaceous, and tufaceous. Classification of Hardened Lavas. — Hardened lava consists essentially of two principal minerals, viz., feld- spar and augite.* If the former predominate, it is called feldspathic ; if the latter, augitic lava. These two min- erals are often not detectable except with the microscope, and yet the two kinds of lavas may usually be distin- guished by the eye. The lighter colored and lighter weighted are usually feldspathic ; the darker and heavier, augitic. The feldspathic lavas are said to be acidic ; the augitic, dasic. Both of these kinds take on the four physical states mentioned above. Feldspathic lava, in the stony condition, is trachyte ; in glassy condition, obsidian ; in scoriaceous condition, pumice j in tufa- ceous, the light-colored tufas. Augitic lava, if stony, is basalt ; if glassy, pitchstone j if scoriaceous and tufa- ceous, blach scoricB and tufas. Gases and Vapors. — The gases ejected from vol- canoes are steam, chlorhydric acid, sulphurous acid, sulphhydric acid, and carbonic acid (H^O, HCl, SO^, H^S, CO3). The first three are characteristic of true eruptions, the others of feeble, secondary volcanic activ- ity. Of all, steam is by far the most abundant. In vol- canoes of the explosive type the quantity of steam is often enormous. This fact strongly suggests this vapor as the main agent of eruption. Flames are often spoken * The pupil ought to be shown specimens of these minerals. 140 DYNAMICAL GEOLOGY. of in eruptions. It is possible that there may be some- times feeble flame from the combustion of II or II^S, but probably the so-called flame is nothing else than the ruddy reflection of the glowing liquid in the crater upon the smoke and cloud hanging in the air. Formation of Volcanoes and their Structure. — It is now generally admitted that volcanic cones are built up mainly by their own eruptions. On this view, their origin and mode of growth may be briefly described as follows ; 1. The increase of heat (by causes which we lit- Fio. 71.— Section across Hawaii. L„ Mauna Loa ; K, Manna Kea. tie understand) at the focus of the volcano thins the crust in that point, until it gives way, and the melted matter is outpoured on the surface around the opening. 2. With every eruption the accumulated material rises higher and spreads farther ; and thus a conical mound is formed. The shape of this mound will depend on the kind of mat- ter erupted. If it be very liquid lava, it will spread far, and the cone will be low in proportion to the base, as in the Hawaiian volcanoes (Fig. 71) ; but if the material be cin- ders, these will pile up into a steep cone (Fig. 72). The repeated lay- ers of lava or cinders produce a stratified appearance ; but this must not be confounded with true stratification. 3. With every eruption, the eruptive throes split the sides of the cone with radiating cracks, which, filling with liquid and hardening, form radiating rocky ribs called dikes (Fig. 73), and these bind the lava or cinder Fig. 72.— Section of cinder cone. IGNEOUS AGENCIES. 141 layers into a stronger mass. 4. When the cone grows very high, eruptions will take place through these fis- I Fig. 73.— Dikes in Etna. Bures, as well as from the top cratcT, and thus will be formed secondary cones or monticles. 5. If any of these monticles cease to erupt, they will be covered up by ejec- tions from the main crater or other secondary craters. All these facts are shown in Fig. 74. 6. From time to Fig. 74.— Ideal section of a volcano, ss, original surface ; mm, monticles ; m'm' extinct monticles ; cr, cr, original stratified crust. 142 DYNAMICAL GEOLOGY, time, at very long intervals, there occur very great erup- tions. If the volcano be of the quiet type, the whole top of the cone is melted, and, after eruption, is ingulfed ; or if of the explosive type, the whole top of the cone is blown into the air, and the mountain is disemboweled. In either case a yawning chasm many miles in extent is left. 7. Within this great crater, by subsequent erup- tions, is built up a smaller cone, and within this again often still smaller cones.- Thus volcanoes often have about their present eruptive cone a great surrounding rampart. This rampart is the remains of the great crater. In Vesuvius (Fig. 75), Mount Somma, s, is the Fig. 75.— Section of Vesuvius, vv, Vesuvius cone ; s. Mount Somma ; «', other side of Somma overflowed by lava from Vesuvius. remains of such a great crater, the other side of it being broken down, and now covered by flows from the present crater. Crater Lake. — An excellent example of this structure is found in Crater Lake. This beautiful lake, with its splendid blue waters, occupies a yawning chasm on the top of an extinct volcano in southern Oregon. The lake itself is about six miles in diameter and 2,000 feet deep (the deepest lake on the American continent), and is sur- rounded by almost perpendicular walls, 1,000 to 2,200 feet high. From the midst of the blue waters, but nearer one side, there rises a beautiful island (Wizard Isle), 800 feet high, which is in fact a cinder cone with a small crater atop. Fig. 76 gives an ideal section of the lake and island, and also, in dotted outline, the supposed form IGNEOUS AGENCIES. 143 and height of the original volcano before ingiilfment. This former volcano has been named Mt. Mazama. ' Mt. Mazama \ Fig. 76.— Ideal section of Crater Lake, Mt. Mazama, and Wizard Isle. (After Diller.) Age of Volcanoes. — From the progressive manner in which volcanoes grow, it would seem that we may esti- mate their age. Such estimates, it is true, must be very rough, yet they are useful in familiarizing the mind with the idea of the great amount of time necessary to account for geological phenomena. For this purpose we will use Etna. There have been, indeed, other volcanic eruptions great enough to build this mountain at once^but the eruptions of Etna itself have been very regular and mod- erate. Etna is 11,000 feet high, and about thirty miles in di- ameter at its base. We will take its circumference at one hundred miles. Now, a lava-stream of triangular shape, one foot thick, reaching to the base, and one mile wide, would, we believe, be an average eruption. It would cover seven square miles, one foot deep, and would be equal to more than 200,000,000 cubic feet. It would take one hundred such eruptions to raise the whole mountain-surface one foot. Taking one such eruption every year (eruptions of Etna for the last 2,000 years have been but one in twenty-five years), it would take a century to raise the mountain-surface one foot. But there is a gorge cut into the side of this mountain, revealing 3,000 feet of lava-layers. To have built up these 3,000 feet would require 300,000 years. That we have been moder- 144 DYNAMICAL GEOLOGY. ate in our estimate is shown by the fact that there are known on the flanks of Etna lava-flows 2,000 years old, which are still not covered by subsequent flows. We are justified, then, in saying that Etna is probably much more than 300,000 years old. But the birth of Etna is a very recent geological event, for it stands, and has been built up, on the latest tertiary formation. Cause of Volcanic Eruptions. 'his question is still very obscure. There are two things to be explained, viz., volcanic force and volcanic heat — the force necessary to raise lava to the lip of the crater, and the heat necessary to melt the lava. (a.) Force. — If we consider the height of volcanic cones, we shall be better able to appreciate the greatness of the force. In the accompanying table we give the heights of some well-known volcanoes and the pressure in atmospheres (one atmosphere = fifteen pounds per square inch, or one ton per square foot) necessary to raise lava (taking the specific gravity at 2. 8) to the lip of the crater. It is true lava is sometimes foamy, and therefore lighter, but, on the other hand, we have taken the focus of vol- canoes at sea-level, while it is probably much deeper, and have supposed the force only sufficient to raise to the lip of the crater, whereas it often ejects with violence many thousand feet in the air. NAME. Height. Pressure in atmospheres. Vesuvius ... 3,900 feet 11,000 " 13,800 " 19,660 " 325 Etna 920 Mauna Loa 1,150 1,638 Cotopaxi What, then, is the agent of this great force ? It is believed that it is the elastic force of compressed gases I IGNEOUS AGENCIES. 145 and vapors, especially steam. The power of these agents is well known ; and gas and steam issne in immense quan- tities during eruptions, especially of the explosive type. On this point there is little difference of opinion. ijb.) Heat. — But the cause of the heat necessary to fuse the rocks is one of the most difficult of all questions con- nected with the physics of the earth. By most geologists it is thought to be connected with the primal heat of the earth, and the supposed universal melted condition of the interior. This view assumes {a) that the earth was once an incandescent, fused mass. This is almost cer- tainly true ; (h) that in cooling it formed a crust, which thickened by additions to its inner face, until it is now about thirty miles thick ; (c) and that this limit between the solid crust and melted interior is the place of the focus of volcanoes. There are many difficulties in the way of acceptance of this view, some of which are given on page 132. All other theories regard the melted matter as locals but, as to the cause of the fusion, there is yet great diver- sity of view. Some attribute it to chemical action ; some to mechanical crushing. It must be remembered in this connection,, however, that in some cases, at least, the amount of heat required is not more than 800° F., for in some lavas the fusion is hydr other mal, and in all cases the access of water seems necessary to supply the force. Secondary Volcanic Phenomena. There are many phenomena which linger after the true eruptions have ceased. The chief of these are hot springs, carbonated springs, lime-depositing springs, solfataras, fumaroles, mud-volcanoes, and geysers. These all seem to be the result of circulation of water through lavas which still retain their heat, and are therefore properly called secondary volcanic phenomena. The lavas, outpoured by primary or true eruptions, remain hot in their interior for Lk Conte, Geol. 10 146 DYNAMICAL GEOLOGY. an indefinite time. If waters^ percolating through these, come up again * after taking up only heat, they form hot springs. If, in addition, they take up CO,,, they form carbonated springs. If lime be taken and deposited on the surface, they form lime-depositing springs (p. 73). If the heat be great, so that vapors are given off and con- densed as clouds, they are called f umaroles. If the waters contain H.^S, and AlkS, they are called solfataras. If mud is brought up and deposited about the vent, they are mud-volcanoes. Finally, if the springs are periodi- cally and violently eruptive, they are called geysers. The only variety of these springs which need detain us here is \ ■^ Geysers. Geysers may be defined as periodically eruptive springs. They seem also usually, if not always, to deposit silica. They are found only in Iceland, in New Zealand, and in Yellowstone Park. The so-called California geysers are solfataric fumaroles. Steamboat Springs in Nevada may possibly be classed with geysers, but their erup- tions are feeble. The phenomena of true geysers are so splendid that a somewhat full account of them is neces- sary. As they were first studied in Iceland, and the cause of their eruption was first understood there, we will speak of these first. Geysers of Iceland. — Iceland may be briefly described as a plateau 2,000 feet high, studded with volcanic peaks, with margin sloping gently to the sea. Only the marginal area is to any extent inhabited. The interior is a scene of desolation, where every form of volcanic phenomena exists in the greatest activity— volcanoes, hot springs, boiling springs, fumaroles, solfataras, and geysers. Of these last there are very many in various degrees of activity. The most celebrated of these is the Great Geyser. IGNEOUS AGENCIES. 147 Great Geyser. — This is a low mound, with a basin- shaped depression at top, from the bottom of which de- scends a tube or well to unknown depth, but may be sounded to eighty feet or more. The basin is fifty feet across, and the tube or throat ten feet in diameter at the top but narrowing downward. Both the basin and the throat are lined with silica deposited from the water, and doubtless the mound itself was built up by similar deposits. In the intervals between eruptions the basin is filled to near the brim with water at 180°. Phenomena of an Eruption. — As the time for the eruption approaches, the first thing observed is a series of explosions in the bottom of the throat like subterranean cannonading ; then bubbles of vapor are seen to rise and burst on the surface ; then the water of the surface bulges up and overflows. Immediately thereafter the whole of the water in the throat and basin is ejected with violence one hundred feet into the air, forming a fountain of daz- zling splendor, followed by the roaring escape of steam. As the water falls back, it is again ejected, and the foun- tain continues to play several minutes until the steam has all escaped and the water partly cooled ; then all is quiet again until another eruption. The interval between erup- tions is irregular. An eruption may be brought on pre- maturely by throwing large stones down the throat of the geyser. Yellowstone Geysers. — But in splendor of eruption the Icelandic geysers are far surpassed by those of Yellow- stone Park. This, like Iceland, is a volcanic region, but, unlike Iceland, primary volcanic phenomena are all ex- tinct. The geyser phenomena here occur in a narrow valley surrounded on all sides with volcanic rocks of great thickness, of comparatively recent origin, and doubtless, therefore, still hot in their interior. In tliis little valley there are no less than 10,000 vents of all kinds, hot springs, boiling springs, mud-volcanoes, lime-depositing springs. 148 DYNAMICAL GEOLOGY. and geysers. On Gardiner^s River the vents are mostly hot carbonated springs, depositing lime ; on Firehole River they are geysers, depositing silica. In Yellowstone Park alone there are in all 3,000 vents, of which sixty-two are eruptive geysers. In the lime carbonate springs the de- posits on hillsides have given rise to a succession of terraces (Fig. 41, page 75), and sometimes the water descending through a succes- sion of pools from terrace to terrace Fig. 77.— Deposits from carbonated springs. gives rise to beautiful stalactitic forms (Fig. 77). In the geysers the hot alkaline waters collect in pools, and deposit the silica first in a gelatinous condition, which afterward concretes into all kinds of fantastic forms (Fig. 78). The deposit immediately about the eruptive IGNEOUS AOENCIES. 149 150 DYNAMICAL GEOLOGY. vents builds up moundlike, hivelike, and cliimneylike forms (Fig. 79). The silica-charged waters, trickling Fig. 79.— Crater of Castle Geyser, Yellowstone Park. slowly over the mounds, give rise by deposit to patterns of exquisite and delicate beauty, compared by Hayden to embroidered lace-work with edging-fringe and pendent tassels, and studded with pearls. Similar deposits are formed also in l^ew Zealand ; we give an example in Fig. 80. Only a few of the grandest of these geysers can be mentioned : 1. The Grand Geyser throws up a column of water six feet in diameter to the height of 200 feet, while the steam ascends 1,000 feet or more. The eruption is repeated every thirty-two hours, and lasts twenty minutes. 2. The Giant (Fig. 81) throws a column five feet in diameter 140 feet in the air, and plays continuously for three hours. 3. The Giantess, the greatest of all, throws up a huge IGNEOUS AGENCIES, 161 column twenty feet in diameter to the height of sixty feet, and through this great mass it shoots up several lesser jets Pig. 80.— Pink terraces, New Zealand. (After Peale.) to the height of 250 feet. It erupts about once in eleven hours, and plays twenty minutes. 4. The Beehive, so called from the shape of its mound, shoots up a splendid column two to three feet in diameter to the height of 219 feet, and plays fifteen minutes (Fiff. 82). 5. Old Faithful, so called from tlie frequency and regu- larity of its eruptions, throws up a column six feet in diameter to the height of 100 to 150 feet, and plays fifteen minutes (Fig. 83). Cause of Geyser Eruption.*— This may be explained, in a very general way, as follows : Experiments show that the heat of the water rapidly increases as we pass down the geyser-throat. There is no doubt, therefore, that in spite of the increasing pressure (which raises the boiling-point) * For a complete discussion of this interesting subject, see author's " Elements," pp. 99-104. 152 DYNAMICAL GEOLOGY. Fig. 81.— Giant geyser. (After Hayden.) 1 i IGNEOUS AGENCIES. 153 Fig. 83. Bi-ehive geyser. (From a drawing by Holmes.) 154 DYNAMICAL GEOLOGY. the boiling-point is reached and a large quantity of steam is formed first, at some point deep below. The water above is immediately ejected, and the fountain continues Fig. 83.-01(1 Faithful geyser in action. (After Hayden.) to play until all the steam escapes and the water is some- what cooled. Then all is quiet until the water again heats up to the boiling-point. Section II. — Earthquakes. When we consider the suddenness with which earth- quakes occur, the terror they inspire, and the place of their origin, deep in the interior of the earth, and hidden from observation, it is not surprising that we know so I IGNEOUS AGENCIES. 155 little about their cause. In fact, until about forty years ago no attempt had been made to study them scientifi- cally. Now, however, it is believed the foundations of a true science of earthquakes* (seismology) have been laid, and a true progress has been made. The basis has been laid by Mr. Mallet, and progress has been made possible by the use of self -registering seismometers. Frequency of Earthquakes. — The slow development of earthquake-science is not due to want of material, but, as has already been stated, partly to the difficulty of the subject, and partly to the terror produced — unfitting the mind for scientific observation. The earthquake catalogue of Alexis Perrey records 18,000 in thirty years (1843- 1873), or nearly two a day. When we remember that three-fourths of the earth^s surface is covered with the sea, that a large portion of the land-surface is inhabited by uncivilized races, and that even in civilized countries many slight tremors are unrecorded, it will not seem extravagant to say that, probably, there is not an hour of any day in which the earth is not shaking in some portion of its surface. Phenomena of an Earthquake. — In brief, the phe- nomena of an earthquake are : 1. Sounds, sometimes like underground cannonading ; sometimes a hollow rumhUng, or claslmig, or gri7iding. 2. Accompanying, or immedi- ately succeeding, comes the movement of the earth, as a slight tremor, or as a violent shaking ; in extreme cases, so violent that the houses of whole cities are shaken down, like card-houses of children, and bodies on the surface are thrown up a hundred feet into the air, as at Eiobamba in 1797. 3. As to direction, the movement may be up and down, or from side to side, or partaking of both, i. e., obliquely, or it may be rotating or twisting, as, for ex- ample, when chimney-tops are twisted about without being upset, or wardrobes and bureaus turned about before upsetting. 4. One thing is always observed and is of 156 DYNAMICAL G£OLOGY. primary importance, viz., that the shake does not occur everywhere at the same time, but on the contrary appears first at one place and spreads thence in all directions, precisely like a system of waves when a stone is thrown into the water. This point of first appearance is called the '' epicentrum," because it is immediately above the origin. The violence of the earthquake is greatest there, and thence decreases precisely like a system of widening circular waves. Velocity of Shock and of Transit. — The velocity of the spread from the center or velocity of travel (transit) must be carefully distinguished from the velocity of the earth-movement (shock). There is no close relation be- tween these. We may best illustrate this by water-waves. Suppose we are in a boat on the surface of a bay traversed by long, low swells. As each sw^ell passes under us, we are slowly heaved up and slowly let down again, but the waves are here, there, and away with great velocity. The velocity of oscillation is small, the velocity of transit is great. But if the surface of the bay be agitated by short, high waves, the oscillation or shaking is more rapid, but the transit is comparatively slow. So in earthquakes, the movement may be only a slow heaving up and down^ or swinging back and forth, and yet this movement may travel from place to place with great velocity. JSTow, as in water-waves generated by a stone thrown in still water, so in earthquakes, the velocity and amount of movement (which is equivalent to the wave-height) is greatest at the center (epicentrum), and diminishes as it spreads, but the velocity of the transit or travel is nearly or quite uniform. Now, the velocity of transit has been determined in many earthquakes by noting the time of arrival at different places. It varies with the kind of rock, being greatest in the hardest, and also with the depth of the origin, being greater for very deep earthquakes. In some cases it is only ten miles per minute ; sometimes fifteen, twenty. IGNEOUS AGENCIES. 157 thirty miles per minute, or even much more. Sometimes the spread is equally rapid in all directions, and the spread- ing wave is circular , or nearly so ; sometimes it is more rapid in one direction than another, and the spreading wave is elliptical. Cause of Earthquakes. — The origin of earthquakes being deep beneath the surface and hidden from obser- vation, their cause is very obscure. Yet their association with other forms of igneous agency suggests prohahU causes : 1. Volcanic eruptions, especially of the explosive type, are always accompanied by slight and sometimes by seri- ous earthquakes. This fact suggests the sudden formation of gases or the sudden collapse of vapors as a possible cause. On this view an earthquake would be like the earth-jar produced by a mine-explosion, or by the explo- sion of large quantities of gunpowder or nitro-glycerine. 2. But great earthquakes are oftener associated with bodily movements of extensive areas of the earth-crust. Thus, for example, in 1835, after a severe earthquake on the western coast of South America, it was found that the whole coast-line of Chili and Patagonia was raised from two to ten feet above sea-level. Again, in 182.2, the same phenomenon was observed in the same region after a great earthquake. Again, in 1819, after a severe earthquake which shook the delta of the Indus, a tract of land fifty miles long and sixteen miles wide was raised ten feet, and an adjacent area of 2,000 square miles was sunk, and became a lagoon. In commemoration of the wonderful event, the elevated tract was called Ullah bund, or, the mound of God. Again, in 1811, a severe earthquake — perhaps the severest (except the Charleston earthquake of August, 1886) ever felt in the United States— shook the valley of the Mississippi. Coincidently with the shock, large areas of tlie river-swamp sank bodily, and have ever since been covered with water. In commemoration of the 158 DYI^AMIGAL GEOLOGY. event, this area is still called the sunken country. In all these cases, probably, and in the last two certainly, there was a great fissure of the earth-crust, and a slipping of one side on the other. Now, these facts suggest another and, we believe, a more probable cause of earthquakes. It is well known that there are operating within the earth forces elevating or depressing or crushing together portions of the crust. We will discuss the nature of these forces in Part II. Suffice it to say now that it is in this way that continents are elevated and mountain-ranges are formed. Now, suppose such forces operating to raise or depress large areas of the crust — e. g., the southern end of South America — it is evident that, the interior forces lifting and the stiff crust resisting, there would come a time when the crust would break — i. e., form a great fissure. Such a sudden break would produce an earth- jar which would propagate itself from the fissure as focus in all directions as an earthquake. Or, again, after such a fissure is formed, the two walls may at any time slip on each other and pro- duce an earth-jar. Now, this is not mere speculation. We find such great fissures intersecting the earth in many places ; they break through miles of thickness of rock, and in many cases the two walls are slipped on each other several thousand feet vertically. It is almost certain that earthquakes are produced hy the formation or the slipping of such fissures. In 1873 there was ^ severe earthquake in Inyo County, California, just at the eastern base of the Sierra. Now, there is on that side of the range a great fissure and a slip of several thousand feet. It is almost certain that the earthquake was produced by a slight readjustment of the position of the walls of this fissure. Moreover, the thorough investigations very recently of several earthquakes have seemed to establish the fact that they originated in the formation or the readjustment of a fissure. IGNEOUS AGUKCIES. 159 Nature of Eartliqiiake-Waves. — In any case, it is evident that an earthquake is produced by concussion of some kind somewhere in the interior of the earth, usually at a depth of from six to ten miles. The concussion gives rise to a series of elastic earth-waves, spreading in all directions spherically, like sound-waves, until they reach the surface, and then spread in all directions on the surface as a circular wave, as in Fig. 84. The interior Pig. 84.— Section and perspective of a portion of tlie eartii's crust shaken by an earthquake, showing origin, x; section of the spherical waves, a', b', c', etc., and perspective of the outcropping surface waves, a, 6, c, etc. point of origin (x) is called the focus, or centrum ; the point of first emergence {a), the epicentrum. It is the passage of a series of these circular waves beneath the feet of the observer at any point (d) that gives rise to the actual observed phenomena ; so that the scientific discussion of earthquake phenomena is little else than the discussion of such earth-waves emerging and spreading on the surface. Earthquakes occurring- beneath the Sea. — We have thus far spoken of earthquakes occurring beneath the land ; but three fourths of the earth-crust is covered with water, and therefore it is probable that the larger number of earthquakes have their origin beneath the sea-bed. Besides, as we shall see hereafter in treating of mountain- chains, marginal sea-bottoms are particularly liable to movements. When an earthquake occurs beneath the sea-bed, there are some additional phenomena, which must now be discussed. 160 DYNAMICAL OEOLOGY. Suppose, then, a concussion, from any cause, beneath the sea-bed. There would be formed, as before, about the focus, a series of spherical earth-waves, which, by en- largement, would emerge on the surface of the sea-bed as circular surface-waves. These, spreading beneath the sea, would reach the nearest shore, and produce their de- structive effects there. Some time afterward, perhaps a half -hour or more, there comes rolling in on shore a prodigious water wave, or perhaps a series of water waves, thirty to sixty feet high, deluging the whole shore region, and completing the destruction commenced by the earth- wave. ~^-^ The Great Sea Wave. — This very destructive accom- paniment of earthquakes occurring beneath offshore sea- beds may be explained as follows : The bed of the sea at the epicentrum is lifted up perhaps several times. This lifts the whole sea water above, so that the surface is raised into a water mound. This mound immediately sinks as much below the sea level as it was before raised above it, and thus gives origin to a circular water wave (or series of such waves) which spreads exactly like any other water wave, growing lower as its spreads, until it breaks on the nearest shore. Out at sea such great low waves would pass under a ship unobserved, heaving it slowly up and letting it down again. But when they approach shore, on account of their great size, often fifty feet high and one hundred to two hundred miles across the base, they rush forward as a tide fifty feet high and" devastate the whole coast within their reach. They are, therefore, sometimes called tidal waves, although they have nothing to do with tides. Though originating at the same place, the great sea wave moves much less rapidly than the earth-wave, and therefore reaches the shore later. Examples of the Great Sea Wave. — 1. In 1755 a terrible earthquake destroyed Lisbon, and, it is said, forty thousand people. The focus of this earthquake was IGNEOUS AGENCIES. 161 beneath the seii-bed, perhaps one hundred miles off shore. The arrival of the earth-wave shook down the houses. Then, after a half-hour, when all was quiet, there came great sea waves sixty feet high and completed the destruc- tion of the city. These waves were sixty feet high at Lisbon, thirty feet at Cadiz, eighteen feet at Madeira, and five feet on the coast of Ireland. They were also felt on the coast of Norway and on the West India Islands, after having traversed the breadth of the Atlantic. 2. In 1854 an earthquake shook the coast ,of Japan. A half -hour afterward a great wave, thirty feet high, came in and swept the town of Simoda clean away. The epi- centrum was probably a hundred miles off shore. The wave, spreading in all directions, was highest on the coast of Japan, because this was near the epicentrum. But in the other direction it was observed at the Bonin Islands fifteen feet high, and — after traversing the Pacific and being nearly exhausted — on the California coast, only eight inches high at San Francisco and six inches at San Diego. 3. In August, 1868, a very destructive earthquake shook the coast of Peru, severest about Arica. The epi- centrum was not far off shore, for in five minutes after- ward there came in great sea waves sixty feet high and desolated the whole coast, carrying ships far inland and stranding them high up on the mountain slopes. These great waves were traced southward to Coquimbo and be- yond, northward to San Francisco, Astoria, and Sitka, southwestward to Australia and New Zealand, and west- ward to Hawaii and Japan, thus having traversed the whole breadth of the Pacific. Were it not for the obstruct- ing continents, there is no doubt that they would have encompassed the earth in their widening circles. In regard to these waves, there are several points worthy of notice : a. Their velocity, though less than that of earth-waves, Lk Contb, Geol. 11 162 DYNAMICAL GEOLOGY. is enormously great for water waves. The wave of 1854 traversed the Pacific, from Japan to San Francisco, a dis- tance of 4,500 miles, in about twelve hours, or at a rate of 370 miles an hour. The wave of 1868 ran across the Pacific with even greater speed. The reason of their great velocity is their enormous size. h. The size of the great sea wave is determined by the principle that every wave runs its own length in the time of one -oscillation. If a boat be lying on smooth water, and a series of water waves passes under it, the boat will be moved up and down once while the waves run the length of one wave ; i. e., from trough to trough. Now, the time of oscillation of the great sea waves of 1854 was about thirty-three minutes. If, then, the waves run 370 miles in an hour (60 minutes), how much did they run in 33 minutes— 60 : 33 : : 370 : 203. Therefore, these waves were 203 miles from trough to trough. c. Hie mean depth of the ocean may be determined by these waves. The principle on which this is done is as follows : Every one has observed that waves coming in from deep water on to a flat, shelving shore, at a certain depth begin to drag bottom, and are impeded thereby ; also, that the larger the wave, the deeper the water in which it begins to drag. Now, in the case of these enor- mous earthquake sea waves, the ocean itself is not deep enough to prevent them from dragging bottom. As they run over the sea their velocity is impeded everywhere, but more or less according to the varying depth of the ocean. Now, the normal or unimpeded velocity of a wave may be accurately calculated, since it varies as the square root of the wave-length (v ex VL)- Therefore, the amount of retardation will give the depth of the ocean over which it passes. The mean depth of the ocean between Japan and San Francisco, as thus determined, is 12,000 feet ; between Arica and Hawaii it is 18,000 feet. IGNEOUS AGENCIES. 163 Determination of the Epicentruni and Centrum. — By means of seismometers the direction of the earth's motion may be determined. If this be taken in many places, and the lines of direction be protracted, they will be found to meet at some point from which all seem to radiate. This is the center of the circular surface-waves or epicentrum. Or, by accurate clocks in many stations, the time of arrival of the shock may be recorded. If, now, we draw a line through all the places where the time of arrival was the same, we shall have a curve which represents the form of the wave and the center of which, a, is the epicen- trum. Such lines of simul- taneous arrival of shock are called coseismal lines {c s, Fig. 85). The position of the cen- trum or origin is much more difficult to find, but has been approximately found for several earthquakes. The general conclusion thus arrived at is that an earthquake focus (centrum) is usually only six to ten miles in depth, and that the shock is a jar produced by the formation of a great fissure. Connection of Earthquakes with Phases of the Moon. — By careful comparison of the times of occur- rence of thousands of earthquakes, it has been shown— -1. That they are a little more frequent when the moon is on the meridian than when on the horizon. 2. Also at new and full moon than at half moons. 3. Also when the moon is nearest the earth than when she is farthest away. Now, these are the times of flood-tide, and of high flood-tides, and of highest flood-tides. Some have imagined that these facts prove the existence in the Fig. 85. 164 DYNAMICAL GEOLOGY. interior of the earth of a general liquid subject to tides. But the argument is evidently valueless, for any force tending to lift and break up the crust of the earth would be assisted by the gravitation or lifting power of the moon in passing the meridian, and this lifting power would be greatest at the times indicated above. Suppose, then, an interior force, tending to elevate and break the crust, constantly increasing but resisted by the rigidity of the crust : it is evident that, when the two forces are nearly balanced, the lifting force of the passing moon might well determine the moment of fracture. The moon does not produce the earthquake, but only deter- mines the moment of its occurrence — only adds the last feather that breaks the cameFs back. Connection with Season and Weather. — By the discussion of the times of occurrence of a large number of earthquakes it is found that they are a little more fre- quent in winter than in summer. No cause for this is known. Again : It is a popular belief that the occurrence is usually associated with an oppressive feeling of the atmos- phere, or with storms. These meteorological phenomena are usually attended with a low condition of the barome- ter. Now, a low barometer means diminished pressure of the atmosphere, and this, again, might determine the moment of fracture of the crust. But this, like the attraction of the moon, must be regarded, not as the cause of the earthquake (which undoubtedly lies wholly within the earth itself), but only as sometimes determin- ing the moment of its occurrence. SECTioif III. — Gradual Oscillations of the Earth- Crust. The movements included under this head are on a grand scale, perhaps affecting whole continents, but usu- J IGNEOUS AGENCIES. 165 ally so slow as to escape popular observation. But, though so inconspicuous, they are the most important of all forms of igneous agency, since it is by movements such as these that continents and sea-bottoms, mountains and great valleys, have been formed. Volcanoes and earthquakes occur suddenly, fill the mind with terror, and pass away, leaving behind little eifect on the config- uration of the earth ; but gradual movements of the crust, acting over large areas, and without ceasing, through inconceivable ages, have produced all the great inequalities of the earth's surface. Thus is it always — the causes producing the most far-reaching effects are ever those which, acting slowly, but everywhere and at all times, are scarcely recognized except by the thoughtful mind. But although the effects of this form of igneous agency are so important, yet they are so obscure, and so little has been accomplished by them in the present geological epoch, that little is known of them, and our account must therefore be brief. It is their accumulated effects through all geological times, as shown in the structure and config- uration of the earth, that alone are conspicuous. These we shall treat of in Part II. In the meantime, however, a few examples of their action now will prepare us for the discussion of these effects. Elevation. — 1. South America. — We have already mentioned (page 157) that in 1822 and again in 1835, after severe earthquakes, the southwest coast of South America was elevated several feet along a line of many hundreds jDf miles. It is not probable that very much is accomplished in this paroxysmal way, but the fact is important as showing the connection of earthquakes with bodily elevation of large tracts. Suppose, then, any force beneath tending to elevate the southern end of the South American Continent, but resisted by the stiffness of the crust : if the crust yielded gradually as the force 166 DYNAMICAL GEOLOGY. accumulated, only gradual elevation would take place ; but if the stiffness was very great, the yielding might take place paroxysmally, by fracture, earthquake, and sudden elevation. The normal process is, gradual eleva- tion by gradual yielding. Earthquakes are but occasional accidents in the slow march of these grand effects. But, besides these sudden elevations, there has been during an immense time a gradual elevation of the whole southern part of the South American Continent out of the sea. The evidence of this is seen in the old beach- marks one above another to the height of 1,300 feet above the sea and extending along shore 2,000 miles on the western and 1,100 miles along the eastern coast. More recently, A. Agassiz has found on the same coast dead corals of recent species sticking to the rocks 3,000 feet above sea. Here, then, we have continent-making forces at work on a grand scale. It is not probable that the whole of these effects was accomplished during the present geological epoch, but they are the more interest- ing on that very account, since we here trace geological causes directly into causes now in operation. 2. Italy. — The most carefully observed example of gradual elevation is that at the Bay of Baise near Naples. Fig. 86 is a map of the Bay of Baiae. From the present shore-line there runs back a flat plain of stratified vol- canic matter sloping gently to the sea, called the 8tarza; this is terminated by a perpendicular cliff. In the vicin- ity are evidences of volcanic action in the form of vol- canic cones and solfataras of very recent origin. Fig. 87 is a section of the same. Now, there is abundant proof that this coast has slowly sunk and risen again at least twenty feet, and that this has all taken place certainly since Eoman times, and probably since 1200 A. D. The evidence is briefly as follows : 1. The Starza consists of stratified material con- taining recent Mediterranean shells. 2. The cliff which IGNEOUS AGENCIES. 167 terminates the Starza is obviously an old shore-cliff. 3. The face of this cliff up to a line twenty feet above \ "Sozzuoli Pig. 86.— Map of Bay of Baise. sea-level is riddled with holes bored by lithodomi, a spe- cies of marine-boring shell. 4. On the Starza have been found the remains of an ancient Roman temple. When found, only the upper parts of three fine columns were visible, but, by removal of the soil twelve feet deep, a beautiful tessellated pavement and many broken columns were exposed. The pave- ment and buried por- tions of the columns were smooth and well preserved ; then fol- lowed nine feet riddled with lithodomi, above which it was again smooth. The uppermost borings were on the same level as those on the cliff, and therefore mark the former level of the sea. Inscriptions on the pavement show that the temple was repaired in the third century, and it was then, therefore, alove sea-level. The limit of the borings shows that it subsequently sank twenty-one feet, and again rose slowly to the original level, for the floor is now above sea- FiG. 87.— Section of map of Bay of Baise. 168 DYNAMICAL GEOLOGY. level. All this was done so quietly that it was unre- marked by contemporaneous writers. There is good reason to think that the whole took place between a. d. 1200 and IGOO. Writers of the six- teenth century say that in 1530 one might stand on the cliff, b, and fish in the sea ; this, therefore, was during the period of subsidence. Now, in 1198 a great earth- quake destroyed Pozzuoli, and in 1535 Monte Nuovo was formed by eruption. It is probable, therefore, that the history of events was briefly this : After the earthquake of 1198, the sinking commenced, and continued until it reached twenty-one feet ; it remained in this condition until the eruption of 1535, when it began to rise again. During the interval of subsidence, sediments, volcanic ashes, etc., filled up the bottom twelve feet deep, and protected the lower part of the columns, and only the part representing clear water was bored. Other evidences of movements up or down are found all along the coasts of the Mediterranean. The ruins of the Temple of the Nymphs are now in water. The bridge of Caligula is bored several feet above the sea-level, etc. 3. Sweden and Norway. — The examples thus far given are in volcanic countries, and possibly caused by volcanic forces ; but such movements are by no means always asso- ciated with volcanism ; for example, Scandinavia is re- markably free from volcanism, and yet the whole coast, both on the Atlantic and the Baltic side, has been for a long time, and is still, rising out of the sea. The rate is less in the southern part and increases northward, the average being about two to three feet per century. That this has been going on for a long time is shown by old beach-marks at various levels up to six hundred feet above sea-level, showing an elevation to tliat extent, and tliat during the present geological epoch. At the rate of two and a half feet per century, this would require two Imn- dred and forty centuries^ or twenty-four thousand years. IGNEOUS AGENCIES. 169 This is of course only an approximate estimate, but we may say with confidence that for thousands of years the whole of Scandinavia, and perhaps much more, has been rising bodily out of the ocean. Subsidence. — 1. Greenland. — The coast of Greenland, for six hundred miles, is now subsiding, but at what rate is not known. The subsidence is proved by the fact that the houses built by the early Norwegian discoverers are now partially submerged. The fact is so well recognized by the Eskimos that they never build near the sea-level. 2. River Deltas. — In all great river deltas and perhaps we might say in all places where abundant sediments are accumulating, the earth-crust subsides as if weighted down with the ever-increasing load. In digging or boring into the delta of the Mississippi, the Ganges, or the Po, the deposit is found to consist of an alternation of river sediments with old forest-grounds, and sometimes peat several feet thick, and occasional layers of limestone. This is represented in Fig. 88, in which s s is the surface. i: rs -1..-.J. - — — 1 - 1 =-1 - 1 ^ =tr — -r- rs rs '^k'-' \ ^^3 ii "' '- — ^ «^ Fig. 88. -Section of river delta. ^«(s, surface ; rs, river-silt ; fg, forest-ground ; I, limestone. with growing vegetation and accumulated vegetable mold, and perhaps peat. As we go down we pass through river- silt, r s, then an old submerged forest-ground, /^, with black mold and stumps in place, as they grew, sometimes 170 DYNAMICAL GEOLOGY, with a considerable layer of peat, then more river-silt, with an occasional layer of limestone, and so on, several times repeated. Such old forest-grounds have been found in the Mississippi delta fifty feet below sea-level, and in the Ganges layers of peat fifty feet below sea-level, and fresh-water shells and river-silt near four hundred feet. In the delta of the Po, peaty layers are found four hun- dred feet below sea-level (Lyell). Now, the only way possible to explain these facts is to suppose a slow subsidence on the one hand and the up- building by sedimentation on the other, but not always absolutely at the same rate. When the upbuilding pre- vailed, the area was reclaimed and overgrown with forest. When the subsidence prevailed, the trees were submerged and destroyed^ rotted to stumps and buried in sediments. Sometimes the subsidence was so rapid that salt-water conditions prevailed and limestones were formed. Sub- merged forests are found not only in deltas, but also on many coast-lines, and are among the surest signs of sub- mergence. 3. Mid-Pacific Bottom. — But the grandest example of subsidence, still in progress, is undoubtedly that already discussed under coral reefs. As already shown, we have evidence that over an area of 10,000,000 square miles in mid-Pacific there has been, in comparatively recent geo- logical times, a subsidence of 10,000 feet, and that the subsidence is still going on. Surely, in this case, we have changes now in progress which are of the nature of those by which continents and sea-bottoms were formed. 4. River-beds. — Our examples thus far are all from the coast region. The phenomena are plainest there, because we have the sea-level as a term of comparison. But in the interior of continents we have river beds as indicators of movement. We have seen (p. 28) that in a rising country rivers cut deeper, while in a sinking country they build up by deposit. IQJSEOUS AOENCIES. 171 Cause of Crust Movements. It is evident that the thing actually observed is only changes in the relative level of sea and land. In the inte- rior of continents we have no means of determining such movements, except by river beds, as just explained. The cause of these slow changes is very obscure and can not be discussed here. * Suffice it to say that the great inequali- ties of the earth^s crust, such as continents, ocean basins, and mountain chains, are probably due to the slow cooling, unequal shrinking, and consequent slight deformation o| the whole earth, progressive through all geological time. General Retrospect. We have discussed briefly the agencies now in operation on the earth^s surface, producing structure and form under our eyes. We believe that similar agencies have been at work through all time, and left their effects in the structure and surface forms which we actually find. We study the small and insignificant effects now produced in order that we may throw light on those greater effects which, accumulating through all geological times, are now embodied in the earth^s structure. We are now in a posi- tion to examine the actual structure and forms of the earth, and to interpret them by the light of the previous discussions. Again : Of the agencies which we have been discussing there are manifestly two groups. Atmospheric, aqueous, and organic agencies constitute the one, and igneous agencies the other. The one group tends to reduce the inequalities of the surface, and, acting alone, would event- ually bring all to sea-level, and are therefore called level- ing agencies. The other originally caused, and has ever tended to increase, the inequalities of the surface, and, * For fuller discussion, see the author's ** Elements of Geology," p. 131. 173 DYNAMICAL GEOLOGY. acting alone, would ere this have made them of incredible dimensions, and are therefore called elevating agencies. The state of the contest between these two opposite forces at any time, determined the distribution of land and water, the height of continents and mountains, and depth of seas, at that time. The one group roughhews, the other shapes, the forms of the earth. I PART II. STRUCTURAL GEOLOGY. CHAPTER L GEN"ERAL FORM AN^D STRUCTURE OF THE EARTH. General Form. The general form of the earth is that of an ohlate spheroid flattened a little at the poles. In other words, it is an ellipsoid of revolution about its minor axis. The equatorial diameter is about twenty-six miles greater than the polar diameter. This general form is taken at sea- level, the land-surfaces rising above and the sea-bottoms sinking below. This form is precisely that which a liquid globe would inevitably assume under the influence of ro- tation. It has, therefore, been somewhat hastily concluded that this general form is demonstrative evidence of the early incandescent liquid condition of the earth. It is certain, however, that the earth would have assumed this form by rotation, whether it were originally liquid or solid.* Therefore, while it is almost certain, from other considerations, that the earth was once liquid, and assumed its oblate spheroid form in that condition, yet this gen- eral form alone can not be regarded as proof of that con- dition. General Structure. — We have already stated (page * This subject is more fully explained in the author's *' Elements of Geology." 173 174 STRUCTURAL GEOLOGY, 132) that the interior temperature of the earth increases 1° for every fifty-three feet in depth, and that at this rate the fusing temperature of rocks would be reached at about thirty miles ; and, finally, that many have thence hastily concluded that the general structure of the earth is that of a globe of fused rock or lava, covered with a thin shell thirty miles thick. But we have also shown there the untenableness of this view. There are only two other views possible, and now held. Some hold that the earth is truly solid throughout, excepting reservoirs of liquid matter forming the foci of volcanoes. Others hold that the earth consists of — 1. A solid nucleus, which forms its greatest part ; 2. A solid crust, comparatively thin ; and, 3. Separating these, a suh-crust layer of liquid or semi- liquid matter, if not universal, at least over large areas. There are many geological phenomena which seem to make this last view most probable. Density of the Earth. — The mean density of the earth, taken as a whole, is 5.6. The density of the crust is about 2.5. Therefore the density of the central parts must be very much greater than 5.6. It is probably not less than 15 to 16. This greater interior density is due partly to a difference of material (the denser settling toward the center, while the earth was still in a fused con- dition), and partly to condensation hy pressure. Crust of the Earth. — The surface portion of the earth differs in many respects from the interior, and is, therefore, properly called a crust: 1. It is certainly a lighter portion covering a denser interior. 2. It is a cooler portion, covering an incandescent interior. 3. It is, as we shall see hereafter, a stratified portion covering an unstratified interior. 4. It is probably an oxidized por- tion covering an unoxidized or less oxidized interior (for oxidation comes by contact with air and water). 5. It is probably a solid shell covering a liquid or semi-liquid sub- crust layer. It is this idea of a solid shell covering a FORM AND STRUCTURE OF THE EARTH. 175 liquid which gave origin to the term crust ; but the word is now used only to signify the superficial portions of the earth, subject to human observation, without any impli- cation as to the interior condition. Means of Geological Observation. — As thus defined, the crust is estimated at from ten to twenty miles in thickness. The manner in which we get a knowledge of the earth to that depth, or the means of geological obser- vation, are — 1. By mines and artesian wells. These pene- trate 4,000 or 5,000 feet. 2. Canons and ravines. These give sections of 6,000 or 7,000 feet. 3. Volcanic ejections. Fig. 89. These bring up matter from unknown but certainly still greater depth. But the most common and effective means of observation is furnished by — 4. Foldings of the crust, and subsequent erosion. In the section (Fig. 89) in which 5 5 is the present surface, we represent one of the com- monest of all geological phenomena. It is seen that from the point a the strata are repeated on the two sides. The dotted lines show how much has been cut away, and what depth of strata has been exposed to view. In this way, in very many places, the character of the rocks ten or more miles deep is revealed. Our direct observation is absolutely confined to this superficial portion. We can only speculate about what is beneath. It would seem, at first sight, that this is an 176 STRUCTURAL GEOLOGY. insignificaut portion of tlic earth upon which to found a science of the earth. But it must be remembered that on this superficial portion has been enacted, and in its struc- ture has been recorded, the whole history of the earth. General Surface Configuration of the Crust. — The crust of the earth is diversified by greater and smaller features. The greater features are due to interior or elevating, the lesser to exterior or leveling agencies. Under the former head come those greatest features, constitut- ing continental surfaces and oceanic bottoms, and those next greatest, viz., mountain-chains and great valleys. Under the latter come all those peaks and ridges, valleys and ravines, which have been produced by subsequent erosion. The mean height above the sea-level of the continents is about 1,200 to 1,300 feet, or less than one fourth mile, and the mean depth of the ocean-bottoms below the same level is about 15,000 or 16,000 feet, or nearly three miles. The ocean-surface being nearly three times as great as the land-surface, it is evident that, if the inequalities of the crust-surface were removed, there is water enough to cover the whole earth more than two miles deep. General Laws of Continental Form. — There are certain general laws of continental form which have a bearing on the question of the origin of continents, and which, therefore, must be briefly mentioned. 1. Continents consist essentially of Interior Basins, with Coast-Chain Kims. — The interior basins are drained by the great rivers of the world. This typical structure is well shown in America, North and South, in Australia, and in Africa. For example, in North America we have the great interior basin drained by the Mississippi River, bordered on the Atlantic side by the Appalachian, and on the Pacific side by the great Rocky Mountain sys- tem or American Cordilleras, consisting of many ranges, of which Colorado, Wahsatch, and the Sierra and Coast FORM AND STRUCTURE OF THE EARTH. 177 Eaiige of California are the most notable (Fig. 90, a). South America has the Andes on one coast, the Brazilian mountains on the other, and the great interior basin drained by the Amazon, La Plata, and Orinoco Rivers (Fig. 90, h). Similarly i the great basin of Africa is- drained by the Nile, Niger, Congo, and Zambesi Rivers. Basin. Plateau. Plains. East and west section of North American Continent : cr, coast range ; SJ., San Joa- quin plain ; S, Sierra ; w, Wahsatch ; c, Colorado range ; Ap, Appalachian. W '^h Andes. East and west section across South America. Brazil " tw*^ WJ!\limimi] m\mmmnmmmimimmmu\m Mi East and west section across Australia. -Sections across North and South America and Australia. Fig. 90. Australia is also a fine example, as shown in Fig. 90, c. Europe and Asia have similar structure, but less perfect. This continent is elongated east and west, and therefore the section must be north and south. 2. The Greater Range faces the Greater Ocean. — In America, the North American Cordilleras and the Andes face the Pacific, while the Appalachian and the Brazilian mountains face the Atlantic. In Africa and Australia, on the contrary, the east range faces the greater ocean, and is the greater. 3. The greater chains are usually the most complex and crumpled in structure, and give evidence of greatest vol- canic activity in the present or in the past. Le Conte, Geol. 12 178 STRUCTURAL GEOLOGY. 4. - Continents and ocean-bottoms have not, as some imagine, frequently changed places. On the contrary, the places of continents have been indicated and their outlines sketched out from the beginning, and their forms have been gradually developed, though with many oscil- lations, throughout all geological times. The origin of continents and ocean-bottoms is very obscure, but it is probably in some way connected with the unequal contraction and therefore deformation of the spheroidal form of the earth, by slow cooling from a former incandescent condition. In such an irregular or deformed spheroid, of course, the water would collect in the hollows, and the protuberances would become conti- nents. The origin of mountains we discuss further on. RocTcs, Definition of Rock. — The term rock is used in popu- lar language to designate any substance of stony hard- ness. Not so in geology. Any substance constituting a portion of the earth^s crust, whether it be hard or soft, is called a roch. No distinction based on hardness alone is of any value. The same sandy bed may be found in one place hard enough for building-stone, and in an- other soft enough to be spaded. The same clay stratum may sometimes be traced from a condition of slaty hard- ness in one place to good brick-earth in another ; the same bed of lime from marble into chalk, and the same volcanic eruption from stony lava into a bed of volcanic ashes. Classes of Rocks. — Rocks are divided, according to their structure and origin, into two principal kinds, viz., stratified and unstratified. Stratified rocks are more or less consolidated sediments, and are therefore aqueous in origin and earthy in structure. Unstratified rocks have been more or less fused, and therefore are igneous in origin and either crystalline or glassy in structure. CHAPTER II. STRATIFIED ROCKS. Section" I. — Their Structure and Position*. Let any one examine the rocks of a quarry of limestone or sandstone, and he will find that the stone lies in regu- lar beds. In some places these beds will lie level (Fig. 91), in other places they may be inclined (Fig. 92). For example, throughout the valley of the Mississippi they are usually level, while in mountain-regions they are usually inclined. The next most conspicuous structure will probably be the cross-divisions called joints, by which the beds are broken into separ- able blocks. These are found in all rocks, are not char- acteristic of strati- fied rocks, and therefore we say nothing more about them now. On ex- amining a little more closely, the beds will be seen to be subdivided by faint lines similiar to those observed in a section of sedi- ments, and known to be produced by the sorting power 179 ±,s sit Fitis. 91, 92.— Sections of horizoutal and inclined strata : *', soil ; ss, sandstone ; sh, shale ; i«, limestone. 180 STRUCTURAL GEOLOGY. of water (page 27). In a word, the mass exposed on a cliff or in a quarry, or any large section of stratified rock, is seen to be divided by parallel planes into thick beds of different kinds of materials, as sandstone, limestone, etc., and each of these, probably, into thinner beds, differing perhaps in grain or color, and finally these again into thin sheets, produced by the sorting of material. Now, the larger beds are called strata, the subdivisions of different color or grain, layers, and the lines of sorted materials are lamincB. These terms are loosely used, but always in the order mentioned, and the word lamina is always used to signify the marks of water-sorting. Now, the structure we have described is called stratification , and such rocks stratified rocks. Extent and Thickness. — Stratified rocks cover at least nine tenths of the land-surface, and even where they do not occur it is only because they have been removed by erosion or else covered by igneous rocks. Since, as we shall see presently, stratified rocks were formed at the bottom of the water, it is evident that there is no portion of the earth which has not been at some time covered by the sea. The extreme thickness of these rocks is proba- bly ten to twenty miles ; the average thickness is certainly several thousand feet. Principal Kinds. — As defined above, stratified rocks fall naturally into three great groups : 1. Arenaceous or sand-rocks ; 2. A rgillaceous or clay-rocks ; and, 3. Cal- careous or lime-rocks. These may be either in a soft or in a stony condition. The sand-rochs, in their soft or incoherent condition, are beds of sand, gravel, and pebbles or shingle. In their coherent or stony condition they are sandstones, grits, and conglomerates. Breccias differ from conglomerates only in having the fragments angular instead of rounded. They consist of rubble, instead of pebbles^ cemented together. STRATIFIED ROCKS. 181 The clay-rocks, in their incoherent condition, are beds of clay, brick-earth, mud, and ooze. In their coherent condition they are the same cemented into shales, or, still harder, into slates. Lime-rochs, in an incoherent condition, are lime-muds, such as exist now in coral lagoons, or in the deep sea (glo- bigerina ooze, page 117) ; in a slightly consolidated con- dition they are chalks, and in a stony condition they are limestones, marbles, and travertines. These different kinds may each produce varieties ol different color and grain. They also pass by mixture insensibly into each other, and thus form infinite varie- ties. Thus we may have an argillaceous or calcareous sandstone or calcareous shale, etc. All that need further be said on the subject of the origin of stratified rocks is best thrown into a series of propositions, very simple and yet underlying all geologi- cal reasonings : 1. Stratified Rocks are more or less Consolidated Sediments. — This has been thus far assumed. We wish now to direct the pupil to the observation of the evidence : a. Every gradation may be traced between muds, clays, and sands, which we know were deposited in water ; and shales and sandstones, which we find forming the strata of mountains. 5. In many cases we may see the process of hardening going on under our eyes. For example, at the mouths of rivers carrying lime in solution, like the Rhine, the river-silts are consolidated into calcareous shales. On the shores of coral reefs we find coral mud, coral sand, and coral breccia consolidated into peculiar limestones (page 108). c. Close examination of many rocks, especially sandstones and shales, clearly shows the sorting of material (water-sorting) along the lines of lam- ination, d. As shells and skeletons of animals are now imbedded in muds of rivers, lakes, and seas, so fossils are found in stratified rocks, e. Other marks, which occur 182 STRUCTURAL GEOLOGY, in recent sediments, such as ripplc-marks, rain-prints, sun-cracks, foot-prints of animals, etc., are also found in the hardest stratified rocks. In a word, it may be said that every marh or peculiarity which has been observed in recent sediments has been found also in stratified rocks. We may assume, then, as certain that stratified rocks are sediments formed originally at the bottom of seas, lakes, rivers, etc., and that when we find them far in the interior of continents and high up the slopes of mountains we have indubitable evidence of great changes of level. Stratified rocks are all deposits in water. Sandstones and shales are the debris of erosion, and are therefore mechanical de2^osits ; and these rocks are often called fragmental rocks, because they are made up of the frag- ments of previous rocks. Limestones, on the other hand, are either organic or chemical deposits. Again, sand- stones, grits, and conglomerates are formed by violent action, and they indicate either rapid currents or exposed shores ; shales indicate quiet seas or bays ; limestones^ open seas. We have already seen (page 27 et seq.) that sediments are transported soils, and (page 10) that soils are disinte- grated rocks. Now, we see that stratified rocks are con- solidated sediments. We have here an example of a per- petually recurring cycle of changes : rocks are decomposed into soils, soils are carried and deposited as sediments, sediments are again consolidated into rocks, to be raised into land-surfaces, and again disintegrated into soils — and so the cycle goes round. The cause of consolidation is sometimes only the pressure of great thickness of sediment ; sometimes the same, aided by gentle heat ; sometimes there is a distinct cementing substance, the most common being lime car- bonate and silica. When there is a cementing substance, the process is often rapid, and may be observed ; as, for example, in the formation of coral rock. But in other STRATIFIED ROCKS, 183 cases the process is very slow, and therefore the newer rocks are often, though not always, imperfectly consolidated. 2. Stratified Rocks have been gradually deposited. — By this we mean that they have not heen formed at once, as some of the older geologists imagined, but by the regular operation of causes similar to those now accumu- lating sediments. The slowness was sometimes extreme. For example : a. We have strata in which the laminae are as thin as paper, and yet each one represents recurring conditions, as ebb and flow of tide, or flood and low water of rivers, h. In some cases we have a shell attached to the inside of another shell (Fig. 93), in such wise that the latter shell must have been dead before the former attached itself. In such cases a half or quarter inch thickness of rock represents the whole life of the second shell, c. We have seen that some limestones are made up of the accumulated remains of successive genera- tions of microscopic shells (page 115). Every inch thick- ness of such deposit must rep- resent a long period of time. And yet such deposits are often hundreds or even thousands of feet in thickness. These are, however, extreme cases of slow- ness. As a general rule, coarser materials are deposited more rapidly than finer — e. g., sands than clays and limestone, but all by regular opera- tion of causes ; and therefore, making due allowance for the nature of the materials, thickness is a rough measure of time. 3. Stratified Rocks were orig-inally horizontal at the Bottom of the Water, — This is a necessary conse- Fift. 93.— Serpuloe on interior of a shell. 184 STRUCTURAL GEOLOGY. quence of the manner in which they were formed. There- fore, when we find them in other positions and at other levels, we conclude that they have come so by subsequent change. We must not imagine, however, that the planes between the strata were ever absolutely horizontal. Strata must not be likened to continuous, even sheets, but rather to extensive cakes, thickest in the middle and thinning on the margins and there interlapping with other strata or cakes (Fig 94). Coarse materials, like sandstones and Fig. 94.— Diagram showing thinning out of beds : a, sandstones and conglomerates; J, limestones. grits, are more local, and thin out more rapidly, while fine materials, like clays, are often very widely continuous. This thinning out of strata, however, does not interfere seriously with their appearance of evenness at any point of observation. Another more important apparent exception to original horizontality is what is called cross-lamination or false- bedding (Fig. 95). These are liable to be mistaken for J^G. 95.— Section on Mississippi Central Railroad at Oxford (after Hilgard) : oblique lamination, STRATIFIED ROCKS. 185 tilted strata. But it will be observed that it is the laminae, and not the strata, which are inclined. And, moreover, their extreme irregularity is sufficient to distinguish them from true inclined strata. They seem always to be pro- duced by deposit from rapid, shifting, overloaded currents, and are, therefore, common in river-deposits. After explaining these apparent exceptions, we come back with still more confidence to the proposition that stratified rocks were originally soft sediments in a hori- zontal position at the bottom of seas, lakes, etc. But we usually find them noiu in an entirely different condition and position. We indeed find them sometimes soft, but more commonly stony ; sometimes, indeed, still horizon- tal, though raised above the sea and in the interior of continents, but more commonly more or less tilted ; some- times, especially in mountain-regions, not only tilted, but folded, crushed, contorted, broken, and dislocated in the most complex manner, so that it is difficult to make out their natural order. Sometimes the contortion is in the lamincBy so that it can be seen in a hand-specimen (Fig. 96). Sometimes a series of strata are folded together. Fig. 9G.— Crumpled lamina\ (After Geikie.) such as may be seen at one view on an exposed cliff (Fig. 97). Sometimes the strata composing the crust of the 186 STRUCTURAL GEOLOGY, Fig. 97.— Contorted strata. (From Logan.) earth, several thousand feefc thick, are folded all togethei- so that their foldings form great mountain-ridges, and can only be made out by extensive surveys (Fig 98). As Fig. 98.— Section of Appalachian chain. might be expected, the strata by such violent movements are usually broken and dislocated, and always, as seen in Eigs. 97 and 99, large portions of their upper parts have Fig. 99. been carried away by erosion, leaving their edges exposed on the surface. Such exposure of strata on the surface is called outcrop. Fig. loa STRATIFIED ROCKS. 187 This important subject must be taken up with some detail, and for this purpose it becomes necessary to define some common geoh)gical terms. l>il> anil Strike. — 'I'he angle of inclination of strata with the horizon is called the dip. There are always two elements to be considered ; viz.^ direction and amount. Thus a stratum may dip northward 30°. The angle of dip varies from to 90° — i. e., from horizontality to verticality. Sometimes strata are even pushed over beyond the vertical — such are called overturn-dips (Fig. 99). Examples are found in all great mountain-chains, especially in the Alps. When strata dip regularly, their thickness may be easily estimated. For example, in walking from a to h (Fig. 100), we pass over strata whose thickness i^h c {= ah . sin h a c). The dip may be accurately determined by means of a clinometer (Fig. 101). Fig. 101.— Clinometer. The direction of strata, or their line of intersection with a horizontal plane, is called the strike. It is always at right angles to the dip. If the dip is so many degrees north or south, the strike will be east and west. If the surface of the ground is level, the strike will be the same as the outcrop, or appearance on the surface, of the strata ; but this is seldom the case. If the strata are plane, the strike will be a straight line. If the strata are folded, the strike may be very sinuous (Fig. 107). In a map view of strata, the dip and strike are represented by the sign 1, in which the heavy line represents the strike. 188 STRUCTURAL GEOLOGY. and the perpendicular the dip (Fig. 105). The perpen- dicular is made shorter, as the dip is at a higher angle. Anticline and Syncline. — When a series of strata dip in one direction in one place, the same series Avill usually be found to dip in a contrary direction in another place. In other words, strata are usually disturbed by lateral pressure, which throws them into folds, sometimes wide and gentle, like undulations, sometimes closely appressed. Thus strata usually occur in alternate saddles and troughs (Figs. 102, 103). The saddles are called anticlines, the troughs syncUnes, An anticlinal axis, then, may be de- FiGi- 102.. fined as a line on either side of which the strata repeat one another, dipping in opposite directions, away from the axis. A synclinal axis is a line on either side of which the strata repeat each other, dipping in opposite directions, but toward the axis. In Figs. 103 and 104, a is an anticline, and s a syncline. In anticlines the strata lie in saddles and in synclines in troughs, but the surface configuration of the ground may or may not correspond. Sometimes the ground is comparatively level, though the foldings are strongly marked (Fig. 102). Sometimes the anticlines are ridges, and the synclines valleys (Fig. 103), and sometimes the Fig. 103. STRATIFIED ROCKS. 189 reverse (Fig. 104). In gently folded strata it is very common to fmd the configuration reversed on the surf ace. Fig. 104. i. e., synclinal ridges and anticlinal valleys. Examples of these are given on page 248. Folded strata, which are tilted only hy folding, will outcrop on level ground in parallel bands, as in Fig. 105, „ 1 1 II III II _^ 1 1 II II l( (1 1 " " r " 1 " ii ''! II II h H II u III 1 " '■1 I " ill II Fig. 105. which is a map view of Fig. 102. But if the whole be again tilted in a direction at right angles to the folds, Fig. 106.— Section of undulating strata. then the map of outcrop will be sinuous. Fig. 106 is a section of folded strata thus tilted, and Fig. 107 is a map of the same. The section is along the line CD. Exam- ination of the signs of dip will explain the map. 190 STRUCTURAL GEOLOQt. Fig 107.— Plan of undulating strata. We have spoken of folded strata and the way in which they outcrop ; but \\\ a survey the process is reversed, i.e., it is the outcrop which is observed, and from this we con- struct the section. Now, when we remember the complex folding, then the tilting after folding, then the displace- ment by fractures, and then, worst of all, the covering of the whole deeply with soil, leaving exposed only patches here and there, we can easily see how difficult a problem it often is to construct a section of the stratified rocks of a country. If the strata be exposed on a cliff or a cafi on- side, there is little difficulty, but, in the absence of such, the geologist takes advantage of every exposed patch, examines every gulch or stream-bed, every quarry or railroad-cutting, and thus constructs an ideal section. Conformity and Unconformity. — We have just seen that the strata composing the country rock of a land- surface are usually tilted and crumpled and always eroded, so that their edges are exposed (see Figs. 106, 107). But we have also seen (pages 164-170) that in some places land-surfaces are now sinking beneath the sea, and in others sea-bottoms are rising to become land-surfaces. The same is true for all geological epochs. N ow, suppose at any time an eroded land-surface sank below sea-level so that sediments were deposited on the eroded edges and filling the erosion-hollows of the strata, and finally the STRATIFIED ROCKS. 191 whole was again raised above sea and exposed to the in- spection of the geologist ; the phenomena which would Fig. 108.— Some cases of unconformity. be observed are represented by Fig. 108, A, B, C. This is what is called unconformity. More commonly in such cases there is a want of parallelism between the two series of strata, as in Fig. 108, A, B. But this is not necessary. Fig. 108, C, represents unconformity no less than A and B, In the one case the strata were raised into land-surface and at the same time folded and tilted, and then eroded ; in the other case, they were raised and eroded without folding or tilting. Sometimes the second raising is also attended with tilting, in which case both series are tilted, but in different degrees, as in D. Definition. — After this explanation, we are prepared to define. When a series of strata are parallel, as if formed continuously under similar conditions, they are 192 STRUCTURAL GEOLOGY. said to be conformable. But if two series are discontinii- ous — i. e., separated by an erosion-surface or old land-sur- face, and therefore formed at different times and under different conditions — they are said to be unconformable. In all the figures the strata of the lower series are con- formable throughout, and so are also those of the upper, but the two series are unconformable with each other, the line of unconformity being an old eroded land-surface. Even so simple sections as Fig. 108, one of the com- monest observed, record many interesting events in the history of the earth, viz. : 1. A long period of quiet, during which the first series of strata was deposited. 2. A period of commotion, during which the sea-bottom here was elevated into land, and perhaps the strata crum- pled. 3. A long period during which it remained land- surface and was deeply eroded and the strata-edges exposed. 4. Another period of commotion, during which it sank again and became sea-bottom. 5. Another long period of quiet, during which the second series of strata was de- posited ; and, 6. Still another period of movement, by Avhich the whole was finally raised and became thus sub- ject to the inspection of the geologist. The following diagrams (Fig. 109) represent the man- ner in which the phenomena may be supposed to have occurred. In ^, we have thick sediments, 8d, accumu- lated on an off-shore sea-bottom. In B, the same have been elevated into land, and crumpled. In (7, they have been eroded and their edges exposed. In D, they have again subsided beneath the sea, and received sediments, 8d, on their eroded edges. Since geological history is mainly recorded in stratified rocks, and since, while a place is land-surface and being eroded, there can be no strata formed there, it is evident that a line of unconformity always indicates a period of which there is no record at that place, although the record may be found elsewhere. Unconformity, therefore, al- STBATIFIED ROCKS. 193 ways represents a gap in the record — ti lost interval of time— which may be very long, viz., the whole time dur- ing which the erosion was going on. Fig. 109.— In all : L, land ; II, sea-level ; Sh, shore-line ; Sb. sea-bottom ; 5a, sediments. A group of conformable strata usually form a geologi- cal formation, and a line of unconformity usually sepa- rates two different geological formations. The division of the strata into formations, however, is based also on other characters, viz., the contained fossils. The subject 'will be taken up again under that head. Cleavage Structure. Stratification is an origi^ial structure, i. e., impressed* at the time of deposit of sediments. Cleavage is a super- induced or suJ)sequent structure, but it so simulates Le Cokte, Geol. 13 194 STRUCTURAL OEOLOOY. stratification that it seems best to take it up here. It is found in many kinds of rocks, but most perfectly in slates, and is therefore often called slaty cleavage. Definition. — Cleavage is easy splitting in certain di- rections. There are many kinds of cleavage due to dif- erent causes. For example, many crystals split perfectly in certain directions. This is called crystalline cleavage, and is due to molecular arrangement. Certain stratified sands split easily into broad flag-stones in the direction of the laminae. This is lamination cleavage, and is due to the arrangement of the grains by the sorting power of water. Again, wood splits easily in the direction of the silver grain. This wood-cleavage is due to the arrange- ment of the wood-cells. Slaty Cleavag-e. — Now, there is also an easy splitting of rocks in definite directions, which occurs on an im- mense scale, and in certain slates is a very marked struc- ture. The direction of cleavage is usually vertical or highly inclined. Whole mountains are thus cleavable from top to bottom, and rocks over thousands of square miles are often made up of such thin sheets. It is by splitting along these lines of easy fracture that roofing- slates, ciphering-slates, and blackboard-slates are made. On casual examination of strata the cleavage-planes are liable to be mistaken for fine lamince, and we are apt to Fig. 110.— Cleavage-planes cutting through strata. think that we are examining a beautiful example of highly inclined strata. But a closer examination will usually show the lines of stratification running in an entirely different direction. In Fig. 110, the strong lines STRATIFIED ROCKS. 195 show the strata strongly folded, while the light lines show the cleavage nearly vertical, cutting through these Fig. 111.— Strata, cleavage-planes, and joints. in parallel planes. In Fig. Ill, three kinds of structure, which should be kept distinct in the mind, are shown. The rectangular block-faces are joints ; the strong lines, s s, slightly inclined to the right, are strata ; while the highly inclined lighter lines are cleavage-planes cutting through both. Cause of Slaty Cleavage. — Slaty cleavage is undoubt- edly caused by a mashing together of the whole rock-mass in a direction at right angles to the cleavage-planes, and an extension in the direction of these planes ; and, since cleavage -planes are usually nearly vertical, it is the result of a mashing together horizontally, and an up-swelling or extension vertically of the whole cleaved mass. Proof. — This is proved (a) in field-observation by the folding of the strata (Fig. 110), and (b) in hand-speci- mens by the crumpling of the finest laminae in the direc- tion indicated above. Fig. 112 represents a block of slate eighteen inches long, in which the lamination-lines are shown crumpled by the pressure. In the position of the block it is evident that the crushing was horizontal. The cleavage-planes, represented by the light lines, are vertical. One cleavage-face, c p, is shown. The same is proved, also (c), by distorted fossils often found in cleaved slates (Fig. 113). By comparing the natural with the 196 STRUCTURAL GEOLOGY, distorted form the direction of pressure is found to be always at right angles to the cleavage-planes, i. e., the Fig. 112.— a block of cleaved elate. (After Jukee.) fossils are shortened in that direction and elongated in the direction of the planes {d). In many slates, especially Fig. 113.— Cardium hillanum : A , natural form ; B and (7, deformed by pressure. the purple Cumberland slates, much used in roofing, oblong greenish spots are common. If they be closely examined, they will be found to be veri/ thin in the direc- i STRATIFIED ROCKS. 197 tion of the thickness of the slate or at right angles to cleavage. On the cleavage surface the shape is broad, elliptical (Fig. 114, A), while on sec- tion the shape is very flat, B. These spots before mashing were round pellets of clay. They have been mashed into an ellipsoid of three unequal diameters, the longest, a h, in the dip of the cleavage, and therefore nearly vertical ; the next, c d, in the strike of the cleavage, and therefore horizontal ; and the smallest, b ^ e f, at right angles to cleavage. This Fig. ii4. -Flattened proves that the whole mass has been "iet";^ J5, sWe-vrw! mashed at right angles to cleavage, and extended in the direction of the dip of cleavage. Micro- scopic examination shows that every constituent granule of the original clay is in the slate mashed into a thin scale, so that the original granular structure is changed into a scaly structure, and it is this which determines the easy splitting. Geological Application. — The amount of mashing to- gether horizontally and extension vertically shown in these different ways is so great that an original cube or sphere in the unsqueezed mass is changed into an oblong, of which the shortest diameter is to the longest as one to three or four, one to five or six, one to nine or ten, and even sometimes as one to fifteen. The average in well- cleaved slates is one to six. Now, when we remember that thousands of square miles and thousands of :feet thickness of rocks are thus affected, it is evident that this slow mashing together horizontally of whole mountain-regions must be an important agent in the elevation of land, and especially in the formation of mountains. We shall speak of this again under the head of mountains. 198 STRUCTURAL OEOLOOY. Concretionary or Nodular Structure. This, also, is a superinduced structure simulating an original structure. As slaty cleavage simulates stratifi- cation, so concretions or nodules simulate and are apt to be mistaken for fossils. In many strata, especially calcareous sandstones and shales, we find rounded masses often of curious shapes, separable from the general mass of the strata, and differ- ing a little from it in hardness and composition. These are called concretions, nodules, septaria, etc. They have evidently been separated out of the general mass after the latter was deposited. This is shown by the fact that the planes of stratification often run right through them (Fig. 115). Forms and Structure. — Inform they are sometimes perfectly spherical, like can- non-balls, and vary in size from that of a marble to many feet or even yards in diame- ter ; sometimes flattened ellipsoidal, and these, when Fig. 115. Fig. 116.— Nodules, fium bUatu STRATIFIED ROCKS, 199 marked with polygonal cracks, simulate very much a turtle-shell, and are called turtle-stones ; sometimes dumb- bell-shaped, sometimes rings, sometimes all sorts of strange and fantastic shapes (Fig. 116). In structure they are sometimes solid, sometimes hollow, sometimes affected with interior cracks, sometimes have a concentric shell-structure, and sometimes a radiated structure. These curious shapes so simulate fossils that even ex- perienced geologists may sometimes be in doubt. By common observers they are very often mistaken for fossil nuts, fossil turtles, etc. They are, however, very inter- esting to the geologist, because they often contain a fossil beautifully preserved in the center. How Formed. — They seem to be formed by the slow aggregation of more soluble or more suspensible matter from a general mass of insoluble matter, an organism Fig. 117.— Chalk-cliffs with flint nodules. often forming the nucleus of aggregation. Thus, if the mass be a calcareous sandstone, the lime will gather in places, forming sandstones containing more lime than the general mass. So calcareous clays form nodules of lime mixed with clay. These are the hydraulic-cement nod- ules. In chalk the disseminated silica seems to gather 200 STRUCTURAL GEOLOGY. into nodules of pure flint, and leave the chalk a pure carbonate of lime deprived of its silica. Hence, chalk usually contains flint nodules, scattered or in layers (Fig. 117). , We speak of this nodular structure not on account of its great importance, but because it is apt to strike the observing eye, and very apt, too, to be mistaken for fossils. Fossils : their Origi7i and Distrihution, Ev^ry one must have observed that in many places the stratified rocks contain the exact forms of organisms, especially shells, though these seem to have turned to stone. These are called fossils. They are of extreme interest to geologists, because they reveal the nature of the former inhabitants of the earth. Stratified rocks are the consolidated sediments of former seas, bays, lakes, and rivers. Then, as now, shells lived in the ooze of sea- bottoms, or were cast up on beaches ; the leaves and branches of trees and carcasses of land-animals were car- ried down by rivers to lakes and estuaries and buried in mud. These have been preserved, with more or less change, to the present day. A fossil, then, may be defined as any evidence of the former existence of a living thing. Next to lamination, they are the most constant characteristic of sedimentary rocks. Degrees and Kinds of Preservation. — There are various degrees and kinds of preservation of organic forms. In some cases not only form and structure, but even the organic matter of soft parts, is preserved. More commonly, however, only the shells and skeletons of ani- mals are preserved, and of these sometimes both the form and structure, and sometimes only the form. We shall speak of these under three heads : 1. Organic Matter preserved. — This, of course, is rare. The only perfect examples are those of carcasses STRATIFIED ROCKS, 201 preserved in ice. In the frozen cliffs and soils of Siberia, the carcasses of extinct elephants and rhinoceroses have been exhumed by the rivers, in a condition so perfect that dogs and wolves fed on the flesh. In peat-bogs are found the perfect skeletons (still retaining the organic matter of the bones) of extinct animals ; and in some cases even the flesh is preserved, but changed into a fatty substance (adipocere). These are all in comparatively recent strata. But, even in the oldest strata, organic matters of once living beings are preserved, though changed into coal, lignite, petroleum, bitumen, etc. 2. Organic Structure preserved. — This is the type of what is called petrifaction ; it is best illustrated by petrified wood. In many strata, but especially in the sub-lava gravels of California (page 395) and the tufa beds of California and the Basin region, drift-wood is found completely changed into stone. In these we have not only the form, not only the general structure — i. e., bark, wood, and pith, concentric rings, medullary rays, and woody wedges — but even the minutest microscopic structure of tissue and markings on the walls of cells, perfectly preserved in the stony matter (usually silica) replacing the wood. Mode of Petrifaction. — It must not be imagined that the wood is turned to stone, but is only replaced by stony matter. As each particle of woody matter passes away by decay, a particle of mineral matter is deposited in its place from solution, thus reproducing its structure per- fectly. Wood best illustrates the process, but in a simi- lar manner the minute structure of bones, teeth, corals, shells, etc., are preserved, even though the original mat- ter is all gone. The most common petrifiers are silica and carbonate of lime. 3. Organic Form only preserved. — In many cases the structure is not preserved, but we find only a mold of the external form, or a cast of the same in stone. This is 202 STRUCTURAL GEOLOGY. best illustrated by the case of shells. The following figure is a diagram showing four different cases, all of which are very common. In the figure the horizontal Fig. 118.— Section of strata containing fossils. lines represent the stony matrix in which the shell is formed, or 7nud in which the shell was originally buried, and the vertical lines represent the subsequent filling with finer material. Explanation. — In case a, the living or recently dead shell was buried in mud, and afterward the whole organ- ism was dissolved and removed, leaving only the hollow mold where it lay. In case h, we have the same, only the mold has been subsequently filled and a cast made by the deposit of silica or carbonate of lime from solution. If the rock be broken, the cast will often drop out of the mold. In c, the dead, empty shell was buried in mud and filled with the same, and afterward the shell was removed Fig. 119.— a, Natural form ; &, Fig. 120.— Trigonia longa, showing cast (a) of the cast of interior and mold of exterior and (6) of the interior of the shelL exterior. STRATIFIED ROCKS. 203 by solution, leaving an empty space corresponding to the thickness of the shell. In d, this hollow space was sub- sequently filled by deposit of soluble matters from perco- lating waters. Cases c and d are represented by Figs. 119 and 120. Sometimes we have only the mold and cast of a small part of an organism, as, for example, impressions of the leaves of plants, or the footprints of animals walking on the mud when it was soft. These, however, are of great value, because they are very characteristic parts of plants and animals. Finally, there are all grades of completeness of the process of replacement. In bones, shells, and teeth, sometimes only the organic matter is partly or wholly replaced. Sometimes, also, the mineral matter is replaced by other mineral matter. Distribution of Fossil Species. The kind of fossils which we find in the strata at any place will depend on three things : 1. On the kind of rock ; 2. On the country ; and, 3. On the age of the rock. Kind of Rock. — We have already said (page 130) that at the present time different depths and bottoms are fre- quented by different marine species. Some live on sand- bottoms, some on mud-bottoms, and some on deep-sea ooze. The same was true in previous epochs, and there- fore we ought to expect and we do find that, in the same country, and in strata of the same age, sandstones will contain different fossils from limestones ; the one being shore and the other open-sea deposit. Again, then as now, lake-deposits contained fresh-water animals, and estuary deposits land plants and animals ; and these are of course different from marine species, though they be of the same age and country. The Country. — In rocks of the same age and same 204 STRUCTURAL GEOLOGY. kind, but in different continents, we shall often find a great difference of species, for we find the same thing true of living species (page 118). But the geographical diversity of fossil species, as a general fact, is not so great as that of living species. Commencing with the earliest times, . the geographical differences of species have in- creased more and more to the present time. The Age. — The distribution of fossil species according to the age of the rocks is the main subject of Part III, or Historical Geology ; but some general notions on this subject are necessary as a basis of classification of strati- fied rocks, and must therefore precede that part. Successive Geological Faunas and Floras. — The fossil species found in rocks, even of the same kind and country, will depend largely on the age of the rocks. The whole earth has been inhabited at different times by entirely different species. All the animals and plants in- habiting the earth at one time are called the fauna and flora of that geological time. Thus we have a fauna and flora of Tertiary times, of Jurassic times, of Devonian times, etc. Definition of Formation and Period. — When the strata are conformable, the change from one geological fauna to another is gradual, but a line of unconformity usually abruptly separates two faunas. A formation, therefore, is a series of conformable strata, in which the fossil species are either the same or change very gradu- ally ; and a geological period is the period during which such a formation has been laid down. There are two tests, therefore, of the limits of a geological formation and a geological period, viz., unconformity of the rock- system and great change in the species. Of these the latter is the more valuable. Law of Gradual Approach to the Present. — It is a fundamental and very important fact that in the suc- cessive changes of geological species there is a steady STRATIFIED liOGKS, 205 approach to living forms, first in families, then in genera, and then in species. Species do not begin to be identical with the living species until the Tertiary period, and thence onward we have an increasing percentage, identical with the living. Now, we determine that rocks belong to the same time, all over the earth, by the general similarity of the fossil species. We find difficulty in applying this rule only in the Tertiary, because then the geographical diversity is beginning to be so great as seriously to interfere with the general similarity. But just here we begin to use another principle, viz., the percentage of the fossil species still living in the immediate vicinity. Similar percentage in- dicates the same age — greater percentage less age, and less percentage greater age.* It is on these principles that is based the classification of stratified rocks. Section II. — Classification" of Stratified Kocks. Geology is a history. Stratified rocks are the leaves of an historical book. Evidently, then, the true basis of classification must be relative age. In classification, the geologist has two objects in view : 1. To arrange all the strata, from lowest to highest, in the order in which they were formed. 2. Then to separate them into groups and sub-groups for convenient treatment — i. e., 1. To arrange the leaves in the order in which they were written, so that the story they contain may be read intelligently. 2. To divide and subdivide into chapters and sections, deter- mined by great events in the history. In a word, he must make first a clironology, and then divide into eras, ages, periods, etc. Chronology ; Order of Superposition. — It is evi- dent, from the manner in which sediments are formed, * The teacher should consult the larger work, for a complete state- ment. 206 STRUCTURAL OEOLOGY, that, if they have not been greatly disturbed, their rela- tive position indicates their relative ages, the uppermost being of course the youngest. If, therefore, we have a natural section of strata (an exposed sea-cliff or caflon- side), either horizontal or regularly inclined, it is easy to make out the relative ages. But often the rocks are folded and crumpled, and pushed over beyond the verti- cal ; they are broken and slipped, and a large part worn away by erosion ; they are covered with soil and hidden from view ; so that to make an ideal section showing their real relation is one of the hardest of geological problems. Nevertheless, if this were all, we might still hope for per- fect success. But all the strata are not represented in any one place — usually only a fraction. Thus, in New York, and all the States westward as far as the Plains, only the older portion of the record is found ; while in California we 'have mostly the later portion. In many places the record is still more fragmentary. The leaves of this book are scattered about — here, perhaps, nearly -a whole vol- ume ; there, one or two chapters ; and yonder, only a few leaves. The geologist must gather these and arrange them according to their paging ; and then divide and subdivide them into volumes, chapters, etc. Therefore, although the order of superposition must, wherever it can be applied, take precedence of every other method, yet it must be supplemented by careful comparison of the rocks in different localities with one another. There are two means of comparison, viz., the character of the rock and the character of the fossils. Comparison by Rock-Character.^This method is of little value except in contiguous localities. Sandstones of similar character belong to nearly all times, and are forming now. So, also, of clays and limestones. Coal was once considered characteristic of a particular age, but now is known to occur in strata of many ages. Chalk was once supposed to be characteristic of the Cretaceous, STRATIFIED ROCKS. 207 but is now known to be forming at present in deep seas. But since, both now and in former times, the same kind of deposits formed over wide areas, rocks of similar kind (for example, sandstones of similar grain and color), and especially a group of similar rocks, in^ contiguous locali- ties, are probably of the same age. But in widely sepa- rated localities, as, for example, in different continents, we can not use this method. To conclude that rocks are of the same age, because they are of similar grain, color, or composition, would almost certainly lead us astray. Comparison of Fossils. — This is the most universal and valuable means of comparison of rocks in all parts of the world. If we find a general similarity of species, we conclude that the rocks belong to the same age. But we must make due allowance — 1. For difference of conditions of deposit, whether shore-deposit or deep-sea deposit, whether fresh-water or marine. 2. We must also make due allowance for geographical diversity. We must ex- pect, in fossils of rocks in different continents, not abso- lute identity, but only general si^nilarity. We shall find little difficulty in applying this, until we come to the Tertiary. But here we have another principle to help us, viz., the percentage of livi7ig invertebrates found in the rock. Vertebrate, and especially mammalian species, may be used in the Tertiary in much the same way as all species in the lower rocks. Construction of Chronology. — By application of these methods, geologists in all countries, working to- gether, have gradually made a nearly complete chronol- ogy. Breaks in one country are filled by strata in an- other. But a really complete chronology can not be expected until the whole surface of the earth has been studied, and perhaps not even then, for some missing links are i^robably concealed beneath the sea. Divisions and Subdivisions. — The next task is to divide and subdivide the whole into primary and second- 208 STRUCTURAL QEOLOQY. ary groups — into volumes, chapters, etc., separated by great changes. As already explained (page 204), there Eras. Ages. Periods. Epochs. 5. Psychozoic. 7. Age of Man. Human. Recent. 4. Cenozoic. 6. Age of Mam- mals. r Quaternary, t Tertiary. ( Terrace. \ Champlain. ( Glacial. I Pliocene. < Miocene. ( Eocene. 3. Mesozoic. Secondary rocJcs. 5. Age of Rep- tiles. i Cretaceous. ■j Jurassic. ( Triassic. Upper- Carboniferous ) rocks. 1 4. Age of Aero- y gens and | Amphibians, j r Permian. J Carboniferous. 1 Subcarbonifer- (^ ous. l 2. Paleozoic. Lower Devonian rocks. 3. Age of Fishes. rCatskill. Chemung. -I Hamilton. Coniiferous. [^ Oriskany. Silurian rocks. 2. Age of Inver- tebrates. Cambrian or 'primordial rocks. f Helderberg. Salina. - Niagara. Trenton. Canadian. • Midd[e. ( Lower. 1. Archaean or Archaeozoic. 1. Archaean rocks. 1 \ Huronian. \ Laurentian. STRATIFIED ROCKS. 209 are two modes of determining the limits of the divisions of the rocks, and corresponding divisions of time, viz., by u7iGonformity of the rocks, and by change of the fossils. These two usually occur together, because they are pro- duced by the same cause, viz., change in physical geogra- phy and climate ; but, if there be discordance between the two, then we follow the change in the fossils rather than unconformity of rocks. By means of the most gen- eral unconformity and greatest change in fossil forms, the primary divisions are established ; and then, by less gen- eral unconformity and less important changes in organic forms, these are divided and subdivided. A generalized schedule of the divisions and subdivisions of the rocks and corresponding divisions of time which will be used in this work, is given on the preceding page. Lb Conte, Geol. 14 CHAPTER III. UNSTRATIFIED OR IGN"EOUS ROCKS. These differ wholly from tlie stratified rocks — 1. By absence of true stratification, i. e., lamination by sorting of material. 2. By absence of fossils. 3. By a crystal- line or else a glassy texture instead of an eartliy texture. 4. By mode of occurrence, as explained below. Origin. — All these characteristics are the result of their mode of origin. They have consolidated from a state of fusion or semi-fusion, and poured out from heloiv, instead of deposited as sediments from above. Their original fused condition is shown by their crystalline or glassy texture, by their occurrence injected into fissures, or even tortuous cracks, and by their effects on the stratified rocks with which they come in contact. Mode of Occurrence. — They occur in three main positions : 1. Underlying the stratified rocks and appear- ing on the surface in great masses, especially in mountain- f/i"'^- ,"- ^ ,"" |miii| Eruptives. ^ ESI Granitics. I::::::! Metamorphic. Palaeozoic. |:->>j Mesozoic. F^g Cenozoic. Fig. 121.— Ideal sectiou of the eaitirB crust. UNSTRATIFIED OR laNEOUS ROCKS. 211 ous regions {a, Fig. 121). 2. In vertical sheets intersect- ing the stratified rocks or other igneous rocks, b. 3. In streams or sheets overlying the stratified, or else between the strata, c c\ 4. Sometimes as tortuous veins, d d, connected with the great underlying masses. All of these are connected with, and are extensions of, the great underlying masses. Extent. — As thus defined, igneous rocks occupy but a small portion, certainly not more than one tenth, of the land-surface. But beneath the stratified rocks they are supposed to form the great mass of the earth. Classification of Igneous Rocks. — Igneous rocks can not be classified, like sedimentaries, by relative age. They are best classified partly by texture and partly by mode of occurrence. They thus fall into two strongly contrasted groups, viz., pliitonlcs and volcanics, or gra- nitics and true eruptives. The rocks of the one group are very coarse-grained and wholly crystalline, of the other, finer-grained or even glassy. The one occurs only in great masses, either underlying the stratified rocks, or appearing on the surface over wide areas, especially in the axes of mountain-ranges ; the other, in sheets injected among the strata, or as streams and sheets outpoured on the surface. The granitics have not usually been erupted at all, although they often form the reservoirs from which eruptions have taken place. It is sometimes convenient to speak of an intermediate group — trappean. If so, then the three kinds correspond to the three positions mentioned above. The granitic (Fig. 121, «) occur beneath ; the trappean, b b, injected among ; the volcanic, cc, outpoured upon, the stratified rocks. I. — The Massive or Graistitic Group. The rocks of this group occur in great masses, not in sheets or streams. They are all very coarse-grained in 212 STRUCTURAL GEOLOGY. texture, and have a speckled or mottled appearance, be- cause composed of crystals of considerable size, and of different colors, aggregated together. The crystals of which they mainly consist are, quartz, feldspar, mica, and hornblende. In such a coarse, speckled rock, the bluish, glassy, transparent spots are quartz ; the opaque, whitish, or rose or greenish crystals, with striated surface, are feldspar ; the black spots are usually hornblende ; the mica may be known by its thin, scaly structure, some- times pearly, sometimes black. The whole group is called granitic, because granite is its best type. In popular language, indeed, all these rocks would be called granite, but sci- ence makes a difference. If the rock consists of quartz, feld- spar, and mica, or else of these with hornblende, then it is granite proper. If it consists of feldspar and hornblende, or Pig. i^s.-'S^aphic g^ite. ^^6^0 with quartz, it is Called syenite. If it consists of only quartz and feldspar, and the quartz be in bent plates, looking, on section, like Hebrew characters, it is called pegmatite (Fig. 122). The feldspar in all these is potash- feldspar, or orthoclase. Diorite is a dark, speckled rock of the same composition as syenite, except that the feld- spar is a soda-lime feldspar or plagiodase. Gahhro and diahase are dark -greenish rocks similar to diorite, except that the hornblende is replaced by augite and olivine."^ Mode of Occurrence. — The mode of occurrence of these rocks has been already explained. They never occur in overflows. They rarely or never occur in in- truded sheets or diJces. They occur only in great masses, or sometimes in tortuous veins closely connected with the * The teacher must have a small collection of rocks and of min- erals for illustration. UNSTRATIFIED OR IGNEOUS ROCKS. 213 great masses, as if forced into cracks by heavy pressure (Fig. 121, d). Their coarsely crystalline texture and their mode of occurrence are well explained by supposing that they have cooled at great depth in large masses, and consequently sloivly. When they appear at the surface, therefore, they have been exposed by extensive erosion. Two Sub-Groups. — All igneous rocks, whether plu- tonic or volcanic, are divisible into two sub-groups, acidic and basic. In the acidic, quartz and potash-feldspar (orthoclase) predominate ; in the basic, hornblende or augite and soda-lime feldspar (plagioclase) predominate. The rocks of the former group are lighter colored and less dense ; of the latter, are darker and heavier ; but the two sub-groups run insensibly into each other. Among the granitics, granite is the best type of the acidics ; and diorite, and especially gabbro or diabase, of the basics. Intermediate Series. Between the true plutonics and true volcanics there is an intermediate series, called trai^pean or intrusives. If the plutonics occur in masses beneath, the volcanics in outpoured streams and sheets upon, these occur in sheets intruded among, the strata, especially of the older rocks. They are finer-grained than the plutonics and more crys- talline than volcanics. The reason, apparently, is that they have cooled more rapidly than the former, and less rapidly than the latter. These are also divisible into acidics and/ ^ — basics. Among the acidics Fig. I23.VA piece of porphyry ,, -^ ., - Vafter Lyell). would come felsite and por- ^ phyry, and, among basics, diorite and diabase, for these occur both massive and intrusive. 214 STRUCTURAL GEOLOGY. Diorite and diabase have already been described. It is only necessary to say that, when occurring intrusive, they are finer-grained than the massive varieties. Felsite is a fine-grained, light-grayish rock, consisting essentially of orthoclaso and quartz. Porphyry is a rock consisting of fine-grained feldspathic paste, with disseminated large crystals of feldspar (Fig. 123). But any rock is said to be porphyritic if it consists of fine-grained paste with large crystals of any kind disseminated. II. — VoLCANics, OR True Eruptives. The rocks of this group are distinguished from those of the other, both by texture and mode of occurrence. By texture they are not only finer-grained (micro-crystal- line), but there is always more or less of uncrystalline or glassy base or cement, showing that the fused mass has cooled too quickly to allow complete crystallization. Often, also, as already explained under volcanoes (page 139), these rocks are in a wholly glassy and even in a scoriaceous and tufaceous condi- tion. The principal rocks of this group are given in the ac- companying table. Trachyte may be taken as a type of the acidics. It is a light-colored rock, with a rough feel (hence the name), consisting essentially of orthoclase with more or less quartz. When the quartz-grains are con- spicuous, it becomes rhyolite. Phonolite is a dense vari- ety, of light-grayish color, which splits into slabs in weathering, and rings under the hammer almost like metal (hence the name). Obsidian and pumice are glassy and scoriaceous varieties of trachyte. VOLCANIC ROCKS. ACIDIC. BASIC. r Rhyolite. Stony. } Trachyte. ( Phonolite. Glassy, i Obsidian. ( Pumice. Basalt. Dolerite. Andesite. Tachylite. Black scorisB. UN STRATIFIED OR IGNEOUS ROCKS. 215 Basalt is the type of the basics. It is a very dark, almost black, heavy rock, scarcely visibly grained to the naked eye, and breaking with conchoidal fracture. It consists of plagioclase with augite, olivine, and magnetite. Dolerite has a similar composition, but more distinctly crystalline texture, and therefore dark-grayish color. Tachylite is the glassy variety, which, if vesicular, be- comes black scoria. The following table is a condensed statement of the composition of the principal kinds of rocks numbered above. The sign x x indicates crystals. IGNEOUS ROCKS. ACIDIC. BASIC. ■fl s a; Rhyolite. Trachyte. Phonolite. Vitreous Vitreous Vitreous base. base. base. + + + X X of X X of X X of nrtwia«e Orthoclase Sanidin, ""fsaSiS. («--din). Nephelin. Andesite. Basalt. Vitreous Vitreous base. base. + + X X of X X of Plagioclase, Plagioclase, Augite, or Augite, Hornblende. Olivine. r Occurring in intrusions. Quartz-porphyry. Micro X X ground- Felslte. mass. + Micro X X of X X of Orttioclase, Orthoclase, Quartz. Quartz. Di(ynte. DiaMse. See below. See below. 1 - Granite. Syenite. ** ^ **^ V V nf SThnH'««P Orthoclase, Orthoclase, Hornblende. DioHte. Diabase. X X of X X of Plagioclase, Plagioclase. Hornblende. Augite. Two Modes of Eruption. — There are two modes of eruption. In the one, the fused mass comes up through chimneys, and flows off in streams (or ejected as cinders and ashes) ; in the other, it comes up through great fissures often hundreds of miles long, and spreads as 216 STRUCTURAL GEOLOGY. extensive sheets. In the one the erupted matters ac- cumulate about the vent as a cone ; in the other they form great lava-fields, or else may be forced between the strata and never come to the surface at all. In the one ih.Q force of ejection is probably the elastic force of vapors, as explained under volcanoes ; in the other the force is more obscure, but probably of the same nature as that which /or^s mountains. The two kinds may be called crater-eruptions and fissure-eruptions. At present only the former kind seems to exist ; and therefore in Part I, while treating of causes now in operation, we treated only of this mode. But in studying erupted materials of all periods, it is plain that by far the larger quantity have come up in the second way. Modes of Occurrence. — Leaving out of view those modes of occurrence already described under volcanoes, viz., chimney-cones with radiating dikes and lava-streams, the principal modes of occurrence of eruptive rocks are : 1. Dikes. 2. Overflow-sheets. 3. Intercalary beds. 1. Dikes. — Dikes are vertical sheets filling great fis- sures in stratified or other igneous rocks. They are the most common of all modes of occurrence of eruptives and intrusives. In all mountain-regions they are found in great numbers. In width they vary from a few feet to hundreds of feet, and may often be traced outcropping over the surface fifty to one hundred miles. But since rocks are usually covered with soil, they arc not always visible at once, but must be looked for wherever the rock is exposed, especially in stream-beds. It is evident that fused matter coming to the surface must overflow, and therefore dikes thus outcropping on the surface are either the exposed roots of former over- flows which have been removed by erosion, or else are the fillings of fissures which never reached the surface at all (Fig. 121, V). In either case, an outcropping dike is the sign of gi3at erosion. If, therefore, the dike is harder UNSTRATIFIED OR IGNEOUS ROCKS. 217 than the country-rock through which it breaks, it will stand above the surface and look like a low, ruined wall Fig. 124.— Dikes. (Fig. 124, a). If, on the contrary, the igneous rock yield more easily to erosion than the country-rock, then it may be traced as a shallow, half-filled ditch (Fig. 124, b). Effect of Dikes on Stratified Rocks. — On both sides of a dike the bounding walls of stratified rock are always changed by the intense heat of the fused matter. Sandstones are changed into a rock resembling gneiss (page 225), clays are baked into porcelain jaspers, lime- stones are changed into crystalline marbles, coal-seams into anthracite and sometimes into coke. In all cases the fossils, if any, are more or less completely destroyed. These metamorphic changes usually extend only -a few feet or yards from the place of contact. 2. Overflows. — This is the next most common form of occurrence. The liquicj matter has come up through great ^i^^^^^Sim^ Fig. 125.— Lava sheets. fissures, such as are made by crust-movements, and spread on the surface as extensive sheets. Often sheet after sheet 218 STRUCTURAL GEOLOGY. is outpoured, one on another, until masses 2,000 to 3,000 feet thick are piled up (Fig. 125). The extent and tliichness of some of these lava-floods are almost incredible. The great lava-flood of the North- west covers the whole of northern California, north- western Nevada, and a great part of Oregon, AVashington, and Idaho, and extends far into Montana and British Columbia. Its area is supposed to be 150,000 square miles, and its thickness, where cut through by the Co- lumbia River, is at least 3,000 feet. There are about a dozen extinct volcanoes dotted, at wide intervals, over this vast area. It seems certain that the lava came up through fissures in the Cascade and Blue Mountains, and spread as sheets which covered the whole intervening space. Afterward eruptive activity continued, in a more feeble form as volcanoes, almost to the present time. The great Deccan lava-field, described by the geologists of India, covers an area of 200,000 square miles, and is in places 6,000 feet thick, and there is no evidence of any crater-eruptions at all. These very extensive sheets are usually basalt. In some parts of the Utah and Nevada Basin region, how- ever, rhyolitic and trachytic lavas are found 7,000 feet thick, but these are far less extensive. As a general rule, the lasiG lavas, like basalt, were very liquid (superfused), and spread out in thin sheets, while the acidic lavas, like trachyte, have been stiffly viscous- (semi-fused), and were squeezed out dome-shaped. Fig. 12G.— Intercalary beds. UNSTRATIFIED OR IGNEOUS ROCKS. 219 3. Intercalary Beds. — Often sheets are found be- tween the strata, sometimes repeated many times. In such cases they may have been poured out on the bed of the sea or lake, and covered with sediment ; or they may have broken through the strata for a certain distance, and then spread between the separated strata (Fig. 126). Both of these eases occur. If the strata both above and below the sheet are changed by heat, then it has been forced between ; but if only the underlying stratum is changed, then it has been outpoured on the bed of the sea or lake, and covered with sediment. Age of Eruptives. — Where two dikes or streams meet, their relative ages may be known. In case of suc- cessive streams, that which covers is of course the later. If one dike intersects another (Fig. 127), the intersecting dike, a, is the younger. The absolute age, i. e., the geo- FiG. 127. logical period when the eruption took place, can be de- termined only by the age of the associated stratified rocks. If igneous rocks break through, or are outpoured upon. Fig. 128. or forced between layers of stratified rocks, then the igneous rock must be younger ; but if intercalary beds 220 STRUCTURAL GEOLOGY. are the result of outpouring on the bed of the sea, and covering it with sediment, then the igneous and the stratified rocks are contemporaneous. Finally, if dikes outcropping on the surface are covered with other strata through which they do not break (Fig. 128), then they are younger than the lower series, a, ai\d older than the upper, b. Some Structures common to Many Eruptives, Columnar Structure. — Many eruptive rocks, espe- cially of the more basic kinds, seem to be wholly made Fig. 129.— Columnar basalt, New South Wales (Dana). up of regular prismatic columns (Fig. 129). This re- markable structure is most common and perfect in basalt, and is therefore often called basaltic structure. The col- umns vary in size from a few inches to several feet in diameter, and in length from a few feet to one hundred feet ; the number of sides from three to seven, more com- FiG. 130.— Basaltic columns (after Geikie). UNSTRATIFIED OR IGNEOUS ROCKS. 321 monly five or six. The cclumns are not usually continu- ous, but short- jointed, like a vertebral column (Fig. 130). The position of the columns is usually perpendicular to the cooling surface. Thus, in vertical sheets, like dikes, they are horizontal, and an outcropping dike often pre- sents the appearance of a pile of corded wood (Fig. 131). Fig. 131.— Columnar dike, Lake Superior. (After Owen.) In overflow-sheets the columns are vertical (Fig. 129), and at the base of a cliif of such rocks are found piles of separated and disjointed columns. The cause of this structure is shrinkage by cooling. Many substances shrink by rhyi^ig, and break into pris- matic columns. Mud thus forms polygonal prisms by sun-cracks. AYet starch, poured into boxes and drying, breaks into prismatic pencils. In the case of lava, the shrinkage is by cooling, instead of drying, and the prisms are far more regular. Examples of this structure are found in every country, and give rise to many remarkable scenes. In Europe, the 222 STRUCTURAL OEOLOOY. Giant's Causeway in Ireland, and Fingal's Cave on the Island of Staffa, are good examples. The Giant's Cause- way is a sea-cliff of columnar basalt, consisting of many layers, with softer material between, and the whole rest- ing on stratified rock. By the action of the sea and air the separated and disjointed columns are undermined and fall to the base of the cliff. In this country, the Pali- sades of the Hudson River, and Mounts Tom and Hoi- yoke in the Connecticut River Valley, are good examples. Fine examples are found also in the trap of Lake Supe- rior (Fig. 132). But the finest in this country are the Fig. 132.— Basaltic columns on sedimentary rock, Lake Superior. (After Owen.) basaltic cliffs of Columbia and Des Chutes Rivers in Ore- gon. On the Des Chutes River at least thirty lava-layers may be counted, one above another, each entirely com- posed of vertical columns. Volcanic Cong-lomerate and Breccia. — If a lava- stream runs down a stream-bed or a shingly beach, it gathers up the pebbles and forms with them a conglome- rate differing from aqueous conglomerate in the fact that the uniting paste is igneous instead of sedimentary. So, also, a lava-stream may gather up rubble and form a volcanic breccia differing in the same way from sedimen- tary breccia. UNSTRATIFIED OR IGNEOUS ROCKS. 223 Amygdaloid. — The upper part of a lava-stream is ve- sicular, or full of air-bubbles. If such a stream be cov- ered by another stream, percolating waters, charged with silica and carbonate of lime gathered from the lava, will fill up the empty spaces with these materials. If the rock be broken or weathered, these amygdules fall out. They looTc somewhat like peblles, and the rock (Fig. 133) might be mistaken for conglomerate, but is formed in an entirely Fig 133.— Amygdaloid. different way. The filling of the cavities takes place slowly, layer within layer, and the layers are often of dif- ferent colors. It is in this way that are formed the most exquisite agate and carnelian nodules. Tufas. — When volcanic materials disintegrate, and are then moved and deposited in water, they form tufas. Sometimes the fragments may be larger and the mass may simulate volcanic breccia. It is, however, an aqueous breccia of volcanic rock. Such are sometimes called vol- canic agglomerates. CHAPTER IV. METAMORPHIC ROCKS. We have now finished both the stratified and the un- stratified rocks, but there is yet an intermediate series which must be described. These are stratified like the stratified rocks, but crystalline in texture, and usually destitute of fossils, like the igneous rocks. They are supposed to have been formed from sediments like strati- fied rocks, but have been subsequently changed by heat and other agencies. They are therefore called 7neta- morphic rocks. They may be traced by gradations, on the one hand, into stratified, and, on the other, into igneous rocks. Extent and Thickness. — They cover large areas, es- pecially among the oldest rocks and along axes of great mountain-chains. The whole of Labrador, the larger por- tion of Canada, the whole eastern slope of the Appala- chian, and also the axes of the Colorado and Sierra, con- sist of them. In Canada they are supposed to be 40,000 to 50,000 feet thick and very much crumpled. Meta- morphism is nearly always associated with great thickness and crumpling. Age. — The oldest rocks are all metamorphic. Hence many regard it as a sign of age. But it is probably n^ore correct to say that metamorphism is found in rocks of all ages if only they be very thick and very much crumpled ; but, since great thickness and complex crumplings are most common in the oldest rocks, so also is metamorphism. 224 METAMORPHIC ROCKS, 225 Kiuds. — The adjoining table shows the principal kinds: Gneiss is a rock having much the appearance and mineral composition of granite — i. e., quartz, feldspar, mica, and hornblende— differing only in a ledded structure. In many places, as, for example, on Manhattan Island, gneiss can be traced by in- sensible gradations into granite. Schists are rocks having a fissile structure through the abundant Gneiss. Mica-schist. Chlorite-schist. Talcose-schist. Hornblende-schist. Clay-slate. Quartzite. Marble. Serpentine. presence of scales of some kind. In mica-schist they are mica ; in the other schists they are chlorite, or talc, or hornblende. c^*;^ ""^ -"-— Quartzite and Marble are both white, crystalline, or granular rocks, looking like loaf-sugar ; but in the one case the granules are quartz, in the other, lime-carbonate. Serpentine is a greenish rock, having usually a schistose structure and a greasy feel like talc. It contains a nota- ble quantity of magnesia. Origin of these Kinds. — Metamorphic rocks are prob- ably changed sandstones, limestones, and clays, and mixtures of these. The infinite variety which we find is the result partly of the original kind and partly of the degree of change. For example, sandstones and lime- stones are often perfectly pure. Now, a metamorphic pure sandstone is quartzite, and a metamorphic pure lime- stone is marble. But clays are nearly always impure, being mixed with sand and lime and iron and other bases. A moderately pure clay with a little sand by metamor- phosis makes gneiss or mica-schist. If it contains much iron, it makes a hornblende-schist ; if magnesia, talcose- schist or serpentine. Serpentine is, however, often a changed eruptive rock. Le Conte, Geol. 15 226 STRUCTURAL GEOLOGY, Cause of Metarnorphism. There are two kinds of metamorphism which must be distinguished, viz., local or co?i^«56'^ metamorphism, andre- gional metamorphism. The former is produced by direct contact with fused matter, as in dikes or intercalary beds (page 217). There can be no doubt as to the cause in 4:his case. It is intense heat. But the effect of the heat extends but a little way from the plane of contact. In regional metamorphism, on the contrary, the change is universal over hundreds of thousands of square miles and thousands of feet of thickness. In these cases there is no evidence of intense heat in every part ; the heat was prob- ably very moderate. It is of this kind that we now wish to explain the cause. The Ag-ents of regional metamorphism are — 1. Heat ; 2. Water ; 3. Alkali ; 4. Pressure ; 5. Crushing. To produce metamorphism by heat alone, i. e., dry heat, would require a temperature of 2,500° to 3,000°, but in the presence of water a very moderate heat will change rocks. At 400° Fahr. (= 205° C), incipient change commences ; and at 800° Fahr., complete hydrothermal fusion takes place. If any alkaline carbonate be present in the water, these effects occur at still lower temperature. The quan- tity of water necessary is only ten to fifteen per cent. ; in other words, the included water of sediments is amply sufficient. Pressure is necessary, because it is impossible to have even such moderate heat in the presence of water, unless the whole be under pressure. Application — Suppose, then, we have sediments ac- cumulating along a shore-line, or at the mouth of a river until a thickness of 10,000, 20,000, or 40,000 feet is reached. It is evident that the isogeotherms (interior isotherms) would rise, and the lower portion of the sedi- ments with their included waters would be invaded by the interior heat of the earth (Fig. 134). At the rate of METAMORPHIC ROCKS. 227 100° increase per mile (page 132), the lower portion of the sediments 20,000 feet thick would be 400° + 60° (mean surface temperature) = 460°, and 40,000 feet of the sediments would be at the bottom 860°. Now, we actually Fig. 134. —s6, original sea-bottom ; s'b\ sea-bottom after sediments, sd, liave accu- mulated ; . . . ., isogeotherms of 800° and 400" ; . — , same after accumula" tion of sediments. have strata 20,000 and 40,000 and even more feet thick. The lower portions of such strata must be completely metamorphic. The figure (Fig. 134) shows how the pro- cess takes place. Crushing. — Pressure alone is a condition, but not a cause of heat. But pressure producing jnotion, or crush- ing, crumpling, is an active cause of heat. Now, we usually find metamorphism associated with most complex crumpling of strata. The heat must have been increased also by this cause. Even igneous rocks, by pressure, mashing, and shear- ing, may be made to assume the appearance of metamor- phic stratified. Many schists, especially gneisses, are formed in this way. CHAPTER V. STRUCTURES COMMON TO ALL ROCKS. We have now given a brief account of all the different kinds of rocks. But there are still some structures which are found in all kinds of rocks, and which could not be described until these kinds had been defined. These are: 1. Joi?its ; 2. Great fissures; and, 3. Mineral veins. Mountain-chains, as involving all kinds of rocks and all kinds of structure — in fact, as summing up all the prin- ciples of dynamical and structural geology — we must take last of all. Sectiok I. — Joints and Fissures. Joints. We have already alluded to joints in stratified rocks (page 179), but without describing them, because not characteristic of these rocks. All rocks — sedimentary, igneous, and metamorphic — are divided by cracks in dif- ferent directions into separable blocks of various sizes and shapes. These cracks are called joints. In stratified rocks, one of the division-planes is between the strata, and the other two nearly at right angles to this. The shape and size of the blocks differ in different kinds of rocks. For example, in sandstone the blocks are usually very large and roughly prismatic ; in limestone, they are usually very regularly cubic (Fig. 135) ; in shale, oblong rhom- boidal ; in slate, small and sharply rhombic ; in granite, sometimes large and roughly cubic, sometimes scaling in STRUCTURES COMMON TO ALL ROCKS. 229 concentric shells, producing domes ; in eriiptives, of many shapes, rough cubic, ball-like, regular columnar, tilelike. Fig. 135.— Regular jointing of limestone. For this reason a cliff, especially of stratified rock, looks like a wall of titanic masonry without mortar. Cause. — These cracks are supposed to have been formed by the shrinkage of the rocks ; in stratified rocks, in con- solidating from sediments ; in igneous and metamorphic rocks, in cooling from a state of fusion or semi-fusion. In stratified rocks they are usually confined to the stra- tum, though some larger joints (master-joints) run through several strata. They are mentioned mainly that the student should not confound them with other kinds of structure. Great Fissures, Joints are probably shrinkage-cracks. Fissures are fractures by crust-movements. Joints are cracks of the individual strata ; fissures are fractures of the earth's crust, extending through many formations, and continu- ing for many miles. Cause. — We shall see hereafter that the earth's crust is subjected to a powerful horizontal pressure, by which 230 STRUCTURAL GEOLOGY it is sometimes maslied together, sometimes thrown into arches and hollows. Such bondings of the crust produce enormous fractures parallel to the axis of the bending, and parallel to mountain-ranges, since mountain-ranges are produced in this way. Sometimes there is a system at right angles to the main system, or in the direction of the cross-valleys of mountains. The characteristics, therefore, of great fissures are — 1. Their occurrence in systems, usually parallel to the axis of elevation. 2. Their length, often extending for hundreds of miles. 3. Their depth, sometimes breaking through miles of thickness of rock. When filled at the moment of formation with fused matter from below, they Fig. 136.— Fault in Southwest Virginia : a, gilnrian : pus, Monkeys, CamiTores, Ungulates, Tillodonts, Rodents, Serpents. Lignite Series. iignite fc &uirosau Pteranodon Beds. Birds -with Teeth, Begperomis, lehthyomit. Pterodactyls, Plesiosaurs. .\tlantosaurus Beds. Dinosaurs, Apatotaurus, Allosaurus, Ifanotaurvs. Turtles. Diploiaurus. Connecticut River Beds. Dinosaur Foot-prints, Amphisaurux. Crocodiles (Belodon). Permian . First BeptUes. Coal-Measures. Sub-carboniferous. First knoim Amphibians (Labyrinthodonts). Corniferous. Schoharie Grit. First Fish Fauna. Upper Silurian. First known Fishes. Lower Silurian. Primordial. Huronian. Laurentian. No Vertebrates known. Fig. 164. -Section of the earth's crust, to illustrate vertebrate life in America. (Slightly modified from Marsh .)^ 262 HISTORICAL OEOLOGY. In the diagram (Fig. 103) the diUerent rock-systems are placed one on top of the other, and the vertical black spaces represent by their breadth the relative dominance of different classes at different times. Periods and Epochs. — The subdivisions of these again into periods and epochs are founded on more local uncon- formities, and especially on less important changes in the species. We have already, on page 204, given a schedule of the most important divisions and subdivisions adopted in this work ; but we shall not treat separately all of these. As in human history, so in geology, the earliest times are little known, and are touched lightly. As we come toward the present, and events thicken, we shall take up subdivisions more and more — first ages, then periods, and, finally, even epochs. We give here also (Fig. 164) a generalized sec- tion of American strata, which will be found useful for reference. It must not be supposed, however, that all these strata occur in any one place. It is an ideal section, in which all the most important American strata occurring in different places are brought together and arranged in the order of time. We are now ready to commence a rapid survey of the history of the earth. But it must be understood that we can commence only where the record commences. Before this is the abyss of the unrecorded, of which we know nothing positive. Before the historic is the prehistoric ; no history can recall its own beginning. CHAPTER II. ARCH^AN SYSTEM AKD ARCHEOZOIC ERA. The events recorded in this oldest system of rocks, in this first volume of the iooh of time, are so few and so imperfectly recorded that their chief interest consists in the fact that they are the first. There is a fascination about the beginning — the mythical period — of all history. The distinctness of this system was for a long time un- recognized. It has now, chiefly by the labors of Ameri- can geologists, been completely established. In no single instance have these rocks been found to graduate into the Paleozoic. There is absolutely everywhere an uncon- formity between them and every other system. No such complete and universal break occurs anywhere else in the rocky series as occurs here (Fig. 165). It is, therefore. 1 2 Fig. 165.— Section showing Primordial unconformable on the Archaean : 1, Archaean or Laurentian ; 2, Primordial or lowest Silurian. (After Logan.) properly called a distinct system and a distinct era — more distinct, in fact, than any other. Here, then, we have the oldest known rocks. Are they, then, absolutely the oldest — the primitive rocks, as some imagine ? By no means. They are stratified rocks, and therefore consolidated sediments, and therefore, also, the 263 2G4 HISTORICAL GEOLOGY. debris of still older rocks, of which we know nothing. Thus, we seek in vain for the absolutely oldest, the primi- tive crust. As already said, no history can write its own beginning. Character of these Rocks. — We can only say, in brief, that they do not differ very conspicuously from metamorphic rocks of other times. They were probably originally sands, clays, and limestones, much like those of other times ; but, in this case, always very highly meta- morphic and strongly crumpled (Fig. 166). The sands are thereby changed into quartzites, the clays into schists, Fig. 166.— Contortion of Laurentian strata. (After Logan.) gneisses, and even granites, and the limestones into mar- bles. Along with these, however, are associated two kinds of beds, which are worthy of note, viz., beds of iron-ore and beds of graphite. In Canada the whole series is not less than 40,000 feet thick. The greatest beds of iron-ore known in any strata are found here. The great iron-ore beds of Sweden, of Lake Superior (Fig. 167), of New Jersey, and the Iron Moun- tain of Missouri, are in these rocks. Recently, in southern Utah, in rock of this age (or possibly later), have been found the greatest iron-de- posits, perhaps, in the world. The strata here stand on edge, and the beds of iron-ore, being very hard, have been left by erosion standing out as black, castellated, inaccessible crags, 300 feet high, 1,000 feet long, and 500 feet thick. In Canada and elsewhere graphite also occurs in immense beds, sometimes pure, sometimes mixed with the rock. Fig. 167. ARCH^AN SYSTEM AND ARCHEOZOIC ERA. 265 Area. — 1. These rocks cover the whole of Labrador, nearly the whole of Canada (passing into New York in the region of the Adirondacks), then extend northwest probably to the Arctic regions. This, the greatest Ar- chaean area in North America, forms a broad, open V, inclosing in its arms Hudson Bay. 2. The next largest area is a broad space extending from New England to Georgia, including the Blue Eidge and the eastern slope of tlie Appalachian. 3. The axes of many of the great mountiiin-ranges, such as the Colorado, Park, and Wah- satch Ranges, and possibly the Sierra Nevada. 4. Some small, isolated spots, one in Texas and one in Missouri. In the map (page 272) these are represented by v • Physical Geography. — These being stratified rocks, it is evident that the whole Archaean area was sea-bottom at that time. Where, then, was the land from which this debris was derived ? Of this we know nothing. Some have thought that it was to the northeastward. We shall see hereafter that the continent developed southward and westward. Amount of Time. — The Archaean rocks are of enor- mous thickness, probably equal to all other subsequent rocks put together. The amount of time represented is, therefore, probably equal to all the rest of recorded his- tory of the earth. And yet how meager the record ! It is the same with the earliest human history. Life. — Did any living thing exist at that time ? This is a very important question, but we can not yet answer it with absolute certainty. There are, however, some good evidences of life : 1. Iron-ore is accumulated 7iow, and therefore probably also in earlier times, only by means of decaying organic matter (page 89), and is, therefore, justly regarded as a sign of life and a measure of its quantity. 2. Graphite is regarded as the highest anthra- citic condition of coal; and coal is a positive sign of organic matter, and therefore of the previous existence 266 HISTORICAL GEOLOGY, of life. Limestone, as we have seen (pages 114-117), is now, and at previous geological times, usually, though not always, of organic origin. Judging from these signs, it would seem that life was not only present, but in large quantity. Can we say any- thing as to its kind ? Are there any fossils ? Here we must answer still more doubtfully. Some curious forms are found which are supposed to be those of the lowest order of animals {compound Protozoa). These have been called eozoon or dawn-animal, and therefore some have called this first era eozoic. Most, however, do not accept this animal, and prefer the name Azoic (no animal life), or, better still, Archaean or Archaeozoic, as carrying no implication. In conclusion, we may say that the existence of the lowest forms of vegetable life is almost certain, and of the lowest forms of animal life probable. CHAPTER III. PALEOZOIC ROCKS AND ERA. Section I. — General Description. The Lost Interval. — Between the xirchaean and Pale- ozoic rocks occurs the greatest and most universal break in the whole stratified series. At this point in time occurred the greatest and most wide-spread changes in physical geography and climate which has ever occurred in the history of the earth. The justification for this statement is found in the fact that everywhere, even in the most distant localities, we find the lowermost Paleo- zoic (Primordial) lying unconformably on the Archaean. Ko one has yet seen the two series continuous. Now, when we remember that unconformity always means a previous eroded land-surface (page 192), and stratified rock a sea-bottom, we easily perceive how wide-spread the changes of physical geography must have been at this time. Again, when we remember that unconformity also always means a lost interval unrecorded at the place ob- served, and that the unconformity exists at all observed places, we at once see that right here is an unrecovered, probably an irrecoverable, lost ititerval of time. During the lost interval wide areas of land existed, which were afterward submerged and covered with Paleozoic sedi- ments. As compared with the early Paleozoic, it was evidently a continental period. Corresponding with the great physical changes here, there was also immense advance in life-forms. During 267 268 HISTORICAL GEOLOGY. the Archaeozoic, as we have already seen, the life, if any, was only of the lowest possible kind. Life-forms had not differentiated into distinct, recognizable species. There was not yet what could justly be called a fauna and flora. Then came the lost interval, represented by the unconformity. Of what took place then we know nothing. AVhen the record opens again with the Paleozoic, we have already an abundant and diversified fauna and flora. Even in the lowest Primordial we find all the great de- partments of Invertebrates, and nearly all the classes of these departments, already represented. It certainly looks like a sudden appearance of somewhat higlily organ- ized animals, without progenitors. But we must not forget the lost interval. It is probable that during this period of rapid physical changes there were also rapid changes in organic forms. It is for these reasons that the Paleozoic is regarded as opening a new era, and, in fact, the most distinct in the history of the earth. We have explained its distinct- ness from the Archsean below, but we shall find hereafter that it is almost equally distinct from the Mesozoic above. It is separated on both sides by unconformity and by changes in life — a distinct volume with, as it were, blank boards on either side. Rock-System. — There is nothing very noteworthy in the character of the rocks of the Paleozoic. Only this may be said : as compared with the Archaean rocks, they are far less universally thick, metamorphic, and crumpled. In mountain-regions, indeed, they are very thick (40,000 feet in the Appalachian), very metamorphic, and very much folded ; but in level regions they are often much thinner, entirely unchanged, and level-lying. For exam- ple, in passing from the Appalachian westward, we find the following four kinds of change : 1. In the Appalachian the Paleozoics are 40,000 feet thick ; they thin out west- ward, until at the Mississippi Eiver they are only 4,000 PALEOZOIC ROCKS AND ERA. 269 feeto 2o In the former, sands, grits, and clays predomi- nate ; in the latter, limestones. 3. In the former the rocks are strongly folded ; these folds die out through gentle undulations to level-lying strata in the latter. 4. In the former the rocks are highly metamorphic ; in the latter they are wholly unchanged. Area in the United States. — lo Eastern Paleozoic Basin. The Paleozoic rocks cover a large continuous area in the very best part of the United States. This area is bounded on the north by the chain of the Great Lakes ; on the east by the Blue Eidge of the Appalachian chain ; on the south by a line running through mid- Alabama, turning northward to the mouth of the Ohio Eiver ; then south through mid-Arkansas and Indian Territory ; on the west by the Western grassy plains. 2. Besides this great area, there are several considerable areas scattered about in the Plateau region and exposures along flanks of mountains of the Plateau and Basin regions. Physical Geography. — The physical geography of the eastern portions of the North American Continent in Paleozoic times can be made out with considerable cer- tainty. In fact, we can in many places trace the Primor- dial shore-line. Immediately in contact with the Canadian Archaean on the north, and the Blue Eidge Archaean on the east, are found patches, or continuous lines of a coarse sandstone, which contain all the marks characteristic of shore-lines, such as worm-tubes, worm-trails, crustacean tracks, ripple-marks, rain-prints, etc. This is the old Primordial beach. At the beginning of Paleozoic times, therefore, the whole Paleozoic basin was covered by a sea which beat against a land-mass to the north (Canadian Archaean area), and a land-mass to the east (Blue Eidge Archaean area). This is called the great interior Paleo- zoic Sea. There was also a large land-mass in the Basin region, and smaller masses, probably islands, in the Colo- rado mountain-region, but the exact limits of these are 270 HISTORICAL GEOLOGY. not known. The map (Fig. 168) represents the present state of our knowledge on this subject. It is probable, however, that the Eastern land-mass (Blue Ridge Archaean area) was larger than represented, having been subse- quently covered by later deposits, and partly, even now, by the Atlantic Ocean. The change in the rocks, in passing westward from the Appalachian region, is completely explained by the posi- tion of the Appalachian region and the subsequent f orma- PiG. 168.— Map of physical geography of Primordial times : existing seas and lakes, black ; continental seas of that time, light shade ; land of that time, white. The white dotted line shows the probable shore-line of 2 at this time. tion of the mountains. This region was then the marginal bottom of the interior sea, receiving abundant and coarse sediments, which became finer and thinner seaward. This thick marginal line then yielded, was strongly folded and highly metamorphosed in the act of mountain- making which took place at the end of the Paleozoic. PALEOZOIC ROCKS AND ERA. 2?1 Growth of the Continent during Paleozoic Times. — The map (Fig. 168) represents the continent at the beginning of the Paleozoic. But during that era there was a steady growth from this nucleus by addition south- ward and westward, until, at the end, the whole of the Paleozoic areas were reclaimed from the sea, and the continent was nearly, though not exactly, that represented on page 349. It will be seen that the continent was already outlined at the beginning of the era, and was steadily developed toward its present form. We shall hereafter trace this development to its completion. Subdivisions of the Paleozoic. — The Paleozoic era and strata are divided into three ages, each represented by corresponding rock-systems : 1. The age of Molluslcs, or of Invertebrates, represented by the Cambrian and Silurian system ; 2. The age of Fishes, by the Devonian ; 3. The age of Acrogen Plants and Amphibian Animals, by the Carboniferous. These three rock-systems, in many parts of the world, are unconformable with each other ; but in the United States they are usually entirely conformable. Nevertheless, their life-systems (organic forms) are here, as everywhere, quite different. All these subdivisions are well represented in the Pa- leozoic basin of the United States (Fig. 169). In the fol- lowing map of the main divisions of the geological strata of the Eastern United States, the rocks representing these three ages are all shown. It is important to study this map well, for it will bo referred to frequently hereafter in connection with more recent strata. Section II. — Lower Paleozoic or Cambrian" and Silurian System. Age of Invertebrates. Bocks ; Name. — These rocks are called Cambrian and Silurian, from the Eoman name for Wales and the Welsh, because they were first studied in Wales, by Sedgwick and 272 HISTORICAL GEOLOGY, PALJ^OZOIC ROCKS AND ERA, -ZTd Murchison. But tliey are far more perfectly represented ill the United States. Ai*ea. — It will be seeii^ by reference to the map. Fig. 1G9, that in the great Paleozoic basin these rocks form an irregular border to the Canadian and Blue Ridge Archaean areas. These borders were marginal sea-bot- toms at the beginning of the Silurian times, and were elevated and reclaimed during and at the end of that time. There are many other smaller areas in the West, but these can not be defined. Physical Geography. — We have already given this for the beginning of the age in the map. Fig. 168. For the end of the age, as just stated, we must add the Silu- rian area to the Archaean area. There was also at the end added a large island of Silurian sea-bottom in Ohio and Tennessee (see map. Fig. 169). Suhdivisions. — The Lower Paleozoic rocks are sub- divided into — 1. Primordial, or Cambrian ; 2. Lower Silurian ; 3. Upper Silurian ; and these, again subdivided, as shown in the following schedule. We simply give these by name for reference, if necessary, but will treat of the whole Cambrian and Silurian together : i Helderberg period 3. Upper Silurian. \ Salina ! Niagara 2. Lower Silurian. 3 Trenton . ^, , . i Canada 1. Cambrian, or \ Primordial. \ Primordial Life- System. We have already spoken of the apparent suddenness of the appearance of a somewhat diversified fauna in the Primordial, and accounted for it by the existence of a lost interval. Immediately after the Primordial the fullness of Paleozoic life became really wonderful. These early Lk Conte, Geol, 18 274 iliSTORlCAL GEOLOGY. seas seem to have swarmed with a life as abundant xis any now existing, but wholly dilferent in species, in genera, and even in families, not only from any noiv living, but from those living in any other geological joeriod. About 20,000 species are described from the Paleozoic, and of these at last one half, i. e., 10,000 species, are from the Silurian ; and of course these are but a very small frac- tion of the number which actually existed. The number being so great, and the forms so unfamiliar to the pupil, it is impossible to do more than mention and figure a few of the most common and striking forms. Plants. The only kind of plants which are found so early are^ allied to sea-weeds.* As it is very difficult to determine these species from the very imperfect impressions of them left in the rocks, we shall call them by the general name Fig. 170. Fig. 171. Figs. 170, 171.— Silurian plants : 170. Sphenothallus angustifolius. 171. Buthotre- phis gracilis. of Fucoids, i. e., /wcws-like plants, from their general resemblance to Fucus (tangle or kelp). We give a few * A few small vascular cryptogams, allied to club-mosses, have been recently found in the Silurian. PALEOZOIC ROCKS AND ERA. 275 (Figs. 170, 171), to show their general appearance. They belong to the lowest order of plants. Animals. These are far more numerous and diversified than the plants. We can mention only such as may be recognized even by the untrained eye. Corals. — These are very abundant, and seem some- times to have formed veritable reefs. There are three very characteristic forms, viz., Cup-aovdX^ {Cyathophyl- loids, Figs. 172, 173), Honeycomb-corals {Favositids, Fig. 174), and C%«^/^-corals {Halysitids, Fig. 175). These Fig. 173. Fig. 172. Figs. 172, 173.— Cyathophylloid corals : 172. Lonsdaleia lloriformis. (After Nichol- son.) 173. Strombodes peutagoiius. (After Hall.) are all characteristic of the Paleozoic, and the last char- acteristic of the Silurian. Now, any one can recognise these, especially the Honeycomb and Chain corals, and therefore wlien these arc found any one may identify Paleozoic or even Silurian rocks. Hydrozoa. — In still, sheltered bays, with fine mud- bottom, are now found, attached to sticks, logs, or shells, fine, feathery things, which look like finely dissected sea- weed or sea-moss. They are, indeed, gathered by ama- 276 HISTORICAL QEOLOQY. teur collectors and pressed as sea- weeds. If they be ex- amined with a lens, they are seen to be composed of Fig. 174. Fig. 175. Figs. 174, 175.— Favositid and halysitid corals : 174. Columnaria alveolata. (After Hall.) 175. Halyeites cateuulata. (After Hall.) hollow, branching stems, set on one or both sides with hollow cups, each containing an animal which, if kept undisturbed in sea-water, quickly spreads its thread-like tentacles. These are the Hydrozoa of the present day (Figs. 176, 177, 178). Now, in fine Silurian shales, which Fig. 176. Fig. 177. Fig. 178. FiGB. 176-178.— Living hydrozoa : 176 and 177. Sertularia. 178. Phnniilaria. were once fine mud, are found impressions of animals probably similar to these. They are called Graptolites. Whatever they be, they are easily recognized and wholly PALEOZOIC ROCKS AND ERA. 277 characteristic of Silurian, and any one may identify Silu- rian by means of them (Figs. 179, 180). 179. Fig. 180. 180.— Silurian hydrozoa : 179. Diplograptus pristis. (After Nicholson.) 180. Graptolites Clintonensis. (After Hall.) Ecliinoderms ; Crinoids. — At the present time^ if we leave out sea-cucumbers (Holo- thurians), because, having no shells, they are not preserved as fossils, Echinoderms are of three orders : 1. Echinoids, or sea-ur- chins ; 2. Asteroids, or star-fishes ; and, 3. Crinoids. The first two are /ree-moving, the last is stemmed. The first two are now very abundant, the last rare. But in Silurian times it was the reverse. The Echinoids did not exist at all, the Asteroids were rare, but the Crinoids extremely abundant, though, of course, of species and genera wholly differ- ent from any now existing (Fig. 181). It is well to ob- serve that the crinoid is a lower form than the other two, Fig. 181. — Living crinoid. Pentacrinus caput-medusae. 278 lUlSTORICAL GEOLOGY. as is shown by the fact that some free echinoderms have stems ill the early stages of life, and afterward throw them off and become free. Deseriptioii of ji Crinoid. — A crinoid has a pear- shaped body, containing the viscera, set npon a jointed Fig, 182. Fig. 183. Fig. 184. Figs. 182-184.— Silurian crinoids : 182. Heterocrinus simplex, (After Meek.) 183. Pleurocystitis squainosus. 184. Lepadocrinus Gebhardii. stem, with mouth on the top of the pear, sometimes sur- rounded by many plumose arms (Fig. 182), sometimes with few simple arms (Fig. 183), sometimes with no arms at all (Fig. 184). Range in Time. — We have said that stemmed echino- derms or crinoids continue from earliest times until noio (though the species and genera change repeatedly), but in diminishing numbers. The free echinoderms, on the contrary, have been constantly increasing. If, then, A B (Fig. 185) represent the course of geological time, and the parallelogram the equal abundance of echinoderms throughout, then the shaded portion below the diagonal would, in a general way, represent the constantly decreas- ing stemmed, and the unshaded space above the diagonal PALEOZOIC ROCKS AND ERA, 279 "the constantly increasing free forms. But are there any characters by which we may easily recognize those pecu- 'PALEOZOIC- SILURIAN. DEVON'^ CAfiBONiP liiiiiil! i^in BLASTIDS mln^^.i:, nniinHiliiiiiiiiniTil 11 STEMMED Fig. 185.— Diagram showing distiibution in tiine of crinoids. liar to the Silurian ? There are. Crinoids are subdi- vided into three main groups, viz. : 1. Crinids, or plumose-armed crinoids (Fig. 182) ; 2. Blastids (Fig. 242, page 316), or bud-crinoids ; 3. Cystids (Figs. 183, 184), or bladder-crinoids. The crinids are not character- istic of Silurian, nor even of Paleozoic ; the blastids are characteristic of Paleozoic, though not of Silurian ; the cystids are characteristic of Silurian alone. This is rep- resented by subdivisions of the shaded space in Fig. 1 85, in relation with the subdivision of the Paleozoic. Mollusks ; Bracliiopods. — Bivalve shells are divided into two great groups, viz. : 1. Common bivalves {Lamel- lihranclis) ; and, 2. Lamp-shells, or Bracliiopods. At present, the former are extremely abundant, and the lat- ter rare. The reverse was true in Silurian times. The distribution in time of the two kinds may be roughly Fig. 186.— Diagram showing the general distribution, in time, of brachiopods and lamellibranclis. represented l)y the diagram (Fig. 180). Now. bracliio- pods are very different from, and much lower than, ordi- nary bivalves. Lamellil)ranchs have a right and left valve — right and left gills, etc. ; in brachiopods the 280 HISTORICAL GEOLOGY. valves are upper and lower, or a back-piece and a breast- plate. The deeper and more projecting valve is the ven- tral. From the point of this valve comes out a fleshy cord, by which it is attached. It is this which gives it the name of lamp-shells, on account of its resemblance to the ancient lamp (Fig. 187). A large portion of the in- terior of the shell is occupied by long, spiral, fringed arms. It is these which give the name of brachiopod (arm-feet), although they are really gills. These are attached to complex, and sometimes spiral, bony pieces. Fig. 188 is a living brachiopod, showing structure. These shells Fig: 187.— Living bra- chiopod. Side view. A living brachiopod Terebratnla flavescens. are so extremely abundant in Paleozoic, especially Silu- rian rocks, that these rocks may often be identified by them. In Figs. 189, 190, we give two of the most common forms. Are there any characters by which Silurian brachiopods can be easily distinguished ? Not by the un- trained eye. Yet the square shoul- dered forms, like those figured here, are very characteristic of Paleozoic, tliough not of Silu- rian. L.aiiiellibraiiclis and Gasteropods. — Tlie ordinary bivalve-shells {Lamellibranchs), and the univalves or gas- FiG. 1S9. -Silurian brachio pods ; Orthis Davidsonii. PALEOZOIC ROCKS AND ERA. 281 teropods, like conchs, whelks, etc., are also found; but, in order to avoid confusing the mind with too many de- FiG. 190.— Silurian brachiopods : Spirifer CumberlandiiB— a, ventral valve ; 6, suture. tails, we shall pass over these and confine ourselves only to the most striking and characteristic forms. Ceplialopods ; Ortlioceratite. — The great class of Cephalopods, including now the squids, cuttle-fishes, and nautilus, were represented, in Silurian times, by 'a very remarkable family called Ortlioceratite (straight-horn). The appropriateness of the name is recognized by the figures on page 282 (Figs. 192, 193). Cephalopods now are, some of them, naked (squid and cuttle-fish), and some shelled (nautilus). When they have a shell, the shell is cham- hered. The animal lives in the outer part, and all the chambers are empty, full of air only, and connected with the animal by a mem- branous tube called the si- phon-tube or siphuncle (Fig. 191). Now, at the present time, nearly all cephalopods are naked. Only one genus of the shelled kind remains, viz., the Nautilus. In Silu- rian times, and indeed long after, there were no naked ones. Only tlie shelled kinds existed. The naked Fig. 191.— Pearly muitiluf (Nautilus pompilius) : a, mantle ; 6, its dor- sal fold ; c, hood ; o, eye ; i, ten- tacles ; /, funnel. 382 HISTORICAL GEOLOGY. kinds are the higher. Again, notv, and throughout all later geological times, all the shelled cephalopods were coiled, like the nautilus. But Paleozoic, and especially Silurian times, were characterized by the abundance of long, tapering, straight, chambered shells. These are the Orthoceratites. They are entirely characteristic of Fig. 192. Fig. 193. Fig. 194. Figs. 192-194.— Silurian cephalopods : 192. Orthoceras multicameratum. (After Hall.) 193. Orthoceras Duseri. (After Hall.) 194. Restoration of orthoceras, the shell being supposed to be divided vertically, and only its upper part being shown — a, arms ; /, muscular tube (" funnel ") by which water is expelled from the mantle-chamber ; i-, air-chambers ; s, siphuncle. (After Nicholson.) Paleozoic, most abundant in the Silurian, and easily rec- ognized by any one. We give figures of a few (Figs. 102-194), and an attempted restoration of the front part of the shell containing the animal. PALEOZOIC ROCKS AND ERA. 283 Orthoceratites were extremely abundant in Silurian times, and, in some cases, reached an enormous size. In the Silurian of the Western States, specimens have been found which were eight to ten inches in diameter, and over fifteen feet long. They were the most formidable animals of these early seas. They came in with the Pri- mordial, reached their maximum in the Mid-Silurian, but continued through the Paleozoic, and then passed away forever. Although the straight, chambered shells were by far the most abundant, yet the coiled kinds were also found. Crustacea; Trilobites. — Passing over the worms, as being of less importance, although their borings, their tracks, their calcareous tubes, and even their teeth, have been found, we come at once to perhaps the most abun- dant and characteristic of all Paleozoic forms — Trilobites. Description. — The shell of these curious creatures was convex above and » flat or, more probably, concave below (Fig. 196). It was divided, like most crustaceans, into many movable joints, but several front joints were always consolidated to make a lucMer, or head-shield, and usually, but not always, several hind joints were con- solidated to form a pygidium, or tail-shield. Longitudi- FiG. 195.— structure of the eye of trilobites: a, Dalmania pleuropteryx; ft, eye slightly magnified; c, eye more highly magnified. (After Hall.) 284 HISTORICAL GEOLOGY. nally, the upper surface of the shell was divided by two depressions into three lobes (hence the name). On each side of the head-shield, in position exact- ly as in the hing-crah (Li- mulus), were placed the eyes; and, strange to say, we find the eye, even at this early time, al- ready a com- plex structure well adapted to form an image (Fig. 195). Eecently have been discovered jointed legs, I, 1, each with two branches, one for crawl- ing and one for swimming; and also slender, many-jointed antennse, a, a, -Restoration of upper side of calymene. (After i,-v^ .^^^^^ ^„„^ Beecher.) ^^^^ many crus- taceans of the present day (Fig. 196). They had the habit, which many crustaceans now have, of folding themselves so as to bring head and tail together in front, as shown in Fig. 199. In the following figures (Figs. 197, 198) we give some examples of Silurian Trilobites. r PALEOZOIC ROCKS AND ERA. 285 Trilobites are found in great numbers, of almost infinite variety of form and markings, and of size varying from a fraction of an inch to twenty inches in length (Fig. 197). Fig. 197. Fig. 198. Fig. 199. Figs. 197-199.— Silurian trilobites: 197. Paradoxides Harlifai, x J (after Rogers), American. 198. Calyraene Blumenbachii. 199. Salme in folded condition. They come in with the earliest Primordial, reach their maximum in Mid-Silurian, but continue through Pala3- ozoic, and pass out forever. They are, therefore, entirely characteristic of the Paleozoic, and especially abundant in Silurian. Although belonging to a distinct order, different from any now living, yet they were more nearly / 286 HISTORICAL GEOLOGY. allied to the horseshoe crab (Limulus) than anything else. They are so abundant, so well preserved, and so easily rec- ognized, even by the untrained eye, that they are a very valuable means of identifying Paleozoic, and especially Silurian, strata. Anticipations of the Next Ag-e. — The most highly organized and most powerful animals of Silurian times were undoubtedly the Orthoceratites and the Trilobites. The Orthoceratites especially were the tyrants and scaven- gers of those early seas ; yet, in the uppermost Silurian are found a few insects, scorpions, and cockroaches, and a iQVf fishes similar to forms far more abundant in Devonian. It is better, therefore, to regard these as anticipations. Section^ III. — Devoniai^ System. The Age of Fishes. Rock-System ; Name. — The name Devonian was given to these rocks because first studied with success in Devon- shire. In Scotland they were called Old Eed Sandstone, by Hugh Miller. In England it is often unconformable on the Silurian, but in the Eastern United States, as al- ready stated, the Paleozoics are conformable throughout. Nevertheless, even in America there is a great change of life-forms at this point of time ; and, moreover, the first introduction here of a new reigning class — viz., fishes, and a new great department of animals — viz., Vertebrata, or backboned animals, is a prodigious advance, and en- titles this to be considered a distinct age. It is well to note, however, that some anticipations of this great ad- vance are found in the Silurian. Area in the United States. — By examining the map on page 272, it will be seen that in general the Devonian rocks border on the Silurian area on the south and west and extend far south in the middle region. In the Rocky Mountain region there are considerable areas of Devonian, but their limits are too little known to be described. Paleozoic rocks and era. 38'}' Physical Oeograpliy. — The land during early Devo- nian times was the Archaean area, increased by the addi- tion of the Cambrian and Silurian area, the Devonian area being then of course sea-bottom. In the middle of the Devonian Sea there was a large island of Silurian rocks occupying what is now mid- Ohio and running down through mid-Tennessee. At the end of the Devonian, the Devonian area was exposed as land and added to that previously existing. Subdivisions. — The American Devonian is subdivided into at least four groups of strata representing four periods, as shown in the schedule. We shall, however, neglect these subdivisions in our general account of the life- system : 4. Chemung period. 3. Hamilton period. 2. Corniferous period. 1. Oriskany. Life- System of Devonian. — Plants. In Silurian times, with the exception of a very few small vascular cryptogams allied to club-moss, we found nothing higher than f ucoids. In addition to these, now, for the first time, land-plants become conspicuous. Here, for the first time, we have a ivMQ forest vegetation. The character of the trees of this first forest is a question of the highest interest. The Devonian forests consisted of the highest cryptogams, vascular cryptogams, and the lowest phenogams, Gymnosperms. More explicitly, there were Ferns, Lepidodendrids, and Sigillarids (gigantic club-mosses), and Catamites (gigantic Equisetce) among vascular cryptogams : and Conifers allied to the yews among gymnospermous phenogams. We shall not describe these now, since they are all much more abundantly represented in the Carboniferous. 288 BISToniCAL GEOLOGY. We shall therefore dismiss them for the present with one or two remtirks. 1. In Nova Scotia, in direct connection with the plant- beds, have also been found many fossil forest-grounds. These are marked by dark seams with stumps and roots in place just as the trees grew. In some cases, also, thin seams of coal lie upon the forest-grounds. Thus, there- fore, we have here in the Devonian an anticipation not only of coal vegetation, but also of the conditions neces- sary for the formation and preservation of coal. 2. We have here a somewhat sudden appearance of land-plants, as if they came without progenitors. But we must remember that we have a feeble beginning of land-plants in the Silurian. It seems probable that in the Devonian we had more favorable conditions, and therefore a rapid development of new forms. Animals, If we bear in mind what we said about Silurian ani- mals, it will be necessary here only to note the great changes, i. e., what old forms pass out, what new forms come in, and what advances are made in the progress of life, dwelling only on the great characteristic of the age, viz., \)iiQ fishes. Fig. 200. Fig. 201. Figs. 300, 201.— Devonian corals : 200. Favosites hemispherica. 201. Zaphrentis Wortheni. (After Meek.) PALEOZOIC MOCKS ANt> EUA, 289 Radiates. — Among corals, the characteristic Silurian chain-corals {Halysitlds) are gone, but the other two forms remain, with different species (Figs. 200, 201). The graptolites are gone, as also the Cystidean crinoids, but the blastids or bud-crinoids are now far more abun- dant, though they reach their maximum only in the Carboniferous (Fig. 242, page 316). Bivalves and Univalves. — Brachiopods still continue in great numbers, of the characteristic Paleozoic, square- shouldered forms (Fig. 202), and both Lamellibranchs Fig. 202. Fig. 203. Fig. 204. Fig. 205. Pigs. 202-205.— Devonian brachiopods . 202. Spirifer perextensus. (After Meek.) Devonian lamellibranchs and gasteropods : 203. Ctenopistha antiqua. (After Meek.) 204. Lucina Ohioensis. (After Meek.) 205. Orthonema Newberryi. (After Meek.) and Gasteropods (univalves) are now more abundant. It is well to note that fresh-water and land forms are now for the first time introduced. Cephalopods. — The Orthoceratites still continue in Devonian times, though in greatly diminished number and size ; but we note here a great advance in the intro- duction of a netv form characteristic of this and the Carboniferous, viz., the Ooniatites (angled stones), so called because the suture or junction of the partition with the shell is angled instead of simple (see Fig. 246, page Le Conte, Geol. 19 290 HISTORICAL OEOLOQY. 317). It should be remembered that this is the first introduction of a family {Ammonite family) which in Mesozoic times became extremely abundant. The family is characterized by the dorsal position of the siplion-tuhe and the complexity of the suture. We shall hereafter trace the increasing complexity of the suture. It only begins in the Goniatite. Crustacea. — Trilobites still continue under new forms (Fig. 206), but in greatly diminished number and size. They have passed their prime. Insects. — Insects are now, and at all previous geologi- cal times have been, closely related to land vegetation. Fig. 207. Fig. 2J6. Figs. 206, 207.— 206. Devonian trilobite and insect: Dalmania punctata; Europe. 207. Wing of platepheraera antiqua ; Devonian, America. (After Dawson.) The first conspicuous land vegetation is found in the Devonian, and in connection with this vegetation are found also the first known insects (Fig. 207). These first insects were most nearly allied to cockroaches and dragon- flies — in fact, a connecting link between these orders. PALEOZOIC ROCKS AND ERA. 291 In some a chirping organ has been found. This shows that an auditory apparatus was already developed. Although these first known insects are among the lower orders of the class, and also are connecting links between two such lower orders, yet their somewhat perfect devel- opment indicates that we must look for the very first insects still lower, i. e., in the Upper Silurian. They have been recently found there. Fishes. — The introduction of fishes must be regarded as a great step in the progress of life, for it is the begin- ning not only of a new and higher class, but of a new great department and the highest, viz., Vertebrata. They commenced first in the lowest Devonian or perhaps even in the uppermost Silurian, few in numbers, small in size, and of strange, un-fish-like forms, but soon developed in size and numbers until these early seas swarmed with them, and they quickly became the rulers of the age. The previous rulers, Orthoceratites and Trilobites, there- fore diminish in number and size, and thus seeh safety in subordination. As examples of the great size of Devonian fishes, we mention a few. The Onchyodus (claw-tooth) had jaws eighteen inches long, armed with teeth two or more inches long ; the Dinichthys (huge fish) was fifteen to eighteen feet long, three feet thick, and had jaws two feet long, armed with curious blade-like teeth. These are from America. The Asterolepis (star-scale) of Europe is believed to have been twenty feet long, and of propor- tionate dimensions. We must not imagine, however, that these fishes were at all like most common fishes of the present day. Neglect- ing some rare and unusual kinds, fishes may be divided into three great orders, viz., 1. Teleosts (complete bone); 2. Ganoids (shining) ; and 3. Elasmohraiichs (plate-gills). The Teleosts include all the ordinary llslies : examples of Ganoids are found in gar-fish and sturgeon ; of Elasmo- branchs, in sharks, skates, and rays. At the present 292 HISTORICAL GEOLOGY, time, nine-tenths of all fishes are Teleosts, but in Devo- nian times all the fishes were Ganoids and sharks^ espe- cially the former, though differing in species and genera from Ganoids and sharks of the present day. But we must give some figures of these strange Devonian fishes before discussing their affinities any further. Description of Some Devonian Fishes. — The Ceph- alaspis (head-shield. Fig. 208) was a small fish, of very Fi(} 208. Figs. 208, 209.— Devonian fishes— Placoderms : 208. Cephalaspis Lyelli. (After Nich- olson.) 209. Pterichthys cornutus. (After Nicholson.) curious shape, with mouth beneath the head-shield. The Pterichthys (winged fish, Fig. 209) was so completely in- cased in bony plates that it must have swum mainly by means of its wing-like anterior fins. The mouth was also beneath. The Coccosteus (berry-bone, Fig. 210) was cov- ered with bony plates in front parts, but the tail was usable for locomotion. The Dinichthys (Fig. 211) was a PaljJ(jZ(jic bocks and era. 293 liuge fish, sometimes eighteen to twenty feet long, very abundant in the Devonian of Ohio. Like the Coccosteus, the anterior tail was covered with broad, bony plates. Fig. 210. Fig. 211. Figs. 210, 211.— Devonian fls^hes— Placoderms : 210. Coccosteus decipiens. (After Owen.) 211. Diuichthys. (After Dean.) The Osteolepis (bony scale. Fig. 212) was covered with a complete coat-of-mail of rhomboidal bony scales, like the gar-fish and polypterus (Fig. 217) of the present day. The Diplacanthus (double spine, Fig. 213) is more fish-like in form, but is also covered with rhomboidal bony scales. We draw attention to the shape of its tail. All these are Ganoids. The sharks, on account of their cartilaginous skeleton and imperfect scales, are known chiefly by their bony spines and by their teeth. A restoration of a Devo- nian shark from Ohio is given in Fig. 214. By examination of the figures, it is seen that Devonian Ganoids are, some of them, wholly or partly covered with large, immovable, bony plates (Figs. 208-211) ; others with smaller, rhomboidal, bony scales (Figs. 212, 213). The former are called Placo-ganoids (plate-ganoids), or 294 HISTORICAL GEOLOGY. Placoderms (plate-skiu) ; the latter, Lepido-ganoids (scale- ganoids). Now, the Placo-ganoids are characteristic of the Devonian^ and the largest Devonian fishes, such as Fig. 212. Fig. 214. Figs. 212-214.— Devonian fishes— Lepido-ganoids : 212. Osteolepis. (After Nichol- son.) 213. Diplacanthus gracilis, (After Nicholson.) Sharlss : 214. Cteuacan- thus vetustus, spine reduced. (After Newberry.) the Dinichthys and Aster olepis, belong to this family. The Lepido-ganoids continued after the Devonian, and are still represented by gar-fishes, etc. The sharks of the Devonian belong, all of them, to a family now almost extinct, called Cestracionts {sharp tool, referring to the spine). AflSnities of Devonian Fishes. — There are no living representatives of the Placo-ganoids, but there are such of the Lepido-ganoids. We herewith give figures of those modern fishes which are most like the Devonian fishes. PALEOZOIC ROCKS AND ERA. 295 The first is an Australian fresh-water fish, the recently discovered Ceratodm (horn-tooth) (Fig. 215). The sec- ond, Lepidosiren (scale-siren), is a very curious animal, intermediate between fish and reptile, found in South America and Africa (Fig. 216). The third is the gar- fish, Polypterus (many fins), from the Nile (Fig. 217). The fourth is the only living representative of cestraciont sharks — the Cestracion of Australian seas (Fig. 218). Bearing- on Evolution. — It is a curious fact that these fishes, which are most nearly allied to Devonian Fig. 215. Fig. 216. Figs. 215, 216. — Nearest living allies of Devonian fishes : 215. Ceratodus Fosterii, x Jj. (After Gunther.) 216. Lepidosiren. fishes, are by no means low in the scale, but, on the contrary, are, in some respects at least, very high. But one thing is vei:y noteworthy, viz., that they all have amphibian characters united with fish characters — they are all connecting links between fish and amphi- hian. For example, it is seen that the vertebral col- umn in these, and still more in their Devonian allies, runs far into, often to the end of, the tail-fin. The tailfin is vertehrated. The tail vertebrae are finned on the sides. This is universal in Devonian fishes. Again, 296 HISTORICAL OEOLOGY. it is observed that in many the paired fins are curiously formed — they are a sort of \\mb^ fringed with fin. Xow, a large number of Devonian fishes (-Fig. 212) have this style of paired fins. In the third place, all the living Fig. 2ir. Fig. 218. Figs. 217, 218.— Nearest living allies of Devonian fishes : 217. Polypterus. 218. Ces- tracion Phillippi (a living cestraciont from Australia). Ganoids given above (Figs. 215, 217) have a more or less perfect lung, and supplement their water-breathing with air-breathing, in the manner of some amphibian reptiles. It is almost certain that the same was true of Devonian Ganoids. And yet, with all these reptilian characters, all Devonian fishes had cartilaginous skeletons like the embryos of Teleosts. We wish now to take advantage of these facts to an- nounce a very general law. The first introduced exam- ples of any family, order, or class, are not typical forms of that family, order, or class, but intermediate forms or connecting links With other families, orders, etc. From such intermediate forms or connecting links have been PALEOZOIC ROCKS AND ERA. 297 afterward developed the more typical forms. To illus- trate : The first fishes were not typical fishes, but con- necting links between fish and amphibian, and from this intermediate form, as from a trunk, true fishes and amphibians were afterward separated and developed as branches. Such intermediate forms we shall hereafter call generalized forms, and the more typical forms into which they seem to be afterward developed, specialized forms. We shall find many illustrations of this law as we proceed. Apparent Suddenness of the Appearance of Fishes. — At a certain time fishes seem suddenly to appear, as if they came without progenitors. But we must re- member that the very lowest forms of fishes have neither bony skeleton nor scales, and their remains are not likely to be preserved. We may yet find evidences of such far down in the Silurian. Nevertheless, there can be little doubt that conditions were favorable for the development of fishes about the beginning of the Devonian, and there- fore the steps of development were exceptionally rapid at that time. Section IV. — Carboniferous System. Age of Acro- GENS AND Amphibians. Subdivisions. — The Carboniferous age is subdivided into three periods : 1. Sui-carhoniferous ; 2. Carbonifer- ous proper, or coal-measures ; 3. Permian. The first may be regarded as the preparation, the second the culmina- tion, and the third the transition to the Mesozoic. The whole carboniferous strata in Nova Scotia is 16,000 feet thick, in Wales 14,000 feet, in Pennsylvania 9,000 feet. The sub-carboniferous strata are mostly limestones ; those of the coal-measures mostly, thougli not wholly, sands and clays. Tlie sub-carboniferous are marine de- posits, the coal-measures mainly fresh-water deposits, 298 HISTORICAL GEOLOGY. The fossils of the former are, therefore, marine animals ; those of the latter mainly land-plants, and fresh-water and land animals. In both Europe and America the sub- carboniferous underlies the coal-measures and outcrops around, and thus forms a penumbral margin about the black areas representing coal-fields on geological maps (see Fig. 169). After this brief comparison and contrast, we shall now concentrate our attention on the coal-measures, because all the characteristics of the Carboniferous age culminate there. In speaking of the fauna, however, we shall take the two together. The Permian will be treated as a transition to the Mesozoic. Carboniferous Proper — Rock-system, or Coal- Measures. Name. — The Carboniferous period is but one of three periods of the Carboniferous age. The Carboniferous age is but one of the three ages of the Paleozoic era. The Paleozoic era is but one of the four great eras, exclusive of the present. The Carboniferous period, therefore, is but a small fraction, certainly not more than one twentieth to one thirtieth of the recorded history of the earth. Yet, during this period were accumulated, and in its strata were preserved, and are now found, nine tenths of all the coal used by man. The name carboniferous, for the period, and coal-measures, for the strata, is surely, there- fore, appropriate. Thickness of the Strata. — Although so small a por- tion of' the whole strata of the earth, these coal-measures are often, locally, of great thickness. In Nova Scotia the coal-measures, exclusive of the sub-carboniferous, are 14,000 feet thick, in Wales 12,000, in Arkansas 25,000 (Branner), and in AVest Virginia 5,00(1. Mode of Occurrence of Coal. — Sucli being tlie tliick- ness, it is evident that but a small portion is coal. In PALEOZOIC ROCKS AND ERA. 299 fact, the cojil-measures consist of alternations of sand- stones, shales, and limestones, like other formations ; but interbedded with these are also seams of coal and beds of iro7i-ore. These five kinds of strata alternate with each other, and are each repeated many times, but without any regular order, as shown in Fig. 219, which is an ideal column from a coal-field. Thus, the strata of a coal-field may be likened to a ream of sheets of five colors, but arranged without order. Only this may be said, that beneath every coal-seam there is al- ways a thin layer of clay, called the under - clay, and above is usually, but not always, a shale, called the hlach shale or roof- shale. It is a rich coal-measure in which we find one foot of coal for fifty feet of rock. Subsequent Changes. — The strata of coal-measures, like all other strata, were horizontal when first laid down ; but, like other strata also, they have been elevated, and tilted and folded and crumpled and broken and faulted, especially in moun- tain-regions. And in all cases, whether horizontal (Fig. 221) or folded (Fig. 220), they have been largely carried away by erosion, and the strata left outcropping on the surface (Figs. 220, 221), and often in isolated patches. Since coal-seams, like other strata, are broken and faulted, it is very important to remember the law of slip mentioned on page 232. Thickness and Number of Seams. — The thickness of seams varies from a few inches to many yards. The mammoth seam of Pennsylvania is over one hundred feet thick. The best thickness for easy working is about six Fig. 219.— Ideal sec- tion, showing alter- nation of different kinds of strata : Ss, sandstone ; Sh, shale ; I, limestone ; », iron : and c, coal. 300 HISTORICAL GEOLOGY. to ten feet. The number of seams in a sjngle field may be a hundred or more, and their aggregate thickness may Fig. 220.— Panther Creek and Summit Hill traverse. (After Dacldovv.) Fig. 221.— Illinois coal-field. (After Daddow.) be, in some cases, one hundred to one hundred and fifty feet of solid coal. Coal-Fields of the United States. — In the map on page 272 the coal-fields of the United States belonging to this period are represented in black. It is seen that there are four of these : 1. The Appalachian coal-field, probably the richest in the world. In a general way it may be said to cover the western slope of the Appalachian chain from Pennsylvania southward. It covers an area of 60,000 square miles. 2. TJie central coal-field. This covers nearly the whole of Illinois, the western portion of Indiana, and northwestern Kentucky, and its area is 47,000 square miles. 3. Tlie great Western coalfield. This cov- ers southern Iowa, northwestern Missouri, eastern Kan- sas, the Indian Territory, western Arkansas, and north- ern Texas. Its area is no less than 78,000 square miles. 4. The Michigan coal-field. This occupies an area of PALEOZOIC ROCKS AND ERA. 301 6,700 square miles in the center of the Michigan Peninsula. Besides these, there is a small area of coal of little value in Rhode Island, and a fine coal-field of 18,000 square miles accessible to us in Nova Scotia. Of the 192,200 square miles of coal within the limits of the United States, 120,000 square miles are estimated as workable. It may be said with confidence that there is no country in the world so liberally supplied with this great agent of modern civilization as our own. Appalachian Central Great Western, Michigan . . . . Rhode Island . . Nova Scotia. Total 00,000 47,000 78,000 G,700 500 192,200 18,000 210,200 Origin of Coal and of its Varieties. There can be no doubt that coal is of vegetable origin. All portions of a coal-seam, even the most structureless to the naked eye, when properly prepared, reveal their vegetable structure to the microscope (Figs. 222, 223). Fig. 222.— Section of anthracite : a, natural size ; b and C| magnified. (After Bailey.) Fig. 223.— Vegetable stracture in coal. (After Dawson.) Varieties of Coal. — Assuming the vegetable origin of coal, how do we account for the varieties ? These varie- ties are of three kinds : 1. Varieties depending on degrees 302 HISTORICAL GEOLOGY, of purity ; 2. On degrees of bituminization ; 3. On the relative proportion of fixed and volatile matter. 1. Varieties depending' on Deg^rees of Purity. — Coal consists of combustible and incombustible matter, or ash. The combustible matter is organic, the ash min- eral. Now, the relative proportion of these varies in every degree. The purest coal may contain only 1 to 2 per cent, of ash ; but coal may contain 5 to 10 per cent., 20 to 30 per cent., 50 to 60 per cent., and so on to 90, 95, 99 per cent. ash. If a coal contains not more than 5 per cent, ash, it is probably pure, i. e., its ash is wholly due to ash of original vegetable matter ; but if it contains more than 10 per cent., it is certainly impure, the excess being due to mud deposited with the vegetable matter. 2. Varieties depending* on tlie Degrees of Bitu- minization. — Coal may be pure, and yet imperfectly bituminized. Such are lignites, Irown coal, and the like. This depends mainly on age, the oldest coals being most completely changed. 3. Varieties depending- on the Relative Propor- tion of Fixed and Volatile Matter. — In pure and per- fect coal there are still varieties depending on the relative amount of fixed carbon and volatile hydrocarbon, and it is mainly this which produces the varieties of good coal, and determines its various uses. If the coal contains only 5 to 10 per cent, volatile matter, it is called anthracite, which is a hard, lustrous variety, breaking with a con- ch oidal fracture, and burning with very little blaze, but great heat. If it contains 15 to 20 per cent, of volatile matter, it is called semi-bituminous, or steam-coal, because of its excellence in rapid formation of steam. It burns with a long blaze, but does not cahe. If it contains 30 to 40 per cent., it is ordinary bituminous caking coal ; if 50 per cent., or more, higlily bituminous, fat, or fusing coal. In this series we might well put graphite, or plum- bago, above anthracite ; for graphite consists of carbon PALEOZOIC ROCKS AND ERA, 303 without iiuy volatilo matter, and, although it is not oalled coal, because incombustible, yet it is but the last term in the above series of varieties. Cause of these Varieties. — Vegetable matter decay- ing out of contact with air, i. e., beneath water or buried in mud, loses a large portion of its material in the form of gases (CO,, CH,, and H,0). These (CO, and CHJ are the gases which escape in bubbles when we stir the bottom of a stagnant pool in which plants are growing. They are also the gases which are constantly escaping in every coal-mine, and form the deadly choice-damp and the still more dreaded ^re-fi?«7??j!? of the mines. Now, the relative proportion in which these are given off determines most of the above varieties. Anthracite and graphite may be regarded as metamor- phic coals. The reasons for so thinking are mainly the following : 1. Coal is often made locally anthracitic by a lava-flow or dike. 2. In the same coal-field, wherever the strata are crumpled and metamorphic, as in eastern Pennsylvania, the coals are anthracitic ; and where the strata are flat-lying and unchanged, as in Ohio, the coal is bituminous. Plants of the Coal. In no other strata have the remains of plants been found in so great abundance and variety as in the coal- measures. We could expect nothing else when we re- member that a coal-seam is a mass of vegetable matter, and that, on account of their economic value, these seams are continually explored. The remains of plants are found in the form of leaves, flattened stems and branches, and sometimes fruits, in connection with the black roof- shale ; and as stumps and roots, in connection with the under-day or floor of the seams. Principal Kinds. — The plants belong mainly to four or five great orders, viz.. Conifers and probably Cycads, 304 HISTORICAL QmhOQY, among gymnosperms, and Ferns, Cliih-mosses, and Equi- setcB, among vascular cryptogams. These orders were anticipated in the Devonian, but culminate here. 1. Conifers and Cycads. — These are found as leafy Fig. 224. — Araucarites gracilis, reduced. (After Dawson.) Fig. 225.- Cordaiics. (Restored by Dawson.) Fig. 227. Fig. 228. -Fruits of coal-plants, probably conifers : Cardiocarpon. (After Newberry and Dawson.) PALEOZOIC ROCKS AND ERA. 305 branches (Fig. 224), as scattered leaves, like those in the restored tree (Fig. 225), as nut-like fruits (Figs. 226-228), near the top of the coal-seams, and sometimes as drift- logs in the sandstones, interstratified with the coal. The trunks are known to be conifers by the microscopic structure of the wood, the cells of which are marked with circular disks on lon- gitudinal section (Fig. 229), and on cross-section the wood is destitute of pores. Now, what kind of coni- fers have such leaves and fruit as these ? None but the yew family. All of these have plum- like fruit with nut-like seeds, and many of them have broad leaves (Fig. 230). The cordaites (Fig. 225) has been found with trunk sixty to seventy feet Fig. 229.— Longitudinal section of wood of a living conifer, niagnilied. Fig. 230.— Living broad-leaved yews. Le Contr, Geol. 20 306 HISTORICAL GEOLOGY. long, crowned with broad leaves, and with a spike of fruit. It is probably a Cycad, or else a broad-leaved conifer like the ginkgo. 2. Ferns. — These are the most abundant, but not the largest, plants of the coal. About one half of all the known species of coal-plants are ferns. They are often beautifully preserved, large, complex fronds spread out and pressed, as if between the leaves of a botanist's her- barium, with even the niicroscopic veining of leaflets visible. They are known to be ferns — 1. By their large, complex fonds (Fig. 231). 2. By the peculiar veining of Fig. 231. Fig. 232. Fig. 233. Figs. 231-233.— Coal -ferns: 231. Megaphyton, a coal-fern restored. (After Dawson.) 232. Callipteris SuUivanti. (After Lesquereux.) 233. Pecopteris Strongii. (After Lesquereux.) the leaves, characteristic of ferns (Fig. 232). 3. By the rows of spore-cases on the under surface of the leaves (Fig. 234). 4. In the case of tree-ferns, by ragged, ovoid marks, leaf-scars left by the fallen fronds. "VYe give a few figures of ferns of the American and French coal-measures. The remaining orders, viz., Lycopods (or club-mosses) and EquisetcB (horse-tails or scouring-rushes), are still more important, for two reasons : 1. They formed the principal mass of the coal. 2. They were very remarkable examples of generalized types or connecting links, and PALEOZOIC ROCKS AND ERA. 307 Fig. 234.— Dactylothe. ca dentata. (After Zeiler.) a, spore case enlarged. possess a high interest on that account. We shall treat of them under three heads, viz., Lepidodendridsy Sigilla- rids, and Calamites. 1. Lepidodendrids. — Every part of these has been found — roots, stems, branches clothed with leaves and tipped with fruit. They may be restored, therefore, with some degree of confi- dence. Imagine, then, a trunk two, three, or even four feet in diameter at its base where it joins the wide-spread- ing roots ; marked with regular rhomboidal figures, which are the leaf -scars (Fig. 236) ; branching widely, but not profusely ; the great branches, clothed with scale-like or needle- like leaves, stretching aloft, like uplifted hairy arms, to the height of fifty or sixty feet, and terminating in scaly cones like club-mosses. The most common findings are flattened stems with beautiful rhomboidal markings (Fig. 236), looking much like rhomboidal scales of a ganoid fish ; hence the name Lepido- dendron, or scale- tree. There can be no doubt that the Lepidodendron was a lyco- pod, or club-moss ; but its inter- nal structure, as well as its great size (club-mosses are now but a few inches, or, at most, a few feet high), ally it strongly with conifers. We may regard it, therefore, as a lycopod, with characters connecting it with conifers. Fig. 235 dodendron E«6toration of a Lepi- (By Dawson.) 308 HISTORICAL GEOLOGY. 2. Sigillarids. — The family name is taken from the type genus, Sigillaria. It includes Sigillaria and Sigil- larialike plants. The name Sigillaria is taken from the Pig. 236. — Lepidodendron Ym. 237. — Sigillarids : Sigil- Fig. 238. — Restora- modulatum. (After Les- laria reticulata. (After Les- tion of Sigillaria. quereux.) quereux.) (By Dawson.) seallike markings (sigilla, a seal) left on the trunk by the falling leaves (Fig. 237). They were the largest of all the coal-trees. Eoot, stem, branches, and leaves have been found. From these it is possible to reconstruct the general appearance of the tree. Imagine, then, a tree four or five feet in diameter at the base, with widely spreading roots ; the trunk regularly fluted like a Corin- thian column, and ornamented with vertical rows of seal- like impressions (leaf-scars), and towering to the height of one hundred to one hundred and fifty feet ; the top branchless, or else with only a few large branches clothed with grasslike or yuccalike leaves. The fruit is not PALEOZOIC ROCKS AND ERA. 309 known with certainty. The general appearance is given in Fig. 238. 3. Calamites. — These are so named from their jointed, reed-like appearance (cala- mus y a reed). They are usu- ally found in the form of flat- tened, jointed, and striated stems. They may be de- scribed as follows : Imagine a straight, hollow, jointed, tapering stem, one to two feet in diameter, and twenty, thirty, or forty feet high, ter- minating in a compact, cone- like fruit (Fig. 240), the joints striated, but the grooves in- terrupted at the joints by whorls of scale-like leaves, or else whorls of jointed, thread- like branches (Fig. 239) about the joints. From the basal joints come out thread-like roots. Fig. 239 is a restora- tion of its appearance. Now, all that we have said applies, word for word, to equisetae, or horse-tails, except the great size. But equi- set88 of the present day are small, rushlike or reedlike plants. Moreover, the internal structure of Calamites shows a close relation with gymnosperms, probably coni- fers. Conclusion. — The general conclusion, then, is that all the plants of the Coal, but especially the Lepidodendrids, the Sigillarids, and the Calamites, were remarkable gen- eralized types, connecting classes and orders now widely separated from each other — viz., the higher or vascular cryptogams with the lowest or gymnospermous pheno- FiG. 239.— Restoration Fig. 240.— Fruit of a Calamite. (Af- of Calamite. ter Dawson,) (After Heer.) 310 mSTORICAL QEOLOOY, gams. The two branches of the tree of life, cryptogam and phenogam, so widely separated now, when traced downward, approach and almost meet here in the Coal period. Mode of Accumulation of Coal. There has been much dispute on this subject, and it is still obscure. There are some things, however, which are reasonably certain. We shall give what is most certain, in the form of three propositions : 1. Coal has been accumulated in the presence of water. — This is indicated [a) by the nature of the plants, which are mostly swamp-plants ; {h) by the inter- stratified sands and clays, which were, of course, deposited in water ; but, more than all (c) by the preservation of the vegetable matter, which would have entirely disinte- grated and passed off, as CO, and HjO, unless completely water-soaked. 2. Coal has been formed by accumulation of vegetable matter "in place" — i. e., where the plants grew — by annual decay of generation after generation, as we see now in peat-bogs and peat-swamps ; and not iy accumulation dy driftage, as we see in rafts. The evi- dence of this is complete. We shall only mention one fact, which is demonstrative : The under-day of every coal is full of stumps and roots in position as they grew. Every under-clay is an old fossil forest-ground, or rather swamp-ground. Imagine, then, an old coal-swamp, with its clay bot- tom full of dead stumps and roots, with its accumulation many feet deep of pure peat, with its surface covered with late-fallen leaves, broken branches, and prostrate trunks, and the still growing vegetation shading all. ]S"ow, imagine this overwhelmed and buried by sediments, subjected to powerful pressure and slow change, and we have all the phenomena of a coal-seam, with its under- PALEOZOIC BOCKS AND ERA. 311 clay full of roots and its roof-shale full of impressions of leaves and flattened branches, etc. 3. Coal lias been accumulated at the mouths of rivers, and therefore subject to overflows and deposits of mud by the river, and to occasional incursions by the sea. This is proved by the alternation of river-sand and clay with marine deposits of limestone. It may be difficult to put these three propositions to- gether and form a clear picture of the precise manner of accumulation, and therefore there is still a large field for the play of fancy. Estimate of Length of the Coal Period, If the sands and clays of a coal-field have been accu- mulated by river-deposit, then we have a means of making a rough approximate estimate of the time embraced by the Coal period. It is true, agencies may have acted then at a different rate from now, but our estimate will be liberal. For this purpose we take the Nova Scotia coal-field, because the evidence of river-deposit is very strong in every part. It has been estimated that there were not less than 54,000 cubic miles of river sediment in the original field. Now, the Mississippi Eiver at present ac- cumulates one twentieth cubic mile per annum, and would therefore take twenty years to accumulate one cubic mile, and 1,080,000 years to accumulate 54,000 cubic miles. But, as already said (page 298), the Coal period is but a small fraction, certainly not more than one twentieth to one thirtieth, of the recorded history of the earth. Therefore, this recorded history can not be less than twenty to thirty millions of years. It is proba- bly much more. We only give this estimate in order to accustom the mind to the great periods of time with Avbich geology deals. 312 HISTORICAL GEOLOGY, Physical Geography and Climate of the Coal Period. Physical Geography. — The Paleozoic era was a time of gradual growth of the continent from the Archaean nucleus by successive additions, first of the Silurian, then of the Devonian, and now of the Carboniferous area. During Carboniferous times the form of the American Continent probably did not differ greatly from that repre- sented on page 349 (Fig. 303) as the Cretaceous conti- nent, except that the areas of coal-measures were not then permanent land, but were in an uncertain state, sometimes swamp-land, sometimes covered with river- sediment, sometimes covered by the sea. Although the continent had greatly grown, still we must imagine it as small and low compared with its present state. The same is probably true of other continents. Climate. — The climate was probably warm, very moist, very uniform, and the air loaded with CO^. The greater warmth and uniformity are shown by the fact that the plants are those of a tropical climate. Tree-ferns, arbo- rescent lycopods, etc., grew then with ultra-tropical lux- uriance, not only in now temperate regions, but in Mel- ville Island and Grinnell Land, 78°-80° north latitude. The prevalence of the great coal-swamps and the charac- ter of the plants are sufficient evidence of greater humid- ity. Finally, when we remember that the whole of the coal in the world represents so much carbon taken from the atmosphere, as CO, with return of the oxygen, we shall be convinced that the quantity of CO, in the air was greater and of oxygen less than now. It is probable, therefore, that in early geological times there were more moisture and CO, and less oxygen than now. This would make a paradise for plants, especially the lower orders, but would be unsuitable for air-breath- ing animals. There has been throughout all geological PALEOZOIC ROCKS AND ERA, 313 times a gradual purification of the air of its superabun- dant moisture by increase of the size and height of con- tinents, and of its superabundant CO^ by its withdrawal in many ways, but during the Coal period especially by the growth of plants and the preservation of the carbon as coal. In this process not only was the COg removed, but oxygen restored, and thus was the air prepared for the use of air-breathing, hot-blooded animals, such as birds and mammals, which were accordingly introduced soon afterward. Petroleum and Bitumen. We take up these here, not because they are peculiar to the coal-measures, for such is not the fact, but because they seem to have been formed from organic matter by a process similar to that of coal, and also because some think they are actually formed from coal by distillation. This, however, is not probable. If bituminous coal, or any organic matter, be heated red-hot, out of contact with air, the volatile matters are driven off, broken up, and recombined, and may be col- lected in a great variety of forms of hydrocarbons — some solid, as coal-pitch J some tarry, as coal-tar ; some liquid, as coal-oil; some volatile, as coal-naphtha; and some gaseous, as coal-gas. Now, a somewhat similar series of hydrocarbons is found in the earth and issuing on its sur- face : some solid, as asphalt, Alhertite, Grahamite, etc. ; some tarry, as bitumen ; some liquid, as petroleum ; some volatile, as rocTc-naphtha ; some gaseous, as the gas of hurning -springs. It is almost certain also that these are of organic origin. Mode of Occurrence. — Petroleum occurs in the strata much as water does, and the two are often associated. Like water, and with water, it is found in porous and fissured strata, such as sandstones and limestones, when these are covered with a stratum of impermeable shale. 314 HISTORICAL GEOLOGY, Like water, and with water, it often oozes on the surface as hillside springs. With water, it collects in fissures and cavities of all kinds, from which, through artesian wells, it issues, in some cases, in great quantities as fountains. But, unlike water, there is no great, continuous, peren- nial supply ; and also, unlike water, the force by which it spouts is not by hydrostatic pressure alone, but hydro- static pressure transmitted through the elastic force of compressed gas always associated with the oil. As gas, oil, and water are nearly always found together, these arrange themselves in the order of their relative specific gravities ; and therefore in a flowing well the water usu- ally appears only after the gas and oil are exhausted. The flow of oil wells is not perennial like water, because the oil is not continually re-supplied. The accumulation of ages is now being exhausted with a rapidity propor- tioned to the abundance of the flow. What is true of oil is much more true of gas. A gas well is necessarily short-lived. Age of Petroleum-bearing Strata. — Petroleum has been found in strata of nearly all ages, but under the two conditions of local abundance of the organic matter from which this substance is formed, and the absence of metamorphism, which always changes it into asphalt. At one time it was supposed to be characteristic of newer rocks, having been found in foreign localities, mostly in Tertiary strata. But in the Eastern United States it is confined to the Paleozoic rocks, while in California it is again found only in the Tertiary. The great petroleum area of the Eastern United States is the Paleozoic basin. In this basin it is found on sev- eral horizons, but always helow the coal-measures. The most celebrated, viz., the Pennsylvania horizon, is in the Upper Devonian. The Canada horizon is in the lowest Devonian. In West Virginia it is in the sub-Carbonifer- PALEOZOIC ROCKS AND ERA, 315 ous. Ill Ohio it is in the Devonian and in the Silurian, especially the latter. In California it is in the Miocene Tertiary of the Coast Eange. Origin of Petroleum. — It is probable that petroleum was formed by a change of organic matter, somewhat similar to that which makes coal, but from a different kind of organic matter, and under different conditions. Land-plants, in the presence of fresh water, form coal ; while marine plants, and sometimes lower animals, in the presence of salt water, form petroleum, bitumen, etc. It has been observed that petroleum is often found in connection with salt. Orig-in of Varieties. — But, however formed in the first instance, there is no doubt that the different varieties or physical conditions are formed from each other by the passing away of gaseous hydrocarbon. In this manner light oil changes into heavy oil, and this into bitumen, and finally into asphalt. Thus there are two series de- rived from organic matter, the coal series and the petro- leum series. By successive changes, coal passes from fat-coals to bituminous, then semi-bituminous, then an- thracite, and finally graphite ; petroleum from light oil to heavy oil, then bitumen, asphalt, jet, and possibly diamond. But the origin of diamond is uncertain. Fauna of the Carhoniferous Age, As already stated, we shall take up the fauna of the sub-Carboniferous and Carboniferous together ; only let it be remembered that the land and fresh- water animals are from the coal-measures, especially the vertebrates, and the marine animals are mostly from the sub-Car- boniferous. We shall touch only the most prominent points. We have nothing characteristic to add about corals, but only draw attention here to an exceedingly curious 316 HISTORICAL GEOLOGY. and characteristic form of coral-making Bryozoan, called from its perfect screw-like form, ArcJiimedes (Fig. 241). This abundant and easily recognized form is wholly characteristic of the sub-Carboniferous. By studying the diagram (Fig. 243) the main facts regarding Ecliinoderms may be easily re- membered. As before (page 279), the lower shaded part represents stemmed, and the up- per unshaded, the free forms. The Cystids, it is seen, are confined to the Silurian, the Blastids commence in the Silurian, continue through the Devonian and Carboniferous, and perish ; while the Crinids continue until now. The Asteroids commence in the Lower Silu- rian, the Echinoids in the Carboniferous, and both continue until now — the species, of course, changing. As Blastids are very abun- dant in the sub-Carboniferous, we give a fig- ure (Fig. 242). Concerning Mollusca, we touch two points : 1. Fresh-water and land shells, which were in- FiG. a4i. — troduced in the Devonian, are more abundant Archimedes (^igs. 244, 245). 2. The Goniatites, first in- Hall.) troduced m the Devonian, numerous here (Fig. 246). Concerning Crustacea, also two points : 1. While Trilobites continue under new forms, ready to perish at the end of this period, Lmiuloids, or horseshoe crabs, a higher type, are here introduced (Fig. 247). The transition from Trilobites to Limuloids may be quite perfectly traced. 2. True typi- cal crustaceans of the long-tailed kind (Ma- ^iq. 242.-Bia8- crourans), such as shrimps and the like, were tid : Pentre- first introduced here (Fig. 248). "^^ P^f^^; Insects, viiich first appeared in the Devo- Haii.) are also more PALEOZOIC ROCKS AND ERA. 317 nian in connection with land vegetation, as might be ex- pected, are much more abundant and in greater variety in S I LUFtlAN OEV PALEOZOIC] N E O Z O I C Fig. 243. Fig. 244.— Dawsonclla Meekii. Fia. 245.— Anthracopupa Ohioensis. (After Bradley.) (After Whitfield.) Fig. 246. — Carboniferous goniatites : Goniatites crenistria (European) ; a, side-view : b, end-view. Fig. 247.— Carboniferous crustacean : En- proOps Danse. (After Meek and Worthen.) the coal-measures. There are spiders, scorpions, centipeds, cockroaches, dragon-flies, and beetles (Figs. 249, 250). It is well to observe that the highest orders of insects. 318 HISTORICAL GEOLOGY. flower-loving, honey-loving, and social, such as flies, but- terflies, bees, and ants {Dipters, Lepidopters, and Hyme- Fig. 248. — Carboniferous crustacean : Anthrapalsemon gracilis. (After Meek and Worthen.) Fig. 249.— Carboniferous insect : Blatta maderse, wing - cases. (After Lesquereux.) nopters), are not yet found, because there are not yet any flowering plants. Fishes. — We have little to add here to what has already been said under the Devo- nian. The same kinds of fishes — viz.. Ganoids and Placoids — still j)re- vail. Of the Ganoids, however, the Placo- derms passed away with the Devonian, but the Lepido- ganoids continue, and some of them become still more reptilian. We note also an advance in the Placoids, in that an intermediate form (the Hybodonts) between the Cestracionts and true sharks {Squalodonts) here appears. Fig. 250.— Carboniferous insects : Zylobius sigillarise. (After Dawson.) a, anterior ; 5, posterior portion, enlarged. PALEOZOIC ROCKS AND ERA. 319 Amphibians. — The introduction of amphibians must be regarded as a great step in the progress of life ; for they are the first true land vertebrates and air-hreathing vertebrates. Yet we must remember, on the one hand, that amphibians, as their name implies, all of them at some period of their life, some of them permanently, breathe both air and water — both by gills and by lungs ; and on the other hand we must remember that Ganoid fishes also supplement their gill-breathing by lung- breathing. The amphibians are intermediate between fishes and true reptiles. They are represented now by frogs, toads, newts, etc. Now, in the Carboniferous, and long afterward, am- phibians were very diiferent from any of those mentioned as still living. They belonged to a peculiar order now long extinct, called Labyrinthodonts, from the labyrin- thine structure of their teeth (Fig. 252). All the living amphibians are small creatures ; these were often of huge size. All the living kinds have soft, moist skin ; these were partly covered with large, ganoidal plates. The early Ganoids, too, had the same labyrin- thine structure of the teeth, thoughless ^ „,, „ , ., ^ ,^^ 1 n /-n- \ X FiG.251.— structure of a ganoid tooth. (After marked (tig. 251). In Agassiz.) fact, the transition from the reptilian Ganoids to the ganoidlike amphibians of the coal-measures is so gradual that it is difficult in some cases to say whether some of these are amphibian reptile or ganoid fish (Fig. 253). Amphibians seem to have been very abundant in the coal-measures — some snakelike forms, with very small or no feet, some lizardlike forms, some fishlike forms, and some huge crocodilian forms, but not with croco- dilian affinities. These huge forms were, however, more 320 HISTORICAL GEOLOGY. common later, i. e., in the Triassic. We will describe only two examples : The Arcliegosaurus {primeval saurian) (Fig., 253) was an animal two to three feet long, with head and body much like a ganoid fish, and covered with ganoid plates and scales. It had probably per- manent gills as well as lungs, and its legs were little more than legged fins, such as are found in some ganoids, and wholly unadapted for locomo- tion on land. It was a re- markable' connecting link be- tween ganoid fish and laby- rinthodont amphibian. The Dendrerpetoii [tree- reptile) was so called because first discovered (by Dawson) in the hollow stump of a sigil- laria tree. It was of lizard-like form, and about two feet long It is a curious fact that these hollow stumps of Fig. 252.— Section of portion of a tooth of a labyrinthodont. Pig. 253.— Archegosaurus : A, plates ; B, section of tootli. sigillaria filled with sandstone (Fig. 254) are very rich in fossils, e. g., skeletons of amphibians, remains of in- sects, and shells of land mollusca. The sigillaria tree was very soft and spongy, but was covered with a hard bark. We may easily picture to ourselves the conditions under PALEOZOIC ROCKS AND ERA. 321 which these remains were entombed. Imagine, then, a large sigillaria tree on the borders of a coal-swamp, rotted down to a hollow stump. A flood then carried thither floating insects, shells, and carcasses of amphibians, which lodged in the hollow stump, and were covered up with sand. The hollow stump changed to coal, the sand to sandstone, and the animal re- mains to fossils. The very earliest amphibian known is recognized by its tracks (Fig. 255). These are found in the sub- Carboniferous of Pennsylvania, in a sand- stone marked with ripple-marks. The animal has been called 8auropus primcevus (primeval reptile-foot). It was evidently a large Labyrinthodont. Not only tracks and ripple-marks, but also rain-prints (Fig. 256) and sun- cracks, are common in the coal-measures. Fig. 254. — Section of hollow sigil- laria stump, filled with sandstone. (After Dawson.) Fig. 'iio5. — Slab of sandstone with siiii-cracks a, and leplilian footprints i, from coal-measures of Pennsylvania ; x |. Some General Observations on the Whole Paleo- zoic. — Before leaving this long and diversified era, we must look back and make some general observations. Progressive Change. — During the whole time we may observe a progressive change going on : 1. There was, as we have seen, a steady growth of the continent Le Conte, Geol. 21 322 HISTORICAL GEOLOGY. from the Archaean nucleus. 2. There was also a pro- gressive change in the constitution of the atmosphere, especially by removal of excess of water and 00^, fitting it for the introduction of higher animals. 3. In connec- tion with these physical changes, there were also progres- sive changes in life-forms.* Appalachian Revolu- tion. — Thus we see a slow, steady, progressive change during the era. But now, at the end, there occurred one of those great and rapid changes in physical geogra- phy and climate which mark the end of the eras, and a corresponding sweeping change in the forms of life. The Appalachian chain was formed at this time, and is its monument, and therefore, by American geologists, it is fitly called the Appalachian Revolutio7i. The place of the Appalachian chain during the Silurian and Devonian eras was the marginal sea- bottom of the great interior Paleozoic Sea, receiving sediments until 30,000 feet were accumulated. During the Carboniferous it was sometimes an inland sea-bottom, sometimes a coal-marsh, and sometimes, perhaps, a lake, but always receiving sediment until 10,000 more feet were accumulated. Now, at last, it yielded to the ever-increas- ing lateral pressure, and was folded and crumbled with all its coal-beds, and swelled up into a great mountain- range. It has since been sculptured by erosion into its present forms. We have said that the change of life-forms produced * For a fuller account of this important point, the teacher is re- ferred to the larger work. Fig. 256. -Fossil rain-prints of the Coal period. PALEOZOIC ROCKS AND ERA, 323 by this revolution was sweeping. When quiet and pros- perous times again commenced, in the Mesozoic, we find an entirely different condition of things. It is almost like a new world. We must not imagine, however, that the change was absolutely sudden. The steps of change here were only more rapid, and the general unconformity and loss of record which occur here make it seem sudden. Transition to the Mesozoic Era. — Permian Period. We have seen (page 267) that the Paleozoic commenced after a great revolution. Now, the Paleozoic was closed also by a similar revolution. We have called this latter the Appalachian Revolution, because this range was made at that time ; but it was a time of widespread oscillations, and, therefore, of great changes in physical geography and climate, marked by universal unconformity and by sweeping changes in life-forms. Now, as already seen on page 193, unconformity always means lost record at that place. Of the lost record between the Archaean and Paleozoic nothing has been certainly found, but of that between the Paleozoic and Mesozoic, certain leaves have been recovered. These are brought together and called the Permia^i. Some have allied the Permian with the Mesozoic under the name of Dyas. Others have allied it with the Paleozoic. The truth is, it ought to be regarded as a period of transition or of revolution between the two. lilfe System. — As might be expected the organisms of the Permian are mainly transitional. Paleozoic forms are passing away and Mesozoic forms are coming in. The very first reptiles are introduced here, but they had not yet attained supremacy. CHAPTER IV. MESOZOIC ERA. AGE OF REPTILES. The Paleozoic era was very long and diversified. It consisted of three ages — the age of Invertebrates, the age of Fishes, and the age of Acrogens and Amphibians. The Mesozoic era, on the contrary, consists of but one age — the age of Reptiles. Never, in the history of the earth, were reptiles so abundant, of such size and variety, or so highly organized, as then. Characteristics of the Age. — The characteristics of this age are the culmination of the class of reptiles, and the class of cephalopod mollusks among animals, and of cycads among plants ; and the first introduction of mam- mals and birds f and, in the last part, of Teleost fishes and Dicotyledonous trees. The most striking characteristic is the culmination of reptiles, and this, therefore, gives it its name. Subdivisions. — The Mesozoic era and Reptilian age is divided into three periods r 3. Cretaceous. — viz. : 1. Triassic, on ac- Mesozoic rocks. -I 2. Jurassic. count of its distinct three- [ 1. Triassic, fold division in Germany, where first well studied. 2. Jurassic, on account of its splendid development in the folded structure of the Jura Mountains. 3. Creta- ceous, on account of the chalk of England and France being one of its members. These three are very distinct periods in Europe, but in America the Trias and Juras are closely connected 324 MESOZOIC ERA.— AGE OF REPTILES. 325 though very distinct from the Cretaceous ; so that, studied ill America alone, it would be most natural to divide the whole age into two periods: 1. Jura- Trias ; and, 2. Cretaceous. Again, the Jura-Trias is much poorer in fossils in this country than in Europe ; so that, if we treated of American strata alone, we should give but a very imperfect picture of the times. Therefore, our plan will be to give a brief sketch of the Trias, and then of the Juras, taking illus- trations chiefly from foreign sources, and then a sketch of the Jura-Trias in America. The Cretaceous can be fully illustrated from American strata. Section^ I. — Triassic Period. As already stated, the lowest Mesozoic (Triassic) is always, or nearly always, unconformable with the Coal. The line of break may be between the Triassic and the Permian, but more commonly between the Permian and the Coal. But the fossils of the Triassic are always very different from those of the Permian. The break in the life system is always greatest here. We will neglect the subdivisions, and take up all together. Life System. — Although the revolution which closed the Paleozoic is passed, and comparative quiet again re- stored, yet it took some time for the old fullness of life to recover itself. Mesozoic life, therefore, is compara- tively poor in the Triassic compared with the Jurassic and Cretaceous. We will, therefore, touch very briefly on Triassic life. The Change. — The most striking fact is the sweep- ing change in life-forms. All the old style corals are replaced by new style ; all the armless crinoids (Blastids and Cystids), the square-shouldered brachiopods, the orthoceratites and trilobites, the lepidodendrids, sigil- larids, and calamites — in a word, all that we found most 3^6 HISTORICAL LiEOLOGW characteristic of the Paleozoic, are gone. They are re- placed by other and very different forms. Plants. — As the grand characteristic of tlie Coal pe- riod was the predominance of the vascnlar Cryptogams, so that of this period is the predominance of the next higher group of plants, viz., GymnospermSy i. e.. Conifers and Cycads, especially Cycads (see diagram on page 259). Ferns and Equisetse, however, still abounded, though of different genera from those of the Coal. But as the pe- culiar flora of the Mesozoic did not culminate until the Jurassic, we shall put off illustrations until that time. Animals. — Although Cystids and Blastids disappear with the Paleozoic, the Crinids are still represented by many beautiful new forms, with plumose arms, which, when expanded, must have presented a truly flower-like appearance, and their fossilized remains are therefore often called stone-lilies. One of these is shown iu Fig. Pig. 257.— EncrinuB lUiformls Fig. 258.— Ceratites nodosus. 257, and on page 331 we give a similar form in expanded condition. The Goniatites have passed away. The Ammonite MESOZOIC ERA.-AGE OF REPTILES, 327 family is here represented by Ceratites. They are easily recognized, and entirely characteristic of the Triassic. The complexity of the suture is increased, as shown in Fig. 258. Pig. 259,— Teeth of Triassic fishes ; Hybodus apicalis, (Af- ter Agaesiz.) Fig. 360.— Mastodonsaurua Jaegeii Among fishes we find still only Ganoids and Placoids, but the Ganoids are assuming more and more the form of ordinary fishes (Teleosts), and the teeth of the Placoids are becoming more shark-like (Fig. 259). Fig. 201.— Triassic reptiles (after Owen)— anoraodouts and tlierodonts ; Dicynodon lacerticeps. 3;i8 HISTORICAL GEOLOGY. Amphibians. — Labyrinthodonts, introduced in the Coal, continue and culminate here (Fig. 260), and soon become extinct. Reptiles. — Reptiles were introduced in the Permian but did not become dominant until the Mesozoic. Cer- tain forms, of which we shall speak hereafter, commence here, but culminate in the Jurassic. But there are also some curious transitional forms entirely characteristic of this period. The Anomodonts (lawless-toothed) were beaked like a turtle, and either toothless or else with long tusks only (Fig. 261), but crocodilian in form. The Therodonts (beast-toothed) were so called because their teeth Avere in three groups, corresponding to i7ici- sors, canines y and molars of mammals (Fig. 262). Both of these curious families had many char- acters allying them with the lowest mammals, i. e., Monotremes (Ornithorhynchus, Echidna, etc.), now found only in Australia. They have been fitly called, by Cope, Theromorpha (beast-like). These beast-like reptiles seem to have been introduced first in the Permian. Mammals. — If beast-like reptiles are found here, we might naturally expect also the lower forms of beasts themselves. In the uppermost Triassic, both of Europe and America, remains of small marsupial mammals have indeed been found ; but as only a feio have been found, and these in the uppermost Triassic, almost passing into the Jurassic, and as similar remains are far more abun- dant in the Jurassic, wo shall put oil their description until that time. Fig. 262.— Lycosaurus. MESOZOIC ERA.- AGE OF REPTILES. 329 No birds have been found. It may seem strange that mammals should have been introduced before birds ; but we find the explanation of this in the fact that birds are a suh-branch of the reptilian branch of the vertebrate stem. Sectioit II. — Jurassic Period. Name. — These strata and the period they represent are called Jurassic, because of their splendid develop- ment in the folded structure of the Jura Mountains (Fig. 145, page 241) and their richness in fossils there. Rock-System. — In England the Jurassic has been subdivided into the Lias, the Oolite, and the Wealden ; but we shall neglect these, and speak only of the whole together. Coal. — One point worthy of note here is the occurrence of coal. The Jurassic coal-fields are far smaller than those of the Carboniferous, but the mode of occurrence of the coal is much the same. Examples of such coal are the Yorkshire and Brora coal of Great Britain, and some of the coals of India and China ; also the coals of eastern Virginia and North Caro- lina. Of these last we shall speak again. Many Jurassic coals are of excellent quality, though the average is inferior to the coal of the Carboniferous. Plants. — The characteristic families of the Jurassic are Ferns, Conifers, and Cycads. Conifers and Cycads, especially Cycads, culminated in this period ; they are found in extreme abundance in connection with the Jurassic coal in the form of leaves, trunks, and roots. Some Jurassic plants and their living allies are shown in Figs. 263-266. Animals. The culmination of the characteristic animals of the Mesozoic, especially reptiles, occurred in this middle 330 HISTORICAL GEOLOGY, period. We shall touch very briefly all except the most important characteristic kinds. Fio. ::2t;3.— Zaiuia spiralis, a li\iu^ cycad of Australia. Fig. 264.— Stem of cycadeoidea megalophylla. Crinoids, beautiful, plumose-armed, and lilylike, are abundant (Fig. 267) ; but so, also, are the free asteroids and echinoids (Fig. 268). The two kinds, stemmed and free, are evenly balanced. Bivalves are, of course, abundant and of characteristic forms, in this as in all geologi- cal times ; but we can only draw special attention to the oyster family (including Os- trea, Gryphea, Trigonia, etc.), which were first introduced here (Figs. 269-271). Ammonites. — The Ammo- nite family were introduced first in the Devonian as Goniatites. These were replaced in the Tri- assic by .Ceratites. Tlie Am- monites proper, the liighest type of the family, were intro- duced in the early Mesozoic, culminated here in the Juras- sic, continued through the Cretaceous, and died out at its . 265.— Jurassic plants : I'tero- pliyllum comptum (a cycad). MESOZOIC ERA.— AGE OF REPTILES. ^331 end. It is, therefore, characteristic of the Mesozoic. In the Jurassic they were of extreme abundance, and of all Fig. ;ibO.— Juiabsic plants- Conifers : Cone of a pine. Fig. 268.— Clypeus Plotii. Fig. 267.— Apiocrinites restored. (After Buckland.) Fig. 269. Fig. 270. Fig. 271. Figs. 269-271.— Jurassic lamellibranchs of England : 269. Trigonia clavellata. 270. Ostrea Sowerbyi. 271. Ostrea Marshii. 332 HISTORICAL GEOLOGY. sizes, from half an inch to three feet in diameter. We give some hgnres of the most characteristic forms (Figs. 272-274). It is interesting to trace the gradual changes in the Fig. 272. Fig. 273. Fig. 274. Figs. 272-274.— Jurassic cephalopods— Ammonites : 272. Ammonites margaritanus. 273. Ammonites Jason : side-view. 274. Ammonites cordatus : a, side-view ; 6, showing suture. form of the suture in shelled cephalopods. In the Silu- rian Orthoceratites the sutures were even; in the Devonian and Carboniferous (jroniatites they were angled ; in the Fig. 27 Figs. 275, 276.-275. Belemnites Owenii. 27G. Belemnites unicanaliculatus. Triassic Ceratites they were scalloped ; finally, here in the Ammonites they were frilled in the most complex patterns. Belemnites. — Now, for the first time, we find the highest order of cephalopods, viz., the naked ones, allied to the squids, cuttle-fishes, etc. This order is represented in Jurassic times by a peculiar form, called Belemnites, MESOZOIG ERA.— AGE OF REPTILES. 533 from the curious, dartlihe bone (Figs. 275, 276), which is often the only part found. Sometimes the soft parts have been found ; the ink-bag (Fig. 277) has been found so per- fect that good ink has been made of it, and the animal has even been drawn with its own fossil ink. From the various parts found it is possible to restore the animal with some confidence. In Fig. 278 we give such a restoration, and in Fig. 279 aliving squid for comparison. Crustaceans and Insects. — There is a steady development. Fig. 277. Fig. 278. Fig. 279. Figs. 277-279.-277. FoBsil ink-bags of Belemnites. 278. Belemnite restored. 279. A living squid. during the Mesozoic, of crustaceans, toward the highest form, viz., the crabs. This, however, was /«^VZ?/ attained only in the Cretaceous, though a spider-crab has been found in the Jurassic. Insects also are far more numerous and diversified (Figs. 280, 281) than heretofore, although even yet the highest forms, such as ants, bees, and butterflies, are not found. There is little of importance to be noted in regard to Fishes. We therefore jmss on to Reptiles. 334 HISTORICAL GEOLOGY. Fig. 280.— Jurassic in- sects : Blattina for- mosa. (After Heer.) Reptiles. — These are the rulers of the age, and cul- minate in this period. We shall therefore dwell a little more fully on them. During the Jurassic there was a truly extraordinary development of this class, in number, size, variety, and degree of organization. They were rulers in every department of Nature : rulers in the sea, in place of whales and sharks of to-day; rulers on the land, in place of beasts ; and rulers in the air, in place of birds. We shall take them up under the three heads indicated, viz.: 1. Ma- rine Saurians (Enaliosaurs). 2. Land Saurians (Dinosaurs). 3. Winged Sau- rians (Pterosaurs). The first were swim- ming, the second walking, the third fly- ing, animals. 1. Marine Saurians. — Among these we shall mention only the two most noted, viz.. Ichthyosaurus and Plesiosaurus. The Ichthyosaurus (fish-reptile) (Fig. 282) was a huge monster, thirty to forty feet long, with thick body, short neck, enormous head, eyes twelve to fifteen inches in diameter, and jaws set with hundreds of conical teeth. The limbs were paddles, suitable for swimming, not for walking. The powerful tail was expanded vertically into a fin at its extremity, and the bodies of the vertebrae were biconcave like those of a fish. Perfect skeletons of this animal have been found ; and even the impressions of its intes- tines, and the contents of its stomach, revealing the nature of its last meal, have been preserved. The Plesiosaurus (lizardlike) (Fig. 283) was a slenderer animal, with a very long neck, small head, short tail, long and powerful paddles, and fishlike vertebrae. 2. Dinosaurs, or Land Saurians. — The hugest of Fig. 281 — Glaphyrop- tera gracilis. (After Heer.) MtJtiOZOlC ERA.— AGE OF REPTILES. 335 reptiles — in fact, the liiigest animals which have ever walked the earth — were of this order. They were also the most highly organized of reptiles ; for, if the marine saurians connected this class with fishes, the dinosaurs Fig. a83.— Jurassic reptiles— Ichthyosaurus and Plesioeaurus : chthyosaurus com- munis, X X5ff- connected it with the higher class of birds. Some of the characters connecting them with birds are the following: 1. Many of them had long, powerful hind-legs, large hip^ bones, and strong sacrum, and very short and small fore- FiG. 283.— Jurassic reptiles— Ichthyosaurus and Plesiosaurus : Plesiosaurus doli- chodeirus, restored, x ^V- legs. These characters show that they walked mainly on their hind-legs, in the manner of birds. 2. Many of them, like some birds, had only three toes on the hind-feet, so that they made tracks which were bird-like. 3. There were peculiarities about their ankle-joints which were still more bird-like. 336 HISTORICAL GEOLOGY. Pig. 284.-^Iguanoclon Bemissartensis. (After Marsh.) The most noted of this order found in Europe are the Iguanodon and the Megalosaurus. The iguanodon (Iguana-tooth), judging from the size of its hones, was probably several times more bulky than the elephant ; and yet a perfect skeleton, recently found in Belgium (Fig. 284), shows that it walked on the hind-legs alone, supporting itself by its massive tail. The neck was long, flexible, and bird-like, and the jaws were beaked in front and set with herbivorous, iguanalike teeth (Fig. 285) behind. The megalosaur (great saurian) was not quite so large, but prob- ably still more formidable, since it was carnivorous. A restoration of a smaller allied form is given in Fig. 286. This also walked mainly od two Fig. 285. -Tooth of an Igua- nodon. MESOZOIC ERA.—A01<: OF REPTILES. 337 legs. Still much larger animals of this order have been found in the United States, as we shall see further on. 3. Pterosaurs, or Winged Sauriaiis. — These are per- haps the most extraordinary of all known animals. They Fig. 286.— Compsognathus, x ^rs- (Restored by Marsh.) combined the stout body with keeled breastbone, the long, flexible neck and beaklike jaws of a bird, with the long arms and membranous flying-web of a bat and the essen- tial characters of a reptile. In some cases they had a short, aborted tail, like a bird, but in others a long tail, with vertical expansion at the tip, which was used as a rudder in flying (Fig. 287). The pterosaurs varied in size from two or three feet to eighteen or twenty feet from tip to tip of the extended, wings. Birds. — We have seen that the reptiles of this time approached birds, but still more remarkably do the earliest birds approach reptiles. There is in Bavaria a peculiar limestone used the world over for lithographic drawings. This lithographic limestone is equally celebrated for its marvelous preservation of fossils. In 1862 the oldest known bird, the Archceopteryx, was found there with even Le Conte, Geol. 22 338 HISTORICAL GEOLOGY. the feathers, and the minute structure of the feathers of the wings and tail, preserved. An undoubted bird, yet Fig. 887.— Restoration of Rhampliorhynchus phylluras. (After Marsh.) One seventh natural size. how different from modern birds ! Instead of the short, aborted tail, bearing feathers radiating almost from one point, as in all modern birds, it had a long reptilian tail with twenty-one joints, and the feathers given off in pairs on the two sides of each joint. Among many other rep- tilian characters are the possession of socketed teeth, and, instead of the hand being wholly consolidated to form the wing, as in modern birds, the three fingers remain free, and are armed with claws (Fig. 288). Another fine specimen of this wonderful bird was found, in 1873, in the same locality, and is now in the Berlin Museum. In the Jurassic dinosaurs and this Jurassic bird we have excellent examples of what we have called generalized or connecting types. These two branches — reptile and bird — which seem so widely dis- tinct now, when traced backward in time, approach more and more, until we find almost their point of union. Mammals. — We have already stated, on page 328, that a few small marsupial mammals are found in the uppermost Triassic, both of Europe and the United States. These we regarded as anticipations, and therefore put off their MESOZOIG ERA.— AGE OF REPTILES. 339 discussion. This anticipation is fully realized in the Ju- rassic. In England there have been found about eighteen species^ and, in the United States, Marsh has found seven- teen species; so that there are now known about thirty-five species of Jurassic and three species of Triassic mammals. 340 HISTORICAL GEOLOGY. But, as the first birds were not true typical birds, but reptilian birds, so also the earliest mammals were not true typical mammals, but reptilian mammals, or marsupials. The marsupials live now almost wholly in Australia. They include the kangaroos, the opossums, the bandicoots, the wombats, etc. In Jurassic times they apparently inhab- ited all parts of the earth in great numbers. Now, the ^^^ marsupials differ so ^^^fc greatly from ordinary ^Bfl^Hk. mammals that they are ^^ ^^ ^^ ^^^ ^^^^^ ^ distinct sub- ■-^^^^3^i^-~ Fig. 289.— Jaw of a Jurassic mammal : Amphitharium Prevostii. class. One striking pe- culiarity about them is that their young are born in an exceedingly imperfect state, so that they are almost egg-bearing, semi-oviparous. But neither were the Jurassic marsupials typical mar- supials, but rather generalized types connecting with In- sectivora, the lowest of the true mammals. They were all small animals, varying in size from that of a mole to that of a skunk. They were not able to contend for mastery with the great reptiles. The reign of mammals had not yet come. We give here (Figs. 289, 290) a jaw of a Ju- FiG. 290.— Myrmecobius fasciatus, banded ant-eater of Australia. MESOZOIC ERA.— AGE OF REPTILES. 341 rassic marsupial, and also a living marsupial most nearly allied to them. Section III. — Jura-Trias in America. Areas; Atlantic Border.— All along the eastern slope of the Appalachian chain, from Nova Scotia to South Caro- lina, in the Archaean region of the map on page 272, are found elongated patches of sandstones and shales which belong to this period. One of these is in Nova Scotia and Prince Edward Island ; the next, going south, is the celebrated Connecticut River Valley sandstone ; the next a long, narrow patch commencing in New York, passing through New Jersey, Pennsylvania, Maryland, and ending in northern Virginia ; then two or three patches in eastern Virginia, about Richmond and Piedmont ; and, lastly, some on the Deep River and the Dan River of North Carolina. They all lie in hollows unconformably on the Archaean gneiss, and therefore their age can not be known except by fossils ; but these, though few, seem to indicate that they represent the whole Jura-Trias, although most writers speak of them as Triassic. In all these patches are found remarkable outbursts of igneous rocks, often columnar in structure, which by erosion have formed the so-called trap-ridges. Such are Mounts Tom and Holy- oke, in the Connecticut Valley patch, and the Palisades of the Hudson River in the New Jersey patch. Interior Region. — Red sandstones, poor in fossils, but probably referable to this period, are found in many places in the Plateau and Basin regions. Pacific Slope. — On both sides of the Sierra, rocks of this age, in a metamorphic condition, form the auriferous slates of this region. Life- System. Life, no doubt, abounded, but the conditions were unfavorable for preservation. We can, therefore, take 343 HISTORICAL GEOLOGY. up only a few of these localities and give, briefly, the findings. 1. Connecticut River Valley. — This celebrated local- ity is classic ground, through the life-long labors of Dr. Hitchcock. The patch is one hundred and fifty miles long and ten to fifteen miles wide, extending from New Haven Bay, on Long Island Sound, through Connecticut and Massachusetts, and mostly on the two sides of the Connecticut River. As the strata dip regularly to the east, their thickness is easily estimated, and seems to be at least 5,000 to 10,000 feet. They consist of red sandstones and shales, and are in some places beautifully fissile. As might be expected from their redness,* they are very poor in fossils proper ; but in certain parts an immense num- ber of tracks of various animals have been found. There are tracks of {a) insects and crustaceans ; (b) of reptiles; (c) possibly, but not probably, of birds. (a) Insects and Crustaceans. — Of the insect and crustacean tracks little can be made out with certainty. ^Ye give an example (Fig. 291). (b) Reptiles. — The reptilian tracks vary in size, from Fig. 291.— Tracks of insects. (After Hitchcock.) those of a lizard to those of the huge Otozoum, twenty- two inches long with a stride of four feet. In character, some are five-toed, some four-toed, some three-toed ; some walked on four feet, some on only two hind-feet ; some had long, dragging tails (Fig. 292), and some short tails, or none at all (Figs. 293, 294). {c) As already said, some of these reptiles walked on two legs only, and had only three functional toes, and * Organic matter decolorizes sandstones. — See page 89. MESOZOIC ERA.— AGE OF REPTILES. 343 some were short-tailed or tailless. These have been re- garded by some as wingless birds. They were probably h I QO> Fig. 292. Fig. 293. Fig. 294. Figs. 292-294.— Reptile tracks (after Hitchcock) : 292. Gigantitherium caudatnm, X ^. 293. Anomoepus minor, x J : a, hind-foot ; 6, fore-foot. 294. Track of Brontozoum giganteuni, x \. all reptiles. One of these wonderful two-legged reptiles is given in Fig. 295. The general conclusion, then, is that all these tracks were those of Dinosaurs and, possibly, Labyrinthodonts. In Jura-Trias times there seems to have been in this place an estuary, into which the tides ebbed and flowed. At low tide, reptiles of many kinds were in the habit of walking on the soft, exposed mud in search of food left by the retreating tide. The incoming tide covered the tracks with line sediment, and preserved them till now, the sediments, meantime, hardening into stone. 2. New Jersey Patch. — In this patch we find llie same redness of the sandstone, and therefore the same poverty of fossils. Of this sandstone have been built all 344 HISTORICAL GEOLOGY. the brownstone houses of New York city. A few bones and teeth of reptiles, however, have been found, and these Fig. 295.— Anchisauras colurus, x ^, from Connecticut sandstone. (After Marsh.) confirm the conclusions already given. A few tridactyl tracks also have been recently found, similar to those of the Connecticut patch. In Fig. 290 we give a restoration of fish from the New Jersey sandstone. Fig. 296.— Biplnms longicandatus, x \. (After Dean.) 3. Virginia and North Caroliua Patches. — These are very different from the Northern patches. They form the Richmond and Piedmont coal-fields of eastern Vir- ginia (Fig. 297) and the Deep Eiver and Dan River coal- MESOZOIC EBA.—AQE OF REPTILES. 345 Fig. 298.— Jaw of Dromatherium sylvestre. Fig 297.— Section across Richmond coal-field. (After Daddow.) fields of North Carolina. In connection with the Coal, plants have been found in considerable abundance. They are those characteristics of the Jura-Trias every- where, viz., ferns, cy- cads, and conifers. In North Carolina the jaw of a small marsupial has been found about the middle of the series (Fig. 298). The coal of these Jura-Trias fields is of good quality, in thick seams, and easily worked. 4. Atlantosaur Beds. — These we describe separately, not only because they are recent discoveries, but also and chiefly because they belong to an entirely different horizon, viz., the uppermost Jurassic passing into the Cretaceous. In these uppermost Jurassic beds, called Atlantosaur ], from their most abundant and characteristic genus, Fig. 299. — Broiitosanrvis excelsis, x ^itj- (Restored by Marsh.) have recently been found, in Wyoming and Colorado, great numbers of most extraordinary reptiles, the largest yet known, and also a bird and seventeen species of small marsupial mammals. 346 HISTORICAL GEOLOGY. Reptiles. — The extraordinary number of dinosaurian reptiles found here have thrown much light on this order. Some of them were reptile-footed {Sauropoda) (Fig. 299), some bird-footed {OrnWiopoda) (Fig. 300), some beast- FiG. 800.— Laosaurus, x ^. (Restored by Marsh.) Fiu. 301.— Stegosaurusimgulatus, x A. (Restored by Marsh.) MESOZOIC ERA.— AGE OF REPTILES. 347 footed {Titer opodct), and some curious plate-covered rep- tiles {Stegosauria) (Fig 301). The Ornithopoda and some Theropoda walked almost wholly on their hind-legs in the manner of birds. The size of some of these reptiles is almost inconceivable. A thigh-bone of an Atlantosaur, found by Marsh, was six or seven feet long, and a vertebra of an Amphicoilias, found by Cope, was six feet high to the top of the spinous process. The Atlantosaur has been estimated to have been seventy to eighty feet long ! In the same beds, as already stated, were found the remains of a bird and of seventeen species of marsupials. A figure of one of these is herewith given (Fig. 302). Fig. 302.— Right lower jaw of Diplocynodon victor (after Marsh), outside view — twice natural size. Disturbances which closed the Jura-Trias Pe- riod. — One of the most important changes which oc- curred at the close of this period was the formation of the Sierra Nevada Range. Until that time the Pacific shore-line was east of the Sierra, and the place of this range was a marginal sea bottom receiving sediment. These sediments finally yielded at the close of this period and were folded and swelled up into this great range. Subsequent erosion sculptured it into its present grand forms. Coincidently with this change in the West, there were on the Atlantic horder outbursts of igneous matter forming the trap ridges. In the interior region there 348 HISTORICAL GEOLOGY. was a downward movement of tlie crust over the whole Plains and Plateau region hy which isolated inland- seas were changed into the great interior Cretaceous sea. The Sierra Nevada Range is the most conspicuous monu- ment of this period of change, and therefore it may be called the Sierra revolution. Sectio:n^ IV. — Cretaceous Rocks and Period. General Characteristics. — The Cretaceous is in some respects a transition to, and a preparation for, the next era. Mesozoic types, such as the great reptiles, the am- monites, etc., continue, but Cenozoic types, like dicoty- ledonous trees and teleost fishes, are introduced, and the two kinds of types coexisted side by side. Rock System ; Areas. — 1. In the Atlantic 'border region, going southward, we find no cretaceans until we reach Long Island. Going south from this, we find a strip running through New Jersey, Delaware, and Maryland, lying directly against the Archaean ; then small, isolated patches exposed by erosion in North Carolina, South Carolina, and Georgia. It doubtless extends all along the Southern coast, but is mostly covered with later Ter- tiary deposits. 2. In the Gulf harder region it forms a broad, crescentic band, commencing in western middle Georgia, passing through middle Alabama, turning north- ward through Mississippi and Tennessee, to near the mouth of the Ohio. It underdips the Tertiary of the Mississippi River region, and reappears on its west side (see map, page 272). 3. It thence passes northward, covering nearly the whole Plains and Plateau region, though largely concealed by the Tertiary. 4. On the Pacific border it is found on the lower foot-hills of the Sierra Nevada in Northern California, and, together with the Tertiary, forming the whole of the Coast Range. Physical Geography. — From this distribution we can MESOZOIC ERA.— AGE OF REPTILES. 349 make out with some confidence the condition of the con- tinent in Cretaceous times. 1. North of New York the Atlantic sJiore-Iine was farther out than now. It crossed the present shore-line near New York, passed along the inner horder of the Cretaceous of New Jersey, Delaware, and Maryland, and southward nearly along the limit of the low countries. 2. The Gulf shore-line went through Fig. 303.~Map of North America in Cretaceous times. middle Alabama, and northward to the mouth of the Ohio, and southward again on the other side of the Mis- sissippi Eiver. 3. Connected with this extended gulf was a great inland sea five to six hundred miles wide, covering the whole Plains and Plateau region (with some islands in the Colorado mountains region), and stretching northward probably even to the Arctic Ocean, and thus dividing the continent into two parts, an Eastern or Appalachian continent and a Western or Basin regio» 350 HISTORICAL GEOLOGY. continent. The place of the Wahsatch Range was then the western marginal bottom of this interior sea. 4. The Pacific shore-line was then east of the Coast Ranges, and its waves beat against the lowest foot-hills of the Sierra. This is shown in the map, Fig. 303. Character of the Rocks.— In regard to the kind of strata, there are two points worthy of passing mention. 1. Chalk. — The period takes its name from the chalk of England and France, which belongs here. Chalk is a soft, snow-white, very pure lime-carbonate, scattered Fig. 304.— View of Iowa chalk under the microscope. (After Calvin.) through which are nodules of flint. On account of its softness, it is worn into strange, castellated forms. Pure chalk, as described, was until recently, supposed to be confined to England, and France, and middle Europe, but has now been found in the Cretaceous of Texas and the Plains. When examined with the microscope, it seems MESOZOIC ERA. -AGE OF REPTILES. 351 to be composed wholly of the remains of low organisms, chiefly foraminifera (Fig. 304). The flints are seen to be composed of shells of Diatoms and spicules of sponges. Now, as already shown (page 117), this is exactly the composition of deep-sea ooze (globigerina ooze), except that the silica has been separated and collected in nod- ules. It seems probable, therefore, that chalk is a deep- sea ooze of the Cretaceous times. 2. Coal. — Coal is found, again, in the Cretaceous, both in the United States and elsewhere. But as most of our later coal belongs to a tra7isitio?i period between the Cre- taceous and the Tertiary, we shall put off the discussion of these for the present. Life-System ; Plants. So great is the change and the advance in plants at this point, that if we were guided by plants alone, we would say that the Cenozoic era commenced with the Cretaceous. Here the present aspect of field and forest seems to begin, for here were introduced for the first time, and in great numbers, dicotyls, or ordinary hard-tvood trees. The sud- denness of their appearance, however, is due, in part at Fig. 305. Fig. 306. Fig. 307. Figs. 305-307.— Cretaceous plants (after Lesquereux): 305. Sassafras araliopsis. 306. Salix proteaifolia. 307. Fagus polyclada. All reduced. 352 HISTORICAL GEOLOGY. least, to a lost interval between the Jura-Trias and the Cretaceous. Of the four hundred and sixty species of plants found in the Middle Cretaceous of the West, four hundred are dicotyls. Nearly all the genera of common trees are represented, although, of course, the species are extinct. There were then, as now, oaks, maples, willows, sassafras, dogwoodg, hickory, beech, poplar, tulip-tree {Liriodendron), walnut, sycamore, sweet-gum {Liquid- ambar), laurels, myrtles, etc. A few of these are given in Figs. 305-307. Animals. Protozoa. — Though these are found in nearly all the strata lieretofore described, we have usually neglected l^'i". 3J8. F.u. 300. Fig. 310. Tigs. 308-310.— Foraniinifera of chalk, magnified : 308. Flabellina nigosa. 309. Lituola nautiloides. 310. Chrysalidina gradata. (After D'Orbigny.) them, because they are inconspicuous. But here in the Cretaceous they are so abundant that they demand attention. Chalk, as already said, is almost wholly made up of for- aminifers (Figs. 308-310), and sponges are also extremely abundant. Of the former, some are identical with living species. Fig. aJl.— Kcliiiioids oT tlie Cretaceous of Eiuopo : Galerites albogalerus. MESOZOIC ERA.— AGE OF REPTILES. 353 Fig. 312. -Hippurites Toucasiana, a large individual with two small ones attached. (After D'Orbigny.) Echinoderms are now almost wholly of free forms. The highest echinoids are especially abundant. And, what is remarkable, those from the chalk are very like those still living in deep seas. The reason of this is that deep-sea conditions, and there- fore species, change far more slowly than those of shallow water and land. Bivalve Shells. — Among the immense number of bivalve species found here, we mention only the oyster family, of which there are many species, and the strange Hippurite family (Fig. 312). Surely no one, from its general' form, would imagine that these latter were bivalves. Fig. 815. Fig. 316. Figs. 313-316.— 313. Toxoceras annulare. 314. Hamitesattenuatus. 315. Ancyloceras spinige- rum. 316. Baculites anceps, x i. (After Wood- ward.) Cephalopods. — The Le Conte, Geol. 23 Ammonites and Belemnites still 354 HISTORICAL GEOLOGY, conf-iiiuaiii great numbers, though they disappear at the end. of the Cretaceous ; but, in addition to the usual form, the Ammonites take on now the most strange and un- accountable shapes (Figs. 313-316). Some are partly uncoiled, as in Scaphites (boat), Toxoceras (bow-horn), Ancyloceras (curved-horn), Ilamites (hook); in some, completely uncoiled and straight, as Baculites (staff). Sometimes they are coiled spirally, like a gasteropod, as Flo. 817.— Cretaceous fishes— Teleosts : Osraeroides Miintelli. in Turrulites. But for the complexity of the suture, no one would imagine a baculite or a turrulite to belong to the Ammonite family. It is probable that rapidly changing and unfav- orable conditions tend to produce new and strange forms. The Am- monite family were on the point of becoming extinct. Fishes. — Ilere we note another great step in the progress of life. The Teleost fishes, the vastly pre- dominant kind at the present day, are here first introduced, and al- most immediately become abun- dant. The Ganoids at once be- come very subordinate. The sharks, however, are abundant and of large size, and of the highest kind, viz., Squalodonts, or true sharks (Fig. 318). Reptiles. — If, in Europe, reptiles seem to have culmi- FiG. 318 — Crutaceoue liBlies— Sharks: Otodus. (After Leidy.) MESOZOIC ERA.— AGE OF REPTILES. 355 nated in the Jurassic, in America they seem to have cul- minated in the uppermost Jurassic and Cretaceous. The great interior Cretaceous sea and adjacent land seems to have swarmed with marine and land reptiles of incredible size. All the kinds already spoken of under the Jurassic were found also in the Cretaceous, and in addition, one order, the Mosasaurs — wholly characteristic of the Cretace- ous. The accompanying sched- ule will give some idea of the number of species, and the kinds, of these reptiles. We shall not again describe most of these, but only mention a few interesting points : The marine saurians were represented in America only by the long-necked kinds ( Plesiosaurs) ; but these were numerous, and some greatly surpassed in size any European species, attaining even fifty feet in length. Plesiosaurs, 13 species. Dinosaurs, 21 Crocodilian s ,14 Pterosaurs, 7 Chelonians, 48 Mosasaurs, 50 Total, 153 <( Fig. 319.— Hypsilophodou, x y^g. (Restored by Marsh.) 356 HISTORICAL GEOLOGY The Dinosaurs were also abun- dant, and of great size. The re- stored skeleton of a European spe- cies (Fig. 319) will give some idea of their general ap- pearance and size. The Ptero- saurs, or winged saurians, of America, were of enormous size in the Cretaceous, some attaining an alar extent of twenty-five feet ; but a striking pe- culiarity of them was the entire ab- sence of teeth. On this account they have been put into a distinct family, Pterano- donta (toothless, winged). The Pteranodon in- had tooth- jaws, four feet long, and wings twenty- two feet from tip to tip (Fig. 320). gens MESOZOtC ERA.— AGE OF R:EPTtL^S. 35t But the Mosasaurs were the most abundant and also the most character- istic of all, being found only in the Cre- taceous. At least fifty species are known, and the remains of 1,400 are now in the Peabody Museum at Yale University. These were long, slender, almost snake- like in form, with limbs in the form of powerful paddles (Fig. 321). They were, therefore, entirely marine in habits, and wholly incapable of locomotion on land. The head was slender, and armed with large, recurved teeth. They were allied most nearly to lizards, and therefore might be called huge sea-lizards; but, like most early animals, they were a gen- eralized type, connecting also with other orders, especially snakes. Some species were seventy to eighty feet long, and had teeth seven inches in length. Birds. — The history of the discovery of fossil birds is interesting. In 1862 the wonderful Jurassic bird, Archmop- teryx, already spoken of (page 338), was discovered. But this stood alone, with- out links connecting it with typical birds. In 1870 commenced the wonder- ful series of discoveries by Marsh, mostly in the Cretaceous of the AVest, which served largely to fill up this gap. About twenty species of Cretaceous birds have been described by him. Of these, about one half were ordinary water- birds, allied to the Rails, Divers, Cor- morants, etc., though of different gen- era, but the other ten were wonderful 358 HISTORICAL GEOLOGY. Toothed-hirds, wholly different from anything now living. These Toothed-birds were, again, of two types. Those of the one class (of which the Resperornis may be taken Fig. 322.— Ichthyomis victor, x J. Fig. 323.— Hesperornis regalis, x ^^. (Re- (Restored by Marsh.) stored by Marsh.) as a type) were flightless swimmers and divers, of great size (five to six feet long), with scarcely a rudiment of wings. Those of the other class (of which the Ichthy- omis is the type) were smaller in size, but powerful fliers. The Hesperornidm had teeth in grooves — a lower condi- tion. The IchthyornidcB had teeth set in distinct sockets. We give herewith (Figs. 322, 323) Marshes restorations of these two types. Mammals. — We found marsupials somewhat abundant in the Jurassic, though no true typical mammals. It is, therefore, somewhat remarkable that no mammal of any kind has yet been found in the Cretaceous, except in the Laramie, which may be regarded as a transition to the MESOZOIC ERA.— AGE OF REPTILES. 359 Tertiary. Yet, doubtless, marsupials did exist throughout the Cretaceous, because they existed in the Jurassic, and again in the Tertiary, and even now ; and it is a law in paleontology that a form, once become extinct, is never revived. Nature never repeats herself. Doubtless, mar- supials existed in some part of the earth, and their remains will yet be discovered. General Observations on the Mesozoic, That this was, in a most wonderful degree, an age of reptiles, is easily shown. In the world, at the present time, there are about six great reptiles — one crocodile in Africa, two gavials in India, three alligators in America, North and South — all of them in tropical and sub-tropical regions, and none more than twenty to twenty-five feet long. Now, take a single epoch, the Wealden — compar- able, therefore, with the present — and only the small area of England. There were in England, in Wealden times, five or six dinosaurs, twenty to sixty feet long ; ten or twelve marine saurians and crocodilians, ten to fifty feet long, besides pterodactyls, turtles, etc. Again, in America, in Cretaceous times, leaving out the turtles, there were more than one hundred species of land, marine, and flying reptiles, the larger number of which were greater than any living crocodile. In the epoch of the Atlantosaur beds, reptiles were probably as numerous, and certainly of still greater size. These are the known ; but, of course, the findings are but a small fraction of the actual fauna. The fact is, reptiles were rulers in every realm of Nature. They stood in place of beasts, as rulers of the land ; of whales and sharks, as rulers of the sea ; and in place of birds, as rulers of the air. They impressed their reptilian character— the fashion of the court — o]i all other higher classes ; the mammals were reptilian, and so were the birds. 360 HISTORICAL GEOLOGY, Disturbances which closed the Cretaceous Period and Mesozoic Era. — Remember that during the Creta- ceous a great sea, stretching from the Gulf of Mexico to the Arctic Ocean, covered the whole Plains and Plateau region, and divided the continent into two continents — an eastern and a western. Now, at the end of the Creta- ceous, this great sea was abolished by the gradual upheaval of this region, and the continent became one. At the same time the western marginal bottom of the great interior sea yielded to horizontal pressure, and was crushed together and swelled up into the Wahsatch Range, At the same time, also, the Colorado Moun- tains, which had been a line of islands in the Cretaceous sea (map, page 349), were pushed up, and the Cretaceous strata sharply uptilted on the flanks. At the same time, also, the Uintah Mountains seem to have been born. Such great changes in physical geography imply corres- ponding changes in climate, and in fauna and flora. AYe ought to, and do, indeed, find the animals and plants very different in the next age (Cenozoic). Laramie or Transition Epoch, The abolition of the great Cretaceous sea, and the unification of the continent, as we have said, were pro- duced by the upheaval of the Plains and Plateau region. When completed, the Plateau region was occupied by great fresh-water lakes, which we shall describe hereafter. But this change took place gradually, passing through intermediate stages of brackish-water seas. AVhen marine conditions prevailed, it was undoubtedly Cretaceous ;, when fresh-water conditions were established, it was undoubtedly Tertiary. But what shall we call the inter- mediate time of brackish water ? This is evidently a transition period. It is the lost interval between tlie Cretaceous and the Tertiary in Europe, recovered here. MESOZOIG ERA.— AGE OF REPTILES, 361 As we might expect, we find Cretaceous types lingering and Tertiary types coming in, and the two coexisting side by side. The Cretaceous dinosaurs linger, but the Tertiary plants are introduced. The paleo-zoologists are disposed to ally it with the Cretaceous, and the paleo- botanists with the Tertiary. It is really a transition be- tween the two. Plants. — Vegetation was luxuriant at this time. More than three hundred species of dicotyls have been described here. But, as the types are wholly Tertiary, we shall illus- trate them under that head. Coal. — The conditions seem to have been favorable, not only for luxuriant vegetation, but for its preserva- tion as coal, and nearly all the Cretaceous coal mentioned on page 351 belong to this transition period, and have therefore been often put in the Tertiary. Next to the Coal-measures, this is the great coal-bearing period of the United States. The largest fields are in the Plains and Plateau region, viz. : 1. A large field, the Marshall coal-field, in western Kansas, about 5,000 square miles. 2. Another large field, in New Mexico, of equal size. 3. A third large field, in Dakota, extending into British America. 4. A large and valuable field, in the Plateau region, on the Laramie Plains, stretching through Wyo- ming to the borders of Utah. These altogether can not be less than 50,000 square miles. On the Pacific slope several coal-fields, probably of the same age, are found : 1. Mount Diablo and Corral Hollow field. 2. Seattle, Carbon Hill, and Bellingham Bay field. 3. Nanaimo or Wellington field on Vancouver's Island. Coal is also found in Arizona and in southern California, but the age is not known. All the later coals are often called lignites, but much of it is an excellent coal, scarcely distinguishable from carboniferous coal. We herewith present in tabulated form all the principal coal-fields of the United States : 362 HISTORICAL GEOLOGY, Carboniferous. Jura-Triassic. Laramie. Appalachian. Central. Western. Michigan. Eastern Virginia. North Carolina. Plains and Plateau. Pacific Slope. 192,000. [ oOO (?). [ 50,000 (?). Animals — Reptiles. — A large number of the most extraordinary reptiles yet discovered (Fig. 324), and also several species of small marsupial mammals, are found in these uppermost Cretaceous beds. Fi«. 3;i4. — Triceratops prorsus, x j^- (After Marsh.) CHAPTER V. CENOZOIC ERA. — AGE OF MAMMALS. This is reckoned a primary division — an Era — because there is just here a very general break in the rock-system, and a very great change in the life-system. It is also called an Age, because a new and higher dominant class appears here. In Europe, the unconformity is universal, and, as might naturally be expected, there is an appa- rently sudden change in the life-system. But in America the Laramie is not only everywhere conformable with the Cretaceous beneath, but in many places also with the Tertiary above ; so that the record is almost continuous. And yet, at the same level, viz., between the Laramie and the Tertiary, we find an enormous change of life- forms. It is impossible to account for this, unless we admit that the steps of progress were quicker at this time. General Characteristics. — In a geological sense, modern history commences here. Modern types of ani- mals and plants, modern aspects of field and forest, were fairly inaugurated. Now was established in broad outline the present order of things — the present rulers on land (except man), in the seas, and in the air ; the present adjustment of the orders of animals and plants. Hence the name, " Cenozoic." Some of these characteristics, however, especially the introduction of Dicotyls, and therefore the aspect of forests, were anticipated in the Cretaceous. As there is now a new and higher dominant 363 364 HISTORICAL GEOLOGY. class, viz., mammals, reptiles must decline in number and size, and thus seek safety in a subordinate position. Subdivisions. — The Cenozoic era and Mammalian age is divided into two periods — Tertiary and Quaternary. In the Tertiary all the mammalian species are extinct, but many invertebrate species are still living, and the percentage of living species increases with time. In the Quaternary, on the contrary, nearly all the invertebrate species, e. g., mollusks, still survive, and some of the mammalian species also survive. These facts are shown in the diagram (Fig. 325). The space above the lines of mollusks and mammals shows proportion of extinct, Fig. 325. — Diagram showing the relative number of species living and extinct. and below the line, of living, species. The dawn of liv- ing species of shells is with the beginning of the Ter- tiary ; the dawn of living mammalian species is in the Quaternary. Both curves show increasing nercentage of living species with time. Section" I. — Tertiary Period. As already stated, the dawn of living molluscan species is in the earliest Tertiary, and thenceforward the per- centage of living species steadily increases ; but no living mammalian species are found there. Subdivisions. — The subdivisions of the Tertiary period into epochs are founded on this percentage of living mol- luscan species. It is thus divided into three epochs — Eocene, Miocene, and Pliocene. If we find a stratum GENOZOIC ERA— AGE OF MAMMALS. 365 which contains not more than 5 to 10 or 15 per cent, of its shells still living in neighboring seas or lakes, we call it Eocene; if 15 to 40 or 50 per cent., we call it Miocene ; if 50 to 80 or 90 per cent., we call it Pliocene. This is graphically illustrated in the diagram (Fig. 325). ( Pliocene, 50 to 90 per cent, living. Tertiary Period. \ Miocene, 15 " 50 ** ** ( Eocene, 5 ** 16 ' " «* Roch'System, Areas in the United States. — 1. On the Atlantic border, going south, we find no Tertiary until we reach New Jersey. Thence to Georgia there is a band of Ter- tiary strata about a hundred miles wide, resting in New Jersey on the Cretaceous, but elsewhere against the Archaean gneiss. It constitutes what are called the low countries of the Southern Atlantic States. The rivers, in passing from the gneissic to the softer Tertiary, make falls or rapids. Here, therefore, is the head of naviga- tion of the Southern rivers, and, therefore, also the posi- tion of many important towns. Richmond and Peters- burg, Virginia ; Raleigh, North Carolina ; Columbia, South Carolina ; Augusta, Milledgeville, and Macon, Georgia — are thus situated. 2. The same broad strip of Tertiary lowlands borders the Gulf, resting there, however, on the Cretaceous (see map, page 272), expands northward to the mouth of the Ohio River, and sweeps southward about the western bor- der of the Gulf into Mexico. 3. On the Pacific border we find Tertiary with Creta- ceous, forming the Coast ranges of California and Ore- gon. All these border Tertiaries — Atlantic, Gulf, and Pacific — are marine deposits. 4. But in the interior regions — i. e.. Plains, Plateau, and Basin — we have extensive fresh- water deposits. Some 366 HISTORICAL GEOLOGY. of these are Eocene, some Miocene, some Pliocene. The Eocene deposits are in the Plateau region north and south of the Uintah Mountains. The Miocene and Pliocene deposits are in the Plains and the Basin regions. These fresh-water deposits of the West are imperfectly lithified, and therefore are sculptured by erosion into the curious forms called Mauvaises Terres, as already ex- plained (page 248, Fig. 152). Physical Geography. — It is easy, from the distribu- tion just given, to reconstruct the physical geography of the American Continent during the Tertiary. It is sim- ply a restatement in another form of what we have already said. On the Atlantic border the New England shore-line was farther out than now, because we have no Tertiary deposits exposed along that coast. The Tertiary shore- line crossed the present shore-line about New York, and thence passed along the line of limit of the Tertiaries of the Southern Atlantic States, the waves beating there against Archsean shore-rocks. On the Gulf border the north shore of the Gulf did not reach quite so far as in Cretaceous times (see map on page 272), but the Gulf waters covered all the flat lands about the Gulf, beating here on Cretaceous rocks ; extending north, as an embay- ment to the mouth of the Ohio Eiver, and then swept southward, covering a broad strip on the west. The Up- per Mississippi (if it existed at all) and the Ohio emptied by separate mouths into the embayment. On the Pacific border the waves of the Pacific beat against the foot-hills of the Sierra, the place of the Coast Eange being then a marginal sea-bottom. Fig. 326 shows the American con- tinent in Early Tertiary. The interior regio7i was occupied by enormous lakes. During the Eocene, the lakes were in the Plateau region ; during Miocene and Pliocene times, in the Basin on the one side and the Plains on the other. Coal, — Lignite is found again in the Tertiary, espe- CENOZOIG ERA.— AGE OF MAMMALS. 367 cially in the Miocene. The Coos Bay coal of Oregon, and the imperfect seams of the Contra Costa Range, Califor- tiia, are Miocene. The lone brown coal of Amador County, California, is still more recent, probably Plio- cene. 368 HISTORICAL OEOLOGY. Life-System. General Character. — This era is called Ceiiozoic be- cause modern life in its main features commences here. We are therefore prepared to find that, among plants and lower animals, the general similarity to present forms is so great that the difference would hardly be recognized by the popular eye. We must touch yery lightly on these lower forms. Plants. — We have already seen that in the Cretaceous many familiar genera of forest-trees were introduced. In fact, so far as trees are concerned, the Oenozoic might be said to commence in the Cretaceous. In the Tertiary nearly all the genera are the same as now, although the species are mostly different. The genera are the same as now, hut not in the same localities. On the contrary, the same genera grew much farther north than now. The vegetation indicated a much warmer temperature than now. In Eocene times, palms and other tropical plants grew all over Europe, and the mean temperature seems to have been 75° to 80°. In Miocene times, evergreens, like those now about the shores of the Mediterranean, flour- ished even to Lapland and Spitzbergen. The mean tem- perature of Europe was 16° to 20° higher than now. In America, during the Eocene, palms and figs and evergreens, in Dakota, show a temperature there about that of Florida now. In Miocene times. Sequoias very like the Big tree and the Eedwood of California, and taxodiums, and magnolias — almost, if not quite, identical with the cypress of the Southern swamps and the Mag- nolia grandijiora of Southern forests — were abundant in Greenland. The temperature of Greenland was then at least 30° higher than now. It is easy to see that polar ice could not have existed, and Arctic expeditions would have been an easy matter, if man had lived at that time. We give some figures of Tertiary plants (Figs. 327-332). CENOZOIC ERA,— AGE OF MAMMALS. 309 But if these highest plants were exceptionally abuii- riant, so were also the lowest of all, vi/., the unicelled Fig. 327. Fig. 328. Fig. 320. f Fig. 3.30. Fig 331. Fig. 3-^2. Figs. 327-332.— American Tertiary plants (after Safford and Lesquereux): 327. Quer- cus crassinervis. 328. Andromeda vaccinifoliae afflnis. 329. Carpolithes irregu- laris. 330. Fagus ferruginea— nut. 331. Fruit of Sequoia Langsdorfii (after Heer). 332. Leaf of Sequoia Langsdorfii (after Heer). diatoms. The great deposits of diatomaceous earths found in many j^arts of the world are Tertiary. In the United States the best known localities are near Rich- mond, Virginia, and in California. These deposits are many miles in extent, and thirty to one hundred feet thick, and made up wholly of the silicious shells of these microscopic plants. Le Coxte, Geol. 24 370 HISTORICAL GEOLOGY. Animals. The similarity in general appearance of most Tertiary invertebrates to living species is so great that we shall only draw brief attention to a few interesting points. MoUusca. — We are all doubtless interested in the fam- ily history of the oyster. The family commenced in the Jurassic^ increased in the Cretaceous, and culminated in the Tertiary, and then declined. The Ostrea Georgiensis and the Caroliiiensis of the Eocene were several times larger than their modern representative. The Ostrea Titan, of the Pacific coast Miocene, was still larger, being thirteen inches long, eight inches wide, and six inches thick. Lest some may regret inconsolably the passing away of these magnificent oysters before the advent of man, I hasten to remind them that what has been lost in size has probably been gained in flavor. Insects. — Insects are always closely associated with land vegetation, and the kinds of the one are determined by the nature of the other. Now, for the first time, the highest flowering plants are abundant, and now, for the first time also, all orders of insects, even the highest flower-loving kinds, such as butterflies, bees, ants, etc., are abundant (Fig. 333). On account of the greater Fig. 333.— Ants and bees of European Miocene. (After Heer.) warmth and moisture, both vegetal and insect life were fuller even than now. We select a few examples of find- ings, by means of which we may reproduce in imagina- CENOZOIC ERA.— AGE OF MAMMALS. 371 tion the conditions of things which prevailed in Tertiary times : 1. In the Miocene fresh-water deposit of Oeningen, a layer two feet thick is black with the remains of insects. It is also full of leaves. About nine hundred species of insects and five hundred species of plants have been made out. The larger number of insects are beetles and ants. We may imagine that in Miocene times there was at Oeningen a lake surrounded Avith a thick forest, whose leaves were scattered on the waters and cast upon the shore. Beetles and flying ants, essaying to fly over the lake, were beaten down by the winds and also cast on the shore. These remains were covered up by mud, and thus preserved. 2. On the shores of the Baltic, bits of amber, derived from Miocene strata outcropping beneath the water, are continually thrown up by the action of the waves. In these are found, sealed up, and in transparent pieces clearly visible, great numbers of insects, often in an exquisitely perfect state of preservation. About eight hundred species of insects and one hundred and fifty species of plants have been described. The insects are mostly winged ants and flies. Amber is known to be the fossil gum of a pine {Pinus succinifer). We may imagine, then, that in Miocene times, in the region now occupied by the southern Baltic, there was a forest, among the trees of which the Pinus succinifer abounded. From these trees a semi-liquid, sticky gum exuded in tears, on which insects alighting stuck fast, and were covered by later exudations. 3. In Auvergne, France, there is a Miocene fresh-water deposit, one layer of which, two to three feet thick, is almost wholly composed of the cast-off cases (indusia) of caddis-worms, and is therefore called indusial limestone. The caddis-worm (larva of the caddis-fly) of to-day is a wingless creature, living wholly in the water. It has the 372 HISTORICAL GEOLOGY. curious habit of gathering bits of wood, small dead shells, or even grains of sand, and webbing them together to form a cylindrical hollow case in which it lives. When it E^G. 334.— Fragment of indusial lime- stone (natural size), showing the caddis-worm cases. Fig. 385 — Recent caddis-worm, with its case. wishes to walk about, it puts out the head and legs for that purpose, as seen in the figure. These cases are left when the worm changes into the caddis-fly. AVe may imagine, then, that in Auvergne, in Miocene times, there Flo. 336. — Prodryas Persephone, (After Scudder.) was a lake in which lived countless generations of caddis- worms, and their cast-off cases accumulated until a deposit, two to three feet thick, was produced. (JENOZOJd ERA.—AaE OF MAMMALS. 373 4. Only very recently a remarkable American locality lias been discovered. At Florissant, Colorado, a fresh- water deposit of Upper Eocene or Lower Miocene age has ])een found, one layer of which is black with the remains of insects of all kinds. Scudder has identified more than a thousand species. We give here (Fig. 336) a beauti- fully preserved butterfly from this locality. Here, then, we have phenomena like those at Oeningen, and explained in the same way. Fishes. — In general appearance. Tertiary fishes are much like those of the present day. Then, as now, Tele- osts vastly predominated (Fig. 337), and Ganoids were Fig. 337.— Tertiary fishes— Teleosts : Lebias cephalotes, Miocene. nearly extinct. Then, as now, sharks were among the chief rulers of the seas. In fact, they seem to have cul- minated in the Tertiary. The Eocene strata of the At- lantic border are in places full of sharks' teeth, some of which are of incredible size. We have seen one of these, of the kind represented in Fig. 338, which would more than cover a page of this book, being nearly seven inches long and six inches wide. The original possessors of such 374 HISTORICAL GEOLOGY. teeth could hardly have been less than sixty to seventy feet long. Reptiles. — The reign of reptiles is past. The Reptilian dynasty is overthrown. This class no longer oc- cupies a prominent place in history. In geological history the ruling class is always the fittest to rule, which can not al- ways be said of the reigning families in hu- man history. The great char- acteristic reptiles of the Mesozoic are all extinct. Among great rep- tiles, the crocodili- ans alone remain. The reptiles of the Tertiary are of the same fam- ilies as now exist, viz., crocodiles, turtles, snakes, lizards, frogs, toads, and salamanders. Snakes seem a low type, and yet were intro- duced only in the Tertiary. But they are not low in the sense of it7idev eloped. They have developed backward — they are an example of a degraded type. The tailless am- phibians (frogs and toads) are undoubtedly the highest among amphibians ; for they pass through the tailed stage (tadpole) in embryonic life. These tailless amphibians were introduced first in the Tertiary. The biggest known turtle (Colossochelys) was found in the Miocene of India. Its shell was twelve feet long, eight feet wide, and seven feet high. • Fig. 338.— Tertiary fishes— Sharks : Carcharodon megalodon, x J. (After Gibbes.) CENOZOIC ERA.— AGE OF MAMMALS. 375 Birds. — It will be remembered that the earliest bird known (the Jurassic Archaeopteryx) was also the most reptilian. In the Cretaceous we found both reptilian toothed-birds and ordinary water-birds. Now, in the Tertiary, as in the present, all the reptilian birds had disappeared, and only typi- cal birds remain ; and not only water-birds, but also the highest, viz., land-birds. In other words, the bird-class had now fairly separated it- self from the reptilian, and the connecting links were all destroyed. Nearly all the families of birds now existing have been found in the Tertiary, but also a few of strange forms. The Gastornis, of the Eocene of Paris (Fig. 339), was a huge bird, ten feet high, and a curious connecting link be- tween waders and ostriches. Besides these curious forms, many birds have been found, in the Tertiary of this coun- try and in Europe, similar to those still living. But in France, especially, the birds, like the plants and insects, show a decided tropic climate. Parrots, trogons, ibises, secretary-birds, and flamingoes inhabited France at that time. Mammals. Remember that, although marsupials or reptilian mam- mals were found in Jura-Trias, and doubtless continued Pig, 339.— Eestoration of Gastomis Eduardsii. (After Meunier.) 376 HISTORICAL GEOLOGY. through the Cretaceous, true, ordinary, or typical mam- mals first appear in the lowest Tertiary, and immediately became the dominant class. Some Preliminary Remarks. — Before describing the Tertiary mammals, there are some points requiring notice : 1. The suddenness of their appearance is very remark- able. In the very lowest Tertiary, without warning and without apparent progenitors, true mammals appear in great numbers, in considerable diversity, and even of the highest order — Primates, or monkey tribe. Now, in Eu- rope, where there is a decided break and a lost interval, this is not so surprising ; but even in America, where the Laramie passes without break into the Tertiary, the same is true. At a certain level the great dinosaurs disappear, and the mammals take their place. A new dynasty and a new age in history commence. It is impossible to account for this by natural causes, unless we admit times of rapid progress. In addition to this, we must also admit that the apparent sudden appearance in a particular place is largely due to migration. 2. We have said that they appeared in great numbers and considerable diversity. All the great branches of the Mammalian class were represented in the first fauna — herbivores, carnivores, and primates or monkeys. Yet these were not so distinctly separated as now. Thei/ were all generalized types. If we represent all the orders and families of mammals as branches and sub-branches of one main trunk, then, as we go backward in time, these be- come less numerous and less widely separated. In the earliest Eocene the branches are few and very near to- gether. The carnivores are but slightly separated from the herbivores — in fact, they are both omnivores. The monkeys, also, were not yet fairly separated as typical monkeys. They are therefore called Prosimiae, or pro- genitors of the true monkey. Manifestly, if these branches CENOZOIC ERA.— AGE OF MAMMALS, 377 have a common origin, it must be sought still lower, prob- ably in the Laramie. Tertiary Lake-Deposits of the West. Nowhere in the world is there so complete a series of Tertiary deposits and of Tertiary mammals as in the lake- deposits of the Plateau and Plains and Basin regions al- ready spoken of (page 365). We shall therefore take most of our illustrations from these. Eocene Lake-Deposits. — In the Lower Eocene de- posits — viz., Puerco beds and Wahsatch or Coryphodon beds — have been found nearly one hundred species of mammals, including carnivores, herbivores, insectivores, and monkeys. Perhaps the most remarkable and charac- teristic animals of the lower Tertiary were the Corypho- donts. These were huge animals with very small brains, plantigrade feet, slow, awkward movements, and very generalized structure. In the Middle Eocene Bridger heds, mammalian life was even still more abundant. More than one hundred species are known, and these are, of course, but a fraction of what actually existed. Perhaps the most remarkable animals of this time were those of the Dinoceras family. The Dinoceras may be taken as a type of the family. This was a heavy-built, sluggish-moving animal of ele- phantine size, with a most singular conformation of head, which was armed with three pairs of horns and a pair of huge tusks, as shown in Fig. 340. Some are supposed to have had a head five feet long. During the Eocene, also. Marsh finds the earliest pro- genitors of the horse. In the early Eocene is found the Eohippus, an animal which had three hoofed toes on the hind-feet and four perfect hoofed toes and a rudimen- tary fifth toe on the fore-feet. This was followed in the Middle Eocene by the Orohipjms, similar to the other^ 378 HISTORICAL GEOLOGY. except that the fifth rudimentary toe is dropped. These animals were about the size of a fox. Fig. 340.— Tinoceras ingens. (After Marsh.) Miocene. — The Eocene lake-deposits are in the Pla- teau region, the Miocene and Pliocene are in the Plains and Basin region. The first thing to be noted here is the complete change of species. It is worthy of note that in the Miocene many existing /am^7m (not species), such as the rhinoceros family, the camel family, the deer family, the dog family, and the cat family commenced to exist. Among the many forms which occur here we can only mention the most remarkable. In this respect, certainly, the Brontothere stands first. This animal was still larger than the tinoceras, and connects the latter with the rhinoceros. The peculiar saddle-shaped head was three feet long. It had only three toes behind, like the rhi- noceros, but four in front. In the Miocene the horse family is represented by the MesoMppus and MioMppus. These had three toes on the hind and the fore foot, and were about the size of a sheep. True tridactyl horses commence here. CENOZOIC ERA.— AGE OF MAMMALS. 379 Pliocene. — Here, again, we have a great change of mammalian species. The animals are much more like existing species. If many existing families commenced Fig. 341.— Broutops. (After Marsh.) in the Miocene, we find many existing genera, such as the horse (equus), the camel {camelus), the elephant (elephas), etc., commencing in the Pliocene. Great numbers of the horse family, Protohippus, Plichippus, and finally Equus, great numbers of the camel family, several elephants and mastodons, roamed in herds over the American Continent. The Protohippus was a three-toed horse, like the Miohip- pus, but the side-toes were shorter. It was very similar to the Hipparion of Europe (Fig. 343). The Pliohippus was very horselike. It was one-toed, like a true horse, the two side-toes having dwindled to splints. Foreign Localities. Paris Basin. — Among foreign Tertiary deposits the most celebrated is the Eocene basin of Paris. The streets of Paris teem with a living generation of men and animals. 380 HISTORICAL GEOLOGY. Its cemeteries are full of the remains of a former genera- tion. But a little deeper down we find another cemetery full of the remains of extinct animals of strange forms. The masterly study of this fauna by the illustrious Cuvier gave an incredible impulse to geology. One striking char- acteristic of this fauna was the great predominance of tapirlike animals. Of fifty species of mammals found here, forty species were of this general kind. The most celebrated of these remains are the Paleothere (Fig. 342) Fig. 342.— Paleotherium magnum, x ^. (After Gaudry.) and the Anoplothere. The Paleothere was a three-hoofed animal allied to the tapir, and perhaps connecting with the horse family. The Anoplothere, on the contrary, was a two-hoofed animal, apparently connecting tapirs with the ruminants. In these two we have the even-toed and the odd-toed hoofed animals almost united. The great bird Gastornis, figured on page 375, was found here. It is probable that during the Eocene the Paris basin was the place of an estuary, and the bodies of animals of that epoch were washed down by a river and buried in sediments at its mouth. In the European Miocene great numbers of remains CENOZOIC ERA.— AGE OF MAMMALS. 381 have been found. Corresponding with the Miohippus and perhaps the Protohippns of the United States, was the graceful tridactyl horse (Hipparion), represented in Fig. 343. The most remarkable animal of this time was the Fig. 343.— Skeleton of Hipparion gracile, restored. (After Gaudry.) huge Dinothere, the earliest of the Proboscidians. It had a proboscis, but not yet developed to the size and strength which this organ attained in the mastodon and the ele- phant. The singular form of the head is shown in Fig. 344. True monkeys were introduced in the Miocene, and that most destructive of carnivores, the saber-toothed tiger (Machairodus), in the Pliocene, though the genus culminated in the Quaternary (see Fig. 356, page 402). Some General Observations on the Tertiary Mam- mals ; Genesis of Mammalian Orders and Families, etc. — We have already said that in the earliest Eocene, the great branches of the mammalian class were very near together, tliough their point of union has not yet 382 HISTORICAL GEOLOGY. been found. As time went on, these separated more and more widely, and gave off sub-branches, which again divided, and so on. In general terms, it may be said that some of the existing orders may be traced back to the Eocene. Many of the exist- ing families commenced in the Mioce^ie ; existing genera in the Pliocene ; but existing species only in the Quater- nary. This is well illustrated by one great branch, the Un- gulates, or hoofed animals. These consist now of many widely separated sub- branches ; but in the earliest Fig. 344.-Head of Dinotheriam rp^^^- ^. ^^^^ ^^ ^^.^^ giganteum, greatly reduced. , •^ •^ . mto one, a primal ungulate. As we go up, this branch separates, even in the Upper Eocene, into odd-toed (perissodactyls) and even-toed (artiodactyls) ungulates. In the Miocene, each of these again separates, the former into the elephant family (Proboscidians) with five toes, the tapir and rhinoceros families with three toes, and the horse family, with three toes passing into 07ie; the latter into the hog and hippo- potamus families with four toes, and the ruminant family (horned animals) with two toes. Genesis of the Horse. — Let us trace one of these branches throughout. We select for this purpose the horse. A most wonderful series representing this family, about forty species in all, has been furnished by the American Tertiaries, and the successive steps traced by Professor Marsh, First of all, in the early Eocene Wah- satch beds, we find the Eohippus (dawn-horse). This little animal (the size of a fox) had three toes on the hind-foot, and four perfect toes and a fifth splint, and perhaps dew-claw, on the fore-foot. Next, in the Middle CENOZOIC ERA.— AGE OF MAM3IALS. 383 a b c d e f g Equus • Qua- ternai-y and Recent. Pliohippus : Pliocene. Protohippus • Lower Plio- Miohlppus • Miocene. Mesohippus Lower Mio- Orohippus : Eocene. Pig. 345 Diagram illustrating gradual changes in the horse family. Throughout, a is fore-foot ; &, hind-foot ; r, fore-arm ; d, shank ; e, molar on side-view ; /and g, grinding surface of upper and lower molars. (After Marsh.) 384 HlSTOniGAL OEOLOQW Eocene, came the Orohippus, about the same size, with three toes behind and four in front — the fifth splint being dropped. Next, in the Miocene, came the Mesohippus and the Miohippus (about the size of a sheep), with tliree toes behind and in front, but the fourth toe of the Oro- hippus still retained as a useless splint. In these the horse family may be said to be fairly established. Then, in the Lower Pliocene, came the Protohippus, about the size of an ass, with three toes on all the feet, but the two side-toes shorter, and the mid-toe larger, than before. Then, lastly, in the uppermost Pliocene, come the Plio- hippus and Equus, in which the side-toes are reduced to useless splints, and the middle toe is greatly enlarged. This is the case in the modern horse ; its side-splints attest its three-toed ancestry. Crust-Movements during and closing the Tertiary Period, Remember that, during the Cretaceous, a great sea covered the whole of the Plains and Plateau region, dividing the continent into two continents. By the gradual elevation of the region, this sea was obliterated and replaced by great lakes. The formation of these lakes inaugurated the Tertiary. The elevation of the same region continuing, these Tertiary lakes were suc- cessively obliterated, and the prodigious general erosion and cafi on-cutting of this region commenced. On the Pacific border, at the end of the Miocene, the Coast Range of California and Oregon was born. From the beginning of the Cretaceous, the place of this range had been marginal sea-bottom receiving sediment. At the end of the Miocene, these yielded to horizontal pres- sure, were crushed together, and swelled up into this great range. Probably at the same time occurred the great lava-flood of the northwest, described on page 218. On the Atlantic border the changes were far less remarkable. There was, however, a gradual increase of CENOZOIC ERA.— AGE OF MAMMALS, 385 the land along the border, until, at the end of the Ter- tiary, the continent was finished, except the southern part of Florida and its keys, and a very narrow strip along the Southern coast generally. The southern point and the keys of Florida are still growing (see page 111). Section^ II. — Quaternary Period. This is one of the most interesting and yet most diffi- cult portions of the history of the earth. It is the last period preceding and preparatory to the present. Characteristics. — The grand characteristic of this pe- riod is the occurrence of wide-spread up-and-down move- ments of the earth's crust in high latitudes or circum- polar regions north and south, attended with great changes of climate from extreme rigor to temperateness, and consequent great changes in species. Also, the age of mammals seems to culminate here, and man appears on the scene, and was doubtless an important agent among others in bringing about the change of species. Nearly all the invertebrate species and some mammals of the Quaternary are still living. A small percentage of the present mammalian species, man among the number, commenced here (see Fig. 325, page 364). Subdivisions. — The Quaternary period is divided into two epochs, founded upon the attitude of the land and the changes of climate. These are — 1. Glacial. 2. Cham- plain. The Glacial epoch was characterized by upward crust-movement in high-latitude regions, until the land there stood 2,000 to 3,000 feet higher than now, was sheeted with ice, and an Arctic rigor of climate extended in America almost to the shores of the Gulf. The Cham- plain epoch was characterized by a downward movement in the same region until the land was 500 to 1,000 feet lower than now, so that many lower parts of the conti- nent were covered with sea ; and by a moderation of tern- Le Conte, Geol. 25 38G HISTORICAL GEOLOGY, peraturCj a melting of ice, and a flooding of lakes and rivers. It was therefore a flooded epoch. Loosened icebergs floated over the flooded seas and lakes. It was therefore, also, an epoch of the reign of icebergs. From this condition the crust gradually rose again to the pres- ent condition of things. Similar changes seem to have occurred everywhere in high-latitude regions, but we are not sure that they were absolutely contemporaneous. Therefore it will be best to take the whole series of changes right through for each locality. We commence with the Eastern United States, because it has been best studied there. QUATERNAKY IK EASTERN" NORTH AMERICA. 1. Glacial Epoch. The Drift. — The phenomena now about to be de- scribed are extremely varied ; but, as they exist all over the Northern United States, we insist that every one observe for himself. What we say is meant 07ily as a guide. All over the northern portion of our country, from 38° to 40° latitude northward, mantling over hill and dale, over mountain and valley, is found a peculiar deposit or soil composed of a heterogeneous mixture of earth, gravel, pebbles, and rock-fragments of all sizes. As this material has evidently been shifted and sometimes brought from a long distance, it is called Drift. It is impossible to make a description which will apply to all cases, but almost everywhere the lower part in contact with the bed-rock consists of stiff clay with disseminated stones rounded or partly rounded, and scratched (Fig. 346). This is called the stony-clay or lowlder-clay. It is exactly like the ground moraine of a glacier, mentioned on page 58. In places are found heaps or dumps of loose materials sim- ilar to the top moraine of glaciers. In places the ma- CENOZOIC ERA.^AGE OF MAMMALS, 387 terials may be irregularly stratified and cross-laminated, as if by water running beneath, or from the snout of a glacier. In places the laminae may be twisted and crum- FiG. 846.— Subangular stone. (Af ter Geikie.) pled, as if by a glacier pushing along on a mud surface. In still other places, especially west of the Appalachian, the upper part is more widely stratified. But this may belong to a later epoch (Champlain). Bowlders. — Over all are scattered rock-fragments and bowlders, of all sizes, both angular and rounded — some- times as thick as hailstones after a storm, and actuall}^ cumbering the earth. These bowlders, whether imbedded in drift or scattered on the surface, are usually entirely different from the country-rock. Great blocks, of thou- sands of cubic feet, are often seen perched where they do not belong, as if stranded by glacier or iceberg. The par- ent ledge from which they were torn can often be found, and thus the direction of their transport is known. By this means it has been ascertained that from the Cana- 388 HISTORICAL GEOLOGY. dian highlands the material has been carried southeast- ward, southward, and southwestward. The distance car- ried has been in some cases several hundred miles. Bed-Rock Surface. — AVherever the drift-mantle is removed, the bed-rock underlying is found to be glaciated, i. e., it presents a smooth, billowy surface, scored with straight parallel marks, precisely like the pathway of a glacier, described on page 61. The general direction of these marks is the same as that of the transport of the bowlders, viz., southeast, south, and southwest. Southern Limit of the Drift ; Ice-Sheet Moraine. — The most characteristic of the phenomena described, viz., the stony clay, the glaciated bed-rock, and the great bowlders, extend over the whole northern portion of the continent, down to about 38° to 40° north latitude. Along this southern limit are found remnants of the termiyial moraine of the ice-sheet. Its position is marked on the map by the strong line. Within this, and marked on the map by the dotted lines, another and later and far distincter terminal moraine is seen sweeping about the Great Lakes and westward in huge festoons (Fig. 347). Explanation. — The simplest explanation of these facts is, that during this epoch the whole northern part of the continent was elevated, so that the Canadian highlands were 1,000 to 2,000 feet above its present level, and com- pletely covered with an ice-mantle several thousand feet thick, as Greenland and the Antarctic Continent are to- day. This ice-mantle, covering everything except per- haps the highest peaks, moved southeastward, southward, and southwestward, scoring the whole surface of the coun- try in its path, and accumulating bowlders and earth be- neath it. At its limit, represented by the strong line seen on the map, the accumulations, being more abundant, formed a moraine. After a while, the ice-limit, by melt- ing, went northward, dropping bowlders in its course to CENOZOIC ERA.— AGE OF MAMMALS. 389 or perhaps beyond the lakes, but again advanced, and formed the deeply lobed moraine marked by the dotted lines. We have given only the limit of the general ice-sheet. But in mountain-regions, e. g., in Colorado, and perhaps Fig. 347.— Map showing limit of the drift and the second ice-sheet moraine. Limit of northern drift represented by heavy line from Long Island to Minnesota ; second ice-sheet moraine represented by triple dotted line. in Virginia, even beyond this limit, there were great separate glaciers, occupying the valleys, as shown by the moraines left by them. The tracing of the course of these old glaciers by their glaciated pathways, perched 390 HISTORICAL GEOLOGY. bowlders, and terminal and lateral moraines gives a fas- cinating interest to travel among these mountains. Contrast of Northern and Southern Soils and Rock-Surfaces. — Nothing can be more striking than the contrast between the soil and underlying rock-surfaces within and beyond the limits of the Drift. Within these limits the covering is a heterogeneous mass of shifted ma- terial lying on sound rock ; south of this limit the soil is stratified, and in many places graduates into the rock be- neath from which it has been formed by rotting in place. Again, the underlying rock in drift-regions is glaciated, I. e., smooth, moutonneed, scored; beyond the drift-region there is either no distinct surface to the rock, or else, if there be, it is a rough, weathered surface. 2. Champlain Epoch. At the end of the Glacial epoch, when the condition of things was such as described above, there commenced a crust-movement in a contrary direction, by which the land in the same region was brought downward 100 to 500 or 1,000 feet below this present level, and the lower parts of the continent became covered with the sea. It was therefore a period of inland seas. The movement was attended with moderation of temperature, by which the ice-sheet was melted and progressively retired northward. The melting ice produced flooded lakes and flooded rivers. It was therefore also di, flooded period. Icebergs, loosened from the northern ice-foot, floated over the inland seas and the great flooded lakes, dropping debris. Some of the great bowlders are probably to be accounted for in this way. It was therefore also a period of iceberg agency. The evidences of this condition of things are found in old elevated sea-margins, lake-margins, and old river flood- plain deposits. Sea-Margins. — Elevated sea-beaches are found in all countries affected with the Drift. The highest one marks CENOZOIG ERA.— AGE OF MAMMALS. 391 the level in the Champlain period. In southern New England it is 50 feet high, in Maine 100 feet high, on the Gulf of St. Lawrence and Labrador 500 feet, and in Greenland 1,000 feet high. The old sea-line may be traced on both sides of the St. Lawrence Eiver, and thence around Lake Champlain nearly 500 feet high, showing that there was a wide bay or sound in this re- gion. It is this which gives name to the epoch. On the bench marking the sea-level about Lake Champlain have been found not only many marine shells, but also the skeleton of a stranded whale. L.ake-Marg'ins. — About all the Great Lakes are found now many terraces or benches rising one above another, the highest marking the greatest extent of the lake. About Ontario the highest is 500 feet ; about Lake Erie, 250 feet ; about Lake Superior, 330 feet. These lakes doubtless at that time ran together, forming a vast sheet of water which drained southward through the Mississippi River into the Gulf. At the same time an enormous lake covered the region about Lake Winnipeg and drained through the Minnesota Eiver into the Mississippi. This ancient lake has been called Lake Agassiz. River-Deposits. — The section. Fig. 348, represents in a r m I _±_A a '" ^^"^B ^L=^ •rS^ R ii II u 1 ." ^ II \ fl a ' 1 m " \ U li ■ " ir^««^--- ^R 1 ' " —L - 1 ' 1 • 1 ./ 1 . .. 1 Fig. 348.— Ideal section across river-bed in drift-region. a general way the condition of the rivers in all the drift- region. Beneath the present river-bed, r, there is a much 392 HISTORICAL GEOLOGY, wider and deeper old river-bed, R R, which is filled up often several hundred feet deep with river-silt, h h, and into this the river is now cutting its bed. The great river- bed, R R, was cut out during the epoch of elevation (Gla- cial) and previous periods. They are preglacial river- beds. The /Z/m^ was' done during this epoch of subsi- dence (Champlain). The river since then has again cut down, but not so deeply. All the rivers in the drift- region, therefore, are bordered on each side by a wide area of old silt, usually much above the present flood- level, and therefore forming high bluffs or terraces, sometimes one, sometimes many, on each side. The Cause of the flooded condition was primarily the great water-supply from melting of the ice-sheet. But it is evident that the subsidence of the land would cause the sea to enter the mouths of many rivers, forming great estuaries ; and also, by diminishing the slope of the river- bed, would tend to increase their floods. From this subsided condition the land gradually rose again, by successive stages, to the present condition. These successive stages are marked by a succession of sea-beaches, lake-terraces, and river-terraces, below the highest just described. As the land rose, successive sea- margins were left ; the outlet of the lakes also cut deeper and deeper, and drained the lakes to lower and lower levels. Also, all the rivers cut deeper and deeper into the old Champlain silts, leaving them as bluffs and ter- races high above the present flood-line (Fig. 348). Some- times there is but one great bluff on each side, as in the Mississippi Eiver. Sometimes there are several terraces, one above the other, as in the case of the Connecticut Eiver. It is evident that when Lake Champlain was first cut off from the sea by elevation it was a salt lake. It was freshened in the manner explained on page 79. CENOZOIC ERA.^AOE OF MAMMALS. 393 Quaternary in the Westerti Part of the Continent, On the Pacific slope the signs of all these movements are clear ; especially are the signs of extensive glaciation magnificent. AYe shall again vary our mode of presenta- tion by tracing the condition of things throughout the Quaternary in seas, glaciers, lahes, and rivers. We take seas first, because by this we establish the oscillations. Seas. — A more elevated condition of land than now exists is plainly shown, not only by the boldness of the Western coast and the existence of a line of bold, rocky islands a little way off shore, a recognized sign of a sunken coast, but also by the remarkable fact that remains of the Quaternary mammoth have been found on one of these islands — the Santa Eosa. When this elephant lived, the island was evidently connected with the mainland. A subsequent subsided condition is demonstrated by sea-margins in many places. We shall describe briefly the condition of the sea. At that time the Bay of San Francisco was enormously enlarged ; for its waters covered the whole of the flat lands about the bay, including the Santa Clara, Napa, and Sonoma Valleys, and then, passing through the Straits of Carquinas, spread all over the great interior valley of California (Sacramento and San Joaquin), forming an inland sea fifty miles wide and three hundred miles long. The old beach-marks may be traced in many places. Lake Tulare is a remnant of this great inland sea. In Oregon the sea went up the Columbia River, and spread over the Willamette Valley, forming a great sound. From this subsided condition the land rose again, making successive terraces down to the present level. Glaciers. — It is still doubtful if the general ice-sheet extended on this coast as far south as California, although abundant evidences are found in British Columbia ; but it 394 HISTORICAL GEOLOGY. is certain that the whole Sierra was at that time covered with perpetual snow, from which ran great glaciers forty to fifty miles long to the valleys below. It is certain that all the valleys and cations which trench the flanks of the Sierra were filled with glaciers of enormous size. Many Fig. 349.— Glaciated surface and scattered bowlders near Lake Tenaya, Cai. (From a photograph by J. N. Le Conte.) of these have been traced in the clearest manner by their polished pathways, their scattered bowlders, and their lateral and terminal moraines (Fig. 349). Lakes. — All the lakes of that time, especially in the Basin region, were greatly enlarged. About Lake Mono, terraces rise, one above another, to 700 feet above the present lake-level, and inclosing an immense area. The lake-waters then washed against the foot of the Sierra, and glaciers ran into its waters and produced icebergs. At the same time, the whole lower part of the Utah and Nevada basins was filled each with a great lake. That which filled the Utah basin, called Lake Bonneville, was GENOZOIG ERA.— AGE OF MAMMALS. 395 100 miles wide and 300 miles long. The traveler on the Union Pacific Railway can hardly fail to observe the old terraces, rising up to 1,000 feet above the present lake- level. It drained at that time into the Snake and Co- lumbia Elvers, then lost its outlet, and dried away to the remnants — Great Salt Lake, Utah Lake, and Sevier Lake — which we now have. The lake which filled the Nevada basin — Lake Laliontan — was of nearly equal size, and its dried-away residues are seen in numerous salt and alkaline lakes, such as Pyramid, Winnemucca, Humboldt, Carson, Walker, etc., which overdot this great area. Rivers. — The old or preglacial river-beds, on the eastern side of the continent, as we have seen (page 391), underlie the present river-beds — i. e., are in the same place, but deeper. In middle California the relation is quite different and peculiar. Here the old river-beds overlook the new — i. e., they are in a different place, and higher. The old river-beds are on the divides between the new. The reason is this : In middle California, at the beginning of the Glacial epoch, the old river-beds .had already been filled up, first with gravel, and then, by igneous outbursts, with lava. The rivers were thus dis- placed, and began to cut new beds. But at the same Fig. 350. — Ideal section through two modem river-beds and table-mountain divide ; ?•', old river-bed ; r, r, present river-beds ; «, slate ; gr, new gravel ; /, lava ; gr'^ old gravel under the lava. time there was a considerable lifting of the whole moun- tain-region, and consequently the rivers now cut deeper than before (Fig. 350). Thus it has come to pass that the new river-beds occupy the places of the old divides. 396 HISTORICAL GEOLOGY, and the old river-beds are now found on the top of the present divides. Phenomena similar to those discussed are found in Europe and in all other high-latitude regions, both north and south of the equator. Some General Results of Glacial Erosion, 1. Fiords. — If one examines an accurate map of coast- lines, he will see that, in the region affected by Quater- nary oscillations, there is a bold, deeply dissected coast- line. In Norway these deep inlets are called fiords, and therefore this structure, wherever found, is called fiord-structure. We find it strongly marked in Green- land and in Alaska. This structure, in Norway, is partly due to the action of waves (page 45), but also, and mainly, to the submergence of old glacial valleys. In Greenland and Alaska they are still partly occupied by glaciers. 2. Lakes. — Examine your map of North America. See how the whole northern part is dotted over with lakes, while the southern part is almost destitute of them. See also that the lake-area is also the area of the drift. Now, although lakes may be formed in many ways, and exist in all parts of the world, yet undoubtedly the small lakes at least, which are so thickly sprinkled over the drift-region, have been produced by glacial agency. There are several ways in which glacial lakes were formed : 1 . They are sometimes roch-hasins, scooped out by glacial erosion. 2. They are often formed by the damming of drainage waters behind old terminal moraines. These two kinds are thickly strewed all over high mountain- regions in the pathways of old glaciers. Standing on the crest of the Sierra, fifty may sometimes be counted at one view. 3. In flat regions, as in northern Minnesota and British America, they are simply hollows produced CENOZOIC ERA.—AOE OF MAMMALS. 397 by inequalities of deposit of the Drift when the ice-sheet retreated. Life-System of the Quaternary, Plants and Invertebrates. — The plants and inver- tebrate animals were mostly identical with those still living. AVe dismiss these, therefore, with one important remark. Quaternary species are indeed still living ; not, however, in the same place, but much farther north. This indicated that the climate was much colder in the Qua- ternary than now. Mammals. — It is only in mammals that we find a striking difference as compared with the present time. Those of the Quaternary are peculiar, differing conspicu- ously both from the Tertiary and the living species. We shall take our first examples from Europe, as they have been best studied there. Quaternary Mammals of Europe. — In Europe they 3,re found sometimes in caves, where in great numbers and of all kinds they have become entombed ; sometimes on river-terraces and old sea-heaches, where their fioating carcasses have been stranded and buried ; sometimes in peat-logSf where, venturing in search of food, they have mired and perished ; and sometimes, as in Arctic regions, in frozen soils, where whole carcasses were sealed up, and are now found perfectly preserved. The Mammalian Age culminates here. — As already said, the mammalian^ age seems to culminate in the Quaternary just before its downfall. For example, in England alone, during this time, there lived a great elephant, the mammoth (Elephas primigenius), much larger than any now living ; two species of the rhinoceros and one of the hippopotamus ; three species of oxen, two of which were of gigantic size ; a wild horse ; several species of deer, among which were the reindeer and the great Irish elk, a magnificent animal, eleven feet high to 398 HISTORICAL GEOLOGY. the top of its elevated jiii tiers mid ten feet between their tips. Of carnivores there were the great cave-bear, larger than the grizzly ; a lion and a tiger as large as the African lion and the Bengal tiger ; a saber-toothed tiger (Machai- 7'odus), more formidable than either, with its saber-like tusks projecting six to eight inches beyond the gums ; hyenas in great abundance ; besides many smaller species. The remains of man have also been found associated with these extinct animals. Mammoth. — This great animal deserves more special mention. During Quaternary times, three great elephants roamed in herds over Europe. The greatest of these — in fact the greatest of all elephants, and the most numerous at this time — was the mammoth {Elephas primiqenius). The remains of these are found everywhere, but the most perfect in Siberia. Here perfectly fresh carcasses have been exposed by the undermining, by the river, of the frozen bluffs of the river-banks. The one represented here (Fig. 351) is in the Museum of St. Petersburg. The dried skin still remains on the feet and portions of the head. It is known from these carcasses that this elephant was covered with a thick wool, and over this long hair. Unlike living elephants, it was adapted to endure cold. The same was true of the Quaternary rhinoceros, the carcasses of which have also been found preserved in the same way. Quaternary Mammals in America. — Great mam- mals were equally abundant in America. There roamed in herds all over this country one species of the mastodon and two species of the elephant, viz., the Elephas primi- geniuSf or mammoth, and the Elephas Americanus. There were also three or four species of the horse, some of gigantic size ; several species of oxen, one of them ten feet from tip to tip of their widely spreading horns ; several species of the e^k, one of them equal to the great Irish elk, and a great number of gigantic edentates, CENOZOIC J^RA.—AGE OF MAMMALS. 399 ground-sloths, and armadillos. Carnivores were not so abundant as in Europe ; but there were several species of the bear, a lion, and a saber-toothed tiger. The Great Mastodon. — The most perfect specimens of the mastodon have been found in the peat-bogs, where, venturing in search of food, they have become mired. 400 HISTORICAL GEOLOGY Fig. 352. — Mastodon Americanns. (After Owen.) In Fig. 352 we give one of the most perfect of these. Any one can dis- tinguish the re- mains of the mas- todon from those of the mammoth, if the jaw-teeth be preserved. The difference is shown in Figs. 353, 354. It is doubtful Fig. 354.— Molar tooth which of theSC twO Fig. 353.— Tooth of Mas- todon AmericanuB. of a Mammoth (Ele- phas primigcniiis), grinding surface. animals was the greater; but either CENOZOIC ERA.~AGE OF 3IAM3IALS. 401 was probably more than twice the bulk of the greatest liv- ing elephant. Quaternary Mammals in South America.^ We shall mention here only the most characteristic. South America now is characterized by sloths, armadillos (eden- tates), and llamas. In Quaternary times it was similarly characterized, but the species were gigantic. Great ground- sloths and cuirassed animals allied to the armadillo, but bigger than an ox, had their homes in South America, but wandered northward into North America as far as Cali- fornia and Pennsylvania. Among the ground-sloths, the best known are the Megatherium (great beast) and the Mylodon. The hugest of these was the Megatherium (Fig. 355). This was as big as a rhinoceros, and had thigh-bones several times the bulk of those of an elephant. The massiveness of the hind-legs, the hip-bones, and the tail, together with the long arms and prodigious hands, seem to indicate that the animal had the power of stand- ing on its hind-legs while it reached up to tear down branches of trees and feed upon them. Fig. 3.55.— Megatherium Cuviori. Among the cuirassed edentates, the best known is the Glyptodon, the shell of which was at least five feet long ; Le Conte, Geoi.. 26 402 HISTORICAL GEOLOGY. but other genera have been found much larger, one as big as a rhinoceros, and another as big as an ox. The saber-toothed tigers were also abundant in South America at this time (Fig. 356). Quaternary Mammals of Australia. — At the present time the mammals of Australia are all ma7'supials. So was it also in Quaternary times ; but the species were, again, gigantic. The Diprotodon, for example. Pig. 356.— BA«^-»*-Machairodu8 was a kaugaroo as big as a rhi- (smil^don) necator, x ^V B*Mmei8terJ Lfter noceros. Many other gigantic species are al4/) found. We see, then, that the present disti/il6ution of mamma- lian forms was already lestablished in/ipe Quaternary, but everywhere the speciesf were giganti^ Sdme Important Generx 1. CaTuse of the /Cold of thrfe Glacial Epoch.— The intense cold which fcharacteri^d khe Gli^^l epoch may have been due to prresjpial or tX/fosmical causes. It seems right that we-should, as faAas possible, account for it by terrestrial causes, and resoit to the other only if these fail. Now, northern efevatwn would probably produce great cold in the nortPkQrnyiiemisphere. This, then, is certainly a probable cause. But the effect has seemed so great and widespread that many think this cause insufficient, and have therefore looked abroad for extra-terrestrial or for cosmical causes. Among the many causes of this kind which have been proposed, the only one which has attracted much attention is that brought forward by Mr. Croll, which attributes it to slow changes in the form and position of the earth's orbit.* * For a discussion of this subject, see " Elements," p. 575. CENOZOIC ERA.— AGE OF^MA^UIALS. 403 2. Migrations during- the Glacial Epoch and their Effect on the Geographical Distribution of Organisms. — The oscillations of the earth's crust during glacial times produced great changes in Physical Geog- raphy, elevation enlarging and subsidence' diminishing the area of the continents. In this manner gateways were opened permitting migrations from one conti- nent to another, as for example between North America and Asia through Bering Straits, and between Europe and Africa through the Mediterranean. Again, the great changes of climate from subtropical mildness to extreme arctic rigor, and back again to temperateness, enforced migrations southward and northward, perhaps several times. These migrations, whether permitted or enforced, produced a mixing of different faunas and flo'tas on the same gm»«id ; and the severe competitiw^ struggles amo4g^hem,\tDgether with the great chajig^ of climaty conditions, c»ufed^U Bany chan gfiar^lSartlv hj extinctic(n indi partly hy\ modification. After these migrations, minglings, struggles, and consequent modmcations/ the resulting f a^mas and floras were again in\iaa«y cases reisolatpdr'iji their new homes by subsidence. In these "iSUtS^ed new homes they have undergone slow changes by evolution until Ahe present time. Thus have come about the present Geographical faunas spoken of in chap- ter iii., section 4, ql Part I. (P. 118). Now, as the Glacial epoch is a comparatively recent geological event, it is evident that the migrations of that time furnish a key to the present distribution of organisms ; and conversely, the present distribution of organisms is a key to direction of migrations during that time. AVe give a few striking examples illustrating this very interesting subject, and completing the explanations given in Part I. 1. Alpine Species. — It is a curious fact that alpine species of plants and insects (i. e., species which live on mountains near the snow line) are very similar in all parts 404 HISTORICAL QEOLOOY. of the world (as for example in North America and Europe), although they are so far separated from one another and completely isolated. It must be observed, however, that they are also very similar to Arctic species. The explanation is found in the migrations of the glacial times. At that time Arctic species were pushed south- ward on both continents — to the shores of the Mediter- ranean in one and of the Gulf of Mexico in the other. On the return of a temperate climate most of them fol- lowed the retreating ice-foot back to their Arctic home ; but some followed arctic conditions upward to the tops of high mountains, and were stranded there in alpine isola- tion till now. It is true they have been slowly changing since then — some in one direction, some in another — in accordance with a universal law in the case of isolated species ; but the time has been too short to effect any great changes. 2. South Africa. — Africa, south of Sahara, is inhabited by two very distinct groups of mammals. The first group consists of small animals of very low organization, such as insectivores, but very different from those found any- where else. These we shall call indigenes. The other group consists of very large and highly organized ani- mals, mostly also peculiar to Africa, but similar in gen- eral character to those found in Eurasia, especially those of Pliocene times. These we shall call invaders. Now, before glacial times, Africa was isolated from the rest of the world and inhabited by the indigenes only. Then came the glacial elevation, opening gateways through the Mediterranean and into Africa, and the glacial cold driving the Pliocene mammals southward into Africa, where they were shut up by the closing of the passages through the Mediterranean and by the formation of the Desert of Sahara. The subsequent struggles between invaders and indigenes, and the effect of a new environ- ment on the invaders, have greatly changed both, but CENOZOIG ERA.— AGE OF MAMMALS. 405 especially the weaker indigenes. Thus have resulted the mammalian fauna of Africa. 3. The British Isles. — The fauna and flora of the British Isles are almost identical with those of Europe, but not quite. They differ in two respects, a. There are varietal, though perhaps not specific, differences of form. h. The number of species is much less than on the continent. This is especially true of Ireland. Thus, of 90 species of European mammals only 40 are found in England and 22 in Ireland. Of 22 European species of reptiles and amphibians only 13 are found in England and 4 in Ireland. The migrations of glacial times com- pletely explain this. Before glacial times Great Britain was a part of Europe and had the same fauna and flora. During the glacial times it was covered with the ice-sheet, and all life destroyed or driven southward. After the glacial times it was still connected with the continent, and began to be recolonized by migration from Europe. But before the colonization was completed, especially for more distant Ireland, it was again separated by subsidence from the continent, and at the same time Ireland was separated from England. The time since has not been sufficient to make species, although it has been enough to make incipient species, i. e., geographical varieties. 4. Coast Islands of California. — The flora of Cali- fornia consists of two groups of species, the one charac- teristically Californian, the other more widely diffused. The first is undoubtedly indigenous, the second is prob- ably composed of invaders from the north. Now, off the coast of Southern California there is a string of bold, rocky islands about 2,000 feet high and separated from the mainland by a deep channel 50 miles wide. The flora of these islands is very peculiar. Of 300 known species about 50 are wholly peculiar to the islands and not found elsewhere in the world. The remaining 250 are all characteristic California species. Now the explanation. 406 HISTORICAL GEOLOGY, Before and during the early part of the Glacial epoch the islands were a part of the continent. We have already given proof of this on page 393. At that time all was inhabited by the same flora, viz., the indigenous. Before the invasion from the north the islands were separated from the mainland. Then came the northern invasion and consequent struggle between native and invading species — the destruction of some natives and the modifi- cation of others — and the final result was the California flora as we now know it. But the island flora was spared this conflict, and therefore retained more nearly the orig- inal character of both. In the flora of these islands, therefore, we see a near approach to the flora of both mainland and islands before the separation. CHAPTER VI. PSYCHOZOIC ERA. — AGE OF MAN. I]S" all previous ages there ruled brute force and ferocity. In this age alone Reason appears as ruler. The order of Nature must be adjusted to this keynote. Therefore, the great ruling mammals of the previous age must become extinct, and the mammalian class must become subordi- nate ; noxious animals and plants must diminish, and useful ones be preserved. Although in length of time this is not to be compared to an era, nor to an age, nor to a period, nor even to an epoch, yet it deserves to be made one of the primary divisions of time, not only on account of the dignity of man, but also, and mainly, because through his agency there is now going on in organic forms a change as sweep- ing as any which has ever taken place. This change has been going on ever since the introduction of man, and is going on now, but will not be complete until civilized man occupies the whole earth. It is interesting to mark some of the steps of this change. The disappearance of the mammoth, the mas- todon, the cave-bear, and the saber-toothed tiger was due, partly at least, to man. These are among the first. Some of the gigantic oxen of Europe (urns) lingered until Roman times. One species (aurochs) still lingers, being preserved by royal edict in the forests of Lithuania. The bison or buffalo of our Western plains is doomed to speedy extinction, unless saved by domestication. In fact, 407 408 HISTORICAL GEOLOGY. nearly all our domesticated animals and useful plants have been thus saved. A remarkable example of recent extinction of the Qua- ternary species is found in the gigantic wingless birds of New Zealand and Mada- gascar. The bones of the Dinornis and the Epiornis are very abundant in these islands. The Dinornis giganteus (Fig. 357) was twelve feet high. The drumstick was a yard long^ and as big as the leg- bone of a horse. A perfect egg of the Epiornis has been found, six times as big as the Qgg of an ostrich. The ex- tinction of these birds, although it occurred before the discovery of these islands by civilized man, was so recent that the feet have been found with dried skin upon them, and eggs with the skeletons of chicks within. Now, in this gradual change from the Quaternary to the present fauna and flora, when did man first appear upon the scene and become an agent of clumge ? Km\ ivhat hind of man was this 'primeval man? These are tjuestions of Inm- scendent importance. Fig 357.— Dinornis giganteus, x ^. (From a pho- tograpli of a skeleton in Christchurch Museum, New Zealand.) PSYCHOZOIC ERA.—AOE OF MAN. 409 Antiquity of Man, On this important question, history, archaeology, and geology meet and cooperate ; and it is to the introduc- tion of geological methods that we must attribute the rapid advances in recent times. Archaeologists long ago divided the history of human progress, according to the nature of the implements used, into three ages — a stone age, a bronze age, and an iron age. Again, by closer study, they subdivided the stone age into an older stone {Paleolithic) and a newer stone {Neolithic) age. In the one, the stone implements are chipped ; in the other, polished. Again, under the guid- ance of geology, the Paleolithic has been subdivided into the mammoth age and the reindeer age. In the former, man was contemporaneous with the mammoth, the cave- bear, and other extinct Quaternary animals ; in the latter, the mammoth had nearly disappeared, but the reindeer was abundant over all middle and southern Europe. The flint implements in the former were so rude that they might well be called flint-flakes ; in the latter they were carefully chipped. The former was coincident with the Mid-Quaternary, i. e., Champlain, or iperhni^s Interglacial ; the latter with the second Glacial or the early Post-glacial. 3. Iron age ) 2. Bronze age [• Psychozoic. ( Neolithic — Domestic animals. ) 1. Stone age \ ( Reindeer — Late Quaternary. ( Paleolithic ■< Mammoth — Mid-Quaternary. As seen by the schedule above, the Psychozoic era and age of man commences with the Neolithic. Before that time, man existed, indeed, but contended doubtfully for mastery with the great Quaternary animals. From tluit time liis victory is assured and his reign begins. 410 HISTORICAL GEOLOGY. Primeval Man in Europe. According to our schedule, man is traced back to the Mid-Quaternary. Some geologists think that there are signs of his existence still earlier, viz., in the Tertiary ; but the evidence is acknowledged to be unsatisfactory. We shall confine ourselves, therefore, to Quaternary man. We shall commence with Europe, as the evidence is more complete, and all the steps represented. Quaternary Man ; Mammoth Age ; the River- Drift Man. — Some twenty years ago, M. Boucher de Perthes found, in the undisturbed gravels of the upper terraces of the river Somme, the implements of man asso- ciated with the bones of many extinct Quaternary animals, such as the mammoth, the rhinoceros, the hippopotamus, the hyena, the horse, the Irish elk, the cave-lion, etc. The doubts which were at first entertained by the more cautious geologists have been entirely removed by careful examination. We give this as only one example of very many. In all cases the implements are of the rudest kind of flaked flints, like those figured on page 414. The Cave -Man. — In Quaternary times, man un- doubtedly contested with the hyena, the lion, the saber- toothed tiger, and the cave-bear the right to occupy the caves as homes. The evidence of this is found in the association of his implements, and even his bones, with those of all the extinct carnivores mentioned, under con- ditions which admit of no doubt of their contempo- raneousness. They are sometimes entombed together, and covered with stalagmitic crust, which has never been broken from Quaternary times until rifled by the geolo- gist. We give a single example. The Mentone Man. — In a cave at Mentone, near Nice, has been recently found the almost perfect skeleton of an old man, of more than average height, lying on his side in an easy position, and about him chipped im- PSYCHOZOIC ERA.— AGE OF MAN. 411 plements and bones of extinct animals, among which were many pierced reindeer's teeth. All of these were perfectly preserved by a stalagmitic crust. We may well imagine that this old hunter, finding his end approaching, retired to Jiis cave-home, laid himself quietly down, with the implements and trophies of successful chase about him, and gave up the ghost. Good Mother Nature then slowly buried his remains, and sealed them up beneath a crust of stalagmite. The Primeval Aquitanians. — In southwestern France, on the river Vizere, a branch of the Dordogne, are found many caves which were inhabited by a more peaceful race. They were not only hunters, but also fishers ; for we find, besides stone implements, many im- plements made of bone, among which are rude fishhooks. They also show evidence of some skill in drawing and carving. Among the bone implements found there are many drawings of extinct animals. Fig. 358 represents a rude but very characteristic sketch of a mammoth. Fig. 358.— Drawing of a Mammoth by cont'juiporaneous man made by contemporaneous man. In these caves we find a gradual transition from the mammoth to the reindeer age. General Conclusions. — These all belong to the Qua- ternary. In Europe, therefore, man certainly saw the 41^ HISTORICAL GEOLOGY. flooded rivers and lakes,, and probably the great glaciers. He certainly hunted the great extinct Quaternary animals, the mammoth, the cave-bear, the cave-lion, the great Irish elk, and the reindeer. All the evidence points to an ex- tremely low, savage state, with little or no tribal organi- zation. There is no evidence yet of either domestic ani- mals or of agriculture. Neolithic Man. Kitchen-Middens ; Refuse-Heaps ; Shell-Mounds. — In many parts of Europe, especially in Denmark and Sweden, are found mounds, composed wholly of shells and other refuse of tribal gatherings and feastings. The men of that time seem to have had the habit of gathering annually at some place where food was abundant, usually on the seashore, at the mouth of a river. From year to year the refuse of such gatherings accumulated until mounds of great extent were gradually formed. In these mounds are found the bones of men and animals and the implements of men, and from these we may form a good idea of the character and habits of the men. Here, then, we find a great and somewhat sudden change : 1. There are no longer any extinct Quaternary animals. 2. We find here, for the first time, domestic animals, viz., the dog, the ox, the sheep, etc., and also evidences of agriculture. 3. The implements are no longer only chipped, but are often carefully polished by rubbing. Rude pottery is also found. 4. We have here for the first time the evidence of tribal organization, similar to the savage races of the present day. 5. The conformation of the skull shows a diiferent race from that of the cave and river-drift men. In a word, we have here the appearance in Europe, probably by migration, of a different and higher race. Until this time man in Europe seems to have contended doubtfully with wild animals : now he seems to have established his su- PSYCHOZOIC ERA.— AGE OF MAN. 413 premacy. The Psychozoic era and age of man, there- fore, rightly commence here, and all that follows may be claimed by archseology and history. Nevertheless, we shall give a very brief sketch of further progress. Transition to the Bronze Age. Lake-Dwellers. — In 1850 the lakes of Switzerland be- came very low, and a great number of wooden piles were exposed. Interest being excited, the same was found to be true of all the lakes of middle Europe. By dredging, implements of war, of the chase, of husbandry, and orna- ments and trinkets of all kinds were found in great abun- dance. Some of these were polished stone, but most were bronze, and often beautifully finished. Remnants of grain and fruits of several kinds were also found. From these findings the houses (Fig. 359), the habits, and the mode Fig. 359.— Lake-dwellings, lebtored. (After Mortillet.) of life of this people have been reconstructed, and even a novel embodying their life has been written.* Thus we might continue, by means of remains alone, to trace progress, through Roman graves, Roman roads * ' • Realmar, " by Arthur Helps, 414 HISTORICAL GEOLOGY. and implements, etc., to the graves in our own church- yards and the machinery of our own times. This all be- longs to history. Thus we trace geology into archaeology, and archaeology into history. Primeval Man in America. It must be remembered that the different men we have described in Europe represent different stages of progress there. The progress has not been at the same rate every- where, and therefore the different stages are not necessa- rily contemporaneous. When America was discovered, the native tribes were still in the stone age, and many savages are only in this stage of advance now. The advance was more rapid in Europe, apparently because of the frequent and extensive migrations and conflict of races there. Nevertheless, the rudest state (Paleolithic age) Seems to have been nearly contemporaneous in America and Europe, and probably elsewhere. Quaternary River-Drift Man in America. — There are many examples of rude flint-flakes in the river-gravels of California and in the glacial drift of New Jersey and Ohio. These were, it is believed, the work of a race cor- PiG. 360.— Paleolith found by Abbott in New Jersey, slightly reduced. (After Wright.) PSYCH020IC ERA.— AGE OF MAN. 415 responding to and contemporaneous with the river-drift man of Europe (Fig. 360). Some doubts have been recently thrown on the antiquity of these findings. For this reason we will not dwell on Grlacial man in America. Neolithic Man in America. — The Neolithic age is represented here, as in Europe, by refuse-heaps, which were evidently made in the same way as those already described, and have similar contents. They are abun- dant on the seacoasts everywhere, and some of them are probably no older than the discovery of America ; for, as already said, the native tribes were then still in the stone age. Mound-Builders. — The bronze age is probably, though imperfectly, represented by the mound-builders. In many places, especially in the valley of the Mississippi, are found mounds of enormous size, and fortifications and communal houses of somewhat elaborate construc- tion. In connection with these have also been found not only highly polished stone implements, but also imple- ments of hammered copper. The copper-mines of Lake Superior were evidently worked by them, as the old work- ings have been found. The mound-builders were prob- ably a different race from the hunter tribes of Indians, and more advanced, although many now think they are the same. Cliff-Dwellers. — In the dry regions of New Mexico and Arizona the almost perpendicular cliffs bordering the mesas are studded with remains of many-storied com- munal houses of stone. There are small remnants of sev- eral tribes in that region — Pueblos, Moquis, and Zuflis — that live now in similar dwellings, on the flat tops of almost inaccessible mesas. One dwelling with many rooms is occupied by a whole community. These also are entirely different from the roving tribes, and by many are connected with the Aztecs on the one hand, and the mound-builders on the other. 416 HISTORICAL GEOLOGY. It is needless to repeat that these last three heads be- long to the present epoch. Conclusions. 1. We have thus traced man back to the Mid-Quater- nary. It is possible that he may hereafter be traced still further back ; but this seems very improbable. No mam- malian species now living can be traced further back than the Quaternary. Man belongs to the present mamma- lian fauna, and probably came in with other mammalian species in the Quaternary. 2. We have not yet been able to find any undoubted transition forms or connecting links between man and the highest animals.* The earliest known man, the river- drift man, though in a low state of civilization, was as thoroughly human as any of us. 3. The amount of time which has elapsed since man first appeared is still doubtful. Some estimate it at more than a hundred thousand years — some only ten thousand. The question should not be regarded as of any impor- tance, except as a question of science. * Such a link is supposed, by many, to have been recently found in Java, and named Pithecanthropus. We wait for more evidence. INDEX Acrogens (point-growers or apex- growers), age of, 395. African fauna explained, 404. Agencies, geological, 9. leveling and elevating, 131. Ages, 360. Air, chemical action of, 14. mechanical action of, 15. Albertite ; a form of asphalt, 313, Alkaline lakes, deposits in, 77. Alpine species, 403. Ammonite (horn of ammon stone), 330. Amphibians (living in both [air and water] ), called also batra- chians, 319-328. age of, 297. Amphicoelias (amphi, both sides; koilos, hollow ; double con- cave), 347. Amygdaloid, 223. Anchisaurus, 344. Ancyloceras (curved horn), 353. Andesite, 214. Angustifolius (narrow leaf), 274. Anomodont (lawless tooth), 328. Anoraoepus (unlike feet), 343. Anoplotherium (unarmed beast), 380. Anticline and syncline defined, 188. Apiocrinus (pear crinoid), 331. Appalachian revolution, 322. Aqueous agencies, 17. Aquitanians, primeval, 411. Archaean (relating to earliest times), 259. rocks, area of, in the United States, 265. rocks, character of, 264. Lb Conte, Geoi-. 27 Archaean system, 263. times, life of, 265. times, physical geography of, 265. Archaeozoic (primeval life), 259. Archegosaurus (primordial liz- ard), 320. ArchaBopteryx (primordial winged creature), 337. Artesian wells, 70. Asteroid (star-like), 277. Asterolepis (star-scale), 291. Atlantosaur (great lizard) beds, 345. Atmosphere, chemical action of, 14. mechanical action of, 15. Atmospheric agencies, 10. Baculite (stone-staff), 354 Bad Lands, 248, 366. Banks, Bahama, 50. in North Sea, 50. of Newfoundland, 50, submarine, 49. Bars, how formed, 38. position of, 38. removal of, 40. Basalt, 139, 215. columnar structure of, 220. Base level of erosion, 28. Bed-rock surface, 387. Belemnite (stone dart), 332. Birds, 337. Bitumen and petroleum, 313. Blastids (bud-like), 279. Borax lakes, 77. Botanical regions, 119. Bowlders, 387. of disintegration, 12. 417 418 INUJ^X. Brachiopod (arm-foot), 279. Breccia, 180. volcanic, 222. British Isles, fauna of, 405. BrontosHur (giant lizard), 345. Brontotherium (giant beast), 378. Brontozoum (giant animal), 343. Bronze age, 409, 413. Bryozoon (moss-animal, called also Polyzoon; an order of com- pound moUuscoid animals), 316. Buthotrephis (reared in the deep), 274. Butterfly, a fossil, 372. Calamite (stone reed : a family of coal-plants allied to equisetae), 309. California coast isles, 405. Cambrian, 271. Canons, ravines, gorges, 23. examples of, 24. Carboniferous (coal-bearing) age, 298. age, fauna of, 315. age, subdivisions of, 297. Caves, limestone, 71. Cenozoic (pertaining to recent ani- mal life) era, 363. era, characteristics of, 363. era, subdivisions of, 364. Cephalaspis (head-shield), 292. Cephalopod (head-foot : refer- ring to position of limbs), 281. Ceratite (stone-horn: a family of shelled cephalopods), 327. Ceratodus (horn-tooth), 295. Cestracion (sharp tool: referring to the spine), 296. Chalk, 350. Cham plain epoch, 390. Chemical deposits in lakes, 76. deposits in springs, 72. deposits of iron oxide, 75. deposits of lime carbonate, 73. deposits of silica, 76. deposits of sulphur, 76. Chronology, construction of geo- logical, 207. Cinders, ashes, etc., 135. Cleavage, slaty, 194. Cleavage, structure, 193. Clilf-dwellers, 415. Coal-fields of the United States, 300. of Eastern Virginia and North Carolina, 344. Coal measures, 298. mode of accumulation of, 310. origin of, 301. period, climate of, 312. period, length of, 303. period, physical geography of, 312. plants of the, 303. varieties of, 301. Coccosteus (berry-bone), 292. Colorado Canon, 25. Columbia River and tributaries, 22. Columnaria alveolata (cellular columns), 276. Columnar structure, 220. structure, cause of, 221. Compsognathus (handsome jaw), 337. Concretionary or nodular struc- ture, 198. Concretions, how formed, 199. Conformity and unconformity, 190. Conglomerate, volcanic, 222. Connecticut River Valley tracks, 342. Continental faunas and floras, 123. faunas and floras, subdivisions of, 126. form, general laws of, 176. Continents and sea-bottoms, ori- gin of, 178. mean height of, 176. Coral, compound, mode of growth, 95. conditions of growth, 98. forest, how formed, 96. islands, closed lagoons, 103. islands, how formed, 97. islands, lagoonless, 103. islands of the Pacific, 98. polyp, structure of, 93. reef-rock, 97. reef- rock, different kinds of,108. reefs, 97. reefs, atolls, 102. INDEX. 419 Coral reefs, barrier, 101. reefs, fringing, 100. reefs, how formed, 97. reefs of Florida, 109. reefs of the Pacific, 99. reefs, theories of barriers and atolls, 103. Corals in Paleozoic rocks, 275. Cordaites (a coal-plant named after Corda), 304. Coryphodon (peak-tooth), 377. Co-seismal lines (lines connecting points which feel a shock at the same moment), 103. Crater lake, 142. Cretaceous period, 348. period, animals of, 352. period, areas of rocks of, .348. period, coal of, 351. period, physical geography of, 348. period, plants of, 351. Crinoid (lilylike stone), 277. range in time, 278. Crust of the earth, 174. general configuration of, 176. Cyathophylloid (cup-leaf like), 275. Cycads : plants of cycas, or sago- palm family, 329. Cystid (baglike), 279. Darwin's subsidence theory of atolls, 104. Deltas, how formed, 33. age of, 36. subsidence of, 169. Dendrerpeton (tree-reptile), 320. Denudation, or general erosion, 252. modes of determining amount of, 253. Deposits, chemical, 72, 182. deep-sea, 117. made by waA^es, 50. mechanical, 182. organic, 182. Devonian age, 286. age, animals of, 288. age, fishes of, 291. age, life-system of, 287. age, physical geography of, 287. age, plants of, 286. Devonian age, rocks of, 286. Devonian fishes, sudden appear- ance of, 297. Diatoms (5^«'^o/^o?, cut in two) : microscopic plants which multiply by dividing in two, 115, 352. shell deposits of, 116. Diabase, 212, 213. Dicotyls. contraction for dicoty- ledons : plants having two seed-leaves, 351. Dicynodon (two canine-toothed), 327 Dikes" 141, 216. effect on stratified rocks, 217. Dinichthys (huge fish), 293. Dinoceras (huge horned animal), 377. Dinosaur (huge lizard), 334. Dinotherium (huge beast), 381. Diorite, 217. Dip and strike defined, 187. Diplacanthus (double spine), 298. Diplocynodon (double canine- teeth), 347. Diprotodon (two front teeth), 402. Disintegration, rate of, 13. Dolerite, 214. Drift, 386. Drift-timber, 88. Dromatherium (running beast), 345. Dynamical geology, 9. Earth, crust of, 174. crust, cause of inequalities in, 178-240. crust, cause of movement of, 171. crust, gradual oscillation of, 164. density of, 174. general form of, 173. general structure of, 173. general structure of, means of observing, 175. internal heat of, 131. Earthquake, epicentrum of, 156. focus, mode of determining, 163. wave, nature of, 158. wave, velocity of, 156. 420 INDEX. Earthquakes, 154. beneath the sea-bed, 159. cause of, 157. connection of, with phases of the moon, and with the weather, 163. frequency of, 155. great sea- wave of, 160. phenomena of, 155. Echinoderm (spiny skin), 277. Echinoid (urchin-like), 277, 352. Echinus (hedgehog or urchin) : a sea-urchin, 277. Elasmobranchs, 291. Elevation and subsidence, cause of, 171. of crust, gradual, 165. Eocene (dawn of recency), 364. Eohippus (dawn of earliest horsey, 377. Eozoon (dawn animal), 266. Epicentrum (upon the center), 156. of an earthquake, mode of de- termining, 163. Epochs, 262. EquisetaB : horse-tails, scouring- rush, 287, 306, 326. Eras, 259. Erosion, agents of, 252. amount of, 253. average rate of, 19. general, or denudation, 252. general results of glacial, 396. of rain and rivers, 18. Eruptive rocks, true, 214. Estuaries, deposits in, 38. how formed, 37. Evolution, bearing of Devonian fishes on, 295. Falls, Niagara, 20. of St. Anthony, 22. Minnehaha, 22. Yosemite, 23. False bedding, 184. Faults, amount of displacement, 230. kinds of, 232. law of slip, 232. Faunas and floras, continental, 123. defined, 118. marine, 129. Faunas and floras, geographical, explained, 403. Favositid (honeycomblike stone), 275. Felsite, 213. Ferns, 306. Fiords (Norwegian term for deep inlets between high head- lands), 45. origin of, 396. Fishes, age of, 286. Fissures, great, 229. great characteristics of, 230. Flood-plain, 30. ' of the Mississippi, 31 . of the Nile, age of, 31. Floras, defined, 118. Florida, reefs and keys of, 109. Floriformis (flower-like), 275. Folded strata, 186. Foraminifera (full of holes : pro- tozoan animals with perfo- rated shells), 351. Formation defined, 204. geological, 193. Fossils, 200. degrees of preservation of, 200. Fossil species, distribution of, 203. Frost, action of, in soil-making, 15. Fucoid (resembling tangle), 274. Fucus (tangle or wrack), 274. Fumaroles (smoking vents), 145. Gabbro, 212. Ganoid (shining : referring to the scales), 291. Gasteropod (belly-foot : referring to mode of walking), 280. Gastornis (Gaston's bird), 374. Geographical distribution of spe- cies, 118. diversity of species, origin of, 130. Geological and human history, correspondence of great prin- ciples of, 256. chronology, construction of, 207. formation, 193. history, divisions of, 259. Geology, definition of, 7. dynamical, 9. INDEX. 421 Geology, great divisions of, 8. historical, 256. structural, 173. Geyser, Great, 147. Great, phenomena of eruption of, 147. Geysers defined, 14C. cause of eruption of, 151. of Iceland, 146. of Yellowstone Park, 147. Gigantitherium (gigantic beast), 343. Glacial (icy), 385. cold, cause of, 402. epoch, 386. epoch, explanation of phenom- ena of, 358. erosion, general results of, 396. Glaciers as a geological agent, 60. characteristic signs of, 62. defined, 52. erosion of, 61. evidences of former greater ex- tension of, 62. in the Sierra, 56. lower limit of, 53. motion of, 58. size of, in various regions, 56. structure of, 56. transportation and deposit by, 61, 62. Globigerina (globule-bearing),117. ooze, 117. Glyptodon (sculptured tooth), 400. Goniatite (angled stone : a family of shelled cephalopods with angled sutures), 290, 317. Gorge formed by recession of falls, 21. Gracilis (graceful), 274. Gradual oscillation of the earth- crust, 164. Grahamite : a form of asphalt, 313. Granite, 212. Granitic rocks, composition of, 211. rocks, mode of occurrence, 212. Graphic granite, 212. Graptolites (stone-writing), 276. Ground-water, perpetual, 68. Gulf Stream, geological agency of, 48. Gulf Stream, origin and cause of, 47. Gymnosperm (naked seed: a class of plants including conifers and cycads), 287, 304. Halysites (stone chain) catenulata (like a little chain), 276. Halysitid (stone chain), 275. Hamite (stone hook), 354. Hesperornis (western bird), 358. Hipparion (little horse), 381. Hippurite ^horse-tail), 353. Historical geology, 256. History, general principles of, 256. geological, divisions of, 259. Horse, genesis of, 382. Hybodont (hybodus, hump-tooth : a family of shark-like fishes), 318. » Hydrothermal fusion (fusion by heat and water), 133. Hydrozoa (water-animals), 275. Hypsilophodon, 355. Ice, agency of, 52. Icebergs as a geological agent, QQ. effect of, compared with gla- ciers, 66. how formed, 64. of Greenland, 64. of the Antarctic, 66. Ice-sheet moraine, 388. Ichthyornis (fish-bird), 358. Ichthyosaur (fish-lizard), 334. Ideal section of earth-crust, 261. Igneous agencies, 131. rocks, 210. rocks, characteristics of, 210. rocks, classification of, 211. rocks, extent of, on the surface, 211. rocks, modes of occurrence of, 211, 216. rocks, origin of, 210. rocks, sub-groups of, 213. Iguanodon (iguana-toothed), 336. Indusium (an inner garment), 371. Insects, 290. Intercalary beds, 219. Invertebrates, age of, 271. Iron accumulations, 88. 422 INDEX. Iron accumulations, mode of formation of, 89. bog-ore, 89. oxide, deposits of, 75. Islands : coastislands, how formed, 51. Joints, 328. Jurassic period, 329. period, animals of, 329. period, coal of, 329. period, plants of, 329. Jura-Trias, disturbances which closed, 347. in America, 341. life-system of, 341. Kitchen-middens, 412. Labyrinthodont (labyrinthine tooth : a family of extinct amphibians), 3.19. Lake Agassiz, 391. Bonneville, 394. dwellers, 413. Lahontan, 395. margins, 391. Lakes, alkaline, 77. borax, 77. chemical deposits in, 80. crater, 142. glacial origin of, 396. salt, 76. Lamellibranch (plate-gill), 379. Lamination, cross or oblique, 184. Land, mean height of, 176. Laosaurus, 346. Laramie epoch, 360. epoch, coal of, 361. Lava, classification of, 139. kinds of, 137. sheets, extent of, 217. Lepidodendrid, 287, 307. Lepidodendron (scale-tree), 307. Lepidoganoid (scale-ganoid), 293. Lepidosiren (scaly siren : an am- phibious fish), 295. Levees, artificial, 32. natural, 31. Lime accumulations, 91. carbonate, deposits of, 73. sinks, 73. Limestone caves, how formed, 71. Limestone shell, 114. Limuloids : Limulus family, 316. Limulus ; horseshoe-crab, or king- crab. 284, 316. Lithodomi {lithos, stone, domiis, house : a species of shell-fiirh which burrow in rocks), 167. Lost intervals explained, 267. Lycopod (wolfs toot : an order of club-mosses), 307. Machairodus (saber-toothed), 381, 402. Mammals, age of, 363. genesis of orders, 382. of the Tertiary period, 374. Mammoth, 398.' Man, antiquity of, 409. Neolithic, 412. of the caves, 410. of the river-drift, 410. primeval, in America, 414. primeval, in Europe, 410. Marmites des geants, 63. Mastodon (nipple-toothed), 399. Mastodonsaur (teat-toothed liz- ard), 327. Mauvaises Terres, 248, 366. Megalosaur (great lizard), 836. Megatherium (great beast), 401. Mentone man, 410. Mesohippus (mid-horse), 379. Mesozoic (pertaining to middle animal life) era, 324. era, characteristics of, 324. era, disturbance which closed, 360. era, general observations on, 359. era, subdivisions of, 324. Metamorphic rocks, 224. Metamorphism, cause of, 226. agents of, 226. Migrations during Glacial epoch, 403. Mineral springs, 71. veins, 233. iMiocene (less recent), 364. Miohippus (less horse-like), 378. Mississippi delta, 34. Mode of accumulation of coal, 310. Mollusks, 279. inde: 423 Mono Lake in Quaternary period, 394. Monotremes (one vent : the lowest order of mammals, including ornithorhynchus and echid- na), 328. Monticles (little mountains), 136. Moraines, 57. Mosasaur (Meuse lizard), 357. Mound-builders, 415. Mountain life, different stages of, 245. sculpture, 246. sculpture forms of, 247. strata, thickness of, 244. Mountains, defined, 238. structure and origin of, 238. Murray's theory of atolls, 105. Myrmecobius (ant-liver), 340. Nautilus, 281. Neolithic (new stone), 412. Niagara Falls, recession of, 21. gorge, origin of, 21. Nodules, forms of, 198. how formed, 199. Obsidian, 214. Ocean, agency of, 41. mean depth of, 176. why is it salt ? 79. Oceanic currents, 47. Organic agencies, 83. agencies, subdivisions of, 83. Orohippus (mountain-horse), 377. Orthoceratite (straight stone horn), 282. Orthoclase (right cleavage), 212, 215. Osteolepis (bony scale), 294. Otozoum (giant animal), 342. Outcrop, 186. Overflows, 217. Pacificbottora, subsiding area,106. Paleolithic (old stone), 409. Paleotherium (old beast), 380. Paleozoic (pertaining to old or ancient animal life), 259. era, general observations on, 321. era, progressive changes during, 321. Paleozoic era, subdivisions of, 271. rocks, 268. rocks and era, 267. rocks, area of, in the United States, 269. system, unconformity of, with Archaean, 267. times, growth of the continent in, 271. times, physical geography of, 269. Peat, antiseptic property of, 85. bogs, 84. bogs, rate of growth of, 87. bogs, structure of, 87. composition of, 84. mode of accumulation of, 85. swamps, 84. Pegmatite, 212. Peneplain, 28. Period, geological, defined, 204. Periods and epochs, 262. Permian period, 323. Petroleum (rock-oil), 313. age of strata of, 314. origin of, 315. Phonolite (ringing-stone), 214. Pithecanthropus, 416. Placoderm (plate-skin), 293. Placoganoid (plate-ganoid), 293. Plagioclase (oblique cleavage), 212, 215. Plesiosaur (near to a lizard), 334. Pliocene (more recent), 365. Pliohippus (more horse-like), 379. Plumularia (plume-like), 276. Plutonic rocks, composition of, 212. Polypterus (many-finned), 296. Porphyry, 213. Potholes, 26, 63. Primordial beach, 269. Protohippus (first horse), 379. Protozoa (first and lowest living things), 266, 352. Psychozoic (pertaining to rational life), 260. era, 407. Pteranodon (winged-toothless), 356. Pterichthys (winged fish), 292. 424 INDEX. Pterosaur (winged lizard), 337. Pumice, 214. Quaternary period, 385. period in Eastern North Amer- ica, 386. period in Western North Amer- ica, 393. period, life-system of, 397. period, mammals of, in Amer- ica, 398. period, mammals of, in Aus- tralia, 402. period, mammals of, in Eu- rope, 397. period, mammals of, in South America, 401. period, subdivisions of, 385. Rafts, 88. Rain and rivers, erosive action of, 18. final effect, 28. rate of erosion by, 19. Ramphorhynchus (beak-snout), 338. Range of species, genera,etc., 120. Ravines, gorges, caSons, 23. Reefs and keys of Florida, 109. how formed. 111. Reefs of Pacific, 97. Regions, botanical, 119. definition of, 120. primary, 128. primary, subdivisions of, 128. zoological, 122. Reptiles, age of, 324. Rhyolite, 314. River-beds of California, old, 395. as indicators of crust-move- ments, 28, 170. deltas, subsidence of, 169, drift-man in America, 414. Rivers, deposits at the mouths of, 38. deposits of old, 392. erosive action of, 30. flood-plain deposits of, 30. winding course of, 29. Rock disintegration, rate of, 13. disintegration, explanation of, 14. Rocks, classes of, 178. Rocks, defined, 178. igneous, 210. metamorphic, 224. stratified, cause of consolida- tion of, 182. stratified, classification of, 205. stratified, description of, 179. stratified, extent of, 180. stratified, origin of, 181. stratified, principal kinds of, 180. structures common to all, 228. unstratified, 210. St. Anthony, Falls of, 22. Saline lakes, how formed, 76. lakes, chemical deposits in, 76. Salt lakes, how formed, 77. Sauropus (lizard-foot), 321. Scaphites (stone-boat), 353. Sea-beaches, elevation of, 391. Section of earth-crust, ideal, 261. Sediments, transportation and distribution of, 26. Sequoia : genus of conifers, in- cluding Redwoods and Big Trees of California, 368. Sertularia (a little garland), 276. Sharks, 291, 318, 354, 373. Shell limestone, how formed, 114. mounds, 412. Sierra Nevada range, formation of, 348. Sigillaria (from sigillum, a seal), 308. Sigillarid, 287, 308. Silica, deposits of, 76. Silurian animals, 275. rocks, area of, in the United States, 273. system, 271. times, life-system of, 273. times, plants of, 274. times, subdivisions of, 273. times, physical geography of, 273. Slaty cleavage, 194. Snow-line, 53. Soil, depth of, 12. origin of, 10. Solfataras (hot sulphur springs), 145. Sorting power of water, 27. INDEX. 425 Species, geographical distribu- tion of, 118. origin of geographical diversity of, 130. Sphenothallus (wedge frond), 274. Springs, 69. great, 69. mineral, 71. Squalodont (shark - toothed : a family of true sharks), 318. Stalactites and stalagmites, 72. Stegosaur (covered lizard), 846. Strata, crumpling of, 185. folding of, 185. Stratification explained, 27. Stratified rocks, classification of, 205. rocks, divisions and subdivi- sions of, 207. rocks, how relative age of, is determined, 205. Strike and dip defined, 187. Strombodes pentagonus (five- angled strorabus-like ani- mal), 275. Structural geology, 173. Sub-carboniferous, 297. Submarine banks, 41. Subsidence of crust, gradual, 169. of Pacific bottom, amount of, 107. of Pacific bottom, time of, 107. of river deltas, 169. Succinifer (amber-bearing), 371. Sulphur, deposits of, 76. Swamp, Great Dismal, 86. Syenite, 212. Syncline and anticline defined, 188. Table-mountains, 247. Tachylite, 214. Taxodium, bald cypress of South- ern swamps, 368. Teleost (complete bone), 291. Terraces, 392. Tertiary period, 364. period, animals of, 370. period, coal of, 366. period, crust-movements during and closing, 384. Tei'tiary period, lake deposits of, 377. period, life-system of, 368. period, mammals of, 375. period, physical geography of, 366. period, plants of, 368. period, subdivisions of, 364. system, areas of, 365. Theromorpha (beast-like), 328. Tides and waves, agency of, 41. and waves, effect on coast-line, 42. Toxoceras (bow-horn), 354. Trachyte, 139, 214. Transporting powder of water, 26. Trappean rocks, 213. Triceratops (three-horned face), 362. Triassic period, 325. period, animals of, 326. period, life-system of, 325, period, plants of, 326. Trigonia (three-angled), 331. Trilobite (three-lobed stone), 283. Tufa, 139, 223. Turrulite (stone tower), 354, Unconformity, 190. Unstratified rocks, 210. Vegetable accumulations, 84. Veins, age of, 236. contents of, 234. fissure, 234. irregularities of, 236. metalliferous, 234. mineral, 233. origin of, 237. structure of, 235. surface changes of, 237. Volcanic cinders, ashes, etc., 135. dikes, 141. eruptions, cause of, 144. eruptions, phenomena of, 135. gases and vapors, 139. phenomena, secondary, 145. rocks, 214. rocks, age of, 219. rocks, different kinds of, 215. rocks, intercalary beds of, 219. rocks, modes of eruption, 215. rocks, modes of occurrence, 216. 4:2Q INDEX. Volcauic rocks, sub-groups of, 215. Volcanoes, 133. age of, 143. erupted matters of, 136. mode of formation of, 140. number, size, and distribution of, 134. two types of, 135. Water, agencies of, 17. chemical agency of, 67. mechanical agency of, 18. perpetual ground, 68. sorting power of, 27. transporting power of, 26. Water, underground, 67. volcanic, 68. Waterfalls, recession of, 20. Waves, land formed by, 50. nature of deposits by, 46. and tides, agency of, 41. transportation and deposit by, 46. Wells, artesian, 70. Winds, action of, 16. Yosemite Falls, 23. Zaphrentis (proper name), 288. 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TODD, M.A., Ph.D. Professor of Astronomy and Director of the Observatory, Amherst College. Cloth, 1 2mo, 480 pages. Illustrated - - Price, $1.30 This book is designed for classes pursuing the study in High Schools, Academies, and Colleges. The author's long experience as a director in astronomical observatories and in teaching the subject has given him unusual qualifi- cations and advantages for preparing an ideal text-book. The noteworthy feature which distinguishes this from other text-books on Astronomy is the practical way in which the subjects treated are enforced by laboratory experiments and methods. In this the author follows the principle that Astronomy is preeminently a science of observation and should be so taught. By placing more importance on the physical than on the mathematical facts of Astronomy the author has made every page of the book deeply interesting to the student and the general reader. 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