diff options
Diffstat (limited to '40404-0.txt')
| -rw-r--r-- | 40404-0.txt | 14568 |
1 files changed, 14568 insertions, 0 deletions
diff --git a/40404-0.txt b/40404-0.txt new file mode 100644 index 0000000..948de83 --- /dev/null +++ b/40404-0.txt @@ -0,0 +1,14568 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 40404 *** + +[Transcriber's Note: + + _Text_ and =Text= represent italic and bold text respectively. + + Subscripts are displayed as an underscore followed by the number + or text in braces: SiO_{2}.] + + + * * * * * + + + [Illustration: A Valley with Rocky Ledges cut in the Horizontal + Strata, Scotland] + + + + +THE ELEMENTS OF GEOLOGY + +BY + +WILLIAM HARMON NORTON + +PROFESSOR OF GEOLOGY IN CORNELL COLLEGE + + + + +GINN & COMPANY + +BOSTON * NEW YORK * CHICAGO * LONDON + + + + +Copyright, 1905, 1921, by + +WILLIAM HARMON NORTON + +ALL RIGHTS RESERVED + +5511 + + +The Atheneum Press + +GINN & COMPANY PROPRIETORS + +BOSTON * U.S.A. + + + + +PREFACE + + +Geology is a science of such rapid growth that no apology is expected +when from time to time a new text-book is added to those already in the +field. The present work, however, is the outcome of the need of a +text-book of very simple outline, in which causes and their consequences +should be knit together as closely as possible,--a need long felt by the +author in his teaching, and perhaps by other teachers also. The author +has ventured, therefore, to depart from the common usage which +subdivides geology into a number of departments,--dynamical, structural, +physiographic, and historical,--and to treat in immediate connection +with each geological process the land forms and the rock structures +which it has produced. + +It is hoped that the facts of geology and the inferences drawn from +them have been so presented as to afford an efficient discipline in +inductive reasoning. Typical examples have been used to introduce many +topics, and it has been the author's aim to give due proportion to +both the wide generalizations of our science and to the concrete facts +on which they rest. + +There have been added a number of practical exercises such as the +author has used for several years in the class room. These are not +made so numerous as to displace the problems which no doubt many +teachers prefer to have their pupils solve impromptu during the +recitation, but may, it is hoped, suggest their use. + +In historical geology a broad view is given of the development of the +North American continent and the evolution of life upon the planet. +Only the leading types of plants and animals are mentioned, and +special attention is given to those which mark the lines of descent of +forms now living. + +By omitting much technical detail of a mineralogical and +palæontological nature, and by confining the field of view almost +wholly to our own continent, space has been obtained to give to what +are deemed for beginners the essentials of the science a fuller +treatment than perhaps is common. + +It is assumed that field work will be introduced with the commencement +of the study. The common rocks are therefore briefly described in the +opening chapters. The drift also receives early mention, and teachers +in the northern states who begin geology in the fall may prefer to +take up the chapter on the Pleistocene immediately after the chapter +on glaciers. + +Simple diagrams have been used freely, not only because they are often +clearer than any verbal statement, but also because they readily lend +themselves to reproduction on the blackboard by the pupil. The text +will suggest others which the pupil may invent. It is hoped that the +photographic views may also be used for exercises in the class room. + +The generous aid of many friends is recognized with special pleasure. +To Professor W. M. Davis of Harvard University there is owing a large +obligation for the broad conceptions and luminous statements of +geologic facts and principles with which he has enriched the +literature of our science, and for his stimulating influence in +education. It is hoped that both in subject-matter and in method the +book itself makes evident this debt. But besides a general obligation +shared by geologists everywhere, and in varying degrees by perhaps all +authors of recent American text-books in earth science, there is owing +a debt direct and personal. The plan of the book, with its use of +problems and treatment of land forms and rock structures in immediate +connection with the processes which produce them, was submitted to +Professor Davis, and, receiving his approval, was carried into effect, +although without the sanction of precedent at the time. Professor +Davis also kindly consented to read the manuscript throughout, and his +many helpful criticisms and suggestions are acknowledged with sincere +gratitude. + +Parts of the manuscript have been reviewed by Dr. Samuel Calvin and +Dr. Frank M. Wilder of the State University of Iowa; Dr. S. W. Beyer +of the Iowa College of Agriculture and Mechanic Arts; Dr. U. S. Grant +of Northwestern University; Professor J. A. Udden of Augustana +College, Illinois; Dr. C. H. Gordon of the New Mexico State School of +Mines; Principal Maurice Ricker of the High School, Burlington, Iowa; +and the following former students of the author who are engaged in the +earth sciences: Dr. W. C. Alden of the United States Geological Survey +and the University of Chicago; Mr. Joseph Sniffen, instructor in the +Academy of the University of Chicago, Morgan Park; Professor Martin +Iorns, Fort Worth University, Texas; Professor A. M. Jayne, Dakota +University; Professor G. H. Bretnall, Monmouth College, Illinois; +Professor Howard E. Simpson, Colby College, Maine; Mr. E. J. Cable, +instructor in the Iowa State Normal College; Principal C. C. Gray of +the High School, Fargo, North Dakota; and Mr. Charles Persons of the +High School, Hannibal, Missouri. A large number of the diagrams of the +book were drawn by Mr. W. W. White of the Art School of Cornell +College. To all these friends, and to the many who have kindly +supplied the illustrations of the text, whose names are mentioned in +an appended list, the writer returns his heartfelt thanks. + +WILLIAM HARMON NORTON + +Cornell College, Mount Vernon, Iowa + +July, 1905 + + + + +INTRODUCTORY NOTE + + +During the preparation of this book Professor Norton has +frequently discussed its plan with me by correspondence, and we +have considered together the matters of scope, arrangement, and +presentation. + +As to scope, the needs of the young student and not of the expert +have been our guide; the book is therefore a text-book, not a +reference volume. + +In arrangement, the twofold division of the subject was chosen +because of its simplicity and effectiveness. The principles of +physical geology come first; the several chapters are arranged in +what is believed to be a natural order, appropriate to the +greatest part of our country, so that from a simple beginning a +logical sequence of topics leads through the whole subject. The +historical view of the science comes second, with many specific +illustrations of the physical processes previously studied, but +now set forth as part of the story of the earth, with its many +changes of aspect and its succession of inhabitants. Special +attention is here given to North America, and care is taken to +avoid overloading with details. + +With respect to method of presentation, it must not be forgotten +that the text-book is only one factor in good teaching, and that +in geology, as in other sciences, the teacher, the laboratory, and +the local field are other factors, each of which should play an +appropriate part. The text suggests observational methods, but it +cannot replace observation in field or laboratory; it offers +certain exercises, but space cannot be taken to make it a +laboratory manual as well as a book for study; it explains many +problems, but its statements are necessarily more terse than the +illustrative descriptions that a good and experienced teacher +should supply. Frequent use is made of induction and inference in +order that the student may come to see how reasonable a science is +geology, and that he may avoid the too common error of thinking +that the opinions of "authorities" are reached by a private road +that is closed to him. The further extension of this method of +presentation is urged upon the teacher, so that the young +geologist may always learn the evidence that leads to a +conclusion, and not only the conclusion itself. + +W. M. DAVIS + +Harvard University, Cambridge, Mass. + +July, 1905 + + + + +ACKNOWLEDGMENT OF ILLUSTRATIONS + + + Adams, Professor F. D., McGill University, Canada, 241. + Alden, Dr. W. C., Washington, D.C., 353. + American Museum of Natural History, New York, 344. + Ash, H. C., Galesburg, Ill., 133. + Beyer, Dr. S. W., Iowa College of Agriculture, 363. + Calvin, Dr. Samuel, Iowa State University, 45, 295, 317, 325, 371. + Carney, Frank, Ithaca, N.Y., 356. + Clark, Dr. Wm. B., Maryland Geological Survey, 43. + Borne, Dr. Georg v. d., Jena, Germany, 5, 6. + Daly, Dr. R. A., Ottawa, Canada, 164. + Defieux, C. A., Liverpool, England, 154. + * Detroit Photographic Co., 235, 236. + * Ellis, W. M., Edna, Kan., 13. + Fairchild, Professor H. L., University of Rochester, 141, 357. + Field Columbian Museum, Chicago, 87. + Forster, Dr. A. E., University of Vienna, 32. + Gardner, J. L., Boston, 12, 140, 352. + Geological Survey of Canada, 256. + Gilbert, Dr. G. K., by courtesy of the American Book Company, 39. + * Haines, Ben, New Albany, Ind., 33. + * Haynes, F. J., St. Paul, Minn., 52, 95, 233. + Henderson, Judge Julius, Boulder, Col., 94. + James, George Wharton, Pasadena, Cal., 16, 127, 215, 229. + Johnston-Lavis, Professor H. J., Beaulieu, France, 216. + King, J. Harding, Stourbridge, England, 119. + Lawson, Dr. Andrew C., University of California, 113. + Le Conte, Professor J. N., University of California, 8. + Libbey, Dr. William, Princeton University, 92. + * McAllister, T. H., New York, 242. + * Meyers, H. C., Boise, Id., 19. + Mills, Professor H. A., Cornell College, 208, 304. + Norton, Professor W. H., Cornell College, 14, 35, 59, 88, 128, + 183, 226, 234, 255, 340, 364, 367. + * Notman, Wm. & Son, Montreal, Canada, 98, 181. + Obrutschew, Dr. W., Tomsk Technological Institute, Siberia, 73. + Oldham, Dr. R. D., Geological Survey of India, 120. + * Peabody, H. C., Pasadena, Cal., 54. + * Pierce, C. C. & Co., Los Angeles, Cal., 15. + Pillsbury, Arthur, San Francisco, Cal., .115. + * Rau, Wm., Philadelphia, 18, 21, 122, 123, 218. + Reusch, Dr. Hans, Geological Survey of Norway, 112. + Reynolds, Professor S. H., University College, Bristol, England, 202. + Ricker, Principal Maurice, Burlington, Iowa, 48, 89. + * Shepard, E. A., Minneapolis, Minn., 105. + Smith, W. S. Tangier, Los Gatos, Cal., 186. + * Soule Photographic Co., Boston, 131. + U. S. Geological Survey, 3, 4, 23, 25, 34, 41, 63, 69, 78, 79, + 80, 110, 111, 114, 125, 126, 129, 130, 142, 151, 153, 169, + 172,177, 178, 188, 211, 212, 214, 228, 237, 238, 239, 243, 244, + 254, 257, 340, 341, 353, 355. + U. S. National Museum, 149, 220, 221, 222, 225, 332. + * Valentine & Sons, Dundee, Scotland, 40, 136, 227. + Vroman, A. C., Pasadena, Cal., 17. + * Ward's Natural Science Establishment, Rochester, N.Y., 152. + * Welch, R., Belfast, Ireland, 1, 37. + * Westgate, Dr. L. G., Ohio Wesleyan University, 66. + Whymper, Edward, London, England, 106. + * Wilcox, W. D., Washington, D.C., 20. + * Wilson, Dr. A. W. G., McGill University, Canada, 68. + * Wilson, G. W., & Co., Aberdeen, Scotland, 82, 213. + * Worsley-Benison, F. H., Cheapstow, England, 170. + + * Dealer in photographs or lantern slides. + + + + +CONTENTS + + + Page + + Introduction.--The Scope And Aim Of Geology 1 + + PART I + + EXTERNAL GEOLOGICAL AGENCIES + + Chapter + I. The Work Of The Weather 5 + II. The Work Of Ground Water 39 + III. Rivers And Valleys 54 + IV. River Deposits 93 + V. The Work Of Glaciers 113 + VI. The Work Of The Wind 144 + VII. The Sea And Its Shores 155 + VIII. Offshore And Deep-Sea Deposits 174 + + PART II + + INTERNAL GEOLOGICAL AGENCIES + + IX. Movements Of The Earth's Crust 195 + X. Earthquakes 233 + XI. Volcanoes 238 + XII. Underground Structures Of Igneous Origin 265 + XIII. Metamorphism And Mineral Veins 281 + + PART III + + HISTORICAL GEOLOGY + + XIV. The Geological Record 291 + XV. The Pre-Cambrian Systems 304 + XVI. The Cambrian 315 + XVII. The Ordovician And Silurian 327 + XVIII. The Devonian 341 + XIX. The Carboniferous 350 + XX. The Mesozoic 368 + XXI. The Tertiary 394 + XXII. The Quaternary 416 + + INDEX 451 + + + + +THE ELEMENTS OF GEOLOGY + + + + +INTRODUCTION + + +THE SCOPE AND AIM OF GEOLOGY + +Geology deals with the rocks of the earth's crust. It learns from +their composition and structure how the rocks were made and how they +have been modified. It ascertains how they have been brought to their +present places and wrought to their various topographic forms, such as +hills and valleys, plains and mountains. It studies the vestiges which +the rocks preserve of ancient organisms which once inhabited our +planet. Geology is the history of the earth and its inhabitants, as +read in the rocks of the earth's crust. + +To obtain a general idea of the nature and method of our science +before beginning its study in detail, we may visit some valley, such +as that illustrated in the frontispiece, on whose sides are rocky +ledges. Here the rocks lie in horizontal layers. Although only their +edges are exposed, we may infer that these layers run into the upland +on either side and underlie the entire district; they are part of the +foundation of solid rock which everywhere is found beneath the loose +materials of the surface. + +The ledges of the valley of our illustration are of sandstone. Looking +closely at the rock we see that it is composed of myriads of grains of +sand cemented together. These grains have been worn and rounded. They +are sorted also, those of each layer being about of a size. By some +means they have been brought hither from some more ancient source. +Surely these grains have had a history before they here found a +resting place,--a history which we are to learn to read. + +The successive layers of the rock suggest that they were built one +after another from the bottom upward. We may be as sure that each +layer was formed before those above it as that the bottom courses of +stone in a wall were laid before the courses which rest upon them. + +We have no reason to believe that the lowest layers which we see here +were the earliest ever formed. Indeed, some deep boring in the +vicinity may prove that the ledges rest upon other layers of rock +which extend downward for many hundreds of feet below the valley +floor. Nor may we conclude that the highest layers here were the +latest ever laid; for elsewhere we may find still later layers lying +upon them. + +A short search may find in the rock relics of animals, such as the +imprints of shells, which lived when it was deposited; and as these +are of kinds whose nearest living relatives now have their home in the +sea, we infer that it was on the flat sea floor that the sandstone was +laid. Its present position hundreds of feet above sea level proves +that it has since emerged to form part of the land; while the flatness +of the beds shows that the movement was so uniform and gentle as not +to break or strongly bend them from their original attitude. + +The surface of some of these layers is ripple-marked. Hence the sand +must once have been as loose as that of shallow sea bottoms and sea +beaches to-day, which is thrown into similar ripples by movements of +the water. In some way the grains have since become cemented into firm +rock. + +Note that the layers on one side of the valley agree with those on the +other, each matching the one opposite at the same level. Once they +were continuous across the valley. Where the valley now is was once a +continuous upland built of horizontal layers; the layers now show +their edges, or _outcrop_, on the valley sides because they have been +cut by the valley trench. + +The rock of the ledges is crumbling away. At the foot of each step of +rock lie fragments which have fallen. Thus the valley is slowly +widening. It has been narrower in the past; it will be wider in the +future. + +Through the valley runs a stream. The waters of rains which have +fallen on the upper parts of the stream's basin are now on their way +to the river and the sea. Rock fragments and grains of sand creeping +down the valley slopes come within reach of the stream and are washed +along by the running water. Here and there they lodge for a time in +banks of sand and gravel, but sooner or later they are taken up again +and carried on. The grains of sand which were brought from some +ancient source to form these rocks are on their way to some new goal. +As they are washed along the rocky bed of the stream they slowly rasp +and wear it deeper. The valley will be deeper in the future; it has +been less deep in the past. + +In this little valley we see slow changes now in progress. We find +also in the composition, the structure, and the attitude of the rocks, +and the land forms to which they have been sculptured, the record of a +long succession of past changes involving the origin of sand grains +and their gathering and deposit upon the bottom of some ancient sea, +the cementation of their layers into solid rock, the uplift of the +rocks to form a land surface, and, last of all, the carving of a +valley in the upland. + +Everywhere, in the fields, along the river, among the mountains, by the +seashore, and in the desert, we may discover slow changes now in +progress and the record of similar changes in the past. Everywhere we +may catch glimpses of a process of gradual change, which stretches +backward into the past and forward into the future, by which the forms +and structures of the face of the earth are continually built and +continually destroyed. The science which deals with this long process is +geology. Geology treats of the natural changes now taking place upon the +earth and within it, the agencies which produce them, and the land forms +and rock structures which result. It studies the changes of the present +in order to be able to read the history of the earth's changes in the +past. + +The various agencies which have fashioned the face of the earth may. +be divided into two general classes. In Part I we shall consider those +which work upon the earth from without, such as the weather, running +water, glaciers, the wind, and the sea. In Part II we shall treat of +those agencies whose sources are within the earth, and among whose +manifestations are volcanoes and earthquakes and the various movements +of the earth's crust. As we study each agency we shall notice not only +how it does its work, but also the records which it leaves in the rock +structures and the land forms which it produces. With this preparation +we shall be able in Part III to read in the records of the rocks the +history of our planet and the successive forms of life which have +dwelt upon it. + + + + +Part I + +EXTERNAL GEOLOGICAL AGENCIES + + +CHAPTER I + +THE WORK OF THE WEATHER + + +In our excursion to the valley with sandstone ledges we witnessed a +process which is going forward in all lands. Everywhere the rocks are +crumbling away; their fragments are creeping down hillsides to the +stream ways and are carried by the streams to the sea, where they are +rebuilt into rocky layers. When again the rocks are lifted to form +land the process will begin anew; again they will crumble and creep +down slopes and be washed by streams to the sea. Let us begin our +study of this long cycle of change at the point where rocks +disintegrate and decay under the action of the weather. In studying +now a few outcrops and quarries we shall learn a little of some common +rocks and how they weather away. + +=Stratification and jointing.= At the sandstone ledges we saw that the +rock was divided into parallel layers. The thicker layers are known as +_strata_, and the thin leaves into which each stratum may sometimes be +split are termed _laminæ_. To a greater or less degree these layers +differ from each other in fineness of grain, showing that the material +has been sorted. The planes which divide them are called _bedding +planes_. + +Besides the bedding planes there are other division planes, which cut +across the strata from top to bottom. These are found in all rocks and +are known as _joints_ (Fig. 1). Two sets of joints, running at about +right angles to each other, together with the bedding planes, divide +the sandstone into quadrangular blocks. + + [Illustration: Fig. 1. Cliff of Sandstone, Ireland] + +=Sandstone.= Examining a piece of sandstone we find it composed of +grains quite like those of river sand or of sea beaches. Most of the +grains are of a clear glassy mineral called quartz. These quartz +grains are very hard and will scratch the steel of a knife blade. They +are not affected by acid, and their broken surfaces are irregular like +those of broken glass. + +The grains of sandstone are held together by some cement. This may be +_calcareous_, consisting of soluble carbonate of lime. In brown +sandstones the cement is commonly _ferruginous_,--hydrated iron oxide, +or iron rust, forming the bond, somewhat as in the case of iron nails +which have rusted together. The strongest and most lasting cement is +_siliceous_, and sand rocks whose grains are closely cemented by +silica, the chemical substance of which quartz is made, are known as +quartzites. + +We are now prepared to understand how sandstone is affected by the +action of the weather. On ledges where the rock is exposed to view its +surface is more or less discolored and the grains are loose and may be +rubbed off with the finger. On gentle slopes the rock is covered with +a soil composed of sand, which evidently is crumbled sandstone, and +dark carbonaceous matter derived from the decay of vegetation. Clearly +it is by the dissolving of the cement that the rock thus breaks down +to loose sand. A piece of sandstone with calcareous cement, or a bit +of old mortar, which is really an artificial stone also made of sand +cemented by lime, may be treated in a test tube with hydrochloric acid +to illustrate the process. + + [Illustration: Fig. 2. Section of Limestone Quarry + + Scale, 1 in. = 30 ft. _a_, red residual clay; _mn_, pitted + surface of rotted limestone; _bb_, limestone divided into thin + layers; _c_, thick layers of laminated limestone, the laminæ + being firmly cemented together; _j_, _j_, _j_, joints. Is _bb_ + thin-layered because originally so laid, or because it has been + broken up by weathering, although once like _c_ thick-layered?] + +A limestone quarry. Here also we find the rock stratified and jointed +(Fig. 2). On the quarry face the rock is distinctly seen to be altered +for some distance from its upper surface. Below the altered zone the +rock is sound and is quarried for building; but the altered upper +layers are too soft and broken to be used for this purpose. If the +limestone is laminated, the laminae here have split apart, although +below they hold fast together. Near the surface the stone has become +rotten and crumbles at the touch, while on the top it has completely +broken down to a thin layer of limestone meal, on which rests a fine +reddish clay. + +Limestone is made of minute grains of carbonate of lime all firmly +held together by a calcareous cement. A piece of the stone placed in a +test tube with hydrochloric acid dissolves with brisk effervescence, +leaving the insoluble impurities, which were disseminated through it, +at the bottom of the tube as a little clay. + +We can now understand the changes in the upper layers of the quarry. +At the surface of the rock the limestone has completely dissolved, +leaving the insoluble residue as a layer of reddish clay. Immediately +below the clay the rock has disintegrated into meal where the cement +between the limestone grains has been removed, while beneath this the +laminae are split apart where the cement has been dissolved only along +the planes of lamination where the stone is more porous. As these +changes in the rock are greatest at the surface and diminish downward, +we infer that they have been caused by agents working downward from +the surface. + +At certain points these agencies have been more effective than +elsewhere. The upper rock surface is pitted. Joints are widened as +they approach the surface, and along these seams we may find that the +rock is altered even down to the quarry floor. + +=A shale pit.= Let us now visit some pit where shale--a laminated and +somewhat hardened clay--is quarried for the manufacture of brick. The +laminae of this fine-grained rock may be as thin as cardboard in +places, and close joints may break the rock into small rhombic blocks. +On the upper surface we note that the shale has weathered to a clayey +soil in which all traces of structure have been destroyed. The clay +and the upper layers of the shale beneath it are reddish or yellow, +while in many cases the color of the unaltered rock beneath is blue. + +=The sedimentary rocks.= The three kinds of layered rocks whose +acquaintance we have made--sandstone, limestone, and shale--are the +leading types of the great group of stratified, or sedimentary, rocks. +This group includes all rocks made of sediments, their materials +having settled either in water upon the bottoms of rivers, lakes, or +seas, or on dry land, as in the case of deposits made by the wind and +by glaciers. Sedimentary rocks are divided into the fragmental +rocks--which are made of fragments, either coarse or fine--and the far +less common rocks which are constituted of chemical precipitates. + + [Illustration: Fig. 3. Conglomerate] + +The sedimentary rocks are divided according to their composition into +the following classes: + +1. The arenaceous, or quartz rocks, including beds of loose sand and +gravel, sandstone, quartzite, and conglomerate (a rock made of +cemented rounded gravel or pebbles). + +2. The calcareous, or lime rocks, including limestone and a soft white +rock formed of calcareous powder known as chalk. + +3. The argillaceous, or clay rocks, including muds, clays, and shales. +These three classes pass by mixture into one another. Thus there are +limy and clayey sandstones, sandy and clayey limestones, and sandy and +limy shales. + +=Granite.= This familiar rock may be studied as an example of the +second great group of rocks,--_the unstratified_, or _igneous rocks_. +These are not made of cemented sedimentary grains, but of interlocking +crystals which have crystallized from a molten mass. Examining a piece +of granite, the most conspicuous crystals which meet the eye are those +of feldspar. They are commonly pink, white, or yellow, and break along +smooth cleavage planes which reflect the light like tiny panes of +glass. Mica may be recognized by its glittering plates, which split +into thin elastic scales. A third mineral, harder than steel, breaking +along irregular surfaces like broken glass, we identify as quartz. + +How granite alters under the action of the weather may be seen in +outcrops where it forms the bed rock, or country rock, underlying the +loose formations of the surface, and in many parts of the northern +states where granite bowlders and pebbles more or less decayed may be +found in a surface sheet of stony clay called the drift. Of the +different minerals composing granite, quartz alone remains unaltered. +Mica weathers to detached flakes which have lost their elasticity. The +feldspar crystals have lost their luster and hardness, and even have +decayed to clay. Where long-weathered granite forms the country rock, +it often may be cut with spade or trowel for several feet from the +surface, so rotten is the feldspar, and here the rock is seen to break +down to a clayey soil containing grains of quartz and flakes of mica. + +These are a few simple illustrations of the surface changes which some +of the common kinds of rocks undergo. The agencies by which these +changes are brought about we will now take up under two +divisions,--_chemical agencies_ producing rock decay and _mechanical +agencies_ producing rock disintegration. + + +The Chemical Work Of Water + +As water falls on the earth in rain it has already absorbed from the +air carbon dioxide (carbonic acid gas) and oxygen. As it sinks into +the ground and becomes what is termed ground water, it takes into +solution from the soil humus acids and carbon dioxide, both of which +are constantly being generated there by the decay of organic matter. +So both rain and ground water are charged with active chemical agents, +by the help of which they corrode and rust and decompose all rocks to +a greater or less degree. We notice now three of the chief chemical +processes concerned in weathering,--solution, the formation of +carbonates, and oxidation. + +=Solution.= Limestone, although so little affected by pure water that +five thousand gallons would be needed to dissolve a single pound, is +easily dissolved in water charged with carbon dioxide. In limestone +regions well water is therefore "hard." On boiling the water for some +time the carbon dioxide gas is expelled, the whole of the lime +carbonate can no longer be held in solution, and much of it is thrown +down to form a crust or "scale" in the kettle or in the tubes of the +steam boiler. All waters which flow over limestone rocks or soak +through them are constantly engaged in dissolving them away, and in +the course of time destroy beds of vast extent and great thickness. + + [Illustration: Fig. 4. Surface of Limestone furrowed by + Weathering, Montana] + +The upper surface of limestone rocks becomes deeply pitted, as we saw +in the limestone quarry, and where the mantle of waste has been +removed it may be found so intricately furrowed that it is difficult +to traverse (Fig. 4). + +Beds of _rock salt_ buried among the strata are dissolved by seeping +water, which issues in salt springs. _Gypsum_, a mineral composed of +hydrated sulphate of lime, and so soft that it may be scratched with +the finger nail, is readily taken up by water, giving to the water of +wells and springs a peculiar hardness difficult to remove. + +The dissolving action of moisture may be noted on marble tombstones of +some age, marble being a limestone altered by heat and pressure and +composed of crystalline grains. By assuming that the date on each +monument marks the year of its erection, one may estimate how many +years on the average it has taken for weathering to loosen fine grains +on the polished surface, so that they may be rubbed off with the +finger, to destroy the polish, to round the sharp edges of tool marks +in the lettering, and at last to open cracks and seams and break down +the stone. We may notice also whether the gravestones weather more +rapidly on the sunny or the shady side, and on the sides or on the +top. + +The weathered surface of granular limestone containing shells shows +them standing in relief. As the shells are made of crystalline +carbonate of lime, we may infer whether the carbonate of lime is less +soluble in its granular or in its crystalline condition. + +=The formation of carbonates.= In attacking minerals water does more +than merely take them into solution. It decomposes them, forming new +chemical compounds of which the carbonates are among the most +important. Thus feldspar consists of the insoluble silicate of +alumina, together with certain alkaline silicates which are broken up +by the action of water containing carbon dioxide, forming alkaline +carbonates. These carbonates are freely soluble and contribute potash +and soda to soils and river waters. By the removal of the soluble +ingredients of feldspar there is left the silicate of alumina, united +with water or hydrated, in the condition of a fine plastic clay which, +when white and pure, is known as _kaolin_ and is used in the +manufacture of porcelain. Feldspathic rocks which contain no iron +compounds thus weather to whitish crusts, and even apparently sound +crystals of feldspar, when ground to thin slices and placed under the +microscope, may be seen to be milky in color throughout because an +internal change to kaolin has begun. + + [Illustration: Fig. 5. Bowlder split by Heat and Cold, + Western Texas] + +=Oxidation.= Rocks containing compounds of iron weather to reddish +crusts, and the seams of these rocks are often lined with rusty films. +Oxygen and water have here united with the iron, forming hydrated iron +oxide. The effects of oxidation may be seen in the alteration of many +kinds of rocks and in red and yellow colors of soils and subsoils. + +_Pyrite_ is a very hard mineral of a pale brass color, found in +scattered crystals in many rocks, and is composed of iron and sulphur +(iron sulphide). Under the attack of the weather it takes up oxygen, +forming iron sulphate (green vitriol), a soluble compound, and +insoluble hydrated iron oxide, which as a mineral is known as +limonite. Several large masses of iron sulphide were placed some years +ago on the lawn in front of the National Museum at Washington. The +mineral changed so rapidly to green vitriol that enough of this +poisonous compound was washed into the ground to kill the roots of the +surrounding grass. + + +Agents Of Mechanical Disintegration + +=Heat and cold.= Rocks exposed to the direct rays of the sun become +strongly heated by day and expand. After sunset they rapidly cool and +contract. When the difference in temperature between day and night is +considerable, the repeated strains of sudden expansion and contraction +at last become greater than the rocks can bear, and they break, for +the same reason that a glass cracks when plunged into boiling water +(Fig. 5). + +Rocks are poor conductors of heat, and hence their surfaces may become +painfully hot under the full blaze of the sun, while the interior +remains comparatively cool. By day the surface shell expands and tends +to break loose from the mass of the stone. In cooling in the evening +the surface shell suddenly contracts on the unyielding interior and in +time is forced off in scales (Fig. 6). + + [Illustration: Fig. 6. Bowlders scaling off under Heat and Cold, + Western Texas] + +Many rocks, such as granite, are made up of grains of various minerals +which differ in color and in their capacity to absorb heat, and which +therefore contract and expand in different ratios. In heating and +cooling these grains crowd against their neighbors and tear loose from +them, so that finally the rock disintegrates into sand. + +The conditions for the destructive action of heat and cold are most +fully met in arid regions when vegetation is wanting for lack of +sufficient rain. The soil not being held together by the roots of +plants is blown away over large areas, leaving the rocks bare to the +blazing sun in a cloudless sky. The air is dry, and the heat received +by the earth by day is therefore rapidly radiated at night into space. +There is a sharp and sudden fall of temperature after sunset, and the +rocks, strongly heated by day, are now chilled perhaps even to the +freezing point. + +In the Sahara the thermometer has been known to fall 131° F. within a +few hours. In the light air of the Pamir plateau in central Asia a +rise of 90° F. has been recorded from seven o'clock in the morning to +one o'clock in the afternoon. On the mountains of southwestern Texas +there are frequently heard crackling noises as the rocks of that arid +region throw off scales from a fraction of an inch to four inches in +thickness, and loud reports are made as huge bowlders split apart. +Desert pebbles weakened by long exposure to heat and cold have been +shivered to fine sharp-pointed fragments on being placed in sand +heated to 180 degrees F. Beds half a foot thick, forming the floor of +limestone quarries in Wisconsin, have been known to buckle and arch +and break to fragments under the heat of the summer sun. + +=Frost.= By this term is meant the freezing and thawing of water +contained in the pores and crevices of rocks. All rocks are more or +less porous and all contain more or less water in their pores. Workers +in stone call this "quarry water," and speak of a stone as "green" +before the quarry water has dried out. Water also seeps along joints +and bedding planes and gathers in all seams and crevices. Water +expands in freezing, ten cubic inches of water freezing to about +eleven cubic inches of ice. As water freezes in the rifts and pores of +rocks it expands with the irresistible force illustrated in the +freezing and breaking of water pipes in winter. The first rift in the +rock, perhaps too narrow to be seen, is widened little by little by +the wedges of successive frosts, and finally the rock is broken into +detached blocks, and these into angular chip-stone by the same +process. + +It is on mountain tops and in high latitudes that the effects of frost +are most plainly seen. "Every summit" says Whymper, "amongst the rock +summits upon which I have stood has been nothing but a piled-up heap +of fragments" (Fig. 7). In Iceland, in Spitzbergen, in Kamchatka, and +in other frigid lands large areas are thickly strewn with sharp-edged +fragments into which the rock has been shattered by frost. + + [Illustration: Fig. 7. Rocks broken by Frost, Summit of the + Eggischhorn, Switzerland] + +=Organic agents.= We must reckon the roots of plants and trees among +the agents which break rocks into pieces. The tiny rootlet in its +search for food and moisture inserts itself into some minute rift, and +as it grows slowly wedges the rock apart. Moreover, the acids of the +root corrode the rocks with which they are in contact. One may +sometimes find in the soil a block of limestone wrapped in a mesh of +roots, each of which lies in a little furrow where it has eaten into +the stone. + +Rootless plants called _lichens_ often cover and corrode rocks as yet +bare of soil; but where lichens are destroying the rock less rapidly +than does the weather, they serve in a way as a protection. + +=Conditions favoring disintegration and decay.= The disintegration of +rocks under frost and temperature changes goes on most rapidly in cold +and arid climates, and where vegetation is scant or absent. On the +contrary, the decay of rocks under the chemical action of water is +favored by a warm, moist climate and abundant vegetation. Frost and +heat and cold can only act within the few feet from the surface to +which the necessary temperature changes are limited, while water +penetrates and alters the rocks to great depths. + +The pupil may explain. + +In what ways the presence of joints and bedding planes assists in the +breaking up and decay of rocks under the action of the weather. + +Why it is a good rule of stone masons never to lay stones on edge, but +always on their natural bedding planes. + +Why stones fresh from the quarry sometimes go to pieces in early +winter, when stones which have been quarried for some months remain +uninjured. + +Why quarrymen in the northern states often keep their quarry floors +flooded during winter. + +Why laminated limestone should not be used for curbstone. + +Why rocks composed of layers differing in fineness of grain and in +ratios of expansion do not make good building stone. + +Fine-grained rocks with pores so small that capillary attraction keeps +the water which they contain from readily draining away are more apt +to hold their pores ten elevenths full of water than are rocks whose +pores are larger. Which, therefore, are more likely to be injured by +frost? + +Which is subject to greater temperature changes, a dark rock or one of +a light color? the north side or the south side of a valley? + + +The Mantle of Rock Waste + +We have seen that rocks are everywhere slowly wasting away. They are +broken in pieces by frost, by tree roots, and by heat and cold. They +dissolve and decompose under the chemical action of water and the +various corrosive substances which it contains, leaving their +insoluble residues as residual clays and sands upon the surface. As a +result there is everywhere forming a mantle of rock waste which covers +the land. It is well to imagine how the country would appear were this +mantle with its soil and vegetation all scraped away or had it never +been formed. The surface of the land would then be everywhere of bare +rock as unbroken as a quarry floor. + +=The thickness of the mantle.= In any locality the thickness of the +mantle of rock waste depends as much on the rate at which it is +constantly being removed as on the rate at which it is forming. On the +face of cliffs it is absent, for here waste is removed as fast as it +is made. Where waste is carried away more slowly than it is produced, +it accumulates in time to great depth. + +The granite of Pikes Peak is disintegrated to a depth of twenty feet. +In the city of Washington granite rock is so softened to a depth of +eighty feet that it can be removed with pick and shovel. About +Atlanta, Georgia, the rocks are completely rotted for one hundred feet +from the surface, while the beginnings of decay may be noticed at +thrice that depth. In places in southern Brazil the rock is decomposed +to a depth of four hundred feet. + +In southwestern Wisconsin a reddish residual clay has an average depth +of thirteen feet on broad uplands, where it has been removed to the +least extent. The country rock on which it rests is a limestone with +about ten per cent of insoluble impurities. At least how thick, then, +was that portion of the limestone which has rotted down to the clay? + +=Distinguishing characteristics of residual waste.= We must learn to +distinguish waste formed in place by the action of the weather from +the products of other geological agencies. Residual waste is +unstratified. It contains no substances which have not been derived +from the weathering of the parent rock. There is a gradual transition +from residual waste into the unweathered rock beneath. Waste resting +on sound rock evidently has been shifted and was not formed in place. + +In certain regions of southern Missouri the land is covered with a +layer of broken flints and red clay, while the country rock is +limestone. The limestone contains nodules of flint, and we may infer +that it has been by the decay and removal of thick masses of limestone +that the residual layer of clay and flints has been left upon the +surface. Flint is a form of quartz, dull-lustered, usually gray or +blackish in color, and opaque except on thinnest edges, where it is +translucent. + +Over much of the northern states there is spread an unstratified stony +clay called the _drift_. It often rests on sound rocks. It contains +grains of sand, pebbles, and bowlders composed of many different +minerals and rocks that the country rock cannot furnish. Hence the +drift cannot have been formed by the decay of the rock of the region. +A shale or limestone, for example, cannot waste to a clay containing +granite pebbles. The origin of the drift will be explained in +subsequent chapters. + +The differences in rocks are due more to their soluble than to their +insoluble constituents. The latter are few in number and are much the +same in rocks of widely different nature, being chiefly quartz, +silicate of alumina, and iron oxide. By the removal of their soluble +parts very many and widely different rocks rot down to a residual clay +gritty with particles of quartz and colored red or yellow with iron +oxide. + +In a broad way the changes which rocks undergo in weathering are an +adaptation to the environment in which they find themselves at the +earth's surface,--an environment different from that in which they +were formed under sea or under ground. In open air, where they are +attacked by various destructive agents, few of the rock-making +minerals are stable compounds except quartz, the iron oxides, and the +silicate of alumina; and so it is to one or more of these +comparatively insoluble substances that most rocks are reduced by long +decay. + +Which produces a mantle of finer waste, frost or chemical decay? which +a thicker mantle? In what respects would you expect that the mantle of +waste would differ in warm humid lands like India, in frozen countries +like Alaska, and in deserts such as the Sahara? + +=The soil.= The same agencies which produce the mantle of waste are +continually at work upon it, breaking it up into finer and finer +particles and causing its more complete decay. Thus on the surface, +where the waste has weathered longest, it is gradually made fine +enough to support the growth of plants, and is then known as _soil_. +The coarser waste beneath is sometimes spoken of as subsoil. Soil +usually contains more or less dark, carbonaceous, decaying organic +matter, called humus, and is then often termed the _humus layer_. Soil +forms not only on waste produced in place from the rock beneath, but +also on materials which have been transported, such as sheets of +glacial drift and river deposits. + +Until rocks are reduced to residual clays the work of the weather is +more rapid and effective on the fragments of the mantle of waste than +on the rocks from which waste is being formed. Why? + +Any fresh excavation of cellar or cistern, or cut for road or railway, +will show the characteristics of the humus layer. It may form only a +gray film on the surface, or we may find it a layer a foot or more +thick, dark, or even black, above, and growing gradually lighter in +color as it passes by insensible gradations into the subsoil. In some +way the decaying vegetable matter continually forming on the surface +has become mingled with the material beneath it. + +=How humus and the subsoil are mingled.= The mingling of humus and the +subsoil is brought about by several means. The roots of plants +penetrate the waste, and when they die leave their decaying substance +to fertilize it. Leaves and stems falling on the surface are turned +under by several agents. Earthworms and other animals whose home is in +the waste drag them into their burrows either for food or to line +their nests. Trees overthrown by the wind, roots and all, turn over +the soil and subsoil and mingle them together. Bacteria also work in +the waste and contribute to its enrichment. The animals living in the +mantle do much in other ways toward the making of soil. They bring the +coarser fragments from beneath to the surface, where the waste +weathers more rapidly. Their burrows allow air and water to penetrate +the waste more freely and to affect it to greater depths. + +=Ants.= In the tropics the mantle of waste is worked over chiefly by +ants. They excavate underground galleries and chambers, extending +sometimes as much as fourteen feet below the surface, and build mounds +which may reach as high above it. In some parts of Paraguay and +southern Brazil these mounds, like gigantic potato hills, cover tracts +of considerable area. + +In search for its food--the dead wood of trees--the so-called white +ant constructs runways of earth about the size of gas pipes, reaching +from the base of the tree to the topmost branches. On the plateaus of +central Africa explorers have walked for miles through forests every +tree of which was plastered with these galleries of mud. Each grain of +earth used in their construction is moistened and cemented by slime as +it is laid in place by the ant, and is thus acted on by organic +chemical agents. Sooner or later these galleries are beaten down by +heavy rains, and their fertilizing substances are scattered widely by +the winds. + +=Earthworms.= In temperate regions the waste is worked over largely by +earthworms. In making their burrows worms swallow earth in order to +extract from it any nutritive organic matter which it may contain. +They treat it with their digestive acids, grind it in their stony +gizzards, and void it in castings on the surface of the ground. It was +estimated by Darwin that in many parts of England each year, on every +acre, more than ten tons of earth pass through the bodies of +earthworms and are brought to the surface, and that every few years +the entire soil layer is thus worked over by them. + +In all these ways the waste is made fine and stirred and enriched. +Grain by grain the subsoil with its fresh mineral ingredients is +brought to the surface, and the rich organic matter which plants and +animals have taken from the atmosphere is plowed under. Thus Nature +plows and harrows on "the great world's farm" to make ready and ever +to renew a soil fit for the endless succession of her crops. + +The world processes by which rocks are continually wasting away are +thus indispensable to the life of plants and animals. The organic +world is built on the ruins of the inorganic, and because the solid +rocks have been broken down into soil men are able to live upon the +earth. + +=Solar energy.= The source of the energy which accomplishes all this +necessary work is the sun. It is the radiant energy of the sun which +causes the disintegration of rocks, which lifts vapor into the +atmosphere to fall as rain, which gives life to plants and animals. +Considering the earth in a broad way, we may view it as a globe of +solid rock,--_the lithosphere_,--surrounded by two mobile envelopes: +the envelope of air,--_the atmosphere_; and the envelope of +water,--_the hydrosphere_. Under the action of solar energy these +envelopes are in constant motion. Water from the hydrosphere is +continually rising in vapor into the atmosphere, the air of the +atmosphere penetrates the hydrosphere,--for its gases are dissolved in +all waters,--and both air and water enter and work upon the solid +earth. By their action upon the lithosphere they have produced a third +envelope,--the mantle of rock waste. + +This envelope also is in movement, not indeed as a whole, but particle +by particle. The causes which set its particles in motion, and the +different forms which the mantle comes to assume, we will now proceed +to study. + + +Movements of the Mantle of Rock Waste + +At the sandstone ledges which we first visited we saw not only that +the rocks were crumbling away, but also that grains and fragments of +them were creeping down the slopes of the valley to the stream and +were carried by it onward toward the sea. This process is going on +everywhere. Slowly it may be, and with many interruptions, but surely, +the waste of the land moves downward to the sea. We may divide its +course into two parts,--the path to the stream, which we will now +consider, and its carriage onward by the stream, which we will defer +to a later chapter. + +=Gravity.= The chief agent concerned in the movement of waste is +gravity. Each particle of waste feels the unceasing downward pull of +the earth's mass and follows it when free to do so. All agencies which +produce waste tend to set its particles free and in motion, and +therefore coöperate with gravity. On cliffs, rocks fall when wedged +off by frost or by roots of trees, and when detached by any other +agency. On slopes of waste, water freezes in chinks between stones, +and in pores between particles of soil, and wedges them apart. Animals +and plants stir the waste, heat expands it, cold contracts it, the +strokes of the raindrops drive loose particles down the slope and the +wind lifts and lets them fall. Of all these movements, gravity assists +those which are downhill and retards those which are uphill. On the +whole, therefore, the downhill movements prevail, and the mantle of +waste, block by block and grain by grain, creeps along the downhill +path. + +A slab of sandstone laid on another of the same kind at an angle of +17° and left in the open air was found to creep down the slope at the +rate of a little more than a millimeter a month. Explain why it did +so. + +=Rain.= The most efficient agent in the carriage of waste to the +streams is the rain. It moves particles of soil by the force of the +blows of the falling drops, and washes them down all slopes to within +reach of permanent streams. On surfaces unprotected by vegetation, as +on plowed fields and in arid regions, the rain wears furrows and +gullies both in the mantle of waste and in exposures of unaltered rock +(Fig. 17). + +At the foot of a hill we may find that the soil has accumulated by +creep and wash to the depth of several feet; while where the hillside +is steepest the soil may be exceedingly thin, or quite absent, because +removed about as fast as formed. Against the walls of an abbey built +on a slope in Wales seven hundred years ago, the creeping waste has +gathered on the uphill side to a depth of seven feet. The slow-flowing +sheet of waste is often dammed by fences and walls, whose uphill side +gathers waste in a few years so as to show a distinctly higher surface +than the downhill side, especially in plowed fields where the movement +is least checked by vegetation. + +=Talus.= At the foot of cliffs there is usually to be found a slope of +rock fragments which clearly have fallen from above (Fig. 8). Such a +heap of waste is known as _talus_. The amount of talus in any place +depends both on the rate of its formation and the rate of its removal. +Talus forms rapidly in climates where mechanical disintegration is +most effective, where rocks are readily broken into blocks because +closely jointed and thinly bedded rather than massive, and where they +are firm enough to be detached in fragments of some size instead of in +fine grains. Talus is removed slowly where it decays slowly, either +because of the climate or the resistance of the rock. It may be +rapidly removed by a stream flowing along its base. + + [Illustration: Fig. 8. Talus at Foot of Granite Cliffs, Sierra + Nevada Mountains] + +In a moist climate a soluble rock, such as massive limestone, may form +talus little if any faster than the talus weathers away. A +loose-textured sandstone breaks down into incoherent sand grains, +which in dry climates, where unprotected by vegetation, may be blown +away as fast as they fall, leaving the cliff bare to the base. Cliffs +of such slow-decaying rocks as quartzite and granite when closely +jointed accumulate talus in large amounts. + + [Illustration: Fig. 9. Diagram Illustrating Retreat of Cliff, + _c_, and Talus, _t_] + +Talus slopes may be so steep as to reach _the angle of repose_, i.e. +the steepest angle at which the material will lie. This angle varies +with different materials, being greater with coarse and angular +fragments than with fine rounded grains. Sooner or later a talus +reaches that equilibrium where the amount removed from its surface +just equals that supplied from the cliff above. As the talus is +removed and weathers away its slope retreats together with the retreat +of the cliff, as seen in Figure 9. + +=Graded slopes.= Where rocks weather faster than their waste is +carried away, the waste comes at last to cover all rocky ledges. On +the steeper slopes it is coarser and in more rapid movement than on +slopes more gentle, but mountain sides and hills and plains alike come +to be mantled with sheets of waste which everywhere is creeping toward +the streams. Such unbroken slopes, worn or built to the least +inclination at which the waste supplied by weathering can be urged +onward, are known as _graded slopes_. + +Of far less importance than the silent, gradual creep of waste, which +is going on at all times everywhere about us, are the startling local +and spasmodic movements which we are now to describe. + +=Avalanches.= On steep mountain sides the accumulated snows of winter +often slip and slide in avalanches to the valleys below. These rushing +torrents of snow sweep their tracks clean of waste and are one of +Nature's normal methods of moving it along the downhill path. + + [Illustration: Fig. 10. A Landslide, Quebec] + +=Landslides.= Another common and abrupt method of delivering waste to +streams is by slips of the waste mantle in large masses. After long +rains and after winter frosts the cohesion between the waste and the +sound rock beneath is loosened by seeping water underground. The waste +slips on the rock surface thus lubricated and plunges down the +mountain side in a swift roaring torrent of mud and stones. + + [Illustration: Fig. 11. Diagram Illustrating Conditions favorable + to a Landslide + + _lm_, limestone dipping toward valley of river, _r_; _sh_, shale] + +We may conveniently mention here a second type of landslide, where +masses of solid rock as well as the mantle of waste are involved in +the sudden movement. Such slips occur when valleys have been rapidly +deepened by streams or glaciers and their sides have not yet been +graded. A favorable condition is where the strata dip (i.e. incline +downwards) towards the valley (Fig. 11), or are broken by joint planes +dipping in the same direction. The upper layers, including perhaps the +entire mountain side, have been cut across by the valley trench and +are left supported only on the inclined surface of the underlying +rocks. Water may percolate underground along this surface and loosen +the cohesion between the upper and the underlying strata by converting +the upper surface of a shale to soft wet clay, by dissolving layers of +a limestone, or by removing the cement of a sandstone and converting +it into loose sand. When the inclined surface is thus lubricated the +overlying masses may be launched into the valley below. The solid +rocks are broken and crushed in sliding and converted into waste +consisting, like that of talus, of angular unsorted fragments, blocks +of all sizes being mingled pell-mell with rock meal and dust. The +principal effects of landslides may be gathered from the following +examples. + +At Gohna, India, in 1893, the face of a spur four thousand feet high, +of the lower ranges of the Himalayas, slipped into the gorge of the +headwaters of the Ganges River in successive rock falls which lasted +for three days. Blocks of stone were projected for a mile, and clouds +of limestone dust were spread over the surrounding country. The débris +formed a dam one thousand feet high, extending for two miles along the +valley. A lake gathered behind this barrier, gradually rising until it +overtopped it in a little less than a year. The upper portion of the +dam then broke, and a terrific rush of water swept down the valley in +a wave which, twenty miles away, rose one hundred and sixty feet in +height. A narrow lake is still held by the strong base of the dam. + +In 1896, after forty days of incessant rain, a cliff of sandstone +slipped into the Yangtse River in China, reducing the width of the +channel to eighty yards and causing formidable rapids. + + [Illustration: Fig. 12. Bowlders of Weathering, Granite Quarry, + Cape Ann, Massachusetts] + +At Flims, in Switzerland, a prehistoric landslip flung a dam eighteen +hundred feet high across the headwaters of the Rhine. If spread evenly +over a surface of twenty-eight square miles, the material would cover +it to a depth of six hundred and sixty feet. The barrier is not yet +entirely cut away, and several lakes are held in shallow basins on its +hummocky surface. + +A slide from the precipitous river front of the citadel hill of +Quebec, in 1889, dashed across Champlain Street, wrecking a number of +houses and causing the death of forty-five persons. The strata here +are composed of steeply dipping slate (Fig. 10). + +In lofty mountain ranges there may not be a single valley without its +traces of landslides, so common there is this method of the movement +of waste, and of building to grade over-steepened slopes. + + +Rock Sculpture By Weathering + +We are now to consider a few of the forms into which rock masses are +carved by the weather. + + [Illustration: Fig. 13. Differential Weathering on a Monument, + Colorado] + +=Bowlders of weathering.= In many quarries and outcrops we may see +that the blocks into which one or more of the uppermost layers have +been broken along their joints and bedding planes are no longer +angular, as are those of the layers below. The edges and corners of +these blocks have been worn away by the weather. Such rounded cores, +known as bowlders of weathering, are often left to strew the surface. + +=Differential weathering.= This term covers all cases in which a rock +mass weathers differently in different portions. Any weaker spots or +layers are etched out on the surface, leaving the more resistant in +relief. Thus massive limestones become pitted where the weather drills +out the weaker portions. In these pits, when once they are formed, +moisture gathers, a little soil collects, vegetation takes root, and +thus they are further enlarged until the limestone may be deeply +honeycombed. + + [Illustration: Fig. 14. Honeycombed Limestone, Iowa] + + [Illustration: Fig. 15. Cliffs and Slopes on North Wall of the + Grand Canyon of the Colorado River, Arizona] + +On the sides of canyons, and elsewhere where the edges of strata are +exposed, the harder layers project as cliffs, while the softer weather +back to slopes covered with the talus of the harder layers above them. +It is convenient to call the former _cliff makers_ and the latter +_slope makers_ (Fig. 15). + +Differential weathering plays a large part in the sculpture of the +land. Areas of weak rock are wasted to plains, while areas of hard +rock adjacent are still left as hills and mountain ridges, as in the +valleys and mountains of eastern Pennsylvania. But in such instances +the lowering of the surface of the weaker rock is also due to the wear +of streams, and especially to the removal by them from the land of the +waste which covers and protects the rocks beneath. + + [Illustration: Fig. 16. Taverlone Mesa, New Mexico] + +Rocks owe their weakness to several different causes. Some, such as +beds of loose sand, are soft and easily worn by rains; some, as +limestone and gypsum for example, are soluble. Even hard insoluble +rocks are weak under the attack of the weather when they are closely +divided by joints and bedding planes and are thus readily broken up +into blocks by mechanical agencies. + + [Illustration: Fig. 17. Monuments, Arizona + + Note the rain furrows on the slope at the foot of the monuments. + In the foreground are seen fragments of petrified trunks of trees, + composed of silica and extremely resistant to the weather. On the + removal of the rock layers in which these fragments were imbedded + they are left to strew the surface in the same way as are the + residual flints of southern Missouri.] + +=Outliers and monuments.= As cliffs retreat under the attack of the +weather, portions are left behind where the rock is more resistant or +where the attack for any reason is less severe. Such remnant masses, +if large, are known as outliers. When flat-topped, because of the +protection of a resistant horizontal capping layer, they are termed +_mesas_ (Fig. 16),--a term applied also to the flat-topped portions of +dissected plateaus (Fig. 129). Retreating cliffs may fall back a +number of miles behind their outliers before the latter are finally +consumed. + + [Illustration: Fig. 18. Undercut Monuments, Colorado] + +Monuments are smaller masses and may be but partially detached from +the cliff face. In the breaking down of sheets of horizontal strata, +outliers grow smaller and smaller and are reduced to massive +rectangular monuments resembling castles (Fig. 17). The rock castle +falls into ruin, leaving here and there an isolated tower; the tower +crumbles to a lonely pillar, soon to be overthrown. The various and +often picturesque shapes of monuments depend on the kind of rock, the +attitude of the strata, and the agent by which they are chiefly +carved. Thus pillars may have a capital formed of a resistant stratum. +Monuments may be undercut and come to rest on narrow pedestals, +wherever they weather more rapidly near the ground, either because of +the greater moisture there, or--in arid climates--because worn at +their base by drifting sands. + +Stony clays disintegrating under the rain often contain bowlders +which protect the softer material beneath from the vertical blows +of raindrops, and thus come to stand on pedestals of some height +(Fig. 19). One may sometimes see on the ground beneath dripping eaves +pebbles left in the same way, protecting tiny pedestals of sand. + +=Mountain peaks and ridges.= Most mountains have been carved out of +great broadly uplifted folds and blocks of the earth's crust. Running +water and glacier ice have cut these folds and blocks into masses +divided by deep valleys; but it is by the weather, for the most part, +that the masses thus separated have been sculptured to the present +forms of the individual peaks and ridges. + + [Illustration: Fig. 19. Roosevelt Column, Idaho + + An erosion pillar 70 feet high. How was it produced? Why + quadrangular? What does it show as to the recent height of the + hillside surface?] + +Frost and heat and cold sculpture high mountains to sharp, tusklike +peaks and ragged, serrate crests, where their waste is readily removed +(Fig. 8). + +The Matterhorn of the Alps is a famous example of a mountain peak +whose carving by the frost and other agents is in active progress. On +its face "scarcely a rock anywhere is firmly attached," and the fall +of loosened stones is incessant. Mountain climbers who have camped at +its base tell how huge rocks from time to time come leaping down its +precipices, followed by trains of dislodged smaller fragments and rock +dust; and how at night one may trace the course of the bowlders by the +sparks which they strike from the mountain walls. Mount Assiniboine, +Canada (Fig. 20), resembles the Matterhorn in form and has been carved +by the same agencies. + +"The Needles" of Arizona are examples of sharp mountain peaks in a +warm arid region sculptured chiefly by temperature changes. + +Chemical decay, especially when carried on beneath a cover of waste +and vegetation, favors the production of rounded knobs and dome-shaped +mountains. + +=The weather curve.= We have seen that weathering reduces the angular +block quarried by the frost to a rounded bowlder by chipping off its +corners and smoothing away its edges. In much the same way weathering +at last reduces to rounded hills the earth blocks cut by streams or +formed in any other way. High mountains may at first be sculptured by +the weather to savage peaks (Fig. 181), but toward the end of their +life history they wear down to rounded hills (Fig. 182). The weather +curve, which may be seen on the summits of low hills (Fig. 21), is +convex upward. + + [Illustration: Fig. 20. Mount Assiniboine, Canada] + + [Illustration: Fig. 21. Big Round Top and Little Round Top, + Gettysburg, Pennsylvania] + +In Figure 22, representing a cubic block of stone whose faces are a +yard square, how many square feet of surface are exposed to the +weather by a cubic foot at a corner _a_; by one situated in the middle +of an edge _b_; by one in the center of a side _c_? How much faster +will _a_ and _b_ weather than _c_, and what will be the effect on the +shape of the block? + + [Illustration: Fig. 22] + +=The coöperation of various agencies in rock sculpture.= For the sake +of clearness it is necessary to describe the work of each geological +agent separately. We must not forget, however, that in Nature no agent +works independently and alone; that every result is the outcome of a +long chain of causes. Thus, in order that the mountain peak may be +carved by the agents of disintegration, the waste must be rapidly +removed,--a work done by many agents, including some which we are yet +to study; and in order that the waste may be removed as fast as +formed, the region must first have been raised well above the level of +the sea, so that the agents of transportation could do their work +effectively. The sculpture of the rocks is accomplished only by the +coöperation of many forces. + +The constant removal of waste from the surface by creep and wash and +carriage by streams is of the highest importance, because it allows +the destruction of the land by means of weathering to go on as long as +any land remains above sea level. If waste were not removed, it would +grow to be so thick as to protect the rock beneath from further +weathering, and the processes of destruction which we have studied +would be brought to an end. The very presence of the mantle of waste +over the land proves that on the whole rocks weather more rapidly than +their waste is removed. The destruction of the land is going on as +fast as the waste can be carried away. + +We have now learned to see in the mantle of waste the record of the +destructive action of the agencies of weathering on the rocks of the +land surface. Similar records we shall find buried deeply among the +rocks of the crust in old soils and in rocks pitted and decayed, +telling of old land surfaces long wasted by the weather. Ever since +the dry land appeared these agencies have been as now quietly and +unceasingly at work upon it, and have ever been the chief means of the +destruction of its rocks. The vast bulk of the stratified rocks of the +earth's crust is made up almost wholly of the waste thus worn from +ancient lands. + + [Illustration: Fig. 23. Mount Sneffels, Colorado + + Describe and account for what you see in this view. What + changes may the mountain be expected to undergo in the future + from the agencies now at work upon it?] + +In studying the various geological agencies we must remember the +almost inconceivable times in which they work. The slowest process +when multiplied by the immense time in which it is carried on produces +great results. The geologist looks upon the land forms of the earth's +surface as monuments which record the slow action of weathering and +other agents during the ages of the past. The mountain peak, the +rounded hill, the wide plain which lies where hills and mountains once +stood, tell clearly of the great results which slow processes will +reach when given long time in which to do their work. We should +accustom ourselves also to think of the results which weathering will +sooner or later bring to pass. The tombstone and the bowlder of the +field, which each year lose from their surfaces a few crystalline +grains, must in time be wholly destroyed. The hill whose rocks are +slowly rotting underneath a cover of waste must become lower and lower +as the centuries and millenniums come and go, and will finally +disappear. Even the mountains are crumbling away continually, and +therefore are but fleeting features of the landscape. + + + + +CHAPTER II + +THE WORK OF GROUND WATER + + +=Land waters.= We have seen how large is the part that water plays at +and near the surface of the land in the processes of weathering and in +the slow movement of waste down all slopes to the stream ways. We now +take up the work of water as it descends beneath the ground,--a +corrosive agent still, and carrying in solution as its load the +invisible waste of rocks derived from their soluble parts. + +Land waters have their immediate source in the rainfall. By the heat +of the sun water is evaporated from the reservoir of the ocean and +from moist surfaces everywhere. Mingled as vapor with the air, it is +carried by the winds over sea and land, and condensed it returns to +the earth as rain or snow. That part of the rainfall which descends on +the ocean does not concern us, but that which falls on the land +accomplishes, as it returns to the sea, the most important work of all +surface geological agencies. + +The rainfall may be divided into three parts: the first _dries up_, +being discharged into the air by evaporation either directly from the +soil or through vegetation; the second _runs off_ over the surface to +flood the streams; the third _soaks in_ the ground and is henceforth +known as _ground_ or _underground water_. + +=The descent of ground water.= Seeping through the mantle of waste, +ground water soaks into the pores and crevices of the underlying rock. +All rocks of the upper crust of the earth are more or less porous, and +all drink in water. _Impervious rocks_, such as granite, clay, and +shale, have pores so minute that the water which they take in is held +fast within them by capillary attraction, and none drains through. +_Pervious rocks_, on the other hand, such as many sandstones, have +pore spaces so large that water filters through them more or less +freely. Besides its seepage through the pores of pervious rocks, water +passes to lower levels through the joints and cracks by which all +rocks, near the surface are broken. + +Even the closest-grained granite has a pore space of 1 in 400, while +sandstone may have a pore space of 1 in 4. Sand is so porous that it +may absorb a third of its volume of water, and a loose loam even as +much as one half. + + [Illustration: Fig. 24. Diagram Illustrating the Relation of the + Ground-Water Surface to the Surface of the Ground + + The dotted line represents the ground-water surface, and the + arrows indicate the direction of the movements of ground-water. + _m_, marsh; _w_, well; _r_, river] + +=The ground-water surface= is the name given the upper surface of +ground water, the level below which all rocks are saturated. In dry +seasons the ground-water surface sinks. For ground water is constantly +seeping downward under gravity, it is evaporated in the waste and its +moisture is carried upward by capillarity and the roots of plants to +the surface to be evaporated in the air. In wet seasons these constant +losses are more than made good by fresh supplies from that part of the +rainfall which soaks into the ground, and the ground-water surface +rises. + +In moist climates the ground-water surface (Fig. 24) lies, as a rule, +within a few feet of the land surface and conforms to it in a general +way, although with slopes of less inclination than those of the hills +and valleys. In dry climates permanent ground water may be found only +at depths of hundreds of feet. Ground water is held at its height by +the fact that its circulation is constantly impeded by capillarity and +friction. If it were as free to drain away as are surface streams, it +would sink soon after a rain to the level of the deepest valleys of +the region. + +=Wells and springs.= Excavations made in permeable rocks below the +ground-water surface fill to its level and are known as wells. Where +valleys cut this surface permanent streams are formed, the water +either oozing forth along ill-defined areas or issuing at definite +points called springs, where it is concentrated by the structure of +the rocks. A level tract where the ground-water surface coincides with +the surface of the ground is a swamp or marsh. + +By studying a spring one may learn much of the ways and work of ground +water. Spring water differs from that of the stream into which it +flows in several respects. If we test the spring with a thermometer +during successive months, we shall find that its temperature remains +much the same the year round. In summer it is markedly cooler than the +stream; in winter it is warmer and remains unfrozen while the latter +perhaps is locked in ice. This means that its underground path must +lie at such a distance from the surface that it is little affected by +summer's heat and winter's cold. + +While the stream is often turbid with surface waste washed into it by +rains, the spring remains clear; its water has been filtered during +its slow movement through many small underground passages and the +pores of rocks. Commonly the spring differs from the stream in that it +carries a far larger load of dissolved rock. Chemical analysis proves +that streams contain various minerals in solution, but these are +usually in quantities so small that they are not perceptible to the +taste or feel. But the water of springs is often well charged with +soluble minerals; in its slow, long journey underground it has +searched out the soluble parts of the rocks through which it seeps and +has dissolved as much of them as it could. When spring water is boiled +away, the invisible load which it has carried is left behind, and in +composition is found to be practically identical with that of the +soluble ingredients of the country rock. Although to some extent the +soluble waste of rocks is washed down surface slopes by the rain, by +far the larger part is carried downward by ground water and is +delivered to streams by springs. + +In limestone regions springs are charged with calcium carbonate (the +carbonate of lime), and where the limestone is magnesian they contain +magnesium carbonate also. Such waters are "hard"; when used in +washing, the minerals which they contain combine with the fatty acids +of soap to form insoluble curdy compounds. When springs rise from +rocks containing gypsum they are hard with calcium sulphate. In +granite regions they contain more or less soda and potash from the +decay of feldspar. + +The flow of springs varies much less during the different seasons of +the year than does that of surface streams. So slow is the movement of +ground water through the rocks that even during long droughts large +amounts remain stored above the levels of surface drainage. + +=Movements of ground water.= Ground water is in constant movement +toward its outlets. Its rate varies according to many conditions, but +always is extremely slow. Even through loose sands beneath the beds of +rivers it sometimes does not exceed a fifth of a mile a year. + + [Illustration: Fig. 26. Geological Conditions favorable to + Strong Springs + + _a_, limestone; _b_, shale; _c_, coarse sandstone; _d_, + limestone; _e_, sandstone; _f_, fissure. The strata dip toward + the South, _S_. Redraw the diagram, marking the points at which + strong springs (_ss_) may be expected.] + +In any region two zones of flow may be distinguished. The _upper zone +of flow_ extends from the ground-water surface downward through the +waste mantle and any permeable rocks on which the mantle rests, as far +as the first impermeable layer, where the descending movement of the +water is stopped. The =deep zones of flow= occupy any pervious rocks +which may be found below the impervious layer which lies nearest to +the surface. The upper zone is a vast sheet of water saturating the +soil and rocks and slowly seeping downward through their pores and +interstices along the slopes to the valleys, where in part it +discharges in springs and often unites also in a wide underflowing +stream which supports and feeds the river (Fig. 24). + + [Illustration: Fig. 27. Diagram of Well which goes dry in + Drought, _a_, and of of Unfailing Well, _b_ + + Redraw the diagram, showing by dotted line the normal + ground-water surface and by broken line the ground-water + surface at times of drought] + + [Illustration: Fig. 28. Diagram of Wet Weather Stream, _a_, and + of Permanent Stream, _b_ + + Redraw the diagram, showing ground-water surface by dotted line] + +A city in a region of copious rains, built on the narrow flood plain +of a river, overlooked by hills, depends for its water supply on +driven wells, within the city limits, sunk in the sand a few yards +from the edge of the stream. Are these wells fed by water from the +river percolating through the sand, or by ground water on its way to +the stream and possibly contaminated with the sewage of the town? + +At what height does underground water stand in the wells of your +region? Does it vary with the season? Have you ever known wells to go +dry? It may be possible to get data from different wells and to draw a +diagram showing the ground-water surface as compared with the surface +of the ground. + +=Fissure springs and artesian wells.= The _deeper zones of flow_ lie +in pervious strata which are overlain by some impervious stratum. Such +layers are often carried by their dip to great depths, and water may +circulate in them to far below the level of the surface streams and +even of the sea. When a fissure crosses a water-bearing stratum, or +_aquifer, water is forced upward by the pressure of the weight of +the water contained in the higher parts of the stratum, and may reach +the surface as a fissure spring. A boring which taps such an aquifer +is known as an artesian well, a name derived from a province in France +where wells of this kind have been long in use. The rise of the water +in artesian wells, and in fissure springs also, depends on the +following conditions illustrated in Figure 29. The aquifer dips toward +the region of the wells from higher ground, where it outcrops and +receives its water. It is inclosed between an impervious layer above +and water-tight or water-logged layers beneath. The weight of the +column of water thus inclosed in the aquifer causes water to rise in +the well, precisely as the weight of the water in a standpipe forces +it in connected pipes to the upper stories of buildings. + + [Illustration: Fig. 29. Section across South Dakota from the + Black Hills to Sioux Falls (S), Illustrating the Conditions + of Artesian Wells + + _a_, crystalline impervious rocks; _b_, sedimentary rocks, + shales, limestones, and sandstones; _c_, pervious sandstone, + the aquifer; _d_, impervious shales; _w_, _w_, _w_, artesian wells.] + +Which will supply the larger region with artesian wells, an aquifer +whose dip is steep or one whose dip is gentle? Which of the two +aquifers, their thickness being equal, will have the larger outcrop +and therefore be able to draw upon the larger amount of water from the +rainfall? Illustrate with diagrams. + +=The zone of solution.= Near the surface, where the circulation of +ground water is most active, it oxidizes, corrodes, and dissolves the +rocks through which it passes. It leaches soils and subsoils of their +lime and other soluble minerals upon which plants depend for their +food. It takes away the soluble cements of rocks; it widens fissures +and joints and opens winding passages along the bedding planes; it may +even remove whole beds of soluble rocks, such as rock salt, limestone, +or gypsum. The work of ground water in producing landslides has +already been noticed. The zone in which the work of ground water is +thus for the most part destructive we may call the zone of solution. + + [Illustration: Fig. 30. Diagram of Caverns and Sink Holes] + +=Caves.= In massive limestone rocks, ground water dissolves channels +which sometimes form large caves (Fig. 30). The necessary conditions +for the excavation of caves of great size are well shown in central +Kentucky, where an upland is built throughout of thick horizontal beds +of limestone. The absence of layers of insoluble or impervious rock in +its structure allows a free circulation of ground water within it by +the way of all natural openings in the rock. These water ways have +been gradually enlarged by solution and wear until the upland is +honeycombed with caves. Five hundred open caverns are known in one +county. + +Mammoth Cave, the largest of these caverns, consists of a labyrinth of +chambers and winding galleries whose total length is said to be as +much as thirty miles. One passage four miles long has an average width +of about sixty feet and an average height of forty feet. One of the +great halls is three hundred feet in width and is overhung by a solid +arch of limestone one hundred feet above the floor. Galleries at +different levels are connected by well-like pits, some of which +measure two hundred and twenty-five feet from top to bottom. Through +some of the lowest of these tunnels flows Echo River, still at work +dissolving and wearing away the rock while on its dark way to appear +at the surface as a great spring. + +=Natural bridges.= As a cavern enlarges and the surface of the land +above it is lowered by weathering, the roof at last breaks down and +the cave becomes an open ravine. A portion of the roof may for a while +remain, forming a "natural bridge." + +=Sink holes.= In limestone regions channels under ground may become so +well developed that the water of rains rapidly drains away through +them. Ground water stands low and wells must be sunk deep to find it. +Little or no surface water is left to form brooks. + + [Illustration: Fig. 31. Sink Holes in the Karst, Austria] + +Thus across the limestone upland of central Kentucky one meets but +three surface streams in a hundred miles. Between their valleys +surface water finds its way underground by means of sink holes. These +are pits, commonly funnel shaped, formed by the enlargement of crevice +or joint by percolating water, or by the breakdown of some portion of +the roof of a cave. By clogging of the outlet a sink hole may come to +be filled by a pond. + +Central Florida is a limestone region with its drainage largely +subterranean and in part below the level even of the sea. Sink holes +are common, and many of them are occupied by lakelets. Great springs +mark the point of issue of underground streams, while some rise from +beneath the sea. Silver Spring, one of the largest, discharges from a +basin eight hundred feet wide and thirty feet deep a little river +navigable for small steamers to its source. About the spring there are +no surface streams for sixty miles. + + [Illustration: Fig. 32. Underground Stream Issuing from Base of + Cliff, the Karst, Austria] + +=The Karst.= Along the eastern coast of the Adriatic, as far south as +Montenegro, lies a belt of limestone mountains singularly worn and +honeycombed by the solvent action of water. Where forests have been +cut from the mountain sides and the red soil has washed away, the +surface of the white limestone forms a pathless desert of rock where +each square rod has been corroded into an intricate branch work of +shallow furrows and sharp ridges. Great sink holes, some of them six +hundred feet deep and more, pockmark the surface of the land. The +drainage is chiefly subterranean. Surface streams are rare and a +portion of their courses is often under ground. Fragmentary valleys +come suddenly to an end at walls of rock where the rivers which occupy +the valleys plunge into dark tunnels to reappear some miles away. +Ground water stands so far below the surface that it cannot be reached +by wells, and the inhabitants depend on rain water stored for +household uses. The finest cavern of Europe, the Adelsberg Grotto, is +in this region. Karst, the name of a part of this country, is now used +to designate any region or landscape thus sculptured by the chemical +action of surface and ground water. We must remember that Karst +regions are rare, and striking as is the work of their subterranean +streams, it is far less important than the work done by the sheets of +underground water slowly seeping through all subsoils and porous rocks +in other regions. + +Even when gathered into definite channels, ground water does not have +the erosive power of surface streams, since it carries with it little +or no rock waste. Regions whose underground drainage is so perfect +that the development of surface streams has been retarded or prevented +escape to a large extent the leveling action of surface running +waters, and may therefore stand higher than the surrounding country. +The hill honeycombed by Luray Cavern, Virginia, has been attributed to +this cause. + + [Illustration: Fig. 33. Stalactites and Stalagmites, Marengo + Cavern, Indiana] + +=Cavern deposits.= Even in the zone of solution water may under +certain circumstances deposit as well as erode. As it trickles from +the roof of caverns, the lime carbonate which it has taken into +solution from the layers of limestone above is deposited by +evaporation in the air in icicle-like pendants called _stalactites_. +As the drops splash on the floor there are built up in the same way +thicker masses called _stalagmites_, which may grow to join the +stalactites above, forming pillars. A stalagmitic crust often seals +with rock the earth which accumulates in caverns, together with +whatever relics of cave dwellers, either animals or men, it may +contain. + +Can you explain why slender stalactites formed by the drip of single +drops are often hollow pipes? + +=The zone of cementation.= With increasing depth subterranean water +becomes more and more sluggish in its movements and more and more +highly charged with minerals dissolved from the rocks above. At such +depths it deposits these minerals in the pores of rocks, cementing +their grains together, and in crevices and fissures, forming mineral +veins. Thus below the zone of solution where the work of water is to +dissolve, lies the zone of cementation where its work is chemical +deposit. A part of the invisible load of waste is thus transferred +from rocks near the surface to those at greater depths. + +As the land surface is gradually lowered by weathering and the work of +rain and streams, rocks which have lain deep within the zone of +cementation are brought within the zone of solution. Thus there are +exposed to view limestones, whose cracks were filled with calcite +(crystallized carbonate of lime), with quartz or other minerals, and +sandstones whose grains were well cemented many feet below the +surface. + +=Cavity filling.= Small cavities in the rocks are often found more or +less completely filled with minerals deposited from solution by water +in its constant circulation underground. The process may be +illustrated by the deposit of salt crystals in a cup of evaporating +brine, but in the latter instance the solution is not renewed as in +the case of cavities in the rocks. A cavity thus lined with +inward-pointing crystals is called a _geode_. + +=Concretions.= Ground water seeping through the pores of rocks may +gather minerals disseminated throughout them into nodular masses +called concretions. Thus silica disseminated through limestone is +gathered into nodules of flint. While geodes grow from the outside +inwards, concretions grow outwards from the center. Nor are they +formed in already existing cavities as are geodes. In soft clays +concretions may, as they grow, press the clay aside. In many other +rocks concretions are made by the process of _replacement_. Molecule +by molecule the rock is removed and the mineral of the concretion +substituted in its place. The concretion may in this way preserve +intact the lamination lines or other structures of the rock (Fig. 34). +Clays and shales often contain concretions of lime carbonate, of iron +carbonate, or of iron sulphide. Some fossil, such as a leaf or shell, +frequently forms the nucleus around which the concretion grows. + +Why are building stones more easily worked when "green" than after +their quarry water has dried out? + + [Illustration: Fig. 34. Concretions in Sandstone, Wyoming] + +=Deposits of ground water in arid regions.= In arid lands where ground +water is drawn by capillarity to the surface and there evaporates, it +leaves as surface incrustations the minerals held in solution. White +limy incrustations of this nature cover considerable tracts in +northern Mexico. Evaporating beneath the surface, ground water may +deposit a limy cement in beds of loose sand and gravel. Such firmly +cemented layers are not uncommon in western Kansas and Nebraska, where +they are known as "mortar beds." + +=Thermal springs.= While the lower limit of surface drainage is sea +level, subterranean water circulates much below that depth, and is +brought again to the surface by hydrostatic pressure. In many +instances springs have a higher temperature than the average annual +temperature of the region, and are then known as thermal springs. In +regions of present or recent volcanic activity, such as the +Yellowstone National Park, we may believe that the heat of thermal +springs is derived from uncooled lavas, perhaps not far below the +surface. But when hot springs occur at a distance of hundreds of miles +from any volcano, as in the case of the hot springs of Bath, England, +it is probable that their waters have risen from the heated rocks +of the earth's interior. The springs of Bath have a temperature of +120° F., 70° above the average annual temperature of the place. If +we assume that the rate of increase in the earth's internal heat is +here the average rate, 1° F. to every sixty feet of descent, we may +conclude that the springs of Bath rise from at least a depth of +forty-two hundred feet. + +Water may descend to depths from which it can never be brought back by +hydrostatic pressure. It is absorbed by highly heated rocks deep below +the surface. From time to time some of this deep-seated water may be +returned to open air in the steam of volcanic eruptions. + + [Illustration: Fig. 35. Calcareous Deposits from Hot Springs, + Yellowstone National Park] + +=Surface deposits of springs.= Where subterranean water returns to the +surface highly charged with minerals in solution, on exposure to the +air it is commonly compelled to lay down much of its invisible load in +chemical deposits about the spring. These are thrown down from +solution either because of cooling, evaporation, the loss of carbon +dioxide, or the work of algae. + +Many springs have been charged under pressure with carbon dioxide from +subterranean sources and are able therefore to take up large +quantities of lime carbonate from the limestone rocks through which +they pass. On reaching the surface the pressure is relieved, the gas +escapes, and the lime carbonate is thrown down in deposits called +_travertine_. The gas is sometimes withdrawn and the deposit produced +in large part by the action of algae and other humble forms of plant +life. + +At the Mammoth Hot Springs in the valley of the Gardiner River, +Yellowstone National Park, beautiful terraces and basins of travertine +(Fig. 35) are now building, chiefly by means of algae which cover the +bottoms, rims, and sides of the basins and deposit lime carbonate upon +them in successive sheets. The rock, snow-white where dry, is coated +with red and orange gelatinous mats where the algae thrive in the +over-flowing waters. + +Similar terraces of travertine are found to a height of fourteen +hundred feet up the valley side. We may infer that the springs which +formed these ancient deposits discharged near what was then the bottom +of the valley, and that as the valley has been deepened by the river +the ground water of the region has found lower and lower points of +issue. + +In many parts of the country calcareous springs occur which coat with +lime carbonate mosses, twigs, and other objects over which their +waters flow. Such are popularly known as petrifying springs, although +they merely incrust the objects and do not convert them into stone. + +Silica is soluble in alkaline waters, especially when these are hot. +Hot springs rising through alkaline siliceous rocks, such as lavas, +often deposit silica in a white spongy formation known as _siliceous +sinter_, both by evaporation and by the action of algae which secrete +silica from the waters. It is in this way that the cones and mounds of +the geysers in the Yellowstone National Park and in Iceland have been +formed (Fig. 234). + +Where water oozes from the earth one may sometimes see a rusty deposit +on the ground, and perhaps an iridescent scum upon the water. The scum +is often mistaken for oil, but at a touch it cracks and breaks, as oil +would not do. It is a film of hydrated iron oxide, or _limonite_, and +the spring is an iron, or chalybeate, spring. Compounds of iron have +been taken into solution by ground water from soil and rocks, and are +now changed to the insoluble oxide on exposure to the oxygen of the +air. + +In wet ground iron compounds leached by ground water from the soil +often collect in reddish deposits a few feet below the surface, where +their downward progress is arrested by some impervious clay. At the +bottom of bogs and shallow lakes iron ores sometimes accumulate to a +depth of several feet. + +Decaying organic matter plays a large part in these changes. In its +presence the insoluble iron oxides which give color to most red and +yellow rocks are decomposed, leaving the rocks of a gray or bluish +color, and the soluble iron compounds which result are readily leached +out,--effects seen where red or yellow clays have been bleached about +some decaying tree root. + +The iron thus dissolved is laid down as limonite when oxidized, as +about a chalybeate spring; but out of contact with the air and in the +presence of carbon dioxide supplied by decaying vegetation, as in a +peat bog, it may be deposited as iron carbonate, or _siderite_. + +=Total amount of underground waters.= In order to realize the vast work +in solution and cementation which underground waters are now doing and +have done in all geological ages, we must gain some conception of their +amount. At a certain depth, estimated at about six miles, the weight of +the crust becomes greater than the rocks can bear, and all cavities and +pores in them must be completely closed by the enormous pressure which +they sustain. Below a depth, therefore, water cannot go. Above it all +rocks are water-soaked, up to the limit of their capacity, to within a +few feet of the surface. Estimating the average pore space of the rocks +above a depth of six miles at from two and a half per cent to five per +cent of their volume, it is found that the total amount of ground water +may be great enough to cover the entire surface of the earth to a depth +of from eight hundred to sixteen hundred feet. + + + + +CHAPTER III + +RIVERS AND VALLEYS + + +=The run-off.= We have traced the history of that portion of the +rainfall which soaks into the ground; let us now return to that part +which washes along the surface and is known as the _run-off_. Fed by +rains and melting snows, the run-off gathers into courses, perhaps but +faintly marked at first, which join more definite and deeply cut +channels, as twigs their stems. In a humid climate the larger ravines +through which the run-off flows soon descend below the ground-water +surface. Here springs discharge along the sides of the little valleys +and permanent streams begin. The water supplied by the run-off here +joins that part of the rainfall which had soaked into the soil, and +both now proceed together by way of the stream to the sea. + +=River floods.= Streams vary greatly in volume during the year. At +stages of flood they fill their immediate banks, or overrun them and +inundate any low lands adjacent to the channel; at stages of low water +they diminish to but a fraction of their volume when at flood. + +At times of flood, rivers are fed chiefly by the run-off; at times of +low water, largely or even wholly by springs. + +How, then, will the water of streams differ at these times in +turbidity and in the relative amount of solids carried in solution? + +In parts of England streams have been known to continue flowing after +eighteen months of local drought, so great is the volume of water +which in humid climates is stored in the rocks above the drainage +level, and so slowly is it given off in springs. + +In Illinois and the states adjacent, rivers remain low in winter and a +"spring freshet" follows the melting of the winter's snows. A "June +rise" is produced by the heavy rains of early summer. Low water +follows in July and August, and streams are again swollen to a +moderate degree under the rains of autumn. + +=The discharge of streams.= The per cent of rainfall discharged by +rivers varies with the amount of rainfall, the slope of the drainage +area, the texture of the rocks, and other factors. With an annual +rainfall of fifty inches in an open country, about fifty per cent is +discharged; while with a rainfall of twenty inches only fifteen per +cent is discharged, part of the remainder being evaporated and part +passing underground beyond the drainage area. Thus the Ohio discharges +thirty per cent of the rainfall of its basin, while the Missouri +carries away but fifteen per cent. A number of the streams of the +semi-arid lands of the West do not discharge more than five per cent +of the rainfall. + +Other things being equal, which will afford the larger proportion of +run-off, a region underlain with granite rock or with coarse +sandstone? grass land or forest? steep slopes or level land? a +well-drained region or one abounding in marshes and ponds? frozen or +unfrozen ground? Will there be a larger proportion of run-off after +long rains or after a season of drought? after long and gentle rains, +or after the same amount of precipitation in a violent rain? during +the months of growing vegetation, from June to August, or during the +autumn months? + + [Illustration: Fig. 36. Rise of Ground-Water Surface (broken + line) beneath Valley (_V_) in Arid Region] + +=Desert streams.= In arid regions the ground-water surface lies so low +that for the most part stream ways do not intersect it. Streams +therefore are not fed by springs, but instead lose volume as their +waters soak into the thirsty rocks over which they flow. They +contribute to the ground water of the region instead of being +increased by it. Being supplied chiefly by the run-off, they wither at +times of drought to a mere trickle of water, to a chain of pools, or +go wholly dry, while at long intervals rains fill their dusty beds +with sudden raging torrents. Desert rivers therefore periodically +shorten and lengthen their courses, withering back at times of drought +for scores of miles, or even for a hundred miles from the point +reached by their waters during seasons of rain. + +=The geological work of streams.= The work of streams is of three +kinds,--transportation, erosion, and deposition. Streams _transport_ +the waste of the land; they wear, or _erode_, their channels both on +bed and banks; and they _deposit_ portions of their load from time to +time along their courses, finally laying it down in the sea. Most of +the work of streams is done at times of flood. + + +Transportation + +=The invisible load of streams.= Of the waste which a river transports +we may consider first the invisible load which it carries in solution, +supplied chiefly by springs but also in part by the run-off and from +the solution of the rocks of its bed. More than half the dissolved +solids in the water of the average river consists of the carbonates of +lime and magnesia; other substances are gypsum, sodium sulphate +(Glauber's salts), magnesium sulphate (Epsom salts), sodium chloride +(common salt), and even silica, the least soluble of the common +rock-making minerals. The amount of this invisible load is +surprisingly large. The Mississippi, for example, transports each year +113,000,000 tons of dissolved rock to the Gulf. + +=The visible load of streams.= This consists of the silt which the +stream carries in suspension, and the sand and gravel and larger +stones which it pushes along its bed. Especially in times of flood one +may note the muddy water, its silt being kept from settling by the +rolling, eddying currents; and often by placing his ear close to the +bottom of a boat one may hear the clatter of pebbles as they are +hurried along. In mountain torrents the rumble of bowlders as they +clash together may be heard some distance away. The amount of the load +which a stream can transport depends on its velocity. A current of two +thirds of a mile per hour can move fine sand, while one of four miles +per hour sweeps along pebbles as large as hen's eggs. The transporting +power of a stream varies as the sixth power of its velocity. If its +velocity is multiplied by two, its transporting power is multiplied by +the sixth power of two: it can now move stones sixty-four times as +large as it could before. + +Stones weigh from two to three times as much as water, and in water +lose the weight of the volume of water which they displace. What +proportion, then, of their weight in air do stones lose when +submerged? + +=Measurement of stream loads.= To obtain the total amount of waste +transported by a river is an important but difficult matter. The +amount of water discharged must first be found by multiplying the +number of square feet in the average cross section of the stream by +its velocity per second, giving the discharge per second in cubic +feet. The amount of silt to a cubic foot of water is found by +filtering samples of the water taken from different parts of the +stream and at different times in the year, and drying and weighing the +residues. The average amount of silt to the cubic foot of water, +multiplied by the number of cubic feet of water discharged per year, +gives the total load carried in suspension during that time. Adding to +this the estimated amount of sand and gravel rolled along the bed, +which in many swift rivers greatly exceeds the lighter material held +in suspension, and adding also the total amount of dissolved solids, +we reach the exceedingly important result of the total load of waste +discharged by the river. Dividing the volume of this load by the area +of the river basin gives another result of the greatest geological +interest,--the rate at which the region is being lowered by the +combined action of weathering and erosion, or the rate of denudation. + +=The rate of denudation of river basins.= This rate varies widely. The +Mississippi basin may be taken as a representative land surface +because of the varieties of surface, altitude and slope, climate, and +underlying rocks which are included in its great extent. Careful +measurements show that the Mississippi basin is now being lowered at a +rate of one four-thousandth of a foot a year, or one foot in four +thousand years. Taking this as the average rate of denudation for the +land surfaces of the globe, estimates have been made of the length of +time required at this rate to wash and wear the continents to the +level of the sea. As the average elevation of the lands of the globe +is reckoned at 2411 feet, this result would occur in nine or ten +million years, if the present rate of denudation should remain +unchanged. But even if no movements of the earth's crust should lift +or depress the continents, the rate of wear and the removal of waste +from their surfaces will not remain the same. It must constantly +decrease as the lands are worn nearer to sea level and their slopes +become more gentle. The length of time required to wear them away is +therefore far in excess of that just stated. + +The drainage area of the Potomac is 11,000 square miles. The silt +brought down in suspension in a year would cover a square mile to the +depth of four feet. At what rate is the Potomac basin being lowered +from this cause alone? + +It is estimated that the Upper Ganges is lowering its basin at the +rate of one foot in 823 years, and the Po one foot in 720 years. Why +so much faster than the Potomac and the Mississippi? + +=How streams get their loads.= The load of streams is derived from a +number of sources, the larger part being supplied by the weathering of +valley slopes. We have noticed how the mantle of waste creeps and +washes to the stream ways. Watching the run-off during a rain, as it +hurries muddy with waste along the gutter or washes down the hillside, +we may see the beginning of the route by which the larger part of +their load is delivered to rivers. Streams also secure some of their +load by wearing it from their beds and banks,--a process called +erosion. + + +Erosion + +Streams erode their beds chiefly by means of their bottom load,--the +stones of various sizes and the sand and even the fine mud which they +sweep along. With these tools they smooth, grind, and rasp the rock of +their beds, using them in much the fashion of sandpaper or a file. + + [Illustration: Fig. 37. Pothole in Bed of Stream, Ireland] + +=Weathering of river beds.= The erosion of stream beds is greatly +helped by the work of the weather. Especially at low water more or +less of the bed is exposed to the action of frost and heat and cold, +joints are opened, rocks are pried loose and broken up and made ready +to be swept away by the stream at time of flood. + +=Potholes.= In rapids streams also drill out their rocky beds. Where +some slight depression gives rise to an eddy, the pebbles which gather +in it are whirled round and round, and, acting like the bit of an +auger, bore out a cylindrical pit called a pothole. Potholes sometimes +reach a depth of a score of feet. Where they are numerous they aid +materially in deepening the channel, as the walls between them are +worn away and they coalesce. + +=Waterfalls.= One of the most effective means of erosion which the +river possesses is the waterfall. The plunging water dislodges stones +from the face of the ledge over which it pours, and often undermines +it by excavating a deep pit at its base. Slice after slice is thus +thrown down from the front of the cliff, and the cataract cuts its way +upstream leaving a gorge behind it. + + [Illustration: Fig. 38. Map of the Gorge of the Niagara River] + +=Niagara Falls.= The Niagara River flows from Lake Erie at Buffalo in +a broad channel which it has cut but a few feet below the level of the +region. Some thirteen miles from the outlet it plunges over a ledge +one hundred and seventy feet high into the head of a narrow gorge +which extends for seven miles to the escarpment of the upland in which +the gorge is cut. The strata which compose the upland dip gently +upstream and consist at top of a massive limestone, at the Falls about +eighty feet thick, and below of soft and easily weathered shale. +Beneath the Falls the underlying shale is cut and washed away by the +descending water and retreats also because of weathering, while the +overhanging limestone breaks down in huge blocks from time to time. + +Niagara is divided by Goat Island into the Horseshoe Falls and the +American Falls. The former is supplied by the main current of the +river, and from the semicircular sweep of its rim a sheet of water in +places at least fifteen or twenty feet deep plunges into a pool a +little less than two hundred feet in depth. Here the force of the +falling water is sufficient to move about the fallen blocks of +limestone and use them in the excavation of the shale of the bed. At +the American Falls the lesser branch of the river, which flows along +the American side of Goat Island, pours over the side of the gorge and +breaks upon a high talus of limestone blocks which its smaller volume +of water is unable to grind to pieces and remove. + +A series of surveys have determined that from 1842 to 1890 the +Horseshoe Falls retreated at the rate of 2.18 feet per year, while the +American Falls retreated at the rate of 0.64 feet in the same period. +We cannot doubt that the same agency which is now lengthening the +gorge at this rapid rate has cut it back its entire length of seven +miles. + +While Niagara Falls have been cutting back a gorge seven miles long +and from two hundred to three hundred feet deep, the river above the +Falls has eroded its bed scarcely below the level of the upland on +which it flows. Like all streams which are the outlets of lakes, the +Niagara flows out of Lake Erie clear of sediment, as from a settling +basin, and carries no tools with which to abrade its bed. We may infer +from this instance how slight is the erosive power of clear water on +hard rock. + + [Illustration: Fig. 39. Longitudinal Section of Niagara Gorge + + Black, water; _F_, falls; _R_, rapids; _W_, whirlpool; + _E_, escarpment; _N_, north; _S_, south] + +Assuming that the rate of recession of the combined volumes of the +American and Horseshoe Falls was three feet a year below Goat Island, +and _assuming that this rate has been uniform in the past_, how long +is it since the Niagara River fell over the edge of the escarpment +where now is the mouth of the present gorge? + +The profile of the bed of the Niagara along the gorge (Fig. 39) shows +alternating deeps and shallows which cannot be accounted for, except +in a single instance, by the relative hardness of the rocks of the +river bed. The deeps do not exceed that at the foot of the Horseshoe +Falls at the present time. When the gorge was being cut along the +shallows, how did the Falls compare in excavating power, in force, and +volume with the Niagara of to-day? How did the rate of recession at +those times compare with the present rate? Is the assumption made +above that the rate of recession has been uniform correct? + +The first stretch of shallows below the Falls causes a tumultuous +rapid impossible to sound. Its depth has been estimated at thirty-five +feet. From what data could such an estimate be made? + +Suggest a reason why the Horseshoe Falls are convex upstream. + +At the present rate of recession which will reach the head of Goat +Island the sooner, the American or the Horseshoe Falls? What will be +the fate of the Falls left behind when the other has passed beyond the +head of the island? + +The rate at which a stream erodes its bed depends in part upon the +nature of the rocks over which it flows. Will a stream deepen its +channel more rapidly on massive or on thin-bedded and close-jointed +rocks? on horizontal strata or on strata steeply inclined? + + [Illustration: Fig. 40. A Stream in Scotland + + In what ways is the bed now being deepened?] + + +Deposition + +While the river carries its invisible load of dissolved rock on +without stop to the sea, its load of visible waste is subject to many +delays en route. Now and again it is laid aside, to be picked up later +and carried some distance farther on its way. One of the most striking +features of the river therefore is the waste accumulated along its +course, in bars and islands in the channel, beneath its bed, and in +flood plains along its banks. All this _alluvium_, to use a general +term for river deposits, with which the valley is cumbered is really +en route to the sea; it is only temporarily laid aside to resume its +journey later on. Constantly the river is destroying and rebuilding +its alluvial deposits, here cutting and there depositing along its +banks, here eroding and there building a bar, here excavating its bed +and there filling it up, and at all times carrying the material picked +up at one point some distance on downstream before depositing it at +another. + + [Illustration: Fig. 41. Sand Bar deposited by Stream, showing + Cross Bedding] + +These deposits are laid down by slackening currents where the velocity +of the stream is checked, as on the inner side of curves, and where +the slope of the bed is diminished, and in the lee of islands, bridge +piers and projecting points of land. How slight is the check required +to cause a current to drop a large part of its load may be inferred +from the law of the relation of the transporting power to the +velocity. If the velocity is decreased one half, the current can move +fragments but one sixty-fourth the size of those which it could move +before, and must drop all those of larger size. + +Will a river deposit more at low water or at flood? when rising or +when falling? + +=Stratification.= River deposits are stratified, as may be seen in any +fresh cut in banks or bars. The waste of which they are built has been +sorted and deposited in layers, one above another; some of finer and +some of coarser material. The sorting action of running water depends +on the fact that its transporting power varies with the velocity. A +current whose diminishing velocity compels it to drop coarse gravel, +for example, is still able to move all the finer waste of its load, +and separating it from the gravel, carries it on downstream; while at +a later time slower currents may deposit on the gravel bed layers of +sand, and, still later, slack water may leave on these a layer of mud. +In case of materials lighter than water the transporting power does +not depend on the velocity, and logs of wood, for instance, are +floated on to the sea on the slowest as well as on the most rapid +currents. + + [Illustration: Fig. 42. Longitudinal Section of a River Bar] + +=Cross bedding.= A section of a bar exposed at low water may show that +it is formed of layers of sand, or coarser stuff, inclined downstream +as steeply often as the angle of repose of the material. From a boat +anchored over the lower end of a submerged sand bar we may observe the +way in which this structure, called cross bedding, is produced. Sand +is continually pushed over the edge of the bar at _b_ (Fig. 42) and +comes to rest in successive layers on the sloping surface. At the same +time the bar may be worn away at the upper end, _a_, and thus slowly +advance down stream. While the deposit is thus cross bedded, it +constitutes as a whole a stratum whose upper and lower surfaces are +about horizontal. In sections of river banks one may often see a +vertical succession of cross-bedded strata, each built in the way +described. + +=Water wear.= The coarser material of river deposits, such as +cobblestones, gravel, and the larger grains of sand, are _water worn_, +or rounded, except when near their source. Rolling along the bottom +they have been worn round by impact and friction as they rubbed +against one another and the rocky bed of the stream. + +Experiments have shown that angular fragments of granite lose nearly +half their weight and become well rounded after traveling fifteen +miles in rotating cylinders partly filled with water. Marbles are +cheaply made in Germany out of small limestone cubes set revolving in +a current of water between a rotating bed of stone and a block of oak, +the process requiring but about fifteen minutes. It has been found +that in the upper reaches of mountain streams a descent of less than a +mile is sufficient to round pebbles of granite. + + [Illustration: Fig. 43. Water-Worn Pebbles, Upper Potomac River, + Maryland] + + +Land Forms Due To River Erosion + +=River valleys.= In their courses to the sea, rivers follow valleys of +various forms, some shallow and some deep, some narrow and some wide. +Since rivers are known to erode their beds and banks, it is a fair +presumption that, aided by the weather, they have excavated the +valleys in which they flow. + +Moreover, a bird's-eye view or a map of a region shows the significant +fact that the valleys of a system unite with one another in a branch +work, as twigs meet their stems and the branches of a tree its trunk. +Each valley, from that of the smallest rivulet to that of the master +stream, is proportionate to the size of the stream which occupies it. +With a few explainable exceptions the valleys of tributaries join that +of the trunk stream at a level; there is no sudden descent or break in +the bed at the point of juncture. These are the natural consequences +which must follow if the land has long been worked upon by streams, +and no other process has ever been suggested which is competent to +produce them. We must conclude that valley systems have been formed by +the river systems which drain them, aided by the work of the weather; +they are not gaping fissures in the earth's crust, as early observers +imagined, but are the furrows which running water has drawn upon the +land. + +As valleys are made by the slow wear of streams and the action of the +weather, they pass in their development through successive stages, +each of which has its own characteristic features. We may therefore +classify rivers and valleys according to the stage which they have +reached in their life history from infancy to old age. + + +Young River Valleys + +=Infancy.= The Red River of the North. A region in northwestern +Minnesota and the adjacent portions of North Dakota and Manitoba was +so recently covered by the waters of an extinct lake, known as Lake +Agassiz, that the surface remains much as it was left when the lake +was drained away. The flat floor, spread smooth with lake-laid silts, +is still a plain, to the eye as level as the sea. Across it the Red +River of the North and its branches run in narrow, ditch-like +channels, steep-sided and shallow, not exceeding sixty feet in depth, +their gradients differing little from the general slopes of the +region. The trunk streams have but few tributaries; the river system, +like a sapling with few limbs, is still undeveloped. Along the banks +of the trunk streams short gullies are slowly lengthening headwards, +like growing twigs which are sometime to become large branches. + + [Illustration: Fig. 44. A Young Lacustrine Plain; the Red River + of the North + + Scale 5 inches = about 11 miles. Contour interval, 20 feet] + +The flat interstream areas are as yet but little scored by drainage +lines, and in wet weather water lingers in ponds in any initial +depressions on the plain. + + [Illustration: Fig. 45. A Young River, Iowa + + Note that it has hardly begun to cut in the plain of glacial + drift on which it flows] + +=Contours.= In order to read the topographic maps of the text-book and +the laboratory the student should know that contours are lines drawn +on maps to represent relief, all points on any given contour being of +equal height above sea level. The _contour interval_ is the uniform +vertical distance between two adjacent contours and varies on +different maps. To express regions of faint relief a contour interval +of ten or twenty feet is commonly selected; while in mountainous +regions a contour interval of two hundred and fifty, five hundred, or +even one thousand feet may be necessary in order that the contours may +not be too crowded for easy reading. + +Whether a river begins its life on a lake plain, as in the example +just cited, or upon a coastal plain lifted from beneath the sea or on +a spread of glacial drift left by the retreat of continental ice +sheets, such as covers much of Canada and the northeastern parts of +the United States, its infantile stage presents the same +characteristic features,--a narrow and shallow valley, with +undeveloped tributaries and undrained interstream areas. Ground water +stands high, and, exuding in the undrained initial depressions, forms +marshes and lakes. + + [Illustration: Fig. 46. A Young Drift Region in Wisconsin + + Describe this area. How high are the hills? Are they such in form + and position as would be left by stream erosion? Consult a map of + the entire state and notice that the Fox River finds its way to Lake + Michigan, while the Wisconsin empties into the Mississippi. Describe + that portion of the divide here shown between the Mississippi and + the St. Lawrence systems. Which is the larger river, the Wisconsin + or the Fox? Other things being equal, which may be expected to + deepen its bed the more rapidly? What changes are likely to occur + when one of these rivers comes to flow at a lower level than the + other? Why have not these changes occurred already?] + +=Lakes.= Lakes are perhaps the most obvious of these fleeting features +of infancy. They are short-lived, for their destruction is soon +accomplished by several means. As a river system advances toward +maturity the deepening and extending valleys of the tributaries lower +the ground-water surface and invade the undrained depressions of the +region. Lakes having outlets are drained away as their basin rims are +cut down by the outflowing streams,--a slow process where the rim is +of hard rock, but a rapid one where it is of soft material such as +glacial drift. + +Lakes are effaced also by the filling of their basins. Inflowing +streams and the wash of rains bring in waste. Waves abrade the shore +and strew the débris worn from it over the lake bed. Shallow lakes are +often filled with organic matter from decaying vegetation. + +Does the outflowing stream, from a lake carry sediment? How does this +fact affect its erosive power on hard rock? on loose material? + +Lake Geneva is a well-known example of a lake in process of +obliteration. The inflowing Rhone has already displaced the waters of +the lake for a length of twenty miles with the waste brought down from +the high Alps. For this distance there extends up the Rhone Valley an +alluvial plain, which has grown lakeward at the rate of a mile and a +half since Roman times, as proved by the distance inland at which a +Roman port now stands. + + [Illustration: Fig. 47. A Small Lake being broadened and shoaled + by Wave Wear + + _ls_, lake surface; dotted line, initial shore; + _b_, fill made of material taken from _a_] + +How rapidly a lake may be silted up under exceptionally favorable +conditions is illustrated by the fact that over the bottom of the +artificial lake, of thirty-five square miles, formed behind the great +dam across the Colorado River at Austin, Texas, sediments thirty-nine +feet deep gathered in seven years. + +Lake Mendota, one of the many beautiful lakes of southern Wisconsin, +is rapidly cutting back the soft glacial drift of its shores by means +of the abrasion of its waves. While the shallow basin is thus +broadened, it is also being filled with the waste; and the time is +brought nearer when it will be so shoaled that vegetation can complete +the work of its effacement. + + [Illustration: Fig. 48. A Lake well-nigh effaced, Montana + + By what means is the lake bed being filled?] + +Along the margin of a shallow lake mosses, water lilies, grasses, and +other water-loving plants grow luxuriantly. As their decaying remains +accumulate on the bottom, the ring of marsh broadens inwards, the lake +narrows gradually to a small pond set in the midst of a wide bog, and +finally disappears. All stages in this process of extinction may be +seen among the countless lakelets which occupy sags in the recent +sheets of glacial drift in the northern states; and more numerous than +the lakes which still remain are those already thus filled with +carbonaceous matter derived from the carbon dioxide of the atmosphere. +Such fossil lakes are marked by swamps or level meadows underlain with +muck. + + [Illustration: Fig. 49. A Level Meadow, Scotland + + Explain its origin. What will be its future?] + +=The advance to maturity.= The infantile stage is brief. As a river +advances toward maturity the initial depressions, the lake basins of +its area, are gradually effaced. By the furrowing action of the rain +wash and the head ward lengthening, of tributaries a branchwork of +drainage channels grows until it covers the entire area, and not an +acre is left on which the fallen raindrop does not find already cut +for it an uninterrupted downward path which leads it on by way of +gully, brook, and river to the sea. The initial surface of the land, +by whatever agency it was modeled, is now wholly destroyed; the region +is all reduced to valley slopes. + + [Illustration: Fig. 50. Drainage Maps + + _A_, an area in its infancy, Buena Vista County, Iowa; + _B_, an area in its maturity, Ringgold County, Iowa] + + [Illustration: Fig. 51. Successive Longitudinal Profiles of a + Stream + + _am_, initial profile, with waterfall at _w_, and basins at _l_ + and _l´_, which at first are occupied by lakes and later are + filled or drained; _b_, _c_, _d_, and _e_, profiles established + in succession as the stream advances from infancy toward old + age. Note that these profiles are concave toward the sky. This + is the _erosion curve_. What contrasting form has the weather + weather curve (p. 34)?] + +=The longitudinal profile of a stream.= This at first corresponds with +the initial surface of the region on which the stream begins to flow, +although its way may lead through basins and down steep descents. The +successive profiles to which it reduces its bed are illustrated in +Figure 51. As the gradient, or rate of descent of its bed, is lowered, +the velocity of the river is decreased until its lessening energy is +wholly consumed in carrying its load and it can no longer erode its +bed. The river is now _at grade_, and its capacity is just equal to +its load. If now its load is increased the stream deposits, and thus +builds up, or _aggrades_, its bed. On the other hand, if its load is +diminished it has energy to spare, and resuming its work of erosion, +_degrades_ its bed. In either case the stream continues aggrading or +degrading until a new gradient is found where the velocity is just +sufficient to move the load, and here again it reaches grade. + + [Illustration: Fig. 52. A V-Valley,--the Canyon of the + Yellowstone + + Note the steep sides. What processes are at work upon them? How + wide is the valley at the base compared with the width of the + stream? Do you see any river deposits along the banks? Is the + stream flowing swiftly over a rock bed, or quietly over a bed + which it has built up? Is it graded or ungraded? Note that the + canyon walls project in interlocking spurs] + +=V-Valleys.= Vigorous rivers well armed with waste make short work of +cutting their beds to grade, and thus erode narrow, steep-sided gorges +only wide enough at the base to accommodate the stream. The steepness +of the valley slopes depends on the relative rates at which the bed is +cut down by the stream and the sides are worn back by the weather. In +resistant rock a swift, well-laden stream may saw out a gorge whose +sides are nearly or even quite vertical, but as a rule young valleys +whose streams have not yet reached grade are V-shaped; their sides +flare at the top because here the rocks have longest been opened up to +the action of the weather. Some of the deepest canyons may be found +where a rising land mass, either mountain range or plateau, has long +maintained by its continued uplift the rivers of the region above +grade. + + [Illustration: Fig. 53. Section of the Yellowstone Canyon + + This canyon is 100 feet deep, 2500 feet wide at the top, and + about 250 feet wide at the bottom. Neglecting any cutting of the + river against the banks, estimate what part of the excavation + of the canyon is due to the vertical erosion of its bed by the + river and what to weathering and rain wash on the canyon sides] + +In the northern hemisphere the north sides of river valleys are +sometimes of more gentle slope than the south sides. Can you suggest a +reason? + +=The Grand Canyon of the Colorado River in Arizona.= The Colorado +River trenches the high plateau of northern Arizona with a colossal +canyon two hundred and eighteen miles long and more than a mile in +greatest depth (Fig. 15). The rocks in which the canyon is cut are for +the most part flat-lying, massive beds of limestones and sandstones, +with some shales, beneath which in places harder crystalline rocks are +disclosed. Where the canyon is deepest its walls have been profoundly +dissected. Lateral ravines have widened into immense amphitheaters, +leaving between them long ridges of mountain height, buttressed +and rebuttressed with flanking spurs and carved into majestic +architectural forms. From the extremity of one of these promontories +it is two miles or more across the gulf to the point of the one +opposite, and the heads of the amphitheaters are thirteen miles apart. + + [Illustration: Fig. 54. Grand Canyon of the Colorado River, + Arizona] + +The lower portion of the canyon is much narrower (Fig. 54) and its +walls of dark crystalline rock sink steeply to the edge of the river, +a swift, powerful stream a few hundred feet wide, turbid with reddish +silt, by means of which it continually rasps its rocky bed as it +hurries on. The Colorado is still deepening its gorge. In the Grand +Canyon its gradient is seven and one half feet to the mile, but, as in +all ungraded rivers, the descent is far from uniform. Graded reaches +in soft rock alternate with steeper declivities in hard rock, forming +rapids such as, for example, a stretch of ten miles where the fall +averages twenty-one feet to the mile. Because of these dangerous +rapids the few exploring parties who have traversed the Colorado +canyon have done so at the hazard of their lives. + +The canyon has been shaped by several agencies. Its depth is due to +the river which has sawed its way far toward the base of a lofty +rising plateau. Acting alone this would have produced a slitlike gorge +little wider than the breadth of the stream. The impressive width of +the canyon and the magnificent architectural masses which fill it are +owing to two causes. Running water has gulched the walls and +weathering has everywhere attacked and driven them back. The +horizontal harder beds stand out in long lines of vertical cliffs, +often hundreds of feet in height, at whose feet talus slopes conceal +the outcrop of the weaker strata (Fig. 15). As the upper cliffs have +been sapped and driven back by the weather, broad platforms are left +at their bases and the sides of the canyon descend to the river by +gigantic steps. Far up and down the canyon the eye traces these +horizontal layers, like the flutings of an elaborate molding, +distinguishing each by its contour as well as by its color and +thickness. + + [Illustration: Fig. 55. Diagrams illustrating Conditions which + produce Falls or Rapids + + _A_, vertical succession of harder and softer rocks; + _B_, horizontal succession of the same. In _A_ the stream _ab_ + in sinking its bed through a mass of strata of different degrees + of hardness has discovered the weak layer _s_ beneath the hard + layer _h_. It rapidly cuts its way in _s_, while in _A_ its + work is delayed. Thus the profile _afb´_ is soon reached, with + falls at _f_. In _B_ the initial profile is shown by dotted + line.] + +The Grand Canyon of the Colorado is often and rightly cited as an +example of the stupendous erosion which may be accomplished by a +river. And yet the Colorado is a young stream and its work is no more +than well begun. It has not yet wholly reached grade, and the great +task of the river and its tributaries--the task of leveling the lofty +plateau to a low plain and of transporting it grain by grain to the +sea--still lies almost entirely in the future. + + [Illustration: Fig. 56. Longitudinal Section of Yellowstone + River at Lower Fall, _F_, and Upper Fall, _F´_, Yellowstone + National Park + + _la_, lava deeply decayed through action of thermal waters; _m_ + and _m´_, masses of decayed lavas to whose hardness the falls + are due. Which fall will be worn away the sooner? How far + upstream will each fall migrate? Draw profile of the river when + one fall has disappeared] + + [Illustration: Fig. 57. Diagram illustrating Migration of a + Fall due to a Hard Layer _H_, in the Midst of Soft Layers + _S_ and _S_, all dipping upstream + + _a_, _b_, _c_, _d_, and _e_, successive positions of the fall; + _r_, rapid to which the fall is reduced. Draw diagram showing + migration of fall in strata dipping _downstream_. Under what + conditions of inclination of the strata will a fall migrate the + farthest and have the longest life? Under what conditions will + it migrate the least distance and soonest be destroyed?] + +=Waterfalls and rapids.= Before the bed of a stream is reduced to +grade it may be broken by abrupt descents which give rise to +waterfalls and rapids. Such breaks in a river's bed may belong to the +initial surface over which it began its course; still more commonly +are they developed in the rock mass through which it is cutting its +valley. Thus, wherever a stream leaves harder rocks to flow over +softer ones the latter are quickly worn below the level of the former, +and a sharp change in slope, with a waterfall or rapid, results. + +At time of flood young tributaries with steeper courses than that of +the trunk stream may bring down stones and finer waste, which the +gentler current cannot move along, and throw them as a dam across its +way. The rapids thus formed are also ephemeral, for as the gradient of +the tributaries is lowered the main stream becomes able to handle the +smaller and finer load which they discharge. + +A rare class of falls is produced where the minor tributaries of a +young river are not able to keep pace with their master stream in the +erosion of their beds because of their smaller volume, and thus join +it by plunging over the side of its gorge. But as the river approaches +grade and slackens its down cutting, the tributaries sooner or later +overtake it, and effacing their falls, unite with it on a level. + + [Illustration: Fig. 58. Maturely Dissected Plateau near + Charleston, West Virginia + + Compare the number of streams in any given number of square + miles with the number on an area of the same size in the Red + River Valley (Fig. 44). What is the shape of the ridges? Are + their summits broad or narrow? Are their crests even or broken + by knobs and cols (the depressions on the crest line)? If the + latter, how deeply have the cols been worn beneath the summits + of the knobs?] + +Waterfalls and rapids of all kinds are evanescent features of a +river's youth. Like lakes they are soon destroyed, and if any long +time had already elapsed since their formation they would have been +obliterated already. + +=Local baselevels.= That balanced condition called grade, where a +river neither degrades its bed by erosion nor aggrades it by +deposition, is first attained along reaches of soft rocks, ungraded +outcrops of hard rocks remaining as barriers which give rise to rapids +or falls. Until these barriers are worn away they constitute local +baselevels, below which level the stream, up valley from them, cannot +cut. They are eroded to grade one after another, beginning with the +least strong, or the one nearest the mouth of the stream. In a similar +way the surface of a lake in a river's course constitutes for all +inflowing streams a local baselevel, which disappears when the basin +is filled or drained. + + [Illustration: Fig. 59. A Maturity Dissected Region of Slight + Relief, Iowa] + + +Mature And Old Rivers + +Maturity is the stage of a river's complete development and most +effective work. The river system now has well under way its great task +of wearing down the land mass which it drains and carrying it particle +by particle to the sea. The relief of the land is now at its greatest; +for the main channels have been sunk to grade, while the divides +remain but little worn below their initial altitudes. Ground water now +stands low. The run-off washes directly to the streams, with the least +delay and loss by evaporation in ponds and marches; the discharge of +the river is therefore at its height. The entire region is dissected +by stream ways. The area of valley slopes is now largest and sheds to +the streams a heavier load of waste than ever before. At maturity the +river system is doing its greatest amount of work both in erosion and +in the carriage of water and of waste to the sea. + + [Illustration: Fig. 60. Successive Stages, _A_, _B_, _C_, and + _D_, in Valley-Widening by Planation + + Describe valley _A_. What changes have taken place in _B_, _C_, + and _D_? Do the river bends remain stationary or move up or + down valley? With what effect on the projecting spurs of the + valley sides? Draw diagrams showing a still later stage than _D_] + +=Lateral erosion.= On reaching grade a river ceases to scour its bed, +and it does not again begin to do so until some change in load or +volume enables it to find grade at a lower level. On the other hand, a +stream erodes its banks at all stages in its history, and with graded +rivers this process, called lateral erosion, or _planation_, is +specially important. The current of a stream follows the outer side of +all curves or bends in the channel, and on this side it excavates its +bed the deepest and continually wears and saps its banks. On the inner +side deposition takes place in the more shallow and slower-moving +water. The inner bank of bends is thus built out while the outer bank +is worn away. By swinging its curves against the valley sides a graded +river continually cuts a wider and wider floor. The V-valley of youth +is thus changed by planation to a flat-floored valley with flaring +sides which gradually become subdued by the weather to gentle slopes. +While widening their valleys streams maintain a constant width of +channel, so that a wide-floored valley does not signify that it ever +was occupied by a river of equal width. + +=The gradient.= The gradients of graded rivers differ widely. A large +river with a light load reaches grade on a faint slope, while a +smaller stream heavily burdened with waste requires a steep slope to +give it velocity sufficient to move the load. + +The Platte, a graded river of Nebraska with its headwaters in the +Rocky Mountains, is enfeebled by the semi-arid climate of the Great +Plains and surcharged with the waste brought down both by its branches +in the mountains and by those whose tracks lie over the soft rocks of +the plains. It is compelled to maintain a gradient of eight feet to +the mile in western Nebraska. The Ohio reaches grade with a slope of +less than four inches to the mile from Cincinnati to its mouth, and +the powerful Mississippi washes along its load with a fall of but +three inches per mile from Cairo to the Gulf. + +Other things being equal, which of graded streams will have the +steeper gradient, a trunk stream or its tributaries? a stream supplied +with gravel or one with silt? + +Other factors remaining the same, what changes would occur if the +Platte should increase in volume? What changes would occur if the load +should be increased in amount or in coarseness? + + + [Illustration: Fig. 61. Successive Cross Sections of a Region as + it advances from Infancy _a_, to Old Age _e_] + +_The old age of rivers._ As rivers pass their prime, as denudation +lowers the relief of the region, less waste and finer is washed over +the gentler slopes of the lowering hills. With smaller loads to carry, +the rivers now deepen their valleys and find grade with fainter +declivities nearer the level of the sea. This limit of the level of +the sea beneath which they cannot erode is known as _baselevel_.[1] As +streams grow old they approach more and more closely to baselevel, +although they are never able to attain it. Some slight slope is needed +that water may flow and waste be transported over the land. Meanwhile +the relief of the land has ever lessened. The master streams and their +main tributaries now wander with sluggish currents over the broad +valley floors which they have planed away; while under the erosion of +their innumerable branches and the wear of the weather the divides +everywhere are lowered and subdued to more and more gentle slopes. +Mountains and high plateaus are thus reduced to rolling hills, and at +last to plains, surmounted only by such hills as may still be +unreduced to the common level, because of the harder rocks of which +they are composed or because of their distance from the main erosion +channels. Such regions of faint relief, worn down to near base level +by subaërial agencies, are known as _peneplains_ (almost plains). +Any residual masses which rise above them are called _monadnocks_, +from the name of a conical peak of New Hampshire which overlooks the +now uplifted peneplain of southern New England. + + [1] The term "baselevel" is also used to designate the close + approximation to sea level to which streams are able to + subdue the land. + +In its old age a region becomes mantled with thick sheets of fine and +weathered waste, slowly moving over the faint slopes toward the water +ways and unbroken by ledges of bare rock. In other words, the waste +mantle also is now graded, and as waterfalls have been effaced in the +river beds, so now any ledges in the wide streams of waste are worn +away and covered beneath smooth slopes of fine soil. Ground water +stands high and may exude in areas of swamp. In youth the land mass +was roughhewn and cut deep by stream erosion. In old age the faint +reliefs of the land dissolve away, chiefly under the action of the +weather, beneath their cloak of waste. + + [Illustration: Fig. 62. Peneplain surrounded by Monadnocks, + Piedmont Belt, Virginia + + From Davis' _Elementary Physical Geography] + +=The cycle of erosion.= The successive stages through which a land +mass passes while it is being leveled to the sea constitute together a +cycle of erosion. Each stage of the cycle from infancy to old age +leaves, as we have seen, its characteristic records in the forms +sculptured on the land, such as the shapes of valleys and the contours +of hills and plains. The geologist is thus able to determine by the +land forms of any region the stage in the erosion cycle to which it +now belongs, and knowing what are the earlier stages of the cycle, to +read something of the geological history of the region. + +=Interrupted cycles.= So long a time is needed to reduce a land mass +to baselevel that the process is seldom if ever completed during a +single uninterrupted cycle of erosion. Of all the various +interruptions which may occur the most important are gradual movements +of the earth's crust, by which a region is either depressed or +elevated relative to sea level. + + [Illustration: Fig. 63. Young Inner Gorge in Wide Older Valley, + Alaska] + +The _depression_ of a region hastens its old age by decreasing the +gradient of streams, by destroying their power to excavate their beds +and carry their loads to a degree corresponding to the amount of the +depression, and by lessening the amount of work they have to do. The +slackened river currents deposit their waste in Hood plains which +increase in height as the subsidence continues. The lower courses of +the rivers are invaded by the sea and become estuaries, while the +lower tributaries are cut off from the trunk stream. + +_Elevation_, on the other hand, increases the activity of all agencies +of weathering, erosion, and transportation, restores the region to its +youth, and inaugurates a new cycle of erosion. Streams are given a +steeper gradient, greater velocity, and increased energy to carry +their loads and wear their beds. They cut through the alluvium of +their flood plains, leaving it on either bank as successive terraces, +and intrench themselves in the underlying rock. In their older and +wider valleys they cut narrow, steep-walled inner gorges, in which +they flow swiftly over rocky floors, broken here and there by falls +and rapids where a harder layer of rock has been discovered. Winding +streams on plains may thus incise their meanders in solid rock as the +plains are gradually uplifted. Streams which are thus restored to +their youth are said to be _revived_. + + [Illustration: Fig. 64. Incised Meanders of Oneota River, Iowa] + +As streams cut deeper and the valley slopes are steepened, the mantle +of waste of the region undergoing elevation is set in more rapid +movement. It is now removed particle by particle faster than it forms. +As the waste mantle thins, weathering attacks the rocks of the region +more energetically until an equilibrium is reached again; the rocks +waste rapidly and their waste is as rapidly removed. + +=Dissected peneplains.= When a rise of the land brings one cycle to an +end and begins another, the characteristic land forms of each cycle +are found together and the topography of the region is composite until +the second cycle is so far advanced that the land forms of the first +cycle are entirely destroyed. The contrast between the land surfaces +of the later and the earlier cycles is most striking when the earlier +had advanced to age and the later is still in youth. Thus many +peneplains which have been elevated and dissected have been recognized +by the remnants of their ancient erosion surfaces, and the length of +time which has elapsed since their uplift has been measured by the +stage to which the new cycle has advanced. + + [Illustration: Fig. 65. + + Describe the valley of stream _a_. Is it young or old? How does + the valley of _b_ differ from that of _a_? Compare as to form + and age the inner valley of _b_ with the outer valley and with + the valley of _a_. Account for the inner valley. Why does it + not extend to the upper portion of the course of _b_? Will it + ever do so? Draw longitudinal profile of _b_, showing the + different gradient of upper and lower portions of its course + not here seen. As the inner valley of tributary _c_ extends + headward it may invade the valley of _a_ before the inner + valley of _a_ has worked upstream to the area seen in the + diagram. With what results?] + +=The Piedmont Belt.= As an example of an ancient peneplain uplifted +and dissected we may cite the Piedmont Belt, a broad upland lying +between the Appalachian Mountains and the Atlantic coastal plain. The +surface of the Piedmont is gently rolling. The divides, which are +often smooth areas of considerable width, rise to a common plane, and +from them one sees in every direction an even sky line except where in +places some lone hill or ridge may lift itself above the general level +(Fig. 62). The surface is an ancient one, for the mantle of residual +waste lies deep upon it, soils are reddened by long oxidation, and the +rocks are rotted to a depth of scores of feet. + +At present, however, the waste mantle is not forming so rapidly as it +is being removed. The streams of the upland are actively engaged in +its destruction. They flow swiftly in narrow, rock-walled valleys over +rocky beds. This contrast between the young streams and the aged +surface which they are now so vigorously dissecting can only be +explained by the theory that the region once stood lower than at +present and has recently been upraised. If now we imagine the valleys +refilled with the waste which the streams have swept away, and the +upland lowered, we restore the Piedmont region to the condition in +which it stood before its uplift and dissection,--a gently rolling +plain, surmounted here and there by isolated hills and ridges. + + [Illustration: Fig. 66. Dissected Peneplain of Southern New + England] + +The surface of the ancient Piedmont plain, as it may be restored from +the remnants of it found on the divides, is not in accordance with the +structures of the country rocks. Where these are exposed to view they +are seen to be far from horizontal. On the walls of river gorges they +dip steeply and in various directions and the streams flow over their +upturned edges. As shown in Figure 67, the rocks of the Piedmont have +been folded and broken and tilted. + + [Illustration: Fig. 67. Section in Piedmont Belt + _M_, a monadnock] + +It is not reasonable to believe that when the rocks of the Piedmont +were thus folded and otherwise deformed the surface of the region was +a plain. The upturned layers have not always stopped abruptly at the +even surface of the Piedmont plain which now cuts across them. They +are the bases of great folds and tilted blocks which must once have +risen high in air. The complex and disorderly structures of the +Piedmont rocks are those seen in great mountain ranges, and there is +every reason to believe that these rocks after their deformation rose +to mountain height. + + [Illustration: Fig. 68. The area of the Laurentian Peneplain + (shaded)] + +The ancient Piedmont plain cuts across these upturned rocks as +independently of their structure as the even surface of the sawed +stump of some great tree is independent of the direction of its +fibers. Hence the Piedmont plain as it was before its uplift was not a +coastal plain formed of strata spread in horizontal sheets beneath the +sea and then uplifted; nor was it a structural plain, due to the +resistance to erosion of some hard, flat-lying layer of rock. Even +surfaces developed on rocks of discordant structure, such as the +Piedmont shows, are produced by long denudation, and we may consider +the Piedmont as a peneplain formed by the wearing down of mountain +ranges, and recently uplifted. + +=The Laurentian peneplain.= This is the name given to a denuded +surface on very ancient rocks which extends from the Arctic Ocean to +the St. Lawrence River and Lake Superior, with small areas also in +northern Wisconsin and New York. Throughout this U-shaped area, which +incloses Hudson Bay within its arms, the country rocks have the +complicated and contorted structures which characterize mountain +ranges (see Fig. 179, P. 211). But the surface of the area is by no +means mountainous. The sky line when viewed from the divides is +unbroken by mountain peaks or rugged hills. The surface of the arm +west of Hudson Bay is gently undulating and that of the eastern arm +has been roughened to low-rolling hills and dissected in places by +such deep river gorges as those of the Ottawa and Saguenay. This +immense area may be regarded as an ancient peneplain truncating the +bases of long-vanished mountains and dissected after elevation. + +In the examples cited the uplift has been a broad one and to +comparatively little height. Where peneplains have been uplifted to +great height and have since been well dissected, and where they have +been upfolded and broken and uptilted, their recognition becomes more +difficult. Yet recent observers have found evidences of ancient +lowland surfaces of erosion on the summits of the Allegheny ridges, +the Cascade Mountains (Fig. 69), and the western slope of the Sierra +Nevadas. + + [Illustration: Fig. 69. View in the Cascade Mountains, Washington + + The general level to which these ridges rise may be accounted + for by the uplift and dissection of a once low-lying peneplain] + +=The southern Appalachian region.= We have here an example of an area +the latter part of whose geological history may be deciphered by means +of its land forms. The generalized section of Figure 70, which passes +from west to east across a portion of the region in eastern Tennessee, +shows on the west a part of the broad Cumberland plateau. On the east +is a roughened upland platform, from which rise in the distance the +peaks of the Great Smoky Mountains. The plateau, consisting of strata +but little changed from their original flat-lying attitude, and the +platform, developed on rocks of disordered structure made crystalline +by heat and pressure, both stand at the common level of the line AB. +They are separated by the Appalachian valley, forty miles wide, cut in +strata which have been folded and broken into long narrow blocks. The +valley is traversed lengthwise by long, low ridges, the outcropping +edges of the harder strata, which rise to about the same level,--that +of the line _cd_. Between these ridges stretch valley lowlands at the +level _ef_ excavated in the weaker rocks, while somewhat below them lie +the channels of the present streams now busily engaged in deepening +their beds. + +_The valley lowlands._ Were they planed by graded or ungraded streams? +Have the present streams reached grade? Why did the streams cease +widening the floors of the valley lowlands? How long since? When will +they begin anew the work of lateral planation? What effect will this +have on the ridges if the present cycle of erosion continues long +uninterrupted? + + [Illustration: Fig. 70. Generalized Section of the Southern + Appalachian Region in Eastern Tennessee] + +_The ridges of the Appalachian valley._ Why do they stand above the +valley lowlands? Why do their summits lie in about the same plane? +Refilling the valleys intervening between these ridges with the +material removed by the streams, what is the nature of the surface +thus restored? Does this surface _cd_ accord with the rock structures +on which it has been developed? How may it have been made? At what +height did the land stand then, compared with its present height? What +elevations stood above the surface _cd_? Why? What name may you use to +designate them? How does the length of time needed to develop the +surface _cd_ compare with that needed to develop the valley lowlands? + +_The Platform And Plateau._ Why do they stand at a common level ab? Of +what surface may they be remnants? Is it accordant with the rock +structure? How was it produced? What unconsumed masses overlooked it? +Did the rocks of the Appalachian valley stand above this surface when +it was produced? Did they then stand below it? Compare the time needed +to develop this surface with that needed to develop _cd_. Which surface +is the older? + +How many cycles of erosion are represented here? Give the erosion +history of the region by cycles, beginning with the oldest, the work +done in each and the work left undone, what brought each cycle to a +close, and how long relatively it continued. + + + + +CHAPTER IV + +RIVER DEPOSITS + + +The characteristic features of river deposits and the forms which they +assume may be treated under three heads: (1) valley deposits, (2) +basin deposits, and (3) deltas. + + +Valley Deposits + +=Flood plains.= The deposits which streams build along their courses +at times of flood are known as flood plains. A swift current then +sweeps along the channel, while a shallow sheet of water moves slowly +over the flood plain, spreading upon it a thin layer of sediment. It +has been estimated that each inundation of the Nile leaves a layer of +fertilizing silt three hundredths of an inch thick over the flood +plain of Egypt. + +Flood plains may consist of a thin spread of alluvium over the flat +rock floor of a valley which is being widened by the lateral erosion +of a graded stream (Fig. 60). Flood-plain deposits of great thickness +may be built by aggrading rivers even in valleys whose rock floors +have never been thus widened (Fig. 368). + + [Illustration: Fig. 71. Cross Section of a Flood Plain] + +A cross section of a flood plain (Fig. 71) shows that it is highest +next the river, sloping gradually thence to the valley sides. These +wide natural embankments are due to the fact that the river deposit is +heavier near the bank, where the velocity of the silt-laden channel +current is first checked by contact with the slower-moving overflow. + + [Illustration: Fig. 72. Waste-filled Valley and Braided + Channels of the Upper Mississippi] + +Thus banked off from the stream, the outer portions of a flood plain +are often ill-drained and swampy, and here vegetal deposits, such as +peat, may be interbedded with river silts. + +A map of a wide flood plain, such as that of the Mississippi or the +Missouri (Fig. 77), shows that the courses of the tributaries on +entering it are deflected downstream. Why? + +The aggrading streams by which flood plains are constructed gradually +build their immediate banks and beds to higher and higher levels, and +therefore find it easy at times of great floods to break their natural +embankments and take new courses over the plain. In this way they +aggrade each portion of it in turn by means of their shifting +channels. + +=Braided channels.= A river actively engaged in aggrading its valley +with coarse waste builds a flood plain of comparatively steep gradient +and often flows down it in a fairly direct course and through a +network of braided channels. From time to time a channel becomes +choked with waste, and the water no longer finding room in it breaks +out and cuts and builds itself a new way which reunites down valley +with the other channels. Thus there becomes established a network of +ever-changing channels inclosing low islands of sand and gravel. + + [Illustration: Fig. 73. Terraced Valley of River in Central Asia] + + [Illustration: Fig. 74. Terraces carved in Alluvial Deposits] + + Which is older, the rock floor of the valley or the river + deposits which fill it? What are the relative ages of terraces + _a_, _b_, _c_, and _e_? It will be noted that the remnants of + the higher flood plains have not been swept away by the + meandering river, as it swung from side to side of the valley + at lower levels, because they have been defended by ledges of + hard rock in the projecting spurs of the initial valley. The + stream has encountered such defending ledges at the point + marked _d_] + + [Illustration: Fig. 75. River Terraces of Rock covered with + Alluvium + + _c_, recent flood plain of the river. To what processes is it + due? Account for the alluvium at _a_ and _b_ and on the + opposite side of the valley at the same levels. Which is the + older? Account for the flat rock floors on which these deposits + of alluvium rest. Give the entire history which may be read in + the section] + +=Terraces.= While aggrading streams thus tend to shift their channels, +degrading streams, on the contrary, become more and more deeply +intrenched in their valleys. It often occurs that a stream, after +having built a flood plain, ceases to aggrade its bed because of a +lessened load or for other reasons, such as an uplift of the region, +and begins instead to degrade it. It leaves the original flood plain +out of reach of even the highest floods. When again it reaches grade +at a lower level it produces a new flood plain by lateral erosion in +the older deposits, remnants of which stand as terraces on one or both +sides of the valley. In this way a valley may be lined with a +succession of terraces at different levels, each level representing an +abandoned flood plain. + + [Illustration: Fig. 76. Development of a Meander + + The dotted line in _a_, _b_, and _c_ shows the stage preceding that indicate by the unbroken line] + +=Meanders.= Valleys aggraded with fine waste form well-nigh level +plains over which streams wind from side to side of a direct course in +symmetric bends known as meanders, from the name of a winding river of +Asia Minor. The giant Mississippi has developed meanders with a radius +of one and one half miles, but a little creek may display on its +meadow as perfect curves only a rod or so in radius. On the flood +plain of either river or creek we may find examples of the successive +stages in the development of the meander, from its beginning in the +slight initial bend sufficient to deflect the current against the +outer side. Eroding here and depositing on the inner side of the bend, +it gradually reaches first the open bend (Fig. 76, _a_) whose width +and length are not far from equal, and later that of the horseshoe +meander (Fig. 76, _b_) whose diameter transverse to the course of the +stream is much greater than that parallel with it. Little by little +the neck of land projecting into the bend is narrowed, until at last +it is cut through and a "cut-off" is established. The old channel is +now silted up at both ends and becomes a crescentic lagoon (Fig. 76, +_c_), or oxbow lake, which fills gradually to an arc-shaped shallow +depression. + + [Illustration: Fig. 77. Map of a portion of the Flood Plain of + the Missouri River + + Each small square represents one square mile. How wide is the + flood plain of the Missouri? How wide is the flood plain of the + Big Sioux? Why is the latter river deflected down valley on + entering the flood plain of the master stream? How do the + meanders of the two rivers compare in size? How does the width + of each flood plain compare with the width of the belt occupied + by the meanders of the river? Do you find traces of any former + channels?] + +=Flood plains characteristic of mature rivers.= On reaching grade a +stream planes a flat floor for its continually widening valley. Ever +cutting on the outer bank of its curves, it deposits on the inner bank +scroll-like flood-plain patches (Fig 60). For a while the valley bluffs +do not give its growing meanders room to develop to their normal size, +but as planation goes on, the bluffs are driven back to the full width +of the meander belt and still later to a width which gives room for +broad stretches of flood plain on either side (Fig. 77). + +Usually a river first attains grade near its mouth, and here first sinks +its bed to near baselevel. Extending its graded course upstream by +cutting away barrier after barrier, it comes to have a widened and +mature valley over its lower course, while its young headwaters are +still busily eroding their beds. Its ungraded branches may thus bring +down to its lower course more waste than it is competent to carry on to +the sea, and here it aggrades its bed and builds a flood plain in order +to gain a steeper gradient and velocity enough to transport its load. + +As maturity is past and the relief of the land is lessened, a smaller +and smaller load of waste is delivered to the river. It now has energy +to spare and again degrades its valley, excavating its former flood +plains and leaving them in terraces on either side, and at last in its +old age sweeping them away. + + [Illustration: Fig. 78. Alluvial Cones, Wyoming] + +=Alluvial cones and fans.= In hilly and mountainous countries one often +sees on a valley side a conical or fan-shaped deposit of waste at the +mouth of a lateral stream. The cause is obvious: the young branch has +not been able as yet to wear its bed to accordant level with the already +deepened valley of the master stream. It therefore builds its bed to +grade at the point of juncture by depositing here its load of waste,--a +load too heavy to be carried along the more gentle profile of the trunk +valley. + + [Illustration: Fig. 79. Tributaries and Distributaries of a + Fan-Building Stream] + +Where rivers descend from a mountainous region upon the plain they may +build alluvial fans of exceedingly gentle slope. Thus the rivers of +the western side of the Sierra Nevada Mountains have spread fans with +a radius of as much as forty miles and a slope too slight to be +detected without instruments, where they leave the rock-cut canyons in +the mountains and descend upon the broad central valley of California. + +As a river flows over its fan it commonly divides into a branchwork of +shifting channels called _distributaries_, since they lead off the +water from the main stream. In this way each part of the fan is +aggraded and its symmetric form is preserved. + +=Piedmont plains.= Mountain streams may build their confluent fans +into widespread piedmont (foot of the mountain) alluvial plains. These +are especially characteristic of arid lands, where the streams wither +as they flow out upon the thirsty lowlands and are therefore compelled +to lay down a large portion of their load. In humid climates +mountain-born streams are usually competent to carry their loads of +waste on to the sea, and have energy to spare to cut the lower +mountain slopes into foothills. In arid regions foothills are commonly +absent and the ranges rise, as from pedestals, above broad, sloping +plains of stream-laid waste. + + [Illustration: Fig. 80. Section from the Rocky Mountains Eastward + River deposits dotted] + +=The High Plains.= The rivers which flow eastward from the Rocky +Mountains have united their fans in a continuous sheet of waste which +stretches forward from the base of the mountains for hundreds of miles +and in places is five hundred feet thick (Fig. 80). That the deposit +was made in ancient times on land and not in the sea is proved by the +remains which it contains of land animals and plants of species now +extinct. That it was laid by rivers and not by fresh-water lakes is +shown by its structure. Wide stretches of flat-lying, clays and sands +are interrupted by long, narrow belts of gravel which mark the +channels of the ancient streams. Gravels, and sands are often cross +bedded, and their well worn pebbles may be identified with the rocks +of the mountains. After building this sheet of waste the streams +ceased to aggrade and began the work of destruction. Large uneroded +remnants, their surfaces flat as a floor, remain as the High Plains of +western Kansas and Nebraska. + +=River deposits in subsiding troughs.= To a geologist the most +important river deposits are those which gather in areas of gradual +subsidence; they are often of vast extent and immense thickness, and +such deposits of past geological ages have not infrequently been +preserved, with all their records of the times in which they were +built, by being carried below the level of the sea, to be brought to +light by a later uplift. On the other hand, river deposits which +remain above baselevels of erosion are swept away comparatively soon. + +=The Great Valley Of California= is a monotonously level plain of +great fertility, four hundred miles in length and fifty miles in +average width, built of waste swept down by streams from the mountain +ranges which inclose it,--the Sierra Nevada on the east and the Coast +Range on the west. On the waste slopes at the foot of the bordering +hills coarse gravels and even bowlders are left, while over the +interior the slow-flowing streams at times of flood spread wide sheets +of silt. Organic deposits are now forming by the decay of vegetation +in swampy tule (reed) lands and in shallow lakes which occupy +depressions left by the aggrading streams. + +Deep borings show that this great trough is filled to a depth of at +least two thousand feet below sea level with recent unconsolidated +sands and silts containing logs of wood and fresh-water shells. These +are land deposits, and the absence of any marine deposits among them +proves that the region has not been invaded by the sea since the +accumulation began. It has therefore been slowly subsiding and its +streams, although continually carried below grade, have yet been able +to aggrade the surface as rapidly as the region sank, and have +maintained it, as at present, slightly above sea level. + +=The Indo-Gangetic Plain=, spread by the Brahmaputra, the Ganges, and +the Indus river systems, stretches for sixteen hundred miles along the +southern base of the Himalaya Mountains and occupies an area of three +hundred thousand square miles (Fig. 342). It consists of the flood +plains of the master streams and the confluent fans of the tributaries +which issue from the mountains on the north. Large areas are subject +to overflow each season of flood, and still larger tracts mark +abandoned flood plains below which the rivers have now cut their beds. +The plain is built of far-stretching beds of clay, penetrated by +streaks of sand, and also of gravel near the mountains. Beds of impure +peat occur in it, and it contains fresh-water shells and the bones of +land animals of species now living in northern India. At Lucknow an +artesian well was sunk to one thousand feet below sea level without +reaching the bottom of these river-laid sands and silts, proving a +slow subsidence with which the aggrading rivers have kept pace. + +=Warped valleys.= It is not necessary that an area should sink below +sea level in order to be filled with stream-swept waste. High valleys +among growing mountain ranges may suffer warping, or may be blockaded +by rising mountain folds athwart them. Where the deformation is rapid +enough, the river may be ponded and the valley filled with lake-laid +sediments. Even when the river is able to maintain its right of way it +may yet have its declivity so lessened that it is compelled to aggrade +its course continually, filling the valley with river deposits which +may grow to an enormous thickness. + +Behind the outer ranges of the Himalaya Mountains lie several +waste-filled valleys, the largest of which are Kashmir and Nepal, the +former being an alluvial plain about as large as the state of +Delaware. The rivers which drain these plains have already cut down +their outlet gorges sufficiently to begin the task of the removal of +the broad accumulations which they have brought in from the +surrounding mountains. Their present flood plains lie as much as some +hundreds of feet below wide alluvial terraces which mark their former +levels. Indeed, the horizontal beds of the Hundes Valley have been +trenched to the depth of nearly three thousand feet by the Sutlej +River. These deposits are recent or subrecent, for there have been +found at various levels the remains of land plants and land and +fresh-water shells, and in some the bones of such animals as the hyena +and the goat, of species or of genera now living. Such soft deposits +cannot be expected to endure through any considerable length of future +time the rapid erosion to which their great height above the level of +the sea will subject them. + + [Illustration: Fig. 81. Cross Section of Aggraded Valley, + showing Structure of River Deposits] + +=Characteristics of river deposits.= The examples just cited teach +clearly the characteristic features of extensive river deposits. These +deposits consist of broad, flat-lying sheets of clay and fine sand +left by the overflow at time of flood, and traversed here and there by +long, narrow strips of coarse, cross-bedded sands and gravels thrown +down by the swifter currents of the shifting channels. Occasional beds +of muck mark the sites of shallow lakelets or fresh-water swamps. The +various strata also contain some remains of the countless myriads of +animals and plants which live upon the surface of the plain as it is +in process of building. River shells such as the mussel, land shells +such as those of snails, the bones of fishes and of such land animals +as suffer drowning at times of flood or are mired in swampy places, +logs of wood, and the stems and leaves of plants are examples of the +variety of the remains of land and fresh-water organisms which are +entombed in river deposits and sealed away as a record of the life of +the time, and as proof that the deposits were laid by streams and not +beneath the sea. + + +Basin Deposits + +=Deposits in dry basins.= On desert areas without outlet to the sea, +as on the Great Basin of the United States and the deserts of central +Asia, stream-swept waste accumulates indefinitely. The rivers of the +surrounding mountains, fed by the rains and melting snows of these +comparatively moist elevations, dry and soak away as they come down +upon the arid plains. They are compelled to lay aside their entire +load of waste eroded from the mountain valleys, in fans which grow to +enormous size, reaching in some instances thousands of feet in +thickness. + +The monotonous levels of Turkestan include vast alluvial tracts now in +process of building by the floods of the frequently shifting channels +of the Oxus and other rivers of the region. For about seven hundred +miles from its mouth in Aral Lake the Oxus receives no tributaries, +since even the larger branches of its system are lost in a network of +distributaries and choked with desert sands before they reach their +master stream. These aggrading rivers, which have channels but no +valleys, spread their muddy floods--which in the case of the Oxus +sometimes equal the average volume of the Mississippi--far and wide +over the plain, washing the bases of the desert dunes. + +=Playas.= In arid interior basins the central depressions may be +occupied by playas,--plains of fine mud washed forward from the +margins. In the wet season the playa is covered with a thin sheet of +muddy water, a playa lake, supplied usually by some stream at flood. +In the dry season the lake evaporates, the river which fed it +retreats, and there is left to view a hard, smooth, level floor of +sun-baked and sun-cracked yellow clay utterly devoid of vegetation. + +In the Black Rock desert of Nevada a playa lake spreads over an area +fifty miles long and twenty miles wide. In summer it disappears; the +Quinn River, which feeds it, shrinks back one hundred miles toward its +source, leaving an absolutely barren floor of clay, level as the sea. + +=Lake deposits.= Regarding lakes as parts of river systems, we may now +notice the characteristic features of the deposits in lake basins. +Soundings in lakes of considerable size and depth show that their +bottoms are being covered with tine clays. Sand and gravel are found +along; their margins, being brought in by streams and worn by waves +from the shore, but there are no tidal or other strong currents to +sweep coarse waste out from shore to any considerable distance. Where +fine clays are now found on the land in even, horizontal layers +containing the remains of fresh-water animals and plants, uncut by +channels tilled with cross-bedded gravels and sands and bordered by +beach deposits of coarse waste, we may safely infer the existence of +ancient lakes. + +=Marl.= Marl is a soft, whitish deposit of carbonate of lime, mingled +often with more or less of clay, accumulated in small lakes whose +feeding springs are charged with carbonate of lime and into which +little waste is washed from the land. Such lakelets are not infrequent +on the surface of the younger drift sheets of Michigan and northern +Indiana, where their beds of marl--sometimes as much as forty feet +thick--are utilized in the manufacture of Portland cement. The deposit +results from the decay of certain aquatic plants which secrete lime +carbonate from the water, from the decomposition of the calcareous +shells of tiny mollusks which live in countless numbers on the lake +floor, and in some cases apparently from chemical precipitation. + +=Peat.= We have seen how lakelets are extinguished by the decaying +remains of the vegetation which they support. A section of such a +fossil lake shows that below the growing mosses and other plants of +the surface of the bog lies a spongy mass composed of dead vegetable +tissue, which passes downward gradually into _peat_,--a dense, dark +brown carbonaceous deposit in which, to the unaided eye, little or no +trace of vegetable structure remains. When dried, peat forms a fuel of +some value and is used either cut into slabs and dried or pressed into +bricks by machinery. + + [Illustration: Fig. 82. Digging Peat, Scotland] + +When vegetation decays in open air the carbon of its tissues, taken +from the atmosphere by the leaves, is oxidized and returned to it in +its original form of carbon dioxide. But decomposing in the presence +of water, as in a bog, where the oxygen of the air is excluded, the +carbonaceous matter of plants accumulates in deposits of peat. + +Peat bogs are numerous in regions lately abandoned by glacier ice, +where river systems are so immature that the initial depressions left +in the sheet of drift spread over the country have not yet been +drained. One tenth of the surface of Ireland is said to be covered +with peat, and small bogs abound in the drift-covered area of New +England and the states lying as far west as the Missouri River. In +Massachusetts alone it has been reckoned that there are fifteen +billion cubic feet of peat, the largest bog occupying several thousand +acres. + +Much larger swamps occur on the young coastal plain of the Atlantic +from New Jersey to Florida. The Dismal Swamp, for example, in Virginia +and North Carolina is forty miles across. It is covered with a dense +growth of water-loving trees such as the cypress and black gum. The +center of the swamp is occupied by Lake Drummond, a shallow lake seven +miles in diameter, with banks of pure-peat, and still narrowing from +the encroachment of vegetation along its borders. + +=Salt lakes.= In arid climates a lake rarely receives sufficient +inflow to enable it to rise to the basin rim and find an outlet. +Before this height is reached its surface becomes large enough to +discharge by evaporation into the dry air the amount of water that is +supplied by streams. As such a lake has no outlet, the minerals in +solution brought into it by its streams cannot escape from the basin. +The lake water becomes more and more heavily charged with such +substances as common salt and the sulphates and carbonates of lime, of +soda, and of potash, and these are thrown down from solution one after +another as the point of saturation for each mineral is reached. +Carbonate of lime, the least soluble and often the most abundant +mineral brought in, is the first to be precipitated. As concentration +goes on, gypsum, which is insoluble in a strong brine, is deposited, +and afterwards common salt. As the saltness of the lake varies with +the seasons and with climatic changes, gypsum and salt are laid in +alternate beds and are interleaved with sedimentary clays spread from +the waste brought in by streams at times of flood. Few forms of life +can live in bodies of salt water so concentrated that chemical +deposits take place, and hence the beds of salt, gypsum, and silt of +such lakes are quite barren of the remains of life. Similar deposits +are precipitated by the concentration of sea water in lagoons and arms +of the sea cut off from the ocean. + + [Illustration: Fig. 83. Map of Lake Bonneville and Lahontan + + From Davis' _Physical Geography_] + +=Lakes Bonneville and Lahontan.= These names are given to extinct +lakes which once occupied large areas in the Great Basin, the former +in Utah, the latter in northwestern Nevada. Their records remain in +old horizontal beach lines which they drew along their mountainous +shores at the different levels at which they stood, and in the +deposits of their beds. At its highest stage Lake Bonneville, then one +thousand feet deep, overflowed to the north and was a fresh-water +lake. As it shrank below the outlet it became more and more salty, and +the Great Salt Lake, its withered residue, is now depositing salt +along its shores. In its strong brine lime carbonate is insoluble, and +that brought in by streams is thrown down at once in the form of +travertine. + + [Illustration: Fig. 84. Section of Deposits in Beds of Lakes + Bonneville and Lahontan] + +Lake Lahontan never had an outlet. The first chemical deposits to be +made along its shores were deposits of travertine, in places eighty +feet thick. Its floor is spread with fine clays, which must have been +laid in deep, still water, and which are charged with the salts +absorbed by them as the briny water of the lake dried away. These +sedimentary clays are in two divisions, the upper and lower, each +being about one hundred feet thick (_a_ and _c_, Fig. 84). They are +separated by heavy deposits of well-rounded, cross-bedded gravels and +sands (_b_, Fig. 84), similar to those spread at the present time by +the intermittent streams of arid regions. A similar record is shown in +the old floors of Lake Bonneville. What conclusions do you draw from +these facts as to the history of these ancient lakes? + + +Deltas + +In the river deposits which are left above sea level particles of +waste are allowed to linger only for a time. From alluvial fans and +flood plains they are constantly being taken up and swept farther on +downstream. Although these land forms may long persist, the particles +which compose them are ever changing. We may therefore think of the +alluvial deposits of a valley as a stream of waste fed by the waste +mantle as it creeps and washes down the valley sides, and slowly +moving onwards to the sea. + +In basins waste finds a longer rest, but sooner or later lakes and dry +basins are drained or filled, and their deposits, if above sea level, +resume their journey to their final goal. It is only when carried +below the level of the sea that they are indefinitely preserved. + +On reaching this terminus, rivers deliver their load to the ocean. In +some cases the ocean is able to take it up by means of strong tidal +and other currents, and to dispose of it in ways which we shall study +later. But often the load is so large, or the tides are so weak, that +much of the waste which the river brings in settles at its mouth, +there building up a deposit called the _delta_, from the Greek letter +(D) of that name, whose shape it sometimes resembles. + +Deltas and alluvial fans have many common characteristics. Both owe +their origin to a sudden check in the velocity of the river, +compelling a deposit of the load; both are triangular in outline, the +apex pointing upstream; and both are traversed by distributaries which +build up all parts in turn. + +In a delta we may distinguish deposits of two distinct kinds,--the +submarine and the subaërial. In part a delta is built of waste +brought down by the river and redistributed and spread by waves and +tides over the sea bottom adjacent to the river's mouth. The origin of +these deposits is recorded in the remains of marine animals and plants +which they contain. + + [Illustration: Fig. 85. Delta of the Mississippi River] + +As the submarine delta grows near to the level of the sea the +distributaries of the river cover it with subaërial deposits +altogether similar to those of the flood plain, of which indeed the +subaërial delta is the prolongation. Here extended deposits of peat +may accumulate in swamps, and the remains of land and fresh-water +animals and plants swept down by the stream are imbedded in the silts +laid at times of flood. + +Borings made in the deltas of great rivers such as the Mississippi, +the Ganges, and the Nile, show that the subaërial portion often +reaches a surprising thickness. Layers of peat, old soils, and forest +grounds with the stumps of trees are discovered hundreds of feet below +sea level. In the Nile delta some eight layers of coarse gravel were +found interbedded with river silts, and in the Ganges delta at +Calcutta a boring nearly five hundred feet in depth stopped in such a +layer. + +The Mississippi has built a delta of twelve thousand three hundred +square miles, and is pushing the natural embankments of its chief +distributaries into the Gulf at a maximum rate of a mile in sixteen +years. Muddy shoals surround its front, shallow lakes, e.g. lakes +Pontchartrain and Borgne, are formed between the growing delta and the +old shore line, and elongate lakes and swamps are inclosed between the +natural embankments of the distributaries. + +The delta of the Indus River, India, lies so low along shore that a +broad tract of country is overflowed by the highest tides. The +submarine portion of the delta has been built to near sea level over +so wide a belt offshore that in many places large vessels cannot come +even within sight of land because of the shallow water. + + [Illustration: Fig. 86. Radial Section of a Delta + + This section of a delta illustrates the structure of the + platform which swift streams well loaded with coarse waste + build in the water bodies into which they empty. Three members + may be distinguished: the _bottom set beds_, _a_: the _fore set + beds_, _b_; and the _top set beds_, _c_. Account for the slope + of each of these. Why are the bottom set beds of the finer + material and why do they extend beyond the others? How does the + profile of this delta differ from that of an alluvial cone and + why?] + +A former arm of the sea, the Rann of Cutch, adjoining the delta on the +east has been silted up and is now an immense barren flat of sandy mud +two hundred miles in length and one hundred miles in greatest breadth. +Each summer it is flooded with salt water when the sea is brought in +by strong southwesterly monsoon winds, and the climate during the +remainder of the year is hot and dry. By the evaporation of sea water +the soil is thus left so salty that no vegetation can grow upon it, +and in places beds of salt several feet in thickness have accumulated. +Under like conditions salt beds of great thickness have been formed in +the past and are now found buried among the deposits of ancient +deltas. + +=Subsidence of great deltas.= As a rule great deltas are slowly +sinking. In some instances upbuilding by river deposits has gone on as +rapidly as the region has subsided. The entire thickness of the Ganges +delta, for example, so far as it has been sounded, consists of +deposits laid in open air. In other cases interbedded limestones and +other sedimentary rocks containing marine fossils prove that at times +subsidence has gained on the upbuilding and the delta has been covered +with the sea. + +It is by gradual depression that delta deposits attain enormous +thickness, and, being lowered beneath the level of the sea, are safely +preserved from erosion until a movement of the earth's crust in the +opposite direction lifts them to form part of the land. We shall read +later in the hard rocks of our continent the records of such ancient +deltas, and we shall not be surprised to find them as thick as are +those now building at the mouths of great rivers. + +=Lake deltas.= Deltas are also formed where streams lose their +velocity on entering the still waters of lakes. The shore lines of +extinct lakes, such as Lake Agassiz and Lakes Bonneville and Lahontan, +may be traced by the heavy deposits at the mouths of their tributary +streams. + + * * * * * + +We have seen that the work of streams is to drain the lands of the +water poured upon them by the rainfall, to wear them down, and to +carry their waste away to the sea, there to be rebuilt by other agents +into sedimentary rocks. The ancient strata of which the continents are +largely made are composed chiefly of material thus worn from still +more ancient lands--lands with their hills and valleys like those of +to-day--and carried by their rivers to the ocean. In all geological +times, as at the present, the work of streams has been to destroy the +lands, and in so doing to furnish to the ocean the materials from +which the lands of future ages were to be made. Before we consider how +the waste of the land brought in by streams is rebuilt upon the ocean +floor, we must proceed to study the work of two agents, glacier ice +and the wind, which coöperate with rivers in the denudation of the +land. + + [Illustration: Fig. 87. Section of Undifferentiated Drift near + Chicago] + + + + +CHAPTER V + +THE WORK OF GLACIERS + + +=The drift.= The surface of northeastern North America, as far south +as the Ohio and Missouri rivers, is generally covered by the drift,--a +formation which is quite unlike any which we have so far studied. A +section of it, such as that illustrated in Figure 87, shows that for +the most part it is unstratified, consisting of clay, sand, pebbles, +and even large bowlders, all mingled pell-mell together. The agent +which laid the drift is one which can carry a load of material of all +sizes, from the largest bowlder to the finest clay, and deposit it +without sorting. + + [Illustration: Fig. 88. Characteristic Pebbles from the Drift + + No. 1 has six facets; No. 4, originally a rounded river + pebble, has been nibbled down to one flat face; Nos. 3 + and 5 are battered subangular fragments on one side only] + +The stones of the drift are of many kinds. The region from which it +was gathered may well have been large in order to supply these many +different varieties of rocks. Pebbles and bowlders have been left far +from their original homes, as may be seen in southern Iowa, where the +drift contains nuggets of copper brought from the region about Lake +Superior. The agent which laid the drift is one able to gather its +load over a large area and carry it a long way. + + [Illustration: Fig. 89. Smoothed and Scored Rock Surface exposed + to View by the Removal of Overlying Drift, Iowa] + +The pebbles of the drift are unlike those rounded by running water or +by waves. They are marked with scratches. Some are angular, many have +had their edges blunted, while others have been ground flat and smooth +on one or more sides, like gems which have been faceted by being held +firmly against the lapidary's wheel (Fig. 88). In many places the +upper surface of the country rock beneath the drift has been swept +clean of residual clays and other waste. All rock rotten has been +planed away, and the ledges of sound rock to which the surface has +been cut down have been rubbed smooth and scratched with long, +straight, parallel lines (Fig. 89). The agent which laid the drift can +hold sand and pebbles firmly in its grasp and can grind them against +the rock beneath, thus planing it down and scoring it, while faceting +the pebbles also. + +Neither water nor wind can do these things. Indeed, nothing like the +drift is being formed by any process now at work anywhere in the +eastern United States. To find the agent which has laid this extensive +formation we must go to a region of different climatic conditions. + + [Illustration: Fig. 90. Map of Greenland + + Glacier ice covers all but the areas shaded] + +=The inland ice of Greenland.= Greenland is about fifteen hundred +miles long and nearly seven hundred miles in greatest width. With the +exception of a narrow fringe of mountainous coast land, it is +completely buried beneath a sheet of ice, in shape like a vast white +shield, whose convex surface rises to a height of nine thousand feet +above the sea. The few explorers who have crossed the ice cap found it +a trackless desert destitute of all life save such lowly forms as the +microscopic plant which produces the so-called "red snow." On the +smooth plain of the interior no rock waste relieves the snow's +dazzling whiteness; no streams of running water are seen; the silence +is broken only by howling storm winds and the rustle of the surface +snow which they drive before them. Sounding with long poles, explorers +find that below the powdery snow of the latest snowfall lie successive +layers of earlier snows, which grow more and more compact downward, +and at last have altered to impenetrable ice. The ice cap formed by +the accumulated snows of uncounted centuries may well be more than a +mile in depth. Ice thus formed by the compacting of snow is +distinguished when in motion as _glacier ice_. + + [Illustration: Fig. 91. Hypothetical Cross Section of Greenland] + +The inland ice of Greenland moves. It flows with imperceptible +slowness under its own weight, like, a mass of some viscous or plastic +substance, such as pitch or molasses candy, in all directions outward +toward the sea. Near the edge it has so thinned that mountain peaks +are laid bare, these islands in the sea of ice being known as +_nunataks_. Down the valleys of the coastal belt it drains in separate +streams of ice, or _glaciers_. The largest of these reach the sea at +the head of inlets, and are therefore called _tide glaciers_. Their +fronts stand so deep in sea water that there is visible seldom more +than three hundred feet of the wall of ice, which in many glaciers +must be two thousand and more feet high. From the sea walls of tide +glaciers great fragments break off and float away as icebergs. Thus +snows which fell in the interior of this northern land, perhaps many +thousands of years ago, are carried in the form of icebergs to melt at +last in the North Atlantic. + +Greenland, then, is being modeled over the vast extent of its interior +not by streams of running water, as are regions in warm and humid +climates, nor by currents of air, as are deserts to a large extent, +but by a sheet of flowing ice. What the ice sheet is doing in the +interior we may infer from a study of the separate glaciers into which +it breaks at its edge. + +=The smaller Greenland glaciers.= Many of the smaller glaciers of +Greenland do not reach the sea, but deploy on plains of sand and +gravel. The edges of these ice tongues are often as abrupt as if +sliced away with a knife (Fig. 92), and their structure is thus +readily seen. They are stratified, their layers representing in part +the successive snowfalls of the interior of the country. The upper +layers are commonly white and free from stones; but the lower layers, +to the height of a hundred feet or more, are dark with debris which is +being slowly carried on. So thickly studded with stones is the base of +the ice that it is sometimes difficult to distinguish it from the rock +waste which has been slowly dragged beneath the glacier or left about +its edges. The waste beneath and about the glacier is unsorted. The +stones are of many kinds, and numbers of them have been ground to flat +faces. Where the front of the ice has retreated the rock surface is +seen to be planed and scored in places by the stones frozen fast in +the sole of the glacier. + + [Illustration: Fig. 92. A Greenland Glacier] + +We have now found in glacier ice an agent able to produce the drift of +North America. The ice sheet of Greenland is now doing what we have +seen was done in the recent past in our own land. It is carrying for +long distances rocks of many kinds gathered, we may infer, over a +large extent of country. It is laying down its load without assortment +in unstratified deposits. It grinds down and scores the rock over +which it moves, and in the process many of the pebbles of its load are +themselves also ground smooth and scratched. Since this work can be +done by no other agent, we must conclude that the northeastern part of +our own continent was covered in the recent past by glacier ice, as +Greenland is to-day. + + +Valley Glaciers + +The work of glacier ice can be most conveniently studied in the +separate ice streams which creep down mountain valleys in many regions +such as Alaska, the western mountains of the United States and Canada, +the Himalayas, and the Alps. As the glaciers of the Alps have been +studied longer and more thoroughly than any others, we shall describe +them in some detail as examples of valley glaciers in all parts of the +world. + +=Conditions of glacier formation.= The condition of the great +accumulation of snow to which glaciers are due--that more or less of +each winter's snow should be left over unmelted and unevaporated to +the next--is fully met in the Alps. There is abundant moisture brought +by the winds from neighboring seas. The currents of moist air driven +up the mountain slopes are cooled by their own expansion as they rise, +and the moisture which they contain is condensed at a temperature at +or below 32° F., and therefore is precipitated in the form of snow. +The summers are cool and their heat does not suffice to completely +melt the heavy snow of the preceding winter. On the Alps the _snow +line_--the lower limit of permanent snow--is drawn at about eight +thousand five hundred feet above sea level. Above the snow line on the +slopes and crests, where these are not too steep, the snow lies the +year round and gathers in valley heads to a depth of hundreds of feet. + + [Illustration: Fig. 93. Glaciers heading in Snow-filled + Amphitheaters, the Alps] + + [Illustration: Fig. 94. Bergschrund of a Glacier in Colorado] + +This is but a small fraction of the thickness to which snow would be +piled on the Alps were it not constantly being drained away. Below the +snow fields which mantle the heights the mountain valleys are occupied +by glaciers which extend as much as a vertical mile below the snow +line. The presence in the midst of forests and meadows and cultivated +fields of these tongues of ice, ever melting and yet from year to year +losing none of their bulk, proves that their loss is made good in the +only possible way. They are fed by snow fields above, whose surplus of +snow they drain away in the form of ice. The presence of glaciers +below the snow line is a clear proof that, rigid and motionless as +they appear, glaciers really are in constant motion down valley. + +=The névé field.= The head of an Alpine valley occupied by a glacier +is commonly a broad amphitheater deeply filled with snow (Fig. 93). +Great peaks tower above it, and snowy slopes rise on either side on +the flanks of mountain spurs. From these heights fierce winds drift +the snows into the amphitheater, and avalanches pour in their torrents +of snow and waste. The snow of the amphitheater is like that of drifts +in late winter after many successive thaws and freezings. It is made +of hard grains and pellets and is called _névé_. Beneath the surface +of the névé field and at its outlet the granular névé has been +compacted to a mass of porous crystalline ice. Snow has been changed +to névé, and névé to glacial ice, both by pressure, which drives the +air from the interspaces of the snowflakes, and also by successive +meltings and freezings, much as a snowball is packed in the warm hand +and becomes frozen to a ball of ice. + + [Illustration: Fig. 95. Sea Wall of the Muir Glacier, Alaska] + +=The bergschrund.= The névé is in slow motion. It breaks itself loose +from the thinner snows about it, too shallow to share its motion, and +from the rock rim which surrounds it, forming a deep fissure called +the bergschrund, sometimes a score and more feet wide (Fig. 94). + +=Size of glaciers.= The ice streams of the Alps vary in size according +to the amount of precipitation and the area of the névé fields which +they drain. The largest of Alpine glaciers, the Aletsch, is nearly ten +miles long and has an average width of about a mile. The thickness of +some of the glaciers of the Alps is as much as a thousand feet. Giant +glaciers more than twice the length of the longest in the Alps occur +on the south slope of the Himalaya Mountains, which receive frequent +precipitations of snow from moist winds from the Indian Ocean. The +best known of the many immense glaciers of Alaska, the Muir, has an +area of about eight hundred square miles (Fig. 95). + + [Illustration: Fig. 96. Diagram showing Movement of Row of + + Stakes _a_, set in a direct line across the surface of a glacier; + _b_, _c_, and _d_, successive later positions of the stakes] + + [Illustration: Fig. 97. Diagram showing Movement of Vertical + Row of Stakes _a_, set on side of glacier] + +=Glacier motion.= The motion of the glaciers of the Alps seldom +exceeds one or two feet a day. Large glaciers, because of the enormous +pressure of their weight and because of less marginal resistance, move +faster than small ones. The Muir advances at the rate of seven feet a +day, and some of the larger tide glaciers of Greenland are reported to +move at the exceptional rate of fifty feet and more in the same time. +Glaciers move faster by day than by night, and in summer than in +winter. Other laws of glacier motion may be discovered by a study of +Figures 96 and 97. It is important to remember that glaciers do not +slide bodily over their beds, but urged by gravity move slowly down +valley in somewhat the same way as would a stream of thick mud. +Although small pieces of ice are brittle, the large mass of granular +ice which composes a glacier acts as a viscous substance. + + [Illustration: Fig. 98. Crevasses of a Glacier, Canada] + +=Crevasses.= Slight changes of slope in the glacier bed, and the +different rates of motion in different parts, produce tensions under +which the ice cracks and opens in great fissures called crevasses. At +an abrupt descent in the bed the ice is shattered into great +fragments, which unite again below the icefall. Crevasses are opened +on lines at right angles to the direction of the tension. _Transverse +crevasses_ are due to a convexity in the bed which stretches the ice +lengthwise (Fig. 99). _Marginal crevasses_ are directed upstream and +inwards; _radial crevasses_ are found where the ice stream deploys +from some narrow valley and spreads upon some more open space. What is +the direction of the tension which causes each and to what is it due? +(Figs. 100 and 101). + + [Illustration: Fig. 99. Longitudinal Section of a Portion of a + Glacier, showing Traverse Crevasses] + + [Illustration: Fig. 100. Map view of Marginal Crevasses] + + [Illustration: Fig. 101. The Rhone Glacier, showing Radial + Crevasses, the Alps] + + [Illustration: Fig. 102. Map View of the Junction of Two + Branches of a Glacier + + The moraines are represented by broken lines] + +=Lateral and medial moraines.= The surface of a glacier is striped +lengthwise by long dark bands of rock debris. Those in the center are +called the medial moraines. The one on either margin is a lateral +moraine, and is clearly formed of waste which has fallen on the edge +of the ice from the valley slopes. A medial moraine cannot be formed +in this way, since no rock fragments can fall so far out from the +sides. But following it up the glacial stream, one finds that a medial +moraine takes its beginning at the junction of the glacier and some +tributary and is formed by the union of their two adjacent lateral +moraines (Fig. 102). Each branch thus adds a medial moraine, and by +counting the number of medial moraines of a trunk stream one may learn +of how many branches it is composed. + + [Illustration: Fig. 103. Cross Section of a Glacier showing + Lateral Moraines + + _l_, _l_, and Medial Moraines _m_, _m_] + +Surface moraines appear in the lower course of the glacier as ridges, +which may reach the exceptional height of one hundred feet. The bulk +of such a ridge is ice. It has been protected from the sun by the +veneer of moraine stuff; while the glacier surface on either side has +melted down at least the distance of the height of the ridge. In +summer the lowering of the glacial surface by melting goes on rapidly. +In Swiss glaciers it has been estimated that the average lowering of +the surface by melting and evaporation amounts to ten feet a year. As +a moraine ridge grows higher and more steep by the lowering of the +surface of the surrounding ice, the stones of its cover tend to slip +down its sides. Thus moraines broaden, until near the terminus of a +glacier they may coalesce in a wide field of stony waste. + + [Illustration: Fig. 104. Glacier with Medial Moraines, the Alps + + Is the ice moving from or towards the observer?] + +=Englacial drift.= This name is applied to whatever debris is carried +within the glacier. It consists of rock waste fallen on the névé and +there buried by accumulations of snow, and of that engulfed in the +glacier where crevasses have opened beneath a surface moraine. As the +surface of the glacier is lowered by melting, more or less englacial +drift is brought again to open air, and near the terminus it may help +to bury the ice from view beneath a sheet of debris. + +=The ground moraine.= The drift dragged along at the glacier's base +and lodged beneath it is known as the ground moraine. Part of the +material of it has fallen down deep crevasses and part has been torn +and worn from the glacier's bed and banks. While the stones of the +surface moraines remain as angular as when they lodged on the ice, +many of those of the ground moraine have been blunted on the edges and +faceted and scratched by being ground against one another and the +rocky bed. + +In glaciers such as those of Greenland, whose basal layers are well +loaded with drift and whose surface layers are nearly clean, different +layers have different rates of motion, according to the amount of +drift with which they are clogged. One layer glides over another, and +the stones inset in each are ground and smoothed and scratched. +Usually the sides of glaciated pebbles are more worn than the ends, +and the scratches upon them run with the longer axis of the stone. +Why? + +=The terminal moraine.= As a glacier is in constant motion, it brings +to its end all of its load except such parts of the ground moraine as +may find permanent lodgment beneath the ice. Where the glacier front +remains for some time at one place, there is formed an accumulation of +drift known as the terminal moraine. In valley glaciers it is shaped +by the ice front to a crescent whose convex side is downstream. Some +of the pebbles of the terminal moraine are angular, and some are +faceted and scored, the latter having come by the hard road of the +ground moraine. The material of the dump is for the most part +unsorted, though the water of the melting ice may find opportunity to +leave patches of stratified sands and gravels in the midst of the +unstratified mass of drift, and the finer material is in places washed +away. + + [Illustration: Fig. 105. Terminal Moraine of a Glacier in Montana + + The ice has melted back from the morainic ridge on the left and + is building another on the right. The hollow between the ridges + is occupied by a lakelet.] + +=Glacier drainage.= The terminal moraine is commonly breached by a +considerable stream, which issues from beneath the ice by a tunnel +whose portal has been enlarged to a beautiful archway by melting +in the sun and the warm air (Fig. 107). The stream is gray with +silt and loaded with sand and gravel washed from the ground +moraine. "Glacier milk" the Swiss call this muddy water, the gray +color of whose silt proves it rock flour freshly ground by the ice +from the unoxidized sound rock of its bed, the mud of streams +being yellowish when it is washed from the oxidized mantle of +waste. Since glacial streams are well loaded with waste due to +vigorous ice erosion, the valley in front of the glacier is +commonly aggraded to a broad, flat floor. These outwash deposits +are known as _valley drift_. + + [Illustration: Fig. 106. Heavy Moraine about the Terminus of a + Glacier in the Rocky Mountains of Canada + + Account for the fact that the morainic ridge rises considerably + above the surface of the ice] + +The sand brought out by streams from beneath a glacier differs from +river sand in that it consists of freshly broken angular grains. Why? + +The stream derives its water chiefly from the surface melting of the +glacier. As the ice is touched by the rays of the morning sun in +summer, water gathers in pools, and rills trickle and unite in +brooklets which melt and cut shallow channels in the blue ice. The +course of these streams is short. Soon they plunge into deep wells cut +by their whirling waters where some crevasse has begun to open across +their path. These wells lead into chambers and tunnels by which sooner +or later their waters find way to the rock floor of the valley and +there unite in a subglacial stream. + + [Illustration: Fig. 107. Subglacial Stream Issuing from Tunnel + in the Ice, Norway] + +=The lower limit of glaciers.= The glaciers of a region do not by any +means end at a uniform height above sea level. Each terminates where +its supply is balanced by melting. Those therefore which are fed by +the largest and deepest névés and those also which are best protected +from the sun by a northward exposure or by the depth of their +inclosing valleys flow to lower levels than those whose supply is less +and whose exposure to the sun is greater. + +A series of cold, moist years, with an abundant snowfall, causes +glaciers to thicken and advance; a series of warm, dry years causes +them to wither and melt back. The variation in glaciers is now +carefully observed in many parts of the world. The Muir glacier has +retreated two miles in twenty years. The glaciers of the Swiss Alps +are now for the most part melting back, although a well-known glacier +of the eastern Alps, the Vernagt, advanced five hundred feet in the +year 1900, and was then plowing up its terminal moraine. + +How soon would you expect a glacier to advance after its névé fields +have been swollen with unusually heavy snows, as compared with the +time needed for the flood of a large river to reach its mouth after +heavy rains upon its headwaters? + + [Illustration: Fig. 108. A Glacier Table] + +On the surface of glaciers in summer time one may often see large +stones supported by pillars of ice several feet in height (Fig. 108). +These "glacier tables" commonly slope more or less strongly to the +south, and thus may be used to indicate roughly the points of the +compass. Can you explain their formation and the direction of their +slope? On the other hand, a small and thin stone, or a patch of dust, +lying on the ice, tends to sink a few inches into it. Why? + +In what respects is a valley glacier like a mountain stream which +flows out upon desert plains? + +Two confluent glaciers do not mingle their currents as do two +confluent rivers. What characteristics of surface moraines prove this +fact? + +What effect would you expect the laws of glacier motion to have on the +slant of the sides of transverse crevasses? + + [Illustration: Fig. 109. Map of Malaspina Glacier, Alaska] + +A trunk glacier has four medial moraines. Of how many tributaries is +it composed? Illustrate by diagram. + +State all the evidences which you have found that glaciers move. + +If a glacier melts back with occasional pauses up a valley, what +records are left of its retreat? + + [Illustration: Fig. 110. Outwash Plain, the Delta of the Yahtse + River, Alaska] + + +Piedmont Glaciers + +=The Malaspina glacier.= Piedmont (foot of the mountain) glaciers are, +as the name implies, ice fields formed at the foot of mountains by the +confluence of valley glaciers. The Malaspina glacier of Alaska, the +typical glacier of this kind, is seventy miles wide and stretches for +thirty miles from the foot of the Mount Saint Elias range to the shore +of the Pacific Ocean. The valley glaciers which unite and spread to +form this lake of ice lie above the snow line and their moraines are +concealed beneath névé. The central area of the Malaspina is also +free from debris; but on the outer edge large quantities of englacial +drift are exposed by surface melting and form a belt of morainic waste +a few feet thick and several miles wide, covered in part with a +luxuriant forest, beneath which the ice is in places one thousand feet +in depth. The glacier here is practically stagnant, and lakes a few +hundred yards across, which could not exist were the ice in motion and +broken with crevasses, gather on their beds sorted waste from the +moraine. The streams which drain the glacier have cut their courses in +englacial and subglacial tunnels; none flow for any distance on the +surface. The largest, the Yahtse River, issues from a high archway in +the ice,--a muddy torrent one hundred feet wide and twenty feet deep, +loaded with sand and stones which it deposits in a broad outwash plain +(Fig. 110). Where the ice has retreated from the sea there is left a +hummocky drift sheet with hollows filled with lakelets. These deposits +help to explain similar hummocky regions of drift and similar plains +of coarse, water-laid material often found in the drift-covered area +of the northeastern United States. + + +The Geological Work Of Glacier Ice + +The sluggish glacier must do its work in a different way from the +agile river. The mountain stream is swift and small, and its channel +occupies but a small portion of the valley. The glacier is slow and +big; its rate of motion may be less than a millionth of that of +running water over the same declivity, and its bulk is proportionately +large and fills the valley to great depth. Moreover, glacier ice is a +solid body plastic under slowly applied stresses, while the water of +rivers is a nimble fluid. + +=Transportation.= Valley glaciers differ from rivers as carriers in +that they float the major part of their load upon their surface, +transporting the heaviest bowlder as easily as a grain of sand; while +streams push and roll much of their load along their beds, and their +power of transporting waste depends solely upon their velocity. The +amount of the surface load of glaciers is limited only by the amount +of waste received from the mountain slopes above them. The moving +floor of ice stretched high across a valley sweeps along as lateral +moraines much of the waste which a mountain stream would let +accumulate in talus and alluvial cones. + +While a valley glacier carries much of its load on top, an ice sheet, +such as that of Greenland, is free from surface debris, except where +moraines trail away from some nunatak. If at its edge it breaks into +separate glaciers which drain down mountain valleys, these tongues of +ice will carry the selvages of waste common to valley glaciers. Both +ice sheets and valley glaciers drag on large quantities of rock waste +in their ground moraines. + +Stones transported by glaciers are sometimes called erratics. Such are +the bowlders of the drift of our northern states. Erratics may be set +down in an insecure position on the melting of the ice. + +=Deposit.= Little need be added here to what has already been said of +ground and terminal moraines. All strictly glacial deposits are +unstratified. The load laid down at the end of a glacier in the +terminal moraine is loose in texture, while the drift lodged beneath +the glacier as ground moraine is often an extremely dense, stony clay, +having been compacted under the pressure of the overriding ice. + +=Erosion.= A glacier erodes its bed and banks in two ways,--by +abrasion and by plucking. + +The rock bed over which a glacier has moved is seen in places to have +been abraded, or ground away, to smooth surfaces which are marked by +long, straight, parallel scorings aligned with the line of movement of +the ice and varying in size from hair lines and coarse scratches to +exceptional furrows several feet deep. Clearly this work has been +accomplished by means of the sharp sand, the pebbles, and the larger +stones with which the base of the glacier is inset, and which it holds +in a firm grasp as running water cannot. Hard and fine-grained rocks, +such as granite and quartzite, are often not only ground down to a +smooth surface but are also highly polished by means of fine rock +flour worn from the glacier bed. + +In other places the bed of the glacier is rough and torn. The rocks +have been disrupted and their fragments have been carried away,--a +process known as _plucking_. Moving under immense pressure the ice +shatters the rock, breaks off projections, presses into crevices and +wedges the rocks apart, dislodges the blocks into which the rock is +divided by joints and bedding planes, and freezing fast to the +fragments drags them on. In this work the freezing and thawing of +subglacial waters in any cracks and crevices of the rock no doubt play +an important part. Plucking occurs especially where the bed rock is +weak because of close jointing. The product of plucking is bowlders, +while the product of abrasion is fine rock flour and sand. + +Is the ground moraine of Figure 87 due chiefly to abrasion or to +plucking? + + [Illustration: Fig. 111. Roches Moutonnés, Bronx Park, New York] + +=Roches moutonnées and rounded hills.= The prominences left between +the hollows due to plucking are commonly ground down and rounded on +the stoss side,--the side from which the ice advances,--and sometimes +on the opposite, the lee side, as well. In this way the bed rock often +comes to have a billowy surface known as roches moutonnées (sheep +rocks). Hills overridden by an ice sheet often have similarly rounded +contours on the stoss side, while on the lee side they may be craggy, +either because of plucking or because here they have been less worn +from their initial profile (Fig. 112). + +=The direction of glacier movement.= The direction of the flow of +vanished glaciers and ice sheets is recorded both in the differences +just mentioned in the profiles of overridden hills and also in the +minute details of the glacier trail. + +Flint nodules or other small prominences in the bed rock are found +more worn on the stoss than on the lee side, where indeed they may +have a low cone of rock protected by them from abrasion. Cavities, on +the other hand, have their edges worn on the lee side and left sharp +upon the stoss. + +Surfaces worn and torn in the ways which we have mentioned are said to +be glaciated. But it must not be supposed that a glacier everywhere +glaciates its bed. Although in places it acts as a rasp or as a pick, +in others, and especially where its pressure is least, as near the +terminus, it moves over its bed in the manner of a sled. Instances are +known where glaciers have advanced over deposits of sand and gravel +without disturbing them to any notable degree. Like a river, a glacier +does not everywhere erode. In places it leaves its bed undisturbed and +in places aggrades it by deposits of the ground moraine. + + [Illustration: Fig. 112. A Glaciated Hill, Norway. Sharp + Weathered Mountain Peaks in the Distance] + +=Cirques.= Valley glaciers commonly head as we have seen, in broad +amphitheaters deeply filled with snow and ice. On mountains now +destitute of glaciers, but whose glaciation shows that they have +supported glaciers in the past, there are found similar crescentic +hollows with high, precipitous walls and glaciated floors. Their +floors are often basined and hold lakelets whose deep and quiet waters +reflect the sheltering ramparts of rugged rock which tower far above +them. Such mountain hollows are termed _cirques_. As a powerful spring +wears back a recess in the valley side where it discharges, so the +fountain head of a glacier gradually wears back a cirque. In its slow +movement the névé field broadly scours its bed to a flat or basined +floor. Meanwhile the sides of the valley head are steepened and driven +back to precipitous walls. For in winter the crevasse of the +bergschrund which surrounds the névé field is filled with snow and the +névé is frozen fast to the rocky sides of the valley. In early summer +the névé tears itself free, dislodging and removing any loosened +blocks, and the open fissure of the bergschrund allows frost and other +agencies of weathering to attack the unprotected rock. As cirques are +thus formed and enlarged the peaks beneath which they lie are +sharpened, and the mountain crests are scalloped and cut back from +either side to knife-edged ridges (Figs. 113 and 93). + + [Illustration: Fig. 113. Cirques, Sierra Nevada Mountains] + +In the western mountains of the United States many cirques, now empty +of névé and glacier ice, and known locally as "basins," testify to the +fact that in recent times the snow line stood beneath the levels of +their floors, and thus far below its present altitude. + + [Illustration: Fig. 114. A Glacier Trough, Montana] + +=Glacier troughs.= The channel worn to accommodate the big and clumsy +glacier differs markedly from the river valley cut as with a saw by +the narrow and flexible stream and widened by the weather and the wash +of rains. The valley glacier may easily be from one thousand to three +thousand feet deep and from one to three miles wide. Such a ponderous +bulk of slowly moving ice does not readily adapt itself to sharp turns +and a narrow bed. By scouring and plucking all resisting edges it +develops a fitting channel with a wide, flat floor, and steep, smooth +sides, above which are seen the weathered slopes of stream-worn +mountain valleys. Since the trunk glacier requires a deeper channel +than do its branches, the bed of a branch glacier enters the main +trough at some distance above the floor of the latter, although the +surface of the two ice streams may be accordant. Glacier troughs can +be studied best where large glaciers have recently melted completely +away, as is the case in many valleys of the mountains of the western +United States and of central and northern Europe (Fig. 114). The +typical glacier trough, as shown in such examples, is U-shaped, with a +broad, flat floor, and high, steep walls. Its walls are little broken +by projecting spurs and lateral ravines. It is as if a V-valley cut by +a river had afterwards been gouged deeper with a gigantic chisel, +widening the floor to the width of the chisel blade, cutting back the +spurs, and smoothing and steepening the sides. A river valley could +only be as wide-floored as this after it had long been worn down to +grade. + + [Illustration: Fig. 115 Lynn Canal, Alaska, a Fjord] + +But the floor of a glacier trough may not be graded; it is often +interrupted by irregular steps perhaps hundreds and even a thousand +feet in height, over which the stream that now drains the valley +tumbles in waterfalls. Reaches between the steps are often basined. +Lakelets may occupy hollows excavated in solid rock, and other lakes +may be held behind terminal moraines left as dams across the valley at +pauses in the retreat of the glacier. + +=Fjords= are glacier troughs now occupied in part or wholly by the +sea, either because they were excavated by a tide glacier to their +present depth below sea level, or because of a submergence of the +land. Their characteristic form is that of a long, deep, narrow bay +with steep rock walls and basined floor (Fig. 115). Fjords are found +only in regions which have suffered glaciation, such as Norway and +Alaska. + + [Illustration: Fig. 116. _A_, V-River Valley, with Valley of + Tributary joining it a Accordant Level; _B_, the Same changed + after Long Glaciation to a Trough with Hanging Valley] + +=Hanging valleys.= These are lateral valleys which open on their main +valley some distance above its floor. They are conspicuous features of +glacier troughs from which the ice has vanished; for the trunk glacier +in widening and deepening its channel cut its bed below the bottoms of +the lateral valleys (Fig. 116). + +Since the mouths of hanging valleys are suspended on the walls of the +glacier trough, their streams are compelled to plunge down its steep, +high sides in waterfalls. Some of the loftiest and most beautiful +waterfalls of the world leap from hanging valleys,--among them the +celebrated Staubbach of the Lauterbrunnen valley of Switzerland, and +those of the fjords of Norway and Alaska (Fig. 117). + + [Illustration: Fig. 117. Hanging Valley on the Wall of a Fjord, + Norway] + +Hanging valleys are found also in river gorges where the smaller +tributaries have not been able to keep pace with a strong master +stream in cutting down their beds. In this case, however, they are a +mark of extreme youth; for, as the trunk stream approaches grade and +its velocity and power to erode its bed decrease, the side streams +soon cut back their falls and wear their beds at their mouths to a +common level with that of the main river. The Grand Canyon of the +Colorado must be reckoned a young valley. At its base it narrows to +scarcely more than the width of the river, and yet its tributaries, +except the very smallest, enter it at a common level. + +Why could not a wide-floored valley, such as a glacier trough, with +hanging valleys opening upon it, be produced in the normal development +of a river valley? + +=The troughs of young and of mature glaciers.= The features of a +glacier trough depend much on the length of time the preexisting +valley was occupied with ice. During the infancy of a glacier, we may +believe, the spurs of the valley which it fills are but little blunted +and its bed is but little broken by steps. In youth the glacier +develops icefalls, as a river in youth develops waterfalls, and its +bed becomes terraced with great stairs. The mature glacier, like the +mature river, has effaced its falls and smoothed its bed to grade. It +has also worn back the projecting spurs of its valley, making itself a +wide channel with smooth sides. The bed of a mature glacier may form a +long basin, since it abrades most in its upper and middle course, +where its weight and motion are the greatest. Near the terminus, where +weight and motion are the least, it erodes least, and may instead +deposit a sheet of ground moraine, much as a river builds a flood +plain in the same part of its course as it approaches maturity. The +bed of a mature glacier thus tends to take the form of a long, +relatively narrow basin, across whose lower end may be stretched the +dam of the terminal moraine. On the disappearance of the ice the basin +is rilled with a long, narrow lake, such as Lake Chelan in Washington +and many of the lakes in the Highlands of Scotland. + +Piedmont glaciers apparently erode but little. Beneath their lake-like +expanse of sluggish or stagnant ice a broad sheet of ground moraine is +probably being deposited. + +Cirques and glaciated valleys rapidly lose their characteristic forms +after the ice has withdrawn. The weather destroys all smoothed, +polished, and scored surfaces which are not protected beneath glacial +deposits. The over-steepened sides of the trough are graded by +landslips, by talus slopes, and by alluvial cones. Morainic heaps of +drift are dissected and carried away. Hanging valleys and the +irregular bed of the trough are both worn down to grade by the streams +which now occupy them. The length of time since the retreat of the ice +from a mountain valley may thus be estimated by the degree to which +the destruction of the characteristic features of the glacier trough +has been carried. + +In Figure 104 what characteristics of a glacier trough do you notice? +What inference do you draw as to the former thickness of the glacier? + +Name all the evidences you would expect to find to prove the fact that +in the recent geological past the valleys of the Alps contained far +larger glaciers than at present, and that on the north of the Alps the +ice streams united in a piedmont glacier which extended across the +plains of Switzerland to the sides of the Jura Mountains. + +=The relative importance of glaciers and of rivers.= Powerful as +glaciers are, and marked as are the land forms which they produce, it +is easy to exaggerate their geological importance as compared with +rivers. Under present climatic conditions they are confined to lofty +mountains or polar lands. Polar ice sheets are permanent only so long +as the lands remain on which they rest. Mountain glaciers can stay +only the brief time during which the ranges continue young and high. +As lofty mountains, such as the Selkirks and the Alps, are lowered by +frost and glacier ice, the snowfall will decrease, the line of +permanent snow will rise, and as the mountain hollows in which snow +may gather are worn beneath the snow line, the glaciers must +disappear. Under present climatic conditions the work of glaciers is +therefore both local and of short duration. + + [Illustration: Fig. 118. Longitudinal Section of a Tide Glacier + occupying a Fjord and discharging Icebergs + Dotted Line, sea level] + +Even the glacial epoch, during which vast ice sheets deposited drift +over northeastern North America, must have been brief as well as +recent, for many lofty mountains, such as the Rockies and the Alps, +still bear the marks of great glaciers which then filled their +valleys. Had the glacial epoch been long, as the earth counts time, +these mountains would have been worn low by ice; had the epoch been +remote, the marks of glaciation would already have been largely +destroyed by other agencies. + +On the other hand, rivers are well-nigh universally at work over the +land surfaces of the globe, and ever since the dry land appeared they +have been constantly engaged in leveling the continents and in +delivering to the seas the waste which there is built into the +stratified rocks. + +=Icebergs.= Tide glaciers, such as those of Greenland and Alaska, are +able to excavate their beds to a considerable distance below sea +level. From their fronts the buoyancy of sea water raises and breaks +away great masses of ice which float out to sea as icebergs. Only +about one seventh of a mass of glacier ice floats above the surface, +and a berg three hundred feet high may be estimated to have been +detached from a glacier not less than two thousand feet thick where it +met the sea. + +Icebergs transport on their long journeys whatever drift they may have +carried when part of the glacier, and scatter it, as they melt, over +the ocean floor. In this way pebbles torn by the inland ice from the +rocks of the interior of Greenland and glaciated during their carriage +in the ground moraine are dropped at last among the oozes of the +bottom of the North Atlantic. + + + + +CHAPTER VI + +THE WORK OF THE WIND + + + [Illustration: Fig. 119. A sandy Region in a Desert, the Sahara] + +We are now to study the geological work of the currents of the +atmosphere, and to learn how they erode, and transport and deposit +waste as they sweep over the land. Illustrations of the wind's work +are at hand in dry weather on any windy day. + +Clouds of dust are raised from the street and driven along by the +gale. Here the roadway is swept bare; and there, in sheltered places, +the dust settles in little windrows. The erosive power of waste-laden +currents of air is suggested as the sharp grains of flying sand sting +one's face or clatter against the window. In the country one sometimes +sees the dust whirled in clouds from dry, plowed fields in spring and +left in the lee of fences in small drifts resembling in form those of +snow in winter. + +=The essential conditions= for the wind's conspicuous work are +illustrated in these simple examples; they are aridity and the absence +of vegetation. In humid climates these conditions are only rarely and +locally met; for the most part a thick growth of vegetation protects +the moist soil from the wind with a cover of leaves and stems and a +mattress of interlacing roots. But in arid regions either vegetation +is wholly lacking, or scant growths are found huddled in detached +clumps, leaving interspaces of unprotected ground (Fig. 119). Here, +too, the mantle of waste, which is formed chiefly under the action of +temperature changes, remains dry and loose for long periods. Little or +no moisture is present to cause its particles to cohere, and they are +therefore readily lifted and drifted by the wind. + + +Transportation By The Wind + +In the desert the finer waste is continually swept to and fro by the +ever-shifting wind. Even in quiet weather the air heated by contact +with the hot sands rises in whirls, and the dust is lifted in stately +columns, sometimes as much as one thousand feet in height, which march +slowly across the plain. In storms the sand is driven along the ground +in a continuous sheet, while the air is tilled with dust. Explorers +tell of sand storms in the deserts of central Asia and Africa, in +which the air grows murky and suffocating. Even at midday it may +become dark as night, and nothing can be heard except the roar of the +blast and the whir of myriads of grains of sand as they fly past the +ear. + +Sand storms are by no means uncommon in the arid regions of the +western United States. In a recent year, six were reported from Yuma, +Arizona. Trains on transcontinental railways are occasionally +blockaded by drifting sand, and the dust sifts into closed passenger +coaches, covering the seats and floors. After such a storm thirteen +car loads of sand were removed from the platform of a station on a +western railway. + +=Dust falls.= Dust launched by upward-whirling winds on the swift +currents of the upper air is often blown for hundreds of miles beyond +the arid region from which it was taken. Dust falls from western +storms are not unknown even as far east as the Great Lakes. In 1896 a +"black snow" fell in Chicago, and in another dust storm in the same +decade the amount of dust carried in the air over Rock Island, Ill., +was estimated at more than one thousand tons to the cubic mile. + + [Illustration: Fig. 120. A Tract of Rocky Desert, Arabia + By what process have these rocks been broken up? + Why is finer waste here absent?] + +In March, 1901, a cyclonic storm carried vast quantities of dust from +the Sahara northward across the Mediterranean to fall over southern +and central Europe. On March 8th dust storms raged in southern +Algeria; two days later the dust fell in Italy; and on the 11th it had +reached central Germany and Denmark. It is estimated that in these few +days one million eight hundred thousand tons of waste were carried +from northern Africa and deposited on European soil. + +We may see from these examples the importance of the wind as an agent +of transportation, and how vast in the aggregate are the loads which +it carries. There are striking differences between air and water as +carriers of waste. Rivers flow in fixed and narrow channels to +definite goals. The channelless streams of the air sweep across broad +areas, and, shifting about continually, carry their loads back and +forth, now in one direction and now in another. + + +Wind Deposits + +The mantle of waste of deserts is rapidly sorted by the wind. The +coarser rubbish, too heavy to be lifted into the air, is left to strew +wide tracts with residual gravels (Fig. 120). The sand derived from +the disintegration of desert rocks gathers in vast fields. About one +eighth of the surface of the Sahara is said to be thus covered with +drifting sand. In desert mountains, as those of Sinai, it lies like +fields of snow in the high valleys below the sharp peaks. On more +level tracts it accumulates in seas of sand, sometimes, as in the +deserts of Arabia, two hundred and more feet deep. + + [Illustration: Fig. 121. Longitudinal Dunes, Desert of + Northwestern India + Scale, 1 in = 3 miles] + +=Dunes.= The sand thus accumulated by the wind is heaped in wavelike +hills called dunes. In the desert of northwestern India, where the +prevalent wind is of great strength, the sand is laid in longitudinal +dunes, i.e. in stripes running parallel with the direction of the +wind; but commonly dunes lie, like ripple marks, transverse to the +wind current. On the windward side they show a long, gentle slope, up +which grains of sand can readily be moved; while to the lee their +slope is frequently as great as the angle of repose (Fig. 122). Dunes +whose sands are not fixed by vegetation travel slowly with the wind; +for their material is ever shifted forward as the grains are driven up +the windward slope and, falling over the crest, are deposited in +slanting layers in the quiet of the lee. + + [Illustration: Fig. 122. A Transverse Dune, Seven Mile Beach, + New Jersey + Account for the difference of slope in the two sides of the + dune. Is the dune marching? In what direction? With what + effect? Do the ridges of the ripple marks upon the dune extend + along it or athwart it? Why?] + +Like river deposits, wind-blown sands are stratified, since they are +laid by currents of air varying in intensity, and therefore in +transporting power, which carry now finer and now coarser materials +and lay them down where their velocity is checked (Fig. 123). Since +the wind varies in direction, the strata dip in various directions. +They also dip at various angles, according to the inclination of the +surface on which they were laid. + + [Illustration: Fig. 123. Stratified Wind-Blown Sands, Bermuda + Islands + These islands are made wholly of limestone, the product of + reef-building corals, and of lime from the sea water. The + limestone sand of the beaches has been blown up into great + dunes, some more than two hundred feet in height. Much of the + loose dune sand has been changed to firm rock by percolating + waters, which have dissolved some of the limestone and + deposited it again as a cement between the grains] + +Dunes occur not only in arid regions, but also wherever loose sand +lies unprotected by vegetation from the wind. From the beaches of sea +and lake shores the wind drives inland the surface sand left dry +between tides and after storms, piling it in dunes which may invade +forests and fields and bury villages beneath their slowly advancing +waves. On flood plains during summer droughts river deposits are often +worked over by the wind; the sand is heaped in hummocks and much of +the fine silt is caught and held by the forests and grassy fields of +the bordering hills. + + [Illustration: Fig. 124. Cross Section of Transverse Dune after + Reversal of Wind + + Redraw diagram, showing by dotted line the original outline of + the dune] + +The sand of shore dunes differs little in composition and the shape of +its grains from that of the beach from which it was derived. But in +deserts, by the long wear of grain on grain as they are blown hither +and thither by the wind, all soft minerals are ground to powder and +the sand comes to consist almost wholly of smooth round grams of hard +quartz. + + [Illustration: Fig. 125. Dune Sands, Shore of Lake Michigan + + Account for the dead forest, for its leaning tree trunks. Is + the lake shore to the right or left? What has been the history + of the landscape?] + +Some marine sandstones, such as the St. Peter sandstone of the upper +Mississippi valley, are composed so entirely of polished spherules of +quartz that it has been believed by some that their grains were long +blown about in ancient deserts before they were deposited in the sea. + + [Illustration: Fig. 126. Crescentic Sand Dunes, Valley of the + Columbia River + + Did the wind which shaped them blow from the left or from the + right?] + +=Dust deposits.= As desert sands are composed almost wholly of quartz, +we may ask what has become of the softer minerals of which the rocks +whose disintegration has supplied the sand were in part, and often in +large part, composed. The softer minerals have been ground to powder, +and little by little the quartz sand also is worn by attrition to fine +dust. Yet dust deposits are scant and few in great deserts such as the +Sahara. The finer waste is blown beyond its limits and laid in +adjacent oceans, where it adds to the muds and oozes of their floors, +and on bordering steppes and forest lands, where it is bound fast by +vegetation and slowly accumulates in deposits of unstratified loose +yellow earth. The fine waste of the Sahara has been identified in +dredgings from the bottom of the Atlantic Ocean, taken hundreds of +miles from the coast of Africa. + +=Loess.= In northern China an area as large as France is deeply +covered with a yellow pulverulent earth called loess (German, loose), +which many consider a dust deposit blown from the great Mongolian +desert lying to the west. Loess mantles the recently uplifted +mountains to the height of eight thousand feet and descends on the +plains nearly to sea level. Its texture and lack of stratification +give it a vertical cleavage; hence it stands in steep cliffs on the +sides of the deep and narrow trenches which have been cut in it by +streams. + +On loess hillsides in China are thousands of villages whose eavelike +dwellings have been excavated in this soft, yet firm, dry loam. While +dust falls are common at the present time in this region, the loess is +now being rapidly denuded by streams, and its yellow silt gives name +to the muddy Hwang-ho (Yellow River), and to the Yellow Sea, whose +waters it discolors for scores of miles from shore. + +Wind deposits both of dust and of sand may be expected to contain the +remains of land shells, bits of wood, and bones of land animals, +testifying to the fact that they were accumulated in open air and not +in the sea or in bodies of fresh water. + + +Wind Erosion + + [Illustration: Fig. 127. Wind-Carved Rocks, Arizona] + +Sand-laden currents of air abrade and smooth and polish exposed rock +surfaces, acting in much the same way as does the jet of steam fed +with sharp sand, which is used in the manufacture of ground glass. +Indeed, in a single storm at Cape Cod a plate glass of a lighthouse +was so ground by flying sand that its transparency was destroyed and +its removal made necessary. + + [Illustration: Fig. 128. A Wind-Carved Pebble, Cape Cod] + +Telegraph poles and wires whetted by wind-blown sands are destroyed +within a few years. In rocks of unequal resistance the harder parts +are left in relief, while the softer are etched away. Thus in the pass +of San Bernardino, Cal., through which strong winds stream from the +west, crystals of garnet are left projecting on delicate rock fingers +from the softer rock in which they were imbedded. + +Wind-carved pebbles are characteristically planed, the facets meeting +along a summit ridge or at a point like that of a pyramid. We may +suppose that these facets were ground by prevalent winds from certain +directions, or that from time to time the stone was undermined and +rolled over as the sand beneath it was blown away on the windward +side, thus exposing fresh surfaces to the driving sand. Such +wind-carved pebbles are sometimes found in ancient rocks and may be +accepted as evidence that the sands of which the rocks are composed +were blown about by the wind. + +=Deflation.= In the denudation of an arid region, wind erosion is +comparatively ineffective as compared with deflation (Latin, _de_, +from; _flare_, to blow),--a term by which is meant the constant +removal of waste by the wind, leaving the rocks bare to the continuous +attack of the weather. In moist climates denudation is continually +impeded by the mantle of waste and its cover of vegetation, and the +land surface can be lowered no faster than the waste is removed by +running water. Deep residual soils come to protect all regions of +moderate slope, concealing from view the rock structure, and the +various forms of the land are due more to the agencies of erosion and +transportation than to differences in the resistance of the underlying +rocks. + + [Illustration: Fig. 129. Mesa Verde, Colorado + + In the distance on the left are high volcanic mountains. On the + extreme right are seen outliers of strata which once covered + the region of the mesa] + +But in arid regions the mantle is rapidly removed, even from well-nigh +level plains and plateaus, by the sweep of the wind and the wash of +occasional rains. The geological structure of these regions of naked +rock can be read as far as the eye can see, and it is to this +structure that the forms of the land are there largely due. In a land +mass of horizontal strata, for example, any softer surface rocks wear +down to some underlying, resistant stratum, and this for a while forms +the surface of a level plateau (Fig. 129). The edges of the capping +layer, together with those of any softer layers beneath it, wear back +in steep cliffs, dissected by the valleys of wet-weather streams and +often swept bare to the base by the wind. As they are little protected +by talus, which commonly is removed about as fast as formed, these +escarpments and the walls of the valleys retreat indefinitely, +exposing some hard stratum beneath which forms the floor of a widening +terrace. + +The high plateaus of northern Arizona and southern Utah (Fig. 130), +north of the Grand Canyon of the Colorado River, are composed of +stratified rocks more than ten thousand feet thick and of very gentle +inclination northward. From the broad plat form in which the canyon +has been cut rises a series of gigantic stairs, which are often more +than one thousand feet high and a score or more of miles in breadth. +The retreating escarpments, the cliffs of the mesas and buttes which +they have left behind as outliers, and the walls of the ravines are +carved into noble architectural forms--into cathedrals, pyramids, +amphitheaters, towers, arches, and colonnades--by the processes of +weathering aided by deflation. It is thus by the help of the action of +the wind that great plateaus in arid regions are dissected and at last +are smoothed away to waterless plains, either composed of naked rock, +or strewed with residual gravels, or covered with drifting residual +sand. + + [Illustration: Fig. 130. North-South Section, Eighty-Five Miles + Long, across the Plateau North of the Grand Canyon of the + Colorado River, Arizona, showing Retreating Escarpments + + _O_, outliers; _V_, canyon of the Colorado; _A-H_, rock systems + from the Archean to the Tertiary; _P_, platform of the plateau + from which the once overlying rocks have been stripped; dotted + lines indicate probable former extension of the strata. How + thick is the mass of strata which has been removed from over + the platform? Has this work been accomplished while the + Colorado River has been cutting its present canyon?] + +The specific gravity of air is 1/823 that of water. How does this fact +affect the weight of the material which each can carry at the same +velocity? + +If the rainfall should lessen in your own state to from five to ten +inches a year, what changes would take place in the vegetation of the +country? in the soil? in the streams? in the erosion of valleys? in +the agencies chiefly at work in denuding the land? + +In what way can a wind-carved pebble be distinguished from a +river-worn pebble? from a glaciated pebble? + + + + +CHAPTER VII + +THE SEA AND ITS SHORES + + + [Illustration: Fig. 131. Sea Cliff and Rock Bench Cut in Chalk, + Dover, England] + +We have already seen that the ocean is the goal at which the waste of +the land arrives. The mantle of rock waste, creeping down slopes, is +washed to the sea by streams, together with the material which the +streams have worn from their beds and that dissolved by underground +waters. In arid regions the winds sweep waste either into bordering +oceans or into more humid regions where rivers take it up and carry it +on to the sea. Glaciers deliver the load of their moraines either +directly to the sea or leave it for streams to transport to the same +goal. All deposits made on the land, such as the flood plains of +rivers, the silts of lake beds, dune sands, and sheets of glacial +drift, mark but pauses in the process which is to bring all the +materials of the land now above sea level to rest upon the ocean bed. + +But the sea is also at work along all its shores as an agent of +destruction, and we must first take up its work in erosion before we +consider how it transports and deposits the waste of the land. + + +Sea Erosion + +=The sea cliff and the rock bench.= On many coasts the land fronts the +ocean in a line of cliffs (Fig. 131). To the edge of the cliffs there +lead down valleys and ridges, carved by running water, which, if +extended, would meet the water surface some way out from shore. +Evidently they are now abruptly cut short at the present shore line +because the land has been cut back. + + [Illustration: Fig. 132. Diagram of Sea Cliff _sc_, and Rock Bench _rb_ + + The broken line indicates the former extent of the land.] + +Along the foot of the cliff lies a gently shelving bench of rock, more +or less thickly veneered with sand and shingle. At low tide its inner +margin is laid bare, but at high tide it is covered wholly, and the +sea washes the base of the cliffs. A notch, of which the _sea cliff_ +and the _rock bench_ are the two sides, has been cut along the shore +(Fig. 132). + +=Waves.= The position of the rock bench, with its inner margin +slightly above low tide, shows that it has been cut by some agent +which acts like a horizontal saw set at about sea level. This agent is +clearly the surface agitation of the water; it is the wind-raised +wave. + +As a wave comes up the shelving bench the crest topples forward and +the wave "breaks," striking a blow whose force is measured by the +momentum of all its tons of falling water (Fig. 133). On the coast of +Scotland the force of the blows struck by the waves of the heaviest +storms has sometimes exceeded three tons to the square foot. But even +a calm sea constantly chafes the shore. It heaves in gentle +undulations known as the ground swell, the result of storms perhaps a +thousand miles distant, and breaks on the shore in surf. + + [Illustration: Fig. 133. Breaking Wave, Lake Superior] + +The blows of the waves are not struck with clear water only, else they +would have little effect on cliffs of solid rock. Storm waves arm +themselves with the sand and gravel, the cobbles, and even the large +bowlders which lie at the base of the cliff, and beat against it with +these hammers of stone. + +Where a precipice descends sheer into deep water, waves swash up and +down the face of the rocks but cannot break and strike effective +blows. They therefore erode but little until the talus fallen from the +cliff is gradually built up beneath the sea to the level at which the +waves drag bottom upon it and break. + +Compare the ways in which different agents abrade. The wind lightly +brushes sand and dust over exposed surfaces of rock. Running water +sweeps fragments of various sizes along its channels, holding them +with a loose hand. Glacial ice grinds the stones of its ground moraine +against the underlying rock with the pressure of its enormous weight. +The wave hurls fragments of rock against the sea cliff, bruising and +battering it by the blow. It also rasps the bench as it drags sand and +gravel to and fro upon it. + +=Weathering of sea cliffs.= The sea cliff furnishes the weapons for +its own destruction. They are broken from it not only by the wave but +also by the weather. Indeed the sea cliff weathers more rapidly, as a +rule, than do rock ledges inland. It is abundantly wet with spray. +Along its base the ground water of the neighboring land finds its +natural outlet in springs which under mine it. Moreover, it is +unprotected by any shield of talus. Fragments of rock as they fall +from its face are battered to pieces by the waves and swept out to +sea. The cliff is thus left exposed to the attack of the weather, and +its retreat would be comparatively rapid for this reason alone. + + [Illustration: Fig. 134. Sea Caves, La Jolla, California + + Copyright, 1899, by the Detroit Photography Company] + +Sea cliffs seldom overhang, but commonly, as in Figure 134, slope +seaward, showing that the upper portion has retreated at a more rapid +rate than has the base. Which do you infer is on the whole the more +destructive agent, weathering or the wave? + +Draw a section of a sea cliff cut in well jointed rocks whose joints +dip toward the land. Draw a diagram of a sea cliff where the joints +dip toward the sea. + +=Sea caves.= The wave does not merely batter the face of the cliff. +Like a skillful quarryman it inserts wedges in all natural fissures, +such as joints, and uses explosive forces. As a wave flaps against a +crevice it compresses the air within with the sudden stroke; as it +falls back the air as suddenly expands. On lighthouses heavily barred +doors have been burst outward by the explosive force of the air +within, as it was released from pressure when a partial vacuum was +formed by the refluence of the wave. Where a crevice is filled with +water the entire force of the blow of the wave is transmitted by +hydraulic pressure to the sides of the fissure. Thus storm waves +little by little pry and suck the rock loose, and in this way, and by +the blows which they strike with the stones of the beach, they quarry +out about a joint, or wherever the rock may be weak, a recess known as +a _sea cave_, provided that the rock above is coherent enough to form +a roof. Otherwise an open chasm results. + + [Illustration: Fig. 135. A Sea Arch, California + + Copyright, 1899, by the Detroit Photography Company] + +=Blowholes and sea arches.= As a sea cave is drilled back into the +rock, it may encounter a joint or crevice opened to the surface by +percolating water. The shock of the waves soon enlarges this to a +blowhole, which one may find on the breezy upland, perhaps a hundred +yards and more back from the cliff's edge. In quiet weather the +blowhole is a deep well; in storm it plays a fountain as the waves +drive through the long tunnel below and spout their spray high in air +in successive jets. As the roof of the cave thus breaks down in the +rear, there may remain in front for a while a sea arch, similar to the +natural bridges of land caverns (Fig. 135). + + [Illustration: Fig. 136. Chasms worn by Waves, Coast of Scotland] + +=Stacks and wave-cut islands.= As the sea drives its tunnels and open +drifts into the cliff, it breaks through behind the intervening +portions and leaves them isolated as stacks, much as monuments are +detached from inland escarpments by the weather; and as the sea cliff +retreats, these remnant masses may be left behind as rocky islets. +Thus the rock bench is often set with stacks, islets in all stages of +destruction, and sunken reefs,--all wrecks of the land testifying to +its retreat before the incessant attack of the waves. + + [Illustration: Fig. 137. A Stack, Scotland] + + [Illustration: Fig. 138. Wave-Cut Islands, Scotland + + How far did the land once extend?] + +=Coves.= Where zones of soft or closely jointed rock outcrop along a +shore, or where minor water courses conic down to the sea and aid in +erosion, the shore is worn back in curved reëntrants called coves; +while the more resistant rocks on either hand are left projecting as +headlands (Fig. 139). After coves are cut back a short distance by the +waves, the headlands come to protect them, as with breakwaters, and +prevent their indefinite retreat. The shore takes a curve of +equilibrium, along which the hard rock of the exposed headland and the +weak rock of the protected cove wear back at an equal rate. + + [Illustration: Fig. 139. Coves formed in Softer Strata _S_, _S_; + while the Harder Strata _H_, _H_, are left as Headlands] + +=Rate of recession.= The rate at which a shore recedes depends on +several factors. In soft or incoherent rocks exposed to violent storms +the retreat is so rapid as to be easily measured. The coast of +Yorkshire, England, whose cliffs are cut in glacial drift, loses seven +feet a year on the average, and since the Norman conquest a strip a +mile wide, with farmsteads and villages and historic seaports, has +been devoured by the sea. The sandy south shore of Martha's Vineyard +wears back three feet a year. But hard rocks retreat so slowly that +their recession has seldom been measured by the records of history. + + [Illustration: Fig. 140. A Pebble Beach, Cape Ann, Massachusetts] + + +Shore Drift + +=Bowlder and pebble beaches.= About as fast as formed the waste of the +sea cliff is swept both along the shore and out to sea. The road of +waste along shore is the _beach_. We may also define the beach as the +exposed edge of the sheet of sediment formed by the carriage of land +waste out to sea. At the foot of sea cliffs, where the waves are +pounding hardest, one commonly finds the rock bench strewn on its +inner margin with large stones, dislodged by the waves and by the +weather and somewhat worn on their corners and edges. From this +_bowlder beach_ the smaller fragments of waste from the cliff and the +fragments into which the bowlders are at last broken drift on to more +sheltered places and there accumulate in a _pebble beach_, made of +pebbles well rounded by the wear which they have suffered. Such +beaches form a mill whose raw material is constantly supplied by the +cliff. The breakers of storms set it in motion to a depth of several +feet, grinding the pebbles together with a clatter to be heard above +the roar of the surf. In such a rock crusher the life of a pebble is +short. Where ships have stranded on our Atlantic coast with cargoes of +hard-burned brick or of coal, a year of time and a drift of five miles +along the shore have proved enough to wear brick and coal to powder. +At no great distance from their source, therefore, pebble beaches give +place to beaches of sand, which occupy the more sheltered reaches of +the shore. + +=Sand beaches.= The angular sand grains of various minerals into which +pebbles are broken by the waves are ground together under the beating +surf and rounded, and those of the softer minerals are crushed to +powder. The process, however, is a slow one, and if we study these +sand grains under a lens we may be surprised to see that, though their +corners and edges have been blunted, they are yet far from the +spherical form of the pebbles from which they were derived. The grains +are small, and in water they have lost about half their weight in +air; the blows which they strike one another are therefore weak. +Besides, each grain of sand of the wet beach is protected by a cushion +of water from the blows of its neighbors. + +The shape and size of these grains and the relative proportion of +grains of the softer minerals which still remain give a rough measure +of the distance in space and time which they have traveled from their +source. The sand of many beaches, derived from the rocks of adjacent +cliffs or brought in by torrential streams from neighboring highlands, +is dark with grains of a number of minerals softer than quartz. The +white sand of other beaches, as those of the east coast of Florida, is +almost wholly composed of quartz grains; for in its long travel down +the Atlantic coast the weaker minerals have been worn to powder and +the hardest alone survive. + +How does the absence of cleavage in quartz affect the durability of +quartz sand? + +=How shore drift migrates.= It is under the action of waves and +currents that shore drift migrates slowly along a coast. Where waves +strike a coast obliquely they drive the waste before them little by +little along the shore. Thus on a north-south coast, where the +predominant storms are from the northeast, there will be a migration +of shore drift southwards. + +All shores are swept also by currents produced by winds and tides. +These are usually far too gentle to transport of themselves the coarse +materials of which beaches are made. But while the wave stirs the +grains of sand and gravel, and for a moment lifts them from the +bottom, the current carries them a step forward on their way. The +current cannot lift and the wave cannot carry, but together the two +transport the waste along the shore. The road of shore drift is +therefore the zone of the breaking waves. + + [Illustration: Fig. 141. A Bay Bar, Lake Ontario] + +=The bay-head beach.= As the waste derived from the wear of waves and +that brought in by streams is trailed along a coast it assumes, under +varying conditions, a number of distinct forms. When swept into the +head of a sheltered bay it constitutes the bay-head beach. By the +highest storm waves the beach is often built higher than the ground +immediately behind it, and forms a dam inclosing a shallow pond or +marsh. + +=The bay bar.= As the stream of shore drift reaches the mouth of a bay +of some size it often occurs that, instead of turning in, it sets +directly across toward the opposite headland. The waste is carried out +from shore into the deeper waters of the bay mouth; where it is no +longer supported by the breaking waves, and sinks to the bottom. The +dump is gradually built to the surface as a stubby spur, pointing +across the bay, and as it reaches the zone of wave action current and +wave can now combine to carry shore drift along it, depositing their +load continually at the point of the spur. An embankment is thus +constructed in much the same manner as a railway fill, which, while it +is building, serves as a roadway along which the dirt from an adjacent +cut is carted to be dumped at the end. When the embankment is +completed it bridges the bay with a highway along which shore drift +now moves without interruption, and becomes a bay bar. + + [Illustration: Fig. 142. A Hook, Lake Michigan] + +=Incomplete bay bars.= Under certain conditions the sea cannot carry +out its intention to bridge a bay. Rivers discharging in bays demand +open way to the ocean. Strong tidal currents also are able to keep +open channels scoured by their ebb and flow. In such cases the most +that land waste can do is to build spits and shoals, narrowing and +shoaling the channel as much as possible. Incomplete bay bars +sometimes have their points recurved by currents setting at right +angles to the stream of shore drift and are then classified as _hooks_ +(Fig. 142). + + [Illustration: Fig. 143. Cross Section of Sand Reef _sr_, and + Lagoon; _sl_, Sea Level] + +=Sand reefs.= On low coasts where shallow water extends some distance +out, the highway of shore drift lies along a low, narrow ridge, termed +the sand reef, separated from the land by a narrow stretch of shallow +water called the _lagoon_ (Fig. 143). At intervals the reef is held +open by _inlets_,--gaps through which the tide flows and ebbs, and by +which the water of streams finds way to the sea. + + [Illustration: Fig. 144. Sand Reef and Lagoon, Texas] + +No finer example of this kind of shore line is to be found in the +world than the coast of Texas. From near the mouth of the Rio Grande a +continuous sand reef draws its even curve for a hundred miles to +Corpus Christi Pass, and the reefs are but seldom interrupted by +inlets as far north as Galveston Harbor. On this coast the tides are +variable and exceptionally weak, being less than one foot in height, +while the amount of waste swept along the shore is large. The lagoon +is extremely shallow, and much of it is a mud flat too shoal for even +small boats. On the coast of New Jersey strong tides are able to keep +open inlets at intervals of from two to twenty miles in spite of a +heavy alongshore drift. + +Sand reefs are formed where the water is so shallow near shore that +storm waves cannot run in it and therefore break some distance out +from land. Where storm waves first drag bottom they erode and deepen +the sea floor, and sweep in sediment as far as the line where they +break. Here, where they lose their force, they drop their load and +beat up the ridge which is known as the sand reef when it reaches the +surface. + + +Shores of Elevation and Depression + +Our studies have already brought to our notice two distinct forms of +strand lines,--one the high, rocky coast cut back to cliffs by the +attack of the waves, and the other the low, sandy coast where the +waves break usually upon the sand reef. To understand the origin of +these two types we must know that the meeting place of sea and land is +determined primarily by movements of the earth's crust. Where a coast +land emerges the--shore line moves seaward; where it is being +submerged the shore line advances on the land. + +=Shores of elevation.= The retreat of the sea, either because of a +local uplift of the land or for any other reason, such as the lowering +of any portion of ocean bottom, lays bare the inner margin of the sea +floor. Where the sea floor has long received the waste of the land it +has been built up to a smooth, subaqueous plain, gently shelving from +the land. Since the new shore line is drawn across this even surface +it is simple and regular, and is bordered on the one side by shallow +water gradually deepening seaward, and on the other by low land +composed of material which has not yet thoroughly consolidated to firm +rock. A sand reef is soon beaten up by the waves, and for some time +conditions will favor its growth. The loss of sand driven into the +lagoon beyond, and of that ground to powder by the surf and carried +out to sea, is more than made up by the stream of alongshore drift, +and especially by the drag of sediments to the reef by the waves as +they deepen the sea floor on its seaward side. + +Meanwhile the lagoon gradually fills with waste from the reef and from +the land. It is invaded by various grasses and reeds which have +learned to grow in salt and brackish water; the marsh, laid bare only +at low tide, is built above high tide by wind drift and vegetable +deposits, and becomes a meadow, soldering the sand reef to the +mainland. + +While the lagoon has been filling, the waves have been so deepening +the sea floor off the sand reef that at last they are able to attack +it vigorously. They now wear it back, and, driving the shore line +across the lagoon or meadow, cut a line of low cliffs on the mainland. +Such a shore is that of Gascony in southwestern France,--a low, +straight, sandy shore, bordered by dunes and unprotected by reefs from +the attack of the waves of the Bay of Biscay. + + [Illustration: Fig. 145. Map of New Jersey, with that Portion of + the State one Hundred Feet and more above Sea Level shaded + + Describe the coast line which the state would have if depressed + one hundred feet. Compare it with the present coastline] + +We may say, then, that on shores of elevation the presence of sand +reefs and lagoons indicates the stage of youth, while the absence of +these features and the vigorous and unimpeded attack by the sea upon +the mainland indicate the stage of maturity. Where much waste is +brought in by rivers the maturity of such a coast may be long delayed. +The waste from the land keeps the sea shallow offshore and constantly +renews the sand reef. The energy of the waves is consumed in handling +shore drift, and no energy is left for an effective attack upon the +land. Indeed, with an excessive amount of waste brought down by +streams the land may be built out and encroach temporarily upon the +sea; and not until long denudation has lowered the land, and thus +decreased the amount of waste from it, may the waves be able to cut +through the sand reef and thus the coast reach maturity. + + +Shores of Depression + +Where a coastal region is undergoing submergence the shore line moves +landward. The horizontal plane of the sea now intersects an old land +surface roughened by subaërial denudation. The shore line is irregular +and indented in proportion to the relief of the land and the amount of +the submergence which the land has suffered. It follows up partially +submerged valleys, forming bays, and bends round the divides, leaving +them to project as promontories and peninsulas. The outlines of shores +of depression are as varied as are the forms of the land partially +submerged. We give a few typical illustrations. + + [Illustration: Fig. 146. Chesapeake Bay + + Draw a sketch of this area before its depression] + +The characteristics of the coast of Maine are due chiefly to the fact +that a mountainous region of hard rocks, once worn to a peneplain, and +after a subsequent elevation deeply dissected by north-south valleys, +has subsided, the depression amounting on its southern margin to as +much as six hundred feet below sea level. Drowned valleys penetrate +the land in long, narrow bays, and rugged divides project in long, +narrow land arms prolonged seaward by islands representing the high +portions of their extremities. Of this exceedingly ragged shore there +are said to be two thousand miles from the New Brunswick boundary as +far west as Portland,--a straight-line distance of but two hundred +miles. Since the time of its greatest depression the land is known to +have risen some three hundred feet; for the bays have been shortened, +and the waste with which their floors were strewn is now in part laid +bare as clay plains about the bay heads and in narrow selvages about +the peninsulas and islands. + +The coast of Dalmatia, on the Adriatic Sea, is characterized by long +land arms and chains of long and narrow islands, all parallel to the +trend of the coast. A region of parallel mountain ranges has been +depressed, and the longitudinal valleys which lie between them are +occupied by arms of the sea. + +Chesapeake Bay is a branching bay due to the depression of an ancient +coastal plain which, after having emerged from the sea, was channeled +with broad, shallow valleys. The sea has invaded the valley of the +trunk stream and those of its tributaries, forming a shallow bay whose +many branches are all directed toward its axis (Fig. 146). + +Hudson Bay, and the North, the Baltic, and the Yellow seas are +examples where the sinking of the land has brought the sea in over low +plains of large extent, thus deeply indenting the continental +outline. The rise of a few hundred feet would restore these submerged +plains to the land. + +=The cycle of shores of depression.= In its _infantile stage_ the +outline of a shore of depression depends almost wholly on the previous +relief of the land, and but little on erosion by the sea. Sea cliffs +and narrow benches appear where headlands and outlying islands have +been nipped by the waves. As yet, little shore waste has been formed. +The coast of Maine is an example of this stage. + +In _early youth_ all promontories have been strongly cliffed, and under +a vigorous attack of the sea the shore of open bays may be cut back +also. Sea stacks and rocky islets, caves and coves, make the shore +minutely ragged. The irregularity of the coast, due to depression, is +for a while increased by differential wave wear on harder and softer +rocks. The rock bench is still narrow. Shore waste, though being +produced in large amounts, is for the most part swept into deeper +water and buried out of sight. Examples of this stage are the east +coast of Scotland and the California coast near San Francisco. + +_Later youth_ is characterized by a large accumulation of shore waste. +The rock bench has been cut back so that it now furnishes a good +roadway for shore drift. The stream of alongshore drift grows larger +and larger, filling the heads of the smaller bays with beaches, +building spits and hooks, and tying islands with sand bars to the +mainland. It bridges the larger bays with bay bars, while their length +is being reduced as their inclosing promontories are cut back by the +waves. Thus there comes to be a straight, continuous, and easy road, +no longer interrupted by headlands and bays, for the transportation of +waste alongshore. The Baltic coast of Germany is in this stage. + + [Illustration: Fig. 147. Portion of the Northwest Coast of France] + +All this while streams have been busy filling with delta deposits the +bays into which they empty. By these steps a coast gradually advances +to _maturity_, the stage when the irregularities due to depression +have been effaced, when outlying islands formed by subsidence have +been planed away, and when the shore line has been driven back behind +the former bay heads. The sea now attacks the land most effectively +along a continuous and fairly straight line of cliffs. Although the +first effect of wave wear was to increase the irregularities of the +shore, it sooner or later rectifies it, making it simple and smooth. +The northwest coast of France is often cited as an example of a coast +which has reached this stage of development (Fig. 147). + +In the _old age_ of coasts the rock bench is cut back so far that the +waves can no longer exert their full effect upon the shore. Their +energy is dissipated in moving shore drift hither and thither and in +abrading the bench when they drag bottom upon it. Little by little the +bench is deepened by tidal currents and the drag of waves; but this +process is so slow that meanwhile the sea cliffs melt down under the +weather, and the bench becomes a broad shoal where waves and tides +gradually work over the waste from the land to greater fineness and +sweep it out to sea. + + [Illustration: Fig. 148. The South Shore of Martha's Vineyard + + The land is shaded. To what class of coasts does this belong? + What stage has it reached, and by what process? What changes + will take place in the future?] + +=Plains of marine abrasion.= While subaërial denudation reduces the +land to baselevel, the sea is sawing its edges to _wave base_, i.e. +the lowest limit of the wave's effective wear. The widened rock bench +forms when uplifted a plain of marine abrasion, which like the +peneplain bevels across strata regardless of their various +inclinations and various degrees of hardness. + +How may a plain of marine abrasion be expected to differ from a +peneplain in its mantle of waste? + +Compared with subaërial denudation, marine abrasion is a comparatively +feeble agent. At the rate of five feet per century--a higher rate than +obtains on the youthful rocky, coast of Britain--it would require more +than ten million years to pare a strip one hundred miles wide from the +margin of a continent, a time sufficient, at the rate at which the +Mississippi valley is now being worn away, for subaërial denudation to +lower the lands of the globe to the level of the sea. + +Slow submergence favors the cutting of a wide rock bench. The water +continually deepens upon the bench; storm waves can therefore always +ride in to the base of the cliffs and attack them with full force; +shore waste cannot impede the onset of the waves, for it is +continually washed out in deeper water below wave base. + +=Basal conglomerates.= As the sea marches across the land during a +slow submergence, the platform is covered with sheets of sea-laid +sediments. Lowest of these is a conglomerate,--the bowlder and pebble +beach, widened indefinitely by the retreat of the cliffs at whose base +it was formed, and preserved by the finer deposits laid upon it in +the constantly deepening water as the land subsides. Such basal +conglomerates are not uncommon among the ancient rocks of the land, +and we may know them by their rounded pebbles and larger stones, +composed of the same kind of rock as that of the abraded and evened +surface on which they lie. + + + + +CHAPTER VIII + +OFFSHORE AND DEEP-SEA DEPOSITS + + +The alongshore deposits which we have now studied are the exposed edge +of a vast subaqueous sheet of waste which borders the continents and +extends often for as much as two or three hundred miles from land. +Soundings show that offshore deposits are laid in belts parallel to +the coast, the coarsest materials lying nearest to the land and the +finest farthest out. The pebbles and gravel and the clean, coarse sand +of beaches give place to broad stretches of sand, which grows finer +and finer until it is succeeded by sheets of mud. Clearly there is an +offshore movement of waste by which it is sorted, the coarser being +sooner dropped and the finer being carried farther out. + + +Offshore Deposits + +The debris torn by waves from rocky shores is far less in amount than +the waste of the land brought down to the sea by rivers, being only +one thirty-third as great, according to a conservative estimate. Both +mingle alongshore in all the forms of beach and bar that have been +described, and both are together slowly carried out to sea. On the +shelving ocean floor waste is agitated by various movements of the +unquiet water,--by the undertow (an outward-running bottom current +near the shore), by the ebb and flow of tides, by ocean currents where +they approach the land, and by waves and ground swells, whose effects +are sometimes felt to a depth of six hundred feet. By all these means +the waste is slowly washed to and fro, and as it is thus ground finer +and finer and its soluble parts are more and more dissolved, it drifts +farther and farther out from land. It is by no steady and rapid +movement that waste is swept from the shore to its final resting +place. Day after day and century after century the grains of sand and +particles of mud are shifted to and fro, winnowed and spread in +layers, which are destroyed and rebuilt again and again before they +are buried safe from further disturbance. + +These processes which are hidden from the eye are among the most +important of those with which our science has to do; for it is they +which have given shape to by far the largest part of the stratified +rocks of which the land is made. + +=The continental delta.= This fitting term has been recently suggested +for the sheet of waste slowly accumulating along the borders of the +continents. Within a narrow belt, which rarely exceeds two or three +hundred miles, except near the mouths of muddy rivers such as the +Amazon and Congo, nearly all the waste of the continent, whether worn +from its surface by the weather, by streams, by glaciers, or by the +wind, or from its edge by the chafing of the waves, comes at last to +its final resting place. The agencies which spread the material of the +continental delta grow more and more feeble as they pass into deeper +and more quiet water away from shore. Coarse materials are therefore +soon dropped along narrow belts near land. Gravels and coarse sands +lie in thick, wedge-shaped masses which thin out seaward rapidly and +give place to sheets of finer sand. + +=Sea muds.= Outermost of the sediments derived from the waste of the +continents is a wide belt of mud; for fine clays settle so slowly, +even in sea water,--whose saltness causes them to sink much faster +than they would in fresh water,--that they are wafted far before they +reach a bottom where they may remain undisturbed. Muds are also found +near shore, carpeting the floors of estuaries, and among stretches of +sandy deposits in hollows where the more quiet water has permitted the +finer silt to rest. + +Sea muds are commonly bluish and consolidate to bluish shales; the red +coloring matter brought from land waste--iron oxide--is altered to +other iron compounds by decomposing organic matter in the presence of +sea water. Yellow and red muds occur where the amount of iron oxide in +the silt brought down to the sea by rivers is too great to be reduced, +or decomposed, by the organic matter present. + +Green muds and green sand owe their color to certain chemical changes +which take place where waste from the land accumulates on the sea +floor with extreme slowness. A greenish mineral called _glauconite_--a +silicate of iron and alumina--is then formed. Such deposits, known as +_green sand_, are now in process of making in several patches off the +Atlantic coast, and are found on the coastal plain of New Jersey among +the offshore deposits of earlier geological ages. + +=Organic deposits.= Living creatures swarm along the shore and on the +shallows out from land as nowhere else in the ocean. Seaweed often +mantles the rock of the sea cliff between the levels of high and low +tide, protecting it to some degree from the blows of waves. On the +rock bench each little pool left by the ebbing tide is an aquarium +abounding in the lowly forms of marine life. Below low-tide level +occur beds of molluscous shells, such as the oyster, with countless +numbers of other humble organisms. Their harder parts--the shells of +mollusks, the white framework of corals, the carapaces of crabs and +other crustaceans, the shells of sea urchins, the bones and teeth of +fishes--are gradually buried within the accumulating sheets of +sediment, either whole or, far more often, broken into fragments by +the waves. + +By means of these organic remains each layer of beach deposits and +those of the continental delta may contain a record of the life of the +time when it was laid. Such a record has been made ever since living +creatures with hard parts appeared upon the globe. We shall find it +sealed away in the stratified rocks of the continents,--parts of +ancient sea deposits now raised to form the dry land. Thus we have in +the traces of living creatures found in the rocks, i.e. in fossils, a +history of the progress of life upon the planet. + + [Illustration: Fig. 149. Coquina, Florida] + +=Molluscous shell deposits.= The forms of marine life of importance in +rock making thrive best in clear water, where little sediment is being +laid, and where at the same time the depth is not so great as to +deprive them of needed light, heat, and of sufficient oxygen absorbed +by sea water from the air. In such clear and comparatively shallow +water there often grow countless myriads of animals, such as mollusks +and corals, whose shells and skeletons of carbonate of lime gradually +accumulate in beds of limestone. + +A shell limestone made of broken fragments cemented together is +sometimes called _coquina_, a local term applied to such beds recently +uplifted from the sea along the coast of Florida (Fig. 149). + +_Oölitic_ limestone (_öon_, an egg; _lithos_, a stone) is so named +from the likeness of the tiny spherules which compose it to the roe of +fish. Corals and shells have been pounded by the waves to calcareous +sand, and each grain has been covered with successive concentric +coatings of lime carbonate deposited about it from solution. + +The impalpable powder to which calcareous sand is ground by the waves +settles at some distance from shore in deeper and quieter water as a +limy silt, and hardens into a dense, fine-grained limestone in which +perhaps no trace of fossil is found to suggest the fact that it is of +organic origin. + +From Florida Keys there extends south to the trough of Florida Straits +a limestone bank covered by from five hundred and forty to eighteen +hundred feet of water. The rocky bottom consists of limestone now +slowly building from the accumulation of the remains of mollusks, +small corals, sea urchins, worms with calcareous tubes, and +lime-secreting seaweed, which live upon its surface. + +Where sponges and other silica-secreting organisms abound on limestone +banks, silica forms part of the accumulated deposit, either in its +original condition, as, for example, the spicules of sponges, or +gathered into concretions and layers of flint. + +Where considerable mud is being deposited along with carbonate of lime +there is in process of making a clayey limestone or a limy shale; +where considerable sand, a sandy limestone or a limy sandstone. + +=Consolidation of offshore deposits.= We cannot doubt that all these +loose sediments of the sea floor are being slowly consolidated to +solid rock. They are soaked with water which carries in solution lime +carbonate and other cementing substances. These cements are deposited +between the fragments of shells and corals, the grains of sand and +the particles of mud, binding them together into firm rock. Where +sediments have accumulated to great thickness the lower portions tend +also to consolidate under the weight of the overlying beds. Except in +the case of limestones, recent sea deposits uplifted to form land are +seldom so well cemented as are the older strata, which have long been +acted upon by underground waters deep below the surface within the +zone of cementation, and have been exposed to view by great erosion. + + [Illustration: Fig. 150. Ripple Marks on Layers of Ancient + Sandstone, Wisconsin] + +=Ripple marks, sun cracks, etc.= The pulse of waves and tidal currents +agitates the loose material of offshore deposits, throwing it into +fine parallel ridges called ripple marks. One may see this beautiful +ribbing imprinted on beach sands uncovered by the outgoing tide, and +it is also produced where the water is of considerable depth. While +the tide is out the surface of shore deposits may be marked by the +footprints of birds and other animals, or by the raindrops of a +passing shower (Fig. 153). The mud of flats, thus exposed to the sun +and dried, cracks in a characteristic way (Figs. 151 and 152). Such +markings may be covered over with a thin layer of sediment at the next +flood tide and sealed away as a lasting record of the manner and place +in which the strata were laid. In Figure 150 we have an illustration +of a very ancient ripple-marked sand consolidated to hard stone, +uplifted and set on edge by movements of the earth's crust, and +exposed to open air after long erosion. + + [Illustration: Fig. 151. Sun Cracks] + +=Stratification.= For the most part the sheet of sea-laid waste is +hidden from our sight. Where its edge is exposed along the shore we +may see the surface markings which have just been noticed. Soundings +also, and the observations made in shallow waters by divers, tell +something of its surface; but to learn more of its structures we must +study those ancient sediments which have been lifted from the sea and +dissected by subaërial agencies. From them we ascertain that sea +deposits are stratified. They lie in distinct layers which often +differ from one another in thickness, in size of particles, and +perhaps in color. They are parted by bedding planes, each of which +represents either a change in material or a pause during which +deposition ceased and the material of one layer had time to settle and +become somewhat consolidated before the material of the next was laid +upon it. Stratification is thus due to intermittently acting forces, +such as the agitation of the water during storms, the flow and ebb of +the tide, and the shifting channels of tidal currents. Off the mouths +of rivers, stratification is also caused by the coarser and more +abundant material brought down at time of floods being laid on the +finer silt which is discharged during ordinary stages. + + [Illustration: Fig. 152. The Under Side of a Layer deposited + upon a Sun-Cracked Surface, showing Casts of the Cracks] + + [Illustration: Fig. 153. Rain Prints] + +How stratified deposits are built up is well illustrated in the flats +which border estuaries, such as the Bay of Fundy. Each advance of the +tide spreads a film of mud, which dries and hardens in the air during +low water before another film is laid upon it by the next incoming +tidal flood. In this way the flats have been covered by a clay which +splits into leaves as thin as sheets of paper. + +It is in fine material, such as clays and shales and limestones, that +the thinnest and most uniform layers, as well as those of widest +extent, occur. On the other hand, coarse materials are commonly laid +in thick beds, which soon thin out seaward and give place to deposits +of finer stuff. In a general way strata are laid in well-nigh +horizontal sheets, for the surface on which they are laid is generally +of very gentle inclination. Each stratum, however, is lenticular, or +lenslike, in form, having an area where it is thickest, and thinning +out thence to its edges, where it is overlapped by strata similar in +shape. + + [Illustration: Fig. 154. Cross Bedding in Sandstone, England] + +=Cross bedding.= There is an apparent exception to this rule where +strata whose upper and lower surfaces may be about horizontal are made +up of layers inclined at angles which may be as high as the angle of +repose. In this case each stratum grew by the addition along its edge +of successive layers of sediment, precisely as does a sand bar in a +river, the sand being pushed continuously over the edge and coming to +rest on a sloping surface. Shoals built by strong and shifting tidal +currents often show successive strata in which the cross bedding is +inclined in different directions. + +=Thickness of sea deposits.= Remembering the vast amount of material +denuded from the land and deposited offshore, we should expect that +with the lapse of time sea deposits would have grown to an enormous +thickness. It is a suggestive fact that, as a rule, the profile of the +ocean bed is that of a soup plate,--a basin surrounded by a flaring +rim. On the _continental shelf_, as the rim is called, the water is +seldom more than six hundred feet in depth at the outer edge, and +shallows gradually towards shore. Along the eastern coast of the +United States the continental shelf is from fifty to one hundred and +more miles in width; on the Pacific coast it is much narrower. So far +as it is due to upbuilding, a wide continental shelf, such as that of +the Atlantic coast, implies a massive continental delta thousands of +feet in thickness. The coastal plain of the Atlantic states may be +regarded as the emerged inner margin of this shelf, and borings made +along the coast probe it to the depth of as much as three thousand +feet without finding the bottom of ancient offshore deposits. +Continental shelves may also be due in part to a submergence of the +outer margin of a continental plateau and to marine abrasion. + +=Deposition of sediments and subsidence.= The stratified rocks of the +land show in many places ancient sediments which reach a thickness +which is measured in miles, and which are yet the product of well-nigh +continuous deposition. Such strata may prove by their fossils and by +their composition and structure that they were all laid offshore in +shallow water. We must infer that, during the vast length of time +recorded by the enormous pile, the floor of the sea along the coast +was slowly sinking, and that the trough was constantly being filled, +foot by foot, as fast as it was depressed. Such gradual, quiet +movements of the earth's crust not only modify the outline of coasts, +as we have seen, but are of far greater geological importance in that +they permit the making of immense deposits of stratified rock. + +A slow subsidence continued during long time is recorded also in the +succession of the various kinds of rock that come to be deposited in +the same area. As the sea transgresses the land, i.e. encroaches upon +it, any given part of the sea bottom is brought farther and farther +from the shore. The basal conglomerate formed by bowlder and pebble +beaches comes to be covered with sheets of sand, and these with layers +of mud as the sea becomes deeper and the shore more remote; while +deposits of limestone are made when at last no waste is brought to the +place from the now distant land, and the water is left clear for the +growth of mollusks and other lime-secreting organisms. + + [Illustration: Fig. 155. Succession of Deposits recording a + Transgressing Sea + + _c_, conglomerate; _ss_, sandstone; _sh_, shale; _lm_, limestone] + +=Rate of deposition.= As deposition in the sea corresponds to +denudation on the land, we are able to make a general estimate of the +rate at which the former process is going on. Leaving out of account +the soluble matter removed, the Mississippi is lowering its basin at +the rate of one foot in five thousand years, and we may assume this as +the average rate at which the earth's land surface of fifty-seven +million square miles is now being denuded by the removal of its +mechanical waste. But sediments from the land are spread within a zone +but two or three hundred miles in width along the margin of the +continents, a line one hundred thousand miles long. As the area of +deposition--about twenty-five million square miles--is about one half +the area of denudation, the average rate of deposition must be twice +the average rate of denudation, i.e. about one foot in twenty-five +hundred years. If some deposits are made much more rapidly than this, +others are made much more slowly. If they were laid no faster than the +present average rate, the strata of ancient sea deposits exposed in a +quarry fifty feet deep represent a lapse of at least one hundred and +twenty-five thousand years, and those of a formation five hundred feet +thick required for their accumulation one million two hundred and +fifty thousand years. + + [Illustration: Fig. 156. Thick Offshore Deposits of Coarse Waste + recording the Presence of a Young Mountain Range near Shore] + +=The sedimentary record and the denudation cycle.= We have seen that +the successive stages in a cycle of denudation, such as that by which +a land mass of lofty mountains is worn to low plains, are marked each +by its own peculiar land forms, and that the forms of the earlier +stages are more or less completely effaced as the cycle draws toward +an end. Far more lasting records of each stage are left in the +sedimentary deposits of the continental delta. Thus, in the youth of +such a land mass as we have mentioned, torrential streams flowing down +the steep mountain sides deliver to the adjacent sea their heavy loads +of coarse waste, and thick offshore deposits of sand and gravel (Fig. +156) record the high elevation of the bordering land. As the land is +worn to lower levels, the amount and coarseness of the waste brought +to the sea diminishes, until the sluggish streams carry only a fine +silt which settles on the ocean floor near to land in wide sheets of +mud which harden into shale. At last, in the old age of the region +(Fig. 157), its low plains contribute little to the sea except the +soluble elements of the rocks, and in the clear waters near the land +lime-secreting organisms flourish and their remains accumulate in beds +of limestone. When long-weathered lands mantled with deep, +well-oxidized waste are uplifted by a gradual movement of the earth's +crust, and the mantle is rapidly stripped off by the revived streams, +the uprise is recorded in wide deposits of red and yellow clays and +sands upon the adjacent ocean floor. + +Where the waste brought in is more than the waves can easily +distribute, as off the mouths of turbid rivers which drain highlands +near the sea, deposits are little winnowed, and are laid in rapidly +alternating, shaly sandstones and sandy shales. + + [Illustration: Fig. 157. Offshore Deposits recording Old Age of + the Adjacent Land + + _ss_, sandstone; _sh_, shale; _lm_, limestone] + +Where the highlands are of igneous rock, such as granite, and +mechanical disintegration is going on more rapidly than chemical +decay, these conditions are recorded in the nature of the deposits +laid offshore. The waste swept in by streams contains much feldspar +and other minerals softer and more soluble than quartz, and where the +waves have little opportunity to wear and winnow it, it comes to rest +in beds of sandstone in which grains of feldspar and other soft +minerals are abundant. Such feldspathic sandstones are known as +_arkose_. + +On the other hand, where the waste supplied to the sea comes chiefly +from wide, sandy, coastal plains, there are deposited offshore clean +sandstones of well-worn grains of quartz alone. In such coastal plains +the waste of the land is stored for ages. Again and again they are +abandoned and invaded by the sea as from time to time the land slowly +emerges and is again submerged. Their deposits are long exposed to the +weather, and sorted over by the streams, and winnowed and worked over +again and again by the waves. In the course of long ages such deposits +thus become thoroughly sorted, and the grains of all minerals softer +than quartz are ground to mud. + + [Illustration: Fig. 158. Globigerina Ooze under the Microscope] + + +Deep-Sea Oozes and Clays + +=Globigerina ooze.= Beyond the reach of waste from the land the bottom +of the deep sea is carpeted for the most part with either chalky ooze +or a fine red clay. The surface waters of the warm seas swarm with +minute and lowly animals belonging to the order of the _Foraminifera_, +which secrete shells of carbonate of lime. At death these tiny white +shells fall through the sea water like snowflakes in the air, and, +slowly dissolving, seem to melt quite away before they can reach +depths greater than about three miles. Near shore they reach bottom, +but are masked by the rapid deposit of waste derived from the land. At +intermediate depths they mantle the ocean floor with a white, soft +lime deposit known as _Globigerina ooze_, from a genus of the +Foraminifera which contributes largely to its formation. + +=Red clay.= Below depths of from fifteen to eighteen thousand feet the +ocean bottom is sheeted with red or chocolate colored clay. It is the +insoluble residue of seashells, of the debris of submarine volcanic +eruptions, of volcanic dust wafted by the winds, and of pieces of +pumice drifted by ocean currents far from the volcanoes from which +they were hurled. The red clay builds up with such inconceivable +slowness that the teeth of sharks and the hard ear bones of whales may +be dredged in large numbers from the deep ocean bed, where they have +lain unburied for thousands of years; and an appreciable part of the +clay is also formed by the dust of meteorites consumed in the +atmosphere,--a dust which falls everywhere on sea and land, but which +elsewhere is wholly masked by other deposits. + +The dark, cold abysses of the ocean are far less affected by change +than any other portion of the surface of the lithosphere. These vast, +silent plains of ooze lie far below the reach of storms. They know no +succession of summer and winter, or of night and day. A mantle of deep +and quiet water protects them from the agents of erosion which +continually attack, furrow, and destroy the surface of the land. While +the land is the area of erosion, the sea is the area of deposition. +The sheets of sediment which are slowly spread there tend to efface +any inequalities, and to form a smooth and featureless subaqueous +plain. + +With few exceptions, the stratified rocks of the land are proved by +their fossils and composition to have been laid in the sea; but in the +same way they are proved to be offshore, shallow-water deposits, akin +to those now making on continental shelves. Deep-sea deposits are +absent from the rocks of the land, and we may therefore infer that the +deep sea has never held sway where the continents now are,--that the +continents have ever been, as now, the elevated portions of the +lithosphere, and that the deep seas of the present have ever been its +most depressed portions. + + +The Reef-Building Corals + +In warm seas the most conspicuous of rock-making organisms are the +corals known as the reef builders. Floating in a boat over a coral +reef, as, for example, off the south coast of Florida or among the +Bahamas, one looks down through clear water on thickets of branching +coral shrubs perhaps as much as eight feet high, and hemispherical +masses three or four feet thick, all abloom with countless minute +flowerlike coral polyps, gorgeous in their colors of yellow, orange, +green, and red. In structure each tiny polyp is little more than a +fleshy sac whose mouth is surrounded with petal-like tentacles, or +feelers. From the sea water the polyps secrete calcium carbonate and +build it up into the stony framework which supports their colonies. +Boring mollusks, worms, and sponges perforate and honeycomb this +framework even while its surface is covered with myriads of living +polyps. It is thus easily broken by the waves, and white fragments of +coral trees strew the ground beneath. Brilliantly colored fishes live +in these coral groves, and countless mollusks, sea urchins, and other +forms of marine life make here their home. With the debris from all +these sources the reef is constantly built up until it rises to +low-tide level. Higher than this the corals cannot grow, since they +are killed by a few hours' exposure to the air. + + [Illustration: Fig. 159. Patch of Growing Corals exposed at an + Exceptionally Low Tide, Great Barrier Reef, Australia] + +When the reef has risen to wave base, the waves abrade it on the +windward side and pile to leeward coral blocks torn from their +foundation, filling the interstices with finer fragments. Thus they +heap up along the reef low, narrow islands (Fig. 160). + +Reef building is a comparatively rapid progress. It has been estimated +that off Florida a reef could be built up to the surface from a depth +of fifty feet in about fifteen hundred years. + + [Illustration: Fig. 160. Wave-Built Island on Coral Reef + + _r_, reef; _sl_, sea level] + +=Coral limestones.= Limestones of various kinds are due to the reef +builders. The reef rock is made of corals in place and broken +fragments of all sizes, cemented together with calcium carbonate from +solution by infiltrating waters. On the island beaches coral sand is +forming oolitic limestone, and the white coral mud with which the sea +is milky for miles about the reef in times of storm settles and +concretes into a compact limestone of finest grain. Corals have been +among the most important limestone builders of the sea ever since they +made their appearance in the early geological ages. + +The areas on which coral limestone is now forming are large. The Great +Barrier Reef of Australia, which lies off the northeastern coast, is +twelve hundred and fifty miles long, and has a width of from ten to +ninety miles. Most of the islands of the tropics are either skirted +with coral reefs or are themselves of coral formation. + +=Conditions of coral growth.= Reef-building corals cannot live except +in clear salt water less, as a rule, than one hundred and fifty feet +in depth, with a winter temperature not lower than 68° F. An important +condition also is an abundant food supply, and this is best secured in +the path of the warm oceanic currents. + +Coral reefs may be grouped in three classes,--fringing reefs, barrier +reefs, and atolls. + +=Fringing reefs.= These take their name from the fact that they are +attached as narrow fringes to the shore. An example is the reef which +forms a selvage about a mile wide along the northeastern coast of +Cuba. The outer margin, indicated by the line of white surf, where the +corals are in vigorous growth, rises from about forty feet of water. +Between this and the shore lies a stretch of shoal across which one +can wade at low water, composed of coral sand with here and there a +clump of growing coral. + +=Barrier reefs.= Reefs separated from the shore by a ship channel of +quiet water, often several miles in width and sometimes as much as +three hundred feet in depth, are known as barrier reefs. The seaward +face rises abruptly from water too deep for coral growth. Low islands +are cast up by the waves upon the reef, and inlets give place for the +ebb and flow of the tides. Along the west coast of the island of New +Caledonia a barrier reef extends for four hundred miles, and for a +length of many leagues seldom approaches within eight miles of the +shore. + +=Atolls.= These are ring-shaped or irregular coral islands, or +island-studded reefs, inclosing a central lagoon. The narrow zone of +land, like the rim of a great bowl sunken to the water's edge, rises +hardly more than twenty feet at most above the sea, and is covered +with a forest of trees such as the cocoanut, whose seeds can be +drifted to it uninjured from long distances. The white beach of coral +sand leads down to the growing reef, on whose outer margin the surf is +constantly breaking. The sea face of the reef falls off abruptly, +often to depths of thousands of feet, while the lagoon varies in depth +from a few feet to one hundred and fifty or two hundred, and +exceptionally measures as much as three hundred and fifty feet. + +=Theories of coral reefs.= Fringing reefs require no explanation, +since the depth of water about them is not greater than that at which +coral can grow; but barrier reefs and atolls, which may rise from +depths too great for coral growth demand a theory of their origin. + + [Illustration: Fig. 161. Diagram illustrating the Subsidence + Theory of Coral Reefs] + +Darwin's theory holds that barrier reefs and atolls are formed from +fringing reefs by _subsidence_. The rate of sinking cannot be greater +than that of the upbuilding of the reef, since otherwise the corals +would be carried below their depth and drowned. The process is +illustrated in Figure 161, where v represents a volcanic island in mid +ocean undergoing slow depression, and _ss_ the sea level before the +sinking began, when the island was surrounded by a fringing reef. As +the island slowly sinks, the reef builds up with equal pace. It rears +its seaward face more steep than the island slope, and thus the +intervening space between the sinking, narrowing land and the outer +margin of the reef constantly widens. In this intervening space the +corals are more or less smothered with silt from the outer reef and +from the land, and are also deprived in large measure of the needful +supply of food and oxygen by the vigorous growth of the corals on the +outer rim. The outer rim thus becomes a barrier reef and the inner +belt of retarded growth is deepened by subsidence to a ship channel, +_s´s´_ representing sea level at this time. The final stage, where the +island has been carried completely beneath the sea and overgrown by +the contracting reef, whose outer ring now forms an atoll, is +represented by _s´´s´´_. + + [Illustration: Fig. 162. Barrier Reef formed without Subsidence + + _a_, zone of coral growth; _f_, former fringing reef; _t_, + talus; _b_, barrier reef] + +In very many instances, however, atolls and barrier reefs may be +explained without subsidence. Thus a barrier reef may be formed by the +seaward growth of a fringing reef upon the talus of its sea face. In +Figure 162, _f_ is a fringing reef whose outer wall rises from about +one hundred and fifty feet, the lower limit of the reef-building +species. At the foot of this submarine cliff a talus of fallen blocks +t accumulates, and as it reaches the zone of coral growth becomes the +foundation on which the reef is steadily extended seaward. As the reef +widens, the polyps of the circumference flourish, while those of the +inner belt are retarded in their growth and at last perish. The coral +rock of the inner belt is now dissolved by sea water and scoured out +by tidal currents until it gives place to a gradually deepening ship +channel, while the outer margin is left as a barrier reef. + + [Illustration: Fig. 163. Section of Atoll on a Shoal which has + been built up to near the Surface by Organic Deposits upon a + Submarine Volcanic Peak + + _v_, volcano; _f_, foraminiferal deposits; _m_, molluscous shell + deposits; _c_, coral reef; _sl_, sea level] + +In much the same way atolls may be built on any shoal which lies +within the zone of coral growth. Such shoals may be produced when +volcanic islands are leveled by waves and ocean currents, and when +submarine plateaus, ridges, and peaks are built up by various organic +agencies, such as molluscous and foraminiferal shell deposits (Fig. +163). The reef-building corals, whose eggs are drifted widely over the +tropic seas by ocean currents, colonize such submarine foundations +wherever the conditions are favorable for their growth. As the reef +approaches the surface the corals of the inner area are smothered by +silt and starved, and their Submarine Volcanic Peak hard parts are +dissolved and scoured away; while those of the circumference, with +abundant food supply, nourish and build the ring of the atoll. Atolls +may be produced also by the backward drift of sand from either end of +a crescentic coral reef or island, the spits uniting in the quiet +water of the lee to inclose a lagoon. In the Maldive Archipelago all +gradations between crescent-shaped islets and complete atoll rings +have been observed. + +In a number of instances where coral reefs have been raised by +movements of the earth's crust, the reef formation is found to be a +thin veneer built upon a foundation of other deposits. Thus Christmas +Island, in the Indian Ocean, is a volcanic pile rising eleven hundred +feet above sea level and fifteen thousand five hundred feet above the +bottom of the sea. The summit is a plateau surrounded by a rim of +hills of reef formation, which represent the ring of islets of an +ancient atoll. Beneath the reef are thick beds of limestone, composed +largely of the remains of foraminifers, which cover the lavas and +fragmental materials of the old submarine volcano. + +Among the ancient sediments which now form the stratified rocks of the +land there occur many thin reef deposits, but none are known of the +immense thickness which modern reefs are supposed to reach according +to the theory of subsidence. + +Barrier and fringing reefs are commonly interrupted off the mouths of +rivers. Why? + +=Summary.= We have seen that the ocean bed is the goal to which the +waste of the rocks of the land at last arrives. Their soluble parts, +dissolved by underground waters and carried to the sea by rivers, are +largely built up by living creatures into vast sheets of limestone. +The less soluble portions--the waste brought in by streams and the +waste of the shore--form the muds and sands of continental deltas. All +of these sea deposits consolidate and harden, and the coherent rocks +of the land are thus reconstructed on the ocean floor. But the +destination is not a final one. The stratified rocks of the land are +for the most part ancient deposits of the sea, which have been lifted +above sea level; and we may believe that the sediments now being laid +offshore are the "dust of continents to be," and will some time emerge +to form additions to the land. We are now to study the movements of +the earth's crust which restore the sediments of the sea to the light +of day, and to whose beneficence we owe the habitable lands of the +present. + + + + +PART II + +INTERNAL GEOLOGICAL AGENCIES + + +CHAPTER IX + +MOVEMENTS OF THE EARTH'S CRUST + + +The geological agencies which we have so far studied--weathering, +streams, underground waters, glaciers, winds, and the ocean--all work +upon the earth from without, and all are set in motion by an energy +external to the earth, namely, the radiant energy of the sun. All, +too, have a common tendency to reduce the inequalities of the earth's +surface by leveling the lands and strewing their waste beneath the +sea. + +But despite the unceasing efforts of these external agencies, they +have not destroyed the continents, which still rear their broad plains +and great plateaus and mountain ranges above the sea. Either, then, +the earth is very young and the agents of denudation have not yet had +time to do their work, or they have been opposed successfully by other +forces. + +We enter now upon a department of our science which treats of forces +which work upon the earth from within, and increase the inequalities +of its surface. It is they which uplift and recreate the lands which +the agents of denudation are continually destroying; it is they which +deepen the ocean bed and thus withdraw its waters from the shores. At +times also these forces have aided in the destruction of the lands by +gradually lowering them and bringing in the sea. Under the action of +forces resident within the earth the crust slowly rises or sinks; from +time to time it has been folded and broken; while vast quantities of +molten rock have been pressed up into it from beneath and outpoured +upon its surface. We shall take up these phenomena in the following +chapters, which treat of upheavals and depressions of the crust, +foldings and fractures of the crust, earthquakes, volcanoes, the +interior conditions of the earth, mineral veins, and metamorphism. + + +Oscillations of the Crust + +Of the various movements of the crust due to internal agencies we will +consider first those called oscillations, which lift or depress large +areas so slowly that a long time is needed to produce perceptible +changes of level, and which leave the strata in nearly their original +horizontal attitude. These movements are most conspicuous along +coasts, where they can be referred to the datum plane of sea level; we +will therefore take our first illustrations from rising and sinking +shores. + +=New Jersey.= Along the coasts of New Jersey one may find awash at +high tide ancient shell heaps, the remains of tribal feasts of +aborigines. Meadows and old forest grounds, with the stumps still +standing, are now overflowed by the sea, and fragments of their turf +and wood are brought to shore by waves. Assuming that the sea level +remains constant, it is clear that the New Jersey coast is now +gradually sinking. The rate of submergence has been estimated at about +two feet per century. + +On the other hand, the wide coastal plain of New Jersey is made of +stratified sands and clays, which, as their marine fossils show, were +outspread beneath the sea. Their present position above sea level +proves that the land now subsiding emerged in the recent past. + +The coast of New Jersey is an example of the slow and tranquil +oscillations of the earth's unstable crust now in progress along many +shores. Some are emerging from the sea, some are sinking beneath it; +and no part of the land seems to have been exempt from these changes +in the past. + +=Evidences of changes of level.= Taking the surface of the sea as a +level of reference, we may accept as proofs of relative upheaval +whatever is now found in place above sea level and could have been +formed only at or beneath it, and as proofs of relative subsidence +whatever is now found beneath the sea and could only have been formed +above it. + +Thus old strand lines with sea cliffs, wave-cut rock benches, and +beaches of wave-worn pebbles or sand, are striking proofs of recent +emergence to the amount of their present height above tide. No less +conclusive is the presence of sea-laid rocks which we may find in the +neighboring quarry or outcrop, although it may have been long ages +since they were lifted from the sea to form part of the dry land. + +Among common proofs of subsidence are roads and buildings and other +works of man, and vegetal growths and deposits, such as forest grounds +and peat beds, now submerged beneath the sea. In the deltas of many +large rivers, such as the Po, the Nile, the Ganges, and the +Mississippi, buried soils prove subsidences of hundreds of feet; and +in several cases, as in the Mississippi delta, the depression seems to +be now in progress. + +Other proofs of the same movement are drowned land forms which are +modeled only in open air. Since rivers cannot cut their valleys +farther below the baselevel of the sea than the depths of their +channels, _drowned valleys_ are among the plainest proofs of +depression. To this class belong Narragansett, Delaware, Chesapeake, +Mobile, and San Francisco bays, and many other similar drowned valleys +along the coasts of the United States. Less conspicuous are the +_submarine channels_ which, as soundings show, extend from the mouths +of a number of rivers some distance out to sea. Such is the submerged +channel which reaches from New York Bay southeast to the edge of the +continental shelf, and which is supposed to have been cut by the +Hudson River when this part of the shelf was a coastal plain. + +=Warping.= In a region undergoing changes of level the rate of +movement commonly varies in different parts. Portions of an area may +be rising or sinking, while adjacent portions are stationary or moving +in the opposite direction. In this way a land surface becomes +_warped_. Thus, while Nova Scotia and New Brunswick are now rising +from the level of the sea, Prince Edward Island and Cape Breton Island +are sinking, and the sea now flows over the site of the famous old +town of Louisburg destroyed in 1758. + +Since the close of the glacial epoch the coasts of Newfoundland and +Labrador have risen hundreds of feet, but the rate of emergence has +not been uniform. The old strand line, which stands at five hundred +and seventy-five feet above tide at St. John's, Newfoundland, declines +to two hundred and fifty feet near the northern point of Labrador +(Fig. 164). + + [Illustration: Fig. 164. Warped Strand Line from St. John's, + Newfoundland, to Nachvak, Labrador] + +=The Great Lakes= is now undergoing perceptible warping. Rivers enter +the lakes from the south and west with sluggish currents and deep +channels resembling the estuaries of drowned rivers; while those that +enter from opposite directions are swift and shallow. At the western +end of Lake Erie are found submerged caves containing stalactites, and +old meadows and forest grounds are now under water. It is thus seen +that the water of the lakes is rising along their southwestern shores, +while from their northeastern shores it is being withdrawn. The +region of the Great Lakes is therefore warping; it is rising in the +northeast as compared with the southwest. + +From old bench marks and records of lake levels it has been estimated +that _the rate of warping_ amounts to five inches a century for every +one hundred miles. It is calculated that the water of Lake Michigan is +rising at Chicago at the rate of nine or ten inches per century. The +divide at this point between the tributaries of the Mississippi and +Lake Michigan is but eight feet above the mean stage of the lake. If +the canting of the region continues at its present rate, in a thousand +years the waters of the lake will here overflow the divide. In three +thousand five hundred years all the lakes except Ontario will +discharge by this outlet, via the Illinois and Mississippi rivers, +into the Gulf of Mexico. The present outlet by the Niagara River will +be left dry, and the divide between the St. Lawrence and the +Mississippi systems will have shifted from Chicago to the vicinity of +Buffalo. + +=Physiographic effects of oscillations.= We have already mentioned +several of the most important effects of movements of elevation and +depression, such as their effects on rivers, the mantle of waste (pp. +85, 86), and the forms of coasts (p. 166). Movements of +elevation--including uplifts by folding and fracture of the crust to +be noticed later--are the necessary conditions for erosion by whatever +agent. They determine the various agencies which are to be chiefly +concerned m the wear of any land,--whether streams or glaciers, +weathering or the wind,--and the degree of their efficiency. The lands +must be uplifted before they can be eroded, and since they must be +eroded before their waste can be deposited, movements of elevation are +a prerequisite condition for sedimentation also. Subsidence is a +necessary condition for deposits of great thickness, such as those of +the Great Valley of California and the Indo-Gangetic plain (p. 101), +the Mississippi delta (p. 109), and the still more important +formations of the continental delta in gradually sinking troughs (p. +183). It is not too much to say that the character and thickness of +each formation of the stratified rocks depend primarily on these +crustal movements. + +Along the Baltic coast of Sweden, bench marks show that the sea is +withdrawing from the land at a rate which at the north amounts to +between three and four feet per century; Towards the south the rate +decreases. South of Stockholm, until recent years, the sea has gained +upon the land, and here in several seaboard towns streets by the shore +are still submerged. The rate of oscillation increases also from the +coast inland. On the other hand, along the German coast of the Baltic +the only historic fluctuations of sea level are those which may be +accounted for by variations due to changes in rainfall. In 1730 +Celsius explained the changes of level of the Swedish coast as due to +a lowering of the Baltic instead of to an elevation of the land. Are +the facts just stated consistent with his theory? + + [Illustration: Fig. 165. Old Strand Lines, Tadousac, Quebec] + +At the little town of Tadousac--where the Saguenay River empties into +the St. Lawrence--there are terraces of old sea beaches, some almost +as fresh as recent railway fills, the highest standing two hundred and +thirty feet above the river (Fig. 165). Here the Saguenay is eight +hundred and forty feet in depth, and the tide ebbs and flows far up +its stream. Was its channel cut to this depth by the river when the +land was at its present height? What oscillations are here recorded, +and to what amount? + + [Illustration: Fig. 166. Diagram showing Ruins of Temple, North + of Naples + + _C_, ancient sea cliff; _m_, marble pillars, dotted where bored + by mollusks; _sl_, sea level] + +A few miles north of Naples, Italy, the ruins of an ancient Roman +temple lie by the edge of the sea, on a narrow plain which is +overlooked in the rear by an old sea cliff (Fig. 166). Three marble +pillars are still standing. For eleven feet above their bases these +columns are uninjured, for to this height they were protected by an +accumulation of volcanic ashes; but from eleven to nineteen feet they +are closely pitted with the holes of boring marine mollusks. From +these facts trace the history of the oscillations of the region. + + [Illustration: Fig. 167. Section in a Region of Folded Rocks] + + +Foldings of the Crust + +The oscillations which we have just described leave the strata not far +from their original horizontal attitude. Figure 167 represents a +region in which movements of a very different nature have taken place. +Here, on either side of the valley _v_, we find outcrops of layers +tilted at high angles. Sections along the ridge _r_ show that it is +composed of layers which slant inward from either side. In places the +outcropping strata stand nearly on edge, and on the right of the +valley they are quite overturned; a shale _sh_ has come to overlie a +limestone _lm_ although the shale is the older rock, whose original +position was beneath the limestone. + + [Illustration: Fig. 168. Dip and Strike] + +It is not reasonable to suppose that these rocks were deposited in the +attitude in which we find them now; we must believe that, like other +stratified rocks, they were outspread in nearly level sheets upon the +ocean floor. Since that time they must have been deformed. Layers of +solid rock several miles in thickness have been crumpled and folded +like soft wax in the hand, and a vast denudation has worn away the +upper portions of the folds, in part represented in our section by +dotted lines. + +=Dip and strike.= In districts where the strata have been disturbed it +is desirable to record their attitude. This is most easily done by +taking the angle at which the strata are inclined and the compass +direction in which they slant. It is also convenient to record the +direction in which the outcrop of the strata trends across the +country. + + [Illustration: Fig. 169. An Anticline, Maryland] + +The inclination of a bed of rocks to the horizon is its _dip_ (Fig. +168). The amount of the dip is the angle made with a horizontal plane. +The dip of a horizontal layer is zero, and that of a vertical layer +is 90°. The direction of the dip is taken with the compass. Thus a +geologist's notebook in describing the attitude of outcropping strata +contains many such entries as these: dip 32° north, or dip 8° south 20° +west,--meaning in the latter case that the amount of the dip is 8° and +the direction of the dip bears 20° west of south. + +The line of intersection of a layer with the horizontal plane is the +_strike_. The strike always runs at right angles to the dip. + +Dip and strike may be illustrated by a book set aslant on a shelf. The +dip is the acute angle made with the shelf by the side of the book, +while the strike is represented by a line running along the book's +upper edge. If the dip is north or south, the strike runs east and +west. + + [Illustration: Fig. 170. Folded Strata, Coast of England + + A syncline in the center, with an anticline on either side] + +=Folded structures.= An upfold, in which the strata dip away from a +line drawn along the crest and called the axis of the fold, is known +as an _anticline_ (Fig. 169). A downfold, where the strata dip from +either side toward the axis of the trough, is called a _syncline_ +(Fig. 170). There is sometimes seen a downward bend in horizontal or +gently inclined strata, by which they descend to a lower level. Such a +single flexure is a _monocline_ (Fig. 171). + + [Illustration: Fig. 171. A Monocline] + +=Degrees of folding.= Folds vary in degree from broad, low swells, +which can hardly be detected, to the most highly contorted and +complicated structures. In _symmetric_ folds (Figs. 169 and 180) the +dips of the rocks on each side the axis of the fold are equal. In +_unsymmetrical_ folds one limb is steeper than the other, as in the +anticline in Figure 167. In _overturned_ folds (Figs. 167 and 172) one +limb is inclined beyond the perpendicular. _Fan folds_ have been so +pinched that the original anticlines are left broader at the top than +at the bottom (Fig. 173). + + [Illustration: Fig. 172. Overturned Fold, Vermont] + +In folds where the compression has been great the layers are often +found thickened at the crest and thinned along the limbs (174). Where +strong rocks such as heavy limestones are folded together with weak +rocks such as shales, the strong rocks are often bent into great +simple folds, while the weak rocks are minutely crumpled. + + [Illustration: Fig. 173. Fan Folds, the Alps] + +=Systems of folds.= As a rule, folds occur in systems. Over the +Appalachian mountain belt, for example, extending from northeastern +Pennsylvania to northern Alabama and Georgia, the earth's crust has +been thrown into a series of parallel folds whose axes run from +northeast to southwest (Fig. 175). In Pennsylvania one may count a +score or more of these earth waves,--some but from ten to twenty miles +in length, and some extending as much as two hundred miles before they +die away. On the eastern part of this belt the folds are steeper and +more numerous than on the western side. + + [Illustration: Fig. 174. Folds with Layers thickened at the + Crest and thinned along the Limbs] + +=Cause and conditions of folding.= The sections which we have studied +suggest that rocks are folded by lateral pressure. While a single, +simple fold might be produced by a heave, a series of folds, including +overturns, fan folds, and folds thickened on their crests at the +expense of their limbs, could only be made in one way,--by pressure +from the side. Experiment has reproduced all forms of folds by +subjecting to lateral thrust layers of plastic material such as wax. + +Vast as the force must have been which could fold the solid rocks of +the crust as one may crumple the leaves of a magazine in the fingers, +it is only under certain conditions that it could have produced the +results which we see. Rocks are brittle, and it is only when under a +_heavy load_ and by _great pressure slowly applied_, that they can +thus be folded and bent instead of being crushed to pieces. Under +these conditions, experiments prove that not only metals such as +steel, but also brittle rocks such as marble, can be deformed and +molded and made to flow like plastic clay. + + [Illustration: Fig. 175. Relief Map of the Northern Appalachian + Region + + From Bingham's _Geographic Influences in American History_] + +=Zone of flow, zone of flow and fracture, and zone of fracture.= We +may believe that at depths which must be reckoned in tens of thousands +of feet the load of overlying rocks is so great that rocks of all +kinds yield by folding to lateral pressure, and flow instead of +breaking. Indeed, at such profound depths and under such inconceivable +weight no cavity can form, and any fractures would be healed at once +by the welding of grain to grain. At less depths there exists a zone +where soft rocks fold and flow under stress, and hard rocks are +fractured; while at and near the surface hard and soft rocks alike +yield by fracture to strong pressure. + + +Structures developed in Compressed Rocks + +Deformed rocks show the effects of the stresses to which they have +yielded, not only in the immense folds into which they have been +thrown but in their smallest parts as well. A hand specimen of slate, +or even a particle under the microscope, may show plications similar +in form and origin to the foldings which have produced ranges of +mountains. A tiny flake of mica in the rocks of the Alps may be +puckered by the same resistless forces which have folded miles of +solid rock to form that lofty range. + +=Slaty cleavage.= Rocks which have yielded to pressure often split +easily in a certain direction across the bedding planes. This cleavage +is known as slaty cleavage, since it is most perfectly developed in +fine-grained, homogeneous rocks, such as slates, which cleave to the +thin, smooth-surfaced plates with which we are familiar in the slates +used in roofing and for ciphering and blackboards. In coarse-grained +rocks, pressure develops more distant partings which separate the +rocks into blocks. + +Slaty cleavage cannot be due to lamination, since it commonly crosses +bedding planes at an angle, while these planes have been often +well-nigh or quite obliterated. Examining slate with a microscope, we +find that its cleavage is due to the grain of the rock. Its particles +are flattened and lie with their broad faces in parallel planes, along +which the rock naturally splits more easily than in any other +direction. The irregular grains of the mud which has been altered to +slate have been squeezed flat by a pressure exerted at right angles to +the plane of cleavage. Cleavage is found only in folded rocks, and, as +we may see in Figure 176, the strike of the cleavage runs parallel to +the strike of the strata and the axis of the folds. The dip of the +cleavage is generally steep, hence the pressure was nearly horizontal. +The pressure which has acted at right angles to the cleavage, and to +which it is due, is the same lateral pressure which has thrown the +strata into folds. + + [Illustration: Fig. 176. Slaty Cleavage] + +We find additional proof that slates have undergone compression at +right angles to their cleavage in the fact that any inclusions in +them, such as nodules and fossils, have been squeezed out of shape and +have their long diameters lying in the planes of cleavage. + +That pressure is competent to cause cleavage is shown by experiment. +Homogeneous material of fine grain, such as beeswax, when subjected to +heavy pressure cleaves at right angles to the direction of the +compressing force. + +=Rate of folding.= All the facts known with regard to rock deformation +agree that it is a secular process, taking place so slowly that, like +the deepening of valleys by erosion, it escapes the notice of the +inhabitants of the region. It is only under stresses slowly applied +that rocks bend without breaking. The folds of some of the highest +mountains have risen so gradually that strong, well-intrenched rivers +which had the right of way across the region were able to hold to +their courses, and as a circular saw cuts its way through the log +which is steadily driven against it, so these rivers sawed their +gorges through the fold as fast as it rose beneath them. Streams which +thus maintain the course which they had antecedent to a deformation of +the region are known as _antecedent_ streams. Examples of such are the +Sutlej and other rivers of India, whose valleys trench the outer +ranges of the Himalayas and whose earlier river deposits have been +upturned by the rising ridges. On the other hand, mountain crests are +usually divides, parting the head waters of different drainage +systems. In these cases the original streams of the region have been +broken or destroyed by the uplift of the mountain mass across their +paths. + +On the whole, which have worked more rapidly, processes of deformation +or of denudation? + + [Illustration: Fig. 177. An Unroofed Anticline] + + +Land Forms due to Folding + +As folding goes on so slowly, it is never left to form surface +features unmodified by the action of other agencies. An anticlinal +fold is attacked by erosion as soon as it begins to rise above the +original level, and the higher it is uplifted, and the stronger are +its slopes, the faster is it worn away. Even while rising, a young +upfold is often thus unroofed, and instead of appearing as a long, +Smooth, boat-shaped ridge, it commonly has had opened along the rocks +of the axis, when these are weak, a valley which is overlooked by the +infacing escarpments of the hard layers of the sides of the fold (Fig. +177). Under long-continued erosion, anticlines may be degraded to +valleys, while the synclines of the same system may be left in relief +as ridges (Fig. 167). + +=Folded mountains.= The vastness of the forces which wrinkle the crust +is best realized in the presence of some lofty mountain range. All +mountains, indeed, are not the result of folding. Some, as we shall +see, are due to upwarps or to fractures of the crust; some are piles +of volcanic material; some are swellings caused by the intrusion of +molten matter beneath the surface; some are the relicts left after the +long denudation of high plateaus. + + [Illustration: Fig. 178. Mountain Peaks carved in Folded + Strata, Rocky Mountains, Montana] + +But most of the mountain ranges of the earth, and some of the +greatest, such as the Alps and the Himalayas, were originally +mountains of folding. The earth's crust has wrinkled into a fold; or +into a series of folds, forming a series of parallel ridges and +intervening valleys; or a number of folds have been mashed together +into a vast upswelling of the crust, in which the layers have been so +crumpled and twisted, overturned and crushed, that it is exceedingly +difficult to make out the original structure. + +The close and intricate folds seen in great mountain ranges were +formed, as we have seen, deep below the surface, within the zone of +folding. Hence they may never have found expression in any individual +surface features. As the result of these deformations deep under +ground the surface was broadly lifted to mountain height, and the +crumpled and twisted mountain structures are now to be seen only +because erosion has swept away the heavy cover of surface rocks under +whose load they were developed. + + [Illustration: Fig. 179. Section of a Portion of the Alps] + +When the structure of mountains has been deciphered it is possible to +estimate roughly the amount of horizontal compression which the region +has suffered. If the strata of the folds of the Alps were smoothed +out, they would occupy a belt seventy-four miles wider than that to +which they have been compressed, or twice their present width. A +section across the Appalachian folds in Pennsylvania shows a +compression to about two thirds the original width; the belt has been +shortened thirty-five miles in every hundred. + +Considering the thickness of their strata, the compression which +mountains have undergone accounts fully for their height, with enough +to spare for all that has been lost by denudation. + +The Appalachian folds involve strata thirty thousand feet in +thickness. Assuming that the folded strata rested on an unyielding +foundation, and that what was lost in width was gained in height, what +elevation would the range have reached had not denudation worn it as +it rose? + +=The life history of mountains.= While the disturbance and uplift of +mountain masses are due to deformation, their sculpture into ridges +and peaks, valleys and deep ravines, and all the forms which meet the +eye in mountain scenery, excepting in the very youngest ranges, is due +solely to erosion. We may therefore classify mountains according to +the degree to which they have been dissected. The Juras are an example +of the stage of early youth, in which the anticlines still persist as +ridges and the synclines coincide with the valleys; this they owe as +much to the slight height of their uplift as to the recency of its +date (Fig. 180). + + [Illustration: Fig. 180. Section of a Portion of the Jura + Mountains] + +The Alps were upheaved at various times (Fig. 399), the last uplift +being later than the uplift of the Juras, but to so much greater +height that erosion has already advanced them well on towards +maturity. The mountain mass has been cut to the core, revealing +strange contortions of strata which could never have found expression +at the surface. Sharp peaks, knife-edged crests, deep valleys with +ungraded slopes subject to frequent landslides, are all features of +Alpine scenery typical of a mountain range at this stage in its life +history. They represent the survival of the hardest rocks and the +strongest structures, and the destruction of the weaker in their long +struggle for existence against the agents of erosion. Although miles +of rock have been removed from such ranges as the Alps, we need not +suppose that they ever stood much, if any, higher than at present. All +this vast denudation may easily have been accomplished while their +slow upheaval was going on; in several mountain ranges we have +evidence that elevation has not yet ceased. + + [Illustration: Fig. 181. Young Mountains, Rocky Mountains of + Canada] + +Under long denudation mountains are subdued to the forms +characteristic of old age. The lofty peaks and jagged crests of their +earlier life are smoothed down to low domes and rounded crests. The +southern Appalachians and portions of the Hartz Mountains in Germany +(Fig. 182) are examples of mountains which have reached this stage. + + [Illustration: Fig. 182. Subdued Mountains, the Hartz + Mountains, Germany] + +There are numerous regions of upland and plains in which the rocks are +found to have the same structure that we have seen in folded +mountains; they are tilted, crumpled, and overturned, and have clearly +suffered intense compression. We may infer that their folds were once +lifted to the height of mountains and have since been wasted to +low-lying lands. Such a section as that of Figure 67 illustrates how +ancient mountains may be leveled to their roots, and represents the +final stage to which even the Alps and the Himalayas must sometime +arrive. Mountains, perhaps of Alpine height, once stood about Lake +Superior; a lofty range once extended from New England and New Jersey +southwestward to Georgia along the Piedmont belt. In our study of +historic geology we shall see more clearly how short is the life of +mountains as the earth counts time, and how great ranges have been +lifted, worn away, and again upheaved into a new cycle of erosion. + +=The sedimentary history of folded mountains.= We may mention here +some of the conditions which have commonly been antecedent to great +foldings of the crust. + +1. Mountain ranges are made of belts of enormously and exceptionally +thick sediments. The strata of the Appalachians are thirty thousand +feet thick, while the same formations thin out to five thousand feet +in the Mississippi valley. The folds of the Wasatch Mountains involve +strata thirty thousand feet thick, which thin to two thousand feet in +the region of the Plains. + +2. The sedimentary strata of which mountains are made are for the most +part the shallow-water deposits of continental deltas. Mountain ranges +have been upfolded along the margins of continents. + +3. Shallow-water deposits of the immense thickness found in mountain +ranges can be laid only in a gradually sinking area. A profound +subsidence, often to be reckoned in tens of thousands of feet, +precedes the upfolding of a mountain range. + +Thus the history of mountains of folding is as follows: For long ages +the sea bottom off the coast of a continent slowly subsides, and the +great trough, as fast as it forms, is filled with sediments, which at +last come to be many thousands of feet thick. The downward movement +finally ceases. A slow but resistless pressure sets in, and gradually, +and with a long series of many intermittent movements, the vast mass +of accumulated sediments is crumpled and uplifted into a mountain +range. + + +Fractures and Dislocations of the Crust + +Considering the immense stresses to which the rocks of the crust are +subjected, it is not surprising to find that they often yield by +fracture, like brittle bodies, instead of by folding and flowing, like +plastic solids. Whether rocks bend or break depends on the character +and condition of the rocks, the load of overlying rocks which they +bear, and the amount of the force and the slowness with which it is +applied. + +=Joints.= At the surface, where their load is least, we find rocks +universally broken into blocks of greater or less size by partings +known as joints. Under this name are included many division planes +caused by cooling and drying; but it is now generally believed that +the larger and more regular joints, especially those which run +parallel to the dip and strike of the strata, are fractures due to +up-and-down movements and foldings and twistings of the rocks. + + [Illustration: Fig. 183. Joints utilized by a River in widening + its Valley, Iowa] + +Joints are used to great advantage in quarrying, and we have seen how +they are utilized by the weather in breaking up rock masses, by rivers +in widening their valleys, by the sea in driving back its cliffs, by +glaciers in plucking their beds, and how they are enlarged in soluble +rocks to form natural passageways for underground waters. The ends of +the parted strata match along both sides of joint planes; in. joints +there has been little or no displacement of the broken rocks. + + [Illustration: Fig. 184. A Normal Fault] + +=Faults.= In Figure 184 the rocks have been both broken and dislocated +along the plane _ff´_. One side must have been moved up or down past +the other. Such a dislocation is called a fault. The amount of the +displacement, as measured by the vertical distance between the ends of +a parted layer, is the _throw_ (_cd_). The angle (_ff´v_) which the +fault plane makes with the vertical is the _hade_. In Figure 184 the +right side has gone down relatively to the left; the right is the side +of the downthrow, while the left is the side of the upthrow. Where the +fault plane is not vertical the surfaces on the two sides may be +distinguished as the _hanging wall_ (that on the right of Figure 184) +and the _foot wall_ (that on the left of the same figure). Faults +differ in throw from a fraction of an inch to many thousands of feet. + +=Slickensides.= If we examine the walls of a fault, we may find +further evidence of movement in the fact that the surfaces are +polished and grooved by the enormous friction which they have suffered +as they have ground one upon the other. These appearances, called +slickensides, have sometimes been mistaken for the results of glacial +action. + +=Normal faults.= Faults are of two kinds,--normal faults and thrust +faults. Normal faults, of which Figure 184 is an example, hade to the +downthrow; the hanging wall has gone down. The total length of the +strata has been increased by the displacement. It seems that the +strata have been stretched and broken, and that the blocks have +readjusted themselves under the action of gravity as they settled. + +=Thrust faults.= Thrust faults hade to the upthrow; the hanging wall +has gone up. Clearly such faults, where the strata occupy less space +than before, are due to lateral thrust. Folds and thrust faults are +closely associated. Under lateral pressure strata may fold to a +certain point and then tear apart and fault along the surface of least +resistance. Under immense pressure strata also break by shear without +folding. Thus, in Figure 185, the rigid earth block under lateral +thrust has found it easier to break along the fault plane than to +fold. Where such faults are nearly horizontal they are distinguished +as _thrust planes_. + + [Illustration: Fig. 185. A Thrust Fault] + +In all thrust faults one mass has been pushed over another, so as to +bring the underlying and older strata upon younger beds; and when the +fault planes are nearly horizontal, and especially when the rocks have +been broken into many slices which have slidden far one upon another, +the true succession of strata is extremely hard to decipher. + +In the Selkirk Mountains of Canada the basement rocks of the region +have been driven east for seven miles on a thrust plane, over rocks +which originally lay thousands of feet above them. + +Along the western Appalachians, from Virginia to Georgia, the mountain +folds are broken by more than fifteen parallel thrust planes, running +from northeast to southwest, along which the older strata have been +pushed westward over the younger. The longest continuous fault has +been traced three hundred and seventy-five miles, and the greatest +horizontal displacement has been estimated at not less than eleven +miles. + +=Crush breccia.= Rocks often do not fault with a clean and simple +fracture, but along a zone, sometimes several yards in width, in which +they are broken to fragments. It may occur also that strata which as a +whole yield to lateral thrust by folding include beds of brittle +rocks, such as thin-layered limestones, which are crushed to pieces by +the strain. In either case the fragments when recemented by +percolating waters form a rock known as a _crush breccia_ (pronounced +_bretcha_)(Fig. 186). + + [Illustration: Fig. 186. Breccia] + +Breccia is a term applied to any rock formed of cemented _angular_ +fragments. This rock may be made by the consolidation of volcanic +cinders, of angular waste at the foot of cliffs, or of fragments of +coral torn by the waves from coral reefs, as well as of strata crushed +by crustal movements. + + +Surface Features due to Dislocations + +=Fault scarps.= A fault of recent date may be marked at surface by a +scarp, because the face of the upthrown block has not yet been worn to +the level of the downthrow side. + +After the upthrown block has been worn down to this level, +differential erosion produces fault scarps wherever weak rocks and +resistant rocks are brought in contact along the fault plane; and the +harder rocks, whether on the upthrow or the downthrow side, emerge in +a line of cliffs. Where a fault is so old that no abrupt scarps +appear, its general course is sometimes marked by the line of division +between highland and lowland or hill and plain. Great faults have +sometimes brought ancient crystalline rocks in contact with weaker and +younger sedimentary rocks, and long after erosion has destroyed all +fault scarps the harder crystallines rise in an upland of rugged or +mountainous country which meets the lowland along the line of +faulting. + + [Illustration: Fig. 187. A Concealed Fault + + This fault may be inferred from the changes in strata in + passing along the strike, as from _b_ to _a´_ and from + _c_ to _b´_] + +The vast majority of faults give rise to no surface features. The +faulted region may be old enough to have been baseleveled, or the +rocks on both sides of the line of dislocation may be alike in their +resistance to erosion and therefore have been worn down to a common +slope. The fault may be entirely concealed by the mantle of waste, and +in such cases it can be inferred from abrupt changes in the character +or the strike and dip of the strata where they may outcrop near it +(Fig. 187). + + [Illustration: Fig. 188. East-West Section across the Broken + Plateau north of the Grand Canyon of the Colorado River, + Arizona] + +The plateau trenched by the Grand Canyon of the Colorado River +exhibits a series of magnificent fault scarps whose general course is +from north to south, marking the edges of the great crust blocks into +which the country has been broken. The highest part of the plateau is +a crust block ninety miles long and thirty-five miles in maximum +width, which has been hoisted to nine thousand three hundred feet +above, sea level. On the east it descends four thousand feet by a +monoclinal fold, which passes into a fault towards the north. On the +west it breaks down by a succession of terraces faced by fault scarps. +The throw of these faults varies from seven hundred feet to more than +a mile. The escarpments, however, are due in a large degree to the +erosion of weaker rock on the downthrow side. + + [Illustration: Fig. 189. The Fault separating the Highlands and + the Lowlands, Scotland] + +The Highlands of Scotland (Fig. 189) meet the Lowlands on the south +with a bold front of rugged hills along a line of dislocation which +runs across the country from sea to sea. On the one side are hills of +ancient crystalline rocks whose crumpled structures prove that they +are but the roots of once lofty mountains; on the other lies a lowland +of sandstone and other stratified rocks formed from the waste of those +long-vanished mountain ranges. Remnants of sandstone occur in places +on the north of the great fault, and are here seen to rest on the worn +and fairly even surface of the crystallines. We may infer that these +ancient mountains were reduced along their margins to low plains, +which were slowly lowered beneath the sea to receive a cover of +sedimentary rocks. Still later came an uplift and dislocation. On the +one side erosion has since stripped off the sandstones for the most +part, but the hard crystalline rocks yet stand in bold relief. On the +other side the weak sedimentary rocks have been worn down to lowlands. + +=Rift valleys.= In a broken region undergoing uplift or the unequal +settling which may follow, a slice inclosed between two fissures may +sink below the level of the crust blocks on either side, thus forming +a linear depression known as a rift valley, or valley of fracture. + + [Illustration: Fig. 190. Section from the Mountains of + Palestine to the Mountains of Moab across the Dead Sea + + _a_, ancient schists; _b_, Carboniferous strata; _c_, _d_, and + _e_, Cretaceous strata] + +One of the most striking examples of this rare type of valley is the +long trough which runs straight from the Lebanon Mountains of Syria on +the north to the Red Sea on the south, and whose central portion is +occupied by the Jordan valley and the Dead Sea. The plateau which it +gashes has been lifted more than three thousand feet above sea level, +and the bottom of the trough reaches a depth of two thousand six +hundred feet below that level in parts of the Dead Sea. South of the +Dead Sea the floor of the trough rises somewhat above sea level, and +in the Gulf of Akabah again sinks below it. This uneven floor could be +accounted for either by the profound warping of a valley of erosion or +by the unequal depression of the floor of a rift valley. But that the +trough is a true valley of fracture is proved by the fact that on +either side it is bounded by fault scarps and monoclinal folds. The +keystone of the arch has subsided. Many geologists believe that the +Jordan-Akabah trough, the long narrow basin of the Red Sea, and the +chain of down-faulted valleys which in Africa extends from the strait +of Bab-el-Mandeb as far south as Lake Nyassa--valleys which contain +more than thirty lakes--belong to a single system of dislocation. + +Should you expect the lateral valleys of a rift valley at the time of +its formation to enter it as hanging valleys or at a common level? + +=Block mountains.= Dislocations take place on so grand a scale that by +the upheaval of blocks of the earth's crust or the downfaulting of +the blocks about one which is relatively stationary, mountains known +as block mountains are produced. A tilted crust block may present a +steep slope on the side upheaved and a more gentle descent on the side +depressed. + + [Illustration: Fig. 191. Block Mountains, Southern Oregon] + +=The Basin ranges.= The plateaus of the United States bounded by the +Rocky Mountains on the east, and on the west by the ranges which +front the Pacific, have been profoundly fractured and faulted. The +system of great fissures by which they are broken extends north and +south, and the long, narrow, tilted crust blocks intercepted between +the fissures give rise to the numerous north-south ranges of the +region. Some of the tilted blocks, as those of southern Oregon, are as +yet but moderately carved by erosion, and shallow lakes lie on the +waste that has been washed into the depressions between them (Fig. +191). We may therefore conclude that their displacement is somewhat +recent. Others, as those of Nevada, are so old that they have been +deeply dissected; their original form has been destroyed by erosion, +and the intermontane depressions are occupied by wide plains of waste. + +=Dislocations and river valleys.= Before geologists had proved that +rivers can by their own unaided efforts cut deep canyons, it was +common to consider any narrow gorge as a gaping fissure of the crust. +This crude view has long since been set aside. A map of the plateaus +of northern Arizona shows how independent of the immense faults of the +region is the course of the Colorado River. In the Alps the tunnels on +the Saint Gotthard railway pass six times beneath the gorge of the +Reuss, but at no point do the rocks show the slightest trace of a +fault. + + [Illustration: Fig. 192. Fault crossing Valley in Japan] + +=Rate of dislocation.= So far as human experience goes, the earth +movements which we have just studied, some of which have produced +deep-sunk valleys and lofty mountain ranges, and faults whose throw is +to be measured in thousands of feet, are slow and gradual. They are +not accomplished by a single paroxysmal effort, but by slow creep and +a series of slight slips continued for vast lengths of time. + +In the Aspen mining district in Colorado faulting is now going on at a +comparatively rapid rate. Although no sudden slips take place, the +creep of the rock along certain planes of faulting gradually bends out +of shape the square-set timbers in horizontal drifts and has closed +some vertical shafts by shifting the upper portion across the lower. +Along one of the faults of this region it is estimated that there has +been a movement of at least four hundred feet since the Glacial epoch. +More conspicuous are the instances of active faulting by means of +sudden slips. In 1891 there occurred along an old fault plane in Japan +a slip which produced an earth rent traced for fifty miles (Fig. 192). +The country on one side was depressed in places twenty feet below that +on the other, and also shifted as much as thirteen feet horizontally +in the direction of the fault line. + +In 1872 a slip occurred for forty miles on the great line of +dislocation which runs along the eastern base of the Sierra Nevada +Mountains. In the Owens valley, California, the throw amounted to +twenty-five feet in places, with a horizontal movement along the fault +line of as much as eighteen feet. Both this slip and that in Japan +just mentioned caused severe earthquakes. + +For the sake of clearness we have described oscillations, foldings, +and fractures of the crust as separate processes, each giving rise to +its own peculiar surface features, but in nature earth movements are +by no means so simple,--they are often implicated with one another: +folds pass into faults; in a deformed region certain rocks have bent, +while others under the same strain, but under different conditions of +plasticity and load, have broken; folded mountains have been worn to +their roots, and the peneplains to which they have been denuded have +been upwarped to mountain height and afterwards dissected,--as in the +case of the Allegheny ridges, the southern Carpathians, and other +ranges,--or, as in the case of the Sierra Nevada Mountains, have been +broken and uplifted as mountains of fracture. + +Draw the following diagrams, being careful to show the direction +in which the faulted blocks have moved, by the position of the two +parts of some well-defined layer of limestone, sandstone, or +shale, which occurs on each side of the fault plane, as in Figure +184. + +1. A normal fault with a hade of 15°, the original fault +scarp remaining. + +2. A normal fault with a hade of 50°, the original fault +scarp worn away, showing cliffs caused by harder strata on the +downthrow side. + +3. A thrust fault with a hade of 30°, showing cliffs due to +harder strata outcropping on the downthrow. + +4. A thrust fault with a hade of 80°, with surface +baseleveled. + +5. In a region of normal faults a coal mine is being worked along +the seam of coal _AB_ (Fig. 193). At _B_ it is found broken by a fault +f which hades toward _A_. To find the seam again, should you advise +tunneling up or down from _B_? + + [Illustration: Fig. 193] + +6. In a vertical shaft of a coal mine the same bed of coal is +pierced twice at different levels because of a fault. Draw a +diagram to show whether the fault is normal or a thrust. + + [Illustration: Fig. 194. Ridges to be explained by Faulting] + +7. Copy the diagram in Figure 194, showing how the two ridges may +be accounted for by a single resistant stratum dislocated by a +fault. Is the fault a _strike fault_, i.e. one running parallel with +the strike of the strata, or a _dip fault_, one running parallel +with the direction of the dip? + + [Illustration: Fig. 195. Earth Block of Tilted Strata, with + Included Seam of Coal _cc_] + +8. Draw a diagram of the block in Figure 195 as it would appear if +dislocated along the plane _efg_ by a normal fault whose throw equals +one fourth the height of the block. Is the fault a strike or a dip +fault? Draw a second diagram showing the same block after denudation +has worn it down below the center of the upthrown side. Note that the +outcrop of the coal seam is now deceptively repeated. This exercise +may be done in blocks of wood instead of drawings. + + [Illustration: Fig. 196. _A_ and _B_. Repeated Outcrops of Same + Strata] + +9. Draw diagrams showing by dotted lines the conditions both of _A_ +and _B_, Figure 196, after deformation had given the strata their +present attitude. + + [Illustration: Fig. 197. A Block Mountain] + +10. What is the attitude of the strata of this earth block, Figure +197? What has taken place along the plane _baf_? When did the +dislocation occur compared with the folding of the strata? With the +erosion of the valleys on the right-hand side of the mountain? With +the deposition of the sediments _efg_? Do you find any remnants of the +original surface _baf_ produced by the dislocation? From the left-hand +side of the mountain infer what was the relief of the region before +the dislocation. Give the complete history recorded in the diagram +from the deposition of the strata to the present. + + [Illustration: Fig. 198. A Faulted Lava Flow _aa´_] + +11. Which is the older fault, in Figure 198, _f_ or _f´_? When did the +lava flow occur? How long a time elapsed between the formation of the +two faults as measured in the work done in the interval? How long a +time since the formation of the later fault? + + [Illustration: Fig. 199. Measurement of the Thickness of + Inclined Strata] + +12. Measure by the scale the thickness _bc_ of the coal-bearing strata +outcropping from _a_ to _b_ in Figure 199. On any convenient scale +draw a similar section of strata with a dip of 30° outcropping along a +horizontal line normal to the strike one thousand feet in length, and +measure the thickness of the strata by the scale employed. The +thickness may also be calculated by trigonometry. + + [Illustration: Fig. 200. Unconformity between Parallel Strata] + + [Illustration: Fig. 201. Unconformity between Non-parallel + Strata] + + +Unconformity + +Strata deposited one upon, another in an unbroken succession are said +to be _conformable_. But the continuous deposition of strata is often +interrupted by movements of the earth's crust, Old sea floors are +lifted to form land and are again depressed beneath the sea to receive +a cover of sediments only after an interval during which they were +carved by subaërial erosion. An erosion surface which thus parts older +from younger strata is known as an _unconformity_, and the strata +above it are said to be _unconformable_ with the rocks below, or to +rest unconformably upon them. An unconformity thus records movements +of the crust and a consequent break in the deposition of the strata. +It denotes a period of land erosion of greater or less length, which +may sometimes be roughly measured by the stage in the erosion cycle +which the land surface had attained before its burial. Unconformable +strata may be _parallel_, as in Figure 200, where the record includes +the deposition of strata _a_, their emergence, the erosion of the land +surface _ss_, a submergence and the deposit of the strata _b_, and +lastly, emergence and the erosion of the present surface _s´s´_. + + [Illustration: Fig. 202. Carboniferous Limestone resting + unconformably on Early Silurian Slates, Yorkshire, England] + +Often the earth movements to which the uplift or depression was due +involved tilting or folding of the earlier strata, so that the strata +are now nonparallel as well as unconformable. In Figure 201, for +example, the record includes deposition, uplift, and _tilting_ of _a_; +erosion, depression, the deposit of _b_; and finally the uplift which +has brought the rocks to open air and permitted the dissection by +which the unconformity is revealed. + +From this section we infer that during early Silurian times the area +was sea, and thick sea muds were laid upon it. These were later +altered to hard slates by pressure and upfolded into mountains. During +the later Silurian and the Devonian the area was land and suffered +vast denudation. In the Carboniferous period it was lowered beneath +the sea and received a cover of limestone. + + [Illustration: Fig. 203. Diagram Illustrating how the Age of + Mountains is determined] + +=The age of mountains.= It is largely by means of unconformities that +we read the history of mountain making and other deformations and +movements of the crust. In Figure 203, for example, the deformation +which upfolded the range of mountains took place after the deposit of +the series of strata a of which the mountains are composed, and before +the deposit of the stratified rocks, which rest unconformably on a and +have not shared their uplift. + + [Illustration: Fig. 204. Section of Mountain Range showing + repeated Uplifts + + _a_, strata whose folding formed a mountain range; on, + baseleveled surface produced by long denudation of the + mountains; _b_, tilted strata resting unconformably on _a_; + _c_, horizontal strata parted from _b_ by the unconformity + _u´u´_. The first uplift of the range preceded the period of + time when _b_ was deposited. The and uplift, to which the + present mountains owe their height, was later than this period + but earlier than the period when strata _c_ were laid] + +Most great mountain ranges, like the Sierra Nevada and the Alps, mark +lines of weakness along which the earth's crust has yielded again and +again during the long ages of geological time. The strata deposited at +various times about their flanks have been infolded by later +crumplings with the original mountain mass, and have been repeatedly +crushed, inverted, faulted, intruded with igneous rocks, and denuded. +The structure of great mountain ranges thus becomes exceedingly +complex and difficult to read. A comparatively simple case of repeated +uplift is shown in Figure 204. In the section of a portion of the Alps +shown in Figure 179 a far more complicated history may be deciphered. + + [Illustration: Fig. 205. Unconformity showing Buried Valleys + + _lm_, limestone; _sh_, shale; _r_, _r´_, and _r´´_, river silts + filling eroded valleys in the limestone. The upper surface of + the limestone is evidently a land surface developed by erosion. + The valleys which trench it are narrow and steep-sided; hence + the land surface had not reached maturity. The sands and muds, + now hardened to firm rock, which fill these valleys, _r_, _r´_, + and _r´´_, contain no relics of the sea, but Instead the remains + of land animals and plants. They are river deposits, and we may + infer that owing to a subsidence the young rivers ceased to + degrade their channels and slowly filled their gorges with + sands and silts. The overlying shale records a further + depression which brought the lanes below the level of the sea. + A section similar to this is to be seen in the coal mines of + Bernissant, Belgium, where a gorge twice as deep as that of + Niagara was discovered within whose ancient river deposits were + found entombed the skeletons of more than a score of the huge + reptiles characteristic of the age when the gorge was cut and + filled] + + [Illustration: Fig. 206. Unconformity showing Buried Mountains, + Scotland + + _gn_, ancient crystalline rocks; _ss_, marine sandstones. The + surface _bb_ of the ancient crystalline rocks is mountainous, + with peaks rising to a height of as much as three thousand + feet. It is one of the most ancient land surfaces on the planet + and is covered unconformably with pre-Cambrian sandstones + thousands of feet in thickness, in which the Torridonian + Mountains of Scotland have been carved. What has been the + history of the region since the mountainous surface _bb_ was + produced by erosion?] + +=Unconformities in the Colorado Canyon, Arizona.= How geological +history may be read in unconformities is further illustrated in +Figures 207 and 208>. The dark crystalline rocks _a_ at the bottom of +the canyon are among the most ancient known, and are overlain +unconformably by a mass of tilted coarse marine sandstones _b_, whose +total thickness is not seen in the diagram and measures twelve +thousand feet perpendicularly to the dip. Both _a_ and _b_ rise to a +common level _nn´_ and upon them rest the horizontal sea-laid strata +_c_, in which the upper portion of the canyon has been cut. + + [Illustration: Fig. 207. Diagram of Wall of the Colorado + Canyon, Arizona, showing Unconformities] + +Note that the crystalline rocks a have been crumpled and crushed. +Comparing their structure with that of folded mountains, what do you +infer as to their relief after their deformation? To which surface +were they first worn down, _mm´_ or _nm_? Describe and account for the +surface _mm´_. How does it differ from the surface of the crystalline +rocks seen in the Torridonian Mountains (Fig. 206), and why? This +surface _mm´_ is one of the oldest land surfaces of which any vestige +remains. It is a bit of fossil geography buried from view since the +earliest geological ages and recently brought to light by the erosion +of the canyon. + + [Illustration: Fig. 208. View of the North Wall of the Grand + Canyon of the Colorado River, Arizona, showing the + Unconformities illustrated in Figure 207] + +How did the surface _mm´_ come to receive its cover of sandstones _b_? +From the thickness and coarseness of these sediments draw inferences +as to the land mass from which they were derived. Was it rising or +subsiding? high or low? Were its streams slow or swift? Was the amount +of erosion small or great? + +Note the strong dip of these sandstones _b_. Was the surface _mm´_ +tilted as now when the sandstones were deposited upon it? When was it +tilted? Draw a diagram showing the attitude of the rocks after this +tilting occurred, and their height relative to sea level. + +The surface _nn´_ is remarkably even, although diversified by some low +hills which rise into the bedded rocks of _c_, and it may be traced +for long distances up and down the canyon. Were the layers of _b_ and +the surface _mm´_ always thus cut short by _nn´_ as now? What has made +the surface _nn´_ so even? How does it come to cross the hard +crystalline rocks a and the weaker sandstones _b_ at the same +impartial level? How did the sediments of _c_ come to be laid upon it? +Give now the entire history recorded in the section, and in addition +that involved in the production of the platform _P_, shown in Figure +130, and that of the cutting of the canyon. How does the time involved +in the cutting of the canyon compare with that required for the +production of the surfaces _mm´_, _nn´_, and _P_? + + + + +CHAPTER X + +EARTHQUAKES + + +Any sudden movement of the rocks of the crust, as when they tear apart +when a fissure is formed or extended, or slip from time to time along +a growing fault, produces a jar called an earthquake, which spreads in +all directions from the place of disturbance. + +=The Charleston earthquake.= On the evening of August 31, 1886, the +city of Charleston, S.C., was shaken by one of the greatest +earthquakes which has occurred in the United States. A slight tremor +which rattled the windows was followed a few seconds later by a roar, +as of subterranean thunder, as the main shock passed beneath the city. +Houses swayed to and fro, and their heaving floors overturned +furniture and threw persons off their feet as, dizzy and nauseated, +they rushed to the doors for safety. In sixty seconds a number of +houses were completely wrecked, fourteen thousand chimneys were +toppled over, and in all the city scarcely a building was left without +serious injury. In the vicinity of Charleston railways were twisted +and trains derailed. Fissures opened in the loose superficial +deposits, and in places spouted water mingled with sand from shallow +underlying aquifers. + +The point of origin, or _focus_, of the earthquake was inferred from +subsequent investigations to be a rent in the rocks about twelve miles +beneath the surface. From the center of greatest disturbance, which +lay above the focus, a few miles northwest of the city, the surface +shock traveled outward in every direction, with decreasing effects, at +the rate of nearly two hundred miles per minute. It was felt from +Boston to Cuba, and from eastern Iowa to the Bermudas, over a circular +area whose diameter was a thousand miles. + +An earthquake is transmitted from the focus through the elastic rocks +of the crust, as a wave, or series of waves, of compression and +rarefaction, much as a sound wave is transmitted through the elastic +medium of the air. Each earth particle vibrates with exceeding +swiftness, but over a very short path. The swing of a particle in firm +rock seldom exceeds one tenth of an inch in ordinary earthquakes, and +when it reaches one half an inch and an inch, the movement becomes +dangerous and destructive. + + [Illustration: Fig. 210. Block of the Earth's Crust shaken by + an Earthquake + + _x_, focus; _a_, _b_, _c_, _d_, successive spheroidal waves in + the crust; _a´_, _b´_, _c´_, _d´_, successive surface waves + produced by the outcropping of _a_, _b_, _c_, and _d_] + +The velocity of earthquake waves, like that of all elastic waves, +varies with the temperature and elasticity of the medium. In the deep, +hot, elastic rocks they speed faster than in the cold and broken rocks +near the surface. The deeper the point of origin and the more violent +the initial shock, the faster and farther do the vibrations run. + +Great earthquakes, caused by some sudden displacement or some violent +rending of the rocks, shake the entire planet. Their waves run through +the body of the earth at the rate of about three hundred and fifty +miles a minute, and more slowly round its circumference, registering +their arrival at opposite sides of the globe on the exceedingly +delicate instruments of modern earthquake observatories. + +=Geological effects.= Even great earthquakes seldom produce geological +effects of much importance. Landslides may be shaken down from the +sides of mountains and hills, and cracks may be opened in the surface +deposits of plains; but the transient shiver, which may overturn +cities and destroy thousands of human lives, runs through the crust +and leaves it much the same as before. + +=Earthquakes attending great displacements.= Great earthquakes +frequently attend the displacement of large masses of the rocks of the +crust. In 1822 the coast of Chile was suddenly raised three or four +feet, and the rise was five or six feet a mile inland. In 1835 the +same region was again upheaved from two to ten feet. In each instance +a destructive earthquake was felt for one thousand miles along the +coast. + +The great California earthquake of 1906.= A sudden dislocation +occurred in 1906 along an ancient fault plane which extends for 300 +miles through western California. The vertical displacement did not +exceed four feet, while the horizontal shifting reached a maximum of +twenty feet. Fences, rows of trees, and roads which crossed the fault +were broken and offset. The latitude and longitude of all points over +thousands of square miles were changed. On each side of the fault the +earth blocks moved in opposite directions, the block on the east +moving southward and that on the west moving northward and to twice +the distance. East and west of the fault the movements lessened with +increasing distance from it. + +This sudden slip set up an earthquake lasting sixty-five seconds, +followed by minor shocks recurring for many days. In places the jar +shook down the waste on steep hillsides, snapped off or uprooted +trees, and rocked houses from their foundations or threw down their +walls or chimneys. The water mains of San Francisco were broken, and +the city was thus left defenseless against a conflagration which +destroyed $500,000,000 worth of property. The destructive effects +varied with the nature of the ground. Buildings on firm rock suffered +least, while those on deep alluvium were severely shaken by the +undulations, like water waves, into which the loose material was +thrown. Well-braced steel structures, even of the largest size, were +earthquake proof, and buildings of other materials, when honestly +built and intelligently designed to withstand earthquake shocks, +usually suffered little injury. The length of the intervals between +severe earthquakes in western California shows that a great +dislocation so relieves the stresses of the adjacent earth blocks that +scores of years may elapse before the stresses again accumulate and +cause another dislocation. + +Perhaps the most violent earthquake which ever visited the United +States attended the depression, in 1812, of a region seventy-five +miles long and thirty miles wide, near New Madrid, Mo. Much of the +area was converted into swamps and some into shallow lakes, while a +region twenty miles in diameter was bulged up athwart the channel of +the Mississippi. Slight quakes are still felt in this region from time +to time, showing that the strains to which the dislocation was due +have not yet been fully relieved. + +=Earthquakes originating beneath the sea.= Many earthquakes originate +beneath the sea, and in a number of examples they seem to have been +accompanied, as soundings indicate, by local subsidences of the ocean +bottom. There have been instances where the displacement has been +sufficient to set the entire Pacific Ocean pulsating for many hours. +In mid ocean the wave thus produced has a height of only a few feet, +while it may be two hundred miles in width. On shores near the point +of origin destructive waves two or three score feet in height roll in, +and on coasts thousands of miles distant the expiring undulations may +be still able to record themselves on tidal gauges. + +=Distribution of earthquakes.= Every half hour some considerable area +of the earth's surface is sensibly shaken by an earthquake, but +earthquakes are by no means uniformly distributed over the globe. As +we might infer from what we know as to their causes, earthquakes are +most frequent in regions now undergoing deformation. Such are young +rising mountain ranges, fault lines where readjustments recur from +time to time, and the slopes of suboceanic depressions whose steepness +suggests that subsidence may there be in progress. + +Earthquakes, often of extreme severity, frequently visit the lofty and +young ranges of the Andes, while they are little known in the subdued +old mountains of Brazil. The Highlands of Scotland are crossed by a +deep and singularly straight depression called the Great Glen, which +has been excavated along a very ancient line of dislocation. The +earthquakes which occur from time to time in this region, such as the +Inverness earthquake in 1891, are referred to slight slips along this +fault plane. + +In Japan, earthquakes are very frequent. More than a thousand are +recorded every year, and twenty-nine world-shaking earthquakes +occurred in the three years ending with 1901. They originate, for the +most part, well down on the eastern flank of the earth fold whose +summit is the mountainous crest of the islands, and which plunges +steeply beneath the sea to the abyss of the Tuscarora Deep. + +=Minor causes of earthquakes.= Since any concussion within the crust +sets up an earth jar, there are several minor causes of earthquakes, +such as volcanic explosions and even the collapse of the roofs of +caves. The earthquakes which attend the eruption of volcanoes are +local, even in the case of the most violent volcanic paroxysms known. +When the top of a volcano has been blown to fragments, the +accompanying earth shock has sometimes not been felt more than +twenty-five miles away. + +=Depth of focus.= The focus of the Charleston earthquake, estimated at +about twelve miles below the surface, was exceptionally deep. Volcanic +earthquakes are particularly shallow, and probably no earthquakes +known have started at a greater depth than fifteen or twenty miles. +This distance is so slight compared with the earth's radius that we +may say that earthquakes are but skin-deep. + +Should you expect the velocity of an earthquake to be greater in a +peneplain or in a river delta? + +After an earthquake, piles on which buildings rested were found driven +into the ground, and chimneys crushed at base. From what direction did +the shock come? + +Chimneys standing on the south walls of houses toppled over on the +roof. Should you infer that the shock in this case came from the north +or south? + +How should you expect a shock from the east to affect pictures hanging +on the east and the west walls of a room? how the pictures hanging on +the north and the south walls? + +In parts of the country, as in southwestern Wisconsin, slender erosion +pillars, or "monuments," are common. What inference could you draw as +to the occurrence in such regions of severe earthquakes in the recent +past? + + + + +CHAPTER XI + +VOLCANOES + + +Connected with movements of the earth's crust which take place so +slowly that they can be inferred only from their effects is one of the +most rapid and impressive of all geological processes,--the extrusion +of molten rock from beneath the surface of the earth, giving rise to +all the various phenomena of volcanoes. + +In a volcano, molten rock from a region deep below, which we may call +its reservoir, ascends through a pipe or fissure to the surface. The +materials erupted may be spread over vast areas, or, as is commonly +the case, may accumulate about the opening, forming a conical pile +known as the volcanic cone. It is to this cone that popular usage +refers the word _volcano_; but the cone is simply a conspicuous part +of the volcanic mechanism whose still more important parts, the +reservoir and the pipe, are hidden from view. + +Volcanic eruptions are of two types,--_effusive_ eruptions, in which +molten rock wells up from below and flows forth in streams of _lava_ +(a comprehensive term applied to all kinds of rock emitted from +volcanoes in a molten state), and _explosive_ eruptions, in which the +rock is blown out in fragments great and small by the expansive force +of steam. + + +Eruptions of the Effusive Type + +=The Hawaiian volcanoes.= The Hawaiian Islands are all volcanic +in origin, and have a linear arrangement characteristic of many +volcanic groups in all parts of the world. They are strung along a +northwest-southeast line, their volcanoes standing in two parallel +rows as if reared along two adjacent lines of fracture or folding. In +the northwestern islands the volcanoes have long been extinct and are +worn low by erosion. In the southeastern island. Hawaii, three +volcanoes are still active and in process of building. Of these Mauna +Loa, the monarch of volcanoes, with a girth of two hundred miles and a +height of nearly fourteen thousand feet above sea level, is a lava +dome the slope of whose sides does not average more than five degrees. +On the summit is an elliptical basin ten miles in circumference and +several hundred feet deep. Concentric cracks surround the rim, and +from time to time the basin is enlarged as great slices are detached +from the vertical walls and engulfed. + +Such a volcanic basin, formed by the insinking of the top of the cone, +is called a _caldera_. + + [Illustration: Fig. 211. Mauna Loa] + + [Illustration: Fig. 212. Caldera of Mauna Loa] + +On the flanks of Mauna Loa, four thousand feet above sea level, lies the +caldera of Kilauea, an independent volcano whose dome has been joined to +the larger mountain by the gradual growth of the two. In each caldera +the floor, which to the eye is a plain of black lava, is the congealed +surface of a column of molten rock. At times of an eruption lakes of +boiling lava appear which may be compared to air holes in a frozen +river. Great waves surge up, lifting tons of the fiery liquid a score of +feet in air, to fall back with a mighty plunge and roar, and +occasionally the lava rises several hundred feet in fountains of +dazzling brightness. The lava lakes may flood the floor of the basin, +but in historic times have never been known to fill it and overflow the +rim. Instead, the heavy column of lava breaks way through the sides of +the mountain and discharges in streams which flow down the mountain +slopes for a distance sometimes of as much as thirty-five miles. With +the drawing off of the lava the column in the duct of the volcano +lowers, and the floor of the caldera wholly or in part subsides. A black +and steaming abyss marks the place of the lava lakes (Fig. 213). After a +time the lava rises in the duct, the floor is floated higher, and the +boiling lakes reappear. + + [Illustration: Fig. 213. Portion of Caldera of Kilauea after + Collapse following an Eruption] + +The eruptions of the Hawaiian volcanoes are thus of the effusive type. +The column of lava rises, breaks through the side of the mountain, and +discharges in lava streams. There are no explosions, and usually no +earthquakes, or very slight ones, accompany the eruptions. The lava in +the calderas boils because of escaping steam, but the vapor emitted is +comparatively little, and seldom hangs above the summits in heavy +clouds. We see here in its simplest form the most impressive and +important fact in all volcanic action, molten rock has been driven +upward to the surface from some deep-lying source. + +=Lava flows.= As lava issues from the side of a volcano or overflows +from the summit, it flows away in a glowing stream resembling molten +iron drawn white-hot from an iron furnace. The surface of the stream +soon cools and blackens, and the hard crust of nonconducting rock may +grow thick and firm enough to form a tunnel, within which the fluid +lava may flow far before it loses its heat to any marked degree. Such +tunnels may at last be left as caves by the draining away of the lava, +and are sometimes several miles in length. + + [Illustration: Fig. 214. Pahoehoe Lava, Hawaii] + +=Pahoehoe and aa.= When the crust of highly fluid lava remains unbroken +after its first freezing, it presents a smooth, hummocky, and ropy +surface known by the Hawaiian term _pahoehoe_ (Fig. 214). On the other +hand, the crust of a viscid flow may be broken and splintered as it is +dragged along by the slowly moving mass beneath. The stream then appears +as a field of stones clanking and grinding on, with here and there from +some chink a dull red glow or a wisp of steam. It sets to a surface +called _aa_, of broken, sharp-edged blocks, which is often both +difficult and dangerous to traverse (Fig. 215). + + [Illustration: Fig. 215. Lava Flow of the _Aa_ Type, Cinder + Cones in the Distance, Arizona] + +=Fissure eruptions.= Some of the largest and most important outflows +of lava have not been connected with volcanic cones, but have been +discharged from fissures, flooding the country far and wide with +molten rock. Sheet after sheet of molten rock has been successively +outpoured, and there have been built up, layer upon layer, plateaus of +lava thousands of feet in thickness and many thousands of square miles +in area. + +=Iceland.= This island plateau has been rent from time to time by +fissures from which floods of lava have outpoured. In some instances +the lava discharges along the whole length of the fissure, but more +often only at certain points upon it. The Laki fissure, twenty miles +long, was in eruption in 1783 for seven months. The inundation of +fluid rock which poured from it is the largest of historic record, +reaching a distance of forty-seven miles and covering two hundred and +twenty square miles to an average depth of a hundred feet. At the +present time the fissure is traced by a line of several hundred +insignificant mounds of fragmental materials which mark where the lava +issued (Fig. 216). + +The distance to which the fissure eruptions of Iceland flow on slopes +extremely gentle is noteworthy. One such stream is ninety miles in +length, and another seventy miles long has a slope of little more than +one half a degree. + +Where lava is emitted at one point and flows to a less distance there +is gradually built up a dome of the shape of an inverted saucer with +an immense base but comparatively low. Many _lava domes_ have been +discovered in Iceland, although from their exceedingly gentle slopes, +often but two or three degrees, they long escaped the notice of +explorers. + +The entire plateau of Iceland, a region as large as Ohio, is composed +of volcanic products,--for the most part of successive sheets of lava +whose total thickness falls little short of two miles. The lava sheets +exposed to view were outpoured in open air and not beneath the sea; +for peat bogs and old forest grounds are interbedded with them, and +the fossil plants of these vegetable deposits prove that the plateau +has long been building and is very ancient. On the steep sea cliffs of +the island, where its structure is exhibited, the sheets of lava are +seen to be cut with many _dikes_,--fissures which have been filled by +molten rock,--and there is little doubt that it was through these +fissures that the lava outwelled in successive flows which spread far +and wide over the country and gradually reared the enormous pile of +the plateau. + + +Eruptions of the Explosive Type + +In the majority of volcanoes the lava which rises in the pipe is at +least in part blown into fragments with violent explosions and shot +into the air together with vast quantities of water vapor and various +gases. The finer particles into--which the lava is exploded are called +_volcanic dust_ or _volcanic ashes_, and are often carried long +distances by the wind before they settle to the earth. The coarser +fragments fall about the vent and there accumulate in a steep, +conical, volcanic mountain. As successive explosions keep open the +throat of the pipe, there remains on the summit a cup-shaped +depression called the _crater_. + +=Stromboli.= To study the nature of these explosions we may visit +Stromboli, a low volcano built chiefly of fragmental materials, which +rises from the sea off the north coast of Sicily and is in constant +though moderate action. + +Over the summit hangs a cloud of vapor which strikingly resembles the +column of smoke puffed from the smokestack of a locomotive, in that it +consists of globular masses, each the product of a distinct explosion. +At night the cloud of vapor is lighted with a red glow at intervals of +a few minutes, like the glow on the trail of smoke behind the +locomotive when from time to time the fire box is opened. Because of +this intermittent light flashing thousands of feet above the sea, +Stromboli has been given the name of the Lighthouse of the +Mediterranean. + +Looking down into the crater of the volcano, one sees a viscid lava +slowly seething. The agitation gradually increases. A great bubble +forms. It bursts with an explosion which causes the walls of the +crater to quiver with a miniature earthquake, and an outrush of steam +carries the fragments of the bubble aloft for a thousand feet to fall +into the crater or on the mountain side about it. With the explosion +the cooled and darkened crust of the lava is removed, and the light of +the incandescent liquid beneath is reflected from the cloud of vapor +which overhangs the cone. + +At Stromboli we learn the lesson that the explosive force in volcanoes +is that of steam. The lava in the pipe is permeated with it much as is +a thick boiling porridge. The steam in boiling porridge is unable to +escape freely and gathers into bubbles which in breaking spurt out +drops of the pasty substance; in the same way the explosion of great +bubbles of steam in the viscid lava shoots clots and fragments of it +into the air. + +=Krakatoa.= The most violent eruption of history, that of Krakatoa, a +small volcanic island in the strait between Sumatra and Java, occurred +in the last week of August, 1883. Continuous explosions shot a column +of steam and ashes. seventeen miles in air. A black cloud, beneath +which was midnight darkness and from which fell a rain of ashes and +stones, overspread the surrounding region to a distance of one hundred +and fifty miles. Launched on the currents of the upper air, the dust +was swiftly carried westward to long distances. Three days after the +eruption it fell on the deck of a ship sixteen hundred miles away, and +in thirteen days the finest impalpable powder from the volcano had +floated round the globe. For many months the dust hung over Europe and +America as a faint lofty haze illuminated at sunrise and sunset with +brilliant crimson. In countries nearer the eruption, as in India and +Africa, the haze for some time was so thick that it colored sun and +moon with blue, green, and copper-red tints and encircled them with +coronas. + +At a distance of even a thousand miles the detonations of the eruption +sounded like the booming of heavy guns a few miles away. In one +direction they were audible for a distance as great as that from San +Francisco to Cleveland. The entire atmosphere was thrown into +undulations under which all barometers rose and fell as the air waves +thrice encircled the earth. The shock of the explosions raised sea +waves which swept round the adjacent shores at a height of more than +fifty feet, and which were perceptible halfway around the globe. + +At the close of the eruption it was found that half the mountain had +been blown away, and that where the central part of the island had +been the sea was a thousand feet deep. + +=Martinique and St. Vincent.= In 1902 two dormant volcanoes of the +West Indies, Mt. Pelee in Martinique and Soufrière in St. Vincent, +broke into eruption simultaneously. No lava was emitted, but there +were blown into the air great quantities of ashes, which mantled the +adjacent parts of the islands with a pall as of gray snow. In early +stages of the eruption lakes which occupied old craters were +discharged and swept down the ash-covered mountain valleys in torrents +of boiling mud. + +On several occasions there was shot from the crater of each volcano a +thick and heavy cloud of incandescent ashes and steam, which rushed +down the mountain side like an avalanche, red with glowing stones and +scintillating with lightning flashes. Forests and buildings in its +path were leveled as by a tornado, wood was charred and set on fire by +the incandescent fragments, all vegetation was destroyed, and to +breathe the steam and hot, suffocating dust of the cloud was death to +every living creature. On the morning of the 8th of May, 1902, the +first of these peculiar avalanches from Mt. Pelee fell on the city of +St. Pierre and instantly destroyed the lives of its thirty thousand +inhabitants. + + [Illustration: Fig. 219. An Eruption of Vesuvius, 1872] + +The eruptions of many volcanoes partake of both the effusive and the +explosive types: the molten rock in the pipe is in part blown into the +air with explosions of steam, and in part is discharged in streams of +lava over the lip of the crater and from fissures in the sides of the +cone. Such are the eruptions of Vesuvius, one of which is illustrated +in Figure 219. + +=Submarine eruptions.= The many volcanic islands of the ocean and the +coral islands resting on submerged volcanic peaks prove that eruptions +have often taken place upon the ocean floor and have there built up +enormous piles of volcanic fragments and lava. The Hawaiian volcanoes +rise from a depth of eighteen thousand feet of water and lift their +heads to about thirty thousand feet above the ocean bed. Christmas +Island (see p. 194), built wholly beneath the ocean, is a coral-capped +volcanic peak, whose total height, as measured from the bottom of the +sea, is more than fifteen thousand feet. Deep-sea soundings have +revealed the presence of numerous peaks which fail to reach sea level +and which no doubt are submarine volcanoes. A number of volcanoes on +the land were submarine in their early stages, as, for example, the +vast pile of Etna, the celebrated Sicilian volcano, which rests on +stratified volcanic fragments containing marine shells now uplifted +from the sea. + +Submarine outflows of lava and deposits of volcanic fragments become +covered with sediments during the long intervals between eruptions. +Such volcanic deposits are said to be _contemporaneous_, because they +are formed during the same period as the strata among which they are +imbedded. Contemporaneous lava sheets may be expected to bake the +surface of the stratum on which they rest, while the sediments +deposited upon them are unaltered by their heat. They are among the +most permanent records of volcanic action, far outlasting the greatest +volcanic mountains built in open air. + +From upraised submarine volcanoes, such as Christmas Island, it is +learned that lava flows which are poured out upon the bottom of the +sea do not differ materially either in composition or texture from +those of the land. + + +Volcanic Products + +Vast amounts of steam are, as we have seen, emitted from volcanoes, +and comparatively small quantities of other vapors, such as various +acid and sulphurous gases. The rocks erupted from volcanoes differ +widely in chemical composition and in texture. + + [Illustration: Fig. 220. Cellular Lava] + +=Acidic and basic lavas.= Two classes of volcanic rocks may be +distinguished,--those containing a large proportion of silica (silicic +acid, SiO_{2}) and therefore called _acidic_, and those containing less +silica and a larger proportion of the bases (lime, magnesia, soda, +etc.) and therefore called _basic_. The acidic lavas, of which +_rhyolite_ and _thrachyte_ are examples, are comparatively light in +color and weight, and are difficult to melt. The basic lavas, of which +_basalt_ is a type, are dark and heavy and melt at a lower +temperature. + +=Scoria and pumice.= The texture of volcanic rocks depends in part on +the degree to which they were distended by the steam which permeated +them when in a molten state. They harden into compact rock where the +steam cannot expand. Where the steam is released from pressure, as on +the surface of a lava stream, it forms bubbles (steam blebs) of +various sizes, which give the hardened rock a cellular structure +(Fig. 220), In this way are formed the rough slags and clinkers called +_scoria_, which are found on the surface of flows and which are also +thrown out as clots of lava in explosive eruptions. + +On the surface of the seething lava in the throat of the volcano there +gathers a rock foam, which, when hurled into the air, is cooled and +falls as _pumice_,--a spongy gray rock so light that it floats on +water. + + [Illustration: Fig. 221. Amygdules in Lava] + +=Amygdules.= The steam blebs of lava flows are often drawn out from a +spherical to an elliptical form resembling that of an almond, and +after the rock has cooled these cavities are gradually filled with +minerals deposited from solution by underground water. From their +shape such casts are called amygdules (Greek, _amygdalon_, an almond). +Amygdules are commonly composed of silica. Lavas contain both silica +and the alkalies, potash and soda, and after dissolving the alkalies, +percolating water is able to take silica also into solution. Most +_agates_ are banded amygdules in which the silica has been laid in +varicolored, concentric layers (Fig. 222). + + [Illustration: Fig. 222. Polished Section of an Agate] + + [Illustration: Fig. 223. Microsection showing the Beginnings of + Crystal Growth in Glassy Lava] + +=Glassy and stony lavas.= Volcanic rocks differ in texture according +also to the rate at which they have solidified. When rapidly cooled, +as on the surface of a lava flow, molten rock chills to a glass, +because the minerals of which it is composed have not had time to +separate themselves from the fused mixture and form crystals. Under +slow cooling, as in the interior of the flow, it becomes a stony mass +composed of crystals set in a glassy paste. In thin slices of volcanic +glass one may see under the microscope the beginnings of crystal +growth in filaments and needles and feathery forms, which are the +rudiments of the crystals of various minerals. + +Spherulites, which also mark the first changes of glassy lavas toward +a stony condition, are little balls within the rock, varying from +microscopic size to several inches in diameter, and made up of +radiating fibers. + +Perlitic structure, common among glassy lavas, consists of microscopic +curving and interlacing cracks, due to contraction. + + [Illustration: Fig. 224. Perlitic Structure and Spherulites, + _a_, _a_] + + [Illustration: Fig. 225. Flow Lines in Lava] + +=Flow lines= are exhibited by volcanic rocks both to the naked eye and +under the microscope. Steam blebs, together with crystals and their +embryonic forms, are left arranged in lines and streaks by the +currents of the flowing lava as it stiffened into rock. + + [Illustration: Fig. 226. Porphyritic Structure] + +=Porphyritic structure.= Rocks whose ground mass has scattered through +it large conspicuous crystals (Fig. 226) are said to be _porphyritic_, +and it is especially among volcanic rocks that this structure occurs. +The ground mass of porphyries either may be glassy or may consist in +part of a felt of minute crystals; in either case it represents the +consolidation of the rock after its outpouring upon the surface. On +the other hand, the large crystals of porphyry have slowly formed deep +below the ground at an earlier date. + +=Columnar structure.= Just as wet starch contracts on drying to +prismatic forms, so lava often contracts on cooling to a mass of +close-set, prismatic, and commonly six-sided columns, which stand at +right angles to the cooling surface. The upper portion of a flow, on +rapid cooling from the surface exposed to the air, may contract to a +confused mass of small and irregular prisms; while the remainder forms +large and beautifully regular columns, which have grown upward by slow +cooling from beneath (Fig. 227). + + +Fragmental Materials + +Rocks weighing many tons are often thrown from a volcano at the +beginning of an outburst by the breaking up of the solidified floor of +the crater; and during the progress of an eruption large blocks may be +torn from the throat of the volcano by the outrush of steam. But the +most important fragmental materials are those derived from the lava +itself. As lava rises in the pipe, the steam which permeates it is +released from pressure and explodes, hurling the lava into the air in +fragments of all sizes,--large pieces of scoria, _lapilli_ (fragments +the size of a pea or walnut), volcanic "sand" and volcanic "ashes." +The latter resemble in appearance the ashes of wood or coal, but they +are not in any sense, like them, a residue after combustion. + + [Illustration: Fig. 227. Columnar Structure in Basaltic Lava, + Scotland] + +Volcanic ashes are produced in several ways: lava rising in the +volcanic duct is exploded into fine dust by the steam which permeates +it; glassy lava, hurled into the air and cooled suddenly, is brought +into a state of high strain and tension, and, like Prince Rupert's +drops, flies to pieces at the least provocation. The clash of rising +and falling projectiles also produces some dust, a fair sample of +which may be made by grating together two pieces of pumice. + +Beds of volcanic ash occur widely among recent deposits in the western +United States. In Nebraska ash beds are found in twenty counties, and +are often as white as powdered pumice. The beds grow thicker and +coarser toward the southwestern part of the state, where their +thickness sometimes reaches fifty feet. In what direction would you +look for the now extinct volcano whose explosive eruptions are thus +recorded? + +=Tuff.= This is a convenient term designating any rock composed of +volcanic fragments. Coarse tuffs of angular fragments are called +_volcanic breccia_, and when the fragments have been rounded and sorted +by water the rock is termed a _volcanic conglomerate_. Even when +deposited in the open air, as on the slopes of a volcano, tuffs may be +rudely bedded and their fragments more or less rounded, and unless +marine shells or the remains of land plants and animals are found as +fossils in them, there is often considerable difficulty in telling +whether they were laid in water or in air. In either case they soon +become consolidated. Chemical deposits from percolating waters fill +the interstices, and the bed of loose fragments is cemented to hard +rock. + +The materials of which tuffs are composed are easily recognized as +volcanic in their origin. The fragments are more or less cellular, +according to the degree to which they were distended with steam when +in a molten state, and even in the finest dust one may see the glass +or the crystals of lava from which it was derived. Tuffs often contain +_volcanic bombs_,--balls of lava which took shape while whirling in +the air, and solidified before falling to the ground. + + [Illustration: Fig. 228. Volcanic Bombs, Cinder Cone, California] + + [Illustration: Fig. 229. A Volcanic Cone, Arizona] + +=Ancient volcanic rocks.= It is in these materials and structures +which we have described that volcanoes leave some of their most +enduring records. Even the volcanic rocks of the earliest geological +ages, uplifted after long burial beneath the sea and exposed to view +by deep erosion, are recognized and their history read despite the +many changes which they may have undergone. A sheet of ancient lava +may be distinguished by its composition from the sediments among which +it is imbedded. The direction of its flow lines may be noted. The +cellular and slaggy surface where the pasty lava was distended by +escaping steam is recognized by the amygdules which now fill the +ancient steam blebs. In a pile of successive sheets of lava each flow +may be distinguished and its thickness measured; for the surface of +each sheet is glassy and scoriaceous, while beneath its upper portions +the lava of each flow is more dense and stony. The length of time +which elapsed before a sheet was buried beneath the materials of +succeeding eruptions may be told by the amount of weathering which it +had undergone, the depth of ancient soil--now baked to solid +rock--upon it, and the erosion which it had suffered in the interval. + +If the flow occurred from some submarine volcano, we may recognize the +fact by the sea-laid sediments which cover it, filling the cracks and +crevices of its upper surface and containing pieces of lava washed +from it in their basal layers. + +Long-buried glassy lavas devitrify, or pass to a stony condition, +under the unceasing action of underground waters; but their flow lines +and perlitic and spherulitic structures remain to tell of their +original state. + +Ancient tuffs are known by the fragmental character of their volcanic +material, even though they have been altered to firm rock. Some +remains of land animals and plants may be found imbedded to tell that +the beds were laid in open air; while the remains of marine organisms +would prove as surely that the tuffs were deposited in the sea. + +In these ways ancient volcanoes have been recognized near Boston, in +southeastern Pennsylvania, about Lake Superior, and in other regions +of the United States. + + +The Life History of a Volcano + +The invasion of a region by volcanic forces is attended by movements +of the crust heralded by earthquakes. A fissure or a pipe is opened +and the building of the cone or the spreading of wide lava sheets is +begun. + +=Volcanic cones.= The shape of a volcanic cone depends chiefly on the +materials erupted. Cones made of fragments may have sides as steep as +the angle of repose, which in the case of coarse scoria is sometimes +as high as thirty or forty degrees. About the base of the mountain the +finer materials erupted are spread in more gentle slopes, and are also +washed forward by rains and streams. The normal profile is thus a +symmetric cone with a flaring base. + + [Illustration: Fig. 230. Sarcoui, a Trachyte Dome, France] + +Cones built of lava vary in form according to the liquidity of the +lava. Domes of gentle slope, as those of Hawaii, for example, are +formed of basalt, which flows to long distances before it congeals. +When superheated and emitted from many vents, this easily melted lava +builds great plateaus, such as that of Iceland. On the other hand, +lavas less fusible, or poured out at a lower temperature, stiffen when +they have flowed but a short distance, and accumulate in a steep cone. +Trachyte has been extruded in a state so viscid that it has formed +steep-sided domes like that of Sarcoui (Fig. 230). + +Most volcanoes are built, like Vesuvius, both of lava flows and of +tuffs, and sections show that the structure of the cone consists of +outward-dipping, alternating layers of lava, scoria, and ashes. + + [Illustration: Fig. 231. Section of Vesuvius + + _V_, Vesuvius; _S_, Somma, a mountainous rampart half encircling + Vesuvius, and like it built of outward-dipping sheets of tuff and + lava; _a_, crystalline rocks; _b_, marine strata; _c_, tuffs + containing seashells. Which is the older mountain, Vesuvius or + Somma? Of what is Somma a remnant? Draw a diagram showing its + original outline. Suggest what processes may have brought it to its + present form. What record do you find of the earliest volcanic + activity? What do you infer as to the beginnings of the volcano?] + +From time to time the cone is rent by the violence of explosions and +by the weight of the column of lava in the pipe. The fissures are +filled with lava and some discharge on the sides of the mountain, +building parasitic cones, while all form dikes, which strengthen the +pile with ribs of hard rock and make it more difficult to rend. + +Great catastrophes are recorded in the shape of some volcanoes which +consist of a circular rim perhaps miles in diameter, inclosing a vast +crater or a caldera within which small cones may rise. We may infer +that at some time the top of the mountain has been blown off, or has +collapsed and been engulfed because some reservoir beneath had been +emptied by long-continued eruptions (Fig. 230). + +The cone-building stage may be said to continue until eruptions of +lava and fragmental materials cease altogether. Sooner or later the +volcanic forces shift or die away, and no further eruptions add to the +pile or replace its losses by erosion during periods of repose. Gases +however are still emitted, and, as sulphur vapors are conspicuous +among them, such vents are called _solfataras_. Mount Hood, in Oregon, +is an example of a volcano sunk to this stage. From a steaming rift on +its side there rise sulphurous fumes which, half a mile down the wind, +will tarnish a silver coin. + + [Illustration: Fig. 232. Crater Lake, Oregon + + How wide and deep is the basin which holds the lake? The + mountain walls which enclose it are made of outward-dipping + sheets of lava. Draw a diagram restoring the volcano of which + they are the remnant. No volcanic fragments of the same nature + as the materials of which the volcano is built are found about + the region. What theory of the destruction of the cone does + this fact favor? _W´_, Wizard Island, is a cinder cone. When was + it built?] + +=Geysers and hot springs.= The hot springs of volcanic regions are +among the last vestiges of volcanic heat. Periodically eruptive +boiling springs are termed geysers. In each of the geyser regions of +the earth--the Yellowstone National Park, Iceland, and New +Zealand--the ground water of the locality is supposed to be heated by +ancient lavas that, because of the poor conductivity of the rock, +still remain hot beneath the surface. + + [Illustration: Fig. 233. Old Faithful Geyser in Eruption, + Yellowstone National Park] + +=Old Faithful=, one of the many geysers of the Yellowstone National +Park, plays a fountain of boiling water a hundred feet in air; while +clouds of vapor from the escaping steam ascend to several times that +height. The eruptions take place at intervals of from seventy to +ninety minutes. In repose the geyser is a quiet pool, occupying a +craterlike depression in a conical mound some twelve feet high. The +conduit of the spring is too irregular to be sounded. The mound is +composed of porous silica deposited by the waters of the geyser. + +Geysers erupt at intervals instead of continuously boiling, because +their long, narrow, and often tortuous conduits do not permit a free +circulation of the water. After an eruption the tube is refilled and +the water again gradually becomes heated. Deep in the tube where it is +in contact with hot lavas the water sooner or later reaches the +boiling point, and bursting into steam shoots the water above it high +in air. + + [Illustration: Fig. 234. Terrace and Cones of Siliceous Sinter + deposited by Geysers, Yellowstone National Park] + +=Carbonated springs.= After all the other signs of life have gone, the +ancient volcano may emit carbon dioxide as its dying breath. The +springs of the region may long be charged with carbon dioxide, or +carbonated, and where they rise through limestone may be expected to +deposit large quantities of travertine. We should remember, however, +that many carbonated springs, and many hot springs, are wholly +independent of volcanoes. + + [Illustration: Fig. 235. Mount Shasta, California] + + [Illustration: Fig. 236. Mount Hood, Oregon] + +=The destruction of the cone.= As soon as the volcanic cone ceases to +grow by eruptions the agents of erosion begin to wear it down, and the +length of time that has elapsed since the period of active growth may +be roughly measured by the degree to which the cone has been +dissected. We infer that Mount Shasta, whose conical shape is still +preserved despite the gullies one thousand feet deep which trench its +sides (Fig. 235), is younger than Mount Hood, which erosive agencies +have carved to a pyramidal form (Fig. 236). The pile of materials +accumulated about a volcanic vent, no matter how vast in bulk, is at +last swept entirely away. The cone of the volcano, active or extinct, +is not old as the earth counts time; volcanoes are short-lived +geological phenomena. + + [Illustration: Fig. 237. Crandall Volcano] + +=Crandall volcano.= This name is given to a dissected ancient volcano +in the Yellowstone National Park, which once, it is estimated, reared +its head thousands of feet above the surrounding country and greatly +exceeded in bulk either Mount Shasta or Mount Etna. Not a line of the +original mountain remains; all has been swept away by erosion except +some four thousand feet of the base of the pile. This basal wreck now +appears as a rugged region about thirty miles in diameter, trenched by +deep valleys and cut into sharp peaks and precipitous ridges. In the +center of the area is found the nucleus (_N_, Fig. 237),--a mass of +coarsely crystalline rock that congealed deep in the old volcanic +pipe. From it there radiate in all directions, like the spokes of a +wheel, long dikes whose rock grows rapidly finer of grain as it leaves +the vicinity of the once heated core. The remainder of the base of the +ancient mountain is made of rudely bedded tuffs and volcanic breccia, +with occasional flows of lava, some of the fragments of the breccia +measuring as much as twenty feet in diameter. On the sides of canyons +the breccia is carved by rain erosion to fantastic pinnacles. At +different levels in the midst of these beds of tuff and lava are many +old forest grounds. The stumps and trunks of the trees, now turned to +stone, still in many cases stand upright where once they grew on the +slopes of the mountain as it was building (Fig. 238). The great size +and age of some of these trees indicate, the lapse of time between the +eruption whose lavas or tuffs weathered to the soil on which they grew +and the subsequent eruption which buried them beneath showers of +stones and ashes. + +Near the edge of the area lies Death Gulch, in which carbon dioxide is +given off in such quantities that in quiet weather it accumulates in a +heavy layer along the ground and suffocates the animals which may +enter it. + + [Illustration: Fig. 238. Fossil Tree Trunks, Yellowstone National Park] + + + + +CHAPTER XII + +UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN + + +It is because long-continued erosion lays bare the innermost anatomy +of an extinct volcano, and even sweeps away the entire pile with much +of the underlying strata, thus leaving the very roots of the volcano +open to view, that we are able to study underground volcanic +structures. With these we include, for convenience, intrusions of +molten rock which have been driven upward into the crust, but which +may not have succeeded in breaking way to the surface and establishing +a volcano. All these structures are built of rock forced when in a +fluid or pasty state into some cavity which it has found or made, and +we may classify them therefore, according to the shape of the molds in +which the molten rock has congealed, as (1) dikes, (2) volcanic necks, +(3) intrusive sheets, and (4) intrusive masses. + +=Dikes.= The sheet of once molten rock with which a fissure has been +filled is known as a dike. Dikes are formed when volcanic cones are +rent by explosions or by the weight of the lava column in the duct, +and on the dissection of the pile they appear as radiating vertical +ribs cutting across the layers of lava and tuff of which the cone is +built. In regions undergoing deformation rocks lying deep below the +ground are often broken and the fissures are filled with molten rock +from beneath, which finds no outlet to the surface. Such dikes are +common in areas of the most ancient rocks, which have been brought to +light by long erosion. + +In exceptional cases dikes may reach the length of fifty or one +hundred miles. They vary in width from a fraction of a foot to even as +much as three hundred feet. + + [Illustration: Fig. 239. Dikes, Spanish Peaks, Colorado] + +Dikes are commonly more fine of grain on the sides than in the center, +and may have a glassy and crackled surface where they meet the +inclosing rock. Can you account for this on any principle which you +have learned? + + [Illustration: Fig. 240. A Dissected Volcanic Cone + + _N_, volcanic neck; _l_, _l_, lava-topped table mountains; + _t_, _t_, beds of tuff; _d_, _d_, dikes; dotted lines indicate + the initial profile] + +=Volcanic necks.= The pipe of a volcano rises from far below the base +of the cone,--from the deep reservoir from which its eruptions are +supplied. When the volcano has become extinct this great tube remains +filled with hardened lava. It forms a cylindrical core of solid rock, +except for some distance below the ancient crater, where it may +contain a mass of fragments which had fallen back into the chimney +after being hurled into the air. + + [Illustration: Fig. 241. Mount Johnson, a Volcanic Neck near + Montreal] + +As the mountain is worn down, this central column known as the +_volcanic neck_ is left standing as a conical hill (Fig. 240). Even +when every other trace of the volcano has been swept away, erosion +will not have passed below this great stalk on which the volcano was +borne as a fiery flower whose site it remains to mark. In volcanic +regions of deep denudation volcanic necks rise solitary and abrupt +from the surrounding country as dome-shaped hills. They are marked +features in the landscape in parts of Scotland and in the St. Lawrence +valley about Montreal (Fig. 241). + + [Illustration: Fig. 242. The Palisades of the Hudson, New Jersey] + +=Intrusive sheets.= Sheets of igneous rocks are sometimes found +interleaved with sedimentary strata, especially in regions where the +rocks have been deformed and have suffered from volcanic action. In +some instances such a sheet is seen to be _contemporaneous_ (p. 248). +In other instances the sheet must be _intrusive_. The overlying +stratum, as well as that beneath, has been affected by the heat of the +once molten rock. We infer that the igneous rock when in a molten +state was forced between the strata, much as a card may be pushed +between the leaves of a closed book. The liquid wedged its way between +the layers, lifting those above to make room for itself. The source of +the intrusive sheet may often be traced to some dike (known therefore +as the _feeding dike_), or to some mass of igneous rock. + +Intrusive sheets may extend a score and more of miles, and, like the +longest surface flows, the most extensive sheets consist of the more +fusible and fluid lavas,--those of the basic class of which basalt is +an example. Intrusive sheets are usually harder than the strata in +which they lie and are therefore often left in relief after long +denudation of the region (Fig. 315). + + [Illustration: Fig. 243. Diagram of the Palisades of the Hudson + + _i_, intrusive sheet; _s_, sandstone; _d_, feeding dike; + _HR_, Hudson River] + +On the west bank of the Hudson there extends from New York Bay north +for thirty miles a bold cliff several hundred feet high,--the +_Palisades of the Hudson_. It is the outcropping edge of a sheet of +ancient igneous rock, which rests on stratified sandstones and is +overlain by strata of the same series. Sandstones and lava sheet +together dip gently to the west and the latter disappears from view +two miles back from the river. + +It is an interesting question whether the Palisades sheet is +_contemporaneous_ or _intrusive_. Was it outpoured on the sandstones +beneath it when they formed the floor of the sea, and covered +forthwith by the sediments of the strata above, or was it intruded +among these beds at a later date? + + [Illustration: Fig. 244. Section of Electric Peak. E. and Gray + Peak, G, Yellowstone National Park + + Intrusive sheets and masses of igneous rock are drawn in black] + +The latter is the case: for the overlying stratum is intensely baked +along the zone of contact. At the west edge of the sheet is found the +dike in which the lava rose to force its way far and wide between the +strata. + +_Electric Peak_, one of the prominent mountains of the Yellowstone +National Park, is carved out of a mass of strata into which many +sheets of molten rock have been intruded. The western summit consists +of such a sheet several hundred feet thick. Studying the section of +Figure 244, what inference do you draw as to the source of these +intrusive sheets? + + [Illustration: Fig. 245. Stone Mountain, Georgia, a Granite Boss] + + +Intrusive Masses + +=Bosses.= This name is generally applied to huge irregular masses of +coarsely crystalline igneous rock lying in the midst of other +formations. Bosses vary greatly in size and may reach scores of miles +in extent. Seldom are there any evidences found that bosses ever had +connection with the surface. On the other hand, it is often proved +that they have been driven, or have melted their way, upward into the +formations in which they lie; for they give off dikes and intrusive +sheets, and have profoundly altered the rocks about them by their +heat. + + [Illustration: Fig. 246. Map of Granite Bosses near Baltimore + (areas horizontally Lined) + +The texture of the rock of bosses proves that consolidation proceeded +slowly and at great depths, and it is only because of vast denudation +that they are now exposed to view. Bosses are commonly harder than the +rocks about them, and stand up, therefore, as rounded hills and +mountainous ridges long after the surrounding country has worn to a +low plain (Fig. 245). + +Figure 246 exhibits a few small bosses of granite near Baltimore as +examples of numerous areas of igneous rock within the Piedmont Belt +which represent bodies of molten rock which solidified deep below the +surface. + +The _Spanish Peaks_ of southeastern Colorado were formed by the +upthrust of immense masses of igneous rock, bulging and breaking the +overlying strata. On one side of the mountains the throw of the fault +is nearly a mile, and fragments of deep-lying beds were dragged upward +by the rising masses. The adjacent rocks were altered by heat to a +distance of several thousand feet. No evidence appears that the molten +rock ever reached the surface, and if volcanic eruptions ever took +place either in lava flows or fragmental materials, all traces of them +have been effaced. The rock of the intrusive masses is coarsely +crystalline, and no doubt solidified slowly under the pressure of vast +thicknesses of overlying rock, now mostly removed by erosion. + +A magnificent system of dikes radiates from the Peaks to a distance of +fifteen miles, some now being left by long erosion as walls a hundred +feet in height (Fig. 239). Intrusive sheets fed by the dikes penetrate +the surrounding strata, and their edges are cut by canyons as much as +twenty-five miles from the mountain. In these strata are valuable beds +of lignite, an imperfect coal, which the heat of dikes and sheets has +changed to coke. + + [Illustration: Fig. 247. Section of a Laccolith] + +=Laccoliths.= The laccolith (Greek laccos, cistern; lithos, stone) is +a variety of intrusive masses in which molten rock has spread between +the strata, and, lifting the strata above it to a dome-shaped form, +has collected beneath them in a lens-shaped body with a flat base. + +The _Henry Mountains_, a small group of detached peaks in southern +Utah, rise from a plateau of horizontal rocks. Some of the peaks are +carved wholly in separate domelike uplifts of the strata of the +plateau. In others, as Mount Hillers, the largest of the group, there +is exposed on the summit a core of igneous rock from which the +sedimentary rocks of the flanks dip steeply outward in all directions. +In still others erosion has stripped off the covering strata and has +laid bare the core to its base; and its shape is here seen to be that +of a plano-convex lens or a baker's bun, its flat base resting on the +undisturbed bedded rocks beneath. The structure of Mount Hillers is +shown in Figure 248. The nucleus of igneous rock is four miles in +diameter and more than a mile in depth. + + [Illustration: Fig. 248. Section of Mount Hillers] + +=Regional intrusions.= These vast bodies of igneous rock, which may +reach hundreds of miles in diameter, differ little from bosses except +in their immense bulk. Like bosses, regional intrusions give off dikes +and sheets and greatly change the rocks about them by their heat. They +are now exposed to view only because of the profound denudation which +has removed the upheaved dome of rocks beneath which they slowly +cooled. Such intrusions are accompanied--whether as cause or as +effect is still hardly known--by deformations, and their masses of +igneous rock are thus found as the core of many great mountain ranges. +The granitic masses of which the Bitter Root Mountains and the Sierra +Nevadas have been largely carved are each more than three hundred +miles in length. Immense regional intrusions, the cores of once lofty +mountain ranges, are found upon the Laurentian peneplain. + +=Physiographic effects of intrusive masses.= We have already seen +examples of the topographic effects of intrusive masses in Mount +Hillers, the Spanish Peaks, and in the great mountain ranges mentioned +in the paragraph on regional intrusions, although in the latter +instances these effects are entangled with the effects of other +processes. Masses of igneous rock cannot be intruded within the crust +without an accompanying deformation on a scale corresponding to the +bulk of the intruded mass. The overlying strata are arched into hills +or mountains, or, if the molten material is of great extent, the +strata may conceivably be floated upward to the height of a plateau. +We may suppose that the transference of molten matter from one region +to another may be among the causes of slow subsidences and elevations. +Intrusions give rise to fissures, dikes, and intrusive sheets, and +these dislocations cannot fail to produce earthquakes. Where intrusive +masses open communication with the surface, volcanoes are established +or fissure eruptions occur such as those of Iceland. + + +The Intrusive Rocks + +The igneous rocks are divided into two general classes,--the +_volcanic_ or _eruptive_ rocks, which have been outpoured in open air +or on the floor of the sea, and the _intrusive_ rocks, which have been +intruded within the rocks of the crust and have solidified below the +surface. The two classes are alike in chemical composition and may be +divided into acidic and basic groups. In texture the intrusive rocks +differ from the volcanic rocks because of the different conditions +under which they have solidified. They cooled far more slowly beneath +the cover of the rocks into which they were pressed than is permitted +to lava flows in open air. Their constituent minerals had ample +opportunity to sort themselves and crystallize from the fluid mixture, +and none of that mixture was left to congeal as a glassy paste. + +They consolidated also under pressure. They are never scoriaceous, for +the steam with which they were charged was not allowed to expand and +distend them with steam blebs. In the rocks of the larger intrusive +masses one may see with a powerful microscope exceedingly minute +cavities, to be counted by many millions to the cubic inch, in which +the gaseous water which the mass contained was held imprisoned under +the immense pressure of the overlying rocks. + +Naturally these characteristics are best developed in the intrusives +which cooled most slowly, i.e. in the deepest-seated and largest +masses; while in those which cooled more rapidly, as in dikes and +sheets, we find gradations approaching the texture of surface flows. + +=Varieties of the intrusive rocks.= We will now describe a few of the +varieties of rocks of deep-seated intrusions. All are even grained, +consisting of a mass of crystalline grains formed during one +continuous stage of solidification, and no porphyritic crystals appear +as in lavas. + +_Granite_, as we have learned already, is composed of three +minerals,--quartz, feldspar, and mica. According to the color of the +feldspar the rock may be red, or pink, or gray. Hornblende--a black or +dark green mineral, an iron-magnesian silicate, about as hard as +feldspar--is sometimes found as a fourth constituent, and the rock is +then known as _hornblendic granite_. Granite is an acidic rock +corresponding to rhyolite in chemical composition. We may believe that +the same molten mass which supplies this acidic lava in surface flows +solidifies as granite deep below ground in the volcanic reservoir. + +_Syenite_, composed of feldspar and mica, has consolidated from a less +siliceous mixture than has granite. + +_Diorite_, still less siliceous, is composed of hornblende and +feldspar,--the latter mineral being of different variety from the +feldspar of granite and syenite. + +_Gabbro_, a typical basic rock, corresponds to basalt in chemical +composition. It is a dark, heavy, coarsely crystalline aggregate of +feldspar and _augite_ (a dark mineral allied to hornblende). It often +contains _magnetite_ (the magnetic black oxide of iron) and _olivine_ +(a greenish magnesian silicate). + +In the northern states all these types, and many others also of the +vast number of varieties of intrusive rocks, can be found among the +rocks of the drift brought from the areas of igneous rock in Canada +and the states of our northern border. + + [Illustration: Fig. 249. Ground Plan of Dikes in Granite. + (Scale 80 feet to the inch) + + What is the relative age of the dikes _aa_, _bb_, and _cc_?] + + [Illustration: Fig. 250. _A_ and _B_. Mountains of coarsely + Crystalline Igneous _i_, surrounded by Sedimentary Strata _s_ + and _s´_ + + Copy each diagram and complete it, so as to show whether the + mass of igneous rock is a volcanic neck, a boss, or a laccolith] + +=Summary.= The records of geology prove that since the earliest of +their annals tremendous forces have been active in the earth. In all +the past, under pressures inconceivably great, molten rock has been +driven upward into the rocks of the crust. It has squeezed into +fissures forming dikes; it has burrowed among the strata as intrusive +sheets; it has melted the rocks away or lifted the overlying strata, +filling the chambers which it has made with intrusive masses. During +all geological ages molten rock has found way to the surface, and +volcanoes have darkened the sky with clouds of ashes and poured +streams of glowing lava down their sides. The older strata,--the +strata which have been most deeply buried,--and especially those which +have suffered most from folding and from fracture, show the largest +amount of igneous intrusions. The molten rock which has been driven +from the earth's interior to within the crust or to the surface during +geologic time must be reckoned in millions of cubic miles. + + [Illustration: Fig. 251. + + 1, limestone; 2, tuff; 3, 5, 7, shale with marine shells; 4, 6, + lava, dotted portions scoriaceous. Give the history recorded in + this section] + + [Illustration: Fig. 252. + + _a_, sedimentary strata with intrusive sheets; _b_, sedimentary + strata; _c_, lava flow; _d_, dike. Give the succession of + events recorded in this section] + + [Illustration: Fig. 253. + + Which of the lava sheets of this section are contemporaneous + anti which intrusive,--_A_, whose upper surface is overlain + with a conglomerate of rolled lava pebbles; _B_, the cracks and + seams of whose upper surface are filled with the material of + the overlying sandstone; _C_, which breaks across the strata in + which it is imbedded; _D_, which includes fragments of both the + underlying and overlying strata and penetrates their crevices + and seams?] + + [Illustration: Fig. 254. Mato Tepee, Wyoming + + This magnificent tower of igneous rock three hundred feet in + height has been called by some a volcanic neck. Is the + direction of the columns that which would obtain in the + cylindrical pipe of a volcano? The tower is probably the + remnant of a small laccolith, an outlying member of a group of + laccoliths situated not far distant] + + +The Interior Condition of the Earth and Causes of Vulcanism and +Deformation + +The problems of volcanoes and of deformation are so closely connected +with that of the earth's interior that we may consider them together. +Few of these problems are solved, and we may only state some known +facts and the probable conclusions which may be drawn as inferences +from them. + +=The interior of the earth is hot.= Volcanoes prove that in many parts +of the earth there exist within reach of the surface regions of such +intense heat that the rock is in a molten condition. Deep wells and +mines show everywhere an increase in temperature below the surface +shell affected by the heat of summer and the cold of winter,--a shell +in temperate latitudes sixty or seventy feet thick. Thus in a boring +more than a mile deep at Schladebach, Germany, the earth grows warmer +at the rate of 1° F. for every sixty-seven feet as we descend. Taking +the average rate of increase at one degree for every sixty feet of +descent, and assuming that this rate, observed at the moderate +distances open to observation, continues to at least thirty-five +miles, the temperature at that depth must be more than three thousand +degrees,--a temperature at which all ordinary rocks would melt at the +earth's surface. The rate of increase in temperature probably lessens +as we go downward, and it may not be appreciable below a few hundred +miles. But there is no reason to doubt that _the interior of the earth +is intensely hot_. Below a depth of one or two score miles we may +imagine the rocks everywhere glowing with heat. + +Although the heat of the interior is great enough to melt all rocks at +atmospheric pressure, it does not follow that the interior is fluid. +Pressure raises the fusing point of rocks, and the weight of the crust +may keep the interior in what may be called a solid state, although so +hot as to be a liquid or a gas were the pressure to be removed. + +=The interior of the earth is dense and heavy.= The earth behaves as a +globe more rigid than glass under the strains to which it is subjected +by the attractions of the sun and moon and other heavenly bodies. The +jar of world-shaking earthquakes passes through the earth's interior +with nearly twice the velocity with which it would traverse solid steel, +and since the speed of elastic waves depends on the density and +elasticity of the medium, it follows that the globe is as a whole more +dense and rigid than steel. _The interior of the earth is extremely +dense and rigid._ + +The common rocks of the crust are about two and a half times heavier +than water, while the earth as a whole weighs five and six-tenths +times as much as a globe of water of the same size. _The interior is +therefore much more heavy than the crust._ This may be caused in part +by compression of the interior under the enormous weight of the crust, +and in part also by an assortment of material, the heavier substances, +such as the heavy metals, having gravitated towards the center. + +Between the crust, which is solid because it is cool, and the +interior, which is hot enough to melt were it not for the pressure +which keeps it dense and rigid, there may be an intermediate zone in +which heat and pressure are so evenly balanced that here rock +liquefies whenever and wherever the pressure upon it may be relieved +by movements of the crust. It is perhaps from such a subcrustal layer +that the lava of volcanoes is supplied. + +=The causes of volcanic action.= It is now generally believed that the +_heat_ of volcanoes is that of the earth's interior. Other causes, +such as friction and crushing in the making of mountains and the +chemical reactions between oxidizing agents of the crust and the +unoxidized interior, have been suggested, but to most geologists they +seem inadequate. + +There is much difference of opinion as to the _force_ which causes +molten rock to rise to the surface in the ducts of volcanoes. Steam is +so evidently concerned in explosive eruptions that many believe that +lava is driven upward by the expansive force of the steam with which +it is charged, much as a viscid liquid rises and boils over in a test +tube or kettle. + +But in quiet eruptions, and still more in the irruption of intrusive +sheets and masses, there is little if any evidence that steam is the +driving force. It is therefore believed by many geologists that it is +_pressure due to crustal movements and internal stresses_ which +squeezes molten rock from below into fissures and ducts in the crust. +It is held by some that where considerable water is supplied to the +rising column of lava, as from the ground water of the surrounding +region, and where the lava is viscid so that steam does not readily +escape, the eruption is of the explosive type; when these conditions +do not obtain, the lava outwells quietly, as in the Hawaiian +volcanoes. It is held by others not only that volcanoes are due to the +outflow of the earth's deep-seated heat, but also that the steam and +other emitted gases are for the most part native to the earth's +interior and never have had place in the circulation of atmospheric +and ground waters. + +=Volcanic action and deformation.= Volcanoes do not occur on wide +plains or among ancient mountains. On the other hand, where movements +of the earth's crust are in progress in the uplift of high plateaus, +and still more in mountain making, molten rock may reach the surface, +or may be driven upward toward it forming great intrusive masses. Thus +extensive lava flows accompanied the upheaval of the block mountains +of western North America and the uplift of the Colorado plateau. A +line of recent volcanoes may be traced along the system of rift +valleys which extends from the Jordan and Dead Sea through eastern +Africa to Lake Nyassa. The volcanoes of the Andes show how conspicuous +volcanic action may be in young rising ranges. Folded mountains often +show a core of igneous rock, which by long erosion has come to form +the axis and the highest peaks of the range, as if the molten rock had +been squeezed up under the rising upfolds. As we decipher the records +of the rocks in historical geology we shall see more fully how, in all +the past, volcanic action has characterized the periods of great +crustal movements, and how it has been absent when and where the +earth's crust has remained comparatively at rest. + +=The causes of deformation.= As the earth's interior, or nucleus, is +highly heated it must be constantly though slowly losing its heat by +conduction through the crust and into space; and since the nucleus is +cooling it must also be contracting. The nucleus has contracted also +because of the extrusion of molten matter, the loss of constituent +gases given off in volcanic eruptions, and (still more important) the +compression and consolidation of its material under gravity. As the +nucleus contracts, it tends to draw away from the cooled and solid +crust, and the latter settles, adapting itself to the shrinking +nucleus much as the skin of a withering apple wrinkles down upon the +shrunken fruit. The unsupported weight of the spherical crust develops +enormous tangential pressures, similar to the stresses of an arch or +dome, and when these lateral thrusts accumulate beyond the power of +resistance the solid rock is warped and folded and broken. + +Since the planet attained its present mass it has thus been lessening +in volume. Notwithstanding local and relative upheavals the earth's +surface on the whole has drawn nearer and nearer to the center. The +portions of the lithosphere which have been carried down the farthest +have received the waters of the oceans, while those portions which +have been carried down the least have emerged as continents. + +Although it serves our convenience to refer the movements of the crust +to the sea level as datum plane, it is understood that this level is +by no means fixed. Changes in the ocean basins increase or reduce +their capacity and thus lower or raise the level of the sea. But since +these basins are connected, the effect of any change upon the water +level is so distributed that it is far less noticeable than a +corresponding change would be upon the land. + + + + +CHAPTER XIII + +METAMORPHISM AND MINERAL VEINS + + +Under the action of internal agencies rocks of all kinds may be +rendered harder, more firmly cemented, and more crystalline. These +processes are known as _metamorphism_, and the rocks affected, whether +originally sedimentary or igneous, are called _metamorphic rocks_. We +may contrast with metamorphism the action of external agencies in +weathering, which render rocks less coherent by dissolving their +soluble parts and breaking down their crystalline grains. + +=Contact metamorphism.= Rocks beneath a lava flow or in contact with +igneous intrusions are found to be metamorphosed to various degrees by +the heat of the cooling mass. The adjacent strata may be changed only +in color, hardness, and texture. Thus, next to a dike, bituminous coal +may be baked to coke or anthracite, and chalk and limestone to +crystalline marble. Sandstone may be converted into quartzite, and +shale into _argillite_, a compact, massive clay rock. New minerals may +also be developed. In sedimentary rocks there may be produced crystals +of mica and of _garnet_ (a mineral as hard as quartz, commonly +occurring in red, twelve-sided crystals). Where the changes are most +profound, rocks may be wholly made over in structure and mineral +composition. + +In contact metamorphism, thin sheets of molten rock produce less +effect than thicker ones. The strongest heat effects are naturally +caused by bosses and regional intrusions, and the zone of change about +them may be several miles in width. In these changes heated waters and +vapors from the masses of igneous rocks undoubtedly play a very +important part. + +Which will be more strongly altered, the rocks about a closed dike in +which lava began to cool as soon as it filled the fissure, or the +rocks about a dike which opened on the surface and through which the +molten rock flowed for some time? + +Taking into consideration the part played by heated waters, which will +produce the most far-reaching metamorphism, dikes which cut across the +bedding planes or intrusive sheets which are thrust between the +strata? + +=Regional metamorphism.= Metamorphic rocks occur widespread in many +regions, often hundreds of square miles in area, where such extensive +changes cannot be accounted for by igneous intrusions. Such are the +dissected cores of lofty mountains, as the Alps, and the worn-down +bases of ancient ranges, as in New England, large areas in the +Piedmont Belt, and the Laurentian peneplain. + +In these regions the rocks have yielded to immense pressure. They have +been folded, crumpled, and mashed, and even their minute grains, as +one may see with a microscope, have often been puckered, broken, and +crushed to powder. It is to these mechanical movements and strains +which the rocks have suffered in every part that we may attribute +their metamorphism, and the degree to which they have been changed is +in direct proportion to the degree to which they have been deformed +and mashed. + +Other factors, however, have played important parts. Rock crushing +develops heat, and allows a freer circulation of heated waters and +vapors. Thus chemical reactions are greatly quickened; minerals are +dissolved and redeposited in new positions, or their chemical +constituents may recombine in new minerals, entirely changing the +nature of the rock, as when, for example, feldspar recrystallizes as +quartz and mica. + +Early stages of metamorphism are seen in _slate_. Pressure has +hardened the marine muds, the arkose (p. 186), or the volcanic ash +from which slates are derived, and has caused them to cleave by the +rearrangement of their particles. + +Under somewhat greater pressure, slate becomes _phyllite_, a clay +slate whose cleavage surfaces are lustrous with flat-lying mica +flakes. The same pressure which has caused the rock to cleave has set +free some of its mineral constituents along the cleavage planes to +crystallize there as mica. + + [Illustration: Fig. 255. A Foliated Rock] + +=Foliation.= Under still stronger pressure the whole structure of the +rock is altered. The minerals of which it is composed, and the new +minerals which develop by heat and pressure, arrange themselves along +planes of cleavage or of shear in rudely parallel leaves, or _folia_. +Of this structure, called _foliation_, we may distinguish two +types,--a coarser feldspathic type, and a fine type in which other +minerals than feldspar predominate. + +_Gneiss_ is the general name under which are comprised coarsely +foliated rocks banded with irregular layers of feldspar and other +minerals. The gneisses appear to be due in many cases to the crushing +and shearing of deep-seated igneous rocks, such as granite and gabbro. + +_The crystalline schists_, representing the finer types of foliation, +consist of thin, parallel, crystalline leaves, which are often +remarkably crumpled. These folia can be distinguished from the laminae +of sedimentary rocks by their lenticular form and lack of continuity, +and especially by the fact that they consist of platy, crystalline +grains, and not of particles rounded by wear. + +_Mica schist_, the most common of schists, and in fact of all +metamorphic rocks, is composed of mica and quartz in alternating wavy +folia. All gradations between it and phyllite may be traced, and in +many cases we may prove it due to the metamorphism of slates and +shales. It is widespread in New England and along the eastern side of +the Appalachians. _Talc schist_ consists of quartz and _talc_, a +light-colored magnesian mineral of greasy feel, and so soft that it +can be scratched with the thumb nail. + +_Hornblende schist_, resulting in many cases from the foliation of +basic igneous rocks, is made of folia of hornblende alternating with +bands of quartz and feldspar. Hornblende schist is common over large +areas in the Lake Superior region. + +_Quartz schist_ is produced from quartzite by the development of fine +folia of mica along planes of shear. All gradations may be found +between it and unfoliated quartzite on the one hand and mica schist on +the other. + +Under the resistless pressure of crustal movements almost any rocks, +sandstones, shales, lavas of all kinds, granites, diorites, and +gabbros may be metamorphosed into schists by crushing and shearing. +Limestones, however, are metamorphosed by pressure into _marble_, the +grains of carbonate of lime recrystallizing freely to interlocking +crystals of calcite. + +These few examples must suffice of the great class of metamorphic +rocks. As we have seen, they owe their origin to the alteration of +both of the other classes of rocks--the sedimentary and the +igneous--by heat and pressure, assisted usually by the presence of +water. The fact of change is seen in their hardness arid cementation, +their more or less complete recrystallization, and their foliation; +but the change is often so complete that no trace of their original +structure and mineral composition remains to tell whether the rocks +from which they were derived were sedimentary or igneous, or to what +variety of either of these classes they belonged. + + [Illustration: Fig. 256. Contorted Gneiss, the Ottawa River, + Canada] + + [Illustration: Fig. 257. Quartz Veins in Slate] + +In many cases, however, the early history of a metamorphic rock can be +deciphered. Fossils not wholly obliterated may prove it originally +water-laid. Schists may contain rolled-out pebbles, showing their +derivation from a conglomerate. Dikes of igneous rocks may be followed +into a region where they have been foliated by pressure. The most +thoroughly metamorphosed rocks may sometimes be traced out into +unaltered sedimentary or igneous rocks, or among them may be found +patches of little change where their history maybe read. + +Metamorphism is most common among rocks of the earlier geological +ages, and most rare among rocks of recent formation. No doubt it is +now in progress where deep-buried sediments are invaded by heat either +from intrusive igneous masses or from the earth's interior, or are +suffering slow deformation under the thrust of mountain-making forces. + +Suggest how rocks now in process of metamorphism may sometimes be +exposed to view. Why do metamorphic rocks appear on the surface +to-day? + + +Mineral Veins + +In regions of folded and broken rocks fissures are frequently found to +be filled with sheets of crystalline minerals deposited from solution +by underground water, and fissures thus filled are known as _mineral +veins_. Much of the importance of mineral veins is due to the fact +that they are often metalliferous, carrying valuable native metals and +metallic ores disseminated in fine particles, in strings, and +sometimes in large masses in the midst of the valueless nonmetallic +minerals which make up what is known as the _vein stone_. + +The most common vein stones are _quartz_ and _calcite_. _fluorite_ +(calcium fluoride), a mineral harder than calcite and crystallizing in +cubes of various colors, and _barite_ (barium sulphate), a heavy white +mineral, are abundant in many veins. + + [Illustration: Fig. 258. Placer Deposits in California + + _g_, gold-bearing gravels in present river beds; _g´_, ancient + gold-bearing river gravels; _a_, _a_, lava flows capping table + mountains; _s_, slate. Draw a diagram showing by dotted lines + conditions before the lava flows occurred. What changes have + since taken place?] + +The gold-bearing quartz veins of California traverse the metamorphic +slates of the Sierra Nevada Mountains. Below the zone of solution (p. +45) these veins consist of a vein stone of quartz mingled with pyrite +(p. 13), the latter containing threads and grains of native gold. But +to the depth of about fifty feet from the surface the pyrite of the +vein has been dissolved, leaving a rusty, cellular quartz with grains +of the insoluble gold scattered through it. + +The _placer deposits_ of California and other regions are gold-bearing +deposits of gravel and sand in river beds. The heavy gold is apt to be +found mostly near or upon the solid rock, and its grains, like those +of the sand, are always rounded. How the gold came in the placers we +may leave the pupil to suggest. + +Copper is found in a number of ores, and also in the native metal. +Below the zone of surface changes the ore of a copper vein is often a +double sulphide of iron and copper called _chalcopyrite_, a mineral +softer than pyrite--it can easily be scratched with a knife--and +deeper yellow in color. For several score of feet below the ground the +vein may consist of rusty quartz from which the metallic ores have +been dissolved; but at the base of the zone of solution we may find +exceedingly rich deposits of copper ores,--copper sulphides, red and +black copper oxides, and green and blue copper carbonates, which have +clearly been brought down in solution from the leached upper portion +of the vein. + +=Origin of mineral veins.= Both vein stones and ores have been +deposited slowly from solution in water, much as crystals of salt are +deposited on the sides of a jar of saturated brine. In our study of +underground water we learned that it is everywhere circulating through +the permeable rocks of the crust, descending to profound depths under +the action of gravity and again driven to the surface by hydrostatic +pressure. Now fissures, wherever they occur, form the trunk channels +of the underground circulation. Water descends from the surface along +these rifts; it moves laterally from either side to the fissure plane, +just as ground water seeps through the surrounding rocks from every +direction to a well; and it ascends through these natural water ways +as in an artesian well, whenever they intersect an aquifer in which +water is under hydrostatic pressure. + +The waters which deposit vein stones and ores are commonly hot, and in +many cases they have derived their heat from intrusions of igneous +rock still uncooled within the crust. The solvent power of the water +is thus greatly increased, and it takes up into solution various +substances from the igneous and sedimentary rocks which it traverses. +For various reasons these substances stances are deposited in the vein +as ores and vein stones. On rising through the fissure the water cools +and loses pressure, and its capacity to hold minerals in solution is +therefore lessened. Besides, as different currents meet in the +fissure, some ascending, some descending, and some coming in from the +sides, the chemical reaction of these various weak solutions upon one +another and upon the walls of the vein precipitates the minerals of +vein stuffs and ores. + +As an illustration of the method of vein deposits we may cite the case +of a wooden box pipe used in the Comstock mines, Nevada, to carry the +hot water of the mine from one level to another, which in ten years +was lined with calcium carbonate more than half an inch thick. + +The Steamboat Springs, Nevada, furnish examples of mineral veins in +process of formation. The steaming water rises through fissures in +volcanic rocks and is now depositing in the rifts a vein stone of +quartz, with metallic ores of iron, mercury, lead, and other metals. + +=Reconcentration.= Near the base of the zone of solution veins are +often stored with exceptionally large and valuable ore deposits. This +local enrichment of the vein is due to the reconcentration of its +metalliferous ores. As the surface of the land is slowly lowered by +weathering and running water, the zone of solution is lowered at an +equal rate and encroaches constantly on the zone of cementation. The +minerals of veins are therefore constantly being dissolved along their +upper portions and carried down the fissures by ground water to lower +levels, where they are redeposited. + +Many of the richest ore deposits are thus due to successive +concentrations: the ores were leached originally from the rocks to a +large extent by laterally seeping waters; they were concentrated in +the ore deposits of the vein chiefly by ascending currents; they have +been reconcentrated by descending waters in the way just mentioned. + +=The original source of the metals.= It is to the igneous rocks that +we may look for the original source of the metals of veins. Lavas +contain minute percentages of various metallic compounds, and no doubt +this was the case also with the igneous rocks which formed the +original earth crust. By the erosion of the igneous rocks the metals +have been distributed among sedimentary strata, and even the sea has +taken into solution an appreciable amount of gold and other metals, +but in this widely diffused condition they are wholly useless to man. +The concentration which has made them available is due to the +interaction of many agencies. Earth movements fracturing deeply the +rocks of the crust, the intrusion of heated masses, the circulation of +underground waters, have all coöperated in the concentration of the +metals of mineral veins. + +While fissure veins are the most important of mineral veins, the +latter term is applied also to any water way which has been filled by +similar deposits from solution. Thus in soluble rocks, such as +limestones, joints enlarged by percolating water are sometimes filled +with metalliferous deposits, as, for example, the lead and zinc +deposits of the upper Mississippi valley. Even a porous aquifer may be +made the seat of mineral deposits, as in the case of some +copper-bearing and silver-bearing sandstones of New Mexico. + + * * * * * + + [Illustration: Fig. 260. Geological Map of the United states + and Part of Canada] + + * * * * * + + + + +PART III + +HISTORICAL GEOLOGY + + +CHAPTER XIV + +THE GEOLOGICAL RECORD + + +=What a formation records.= We have already learned that each +individual body of stratified rock, or formation, constitutes a record +of the time when it was laid. The structure and the character of the +sediments of each formation tell whether the area was land or sea at +the time when they were spread; and if the former, whether the land +was river plain, or lake bed, or was covered with wind-blown sands, or +by the deposits of an ice sheet. If the sediments are marine, we may +know also whether they were laid in shoal water near the shore or in +deeper water out at sea, and whether during a period of emergence, or +during a period of subsidence when the sea transgressed the land. By +the same means each formation records the stage in the cycle of +erosion of the land mass from which its sediments were derived (p. +185). An unconformity between two marine formations records the fact +that between the periods when they were deposited in the sea the area +emerged as land and suffered erosion (p. 227). The attitude and +structure of the strata tell also of the foldings and fractures, +the deformation and the metamorphism, which they have suffered; and +the igneous rocks associated with them as lava flows and igneous +intrusions add other details to the story. Each formation is thus a +separate local chapter in the geological history of the earth, and its +strata are its leaves. It contains an authentic record of the physical +conditions--the geography--of the time and place when and where its +sediments were laid. + +=Past cycles of erosion.= These chapters in the history of the planet +are very numerous, although much of the record has been destroyed in +various ways. A succession of different formations is usually seen in +any considerable section of the crust, such as a deep canyon or where +the edges of upturned strata are exposed to view on the flanks of +mountain ranges; and in any extensive area, such as a state of the +Union or a province of Canada, the number of formations outcropping on +the surface is large. + +It is thus learned that our present continent is made up for the most +part of old continental deltas. Some, recently emerged as the strata +of young coastal plains, are the records of recent cycles of erosion; +while others were deposited in the early history of the earth, and in +many instances have been crumpled into mountains, which afterwards +were leveled to their bases and lowered beneath the sea to receive a +cover of later sediments before they were again uplifted to form land. + +The cycle of erosion now in progress and recorded in the layers of +stratified rock being spread beneath the sea in continental deltas has +therefore been preceded by many similar cycles. Again and again +movements of the crust have brought to an end one cycle--sometimes +when only well under way, and sometimes when drawing toward its +close--and have begun another. Again and again they have added to the +land areas which before were sea, with all their deposition records of +earlier cycles, or have lowered areas of land beneath the sea to +receive new sediments. + +=The age of the earth.= The thickness of the stratified rocks now +exposed upon the eroded surface of the continents is very great. In +the Appalachian region the strata are seven or eight miles thick, and +still greater thicknesses have been measured in several other mountain +ranges. The aggregate thickness of all the formations of the +stratified rocks of the earth's crust, giving to each formation its +maximum thickness wherever found, amounts to not less than forty +miles. Knowing how slowly sediments accumulate upon the sea floor +(p. 184), we must believe that the successive cycles which the earth +has seen stretch back into a past almost inconceivably remote, and +measure tens of millions and perhaps even hundreds of millions of +years. + +=How the formations are correlated and the geological record made up.= +Arranged in the order of their succession, the formations of the +earth's crust would constitute a connected record in which the +geological history of the planet may be read, and therefore known as +the _geological record_. But to arrange the formations in their +natural order is not an easy task. A complete set of the volumes of +the record is to be found in no single region. Their leaves and +chapters are scattered over the land surface of the globe. In one area +certain chapters may be found, though perhaps with many missing +leaves, and with intervening chapters wanting, and these absent parts +perhaps can be supplied only after long search through many other +regions. + +Adjacent strata in any region are arranged according to the _law of +superposition_, i.e. any stratum is younger than that on which it was +deposited, just as in a pile of paper, any sheet was laid later than +that on which it rests. Where rocks have been disturbed, their +original attitude must be determined before the law can be applied. +Nor can the law of superposition be used in identifying and comparing +the strata of different regions where the formations cannot be traced +continuously from one region to the other. + +The formations of different regions are arranged in their true order +by the _law of included organisms_; i.e. formations, however widely +separated, which contain a similar assemblage of fossils are +equivalent and belong to the same division of geological time. + +The correlation of formations by means of fossils may be explained by +the formations now being deposited about the north Atlantic. +Lithologically they are extremely various. On the continental shelf of +North America limestones of different kinds are forming off Florida, +and sandstones and shales from Georgia northward. Separated from them +by the deep Atlantic oozes are other sedimentary deposits now +accumulating along the west coast of Europe. If now all these offshore +formations were raised to open air, how could they be correlated? +Surely not by lithological likeness, for in this respect they would be +quite diverse. All would be similar, however, in the fossils which +they contain. Some fossil species would be identical in all these +formations and others would be closely allied. Making all due +allowance for differences in species due to local differences in +climate and other physical causes, it would still be plain that plants +and animals so similar lived at the same period of time, and that the +formations in which their remains were imbedded were contemporaneous +in a broad way. The presence of the bones of whales and other marine +mammals would prove that the strata were laid after the appearance of +mammals upon earth, and imbedded relics of man would give a still +closer approximation to their age. In the same way we correlate the +earlier geological formations. + +For example, in 1902 there were collected the first fossils ever found +on the antarctic continent. Among the dozen specimens obtained were +some fossil ammonites (a family of chambered shells) of genera which +are found on other continents in certain formations classified as the +Cretaceous system, and which occur neither above these formations nor +below them. On the basis of these few fossils we may be confident that +the strata in which they were found in the antarctic region were laid +in the same period of geologic time as were the Cretaceous rocks of +the United States and Canada. + +=The record as a time scale.= By means of the law of included +organisms and the law of superposition the formations of different +countries and continents are correlated and arranged in their natural +order. When the geological record is thus obtained it may be used as a +universal time scale for geological history. Geological time is +separated into divisions corresponding to the times during which the +successive formations were laid. The largest assemblages of formations +are known as groups, while the corresponding divisions of time are +known as eras. Groups are subdivided into systems, and systems into +series. Series are divided into stages and substages,--subdivisions +which do not concern us in this brief treatise. The corresponding +divisions of time are given in the following table. + + _Strata_ _Time_ + + Group Era + System Period + Series Epoch + +The geologist is now prepared to read the physical history--the +geographical development--of any country or of any continent by means +of its formations, when he has given each formation its true place in +the geological record as a time scale. + +The following chart exhibits the main divisions of the record, the +name given to each being given also to the corresponding time +division. Thus we speak of the _Cambrian system_, meaning a certain +succession of formations which are classified together because of +broad resemblances in their included organisms; and of the _Cambrian +period_, meaning the time during which these rocks were deposited. + + _Group and Era_ _System and Period_ _Series and Epoch_ + + { Recent + { Quaternary . . . . { Pleistocene + { + Cenozoic . . . . { { Pliocene + { Tertiary . . . . { Miocene + { Eocene + + { Cretaceous + Mesozoic . . . . { Jurassic + { Triassic + + { Permian + { Carboniferous . . { Pennsylvanian + { { Mississippian + Paleozoic . . . . { Devonian + { Silurian + { Ordovician + { Cambrian + + Algonkian + Archean + + +Fossils and what they teach + +The geological formations contain a record still more important than +that of the geographical development of the continents; the fossils +imbedded in the rocks of each formation tell of the kinds of animals +and plants which inhabited the earth at that time, and from these +fossils we are therefore able to construct the history of life upon +the earth. + +=Fossils.= These remains of organisms are found in the strata in all +degrees of perfection, from trails and tracks and fragmentary +impressions, to perfectly preserved shells, wood, bones, and complete +skeletons. As a rule, it is only the hard parts of animals and plants +which have left any traces in the rocks. Sometimes the original hard +substance is preserved, but more often it has been replaced by some +less soluble material. Petrifaction, as this process of slow +replacement is called, is often carried on in the most exquisite +detail. When wood, for example, is undergoing petrifaction, the woody +tissue may be replaced, particle by particle, by silica in solution +through the action of underground waters, even the microscopic +structures of the wood being perfectly reproduced. In shells +originally made of _aragonite_, a crystalline form of carbonate of +lime, that mineral is usually replaced by _calcite_, a more stable +form of the same substance. The most common petrifying materials are +calcite, silica, and pyrite. + +Often the organic substance has neither been preserved nor replaced, +but the _form_ has been retained by means of molds and casts. +Permanent impressions, or molds, may be made in sediments not only by +the hard parts of organisms, but also by such soft and perishable +parts as the leaves of plants, and, in the rarest instances, by the +skin of animals and the feathers of birds. In fine-grained limestones +even the imprints of jellyfish have been retained. + +The different kinds of molds and casts may be illustrated by means of +a clam shell and some moist clay, the latter representing the +sediments in which the remains of animals and plants are entombed. +Imbedding the shell in the clay and allowing the clay to harden, we +have a _mold of the exterior_ of the shell, as is seen on cutting the +clay matrix in two and removing the shell from it. Filling this mold +with clay of different color, we obtain a _cast of the exterior_, +which represents accurately the original form and surface markings of +the shell. In nature, shells and other relics of animals or plants are +often removed by being dissolved by percolating waters, and the molds +are either filled with sediments or with minerals deposited from +solution. + +Where the fossil is hollow, a _cast of the interior_ is made in the +same way. Interior casts of shells reproduce any markings on the +inside of the valves, and casts of the interior of the skulls of +ancient vertebrates show the form and size of their brains. + +=Imperfection of the life record.= At the present time only the +smallest fraction of the life on earth ever gets entombed in rocks now +forming. In the forest great fallen tree trunks, as well as dead +leaves, decay, and only add a little to the layer of dark vegetable +mold from which they grew. The bones of land animals are, for the most +part, left unburied on the surface and are soon destroyed by chemical +agencies. Even where, as in the swamps of river, flood plains and in +other bogs, there are preserved the remains of plants, and sometimes +insects, together with the bones of some animal drowned or mired, in +most cases these swamp and bog deposits are sooner or later destroyed +by the shifting channels of the stream or by the general erosion of +the land. + +In the sea the conditions for preservation are more favorable than on +land; yet even here the proportion of animals and plants whose hard +parts are fossilized is very small compared with those which either +totally decay before they are buried in slowly accumulating sediments +or are ground to powder by waves and currents. + +We may infer that during each period of the past, as at the present, +only a very insignificant fraction of the innumerable organisms of sea +and land escaped destruction and left in continental and oceanic +deposits permanent records of their existence. Scanty as these +original life records must have been, they have been largely destroyed +by metamorphism of the rocks in which they were imbedded, by solution +in underground waters, and by the vast denudation under which the +sediments of earlier periods have been eroded to furnish materials for +the sedimentary records of later times. Moreover, very much of what +has escaped destruction still remains undiscovered. The immense bulk +of the stratified rocks is buried and inaccessible, and the records of +the past which it contains can never be known. Comparatively few +outcrops have been thoroughly searched for fossils. Although new +species are constantly being discovered, each discovery may be +considered as the outcome of a series of happy accidents,--that the +remains of individuals of this particular species happened to be +imbedded and fossilized, that they happened to escape destruction +during long ages, and that they happened to be exposed and found. + +=Some inferences from the records of the history of life upon the +planet.= Meager as are these records, they set forth plainly some +important truths which we will now briefly mention. + +1. Each series of the stratified rocks, except the very deepest, +contains vestiges of life. Hence _the earth was tenanted by living +creatures for an uncalculated length of time before human history +began_. + +2. _Life on the earth has been ever-changing._ The youngest strata hold +the remains of existing species of animals and plants and those of +species and varieties closely allied to them. Strata somewhat older +contain fewer existing species, and in strata of a still earlier, but +by no means an ancient epoch, no existing species are to be found; the +species of that epoch and of previous epochs have vanished from the +living world. During all geological time since life began on earth old +species have constantly become extinct and with them the genera and +families to which they belong, and other species, genera, and families +have replaced them. The fossils of each formation differ on the whole +from those of every other. The assemblage of animals and plants (the +_fauna-flora_) of each epoch differs from that of every other epoch. + +In many cases the extinction of a type has been gradual; in other +instances apparently abrupt. There is no evidence that any organism +once become extinct has ever reappeared. The duration of a species in +time, or its "vertical range" through the strata, varies greatly. Some +species are limited to a stratum a few feet in thickness; some may +range through an entire formation and be found but little modified in +still higher beds. A formation may thus often be divided into zones, +each characterized by its own peculiar species. As a rule, the simpler +organisms have a longer duration as species, though not as +individuals, than the more complex. + +3. _The larger zoölogical and botanical groupings survive longer than +the smaller._ Species are so short-lived that a single geological +epoch may be marked by several more or less complete extinctions of +the species of its fauna-flora and their replacement by other species. +A genus continues with new species after all the species with which it +began have become extinct. Families survive genera, and orders +families. Classes are so long-lived that most of those which are known +from the earliest formations are represented by living forms, and no +subkingdom has ever become extinct. + +Thus, to take an example from the stony corals,--the +_zoantharia_,--the particular characters--which constituted a certain +_species_--_Facosites niagarensis_--of the order are confined to the +Niagara series. Its _generic_ characters appeared in other species +earlier in the Silurian and continued through the Devonian. Its +_family_ characters, represented in different genera and species, +range from the Ordovician to the close of the Paleozoic; while the +characters which it shares with all its order, the Zoantharia, began +in the Cambrian and are found in living species. + +4. _The change in organisms has been gradual._ The fossils of each +life zone and of each formation of a conformable series closely +resemble, with some explainable exceptions, those of the beds +immediately above and below. The animals and plants which tenanted the +earth during any geological epoch are so closely related to those of +the preceding and the succeeding epochs that we may consider them to +be the descendants of the one and the ancestors of the other, thus +accounting for the resemblance by heredity. It is therefore believed +that the species of animals and plants now living on the earth are the +descendants of the species whose remains we find entombed in the +rocks, and that the chain of life has been unbroken since its +beginning. + +5. _The change in species has been a gradual differentiation._ Tracing +the lines of descent of various animals and plants of the present +backward through the divisions of geologic time, we find that these +lines of descent converge and unite in simpler and still simpler +types. The development of life may be represented by a tree whose +trunk is found in the earliest ages and whose branches spread and +subdivide to the growing twigs of present species. + +6. _The change in organisms throughout geologic time has been a +progressive change._ In the earliest ages the only animals and plants +on the earth were lowly forms, simple and generalized in structure; +while succeeding ages have been characterized by the introduction of +types more and more specialized and complex, and therefore of higher +rank in the scale of being. Thus the Algonkian contains the remains of +only the humblest forms of the invertebrates. In the Cambrian, +Ordovician, and Silurian the invertebrates were represented in all +their subkingdoms by a varied fauna. In the Devonian, fishes--the +lowest of the vertebrates--became abundant. Amphibians made their +entry on the stage in the Carboniferous, and reptiles came to rule the +world in the Mesozoic. Mammals culminated in the Tertiary in strange +forms which became more and more like those of the present as the long +ages of that era rolled on; and latest of all appeared the noblest +product of the creative process, man. + +Just as growth is characteristic of the individual life, so gradual, +progressive change, or evolution, has characterized the history of +life upon the planet. The evolution of the organic kingdom from its +primitive germinal forms to the complex and highly organized +fauna-flora of to-day may be compared to the growth of some noble oak +as it rises from the acorn, spreading loftier and more widely extended +branches as it grows. + +7. While higher and still higher types have continually been evolved, +until man, the highest of all, appeared, _the lower and earlier types +have generally persisted_. Some which reached their culmination early +in the history of the earth have since changed only in slight +adjustments to a changing environment. Thus the brachiopods, a type of +shellfish, have made no progress since the Paleozoic, and some of +their earliest known genera are represented by living forms hardly to +be distinguished from their ancient ancestors. The lowest and earliest +branches of the tree of life have risen to no higher levels since they +reached their climax of development long ago. + +8. A strange parallel has been found to exist between the evolution of +organisms and the development of the individual. In the embryonic +stages of its growth the individual passes swiftly through the +successive stages through which its ancestors evolved during the +millions of years of geologic time. _The development of the individual +recapitulates the evolution of the race._ + + * * * * * + +The frog is a typical amphibian. As a tadpole it passes through a +stage identical in several well-known features with the maturity of +fishes; as, for example, its aquatic life, the tail by which it swims, +and the gills through which it breathes. It is a fair inference that +the tadpole stage in the life history of the frog represents a stage +in the evolution of its kind,--that the Amphibia are derived from +fishlike ancestral forms. This inference is amply confirmed in the +geological record; fishes appeared before Amphibia and were connected +with them by transitional forms. + +=The great length of geologic time inferred from the slow change of +species.= Life forms, like land forms, are thus subject to change +under the influence of their changing environment and of forces acting +from within. How slowly they change may be seen in the apparent +stability of existing species. In the lifetime of the observer and +even in the recorded history of man, species seem as stable as the +mountain and the river. But life forms and land forms are alike +variable, both in nature and still more under the shaping hand of man. +As man has modified the face of the earth with his great engineering +works, so he has produced widely different varieties of many kinds of +domesticated plants and animals, such as the varieties of the dog and +the horse, the apple and the rose, which may be regarded in some +respects as new species in the making. We have assumed that land forms +have changed in the past under the influence of forces now in +operation. Assuming also that life forms have always changed as they +are changing at present, we come to realize something of the immensity +of geologic time required for the evolution of life from its earliest +lowly forms up to man. + +It is because the onward march of life has taken the same general +course the world over that we are able to use it as a _universal time +scale_ and divide geologic time into ages and minor subdivisions +according to the ruling or characteristic organisms then living on the +earth. Thus, since vertebrates appeared, we have in succession the Age +of Fishes, the Age of Amphibians, the Age of Reptiles, and the Age of +Mammals. + +The chart given on page 295 is thus based on the law of superposition +and the law of the evolution of organisms. The first law gives the +succession of the formations in local areas. The fossils which they +contain demonstrate the law of the progressive appearance of +organisms, and by means of this law the formations of different +countries are correlated and set each in its place in a universal time +scale and grouped together according to the affinities of their +imbedded organic remains. + +=Geologic time divisions compared with those of human history.= We may +compare the division of geologic time into eras, periods, and other +divisions according to the dominant life of the time, to the +ill-defined ages into which human history is divided according to the +dominance of some nation, ruler, or other characteristic feature. Thus +we speak of the _Dark Ages_, the _Age of Elizabeth_, and the _Age of +Electricity_. These crude divisions would be of much value if, as in +the case of geologic time, we had no exact reckoning of human history +by years. + +And as the course of human history has flowed in an unbroken stream +along quiet reaches of slow change and through periods of rapid change +and revolution, so with the course of geologic history. Periods of +quiescence, in which revolutionary forces are perhaps gathering head, +alternate with periods of comparatively rapid change in physical +geography and in organisms, when new and higher forms appear which +serve to draw the boundary line of new epochs. Nevertheless, +geological history is a continuous progress; its periods and epochs +shade into one another by imperceptible gradations, and all our +subdivisions must needs be vague and more or less arbitrary. + +=How fossils tell of the geography of the past.= Fossils are used not +only as a record of the development of life upon the earth, but also +in testimony to the physical geography of past epochs. They indicate +whether in any region the climate was tropical, temperate, or arctic. +Since species spread slowly from some center of dispersion where they +originate until some barrier limits their migration farther, the +occurrence of the same species in rocks of the same system in +different countries implies the absence of such barriers at the +period. Thus in the collection of antarctic fossils referred to on +page 294 there were shallow-water marine shells identical in species +with Mesozoic shells found in India and in the southern extremity of +South America. Since such organisms are not distributed by the +currents of the deep sea and cannot migrate along its bottom, we infer +a shallow-water connection in Mesozoic times between India, South +America, and the antarctic region. Such a shallow-water connection +would be offered along the marginal shelf of a continent uniting these +now widely separated countries. + + + + +CHAPTER XV + +THE PRE-CAMBRIAN SYSTEMS + + +=The earth's beginnings.= The geological record does not tell us of +the beginnings of the earth. The history of the planet, as we have +every reason to believe, stretches far back beyond the period of the +oldest stratified rocks, and is involved in the history of the solar +system and of the nebula,--the cloud of glowing gases or of cosmic +dust,--from which the sun and planets are believed to have been +derived. + +=The nebular hypothesis.= It was long held that the earth began as +a vaporous, shining sphere, formed by the gathering together of the +material of a gaseous ring which had been detached from a cooling +and shrinking nebula. Such a vaporous sphere would condense to a +liquid fiery globe, whose surface would become cold and solid, while +the interior would long remain intensely hot because of the slow +conductivity of the crust. Under these conditions the primeval +atmosphere of the earth must have contained in vapor the water now +belonging to the earth's crust and surface. It also held all the oxygen +since locked up in rocks by their oxidation, and all the carbon dioxide +which has since been laid away in limestones, besides that corresponding +to the carbon of carbonaceous deposits, such as peat, coal, and +petroleum. On this hypothesis the original atmosphere was dense, dark, +and noxious, and enormously heavier than the atmosphere at present. + +=The accretion hypothesis.= On the other hand, it has been recently +suggested that the earth may have grown to its present size by the +gradual accretion of meteoritic masses. Such cold, stony bodies might +have come together at so slow a rate that the heat caused by their +impact would not raise sensibly the temperature of the growing planet. +Thus the surface of the earth may never have been hot and luminous; but +as the loose aggregation of stony masses grew larger and was more and +more compressed by its own gravitation, the heat thus generated raised +the interior to high temperatures, while from time to time molten rock +was intruded among the loose, cold meteoritic masses of the crust and +outpoured upon the surface. + +Such a spiral nebula might be formed by the close approach of one star +to another,--of a passing star to our own sun, for example, before the +birth of the solar system. As the pull of the moon raises the tides on +opposite sides of the earth, so, it is supposed, the pull of the +passing star released the explosive forces of the sun, and two streams +of matter were flung out from it. The knots in the arms formed the +nuclei of the planets. The gaseous matter scattered outside the knots +cooled into small stony masses, revolving about a central mass and +hence called planetesimals (little planets). Like the meteorites which +still fall upon the earth, the planetesimals were gradually gathered +in by the nuclear knots, which thus grew to the present planets. + +It is supposed that the meteorites of which the earth was built +brought to it, as meteorites do now, various gases shut up within +their pores. As the heat of the interior increased, these gases +transpired to the surface and formed the primitive atmosphere and +hydrosphere. The atmosphere has therefore grown slowly from the +smallest beginnings. Gases emitted from the interior in volcanic +eruptions and in other ways have ever added to it, and are adding to +it now. On the other hand, the atmosphere has constantly suffered +loss, as it has been robbed of oxygen by the oxidation of rocks in +weathering, and of carbon dioxide in the making of limestones and +carbonaceous deposits. + +While all hypotheses of the earth's beginnings are as yet unproved +speculations, they serve to bring to mind one of the chief lessons +which geology has to teach,--that the duration of the earth in time, +like the extension of the universe in space, is vastly beyond the +power of the human mind to realize. Behind the history recorded in the +rocks, which stretches back for many million years, lies the long +unrecorded history of the beginnings of the planet; and still farther +in the abysses of the past are dimly seen the cycles of the evolution +of the solar system and of the nebula which gave it birth. + +We pass now from the dim realm of speculation to the earliest era of +the recorded history of the earth, where some certain facts may be +observed and some sure inferences from them may be drawn. + + +The Archean + +The oldest known sedimentary strata, wherever they are exposed by uplift +and erosion, are found to be involved with a mass of crystalline rocks +which possesses the same characteristics in all parts of the world. It +consists of foliated rocks, gneisses, and schists of various kinds, +which have been cut with dikes and other intrusions of molten rock, and +have been broken, crumpled, and crushed, and left in interlocking masses +so confused that their true arrangement can usually be made out only +with the greatest difficulty if at all. The condition of this body of +crystalline rocks is due to the fact that they have suffered not only +from the faultings, foldings, and igneous intrusions of their time, but +necessarily, also, from those of all later geological ages. + +At present three leading theories are held as to the origin of these +basal crystalline rocks. + +1. They are considered by perhaps the majority of the geologists who +have studied them most carefully to be igneous rocks intruded in a +molten state among the sedimentary rocks involved with them. In many +localities this relation is proved by the phenomena of contact (p. 268); +but for the most part the deformations which the rocks have since +suffered again and again have been sufficient to destroy such evidence +if it ever existed. + +2. An older view regards them as profoundly altered sedimentary strata, +the most ancient of the earth. + +3. According to a third theory they represent portions of the earth's +original crust; not, indeed, its original surface, but deeper portions +uncovered by erosion and afterwards mantled with sedimentary deposits. +All these theories agree that the present foliated condition of these +rocks is due to the intense metamorphism which they have suffered. + +It is to this body of crystalline rocks and the stratified rocks +involved with it, which form a very small proportion of its mass, +that the term _Archean_ (Greek, arche, beginning) is applied by +many geologists. + + +The Algonkian + +In some regions there rests unconformably on the Archean an immense +body of stratified rocks, thousands and in places even scores +of thousands of feet thick, known as the _Algonkian_. Great +unconformities divide it into well-defined systems, but as only +the scantiest traces of fossils appear here and there among its strata, +it is as yet impossible to correlate the formations of different +regions and to give them names of more than local application. We +will describe the Algonkian rocks of two typical areas. + +=The Grand Canyon of the Colorado.= We have already studied a very +ancient peneplain whose edge is exposed to view deep on the walls of +the Colorado Canyon (_nu´_, Fig. 207). The formation of flat-lying +sandstone which covers this buried land surface is proved by its +fossils to belong to the Cambrian,--the earliest period of the +Paleozoic era. The tilted rocks (_b_, Fig. 207). on whose upturned +edges the Cambrian sandstone rests are far older, for the physical +break which separates them from it records a time interval during +which they were upheaved to mountainous ridges and worn down to a low +plain. They are therefore classified as Algonkian. They comprise two +immense series. The upper is more than five thousand feet thick and +consists of shales and sandstones with some limestones. Separated from +it by an unconformity which does not appear in Figure 207, the lower +division, seven thousand feet thick, consists chiefly of massive +reddish sandstones with seven or more sheets of lava interbedded. The +lowest member is a basal conglomerate composed of pebbles derived from +the erosion of the dark crumpled schists beneath,--schists which are +supposed to be Archean. As shown in Figure 207, a strong unconformity +(_nm´_, Fig. 207) parts the schists and the Algonkian. The floor on +which the Algonkian rests is remarkably even, and here again is proved +an interval of incalculable length, during which an ancient land mass +of Archean rocks was baseleveled before it received the cover of the +sediments of the later age. + +=The Lake Superior region.= In eastern Canada an area of pre-Cambrian +rocks, Archean and Algonkian, estimated at two million square miles, +stretches from the Great Lakes and the St. Lawrence River northward to +the confines of the continent, inclosing Hudson Bay in the arms of a +gigantic U. This immense area, which we have already studied as the +Laurentian peneplain (p. 89), extends southward across the Canadian +border into northern Minnesota, Wisconsin, and Michigan. The rocks of +this area are known to be pre-Cambrian; for the Cambrian strata, +wherever found, lie unconformably upon them. + + [Illustration: Fig. 262. Ideal Section in the Lake Superior + Region] + +The general relations of the formations of that portion of the area +which lies about Lake Superior are shown in Figure 262. Great +unconformities, _UU´_ separate the Algonkian both from the Archean and +from the Cambrian, and divide it into three distinct systems,--the +_Lower Huronian_, the _Upper Huronian_, and the _Keweenawan_. The +Lower and the Upper Huronian consist in the main of old sea muds and +sands and limy oozes now changed to gneisses, schists, marbles, +quartzites, slates, and other metamorphic rocks. The Keweenawan is +composed of immense piles of lava, such as those of Iceland, overlain +by bedded sandstones. What remains of these rock systems after the +denudation of all later geologic ages is enormous. The Lower Huronian +is more than a mile thick, the Upper Huronian more than two miles +thick, while the Keweenawan exceeds nine miles in thickness. The vast +length of Algonkian time is shown by the thickness of its marine +deposits and by the cycles of erosion which it includes. In Figure 262 +the student may read an outline of the history of the Lake Superior +region, the deformations which it suffered, their relative severity, +the times when they occurred, and the erosion cycles marked by the +successive unconformities. + +=Other pre-Cambrian areas in North America.= Pre-Cambrian rocks are +exposed in various parts of the continent, usually by the erosion of +mountain ranges in which their strata were infolded. Large areas occur +in the maritime provinces of Canada. The core of the Green Mountains +of Vermont is pre-Cambrian, and rocks of these systems occur in +scattered patches in western Massachusetts. Here belong also the +oldest rocks of the Highlands of the Hudson and of New Jersey. The +Adirondack region, an outlier of the Laurentian region, exposes +pre-Cambrian rocks, which have been metamorphosed and tilted by the +intrusion of a great boss of igneous rock out of which the central +peaks are carved. The core of the Blue Ridge and probably much of the +Piedmont Belt are of this age. In the Black Hills the irruption of an +immense mass of granite has caused or accompanied the upheaval of +pre-Cambrian strata and metamorphosed them by heat and pressure into +gneisses, schists, quartzites, and slates. In most of these +mountainous regions the lowest strata are profoundly changed by +metamorphism, and they can be assigned to the pre-Cambrian only where +they are clearly overlain unconformably by formations proved to be +Cambrian by their fossils. In the Belt Mountains of Montana, however, +the Cambrian is underlain by Algonkian sediments twelve thousand feet +thick, and but little altered. + +=Mineral wealth of the pre-Cambrian rocks.= The pre-Cambrian rocks are +of very great economic importance, because of their extensive +metamorphism and the enormous masses of igneous rock which they +involve. In many parts of the country they are the source of supply of +granite, gneiss, marble, slate, and other such building materials. +Still more valuable are the stores of iron and copper and other metals +which they contain. + +At the present time the pre-Cambrian region about Lake Superior leads +the world in the production of iron ore, its output for 1903 being +more than five sevenths of the entire output of the whole United +States, and exceeding that of any foreign country. The ore bodies +consist chiefly of the red oxide of iron (hematite) and occur in +troughs of the strata, underlain by some impervious rock. A theory +held by many refers the ultimate source of the iron to the igneous +rocks of the Archean. When these rocks were upheaved and subjected to +weathering, their iron compounds were decomposed. Their iron was +leached out and carried away to be laid in the Algonkian water bodies +in beds of iron carbonate and other iron compounds. During the later +ages, after the Algonkian strata had been uplifted to form part of the +continent, a second concentration has taken place. Descending +underground waters charged with oxygen have decomposed the iron +carbonate and deposited the iron, in the form of iron oxide, in +troughs of the strata where their downward progress was arrested by +impervious floors. + +The pre-Cambrian rocks of the eastern United States also are rich in +iron. In certain districts, as in the Highlands of New Jersey, the +black oxide of iron (magnetite) is so abundant in beds and +disseminated grains that the ordinary surveyor's compass is useless. + +The pre-Cambrian copper mines of the Lake Superior region are among +the richest on the globe. In the igneous rocks copper, next to iron, +is the most common of all the useful metals, and it was especially +abundant in the Keweenawan lavas. After the Keweenawan was uplifted to +form land, percolating waters leached out much of the copper diffused +in the lava sheets and deposited it within steam blebs as amygdules of +native copper, in cracks and fissures, and especially as a cement, or +matrix, in the interbedded gravels which formed the chief aquifers of +the region. The famous Calumet and Hecla mine follows down the dip of +the strata to the depth of nearly a mile and works such an ancient +conglomerate whose matrix is pure copper. + + [Illustration: Fig. 263. Successive Stages in the Development + of the Ovum to the Gastrula Stage] + +=The appearance of life.= Sometime during the dim ages preceding the +Cambrian, whether in the Archean or in the Algonkian we know not, +occurred one of the most important events in the history of the earth. +Life appeared for the first time upon the planet. Geology has no +evidence whatever to offer as to whence or how life came. All +analogies lead us to believe that its appearance must have been +sudden. Its earliest forms are unknown, but analogy suggests that as +every living creature has developed from a single cell, so the +earliest organisms upon the globe--the germs from which all later life +is supposed to have been evolved--were tiny, unicellular masses of +protoplasm, resembling the amoeba of to-day in the simplicity of their +structure. + +Such lowly forms were destitute of any hard parts and could leave no +evidence of their existence in the record of the rocks. And of their +supposed descendants we find so few traces in the pre-Cambrian strata +that the first steps in organic evolution must be supplied from such +analogies in embryology as the following. The fertilized ovum, the +cell with which each animal begins its life, grows and multiplies by +cell division, and develops into a hollow globe of cells called the +_blastosphere_. This stage is succeeded by the stage of the +_gastrula_,--an ovoid or cup-shaped body with a double wall of cells +inclosing a body cavity, and with an opening, the primitive mouth. +Each of these early embryological stages is represented by living +animals,--the undivided cell by the _protozoa_, the blastosphere by +some rare forms, and the gastrula in the essential structure of the +_coelenterates_,--the subkingdom to which the fresh-water hydra and +the corals belong. All forms of animal life, from the coelenterates to +the mammals, follow the same path in their embryological development +as far as the gastrula stage, but here their paths widely diverge, +those of each subkingdom going their own separate ways. + +We may infer, therefore, that during the pre-Cambrian periods organic +evolution followed the lines thus dimly traced. The earliest +one-celled protozoa were probably succeeded by many-celled animals of +the type of the blastosphere, and these by gastrula-like organisms. +From the gastrula type the higher subdivisions of animal life +probably diverged, as separate branches from a common trunk. Much or +all of this vast differentiation was accomplished before the opening +of the next era; for all the subkingdoms are represented in the +Cambrian except the vertebrates. + +=Evidences of pre-Cambrian life.= An indirect evidence of life during +the pre-Cambrian periods is found in the abundant and varied fauna of +the next period; for, if the theory of evolution is correct, the +differentiation of the Cambrian fauna was a long process which might +well have required for its accomplishment a large part of pre-Cambrian +time. + +Other indirect evidences are the pre-Cambrian limestones, iron ores, +and graphite deposits, since such minerals and rocks have been formed +in later times by the help of organisms. If the carbonate of lime of +the Algonkian limestones and marbles was extracted from sea water by +organisms, as is done at present by corals, mollusks, and other humble +animals and plants, the life of those ancient seas must have been +abundant. Graphite, a soft black mineral composed of carbon and used +in the manufacture of lead pencils and as a lubricant, occurs widely +in the metamorphic pre-Cambrian rocks. It is known to be produced in +some cases by the metamorphism of coal, which itself is formed of +decomposed vegetal tissues. Seams of graphite may therefore represent +accumulations of vegetal matter such as seaweed. But limestone, iron +ores, and graphite can be produced by chemical processes, and their +presence in the pre-Cambrian makes it only probable, and not certain, +that life existed at that time. + +=Pre-Cambrian fossils.= Very rarely has any clear trace of an organism +been found in the most ancient chapters of the geological record, so +many of their leaves have been destroyed and so far have their pages +been defaced. Omitting structures whose organic nature has been +questioned, there are left to mention a tiny seashell of one of the +most lowly types,--a _Discina_ from the pre-Cambrian rocks of the +Colorado Canyon,--and from the pre-Cambrian rocks of Montana trails of +annelid worms and casts of their burrows in ancient beaches, and +fragments of the tests of crustaceans. These diverse forms indicate +that before the Algonkian had closed, life was abundant and had widely +differentiated. We may expect that other forms will be discovered as +the rocks are closely searched. + +=Pre-Cambrian geography.= Our knowledge is far too meager to warrant +an attempt to draw the varying outlines of sea and land during the +Archean and Algonkian eras. Pre-Cambrian time probably was longer than +all later geological time down to the present, as we may infer from +the vast thicknesses of its rocks and the unconformities which part +them. We know that during its long periods land masses again and again +rose from the sea, were worn low, and were submerged and covered with +the waste of other lands. But the formations of separated regions +cannot be correlated because of the absence of fossils, and nothing +more can be made out than the detached chapters of local histories, +such as the outline given of the district about Lake Superior. + +The pre-Cambrian rocks show no evidence of any forces then at work +upon the earth except the forces which are at work upon it now. The +most ancient sediments known are so like the sediments now being laid +that we may infer that they were formed under conditions essentially +similar to those of the present time. There is no proof that the sands +of the pre-Cambrian sandstones were swept by any more powerful waves +and currents than are offshore sands to-day, or that the muds of the +pre-Cambrian shales settled to the sea floor in less quiet water than +such muds settle in at present. The pre-Cambrian lands were, no doubt, +worn by wind and weather, beaten by rain, and furrowed by streams as +now, and, as now, they fronted the ocean with beaches on which waves +dashed and along which tidal currents ran. + +Perhaps the chief difference between the pre-Cambrian and the present +was the absence of life upon the land. So far as we have any +knowledge, no forests covered the mountain sides, no verdure carpeted +the plains, and no animals lived on the ground or in the air. It is +permitted to think of the most ancient lands as deserts of barren rock +and rock waste swept by rains and trenched by powerful streams. We may +therefore suppose that the processes of their destruction went on more +rapidly than at present. + + + + +CHAPTER XVI + +THE CAMBRIAN + + +=The Paleozoic era.= The second volume of the geological record, +called the Paleozoic (Greek, _palaios_, ancient; _zoe_, life), has +come down to us far less mutilated and defaced than has the first +volume, which contains the traces of the most ancient life of the +globe. Fossils are far more abundant in the Paleozoic than in the +earlier strata, while the sediments in which they were entombed have +suffered far less from metamorphism and other causes, and have been +less widely buried from view, than the strata of the pre-Cambrian +groups. By means of their fossils we can correlate the formations of +widely separated regions from the beginning of the Paleozoic on, and +can therefore trace some outline of the history of the continents. + +Paleozoic time, although shorter than the pre-Cambrian as measured by +the thickness of the strata, must still be reckoned in millions of +years. During this vast reach of time the changes in organisms were +very great. It is according to the successive stages in the advance of +life that the Paleozoic formations are arranged in five systems,--the +_Cambrian_, the _Ordovician_, the _Silurian_, the _Devonian_, and the +_Carboniferous_. On the same basis the first three systems are grouped +together as the older Paleozoic, because they alike are characterized +by the dominance of the invertebrates; while the last two systems are +united in the later Paleozoic, and are characterized, the one by the +dominance of fishes, and the other by the appearance of amphibians and +reptiles. + +Each of these systems is world-wide in its distribution, and may be +recognized on any continent by its own peculiar fauna. The names first +given them in Great Britain have therefore come into general use, +while their subdivisions, which often cannot be correlated in +different countries and different regions, are usually given local +names. + +The first three systems were named from the fact that their strata are +well displayed in Wales. The Cambrian carries the Roman name of Wales, +and the Ordovician and Silurian the names of tribes of ancient Britons +which inhabited the same country. The Devonian is named from the +English county Devon, where its rocks were early studied. The +Carboniferous was so called from the large amount of coal which it was +found to contain in Great Britain and continental Europe. + + +The Cambrian + +=Distribution of strata.= The Cambrian rocks outcrop in narrow belts +about the pre-Cambrian areas of eastern Canada and the Lake Superior +region, the Adirondacks and the Green Mountains. Strips of Cambrian +formations occupy troughs in the pre-Cambrian rocks of New England and +the maritime provinces of Canada; a long belt borders on the west the +crystalline rocks of the Blue Ridge; and on the opposite side of the +continent the Cambrian reappears in the mountains of the Great Basin +and the Canadian Rockies. In the Mississippi valley it is exposed in +small districts where uplift has permitted the stripping off of +younger rocks. Although the areas of outcrop are small, we may infer +that Cambrian rocks were widely deposited over the continent of North +America. + +=Physical geography.= The Cambrian system of North America comprises +three distinct series, the _Lower Cambrian_, the _Middle Cambrian_, +and the _Upper Cambrian_, each of which is characterized by its own +peculiar fauna. In sketching the outlines of the continent as it was +at the beginning of the Paleozoic, it must be remembered that wherever +the Lower Cambrian formations now are found was certainly then sea +bottom, and wherever the Lower Cambrian are wanting, and the next +formations rest directly on pre-Cambrian rocks, was probably then +land. + + [Illustration: Fig. 264. Hypothetical Map of Eastern North + America at the Beginning of Cambrian Time + + Unshaded areas, probable land] + +=Early Cambrian geography.= In this way we know that at the opening of +the Cambrian two long, narrow mediterranean seas stretched from north +to south across the continent. The eastern sea extended from the Gulf +of St. Lawrence down the Champlain-Hudson valley and thence along the +western base of the Blue Ridge south at least to Alabama. The western +sea stretched from the Canadian Rockies over the Great Basin and at +least as far south as the Grand Canyon of the Colorado in Arizona. + +Between these mediterraneans lay a great central land which included +the pre-Cambrian U-shaped area of the Laurentian peneplain, and +probably extended southward to the latitude of New Orleans. To the +east lay a land which we may designate as _Appalachia_, whose western +shore line was drawn along the site of the present Blue Ridge, but +whose other limits are quite unknown. The land of Appalachia must have +been large, for it furnished a great amount of waste during the entire +Paleozoic era, and its eastern coast may possibly have lain even +beyond the edge of the present continental shelf. On the western side +of the continent a narrow land occupied the site of the Sierra Nevada +Mountains. + +Thus, even at the beginning of the Paleozoic, the continental plateau +of North America had already been left by crustal movements in relief +above the abysses of the great oceans on either side. The +mediterraneans which lay upon it were shallow, as their sediments +prove. They were _epicontinental seas_; that is, they rested _upon_ +(Greek, _epi_) the submerged portion of the continental plateau. We +have no proof that the deep ocean ever occupied any part of where +North America now is. + +The Middle and Upper Cambrian strata are found together with the Lower +Cambrian over the area of both the eastern and the western +mediterraneans, so that here the sea continued during the entire +period. The sediments throughout are those of shoal water. Coarse +cross-bedded sandstones record the action of strong shifting currents +which spread coarse waste near shore and winnowed it of finer stuff. +Frequent ripple marks on the bedding planes of the strata prove that +the loose sands of the sea floor were near enough to the surface to be +agitated by waves and tidal currents. Sun cracks show that often the +outgoing tide exposed large muddy flats to the drying action of the +sun. The fossils, also, of the strata are of kinds related to those +which now live in shallow waters near the shore. + +The sediments which gathered in the mediterranean seas were very +thick, reaching in places the enormous depth of ten thousand feet. +Hence the bottoms of these seas were sinking troughs, ever filling +with waste from the adjacent land as fast as they subsided. + +=Late Cambrian geography.= The formations of the Middle and Upper +Cambrian are found resting unconformably on the pre-Cambrian rocks +from New York westward into Minnesota and at various points in the +interior, as in Missouri and in Texas. Hence after earlier Cambrian +time the central land subsided, with much the same effect as if the +Mississippi valley were now to lower gradually, and the Gulf of Mexico +to spread northward until it entered Lake Superior. The Cambrian seas +transgressed the central land and strewed far and wide behind their +advancing beaches the sediments of the later Cambrian upon an eroded +surface of pre-Cambrian rocks. + +The succession of the Cambrian formations in North America records +many minor oscillations and varying conditions of physical geography; +yet on the whole it tells of widening seas and lowering lands. Basal +conglomerates and coarse sandstones which must have been laid near +shore are succeeded by shaly sandstones, sandy shales, and shales. +Toward the top of the series heavy beds of limestone, extending from +the Blue Ridge to Missouri, speak of clear water, and either of more +distant shores or of neighboring lands which were worn or sunk so low +that for the most part their waste was carried to the sea in solution. + +In brief, the Cambrian was a period of submergence. It began with the +larger part of North America emerged as great land masses. It closed +with most of the interior of the continental plateau covered with a +shallow sea. + + +The Life of the Cambrian Period + +It is now for the first time that we find preserved in the offshore +deposits of the Cambrian seas enough remains of animal life to be +properly called a fauna. Doubtless these remains are only the most +fragmentary representation of the life of the time, for the Cambrian +rocks are very old and have been widely metamorphosed. Yet the five +hundred and more species already discovered embrace all the leading +types of invertebrate life, and are so varied that we must believe +that their lines of descent stretch far back into the pre-Cambrian +past. + +=Plants.= No remains of plants have been found in Cambrian strata, +except some doubtful markings, as of seaweed. + +=Sponges.= The sponges, the lowest of the multicellular animals, were +represented by several orders. Their fossils are recognized by the +siliceous spicules, which, as in modern sponges, either were scattered +through a mass of horny fibers or were connected in a flinty +framework. + + [Illustration: Fig. 265. Sponge Spicules as seen in Flint under + the Microscope] + +=Coelenterates.= This subkingdom includes two classes of interest to +the geologist,--the _Hydrozoa_, such as the fresh-water hydra and the +jellyfish, and the _corals_. Both classes existed in the Cambrian. + + [Illustration: Fig. 266. Graptolites] + +The Hydrozoa were represented not only by jellyfish but also by the +_graptolite_, which takes its name from a fancied resemblance of some +of its forms to a quill pen. It was a composite animal with a horny +framework, the individuals of the colony living in cells strung on one +or both sides along a hollow stem, and communicating by means of a +common flesh in this central tube. Some graptolites were straight, and +some curved or spiral; some were single stemmed, and others consisted +of several radial stems united. Graptolites occur but rarely in the +Upper Cambrian. In the Ordovician and Silurian they are very +plentiful, and at the close of the Silurian they pass out of +existence, never to return. + +=Corals= are very rarely found in the Cambrian, and the description of +their primitive types is postponed to later chapters treating of +periods when they became more numerous. + +=Echinoderms.= This subkingdom comprises at present such familiar +forms as the crinoid, the starfish, and the sea urchin. The structure +of echinoderms is radiate. Their integument is hardened with plates or +particles of carbonate of lime. + + [Illustration: Fig. 267. Cystoids, one showing Two Rudimentary + Arms] + +Of the free echinoderms, such as the starfish and the sea urchin, the +former has been found in the Cambrian rocks of Europe, but neither +have so far been discovered in the strata of this period in North +America. The stemmed and lower division of the echinoderms was +represented by a primitive type, the _cystoid_, so called from its +saclike form, A small globular or ovate "calyx" of calcareous plates, +with an aperture at the top for the mouth, inclosed the body of the +animal, and was attached to the sea bottom by a short flexible stalk +consisting of disks of carbonate of lime held together by a central +ligament. + +=Arthropods.= These segmented animals with "jointed feet," as their +name suggests, may be divided in a general way into water breathers +and air breathers. The first-named and lower division comprises the +class of the _Crustacea_,--arthropods protected by a hard exterior +skeleton, or "crust,"--of which crabs, crayfish, and lobsters are +familiar examples. The higher division, that of the air breathers, +includes the following classes: spiders, scorpions, centipedes, and +insects. + +=The trilobite.= The aquatic arthropods, the Crustacea, culminated +before the air breathers; and while none of the latter are found in +the Cambrian, the former were the dominant life of the time in +numbers, in size, and in the variety of their forms. The leading +crustacean type is the _trilobite_, which takes its name from the +three lobes into which its shell is divided longitudinally. There are +also three cross divisions,--the head shield, the tail shield, and +between the two the thorax, consisting of a number of distinct and +unconsolidated segments. The head shield carries a pair of large, +crescentic, compound eyes, like those of the insect. The eye varies +greatly in the number of its lenses, ranging from fourteen in some +species to fifteen thousand in others. Figure 268, C, is a restoration +of the trilobite, and shows the appendages, which are found preserved +only in the rarest cases. + + [Illustration: Fig. 268. Trilobites + + A, a Cambrian species; B, a Devonian species showing eyes; + C, restoration of an Ordovician species] + +During the long ages of the Cambrian the trilobite varied greatly. +Again and again new species and genera appeared, while the older types +became extinct. For this reason and because of their abundance, +trilobites are used in the classification of the Cambrian system. The +Lower Cambrian is characterized by the presence of a trilobitic fauna +in which the genus Olenellus is predominant. This, the _Olenellus +Zone_, is one of the most important platforms in the entire geological +series; for, the world over, it marks the beginning of Paleozoic time, +while all underlying strata are classified as pre-Cambrian. The Middle +Cambrian is marked by the genus Paradoxides, and the Upper Cambrian by +the genus Olenus. Some of the Cambrian trilobites were giants, +measuring as much as two feet long, while others were the smallest of +their kind, a fraction of an inch in length. + +Another type of crustacean which lived in the Cambrian and whose order +is still living is illustrated in Figure 269. + + [Illustration: Fig. 269. A Phyllopod] + +=Worms.= Trails and burrows of worms have been left on the sea beaches +and mud flats of all geological times from the Algonkian to the +present. + +=Brachiopods.= These soft-bodied animals, with bivalve shells and two +interior armlike processes which served for breathing, appeared in the +Algonkian, and had now become very abundant. The two valves of the +brachiopod shell are unequal in size, and in each valve a line drawn +from the beak to the base divides the valve into two equal parts +(Fig. 270). It may thus be told from the pelecypod mollusk, such as the +clam, whose two valves are not far from equal in size, each being +divided into unequal parts by a line dropped from the beak (Fig.272). + + [Illustration: Fig. 270. A Cambrian Articulate Brachiopod, Orthis] + + [Illustration: Fig. 271. Cambrian Inarticulate Brachiopods + + A, Lingulella; B, Discina] + +Brachiopods include two orders. In the most primitive order--that of +the _inarticulate_ brachiopods--the two valves are held together only +by muscles of the animal, and the shell is horny or is composed of +phosphate of lime. The _Discina_, which began in the Algonkian, is of +this type, as is also the _Lingulella_ of the Cambrian (Fig. 271). Both +of these genera have lived on during the millions of years of geological +time since their introduction, handing down from generation to +generation with hardly any change to their descendants now living off +our shores the characters impressed upon them at the beginning. + +The more highly organized _articulate_ brachiopods have valves of +carbonate of lime more securely joined by a hinge with teeth and +sockets (Fig. 270). In the Cambrian the inarticulates predominate, +though the articulates grow common toward the end of the period. + +=Mollusks.= The three chief classes of mollusks--the _pelecypods_ +(represented by the oyster and clam of to-day), the _gastropods_ +(represented now by snails, conches, and periwinkles), and the +_cephalopods_ (such as the nautilus, cuttlefish, and squids)--were all +represented in the Cambrian, although very sparingly. + + [Illustration: Fig. 272. A Cambrian Pelecypod] + + [Illustration: Fig. 273. Gastropods] + +Pteropods, a suborder of the gastropods, appeared in this age. Their +papery shells of carbonate of lime are found in great numbers from +this time on. + + [Illustration: Fig. 274. Cambrian Pteropods] + +Cephalopods, the most highly organized of the mollusks, started into +existence, so far as the record shows, toward, the end of the Cambrian, +with the long extinct _Orthoceras_ (_straighthorn_) and the allied +genera of its family. The Orthoceras had a long, straight, and tapering +shell, divided by cross partitions into chambers. The animal lived in +the "body chamber" at the larger end, and walled off the other chambers +from it in succession during the growth of the shell. A central tube, +the _siphuncle_ (_s_, Fig. 275, _B_), passed through from the body +chamber to the closed tip of the cone. + + [Illustration: Fig. 275. Orthoceras + + A, fossil; B, restoration] + +The seashells, both brachiopods and mollusks, are in some respects the +most important to the geologist of all fossils. They have been so +numerous, so widely distributed, and so well preserved because of +their durable shells and their station in growing sediments, that +better than any other group of organisms they can be used to correlate +the strata of different regions and to mark by their slow changes the +advance of geological time. + +=Climate.= The life of Cambrian times in different countries contains +no suggestion of any marked climatic zones, and as in later periods a +warm climate probably reached to the polar regions. + + + + +CHAPTER XVII + +THE ORDOVICIAN[2] AND SILURIAN + +[2] Often known as the Lower Silurian. + + +The Ordovician + +In North America the Ordovician rocks lie conformably on the Cambrian. +The two periods, therefore, were not parted by any deformation, either +of mountain making or of continental uplift. The general submergence +which marked the Cambrian continued into the succeeding period with +little interruption. + +=Subdivisions and distribution of strata.= The Ordovician series, as +they have been made out in New York, are given for reference in the +following table, with the rocks of which they are chiefly composed: + + 5 Hudson ..... shales + 4 Utica ..... shales + 3 Trenton ..... limestones + 2 Chazy ..... limestones + 1 Calciferous ..... sandy limestones + +These marine formations of the Ordovician outcrop about the Cambrian +and pre-Cambrian areas, and, as borings show, extend far and wide over +the interior of the continent beneath more recent strata. The +Ordovician sea stretched from Appalachia across the Mississippi +valley. It seems to have extended to California, although broken +probably by several mountainous islands in the west. + +=Physical geography.= The physical history of the period is recorded +in the succession of its formations. The sandstones of the Upper +Cambrian, as we have learned, tell of a transgressing sea which +gradually came to occupy the Mississippi valley and the interior of +North America. The limestones of the early and middle Ordovician show +that now the shore had become remote and the lands had become more +low. The waters now had cleared. Colonies of brachiopods and other +lime-secreting animals occupied the sea bottom, and their debris +mantled it with sheets of limy ooze. The sandy limestones of the +Calciferous record the transition stage from the Cambrian when some +sand was still brought in from shore. The highly fossiliferous +limestones of the Trenton tell of clear water and abundant life. We +need not regard this epicontinental sea as deep. No abysmal deposits +have been found, and the limestones of the period are those which +would be laid in clear, warm water of moderate depth like that of +modern coral seas. + + [Illustration: Fig. 276. Hypothetical Map of the Eastern United + States in Ordovician Time + + Shaded areas, probable sea; broken lines, approximate shore lines] + +The shales of the Utica and Hudson show that the waters of the sea now +became clouded with mud washed in from land. Either the land was +gradually uplifted, or perhaps there had arrived one of those periodic +crises which, as we may imagine, have taken place whenever the +crust of the shrinking earth has slowly given way over its great +depressions, and the ocean has withdrawn its waters into deepening +abysses. The land was thus left relatively higher and bordered with +new coastal plains. The epicontinental sea was shoaled and narrowed, +and muds were washed in from the adjacent lands. + +=The Taconic deformation.= The Ordovician was closed by a deformation +whose extent and severity are not yet known. From the St. Lawrence +River to New York Bay, along the northwestern and western border of +New England, lies a belt of Cambrian-Ordovician rocks more than a mile +in total thickness, which accumulated during the long ages of those +periods in a gradually subsiding trough between the Adirondacks and a +pre-Cambrian range lying west of the Connecticut River. But since +their deposition these ancient sediments have been crumpled and +crushed, broken with great faults, and extensively metamorphosed. The +limestones have recrystallized into marbles, among them the famous +marbles of Vermont; the Cambrian sandstones have become quartzites, +and the Hudson shale has been changed to a schist exposed on Manhattan +Island and northward. + +In part these changes occurred at the close of the Ordovician, for in +several places beds of Silurian age rest unconformably on the upturned +Ordovician strata; but recent investigations have made it probable +that the crustal movements recurred at later times, and it was perhaps +in the Devonian and at the close of the Carboniferous that the greater +part of the deformation and metamorphism was accomplished. As a result +of these movements,--perhaps several times repeated,--a great mountain +range was upridged, which has been long since leveled by erosion, but +whose roots are now visible in the Taconic Mountains of western New +England. + +=The Cincinnati anticline.= Over an oval area in Ohio, Indiana, and +Kentucky, whose longer axis extends from north to south through +Cincinnati, the Ordovician strata rise in a very low, broad swell, +called the Cincinnati anticline. The Silurian and Devonian strata thin +out as they approach this area and seem never to have deposited upon +it. We may regard it, therefore, as an island upwarped from the sea at +the close of the Ordovician or shortly after. + +=Petroleum and natural gas.= These valuable illuminants and fuels are +considered here because, although they are found in traces in older +strata, it is in the Ordovician that they occur for the first time in +large quantities. They range throughout later formations down to the +most recent. + + [Illustration: Fig. 277. Diagram Illustrating the Conditions of + Accumulation of Oil and Gas + + _a_, source; _b_, reservoir; _c_, cover. What would be the result + of boring to the reservoir rock at _d_? at _d´_? at _d´´_?] + +The oil horizons of California and Texas are Tertiary; those of +Colorado, Cretaceous; those of West Virginia, Carboniferous; those of +Pennsylvania, Kentucky, and Canada, Devonian; and the large field of +Ohio and Indiana belongs to the Ordovician and higher systems. + +Petroleum and natural gas, wherever found, have probably originated +from the decay of organic matter when buried in sedimentary deposits, +just as at present in swampy places the hydrogen and carbon of +decaying vegetation combine to form marsh gas. The light and heat of +these hydrocarbons we may think of, therefore, as a gift to the +civilized life of our race from the humble organisms, both animal and +vegetable, of the remote past, whose remains were entombed in the +sediments of the Ordovician and later geological ages. + +Petroleum is very widely disseminated throughout the stratified rocks. +Certain limestones are visibly greasy with it, and others give off its +characteristic fetid odor when struck with a hammer. Many shales are +bituminous, and some are so highly charged that small flakes may be +lighted like tapers, and several gallons of oil to the ton may be +obtained by distillation. + +But oil and gas are found in paying quantities only when certain +conditions meet: + +1. A _source_ below, usually a bituminous shale, from whose organic +matter they have been derived by slow change. + +2. A _reservoir_ above, in which they have gathered. This is either a +porous sandstone or a porous or creviced limestone. + +3. Oil and gas are lighter than water, and are usually under pressure +owing to artesian water. Hence, in order to hold them from escaping to +the surface, the reservoir must have the shape of an _anticline_, +_dome_, or _lens_. + +4. It must also have an _impervious cover_, usually a shale. In these +reservoirs gas is under a pressure which is often enormous, reaching +in extreme cases as high as a thousand five hundred pounds to the +square inch. When tapped it rushes out with a deafening roar, +sometimes flinging the heavy drill high in air. In accounting for this +pressure we must remember that the gas has been compressed within the +pores of the reservoir rock by artesian water, and in some cases also +by its own expansive force. It is not uncommon for artesian water to +rise in wells after the exhaustion of gas and oil. + + +_Life of the Ordovician_ + +During the ages of the Ordovician, life made great advances. Types +already present branched widely into new genera and species, and new +and higher types appeared. + +Sponges continued from the Cambrian. Graptolites now reached their +climax. + + [Illustration: Fig. 278. Stromatopora] + +=Stromatopora=--colonies of minute hydrozoans allied to corals--grew +in places on the sea floor, secreting stony masses composed of thin, +close, concentric layers, connected by vertical rods. The Stromatopora +are among the chief limestone builders of the Silurian and Devonian +periods. + +=Corals= developed along several distinct lines, like modern corals +they secreted a calcareous framework, in whose outer portions the +polyps lived. In the Ordovician, corals were represented chiefly by +the family of the _Chætetes_, all species of which are long since +extinct. The description of other types of corals will be given under +the Silurian, where they first became abundant. + +=Echinoderms.= The cystoid reaches its climax, but there appear now +two higher types of echinoderms,--the crinoid and the starfish. The +_crinoid_, named from its resemblance to the lily, is like the cystoid +in many respects, but has a longer stem and supports a crown of +plumose arms. Stirring the water with these arms, it creates currents +by which particles of food are wafted to its mouth. Crinoids are rare +at the present time, but they grew in the greatest profusion in the +warm Ordovician seas and for long ages thereafter. In many places the +sea floor was beautiful with these graceful, flowerlike forms, as with +fields of long-stemmed lilies. Of the higher, free-moving classes of +the echinoderms, starfish are more numerous than in the Cambrian, and +sea urchins make their appearance in rare archaic forms. + + [Illustration: Fig. 279. Crinoid, a Jurassic Species] + + [Illustration: Fig. 280. An Ordovician Starfish] + + [Illustration: Fig. 281. An Ordovician Sea Urchin] + + [Illustration: Fig. 282. Eurypterus] + +=Crustaceans.= Trilobites now reach their greatest development and +more than eleven hundred species have been described from the rocks of +this period. It is interesting to note that in many species the +segments of the thorax have now come to be so shaped that they move +freely on one another. Unlike their Cambrian ancestors, many of the +Ordovician trilobites could roll themselves into balls at the approach +of danger. It is in this attitude, taken at the approach of death, +that trilobites are often found in the Ordovician and later rocks. The +gigantic crustaceans called the _eurypterids_ were also present in +this period (Fig. 282). + +The arthropods had now seized upon the land. Centipedes and insects of +a low type, the earliest known land animals, have been discovered in +strata of this system. + + [Illustration: Fig. 283. A Bryozoan] + +=Bryozoans.= No fossils are more common in the limestones of the time +than the small branching stems and lacelike mats of the +bryozoans,--the skeletons of colonies of a minute animal allied in +structure to the brachiopod. + + [Illustration: Fig. 284. Ordovician Brachiopods] + +=Brachiopods.= These multiplied greatly, and in places their shells +formed thick beds of coquina. They still greatly surpassed the +mollusks in numbers. + +=Cephalopods.= Among the mollusks we must note the evolution of the +cephalopods. The primitive straight Orthoceras has now become +abundant. But in addition to this ancestral type there appears a +succession of forms more and more curved and closely coiled, as +illustrated in Figure 285. The nautilus, which began its course in +this period, crawls on the bottom of our present seas. + + [Illustration: Fig. 285. A, Cyrtoceras; B, Trochoceras; C, Lituites] + + [Illustration: Fig. 286. Nautilus] + +=Vertebrates.= The most important record of the Ordovician is that of +the appearance of a new and higher type, with possibilities of +development lying hidden in its structure that the mollusk and the +insect could never hope to reach. Scales and plates of minute fishes +found in the Ordovician rocks near Canon City, Colorado, show that the +humblest of the vertebrates had already made its appearance. But it is +probable that vertebrates had been on the earth for ages before this +in lowly types, which, being destitute of hard parts, would leave no +record. + + +The Silurian + +The narrowing of the seas and the emergence of the lands which +characterized the closing epoch of the Ordovician in eastern North +America continue into the succeeding period of the Silurian. New +species appear and many old species now become extinct. + +=The Appalachian region.= Where the Silurian system is most fully +developed, from New York southward along the Appalachian Mountains, it +comprises four series: + + 4 Salina ..... shales, impure limestones, gypsum, salt + 3 Niagara ..... chiefly limestones + 2 Clinton ..... sandstones, shales, with some limestones + 1 Medina ..... conglomerates, sandstones + +The rocks of these series are shallow-water deposits and reach the +total thickness of some five thousand feet. Evidently they were laid +over an area which was on the whole gradually subsiding, although with +various gentle oscillations which are recorded in the different +formations. The coarse sands of the heavy Medina formations record a +period of uplift of the oldland of Appalachia, when erosion went on +rapidly and coarse waste in abundance was brought down from the hills +by swift streams and spread by the waves in wide, sandy flats. As the +lands were worn lower the waste became finer, and during an epoch of +transition--the Clinton--there were deposited various formations of +sandstones, shales, and limestones. The Niagara limestones testify to +a long epoch of repose, when low-lying lands sent little waste down to +the sea. + +The gypsum and salt deposits of the Salina show that toward the close +of the Silurian period a slight oscillation brought the sea floor +nearer to the surface, and at the north cut off extensive tracts from +the interior sea. In these wide lagoons, which now and then regained +access to the open sea and obtained new supplies of salt water, beds +of salt and gypsum were deposited as the briny waters became +concentrated by evaporation under a desert climate. Along with these +beds there were also laid shales and impure limestones. + +In New York the "salt pans" of the Salina extended over an area one +hundred and fifty miles long from east to west and sixty miles wide, +and similar salt marshes occurred as far west as Cleveland, Ohio, and +Goderich on Lake Huron. At Ithaca, New York, the series is fifteen +hundred feet thick, and is buried beneath an equal thickness of later +strata. It includes two hundred and fifty feet of solid salt, in +several distinct beds, each sealed within the shales of the series. + +Would you expect to find ancient beds of rock salt inclosed in beds of +pervious sandstone? + +The salt beds of the Salina are of great value. They are reached by +well borings, and their brines are evaporated by solar heat and by +boiling. The rock salt is also mined from deep shafts. + +Similar deposits of salt, formed under like conditions, occur in the +rocks of later systems down to the present. The salt beds of Texas are +Permian, those of Kansas are Permian, and those of Louisiana are +Tertiary. + +=The Mississippi valley.= The heavy near-shore formations of the +Silurian in the Appalachian region thin out toward the west. The +Medina and the Clinton sandstones are not found west of Ohio, where +the first passes into a shale and the second into a limestone. The +Niagara limestone, however, spreads from the Hudson River to beyond +the Mississippi, a distance of more than a thousand miles. During the +Silurian period the Mississippi valley region was covered with a +quiet, shallow, limestone-making sea, which received little waste from +the low lands which bordered it. + +The probable distribution of land and sea in eastern North America and +western Europe is shown in Figure 287. The fauna of the interior +region and of eastern Canada are closely allied with that of western +Europe, and several species are identical. We can hardly account for +this except by a shallow-water connection between the two ancient +epicontinental seas. It was perhaps along the coastal shelves of a +northern land connecting America and Europe by way of Greenland and +Iceland that the migration took place, so that the same species came +to live in Iowa and in Sweden. + + [Illustration: Fig. 287. Hypothetical Map of Parts of North America + and Europe in Silurian Time. + + Shaded areas, probably seas; broken lines, approximate shore lines] + +=The western United States.= So little is found of the rocks of the +system west of the Missouri River that it is quite probable that the +western part of the United States had for the most part emerged from +the sea at the close of the Ordovician and remained land during the +Silurian. At the same time the western land was perhaps connected with +the eastern land of Appalachia across Arkansas and Mississippi; for +toward the south the Silurian sediments indicate an approach to shore. + + +_Life of the Silurian_ + +In this brief sketch it is quite impossible to relate the many changes +of species and genera during the Silurian. + +=Corals.= Some of the more common types are familiarly known as cup +corals, honeycomb corals, and chain corals. In the _cup corals_ the +most important feature is the development of radiating vertical +partitions, or _septa_, in the cell of the polyp. Some of the cup +corals grew in hemispherical colonies (Fig. 288), while many were +separate individuals (Fig. 289), building a single conical, or +horn-shaped cell, which sometimes reached the extreme size of a foot +in length and two or three inches in diameter. + + [Illustration: Fig. 288. A Compound Cup Coral] + + [Illustration: Fig. 289. A Simple Cup Coral] + + [Illustration: Fig. 290. Honeycomb Corals] + + [Illustration: Fig. 291. A Chain Coral] + + [Illustration: Fig. 292. A Syringopora Coral] + +_Honeycomb corals_ consist of masses of small, close-set prismatic +cells, each crossed by horizontal partitions, or _tabulæ_, while the +septa are rudimentary, being represented by faintly projecting ridges +or rows of spines. + +_Chain corals_ are also marked by tabulæ. Their cells form elliptical +tubes, touching each other at the edges, and appearing in cross +section like the links of a chain. They became extinct at the end of +the Silurian. + +The corals of the _Syringopora_ family are similar in structure to +chain corals, but the tubular columns are connected only in places. + + [Illustration: Fig. 293. A Blastoid: A, side view, showing + portion of the stem; B, summit of calyx (species + Carboniferous)] + + [Illustration: Fig. 294. A Silurian Scorpion] + +To the echinoderms there is now added the _blastoid_ (bud-shaped). The +blastoid is stemmed and armless, and its globular "head" or "calyx," +with its five petal-like divisions, resembles a flower bud. The +blastoids became more abundant in the Devonian, culminated in the +Carboniferous, and disappeared at the end of the Paleozoic. + +The great eurypterids--some of which were five or six feet in +length--and the cephalopods were still masters of the seas. Fishes +were as yet few and small; trilobites and graptolites had now passed +their prime and had diminished greatly in numbers. Scorpions are found +in this period both in Europe and in America. The limestone-making +seas of the Silurian swarmed with corals, crinoids, and brachiopods. + +With the end of the Silurian period the _Age of Invertebrates_ comes +to a close, giving place to the Devonian, the _Age of Fishes_. + + [Illustration: Fig. 295. Block of Limestone showing Interior Casts of + _Pentamerus oblongus_, a Common Silurian Brachiopod] + + + + +CHAPTER XVIII + +THE DEVONIAN + + +In America the Silurian is not separated from the Devonian by any +mountain-making deformation or continental uplift. The one period +passed quietly into the other. Their conformable systems are so +closely related, and the change in their faunas is so gradual, that +geologists are not agreed as to the precise horizon which divides +them. + +=Subdivisions and physical geography.= The Devonian is represented in +New York and southward by the following five series. We add the rocks +of which they are chiefly composed. + + 5 Chemung ..... sandstones and sandy shales + 4 Hamilton ..... shales and sandstones + 3 Corniferous ..... limestones + 2 Oriskany ..... sandstones + 1 Helderberg ..... limestones + +The Helderberg is a transition epoch referred by some geologists to +the Silurian. The thin sandstones of the Oriskany mark an epoch when +waves worked over the deposits of former coastal plains. The +limestones of the Corniferous testify to a warm and clear wide sea +which extended from the Hudson to beyond the Mississippi. Corals +throve luxuriantly, and their remains, with those of mollusks and +other lime-secreting animals, built up great beds of limestone. The +bordering continents, as during the later Silurian, must now have been +monotonous lowlands which sent down little of even the finest waste to +the sea. + +In the Hamilton the clear seas of the previous epoch became clouded +with mud. The immense deposits of coarse sandstones and sandy shales +of the Chemung, which are found off what was at the time the west +coast of Appalachia, prove an uplift of that ancient continent. + +The Chemung series extends from the Catskill Mountains to northeastern +Ohio and south to northeastern Tennessee, covering an area of not less +than a hundred thousand square miles. In eastern New York it attains +three thousand feet in thickness; in Pennsylvania it reaches the +enormous thickness of two miles; but it rapidly thins to the west. +Everywhere the Chemung is made of thin beds of rapidly alternating +coarse and fine sands and clays, with an occasional pebble layer, and +hence is a shallow-water deposit. The fine material has not been +thoroughly winnowed from the coarse by the long action of strong waves +and tides. The sands and clays have undergone little more sorting than +is done by rivers. We must regard the Chemung sandstones as deposits +made at the mouths of swift, turbid rivers in such great amount that +they could be little sorted and distributed by waves. + +Over considerable areas the Chemung sandstones bear little or no trace +of the action of the sea. The Catskill Mountains, for example, have as +their summit layers some three thousand feet of coarse red sandstones +of this series, whose structure is that of river deposits, and whose +few fossils are chiefly of fresh-water types. The Chemung is therefore +composed of delta deposits, more or less worked over by the sea. The +bulk of the Chemung equals that of the Sierra Nevada Mountains. To +furnish this immense volume of sediment a great mountain range, or +highland, must have been upheaved where the Appalachian lowland long +had been. To what height the Devonian mountains of Appalachia attained +cannot be told from the volume of the sediments wasted from them, for +they may have risen but little faster than they were worn down by +denudation. We may infer from the character of the waste which they +furnished to the Chemung shores that they did not reach an Alpine +height. The grains of the Chemung sandstones are not those which would +result from mechanical disintegration, as by frost on high mountain +peaks, but are rather those which would be left from the long chemical +decay of siliceous crystalline rocks; for the more soluble minerals +are largely wanting. The red color of much of the deposits points to +the same conclusion. Red residual clays accumulated on the mountain +sides and upland summits, and were washed as ocherous silt to mingle +with the delta sands. The iron-bearing igneous rocks of the oldland +also contributed by their decay iron in solution to the rivers, to be +deposited in films of iron oxide about the quartz grains of the +Chemung sandstones, giving them their reddish tints. + + +Life of the Devonian + +=Plants.= The lands were probably clad with verdure during Silurian +times, if not still earlier; for some rare remains of ferns and other +lowly types of vegetation have been found in the strata of that +system. But it is in the Devonian that we discover for the first time +the remains of extensive and luxuriant forests. This rich flora +reached its climax in the Carboniferous, and it will be more +convenient to describe its varied types in the next chapter. + +=Rhizocarps.= In the shales of the Devonian are found microscopic +spores of rhizocarps in such countless numbers that their weight must +be reckoned in hundreds of millions of tons. It would seem that these +aquatic plants culminated in this period, and in widely distant +portions of the earth swampy flats and shallow lagoons were filled +with vegetation of this humble type, either growing from the bottom or +floating free upon the surface. It is to the resinous spores of the +rhizocarps that the petroleum and natural gas from Devonian rocks are +largely due. The decomposition of the spores has made the shales +highly bituminous, and the oil and gas have accumulated in the +reservoirs of overlying porous sandstones. + +=Invertebrates.= We must pass over the ever-changing groups of the +invertebrates with the briefest notice. Chain corals became extinct at +the close of the Silurian, but other corals were extremely common in +the Devonian seas. At many places corals formed thin reefs, as at +Louisville, Kentucky, where the hardness of the reef rock is one of +the causes of the Falls of the Ohio. + +Sponges, echinoderms, brachiopods, and mollusks were abundant. The +cephalopods take a new departure. So far in all their various forms, +whether straight, as the Orthoceras, or curved, or close-coiled as in +the nautilus, the septum, or partition dividing the chambers, met the +inner shell along a simple line, like that of the rim of a saucer. +There now begins a growth of the septum by which its edges become +sharply corrugated, and the suture, or line of juncture of the septum +and the shell, is thus angled. The group in which this growth of the +septum takes place is called the _Goniatite_ (Greek _gonia_, angle). + + [Illustration: Fig. 296. A Goniatite] + +=Vertebrates.= It is with the greatest interest that we turn now to +study the backboned animals of the Devonian; for they are believed to +be the ancestors of the hosts of vertebrates which have since +dominated the earth. Their rudimentary structures foreshadowed what +their descendants were to be, and give some clue to the earliest +vertebrates from which they sprang. Like those whose remains are found +in the lower Paleozoic systems, all of these Devonian vertebrates were +aquatic and go under the general designation of fishes. + +The lowest in grade and nearest, perhaps, to the ancestral type of +vertebrates, was the problematic creature, an inch or so long, of +Figure 297. Note the circular mouth not supplied with jaws, the lack +of paired fins, and the symmetric tail fin, with the column of +cartilaginous, ringlike vertebræ running through it to the end. The +animal is probably to be placed with the jawless lampreys and hags,--a +group too low to be included among true fishes. + + [Illustration: Fig. 297. Palæospondylus] + +=Ostracoderms.= This archaic group, long since extinct, is also too +lowly to rank among the true fishes, for its members have neither jaws +nor paired fins. These small, fishlike forms were cased in front with +bony plates developed in the skin and covered in the rear with scales. +The vertebræ were not ossified, for no trace of them has been found. + + [Illustration: Fig. 298. An Ostracoderm] + +=Devonian fishes.= The _true fishes_ of the Devonian can best be +understood by reference to their descendants now living. Modern fishes +are divided into several groups: _sharks_ and their allies; +_dipnoans_; _ganoids_, such as the sturgeon and gar; and +_teleosts_,--most common fishes, such as the perch and cod. + + [Illustration: Fig. 299. A Paleozoic Shark] + +=Sharks.= Of all groups of living fishes the sharks are the oldest and +still retain most fully the embryonic characters of their Paleozoic +ancestors. Such characters are the cartilaginous skeleton, and the +separate gill slits with which the throat wall is pierced and which +are arranged in line like the gill openings of the lamprey. The sharks +of the Silurian and Devonian are known to us chiefly by their teeth +and fin spines, for they were unprotected by scales or plates, and +were devoid of a bony skeleton. Figure 299 is a restoration of an +archaic shark from a somewhat higher horizon. Note the seven gill +slits and the lappetlike paired fins. These fins seem to be remnants +of the continuous fold of skin which, as embryology teaches, passed +from fore to aft down each side of the primitive vertebrate. + +Devonian sharks were comparatively small. They had not evolved into +the ferocious monsters which were later to be masters of the seas. + + [Illustration: Fig. 300. A Devonian Dipnoan] + +=Dipnoans, or lung fishes.= These are represented to-day by a few +peculiar fishes and are distinguished by some high structures which +ally them with amphibians. An air sac with cellular spaces is +connected with the gullet and serves as a rudimentary lung. It +corresponds with the swim bladder of most modern fishes, and appears +to have had a common origin with it. We may conceive that the +primordial fishes not only had gills used in breathing air dissolved +in water, but also developed a saclike pouch off the gullet. This sac +evolved along two distinct lines. On the line of the ancestry of most +modern fishes its duct was closed and it became the swim bladder used +in flotation and balancing. On another line of descent it was left +open, air was swallowed into it, and it developed into the rudimentary +lung of the dipnoans and into the more perfect lungs of the amphibians +and other air-breathing vertebrates. + +One of the ancient dipnoans is illustrated in Figure 300. Some of the +members of this order were, like the ostracoderms, cased in armor, but +their higher rank is shown by their powerful jaws and by other +structures. Some of these armored fishes reached twenty-five feet in +length and six feet across the head. They were the tyrants of the +Devonian seas. + + [Illustration: Fig. 301. A Devonian Fringe-Finned Ganoid] + +=Ganoids.= These take their name from their enameled plates or scales +of bone. The few genera now surviving are the descendants of the +tribes which swarmed in the Devonian seas. A restoration of one of a +leading order, the _fringe-finned_ ganoids, is given in Figure 301. +The side fins, which correspond to the limbs of the higher +vertebrates, are quite unlike those of most modern fishes. Their rays, +instead of radiating from a common base, fringe a central lobe which +contains a cartilaginous axis. The teeth of the Devonian ganoids show +a complicated folded structure. + +=General characteristics of Devonian fishes.= _The notochord is +persistent._ The notochord is a continuous rod of cartilage, or +gristle, which in the embryological growth of vertebrate animals +supports the spinal nerve cord before the formation of the vertebræ. +In most modern fishes and in all higher vertebrates the notochord is +gradually removed as the bodies of the vertebræ are formed about it; +but in the Devonian fishes it persists through maturity and the +vertebræ remain incomplete. + +=The skeleton is cartilaginous.= This also is an embryological +characteristic. In the Devonian fishes the vertebræ, as well as the +other parts of the skeleton, have not ossified, or changed to bone, +but remain in their primitive cartilaginous condition. + + [Illustration: Fig. 302. Vertebræ of Sturgeon in side view _A_; + and vertical transverse section _B_, showing Notochord _ch_, and + Neural Canal _m_] + +=The tail fin is vertebrated.= The backbone runs through the fin and is +fringed above and below with its vertical rays. In some fishes with +vertebrated tail fins the fin is symmetric (Fig. 300), and this seems +to be the primitive type. In others the tail fin is unsymmetric: the +backbone runs into the upper lobe, leaving the two lobes of unequal +size. In most modern fishes (the _teleosts_) the tail fin is not +vertebrated: the spinal column ends in a broad plate, to which the +diverging fin rays are attached. + +But along with these embryonic characters, which were common to all +Devonian fishes, there were other structures in certain groups which +foreshadowed the higher structures of the land vertebrates which were +yet to come: air sacs which were to develop into lungs, and +cartilaginous axes in the side fins which were a prophecy of limbs. +The vertebrates had already advanced far enough to prove the +superiority of their type of structure to all others. Their internal +skeleton afforded the best attachment for muscles and enabled them to +become the largest and most powerful creatures of the time. The +central nervous system, with the predominance given to the ganglia at +the fore end of the nerve cord,--the brain,--already endowed them with +greater energy than the invertebrates; and, still more important, +these structures contained the possibility of development into the +more highly organized land vertebrates which were to rule the earth. + +=Teleosts.= The great group of fishes called the teleosts, or those +with complete bony skeletons, to which most modern fishes belong, may +be mentioned here, although in the Devonian they had not yet appeared. +The teleosts are a highly specialized type, adapted most perfectly to +their aquatic environment. Heavy armor has been discarded, and +reliance is placed instead on swiftness. The skeleton is completely +ossified and the notochord removed. The vertebræ have been +economically withdrawn from the tail, and the cartilaginous axis of +the side fins has been found unnecessary. The air sac has become a +swim bladder. In this complete specialization they have long since +lost the possibility of evolving into higher types. + +It is interesting to note that the modern teleosts in their +embryological growth pass through the stages which characterized the +maturity of their Devonian ancestors; their skeleton is cartilaginous +and their tail fin vertebrated. + + + + +CHAPTER XIX + +THE CARBONIFEROUS + + +The Carboniferous system is so named from the large amount of coal +which it contains. Other systems, from the Devonian on, are coal +bearing also, but none so richly and to so wide an extent. Never +before or since have the peculiar conditions been so favorable for the +formation of extensive coal deposits. + +With few exceptions the Carboniferous strata rest on those of the +Devonian without any marked unconformity; the one period passed +quietly into the other, with no great physical disturbances. + +The Carboniferous includes three distinct series. The lower is called +the _Mississippian_, from the outcrop of its formations along the +Mississippi River in central and southern Illinois and the adjacent +portions of Iowa and Missouri. The middle series is called the +_Pennsylvanian_ (or Coal Measures), from its wide occurrence over +Pennsylvania. The upper series is named the _Permian_, from the +province of Perm in Russia. + +=The Mississippian series.= In the interior the Mississippian is +composed chiefly of limestones, with some shales, which tell of a +clear, warm, epicontinental sea swarming with crinoids, corals, and +shells, and occasionally clouded with silt from the land. + +In the eastern region, New York had been added by uplift to the +Appalachian land which now was united to the northern area. From +eastern Pennsylvania southward there were laid in a subsiding trough, +first, thick sandstones (the Pocono sandstone), and later still +heavier shales,--the two together reaching the thickness of four +thousand feet and more. We infer a renewed uplift of Appalachia +similar to that of the later epochs of the Devonian, but as much less +in amount as the volume of sediments is smaller. + + +The Pennsylvanian Series + +The Mississippian was brought to an end by a quiet oscillation which +lifted large areas slightly above the sea, and the Pennsylvanian began +with a movement in the opposite direction. The sea encroached on the +new land, and spread far and wide a great basal conglomerate and +coarse sandstones. On this ancient beach deposit a group of strata +rests which we must now interpret. They consist of alternating shales +and sandstones, with here and there a bed of limestone and an +occasional seam of coal. A stratum of fire clay commonly underlies a +coal seam, and there occur also beds of iron ore. We give a typical +section of a very small portion of the series at a locality in +Pennsylvania. Although some of the minor changes are omitted, the +section shows the rapid alternation of the strata: + + 9 Sandstone and shale ..... 25 + 8 Limestone ..... 18 + 7 Sandstone ..... 10 + 6 Coal ..... 1-6 + 5 Shale ..... 0-2 + 4 Sandstone ..... 40 + 3 Limestone ..... 10 + 2 Coal ..... 5-12 + 1 Fire clay ..... 3 + +This section shows more coal than is usual; on the whole, coal seams +do not take up more than one foot in fifty of the Coal Measures. They +vary also in thickness more than is seen in the section, some +exceptional seams reaching the thickness of fifty feet. + +=How coal was made.= 1. Coal is of vegetable origin. Examined under +the microscope even anthracite, or hard coal, is seen to contain +carbonized vegetal tissues. There are also all gradations connecting +the hardest anthracite--through semibituminous coal, bituminous or +soft coal, lignite (an imperfect coal in which sometimes woody fibers +may be seen little changed)--with peat and decaying vegetable tissues. +Coal is compressed and mineralized vegetal matter. Its varieties +depend on the perfection to which the peculiar change called +bituminization has been carried, and also, as shown in the table +below, on the degree to which the volatile substances and water have +escaped, and on the per cent of carbon remaining. + + Peat Bituminous + Dismal Lignite Coal Anthracite + Swamp Texas Penn. Penn. + + Moisture 78.89 14.67 1.30 2.74 + Volatile matter 13.84 37.32 20.87 4.25 + Fixed carbon 6.49 41.07 67.20 81.51 + Ash 0.78 6.69 8.80 10.87 + +2. The vegetable remains associated with coal are those of land +plants. + +3. Coal accumulated in the presence of water; for it is only when thus +protected from the air that vegetal matter is preserved. + +4. The vegetation of coal accumulated for the most part where it grew; +it was not generally drifted and deposited by waves and currents. +Commonly the fire clay beneath the seam is penetrated with roots, and +the shale above is packed with leaves of ferns and other plants as +beautifully pressed as in a herbarium. There often is associated with +the seam a fossil forest, with the stumps, which are still standing +where they grew, their spreading roots, and the soil beneath, all +changed to stone. In the Nova Scotia field, out of seventy-six +distinct coal seams, twenty are underlain by old forest grounds. + +The presence of fire clay beneath a seam points in the same direction. +Such underclays withstand intense heat and are used in making fire +brick, because their alkalies have been removed by the long-continued +growth of vegetation. + +Fuel coal is also too pure to have been accumulated by driftage. In +that case we should expect to find it mixed with mud, while in fact it +often contains no more ash than the vegetal matter would furnish from +which it has been compressed. + + [Illustration: Fig. 303. Fossil Tree Stumps of a Carboniferous + Forest, Scotland] + +These conditions are fairly met in the great swamps of river plains +and deltas and of coastal plains, such as the great Dismal Swamp, +where thousands of generations of forests with their undergrowths +contribute their stems and leaves to form thick beds of peat. A coal +seam is a fossil peat bed. + +=Geographical conditions during the Pennsylvanian.= The Carboniferous +peat swamps were of vast extent. A map of the Coal Measures (Fig. 260) +shows that the coal marshes stretched, with various interruptions of +higher ground and straits of open water, from eastern Pennsylvania +into Alabama, Texas, and Kansas. Some individual coal beds may still +be traced over a thousand square miles, despite the erosion which they +have suffered. It taxes the imagination to conceive that the varied +region included within these limits was for hundreds of thousands of +years a marshy plain covered with tropical jungles such as that +pictured in Figure 304. + +On the basis that peat loses four fifths of its bulk in changing to +coal, we may reckon the thickness of these ancient peat beds. Coal +seams six and ten feet thick, which are not uncommon, represent peat +beds thirty and fifty feet in thickness, while mammoth coal seams +fifty feet thick have been compressed from peat beds two hundred and +fifty feet deep. + +At the same time, the thousands of feet of marine and fresh-water +sediments, with their repeated alternations of limestones, sandstones, +and shales, in which the seams of coal occur, prove a slow subsidence, +with many changes in its rate, with halts when the land was at a +stillstand, and with occasional movements upward. + +When subsidence was most rapid and long continued the sea encroached +far and wide upon the lowlands and covered the coal swamps with sands +and muds and limy oozes. When subsidence slackened or ceased the land +gained on the sea. Bays were barred, and lagoons as they gradually +filled with mud became marshes. River deltas pushed forward, burying +with their silts the sunken peat beds of earlier centuries, and at the +surface emerged in broad, swampy flats,--like those of the deltas of +the Mississippi and the Ganges,--which soon were covered with +luxuriant forests. At times a gentle uplift brought to sea level great +coastal plains, which for ages remained mantled with the jungle, their +undeveloped drainage clogged with its debris, and were then again +submerged. + + [Illustration: Fig. 304. Ideal Landscape of the Pennsylvanian + Epoch] + +=Physical geography of the several regions.= _The Acadian region_ lay +on the eastern side of the northern land, where now are New Brunswick +and Nova Scotia, and was an immense river delta. Here river deposits +rich in coal accumulated to a depth of sixteen thousand feet. The area +of this coal field is estimated at about thirty-six thousand square +miles. + +_The Appalachian region_ skirts the Appalachian oldland on the west +from the southern boundary of New York to northern Alabama, extending +west into eastern Ohio. The Cincinnati anticline was now a peninsula, +and the broad gulf which had lain between it and Appalachia was +transformed at the beginning of the Pennsylvanian into wide marshy +plains, now sinking beneath the sea and now emerging from it. This +area subsided during the Carboniferous period to a depth of nearly ten +thousand feet. + +_The Central region_ lay west of the peninsula of the Cincinnati +anticline, and extended from Indiana west into eastern Nebraska, and +from central Iowa and Illinois southward about the ancient island +in Missouri and Arkansas into Oklahoma and Texas. On the north +the subsidence in this area was comparatively slight, for the +Carboniferous strata scarcely exceed two thousand feet in thickness. +But in Arkansas and Indian Territory the downward movement amounted to +four and five miles, as is proved by shoal water deposits of that +immense thickness. + +The coal fields of Indiana, and Illinois are now separated by erosion +from those lying west of the Mississippi River. At the south the +Appalachian land seems still to have stretched away to the west across +Louisiana and Mississippi into Texas, and this westward extension +formed the southern boundary of the coal marshes of the continent. + +The three regions just mentioned include the chief Carboniferous coal +fields of North America. Including a field in central Michigan +evidently formed in an inclosed basin (Fig. 260), and one in Rhode +Island, the total area of American coal fields has been reckoned at +not less than two hundred thousand square miles. We can hardly +estimate the value of these great stores of fossil fuel to an +industrial civilization. The forests of the coal swamps accumulated in +their woody tissues the energy which they received from the sun in +light and heat, and it is this solar energy long stored in coal seams +which now forms the world's chief source of power in manufacturing. + +=The western area.= On the Great Plains beyond the Missouri River the +Carboniferous strata pass under those of more recent systems. Where +they reappear, as about dissected mountain axes or on stripped +plateaus, they consist wholly of marine deposits and are devoid of +coal. The rich coal fields of the West are of later date. + +On the whole the Carboniferous seems to have been a time of subsidence +in the West. Throughout the period a sea covered the Great Basin and +the plateaus of the Colorado River. At the time of the greatest +depression the sites of the central chains of the Rockies were +probably islands, but early in the period they may have been connected +with the broad lands to the south and east. Thousands of feet of +Carboniferous sediments were deposited where the Sierra Nevada +Mountains now stand. + +=The Permian.= As the Carboniferous period drew toward its close the +sea gradually withdrew from the eastern part of the continent. Where +the sea lingered in the deepest troughs, and where inclosed basins +were cut off from it, the strata of the Permian were deposited. Such +are found in New Brunswick, in Pennsylvania and West Virginia, in +Texas, and in Kansas. In southwestern Kansas extensive Permian beds of +rock salt and gypsum show that here lay great salt lakes in which +these minerals were precipitated as their brines grew dense and dried +away. + +In the southern hemisphere the Permian deposits are so extraordinary +that they deserve a brief notice, although we have so far omitted +mention of the great events which characterized the evolution of other +continents than our own. The Permian fauna-flora of Australia, India, +South Africa, and the southern part of South America are so similar +that the inference is a reasonable one that these widely separated +regions were then connected together, probably as extensions of a +great antarctic continent. + +Interbedded with the Permian strata of the first three countries named +are extensive and thick deposits of a peculiar nature which are +clearly ancient ground moraines. Clays and sand, now hardened to firm +rock, are inset with unsorted stones of all sizes, which often are +faceted and scratched. Moreover, these bowlder clays rest on rock +pavements which are polished and scored with glacial markings. Hence +toward the close of the Paleozoic the southern lands of the eastern +hemisphere were invaded by great glaciers or perhaps by ice sheets +like that which now shrouds Greenland. + +These Permian ground moraines are not the first traces of the work of +glaciers met with in the geological record. Similar deposits prove +glaciation in Norway succeeding the pre-Cambrian stage of elevation, +and Cambrian glacial drift has recently been found in China. + +=The Appalachian deformation.= We have seen that during Paleozoic +times a long, narrow trough of the sea lay off the western coast of +the ancient land of Appalachia, where now are the Appalachian +Mountains. During the long ages of this era the trough gradually +subsided, although with many stillstands and with occasional slight +oscillations upward. Meanwhile the land lying to the east was +gradually uplifted at varying rates and with long pauses. The waste of +the rising land was constantly transferred to the sinking marginal sea +bottom, and on the whole the trough was filled with sediments as +rapidly as it subsided. The sea was thus kept shallow, and at times, +especially toward the close of the era, much of the area was upbuilt +or raised to low, marshy, coastal plains. When the Carboniferous was +ended the waste which had been removed from the land and laid along +its margin in the successive formations of the Paleozoic had reached a +thickness of between thirty and forty thousand feet. + +Both by sedimentation and by subsidence the trough had now become a +belt of weakness in the crust of the earth. Here the crust was now +made of layers to the depth of six or seven miles. In comparison with +the massive crystalline rocks of Appalachia on the east, the layered +rock of the trough was weak to resist lateral pressure, as a ream of +sheets of paper is weak when compared with a solid board of the same +thickness. It was weaker also than the region to the west, since there +the sediments were much thinner. Besides, by the long-continued +depression the strata of the trough had been bent from the flat-lying +attitude in which they were laid to one in which they were less able +to resist a horizontal thrust. + +There now occurred one of the critical stages in the history of the +planet, when the crust crumples under its own weight and shrinks down +upon a nucleus which is diminishing in volume and no longer able to +support it. Under slow but resistless pressure the strata of the +Appalachian trough were thrust against the rigid land, and slowly, +steadily bent into long folds whose axes ran northeast-southwest +parallel to the ancient coast line. It was on the eastern side next +the buttress of the land that the deformation was the greatest, and +the folds most steep and close. In central Pennsylvania and West +Virginia the folds were for the most part open. South of these states +the folds were more closely appressed, the strata were much broken, +and the great thrust faults were formed which have been described +already. In eastern Pennsylvania seams of bituminous coal were altered +to anthracite, while outside the region of strong deformation, as in +western Pennsylvania, they remained unchanged. An important factor in +the deformation was the massive limestones of the Cambrian-Ordovician. +Because of these thick, resistant beds the rocks were bent into wide +folds and sheared in places with great thrust faults. Had the strata +been weak shales, an equal pressure would have crushed and mashed +them. + +Although the great earth folds were slowly raised, and no doubt eroded +in their rising, they formed in all probability a range of lofty +mountains, with a width of from fifty to a hundred and twenty-five +miles, which stretched from New York to central Alabama. + +From their bases lowlands extended westward to beyond the Missouri +River. At the same time ranges were upridged out of thick Paleozoic +sediments both in the Bay of Fundy region and in the Indian Territory. +The eastern portion of the North American continent was now well-nigh +complete. + +The date of the Appalachian deformation is told in the usual way. The +Carboniferous strata, nearly two miles thick, are all infolded in the +Appalachian ridges, while the next deposits found in this +region--those of the later portion of the first period (the Trias) of +the succeeding era--rest unconformably on the worn edges of the +Appalachian folded strata. The deformation therefore took place about +the close of the Paleozoic. It seems to have begun in the Permian, in, +eastern Pennsylvania,--for here the Permian strata are wanting,--and +to have continued into the Trias, whose earlier formations are absent +over all the area. + +With this wide uplift the subsidence of the sea floor which had so +long been general in eastern North America came to an end. Deposition +now gave place to erosion. The sedimentary record of the Paleozoic was +closed, and after an unknown lapse of time, here unrecorded, the +annals of the succeeding era were written under changed conditions. + +In western North America the closing stages of the Paleozoic were +marked by important oscillations. The Great Basin, which had long been +a mediterranean sea, was converted into land over western Utah and +eastern Nevada, while the waves of the Pacific rolled across +California and western Nevada. + +The absence of tuffs and lavas among the Carboniferous strata of North +America shows that here volcanic action was singularly wanting during +the entire period. Even the Appalachian deformation was not +accompanied by any volcanic outbursts. + + [Illustration: Fig. 305. Carboniferous Ferns] + + [Illustration: Fig. 306. Calamites] + + +Life of the Carboniferous + +=Plants.= The gloomy forests and dense undergrowths of the +Carboniferous jungles would appear unfamiliar to us could we see them +as they grew, and even a botanist would find many of their forms +perplexing and hard to classify. None of our modern trees would meet +the eye. Plants with conspicuous flowers of fragrance and beauty were +yet to come. Even mosses and grasses were still absent. + +Tree ferns lifted their crowns of feathery fronds high in air on +trunks of woody tissue; and lowly herbaceous ferns, some belonging to +existing families, carpeted the ground. Many of the fernlike forms, +however, have distinct affinities with the cycads, of which they may +be the ancestors, and some bear seeds and must be classed as +gymnosperms. + +Dense thickets, like cane or bamboo brakes, were composed of thick +clumps of _Calamites_, whose slender, jointed stems shot up to a +height of forty feet, and at the joints bore slender branches set +with whorls of leaves. These were close allies of the Equiseta or +"horsetails," of the present; but they bore characteristics of higher +classes in the woody structures of their stems. + +There were also vast monotonous forests, composed chiefly of trees +belonging to the lycopods, and whose nearest relatives to-day are the +little club mosses of our eastern woods. Two families of lycopods +deserve special mention,--the Lepidodendrons and the Sigillaria. + + [Illustration: Fig. 307. Lepidodendron] + + [Illustration: Fig. 308. Sigillaria] + +The _Lepidodendron_, or "scale tree," was a gigantic club moss fifty +and seventy-five feet high, spreading toward the top into stout +branches, at whose ends were borne cone-shaped spore cases. The +younger parts of the tree were clothed with stiff needle-shaped +leaves, but elsewhere the trunk and branches were marked with +scalelike scars, left by the fallen leaves, and arranged in spiral +rows. + +The _Sigillaria_, or "seal tree," was similar to the Lepidodendron, +but its fluted trunk divided into even fewer branches, and was dotted +with vertical rows of leaf scars, like the impressions of a seal. + +Both Lepidodendron and Sigillaria were anchored by means of great +cablelike underground stems, which ran to long distances through the +marshy ground. The trunks of both trees had a thick woody rind, +inclosing loose cellular tissue and a pith. Their hollow stumps, +filled with sand and mud, are common in the Coal Measures, and in them +one sometimes finds leaves and stems, land shells, and the bones of +little reptiles of the time which made their home there. + +It is important to note that some of these gigantic lycopods, which +are classed with the _cryptogams_, or flowerless plants, had pith and +medullary rays dividing their cylinders into woody wedges. These +characters connect them with the _phanerogams_, or flowering plants. +Like so many of the organisms of the remote past, they were connecting +types from which groups now widely separated have diverged. + +Gymnosperms, akin to the cycads, were also present in the +Carboniferous forests. Such were the different species of _Cordaites_, +trees pyramidal in shape, with strap-shaped leaves and nutlike fruit. +Other gymnosperms were related to the yews, and it was by these that +many of the fossil nuts found in the Coal Measures were borne. It is +thought by some that the gymnosperms had their station on the drier +plains and higher lands. + +The Carboniferous jungles extended over parts of Europe and of Asia, +as well as eastern North America, and reached from the equator to +within nine degrees of the north pole. Even in these widely separated +regions the genera and species of coal plants are close akin and often +identical. + +=Invertebrates.= Among the echinoderms, crinoids are now exceedingly +abundant, sea urchins are more plentiful, and sea cucumbers are found +now for the first time. Trilobites are rapidly declining, and pass +away forever with the close of the period. Eurypterids are common; +stinging scorpions are abundant; and here occur the first-known +spiders. + +We have seen that the arthropods were the first of all animals to +conquer the realm of the air, the earliest insects appearing in the +Ordovician. Insects had now become exceedingly abundant, and the +Carboniferous forests swarmed with the ancestral types of dragon +flies,--some with a spread of wing of more than two feet,--May flies, +crickets, and locusts. Cockroaches infested the swamps, and one +hundred and thirty-three species of this ancient order have been +discovered in the Carboniferous of North America. The higher +flower-loving insects are still absent; the reign of the flowering +plants has not yet begun. The Paleozoic insects were generalized types +connecting the present orders. Their fore wings were still membranous +and delicately veined, and used in flying; they had not yet become +thick, and useful only as wing covers, as in many of their +descendants. + + [Illustration: Fig. 309. Carboniferous Brachiopods + + _A_, Productus; _B_, Spirifer, the right-hand figure showing the + interior with the calcareous spires for the support of the arms] + +=Fishes= still held to the Devonian types, with the exception that the +strange ostracoderms now had perished. + +=Amphibians.= The vertebrates had now followed the arthropods and the +mollusks upon the land, and developed a higher type adapted to the new +environment. Amphibians--the class to which frogs and salamanders +belong--now appear, with lungs for breathing air and with limbs for +locomotion on the land. Most of the Carboniferous amphibians were shaped +like the salamander, with weak limbs adapted more for crawling than for +carrying the body well above the ground. some legless, degenerate forms +were snakelike in shape. + + [Illustration: Fig. 310. A Carboniferous Dragon Fly + + One tenth natural size] + +The earliest amphibians differ from those of to-day in a number of +respects. They were connecting types linking together fishes, from +which they were descended, with reptiles, of which they were the +ancestors. They retained the evidence of their close relationship with +the Devonian fishes in their cold blood, their gills and aquatic habit +during their larval stage, their teeth with dentine infolded like +those of the Devonian ganoids but still more intricately, and their +biconcave vertebræ which never completely ossified. These, the +highest vertebrates of the time, had not yet advanced beyond the +embryonic stage of the more or less cartilaginous skeleton and the +persistent notochord. + + [Illustration: Fig. 311. A Carboniferous Amphibian] + + [Illustration: Fig. 312. Transverse Section of + Segment of Tooth of Carboniferous Amphibian] + +On the other hand, the skull of the Carboniferous amphibians was made +of close-set bony plates, like the skull of the reptile, rather than +like that of the frog, with its open spaces (Figs. 313 and 314). +Unlike modern amphibians, with their slimy skin, the Carboniferous +amphibians wore an armor of bony scales over the ventral surface and +sometimes over the back as well. + + [Illustration: Fig. 313. Skull of a Permian Amphibian from Texas] + + [Illustration: Fig. 314. Skull of a Frog] + +It is interesting to notice from the footprints and skeletons of these +earliest-known vertebrates of the land what was the primitive number +of digits. The Carboniferous amphibians had five-toed feet, the +primitive type of foot, from which their descendants of higher orders, +with a smaller number of digits, have diverged. + +The Carboniferous was the age of lycopods and amphibians, as the +Devonian had been the age of rhizocarps and fishes. + +=Life of the Permian.= The close of the Paleozoic was, as we have +seen, a time of marked physical changes. The upridging of the +Appalachians had begun and a wide continental uplift--proved by the +absence of Permian deposits over large areas where sedimentation had +gone on before--opened new lands for settlement to hordes of +air-breathing animals. Changes of climate compelled extensive +migrations, and the fauna of different regions were thus brought into +conflict. The Permian was a time of pronounced changes in plant and +animal life, and a transitional period between two great eras. The +somber forests of the earlier Carboniferous, with their gigantic club +mosses, were now replaced by forests of cycads, tree ferns, and +conifers. Even in the lower Permian the Lepidodendron and Sigillaria +were very rare, and before the end of the epoch they and the Calamites +also had become extinct. Gradually the antique types of the Paleozoic +fauna died out, and in the Permian rocks are found the last survivors +of the cystoid, the trilobite, and the eurypterid, and of many +long-lived families of brachiopods, mollusks, and other invertebrates. +The venerable Orthoceras and the Goniatite linger on through the epoch +and into the first period of the succeeding era. Forerunners of the +great ammonite family of cephalopod mollusks now appear. The antique +forms of the earlier Carboniferous amphibians continue, but with many +new genera and a marked increase in size. + +A long forward step had now been taken in the evolution of the +vertebrates. A new and higher type, the reptiles, had appeared, and in +such numbers and variety are they found in the Permian strata that +their advent may well have occurred in a still earlier epoch. It will +be most convenient to describe the Permian reptiles along with their +descendants of the Mesozoic. + + + + +CHAPTER XX + +THE MESOZOIC + + +With the close of the Permian the world of animal and vegetable +life had so changed that the line is drawn here which marks the +end of the old order and the beginning of the new and separates +the Paleozoic from the succeeding era,--the Mesozoic, the Middle +Age of geological history. Although the Mesozoic era is shorter +than the Paleozoic, as measured by the thickness of their strata, +yet its duration must be reckoned in millions of years. Its +predominant life features are the culmination and the beginning of +the decline of reptiles, amphibians, cephalopod mollusks, and +cycads, and the advent of marsupial mammals, birds, teleost +fishes, and angiospermous plants. The leading events of the long +ages of the era we can sketch only in the most summary way. + +The Mesozoic comprises three systems,--the _Triassic_, named from +its threefold division in Germany; the _Jurassic_, which is well +displayed in the Jura Mountains; and the _Cretaceous_, which +contains the extensive chalk (Latin, _creta_) deposits of Europe. + +In eastern North America the Mesozoic rocks are much less +important than the Paleozoic, for much of this portion of the +continent was land during the Mesozoic era, and the area of the +Mesozoic rocks is small. In western North America, on the other +hand, the strata of the Mesozoic--and of the Cenozoic also--are +widely spread. The Paleozoic rocks are buried quite generally from +view except where the mountain makings and continental uplifts of +the Mesozoic and Cenozoic have allowed profound erosion to bring +them to light, as in deep canyons and about mountain axes. The +record of many of the most important events in the development of +the continent during the Mesozoic and Cenozoic eras is found in +the rocks of our western states. + + +The Triassic and Jurassic + +=Eastern North America.= The sedimentary record interrupted by the +Appalachian deformation was not renewed in eastern North America +until late in the Triassic. Hence during this long interval the +land stood high, the coast was farther out than now, and over our +Atlantic states geological time was recorded chiefly in erosion +forms of hill and plain which have long since vanished. The area +of the later Triassic rocks of this region, which take up again +the geological record, is seen in the map of Figure 260. They lie +on the upturned and eroded edges of the older rocks and occupy +long troughs running for the most part parallel to the Atlantic +coast. Evidently subsidence was in progress where these rocks were +deposited. The eastern border of Appalachia was now depressed. The +oldland was warping, and long belts of country lying parallel to +the shore subsided, forming troughs in which thousands of feet of +sediment now gathered. + +These Triassic rocks, which are chiefly sandstones, hold no marine +fossils, and hence were not laid in open arms of the sea. But +their layers are often ripple-marked, and contain many tracks of +reptiles, imprints of raindrops, and some fossil wood, while an +occasional bed of shale is filled with the remains of fishes. We +may conceive, then, of the Connecticut valley and the larger +trough to the southwest as basins gradually sinking at a rate +perhaps no faster than that of the New Jersey coast to-day, and as +gradually aggraded by streams from the neighboring uplands. Their +broad, sandy flats were overflowed by wandering streams, and when +subsidence gained on deposition shallow lakes overspread the +alluvial plains. Perhaps now and then the basins became long, +brackish estuaries, whose low shores were swept by the incoming +tide and were in turn left bare at its retreat to receive the rain +prints of passing showers and the tracks of the troops of reptiles +which inhabited these valleys. + +The Triassic rocks are mainly red sandstones,--often feldspathic, +or arkose, with some conglomerates and shales. Considering the +large amount of feldspathic material in these rocks, do you infer +that they were derived from the adjacent crystalline and +metamorphic rocks of the oldland of Appalachia, or from the +sedimentary Paleozoic rocks which had been folded into mountains +during the Appalachian deformation? If from the former, was the +drainage of the northern Appalachian mountain region then, as now, +eastward and southeastward toward the Atlantic? The Triassic +sandstones are voluminous, measuring at least a mile in thickness, +and are largely of coarse waste. What do you infer as to the +height of the lands from which the waste was shed, or the +direction of the oscillation which they were then undergoing? In +the southern basins, as about Richmond, Virginia, are valuable +beds of coal; what was the physical geography of these areas when +the coal was being formed? + + [Illustration: Fig. 315. Section of Triassic Sandstones of the + Connecticut Valley + + _ss_, sandstones; _ll_, lava sheets; _cc_, crystalline igneous + and metamorphic rocks] + +Interbedded with the Triassic sandstones are contemporaneous lava +beds which were fed from dikes. Volcanic action, which had been +remarkably absent in eastern North America during Paleozoic times, +was well-marked in connection with the warping now in progress. +Thick intrusive sheets have also been driven in among the strata, +as, for example, the sheet of the Palisades of the Hudson, +described on page 269. + +The present condition of the Triassic sandstones of the +Connecticut valley is seen in Figure 315. Were the beds laid in +their present attitude? What was the nature of the deformation +which they have suffered? When did the intrusion of lava sheets +take place relative to the deformation? What effect have these +sheets on the present topography, and why? Assuming that the +Triassic deformation went on more rapidly than denudation, what +was its effect on the topography of the time? Are there any of its +results remaining in the topography of to-day? Do the Triassic +areas now stand higher or lower than the surrounding country, and +why? How do the Triassic sandstones and shales compare in hardness +with the igneous and metamorphic rocks about them? The Jurassic +strata are wanting over the Triassic areas and over all of eastern +North America. Was this region land or sea, an area of erosion or +sedimentation, during the Jurassic period? In New Jersey, +Pennsylvania, and farther southwest the lowest strata of the next +period, the Cretaceous, rest on the eroded edges of the earlier +rocks. The surface on which they lie is worn so even that we must +believe that at the opening of the Cretaceous the oldland of +Appalachia, including the Triassic areas, had been baseleveled at +least near the coast. When, therefore, did the deformation of the +Triassic rocks occur? + +=Western North America.= Triassic strata infolded in the Sierra +Nevada Mountains carry marine fossils and reach a thickness of +nearly five thousand feet. California was then under water, and +the site of the Sierra was a subsiding trough slowly filling with +waste from the Great Basin land to the east. + +Over a long belt which reaches from Wyoming across Colorado into +New Mexico no Triassic sediments are found, nor is there any +evidence that they were ever present; hence this area was high +land suffering erosion during the Triassic. On each side of it, in +eastern Colorado and about the Black Hills, in western Texas, in +Utah, over the site of the Wasatch Mountains, and southward into +Arizona over the plateaus trenched by the Colorado River, are +large areas of Triassic rocks, sandstones chiefly, with some rock +salt and gypsum. Fossils are very rare and none of them marine. +Here, then, lay broad shallow lakes often salt, and warped basins, +in which the waste of the adjacent uplands gathered. To this +system belong the sandstones of the Garden of the Gods in +Colorado, which later earth movements have upturned with the +uplifted mountain flanks. + +The Jurassic was marked with varied oscillations and wide changes +in the outline of sea and land. + +Jurassic shales of immense thickness--now metamorphosed into +slates--are found infolded into the Sierra Nevada Mountains. Hence +during Jurassic times the Sierra trough continued to subside, and +enormous deposits of mud were washed into it from the land lying +to the east. Contemporaneous lava flows interbedded with the +strata show that volcanic action accompanied the downwarp, and +that molten rock was driven upward through fissures in the crust +and outspread over the sea floor in sheets of lava. + +=The Sierra deformation.= Ever since the middle of the Silurian, the +Sierra trough had been sinking, though no doubt with halts and +interruptions, until it contained nearly twenty-five thousand feet +of sediment. At the close of the Jurassic it yielded to lateral +pressure and the vast pile of strata was crumpled and upheaved +into towering mountains. The Mesozoic muds were hardened and +squeezed into slates. The rocks were wrenched and broken, and +underground waters began the work of filling their fissures with +gold-bearing quartz, which was yet to wait millions of years +before the arrival of man to mine it. Immense bodies of molten +rock were intruded into the crust as it suffered deformation, and +these appear in the large areas of granite which the later +denudation of the range has brought to light. + +The same movements probably uplifted the rocks of the Coast Range +in a chain of islands. The whole western part of the continent was +raised and its seas and lakes were for the most part drained away. + +=The British Isles.= The Triassic strata of the British Isles are +continental, and include breccia beds of cemented talus, deposits +of salt and gypsum, and sandstones whose rounded and polished +grains are those of the wind-blown sands of deserts. In Triassic +times the British Isles were part of a desert extending over much +of northwestern Europe. + + +The Cretaceous + +The third great system of the Mesozoic includes many formations, +marine and continental, which record a long and complicated +history marked by great oscillations of the crust and wide changes +in the outlines of sea and land. + +=Early Cretaceous.= In eastern North America the lowest Cretaceous +series comprises fresh-water formations which are traced from +Nantucket across Martha's Vineyard and Long Island, and through +New Jersey southward into Georgia. They rest unconformably on the +Triassic sandstones and the older rocks of the region. The +Atlantic shore line was still farther out than now in the northern +states. Again, as during the Triassic, a warping of the crust +formed a long trough parallel to the coast and to the Appalachian +ridges, but cut off from the sea; and here the continental +deposits of the early Cretaceous were laid. + +Along the Gulf of Mexico the same series was deposited under like +conditions over the area known as the Mississippi embayment, +reaching from Georgia northwestward into Tennessee and thence +across into Arkansas and southward into Texas. + +In the Southwest the subsidence continued until the transgressing +sea covered most of Mexico and Texas and extended a gulf northward +into Kansas. In its warm and quiet waters limestones accumulated +to a depth of from one thousand to five thousand feet in Texas, +and of more than ten thousand feet in Mexico. Meanwhile the +lowlands, where the Great Plains are now, received continental +deposits; coal swamps stretched from western Montana into British +Columbia. + +=The Middle Cretaceous.= This was a land epoch. The early Cretaceous +sea retired from Texas and Mexico, for its sediments are overlain +unconformably by formations of the Upper Cretaceous. So long was the +time gap between the two series that no species found in the one +occurs in the other. + +=The Upper Cretaceous.= There now began one of the most remarkable +events in all geological history,--the great Cretaceous subsidence. +Its earlier warpings were recorded in continental deposits,--wide +sheets of sandstone, shale, and some coal,--which were spread from +Texas to British Columbia. These continental deposits are overlain by +a succession of marine formations whose vast area is shown on the map, +Figure 260. We may infer that as the depression of the continent +continued the sea came in far and wide over the coast lands and the +plains worn low during the previous epochs. Upper Cretaceous +formations show that south of New England the waters of the Atlantic +somewhat overlapped the crystalline rocks of the Piedmont Belt and +spread their waste over the submerged coastal plain. The Gulf of +Mexico again covered the Mississippi embayment, reaching as far north +as southern Illinois, and extended over Texas. A mediterranean sea now +stretched from the Gulf to the arctic regions and from central Iowa to +the eastern shore of the Great Basin land at about the longitude of +Salt Lake City, the Colorado Mountains rising from it in a chain of +islands. Along with minor oscillations there were laid in the interior +sea various formations of sandstones, shales, and limestones, and from +Kansas to South Dakota beds of white chalk show that the clear, warm +waters swarmed at times with foraminiferal life whose disintegrating +microscopic shells accumulated in this rare deposit. + + [Illustration: Fig. 316. Hypothetical Map of Upper Cretaceous + Epicontinental Seas + + Shaded areas, probable seas; broken lines, approximate shore + lines] + + [Illustration: Fig. 317. Foraminifera from Cretaceous Chalk, + Iowa] + +At this epoch a wide sea, interrupted by various islands, stretched +across Eurasia from Wales and western Spain to China, and spread +southward over much of the Sahara. To the west its waters were clear +and on its floor the crumbled remains of foraminifers gathered in +heavy accumulations of calcareous ooze,--the white chalk of France and +England. Sea urchins were also abundant, and sponges contributed their +spicules to form nodules of flint. + +=The Laramie.= The closing stage of the Cretaceous was marked in +North America by a slow uplift of the land. As the interior sea +gradually withdrew, the warping basins of its floor were filled with +waste from the rising lands about them, and over this wide area +there were spread continental deposits in fresh-water lakes like the +Great Lakes of the present, in brackish estuaries, and in river +plains, while occasional oscillations now and again let in the sea. +There were vast marshes in which there accumulated the larger part +of the valuable coal seams of the West. The Laramie is the +coal-bearing series of the West, as the Pennsylvanian is of the +eastern part of our country. + +=The Rocky Mountain deformation.= At the close of the Cretaceous we +enter upon an epoch of mountain-making far more extensive than any which +the continent had witnessed. The long belt lying west of the ancient +axes of the Colorado Islands and east of the Great Basin land had been +an area of deposition for many ages, and in its subsiding troughs +Paleozoic and Mesozoic sediments had gathered to the depth of many +thousand feet. And now from Mexico well-nigh to the Arctic Ocean this +belt yielded to lateral pressure. The Cretaceous limestones of Mexico +were folded into lofty mountains. A massive range was upfolded where the +Wasatch Mountains now are, and various ranges of the Rockies in Colorado +and other states were upridged. However slowly these deformations were +effected they were no doubt accompanied by world-shaking earthquakes, +and it is known that volcanic eruptions took place on a magnificent +scale. Outflows of lava occurred along the Wasatch, the laccoliths of +the Henry Mountains (p. 271) were formed, while the great masses of +igneous rock which constitute the cores of the Spanish Peaks (p. 271) +and other western mountains were thrust up amid the strata. The high +plateaus from which many of these ranges rise had not yet been uplifted, +and the bases of the mountains probably stood near the level of the sea. + +North America was now well-nigh completed. The mediterranean seas +which so often had occupied the heart of the land were done away +with, and the continent stretched unbroken from the foot of the +Sierras on the west to the Fall Line of the Atlantic coastal plain +on the east. + +=The Mesozoic peneplain.= The immense thickness of the Mesozoic +formations conveys to our minds some idea of the vast length of time +involved in the slow progress of its successive ages. The same +lesson is taught as plainly by the amount of denudation which the +lands suffered during the era. + +The beginning of the Mesozoic saw a system of lofty mountain ranges +stretching from New York into central Alabama. The end of this long +era found here a wide peneplain crossed by sluggish wandering rivers +and overlooked by detached hills as yet unreduced to the general +level. The Mesozoic era was long enough for the Appalachian +Mountains, upridged at its beginning, to have been weathered and +worn away and carried grain by grain to the sea. The same plain +extended over southern New England. The Taconic range, uplifted +partially at least at the close of the Ordovician, and the block +mountains of the Triassic, together with the pre-Cambrian mountains +of ancient Appalachia, had now all been worn to a common level with +the Allegheny ranges. The Mesozoic peneplain has been upwarped by +later crustal movements and has suffered profound erosion, but the +remnants of it which remain on the upland of southern New England +and the even summits of the Allegheny ridges suffice to prove that +it once existed. The age of the Mesozoic peneplain is determined +from the fact that the lower Tertiary sediments were deposited on +its even surface when at the close of the era the peneplain was +depressed along its edges beneath the sea. + + +Life of the Mesozoic + +=Plant life of the Triassic and Jurassic.= The Carboniferous forests +of lepidodendrons and sigillarids had now vanished from the earth. +The uplands were clothed with conifers, like the Araucarian pines +of South America and Australia. Dense forests of tree ferns throve +in moist regions, and canebrakes of horsetails of modern type, but +with stems reaching four inches in thickness, bordered the lagoons +and marshes. Cycads were exceedingly abundant. These gymnosperms, +related to the pines and spruces in structure and fruiting, but +palmlike in their foliage, and uncoiling their long leaves after +the manner of ferns, culminated in the Jurassic. From the view +point of the botanist the Mesozoic is the Age of Cycads, and after +this era they gradually decline to the small number of species now +existing in tropical latitudes. + + [Illustration: Fig. 318. A Living Cycad of Australia] + + [Illustration: Fig. 319. Stem of a Mesozoic Cycad] + +=Plant life of the Cretaceous.= In the Lower Cretaceous the woodlands +continued of much the same type as during the Jurassic. The +forerunners now appeared of the modern dicotyls (plants with two seed +leaves), and in the Middle Cretaceous the monocotyledonous group of +palms came in. Palms are so like cycads that we may regard them as the +descendants of some cycad type. + +In the _Upper Cretaceous_, cycads become rare. The highest types of +flowering plants gain a complete ascendency, and forests of modern +aspect cover the continent from the Gulf of Mexico to the Arctic +Ocean. Among the kinds of forest trees whose remains are found in +the continental deposits of the Cretaceous are the magnolia, the +myrtle, the laurel, the fig, the tulip tree, the chestnut, the +oak, beech, elm, poplar, willow, birch, and maple. Forests of +Eucalyptus grew along the coast of New England, and palms on the +Pacific shores of British Columbia. Sequoias of many varieties +ranged far into northern Canada. In northern Greenland there were +luxuriant forests of magnolias, figs, and cycads; and a similar +flora has been disinterred from the Cretaceous rocks of Alaska and +Spitzbergen. Evidently the lands within the Arctic Circle enjoyed +a warm and genial climate, as they had done during the Paleozoic. +Greenland had the temperature of Cuba and southern Florida, and +the time was yet far distant when it was to be wrapped in glacier +ice. + + [Illustration: Fig. 320. A Jurassic Long-Tailed Crustacean] + +=Invertebrates.= During the long succession of the ages of the +Mesozoic, with their vast geographical changes, there were many +and great changes in organisms. Species were replaced again and +again by others better fitted to the changing environment. During +the Lower Cretaceous alone there were no less than six successive +changes in the faunas which inhabited the limestone-making sea +which then covered Texas. We shall disregard these changes for the +most part in describing the life of the era, and shall confine our +view to some of the most important advances made in the leading +types. + +Stromatopora have disappeared. Protozoans and sponges are +exceedingly abundant, and all contribute to the making of Mesozoic +strata. Corals have assumed a more modern type. Sea urchins have +become plentiful; crinoids abound until the Cretaceous, where they +begin their decline to their present humble station. + + [Illustration: Fig. 321. A Fossil Crab] + + [Illustration: Fig. 322. Cretaceous Mollusks + + _A_, Ostrea (oyster); _B_, Exogyra; _C_. Gryphæa] + +Trilobites and eurypterids are gone. Ten-footed crustaceans abound of +the primitive long-tailed type (represented by the lobster and the +crayfish), and in the Jurassic there appears the modern short-tailed +type represented by the crabs. The latter type is higher in +organization and now far more common. In its embryological development +it passes through the long-tailed stage; connecting links in the +Mesozoic also indicate that the younger type is the offshoot of the +older. + +Insects evolve along diverse lines, giving rise to beetles, ants, +bees, and flies. + +Brachiopods have dwindled greatly in the number of their species, +while mollusks have correspondingly increased. The great oyster family +dates from here. + +Cephalopods are now to have their day. The archaic Orthoceras lingers +on into the Triassic and becomes extinct, but a remarkable development +is now at hand for the more highly organized descendants of this +ancient line. We have noticed that in the Devonian the sutures of some +of the chambered shells become angled, evolving the Goniatite type. +The sutures now become lobed and _corrugated_ in _Ceratites_. The process +was carried still farther, and the sutures were elaborately frilled in +the great order of the Ammonites. It was in the Jurassic that the +Ammonites reached their height. No fossils are more abundant or +characteristic of their age. Great banks of their shells formed beds +of limestone in warm seas the world over. + + [Illustration: Fig. 323. Ceratites] + + [Illustration: Fig. 324. An Ammonite + + A portion of the shell is removed to show frilling of suture] + + [Illustration: Fig. 325. Slab of Rock covered with + Ammonites,--a Bit of a Mesozoic Sea Bottom] + + [Illustration: Fig. 326. Representative Species of Different + Families of Ammonites] + +The ammonite stem branched into a most luxuriant variety of forms. +The typical form was closely coiled like a nautilus. In others the +coil was more or less open, or even erected into a spiral. Some +were hook-shaped, and there were members of the order in which the +shell was straight, and yet retained all the internal structures +of its kind. At the end of the Mesozoic the entire tribe of +ammonites became extinct. + +The Belemnite (Greek, _belemnon_, a dart) is a distinctly higher +type of cephalopod which appeared in the Triassic, became numerous +and varied in the Jurassic and Cretaceous, and died out early in +the Tertiary. Like the squids and cuttlefish, of which it was the +prototype, it had an internal calcareous shell. This consisted of +a chambered and siphuncled cone, whose point was sheathed in a +long solid guard somewhat like a dart. The animal carried an ink +sac, and no doubt used it as that of the modern cuttlefish is +used,--to darken the water and make easy an escape from foes. +Belemnites have sometimes been sketched with fossil sepia, or +india ink, from their own ink sacs. In the belemnites and their +descendants, the squids and cuttlefish, the cephalopods made the +radical change from external to the internal shell. They abandoned +the defensive system of warfare and boldly took up the offensive. +No doubt, like their descendants, the belemnites were exceedingly +active and voracious creatures. + + [Illustration: Fig. 327. Internal Shell of Belemnite] + +=Fishes and amphibians.= In the Triassic and Jurassic, little +progress was made among the fishes, and the ganoid was still the +leading type. In the Cretaceous the teleosts, or bony fishes, made +their appearance, while ganoids declined toward their present +subordinate place. + +The amphibians culminated in the Triassic, some being formidable +creatures as large as alligators. They were still of the primitive +Paleozoic types. Their pygmy descendants of more modern types are +not found until later, salamanders appearing first in the +Cretaceous, and frogs at the beginning of the Cenozoic. + +No remains of amphibians have been discovered in the Jurassic. Do +you infer from this that there were none in existence at that +time? + + +Reptiles of the Mesozoic + +The great order of Reptiles made its advent in the Permian, culminated +in the Triassic and Jurassic, and began to decline in the Cretaceous. +The advance from the amphibian to the reptile was a long forward step +in the evolution of the vertebrates. In the reptile the vertebrate +skeleton now became completely ossified. Gills were abandoned and +breathing was by lungs alone. The development of the individual from +the egg to maturity was uninterrupted by any metamorphosis, such as +that of the frog when it passes from the tadpole stage. Yet in +advancing from the amphibian to the reptile the evolution of the +vertebrate was far from finished. The cold-blooded, clumsy and +sluggish, small-brained and unintelligent reptile is as far inferior +to the higher mammals, whose day was still to come, as it is superior +to the amphibian and the fish. + +The reptiles of the Permian, the earliest known, were much like +lizards in form of body. Constituting a transition type between the +amphibians on the one hand, and both the higher reptiles and the +mammals on the other, they retained the archaic biconcave vertebra of +the fish and in some cases the persistent notochord, while some of +them, the theromorphs, possessed characters allying them with mammals. +In these the skull was remarkably similar to that of the carnivores, +or flesh-eating mammals, and the teeth, unlike the teeth of any later +reptiles, were divisible into incisors, canines, and molars, as are +the teeth of mammals (Fig. 328). + + [Illustration: Fig. 328. Skull of a Permian Theromorph] + +At the opening of the Mesozoic era reptiles were the most highly +organized and powerful of any animals on the earth. New ranges of +continental extent were opened to them, food was abundant, the climate +was congenial, and they now branched into very many diverse types +which occupied and ruled all fields,--the land, the air, and the sea. +The Mesozoic was the Age of Reptiles. + +=The ancestry of surviving reptilian types.= We will consider first +the evolution of the few reptilian types which have survived to the +present. + +Crocodiles, the highest of existing reptiles, are a very ancient +order, dating back to the lower Jurassic, and traceable to earlier +ancestral, generalized forms, from which sprang several other orders +also. + +Turtles and tortoises are not found until the early Jurassic, when +they already possessed the peculiar characteristics which set them off +so sharply from other reptiles. They seem to have lived at first in +shallow water and in swamps, and it is not until after the end of the +Mesozoic that some of the order became adapted to life on the land. + +The largest of all known turtles, _Archelon_, whose home was the great +interior Cretaceous sea, was fully a dozen feet in length and must +have weighed at least two tons. The skull alone is a yard long. + +Lizards and snakes do not appear until after the close of the +Mesozoic, although their ancestral lines may be followed back into the +Cretaceous. + +We will now describe some of the highly specialized orders peculiar to +the Mesozoic. + +=Land reptiles.= The _dinosaurs_ (terrible reptiles) are an extremely +varied order which were masters of the land from the late Trias until +the close of the Mesozoic era. Some were far larger than elephants, +some were as small as cats; some walked on all fours, some were +bipedal; some fed on the luxuriant tropical foliage, and others on the +flesh of weaker reptiles. They may be classed in three divisions,--the +_flesh-eating dinosaurs_, the _reptile-footed dinosaurs_, and the +_beaked dinosaurs_,--the latter two divisions being herbivorous. + +The _flesh-eating dinosaurs_ are the oldest known division of the +order, and their characteristics are shown in Figure 329. As a class, +reptiles are egg layers (_oviparous_); but some of the flesh-eating +dinosaurs are known to have been _viviparous_, i.e. to have brought +forth their young alive. This group was the longest-lived of any of +the three, beginning in the Trias and continuing to the close of the +Mesozoic era. + + [Illustration: Fig. 329. Ceratosaurus] + +Contrast the small fore limbs, used only for grasping, with the +powerful hind limbs on which the animal stalked about. Some of the +species of this group seem to have been able to progress by +leaping in kangaroo fashion. Notice the sharp claws, the ponderous +tail, and the skull set at right angles with the spinal column. +The limb bones are hollow. The ceratosaurs reached a length of +some fifteen feet, and were not uncommon in Colorado and the +western lands in Jurassic times. + + [Illustration: Fig. 330. Diplodocus] + +The _reptile-footed dinosaurs_ (Sauropoda) include some of the +biggest brutes which ever trod the ground. One of the largest, +whose remains are found entombed in the Jurassic rocks of Wyoming +and Colorado, is shown in Figure 330. + +Note the five digits on the hind feet, the quadrupedal gait, the +enormous stretch of neck and tail, the small head aligned with the +vertebral column. Diplodocus was fully sixty-five feet long and +must have weighed about twenty tons. The thigh bones of the +Sauropoda are the largest bones which ever grew. That of a genus +allied to the Diplodocus measures six feet and eight inches, and +the total length of the animal must have been not far from eighty +feet, the largest land animal known. + +The Sauropoda became extinct when their haunts along the rivers +and lakes of the western plains of Jurassic times were invaded by +the Cretaceous interior sea. + +The _beaked dinosaurs_ (Predentata) were distinguished by a beak +sheathed with horn carried in front of the tooth-set jaw, and +used, we may imagine, in stripping the leaves and twigs of trees +and shrubs. We may notice only two of the most interesting types. + + [Illustration: Fig. 331. Stegosaurus] + +_Stegosaurus_ (plated reptile) takes its name from the double row of +bony plates arranged along its back. The powerful tail was armed +with long spines, and the thick skin was defended with irregular +bits of bone implanted in it. The brain of the stegosaur was +smaller than that of any land vertebrate, while in the sacrum the +nerve canal was enlarged to ten times the capacity of the brain +cavity of the skull. Despite their feeble wits, this well-armored +family lived on through millions of years which intervened between +their appearance, at the opening of the Jurassic, and the close of +the Cretaceous, when they became extinct. + +A less stupid brute than the stegosaur was _Triceratops_, the +dinosaur of the three horns,--one horn carried on the nose, and a +massive pair set over the eyes (Fig. 332). Note the enormous wedge-shaped +skull, with its sharp beak, and the hood behind resembling a +fireman's helmet. Triceratops was fully twenty-five feet long, and +of twice the bulk of an elephant. The family appeared in the Upper +Cretaceous and became extinct at its close. Their bones are found +buried in the fresh-water deposits of the time from Colorado to +Montana and eastward to the Dakotas. + + [Illustration: Fig. 332. Restoration of Triceratops + + By courtesy of the American Museum of Natural History] + +=Marine reptiles.= In the ocean, reptiles occupied the place now +held by the aquatic mammals, such as whales and dolphins, and +their form and structure were similarly modified to suit their +environment. In the Ichthyosaurus (fish reptile), for example, the +body was fishlike in form, with short neck and large, pointed head +(Fig. 333). + + [Illustration: Fig. 333. Ichthyosaurus] + +A powerful tail, whose flukes were set vertical, and the lower one +of which was vertebrated, served as propeller, while a large +dorsal fin was developed as a cutwater. The primitive biconcave +vertebræ of the fish and of the early land vertebrates were +retained, and the limbs degenerated into short paddles. The skin +of the ichthyosaur was smooth like that of a whale, and its food +was largely fish and cephalopods, as the fossil contents of its +stomach prove. + +These sea monsters disported along the Pacific shore over northern +California in Triassic times, and the bones of immense members of +the family occur in the Jurassic strata of Wyoming. Like whales +and seals, the ichthyosaurs were descended from land vertebrates +which had become adapted to a marine habitat. + + [Illustration: Fig. 334. Plesiosaurus] + +_Plesiosaurs_ were another order which ranged throughout the +Mesozoic. Descended from small amphibious animals, they later +included great marine reptiles, characterized in the typical genus +by long neck, snakelike head, and immense paddles. They swam in +the Cretaceous interior sea of western North America. + + [Illustration: Fig. 335. Restoration of a Mosasaur] + +_Mosasaurs_ belong to the same order as do snakes and lizards, and +are an offshoot of the same ancestral line of land reptiles. These +snakelike creatures--which measured as much as forty-five feet in +length--abounded in the Cretaceous seas. They had large conical +teeth, and their limbs had become stout paddles. + +The lower jaw of the mosasaur was jointed; the quadrate bone, +which in all reptiles connects the bone of the lower jaw with the +skull, was movable, and as in snakes the lower jaw could be used +in thrusting prey down the throat. The family became extinct at +the end of the Mesozoic, and left no descendants. One may imitate +the movement of the lower jaw of the mosasaur by extending the +arms, clasping the hands, and bending the elbows. + +=Flying reptiles.= The atmosphere, which had hitherto been tenanted +only by insects, was first conquered by the vertebrates in the +Mesozoic. _Pterosaurs_, winged reptiles, whose whole organism was +adapted for flight through the air, appeared in the Jurassic and +passed off the stage of existence before the end of the +Cretaceous. The bones were hollow, as are those of birds. The +sternum, or breastbone, was given a keel for the attachment of the +wing muscles. The fifth finger, prodigiously lengthened, was +turned backward to support a membrane which was attached to the +body and extended to the base of the tail. The other fingers were +free, and armed with sharp and delicate claws, as shown in Figures +336 and 337. + + [Illustration: Fig. 336. Restoration of a Pterosaur] + + [Illustration: Fig. 337. Skeletons of Pterosaur Ornithostoma, + _A_, and of the Condor, _B_ + + After Lucas] + +These "dragons of the air" varied greatly in size; some were as +small as sparrows, while others surpassed in stretch of wing the +largest birds of the present day. They may be divided into two +groups. The earliest group comprises genera with jaws set with +teeth, and with long tails sometimes provided with a rudderlike +expansion at the end. In their successors of the later group the +tail had become short, and in some of the genera the teeth had +disappeared. Among the latest of the flying reptiles was +_Ornithostoma_ (bird beak), the largest creature which ever flew, +and whose remains are imbedded in the offshore deposits of the +Cretaceous sea which held sway over our western plains. +Ornithostoma's spread of wings was twenty feet. Its bones were a +marvel of lightness, the entire skeleton, even in its petrified +condition, not weighing more than five or six pounds. The sharp +beak, a yard long, was toothless and bird-like, as its name +suggests. + + [Illustration: Fig. 338. Archæopteryx] + +=Birds.= The earliest known birds are found in the Jurassic, and +during the remainder of the Mesozoic they contended with the +flying reptiles for the empire of the air. The first feathered +creatures were very different from the birds of to-day. Their +characteristics prove them an offshoot of the dinosaur line of +reptiles. _Archæopteryx_ (_ancient bird_) (Fig. 338) exhibits a +strange mingling of bird and reptile. Like birds, it was fledged +with perfect feathers, at least on wings and tail, but it retained +the teeth of the reptile, and its long tail was vertebrated, +a pair of feathers springing from each joint. Throughout the +Jurassic and Cretaceous the remains of birds are far less common +than those of flying reptiles, and strata representing hundreds of +thousands of years intervene between Archæopteryx and the next +birds of which we know, whose skeletons occur in the Cretaceous +beds of western Kansas. + +=Mammals.= So far as the entries upon the geological record show, +mammals made their advent in a very humble way during the Trias. +These earliest of vertebrates which suckle their young were no +bigger than young kittens, and their strong affinities with the +theromorphs suggest that their ancestors are to be found among +some generalized types of that order of reptiles. + + [Illustration: Fig. 339. Jawbone of a Jurassic Mammal] + +During the long ages of the Mesozoic, mammals continued small and +few, and were completely dominated by the reptiles. Their remains +are exceedingly rare, and consist of minute scattered teeth,--with +an occasional detached jaw,--which prove them to have been flesh +or insect eaters. In the same way their affinities are seen to be +with the lowest of mammals,--the _monotremes_ and _marsupials_. +The monotremes,--such as the duckbill mole and the spiny ant-eater +of Australia, reproduce by means of eggs resembling those of +reptiles; the marsupials, such as the opossum and the kangaroo, +bring forth their young alive, but in a very immature condition, +and carry them for some time after birth in the marsupium, a pouch +on the ventral side of the body. + + + + +CHAPTER XXI + +THE TERTIARY + + +=The Cenozoic era.= The last stages of the Cretaceous are marked by a +decadence of the reptiles. By the end of that period the reptilian +forms characteristic of the time had become extinct one after another, +leaving to represent the class only the types of reptiles which +continue to modern times. The day of the ammonite and the belemnite +also now drew to a close, and only a few of these cephalopods were +left to survive the period. It is therefore at the close of the +Cretaceous that the line is drawn which marks the end of the Middle +Age of geology and the beginning of the Cenozoic era, the era of +modern life,--the Age of Mammals. + +In place of the giant reptiles, mammals now become masters of the +land, appearing first in generalized types which, during the long ages +of the era, gradually evolve to higher forms, more specialized and +ever more closely resembling the mammals of the present. In the +atmosphere the flying dragons of the Mesozoic give place to birds and +bats. In the sea, whales, sharks, and teleost fishes of modern types +rule in the stead of huge swimming reptiles. The lower vertebrates, +the invertebrates of land and sea, and the plants of field and forest +take on a modern aspect, and differ little more from those of to-day +than the plants and animals of different countries now differ from one +another. From the beginning of the Cenozoic era until now there is a +steadily increasing number of species of animals and plants which have +continued to exist to the present time. + +The Cenozoic era comprises two divisions,--the _Tertiary_ period and +the _Quaternary_ period. + +In the early days of geology the formations of the entire geological +record, so far as it was then known, were divided into three +groups,--the _Primary_, the _Secondary_ (now known as the Mesozoic), +and the _Tertiary_, When the third group was subdivided into two +systems, the term Tertiary was retained for the first system of the +two, while the term _Quaternary_ was used to designate the second. + +=Divisions of the Tertiary.= The formations of the Tertiary are +grouped in three divisions,--the _Pliocene_ (more recent), the +_Miocene_ (less recent), and the _Eocene_ (the dawn of the recent). +Each of these epochs is long and complex. Their various subdivisions +are distinguished each by its own peculiar organisms, and the changes +of physical geography recorded in their strata. In the rapid view +which we are compelled to take we can note only a few of the most +conspicuous events of the period. + +=Physical geography of the Tertiary in eastern North America.= The +Tertiary rocks of eastern North America are marine deposits and occupy +the coastal lowlands of the Atlantic and Gulf states (Fig. 260). In +New England, Tertiary beds occur on the island of Martha's Vineyard, +but not on the mainland; hence the shore line here stood somewhat +farther out than now. From New Jersey southward the earliest Tertiary +sands and clays, still unconsolidated, leave only a narrow strip of +the edge of the Cretaceous between them and the Triassic and +crystalline rocks of the Piedmont oldland; hence the Atlantic shore +here stood farther in than now, and at the beginning of the period the +present coastal plain was continental delta. A broad belt of Tertiary +sea-laid limestones, sandstones, and shales surrounds the Gulf of +Mexico and extends northward up the Mississippi embayment to the mouth +of the Ohio River; hence the Gulf was then larger than at present, and +its waters reached in a broad bay far up the Mississippi valley. + +Along the Atlantic coast the Mesozoic peneplain may be traced +shoreward to where it disappears from view beneath an unconformable +cover of early Tertiary marine strata. The beginning of the Tertiary +was therefore marked by a subsidence. The wide erosion surface which +at the close of the Mesozoic lay near sea level where the Appalachian +Mountains and their neighboring plateaus and uplands now stand was +lowered gently along its seaward edge beneath the Tertiary Atlantic to +receive a cover of its sediments. + +As the period progressed slight oscillations occurred from time to +time. Strips of coastal plain were added to the land, and as early as +the close of the Miocene the shore lines of the Atlantic and Gulf +states had reached well-nigh their present place. Louisiana and +Florida were the last areas to emerge wholly from the sea,--Florida +being formed by a broad transverse upwarp of the continental delta at +the opening of the Miocene, forming first an island, which afterwards +was joined to the mainland. + +=The Pacific coast.= Tertiary deposits with marine fossils occur along +the western foothills of the Sierra Nevadas, and are crumpled among +the mountain masses of the Coast Ranges; it is hence inferred that the +Great Valley of California was then a border sea, separated from the +ocean by a chain of mountainous islands which were upridged into the +Coast Ranges at a still later time. Tertiary marine strata are spread +over the lower Columbia valley and that of Puget Sound, showing that +the Pacific came in broadly there. + +=The interior of the western United States.= The closing stages of the +Mesozoic were marked, as we have seen, by the upheaval of the Rocky +Mountains and other western ranges. The bases of the mountains are now +skirted by widespread Tertiary deposits, which form the highest strata +of the lofty plateaus from the level of whose summits the mountains +rise. Like the recent alluvium of the Great Valley of California (p. +101), these deposits imply low-lying lands when they were laid, and +therefore at that time the mountains rose from near sea level. But the +height at which the Tertiary strata now stand--five thousand feet +above the sea at Denver, and twice that height in the plateaus of +southern Utah--proves that the plateaus of which the Tertiary strata +form a part have been uplifted during the Cenozoic. During their +uplift, warping formed extensive basins both east and west of the +Rockies, and in these basins stream-swept and lake-laid waste gathered +to depths of hundreds and thousands of feet, as it is accumulating at +present in the Great Valley of California and on the river plains of +Turkestan (p. 103). The Tertiary river deposits of the High Plains +have already been described (p. 100). How widespread are these ancient +river plains and beds of fresh-water lakes may be seen in the map of +Figure 260. + + [Illustration: Fig. 340. View in the Bad Lands of South Dakota] + +=The Bad Lands.= In several of the western states large areas of +Tertiary fresh-water deposits have been dissected to a maze of hills +whose steep sides are cut with innumerable ravines. The deposits of +these ancient river plains and lake beds are little cemented and +because of the dryness of the climate are unprotected by vegetation; +hence they are easily carved by the wet-weather rills of scanty and +infrequent rains. These waterless, rugged surfaces were named by the +early French explorers the _Bad Lands_ because they were found so +difficult to traverse. The strata of the Bad Lands contain vast +numbers of the remains of the animals of Tertiary times, and the large +amount of barren surface exposed to view makes search for fossils easy +and fruitful. These desolate tracts are therefore frequently visited +by scientific collecting expeditions. + +=Mountain making in the Tertiary.= The Tertiary period included epochs +when the earth's crust was singularly unquiet. From time to time on +all the continents subterranean forces gathered head, and the crust +was bent and broken and upridged in lofty mountains. + +The Sierra Nevada range was formed, as we have seen, by strata +crumpling at the end of the Jurassic. But since that remote time the +upfolded mountains had been worn to plains and hilly uplands, the +remnants of whose uplifted erosion surfaces may now be traced along +the western mountain slopes. Beginning late in the Tertiary, the +region was again affected by mountain-making movements. A series of +displacements along a profound fault on the eastern side tilted the +enormous earth block of the Sierras to the west, lifting its eastern +edge to form the lofty crest and giving to the range a steep eastern +front and a gentle descent toward the Pacific. + +The Coast Ranges also have had a complex history with many +vicissitudes. The earliest foldings of their strata belong to the +close of the Jurassic, but it was not until the end of the Miocene +that the line of mountainous islands and the heavy sediments which had +been deposited on their submerged flanks were crushed into a +continuous mountain chain. Thick Pliocene beds upon their sides prove +that they were depressed to near sea level during the later Tertiary. +At the close of the Pliocene the Coast Ranges rose along with the +upheaval of the Sierra, and their gradual uplift has continued to the +present time. + +The numerous north-south ranges of the Great Basin and the Mount Saint +Elias range of Alaska were also uptilted during the Tertiary. + +During the Tertiary period many of the loftiest mountains of the +earth--the Alps, the Apennines, the Pyrenees, the Atlas, the Caucasus, +and the Himalayas--received the uplift to which they owe most of their +colossal bulk and height, as portions of the Tertiary sea beds now +found high upon their flanks attest. In the Himalayas, Tertiary marine +limestones occur sixteen thousand five hundred feet above sea level. + +=Volcanic activity in the tertiary.= The vast deformations of the +Tertiary were accompanied on a corresponding scale by outpourings of +lava, the outburst of volcanoes, and the intrusion of molten masses +within the crust. In the Sierra Nevadas the Miocene river gravels of +the valleys of the western slope, with their placer deposits of gold, +were buried beneath streams of lava and beds of tuff (Fig. 258). +Volcanoes broke forth along the Rocky Mountains and on the plateaus of +Utah, New Mexico, and Arizona. + +Mount Shasta and the immense volcanic piles of the Cascades date from +this period. The mountain basin of the Yellowstone Park was filled to +a depth of several thousand feet with tuffs and lavas, the oldest +dating as far back as the beginning of the Tertiary. Crandall +volcano (Fig. 263) was reared in the Miocene and the latest eruptions +of the Park are far more recent. + + [Illustration: Fig. 341. Lava Plateau with Lava Domes in the + Distance] + +=The Columbia and Snake River lavas.= Still more important is the +plateau of lava, more than two hundred thousand square miles in area, +extending from the Yellowstone Park to the Cascade Mountains, which +has been built from Miocene times to the present. + +Over this plateau, which occupies large portions of Idaho, Washington, +and Oregon, and extends into northern California and Nevada, the +country rock is basaltic lava. For thousands of square miles the +surface is a lava plain which meets the boundary mountains as a lake +or sea meets a rugged and deeply indented coast. The floods of molten +rock spread up the mountain valleys for a score of miles and more, the +intervening spurs rising above the lava like long peninsulas, while +here and there an isolated peak was left to tower above the inundation +like an island off a submerged shore. + +The rivers which drain the plateau--the Snake, the Columbia, and their +tributaries--have deeply trenched it, yet their canyons, which reach the +depth of several thousand feet, have not been worn to the base of the +lava except near the margin and where they cut the summits of mountains +drowned beneath the flood. Here and there the plateau has been deformed. +It has been upbent into great folds, and broken into immense blocks of +bedded lava, forming mountain ranges, which run parallel with the +Pacific coast line. On the edges of these tilted blocks the thickness of +the lava is seen to be fully five thousand feet. The plateau has been +built, like that of Iceland (p. 242), of innumerable overlapping sheets +of lava. On the canyon walls they weather back in horizontal terraces +and long talus slopes. One may distinguish each successive flow by its +dense central portion, often jointed with large vertical columns, and +the upper portion with its mass of confused irregular columns and +scoriaceous surface. The average thickness of the flows seems to be +about seventy-five feet. + +The plateau was long in building. Between the layers are found in +places old soil beds and forest grounds and the sediments of lakes. +Hence the interval between the flows in any locality was sometimes +long enough for clays to gather in the lakes which filled depressions +in the surface. Again and again the surface of the black basalt was +reddened by oxidation and decayed to soil, and forests had time to +grow upon it before the succeeding inundation sealed the sediments and +soils away beneath a sheet of stone. Near the edges of the lava plain, +rivers from the surrounding mountains spread sheets of sand and gravel +on the surface of one flow after another. These pervious sands, +interbedded with the lava, become the aquifers of artesian wells. + +In places the lavas rest on extensive lake deposits, one thousand feet +deep, and Miocene in age as their fossils prove. It is to the middle +Tertiary, then, that the earliest flows and the largest bulk of the +great inundation belong. So ancient are the latest floods in the +Columbia basin that they have weathered to a residual yellow clay from +thirty to sixty feet in depth and marvelously rich in the mineral +substances on which plants feed. + +In the Snake River valley the latest lavas are much younger. Their +surfaces are so fresh and undecayed that here the effusive eruptions +may well have continued to within the period of human history. Low +lava domes like those of Iceland mark where last the basalt outwelled +and spread far and wide before it chilled (Fig. 341). In places small +mounds of scoria show that the eruptions were accompanied to a slight +degree by explosions of steam. So fluid was this superheated lava that +recent flows have been traced for more than fifty miles. + +The rocks underlying the Columbia lavas, where exposed to view, are +seen to be cut by numerous great dikes of dense basalt, which mark the +fissures through which the molten rock rose to the surface. + +The Tertiary included times of widespread and intense volcanic action +in other continents as well as in North America. In Europe, +Vesuvius (p. 231) and Etna began their career as submarine volcanoes in +connection with earth movements which finally lifted Pliocene deposits +in Sicily to their present height,--four thousand feet above the sea. +Volcanoes broke forth in central France and southern Germany, in +Hungary and the Carpathians. Innumerable fissures opened in the crust +from the north of Ireland and the western islands of Scotland to the +Faroes, Iceland, and even to arctic Greenland; and here great plateaus +were built of flows of basalt similar to that of the Columbia River. +In India, at the opening of the Tertiary, there had been an outwelling +of basalt, flooding to a depth of thousands of feet two hundred +thousand square miles of the northwestern part of the peninsula (Fig. +342), and similar inundations of lava occurred where are now the +table-lands of Abyssinia. From the middle Tertiary on, Asia Minor, +Arabia, and Persia were the scenes of volcanic action. In Palestine +the rise of the uplands of Judea at the close of the Eocene, and the +downfaulting of the Jordan valley (p. 221) were followed by volcanic +outbursts. In comparison with the middle Tertiary, the present is a +time of volcanic inactivity and repose. + + [Illustration: Fig. 342. Map showing the Lava Sheet + (shaded area) of Western India] + +=Erosion of Tertiary mountains and plateaus.= The mountains and +plateaus built at various times during the Tertiary and at its +commencement have been profoundly carved by erosive agents. The Sierra +Nevada Mountains have been dissected on the western slope by such +canyons as those of King's River and the Yosemite. Six miles of strata +have been denuded from parts of the Wasatch Mountains since their rise +at the beginning of the era. From the Colorado plateaus, whose uplift +dates from the same time, there have been stripped off ten thousand +feet of strata over thousands of square miles, and the colossal canyon +of the Colorado has been cut after this great denudation had been +mostly accomplished. + +On the eastern side of the continent, as we have seen, a broad +peneplain had been developed by the close of the Cretaceous. The +remnants of this old erosion surface are now found upwarped to various +heights in different portions of its area. In southern New England it +now stands fifteen hundred feet above the sea in western +Massachusetts, declining thence southward and eastward to sea level at +the coast. In southwestern Virginia it has been lifted to four +thousand feet above the sea. Manifestly this upwarp occurred since the +peneplain was formed; it is later than the Mesozoic, and the vast +dissection which the peneplain has suffered since its uplift must +belong to the successive cycles of Cenozoic time. + +Revived by the uplift, the streams of the area trenched it as deeply +as its elevation permitted, and reaching grade, opened up wide valleys +and new peneplains in the softer rocks. The Connecticut valley is +Tertiary in age, and in the weak Triassic sandstones has been widened +in places to fifteen miles. Dating from the same time are the valleys +of the Hudson, the Susquehanna, the Delaware, the Potomac, and the +Shenandoah. + +In Pennsylvania and the states lying to the south the Mesozoic +peneplain lies along the summits of the mountain ridges. On the +surface of this ancient plain, Tertiary erosion etched out the +beautifully regular pattern of the Allegheny mountain ridges and their +intervening valleys. The weaker strata of the long, regular folds were +eroded into longitudinal valleys, while the hard Paleozoic sandstones, +such as the Medina (p. 335) and the Pocono (p. 350), were left in +relief as bold mountain walls whose even crests rise to the common +level of the ancient plain. From Virginia far into Alabama the great +Appalachian valley was opened to a width in places of fifty miles and +more, along a belt of intensely folded and faulted strata where once +was the heart of the Appalachian Mountains. In Figure 70, the summit of +the Cumberland plateau (ab) marks the level of the Mesozoic peneplain, +while the lower erosion levels are Tertiary and Quaternary in age. + + [Illustration: Fig. 343. Diagram of the Allegheny Mountains, + Pennsylvania + + From Davis' Elementary Physical Geography] + + +Life of the Tertiary Period + +=Vegetation and climate.= The highest plants in structure, the +_dicotyls_ (such as our deciduous forest trees) and the _monocotyls_ +(represented by the palms), were introduced during the Cretaceous. The +vegetable kingdom reached its culmination before the animal kingdom, +and if the dividing line between the Mesozoic and the Cenozoic were +drawn according to the progress of plant life, the Cretaceous instead +of the Tertiary would be made the opening period of the modern era. + +The plants of the Tertiary belonged, for the most part, to genera +now living; but their distribution was very different from that of +the flora of to-day. In the earlier Tertiary, palms flourished over +northern Europe, and in the northwestern United States grew the +magnolia and laurel, along with the walnut, oak, and elm. Even in +northern Greenland and in Spitzbergen there were lakes covered with +water lilies and surrounded by forests of maples, poplars, limes, the +cypress of our southern states, and noble sequoias similar to the +"big trees" and redwoods of California. A warm climate like that of +the Mesozoic, therefore, prevailed over North America and Europe, +extending far toward the pole. In the later Tertiary the climate +gradually became cooler. Palms disappeared from Europe, and everywhere +the aspect of forests and open lands became more like that of to-day. +Grasses became abundant, furnishing a new food for herbivorous +animals. + +=Animal life of the Tertiary.= Little needs to be said of the Tertiary +invertebrates, so nearly were they like the invertebrates of the +present. Even in the Eocene, about five per cent of marine shells were +of species still living, and in the Pliocene the proportion had risen +to more than one half. + +Fishes were of modern types. Teleosts were now abundant. The ocean +teemed with sharks, some of them being voracious monsters seventy-five +feet and even more in length, with a gape of jaw of six feet, as +estimated by the size of their enormous sharp-edged teeth. + +Snakes are found for the first time in the early Tertiary. These +limbless reptiles, evolved by degeneration from lizardlike ancestors, +appeared in nonpoisonous types scarcely to be distinguished from those +of the present day. + +=Mammals of the early Tertiary.= The fossils of continental deposits +of the earliest Eocene show that a marked advance had now been made in +the evolution of the Mammalia. The higher mammals had appeared, and +henceforth the lower mammals--the monotremes and the marsupials--are +reduced to a subordinate place. + + [Illustration: Fig. 344. Phenacodus] + +These first true mammals were archaic and generalized in structure. +Their feet were of the primitive type, with five toes of about equal +length. They were also _plantigrades_,--that is, they touched the +ground with the sole of the entire foot from toe to heel. No foot had +yet become adapted to swift running by a decrease in the number of +digits and by lifting the heel and sole so that only the toes touch +the ground,--a tread called _digitigrade_. Nor was there yet any foot +like that of the cats, with sharp retractile claws adapted to seizing +and tearing the prey. The forearm and the lower leg each had still +two separate bones (ulna and radius, fibula and tibia), neither pair +having been replaced with a single strong bone, as in the leg of the +horse. The teeth also were primitive in type and of full number. The +complex heavy grinders of the horse and elephant, the sharp cutting +teeth of the carnivores, and the cropping teeth of the grass eaters +were all still to come. + +Phenacodus is a characteristic genus of the early Eocene, whose +species varied in size from that of a bulldog to that of an animal a +little larger than a sheep. Its feet were primitive, and their five +toes bore nails intermediate in form between a claw and a hoof. The +archaic type of teeth indicates that the animal was omnivorous in +diet. A cast of the brain cavity shows that, like its associates of +the time, its brain was extremely small and nearly smooth, having +little more than traces of convolutions. + +The long ages of the Eocene and the following epochs of the Tertiary +were times of comparatively rapid evolution among the Mammalia. +The earliest forms evolved along diverging lines toward the various +specialized types of hoofed mammals, rodents, carnivores, +proboscidians, the primates, and the other mammalian orders as we know +them now. We must describe the Tertiary mammals very briefly, tracing +the lines of descent of only a few of the more familiar mammals of the +present. + +=The horse.= The pedigree of the horse runs back into the early Eocene +through many genera and species to a five-toed,[3] short-legged ancestor +little bigger than a cat. Its descendants gradually increased in stature +and became better and better adapted to swift running to escape their +foes. The leg became longer, and only the tip of the toes struck the +ground. The middle toe (digit number three), originally the longest of +the five, steadily enlarged, while the remaining digits dwindled and +disappeared. The inner digit, corresponding to the great toe and thumb, +was the first to go. Next number five, the little finger, was also +dropped. By the end of the Eocene a three-toed genus of the horse +family had appeared, as large as a sheep. The hoof of digit number +three now supported most of the weight, but the slender hoofs of +digits two and four were still serviceable. In the Miocene the stature +of the ancestors of the horse increased to that of a pony. The feet +were still three-toed, but the side hoofs were now mere dewclaws and +scarcely touched the ground. The evolution of the family was completed +in the Pliocene. The middle toe was enlarged still more, the side toes +were dropped, and the palm and foot bones which supported them were +reduced to splints. + + [3] Or, more accurately, with four perfect toes and a + rudimentary fifth corresponding to the thumb. + + [Illustration: Fig. 345. Development of Forefoot (A), the + Forearm (B), the Molar (C), of the Horse Family] + +While these changes were in progress the radius and ulna of the fore +limb became consolidated to a single bone; and in the hind limb the +fibula dwindled to a splint, while the tibia was correspondingly +enlarged. The molars, also gradually lengthened, and became more and +more complex on their grinding surface; the neck became longer; the +brain steadily increased in size and its convolutions became more +abundant. The evolution of the horse has made for greater fleetness +and intelligence. + +=The rhinoceros and tapir.= These animals, which are grouped with the +horse among the _odd-toed_ (perissodactyl) mammals, are now verging +toward extinction. In the rhinoceros, evolution seems to have taken +the opposite course from that of the horse. As the animal increased in +size it became more clumsy, its limbs became shorter and more massive, +and, perhaps because of its great weight, the number of digits were +not reduced below the number three. Like other large herbivores, the +rhinoceros, too slow to escape its enemies by flight, learned to +withstand them. It developed as its means of defense a nasal horn. + +Peculiar offshoots of the line appeared at various times in the +Tertiary. A rhinoceros, semiaquatic in habits, with curved tusks, +resembling in aspect the hippopotamus, lived along the water courses +of the plains east of the Rockies, and its bones are now found by the +thousands in the Miocene of Kansas. Another developed along a line +parallel to that of the horse, and herds of these light-limbed and +swift-footed running rhinoceroses ranged the Great Plains from the +Dakotas southward. + +The tapirs are an ancient family which has changed but little since it +separated from the other perissodactyl stocks in the early Tertiary. +At present, tapirs are found only in South America and southern +Asia,--a remarkable distribution which we could not explain were it +not that the geological record shows that during Tertiary times tapirs +ranged throughout the northern hemisphere, making their way to South +America late in that period. During the Pleistocene they became +extinct over all the intervening lands between the widely separated +regions where now they live. The geographic distribution of animals, +as well as their relationships and origins, can be understood only +through a study of their geological history. + + [Illustration: Fig. 346. A Tertiary Mastodon] + + [Illustration: Fig. 347. Head of Dinothere] + +=The proboscidians.= This unique order of hoofed mammals, of which the +elephant is the sole survivor, has been traced back to the close of +the Eocene. In the middle and later Tertiary it was represented by +huge creatures so nearly akin to the mastodons of the Pleistocene that +they are often included in that genus. The Tertiary _Mastodon_ was +furnished with a long, flexible proboscis, and armed with two pairs of +long, straight ivory tusks, the pair of the lower jaw being smaller. + +The _Dinothere_ was a curious offshoot of the line, which developed in +the Miocene in Europe. In this immense proboscidian, whose skull was +three feet long, the upper pair of tusks had disappeared, and those of +the lower jaw were bent down with a backward curve in walrus fashion. + + [Illustration: Fig. 348. Crown of Mastodon Tooth] + +In the true _elephants_, which do not appear until near the close of +the Tertiary, the lower jaw loses its tusks and the grinding teeth +become exceedingly complex in structure. The grinding teeth of the +mastodon had long roots and low crowns crossed by four or five peaked +enameled ridges. In the teeth of the true elephants the crown has +become deep, and the ridges of enamel have changed to numerous +upright, platelike folds, their interspaces filled with cement. The +two genera--Mastodon and Elephant--are connected by species whose +teeth are intermediate in pattern. The proboscidians culminated in the +Pliocene, when some of the giant elephants reached a height of +fourteen feet. + + [Illustration: Fig. 349. Tooth of an Extinct Elephant, the Mammoth] + + [Illustration: Fig. 350. Evolution of the Artiodactyl Foot, + Illustrated by Existing Families + + _A_, pig; _B_, roebuck; _C_, sheep; _D_, camel] + +=The artiodactyls= comprise the hoofed Mammalia which have an even +number of toes, such as cattle, sheep, and swine. Like the +perissodactyls, they are descended from the primitive five-toed +plantigrade mammals of the lowest Eocene. In their evolution, digit +number one was first dropped, and the middle pair became larger and +more massive, while the side digits, numbers two and five, became +shorter, weaker, and less serviceable. The _four-toed artiodactyls_ +culminated in the Tertiary; at present they are represented only by +the hippopotamus and the hog. Along the main line of the evolution of +the artiodactyls the side toes, digits two and five, disappeared, +leaving as proof that they once existed the corresponding bones of +palm and sole as splints. The _two-toed artiodactyls_, such as the +camels, deer, cattle, and sheep, are now the leading types of the +herbivores. + +_Swine and peccaries_ are two branches of a common stock, the first +developing in the Old World and the second in the New. In the Miocene +a noticeable offshoot of the line was a gigantic piglike brute, a root +eater, with a skull a yard in length, whose remains are now found in +Colorado and South Dakota. + +=Camels and llamas.= The line of camels and llamas developed in North +America, where the successive changes from an early Eocene ancestor, +no larger than a rabbit, are traced step by step to the present forms, +as clearly as is the evolution of the horse. In the late Miocene some +of the ancestral forms migrated to the Old World by way of a land +connection where Bering Strait now is, and there gave rise to the +camels and dromedaries. Others migrated into South America, which had +now been connected with our own continent, and these developed into +the llamas and guanacos, while those of the race which remained in +North America became extinct during the Pleistocene. + +Some peculiar branches of the camel stem appeared in North America. In +the Pliocene arose a llama with the long neck and limbs of a giraffe, +whose food was cropped from the leaves and branches of trees. Far more +generalized in structure was the _Oreodon_, an animal related to the +camels, but with distinct affinities also with other lines, such as +those of the hog and deer. These curious creatures were much like the +peccary in appearance, except for their long tails. In the middle +Eocene they roamed in vast herds from Oregon to Kansas and Nebraska. + +=The ruminants.= This division of the artiodactyls includes antelopes, +deer, oxen, bison, sheep, and goats,--all of which belong to a common +stock which took its rise in Europe in the upper Eocene from ancestral +forms akin to those of the camels. In the Miocene the evolution of the +two-toed artiodactyl foot was well-nigh completed. Bonelike growths +appeared on the head, and the two groups of the ruminants became +specialized,--the deer with bony antlers, shed and renewed each year, +and the ruminants with hollow horns, whose two bony knobs upon the +skull are covered with permanent, pointed, horny sheaths. + +The ruminants evolved in the Old World, and it was not until the later +Miocene that the ancestors of the antelope and of some deer found +their way to North America. Mountain sheep and goats, the bison and +most of the deer, did not arrive until after the close of the +Tertiary, and sheep and oxen were introduced by man. + +The hoofed mammals of the Tertiary included many offshoots from the +main lines which we have traced. Among them were a number of genera of +clumsy, ponderous brutes, some almost elephantine in their bulk. + +=The carnivores.= The ancestral lines of the families of the flesh +eaters--such as the cats (lions, tigers, etc.), the bears, the hyenas, +and the dogs (including wolves and foxes)--converge in the creodonts +of the early Eocene,--an order so generalized that it had affinities +not only with the carnivores but also with the insect eaters, the +marsupials, and the hoofed mammals as well. From these primitive flesh +eaters, with small and simple brains, numerous small teeth, and +plantigrade tread, the different families of the carnivores of the +present have slowly evolved. + +=Dogs and bears.= The dog family diverged from the creodonts late in +the Eocene, and divided into two branches, one of which evolved the +wolves and the other the foxes. An offshoot gave rise to the family +of the bears, and so closely do these two families, now wide apart, +approach as we trace them back in Tertiary times that the Amphicyon, +a genus doglike in its teeth and bearlike in other structures, is +referred by some to the dog and by others to the bear family. The +well-known plantigrade tread of bears is a primitive characteristic +which has survived from their creodont ancestry. + +=Cats.= The family of the cats, the most highly specialized of all the +carnivores, divided in the Tertiary into two main branches. One, the +saber-tooth tigers (Fig. 351), which takes its name from their long, +saberlike, sharp-edged upper canine teeth, evolved a succession of +genera and species, among them some of the most destructive beasts of +prey which ever scourged the earth. They were masters of the entire +northern hemisphere during the middle Tertiary, but in Europe during +the Pliocene they declined, from unknown causes, and gave place to the +other branch of cats,--which includes the lions, tigers, and leopards. +In the Americas the saber-tooth tigers long survived the epoch. + + [Illustration: Fig. 351. Saber-Tooth Tiger] + +=Marine mammals.= The carnivorous mammals of the sea--whales, seals, +walruses, etc.--seem to have been derived from some of the creodonts +of the early Tertiary by adaptation to aquatic life. Whales evolved +from some land ancestry at a very early date in the Tertiary; in the +marine deposits of the Eocene are found the bones of the Zeuglodon, a +whalelike creature seventy feet in length. + +=Primates.= This order, which includes lemurs, monkeys, apes, and man, +seems to have sprung from a creodont or insectivorous ancestry in +the lower Eocene. Lemur-like types, with small, smooth brains, were +abundant in the United States in the early Tertiary, but no primates +have been found here in the middle Tertiary and later strata. In +Europe true monkeys were introduced in the Miocene, and were abundant +until the close of the Tertiary, when they were driven from the +continent by the increasing cold. + +=Advance of the mammalia during the tertiary.= During the several +millions of years comprised in Tertiary time the mammals evolved from +the lowly, simple types which tenanted the earth at the beginning of +the period, into the many kinds of highly specialized mammals of the +Pleistocene and the present, each with the various structures of the +body adapted to its own peculiar mode of life. The swift feet of the +horse, the horns of cattle and the antlers of the deer, the lion's +claws and teeth, the long incisors of the beaver, the proboscis of the +elephant, were all developed in Tertiary times. In especial the brain +of the Tertiary mammals constantly grew larger relatively to the +size of body, and the higher portion of the brain--the cerebral +lobes--increased in size in comparison with the cerebellum. Some +of the hoofed mammals now have a brain eight or ten times the size +of that of their early Tertiary predecessors of equal bulk. Nor +can we doubt that along with the increasing size of brain went a +corresponding increase in the keenness of the senses, in activity +and vigor, and in intelligence. + + + + +CHAPTER XXII + +THE QUATERNARY + + +The last period of geological history, the Quaternary, may be said to +have begun when all, or nearly all, living species of mollusks and +most of the existing mammals had appeared. + +It is divided into two great epochs. The first, the _Pleistocene_ or +_Glacial epoch_, is marked off from the Tertiary by the occupation of +the northern parts of North America and Europe by vast ice sheets; the +second, the _Recent epoch_, began with the disappearance of the ice +sheets from these continents, and merges into the present time. + + +The Pleistocene Epoch + +We now come to an episode of unusual interest, so different was it +from most of the preceding epochs and from the present, and so largely +has it influenced the conditions of man's life. + +The records of the Glacial epoch are so plain and full that +we are compelled to believe what otherwise would seem almost +incredible,--that following the mild climate of the Tertiary came a +succession of ages when ice fields, like that of Greenland, shrouded +the northern parts of North America and Europe and extended far into +temperate latitudes. + +=The drift.= Our studies of glaciers have prepared us to decipher and +interpret the history of the Glacial epoch, as it is recorded in the +surface deposits known as the drift. Over most of Canada and the +northern states this familiar formation is exposed to view in nearly +all cuttings which pass below the surface soil. The drift includes two +distinct classes of deposits,--the unstratified drift laid down by +glacier ice, and the stratified drift spread by glacier waters. + +The materials of the drift are in any given place in part unlike the +rock on which it rests. They cannot be derived from the underlying +rock by weathering, but have been brought from elsewhere. Thus where a +region is underlain by sedimentary rocks, as is the drift-covered area +from the Hudson River to the Missouri, the drift contains not only +fragments of limestone, sandstone, and shale of local derivation, but +also pebbles of many igneous and metamorphic rocks, such as granites, +gneisses, schists, dike rocks, quartzites, and the quartz of mineral +veins, whose nearest source is the Archean area of Canada and the +states of our northern border. The drift received its name when it was +supposed that the formation had been drifted by floods and icebergs +from outside sources,--a theory long since abandoned. + + [Illustration: Fig. 352. Stratified Drift overlaying + Unstratified Drift, Massachusetts] + +The distribution also of the drift points clearly to its peculiar +origin. Within the limits of the glaciated area it covers the country +without regard to the relief, mantling with its debris not only +lowlands and valleys but also highlands and mountain slopes. + +The boundary of the drift is equally independent of the relief of +the land, crossing hills and plains impartially, unlike water-laid +deposits, whose margins, unless subsequently deformed, are horizontal. +The boundary of the drift is strikingly lobate also, bending outward +in broad, convex curves, where there are no natural barriers in the +topography of the country to set it such a limit. Under these +conditions such a lobate margin cannot belong to deposits of rivers, +lakes, or ocean, but is precisely that which would mark the edge of a +continental glacier which deployed in broad tongues of ice. + +=The rock surface underlying the drift.= Over much of its area the +drift rests on firm, fresh rock, showing that both the preglacial +mantle of residual waste and the partially decomposed and broken rock +beneath it have been swept away. The underlying rock, especially if +massive, hard, and of a fine grain, has often been ground down to a +smooth surface and rubbed to a polish as perfect as that seen on the +rock beside an Alpine glacier where the ice has recently melted back. +Frequently it has been worn to the smooth, rounded hummocks known as +roches moutonnées, and even rocky hills have been thus smoothed to +flowing outlines like roches moutonnées on a gigantic scale. The rock +pavement beneath the drift is also marked by long, straight, parallel +scorings, varying in size from deep grooves to fine striae as delicate +as the hair lines cut by an engraver's needle. Where the rock is soft +or closely jointed it is often shattered to a depth of several feet +beneath the drift, while stony clay has been thrust in among the +fragments into which the rock is broken. + +In the presence of these glaciated surfaces we cannot doubt that the +area of the drift has been overridden by vast sheets of ice which, in +their steady flow, rasped and scored the rock bed beneath by means of +the stones with which their basal layers were inset, and in places +plucked and shattered it. + +=Till.= The unstratified portion of the drift consists chiefly of +sheets of dense, stony clay called till, which clearly are the ground +moraines of ancient continental glaciers. Till is an unsorted mixture +of materials of all sizes, from fine clay and sand, gravel, pebbles, +and cobblestones, to large bowlders. The stones of the till are of +many kinds, some having been plucked from the bed rock of the locality +where they are found, and others having been brought from outside and +often distant places. Land ice is the only agent known which can +spread unstratified material in such extensive sheets. + +The _fine material_ of the till comes from two different sources. In +part it is derived from old residual clays, which in the making had +been leached of the lime and other soluble ingredients of the rock +from which they weathered. In part it consists of sound rock ground +fine; a drop of acid on fresh, clayey till often proves by brisk +effervescence that the till contains much undecayed limestone flour. +The ice sheet, therefore, both scraped up the mantle of long-weathered +waste which covered the country before its coming, and also ground +heavily upon the sound rock underneath, and crushed and wore to rock +flour the fragments which it carried. + +The color of unweathered till depends on that of the materials of +which it is composed. Where red sandstones have contributed largely to +its making, as over the Triassic sandstones of the eastern states and +the Algonkian sandstones about Lake Superior, the drift is reddish. +When derived in part from coaly shales, as over many outcrops of the +Pennsylvanian, it may when moist be almost black. Fresh till is +normally a dull gray or bluish, so largely is it made up of the +grindings of unoxidized rocks of these common colors. + +Except where composed chiefly of sand or coarser stuff, unweathered +till is often exceedingly dense. Can you suggest by what means it has +been thus compacted? Did the ice fields of the Glacial epoch bear +heavy surface moraines like the medial and lateral moraines of valley +glaciers? Where was the greater part of the load of these ice fields +carried, judging from what you know of the glaciers of Greenland? + +=Bowlders of the drift.= The pebbles and bowlders of the drift are in +part stream gravels, bowlders of weathering, and other coarse rock +waste picked up from the surface of the country by the advancing ice, +and in part are fragments plucked from ledges of sound rock after the +mantle of waste had been removed. Many of the stones of the till are +dressed as only glacier ice can do; their sharp edges have been +blunted and their sides faceted and scored. + +We may easily find all stages of this process represented among the +pebbles of the till. Some are little worn, even on their edges; some +are planed and scored on one side only; while some in their long +journey have been ground down to many facets and have lost much of +their original bulk. Evidently the ice played fast and loose with a +stone carried in its basal layers, now holding it fast and rubbing it +against the rock beneath, now loosening its grasp and allowing the +stone to turn. + +Bowlders of the drift are sometimes found on higher ground than their +parent ledges. Thus bowlders have been left on the sides of Mount +Katahdin, Maine, which were plucked from limestone ledges twelve miles +distant and three thousand feet lower than their resting place. In +other cases stones have been carried over mountain ranges, as in +Vermont, where pebbles of Burlington red sandstone were dragged over +the Green Mountains, three thousand feet in height, and left in the +Connecticut valley sixty miles away. No other geological agent than +glacier ice could do this work. + +The bowlders of the drift are often large. Bowlders ten and twenty +feet in diameter are not uncommon, and some are known whose diameter +exceeds fifty feet. As a rule the average size of bowlders decreases +with increasing distance from their sources. Why? + +=Till plains.= The surface of the drift, where left in its initial +state, also displays clear proof of its glacial origin. Over large +areas it is spread in level plains of till, perhaps bowlder-dotted, +similar to the plains of stony clay left in Spitzbergen by the recent +retreat of some of the glaciers of that island. In places the +unstratified drift is heaped in hills of various kinds, which we will +now describe. + + [Illustration: Fig. 354. Map of a portion of a Drumlin Area near + Oswego, New York] + +=Drumlins.= Drumlins are smooth, rounded hills composed of till, +elliptical in base, and having their longer axes parallel to the +movement of the ice as shown by glacial scorings. They crowd certain +districts in central New York and in southern Wisconsin, where they +may be counted by the thousands. Among the numerous drumlins about +Boston is historic Bunker Hill. + +Drumlins are made of ground moraine. They were accumulated and given +shape beneath the overriding ice, much as are sand bars in a river, or +in some instances were carved, like roches moutonnées, by an ice sheet +out of the till left by an earlier ice invasion. + +=Terminal moraines.= The glaciated area is crossed by belts of +thickened drift, often a mile or two, and sometimes even ten miles +and more, in breadth, which lie transverse to the movement of the ice +and clearly are the terminal moraines of ancient ice sheets, marking +either the limit of their farthest advance or pauses in their general +retreat. + + [Illustration: Fig. 355. Terminal Moraine, Staten Island] + +The surface of these moraines is a jumble of elevations and +depressions, which vary from low, gentle swells and shallow sags to +sharp hills, a hundred feet or so in height, and deep, steep-sided +hollows. Such tumultuous hills and hummocks, set with depressions of +all shapes, which usually are without outlet and are often occupied by +marshes, ponds, and lakes, surely cannot be the work of running water. +The hills are heaps of drift, lodged beneath the ice edge or piled +along its front. The basins were left among the tangle of morainic +knolls and ridges (Fig. 105) as the margin of the ice moved back and +forth. Some bowl-shaped basins were made by the melting of a mass of +ice left behind by the retreating glacier and buried in its debris. + + [Illustration: Fig. 356. Esker, New York] + +=The stratified drift.= Like modern glaciers the ice sheets of the +Pleistocene were ever being converted into water about their margins. +Their limits on the land were the lines where their onward flow was +just balanced by melting and evaporation. On the surface of the ice +along the marginal zone, rivulets no doubt flowed in summer, and found +their way through crevasses to the interior of the glacier or to +the ground. Subglacial streams, like those of the Malaspina glacier, +issued from tunnels in the ice, and water ran along the melting ice +front as it is seen to do about the glacier tongues of Greenland. All +these glacier waters flowed away down the chief drainage channels in +swollen rivers loaded with glacial waste. + +It is not unexpected therefore that there are found, over all the +country where the melting ice retreated, deposits made of the same +materials as the till, but sorted and stratified by running water. +Some of these were deposited behind the ice front in ice-walled +channels, some at the edge of the glaciers by issuing streams, and +others were spread to long distances in front of the ice edge by +glacial waters as they flowed away. + +_Eskers_ are narrow, winding ridges of stratified sand and gravel +whose general course lies parallel with the movement of the glacier. +These ridges, though evidently laid by running water, do not follow +lines of continuous descent, but may be found to cross river valleys +and ascend their sides. Hence the streams by which eskers were laid +did not flow unconfined upon the surface of the ground. We may infer +that eskers were deposited in the tunnels and ice-walled gorges of +glacial streams before they issued from the ice front. + + [Illustration: Fig. 357. Kames, New York] + +_Kames_ are sand and gravel knolls, associated for the most part +with terminal moraines, and heaped by glacial waters along the +margin of the ice. + + [Illustration: Fig. 358. Diagram Illustrating the Formation of + Kame Terraces + + _i_, glacier ice; _t_, _t_, terraces] + +_Kame terraces_ are hummocky embankments of stratified drift sometimes +found in rugged regions along the sides of valleys. In these valleys +long tongues of glacier ice lay slowly melting. Glacial waters took +their way between the edges of the glaciers and the hillside, and here +deposited sand and gravel in rude terraces. + +_Outwash plains_ are plains of sand and gravel which frequently border +terminal moraines on their outward face, and were spread evidently by +outwash from the melting ice. Outwash plains are sometimes pitted by +bowl-shaped basins where ice blocks were left buried in the sand by +the retreating glacier. + +_Valley trains_ are deposits of stratified drift with which river +valleys have been aggraded. Valleys leading outward from the ice front +were flooded by glacial waters and were filled often to great depths +with trains of stream-swept drift. Since the disappearance of the ice +these glacial flood plains have been dissected by the shrunken rivers +of recent times and left on either side the valley in high terraces. +Valley trains head in morainic plains, and their material grows finer +down valley and coarser toward their sources. Their gradient is +commonly greater than that of the present rivers. + +=The extent of the drift.= The extent of the drift of North America +and its southern limits are best seen in Figure 359. Its area is +reckoned at about four million square miles. The ice fields which once +covered so much of our continent were all together ten times as large +as the inland ice of Greenland, and about equal to the enormous ice +cap which now covers the antartic regions. + +The ice field of Europe was much smaller, measuring about seven +hundred and seventy thousand square miles. + +=Centers of dispersion.= The direction of the movement of the ice is +recorded plainly in the scorings of the rock surface, in the shapes of +glaciated hills, in the axes of drumlins and eskers, and in trains of +bowlders, when the ledges from which they were plucked can be +discovered. In these ways it has been proved that in North America +there were three centers where ice gathered to the greatest depth, +and from which it flowed in all directions outward. There were thus +three vast ice fields,--one the _Cordilleran_, which lay upon the +Cordilleras of British America; one the _Keewatin_, which flowed +out from the province of Keewatin, west of Hudson Bay; and one the +_Labrador_ ice field, whose center of dispersion was on the highlands +of the peninsula of Labrador. As shown in Figure 359, the western ice +field extended but a short way beyond the eastern foothills of the +Rocky Mountains, where perhaps it met the far-traveled ice from the +great central field. The Keewatin and the Labrador ice fields flowed +farthest toward the south, and in the Mississippi valley the one +reached the mouth of the Missouri and the other nearly to the mouth of +the Ohio. In Minnesota and Wisconsin and northward they merged in one +vast field. + + [Illustration: Fig. 359. Hypothetical Map of the Pleistocene Ice Sheets + of North America + + From Salisbury's _Glacial Geology of New Jersey_] + +The thickness of the ice was so great that it buried the highest +mountains of eastern North America, as is proved by the transported +bowlders which have been found upon their summits. If the land then +stood at its present height above sea level, and if the average slope +of the ice were no more than ten feet to the mile,--a slope so gentle +that the eye could not detect it and less than half the slope of the +interior of the inland ice of Greenland,--the ice plateaus about +Hudson Bay must have reached a thickness of at least ten thousand +feet. + +In Europe the Scandinavian plateau was the chief center of dispersion. +At the time of greatest glaciation a continuous field of ice extended +from the Ural Mountains to the Atlantic, where, off the coasts of +Norway and the British Isles, it met the sea in an unbroken ice wall. +On the south it reached to southern England, Belgium, and central +Germany, and deployed on the eastern plains in wide lobes over Poland +and central Russia (Fig. 360). + + [Illustration: Fig. 360. Hypothetical Map of the Pleistocene + Ice Sheet of Europe] + +At the same time the Alps supported giant glaciers many times the size +of the surviving glaciers of to-day, and a piedmont glacier covered +the plains of northern Switzerland. + +=The thickness of the drift.= The drift is far from uniform in +thickness. It is comparatively thin and scanty over the Laurentian +highlands and the rugged regions of New England, while from southern +New York and Ontario westward over the Mississippi valley, and on the +great western plains of Canada, it exceeds an average of one hundred +feet over wide areas, and in places has five and six times that +thickness. It was to this marginal belt that the ice sheets brought +their loads, while northwards, nearer the centers of dispersion, +erosion was excessive and deposition slight. + +=Successive ice invasions and their drift sheets.= Recent studies of +the drift prove that it does not consist of one indivisible formation, +but includes a number of distinct drift sheets, each with its own +peculiar features. The Pleistocene epoch consisted, therefore, of +several glacial stages,--during each of which the ice advanced far +southward,--together with the intervening interglacial stages when, +under a milder climate, the ice melted back toward its sources or +wholly disappeared. + + [Illustration: Fig. 361. Diagram illustrating Criteria by which + Different Drift Sheets are distinguished] + +The evidences of such interglacial stages, and the means by which the +different drift sheets are told apart, are illustrated in Figure 361. +Here the country from N to S is wholly covered by drift, but the drift +from N to _m_ is so unlike that from _m_ to S that we may believe it +the product of a distinct ice invasion and deposited during another +and far later glacial stage. The former drift is very young, for its +drainage is as yet immature, and there are many lakes and marshes +upon its surface; the latter is far older, for its surface has been +thoroughly dissected by its streams. The former is but slightly +weathered, while the latter is so old that it is deeply reddened by +oxidation and is leached of its soluble ingredients such as lime. +The younger drift is bordered by a distinct terminal moraine, while +the margin of the older drift is not thus marked. Moreover, the two +drift sheets are somewhat unlike in composition, and the different +proportion of pebbles of the various kinds of rocks which they contain +shows that their respective glaciers followed different tracks and +gathered their loads from different regions. Again, in places beneath +the younger drift there is found the buried land surface of an older +drift with old soils, forest grounds, and vegetable deposits, +containing the remains of animals and plants, which tell of the +climate of the interglacial stage in which they lived. + +By such differences as these the following drift sheets have been made +out in America, and similar subdivisions have been recognized in +Europe. + + 5 The Wisconsin formation + 4 The Iowan formation + 3 The Illinoian formation + 2 The Kansan formation + 1 The pre-Kansan or Jerseyan formation + +In New Jersey and Pennsylvania the edge of a deeply weathered and +eroded drift sheet, the Jerseyan, extends beyond the limits of a much +younger overlying drift. It may be the equivalent of a deep-buried +basal drift sheet found in the Mississippi valley beneath the Kansan +and parted from it by peat, old soil, and gravel beds. + +The two succeeding stages mark the greatest snowfall of the Glacial +epoch. In Kansan times the Keewatin ice field slowly grew southward +until it reached fifteen hundred miles from its center of dispersion +and extended from the Arctic Ocean to northeastern Kansas. In the +Illinoian stage the Labrador ice field stretched from Hudson Straits +nearly to the Ohio River in Illinois. In the Iowan and the Wisconsin, +the closing stages of the Glacial epoch, the readvancing ice fields +fell far short of their former limits in the Mississippi valley, but +in the eastern states the Labrador ice field during Wisconsin times +overrode for the most part all earlier deposits, and, covering New +England, probably met the ocean in a continuous wall of ice which set +its bergs afloat from Massachusetts to northern Labrador. + +We select for detailed description the Kansan and the Wisconsin +formations as representatives, the one of the older and the other of +the younger drift sheets. + + [Illustration: Fig. 362. Photograph of Relief Map of the United + States at the Time of the Wisconsin Ice Invasion + + By the courtesy of E. E. Howell, Washington, D.C.] + +=The Kansan formation.= The Kansan drift consists for the most part of +a sheet of clayey till carrying smaller bowlders than the later drift. +Few traces of drumlins, kames, or terminal moraines are found upon the +Kansan drift, and where thick enough to mask the preexisting surface, +it seems to have been spread originally in level plains of till. + +The initial Kansan plain has been worn by running water until there +are now left only isolated patches and the narrow strips and crests of +the divides, which still rise to the ancient level. The valleys of the +larger streams have been opened wide. Their well-developed tributaries +have carved nearly the entire plain to valley slopes (Figs. 50 B, and +59). The lakes and marshes which once marked the infancy of the region +have long since been effaced. The drift is also deeply weathered. The +till, originally blue in color, has been yellowed by oxidation to +a depth of ten and twenty feet and even more, and its surface is +sometimes rusted to terra-cotta red. To a somewhat less depth it has +been leached of its lime and other soluble ingredients. In the +weathered zone its pebbles, especially where the till is loose in +texture, are sometimes so rotted that granites may be crumbled with +the fingers. The Kansan drift is therefore old. + + [Illustration: Fig. 363. Plain of Wisconsin Drift, Iowa] + +=The Wisconsin formation.= The Wisconsin drift sheet is but little +weathered and eroded, and therefore is extremely young. Oxidation has +effected it but slightly, and lime and other soluble plant foods +remain undissolved even at the grass roots. Its river systems are +still in their infancy (Fig. 50, A). Swamps and peat bogs are abundant +on its undrained surface, and to this drift sheet belong the lake +lands of our northern states and of the Laurentian peneplain of +Canada. + +The lake basins of the Wisconsin drift are of several different +classes. Many are shallow sags in the ground moraine. Still more +numerous are the lakes set in hollows among the hills of the terminal +moraines; such as the thousands of lakelets of eastern Massachusetts. +Indeed, the terminal moraines of the Wisconsin drift may often be +roughly traced on maps by means of belts of lakes and ponds. Some +lakes are due to the blockade of ancient valleys by morainic débris, +and this class includes many of the lakes of the Adirondacks, the +mountain regions of New England, and the Laurentian area. Still other +lakes rest in rock basins scooped out by glaciers. In many cases lakes +are due to more than one cause, as where preglacial valleys have both +been basined by the ice and blockaded by its moraines. The Finger +lakes of New York, for example, occupy such glacial troughs. + +Massive _terminal moraines_, which mark the farthest limits to which +the Wisconsin ice advanced, have been traced from Cape Cod and +the islands south of New England, across the Appalachians and the +Mississippi valley, through the Dakotas, and far to the north over the +plains of British America. Where the ice halted for a time in its +general retreat, it left _recessional moraines_, as this variety of +the terminal moraine is called. The moraines of the Wisconsin drift +lie upon the country like great festoons, each series of concentric +loops marking the utmost advance of broad lobes of the ice margin and +the various pauses in their recession. + +Behind the terminal moraines lie wide till plains, in places studded +thickly with drumlins, or ridged with an occasional esker. Great +outwash plains of sand and gravel lie in front of the moraine belts, +and long valley trains of coarse gravels tell of the swift and +powerful rivers of the time. + +=The loess of the Mississippi valley.= A yellow earth, quite like +the loess of China, is laid broadly as a surface deposit over +the Mississippi valley from eastern Nebraska to Ohio outside the +boundaries of the Iowan and the Wisconsin drift. Much of the loess was +deposited in Iowan times. It is younger than the earlier drift sheets, +for it overlies their weathered and eroded surfaces. It thickens to +the Iowan drift border, but is not found upon that drift. It is older +than the Wisconsin, for in many places it passes underneath the +Wisconsin terminal moraines. In part the loess seems to have been +washed from glacial waste and spread in sluggish glacial waters, and +in part to have been distributed by the wind from plains of aggrading +glacial streams. + + [Illustration: Fig. 364. Bank of Loess, Iowa] + +=The effects of the ice invasions on rivers.= The repeated ice +invasions of the Pleistocene profoundly disarranged the drainage +systems of our northern states. In some regions the ancient valleys +were completely filled with drift. On the withdrawal of the ice the +streams were compelled to find their way, as best they could, over a +fresh land surface, where we now find them flowing on the drift in +young, narrow channels. But hundreds of feet below the ground the +well driller and the prospector for coal and oil discover deep, +wide, buried valleys cut in rock,--the channels of preglacial and +interglacial streams. In places the ancient valleys were filled with +drift to a depth of a hundred feet, and sometimes even to a depth of +four hundred and five hundred feet. In such valleys, rivers now flow +high above their ancient beds of rock on floors of valley drift. Many +of the valleys of our present rivers are but patchworks of preglacial, +interglacial, and postglacial courses (Fig. 366). Here the river winds +along an ancient valley with gently sloping sides and a wide alluvial +floor perhaps a mile or so in width, and there it enters a young, +rock-walled gorge, whose rocky bed may be crossed by ledges over which +the river plunges in waterfalls and rapids. + + [Illustration: Fig. 365. Preglacial Drainage, Upper Ohio Valley + + After Chamberlain and Leverett] + + [Illustration: Fig. 366. A Patchwork Valley + + _a_ and _a´_, ancient courses still occupied by the river; + _b_, postglacial gorge; _c_, ancient course now filled with drift] + +In such cases it is possible that the river was pushed to one side +of its former valley by a lobe of ice, and compelled to cut a new +channel in the adjacent uplands. A section of the valley may have been +blockaded with morainic waste, and the lake formed behind the barrier +may have found outlet over the country to one side of the ancient +drift-filled valley. In some instances it would seem that during the +waning of the ice sheets, glacial streams, while confined within walls +of stagnant ice, cut down through the ice and incised their channels +on the underlying country, in some cases being let down on old river +courses, and in other cases excavating gorges in adjacent uplands. + +=Pleistocene lakes.= Temporary lakes were formed wherever the ice +front dammed the natural drainage of the region. Some, held in the +minor valleys crossed by ice lobes, were small, and no doubt many were +too short-lived to leave lasting records. Others, long held against +the northward sloping country by the retreating ice edge, left in +their beaches their clayey beds, and their outlet channels permanent +evidences of their area and depth. Some of these glacial lakes are +thus known to have been larger than any present lake. + +Lake Agassiz, named in honor of the author of the theory of +continental glaciation, is supposed to have been held by the united +front of the Keewatin and the Labrador ice fields as they finally +retreated down the valley of the Red River of the North and the +drainage basin of Lake Winnipeg. From first to last Lake Agassiz +covered a hundred and ten thousand square miles in Manitoba and the +adjacent parts of Minnesota and North Dakota,--an area larger than all +the Great Lakes combined. It discharged its waters across the divide +which held it on the south, and thus excavated the valley of the +Minnesota River. The lake bed--a plain of till--was spread smooth and +level as a floor with lacustrine silts. Since Lake Agassiz vanished +with the melting back of the ice beyond the outlet by the Nelson River +into Hudson Bay, there has gathered on its floor a deep humus, rich in +the nitrogenous elements so needful for the growth of plants, and it +is to this soil that the region owes its well-known fertility. + +=The Great Lakes.= The basins of the Great Lakes are broad preglacial +river valleys, warped by movements of the crust still in progress, +enlarged by the erosive action of lobes of the continental ice sheets, +and blockaded by their drift. The complicated glacial and postglacial +history of the lakes is recorded in old strand lines which have been +traced at various heights about them, showing their areas and the +levels at which their waters stood at different times. + +With the retreat of the lobate Wisconsin ice sheet toward the north +and east, the southern and western ends of the basins of the Great +Lakes were uncovered first; and here, between the receding ice front +and the slopes of land which faced it, lakes gathered which increased +constantly in size. + +The lake which thus came to occupy the western end of the Lake +Superior basin discharged over the divide at Duluth down the St. Croix +River, as an old outlet channel proves; that which held the southern +end of the basin of Lake Michigan sent its overflow across the divide +at Chicago via the Illinois River to the Mississippi; the lake which +covered the lowlands about the western end of Lake Erie discharged its +waters at Fort Wayne into the Wabash River. + +The ice still blocked the Mohawk and St. Lawrence valleys on the east, +while on the west it had retreated far to the north. The lakes become +confluent in wide expanses of water, whose depths and margins, as +shown by their old lake beaches, varied at different times with the +position of the confining ice and with warpings of the land. These +vast water bodies, which at one or more periods were greater than all +the Great Lakes combined, discharged at various times across the +divide at Chicago, near Syracuse, New York, down the Mohawk valley, +and by a channel from Georgian Bay into the Ottawa River. Last of all +the present outlet by the St. Lawrence was established. + +The beaches of the glacial lakes just mentioned are now far from +horizontal. That of the lake which occupied the Ontario basin has an +elevation of three hundred and sixty-two feet above tide at the west +and of six hundred and seventy-five feet at the northeast, proving +here a differential movement of the land since glacial times amounting +to more than three hundred feet. The beaches which mark the successive +heights of these glacial lakes are not parallel; hence the warping +began before the Glacial epoch closed. We have already seen that the +canting of the region is still in progress. + +=The Champlain subsidence.= As the Glacial epoch approached its end, +and the Labrador ice field melted back for the last time to near its +source, the land on which the ice had lain in eastern North America +was so depressed that the sea now spread far and wide up the St. +Lawrence valley. It joined with Lake Ontario, and extending down the +Champlain and Hudson valleys, made an island of New England and the +maritime provinces of Canada. + +The proofs of this subsidence are found in old sea beaches and +sea-laid clays resting on Wisconsin till. At Montreal such terraces +are found six hundred and twenty feet above sea level, and along Lake +Champlain--where the skeleton of a whale was once found among them--at +from five hundred to four hundred feet. The heavy delta which the +Mohawk River built at its mouth in this arm of the sea now stands +something more than three hundred feet above sea level. The clays of +the Champlain subsidence pass under water near the mouth of the +Hudson, and in northern New Jersey they occur two hundred feet below +tide. In these elevations we have measures of the warping of the +region since glacial times. + +=The western United States in glacial times.= The western United +States was not covered during the Pleistocene by any general ice +sheet, but all the high ranges were capped with permanent snow and +nourished valley glaciers, often many times the size of the existing +glaciers of the Alps. In almost every valley of the Sierras and the +Rockies the records of these vanished ice streams may be found in +cirques, glacial troughs, roches moutonnées, and morainic deposits. + +It was during the Glacial epoch that Lakes Bonneville and Lahontan +were established in the Great Basin, whose climate must then have been +much more moist than now. + + [Illustration: Fig. 367. A Valley in the Driftless Area] + +=The driftless area.= In the upper Mississippi valley there is an +area of about ten thousand square miles in southwestern Wisconsin +and the adjacent parts of Iowa and Minnesota, which escaped the ice +invasions. The rocks are covered with residual clays, the product of +long preglacial weathering. The region is an ancient peneplain, +uplifted and dissected in late Tertiary times, with mature valleys +whose gentle gradients are unbroken by waterfalls and rapids. Thus the +driftless area is in strong contrast with the immature drift topography +about it, where lakes and waterfalls are common. It is a bit of +preglacial landscape, showing the condition of the entire region before +the Glacial epoch. + +The driftless area lay to one side of the main track of both the +Keewatin and the Labrador ice fields, and at the north it was +protected by the upland south of Lake Superior, which weakened and +retarded the movement of the ice. + +South of the driftless area the Mississippi valley was invaded at +different times by ice sheets from the west,--the Kansan and the +Iowan,--and again by the Illinoian ice sheet from the east. Again and +again the Mississippi River was pushed to one side or the other of its +path. The ancient channel which it held along the Illinoian ice front +has been traced through southeastern Iowa for many miles. + + [Illustration: Fig. 368. Cross Section of a Valley in Eastern Iowa + + _a_, country rock; _b_, Kansan till; _c_, loess; _t_, terrace + of reddish sands and decayed pebbles above reach of present + stream; _s_, stream; _fp_, flood plain of _s_. What is the age + of rock-cut valley and of the alluvium which partially fills + it, compared with that of the Kansan till? with that of the + loess? Give the complete history recorded in the section.] + +=Benefits of glaciation.= Like the driftless area, the preglacial +surface over which the ice advanced seems to have been well dissected +after the late Tertiary uplifts, and to have been carved in many +places to steep valley slopes and rugged hills. The retreating ice +sheets, which left smooth plains and gently rolling country over the +wide belt where glacial deposition exceeded glacial erosion, have made +travel and transportation easier than they otherwise would have been. + +The preglacial subsoils were residual clays and sands, composed of the +insoluble elements of the country rock of the locality, with some +minglings of its soluble parts still undissolved. The glacial subsoils +are made of rocks of many kinds, still undecayed and largely ground to +powder. They thus contain an inexhaustible store of the mineral foods +of plants, and in a form made easily ready for plant use. + +On the preglacial hillsides the humus layer must have been +comparatively thin, while the broad glacial plains have gathered deep +black soils, rich in carbon and nitrogen taken from the atmosphere. +To these soils and subsoils a large part of the wealth and prosperity +of the glaciated regions of our country must be attributed. + +The ice invasions have also added very largely to the water power of +the country. The rivers which in preglacial times were flowing over +graded courses for the most part, were pushed from their old valleys +and set to flow on higher levels, where they have developed waterfalls +and rapids. This power will probably be fully utilized long before the +coal beds of the country are exhausted, and will become one of the +chief sources of the national wealth. + +=The Recent epoch.= The deposits laid since glacial times graduate +into those now forming along the ocean shores, on lake beds, and in +river valleys. Slow and comparatively slight changes, such as the +warpings of the region of the Great Lakes, have brought about the +geographical conditions of the present. The physical history of the +Recent epoch needs here no special mention. + + +The Life of the Quaternary + +During the entire Quaternary, invertebrates and plants suffered little +change in species,--so slowly are these ancient and comparatively +simple organisms modified. The Mammalia, on the other hand, have +changed much since the beginning of Quaternary time: the various +species of the present have been evolved, and some lines have become +extinct. These highly organized vertebrates are evidently less stable +than are lower types of animals, and respond more rapidly to changes +in the environment. + +=Pleistocene mammals.= In the Pleistocene the Mammalia reached their +culmination both in size and in variety of forms, and were superior +in both these respects to the mammals of to-day. In Pleistocene times +in North America there were several species of bison,--one whose +widespreading horns were ten feet from tip to tip,--a gigantic moose +elk, a giant rodent (Castoroides) five feet long, several species of +musk oxen, several species of horses,--more akin, however, to zebras +than to the modern horse,--a huge lion, several saber-tooth tigers, +immense edentates of several genera, and largest of all the mastodon +and mammoth. + + [Illustration: Fig. 369. Megatherium] + + [Illustration: Fig. 370. Glyptodon] + +The largest of the edentates was the Megatherium, a. clumsy ground +sloth bigger than a rhinoceros. The bones of the Megatherium are +extraordinarily massive,--the thigh bone being thrice as thick as +that of an elephant,--and the animal seems to have been well able to +get its living by overthrowing trees and stripping off their leaves. +The Glyptodon was a mailed edentate, eight feet long, resembling the +little armadillo. These edentates survived from Tertiary times, and in +the warmer stages of the Pleistocene ranged north as far as Ohio and +Oregon. + +The great proboscidians of the Glacial epoch were about the size of +modern elephants, and somewhat smaller than their ancestral species in +the Pliocene. The _Mastodon_ ranged over all North America south of +Hudson Bay, but had become extinct in the Old World at the end of the +Tertiary. The elephants were represented by the _Mammoth_, which +roamed in immense herds from our middle states to Alaska, and from +Arctic Asia to the Mediterranean and Atlantic. + +It is an oft-told story how about a century ago, near the Lena River +in Siberia, there was found the body of a mammoth which had been +safely preserved in ice for thousands of years, how the flesh was +eaten by dogs and bears, and how the eyes and hoofs and portions of +the hide were taken with the skeleton to St. Petersburg. Since then +several other carcasses of the mammoth, similarly preserved in ice, +have been found in the same region,--one as recently as 1901. We know +from these remains that the animal was clothed in a coat of long, +coarse hair, with thick brown fur beneath. + + [Illustration: Fig. 371. Skull of Musk Ox, from Pleistocene + Deposits, Iowa] + +=The distribution of animals and plants.= The distribution of species +in the Glacial epoch was far different from that of the present. In +the glacial stages arctic species ranged south into what are now +temperate latitudes. The walrus throve along the shores of Virginia +and the musk ox grazed in Iowa and Kentucky. In Europe the reindeer +and arctic fox reached the Pyrenees. During the Champlain depression +arctic shells lived along the shore of the arm of the sea which +covered the St. Lawrence valley. In interglacial times of milder +climate the arctic fauna-flora retreated, and their places were taken +by plants and animals from the south. Peccaries, now found in Texas, +ranged into Michigan and New York, while great sloths from South +America reached the middle states. Interglacial beds at Toronto, +Canada, contain remains of forests of maple, elm, and papaw, with +mollusks now living in the Mississippi basin. + +What changes in the forests of your region would be brought about, and +in what way, if the climate should very gradually grow colder? What +changes if it should grow warmer? + +On the Alps and the highest summits of the White Mountains of New +England are found colonies of arctic species of plants and insects. +How did they come to be thus separated from their home beyond the +arctic circle by a thousand miles and more of temperate climate +impossible to cross? + +=Man.= Along with the remains of the characteristic animals of the +time which are now extinct there have been found in deposits of the +Glacial epoch in the Old World relics of Pleistocene _Man_, his bones, +and articles of his manufacture. In Europe, where they have best been +studied, human relics occur chiefly in peat bogs, in loess, in caverns +where man made his home, and in high river terraces sometimes eighty +and a hundred feet above the present flood plains of the streams. + +In order to understand the development of early man, we should know +that prehistoric peoples are ranked according to the materials of +which their tools were made and the skill shown in their manufacture. +There are thus four well-marked stages of human culture preceding the +written annals of history: + + 4 The Iron stage. + 3 The Bronze stage. + 2 The Neolithic (recent stone) stage. + 1 The Paleolithic (ancient stone) stage. + +In the Neolithic stage the use of the metals had not yet been learned, +but tools of stone were carefully shaped and polished. To this stage +the North American Indian belonged at the time of the discovery of the +continent. In the Paleolithic stage, stone implements were chipped to +rude shapes and left unpolished. This, the lowest state of human +culture, has been outgrown by nearly every savage tribe now on earth. +A still earlier stage may once have existed, when man had not learned +so much as to shape his weapons to his needs, but used chance pebbles +and rock splinters in their natural forms; of such a stage, however, +we have no evidence. + + [Illustration: Fig. 372. Paleolithic Implement from Great Britain] + +=Paleolithic man in Europe.= It was to the Paleolithic stage that the +earliest men belonged whose relics are found in Europe. They had +learned to knock off two-edged flakes from flint pebbles, and to work +them into simple weapons. The great discovery had been made that fire +could be kindled and made use of, as the charcoal and the stones +discolored by heat of their ancient hearths attest. Caves and shelters +beneath overhanging cliffs were their homes or camping places. +Paleolithic man was a savage of the lowest type, who lived by hunting +the wild beasts of the time. + +Skeletons found in certain caves in Belgium and France represent +perhaps the earliest race yet found in Europe. These short, +broad-shouldered men, muscular, with bent knees and stooping gait, +low-browed and small of brain, were of little intelligence and yet +truly human. + +The remains of Pleistocene man are naturally found either in caverns, +where they escaped destruction by the ice sheets, or in deposits +outside the glaciated area. In both cases it is extremely difficult, +or quite impossible, to assign the remains to definite glacial or +interglacial times. Their relative age is best told by the fauna with +which they are associated. Thus the oldest relics of man are found +with the animals of the late Tertiary or early Quaternary, such as a +species of hippopotamus and an elephant more ancient than the mammoth. +Later in age are the remains found along with the mammoth, cave bear +and cave hyena, and other animals of glacial time which are now +extinct; while more recent still are those associated with the +reindeer, which in the last ice invasion roamed widely with the +mammoth over central Europe. + + [Illustration: Fig. 373. Paleolithic Sketch on Ivory of the Mammoth] + +=The caves of southern France.= These contain the fullest records of +the race, much like the Eskimos in bodily frame, which lived in +western Europe at the time of the mammoth and the reindeer. The floors +of these caves are covered with a layer of bone fragments, the remains +of many meals, and here are found also various articles of handicraft. +In this way we know that the savages who made these caves their homes +fished with harpoons of bone, and hunted with spears and darts tipped +with flint and horn. The larger bones are split for the extraction of +the marrow. Among such fragments no split human bones are found; this +people, therefore, were not cannibals. Bone needles imply the art of +sewing, and therefore the use of clothing, made no doubt of skins; +while various ornaments, such as necklaces of shells, show how ancient +is the love of personal adornment. Pottery was not yet invented. There +is no sign of agriculture. No animals had yet been domesticated; not +even man's earliest friend, the dog. Certain implements, perhaps used +as the insignia of office, suggest a rude tribal organization and the +beginnings of the state. The remains of funeral feasts in front of +caverns used as tombs point to a religion and the belief in a life +beyond the grave. In the caverns of southern France are found also the +beginnings of the arts of painting and of sculpture. With surprising +skill these Paleolithic men sketched on bits of ivory the mammoth with +his long hair and huge curved tusks, frescoed their cavern walls with +pictures of the bison and other animals, and carved reindeer on their +dagger heads. + + [Illustration: Fig. 374. Restoration of Head of Pithecanthropus + erectus] + +=Early man on other continents.= Paleolithic flints curiously like +those of western Europe are found also in many regions of the Old +World,--in India, Egypt, and Asia Minor,--beneath the earliest +vestiges of the civilization of those ancient seats, and sometimes +associated with the fauna of the Glacial epoch. + +In Java there were found in 1891, in strata early Quaternary or late +Pliocene in age, parts of a skeleton of lower grade, if not of greater +antiquity, than any human remains now known. _Pithecanthropus erectus_, +as the creature has been named, walked erect, as its thigh bone shows, +but the skull and teeth indicate a close affinity with the ape. + +In North America there have been reported many finds of human relics +in valley trains, loess, old river gravels buried beneath lava flows, +and other deposits of supposed glacial age; but in the opinion of some +geologists sufficient proof of the existence of man in America in +glacial times has not as yet been found. + +These finds in North America have been discredited for various +reasons. Some were not made by scientific men accustomed to the +closest scrutiny of every detail. Some were reported after a number of +years, when the circumstances might not be accurately remembered; +while in a number of instances it seems possible that the relics might +have been worked into glacial deposits by natural causes from the +surface. + +Man, we may believe, witnessed the great ice fields of Europe, if not +of America, and perhaps appeared on earth under the genial climate +of preglacial times. Nothing has yet been found of the line of man's +supposed descent from the primates of the early Tertiary, with the +possible exception of the Java remains just mentioned. The structures +of man's body show that he is not descended from any of the existing +genera of apes. And although he may not have been exempt from the law +of evolution,--that method of creation which has made all life on +earth akin,--yet his appearance was an event which in importance +ranks with the advent of life upon the planet, and marks a new +manifestation of creative energy upon a higher plane. There now +appeared intelligence, reason, a moral nature, and a capacity for +self-directed progress such as had never been before on earth. + +=The Recent epoch.= The Glacial epoch ends with the melting of the +ice sheets of North America and Europe, and the replacement of the +Pleistocene mammalian fauna by present species. How gradually the one +epoch shades into the other is seen in the fact that the glaciers +which still linger in Norway and Alaska are the lineal descendants or +the renewed appearances of the ice fields of glacial times. + +Our science cannot foretell whether all traces of the Great Ice Age +are to disappear, and the earth is to enjoy again the genial climate +of the Tertiary, or whether the present is an interglacial epoch and +the northern lands are comparatively soon again to be wrapped in ice. + +=Neolithic man.= The wild Paleolithic men vanished from Europe with +the wild beasts which they hunted, and their place was taken by +tribes, perhaps from Asia, of a higher culture. The remains of +Neolithic man are found, much as are those of the North American +Indians, upon or near the surface, in burial mounds, in shell heaps +(the refuse heaps of their settlements), in peat bogs, caves, recent +flood-plain deposits, and in the beds of lakes near shore where they +sometimes built their dwellings upon piles. + +The successive stages in European culture are well displayed in the +peat bogs of Denmark. The lowest layers contain the polished _stone_ +implements of Neolithic man, along with remains of the _Scotch fir_. +Above are _oak_ trunks with implements of _bronze_, while the higher +layers hold _iron_ weapons and the remains of a _beech_ forest. + +Neolithic man in Europe had learned to make pottery, to spin and weave +linen, to hew timbers and build boats, and to grow wheat and barley. +The dog, horse, ox, sheep, goat, and hog had been domesticated, and, +as these species are not known to have existed before in Europe, it is +a fair inference that they were brought by man from another continent +of the Old World. Neolithic man knew nothing of the art of extracting +the metals from their ores, nor had he a written language. + +The Neolithic stage of culture passes by insensible gradations into +that of the age of bronze, and thus into the Recent epoch. + +In the Recent epoch the progress of man in language, in social +organization, in the arts of life, in morals and religion, has left +ample records which are for other sciences than ours to read; here, +therefore, geology gives place to archæology and history. + +Our brief study of the outlines of geology has given us, it is hoped, +some great and lasting good. To conceive a past so different from the +present has stimulated the imagination, and to follow the inferences +by which the conclusions of our science have been reached has +exercised one of the noblest faculties of the mind,--the reason. We +have learned to look on nature in new ways: every landscape, every +pebble now has a meaning and tells something of its origin and +history, while plants and animals have a closer interest since we have +traced the long lines of their descent. The narrow horizons of human +life have been broken through, and we have caught glimpses of that +immeasurable reach of time in which nebulae and suns and planets run +their courses. Moreover, we have learned something of that orderly and +world-embracing progress by which the once uninhabitable globe has +come to be man's well-appointed home, and life appearing in the +lowliest forms has steadily developed higher and still higher types. +Seeing this process enter human history and lift our race continually +to loftier levels, we find reason to believe that the onward, upward +movement of the geological past is the manifestation of the same wise +Power which makes for righteousness and good and that this unceasing +purpose will still lead on to nobler ends. + + + + +INDEX + + + Aa, lava, 241 + Acadian coal field, 354 + Accretion hypothesis, 304 + Acidic rocks, 249 + Adelsberg grotto, 47 + Adirondacks, 309, 316 + Africa, 357 + Agassiz, Lake, 67, 111, 435 + Agates, 251 + Alabama, 317, 360 + Alaska, 85, 138, 140, 378 + Aletsch glacier, 121 + Algæ, 51, 52 + Algonkian era, 306, 310 + Allegheny Mountains, 90, 224, 326, 403 + Alluvial cones, 98 + Alluvium, 62 + Alps, 118, 121, 141, 210, 211, 212, 223, 229, 349, 427, 443 + Amazon River, 175 + Ammonites, 294, 367, 380, 382 + Amphibians, 364, 383 + Amphicyon, 413 + Amygdules, 250 + Andes, 236, 279 + Angle of repose, 25 + Antarctic continent, 294 + Antecedent streams, 209 + Antelope, 413 + Anthracite, 281 + Anticlinal folds, 203, 209 + Ants, 20 + Apennine Mountains, 399 + Appalachia, 317, 351, 358 + Appalachian coal field, 356 + Appalachian deformation, 358 + Appalachian Mountains, 211, 214, 218, 292 + Aquifer, 44 + Aragonite, 296 + Archæopteryx, 393 + Archean era, 305 + Arenaceous rocks, 9 + Argillaceous rocks, 9 + Arizona, 32, 76, 140, 151, 164, 220, 229, 249, 257, 371, 390 + Arkansas, 337, 356, 373 + Arkose, 186, 282, 370 + Artesian wells, 44 + Arthropods, 322 + Artiodactyls, 411 + Assiniboine, Mount, 34 + Atlas Mountains, 399 + Atmosphere, 304, 305 + Atolls, 191, 193 + Augite, 274 + Austin, Tex., 71 + Australia, 190, 357 + Avalanches, 26 + + Bad Lands, 397, 398 + Baltic Sea, 170, 171, 199 + Barite, 287 + Barrier Reefs, 191, 192 + Basal conglomerate, 173, 184 + Basalt, 249 + Baselevel, 80, 83 + Basic rocks, 249 + Basin deposits, 103 + Bay bars, 164 + Beaches, 162, 164 + Bears, 413 + Bedding planes, 5 + Belemnites, 382 + Belt Mountains, 309 + Bergschrund, 121, 135, 137 + Bermudas, 148 + Birds, 392 + Bison, 413 + Bitter Root Mountains, 272 + Black Hills, 309, 371 + Blastoids, 339 + Blastosphere, 311 + Block mountains, 222, 226 + Blowholes, 159 + Blue Ridge, 309, 316 + Bomb, volcanic, 256 + Bonneville, Lake, 107, 488 + Bosses, 270 + Bowlders, erratic, 420 + of weathering, 28 + Brachiopods, 328, 383, 343, 364, 380 + Brazil, 18, 286 + Breccia, 218, 255, 264 + British Columbia, 373, 378 + Bronze stage, 443, 448 + Bryozoans, 333 + Bunker Hill, 422 + + Calamites, 361, 367 + Calcareous rocks, 9 + Calciferous series, 327 + Calcite, 290 + Caldera, 239 + California, 24, 99, 186, 152, 158, 169, 170, 197, 224, 256, 262, 287, + 357, 360, 371, 400 + Great Valley of, 101, 199, 372, 396 + Cambrian period, 315 + glaciation in, 358 + life of, 319 + Camels, 412 + Canada, 28, 86, 67, 69, 90, 182, 198, 200, 218, 218, 267, 307, 309, + 316, 336, 364, 367, 482, 487 + Cape Breton Island, 198 + Cape Cod, 162 + Carbonated springs, 261 + Carbonates, formation of, 12 + Carboniferous period, 350 + life of, 301 + Carnivores, 418 + Cascade Mountains, 00, 400 + Cats, 418 + Catskill Mountains, 342 + Caucasus Mountains, 399 + Caverns, 46, 241 + Cenozoic era, 394 + Centipedes, 388 + Cephalopods, 324, 388, 389, 344, 367, 380 + Ceratites, 380 + Ceratosaurus, 385 + Chain coral, 389, 843 + Chalcopyrite, 287 + Chalk, 9, 374, 375 + Chalybeate springs, 62 + Champlain subsidence, 487 + Charleston earthquake, 288 + Chazy series, 327 + Chelan, Lake, 141 + Chemung series, 341, 342 + Chesapeake Bay, 169, 170, 197 + Chicago, 146, 198, 486 + Chile, 286 + China, 28, 161 + Christmas Island, 194, 248 + Cincinnati anticline, 329, 366 + Cirques, 136 + Clinton series, 336 + Coal, 362, 370, 375 + Coal Measures, 351 + Coast Range, 101, 372, 399 + Coastal plain, Atlantic, 188 + Coelenterates, 320 + Coke, 271 + Colorado, 18, 29, 88, 87, 158, 288, 266, 271, 334 + Colorado plateaus, 357, 403 + Colorado River, 80, 76, 140, 154, 228, 307, 318, 317 + Columbia lavas, 400 + Columnar structure, 268 + Concretions, 49 + Cones, alluvial, 98 + volcanic, 267 + Conglomerate, 9, 178 + Congo River, 175 + Conifers, 377 + Connecticut, 370 + valley, 408 + Contemporaneous lava sheets, 248, 268 + Continental delta, 176, 183 + Continental shelf, 183 + Continents, 188 + Contours, 60 + Copper, 287, 310 + Coquina, 177 + Coral reefs, 188 + Corals, ancient, 321, 332, 338, 379 + Cordaites, 363 + Cordilleran ice field, 426 + Corniferous series, 341 + Coves, 161 + Crabs, 379 + Crandall volcano, 268, 400 + Crater Lake, 259 + Creodonts, 418 + Cretaceous period, 372 + Crinoids, 382, 303, 379 + Crocodiles, 384 + Cross bedding, 65, 182 + Crustacea, 322, 382, 368, 379 + Crustal movements, 195 + Cumberland plateau, 90 + Cup corals, 388 + Cycads, 377, 378 + Cycle of erosion, 84, 185, 292 + Cystoids, 321, 382, 367 + + Dalmatia, 170 + Darwin's theory of coral reefs, 191 + Dead Sea, 221, 279 + Death Gulch, 264 + Deep-sea deposits, 187 + Deer, 413 + Deflation, 152 + Deformation, 279 + Delaware River, 197, 403 + Deltas, 108, 111, 197 + of Ganges, 109 + of Indus, 110 + of Mississippi, 109, 197 + Denudation, 57 + Denver, 398 + Desert, 15, 55 + Devitrification, 257 + Devonian period, 316, 341 + Dicotyls, 377, 404 + Digitigrade, 406 + Dikes, 244, 265 + Dinosaurs, 385 + Dinothere, 410 + Diorite, 274 + Dip, 202 + Dip fault, 225 + Diplodocus, 286 + Dipnoans, 346 + Discina, 324 + Dismal Swamp, 106 + Dogs, 413 + Dragon flies, 364 + Drift, 18, 113, 416 + bowlders of, 420 + englacial, 125 + extent of, 425 + pebbles of, 114, 420 + stratified, 423 + thickness of, 429 + Driftless area, 438 + Drowned valleys, 197 + Drumlins, 421 + Duluth, 436 + Dunes, 147 + Dust falls, 145 + + Earth, age of, 292, 298, 302 + interior of, 276 + Earthquakes, 224, 233 + causes of, 233, 237 + Charleston, 233 + distribution of, 236 + geological effects of, 234 + India, 236 + Japan, 237 + New Madrid, 236 + Earthworms, 20, 21 + Echinoderms, 321, 332, 333, 343, 363 + Edentates, 441 + Egypt, 98 + Electric Peak, 269 + Elephants, 410 + Elevation, effects of, 85 + movements of, 197 + Eocene epoch, 395 + Epicontinental seas, 318 + Erratics, 133, 420 + Eskers, 424 + Etna, 248, 402 + Europe, Pleistocene ice sheet of, 427 + Eurypterids, 333, 339, 363, 367 + Evolution, 300, 447 + + Faceted pebbles, 113, 114, 420 + Falls of the Ohio, 343 + Fan folds, 205 + Fault scarps, 219 + Faults, 217 + Faunas, 299 + Feldspar, 9, 10, 42 + Ferns, 361 + Finger lakes, 432 + Fire clay, 353 + Fishes, 334, 339, 345, 364, 405 + Fissure eruptions, 242 + Fissure springs, 44 + Fjords, 139, 142 + Flint, 18, 375 + Flood plains, 85, 93 + Floods, 54 + Floras, 299 + Florida, 46, 163, 177, 178, 188, 396 + Flow lines, 252 + Fluorite, 287 + Folded mountains, 210 + Folds, 201, 208 + Foliation, 283 + Foraminifera, 187, 374: + Forests, Carboniferous, 354, 361 + Cretaceous, 377, 378 + Devonian, 343 + Tertiary, 404 + Fort Wayne, 436 + Fossils, 177, 296 + Fractures, 215 + Fragmental rocks, 8 + France, 167, 171 + cave men of, 445 + Fringing reefs, 190 + Frogs, 383 + Frost, 15 + Fundy, Bay of, 182 + + Gabbro, 274 + Ganges, 58, 109, 197 + Ganoids, 347 + Garnet, 281 + Gases, volcanic, 244 + Gastropods, 324 + Gastrula, 311 + Geneva, Lake, 71 + Geodes, 49 + Geological time, divisions of, 295 + Geology, definition of, 1, 3 + departments of, 4 + Georgia, 18, 373 + Geysers, 52, 260 + Glacial epoch, 142, 416 + Glaciers, 113 + abrasion by, 133 + Alpine, 118 + compared with rivers, 137, 142 + crevasses of, 123 + deposition by, 138 + Greenland, 116 + lower limit of, 129 + melting of, 126 + mode of formation, 118 + moraines, 124 + motion of, 120, 122, 134 + piedmont, 131, 141 + plucking by, 133 + tables, 130 + transportation by, 132 + troughs, 137 + wells, 129 + young and mature, 129 + Glauconite, 176 + Globigerina ooze, 187 + Glyptodon, 441 + Gneiss, 283 + Goats, 413 + Gold, 287, 372 + Goniatite, 344, 367 + Graded slopes, 25 + Granite, 9, 274 + Graphite, 312 + Graptolites, 320, 339 + Gravitation, 22 + Great Basin, 357, 360, 374, 376 + Great Lakes, 198, 436 + Great Plains, 82 + Great Salt Lake, 107 + Greenland, 115, 126, 378 + Green Mountains, 309, 316, 420 + Green sand, 176 + Ground water, 39 + Ground water surface, 40 + Gryphæa, 379 + Gymnosperms, 363, 377 + Gypsum, 12, 335, 357, 371 + + Hade, 217 + Hamilton series, 341 + Hanging valley, 1389 + Hanging wall, 217 + Hartz Mountains, 214 + Hawaiian volcanoes, 238, 248, 258, 279 + Heat and cold, 13 + Helderberg series, 341 + Hematite, 310 + Henry Mountains, 271, 376 + High Plains, 100, 398 + Hillers Mountain, 271 + Himalaya Mountains, 122, 209, 210, 399 + Historical geology, 4, 291 + Honeycomb corals, 339 + Hood, Mount, 260, 262 + Hooks, 165 + Hornblende, 274 + Hornblende schist, 284 + Hudson Bay, 90, 170 + Hudson River, 197, 417 + Hudson series, 327, 329 + Humus acids, 10 + Humus layer, 19 + Huronian systems, 308 + Hwang-ho River, 151 + Hydrosphere, 22 + Hydrozoa, 320 + + Icebergs, 116, 148 + Iceland, 242, 258 + Ichthyosaurus, 389 + Idaho, 34, 400 + Igneous rocks, 9, 249, 250, 251, 273 + Illinoian formation, 429 + Illinois, 54, 146, 356, 374 + India, 28, 102, 147, 235, 357, 402 + Indian Territory, 356 + Indiana, 48, 104 + Indo-Gangetic plain, 101 + Indus River, 101, 110 + Insects, 333, 364, 380 + Interior of earth, 276 + Internal geological agencies, 195 + Intrusive masses, 270 + Intrusive rocks, 273 + Intrusive sheets, 268 + Inverness earthquake, 236 + Iowa, 29, 69, 73, 80, 86, 336, 356, 374, 431, 433, 439, 442 + Iowan formation, 429 + Iron ores, 13, 53, 279, 310 + Islands, coral, 188 + wave cut, 159, 161 + + Japan, 223, 224, 237 + Joints, 5, 31, 216 + Jordan valley, 279 + Jura Mountains, 141, 212 + Jurassic period, 369 + + Kame terraces, 424 + Kames, 424 + Kansan formation, 429 + Kansas, 41, 50, 100, 336, 357, 373, 374, 429 + Kaolin, 12 + Karst, 47 + Katahdin, Mount, 420 + Keewatin ice field, 425 + Kentucky, 45, 46, 343, 442 + Keweenawan system, 308, 310 + Kilauea, 239 + Kings River Canyon, 403 + Krakatoa, 245 + + Labrador, 198 + Labrador ice field, 426 + Laccolith, 271 + Lagoon, 165, 167 + Lahontan, Lake, 107, 438 + Lake Chelan, 141 + Lake dwellings, 448 + Lake Geneva, 71 + Lake Superior region, 284, 308, 310 + Lakes, 70, 222, 432 + basins, 97, 110, 127, 139, 141, 164, 165, 167, 191, 221, 222, 235, + 259, 423, 432, 435 + deposits, 104 + glacial, 127, 139, 141, 423, 432, 435 + Pleistocene, 435 + salt, 106 + Laminæ, 5 + Landslides, 26, 234 + Lapilli, 255 + Laramie series, 375 + Laurentian peneplain, 84, 308, 432 + Lava, 238, 241 + Lava domes, 243, 400 + Lepidodendron, 362, 367 + Lichens, 16 + Lignite, 271 + Limestone, 7, 177, 178, 190 + Limonite, 13 + Lingulella, 324 + Lithosphere, 21 + Lizards, 384 + Llamas, 412 + Loess, 150, 433 + Long Island, 373 + Louisiana, 336, 396 + Lower Silurian period, 327 + Luray Cavern, 48 + Lycopods, 362 + + Magnetite, 279, 310 + Maine, 169, 420 + Malaspina glacier, 181 + Maldive Archipelago, 198 + Mammals, 393, 406, 440 + Mammoth, 442 + Mammoth Cave, 46 + Mammoth Hot Springs, 52 + Man, 414, 443 + Mantle of waste, 17 + Marble, 284, 329 + Marengo Cavern, 48 + Marl, 104 + Marsupials, 393, 406 + Martha's Vineyard, 161, 373, 395 + Maryland, 56, 270 + Massachusetts, 106, 162, 257, 309, 408, 417, 429 + Mastodon, 410, 441, 442 + Matterhorn, 34 + Maturity of land forms, 80 + Mauna Loa, 239 + Meanders, 96 + Medina series, 335, 403 + Megatherium, 441 + Mendota, Lake, 71 + Mesa, 31, 32, 153 + Mesozoic era, 369 + Mesozoic peneplain, 376, 403 + Metamorphism, 281 + Mexico, 373, 375 + Mica, 9 + Mica schist, 284 + Michigan, 104, 356, 443 + Michigan, Lake, 149, 198 + Mineral veins, 49, 286 + Minnesota, 97, 426 + Miocene series, 395 + Mississippi, 337 + Mississippi embayment, 373, 374, 395 + Mississippi River, 56, 57, 82, 94, 96, 109 + Mississippian series, 350 + Missouri, 18, 236 + Missouri River, 55, 97 + Mobile Bay, 197 + Mohawk valley, 436, 437 + Molluscous shell deposits, 177 + Mollusks, 324 + Monadnock,83 + Monkeys, 414 + Monoclinal fold, 204 + Monocotyls, 377, 404 + Monotremes, 393, 406 + Montana, 71, 309, 313, 373 + Montreal, 268, 437 + Monuments, 33 + Moraines, 124 + Mosasaurs, 390 + Mountain sheep, 413 + Mountains, age of, 229 + life history of, 212, 215 + origin of, 90, 210, 222 + sculpture of, 33, 137 + Movements of crust, 195 + Muir glacier, 122, 129 + + Nantucket, 373 + Naples, 201 + Narragansett Bay, 197 + Natural bridges, 46 + Natural gas, 330 + Natural levees, 93 + Nautilus, 334 + Nebraska, 50, 82, 100, 255, 356 + Nebular hypothesis, 304 + Neolithic man, 443, 448 + Nevada, 104, 107, 222, 288, 289, 360, 400 + Névé, 120 + New Brunswick, 198 + New England, 88, 373, 376, 378, 395, 403, 429, 432, 437 + Newfoundland, 198 + New Jersey, 148, 166, 168, 176, 196, 268, 269, 309, 310, 373, 437 + New Madrid earthquake, 236 + New Mexico, 31, 371, 399 + New York, 60, 90, 309, 327, 329, 335, 336, 360, 421, 422, 423, 424, + 432, 448 + Niagara Falls, 60, 199 + Niagara series, 335 + Nile, 93, 109, 197 + Normal fault, 217 + North Carolina, 106 + North Dakota, 67 + North Sea, 170 + Notochord, 347 + Nova Scotia, 198 + Nunatak, 116, 132 + + Ohio, 82, 198, 329, 335, 441 + Ohio River, 55, 82 + Oil, 330 + Olenellus zone, 328 + Olivine, 274 + Oolitic limestone, 178 + Ooze, deep-sea, 131 + Ordovician period, 316, 327 + life of, 331 + Oregon, 222, 262, 400 + Oreodon, 412 + Ores, 287, 290 + Organisms, work of, 16 + Oriskany series, 341 + Ornithostoma, 392 + Orthoceras, 325, 367, 380 + Oscillations, 196 + a cause of, 273 + effect on drainage, 85 + Ostracoderms, 344 + Ottawa River, 90 + Outcrop, 2 + Outliers, 31 + Outwash plains, 425 + Oxidation, 13 + Oyster, 379, 380 + + Pahoehoe lava,241 + Palæospondylus, 344 + Paleolithic man, 444 + Paleozoic era, 315 + Palisades of Hudson, 268 + Palms, 377 + Pamir, 15 + Peat, 94, 104 + Peccaries, 412 + Pelecypods, 324 + Pelée, Mt., 246 + Peneplain, 83 + dissected, 86 + Laurentian, 89, 308, 402 + Mesozoic, 376, 403 + Pennsylvania, 35, 211, 257, 357, 359, 403 + Pennsylvanian series, 350, 351 + Perissodactyl, 408 + Perlitic structure, 252 + Permian series, 350, 357, 360, 366 + Petrifaction, 296 + Petroleum, 330, 343 + Phenacodus, 406 + Phyllite, 283 + Phyllopod, 323 + Piedmont Belt, 87, 214, 309, 374 + Piedmont plains, 99 + Pikes Peak, 18 + _Pithecanthropus erectus_, 446 + Placers, 287 + Plains of marine abrasion, 172 + Planation, 81 + Plantigrade, 406 + Platte River, 82 + Playa, 103 + Playa lakes, 104 + Pleistocene epoch, 416 + Plesiosaurus, 389, 390 + Pliocene epoch, 395 + Plucking, 133 + Po River, 58, 197 + Pocono sandstone, 350, 404 + Porosity of rocks, 40 + Porphyritic structure, 252 + Potholes, 59 + Potomac River, 58, 66, 403 + Predentata, 386 + Pre-Kansan formation, 429 + Primates, 414 + Prince Edward Island, 198 + Proboscidians, 410, 441, 442 + Pteropods, 325 + Pterosaurs, 391 + Puget Sound, 396 + Pumice, 250 + Pyrite, 13 + + Quarry water, 15 + Quartz, 6, 9 + Quartz schist, 284 + Quaternary period, 395, 416 + Quebec, 28 + + Rain, erosion, 23 + Rain prints, 181 + Recent epoch, 416, 440, 447 + Reconcentration of ores, 289 + Record, the geological, 291 + Red clay, 187 + Red River of the North, 67 + Red Sea, 221 + Red snow, 115 + Reefs, coral, 188 + Regional intrusions, 272 + Reptiles, 367, 383 + Rhinoceros, 408 + Rhizocarp, 343 + Rhode Island, 356 + Rhone glacier, 123 + Rhyolite, 240 + Richmond, Va., 370 + Rift valleys, 221 + Ripple marks, 180 + Rivers, 54 + bars, 65 + braided channels, 94 + deltas, 108 + deposition, 62 + discharge, 55 + erosion, 59 + estuaries, 85 + flood plains, 93 + floods, 54 + graded, 74 + gradients, 82 + load of, 56 + mature, 72, 80, 97, 98 + meanders, 96 + plains, 99 + profile of, 73 + revived, 85 + run-off, 54 + structure of deposits, 102 + terraces, 96 + transportation, 56, 64 + waterfalls, 78 + young, 67 + Roches moutonnées, 134, 418 + Rock bench, 156 + Hock salt, 12, 357, 371 + Rocky Mountains, 375, 399, 437 + Ruminants, 412 + + Saber-tooth tiger, 413 + Saguenay River, 90, 201 + Sahara, 15, 146, 150 + St. Elias Range, 399 + St. Peter sandstone, 150 + Salamanders, 383 + Salina series, 335 + Salt, common, 106, 335 + Salt lakes, 106 + San Francisco Bay, 197 + Sand, beach, 163 + of deserts, 149 + reefs, 165, 167 + storms, 145 + Sandstone, 6, 7, 186 + Sarcoui, 258 + Sauropoda, 386 + Schist, 283 + Schladebach, 277 + Scoria, 250, 255 + Scorpions, 339, 340, 363 + Scotland, 170, 220, 402 + Sea, 155 + erosion, 156 + deposition, 174 + transportation, 162 + Sea arch, 159 + Sea cave, 158 + Sea cliff, 156, 157 + Sea cucumber, 363 + Seals, 414 + Sea stacks, 169 + Sea urchin, 332, 379 + Seaweed, 176 + Sedimentary rocks, 8, 9 + Selkirk Mountains, 218 + Septa, 338 + Sequoia, 378 + Shale, 8, 9 + Sharks, 345, 405 + Shasta, Mount, 262, 400 + Sheep, 413 + Shenandoah valley, 403 + Shores of elevation, 167 + Shores of depression, 169 + Siderite, 63 + Sierra Nevada Mountains, 24, 90, 99, 224, 229, 272, 287, 318, 357, + 371, 372, 396, 398, 390, 402, 437 + Sigillaria, 362, 367 + Silica, 6, 178 + Silurian period, 316, 334 + life of, 338 + Sink hole, 46 + Slate, 207, 282 + Slaty cleavage, 207 + Slickensides, 217 + Snake River lavas, 400, 401 + Snakes, 384, 405 + Soil, 19 + Solfatara, 260 + Solution, 11 + Soufriére, 246 + South America, 357 + South Carolina, 233 + South Dakota, 276, 374, 397 + Spanish Peaks, 271, 376 + Spherulites, 252 + Spiders, 363 + Spitzbergen, 378 + Sponges, 320, 379 + Springs, 41 + thermal, 50 + Stalactite, 48 + Stalagmite, 48 + Starfishes, 332 + Staubbach, 140 + Stegosaurus, 387 + Stoss side, 134 + Stratification, 5, 64, 180 + Striæ, glacial, 114, 133, 418 + Strike, 203 + Strike fault, 225 + Stromatopora, 331, 379 + Stromboli, 244 + Subsidence, 85, 183, 197 + Sun cracks, 180 + Superior, Lake, 257 + Superposition, law of, 293 + Susquehanna River, 403 + Sutlej River, 209 + Sweden, 199 + Swine, 412 + Switzerland, 28, 427 + Syenite, 274 + Synclinal fold, 204 + Syracuse, N.Y., 436 + Syringopora, 339 + + Tabulæ, 339 + Taconic deformation, 329 + Taconic Mountains, 376 + Talc, 284 + Talc schist, 284 + Talus, 23 + Tapir, 409 + Teleost fishes, 349, 382, 405 + Tennessee, 90, 373 + Terminal moraines, 126, 422, 432 + Terraces, 86, 96 + Tertiary period, 395 + Texas, 15, 69, 71, 166, 336, 356, 357, 371, 373, 374, 378 + Theromorphs, 383 + Throw, 217 + Thrust faults, 217 + Till, 418 + Till plains, 420 + Toronto, 443 + Trachyte, 249, 258 + Travertine, 52 + Trenton series, 327 + Triassic period, 369 + Triceratops, 387 + Trilobites, 322, 332, 339, 363, 367 + Tuff, 255 + Turkestan, 103 + Turtles, 384 + + Unconformity, 227 + Undertow, 174 + Utah, 107, 271, 360, 371, 396, 399 + Utica series, 327 + + V-Valleys, 74 + Valley drift, 128 + Valley trains, 425 + Valleys, 66 + Vermont, 309, 329, 420 + Vernagt glacier, 129 + Vertebrates, 334, 349 + Vesuvius, 247, 259, 402 + Virginia, 48, 84, 106, 370, 403, 442 + Volcanic ashes, 244, 255 + cones, 257 + necks, 267 + rocks, 249 + Volcanoes, 238 + causes of, 278 + decadent, 260 + submarine, 248 + tertiary, 399 + + Walrus, 414 + Warped valleys, 101 + Warping, 198 + Wasatch Mountains, 375 + Washington, 18, 91, 150, 400 + Waterfalls, 59, 78 + Waves, 156 + Weathering, 5 + chemical, 10 + differential, 29 + mechanical, 13 + Wells, 41 + artesian, 44 + West Virginia, 79, 357, 359 + White Mountains, 443 + Wind, 144 + deposition, 147 + erosion, 151 + pebbles carved by, 152 + transportation, 145 + Wisconsin, 15, 18, 70, 71, 90, 94, 422, 426 + Wisconsin formation, 429, 431 + Wyoming, 50, 98, 371 + + Yahtse River, 131 + Yellow Sea, 151, 170 + Yellowstone canyon, 74 + Yellowstone National Park, 50, 51, 52, 260, 261, 263, 269, 400 + Yosemite, 403 + + Zeuglodon, 414 + Zone of cementation, 49, 180 + Zone of solution, 45 + Zones of flow and of fracture, 207 + + + * * * * * + + + + +ANNOUNCEMENTS + + + + +================================================================= + +TEXT BOOKS ON SCIENCE + +FOR HIGHER SCHOOLS AND COLLEGES + + _List _Mailing + price_ price_ + + Bergen's Elements of Botany. (Revised Edition) $1.30 $1.45 + Bergen's Foundations of Botany 1.50 1.70 + Blaisdell's Life and Health .90 1.00 + Blaisdell's Practical Physiology 1.10 1.20 + Brown's Physiology for the Laboratory .75 .85 + Byrd's Laboratory Manual in Astronomy 1.25 1.35 + Davis' Elementary Meteorology 2.50 2.70 + Davis' Elementary Physical Geography 1.25 1.40 + Davis' Physical Geography 1.25 1.40 + Dennis and Whittelsey's Qualitative Analysis 1.00 1.10 + Dolbear's First Principles of Natural Philosophy 1.00 1.10 + Evans' Introductory Course in Quantitative Chemical + Analysis .50 .55 + Frost's Scheiner's Astronomical Spectroscopy 4.75 5.00 + Gage's Principles of Physics 1.30 1.45 + Gage's Elements of Physics. (Revised Edition) 1.12 1.20 + Gage's Physical Experiments .35 .45 + Gage's Physical Laboratory Manual and Notebook .35 .45 + Gage's Introduction to Physical Science 1.00 1.10 + Gage's Introduction to Physical Science. (Revised) 1.00 1.10 + Hastings and Beach's General Physics 2.75 2.95 + Higgins' Lessons in Physics .90 1.00 + Lincoln's Hygienic Physiology .80 .90 + Meier's Herbarium and Plant Description. With directions + for collecting, pressing, and mounting specimens .60 .70 + Moore's Laboratory Directions for Beginners in + Bacteriology 1.00 1.05 + Nichols, Smith, and Turton's Manual of Experimental + Physics. 1.00 .90 + Pratt's Invertebrate Zoölogy. 1.25 1.35 + Sabine's Laboratory Course in Physical Measurements 1.25 1.35 + Sellers' Elementary Treatise on Qualitative Chemical + Analysis .75 .80 + Snyder and Palmer's One Thousand Problems in Physics .50 .55 + Stone's Experimental Physics 1.00 1.10 + Thorp's Inorganic Chemical Preparations 1.50 1.60 + Upton's Star Atlas 2.00 2.15 + Ward's Practical Exercises in Elementary Meteorology 1.12 1.25 + Wentworth and Hill's Text-Book of Physics 1.15 1.25 + Wentworth and Hill's Laboratory Exercises in Elementary + Physics .25 .27 + White's Elementary Chemistry 1.00 1.10 + Williams' Chemical Experiments .60 .50 + Williams' Chemical Exercises .35 .30 + Williams' Elements of Chemistry 1.10 1.20 + Williams' Introduction to Chemical Science .80 .90 + Williams' Laboratory Manual of Inorganic Chemistry .35 .30 + Williams' Laboratory Manual of General Chemistry .25 .30 + Young's Elements of Astronomy 1.60 1.75 + Young's General Astronomy 2.75 3.00 + Young's Lessons in Astronomy. (Revised Edition) 1.25 1.40 + Young's Manual of Astronomy 2.25 2.45 + + +================================================================= + +GINN & COMPANY PUBLISHERS + + + + +Davis' Elementary Physical Geography + +================================================================= + +By WILLIAM MORRIS DAVIS + +_Author of Davis' "Physical Geography" and Professor of Geology in +Harvard University_ + +12mo. Cloth. viii + 401 pages + 6 color charts + 16 pages of maps. +Illustrated. List price, $1.25. + +The "Elementary Physical Geography" is a new book by Professor W. M. +Davis, the first authority in the United States in the field of physical +geography. It retains the characteristic features of the author's +earlier work, "Physical Geography," but is much less difficult and is +admirably adapted for use with younger pupils. + +The plan of this volume, like that of its predecessor, is to give the +problems of physical geography a rational treatment. The object of this +method is not simply to explain physiographic facts, but to increase the +appreciation of the facts themselves by associating them with their +causes and their consequences. This relation is not presented merely +as an afterthought in a detached chapter at the end of the book; it +accompanies the presentation of the facts themselves. + +The chapter on the Atmosphere has been considerably expanded, and an +entirely new chapter has been added on the Distribution of Plants, +Animals, and Man, considered from a physiographic standpoint. The +questions at the end of each chapter, prepared by an experienced +high-school teacher, will be found most useful in class-room work with +this book. + +Especially notable are the illustrations, which include nearly two +hundred splendid woodcuts, five pages of charts in color, and nineteen +full-page half-tone plates from rare photographs. + + * * * * * + +The Nation, July 3, 1902: Taken all in all, this seems the most +satisfactory elementary text-book in physical geography yet published. +Certainly in its treatment of the land it has not been surpassed unless, +perhaps, by the author's larger work ["Physical Geography"]. + +================================================================= + +GINN & COMPANY Publishers + + + + +================================================================= + +GEOGRAPHIC INFLUENCES IN AMERICAN HISTORY + +By ALBERT PERRY BRIGHAM + +Professor of Geology in Colgate University. 12mo. Cloth. 366 pages. +Illustrated. List price, $1.25; mailing price, $1.40 + + * * * * * + +In this new book Professor Brigham has presented vividly and clearly +those physiographic features of America which have been important +in guiding the unfolding of our industrial and national life. The +arrangement is mainly geographical. Among the themes receiving special +treatment are: The Eastern Gateway of the United States, the Appalachian +Barrier, the Great Lakes and American Commerce, the Civil War, and +Mines and Mountain Life. Closing chapters deal with the unity and +diversity of American life and with physiography as affecting American +destiny. + +The book will be found particularly interesting and valuable to +students and teachers of geography and history, but it will also +appeal to the general reader. The very large number of rare and +attractive photographs and the numerous maps are of importance +in vivifying and explaining the text. + +================================================================= + +GINN & COMPANY PUBLISHERS + + + + +================================================================= + +The best example in this country of the kind of books that geography +needs.--_The Nation_ + +THE LAKES OF NORTH AMERICA + +By ISRAEL C. RUSSELL + +Professor of Geology in the University of Michigan + +8vo. Cloth, xi + 125 pages. Illustrated. List price, $1.50; +mailing price, $1.65 + +Recent advances have made physical geography almost a new science. +Professor Russell here presents a single phase, but one of great +importance. The work is based upon his original investigations, and +is at once thoroughly scientific in substance and enjoyably popular +in style. + +As a text-book or reading book for students in geography and geology +the "Lakes of North America" possesses a unique value. Its interest +to the general reader is enhanced by its illustrations and its happy +descriptions of lake scenery. + + +GLACIERS OF NORTH AMERICA + +By ISRAEL C. RUSSELL, Professor of Geology in the University of Michigan +Author of "Lakes of North America" + +8vo. Cloth. x + 210 pages., Illustrated. List price, $1.75; +mailing price, $1.90 + + +Recent explorations have shown that North America contains thousands of +glaciers, some of which are not only vastly larger than any in Europe, +but belong to types of ice bodies not there represented. In the study +of the glaciers of North America, and especially of those in Alaska, +Professor Russell has taken an active part; and this book not only +presents the results of his own explorations but also a condensed and +accurate statement of the present status of glacial investigations. Its +popular character and numerous illustrations will make it of interest to +the general reader. + + * * * * * + +_DEPARTMENT OF SPECIAL PUBLICATION_ + +================================================================= + +GINN & COMPANY PUBLISHERS + + + + + + * * * * * + + +Transcriber's Notes + + +This transcription was derived from the 1905 publication obtained from +The Internet Archive. As the Index of the original 1905 book is missing +entries for U and V, the Index from the 1921 version was used to add the +missing sections. + +One error was noted in preparing this revision (page 493 under Hudson +River should have been 417). Several hyphenated vs. unhyphenated forms +were standardized to the most prevalent. Minor corrections were made +where periods, commas, etc. were missing. A number of paragraphs which +were split by images were rejoined. In some cases, the text was moved to +the preceding or following page. As ALL CAPS was employed for the +Chapter Titles in the original book, the small caps subchapter headings +and Figure captions were not converted to all caps. The oe ligature in +Coelenterates was converted to "oe". + + + + + + + + +End of Project Gutenberg's The Elements of Geology, by William Harmon Norton + +*** END OF THE PROJECT GUTENBERG EBOOK 40404 *** |