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diff --git a/40404.txt b/40404.txt deleted file mode 100644 index c123a0e..0000000 --- a/40404.txt +++ /dev/null @@ -1,14954 +0,0 @@ -Project Gutenberg's The Elements of Geology, by William Harmon Norton - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org/license - - -Title: The Elements of Geology - -Author: William Harmon Norton - -Release Date: August 4, 2012 [EBook #40404] - -Language: English - -Character set encoding: ASCII - -*** START OF THIS PROJECT GUTENBERG EBOOK THE ELEMENTS OF GEOLOGY *** - - - - -Produced by Tom Cosmas - - - - - - -[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 -palaeontological 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 _laminae_. 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 laminae - 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 deg. F. within a -few hours. In the light air of the Pamir plateau in central Asia a -rise of 90 deg. 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 cooeperate 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 deg. 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 debris -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 cooeperation 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 -cooeperation 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 deg. F., 70 deg. 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 deg. 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 debris 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 subaerial 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 subaerial. 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 subaerial deposits -altogether similar to those of the flood plain, of which indeed the -subaerial 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 subaerial 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 cooeperate 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 deg. 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 neve 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 _neve_. Beneath the surface -of the neve field and at its outlet the granular neve has been -compacted to a mass of porous crystalline ice. Snow has been changed -to neve, and neve 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 neve 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 neve 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 neve 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 neves 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 neve 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 neve. 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 Moutonnes, Bronx Park, New York] - -=Roches moutonnees 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 moutonnees (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 neve 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 neve field is filled with snow and the -neve is frozen fast to the rocky sides of the valley. In early summer -the neve 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 neve 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 reentrants 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 subaerial 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 subaerial 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 subaerial 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 subaerial 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). - -_Ooelitic_ limestone (_oeon_, 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 subaerial 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 deg. 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 deg.. 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 deg. north, or dip 8 deg. south 20 deg. -west,--meaning in the latter case that the amount of the dip is 8 deg. and -the direction of the dip bears 20 deg. 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 deg., the original fault -scarp remaining. - -2. A normal fault with a hade of 50 deg., 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 deg., showing cliffs due to -harder strata outcropping on the downthrow. - -4. A thrust fault with a hade of 80 deg., 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 deg. 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 subaerial 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 Soufriere 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 deg. 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 cooeperated 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 zooelogical 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 _Chaetetes_, 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 _tabulae_, while the -septa are rudimentary, being represented by faintly projecting ridges -or rows of spines. - -_Chain corals_ are also marked by tabulae. 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 vertebrae 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. Palaeospondylus] - -=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 vertebrae 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 vertebrae. -In most modern fishes and in all higher vertebrates the notochord is -gradually removed as the bodies of the vertebrae are formed about it; -but in the Devonian fishes it persists through maturity and the -vertebrae remain incomplete. - -=The skeleton is cartilaginous.= This also is an embryological -characteristic. In the Devonian fishes the vertebrae, 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. Vertebrae 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 vertebrae 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 vertebrae 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_. Gryphaea] - -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 -vertebrae 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. Archaeopteryx] - -=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. _Archaeopteryx_ (_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 Archaeopteryx 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 moutonnees, and even rocky hills have been thus smoothed to -flowing outlines like roches moutonnees 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 moutonnees, 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 debris, -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 moutonnees, 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 archaeology 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 - Algae, 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 - Archaeopteryx, 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 - Gryphaea, 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 - Laminae, 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 - Neve, 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 - Palaeospondylus, 344 - Paleolithic man, 444 - Paleozoic era, 315 - Palisades of Hudson, 268 - Palms, 377 - Pamir, 15 - Peat, 94, 104 - Peccaries, 412 - Pelecypods, 324 - Pelee, 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 moutonnees, 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 - Soufriere, 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 - Striae, 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 - - Tabulae, 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. 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(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). 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