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diff --git a/4204.txt b/4204.txt new file mode 100644 index 0000000..b3cf973 --- /dev/null +++ b/4204.txt @@ -0,0 +1,12776 @@ +The Project Gutenberg Etext of The Elements of Geology, by W. H. Norton + +Copyright laws are changing all over the world. Be sure to check the +copyright laws for your country before distributing this or any other +Project Gutenberg file. + +We encourage you to keep this file, exactly as it is, on your +own disk, thereby keeping an electronic path open for future +readers. Please do not remove this. + +This header should be the first thing seen when anyone starts to +view the etext. 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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 +paleontological 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 + + + + + +CONTENTS + +INTRODUCTION.--THE SCOPE AND AIM OF GEOLOGY + +PART I + +EXTERNAL GEOLOGICAL AGENCIES + + I. THE WORK OF THE WEATHER + II. THE WORK OF GROUND WATER + III. RIVERS AND VALLEYS + IV. RIVER DEPOSITS + V. THE WORK OF GLACIERS + VI. THE WORK OF THE WIND + VII. THE SEA AND ITS SHORES + VIII. OFFSHORE AND DEEP-SEA DEPOSITS + +PART II + +INTERNAL GEOLOGICAL AGENCIES + + IX. MOVEMENTS OF THE EARTH'S CRUST + X. EARTHQUAKES + XI. VOLCANOES + XII. UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN + XIII. METAMORPHISM AND MINERAL VEINS + +PART III + +HISTORICAL GEOLOGY + + XIV. THE GEOLOGICAL RECORD + XV. THE PRE-CAMBRIAN SYSTEMS + XVI. THE CAMBRIAN + XVII. THE ORDOVICIAN AND SILURIAN + XVIII. THE DEVONIAN + XIX. THE CARBONIFEROUS + XX. THE MESOZOIC + XXI. THE TERTIARY + XXII. THE QUATERNARY + INDEX + + + + + +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. Two sets of joints, +running at about right angles to each other, together with the +bedding planes, divide the sandstone into quadrangular blocks. + +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. + +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. + +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. + +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. + +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. + +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. + +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 degrees +F. within a few hours. In the light air of the Pamir plateau in +central Asia a rise of 90 degrees 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 Spitsbergen, +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. + +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 cooperate 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 degrees 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. 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. + +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. + +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. + +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. + +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 pellmell 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. + +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. + +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. + +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. + +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. + +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. + +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. + +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 + +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. flat-topped, because of the +protection of a resistant horizontal capping layer, they are +termed mesas,--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. + +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. +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. + +Frost and heat and cold sculpture high mountains to sharp, +tusklike peaks and ragged, serrate crests, where their waste is +readily removed. + +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. + +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? + +THE COOPERATION 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 cooperation 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. + +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. + +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. + +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). + +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 AQUIFIER, 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. + +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. + +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. + +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. + +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. + +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. 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? + +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 degrees F., 70 degrees 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 degree 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. + +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 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. + +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 +of even three or four miles it is believed that ground water +cannot circulate. Estimating the average pore spaces of the +different rocks of the earth's crust above this depth, and the +average per cents of their pore spaces occupied by water, it has +been recently computed that the total amount of ground water is +equal to a sheet of water one hundred feet deep, covering the +entire surface of the earth. + + + + + +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? + +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. + +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. + +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. + +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? + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. 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. + +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. +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. + +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. + +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. + +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. + +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. + +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? + +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. [Footnote: The term "baselevel" is also used to +designate the close approximation to sea level to which streams +are able to subdue the land.] 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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. 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. + +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? + +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 are the surfaces of the alluvial deposits which +streams build along their courses at times of flood. 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. + +A cross section of a flood plain 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. + +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. + +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. + +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 whose width and length are not far from equal, +and later that of the horseshoe meander 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, or oxbow lake, which fills gradually +to an arc-shaped shallow depression. + +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. 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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. They are separated +by heavy deposits of well-rounded, cross-bedded gravels and sands, +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 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. + +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. + +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 cooperate with rivers in the denudation of the land. + + + + + +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. + +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. + +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. 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. 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. + +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. + +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. + +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 degrees 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. + +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. 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. + +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. + +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). + +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. + +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? + +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. 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. + +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. + +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. + +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. + +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. + +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? + +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? + +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? + +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? + +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. + +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. + +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. + +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. + +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. + +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. Fjords are found only +in regions which have suffered glaciation, such as Norway and +Alaska. + +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. + +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. + +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 oversteepened 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. + +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 + + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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 + +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. + +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. + +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, 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. + +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 + + +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. 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. + +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. + +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. 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. + +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. + +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. + +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. + +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. + +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. + +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. + +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 some-what 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 weiglit 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. + +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. + +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). + +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. 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. + +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. + +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. + +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 +out-line. 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. + +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. Northwestern France may be cited as +an upland plain, dissected and depressed, whose coast has reached +maturity. + +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. + +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 CONGOLMERATES. 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. + +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). + +OOLITIC limestone (oon, 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. + +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. + +The mud of flats, thus exposed to the sun and dried, cracks in a +characteristic way. 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. + +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. + +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. + +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. + +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. + +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. + +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 off- +shore 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. + +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. + +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. + +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 north-eastern +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 +degrees 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. + +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". + +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. + +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. 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 fragraental 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. + +THE GREAT LAKES is now under-going 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 north-eastern +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, and the forms of coasts. 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? + +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. 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? + +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. + +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. + +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. + +The inclination of a bed of rocks to the horizon is its DIP. 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 degrees. 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 +degrees north, or dip 8 degrees south 20 degrees west,--meaning in +the latter case that the amount of the dip is 8 degrees and the +direction of the dip bears 20 degrees 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. + +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. A downfold, where the strata dip from +either side toward the axis of the trough, is called a SYNCLINE. +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. + +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 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 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. + +In folds where the compression has been great the layers are often +found thickened at the crest and thinned along the limbs. 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. + +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. + +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. + +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. + +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? + +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. Under long-continued erosion, anticlines may be +degraded to valleys, while the synclines of the same system may be +left in relief as ridges. + +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. + +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. + +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 +Pennyslvania 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. + +The Alps were upheaved at various times, 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. + +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 are examples of mountains which have reached +this stage. + +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. + +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. + +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. The angle 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 and the FOOT +WALL. 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 sliekensides, 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. + +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). + +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. + +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. + +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. + +The Highlands of Scotland 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. + +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 down- +faulting 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. + +THE BASIN RANGES. The plateaus of the United States bounded by the +Rocky Mouirtains 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. 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. + +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 Alleghany 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 degrees, the original fault +scarp remaining. + +2. A normal fault with a hade of 50 degrees, 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 degrees, showing cliffs due to +harder strata outcropping on the downthrow. + +4. A thrust fault with a hade of 80 degrees, 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? + +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. + +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? + +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. + +9. Draw diagrams showing by dotted lines the conditions both of A +and of B, Figure 196, after deformation had given the strata their +present attitude. + +10. What is the attitude of the strata of this earth block, Figure +197? What has taken place along the plane bef? 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? 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. + +11. Which is the older fault, in Figure 198, or 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? + +12. Measure by the scale the thickness lie 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 degrees +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. + +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, their emergence, the erosion of the land +surface, a submergence and the deposit of the strata, and lastly, +emergence and the erosion of the present surface. + +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 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. + +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. + +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. + +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. + +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, 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. + +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. + +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. + +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. After a time the lava rises in the duct, the floor is +floated higher, and the boiling lakes reappear. + +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. + +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. 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. + +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. + +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 bos +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. + +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. + +ACIDIC AND BASIC LAVAS. Two classes of volcanic rocks may be +distinguished,--those containing a large proportion of silica +(silicic acid, SiO2) 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. + +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. + +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. + +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. + +PORPHYRITIC STRUCTURE. Rocks whose ground mass has scattered +through it large conspicuous crystals 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. + +FRAGMENTAL MATERIALS + +Rocks weighing many tons are often thrown from a volcano at the +beginning of an outburst by the breaking up of the solidofied +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. + +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 BRECIA, 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 VOCLANIC BOMBS,--balls of lava which took shape +while whirling in the air, and solidified before falling to the +ground. + +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. + +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 steepsided domes like that of +Sarcoui. + +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. + +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. + +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. + +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. + +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. + +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. + +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, is younger than Mount Hood, which erosive +agencies have carved to a pyramidal form. 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. + +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. + + + + + +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. + +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? + +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. + +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). + +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). + +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 arid 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? + +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? + +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. + +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. + +The base of bosses is indefinite or undetermined, and in this +respect they differ from laccoliths. Some bosses have broken and +faulted the overlying beds; some have forced the rocks aside and +melted them away. + +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. + +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. + +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. + +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. + +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 degrees 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 RIGID AND HEAVY. The earth behaves as +a globe more rigid than glass under the attractions of the sun and +moon. It is not deformed by these stresses as is the ocean in the +tides, proving that it is not a fluid ball covered with a yielding +crust a few miles thick. Earthquakes pass through the earth faster +than they would were it of solid steel. Hence the rocks of the +interior are highly elastic, being brought by pressure to a +compact, continuous condition unbroken by the cracks and vesicles +of surface rocks. THE INTERIOR OF THE EARTH IS 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 wide-spread 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, 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. + +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. + +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. + +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 +cooperated 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. + + + + + +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. 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. 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, 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 + + |Quaternary-----|Recent +Cenozoic------| |Pleistocene + | + |Tertiary-------|Pliocene + |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 EVERCHANGING. 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 ZOOLOGICAL 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 sub-kingdom 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 is possible 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 held also +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. + +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; 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. 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 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 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, 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. + +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. + +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 sub- +divisions 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. + +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. + +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. + +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. + +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. + +ARTHOPODS. 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. + +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. + +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. 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. + +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. 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. + +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. + +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, passed through from the +body chamber to the closed tip of the cone. + +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 AND SILURIAN +[Footnote: 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. + +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. + +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. + +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 CHOETETES, 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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + + + + + +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). + +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. + +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. + +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. + +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. + +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. + +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. + +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, 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 fotfoid 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 Pennyslvania. Although some +of the minor changes are omitted, the section shows the rapid +alternation of the strata: + + Feet + 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 Lignite Bituminous Coal +Anthracite + Dismal 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. + +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 freshwater +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. + +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 +Pennyslvania, 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. + +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. + +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. + +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 had evolved 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. + +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. + +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. + +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? + +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. + +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 were formed, while the great masses of +igneous rock which constitute the cores of the Spanish Peaks 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 sigillafids 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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. 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. + +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). + +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. + +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. + +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. + +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 + +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. + +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 sub- +divisions 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, 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. The Tertiary river deposits of the High Plains have +already been described. How widespread are these ancient river +plains and beds of fresh-water lakes may be seen in the map of +Figure 260. + +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. 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 was reared in the Miocene and the latest +eruptions of the Park are far more recent. + +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, 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 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, 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 were followed by volcanic +outbursts. In comparison with the middle Tertiary, the present is +a time of volcanic inactivity and repose. + +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 and the Pocono, 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. + +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. + +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, [Footnote: +Or, more accurately, with four perfect toes and a rudimentary +fifth corresponding to the thumb.] 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. + +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. + +THE PROBOSCIDIANS. This unique order of hoofed mammals, of which +the elephant is the sole survivor, began, so far as known, in the +Eocene, in Egypt, with a piglike ancestor the size of a small +horse, with cheek teeth like the Mastodon's, but wanting both +trunk and tusks. A proboscidian came next with four short tusks, +and in the Miocene there followed a Mastodon (Fig. 346) armed with +two pairs of long, straight tusks on which rested a flexible +proboscis. + +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. + +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. + +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. + +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. + +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 coun try 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. + +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. + +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 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. + +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. + +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. + +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. + +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). + +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. + +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. + +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. + +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 delms, 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. + +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. + +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 moutonnecs, 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. + +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. + +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. + +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. + +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. + +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. + +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. + +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. + + +End of the Project Gutenberg Etext of The Elements of Geology, by W. H. Norton + |