,ver4#aj_/jd_/ja99;,,end99 ,this etext was produced by ,robert ,rowe1 ,charles ,franks and the ,online ,distributed ,proofreading ,team ,,the ,,elements ,,of ,,geology ,,by ,,william ,,harmon ,,norton ,,professor ,,of ,,geology ,,in ,,cornell ,,college ,,preface ,geology is a science of such rapid growth that no apology is expected when from time to time a new text-book is added to those already in the field4 ,the present work1 however1 is the outcome of the need of a text-book of very simple outline1 in which causes and their consequences should be knit together as closely as possible1--a need long felt by the author in his teaching1 and perhaps by other teachers also4 ,the author has ventured1 therefore1 to depart from the common usage which subdivides geology into a number of departments1--dynamical1 structural1 physiographic1 and historical1--and to treat in immediate connection with each geological process the land forms and the rock structures which it has produced4 ,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 reasoning4 ,typical examples have been used to introduce many topics1 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 rest4 ,there have been added a number of practical exercises such as the author has used for several years in the class room4 ,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 recitation1 but may1 it is hoped1 suggest their use4 ,in historical geology a broad view is given of the development of the ,north ,american continent and the evolution of life upon the planet4 ,only the leading types of plants and animals are mentioned1 and special attention is given to those which mark the lines of descent of forms now living4 ,by omitting much technical detail of a mineralogical and paleontological nature1 and by confining the field of view almost wholly to our own continent1 space has been obtained to give to what are deemed for beginners the essentials of the science a fuller treatment than perhaps is common4 ,it is assumed that field work will be introduced with the commencement of the study4 ,the common rocks are therefore briefly described in the opening chapters4 ,the drift also receives early mention1 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 glaciers4 ,simple diagrams have been used freely1 not only because they are often clearer than any verbal statement1 but also because they readily lend themselves to reproduction on the blackboard by the pupil4 ,the text will suggest others which the pupil may invent4 ,it is hoped that the photographic views may also be used for exercises in the class room4 ,the generous aid of many friends is recognized with special pleasure4 ,to ,professor ,w4 ,m4 ,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 science1 and for his stimulating influence in education4 ,it is hoped that both in subject-matter and in method the book itself makes evident this debt4 ,but besides a general obligation shared by geologists everywhere1 and in varying degrees by perhaps all authors of recent ,american text- books in earth science1 there is owing a debt direct and personal4 ,the plan of the book1 with its use of problems and treatment of land forms and rock structures in immediate connection with the processes which produce them1 was submitted to ,professor ,davis1 and1 receiving his approval1 was carried into effect1 although without the sanction of precedent at the time4 ,professor ,davis also kindly consented to read the manuscript throughout1 and his many helpful criticisms and suggestions are acknowledged with sincere gratitude4 ,parts of the manuscript have been reviewed by ,dr4 ,samuel ,calvin and ,dr4 ,frank ,m4 ,wilder of the ,state ,university of ,iowa2 ,dr4 ,s4 ,w4 ,beyer of the ,iowa ,college of ,agriculture and ,mechanic ,arts2 ,dr4 ,u4 ,s4 ,grant of ,northwestern ,university2 ,professor ,j4 ,a4 ,udden of ,augustana ,college1 ,illinois2 ,dr4 ,c4 ,h4 ,gordon of the ,new ,mexico ,state ,school of ,mines2 ,principal ,maurice ,ricker of the ,high ,school1 ,burlington1 ,iowa2 and the following former students of the author who are engaged in the earth sciences3 ,dr4 ,w4 ,c4 ,alden of the ,united ,states ,geological ,survey and the ,university of ,chicago2 ,mr4 ,joseph ,sniffen1 instructor in the ,academy of the ,university of ,chicago1 ,morgan ,park2 ,professor ,martin ,iorns1 ,fort ,worth ,university1 ,texas2 ,professor ,a4 ,m4 ,jayne1 ,dakota ,university2 ,professor ,g4 ,h4 ,bretnall1 ,monmouth ,college1 ,illinois2 ,professor ,howard ,e4 ,simpson1 ,colby ,college1 ,maine2 ,mr4 ,e4 ,j4 ,cable1 instructor in the ,iowa ,state ,normal ,college2 ,principal ,c4 ,c4 ,gray of the ,high ,school1 ,fargo1 ,north ,dakota2 and ,mr4 ,charles ,persons of the ,high ,school1 ,hannibal1 ,missouri4 ,a large number of the diagrams of the book were drawn by ,mr4 ,w4 ,w4 ,white of the ,art ,school of ,cornell ,college4 ,to all these friends1 and to the many who have kindly supplied the illustrations of the text1 whose names are mentioned in an appended list1 the writer returns his heartfelt thanks4 ,,william ,,harmon ,,norton ,,cornell ,,college1 ,,mount ,,vernon1 ,,iowa ,,july1 #aije ,,introductory ,,note ,during the preparation of this book ,professor ,norton has frequently discussed its plan with me by correspondence1 and we have considered together the matters of scope1 arrangement1 and presentation4 ,as to scope1 the needs of the young student and not of the expert have been our guide2 the book is therefore a text-book1 not a reference volume4 ,in arrangement1 the twofold division of the subject was chosen because of its simplicity and effectiveness4 ,the principles of physical geology come first2 the several chapters are arranged in what is believed to be a natural order1 appropriate to the greatest part of our country1 so that from a simple beginning a logical sequence of topics leads through the whole subject4 ,the historical view of the science comes second1 with many specific illustrations of the physical processes previously studied1 but now set forth as part of the story of the earth1 with its many changes of aspect and its succession of inhabitants4 ,special attention is here given to ,north ,america1 and care is taken to avoid overloading with details4 ,with respect to method of presentation1 it must not be forgotten that the text-book is only one factor in good teaching1 and that in geology1 as in other sciences1 the teacher1 the laboratory1 and the local field are other factors1 each of which should play an appropriate part4 ,the text suggests observational methods1 but it cannot replace observation in field or laboratory2 it offers certain exercises1 but space cannot be taken to make it a laboratory manual as well as a book for study2 it explains many problems1 but its statements are necessarily more terse than the illustrative descriptions that a good and experienced teacher should supply4 ,frequent use is made of induction and inference in order that the student may come to see how reasonable a science is geology1 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 him4 ,the further extension of this method of presentation is urged upon the teacher1 so that the young geologist may always learn the evidence that leads to a conclusion1 and not only the conclusion itself4 ,w4 ,m4 ,,davis ,,harvard ,,university1 ,,cambridge1 ,,mass4 ,,july1 #aije ,,contents ,,introduction4--,,the ,,scope ,,and ,,aim ,,of ,,geology ,,part ,i ,,external ,,geological ,,agencies ,i4 ,,the ,,work ,,of ,,the ,,weather ,,ii4 ,,the ,,work ,,of ,,ground ,,water ,,iii4 ,,rivers ,,and ,,valleys ,,iv4 ,,river ,,deposits ,v4 ,,the ,,work ,,of ,,glaciers ,,vi4 ,,the ,,work ,,of ,,the ,,wind ,,vii4 ,,the ,,sea ,,and ,,its ,,shores ,,viii4 ,,offshore ,,and ,,deep-,,sea ,,deposits ,,part ,,ii ,,internal ,,geological ,,agencies ,,ix4 ,,movements ,,of ,,the ,,earth',s ,,crust ,x4 ,,earthquakes ,,xi4 ,,volcanoes ,,xii4 ,,underground ,,structures ,,of ,,igneous ,,origin ,,xiii4 ,,metamorphism ,,and ,,mineral ,,veins ,,part ,,iii ,,historical ,,geology ,,xiv4 ,,the ,,geological ,,record ,,xv4 ,,the ,,pre-,,cambrian ,,systems ,,xvi4 ,,the ,,cambrian ,,xvii4 ,,the ,,ordovician ,,and ,,silurian ,,xviii4 ,,the ,,devonian ,,xix4 ,,the ,,carboniferous ,,xx4 ,,the ,,mesozoic ,,xxi4 ,,the ,,tertiary ,,xxii4 ,,the ,,quaternary ,,index ,,the ,,elements ,,of ,,geology ,,introduction ,,the ,,scope ,,and ,,aim ,,of ,,geology ,geology deals with the rocks of the earth's crust4 ,it learns from their composition and structure how the rocks were made and how they have been modified4 ,it ascertains how they have been brought to their present places and wrought to their various topographic forms1 such as hills and valleys1 plains and mountains4 ,it studies the vestiges which the rocks preserve of ancient organisms which once inhabited our planet4 ,geology is the history of the earth and its inhabitants1 as read in the rocks of the earth's crust4 ,to obtain a general idea of the nature and method of our science before beginning its study in detail1 we may visit some valley1 such as that illustrated in the frontispiece1 on whose sides are rocky ledges4 ,here the rocks lie in horizontal layers4 ,although only their edges are exposed1 we may infer that these layers run into the upland on either side and underlie the entire district2 they are part of the foundation of solid rock which everywhere is found beneath the loose materials of the surface4 ,the ledges of the valley of our illustration are of sandstone4 ,looking closely at the rock we see that it is composed of myriads of grains of sand cemented together4 ,these grains have been worn and rounded4 ,they are sorted also1 those of each layer being about of a size4 ,by some means they have been brought hither from some more ancient source4 ,surely these grains have had a history before they here found a resting place1--a history which we are to learn to read4 ,the successive layers of the rock suggest that they were built one after another from the bottom upward4 ,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 them4 ,we have no reason to believe that the lowest layers which we see here were the earliest ever formed4 ,indeed1 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 floor4 ,nor may we conclude that the highest layers here were the latest ever laid2 for elsewhere we may find still later layers lying upon them4 ,a short search may find in the rock relics of animals1 such as the imprints of shells1 which lived when it was deposited2 and as these are of kinds whose nearest living relatives now have their home in the sea1 we infer that it was on the flat sea floor that the sandstone was laid4 ,its present position hundreds of feet above sea level proves that it has since emerged to form part of the land2 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 attitude4 ,the surface of some of these layers is ripple-marked4 ,hence the sand must once have been as loose as that of shallow sea bottoms and sea beaches to-day1 which is thrown into similar ripples by movements of the water4 ,in some way the grains have since become cemented into firm rock4 ,note that the layers on one side of the valley agree with those on the other1 each matching the one opposite at the same level4 ,once they were continuous across the valley4 ,where the valley now is was once a continuous upland built of horizontal layers2 the layers now show their edges1 or ,,outcrop1 on the valley sides because they have been cut by the valley trench4 ,the rock of the ledges is crumbling away4 ,at the foot of each step of rock lie fragments which have fallen4 ,thus the valley is slowly widening4 ,it has been narrower in the past2 it will be wider in the future4 ,through the valley runs a stream4 ,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 sea4 ,rock fragments and grains of sand creeping down the valley slopes come within reach of the stream and are washed along by the running water4 ,here and there they lodge for a time in banks of sand and gravel1 but sooner or later they are taken up again and carried on4 ,the grains of sand which were brought from some ancient source to form these rocks are on their way to some new goal4 ,as they are washed along the rocky bed of the stream they slowly rasp and wear it deeper4 ,the valley will be deeper in the future2 it has been less deep in the past4 ,in this little valley we see slow changes now in progress4 ,we find also in the composition1 the structure1 and the attitude of the rocks1 and the land forms to which they have been sculptured1 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 sea1 the cementation of their layers into solid rock1 the uplift of the rocks to form a land surface1 and1 last of all1 the carving of a valley in the upland4 ,everywhere1 in the fields1 along the river1 among the mountains1 by the seashore1 and in the desert1 we may discover slow changes now in progress and the record of similar changes in the past4 ,everywhere we may catch glimpses of a process of gradual change1 which stretches backward into the past and forward into the future1 by which the forms and structures of the face of the earth are continually built and continually destroyed4 ,the science which deals with this long process is geology4 ,geology treats of the natural changes now taking place upon the earth and within it1 the agencies which produce them1 and the land forms and rock structures which result4 ,it studies the changes of the present in order to be able to read the history of the earth's changes in the past4 ,the various agencies which have fashioned the face of the earth may4 be divided into two general classes4 ,in ,part ,i we shall consider those which work upon the earth from without1 such as the weather1 running water1 glaciers1 the wind1 and the sea4 ,in ,part ,,ii we shall treat of those agencies whose sources are within the earth1 and among whose manifestations are volcanoes and earthquakes and the various movements of the earth's crust4 ,as we study each agency we shall notice not only how it does its work1 but also the records which it leaves in the rock structures and the land forms which it produces4 ,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 it4 ,,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 lands4 ,everywhere the rocks are crumbling away2 their fragments are creeping down hillsides to the stream ways and are carried by the streams to the sea1 where they are rebuilt into rocky layers4 ,when again the rocks are lifted to form land the process will begin anew2 again they will crumble and creep down slopes and be washed by streams to the sea4 ,let us begin our study of this long cycle of change at the point where rocks disintegrate and decay under the action of the weather4 ,in studying now a few outcrops and quarries we shall learn a little of some common rocks and how they weather away4 ,,stratification ,,and ,,jointing4 ,at the sandstone ledges we saw that the rock was divided into parallel layers4 ,the thicker layers are known as ,,strata1 and the thin leaves into which each stratum may sometimes be split are termed ,,laminae4 ,to a greater or less degree these layers differ from each other in fineness of grain1 showing that the material has been sorted4 ,the planes which divide them are called ,,bedding ,,planes4 ,besides the bedding planes there are other division planes1 which cut across the strata from top to bottom4 ,these are found in all rocks and are known as joints4 ,two sets of joints1 running at about right angles to each other1 together with the bedding planes1 divide the sandstone into quadrangular blocks4 ,,sandstone4 ,examining a piece of sandstone we find it composed of grains quite like those of river sand or of sea beaches4 ,most of the grains are of a clear glassy mineral called quartz4 ,these quartz grains are very hard and will scratch the steel of a knife blade4 ,they are not affected by acid1 and their broken surfaces are irregular like those of broken glass4 ,the grains of sandstone are held together by some cement4 ,this may be calcareous1 consisting of soluble carbonate of lime4 ,in brown sandstones the cement is commonly ferruginous1--hydrated iron oxide1 or iron rust1 forming the bond1 somewhat as in the case of iron nails which have rusted together4 ,the strongest and most lasting cement is siliceous1 and sand rocks whose grains are closely cemented by silica1 the chemical substance of which quartz is made1 are known as quartzites4 ,we are now prepared to understand how sandstone is affected by the action of the weather4 ,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 finger4 ,on gentle slopes the rock is covered with a soil composed of sand1 which evidently is crumbled sandstone1 and dark carbonaceous matter derived from the decay of vegetation4 ,clearly it is by the dissolving of the cement that the rock thus breaks down to loose sand4 ,a piece of sandstone with calcareous cement1 or a bit of old mortar1 which is really an artificial stone also made of sand cemented by lime1 may be treated in a test tube with hydrochloric acid to illustrate the process4 ,a ,,limestone ,,quarry4 ,here also we find the rock stratified and jointed 7,fig4 #b74 ,on the quarry face the rock is distinctly seen to be altered for some distance from its upper surface4 ,below the altered zone the rock is sound and is quarried for building2 but the altered upper layers are too soft and broken to be used for this purpose4 ,if the limestone is laminated1 the laminae here have split apart1 although below they hold fast together4 ,near the surface the stone has become rotten and crumbles at the touch1 while on the top it has completely broken down to a thin layer of limestone meal1 on which rests a fine reddish clay4 ,limestone is made of minute grains of carbonate of lime all firmly held together by a calcareous cement4 ,a piece of the stone placed in a test tube with hydrochloric acid dissolves with brisk effervescence1 leaving the insoluble impurities1 which were disseminated through it1 at the bottom of the tube as a little clay4 ,we can now understand the changes in the upper layers of the quarry4 ,at the surface of the rock the limestone has completely dissolved1 leaving the insoluble residue as a layer of reddish clay4 ,immediately below the clay the rock has disintegrated into meal where the cement between the limestone grains has been removed1 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 porous4 ,as these changes in the rock are greatest at the surface and diminish downward1 we infer that they have been caused by agents working downward from the surface4 ,at certain points these agencies have been more effective than elsewhere4 ,the upper rock surface is pitted4 ,joints are widened as they approach the surface1 and along these seams we may find that the rock is altered even down to the quarry floor4 ,a ,,shale ,,pit4 ,let us now visit some pit where shale--a laminated and somewhat hardened clay--is quarried for the manufacture of brick4 ,the laminae of this fine-grained rock may be as thin as cardboard in places1 and close joints may break the rock into small rhombic blocks4 ,on the upper surface we note that the shale has weathered to a clayey soil in which all traces of structure have been destroyed4 ,the clay and the upper layers of the shale beneath it are reddish or yellow1 while in many cases the color of the unaltered rock beneath is blue4 ,,the ,,sedimentary ,,rocks4 ,the three kinds of layered rocks whose acquaintance we have made--sandstone1 limestone1 and shale--are the leading types of the great group of stratified1 or sedimentary1 rocks4 ,this group includes all rocks made of sediments1 their materials having settled either in water upon the bottoms of rivers1 lakes1 or seas1 or on dry land1 as in the case of deposits made by the wind and by glaciers4 ,sedimentary rocks are divided into the fragmental rocks--which are made of fragments1 either coarse or fine--and the far less common rocks which are constituted of chemical precipitates4 ,the sedimentary rocks are divided according to their composition into the following classes3 #a4 ,the arenaceous1 or quartz rocks1 including beds of loose sand and gravel1 sandstone1 quartzite1 and conglomerate 7a rock made of cemented rounded gravel or pebbles74 #b4 ,the calcareous1 or lime rocks1 including limestone and a soft white rock formed of calcareous powder known as chalk4 #c4 ,the argillaceous1 or clay rocks1 including muds1 clays1 and shales4 ,these three classes pass by mixture into one another4 ,thus there are limy and clayey sandstones1 sandy and clayey limestones1 and sandy and limy shales4 ,,granite4 ,this familiar rock may be studied as an example of the second great group of rocks1--the unstratified1 or igneous rocks4 ,these are not made of cemented sedimentary grains1 but of interlocking crystals which have crystallized from a molten mass4 ,examining a piece of granite1 the most conspicuous crystals which meet the eye are those of feldspar4 ,they are commonly pink1 white1 or yellow1 and break along smooth cleavage planes which reflect the light like tiny panes of glass4 ,mica may be recognized by its glittering plates1 which split into thin elastic scales4 ,a third mineral1 harder than steel1 breaking along irregular surfaces like broken glass1 we identify as quartz4 ,how granite alters under the action of the weather may be seen in outcrops where it forms the bed rock1 or country rock1 underlying the loose formations of the surface1 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 drift4 ,of the different minerals composing granite1 quartz alone remains unaltered4 ,mica weathers to detached flakes which have lost their elasticity4 ,the feldspar crystals have lost their luster and hardness1 and even have decayed to clay4 ,where long- weathered granite forms the country rock1 it often may be cut with spade or trowel for several feet from the surface1 so rotten is the feldspar1 and here the rock is seen to break down to a clayey soil containing grains of quartz and flakes of mica4 ,these are a few simple illustrations of the surface changes which some of the common kinds of rocks undergo4 ,the agencies by which these changes are brought about we will now take up under two divisions1--,,chemical ,,agencies producing rock decay and ,,mechanical ,,agencies producing rock disintegration4 ,,the ,,chemical ,,work ,,of ,,water ,as water falls on the earth in rain it has already absorbed from the air carbon dioxide 7carbonic acid gas7 and oxygen4 ,as it sinks into the ground and becomes what is termed ground water1 it takes into solution from the soil humus acids and carbon dioxide1 both of which are constantly being generated there by the decay of organic matter4 ,so both rain and ground water are charged with active chemical agents1 by the help of which they corrode and rust and decompose all rocks to a greater or less degree4 ,we notice now three of the chief chemical processes concerned in weathering1-- solution1 the formation of carbonates1 and oxidation4 ,,solution4 ,limestone1 although so little affected by pure water that five thousand gallons would be needed to dissolve a single pound1 is easily dissolved in water charged with carbon dioxide4 ,in limestone regions well water is therefore hard4 ,on boiling the water for some time the carbon dioxide gas is expelled1 the whole of the lime carbonate can no longer be held in solution1 and much of it is thrown down to form a crust or scale in the kettle or in the tubes of the steam boiler4 ,all waters which flow over limestone rocks or soak through them are constantly engaged in dissolving them away1 and in the course of time destroy beds of vast extent and great thickness4 ,the upper surface of limestone rocks becomes deeply pitted1 as we saw in the limestone quarry1 and where the mantle of waste has been removed it may be found so intricately furrowed that it is difficult to traverse4 ,beds of rock salt buried among the strata are dissolved by seeping water1 which issues in salt springs4 ,gypsum1 a mineral composed of hydrated sulphate of lime1 and so soft that it may be scratched with the finger nail1 is readily taken up by water1 giving to the water of wells and springs a peculiar hardness difficult to remove4 ,the dissolving action of moisture may be noted on marble tombstones of some age1 marble being a limestone altered by heat and pressure and composed of crystalline grains4 ,by assuming that the date on each monument marks the year of its erection1 one may estimate how many years on the average it has taken for weathering to loosen fine grains on the polished surface1 so that they may be rubbed off with the finger1 to destroy the polish1 to round the sharp edges of tool marks in the lettering1 and at last to open cracks and seams and break down the stone4 ,we may notice also whether the gravestones weather more rapidly on the sunny or the shady side1 and on the sides or on the top4 ,the weathered surface of granular limestone containing shells shows them standing in relief4 ,as the shells are made of crystalline carbonate of lime1 we may infer whether the carbonate of lime is less soluble in its granular or in its crystalline condition4 ,,the ,,formation ,,of ,,carbonates4 ,in attacking minerals water does more than merely take them into solution4 ,it decomposes them1 forming new chemical compounds of which the carbonates are among the most important4 ,thus feldspar consists of the insoluble silicate of alumina1 together with certain alkaline silicates which are broken up by the action of water containing carbon dioxide1 forming alkaline carbonates4 ,these carbonates are freely soluble and contribute potash and soda to soils and river waters4 ,by the removal of the soluble ingredients of feldspar there is left the silicate of alumina1 united with water or hydrated1 in the condition of a fine plastic clay which1 when white and pure1 is known as ,,kaolin and is used in the manufacture of porcelain4 ,feldspathic rocks which contain no iron compounds thus weather to whitish crusts1 and even apparently sound crystals of feldspar1 when ground to thin slices and placed under the microscope1 may be seen to be milky in color throughout because an internal change to kaolin has begun4 ,,oxidation4 ,rocks containing compounds of iron weather to reddish crusts1 and the seams of these rocks are often lined with rusty films4 ,oxygen and water have here united with the iron1 forming hydrated iron oxide4 ,the effects of oxidation may be seen in the alteration of many kinds of rocks and in red and yellow colors of soils and subsoils4 ,pyrite is a very hard mineral of a pale brass color1 found in scattered crystals in many rocks1 and is composed of iron and sulphur 7iron sulphide74 ,under the attack of the weather it takes up oxygen1 forming iron sulphate 7green vitriol71 a soluble compound1 and insoluble hydrated iron oxide1 which as a mineral is known as limonite4 ,several large masses of iron sulphide were placed some years ago on the lawn in front of the ,national ,museum at ,washington4 ,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 grass4 ,,agents ,,of ,,mechanical ,,disintegration ,,heat ,,and ,,cold4 ,rocks exposed to the direct rays of the sun become strongly heated by day and expand4 ,after sunset they rapidly cool and contract4 ,when the difference in temperature between day and night is considerable1 the repeated strains of sudden expansion and contraction at last become greater than the rocks can bear1 and they break1 for the same reason that a glass cracks when plunged into boiling water 7,fig4 #e74 ,rocks are poor conductors of heat1 and hence their surfaces may become painfully hot under the full blaze of the sun1 while the interior remains comparatively cool4 ,by day the surface shell expands and tends to break loose from the mass of the stone4 ,in cooling in the evening the surface shell suddenly contracts on the unyielding interior and in time is forced off in scales4 ,many rocks1 such as granite1 are made up of grains of various minerals which differ in color and in their capacity to absorb heat1 and which therefore contract and expand in different ratios4 ,in heating and cooling these grains crowd against their neighbors and tear loose from them1 so that finally the rock disintegrates into sand4 ,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 rain4 ,the soil not being held together by the roots of plants is blown away over large areas1 leaving the rocks bare to the blazing sun in a cloudless sky4 ,the air is dry1 and the heat received by the earth by day is therefore rapidly radiated at night into space4 ,there is a sharp and sudden fall of temperature after sunset1 and the rocks1 strongly heated by day1 are now chilled perhaps even to the freezing point4 ,in the ,sahara the thermometer has been known to fall #aca degrees ,f4 within a few hours4 ,in the light air of the ,pamir plateau in central ,asia a rise of #ij degrees ,f4 has been recorded from seven o'clock in the morning to one o'clock in the afternoon4 ,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 thickness1 and loud reports are made as huge bowlders split apart4 ,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 #ahj degrees ,f4 ,beds half a foot thick1 forming the floor of limestone quarries in ,wisconsin1 have been known to buckle and arch and break to fragments under the heat of the summer sun4 ,,frost4 ,by this term is meant the freezing and thawing of water contained in the pores and crevices of rocks4 ,all rocks are more or less porous and all contain more or less water in their pores4 ,workers in stone call this quarry water1 and speak of a stone as green before the quarry water has dried out4 ,water also seeps along joints and bedding planes and gathers in all seams and crevices4 ,water expands in freezing1 ten cubic inches of water freezing to about eleven cubic inches of ice4 ,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 winter4 ,the first rift in the rock1 perhaps too narrow to be seen1 is widened little by little by the wedges of successive frosts1 and finally the rock is broken into detached blocks1 and these into angular chip-stone by the same process4 ,it is on mountain tops and in high latitudes that the effects of frost are most plainly seen4 ,every summit says ,whymper1 amongst the rock summits upon which ,i have stood has been nothing but a piled-up heap of fragments 7,fig4 #g74 ,in ,iceland1 in ,spitsbergen1 in ,kamchatka1 and in other frigid lands large areas are thickly strewn with sharp-edged fragments into which the rock has been shattered by frost4 ,,organic ,,agents ,we must reckon the roots of plants and trees among the agents which break rocks into pieces4 ,the tiny rootlet in its search for food and moisture inserts itself into some minute rift1 and as it grows slowly wedges the rock apart4 ,moreover1 the acids of the root corrode the rocks with which they are in contact4 ,one may sometimes find in the soil a block of limestone wrapped in a mesh of roots1 each of which lies in a little furrow where it has eaten into the stone4 ,rootless plants called lichens often cover and corrode rocks as yet bare of soil2 but where lichens are destroying the rock less rapidly than does the weather1 they serve in a way as a protection4 ,,conditions ,,favoring ,,disintegration ,,and ,,decay4 ,the disintegration of rocks under frost and temperature changes goes on most rapidly in cold and arid climates1 and where vegetation is scant or absent4 ,on the contrary1 the decay of rocks under the chemical action of water is favored by a warm1 moist climate and abundant vegetation4 ,frost and heat and cold can only act within the few feet from the surface to which the necessary temperature changes are limited1 while water penetrates and alters the rocks to great depths4 ,the pupil may explain4 ,in what ways the presence of joints and bedding planes assists in the breaking up and decay of rocks under the action of the weather4 ,why it is a good rule of stone masons never to lay stones on edge1 but always on their natural bedding planes4 ,why stones fresh from the quarry sometimes go to pieces in early winter1 when stones which have been quarried for some months remain uninjured4 ,why quarrymen in the northern states often keep their quarry floors flooded during winter4 ,why laminated limestone should not be used for curbstone4 ,why rocks composed of layers differing in fineness of grain and in ratios of expansion do not make good building stone4 ,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 larger4 ,which1 therefore1 are more likely to be injured by frost8 ,which is subject to greater temperature changes1 a dark rock or one of a light color8 the north side or the south side of a valley8 ,,the ,,mantle ,,of ,,rock ,,waste ,we have seen that rocks are everywhere slowly wasting away4 ,they are broken in pieces by frost1 by tree roots1 and by heat and cold4 ,they dissolve and decompose under the chemical action of water and the various corrosive substances which it contains1 leaving their insoluble residues as residual clays and sands upon the surface4 ,as a result there is everywhere forming a mantle of rock waste which covers the land4 ,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 formed4 ,the surface of the land would then be everywhere of bare rock as unbroken as a quarry floor4 ,,the ,,thickness ,,of ,,the ,,mantle4 ,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 forming4 ,on the face of cliffs it is absent1 for here waste is removed as fast as it is made4 ,where waste is carried away more slowly than it is produced1 it accumulates in time to great depth4 ,the granite of ,pikes ,peak is disintegrated to a depth of twenty feet4 ,in the city of ,washington granite rock is so softened to a depth of eighty feet that it can be removed with pick and shovel4 ,about ,atlanta1 ,georgia1 the rocks are completely rotted for one hundred feet from the surface1 while the beginnings of decay may be noticed at thrice that depth4 ,in places in southern ,brazil the rock is decomposed to a depth of four hundred feet4 ,in southwestern ,wisconsin a reddish residual clay has an average depth of thirteen feet on broad uplands1 where it has been removed to the least extent4 ,the country rock on which it rests is a limestone with about ten per cent of insoluble impurities4 ,at least how thick1 then1 was that portion of the limestone which has rotted down to the clay8 ,,distinguishing ,,characteristics ,,of ,,residual ,,waste4 ,we must learn to distinguish waste formed in place by the action of the weather from the products of other geological agencies4 ,residual waste is unstratified4 ,it contains no substances which have not been derived from the weathering of the parent rock4 ,there is a gradual transition from residual waste into the unweathered rock beneath4 ,waste resting on sound rock evidently has been shifted and was not formed in place4 ,in certain regions of southern ,missouri the land is covered with a layer of broken flints and red clay1 while the country rock is limestone4 ,the limestone contains nodules of flint1 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 surface4 ,flint is a form of quartz1 dull-lustered1 usually gray or blackish in color1 and opaque except on thinnest edges1 where it is translucent4 ,over much of the northern states there is spread an unstratified stony clay called the drift4 ,it often rests on sound rocks4 ,it contains grains of sand1 pebbles1 and bowlders composed of many different minerals and rocks that the country rock cannot furnish4 ,hence the drift cannot have been formed by the decay of the rock of the region4 ,a shale or limestone1 for example1 cannot waste to a clay containing granite pebbles4 ,the origin of the drift will be explained in subsequent chapters4 ,the differences in rocks are due more to their soluble than to their insoluble constituents4 ,the latter are few in number and are much the same in rocks of widely different nature1 being chiefly quartz1 silicate of alumina1 and iron oxide4 ,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 oxide4 ,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 surface1--an environment different from that in which they were formed under sea or under ground4 ,in open air1 where they are attacked by various destructive agents1 few of the rock- making minerals are stable compounds except quartz1 the iron oxides1 and the silicate of alumina2 and so it is to one or more of these comparatively insoluble substances that most rocks are reduced by long decay4 ,which produces a mantle of finer waste1 frost or chemical decay8 which a thicker mantle8 ,in what respects would you expect that the mantle of waste would differ in warm humid lands like ,india1 in frozen countries like ,alaska1 and in deserts such as the ,sahara8 ,,the ,,soil4 ,the same agencies which produce the mantle of waste are continually at work upon it1 breaking it up into finer and finer particles and causing its more complete decay4 ,thus on the surface1 where the waste has weathered longest1 it is gradually made fine enough to support the growth of plants1 and is then known as soil4 ,the coarser waste beneath is sometimes spoken of as subsoil4 ,soil usually contains more or less dark1 carbonaceous1 decaying organic matter1 called humus1 and is then often termed the humus layer4 ,soil forms not only on waste produced in place from the rock beneath1 but also on materials which have been transported1 such as sheets of glacial drift and river deposits4 ,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 formed4 ,why8 ,any fresh excavation of cellar or cistern1 or cut for road or railway1 will show the characteristics of the humus layer4 ,it may form only a gray film on the surface1 or we may find it a layer a foot or more thick1 dark1 or even black1 above1 and growing gradually lighter in color as it passes by insensible gradations into the subsoil4 ,in some way the decaying vegetable matter continually forming on the surface has become mingled with the material beneath it4 ,,how ,,humus ,,and ,,the ,,subsoil ,,are ,,mingled4 ,the mingling of humus and the subsoil is brought about by several means4 ,the roots of plants penetrate the waste1 and when they die leave their decaying substance to fertilize it4 ,leaves and stems falling on the surface are turned under by several agents4 ,earthworms and other animals whose home is in the waste drag them into their burrows either for food or to line their nests4 ,trees overthrown by the wind1 roots and all1 turn over the soil and subsoil and mingle them together4 ,bacteria also work in the waste and contribute to its enrichment4 ,the animals living in the mantle do much in other ways toward the making of soil4 ,they bring the coarser fragments from beneath to the surface1 where the waste weathers more rapidly4 ,their burrows allow air and water to penetrate the waste more freely and to affect it to greater depths4 ,,ants4 ,in the tropics the mantle of waste is worked over chiefly by ants4 ,they excavate underground galleries and chambers1 extending sometimes as much as fourteen feet below the surface1 and build mounds which may reach as high above it4 ,in some parts of ,paraguay and southern ,brazil these mounds1 like gigantic potato hills1 cover tracts of considerable area4 ,in search for its food--the dead wood of trees--the so-called white ant constructs runways of earth about the size of gas pipes1 reaching from the base of the tree to the topmost branches4 ,on the plateaus of central ,africa explorers have walked for miles through forests every tree of which was plastered with these galleries of mud4 ,each grain of earth used in their construction is moistened and cemented by slime as it is laid in place by the ant1 and is thus acted on by organic chemical agents4 ,sooner or later these galleries are beaten down by heavy rains1 and their fertilizing substances are scattered widely by the winds4 ,,earthworms4 ,in temperate regions the waste is worked over largely by earthworms4 ,in making their burrows worms swallow earth in order to extract from it any nutritive organic matter which it may contain4 ,they treat it with their digestive acids1 grind it in their stony gizzards1 and void it in castings on the surface of the ground4 ,it was estimated by ,darwin that in many parts of ,england each year1 on every acre1 more than ten tons of earth pass through the bodies of earthworms and are brought to the surface1 and that every few years the entire soil layer is thus worked over by them4 ,in all these ways the waste is made fine and stirred and enriched4 ,grain by grain the subsoil with its fresh mineral ingredients is brought to the surface1 and the rich organic matter which plants and animals have taken from the atmosphere is plowed under4 ,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 crops4 ,the world processes by which rocks are continually wasting away are thus indispensable to the life of plants and animals4 ,the organic world is built on the ruins of the inorganic1 and because the solid rocks have been broken down into soil men are able to live upon the earth4 ,,solar ,,energy4 ,the source of the energy which accomplishes all this necessary work is the sun4 ,it is the radiant energy of the sun which causes the disintegration of rocks1 which lifts vapor into the atmosphere to fall as rain1 which gives life to plants and animals4 ,considering the earth in a broad way1 we may view it as a globe of solid rock1--the lithosphere1--surrounded by two mobile envelopes3 the envelope of air1--,,the ,,atmosphere1 and the envelope of water1--,,the ,,hydrosphere4 ,under the action of solar energy these envelopes are in constant motion4 ,water from the hydrosphere is continually rising in vapor into the atmosphere1 the air of the atmosphere penetrates the hydrosphere1--for its gases are dissolved in all waters1--and both air and water enter and work upon the solid earth4 ,by their action upon the lithosphere they have produced a third envelope1--the mantle of rock waste4 ,this envelope also is in movement1 not indeed as a whole1 but particle by particle4 ,the causes which set its particles in motion1 and the different forms which the mantle comes to assume1 we will now proceed to study4 ,,movements ,,of ,,the ,,mantle ,,of ,,rock ,,waste ,at the sandstone ledges which we first visited we saw not only that the rocks were crumbling away1 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 sea4 ,this process is going on everywhere4 ,slowly it may be1 and with many interruptions1 but surely1 the waste of the land moves downward to the sea4 ,we may divide its course into two parts1--the path to the stream1 which we will now consider1 and its carriage onward by the stream1 which we will defer to a later chapter4 ,,gravity4 ,the chief agent concerned in the movement of waste is gravity4 ,each particle of waste feels the unceasing downward pull of the earth's mass and follows it when free to do so4 ,all agencies which produce waste tend to set its particles free and in motion1 and therefore cooperate with gravity4 ,on cliffs1 rocks fall when wedged off by frost or by roots of trees1 and when detached by any other agency4 ,on slopes of waste1 water freezes in chinks between stones1 and in pores between particles of soil1 and wedges them apart4 ,animals and plants stir the waste1 heat expands it1 cold contracts it1 the strokes of the raindrops drive loose particles down the slope and the wind lifts and lets them fall4 ,of all these movements1 gravity assists those which are downhill and retards those which are uphill4 ,on the whole1 therefore1 the downhill movements prevail1 and the mantle of waste1 block by block and grain by grain1 creeps along the downhill path4 ,a slab of sandstone laid on another of the same kind at an angle of #ag 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 month4 ,explain why it did so4 ,,rain4 ,the most efficient agent in the carriage of waste to the streams is the rain4 ,it moves particles of soil by the force of the blows of the falling drops1 and washes them down all slopes to within reach of permanent streams4 ,on surfaces unprotected by vegetation1 as on plowed fields and in arid regions1 the rain wears furrows and gullies both in the mantle of waste and in exposures of unaltered rock 7,fig4 #ag74 ,at the foot of a hill we may find that the soil has accumulated by creep and wash to the depth of several feet2 while where the hillside is steepest the soil may be exceedingly thin1 or quite absent1 because removed about as fast as formed4 ,against the walls of an abbey built on a slope in ,wales seven hundred years ago1 the creeping waste has gathered on the uphill side to a depth of seven feet4 ,the slow-flowing sheet of waste is often dammed by fences and walls1 whose uphill side gathers waste in a few years so as to show a distinctly higher surface than the downhill side1 especially in plowed fields where the movement is least checked by vegetation4 ,,talus4 ,at the foot of cliffs there is usually to be found a slope of rock fragments which clearly have fallen from above4 ,such a heap of waste is known as talus4 ,the amount of talus in any place depends both on the rate of its formation and the rate of its removal4 ,talus forms rapidly in climates where mechanical disintegration is most effective1 where rocks are readily broken into blocks because closely jointed and thinly bedded rather than massive1 and where they are firm enough to be detached in fragments of some size instead of in fine grains4 ,talus is removed slowly where it decays slowly1 either because of the climate or the resistance of the rock4 ,it may be rapidly removed by a stream flowing along its base4 ,in a moist climate a soluble rock1 such as massive limestone1 may form talus little if any faster than the talus weathers away4 ,a loose-textured sandstone breaks down into incoherent sand grains1 which in dry climates1 where unprotected by vegetation1 may be blown away as fast as they fall1 leaving the cliff bare to the base4 ,cliffs of such slow-decaying rocks as quartzite and granite when closely jointed accumulate talus in large amounts4 ,talus slopes may be so steep as to reach ,,the ,,angle ,,of ,,repose1 i4e4 the steepest angle at which the material will lie4 ,this angle varies with different materials1 being greater with coarse and angular fragments than with fine rounded grains4 ,sooner or later a talus reaches that equilibrium where the amount removed from its surface just equals that supplied from the cliff above4 ,as the talus is removed and weathers away its slope retreats together with the retreat of the cliff1 as seen in ,figure #i4 ,,graded ,,slopes4 ,where rocks weather faster than their waste is carried away1 the waste comes at last to cover all rocky ledges4 ,on the steeper slopes it is coarser and in more rapid movement than on slopes more gentle1 but mountain sides and hills and plains alike come to be mantled with sheets of waste which everywhere is creeping toward the streams4 ,such unbroken slopes1 worn or built to the least inclination at which the waste supplied by weathering can be urged onward1 are known as ,,graded ,,slopes4 ,of far less importance than the silent1 gradual creep of waste1 which is going on at all times everywhere about us1 are the startling local and spasmodic movements which we are now to describe4 ,,avalanches4 ,on steep mountain sides the accumulated snows of winter often slip and slide in avalanches to the valleys below4 ,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 path4 ,,landslides4 ,another common and abrupt method of delivering waste to streams is by slips of the waste mantle in large masses4 ,after long rains and after winter frosts the cohesion between the waste and the sound rock beneath is loosened by seeping water underground4 ,the waste slips on the rock surface thus lubricated and plunges down the mountain side in a swift roaring torrent of mud and stones4 ,we may conveniently mention here a second type of landslide1 where masses of solid rock as well as the mantle of waste are involved in the sudden movement4 ,such slips occur when valleys have been rapidly deepened by streams or glaciers and their sides have not yet been graded4 ,a favorable condition is where the strata dip 7i4e4 incline downwards7 towards the valley 7,fig4 #aa71 or are broken by joint planes dipping in the same direction4 ,the upper layers1 including perhaps the entire mountain side1 have been cut across by the valley trench and are left supported only on the inclined surface of the underlying rocks4 ,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 clay1 by dissolving layers of a limestone1 or by removing the cement of a sandstone and converting it into loose sand4 ,when the inclined surface is thus lubricated the overlying masses may be launched into the valley below4 ,the solid rocks are broken and crushed in sliding and converted into waste consisting1 like that of talus1 of angular unsorted fragments1 blocks of all sizes being mingled pellmell with rock meal and dust4 ,the principal effects of landslides may be gathered from the following examples4 ,at ,gohna1 ,india1 in #ahic1 the face of a spur four thousand feet high1 of the lower ranges of the ,himalayas1 slipped into the gorge of the headwaters of the ,ganges ,river in successive rock falls which lasted for three days4 ,blocks of stone were projected for a mile1 and clouds of limestone dust were spread over the surrounding country4 ,the debris formed a dam one thousand feet high1 extending for two miles along the valley4 ,a lake gathered behind this barrier1 gradually rising until it overtopped it in a little less than a year4 ,the upper portion of the dam then broke1 and a terrific rush of water swept down the valley in a wave which1 twenty miles away1 rose one hundred and sixty feet in height4 ,a narrow lake is still held by the strong base of the dam4 ,in #ahif1 after forty days of incessant rain1 a cliff of sandstone slipped into the ,yangtse ,river in ,china1 reducing the width of the channel to eighty yards and causing formidable rapids4 ,at ,flims1 in ,switzerland1 a prehistoric landslip flung a dam eighteen hundred feet high across the headwaters of the ,rhine4 ,if spread evenly over a surface of twenty-eight square miles1 the material would cover it to a depth of six hundred and sixty feet4 ,the barrier is not yet entirely cut away1 and several lakes are held in shallow basins on its hummocky surface4 ,a slide from the precipitous river front of the citadel hill of ,quebec1 in #ahhi1 dashed across ,champlain ,street1 wrecking a number of houses and causing the death of forty-five persons4 ,the strata here are composed of steeply dipping slate4 ,in lofty mountain ranges there may not be a single valley without its traces of landslides1 so common there is this method of the movement of waste1 and of building to grade over-steepened slopes4 ,,rock ,,sculpture ,,by ,,weathering ,we are now to consider a few of the forms into which rock masses are carved by the weather4 ,,bowlders ,,of ,,weathering4 ,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 angular1 as are those of the layers below4 ,the edges and corners of these blocks have been worn away by the weather4 ,such rounded cores1 known as bowlders of weathering1 are often left to strew the surface4 ,,differential ,,weathering4 ,this term covers all cases in which a rock mass weathers differently in different portions4 ,any weaker spots or layers are etched out on the surface1 leaving the more resistant in relief4 ,thus massive limestones become pitted where the weather drills out the weaker portions4 ,in these pits1 when once they are formed1 moisture gathers1 a little soil collects1 vegetation takes root1 and thus they are further enlarged until the limestone may be deeply honeycombed4 ,on the sides of canyons1 and elsewhere where the edges of strata are exposed1 the harder layers project as cliffs1 while the softer weather back to slopes covered with the talus of the harder layers above them4 ,it is convenient to call the former cliff makers and the latter slope makers4 ,differential weathering plays a large part in the sculpture of the land4 ,areas of weak rock are wasted to plains1 while areas of hard rock adjacent are still left as hills and mountain ridges1 as in the valleys and mountains of eastern ,pennsylvania4 ,but in such instances the lowering of the surface of the weaker rock is also due to the wear of streams1 and especially to the removal by them from the land of the waste which covers and protects the rocks beneath4 ,rocks owe their weakness to several different causes4 ,some1 such as beds of loose sand1 are soft and easily worn by rains2 some1 as limestone and gypsum for example1 are soluble4 ,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 agencies4 ,,outliers ,,and ,,monuments4 ,as cliffs retreat under the attack of the weather1 portions are left behind where the rock is more resistant or where the attack for any reason is less severe4 ,such remnant masses1 if large1 are known as outliers4 ,when ,note the rain furrows on the slope at the foot of the monuments4 ,in the foreground are seen fragments of petrified trunks of trees1 composed of silica and extremely resistant to the weather4 ,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 ,missouri4 flat-topped1 because of the protection of a resistant horizontal capping layer1 they are termed mesas1--a term applied also to the flat-topped portions of dissected plateaus 7,fig4 #abi74 ,retreating cliffs may fall back a number of miles behind their outliers before the latter are finally consumed4 ,monuments are smaller masses and may be but partially detached from the cliff face4 ,in the breaking down of sheets of horizontal strata1 outliers grow smaller and smaller and are reduced to massive rectangular monuments resembling castles 7,fig4 #ag74 ,the rock castle falls into ruin1 leaving here and there an isolated tower2 the tower crumbles to a lonely pillar1 soon to be overthrown4 ,the various and often picturesque shapes of monuments depend on the kind of rock1 the attitude of the strata1 and the agent by which they are chiefly carved4 ,thus pillars may have a capital formed of a resistant stratum4 ,monuments may be undercut and come to rest on narrow pedestals1 wherever they weather more rapidly near the ground1 either because of the greater moisture there1 or--in arid climates--because worn at their base by drifting sands4 ,stony clays disintegrating under the rain often contain bowlders which protect the softer material beneath from the vertical blows of raindrops1 and thus come to stand on pedestals of some height4 ,one may sometimes see on the ground beneath dripping eaves pebbles left in the same way1 protecting tiny pedestals of sand4 ,,mountain ,,peaks ,,and ,,ridges4 ,most mountains have been carved out of great broadly uplifted folds and blocks of the earth's crust4 ,running water and glacier ice have cut these folds and blocks into masses divided by deep valleys2 but it is by the weather1 for the most part1 that the masses thus separated have been sculptured to the present forms of the individual peaks and ridges4 ,frost and heat and cold sculpture high mountains to sharp1 tusklike peaks and ragged1 serrate crests1 where their waste is readily removed4 ,the ,matterhorn of the ,alps is a famous example of a mountain peak whose carving by the frost and other agents is in active progress4 ,on its face scarcely a rock anywhere is firmly attached1 and the fall of loosened stones is incessant4 ,mountain climbers who have camped at its base tell how huge rocks from time to time come leaping down its precipices1 followed by trains of dislodged smaller fragments and rock dust2 and how at night one may trace the course of the bowlders by the sparks which they strike from the mountain walls4 ,mount ,assiniboine1 ,canada 7,fig4 #bj71 resembles the ,matterhorn in form and has been carved by the same agencies4 ,the ,needles of ,arizona are examples of sharp mountain peaks in a warm arid region sculptured chiefly by temperature changes4 ,chemical decay1 especially when carried on beneath a cover of waste and vegetation1 favors the production of rounded knobs and dome-shaped mountains4 ,,the ,,weather ,,curve4 ,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 edges4 ,in much the same way weathering at last reduces to rounded hills the earth blocks cut by streams or formed in any other way4 ,high mountains may at first be sculptured by the weather to savage peaks 7,fig4 #aha71 but toward the end of their life history they wear down to rounded hills 7,fig4 #ahb74 ,the weather curve1 which may be seen on the summits of low hills 7,fig4 #ba71 is convex upward4 ,in ,figure #bb1 representing a cubic block of stone whose faces are a yard square1 how many square feet of surface are exposed to the weather by a cubic foot at a corner a2 by one situated in the middle of an edge b2 by one in the center of a side c8 ,how much faster will a and b weather than c1 and what will be the effect on the shape of the block8 ,,the ,,cooperation ,,of ,,various ,,agencies ,,in ,,rock ,,sculpture4 ,for the sake of clearness it is necessary to describe the work of each geological agent separately4 ,we must not forget1 however1 that in ,nature no agent works independently and alone2 that every result is the outcome of a long chain of causes4 ,thus1 in order that the mountain peak may be carved by the agents of disintegration1 the waste must be rapidly removed1--a work done by many agents1 including some which we are yet to study2 and in order that the waste may be removed as fast as formed1 the region must first have been raised well above the level of the sea1 so that the agents of transportation could do their work effectively4 ,the sculpture of the rocks is accomplished only by the cooperation of many forces4 ,the constant removal of waste from the surface by creep and wash and carriage by streams is of the highest importance1 because it allows the destruction of the land by means of weathering to go on as long as any land remains above sea level4 ,if waste were not removed1 it would grow to be so thick as to protect the rock beneath from further weathering1 and the processes of destruction which we have studied would be brought to an end4 ,the very presence of the mantle of waste over the land proves that on the whole rocks weather more rapidly than their waste is removed4 ,the destruction of the land is going on as fast as the waste can be carried away4 ,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 surface4 ,similar records we shall find buried deeply among the rocks of the crust in old soils and in rocks pitted and decayed1 telling of old land surfaces long wasted by the weather4 ,ever since the dry land appeared these agencies have been as now quietly and unceasingly at work upon it1 and have ever been the chief means of the destruction of its rocks4 ,the vast bulk of the stratified rocks of the earth's crust is made up almost wholly of the waste thus worn from ancient lands4 ,in studying the various geological agencies we must remember the almost inconceivable times in which they work4 ,the slowest process when multiplied by the immense time in which it is carried on produces great results4 ,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 past4 ,the mountain peak1 the rounded hill1 the wide plain which lies where hills and mountains once stood1 tell clearly of the great results which slow processes will reach when given long time in which to do their work4 ,we should accustom ourselves also to think of the results which weathering will sooner or later bring to pass4 ,the tombstone and the bowlder of the field1 which each year lose from their surfaces a few crystalline grains1 must in time be wholly destroyed4 ,the hill whose rocks are slowly rotting underneath a cover of waste must become lower and lower as the centuries and millenniums come and go1 and will finally disappear4 ,even the mountains are crumbling away continually1 and therefore are but fleeting features of the landscape4 ,,chapter ,,ii ,,the ,,work ,,of ,,ground ,,water ,,land ,,waters4 ,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 ways4 ,we now take up the work of water as it descends beneath the ground1--a corrosive agent still1 and carrying in solution as its load the invisible waste of rocks derived from their soluble parts4 ,land waters have their immediate source in the rainfall4 ,by the heat of the sun water is evaporated from the reservoir of the ocean and from moist surfaces everywhere4 ,mingled as vapor with the air1 it is carried by the winds over sea and land1 and condensed it returns to the earth as rain or snow4 ,that part of the rainfall which descends on the ocean does not concern us1 but that which falls on the land accomplishes1 as it returns to the sea1 the most important work of all surface geological agencies4 ,the rainfall may be divided into three parts3 the first ,,dries ,,up1 being discharged into the air by evaporation either directly from the soil or through vegetation2 the second ,,runs ,,off over the surface to flood the streams2 the third ,,soaks ,,in the ground and is henceforth known as ,,ground or ,,underground ,,water4 ,,the ,,descent ,,of ,,ground ,,water4 ,seeping through the mantle of waste1 ground water soaks into the pores and crevices of the underlying rock4 ,all rocks of the upper crust of the earth are more or less porous1 and all drink in water4 ,,impervious ,,rocks1 such as granite1 clay1 and shale1 have pores so minute that the water which they take in is held fast within them by capillary attraction1 and none drains through4 ,,pervious ,,rocks1 on the other hand1 such as many sandstones1 have pore spaces so large that water filters through them more or less freely4 ,besides its seepage through the pores of pervious rocks1 water passes to lower levels through the joints and cracks by which all rocks1 near the surface are broken4 ,even the closest-grained granite has a pore space of #a in #djj1 while sandstone may have a pore space of #a in #d4 ,sand is so porous that it may absorb a third of its volume of water1 and a loose loam even as much as one half4 ,,the ,,ground-,,water ,,surface is the name given the upper surface of ground water1 the level below which all rocks are saturated4 ,in dry seasons the ground-water surface sinks4 ,for ground water is constantly seeping downward under gravity1 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 air4 ,in wet seasons these constant losses are more than made good by fresh supplies from that part of the rainfall which soaks into the ground1 and the ground-water surface rises4 ,in moist climates the ground-water surface 7,fig4 #bd7 lies1 as a rule1 within a few feet of the land surface and conforms to it in a general way1 although with slopes of less inclination than those of the hills and valleys4 ,in dry climates permanent ground water may be found only at depths of hundreds of feet4 ,ground water is held at its height by the fact that its circulation is constantly impeded by capillarity and friction4 ,if it were as free to drain away as are surface streams1 it would sink soon after a rain to the level of the deepest valleys of the region4 ,,wells ,,and ,,springs4 ,excavations made in permeable rocks below the ground-water surface fill to its level and are known as wells4 ,where valleys cut this surface permanent streams are formed1 the water either oozing forth along ill-defined areas or issuing at definite points called springs1 where it is concentrated by the structure of the rocks4 ,a level tract where the ground-water surface coincides with the surface of the ground is a swamp or marsh4 ,by studying a spring one may learn much of the ways and work of ground water4 ,spring water differs from that of the stream into which it flows in several respects4 ,if we test the spring with a thermometer during successive months1 we shall find that its temperature remains much the same the year round4 ,in summer it is markedly cooler than the stream2 in winter it is warmer and remains unfrozen while the latter perhaps is locked in ice4 ,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 cold4 ,while the stream is often turbid with surface waste washed into it by rains1 the spring remains clear2 its water has been filtered during its slow movement through many small underground passages and the pores of rocks4 ,commonly the spring differs from the stream in that it carries a far larger load of dissolved rock4 ,chemical analysis proves that streams contain various minerals in solution1 but these are usually in quantities so small that they are not perceptible to the taste or feel4 ,but the water of springs is often well charged with soluble minerals2 in its slow1 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 could4 ,when spring water is boiled away1 the invisible load which it has carried is left behind1 and in composition is found to be practically identical with that of the soluble ingredients of the country rock4 ,although to some extent the soluble waste of rocks is washed down surface slopes by the rain1 by far the larger part is carried downward by ground water and is delivered to streams by springs4 ,in limestone regions springs are charged with calcium carbonate 7the carbonate of lime71 and where the limestone is magnesian they contain magnesium carbonate also4 ,such waters are hard2 when used in washing1 the minerals which they contain combine with the fatty acids of soap to form insoluble curdy compounds4 ,when springs rise from rocks containing gypsum they are hard with calcium sulphate4 ,in granite regions they contain more or less soda and potash from the decay of feldspar4 ,the flow of springs varies much less during the different seasons of the year than does that of surface streams4 ,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 drainage4 ,,movements ,,of ,,ground ,,water4 ,ground water is in constant movement toward its outlets4 ,its rate varies according to many conditions1 but always is extremely slow4 ,even through loose sands beneath the beds of rivers it sometimes does not exceed a fifth of a mile a year4 ,in any region two zones of flow may be distinguished4 ,the ,,upper ,,zone ,,of ,,flow extends from the ground-water surface downward through the waste mantle and any permeable rocks on which the mantle rests1 as far as the first impermeable layer1 where the descending movement of the water is stopped4 ,the ,,deep ,,zones ,,of ,,flow occupy any pervious rocks which may be found below the impervious layer which lies nearest to the surface4 ,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 valleys1 where in part it discharges in springs and often unites also in a wide underflowing stream which supports and feeds the river 7,fig4 #bd74 ,a city in a region of copious rains1 built on the narrow flood plain of a river1 overlooked by hills1 depends for its water supply on driven wells1 within the city limits1 sunk in the sand a few yards from the edge of the stream4 ,are these wells fed by water from the river percolating through the sand1 or by ground water on its way to the stream and possibly contaminated with the sewage of the town8 ,at what height does underground water stand in the wells of your region8 ,does it vary with the season8 ,have you ever known wells to go dry8 ,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 ground4 ,,fissure ,,springs ,,and ,,artesian ,,wells4 ,the ,,deeper ,,zones ,,of ,,flow lie in pervious strata which are overlain by some impervious stratum4 ,such layers are often carried by their dip to great depths1 and water may circulate in them to far below the level of the surface streams and even of the sea4 ,when a fissure crosses a water- bearing stratum1 or ,,aquifier1 water is forced upward by the pressure of the weight of the water contained in the higher parts of the stratum1 and may reach the surface as a fissure spring4 ,a boring which taps such an aquifer is known as an artesian well1 a name derived from a province in ,france where wells of this kind have been long in use4 ,the rise of the water in artesian wells1 and in fissure springs also1 depends on the following conditions illustrated in ,figure #bi4 ,the aquifer dips toward the region of the wells from higher ground1 where it outcrops and receives its water4 ,it is inclosed between an impervious layer above and water- tight or water-logged layers beneath4 ,the weight of the column of water thus inclosed in the aquifer causes water to rise in the well1 precisely as the weight of the water in a standpipe forces it in connected pipes to the upper stories of buildings4 ,which will supply the larger region with artesian wells1 an aquifer whose dip is steep or one whose dip is gentle8 ,which of the two aquifers1 their thickness being equal1 will have the larger outcrop and therefore be able to draw upon the larger amount of water from the rainfall8 ,illustrate with diagrams4 ,,the ,,zone ,,of ,,solution4 ,near the surface1 where the circulation of ground water is most active1 it oxidizes1 corrodes1 and dissolves the rocks through which it passes4 ,it leaches soils and subsoils of their lime and other soluble minerals upon which plants depend for their food4 ,it takes away the soluble cements of rocks2 it widens fissures and joints and opens winding passages along the bedding planes2 it may even remove whole beds of soluble rocks1 such as rock salt1 limestone1 or gypsum4 ,the work of ground water in producing landslides has already been noticed4 ,the zone in which the work of ground water is thus for the most part destructive we may call the zone of solution4 ,,caves4 ,in massive limestone rocks1 ground water dissolves channels which sometimes form large caves 7,fig4 #cj74 ,the necessary conditions for the excavation of caves of great size are well shown in central ,kentucky1 where an upland is built throughout of thick horizontal beds of limestone4 ,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 rock4 ,these water ways have been gradually enlarged by solution and wear until the upland is honeycombed with caves4 ,five hundred open caverns are known in one county4 ,mammoth ,cave1 the largest of these caverns1 consists of a labyrinth of chambers and winding galleries whose total length is said to be as much as thirty miles4 ,one passage four miles long has an average width of about sixty feet and an average height of forty feet4 ,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 floor4 ,galleries at different levels are connected by well-like pits1 some of which measure two hundred and twenty-five feet from top to bottom4 ,through some of the lowest of these tunnels flows ,echo ,river1 still at work dissolving and wearing away the rock while on its dark way to appear at the surface as a great spring4 ,,natural ,,bridges4 ,as a cavern enlarges and the surface of the land above it is lowered by weathering1 the roof at last breaks down and the cave becomes an open ravine4 ,a portion of the roof may for a while remain1 forming a natural bridge4 ,,sink ,,holes4 ,in limestone regions channels under ground may become so well developed that the water of rains rapidly drains away through them4 ,ground water stands low and wells must be sunk deep to find it4 ,little or no surface water is left to form brooks4 ,thus across the limestone upland of central ,kentucky one meets but three surface streams in a hundred miles4 ,between their valleys surface water finds its way underground by means of sink holes4 ,these are pits1 commonly funnel shaped1 formed by the enlargement of crevice or joint by percolating water1 or by the breakdown of some portion of the roof of a cave4 ,by clogging of the outlet a sink hole may come to be filled by a pond4 ,central ,florida is a limestone region with its drainage largely subterranean and in part below the level even of the sea4 ,sink holes are common1 and many of them are occupied by lakelets4 ,great springs mark the point of issue of underground streams1 while some rise from beneath the sea4 ,silver ,spring1 one of the largest1 discharges from a basin eight hundred feet wide and thirty feet deep a little river navigable for small steamers to its source4 ,about the spring there are no surface streams for sixty miles4 ,,the ,,karst4 ,along the eastern coast of the ,adriatic1 as far south as ,montenegro1 lies a belt of limestone mountains singularly worn and honeycombed by the solvent action of water4 ,where forests have been cut from the mountain sides and the red soil has washed away1 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 ridges4 ,great sink holes1 some of them six hundred feet deep and more1 pockmark the surface of the land4 ,the drainage is chiefly subterranean4 ,surface streams are rare and a portion of their courses is often under ground4 ,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 away4 ,ground water stands so far below the surface that it cannot be reached by wells1 and the inhabitants depend on rain water stored for household uses4 ,the finest cavern of ,europe1 the ,adelsberg ,grotto1 is in this region4 ,karst1 the name of a part of this country1 is now used to designate any region or landscape thus sculptured by the chemical action of surface and ground water4 ,we must remember that ,karst regions are rare1 and striking as is the work of their subterranean streams1 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 regions4 ,even when gathered into definite channels1 ground water does not have the erosive power of surface streams1 since it carries with it little or no rock waste4 ,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 waters1 and may therefore stand higher than the surrounding country4 ,the hill honeycombed by ,luray ,cavern1 ,virginia1 has been attributed to this cause4 ,,cavern ,,deposits4 ,even in the zone of solution water may under certain circumstances deposit as well as erode4 ,as it trickles from the roof of caverns1 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 ,,stalactites4 ,as the drops splash on the floor there are built up in the same way thicker masses called ,,stalagmites1 which may grow to join the stalactites above1 forming pillars4 ,a stalagmitic crust often seals with rock the earth which accumulates in caverns1 together with whatever relics of cave dwellers1 either animals or men1 it may contain4 ,can you explain why slender stalactites formed by the drip of single drops are often hollow pipes8 ,,the ,,zone ,,of ,,cementation4 ,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 above4 ,at such depths it deposits these minerals in the pores of rocks1 cementing their grains together1 and in crevices and fissures1 forming mineral veins4 ,thus below the zone of solution where the work of water is to dissolve1 lies the zone of cementation where its work is chemical deposit4 ,a part of the invisible load of waste is thus transferred from rocks near the surface to those at greater depths4 ,as the land surface is gradually lowered by weathering and the work of rain and streams1 rocks which have lain deep within the zone of cementation are brought within the zone of solution4 ,thus there are exposed to view limestones1 whose cracks were filled with calcite 7crystallized carbonate of lime71 with quartz or other minerals1 and sandstones whose grains were well cemented many feet below the surface4 ,,cavity ,,filling4 ,small cavities in the rocks are often found more or less completely filled with minerals deposited from solution by water in its constant circulation underground4 ,the process may be illustrated by the deposit of salt crystals in a cup of evaporating brine1 but in the latter instance the solution is not renewed as in the case of cavities in the rocks4 ,a cavity thus lined with inward-pointing crystals is called a ,,geode4 ,,concretions4 ,ground water seeping through the pores of rocks may gather minerals disseminated throughout them into nodular masses called concretions4 ,thus silica disseminated through limestone is gathered into nodules of flint4 ,while geodes grow from the outside inwards1 concretions grow outwards from the center4 ,nor are they formed in already existing cavities as are geodes4 ,in soft clays concretions may1 as they grow1 press the clay aside4 ,in many other rocks concretions are made by the process of ,,replacement4 ,molecule by molecule the rock is removed and the mineral of the concretion substituted in its place4 ,the concretion may in this way preserve intact the lamination lines or other structures of the rock4 ,clays and shales often contain concretions of lime carbonate1 of iron carbonate1 or of iron sulphide4 ,some fossil1 such as a leaf or shell1 frequently forms the nucleus around which the concretion grows4 ,why are building stones more easily worked when green than after their quarry water has dried out8 ,,deposits ,,of ,,ground ,,water ,,in ,,arid ,,regions4 ,in arid lands where ground water is drawn by capillarity to the surface and there evaporates1 it leaves as surface incrustations the minerals held in solution4 ,white limy incrustations of this nature cover considerable tracts in northern ,mexico4 ,evaporating beneath the surface1 ground water may deposit a limy cement in beds of loose sand and gravel4 ,such firmly cemented layers are not uncommon in western ,kansas and ,nebraska1 where they are known as mortar beds4 ,,thermal ,,springs4 ,while the lower limit of surface drainage is sea level1 subterranean water circulates much below that depth1 and is brought again to the surface by hydrostatic pressure4 ,in many instances springs have a higher temperature than the average annual temperature of the region1 and are then known as thermal springs4 ,in regions of present or recent volcanic activity1 such as the ,yellowstone ,national ,park1 we may believe that the heat of thermal springs is derived from uncooled lavas1 perhaps not far below the surface4 ,but when hot springs occur at a distance of hundreds of miles from any volcano1 as in the case of the hot springs of ,bath1 ,england1 it is probable that their waters have risen from the heated rocks of the earth's interior4 ,the springs of ,bath have a temperature of #abj degrees ,f41 #gj degrees above the average annual temperature of the place4 ,if we assume that the rate of increase in the earth's internal heat is here the average rate1 #a degree ,f4 to every sixty feet of descent1 we may conclude that the springs of ,bath rise from at least a depth of forty-two hundred feet4 ,water may descend to depths from which it can never be brought back by hydrostatic pressure4 ,it is absorbed by highly heated rocks deep below the surface4 ,from time to time some of this deep- seated water may be returned to open air in the steam of volcanic eruptions4 ,,surface ,,deposits ,,of ,,springs4 ,where subterranean water returns to the surface highly charged with minerals in solution1 on exposure to the air it is commonly compelled to lay down much of its invisible load in chemical deposits about the spring4 ,these are thrown down from solution either because of cooling1 evaporation1 the loss of carbon dioxide1 or the work of algae4 ,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 pass4 ,on reaching the surface the pressure is relieved1 the gas escapes1 and the lime carbonate is thrown down in deposits called ,,travertine4 ,the gas is sometimes withdrawn and the deposit produced in large part by the action of algae and other humble forms of plant life4 ,at the ,mammoth ,hot ,springs in the valley of the ,gardiner ,river1 ,yellowstone ,national ,park1 beautiful terraces and basins of travertine are now building1 chiefly by means of algae which cover the bottoms1 rims1 and sides of the basins and deposit lime carbonate upon them in successive sheets4 ,the rock1 snow-white where dry1 is coated with red and orange gelatinous mats where the algae thrive in the over-flowing waters4 ,similar terraces of travertine are found to a height of fourteen hundred feet up the valley side4 ,we may infer that the springs which formed these ancient deposits discharged near what was then the bottom of the valley1 and that as the valley has been deepened by the river the ground water of the region has found lower and lower points of issue4 ,in many parts of the country calcareous springs occur which coat with lime carbonate mosses1 twigs1 and other objects over which their waters flow4 ,such are popularly known as petrifying springs1 although they merely incrust the objects and do not convert them into stone4 ,silica is soluble in alkaline waters1 especially when these are hot4 ,hot springs rising through alkaline siliceous rocks1 such as lavas1 often deposit silica in a white spongy formation known as ,,siliceous ,,sinter1 both by evaporation and by the action of algae which secrete silica from the waters4 ,it is in this way that the cones and mounds of the geysers in the ,yellowstone ,national ,park and in ,iceland have been formed4 ,where water oozes from the earth one may sometimes see a rusty deposit on the ground1 and perhaps an iridescent scum upon the water4 ,the scum is often mistaken for oil1 but at a touch it cracks and breaks1 as oil would not do4 ,it is a film of hydrated iron oxide1 or ,,limonite1 and the spring is an iron1 or chalybeate1 spring4 ,compounds of iron have been taken into solution by ground water from soil and rocks1 and are now changed to the insoluble oxide on exposure to the oxygen of the air4 ,in wet ground iron compounds leached by ground water from the soil often collect in reddish deposits a few feet below the surface1 where their downward progress is arrested by some impervious clay4 ,at the bottom of bogs and shallow lakes iron ores sometimes accumulate to a depth of several feet4 ,decaying organic matter plays a large part in these changes4 ,in its presence the insoluble iron oxides which give color to most red and yellow rocks are decomposed1 leaving the rocks of a gray or bluish color1 and the soluble iron compounds which result are readily leached out1--effects seen where red or yellow clays have been bleached about some decaying tree root4 ,the iron thus dissolved is laid down as limonite when oxidized1 as about a chalybeate spring2 but out of contact with the air and in the presence of carbon dioxide supplied by decaying vegetation1 as in a peat bog1 it may be deposited as iron carbonate1 or ,,siderite4 ,,total ,,amount ,,of ,,underground ,,waters4 ,in order to realize the vast work in solution and cementation which underground waters are now doing and have done in all geological ages1 we must gain some conception of their amount4 ,at a certain depth1 estimated at about six miles1 the weight of the crust becomes greater than the rocks can bear1 and all cavities and pores in them must be completely closed by the enormous pressure which they sustain4 ,below a depth of even three or four miles it is believed that ground water cannot circulate4 ,estimating the average pore spaces of the different rocks of the earth's crust above this depth1 and the average per cents of their pore spaces occupied by water1 it has been recently computed that the total amount of ground water is equal to a sheet of water one hundred feet deep1 covering the entire surface of the earth4 ,,chapter ,,iii ,,rivers ,,and ,,valleys ,,the ,,run-,,off4 ,we have traced the history of that portion of the rainfall which soaks into the ground2 let us now return to that part which washes along the surface and is known as the ,,run-,,off4 ,fed by rains and melting snows1 the run-off gathers into courses1 perhaps but faintly marked at first1 which join more definite and deeply cut channels1 as twigs their stems4 ,in a humid climate the larger ravines through which the run-off flows soon descend below the ground-water surface4 ,here springs discharge along the sides of the little valleys and permanent streams begin4 ,the water supplied by the run-off here joins that part of the rainfall which had soaked into the soil1 and both now proceed together by way of the stream to the sea4 ,,river ,,floods4 ,streams vary greatly in volume during the year4 ,at stages of flood they fill their immediate banks1 or overrun them and inundate any low lands adjacent to the channel2 at stages of low water they diminish to but a fraction of their volume when at flood4 ,at times of flood1 rivers are fed chiefly by the run-off2 at times of low water1 largely or even wholly by springs4 ,how1 then1 will the water of streams differ at these times in turbidity and in the relative amount of solids carried in solution8 ,in parts of ,england streams have been known to continue flowing after eighteen months of local drought1 so great is the volume of water which in humid climates is stored in the rocks above the drainage level1 and so slowly is it given off in springs4 ,in ,illinois and the states adjacent1 rivers remain low in winter and a spring freshet follows the melting of the winter's snows4 ,a ,june rise is produced by the heavy rains of early summer4 ,low water follows in ,july and ,august1 and streams are again swollen to a moderate degree under the rains of autumn4 ,,the ,,discharge ,,of ,,streams4 ,the per cent of rainfall discharged by rivers varies with the amount of rainfall1 the slope of the drainage area1 the texture of the rocks1 and other factors4 ,with an annual rainfall of fifty inches in an open country1 about fifty per cent is discharged2 while with a rainfall of twenty inches only fifteen per cent is discharged1 part of the remainder being evaporated and part passing underground beyond the drainage area4 ,thus the ,ohio discharges thirty per cent of the rainfall of its basin1 while the ,missouri carries away but fifteen per cent4 ,a number of the streams of the semi-arid lands of the ,west do not discharge more than five per cent of the rainfall4 ,other things being equal1 which will afford the larger proportion of run-off1 a region underlain with granite rock or with coarse sandstone8 grass land or forest8 steep slopes or level land8 a well-drained region or one abounding in marshes and ponds8 frozen or unfrozen ground8 ,will there be a larger proportion of run-off after long rains or after a season of drought8 after long and gentle rains1 or after the same amount of precipitation in a violent rain8 during the months of growing vegetation1 from ,june to ,august1 or during the autumn months8 ,,desert ,,streams4 ,in arid regions the ground-water surface lies so low that for the most part stream ways do not intersect it4 ,streams therefore are not fed by springs1 but instead lose volume as their waters soak into the thirsty rocks over which they flow4 ,they contribute to the ground water of the region instead of being increased by it4 ,being supplied chiefly by the run-off1 they wither at times of drought to a mere trickle of water1 to a chain of pools1 or go wholly dry1 while at long intervals rains fill their dusty beds with sudden raging torrents4 ,desert rivers therefore periodically shorten and lengthen their courses1 withering back at times of drought for scores of miles1 or even for a hundred miles from the point reached by their waters during seasons of rain4 ,,the ,,geological ,,work ,,of ,,streams4 ,the work of streams is of three kinds1--transportation1 erosion1 and deposition4 ,streams ,,transport the waste of the land2 they wear1 or ,,erode1 their channels both on bed and banks2 and they ,,deposit portions of their load from time to time along their courses1 finally laying it down in the sea4 ,most of the work of streams is done at times of flood4 ,,transportation ,,the ,,invisible ,,load ,,of ,,streams4 ,of the waste which a river transports we may consider first the invisible load which it carries in solution1 supplied chiefly by springs but also in part by the run-off and from the solution of the rocks of its bed4 ,more than half the dissolved solids in the water of the average river consists of the carbonates of lime and magnesia2 other substances are gypsum1 sodium sulphate 7,glauber's salts71 magnesium sulphate 7,epsom salts71 sodium chloride 7common salt71 and even silica1 the least soluble of the common rock-making minerals4 ,the amount of this invisible load is surprisingly large4 ,the ,mississippi1 for example1 transports each year #aac1jjj1jjj tons of dissolved rock to the ,gulf4 ,,the ,,visible ,,load ,,of ,,streams4 ,this consists of the silt which the stream carries in suspension1 and the sand and gravel and larger stones which it pushes along its bed4 ,especially in times of flood one may note the muddy water1 its silt being kept from settling by the rolling1 eddying currents2 and often by placing his ear close to the bottom of a boat one may hear the clatter of pebbles as they are hurried along4 ,in mountain torrents the rumble of bowlders as they clash together may be heard some distance away4 ,the amount of the load which a stream can transport depends on its velocity4 ,a current of two thirds of a mile per hour can move fine sand1 while one of four miles per hour sweeps along pebbles as large as hen's eggs4 ,the transporting power of a stream varies as the sixth power of its velocity4 ,if its velocity is multiplied by two1 its transporting power is multiplied by the sixth power of two3 it can now move stones sixty-four times as large as it could before4 ,stones weigh from two to three times as much as water1 and in water lose the weight of the volume of water which they displace4 ,what proportion1 then1 of their weight in air do stones lose when submerged8 ,,measurement ,,of ,,stream ,,loads4 ,to obtain the total amount of waste transported by a river is an important but difficult matter4 ,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 second1 giving the discharge per second in cubic feet4 ,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 year1 and drying and weighing the residues4 ,the average amount of silt to the cubic foot of water1 multiplied by the number of cubic feet of water discharged per year1 gives the total load carried in suspension during that time4 ,adding to this the estimated amount of sand and gravel rolled along the bed1 which in many swift rivers greatly exceeds the lighter material held in suspension1 and adding also the total amount of dissolved solids1 we reach the exceedingly important result of the total load of waste discharged by the river4 ,dividing the volume of this load by the area of the river basin gives another result of the greatest geological interest1-- the rate at which the region is being lowered by the combined action of weathering and erosion1 or the rate of denudation4 ,,the ,,rate ,,of ,,denudation ,,of ,,river ,,basins4 ,this rate varies widely4 ,the ,mississippi basin may be taken as a representative land surface because of the varieties of surface1 altitude and slope1 climate1 and underlying rocks which are included in its great extent4 ,careful measurements show that the ,mississippi basin is now being lowered at a rate of one four-thousandth of a foot a year1 or one foot in four thousand years4 ,taking this as the average rate of denudation for the land surfaces of the globe1 estimates have been made of the length of time required at this rate to wash and wear the continents to the level of the sea4 ,as the average elevation of the lands of the globe is reckoned at #bdaa feet1 this result would occur in nine or ten million years1 if the present rate of denudation should remain unchanged4 ,but even if no movements of the earth's crust should lift or depress the continents1 the rate of wear and the removal of waste from their surfaces will not remain the same4 ,it must constantly decrease as the lands are worn nearer to sea level and their slopes become more gentle4 ,the length of time required to wear them away is therefore far in excess of that just stated4 ,the drainage area of the ,potomac is #aa1jjj square miles4 ,the silt brought down in suspension in a year would cover a square mile to the depth of four feet4 ,at what rate is the ,potomac basin being lowered from this cause alone8 ,it is estimated that the ,upper ,ganges is lowering its basin at the rate of one foot in #hbc years1 and the ,po one foot in #gbj years4 ,why so much faster than the ,potomac and the ,mississippi8 ,,how ,,streams ,,get ,,their ,,loads4 ,the load of streams is derived from a number of sources1 the larger part being supplied by the weathering of valley slopes4 ,we have noticed how the mantle of waste creeps and washes to the stream ways4 ,watching the run-off during a rain1 as it hurries muddy with waste along the gutter or washes down the hillside1 we may see the beginning of the route by which the larger part of their load is delivered to rivers4 ,streams also secure some of their load by wearing it from their beds and banks1--a process called erosion4 ,,erosion ,streams erode their beds chiefly by means of their bottom load1-- the stones of various sizes and the sand and even the fine mud which they sweep along4 ,with these tools they smooth1 grind1 and rasp the rock of their beds1 using them in much the fashion of sandpaper or a file4 ,,weathering ,,of ,,river ,,beds4 ,the erosion of stream beds is greatly helped by the work of the weather4 ,especially at low water more or less of the bed is exposed to the action of frost and heat and cold1 joints are opened1 rocks are pried loose and broken up and made ready to be swept away by the stream at time of flood4 ,,potholes4 ,in rapids streams also drill out their rocky beds4 ,where some slight depression gives rise to an eddy1 the pebbles which gather in it are whirled round and round1 and1 acting like the bit of an auger1 bore out a cylindrical pit called a pothole4 ,potholes sometimes reach a depth of a score of feet4 ,where they are numerous they aid materially in deepening the channel1 as the walls between them are worn away and they coalesce4 ,,waterfalls4 ,one of the most effective means of erosion which the river possesses is the waterfall4 ,the plunging water dislodges stones from the face of the ledge over which it pours1 and often undermines it by excavating a deep pit at its base4 ,slice after slice is thus thrown down from the front of the cliff1 and the cataract cuts its way upstream leaving a gorge behind it4 ,,niagara ,,falls4 ,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 region4 ,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 cut4 ,the strata which compose the upland dip gently upstream and consist at top of a massive limestone1 at the ,falls about eighty feet thick1 and below of soft and easily weathered shale4 ,beneath the ,falls the underlying shale is cut and washed away by the descending water and retreats also because of weathering1 while the overhanging limestone breaks down in huge blocks from time to time4 ,niagara is divided by ,goat ,island into the ,horseshoe ,falls and the ,american ,falls4 ,the former is supplied by the main current of the river1 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 depth4 ,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 bed4 ,at the ,american ,falls the lesser branch of the river1 which flows along the ,american side of ,goat ,island1 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 remove4 ,a series of surveys have determined that from #ahdb to #ahij the ,horseshoe ,falls retreated at the rate of #b4ah feet per year1 while the ,american ,falls retreated at the rate of #j4fd feet in the same period4 ,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 miles4 ,while ,niagara ,falls have been cutting back a gorge seven miles long and from two hundred to three hundred feet deep1 the river above the ,falls has eroded its bed scarcely below the level of the upland on which it flows4 ,like all streams which are the outlets of lakes1 the ,niagara flows out of ,lake ,erie clear of sediment1 as from a settling basin1 and carries no tools with which to abrade its bed4 ,we may infer from this instance how slight is the erosive power of clear water on hard rock4 ,assuming that the rate of recession of the combined volumes of the ,american and ,horseshoe ,falls was three feet a year below ,goat ,island1 and ,,assuming ,,that ,,this ,,rate ,,has ,,been ,,uniform ,,in ,,the ,,past1 how long is it since the ,niagara ,river fell over the edge of the escarpment where now is the mouth of the present gorge8 ,the profile of the bed of the ,niagara along the gorge 7,fig4 #ci7 shows alternating deeps and shallows which cannot be accounted for1 except in a single instance1 by the relative hardness of the rocks of the river bed4 ,the deeps do not exceed that at the foot of the ,horseshoe ,falls at the present time4 ,when the gorge was being cut along the shallows1 how did the ,falls compare in excavating power1 in force1 and volume with the ,niagara of to-day8 ,how did the rate of recession at those times compare with the present rate8 ,is the assumption made above that the rate of recession has been uniform correct8 ,the first stretch of shallows below the ,falls causes a tumultuous rapid impossible to sound4 ,its depth has been estimated at thirty- five feet4 ,from what data could such an estimate be made8 ,suggest a reason why the ,horseshoe ,falls are convex upstream4 ,at the present rate of recession which will reach the head of ,goat ,island the sooner1 the ,american or the ,horseshoe ,falls8 ,what will be the fate of the ,falls left behind when the other has passed beyond the head of the island8 ,the rate at which a stream erodes its bed depends in part upon the nature of the rocks over which it flows4 ,will a stream deepen its channel more rapidly on massive or on thin-bedded and close- jointed rocks8 on horizontal strata or on strata steeply inclined8 ,,deposition ,while the river carries its invisible load of dissolved rock on without stop to the sea1 its load of visible waste is subject to many delays en route4 ,now and again it is laid aside1 to be picked up later and carried some distance farther on its way4 ,one of the most striking features of the river therefore is the waste accumulated along its course1 in bars and islands in the channel1 beneath its bed1 and in flood plains along its banks4 ,all this alluvium1 to use a general term for river deposits1 with which the valley is cumbered is really en route to the sea2 it is only temporarily laid aside to resume its journey later on4 ,constantly the river is destroying and rebuilding its alluvial deposits1 here cutting and there depositing along its banks1 here eroding and there building a bar1 here excavating its bed and there filling it up1 and at all times carrying the material picked up at one point some distance on downstream before depositing it at another4 ,these deposits are laid down by slackening currents where the velocity of the stream is checked1 as on the inner side of curves1 and where the slope of the bed is diminished1 and in the lee of islands1 bridge piers and projecting points of land4 ,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 velocity4 ,if the velocity is decreased one half1 the current can move fragments but one sixty-fourth the size of those which it could move before1 and must drop all those of larger size4 ,will a river deposit more at low water or at flood8 when rising or when falling8 ,,stratification4 ,river deposits are stratified1 as may be seen in any fresh cut in banks or bars4 ,the waste of which they are built has been sorted and deposited in layers1 one above another2 some of finer and some of coarser material4 ,the sorting action of running water depends on the fact that its transporting power varies with the velocity4 ,a current whose diminishing velocity compels it to drop coarse gravel1 for example1 is still able to move all the finer waste of its load1 and separating it from the gravel1 carries it on downstream2 while at a later time slower currents may deposit on the gravel bed layers of sand1 and1 still later1 slack water may leave on these a layer of mud4 ,in case of materials lighter than water the transporting power does not depend on the velocity1 and logs of wood1 for instance1 are floated on to the sea on the slowest as well as on the most rapid currents4 ,,cross ,,bedding4 ,a section of a bar exposed at low water may show that it is formed of layers of sand1 or coarser stuff1 inclined downstream as steeply often as the angle of repose of the material4 ,from a boat anchored over the lower end of a submerged sand bar we may observe the way in which this structure1 called cross bedding1 is produced4 ,sand is continually pushed over the edge of the bar at b 7,fig4 #db7 and comes to rest in successive layers on the sloping surface4 ,at the same time the bar may be worn away at the upper end1 a1 and thus slowly advance down stream4 ,while the deposit is thus cross bedded1 it constitutes as a whole a stratum whose upper and lower surfaces are about horizontal4 ,in sections of river banks one may often see a vertical succession of cross-bedded strata1 each built in the way described4 ,,water ,,wear4 ,the coarser material of river deposits1 such as cobblestones1 gravel1 and the larger grains of sand1 are ,,water ,,worn1 or rounded1 except when near their source4 ,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 stream4 ,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 water4 ,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 oak1 the process requiring but about fifteen minutes4 ,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 granite4 ,,land ,,forms ,,due ,,to ,,river ,,erosion ,,river ,,valleys4 ,in their courses to the sea1 rivers follow valleys of various forms1 some shallow and some deep1 some narrow and some wide4 ,since rivers are known to erode their beds and banks1 it is a fair presumption that1 aided by the weather1 they have excavated the valleys in which they flow4 ,moreover1 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 work1 as twigs meet their stems and the branches of a tree its trunk4 ,each valley1 from that of the smallest rivulet to that of the master stream1 is proportionate to the size of the stream which occupies it4 ,with a few explainable exceptions the valleys of tributaries join that of the trunk stream at a level2 there is no sudden descent or break in the bed at the point of juncture4 ,these are the natural consequences which must follow if the land has long been worked upon by streams1 and no other process has ever been suggested which is competent to produce them4 ,we must conclude that valley systems have been formed by the river systems which drain them1 aided by the work of the weather2 they are not gaping fissures in the earth's crust1 as early observers imagined1 but are the furrows which running water has drawn upon the land4 ,as valleys are made by the slow wear of streams and the action of the weather1 they pass in their development through successive stages1 each of which has its own characteristic features4 ,we may therefore classify rivers and valleys according to the stage which they have reached in their life history from infancy to old age4 ,,young ,,river ,,valleys ,,infancy4 ,the ,red ,river of the ,north4 ,a region in northwestern ,minnesota and the adjacent portions of ,north ,dakota and ,manitoba was so recently covered by the waters of an extinct lake1 known as ,lake ,agassiz1 that the surface remains much as it was left when the lake was drained away4 ,the flat floor1 spread smooth with lake-laid silts1 is still a plain1 to the eye as level as the sea4 ,across it the ,red ,river of the ,north and its branches run in narrow1 ditch-like channels1 steep-sided and shallow1 not exceeding sixty feet in depth1 their gradients differing little from the general slopes of the region4 ,the trunk streams have but few tributaries2 the river system1 like a sapling with few limbs1 is still undeveloped4 ,along the banks of the trunk streams short gullies are slowly lengthening headwards1 like growing twigs which are sometime to become large branches4 ,the flat interstream areas are as yet but little scored by drainage lines1 and in wet weather water lingers in ponds in any initial depressions on the plain4 ,,contours4 ,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 relief1 all points on any given contour being of equal height above sea level4 ,the ,,contour ,,interval is the uniform vertical distance between two adjacent contours and varies on different maps4 ,to express regions of faint relief a contour interval of ten or twenty feet is commonly selected2 while in mountainous regions a contour interval of two hundred and fifty1 five hundred1 or even one thousand feet may be necessary in order that the contours may not be too crowded for easy reading4 ,whether a river begins its life on a lake plain1 as in the example just cited1 or upon a coastal plain lifted from beneath the sea or on a spread of glacial drift left by the retreat of continental ice sheets1 such as covers much of ,canada and the northeastern parts of the ,united ,states1 its infantile stage presents the same characteristic features1--a narrow and shallow valley1 with undeveloped tributaries and undrained interstream areas4 ,ground water stands high1 and1 exuding in the undrained initial depressions1 forms marshes and lakes4 ,,lakes4 ,lakes are perhaps the most obvious of these fleeting features of infancy4 ,they are short-lived1 for their destruction is soon accomplished by several means4 ,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 region4 ,lakes having outlets are drained away as their basin rims are cut down by the outflowing streams1--a slow process where the rim is of hard rock1 but a rapid one where it is of soft material such as glacial drift4 ,lakes are effaced also by the filling of their basins4 ,inflowing streams and the wash of rains bring in waste4 ,waves abrade the shore and strew the debris worn from it over the lake bed4 ,shallow lakes are often filled with organic matter from decaying vegetation4 ,does the outflowing stream1 from a lake carry sediment8 ,how does this fact affect its erosive power on hard rock8 on loose material8 ,lake ,geneva is a well-known example of a lake in process of obliteration4 ,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 ,alps4 ,for this distance there extends up the ,rhone ,valley an alluvial plain1 which has grown lakeward at the rate of a mile and a half since ,roman times1 as proved by the distance inland at which a ,roman port now stands4 ,how rapidly a lake may be silted up under exceptionally favorable conditions is illustrated by the fact that over the bottom of the artificial lake1 of thirty-five square miles1 formed behind the great dam across the ,colorado ,river at ,austin1 ,texas1 sediments thirty-nine feet deep gathered in seven years4 ,lake ,mendota1 one of the many beautiful lakes of southern ,wisconsin1 is rapidly cutting back the soft glacial drift of its shores by means of the abrasion of its waves4 ,while the shallow basin is thus broadened1 it is also being filled with the waste2 and the time is brought nearer when it will be so shoaled that vegetation can complete the work of its effacement4 ,along the margin of a shallow lake mosses1 water lilies1 grasses1 and other water-loving plants grow luxuriantly4 ,as their decaying remains accumulate on the bottom1 the ring of marsh broadens inwards1 the lake narrows gradually to a small pond set in the midst of a wide bog1 and finally disappears4 ,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 states2 and more numerous than the lakes which still remain are those already thus filled with carbonaceous matter derived from the carbon dioxide of the atmosphere4 ,such fossil lakes are marked by swamps or level meadows underlain with muck4 ,,the ,,advance ,,to ,,maturity4 ,the infantile stage is brief4 ,as a river advances toward maturity the initial depressions1 the lake basins of its area1 are gradually effaced4 ,by the furrowing action of the rain wash and the head ward lengthening1 of tributaries a branchwork of drainage channels grows until it covers the entire area1 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 gully1 brook1 and river to the sea4 ,the initial surface of the land1 by whatever agency it was modeled1 is now wholly destroyed2 the region is all reduced to valley slopes4 ,,the ,,longitudinal ,,profile ,,of ,a ,,stream4 ,this at first corresponds with the initial surface of the region on which the stream begins to flow1 although its way may lead through basins and down steep descents4 ,the successive profiles to which it reduces its bed are illustrated in ,figure #ea4 ,as the gradient1 or rate of descent of its bed1 is lowered1 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 bed4 ,the river is now ,,at ,,grade1 and its capacity is just equal to its load4 ,if now its load is increased the stream deposits1 and thus builds up1 or ,,aggrades1 its bed4 ,on the other hand1 if its load is diminished it has energy to spare1 and resuming its work of erosion1 ,,degrades its bed4 ,in either case the stream continues aggrading or degrading until a new gradient is found where the velocity is just sufficient to move the load1 and here again it reaches grade4 ,v-,,valleys4 ,vigorous rivers well armed with waste make short work of cutting their beds to grade1 and thus erode narrow1 steep-sided gorges only wide enough at the base to accommodate the stream4 ,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 weather4 ,in resistant rock a swift1 well-laden stream may saw out a gorge whose sides are nearly or even quite vertical1 but as a rule young valleys whose streams have not yet reached grade are ,v-shaped2 their sides flare at the top because here the rocks have longest been opened up to the action of the weather4 ,some of the deepest canyons may be found where a rising land mass1 either mountain range or plateau1 has long maintained by its continued uplift the rivers of the region above grade4 ,in the northern hemisphere the north sides of river valleys are sometimes of more gentle slope than the south sides4 ,can you suggest a reason8 ,,the ,,grand ,,canyon ,,of ,,the ,,colorado ,,river ,,in ,,arizona4 ,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 depth4 ,the rocks in which the canyon is cut are for the most part flat-lying1 massive beds of limestones and sandstones1 with some shales1 beneath which in places harder crystalline rocks are disclosed4 ,where the canyon is deepest its walls have been profoundly dissected4 ,lateral ravines have widened into immense amphitheaters1 leaving between them long ridges of mountain height1 buttressed and rebuttressed with flanking spurs and carved into majestic architectural forms4 ,from the extremity of one of these promontories it is two miles or more across the gulf to the point of the one opposite1 and the heads of the amphitheaters are thirteen miles apart4 ,the lower portion of the canyon is much narrower 7,fig4 #ed7 and its walls of dark crystalline rock sink steeply to the edge of the river1 a swift1 powerful stream a few hundred feet wide1 turbid with reddish silt1 by means of which it continually rasps its rocky bed as it hurries on4 ,the ,colorado is still deepening its gorge4 ,in the ,grand ,canyon its gradient is seven and one half feet to the mile1 but1 as in all ungraded rivers1 the descent is far from uniform4 ,graded reaches in soft rock alternate with steeper declivities in hard rock1 forming rapids such as1 for example1 a stretch of ten miles where the fall averages twenty-one feet to the mile4 ,because of these dangerous rapids the few exploring parties who have traversed the ,colorado canyon have done so at the hazard of their lives4 ,the canyon has been shaped by several agencies4 ,its depth is due to the river which has sawed its way far toward the base of a lofty rising plateau4 ,acting alone this would have produced a slitlike gorge little wider than the breadth of the stream4 ,the impressive width of the canyon and the magnificent architectural masses which fill it are owing to two causes43 ,running water has gulched the walls and weathering has everywhere attacked and driven them back4 ,the horizontal harder beds stand out in long lines of vertical cliffs1 often hundreds of feet in height1 at whose feet talus slopes conceal the outcrop of the weaker strata4 ,as the upper cliffs have been sapped and driven back by the weather1 broad platforms are left at their bases and the sides of the canyon descend to the river by gigantic steps4 ,far up and down the canyon the eye traces these horizontal layers1 like the flutings of an elaborate molding1 distinguishing each by its contour as well as by its color and thickness4 ,the ,grand ,canyon of the ,colorado is often and rightly cited as an example of the stupendous erosion which may be accomplished by a river4 ,and yet the ,colorado is a young stream and its work is no more than well begun4 ,it has not yet wholly reached grade1 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 future4 ,,waterfalls ,,and ,,rapids4 ,before the bed of a stream is reduced to grade it may be broken by abrupt descents which give rise to waterfalls and rapids4 ,such breaks in a river's bed may belong to the initial surface over which it began its course2 still more commonly are they developed in the rock mass through which it is cutting its valley4 ,thus1 wherever a stream leaves harder rocks to flow over softer ones the latter are quickly worn below the level of the former1 and a sharp change in slope1 with a waterfall or rapid1 results4 ,at time of flood young tributaries with steeper courses than that of the trunk stream may bring down stones and finer waste1 which the gentler current cannot move along1 and throw them as a dam across its way4 ,the rapids thus formed are also ephemeral1 for as the gradient of the tributaries is lowered the main stream becomes able to handle the smaller and finer load which they discharge4 ,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 volume1 and thus join it by plunging over the side of its gorge4 ,but as the river approaches grade and slackens its down cutting1 the tributaries sooner or later overtake it1 and effacing their falls1 unite with it on a level4 ,waterfalls and rapids of all kinds are evanescent features of a river's youth4 ,like lakes they are soon destroyed1 and if any long time had already elapsed since their formation they would have been obliterated already4 ,,local ,,baselevels4 ,that balanced condition called grade1 where a river neither degrades its bed by erosion nor aggrades it by deposition1 is first attained along reaches of soft rocks1 ungraded outcrops of hard rocks remaining as barriers which give rise to rapids or falls4 ,until these barriers are worn away they constitute local baselevels1 below which level the stream1 up valley from them1 cannot cut4 ,they are eroded to grade one after another1 beginning with the least strong1 or the one nearest the mouth of the stream4 ,in a similar way the surface of a lake in a river's course constitutes for all inflowing streams a local baselevel1 which disappears when the basin is filled or drained4 ,,mature ,,and ,,old ,,rivers ,maturity is the stage of a river's complete development and most effective work4 ,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 sea4 ,the relief of the land is now at its greatest2 for the main channels have been sunk to grade1 while the divides remain but little worn below their initial altitudes4 ,ground water now stands low4 ,the run-off washes directly to the streams1 with the least delay and loss by evaporation in ponds and marches2 the discharge of the river is therefore at its height4 ,the entire region is dissected by stream ways4 ,the area of valley slopes is now largest and sheds to the streams a heavier load of waste than ever before4 ,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 sea4 ,,lateral ,,erosion4 ,on reaching grade a river ceases to scour its bed1 and it does not again begin to do so until some change in load or volume enables it to find grade at a lower level4 ,on the other hand1 a stream erodes its banks at all stages in its history1 and with graded rivers this process1 called lateral erosion1 or ,,planation1 is specially important4 ,the current of a stream follows the outer side of all curves or bends in the channel1 and on this side it excavates its bed the deepest and continually wears and saps its banks4 ,on the inner side deposition takes place in the more shallow and slower-moving water4 ,the inner bank of bends is thus built out while the outer bank is worn away4 ,by swinging its curves against the valley sides a graded river continually cuts a wider and wider floor4 ,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 slopes4 ,while widening their valleys streams maintain a constant width of channel1 so that a wide-floored valley does not signify that it ever was occupied by a river of equal width4 ,,the ,,gradient4 ,the gradients of graded rivers differ widely4 ,a large river with a light load reaches grade on a faint slope1 while a smaller stream heavily burdened with waste requires a steep slope to give it velocity sufficient to move the load4 ,the ,platte1 a graded river of ,nebraska with its headwaters in the ,rocky ,mountains1 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 plains4 ,it is compelled to maintain a gradient of eight feet to the mile in western ,nebraska4 ,the ,ohio reaches grade with a slope of less than four inches to the mile from ,cincinnati to its mouth1 and the powerful ,mississippi washes along its load with a fall of but three inches per mile from ,cairo to the ,gulf4 ,other things being equal1 which of graded streams will have the steeper gradient1 a trunk stream or its tributaries8 a stream supplied with gravel or one with silt8 ,other factors remaining the same1 what changes would occur if the ,platte should increase in volume8 ,what changes would occur if the load should be increased in amount or in coarseness8 ,,the ,,old ,,age ,,of ,,rivers4 ,as rivers pass their prime1 as denudation lowers the relief of the region1 less waste and finer is washed over the gentler slopes of the lowering hills4 ,with smaller loads to carry1 the rivers now deepen their valleys and find grade with fainter declivities nearer the level of the sea4 ,this limit of the level of the sea beneath which they cannot erode is known as baselevel4 ,footnote3 ,the term baselevel is also used to designate the close approximation to sea level to which streams are able to subdue the land4 ,as streams grow old they approach more and more closely to baselevel1 although they are never able to attain it4 ,some slight slope is needed that water may flow and waste be transported over the land4 ,meanwhile the relief of the land has ever lessened4 ,the master streams and their main tributaries now wander with sluggish currents over the broad valley floors which they have planed away2 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 slopes4 ,mountains and high plateaus are thus reduced to rolling hills1 and at last to plains1 surmounted only by such hills as may still be unreduced to the common level1 because of the harder rocks of which they are composed or because of their distance from the main erosion channels4 ,such regions of faint relief1 worn down to near base level by subaerial agencies1 are known as ,,peneplains 7almost plains74 ,any residual masses which rise above them are called ,,monadnocks1 from the name of a conical peak of ,new ,hampshire which overlooks the now uplifted peneplain of southern ,new ,england4 ,in its old age a region becomes mantled with thick sheets of fine and weathered waste1 slowly moving over the faint slopes toward the water ways and unbroken by ledges of bare rock4 ,in other words1 the waste mantle also is now graded1 and as waterfalls have been effaced in the river beds1 so now any ledges in the wide streams of waste are worn away and covered beneath smooth slopes of fine soil4 ,ground water stands high and may exude in areas of swamp4 ,in youth the land mass was roughhewn and cut deep by stream erosion4 ,in old age the faint reliefs of the land dissolve away1 chiefly under the action of the weather1 beneath their cloak of waste4 ,,the ,,cycle ,,of ,,erosion4 ,the successive stages through which a land mass passes while it is being leveled to the sea constitute together a cycle of erosion4 ,each stage of the cycle from infancy to old age leaves1 as we have seen1 its characteristic records in the forms sculptured on the land1 such as the shapes of valleys and the contours of hills and plains4 ,the geologist is thus able to determine by the land forms of any region the stage in the erosion cycle to which it now belongs1 and knowing what are the earlier stages of the cycle1 to read something of the geological history of the region4 ,,interrupted ,,cycles4 ,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 erosion4 ,of all the various interruptions which may occur the most important are gradual movements of the earth's crust1 by which a region is either depressed or elevated relative to sea level4 ,the ,,depression of a region hastens its old age by decreasing the gradient of streams1 by destroying their power to excavate their beds and carry their loads to a degree corresponding to the amount of the depression1 and by lessening the amount of work they have to do4 ,the slackened river currents deposit their waste in ,hood plains which increase in height as the subsidence continues4 ,the lower courses of the rivers are invaded by the sea and become estuaries1 while the lower tributaries are cut off from the trunk stream4 ,,elevation1 on the other hand1 increases the activity of all agencies of weathering1 erosion1 and transportation1 restores the region to its youth1 and inaugurates a new cycle of erosion4 ,streams are given a steeper gradient1 greater velocity1 and increased energy to carry their loads and wear their beds4 ,they cut through the alluvium of their flood plains1 leaving it on either bank as successive terraces1 and intrench themselves in the underlying rock4 ,in their older and wider valleys they cut narrow1 steep-walled inner gorges1 in which they flow swiftly over rocky floors1 broken here and there by falls and rapids where a harder layer of rock has been discovered4 ,winding streams on plains may thus incise their meanders in solid rock as the plains are gradually uplifted4 ,streams which are thus restored to their youth are said to be ,,revived4 ,as streams cut deeper and the valley slopes are steepened1 the mantle of waste of the region undergoing elevation is set in more rapid movement4 ,it is now removed particle by particle faster than it forms4 ,as the waste mantle thins1 weathering attacks the rocks of the region more energetically until an equilibrium is reached again2 the rocks waste rapidly and their waste is as rapidly removed4 ,,dissected ,,peneplains4 ,when a rise of the land brings one cycle to an end and begins another1 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 destroyed4 ,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 youth4 ,thus many peneplains which have been elevated and dissected have been recognized by the remnants of their ancient erosion surfaces1 and the length of time which has elapsed since their uplift has been measured by the stage to which the new cycle has advanced4 ,,the ,,piedmont ,,belt4 ,as an example of an ancient peneplain uplifted and dissected we may cite the ,piedmont ,belt1 a broad upland lying between the ,appalachian ,mountains and the ,atlantic coastal plain4 ,the surface of the ,piedmont is gently rolling4 ,the divides1 which are often smooth areas of considerable width1 rise to a common plane1 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 7,fig4 #fb74 ,the surface is an ancient one1 for the mantle of residual waste lies deep upon it1 soils are reddened by long oxidation1 and the rocks are rotted to a depth of scores of feet4 ,at present1 however1 the waste mantle is not forming so rapidly as it is being removed4 ,the streams of the upland are actively engaged in its destruction4 ,they flow swiftly in narrow1 rock- walled valleys over rocky beds4 ,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 upraised4 ,if now we imagine the valleys refilled with the waste which the streams have swept away1 and the upland lowered1 we restore the ,piedmont region to the condition in which it stood before its uplift and dissection1--a gently rolling plain1 surmounted here and there by isolated hills and ridges4 ,the surface of the ancient ,piedmont plain1 as it may be restored from the remnants of it found on the divides1 is not in accordance with the structures of the country rocks4 ,where these are exposed to view they are seen to be far from horizontal4 ,on the walls of river gorges they dip steeply and in various directions and the streams flow over their upturned edges4 ,as shown in ,figure #fg1 the rocks of the ,piedmont have been folded and broken and tilted4 ,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 plain4 ,the upturned layers have not always stopped abruptly at the even surface of the ,piedmont plain which now cuts across them4 ,they are the bases of great folds and tilted blocks which must once have risen high in air4 ,the complex and disorderly structures of the ,piedmont rocks are those seen in great mountain ranges1 and there is every reason to believe that these rocks after their deformation rose to mountain height4 ,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 fibers4 ,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 uplifted2 nor was it a structural plain1 due to the resistance to erosion of some hard1 flat-lying layer of rock4 ,even surfaces developed on rocks of discordant structure1 such as the ,piedmont shows1 are produced by long denudation1 and we may consider the ,piedmont as a peneplain formed by the wearing down of mountain ranges1 and recently uplifted4 ,,the ,,laurentian ,,peneplain4 ,this is the name given to a denuded surface on very ancient rocks which extends from the ,arctic ,ocean to the ,st4 ,lawrence ,river and ,lake ,superior1 with small areas also in northern ,wisconsin and ,new ,york4 ,throughout this ,u-shaped area1 which incloses ,hudson ,bay within its arms1 the country rocks have the complicated and contorted structures which characterize mountain ranges4 ,but the surface of the area is by no means mountainous4 ,the sky line when viewed from the divides is unbroken by mountain peaks or rugged hills4 ,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 ,saguenay4 ,this immense area may be regarded as an ancient peneplain truncating the bases of long-vanished mountains and dissected after elevation4 ,in the examples cited the uplift has been a broad one and to comparatively little height4 ,where peneplains have been uplifted to great height and have since been well dissected1 and where they have been upfolded and broken and uptilted1 their recognition becomes more difficult4 ,yet recent observers have found evidences of ancient lowland surfaces of erosion on the summits of the ,allegheny ridges1 the ,cascade ,mountains 7,fig4 #fi71 and the western slope of the ,sierra ,nevadas4 ,,the ,,southern ,,appalachian ,,region4 ,we have here an example of an area the latter part of whose geological history may be deciphered by means of its land forms4 ,the generalized section of ,figure #gj1 which passes from west to east across a portion of the region in eastern ,tennessee1 shows on the west a part of the broad ,cumberland plateau4 ,on the east is a roughened upland platform1 from which rise in the distance the peaks of the ,great ,smoky ,mountains4 ,the plateau1 consisting of strata but little changed from their original flat-lying attitude1 and the platform1 developed on rocks of disordered structure made crystalline by heat and pressure1 both stand at the common level of the line ,,ab4 ,they are separated by the ,appalachian valley1 forty miles wide1 cut in strata which have been folded and broken into long narrow blocks4 ,the valley is traversed lengthwise by long1 low ridges1 the outcropping edges of the harder strata1 which rise to about the same level1--that of the line cd4 ,between these ridges stretch valley lowlands at the level ef excavated in the weaker rocks1 while somewhat below them lie the channels of the present streams now busily engaged in deepening their beds4 ,,the ,,valley ,,lowlands4 ,were they planed by graded or ungraded streams8 ,have the present streams reached grade8 ,why did the streams cease widening the floors of the valley lowlands8 ,how long since8 ,when will they begin anew the work of lateral planation8 ,what effect will this have on the ridges if the present cycle of erosion continues long uninterrupted8 ,,the ,,ridges ,,of ,,the ,,appalachian ,,valley4 ,why do they stand above the valley lowlands8 ,why do their summits lie in about the same plane8 ,refilling the valleys intervening between these ridges with the material removed by the streams1 what is the nature of the surface thus restored8 ,does this surface cd accord with the rock structures on which' it has been developed8 ,how may it have been made8 ,at what height did the land stand then1 compared with its present height8 ,what elevations stood above the surface cd8 ,why8 ,what name may you use to designate them8 ,how does the length of time needed to develop the surface cd compare with that needed to develop the valley lowlands8 ,,the ,,platform ,,and ,,plateau4 ,why do they stand at a common level ab8 ,of what surface may they be remnants8 ,is it accordant with the rock structure8 ,how was it produced8 ,what unconsumed masses overlooked it8 ,did the rocks of the ,appalachian valley stand above this surface when it was produced8 ,did they then stand below it8 ,compare the time needed to develop this surface with that needed to develop cd4 ,which surface is the older8 ,how many cycles of erosion are represented here8 ,give the erosion history of the region by cycles1 beginning with the oldest1 the work done in each and the work left undone1 what brought each cycle to a close1 and how long relatively it continued4 ,,chapter ,,iv ,,river ,,deposits ,the characteristic features of river deposits and the forms which they assume may be treated under three heads3 7#a7 valley deposits1 7#b7 basin deposits1 and 7#c7 deltas4 ,,valley ,,deposits ,,flood ,,plains are the surfaces of the alluvial deposits which streams build along their courses at times of flood4 ,a swift current then sweeps along the channel1 while a shallow sheet of water moves slowly over the flood plain1 spreading upon it a thin layer of sediment4 ,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 ,egypt4 ,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 7,fig4 #fj74 ,flood-plain deposits of great thickness may be built by aggrading rivers even in valleys whose rock floors have never been thus widened4 ,a cross section of a flood plain shows that it is highest next the river1 sloping gradually thence to the valley sides4 ,these wide natural embankments are due to the fact that the river deposit is heavier near the bank1 where the velocity of the silt-laden channel current is first checked by contact with the slower-moving overflow4 ,thus banked off from the stream1 the outer portions of a flood plain are often ill-drained and swampy1 and here vegetal deposits1 such as peat1 may be interbedded with river silts4 ,a map of a wide flood plain1 such as that of the ,mississippi or the ,missouri 7,fig4 #gg71 shows that the courses of the tributaries on entering it are deflected downstream4 ,why8 ,the aggrading streams by which flood plains are constructed gradually build their immediate banks and beds to higher and higher levels1 and therefore find it easy at times of great floods to break their natural embankments and take new courses over the plain4 ,in this way they aggrade each portion of it in turn by means of their shifting channels1 ,,braided ,,channels4 ,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 channels4 ,from time to time a channel becomes choked with waste1 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 channels4 ,thus there becomes established a network of ever-changing channels inclosing low islands of sand and gravel4 ,,terraces4 ,while aggrading streams thus tend to shift their channels1 degrading streams1 on the contrary1 become more and more deeply intrenched in their valleys4 ,it often occurs that a stream1 after having built a flood plain1 ceases to aggrade its bed because of a lessened load or for other reasons1 such as an uplift of the region1 and begins instead to degrade it4 ,it leaves the original flood plain out of reach of even the highest floods4 ,when again it reaches grade at a lower level it produces a new flood plain by lateral erosion in the older deposits1 remnants of which stand as terraces on one or both sides of the valley4 ,in this way a valley may be lined with a succession of terraces at different levels1 each level representing an abandoned flood plain4 ,,meanders4 ,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 meanders1 from the name of a winding river of ,asia ,minor4 ,the giant ,mississippi has developed meanders with a radius of one and one half miles1 but a little creek may display on its meadow as perfect curves only a rod or so in radius4 ,on the flood plain of either river or creek we may find examples of the successive stages in the development of the meander1 from its beginning in the slight initial bend sufficient to deflect the current against the outer side4 ,eroding here and depositing on the inner side of the bend1 it gradually reaches first the open bend whose width and length are not far from equal1 and later that of the horseshoe meander whose diameter transverse to the course of the stream is much greater than that parallel with it4 ,little by little the neck of land projecting into the bend is narrowed1 until at last it is cut through and a cut-off is established4 ,the old channel is now silted up at both ends and becomes a crescentic lagoon1 or oxbow lake1 which fills gradually to an arc-shaped shallow depression4 ,,flood ,,plains ,,characteristic ,,of ,,mature ,,rivers4 ,on reaching grade a stream planes a flat floor for its continually widening valley4 ,ever cutting on the outer bank of its curves1 it deposits on the inner bank scroll-like flood-plain patches4 ,for a while the valley bluffs do not give its growing meanders room to develop to their normal size1 but as planation goes on1 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 side4 ,usually a river first attains grade near its mouth1 and here first sinks its bed to near baselevel4 ,extending its graded course upstream by cutting away barrier after barrier1 it comes to have a widened and mature valley over its lower course1 while its young headwaters are still busily eroding their beds4 ,its ungraded branches may thus bring down to its lower course more waste than it is competent to carry on to the sea1 and here it aggrades its bed and builds a flood plain in order to gain a steeper gradient and velocity enough to transport its load4 ,as maturity is past and the relief of the land is lessened1 a smaller and smaller load of waste is delivered to the river4 ,it now has energy to spare and again degrades its valley1 excavating its former flood plains and leaving them in terraces on either side1 and at last in its old age sweeping them away4 ,,alluvial ,,cones ,,and ,,fans4 ,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 stream4 ,the cause is obvious3 the young branch has not been able as yet to wear its bed to accordant level with the already deepened valley of the master stream4 ,it therefore builds its bed to grade at the point of juncture by depositing here its load of waste1--a load too heavy to be carried along the more gentle profile of the trunk valley4 ,where rivers descend from a mountainous region upon the plain they may build alluvial fans of exceedingly gentle slope4 ,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 instruments1 where they leave the rock-cut canyons in the mountains and descend upon the broad central valley of ,california4 ,as a river flows over its fan it commonly divides into a branchwork of shifting channels called ,,distributaries1 since they lead off the water from the main stream4 ,in this way each part of the fan is aggraded and its symmetric form is preserved4 ,,piedmont ,,plains4 ,mountain streams may build their confluent fans into widespread piedmont 7foot of the mountain7 alluvial plains4 ,these are especially characteristic of arid lands1 where the streams wither as they flow out upon the thirsty lowlands and are therefore compelled to lay down a large portion of their load4 ,in humid climates mountain-born streams are usually competent to carry their loads of waste on to the sea1 and have energy to spare to cut the lower mountain slopes into foothills4 ,in arid regions foothills are commonly absent and the ranges rise1 as from pedestals1 above broad1 sloping plains of stream-laid waste4 ,,the ,,high ,,plains4 ,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 7,fig4 #hj74 ,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 extinct4 ,that it was laid by rivers and not by fresh-water lakes is shown by its structure4 ,wide stretches of flat-lying1 clays and sands are interrupted by long1 narrow belts of gravel which mark the channels of the ancient streams4 ,gravels1 and sands are often cross bedded1 and their well worn pebbles may be identified with the rocks of the mountains4 ,after building this sheet of waste the streams ceased to aggrade and began the work of destruction4 ,large uneroded remnants1 their surfaces flat as a floor1 remain as the ,high ,plains of western ,kansas and ,nebraska4 ,,river ,,deposits ,,in ,,subsiding ,,troughs4 ,to a geologist the most important river deposits are those which gather in areas of gradual subsidence2 they are often of vast extent and immense thickness1 and such deposits of past geological ages have not infrequently been preserved1 with all their records of the times in which they were built1 by being carried below the level of the sea1 to be brought to light by a later uplift4 ,on the other hand1 river deposits which remain above baselevels of erosion are swept away comparatively soon4 ,,the ,,great ,,valley ,,of ,,california is a monotonously level plain of great fertility1 four hundred miles in length and fifty miles in average width1 built of waste swept down by streams from the mountain ranges which inclose it1--the ,sierra ,nevada on the east and the ,coast ,range on the west4 ,on the waste slopes at the foot of the bordering hills coarse gravels and even bowlders are left1 while over the interior the slow-flowing streams at times of flood spread wide sheets of silt4 ,organic deposits are now forming by the decay of vegetation in swampy tule 7reed7 lands and in shallow lakes which occupy depressions left by the aggrading streams4 ,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 shells4 ,these are land deposits1 and the absence of any marine deposits among them proves that the region has not been invaded by the sea since the accumulation began4 ,it has therefore been slowly subsiding and its streams1 although continually carried below grade1 have yet been able to aggrade the surface as rapidly as the region sank1 and have maintained it1 as at present1 slightly above sea level4 ,,the ,,indo-,,gangetic ,,plain1 spread by the ,brahmaputra1 the ,ganges1 and the ,indus river systems1 stretches for sixteen hundred miles along the southern base of the ,himalaya ,mountains and occupies an area of three hundred thousand square miles 7,fig4#cdb74 ,it consists of the flood plains of the master streams and the confluent fans of the tributaries which issue from the mountains on the north4 ,large areas are subject to overflow each season of flood1 and still larger tracts mark abandoned flood plains below which the rivers have now cut their beds4 ,the plain is built of far- stretching beds of clay1 penetrated by streaks of sand1 and also of gravel near the mountains4 ,beds of impure peat occur in it1 and it contains fresh-water shells and the bones of land animals of species now living in northern ,india4 ,at ,lucknow an artesian well was sunk to one thousand feet below sea level without reaching the bottom of these river-laid sands and silts1 proving a slow subsidence with which the aggrading rivers have kept pace4 ,,warped ,,valleys4 ,it is not necessary that an area should sink below sea level in order to be filled with stream-swept waste4 ,high valleys among growing mountain ranges may suffer warping1 or may be blockaded by rising mountain folds athwart them4 ,where the deformation is rapid enough1 the river may be ponded and the valley filled with lake-laid sediments4 ,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 continually1 filling the valley with river deposits which may grow to an enormous thickness4 ,behind the outer ranges of the ,himalaya ,mountains lie several waste-filled valleys1 the largest of which are ,kashmir and ,nepal1 the former being an alluvial plain about as large as the state of ,delaware4 ,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 mountains4 ,their present flood plains lie as much as some hundreds of feet below wide alluvial terraces which mark their former levels4 ,indeed1 the horizontal beds of the ,hundes ,valley have been trenched to the depth of nearly three thousand feet by the ,sutlej ,river4 ,these deposits are recent or subrecent1 for there have been found at various levels the remains of land plants and land and fresh-water shells1 and in some the bones of such animals as the hyena and the goat1 of species or of genera now living4 ,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 them4 ,,characteristics ,,of ,,river ,,deposits4 ,the examples just cited teach clearly the characteristic features of extensive river deposits4 ,these deposits consist of broad1 flat-lying sheets of clay and fine sand left by the overflow at time of flood1 and traversed here and there by long1 narrow strips of coarse1 cross-bedded sands and gravels thrown down by the swifter currents of the shifting channels4 ,occasional beds of muck mark the sites of shallow lakelets or fresh-water swamps4 ,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 building4 ,river shells such as the mussel1 land shells such as those of snails1 the bones of fishes and of such land animals as suffer drowning at times of flood or are mired in swampy places1 logs of wood1 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 time1 and as proof that the deposits were laid by streams and not beneath the sea4 ,,basin ,,deposits ,,deposits ,,in ,,dry ,,basins4 ,on desert areas without outlet to the sea1 as on the ,great ,basin of the ,united ,states and the deserts of central ,asia1 stream-swept waste accumulates indefinitely4 ,the rivers of the surrounding mountains1 fed by the rains and melting snows of these comparatively moist elevations1 dry and soak away as they come down upon the arid plains4 ,they are compelled to lay aside their entire load of waste eroded from the mountain valleys1 in fans which grow to enormous size1 reaching in some instances thousands of feet in thickness4 ,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 region4 ,for about seven hundred miles from its mouth in ,aral ,lake the ,oxus receives no tributaries1 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 stream4 ,these aggrading rivers1 which have channels but no valleys1 spread their muddy floods--which in the case of the ,oxus sometimes equal the average volume of the ,mississippi--far and wide over the plain1 washing the bases of the desert dunes4 ,,playas4 ,in arid interior basins the central depressions may be occupied by playas1--plains of fine mud washed forward from the margins4 ,in the wet season the playa is covered with a thin sheet of muddy water1 a playa lake1 supplied usually by some stream at flood4 ,in the dry season the lake evaporates1 the river which fed it retreats1 and there is left to view a hard1 smooth1 level floor of sun-baked and sun-cracked yellow clay utterly devoid of vegetation4 ,in the ,black ,rock desert of ,nevada a playa lake spreads over an area fifty miles long and twenty miles wide4 ,in summer it disappears2 the ,quinn ,river1 which feeds it1 shrinks back one hundred miles toward its source1 leaving an absolutely barren floor of clay1 level as the sea4 ,,lake ,,deposits4 ,regarding lakes as parts of river systems1 we may now notice the characteristic features of the deposits in lake basins4 ,soundings in lakes of considerable size and depth show that their bottoms are being covered with tine clays4 ,sand and gravel are found along2 their margins1 being brought in by streams and worn by waves from the shore1 but there are no tidal or other strong currents to sweep coarse waste out from shore to any considerable distance4 ,where fine clays are now found on the land in even1 horizontal layers containing the remains of fresh-water animals and plants1 uncut by channels tilled with cross-bedded gravels and sands and bordered by beach deposits of coarse waste1 we may safely infer the existence of ancient lakes4 ,,marl4 ,marl is a soft1 whitish deposit of carbonate of lime1 mingled often with more or less of clay1 accumulated in small lakes whose feeding springs are charged with carbonate of lime and into which little waste is washed from the land4 ,such lakelets are not infrequent on the surface of the younger drift sheets of ,michigan and northern ,indiana1 where their beds of marl--sometimes as much as forty feet thick--are utilized in the manufacture of ,portland cement4 ,the deposit results from the decay of certain aquatic plants which secrete lime carbonate from the water1 from the decomposition of the calcareous shells of tiny mollusks which live in countless numbers on the lake floor1 and in some cases apparently from chemical precipitation4 ,,peat4 ,we have seen how lakelets are extinguished by the decaying remains of the vegetation which they support4 ,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 tissue1 which passes downward gradually into ,,peat1--a dense1 dark brown carbonaceous deposit in which1 to the unaided eye1 little or no trace of vegetable structure remains4 ,when dried1 peat forms a fuel of some value and is used either cut into slabs and dried or pressed into bricks by machinery4 ,when vegetation decays in open air the carbon of its tissues1 taken from the atmosphere by the leaves1 is oxidized and returned to it in its original form of carbon dioxide4 ,but decomposing in the presence of water1 as in a bog1 where the oxygen of the air is excluded1 the carbonaceous matter of plants accumulates in deposits of peat4 ,peat bogs are numerous in regions lately abandoned by glacier ice1 where river systems are so immature that the initial depressions left in the sheet of drift spread over the country have not yet been drained4 ,one tenth of the surface of ,ireland is said to be covered with peat1 and small bogs abound in the drift-covered area of ,new ,england and the states lying as far west as the ,missouri ,river4 ,in ,massachusetts alone it has been reckoned that there are fifteen billion cubic feet of peat1 the largest bog occupying several thousand acres4 ,much larger swamps occur on the young coastal plain of the ,atlantic from ,new ,jersey to ,florida4 ,the ,dismal ,swamp1 for example1 in ,virginia and ,north ,carolina is forty miles across4 ,it is covered with a dense growth of water-loving trees such as the cypress and black gum4 ,the center of the swamp is occupied by ,lake ,drummond1 a shallow lake seven miles in diameter1 with banks of pure-peat1 and still narrowing from the encroachment of vegetation along its borders4 ,,salt ,,lakes4 ,in arid climates a lake rarely receives sufficient inflow to enable it to rise to the basin rim and find an outlet4 ,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 streams4 ,as such a lake has no outlet1 the minerals in solution brought into it by its streams cannot escape from the basin4 ,the lake water becomes more and more heavily charged with such substances as common salt and the sulphates and carbonates of lime1 of soda1 and of potash1 and these are thrown down from solution one after another as the point of saturation for each mineral is reached4 ,carbonate of lime1 the least soluble and often the most abundant mineral brought in1 is the first to be precipitated4 ,as concentration goes on1 gypsum1 which is insoluble in a strong brine1 is deposited1 and afterwards common salt4 ,as the saltness of the lake varies with the seasons and with climatic changes1 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 flood4 ,few forms of life can live in bodies of salt water so concentrated that chemical deposits take place1 and hence the beds of salt1 gypsum1 and silt of such lakes are quite barren of the remains of life4 ,similar deposits are precipitated by the concentration of sea water in lagoons and arms of the sea cut off from the ocean4 ,,lakes ,,bonneville ,,and ,,lahontan4 ,these names are given to extinct lakes which once occupied large areas in the ,great ,basin1 the former in ,utah1 the latter in northwestern ,nevada4 ,their records remain in old horizontal beach lines which they drew along their mountainous shores at the different levels at which they stood1 and in the deposits of their beds4 ,at its highest stage ,lake ,bonneville1 then one thousand feet deep1 overflowed to the north and was a fresh-water lake4 ,as it shrank below the outlet it became more and more salty1 and the ,great ,salt ,lake1 its withered residue1 is now depositing salt along its shores4 ,in its strong brine lime carbonate is insoluble1 and that brought in by streams is thrown down at once in the form of travertine4 ,lake ,lahontan never had an outlet4 ,the first chemical deposits to be made along its shores were deposits of travertine1 in places eighty feet thick4 ,its floor is spread with fine clays1 which must have been laid in deep1 still water1 and which are charged with the salts absorbed by them as the briny water of the lake dried away4 ,these sedimentary clays are in two divisions1 the upper and lower1 each being about one hundred feet thick4 ,they are separated by heavy deposits of well-rounded1 cross-bedded gravels and sands1 similar to those spread at the present time by the intermittent streams of arid regions4 ,a similar record is shown in the old floors of ,lake ,bonneville4 ,what conclusions do you draw from these facts as to the history of these ancient lakes8 ,,deltas ,in the river deposits which are left above sea level particles of waste are allowed to linger only for a time4 ,from alluvial fans and flood plains they are constantly being taken up and swept farther on downstream4 ,although these land forms may long persist1 the particles which compose them are ever changing4 ,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 sides1 and slowly moving onwards to the sea4 ,in basins waste finds a longer rest1 but sooner or later lakes and dry basins are drained or filled1 and their deposits1 if above sea level1 resume their journey to their final goal4 ,it is only when carried below the level of the sea that they are indefinitely preserved4 ,on reaching this terminus1 rivers deliver their load to the ocean4 ,in some cases the ocean is able to take it up by means of strong tidal and other currents1 and to dispose of it in ways which we shall study later4 ,but often the load is so large1 or the tides are so weak1 that much of the waste which the river brings in settles at its mouth1 there building up a deposit called the ,,delta1 from the ,greek letter of that name1 whose shape it sometimes resembles4 ,deltas and alluvial fans have many common characteristics4 ,both owe their origin to a sudden check in the velocity of the river1 compelling a deposit of the load2 both are triangular in outline1 the apex pointing upstream2 and both are traversed by distributaries which build up all parts in turn4 ,in a delta we may distinguish deposits of two distinct kinds1-- the submarine and the subaerial4 ,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 mouth4 ,the origin of these deposits is recorded in the remains of marine animals and plants which they contain4 ,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 plain1 of which indeed the subaerial delta is the prolongation4 ,here extended deposits of peat may accumulate in swamps1 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 flood4 ,borings made in the deltas of great rivers such as the ,mississippi1 the ,ganges1 and the ,nile1 show that the subaerial portion often reaches a surprising thickness4 ,layers of peat1 old soils1 and forest grounds with the stumps of trees are discovered hundreds of feet below sea level4 ,in the ,nile delta some eight layers of coarse gravel were found interbedded with river silts1 and in the ,ganges delta at ,calcutta a boring nearly five hundred feet in depth stopped in such a layer4 ,the ,mississippi has built a delta of twelve thousand three hundred square miles1 and is pushing the natural embankments of its chief distributaries into the ,gulf at a maximum rate of a mile in sixteen years4 ,muddy shoals surround its front1 shallow lakes1 e4g4 lakes ,pontchartrain and ,borgne1 are formed between the growing delta and the old shore line1 and elongate lakes and swamps are inclosed between the natural embankments of the distributaries4 ,the delta of the ,indus ,river1 ,india1 lies so low along shore that a broad tract of country is overflowed by the highest tides4 ,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 water4 ,a former arm of the sea1 the ,rann of ,cutch1 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 breadth4 ,each summer it is flooded with salt water when the sea is brought in by strong southwesterly monsoon winds1 and the climate during the remainder of the year is hot and dry4 ,by the evaporation of sea water the soil is thus left so salty that no vegetation can grow upon it1 and in places beds of salt several feet in thickness have accumulated4 ,under like conditions salt beds of great thickness have been formed in the past and are now found buried among the deposits of ancient deltas4 ,,subsidence ,,of ,,great ,,deltas4 ,as a rule great deltas are slowly sinking4 ,in some instances upbuilding by river deposits has gone on as rapidly as the region has subsided4 ,the entire thickness of the ,ganges delta1 for example1 so far as it has been sounded1 consists of deposits laid in open air4 ,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 sea4 ,it is by gradual depression that delta deposits attain enormous thickness1 and1 being lowered beneath the level of the sea1 are safely preserved from erosion until a movement of the earth's crust in the opposite direction lifts them to form part of the land4 ,we shall read later in the hard rocks of our continent the records of such ancient deltas1 and we shall not be surprised to find them as thick as are those now building at the mouths of great rivers4 ,,lake ,,deltas4 ,deltas are also formed where streams lose their velocity on entering the still waters of lakes4 ,the shore lines of extinct lakes1 such as ,lake ,agassiz and ,lakes ,bonneville and ,lahontan1 may be traced by the heavy deposits at the mouths of their tributary streams4 ,we have seen that the work of streams is to drain the lands of the water poured upon them by the rainfall1 to wear them down1 and to carry their waste away to the sea1 there to be rebuilt by other agents into sedimentary rocks4 ,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 ocean4 ,in all geological times1 as at the present1 the work of streams has been to destroy the lands1 and in so doing to furnish to the ocean the materials from which the lands of future ages were to be made4 ,before we consider how the waste of the land brought in by streams is rebuilt upon the ocean floor1 we must proceed to study the work of two agents1 glacier ice and the wind1 which cooperate with rivers in the denudation of the land4 ,,chapter ,v ,,the ,,work ,,of ,,glaciers ,,the ,,drift4 ,the surface of northeastern ,north ,america1 as far south as the ,ohio and ,missouri rivers1 is generally covered by the drift1--a formation which is quite unlike any which we have so far studied4 ,a section of it1 such as that illustrated in ,figure #hg1 shows that for the most part it is unstratified1 consisting of clay1 sand1 pebbles1 and even large bowlders1 all mingled pell- mell together4 ,the agent which laid the drift is one which can carry a load of material of all sizes1 from the largest bowlder to the finest clay1 and deposit it without sorting4 ,the stones of the drift are of many kinds4 ,the region from which it was gathered may well have been large in order to supply these many different varieties of rocks4 ,pebbles and bowlders have been left far from their original homes1 as may be seen in southern ,iowa1 where the drift contains nuggets of copper brought from the region about ,lake ,superior4 ,the agent which laid the drift is one able to gather its load over a large area and carry it a long way4 ,the pebbles of the drift are unlike those rounded by running water or by waves4 ,they are marked with scratches4 ,some are angular1 many have had their edges blunted1 while others have been ground flat and smooth on one or more sides1 like gems which have been faceted by being held firmly against the lapidary's wheel4 ,in many places the upper surface of the country rock beneath the drift has been swept clean of residual clays and other waste4 ,all rock rotten has been planed away1 and the ledges of sound rock to which the surface has been cut down have been rubbed smooth and scratched with long1 straight1 parallel lines4 ,the agent which laid the drift can hold sand and pebbles firmly in its grasp and can grind them against the rock beneath1 thus planing it down and scoring it1 while faceting the pebbles also4 ,neither water nor wind can do these things4 ,indeed1 nothing like the drift is being formed by any process now at work anywhere in the eastern ,united ,states4 ,to find the agent which has laid this extensive formation we must go to a region of different climatic conditions4 ,,the ,,inland ,,ice ,,of ,,greenland4 ,greenland is about fifteen hundred miles long and nearly seven hundred miles in greatest width4 ,with the exception of a narrow fringe of mountainous coast land1 it is completely buried beneath a sheet of ice1 in shape like a vast white shield1 whose convex surface rises to a height of nine thousand feet above the sea4 ,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 snow4 ,on the smooth plain of the interior no rock waste relieves the snow's dazzling whiteness2 no streams of running water are seen2 the silence is broken only by howling storm winds and the rustle of the surface snow which they drive before them4 ,sounding with long poles1 explorers find that below the powdery snow of the latest snowfall lie successive layers of earlier snows1 which grow more and more compact downward1 and at last have altered to impenetrable ice4 ,the ice cap formed by the accumulated snows of uncounted centuries may well be more than a mile in depth4 ,ice thus formed by the compacting of snow is distinguished when in motion as ,,glacier ,,ice4 ,the inland ice of ,greenland moves4 ,it flows with imperceptible slowness under its own weight1 like1 a mass of some viscous or plastic substance1 such as pitch or molasses candy1 in all directions outward toward the sea4 ,near the edge it has so thinned that mountain peaks are laid bare1 these islands in the sea of ice being known as ,,nunataks4 ,down the valleys of the coastal belt it drains in separate streams of ice1 or ,,glaciers4 ,the largest of these reach the sea at the head of inlets1 and are therefore called ,,tide ,,glaciers4 ,their fronts stand so deep in sea water that there is visible seldom more than three hundred feet of the wall of ice1 which in many glaciers must be two thousand and more feet high4 ,from the sea walls of tide glaciers great fragments break off and float away as icebergs4 ,thus snows which fell in the interior of this northern land1 perhaps many thousands of years ago1 are carried in the form of icebergs to melt at last in the ,north ,atlantic4 ,greenland1 then1 is being modeled over the vast extent of its interior not by streams of running water1 as are regions in warm and humid climates1 nor by currents of air1 as are deserts to a large extent1 but by a sheet of flowing ice4 ,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 edge4 ,,the ,,smaller ,,greenland ,,glaciers4 ,many of the smaller glaciers of ,greenland do not reach the sea1 but deploy on plains of sand and gravel4 ,the edges of these ice tongues are often as abrupt as if sliced away with a knife 7,fig4 #ib71 and their structure is thus readily seen4 ,they are stratified1 their layers representing in part the successive snowfalls of the interior of the country4 ,the upper layers are commonly white and free from stones2 but the lower layers1 to the height of a hundred feet or more1 are dark with debris which is being slowly carried on4 ,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 edges4 ,the waste beneath and about the glacier is unsorted4 ,the stones are of many kinds1 and numbers of them have been ground to flat faces4 ,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 glacier4 ,we have now found in glacier ice an agent able to produce the drift of ,north ,america4 ,the ice sheet of ,greenland is now doing what we have seen was done in the recent past in our own land4 ,it is carrying for long distances rocks of many kinds gathered1 we may infer1 over a large extent of country4 ,it is laying down its load without assortment in unstratified deposits4 ,it grinds down and scores the rock over which it moves1 and in the process many of the pebbles of its load are themselves also ground smooth and scratched4 ,since this work can be done by no other agent1 we must conclude that the northeastern part of our own continent was covered in the recent past by glacier ice1 as ,greenland is to-day4 ,,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 ,alaska1 the western mountains of the ,united ,states and ,canada1 the ,himalayas1 and the ,alps4 ,as the glaciers of the ,alps have been studied longer and more thoroughly than any others1 we shall describe them in some detail as examples of valley glaciers in all parts of the world4 ,,conditions ,,of ,,glacier ,,formation4 ,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 ,alps4 ,there is abundant moisture brought by the winds from neighboring seas4 ,the currents of moist air driven up the mountain slopes are cooled by their own expansion as they rise1 and the moisture which they contain is condensed at a temperature at or below #cb degrees ,f41 and therefore is precipitated in the form of snow4 ,the summers are cool and their heat does not suffice to completely melt the heavy snow of the preceding winter4 ,on the ,alps the ,,snow ,,line--the lower limit of permanent snow--is drawn at about eight thousand five hundred feet above sea level4 ,above the snow line on the slopes and crests1 where these are not too steep1 the snow lies the year round and gathers in valley heads to a depth of hundreds of feet4 ,this is but a small fraction of the thickness to which snow would be piled on the ,alps were it not constantly being drained away4 ,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 line4 ,the presence in the midst of forests and meadows and cultivated fields of these tongues of ice1 ever melting and yet from year to year losing none of their bulk1 proves that their loss is made good in the only possible way4 ,they are fed by snow fields above1 whose surplus of snow they drain away in the form of ice4 ,the presence of glaciers below the snow line is a clear proof that1 rigid and motionless as they appear1 glaciers really are in constant motion down valley4 ,,the ,,neve ,,field4 ,the head of an ,alpine valley occupied by a glacier is commonly a broad amphitheater deeply filled with snow4 ,great peaks tower above it1 and snowy slopes rise on either side on the flanks of mountain spurs4 ,from these heights fierce winds drift the snows into the amphitheater1 and avalanches pour in their torrents of snow and waste4 ,the snow of the amphitheater is like that of drifts in late winter after many successive thaws and freezings4 ,it is made of hard grains and pellets and is called ,,neve4 ,beneath the surface of the neve field and at its outlet the granular neve has been compacted to a mass of porous crystalline ice4 ,snow has been changed to neve1 and neve to glacial ice1 both by pressure1 which drives the air from the interspaces of the snowflakes1 and also by successive meltings and freezings1 much as a snowball is packed in the warm hand and becomes frozen to a ball of ice4 ,,the ,,bergschrund4 ,the neve is in slow motion4 ,it breaks itself loose from the thinner snows about it1 too shallow to share its motion1 and from the rock rim which surrounds it1 forming a deep fissure called the bergschrund1 sometimes a score and more feet wide4 ,,size ,,of ,,glaciers4 ,the ice streams of the ,alps vary in size according to the amount of precipitation and the area of the neve fields which they drain4 ,the largest of ,alpine glaciers1 the ,aletsch1 is nearly ten miles long and has an average width of about a mile4 ,the thickness of some of the glaciers of the ,alps is as much as a thousand feet4 ,giant glaciers more than twice the length of the longest in the ,alps occur on the south slope of the ,himalaya ,mountains1 which receive frequent precipitations of snow from moist winds from the ,indian ,ocean4 ,the best known of the many immense glaciers of ,alaska1 the ,muir1 has an area of about eight hundred square miles 7,fig4 #ie74 ,,glacier ,,motion4 ,the motion of the glaciers of the ,alps seldom exceeds one or two feet a day4 ,large glaciers1 because of the enormous pressure of their weight and because of less marginal resistance1 move faster than small ones4 ,the ,muir advances at the rate of seven feet a day1 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 time4 ,glaciers move faster by day than by night1 and in summer than in winter4 ,other laws of glacier motion may be discovered by a study of ,figures #if and #ig4 ,it is important to remember that glaciers do not slide bodily over their beds1 but urged by gravity move slowly down valley in somewhat the same way as would a stream of thick mud4 ,although small pieces of ice are brittle1 the large mass of granular ice which composes a glacier acts as a viscous substance4 ,,crevasses4 ,slight changes of slope in the glacier bed1 and the different rates of motion in different parts1 produce tensions under which the ice cracks and opens in great fissures called crevasses4 ,at an abrupt descent in the bed the ice is shattered into great fragments1 which unite again below the icefall4 ,crevasses are opened on lines at right angles to the direction of the tension4 ,,transverse ,,crevasses are due to a convexity in the bed which stretches the ice lengthwise 7,fig4 #ii74 ,,marginal ,,crevasses are directed upstream and inwards2 ,,radial ,,crevasses are found where the ice stream deploys from some narrow valley and spreads upon some more open space4 ,what is the direction of the tension which causes each and to what is it due8 ,,lateral ,,and ,,medial ,,moraines4 ,the surface of a glacier is striped lengthwise by long dark bands of rock debris4 ,those in the center are called the medial moraines4 ,the one on either margin is a lateral moraine1 and is clearly formed of waste which has fallen on the edge of the ice from the valley slopes4 ,a medial moraine cannot be formed in this way1 since no rock fragments can fall so far out from the sides4 ,but following it up the glacial stream1 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 moraines4 ,each branch thus adds a medial moraine1 and by counting the number of medial moraines of a trunk stream one may learn of how many branches it is composed4 ,surface moraines appear in the lower course of the glacier as ridges1 which may reach the exceptional height of one hundred feet4 ,the bulk of such a ridge is ice4 ,it has been protected from the sun by the veneer of moraine stuff2 while the glacier surface on either side has melted down at least the distance of the height of the ridge4 ,in summer the lowering of the glacial surface by melting goes on rapidly4 ,in ,swiss glaciers it has been estimated that the average lowering of the surface by melting and evaporation amounts to ten feet a year4 ,as a moraine ridge grows higher and more steep by the lowering of the surface of the surrounding ice1 the stones of its cover tend to slip down its sides4 ,thus moraines broaden1 until near the terminus of a glacier they may coalesce in a wide field of stony waste4 ,,englacial ,,drift4 ,this name is applied to whatever debris is carried within the glacier4 ,it consists of rock waste fallen on the neve and there buried by accumulations of snow1 and of that engulfed in the glacier where crevasses have opened beneath a surface moraine4 ,as the surface of the glacier is lowered by melting1 more or less englacial drift is brought again to open air1 and near the terminus it may help to bury the ice from view beneath a sheet of debris4 ,,the ,,ground ,,moraine4 ,the drift dragged along at the glacier's base and lodged beneath it is known as the ground moraine4 ,part of the material of it has fallen down deep crevasses and part has been torn and worn from the glacier's bed and banks4 ,while the stones of the surface moraines remain as angular as when they lodged on the ice1 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 bed4 ,in glaciers such as those of ,greenland1 whose basal layers are well loaded with drift and whose surface layers are nearly clean1 different layers have different rates of motion1 according to the amount of drift with which they are clogged4 ,one layer glides over another1 and the stones inset in each are ground and smoothed and scratched4 ,usually the sides of glaciated pebbles are more worn than the ends1 and the scratches upon them run with the longer axis of the stone4 ,why8 ,,the ,,terminal ,,moraine4 ,as a glacier is in constant motion1 it brings to its end all of its load except such parts of the ground moraine as may find permanent lodgment beneath the ice4 ,where the glacier front remains for some time at one place1 there is formed an accumulation of drift known as the terminal moraine4 ,in valley glaciers it is shaped by the ice front to a crescent whose convex side is downstream4 ,some of the pebbles of the terminal moraine are angular1 and some are faceted and scored1 the latter having come by the hard road of the ground moraine4 ,the material of the dump is for the most part unsorted1 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 drift1 and the finer material is in places washed away4 ,,glacier ,,drainage4 ,the terminal moraine is commonly breached by a considerable stream1 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 7,fig4 #ajg74 ,the stream is gray with silt and loaded with sand and gravel washed from the ground moraine4 ,glacier milk the ,swiss call this muddy water1 the gray color of whose silt proves it rock flour freshly ground by the ice from the unoxidized sound rock of its bed1 the mud of streams being yellowish when it is washed from the oxidized mantle of waste4 ,since glacial streams are well loaded with waste due to vigorous ice erosion1 the valley in front of the glacier is commonly aggraded to a broad1 flat floor4 ,these outwash deposits are known as ,,valley ,,drift4 ,the sand brought out by streams from beneath a glacier differs from river sand in that it consists of freshly broken angular grains4 ,why8 ,the stream derives its water chiefly from the surface melting of the glacier4 ,as the ice is touched by the rays of the morning sun in summer1 water gathers in pools1 and rills trickle and unite in brooklets which melt and cut shallow channels in the blue ice4 ,the course of these streams is short4 ,soon they plunge into deep wells cut by their whirling waters where some crevasse has begun to open across their path4 ,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 stream4 ,,the ,,lower ,,limit ,,of ,,glaciers4 ,the glaciers of a region do not by any means end at a uniform height above sea level4 ,each terminates where its supply is balanced by melting4 ,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 greater4 ,a series of cold1 moist years1 with an abundant snowfall1 causes glaciers to thicken and advance2 a series of warm1 dry years causes them to wither and melt back4 ,the variation in glaciers is now carefully observed in many parts of the world4 ,the ,muir glacier has retreated two miles in twenty years4 ,the glaciers of the ,swiss ,alps are now for the most part melting back1 although a well-known glacier of the eastern ,alps1 the ,vernagt1 advanced five hundred feet in the year #aijj1 and was then plowing up its terminal moraine4 ,how soon would you expect a glacier to advance after its neve fields have been swollen with unusually heavy snows1 as compared with the time needed for the flood of a large river to reach its mouth after heavy rains upon its headwaters8 ,on the surface of glaciers in summer time one may often see large stones supported by pillars of ice several feet in height 7,fig4 #ajh74 ,these glacier tables commonly slope more or less strongly to the south1 and thus may be used to indicate roughly the points of the compass4 ,can you explain their formation and the direction of their slope8 ,on the other hand1 a small and thin stone1 or a patch of dust1 lying on the ice1 tends to sink a few inches into it4 ,why8 ,in what respects is a valley glacier like a mountain stream which flows out upon desert plains8 ,two confluent glaciers do not mingle their currents as do two confluent rivers4 ,what characteristics of surface moraines prove this fact8 ,what effect would you expect the laws of glacier motion to have on the slant of the sides of transverse crevasses8 ,a trunk glacier has four medial moraines4 ,of how many tributaries is it composed8 ,illustrate by diagram4 ,state all the evidences which you have found that glaciers move4 ,if a glacier melts back with occasional pauses up a valley1 what records are left of its retreat8 ,,piedmont ,,glaciers ,,the ,,malaspina ,,glacier4 ,piedmont 7foot of the mountain7 glaciers are1 as the name implies1 ice fields formed at the foot of mountains by the confluence of valley glaciers4 ,the ,malaspina glacier of ,alaska1 the typical glacier of this kind1 is seventy miles wide and stretches for thirty miles from the foot of the ,mount ,saint ,elias range to the shore of the ,pacific ,ocean4 ,the valley glaciers which unite and spread to form this lake of ice lie above the snow line and their moraines are concealed beneath neve4 ,the central area of the ,malaspina is also free from debris2 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 wide1 covered in part with a luxuriant forest1 beneath which the ice is in places one thousand feet in depth4 ,the glacier here is practically stagnant1 and lakes a few hundred yards across1 which could not exist were the ice in motion and broken with crevasses1 gather on their beds sorted waste from the moraine4 ,the streams which drain the glacier have cut their courses in englacial and subglacial tunnels2 none flow for any distance on the surface4 ,the largest1 the ,yahtse ,river1 issues from a high archway in the ice1--a muddy torrent one hundred feet wide and twenty feet deep1 loaded with sand and stones which it deposits in a broad outwash plain 7,fig4 #aaj74 ,where the ice has retreated from the sea there is left a hummocky drift sheet with hollows filled with lakelets4 ,these deposits help to explain similar hummocky regions of drift and similar plains of coarse1 water-laid material often found in the drift-covered area of the northeastern ,united ,states4 ,,the ,,geological ,,work ,,of ,,glacier ,,ice ,the sluggish glacier must do its work in a different way from the agile river4 ,the mountain stream is swift and small1 and its channel occupies but a small portion of the valley4 ,the glacier is slow and big2 its rate of motion may be less than a millionth of that of running water over the same declivity1 and its bulk is proportionately large and fills the valley to great depth4 ,moreover1 glacier ice is a solid body plastic under slowly applied stresses1 while the water of rivers is a nimble fluid4 ,,transportation4 ,valley glaciers differ from rivers as carriers in that they float the major part of their load upon their surface1 transporting the heaviest bowlder as easily as a grain of sand2 while streams push and roll much of their load along their beds1 and their power of transporting waste depends solely upon their velocity4 ,the amount of the surface load of glaciers is limited only by the amount of waste received from the mountain slopes above them4 ,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 cones4 ,while a valley glacier carries much of its load on top1 an ice sheet1 such as that of ,greenland1 is free from surface debris1 except where moraines trail away from some nunatak4 ,if at its edge it breaks into separate glaciers which drain down mountain valleys1 these tongues of ice will carry the selvages of waste common to valley glaciers4 ,both ice sheets and valley glaciers drag on large quantities of rock waste in their ground moraines4 ,stones transported by glaciers are sometimes called erratics4 ,such are the bowlders of the drift of our northern states4 ,erratics may be set down in an insecure position on the melting of the ice4 ,,deposit4 ,little need be added here to what has already been said of ground and terminal moraines4 ,all strictly glacial deposits are unstratified4 ,the load laid down at the end of a glacier in the terminal moraine is loose in texture1 while the drift lodged beneath the glacier as ground moraine is often an extremely dense1 stony clay1 having been compacted under the pressure of the overriding ice4 ,,erosion4 ,a glacier erodes its bed and banks in two ways1--by abrasion and by plucking4 ,the rock bed over which a glacier has moved is seen in places to have been abraded1 or ground away1 to smooth surfaces which are marked by long1 straight1 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 deep4 ,clearly this work has been accomplished by means of the sharp sand1 the pebbles1 and the larger stones with which the base of the glacier is inset1 and which it holds in a firm grasp as running water cannot4 ,hard and fine-grained rocks1 such as granite and quartzite1 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 bed4 ,in other places the bed of the glacier is rough and torn4 ,the rocks have been disrupted and their fragments have been carried away1--a process known as ,,plucking4 ,moving under immense pressure the ice shatters the rock1 breaks off projections1 presses into crevices and wedges the rocks apart1 dislodges the blocks into which the rock is divided by joints and bedding planes1 and freezing fast to the fragments drags them on4 ,in this work the freezing and thawing of subglacial waters in any cracks and crevices of the rock no doubt play an important part4 ,plucking occurs especially where the bed rock is weak because of close jointing4 ,the product of plucking is bowlders1 while the product of abrasion is fine rock flour and sand4 ,is the ground moraine of ,figure #hg due chiefly to abrasion or to plucking8 ,,roches ,,moutonnees ,,and ,,rounded ,,hills4 ,the prominences left between the hollows due to plucking are commonly ground down and rounded on the stoss side1--the side from which the ice advances1--and sometimes on the opposite1 the lee side1 as well4 ,in this way the bed rock often comes to have a billowy surface known as roches moutonnees 7sheep rocks74 ,hills overridden by an ice sheet often have similarly rounded contours on the stoss side1 while on the lee side they may be craggy1 either because of plucking or because here they have been less worn from their initial profile4 ,,the ,,direction ,,of ,,glacier ,,movement4 ,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 trail4 ,flint nodules or other small prominences in the bed rock are found more worn on the stoss than on the lee side1 where indeed they may have a low cone of rock protected by them from abrasion4 ,cavities1 on the other hand1 have their edges worn on the lee side and left sharp upon the stoss4 ,surfaces worn and torn in the ways which we have mentioned are said to be glaciated4 ,but it must not be supposed that a glacier everywhere glaciates its bed4 ,although in places it acts as a rasp or as a pick1 in others1 and especially where its pressure is least1 as near the terminus1 it moves over its bed in the manner of a sled4 ,instances are known where glaciers have advanced over deposits of sand and gravel without disturbing them to any notable degree4 ,like a river1 a glacier does not everywhere erode4 ,in places it leaves its bed undisturbed and in places aggrades it by deposits of the ground moraine4 ,,cirques4 ,valley glaciers commonly head as we have seen1 in broad amphitheaters deeply filled with snow and ice4 ,on mountains now destitute of glaciers1 but whose glaciation shows that they have supported glaciers in the past1 there are found similar crescentic hollows with high1 precipitous walls and glaciated floors4 ,their floors are often basined and hold lakelets whose deep and quiet waters reflect the sheltering ramparts of rugged rock which tower far above them4 ,such mountain hollows are termed ,,cirques4 ,as a powerful spring wears back a recess in the valley side where it discharges1 so the fountain head of a glacier gradually wears back a cirque4 ,in its slow movement the neve field broadly scours its bed to a flat or basined floor4 ,meanwhile the sides of the valley head are steepened and driven back to precipitous walls4 ,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 valley4 ,in early summer the neve tears itself free1 dislodging and removing any loosened blocks1 and the open fissure of the bergschrund allows frost and other agencies of weathering to attack the unprotected rock4 ,as cirques are thus formed and enlarged the peaks beneath which they lie are sharpened1 and the mountain crests are scalloped and cut back from either side to knife-edged ridges4 ,in the western mountains of the ,united ,states many cirques1 now empty of neve and glacier ice1 and known locally as basins1 testify to the fact that in recent times the snow line stood beneath the levels of their floors1 and thus far below its present altitude4 ,,glacier ,,troughs4 ,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 rains4 ,the valley glacier may easily be from one thousand to three thousand feet deep and from one to three miles wide4 ,such a ponderous bulk of slowly moving ice does not readily adapt itself to sharp turns and a narrow bed4 ,by scouring and plucking all resisting edges it develops a fitting channel with a wide1 flat floor1 and steep1 smooth sides1 above which are seen the weathered slopes of stream-worn mountain valleys4 ,since the trunk glacier requires a deeper channel than do its branches1 the bed of a branch glacier enters the main trough at some distance above the floor of the latter1 although the surface of the two ice streams may be accordant4 ,glacier troughs can be studied best where large glaciers have recently melted completely away1 as is the case in many valleys of the mountains of the western ,united ,states and of central and northern ,europe 7,fig4 #aad74 ,the typical glacier trough1 as shown in such examples1 is ,u-shaped1 with a broad1 flat floor1 and high1 steep walls4 ,its walls are little broken by projecting spurs and lateral ravines4 ,it is as if a ,v- valley cut by a river had afterwards been gouged deeper with a gigantic chisel1 widening the floor to the width of the chisel blade1 cutting back the spurs1 and smoothing and steepening the sides4 ,a river valley could only be as wide-floored as this after it had long been worn down to grade4 ,the floor of a glacier trough may not be graded2 it is often interrupted by irregular steps perhaps hundreds and even a thousand feet in height1 over which the stream that now drains the valley tumbles in waterfalls4 ,reaches between the steps are often basined4 ,lakelets may occupy hollows excavated in solid rock1 and other lakes may be held behind terminal moraines left as dams across the valley at pauses in the retreat of the glacier4 ,,fjords are glacier troughs now occupied in part or wholly by the sea1 either because they were excavated by a tide glacier to their present depth below sea level1 or because of a submergence of the land4 ,their characteristic form is that of a long1 deep1 narrow bay with steep rock walls and basined floor4 ,fjords are found only in regions which have suffered glaciation1 such as ,norway and ,alaska4 ,,hanging ,,valleys4 ,these are lateral valleys which open on their main valley some distance above its floor4 ,they are conspicuous features of glacier troughs from which the ice has vanished2 for the trunk glacier in widening and deepening its channel cut its bed below the bottoms of the lateral valleys4 ,since the mouths of hanging valleys are suspended on the walls of the glacier trough1 their streams are compelled to plunge down its steep1 high sides in waterfalls4 ,some of the loftiest and most beautiful waterfalls of the world leap from hanging valleys1-- among them the celebrated ,staubbach of the ,lauterbrunnen valley of ,switzerland1 and those of the fjords of ,norway and ,alaska4 ,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 beds4 ,in this case1 however1 they are a mark of extreme youth2 for1 as the trunk stream approaches grade and its velocity and power to erode its bed decrease1 the side streams soon cut back their falls and wear their beds at their mouths to a common level with that of the main river4 ,the ,grand ,canyon of the ,colorado must be reckoned a young valley4 ,at its base it narrows to scarcely more than the width of the river1 and yet its tributaries1 except the very smallest1 enter it at a common level4 ,why could not a wide-floored valley1 such as a glacier trough1 with hanging valleys opening upon it1 be produced in the normal development of a river valley8 ,,the ,,troughs ,,of ,,young ,,and ,,of ,,mature ,,glaciers4 ,the features of a glacier trough depend much on the length of time the preexisting valley was occupied with ice4 ,during the infancy of a glacier1 we may believe1 the spurs of the valley which it fills are but little blunted and its bed is but little broken by steps4 ,in youth the glacier develops icefalls1 as a river in youth develops waterfalls1 and its bed becomes terraced with great stairs4 ,the mature glacier1 like the mature river1 has effaced its falls and smoothed its bed to grade4 ,it has also worn back the projecting spurs of its valley1 making itself a wide channel with smooth sides4 ,the bed of a mature glacier may form a long basin1 since it abrades most in its upper and middle course1 where its weight and motion are the greatest4 ,near the terminus1 where weight and motion are the least1 it erodes least1 and may instead deposit a sheet of ground moraine1 much as a river builds a flood plain in the same part of its course as it approaches maturity4 ,the bed of a mature glacier thus tends to take the form of a long1 relatively narrow basin1 across whose lower end may be stretched the dam of the terminal moraine4 ,on the disappearance of the ice the basin is rilled with a long1 narrow lake1 such as ,lake ,chelan in ,washington and many of the lakes in the ,highlands of ,scotland4 ,piedmont glaciers apparently erode but little4 ,beneath their lake- like expanse of sluggish or stagnant ice a broad sheet of ground moraine is probably being deposited4 ,cirques and glaciated valleys rapidly lose their characteristic forms after the ice has withdrawn4 ,the weather destroys all smoothed1 polished1 and scored surfaces which are not protected beneath glacial deposits4 ,the oversteepened sides of the trough are graded by landslips1 by talus slopes1 and by alluvial cones4 ,morainic heaps of drift are dissected and carried away4 ,hanging valleys and the irregular bed of the trough are both worn down to grade by the streams which now occupy them4 ,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 carried4 ,in ,figure #ajd what characteristics of a glacier trough do you notice8 ,what inference do you draw as to the former thickness of the glacier8 ,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 present1 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 ,mountains4 ,,the ,,relative ,,importance ,,of ,,glaciers ,,and ,,of ,,rivers4 ,powerful as glaciers are1 and marked as are the land forms which they produce1 it is easy to exaggerate their geological importance as compared with rivers4 ,under present climatic conditions they are confined to lofty mountains or polar lands4 ,polar ice sheets are permanent only so long as the lands remain on which they rest4 ,mountain glaciers can stay only the brief time during which the ranges continue young and high4 ,as lofty mountains1 such as the ,selkirks and the ,alps1 are lowered by frost and glacier ice1 the snowfall will decrease1 the line of permanent snow will rise1 and as the mountain hollows in which snow may gather are worn beneath the snow line1 the glaciers must disappear4 ,under present climatic conditions the work of glaciers is therefore both local and of short duration4 ,even the glacial epoch1 during which vast ice sheets deposited drift over northeastern ,north ,america1 must have been brief as well as recent1 for many lofty mountains1 such as the ,rockies and the ,alps1 still bear the marks of great glaciers which then filled their valleys4 ,had the glacial epoch been long1 as the earth counts time1 these mountains would have been worn low by ice2 had the epoch been remote1 the marks of glaciation would already have been largely destroyed by other agencies4 ,on the other hand1 rivers are well-nigh universally at work over the land surfaces of the globe1 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 rocks4 ,,icebergs4 ,tide glaciers1 such as those of ,greenland and ,alaska1 are able to excavate their beds to a considerable distance below sea level4 ,from their fronts the buoyancy of sea water raises and breaks away great masses of ice which float out to sea as icebergs4 ,only about one seventh of a mass of glacier ice floats above the surface1 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 sea4 ,icebergs transport on their long journeys whatever drift they may have carried when part of the glacier1 and scatter it1 as they melt1 over the ocean floor4 ,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 ,atlantic4 ,,chapter ,,vi ,,the ,,work ,,of ,,the ,,wind ,we are now to study the geological work of the currents of the atmosphere1 and to learn how they erode1 and transport and deposit waste as they sweep over the land4 ,illustrations of the wind's work are at hand in dry weather on any windy day4 ,clouds of dust are raised from the street and driven along by the gale4 ,here the roadway is swept bare2 and there1 in sheltered places1 the dust settles in little windrows4 ,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 window4 ,in the country one sometimes sees the dust whirled in clouds from dry1 plowed fields in spring and left in the lee of fences in small drifts resembling in form those of snow in winter4 ,,the ,,essential ,,conditions for the wind's conspicuous work are illustrated in these simple examples2 they are aridity and the absence of vegetation4 ,in humid climates these conditions are only rarely and locally met2 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 roots4 ,but in arid regions either vegetation is wholly lacking1 or scant growths are found huddled in detached clumps1 leaving interspaces of unprotected ground 7,fig4 #aai74 ,here1 too1 the mantle of waste1 which is formed chiefly under the action of temperature changes1 remains dry and loose for long periods4 ,little or no moisture is present to cause its particles to cohere1 and they are therefore readily lifted and drifted by the wind4 ,,transportation ,,by ,,the ,,wind ,in the desert the finer waste is continually swept to and fro by the ever-shifting wind4 ,even in quiet weather the air heated by contact with the hot sands rises in whirls1 and the dust is lifted in stately columns1 sometimes as much as one thousand feet in height1 which march slowly across the plain4 ,in storms the sand is driven along the ground in a continuous sheet1 while the air is tilled with dust4 ,explorers tell of sand storms in the deserts of central ,asia and ,africa1 in which the air grows murky and suffocating4 ,even at midday it may become dark as night1 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 ear4 ,sand storms are by no means uncommon in the arid regions of the western ,united ,states4 ,in a recent year1 six were reported from ,yuma1 ,arizona4 ,trains on transcontinental railways are occasionally blockaded by drifting sand1 and the dust sifts into closed passenger coaches1 covering the seats and floors4 ,after such a storm thirteen car loads of sand were removed from the platform of a station on a western railway4 ,,dust ,,falls4 ,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 taken4 ,dust falls from western storms are not unknown even as far east as the ,great ,lakes4 ,in #ahif a black snow fell in ,chicago1 and in another dust storm in the same decade the amount of dust carried in the air over ,rock ,island1 ,ill41 was estimated at more than one thousand tons to the cubic mile4 ,in ,march1 #aija1 a cyclonic storm carried vast quantities of dust from the ,sahara northward across the ,mediterranean to fall over southern and central ,europe4 ,on ,march #hth dust storms raged in southern ,algeria2 two days later the dust fell in ,italy2 and on the #aath it had reached central ,germany and ,denmark4 ,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 soil4 ,we may see from these examples the importance of the wind as an agent of transportation1 and how vast in the aggregate are the loads which it carries4 ,there are striking differences between air and water as carriers of waste4 ,rivers flow in fixed and narrow channels to definite goals4 ,the channelless streams of the air sweep across broad areas1 and1 shifting about continually1 carry their loads back and forth1 now in one direction and now in another4 ,,wind ,,deposits ,the mantle of waste of deserts is rapidly sorted by the wind4 ,the coarser rubbish1 too heavy to be lifted into the air1 is left to strew wide tracts with residual gravels 7,fig4 #abj74 ,the sand derived from the disintegration of desert rocks gathers in vast fields4 ,about one eighth of the surface of the ,sahara is said to be thus covered with drifting sand4 ,in desert mountains1 as those of ,sinai1 it lies like fields of snow in the high valleys below the sharp peaks4 ,on more level tracts it accumulates in seas of sand1 sometimes1 as in the deserts of ,arabia1 two hundred and more feet deep4 ,,dunes4 ,the sand thus accumulated by the wind is heaped in wavelike hills called dunes4 ,in the desert of northwestern ,india1 where the prevalent wind is of great strength1 the sand is laid in longitudinal dunes1 i4e4 in stripes running parallel with the direction of the wind2 but commonly dunes lie1 like ripple marks1 transverse to the wind current4 ,on the windward side they show a long1 gentle slope1 up which grains of sand can readily be moved2 while to the lee their slope is frequently as great as the angle of repose 7,fig4 #abb74 ,dunes whose sands are not fixed by vegetation travel slowly with the wind2 for their material is ever shifted forward as the grains are driven up the windward slope and1 falling over the crest1 are deposited in slanting layers in the quiet of the lee4 ,like river deposits1 wind-blown sands are stratified1 since they are laid by currents of air varying in intensity1 and therefore in transporting power1 which carry now finer and now coarser materials and lay them down where their velocity is checked 7,fig4 #abc74 ,since the wind varies in direction1 the strata dip in various directions4 ,they also dip at various angles1 according to the inclination of the surface on which they were laid4 ,dunes occur not only in arid regions1 but also wherever loose sand lies unprotected by vegetation from the wind4 ,from the beaches of sea and lake shores the wind drives inland the surface sand left dry between tides and after storms1 piling it in dunes which may invade forests and fields and bury villages beneath their slowly advancing waves4 ,on flood plains during summer droughts river deposits are often worked over by the wind2 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 hills4 ,the sand of shore dunes differs little in composition and the shape of its grains from that of the beach from which it was derived4 ,but in deserts1 by the long wear of grain on grain as they are blown hither and thither by the wind1 all soft minerals are ground to powder and the sand comes to consist almost wholly of smooth round grams of hard quartz4 ,some marine sandstones1 such as the ,st4 ,peter sandstone of the upper ,mississippi valley1 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 sea4 ,,dust ,,deposits4 ,as desert sands are composed almost wholly of quartz1 we may ask what has become of the softer minerals of which the rocks whose disintegration has supplied the sand were in part1 and often in large part1 composed4 ,the softer minerals have been ground to powder1 and little by little the quartz sand also is worn by attrition to fine dust4 ,yet dust deposits are scant and few in great deserts such as the ,sahara4 ,the finer waste is blown beyond its limits and laid in adjacent oceans1 where it adds to the muds and oozes of their floors1 and on bordering steppes and forest lands1 where it is bound fast by vegetation and slowly accumulates in deposits of unstratified loose yellow earth4 ,the fine waste of the ,sahara has been identified in dredgings from the bottom of the ,atlantic ,ocean1 taken hundreds of miles from the coast of ,africa4 ,,loess4 ,in northern ,china an area as large as ,france is deeply covered with a yellow pulverulent earth called loess 7,german1 loose71 which many consider a dust deposit blown from the great ,mongolian desert lying to the west4 ,loess mantles the recently uplifted mountains to the height of eight thousand feet and descends on the plains nearly to sea level4 ,its texture and lack of stratification give it a vertical cleavage2 hence it stands in steep cliffs on the sides of the deep and narrow trenches which have been cut in it by streams4 ,on loess hillsides in ,china are thousands of villages whose eavelike dwellings have been excavated in this soft1 yet firm1 dry loam4 ,while dust falls are common at the present time in this region1 the loess is now being rapidly denuded by streams1 and its yellow silt gives name to the muddy ,hwang-ho 7,yellow ,river71 and to the ,yellow ,sea1 whose waters it discolors for scores of miles from shore4 ,wind deposits both of dust and of sand may be expected to contain the remains of land shells1 bits of wood1 and bones of land animals1 testifying to the fact that they were accumulated in open air and not in the sea or in bodies of fresh water4 ,,wind ,,erosion ,sand-laden currents of air abrade and smooth and polish exposed rock surfaces1 acting in much the same way as does the jet of steam fed with sharp sand1 which is used in the manufacture of ground glass4 ,indeed1 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 necessary4 ,telegraph poles and wires whetted by wind-blown sands are destroyed within a few years4 ,in rocks of unequal resistance the harder parts are left in relief1 while the softer are etched away4 ,thus in the pass of ,san ,bernardino1 ,cal41 through which strong winds stream from the west1 crystals of garnet are left projecting on delicate rock fingers from the softer rock in which they were imbedded4 ,wind-carved pebbles are characteristically planed1 the facets meeting along a summit ridge or at a point like that of a pyramid4 ,we may suppose that these facets were ground by prevalent winds from certain directions1 or that from time to time the stone was undermined and rolled over as the sand beneath it was blown away on the windward side1 thus exposing fresh surfaces to the driving sand4 ,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 wind4 ,,deflation4 ,in the denudation of an arid region1 wind erosion is comparatively ineffective as compared with deflation 7,latin1 de1 from2 flare1 to blow71--a term by which is meant the constant removal of waste by the wind1 leaving the rocks bare to the continuous attack of the weather4 ,in moist climates denudation is continually impeded by the mantle of waste and its cover of vegetation1 and the land surface can be lowered no faster than the waste is removed by running water4 ,deep residual soils come to protect all regions of moderate slope1 concealing from view the rock structure1 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 rocks4 ,but in arid regions the mantle is rapidly removed1 even from well- nigh level plains and plateaus1 by the sweep of the wind and the wash of occasional rains4 ,the geological structure of these regions of naked rock can be read as far as the eye can see1 and it is to this structure that the forms of the land are there largely due4 ,in a land mass of horizontal strata1 for example1 any softer surface rocks wear down to some underlying1 resistant stratum1 and this for a while forms the surface of a level plateau 7,fig4 #abi74 ,the edges of the capping layer1 together with those of any softer layers beneath it1 wear back in steep cliffs1 dissected by the valleys of wet-weather streams and often swept bare to the base by the wind4 ,as they are little protected by talus1 which commonly is removed about as fast as formed1 these escarpments and the walls of the valleys retreat indefinitely1 exposing some hard stratum beneath which forms the floor of a widening terrace4 ,the high plateaus of northern ,arizona and southern ,utah1 north of the ,grand ,canyon of the ,colorado ,river1 are composed of stratified rocks more than ten thousand feet thick and of very gentle inclination northward4 ,from the broad plat form in which the canyon has been cut rises a series of gigantic stairs1 which are often more than one thousand feet high and a score or more of miles in breadth4 ,the retreating escarpments1 the cliffs of the mesas and buttes which they have left behind as outliers1 and the walls of the ravines are carved into noble architectural forms-- into cathedrals1 pyramids1 amphitheaters1 towers1 arches1 and colonnades--by the processes of weathering aided by deflation4 ,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 plains1 either composed of naked rock1 or strewed with residual gravels1 or covered with drifting residual sand4 ,the specific gravity of air is #a_/hbc that of water4 ,how does this fact affect the weight of the material which each can carry at the same velocity8 ,if the rainfall should lessen in your own state to from five to ten inches a year1 what changes would take place in the vegetation of the country8 in the soil8 in the streams8 in the erosion of valleys8 in the agencies chiefly at work in denuding the land8 ,in what way can a wind-carved pebble be distinguished from a river-worn pebble8 from a glaciated pebble8 ,,chapter ,,vii ,,the ,,sea ,,and ,,its ,,shores ,we have already seen that the ocean is the goal at which the waste of the land arrives4 ,the mantle of rock waste1 creeping down slopes1 is washed to the sea by streams1 together with the material which the streams have worn from their beds and that dissolved by underground waters4 ,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 sea4 ,glaciers deliver the load of their moraines either directly to the sea or leave it for streams to transport to the same goal4 ,all deposits made on the land1 such as the flood plains of rivers1 the silts of lake beds1 dune sands1 and sheets of glacial drift1 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 bed4 ,but the sea is also at work along all its shores as an agent of destruction1 and we must first take up its work in erosion before we consider how it transports and deposits the waste of the land4 ,,sea ,,erosion ,,the ,,sea ,,cliff ,,and ,,the ,,rock ,,bench4 ,on many coasts the land fronts the ocean in a line of cliffs4 ,to the edge of the cliffs there lead down valleys and ridges1 carved by running water1 which1 if extended1 would meet the water surface some way out from shore4 ,evidently they are now abruptly cut short at the present shore line because the land has been cut back4 ,along the foot of the cliff lies a gently shelving bench of rock1 more or less thickly veneered with sand and shingle4 ,at low tide its inner margin is laid bare1 but at high tide it is covered wholly1 and the sea washes the base of the cliffs4 ,a notch1 of which the ,,sea ,,cliff and the ,,rock ,,bench are the two sides1 has been cut along the shore4 ,,waves4 ,the position of the rock bench1 with its inner margin slightly above low tide1 shows that it has been cut by some agent which acts like a horizontal saw set at about sea level4 ,this agent is clearly the surface agitation of the water2 it is the wind-raised wave4 ,as a wave comes up the shelving bench the crest topples forward and the wave breaks1 striking a blow whose force is measured by the momentum of all its tons of falling water4 ,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 foot4 ,but even a calm sea constantly chafes the shore4 ,it heaves in gentle undulations known as the ground swell1 the result of storms perhaps a thousand miles distant1 and breaks on the shore in surf4 ,the blows of the waves are not struck with clear water only1 else they would have little effect on cliffs of solid rock4 ,storm waves arm themselves with the sand and gravel1 the cobbles1 and even the large bowlders which lie at the base of the cliff1 and beat against it with these hammers of stone4 ,where a precipice descends sheer into deep water1 waves swash up and down the face of the rocks but cannot break and strike effective blows4 ,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 break4 ,compare the ways in which different agents abrade4 ,the wind lightly brushes sand and dust over exposed surfaces of rock4 ,running water sweeps fragments of various sizes along its channels1 holding them with a loose hand4 ,glacial ice grinds the stones of its ground moraine against the underlying rock with the pressure of its enormous weight4 ,the wave hurls fragments of rock against the sea cliff1 bruising and battering it by the blow4 ,it also rasps the bench as it drags sand and gravel to and fro upon it4 ,,weathering ,,of ,,sea ,,cliffs4 ,the sea cliff furnishes the weapons for its own destruction4 ,they are broken from it not only by the wave but also by the weather4 ,indeed the sea cliff weathers more rapidly1 as a rule1 than do rock ledges inland4 ,it is abundantly wet with spray4 ,along its base the ground water of the neighboring land finds its natural outlet in springs which under mine it4 ,moreover1 it is unprotected by any shield of talus4 ,fragments of rock as they fall from its face are battered to pieces by the waves and swept out to sea4 ,the cliff is thus left exposed to the attack of the weather1 and its retreat would be comparatively rapid for this reason alone4 ,sea cliffs seldom overhang1 but commonly1 as in ,figure #acd1 slope seaward1 showing that the upper portion has retreated at a more rapid rate than has the base4 ,which do you infer is on the whole the more destructive agent1 weathering or the wave8 ,draw a section of a sea cliff cut in well jointed rocks whose joints dip toward the land4 ,draw a diagram of a sea cliff where the joints dip toward the sea4 ,,sea ,,caves4 ,the wave does not merely batter the face of the cliff4 ,like a skillful quarryman it inserts wedges in all natural fissures1 such as joints1 and uses explosive forces4 ,as a wave flaps against a crevice it compresses the air within with the sudden stroke2 as it falls back the air as suddenly expands4 ,on lighthouses heavily barred doors have been burst outward by the explosive force of the air within1 as it was released from pressure when a partial vacuum was formed by the refluence of the wave4 ,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 fissure4 ,thus storm waves little by little pry and suck the rock loose1 and in this way1 and by the blows which they strike with the stones of the beach1 they quarry out about a joint1 or wherever the rock may be weak1 a recess known as a ,,sea ,,cave1 provided that the rock above is coherent enough to form a roof4 ,otherwise an open chasm results4 ,,blowholes ,,and ,,sea ,,arches4 ,as a sea cave is drilled back into the rock1 it may encounter a joint or crevice opened to the surface by percolating water4 ,the shock of the waves soon enlarges this to a blowhole1 which one may find on the breezy upland1 perhaps a hundred yards and more back from the cliff's edge4 ,in quiet weather the blowhole is a deep well2 in storm it plays a fountain as the waves drive through the long tunnel below and spout their spray high in air in successive jets4 ,as the roof of the cave thus breaks down in the rear1 there may remain in front for a while a sea arch1 similar to the natural bridges of land caverns4 ,,stacks ,,and ,,wave-,,cut ,,islands4 ,as the sea drives its tunnels and open drifts into the cliff1 it breaks through behind the intervening portions and leaves them isolated as stacks1 much as monuments are detached from inland escarpments by the weather2 and as the sea cliff retreats1 these remnant masses may be left behind as rocky islets4 ,thus the rock bench is often set with stacks1 islets in all stages of destruction1 and sunken reefs1 all wrecks of the land testifying to its retreat before the incessant attack of the waves4 ,,coves4 ,where zones of soft or closely jointed rock outcrop along a shore1 or where minor water courses conic down to the sea and aid in erosion1 the shore is worn back in curved reentrants called coves2 while the more resistant rocks on either hand are left projecting as headlands 7,fig4 #aci74 ,after coves are cut back a short distance by the waves1 the headlands come to protect them1 as with breakwaters1 and prevent their indefinite retreat4 ,the shore takes a curve of equilibrium1 along which the hard rock of the exposed headland and the weak rock of the protected cove wear back at an equal rate4 ,,rate ,,of ,,recession4 ,the rate at which a shore recedes depends on several factors4 ,in soft or incoherent rocks exposed to violent storms the retreat is so rapid as to be easily measured4 ,the coast of ,yorkshire1 ,england1 whose cliffs are cut in glacial drift1 loses seven feet a year on the average1 and since the ,norman conquest a strip a mile wide1 with farmsteads and villages and historic seaports1 has been devoured by the sea4 ,the sandy south shore of ,martha's ,vineyard wears back three feet a year4 ,but hard rocks retreat so slowly that their recession has seldom been measured by the records of history4 ,,shore ,,drift ,,bowlder ,,and ,,pebble ,,beaches4 ,about as fast as formed the waste of the sea cliff is swept both along the shore and out to sea4 ,the road of waste along shore is the ,,beach4 ,we may also define the beach as the exposed edge of the sheet of sediment formed by the carriage of land waste out to sea4 ,at the foot of sea cliffs1 where the waves are pounding hardest1 one commonly finds the rock bench strewn on its inner margin with large stones1 dislodged by the waves and by the weather and some-what worn on their corners and edges4 ,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 ,,beach1 made of pebbles well rounded by the wear which they have suffered4 ,such beaches form a mill whose raw material is constantly supplied by the cliff4 ,the breakers of storms set it in motion to a depth of several feet1 grinding the pebbles together with a clatter to be heard above the roar of the surf4 ,in such a rock crusher the life of a pebble is short4 ,where ships have stranded on our ,atlantic coast with cargoes of hard-burned brick or of coal1 a year of time and a drift of five miles along the shore have proved enough to wear brick and coal to powder4 ,at no great distance from their source1 therefore1 pebble beaches give place to beaches of sand1 which occupy the more sheltered reaches of the shore4 ,,sand ,,beaches4 ,the angular sand grains of various minerals into which pebbles are broken by the waves are ground together under the beating surf and rounded1 and those of the softer minerals are crushed to powder4 ,the process1 however1 is a slow one1 and if we study these sand grains under a lens we may be surprised to see that1 though their corners and edges have been blunted1 they are yet far from the spherical form of the pebbles from which they were derived4 ,the grains are small1 and in water they have lost about half their weiglit in air2 the blows which they strike one another are therefore weak4 ,besides1 each grain of sand of the wet beach is protected by a cushion of water from the blows of its neighbors4 ,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 source4 ,the sand of many beaches1 derived from the rocks of adjacent cliffs or brought in by torrential streams from neighboring highlands1 is dark with grains of a number of minerals softer than quartz4 ,the white sand of other beaches1 as those of the east coast of ,florida1 is almost wholly composed of quartz grains2 for in its long travel down the ,atlantic coast the weaker minerals have been worn to powder and the hardest alone survive4 ,how does the absence of cleavage in quartz affect the durability of quartz sand8 ,,how ,,shore ,,drift ,,migrates4 ,it is under the action of waves and currents that shore drift migrates slowly along a coast4 ,where waves strike a coast obliquely they drive the waste before them little by little along the shore4 ,thus on a north-south coast1 where the predominant storms are from the northeast1 there will be a migration of shore drift southwards4 ,all shores are swept also by currents produced by winds and tides4 ,these are usually far too gentle to transport of themselves the coarse materials of which beaches are made4 ,but while the wave stirs the grains of sand and gravel1 and for a moment lifts them from the bottom1 the current carries them a step forward on their way4 ,the current cannot lift and the wave cannot carry1 but together the two transport the waste along the shore4 ,the road of shore drift is therefore the zone of the breaking waves4 ,,the ,,bay-,,head ,,beach4 ,as the waste derived from the wear of waves and that brought in by streams is trailed along a coast it assumes1 under varying conditions1 a number of distinct forms4 ,when swept into the head of a sheltered bay it constitutes the bay-head beach4 ,by the highest storm waves the beach is often built higher than the ground immediately behind it1 and forms a dam inclosing a shallow pond or marsh4 ,,the ,,bay ,,bar4 ,as the stream of shore drift reaches the mouth of a bay of some size it often occurs that1 instead of turning in1 it sets directly across toward the opposite headland4 ,the waste is carried out from shore into the deeper waters of the bay mouth2 where it is no longer supported by the breaking waves1 and sinks to the bottom4 ,the dump is gradually built to the surface as a stubby spur1 pointing across the bay1 and as it reaches the zone of wave action current and wave can now combine to carry shore drift along it1 depositing their load continually at the point of the spur4 ,an embankment is thus constructed in much the same manner as a railway fill1 which1 while it is building1 serves as a roadway along which the dirt from an adjacent cut is carted to be dumped at the end4 ,when the embankment is completed it bridges the bay with a highway along which shore drift now moves without interruption1 and becomes a bay bar4 ,,incomplete ,,bay ,,bars4 ,under certain conditions the sea cannot carry out its intention to bridge a bay4 ,rivers discharging in bays demand open way to the ocean4 ,strong tidal currents also are able to keep open channels scoured by their ebb and flow4 ,in such cases the most that land waste can do is to build spits and shoals1 narrowing and shoaling the channel as much as possible4 ,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 7,fig4 #adb74 ,,sand ,,reefs4 ,on low coasts where shallow water extends some distance out1 the highway of shore drift lies along a low1 narrow ridge1 termed the sand reef1 separated from the land by a narrow stretch of shallow water called the ,,lagoon4 ,at intervals the reef is held open by ,,inlets1--gaps through which the tide flows and ebbs1 and by which the water of streams finds way to the sea4 ,no finer example of this kind of shore line is to be found in the world than the coast of ,texas4 ,from near the mouth of the ,rio ,grande a continuous sand reef draws its even curve for a hundred miles to ,corpus ,christi ,pass1 and the reefs are but seldom interrupted by inlets as far north as ,galveston ,harbor4 ,on this coast the tides are variable and exceptionally weak1 being less than one foot in height1 while the amount of waste swept along the shore is large4 ,the lagoon is extremely shallow1 and much of it is a mud flat too shoal for even small boats4 ,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 drift4 ,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 land4 ,where storm waves first drag bottom they erode and deepen the sea floor1 and sweep in sediment as far as the line where they break4 ,here1 where they lose their force1 they drop their load and beat up the ridge which is known as the sand reef when it reaches the surface4 ,,shores ,,of ,,elevation ,,and ,,depression ,our studies have already brought to our notice two distinct forms of strand lines1--one the high1 rocky coast cut back to cliffs by the attack of the waves1 and the other the low1 sandy coast where the waves break usually upon the sand reef4 ,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 crust4 ,where a coast land emerges the--shore line moves seaward2 where it is being submerged the shore line advances on the land4 ,,shores ,,of ,,elevation4 ,the retreat of the sea1 either because of a local uplift of the land or for any other reason1 such as the lowering of any portion of ocean bottom1 lays bare the inner margin of the sea floor4 ,where the sea floor has long received the waste of the land it has been built up to a smooth1 subaqueous plain1 gently shelving from the land4 ,since the new shore line is drawn across this even surface it is simple and regular1 and is bordered on the one side by shallow water gradually deepening seaward1 and on the other by low land composed of material which has not yet thoroughly consolidated to firm rock4 ,a sand reef is soon beaten up by the waves1 and for some time conditions will favor its growth4 ,the loss of sand driven into the lagoon beyond1 and of that ground to powder by the surf and carried out to sea1 is more than made up by the stream of alongshore drift1 and especially by the drag of sediments to the reef by the waves as they deepen the sea floor on its seaward side4 ,meanwhile the lagoon gradually fills with waste from the reef and from the land4 ,it is invaded by various grasses and reeds which have learned to grow in salt and brackish water2 the marsh1 laid bare only at low tide1 is built above high tide by wind drift and vegetable deposits1 and becomes a meadow1 soldering the sand reef to the mainland4 ,while the lagoon has been filling1 the waves have been so deepening the sea floor off the sand reef that at last they are able to attack it vigorously4 ,they now wear it back1 and1 driving the shore line across the lagoon or meadow1 cut a line of low cliffs on the mainland4 ,such a shore is that of ,gascony in southwestern ,france1--a low1 straight1 sandy shore1 bordered by dunes and unprotected by reefs from the attack of the waves of the ,bay of ,biscay4 ,we may say1 then1 that on shores of elevation the presence of sand reefs and lagoons indicates the stage of youth1 while the absence of these features and the vigorous and unimpeded attack by the sea upon the mainland indicate the stage of maturity4 ,where much waste is brought in by rivers the maturity of such a coast may be long delayed4 ,the waste from the land keeps the sea shallow offshore and constantly renews the sand reef4 ,the energy of the waves is consumed in handling shore drift1 and no energy is left for an effective attack upon the land4 ,indeed1 with an excessive amount of waste brought down by streams the land may be built out and encroach temporarily upon the sea2 and not until long denudation has lowered the land1 and thus decreased the amount of waste from it1 may the waves be able to cut through the sand reef and thus the coast reach maturity4 ,,shores ,,of ,,depression ,where a coastal region is undergoing submergence the shore line moves landward4 ,the horizontal plane of the sea now intersects an old land surface roughened by subaerial denudation4 ,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 suffered4 ,it follows up partially submerged valleys1 forming bays1 and bends round the divides1 leaving them to project as promontories and peninsulas4 ,the outlines of shores of depression are as varied as are the forms of the land partially submerged4 ,we give a few typical illustrations4 ,the characteristics of the coast of ,maine are due chiefly to the fact that a mountainous region of hard rocks1 once worn to a peneplain1 and after a subsequent elevation deeply dissected by north-south valleys1 has subsided1 the depression amounting on its southern margin to as much as six hundred feet below sea level4 ,drowned valleys penetrate the land in long1 narrow bays1 and rugged divides project in long1 narrow land arms prolonged seaward by islands representing the high portions of their extremities4 ,of this exceedingly ragged shore there are said to be two thousand miles from the ,new ,brunswick boundary as far west as ,portland1--a straight-line distance of but two hundred miles4 ,since the time of its greatest depression the land is known to have risen some three hundred feet2 for the bays have been shortened1 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 islands4 ,the coast of ,dalmatia1 on the ,adriatic ,sea1 is characterized by long land arms and chains of long and narrow islands1 all parallel to the trend of the coast4 ,a region of parallel mountain ranges has been depressed1 and the longitudinal valleys which lie between them are occupied by arms of the sea4 ,chesapeake ,bay is a branching bay due to the depression of an ancient coastal plain which1 after having emerged from the sea1 was channeled with broad1 shallow valleys4 ,the sea has invaded the valley of the trunk stream and those of its tributaries1 forming a shallow bay whose many branches are all directed toward its axis 7,fig4 #adf74 ,hudson ,bay1 and the ,north1 the ,baltic1 and the ,yellow seas are examples where the sinking of the land has brought the sea in over low plains of large extent1 thus deeply indenting the continental out-line4 ,the rise of a few hundred feet would restore these submerged plains to the land4 ,,the ,,cycle ,,of ,,shores ,,of ,,depression4 ,in its infantile stage the outline of a shore of depression depends almost wholly on the previous relief of the land1 and but little on erosion by the sea4 ,sea cliffs and narrow benches appear where headlands and outlying islands have been nipped by the waves4 ,as yet1 little shore waste has been formed4 ,the coast of ,maine is an example of this stage4 ,in early youth all promontories have been strongly cliffed1 and under a vigorous attack of the sea the shore of open bays may be cut back also4 ,sea stacks and rocky islets1 caves and coves1 make the shore minutely ragged4 ,the irregularity of the coast1 due to depression1 is for a while increased by differential wave wear on harder and softer rocks4 ,the rock bench is still narrow4 ,shore waste1 though being produced in large amounts1 is for the most part swept into deeper water and buried out of sight4 ,examples of this stage are the east coast of ,scotland and the ,california coast near ,san ,francisco4 ,later youth is characterized by a large accumulation of shore waste4 ,the rock bench has been cut back so that it now furnishes a good roadway for shore drift4 ,the stream of alongshore drift grows larger and larger1 filling the heads of the smaller bays with beaches1 building spits and hooks1 and tying islands with sand bars to the mainland4 ,it bridges the larger bays with bay bars1 while their length is being reduced as their inclosing promontories are cut back by the waves4 ,thus there comes to be a straight1 continuous1 and easy road1 no longer interrupted by headlands and bays1 for the transportation of waste alongshore4 ,the ,baltic coast of ,germany is in this stage4 ,all this while streams have been busy filling with delta deposits the bays into which they empty4 ,by these steps a coast gradually advances to ,,maturity1 the stage when the irregularities due to depression have been effaced1 when outlying islands formed by subsidence have been planed away1 and when the shore line has been driven back behind the former bay heads4 ,the sea now attacks the land most effectively along a continuous and fairly straight line of cliffs4 ,although the first effect of wave wear was to increase the irregularities of the shore1 it sooner or later rectifies it1 making it simple and smooth4 ,northwestern ,france may be cited as an upland plain1 dissected and depressed1 whose coast has reached maturity4 ,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 shore4 ,their energy is dissipated in moving shore drift hither and thither and in abrading the bench when they drag bottom upon it4 ,little by little the bench is deepened by tidal currents and the drag of waves2 but this process is so slow that meanwhile the sea cliffs melt down under the weather1 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 sea4 ,,plains ,,of ,,marine ,,abrasion4 ,while subaerial denudation reduces the land to baselevel1 the sea is sawing its edges to ,,wave ,,base1 i4e4 the lowest limit of the wave's effective wear4 ,the widened rock bench forms when uplifted a plain of marine abrasion1 which like the peneplain bevels across strata regardless of their various inclinations and various degrees of hardness4 ,how may a plain of marine abrasion be expected to differ from a peneplain in its mantle of waste8 ,compared with subaerial denudation1 marine abrasion is a comparatively feeble agent4 ,at the rate of five feet per century-- a higher rate than obtains on the youthful rocky1 coast of ,britain--it would require more than ten million years to pare a strip one hundred miles wide from the margin of a continent1 a time sufficient1 at the rate at which the ,mississippi valley is now being worn away1 for subaerial denudation to lower the lands of the globe to the level of the sea4 ,slow submergence favors the cutting of a wide rock bench4 ,the water continually deepens upon the bench2 storm waves can therefore always ride in to the base of the cliffs and attack them with full force2 shore waste cannot impede the onset of the waves1 for it is continually washed out in deeper water below wave base4 ,,basal ,,congolmerates4 ,as the sea marches across the land during a slow submergence1 the platform is covered with sheets of sea-laid sediments4 ,lowest of these is a conglomerate1--the bowlder and pebble beach1 widened indefinitely by the retreat of the cliffs at whose base it was formed1 and preserved by the finer deposits laid upon it in the constantly deepening water as the land subsides4 ,such basal conglomerates are not uncommon among the ancient rocks of the land1 and we may know them by their rounded pebbles and larger stones1 composed of the same kind of rock as that of the abraded and evened surface on which they lie4 ,,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 land4 ,soundings show that offshore deposits are laid in belts parallel to the coast1 the coarsest materials lying nearest to the land and the finest farthest out4 ,the pebbles and gravel and the clean1 coarse sand of beaches give place to broad stretches of sand1 which grows finer and finer until it is succeeded by sheets of mud4 ,clearly there is an offshore movement of waste by which it is sorted1 the coarser being sooner dropped and the finer being carried farther out4 ,,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 rivers1 being only one thirty-third as great1 according to a conservative estimate4 ,both mingle alongshore in all the forms of beach and bar that have been described1 and both are together slowly carried out to sea4 ,on the shelving ocean floor waste is agitated by various movements of the unquiet water1--by the undertow 7an outward- running bottom current near the shore71 by the ebb and flow of tides1 by ocean currents where they approach the land1 and by waves and ground swells1 whose effects are sometimes felt to a depth of six hundred feet4 ,by all these means the waste is slowly washed to and fro1 and as it is thus ground finer and finer and its soluble parts are more and more dissolved1 it drifts farther and farther out from land4 ,it is by no steady and rapid movement that waste is swept from the shore to its final resting place4 ,day after day and century after century the grains of sand and particles of mud are shifted to and fro1 winnowed and spread in layers1 which are destroyed and rebuilt again and again before they are buried safe from further disturbance4 ,these processes which are hidden from the eye are among the most important of those with which our science has to do2 for it is they which have given shape to by far the largest part of the stratified rocks of which the land is made4 ,,the ,,continental ,,delta4 ,this fitting term has been recently suggested for the sheet of waste slowly accumulating along the borders of the continents4 ,within a narrow belt1 which rarely exceeds two or three hundred miles1 except near the mouths of muddy rivers such as the ,amazon and ,congo1 nearly all the waste of the continent1 whether worn from its surface by the weather1 by streams1 by glaciers1 or by the wind1 or from its edge by the chafing of the waves1 comes at last to its final resting place4 ,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 shore4 ,coarse materials are therefore soon dropped along narrow belts near land4 ,gravels and coarse sands lie in thick1 wedge-shaped masses which thin out seaward rapidly and give place to sheets of finer sand4 ,,sea ,,muds4 ,outermost of the sediments derived from the waste of the continents is a wide belt of mud2 for fine clays settle so slowly1 even in sea water1--whose saltness causes them to sink much faster than they would in fresh water1--that they are wafted far before they reach a bottom where they may remain undisturbed4 ,muds are also found near shore1 carpeting the floors of estuaries1 and among stretches of sandy deposits in hollows where the more quiet water has permitted the finer silt to rest4 ,sea muds are commonly bluish and consolidate to bluish shales2 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 water4 ,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 reduced1 or decomposed1 by the organic matter present4 ,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 slowness4 ,a greenish mineral called ,,glauconite--a silicate of iron and alumina--is then formed4 ,such deposits1 known as ,,green ,,sand1 are now in process of making in several patches off the ,atlantic coast1 and are found on the coastal plain of ,new ,jersey among the offshore deposits of earlier geological ages4 ,,organic ,,deposits4 ,living creatures swarm along the shore and on the shallows out from land as nowhere else in the ocean4 ,seaweed often mantles the rock of the sea cliff between the levels of high and low tide1 protecting it to some degree from the blows of waves4 ,on the rock bench each little pool left by the ebbing tide is an aquarium abounding in the lowly forms of marine life4 ,below low-tide level occur beds of molluscous shells1 such as the oyster1 with countless numbers of other humble organisms4 ,their harder parts--the shells of mollusks1 the white framework of corals1 the carapaces of crabs and other crustaceans1 the shells of sea urchins1 the bones and teeth of fishes--are gradually buried within the accumulating sheets of sediment1 either whole or1 far more often1 broken into fragments by the waves4 ,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 laid4 ,such a record has been made ever since living creatures with hard parts appeared upon the globe4 ,we shall find it sealed away in the stratified rocks of the continents1-- parts of ancient sea deposits now raised to form the dry land4 ,thus we have in the traces of living creatures found in the rocks1 i4e4 in fossils1 a history of the progress of life upon the planet4 ,,molluscous ,,shell ,,deposits4 ,the forms of marine life of importance in rock making thrive best in clear water1 where little sediment is being laid1 and where at the same time the depth is not so great as to deprive them of needed light1 heat1 and of sufficient oxygen absorbed by sea water from the air4 ,in such clear and comparatively shallow water there often grow countless myriads of animals1 such as mollusks and corals1 whose shells and skeletons of carbonate of lime gradually accumulate in beds of limestone4 ,a shell limestone made of broken fragments cemented together is sometimes called ,,coquina1 a local term applied to such beds recently uplifted from the sea along the coast of ,florida 7,fig4 #adi74 ,,oolitic limestone 7oon1 an egg2 lithos1 a stone7 is so named from the likeness of the tiny spherules which compose it to the roe of fish4 ,corals and shells have been pounded by the waves to calcareous sand1 and each grain has been covered with successive concentric coatings of lime carbonate deposited about it from solution4 ,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 silt1 and hardens into a dense1 fine-grained limestone in which perhaps no trace of fossil is found to suggest the fact that it is of organic origin4 ,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 water4 ,the rocky bottom consists of limestone now slowly building from the accumulation of the remains of mollusks1 small corals1 sea urchins1 worms with calcareous tubes1 and lime-secreting seaweed1 which live upon its surface4 ,where sponges and other silica-secreting organisms abound on limestone banks1 silica forms part of the accumulated deposit1 either in its original condition1 as1 for example1 the spicules of sponges1 or gathered into concretions and layers of flint4 ,where considerable mud is being deposited along with carbonate of lime there is in process of making a clayey limestone or a limy shale2 where considerable sand1 a sandy limestone or a limy sandstone4 ,,consolidation ,,of ,,offshore ,,deposits4 ,we cannot doubt that all these loose sediments of the sea floor are being slowly consolidated to solid rock4 ,they are soaked with water which carries in solution lime carbonate and other cementing substances4 ,these cements are deposited between the fragments of shells and corals1 the grains of sand and the particles of mud1 binding them together into firm rock4 ,where sediments have accumulated to great thickness the lower portions tend also to consolidate under the weight of the overlying beds4 ,except in the case of limestones1 recent sea deposits uplifted to form land are seldom so well cemented as are the older strata1 which have long been acted upon by underground waters deep below the surface within the zone of cementation1 and have been exposed to view by great erosion4 ,,ripple ,,marks1 ,,sun ,,cracks1 ,,etc4 ,the pulse of waves and tidal currents agitates the loose material of offshore deposits1 throwing it into fine parallel ridges called ripple marks4 ,one may see this beautiful ribbing imprinted on beach sands uncovered by the outgoing tide1 and it is also produced where the water is of considerable depth4 ,while the tide is out the surface of shore deposits may be marked by the footprints of birds and other animals1 or by the raindrops of a passing shower4 ,the mud of flats1 thus exposed to the sun and dried1 cracks in a characteristic way4 ,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 laid4 ,in ,figure #aej we have an illustration of a very ancient ripple-marked sand consolidated to hard stone1 uplifted and set on edge by movements of the earth's crust1 and exposed to open air after long erosion4 ,,stratification4 ,for the most part the sheet of sea-laid waste is hidden from our sight4 ,where its edge is exposed along the shore we may see the surface markings which have just been noticed4 ,soundings also1 and the observations made in shallow waters by divers1 tell something of its surface2 but to learn more of its structures we must study those ancient sediments which have been lifted from the sea and dissected by subaerial agencies4 ,from them we ascertain that sea deposits are stratified4 ,they lie in distinct layers which often differ from one another in thickness1 in size of particles1 and perhaps in color4 ,they are parted by bedding planes1 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 it4 ,stratification is thus due to intermittently acting forces1 such as the agitation of the water during storms1 the flow and ebb of the tide1 and the shifting channels of tidal currents4 ,off the mouths of rivers1 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 stages4 ,how stratified deposits are built up is well illustrated in the flats which border estuaries1 such as the ,bay of ,fundy4 ,each advance of the tide spreads a film of mud1 which dries and hardens in the air during low water before another film is laid upon it by the next incoming tidal flood4 ,in this way the flats have been covered by a clay which splits into leaves as thin as sheets of paper4 ,it is in fine material1 such as clays and shales and limestones1 that the thinnest and most uniform layers1 as well as those of widest extent1 occur4 ,on the other hand1 coarse materials are commonly laid in thick beds1 which soon thin out seaward and give place to deposits of finer stuff4 ,in a general way strata are laid in well-nigh horizontal sheets1 for the surface on which they are laid is generally of very gentle inclination4 ,each stratum1 however1 is lenticular1 or lenslike1 in form1 having an area where it is thickest1 and thinning out thence to its edges1 where it is overlapped by strata similar in shape4 ,,cross ,,bedding4 ,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 repose4 ,in this case each stratum grew by the addition along its edge of successive layers of sediment1 precisely as does a sand bar in a river1 the sand being pushed continuously over the edge and coming to rest on a sloping surface4 ,shoals built by strong and shifting tidal currents often show successive strata in which the cross bedding is inclined in different directions4 ,,thickness ,,of ,,sea ,,deposits4 ,remembering the vast amount of material denuded from the land and deposited offshore1 we should expect that with the lapse of time sea deposits would have grown to an enormous thickness4 ,it is a suggestive fact that1 as a rule1 the profile of the ocean bed is that of a soup plate1--a basin surrounded by a flaring rim4 ,on the ,,continental ,,shelf1 as the rim is called1 the water is seldom more than six hundred feet in depth at the outer edge1 and shallows gradually towards shore4 ,along the eastern coast of the ,united ,states the continental shelf is from fifty to one hundred and more miles in width2 on the ,pacific coast it is much narrower4 ,so far as it is due to upbuilding1 a wide continental shelf1 such as that of the ,atlantic coast1 implies a massive continental delta thousands of feet in thickness4 ,the coastal plain of the ,atlantic states may be regarded as the emerged inner margin of this shelf1 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 deposits4 ,continental shelves may also be due in part to a submergence of the outer margin of a continental plateau and to marine abrasion4 ,,deposition ,,of ,,sediments ,,and ,,subsidence4 ,the stratified rocks of the land show in many places ancient sediments which reach a thickness which is measured in miles1 and which are yet the product of well-nigh continuous deposition4 ,such strata may prove by their fossils and by their composition and structure that they were all laid offshore in shallow water4 ,we must infer that1 during the vast length of time recorded by the enormous pile1 the floor of the sea along the coast was slowly sinking1 and that the trough was constantly being filled1 foot by foot1 as fast as it was depressed4 ,such gradual1 quiet movements of the earth's crust not only modify the outline of coasts1 as we have seen1 but are of far greater geological importance in that they permit the making of immense deposits of stratified rock4 ,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 area4 ,as the sea transgresses the land1 i4e4 encroaches upon it1 any given part of the sea bottom is brought farther and farther from the shore4 ,the basal conglomerate formed by bowlder and pebble beaches comes to be covered with sheets of sand1 and these with layers of mud as the sea becomes deeper and the shore more remote2 while deposits of limestone are made when at last no waste is brought to the place from the now distant land1 and the water is left clear for the growth of mollusks and other lime-secreting organisms4 ,,rate ,,of ,,deposition4 ,as deposition in the sea corresponds to denudation on the land1 we are able to make a general estimate of the rate at which the former process is going on4 ,leaving out of account the soluble matter removed1 the ,mississippi is lowering its basin at the rate of one foot in five thousand years1 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 waste4 ,but sediments from the land are spread within a zone but two or three hundred miles in width along the margin of the continents1 a line one hundred thousand miles long4 ,as the area of deposition--about twenty-five million square miles--is about one half the area of denudation1 the average rate of deposition must be twice the average rate of denudation1 i4e4 about one foot in twenty-five hundred years4 ,if some deposits are made much more rapidly than this1 others are made much more slowly4 ,if they were laid no faster than the present average rate1 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 years1 and those of a formation five hundred feet thick required for their accumulation one million two hundred and fifty thousand years4 ,,the ,,sedimentary ,,record ,,and ,,the ,,denudation ,,cycle4 ,we have seen that the successive stages in a cycle of denudation1 such as that by which a land mass of lofty mountains is worn to low plains1 are marked each by its own peculiar land forms1 and that the forms of the earlier stages are more or less completely effaced as the cycle draws toward an end4 ,far more lasting records of each stage are left in the sedimentary deposits of the continental delta4 ,thus1 in the youth of such a land mass as we have mentioned1 torrential streams flowing down the steep mountain sides deliver to the adjacent sea their heavy loads of coarse waste1 and thick offshore deposits of sand and gravel 7,fig4 #aef7 record the high elevation of the bordering land4 ,as the land is worn to lower levels1 the amount and coarseness of the waste brought to the sea diminishes1 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 shale4 ,at last1 in the old age of the region 7,fig4 #aeg71 its low plains contribute little to the sea except the soluble elements of the rocks1 and in the clear waters near the land lime-secreting organisms flourish and their remains accumulate in beds of limestone4 ,when long-weathered lands mantled with deep1 well-oxidized waste are uplifted by a gradual movement of the earth's crust1 and the mantle is rapidly stripped off by the revived streams1 the uprise is recorded in wide deposits of red and yellow clays and sands upon the adjacent ocean floor4 ,where the waste brought in is more than the waves can easily distribute1 as off the mouths of turbid rivers which drain highlands near the sea1 deposits are little winnowed1 and are laid in rapidly alternating1 shaly sandstones and sandy shales4 ,where the highlands are of igneous rock1 such as granite1 and mechanical disintegration is going on more rapidly than chemical decay1 these conditions are recorded in the nature of the deposits laid offshore4 ,the waste swept in by streams contains much feldspar and other minerals softer and more soluble than quartz1 and where the waves have little opportunity to wear and winnow it1 it comes to rest in beds of sandstone in which grains of feldspar and other soft minerals are abundant4 ,such feldspathic sandstones are known as ,,arkose4 ,on the other hand1 where the waste supplied to the sea comes chiefly from wide1 sandy1 coastal plains1 there are deposited off- shore clean sandstones of well-worn grains of quartz alone4 ,in such coastal plains the waste of the land is stored for ages4 ,again and again they are abandoned and invaded by the sea as from time to time the land slowly emerges and is again submerged4 ,their deposits are long exposed to the weather1 and sorted over by the streams1 and winnowed and worked over again and again by the waves4 ,in the course of long ages such deposits thus become thoroughly sorted1 and the grains of all minerals softer than quartz are ground to mud4 ,,deep-,,sea ,,oozes ,,and ,,clays ,,globigerina ,,ooze4 ,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 clay4 ,the surface waters of the warm seas swarm with minute and lowly animals belonging to the order of the ,foraminifera1 which secrete shells of carbonate of lime4 ,at death these tiny white shells fall through the sea water like snowflakes in the air1 and1 slowly dissolving1 seem to melt quite away before they can reach depths greater than about three miles4 ,near shore they reach bottom1 but are masked by the rapid deposit of waste derived from the land4 ,at intermediate depths they mantle the ocean floor with a white1 soft lime deposit known as ,globigerina ooze1 from a genus of the ,foraminifera which contributes largely to its formation4 ,,red ,,clay4 ,below depths of from fifteen to eighteen thousand feet the ocean bottom is sheeted with red or chocolate colored clay4 ,it is the insoluble residue of seashells1 of the debris of submarine volcanic eruptions1 of volcanic dust wafted by the winds1 and of pieces of pumice drifted by ocean currents far from the volcanoes from which they were hurled4 ,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 bed1 where they have lain unburied for thousands of years2 and an appreciable part of the clay is also formed by the dust of meteorites consumed in the atmosphere1--a dust which falls everywhere on sea and land1 but which elsewhere is wholly masked by other deposits4 ,the dark1 cold abysses of the ocean are far less affected by change than any other portion of the surface of the lithosphere4 ,these vast1 silent plains of ooze lie far below the reach of storms4 ,they know no succession of summer and winter1 or of night and day4 ,a mantle of deep and quiet water protects them from the agents of erosion which continually attack1 furrow1 and destroy the surface of the land4 ,while the land is the area of erosion1 the sea is the area of deposition4 ,the sheets of sediment which are slowly spread there tend to efface any inequalities1 and to form a smooth and featureless subaqueous plain4 ,with few exceptions1 the stratified rocks of the land are proved by their fossils and composition to have been laid in the sea2 but in the same way they are proved to be offshore1 shallow-water deposits1 akin to those now making on continental shelves4 ,deep- sea deposits are absent from the rocks of the land1 and we may therefore infer that the deep sea has never held sway where the continents now are1--that the continents have ever been1 as now1 the elevated portions of the lithosphere1 and that the deep seas of the present have ever been its most depressed portions4 ,,the ,,reef-,,building ,,corals ,in warm seas the most conspicuous of rock-making organisms are the corals known as the reef builders4 ,floating in a boat over a coral reef1 as1 for example1 off the south coast of ,florida or among the ,bahamas1 one looks down through clear water on thickets of branching coral shrubs perhaps as much as eight feet high1 and hemispherical masses three or four feet thick1 all abloom with countless minute flowerlike coral polyps1 gorgeous in their colors of yellow1 orange1 green1 and red4 ,in structure each tiny polyp is little more than a fleshy sac whose mouth is surrounded with petal-like tentacles1 or feelers4 ,from the sea water the polyps secrete calcium carbonate and build it up into the stony framework which supports their colonies4 ,boring mollusks1 worms1 and sponges perforate and honeycomb this framework even while its surface is covered with myriads of living polyps4 ,it is thus easily broken by the waves1 and white fragments of coral trees strew the ground beneath4 ,brilliantly colored fishes live in these coral groves1 and countless mollusks1 sea urchins1 and other forms of marine life make here their home4 ,with the debris from all these sources the reef is constantly built up until it rises to low-tide level4 ,higher than this the corals cannot grow1 since they are killed by a few hours' exposure to the air4 ,when the reef has risen to wave base1 the waves abrade it on the windward side and pile to leeward coral blocks torn from their foundation1 filling the interstices with finer fragments4 ,thus they heap up along the reef low1 narrow islands 7,fig4 #afj74 ,reef building is a comparatively rapid progress4 ,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 years4 ,,coral ,,limestones4 ,limestones of various kinds are due to the reef builders4 ,the reef rock is made of corals in place and broken fragments of all sizes1 cemented together with calcium carbonate from solution by infiltrating waters4 ,on the island beaches coral sand is forming oolitic limestone1 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 grain4 ,corals have been among the most important limestone builders of the sea ever since they made their appearance in the early geological ages4 ,the areas on which coral limestone is now forming are large4 ,the ,great ,barrier ,reef of ,australia1 which lies off the north-eastern coast1 is twelve hundred and fifty miles long1 and has a width of from ten to ninety miles4 ,most of the islands of the tropics are either skirted with coral reefs or are themselves of coral formation4 ,,conditions ,,of ,,coral ,,growth4 ,reef-building corals cannot live except in clear salt water less1 as a rule1 than one hundred and fifty feet in depth1 with a winter temperature not lower than #fh degrees ,f4 ,an important condition also is an abundant food supply1 and this is best secured in the path of the warm oceanic currents4 ,coral reefs may be grouped in three classes1--fringing reefs1 barrier reefs1 and atolls4 ,,fringing ,,reefs4 ,these take their name from the fact that they are attached as narrow fringes to the shore4 ,an example is the reef which forms a selvage about a mile wide along the northeastern coast of ,cuba4 ,the outer margin1 indicated by the line of white surf1 where the corals are in vigorous growth1 rises from about forty feet of water4 ,between this and the shore lies a stretch of shoal across which one can wade at low water1 composed of coral sand with here and there a clump of growing coral4 ,,barrier ,,reefs4 ,reefs separated from the shore by a ship channel of quiet water1 often several miles in width and sometimes as much as three hundred feet in depth1 are known as barrier reefs4 ,the seaward face rises abruptly from water too deep for coral growth4 ,low islands are cast up by the waves upon the reef1 and inlets give place for the ebb and flow of the tides4 ,along the west coast of the island of ,new ,caledonia a barrier reef extends for four hundred miles1 and for a length of many leagues seldom approaches within eight miles of the shore4 ,,atolls4 ,these are ring-shaped or irregular coral islands1 or island-studded reefs1 inclosing a central lagoon4 ,the narrow zone of land1 like the rim of a great bowl sunken to the water's edge1 rises hardly more than twenty feet at most above the sea1 and is covered with a forest of trees such as the cocoanut1 whose seeds can be drifted to it uninjured from long distances4 ,the white beach of coral sand leads down to the growing reef1 on whose outer margin the surf is constantly breaking4 ,the sea face of the reef falls off abruptly1 often to depths of thousands of feet1 while the lagoon varies in depth from a few feet to one hundred and fifty or two hundred1 and exceptionally measures as much as three hundred and fifty feet4 ,,theories ,,of ,,coral ,,reefs4 ,fringing reefs require no explanation1 since the depth of water about them is not greater than that at which coral can grow2 but barrier reefs and atolls1 which may rise from depths too great for coral growth demand a theory of their origin4 ,darwin's theory holds that barrier reefs and atolls are formed from fringing reefs by ,,subsidence4 ,the rate of sinking cannot be greater than that of the upbuilding of the reef1 since otherwise the corals would be carried below their depth and drowned4 ,the process is illustrated in ,figure #afa1 where v represents a volcanic island in mid ocean undergoing slow depression1 and ss the sea level before the sinking began1 when the island was surrounded by a fringing reef4 ,as the island slowly sinks1 the reef builds up with equal pace4 ,it rears its seaward face more steep than the island slope1 and thus the intervening space between the sinking1 narrowing land and the outer margin of the reef constantly widens4 ,in this intervening space the corals are more or less smothered with silt from the outer reef and from the land1 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 rim4 ,the outer rim thus becomes a barrier reef and the inner belt of retarded growth is deepened by subsidence to a ship channel1 s's' representing sea level at this time4 ,the final stage1 where the island has been carried completely beneath the sea and overgrown by the contracting reef1 whose outer ring now forms an atoll1 is represented by ss4 ,in very many instances1 however1 atolls and barrier reefs may be explained without subsidence4 ,thus a barrier reef may be formed by the seaward growth of a fringing reef upon the talus of its sea face4 ,in ,figure #afb f is a fringing reef whose outer wall rises from about one hundred and fifty feet1 the lower limit of the reef-building species4 ,at the foot of this submarine cliff a talus of fallen blocks t accumulates1 and as it reaches the zone of coral growth becomes the foundation on which the reef is steadily extended seaward4 ,as the reef widens1 the polyps of the circumference flourish1 while those of the inner belt are retarded in their growth and at last perish4 ,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 channel1 while the outer margin is left as a barrier reef4 ,in much the same way atolls may be built on any shoal which lies within the zone of coral growth4 ,such shoals may be produced when volcanic islands are leveled by waves and ocean currents1 and when submarine plateaus1 ridges1 and peaks are built up by various organic agencies1 such as molluscous and foraminiferal shell deposits4 ,the reef-building corals1 whose eggs are drifted widely over the tropic seas by ocean currents1 colonize such submarine foundations wherever the conditions are favorable for their growth4 ,as the reef approaches the surface the corals of the inner area are smothered by silt and starved1 and their ,submarine ,volcanic ,peak hard parts are dissolved and scoured away2 while those of the circumference1 with abundant food supply1 nourish and build the ring of the atoll4 ,atolls may be produced also by the backward drift of sand from either end of a crescentic coral reef or island1 the spits uniting in the quiet water of the lee to inclose a lagoon4 ,in the ,maldive ,archipelago all gradations between crescent-shaped islets and complete atoll rings have been observed4 ,in a number of instances where coral reefs have been raised by movements of the earth's crust1 the reef formation is found to be a thin veneer built upon a foundation of other deposits4 ,thus ,christmas ,island1 in the ,indian ,ocean1 is a volcanic pile rising eleven hundred feet above sea level and fifteen thousand five hundred feet above the bottom of the sea4 ,the summit is a plateau surrounded by a rim of hills of reef formation1 which represent the ring of islets of an ancient atoll4 ,beneath the reef are thick beds of limestone1 composed largely of the remains of foraminifers1 which cover the lavas and fragraental materials of the old submarine volcano4 ,among the ancient sediments which now form the stratified rocks of the land there occur many thin reef deposits1 but none are known of the immense thickness which modern reefs are supposed to reach according to the theory of subsidence4 ,barrier and fringing reefs are commonly interrupted off the mouths of rivers4 ,why8 ,,summary4 ,we have seen that the ocean bed is the goal to which the waste of the rocks of the land at last arrives4 ,their soluble parts1 dissolved by underground waters and carried to the sea by rivers1 are largely built up by living creatures into vast sheets of limestone4 ,the less soluble portions--the waste brought in by streams and the waste of the shore--form the muds and sands of continental deltas4 ,all of these sea deposits consolidate and harden1 and the coherent rocks of the land are thus reconstructed on the ocean floor4 ,but the destination is not a final one4 ,the stratified rocks of the land are for the most part ancient deposits of the sea1 which have been lifted above sea level2 and we may believe that the sediments now being laid offshore are the dust of continents to be1 and will some time emerge to form additions to the land4 ,we are now to study the movements of the earth's crust which restore the sediments of the sea to the light of day1 and to whose beneficence we owe the habitable lands of the present4 ,,part ,,ii ,,internal ,,geological ,,agencies ,,chapter ,,ix ,,movements ,,of ,,the ,,earth',s ,,crust ,the geological agencies which we have so far studied--weathering1 streams1 underground waters1 glaciers1 winds1 and the ocean--all work upon the earth from without1 and all are set in motion by an energy external to the earth1 namely1 the radiant energy of the sun4 ,all1 too1 have a common tendency to reduce the inequalities of the earth's surface by leveling the lands and strewing their waste beneath the sea4 ,but despite the unceasing efforts of these external agencies1 they have not destroyed the continents1 which still rear their broad plains and great plateaus and mountain ranges above the sea4 ,either1 then1 the earth is very young and the agents of denudation have not yet had time to do their work1 or they have been opposed successfully by other forces4 ,we enter now upon a department of our science which treats of forces which work upon the earth from within1 and increase the inequalities of its surface4 ,it is they which uplift and recreate the lands which the agents of denudation are continually destroying2 it is they which deepen the ocean bed and thus withdraw its waters from the shores4 ,at times also these forces have aided in the destruction of the lands by gradually lowering them and bringing in the sea4 ,under the action of forces resident within the earth the crust slowly rises or sinks2 from time to time it has been folded and broken2 while vast quantities of molten rock have been pressed up into it from beneath and outpoured upon its surface4 ,we shall take up these phenomena in the following chapters1 which treat of upheavals and depressions of the crust1 foldings and fractures of the crust1 earthquakes1 volcanoes1 the interior conditions of the earth1 mineral veins1 and metamorphism4 ,,oscillations ,,of ,,the ,,crust ,of the various movements of the crust due to internal agencies we will consider first those called oscillations1 which lift or depress large areas so slowly that a long time is needed to produce perceptible changes of level1 and which leave the strata in nearly their original horizontal attitude4 ,these movements are most conspicuous along coasts1 where they can be referred to the datum plane of sea level2 we will therefore take our first illustrations from rising and sinking shores4 ,,new ,,jersey4 ,along the coasts of ,new ,jersey one may find awash at high tide ancient shell heaps1 the remains of tribal feasts of aborigines4 ,meadows and old forest grounds1 with the stumps still standing1 are now overflowed by the sea1 and fragments of their turf and wood are brought to shore by waves4 ,assuming that the sea level remains constant1 it is clear that the ,new ,jersey coast is now gradually sinking4 ,the rate of submergence has been estimated at about two feet per century4 ,on the other hand1 the wide coastal plain of ,new ,jersey is made of stratified sands and clays1 which1 as their marine fossils show1 were outspread beneath the sea4 ,their present position above sea level proves that the land now subsiding emerged in the recent past4 ,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 shores4 ,some are emerging from the sea1 some are sinking beneath it2 and no part of the land seems to have been exempt from these changes in the past4 ,,evidences ,,of ,,changes ,,of ,,level4 ,taking the surface of the sea as a level of reference1 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 it1 and as proofs of relative subsidence whatever is now found beneath the sea and could only have been formed above it4 ,thus old strand lines with sea cliffs1 wave-cut rock benches1 and beaches of wave-worn pebbles or sand1 are striking proofs of recent emergence to the amount of their present height above tide4 ,no less conclusive is the presence of sea-laid rocks which we may find in the neighboring quarry or outcrop1 although it may have been long ages since they were lifted from the sea to form part of the dry land4 ,among common proofs of subsidence are roads and buildings and other works of man1 and vegetal growths and deposits1 such as forest grounds and peat beds1 now submerged beneath the sea4 ,in the deltas of many large rivers1 such as the ,po1 the ,nile1 the ,ganges1 and the ,mississippi1 buried soils prove subsidences of hundreds of feet2 and in several cases1 as in the ,mississippi delta1 the depression seems to be now in progress4 ,other proofs of the same movement are drowned land forms which are modeled only in open air4 ,since rivers cannot cut their valleys farther below the baselevel of the sea than the depths of their channels1 ,,drowned ,,valleys are among the plainest proofs of depression4 ,to this class belong ,narragansett1 ,delaware1 ,chesapeake1 ,mobile1 and ,san ,francisco bays1 and many other similar drowned valleys along the coasts of the ,united ,states4 ,less conspicuous are the ,,submarine ,,channels which1 as soundings show1 extend from the mouths of a number of rivers some distance out to sea4 ,such is the submerged channel which reaches from ,new ,york ,bay southeast to the edge of the continental shelf1 and which is supposed to have been cut by the ,hudson ,river when this part of the shelf was a coastal plain4 ,,warping4 ,in a region undergoing changes of level the rate of movement commonly varies in different parts4 ,portions of an area may be rising or sinking1 while adjacent portions are stationary or moving in the opposite direction4 ,in this way a land surface becomes ,,warped4 ,thus1 while ,nova ,scotia and ,new ,brunswick are now rising from the level of the sea1 ,prince ,edward ,island and ,cape ,breton ,island are sinking1 and the sea now flows over the site of the famous old town of ,louisburg destroyed in #ageh4 ,since the close of the glacial epoch the coasts of ,newfoundland and ,labrador have risen hundreds of feet1 but the rate of emergence has not been uniform4 ,the old strand line1 which stands at five hundred and seventy-five feet above tide at ,st4 ,john's1 ,newfoundland1 declines to two hundred and fifty feet near the northern point of ,labrador4 ,,the ,,great ,,lakes is now under-going perceptible warping4 ,rivers enter the lakes from the south and west with sluggish currents and deep channels resembling the estuaries of drowned rivers2 while those that enter from opposite directions are swift and shallow4 ,at the western end of ,lake ,erie are found submerged caves containing stalactites1 and old meadows and forest grounds are now under water4 ,it is thus seen that the water of the lakes is rising along their southwestern shores1 while from their north-eastern shores it is being withdrawn4 ,the region of the ,great ,lakes is therefore warping2 it is rising in the northeast as compared with the southwest4 ,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 miles4 ,it is calculated that the water of ,lake ,michigan is rising at ,chicago at the rate of nine or ten inches per century4 ,the divide at this point between the tributaries of the ,mississippi and ,lake ,michigan is but eight feet above the mean stage of the lake4 ,if the canting of the region continues at its present rate1 in a thousand years the waters of the lake will here overflow the divide4 ,in three thousand five hundred years all the lakes except ,ontario will discharge by this outlet1 via the ,illinois and ,mississippi rivers1 into the ,gulf of ,mexico4 ,the present outlet by the ,niagara ,river will be left dry1 and the divide between the ,st4 ,lawrence and the ,mississippi systems will have shifted from ,chicago to the vicinity of ,buffalo4 ,,physiographic ,,effects ,,of ,,oscillations4 ,we have already mentioned several of the most important effects of movements of elevation and depression1 such as their effects on rivers1 the mantle of waste1 and the forms of coasts4 ,movements of elevation--including uplifts by folding and fracture of the crust to be noticed later-- are the necessary conditions for erosion by whatever agent4 ,they determine the various agencies which are to be chiefly concerned m the wear of any land1--whether streams or glaciers1 weathering or the wind1--and the degree of their efficiency4 ,the lands must be uplifted before they can be eroded1 and since they must be eroded before their waste can be deposited1 movements of elevation are a prerequisite condition for sedimentation also4 ,subsidence is a necessary condition for deposits of great thickness1 such as those of the ,great ,valley of ,california and the ,indo-,gangetic plain 7p4 #aja71 the ,mississippi delta 7p4 #aji71 and the still more important formations of the continental delta in gradually sinking troughs 7p4 #ahc74 ,it is not too much to say that the character and thickness of each formation of the stratified rocks depend primarily on these crustal movements4 ,along the ,baltic coast of ,sweden1 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 century2 ,towards the south the rate decreases4 ,south of ,stockholm1 until recent years1 the sea has gained upon the land1 and here in several seaboard towns streets by the shore are still submerged4 ,the rate of oscillation increases also from the coast inland4 ,on the other hand1 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 rainfall4 ,in #agcj ,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 land4 ,are the facts just stated consistent with his theory8 ,at the little town of ,tadousac--where the ,saguenay ,river empties into the ,st4 ,lawrence--there are terraces of old sea beaches1 some almost as fresh as recent railway fills1 the highest standing two hundred and thirty feet above the river4 ,here the ,saguenay is eight hundred and forty feet in depth1 and the tide ebbs and flows far up its stream4 ,was its channel cut to this depth by the river when the land was at its present height8 ,what oscillations are here recorded1 and to what amount8 ,a few miles north of ,naples1 ,italy1 the ruins of an ancient ,roman temple lie by the edge of the sea1 on a narrow plain which is overlooked in the rear by an old sea cliff 7,fig4 #aff74 ,three marble pillars are still standing4 ,for eleven feet above their bases these columns are uninjured1 for to this height they were protected by an accumulation of volcanic ashes2 but from eleven to nineteen feet they are closely pitted with the holes of boring marine mollusks4 ,from these facts trace the history of the oscillations of the region4 ,,foldings ,,of ,,the ,,crust ,the oscillations which we have just described leave the strata not far from their original horizontal attitude4 ,figure #afg represents a region in which movements of a very different nature have taken place4 ,here1 on either side of the valley ,v1 we find outcrops of layers tilted at high angles4 ,sections along the ridge r show that it is composed of layers which slant inward from either side4 ,in places the outcropping strata stand nearly on edge1 and on the right of the valley they are quite overturned2 a shale ,,sh has come to overlie a limestone ,,lm although the shale is the older rock1 whose original position was beneath the limestone4 ,it is not reasonable to suppose that these rocks were deposited in the attitude in which we find them now2 we must believe that1 like other stratified rocks1 they were outspread in nearly level sheets upon the ocean floor4 ,since that time they must have been deformed4 ,layers of solid rock several miles in thickness have been crumpled and folded like soft wax in the hand1 and a vast denudation has worn away the upper portions of the folds1 in part represented in our section by dotted lines4 ,,dip ,,and ,,strike4 ,in districts where the strata have been disturbed it is desirable to record their attitude4 ,this is most easily done by taking the angle at which the strata are inclined and the compass direction in which they slant4 ,it is also convenient to record the direction in which the outcrop of the strata trends across the country4 ,the inclination of a bed of rocks to the horizon is its ,,dip4 ,the amount of the dip is the angle made with a horizontal plane4 ,the dip of a horizontal layer is zero1 and that of a vertical layer is #ij degrees4 ,the direction of the dip is taken with the compass4 ,thus a geologist's notebook in describing the attitude of outcropping strata contains many such entries as these3 dip #cb degrees north1 or dip #h degrees south #bj degrees west1--meaning in the latter case that the amount of the dip is #h degrees and the direction of the dip bears #bj degrees west of south4 ,the line of intersection of a layer with the horizontal plane is the ,,strike4 ,the strike always runs at right angles to the dip4 ,dip and strike may be illustrated by a book set aslant on a shelf4 ,the dip is the acute angle made with the shelf by the side of the book1 while the strike is represented by a line running along the book's upper edge4 ,if the dip is north or south1 the strike runs east and west4 ,,folded ,,structures4 ,an upfold1 in which the strata dip away from a line drawn along the crest and called the axis of the fold1 is known as an ,,anticline4 ,a downfold1 where the strata dip from either side toward the axis of the trough1 is called a ,,syncline4 ,there is sometimes seen a downward bend in horizontal or gently inclined strata1 by which they descend to a lower level4 ,such a single flexure is a ,,monocline4 ,,degrees ,,of ,,folding4 ,folds vary in degree from broad1 low swells1 which can hardly be detected1 to the most highly contorted and complicated structures4 ,in ,,symmetric folds the dips of the rocks on each side the axis of the fold are equal4 ,in ,,unsymmetrical folds one limb is steeper than the other1 as in the anticline in ,figure #afg4 ,in ,,overturned folds one limb is inclined beyond the perpendicular4 ,,fan ,,folds have been so pinched that the original anticlines are left broader at the top than at the bottom4 ,in folds where the compression has been great the layers are often found thickened at the crest and thinned along the limbs4 ,where strong rocks such as heavy limestones are folded together with weak rocks such as shales1 the strong rocks are often bent into great simple folds1 while the weak rocks are minutely crumpled4 ,,systems ,,of ,,folds4 ,as a rule1 folds occur in systems4 ,over the ,appalachian mountain belt1 for example1 extending from northeastern ,pennsylvania to northern ,alabama and ,georgia1 the earth's crust has been thrown into a series of parallel folds whose axes run from northeast to southwest 7,fig4 #age74 ,in ,pennsylvania one may count a score or more of these earth waves1-- some but from ten to twenty miles in length1 and some extending as much as two hundred miles before they die away4 ,on the eastern part of this belt the folds are steeper and more numerous than on the western side4 ,,cause ,,and ,,conditions ,,of ,,folding4 ,the sections which we have studied suggest that rocks are folded by lateral pressure4 ,while a single1 simple fold might be produced by a heave1 a series of folds1 including overturns1 fan folds1 and folds thickened on their crests at the expense of their limbs1 could only be made in one way1--by pressure from the side4 ,experiment has reproduced all forms of folds by subjecting to lateral thrust layers of plastic material such as wax4 ,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 fingers1 it is only under certain conditions that it could have produced the results which we see4 ,rocks are brittle1 and it is only when under a ,,heavy ,,load and by ,,great ,,pressure ,,slowly ,,applied1 that they can thus be folded and bent instead of being crushed to pieces4 ,under these conditions1 experiments prove that not only metals such as steel1 but also brittle rocks such as marble1 can be deformed and molded and made to flow like plastic clay4 ,,zone ,,of ,,flow1 ,,zone ,,of ,,flow ,,and ,,fracture1 ,,and ,,zone ,,of ,,fracture4 ,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 pressure1 and flow instead of breaking4 ,indeed1 at such profound depths and under such inconceivable weight no cavity can form1 and any fractures would be healed at once by the welding of grain to grain4 ,at less depths there exists a zone where soft rocks fold and flow under stress1 and hard rocks are fractured2 while at and near the surface hard and soft rocks alike yield by fracture to strong pressure4 ,,structures ,,developed ,,in ,,compressed ,,rocks ,deformed rocks show the effects of the stresses to which they have yielded1 not only in the immense folds into which they have been thrown but in their smallest parts as well4 ,a hand specimen of slate1 or even a particle under the microscope1 may show plications similar in form and origin to the foldings which have produced ranges of mountains4 ,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 range4 ,,slaty ,,cleavage4 ,rocks which have yielded to pressure often split easily in a certain direction across the bedding planes4 ,this cleavage is known as slaty cleavage1 since it is most perfectly developed in fine-grained1 homogeneous rocks1 such as slates1 which cleave to the thin1 smooth-surfaced plates with which we are familiar in the slates used in roofing and for ciphering and blackboards4 ,in coarse-grained rocks1 pressure develops more distant partings which separate the rocks into blocks4 ,slaty cleavage cannot be due to lamination1 since it commonly crosses bedding planes at an angle1 while these planes have been often well-nigh or quite obliterated4 ,examining slate with a microscope1 we find that its cleavage is due to the grain of the rock4 ,its particles are flattened and lie with their broad faces in parallel planes1 along which the rock naturally splits more easily than in any other direction4 ,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 cleavage4 ,cleavage is found only in folded rocks1 and1 as we may see in ,figure #agf1 the strike of the cleavage runs parallel to the strike of the strata and the axis of the folds4 ,the dip of the cleavage is generally steep1 hence the pressure was nearly horizontal4 ,the pressure which has acted at right angles to the cleavage1 and to which it is due1 is the same lateral pressure which has thrown the strata into folds4 ,we find additional proof that slates have undergone compression at right angles to their cleavage in the fact that any inclusions in them1 such as nodules and fossils1 have been squeezed out of shape and have their long diameters lying in the planes of cleavage4 ,that pressure is competent to cause cleavage is shown by experiment4 ,homogeneous material of fine grain1 such as beeswax1 when subjected to heavy pressure cleaves at right angles to the direction of the compressing force4 ,,rate ,,of ,,folding4 ,all the facts known with regard to rock deformation agree that it is a secular process1 taking place so slowly that1 like the deepening of valleys by erosion1 it escapes the notice of the inhabitants of the region4 ,it is only under stresses slowly applied that rocks bend without breaking4 ,the folds of some of the highest mountains have risen so gradually that strong1 well-intrenched rivers which had the right of way across the region were able to hold to their courses1 and as a circular saw cuts its way through the log which is steadily driven against it1 so these rivers sawed their gorges through the fold as fast as it rose beneath them4 ,streams which thus maintain the course which they had antecedent to a deformation of the region are known as ,,antecedent streams4 ,examples of such are the ,sutlej and other rivers of ,india1 whose valleys trench the outer ranges of the ,himalayas and whose earlier river deposits have been upturned by the rising ridges4 ,on the other hand1 mountain crests are usually divides1 parting the head waters of different drainage systems4 ,in these cases the original streams of the region have been broken or destroyed by the uplift of the mountain mass across their paths4 ,on the whole1 which have worked more rapidly1 processes of deformation or of denudation8 ,,land ,,forms ,,due ,,to ,,folding ,as folding goes on so slowly1 it is never left to form surface features unmodified by the action of other agencies4 ,an anticlinal fold is attacked by erosion as soon as it begins to rise above the original level1 and the higher it is uplifted1 and the stronger are its slopes1 the faster is it worn away4 ,even while rising1 a young upfold is often thus unroofed1 and instead of appearing as a long1 ,smooth1 boat-shaped ridge1 it commonly has had opened along the rocks of the axis1 when these are weak1 a valley which is overlooked by the infacing escarpments of the hard layers of the sides of the fold4 ,under long-continued erosion1 anticlines may be degraded to valleys1 while the synclines of the same system may be left in relief as ridges4 ,,folded ,,mountains4 ,the vastness of the forces which wrinkle the crust is best realized in the presence of some lofty mountain range4 ,all mountains1 indeed1 are not the result of folding4 ,some1 as we shall see1 are due to upwarps or to fractures of the crust2 some are piles of volcanic material2 some are swellings caused by the intrusion of molten matter beneath the surface2 some are the relicts left after the long denudation of high plateaus4 ,but most of the mountain ranges of the earth1 and some of the greatest1 such as the ,alps and the ,himalayas1 were originally mountains of folding4 ,the earth's crust has wrinkled into a fold2 or into a series of folds1 forming a series of parallel ridges and intervening valleys2 or a number of folds have been mashed together into a vast upswelling of the crust1 in which the layers have been so crumpled and twisted1 overturned and crushed1 that it is exceedingly difficult to make out the original structure4 ,the close and intricate folds seen in great mountain ranges were formed1 as we have seen1 deep below the surface1 within the zone of folding4 ,hence they may never have found expression in any individual surface features4 ,as the result of these deformations deep under ground the surface was broadly lifted to mountain height1 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 developed4 ,when the structure of mountains has been deciphered it is possible to estimate roughly the amount of horizontal compression which the region has suffered4 ,if the strata of the folds of the ,alps were smoothed out1 they would occupy a belt seventy-four miles wider than that to which they have been compressed1 or twice their present width4 ,a section across the ,appalachian folds in ,pennyslvania shows a compression to about two thirds the original width2 the belt has been shortened thirty-five miles in every hundred4 ,considering the thickness of their strata1 the compression which mountains have undergone accounts fully for their height1 with enough to spare for all that has been lost by denudation4 ,the ,appalachian folds involve strata thirty thousand feet in thickness4 ,assuming that the folded strata rested on an unyielding foundation1 and that what was lost in width was gained in height1 what elevation would the range have reached had not denudation worn it as it rose8 ,,the ,,life ,,history ,,of ,,mountains4 ,while the disturbance and uplift of mountain masses are due to deformation1 their sculpture into ridges and peaks1 valleys and deep ravines1 and all the forms which meet the eye in mountain scenery1 excepting in the very youngest ranges1 is due solely to erosion4 ,we may therefore classify mountains according to the degree to which they have been dissected4 ,the ,juras are an example of the stage of early youth1 in which the anticlines still persist as ridges and the synclines coincide with the valleys2 this they owe as much to the slight height of their uplift as to the recency of its date4 ,the ,alps were upheaved at various times1 the last uplift being later than the uplift of the ,juras1 but to so much greater height that erosion has already advanced them well on towards maturity4 ,the mountain mass has been cut to the core1 revealing strange contortions of strata which could never have found expression at the surface4 ,sharp peaks1 knife-edged crests1 deep valleys with ungraded slopes subject to frequent landslides1 are all features of ,alpine scenery typical of a mountain range at this stage in its life history4 ,they represent the survival of the hardest rocks and the strongest structures1 and the destruction of the weaker in their long struggle for existence against the agents of erosion4 ,although miles of rock have been removed from such ranges as the ,alps1 we need not suppose that they ever stood much1 if any1 higher than at present4 ,all this vast denudation may easily have been accomplished while their slow upheaval was going on2 in several mountain ranges we have evidence that elevation has not yet ceased4 ,under long denudation mountains are subdued to the forms characteristic of old age4 ,the lofty peaks and jagged crests of their earlier life are smoothed down to low domes and rounded crests4 ,the southern ,appalachians and portions of the ,hartz ,mountains in ,germany are examples of mountains which have reached this stage4 ,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 mountains2 they are tilted1 crumpled1 and overturned1 and have clearly suffered intense compression4 ,we may infer that their folds were once lifted to the height of mountains and have since been wasted to low-lying lands4 ,such a section as that of ,figure #fg illustrates how ancient mountains may be leveled to their roots1 and represents the final stage to which even the ,alps and the ,himalayas must sometime arrive4 ,mountains1 perhaps of ,alpine height1 once stood about ,lake ,superior2 a lofty range once extended from ,new ,england and ,new ,jersey southwestward to ,georgia along the ,piedmont belt4 ,in our study of historic geology we shall see more clearly how short is the life of mountains as the earth counts time1 and how great ranges have been lifted1 worn away1 and again upheaved into a new cycle of erosion4 ,,the ,,sedimentary ,,history ,,of ,,folded ,,mountains4 ,we may mention here some of the conditions which have commonly been antecedent to great foldings of the crust4 #a4 ,mountain ranges are made of belts of enormously and exceptionally thick sediments4 ,the strata of the ,appalachians are thirty thousand feet thick1 while the same formations thin out to five thousand feet in the ,mississippi valley4 ,the folds of the ,wasatch ,mountains involve strata thirty thousand feet thick1 which thin to two thousand feet in the region of the ,plains4 #b4 ,the sedimentary strata of which mountains are made are for the most part the shallow-water deposits of continental deltas4 ,mountain ranges have been upfolded along the margins of continents4 #c4 ,shallow-water deposits of the immense thickness found in mountain ranges can be laid only in a gradually sinking area4 ,a profound subsidence1 often to be reckoned in tens of thousands of feet1 precedes the upfolding of a mountain range4 ,thus the history of mountains of folding is as follows3 ,for long ages the sea bottom off the coast of a continent slowly subsides1 and the great trough1 as fast as it forms1 is filled with sediments1 which at last come to be many thousands of feet thick4 ,the downward movement finally ceases4 ,a slow but resistless pressure sets in1 and gradually1 and with a long series of many intermittent movements1 the vast mass of accumulated sediments is crumpled and uplifted into a mountain range4 ,,fractures ,,and ,,dislocations ,,of ,,the ,,crust ,considering the immense stresses to which the rocks of the crust are subjected1 it is not surprising to find that they often yield by fracture1 like brittle bodies1 instead of by folding and flowing1 like plastic solids4 ,whether rocks bend or break depends on the character and condition of the rocks1 the load of overlying rocks which they bear1 and the amount of the force and the slowness with which it is applied4 ,,joints4 ,at the surface1 where their load is least1 we find rocks universally broken into blocks of greater or less size by partings known as joints4 ,under this name are included many division planes caused by cooling and drying2 but it is now generally believed that the larger and more regular joints1 especially those which run parallel to the dip and strike of the strata1 are fractures due to up-and-down movements and foldings and twistings of the rocks4 ,joints are used to great advantage in quarrying1 and we have seen how they are utilized by the weather in breaking up rock masses1 by rivers in widening their valleys1 by the sea in driving back its cliffs1 by glaciers in plucking their beds1 and how they are enlarged in soluble rocks to form natural passageways for underground waters4 ,the ends of the parted strata match along both sides of joint planes2 in4 joints there has been little or no displacement of the broken rocks4 ,,faults4 ,in ,figure #ahd the rocks have been both broken and dislocated along the plane ff'4 ,one side must have been moved up or down past the other4 ,such a dislocation is called a fault4 ,the amount of the displacement1 as measured by the vertical distance between the ends of a parted layer1 is the throw4 ,the angle which the fault plane makes with the vertical is the ,,hade4 ,in ,figure #ahd the right side has gone down relatively to the left2 the right is the side of the downthrow1 while the left is the side of the upthrow4 ,where the fault plane is not vertical the surfaces on the two sides may be distinguished as the ,,hanging ,,wall and the ,,foot ,,wall4 ,faults differ in throw from a fraction of an inch to many thousands of feet4 ,,slickensides4 ,if we examine the walls of a fault1 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 other4 ,these appearances1 called sliekensides1 have sometimes been mistaken for the results of glacial action4 ,,normal ,,faults4 ,faults are of two kinds1--normal faults and thrust faults4 ,normal faults1 of which ,figure #ahd is an example1 hade to the downthrow2 the hanging wall has gone down4 ,the total length of the strata has been increased by the displacement4 ,it seems that the strata have been stretched and broken1 and that the blocks have readjusted themselves under the action of gravity as they settled4 ,,thrust ,,faults4 ,thrust faults hade to the upthrow2 the hanging wall has gone up4 ,clearly such faults1 where the strata occupy less space than before1 are due to lateral thrust4 ,folds and thrust faults are closely associated4 ,under lateral pressure strata may fold to a certain point and then tear apart and fault along the surface of least resistance4 ,under immense pressure strata also break by shear without folding4 ,thus1 in ,figure #ahe1 the rigid earth block under lateral thrust has found it easier to break along the fault plane than to fold4 ,where such faults are nearly horizontal they are distinguished as ,,thrust ,,planes4 ,in all thrust faults one mass has been pushed over another1 so as to bring the underlying and older strata upon younger beds2 and when the fault planes are nearly horizontal1 and especially when the rocks have been broken into many slices which have slidden far one upon another1 the true succession of strata is extremely hard to decipher4 ,in the ,selkirk ,mountains of ,canada the basement rocks of the region have been driven east for seven miles on a thrust plane1 over rocks which originally lay thousands of feet above them4 ,along the western ,appalachians1 from ,virginia to ,georgia1 the mountain folds are broken by more than fifteen parallel thrust planes1 running from northeast to southwest1 along which the older strata have been pushed westward over the younger4 ,the longest continuous fault has been traced three hundred and seventy-five miles1 and the greatest horizontal displacement has been estimated at not less than eleven miles4 ,,crush ,,breccia4 ,rocks often do not fault with a clean and simple fracture1 but along a zone1 sometimes several yards in width1 in which they are broken to fragments4 ,it may occur also that strata which as a whole yield to lateral thrust by folding include beds of brittle rocks1 such as thin-layered limestones1 which are crushed to pieces by the strain4 ,in either case the fragments when recemented by percolating waters form a rock known as a ,,crush ,,breccia 7pronounced ,,bretcha74 ,breccia is a term applied to any rock formed of cemented ,,angular fragments4 ,this rock may be made by the consolidation of volcanic cinders1 of angular waste at the foot of cliffs1 or of fragments of coral torn by the waves from coral reefs1 as well as of strata crushed by crustal movements4 ,,surface ,,features ,,due ,,to ,,dislocations ,,fault ,,scarps4 ,a fault of recent date may be marked at surface by a scarp1 because the face of the upthrown block has not yet been worn to the level of the downthrow side4 ,after the upthrown block has been worn down to this level1 differential erosion produces fault scarps wherever weak rocks and resistant rocks are brought in contact along the fault plane2 and the harder rocks1 whether on the upthrow or the downthrow side1 emerge in a line of cliffs4 ,where a fault is so old that no abrupt scarps appear1 its general course is sometimes marked by the line of division between highland and lowland or hill and plain4 ,great faults have sometimes brought ancient crystalline rocks in contact with weaker and younger sedimentary rocks1 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 faulting4 ,the vast majority of faults give rise to no surface features4 ,the faulted region may be old enough to have been baseleveled1 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 slope4 ,the fault may be entirely concealed by the mantle of waste1 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 it4 ,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 south1 marking the edges of the great crust blocks into which the country has been broken4 ,the highest part of the plateau is a crust block ninety miles long and thirty-five miles in maximum width1 which has been hoisted to nine thousand three hundred feet above1 sea level4 ,on the east it descends four thousand feet by a monoclinal fold1 which passes into a fault towards the north4 ,on the west it breaks down by a succession of terraces faced by fault scarps4 ,the throw of these faults varies from seven hundred feet to more than a mile4 ,the escarpments1 however1 are due in a large degree to the erosion of weaker rock on the downthrow side4 ,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 sea4 ,on the one side are hills of ancient crystalline rocks whose crumpled structures prove that they are but the roots of once lofty mountains2 on the other lies a lowland of sandstone and other stratified rocks formed from the waste of those long-vanished mountain ranges4 ,remnants of sandstone occur in places on the north of the great fault1 and are here seen to rest on the worn and fairly even surface of the crystallines4 ,we may infer that these ancient mountains were reduced along their margins to low plains1 which were slowly lowered beneath the sea to receive a cover of sedimentary rocks4 ,still later came an uplift and dislocation4 ,on the one side erosion has since stripped off the sandstones for the most part1 but the hard crystalline rocks yet stand in bold relief4 ,on the other side the weak sedimentary rocks have been worn down to lowlands4 ,,rift ,,valleys4 ,in a broken region undergoing uplift or the unequal settling which may follow1 a slice inclosed between two fissures may sink below the level of the crust blocks on either side1 thus forming a linear depression known as a rift valley1 or valley of fracture4 ,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 south1 and whose central portion is occupied by the ,jordan valley and the ,dead ,sea4 ,the plateau which it gashes has been lifted more than three thousand feet above sea level1 and the bottom of the trough reaches a depth of two thousand six hundred feet below that level in parts of the ,dead ,sea4 ,south of the ,dead ,sea the floor of the trough rises somewhat above sea level1 and in the ,gulf of ,akabah again sinks below it4 ,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 valley4 ,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 folds4 ,the keystone of the arch has subsided4 ,many geologists believe that the ,jordan- ,akabah trough1 the long narrow basin of the ,red ,sea1 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 dislocation4 ,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 level8 ,,block ,,mountains4 ,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 stationary1 mountains known as block mountains are produced4 ,a tilted crust block may present a steep slope on the side upheaved and a more gentle descent on the side depressed4 ,,the ,,basin ,,ranges4 ,the plateaus of the ,united ,states bounded by the ,rocky ,mouirtains on the east1 and on the west by the ranges which front the ,pacific1 have been profoundly fractured and faulted4 ,the system of great fissures by which they are broken extends north and south1 and the long1 narrow1 tilted crust blocks intercepted between the fissures give rise to the numerous north-south ranges of the region4 ,some of the tilted blocks1 as those of southern ,oregon1 are as yet but moderately carved by erosion1 and shallow lakes lie on the waste that has been washed into the depressions between them4 ,we may therefore conclude that their displacement is somewhat recent4 ,others1 as those of ,nevada1 are so old that they have been deeply dissected2 their original form has been destroyed by erosion1 and the intermontane depressions are occupied by wide plains of waste4 ,,dislocations ,,and ,,river ,,valleys4 ,before geologists had proved that rivers can by their own unaided efforts cut deep canyons1 it was common to consider any narrow gorge as a gaping fissure of the crust4 ,this crude view has long since been set aside4 ,a map of the plateaus of northern ,arizona shows how independent of the immense faults of the region is the course of the ,colorado ,river4 ,in the ,alps the tunnels on the ,saint ,gotthard railway pass six times beneath the gorge of the ,reuss1 but at no point do the rocks show the slightest trace of a fault4 ,,rate ,,of ,,dislocation4 ,so far as human experience goes1 the earth movements which we have just studied1 some of which have produced deep-sunk valleys and lofty mountain ranges1 and faults whose throw is to be measured in thousands of feet1 are slow and gradual4 ,they are not accomplished by a single paroxysmal effort1 but by slow creep and a series of slight slips continued for vast lengths of time4 ,in the ,aspen mining district in ,colorado faulting is now going on at a comparatively rapid rate4 ,although no sudden slips take place1 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 lower4 ,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 epoch4 ,more conspicuous are the instances of active faulting by means of sudden slips4 ,in #ahia there occurred along an old fault plane in ,japan a slip which produced an earth rent traced for fifty miles 7,fig4 #aib74 ,the country on one side was depressed in places twenty feet below that on the other1 and also shifted as much as thirteen feet horizontally in the direction of the fault line4 ,in #ahgb a slip occurred for forty miles on the great line of dislocation which runs along the eastern base of the ,sierra ,nevada ,mountains4 ,in the ,owens valley1 ,california1 the throw amounted to twenty-five feet in places1 with a horizontal movement along the fault line of as much as eighteen feet4 ,both this slip and that in ,japan just mentioned caused severe earthquakes4 ,for the sake of clearness we have described oscillations1 foldings1 and fractures of the crust as separate processes1 each giving rise to its own peculiar surface features1 but in nature earth movements are by no means so simple1--they are often implicated with one another3 folds pass into faults2 in a deformed region certain rocks have bent1 while others under the same strain1 but under different conditions of plasticity and load1 have broken2 folded mountains have been worn to their roots1 and the peneplains to which they have been denuded have been upwarped to mountain height and afterwards dissected1--as in the case of the ,alleghany ridges1 the southern ,carpathians1 and other ranges1 --or1 as in the case of the ,sierra ,nevada ,mountains1 have been broken and uplifted as mountains of fracture4 ,draw the following diagrams1 being careful to show the direction in which the faulted blocks have moved1 by the position of the two parts of some well-defined layer of limestone1 sandstone1 or shale1 which occurs on each side of the fault plane1 as in ,figure #ahd4 #a4 ,a normal fault with a hade of #ae degrees1 the original fault scarp remaining4 #b4 ,a normal fault with a hade of #ej degrees1 the original fault scarp worn away1 showing cliffs caused by harder strata on the downthrow side4 #c4 ,a thrust fault with a hade of #cj degrees1 showing cliffs due to harder strata outcropping on the downthrow4 #d4 ,a thrust fault with a hade of #hj degrees1 with surface baseleveled4 #e4 ,in a region of normal faults a coal mine is being worked along the seam of coal ,,ab 7,fig4 #aic74 ,at ,b it is found broken by a fault f which hades toward ,a4 ,to find the seam again1 should you advise tunneling up or down from ,b8 #f4 ,in a vertical shaft of a coal mine the same bed of coal is pierced twice at different levels because of a fault4 ,draw a diagram to show whether the fault is normal or a thrust4 #g4 ,copy the diagram in ,figure #aid1 showing how the two ridges may be accounted for by a single resistant stratum dislocated by a fault4 ,is the fault a ,,strike ,,fault1 i4e4 one running parallel with the strike of the strata1 or a ,,dip ,,fault1 one running parallel with the direction of the dip8 #h4 ,draw a diagram of the block in ,figure #aie as it would appear if dislocated along the plane efg by a normal fault whose throw equals one fourth the height of the block4 ,is the fault a strike or a dip fault8 ,draw a second diagram showing the same block after denudation has worn it down below the center of the upthrown side4 ,note that the outcrop of the coal seam is now deceptively repeated4 ,this exercise may be done in blocks of wood instead of drawings4 #i4 ,draw diagrams showing by dotted lines the conditions both of ,a and of ,b1 ,figure #aif1 after deformation had given the strata their present attitude4 #aj4 ,what is the attitude of the strata of this earth block1 ,figure #aig8 ,what has taken place along the plane bef8 ,when did the dislocation occur compared with the folding of the strata8 ,with the erosion of the valleys on the right-hand side of the mountain8 ,with the deposition of the sediments8 ,do you find any remnants of the original surface baf produced by the dislocation8 ,from the left-hand side of the mountain infer what was the relief of the region before the dislocation4 ,give the complete history recorded in the diagram from the deposition of the strata to the present4 #aa4 ,which is the older fault1 in ,figure #aih1 or ,when did the lava flow occur8 ,how long a time elapsed between the formation of the two faults as measured in the work done in the interval8 ,how long a time since the formation of the later fault8 #ab4 ,measure by the scale the thickness lie of the coal-bearing strata outcropping from a to b in ,figure #aii4 ,on any convenient scale draw a similar section of strata with a dip of #cj degrees outcropping along a horizontal line normal to the strike one thousand feet in length1 and measure the thickness of the strata by the scale employed4 ,the thickness may also be calculated by trigonometry4 ,,unconformity ,strata deposited one upon1 another in an unbroken succession are said to be conformable4 ,but the continuous deposition of strata is often interrupted by movements of the earth's crust1 ,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 erosion4 ,an erosion surface which thus parts older from younger strata is known as an ,,unconformity1 and the strata above it are said to be ,,unconformable with the rocks below1 or to rest unconformably upon them4 ,an unconformity thus records movements of the crust and a consequent break in the deposition of the strata4 ,it denotes a period of land erosion of greater or less length1 which may sometimes be roughly measured by the stage in the erosion cycle which the land surface had attained before its burial4 ,unconformable strata may be parallel1 as in ,figure #bjj1 where the record includes the deposition of strata1 their emergence1 the erosion of the land surface1 a submergence and the deposit of the strata1 and lastly1 emergence and the erosion of the present surface4 ,often the earth movements to which the uplift or depression was due involved tilting or folding of the earlier strata1 so that the strata are now nonparallel as well as unconformable4 ,in ,figure #bja1 for example1 the record includes deposition1 uplift1 and tilting of a2 erosion1 depression1 the deposit of b2 and finally the uplift which has brought the rocks to open air and permitted the dissection by which the unconformity is revealed4 ,from this section infer that during early ,silurian times the area was sea1 and thick sea muds were laid upon it4 ,these were later altered to hard slates by pressure and upfolded into mountains4 ,during the later ,silurian and the ,devonian the area was land and suffered vast denudation4 ,in the ,carboniferous period it was lowered beneath the sea and received a cover of limestone4 ,,the ,,age ,,of ,,mountains4 ,it is largely by means of unconformities that we read the history of mountain making and other deformations and movements of the crust4 ,in ,figure #bjc1 for example1 the deformation which upfolded the range of mountains took place after the deposit of the series of strata a of which the mountains are composed1 and before the deposit of the stratified rocks1 which rest unconformably on a and have not shared their uplift4 ,most great mountain ranges1 like the ,sierra ,nevada and the ,alps1 mark lines of weakness along which the earth's crust has yielded again and again during the long ages of geological time4 ,the strata deposited at various times about their flanks have been infolded by later crumplings with the original mountain mass1 and have been repeatedly crushed1 inverted1 faulted1 intruded with igneous rocks1 and denuded4 ,the structure of great mountain ranges thus becomes exceedingly complex and difficult to read4 ,a comparatively simple case of repeated uplift is shown in ,figure #bjd4 ,in the section of a portion of the ,alps shown in ,figure #agi a far more complicated history may be deciphered4 ,,unconformities ,,in ,,the ,,colorado ,,canyon1 ,,arizona4 ,how geological history may be read in unconformities is further illustrated in ,figures #bjg and #bjh4 ,the dark crystalline rocks a at the bottom of the canyon are among the most ancient known1 and are overlain unconformably by a mass of tilted coarse marine sandstones b1 whose total thickness is not seen in the diagram and measures twelve thousand feet perpendicularly to the dip4 ,both a and b rise to a common level nn and upon them rest the horizontal sea-laid strata c1 in which the upper portion of the canyon has been cut4 ,note that the crystalline rocks a have been crumpled and crushed4 ,comparing their structure with that of folded mountains1 what do you infer as to their relief after their deformation8 ,to which surface were they first worn down1 mm' or nm8 ,describe and account for the surface mm'4 ,how does it differ from the surface of the crystalline rocks seen in the ,torridonian ,mountains1 and why8 ,this surface mm' is one of the oldest land surfaces of which any vestige remains4 ,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 canyon4 ,how did the surface mm' come to receive its cover of sandstones b8 ,from the thickness and coarseness of these sediments draw inferences as to the land mass from which they were derived4 ,was it rising or subsiding8 high or low8 ,were its streams slow or swift8 ,was the amount of erosion small or great8 ,note the strong dip of these sandstones b4 ,was the surface mm' tilted as now when the sandstones were deposited upon it8 ,when was it tilted8 ,draw a diagram showing the attitude of the rocks after this tilting occurred1 and their height relative to sea level4 ,the surface nn' is remarkably even1 although diversified by some low hills which rise into the bedded rocks of c1 and it may be traced for long distances up and down the canyon4 ,were the layers of b and the surface mm' always thus cut short by nn' as now8 ,what has made the surface nn' so even8 ,how does it come to cross the hard crystalline rocks a and the weaker sandstones b at the same impartial level8 ,how did the sediments of c come to be laid upon it8 ,give now the entire history recorded in the section1 and in addition that involved in the production of the platform ,p1 shown in ,figure #acj1 and that of the cutting of the canyon4 ,how does the time involved in the cutting of the canyon compare with that required for the production of the surfaces mm'1 nn'1 and ,p8 ,,chapter ,x ,,earthquakes ,any sudden movement of the rocks of the crust1 as when they tear apart when a fissure is formed or extended1 or slip from time to time along a growing fault1 produces a jar called an earthquake1 which spreads in all directions from the place of disturbance4 ,,the ,,charleston ,,earthquake4 ,on the evening of ,august #ca1 #ahhf1 the city of ,charleston1 ,s4,c41 was shaken by one of the greatest earthquakes which has occurred in the ,united ,states4 ,a slight tremor which rattled the windows was followed a few seconds later by a roar1 as of subterranean thunder1 as the main shock passed beneath the city4 ,houses swayed to and fro1 and their heaving floors overturned furniture and threw persons off their feet as1 dizzy and nauseated1 they rushed to the doors for safety4 ,in sixty seconds a number of houses were completely wrecked1 fourteen thousand chimneys were toppled over1 and in all the city scarcely a building was left without serious injury4 ,in the vicinity of ,charleston railways were twisted and trains derailed4 ,fissures opened in the loose superficial deposits1 and in places spouted water mingled with sand from shallow underlying aquifers4 ,the point of origin1 or ,,focus1 of the earthquake was inferred from subsequent investigations to be a rent in the rocks about twelve miles beneath the surface4 ,from the center of greatest disturbance1 which lay above the focus1 a few miles northwest of the city1 the surface shock traveled outward in every direction1 with decreasing effects1 at the rate of nearly two hundred miles per minute4 ,it was felt from ,boston to ,cuba1 and from eastern ,iowa to the ,bermudas1 over a circular area whose diameter was a thousand miles4 ,an earthquake is transmitted from the focus through the elastic rocks of the crust1 as a wave1 or series of waves1 of compression and rarefaction1 much as a sound wave is transmitted through the elastic medium of the air4 ,each earth particle vibrates with exceeding swiftness1 but over a very short path4 ,the swing of a particle in firm rock seldom exceeds one tenth of an inch in ordinary earthquakes1 and when it reaches one half an inch and an inch1 the movement becomes dangerous and destructive4 ,the velocity of earthquake waves1 like that of all elastic waves1 varies with the temperature and elasticity of the medium4 ,in the deep1 hot1 elastic rocks they speed faster than in the cold and broken rocks near the surface4 ,the deeper the point of origin and the more violent the initial shock1 the faster and farther do the vibrations run4 ,great earthquakes1 caused by some sudden displacement or some violent rending of the rocks1 shake the entire planet4 ,their waves run through the body of the earth at the rate of about three hundred and fifty miles a minute1 and more slowly round its circumference1 registering their arrival at opposite sides of the globe on the exceedingly delicate instruments of modern earthquake observatories4 ,,geological ,,effects4 ,even great earthquakes seldom produce geological effects of much importance4 ,landslides may be shaken down from the sides of mountains and hills1 and cracks may be opened in the surface deposits of plains2 but the transient shiver1 which may overturn cities and destroy thousands of human lives1 runs through the crust and leaves it much the same as before4 ,,earthquakes ,,attending ,,great ,,displacements4 ,great earthquakes frequently attend the displacement of large masses of the rocks of the crust4 ,in #ahbb the coast of ,chile was suddenly raised three or four feet1 and the rise was five or six feet a mile inland4 ,in #ahce the same region was again upheaved from two to ten feet4 ,in each instance a destructive earthquake was felt for one thousand miles along the coast4 ,,the ,,great ,,california ,,earthquake ,,of #aijf4 ,a sudden dislocation occurred in #aijf along an ancient fault plane which extends for #cjj miles through western ,california4 ,the vertical displacement did not exceed four feet1 while the horizontal shifting reached a maximum of twenty feet4 ,fences1 rows of trees1 and roads which crossed the fault were broken and offset4 ,the latitude and longitude of all points over thousands of square miles were changed4 ,on each side of the fault the earth blocks moved in opposite directions1 the block on the east moving southward and that on the west moving northward and to twice the distance4 ,east and west of the fault the movements lessened with increasing distance from it4 ,this sudden slip set up an earthquake lasting sixty-five seconds1 followed by minor shocks recurring for many days4 ,in places the jar shook down the waste on steep hillsides1 snapped off or uprooted trees1 and rocked houses from their foundations or threw down their walls or chimneys4 ,the water mains of ,san ,francisco were broken1 and the city was thus left defenseless against a conflagration which destroyed #ejj1jjj1jjj worth of property4 ,the destructive effects varied with the nature of the ground4 ,buildings on firm rock suffered least1 while those on deep alluvium were severely shaken by the undulations1 like water waves1 into which the loose material was thrown4 ,well-braced steel structures1 even of the largest size1 were earthquake proof1 and buildings of other materials1 when honestly built and intelligently designed to withstand earthquake shocks1 usually suffered little injury4 ,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 dislocation4 ,perhaps the most violent earthquake which ever visited the ,united ,states attended the depression1 in #ahab1 of a region seventy-five miles long and thirty miles wide1 near ,new ,madrid1 ,mo4 ,much of the area was converted into swamps and some into shallow lakes1 while a region twenty miles in diameter was bulged up athwart the channel of the ,mississippi4 ,slight quakes are still felt in this region from time to time1 showing that the strains to which the dislocation was due have not yet been fully relieved4 ,,earthquakes ,,originating ,,beneath ,,the ,,sea4 ,many earthquakes originate beneath the sea1 and in a number of examples they seem to have been accompanied1 as soundings indicate1 by local subsidences of the ocean bottom4 ,there have been instances where the displacement has been sufficient to set the entire ,pacific ,ocean pulsating for many hours4 ,in mid ocean the wave thus produced has a height of only a few feet1 while it may be two hundred miles in width4 ,on shores near the point of origin destructive waves two or three score feet in height roll in1 and on coasts thousands of miles distant the expiring undulations may be still able to record themselves on tidal gauges4 ,,distribution ,,of ,,earthquakes4 ,every half hour some considerable area of the earth's surface is sensibly shaken by an earthquake1 but earthquakes are by no means uniformly distributed over the globe4 ,as we might infer from what we know as to their causes1 earthquakes are most frequent in regions now undergoing deformation4 ,such are young rising mountain ranges1 fault lines where readjustments recur from time to time1 and the slopes of suboceanic depressions whose steepness suggests that subsidence may there be in progress4 ,earthquakes1 often of extreme severity1 frequently visit the lofty and young ranges of the ,andes1 while they are little known in the subdued old mountains of ,brazil4 ,the ,highlands of ,scotland are crossed by a deep and singularly straight depression called the ,great ,glen1 which has been excavated along a very ancient line of dislocation4 ,the earthquakes which occur from time to time in this region1 such as the ,inverness earthquake in #ahia1 are referred to slight slips along this fault plane4 ,in ,japan1 earthquakes are very frequent4 ,more than a thousand are recorded every year1 and twenty-nine world-shaking earthquakes occurred in the three years ending with #aija4 ,they originate1 for the most part1 well down on the eastern flank of the earth fold whose summit is the mountainous crest of the islands1 and which plunges steeply beneath the sea to the abyss of the ,tuscarora ,deep4 ,,minor ,,causes ,,of ,,earthquakes4 ,since any concussion within the crust sets up an earth jar1 there are several minor causes of earthquakes1 such as volcanic explosions and even the collapse of the roofs of caves4 ,the earthquakes which attend the eruption of volcanoes are local1 even in the case of the most violent volcanic paroxysms known4 ,when the top of a volcano has been blown to fragments1 the accompanying earth shock has sometimes not been felt more than twenty-five miles away4 ,,depth ,,of ,,focus4 ,the focus of the ,charleston earthquake1 estimated at about twelve miles below the surface1 was exceptionally deep4 ,volcanic earthquakes are particularly shallow1 and probably no earthquakes known have started at a greater depth than fifteen or twenty miles4 ,this distance is so slight compared with the earth's radius that we may say that earthquakes are but skin-deep4 ,should you expect the velocity of an earthquake to be greater in a peneplain or in a river delta8 ,after an earthquake1 piles on which buildings rested were found driven into the ground1 and chimneys crushed at base4 ,from what direction did the shock come8 ,chimneys standing on the south walls of houses toppled over on the roof4 ,should you infer that the shock in this case came from the north or south8 ,how should you expect a shock from the east to affect pictures hanging on the east and the west walls of a room8 how the pictures hanging on the north and the south walls8 ,in parts of the country1 as in southwestern ,wisconsin1 slender erosion pillars1 or monuments1 are common4 ,what inference could you draw as to the occurrence in such regions of severe earthquakes in the recent past8 ,,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 processes1--the extrusion of molten rock from beneath the surface of the earth1 giving rise to all the various phenomena of volcanoes4 ,in a volcano1 molten rock from a region deep below1 which we may call its reservoir1 ascends through a pipe or fissure to the surface4 ,the materials erupted may be spread over vast areas1 or1 as is commonly the case1 may accumulate about the opening1 forming a conical pile known as the volcanic cone4 ,it is to this cone that popular usage refers the word ,,volcano2 but the cone is simply a conspicuous part of the volcanic mechanism whose still more important parts1 the reservoir and the pipe1 are hidden from view4 ,volcanic eruptions are of two types1--,,effusive eruptions1 in which molten rock wells up from below and flows forth in streams of ,,lava 7a comprehensive term applied to all kinds of rock emitted from volcanoes in a molten state71 and ,,explosive eruptions1 in which the rock is blown out in fragments great and small by the expansive force of steam4 ,,eruptions ,,of ,,the ,,effusive ,,type ,,the ,,hawaiian ,,volcanoes4 ,the ,hawaiian ,islands are all volcanic in origin1 and have a linear arrangement characteristic of many volcanic groups in all parts of the world4 ,they are strung along a northwest-southeast line1 their volcanoes standing in two parallel rows as if reared along two adjacent lines of fracture or folding4 ,in the northwestern islands the volcanoes have long been extinct and are worn low by erosion4 ,in the southeastern island4 ,hawaii1 three volcanoes are still active and in process of building4 ,of these ,mauna ,loa1 the monarch of volcanoes1 with a girth of two hundred miles and a height of nearly fourteen thousand feet above sea level1 is a lava dome the slope of whose sides does not average more than five degrees4 ,on the summit is an elliptical basin ten miles in circumference and several hundred feet deep4 ,concentric cracks surround the rim1 and from time to time the basin is enlarged as great slices are detached from the vertical walls and engulfed4 ,such a volcanic basin1 formed by the insinking of the top of the cone1 is called a ,,caldera4 ,on the flanks of ,mauna ,loa1 four thousand feet above sea level1 lies the caldera of ,kilauea1 an independent volcano whose dome has been joined to the larger mountain by the gradual growth of the two4 ,in each caldera the floor1 which to the eye is a plain of black lava1 is the congealed surface of a column of molten rock4 ,at times of an eruption lakes of boiling lava appear which may be compared to air holes in a frozen river4 ,great waves surge up1 lifting tons of the fiery liquid a score of feet in air1 to fall back with a mighty plunge and roar1 and occasionally the lava rises several hundred feet in fountains of dazzling brightness4 ,the lava lakes may flood the floor of the basin1 but in historic times have never been known to fill it and overflow the rim4 ,instead1 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 miles4 ,with the drawing off of the lava the column in the duct of the volcano lowers1 and the floor of the caldera wholly or in part subsides4 ,a black and steaming abyss marks the place of the lava lakes4 ,after a time the lava rises in the duct1 the floor is floated higher1 and the boiling lakes reappear4 ,the eruptions of the ,hawaiian volcanoes are thus of the effusive type4 ,the column of lava rises1 breaks through the side of the mountain1 and discharges in lava streams4 ,there are no explosions1 and usually no earthquakes1 or very slight ones1 accompany the eruptions4 ,the lava in the calderas boils because of escaping steam1 but the vapor emitted is comparatively little1 and seldom hangs above the summits in heavy clouds4 ,we see here in its simplest form the most impressive and important fact in all volcanic action1 molten rock has been driven upward to the surface from some deep-lying source4 ,,lava ,,flows4 ,as lava issues from the side of a volcano or overflows from the summit1 it flows away in a glowing stream resembling molten iron drawn white-hot from an iron furnace4 ,the surface of the stream soon cools and blackens1 and the hard crust of nonconducting rock may grow thick and firm enough to form a tunnel1 within which the fluid lava may flow far before it loses its heat to any marked degree4 ,such tunnels may at last be left as caves by the draining away of the lava1 and are sometimes several miles in length4 ,,pahoehoe ,,and ,,aa4 ,when the crust of highly fluid lava remains unbroken after its first freezing1 it presents a smooth1 hummocky1 and ropy surface known by the ,hawaiian term ,,pahoehoe4 ,on the other hand1 the crust of a viscid flow may be broken and splintered as it is dragged along by the slowly moving mass beneath4 ,the stream then appears as a field of stones clanking and grinding on1 with here and there from some chink a dull red glow or a wisp of steam4 ,it sets to a surface called ,,aa1 of broken1 sharp-edged blocks1 which is often both difficult and dangerous to traverse4 ,,fissure ,,eruptions4 ,some of the largest and most important outflows of lava have not been connected with volcanic cones1 but have been discharged from fissures1 flooding the country far and wide with molten rock4 ,sheet after sheet of molten rock has been successively outpoured1 and there have been built up1 layer upon layer1 plateaus of lava thousands of feet in thickness and many thousands of square miles in area4 ,,iceland4 ,this island plateau has been rent from time to time by fissures from which floods of lava have outpoured4 ,in some instances the lava discharges along the whole length of the fissure1 but more often only at certain points upon it4 ,the ,laki fissure1 twenty miles long1 was in eruption in #aghc for seven months4 ,the inundation of fluid rock which poured from it is the largest of historic record1 reaching a distance of forty-seven miles and covering two hundred and twenty square miles to an average depth of a hundred feet4 ,at the present time the fissure is traced by a line of several hundred insignificant mounds of fragmental materials which mark where the lava issued4 ,the distance to which the fissure eruptions of ,iceland flow on slopes extremely gentle is noteworthy4 ,one such stream is ninety miles in length1 and another seventy miles long has a slope of little more than one half a degree4 ,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 low4 ,many ,,lava ,,domes have been discovered in ,iceland1 although from their exceedingly gentle slopes1 often but two or three degrees1 they long escaped the notice of explorers4 ,the entire plateau of ,iceland1 a region as large as ,ohio1 is composed of volcanic products1--for the most part of successive sheets of lava whose total thickness falls little short of two miles4 ,the lava sheets exposed to view were outpoured in open air and not beneath the sea2 for peat bogs and old forest grounds are interbedded with them1 and the fossil plants of these vegetable deposits prove that the plateau has long been building and is very ancient4 ,on the steep sea cliffs of the island1 where its structure is exhibited1 the sheets of lava are seen to be cut with many ,,dikes1--fissures which have been filled by molten rock1--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 plateau4 ,,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 gases4 ,the finer particles into--which the lava is exploded are called ,,volcanic ,,dust or ,,volcanic ,,ashes1 and are often carried long distances by the wind before they settle to the earth4 ,the coarser fragments fall about the vent and there accumulate in a steep1 conical1 volcanic mountain4 ,as successive explosions keep open the throat of the pipe1 there remains on the summit a cup-shaped depression called the ,,crater4 ,,stromboli4 ,to study the nature of these explosions we may visit ,stromboli1 a low volcano built chiefly of fragmental materials1 which rises from the sea off the north coast of ,sicily and is in constant though moderate action4 ,over the summit hangs a cloud of vapor which strikingly resembles the column of smoke puffed from the smokestack of a locomotive1 in that it consists of globular masses1 each the product of a distinct explosion4 ,at night the cloud of vapor is lighted with a red glow at intervals of a few minutes1 like the glow on the trail of smoke behind the locomotive when from time to time the fire bos is opened4 ,because of this intermittent light flashing thousands of feet above the sea1 ,stromboli has been given the name of the ,lighthouse of the ,mediterranean4 ,looking down into the crater of the volcano1 one sees a viscid lava slowly seething4 ,the agitation gradually increases4 ,a great bubble forms4 ,it bursts with an explosion which causes the walls of the crater to quiver with a miniature earthquake1 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 it4 ,with the explosion the cooled and darkened crust of the lava is removed1 and the light of the incandescent liquid beneath is reflected from the cloud of vapor which overhangs the cone4 ,at ,stromboli we learn the lesson that the explosive force in volcanoes is that of steam4 ,the lava in the pipe is permeated with it much as is a thick boiling porridge4 ,the steam in boiling porridge is unable to escape freely and gathers into bubbles which in breaking spurt out drops of the pasty substance2 in the same way the explosion of great bubbles of steam in the viscid lava shoots clots and fragments of it into the air4 ,,krakatoa4 ,the most violent eruption of history1 that of ,krakatoa1 a small volcanic island in the strait between ,sumatra and ,java1 occurred in the last week of ,august1 #ahhc4 ,continuous explosions shot a column of steam and ashes4 seventeen miles in air4 ,a black cloud1 beneath which was midnight darkness and from which fell a rain of ashes and stones1 overspread the surrounding region to a distance of one hundred and fifty miles4 ,launched on the currents of the upper air1 the dust was swiftly carried westward to long distances4 ,three days after the eruption it fell on the deck of a ship sixteen hundred miles away1 and in thirteen days the finest impalpable powder from the volcano had floated round the globe4 ,for many months the dust hung over ,europe and ,america as a faint lofty haze illuminated at sunrise and sunset with brilliant crimson4 ,in countries nearer the eruption1 as in ,india and ,africa1 the haze for some time was so thick that it colored sun and moon with blue1 green1 and copper-red tints and encircled them with coronas4 ,at a distance of even a thousand miles the detonations of the eruption sounded like the booming of heavy guns a few miles away4 ,in one direction they were audible for a distance as great as that from ,san ,francisco to ,cleveland4 ,the entire atmosphere was thrown into undulations under which all barometers rose and fell as the air waves thrice encircled the earth4 ,the shock of the explosions raised sea waves which swept round the adjacent shores at a height of more than fifty feet1 and which were perceptible halfway around the globe4 ,at the close of the eruption it was found that half the mountain had been blown away1 and that where the central part of the island had been the sea was a thousand feet deep4 ,,martinique ,,and ,,st4 ,,vincent4 ,in #aijb two dormant volcanoes of the ,west ,indies1 ,mt4 ,pelee in ,martinique and ,soufriere in ,st4 ,vincent1 broke into eruption simultaneously4 ,no lava was emitted1 but there were blown into the air great quantities of ashes1 which mantled the adjacent parts of the islands with a pall as of gray snow4 ,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 mud4 ,on several occasions there was shot from the crater of each volcano a thick and heavy cloud of incandescent ashes and steam1 which rushed down the mountain side like an avalanche1 red with glowing stones and scintillating with lightning flashes4 ,forests and buildings in its path were leveled as by a tornado1 wood was charred and set on fire by the incandescent fragments1 all vegetation was destroyed1 and to breathe the steam and hot1 suffocating dust of the cloud was death to every living creature4 ,on the morning of the #hth of ,may1 #aijb1 the first of these peculiar avalanches from ,mt4 ,pelee fell on the city of ,st4 ,pierre and instantly destroyed the lives of its thirty thousand inhabitants4 ,the eruptions of many volcanoes partake of both the effusive and the explosive types3 the molten rock in the pipe is in part blown into the air with explosions of steam1 and in part is discharged in streams of lava over the lip of the crater and from fissures in the sides of the cone4 ,such are the eruptions of ,vesuvius1 one of which is illustrated in ,figure #bai4 ,,submarine ,,eruptions4 ,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 lava4 ,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 bed4 ,christmas ,island 7see p4 #aid71 built wholly beneath the ocean1 is a coral-capped volcanic peak1 whose total height1 as measured from the bottom of the sea1 is more than fifteen thousand feet4 ,deep-sea soundings have revealed the presence of numerous peaks which fail to reach sea level and which no doubt are submarine volcanoes4 ,a number of volcanoes on the land were submarine in their early stages1 as1 for example1 the vast pile of ,etna1 the celebrated ,sicilian volcano1 which rests on stratified volcanic fragments containing marine shells now uplifted from the sea4 ,submarine outflows of lava and deposits of volcanic fragments become covered with sediments during the long intervals between eruptions4 ,such volcanic deposits are said to be ,,contemporaneous1 because they are formed during the same period as the strata among which they are imbedded4 ,contemporaneous lava sheets may be expected to bake the surface of the stratum on which they rest1 while the sediments deposited upon them are unaltered by their heat4 ,they are among the most permanent records of volcanic action1 far outlasting the greatest volcanic mountains built in open air4 ,from upraised submarine volcanoes1 such as ,christmas ,island1 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 land4 ,,volcanic ,,products ,vast amounts of steam are1 as we have seen1 emitted from volcanoes1 and comparatively small quantities of other vapors1 such as various acid and sulphurous gases4 ,the rocks erupted from volcanoes differ widely in chemical composition and in texture4 ,,acidic ,,and ,,basic ,,lavas4 ,two classes of volcanic rocks may be distinguished1--those containing a large proportion of silica 7silicic acid1 ,si,o#b7 and therefore called ,,acidic1 and those containing less silica and a larger proportion of the bases 7lime1 magnesia1 soda1 etc47 and therefore called ,,basic4 ,the acidic lavas1 of which ,,rhyolite and ,,thrachyte are examples1 are comparatively light in color and weight1 and are difficult to melt4 ,the basic lavas1 of which ,,basalt is a type1 are dark and heavy and melt at a lower temperature4 ,,scoria ,,and ,,pumice4 ,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 state4 ,they harden into compact rock where the steam cannot expand4 ,where the steam is released from pressure1 as on the surface of a lava stream1 it forms bubbles 7steam blebs7 of various sizes1 which give the hardened rock a cellular structure 7,fig4 #bbj71 ,in this way are formed the rough slags and clinkers called ,,scoria1 which are found on the surface of flows and which are also thrown out as clots of lava in explosive eruptions4 ,on the surface of the seething lava in the throat of the volcano there gathers a rock foam1 which1 when hurled into the air1 is cooled and falls as ,,pumice1--a spongy gray rock so light that it floats on water4 ,,amygdules4 ,the steam blebs of lava flows are often drawn out from a spherical to an elliptical form resembling that of an almond1 and after the rock has cooled these cavities are gradually filled with minerals deposited from solution by underground water4 ,from their shape such casts are called amygdules 7,greek1 amygdalon1 an almond74 ,amygdules are commonly composed of silica4 ,lavas contain both silica and the alkalies1 potash and soda1 and after dissolving the alkalies1 percolating water is able to take silica also into solution4 ,most ,,agates are banded amygdules in which the silica has been laid in varicolored1 concentric layers4 ,,glassy ,,and ,,stony ,,lavas4 ,volcanic rocks differ in texture according also to the rate at which they have solidified4 ,when rapidly cooled1 as on the surface of a lava flow1 molten rock chills to a glass1 because the minerals of which it is composed have not had time to separate themselves from the fused mixture and form crystals4 ,under slow cooling1 as in the interior of the flow1 it becomes a stony mass composed of crystals set in a glassy paste4 ,in thin slices of volcanic glass one may see under the microscope the beginnings of crystal growth in filaments and needles and feathery forms1 which are the rudiments of the crystals of various minerals4 ,spherulites1 which also mark the first changes of glassy lavas toward a stony condition1 are little balls within the rock1 varying from microscopic size to several inches in diameter1 and made up of radiating fibers4 ,perlitic structure1 common among glassy lavas1 consists of microscopic curving and interlacing cracks1 due to contraction4 ,,flow ,,lines are exhibited by volcanic rocks both to the naked eye and under the microscope4 ,steam blebs1 together with crystals and their embryonic forms1 are left arranged in lines and streaks by the currents of the flowing lava as it stiffened into rock4 ,,porphyritic ,,structure4 ,rocks whose ground mass has scattered through it large conspicuous crystals are said to be ,,porphyritic1 and it is especially among volcanic rocks that this structure occurs4 ,the ground mass of porphyries either may be glassy or may consist in part of a felt of minute crystals2 in either case it represents the consolidation of the rock after its outpouring upon the surface4 ,on the other hand1 the large crystals of porphyry have slowly formed deep below the ground at an earlier date4 ,,columnar ,,structure4 ,just as wet starch contracts on drying to prismatic forms1 so lava often contracts on cooling to a mass of close-set1 prismatic1 and commonly six-sided columns1 which stand at right angles to the cooling surface4 ,the upper portion of a flow1 on rapid cooling from the surface exposed to the air1 may contract to a confused mass of small and irregular prisms2 while the remainder forms large and beautifully regular columns1 which have grown upward by slow cooling from beneath4 ,,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 crater2 and during the progress of an eruption large blocks may be torn from the throat of the volcano by the outrush of steam4 ,but the most important fragmental materials are those derived from the lava itself4 ,as lava rises in the pipe1 the steam which permeates it is released from pressure and explodes1 hurling the lava into the air in fragments of all sizes1--large pieces of scoria1 ,,lapilli 7fragments the size of a pea or walnut71 volcanic sand and volcanic ashes4 ,the latter resemble in appearance the ashes of wood or coal1 but they are not in any sense1 like them1 a residue after combustion4 ,volcanic ashes are produced in several ways3 lava rising in the volcanic duct is exploded into fine dust by the steam which permeates it2 glassy lava1 hurled into the air and cooled suddenly1 is brought into a state of high strain and tension1 and1 like ,prince ,rupert's drops1 flies to pieces at the least provocation4 ,the clash of rising and falling projectiles also produces some dust1 a fair sample of which may be made by grating together two pieces of pumice4 ,beds of volcanic ash occur widely among recent deposits in the western ,united ,states4 ,in ,nebraska ash beds are found in twenty counties1 and are often as white as powdered pumice4 ,the beds grow thicker and coarser toward the southwestern part of the state1 where their thickness sometimes reaches fifty feet4 ,in what direction would you look for the now extinct volcano whose explosive eruptions are thus recorded8 ,,tuff4 ,this is a convenient term designating any rock composed of volcanic fragments4 ,coarse tuffs of angular fragments are called ,,volcanic ,,brecia1 and when the fragments have been rounded and sorted by water the rock is termed a ,,volcanic ,,conglomerate4 ,even when deposited in the open air1 as on the slopes of a volcano1 tuffs may be rudely bedded and their fragments more or less rounded1 and unless marine shells or the remains of land plants and animals are found as fossils in them1 there is often considerable difficulty in telling whether they were laid in water or in air4 ,in either case they soon become consolidated4 ,chemical deposits from percolating waters fill the interstices1 and the bed of loose fragments is cemented to hard rock4 ,the materials of which tuffs are composed are easily recognized as volcanic in their origin4 ,the fragments are more or less cellular1 according to the degree to which they were distended with steam when in a molten state1 and even in the finest dust one may see the glass or the crystals of lava from which it was derived4 ,tuffs often contain ,,voclanic ,,bombs1--balls of lava which took shape while whirling in the air1 and solidified before falling to the ground4 ,,ancient ,,volcanic ,,rocks4 ,it is in these materials and structures which we have described that volcanoes leave some of their most enduring records4 ,even the volcanic rocks of the earliest geological ages1 uplifted after long burial beneath the sea and exposed to view by deep erosion1 are recognized and their history read despite the many changes which they may have undergone4 ,a sheet of ancient lava may be distinguished by its composition from the sediments among which it is imbedded4 ,the direction of its flow lines may be noted4 ,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 blebs4 ,in a pile of successive sheets of lava each flow may be distinguished and its thickness measured2 for the surface of each sheet is glassy and scoriaceous1 while beneath its upper portions the lava of each flow is more dense and stony4 ,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 undergone1 the depth of ancient soil--now baked to solid rock--upon it1 and the erosion which it had suffered in the interval4 ,if the flow occurred from some submarine volcano1 we may recognize the fact by the sea-laid sediments which cover it1 filling the cracks and crevices of its upper surface and containing pieces of lava washed from it in their basal layers4 ,long-buried glassy lavas devitrify1 or pass to a stony condition1 under the unceasing action of underground waters2 but their flow lines and perlitic and spherulitic structures remain to tell of their original state4 ,ancient tuffs are known by the fragmental character of their volcanic material1 even though they have been altered to firm rock4 ,some remains of land animals and plants may be found imbedded to tell that the beds were laid in open air2 while the remains of marine organisms would prove as surely that the tuffs were deposited in the sea4 ,in these ways ancient volcanoes have been recognized near ,boston1 in southeastern ,pennsylvania1 about ,lake ,superior1 and in other regions of the ,united ,states4 ,,the ,,life ,,history ,,of ,a ,,volcano ,the invasion of a region by volcanic forces is attended by movements of the crust heralded by earthquakes4 ,a fissure or a pipe is opened and the building of the cone or the spreading of wide lava sheets is begun4 ,,volcanic ,,cones4 ,the shape of a volcanic cone depends chiefly on the materials erupted4 ,cones made of fragments may have sides as steep as the angle of repose1 which in the case of coarse scoria is sometimes as high as thirty or forty degrees4 ,about the base of the mountain the finer materials erupted are spread in more gentle slopes1 and are also washed forward by rains and streams4 ,the normal profile is thus a symmetric cone with a flaring base4 ,cones built of lava vary in form according to the liquidity of the lava4 ,domes of gentle slope1 as those of ,hawaii1 for example1 are formed of basalt1 which flows to long distances before it congeals4 ,when superheated and emitted from many vents1 this easily melted lava builds great plateaus1 such as that of ,iceland4 ,on the other hand1 lavas less fusible1 or poured out at a lower temperature1 stiffen when they have flowed but a short distance1 and accumulate in a steep cone4 ,trachyte has been extruded in a state so viscid that it has formed steepsided domes like that of ,sarcoui4 ,most volcanoes are built1 like ,vesuvius1 both of lava flows and of tuffs1 and sections show that the structure of the cone consists of outward-dipping1 alternating layers of lava1 scoria1 and ashes4 ,from time to time the cone is rent by the violence of explosions and by the weight of the column of lava in the pipe4 ,the fissures are filled with lava and some discharge on the sides of the mountain1 building parasitic cones1 while all form dikes1 which strengthen the pile with ribs of hard rock and make it more difficult to rend4 ,great catastrophes are recorded in the shape of some volcanoes which consist of a circular rim perhaps miles in diameter1 inclosing a vast crater or a caldera within which small cones may rise4 ,we may infer that at some time the top of the mountain has been blown off1 or has collapsed and been engulfed because some reservoir beneath had been emptied by long-continued eruptions4 ,the cone-building stage may be said to continue until eruptions of lava and fragmental materials cease altogether4 ,sooner or later the volcanic forces shift or die away1 and no further eruptions add to the pile or replace its losses by erosion during periods of repose4 ,gases however are still emitted1 and1 as sulphur vapors are conspicuous among them1 such vents are called ,,solfataras4 ,mount ,hood1 in ,oregon1 is an example of a volcano sunk to this stage4 ,from a steaming rift on its side there rise sulphurous fumes which1 half a mile down the wind1 will tarnish a silver coin4 ,,geysers ,,and ,,hot ,,springs4 ,the hot springs of volcanic regions are among the last vestiges of volcanic heat4 ,periodically eruptive boiling springs are termed geysers4 ,in each of the geyser regions of the earth--the ,yellowstone ,national ,park1 ,iceland1 and ,new ,zealand--the ground water of the locality is supposed to be heated by ancient lavas that1 because of the poor conductivity of the rock1 still remain hot beneath the surface4 ,,old ,,faithful1 one of the many geysers of the ,yellowstone ,national ,park1 plays a fountain of boiling water a hundred feet in air2 while clouds of vapor from the escaping steam ascend to several times that height4 ,the eruptions take place at intervals of from seventy to ninety minutes4 ,in repose the geyser is a quiet pool1 occupying a craterlike depression in a conical mound some twelve feet high4 ,the conduit of the spring is too irregular to be sounded4 ,the mound is composed of porous silica deposited by the waters of the geyser4 ,geysers erupt at intervals instead of continuously boiling1 because their long1 narrow1 and often tortuous conduits do not permit a free circulation of the water4 ,after an eruption the tube is refilled and the water again gradually becomes heated4 ,deep in the tube where it is in contact with hot lavas the water sooner or later reaches the boiling point1 and bursting into steam shoots the water above it high in air4 ,,carbonated ,,springs4 ,after all the other signs of life have gone1 the ancient volcano may emit carbon dioxide as its dying breath4 ,the springs of the region may long be charged with carbon dioxide1 or carbonated1 and where they rise through limestone may be expected to deposit large quantities of travertine4 ,we should remember1 however1 that many carbonated springs1 and many hot springs1 are wholly independent of volcanoes4 ,,the ,,destruction ,,of ,,the ,,cone4 ,as soon as the volcanic cone ceases to grow by eruptions the agents of erosion begin to wear it down1 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 dissected4 ,we infer that ,mount ,shasta1 whose conical shape is still preserved despite the gullies one thousand feet deep which trench its sides1 is younger than ,mount ,hood1 which erosive agencies have carved to a pyramidal form4 ,the pile of materials accumulated about a volcanic vent1 no matter how vast in bulk1 is at last swept entirely away4 ,the cone of the volcano1 active or extinct1 is not old as the earth counts time2 volcanoes are short- lived geological phenomena4 ,,crandall ,,volcano4 ,this name is given to a dissected ancient volcano in the ,yellowstone ,national ,park1 which once1 it is estimated1 reared its head thousands of feet above the surrounding country and greatly exceeded in bulk either ,mount ,shasta or ,mount ,etna4 ,not a line of the original mountain remains2 all has been swept away by erosion except some four thousand feet of the base of the pile4 ,this basal wreck now appears as a rugged region about thirty miles in diameter1 trenched by deep valleys and cut into sharp peaks and precipitous ridges4 ,in the center of the area is found the nucleus 7,n1 ,fig4 #bcg71--;a mass of coarsely crystalline rock that congealed deep in the old volcanic pipe4 ,from it there radiate in all directions1 like the spokes of a wheel1 long dikes whose rock grows rapidly finer of grain as it leaves the vicinity of the once heated core4 ,the remainder of the base of the ancient mountain is made of rudely bedded tuffs and volcanic breccia1 with occasional flows of lava1 some of the fragments of the breccia measuring as much as twenty feet in diameter4 ,on the sides of canyons the breccia is carved by rain erosion to fantastic pinnacles4 ,at different levels in the midst of these beds of tuff and lava are many old forest grounds4 ,the stumps and trunks of the trees1 now turned to stone1 still in many cases stand upright where once they grew on the slopes of the mountain as it was building 7,fig4 #bch74 ,the great size and age of some of these trees indicate1 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 ashes4 ,near the edge of the area lies ,death ,gulch1 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 it4 ,,chapter ,,xii ,,underground ,,structures ,,of ,,igneous ,,origin ,it is because long-continued erosion lays bare the innermost anatomy of an extinct volcano1 and even sweeps away the entire pile with much of the underlying strata1 thus leaving the very roots of the volcano open to view1 that we are able to study underground volcanic structures4 ,with these we include1 for convenience1 intrusions of molten rock which have been driven upward into the crust1 but which may not have succeeded in breaking way to the surface and establishing a volcano4 ,all these structures are built of rock forced when in a fluid or pasty state into some cavity which it has found or made1 and we may classify them therefore1 according to the shape of the molds in which the molten rock has congealed1 as 7#a7 dikes1 7#b7 volcanic necks1 7#c7 intrusive sheets1 and 7#d7 intrusive masses4 ,,dikes4 ,the sheet of once molten rock with which a fissure has been filled is known as a dike4 ,dikes are formed when volcanic cones are rent by explosions or by the weight of the lava column in the duct1 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 built4 ,in regions undergoing deformation rocks lying deep below the ground are often broken and the fissures are filled with molten rock from beneath1 which finds no outlet to the surface4 ,such dikes are common in areas of the most ancient rocks1 which have been brought to light by long erosion4 ,in exceptional cases dikes may reach the length of fifty or one hundred miles4 ,they vary in width from a fraction of a foot to even as much as three hundred feet4 ,dikes are commonly more fine of grain on the sides than in the center1 and may have a glassy and crackled surface where they meet the inclosing rock4 ,can you account for this on any principle which you have learned8 ,,volcanic ,,necks4 ,the pipe of a volcano rises from far below the base of the cone1--from the deep reservoir from which its eruptions are supplied4 ,when the volcano has become extinct this great tube remains filled with hardened lava4 ,it forms a cylindrical core of solid rock1 except for some distance below the ancient crater1 where it may contain a mass of fragments which had fallen back into the chimney after being hurled into the air4 ,as the mountain is worn down1 this central column known as the ,,volcanic ,,neck is left standing as a conical hill 7,fig4 #bdj74 ,even when every other trace of the volcano has been swept away1 erosion will not have passed below this great stalk on which the volcano was borne as a fiery flower whose site it remains to mark4 ,in volcanic regions of deep denudation volcanic necks rise solitary and abrupt from the surrounding country as dome-shaped hills4 ,they are marked features in the landscape in parts of ,scotland and in the ,st4 ,lawrence valley about ,montreal 7,fig4 #bda74 ,,intrusive ,,sheets4 ,sheets of igneous rocks are sometimes found interleaved with sedimentary strata1 especially in regions where the rocks have been deformed and have suffered from volcanic action4 ,in some instances such a sheet is seen to be ,,contemporaneous 7p4 #bdh74 ,in other instances the sheet must be ,,intrusive4 ,the overlying stratum1 as well as that beneath1 has been affected by the heat of the once molten rock4 ,we infer that the igneous rock when in a molten state was forced between the strata1 much as a card may be pushed between the leaves of a closed book4 ,the liquid wedged its way between the layers1 lifting those above to make room for itself4 ,the source of the intrusive sheet may often be traced to some dike 7known therefore as the ,,feeding ,,dike71 or to some mass of igneous rock4 ,intrusive sheets may extend a score and more of miles1 and1 like the longest surface flows1 the most extensive sheets consist of the more fusible and fluid lavas1--those of the basic class of which basalt is an example4 ,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 7,fig4 #cae74 ,on the west bank of the ,hudson there extends from ,new ,york ,bay north for thirty miles a bold cliff several hundred feet high1-- the ,,palisades ,,of ,,the ,,hudson4 ,it is the outcropping edge of a sheet of ancient igneous rock1 which rests on stratified sandstones and is overlain by strata of the same series4 ,sandstones and lava sheet together dip gently to the west arid the latter disappears from view two miles back from the river4 ,it is an interesting question whether the ,palisades sheet is ,,contemporaneous or ,,intrusive4 ,was it outpoured on the sandstones beneath it when they formed the floor of the sea1 and covered forthwith by the sediments of the strata above1 or was it intruded among these beds at a later date8 ,the latter is the case3 for the overlying stratum is intensely baked along the zone of contact4 ,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 strata4 ,,electric ,,peak1 one of the prominent mountains of the ,yellowstone ,national ,park1 is carved out of a mass of strata into which many sheets of molten rock have been intruded4 ,the western summit consists of such a sheet several hundred feet thick4 ,studying the section of ,figure #bdd1 what inference do you draw as to the source of these intrusive sheets8 ,,intrusive ,,masses ,,bosses4 ,this name is generally applied to huge irregular masses of coarsely crystalline igneous rock lying in the midst of other formations4 ,bosses vary greatly in size and may reach scores of miles in extent4 ,seldom are there any evidences found that bosses ever had connection with the surface4 ,on the other hand1 it is often proved that they have been driven1 or have melted their way1 upward into the formations in which they lie2 for they give off dikes and intrusive sheets1 and have profoundly altered the rocks about them by their heat4 ,the texture of the rock of bosses proves that consolidation proceeded slowly and at great depths1 and it is only because of vast denudation that they are now exposed to view4 ,bosses are commonly harder than the rocks about them1 and stand up1 therefore1 as rounded hills and mountainous ridges long after the surrounding country has worn to a low plain4 ,the base of bosses is indefinite or undetermined1 and in this respect they differ from laccoliths4 ,some bosses have broken and faulted the overlying beds2 some have forced the rocks aside and melted them away4 ,the ,,spanish ,,peaks of southeastern ,colorado were formed by the upthrust of immense masses of igneous rock1 bulging and breaking the overlying strata4 ,on one side of the mountains the throw of the fault is nearly a mile1 and fragments of deep-lying beds were dragged upward by the rising masses4 ,the adjacent rocks were altered by heat to a distance of several thousand feet4 ,no evidence appears that the molten rock ever reached the surface1 and if volcanic eruptions ever took place either in lava flows or fragmental materials1 all traces of them have been effaced4 ,the rock of the intrusive masses is coarsely crystalline1 and no doubt solidified slowly under the pressure of vast thicknesses of overlying rock1 now mostly removed by erosion4 ,a magnificent system of dikes radiates from the ,peaks to a distance of fifteen miles1 some now being left by long erosion as walls a hundred feet in height 7,fig4 #bci74 ,intrusive sheets fed by the dikes penetrate the surrounding strata1 and their edges are cut by canyons as much as twenty-five miles from the mountain4 ,in these strata are valuable beds of lignite1 an imperfect coal1 which the heat of dikes and sheets has changed to coke4 ,,laccoliths4 ,the laccolith 7,greek laccos1 cistern2 lithos1 stone7 is a variety of intrusive masses in which molten rock has spread between the strata1 and1 lifting the strata above it to a dome- shaped form1 has collected beneath them in a lens-shaped body with a flat base4 ,the ,,henry ,,mountains1 a small group of detached peaks in southern ,utah1 rise from a plateau of horizontal rocks4 ,some of the peaks are carved wholly in separate domelike uplifts of the strata of the plateau4 ,in others1 as ,mount ,hillers1 the largest of the group1 there is exposed on the summit a core of igneous rock from which the sedimentary rocks of the flanks dip steeply outward in all directions4 ,in still others erosion has stripped off the covering strata and has laid bare the core to its base2 and its shape is here seen to be that of a plano-convex lens or a baker's bun1 its flat base resting on the undisturbed bedded rocks beneath4 ,the structure of ,mount ,hillers is shown in ,figure #bdh4 ,the nucleus of igneous rock is four miles in diameter and more than a mile in depth4 ,,regional ,,intrusions4 ,these vast bodies of igneous rock1 which may reach hundreds of miles in diameter1 differ little from bosses except in their immense bulk4 ,like bosses1 regional intrusions give off dikes and sheets and greatly change the rocks about them by their heat4 ,they are now exposed to view only because of the profound denudation which has removed the upheaved dome of rocks beneath which they slowly cooled4 ,such intrusions are accompanied --whether as cause or as effect is still hardly known--by deformations1 and their masses of igneous rock are thus found as the core of many great mountain ranges4 ,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 length4 ,immense regional intrusions1 the cores of once lofty mountain ranges1 are found upon the ,laurentian peneplain4 ,,physiographic ,,effects ,,of ,,intrusive ,,masses4 ,we have already seen examples of the topographic effects of intrusive masses in ,mount ,hillers1 the ,spanish ,peaks1 and in the great mountain ranges mentioned in the paragraph on regional intrusions1 although in the latter instances these effects are entangled with the effects of other processes4 ,masses of igneous rock cannot be intruded within the crust without an accompanying deformation on a scale corresponding to the bulk of the intruded mass4 ,the overlying strata are arched into hills or mountains1 or1 if the molten material is of great extent1 the strata may conceivably be floated upward to the height of a plateau4 ,we may suppose that the transference of molten matter from one region to another may be among the causes of slow subsidences and elevations4 ,intrusions give rise to fissures1 dikes1 and intrusive sheets1 and these dislocations cannot fail to produce earthquakes4 ,where intrusive masses open communication with the surface1 volcanoes are established or fissure eruptions occur such as those of ,iceland4 ,,the ,,intrusive ,,rocks ,the igneous rocks are divided into two general classes1--the ,,volcanic or ,,eruptive rocks1 which have been outpoured in open air or on the floor of the sea1 and the ,,intrusive rocks1 which have been intruded within the rocks of the crust and have solidified below the surface4 ,the two classes are alike in chemical composition and may be divided into acidic and basic groups4 ,in texture the intrusive rocks differ from the volcanic rocks because of the different conditions under which they have solidified4 ,they cooled far more slowly beneath the cover of the rocks into which they were pressed than is permitted to lava flows in open air4 ,their constituent minerals had ample opportunity to sort themselves and crystallize from the fluid mixture1 and none of that mixture was left to congeal as a glassy paste4 ,they consolidated also under pressure4 ,they are never scoriaceous1 for the steam with which they were charged was not allowed to expand and distend them with steam blebs4 ,in the rocks of the larger intrusive masses one may see with a powerful microscope exceedingly minute cavities1 to be counted by many millions to the cubic inch1 in which the gaseous water which the mass contained was held imprisoned under the immense pressure of the overlying rocks4 ,naturally these characteristics are best developed in the intrusives which cooled most slowly1 i4e4 in the deepest-seated and largest masses2 while in those which cooled more rapidly1 as in dikes and sheets1 we find gradations approaching the texture of surface flows4 ,,varieties ,,of ,,the ,,intrusive ,,rocks4 ,we will now describe a few of the varieties of rocks of deep-seated intrusions4 ,all are even grained1 consisting of a mass of crystalline grains formed during one continuous stage of solidification1 and no porphyritic crystals appear as in lavas4 ,,granite1 as we have learned already1 is composed of three minerals1--quartz1 feldspar1 and mica4 ,according to the color of the feldspar the rock may be red1 or pink1 or gray4 ,hornblende--a black or dark green mineral1 an iron-magnesian silicate1 about as hard as feldspar--is sometimes found as a fourth constituent1 and the rock is then known as ,,hornblendic ,,granite4 ,granite is an acidic rock corresponding to rhyolite in chemical composition4 ,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 reservoir4 ,,syenite1 composed of feldspar and mica1 has consolidated from a less siliceous mixture than has granite4 ,,diorite1 still less siliceous1 is composed of hornblende and feldspar1--the latter mineral being of different variety from the feldspar of granite and syenite4 ,,gabbro1 a typical basic rock1 corresponds to basalt in chemical composition4 ,it is a dark1 heavy1 coarsely crystalline aggregate of feldspar and ,,augite 7a dark mineral allied to hornblende74 ,it often contains ,,magnetite 7the magnetic black oxide of iron7 and ,,olivine 7a greenish magnesian silicate74 ,in the northern states all these types1 and many others also of the vast number of varieties of intrusive rocks1 can be found among the rocks of the drift brought from the areas of igneous rock in ,canada and the states of our northern border4 ,,summary4 ,the records of geology prove that since the earliest of their annals tremendous forces have been active in the earth4 ,in all the past1 under pressures inconceivably great1 molten rock has been driven upward into the rocks of the crust4 ,it has squeezed into fissures forming dikes2 it has burrowed among the strata as intrusive sheets2 it has melted the rocks away or lifted the overlying strata1 filling the chambers which it has made with intrusive masses4 ,during all geological ages molten rock has found way to the surface1 and volcanoes have darkened the sky with clouds of ashes and poured streams of glowing lava down their sides4 ,the older strata1--the strata which have been most deeply buried1--and especially those which have suffered most from folding and from fracture1 show the largest amount of igneous intrusions4 ,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 miles4 ,,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 together4 ,few of these problems are solved1 and we may only state some known facts and the probable conclusions which may be drawn as inferences from them4 ,,the ,,interior ,,of ,,the ,,earth ,,is ,,hot4 ,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 condition4 ,deep wells and mines show everywhere an increase in temperature below the surface shell affected by the heat of summer and the cold of winter1--a shell in temperate latitudes sixty or seventy feet thick4 ,thus in a boring more than a mile deep at ,schladebach1 ,germany1 the earth grows warmer at the rate of #a degrees ,f4 for every sixty-seven feet as we descend4 ,taking the average rate of increase at one degree for every sixty feet of descent1 and assuming that this rate1 observed at the moderate distances open to observation1 continues to at least thirty-five miles1 the temperature at that depth must be more than three thousand degrees1--a temperature at which all ordinary rocks would melt at the earth's surface4 ,the rate of increase in temperature probably lessens as we go downward1 and it may not be appreciable below a few hundred miles4 ,but there is no reason to doubt that ,,the ,,interior ,,of ,,the ,,earth ,,is ,,intensely ,,hot4 ,below a depth of one or two score miles we may imagine the rocks everywhere glowing with heat4 ,although the heat of the interior is great enough to melt all rocks at atmospheric pressure1 it does not follow that the interior is fluid4 ,pressure raises the fusing point of rocks1 and the weight of the crust may keep the interior in what may be called a solid state1 although so hot as to be a liquid or a gas were the pressure to be removed4 ,,the ,,interior ,,of ,,the ,,earth ,,is ,,rigid ,,and ,,heavy4 ,the earth behaves as a globe more rigid than glass under the attractions of the sun and moon4 ,it is not deformed by these stresses as is the ocean in the tides1 proving that it is not a fluid ball covered with a yielding crust a few miles thick4 ,earthquakes pass through the earth faster than they would were it of solid steel4 ,hence the rocks of the interior are highly elastic1 being brought by pressure to a compact1 continuous condition unbroken by the cracks and vesicles of surface rocks4 ,,the ,,interior ,,of ,,the ,,earth ,,is ,,rigid ,the common rocks of the crust are about two and a half times heavier than water1 while the earth as a whole weighs five and six-tenths times as much as a globe of water of the same size4 ,,the ,,interior ,,is ,,therefore ,,much ,,more ,,heavy ,,than ,,the ,,crust4 ,this may be caused in part by compression of the interior under the enormous weight of the crust1 and in part also by an assortment of material1 the heavier substances1 such as the heavy metals1 having gravitated towards the center4 ,between the crust1 which is solid because it is cool1 and the interior1 which is hot enough to melt were it not for the pressure which keeps it dense and rigid1 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 crust4 ,it is perhaps from such a subcrustal layer that the lava of volcanoes is supplied4 ,,the ,,causes ,,of ,,volcanic ,,action4 ,it is now generally believed that the ,,heat of volcanoes is that of the earth's interior4 ,other causes1 such as friction and crushing in the making of mountains and the chemical reactions between oxidizing agents of the crust and the unoxidized interior1 have been suggested1 but to most geologists they seem inadequate4 ,there is much difference of opinion as to the ,,force which causes molten rock to rise to the surface in the ducts of volcanoes4 ,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 charged1 much as a viscid liquid rises and boils over in a test tube or kettle4 ,but in quiet eruptions1 and still more in the irruption of intrusive sheets and masses1 there is little if any evidence that steam is the driving force4 ,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 crust4 ,it is held by some that where considerable water is supplied to the rising column of lava1 as from the ground water of the surrounding region1 and where the lava is viscid so that steam does not readily escape1 the eruption is of the explosive type2 when these conditions do not obtain1 the lava outwells quietly1 as in the ,hawaiian volcanoes4 ,it is held by others not only that volcanoes are due to the outflow of the earth's deep-seated heat1 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 waters4 ,,volcanic ,,action ,,and ,,deformation4 ,volcanoes do not occur on wide plains or among ancient mountains4 ,on the other hand1 where movements of the earth's crust are in progress in the uplift of high plateaus1 and still more in mountain making1 molten rock may reach the surface1 or may be driven upward toward it forming great intrusive masses4 ,thus extensive lava flows accompanied the upheaval of the block mountains of western ,north ,america and the uplift of the ,colorado plateau4 ,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 ,nyassa4 ,the volcanoes of the ,andes show how conspicuous volcanic action may be in young rising ranges4 ,folded mountains often show a core of igneous rock1 which by long erosion has come to form the axis and the highest peaks of the range1 as if the molten rock had been squeezed up under the rising upfolds4 ,as we decipher the records of the rocks in historical geology we shall see more fully how1 in all the past1 volcanic action has characterized the periods of great crustal movements1 and how it has been absent when and where the earth's crust has remained comparatively at rest4 ,,the ,,causes ,,of ,,deformation4 ,as the earth's interior1 or nucleus1 is highly heated it must be constantly though slowly losing its heat by conduction through the crust and into space2 and since the nucleus is cooling it must also be contracting4 ,the nucleus has contracted also because of the extrusion of molten matter1 the loss of constituent gases given off in volcanic eruptions1 and 7still more important7 the compression and consolidation of its material under gravity4 ,as the nucleus contracts1 it tends to draw away from the cooled and solid crust1 and the latter settles1 adapting itself to the shrinking nucleus much as the skin of a withering apple wrinkles down upon the shrunken fruit4 ,the unsupported weight of the spherical crust develops enormous tangential pressures1 similar to the stresses of an arch or dome1 and when these lateral thrusts accumulate beyond the power of resistance the solid rock is warped and folded and broken4 ,since the planet attained its present mass it has thus been lessening in volume4 ,notwithstanding local and relative upheavals the earth's surface on the whole has drawn nearer and nearer to the center4 ,the portions of the lithosphere which have been carried down the farthest have received the waters of the oceans1 while those portions which have been carried down the least have emerged as continents4 ,although it serves our convenience to refer the movements of the crust to the sea level as datum plane1 it is understood that this level is by no means fixed4 ,changes in the ocean basins increase or reduce their capacity and thus lower or raise the level of the sea4 ,but since these basins are connected1 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 land4 ,,chapter ,,xiii ,,metamorphism ,,and ,,mineral ,,veins ,under the action of internal agencies rocks of all kinds may be rendered harder1 more firmly cemented1 and more crystalline4 ,these processes are known as ,,metamorphism1 and the rocks affected1 whether originally sedimentary or igneous1 are called ,,metamorphic ,,rocks4 ,we may contrast with metamorphism the action of external agencies in weathering1 which render rocks less coherent by dissolving their soluble parts and breaking down their crystalline grains4 ,,contact ,,metamorphism4 ,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 mass4 ,the adjacent strata may be changed only in color1 hardness1 and texture4 ,thus1 next to a dike1 bituminous coal may be baked to coke or anthracite1 and chalk and limestone to crystalline marble4 ,sandstone may be converted into quartzite1 and shale into ,,argillite1 a compact1 massive clay rock4 ,new minerals may also be developed4 ,in sedimentary rocks there may be produced crystals of mica and of ,,garnet 7a mineral as hard as quartz1 commonly occurring in red1 twelve-sided crystals74 ,where the changes are most profound1 rocks may be wholly made over in structure and mineral composition4 ,in contact metamorphism1 thin sheets of molten rock produce less effect than thicker ones4 ,the strongest heat effects are naturally caused by bosses and regional intrusions1 and the zone of change about them may be several miles in width4 ,in these changes heated waters and vapors from the masses of igneous rocks undoubtedly play a very important part4 ,which will be more strongly altered1 the rocks about a closed dike in which lava began to cool as soon as it filled the fissure1 or the rocks about a dike which opened on the surface and through which the molten rock flowed for some time8 ,taking into consideration the part played by heated waters1 which will produce the most far-reaching metamorphism1 dikes which cut across the bedding planes or intrusive sheets which are thrust between the strata8 ,,regional ,,metamorphism4 ,metamorphic rocks occur wide-spread in many regions1 often hundreds of square miles in area1 where such extensive changes cannot be accounted for by igneous intrusions4 ,such are the dissected cores of lofty mountains1 as the ,alps1 and the worn-down bases of ancient ranges1 as in ,new ,england1 large areas in the ,piedmont ,belt1 and the ,laurentian peneplain4 ,in these regions the rocks have yielded to immense pressure4 ,they have been folded1 crumpled1 and mashed1 and even their minute grains1 as one may see with a microscope1 have often been puckered1 broken1 and crushed to powder4 ,it is to these mechanical movements and strains which the rocks have suffered in every part that we may attribute their metamorphism1 and the degree to which they have been changed is in direct proportion to the degree to which they have been deformed and mashed4 ,other factors1 however1 have played important parts4 ,rock crushing develops heat1 and allows a freer circulation of heated waters and vapors4 ,thus chemical reactions are greatly quickened2 minerals are dissolved and redeposited in new positions1 or their chemical constituents may recombine in new minerals1 entirely changing the nature of the rock1 as when1 for example1 feldspar recrystallizes as quartz and mica4 ,early stages of metamorphism are seen in ,,slate4 ,pressure has hardened the marine muds1 the arkose1 or the volcanic ash from which slates are derived1 and has caused them to cleave by the rearrangement of their particles4 ,under somewhat greater pressure1 slate becomes ,,phyllite1 a clay slate whose cleavage surfaces are lustrous with flat-lying mica flakes4 ,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 mica4 ,,foliation4 ,under still stronger pressure the whole structure of the rock is altered4 ,the minerals of which it is composed1 and the new minerals which develop by heat and pressure1 arrange themselves along planes of cleavage or of shear in rudely parallel leaves1 or ,,folia4 ,of this structure1 called ,,foliation1 we may distinguish two types1--a coarser feldspathic type1 and a fine type in which other minerals than feldspar predominate4 ,,gneiss is the general name under which are comprised coarsely foliated rocks banded with irregular layers of feldspar and other minerals4 ,the gneisses appear to be due in many cases to the crushing and shearing of deep-seated igneous rocks1 such as granite and gabbro4 ,,the ,,crystalline ,,schists1 representing the finer types of foliation1 consist of thin1 parallel1 crystalline leaves1 which are often remarkably crumpled4 ,these folia can be distinguished from the laminae of sedimentary rocks by their lenticular form and lack of continuity1 and especially by the fact that they consist of platy1 crystalline grains1 and not of particles rounded by wear4 ,,mica ,,schist1 the most common of schists1 and in fact of all metamorphic rocks1 is composed of mica and quartz in alternating wavy folia4 ,all gradations between it and phyllite may be traced1 and in many cases we may prove it due to the metamorphism of slates and shales4 ,it is widespread in ,new ,england and along the eastern side of the ,appalachians4 ,,talc ,,schist consists of quartz and ,,talc1 a light-colored magnesian mineral of greasy feel1 and so soft that it can be scratched with the thumb nail4 ,,hornblende ,,schist1 resulting in many cases from the foliation of basic igneous rocks1 is made of folia of hornblende alternating with bands of quartz and feldspar4 ,hornblende schist is common over large areas in the ,lake ,superior region4 ,,quartz ,,schist is produced from quartzite by the development of fine folia of mica along planes of shear4 ,all gradations may be found between it and unfoliated quartzite on the one hand and mica schist on the other4 ,under the resistless pressure of crustal movements almost any rocks1 sandstones1 shales1 lavas of all kinds1 granites1 diorites1 and gabbros may be metamorphosed into schists by crushing and shearing4 ,limestones1 however1 are metamorphosed by pressure into marble1 the grains of carbonate of lime recrystallizing freely to interlocking crystals of calcite4 ,these few examples must suffice of the great class of metamorphic rocks4 ,as we have seen1 they owe their origin to the alteration of both of the other classes of rocks--the sedimentary and the igneous--by heat and pressure1 assisted usually by the presence of water4 ,the fact of change is seen in their hardness arid cementation1 their more or less complete recrystallization1 and their foliation2 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 igneous1 or to what variety of either of these classes they belonged4 ,in many cases1 however1 the early history of a metamorphic rock can be deciphered4 ,fossils not wholly obliterated may prove it originally water-laid4 ,schists may contain rolled-out pebbles1 showing their derivation from a conglomerate4 ,dikes of igneous rocks may be followed into a region where they have been foliated by pressure4 ,the most thoroughly metamorphosed rocks may sometimes be traced out into unaltered sedimentary or igneous rocks1 or among them may be found patches of little change where their history maybe read4 ,metamorphism is most common among rocks of the earlier geological ages1 and most rare among rocks of recent formation4 ,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 interior1 or are suffering slow deformation under the thrust of mountain-making forces4 ,suggest how rocks now in process of metamorphism may sometimes be exposed to view4 ,why do metamorphic rocks appear on the surface to-day8 ,,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 water1 and fissures thus filled are known as mineral veins4 ,much of the importance of mineral veins is due to the fact that they are often metalliferous1 carrying valuable native metals and metallic ores disseminated in fine particles1 in strings1 and sometimes in large masses in the midst of the valueless nonmetallic minerals which make up what is known as the ,,vein ,,stone4 ,the most common vein stones are ,,quartz and ,,calcite4 ,,fluorite 7calcium fluoride71 a mineral harder than calcite and crystallizing in cubes of various colors1 and ,,barite 7barium sulphate71 a heavy white mineral1 are abundant in many veins4 ,the gold-bearing quartz veins of ,california traverse the metamorphic slates of the ,sierra ,nevada ,mountains4 ,below the zone of solution 7p4 #de7 these veins consist of a vein stone of quartz mingled with pyrite 7p4 #ac71 the latter containing threads and grains of native gold4 ,but to the depth of about fifty feet from the surface the pyrite of the vein has been dissolved1 leaving a rusty1 cellular quartz with grains of the insoluble gold scattered through it4 ,the ,,placer ,,deposits of ,california and other regions are gold- bearing deposits of gravel and sand in river beds4 ,the heavy gold is apt to be found mostly near or upon the solid rock1 and its grains1 like those of the sand1 are always rounded4 ,how the gold came in the placers we may leave the pupil to suggest4 ,copper is found in a number of ores1 and also in the native metal4 ,below the zone of surface changes the ore of a copper vein is often a double sulphide of iron and copper called ,,chalcopyrite1 a mineral softer than pyrite--it can easily be scratched with a knife--and deeper yellow in color4 ,for several score of feet below the ground the vein may consist of rusty quartz from which the metallic ores have been dissolved2 but at the base of the zone of solution we may find exceedingly rich deposits of copper ores1-- copper sulphides1 red and black copper oxides1 and green and blue copper carbonates1 which have clearly been brought down in solution from the leached upper portion of the vein4 ,,origin ,,of ,,mineral ,,veins4 ,both vein stones and ores have been deposited slowly from solution in water1 much as crystals of salt are deposited on the sides of a jar of saturated brine4 ,in our study of underground water we learned that it is everywhere circulating through the permeable rocks of the crust1 descending to profound depths under the action of gravity and again driven to the surface by hydrostatic pressure4 ,now fissures1 wherever they occur1 form the trunk channels of the underground circulation4 ,water descends from the surface along these rifts2 it moves laterally from either side to the fissure plane1 just as ground water seeps through the surrounding rocks from every direction to a well2 and it ascends through these natural water ways as in an artesian well1 whenever they intersect an aquifer in which water is under hydrostatic pressure4 ,the waters which deposit vein stones and ores are commonly hot1 and in many cases they have derived their heat from intrusions of igneous rock still uncooled within the crust4 ,the solvent power of the water is thus greatly increased1 and it takes up into solution various substances from the igneous and sedimentary rocks which it traverses4 ,for various reasons these substances stances are deposited in the vein as ores and vein stones4 ,on rising through the fissure the water cools and loses pressure1 and its capacity to hold minerals in solution is therefore lessened4 ,besides1 as different currents meet in the fissure1 some ascending1 some descending1 and some coming in from the sides1 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 ores4 ,as an illustration of the method of vein deposits we may cite the case of a wooden box pipe used in the ,comstock mines1 ,nevada1 to carry the hot water of the mine from one level to another1 which in ten years was lined with calcium carbonate more than half an inch thick4 ,the ,steamboat ,springs1 ,nevada1 furnish examples of mineral veins in process of formation4 ,the steaming water rises through fissures in volcanic rocks and is now depositing in the rifts a vein stone of quartz1 with metallic ores of iron1 mercury1 lead1 and other metals4 ,,reconcentration4 ,near the base of the zone of solution veins are often stored with exceptionally large and valuable ore deposits4 ,this local enrichment of the vein is due to the reconcentration of its metalliferous ores4 ,as the surface of the land is slowly lowered by weathering and running water1 the zone of solution is lowered at an equal rate and encroaches constantly on the zone of cementation4 ,the minerals of veins are therefore constantly being dissolved along their upper portions and carried down the fissures by ground water to lower levels1 where they are redeposited4 ,many of the richest ore deposits are thus due to successive concentrations3 the ores were leached originally from the rocks to a large extent by laterally seeping waters2 they were concentrated in the ore deposits of the vein chiefly by ascending currents2 they have been reconcentrated by descending waters in the way just mentioned4 ,,the ,,original ,,source ,,of ,,the ,,metals4 ,it is to the igneous rocks that we may look for the original source of the metals of veins4 ,lavas contain minute percentages of various metallic compounds1 and no doubt this was the case also with the igneous rocks which formed the original earth crust4 ,by the erosion of the igneous rocks the metals have been distributed among sedimentary strata1 and even the sea has taken into solution an appreciable amount of gold and other metals1 but in this widely diffused condition they are wholly useless to man4 ,the concentration which has made them available is due to the interaction of many agencies4 ,earth movements fracturing deeply the rocks of the crust1 the intrusion of heated masses1 the circulation of underground waters1 have all cooperated in the concentration of the metals of mineral veins4 ,while fissure veins are the most important of mineral veins1 the latter term is applied also to any water way which has been filled by similar deposits from solution4 ,thus in soluble rocks1 such as limestones1 joints enlarged by percolating water are sometimes filled with metalliferous deposits1 as1 for example1 the lead and zinc deposits of the upper ,mississippi valley4 ,even a porous aquifer may be made the seat of mineral deposits1 as in the case of some copper-bearing and silver-bearing sandstones of ,new ,mexico4 ,,part ,,iii ,,historical ,,geology ,,chapter ,,xiv ,,the ,,geological ,,record ,,what ,a ,,formation ,,records4 ,we have already learned that each individual body of stratified rock1 or formation1 constitutes a record of the time when it was laid4 ,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 spread2 and if the former1 whether the land was river plain1 or lake bed1 or was covered with wind-blown sands1 or by the deposits of an ice sheet4 ,if the sediments are marine1 we may know also whether they were laid in shoal water near the shore or in deeper water out at sea1 and whether during a period of emergence1 or during a period of subsidence when the sea transgressed the land4 ,by the same means each formation records the stage in the cycle of erosion of the land mass from which its sediments were derived4 ,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 erosion4 ,the attitude and structure of the strata tell also of the foldings and fractures1 the deformation and the metamorphism1 which they have suffered2 and the igneous rocks associated with them as lava flows and igneous intrusions add other details to the story4 ,each formation is thus a separate local chapter in the geological history of the earth1 and its strata are its leaves4 ,it contains an authentic record of the physical conditions--the geography--of the time and place when and where its sediments were laid4 ,,past ,,cycles ,,of ,,erosion4 ,these chapters in the history of the planet are very numerous1 although much of the record has been destroyed in various ways4 ,a succession of different formations is usually seen in any considerable section of the crust1 such as a deep canyon or where the edges of upturned strata are exposed to view on the flanks of mountain ranges2 and in any extensive area1 such as a state of the ,union or a province of ,canada1 the number of formations outcropping on the surface is large4 ,it is thus learned that our present continent is made up for4 the most part of old continental deltas4 ,some1 recently emerged as the strata of young coastal plains1 are the records of recent cycles of erosion2 while others were deposited in the early history of the earth1 and in many instances have been crumpled into mountains1 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 land4 ,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 cycles4 ,again and again movements of the crust have brought to an end one cycle-- sometimes when only well under way1 and sometimes when drawing toward its close--and have begun another4 ,again and again they have added to the land areas which before were sea1 with all their deposition records of earlier cycles1 or have lowered areas of land beneath the sea to receive new sediments4 ,,the ,,age ,,of ,,the ,,earth4 ,the thickness of the stratified rocks now exposed upon the eroded surface of the continents is very great4 ,in the ,appalachian region the strata are seven or eight miles thick1 and still greater thicknesses have been measured in several other mountain ranges4 ,the aggregate thickness of all the formations of the stratified rocks of the earth's crust1 giving to each formation its maximum thickness wherever found1 amounts to not less than forty miles4 ,knowing how slowly sediments accumulate upon the sea floor1 we must believe that the successive cycles which the earth has seen stretch back into a past almost inconceivably remote1 and measure tens of millions and perhaps even hundreds of millions of years4 ,,how ,,the ,,formations ,,are ,,correlated ,,and ,,the ,,geological ,,record ,,made ,,up4 ,arranged in the order of their succession1 the formations of the earth's crust would constitute a connected record in which the geological history of the planet may be read1 and therefore known as the ,,geological ,,record4 ,but to arrange the formations in their natural order is not an easy task4 ,a complete set of the volumes of the record is to be found in no single region4 ,their leaves and chapters are scattered over the land surface of the globe4 ,in one area certain chapters may be found1 though perhaps with many missing leaves1 and with intervening chapters wanting1 and these absent parts perhaps can be supplied only after long search through many other regions4 ,adjacent strata in any region are arranged according to the ,,law ,,of ,,superposition1 i4e4 any stratum is younger than that on which it was deposited1 just as in a pile of paper1 any sheet was laid later than that on which it rests4 ,where rocks have been disturbed1 their original attitude must be determined before the law can be applied4 ,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 other4 ,the formations of different regions are arranged in their true order by the ,,law ,,of ,,included ,,organisms2 i4e4 formations1 however widely separated1 which contain a similar assemblage of fossils are equivalent and belong to the same division of geological time4 ,the correlation of formations by means of fossils may be explained by the formations now being deposited about the north ,atlantic4 ,lithologically they are extremely various4 ,on the continental shelf of ,north ,america limestones of different kinds are forming off ,florida1 and sandstones and shales from ,georgia northward4 ,separated from them by the deep ,atlantic oozes are other sedimentary deposits now accumulating along the west coast of ,europe4 ,if now all these offshore formations were raised to open air1 how could they be correlated8 ,surely not by lithological likeness1 for in this respect they would be quite diverse4 ,all would be similar1 however1 in the fossils which they contain4 ,some fossil species would be identical in all these formations and others would be closely allied4 ,making all due allowance for differences in species due to local differences in climate and other physical causes1 it would still be plain that plants and animals so similar lived at the same period of time1 and that the formations in which their remains were imbedded were contemporaneous in a broad way4 ,the presence of the bones of whales and other marine mammals would prove that the strata were laid after the appearance of mammals upon earth1 and imbedded relics of man would give a still closer approximation to their age4 ,in the same way we correlate the earlier geological formations4 ,for example1 in #aijb there were collected the first fossils ever found on the antarctic continent4 ,among the dozen specimens obtained were some fossil ammonites 7a family of chambered shells7 of genera which are found on other continents in certain formations classified as the ,cretaceous system1 and which occur neither above these formations nor below them4 ,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 ,canada4 ,,the ,,record ,,as ,a ,,time ,,scale4 ,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 order4 ,when the geological record is thus obtained it may be used as a universal time scale for geological history4 ,geological time is separated into divisions corresponding to the times during which the successive formations were laid4 ,the largest assemblages of formations are known as groups1 while the corresponding divisions of time are known as eras4 ,groups are subdivided into systems1 and systems into series4 ,series are divided into stages and substages1--subdivisions which do not concern us in this brief treatise4 ,the corresponding divisions of time are given in the following table4 ,,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 formations1 when he has given each formation its true place in the geological record as a time scale4 ,the following chart exhibits the main divisions of the record1 the name given to each being given also to the corresponding time division4 ,thus we speak of the ,,cambrian ,,system1 meaning a certain succession of formations which are classified together because of broad resemblances in their included organisms2 and of the ,,cambrian ,,period1 meaning the time during which these rocks were deposited4 ,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 continents2 the fossils imbedded in the rocks of each formation tell of the kinds of animals and plants which inhabited the earth at that time1 and from these fossils we are therefore able to construct the history of life upon the earth4 ,,fossils4 ,these remains of organisms are found in the strata in all degrees of perfection1 from trails and tracks and fragmentary impressions1 to perfectly preserved shells1 wood1 bones1 and complete skeletons4 ,as a rule1 it is only the hard parts of animals and plants which have left any traces in the rocks4 ,sometimes the original hard substance is preserved1 but more often it has been replaced by some less soluble material4 ,petrifaction1 as this process of slow replacement is called1 is often carried on in the most exquisite detail4 ,when wood1 for example1 is undergoing petrifaction1 the woody tissue may be replaced1 particle by particle1 by silica in solution through the action of underground waters1 even the microscopic structures of the wood being perfectly reproduced4 ,in shells originally made of ,,aragonite1 a crystalline form of carbonate of lime1 that mineral is usually replaced by ,,calcite1 a more stable form of the same substance4 ,the most common petrifying materials are calcite1 silica1 and pyrite4 ,often the organic substance has neither been preserved nor replaced1 but the ,,form has been retained by means of molds and casts4 ,permanent impressions1 or molds1 may be made in sediments not only by the hard parts of organisms1 but also by such soft and perishable parts as the leaves of plants1 and1 in the rarest instances1 by the skin of animals and the feathers of birds4 ,in fine-grained limestones even the imprints of jellyfish have been retained4 ,the different kinds of molds and casts may be illustrated by means of a clam shell and some moist clay1 the latter representing the sediments in which the remains of animals and plants are entombed4 ,imbedding the shell in the clay and allowing the clay to harden1 we have a ,,mold ,,of ,,the ,,exterior of the shell1 as is seen on cutting the clay matrix in two and removing the shell from it4 ,filling this mold with clay of different color1 we obtain a ,,cast ,,of ,,the ,,exterior1 which represents accurately the original form and surface markings of the shell4 ,in nature1 shells and other relics of animals or plants are often removed by being dissolved by percolating waters1 and the molds are either filled with sediments or with minerals deposited from solution4 ,where the fossil is hollow1 a ,,cast ,,of ,,the ,,interior is made in the same way4 ,interior casts of shells reproduce any markings on the inside of the valves1 and casts of the interior of the skulls of ancient vertebrates show the form and size of their brains4 ,,imperfection ,,of ,,the ,,life ,,record4 ,at the present time only the smallest fraction of the life on earth ever gets entombed in rocks now forming4 ,in the forest great fallen tree trunks1 as well as dead leaves1 decay1 and only add a little to the layer of dark vegetable mold from which they grew4 ,the bones of land animals are1 for the most part1 left unburied on the surface and are soon destroyed by chemical agencies4 ,even where1 as in the swamps of river1 flood plains and in other bogs1 there are preserved the remains of plants1 and sometimes insects1 together with the bones of some animal drowned or mired1 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 land4 ,in the sea the conditions for preservation are more favorable than on land2 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 currents4 ,we may infer that during each period of the past1 as at the present1 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 existence4 ,scanty as these original life records must have been1 they have been largely destroyed by metamorphism of the rocks in which they were imbedded1 by solution in underground waters1 and by the vast denudation under which the sediments of earlier periods have been eroded to furnish materials for the sedimentary records of later times4 ,moreover1 very much of what has escaped destruction still remains undiscovered4 ,the immense bulk of the stratified rocks is buried and inaccessible1 and the records of the past which it contains can never be known4 ,comparatively few outcrops have been thoroughly searched for fossils4 ,although new species are constantly being discovered1 each discovery may be considered as the outcome of a series of happy accidents1--that the remains of individuals of this particular species happened to be imbedded and fossilized1 that they happened to escape destruction during long ages1 and that they happened to be exposed and found4 ,,some ,,inferences ,,from ,,the ,,records ,,of ,,the ,,history ,,of ,,life ,,upon ,,the ,,planet4 ,meager as are these records1 they set forth plainly some important truths which we will now briefly mention4 #a4 ,each series of the stratified rocks1 except the very deepest1 contains vestiges of life4 ,hence ,,the ,,earth ,,was ,,tenanted ,,by ,,living ,,creatures ,,for ,,an ,,uncalculated ,,length ,,of ,,time ,,before ,,human ,,history ,,began4 #b4 ,,life ,,on ,,the ,,earth ,,has ,,been ,,everchanging4 ,the youngest strata hold the remains of existing species of animals and plants and those of species and varieties closely allied to them4 ,strata somewhat older contain fewer existing species1 and in strata of a still earlier1 but by no means an ancient epoch1 no existing species are to be found2 the species of that epoch and of previous epochs have vanished from the living world4 ,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 belong1 and other species1 genera1 and families have replaced them4 ,the fossils of each formation differ on the whole from those of every other4 ,the assemblage of animals and plants 7the ,,fauna- ,,flora7 of each epoch differs from that of every other epoch4 ,in many cases the extinction of a type has been gradual2 in other instances apparently abrupt4 ,there is no evidence that any organism once become extinct has ever reappeared4 ,the duration of a species in time1 or its vertical range through the strata1 varies greatly4 ,some species are limited to a stratum a few feet in thickness2 some may range through an entire formation and be found but little modified in still higher beds4 ,a formation may thus often be divided into zones1 each characterized by its own peculiar species4 ,as a rule1 the simpler organisms have a longer duration as species1 though not as individuals1 than the more complex4 #c4 ,,the ,,larger ,,zoological ,,and ,,botanical ,,groupings ,,survive ,,longer ,,than ,,the ,,smaller4 ,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 species4 ,a genus continues with new species after all the species with which it began have become extinct4 ,families survive genera1 and orders families4 ,classes are so long- lived that most of those which are known from the earliest formations are represented by living forms1 and no sub-kingdom has ever become extinct4 ,thus1 to take an example from the stony corals1--the ,,zoantharia1-- the particular characters--which constituted a certain ,,species-- ,facosites niagarensis--of the order are confined to the ,niagara series4 ,its ,,generic characters appeared in other species earlier in the ,silurian and continued through the ,devonian4 ,its ,,family characters1 represented in different genera and species1 range from the ,ordovician to the close of the ,paleozoic2 while the characters which it shares with all its order1 the ,zoantharia1 began in the ,cambrian and are found in living species4 #d4 ,,the ,,change ,,in ,,organisms ,,has ,,been ,,gradual4 ,the fossils of each life zone and of each formation of a conformable series closely resemble1 with some explainable exceptions1 those of the beds immediately above and below4 ,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 other1 thus accounting for the resemblance by heredity4 ,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 rocks1 and that the chain of life has been unbroken since its beginning4 #e4 ,,the ,,change ,,in ,,species ,,has ,,been ,a ,,gradual ,,differentiation4 ,tracing the lines of descent of various animals and plants of the present backward through the divisions of geologic time1 we find that these lines of descent converge and unite in simpler and still simpler types4 ,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 species4 #f4 ,,the ,,change ,,in ,,organisms ,,throughout ,,geologic ,,time ,,has ,,been ,a ,,progressive ,,change4 ,in the earliest ages the only animals and plants on the earth were lowly forms1 simple and generalized in structure2 while succeeding ages have been characterized by the introduction of types more and more specialized and complex1 and therefore of higher rank in the scale of being4 ,thus the ,algonkian contains the remains of only the humblest forms of the invertebrates4 ,in the ,cambrian1 ,ordovician1 and ,silurian the invertebrates were represented in all their subkingdoms by a varied fauna4 ,in the ,devonian1 fishes--the lowest of the vertebrates--became abundant4 ,amphibians made their entry on the stage in the ,carboniferous1 and reptiles came to rule the world in the ,mesozoic4 ,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 on2 and latest of all appeared the noblest product of the creative process1 man4 ,just as growth is characteristic of the individual life1 so gradual1 progressive change1 or evolution1 has characterized the history of life upon the planet4 ,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 acorn1 spreading loftier and more widely extended branches as it grows4 #g4 ,while higher and still higher types have continually been evolved1 until man1 the highest of all1 appeared1 ,,the ,,lower ,,and ,,earlier ,,types ,,have ,,generally ,,persisted4 ,some which reached their culmination early in the history of the earth have since changed only in slight adjustments to a changing environment4 ,thus the brachiopods1 a type of shellfish1 have made no progress since the ,paleozoic1 and some of their earliest known genera are represented by living forms hardly to be distinguished from their ancient ancestors4 ,the lowest and earliest branches of the tree of life have risen to no higher levels since they reached their climax of development long ago4 #h4 ,a strange parallel has been found to exist between the evolution of organisms and the development of the individual4 ,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 time4 ,,the ,,development ,,of ,,the ,,individual ,,recapitulates ,,the ,,evolution ,,of ,,the ,,race4 ,the frog is a typical amphibian4 ,as a tadpole it passes through a stage identical in several well-known features with the maturity of fishes2 as1 for example1 its aquatic life1 the tail by which it swims1 and the gills through which it breathes4 ,it is a fair inference that the tadpole stage in the life history of the frog represents a stage in the evolution of its kind1--that the ,amphibia are derived from fishlike ancestral forms4 ,this inference is amply confirmed in the geological record2 fishes appeared before ,amphibia and were connected with them by transitional forms4 ,,the ,,great ,,length ,,of ,,geologic ,,time ,,inferred ,,from ,,the ,,slow ,,change ,,of ,,species4 ,life forms1 like land forms1 are thus subject to change under the influence of their changing environment and of forces acting from within4 ,how slowly they change may be seen in the apparent stability of existing species4 ,in the lifetime of the observer and even in the recorded history of man1 species seem as stable as the mountain and the river4 ,but life forms and land forms are alike variable1 both in nature and still more under the shaping hand of man4 ,as man has modified the face of the earth with his great engineering works1 so he has produced widely different varieties of many kinds of domesticated plants and animals1 such as the varieties of the dog and the horse1 the apple and the rose1 which may be regarded in some respects as new species in the making4 ,we have assumed that land forms have changed in the past under the influence of forces now in operation4 ,assuming also that life forms have always changed as they are changing at present1 we come to realize something of the immensity of geologic time required for the evolution of life from its earliest lowly forms up to man4 ,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 earth4 ,thus1 since vertebrates appeared1 we have in succession the ,age of ,fishes1 the ,age of ,amphibians1 the ,age of ,reptiles1 and the ,age of ,mammals4 ,the chart given on page #bie is thus based on the law of superposition and the law of the evolution of organisms4 ,the first law gives the succession of the formations in local areas4 ,the fossils which they contain demonstrate the law of the progressive appearance of organisms1 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 remains4 ,,geologic ,,time ,,divisions ,,compared ,,with ,,those ,,of ,,human ,,history4 ,we may compare the division of geologic time into eras1 periods1 and other divisions according to the dominant life of the time1 to the ill-defined ages into which human history is divided according to the dominance of some nation1 ruler1 or other characteristic feature4 ,thus we speak of the ,,dark ,,ages1 the ,,age ,,of ,,elizabeth1 and the ,,age ,,of ,,electricity4 ,these crude divisions would be of much value if1 as in the case of geologic time1 we had no exact reckoning of human history by years4 ,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 revolution1 so with the course of geologic history4 ,periods of quiescence1 in which revolutionary forces are perhaps gathering head1 alternate with periods of comparatively rapid change in physical geography and in organisms1 when new and higher forms appear which serve to draw the boundary line of new epochs4 ,nevertheless1 geological history is a continuous progress2 its periods and epochs shade into one another by imperceptible gradations1 and all our subdivisions must needs be vague and more or less arbitrary4 ,,how ,,fossils ,,tell ,,of ,,the ,,geography ,,of ,,the ,,past4 ,fossils are used not only as a record of the development of life upon the earth1 but also in testimony to the physical geography of past epochs4 ,they indicate whether in any region the climate was tropical1 temperate1 or arctic4 ,since species spread slowly from some center of dispersion where they originate until some barrier limits their migration farther1 the occurrence of the same species in rocks of the same system in different countries implies the absence of such barriers at the period4 ,thus in the collection of antarctic fossils referred to on page #bid there were shallow-water marine shells identical in species with ,mesozoic shells found in ,india and in the southern extremity of ,south ,america4 ,since such organisms are not distributed by the currents of the deep sea and cannot migrate along its bottom1 we infer a shallow-water connection in ,mesozoic times between ,india1 ,south ,america1 and the antarctic region4 ,such a shallow-water connection would be offered along the marginal shelf of a continent uniting these now widely separated countries4 ,,chapter ,,xv ,,the ,,pre-,,cambrian ,,systems ,,the ,,earth',s ,,beginnings4 ,the geological record does not tell us of the beginnings of the earth4 ,the history of the planet1 as we have every reason to believe1 stretches far back beyond the period of the oldest stratified rocks1 and is involved in the history of the solar system and of the nebula1--the cloud of glowing gases or of cosmic dust1--from which the sun and planets are believed to have been derived4 ,,the ,,nebular ,,hypothesis4 ,it is possible that the earth began as a vaporous1 shining sphere1 formed by the gathering together of the material of a gaseous ring which had been detached from a cooling and shrinking nebula4 ,such a vaporous sphere would condense to a liquid1 fiery globe1 whose surface would become cold and solid1 while the interior would long remain intensely hot because of the slow conductivity of the crust4 ,under these conditions the primeval atmosphere of the earth must have contained in vapor the water now belonging to the earth's crust and surface4 ,it held also all the oxygen since locked up in rocks by their oxidation1 and all the carbon dioxide which has since been laid away in limestones1 besides that corresponding to the carbon of carbonaceous deposits1 such as peat1 coal1 and petroleum4 ,on this hypothesis the original atmosphere was dense1 dark1 and noxious1 and enormously heavier than the atmosphere at present4 ,,the ,,accretion ,,hypothesis4 ,on the other hand1 it has been recently suggested that the earth may have grown to its present size by the gradual accretion of meteoritic masses4 ,such cold1 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 planet4 ,thus the surface of the earth may never have been hot and luminous2 but as the loose aggregation of stony masses grew larger and was more and more compressed by its own gravitation1 the heat thus generated raised the interior to high temperatures1 while from time to time molten rock was intruded among the loose1 cold meteoritic masses of the crust and outpoured upon the surface4 ,it is supposed that the meteorites of which the earth was built brought to it1 as meteorites do now1 various gases shut up within their pores4 ,as the heat of the interior increased1 these gases transpired to the surface and formed the primitive atmosphere and hydrosphere4 ,the atmosphere has therefore grown slowly from the smallest beginnings4 ,gases emitted from the interior in volcanic eruptions and in other ways have ever added to it1 and are adding to it now4 ,on the other hand1 the atmosphere has constantly suffered loss1 as it has been robbed of oxygen by the oxidation of rocks in weathering1 and of carbon dioxide in the making of limestones and carbonaceous deposits4 ,while all hypotheses of the earth's beginnings are as yet unproved speculations1 they serve to bring to mind one of the chief lessons which geology has to teach1--that the duration of the earth in time1 like the extension of the universe in space1 is vastly beyond the power of the human mind to realize4 ,behind the history recorded in the rocks1 which stretches back for many million years1 lies the long unrecorded history of the beginnings of the planet2 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 birth4 ,we pass now from the dim realm of speculation to the earliest era of the recorded history of the earth1 where some certain facts may be observed and some sure inferences from them may be drawn4 ,,the ,,archean4 ,the oldest known sedimentary strata1 wherever they are exposed by uplift and erosion1 are found to be involved with a mass of crystalline rocks which possesses the same characteristics in all parts of the world4 ,it consists of foliated rocks1 gneisses1 and schists of various kinds1 which have been cut with dikes and other intrusions of molten rock1 and have been broken1 crumpled1 and crushed1 and left in interlocking masses so confused that their true arrangement can usually be made out only with the greatest difficulty if at all4 ,the condition of this body of crystalline rocks is due to the fact that they have suffered not only from the faultings1 foldings1 and igneous intrusions of their time1 but necessarily1 also1 from those of all later geological ages4 ,at present three leading theories are held as to the origin of these basal crystalline rocks4 #a4 ,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 them4 ,in many localities this relation is proved by the phenomena of contact2 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 existed4 #b4 ,an older view regards them as profoundly altered sedimentary strata1 the most ancient of the earth4 #c4 ,according to a third theory they represent portions of the earth's original crust2 not1 indeed1 its original surface1 but deeper portions uncovered by erosion and afterwards mantled with sedimentary deposits4 ,all these theories agree that the present foliated condition of these rocks is due to the intense metamorphism which they have suffered4 ,it is to this body of crystalline rocks and the stratified rocks involved with it1 which form a very small proportion of its mass1 that the term ,,archean 7,greek1 ,,arche1 beginning7 is applied by many geologists4 ,,the ,,algonkian ,in some regions there rests unconformably on the ,archean an immense body of stratified rocks1 thousands and in places even scores of thousands of feet thick1 known as the ,,algonkian4 ,great unconformities divide it into well-defined systems1 but as only the scantiest traces of fossils appear here and there among its strata1 it is as yet impossible to correlate the formations of different regions and to give them names of more than local application4 ,we will describe the ,algonkian rocks of two typical areas4 ,,the ,,grand ,,canyon ,,of ,,the ,,colorado4 ,we have already studied a very ancient peneplain whose edge is exposed to view deep on the walls of the ,colorado ,canyon4 ,the formation of flat-lying sandstone which covers this buried land surface is proved by its fossils to belong to the ,cambrian1--the earliest period of the ,paleozoic era4 ,the tilted rocks on whose upturned edges the ,cambrian sandstone rests are far older1 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 plain4 ,they are therefore classified as ,algonkian4 ,they comprise two immense series4 ,the upper is more than five thousand feet thick and consists of shales and sandstones with some limestones4 ,separated from it by an unconformity which does not appear in ,figure #bjg1 the lower division1 seven thousand feet thick1 consists chiefly of massive reddish sandstones with seven or more sheets of lava interbedded4 ,the lowest member is a basal conglomerate composed of pebbles derived from the erosion of the dark crumpled schists beneath1--schists which are supposed to be ,archean4 ,as shown in ,figure #bjg1 a strong unconformity parts the schists and the ,algonkian4 ,the floor on which the ,algonkian rests is remarkably even1 and here again is proved an interval of incalculable length1 during which an ancient land mass of ,archean rocks was baseleveled before it received the cover of the sediments of the later age4 ,,the ,,lake ,,superior ,,region4 ,in eastern ,canada an area of pre- ,cambrian rocks1 ,archean and ,algonkian1 estimated at two million square miles1 stretches from the ,great ,lakes and the ,st4 ,lawrence ,river northward to the confines of the continent1 inclosing ,hudson ,bay in the arms of a gigantic ,u4 ,this immense area1 which we have already studied as the ,laurentian peneplain1 extends southward across the ,canadian border into northern ,minnesota1 ,wisconsin1 and ,michigan4 ,the rocks of this area are known to be pre-,cambrian2 for the ,cambrian strata1 wherever found1 lie unconformably upon them4 ,the general relations of the formations of that portion of the area which lies about ,lake ,superior are shown in ,figure #bfb4 ,great unconformities1 ,,uu' separate the ,algonkian both from the ,archean and from the ,cambrian1 and divide it into three distinct systems1 --the ,,lower ,,huronian1 the ,,upper ,,huronian1 and the ,,keweenawan4 ,the ,lower and the ,upper ,huronian consist in the main of old sea muds and sands and limy oozes now changed to gneisses1 schists1 marbles1 quartzites1 slates1 and other metamorphic rocks4 ,the ,keweenawan is composed of immense piles of lava1 such as those of ,iceland1 overlain by bedded sandstones4 ,what remains of these rock systems after the denudation of all later geologic ages is enormous4 ,the ,lower ,huronian is more than a mile thick1 the ,upper ,huronian more than two miles thick1 while the ,keweenawan exceeds nine miles in thickness4 ,the vast length of ,algonkian time is shown by the thickness of its marine deposits and by the cycles of erosion which it includes4 ,in ,figure #bfb the student may read an outline of the history of the ,lake ,superior region1 the deformations which it suffered1 their relative severity1 the times when they occurred1 and the erosion cycles marked by the successive unconformities4 ,,other ,,pre-,,cambrian ,,areas ,,in ,,north ,,america4 ,pre-,cambrian rocks are exposed in various parts of the continent1 usually by the erosion of mountain ranges in which their strata were infolded4 ,large areas occur in the maritime provinces of ,canada4 ,the core of the ,green ,mountains of ,vermont is pre-,cambrian1 and rocks of these systems occur in scattered patches in western ,massachusetts4 ,here belong also the oldest rocks of the ,highlands of the ,hudson and of ,new ,jersey4 ,the ,adirondack region1 an outlier of the ,laurentian region1 exposes pre-,cambrian rocks1 which have been metamorphosed and tilted by the intrusion of a great boss of igneous rock out of which the central peaks are carved4 ,the core of the ,blue ,ridge and probably much of the ,piedmont ,belt are of this age4 ,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 gneisses1 schists1 quartzites1 and slates4 ,in most of these mountainous regions the lowest strata are profoundly changed by metamorphism1 and they can be assigned to the pre-,cambrian only where they are clearly overlain unconformably by formations proved to be ,cambrian by their fossils4 ,in the ,belt ,mountains of ,montana1 however1 the ,cambrian is underlain by ,algonkian sediments twelve thousand feet thick1 and but little altered4 ,,mineral ,,wealth ,,of ,,the ,,pre-,,cambrian ,,rocks4 ,the pre-,cambrian rocks are of very great economic importance1 because of their extensive metamorphism and the enormous masses of igneous rock which they involve4 ,in many parts of the country they are the source of supply of granite1 gneiss1 marble1 slate1 and other such building materials4 ,still more valuable are the stores of iron and copper and other metals which they contain4 ,at the present time the pre-,cambrian region about ,lake ,superior leads the world in the production of iron ore1 its output for #aijc being more than five sevenths of the entire output of the whole ,united ,states1 and exceeding that of any foreign country4 ,the ore bodies consist chiefly of the red oxide of iron 7hematite7 and occur in troughs of the strata1 underlain by some impervious rock4 ,a theory held by many refers the ultimate source of the iron to the igneous rocks of the ,archean4 ,when these rocks were upheaved and subjected to weathering1 their iron compounds were decomposed4 ,their iron was leached out and carried away to be laid in the ,algonkian water bodies in beds of iron carbonate and other iron compounds4 ,during the later ages1 after the ,algonkian strata had been uplifted to form part of the continent1 a second concentration has taken place4 ,descending underground waters charged with oxygen have decomposed the iron carbonate and deposited the iron1 in the form of iron oxide1 in troughs of the strata where their downward progress was arrested by impervious floors4 ,the pre-,cambrian rocks of the eastern ,united ,states also are rich in iron4 ,in certain districts1 as in the ,highlands of ,new ,jersey1 the black oxide of iron 7magnetite7 is so abundant in beds and disseminated grains that the ordinary surveyor's compass is useless4 ,the pre-,cambrian copper mines of the ,lake ,superior region are among the richest on the globe4 ,in the igneous rocks copper1 next to iron1 is the most common of all the useful metals1 and it was especially abundant in the ,keweenawan lavas4 ,after the ,keweenawan was uplifted to form land1 percolating waters leached out much of the copper diffused in the lava sheets and deposited it within steam blebs as amygdules of native copper1 in cracks and fissures1 and especially as a cement1 or matrix1 in the interbedded gravels which formed the chief aquifers of the region4 ,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 copper4 ,,the ,,appearance ,,of ,,life4 ,sometime during the dim ages preceding the ,cambrian1 whether in the ,archean or in the ,algonkian we know not1 occurred one of the most important events in the history of the earth4 ,life appeared for the first time upon the planet4 ,geology has no evidence whatever to offer as to whence or how life came4 ,all analogies lead us to believe that its appearance must have been sudden4 ,its earliest forms are unknown1 but analogy suggests that as every living creature has developed from a single cell1 so the earliest organisms upon the globe--the germs from which all later life is supposed to have been evolved--were tiny1 unicellular masses of protoplasm1 resembling the amoeba of to-day in the simplicity of their structure4 ,such lowly forms were destitute of any hard parts and could leave no evidence of their existence in the record of the rocks4 ,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 following4 ,the fertilized ovum1 the cell with which each animal begins its life1 grows and multiplies by cell division1 and develops into a hollow globe of cells called the ,,blastosphere4 ,this stage is succeeded by the stage of the ,,gastrula1--an ovoid or cup-shaped body with a double wall of cells inclosing a body cavity1 and with an opening1 the primitive mouth4 ,each of these early embryological stages is represented by living animals1--the undivided cell by the ,,protozoa1 the blastosphere by some rare forms1 and the gastrula in the essential structure of the ,,coelenterates1--the subkingdom to which the fresh-water hydra and the corals belong4 ,all forms of animal life1 from the coelenterates to the mammals1 follow the same path in their embryological development as far as the gastrula stage1 but here their paths widely diverge1 those of each subkingdom going their own separate ways4 ,we may infer1 therefore1 that during the pre-,cambrian periods organic evolution followed the lines thus dimly traced4 ,the earliest one-celled protozoa were probably succeeded by many- celled animals of the type of the blastosphere1 and these by gastrula-like organisms4 ,from the gastrula type the higher sub- divisions of animal life probably diverged1 as separate branches from a common trunk4 ,much or all of this vast differentiation was accomplished before the opening of the next era2 for all the subkingdoms are represented in the ,cambrian except the vertebrates4 ,,evidences ,,of ,,pre-,,cambrian ,,life4 ,an indirect evidence of life during the pre-,cambrian periods is found in the abundant and varied fauna of the next period2 for1 if the theory of evolution is correct1 the differentiation of the ,cambrian fauna was a long process which might well have required for its accomplishment a large part of pre-,cambrian time4 ,other indirect evidences are the pre-,cambrian limestones1 iron ores1 and graphite deposits1 since such minerals and rocks have been formed in later times by the help of organisms4 ,if the carbonate of lime of the ,algonkian limestones and marbles was extracted from sea water by organisms1 as is done at present by corals1 mollusks1 and other humble animals and plants1 the life of those ancient seas must have been abundant4 ,graphite1 a soft black mineral composed of carbon and used in the manufacture of lead pencils and as a lubricant1 occurs widely in the metamorphic pre- ,cambrian rocks4 ,it is known to be produced in some cases by the metamorphism of coal1 which itself is formed of decomposed vegetal tissues4 ,seams of graphite may therefore represent accumulations of vegetal matter such as seaweed4 ,but limestone1 iron ores1 and graphite can be produced by chemical processes1 and their presence in the pre-,cambrian makes it only probable1 and not certain1 that life existed at that time4 ,,pre-,,cambrian ,,fossils4 ,very rarely has any clear trace of an organism been found in the most ancient chapters of the geological record1 so many of their leaves have been destroyed and so far have their pages been defaced4 ,omitting structures whose organic nature has been questioned1 there are left to mention a tiny seashell of one of the most lowly types1--a ,,discina from the pre- ,cambrian rocks of the ,colorado ,canyon1--and from the pre-,cambrian rocks of ,montana trails of annelid worms and casts of their burrows in ancient beaches1 and fragments of the tests of crustaceans4 ,these diverse forms indicate that before the ,algonkian had closed1 life was abundant and had widely differentiated4 ,we may expect that other forms will be discovered as the rocks are closely searched4 ,,pre-,,cambrian ,,geography4 ,our knowledge is far too meager to warrant an attempt to draw the varying outlines of sea and land during the ,archean and ,algonkian eras4 ,pre-,cambrian time probably was longer than all later geological time down to the present1 as we may infer from the vast thicknesses of its rocks and the unconformities which part them4 ,we know that during its long periods land masses again and again rose from the sea1 were worn low1 and were submerged and covered with the waste of other lands4 ,but the formations of separated regions cannot be correlated because of the absence of fossils1 and nothing more can be made out than the detached chapters of local histories1 such as the outline given of the district about ,lake ,superior4 ,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 now4 ,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 time4 ,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-day1 or that the muds of the pre-,cambrian shales settled to the sea floor in less quiet water than such muds settle in at present4 ,the pre-,cambrian lands were1 no doubt1 worn by wind and weather1 beaten by rain1 and furrowed by streams as now1 and1 as now1 they fronted the ocean with beaches on which waves dashed and along which tidal currents ran4 ,perhaps the chief difference between the pre-,cambrian and the present was the absence of life upon the land4 ,so far as we have any knowledge1 no forests covered the mountain sides1 no verdure carpeted the plains1 and no animals lived on the ground or in the air4 ,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 streams4 ,we may therefore suppose that the processes of their destruction went on more rapidly than at present4 ,,chapter ,,xvi ,,the ,,cambrian ,,the ,,paleozoic ,,era4 ,the second volume of the geological record1 called the ,paleozoic 7,greek1 ,,palaios1 ancient2 ,,zoe1 life71 has come down to us far less mutilated and defaced than has the first volume1 which contains the traces of the most ancient life of the globe4 ,fossils are far more abundant in the ,paleozoic than in the earlier strata1 while the sediments in which they were entombed have suffered far less from metamorphism and other causes1 and have been less widely buried from view1 than the strata of the pre-,cambrian groups4 ,by means of their fossils we can correlate the formations of widely separated regions from the beginning of the ,paleozoic on1 and can therefore trace some outline of the history of the continents4 ,paleozoic time1 although shorter than the pre-,cambrian as measured by the thickness of the strata1 must still be reckoned in millions of years4 ,during this vast reach of time the changes in organisms were very great4 ,it is according to the successive stages in the advance of life that the ,paleozoic formations are arranged in five systems1--the ,,cambrian1 the ,,ordovician1 the ,,silurian1 the ,,devonian1 and the ,,carboniferous4 ,on the same basis the first three systems are grouped together as the older ,paleozoic1 because they alike are characterized by the dominance of the invertebrates2 while the last two systems are united in the later ,paleozoic1 and are characterized1 the one by the dominance of fishes1 and the other by the appearance of amphibians and reptiles4 ,each of these systems is world-wide in its distribution1 and may be recognized on any continent by its own peculiar fauna4 ,the names first given them in ,great ,britain have therefore come into general use1 while their subdivisions1 which often cannot be correlated in different countries and different regions1 are usually given local names4 ,the first three systems were named from the fact that their strata are well displayed in ,wales4 ,the ,cambrian carries the ,roman name of ,wales1 and the ,ordovician and ,silurian the names of tribes of ancient ,britons which inhabited the same country4 ,the ,devonian is named from the ,english county ,devon1 where its rocks were early studied4 ,the ,carboniferous was so called from the large amount of coal which it was found to contain in ,great ,britain and continental ,europe4 ,,the ,,cambrian ,,distribution ,,of ,,strata4 ,the ,cambrian rocks outcrop in narrow belts about the pre-,cambrian areas of eastern ,canada and the ,lake ,superior region1 the ,adirondacks and the ,green ,mountains4 ,strips of ,cambrian formations occupy troughs in the pre-,cambrian rocks of ,new ,england and the maritime provinces of ,canada2 a long belt borders on the west the crystalline rocks of the ,blue ,ridge2 and on the opposite side of the continent the ,cambrian reappears in the mountains of the ,great ,basin and the ,canadian ,rockies4 ,in the ,mississippi valley it is exposed in small districts where uplift has permitted the stripping off of younger rocks4 ,although the areas of outcrop are small1 we may infer that ,cambrian rocks were widely deposited over the continent of ,north ,america4 ,,physical ,,geography4 ,the ,cambrian system of ,north ,america comprises three distinct series1 the ,,lower ,,cambrian1 the ,,middle ,,cambrian1 and the ,,upper ,,cambrian1 each of which is characterized by its own peculiar fauna4 ,in sketching the outlines of the continent as it was at the beginning of the ,paleozoic1 it must be remembered that wherever the ,lower ,cambrian formations now are found was certainly then sea bottom1 and wherever the ,lower ,cambrian are wanting1 and the next formations rest directly on pre-,cambrian rocks1 was probably then land4 ,,early ,,cambrian ,,geography4 ,in this way we know that at the opening of the ,cambrian two long1 narrow mediterranean seas stretched from north to south across the continent4 ,the eastern sea extended from the ,gulf of ,st4 ,lawrence down the ,champlain-,hudson valley and thence along the western base of the ,blue ,ridge south at least to ,alabama4 ,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 ,arizona4 ,between these mediterraneans lay a great central land which included the pre-,cambrian ,u-shaped area of the ,laurentian peneplain1 and probably extended southward to the latitude of ,new ,orleans4 ,to the east lay a land which we may designate as ,,appalachia1 whose western shore line was drawn along the site of the present ,blue ,ridge1 but whose other limits are quite unknown4 ,the land of ,appalachia must have been large1 for it furnished a great amount of waste during the entire ,paleozoic era1 and its eastern coast may possibly have lain even beyond the edge of the present continental shelf4 ,on the western side of the continent a narrow land occupied the site of the ,sierra ,nevada ,mountains4 ,thus1 even at the beginning of the ,paleozoic1 the continental plateau of ,north ,america had already been left by crustal movements in relief above the abysses of the great oceans on either side4 ,the mediterraneans which lay upon it were shallow1 as their sediments prove4 ,they were ,,epicontinental ,,seas2 that is1 they rested ,,upon 7,greek1 ,,epi7 the submerged portion of the continental plateau4 ,we have no proof that the deep ocean ever occupied any part of where ,north ,america now is4 ,the ,middle and ,upper ,cambrian strata are found together with the ,lower ,cambrian over the area of both the eastern and the western mediterraneans1 so that here the sea continued during the entire period4 ,the sediments throughout are those of shoal water4 ,coarse cross-bedded sandstones record the action of strong shifting currents which spread coarse waste near shore and winnowed it of finer stuff4 ,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 currents4 ,sun cracks show that often the outgoing tide exposed large muddy flats to the drying action of the sun4 ,the fossils1 also1 of the strata are of kinds related to those which now live in shallow waters near the shore4 ,the sediments which gathered in the mediterranean seas were very thick1 reaching in places the enormous depth of ten thousand feet4 ,hence the bottoms of these seas were sinking troughs1 ever filling with waste from the adjacent land as fast as they subsided4 ,,late ,,cambrian ,,geography4 ,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 interior1 as in ,missouri and in ,texas4 ,hence after earlier ,cambrian time the central land subsided1 with much the same effect as if the ,mississippi valley were now to lower gradually1 and the ,gulf of ,mexico to spread northward until it entered ,lake ,superior4 ,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 rocks4 ,the succession of the ,cambrian formations in ,north ,america records many minor oscillations and varying conditions of physical geography2 yet on the whole it tells of widening seas and lowering lands4 ,basal conglomerates and coarse sandstones which must have been laid near shore are succeeded by shaly sandstones1 sandy shales1 and shales4 ,toward the top of the series heavy beds of limestone1 extending from the ,blue ,ridge to ,missouri1 speak of clear water1 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 solution4 ,in brief1 the ,cambrian was a period of submergence4 ,it began with the larger part of ,north ,america emerged as great land masses4 ,it closed with most of the interior of the continental plateau covered with a shallow sea4 ,,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 fauna4 ,doubtless these remains are only the most fragmentary representation of the life of the time1 for the ,cambrian rocks are very old and have been widely metamorphosed4 ,yet the five hundred and more species already discovered embrace all the leading types of invertebrate life1 and are so varied that we must believe that their lines of descent stretch far back into the pre-,cambrian past4 ,,plants4 ,no remains of plants have been found in ,cambrian strata1 except some doubtful markings1 as of seaweed4 ,,sponges4 ,the sponges1 the lowest of the multicellular animals1 were represented by several orders4 ,their fossils are recognized by the siliceous spicules1 which1 as in modern sponges1 either were scattered through a mass of horny fibers or were connected in a flinty framework4 ,,coelenterates4 ,this subkingdom includes two classes of interest to the geologist1--the ,,hydrozoa1 such as the fresh-water hydra and the jellyfish1 and the ,,corals4 ,both classes existed in the ,cambrian4 ,the ,hydrozoa were represented not only by jellyfish but also by the ,,graptolite1 which takes its name from a fancied resemblance of some of its forms to a quill pen4 ,it was a composite animal with a horny framework1 the individuals of the colony living in cells strung on one or both sides along a hollow stem1 and communicating by means of a common flesh in this central tube4 ,some graptolites were straight1 and some curved or spiral2 some were single stemmed1 and others consisted of several radial stems united4 ,graptolites occur but rarely in the ,upper ,cambrian4 ,in the ,ordovician and ,silurian they are very plentiful1 and at the close of the ,silurian they pass out of existence1 never to return4 ,,corals are very rarely found in the ,cambrian1 and the description of their primitive types is postponed to later chapters treating of periods when they became more numerous4 ,,echinoderms4 ,this subkingdom comprises at present such familiar forms as the crinoid1 the starfish1 and the sea urchin4 ,the structure of echinoderms is radiate4 ,their integument is hardened with plates or particles of carbonate of lime4 ,of the free echinoderms1 such as the starfish and the sea urchin1 the former has been found in the ,cambrian rocks of ,europe1 but neither have so far been discovered in the strata of this period in ,north ,america4 ,the stemmed and lower division of the echinoderms was represented by a primitive type1 the ,,cystoid1 so called from its saclike form1 ,a small globular or ovate calyx of calcareous plates1 with an aperture at the top for the mouth1 inclosed the body of the animal1 and was attached to the sea bottom by a short flexible stalk consisting of disks of carbonate of lime held together by a central ligament4 ,,arthopods4 ,these segmented animals with jointed feet1 as their name suggests1 may be divided in a general way into water breathers and air breathers4 ,the first-named and lower division comprises the class of the ,,crustacea1--arthropods protected by a hard exterior skeleton1 or crust1--of which crabs1 crayfish1 and lobsters are familiar examples4 ,the higher division1 that of the air breathers1 includes the following classes3 spiders1 scorpions1 centipedes1 and insects4 ,,the ,,trilobite4 ,the aquatic arthropods1 the ,crustacea1 culminated before the air breathers2 and while none of the latter are found in the ,cambrian1 the former were the dominant life of the time in numbers1 in size1 and in the variety of their forms4 ,the leading crustacean type is the ,,trilobite1 which takes its name from the three lobes into which its shell is divided longitudinally4 ,there are also three cross divisions1--the head shield1 the tail shield1 and between the two the thorax1 consisting of a number of distinct and unconsolidated segments4 ,the head shield carries a pair of large1 crescentic1 compound eyes1 like those of the insect4 ,the eye varies greatly in the number of its lenses1 ranging from fourteen in some species to fifteen thousand in others4 ,figure #bfh1 ,c1 is a restoration of the trilobite1 and shows the appendages1 which are found preserved only in the rarest cases4 ,during the long ages of the ,cambrian the trilobite varied greatly4 ,again and again new species and genera appeared1 while the older types became extinct4 ,for this reason and because of their abundance1 trilobites are used in the classification of the ,cambrian system4 ,the ,lower ,cambrian is characterized by the presence of a trilobitic fauna in which the genus ,olenellus is predominant4 ,this1 the ,,olenellus ,,zone1 is one of the most important platforms in the entire geological series2 for1 the world over1 it marks the beginning of ,paleozoic time1 while all underlying strata are classified as pre-,cambrian4 ,the ,middle ,cambrian is marked by the genus ,paradoxides1 and the ,upper ,cambrian by the genus ,olenus4 ,some of the ,cambrian trilobites were giants1 measuring as much as two feet long1 while others were the smallest of their kind1 a fraction of an inch in length4 ,another type of crustacean which lived in the ,cambrian and whose order is still living is illustrated in ,figure #bfi4 ,,worms4 ,trails and burrows of worms have been left on the sea beaches and mud flats of all geological times from the ,algonkian to the present4 ,,brachiopods4 ,these soft-bodied animals1 with bivalve shells and two interior armlike processes which served for breathing1 appeared in the ,algonkian1 and had now become very abundant4 ,the two valves of the brachiopod shell are unequal in size1 and in each valve a line drawn from the beak to the base divides the valve into two equal parts4 ,it may thus be told from the pelecypod mollusk1 such as the clam1 whose two valves are not far from equal in size1 each being divided into unequal parts by a line dropped from the beak4 ,brachiopods include two orders4 ,in the most primitive order--that of the ,,inarticulate brachiopods--the two valves are held together only by muscles of the animal1 and the shell is horny or is composed of phosphate of lime4 ,the ,,discina1 which began in the ,algonkian1 is of this type1 as is also the ,,lingulella of the ,cambrian4 ,both of these genera have lived on during the millions of years of geological time since their introduction1 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 beginning4 ,the more highly organized ,,articulate brachiopods have valves of carbonate of lime more securely joined by a hinge with teeth and sockets 7,fig4 #bgj74 ,in the ,cambrian the inarticulates predominate1 though the articulates grow common toward the end of the period4 ,,mollusks4 ,the three chief classes of mollusks--the ,,pelecypods 7represented by the oyster and clam of to-day71 the ,,gastropods 7represented now by snails1 conches1 and periwinkles71 and the ,,cephalopods 7such as the nautilus1 cuttlefish1 and squids7--were all represented in the ,cambrian1 although very sparingly4 ,pteropods1 a suborder of the gastropods1 appeared in this age4 ,their papery shells of carbonate of lime are found in great numbers from this time on4 ,cephalopods1 the most highly organized of the mollusks1 started into existence1 so far as the record shows1 toward1 the end of the ,cambrian1 with the long extinct ,,orthoceras 7,,straighthorn7 and the allied genera of its family4 ,the ,orthoceras had a long1 straight1 and tapering shell1 divided by cross partitions into chambers4 ,the animal lived in the body chamber at the larger end1 and walled off the other chambers from it in succession during the growth of the shell4 ,a central tube1 the ,,siphuncle1 passed through from the body chamber to the closed tip of the cone4 ,the seashells1 both brachiopods and mollusks1 are in some respects the most important to the geologist of all fossils4 ,they have been so numerous1 so widely distributed1 and so well preserved because of their durable shells and their station in growing sediments1 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 time4 ,,climate4 ,the life of ,cambrian times in different countries contains no suggestion of any marked climatic zones1 and as in later periods a warm climate probably reached to the polar regions4 ,,chapter ,,xvii ,,the ,,ordovician ,,and ,,silurian ,footnote3 ,often known as the ,lower ,silurian4 ,,the ,,ordovician ,in ,north ,america the ,ordovician rocks lie conformably on the ,cambrian4 ,the two periods1 therefore1 were not parted by any deformation1 either of mountain making or of continental uplift4 ,the general submergence which marked the ,cambrian continued into the succeeding period with little interruption4 ,,subdivisions ,,and ,,distribution ,,of ,,strata4 ,the ,ordovician series1 as they have been made out in ,new ,york1 are given for reference in the following table1 with the rocks of which they are chiefly composed3 #e ,hudson 4 4 4 4 4 4 4 4 shales #d ,utica 4 4 4 4 4 4 4 4 shales #c ,trenton 4 4 4 4 4 4 4 limestones #b ,chazy 4 4 4 4 4 4 4 4 limestones #a ,calciferous 4 4 4 4 4 sandy limestones ,these marine formations of the ,ordovician outcrop about the ,cambrian and pre-,cambrian areas1 and1 as borings show1 extend far and wide over the interior of the continent beneath more recent strata4 ,the ,ordovician sea stretched from ,appalachia across the ,mississippi valley4 ,it seems to have extended to ,california1 although broken probably by several mountainous islands in the west4 ,,physical ,,geography4 ,the physical history of the period is recorded in the succession of its formations4 ,the sandstones of the ,upper ,cambrian1 as we have learned1 tell of a transgressing sea which gradually came to occupy the ,mississippi valley and the interior of ,north ,america4 ,the limestones of the early and middle ,ordovician show that now the shore had become remote and the lands had become more low4 ,the waters now had cleared4 ,colonies of brachiopods and other lime-secreting animals occupied the sea bottom1 and their debris mantled it with sheets of limy ooze4 ,the sandy limestones of the ,calciferous record the transition stage from the ,cambrian when some sand was still brought in from shore4 ,the highly fossiliferous limestones of the ,trenton tell of clear water and abundant life4 ,we need not regard this epicontinental sea as deep4 ,no abysmal deposits have been found1 and the limestones of the period are those which would be laid in clear1 warm water of moderate depth like that of modern coral seas4 ,the shales of the ,utica and ,hudson show that the waters of the sea now became clouded with mud washed in from land4 ,either the land was gradually uplifted1 or perhaps there had arrived one of those periodic crises which1 as we may imagine1 have taken place whenever the crust of the shrinking earth has slowly given way over its great depressions1 and the ocean has withdrawn its waters into deepening abysses4 ,the land was thus left relatively higher and bordered with new coastal plains4 ,the epicontinental sea was shoaled and narrowed1 and muds were washed in from the adjacent lands4 ,,the ,,taconic ,,deformation4 ,the ,ordovician was closed by a deformation whose extent and severity are not yet known4 ,from the ,st4 ,lawrence ,river to ,new ,york ,bay1 along the northwestern and western border of ,new ,england1 lies a belt of ,cambrian-,ordovician rocks more than a mile in total thickness1 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 ,river4 ,but since their deposition these ancient sediments have been crumpled and crushed1 broken with great faults1 and extensively metamorphosed4 ,the limestones have recrystallized into marbles1 among them the famous marbles of ,vermont2 the ,cambrian sandstones have become quartzites1 and the ,hudson shale has been changed to a schist exposed on ,manhattan ,island and northward4 ,in part these changes occurred at the close of the ,ordovician1 for in several places beds of ,silurian age rest unconformably on the upturned ,ordovician strata2 but recent investigations have made it probable that the crustal movements recurred at later times1 and it was perhaps in the ,devonian and at the close of the ,carboniferous that the greater part of the deformation and metamorphism was accomplished4 ,as a result of these movements1-- perhaps several times repeated1--a great mountain range was upridged1 which has been long since leveled by erosion1 but whose roots are now visible in the ,taconic ,mountains of western ,new ,england4 ,,the ,,cincinnati ,,anticline4 ,over an oval area in ,ohio1 ,indiana1 and ,kentucky1 whose longer axis extends from north to south through ,cincinnati1 the ,ordovician strata rise in a very low1 broad swell1 called the ,cincinnati anticline4 ,the ,silurian and ,devonian strata thin out as they approach this area and seem never to have deposited upon it4 ,we may regard it1 therefore1 as an island upwarped from the sea at the close of the ,ordovician or shortly after4 ,,petroleum ,,and ,,natural ,,gas4 ,these valuable illuminants and fuels are considered here because1 although they are found in traces in older strata1 it is in the ,ordovician that they occur for the first time in large quantities4 ,they range throughout later formations down to the most recent4 ,the oil horizons of ,california and ,texas are ,tertiary2 those of ,colorado1 ,cretaceous2 those of ,west ,virginia1 ,carboniferous2 those of ,pennsylvania1 ,kentucky1 and ,canada1 ,devonian2 and the large field of ,ohio and ,indiana belongs to the ,ordovician and higher systems4 ,petroleum and natural gas1 wherever found1 have probably originated from the decay of organic matter when buried in sedimentary deposits1 just as at present in swampy places the hydrogen and carbon of decaying vegetation combine to form marsh gas4 ,the light and heat of these hydrocarbons we may think of1 therefore1 as a gift to the civilized life of our race from the humble organisms1 both animal and vegetable1 of the remote past1 whose remains were entombed in the sediments of the ,ordovician and later geological ages4 ,petroleum is very widely disseminated throughout the stratified rocks4 ,certain limestones are visibly greasy with it1 and others give off its characteristic fetid odor when struck with a hammer4 ,many shales are bituminous1 and some are so highly charged that small flakes may be lighted like tapers1 and several gallons of oil to the ton may be obtained by distillation4 ,but oil and gas are found in paying quantities only when certain conditions meet3 #a4 ,a ,,source below1 usually a bituminous shale1 from whose organic matter they have been derived by slow change4 #b4 ,a ,,reservoir above1 in which they have gathered4 ,this is either a porous sandstone or a porous or creviced limestone4 #c4 ,oil and gas are lighter than water1 and are usually under pressure owing to artesian water4 ,hence1 in order to hold them from escaping to the surface1 the reservoir must have the shape of an ,,anticline1 ,,dome1 or ,,lens4 #d4 ,it must also have an ,,impervious ,,cover1 usually a shale4 ,in these reservoirs gas is under a pressure which is often enormous1 reaching in extreme cases as high as a thousand five hundred pounds to the square inch4 ,when tapped it rushes out with a deafening roar1 sometimes flinging the heavy drill high in air4 ,in accounting for this pressure we must remember that the gas has been compressed within the pores of the reservoir rock by artesian water1 and in some cases also by its own expansive force4 ,it is not uncommon for artesian water to rise in wells after the exhaustion of gas and oil4 ,,life ,,of ,,the ,,ordovician ,during the ages of the ,ordovician1 life made great advances4 ,types already present branched widely into new genera and species1 and new and higher types appeared4 ,sponges continued from the ,cambrian4 ,graptolites now reached their climax4 ,,stromatopora--colonies of minute hydrozoans allied to corals--grew in places on the sea floor1 secreting stony masses composed of thin1 close1 concentric layers1 connected by vertical rods4 ,the ,stromatopora are among the chief limestone builders of the ,silurian and ,devonian periods4 ,,corals developed along several distinct lines1 like modern corals they secreted a calcareous framework1 in whose outer portions the polyps lived4 ,in the ,ordovician1 corals were represented chiefly by the family of the ,,choetetes1 all species of which are long since extinct4 ,the description of other types of corals will be given under the ,silurian1 where they first became abundant4 ,,echinoderms4 ,the cystoid reaches its climax1 but there appear now two higher types of echinoderms1--the crinoid and the starfish4 ,the ,,crinoid1 named from its resemblance to the lily1 is like the cystoid in many respects1 but has a longer stem and supports a crown of plumose arms4 ,stirring the water with these arms1 it creates currents by which particles of food are wafted to its mouth4 ,crinoids are rare at the present time1 but they grew in the greatest profusion in the warm ,ordovician seas and for long ages thereafter4 ,in many places the sea floor was beautiful with these graceful1 flowerlike forms1 as with fields of long-stemmed lilies4 ,of the higher1 free-moving classes of the echinoderms1 starfish are more numerous than in the ,cambrian1 and sea urchins make their appearance in rare archaic forms4 ,,crustaceans4 ,trilobites now reach their greatest development and more than eleven hundred species have been described from the rocks of this period4 ,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 another4 ,unlike their ,cambrian ancestors1 many of the ,ordovician trilobites could roll themselves into balls at the approach of danger4 ,it is in this attitude1 taken at the approach of death1 that trilobites are often found in the ,ordovician and later rocks4 ,the gigantic crustaceans called the ,,eurypterids were also present in this period4 ,the arthropods had now seized upon the land4 ,centipedes and insects of a low type1 the earliest known land animals1 have been discovered in strata of this system4 ,,bryozoans4 ,no fossils are more common in the limestones of the time than the small branching stems and lacelike mats of the bryozoans1--the skeletons of colonies of a minute animal allied in structure to the brachiopod4 ,,brachiopods4 ,these multiplied greatly1 and in places their shells formed thick beds of coquina4 ,they still greatly surpassed the mollusks in numbers4 ,,cephalopods4 ,among the mollusks we must note the evolution of the cephalopods4 ,the primitive straight ,orthoceras has now become abundant4 ,but in addition to this ancestral type there appears a succession of forms more and more curved and closely coiled1 as illustrated in ,figure #bhe4 ,the nautilus1 which began its course in this period1 crawls on the bottom of our present seas4 ,,vertebrates4 ,the most important record of the ,ordovician is that of the appearance of a new and higher type1 with possibilities of development lying hidden in its structure that the mollusk and the insect could never hope to reach4 ,scales and plates of minute fishes found in the ,ordovician rocks near ,canon ,city1 ,colorado1 show that the humblest of the vertebrates had already made its appearance4 ,but it is probable that vertebrates had been on the earth for ages before this in lowly types1 which1 being destitute of hard parts1 would leave no record4 ,,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 ,silurian4 ,new species appear and many old species now become extinct4 ,,the ,,appalachian ,,region4 ,where the ,silurian system is most fully developed1 from ,new ,york southward along the ,appalachian ,mountains1 it comprises four series3 #d ,salina 4 4 4 shales1 impure limestones1 gypsum1 salt #c ,niagara 4 4 4 chiefly limestones #b ,clinton 4 4 4 sandstones1 shales1 with some limestones #a ,medina 4 4 4 conglomerates1 sandstones ,the rocks of these series are shallow-water deposits and reach the total thickness of some five thousand feet4 ,evidently they were laid over an area which was on the whole gradually subsiding1 although with various gentle oscillations which are recorded in the different formations4 ,the coarse sands of the heavy ,medina formations record a period of uplift of the oldland of ,appalachia1 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 wide1 sandy flats4 ,as the lands were worn lower the waste became finer1 and during an epoch of transition--the ,clinton-- there were deposited various formations of sandstones1 shales1 and limestones4 ,the ,niagara limestones testify to a long epoch of repose1 when low-lying lands sent little waste down to the sea4 ,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 surface1 and at the north cut off extensive tracts from the interior sea4 ,in these wide lagoons1 which now and then regained access to the open sea and obtained new supplies of salt water1 beds of salt and gypsum were deposited as the briny waters became concentrated by evaporation under a desert climate4 ,along with these beds there were also laid shales and impure limestones4 ,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 wide1 and similar salt marshes occurred as far west as ,cleveland1 ,ohio1 and ,goderich on ,lake ,huron4 ,at ,ithaca1 ,new ,york1 the series is fifteen hundred feet thick1 and is buried beneath an equal thickness of later strata4 ,it includes two hundred and fifty feet of solid salt1 in several distinct beds1 each sealed within the shales of the series4 ,would you expect to find ancient beds of rock salt inclosed in beds of pervious sandstone8 ,the salt beds of the ,salina are of great value4 ,they are reached by well borings1 and their brines are evaporated by solar heat and by boiling4 ,the rock salt is also mined from deep shafts4 ,similar deposits of salt1 formed under like conditions1 occur in the rocks of later systems down to the present4 ,the salt beds of ,texas are ,permian1 those of ,kansas are ,permian1 and those of ,louisiana are ,tertiary4 ,,the ,,mississippi ,,valley4 ,the heavy near-shore formations of the ,silurian in the ,appalachian region thin out toward the west4 ,the ,medina and the ,clinton sandstones are not found west of ,ohio1 where the first passes into a shale and the second into a limestone4 ,the ,niagara limestone1 however1 spreads from the ,hudson ,river to beyond the ,mississippi1 a distance of more than a thousand miles4 ,during the ,silurian period the ,mississippi valley region was covered with a quiet1 shallow1 limestone-making sea1 which received little waste from the low lands which bordered it4 ,the probable distribution of land and sea in eastern ,north ,america and western ,europe is shown in ,figure #bhg4 ,the fauna of the interior region and of eastern ,canada are closely allied with that of western ,europe1 and several species are identical4 ,we can hardly account for this except by a shallow-water connection between the two ancient epicontinental seas4 ,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 place1 so that the same species came to live in ,iowa and in ,sweden4 ,,the ,,western ,,united ,,states4 ,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 ,silurian4 ,at the same time the western land was perhaps connected with the eastern land of ,appalachia across ,arkansas and ,mississippi2 for toward the south the ,silurian sediments indicate an approach to shore4 ,,life ,,of ,,the ,,silurian ,in this brief sketch it is quite impossible to relate the many changes of species and genera during the ,silurian4 ,,corals4 ,some of the more common types are familiarly known as cup corals1 honeycomb corals1 and chain corals4 ,in the ,,cup ,,corals the most important feature is the development of radiating vertical partitions1 or ,,septa1 in the cell of the polyp4 ,some of the cup corals grew in hemispherical colonies 7,fig4 #bhh71 while many were separate individuals 7,fig4 #bhi71 building a single conical1 or horn-shaped cell1 which sometimes reached the extreme size of a foot in length and two or three inches in diameter4 ,,honeycomb ,,corals consist of masses of small1 close-set prismatic cells1 each crossed by horizontal partitions1 or ,,tabulae1 while the septa are rudimentary1 being represented by faintly projecting ridges or rows of spines4 ,,chain ,,corals are also marked by tabulae4 ,their cells form elliptical tubes1 touching each other at the edges1 and appearing in cross section like the links of a chain4 ,they became extinct at the end of the ,silurian4 ,the corals of the ,,syringopora family are similar in structure to chain corals1 but the tubular columns are connected only in places4 ,to the echinoderms there is now added the ,,blastoid 7bud-shaped74 ,the blastoid is stemmed and armless1 and its globular head or calyx1 with its five petal-like divisions1 resembles a flower bud4 ,the blastoids became more abundant in the ,devonian1 culminated in the ,carboniferous1 and disappeared at the end of the ,paleozoic4 ,the great eurypterids--some of which were five or six feet in length--and the cephalopods were still masters of the seas4 ,fishes were as yet few and small2 trilobites and graptolites had now passed their prime and had diminished greatly in numbers4 ,scorpions are found in this period both in ,europe and in ,america4 ,the limestone-making seas of the ,silurian swarmed with corals1 crinoids1 and brachiopods4 ,with the end of the ,silurian period the ,,age ,,of ,,invertebrates comes to a close1 giving place to the ,devonian1 the ,,age ,,of ,,fishes4 ,,chapter ,,xviii ,,the ,,devonian ,in ,america the ,silurian is not separated from the ,devonian by any mountain-making deformation or continental uplift4 ,the one period passed quietly into the other4 ,their conformable systems are so closely related1 and the change in their faunas is so gradual1 that geologists are not agreed as to the precise horizon which divides them4 ,,subdivisions ,,and ,,physical ,,geography4 ,the ,devonian is represented in ,new ,york and southward by the following five series4 ,we add the rocks of which they are chiefly composed4 #e ,chemung 4 4 4 4 4 4 sandstones and sandy shales #d ,hamilton 4 4 4 4 4 4 shales and sandstones #c ,corniferous 4 4 4 4 4 4 limestones #b ,oriskany 4 4 4 4 4 4 sandstones #a ,helderberg 4 4 4 4 4 4 limestones ,the ,helderberg is a transition epoch referred by some geologists to the ,silurian4 ,the thin sandstones of the ,oriskany mark an epoch when waves worked over the deposits of former coastal plains4 ,the limestones of the ,corniferous testify to a warm and clear wide sea which extended from the ,hudson to beyond the ,mississippi4 ,corals throve luxuriantly1 and their remains1 with those of mollusks and other lime-secreting animals1 built up great beds of limestone4 ,the bordering continents1 as during the later ,silurian1 must now have been monotonous lowlands which sent down little of even the finest waste to the sea4 ,in the ,hamilton the clear seas of the previous epoch became clouded with mud4 ,the immense deposits of coarse sandstones and sandy shales of the ,chemung1 which are found off what was at the time the west coast of ,appalachia1 prove an uplift of that ancient continent4 ,the ,chemung series extends from the ,catskill ,mountains to northeastern ,ohio and south to northeastern ,tennessee1 covering an area of not less than a hundred thousand square miles4 ,in eastern ,new ,york it attains three thousand feet in thickness2 in ,pennsylvania it reaches the enormous thickness of two miles2 but it rapidly thins to the west4 ,everywhere the ,chemung is made of thin beds of rapidly alternating coarse and fine sands and clays1 with an occasional pebble layer1 and hence is a shallow-water deposit4 ,the fine material has not been thoroughly winnowed from the coarse by the long action of strong waves and tides4 ,the sands and clays have undergone little more sorting than is done by rivers4 ,we must regard the ,chemung sandstones as deposits made at the mouths of swift1 turbid rivers in such great amount that they could be little sorted and distributed by waves4 ,over considerable areas the ,chemung sandstones bear little or no trace of the action of the sea4 ,the ,catskill ,mountains1 for example1 have as their summit layers some three thousand feet of coarse red sandstones of this series1 whose structure is that of river deposits1 and whose few fossils are chiefly of fresh-water types4 ,the ,chemung is therefore composed of delta deposits1 more or less worked over by the sea4 ,the bulk of the ,chemung equals that of the ,sierra ,nevada ,mountains4 ,to furnish this immense volume of sediment a great mountain range1 or highland1 must have been upheaved where the ,appalachian lowland long had been4 ,to what height the ,devonian mountains of ,appalachia attained cannot be told from the volume of the sediments wasted from them1 for they may have risen but little faster than they were worn down by denudation4 ,we may infer from the character of the waste which they furnished to the ,chemung shores that they did not reach an ,alpine height4 ,the grains of the ,chemung sandstones are not those which would result from mechanical disintegration1 as by frost on high mountain peaks1 but are rather those which would be left from the long chemical decay of siliceous crystalline rocks2 for the more soluble minerals are largely wanting4 ,the red color of much of the deposits points to the same conclusion4 ,red residual clays accumulated on the mountain sides and upland summits1 and were washed as ocherous silt to mingle with the delta sands4 ,the iron- bearing igneous rocks of the oldland also contributed by their decay iron in solution to the rivers1 to be deposited in films of iron oxide about the quartz grains of the ,chemung sandstones1 giving them their reddish tints4 ,,life ,,of ,,the ,,devonian ,,plants4 ,the lands were probably clad with verdure during ,silurian times1 if not still earlier2 for some rare remains of ferns and other lowly types of vegetation have been found in the strata of that system4 ,but it is in the ,devonian that we discover for the first time the remains of extensive and luxuriant forests4 ,this rich flora reached its climax in the ,carboniferous1 and it will be more convenient to describe its varied types in the next chapter4 ,,rhizocarps4 ,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 tons4 ,it would seem that these aquatic plants culminated in this period1 and in widely distant portions of the earth swampy flats and shallow lagoons were filled with vegetation of this humble type1 either growing from the bottom or floating free upon the surface4 ,it is to the resinous spores of the rhizocarps that the petroleum and natural gas from ,devonian rocks are largely due4 ,the decomposition of the spores has made the shales highly bituminous1 and the oil and gas have accumulated in the reservoirs of overlying porous sandstones4 ,,invertebrates4 ,we must pass over the ever-changing groups of the invertebrates with the briefest notice4 ,chain corals became extinct at the close of the ,silurian1 but other corals were extremely common in the ,devonian seas4 ,at many places corals formed thin reefs1 as at ,louisville1 ,kentucky1 where the hardness of the reef rock is one of the causes of the ,falls of the ,ohio4 ,sponges1 echinoderms1 brachiopods1 and mollusks were abundant4 ,the cephalopods take a new departure4 ,so far in all their various forms1 whether straight1 as the ,orthoceras1 or curved1 or close- coiled as in the nautilus1 the septum1 or partition dividing the chambers1 met the inner shell along a simple line1 like that of the rim of a saucer4 ,there now begins a growth of the septum by which its edges become sharply corrugated1 and the suture1 or line of juncture of the septum and the shell1 is thus angled4 ,the group in which this growth of the septum takes place is called the ,,goniatite 7,greek ,,gonia1 angle74 ,,vertebrates4 ,it is with the greatest interest that we turn now to study the backboned animals of the ,devonian2 for they are believed to be the ancestors of the hosts of vertebrates which have since dominated the earth4 ,their rudimentary structures foreshadowed what their descendants were to be1 and give some clue to the earliest vertebrates from which they sprang4 ,like those whose remains are found in the lower ,paleozoic systems1 all of these ,devonian vertebrates were aquatic and go under the general designation of fishes4 ,the lowest in grade and nearest1 perhaps1 to the ancestral type of vertebrates1 was the problematic creature1 an inch or so long1 of ,figure #big4 ,note the circular mouth not supplied with jaws1 the lack of paired fins1 and the symmetric tail fin1 with the column of cartilaginous1 ringlike vertebrae running through it to the end4 ,the animal is probably to be placed with the jawless lampreys and hags1--a group too low to be included among true fishes4 ,,ostracoderms4 ,this archaic group1 long since extinct1 is also too lowly to rank among the true fishes1 for its members have neither jaws nor paired fins4 ,these small1 fishlike forms were cased in front with bony plates developed in the skin and covered in the rear with scales4 ,the vertebrae were not ossified1 for no trace of them has been found4 ,,devonian ,,fishes4 ,the ,,true ,,fishes of the ,devonian can best be understood by reference to their descendants now living4 ,modern fishes are divided into several groups3 ,,sharks and their allies2 ,,dipnoans2 ,,ganoids1 such as the sturgeon and gar2 and ,,teleosts1-- most common fishes1 such as the perch and cod4 ,,sharks4 ,of all groups of living fishes the sharks are the oldest and still retain most fully the embryonic characters of their ,paleozoic ancestors4 ,such characters are the cartilaginous skeleton1 and the separate gill slits with which the throat wall is pierced and which are arranged in line like the gill openings of the lamprey4 ,the sharks of the ,silurian and ,devonian are known to us chiefly by their teeth and fin spines1 for they were unprotected by scales or plates1 and were devoid of a bony skeleton4 ,figure #bii is a restoration of an archaic shark from a somewhat higher horizon4 ,note the seven gill slits and the lappetlike paired fins4 ,these fins seem to be remnants of the continuous fold of skin which1 as embryology teaches1 passed from fore to aft down each side of the primitive vertebrate4 ,devonian sharks were comparatively small4 ,they had not evolved into the ferocious monsters which were later to be masters of the seas4 ,,dipnoans1 ,,or ,,lung ,,fishes4 ,these are represented to-day by a few peculiar fishes and are distinguished by some high structures which ally them with amphibians4 ,an air sac with cellular spaces is connected with the gullet and serves as a rudimentary lung4 ,it corresponds with the swim bladder of most modern fishes1 and appears to have had a common origin with it4 ,we may conceive that the primordial fishes not only had gills used in breathing air dissolved in water1 but also developed a saclike pouch off the gullet4 ,this sac evolved along two distinct lines4 ,on the line of the ancestry of most modern fishes its duct was closed and it became the swim bladder used in flotation and balancing4 ,on another line of descent it was left open1 air was swallowed into it1 and it developed into the rudimentary lung of the dipnoans and into the more perfect lungs of the amphibians and other air- breathing vertebrates4 ,one of the ancient dipnoans is illustrated in ,figure #cjj4 ,some of the members of this order were1 like the ostracoderms1 cased in armor1 but their higher rank is shown by their powerful jaws and by other structures4 ,some of these armored fishes reached twenty- five feet in length and six feet across the head4 ,they were the tyrants of the ,devonian seas4 ,,ganoids4 ,these take their name from their enameled plates or scales of bone4 ,the few genera now surviving are the descendants of the tribes which swarmed in the ,devonian seas4 ,a restoration of one of a leading order1 the ,,fringe-,,finned ganoids1 is given in ,figure #cja4 ,the side fins1 which correspond to the limbs of the higher vertebrates1 are quite unlike those of most modern fishes4 ,their rays1 instead of radiating from a common base1 fringe a central lobe which contains a cartilaginous axis4 ,the teeth of the ,devonian ganoids show a complicated folded structure4 ,,general ,,characteristics ,,of ,,devonian ,,fishes4 ,,the ,,notochord ,,is ,,persistent4 ,the notochord is a continuous rod of cartilage1 or gristle1 which in the embryological growth of vertebrate animals supports the spinal nerve cord before the formation of the vertebrae4 ,in most modern fishes and in all higher vertebrates the notochord is gradually removed as the bodies of the vertebrae are formed about it2 but in the ,devonian fishes it persists through maturity and the vertebrae remain incomplete4 ,,the ,,skeleton ,,is ,,cartilaginous4 ,this also is an embryological characteristic4 ,in the ,devonian fishes the vertebrae1 as well as the other parts of the skeleton1 have not ossified1 or changed to bone1 but remain in their primitive cartilaginous condition4 ,,the ,,tail ,,fin ,,is ,,vertebrated4 ,the backbone runs through the fin and is fringed above and below with its vertical rays4 ,in some fishes with vertebrated tail fins the fin is symmetric1 and this seems to be the primitive type4 ,in others the tail fin is unsymmetric3 the backbone runs into the upper lobe1 leaving the two lobes of unequal size4 ,in most modern fishes 7the teleosts7 the tail fin is not vertebrated3 the spinal column ends in a broad plate1 to which the diverging fin rays are attached4 ,but along with these embryonic characters1 which were common to all ,devonian fishes1 there were other structures in certain groups which foreshadowed the higher structures of the land vertebrates which were yet to come3 air sacs which were to develop into lungs1 and cartilaginous axes in the side fins which were a prophecy of limbs4 ,the vertebrates had already advanced far enough to prove the superiority of their type of structure to all others4 ,their internal skeleton afforded the best attachment for muscles and enabled them to become the largest and most powerful creatures of the time4 ,the central nervous system1 with the predominance given to the ganglia at the fore end of the nerve cord1--the brain1-- already endowed them with greater energy than the invertebrates2 and1 still more important1 these structures contained the possibility of development into the more highly organized land vertebrates which were to rule the earth4 ,,teleosts4 ,the great group of fishes called the teleosts1 or those with complete bony skeletons1 to which most modern fishes belong1 may be mentioned here1 although in the ,devonian they had not yet appeared4 ,the teleosts are a highly specialized type1 adapted most perfectly to their aquatic environment4 ,heavy armor has been discarded1 and reliance is placed instead on swiftness4 ,the skeleton is completely ossified and the notochord removed4 ,the vertebrae have been economically withdrawn from the tail1 and the cartilaginous axis of the side fins has been fotfoid unnecessary4 ,the air sac has become a swim bladder4 ,in this complete specialization they have long since lost the possibility of evolving into higher types4 ,it is interesting to note that the modern teleosts in their embryological growth pass through the stages which characterized the maturity of their ,devonian ancestors2 their skeleton is cartilaginous and their tail fin vertebrated4 ,,chapter ,,xix ,,the ,,carboniferous ,the ,carboniferous system is so named from the large amount of coal which it contains4 ,other systems1 from the ,devonian on1 are coal bearing also1 but none so richly and to so wide an extent4 ,never before or since have the peculiar conditions been so favorable for the formation of extensive coal deposits4 ,with few exceptions the ,carboniferous strata rest on those of the ,devonian without any marked unconformity2 the one period passed quietly into the other1 with no great physical disturbances4 ,the ,carboniferous includes three distinct series4 ,the lower is called the ,,mississippian1 from the outcrop of its formations along the ,mississippi ,river in central and southern ,illinois and the adjacent portions of ,iowa and ,missouri4 ,the middle series is called the ,,pennsylvanian 7or ,coal ,measures71 from its wide occurrence over ,pennsylvania4 ,the upper series is named the ,,permian1 from the province of ,perm in ,russia4 ,,the ,,mississippian ,,series4 ,in the interior the ,mississippian is composed chiefly of limestones1 with some shales1 which tell of a clear1 warm1 epicontinental sea swarming with crinoids1 corals1 and shells1 and occasionally clouded with silt from the land4 ,in the eastern region1 ,new ,york had been added by uplift to the ,appalachian land which now was united to the northern area4 ,from eastern ,pennsylvania southward there were laid in a subsiding trough1 first1 thick sandstones 7the ,pocono sandstone71 and later still heavier shales1--the two together reaching the thickness of four thousand feet and more4 ,we infer a renewed uplift of ,appalachia similar to that of the later epochs of the ,devonian1 but as much less in amount as the volume of sediments is smaller4 ,,the ,,pennsylvanian ,,series ,the ,mississippian was brought to an end by a quiet oscillation which lifted large areas slightly above the sea1 and the ,pennsylvanian began with a movement in the opposite direction4 ,the sea encroached on the new land1 and spread far and wide a great basal conglomerate and coarse sandstones4 ,on this ancient beach deposit a group of strata rests which we must now interpret4 ,they consist of alternating shales and sandstones1 with here and there a bed of limestone and an occasional seam of coal4 ,a stratum of fire clay commonly underlies a coal seam1 and there occur also beds of iron ore4 ,we give a typical section of a very small portion of the series at a locality in ,pennyslvania4 ,although some of the minor changes are omitted1 the section shows the rapid alternation of the strata3 ,feet #i ,sandstone and shale 4 4 4 4 4 4 4 4 #be #h ,limestone 4 4 4 4 4 4 4 4 4 4 4 4 4 #ah #g ,sandstone 4 4 4 4 4 4 4 4 4 4 4 4 4 #aj #f ,coal 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 #a-f #e ,shale 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 #j-b #d ,sandstone 4 4 4 4 4 4 4 4 4 4 4 4 4 #dj #c ,limestone 4 4 4 4 4 4 4 4 4 4 4 4 4 #aj #b ,coal 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 #e-ab #a ,fire clay 4 4 4 4 4 4 4 4 4 4 4 4 4 #c ,this section shows more coal than is usual2 on the whole1 coal seams do not take up more than one foot in fifty of the ,coal ,measures4 ,they vary also in thickness more than is seen in the section1 some exceptional seams reaching the thickness of fifty feet4 ,,how ,,coal ,,was ,,made4 #a4 ,coal is of vegetable origin4 ,examined under the microscope even anthracite1 or hard coal1 is seen to contain carbonized vegetal tissues4 ,there are also all gradations connecting the hardest anthracite--through semibituminous coal1 bituminous or soft coal1 lignite 7an imperfect coal in which sometimes woody fibers may be seen little changed7--with peat and decaying vegetable tissues4 ,coal is compressed and mineralized vegetal matter4 ,its varieties depend on the perfection to which the peculiar change called bituminization has been carried1 and also1 as shown in the table below1 on the degree to which the volatile substances and water have escaped1 and on the per cent of carbon remaining4 ,peat ,lignite ,bituminous ,coal ,anthracite ,dismal ,swamp ,texas ,penn4 ,penn4 ,moisture 4 4 4 4 #gh4hi #ad4fg #a4cj #b4gd ,volatile matter 4 #ac4hd #cg4cb #bj4hg #d4be ,fixed carbon 4 4 #f4di #da4jg #fg4bj #ha4ea ,ash 4 4 4 4 4 4 4 #j4gh #f4fi #h4hj #aj4hg #b4 ,the vegetable remains associated with coal are those of land plants4 #c4 ,coal accumulated in the presence of water2 for it is only when thus protected from the air that vegetal matter is preserved4 #d4 ,the vegetation of coal accumulated for the most part where it grew2 it was not generally drifted and deposited by waves and currents4 ,commonly the fire clay beneath the seam is penetrated with roots1 and the shale above is packed with leaves of ferns and other plants as beautifully pressed as in a herbarium4 ,there often is associated with the seam a fossil forest1 with the stumps1 which are still standing where they grew1 their spreading roots1 and the soil beneath1 all changed to stone4 ,in the ,nova ,scotia field1 out of seventy-six distinct coal seams1 twenty are underlain by old forest grounds4 ,the presence of fire clay beneath a seam points in the same direction4 ,such underclays withstand intense heat and are used in making fire brick1 because their alkalies have been removed by the long-continued growth of vegetation4 ,fuel coal is also too pure to have been accumulated by driftage4 ,in that case we should expect to find it mixed with mud1 while in fact it often contains no more ash than the vegetal matter would furnish from which it has been compressed4 ,these conditions are fairly met in the great swamps of river plains and deltas and of coastal plains1 such as the great ,dismal ,swamp1 where thousands of generations of forests with their undergrowths contribute their stems and leaves to form thick beds of peat4 ,a coal seam is a fossil peat bed4 ,,geographical ,,conditions ,,during ,,the ,,pennsylvanian4 ,the ,carboniferous peat swamps were of vast extent4 ,a map of the ,coal ,measures 7,fig4 #bfj7 shows that the coal marshes stretched1 with various interruptions of higher ground and straits of open water1 from eastern ,pennsylvania into ,alabama1 ,texas1 and ,kansas4 ,some individual coal beds may still be traced over a thousand square miles1 despite the erosion which they have suffered4 ,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 #cjd4 ,on the basis that peat loses four fifths of its bulk in changing to coal1 we may reckon the thickness of these ancient peat beds4 ,coal seams six and ten feet thick1 which are not uncommon1 represent peat beds thirty and fifty feet in thickness1 while mammoth coal seams fifty feet thick have been compressed from peat beds two hundred and fifty feet deep4 ,at the same time1 the thousands of feet of marine and freshwater sediments1 with their repeated alternations of limestones1 sandstones1 and shales1 in which the seams of coal occur1 prove a slow subsidence1 with many changes in its rate1 with halts when the land was at a stillstand1 and with occasional movements upward4 ,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 oozes4 ,when subsidence slackened or ceased the land gained on the sea4 ,bays were barred1 and lagoons as they gradually filled with mud became marshes4 ,river deltas pushed forward1 burying with their silts the sunken peat beds of earlier centuries1 and at the surface emerged in broad1 swampy flats1--like those of the deltas of the ,mississippi and the ,ganges1--which soon were covered with luxuriant forests4 ,at times a gentle uplift brought to sea level great coastal plains1 which for ages remained mantled with the jungle1 their undeveloped drainage clogged with its debris1 and were then again submerged4 ,,physical ,,geography ,,of ,,the ,,several ,,regions4 ,,the ,,acadian ,,region lay on the eastern side of the northern land1 where now are ,new ,brunswick and ,nova ,scotia1 and was an immense river delta4 ,here river deposits rich in coal accumulated to a depth of sixteen thousand feet4 ,the area of this coal field is estimated at about thirty-six thousand square miles4 ,,the ,,appalachian ,,region skirts the ,appalachian oldland on the west from the southern boundary of ,new ,york to northern ,alabama1 extending west into eastern ,ohio4 ,the ,cincinnati anticline was now a peninsula1 and the broad gulf which had lain between it and ,appalachia was transformed at the beginning of the ,pennsylvanian into wide marshy plains1 now sinking beneath the sea and now emerging from it4 ,this area subsided during the ,carboniferous period to a depth of nearly ten thousand feet4 ,,the ,,central ,,region lay west of the peninsula of the ,cincinnati anticline1 and extended from ,indiana west into eastern ,nebraska1 and from central ,iowa and ,illinois southward about the ancient island in ,missouri and ,arkansas into ,oklahoma and ,texas4 ,on the north the subsidence in this area was comparatively slight1 for the ,carboniferous strata scarcely exceed two thousand feet in thickness4 ,but in ,arkansas and ,indian ,territory the downward movement amounted to four and five miles1 as is proved by shoal water deposits of that immense thickness4 ,the coal fields of ,indiana1 and ,illinois are now separated by erosion from those lying west of the ,mississippi ,river4 ,at the south the ,appalachian land seems still to have stretched away to the west across ,louisiana and ,mississippi into ,texas1 and this westward extension formed the southern boundary of the coal marshes of the continent4 ,the three regions just mentioned include the chief ,carboniferous coal fields of ,north ,america4 ,including a field in central ,michigan evidently formed in an inclosed basin 7,fig4 #bfj71 and one in ,rhode ,island1 the total area of ,american coal fields has been reckoned at not less than two hundred thousand square miles4 ,we can hardly estimate the value of these great stores of fossil fuel to an industrial civilization4 ,the forests of the coal swamps accumulated in their woody tissues the energy which they received from the sun in light and heat1 and it is this solar energy long stored in coal seams which now forms the world's chief source of power in manufacturing4 ,,the ,,western ,,area4 ,on the ,great ,plains beyond the ,missouri ,river the ,carboniferous strata pass under those of more recent systems4 ,where they reappear1 as about dissected mountain axes or on stripped plateaus1 they consist wholly of marine deposits and are devoid of coal4 ,the rich coal fields of the ,west are of later date4 ,on the whole the ,carboniferous seems to have been a time of subsidence in the ,west4 ,throughout the period a sea covered the ,great ,basin and the plateaus of the ,colorado ,river4 ,at the time of the greatest depression the sites of the central chains of the ,rockies were probably islands1 but early in the period they may have been connected with the broad lands to the south and east4 ,thousands of feet of ,carboniferous sediments were deposited where the ,sierra ,nevada ,mountains now stand4 ,,the ,,permian4 ,as the ,carboniferous period drew toward its close the sea gradually withdrew from the eastern part of the continent4 ,where the sea lingered in the deepest troughs1 and where inclosed basins were cut off from it1 the strata of the ,permian were deposited4 ,such are found in ,new ,brunswick1 in ,pennsylvania and ,west ,virginia1 in ,texas1 and in ,kansas4 ,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 away4 ,in the southern hemisphere the ,permian deposits are so extraordinary that they deserve a brief notice1 although we have so far omitted mention of the great events which characterized the evolution of other continents than our own4 ,the ,permian fauna- flora of ,australia1 ,india1 ,south ,africa1 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 together1 probably as extensions of a great antarctic continent4 ,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 moraines4 ,clays and sand1 now hardened to firm rock1 are inset with unsorted stones of all sizes1 which often are faceted and scratched4 ,moreover1 these bowlder clays rest on rock pavements which are polished and scored with glacial markings4 ,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 ,greenland4 ,these ,permian ground moraines are not the first traces of the work of glaciers met with in the geological record4 ,similar deposits prove glaciation in ,norway succeeding the pre-,cambrian stage of elevation1 and ,cambrian glacial drift has recently been found in ,china4 ,,the ,,appalachian ,,deformation4 ,we have seen that during ,paleozoic times a long1 narrow trough of the sea lay off the western coast of the ancient land of ,appalachia1 where now are the ,appalachian ,mountains4 ,during the long ages of this era the trough gradually subsided1 although with many stillstands and with occasional slight oscillations upward4 ,meanwhile the land lying to the east was gradually uplifted at varying rates and with long pauses4 ,the waste of the rising land was constantly transferred to the sinking marginal sea bottom1 and on the whole the trough was filled with sediments as rapidly as it subsided4 ,the sea was thus kept shallow1 and at times1 especially toward the close of the era1 much of the area was upbuilt or raised to low1 marshy1 coastal plains4 ,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 feet4 ,both by sedimentation and by subsidence the trough had now become a belt of weakness in the crust of the earth4 ,here the crust was now made of layers to the depth of six or seven miles4 ,in comparison with the massive crystalline rocks of ,appalachia on the east1 the layered rock of the trough was weak to resist lateral pressure1 as a ream of sheets of paper is weak when compared with a solid board of the same thickness4 ,it was weaker also than the region to the west1 since there the sediments were much thinner4 ,besides1 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 thrust4 ,there now occurred one of the critical stages in the history of the planet1 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 it4 ,under slow but resistless pressure the strata of the ,appalachian trough were thrust against the rigid land1 and slowly1 steadily bent into long folds whose axes ran northeast-southwest parallel to the ancient coast line4 ,it was on the eastern side next the buttress of the land that the deformation was the greatest1 and the folds most steep and close4 ,in central ,pennsylvania and ,west ,virginia the folds were for the most part open4 ,south of these states the folds were more closely appressed1 the strata were much broken1 and the great thrust faults were formed which have been described already4 ,in eastern ,pennsylvania seams of bituminous coal were altered to anthracite1 while outside the region of strong deformation1 as in western ,pennyslvania1 they remained unchanged4 ,an important factor in the deformation was the massive limestones of the ,cambrian-,ordovician4 ,because of these thick1 resistant beds the rocks were bent into wide folds and sheared in places with great thrust faults4 ,had the strata been weak shales1 an equal pressure would have crushed and mashed them4 ,although the great earth folds were slowly raised1 and no doubt eroded in their rising1 they formed in all probability a range of lofty mountains1 with a width of from fifty to a hundred and twenty-five miles1 which stretched from ,new ,york to central ,alabama4 ,from their bases lowlands extended westward to beyond the ,missouri ,river4 ,at the same time ranges were upridged out of thick ,paleozoic sediments both in the ,bay of ,fundy region and in the ,indian ,territory4 ,the eastern portion of the ,north ,american continent was now well-nigh complete4 ,the date of the ,appalachian deformation is told in the usual way4 ,the ,carboniferous strata1 nearly two miles thick1 are all infolded in the ,appalachian ridges1 while the next deposits found in this region--those of the later portion of the first period 7the ,trias7 of the succeeding era--rest unconformably on the worn edges of the ,appalachian folded strata4 ,the deformation therefore took place about the close of the ,paleozoic4 ,it seems to have begun in the ,permian1 in1 eastern ,pennsylvania1--for here the ,permian strata are wanting1--and to have continued into the ,trias1 whose earlier formations are absent over all the area4 ,with this wide uplift the subsidence of the sea floor which had so long been general in eastern ,north ,america came to an end4 ,deposition now gave place to erosion4 ,the sedimentary record of the ,paleozoic was closed1 and after an unknown lapse of time1 here unrecorded1 the annals of the succeeding era were written under changed conditions4 ,in western ,north ,america the closing stages of the ,paleozoic were marked by important oscillations4 ,the ,great ,basin1 which had long been a mediterranean sea1 was converted into land over western ,utah and eastern ,nevada1 while the waves of the ,pacific rolled across ,california and western ,nevada4 ,the absence of tuffs and lavas among the ,carboniferous strata of ,north ,america shows that here volcanic action was singularly wanting during the entire period4 ,even the ,appalachian deformation was not accompanied by any volcanic outbursts4 ,,life ,,of ,,the ,,carboniferous ,,plants4 ,the gloomy forests and dense undergrowths of the ,carboniferous jungles would appear unfamiliar to us could we see them as they grew1 and even a botanist would find many of their forms perplexing and hard to classify4 ,none of our modern trees would meet the eye4 ,plants with conspicuous flowers of fragrance and beauty were yet to come4 ,even mosses and grasses were still absent4 ,tree ferns lifted their crowns of feathery fronds high in air on trunks of woody tissue2 and lowly herbaceous ferns1 some belonging to existing families1 carpeted the ground4 ,many of the fernlike forms1 however1 have distinct affinities with the cycads1 of which they may be the ancestors1 and some bear seeds and must be classed as gymnosperms4 ,dense thickets1 like cane or bamboo brakes1 were composed of thick clumps of ,,calamites1 whose slender1 jointed stems shot up to a height of forty feet1 and at the joints bore slender branches set with whorls of leaves4 ,these were close allies of the ,equiseta or horsetails1 of the present2 but they bore characteristics of higher classes in the woody structures of their stems4 ,there were also vast monotonous forests1 composed chiefly of trees belonging to the lycopods1 and whose nearest relatives to-day are the little club mosses of our eastern woods4 ,two families of lycopods deserve special mention1--the ,lepidodendrons and the ,sigillaria4 ,the ,,lepidodendron1 or scale tree1 was a gigantic club moss fifty and seventy-five feet high1 spreading toward the top into stout branches1 at whose ends were borne cone-shaped spore cases4 ,the younger parts of the tree were clothed with stiff needle-shaped leaves1 but elsewhere the trunk and branches were marked with scalelike scars1 left by the fallen leaves1 and arranged in spiral rows4 ,the ,,sigillaria1 or seal tree1 was similar to the ,lepidodendron1 but its fluted trunk divided into even fewer branches1 and was dotted with vertical rows of leaf scars1 like the impressions of a seal4 ,both ,lepidodendron and ,sigillaria were anchored by means of great cablelike underground stems1 which ran to long distances through the marshy ground4 ,the trunks of both trees had a thick woody rind1 inclosing loose cellular tissue and a pith4 ,their hollow stumps1 filled with sand and mud1 are common in the ,coal ,measures1 and in them one sometimes finds leaves and stems1 land shells1 and the bones of little reptiles of the time which made their home there4 ,it is important to note that some of these gigantic lycopods1 which are classed with the ,,cryptogams1 or flowerless plants1 had pith and medullary rays dividing their cylinders into woody wedges4 ,these characters connect them with the ,,phanerogams1 or flowering plants4 ,like so many of the organisms of the remote past1 they were connecting types from which groups now widely separated have diverged4 ,gymnosperms1 akin to the cycads1 were also present in the ,carboniferous forests4 ,such were the different species of ,,cordaites1 trees pyramidal in shape1 with strap-shaped leaves and nutlike fruit4 ,other gymnosperms were related to the yews1 and it was by these that many of the fossil nuts found in the ,coal ,measures were borne4 ,it is thought by some that the gymnosperms had their station on the drier plains and higher lands4 ,the ,carboniferous jungles extended over parts of ,europe and of ,asia1 as well as eastern ,north ,america1 and reached from the equator to within nine degrees of the north pole4 ,even in these widely separated regions the genera and species of coal plants are close akin and often identical4 ,,invertebrates4 ,among the echinoderms1 crinoids are now exceedingly abundant1 sea urchins are more plentiful1 and sea cucumbers are found now for the first time4 ,trilobites are rapidly declining1 and pass away forever with the close of the period4 ,eurypterids are common2 stinging scorpions are abundant2 and here occur the first-known spiders4 ,we have seen that the arthropods were the first of all animals to conquer the realm of the air1 the earliest insects appearing in the ,ordovician4 ,insects had now become exceedingly abundant1 and the ,carboniferous forests swarmed with the ancestral types of dragon flies1--some with a spread of wing of more than two feet1-- ,may flies1 crickets1 and locusts4 ,cockroaches infested the swamps1 and one hundred and thirty-three species of this ancient order have been discovered in the ,carboniferous of ,north ,america4 ,the higher flower-loving insects are still absent2 the reign of the flowering plants has not yet begun4 ,the ,paleozoic insects were generalized types connecting the present orders4 ,their fore wings were still membranous and delicately veined1 and used in flying2 they had not yet become thick1 and useful only as wing covers1 as in many of their descendants4 ,,fishes still held to the ,devonian types1 with the exception that the strange ostracoderms now had perished4 ,,amphibians4 ,the vertebrates had now followed the arthropods and the mollusks upon the land1 and had evolved a higher type adapted to the new environment4 ,amphibians--the class to which frogs and salamanders belong--now appear1 with lungs for breathing air and with limbs for locomotion on the land4 ,most of the ,carboniferous amphibians were shaped like the salamander1 with weak limbs adapted more for crawling than for carrying the body well above the ground4 ,some legless1 degenerate forms were snakelike in shape4 ,the earliest amphibians differ from those of to-day in a number of respects4 ,they were connecting types linking together fishes1 from which they were descended1 with reptiles1 of which they were the ancestors4 ,they retained the evidence of their close relationship with the ,devonian fishes in their cold blood1 their gills and aquatic habit during their larval stage1 their teeth with dentine infolded like those of the ,devonian ganoids but still more intricately1 and their biconcave vertebrae which never completely ossified4 ,these1 the highest vertebrates of the time1 had not yet advanced beyond the embryonic stage of the more or less cartilaginous skeleton and the persistent notochord4 ,on the other hand1 the skull of the ,carboniferous amphibians was made of close-set bony plates1 like the skull of the reptile1 rather than like that of the frog1 with its open spaces 7,figs4 #cac and #cad74 ,unlike modern amphibians1 with their slimy skin1 the ,carboniferous amphibians wore an armor of bony scales over the ventral surface and sometimes over the back as well4 ,it is interesting to notice from the footprints and skeletons of these earliest-known vertebrates of the land what was the primitive number of digits4 ,the ,carboniferous amphibians had five- toed feet1 the primitive type of foot1 from which their descendants of higher orders1 with a smaller number of digits1 have diverged4 ,the ,carboniferous was the age of lycopods and amphibians1 as the ,devonian had been the age of rhizocarps and fishes4 ,,life ,,of ,,the ,,permian4 ,the close of the ,paleozoic was1 as we have seen1 a time of marked physical changes4 ,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 animals4 ,changes of climate compelled extensive migrations1 and the fauna of different regions were thus brought into conflict4 ,the ,permian was a time of pronounced changes in plant and animal life1 and a transitional period between two great eras4 ,the somber forests of the earlier ,carboniferous1 with their gigantic club mosses1 were now replaced by forests of cycads1 tree ferns1 and conifers4 ,even in the lower ,permian the ,lepidodendron and ,sigillaria were very rare1 and before the end of the epoch they and the ,calamites also had become extinct4 ,gradually the antique types of the ,paleozoic fauna died out1 and in the ,permian rocks are found the last survivors of the cystoid1 the trilobite1 and the eurypterid1 and of many long-lived families of brachiopods1 mollusks1 and other invertebrates4 ,the venerable ,orthoceras and the ,goniatite linger on through the epoch and into the first period of the succeeding era4 ,forerunners of the great ammonite family of cephalopod mollusks now appear4 ,the antique forms of the earlier ,carboniferous amphibians continue1 but with many new genera and a marked increase in size4 ,a long forward step had now been taken in the evolution of the vertebrates4 ,a new and higher type1 the reptiles1 had appeared1 and in such numbers and variety are they found in the ,permian strata that their advent may well have occurred in a still earlier epoch4 ,it will be most convenient to describe the ,permian reptiles along with their descendants of the ,mesozoic4 ,,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 era1--the ,mesozoic1 the ,middle ,age of geological history4 ,although the ,mesozoic era is shorter than the ,paleozoic1 as measured by the thickness of their strata1 yet its duration must be reckoned in millions of years4 ,its predominant life features are the culmination and the beginning of the decline of reptiles1 amphibians1 cephalopod mollusks1 and cycads1 and the advent of marsupial mammals1 birds1 teleost fishes1 and angiospermous plants4 ,the leading events of the long ages of the era we can sketch only in the most summary way4 ,the ,mesozoic comprises three systems1--the ,,triassic1 named from its threefold division in ,germany2 the ,,jurassic1 which is well displayed in the ,jura ,mountains2 and the ,,cretaceous1 which contains the extensive chalk 7,latin1 creta7 deposits of ,europe4 ,in eastern ,north ,america the ,mesozoic rocks are much less important than the ,paleozoic1 for much of this portion of the continent was land during the ,mesozoic era1 and the area of the ,mesozoic rocks is small4 ,in western ,north ,america1 on the other hand1 the strata of the ,mesozoic--and of the ,cenozoic also--are widely spread4 ,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 light1 as in deep canyons and about mountain axes4 ,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 states4 ,,the ,,triassic ,,and ,,jurassic ,,eastern ,,north ,,america4 ,the sedimentary record interrupted by the ,appalachian deformation was not renewed in eastern ,north ,america until late in the ,triassic4 ,hence during this long interval the land stood high1 the coast was farther out than now1 and over our ,atlantic states geological time was recorded chiefly in erosion forms of hill and plain which have long since vanished4 ,the area of the later ,triassic rocks of this region1 which take up again the geological record1 is seen in the map of ,figure #bfj4 ,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 coast4 ,evidently subsidence was in progress where these rocks were deposited4 ,the eastern border of ,appalachia was now depressed4 ,the oldland was warping1 and long belts of country lying parallel to the shore subsided1 forming troughs in which thousands of feet of sediment now gathered4 ,these ,triassic rocks1 which are chiefly sandstones1 hold no marine fossils1 and hence were not laid in open arms of the sea4 ,but their layers are often ripple-marked1 and contain many tracks of reptiles1 imprints of raindrops1 and some fossil wood1 while an occasional bed of shale is filled with the remains of fishes4 ,we may conceive1 then1 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-day1 and as gradually aggraded by streams from the neighboring uplands4 ,their broad1 sandy flats were overflowed by wandering streams1 and when subsidence gained on deposition shallow lakes overspread the alluvial plains4 ,perhaps now and then the basins became long1 brackish estuaries1 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 valleys4 ,the ,triassic rocks are mainly red sandstones1--often feldspathic1 or arkose1 with some conglomerates and shales4 ,considering the large amount of feldspathic material in these rocks1 do you infer that they were derived from the adjacent crystalline and metamorphic rocks of the oldland of ,appalachia1 or from the sedimentary ,paleozoic rocks which had been folded into mountains during the ,appalachian deformation8 ,if from the former1 was the drainage of the northern ,appalachian mountain region then1 as now1 eastward and southeastward toward the ,atlantic8 ,the ,triassic sandstones are voluminous1 measuring at least a mile in thickness1 and are largely of coarse waste4 ,what do you infer as to the height of the lands from which the waste was shed1 or the direction of the oscillation which they were then undergoing8 ,in the southern basins1 as about ,richmond1 ,virginia1 are valuable beds of coal2 what was the physical geography of these areas when the coal was being formed8 ,interbedded with the ,triassic sandstones are contemporaneous lava beds which were fed from dikes4 ,volcanic action1 which had been remarkably absent in eastern ,north ,america during ,paleozoic times1 was well-marked in connection with the warping now in progress4 ,thick intrusive sheets have also been driven in among the strata1 as1 for example1 the sheet of the ,palisades of the ,hudson1 described on page #bfi4 ,the present condition of the ,triassic sandstones of the ,connecticut valley is seen in ,figure #cae4 ,were the beds laid in their present attitude8 ,what was the nature of the deformation which they have suffered8 ,when did the intrusion of lava sheets take place relative to the deformation8 ,what effect have these sheets on the present topography1 and why8 ,assuming that the ,triassic deformation went on more rapidly than denudation1 what was its effect on the topography of the time8 ,are there any of its results remaining in the topography of to-day8 ,do the ,triassic areas now stand higher or lower than the surrounding country1 and why8 ,how do the ,triassic sandstones and shales compare in hardness with the igneous and metamorphic rocks about them8 ,the ,jurassic strata are wanting over the ,triassic areas and over all of eastern ,north ,america4 ,was this region land or sea1 an area of erosion or sedimentation1 during the ,jurassic period8 ,in ,new ,jersey1 ,pennsylvania1 and farther southwest the lowest strata of the next period1 the ,cretaceous1 rest on the eroded edges of the earlier rocks4 ,the surface on which they lie is worn so even that we must believe that at the opening of the ,cretaceous the oldland of ,appalachia1 including the ,triassic areas1 had been baseleveled at least near the coast4 ,when1 therefore1 did the deformation of the ,triassic rocks occur8 ,,western ,,north ,,america4 ,triassic strata infolded in the ,sierra ,nevada ,mountains carry marine fossils and reach a thickness of nearly five thousand feet4 ,california was then under water1 and the site of the ,sierra was a subsiding trough slowly filling with waste from the ,great ,basin land to the east4 ,over a long belt which reaches from ,wyoming across ,colorado into ,new ,mexico no ,triassic sediments are found1 nor is there any evidence that they were ever present2 hence this area was high land suffering erosion during the ,triassic4 ,on each side of it1 in eastern ,colorado and about the ,black ,hills1 in western ,texas1 in ,utah1 over the site of the ,wasatch ,mountains1 and southward into ,arizona over the plateaus trenched by the ,colorado ,river1 are large areas of ,triassic rocks1 sandstones chiefly1 with some rock salt and gypsum4 ,fossils are very rare and none of them marine4 ,here1 then1 lay broad shallow lakes often salt1 and warped basins1 in which the waste of the adjacent uplands gathered4 ,to this system belong the sandstones of the ,garden of the ,gods in ,colorado1 which later earth movements have upturned with the uplifted mountain flanks4 ,the ,jurassic was marked with varied oscillations and wide changes in the outline of sea and land4 ,jurassic shales of immense thickness--now metamorphosed into slates--are found infolded into the ,sierra ,nevada ,mountains4 ,hence during ,jurassic times the ,sierra trough continued to subside1 and enormous deposits of mud were washed into it from the land lying to the east4 ,contemporaneous lava flows interbedded with the strata show that volcanic action accompanied the downwarp1 and that molten rock was driven upward through fissures in the crust and outspread over the sea floor in sheets of lava4 ,,the ,,sierra ,,deformation4 ,ever since the middle of the ,silurian1 the ,sierra trough had been sinking1 though no doubt with halts and interruptions1 until it contained nearly twenty-five thousand feet of sediment4 ,at the close of the ,jurassic it yielded to lateral pressure and the vast pile of strata was crumpled and upheaved into towering mountains4 ,the ,mesozoic muds were hardened and squeezed into slates4 ,the rocks were wrenched and broken1 and underground waters began the work of filling their fissures with gold-bearing quartz1 which was yet to wait millions of years before the arrival of man to mine it4 ,immense bodies of molten rock were intruded into the crust as it suffered deformation1 and these appear in the large areas of granite which the later denudation of the range has brought to light4 ,the same movements probably uplifted the rocks of the ,coast ,range in a chain of islands4 ,the whole western part of the continent was raised and its seas and lakes were for the most part drained away4 ,,the ,,british ,,isles4 ,the ,triassic strata of the ,british ,isles are continental1 and include breccia beds of cemented talus1 deposits of salt and gypsum1 and sandstones whose rounded and polished grains are those of the wind-blown sands of deserts4 ,in ,triassic times the ,british ,isles were part of a desert extending over much of northwestern ,europe4 ,,the ,,cretaceous ,the third great system of the ,mesozoic includes many formations1 marine and continental1 which record a long and complicated history marked by great oscillations of the crust and wide changes in the outlines of sea and land4 ,,early ,,cretaceous4 ,in eastern ,north ,america the lowest ,cretaceous series comprises fresh-water formations which are traced from ,nantucket across ,martha's ,vineyard and ,long ,island1 and through ,new ,jersey southward into ,georgia4 ,they rest unconformably on the ,triassic sandstones and the older rocks of the region4 ,the ,atlantic shore line was still farther out than now in the northern states4 ,again1 as during the ,triassic1 a warping of the crust formed a long trough parallel to the coast and to the ,appalachian ridges1 but cut off from the sea2 and here the continental deposits of the early ,cretaceous were laid4 ,along the ,gulf of ,mexico the same series was deposited under like conditions over the area known as the ,mississippi embayment1 reaching from ,georgia northwestward into ,tennessee and thence across into ,arkansas and southward into ,texas4 ,in the ,southwest the subsidence continued until the transgressing sea covered most of ,mexico and ,texas and extended a gulf northward into ,kansas4 ,in its warm and quiet waters limestones accumulated to a depth of from one thousand to five thousand feet in ,texas1 and of more than ten thousand feet in ,mexico4 ,meanwhile the lowlands1 where the ,great ,plains are now1 received continental deposits2 coal swamps stretched from western ,montana into ,british ,columbia4 ,,the ,,middle ,,cretaceous4 ,this was a land epoch4 ,the early ,cretaceous sea retired from ,texas and ,mexico1 for its sediments are overlain unconformably by formations of the ,upper ,cretaceous4 ,so long was the time gap between the two series that no species found in the one occurs in the other4 ,,the ,,upper ,,cretaceous4 ,there now began one of the most remarkable events in all geological history1--the great ,cretaceous subsidence4 ,its earlier warpings were recorded in continental deposits1--wide sheets of sandstone1 shale1 and some coal1--which were spread from ,texas to ,british ,columbia4 ,these continental deposits are overlain by a succession of marine formations whose vast area is shown on the map1 ,figure #bfj4 ,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 epochs4 ,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 plain4 ,the ,gulf of ,mexico again covered the ,mississippi embayment1 reaching as far north as southern ,illinois1 and extended over ,texas4 ,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 ,city1 the ,colorado ,mountains rising from it in a chain of islands4 ,along with minor oscillations there were laid in the interior sea various formations of sandstones1 shales1 and limestones1 and from ,kansas to ,south ,dakota beds of white chalk show that the clear1 warm waters swarmed at times with foraminiferal life whose disintegrating microscopic shells accumulated in this rare deposit4 ,at this epoch a wide sea1 interrupted by various islands1 stretched across ,eurasia from ,wales and western ,spain to ,china1 and spread southward over much of the ,sahara4 ,to the west its waters were clear and on its floor the crumbled remains of foraminifers gathered in heavy accumulations of calcareous ooze1-- the white chalk of ,france and ,england4 ,sea urchins were also abundant1 and sponges contributed their spicules to form nodules of flint4 ,,the ,,laramie4 ,the closing stage of the ,cretaceous was marked in ,north ,america by a slow uplift of the land4 ,as the interior sea gradually withdrew1 the warping basins of its floor were filled with waste from the rising lands about them1 and over this wide area there were spread continental deposits in fresh-water lakes like the ,great ,lakes of the present1 in brackish estuaries1 and in river plains1 while occasional oscillations now and again let in the sea4 ,there were vast marshes in which there accumulated the larger part of the valuable coal seams of the ,west4 ,the ,laramie is the coal-bearing series of the ,west1 as the ,pennsylvanian is of the eastern part of our country4 ,,the ,,rocky ,,mountain ,,deformation4 ,at the close of the ,cretaceous we enter upon an epoch of mountain-making far more extensive than any which the continent had witnessed4 ,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 ages1 and in its subsiding troughs ,paleozoic and ,mesozoic sediments had gathered to the depth of many thousand feet4 ,and now from ,mexico well-nigh to the ,arctic ,ocean this belt yielded to lateral pressure4 ,the ,cretaceous limestones of ,mexico were folded into lofty mountains4 ,a massive range was upfolded where the ,wasatch ,mountains now are1 and various ranges of the ,rockies in ,colorado and other states were upridged4 ,however slowly these deformations were effected they were no doubt accompanied by world-shaking earthquakes1 and it is known that volcanic eruptions took place on a magnificent scale4 ,outflows of lava occurred along the ,wasatch1 the laccoliths of the ,henry ,mountains were formed1 while the great masses of igneous rock which constitute the cores of the ,spanish ,peaks and other western mountains were thrust up amid the strata4 ,the high plateaus from which many of these ranges rise had not yet been uplifted1 and the bases of the mountains probably stood near the level of the sea4 ,north ,america was now well-nigh completed4 ,the mediterranean seas which so often had occupied the heart of the land were done away with1 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 east4 ,,the ,,mesozoic ,,peneplain4 ,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 ages4 ,the same lesson is taught as plainly by the amount of denudation which the lands suffered during the era4 ,the beginning of the ,mesozoic saw a system of lofty mountain ranges stretching from ,new ,york into central ,alabama4 ,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 level4 ,the ,mesozoic era was long enough for the ,appalachian ,mountains1 upridged at its beginning1 to have been weathered and worn away and carried grain by grain to the sea4 ,the same plain extended over southern ,new ,england4 ,the ,taconic range1 uplifted partially at least at the close of the ,ordovician1 and the block mountains of the ,triassic1 together with the pre- ,cambrian mountains of ancient ,appalachia1 had now all been worn to a common level with the ,allegheny ranges4 ,the ,mesozoic peneplain has been upwarped by later crustal movements and has suffered profound erosion1 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 existed4 ,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 sea4 ,,life ,,of ,,the ,,mesozoic ,,plant ,,life ,,of ,,the ,,triassic ,,and ,,jurassic4 ,the ,carboniferous forests of lepidodendrons and sigillafids had now vanished from the earth4 ,the uplands were clothed with conifers1 like the ,araucarian pines of ,south ,america and ,australia4 ,dense forests of tree ferns throve in moist regions1 and canebrakes of horsetails of modern type1 but with stems reaching four inches in thickness1 bordered the lagoons and marshes4 ,cycads were exceedingly abundant4 ,these gymnosperms1 related to the pines and spruces in structure and fruiting1 but palmlike in their foliage1 and uncoiling their long leaves after the manner of ferns1 culminated in the ,jurassic4 ,from the view point of the botanist the ,mesozoic is the ,age of ,cycads1 and after this era they gradually decline to the small number of species now existing in tropical latitudes4 ,,plant ,,life ,,of ,,the ,,cretaceous4 ,in the ,lower ,cretaceous the woodlands continued of much the same type as during the ,jurassic4 ,the forerunners now appeared of the modern dicotyls 7plants with two seed leaves71 and in the ,middle ,cretaceous the monocotyledonous group of palms came in4 ,palms are so like cycads that we may regard them as the descendants of some cycad type4 ,in the ,,upper ,,cretaceous1 cycads become rare4 ,the highest types of flowering plants gain a complete ascendency1 and forests of modern aspect cover the continent from the ,gulf of ,mexico to the ,arctic ,ocean4 ,among the kinds of forest trees whose remains are found in the continental deposits of the ,cretaceous are the magnolia1 the myrtle1 the laurel1 the fig1 the tulip tree1 the chestnut1 the oak1 beech1 elm1 poplar1 willow1 birch1 and maple4 ,forests of ,eucalyptus grew along the coast of ,new ,england1 and palms on the ,pacific shores of ,british ,columbia4 ,sequoias of many varieties ranged far into northern ,canada4 ,in northern ,greenland there were luxuriant forests of magnolias1 figs1 and cycads2 and a similar flora has been disinterred from the ,cretaceous rocks of ,alaska and ,spitzbergen4 ,evidently the lands within the ,arctic ,circle enjoyed a warm and genial climate1 as they had done during the ,paleozoic4 ,greenland had the temperature of ,cuba and southern ,florida1 and the time was yet far distant when it was to be wrapped in glacier ice4 ,,invertebrates4 ,during the long succession of the ages of the ,mesozoic1 with their vast geographical changes1 there were many and great changes in organisms4 ,species were replaced again and again by others better fitted to the changing environment4 ,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 ,texas4 ,we shall disregard these changes for the most part in describing the life of the era1 and shall confine our view to some of the most important advances made in the leading types4 ,stromatopora have disappeared4 ,protozoans and sponges are exceedingly abundant1 and all contribute to the making of ,mesozoic strata4 ,corals have assumed a more modern type4 ,sea urchins have become plentiful2 crinoids abound until the ,cretaceous1 where they begin their decline to their present humble station4 ,trilobites and eurypterids are gone4 ,ten-footed crustaceans abound of the primitive long-tailed type 7represented by the lobster and the crayfish71 and in the ,jurassic there appears the modern short- tailed type represented by the crabs4 ,the latter type is higher in organization and now far more common4 ,in its embryological development it passes through the long-tailed stage2 connecting links in the ,mesozoic also indicate that the younger type is the offshoot of the older4 ,insects evolve along diverse lines1 giving rise to beetles1 ants1 bees1 and flies4 ,brachiopods have dwindled greatly in the number of their species1 while mollusks have correspondingly increased4 ,the great oyster family dates from here4 ,cephalopods are now to have their day4 ,the archaic ,orthoceras lingers on into the ,triassic and becomes extinct1 but a remarkable development is now at hand for the more highly organized descendants of this ancient line4 ,we have noticed that in the ,devonian the sutures of some of the chambered shells become angled1 evolving the ,goniatite type4 ,the sutures now become lobed and corrugated in ,ceratites4 ,the process was carried still farther1 and the sutures were elaborately frilled in the great order of the ,ammonites4 ,it was in the ,jurassic that the ,ammonites reached their height4 ,no fossils are more abundant or characteristic of their age4 ,great banks of their shells formed beds of limestone in warm seas the world over4 ,the ammonite stem branched into a most luxuriant variety of forms4 ,the typical form was closely coiled like a nautilus4 ,in others the coil was more or less open1 or even erected into a spiral4 ,some were hook-shaped1 and there were members of the order in which the shell was straight1 and yet retained all the internal structures of its kind4 ,at the end of the ,mesozoic the entire tribe of ammonites became extinct4 ,the ,belemnite 7,greek1 belemnon1 a dart7 is a distinctly higher type of cephalopod which appeared in the ,triassic1 became numerous and varied in the ,jurassic and ,cretaceous1 and died out early in the ,tertiary4 ,like the squids and cuttlefish1 of which it was the prototype1 it had an internal calcareous shell4 ,this consisted of a chambered and siphuncled cone1 whose point was sheathed in a long solid guard somewhat like a dart4 ,the animal carried an ink sac1 and no doubt used it as that of the modern cuttlefish is used1--to darken the water and make easy an escape from foes4 ,belemnites have sometimes been sketched with fossil sepia1 or india ink1 from their own ink sacs4 ,in the belemnites and their descendants1 the squids and cuttlefish1 the cephalopods made the radical change from external to the internal shell4 ,they abandoned the defensive system of warfare and boldly took up the offensive4 ,no doubt1 like their descendants1 the belemnites were exceedingly active and voracious creatures4 ,,fishes ,,and ,,amphibians4 ,in the ,triassic and ,jurassic1 little progress was made among the fishes1 and the ganoid was still the leading type4 ,in the ,cretaceous the teleosts1 or bony fishes1 made their appearance1 while ganoids declined toward their present subordinate place4 ,the amphibians culminated in the ,triassic1 some being formidable creatures as large as alligators4 ,they were still of the primitive ,paleozoic types4 ,their pygmy descendants of more modern types are not found until later1 salamanders appearing first in the ,cretaceous1 and frogs at the beginning of the ,cenozoic4 ,no remains of amphibians have been discovered in the ,jurassic4 ,do you infer from this that there were none in existence at that time8 ,,reptiles ,,of ,,the ,,mesozoic ,the great order of ,reptiles made its advent in the ,permian1 culminated in the ,triassic and ,jurassic1 and began to decline in the ,cretaceous4 ,the advance from the amphibian to the reptile was a long forward step in the evolution of the vertebrates4 ,in the reptile the vertebrate skeleton now became completely ossified4 ,gills were abandoned and breathing was by lungs alone4 ,the development of the individual from the egg to maturity was uninterrupted by any metamorphosis1 such as that of the frog when it passes from the tadpole stage4 ,yet in advancing from the amphibian to the reptile the evolution of the vertebrate was far from finished4 ,the cold-blooded1 clumsy and sluggish1 small- brained and unintelligent reptile is as far inferior to the higher mammals1 whose day was still to come1 as it is superior to the amphibian and the fish4 ,the reptiles of the ,permian1 the earliest known1 were much like lizards in form of body4 ,constituting a transition type between the amphibians on the one hand1 and both the higher reptiles and the mammals on the other1 they retained the archaic biconcave vertebra of the fish and in some cases the persistent notochord1 while some of them1 the theromorphs1 possessed characters allying them with mammals4 ,in these the skull was remarkably similar to that of the carnivores1 or flesh-eating mammals1 and the teeth1 unlike the teeth of any later reptiles1 were divisible into incisors1 canines1 and molars1 as are the teeth of mammals4 ,at the opening of the ,mesozoic era reptiles were the most highly organized and powerful of any animals on the earth4 ,new ranges of continental extent were opened to them1 food was abundant1 the climate was congenial1 and they now branched into very many diverse types which occupied and ruled all fields1--the land1 the air1 and the sea4 ,the ,mesozoic was the ,age of ,reptiles4 ,,the ,,ancestry ,,of ,,surviving ,,reptilian ,,types4 ,we will consider first the evolution of the few reptilian types which have survived to the present4 ,crocodiles1 the highest of existing reptiles1 are a very ancient order1 dating back to the lower ,jurassic1 and traceable to earlier ancestral1 generalized forms1 from which sprang several other orders also4 ,turtles and tortoises are not found until the early ,jurassic1 when they already possessed the peculiar characteristics which set them off so sharply from other reptiles4 ,they seem to have lived at first in shallow water and in swamps1 and it is not until after the end of the ,mesozoic that some of the order became adapted to life on the land4 ,the largest of all known turtles1 ,archelon1 whose home was the great interior ,cretaceous sea1 was fully a dozen feet in length and must have weighed at least two tons4 ,the skull alone is a yard long4 ,lizards and snakes do not appear until after the close of the ,mesozoic1 although their ancestral lines may be followed back into the ,cretaceous4 ,we will now describe some of the highly specialized orders peculiar to the ,mesozoic4 ,,land ,,reptiles4 ,the ,,dinosaurs 7terrible reptiles7 are an extremely varied order which were masters of the land from the late ,trias until the close of the ,mesozoic era4 ,some were far larger than elephants1 some were as small as cats2 some walked on all fours1 some were bipedal2 some fed on the luxuriant tropical foliage1 and others on the flesh of weaker reptiles4 ,they may be classed in three divisions1--the ,,flesh-,,eating ,,dinosaurs1 the ,,reptile-,,footed ,,dinosaurs1 and the ,,beaked ,,dinosaurs1--the latter two divisions being herbivorous4 ,the ,,flesh-,,eating ,,dinosaurs are the oldest known division of the order1 and their characteristics are shown in ,figure #cbi4 ,as a class1 reptiles are egg layers 7oviparous72 but some of the flesh- eating dinosaurs are known to have been ,,viviparous1 i4e4 to have brought forth their young alive4 ,this group was the longest-lived of any of the three1 beginning in the ,trias and continuing to the close of the ,mesozoic era4 ,contrast the small fore limbs1 used only for grasping1 with the powerful hind limbs on which the animal stalked about4 ,some of the species of this group seem to have been able to progress by leaping in kangaroo fashion4 ,notice the sharp claws1 the ponderous tail1 and the skull set at right angles with the spinal column4 ,the limb bones are hollow4 ,the ceratosaurs reached a length of some fifteen feet1 and were not uncommon in ,colorado and the western lands in ,jurassic times4 ,the ,,reptile-,,footed ,,dinosaurs 7,sauropoda7 include some of the biggest brutes which ever trod the ground4 ,one of the largest1 whose remains are found entombed in the ,jurassic rocks of ,wyoming and ,colorado1 is shown in ,figure #ccj4 ,note the five digits on the hind feet1 the quadrupedal gait1 the enormous stretch of neck and tail1 the small head aligned with the vertebral column4 ,diplodocus was fully sixty-five feet long and must have weighed about twenty tons4 ,the thigh bones of the ,sauropoda are the largest bones which ever grew4 ,that of a genus allied to the ,diplodocus measures six feet and eight inches1 and the total length of the animal must have been not far from eighty feet1 the largest land animal known4 ,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 sea4 ,the ,,beaked ,,dinosaurs7,predentata7 were distinguished by a beak sheathed with horn carried in front of the tooth-set jaw1 and used1 we may imagine1 in stripping the leaves and twigs of trees and shrubs4 ,we may notice only two of the most interesting types4 ,,stegosaurus 7plated reptile7 takes its name from the double row of bony plates arranged along its back4 ,the powerful tail was armed with long spines1 and the thick skin was defended with irregular bits of bone implanted in it4 ,the brain of the stegosaur was smaller than that of any land vertebrate1 while in the sacrum the nerve canal was enlarged to ten times the capacity of the brain cavity of the skull4 ,despite their feeble wits1 this well-armored family lived on through millions of years which intervened between their appearance1 at the opening of the ,jurassic1 and the close of the ,cretaceous1 when they became extinct4 ,a less stupid brute than the stegosaur was ,,triceratops1 the dinosaur of the three horns1--one horn carried on the nose1 and a massive pair set over the eyes4 ,note the enormous wedge-shaped skull1 with its sharp beak1 and the hood behind resembling a fireman's helmet4 ,triceratops was fully twenty-five feet long1 and of twice the bulk of an elephant4 ,the family appeared in the ,upper ,cretaceous and became extinct at its close4 ,their bones are found buried in the fresh-water deposits of the time from ,colorado to ,montana and eastward to the ,dakotas4 ,,marine ,,reptiles4 ,in the ocean1 reptiles occupied the place now held by the aquatic mammals1 such as whales and dolphins1 and their form and structure were similarly modified to suit their environment4 ,in the ,ichthyosaurus 7fish reptile71 for example1 the body was fishlike in form1 with short neck and large1 pointed head 7,fig4 #ccc74 ,a powerful tail1 whose flukes were set vertical1 and the lower one of which was vertebrated1 served as propeller1 while a large dorsal fin was developed as a cutwater4 ,the primitive biconcave vertebrae of the fish and of the early land vertebrates were retained1 and the limbs degenerated into short paddles4 ,the skin of the ichthyosaur was smooth like that of a whale1 and its food was largely fish and cephalopods1 as the fossil contents of its stomach prove4 ,these sea monsters disported along the ,pacific shore over northern ,california in ,triassic times1 and the bones of immense members of the family occur in the ,jurassic strata of ,wyoming4 ,like whales and seals1 the ichthyosaurs were descended from land vertebrates which had become adapted to a marine habitat4 ,,plesiosaurs were another order which ranged throughout the ,mesozoic4 ,descended from small amphibious animals1 they later included great marine reptiles1 characterized in the typical genus by long neck1 snakelike head1 and immense paddles4 ,they swam in the ,cretaceous interior sea of western ,north ,america4 ,,mosasaurs belong to the same order as do snakes and lizards1 and are an offshoot of the same ancestral line of land reptiles4 ,these snakelike creatures--which measured as much as forty-five feet in length--abounded in the ,cretaceous seas4 ,they had large conical teeth1 and their limbs had become stout paddles4 ,the lower jaw of the mosasaur was jointed2 the quadrate bone1 which in all reptiles connects the bone of the lower jaw with the skull1 was movable1 and as in snakes the lower jaw could be used in thrusting prey down the throat4 ,the family became extinct at the end of the ,mesozoic1 and left no descendants4 ,one may imitate the movement of the lower jaw of the mosasaur by extending the arms1 clasping the hands1 and bending the elbows4 ,,flying ,,reptiles4 ,the atmosphere1 which had hitherto been tenanted only by insects1 was first conquered by the vertebrates in the ,mesozoic4 ,pterosaurs1 winged reptiles1 whose whole organism was adapted for flight through the air1 appeared in the ,jurassic and passed off the stage of existence before the end of the ,cretaceous4 ,the bones were hollow1 as are those of birds4 ,the sternum1 or breastbone1 was given a keel for the attachment of the wing muscles4 ,the fifth finger1 prodigiously lengthened1 was turned backward to support a membrane which was attached to the body and extended to the base of the tail4 ,the other fingers were free1 and armed with sharp and delicate claws1 as shown in ,figures #ccf and #ccg4 ,these dragons of the air varied greatly in size2 some were as small as sparrows1 while others surpassed in stretch of wing the largest birds of the present day4 ,they may be divided into two groups4 ,the earliest group comprises genera with jaws set with teeth1 and with long tails sometimes provided with a rudderlike expansion at the end4 ,in their successors of the later group the tail had become short1 and in some of the genera the teeth had disappeared4 ,among the latest of the flying reptiles was ,,ornithostoma 7bird beak71 the largest creature which ever flew1 and whose remains are imbedded in the offshore deposits of the ,cretaceous sea which held sway over our western plains4 ,ornithostoma's spread of wings was twenty feet4 ,its bones were a marvel of lightness1 the entire skeleton1 even in its petrified condition1 not weighing more than five or six pounds4 ,the sharp beak1 a yard long1 was toothless and bird-like1 as its name suggests ,,birds4 ,the earliest known birds are found in the ,jurassic1 and during the remainder of the ,mesozoic they contended with the flying reptiles for the empire of the air4 ,the first feathered creatures were very different from the birds of to-day4 ,their characteristics prove them an offshoot of the dinosaur line of reptiles4 ,,archaeopteryx 7,,ancient ,,bird7 7,fig4 #cch7 exhibits a strange mingling of bird and reptile4 ,like birds1 it was fledged with perfect feathers1 at least on wings and tail1 but it retained the teeth of the reptile1 and its long tail was vertebrated1 a pair of feathers springing from each joint4 ,throughout the ,jurassic and ,cretaceous the remains of birds are far less common than those of flying reptiles1 and strata representing hundreds of thousands of years intervene between ,archaeopteryx and the next birds of which we know1 whose skeletons occur in the ,cretaceous beds of western ,kansas4 ,,mammals4 ,so far as the entries upon the geological record show1 mammals made their advent in a very humble way during the ,trias4 ,these earliest of vertebrates which suckle their young were no bigger than young kittens1 and their strong affinities with the theromorphs suggest that their ancestors are to be found among some generalized types of that order of reptiles4 ,during the long ages of the ,mesozoic1 mammals continued small and few1 and were completely dominated by the reptiles4 ,their remains are exceedingly rare1 and consist of minute scattered teeth1--with an occasional detached jaw1--which prove them to have been flesh or insect eaters4 ,in the same way their affinities are seen to be with the lowest of mammals1--the ,,monotremes and ,,marsupials4 ,the monotremes1--such as the duckbill mole and the spiny ant-eater of ,australia1 reproduce by means of eggs resembling those of reptiles2 the marsupials1 such as the opossum and the kangaroo1 bring forth their young alive1 but in a very immature condition1 and carry them for some time after birth in the marsupium1 a pouch on the ventral side of the body4 ,,chapter ,,xxi ,,the ,,tertiary ,,the ,,cenozoic ,,era4 ,the last stages of the ,cretaceous are marked by a decadence of the reptiles4 ,by the end of that period the reptilian forms characteristic of the time had become extinct one after another1 leaving to represent the class only the types of reptiles which continue to modern times4 ,the day of the ammonite and the belemnite also now drew to a close1 and only a few of these cephalopods were left to survive the period4 ,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 era1 the era of modern life1--the ,age of ,mammals4 ,in place of the giant reptiles1 mammals now become masters of the land1 appearing first in generalized types which1 during the long ages of the era1 gradually evolve to higher forms1 more specialized and ever more closely resembling the mammals of the present4 ,in the atmosphere the flying dragons of the ,mesozoic give place to birds and bats4 ,in the sea1 whales1 sharks1 and teleost fishes of modern types rule in the stead of huge swimming reptiles4 ,the lower vertebrates1 the invertebrates of land and sea1 and the plants of field and forest take on a modern aspect1 and differ little more from those of to-day than the plants and animals of different countries now differ from one another4 ,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 time4 ,the ,cenozoic era comprises two divisions1--the ,,tertiary period and the ,,quaternary period4 ,in the early days of geology the formations of the entire geological record1 so far as it was then known1 were divided into three groups1--the ,,primary1 the ,,secondary 7now known as the ,mesozoic71 and the ,,tertiary1 ,when the third group was subdivided into two systems1 the term ,tertiary was retained for the first system of the two1 while the term ,,quaternary was used to designate the second4 ,,divisions ,,of ,,the ,,tertiary4 ,the formations of the ,tertiary are grouped in three divisions1--the ,,pliocene 7more recent71 the ,,miocene 7less recent71 and the ,,eocene 7the dawn of the recent74 ,each of these epochs is long and complex4 ,their various sub- divisions are distinguished each by its own peculiar organisms1 and the changes of physical geography recorded in their strata4 ,in the rapid view which we are compelled to take we can note only a few of the most conspicuous events of the period4 ,,physical ,,geography ,,of ,,the ,,tertiary ,,in ,,eastern ,,north ,,america4 ,the ,tertiary rocks of eastern ,north ,america are marine deposits and occupy the coastal lowlands of the ,atlantic and ,gulf states 7,fig4 #bfj74 ,in ,new ,england1 ,tertiary beds occur on the island of ,martha's ,vineyard1 but not on the mainland2 hence the shore line here stood somewhat farther out than now4 ,from ,new ,jersey southward the earliest ,tertiary sands and clays1 still unconsolidated1 leave only a narrow strip of the edge of the ,cretaceous between them and the ,triassic and crystalline rocks of the ,piedmont oldland2 hence the ,atlantic shore here stood farther in than now1 and at the beginning of the period the present coastal plain was continental delta4 ,a broad belt of ,tertiary sea- laid limestones1 sandstones1 and shales surrounds the ,gulf of ,mexico and extends northward up the ,mississippi embayment to the mouth of the ,ohio ,river2 hence the ,gulf was then larger than at present1 and its waters reached in a broad bay far up the ,mississippi valley4 ,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 strata4 ,the beginning of the ,tertiary was therefore marked by a subsidence4 ,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 sediments4 ,as the period progressed slight oscillations occurred from time to time4 ,strips of coastal plain were added to the land1 and as early as the close of the ,miocene the shore lines of the ,atlantic and ,gulf states had reached well-nigh their present place4 ,louisiana and ,florida were the last areas to emerge wholly from the sea1-- ,florida being formed by a broad transverse upwarp of the continental delta at the opening of the ,miocene1 forming first an island1 which afterwards was joined to the mainland4 ,,the ,,pacific ,,coast4 ,tertiary deposits with marine fossils occur along the western foothills of the ,sierra ,nevadas1 and are crumpled among the mountain masses of the ,coast ,ranges2 it is hence inferred that the ,great ,valley of ,california was then a border sea1 separated from the ocean by a chain of mountainous islands which were upridged into the ,coast ,ranges at a still later time4 ,tertiary marine strata are spread over the lower ,columbia valley and that of ,puget ,sound1 showing that the ,pacific came in broadly there4 ,,the ,,interior ,,of ,,the ,,western ,,united ,,states4 ,the closing stages of the ,mesozoic were marked1 as we have seen1 by the upheaval of the ,rocky ,mountains and other western ranges4 ,the bases of the mountains are now skirted by widespread ,tertiary deposits1 which form the highest strata of the lofty plateaus from the level of whose summits the mountains rise4 ,like the recent alluvium of the ,great ,valley of ,california1 these deposits imply low-lying lands when they were laid1 and therefore at that time the mountains rose from near sea level4 ,but the height at which the ,tertiary strata now stand--five thousand feet above the sea at ,denver1 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 ,cenozoic4 ,during their uplift1 warping formed extensive basins both east and west of the ,rockies1 and in these basins stream-swept and lake-laid waste gathered to depths of hundreds and thousands of feet1 as it is accumulating at present in the ,great ,valley of ,california and on the river plains of ,turkestan4 ,the ,tertiary river deposits of the ,high ,plains have already been described4 ,how widespread are these ancient river plains and beds of fresh-water lakes may be seen in the map of ,figure #bfj4 ,,the ,,bad ,,lands4 ,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 ravines4 ,the deposits of these ancient river plains and lake beds are little cemented and because of the dryness of the climate are unprotected by vegetation2 hence they are easily carved by the wet-weather rills of scanty and infrequent rains4 ,these waterless1 rugged surfaces were named by the early ,french explorers the ,,bad ,,lands because they were found so difficult to traverse4 ,the strata of the ,bad ,lands contain vast numbers of the remains of the animals of ,tertiary times1 and the large amount of barren surface exposed to view makes search for fossils easy and fruitful4 ,these desolate tracts are therefore frequently visited by scientific collecting expeditions4 ,,mountain ,,making ,,in ,,the ,,tertiary4 ,the ,tertiary period included epochs when the earth's crust was singularly unquiet4 ,from time to time on all the continents subterranean forces gathered head1 and the crust was bent and broken and upridged in lofty mountains4 ,the ,sierra ,nevada range was formed1 as we have seen1 by strata crumpling at the end of the ,jurassic4 ,but since that remote time the upfolded mountains had been worn to plains and hilly uplands1 the remnants of whose uplifted erosion surfaces may now be traced along the western mountain slopes4 ,beginning late in the ,tertiary1 the region was again affected by mountain-making movements4 ,a series of displacements along a profound fault on the eastern side tilted the enormous earth block of the ,sierras to the west1 lifting its eastern edge to form the lofty crest and giving to the range a steep eastern front and a gentle descent toward the ,pacific4 ,the ,coast ,ranges also have had a complex history with many vicissitudes4 ,the earliest foldings of their strata belong to the close of the ,jurassic1 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 chain4 ,thick ,pliocene beds upon their sides prove that they were depressed to near sea level during the later ,tertiary4 ,at the close of the ,pliocene the ,coast ,ranges rose along with the upheaval of the ,sierra1 and their gradual uplift has continued to the present time4 ,the numerous north-south ranges of the ,great ,basin and the ,mount ,saint ,elias range of ,alaska were also uptilted during the ,tertiary4 ,during the ,tertiary period many of the loftiest mountains of the earth--the ,alps1 the ,apennines1 the ,pyrenees1 the ,atlas1 the ,caucasus1 and the ,himalayas--received the uplift to which they owe most of their colossal bulk and height1 as portions of the ,tertiary sea beds now found high upon their flanks attest4 ,in the ,himalayas1 ,tertiary marine limestones occur sixteen thousand five hundred feet above sea level4 ,,volcanic ,,activity ,,in ,,the ,,tertiary4 ,the vast deformations of the ,tertiary were accompanied on a corresponding scale by outpourings of lava1 the outburst of volcanoes1 and the intrusion of molten masses within the crust4 ,in the ,sierra ,nevadas the ,miocene river gravels of the valleys of the western slope1 with their placer deposits of gold1 were buried beneath streams of lava and beds of tuff4 ,volcanoes broke forth along the ,rocky ,mountains and on the plateaus of ,utah1 ,new ,mexico1 and ,arizona4 ,mount ,shasta and the immense volcanic piles of the ,cascades date from this period4 ,the mountain basin of the ,yellowstone ,park was filled to a depth of several thousand feet with tuffs and lavas1 the oldest dating as far back as the beginning of the ,tertiary4 ,crandall volcano was reared in the ,miocene and the latest eruptions of the ,park are far more recent4 ,,the ,,columbia ,,and ,,snake ,,river ,,lavas4 ,still more important is the plateau of lava1 more than two hundred thousand square miles in area1 extending from the ,yellowstone ,park to the ,cascade ,mountains1 which has been built from ,miocene times to the present4 ,over this plateau1 which occupies large portions of ,idaho1 ,washington1 and ,oregon1 and extends into northern ,california and ,nevada1 the country rock is basaltic lava4 ,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 coast4 ,the floods of molten rock spread up the mountain valleys for a score of miles and more1 the intervening spurs rising above the lava like long peninsulas1 while here and there an isolated peak was left to tower above the inundation like an island off a submerged shore4 ,the rivers which drain the plateau--the ,snake1 the ,columbia1 and their tributaries--have deeply trenched it1 yet their canyons1 which reach the depth of several thousand feet1 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 flood4 ,here and there the plateau has been deformed4 ,it has been upbent into great folds1 and broken into immense blocks of bedded lava1 forming mountain ranges1 which run parallel with the ,pacific coast line4 ,on the edges of these tilted blocks the thickness of the lava is seen to be fully five thousand feet4 ,the plateau has been built1 like that of ,iceland1 of innumerable overlapping sheets of lava4 ,on the canyon walls they weather back in horizontal terraces and long talus slopes4 ,one may distinguish each successive flow by its dense central portion1 often jointed with large vertical columns1 and the upper portion with its mass of confused irregular columns and scoriaceous surface4 ,the average thickness of the flows seems to be about seventy-five feet4 ,the plateau was long in building4 ,between the layers are found in places old soil beds and forest grounds and the sediments of lakes4 ,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 surface4 ,again and again the surface of the black basalt was reddened by oxidation and decayed to soil1 and forests had time to grow upon it before the succeeding inundation sealed the sediments and soils away beneath a sheet of stone4 ,near the edges of the lava plain1 rivers from the surrounding mountains spread sheets of sand and gravel on the surface of one flow after another4 ,these pervious sands1 interbedded with the lava1 become the aquifers of artesian wells4 ,in places the lavas rest on extensive lake deposits1 one thousand feet deep1 and ,miocene in age as their fossils prove4 ,it is to the middle ,tertiary1 then1 that the earliest flows and the largest bulk of the great inundation belong4 ,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 feed4 ,in the ,snake ,river valley the latest lavas are much younger4 ,their surfaces are so fresh and undecayed that here the effusive eruptions may well have continued to within the period of human history4 ,low lava domes like those of ,iceland mark where last the basalt outwelled and spread far and wide before it chilled 7,fig4 #cda74 ,in places small mounds of scoria show that the eruptions were accompanied to a slight degree by explosions of steam4 ,so fluid was this superheated lava that recent flows have been traced for more than fifty miles4 ,the rocks underlying the ,columbia lavas1 where exposed to view1 are seen to be cut by numerous great dikes of dense basalt1 which mark the fissures through which the molten rock rose to the surface4 ,the ,tertiary included times of widespread and intense volcanic action in other continents as well as in ,north ,america4 ,in ,europe1 ,vesuvius and ,etna began their career as submarine volcanoes in connection with earth movements which finally lifted ,pliocene deposits in ,sicily to their present height1--four thousand feet above the sea4 ,volcanoes broke forth in central ,france and southern ,germany1 in ,hungary and the ,carpathians4 ,innumerable fissures opened in the crust from the north of ,ireland and the western islands of ,scotland to the ,faroes1 ,iceland1 and even to arctic ,greenland2 and here great plateaus were built of flows of basalt similar to that of the ,columbia ,river4 ,in ,india1 at the opening of the ,tertiary1 there had been an outwelling of basalt1 flooding to a depth of thousands of feet two hundred thousand square miles of the northwestern part of the peninsula1 and similar inundations of lava occurred where are now the table-lands of ,abyssinia4 ,from the middle ,tertiary on1 ,asia ,minor1 ,arabia1 and ,persia were the scenes of volcanic action4 ,in ,palestine the rise of the uplands of ,judea at the close of the ,eocene1 and the downfaulting of the ,jordan valley were followed by volcanic outbursts4 ,in comparison with the middle ,tertiary1 the present is a time of volcanic inactivity and repose4 ,,erosion ,,of ,,tertiary ,,mountains ,,and ,,plateaus4 ,the mountains and plateaus built at various times during the ,tertiary and at its commencement have been profoundly carved by erosive agents4 ,the ,sierra ,nevada ,mountains have been dissected on the western slope by such canyons as those of ,king's ,river and the ,yosemite4 ,six miles of strata have been denuded from parts of the ,wasatch ,mountains since their rise at the beginning of the era4 ,from the ,colorado plateaus1 whose uplift dates from the same time1 there have been stripped off ten thousand feet of strata over thousands of square miles1 and the colossal canyon of the ,colorado has been cut after this great denudation had been mostly accomplished4 ,on the eastern side of the continent1 as we have seen1 a broad peneplain had been developed by the close of the ,cretaceous4 ,the remnants of this old erosion surface are now found upwarped to various heights in different portions of its area4 ,in southern ,new ,england it now stands fifteen hundred feet above the sea in western ,massachusetts1 declining thence southward and eastward to sea level at the coast4 ,in southwestern ,virginia it has been lifted to four thousand feet above the sea4 ,manifestly this upwarp occurred since the peneplain was formed2 it is later than the ,mesozoic1 and the vast dissection which the peneplain has suffered since its uplift must belong to the successive cycles of ,cenozoic time4 ,revived by the uplift1 the streams of the area trenched it as deeply as its elevation permitted1 and reaching grade1 opened up wide valleys and new peneplains in the softer rocks4 ,the ,connecticut valley is ,tertiary in age1 and in the weak ,triassic sandstones has been widened in places to fifteen miles4 ,dating from the same time are the valleys of the ,hudson1 the ,susquehanna1 the ,delaware1 the ,potomac1 and the ,shenandoah4 ,in ,pennsylvania and the states lying to the south the ,mesozoic peneplain lies along the summits of the mountain ridges4 ,on the surface of this ancient plain1 ,tertiary erosion etched out the beautifully regular pattern of the ,allegheny mountain ridges and their intervening valleys4 ,the weaker strata of the long1 regular folds were eroded into longitudinal valleys1 while the hard ,paleozoic sandstones1 such as the ,medina and the ,pocono1 were left in relief as bold mountain walls whose even crests rise to the common level of the ancient plain4 ,from ,virginia far into ,alabama the great ,appalachian valley was opened to a width in places of fifty miles and more1 along a belt of intensely folded and faulted strata where once was the heart of the ,appalachian ,mountains4 ,in ,figure #gj the summit of the ,cumberland plateau 7ab7 marks the level of the ,mesozoic peneplain1 while the lower erosion levels are ,tertiary and ,quaternary in age4 ,,life ,,of ,,the ,,tertiary ,,period ,,vegetation ,,and ,,climate4 ,the highest plants in structure1 the ,,dicotyls 7such as our deciduous forest trees7 and the ,,monocotyls 7represented by the palms71 were introduced during the ,cretaceous4 ,the vegetable kingdom reached its culmination before the animal kingdom1 and if the dividing line between the ,mesozoic and the ,cenozoic were drawn according to the progress of plant life1 the ,cretaceous instead of the ,tertiary would be made the opening period of the modern era4 ,the plants of the ,tertiary belonged1 for the most part1 to genera now living2 but their distribution was very different from that of the flora of to-day4 ,in the earlier ,tertiary1 palms flourished over northern ,europe1 and in the northwestern ,united ,states grew the magnolia and laurel1 along with the walnut1 oak1 and elm4 ,even in northern ,greenland and in ,spitzbergen there were lakes covered with water lilies and surrounded by forests of maples1 poplars1 limes1 the cypress of our southern states1 and noble sequoias similar to the big trees and redwoods of ,california4 ,a warm climate like that of the ,mesozoic1 therefore1 prevailed over ,north ,america and ,europe1 extending far toward the pole4 ,in the later ,tertiary the climate gradually became cooler4 ,palms disappeared from ,europe1 and everywhere the aspect of forests and open lands became more like that of to-day4 ,grasses became abundant1 furnishing a new food for herbivorous animals4 ,,animal ,,life ,,of ,,the ,,tertiary4 ,little needs to be said of the ,tertiary invertebrates1 so nearly were they like the invertebrates of the present4 ,even in the ,eocene1 about five per cent of marine shells were of species still living1 and in the ,pliocene the proportion had risen to more than one half4 ,fishes were of modern types4 ,teleosts were now abundant4 ,the ocean teemed with sharks1 some of them being voracious monsters seventy- five feet and even more in length1 with a gape of jaw of six feet1 as estimated by the size of their enormous sharp-edged teeth4 ,snakes are found for the first time in the early ,tertiary4 ,these limbless reptiles1 evolved by degeneration from lizardlike ancestors1 appeared in nonpoisonous types scarcely to be distinguished from those of the present day4 ,,mammals ,,of ,,the ,,early ,,tertiary4 ,the fossils of continental deposits of the earliest ,eocene show that a marked advance had now been made in the evolution of the ,mammalia4 ,the higher mammals had appeared1 and henceforth the lower mammals--the monotremes and the marsupials--are reduced to a subordinate place4 ,these first true mammals were archaic and generalized in structure4 ,their feet were of the primitive type1 with five toes of about equal length4 ,they were also ,,plantigrades1--that is1 they touched the ground with the sole of the entire foot from toe to heel4 ,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 ground1--a tread called ,,digitigrade4 ,nor was there yet any foot like that of the cats1 with sharp retractile claws adapted to seizing and tearing the prey4 ,the forearm and the lower leg each had still two separate bones 7ulna and radius1 fibula and tibia71 neither pair having been replaced with a single strong bone1 as in the leg of the horse4 ,the teeth also were primitive in type and of full number4 ,the complex heavy grinders of the horse and elephant1 the sharp cutting teeth of the carnivores1 and the cropping teeth of the grass eaters were all still to come4 ,phenacodus is a characteristic genus of the early ,eocene1 whose species varied in size from that of a bulldog to that of an animal a little larger than a sheep4 ,its feet were primitive1 and their five toes bore nails intermediate in form between a claw and a hoof4 ,the archaic type of teeth indicates that the animal was omnivorous in diet4 ,a cast of the brain cavity shows that1 like its associates of the time1 its brain was extremely small and nearly smooth1 having little more than traces of convolutions4 ,the long ages of the ,eocene and the following epochs of the ,tertiary were times of comparatively rapid evolution among the ,mammalia4 ,the earliest forms evolved along diverging lines toward the various specialized types of hoofed mammals1 rodents1 carnivores1 proboscidians1 the primates1 and the other mammalian orders as we know them now4 ,we must describe the ,tertiary mammals very briefly1 tracing the lines of descent of only a few of the more familiar mammals of the present4 ,,the ,,horse4 ,the pedigree of the horse runs back into the early ,eocene through many genera and species to a five-toed1 ,footnote3 ,or1 more accurately1 with four perfect toes and a rudimentary fifth corresponding to the thumb4 short-legged ancestor little bigger than a cat4 ,its descendants gradually increased in stature and became better and better adapted to swift running to escape their foes4 ,the leg became longer1 and only the tip of the toes struck the ground4 ,the middle toe 7digit number three71 originally the longest of the five1 steadily enlarged1 while the remaining digits dwindled and disappeared4 ,the inner digit1 corresponding to the great toe and thumb1 was the first to go4 ,next number five1 the little finger1 was also dropped4 ,by the end of the ,eocene a three-toed genus of the horse family had appeared1 as large as a sheep4 ,the hoof of digit number three now supported most of the weight1 but the slender hoofs of digits two and four were still serviceable4 ,in the ,miocene the stature of the ancestors of the horse increased to that of a pony4 ,the feet were still three-toed1 but the side hoofs were now mere dewclaws and scarcely touched the ground4 ,the evolution of the family was completed in the ,pliocene4 ,the middle toe was enlarged still more1 the side toes were dropped1 and the palm and foot bones which supported them were reduced to splints4 ,while these changes were in progress the radius and ulna of the fore limb became consolidated to a single bone2 and in the hind limb the fibula dwindled to a splint1 while the tibia was correspondingly enlarged4 ,the molars1 also gradually lengthened1 and became more and more complex on their grinding surface2 the neck became longer2 the brain steadily increased in size and its convolutions became more abundant4 ,the evolution of the horse has made for greater fleetness and intelligence4 ,,the ,,rhinoceros ,,and ,,tapir4 ,these animals1 which are grouped with the horse among the ,,odd-,,toed 7perissodactyl7 mammals1 are now verging toward extinction4 ,in the rhinoceros1 evolution seems to have taken the opposite course from that of the horse4 ,as the animal increased in size it became more clumsy1 its limbs became shorter and more massive1 and1 perhaps because of its great weight1 the number of digits were not reduced below the number three4 ,like other large herbivores1 the rhinoceros1 too slow to escape its enemies by flight1 learned to withstand them4 ,it developed as its means of defense a nasal horn4 ,peculiar offshoots of the line appeared at various times in the ,tertiary4 ,a rhinoceros1 semiaquatic in habits1 with curved tusks1 resembling in aspect the hippopotamus1 lived along the water courses of the plains east of the ,rockies1 and its bones are now found by the thousands in the ,miocene of ,kansas4 ,another developed along a line parallel to that of the horse1 and herds of these light-limbed and swift-footed running rhinoceroses ranged the ,great ,plains from the ,dakotas southward4 ,the tapirs are an ancient family which has changed but little since it separated from the other perissodactyl stocks in the early ,tertiary4 ,at present1 tapirs are found only in ,south ,america and southern ,asia1--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 hemisphere1 making their way to ,south ,america late in that period4 ,during the ,pleistocene they became extinct over all the intervening lands between the widely separated regions where now they live4 ,the geographic distribution of animals1 as well as their relationships and origins1 can be understood only through a study of their geological history4 ,,the ,,proboscidians4 ,this unique order of hoofed mammals1 of which the elephant is the sole survivor1 began1 so far as known1 in the ,eocene1 in ,egypt1 with a piglike ancestor the size of a small horse1 with cheek teeth like the ,mastodon's1 but wanting both trunk and tusks4 ,a proboscidian came next with four short tusks1 and in the ,miocene there followed a ,mastodon 7,fig4 #cdf7 armed with two pairs of long1 straight tusks on which rested a flexible proboscis4 ,the ,,dinothere was a curious offshoot of the line1 which developed in the ,miocene in ,europe4 ,in this immense proboscidian1 whose skull was three feet long1 the upper pair of tusks had disappeared1 and those of the lower jaw were bent down with a backward curve in walrus fashion4 ,in the true ,,elephants1 which do not appear until near the close of the ,tertiary1 the lower jaw loses its tusks and the grinding teeth become exceedingly complex in structure4 ,the grinding teeth of the mastodon had long roots and low crowns crossed by four or five peaked enameled ridges4 ,in the teeth of the true elephants the crown has become deep1 and the ridges of enamel have changed to numerous upright1 platelike folds1 their interspaces filled with cement4 ,the two genera--,mastodon and ,elephant--are connected by species whose teeth are intermediate in pattern4 ,the proboscidians culminated in the ,pliocene1 when some of the giant elephants reached a height of fourteen feet4 ,,the ,,artiodactyls comprise the hoofed ,mammalia which have an even number of toes1 such as cattle1 sheep1 and swine4 ,like the perissodactyls1 they are descended from the primitive five-toed plantigrade mammals of the lowest ,eocene4 ,in their evolution1 digit number one was first dropped1 and the middle pair became larger and more massive1 while the side digits1 numbers two and five1 became shorter1 weaker1 and less serviceable4 ,the ,,four-,,toed ,,artiodactyls culminated in the ,tertiary2 at present they are represented only by the hippopotamus and the hog4 ,along the main line of the evolution of the artiodactyls the side toes1 digits two and five1 disappeared1 leaving as proof that they once existed the corresponding bones of palm and sole as splints4 ,the ,,two-,,toed ,,artiodactyls1 such as the camels1 deer1 cattle1 and sheep1 are now the leading types of the herbivores4 ,,swine ,,and ,,peccaries are two branches of a common stock1 the first developing in the ,old ,world and the second in the ,new4 ,in the ,miocene a noticeable offshoot of the line was a gigantic piglike brute1 a root eater1 with a skull a yard in length1 whose remains are now found in ,colorado and ,south ,dakota4 ,,camels ,,and ,,llamas4 ,the line of camels and llamas developed in ,north ,america1 where the successive changes from an early ,eocene ancestor1 no larger than a rabbit1 are traced step by step to the present forms1 as clearly as is the evolution of the horse4 ,in the late ,miocene some of the ancestral forms migrated to the ,old ,world by way of a land connection where ,bering ,strait now is1 and there gave rise to the camels and dromedaries4 ,others migrated into ,south ,america1 which had now been connected with our own continent1 and these developed into the llamas and guanacos1 while those of the race which remained in ,north ,america became extinct during the ,pleistocene4 ,some peculiar branches of the camel stem appeared in ,north ,america4 ,in the ,pliocene arose a llama with the long neck and limbs of a giraffe1 whose food was cropped from the leaves and branches of trees4 ,far more generalized in structure was the ,oreodon1 an animal related to the camels1 but with distinct affinities also with other lines1 such as those of the hog and deer4 ,these curious creatures were much like the peccary in appearance1 except for their long tails4 ,in the middle ,eocene they roamed in vast herds from ,oregon to ,kansas and ,nebraska4 ,,the ,,ruminants4 ,this division of the artiodactyls includes antelopes1 deer1 oxen1 bison1 sheep1 and goats1--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 camels4 ,in the ,miocene the evolution of the two-toed artiodactyl foot was well-nigh completed4 ,bonelike growths appeared on the head1 and the two groups of the ruminants became specialized1--the deer with bony antlers1 shed and renewed each year1 and the ruminants with hollow horns1 whose two bony knobs upon the skull are covered with permanent1 pointed1 horny sheaths4 ,the ruminants evolved in the ,old ,world1 and it was not until the later ,miocene that the ancestors of the antelope and of some deer found their way to ,north ,america4 ,mountain sheep and goats1 the bison and most of the deer1 did not arrive until after the close of the ,tertiary1 and sheep and oxen were introduced by man4 ,the hoofed mammals of the ,tertiary included many offshoots from the main lines which we have traced4 ,among them were a number of genera of clumsy1 ponderous brutes1 some almost elephantine in their bulk4 ,,the ,,carnivores4 ,the ancestral lines of the families of the flesh eaters--such as the cats 7lions1 tigers1 etc471 the bears1 the hyenas1 and the dogs 7including wolves and foxes7--converge in the creodonts of the early ,eocene1--an order so generalized that it had affinities not only with the carnivores but also with the insect eaters1 the marsupials1 and the hoofed mammals as well4 ,from these primitive flesh eaters1 with small and simple brains1 numerous small teeth1 and plantigrade tread1 the different families of the carnivores of the present have slowly evolved4 ,,dogs ,,and ,,bears4 ,the dog family diverged from the creodonts late in the ,eocene1 and divided into two branches1 one of which evolved the wolves and the other the foxes4 ,an offshoot gave rise to the family of the bears1 and so closely do these two families1 now wide apart1 approach as we trace them back in ,tertiary times that the ,amphicyon1 a genus doglike in its teeth and bearlike in other structures1 is referred by some to the dog and by others to the bear family4 ,the well-known plantigrade tread of bears is a primitive characteristic which has survived from their creodont ancestry4 ,,cats4 ,the family of the cats1 the most highly specialized of all the carnivores1 divided in the ,tertiary into two main branches4 ,one1 the saber-tooth tigers 7,fig4 #cea71 which takes its name from their long1 saberlike1 sharp-edged upper canine teeth1 evolved a succession of genera and species1 among them some of the most destructive beasts of prey which ever scourged the earth4 ,they were masters of the entire northern hemisphere during the middle ,tertiary1 but in ,europe during the ,pliocene they declined1 from unknown causes1 and gave place to the other branch of cats1--which includes the lions1 tigers1 and leopards4 ,in the ,americas the saber-tooth tigers long survived the epoch4 ,,marine ,,mammals4 ,the carnivorous mammals of the sea--whales1 seals1 walruses1 etc4--seem to have been derived from some of the creodonts of the early ,tertiary by adaptation to aquatic life4 ,whales evolved from some land ancestry at a very early date in the ,tertiary2 in the marine deposits of the ,eocene are found the bones of the ,zeuglodon1 a whalelike creature seventy feet in length4 ,,primates4 ,this order1 which includes lemurs1 monkeys1 apes1 and man1 seems to have sprung from a creodont or insectivorous ancestry in the lower ,eocene4 ,lemur-like types1 with small1 smooth brains1 were abundant in the ,united ,states in the early ,tertiary1 but no primates have been found here in the middle ,tertiary and later strata4 ,in ,europe true monkeys were introduced in the ,miocene1 and were abundant until the close of the ,tertiary1 when they were driven from the continent by the increasing cold4 ,,advance ,,of ,,the ,,mammalia ,,during ,,the ,,tertiary4 ,during the several millions of years comprised in ,tertiary time the mammals evolved from the lowly1 simple types which tenanted the earth at the beginning of the period1 into the many kinds of highly specialized mammals of the ,pleistocene and the present1 each with the various structures of the body adapted to its own peculiar mode of life4 ,the swift feet of the horse1 the horns of cattle and the antlers of the deer1 the lion's claws and teeth1 the long incisors of the beaver1 the proboscis of the elephant1 were all developed in ,tertiary times4 ,in especial the brain of the ,tertiary mammals constantly grew larger relatively to the size of body1 and the higher portion of the brain--the cerebral lobes--increased in size in comparison with the cerebellum4 ,some of the hoofed mammals now have a brain eight or ten times the size of that of their early ,tertiary predecessors of equal bulk4 ,nor can we doubt that along with the increasing size of brain went a corresponding increase in the keenness of the senses1 in activity and vigor1 and in intelligence4 ,,chapter ,,xxii ,,the ,,quaternary ,the last period of geological history1 the ,quaternary1 may be said to have begun when all1 or nearly all1 living species of mollusks and most of the existing mammals had appeared4 ,it is divided into two great epochs4 ,the first1 the ,pleistocene or ,glacial epoch1 is marked off from the ,tertiary by the occupation of the northern parts of ,north ,america and ,europe by vast ice sheets2 the second1 the ,recent epoch1 began with the disappearance of the ice sheets from these continents1 and merges into the present time4 ,,the ,,pleistocene ,,epoch ,we now come to an episode of unusual interest1 so different was it from most of the preceding epochs and from the present1 and so largely has it influenced the conditions of man's life4 ,the records of the ,glacial epoch are so plain and full that we are compelled to believe what otherwise would seem almost incredible1 --that following the mild climate of the ,tertiary came a succession of ages when ice fields1 like that of ,greenland1 shrouded the northern parts of ,north ,america and ,europe and extended far into temperate latitudes4 ,,the ,,drift4 ,our studies of glaciers have prepared us to decipher and interpret the history of the ,glacial epoch1 as it is recorded in the surface deposits known as the drift4 ,over most of ,canada and the northern states this familiar formation is exposed to view in nearly all cuttings which pass below the surface soil4 ,the drift includes two distinct classes of deposits1--the unstratified drift laid down by glacier ice1 and the stratified drift spread by glacier waters4 ,the materials of the drift are in any given place in part unlike the rock on which it rests4 ,they cannot be derived from the underlying rock by weathering1 but have been brought from elsewhere4 ,thus where a region is underlain by sedimentary rocks1 as is the drift-covered area from the ,hudson ,river to the ,missouri1 the drift contains not only fragments of limestone1 sandstone1 and shale of local derivation1 but also pebbles of many igneous and metamorphic rocks1 such as granites1 gneisses1 schists1 dike rocks1 quartzites1 and the quartz of mineral veins1 whose nearest source is the ,archean area of ,canada and the states of our northern border4 ,the drift received its name when it was supposed that the formation had been drifted by floods and icebergs from outside sources1--a theory long since abandoned4 ,the distribution also of the drift points clearly to its peculiar origin4 ,within the limits of the glaciated area it covers the country without regard to the relief1 mantling with its debris not only lowlands and valleys but also highlands and mountain slopes4 ,the boundary of the drift is equally independent of the relief of the land1 crossing hills and plains impartially1 unlike water-laid deposits1 whose margins1 unless subsequently deformed1 are horizontal4 ,the boundary of the drift is strikingly lobate also1 bending outward in broad1 convex curves1 where there are no natural barriers in the topography of the country to set it such a limit4 ,under these conditions such a lobate margin cannot belong to deposits of rivers1 lakes1 or ocean1 but is precisely that which would mark the edge of a continental glacier which deployed in broad tongues of ice4 ,,the ,,rock ,,surface ,,underlying ,,the ,,drift4 ,over much of its area the drift rests on firm1 fresh rock1 showing that both the preglacial mantle of residual waste and the partially decomposed and broken rock beneath it have been swept away4 ,the underlying rock1 especially if massive1 hard1 and of a fine grain1 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 back4 ,frequently it has been worn to the smooth1 rounded hummocks known as roches moutonnees1 and even rocky hills have been thus smoothed to flowing outlines like roches moutonnees on a gigantic scale4 ,the rock pavement beneath the drift is also marked by long1 straight1 parallel scorings1 varying in size from deep grooves to fine striae as delicate as the hair lines cut by an engraver's needle4 ,where the rock is soft or closely jointed it is often shattered to a depth of several feet beneath the drift1 while stony clay has been thrust in among the fragments into which the rock is broken4 ,in the presence of these glaciated surfaces we cannot doubt that the area of the drift has been overridden by vast sheets of ice which1 in their steady flow1 rasped and scored the rock bed beneath by means of the stones with which their basal layers were inset1 and in places plucked and shattered it4 ,,till4 ,the unstratified portion of the drift consists chiefly of sheets of dense1 stony clay called till1 which clearly are the ground moraines of ancient continental glaciers4 ,till is an unsorted mixture of materials of all sizes1 from fine clay and sand1 gravel1 pebbles1 and cobblestones1 to large bowlders4 ,the stones of the till are of many kinds1 some having been plucked from the bed rock of the locality where they are found1 and others having been brought from outside and often distant places4 ,land ice is the only agent known which can spread unstratified material in such extensive sheets4 ,the ,,fine ,,material of the till comes from two different sources4 ,in part it is derived from old residual clays1 which in the making had been leached of the lime and other soluble ingredients of the rock from which they weathered4 ,in part it consists of sound rock ground fine2 a drop of acid on fresh1 clayey till often proves by brisk effervescence that the till contains much undecayed limestone flour4 ,the ice sheet1 therefore1 both scraped up the mantle of long-weathered waste which covered the coun try before its coming1 and also ground heavily upon the sound rock underneath1 and crushed and wore to rock flour the fragments which it carried4 ,the color of unweathered till depends on that of the materials of which it is composed4 ,where red sandstones have contributed largely to its making1 as over the ,triassic sandstones of the eastern states and the ,algonkian sandstones about ,lake ,superior1 the drift is reddish4 ,when derived in part from coaly shales1 as over many outcrops of the ,pennsylvanian1 it may when moist be almost black4 ,fresh till is normally a dull gray or bluish1 so largely is it made up of the grindings of unoxidized rocks of these common colors4 ,except where composed chiefly of sand or coarser stuff1 unweathered till is often exceedingly dense4 ,can you suggest by what means it has been thus compacted8 ,did the ice fields of the ,glacial epoch bear heavy surface moraines like the medial and lateral moraines of valley glaciers8 ,where was the greater part of the load of these ice fields carried1 judging from what you know of the glaciers of ,greenland8 ,,bowlders ,,of ,,the ,,drift4 ,the pebbles and bowlders of the drift are in part stream gravels1 bowlders of weathering1 and other coarse rock waste picked up from the surface of the country by the advancing ice1 and in part are fragments plucked from ledges of sound rock after the mantle of waste had been removed4 ,many of the stones of the till are dressed as only glacier ice can do2 their sharp edges have been blunted and their sides faceted and scored4 ,we may easily find all stages of this process represented among the pebbles of the till4 ,some are little worn1 even on their edges2 some are planed and scored on one side only2 while some in their long journey have been ground down to many facets and have lost much of their original bulk4 ,evidently the ice played fast and loose with a stone carried in its basal layers1 now holding it fast and rubbing it against the rock beneath1 now loosening its grasp and allowing the stone to turn4 ,bowlders of the drift are sometimes found on higher ground than their parent ledges4 ,thus bowlders have been left on the sides of ,mount ,katahdin1 ,maine1 which were plucked from limestone ledges twelve miles distant and three thousand feet lower than their resting place4 ,in other cases stones have been carried over mountain ranges1 as in ,vermont1 where pebbles of ,burlington red sandstone were dragged over the ,green ,mountains1 three thousand feet in height1 and left in the ,connecticut valley sixty miles away4 ,no other geological agent than glacier ice could do this work4 ,the bowlders of the drift are often large4 ,bowlders ten and twenty feet in diameter are not uncommon1 and some are known whose diameter exceeds fifty feet4 ,as a rule the average size of bowlders decreases with increasing distance from their sources4 ,why8 ,,till ,,plains4 ,the surface of the drift1 where left in its initial state1 also displays clear proof of its glacial origin4 ,over large areas it is spread in level plains of till1 perhaps bowlder- dotted1 similar to the plains of stony clay left in ,spitzbergen by the recent retreat of some of the glaciers of that island4 ,in places the unstratified drift is heaped in hills of various kinds1 which we will now describe4 ,,drumlins4 ,drumlins are smooth1 rounded hills composed of till1 elliptical in base1 and having their longer axes parallel to the movement of the ice as shown by glacial scorings4 ,they crowd certain districts in central ,new ,york and in southern ,wisconsin1 where they may be counted by the thousands4 ,among the numerous drumlins about ,boston is historic ,bunker ,hill4 ,drumlins are made of ground moraine4 ,they were accumulated and given shape beneath the overriding ice1 much as are sand bars in a river1 or in some instances were carved1 like roches moutonnees1 by an ice sheet out of the till left by an earlier ice invasion4 ,,terminal ,,moraines4 ,the glaciated area is crossed by belts of thickened drift1 often a mile or two1 and sometimes even ten miles and more1 in breadth1 which lie transverse to the movement of the ice and clearly are the terminal moraines of ancient ice sheets1 marking either the limit of their farthest advance or pauses in their general retreat4 ,the surface of these moraines is a jumble of elevations and depressions1 which vary from low1 gentle swells and shallow sags to sharp hills1 a hundred feet or so in height1 and deep1 steep- sided hollows4 ,such tumultuous hills and hummocks1 set with depressions of all shapes1 which usually are without outlet and are often occupied by marshes1 ponds1 and lakes1 surely cannot be the work of running water4 ,the hills are heaps of drift1 lodged beneath the ice edge or piled along its front4 ,the basins were left among the tangle of morainic knolls and ridges as the margin of the ice moved back and forth4 ,some bowl-shaped basins were made by the melting of a mass of ice left behind by the retreating glacier and buried in its debris4 ,,the ,,stratified ,,drift4 ,like modern glaciers the ice sheets of the ,pleistocene were ever being converted into water about their margins4 ,their limits on the land were the lines where their onward flow was just balanced by melting and evaporation4 ,on the surface of the ice along the marginal zone1 rivulets no doubt flowed in summer1 and found their way through crevasses to the interior of the glacier or to the ground4 ,subglacial streams1 like those of the ,malaspina glacier1 issued from tunnels in the ice1 and water ran along the melting ice front as it is seen to do about the glacier tongues of ,greenland4 ,all these glacier waters flowed away down the chief drainage channels in swollen rivers loaded with glacial waste4 ,it is not unexpected therefore that there are found1 over all the country where the melting ice retreated1 deposits made of the same materials as the till1 but sorted and stratified by running water4 ,some of these were deposited behind the ice front in ice-walled channels1 some at the edge of the glaciers by issuing streams1 and others were spread to long distances in front of the ice edge by glacial waters as they flowed away4 ,,eskers are narrow1 winding ridges of stratified sand and gravel whose general course lies parallel with the movement of the glacier4 ,these ridges1 though evidently laid by running water1 do not follow lines of continuous descent1 but may be found to cross river valleys and ascend their sides4 ,hence the streams by which eskers were laid did not flow unconfined upon the surface of the ground4 ,we may infer that eskers were deposited in the tunnels and ice-walled gorges of glacial streams before they issued from the ice front4 ,,kames are sand and gravel knolls1 associated for the most part with terminal moraines1 and heaped by glacial waters along the margin of the ice4 ,,kame ,,terraces are hummocky embankments of stratified drift sometimes found in rugged regions along the sides of valleys4 ,in these valleys long tongues of glacier ice lay slowly melting4 ,glacial waters took their way between the edges of the glaciers and the hillside1 and here deposited sand and gravel in rude terraces4 ,outwash plains are plains of sand and gravel which frequently border terminal moraines on their outward face1 and were spread evidently by outwash from the melting ice4 ,outwash plains are sometimes pitted by bowl-shaped basins where ice blocks were left buried in the sand by the retreating glacier4 ,valley trains are deposits of stratified drift with which river valleys have been aggraded4 ,valleys leading outward from the ice front were flooded by glacial waters and were filled often to great depths with trains of stream-swept drift4 ,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 terraces4 ,valley trains head in morainic plains1 and their material grows finer down valley and coarser toward their sources4 ,their gradient is commonly greater than that of the present rivers4 ,,the ,,extent ,,of ,,the ,,drift4 ,the extent of the drift of ,north ,america and its southern limits are best seen in ,figure #cei4 ,its area is reckoned at about four million square miles4 ,the ice fields which once covered so much of our continent were all together ten times as large as the inland ice of ,greenland1 and about equal to the enormous ice cap which now covers the antartic regions4 ,the ice field of ,europe was much smaller1 measuring about seven hundred and seventy thousand square miles4 ,,centers ,,of ,,dispersion4 ,the direction of the movement of the ice is recorded plainly in the scorings of the rock surface1 in the shapes of glaciated hills1 in the axes of drumlins and eskers1 and in trains of bowlders1 when the ledges from which they were plucked can be discovered4 ,in these ways it has been proved that in ,north ,america there were three centers where ice gathered to the greatest depth1 and from which it flowed in all directions outward4 ,there were thus three vast ice fields1--one the ,cordilleran1 which lay upon the ,cordilleras of ,british ,america2 one the ,keewatin1 which flowed out from the province of ,keewatin1 west of ,hudson ,bay2 and one the ,,labrador ice field1 whose center of dispersion was on the highlands of the peninsula of ,labrador4 ,as shown in ,figure #cei1 the western ice field extended but a short way beyond the eastern foothills of the ,rocky ,mountains1 where perhaps it met the far-traveled ice from the great central field4 ,the ,keewatin and the ,labrador ice fields flowed farthest toward the south1 and in the ,mississippi valley the one reached the mouth of the ,missouri and the other nearly to the mouth of the ,ohio4 ,in ,minnesota and ,wisconsin and northward they merged in one vast field4 ,the thickness of the ice was so great that it buried the highest mountains of eastern ,north ,america1 as is proved by the transported bowlders which have been found upon their summits4 ,if the land then stood at its present height above sea level1 and if the average slope of the ice were no more than ten feet to the mile1--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 ,greenland1--the ice plateaus about ,hudson ,bay must have reached a thickness of at least ten thousand feet4 ,in ,europe the ,scandinavian plateau was the chief center of dispersion4 ,at the time of greatest glaciation a continuous field of ice extended from the ,ural ,mountains to the ,atlantic1 where1 off the coasts of ,norway and the ,british ,isles1 it met the sea in an unbroken ice wall4 ,on the south it reached to southern ,england1 ,belgium1 and central ,germany1 and deployed on the eastern plains in wide lobes over ,poland and central ,russia 7,fig4 #cfj74 ,at the same time the ,alps supported giant glaciers many times the size of the surviving glaciers of to-day1 and a piedmont glacier covered the plains of northern ,switzerland4 ,,the ,,thickness ,,of ,,the ,,drift4 ,the drift is far from uniform in thickness4 ,it is comparatively thin and scanty over the ,laurentian highlands and the rugged regions of ,new ,england1 while from southern ,new ,york and ,ontario westward over the ,mississippi valley1 and on the great western plains of ,canada1 it exceeds an average of one hundred feet over wide areas1 and in places has five and six times that thickness4 ,it was to this marginal belt that the ice sheets brought their loads1 while northwards1 nearer the centers of dispersion1 erosion was excessive and deposition slight4 ,,successive ,,ice ,,invasions ,,and ,,their ,,drift ,,sheets4 ,recent studies of the drift prove that it does not consist of one indivisible formation1 but includes a number of distinct drift sheets1 each with its own peculiar features4 ,the ,pleistocene epoch consisted1 therefore1 of several glacial stages1--during each of which the ice advanced far southward1--together with the intervening interglacial stages when1 under a milder climate1 the ice melted back toward its sources or wholly disappeared4 ,the evidences of such interglacial stages1 and the means by which the different drift sheets are told apart1 are illustrated in ,figure #cfa4 ,here the country from ,n to ,s is wholly covered by drift1 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 stage4 ,the former drift is very young1 for its drainage is as yet immature1 and there are many lakes and marshes upon its surface2 the latter is far older1 for its surface has been thoroughly dissected by its streams4 ,the former is but slightly weathered1 while the latter is so old that it is deeply reddened by oxidation and is leached of its soluble ingredients such as lime4 ,the younger drift is bordered by a distinct terminal moraine1 while the margin of the older drift is not thus marked4 ,moreover1 the two drift sheets are somewhat unlike in composition1 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 regions4 ,again1 in places beneath the younger drift there is found the buried land surface of an older drift with old soils1 forest grounds1 and vegetable deposits1 containing the remains of animals and plants1 which tell of the climate of the interglacial stage in which they lived4 ,by such differences as these the following drift sheets have been made out in ,america1 and similar subdivisions have been recognized in ,europe4 #e ,the ,wisconsin formation #d ,the ,iowan formation #c ,the ,illinoian formation #b ,the ,kansan formation #a ,the pre-,kansan or ,jerseyan formation ,in ,new ,jersey and ,pennsylvania the edge of a deeply weathered and eroded drift sheet1 the ,jerseyan1 extends beyond the limits of a much younger overlying drift4 ,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 peat1 old soil1 and gravel beds4 ,the two succeeding stages mark the greatest snowfall of the ,glacial epoch4 ,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 ,kansas4 ,in the ,illinoian stage the ,labrador ice field stretched from ,hudson ,straits nearly to the ,ohio ,river in ,illinois4 ,in the ,iowan and the ,wisconsin1 the closing stages of the ,glacial epoch1 the readvancing ice fields fell far short of their former limits in the ,mississippi valley1 but in the eastern states the ,labrador ice field during ,wisconsin times overrode for the most part all earlier deposits1 and1 covering ,new ,england1 probably met the ocean in a continuous wall of ice which set its bergs afloat from ,massachusetts to northern ,labrador4 ,we select for detailed description the ,kansan and the ,wisconsin formations as representatives1 the one of the older and the other of the younger drift sheets4 ,,the ,,kansan ,,formation4 ,the ,kansan drift consists for the most part of a sheet of clayey till carrying smaller bowlders than the later drift4 ,few traces of drumlins1 kames1 or terminal moraines are found upon the ,kansan drift1 and where thick enough to mask the preexisting surface1 it seems to have been spread originally in level plains of till4 ,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 divides1 which still rise to the ancient level4 ,the valleys of the larger streams have been opened wide4 ,their well- developed tributaries have carved nearly the entire plain to valley slopes 7,figs4 #ej ,b1 and #ei74 ,the lakes and marshes which once marked the infancy of the region have long since been effaced4 ,the drift is also deeply weathered4 ,the till1 originally blue in color1 has been yellowed by oxidation to a depth of ten and twenty feet and even more1 and its surface is sometimes rusted to terra-cotta red4 ,to a somewhat less depth it has been leached of its lime and other soluble ingredients4 ,in the weathered zone its pebbles1 especially where the till is loose in texture1 are sometimes so rotted that granites may be crumbled with the fingers4 ,the ,kansan drift is therefore old4 ,,the ,,wisconsin ,,formation4 ,the ,wisconsin drift sheet is but little weathered and eroded1 and therefore is extremely young4 ,oxidation has effected it but slightly1 and lime and other soluble plant foods remain undissolved even at the grass roots4 ,its river systems are still in their infancy 7,fig4 #ej1 ,a74 ,swamps and peat bogs are abundant on its undrained surface1 and to this drift sheet belong the lake lands of our northern states and of the ,laurentian peneplain of ,canada4 ,the lake basins of the ,wisconsin drift are of several different classes4 ,many are shallow sags in the ground moraine4 ,still more numerous are the lakes set in hollows among the hills of the terminal moraines2 such as the thousands of lakelets of eastern ,massachusetts4 ,indeed1 the terminal moraines of the ,wisconsin drift may often be roughly traced on maps by means of belts of lakes and ponds4 ,some lakes are due to the blockade of ancient valleys by morainic delms1 and this class includes many of the lakes of the ,adirondacks1 the mountain regions of ,new ,england1 and the ,laurentian area4 ,still other lakes rest in rock basins scooped out by glaciers4 ,in many cases lakes are due to more than one cause1 as where preglacial valleys have both been basined by the ice and blockaded by its moraines4 ,the ,finger lakes of ,new ,york1 for example1 occupy such glacial troughs4 ,massive ,,terminal ,,moraines1 which mark the farthest limits to which the ,wisconsin ice advanced1 have been traced from ,cape ,cod and the islands south of ,new ,england1 across the ,appalachians and the ,mississippi valley1 through the ,dakotas1 and far to the north over the plains of ,british ,america4 ,where the ice halted for a time in its general retreat1 it left ,,recessional ,,moraines1 as this variety of the terminal moraine is called4 ,the moraines of the ,wisconsin drift lie upon the country like great festoons1 each series of concentric loops marking the utmost advance of broad lobes of the ice margin and the various pauses in their recession4 ,behind the terminal moraines lie wide till plains1 in places studded thickly with drumlins1 or ridged with an occasional esker4 ,great outwash plains of sand and gravel lie in front of the moraine belts1 and long valley trains of coarse gravels tell of the swift and powerful rivers of the time4 ,,the ,,loess ,,of ,,the ,,mississippi ,,valley4 ,a yellow earth1 quite like the loess of ,china1 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 drift4 ,much of the loess was deposited in ,iowan times4 ,it is younger than the earlier drift sheets1 for it overlies their weathered and eroded surfaces4 ,it thickens to the ,iowan drift border1 but is not found upon that drift4 ,it is older than the ,wisconsin1 for in many places it passes underneath the ,wisconsin terminal moraines4 ,in part the loess seems to have been washed from glacial waste and spread in sluggish glacial waters1 and in part to have been distributed by the wind from plains of aggrading glacial streams4 ,,the ,,effects ,,of ,,the ,,ice ,,invasions ,,on ,,rivers4 ,the repeated ice invasions of the ,pleistocene profoundly disarranged the drainage systems of our northern states4 ,in some regions the ancient valleys were completely filled with drift4 ,on the withdrawal of the ice the streams were compelled to find their way1 as best they could1 over a fresh land surface1 where we now find them flowing on the drift in young1 narrow channels4 ,but hundreds of feet below the ground the well driller and the prospector for coal and oil discover deep1 wide1 buried valleys cut in rock1--the channels of preglacial and interglacial streams4 ,in places the ancient valleys were filled with drift to a depth of a hundred feet1 and sometimes even to a depth of four hundred and five hundred feet4 ,in such valleys1 rivers now flow high above their ancient beds of rock on floors of valley drift4 ,many of the valleys of our present rivers are but patchworks of preglacial1 interglacial1 and postglacial courses 7,fig4 #cff74 ,here the river winds along an ancient valley with gently sloping sides and a wide alluvial floor perhaps a mile or so in width1 and there it enters a young1 rock-walled gorge1 whose rocky bed may be crossed by ledges over which the river plunges in waterfalls and rapids4 ,in such cases it is possible that the river was pushed to one side of its former valley by a lobe of ice1 and compelled to cut a new channel in the adjacent uplands4 ,a section of the valley may have been blockaded with morainic waste1 and the lake formed behind the barrier may have found outlet over the country to one side of the ancient drift-filled valley4 ,in some instances it would seem that during the waning of the ice sheets1 glacial streams1 while confined within walls of stagnant ice1 cut down through the ice and incised their channels on the underlying country1 in some cases being let down on old river courses1 and in other cases excavating gorges in adjacent uplands4 ,,pleistocene ,,lakes4 ,temporary lakes were formed wherever the ice front dammed the natural drainage of the region4 ,some1 held in the minor valleys crossed by ice lobes1 were small1 and no doubt many were too short-lived to leave lasting records4 ,others1 long held against the northward sloping country by the retreating ice edge1 left in their beaches their clayey beds1 and their outlet channels permanent evidences of their area and depth4 ,some of these glacial lakes are thus known to have been larger than any present lake4 ,lake ,agassiz1 named in honor of the author of the theory of continental glaciation1 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 ,winnipeg4 ,from first to last ,lake ,agassiz covered a hundred and ten thousand square miles in ,manitoba and the adjacent parts of ,minnesota and ,north ,dakota1--an area larger than all the ,great ,lakes combined4 ,it discharged its waters across the divide which held it on the south1 and thus excavated the valley of the ,minnesota ,river4 ,the lake bed--a plain of till--was spread smooth and level as a floor with lacustrine silts4 ,since ,lake ,agassiz vanished with the melting back of the ice beyond the outlet by the ,nelson ,river into ,hudson ,bay1 there has gathered on its floor a deep humus1 rich in the nitrogenous elements so needful for the growth of plants1 and it is to this soil that the region owes its well-known fertility4 ,,the ,,great ,,lakes4 ,the basins of the ,great ,lakes are broad preglacial river valleys1 warped by movements of the crust still in progress1 enlarged by the erosive action of lobes of the continental ice sheets1 and blockaded by their drift4 ,the complicated glacial and postglacial history of the lakes is recorded in old strand lines which have been traced at various heights about them1 showing their areas and the levels at which their waters stood at different times4 ,with the retreat of the lobate ,wisconsin ice sheet toward the north and east1 the southern and western ends of the basins of the ,great ,lakes were uncovered first2 and here1 between the receding ice front and the slopes of land which faced it1 lakes gathered which increased constantly in size4 ,the lake which thus came to occupy the western end of the ,lake ,superior basin discharged over the divide at ,duluth down the ,st4 ,croix ,river1 as an old outlet channel proves2 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 ,mississippi2 the lake which covered the lowlands about the western end of ,lake ,erie discharged its waters at ,fort ,wayne into the ,wabash ,river4 ,the ice still blocked the ,mohawk and ,st4 ,lawrence valleys on the east1 while on the west it had retreated far to the north4 ,the lakes become confluent in wide expanses of water1 whose depths and margins1 as shown by their old lake beaches1 varied at different times with the position of the confining ice and with warpings of the land4 ,these vast water bodies1 which at one or more periods were greater than all the ,great ,lakes combined1 discharged at various times across the divide at ,chicago1 near ,syracuse1 ,new ,york1 down the ,mohawk valley1 and by a channel from ,georgian ,bay into the ,ottawa ,river4 ,last of all the present outlet by the ,st4 ,lawrence was established4 ,the beaches of the glacial lakes just mentioned are now far from horizontal4 ,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 northeast1 proving here a differential movement of the land since glacial times amounting to more than three hundred feet4 ,the beaches which mark the successive heights of these glacial lakes are not parallel2 hence the warping began before the ,glacial epoch closed4 ,we have already seen that the canting of the region is still in progress4 ,,the ,,champlain ,,subsidence4 ,as the ,glacial epoch approached its end1 and the ,labrador ice field melted back for the last time to near its source1 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 ,st4 ,lawrence valley4 ,it joined with ,lake ,ontario1 and extending down the ,champlain and ,hudson valleys1 made an island of ,new ,england and the maritime provinces of ,canada4 ,the proofs of this subsidence are found in old sea beaches and sea-laid clays resting on ,wisconsin till4 ,at ,montreal such terraces are found six hundred and twenty feet above sea level1 and along ,lake ,champlain--where the skeleton of a whale was once found among them--at from five hundred to four hundred feet4 ,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 level4 ,the clays of the ,champlain subsidence pass under water near the mouth of the ,hudson1 and in northern ,new ,jersey they occur two hundred feet below tide4 ,in these elevations we have measures of the warping of the region since glacial times4 ,,the ,,western ,,united ,,states ,,in ,,glacial ,,times4 ,the western ,united ,states was not covered during the ,pleistocene by any general ice sheet1 but all the high ranges were capped with permanent snow and nourished valley glaciers1 often many times the size of the existing glaciers of the ,alps4 ,in almost every valley of the ,sierras and the ,rockies the records of these vanished ice streams may be found in cirques1 glacial troughs1 roches moutonnecs1 and morainic deposits4 ,it was during the ,glacial epoch that ,lakes ,bonneville and ,lahontan were established in the ,great ,basin1 whose climate must then have been much more moist than now4 ,,the ,,driftless ,,area4 ,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 ,minnesota1 which escaped the ice invasions4 ,the rocks are covered with residual clays1 the product of long preglacial weathering4 ,the region is an ancient peneplain1 uplifted and dissected in late ,tertiary times1 with mature valleys whose gentle gradients are unbroken by waterfalls and rapids4 ,thus the driftless area is in strong contrast with the immature drift topography about it1 where lakes and waterfalls are common4 ,it is a bit of preglacial landscape1 showing the condition of the entire region before the ,glacial epoch4 ,the driftless area lay to one side of the main track of both the ,keewatin and the ,labrador ice fields1 and at the north it was protected by the upland south of ,lake ,superior1 which weakened and retarded the movement of the ice4 ,south of the driftless area the ,mississippi valley was invaded at different times by ice sheets from the west1--the ,kansan and the ,iowan1--and again by the ,illinoian ice sheet from the east4 ,again and again the ,mississippi ,river was pushed to one side or the other of its path4 ,the ancient channel which it held along the ,illinoian ice front has been traced through southeastern ,iowa for many miles4 ,,benefits ,,of ,,glaciation4 ,like the driftless area1 the preglacial surface over which the ice advanced seems to have been well dissected after the late ,tertiary uplifts1 and to have been carved in many places to steep valley slopes and rugged hills4 ,the retreating ice sheets1 which left smooth plains and gently rolling country over the wide belt where glacial deposition exceeded glacial erosion1 have made travel and transportation easier than they otherwise would have been4 ,the preglacial subsoils were residual clays and sands1 composed of the insoluble elements of the country rock of the locality1 with some minglings of its soluble parts still undissolved4 ,the glacial subsoils are made of rocks of many kinds1 still undecayed and largely ground to powder4 ,they thus contain an inexhaustible store of the mineral foods of plants1 and in a form made easily ready for plant use4 ,on the preglacial hillsides the humus layer must have been comparatively thin1 while the broad glacial plains have gathered deep black soils1 rich in carbon and nitrogen taken from the atmosphere4 ,to these soils and subsoils a large part of the wealth and prosperity of the glaciated regions of our country must be attributed4 ,the ice invasions have also added very largely to the water power of the country4 ,the rivers which in preglacial times were flowing over graded courses for the most part1 were pushed from their old valleys and set to flow on higher levels1 where they have developed waterfalls and rapids4 ,this power will probably be fully utilized long before the coal beds of the country are exhausted1 and will become one of the chief sources of the national wealth4 ,,the ,,recent ,,epoch4 ,the deposits laid since glacial times graduate into those now forming along the ocean shores1 on lake beds1 and in river valleys4 ,slow and comparatively slight changes1 such as the warpings of the region of the ,great ,lakes1 have brought about the geographical conditions of the present4 ,the physical history of the ,recent epoch needs here no special mention4 ,,the ,,life ,,of ,,the ,,quaternary ,during the entire ,quaternary1 invertebrates and plants suffered little change in species1--so slowly are these ancient and comparatively simple organisms modified4 ,the ,mammalia1 on the other hand1 have changed much since the beginning of ,quaternary time3 the various species of the present have been evolved1 and some lines have become extinct4 ,these highly organized vertebrates are evidently less stable than are lower types of animals1 and respond more rapidly to changes in the environment4 ,,pleistocene ,,mammals4 ,in the ,pleistocene the ,mammalia reached their culmination both in size and in variety of forms1 and were superior in both these respects to the mammals of to-day4 ,in ,pleistocene times in ,north ,america there were several species of bison1--one whose widespreading horns were ten feet from tip to tip1--a gigantic moose elk1 a giant rodent 7,castoroides7 five feet long1 several species of musk oxen1 several species of horses1-- more akin1 however1 to zebras than to the modern horse1--a huge lion1 several saber-tooth tigers1 immense edentates of several genera1 and largest of all the mastodon and mammoth4 ,the largest of the edentates was the ,megatherium1 a4 clumsy ground sloth bigger than a rhinoceros4 ,the bones of the ,megatherium are extraordinarily massive1--the thigh bone being thrice as thick as that of an elephant1--and the animal seems to have been well able to get its living by overthrowing trees and stripping off their leaves4 ,the ,glyptodon was a mailed edentate1 eight feet long1 resembling the little armadillo4 ,these edentates survived from ,tertiary times1 and in the warmer stages of the ,pleistocene ranged north as far as ,ohio and ,oregon4 ,the great proboscidians of the ,glacial epoch were about the size of modern elephants1 and somewhat smaller than their ancestral species in the ,pliocene4 ,the ,,mastodon ranged over all ,north ,america south of ,hudson ,bay1 but had become extinct in the ,old ,world at the end of the ,tertiary4 ,the elephants were represented by the ,,mammoth1 which roamed in immense herds from our middle states to ,alaska1 and from ,arctic ,asia to the ,mediterranean and ,atlantic4 ,it is an oft-told story how about a century ago1 near the ,lena ,river in ,siberia1 there was found the body of a mammoth which had been safely preserved in ice for thousands of years1 how the flesh was eaten by dogs and bears1 and how the eyes and hoofs and portions of the hide were taken with the skeleton to ,st4 ,petersburg4 ,since then several other carcasses of the mammoth1 similarly preserved in ice1 have been found in the same region1-- one as recently as #aija4 ,we know from these remains that the animal was clothed in a coat of long1 coarse hair1 with thick brown fur beneath4 ,,the ,,distribution ,,of ,,animals ,,and ,,plants4 ,the distribution of species in the ,glacial epoch was far different from that of the present4 ,in the glacial stages arctic species ranged south into what are now temperate latitudes4 ,the walrus throve along the shores of ,virginia and the musk ox grazed in ,iowa and ,kentucky4 ,in ,europe the reindeer and arctic fox reached the ,pyrenees4 ,during the ,champlain depression arctic shells lived along the shore of the arm of the sea which covered the ,st4 ,lawrence valley4 ,in interglacial times of milder climate the arctic fauna-flora retreated1 and their places were taken by plants and animals from the south4 ,peccaries1 now found in ,texas1 ranged into ,michigan and ,new ,york1 while great sloths from ,south ,america reached the middle states4 ,interglacial beds at ,toronto1 ,canada1 contain remains of forests of maple1 elm1 and papaw1 with mollusks now living in the ,mississippi basin4 ,what changes in the forests of your region would be brought about1 and in what way1 if the climate should very gradually grow colder8 ,what changes if it should grow warmer8 ,on the ,alps and the highest summits of the ,white ,mountains of ,new ,england are found colonies of arctic species of plants and insects4 ,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 cross8 ,,man4 ,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 ,,man1 his bones1 and articles of his manufacture4 ,in ,europe1 where they have best been studied1 human relics occur chiefly in peat bogs1 in loess1 in caverns where man made his home1 and in high river terraces sometimes eighty and a hundred feet above the present flood plains of the streams4 ,in order to understand the development of early man1 we should know that prehistoric peoples are ranked according to the materials of which their tools were made and the skill shown in their manufacture4 ,there are thus four well-marked stages of human culture preceding the written annals of history3 #d ,the ,iron stage4 #c ,the ,bronze stage4 #b ,the ,neolithic 7recent stone7 stage4 #a ,the ,paleolithic 7ancient stone7 stage4 ,in the ,neolithic stage the use of the metals had not yet been learned1 but tools of stone were carefully shaped and polished4 ,to this stage the ,north ,american ,indian belonged at the time of the discovery of the continent4 ,in the ,paleolithic stage1 stone implements were chipped to rude shapes and left unpolished4 ,this1 the lowest state of human culture1 has been outgrown by nearly every savage tribe now on earth4 ,a still earlier stage may once have existed1 when man had not learned so much as to shape his weapons to his needs1 but used chance pebbles and rock splinters in their natural forms2 of such a stage1 however1 we have no evidence4 ,,paleolithic ,,man ,,in ,,europe4 ,it was to the ,paleolithic stage that the earliest men belonged whose relics are found in ,europe4 ,they had learned to knock off two-edged flakes from flint pebbles1 and to work them into simple weapons4 ,the great discovery had been made that fire could be kindled and made use of1 as the charcoal and the stones discolored by heat of their ancient hearths attest4 ,caves and shelters beneath overhanging cliffs were their homes or camping places4 ,paleolithic man was a savage of the lowest type1 who lived by hunting the wild beasts of the time4 ,skeletons found in certain caves in ,belgium and ,france represent perhaps the earliest race yet found in ,europe4 ,these short1 broad- shouldered men1 muscular1 with bent knees and stooping gait1 low- browed and small of brain1 were of little intelligence and yet truly human4 ,the remains of ,pleistocene man are naturally found either in caverns1 where they escaped destruction by the ice sheets1 or in deposits outside the glaciated area4 ,in both cases it is extremely difficult1 or quite impossible1 to assign the remains to definite glacial or interglacial times4 ,their relative age is best told by the fauna with which they are associated4 ,thus the oldest relics of man are found with the animals of the late ,tertiary or early ,quaternary1 such as a species of hippopotamus and an elephant more ancient than the mammoth4 ,later in age are the remains found along with the mammoth1 cave bear and cave hyena1 and other animals of glacial time which are now extinct2 while more recent still are those associated with the reindeer1 which in the last ice invasion roamed widely with the mammoth over central ,europe4 ,,the ,,caves ,,of ,,southern ,,france4 ,these contain the fullest records of the race1 much like the ,eskimos in bodily frame1 which lived in western ,europe at the time of the mammoth and the reindeer4 ,the floors of these caves are covered with a layer of bone fragments1 the remains of many meals1 and here are found also various articles of handicraft4 ,in this way we know that the savages who made these caves their homes fished with harpoons of bone1 and hunted with spears and darts tipped with flint and horn4 ,the larger bones are split for the extraction of the marrow4 ,among such fragments no split human bones are found2 this people1 therefore1 were not cannibals4 ,bone needles imply the art of sewing1 and therefore the use of clothing1 made no doubt of skins2 while various ornaments1 such as necklaces of shells1 show how ancient is the love of personal adornment4 ,pottery was not yet invented4 ,there is no sign of agriculture4 ,no animals had yet been domesticated2 not even man's earliest friend1 the dog4 ,certain implements1 perhaps used as the insignia of office1 suggest a rude tribal organization and the beginnings of the state4 ,the remains of funeral feasts in front of caverns used as tombs point to a religion and the belief in a life beyond the grave4 ,in the caverns of southern ,france are found also the beginnings of the arts of painting and of sculpture4 ,with surprising skill these ,paleolithic men sketched on bits of ivory the mammoth with his long hair and huge curved tusks1 frescoed their cavern walls with pictures of the bison and other animals1 and carved reindeer on their dagger heads4 ,,early ,,man ,,on ,,other ,,continents4 ,paleolithic flints curiously like those of western ,europe are found also in many regions of the ,old ,world1--in ,india1 ,egypt1 and ,asia ,minor1--beneath the earliest vestiges of the civilization of those ancient seats1 and sometimes associated with the fauna of the ,glacial epoch4 ,in ,java there were found in #ahia1 in strata early ,quaternary or late ,pliocene in age1 parts of a skeleton of lower grade1 if not of greater antiquity1 than any human remains now known4 ,pithecanthropus erectus1 as the creature has been named1 walked erect1 as its thigh bone shows1 but the skull and teeth indicate a close affinity with the ape4 ,in ,north ,america there have been reported many finds of human relics in valley trains1 loess1 old river gravels buried beneath lava flows1 and other deposits of supposed glacial age2 but in the opinion of some geologists sufficient proof of the existence of man in ,america in glacial times has not as yet been found4 ,these finds in ,north ,america have been discredited for various reasons4 ,some were not made by scientific men accustomed to the closest scrutiny of every detail4 ,some were reported after a number of years1 when the circumstances might not be accurately remembered2 while in a number of instances it seems possible that the relics might have been worked into glacial deposits by natural causes from the surface4 ,man1 we may believe1 witnessed the great ice fields of ,europe1 if not of ,america1 and perhaps appeared on earth under the genial climate of preglacial times4 ,nothing has yet been found of the line of man's supposed descent from the primates of the early ,tertiary1 with the possible exception of the ,java remains just mentioned4 ,the structures of man's body show that he is not descended from any of the existing genera of apes4 ,and although he may not have been exempt from the law of evolution1--that method of creation which has made all life on earth akin1--yet his appearance was an event which in importance ranks with the advent of life upon the planet1 and marks a new manifestation of creative energy upon a higher plane4 ,there now appeared intelligence1 reason1 a moral nature1 and a capacity for self-directed progress such as had never been before on earth4 ,,the ,,recent ,,epoch4 ,the ,glacial epoch ends with the melting of the ice sheets of ,north ,america and ,europe1 and the replacement of the ,pleistocene mammalian fauna by present species4 ,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 times4 ,our science cannot foretell whether all traces of the ,great ,ice ,age are to disappear1 and the earth is to enjoy again the genial climate of the ,tertiary1 or whether the present is an interglacial epoch and the northern lands are comparatively soon again to be wrapped in ice4 ,,neolithic ,,man4 ,the wild ,paleolithic men vanished from ,europe with the wild beasts which they hunted1 and their place was taken by tribes1 perhaps from ,asia1 of a higher culture4 ,the remains of ,neolithic man are found1 much as are those of the ,north ,american ,indians1 upon or near the surface1 in burial mounds1 in shell heaps 7the refuse heaps of their settlements71 in peat bogs1 caves1 recent flood-plain deposits1 and in the beds of lakes near shore where they sometimes built their dwellings upon piles4 ,the successive stages in ,european culture are well displayed in the peat bogs of ,denmark4 ,the lowest layers contain the polished ,,stone implements of ,neolithic man1 along with remains of the ,,scotch ,,fir4 ,above are ,,oak trunks with implements of ,,bronze1 while the higher layers hold iron weapons and the remains of a ,,beech forest4 ,neolithic man in ,europe had learned to make pottery1 to spin and weave linen1 to hew timbers and build boats1 and to grow wheat and barley4 ,the dog1 horse1 ox1 sheep1 goat1 and hog had been domesticated1 and1 as these species are not known to have existed before in ,europe1 it is a fair inference that they were brought by man from another continent of the ,old ,world4 ,neolithic man knew nothing of the art of extracting the metals from their ores1 nor had he a written language4 ,the ,neolithic stage of culture passes by insensible gradations into that of the age of bronze1 and thus into the ,recent epoch4 ,in the ,recent epoch the progress of man in language1 in social organization1 in the arts of life1 in morals and religion1 has left ample records which are for other sciences than ours to read2 here1 therefore1 geology gives place to archaeology and history4 ,our brief study of the outlines of geology has given us1 it is hoped1 some great and lasting good4 ,to conceive a past so different from the present has stimulated the imagination1 and to follow the inferences by which the conclusions of our science have been reached has exercised one of the noblest faculties of the mind1--the reason4 ,we have learned to look on nature in new ways3 every landscape1 every pebble now has a meaning and tells something of its origin and history1 while plants and animals have a closer interest since we have traced the long lines of their descent4 ,the narrow horizons of human life have been broken through1 and we have caught glimpses of that immeasurable reach of time in which nebulae and suns and planets run their courses4 ,moreover1 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 home1 and life appearing in the lowliest forms has steadily developed higher and still higher types4 ,seeing this process enter human history and lift our race continually to loftier levels1 we find reason to believe that the onward1 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 ends4