,transcriber's ,note ,italic text is denoted by underscores and bold text by equal signs4 ,whole and partial numbers are displayed as #b-a_/b4 ,illustration3 ,photo1 ,ridge ,studio2 ,courtesy ,ogden ,chamber of ,commerce ,a great ledge in ,ogden ,canyon near ,ogden1 ,utah4 ,the rock1 still retaining its stratification1 was deposited layer upon layer horizontally mostly as sand upon the floor of a sea which covered the region fully #be1jjj1jjj years ago4 ,that the sea was of very early ,paleozoic 7i4e41 ,cambrian7 age has been proved by fossils in associated strata4 ,long after their deep burial and consolidation within the earth1 the strata were subjected to tremendous mountain-making pressure1 notably altered to a rock called ,quartzite1 raised high above sea level1 and tilted almost vertically4 ,then through long ages 7millions of years7 overlying rocks of great thickness have been cut away 7eroded7 by weathering and stream action1 laying bare the ledge as we see it to-day4 ,popular ,science ,library ,,editor-,,in-,,chief ,,garrett ,p4 ,,serviss ,,authors ,,william ,j4 ,,miller ,,hippolyte ,,gruener ,a4 ,,russell ,,bond ,d4 ,w4 ,,hering ,,loomis ,,havemeyer ,,ernest ,g4 ,,martin ,,arthur ,,selwyn-,,brown ,,robert ,,chenault ,,givler ,,ernest ,,ingersoll ,,wilfred ,,mason ,,barton ,,william ,b4 ,,scott ,,ernest ,j4 ,,streubel ,,norman ,,taylor ,,david ,,todd ,,charles ,,fitzhugh ,,talman ,,robin ,,beach ,,arranged ,,in ,,sixteen ,,volumes ,,with ,a ,,history ,,of ,,science1 ,,glossaries ,,and ,a ,,general ,,index ,,illustrated ,illustration ,,volume ,,three ,p4 ,f4 ,,collier ,,son ,,company ,,new ,,york ,copyright #aibb ,by ,p4 ,f4 ,collier ,son ,company ,,manufactured ,,in ,u4 ,s4 ,a4 ,,geology ,the ,science of the ,earth's ,crust ,,by ,,william ,j4 ,,miller ,professor of ,geology1 ,smith ,college ,illustration ,p4 ,f4 ,,collier ,,son ,,company ,,new ,,york ,,preface ,in the preparation of this book the author has attempted to present1 in popular form1 the salient points of a general survey of the whole great science of geology1 the science which deals with the history of the earth and its inhabitants as revealed in the rocks4 ,the use of technical and unusual terms has been reduced to a minimum compatible with a reasonable understanding of the subject by the layman4 ,each of the relatively few scientific terms is explained where first used in the text1 and a glossary of common geological terms has been appended4 ,the matter of illustrations has received very careful attention1 and only pictures1 maps1 and diagrams are used which actually illustrate important features of the text4 ,a special point has been made to introduce only cuts of simple construction comparatively free from technicalities4 ,nearly every illustration is accompanied by a really explanatory title4 ,a number of the pictures are from the author's collection of photographs1 and many of the line-cuts have either been made or considerably modified by the author4 ,among the numerous sources of illustrations1 special mention should be made of the ,united ,states ,geological ,survey1 the ,new ,york ,state ,museum1 the ,american ,museum of ,natural ,history1 the ,university of ,chicago ,press1 and various individuals1 full credit being given wherever due4 ,william ,j4 ,miller4 ,northampton1 ,mass4 ,,contents ,,chapter ,,page ,i4 ,introduction #i ,,ii4 ,weathering and ,erosion #bb ,,iii4 ,stream ,work #cj ,,iv4 ,the ,sea and ,its ,work #ea ,v4 ,glaciers and ,their ,work #ei ,,vi4 ,the ,action of ,wind #ga ,,vii4 ,instability of the ,earth's ,crust #gf ,,viii4 ,volcanoes and ,igneous ,rocks #ii ,,ix4 ,waters ,within the ,earth #aac ,x4 ,how ,mountains ,come and ,go #acj ,,xi4 ,a ,study of ,lakes #adb ,,xii4 ,how the ,earth ,may ,have ,originated #aeh ,,xiii4 ,very ,ancient ,earth ,history #afd ,,xiv4 ,ancient ,earth ,history #agi ,,xv4 ,medieval ,earth ,history #bjh ,,xvi4 ,modern ,earth ,history #bba ,,xvii4 ,evolution of ,plants #bdi ,,xviii4 ,geological ,history of ,animals 7,excluding ,vertebrates7 #bei ,,xix4 ,geological ,history of ,vertebrate ,animals 7,including ,man7 #bha ,,xx4 ,mineralogy #cjg ,,xxi4 ,economic ,geology #cdb ,glossary of ,common ,geological ,terms #cgg ,,list ,,of ,,illustrations ,ledge of ,rock with ,nearly ,vertical ,strata ,frontispiece ,,plate ,,facing ,,page #a ,granite ,weathering to ,soil ,leaving ,residual ,cores of ,joint ,blocks 7,upper picture7 #cb ,looking-,glass ,rock1 ,utah4 ,stratified ,sandstone ,sculptured by ,wind ,erosion 7,lower picture7 #cb #b ,grand ,canyon of the ,yellowstone ,river4 ,a ,channel ,worn by ,erosion #cc #c ,gorge of the ,niagara ,river ,below the ,falls4 ,a ,sample of ,recent ,erosion #fd #d ,winding ,stream in the ,st4 ,lawrence ,valley with ,flood ,plain 7,upper picture7 #fe ,davidson ,glacier1 ,alaska1 ,showing ,action on the ,valley's ,walls and ,floor 7,lower picture7 #fe #e ,swift ,current ,valley in ,glacier ,national ,park1 ,formed by ,stream and ,glacier ,action #if #f ,yosemite ,valley from ,western ,entrance4 ,the ,result of ,glacial ,action #ig #g ,an ,upbend ,fold or ,anticline in ,maryland #abh #h ,ledge of ,igneous ,rock ,showing ,joints 7,upper picture7 #abi ,fault ,fracture in ,limestone ,formations 7,lower pictures7 #abi #i ,molten ,lava ,flowing ,over a ,cliff in ,hawaii 7,upper picture7 #afj ,dikes of ,granite ,cutting ,old ,rock 7,lower picture7 #afj #aj ,lassen ,peak1 ,california1 in ,eruption 7,left picture7 #afa ,devil's ,tower1 ,wyoming1 ,once a ,mass of ,molten ,rock ,forced ,through ,strata 7,right picture7 #afa #aa ,mammoth ,hot ,springs ,terrace in ,yellowstone ,national ,park 7,upper picture7 #bbd ,crater ,lake1 ,oregon1 ,formed by the ,subsidence of a ,volcano 7,lower picture7 #bbd #ab ,archeozoic ,rock1 ,oldest ,known ,rock ,formation on ,earth 7,upper picture7 #bbe ,twisted ,mass of ,stratified ,limestone1 ,surrounded by ,granite 7,lower picture7 #bbe #ac ,paleozoic ,rock1 ,covered with ,oldest ,known ,animal ,remains #bef #ad ,stone-,lily ,stems in ,ordovician ,strata 7,upper left picture7 #beg ,various ,brachiopods in ,ordovician ,strata 7,upper right picture7 #beg ,stratified ,limestone of ,middle ,ordovician ,era ,containing ,fossils 7,lower picture7 #beg #ae ,a ,landscape of the ,coal ,age 7,upper picture7 #bhh ,fossil ,fern on a ,piece of ,shale 7,lower picture7 #bhh #af ,restoration of ,huge ,animals of the ,mesozoic ,era #bhi #ag ,skeleton of ,two-,legged ,carnivorous ,dinosaur 7,large picture7 #cbj ,earliest ,known ,bird ,form 7,insert7 #cbj #ah ,skeleton of ,large ,flying ,reptile 7,upper picture7 #cba ,skeleton of a ,swimming ,reptile of the ,mesozoic ,era 7,lower picture7 #cba #ai ,appalachians ,along ,new ,river1 ,peneplain ,upraised ,again #ceb #bj ,glacial ,bowlder ,left by ,ice ,sheet 7,upper picture7 #cec ,esker1 ,deposited by a ,stream in the ,great ,glacier 7,lower picture7 #cec ,,chapter ,i ,,introduction ,earth features are not fixed4 ,the person of ordinary intelligence1 surrounded as he is by a great variety of physical features1 is1 unless he has devoted some study to the subject1 very likely to regard those features as practically unchangeable1 and to think that they are now essentially as they were in the beginning of the earth's history4 ,some of the most fundamental ideas taught in this book are that the physical features of the earth1 as we behold them to-day1 represent but a single phase of a very long-continued history2 that significant changes are now going on all around us2 and that we are able to interpret present-day earth features only by an understanding of earth changes in the past4 ,geology1 meaning literally earth science1 deals with the history of the earth and its inhabitants as revealed in the rocks4 ,the science is very broad in its scope4 ,it treats of the processes by which the earth has been1 and is now being1 changed2 the structure of the earth2 the stages through which it has passed2 and the evolution of the organisms which have lived upon it4 ,geography deals with the distribution of the earth's physical features1 in their relation to one another1 to the life of sea and land1 and human life and culture4 ,it is the present and outward expression of geological effects4 ,as a result of the work of many able students of geology during the past century and a quarter1 it is now well established that our planet has a definitely recorded history of many millions of years1 and that during the lapse of those eons1 revolutionary changes in earth features have occurred1 and also that there has been a vast succession of living things which1 from very early times1 have gradually passed from simple into more and more complex forms4 ,the physical changes and the organisms of past ages have left abundant evidence of their character1 and the study of the rock formations has shown that within them we have a fairly complete record of the earth's history4 ,although very much yet remains to be learned about this old earth1 it is a remarkable fact that man1 through the exercise of his highest faculty1 has come to know so much concerning it4 ,the following words1 by the late ,professor ,barrell1 admirably summarize the significance of geological history4 ,the great lesson taught by the study of the outer crust is that the earth mother1 like her children1 has attained her present form through ceaseless change1 which marks the pulse of life and which shall cease only when her internal forces slumber and the cloudy air and surf-bound ocean no more are moving garments4 ,the flowing landscapes of geologic time may be likened to a kinetoscopic panorama4 ,the scenes transform from age to age1 as from act to act2 seas and plains and mountains of different types follow and replace each other through time1 as the traveler sees them succeed each other in space4 ,at times the drama hastens1 and unusual rapidity of geologic action has1 in fact1 marked those epochs since man has been a spectator upon the earth4 ,science demonstrates that mountains are transitory forms1 but the eye of man through all his lifetime sees no change1 and his reason is appalled at the conception of a duration so vast that the milleniums of written history have not accomplished the shifting of even one of the fleeting views which blend into the moving picture4,a ,a ,central ,connecticut in the ,geologic ,past1 pp4 #a-b4 ,or in the words of ,tennyson3 ,there rolls the deep where grew the tree4 ,o1 earth1 what changes hast thou seen6 ,there where the long street roars1 hath been ,the stillness of the central sea4 ,the hills are shadows1 and they flow ,from form to form1 and nothing stands2 ,they melt like mist1 the solid lands1 ,like clouds they shape themselves and go4 ,the following statement of some of the more definite important conclusions regarding earth changes may serve to make still clearer the general scope of the science of geology4 ,the evidences upon which these conclusions are based are discussed in various parts of this book4 ,for untold millions of years the rocks at and near the earth's surface have been crumbling2 streams have been incessantly sawing into the lands2 the sea has been eating into continental masses2 the winds have been sculpturing desert lands2 and1 more intermittently and locally1 glaciers have plowed through mountain valleys1 and even great sheets of ice have spread over considerable portions of continents4 ,throughout geologic time1 the crust of the earth has shown marked instability4 ,slow upward and downward movements of the lands relative to sea level have been very common1 in many cases amounting to even thousands of feet4 ,various parts of the earth have been notably affected by sudden movements 7resulting in earthquakes7 along fractures in the outer crust4 ,during millions of years molten materials have1 at various times1 been forced into the earth's crust1 and in many cases to its surface4 ,mountain ranges have been brought forth and cut down4 ,the site of the ,appalachian ,mountains was1 millions of years ago1 the bottom of a shallow sea4 ,lakes have come and gone4 ,the ,great ,lakes have come into existence very recently 7geologically71 that is to say1 since the great ,ice ,age4 ,a study of stratified rocks of marine origin shows that all1 or nearly all1 of the earth's surface has at some time1 or times1 been covered by sea water4 ,over certain districts the sea has transgressed and retrogressed repeatedly4 ,organisms have inhabited the earth for many millions of years4 ,in earlier known geologic time1 the plants and animals were comparatively simple and low in the scale of organization1 and through the succeeding ages higher and more complex types were gradually evolved until the highly organized forms of the present time1 including the human race1 were produced4 ,the rocks of the earth constitute the special field of study for the geologist because they contain the records of events through which the earth and its inhabitants have passed during the millions of years of time until their present conditions have been reached4 ,all the rocks of the earth's crust may be divided into three great classes3 igneous1 sedimentary1 and metamorphic4 ,igneous rocks comprise all those which have ever been in a molten condition1 and of these we have the volcanic rocks 7for example1 lavas71 which have cooled at or near the surface2 plutonic rocks 7for example1 granites71 which have cooled in great masses at considerable depths below the surface2 and the dike rocks which1 when molten1 have been forced into fissures in the earth's crust and there cooled4 ,sedimentary rocks comprise all those which have been deposited under water1 except some wind-blown deposits1 and they are nearly always arranged in layers 7stratified74 ,such rocks are called strata4 ,they may be of mechanical origin such as clay or mud which hardens to shale2 sand1 which consolidates into sandstone2 and gravel1 which when cemented becomes conglomerate4 ,they may be of organic origin such as limestone1 most of which is formed by the accumulation of calcareous shells2 flint and chert1 which are accumulations of siliceous shells2 or coal1 which is formed by the accumulation of partly decayed organic matter4 ,or1 finally1 they may be formed by chemical precipitation1 as beds of salt1 gypsum1 bog iron ore1 etc4 ,metamorphic rocks include both sedimentary and igneous rocks which have been notably changed from their original condition4 ,traces or remains of plants and animals preserved in the rocks are known as fossils4 ,the term originally meant anything dug out of the earth1 whether organic or inorganic1 but for many years it has been strictly applied to organic remains4 ,many thousands of species of fossils are known from rocks of all ages except the oldest1 and more are constantly being brought to light1 but these represent only a small part of the life of past ages because relatively few organic remains were deposited under conditions favorable for preservation in fossil form4 ,the fossils in the rocks are1 however1 a fair average of the groups of organisms to which they belong4 ,it is really remarkable that such a vast number of fossils are imbedded in the rocks1 and from a study of these many fundamental conclusions regarding the history of life on our planet may be drawn4 ,as early as the fifth century ,b4 ,c41 ,xenophanes is said to have observed fossil shells and plants in the rocks of ,paros1 and to have attributed their presence to incursions of the sea over the land4 ,herodotus1 about a century later1 came to a similar conclusion regarding fossil shells in the mountains of ,egypt4 ,none of the ancients1 however1 seemed to have the slightest conception of the significance of fossils as time markers in the history of the earth4 7,see discussion below47 ,in the ,middle ,ages1 distinguished writers held curious views regarding fossils4 ,thus ,avicenna 7#ihj-ajcg7 believed that fossils represented unsuccessful attempts on the part of nature to change inorganic materials into organisms within the earth by a peculiar creative force 7vis plastica74 ,about two centuries later1 ,albertus ,magnus held a somewhat similar view4 ,leonardo da ,vinci 7#adeb-aeai71 the famous artist1 architect1 and engineer1 while engaged in canal building in northern ,italy1 saw fossils imbedded in the rocks1 and concluded that these were the remains of organisms which actually lived in sea water which spread over the region4 ,during the seventeenth and eighteenth centuries1 many correctly held that fossils were really of organic origin1 but it was commonly taught that all fossils represented remains of organisms of an earlier creation which were buried in the rocks during the great ,deluge 7,noah's ,flood74 ,william ,smith 7#agfi-ahci71 of ,england1 was1 however1 the first to recognize the fundamental significance of fossils for determining the relative ages of sedimentary rocks4 ,this discovery laid the foundation for the determination of earth chronology which is of great importance in the study of the history of the earth4 7,see discussions below47 ,organic remains1 dating as far back as tens of millions of years1 have been preserved in the rocks of the earth in various ways4 ,a very common kind of fossilization is the preservation of only the hard parts of organisms4 ,thus the soft parts have disappeared by decomposition1 while the hard parts1 such as bones1 shells1 etc41 remain4 ,in many cases practically complete skeletons of large and small animals which lived millions of years ago have been found intact in the rocks4 ,fossils which show none of the original material1 but only the shape or form1 are also very abundant4 ,when sediment hardens around an imbedded organism1 and the organism then decomposes or dissolves away1 a cavity or fossil mold only is left4 ,casts of organisms or parts of them are formed by filling shells or molds with sediment or with mineral matter carried in solution by underground water4 ,only rarely have casts of wholly soft animals been found in ancient rocks4 ,in other cases both original form and structure are preserved1 but none of the original material4 ,this is known as petrifaction which takes place when a plant or hard part of an animal has been replaced1 particle by particle1 by mineral matter from solution in underground water4 ,not uncommonly organic matter1 such as wood1 or inorganic matter1 such as carbonate of lime shells1 has been so perfectly replaced that the original structures are preserved almost as in life4 ,the popular idea that petrified wood is wood which has been changed into stone is1 of course1 incorrect4 ,it is doubtful if flesh has ever been truly petrified4 ,in many cases mainly the carbon only of organisms has been preserved4 ,this is also true of plants where1 under conditions of slow chemical change or decomposition1 the hydrogen and oxygen mostly disappear1 leaving much of the carbon with original structures often remarkably preserved4 ,fine examples are fossil plants in the great coal-bearing strata4 ,much more rarely entire organisms have been preserved either by freezing or by natural embalmment4 ,most remarkable are the species of mammoths and rhinoceroses1 extinct for thousands of years1 bodies of which1 with flesh1 hide1 and hair still intact1 have been held in cold storage in the frozen soils of ,siberia1 or other cases4 ,insects have been perfectly preserved in amber1 as1 for example1 in the ,baltic region4 ,this amber is a hardened resin in which the insects were caught while it was still soft and exuding from the trees4 ,finally1 we should mention the preservation of tracks and trails of land and water animals4 ,thousands of tracks of long-extinct great reptiles occur in the sandstones and shales of the ,connecticut ,valley of ,massachusetts4 ,the footprints were made in soft sandy mud which hardened and then became covered with more sediment4 ,few fossils occur in other than the sedimentary rocks4 ,most numerous1 by far1 are fossils in rocks of marine origin1 because on relatively shallow sea bottoms1 where sediments of the geologic ages have largely accumulated1 the conditions for fossilization have been most favorable4 ,among the many conditions which have produced great diversity in numbers and distribution of marine organisms during geologic time are temperature1 depth of water1 clearness of water1 nature of sea bottom1 degree of salinity1 and food supply4 ,river and lake deposits also not uncommonly contain remains of organisms which inhabited the waters1 but also others which were carried in4 ,surrounding trees drop their leaves1 flowers1 and fruit upon the mud flats1 insects fall into the quiet waters1 while quadrupeds are mired in mud or quicksand and soon buried out of sight4 ,flooded streams bring in quantities of vegetable debris1 together with carcasses of land animals drowned by the sudden rise of the flood 7,w4 ,b4 ,scott74 ,in the study of the many changes which have taken place in the history of the earth1 a fundamental consideration is the determination of the relative ages of the rocks1 especially the strata4 ,how can the geologist assign a rock formation of any part of the earth to a particular age in the history of the earth8 ,how can it be proved that certain rock formations in various parts of the earth originated practically at the same time8 ,there are two important criteria4 ,first1 in any region where the strata have not been disturbed from their normal order1 the older strata underlie the younger because the underlying sediments must have been deposited first4 ,now1 the total thickness of the stratified series of the earth has been estimated to be no less than #bjj1jjj feet and only a small part of this is actually present in any given locality or region4 ,it is1 therefore1 evident that the order of superposition of strata is in itself not sufficient for the determination of the relative ages of all the strata in even a considerable portion of a single continent1 not to mention its utter inadequacy in building up the geological column of the whole earth4 ,when1 however1 the second criterion1 namely1 the fossil content of the strata1 is used in direct connection with the order of superposition1 we have the real basis for determining the relative ages of strata for all parts of the earth4 ,the discovery of this method was very largely due to the painstaking field work in ,england by ,william ,smith about the beginning of the nineteenth century4 ,it is a well-established fact that organisms have inhabited the earth for many millions of years and that1 through the geologic ages1 they have continuously changed1 with gradual development of higher and higher types4 ,tens of thousands of species have come and gone4 ,accepting this fact1 it is then clear that strata which were formed at notably different times must contain notably different fossils1 while strata which accumulated at practically the same time contain similar fossils1 allowing1 of course1 for reasonable differences in geographical distribution of organisms as at the present time4 ,each epoch of earth history or series of strata has its characteristic assemblage of organisms4 ,in short1 a geological chronology is constructed by carefully determining1 first of all1 the order of superposition of the stratified rocks1 and next by learning the fossils characteristic of each group of strata4444 ,the order of succession among the fossils is determined from the order of superposition of the strata in which they occur4 ,when that succession has been thus established1 it may be employed as a general standard 7,w4 ,b4 ,scott74 ,it should1 however1 be borne in mind that precise contemporaneity of strata in widely separated districts can rarely1 if ever1 be determined because of the very great length of geologic time and the general slowness of the evolution of organisms4 ,rocks carrying remarkably similar fossils may really be several thousand years different in age2 but this is1 indeed1 a very small limit of error when one considers the vast antiquity of the earth4 ,much very accurate and satisfactory work has been done1 especially in ,europe and ,north ,america1 in correlating strata and assigning them to their places in the geological time table 7see below71 but a vast amount of work yet remains to be done before the task is complete4 ,certain types or species of organisms are much more useful than others in the determination of earth chronology4 ,best of all for world-wide correlations are species which were widely distributed and which persisted for relatively short times4 ,thus any species which lived in the surface waters of the ocean and was easily distributed over wide areas1 while1 at the same time1 it existed as such only a short time1 is the best type of chronologic indicator4 ,the known history of the earth has been more or less definitely divided into great eras and lesser periods and epochs1 constituting what may be called the geologic time scale4 ,in the accompanying table the era and period names1 except those representing earlier time1 are mostly world-wide in their usage4 ,epoch names1 being more or less locally applied1 are omitted from the table4 ,very conservative estimates of the length of time represented by the eras and the most characteristic general features of the life of the main divisions are also given4 ,,principal ,,divisions ,,of ,,geologic ,,time 7,modified after ,u4 ,s4 ,geological ,survey47 ,millions ,era4 ,period4 ,characteristic life4 of years estimated ---------------------------------------- ------------------------------ ,age of man4 ,animals and plants ,quaternary4 of modern types4 ---------------------------------------- ---------- ,cenozoic ,age of mammals4 ,rise of highest #c to #e4 ,tertiary4 animals except man4 ,rise and development of highest orders of plants4 ---------------------------------------- ------------------------------ ,age of reptiles4 ,rise and ,cretaceous4 culmination of huge land reptiles 7dinosaurs71 of shellfish with complexly partitioned coiled --------------shells 7ammonites71 and of great flying reptiles4 ,first appearance ,mesozoic ,jurassic4 7in ,jurassic7 of birds and mammals2 #e to #aj4 of cycads1 an order of palm-like --------------plants 7in ,triassic72 and of angiospermous plants1 among ,triassic4 which are palms and hardwood trees 7in ,cretaceous74 ---------------------------------------- ------------------------------ ,age of amphibians4 ,dominance of ,permian4 club mosses 7lycopods7 and plants --------------of horsetail and fern types4 ,primitive flowering plants and ,pennsylvanian4earliest cone-bearing trees4 ,beginnings of back-boned land --------------animals 7land vertebrates74 ,insects4 ,mississippian4,animals with nautiluslike coiled shells 7ammonites7 and sharks abundant4 ---------------------------------------- ---------- ,age of fishes4 ,shellfish ,devonian4 7mollusks7 also abundant4 ,rise of amphibians and land plants4 ---------------------------------------- ---------- ,a ,shell-forming sea animals dominant1 g especially those related to e the nautilus 7cephalopods74 ,rise ,paleozoic and culmination of the marine #ag to #be4 ,silurian4 o animals sometimes known as sea f lilies 7crinoids7 and of giant scorpionlike crustaceans ,i 7eurypterids74 ,rise of fishes and n of reef-building corals4 -----------n---------------------------- --------- v ,shell-forming sea animals1 e especially cephalopods and ,ordovician4r mollusk-like brachiopods1 abundant4 t ,culmination of the buglike marine e crustaceans known as trilobites4 -----------b---------------------------- --------- r ,trilobites and brachiopods most ,cambrian4 a characteristic animals4 ,seaweeds t 7alg7 abundant4 ,no trace of e land animals found4 ----------------------s----------------- ----------------------------- ,first life that has left distinct ,proterozoic,algonkian4 record4 ,crustaceans1 brachiopods1 and seaweeds4 ---------------------------------------- --------------------- ,organic matter in form of graphite #be to #ej ,archeozoic ,archean4 7black lead71 but no determinable fossils found4 ---------------------------------------- ------------------------------ ,the length of time represented by human history is very short compared to the vast time of known geological history4 ,the one is measured by thousands of years1 while the other must be measured by tens of millions of years4 ,just as we may roughly divide human history into certain ages according to some notable person1 nation1 principle1 or force as1 for example1 the ,age of ,pericles1 the ,roman ,period1 the ,age of the ,french ,revolution1 or the ,age of ,electricity1 so geologic history may be subdivided according to great predominant physical or organic phenomena1 such as the ,appalachian ,mountain ,revolution 7toward the end of the ,paleozoic era71 the ,age of ,fishes 7,devonian period71 or the ,age of ,reptiles 7,mesozoic era74 ,in the study of earth history1 as in the study of human history1 it is important to distinguish between events and records of events4 ,historical events are continuous1 but they are by no means all recorded4 ,records of events are often interrupted and seemingly sharply separated from each other4 ,,chapter ,,ii ,,weathering ,,and ,,erosion ,all rocks at and near the surface of the earth crumble or decay4 ,the term weathering includes all the processes whereby rocks are broken up1 decomposed1 or dissolved4 ,a mass of very hard and seemingly indestructible granite1 taken from a quarry1 will1 in a very short time1 geologically considered1 crumble 7,plate #a74 ,during the short span of the ordinary human life weathering effects are generally of very little consequence1 but during the long ages of geologic time the various processes of weathering have been slowly and ceaselessly at work upon the outer crust of the earth1 and such tremendous quantities of rock material have been broken up that the lands of the earth have everywhere been profoundly affected4 ,most of us have noticed buildings and monuments in which the stones show marked effects of weathering4 ,a good case in point is ,westminster ,abbey1 ,london1 in which many of the stones are badly weathering1 some of the more ornamental parts having crumbled beyond recognition since the building was erected in the thirteenth century4 ,in many countries1 tombstones and monuments only one or two centuries old are so badly weathered that the inscriptions are scarcely if at all legible4 ,what are some of the processes of nature whereby rocks are weathered8 ,in cold countries1 and often in mountains of generally mild climate regions1 the alternate freezing and thawing of water is a potent agency in breaking up rocks where the soils are thin or absent4 ,on freezing1 water expands about one-tenth of its volume and exerts the enormous pressure of over #b1jjj pounds per square inch4 ,nearly all relatively hard rock formations are separated into more or less distinct blocks by natural cracks called joints 7,plate #h74 ,very commonly the rocks also contain minute crevices1 fissures1 and pores4 ,repeated freezing and thawing of water which finds its way into such openings finally causes even the most resistant rocks to break up into smaller and smaller fragments4 ,a very striking example of difference in climatic effect upon a given rock mass is the obelisk in ,central ,park1 ,new ,york4 ,for many centuries this famous monument stood practically without change in the dry1 frostless climate of ,egypt1 but very soon after its removal to the moist1 frosty climate of ,new ,york1 it began to crumble so rapidly that it was necessary to cover it with a coating of glaze to protect it from the atmosphere4 ,temperature change1 especially in dry regions1 is also an important agency for mechanical breaking up of rocks4 ,on high mountains and on deserts1 a daily range of temperature of from #gj degrees to #hj degrees is frequent4 ,due to heat absorption1 rocks in desert regions1 during the day1 not uncommonly reach temperatures of fully #abj degrees1 while during the night1 due to heat radiation1 their temperature falls greatly4 ,during the heating of the outer portion of the rock1 the various minerals each expand differently1 thus setting up a series of stresses and strains tending to cause the minerals to pull apart4 ,the outer portions of the rocks which are subjected to unstable and relatively rapid temperature changes1 often crack or peel off in slabs or flakes1 this process being called exfoliation4 ,stone ,mountain in ,georgia1 and some of the mountains of the southern ,sierra ,nevada range in ,california1 are excellent examples of mountains which are being rounded off by exfoliation4 ,the principle is the same as that which causes the spalling of stones in buildings during fires4 ,masses of dbris consisting of more or less angular rock fragments of all sizes commonly occur at the bases of cliffs and mountains4 ,they represent materials which have weathered off the ledges mainly by frost action and temperature changes4 ,where electrical storms are frequent1 lightning often shatters portions of rock ledges4 ,many such cases have come under the writer's observation in the ,adirondack ,mountains of ,new ,york4 ,the total effect of lightning as a weathering agency is1 however1 relatively small4 ,another minor weathering effect is the mechanical action of plants4 ,the principle is well illustrated by the breaking or tilting of sidewalks by the wedging action of the growing roots of trees4 ,in many places the roots of plants growing in cracks in rocks1 exert powerful pressure causing the rocks or blocks of rocks to wedge apart4 ,let us now briefly consider some of the chemical processes of weathering4 ,the solvent effect of perfectly pure water upon rocks is very slight and slow4 ,but such water is not found in nature because certain atmospheric gases1 especially oxygen and carbonic acid gas1 are always present in it1 and they notably increase the solvent power of the water4 ,such water has the power to slowly but completely dissolve the common rock called limestone which consists of carbonate of lime4 ,this material is then carried away by the streams4 ,rocks1 like certain sandstones which contain carbonate of lime cementing material1 are caused to crumble due to removal of the cement in solution4 ,carbonic acid gas in water also has the power to chemically alter various minerals in many common rocks and thus the rocks fall apart and the carbonates which result from the action usually are carried away in solution4 ,one of the most important changes of this kind takes place when the very common mineral feldspar is attacked by water containing carbonic acid gas and the mineral alters to a soluble carbonate1 kaolin 7or clay7 and silica4 ,the oxygen1 both of the air and that which is contained in water1 is a very important chemical agent of decomposition of many rocks4 ,water at the surface and the upper part of the crust of the earth as well as moisture in the air are also important chemical agents which bring about rock decay4 ,we are all familiar with the rusting of iron which is due to the chemical union of the iron with oxygen1 thus forming an iron oxide which in turn commonly unites with water from air or earth4 ,now1 many rocks contain iron1 not as such1 but held in combination with other substances in the form of various minerals1 and this iron of the rocks1 where subjected to the oxygen and moisture of air or water1 slowly unites with the oxygen and water to form a hydrated iron oxide which is essentially iron-rust4 ,the minerals containing considerable iron are1 therefore1 decomposed and the rocks crumble4 ,there are various iron oxides1 usually more or less hydrated1 ranging in color from red through brown to yellow1 and these constitute probably the most common and striking colors of the rocks of the earth4 ,the gorgeously colored ,grand ,canyon of the ,yellowstone ,river is a very fine example of large scale coloring due to development of much hydrated oxide of iron during the weathering of lava rock1 the process having been aided by the action of heated underground waters4 ,most of the soils of the earth are the direct result of weathering4 ,important exceptions are soils which have been transported by the action of water1 ice1 or wind4 ,although the process of weathering is very slow and relatively superficial1 it is1 nevertheless1 true that in many places1 the products of weathering form faster than they can be carried away4 ,such weathered materials accumulate in their place of origin to form soils4 ,the upper few hundred feet of the earth's crust is everywhere more or less fractured and porous and the rocks are there affected in varying degrees by most of the ordinary agents of weathering4 ,in such cases1 outside the areas which were recently covered by ice during the great ,ice ,age1 it is common to find the loose soil grading downward into rotten rock1 and this in turn into the fresh practically unaltered bedrock4 ,soils of this kind are generally not more than ten or twenty feet deep1 though under exceptional conditions1 as in parts of ,brazil1 they attain depths of several hundred feet4 ,in order to make still clearer some of the above principles of weathering and also to give the reader some understanding of the most common types of residual soils1 we shall consider what happens to a few rather definite types of ordinary rocks when they are subjected to weathering4 ,a very simple case is that of a sandstone1 the mineral grains 7mostly quartz7 of which are held together by carbonate of lime4 ,the lime simply dissolves and is carried away1 while many of the mineral grains may remain to form a soil of nearly pure sand4 ,where oxide of iron forms the cementing material1 the rock yields less readily to weathering1 and the sandy soil will be yellowish brown or red according to the climate4 ,another simple case is that of limestone which when perfectly pure yields no soil because it is all soluble4 ,pure limestone is1 however1 rare1 and the various mineral impurities in it1 being to a considerable degree insoluble1 tend to remain to form a residual soil which may vary from sandy to clayey1 and usually brown or red due to the setting free of oxides of iron4 ,according to one estimate a thickness of about #ajj feet of a certain fairly impure limestone formation in ,virginia must weather to yield a layer of soil one foot thick4 ,soils of this kind1 which are usually rich1 are common in many limestone valleys of the ,appalachian ,mountains4 ,in the case of shale rock1 which is hardened mud1 the cementing materials are removed1 some chemical changes in the minerals may take place1 and the rock crumbles to a claylike soil4 ,what happens to a very hard1 resistant igneous rock like granite when attacked by the weather8 ,such a rock always consists mainly of the two very common minerals feldspar and quartz1 usually with smaller amounts of other minerals such as mica1 hornblende1 augite1 or magnetite4 ,the feldspar1 which when fresh is harder than steel1 slowly yields when attacked by water containing carbonic acid gas and crumbles or decays to a mixture of kaolin 7clay71 carbonate of potash1 and silica 7quartz74 ,clay is an important constituent of most good soils1 while the carbonate of potash is essential as a food for most plants4 ,due to yielding of the grains or crystals of feldspar1 the granite falls apart 7see ,plate #a74 ,the grains of quartz remain chemically unchanged1 though they may be more or less broken by changes of temperature1 and the other minerals1 which are mostly iron-bearing1 yield more or less to weathering1 resulting in a variety of products1 among which are oxides of iron4 ,a typical granite1 therefore1 gives rise to a good heavy soil which is yellow1 brown or red according to climate4 ,such granite soils are common in many parts of the ,piedmont ,plateau from ,maryland to ,georgia4 ,most of the dark-colored igneous rocks1 like ordinary basaltic lava1 contain much feldspar1 various iron-bearing minerals1 and little or no quartz4 ,such rocks yield to the weather like granite but1 because of lack of quartz1 the soils are more clayey4 ,rich soils of this kind occur in the great lava fields of the northwestern ,united ,states and in the ,hawaiian ,islands4 ,the importance of the breaking down of feldspar under the influence of the weather1 as above described1 not only from the standpoint of soil development1 but also as regards the wearing down of the lands of the earth1 is difficult to overemphasize because that mineral is by far the most abundant constituent of the earth's crust4 ,the term erosion is one of the most important in geologic science4 ,it comprises all the processes whereby the lands of the earth are worn down4 ,it involves the breaking up of earth material1 and its transportation through the agency of water1 ice1 or wind4 ,weathering1 including the various subprocesses as above described1 is a very important process of erosion4 ,by this process much rock material is got into condition for transportation4 ,another process of erosion1 called corrasion1 consists in the rubbing or bumping of rocks fragments of all sizes carried by water1 ice1 or wind against the general country rock1 thus causing the latter to be gradually worn away4 ,a fine illustration of exceedingly rapid corrasion of very hard rock was that of the ,sill tunnel in ,austria1 which was paved with granite blocks several feet thick4 ,water carrying large quantities of rock fragments over the pavement at high velocity caused the granite blocks to be worn through in only one year4 ,ordinarily in nature1 however1 the rate of wear is much slower than this4 ,pressure exerted upon the country rock by any agency of transportation may cause relatively loose joint blocks1 into which most rock formations are separated1 to be pushed away4 ,this process1 called plucking1 is especially effective in the case of flowing ice4 ,,chapter ,,iii ,,stream ,,work ,most streams are incessantly at work cutting or eroding their way into the earth's crust and carrying off the products of weathering4 ,by this means the general level of lands is gradually being reduced to nearer and nearer sea level4 ,base level of erosion is reached when any stream has eroded to its greatest possible depth1 and a whole region is said to be base-leveled when1 by the action of streams1 it has been reduced to a practically flat condition4 ,a region of this kind is known as a peneplain4 ,to one who has not seriously considered the matter1 the power of even moderately swift water to transport rock dbris seems incredible4 ,a well-established law of transportation by running water is that the transporting power of a current varies as the sixth power of its velocity4 ,for example1 a current which is just able to move a rock fragment of a given size will1 when its velocity is merely doubled1 be able to move along a piece of similar rock sixty-four times as large6 ,that this must be the case may be readily proved as follows3 ,a current of given velocity is just able to move a block of rock1 say1 of one cubic inch in the form of a cube4 ,a cubic block sixty-four times as large has a face of sixteen square inches4 ,by doubling the velocity of the current1 therefore1 twice as much water must strike each of the sixteen square inches of the face of the larger block with twice the force1 thus exerting sixty-four times the power against the face of the larger block1 or enough to move it along4 ,this surprising law accounts for the fact that in certain floods1 like the one which rushed over ,johnstown1 ,pennsylvania1 in #ahhi1 locomotives1 massive iron bridges1 and great bowlders were swept along with great velocity4 ,it is obvious1 then1 that ordinarily swift rivers in time of flood accomplish far more work of erosion 7especially transportation7 than during many days or even some months of low water4 ,few people have the slightest idea as to the enormous amount of earth material which the rivers are carrying into the sea each year4 ,the burden carried by the ,mississippi ,river has been carefully studied for many years4 ,each year this river discharges about #djj1jjj1jjj tons of material in suspension2 #abj1jjj1jjj tons in solution2 and #dj1jjj1jjj tons rolled along the bottom4 ,this all represents earth material eroded from the drainage basin of the river4 ,it is sufficient to cover a square mile #cbe feet deep1 or if placed in ordinary freight cars it would require a train reaching around the earth several times to contain it4 ,since the drainage basin of the ,mississippi covers about #a1bej1jjj square miles1 it is1 therefore1 evident that this drainage area is being worn down at the average rate of about one foot in #c1hdj years1 and this is perhaps1 a fair average for the rivers of the earth4 ,the ,ganges ,river1 being unusually favorably situated for rapid erosion1 wears down its drainage basin about one foot in #a1gej years4 ,it has been estimated that nearly #hjj1jjj1jjj tons of material are annually carried into the sea by the rivers of the ,united ,states4 ,according to this the country1 as a whole1 is being cut down at the rate of about one foot in #i1jjj years4 ,in arriving at this figure it should1 of course1 be borne in mind that the average level of hundreds of thousands of square miles of the western ,united ,states1 particularly the so-called ,great ,basin1 is practically not being reduced at all because none of the streams there reach the sea4 ,deposition of sediment is an important natural consequence of erosion4 ,the destination of most streams is the sea1 and where tides are relatively slight the sediments discharged mostly accumulate relatively near the mouths of the rivers in the form of flat1 fan-shaped delta deposits4 ,some rivers1 like the ,ganges1 which carry such unusual quantities of sediment1 are able to construct deltas in spite of considerable tides4 ,deltas also form in lakes4 ,in most cases1 however1 rivers enter the sea where there are considerable tides and their loads are more widely spread over the marginal sea bottom4 ,but in many cases some of the sediment does not reach the mouth of the stream4 ,it is1 instead1 deposited along its course either where the velocity is sufficiently checked1 as is the case over many flood-plain areas of rivers1 or where a heavily loaded1 relatively swift stream has its general velocity notably diminished4 ,an excellent example of the latter type of stream is the ,platte ,river1 which is swift and loaded with sediment in its descent from the ,rocky ,mountains1 but1 on reaching the relatively more nearly level ,nebraska country1 it has its current sufficiently checked to force it to deposit sediment and build up its channel along many miles of its course1 and this in spite of the fact that it still maintains a considerable current4 ,in a mountainous arid region a more or less intermittent stream at times of flood becomes heavily loaded with rock dbris and rushes down the mountain side4 ,on reaching the valley floor the velocity is greatly checked and most of the load is deposited at the base of the mountain1 successive accumulations of such materials1 called alluvial cones or fans1 having not uncommonly built up to depths of hundreds1 or even several thousand feet4 ,illustration3 ,plate #a4--7;a7 ,granite ,weathering to ,soil near ,northampton1 ,mass4 ,under the action of weathering all of the once hard1 fresh1 mass of granite has crumbled to soil except the fairly fresh rounded masses which are residual cores of joint blocks4 7,photo by the author47 ,illustration3 ,plate #a4--7;b7 ,looking-,glass ,rock1 ,utah4 ,the rock is stratified sandstone sculptured mainly by wind erosion1 that is1 by the wind driving particles of sand against it4 7,photo by ,cross1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #b4--;,grand ,canyon of the ,yellowstone ,river in ,yellowstone ,national ,park4 ,the great waterfall #cjh feet high is shown4 ,the large swift river has here sunk its channel 7by erosion7 to a maximum depth of #a1bjj feet during very recent geological time1 and the process is still going on4 ,the wonderful coloring is due to iron oxides set free during weathering of the lava rock4 7,photo by ,hillers1 ,u4 ,s4 ,geological ,survey47 ,any newly formed land surface1 like a recently drained lake bed or part of the marginal sea bottom which has been raised into land1 has a drainage system developed upon it4 ,in the early or youthful stage of such a new land area lying well above sea level1 under ordinary climatic conditions a few streams only form and these tend to follow the natural or initial slope of the land4 ,these streams carve out narrow1 steep-sided valleys1 and all of them are actively engaged in cutting down their channels1 or1 in other words1 none of them have reached base level1 and flood plains and meandering curves are therefore lacking4 ,during this youthful stage there are no sharp drainage divides2 gorges and waterfalls are not uncommonly present2 and the relief of the land in general is not rugged4 ,a good example of youthful topography is the region around ,fargo1 ,north ,dakota1 which is part of the bed of a great recently drained lake4 ,the ,grand ,canyon of the ,yellowstone ,river is an excellent illustration of a youthful valley cut in a high plateau of geologically recent origin4 7,plate #b47 ,as time goes on1 a region in youth gradually gives way to a region in maturity1 during which stage the maximum number 7usually a network7 of streams in broader ,v-shaped valleys have developed2 divisions of drainage are sharp2 the maximum ruggedness of relief has developed2 the larger streams only have cut down so near base level that winding 7meandering7 courses and flood plains are well developed along them2 and waterfalls and gorges are rarely present4 ,an almost perfect example of a region in maturity is that around ,charleston1 ,west ,virginia4 ,the old-age stage develops next in the history of the region1 during which only a moderate number of streams remain1 most of these being at or close to base level so that sweeping curves or meanders 7,plate #d7 and cut-off meanders or ox bows and wide flood plains are characteristic and common4 ,the relief is greatly subdued and the term rolling country might be applied to the moderately hilly region4 ,divisions of drainage are1 of course1 not at all sharp and the valleys are wide and shallow4 ,oxbow lakes are common1 but gorges and waterfalls are absent4 ,a region typical of old-age topography is that around ,caldwell1 ,kansas4 ,finally1 after the remaining low hills have been cut down1 the region is in the condition of a broad monotonous plain1 practically devoid of relief1 over which the sluggish streams pursue very winding1 more or less shifting or indefinite courses4 ,for the attainment of this final stage 7called a peneplain7 in the normal cycle of erosion a proportionately very long time is necessary1 because the rate of erosion becomes slower and slower as the region is being cut down4 ,then1 too1 some change of level between the land and the sea is very likely to take place before the peneplain stage is reached4 ,it is doubtful if any extensive region was ever brought to the condition of a perfect peneplain4 ,some masses of more resistant or more favorably situated rocks are almost sure to maintain at least moderate heights above the general plain level4 ,geologically recently upraised1 fairly well developed peneplains are southern ,new ,england and the great region of eastern ,canada4 ,the remarkably even sky lines of these regions mark the peneplain level before the uplift took place1 and occasionally masses1 called monadnocks from ,mount ,monadnock in southern ,new ,hampshire1 rise above the general level4 ,the valleys in such an uplifted peneplain region have been carved out by streams since the uplift began4 ,we have positive evidence that more or less well-developed peneplains of considerable extent existed in various parts of the earth at various times during the many millions of years of known earth history4 ,the normal cycle of erosion which1 as outlined above1 tends toward the peneplain condition may be interrupted at any stage by other processes4 ,an excellent case in point is the upper ,mississippi ,valley1 which had reached the old-age stage1 even approximating a peneplain1 just before the great ,ice ,age4 ,then1 during the withdrawal of the vast sheet of ice from the region toward the close of the ,ice ,age1 extensive deposits 7moraines1 etc47 of glacial dbris were left irregularly strewn over the country1 giving rise to many low hills1 lake basins1 and altered drainage lines1 in some cases with resultant gorge development4 ,some distinct features of a youthful topography are1 therefore1 plastered over what was otherwise a region well along in old age4 ,the general district around the ,dells of ,wisconsin ,river well illustrates this principle4 ,changes in level between land and sea which take place during the erosion of a region may also disturb the normal cycle of erosion4 ,for example1 a region in old age may be considerably upraised so that the streams have their velocities notably increased4 ,such a region is said to be rejuvenated and the streams1 which are revived in activity1 begin to cut youthful valleys in the bottom of the old ones and1 after a time1 the general surface of the region is subjected to vigorous erosion and a new cycle of erosion will be carried out unless interfered with in some way1 as by relative change of level between the land and the sea4 ,in this connection the history of the topography in the general vicinity of ,harrisburg1 ,pennsylvania1 may be of interest by way of illustration of the principle just described4 ,the long1 narrow1 parallel ,appalachian ,mountain ridges there rise to about the same level1 causing a remarkably even sky line as viewed from one of the summits4 ,this even sky line marks approximately the surface of what was a peneplain late in the ,mesozoic era4 ,early in the succeeding ,cenozoic era1 the broad peneplain was notably upraised to nearly the present altitudes of the ridge tops4 ,the revived ,susquehanna ,river left the old course which it had on the peneplain surface1 and began to carve out its present valley1 while tributaries 7subsequent streams7 to it developed along belts of weaker rock and thus they formed the present parallel valleys separated by belts of more resistant rocks which stand out as ridges4 ,in this way1 the mature stage of topography was reached4 ,very recently1 geologically1 the region has been rejuvenated enough to cause the larger streams to appreciably sink their channels below the general valley floors4 ,the reader will find a general discussion of movements of the earth's crust in a succeeding chapter4 ,illustration3 ,fig4 #a4--,the submerged ,hudson ,river channel is clearly shown by the contour lines on the sea floor4 ,figures indicate depth of water in fathoms4 ,geologically recent sinking of the land has caused the drowning of the river valley4 7,coast and ,geodetic ,survey74 ,if1 for example1 a region along the seaboard has reached the mature stage of erosion1 and the land notably subsides relative to sea level1 the tidewater will enter the lower valleys to form estuaries and the valleys are said to be drowned4 ,the large streams1 or at least their lower courses1 are thus obliterated and also the general erosion of the region is distinctly diminished4 ,the recently sunken coast of ,maine well illustrates the idea of drowned valleys4 ,the drowned valley of the lower ,hudson ,river is another fine example4 ,illustration3 ,fig4 #b4--,sketch maps showing how the ,shenandoah ,river captured the upper waters of ,beaverdam ,creek in ,virginia4 ,the abandoned valley of the creek across ,blue ,ridge is now called a wind gap4 7,after ,b4 ,willis47 ,what is termed stream piracy is of special interest in connection with stream work4 ,by this is meant the stealing of one stream or part of a stream by another4 ,we shall here explain only one of the various ways by which stream capture may be effected4 ,one of two fairly active streams1 flowing roughly parallel to each other1 is more favorably situated and has cut its channel deeper4 ,its tributaries are1 therefore1 more favorable to extension of headwaters and1 in time1 one of its tributaries eats back far enough to tap a branch of the less favorably situated stream so that the waters of this branch are diverted into the more favorably situated stream4 ,the ,shenandoah ,river of ,virginia has been such a pirate4 ,this river developed as a tributary of the ,potomac4 ,by headward extension toward the south1 the ,shenandoah finally tapped and diverted the upper waters of the smaller1 less favorably situated ,beaverdam ,creek4 ,the notch or so-called wind gap through which the upper waters of ,beaverdam ,creek formerly flowed across the ,blue ,ridge is still plainly visible4 ,such abandoned water gaps1 known as wind gaps1 are common in the central ,appalachian ,mountain region4 ,a remarkable type of river is one which has been able to maintain its course through a barrier1 even a mountain range1 which has been built across it4 ,thus1 the ,columbia ,river1 after flowing many miles across the great lava plateau1 has maintained its course right across the growing ,cascade ,range by cutting a deep canyon while the mountain uplift has been in progress4 ,in a similar manner the ,ogden ,river of ,utah has kept its westward course by cutting a deep canyon into the ,wasatch ,range which has geologically recently1 though slowly1 risen across its path4 ,in no other way can we possibly explain the fact that such a river1 rising on one side of a high mountain range1 cuts right across it4 ,a feature of minor though considerable popular interest is the development of potholes by stream action4 ,where eddies occur1 in rather active streams1 rock fragments of varying sizes may be whirled around in such manner as to corrode or grind the bedrock1 resulting in the development of cylinder-shaped potholes4 ,such holes vary in diameter up to fifty feet or more in very exceptional cases4 ,in the production of large potholes many rock fragments are worn away and new ones supplied to continue the work4 ,locally1 some stream beds are honeycombed with potholes4 ,illustration3 ,fig4 #c4--;,grand ,canyon1 ,arizona4 7,from ,darton's ,story of the ,grand ,canyon47 ,strikingly narrow and deep valleys1 called gorges and canyons1 are rather exceptional features of stream action4 ,most wonderful of all features of this kind is the ,grand ,canyon of the ,colorado ,river in ,arizona4 ,in fact1 this canyon takes high rank among the most remarkable works of nature4 ,the canyon is over #bjj miles long1 from #d1jjj to #f1jjj feet deep1 and from #h to #ae miles wide4 ,contrary to popular opinion1 this mighty canyon is not a result of some violent process1 such as volcanic action1 or the sudden sinking of part of the earth's crust4 ,nor is it the result of the scouring action of a great glacier4 ,it is simply a result of the operation of the ordinary processes of erosion where the conditions have been exceptionally favorable4 ,some of the favorable conditions have been1 and are1 a large volume of very swift water 7,colorado ,river7 continually charged with an abundance of rock fragments for the work of corrasion1 and a great thickness of rock which the river must cut through before reaching base-level4 ,aridity of climate also tends to preserve the canyon form4 ,the whole work has been accomplished in very late geological time1 and the tremendous volume of rock which has been weathered and eroded to produce the canyon has all been carried away by the ,colorado ,river and accumulated in the great delta deposit near where the river empties into the ,gulf of ,california4 ,even now the canyon is growing deeper and wider because the very active ,colorado ,river is still from #b1jjj to #c1jjj feet above sea level4 ,standing on the southern rim near ,grand ,canyon station at an altitude of nearly #g1jjj feet1 and looking down into the canyon1 one beholds a vast maze of side canyons1 high1 vertical rock walls which follow very sinuous courses1 giving rise to a steplike topography1 and countless rock pinnacles1 towers1 and mesas often of mountain-like proportions4 ,the side canyons are the result of erosion by tributaries to the main river which have gradually developed and worked headward as the main river has cut down4 ,the mountain-like sculptured forms which rise out of the canyon are erosion remnants1 or1 in other words1 masses of rock which were more favorably situated against erosion by either the main river or any of its tributaries4 ,all of the rocks of the broader1 main portions of the canyon are strata of ,paleozoic age1 arranged as a vast pile of almost horizontal layers1 including sandstone1 limestone1 and shale4 ,some of these layers1 being distinctly more resistant than others1 stand out in the canyon wall in the form of conspicuous cliffs1 in some cases hundreds of feet high4 ,the very striking color bands 7mostly light gray1 red1 and greenish gray71 which may be traced in and out along the canyon sides1 represent the outcropping edges of variously colored rock layers4 ,far down in the canyon lies the steep-sided1 ,v-shaped inner gorge1 or canyon which is fully #a1jjj feet deep4 ,the rocks are there not ordinary strata1 but rather metamorphic and igneous rocks1 mostly dark gray1 not in layers1 and about uniformly resistant to erosion4 ,there is reason to believe that this inner gorge has developed mainly since a distinct renewed uplift 7rejuvenation7 of the ,colorado ,plateau after the river began its canyon cutting4 ,the narrow1 steep-sided inner gorge may thus be readily accounted for and the general lack of steplike forms on its sides is due to essential uniformity of the rock material as regards resistance to erosion4 ,illustration3 ,fig4 #d4--,profile and structure section across the line ,a-,a in ,fig4 #c4 ,length of section #aj miles1 vertical scale not exaggerated4 ,the main relief features1 and the relations of the rocks below the surface are shown4 ,the granite and gneiss are of ,archeozoic age1 and the overlying nearly horizontal strata are of ,paleozoic age4 7,after ,darton1 ,u4 ,s4 ,geological ,survey47 ,the wonderful ,king's ,river ,canyon of the southern ,sierras in ,california is remarkable for its combined narrowness and depth4 ,it is a steep ,v-shaped canyon whose maximum depth is #f1ijj feet1 carved out in mostly solid granite by the action of weathering and running water4 ,some idea of the vast antiquity of the earth may be gleaned from the fact that this tremendously deep canyon has been produced by erosion in one of the most resistant of all known rocks in very late geologic time6 ,conditions favorable for cutting this canyon have been volume and swiftness of water and a liberal supply of grinding tools4 ,among the many other great canyons of the western ,united ,states brief mention may be made of the ,grand ,canyon of the ,yellowstone ,river in the ,national ,park4 ,the plateau into which the river has cut its steep-sided1 narrow1 ,v-shaped canyon1 with a maximum depth of #a1bjj feet1 has been geologically recently built up by outpourings of vast sheets of lava4 ,the large volume of very swift water1 aided by decomposition and weakening of the ordinarily very hard rock by the action of the hot springs1 has been able to carve out this deep canyon practically within the last period of earth history4 ,the deepening process is still vigorously in progress4 ,the wonderful coloring of the rock1 mostly in tones of yellow and brown1 is due to the hydrated iron oxides developed during the decay of the iron-bearing minerals of the lava1 the chemical action having been greatly aided by the action of the hot waters4 7,see ,plate #b47 ,in regard to its origin1 the marvelous ,yosemite ,valley1 or canyon1 falls in a somewhat different category1 and it is discussed beyond in connection with the work of ice4 ,suffice it to say here that running water has been a very important factor in its origin4 ,in ,new ,york and ,new ,england there are many gorges which have developed by the action of running water since the ,great ,ice ,age4 ,famous among these are ,ausable ,chasm and ,watkins ,glen of ,new ,york1 and the ,flume in the ,white ,mountains of ,new ,hampshire4 ,illustration3 ,fig4 #e4--,sketch map showing the retreat of the crest of ,niagara ,falls from #ahdb to #aije1 based upon actual surveys4 ,the retreat of the inner part of the ,horseshoe ,fall was more than #cjj feet4 7,modified by the author after ,gilbert1 ,u4 ,s4 ,geological ,survey47 ,before leaving our discussion of the work of running water1 we should briefly consider waterfalls4 ,true waterfalls originate in a number of ways4 ,most common of all is what may be termed the ,niagara type of waterfall4 ,niagara ,falls merit more than passing mention not only because of their scenic grandeur1 but also because of the unusual number of geologic principles which their origin and history so clearly illustrate4 ,niagara ,falls are divided into two main portions1 the ,canadian1 or so-called ,horseshoe ,fall1 and the ,american ,fall1 separated by a large island4 ,the crest of the ,american ,fall is about #a1jjj feet long and nearly straight1 while the crest of the ,canadian ,fall is notably curved inward upstream1 and it is about #c1jjj feet long4 ,the height of the ,falls is #afg feet4 ,downstream from the ,falls there is a very steep-sided gorge about #bjj feet deep and seven miles long4 ,the exposed rocks of the region are nearly horizontal layers of limestone underlain with shales4 ,relatively more resistant limestone forms the crest of the falls1 and directly underneath are the much weaker shales4 ,herein lies the principle of this type of waterfall because1 due to weathering and the swirling action of the water1 the weaker underlying rocks erode faster1 thus causing the overlying rock to overhang so that from time to time blocks of it already more or less separated by cracks 7joints71 fall down and are mostly carried away in the swift current4 ,thus the waterfall maintains itself while it steadily retreats upstream4 ,careful estimates based upon observations made between #ahbg and #aije show that the ,canadian ,fall retreated at the rate of from three to five feet per year1 while the ,american ,fall retreated during the same time at the rate of only several inches per year4 ,it has been well established that ,niagara ,falls came into existence soon after the ice of the great ,ice ,age had retreated from the district4 ,the falls started by plunging over a limestone escarpment1 situated at what is now the mouth of the gorge seven miles downstream from the present falls4 ,if we consider the rate of recession of the falls to have been always five feet per year1 the length of time required to cut the gorge would be something over #g1jjj years4 ,but the problem is not so simple1 because we know that1 at the time of1 or shortly after1 the beginning of the falls1 the upper ,great ,lakes drained farther north and not over the falls2 and that this continued for a considerable1 though unknown1 length of time4 ,during this interval the volume of water in ,niagara ,river was notably diminished1 and hence the recession of the falls must have been slower4 ,on the other hand1 judging by the width of the gorge1 the length of the crest of the falls has generally been considerably less than at present1 which in turn means greater concentration of water over the crest and more rapid wear4 ,various factors considered1 the best estimates for the age of the falls vary from #g1jjj to #ej1jjj years1 an average being about #be1jjj years4 ,although this figure is not precise1 it is1 nevertheless1 of considerable geologic interest because it shows that the age of ,niagara ,falls 7and gorge7 is to be reckoned in some tens of thousands of years1 rather than hundreds of thousands or millions of years4 ,although waterfalls of the ,niagara type are the most common of all1 it is by no means necessary that the particular rocks should be limestone and shale4 ,another common kind of waterfall may be termed the ,yosemite type1 so named from the high falls in the ,yosemite ,valley of ,california4 ,at the great falls of the ,yosemite1 the rock is a homogeneous granite and the undermining process does not operate4 ,yosemite ,creek first plunges vertically over a granite cliff for #a1dcj feet to form the ,upper ,falls1 which must rank among the very highest of all true water falls4 ,the water then descends about #hjj feet by cascading through a narrow gorge1 after which it makes a final vertical plunge of over #cjj feet4 ,a brief history of the falls is about as follows4 ,a great steep-sided ,v-shaped canyon several thousand feet deep had been carved out by the action of the ,merced ,river which now flows through the valley4 ,then1 during the ,ice ,age1 a mighty glacier plowed through the canyon1 filling it to overflowing4 ,the granite of this district having been unusually highly fractured by great vertical joint cracks was relatively easy prey for ice erosion4 ,due to its great weight1 the erosive power of the ice was most potent toward the bottom1 successive joint blocks were removed1 and the valley was thus widened and the sides steepened or even commonly made practically vertical4 7,see ,plate #f47 ,after the melting of the ice1 certain of the streams1 like ,yosemite ,creek and ,bridal ,veil ,creek1 were forced to enter the valley by plunging over great perpendicular1 granite cliffs which are in reality joint faces4 ,this type of waterfall does not retreat1 but it constantly diminishes in height by cutting into the crest4 ,a number of other high falls of this kind occur in the ,yosemite region1 and also in other mountain valleys which formerly contained glaciers1 as in the ,canadian ,rockies1 the ,alps1 and ,norway1 the rocks in these regions being of various kinds4 ,in the case of the ,yellowstone type of waterfall a different principle is involved1 namely1 a distinctly harder or more resistant mass of rock which extends vertically across the channel of the stream4 ,at the ,great ,falls of the ,yellowstone a mass of relatively fresh1 hard lava lies athwart the course of the river1 while just below it the lava has been much weakened by decomposition4 ,the harder rock therefore acts as a barrier1 while1 in the course of time1 the weak rock on the downstream side has been worn away until a waterfall #cjh feet high has developed4 ,this waterfall does not retreat very appreciably1 but it is probably increasing in height1 due both to the scouring action of the water at the base of the fall and the unusual clearness of the river water here1 thus causing little wear at the crest4 ,it should be noted1 in this connection1 that the channel just on the upstream side of the barrier cannot be cut down faster than the top of the barrier itself4 ,the famous ,victoria ,falls of the ,zambezi ,river in ,south ,africa1 represents a relatively uncommon type of waterfall4 ,considering height of the fall1 length of crest1 and volume of water1 this is perhaps the greatest waterfall in the world4 ,the ,zambezi ,river1 a mile wide1 plunges over #djj feet vertically into a chasm only a few hundred feet wide and at right angles to the main course of the stream4 ,the general country rock is hard lava1 but locally a narrow belt of the rock has been highly fractured vertically1 due to earth movements or faulting 7see explanation beyond7 and therefore weakened and more subject to weathering than the general body of the lava rock4 ,this belt of weakened rock has been easy prey for erosion by the ,zambezi ,river and the chasm has there developed4 ,in fact the chasm is still being increased in depth4 ,leaving the chasm toward one end1 the river flows through a narrow zig-zag gorge whose position has been determined by big joint cracks4 ,the mile-wide crest of the falls is interrupted by a good many ledges and even small islands4 ,the thundering noise of this great waterfall is most impressive1 but a good complete view is impossible because most of the chasm is constantly filled with dense spray4 ,still another type of waterfall develops by the removal of joint blocks by the action of running water4 ,falls of this type are fairly common though they seldom attain really great heights4 ,where the rock in the bed of a stream is traversed by well-developed vertical joint cracks1 slabs of rock cleaved by the joints may fall away due to weathering or they may be pushed away by pressure of the water4 ,such a fall retreats upstream by removal of joint blocks even in comparatively homogeneous rocks4 ,taughannock ,falls1 #bae feet high1 in southern central ,new ,york1 has developed by this manner in a shaly sandstone4 ,the several falls 7one #ej feet high7 in the famous gorge at ,trenton ,falls1 in central ,new ,york1 have developed in this way in limestone4 ,,chapter ,,iv ,,the ,,sea ,,and ,,its ,,work ,it is well known that the waters of the sea cover nearly three-fourths of the surface of the earth4 ,we think of the ,united ,states as being a large piece of land of over three million square miles--but the sea is about forty-five times as large1 that is1 it covers approximately #adj1jjj1jjj square miles4 ,it is a remarkable fact that the average depth of the great oceans of the earth is nearly two and one-half miles4 ,if the sea were universally present everywhere with the same depth1 it would be almost two miles deep4 ,yet this vast body of water is an extremely thin layer when compared with the earth's diameter of #h1jjj miles4 ,the ,pacific is the deepest of the oceans with an average depth of about two and three-fourths miles4 ,the deepest ocean water ever sounded is #cb1aad feet 7over six miles71 not far from the ,philippine ,islands4 ,this is known as the ,planet ,deep1 and was discovered in #aiab4 ,second deepest is #cj1icj feet near the island of ,guam4 ,in the ,pacific ,ocean there are five places where the water is over five miles deep and eleven places were it is over four miles deep4 ,the deepest sounding ever made in the ,atlantic ,ocean was #bg1igb feet1 not far from ,porto ,rico4 ,many substances are known to be in solution in sea water1 but in spite of this the composition is remarkably uniform4 ,the most abundant substance by far in solution is common salt4 ,in every #ajj pounds of sea water1 there are #c4e pounds of mineral matter of various kinds dissolved4 ,nearly #gh per cent of the dissolved matter is common salt4 ,the principal other constituents in solution are chloride and sulphate of magnesia1 and the sulphates of lime and potash4 ,all other dissolved mineral substances together make up less than one per cent of the total4 ,it has been estimated that if all the dissolved mineral matter should be brought together1 it would form a layer #age feet thick over the whole sea bottom4 ,the salts of the sea have been mostly supplied by the rivers1 which in turn have derived them from the disintegration and chemical decay of the rocks4 ,if we make a general comparison with the surface of the land1 the floor of the ocean is a vast monotonous plain4 ,none of the sea bottom compares with the ruggedness of mountains1 and even the more level portions of land surface show many sharp minor irregularities such as stream trenches4 ,but the sea bottom is characterized by its smoothness of surface4 ,there are under the sea1 however1 mountain-like ridges1 plateaus1 submarine volcanoes and valleys known as deeps4 ,but these rarely show ruggedness of relief like similar features on land4 ,one of the most remarkable relief features of the ocean bottom is the so-called continental shelf4 ,this is a relatively narrow platform covered by shallow water bordering nearly all the lands of the earth4 ,seaward1 the depth of water is greatest1 and it is seldom over #fjj or #hjj feet4 ,the continental shelves of the world cover about #aj1jjj1jjj square miles or about one-fourteenth of the area of the sea4 ,viewed in a broad way1 there are two great classes of marine deposits2 first1 those laid down comparatively near the borders of the land1 that is1 on the continental shelf and continental slope1 and second1 the abysmal deposits laid down on the bottom of the deep ocean4 ,those found along and near the continental borders are largely land-derived materials1 that is to say1 they are mostly sediments carried from the land into the sea by rivers1 and to a lesser extent rock material broken up by waves along many shores4 ,practically all such land-derived material is deposited within #ajj to #cjj miles of the shores4 ,the continental border deposits are extremely variable4 ,near shore they are chiefly gravel and sands1 while farther out they become gradually finer1 and on the continental slope only very fine muds are deposited4 ,these deposits usually contain more or less organic materials and shells or skeletons of organisms4 ,in some cases the shells or skeletons of organisms predominate or even exist to the exclusion of nearly all other material1 as is true of the coral deposits or reefs which form only in shallow water4 ,deposits like those just described as accumulating on the bottom of the shallow sea1 comparatively near the lands1 are of great significance to the geologist because just such marine deposits now consolidated into sandstone1 conglomerate1 shale1 and limestone1 are so widely exposed over the various continents4 ,a knowledge of the conditions under which shallow sea deposits are now forming1 is1 therefore1 of great value in interpreting events of earth history as they are recorded in similar rocks which have been accumulating through millions of years of time4 ,one specific instance will make this matter clearer4 ,using the method outlined in ,chapter ,i for the determination of earth chronology1 and our knowledge of present conditions under which shallow sea deposits are formed1 it has been well established that a shallow sea spread over fully four-fifths of the area of ,north ,america during the middle ,ordovician period of the early ,paleozoic era4 ,beyond this main conclusion1 a careful study of these rocks has revealed many important facts regarding the physical geography1 life1 and climate of that time4 ,the importance of this whole matter is still further emphasized by the statement that five-sixths of the exposed rocks of the earth are strata--mostly of shallow sea origin4 ,the deposits on the deep sea bottom are very largely either organic or the shells and skeletons of organisms which have fallen to the bottom from near the surface as already explained4 ,most common of these are the deep sea oozes which are made up of the remains and shells of tiny organisms called foraminifers4 ,these oozes cover about #ej million square miles of the sea bottom down to depths of from two to three miles4 ,at depths greater than from two to three miles1 a peculiar red clay is the prevailing deposit4 ,this is most extensive of all1 covering an area of #ee million square miles1 or nearly the total area of lands of the earth4 ,some remains of organisms are mixed with this clay1 but since most of the shells are of carbonate of lime and very thin1 they are dissolved without reaching the bottom in the deep sea water which is under great pressure and rich in carbonic acid gas4 ,the deep sea deposits1 both oozes and red clay1 do1 however1 contain some land-derived and other materials4 ,thus off the west coast of ,africa some dust carried by the prevailing winds from the ,sahara ,desert1 is known to fall in the deep sea several hundred miles from shore4 ,volcanic dust is carried for many miles and deposited in the deep sea--particularly in the ,south ,pacific ,ocean4 ,bits of porous volcanic rock called pumice sometimes float long distances out over the deep sea1 before becoming water soaked4 ,icebergs often drift far out from the polar regions over the deep sea1 and on melting the rock dbris which they carry is dropped to the sea bottom4 ,also1 particles of iron and dust from meteorites 7shooting stars7 have been dredged from the deep sea4 ,one important geological significance of the deep sea deposits is the proof which they furnish that1 from at least as far back as the beginning of the ,paleozoic era1 fully twenty-five million years ago1 to the present time1 the two great deep ocean basins--the ,atlantic and the ,pacific--have maintained essentially the same positions on the earth4 ,this is proved by the fact that nowhere1 on any continent among the rocks of all ages1 as old at least as the early ,paleozoic1 do we find any really typical deep-sea deposits4 ,there is then no evidence that a deep sea ever spread over any considerable part of any continent1 and this in spite of the fact that marine deposits of shallow water origin furnish abundant evidence of former sea extensions4 ,the shallow seas have at various times spread over large portions of the continents4 ,on many rocky coasts the waves are incessantly pounding and wearing away the rocks4 ,in such places the sea1 like a mighty horizontal saw1 is cutting into the borders of the lands4 ,the finer materials produced by the grinding up of the rocks are carried seaward by the undertow4 ,but1 if the land remains stationary with reference to the sea1 this landward cutting by the waves reaches a limit4 ,since even big waves have very little effect in water #ajj or #bjj feet deep1 a shelf is cut by the waves and this shelf1 not many miles wide1 is covered by shallow water4 ,the finer ground-up rock materials carried out by the undertow are dumped just beyond the edge of the shelf which is thus built out seaward as a terrace4 ,in traveling over this shelf and terrace1 the waves1 due to friction1 lose their power4 ,with gradually sinking land1 a much wider shelf may be cut1 because the power of the waves is then allowed to continue4 ,it might be of interest to cite a few cases of relatively rapid coast destruction by the waves which have come under human observation4 ,a remarkable example is the island of ,heligoland on which is 7or was7 located the powerful ,german fort which guards the entrance to the ,kiel ,canal4 ,in the year #hjj ,a4 ,d4 this island had #abj miles of shore line2 in #acjj it had #de miles of shore2 in #afdi only #h miles2 and in #aijj but #c miles of shore line remained4 ,in southeastern ,england whole farms and villages have been washed away in the last few centuries1 the sea cliffs retreating from #g to #ae feet a year4 ,a church located a mile from the sea shore near the mouth of the ,thames river1 in the sixteenth century1 now stands on a cliff overlooking the sea4 ,an island in ,chesapeake ,bay covered over #djj acres in #ahdh1 and the waves have since reduced it to about fifty acres4 ,study showed that the relatively soft unconsolidated strata of the ,nashaquitsa ,cliffs on the island of ,martha's ,vineyard1 were cut back at the rate of #e-a_/b feet per year1 between #ahdf and #ahhf4 ,if part of the relatively smooth sea bottom should be raised into land1 the resulting shore line would of course1 be regular and free from indentations or sharp embayments4 ,examples of such coast which are very young are at ,cape ,nome1 ,alaska2 the northern coast of ,spain2 and the west coast of northern ,south ,america4 ,soon1 however1 such a shore line is attacked1 and1 either where the waves are greatest or the rocks are weakest1 indentations will result and the whole coast is gradually eaten back until the power of the waves is largely spent in traveling across the shallow water shelf4 ,sand bars are then built across the mouths of the bays or indentations which later the rivers gradually fill up with sediment4 ,the result is a relatively straight or regular old shore line4 ,the coast of ,texas has about reached this stage4 ,if a portion of the relatively rugged land surface should become submerged under the sea1 a very irregular1 deeply indented shore line would result1 due to the entrance of tidewater into the valleys4 ,the deeply indented coast of ,maine is a fine example of a very irregular youthful shore line produced by geologically recent sinking of a rugged1 hilly region so that tidewater backs for miles into the lower reaches of the river valleys4 ,the promontories and islands are undergoing rapid wear1 and the development of bars across the inlets has scarcely begun4 ,other excellent examples are the coasts of ,norway and southern ,alaska4 ,such a coast is then attacked by the ocean waves and the promontories are cut back until the broad shallow water shelf is formed1 after which sand bars are built across the remaining embayments and the shore line becomes relatively regular4 ,it is1 then1 a remarkable fact that1 whether shore lines originate by emergence of sea bottom1 or by sinking of land1 there is a very strong tendency on the part of nature to develop regular shore lines4 ,it should be stated that the principles of wave work and shore-form development just outlined apply almost equally well to lakes1 especially large ones4 ,before leaving this subject of shore-line development1 mention should be made of the fact that bars and beaches are often built part way or wholly across embayments of the coast with surprising rapidity4 ,to illustrate1 ,sandy ,hook1 ,new ,jersey1 is advancing northward1 while ,rockaway ,beach1 ,new ,york1 is extending westward1 the tendency being to close up the entrance to ,new ,york harbor and to make the line of seashore more nearly regular4 ,records show that ,rockaway ,beach actually advanced westward more than three miles between the years #ahce and #aijh4 ,,chapter ,v ,,glaciers ,,and ,,their ,,work ,a glacier may be defined as a mass of flowing ice4 ,the motion may not be that of flowage in the usually accepted sense of the term4 ,a discussion of the various theories of glacier motion will not here be attempted4 ,glaciers form only in regions of perpetual snow1 but they commonly move down far below the line of perpetual snow of any given region4 ,in the polar regions they may form near sea level1 while in the tropics they form at altitudes of two to three miles1 and there only rarely4 ,in southern ,alaska1 the lower limit of perpetual snow is about #e1jjj feet above sea level1 and many of the glaciers come down to sea 7,plate #d71 while in the ,alps1 the lower limit of perpetual snow is at about #i1jjj feet1 and the glaciers descend as much as #e1jjj feet below it4 ,in regions of perpetual snow there is a tendency for more or less snow to accumulate faster than it can be removed by evaporation or melting4 ,as such snow accumulates it gradually undergoes a change1 especially in its lower parts1 first into granulated snow 7so-called nv7 and then into solid ice4 ,snow drifts in the northern ,united ,states often undergo similar transformation1 after a few months first to nv1 and then to ice4 ,this transformation seems to be brought about mainly by weight of overlying snow which compacts the snow crystals2 by rain or melting snow percolating into the snow to freeze and fill spaces between the snow crystals2 and by the actual growth of the crystals themselves4 ,when ice of sufficient thickness has accumulated 7probably at best several hundred feet71 the spreading action or flowage begins and a glacier has developed4 ,renewed snowfalls over the gathering ground keep up the supply of ice4 ,there are several types of glaciers3 valley or alpine glaciers2 cliff or hanging glaciers2 piedmont glaciers2 ice caps2 and continental ice sheets4 ,a valley or alpine glacier consists essentially of a stream of ice slowly flowing down a valley and fed from a catchment basin of snow within a region of perpetual snow4 ,in the ,alps1 where glaciers of this sort are very typically shown1 they vary in length up to eight or nine miles4 ,perhaps the grandest display of great valley glaciers is in southern ,alaska where they attain lengths up to forty or fifty miles and widths of one or two miles 7,plate #d74 ,hanging or cliff glaciers are in many ways like valley glaciers1 but they are generally smaller2 they develop in snow-filled basins above the snow line usually on steep mountain sides2 and they do not reach down into well-defined valleys4 ,most of the glaciers of the ,glacier ,national ,park in ,montana and many of those in the ,cascade ,mountains are of this type4 ,mount ,rainier in ,washington is one of the most remarkable single large mountain peaks in the world1 in regard to development of glaciers over it4 ,great tongues of ice1 starting mostly at #h1jjj to #aj1jjj feet above sea level1 flow down the sides of the mountain for distances of to four and even six miles4 ,the total area of ice in this remarkable system of radiating glaciers on this one mountain is over forty square miles4 ,these ,mount ,rainier glaciers are in general best classified as intermediate in type between valley and hanging glaciers4 ,illustration3 ,fig4 #f4--,map of ,mount ,rainier1 ,washington1 showing its wonderful system of glaciers which covers more than #dj square miles4 ,dotted portions represent moraines4 7,u4 ,s4 ,geological ,survey47 ,in some high latitude areas1 as in ,iceland and ,spitzbergen1 snow and ice may accumulate on relatively level plains or plateaus and slowly spread or flow radially from their centers4 ,these are called ice caps4 ,ordinary ice caps usually do not cover more than some hundreds of square miles4 ,continental glaciers or ice sheets are1 in principle1 much like ice caps1 but they are larger4 ,greenland is buried under an ice sheet of moderate size 7about #ejj1jjj square miles71 the motion being outward in all directions toward the sea4 ,tongues of ice1 like valley glaciers1 are commonly sent off from the main body of ice across the land border of ,greenland into the sea4 ,the size of the great ice sheet of ,antarctica is not definitely known1 but it covers probably at least several million square miles4 ,two continental ice sheets of special interest to the geologist are those which existed during the great ,ice ,age of the ,quaternary period4 ,one of these then covered nearly #d1jjj1jjj square miles of ,north ,america1 while the other covered about #fjj1jjj square miles of northern ,europe4 ,the main facts regarding the ,ice ,age are given in a succeeding chapter4 ,the facts brought out in the present discussion of existing glaciers will greatly aid in understanding the ,ice ,age4 ,how fast do glaciers flow8 ,based upon many observations1 we may say that an average rate of flow for the glaciers of the world is not more than a few feet per day4 ,a very exceptional case is a large glacier1 branching off as a tongue from the ice sheet of ,greenland1 which is said to move sixty to seventy-five feet per day4 ,some of the great ,alaskan glaciers have been found to flow from four to forty feet per day4 ,most glaciers of the ,alps move only one to two feet per day4 ,a glacier advances only when the rate of motion is greater than the rate of melting of its lower end and vice versa in the case of retreat4 ,thus it is true1 though seemingly paradoxical1 to assert that a glacier has a constant forward motion even when it is retreating by melting4 ,by watching the changing position of marked objects placed in the ice1 it has been proved that1 in a valley glacier1 the top moves faster than the bottom2 the middle moves faster than the sides2 the rate of motion increases with thickness of ice1 slope of floor over which it moves1 and temperature4 ,ice1 like molasses candy1 tends to crack when subjected to a relatively sudden force1 and where the ice rides over a salient on the bed of the glacier1 transverse cracks or fissures often develop4 ,due to more rapid motion of the central part of a valley glacier1 stresses and strains are set up and crevasses are formed1 usually pointing obliquely upstream4 ,where the ice tends to spread laterally in a broad portion of a valley1 longitudinal cracks may develop4 ,crevasses vary in size up to several feet in width and hundreds of feet in depth4 ,owing to the forward motion of the ice1 old fissures tend to close up and new ones form1 and1 aided by uneven melting1 the surface of a glacier is generally very rough4 ,like running water1 ice may have considerable erosive power when it is properly supplied with tools4 ,the total erosive effect which has been1 and is now being1 accomplished by ice compared with that of running water is1 however1 slight4 ,one of the main processes by which ice erosion is accomplished is corrasion due to the rubbing or grinding action of hard rock fragments frozen into the bottom and sides of the glacier4 ,thick ice1 shod with hard rock fragments and flowing through a deep1 narrow valley of soft rock1 is especially powerful as an erosive agent because the abrasive tools are supplied2 the work to be done is easy2 and the deep ice causes great pressure on the bottom and lower sides of the valley4 ,rock surfaces which have been thus subjected to ice erosion are characteristically smoothed and more or less scratched1 striated1 or ground due to the corrosive effects of small and large rock fragments4 ,this affords one of the best means of proving the former presence of a glacier over a region or in a valley4 ,a typical ,v-shaped stream cut 7eroded7 valley is changed into one with a ,u-shaped profile or cross section by glacier erosion 7,plate #e74 ,another important process of ice erosion is plucking1 which consists in pushing among already more or less loosened joint blocks by the pressure of the moving ice4 ,the pressure thus exerted1 especially by a deep valley glacier1 may be enormous4 ,this process was an important factor in the development of the famous ,yosemite ,valley1 a very brief account of whose history it will now be instructive to give4 ,illustration3 ,plate #c4--,the ,gorge of ,niagara ,river ,below the ,great ,falls4 ,the strata 7containing fossils7 were accumulated on the bottom of the ,silurian sea which overspread the region at least #ah1jjj1jjj years ago4 ,since the ,ice ,age or within #bj1jjj to #dj1jjj years1 the river has carved out the gorge4 7,courtesy of the ,haines ,photo ,company1 ,conneaut1 ,ohio47 ,illustration3 ,plate #d4--7;a7 ,a ,winding ,stream in the ,st4 ,lawrence ,valley of ,new ,york4 ,due to its low velocity the stream cuts its channel down very little1 but it swings or meanders slowly from one side of its valley to the other1 developing a wide flood plain4 ,the stream once flowed against the valley wall shown at the middle left4 7,photo by the author47 ,illustration3 ,plate #d4--7;b7 ,davidson ,glacier1 ,alaska4 ,this glacier is at work slowly grinding down the valley floor and cutting back its walls1 thus changing the original stream-cut1 ,v-shaped profile1 like that of ,plate #e4 7,photo by ,wright1 ,u4 ,s4 ,geological ,survey47 ,the ,yosemite ,valley1 about #g miles long1 less than one mile wide1 and from #b1jjj to #d1jjj feet deep1 lies on the western slope of the ,sierra ,nevada ,mountains of ,california4 ,great cliffs of granite1 mostly from #a1jjj to over #c1jjj feet high1 bound the valley on either side4 ,the floor of the valley is wide and remarkably flat 7,plate #f74 ,just prior to the ,ice ,age1 by the processes of erosion already set forth1 the ,merced ,river had carved out a great steep-sided ,v-shaped canyon commonly from #a1jjj to #c1jjj feet deep4 ,during the ,ice ,age1 two glaciers joined to form an extra deep powerful glacier1 which flowed through a deep part of the ,merced ,canyon and modified it into the ,yosemite ,valley1 essentially as we see it to-day4 ,because the ice was shod with many fragments of hard rock 7granite71 and the pressure at the bottom and lower sides of the glacier 7several thousand feet thick7 was so great1 the ,v-shaped stream-cut canyon was changed to a ,u-shaped canyon with very steep to even vertical walls4 ,a factor of great importance which notably aided the erosive power of the glacier in this case was the existence of an unusual number of large vertical joint cracks in the granite in this local region4 ,the plucking action of the ice was thus very greatly facilitated and great slabs of rock1 separated by the vertical joints1 especially toward the lower sides and bottom of the valley1 were pushed away one after another by the ice4 ,when the ice disappeared1 great precipitous joint faces from #a1jjj to #c1jjj feet high were left along the valley sides4 ,at its lower end the glacier left a dam of glacial dbris 7moraine7 across the valley1 thus causing a lake to form over the valley floor4 ,the wide flat bottom of the valley was caused by filling up of the lake with sediment4 ,the uniqueness of the ,yosemite ,valley is1 then1 due to a remarkable combination of several main factors2 one1 the presence of a large swift river well supplied with tools which carved out a deep ,v-shaped canyon2 two1 a mighty glacier which plowed its way through this canyon and converted it by erosion into a ,u-shaped canyon2 three1 the weakening of the rock by many joint cracks1 thus greatly facilitating the ice erosion2 and four1 a postglacial lake covering the valley floor which became filled with sediment4 ,as a result of the ice work1 several streams1 tributary to the main stream 7,merced ,river7 which flows through the bottom of the valley1 were forced to plunge over great vertical rock walls 7joint faces71 thus producing high and beautiful true waterfalls1 including the very high ,upper ,yosemite ,fall where ,yosemite ,creek makes a straight drop of #a1dcj feet4 ,a tributary valley like that of ,yosemite ,creek1 which ends abruptly well above the main valley1 is known as a hanging valley4 ,the valley of ,bridal ,veil ,creek is another good example4 7,see ,plate #f47 ,valleys which were once occupied by active glaciers are generally characterized by their ,u-shaped cross sections and their hanging 7tributary7 valleys1 but the great height and steepness of the valley walls in ,yosemite are exceptional4 ,a type of glacial erosion which is of special interest is the sculpturing of so-called cirques or amphitheaters in mountains within the region of perpetual snow4 ,where the main mass of snow and ice in the catchment basin or gathering ground of a valley glacier pulls away from the snow and nv on the upper slopes1 the rock wall is more or less exposed in the deep crevasse4 ,during warm days water fills the joint cracks in the rocks down in this crevasse 7so-called ,bergschrund71 and during cold nights the water freezes and forces the blocks of rock apart4 ,this is greatest toward the bottom of the crevasse and so1 by this excavating or quarrying process1 vertical or very steep walls are developed around a great bowlike basin or cirque4 ,such cirques1 now free from glacial ice1 with precipitous walls #ejj to #b1jjj feet high and one-fourth of a mile to one-half of a mile across1 are common in the ,sierra ,nevada and ,cascade ,ranges and in the ,rocky ,mountains4 ,what becomes of the materials eroded by the ice8 ,an answer to this question involves at least a brief discussion of the deposition of glacial dbris1 this constituting an important feature of the work of ice4 ,the dbris transported by a glacier is carried either on its surface or within it1 or pushed along under it4 ,it is generally heterogeneous material ranging from the finest clay through sand and gravel1 to bowlders of many tons' weight4 ,various types of glacial deposits are abundantly illustrated by dbris left strewn over much of the northeastern ,united ,states and some reference to these will be made4 ,most valley glaciers carry considerable dbris on their surfaces1 this representing material which falls or is carried down from the valley walls upon the margins of the ice1 thus forming marginal moraines4 ,when two glaciers flow together1 one marginal moraine from each will coalesce to form a medial moraine4 ,the material carried along at the bottom of a glacier is called the ground moraine4 ,where it contains much very fine grained material with pebbles or bowlders scattered through its mass1 it is called till or bowlder clay4 ,the pebbles or bowlders of the ground moraine are commonly facetted and striated as a result of having been rubbed against the bedrock on which the glacier moved4 ,ground moraine material is the most extensively developed of all glacial deposits4 ,it is so widely scattered over the glaciated northeastern portion of the ,united ,states that most of the soils consist of it1 having been left strewn over the country during the melting of the vast ice sheet4 ,when a glacier remains practically stationary for some time1 more or less material which it carries is piled up at its lower end to form a terminal moraine4 ,repeated pauses during general glacier retreat permit the accumulations of so-called recessional moraines4 ,a wonderful display of recessional moraines occurs from the ,great ,lakes south1 where they are festooned one within another and remain almost exactly as they were formed during pauses in retreat of great lobes of ice during the closing stages of the ,ice ,age4 ,a great terminal moraine marks the southernmost limit of the ice sheet during the ,ice ,age1 a very fine illustration being the ridge of low irregular hills extending the whole length of ,long ,island4 ,some of the material in that morainic ridge was transported by the ice from northern ,new ,england4 ,considerable rock dbris is transported within the ice1 and such englacial material in part results from rock dbris which falls on the surface in the catchment basin and becomes buried under new snowfalls which change to ice1 and in part from material which falls into the crevasses in the glacier farther down the valley4 ,marked objects thrown into the catchment basin have1 after many years1 emerged at or near the end of the glacier2 thus the rate of motion can be very accurately told4 ,a very remarkable case of transportation through the body of a glacier is the following3 ,in #ahbj1 three men were buried under an avalanche in the catchment basin of the ,bossons ,glacier in the ,alps4 ,forty-one years later several parts of the bodies1 including the three heads together with some pieces of clothing1 emerged at the foot of the glacier after traveling most of its length at the rate of eight inches per day4 ,the heads were so perfectly preserved after their remarkable journey in cold storage that they were clearly recognized by former friends6 ,where a valley floor slopes downward away from the end of a glacier1 waters emerging from the ice1 heavily loaded with rock dbris1 cause more or less deposition of the dbris on the valley floor often for miles beyond the ice front4 ,such a deposit is called a valley train4 ,when the ice front pauses for a considerable time upon a rather flat surface1 the dbris-laden waters emerging from the ice develop an outwash plain by deposition of sediment rather uniformly over the flat surface4 ,a very fine example is the plain which constitutes most of the southern half of ,long ,island just beyond the southern limit of the great terminal moraine ridge4 ,a type of glacial deposit of particular interest is the drumlin which is1 in reality1 only a special form of ground moraine material 7commonly till71 and1 therefore1 essentially unstratified4 ,typical drumlins are low1 rounded mounds of till with roughly elliptical bases and steeper fronts facing the direction from which the ice flowed4 ,their long axes are always parallel to the direction of ice movement4 ,in height they commonly range from #ej to #bjj feet4 ,their mode of origin is not yet definitely known1 but they form near the margins of broad lobes of ice either by erosion of earlier glacial deposits1 or by accumulation beneath the ice under peculiarly favorable conditions1 as perhaps in the longitudinal crevasses4 ,one of the finest and most extensive exhibitions of drumlins in the world is in western ,new ,york between ,syracuse and ,rochester4 ,thousands of drumlins there rise above the general level of the ,ontario plain1 the ,new ,york ,central ,railroad passing through the very midst of them4 ,drumlins are also abundant in eastern ,wisconsin4 ,another type of glacial deposit in the form of low hills is the kame which1 unlike the drumlin1 always consists of more or less stratified material4 ,kames are seldom over #bjj feet high1 and they are of various shapes4 ,in many cases they form irregular groups of hills1 and in other cases fairly well defined kame ridges4 ,kames form as deposits from dbris-laden streams emerging from the margins of glaciers1 the water sometimes rising as great fountains because of the pressure4 ,such deposits are now actually in process of formation along the edge of the great ,malaspina ,glacier of ,alaska4 ,kames are commonly associated with terminal and recessional moraines4 ,eskers are similar except that they are long winding low ridges of stratified material deposited by dbris-laden streams1 probably in longitudinal fissures in the ice near its margin4 7,see ,plate #bj47 ,glacial bowlders1 or erratics are blocks of rock or bowlders left strewn over the country during the melting of the ice4 ,they vary in size from small pebbles to those of many tons of weight1 and most of them were derived from ledges of relatively hard1 resistant rocks4 7,see ,plate #bj47 ,erratics have very commonly been carried a few miles from their parent ledges1 while more rarely they have traveled even hundreds of miles4 ,they are extremely abundant in ,new ,york and ,new ,england1 many occurring even high up on the mountains4 ,in some cases erratics of ten or more tons' weight have been left in such remarkably balanced positions on bedrock that a child can cause one of them to swing back and forth slightly4 ,such a bowlder is literally a rocking stone4 ,in the ,adirondack ,mountains the writer recently observed a rounded erratic of very hard rock fourteen feet in diameter resting in a very remarkably balanced position on top of another large round glacial bowlder4 ,,chapter ,,vi ,,the ,,action ,,of ,,wind ,only during the last quarter of a century have geologists come to properly appreciate the really important geological work of the wind4 ,one reason for this is the fact that people live mostly in humid regions where the soils are largely effectually protected against wind action by the vegetation4 ,but even in such regions1 wind action is by no means negligible4 ,one has but to observe the great clouds of dust raised by strong wind from freshly cultivated fields during a little dry weather in the late spring4 ,much of this dust is carried considerable distances1 often miles1 and in some cases young crops are injured by removal of soil from around the roots1 while in other cases young plants are buried by deposition of the wind-blown material over them4 ,in humid regions1 the action of the wind is perhaps most strikingly exhibited along and near shores of sea and lakes1 where loose dry sands are picked up and transported in great quantities1 often depositing them as sand dunes1 which may form groups of hills covering considerable areas4 ,very conspicuous examples are the sand dunes of ,dune ,park in northern ,indiana1 and the dunes along the coast of ,new ,jersey4 ,but the action of wind is most strikingly effective in desert and semiarid regions4 ,the importance of the work of wind is made more impressive when we realize that about one-fifth of the land of the earth is desert4 ,in deserts some of the ordinary agents of weathering and erosion are either absent or notably reduced in effectiveness4 ,thus1 stream action is1 in general1 reduced to a minimum2 weathering effects due to moisture in the air are notably reduced1 and either frost action1 or wedge work of ice1 is relatively unimportant due to lack of water4 ,change of temperature between night and day is1 however1 unusually important as a process whereby rocks are broken up due to relatively rapid expansion and contraction in deserts because such temperature changes are exceptionally great1 and rocks and soils are almost everywhere directly exposed1 being free from vegetation4 ,the finer grained materials1 especially sand grains1 in deserts are picked up by the wind and driven1 often with great velocity1 against barren rock ledges and large and small rock fragments4 ,by this process 7corrasion7 the rocks are worn and often polished by the materials blown against them4 ,the principle is that of the artificial sand-blast1 used in etching glass1 or cleaning and polishing building and decorative stones4 ,under favorable conditions wind-driven sand accomplishes noticeable erosion in a surprisingly short time4 ,thus1 in a hard wind storm1 a plate glass window in a lighthouse on ,cape ,cod was worn to opaqueness1 while in a few weeks or months the directly exposed window glass may there be worn through4 ,the great erosive effect of wind-driven sand is relatively close to the ground because the larger and heavier fragments are not lifted to very considerable heights4 ,for this reason ordinary telegraph poles are difficult to maintain in desert regions because1 unless they are specially protected1 they are soon cut down by sand swept against their bases4 ,in the desert regions of our ,southwestern ,states cliffs rising above the general level of the country are often undercut by wind erosion1 sometimes with the development of large caverns4 7,see ,plate #a47 ,even the high portions of great ledges are there more or less fantastically sculptured by wind erosion1 the softer portions being more deeply cut into than the harder4 ,the famous sphinx of ,egypt has been notably roughened by action of this kind4 ,the enormous power of high winds to transport rock material in desert regions is strikingly illustrated by the great sand storms of the ,sahara ,desert1 where sand and dust1 forming clouds with cubic miles of volume1 sweep for many miles across the country4 ,some one has estimated that every cubic mile of air in such a storm contains more than #ajj1jjj tons of rock material4 ,it is said that an army of #ej1jjj men under ,cambyses was buried under the sands of a storm in the desert of northern ,africa4 ,dust from some of these storms is known to be driven hundreds of miles out over the ,atlantic ,ocean1 there to settle in the sea4 ,in mountainous desert regions1 like the ,great ,basin of our ,western ,states1 the general tendency is for the rock materials wind-eroded from the mountains to be carried into the intermontane basins or valleys4 ,some basins of this sort are believed to contain depths of #a1jjj to #b1jjj feet of wind-blown material4 ,a special kind of wind-blown material called loess1 is a sort of fine-grained yellow1 or brown loam which1 though relatively unconsolidated1 has a remarkable property of standing out as high steep cliffs or bluffs along the banks of streams4 ,many thousands of square miles of northern ,china are covered with loess4 ,among many other regions1 thousands of square miles of parts of the ,states of ,iowa1 ,nebraska1 and ,kansas are covered with loess1 which1 in this case1 is believed to be fine material gathered by winds from the region just after the retreat of one of the ice sheets of the great ,ice ,age1 when there was very little vegetation to hold down the loose soils of glacial origin4 ,much as snowdrifts are formed1 so1 in many places1 the wind-driven sands are built up into sand hills or so-called dunes4 ,dunes are very common in many places1 as for example1 along our middle ,atlantic coast2 in ,dune ,park of northern ,indiana2 and in the great arid and semiarid regions of the ,western ,states4 ,where there is a distinctly prevailing direction of wind1 the sand is blown to the leeward side from the windward side1 and the dunes are caused to migrate in the direction of the wind4 ,the burial and destruction of forests1 and the uncovering of the dead trees is not uncommonly caused by migration of sand dunes1 all stages of this phenomenon being well exhibited in ,dune ,park1 ,indiana4 ,the rate of dune migration is very variable1 but study in a number of places has shown a rate of from a few feet to more than #ajj feet per year4 ,arable lands1 buildings1 and even towns have been encroached upon and buried under drifting sand4 ,an interesting example is a church in the village of ,kunzen1 on the ,baltic seashore which1 in a period of sixty years1 became completely buried under a dune and then completely uncovered by migration of the dune4 ,much destruction has been wrought by shifting sands on the ,bay of ,biscay1 where farms and even villages have been overwhelmed4 ,the ruins of the ancient cities of ,babylon and ,nineveh are buried mostly under wind-blown sand and dust4 ,there is good reason to believe that the climate of central and western ,asia is now notably drier than it was a few thousand years ago1 and this may help to explain the burial of many old cities and villages there under wind-blown deposits4 ,,chapter ,,vii ,,instability ,,of ,,the ,,earth',s ,,crust ,the crust of the earth is unstable4 ,to the modern student of geology the old notion of a terra firma is outworn4 ,the idea of an unshakable1 immovable earth could never have emanated from the inhabitants of an earthquake country4 ,in general we may recognize two types of crustal movements--slow and sudden4 ,to most people the sudden movements accompanied by earthquakes are more significant and impressive because they are more localized and evident1 and often accompanied by destruction of property1 or quick1 though minor1 changes in the landscape4 ,but movements which take place slowly and quietly are often of far greater significance in the interpretation of the profound physical changes which have affected the earth during its millions of years of known history4 ,illustration3 ,fig4 #g4--,structure section across the ,hudson ,river ,valley near ,west ,point1 ,new ,york4 ,the shafts and tunnel1 #a1bjj feet below sea level1 in solid rock1 show the position of the ,new ,york ,city aqueduct from the ,catskills4 ,the ,preglacial valley has been submerged and filled with ,postglacial sediment to a depth of nearly #hjj feet4 7,redrawn by the author after ,berkey1 from ,new ,york ,state ,museum ,bulletin47 ,a few well-known examples will serve to prove that upward1 downward1 and differential movements of the earth's crust have actually taken place not only in the remote ages of geologic time1 but also that such movements have geologically recently taken place1 and that similar movements are still going on4 ,it is very important that the reader thoroughly appreciate the fact that crustal disturbances1 often profound ones1 do take place1 because this is one of the most fundamental tenets of geologic science4 ,let us consider the case of the ,hudson-,champlain-,st4 ,lawrence ,valley region4 ,that the whole region was once notably higher 7at least #a1jjj feet7 than at present is proved by the drowned character of the ,hudson ,valley1 in which tidewater extends northward for #aej miles to near ,troy4 ,where the ,new ,york ,city ,aqueduct passes under the ,hudson ,river near ,newburgh1 the bedrock bottom of the old river channel is now about #hjj feet below sea level as determined by drilling4 ,this old channel is there filled up nearly to sea level with glacial and postglacial rock dbris1 which shows that the old channel must have been cut before the oncoming of the ice of the great ,ice ,age4 ,before the ,ice ,age1 then1 the lower ,hudson ,valley must have been considerably more than #hjj feet higher than at present1 because it then contained a river with sufficient current to be an active agent of erosion1 carving out the canyonlike valley in the vicinity of ,west ,point4 ,this conclusion is strongly reenforced by the fact that the old valley of the ,hudson ,river has been definitely traced as a distinct trench across the shallow sea bottom for about #ajj miles eastward from the entrance to ,new ,york harbor4 ,toward the eastern end of this trench the depth of water is now considerably over #a1jjj feet1 and thus it is obvious that1 preceding the ,ice ,age1 the earth's crust in the vicinity of ,new ,york ,city must have been much higher than at present1 so that the ,hudson ,river was able to erode its now completely drowned channel4 ,somewhat similar evidence has also established the fact that the lower ,st4 ,lawrence ,valley region was much higher before the ,ice ,age4 ,it is evident1 therefore1 that the general ,hudson-,st4 ,lawrence ,valley region is now notably lower with reference to sea level than it was before the ,ice ,age4 ,that this was caused by actual sinking of the earth's crust rather than by a rise of sea level is proved by the fact that similar changes of level between land and sea did not take place at the same time even along the ,atlantic and ,gulf coast of our ,southern ,states4 ,we shall now proceed to the next step in the geologically recent history of earth-crust movements in the ,hudson-,champlain-,st4 ,lawrence ,valley region by asserting that1 since the ,ice ,age1 the land was actually notably lower than at present4 ,in fact1 the land was enough lower to allow tidewater to extend up the ,st4 ,lawrence ,valley into the ,ontario basin1 and all through the ,champlain-,hudson ,valley4 ,many beaches1 bars1 and delta deposits formed in these arms of the sea are still plainly preserved1 in some cases with shells and bones of marine animals in them1 now hundreds of feet above sea level4 ,these marine deposits are highest above sea level in the northern portion of the ,champlain ,valley1 where they lie at an altitude of #gjj feet or more and their altitude steadily diminishes southward to about #cjj to #djj feet in the general vicinity of ,albany1 and to near sea level in the general vicinity of ,new ,york ,city4 ,obviously1 then1 the land stood lower during part or all of the interval of not more than a few tens of thousands of years since the ,ice ,age than at present4 ,this leads us to the third important conclusion regarding earth movements in this region1 namely1 that still later the land has undergone a differential uplift1 the rate having steadily increased toward the north where the total uplift is many hundreds of feet4 ,we have discussed this region somewhat in detail because the principles of slow up and down movements of the earth's crust are there so plainly recorded4 ,among many other regions where earth movements similar to those above described have taken place1 brief mention may be made of ,norway4 ,the great fjords of ,norway were1 just before the ,ice ,age1 stream-cut valleys which were then more or less modified by glacial erosion1 and after the ,ice ,age the rivers in them were drowned due to land subsidence4 ,the kind of evidence is like that above given for the lower ,hudson ,river4 ,since the subsidence there has been partial reelevation1 as proved by the fact that along the sides of the larger fjords marine terraces and beaches may be traced with gradually increasing altitude for many miles 7#aej or more7 back into the country where they are hundreds of feet above tidewater4 ,scandinavia is of still further special interest because very appreciable earth movements have there come under human observation4 ,marks carefully placed along the shores of ,sweden by the government have proved that during the last #aej years the southern end of the country has actually subsided several feet1 while from ,stockholm north the land has risen in increasing amount1 reaching a maximum of seven or eight feet4 ,in southern ,sweden1 at ,malmo1 a certain street now at times becomes covered by wind-driven high water1 and during excavations made some years ago an older street eight feet below the present one was found4 ,a theory which appears to be in perfect harmony with the facts to account for the subsidence and partial reelevation of central eastern ,north ,america and ,scandinavia since the beginning of the ,ice ,age is that the great weight of ice during the ,ice ,age pressed the land down1 and that since the removal of the ice there has been an appreciable tendency for the land to spring back4 ,certain crustal movements which have occurred about the ,bay of ,naples are of very special interest because actual human history dates can be placed upon them4 ,most remarkable are the records in connection with the temple of ,jupiter ,serapis which was built near the shore before the ,christian era4 ,the land sank about five feet and a new pavement had to be constructed2 then1 by the middle of the third century ,a4 ,d41 the temple rose to well above sea level4 ,by about the ninth century the land had subsided fully thirty feet1 so that marble columns of the temple were bored full of holes as high as twenty-one feet above their bases by marine-shelled animals1 species of which still live in the bay4 ,then a slow uplift of twenty-three feet began1 bringing the bases of the columns two feet above sea level by #agdi4 ,since that time a slight sinking has taken place and this seems to be still going on4 ,three of the marble columns with the borings still stand in upright position4 ,while the movements just described were taking place1 the island of ,capri1 twenty miles across the ,bay of ,naples1 has slowly sunk to an amount estimated at thirty or forty feet as proved by evidence from the famous ,blue ,grotto4 ,about the beginning of the ,christian era a large ancient wave-cut cave1 part of which is now called the ,blue ,grotto1 had its floor above sea level1 and it was used by certain ,romans as a cool place to retire to from the heat4 ,in order to obtain better light an opening was cut through its upper portion4 ,the land has sunk so much that at the present time even part of the artificial opening 7through which tourists pass7 is now under water4 ,by way of illustrating remarkable contrasts in direction of crustal movements on very considerable scales in a given region1 we shall briefly mention some facts regarding part of the coast of southern ,california and the neighboring islands of ,santa ,catalina and ,san ,clemente1 respectively twenty-five and fifty miles offshore4 ,those movements were not1 however1 checked up by human history records4 ,the mainland at ,san ,pedro has clearly risen #a1bdj feet1 as proved by the presence of unusually perfect coast terraces 7so-called raised beaches71 while ,san ,clemente has risen #a1ejj feet as proved by the raised beaches into which deep1 youthful ,v-shaped stream-cut valleys have been sunk1 and a shore line characteristic of recent notable uplift4 ,it is a remarkable fact that at the same time the intervening island 7,santa ,catalina7 has notably sunk1 as proved by the nature of its shore line1 and the distinctly more mature character of its topography4 ,we are1 however1 by no means dependent upon lands along sea shores for evidences of slow rising and sinking of land4 ,thus1 by careful measurements it has been shown that the general region of the ,great ,lakes is now differentially rising toward the northeast at the rate of about five inches per #ajj miles per century4 ,at ,chicago the rise of water is estimated at about nine inches per century1 which means increase of flowage through the ,chicago ,canal4 ,at this rate the upper lakes would1 in some thousands of years1 drain through this canal to the ,mississippi4 ,a well-preserved shore line of the large ancestor of ,lake ,ontario shows a steady increase in altitude at the rate of several feet per mile toward the northeast from near ,niagara to the ,st4 ,lawrence ,valley1 thus proving a tilting of the land since the shore line was formed4 ,shore lines of the great ancestor of ,great ,salt ,lake also show warping of the earth's crust1 some parts of a definite shore line being several hundred feet higher than others4 ,very significant evidence pointing to profound crustal movements consist in the finding of fossil remains of marine animals in the strata high above sea level1 very commonly from one to three miles1 in many parts of the world1 especially in the high mountains4 ,in ,wyoming1 nearly horizontal strata of the ,mesozoic ,age carrying marine fossils lie two miles or more above sea level4 ,the fact that given formations1 carrying marine fossils representing certain definite portions of geologic time1 are found at various altitudes up to several miles in many parts of the world1 shows that the land in those places has really risen relative to sea level4 ,it should not be presumed from the above discussion that the sea level itself has never changed4 ,thus1 the vast areas of thick ice sheets in both ,north ,america and ,europe during the ,great ,ice ,age represented sufficient water withdrawn from the sea to very appreciably lower its level4 ,all land-derived materials1 carried into the sea mainly by rivers1 displace sea water1 with consequent rise of its level4 ,if all existing lands were worn down and carried into the sea1 its level would be raised some hundreds of feet4 ,subsidence of any part of the ocean bottom would cause a lowering of sea level4 ,there is a strong reason to believe that some such shiftings of sea level have occurred during the vast lapse of geologic time4 ,during certain periods erosion of the land predominated1 and during other periods building up of the land predominated1 as pointed out in the chapters on geologic history4 ,it is not thought that shifting of sea level has ever amounted to more than a few hundred feet1 at least not during the millions of years of the more clearly recorded earth history4 ,we have thus far considered slow upward and downward movements of the earth's crust without notable structural changes in the rocks4 ,another type of crustal disturbance causes more or less profound changes in the structures of the rocks themselves4 ,just how the earth originated is a matter of uncertainty1 but we can be sure that for many millions of years it has been a shrinking body4 ,the outer1 or crustal1 portion of the earth1 in adjusting itself to the contracting interior1 has had many pressures1 stresses1 and strains set up within it4 ,as results of such forces the rocks at and near the earth's surface have in various places1 and at various times1 been broken 7faulted7 and subjected to sudden movements 7see discussion beyond71 while those well within the crustal portion1 that is to say a few miles or more down1 have1 in many cases1 been bent 7folded71 or even crumpled4 ,for these reasons the surface and near-surface crustal portions are called the zone of fracture1 while the more deeply buried portions comprise the zone of flowage4 ,in the zone of flowage the rocks1 where subjected to great lateral pressure1 act like plastic materials and therefore bend rather than break1 because of the great weight of overlying materials4 ,laboratory experiments have confirmed the findings of geologists in this regard4 ,small masses of rocks properly inclosed in nickel-steel cylinders have been subjected to slow differential pressures equivalent to those which obtain twenty to forty miles within the earth4 ,under such conditions rocks have been made to change shape very notably without fracturing4 ,both geological observations and experiments have led us to conclude that not even small fractures or crevices can remain open at a depth greater than ten or twelve miles even in the hardest rocks4 ,from time to time1 during the long history of the earth1 forces of lateral pressure have been slowly exerted along more or less localized zones or belts within the earth's crust1 and the rocks have been deformed chiefly by bending or folding1 especially in those regions where mountains of the folded type have developed4 ,movements of this type are considered beyond in the chapter on mountains4 ,rock folds vary in size from microscopic to miles across1 and they exhibit many shapes4 ,plate #g will give the reader a good idea of actual rock folds of common sizes and shapes in various places4 ,folded structures are most clearly discernible in sedimentary rocks1 because of their stratified 7layered7 arrangement4 ,since folds in hard rocks rarely1 if ever1 develop except at a depth of some miles within the earth1 they show at the surface only where great thicknesses of overlying materials have been stripped off by erosion4 ,illustration3 ,fig4 #h4--;,an outcrop of stratified crystalline limestone 7or marble7 exhibiting two small sharp folds--a syncline on the left and an anticline on the right--near ,lenox1 ,mass1 ,these folds developed during the great mountain-making disturbance at the end of the ,ordovician period fully #bj1jjj1jjj years ago4 7,after ,dale1 ,u4 ,s4 ,geological ,survey47 ,from the standpoint of our consideration of slow earth-crust movements1 it is important to bear in mind that lateral pressure in the zone of flowage has not only notably deformed rocks1 but that1 as a result of the buckling forces1 given rock masses have1 in many cases1 been notably shifted downward or upward--mainly upward--from their original positions4 ,folded strata carrying shells of sea animals are commonly found thousands of feet above sea level in many of the great mountain ranges of the world4 ,during the process of folding on a large scale the crust of the earth is very appreciably shortened at right angles to the direction of applied pressure1 due to squeezing or bending of the strata4 ,in the case of the ,appalachian mountains of ,pennsylvania it has been estimated that such shortening amounts to about twenty-six miles or1 in other words1 that the strata originally spread out horizontally across an area whose width was about #ajj miles have been squeezed or folded into an area whose width is twenty-six miles less4 ,illustration3 ,fig4 #i4--,structure section showing the profile of the mountains and relations of rocks below the surface near ,livingston1 ,montana4 ,the strata were crowded together until they bent into great sharply defined folds at the time of the ,rocky ,mountain ,revolution several million years ago4 ,then the rocks broke along the fault fracture and the mass on the right was shoved over upon the mass on the left4 7,after ,u4 ,s4 ,geological ,survey47 ,we shall now turn to a consideration of sudden earth movements and some of their effects1 including earthquakes4 ,mention has already been made of the fact that1 when pressures and strains are set up in the outer portion 7zone of fracture7 of the earth's crust1 the rocks yield mainly by breaking or fracturing because the rocks not being under a great load of overlying material are there brittle4 ,the earth's crust has been fractured on small and large scales in many places during the long space of geologic time4 ,where one block of earth's crust has slipped or moved past another along a fracture we have what is called a fault4 ,such displacements of rock masses vary in amount from less than an inch to some miles1 and they constitute one of the most important features of the architecture of the outer portion of the earth4 ,there are two types of faults fundamentally different as to cause4 ,in one type 7so-called normal fault7 the rocks suddenly yield to a force of tension2 a fracture develops and the earth block on one side of the fracture or fault drops with reference to that on the other4 ,in the other type 7so-called thrust faults7 the rocks yield suddenly to a force of compression or lateral thrust1 and one block of earth is pushed or thrust partly over another along the surface of fracture or fault4 7,see ,plate #h47 ,faults range in length up to hundreds of miles1 those from one to twenty miles in length being very common4 ,where an earth block has been displaced thousands of feet along a fault surface1 it is not to be understood that the whole displacement resulted from a single movement1 but rather from a series of sudden movements separated by greater or less intervals of time4 ,each sudden movement along a fault surface produces a vibration of the earth near by4 ,many such sudden movements are known to have caused violent earthquakes4 ,displacements of twenty to fifty feet1 as a result of single movements1 are definitely known to have taken place in various regions during the last fifty years2 and rarely1 if ever1 has any sudden displacement of as much as several hundred feet occurred4 ,cliffs and steep slopes very commonly result from faulting1 but1 because of the long lapse of time required for the repeated movements in the case of great faults1 the cliffs or steep slopes begin to wear back and become more or less subdued long before the last of the movements take place4 ,in regions where movements along great faults have long since ceased1 the original steep slopes may be completely obliterated by erosion4 ,illustration3 ,fig4 #aj4--,vertical sections through strata illustrating common kinds of faults3 a1 normal faults where one mass simply sinks below another2 b1 a thrust fault where one mass is shoved over another4 7,after ,u4 ,s4 ,geological ,survey47 ,how does the geologist determine the actual amount of displacement1 especially in the case of a large fault in stratified rocks8 ,first1 the various formations of the region1 where unaffected by faulting1 are carefully studied1 especially in regard to the character and thickness of each1 and their relative geologic ages or normal order as they were deposited one layer above the other4 ,then1 in the simple case of a normal-fault surface at right angles to horizontal strata1 it is only necessary to find out what two formations or parts of formations come together along the fault fracture1 and the actual amount of displacement is readily determined4 ,where strata and normal fault surfaces lie at various angles1 and also in thrust faults1 those angles must be determined in addition to the data above named4 ,in many mining regions1 where valuable deposits are affected by faulting1 accurate knowledge of the direction and amount of displacements of faults is of great economic importance4 ,a few examples of normal faults from well-known districts will now be briefly described4 ,the whole eastern front of the central and southern ,sierra ,nevada ,range of ,california is a great1 steep fault slope1 from a few thousand to ten thousand or more feet high and hundreds of miles long1 of such recent geologic age that it has been only moderately affected by erosion4 ,in fact1 it is well known that the southern two-thirds of the range is a great tilted fault block1 the total displacement having resulted from repeated sudden movements since about the middle of the present geologic era4 ,a great fault also extends along the eastern base of the great ,wasatch ,range of ,utah and the steep slope thousands of feet high is a fault scarp only slightly modified by erosion4 ,renewed movements along this profound fault have very recently taken place as proved by the presence of fresh fault scarps in loose deposits which have accumulated across the mouths of some of the canyons1 as1 for example1 near ,ogden4 ,in fact1 practically all of the north-south ranges of the ,great ,basin from ,utah to ,california are essentially a series of tilted fault blocks4 ,another great fault1 less conspicuous from the topographic standpoint1 is hundreds of miles long in the ,coast ,range ,mountains of ,california4 ,at the time of the ,san ,francisco earthquake of #aijf there was a renewed sudden movement along this great fracture4 ,the eastern one-half of the ,adirondack ,mountains of ,new ,york is literally a mosaic of hundreds of fault blocks4 ,many of these faults are from two to thirty miles long and they commonly show displacements of from a few hundred to #b1jjj or more feet4 ,a glance at the geological map 7in colors7 of the vicinity of the great copper mines at ,bisbee1 ,arizona1 shows most of that region to contain a network of normal faults which separate it into a mosaic of fault blocks4 ,in each of the examples of faults just given a block of earth has sunk relative to the other1 or in other words1 each is a normal fault4 ,we shall now turn to some large scale cases of faults in which great masses of earth have been pushed one over another--so-called thrust faults4 ,in the southern ,appalachian ,range1 and especially well exhibited in the vicinity of ,rome1 ,georgia1 one portion of the mountain mass has literally been shoved over another1 at a low angle over a fault surface many miles long1 for fully seven miles westward4 ,both the tremendous weight of rock material actually translated and the number of sudden movements required in the operation stagger the imagination4 ,it is safe to say that during the long time of this great operation violent earthquakes were not uncommon4 ,in the ,rocky ,mountains of the northern ,united ,states and southern ,canada there is the greatest known thrust fault on the continent4 ,it is hundreds of miles long1 and the actual displacement is commonly at least several miles4 ,in the ,glacier ,national ,park of ,montana it has been established that the front range portion of the ,rockies has actually been pushed at least seven miles1 and possibly as much as twenty miles1 eastward over a fault surface1 and out upon the ,great ,plains4 ,in some cases rocks of the ,prepaleozoic ,age have there been pushed upon rocks of the late ,mesozoic ,age1 thus locally upsetting the geologic column4 ,illustration3 ,fig4 #aa4--;,east-west profile and vertical structure sections fifty-two miles long in the ,mohawk ,valley region of ,new ,york1 showing numerous tilted fault blocks which notably influence the topography4 ,vertical scale exaggerated4 ,the rocks are ,prepaleozoic and early ,paleozoic in age4 7,modified by the author after ,darton1 ,new ,york ,state ,museum47 ,the ,wasatch ,range of ,utah1 in addition to the great normal fault along its western base1 contains a remarkable system of thrust faults4 ,in the region now occupied by the ,wasatch ,mountains a number of parallel 7thrust7 faults were developed close together and the broken pieces of the earth's crust between them were pushed up1 the rocks on one side of each crack riding up over those on the other side until a great mountain range was formed where once lay a plain4 ,in the ,ogden ,canyon one great earth block of ,prepaleozoic 7,algonkian7 ,age has been shoved thousands of feet over late ,paleozoic 7,carboniferous7 rock1 which latter has in turn been thrust over early ,paleozoic 7,cambrian7 rock4 ,this thrust faulting was accomplished before the development of the geologically recent normal fault along the western base of the range4 ,illustration3 ,fig4 #ab4--,vertical 7structure7 section through a part of the earth's crust several miles long in ,ogden ,canyon1 ,utah1 showing the system of great thrust faults4 ,prepaleozoic 7,algonkian7 rocks have been pushed far over upon late ,paleozoic 7,carboniferous7 strata1 which latter have in turn been shoved over early ,paleozoic 7,cambrian7 strata1 etc4 7,after ,u4 ,s4 ,geological ,survey47 ,any sudden movement of part of the crust of the earth1 due to a natural cause1 produces a trembling or shaking called an earthquake4 ,though earthquakes are generally classed among the most terrifying of all natural phenomena1 those which have occurred during human historic times have had scarcely any geological or topographical effects of real consequence on the face of the earth4 ,locally1 the effects may be notable and the destruction of life and property may be great4 ,the earth may be locally cracked and rent1 small fault scarps may develop1 landslides and avalanches may result from the shaking of the earth1 buildings may be demolished1 and sea waves may be rolled upon the land4 ,on the other hand1 many earthquakes1 called tremors1 are too slight to be noticed by people1 though they are recorded by specially constructed instruments called seismographs4 ,we have already stated that actual sudden displacements causing earthquakes have amounted to twenty or even fifty feet right along fault fractures1 but during the vibrations or quakings1 which are often so destructively sent out into the neighboring country1 the earth's surface rarely actually moves more than a small fraction of an inch4 ,because of the suddenness of the movement objects on the surface may be moved inches or even feet4 ,violent shocks may last one or two minutes and cause the whole earth to tremble1 though at distant points only seismographs record the movement4 ,it is probably true that some part of the earth is shaking all the time4 ,studies during the last fifty years have made it certain that the main cause of earthquakes is the sudden slipping of earth blocks past each other along fault fractures1 the sudden slipping furnishing the impulse which sends out the vibrations into the surrounding more or less elastic crust of the earth4 ,the low rumbling to roaring sound1 which sometimes immediately precedes an earthquake1 is probably due to the grinding of the rocks together below the surface4 ,earthquakes generally accompany volcanic outbursts of the violent or explosive type1 and in such cases subterranean explosions cause both the eruptions and the quakings of the earth4 ,it is well known that the principal volcanic districts or belts of the earth are also the belts of most frequent earthquakes1 but this does not mean that volcanic action causes most of the earthquakes4 ,active volcanoes and earthquakes are so commonly associated in the same belts because those belts no doubt represent portions of the crust which are now most actively yielding to the forces directly resulting from the shrinkage of the earth4 ,within the volcanic belts many earthquakes take place unaccompanied by any volcanic action1 and many others take place far from volcanoes4 ,some earthquakes have been caused by the impact of great landslides or avalanches1 or by the sudden caving in of underground openings4 ,brief descriptions of a few typical carefully studied earthquakes during recent years will serve to make the main features of earthquakes still clearer to the reader4 ,the violent ,japanese earthquake of #ahia was caused by the sinking of a block of earth forty miles long from two to thirty feet below that on the other side of a fault fracture4 ,there was also considerable horizontal shifting1 and cracks developed in the adjacent region4 ,a distinct fault scarp1 fifteen to twenty feet high1 developed1 and in some cases extended right across cultivated fields4 ,illustration3 ,fig4 #ac4--,map of the ,united ,states1 showing the large areas over which three of the greatest of our earthquakes were actually felt by people4 ,these earthquakes were recorded in many parts of the world by delicate instruments3 ,new ,madrid1 #ahaa2 ,charleston1 #ahhf2 ,san ,francisco1 #aijf4 ,illustration3 ,fig4 #ad4--,sketch map showing the trace of the great fault fracture along which a renewed sudden movement of as much as twenty feet took place to cause the ,san ,francisco earthquake of #aijf4 7,after ,u4 ,s4 ,geological ,survey47 ,the ,san ,francisco earthquake of #aijf was produced by renewed movement along the great fault which extends lengthwise through the ,coast ,range ,mountains for several hundred miles4 ,it is literally correct to say that1 for #bej miles along this great earth fracture1 one part of the ,coast ,range instantaneously slipped from two to twenty-two feet past the other4 ,more or less of the movement extended at least several thousand feet down into the earth4 ,in this case both sides slipped and the movement was very largely horizontal rather than vertical4 ,the land on the east side of the fault moved south and that on the west side moved north1 the amount diminishing away from the fault on each side so that some miles out the actual crustal movement was only a few inches4 ,when one thinks of the tremendous volumes of earth material involved in this shifting of the earth's crust1 is it any wonder that such destructive earthquake waves were produced8 ,many buildings were wrecked1 several hundred people were killed1 the disastrous ,san ,francisco fire resulted1 water mains were broken1 and fences and roads crossed by the fault were dislocated as much as fifteen to twenty feet4 ,illustration3 ,plate #e4--,swift ,current ,valley in ,glacier ,national ,park1 ,montana4 ,this was once a deep ,v-shaped canyon carved out 7eroded7 by stream action4 ,then a great valley glacier slowly plowed its way through it during the ,ice ,age and1 by ice erosion1 the present nearly straight ,u-shaped canyon has resulted4 7,photo by ,campbell1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #f4--,view in the ,yosemite ,valley from ,near the ,western ,entrance4 ,the great rock called ,el ,capitan1 on the left rises #c1ejj feet above the river1 and ,bridal ,veil ,falls on the right is #fbj feet high4 ,all the rock is granite1 the nearly vertical walls of which have resulted from the action of a great glacier which plowed its way through the valley during the ,ice ,age2 the valley walls have been cut back by the removal of large vertical joint blocks4 ,the flat bottom of the valley has resulted from the filling with sediment of a postglacial lake in the valley4 7,photo by ,f4 ,n4 ,kneeland1 ,northampton1 ,mass47 ,during the great earthquake on the coast of ,alaska in #ahii notable changes took place along the shore for some miles1 one portion having suddenly risen as much as forty-seven feet1 while another portion sank below sea level4 ,illustration3 ,fig4 #ae4--,map showing the principal earthquake regions of the world4 ,in #ahhf the earthquake centering near ,charleston1 ,s4 ,c41 was preceded by rumbling and roaring noises and the slight quaking increased to violent shaking which lasted more than a minute4 ,eight minutes later a rather violent earthquake shock took place1 followed during the next ten or twelve hours by less severe shocks4 ,most buildings in the city were wrecked or more or less badly damaged1 and some people were killed4 ,the shocks were so violent that the quaking was actually felt by people over an area of more than #b1jjj1jjj square miles1 the disturbance having spread at the rate of about #aej miles per minute4 ,near ,charleston openings and fissures were formed through which sand and muddy water were ejected1 but the cause of the disturbance was most likely slipping of the old very hard rocks below the loose deposits of the ,coastal ,plain4 ,from #ahaa to #ahac a series of violent earthquakes developed in the general vicinity of ,new ,madrid1 ,missouri4 ,in an area of over #b1jjj square miles1 now called the sunk country1 many portions suddenly sank giving rise to small fault scarps or cliffs1 and various lake basins were formed4 ,development of a fissure caused a local change in the course of the ,mississippi ,river4 ,in #ahig1 ,assam1 ,india1 was shaken by an earthquake of unusual magnitude1 which lasted #b-a_/b minutes4 ,an area of #aej1jjj square miles was disastrously shaken1 and the shocks were distinctly felt over an area of #gej1jjj square miles4 ,a number of notable fault scarps developed1 the movement on one having been thirty-five feet4 ,,chapter ,,viii ,,volcanoes ,,and ,,igneous ,,rocks ,not only because of the great power and terrifying grandeur of violent eruptions1 but also because of their destruction of life and property1 volcanoes stand out in the popular mind as among the most real and important of all geological phenomena4 ,but great volcanic outbursts1 like violent earthquakes1 are in truth only outward1 sensible1 relatively minor manifestations of the tremendous earth-changing forces below the surface4 ,they are far less important as geological agencies than the mighty interior forces which cause parts of continents to be slowly upraised and the rocks folded1 or even than the incessant action of streams whereby the lands are cut down4 ,even as an igneous agency1 volcanoes are notably less important than the development and shifting of molten materials within the earth's crust4 ,volcanic action is1 however1 not only conspicuous1 but it is also of real significance as a means of changing the earth1 such action having taken place since very early known geologic time4 ,after bringing out the main facts and principles of volcanoes1 aided by descriptions of specific eruptions1 we shall turn to a consideration of igneous activity within the earth's crust4 ,mount ,vesuvius in ,italy is perhaps the most famous active volcano in the world4 ,its eruptions have been more or less carefully studied for a longer time than any other4 ,the eruption in the year #gi ,a4 ,d4 was really a tremendous explosion causing a large part of the old crater to be blown away1 and sending immense volumes of rock fragments1 mostly finely divided 7so-called ashes7 into the air which completely buried the small city of ,pompeii4 ,water from the great clouds of condensing steam1 mixed with ashes1 formed muddy floods which overwhelmed ,herculaneum4 ,little or no lava was erupted4 ,since that time the crater has been more or less active and the present cone1 #d1jjj feet high1 has been built up4 ,during the last fifty years the greatest eruptions took place in #ahgb and #aijf1 when1 streams of molten rock flowed down the sides of the mountain4 ,illustration3 ,fig4 #af4--,map showing the distribution of active and recently active volcanoes of the world4 ,one of the greatest volcanic explosions ever recorded was that of the island of ,krakatoa1 between ,sumatra and ,java1 in #ahhc4 ,the greater part of the island was blown away and there was water #a1jjj feet deep1 where just before the island stood hundreds of feet high4 ,about a cubic mile of rock material was sent into the air mostly in the form of fine dust--some of it for seventeen miles--and completely hid the sun1 causing total darkness during the eruption4 ,dust fell over an area of several hundred thousand square miles4 ,several days after the explosion ships more than #a1jjj miles away were dust covered4 ,such enormous quantities of a light porous lava called pumice fell and floated upon the sea that navigation was badly obstructed many miles from the volcano4 ,extremely fine dust gradually spread through the whole earth's atmosphere1 causing the extraordinary red sunsets for several months4 ,the sound of the explosion was heard for hundreds of miles4 ,great sea waves #ej to #ajj feet high were stirred up and they swept inland for several miles over the low-lying coast lands of neighboring ,java and ,sumatra1 overwhelming hundreds of villages and drowning tens of thousands of people4 ,illustration3 ,fig4 #ag4--,the great hole left after the top of ,mt4 ,katmai in southern ,alaska was blown off in #aiab by one of the most tremendous volcanic explosions in the annals of human history4 ,the water in the lake is hot4 7,after ,griggs1 ,national ,geographic ,magazine47 ,one of the greatest explosions on record was that of ,katmai volcano1 several thousand feet high1 on the coast of ,alaska1 in ,june1 #aiab4 ,not only was the top of the mountain completely blown off1 but also a great crater pit1 three miles wide across the top and several thousand feet deep1 was developed in the stump of the former mountain4 ,volcanic dust fell to a depth of several feet within twenty-five to fifty miles of the mountain4 ,dust accumulated to a depth of nearly a foot in the village of ,kodiak1 #ajj miles east of the mountain1 where total darkness prevailed for more than two days4 ,a lake of very hot water now occupies the bottom of the great new crater4 ,the noise of the explosion was heard for at least #gej miles4 ,illustration3 ,fig4 #ah4--;,diagrammatic vertical or structure section through a portion of the earth illustrating the common modes of occurrence of igneous rocks4 ,p1 deep-seated 7plutonic7 igneous rock2 ,s1 strata2 ,d1 dikes2 ,m1 mass of igneous rock forced between strata bending them upward2 ,f1 feeding channel of volcano2 ,v1 volcano2 ,l1 lava sheets4 7,by the author47 ,one of the most frightful volcanic catastrophes of recent years was the eruption of ,mont ,pele1 island of ,martinique1 ,west ,indies1 in #aijb4 ,in this case1 also1 no lava was poured out1 but violent explosions sent great clouds of very highly heated gases and vapors1 mingled with incandescent dust1 thousands of feet into the air4 ,one of these great clouds rushed down the mountain at hurricane speed and destroyed the city of ,st4 ,pierre with its #cj1jjj inhabitants4 ,after the main eruption a spine or core of hard rock began to rise out of the crater and it slowly grew to a height of #a1jjj feet in several months1 after which it began to crumble away4 ,this spine probably represented nearly frozen lava which solidified as it was gradually forced out of the mountain4 ,of special interest to us1 though not of great importance is the only active volcano in the ,united ,states4 ,in ,may1 #aiad1 ,mount ,lassen 7or ,lassen ,peak71 a long inactive volcano in northern ,california1 suddenly burst forth explosively and during the next several years hundreds of eruptions occurred4 ,little or no lava appeared1 but great clouds of steam and dust often shot into the air from one to three miles above the top of the mountain1 which lies over #aj1jjj feet above sea level4 7,plate #aj47 ,great quantities of dust have accumulated for miles around the mountain4 ,at this writing 7,october1 #aibj7 the mountain is again active4 ,it should not be presumed1 however1 that all1 or nearly all1 volcanoes are of the explosive type4 ,others of the more quiet type are well exemplified by the two great ,hawaiian volcanoes1 ,mauna ,loa and ,kilauea4 ,any but relatively very minor explosions rarely1 if ever1 occur1 the product of such volcanoes being almost wholly lava1 which flows down the mountainsides in molten streams4 ,the ,hawaiian ,islands have1 in fact1 been almost entirely built up by successive eruptions of lava1 the building-up process having begun well below sea level4 ,mauna ,loa rises to nearly #ad1jjj feet above the sea1 but1 due to the fact that the streams of lava have spread so far1 the mountain has an exceptionally low angle of slope which makes it difficult to realize that it is so high4 ,considering its submarine portion1 ,mauna ,loa really rises nearly #cj1jjj feet above the sea floor4 ,although ,kilauea lies nearly #d1jjj feet above sea level on the flank of ,mauna ,loa1 and only twenty miles distant from it1 the two volcanoes are singularly independent in regard to their eruptions4 ,each mountain has a crater irregularly oval in shape1 nearly three miles long1 bounded by almost vertical walls of hard lava1 in some cases arranged in terraces4 ,the floors of the great crater pits are relatively level1 and consist of black lava in which are lakes of molten and even boiling lava4 ,the black lava floor is1 in each case1 only a frozen or hardened crust upon a great column of molten lava extending down into the mountain4 ,prior to an eruption of ,mauna ,loa the lava column rises hundreds of feet in the crater1 but during recent years the lava seldom1 if ever1 flows out over the crater rim4 ,instead1 it breaks through the mountainsides at various altitudes1 the great flow of #aiai having started at an altitude of about #h1jjj feet4 ,this stream of liquid rock1 fully one-half of a mile wide1 flowed for weeks down the mountainside and into the ocean1 the waters of which1 in contact with the highly heated lava1 were thrown into terrific commotion4 ,in #ahhe a stream of lava several miles wide flowed forty-five miles4 ,in one case1 lava traveled the first fifteen miles in two hours1 but this is an unusually great rate of speed4 ,lava streams in general seldom move faster than one or two miles per hour1 and as the liquid rock gradually cools and becomes more and more viscous1 the speed diminishes to zero4 ,almost incredible volumes of steam emanate from streams of molten lava4 ,in #ahdj an outflow of lava took place from the side of ,kilauea ,mountain and ran into the sea4 ,since that time the floor of the great crater pit 7quoting ,professor ,w4 ,h4 ,hobbs7 has been essentially a movable platform of frozen lava of unknown and doubtless variable thickness which has risen and descended 7hundreds of feet7 like the floor of an elevator car between its guiding ways4 ,the floor has1 however1 never been complete1 for one or more open lakes are always to be seen1 that of ,halemaumau1 located near the southwestern margin1 having been much the most persistent4 ,within the open lakes the boiling lava is apparently white hot at a depth of but a few inches below the surface1 and in the overturnings of the mass these hotter portions are brought to the surface and appear as white streaks marking the redder surface portions4 ,from time to time the surface freezes over1 the cracks open and erupt at favored points along the fissures1 sending up jets and fountains of lava1 the material of which falls in pasty fragments that build up driblet cones4 ,small fluid clots are shot out1 carrying threadlike lines of lava glass behind them1 the well-known ',pele's hair4' ,sometimes the open lakes build up congealed walls1 rising above the general level of the pit1 and from their rim the lava spills over in cascades to spread out upon the frozen floor4 ,in some regions1 like the ,columbian ,plateau of the northwestern ,united ,states and the ,deccan of ,india1 each covering about #bjj1jjj square miles1 vast quantities of lava have been poured out layer upon layer to depths of even thousands of feet4 ,distinct volcanic cones or mountains in those regions are either absent or too scarce to look to as sources of so much lava4 ,such lava floods were probably mostly erupted from great fissures in the earth's crust1 the fluidity to spread many miles4 ,some idea of the quantitative geological importance of volcanism may be conveyed to the reader when we assert that1 according to a conservative estimate1 fully one-half of a million cubic miles of molten rocks have been poured out upon the surface of the earth through volcanic action in relatively recent geological time6 ,the ,cascade ,range with its lofty peaks1 including ,mount ,shasta and ,mount ,rainier1 each rising more than #ad1jjj feet above the sea1 has been built up very largely by volcanic action during the last era of geologic time4 ,many other mountain peaks and various ranges have been similarly developed either wholly or in part4 ,the great chain of ,aleutian ,islands extending hundreds of miles into the sea1 is the scene of much volcanic activity where a great mountain range is now literally being born out of the sea by the processes of vulcanism4 ,before this the reader has more than likely wondered about the source of the heat1 vapors 7mainly water71 and power involved in volcanic action4 ,answers to these questions are closely tied up with the precise cause 7or causes7 of volcanic action which remains one of the most uncertain of the larger problems of geologic science4 ,before briefly discussing the causes1 a few additional facts should be stated4 ,first1 in regard to the heat1 a careful determination of the temperature of the molten lava of ,kilauea in #aiaa showed it to be #a1bfj degrees ,centigrade1 or #b1cjj degrees ,f4 ,this is1 however1 a relatively low temperature1 because many lavas from other regions show melting points all the way up to at least #b1jjj degrees ,centigrade 7#c1fjj degrees ,f474 ,water in the form of steam is quantitatively one of the greatest products of volcanoes4 ,a fair idea of the tremendous volumes of water involved may be gained from the statement that a careful estimate shows that fully #dfj1jjj1jjj gallons of water in the form of steam erupted from a single secondary cone of ,mount ,etna during a period of #ajj days4 ,among other gases which are given off in greater or less amounts during volcanic activity are carbonic acid gas1 sulphureted hydrogen1 sulphur dioxide1 and hydrochloric acid4 ,some idea of the power back of volcanoes may be gained not only from the tremendous explosions such as those above described1 but also from the fact that the pressure necessary to raise the column of lava from sea level to the top of ,mauna ,loa 7nearly #ad1jjj feet7 is about #a1aej atmospheres1 or about #ag1jjj pounds per square inch4 ,the actual pressure must there be much greater because the lava is forced up from far below sea level4 ,a long-held idea that a relatively thin crust covers a molten interior1 and that downward pressure of this crust due to earth contraction causes molten rocks to be forced out1 has been too thoroughly disproved to now be at all seriously entertained4 ,the fact that near-by volcanoes commonly erupt entirely independently1 as in the case of ,mauna ,loa and ,kilauea1 shows that there can be no universal liquid beneath a relatively thin crust4 ,other arguments against liquidity of the earth's interior are that the earth acts like a body nearly as rigid as steel against the powerful tide-producing forces1 and that earthquake waves which pass through the earth to a depth of at least #b1jjj miles are the kind which require a solid medium for transmission4 ,let us then briefly consider more plausible views regarding the cause of volcanic action4 ,first of all we may be sure that the earth is highly heated inside4 ,measurements in many deep borings show that the temperature increases at the rate of about #a degree ,f4 for each #ej to #fj feet downward1 to depths greater than a mile4 ,accordingly1 on the basis of #a degree rise in #ej feet1 at depths of #bj to #ce miles1 the temperature must be great enough 7#b1abj degrees to #c1eij degrees ,f471 to cause all ordinary rocks to melt if they were at the surface4 ,at such depths1 however1 the downward pressure upon the rocks is so great that their melting points are notably raised1 and there is every reason to believe that under ordinary conditions the rocks #bj to #ce miles down are not molten4 ,if we adhere to the older 7nebular7 hypothesis of earth origin1 the interior heat of the earth is left over from the cooling1 once molten1 earth4 ,on the basis of another 7planetesimal7 hypothesis1 the earth's heat is due to the steady1 powerful action of gravity causing the earth to contract4 ,in any case1 the earth is hot inside as proved by deep well records and igneous phenomena in general1 and it is a contracting or shrinking body as proved by the many large scale zones of wrinkling or folding of rocks4 ,if1 then1 highly heated solid rocks at reasonable distances down in any part of the earth are subjected to relief of pressure by an earth movement such as upward crumpling of the crust1 or by readjustment of large fault blocks1 such heated solid rocks would become molten4 ,the very earth movement which brings about relief of pressure and melting may very reasonably be regarded as the power which forces some of the newly formed molten material higher up into the earth's crust1 and even out upon the surface4 ,this view harmonizes with the well-known fact1 already mentioned1 that the main belts of active volcanoes are also the main belts of active earth movements1 such as earthquakes4 ,another source of power behind volcanic action is steam pressure4 ,we have already mentioned the fact that vast amounts of water in the form of steam escape from volcanoes or even from streams of molten lava4 ,the violent volcanic explosions are quite certainly all1 or nearly all1 direct results of sudden giving way of volcanoes to steam pressure which accumulates during greater or less periods of time1 and with little or no possibility of escape1 without rupturing the mountain4 ,steam alone1 or combined with some of the other gases so common as volcanic products1 may also aid in forcing out molten rock4 ,what is the source of the steam and other gases or vapors8 ,according to one view they were originally in the earth1 while according to another view the water at least has been absorbed by the molten rocks from surface waters which worked their way downward4 ,at least two arguments oppose the second hypothesis3 first1 that not a few volcanoes are really many miles from the sea or other bodies of water1 while downward percolation of rain water would fall far short of supplying the tremendous quantities of water ejected1 and second1 any water taken up by molten rock must be absorbed within a very few miles of the surface because1 as we have learned1 farther down there are no openings large enough to permit the downward passage of water1 but1 as a matter of fact1 the very upper part of the earth's crust is just the place where molten rocks begin to give up their water1 often with terrific violence4 ,we may now turn to a consideration of the other very important kind of igneous activity1 namely1 the rise and transfer of molten materials within the earth's crust1 but not to the surface4 ,the quantity of such deep-seated 7so-called plutonic7 igneous rock material which has been intruded into the earth's crust within known geologic time1 is far greater than that which has been forced to surface1 that is the so-called volcanic material4 ,the plutonic rocks are always thoroughly crystallized1 and they are generally coarser grained than the volcanic rocks4 ,where molten materials have been forced into cracks or fissures in the crust of the earth and there congealed1 we have a very common mode of occurrence called dikes 7,plate #i74 ,in many regions often one set of dikes was formed1 after which one or more succeeding injections from the same or different deep-seated bodies of molten rock took place1 and some of the later dikes were forced to cut across earlier ones4 ,dikes of all lengths up to at least thirty miles1 and of all widths up to many hundreds of feet1 are known1 but they are generally less than a mile long and not more than a few feet or rods wide4 ,they have been intruded into all kinds of rock formations--igneous1 sedimentary1 and metamorphic4 ,dikes are common in many parts of the world and they often excite the interest of lay-men4 ,they are wonderfully displayed along the southern coast of ,maine4 ,plate #i shows small dikes where the molten material was forced from a larger mass into a body of older dark rock4 ,the ,palisades of the ,hudson ,river1 just north of ,new ,york ,city1 consists of a layer of igneous rock several hundred feet thick which1 in the molten condition1 was forced nearly horizontally between layers of sandstone millions of years ago1 that is in the early ,mesozoic era4 ,the palisade or columnar structure was caused by cracking of the rock during the cooling and contraction4 ,this is the explanation of most columnar structures of igneous rocks1 exceptionally fine exhibitions being at the ,giant's ,causeway in ,ireland1 and ,devil's ,tower1 ,wyoming 7,plate #aj74 ,a type of occurrence not so common1 but of special interest1 is where a body of molten rock rising in nearly horizontal strata becomes cooler and therefore stiffer or more viscous and1 losing its power to penetrate1 forces its way between the layers causing the strata to be arched or domed over it4 ,sufficient removal of overlying material by erosion has revealed many fine examples of this type of occurrence4 ,another type of interest is the volcanic neck1 which is the core or plug filling the feeding channel of a volcano4 ,in certain regions1 like parts of ,arizona and ,new ,mexico1 extinct volcanic mountains may be all cut away by erosion1 except the central cores or necks which1 both because they are more resistant and are last to be reached by erosion1 stand out conspicuously as great towers on the landscape 7,plate #i74 ,most important of all from the quantitative standpoint1 however1 are the great bodies of igneous rocks1 ranging up to many miles across1 which1 in a molten condition1 were forced irregularly into the earth's crust from unknown depths4 ,the common rock called granite belongs in this category of rocks1 which are the best and most extensively developed of all igneous types4 ,the roots or cores of great mountain ranges often consist of such rocks which are exposed to view only after removal of great thickness of overlying material4 ,immense areas of granite and other plutonic rocks of extra deep-seated origin are exposed1 because of removal of overlying material by erosion1 in southeastern ,canada1 the ,adirondack ,mountains1 ,new ,england1 the ,piedmont ,plateau of the ,atlantic ,coast1 and in the ,sierra ,nevada ,mountains4 ,all the rock forming the lofty walls of ,yosemite ,valley is granite1 which was forced into the earth's crust in relatively late ,mesozoic time1 and which has since been laid bare by erosion4 ,,chapter ,,ix ,,waters ,,within ,,the ,,earth ,it has been estimated that approximately #a1ejj cubic miles of water fall upon the surface of the ,united ,states each year4 ,about one-half of this goes back into the atmosphere by evaporation2 about one-third of it flows away in surface streams2 and the remaining one-sixth enters the crust of the earth4 ,considerable water which enters the earth returns to the surface as springs1 by capillarity of soils and rocks1 or by being drawn up into plants and evaporated4 ,some idea of the amount of ground water may be gleaned from the statement1 based upon a careful estimate1 that all the water in the rocks and soils of the first #ajj feet below the surface of the ,united ,states would make a layer seventeen feet thick4 ,in most humid regions the soils and loose rock formations are saturated with water at greater or less depths 7usually less than #ajj feet7 below the surface4 ,the surface of this saturated layer is called the ground-water level1 or more familiarly the water table4 ,the water table shifts up and down more or less according to variation in rainfall4 ,in addition to the water held in the loose rocks and soils near the earth's surface1 large quantities occur in definite layers 7usually strata7 of porous rocks which very commonly extend at various angles1 hundreds or even thousands of feet into the earth4 ,a very fine illustration of this principle is the case of the ,dakota sandstone formation of ,nebraska4 ,almost anywhere across the ,state a well drilled through a bed of clay and into the porous sandstone layer encounters water4 7,figure #ai47 ,another principle is also well illustrated1 namely1 that water in such a porous layer may actually travel hundreds of miles1 water obtained from a well sunk to the ,dakota sandstone having actually traveled under the surface of the ,state all the way from the eastern face of the ,rocky ,mountains1 where rain and melting snow entered the upturned and exposed porous rock layer4 ,another good example is ,iowa1 where certain porous rock layers outcropping in the northwestern and northeastern corners of that and adjacent ,states gradually bend down under the ,state1 reaching the greatest depths 7up to #c1jjj feet7 far in the interior4 ,from wells #c1jjj feet deep near ,boone1 ,iowa1 it is1 therefore1 a fact that some of the water pumped out of the earth actually traveled underground all the way from beyond the corners of the ,state4 ,this sort of travel of underground water is common in many parts of the world4 ,it should be clearly understood that such water does not flow freely as in a pipe along subterranean passageways1 but rather it slowly works its way between the grains of porous rock4 ,where such water moves distinctly downward1 and the porous layer has both above and below it an impervious rock layer like shale or clay1 it gradually gets under greater and greater pressure4 ,in some cases such pressure has actually been found by deep drilling to be equivalent to that of a column of water several thousand feet high4 ,the rate of motion of water in porous underground rock layers is very slow1 data from various sources indicating a rate of speed of not more than one-fifth of a mile a year in coarse porous sandstone1 while in many rocks it cannot be more than ten to fifty feet per year4 ,illustration3 ,fig4 #ai4--,vertical 7structure7 section from the ,rocky ,mountains to ,omaha1 ,nebraska1 illustrating a widespread underground porous rock layer 7the ,dakota sandstone7 charged with water under pressure1 the clay formation acting as a cap rock4 7,after ,darton1 ,u4 ,s4 ,geological ,survey47 ,we still have to consider a third mode of occurrence of waters within the earth4 ,many formations1 like granite and other types of crystalline rocks are neither in definite layers1 nor are they sufficiently porous to allow water to really flow through them4 ,where such rocks extend far down from or near the surface1 how does rain water descend8 ,it does so along cracks or fractures 7both joints and faults7 which we have learned are almost universally abundantly present in all hard rocks in the upper 7or zone of fracture7 portion of the earth's crust4 ,joint cracks are generally very irregular in direction and spacing1 while fault fractures are usually fairly regular and straight4 ,many cracks are not wide enough to allow anything like good passageways for water1 while others are sufficiently open to allow water to travel along them for hundreds1 or even thousands of feet4 ,in canyons of the ,west1 springs not rarely emerge from the bottoms of great1 nearly vertical ledges of granite and other hard crystalline rocks1 the waters certainly having entered the rocks hundreds1 or even some thousands of feet1 higher4 ,in rocks of the kind here considered it is evident1 then1 that the movements of subterranean waters must be mostly exceedingly irregular and usually not in great quantities4 ,in many deep mines of the world1 underground water causes little or no trouble except often near the surface4 ,occasionally a shaft or tunnel strikes a prominent joint or fault fracture filled with water4 ,what we might really call underground streams may occur only under exceptional conditions in rocks other than limestone1 but in limestone they are not uncommon because the slow solubility of the rock allows underground waters to slowly enlarge the passageways to form distinct channels4 ,echo ,river1 which flows through ,mammoth ,cave1 is a fine case in point4 ,most water by far which emerges as springs1 was at one time surface water4 ,a simple1 but common case is where rain water soaking through porous soil 7e4g41 sand7 or rock1 sinks to the top of an underlying impervious layer 7e4g41 clay7 along whose surface it flows until it reaches the side of a valley where a spring results4 ,in fact1 wherever the water table is crossed by the surface of the ground1 water must either seep or flow out4 ,where underground streams which are common in limestone regions reach the surface on hill or valley sides1 springs result4 ,another source of springs is where under proper conditions of slope a porous rock layer1 charged with water well below the surface1 appears at a lower level than its source of water4 ,still another type of spring is where a fissure or fracture crosses a water-bearing layer in which the pressure is great enough to cause the water to rise to the surface along the relatively open fissure or fracture4 ,in various localities we hear of springs in seemingly paradoxical situations on tops of hills and even mountains4 ,such a mystery is not difficult to clear up4 ,in the first place1 such springs are rarely at the summit of the hill or mountain4 ,a case well known to many persons is the small1 but never-failing spring a little below the summit of ,mount ,whiteface1 a peak in the ,adirondacks1 rising #c1jjj feet above the general level of the immediately surrounding country4 ,in this case1 a mass of highly fractured rock1 subjected to much rainfall and lying above the level of the spring1 is sufficiently large easily to contain and give forth enough water to account for several such springs4 ,in rare cases1 however1 springs or flowing wells are located on summits1 and in such places it is only necessary to bear in mind some of the principles above set forth1 but mainly the facts that water may travel under pressure long distances underground1 and that the point of emergence may be on a hill which is actually lower than the source of the water far away4 ,the economic significance of underground waters is forcibly brought to our attention when we realize that #ge per cent of the people of the ,united ,states depend upon wells for their water supply4 ,many city supplies1 most farm supplies1 and much irrigation water come from wells4 ,the #c1jjj1jjj people of ,iowa1 for example1 are dependent upon underground waters from wells varying in depth from a few feet to several thousand feet4 ,illustration3 ,fig4 #bj4--;,ideal section illustrating the chief requisite conditions of artesian wells4 ,a1 a porous stratum2 ,b and ,c1 impervious beds below and above ,a1 acting as confining strata2 ,f1 the height of the water level in the porous bed ,a1 or1 in other words1 the height of the reservoir or fountainhead2 ,d and ,e1 flowing wells springing from the porous water-filled bed ,a4 7,after ,u4 ,s4 ,geological ,survey47 ,most wells are simply dug to depths a little below the water table4 ,in humid climate regions the depths seldom exceed fifty feet4 ,the water encountered in such wells is rarely under pressure4 ,in some regions of deep soils or loose formations1 wells are actually bored with an auger to depths of as great as #bjj feet4 ,deep wells in relatively hard rocks are always drilled to depths of even thousands of feet4 ,in such cases the purpose is to strike either a porous rock layer charged with water1 or a crack or fissure filled with water1 the water almost always being under pressure 7sometimes very great71 under such conditions4 ,these are called artesian wells whether the water under pressure actually flows out at the surface or not4 ,we may now inquire as to the necessary conditions for artesian wells4 ,this may best be done by the aid of diagrams4 ,figure #bj illustrates a very common case where a porous layer1 lying between impervious layers1 passing under a valley1 comes to the surface of the hills on each side where the water enters the porous layer4 ,on sinking a well to the water-charged layer1 the water rushes through the hole to a greater or less distance above the surface4 ,in ,figure #ba the porous and impervious layers are simply tilted1 and the water under pressure rises through the free opening to the surface4 ,wells of this kind are also common in the ,atlantic ,coastal ,plain of the ,united ,states4 ,in another case1 less comprehensible to the layman1 the porous water-bearing stratum curves downward under a hill or mountain1 water entering it where it is exposed on each side4 ,under such conditions a flowing artesian well cannot be drilled at or near the summit1 but since the water is under pressure it will rise in the hole to a level approaching that of the lowest part of the outcrop of the porous layer on either side of the hill or mountain4 ,this is essentially the condition of things toward the interior of ,iowa1 where water from the deeper wells rises #b1jjj feet or more in the holes1 but does not reach the surface4 ,illustration3 ,fig4 #ba4--,section illustrating the thinning out of a porous water-bearing bed4 ,a1 inclosed between impervious beds ,b and ,c1 thus furnishing the necessary conditions for an artesian fountain at ,d4 7,after ,u4 ,s4 ,geological ,survey47 ,the drilling of deep wells1 where records1 including samples of rock materials brought up1 have been kept1 has been a great aid to the geologist in determining1 or rendering more precise1 the knowledge of not only the kinds of rocks underground1 but also the thicknesses and structural relations of the formations4 ,in yet another way deep wells are of special significance1 that is in regard to the light which they throw upon the subterranean temperature of the earth4 ,very recently the deepest well in the world was drilled near ,fairmont1 ,west ,virginia1 to a depth of #g1egi feet1 in quest of oil or gas4 ,at a depth of #g1ejj feet1 the temperature was found to be #afh degrees ,f4 ,allowing for a near-surface temperature #ej degrees1 this means an average rate of increase downward of #a degree in #fb feet4 ,the second deepest well is near ,clarksburg1 ,west ,virginia1 sunk to a depth of #g1chf feet1 with a temperature of #agb degrees at the #g1jjj-;foot level1 or at the rate ,of #a degree in #eg feet1 allowing for a near-surface temperature of #ej degrees4 ,it is a remarkable fact1 that little or no water was encountered all the way down4 ,a well #g1cdh feet deep in southeastern ,germany gave a temperature of #ahf degrees at the bottom1 or a rate of increase of #a degree in #ed feet4 ,these three records are about the average for the deep holes of the world4 ,next to the deepest mining shafts in the world are in the copper mining region of northern ,michigan1 where over #e1jjj feet 7counted vertically1 not down the slope7 down the temperature is nearly #ij degrees the year round4 ,the rate of increase is here less than in most wells of such depth1 because of the cooling air currents4 ,many years ago a rather remarkable experiment in well drilling was tried by the city of ,budapest1 ,hungary1 the attempt being to get a supply of water at the brewing temperature of #agf degrees in order to encourage the manufacture of beer4 ,after getting a good supply of water at a depth of #c1abj feet and a temperature of #aeh degrees1 work was stopped4 ,in building the two great tunnels 7,st4 ,gotthard and ,simplon7 through portions of the ,alps1 such high temperatures were encountered that work was continued only under great difficulties4 ,in the famous ,comstock gold and silver mine of ,nevada1 over forty years ago1 temperatures as high as #aeg degrees were encountered in the shafts at a depth of only #b1jjj feet or a little over1 the exceptional temperature for such depth no doubt being due to occurrence of the ores in geologically recent igneous rocks which have not yet cooled to the normal temperature for the depth of #b1jjj feet4 ,from the sanitary standpoint1 wells are of very great significance1 especially in view of the fact that such a large proportion of people depend upon well water4 ,it is generally understood that typhoid fever is more common in the country than in cities1 in spite of what might reasonably be expected4 ,what are some of the causes leading up to such a situation8 ,the idea that water purifies itself after flowing a relatively short distance is1 in many cases1 far from being true1 especially when we are dealing with underground water4 ,actual observations prove that germ-laden water may travel surprisingly far underground4 ,germ-laden water from barns1 cesspools1 or outhouses spreads notably on sinking to the water table and it is easy to see how so many wells become contaminated4 ,on general principles1 a geologist is especially wary of water from a well in a barn yard4 ,the well for human use at least should be located out of reasonable range of such contamination4 ,under the condition of the diagram a well or spring some distance down the side of the hill may actually be unfit for use1 though a serious situation is much less likely to develop there4 ,nor should one assume that by locating the well on the uphill side of the house1 and the outhouses or cesspool on the downhill side1 safety is assured4 ,from what we have learned in regard to earth movements1 and the tilting of strata from their original positions1 we know how the movement of water in the saturated zone near the surface may be downhill roughly following the hill slope1 while in a tilted porous layer of rock farther below the surface the movement of water may be in just the opposite direction4 ,a well drilled into the solid rock for safety on the uphill side of a house might derive its water from this very same porous layer1 whose water has been contaminated from a cesspool or other source down the side of the hill4 ,such a case is by no means a theory or a rarity4 ,there is also real danger of contamination in cases where the water flows more like streams underground through cracks or fissures in hard or dense rocks1 or through channels developed by solution in limestone4 ,it may happen that water becoming contaminated from barn sites1 cesspools1 or outhouses finds its way along such a channel to the side or bottom of a well4 ,the author well remembers the case of a farmer whose house1 barn1 and well were close together on a little limestone terrace and who continued to use the well water although he complained of its disagreeable taste1 especially after a rain when he could taste the barn in it4 ,illustration3 ,fig4 #bb4--;,diagram illustrating a danger of contamination of wells by impure underground water4 ,s1 soil2 ,b1 bedrock2 ,p1 porous stratum4 ,impure water from cesspool moves through porous layer to bottom of well4 7,by the author47 ,finally1 in this connection1 it may be said that wells should be located in the light of the principles above explained1 best of all upon the advice of some one with geological training1 and that1 to insure safety to health from the well water1 sanitary analyses 7at small cost7 should be made once or twice a year4 ,a bad well should be abandoned and a new one sunk4 ,a large amount of money has been wasted upon1 and much mystery and superstition has surrounded so-called water witches1 or those who claim some special or supernatural power of locating supplies of underground water4 ,most common of all devices used is the so-called divining rod1 which is a forked stick of willow1 witch-hazel1 or other wood1 according to the seemingly special requirement of the operator4 ,certain mechanical and electrical devices are also employed4 ,with one fork of the divining rod grasped in each hand and the main part of the stick upright1 the operator walks about until1 due to some mysterious influence1 a place is found where underground water pulls the upright portion of the stick downward in spite of the grasp of the holder4 ,some operators even claim to know just how deep a well must be sunk4 ,without any attempt to question the honesty of all operators1 geologists are in full accord with the following quotation from a paper published by ,m4 ,l4 ,fuller1 for the ,united ,states ,geological ,survey3 ,the uselessness of the divining rod is indicated by the facts that it may be worked at will by the operator1 that he fails to detect strong water currents in tunnels and other channels that afford no surface indications of water1 and that his locations in limestone regions where water flows in well-defined channels are no more successful than those dependent upon mere guesses4 ,in fact1 its operators are successful only in regions in which ground water occurs in definite sheets in porous material1 or in more or less clayey deposits1 such as pebbly clay or till4 ,in such regions 7which are extremely common7 few failures can occur1 for wells can get water almost anywhere4 ,ground water occurs under certain definite conditions1 and just as surface streams may be expected wherever there is a valley1 so ground water may be found where certain rocks and conditions exist4 ,no appliance1 either mechanical or electrical1 has yet been devised that will detect water in places where plain common sense will not show its presence just as well4 ,the only advantage of employing a 'water witch1' as the operator of the divining rod is sometimes called1 is that crudely skilled services are thus occasionally obtained1 since the men so employed1 if endowed with any natural shrewdness1 become1 through their experience in locating wells1 better observers of the occurrences and movements of ground water than the average person4 ,it should not be assumed1 however1 from the above statement that the location or foretelling of underground water is mostly hopeless from a scientific point of view4 ,in most regions the kinds of rocks which would be pierced by wells can be more or less accurately foretold by careful studies of the rocks exposed at the surface4 ,but foretelling the underground water is often much more uncertain4 ,where the geologic structure or arrangement of rocks in a region is fairly regular1 as in the case of most sedimentary rocks1 and a few scattering deep wells have been drilled1 with records preserved1 the geologist1 by combining such data with his surface studies1 can do much toward putting the facts regarding the underground waters of the region on a scientific basis4 ,there are many such regions1 an excellent case in point being ,iowa1 regarding which ,state the ,united ,states ,geological ,survey has published a report containing data by the use of which it is possible to foretell almost exactly what formations would be pierced by drilling from #a1jjj to #c1jjj feet or more1 the thickness of each1 which ones are water-bearing and1 in many cases1 even the character of the mineralization of the water for almost any part of the ,state4 ,such knowledge1 through the years1 is worth untold millions of dollars to the ,state4 ,where the rocks are igneous and rather uniformly dense1 usually little or nothing can be accurately foretold about the underground water supplies1 because in such rocks the water follows exceedingly variable and irregular cracks and fissures4 ,in metamorphic rocks the difficulties are usually about as great4 ,in limestone regions1 with humid climate1 much water travels in channels underground1 but these are so exceedingly irregular that there is no way of locating them by surface studies4 ,in humid climates it seldom happens1 however1 that a well does not reach at least a fair supply of water within a few thousand feet even in rock formations in which the water travels along irregular cracks and channels4 ,certain other important features of the geological work of underground water should be brought to the attention of the reader4 ,one of these is its power to dissolve mineral substances of many kinds more or less rapidly4 ,as already pointed out1 limestone is especially susceptible to solution in water1 both surface and underground4 ,the carbonate of lime taken into solution from limestone is the principal substance which causes so-called hard water4 ,most of the solution takes place in the upper part of the zone of fracture of the earth's crust and the dissolved substances are carried along generally to the lower levels where they tend to deposit 7and crystallize71 filling fissures1 cracks1 and even tiny spaces between mineral grains4 ,cracks and fissures thus filled by mineral matter from solution are called veins4 ,in many mining regions valuable ores and other substances have been deposited from underground water solutions and concentrated in veins4 ,in many places underground waters with certain substances in solution travel through various rocks or encounter solutions of other substances and1 as a result of chemical action1 many new mineral combinations result4 ,such actions through the millions of years of geologic time have effected great changes in many rock formations4 ,in the case of petrification1 like that of petrified wood1 the buried organism slowly decomposes cell by cell1 and particle by particle it is replaced by mineral matter from underground water solutions4 ,in this manner the remarkable so-called petrified forests 7not really forests7 of ,arizona and the ,yellowstone ,park were formed1 the petrifying material there having been the very common substance called silica which is the same in composition as the familiar mineral quartz4 ,mineral matter carried in solution in surface streams is derived from ground waters which reach the surface4 ,an idea of the tremendous quantity of mineral matter thus removed may be gained from the statement that by careful determination the ,mississippi ,river carries #abj1jjj1jjj tons in solution into the ,gulf of ,mexico each year4 ,illustration3 ,fig4 #bc4--,structure section and part of landscape in a limestone region showing how caves and natural bridges are formed by the dissolving action of underground water4 ,,aa1 limestone2 ,,bb1 sink holes2 ,,dd1 caves and galleries2 and an arch 7natural bridge7 which is the remnant of a large cave4 7,after ,shaler1 ,u4 ,s4 ,geological ,survey47 ,one interesting effect of the dissolving power of underground water in limestone regions is the development of caves or caverns4 ,most remarkable of all is ,mammoth ,cave1 ,kentucky1 with its hundreds of miles of passageways and galleries4 ,this marvelous work of nature is all a result of the action of underground water which has dissolved and carried away vast quantities of limestone4 ,echo ,river1 which flows through the cavern1 is still carrying on the work aided by various underground tributaries4 ,the stalactites and stalagmites1 which are so strikingly displayed in many caves1 as at ,luray1 ,virginia1 in which water with carbonate of lime drips or oozes from the roof and1 due mainly to evaporation1 deposits the lime4 ,many wonderful and fantastic effects are thus produced4 ,where part of the roof of a cave is dissolved out1 or falls in1 a sink hole results4 ,where all but a portion of the roof of a cave or underground channel has fallen in1 a natural bridge1 like the famous one in ,virginia1 results1 though natural bridges are also formed by other means4 ,in concluding this chapter we shall briefly discuss hot underground waters1 hot springs1 and geysers4 ,there are two well-known ways by which underground waters may become heated4 ,one is by the movement of water downward into the normally heated portion of the earth1 the rate of increase downward being1 as above stated1 #a degree ,f4 for about #ej to #fj feet4 ,water descending two miles would1 therefore1 attain a temperature of about #bjj degrees ,f4 ,in some regions such a temperature may be reached at depths considerably less4 ,such water 7under pressure7 taking a short course to the surface 7forming springs7 at a lower level would retain much of its heat taken up far below the surface4 ,in regions where there are great down-folds of the strata 7i4e41 synclines71 as in the central to southern ,appalachians1 conditions appear to be favorable for such warm or hot springs1 as1 for example1 at ,hot ,springs1 ,virginia4 ,a second cause of the heating of underground water is by the descent of surface waters into contact with masses of still hot igneous rock of relatively recent geologic age4 ,in some such cases the water does not go more than some hundreds of feet down and when1 under proper conditions1 it returns to the surface hot and even boiling springs may result4 ,illustration3 ,plate #g4--;,an ,upbend ,fold 7anticline7 in the ,appalachian ,mountain ,strata ,near ,hancock1 ,maryland4 ,the strata were deposited in horizontal layers upon the sea bottom1 covering the region many millions of years ago in middle ,paleozoic time4 ,at the time of the ,appalachian ,mountain revolution1 near the end of ,paleozoic time1 this and many other folds developed well below the surface4 ,removal of overlying material by erosion has laid bare the fold as we see it to-day4 7,photo by ,russell1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #h4--7;a7 ,a ,ledge of ,igneous ,rock 7,granite7 in ,northern ,new ,york4 ,this illustrates so-called joints or natural cracks1 commonly separating most hard rock masses into more or less prismatic blocks4 7,photo by the author47 ,illustration3 ,plate #h4--7;b7 ,a ,fault ,fracture in a ,ledge at ,east ,canada ,creek in the ,mohawk ,valley1 ,new ,york4 ,the ,ordovician limestone formation in thin layers on the right has sunk hundreds of feet along vertical fault to the left of middle1 bringing it sharply against the older 7,cambrian7 massive formation on the left4 ,the hole is artificial4 7,photo by ,darton1 ,u4 ,s4 ,geological ,survey47 ,geysers are periodically eruptive hot springs found only in a few of the volcanic regions of the world4 ,they are most wonderfully displayed in the ,yellowstone ,national ,park1 where they send columns of hot water to all heights up to #bej feet at various intervals of time4 ,almost incredible amounts of hot water are sent into the air every day in the geyser basins of ,yellowstone ,park4 ,the single geyser ,old ,faithful1 which erupts at intervals of about seventy minutes1 sends a column of water several feet in diameter to heights of from #abe to #aej feet4 ,during each eruption about #a1ejj1jjj gallons of water are sent forth1 or every day enough to supply the need of a fairly large city4 ,a very brief explanation of the cause of geyser eruptions may be stated as follows3 ,the very irregular1 narrow1 geyser tube extends nearly vertically downward into yet uncooled lava4 ,the tube is more or less rapidly filled by underground water4 ,the bottom1 or near-bottom1 portion of the water gradually becomes heated by the lava until finally the boiling point is reached for that depth4 ,but1 because of the pressure of the overlying water column1 the boiling point at that depth is considerably greater than for the surface4 ,a little steam develops far down and this causes the whole column of water above it to lift slightly1 thus relieving the pressure on the superheated water far down4 ,this relief of pressure allows much of the superheated water far down to flash into steam1 which violently forces the column of water out of the geyser tube4 ,,chapter ,x ,,how ,,mountains ,,come ,,and ,,go ,mountains constitute the grandest relief features of the earth1 and some of the most profound lessons of earth changes may be learned by studying them4 ,to the layman who views great mountains in all their grandeur and massiveness1 the expression everlasting hills seems appropriate4 ,but the geologist knows that even the loftiest mountains are only temporary features on the face of the earth4 ,like organisms1 they come and go4 ,for example1 where the great ,rocky ,mountains now stand was only a few million years ago 7in late ,mesozoic time7 the bottom of an interior sea4 ,where the ,appalachians now stand there were no mountains late in the ,paleozoic era 7not less than ten or twelve million years ago71 but instead sea water covered the district4 ,then the ,appalachians were formed1 lifting their heads much higher than at present1 after which they were cut down almost to sea level1 and then once more upraised4 ,the ,coast ,range ,mountains of our ,pacific ,coast have come into existence since the middle of the present 7,cenozoic7 geologic era4 ,every mountain1 like every organism1 has a life history1 in some cases simple1 and other cases complex4 ,all pass through stages of birth1 youth1 maturity1 old age1 and death4 ,some rear their heads and disappear after a short 7geological7 existence4 ,others continue their growth and persist much longer1 while still others undergo periods of profound rejuvenation4 ,among the various processes by which mountain ranges have been formed1 the folding and accompanying general uplift of strata are the most important4 ,in fact1 in most of the great mountain ranges of the world the folded structure is conspicuously developed1 so much so that they may well be called folded mountains4 ,very commonly1 however1 mountains of this type have also been subjected to more or less fracturing of the rocks 7faulting71 and not uncommonly they have also been subjected to igneous activity1 including both intrusion and extrusion of molten material4 ,it is among the folded mountains of greater or less degree of complexity that the greatest exhibitions of geologic phenomena are seen and the lessons which geology as a sciences teaches may be learned4 ,if one desires to know the history of a region1 one turns naturally to its mountain ranges1 for here may be found the upturned and dissected strata1 a study of whose kinds1 thickness1 and fossils throws light upon past events1 while their foldings and dislocations show the nature and results of those great dynamic agencies which1 from time to time1 have operated upon the outer portion of the earth1 and given to it the broad distinctive features which characterize it to-day4 7,l4 ,v4 ,pirsson47 ,among the great mountains we may also see wonderful exhibitions of the results of weathering and erosion1 especially the work of rivers and glaciers4 ,we can1 perhaps1 best convey to the reader some of the main facts and principles regarding folded mountains by considering certain observations which may be readily made in a short trip across a folded range of not too complex kind--for example1 across the ,appalachian range along the line of the ,baltimore and ,ohio ,railroad1 west of ,washington1 or the ,pennsylvania ,railroad1 west of ,philadelphia4 ,it would be most evident that the mountains consist of strata1 that is sedimentary rocks1 such as sandstone1 shale and limestone1 which were deposited under water4 ,a few measurements would reveal the fact that thousands of feet in thickness of strata are represented4 ,careful measurements by geologists have1 in fact1 shown that the strata were originally piled up layer upon layer to a thickness of #be1jjj to #cj1jjj feet4 ,the fact that they are strata of such great thickness proves that sediments must there have accumulated under water for some millions of years at least4 ,closer examination of a few good exposures 7i4e41 outcrops7 would further reveal the presence of fossil shells and impressions of marine organisms1 thus definitely leading us to conclude that the strata were accumulated under sea water1 which1 of course1 means that the present site of the mountain range was once sea floor4 ,examination of the rock materials also establishes the fact that the strata are such as were deposited in relatively shallow sea water--that is to say1 none are at all of the sort which are now forming under really deep ocean water4 ,most of the strata represent original sands 7and even gravels7 and muds which could have accumulated only relatively near shore1 that is within about #ajj miles1 which harmonizes with a statement made in a preceding chapter to the effect that very little land-derived sediment is at present depositing more than #ajj miles out from shore4 ,the coarse materials 7sands and gravels7 could not1 of course1 be carried many miles out1 while many of the strata are covered with ripple marks1 thus positively proving their shallow-water origin4 ,we conclude1 therefore1 that the ,appalachian strata are of marine1 shallow-water origin4 ,but we have already stated that these strata are at least #be1jjj feet thick4 ,how1 then1 do we reconcile these two seemingly paradoxical statements8 ,all that is necessary is to realize that the floor of the shallow sea1 in which the sediments eroded from adjacent land were being deposited1 slowly1 though more or less irregularly1 subsided or sank during the long ages 7millions of years7 of their accumulation4 ,it would carry us too far afield to really attempt an explanation of this remarkable type of geologic phenomenon1 and it must suffice to suggest that1 starting with the earth's crust in equilibrium1 the very weight of accumulating strata would tend to destroy that equilibrium and so cause subsidence4 ,in our trip across the mountains it would be strikingly evident that the strata are no longer in their original horizontal position1 as they were piled up layer upon layer1 but that they have been notably disturbed and thrown into folds 7,plate #g71 large and small1 some masses of the strata having been bent upward 7anticlines7 and others downward 7synclines74 ,such folded structures could have been developed only by a great force of lateral compression in the earth's crust within the zone of flowage4 ,under compression the strata were mashed together1 notably bent into curves 7folds71 and more or less upraised4 ,it would also be readily observed that the main axes of the folds extend essentially parallel to the main trend of the mountain range1 thus proving that the force of compression was exerted at right angles to the trend of the range4 ,illustration3 ,fig4 #bd4--;,diagrammatic sections illustrating the development of a typical folded mountain range4 ,upper figure3 ,a1 the old land eroded to furnish sediments deposited under the adjacent sea at ,c4 ,middle figure3 strata 7,c7 folded as they would appear if unaffected by erosion1 and a down-warp 7,b7 between ,a and ,c4 ,lower figure3 condition after profound erosion1 and filling of ,b with sediment4 7,drawn by the author47 ,using a biological analogy1 a brief history of a typical folded mountain range may be stated as follows3 ,first1 there is the prenatal or embryonic stage when the materials of the range are gathering1 that is when the sediments are piling up layer upon layer relatively near shore on a sinking sea bottom4 ,next comes the birth of the range when1 due to the great lateral compressive force1 the strata are thrown into folds and forced to appear above sea level1 the range thus literally being born out of the sea4 ,during the next1 or youthful stage1 the range grows 7with increasing altitudes7 because of continued application of the compressive force4 ,even during the youthful growing stage weathering and erosion attack the range and tend to reduce it4 ,then comes the stage of maturity1 when the upbuilding 7compressive7 force and the tearing down 7erosive7 force about counterbalance each other4 ,at this time the range has reached its maximum height and ruggedness of relief1 with ridges and valleys higher and deeper than at any other time4 ,the old-age stage sets in when the upbuilding power wanes or actually ceases1 and erosion dominates or reigns supreme4 ,slowly but surely1 unless there be a renewal by an upbuilding power1 the range is cut down until little or nothing of it remains well above sea level1 or1 in other words1 until a peneplain is developed4 ,this last stage may truly be called the death of the range4 ,usually1 however1 some local portions of the disappearing range1 which are more resistant or more favorably situated against erosion1 are left standing to at least moderate heights above the general level of the plain of erosion4 ,the above normal order of events may be disturbed at any stage1 especially after maturity1 by renewed uplift when the streams are revived in activity and increased ruggedness results4 ,even after the whole range as a relief feature has been planed away1 the site of the range may be uplifted and a new cycle of erosion started4 ,by the use of two well-known examples we shall not only illustrate the above principles of mountain history1 but also show that no less than a few million years must be allowed for the growth and decay of a great folded range4 ,during the last 7,permian7 period of the ,paleozoic era the ,appalachian strata began to buckle and the yielding to pressure continued till well into the succeeding 7,triassic7 period4 ,the climax was reached about the close of the ,permian4 ,then1 throughout the ,mesozoic era1 erosion reduced the central ,appalachians to a great plain 7peneplain7 near sea level1 after which1 about the beginning of the present 7,cenozoic era71 the site of the former range was distinctly upraised 7without folding of the rocks71 causing the revived streams to begin their work of carving out the present ridges and valleys1 this work still being in progress4 ,in the case of the ,sierra ,nevadas1 the strata were folded into a lofty mountain range relatively late in the ,mesozoic era and1 by the middle of the ,cenozoic era1 the old-age stage of erosion was well advanced when the range was not more than a few thousand feet high4 ,then 7in the middle of the ,cenozoic era7 uplift1 accompanied by faulting on a large scale1 but not by folding1 took place1 and the range was notably rejuvenated to about its present height4 ,all the remarkably deep canyons of the ,sierras have been carved out since the rejuvenation4 ,how is the geological birthday of a mountain range determined8 ,in the preceding paragraph we stated that the ,appalachians were folded and born out of the sea about the close of the ,paleozoic era4 ,this is readily proved by calling attention to two facts4 ,first1 the youngest strata involved in the folding are of ,permian1 or late ,paleozoic ,age in the geologic column1 as proved by their fossil content1 etc41 and obviously the folding must have taken place after they had been deposited4 ,clearly1 then1 the folding could not have taken place before very late ,paleozoic time4 ,second1 the oldest strata resting upon the folded rocks are of early 7not the very earliest7 ,mesozoic ,age1 and these strata are somewhat tilted but not folded4 ,obviously1 then1 the folding must have occurred before the nonfolded strata were deposited1 which means that the folding must have been essentially completed in not later than early ,mesozoic time4 ,or1 in the case of the ,rocky ,mountains1 we know that strata were there folded late in the ,mesozoic era or very early in the ,cenozoic era1 because folded rocks as late in age as late ,mesozoic 7,cretaceous7 have resting upon them1 in some places1 nonfolded strata of early ,cenozoic 7,tertiary7 ,age4 ,the figure clearly shows how the ,ordovician strata must have been folded before the next 7,silurian and ,devonian7 strata were deposited upon them in southeastern ,new ,york4 ,illustration3 ,fig4 #be4--;,diagram illustrating the topography and folded structure of the ,appalachian ,mountains west of ,harrisburg1 ,pennsylvania4 ,the valleys have been etched out of belts of weak rocks1 while outcropping resistant rocks stand out to form ridges4 ,note the course of the ,susquehanna ,river across the mountain ridges1 this being a superimposed river 7see text1 p4 #bcc74 7,drawn by ,a4 ,k4 ,lobeck47 ,illustration3 ,fig4 #bf4--,only slightly tilted strata of ,silurian and ,devonian ages resting upon folded strata of ,cambrian and ,ordovician ages in an east-west section across the ,catskill ,mountains and ,hudson ,valley of ,new ,york4 ,the folding took place at the time of the ,taconic ,mountain ,revolution toward the end of the ,ordovician period4 7,drawn by the author47 ,as already suggested1 however1 folding is not the only method by which mountains are formed4 ,many ranges are either entirely due to the tilting of earth blocks by faulting or fracturing of the earth1 or their present altitude1 at least1 is a direct result of faulting4 ,such may be called block mountains4 ,they are wonderfully represented by the various north-south ranges rising some thousands of feet above the general level of the ,great ,basin region of ,utah and ,nevada4 ,these ranges are1 in short1 somewhat eroded edges of approximately parallel-tilted fault blocks lying between the ,sierra ,nevada ,range and the ,wasatch ,range4 ,in southeastern ,oregon a series of nearly parallel block mountains1 up to forty miles in length and over #a1jjj feet in height1 show very steep eastern fronts only slightly modified by erosion4 ,another mode of origin of mountains is by the rise of molten material to the surface1 especially where a chain of volcanoes is located4 ,thus the ,cascade ,mountains from northern ,california through ,oregon and ,washington1 including ,mounts ,lassen1 ,shasta1 ,pitt1 ,baker1 ,st4 ,helens1 and ,rainier1 are very largely the result of volcanic action4 ,the long chain of ,aleutian ,islands of ,alaska1 referred to in our study of volcanoes1 is an excellent example of a great mountain range now being built up out of the sea by volcanic action4 ,more locally molten rocks under pressure may not reach the surface but instead simply bulge or dome the strata over them1 as in the case of the group known as the ,henry ,mountains of ,utah1 and also in other parts of the ,west4 ,in still other cases mountains of considerable area and altitude have resulted from erosion of uplifted regions where the uplift has been practically unaccompanied by either folding1 faulting1 or igneous activity4 ,any low-lying area1 regardless of the character of its rocks1 structure1 or previous history1 may be notably upraised and simply subjected to erosion4 ,an excellent illustration is afforded by the ,catskill ,mountains of ,new ,york1 where numerous deep valleys and narrow ridges have been carved out of upraised nearly horizontal strata4 ,the so-called ,bad ,lands region of parts of ,south ,dakota and ,wyoming is also essentially of this type1 where deep1 narrow valleys and sharp ridges have been etched out of high1 relatively soft1 nearly horizontal strata1 resulting in an almost impassable maze of mountains4 ,in the high1 recently upraised ,colorado ,plateau of parts of ,arizona1 ,new ,mexico1 ,colorado1 and ,utah1 nearly horizontal strata are being etched out1 the result being numerous buttes1 mesas 7flat-topped hills and mountains7 and deep canyons1 including the ,grand ,canyon with its maze of peaks and pinnacles1 many of them rising like mountains out of the canyon depths4 ,mountains of the pure types just described are not the prevailing ones of the earth4 ,most mountains and their structures1 as we see them to-day1 are products of two or more of the processes of folding1 faulting1 igneous action1 and erosion4 ,a few well-known examples will suffice to make this matter clearer4 ,thus1 the ,appalachian ,mountains originally developed by severe folding of thick strata4 ,after considerable erosion1 numerous small and large thrust faults developed1 some of the dislocations amounting to miles4 ,then the whole range was cut down nearly to sea level by erosion1 after which the district was upraised 7without folding7 mostly from #b1jjj to #d1jjj feet1 and the present long1 narrow mountain ridges and valleys have been carved out by stream erosion4 ,thus folding1 faulting1 and erosion all enter into the height and structure of the ,appalachians4 ,a lofty mountain range still more complex in its history is the ,sierra ,nevada of ,california4 ,first1 thick strata were highly folded1 upraised1 and intruded by great masses of molten granite4 ,erosion then proceeded to cut the range down to hills1 after which a great fracture 7fault7 developed along the eastern side and the ,sierra ,nevada earth block was notably tilted with steep eastern front and long western slope4 ,erosion has considerably modified the eastern fault face1 and the deep canyons like ,yosemite1 ,king's ,river and ,american ,river1 have been carved out of the western slope of the great tilted fault block4 ,geologically recently the central to northern portion of the range has been affected by volcanic action1 streams of lava in some cases having flowed down the valleys4 ,,chapter ,,xi ,a ,,study ,,of ,,lakes ,lakes are ephemeral features on the face of the earth4 ,compared to the tens of millions of years of known earth history1 lakes1 even large ones1 are very short lived4 ,they may1 in truth1 be regarded as merely results of the temporary obstructions to drainage4 ,lake basins are known to originate in many ways1 and there are various means by which they are destroyed4 ,not attempting an exhaustive1 scientific treatment of the subject1 our present purpose may be well served by describing and explaining some of the better known and more remarkable lakes of the world4 ,even a cursory examination of a large map of the world reveals the fact that the regions of most numerous lakes are those which were recently occupied by glaciers--either the vast ice sheets of the ,glacial epoch or mountain 7or valley7 glaciers4 ,this is because more lakes of the present time have come into existence as direct or indirect results of glaciation than by any other cause4 ,a considerable number of these lakes occupy rock basins which have been eroded or excavated by the direct action of flowing ice4 ,small lakes of this sort are commonly found in the upper parts of valleys formerly occupied by mountain or so-called ,alpine glaciers1 because there the excavating power of such glaciers was especially effective4 ,more rarely rock basins have been scoured out by glaciers farther down their valleys4 ,many lakes occupy rock basins excavated by ice in the high ,sierra ,nevada and ,cascade ,ranges1 in the ,rocky ,mountains from ,colorado into ,canada1 in the ,alps1 and in the mountains of ,norway4 ,few1 if any of them are1 however1 large or famous4 ,other lakes1 some of very considerable size1 occupy rock basins scoured out by the passage of the great ice sheets of the ,glacial epoch in ,north ,america and ,europe1 though they are less common than formerly supposed4 ,some of the many lake basins of ,ontario1 ,canada1 are quite certainly of this origin1 as might well be expected1 because the power of the great ice sheet was there in general notably greater than south of the ,great ,lakes where the tendency was to unload or deposit the eroded materials as shown by the great accumulations of glacial dbris 7moraines74 ,where the ice walls of certain existing glaciers form dams across valleys1 waters are ponded1 a small lake of this kind occurring alongside the ,great ,aletsch ,glacier of the ,alps1 where its wall is slowly moving past a tributary valley4 ,lakes of this kind also occur in ,greenland and in ,alaska1 but none are of considerable size4 ,during the ,great ,ice ,age1 however1 literally thousands of large and small lakes were formed1 both during the advance and the retreat of the ice1 wherever the glacier wall blocked valleys which sloped downward toward the ice4 ,new ,york ,state furnishes many fine examples of large and small lakes of this sort4 ,thus1 when the great glacier was melting in northern ,new ,york1 waters hundreds of feet deep and many miles long were ponded between two ice lobes--one retreating eastward and the other westward from the ,mohawk ,valley4 ,an ice dam lake was also formed a little later1 when an ice wall blocked the northern part of the ,black ,river ,valley just west of the ,adirondack ,mountains and caused a lake covering about #bjj square miles4 ,one of the largest of all known ice dam lakes has been called ,lake ,agassiz1 which attained a maximum length of over #gjj miles and a width of #bej miles in the ,red ,river of the ,north region of eastern ,north ,dakota1 western and northwestern ,minnesota1 and northward into ,canada1 most of its area having been in ,canada4 ,it began as a small lake with southward drainage into the ,mississippi when the great northward retreating ice sheet formed a dam across the valley of the ,red ,river of the ,north4 ,the retreating ice continued to block the northward drainage until the vast lake1 covering a greater territory than all of the present ,great ,lakes combined1 was developed4 ,beaches1 bars1 deltas and the outflow channel of this remarkable lake are wonderfully well preserved4 ,lake ,winnipeg is a mere remnant of great ,lake ,agassiz4 ,many ponds and small lakes occupy basins formed by irregular accumulations of glacial 7morainic7 materials4 ,still others lie in depressions which formed by the melting of masses of ice which became wholly or partly buried by ice deposits1 or by sediments washed into bodies of water which were held up by ice dams4 ,depressions of the latter kind are commonly found as pits or so-called kettle holes below the general level of sand flats or sand plains of glacial lake origin4 ,most common of all lake basins of glacial origin are those formed by accumulation of glacial dbris or morainic materials acting as natural dams across valleys4 ,this is1 in fact1 the most common of all ways by which existing lake basins1 some of them very large1 have been formed4 ,most of the thousands of ponds and lakes of ,minnesota1 ,wisconsin1 and northern ,new ,york belong in this category4 ,in the ,adirondack ,mountains1 for example1 most of the lakes1 like the well-known ,lake ,placid1 ,saranac ,lakes1 ,long ,lake1 and ,schroon ,lake1 have their waters ponded by single dams of glacial dbris across valleys4 ,in some cases a series of such dams blockades a valley and forms a chain of lakes like the well-known ,fulton ,chain in the ,adirondacks4 ,less commonly the lake may have its waters ponded by two natural dams of glacial dbris1 one across a valley at each end of a lake4 ,a very fine1 large scale example of the last-named type is the famous ,lake ,george in the southeastern ,adirondacks4 ,it is over #cj miles long and from #a to #b-a_/b miles wide4 ,it lies in the bottom of a deep1 narrow mountain valley1 mountain sides rising very steeply from a few hundred feet to #b1jjj or more feet above its shores4 ,there are many islands1 especially in the so-called ,narrows1 thus greatly enhancing the scenic effect4 ,the valley itself has been produced by a combination of faulting and erosion4 ,there was a preglacial stream divide at the present location of the ,narrows4 ,this divide was somewhat reduced by ice erosion when the deep1 narrow body of ice plowed its way through the valley during the ,ice ,age4 ,during the retreat of the ice heavy morainic accumulations were left as dams across the valley at each end of the lake4 ,another remarkable body of water1 similar to ,lake ,george in its origin1 is ,chautauqua ,lake of western ,new ,york1 famous for its ,chautauqua assemblies4 ,it lies #a1cch feet above sea level1 with its northern end near the edge of the steep front of the plateau overlooking ,lake ,erie4 ,chautauqua ,lake really consists of parts of two valleys1 one sloping north and the other sloping south1 each dammed by glacial deposits4 ,the famous ,alpine lakes--,garda1 ,como1 and ,maggiore--have resulted from deposition of glacial morainic materials under conditions different from those above described4 ,in these cases great mountain or valley glaciers once flowed down the valleys and spread out part way upon the ,italian plain4 ,great accumulations of glacial dbris took place around the borders of the glacier lobes1 and1 after retreat of the ice1 the glacial deposits acted as dams ponding the waters far back into the mountain valleys4 ,the origin and history of the ,great ,lakes constitutes one of the most interesting and remarkable chapters in the recent geological history of ,north ,america4 ,most of the salient points have been well worked out and they may be very briefly summarized1 as follows3 ,before the ,ice ,age the ,great ,lakes did not exist1 because the region1 prior to that time1 had been land subjected to erosion for millions of years--a time altogether too long for any lake to survive4 ,their sites were occupied by broad1 low1 stream-cut valleys which were quite certainly locally somewhat deepened by ice erosion during the ,ice ,age4 ,ice erosion is1 however1 altogether insufficient to account for the great closed basins4 ,the two most important factors entering into the formation of the basins of the ,great ,lakes were doubtless the great glacial 7morainic7 accumulations acting as dams along the south side1 and the tilting of the land downward on the north side of the region4 ,in support of this explanation it has been established that the great dumping ground of ice-transported materials from the north was in general along the southern side of the ,great ,lakes and southward4 ,it has also been well established that1 late in the ,ice ,age1 the land on the southern side of the ,great ,lakes region was lower than at present1 as proved by the tilted character of beaches of the well-known extinct glacial lakes which were the ancestors of the present lakes4 ,such a down-warp of the land must have helped to form the closed basins by tending to stop the southward and southwestward drainage of the region4 ,illustration3 ,fig4 #bg4--,sketch map showing a very early stage in the history of the ,great ,lakes when two relatively small lakes in front of the ice wall separately drained into the ,mississippi ,river4 7,drawn by the author from map by ,taylor ,leverett47 ,illustration3 ,fig4 #bh4--,lake ,whittlesey stage of the ,great ,lakes history when the ice had retreated far enough to allow the eastern and western ice margin waters to join with a single outlet past ,chicago4 7,drawn by the author from a map by ,taylor ,leverett47 ,illustration3 ,fig4 #bi4--,the ,algonquin-,iroquois stage of the ,great ,lakes when their whole area was ice-free1 and all their waters drained through the ,mohawk-,hudson ,valleys of ,new ,york into the ,atlantic ,ocean4 7,after ,taylor1 published by ,new ,york ,state ,museum47 ,we shall now very briefly trace out the principal stages in the history of the ,great ,lakes during the final retreat of the vast ice sheet4 ,this may best be done by the aid of maps which need only brief explanation4 ,when the ice sheet had retreated far enough northward to uncover the very southern end of the ,lake ,michigan basin and a little beyond1 a small glacial lake 7,lake ,chicago7 developed against the ice wall4 ,its outlet was through the ,illinois ,river and thence into the ,mississippi4 ,at the same time a larger glacial lake1 held up by the ice wall1 developed over the western part of the ,erie basin and beyond4 ,its outlet was through the ,wabash ,river4 ,with further retreat of the ice a large lake 7,whittlesey7 covering considerably more than the area of ,lake ,erie developed1 with outlet westward across ,michigan into the enlarged ,lake ,chicago which continued to drain into the ,illinois ,river4 ,during a still later stage of ice withdrawal the remarkable set of three glacial lakes existed--,lakes ,duluth1 ,chicago1 and ,lundy4 ,each of these large lakes had its own outlet4 ,lake ,duluth covered about half of the ,lake ,superior basin and drained through the ,st4 ,croix ,river into the ,mississippi4 ,lake ,chicago expanded to cover nearly all of the ,michigan basin and continued to drain through the ,illinois ,river4 ,lake ,lundy covered not only more than the area of the ,erie basin1 but also considerable territory north of ,detroit1 and drained eastward alongside the ice lobe of the ,ontario basin through the ,mohawk and ,hudson valleys of ,new ,york1 and into the ,atlantic ,ocean4 ,just after the ice completely withdrew from the area now occupied by the ,great ,lakes1 but still blocked the ,st4 ,lawrence ,valley1 the vast body of water called ,lake ,algonquin more than covered the sites of the present ,superior1 ,michigan1 and ,huron4 ,at this time the land was distinctly lower toward the northeast than at present1 causing the outlets to the west to be abandoned4 ,the great ,lake ,algonquin poured its waters eastward through the ,trent ,river channel of ,ontario1 ,canada1 into glacial ,lake ,iroquois1 which was the great ancestor of ,lake ,ontario4 ,lake ,iroquois1 in turn1 had its outlet eastward through the ,mohawk and ,hudson ,valleys of ,new ,york4 ,for part of the time at least1 ,lake ,erie maintained a separate existence discharging into ,lake ,iroquois near ,buffalo4 ,during the ,algonquin-,iroquois stage the combined area of all the lakes was notably greater than the present area of the ,great ,lakes4 ,the volume of water discharged by the lakes through the ,mohawk ,valley of ,new ,york was doubtless greater than that which now goes over ,niagara ,falls4 ,gradually1 as the ,st4 ,lawrence ice lobe waned1 the outlet waters of the lakes began to move alongside the ice through the ,st4 ,lawrence ,valley4 ,finally the ice withdrew far enough to free the ,st4 ,lawrence ,valley and the waters of the ,great ,lakes region dropped to a still lower level1 bringing about the ,nipissing ,great ,lakes stage not greatly different from the present4 ,east and northeast of the ,lakes the land was low enough to allow tidewater 7the so-called ,champlain ,sea7 to extend through the ,hudson1 ,champlain1 and ,st4 ,lawrence ,valleys1 and possibly into the ,ontario basin1 as proved by the occurrence of marine beaches and fossils4 ,the waters in the ,erie and ,ontario basins covered about the present areas1 while the ,nipissing ,lakes1 which covered a little more than the present areas of the three upper ,great ,lakes1 had their outlet through the ,ottawa ,river channel into tidewater 7,champlain ,sea74 ,postglacial warping of the land has brought the whole region to the present condition4 ,illustration3 ,fig4 #cj4--,map showing next to the present stage of the ,great ,lakes history when the land was lower on the north and the upper 7,nipissing7 lakes drained through the ,ottawa ,river ,valley into an arm of the sea 7,champlain ,sea7 which reached through the ,champlain and ,hudson ,valleys4 7,after ,taylor1 published by ,new ,york ,state ,museum47 ,many lakes1 including some remarkable ones1 occupy basins which are directly due to movements of the earth's crust--either faulting or warping4 ,an example of a lake occupying part of a fault basin is the famous ,dead ,sea of ,palestine4 ,this lake lies in the lowest part of the ,jordan ,valley1 which has geologically recently come into existence by the sinking of a long1 narrow block of earth for several thousand feet between two great earth fractures 7faults74 ,the ,dead ,sea covers about #ejj square miles and its surface lies about #a1cjj feet below sea level1 which makes it the lowest lake in the world4 ,almost equally remarkable is the fact that its depth is about #a1cjj feet1 so that the lowest part of the lake basin is #b1fjj feet below sea level4 ,the lake contains approximately #bd per cent salt1 mostly common table salt1 causing it to be a thick brine in which there is neither plant nor animal life--hence the name ,dead ,sea4 ,at one time1 probably just after the ,ice ,age1 the lake was much larger and deeper1 when it filled a considerable part of the ,jordan ,valley and had an outlet to the south4 ,during the high-level stage the water was fresh1 but gradually1 as the climate became drier1 evaporation was greater than intake1 the outlet was abandoned1 and the mineral matter 7mostly chloride of magnesia and common table salt7 carried by the streams in solution into the shrinking lake steadily accumulated until the high degree of salinity of the present time has been reached4 ,great ,salt ,lake1 ,utah1 is a remarkable lake whose history has been carefully studied4 ,it occupies the lowest position of an extensive basin which1 in turn1 forms but part of the whole great district of ,utah which has geologically recently sunk thousands of feet on the west side of the great fault already described as occurring along the western base of the ,wasatch ,mountains4 ,at present the lake covers about #b1jjj square miles1 but its area fluctuates considerably4 ,it is scarcely believable that this big lake has an average depth of only fifteen feet and a maximum depth of only fifty feet4 ,it lies #d1bjj feet above sea level1 and it carries about #ah per cent salts in solution4 ,most abundant by far is common table salt1 of which there are no less than #e1jjj1jjj1jjj tons in solution4 ,the waters also contain about #ijj1jjj1jjj tons of other salts4 ,should the lake completely disappear by evaporation1 these salts would be deposited4 ,allowing for cars #dj feet long and of #dj tons capacity1 a train more than #a1jjj1jjj miles long would be required to carry the salts4 ,what has been the source of these salts8 ,great ,salt ,lake is not1 as supposed by some1 a remnant of an ocean once covering the region4 ,briefly1 the explanation is as follows3 ,at one time1 when the climate was moister1 the basin now only in part occupied by the lake was filled to overflowing with an outlet north into the ,snake and ,columbia rivers4 ,that great body of water 7called ,lake ,bonneville7 covered nearly #bj1jjj square miles and its depth was about #a1jjj feet deeper than now1 the present depth being very small4 ,because it had an outlet that lake was1 of course1 fresh4 ,beaches and shore lines #a1jjj feet above the present lake1 and at various lower levels1 are still wonderfully well preserved4 ,when1 due to climatic change1 evaporation exceeded intake by streams1 the outlet was cut off4 ,but slowly1 as the lake shrank1 streams 7especially the ,jordan ,river7 carried a little salt in solution1 the percentage of salt increasing until the present stage has been reached4 ,in a real sense1 much of the salt was once in the sea1 because it has been dissolved out of strata which accumulated under sea water long before the basin of ,great ,salt ,lake came into existence4 ,another famous lake1 which also occupies part of a basin due to faulting1 is ,lake ,tahoe in the ,sierra ,nevada ,mountains1 near ,truckee1 ,california4 ,this lake1 whose length is #ba miles1 and width #ab miles1 lies #f1bbe feet above sea level4 ,on almost all sides steep mountains rise several thousand feet above its waters4 ,its great depth of #a1fce feet makes it1 so far as known1 the second deepest lake in ,north ,america1 ,crater ,lake1 ,oregon1 only outranking it4 ,the water is exceedingly clear4 ,an experiment some years ago showed that a white disk eight inches in diameter could actually be seen through a thickness of #baf feet of its water4 ,the statement sometimes made that ',tahoe is an old volcanic crater' is not true4 ,the region about the lake shows evidences of volcanic activity of various kinds1 and the lake waters themselves have probably been dammed at times by outpourings of lava4 ,a lava flow appears to have temporarily filled the outlet channel below ,tahoe ,city4 ,the lake1 however1 lies in a structural depression--a dropped 7fault7 block in the earth's crust4 7,u4 ,s4 ,geological ,survey47 ,the basin of the largest lake in the world--the ,caspian ,sea--has resulted from warping of the earth's crust4 ,it has an area of #agj1jjj square miles1 a maximum depth of #c1bjj feet1 and its surface is about #ij feet below sea level4 ,the composition of its water and some of its animal life indicate that it was once an arm of the sea4 ,it has been detached or cut off by an upwarp of the land between it and the ,black ,sea region4 ,if this great lake is a cut-off arm of the sea1 with no outlet1 how do we explain the fact that its salinity is much less than that of the ocean8 ,toward the north1 where it is shallow and fed by so much river water1 it is1 in fact1 almost fresh water4 ,even the southern one-half carries not over #a per cent of salt4 ,the explanation is that a steady current passes through a narrow passageway into a gulf or bay on its eastern side where evaporation is much greater than over the general surface of the ,caspian4 ,the salt is1 therefore1 gradually accumulating at the estimated rate of #cej1jjj tons per day in this gulf1 while the sea itself is becoming fresher4 ,the basin of ,lake ,champlain1 about #ajj miles long1 was occupied by tidewater geologically very recently 7that is1 since the ,ice ,age71 but it has been cut off by uplift of the land on the north1 since which time the waters of the lake have been completely rinsed out and freshened4 ,many lake basins directly result from volcanic action4 ,in many parts of the world lakes1 usually of small size1 occupy craters of volcanoes as1 for example1 in the ,eifel region of ,germany1 the ,auvergne district of ,france1 and near ,rome and ,naples in ,italy4 ,such a lake of exceptional interest fills part of the great crater1 several thousand feet deep1 which resulted from the explosion of ,mt4 ,katmai1 ,alaska1 in #aiab4 ,the water of this lake1 more than a mile wide and of unknown depth1 is hot4 ,one of the most unique and beautiful lakes of the world is ,crater ,lake in the ,cascade ,mountains of southern ,oregon4 ,it partly fills a great1 nearly circular hole1 six miles in diameter1 with a maximum depth of about #d1jjj feet1 in the top of a mountain 7,plate #aa74 ,the lake is over five miles in diameter and nearly #b1jjj feet deep1 making it the deepest in ,north ,america4 ,its surface is about #f1bjj feet above sea level4 ,precipitous rock walls rising #ejj to #b1jjj feet completely encircle the lake1 the main body of whose water is of a marvelous deep1 sapphire-blue color1 while the shallow portions around some of the shore are of emerald-green4 ,crater ,lake has very little intake except direct rainfall and snowfall1 and its water is fresh4 ,the great hole was not produced by an explosion like that of ,katmai1 but rather by the sinking of the top of a once much greater mountain4 ,that the mountain was once about the size and shape of ,mt4 ,shasta is proved by the fact that deep glaciated valleys lead up the slopes and end abruptly at the very rim of the present mountain4 ,obviously these valleys were scoured out in recent geologic time by glaciers whose sources were several thousand feet up on a former cone-shaped mountain4 ,that the mountain top sank rather than exploded is proved by the absence of volcanic dbris over the sides and base of the mountain4 ,still another way by which lakes are formed by volcanic action is by streams of lava blocking valleys4 ,the famous ,sea of ,galilee in ,palestine was thus formed by a stream of lava1 which geologically recently flowed down from the east into the ,jordan ,valley and across it1 where it cooled to form a dam ponding the waters of the ,jordan ,river4 ,because the river flows through the lake1 its water is fresh4 ,one of the most remarkable facts about this lake is that its surface lies nearly #gjj feet below sea level4 ,a number of lava-dam lakes are known in the ,sierra ,nevada and ,cascade ,mountains4 ,a very interesting case of a lake basin1 formed by cutting off an arm of the sea without any movement of the earth's crust1 is the ,salton ,sink of southern ,california4 ,this basin1 many miles long and wide1 lies below sea level1 its lowest point being #bhg feet below tide4 ,the ,gulf of ,california formerly reached much farther north and into ,california where it covered the site of the ,salton ,sink4 ,gradually the ,colorado ,river1 always loaded with sediment1 built a broad delta deposit right across the gulf1 the northern end of which thus became cut off1 leaving a big salt lake4 ,but the river flowed into the gulf1 while in the dry climate the evaporation was great enough to gradually dry away the salt lake4 ,this was the condition of things until #aijd1 when much of the river at a time of flood got out of control and1 following the general course of a great irrigating canal1 it flowed for several years into the lowest part of the ,salton ,sink1 partly filling it to form a lake #de miles long1 #ag miles wide1 and #hc feet deep4 ,since #aijg the lake has been notably decreasing in size1 and it may entirely disappear4 ,other ways by which lakes1 mostly relatively small ones1 may develop are by landslides blocking valley drainages2 by streams cutting across winding curves leaving so-called oxbow lakes which are common1 for example1 along the lower ,mississippi ,river2 by wave and wind action along shores of lakes or sea2 by filling so-called sink holes which result from dissolving or falling in of roofs of caves2 and by beavers through whose industry dams are built across valleys or streams4 ,some of the most common ways by which lakes may be destroyed are the following3 by being filled with sediment carried in by streams1 or by vegetation1 or by both2 by cutting down outlets2 by evaporation due to a change in climate2 by removal of the ice dam in certain types of glacial lakes2 and by movements or warping of the earth's crust4 ,,chapter ,,xii ,,how ,,the ,,earth ,,may ,,have ,,originated ,the problem of the origin of the earth is essentially astronomical rather than geological1 because geological history is considered to have begun when common earth processes1 such as erosion1 deposition1 and transportation of sediments1 etc41 were brought into play4 ,it is quite certain1 however1 that the earth in its pregeologic state gradually merged into its geological condition4 ,for this reason the geologist is interested in the more important doctrines or hypotheses which have been put forth to account for the origin of the earth4 ,in fact1 one of the few hypotheses which must be taken seriously is largely the work of a geologist4 ,the most acceptable hypothesis not only best satisfies the facts regarding the earth's astronomical relationships1 but also best harmonizes with our knowledge of the oldest known rocks and their history4 ,since the problem of the origin of the earth is an essential part of the problem of the origin of the solar system1 the following well-known facts should be clearly in the mind of the reader4 ,eight planets1 including the earth1 revolve in nearly circular paths around the central sun1 whose diameter is #hff1jjj miles4 ,the radius of the solar system is at least #b1hjj1jjj1jjj miles1 this being the distance of the outermost known planet 7,neptune7 from the ,sun4 ,neptune requires #afd years for a trip around the sun1 while the earth1 which averages about #ic1jjj1jjj miles from the sun1 makes its circuit once a year4 ,the planets all revolve around the sun in the same direction1 and in nearly the same plane4 ,the sun and all eight planets rotate on their axes in the same direction1 the earth's rotation being accomplished every twenty-four hours4 ,most of the planets have one or more smaller bodies called satellites revolving about them1 such as ,earth1 with its one satellite 7the moon71 and ,saturn1 with its eight satellites1 etc4 ,it is well known that this solar system is only a very small part of the vast universe1 as shown by the facts that no star is nearer the earth than several trillion miles1 and that some stars are so far away that light traveling at the rate of #ahf1jjj miles per second requires a thousand years to reach the earth6 ,toward the end of the eighteenth century the famous nebular or ring hypothesis was set forth by the astronomer named ,laplace4 ,this assumes an original very hot incandescent mass of gas spheroidal in shape and greater in diameter than the present solar system4 ,this mass rotated in the direction of rotation of our sun and its planets4 ,loss of heat by radiation caused the mass to shrink1 and this in turn not only made it rotate faster1 but also caused the centrifugal force 7i4e41 the force whose direction was from the center7 in its equatorial portion to gradually become stronger4 ,finally a time came when the force of gravity 7i4e41 the force whose direction was toward the center7 and the centrifugal force became equal and a ring was left 7not thrown7 off1 while the rest of the mass of gas continued to shrink4 ,after a time the material of the ring collected to form the outermost planet4 ,the other planets were similarly formed from other rings which were left off as contraction of the great mass of gas went on4 ,the sun represents the remainder of the great mass of rotating gas4 ,what is the bearing of this nebular hypothesis upon the early geological history of the earth8 ,according to the hypothesis the earth must once have been much more highly heated and larger than now4 ,it condensed to a liquid and then it cooled enough to permit the formation of a solid crust over a liquid interior4 ,it then had a hot dense atmosphere containing all the water of the earth in the form of vapor1 and this atmosphere steadily became thinner due to absorption by the earth4 ,when the pressure and temperature conditions became favorable1 much of the water vapor condensed to form the ocean and the atmosphere gradually changed to its present condition4 ,according to this view the oldest rocks of the earth must have been igneous because they resulted from the solidification of the outer part of the molten globe4 ,within recent years certain serious objections to the nebular hypothesis have been raised1 and ,chamberlin and ,moulton have formulated the planetesimal or spiral hypothesis as an attempt at a more rational explanation of the origin of the solar system4 ,some of the objections to the older doctrine are that among the many thousands of known nebul in the universe very few only are of the ,laplacian or ring type1 while spiral forms are abundant4 ,spectroscopic study shows that the nebul are not gaseous1 but made up of either liquid or solid particles1 and that the leaving off of rings would necessitate the assumption of an intermittent process which could scarcely have operated under the conditions of the hypothesis4 ,illustration3 ,plate #i4--7;a7 ,molten ,lava ,flowing ,over a ,cliff ,into ,water in the ,hawaiian ,islands4 7,after ,diller1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #i4--7;b7 ,dikes of ,granite 7,light ,gray7 ,cutting an ,old ,dark ,rock4 ,while the granite on the right was being forced in molten condition upward into the earth's crust1 tongues of it 7dikes7 were sent off into the adjacent rock4 7,photo by ,howe1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #aj4--7;a7 ,lassen ,peak1 ,northern ,california1 in ,eruption ,august #bb1 #aiad4 ,the great cloud of steam and volcanic ash rose several miles4 ,this is the only active volcano in the ,united ,states proper1 and it is now included in ,lassen ,volcano ,national ,park4 7,by permission of ,r4 ,e4 ,stinson1 ,red ,bluff1 ,cal47 ,illustration3 ,plate #aj4--7;b7 ,devil's ,tower1 ,wyoming4 ,this great mass of rock was forced in molten condition through strata which1 because of their weakness1 have been eroded away all around the hard igneous rock4 ,this is probably the core or neck of a former volcano4 7,photo by ,darton1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,fig4 #ca4--;,diagram showing the origin and character of a spiral nebula according to the planetesimal hypothesis of the origin of the solar system4 7,modified after ,moulton47 ,anything like a full understanding of the planetesimal hypothesis would be difficult to obtain1 and1 in the brief space at our disposal1 we shall attempt to make clear only a few of the salient points4 ,according to this hypothesis the solar system was1 during a previous stage of its evolution1 a great1 flat1 spiral nebula1 made up of finely divided solid or possibly liquid particles called planetesimals1 among which were scattered some larger knots or masses4 ,each tiny particle and larger mass or knot is considered to have traveled in its own particular orbit or path about a central very large mass--the future sun4 ,it is even suggested that the spiral nebula originated by disruption of one star by a swift-moving passing star4 ,each disrupted particle and large mass at first started straight for the large passing star1 but because of change of position of the latter the particles and larger masses were gradually pulled around so that their paths curved into spirals4 ,because of crossing of paths1 the larger masses or knots gradually increased in size by accretion of the small particles or planetesimals4 ,meteors 7so-called shooting stars7 which now strike the earth are thought to be disrupted materials still being gathered in1 though very slowly at present4 ,after the passing star got well out of range1 the spiral paths of the disrupted masses gradually changed to nearly circular1 due to a wrapping-up process around the central body 7sun7 which then controlled the movements of the both larger masses 7future planets7 and small masses 7planetesimals74 ,let us now inquire briefly into the bearing of this planetesimal hypothesis upon the early geological history of the earth4 ,according to this doctrine the earth was never in the form of a highly heated gas1 nor was it ever necessarily hotter than now4 ,instead of beginning as a much larger body which has gradually diminished in size1 the earth steadily grew1 up to a certain stage1 by ingathering of planetesimals4 ,increase in size caused the force of gravity to increase and this caused not only steady contraction of the earth's matter1 but also a development of greater internal heat4 ,the earth has been getting smaller ever since the force of compression has predominated over the building-up process1 because of the diminishing supply of planetesimals4 ,due to steadily increasing internal pressure and heat the various gases1 including water vapor1 have been driven out of the earth to form an atmosphere which has gradually become larger and denser4 ,after sufficient accumulation of water vapor1 condensation and rainfall took place2 the waters of the earth began to gather to form the oceans2 and the ordinary geologic processes of erosion and deposition of strata were initiated4 ,according to this view stratified rocks could have been formed very early in the history of the earth1 and in this connection it is interesting to note that the oldest known rocks are actually of sedimentary origin4 ,,chapter ,,xiii ,,very ,,ancient ,,earth ,,history 7,archeozoic and ,proterozoic ,eras7 ,we shall now consider the older rocks of the ,earth1 including those of ,archeozoic1 ,proterozoic1 and ,paleozoic ages4 ,what are the salient points in the very early history of the earth 7not including the evolution of organisms7 shown by these very ancient rocks8 ,beginning with the oldest known rocks1 it will be our purpose to trace out the principal recorded events of earth history in the regular order of their occurrence4 ,as in human history1 so in earth history the recorded events of very early times are fewest and most difficult of all to understand4 ,in spite of this difficulty it is best to begin with the oldest known rocks or1 as ,le ,conte has said1 to follow the natural order of events4 ,this has the great advantage of bringing out the philosophy of the history--the law of evolution4 ,because of limitation of space we shall give special attention to the physical history of ,north ,america1 but the general principles brought out apply almost equally well to the other continents4 ,the ,archeozoic rocks contain the earliest known records of geological history1 or1 in other words1 the oldest recorded ordinary geological processes such as weathering and erosion1 deposition of strata1 igneous activity1 etc4 ,although we are here dealing with the most obscure records of any great rock system1 partly because the rocks have been so profoundly altered 7metamorphosed71 and partly because of the absence of anything like definitely determinable fossils1 it is1 nevertheless1 true that certain very important conclusions have been reached regarding this very ancient geological era4 ,among the very oldest of all known rocks of ,north ,america are the ,grenville strata1 so named from a town in the ,st4 ,lawrence ,valley4 ,in fact1 no rocks elsewhere in the world have been proved to be more ancient4 ,the ,grenville series consists of a great mass of sediments 7strata7--original muds1 sands1 and limes--which were deposited layer upon layer under water 7,plate #ab74 ,the widespread extent and character of the series in southeastern ,canada and the ,adirondacks1 and more than likely far beyond these limits1 make it certain that the ,grenville strata were accumulated on the bottom of a relatively shallow sea very much as sediments are now piling up on shallow sea bottoms4 ,thus1 the most ancient definitely known condition of the region where the ,grenville strata are exposed was an expanse of the sea covering the whole area4 ,wherever1 in other parts of the world1 the ,archeozoic rocks have been studied1 stratified rocks also seem to be the very oldest which are recognizable1 but up to the present time no such rocks have been proved to be any older than1 or even as old as1 the ,grenville series4 ,it may occur to the reader to ask1 how long ago did the ,grenville ocean exist8 ,there are grave difficulties in the way of answering this question in terms of years since we have nothing like an exact standard for such a measurement or comparison4 ,although we must concede that not even approximate figures can be given1 it can1 nevertheless1 be demonstrated by several independent lines of reasoning that the time must be measured by at least tens of millions of years1 a very conservative estimate of the minimum time which must be allowed being about #ej million years4 ,in any case1 the time is so utterly inconceivable to us that the important thing to bear in mind is that the great well-known events of earth history1 which have transpired since the existence of the ,grenville ocean1 require a lapse of many millions of years1 as shown by revolutionary changes in geographic and geologic conditions such as the long periods of erosion1 the enormous accumulations of sediment1 the repeated spreading out and disappearance of sea water over many portions of the earth1 and the building up and tearing down of great mountain ranges at various times4 ,the ideas here expressed will be much better appreciated by the reader after following through the salient points in the history of ,north ,america as set forth in the succeeding pages4 ,again1 the reader may ask1 by what line of reasoning do we conclude that these stratified rocks are so exceedingly ancient8 ,all rocks of ,archeozoic ,age1 including strata as well as certain younger igneous rocks 7see below71 invariably occupy a basal position in relation to all other rock systems4 ,they constitute a complex lot of crystalline metamorphic rocks1 combining certain characteristics which lie below the base of the determined sedimentary succession4 ,where rocks with the characteristics of the ,archeozoic are separated from the oldest ,paleozoic 7,cambrian7 strata by the great sedimentary or metamorphic system known as the ,proterozoic 7see below71 we may be sure that we are dealing with ,archeozoic rocks4 ,if the series of rocks in question belongs in the ,archeozoic system1 all that remains is to determine its age position in that system4 ,this can usually readily be done because wherever they have been studied the ,archeozoic rocks may be subdivided into two groups of rocks1 a sedimentary and an igneous4 ,where the igneous rocks1 mainly granites and related types1 occur associated with the sedimentary rocks 7e4g41 ,grenville71 they very clearly were forced or intruded1 while molten1 into the sedimentary rocks1 thus proving these latter to be the older4 ,since the ,archeozoic strata of the ,adirondack ,mountains1 southeastern ,canada1 and also all1 or nearly all1 other known districts are mostly badly disturbed1 tilted1 and more or less bent or folded1 and since neither top nor bottom of the piles of strata has ever been recognized as such1 it is impossible to give anything like an exact figure for the thickness of the series4 ,continuous successions of strata have1 however1 been observed in enough places to show that they were commonly deposited layer upon layer to a thickness of at least some tens of thousands of feet4 ,a thickness of over #ajj1jjj feet has been reported from southeastern ,canada4 ,the clear implication is that the ,archeozoic sea which received sediments must have existed for a vast length of time which must be measured by at least some millions of years1 because in the light of all our knowledge regarding the rate of accumulation of sediments a very long time was necessary for the piling up of such thick masses of strata4 ,it does not1 however1 necessarily follow that the ,grenville ocean was many thousands of feet deep where deposition took place4 ,in fact1 the very character of the original sediments 7muds1 sands1 and limes7 clearly indicates that the ,archeozoic sea in which they accumulated was1 for most part at least1 of shallow water because such sediments have rarely1 if ever1 been carried out into an ocean of deep water4 ,the great ocean abysses of to-day are not receiving any appreciable amount of land-derived sediment4 ,thus we are forced to conclude that in ,archeozoic time1 as well as many times in later ages1 the shallow sea bottom gradually sank while the sediments accumulated4 ,even more conclusive proof of such subsidence has been obtained from the study of so-called folded mountain ranges of ,paleozoic and later time1 an excellent example being the ,appalachian ,range4 ,having established the sedimentary origin and great antiquity of the ,grenville series1 we are led to the interesting and important conclusion that these oldest known rocks are not the most ancient which ever existed1 because the ,grenville strata must have been deposited layer upon layer1 upon a floor of still older rocks4 ,if such still older rocks are anywhere exposed to view1 they have never been recognized as such4 ,again1 the fact that the most ancient known rocks were deposited under water carries with it the corollary that there must have been lands at no great distances from the areas of deposition because1 then as now1 such sediments as muds and sands could have been derived only from the wear or erosion of lands1 and have been deposited in layers under water adjacent to those lands4 ,but we are utterly in the dark regarding any knowledge of the location or character of such very ancient lands4 ,the most ancient known strata1 as we see them to-day1 do not look like ordinary sediments such as shales1 sandstones1 and limestones4 ,they have been profoundly changed from their original condition1 that is to say1 they have undergone metamorphism4 ,the ,archeozoic strata now exposed to view were formerly buried at least some miles below the earth's surface1 the overlying younger rocks having since been removed by erosion through the millions of years of time4 ,far below the earth's surface1 under conditions of relatively high temperature1 pressure1 and moisture1 the materials of the strata were completely crystallized into various minerals4 ,the surfaces of separation of the very ancient layers of sediment are still usually more or less clearly present 7,plate #ab74 ,original limestone has been changed into crystalline limestone or marble2 sandstone has been changed into quartzite1 and shale1 sandy shale1 and shaly sandstone have been changed into various schists and gneisses4 ,in western ,ontario there are also stratified rocks 7called the ,keewatin series7 which seem to be of about the same age as the ,grenville strata farther east4 ,a point of special interest in connection with the ,keewatin strata is the presence of layers of lava in portions of the series1 thus proving that molten rock materials were poured out on the earth's surface during the most ancient known era of the earth's history4 ,after the accumulation of the very ancient ,archeozoic sediments igneous activity took place on grand scales when great masses of molten rock were forced 7intruded7 into the sediments from below4 ,masses of molten materials are known to have been thus intruded at several different times1 but of these the most common by far cooled to form a great series of granite and closely related rocks4 ,the general effect was to break the old strata up into patches or masses of varying sizes as clearly shown by the present distribution and modes of occurrence of these igneous rocks4 ,in most cases the strata were pushed aside by1 or tilted or domed over1 the upwelling molten floods--in many cases the molten materials were1 under great pressure1 intimately forced or injected into the strata2 numerous large and small masses of strata were caught up or enveloped 7as inclusions7 in the molten floods2 in some cases there was local digestion or assimilation of the strata by the molten materials1 while in still other places large bodies of strata seem to have been left practically intact and undisturbed4 ,such igneous rocks1 which are very widespread1 are all of the plutonic or deep-seated types2 that is1 they were never forced up to the earth's surface like lavas1 but they solidified at considerable depths 7at least some thousands of feet7 below the surface4 ,we see them exposed to-day only because a tremendous amount of overlying rock materials has been removed by erosion4 ,these igneous rocks are generally easily distinguished from the old sediments of ,grenville age because of their more general homogeneity in large masses1 and their lack of sharply defined bands or layers of varying composition4 ,the fact that the minerals have always crystallized to form medium to coarse-grained rocks shows that these rocks solidified under deep-seated conditions1 since it is well known that surface flows 7lavas7 are much finer grained commonly with more or less of the rock not crystallized at all4 ,slow cooling under great pressure favors more complete crystallization with growth of larger crystals4 ,as we have just learned1 the very character and structure of the ,archeozoic rocks now exposed to view show conclusively that they were formerly deeply buried1 and the inference is perfectly plain that the overlying rock materials were removed by erosion4 ,profound erosion of any land mass means that the land must have stood well above sea level1 and thus we come to the important conclusion that the great mass of ,archeozoic rocks 7both strata and igneous rocks7 were upraised well above sea level4 ,just when the uplift occurred cannot be positively stated1 but in every region where the matter has been studied it took place before the strata of the next geological era began to deposit as shown by the fact that such later strata rest upon the profoundly eroded surface of the ,archeozoic rocks4 ,such an erosion surface1 called an unconformity1 marks a gap in the geological record of the district where it occurs4 ,there is much to support the view that the uplift was concomitant with the great igneous intrusions1 especially the granite4 ,it is reasonable to believe that the same great force which caused the welling up of such tremendous bodies of liquid rock into the earth's crust might easily have caused a decided uplift of a whole large region1 but even so the process must have been geologically slow4 ,in regard to the height of those ancient lands1 the character of the topography1 and the drainage lines we are as yet utterly in the dark4 ,the fact that many thousands of feet in thickness of materials were removed by erosion to expose the once deeply buried rocks1 does not necessarily imply that the lands at any time had great height1 because it is possible that while elevation slowly progressed1 much material was steadily removed by erosion4 ,in the light of our knowledge of the origin and growth of mountain ranges of later time there is little doubt that at least some of the ,archeozoic lands were raised to such mountain heights4 ,thus far in our study of the ,archeozoic rocks attention has been mainly directed to southeastern ,canada and the ,adirondack mountains1 where careful studies have been made4 ,in all parts of the world where the most ancient known 7,archeozoic7 rocks have been studied in detail the same general principles apply4 ,particular attention has been given to the ,archeozoic rocks south of ,lake ,superior1 and in the ,piedmont ,plateau of the eastern ,united ,states4 ,in the accompanying map ,archeozoic rocks are widely exposed to view within the areas shown in black4 ,it has been estimated that ,archeozoic rocks appear at the surface over about one-fifth of the land area of the earth4 ,where they are not at the surface it is believed that they everywhere exist under cover of later rocks4 ,in other words1 ,archeozoic rocks are considered to be almost universally present either at or under the earth's surface4 ,this is true of the rocks of no other age4 ,special mention should be made of the fine exhibitions of ,archeozoic rocks in ,scandinavia and the ,highlands of ,scotland4 ,illustration3 ,fig4 #cb4--,map showing the surface distribution of ,archeozoic and ,proterozoic rocks in ,north ,america4 7,redrawn by the author after ,u4 ,s4 ,geological ,survey47 ,all known evidence leads us to the remarkable conclusion that the climate of much1 or possibly all1 of ,archeozoic time was not fundamentally different from that of to-day4 ,there must have been weathering of rocks1 rainfall1 and streams much as at present as proved by the character and composition of the stratified rocks which formed in that remote era4 ,the presence of graphite 7black lead7 in crystalline flakes scattered through many of the strata shows that the climate must have been favorable to some form of life1 because graphite thus occurring quite certainly represents the remains of organisms1 this matter being more fully discussed in a succeeding chapter4 ,in passing it may be stated that climatic zones were then probably scarcely if at all marked off1 as they quite certainly were not even during ,paleozoic time4 ,one of the great contributions of geology to human knowledge is that during the tens of millions of years from ,archeozoic times to the present the earth's climate has undergone no fundamental change or evolution4 ,in the earlier ages there was greater uniformity of climate over the earth1 and1 during known geologic time there have been rather localized relatively minor fluctuations giving rise to glaciers1 deserts1 etc41 but there has been no real evolution of climate at all comparable to the marvelous evolution of organisms--both animals and plants4 ,we shall now turn our attention briefly to a consideration of the second great subdivision of geologic time--the ,proterozoic era4 ,rocks of ,proterozoic ,age comprise all of those which were formed after the ,archeozoic rocks and before the deposition of the earliest ,paleozoic 7,cambrian7 strata1 these latter being rather definitely recognizable because they contain fossils characteristic of the time4 ,cambrian strata are1 in fact1 the oldest rocks which contain anything like an abundance of fossils1 so that the separation of rocks of either ,archeozoic or ,proterozoic ,age from the earliest ,paleozoic is seldom difficult4 ,but how may we separate the ,proterozoic rocks from the ,archeozoic8 ,fossils afford us no aid whatever1 because no determinable fossils have been found in rocks as old even as the earlier ,proterozoic4 ,the two great groups of very ancient rocks do1 however1 show a number of differences which must be considered together4 ,thus1 igneous rocks distinctly predominate in the ,archeozoic1 while stratified rocks predominate in the ,proterozoic4 ,all ,archeozoic strata are thoroughly metamorphosed 7changed from their original condition71 while large masses of the ,proterozoic strata are only moderately metamorphosed1 or even unaltered1 and therefore look much like ordinary strata of later ages4 ,archeozoic rocks have almost invariably been notably deformed by more or less folding1 tilting1 etc41 while the ,proterozoic rocks show relatively much less deformation4 ,another important criterion is the fact that the ,proterozoic rocks1 wherever they have been studied in relation to the ,archeozoic rocks1 always rest upon a profoundly eroded surface of the latter1 that is1 an unconformity separates the two great sets of rocks4 ,this erosion surface is of still further interest because it is the very oldest one known1 none having been recognized within the ,archeozoic group itself4 ,even where the ,proterozoic strata have been considerably metamorphosed and deformed1 this old erosion surface may be recognized1 and if the rocks below that surface possess the characteristics of the ,archeozoic rocks as described above1 the two great very ancient rock groups may be distinguished4 ,one of the triumphs of geology during the last #be to #cj years has been the recognition of the great rock group 7,proterozoic7 between the ,archeozoic and ,paleozoic1 thus bringing to light the records of an era which lasted many millions of years4 ,the length of time represented by the ,proterozoic era is by many believed to have been fully as long as all succeeding eras--,paleozoic1 ,mesozoic1 and ,cenozoic--combined4 ,twenty million years would be a very conservative estimate for the duration of the era4 ,what is the nature of the evidence as recorded in the rocks which lead us to conclude that the ,proterozoic era lasted such a vast length of time8 ,the great thickness of ,proterozoic strata 7over #cj1jjj feet in the ,lake ,superior region71 in the light of what we have already learned regarding the present rate of wear 7erosion7 of lands and deposition of the eroded materials under ordinary conditions1 clearly implies millions of years of time for their accumulation4 ,but the ,proterozoic strata as we now see them are in most places not a continuous pile1 that is they were not accumulated layer upon layer without notable interruption4 ,thus1 the thick ,proterozoic group of the ,lake ,superior region has been divided into four distinct1 mainly sedimentary series separated from each other by erosion surfaces 7unconformities74 ,each erosion surface represents a long time when the area was elevated and underwent profound wear before the next series of strata accumulated on the worn surface4 ,that such times of erosion were geologically long is proved not only by the profound alteration 7metamorphism7 of one set of strata before another accumulated1 but also by the fact that granite1 which1 as we have learned1 is never exposed except where much overlying material has been eroded1 actually formed parts of surfaces of earlier ,proterozoic rocks upon which later ones were deposited4 ,in the ,lake ,superior region there are not only three great erosion surfaces 7unconformities7 within the ,proterozoic group1 but also one at the base separating it as a whole from the ,archeozoic group1 and another at the top separating it from the ,paleozoic group4 ,it is1 therefore1 fair to conclude that the amount of time 7millions of years7 represented by these great erosion intervals was fully as great as the time needed for deposition of the existing ,proterozoic strata4 ,in the ,lake ,superior region the older ,proterozoic strata are nearly all more or less folded and altered 7metamorphosed71 and they have been intruded by considerable bodies of molten rock1 mostly granite4 ,the later ,proterozoic strata have been much less deformed and in many cases they are practically unaltered4 ,in this region a very remarkable event took place in late ,proterozoic time4 ,this was volcanic activity on a grand scale4 ,we may gain some idea of the stupendous and long-continued volcanic outpourings from the fact that1 based upon actual measurements of thickness1 lava sheets1 averaging about #ajj feet thick1 poured out one upon another until a pile about six miles high had accumulated4 ,in parts of the ,grand ,canyon of the ,colorado tilted ,proterozoic strata may be seen resting upon the profoundly eroded surface of the ,archeozoic rocks of the inner gorge4 ,the ,proterozoic strata1 #ab1jjj feet thick1 consist of practically unaltered sandstones1 shales1 and limestones1 associated with some layers of basaltic lava4 ,an erosion surface 7unconformity7 separates the whole group into two distinct series1 and the group is separated from the overlying nearly horizontal ,paleozoic 7,cambrian7 strata in the walls of the ,canyon by another erosion surface4 ,more recently the ,proterozoic strata so finely displayed in the ,rocky ,mountains of ,montana and southern ,canada have been studied4 ,these strata1 at least two or three miles thick1 are mostly unaltered sandstones1 shales1 and limestones1 associated with some metamorphic and igneous rocks4 ,as usual1 these strata rest upon the eroded ,archeozoic4 ,they were more or less upturned and folded before deposition of the succeeding ,paleozoic strata4 ,satisfactory subdivisions have not yet been worked out4 ,in ,north ,america most of the areas shown on the accompanying map contain more or less ,proterozoic rocks4 ,rocks of this age are known to some extent in all continents where their general relationships seem to be much like those of ,north ,america4 ,they have perhaps been most carefully studied in ,scandinavia and the ,highlands of ,scotland1 where the strata portions are about two miles thick4 ,the climate of ,proterozoic time must1 for most part1 have been about like that of to-day except1 of course1 for its much greater uniformity over the earth4 ,about a dozen years ago very typical glacial deposits were discovered within the early ,proterozoic rocks of western ,ontario1 ,canada4 ,a climatic condition favorable for the development of glaciers so early in the history of the earth is1 to say the least1 directly opposed to an idea 7based upon the nebular hypothesis7 long held that the climate of early geologic time must have been much warmer than that of the present4 ,,chapter ,,xiv ,,ancient ,,earth ,,history 7,paleozoic ,era7 ,beginning with the earliest ,paleozoic1 the legible records of events of earth history are far more abundant and less defaced than those of earlier times4 ,stratified rocks of the ordinary kinds greatly predominate over the igneous and metamorphic rocks1 and the strata are in general far less disturbed than those of the ,archeozoic and ,proterozoic groups4 ,from the earliest ,paleozoic we have also the first abundant records 7fossils7 of the life of the earth1 so that the ordinary methods of subdividing and determining the relative ages of the ,paleozoic and later strata1 as well as correlating the subdivisions 7formations7 in widely separated regions1 can be used4 ,from here on in our discussion of earth history we shall be able to trace the salient features of the changing outlines of the face of the earth1 the coming and going of the seas over the lands1 and the evolution of animals and plants with a considerable degree of definiteness and satisfaction4 ,first1 we shall trace out1 in the regular order of their occurrence1 the main physical history events of ,paleozoic time1 leaving a consideration of the evolution of life for other chapters4 ,because of limitation of space1 our attention will be almost wholly centered upon the continent of ,north ,america1 but the reader should bear in mind that the general principles and facts set forth apply with about equal force to most other continents4 ,in ,europe the wonderful records of ,paleozoic history are found in strata1 whose estimated maximum thickness is about #ajj1jjj feet6 ,it must not be thought1 however1 that all these strata are piled up in a single locality1 but the figure does actually represent the sum total of the greatest thickness of the many subdivisions 7formations7 of the ,paleozoic rocks in different portions of the continent4 ,in ,north ,america the maximum thickness of all ,paleozoic rocks seems to be no less than #ej1jjj feet4 ,more than #be1jjj feet of strata may actually be observed piled layer upon layer in the highly folded and deeply eroded central ,appalachian ,mountains4 ,the great thickness of the strata1 combined with the facts that the fossils show that many marvelous1 mostly progressive1 changes took place among living things1 that seas came and went repeatedly over many parts of the continent1 and that great changes took place in the configuration of the land1 force us to conclude that ,paleozoic time must have lasted for many millions of years4 ,just before the opening of the ,paleozoic era practically all of ,north ,america appears to have been dry land1 which had undergone so much erosion that it was low and far less rugged in relief than at present4 ,this we know1 because the rather widespread early ,paleozoic 7,cambrian7 strata almost everywhere rest upon deeply eroded rocks of either ,archeozoic or ,proterozoic age4 ,considering both the time involved and the wide area affected1 we have no record of anything like such a profound erosion interval since the beginning of the ,paleozoic era4 ,it seems that the constructive or upbuilding forces within the earth were then remarkably quiescent1 while the destructive forces 7erosion7 were almost unhampered in their work of cutting down the land4 ,have we any definite idea of the relations of land and water in ,north ,america during the first or ,cambrian period of the ,paleozoic era8 ,in the affirmative answer to this question1 certain principles will be brought out which the reader should keep in mind as we trace out the succeeding great physical changes in the history of ,north ,america4 ,it should1 however1 be remembered that1 in the brief space at our disposal1 only the most general1 or the most significant localized1 physical changes in the long and intricate known history of the continent since the opening of the ,paleozoic era can be brought out4 ,in early ,cambrian time a narrow arm of the sea 7like a strait7 extended from the ,gulf of ,st4 ,lawrence southward across eastern ,new ,york and over the site of the present ,appalachian ,mountains connecting with the ,gulf of ,mexico on the south4 ,on the west1 a much larger and broader arm of the sea 7like a mediterranean7 extended from ,alaska southward over the site of the ,rocky ,mountains of ,canada and across the sites of the ,columbia ,plateau to ,great ,basin of the western ,united ,states4 ,all the rest of the continent was land1 apparently almost or wholly devoid of high mountains4 ,by what process of reasoning do we conclude that arms of the early ,cambrian sea reached across eastern and western ,north ,america8 ,first1 wherever marine strata of definitely determined early ,cambrian age now occur1 the early ,cambrian sea must have existed because those strata were obviously deposited in that sea4 ,second1 to those areas we must add others from which it can be demonstrated that early ,cambrian marine strata have been removed by erosion4 ,enough field work along these lines has been done in ,north ,america to render it practically certain that the relations of land and water during early ,cambrian time were essentially as above outlined4 ,illustration3 ,fig4 #cc4--,map showing the relations of land and water in ,north ,america during early ,cambrian time1 at least #be1jjj1jjj years ago4 ,lined areas represent land4 7,principal data from a map by ,willis published in the ,journal of ,geology47 ,illustration3 ,fig4 #cd4--,map showing the relations of land and water in ,north ,america during ,middle ,ordovician time4 ,lined areas represent land4 7,principal data from a map published by ,willis in the ,journal of ,geology47 ,as ,cambrian time went on1 the marine waters gradually spread from south to north across most of the ,mississippi ,valley area1 causing the eastern and western arms of the sea to be connected1 thus forming an interior continental sea4 ,otherwise the relations of land and water were much as in early ,cambrian time4 ,we know that the sea transgressed northward across the ,mississippi ,valley district because1 on the south1 the whole ,cambrian system of strata 7lower1 middle1 and upper7 is present1 while1 farther north1 only middle and upper ,cambrian are present1 and1 farthest north1 only upper ,cambrian strata occur4 ,this progressive northward overlap of younger and younger 7later7 ,cambrian strata upon the old rock floor proves1 that the ,cambrian sea steadily spread farther and farther northward over the ,mississippi ,valley area4 ,that this spreading sea was shallow is amply demonstrated by the deposits it left1 such as shales1 conglomerates 7i4e41 consolidated gravels7 and sandstones1 often ripple-marked4 ,the ,cambrian strata of ,north ,america vary in thickness from less than #a1jjj feet to about #ab1jjj feet4 ,in the ,mississippi ,valley the ,cambrian strata are unaltered and almost undisturbed from their original horizontal position4 ,in the ,appalachian ,mountains of the east1 and the ,rocky ,mountains of the west1 the strata are commonly notably folded and faulted4 ,in some places1 as in western ,new ,england1 the strata have been notably altered 7metamorphosed74 ,the best estimates for the duration of the ,cambrian period range from #b1jjj1jjj to #c1jjj1jjj years4 ,it is a remarkable fact that1 during this great lapse of time1 ,north ,america was unaffected by any great physical disturbances such as mountain making1 emergence of large tracts of land1 or igneous activity4 ,the one great physical event of the ,cambrian was the gradual submergence of a considerable portion of the continent4 ,that the climate of the earliest ,cambrian was at least locally favorable for the existence of glaciers1 is proved by the occurrences of true glacial deposits in rocks of that age in ,china1 ,norway1 and ,australia4 ,it is a remarkable fact that the glacial materials of ,china occur along the ,yangtse ,river1 thus demonstrating that conditions for glaciers then existed at a latitude as far south as ,new ,orleans4 ,these evidences of glaciation directly refute the old idea1 based upon the nebular hypothesis1 that the climate of the ,paleozoic was distinctly warmer than now4 ,the glacial evidence1 added to our knowledge of the character and world-wide distribution of many identical species of animals1 leads us to conclude that early ,paleozoic climate was not essentially different from that of very recent geologic time1 but that the climate was then much more uniform than at present4 ,during the second or ,ordovician period of the ,paleozoic era1 the progressive submergence of ,cambrian time continued until a climax was reached toward the middle of the period when fully four-fifths of the continent was submerged under shallow sea water4 ,since middle ,ordovician marine strata are more widespread than the rocks of any succeeding age1 we can be reasonably sure that so much of the continent was never again covered by the sea4 ,in fact1 so far as the records have been interpreted1 this came nearest to being a universal flood in the whole known history of the continent4 ,by the very character of the rocks deposited 7seldom over a few thousand feet thick71 we can be sure that the middle ,ordovician continental sea was everywhere far shallower than the great ocean abysses of to-day4 ,because the lands were so low and restricted1 relatively little land-derived sediment washed into the sea4 ,but the shallow sea water was inhabited by millions of animals1 the shells of many of which slowly accumulated to build up the thick bodies of limestone strata 7,plate #ad7 which constitute the main bulk of rock of early and middle ,ordovician age4 ,the famous ,trenton limestone1 named from a locality in central ,new ,york1 with its great abundance of fossils1 was formed mostly by the accumulation of shells of animals during middle ,ordovician time4 ,later in the ,ordovician there was a considerable shift in level between land and water causing a withdrawal of much of the widespread sea4 ,as a result of the generally more elevated lands1 erosion proceeded more vigorously1 and sands and muds were more abundantly deposited in the restricted sea1 these sediments having consolidated to form the shales and sandstones which predominate among the upper ,ordovician rocks4 ,a principle above briefly explained in the discussion of the ,cambrian may be reemphasized here4 ,it is as follows3 ,in making a map to show the relations of land and water1 say during middle ,ordovician time1 the geologist is by no means dependent only upon actual surface exposures of middle ,ordovician strata4 ,such exposures fall far short of giving an adequate conception of the former or even present real extent of such strata4 ,in many places originally present ,ordovician strata have been removed by erosion4 ,an excellent case in point is the ,adirondack region of northern ,new ,york4 ,on the west side of the ,adirondacks a great pile of marine ,ordovician strata #a1ejj feet thick end abruptly on the gently sloping flank of the mountains1 thus clearly proving that the strata formerly extended at least twenty to thirty miles eastward4 ,again1 in the southern ,adirondacks a small area of very typical marine middle ,ordovician strata lies fully fifteen miles from the general area of such rocks to the south4 ,this small body of rock is very clearly only an erosion remnant of a general sheet of middle ,ordovician rock which once covered the whole intervening district4 ,in many other regions the middle ,ordovician strata are definitely known to be concealed under cover of later rocks1 as in the ,mississippi ,valley1 where the actual surface exposures constitute only a fraction of the middle ,ordovician strata which underlie nearly all the valley1 as proved by deep well drillings1 study of the scattering outcrops1 etc4 ,in still other places1 middle ,ordovician strata1 associated with other rocks1 are highly folded1 as in the ,appalachians1 where such strata outcrop in only narrow belts following the trend of the folds4 ,in short1 then1 wherever it can be proved that middle ,ordovician marine strata are visible at the surface1 or are concealed under other rocks1 or were once present1 we can be sure that the middle ,ordovician sea existed4 ,exactly this principle applies to any subdivision of geologic time4 ,illustration3 ,fig4 #ce4--,structure section showing rocks representing three geologic eras separated by millions of years of time4 ,length of section #ab miles1 vertical scale much exaggerated4 ,at the bottom are ,archeozoic 7,precambric7 rocks and resting upon them on the left are early ,paleozoic strata #a1ejj feet thick4 ,a glacial lake deposit of late ,cenozoic age lies on the ,archeozoic rock toward the right4 ,it is evident that the ,paleozoic strata formerly extended much farther eastward4 7,by the author as published in a ,new ,york ,state ,museum ,bulletin47 ,the ,ordovician period closed with a great mountain-making disturbance in eastern ,north ,america1 and at the same time all1 or nearly all1 of the continent was land4 ,throughout most of the ,cambrian and ,ordovician periods1 the strata accumulated to a thickness of thousands of feet in the marine waters which spread over the eastern border of ,new ,york1 the sites of the ,green ,mountains of ,vermont1 the ,berkshire ,hills of ,massachusetts1 and southward at least as far as ,virginia1 over the area of the ,piedmont ,plateau4 ,at1 or toward the close of the ,ordovician period1 a great compressive force in the earth's crust was brought to bear upon the mass of strata and they were tilted1 highly folded1 and raised above sea level into a great mountain range known to geologists as the ,taconic ,range4 ,it is quite the rule throughout this region of ,taconic disturbance to find the strata either on edge or making high angles with the plane of the horizon4 ,many of the folds were actually overturned1 and in some cases notable thrust faults developed1 that is1 the upper strata broke across and great masses were shoved over each other4 ,these facts all go to show that the mountain-making compressive force applied to the region was of rather an extreme type4 ,since the origin of the ,taconic ,range a tremendous amount of erosion has taken place1 so that literally only the roots of the range are now exposed in the ,green ,mountains1 ,berkshire ,hills1 ,highlands-of-the-,hudson1 and the northern ,piedmont ,plateau4 ,how do we know that the ,taconic disturbance took place toward the close of the ,ordovician period8 ,by way of answer to this question two facts need to be considered4 ,first1 relatively late 7or young7 ,ordovician strata are involved with the folds1 thus proving that the folds formed after those late ,ordovician sediments were deposited4 ,second1 undisturbed strata formed during the middle of the next 7,silurian7 period1 rest upon the eroded edges of the folds1 which proves that the folds must have developed well before middle ,silurian time because the only time they were subjected to erosion must have been during early ,silurian time4 ,illustration3 ,fig4 #cf4--,structure section showing profile and underground relations of the rocks across part of the ,highlands-of-the-,hudson region in southeastern ,new ,york4 ,length of section1 sixteen miles4 ,the rocks are mostly of ,prepaleozoic ,age1 but with belts of highly infolded early ,paleozoic strata toward the middle right4 7,after ,berkey1 ,new ,york ,museum ,report47 ,mention should also be made of the profound metamorphism 7alteration7 of the ,cambrian and ,ordovician strata along the main axis of the range1 where the intense compression1 aided by heat and moisture1 caused the deeply buried portions of the strata to become plastic1 and hence they became more or less foliated 7cleavable7 and crystallized into various metamorphic rock types1 the limestone having changed to marble1 the shale to slate or schist1 and the sandstone to quartzite4 ,thus we explain the rocks of the extensive marble quarries of ,vermont and western ,massachusetts1 the slate quarries of central eastern ,new ,york1 and the ,berkshire schist of the ,berkshire ,hills of ,massachusetts4 ,one of the grandest and most significant of all the profound geological processes is the birth and history of a great folded mountain range4 ,since the ,taconic ,range affords us such an excellent example of a large-scale1 well-understood folded range of great antiquity we may do well to consider it in the light of certain other broad relationships4 ,the great compressive force which folded and upraised the ,taconic ,mountains did not accomplish its work rapidly in the ordinary human history sense of the word4 ,the force was slowly and irresistibly applied1 and the strata well below the surface were gradually bulged or folded1 or fractured where near the surface1 the length of time required for the operation having been1 at the very least calculation1 some hundreds of thousands of years1 and more than likely a million years or more4 ,such a length of time is1 however1 so short compared with all known earth history1 that we are accustomed to refer to the formation of such a mountain range as simply an event of geological history4 ,even before such a range attains its maximum height a very considerable amount of erosion has already taken place4 ,when the first fold appears above sea level1 erosion begins its work and continues with increasing vigor as the mountain masses get higher and higher4 ,thus we have warfare between two great natural processes--the building up and the tearing down4 ,after a time the building-up process wanes and then ceases1 while the tearing-down process 7erosion7 continues either until the whole range has been completely worn down or until some rejuvenating force causes a renewed uplift4 ,here is an example of one of the remarkable procedures of nature4 ,after millions of years of work causing the deposition of thousands of feet of strata1 piled layer upon layer on the sea floor1 a force of lateral pressure is brought to bear and a mountain range is literally born out of the sea4 ,no sooner is the range well formed than the destructive processes 7erosion7 unceasingly set to work to destroy this marvelous work4 ,but the sediments derived from the wear of the range are carried into the nearest ocean again to accumulate and1 perchance1 after long ages1 to be raised into another range2 and so the process may be often repeated4 ,from this we learn that the mountain ranges of the earth are by no means all of the same age4 ,the original ,adirondacks were formed long before the ,taconics1 which originated millions of years before the ,appalachians1 these latter having been folded up long before the ,sierras4 ,the ,rockies1 followed by the ,coast ,ranges1 are each younger than the ,sierras as regards their original folding and uplift4 ,among foreign countries special mention should be made of the ,british ,isles1 where ,ordovician strata thousands of feet thick were1 late in the period1 notably folded and upraised1 the crustal disturbance having been accompanied by great intrusions of molten rocks and vast outpourings of lavas1 so that this region ranks among the greatest of the ancient volcanic areas of ,europe4 ,illustration3 ,fig4 #cg4--,map showing the general relations of land and water in ,north ,america during middle and late middle ,devonian time fully #ae1jjj1jjj years ago4 7,after ,willis1 courtesy of the ,journal of ,geology47 ,we shall now turn our attention to a very brief consideration of the salient points in the physical history of ,north ,america during the next great period 7,silurian7 of the ,paleozoic era4 ,as a result of the physical disturbance late in the ,ordovician the great interior sea was largely or wholly expelled from the continent1 and this was essentially the condition of the continent at the beginning of the ,silurian4 ,but this condition was of short duration1 for early in the ,silurian the sea again began to spread1 gradually increasing in extent to a climax in about the middle of the period4 ,at this time the famous and extensive ,niagara limestone1 so named from the rock at the crest of ,niagara ,falls1 was deposited4 ,except for the newly formed ,taconic ,range1 standing out as a bold topographic feature along the middle ,atlantic ,coast1 and a somewhat wider extent of land1 the condition of the continent during middle ,silurian time was very similar to that of middle ,ordovician time4 ,illustration3 ,fig4 #ch4--,map showing the general relations of land and water in ,north ,america during middle ,mississippian time4 7,after ,willis1 courtesy of the ,journal of ,geology47 ,soon after mid-,silurian time the seas became greatly restricted almost to disappearance as such4 ,in the eastern ,united ,states and southeastern ,canada strata of that particular age are found only in parts of ,ontario1 ,new ,york1 ,ohio1 ,michigan1 and from ,pennsylvania southward to ,west ,virginia1 where they are characterized by red shales and sandstones1 and salt and gypsum deposits4 ,such materials containing few fossils very clearly indicate deposition in either extensive lagoons or more or less cut-off arms of the sea under arid climate conditions rather than in ordinary marine water4 ,still later in the ,silurian the interior seas were partially restored1 as shown by the fact that true marine strata corresponding to that age not only cover the salt and gypsum deposits1 but are notably more extensive than they4 ,about the close of the ,silurian period almost all of the continent was dry land4 ,unlike the ,ordovician period1 the ,silurian closed without any mountain-making disturbance or great uplift of land4 ,the ,silurian period1 like the preceding ,ordovician and ,cambrian1 seems to have been free from any more than slight igneous activity as1 for example1 in ,maine and ,new ,brunswick4 ,the total thickness of ,silurian strata in ,north ,america is seldom more than a few thousand feet4 ,the salient features of the physical history of the next1 or ,devonian period1 are much like those of the preceding ,silurian4 ,at the beginning of the ,devonian almost all of the continent was dry land1 but soon a long1 narrow arm of the sea extended across the eastern side of the continent from the ,gulf of ,st4 ,lawrence southward through western ,new ,england1 southeastern ,new ,york and throughout the ,appalachian district1 thus reminding us of the long1 narrow sound which occupied almost exactly the same territory during the early part of the ,cambrian period4 ,in the west the only water was a small embayment reaching across southern ,california into ,nevada4 ,by middle ,devonian time these water areas had considerably expanded4 ,during relatively late ,devonian time the sea was so expanded as to cover much of the ,mississippi ,valley area1 the ,appalachian ,mountains and ,st4 ,lawrence ,valley areas1 and most of the site of the ,rocky ,mountains1 except for an island of considerable size reaching from ,new ,mexico through ,wyoming4 ,the main lands were most of northeastern ,north ,america1 a large land area extending from ,florida to ,nova ,scotia1 and a large area on the western side of the continent from ,california to ,alaska4 ,a remarkable formation of late ,devonian ,age should be briefly described4 ,in southeastern ,new ,york and the northern ,appalachian region there was a tremendous accumulation of sediments which have consolidated into sandstone1 together with some shale and conglomerate4 ,this so-called ,catskill formation is from #a1ejj to #h1jjj feet thick and is well shown as the main body of rock in the ,catskill ,mountains4 ,it is largely a shallow-water deposit of essentially nonmarine origin1 as proved by coarseness of material1 ripple marks1 and nonmarine fossils4 ,all evidence points to the origin of this remarkable formation as a great delta deposit built out into the shallow interior sea4 ,notable thinning toward the west1 with increasing fineness of grain of material1 shows that the sediment came from the east1 no doubt carried by a large river from the small continental land mass 7called ,appalachia7 on the eastern side of ,north ,america4 ,the maximum thickness of the ,north ,american ,devonian seems to be about #ae1jjj feet in the northern ,appalachian region1 but elsewhere it generally ranges from #a1jjj to #d1jjj feet thick4 ,in ,north ,america the subdivisions of the ,devonian strata of ,new ,york are taken as a standard for comparison1 both because of the wonderful completeness and almost undisturbed character of the rocks there1 and because they have been so carefully studied4 ,the ,devonian system is there fully #d1jjj feet thick1 with scarcely a minor subdivision missing1 and it covers a wide area 7one-third of the ,state7 with many excellent outcrops4 ,there was practically uninterrupted deposition of ,devonian strata in southern ,new ,york4 ,it is doubtful if there is greater refinement of knowledge regarding the ,devonian or any other ,paleozoic system of strata anywhere else in ,north ,america4 ,during middle to late ,devonian time the region from southern ,new ,england to ,nova ,scotia and the ,st4 ,lawrence ,valley was notably disturbed by earth movements1 the lands having been considerably elevated and the rocks more or less folded4 ,the great delta deposit of late ,devonian time1 already described as being thousands of feet thick in ,new ,york and ,pennsylvania1 was formed by one or more streams which carved much sediment from the newly upraised lands4 ,accompanying the uplift and folding of the rocks considerable masses of molten granite were forced into the earth's crust and some molten rock was forced to the surface1 producing volcanoes4 ,much of the granite may now be seen at the surface in various portions of the region1 while deeply eroded volcanoes occur near the city of ,montreal4 ,except for the disturbance of the region from ,new ,england to the ,st4 ,lawrence1 the ,devonian period seems to have closed rather quietly1 with fairly widespread sea water over the land as already outlined4 ,this is proved by the fact that the early strata of the next period mostly rest in regular order upon the undisturbed late ,devonian strata4 ,for many years the term ,carboniferous period was used to designate a single period of geologic time which1 in ,america at least1 is now divided into two periods--the ,mississippian and ,pennsylvanian--corresponding1 respectively1 to the earliest and latest ,carboniferous4 ,in regard to the relations of land and water during the ,mississippian period1 the general statement may be made that the sea1 already fairly extensive in the late ,devonian1 continued to spread until during the second half of the ,mississippian1 when most of the ,united ,states west of the eastern border of the ,appalachians 7except the ,pacific ,northwest71 and also the ,rocky ,mountain region through ,canada1 were submerged4 ,a significant physical change marked the close of the ,mississippian4 ,this was the withdrawal of sea water from nearly all of the continent1 the emergence of the land having been generally sufficient to allow considerable erosion4 ,the fact that the ,mississippian and the next1 or ,pennsylvanian1 strata are separated by the most extensive distinct erosion surface in the whole ,paleozoic group of rocks is the chief reason for considering those two sets of strata to have formed during separate periods of geologic time4 ,in eastern ,north ,america the ,mississippian strata vary in thickness from a few hundred feet to a maximum of about #e1jjj feet in eastern ,pennsylvania4 ,in the ,west1 where the thickness is commonly several thousand feet1 limestone greatly predominates4 ,there appears to have been vigorous volcanic activity during the period from northern ,california to ,alaska4 ,certain profound crustal disturbances marked the close of the period in western ,europe1 resulting in upturning and folding of rocks during the process of mountain forming from ,ireland to ,germany1 and from ,bohemia to southern ,france4 ,abundant intrusions and extrusions of molten rocks accompanied the disturbances4 ,we turn next to a consideration of the ,pennsylvanian period1 which is of very special interest1 because within the rocks of that age in ,north ,america1 ,europe1 and ,china occur the greatest known coal deposits4 ,the period opened with almost all of ,north ,america dry land undergoing more or less erosion4 ,early in ,pennsylvanian time marine water began to overspread the western side of the continent1 especially most of the western two-thirds of the area of the ,united ,states1 where strata thousands of feet thick piled up4 ,the sea was most widespread before the middle of the period1 when the relations of land and water were about as shown by ,figure #ci4 ,over the site of the ,appalachians and most of the eastern half of the ,mississippi ,valley area the land either stood near sea level and was often swampy or marshy1 or at other times it was a little below sea level1 allowing tidewater to overspread the area4 ,such conditions alternated repeatedly1 usually more or less locally1 over different parts of the districts in which the great coal mines of the east are located4 ,under such conditions strata from #a1jjj to #h1jjj feet thick accumulated4 ,remarkable physical geography of this kind resulted in the growth and accumulation of vast quantities of vegetable matter which has changed into the world's greatest coal beds4 ,similar conditions prevailed over parts of ,nova ,scotia1 ,new ,brunswick1 and ,rhode ,island1 where strata fully #ac1jjj feet thick accumulated4 ,perhaps the most perfect resemblance to coal-forming condition is that now found on such coastal plains as that of southern ,florida and the ,dismal ,swamps of ,virginia and ,north ,carolina4 ,both of these areas are very level1 though with slight depressions in which there is either standing water or swamp condition4 ,in both regions there is such general interference with free drainage that there are extensive areas of swamp1 and in both there are beds of vegetable accumulations4 ,in each of these areas there is a general absence of sediment and therefore a marked variety of vegetable deposit4 ,if either of these areas were submerged beneath the sea1 the vegetable remains would be buried and a further step made toward the formation of a coal bed4 ,reelevation1 making a coastal plain1 would permit the accumulation of another coal bed above the first1 and this process might be continued again and again4 7,h4 ,ries47 ,but it is not necessary to assume repeated oscillations of a swamp area up and down as the only way of accounting for a succession of coal beds one above another in a given region1 because a general1 but intermittent1 subsidence1 with possibly some upward movements1 would occasionally cause the prolific plant life of a swamp to be killed1 after which sediment would deposit over the site4 ,shoaling of water by accumulation of sediment would permit the development of more swamp plant life4 ,illustration3 ,fig4 #ci4--,map showing the general relations of land and water1 including the great coal-plant swamp areas 7vertical lines71 in ,north ,america during the ,pennsylvanian period at least #aj1jjj1jjj years ago4 ,lined areas represent land4 7,after ,willis1 courtesy of the ,journal of ,geology47 ,in most coal-mining districts there are at least several coal beds1 one above another4 ,in ,illinois there are nine2 in ,pennsylvania at least twenty2 in ,alabama1 thirty-five2 and in ,nova ,scotia seventy-six1 but not all are important commercially4 ,each coal bed in such a region represents a swamp which existed in ,pennsylvanian time at least ten or twelve million years ago1 and in which there grew a luxuriant vegetation4 ,many individual swamps of that time were of wide extent4 ,the famous ,pittsburgh bituminous coal bed represents probably the largest one of all4 ,it extends from western ,pennsylvania into parts of ,ohio and ,west ,virginia over an area of fully #ae1jjj square miles4 ,more than #f1jjj square miles of it are being worked and the coal bed averages seven feet in thickness over an area of #b1jjj square miles4 ,among the various anthracite coal beds of the same age in eastern ,pennsylvania the ,mammoth bed is exceptionally thick1 reaching a maximum of fifty feet or more4 ,in order that the reader may not gain the impression that coal beds make up a very considerable bulk of the strata in coal-mining regions1 we should state that1 on the average1 coal actually constitutes less than #b per cent of the containing strata4 ,some idea of the tremendous length of the geologic ages may be gained by a consideration of the time which must reasonably be allowed for the accumulation of so many coal beds and their containing strata4 ,it has been estimated that a luxuriant growth of vegetation would produce #ajj tons of dried organic matter per hundred years4 ,compressed to the specific gravity of coal 7#a4d7 this would form a layer less than two-thirds of an inch deep on an acre4 ,during the chemical alteration of vegetable matter to coal about four-fifths of the organic matter disappears in the form of gases4 ,on this basis1 then1 it would take about #aj1jjj years to accumulate the vegetable matter represented in a coal bed one foot thick4 ,when we realize that the total thickness of the coal beds of the ,pennsylvanian system of strata in the great mining regions is commonly from #ajj to #bej feet1 we conclude that the time they represent is from #a1jjj1jjj to #b1ejj1jjj years4 ,it seems most reasonable that the time necessary for the deposition of the containing strata must have been at least as long4 ,it is1 therefore1 a fair conclusion that the ,pennsylvanian period lasted from #b1jjj1jjj to #e1jjj1jjj years4 ,that the climate of the great ,coal ,age was warm 7not tropical71 very moist1 and uniform1 is borne out by such facts as the following1 according to ,d4 ,white3 ,the succulent nature of the plants with their spongy leaves indicates prolific growth in moist1 mild climate2 lack of yearly rings of growth points to lack of distinct seasons2 as in the case of many existing plants the aerial roots signify a warm1 moist climate2 plants of to-day nearest like the coal plants thrive best in warm1 moist regions2 vegetable matter at present accumulates best in temperate rather than tropical climates1 because there decay is not so rapid2 and the remarkable uniformity of climate over the earth is clearly indicated by finding fossil plants of almost or exactly identical types in rocks of ,pennsylvanian ,age from the ,polar regions to the ,tropics4 ,the more remarkable plants of the great ,coal ,age time are described in the chapter on the evolution of plants 7,plate #ae74 ,during the last 7,permian7 period of the ,paleozoic era the marine waters of the west1 and the alternating shallow tidewater1 swamps1 and near sea level lands of the east gradually gave way to dry lands1 so that by the close of the period marine water covered only a small part of the ,southwest from ,oklahoma across central ,texas to southern ,california and northwestern ,mexico1 where strata as much as several thousand feet thick formed4 ,in the middle western part of the area of the ,united ,states1 especially from northern ,texas to ,nebraska and ,wyoming1 the climate was arid and red strata 7so-called ,red ,beds71 salt1 and gypsum were extensively deposited on land and in great salt lakes or more or less cut-off arms of the sea4 ,strata commonly from #b1jjj to #g1jjj feet thick were there deposited4 ,similar conditions prevailed in parts of ,nova ,scotia1 ,new ,brunswick1 and ,newfoundland1 where strata #h1jjj feet thick accumulated4 ,over the site of most of the ,appalachians the coal swamp conditions1 with local sea incursions1 continued from the preceding period1 as shown by the character of the strata 7#a1jjj feet thick7 containing some coal4 ,vigorous volcanic activity which1 as already mentioned1 began in the ,mississippian period from northern ,california to ,alaska continued not only through the ,pennsylvanian and ,permian but also into the early ,mesozoic era1 as shown by the great quantities of volcanic materials associated with rocks of those ages4 ,the ,permian presents a puzzling combination of climatic conditions which causes it to stand out in marked contrast against the generally mild and uniform climates of nearly all of preceding ,paleozoic time4 ,most remarkable of all are the records of a great ,ice ,age during early ,permian time4 ,one surprising fact is the widespread distribution of the glacial deposits in both the north and south temperate zones1 and even well within the torrid zone4 ,they are perhaps most extensive and best known in ,australia1 ,south ,africa1 ,india1 and ,brazil4 ,glacial deposits almost certainly of the same age on smaller scales occur in eastern ,massachusetts1 southern ,england1 eastern ,russia and the ,caucasus region4 ,although the areas occupied by the ,permian glaciers1 which in many cases must have been extensive ice sheets1 cannot be accurately delimited1 it is1 nevertheless1 quite certain that the ice was notably more extensively developed than it was during the great ,ice ,age of late 7,quaternary7 geologic time4 ,another surprising fact is that certain of the glaciers must have come down to1 or nearly to1 sea level1 as shown by the direct association of marine strata with glacial deposits4 ,thus1 in southern ,australia at least eight beds of glacial materials 7some of them #ajj to #bjj feet thick7 occur within true marine strata #b1jjj feet thick4 ,a third remarkable fact is that the ,permian ,ice ,age1 like the ,quaternary ,ice ,age1 had interglacial epochs of relatively mild climate1 as proved by the occurrences of beds of coal between certain of the layers of glacial materials in ,australia1 ,south ,africa1 and ,brazil4 ,during much of ,permian time the climate was arid over large areas as1 for example1 much of the western interior of the ,united ,states1 from ,ireland to central ,germany1 and in eastern ,russia1 as proved by great deposits of salt1 gypsum1 and red sediments4 ,during late ,permian time the greatest salt beds in the world were deposited in northern ,germany1 a well near ,berlin having penetrated a practically solid body of salt associated with certain potash and magnesia salts to a depth of about #d1jjj feet without reaching the bottom4 ,the occurrence of some coal beds1 especially in the earlier ,permian rocks shows that1 temporarily at least1 climatic conditions must have favored luxurious growths of coal-forming plants in ,south ,africa1 ,brazil1 ,australia1 and our own ,appalachian district4 ,from the above facts we see that the ,permian represents a remarkable combination of very extensive glaciation1 widespread aridity1 and warmth and moisture favorable to prolific plant growth all in a single period of geologic time4 ,the ,permian period1 and1 therefore1 the great ,paleozoic era1 was brought to a close by one of the most profound physical disturbances in the known history of ,north ,america4 ,this has been called the ,appalachian ,revolution because at that time the ,appalachian ,mountain range was born out of the sea by folding and upheaval of the strata4 ,in fact1 the ,appalachian ,revolution was one of the most critical periods in the history of the earth1 and may have been the greatest of them all in its results4 7,c4 ,schuchert74 ,mountains were brought forth in all the continents1 including ,australia4 ,all of the mountains which were formed late in the ,paleozoic have since been profoundly affected by erosion1 and the only ones 7e4g41 ,appalachians7 which now show considerable altitudes are those which have been rejuvenated by relatively 7geologically7 recent earth movements4 ,we shall now turn our attention to the origin of the ,appalachian ,range4 ,all through the vast time 7probably fully #bj1jjj1jjj years7 of the ,paleozoic era a large land mass was remarkably persistent along the eastern side of ,north ,america4 ,this land1 which has been called ,appalachia1 had its western boundary approximately along the eastern border of the sites of the ,appalachian ,range and the western part of ,new ,england4 ,it extended east of the present coast line at least to the border of the continental shelf from #ajj to #bjj miles out4 ,concerning the actual altitude and topography of ,appalachia we know little or nothing1 but the tremendous quantities of sediment derived from its erosion show that it was high enough during nearly all of its history to undergo vigorous erosion4 ,barring certain minor oscillations of level1 the region just west of ,appalachia was mostly occupied by sea water throughout much of ,paleozoic time1 and sediments derived from the erosion of ,appalachia were laid down layer upon layer as strata upon that sea bottom4 ,in general1 the coarsest and greatest thickness of sediments accumulated relatively near the land1 while finer materials1 in thinner sheets1 deposited well out over much of the eastern ,mississippi ,valley area in the shallow seas which were there so commonly present4 ,by actual measurement we know that the thickness of strata deposited over the site of the ,appalachians was at least #be1jjj feet4 ,since these latter strata are mostly of comparatively shallow sea-water origin1 as proved by coarseness of grain of material1 ripple marks1 fossil coral reefs1 etc41 we are forced to conclude that this marginal sea bottom gradually sank while the process of sedimentation was in progress4 ,otherwise we cannot possibly explain the great pile of strata of shallow water origin4 ,the very weight of accumulating strata may either have aided or actually caused the sinking of the long1 relatively narrow trough4 ,finally1 toward the close of the ,paleozoic era1 sinking of the marginal sea floor and deposition of sediments gave way to a yielding of the earth's crust by a great force of lateral compression1 causing the strata to be thrown into folds well below the surface and more or less fractured in their upper portion4 ,thus1 along the eastern side of the site of the great interior ,paleozoic sea1 the ,appalachian ,mountains rose out of what for millions of years had been a long1 narrow1 sinking sea floor4 ,there was more or less folding from the ,gulf of ,st4 ,lawrence to central ,alabama4 ,figure #bd diagrammatically represents the principal stages in the history of the ,appalachian ,range4 ,while the most pronounced earth disturbance occurred through the long ,appalachian belt1 the whole eastern side of the continent was profoundly affected4 ,thus the ,mississippi ,valley area east of the ,great ,plains was considerably upraised never again to be submerged except along the ,gulf ,coast1 and an eastern interior sea has never since overspread the region which was repeatedly sea-covered during ,paleozoic time4 ,,chapter ,,xv ,,medieval ,,earth ,,history 7,mesozoic ,era7 ,what was the condition of ,north ,america during the first or ,triassic period of the ,mesozoic era1 approximately #h or #aj million years ago8 ,as a result of the ,appalachian ,revolution the sea was excluded from all the land except along much of the western side from southern ,california to parts of ,alaska4 ,on this western side of the continent the ,appalachian ,revolution had little or no effect and the ,permian conditions continued1 essentially without change through the ,triassic4 ,the ,triassic strata up to #d1jjj feet thick are there of typical marine origin4 ,in ,british ,columbia and ,alaska there was much igneous activity4 ,throughout much of the ,rocky ,mountains and ,great ,plains region of the western ,united ,states there are extensive deposits of red sediments 7so-called ,red ,beds71 containing layers of salt and gypsum1 from #bjj to #a1jjj or more feet thick4 ,these strata commonly rest in regular order on ,permian ,red ,beds1 so that conditions of deposition of ,permian time continued through ,triassic time1 that is continental deposits formed mostly in salt lakes1 fresh lakes1 along stream courses1 and on land in part by the action of wind4 ,illustration3 ,fig4 #dj4--,map showing the general relations of land and water in ,north ,america during the ,triassic period4 ,lined areas represent land2 vertical-lined areas1 basins in which continental deposits formed4 7,based upon map by ,willis2 courtesy of the ,journal of ,geology47 ,in the eastern half of ,north ,america there is no record of accumulation of any marine strata whatever1 because1 as a result of the ,appalachian ,revolution1 the land was brought well above sea level4 ,there was1 however1 deposition of a remarkable series of nonmarine strata in several long1 narrow1 troughlike depressions whose trend was parallel to1 and just east of1 the main axis of the newly formed ,appalachian ,range4 ,these troughs lay between the ,appalachians and the very persistent old land mass called ,appalachia which we have already described4 ,the facts that these troughs are truly down-warps2 that they so perfectly follow the trend of the ,appalachian ,mountain folds2 and that the strata in them are of late ,triassic ,age1 make it certain that they were formed by a great lateral pressure which must have been a continuation of the ,appalachian ,revolution4 ,thus the ,appalachian ,mountains continued to grow well into the ,triassic period1 and1 while the ,paleozoic strata were being folded1 the surface of old ,appalachia 7including part of the ,taconic ,mountain region7 was down-warped to form the troughs in which the late ,triassic strata accumulated4 ,one trough extended through the ,connecticut ,valley2 another 7the largest7 from southeastern ,new ,york through northern ,new ,jersey1 southeastern ,pennsylvania1 ,maryland1 and into ,virginia2 while several smaller ones occurred in ,virginia and ,north ,carolina4 ,the down-warps or troughlike basins were very favorably situated for rapid accumulation of thick sedimentary deposits because of their position just between large1 high land masses which were being vigorously eroded4 ,the sediments derived from the erosion of the young ,appalachians were especially abundant because of the vigorous wearing down of the newly formed high mountains4 ,a thickness of from #e1jjj to fully #ae1jjj feet of mostly red sandstones and shales accumulated in these down-warps1 the character and great thickness of the strata strongly pointing to gradual down-warping as the deposition of the sediments went on4 ,it is often stated that these strata were formed in estuaries1 but1 in the northern areas1 at least from ,massachusetts to ,maryland1 many of the layers show ripple marks1 sun cracks1 rain-drop pits1 fossil plants1 and fossil bones and tracks of land reptiles4 ,such strata may well have formed in very shallow water1 such as river-flood plains or temporary lakes1 where changing conditions frequently allowed the surface layers to lie exposed to the sun4 ,illustration3 ,fig4 #da4--;,block diagram of the region westward from ,new ,york ,city and vicinity1 showing the main relief features1 the underground relations of rocks of widely different ages1 and the relation of the relief to the rock formations4 7,part of larger drawing by ,a4 ,k4 ,lobeck47 ,during the time of the accumulation of the late ,triassic strata in the down-warp basins there was considerable igneous activity1 as proved by the occurrence of sheets of igneous rock within the body of strata4 ,in some cases true lava flows with cindery tops were forced out on the surface and then buried under later sediments1 while in other cases the sheets of molten rock were forced up either between the strata or obliquely through them1 thus proving their intrusive character4 ,as a result of subsequent erosion1 these very resistant lava masses often stand out conspicuously as relief features4 ,perhaps the most noteworthy example is the great layer of such intrusive igneous rock1 part of which outcrops for seventy miles mostly as a bold cliff forming the famous ,palisades of the ,hudson1 near ,new ,york ,city4 ,during the process of cooling and solidification of the molten mass there was contraction which expressed itself by breaking the rock mass into great1 crude1 nearly vertical columns1 and hence the origin of the name ,palisades4 ,the cliff character of the outcrop is due to the fact that the lava is much more resistant to erosion than the sandstone above and below it4 ,in the ,connecticut ,valley of ,massachusetts a layer of lava several hundred feet thick boldly outcrops1 forming the crest of the well-known ,mount ,tom-,mount ,holyoke ,range4 ,the close of the ,triassic period was marked by enough uplift to leave the whole eastern two-thirds of the continent dry land undergoing erosion4 ,the ,triassic deposits of the ,atlantic ,coast are much broken up into large fault blocks1 and this faulting probably took place as a result of the crustal disturbances toward the end of the period4 ,in the west the ,triassic conditions seem to have continued without much change into the next 7or ,jurassic7 period4 ,during the ,jurassic period the relations of land and water in ,north ,america were very simple4 ,in the earlier ,jurassic all was dry land except portions of the western fringe of the continent from southern ,california to ,alaska1 where marine strata #b1jjj to #aj1jjj feet thick accumulated4 ,late in the period the conditions were the same1 except for a long1 narrow arm of the sea or mediterranean which extended from the ,arctic ,ocean southward across the site of the ,rocky ,mountains to ,arizona4 ,there is no evidence for the existence of anything like real mountains anywhere on the continent during the period4 ,illustration3 ,fig4 #db4--,structure section showing profile and underground relations of rocks across the ,connecticut ,valley 7through ,mount ,tom7 of ,massachusetts4 ,js and ,jl are sandstone strata1 with included lava sheets 7in black7 resting upon ,paleozoic rocks on either side4 ,the rocks have been notably tilted and faulted4 7,after ,emerson1 ,u4 ,s4 ,geological ,survey47 ,profound crustal disturbances marked the close of the ,jurassic period in the western part of the continent4 ,strata which had accumulated to great thickness during millions of years of time1 mainly over the sites of the ,sierra ,nevada and ,cascade ,mountains1 finally yielded to a tremendous force of lateral compression1 especially in the ,sierra region1 and were folded1 crumpled1 and upraised4 ,thus the ,sierra-,cascade district was originally built up into a high mountain range4 ,since that time the ,sierras have been much cut down by erosion and they have been rejuvenated by faulting and tilting of the great earth block4 ,the ,cascade ,range from northern ,california into ,british ,columbia was apparently not so profoundly raised1 and its present height is mainly due to subsequent volcanic activity4 ,the rocks of the ,klamath ,mountains of northwestern ,california1 and of the ,humboldt ,range of ,nevada1 were also folded at that time4 ,during the mountain-making disturbances on the western side of the continent great quantities of molten granite were forced up into the lower portions of the folding strata4 ,because of profound subsequent erosion this granite is now widely exposed as1 for example1 in the great walls of the ,yosemite ,valley4 ,during the earlier half of the last period 7the ,cretaceous7 of the ,mesozoic era1 sea water spread from ,mississippi northwestward to the site of ,denver and southward over ,texas and much of ,mexico4 ,at the same time much of the western margin of the continent from ,alaska to ,california was submerged4 ,all the rest of the continent was land4 ,during this time sediments accumulated on low lands just east of the site of the present ,rocky ,mountains1 and also east of the ,appalachians1 as proved by the numerous fossils of land plants found in these deposits4 ,illustration3 ,fig4 #dc4--,map ,showing the general rotations of land and water in ,north ,america during later ,cretaceous time1 several million years ago4 ,lined areas represent land2 vertical lines1 mainly continental deposits4 7,principal data from a map by ,willis1 published in the ,journal of ,geology47 ,as ,cretaceous time went on the marine waters gradually spread until the whole ,atlantic and ,gulf coastal plain regions from ,long ,island1 ,new ,york1 to ,mexico became submerged under marine water1 and a wide arm of the sea1 or great mediterranean1 spread from ,texas north to the mouth of the ,mackenzie ,river4 ,the ,gulf of ,mexico was thus directly connected with the ,arctic ,ocean4 ,this great interior sea was nowhere connected with the ,pacific ,ocean1 though portions of the ,pacific border of the continent were submerged4 ,this vast interior sea was not only the largest of any which reached well into the continent since the ,mississippian period of the ,paleozoic era1 but it was the last body of marine water which ever extended well into the continent4 ,it should be stated that the later ,cretaceous was also a time of unusually widespread submergence of the continents1 when most of southern ,europe and southeastern ,asia1 as well as about one-half of both ,africa and ,south ,america were submerged4 ,over much of the site of the ,rocky ,mountains during the late ,cretaceous there were low lands receiving continental deposits1 and extensive marshes supporting prolific vegetation were common4 ,much of this vegetable matter became buried1 and has since been converted into workable coal4 ,the maximum thickness of strata accumulated during all of ,cretaceous time over the ,atlantic coastal plain area was about #a1gjj feet2 over the ,gulf coastal plain region fully #g1ejj feet2 over the western interior #aj1jjj to #ae1jjj2 and over parts of the ,pacific border #be1jjj to #cj1jjj feet1 as in ,california4 ,the last-named figures are truly phenomenal1 representing a thickness about equal to the total thickness of all the strata accumulated during the whole ,paleozoic era 7seven periods7 and piled up in the ,appalachian ,mountain region4 ,this great deposit of strata of mostly early ,cretaceous ,age is readily accounted for when we realize that these sediments1 which accumulated in the marginal sea bottom1 were derived from the very rapidly eroding1 newly formed lofty ,sierra ,nevada ,range4 ,illustration3 ,fig4 #dd4--,sketch of a mountain range along ,skolat ,creek1 ,alaska1 showing ,tertiary lava beds resting upon deeply eroded tilted limestones and lavas of late ,paleozoic 7,carboniferous7 ,age4 ,the present topography has been produced by erosion since the ,tertiary lavas flowed out4 7,after ,u4 ,s4 ,geological ,survey47 ,especially in ,alabama and ,texas the ,cretaceous system is remarkable for its richness in chalk deposits4 ,in ,alabama a widespread formation of late ,cretaceous ,age1 about #a1jjj feet thick1 contains much nearly pure white chalk1 and in ,texas a similarly constituted formation of early middle ,cretaceous ,age is from #a1jjj to #e1jjj feet thick4 ,these chalk deposits consist almost wholly of carbonate of lime shells or very tiny single-celled animals which accumulated under exceptionally clear sea water which spread over those parts of ,alabama and ,texas where the chalk now occurs4 ,here again we have a bit of evidence supporting the fact of very long geologic time4 ,think of how long it must have taken for the tiny 7even microscopic7 shells to form a widespread layer of chalk nearly a mile thick6 ,the close of the ,cretaceous period1 or what is the same1 the close of the ,mesozoic era1 was marked by some of the grandest crustal disturbances in the known history of the earth4 ,in fact1 it is not known that the western hemisphere was ever affected by more profound and widespread mountain-making disturbances than those which took place toward the close of the ,mesozoic era1 and continued into the succeeding ,tertiary period4 ,these disturbances were of three kinds3 folding of strata1 volcanic activity1 and renewed uplift of old mountains without folding of the rocks4 ,greatest of all was the ,rocky ,mountain ,revolution1 during which the thick strata1 which accumulated during the ,paleozoic and ,mesozoic eras over the site of the ,rockies1 yielded to vigorous deformation when they were more or less folded and dislocated from ,alaska to ,central ,america4 ,this was in truth the birth of the ,rocky ,mountains1 although their existing altitude and configuration have1 to a very considerable degree1 resulted from later uplift and erosion4 ,in the northern ,united ,states and southern ,canada the ,rocky ,mountain strata1 up to over #dj1jjj feet thick1 were most severely folded and fractured1 forming a range which quite certainly was fully #bj1jjj feet high4 ,in this district a great thrust fault1 hundreds of miles long1 developed1 and rocks as old as the ,proterozoic were shoved at least seven miles1 and probably as much as twenty miles1 westward1 over ,cretaceous and other rocks much later than the ,proterozoic4 ,at the same time the ,andes ,mountains throughout ,south ,america were notably upraised and the rocks folded4 ,the second type of physical disturbance was volcanic activity which took place on a tremendous scale1 and which appears to have started as a direct accompaniment of the ,rocky ,mountain ,revolution4 ,this igneous activity took place not only in the ,rocky ,mountains but also westward to and in the ,sierra-,cascade ,range1 as well as in the mountains of western ,british ,columbia and ,alaska4 ,this activity continued well into the succeeding ,cenozoic era1 and it is more fully considered in the next chapter4 ,the third type of crustal disturbance took place on a large scale when the ,appalachian ,mountains1 which had been almost wholly planed away by erosion during ,mesozoic time1 were reelevated from #a1jjj to #c1jjj feet by an uplifting force not accompanied by folding4 ,all or nearly all of ,new ,york and ,new ,england1 as well as much of southeastern ,canada1 were similarly upraised at the same time4 ,this notable uplift of so much of eastern ,north ,america is a matter of great importance because the major relief features of that area have been produced by erosion or dissection of the upraised surface since late ,mesozoic or early ,cenozoic time4 ,in view of the fact that this work of erosion took place almost wholly during the ,cenozoic era1 it will be discussed in the next chapter4 ,in conclusion1 brief mention may be made of the kind of climate of the ,mesozoic era4 ,as shown by the character and distribution of fossil plants and animals1 the ,mesozoic climate was in general mild and rather uniform over the earth1 but with some distinction of climatic zones4 ,such distinction of climatic zones is unknown for the ,paleozoic era1 while it was notably less than at present4 ,,chapter ,,xvi ,,modern ,,earth ,,history 7,cenozoic ,era7 ,since the ,cenozoic era is the last one of geologic time1 it will be of particular interest to trace out the main events which have led up to the present day conditions1 especially in ,north ,america4 ,both because of the recency of the time and the unusual accessibility of the rocks1 which are mostly at or near the surface1 our knowledge of the ,cenozoic era is exceptionally detailed and accurate4 ,it will1 therefore1 be more necessary than ever to select only the very significant features of this history for our brief discussion4 ,during the first half of the ,tertiary period portions only of the ,atlantic coastal plain were submerged under shallow water1 but soon after the middle of the period 7,miocene epoch7 the sea spread over practically the whole ,atlantic coastal plain area from ,martha's ,vineyard south to and including ,florida4 ,during the late ,tertiary the marine waters had become greatly restricted1 and by the close of the period the sea was entirely excluded from the ,atlantic seaboard4 ,the total thickness of these ,tertiary strata is less than #a1jjj feet1 and they all tilt downward gently toward the sea4 ,the strata consist mostly of unconsolidated sands1 gravels1 clays1 marls1 etc4 ,the ,gulf coastal plain area from ,florida through ,texas and south through eastern ,mexico was largely overspread by the sea during most of ,tertiary time1 except the latest4 ,during early ,tertiary time an arm of the ,gulf reached north to the mouth of the ,ohio ,river4 ,late in the period but little of the ,gulf ,plain was submerged1 and at its close sea water was wholly excluded4 ,on the ,gulf ,coast the ,tertiary strata from #b1jjj to #d1jjj feet thick are also mainly sands1 gravels1 clays1 and marls4 ,they are commonly rich in fossils1 and they show a gentle tilt downward toward the ,gulf4 ,throughout ,tertiary time local portions of the ,pacific border of the continent were submerged1 this having been especially true of portions of ,california1 ,oregon1 and ,washington4 ,in spite of the very restricted marine waters1 the ,tertiary strata of the ,pacific ,coast1 especially in ,california1 are remarkably thick1 #aj1jjj to #bj1jjj feet being common1 while the maximum thickness is fully #cj1jjj feet4 ,such great thicknesses are readily explained when we realize that erosion was notably speeded up by pronounced uplifts resulting from crustal disturbances toward the close of the preceding period1 and again in the midst of the ,tertiary period itself4 ,to summarize the ,tertiary relations of sea and land for ,north ,america we may say that only local portions of the continental border ever became submerged1 and that1 by late ,tertiary time1 practically the whole continent was a land area4 ,at the close of the period the continent was1 as we shall see1 even larger than now because the continental shelves of the ocean were then also largely above water4 ,illustration3 ,fig4 #de4--,map showing the general relations of land and water in ,north ,america during part of the middle ,tertiary period4 7,after ,willis1 courtesy of the ,journal of ,geology47 ,the whole of the ,cenozoic era1 including both the ,tertiary and ,quaternary periods1 has been a time of profound crustal disturbances throughout much of the continent1 certain of these movements having continued right up to the present time1 with positive evidence that some of them are still continuing4 ,these great movements have included notable foldings of strata1 uplifts without folding1 faulting1 and igneous activity1 the whole effect having been to greatly increase the general altitude and ruggedness of the continent4 ,in fact1 ,north ,america is not known ever to have been at once higher1 broader1 and more rugged than it was very late in the ,tertiary1 or early in the ,quaternary1 period4 ,since that time the only notable change 7barring the great ,ice ,age and its effects7 has been a restriction of the area of the continent to its present size by spreading of sea waters over the borders of the continent1 that is over the continental shelves4 ,we shall now rather systematically consider the more profound earth changes which have affected the continent1 producing the existing major relief features1 from west to east4 ,the ,coast ,range ,revolution took place in the midst of the ,tertiary period4 ,over the site of the ,coast ,ranges1 strata had accumulated1 especially during ,cretaceous and earlier ,tertiary times1 to a thickness of thousands of feet4 ,in middle ,tertiary time these strata were subjected to a mountain-making force of compression and more or less folded1 faulted 7fractured71 and uplifted into the ,coast ,range ,mountains4 ,some portions of the range were intensely folded and faulted and upraised many thousands of feet1 while other portions were only moderately folded and uplifted4 ,it is an interesting fact that the great ,san ,francisco earthquake rift or fault originated at this time4 ,it was a renewed1 sudden movement of a few feet along this fault which caused the disastrous earthquake of #aijf4 ,still other considerable earth movements took place in the ,coast ,range region during late ,tertiary and ,quaternary times1 as1 for example1 uplift without folding1 as proved by distinct sea-cut terraces at altitudes of more than a thousand feet1 like those north of ,san ,francisco and south of ,los ,angeles4 ,a moderate amount of still later subsidence has caused the development of ,san ,francisco ,bay4 ,the large islands off the coast of southern ,california have in very recent geologic time 7probably ,quaternary7 been cut off from the mainland by sinking of the land4 ,illustration3 ,plate #aa4--7;a7 ,part of the ,mammoth ,hot ,springs ,terrace in the ,yellowstone ,park4 ,the view shows the deposit with boiling water flowing over it4 ,the water enters the earth back on the mountain1 travels underground in contact with hot lava1 rises through limestone1 from which the boiling water takes into solution much carbonate of lime which is deposited when the water reaches the surface4 7,photo by ,jackson1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #aa4--7;b7 ,view ,across ,part of ,crater ,lake1 ,oregon4 ,this great hole1 #c1jjj to #d1jjj feet deep and #f miles in diameter and now partly filled with a lake #b1jjj feet deep1 was formed by a subsidence of the top of a once great cone-shaped volcano fully #ad1jjj feet high above the sea4 ,the high rock in the distance rises #b1jjj feet above the lake which is over #f1jjj feet above sea level4 ,the island is a small volcano of recent origin4 7,photo by ,russell1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #ab4--7;a7 ,detailed ,view of ,part of the ,very ,oldest ,known 7,archeozoic7 ,rock ,formation of the ,earth4 ,the rock is distinctly stratified and represents sands and muds deposited layer upon layer upon a sea floor at least #ej1jjj1jjj years ago4 ,the sands and muds first consolidated into sandstone and shale below the earth's surface4 ,then1 under conditions of heat1 moisture1 and pressure1 they were notably altered1 mainly by crystallization of minerals1 and raised high above sea level4 ,finally the strata were laid bare by erosion4 7,photo by the author47 ,illustration3 ,plate #ab4--7;b7 ,a ,twisted ,mass of ,stratified ,archeozoic ,limestone ,surrounded by ,granite in ,northern ,new ,york4 ,the limestone was enveloped in the granite while it was being forced in molten condition into the earth's crust4 7,photo by the author47 ,the ,sierra ,nevada ,range1 which originated by intense folding of rocks late in the ,jurassic period1 underwent profound erosion until about the middle of the ,tertiary period1 by which time it had been cut down to a range of hills or low mountains4 ,then the great fault 7fracture7 previously described began to develop along the eastern side4 ,as a result of many sudden movements along this fault1 which is hundreds of miles long1 the vast earth block has been tilted westward with a very steep eastern face and a long1 more gradual western slope1 the crest of the fault block forming the summit of the range4 ,the amount of nearly vertical displacement along this fault has been commonly from #aj1jjj to #bj1jjj feet1 and1 in spite of considerable erosion of the top of the fault block and accumulation of sediment at its eastern base1 the modified fault face now usually stands out boldly from #b1jjj to #aj1jjj feet high4 ,as an evidence that this movement of faulting has not yet ceased we may cite the ,inyo earthquake of #ahgb1 when there was a sudden renewal of movement of ten to twenty-five feet along this fault for many miles4 ,since the great ,sierra block began to tilt1 the many mighty canyons1 like ,yosemite1 ,hetch-,hetchy1 ,king's ,river1 and ,feather ,river1 have been carved out by the action of streams1 in some cases aided by former glaciers4 ,king's ,river canyon has been sunk to a maximum depth of #f1ijj feet in solid granite solely by the erosive action of the river6 ,the ,cascade ,mountains1 too1 were reduced to nearly a peneplain condition by late ,tertiary time when they began to be rejuvenated by arching or bowing of the surface unaccompanied by great faulting or fracturing1 and many canyons1 like that of the ,columbia ,river1 have since been carved out4 ,mention should now be made of the vigorous volcanic activity which took place in the ,cascade and ,sierra ,nevada ,ranges4 ,most of this activity occurred during ,tertiary time 7particularly in the latter part7 and it has continued with diminishing force practically to the present time4 ,in ,california streams of lava buried many gold-bearing river gravels which have yielded rich mines4 ,many well-known mountain peaks1 such as ,shasta1 ,lassen1 ,pitt1 ,hood1 and ,rainier1 from northern ,california to ,washington1 are great volcanic cones which date from ,tertiary time1 and which are now mostly inactive4 ,that this volcanic activity has not yet altogether ceased is shown by renewed eruptions of ,mount ,lassen 7or ,lassen ,peak1 altitude #aj1dcg feet7 in northern ,california4 ,since the beginning of this renewed activity in #aiad1 several hundred outbursts have occurred4 ,no molten rock has flowed out1 but large quantities of rock fragments1 dust and steam have been erupted1 in many cases forming great clouds two or three miles high over the top of the mountain 7,plate #aj74 ,at this writing 7,october1 #aibj71 ,mount ,lassen is still showing vigorous activity4 ,at ,cinder ,cone1 only ten miles from ,mount ,lassen1 there were two eruptions of cinders and a considerable outpouring of lava within the last #bjj years4 ,still other very recent cinder cones occur in southeastern ,california and ,arizona4 ,illustration3 ,fig4 #df4--,sketch map showing the distribution of volcanic rocks of ,cenozoic 7mostly ,tertiary7 ,age in western ,north ,america4 ,only one volcano 7,mount ,lassen1 ,california7 is now active in the ,united ,states proper1 but a number are more or less active in ,mexico and ,central ,america4 7,data from ,willis1 ,u4 ,s4 ,geological ,survey47 ,one of the greatest lava fields in the world forms the ,columbian ,plateau between western ,wyoming 7including the ,yellowstone ,national ,park7 and the ,cascade ,mountains from northeastern ,california to northern ,washington4 ,it covers fully #bjj1jjj square miles and is really considerably larger than shown on the map because the lava in parts of the plateau region are covered by very recent sedimentary materials4 ,the great lava fields of the ,deccan1 ,india1 and of the plateau region of western ,mexico are comparable in size to the ,columbian field and these lava fields are all of the same age4 ,in the ,columbian ,plateau most of the lava was poured out during later ,tertiary time4 ,sheets of molten rock1 averaging fifty to one hundred feet in thickness1 spread out over various parts of the region and piled up by overlapping layers one over another until the lava plateau more than a mile high was built up4 ,many hills and low mountains were completely buried under the molten floods1 and in other places the liquid rock masses flowed against the higher mountains4 ,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 coast4444 ,the plateau was long in building4 ,between the layers are found in places old soil beds and forest grounds and the sediments of lakes4444 ,so ancient are the latest floods in the ,columbia ,river ,basin that they have weathered to a residual yellow clay from thirty to sixty feet in depth1 and marvelously rich in the mineral substances on which plants feed4 ,in the ,snake ,river ,valley the latest lavas are much younger 7,quaternary74 ,their surfaces are so fresh and undecayed that here the effusive eruptions may have continued to within the period of human history4 7,w4 ,h4 ,norton47 ,many of the lava layers are plainly visible where the ,columbia ,river has cut its great gorge or canyon4 ,the ,snake ,river in places has sunk its channel several thousand feet into the lava plateau without reaching underlying rock4 ,both north and south of the ,columbian ,plateau there was also much volcanic activity in the ,rocky ,mountain region during ,tertiary time4 ,a single formation in ,colorado consists mostly of volcanic ash or dust over #b1jjj feet thick4 ,there was also much volcanic activity over the ,colorado ,plateau area of southern ,utah1 ,new ,mexico1 and ,arizona4 ,the volcanoes there exhibit all stages from those which are very recent and practically unaffected by erosion to others which have been completely cut away with the exception of the cores or volcanic necks4 ,during the second half of the ,tertiary period the whole region known as the ,great ,basin1 between the ,sierra ,nevada ,mountains of ,california and the ,wasatch ,mountains of ,utah1 began to be affected by profound faulting or fracturing and tilting of portions of the earth's crust4 ,the two largest faults1 one on the western side of the ,wasatch ,range and the other on the eastern side of the ,sierra ,range1 are each hundreds of miles long4 ,each of these ranges owes most of its present altitude to the uptilting of great fault blocks1 and most of the many nearly north-south ,basin ,ranges of ,nevada and ,utah are in reality recently tilted fault blocks4 ,turning now to the ,colorado ,plateau1 studies have shown that region to have been more or less periodically raised fully #bj1jjj feet since the beginning of ,tertiary time1 but because of profound erosion in the meantime its present altitude is only #f1jjj to #i1jjj feet4 ,during late ,tertiary time the land stood at a much lower level than to-day1 so that1 practically during the last period 7,quaternary7 of geologic time1 the region has been elevated to its present position4 ,as a direct result of this profound rejuvenation the ,colorado ,river has had its erosive activity tremendously increased1 and it has carved out the mightiest of all existing canyons--the ,grand ,canyon4 ,the work of deepening and widening the canyon is still proceeding at a rapid geologic rate4 ,as we have learned1 the ,rocky ,mountains and many of its subsidiary ranges were formed by folding and uplift of strata toward the close of the ,mesozoic era 7,cretaceous period74 ,during much of ,tertiary time the newly formed mountains had been considerably reduced by erosion4 ,then1 late in the ,tertiary period1 much of the ,rocky ,mountain region1 as well as much of the ,great ,plains area just east of the mountains1 became rejuvenated by differential uplift without any notable folding of strata4 ,we can tell that this general uplift amounted to at least several thousand feet because definite formations of relatively late ,tertiary strata1 originally horizontally deposited under inland bodies of water1 gradually rise so that at the base of the ,front ,range of the ,rockies they are fully #c1jjj feet higher than they are #bjj miles or more farther east4 ,thus1 the original folding and faulting of the ,rockies1 ,tertiary volcanic activity1 late ,tertiary rejuvenation1 and subsequent erosion account for the present altitude and relief features of the great ,rocky ,mountain system4 ,portions of the rejuvenated ,great ,plains region have been notably dissected by erosion since the late ,tertiary1 this being particularly true of the so-called ,bad ,lands1 especially in parts of ,wyoming and ,south ,dakota1 where mostly relatively soft ,tertiary strata have been cut to pieces4 ,turning our attention now to the eastern half of the continent we find that all1 or nearly all1 of it was more or less raised toward the close of the ,tertiary period4 ,practically the whole ,mississippi ,valley east of the ,great ,plains1 as well as much of the country to the north in ,canada1 was elevated some hundreds of feet and the streams have since the late ,tertiary uplift 7except where the land was ice-covered during the ,ice ,age7 been at work sinking their channels below the newly upraised surface4 ,as already pointed out1 the lowlands of the ,atlantic and ,gulf ,coastal ,plains were mostly submerged under the sea during early middle ,tertiary time4 ,by the close of the period they had emerged practically to their present positions1 and they have been only moderately affected by erosion4 ,we have still to explain the existing topography or relief of a large and important part of eastern ,north ,america1 including the whole of the ,appalachian ,mountains1 ,allegheny ,plateau1 ,piedmont ,plateau1 ,new ,york1 ,new ,england1 and the ,canadian region to the north4 ,as a starting point in this discussion we should recall the fact that1 after the great ,appalachian ,mountain ,revolution toward the close of the ,paleozoic era1 the predominant geologic process which affected the region under consideration was erosion throughout the succeeding ,mesozoic era4 ,by about the close of the ,mesozoic 7,cretaceous period7 the whole region1 with some local exceptions1 has been worn down to a comparatively smooth plain 7peneplain7 not far above sea level4 ,local exceptions were mainly in the ,new ,york and ,new ,england region as1 for example1 some of the higher parts of the ,adirondack and ,white ,mountains1 ,mount ,monadnock in southern ,new ,hampshire1 and ,mount ,greylock in western ,massachusetts4 ,these and other masses rose rather conspicuously above the general level of the great plain of erosion commonly called the ,cretaceous peneplain because it is believed to have been well developed by the close of that period4 ,the uplift of the vast ,cretaceous peneplain about the beginning of the ,cenozoic era 7,tertiary period7 was an event of prime importance in the recent geological history of eastern ,north ,america because it was literally the initial step in bringing about nearly all of the existing major relief features of the ,appalachian-,new ,york-,new ,england-,st4 ,lawrence region4 ,the amount of uplift 7unaccompanied by folding7 of the peneplain was commonly from a few hundred to a few thousand feet with the greatest amount in general along the main trend of the ,appalachians4 ,the fact should be emphasized that nearly all the principal topographic features of the great upraised region have been produced by dissection 7erosion7 of the uplifted peneplain surface4 ,thus nearly all the valleys1 small and large1 including those of the ,st4 ,lawrence1 ,hudson1 ,mohawk1 ,connecticut1 and ,susquehanna1 have been carved out by streams since the uplift of the great peneplain4 ,the streams which flowed upon the old low-lying peneplain surface meandered sluggishly over deep alluvial or flood-plain deposits1 and their courses were little if any determined by the character and structure of the underlying rocks1 because1 with few exceptions1 all rocks were worn down to the general plain level4 ,the uplift of the peneplain1 however1 caused great revival of activity of erosive power by the streams1 the larger ones of which soon cut through the loose superficial alluvial deposits and then into the underlying bedrock4 ,thus the large1 original streams had their courses well determined in the overlying deposits1 and when the underlying rocks were reached the same courses had to be pursued entirely without reference to the underlying rock character and structure4 ,such streams are said to be superimposed because they have1 so to speak1 been let down upon and into the underlying rock masses4 ,as ,professor ,berkey has well said3 ,the larger rivers1 the great master streams1 of the superimposed drainage system1 in some cases were so efficient in the corrosion of their channels that the discovery of discordant structures 7in the underlying rocks7 has not been of sufficient influence to displace them1 or reverse them1 or even to shift them very far from their original direct course to the sea4 ,they cut directly across mountain ridges because they flowed over the plain out of which these ridges have been carved1 and because their own erosive and transporting power have exceeded those of any of their tributaries or neighbors4 ,fine examples of such superimposed streams which are now entirely out of harmony with the structure of regions through which they flow are the ,susquehanna1 ,delaware1 and ,hudson4 ,thus the ,susquehanna cuts across a whole succession of ,appalachian ridges while1 in accordance with the same explanation1 the ,delaware cuts through the ,kittatiny range or ridge at the famous ,delaware ,water ,gap4 ,the ridges are explained as follows3 while the great master streams were cutting deep trenches or channels in hard and soft rock alike1 numerous side streams 7tributaries7 came into existence and naturally mostly developed along belts of weak1 easily eroded rock parallel to geologic 7folded7 structure4 ,thus the ,appalachian valleys have been1 and are being1 formed1 while the ridges represent the more resistant rock formations which have more effectually stood out against erosion4 ,the lower ,hudson ,river flows at a considerable angle across folded formations above the ,highlands1 after which it passes though a deep gorge which it has cut into the hard granite and other rocks of the ,highlands4 ,the simple explanation is that the ,hudson had its course determined upon the surface of the upraised ,cretaceous peneplain1 and that it has been able to keep that course in spite of discordant structure and character of the underlying rocks4 ,in a similar manner we may readily account for the passage of the ,connecticut ,river through a great gap in the ,holyoke ridge or range of hard lava in western ,massachusetts4 ,before leaving this part of our discussion we shall briefly present some evidence showing that the ,new ,york-,new ,england-,st4 ,lawrence region at least must have been considerably higher shortly before the ,ice ,age 7,quaternary period74 ,an old channel of the ,hudson ,river has been traced about #ajj miles eastward beyond the present mouth of the river and it forms a distinct trench under the shallow sea in the continental shelf4 ,even in the ,hudson ,valley1 many miles above ,new ,york ,city1 the bedrock bottom of the river lies hundreds of feet 7near ,west ,point1 #hjj feet7 below sea level4 ,obviously this submerged channel must have been cut when the land in the general vicinity of ,new ,york ,city was fully #a1jjj feet higher than at present4 ,that the land thus stood higher late in the ,tertiary and possibly early in the ,quaternary periods is proved as follows3 7#a7 because most of ,tertiary time must have been needed for the river to erode such a deep valley after the initial uplift of the peneplain about the beginning of the period2 and 7#b7 because glacial deposits of ,quaternary age filled the former channel to a considerable depth4 ,the valleys of the coast of ,maine1 and the submerged lower ,st4 ,lawrence ,valley 7,gulf of ,st4 ,lawrence71 in a similar way lead us to conclude that the region farther north was also notably higher just before the ,ice ,age4 ,in the eastern hemisphere early in the ,tertiary period a great submergence set in and marine waters spread over much of western and southern ,europe1 northern ,africa1 and southern ,asia4 ,the sites of the ,himalayas1 ,alps1 ,pyrenees1 ,apennines and other mountains were then mostly submerged4 ,a very remarkable marine deposit1 made up almost wholly of carbonate of lime shells of a single-celled animal called ,nummulites1 formed on the floor of this vastly expanded early ,tertiary mediterranean4 ,this rock attains a thickness of several thousand feet4 ,it is doubtful if any other single formation made up almost entirely of the shells of but one species is at once so widespread and thick4 ,in the ,alps this remarkable marine deposit may be seen #aj1jjj feet above sea level1 and in ,tibet fully #bj1jjj feet4 ,much of the rock in the ,egyptian pyramids was quarried from this formation4 ,later in the ,tertiary in ,eurasia and ,africa the marine waters gradually became very restricted1 so that by the close of the period the relations of land and sea were not strikingly different from the present1 although northwestern ,europe1 like northeastern ,north ,america1 was notably higher just before the ,ice ,age than it is to-day4 ,eurasia witnessed tremendous crustal disturbances during the middle and later ,tertiary time when1 due to intense folding and uplift of great zones1 the ,himalayas1 ,caucasus1 ,alps1 ,pyrenees1 ,apennines1 and other great ranges were formed4 ,the crustal disturbance was most remarkable in the region of the ,alps1 where the movement resulted in elevating and folding the ,tertiary and older strata into overturned1 recumbent1 and nearly horizontal folds1 and pushing the southern or ,lepontine ,alps about sixty miles 7over a low angle fault fracture7 to the northward into the ,helvetic region4 ,erosion has since carved up these overthrust sheets1 leaving remnants lying on foundations which belong to a more northern portion of the ancient 7early ,tertiary7 sea4 ,most noted of these residuals of overthrust masses is the ,matterhorn1 a mighty mountain without roots1 a stranger in a foreign geologic environment4 7,c4 ,schuchert47 ,the last period of geological time--the ,quaternary--was ushered in by the spreading of vast sheets of ice over much of northern ,north ,america and northern ,europe1 and this ranks among the most interesting and remarkable events of known geological time4 ,on first thought the former existence of such vast ice sheets seems unbelievable1 but the ,ice ,age occurred so short a time ago that the records of the event are perfectly clear and conclusive4 ,the fact of this great ,ice ,age was discovered by ,louis ,agassiz in #ahcg1 and fully announced before the ,british ,scientific ,association in #ahdj4 ,for some years the idea was opposed1 especially by advocates of the so-called iceberg theory4 ,now1 however1 no important event of earth history is more firmly established1 and no student of the subject ever questions the fact of the ,quaternary ,ice ,age4 ,some of the proofs of the former presence of the great ice sheet are as follows3 7#a7 polished and striated rock surfaces which are precisely like those produced by existing glaciers1 and which could not possibly have been produced by any other agency2 7#b7 glacial bowlders or erratics which are often somewhat rounded and scratched1 and which have often been transported many miles from their parent rock ledges 7,plate #bj72 7#c7 true glacial moraines1 especially terminal moraines1 like that which extends the full length of ,long ,island and marks the southernmost limit of the great ice sheet2 and 7#d7 the generally widespread distribution over most of the glaciated area of heterogeneous glacial dbris1 both unstratified and stratified1 which is clearly transported material and typically rests upon the bedrock by sharp contact4 ,the best known existing great ice sheets are those of ,greenland and ,antarctica1 especially the former1 which covers about #ejj1jjj square miles4 ,this glacier is so large and deep that only an occasional high rocky mountain projects above its surface1 and the ice is known to be slowly moving outward in all directions from the interior to the margins of ,greenland4 ,along the margins1 where melting is more rapid1 some land is exposed1 and often the ice flows out into the ocean where it breaks off to form large icebergs4 ,illustration3 ,fig4 #dg4--,map of ,north ,america showing the area buried under ice during the ,great ,ice ,age of the ,quaternary period2 the three great glacial centers2 and the extent of mountain glaciers in the west4 7,after ,u4 ,s4 ,geological ,survey47 ,the accompanying map shows the area of nearly #d1jjj1jjj square miles of ,north ,america covered by ice at the time of maximum glaciation1 and also the three great centers of accumulation and dispersal of the ice4 ,the directions of flow from these centers have been determined by the study of the directions of many thousands of glacial scratches on rock ledges4 ,the ,labradorean 7or ,laurentide7 glacier spread out #a1fjj miles to the south to ,long ,island and near the mouth of the ,ohio ,river4 ,the vast ,keewatin glacier sent a great lobe of ice nearly as far south1 that is into northern ,missouri4 ,one of the most marvelous features of the ice dispersion was the great extension of the ,keewatin sheet from a low flat center westward and southward over what is now a semiarid plain1 rising in the direction in which the ice moved1 while the mountain glaciers on the west 7,cordilleran region71 where now known1 pushed eastward but little beyond the foot-hills4 7,chamberlin and ,salisbury47 ,the ,labradorean and ,keewatin ice sheets everywhere coalesced except in two places4 ,one of these is an area of about #aj1jjj square miles mostly in southwestern ,wisconsin4 ,in spite of several ice invasions during the ,ice ,age1 this area1 hundreds of miles north of the southern limit of the ice sheets1 was never ice-covered4 ,there is a total absence of records of glaciation within this area1 and so we here have an excellent sample of the kind of topography which prevailed over the northern ,mississippi ,valley just before the advent of the ice4 ,a much smaller1 nonglaciated area occurs in northeastern ,missouri near the southern limit of ice extension4 ,the ,cordilleran ice sheet was the smallest of the three1 and it was probably not such a continuous mass of ice1 the higher mountains projecting above its surface4 ,a surprising fact is that neither this ice sheet nor any other overspread northern ,alaska1 which is well within the ,arctic ,circle1 during the ,ice ,age4 ,more than likely the temperature was low enough1 but precipitation of snow was not sufficient to permit the building up of a great glacier4 ,at the same time that nearly #d1jjj1jjj square miles of ,north ,america were ice-covered1 about #fjj1jjj square miles of northern ,europe were buried under ice which spread from the one great center over ,scandinavia southwest1 south1 and southeast over most of the ,british ,isles1 well into ,germany1 and well into ,russia4 ,in both ,north ,america and ,europe the high mountains1 well south of the great glacier limits1 especially the ,sierras1 ,rockies1 ,alps1 ,pyrenees1 and ,caucasus1 supported many large local glaciers in valleys which now contain none at all or only relatively small ones4 ,records of glaciation1 such as glacial scratches1 bowlders1 lakes1 etc41 occur high up in the ,white and ,green ,mountains1 ,adirondacks1 ,catskills1 and the ,berkshire ,hills1 thus proving that the ice must have been at least some thousands of feet thick over ,new ,england and ,new ,york4 ,we have good reason to believe that even the highest summits1 except possibly in the ,catskills1 from #d1jjj to over #f1jjj feet above sea level1 were completely submerged under the ice4 ,on top of a mountain of ,archeozoic granite nearly #d1jjj feet in altitude1 facing the ,st4 ,lawrence ,valley in northern ,new ,york1 the writer has found many fragments of sandstone which were picked off by the ice in the low valley1 moved southward a good many miles1 and uphill several thousand feet to the top of the mountain4 ,the reader may wonder how a great glacier at least a mile thick in northern ,new ,york could have thinned out to disappearance within the short distance to the southern border of the ,state1 but observations on existing large glaciers show that it is quite the habit for them to thin out very rapidly near their margins1 thus producing steep ice fronts4 ,the fact that glacial ice flows as though it were a viscous substance is well known from studies of valley glaciers in the ,alps and ,alaska1 and the great ice sheet of ,greenland4 ,a common assumption1 either that the land at one of the great centers of ice accumulation during the ,ice ,age must have been many thousands of feet higher1 or that the ice must there have been immensely thick1 in order to permit ice flowage so far out from the center1 is not necessary4 ,viscous tar slowly poured upon a level surface will gradually flow out in all directions1 and at no time need the tar at the center of accumulation be very much thicker than elsewhere4 ,the movement of glacial ice from the great centers of dispersal during the ,ice ,age was much the same in principle1 only in the case of the glaciers the accumulations of snow and ice were by no means confined to the immediate centers4 ,the fronts of the vast ice sheets1 like those of ordinary valley glaciers1 must have undergone many advances and retreats of greater or less consequence4 ,in the northern ,mississippi ,valley1 and also in ,europe1 there is positive proof for five or six important advances and retreats of the ice which gave rise to the true interglacial stages4 ,the strongest evidence is the presence of successive layers of glacial 7morainic7 dbris piled one upon another1 a given layer often having been oxidized1 eroded1 and even covered with plant life before the next or overlying layer was deposited4 ,such is the condition of things throughout much of ,iowa1 where wells sunk into the glacial deposits commonly pass through layers of partly decomposed vegetable matter at depths of from #ajj to #cjj feet4 ,near ,toronto1 ,canada1 the finding of warm climate plants between two glacial deposits proves that the climate there during an interglacial stage was much like that of the southern ,states to-day4 ,during the great interglacial stages the vast glaciers were notably restricted in size1 and in some or possibly all1 cases they may have wholly disappeared from the continent4 ,in former years there was a tendency to ascribe mighty erosive power to the vast slow-moving ice sheets1 but to-day scarcely any geologist would hold that the ice really produced large valleys solely by ice erosion1 or that mountains were notably cut down4 ,throughout the glaciated region1 especially toward the north1 the deep preglacial residual soils and rotten rocks were nearly all scoured off by the passage of the ice4 ,that the ice1 where properly shod with rock fragments1 actually eroded to at least little depths into hard and fresh rocks is well known1 but the evidence is clear and conclusive that the preglacial hills and mountains1 and most of the valleys 7including all the large ones71 were rarely more than a little modified in shape and size4 ,one of the principal effects of the ,ice ,age is the widespread distribution of glacial deposits1 and other deposits which were formed under water in direct association with the ice4 ,such materials have been described in the chapter on ,glaciers and ,their ,work4 ,as a direct result of the ,ice ,age1 many thousands of lakes came into existence throughout the glaciated region where few1 if any1 previously existed4 ,many of these lasted only while the ice was present because their waters were held up by walls of ice acting as dams4 ,thousands of others still persist1 most of these having their water levels maintained by dams of glacial dbris left by the ice across valleys4 ,good examples of lakes of both types1 including a summary of the remarkable history of the ,great ,lakes1 are considered in the chapter on ,a ,study of ,lakes4 ,many drainage changes1 gorges1 and waterfalls have also directly resulted from the great ,ice ,age4 ,in fact it is not too much to say that practically all true gorges and waterfalls of the glaciated region have originated as a direct result of the ,ice ,age4 ,the most remarkable combination of waterfall and gorge thus produced is that of the world-famous ,niagara1 described in the chapter on ,stream ,work4 ,not only are ,niagara ,falls and gorge of postglacial origin but there was no ,niagara ,river as such before the ,ice ,age4 ,in ,new ,york the well-known ,ausable ,chasm1 ,trenton ,falls ,gorge1 and ,watkins ,glen are all excellent examples of gorges cut since the ,ice ,age by streams which1 because their old valleys were filled with glacial dbris1 have been forced to take new courses4 ,a gorge of very special interest is that at ,little ,falls in central ,new ,york4 ,this gorge1 two miles long1 with its precipitous walls hundreds of feet high1 is the most important gateway for traffic between the ,atlantic border and the ,great ,lakes region4 ,the bottom of this defile contains six tracks of the ,new ,york ,central and ,west ,shore ,railroads1 the ,barge ,canal1 an important highway1 and the ,mohawk ,river4 ,before the ,ice ,age there was a stream divide instead of a gorge1 several hundred feet above the present river level4 ,during a late stage of the ,ice ,age1 when the ,great ,lakes drained through the ,mohawk ,valley1 a tremendous volume of water passed over the divide and cut it down to form nearly all of the gorge except the inner or bottom trench which has since been eroded by the ,mohawk ,river4 ,illustration3 ,fig4 #dh4--,sketch map of the region between ,lake ,george and ,schenectady1 ,new ,york1 showing how certain of the main drainage courses have been revolutionized by the great retreating ice sheet and the deposits it left4 ,preglacial courses shown by dotted lines only where essentially different from the present streams4 7,by the author1 as published by ,new ,york ,state ,museum47 ,only a few of the numerous stream changes directly due to the ,ice ,age will be briefly referred to4 ,certain of the principles involved are exceptionally well illustrated in the general vicinity of ,saratoga ,springs and ,lake ,george1 ,new ,york4 ,during the retreat of the great glacier a lobe of ice occupied the ,lake ,george ,valley and forced the ,hudson ,river west over a divide at ,stony ,creek4 ,then1 because of heavy glacial deposits near ,corinth1 the ,hudson could not continue south through what had been the preglacial valley of ,luzerne ,river1 but it was forced eastward over a divide in a low mountain ridge to ,glens ,falls4 ,the remarkable shift of the ,sacandaga ,river from its preglacial channel was caused by the building up of a great morainic ridge across the valley in the vicinity of ,broadalbin4 ,the drainage of the basin of the upper ,ohio ,river has also been revolutionized as a result of the glaciation4 ,all the drainage of western ,pennsylvania passed northward into ,lake ,erie just before the ,ice ,age instead of southwestward through the ,ohio ,river as at present4 ,rivers as large as the ,mississippi and the ,missouri were also more or less locally deflected from their preglacial courses4 ,thus the ,missouri1 which in preglacial time followed the ,james ,river ,valley of eastern ,south ,dakota1 was forced1 by a great lobe of retreating ice1 to find its present course many miles farther west4 ,how long ago did the ,ice ,age end8 ,in seeking an answer to this question we should bear in mind not only the fact that the ,ice ,age ended at different times1 according to latitude1 the more southern districts having been first freed from ice1 but also the fact that approximately #d1jjj1jjj square miles of the polar regions are now ice-covered1 so that in a real sense those portions of the earth are still in an ,ice ,age4 ,some of the best estimates of the length of postglacial time for a given place are based upon the rate of recession of ,niagara ,falls1 the average of the estimates being about #be1jjj years4 ,the evidence for this conclusion is briefly set forth in ,chapter ,,iii4 ,a careful study of the rate of recession of ,st4 ,anthony ,falls1 ,minnesota1 has led to the conclusion that the last retreat of the ice occurred there from #aj1jjj to #af1jjj years ago4 ,certain clays deposited under tidewater since the last withdrawal of ice in ,sweden show a remarkable succession of alternating layers thought to represent seasonal changes4 ,by counting the layers it has been estimated that ,stockholm was freed from ice only #i1jjj years ago4 ,although the actual duration of the ,ice ,age is by no means accurately known1 we can be quite sure that the time represented is far longer than that of postglacial time4 ,that it must have lasted fully #ejj1jjj years seems certain when due consideration is given to amount of time necessary to bring about the repeated changes of climate between the glacial and interglacial stages2 the amount of plant accumulation during the interglacial stages2 the amount of weathering and erosion of the various layers of glacial deposits4 ,some estimates run as high as #a1ejj1jjj years for the duration of the ,ice ,age1 and an average is about #a1jjj1jjj years1 which probably indicates1 at least roughly1 the order of magnitude of the time involved4 ,when it is considered not only that the fact of the great ,ice ,age was not even thought of until #ahcg1 but also that many factors enter into the general problem of the climate of geologic time1 it is not surprising that the cause 7or causes7 of the glacial climate is still not definitely known4 ,a few of the various hypotheses which have been advocated to account for the glacial climate will now be very briefly referred to4 ,one is that the increased cold 7not more than #aj to #ae degrees for the yearly average7 was brought about by the notably increased altitudes of late ,tertiary and early ,quaternary times in northern ,north ,america and ,europe4 ,in this connection it is interesting to note that the four times of real glaciation during geologic time 7mid-,proterozoic1 early ,paleozoic1 late ,paleozoic1 and early ,cenozoic7 did occur directly after great crustal disturbances and notable uplifts of land4 ,according to this hypothesis the interglacial stages would have to be explained by a rather unreasonable assumption of repeated rising and sinking of the glaciated lands4 ,another hypothesis1 long held in favor1 is based upon certain astronomical considerations4 ,thus we now have winter in the northern hemisphere when the earth is nearest the sun1 but in about #aj1ejj years1 due to wobbling of the earth on its axis1 our winter will occur when the earth is farthest from the sun1 thus making the winters longer and colder1 and the summers shorter and hotter4 ,after a much longer period of time the earth will be millions of miles farther from the sun in winter than in summer and this would still further accentuate the length and coldness of the winters4 ,the interglacial stages represent the #aj1ejj year periods when the earth in winter 7northern hemisphere7 is nearest the sun4 ,a difficulty in the way of accepting this hypothesis is that it is inconceivable that each glacial and interglacial stage lasted only #aj1ejj years4 ,another objection to the hypothesis as an explanation of ,ice ,ages is that it is directly opposed by the fact of widespread glaciation at low latitudes either side of the equator during the late ,paleozoic ,ice ,age4 ,another hypothesis is based upon variations in quantity of carbonic acid gas and water vapor in the air4 ,increase or decrease of these constituents causes increase or decrease of temperature because they have high capacities for absorbing heat4 ,the great elevation of the land at the close of the ,tertiary seems to afford conditions favorable both for the consumption of carbon dioxide in large quantities 7by weathering of rocks7 and for the reduction of the water content of the air4 ,depletion of these heat-absorbing elements was equivalent to the thinning of the thermal blanket which they constitute4 ,if it was thinned1 the temperature was reduced4444 ,by variations in the consumption of carbon dioxide1 especially in its absorption and escape from the ocean1 the hypothesis attempts to explain the periodicity of glaciation 7i4e41 glacial and interglacial stages74 7,chamberlin and ,salisbury47 ,still another suggested explanation is based upon variability of amount of heat radiated by the sun4 ,slight variations are now known to take place1 and possibly in the past during certain periods of time these variations may have been sufficiently great to cause a glacial climate with interglacial stages4 ,here1 as in the case of so many other great natural phenomena1 a single1 simple explanation does not seem sufficient to account for all the features of the several well-known glacial epochs of geologic time4 ,two or more hypotheses1 or parts of hypotheses1 must more than likely be combined to explain a particular ,ice ,age4 ,,chapter ,,xvii ,,evolution ,,of ,,plants ,have we any knowledge regarding the beginning of life on our planet8 ,our answer to this question must be decidedly in the negative4 ,we can1 however1 be very positive in regard to two important matters concerning life in early geological time1 namely1 that plants must have existed before animals1 and that the very oldest known 7,archeozoic7 rocks of the earth contain vestiges of organisms4 ,we may be sure that plants preceded animals because animal life ultimately depends upon plants for its food supply or1 in other words1 all animals could never have been carnivorous4 ,now1 if we can prove that organisms existed during ,archeozoic time1 it is evident that plants at least must have lived in that oldest known era of earth history4 ,that living things did then exist is proved by the common occurrence of graphite1 a crystallized form of carbon1 in the oldest known of the ,archeozoic rocks4 ,the facts that flakes of graphite are abundantly scattered through many layers of strata of ,archeozoic ,age1 and that adjacent layers of strata contain such varying amounts of graphite1 render it practically certain that such graphite represents the carbon of organisms4 ,graphite existing under such conditions could not be of igneous origin4 ,carbonaceous or bituminous strata1 so called because they contain more or less decomposed organic matter1 would1 when crystallized under conditions of metamorphism1 yield graphite-bearing rocks exactly like those of ,archeozoic ,age1 and there is every reason to believe that this was their origin4 ,but1 since only graphite 7carbon7 of the ,archeozoic organisms remains1 the rest having disappeared through chemical change or decomposition1 it is impossible to say whether much or all of it represents original plants or animals4 ,in any case we can be very sure about the existence of plants 7probably very simple or primitive types7 in ,archeozoic time1 but the presence of any form of animal life has not been proved4 ,in the next1 or ,proterozoic era1 some plants and animals of definite types are known to have existed and1 from here on in the present chapter1 it is our purpose to consider the salient points in the geological history of plants1 taking up the main types in the regular order of their appearance from the remote ,proterozoic days to the present4 ,the very oldest known definitely determinable fossils of any kind are the more or less rounded masses of crudely concentric layers of carbonate of lime from one to fifteen inches in diameter found in middle ,proterozoic limestone of western ,ontario1 ,canada4 ,similar forms are abundant in late ,proterozoic strata of ,montana4 ,they occur in large numbers as layers or reefs1 in many cases repeating themselves through hundreds or even thousands of feet of strata4 ,careful studies have shown that these forms are the limey secretions of some of the very simplest types of plants1 that is thallophytes 7e4g41 seaweeds71 which lived in water4 ,before proceeding to describe the plants of ,paleozoic and later time1 the reader should be impressed with the important fact that plants of higher and higher types came into existence throughout geological time in almost exactly the botanical order of their classification1 that is to say1 from the very simplest types 7thallophytes7 of ,proterozoic time there were gradually evolved1 through the long geological ages1 higher and higher plant forms reaching a climax in the complex and highly organized plants of the present time4 ,this is the most significant general fact in regard to the geological history of plants4 ,for the convenience of the reader the largest subdivisions in the classification of plants are here given4 ,,outline ,,classification ,,of ,,plants ,i4 ,cryptogams #a4 ,thallophytes 7e4g41 seaweeds1 mushrooms7 7seedless and #b4 ,bryophytes 7e4g41 mosses7 flowerless7 #c4 ,pteridophytes 7e4g41 club mosses1 horsetails1 ferns7 ,,ii4 ,pteridosperms 7seed-bearing1 7e4g41 seed ferns--wholly extinct7 flowerless7 ,,iii4 ,phanerogams #a4 ,gymnosperms 7e4g41 cycads1 conifers7 7seed-bearing1 #b4 ,angiosperms 7e4g41 grasses1 lilies1 flowering7 oaks1 roses7 ,throughout the first two periods--,cambrian and ,ordovician--of the ,paleozoic era1 plant life appears to have made little or no progress toward higher forms4 ,the very simple ,thallophytes 7e4g41 seaweeds7 continued to secrete concentric layers of carbonate of lime in almost exactly the same way as during the middle and late ,proterozoic era4 ,remarkable reefs of such forms occur in the late ,cambrian limestone near ,saratoga ,springs1 ,new ,york1 where one locality has been set aside as a state park4 ,during the ,ordovician there were seaweeds of the more familiar branching types without carbonate of lime supports1 and these have left very perfect impressions in some of the ,ordovician strata4 ,during the ,silurian period seaweeds continued1 as1 in fact1 they did throughout succeeding geologic time to the present4 ,the ,silurian strata seem to contain some vestiges of the first-known land plants1 though the records are meager and some of the specimens are of a doubtful character4 ,most interesting of all is a fern or fernlike plant found in ,france4 ,when we consider the profusion of land plants 7all of relatively simple types7 of the next or ,devonian period1 it seems certain that their progenitors must have existed in the ,silurian1 and their remains may very likely be discovered4 ,beginning with the ,devonian period of the ,paleozoic era the records show that important advances had taken place in the evolution of the plant kingdom4 ,among the very simple ,thallophyte plants some seaweeds of unusually large size occur in fossil form1 but the important fact is that all the principal subdivisions of the typical higher non-flowering plants 7,pteridophytes7 as well as ,pteridosperms1 and even some primitive representatives of the lower order flowering plants 7gymnosperms7 were well represented in the ,devonian4 ,our knowledge of land plants earlier than the ,devonian amounts to almost nothing and they certainly could not have been at all prominent1 but the fossil records make it very clear that many ,devonian land areas were clad with rich and diversified plant life4 ,there were even forests1 probably the first on earth1 but they were far different1 both in general and in particular1 from those of to-day because the trees were all of exceptionally low organization types4 ,during the next two periods--,mississippian and ,pennsylvanian--there was no really important progress in the evolution of plants1 and since these remarkable types of land plants have left such wonderfully preserved records in strata of the ,pennsylvanian or great ,coal ,age1 we shall proceed to descriptions of the main types of that time1 especially those which contributed to the formation of beds of coal4 ,as shown by the abundant records1 the land plant life of ,pennsylvanian time must have been not only prolific but exceedingly varied4 ,thousands of species have been unearthed from the coal-bearing formations alone1 and these must represent only a fraction of all species of plants which lived during the period4 ,most prominent of all were the giant ,lycopods constituting the lowest main subdivision of the ,pteridophytes 7see above classification74 ,these great1 non-flowering plants were at once the biggest1 most common and conspicuous trees of the extensive swamp forests1 and they were the greatest contributors to the formation of coal 7,plate #ae74 ,many species have been described4 ,they commonly attained heights of #ej to #ajj feet and diameters of #b to #f feet4 ,in one important type the fairly numerous branches bristled with stiff1 needle-shaped leaves4 ,when the leaves dropped off the older or trunk portions1 scars were left spirally arranged around the trunks of the trees4 ,in another important type the leaf scars were vertically arranged on the lower portions of the tree trunks4 ,the upper portions of the trunks 7rarely branched7 were thickly set with long1 slender leaves1 which in some species were two or three feet long4 ,an interesting fact is that the inner parts of the trunks of the great ,lycopods were filled with soft1 pithy material4 ,this explains why the fossil trees are nearly always flattened out1 as a result of burial within the earth4 ,the nonbranching type of ,lycopod has been totally extinct for millions of years1 while the branching type is to-day represented only by small1 mostly delicate1 trailing plants familiarly known as club mosses and ground pines4 ,the most conspicuous trees of the great ,pennsylvanian lowlands and swamps have1 indeed1 left meager modern representatives1 and here we have an excellent illustration of a once prominent group of plants which has dwindled away almost to extinction4 ,another common type of ,pennsylvanian vegetation was the so-called horsetail plant or giant rush4 ,the much smaller scouring rush1 represented by several species to-day1 is the direct descendant of this type which1 during later ,paleozoic time1 grew to be #ej to #ij feet high and #a to #b feet in diameter4 ,the long1 slender trunks filled with pith were segmented with variously shaped leaves arranged in whorls around the joints4 ,a fine1 vertical-fluted structure without leaf scars characterized the surfaces of the trunk4 ,recent study has shown that many of the ,pennsylvanian plants1 long classed as true ferns1 were really seed ferns1 as described below4 ,many of the true ferns grew to be real trees up to fifty or sixty feet high1 but all ,paleozoic types were primitive in structure as compared with modern ferns4 ,very remarkable among the later ,paleozoic plants were the ,pteridosperms1 represented by the so-called seed ferns4 ,these now wholly extinct plants seem to have formed the connecting link between the seedless1 flowerless plants 7,cryptogams7 and the seed-bearing1 flowering plants 7,phanerogams71 because they bore seeds but not flowers4 ,many of them were small and herbaceous1 but others were tall trees1 in general appearance resembling the tree ferns4 ,seed ferns1 which play such an important part in the evolution of plants1 are not known to have existed after ,paleozoic time4 ,during the latter half of the ,paleozoic era some very primitive types of flowering plants 7,gymnosperms7 existed4 ,most abundant of these were the so-called ,cordaites1 which were the tallest trees of the time1 some having reached heights of over #ajj feet4 ,the upper portions only bore numerous branches supplied with many simple1 parallel-veined1 strap-shaped leaves up to six feet long and six inches wide4 ,excepting the pithy cores the trunks of these trees were of real wood covered with thick bark4 ,trees of this kind became extinct in the early ,mesozoic era4 ,very late in the ,paleozoic 7,permian period7 two other types of the simple flowering plants 7,gymnosperms7 made their appearance4 ,these were the cycads and conifers1 which were the most conspicuous trees during the first two periods of the ,mesozoic era4 ,the cycads reached their culmination in the ,jurassic period1 but they still exist in modified form in some parts of the world4 ,the short1 stout trunk was crowned with long1 stiff1 palm-like leaves4 ,in fact1 the cycads are distantly related to the palms1 which belong to a higher group of plants4 ,some specimens of cycads1 especially from the ,mesozoic strata of ,south ,dakota1 are so wonderfully fossilized that even the detailed structures of trunks1 leaves1 flowers1 and seeds are so perfectly preserved that almost as much is known about these plants of millions of years ago as though they were living forms4 ,the conifers1 with which are classed present-day pines1 spruces1 and many other evergreen trees1 gradually took on a more modern aspect1 so that late in the ,mesozoic era they were much like those now living4 ,among the most interesting trees were the sequoias1 to which the living big trees and red-woods of ,california belong4 ,these began in relatively late ,mesozoic time1 reached their climax in numbers1 variety of species1 and widespread distribution in the early ,cenozoic era2 and are now almost extinct1 being represented by only two species in local portions of ,california4 ,cordaites1 trees which were so large and abundant in later ,paleozoic time1 were reduced to extinction in the early ,mesozoic era4 ,during ,mesozoic time the ,thallophytes1 represented by seaweeds1 were common4 ,among the ,pteridophytes the ferns and horsetail plants were fairly common1 but the very large forms gradually gave way to much smaller ones during ,mesozoic time4 ,the giant ,lycopods of later ,paleozoic time dwindled almost to extinction even in early ,mesozoic time1 so that from that time to the present they have been very small and relatively insignificant4 ,tens of millions of years of earth history had passed before the true flowering plants--the ,angiosperms--appeared upon the earth4 ,the ,cretaceous period marks their advent4 ,so far as known1 these plants originated along the eastern side of ,north ,america1 and very soon after their establishment they spread over the earth with amazing rapidity and dominated the vegetation as they do to-day1 more than half of the existing species of plants being ,angiosperms4 ,among the common types which have been unearthed from ,cretaceous strata are palms1 grasses1 maples1 oaks1 elms1 figs1 magnolias1 willows1 beeches1 chestnuts1 and poplars4 ,illustration3 ,plate #ac4--;,a ,slab of ,very ,early ,paleozoic 7,cambrian7 ,rock1 ,covered with ,some of the ,oldest ,known ,definitely ,determinable ,animal ,remains4 ,these creatures lived in a sea which overspread the site of the ,rocky ,mountains of southern ,canada fully #be1jjj1jjj years ago4 ,most of the fossils are trilobites 7including some very small ones7 and other related crustacean forms 7lighter portions74 7,after ,c4 ,d4 ,wolcott1 ,smithsonian ,institution1 ,washington1 ,d4 ,c47 ,illustration3 ,plate #ad4--7;a7 ,photographs of ,small ,slabs of ,ordovician ,strata ,full of ,fossils4 ,these slabs are actual bits of sea bottom at least #bj1jjj1jjj years old4 ,the left picture shows stone-lily stems1 so-called sea mosses1 brachiopods4 ,right picture shows various species of brachiopods4 7,photo by the author47 ,illustration3 ,plate #ad4--7;b7 ,an ,outcrop of ,middle ,ordovician ,stratified ,limestone in ,northern ,new ,york4 ,this ledge is full of fossils similar to those above4 ,the material was deposited on the floor of the ,ordovician sea which overspread much of the continent4 7,photo by the author47 ,the introduction of the higher flowering plants 7,angiosperms7 was1 perhaps1 the most important and far-reaching event in the whole history of vegetation1 not only because they almost immediately became dominant1 but also because of their influence upon the animal life of the succeeding periods4 ,hardly had flowers appeared1 before a great horde of insects1 which fed upon their honey or pollen1 seem to have sprung into existence4 ,the nutritious grasses and the various nuts1 seeds1 and fruits afforded a better food for noncarnivores than ever before in the history of the world4 ,it was to be expected1 therefore1 that some new type of animal life would be developed to take advantage of this superior food supply4 ,as we shall see in the discussion of the ,tertiary 7next chapter71 the mammals1 which kept a subordinate position throughout the ,mesozoic1 rapidly took on bulk and variety and acquired possession of the earth as soon as they became adapted to this new food1 quickly supplanting the great reptiles of the ,mesozoic4 7,cleland47 ,during the present or ,cenozoic era vegetation gradually took on a more and more modern aspect until the existing species were developed4 ,the grasses especially developed and spread rapidly1 but the cereals did not evolve until late in the era4 ,certain single-celled plants1 called diatoms1 may be especially mentioned1 for they must have literally swarmed in some of the ,tertiary seas which spread over parts of the present lands4 ,the microscopic plants which form siliceous shells1 called diatoms1 make extensive deposits in some places4 ,one stratum near ,richmond1 ,virginia1 is thirty feet thick and is many miles in extent2 another1 near ,monterey1 ,california1 is fifty feet thick1 and the material is as white and fine as chalk1 which it resembles in appearance2 another1 near ,bilin1 in ,bohemia1 is fourteen feet thick4444 ,ehrenberg has calculated that a cubic inch of the fine1 earthy rock contains about forty-one thousand millions of organisms4 ,such accumulations of diatoms are made both in fresh waters and salt1 and in those of the ocean at all depths4 7,j4 ,d4 ,dana47 ,,chapter ,,xviii ,,geological ,,history ,,of ,,animals 7,,excluding ,,vertebrates7 ,a study of the animals of the past is not only of great interest in itself1 but also it furnishes a mainstay of the great doctrine of organic evolution4 ,at the very outset of our discussion the reader should have already in mind at least the main subdivisions of the animal kingdom in order to reasonably well understand where the important animal types of the different geological ages fit in1 and how those types bear upon the doctrine of evolution4 ,the accompanying1 very brief1 general classification includes the usually recognized subkingdoms with special reference to representatives of those which are of most geological and evolutionary significance4 ,reading downward in this classification1 the degree of complexity of organization steadily increases from single-celled animals to man himself4 ,i4 ,protozoans1 e4g4 foraminifers 7with lime carbonate shells7 ,sponges ,,ii4 ,coelenterates1 e4g4 ,so-called jellyfishes1 graptolites ,corals ,so-called sea lilies ,,iii4 ,echinoderms1 e4g4 ,so-called starfishes ,so-called sea urchins ,,iv4 ,worms1 ,v4 ,molluscoids1 e4g4 ,so-called sea mosses ,brachiopods ,clams1 oysters ,snails ,pearly nautilus1 ,,vi4 ,mollusks1 e4g4 ammonites ,cephalopods1 e4g4 ,so-called cuttle fishes ,trilobites ,,vii4 ,arthropods1 e4g4 ,crabs1 lobsters ,so-called sea scorpions ,insects ,ostracoderms ,fishes ,,viii4 ,vertebrates1 e4g4 ,amphibians ,reptiles ,birds ,mammals 7including man7 ,before entering into a brief but rather systematic discussion of some of the most important types of animals which lived during geological time1 it may be well for the reader to have in mind some of the most important conclusions which have been reached as a result of the study of the fossil animal records4 ,these conclusions may be summarized as follows3 #a4 ,animal life existed many millions of years ago4 #b4 ,not only the animals of to-day1 but also those of any given geological period1 directly descended from those of preceding geological periods4 #c4 ,animal life has undergone continuous change since its introduction upon the earth1 so that each group of strata1 representing a particular geological age1 contains a characteristic assemblage of fossil animals4 #d4 ,many of the changes in the history of animals have been progressive or evolutionary1 so that strata of early geological time contain distinctly more primitive or lower order forms than the strata of late geological time4 ,but1 while the line of evolution has been maintained without a break1 culminating in man1 there have been many offshoots of a retrogressive nature4 #e4 ,even as far along in geological time as the early ,paleozoic era1 the highest subkingdom--vertebrates--had no representative whatever4 ,in other words1 all the important subdivisions of animal life from a little below fishes to man have been evolved since about the close of the ,ordovician period4 #f4 ,any species of animal which ever became extinct has never been known to reappear1 and literally tens of thousands of species are known to have become extinct4 #g4 ,no species like those now living are found in the more ancient strata1 such being confined to the strata of relatively recent geological dates4 #h4 ,while more and more highly organized animals have continuously been evolved1 many of the earlier and simpler types have persisted1 a remarkable case in point being the single-celled animals called foraminifers which may be traced1 without very notable change1 through the tens of millions of years of geological time from the late ,proterozoic era to the present day4 #i4 ,many species have been able to maintain themselves practically without change through long stretches of geological time1 while others have had only very brief existence4 ,when did animal life begin on the earth1 and what were the first forms like8 ,we can only partially answer the first question by saying that animals have existed for tens of millions of years1 certainly as early at least as ,proterozoic time4 ,up to the present time we are utterly in the dark as to what the earliest animal forms looked like1 but we have positive knowledge that the oldest forms found as fossils in the rocks represent creatures which were far more primitive and lower in organization than many animals of to-day1 and that since those oldest known forms lived1 the animal kingdom has undergone various profound alterations4 ,in view of the above statements1 and also the fact that the oldest known plant forms were extremely simple or single-celled1 it is more than likely that the first animal life of the earth was single-celled4 ,in harmony with this view is the fact that fossil single-celled animals are found in the very oldest 7,proterozoic7 rocks which contain any definitely determinable fossil animals4 ,do the most ancient known rocks show that animal life existed during ,archeozoic time8 ,in the preceding chapter we pointed out the fact that the carbon 7in the form of graphite71 so commonly present in those most ancient known strata1 proves the existence of life of some kind during ,archeozoic time4 ,but because nothing like definitely determinable fossil forms have thus far been discovered in those rocks1 we cannot be sure whether the carbon represents plant or animal life or both1 though certainly plants of very low order at least must have existed4 ,because of the intense alteration 7metamorphism7 of those very old strata1 all definite forms have long since been obliterated as such4 ,we may1 however1 in the light of the vast evolution which took place through succeeding geological time1 be very sure that any animals which may have existed during ,archeozoic time were in general much simpler forms than those of even early ,paleozoic time4 ,the early and middle ,proterozoic strata throw no more light upon the early history of animal life than do the ,archeozoic strata4 ,the upper or later ,proterozoic rocks1 however1 contain the oldest recognizable animal fossils4 ,very recently fossil remains of single-celled1 shell-bearing ,protozoans have been found in northern ,france1 while the upper ,proterozoic strata of the ,rocky ,mountains in ,montana1 and the ,grand ,canyon of ,arizona have yielded worm tracks1 a ,molluscoid 7brachiopod7 and fragments of lower forms of ,arthropods4 ,this record1 although very meager1 clearly proves that animal life was so well advanced by late ,proterozoic time1 that next to the highest subkingdom was actually represented 7see above classification71 and that there must have been a long line of simpler and simpler ancestors1 probably extending far back into the ,archeozoic era4 ,when we stop to consider that ,archeozoic and ,proterozoic time was fully as long as all succeeding geological time1 it is not so surprising that fairly highly developed animals 7except ,vertebrates7 had been evolved before the close of the ,proterozoic era4 ,in regard to abundance of fossil animals the oldest 7,cambrian7 ,paleozoic strata stand out in marked contrast to the ,proterozoic4 ,many hundreds of species of animal fossils have been described from ,cambrian strata1 and a great many others yet remain to be discovered4 ,cambrian fossils are remarkably numerous1 varied in species1 and complex in organization 7,plate #ac74 ,all subkingdoms of animals except the ,vertebrates were represented1 though usually only by the simpler types in each subkingdom4 ,it is quite generally agreed that no less than #ej per cent of animal evolution had taken place before the beginning of the ,cambrian period4 ,the reader should1 however1 clearly bear in mind that tremendous advances in evolution have taken place since early ,cambrian time when not only all forms from lower scale ,arthropods to the highest mammals 7including man7 have evolved1 but also when many thousands of species of lower subkingdom animals developed4 ,why are the very early ,paleozoic strata so rich in fossils1 while the immediately preceding ,proterozoic rocks show so few8 ,the seemingly sudden appearance of so many highly developed animals in earliest ,paleozoic 7,cambrian7 time is one of the most important considerations in the history of animal life1 and it is by no means definitely understood4 ,the following statements bear directly upon the problem3 ,the early animal forms were probably soft or gelatinous without shells and lived mostly in the open sea where food 7seaweeds1 etc47 was abundant4 ,such animals were very unfavorable for preservation in fossil form4 ,then1 late in ,proterozoic time or very early in the ,paleozoic1 a severe struggle for existence set in1 probably due to crowding along shores1 and hard parts began to develop both for support and defensive purposes4 ,such hard parts or shells were commonly favorable for fossilization4 ,this view is strongly supported by the fact that very thin shells only are known from late ,proterozoic rocks1 and mostly very thin shells from the earliest ,cambrian1 the heavier shells having been evolved later4 ,a fact of importance to bear in mind in this connection is that just at the critical time 7late ,proterozoic7 in shell development1 the lands of the earth were undergoing widespread and deep erosion as pointed out early in the chapter on ,ancient ,earth ,history4 ,the earliest ,cambrian strata1 therefore1 nearly everywhere rest upon the deeply eroded surface of the ,proterozoic rocks so that the transition strata--the very ones which would contain most fossils of the early shell development stage--are nearly everywhere missing4 ,finally1 mention should be made of the fact1 that all ,archeozoic strata are profoundly altered 7metamorphosed71 and so are nearly all ,proterozoic strata1 except the later4 ,fossils once present in those rocks would of course have been obliterated by the process of metamorphism1 but the fact remains that very considerable thicknesses of practically unaltered ,proterozoic strata show few if any animal fossils4 ,we shall now proceed to a rather systematic consideration of the most interesting and significant types of creatures which have inhabited the earth since the beginning of ,paleozoic time at least twenty-five million years ago4 ,it is our purpose to bring out the salient features in the history of each subkingdom of animals1 beginning with the lowest or simplest1 and taking up in turn the higher and higher subkingdoms4 ,by this method the reader may easily follow the main thread of organic evolution or progressive change which runs through most of the known history of animal life of our planet1 and which is so important in the science of geology4 ,protozoans1 which include all the tiny single-celled animals1 are known in fossil form even in late ,proterozoic rocks and1 as proved by the fossil records1 they have been more or less abundant ever since1 even now swarming in large portions of the surface sea waters4 ,one of the most remarkable facts in the history of animal life is1 that such exceedingly simple creatures persisted almost without change through the tens of millions of years when such profound and even revolutionary changes took place in the animal kingdom in general4 ,the only fossil ,protozoans are those which developed delicate shells either of carbonate of lime 7the foraminifers7 or silica4 ,special mention should be made of the ,cretaceous period when foraminifers must have been exceedingly profuse in clear sea waters which spread over the ,gulf ,coastal ,plain of the ,united ,states1 parts of southern ,england1 much of ,france1 and other areas1 as proved by their accumulated shells which make up formations of chalk hundreds of feet in thickness and many miles in extent4 ,illustration3 ,fig4 #di4--;,a compound colony of fossil graptolites characteristic of late ,ordovician time1 fully #bj1jjj1jjj years ago4 ,each little prong once held a tiny individual living graptolite which was a very simple type of animal belonging to the subkingdom called ,coelenterates4 7,modified after ,ruedemann47 ,the ,coelenterates1 which comprise the simplest of the many-celled animals1 are saclike forms with mouth openings1 but with few other differentiations of parts4 ,all are marine animals4 ,of these the sponges are porous1 and the other types 7including corals7 have tentacles around their mouths4 ,sponges have been more or less common from early ,paleozoic time to the present1 and they have undergone relatively little change4 ,jellyfishes1 which are in truth not fishes at all1 are wholly soft or gelatinous ,coelenterates which have left some very remarkable impressions and casts in strata of very early ,paleozoic age1 those very ancient forms evidently having been almost exactly like those of to-day4 ,graptolites were slender1 plumelike1 delicate forms consisting of colonies of tiny individuals1 in many cases in branching or radiating combinations4 ,they existed only during the first half of the ,paleozoic era4 ,both because they floated in the open sea1 thus permitting widespread distribution1 and because they underwent many distinct species changes during short geologic intervals1 they are among the most useful fossils for separating the various subdivisions of strata of the earlier ,paleozoic4 ,illustration3 ,fig4 #ej4--;,corals1 representing the very simple subkingdom of animals called ,coelenterates3 a1 fossil shell of an individual cup coral found only in ,paleozoic strata2 b1 a compound or chain coral skeleton found only in relatively old ,paleozoic strata2 and c1 part of a modern coral colony showing living corals4 ,corals comprise another important branch of the ,coelenterates4 ,during the ,cambrian period there were corallike sponges and possibly simple corals1 but from the early ,ordovician to the present true corals have been common1 especially in the clearer1 warmer seas4 ,their carbonate of lime skeletons have accumulated to help build up great limestone formations representing almost every geologic age from early ,paleozoic time to the present4 ,paleozoic corals were in general notably different from those of later time4 ,there were three main types including the compound honeycomb and chain types1 and the solitary or compound cup type4 ,they all had four1 or multiples of four1 radiating partitions2 were rarely branched2 and were generally large1 some individual cup corals ranging in length from half an inch to a foot or more4 ,modern corals 7beginning with the ,mesozoic7 have six or eight partitions2 are nearly all profusely branched2 and are mostly tiny individuals4 ,echinoderms are all marine animals1 including the so-called starfishes1 which are not really fishes4 ,they have body cavity1 with digestive canal1 low order nervous system1 and a water circulatory system4 ,most of them have radially segmented shells or skeletons4 ,the oldest fossil forms are found in ,cambrian strata1 these being very simple or primitive types1 with a bladderlike head set on the end of a segmented stem1 both head and stem having been supported by carbonate of lime4 ,such forms lived only to middle ,paleozoic time4 ,ordovician strata contain representatives of all the main types of ,echinoderms in well-fossilized forms4 ,illustration3 ,fig4 #ea4--;,fossil ,echinoderms or so-called starfishes3 a1 simple type known as the stone lily with head1 stem1 and roots intact from ,silurian strata2 b and c1 irregular and regular higher type ,echinoderms called sea urchins from ,cretaceous strata4 ,a stemmed ,echinoderm of special interest1 first known from the ,ordovician1 has persisted to the present day4 ,it is the so-called sea lily or stone lily1 consisting of a complex1 headlike portion attached to the sea bottom by a long segmented stem1 the whole being supported by lime carbonate4 ,they were very numerous during the ,silurian1 but they seem to have culminated in variety of species and numbers of individuals during the ,mississippian period when they were exceedingly profuse4 ,hundreds of species of stone lilies are known from ,mississippian strata alone1 and in certain localities1 as at ,crawfordsville1 ,ind41 and ,burlington1 ,ia41 the stone lily remains are so numerous that when living they must have literally forested parts of the sea bottom4 ,from ,mississippian time to middle ,mesozoic time they occupied a relatively subordinate position when they again developed in great profusion4 ,the ,mesozoic forms were distinctly more like those of to-day1 and it scarcely seems credible that any creature could have contained such a multiplicity of hard parts1 more than #fjj1jjj segments having been counted in a single fossil from ,jurassic strata4 ,the sea lilies of to-day are relatively unimportant4 ,the familiar five-pointed starfishes1 so common along our seacoasts1 are first known from the ,ordovician1 and they persisted through the many millions of years to the present time with remarkably little change4 ,the so-called sea urchins live in rounded1 segmented lime-carbonate shells bristling with movable spines4 ,sea urchins are first known from the ,ordovician1 but they did not become abundant and diversified until ,mesozoic time1 when many of them took on a very modern aspect4 ,worms are known to have existed ever since late ,proterozoic time1 as proved by the occurrence of tracks1 borings and more rarely delicate impressions on rock surfaces4 ,because of their softness they have rarely been well fossilized and are1 therefore1 of no great evolutionary or geological importance4 ,illustration3 ,fig4 #eb4--;,fossil brachiopods belonging to the subkingdom of animals known as ,molluscoids3 a1 b1 c1 forms characteristic of the ,ordovician1 ,devonian1 and ,triassic periods1 respectively4 ,the subkingdom ,molluscoids has been richly represented by both the so-called sea mosses and brachiopods4 ,the sea mosses form colonies of tiny mosslike tufts1 resembling corals outwardly1 though they are much more highly organized4 ,they have been common from ,ordovician time to the present1 their carbonate of lime skeletons often having contributed to the building of limestone formations4 ,brachiopods always have two external shells or valves1 in most cases working on a hinge1 and also a pair of long1 spiral-fringed arms associated with the soft part of the animal inside the shells4 ,they differ from the other type of bivalve 7e4g41 clam1 oyster7 in that they are symmetrical with reference to a plane passed through the middle of the shells at right angles to the hinge line4 ,they have rarely grown to be more than a few inches long4 ,a few scant brachiopod remains are known from the late ,proterozoic1 but throughout known geologic time they reached their greatest development in the ,paleozoic era1 more especially in the ,devonian period4 ,combining number of species and number of individuals1 the brachiopods probably hold the record of all important groups of fossil animals1 more than #g1jjj species being known4 ,many layers of rock are filled with their shells 7,plate #ad74 ,since the close of the ,paleozoic they have fallen off notably1 and are now represented by relatively few small forms4 ,from the standpoint of evolution it is interesting to note that in very early ,paleozoic time the brachiopods were mostly small1 of relatively simple organization1 and their thin shells were not joined by hinges4 ,later they became larger and more complex and their thicker shells worked on hinges4 ,nearly all the ,paleozoic forms had long1 straight hinge lines1 which made it difficult for their enemies to open them4 ,along with the change to narrower1 curved hinge lines came the decline of the tribe4 ,they have been of great value to the geologist in subdividing the geological column of strata into its many formations4 ,the ,mollusks1 which are more highly organized than the ,molluscoids1 have more or less distinctly developed heads and locomotive organs4 ,many thousands of species are now extinct1 the classes of most geological importance being represented by clams1 snails1 and the pearly nautilus4 ,most of them have shells and gills for breathing4 ,the members of the simplest group1 well represented by the clam tribe1 possess two similar shells working on hinges1 so that in this regard they are much like brachiopods1 but1 unlike the latter1 they are not symmetrical with reference to a plane at right angles to the hinge line4 ,cambrian strata contain the oldest known of the fossil forms where they are small1 relatively thin-shelled1 and rare4 ,in marked contrast to the brachiopods these bivalves have rather steadily increased in numbers of species and individuals to the present time1 now being represented by thousands of forms4 ,during the ,mesozoic era they greatly out-numbered the brachiopod bivalves and took on a more distinctly modern aspect1 when the oyster tribe and closely related types were prominently developed4 ,culmination in size and thickness of shell seem to have been reached in early ,cenozoic time1 strata of that age in certain places1 for example in ,georgia and southern ,california1 being filled with oyster shells #aj to #bj inches long and #d to #f inches thick6 ,in addition to their gigantic size and thickness1 many of the shells were fluted or ribbed1 and so they represented an extreme type of defensive armor among the lower animals4 ,snails have existed from the earliest ,paleozoic era to the present time1 and the outstanding fact of interest concerning them is that they furnish one of the finest illustrations of an important class of animals which has undergone practically no conspicuous change or evolution during all those millions of years of time4 ,illustration3 ,fig4 #ec4--,sketches of chambered cephalopods showing the main steps in the evolution of the shell forms and compartment partitions3 a1 b1 the only kinds in ,cambrian time2 c1 d1 forms added in the ,ordovician2 e1 added in the ,devonian2 f1 added in the late ,paleozoic2 g1 h1 characteristic of the ,mesozoic era2 and i1 a living form 7pearly nautilus7 cut through4 7,drawn by the author47 ,we shall now turn our attention to the highest order of ,mollusks--the cephalopods4 ,these creatures1 whose heads are armed with powerful tentacles and supplied with complex eyes1 propel themselves by forcible ejection of water4 ,one general type--the chambered cephalopod--has a shell divided into compartments 7e4g41 modern pearly nautilus7 which are successively built up and abandoned by the animal as it grows larger4 ,these chamber-shelled cephalopods constitute one of the most remarkable and instructive illustrations of evolutionary change within any important subgroup of invertebrate animals1 ranging from early ,paleozoic to the present4 ,both because of the abundance of fossil forms in rocks of all these periods of geological times1 and because certain of the evolutionary changes are so clearly expressed in the well preserved shell portions1 they are specially adapted for study4 ,in the late ,cambrian only straight and slightly curved forms with smooth1 nearly straight chamber partitions existed4 ,notable advance took place during the next 7,ordovician7 period when there were straight1 curved1 open-coiled1 and even close-coiled forms4 ,all had simple partitions1 and the straighter forms predominated4 ,the size attained by the ,ordovician cephalopods was probably never surpassed by representatives of the class4 ,some of the 7straight7 shells were twelve to fifteen feet in length1 and a foot in diameter4 ,from this great size they ranged down to or below the size of a pipe stem4 7,chamberlin and ,salisbury47 ,they were more than likely the undisputed masters of the ,ordovician seas4 ,silurian time marked no important change in their structures1 but the coiled forms predominated for the first time4 ,during the second half of the ,paleozoic era all preceding types with simple partitions persisted1 but in some forms the simple partitions gradually became angled and finally rather complexly curved4 ,during the ,mesozoic era the partition lines of the close-coiled forms evolved until a most remarkable degree of complexity was attained1 comparable1 indeed1 to the sutures of the human skull plates4 ,these remarkable forms called ammonites1 of which more than #b1jjj species are known1 began with the ,mesozoic1 reached their climax1 and passed out of existence toward the close of the same era4 ,certain strata of ,jurassic age are literally filled with ammonites1 some shells being several feet in diameter4 ,various eccentric changes took place in the ammonites shortly before their extinction4 ,some shells became uncoiled and even straight1 thus outwardly at least showing reversion to the original early ,paleozoic ancestors1 but with retention of the complex partitions4 ,others assumed spiral shapes and still others became curved or coiled at each end4 ,while these extraordinary evolutionary changes were going on among the chambers of cephalopods during ,mesozoic time1 some of the ancient close-coiled forms with very simple partitions managed to persist4 ,in fact this simple type1 almost exactly like its early ,paleozoic ancestor1 has been the only one out of this whole remarkable class of animals to persist to the present time1 being now barely represented by the well-known pearly nautilus of the ,indian ,ocean4 ,during the ,mesozoic era the highest type of cephalopod1 represented by modern squids and so-called cuttlefishes1 branched off and developed in great profusion4 ,these had slender internal shells1 but no external chambered shells4 ,an inky black liquid secreted in a bag was forced out to cloud the water when the animal was escaping its enemy1 thus antedating by millions of years the principle of smoke screen so effectively used by ships during the ,world ,war4 ,some ,jurassic species got to be over two feet long1 and a few specimens of that age have been found in such perfect state of preservation that drawings of the fossils have actually been made with the ink 7after moistening7 taken from their own ink bags4 ,illustration3 ,fig4 #ed4--;,a fossil nonchambered cephalopod of ,jurassic age4 ,it was closely related to the modern squid1 and its ink bag is well shown just to the left of the middle4 7,modified after ,mantell47 ,before concluding this chapter we shall take up the salient points in the geological history of ,arthropods which constitute the highest subkingdom of all animals except the ,vertebrates4 ,they are now very abundant and varied1 familiar examples being crabs and insects4 ,a few scant remains of simpler forms are known from the ,proterozoic1 but since very early ,paleozoic time they have been very common and have undergone great evolutionary changes4 ,a few striking examples only will be dwelt upon4 ,among the most common and interesting of all ,paleozoic animals were the trilobites1 distantly related to modern lobsters and crabs4 ,illustration3 ,fig4 #ee4--,restorations of trilobites based upon actual fossils characteristic of earlier ,paleozoic time3 a1 ,cambrian2 b1 ,ordovician2 c1 ,devonian2 b shows the appendages4 ,some of these grew to be two feet long1 but usually they were only one or two inches long4 ,first known from the earliest ,paleozoic1 they reached their culmination relatively early in the era and then dwindled away to utter extinction before its close4 ,they were characteristic of the ,paleozoic era1 beginning in great variety in the ,lower ,cambrian and dominating the seas of the ,cambrian 7#cjj species7 and ,ordovician 7#iej species74 ,in the ,silurian1 though they were still common1 the trilobites were nevertheless on the decline 7#dhe species71 and this ebbing of their vital force is seemingly shown in many picturesque forms replete with protuberances1 spines1 and exaggeration of parts4 ,as a rule1 in evolution1 one finds that when an organic stock is losing its vital force there arises in it an exaggeration of parts1 as if heroic efforts were being made to maintain the race4 ,spinosity in animals is often the prophecy of tribal death4 ,in the ,devonian1 the variety and number of the trilobites were greatly reduced 7#aje species71 at a time when the ancient types of fishes1 which undoubtedly fed on these crustaceans 7trilobites71 began to be common in the seas4 ,in the later ,paleozoic seas1 the trilobites were relics1 or animals surviving from a time better suited to their needs1 and one by one they vanished1 until a little before the close of the ,paleozoic era none were left4 7,schuchert47 ,illustration3 ,fig4 #ef4--;,a giant1 sea scorpion of ,devonian time4 ,length nearly #c feet4 7,after ,clarke and ,ruedemann1 ,new ,york ,state ,museum47 ,an extraordinary type of ,arthropod which ranged throughout ,paleozoic time and became extinct at its close was the so-called sea scorpion1 closely related to the modern scorpion4 ,their five or six pairs of appendages all came out from the head portion1 one pair in some cases having been developed as powerful pincers4 ,their culmination in size was reached during the ,devonian when some forms grew to the astonishing length of over eight feet6 ,such gigantic creatures must have been tyrants of the seas until they were subdued by the oncoming powerful fishes4 ,true scorpions are known from rocks as old as the ,silurian4 ,lobsters and crabs made their appearance during the ,mesozoic era4 ,since insects constitute the highest subdivision of ,arthropods1 they include the very highest forms of animal life except the ,vertebrates4 ,the oldest known fossil insects are from ,pennsylvanian strata1 more than #a1jjj species having been described from rocks of that age4 ,they were all simple or primitive types like cockroaches and dragon flies1 and were remarkable for size4 ,giant cockroaches got to be four inches long4 ,one form of dragon fly1 with a spread of wing of over two feet1 was probably the largest insect which ever lived 7,plate #ae74 ,development of insect life was especially favored during the great ,coal ,age because of the prolific vegetation1 but more than likely insects originated somewhat earlier4 ,early in the ,mesozoic era a great progressive change began to come over insect life and higher forms gradually evolved until by the close of the era many of the highest types like flies1 ants1 and bees were common4 ,as might be expected1 the highest insects did not develop until after the appearance of the true flowering plants in later ,mesozoic time1 butterflies apparently not having evolved until early in ,cenozoic time4 ,many of the thousands of known species of fossil insects are from strata of ,tertiary age during which time they may have been even more numerous than to-day1 although there are about #djj1jjj species now living4 ,an almost incredible case is a ,tertiary stratum only a few feet thick in ,switzerland from which nearly #a1jjj species of insects have been unearthed4 ,another famous locality is ,florissant1 ,colorado1 where during early ,tertiary time there was a small lake into which showers of fine volcanic dust fell and entombed vast numbers of insects1 more than #b1jjj species having been unearthed4 ,still another extraordinary occurrence is along the shores of the southern ,baltic ,sea where more than #b1jjj species of insects have been found in a fossil resin called amber4 ,the insects were caught in the still soft sticky resin while it was exuding from the trees1 and thus we have the insects1 fully two or three million years old1 literally embalmed and marvelously preserved1 often in beautifully transparent amber4 ,,chapter ,,xix ,,geological ,,history ,,of ,,vertebrate ,,animals 7,,including ,,man7 ,vertebrates comprise the highest subkingdom of all animals with man himself at the very top4 ,they are characterized by the possession of a vertebral column1 which1 in all but the very simple or primitive forms1 is an ossified backbone4 ,their main subdivisions are given in the classification table near the beginning of the preceding chapter4 ,the oldest known ,vertebrates1 found in fossil form in middle ,ordovician strata1 were represented by curious and bizarre creatures called ostracoderms1 or more popularly armor fishes4 ,they were not true fishes because they were really somewhat lower in the scale of organization than fishes4 ,some were distinctly fishlike in appearance1 and others notably resembled certain of the ,arthropods1 so that some students consider them to have formed the connecting link between the highest ,invertebrates 7,arthropods7 and low order fishes of the ,vertebrates4 ,the vertebral column always consisted of cartilage or gristle and1 in most forms1 it extended through tail fin4 ,none had true side fins like fishes1 but many were provided with a pair of jointed flappers or paddles4 ,the jawlike portions of the heads moved over each other sidewise as1 for example1 in beetles and not up and down in true ,vertebrate fashion4 ,two eyes were always very close together4 ,one of the most striking features was the protection of the head and fore part of the body by an armor of bony plates1 while the rest of the body had scales4 ,they seldom grew to be more than six or seven inches long4 ,beginning in the ,devonian1 they remained rare during the ,silurian1 and then in the ,devonian period they reached their climax of development only to become extinct at its close4 ,many species were abundantly represented in many parts of the world4 ,by some the ,ostracoderms are thought to have been a primitive 7sharklike7 fish development in the wrong direction1 and hence they became extinct4 ,illustration3 ,fig4 #eg4--,two restored forms of very primitive and ancient 7,devonian7 types of ,vertebrates called ostracoderms4 ,they were lower in organization than true fishes4 7,after ,dean-,woodward and ,british ,museum1 respectively47 ,fishes1 represented only by very primitive sharks1 are known to have existed as early as the ,silurian period1 but the remains are scant4 ,during the ,devonian period1 however1 they showed a marvelous development into many species and countless myriads of individuals4 ,the ,devonian is1 therefore1 commonly called the ,age of ,fishes4 ,these very ancient 7,devonian7 primitive 7fish7 types of ,vertebrate animal life are of profound significance in organic evolution because they were the direct progenitors of the great groups of still higher ,vertebrates which since later ,paleozoic time gradually increased in diversity and complexity of structure through amphibians1 reptiles1 birds1 and mammals finally to man himself4 ,illustration3 ,fig4 #eh4--,restorations of characteristic ,devonian fishes1 based upon actual fossils3 a1 a lung fish with leglike fins 7after ,huasakof72 b1 a ganoid4 7,after ,nicholson47 ,in marked contrast to the most typical and highly organized fishes so abundant to-day1 all ,devonian fishes were of simple types with cartilaginous skeletons and vertebrated tails4 ,many of them were also generalized types1 that is1 associated with their clearly defined fish characters were others connecting them with certain higher ,vertebrates1 as1 for example1 amphibians and reptiles4 ,thus all their tail fins were vertebrated as in reptiles2 their labyrinthine1 internal tooth structure was to be an amphibian feature when those creatures evolved2 many had protective armor or bony scales like most early amphibians and many modern reptiles2 and many had paired fins which were something like jointed legs4 ,most abundant and highly organized of the ,devonian fishes were ganoids1 characterized by a covering of small plates or bony scales set together but not overlapping like in typical modern fishes4 ,their intricate tooth structure and limblike fins strongly suggest the amphibians of later ,paleozoic time4 ,the skeleton of cartilage gradually became somewhat ossified during succeeding geologic periods4 ,from their great profusion and diversity in the ,devonian period the ganoids have steadily fallen away until they now have very few descendants like the gar pike4 ,another important group of remarkable fishes1 now totally extinct1 but common in ,devonian and somewhat later time1 had heavy1 bony armor plates over the fore part of the body4 ,those which grew to be fifteen to twenty-five feet long were probably the rulers of the middle ,paleozoic seas4 ,another remarkable ,devonian fish was able to breathe in both water and air because1 like their few modern descendants1 they had both gills and lungs4 ,because of their leglike fins and lung sac1 it is commonly believed that they were progenitors of the later ,paleozoic amphibians4 ,the simplest of all fishes1 the sharks1 began in the ,silurian1 underwent no important change through the millions of years since1 and are now of course well represented4 ,during early ,cenozoic time the sharks seem to have reached culmination in size--sixty to eighty feet long1 with teeth five or six inches long4 ,among modern fishes the most abundant by far1 and the most highly organized1 are the true bony fishes1 called the teleosts1 which made their first appearance in the middle of the ,mesozoic era4 ,those earliest forms clearly show their descent from the ganoids4 ,apparently they have not yet passed their prime4 ,we shall now consider the next higher group of ,vertebrates1 the amphibians1 which breathe by gills when young and later develop lungs4 ,many live both on land and in water like the frogs4 ,unlike fishes they have legs with toes and not fins4 ,beginning probably in the ,devonian as a branch of the fishes1 amphibians showed a marvelous development during later ,paleozoic and very early ,mesozoic times when they reached their climax1 after which they fell off remarkably1 being now relatively unimportant like the frogs and salamanders4 ,they are of special significance because they were the first of all the back-boned animals 7,vertebrates7 to inhabit the land which they dominated only until the great rise of reptiles of ,mesozoic time4 ,the reptiles in fact evolved from the amphibians in the late ,paleozoic when many transition forms occurred4 7,plate #ae47 ,during those ancient days the numerous and very diversified amphibians were like giant salamanders1 commonly five to eight feet long1 with one ,triassic form fifteen to twenty feet long1 and with heavily armored skulls two to four feet long4 ,turning now to the reptiles we find that they are much more distinctly land animals than the preceding types of ,vertebrates4 ,reptilian life of the earth began in late ,paleozoic time as an evolutionary branch of the amphibians4 ,the earliest forms were in many ways much like the amphibians1 but gradually they diversified and progressed so that before the close of the ,mesozoic era1 which has long been called the ,age of ,reptiles1 they were the rulers of the world4 ,they covered the land with gigantic herbivorous and carnivorous forms2 they swarmed in the sea1 and1 as literal dragons1 they dominated the air4 7,scott47 ,mesozoic reptiles are of special interest and significance not only in themselves1 but also because from one of their branches the birds were evolved1 and from another the mammals4 ,in advancing from the amphibian to the reptile the evolution of the ,vertebrates 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 7,norton47 ,since the reptiles of the ,mesozoic era constitute one of the few most remarkable and diversified classes of animals which ever inhabited the earth1 we shall attempt to give the reader a fair idea of the most typical groups which have been totally extinct since the close of the ,mesozoic era some millions of years ago4 ,of the swimming reptiles which lived in the seas many types are known and only a few will be described4 ,among these one important type was the ichthyosaur1 a fishlike form which not uncommonly grew to be twenty to even forty feet long 7,plate #ah74 ,the large head1 sometimes four or five feet long1 contained as many as #bjj big sharp teeth and enormous eyes up to a foot in diameter4 ,the body was heavy set1 and the neck very short4 ,there were four short1 stout swimming paddles1 and the tail was vertebrated4 ,some specimens of ichthyosaurs have been so perfectly preserved in ,mesozoic strata that even the unborn young are plainly seen in the bodies6 ,in some cases it is actually possible to tell what was the last meal of a particular ichthyosaur those millions of years ago2 in one specimen1 for example1 remains of #bjj creatures of the cuttle-fish tribe having been found in the exact position of the stomach4 ,illustration3 ,fig4 #ei4--;,chart showing the main branches in the history of ,vertebrate 7back-boned7 animal life reaching its culmination in man4 7,by the author1 in part after ,cleland47 ,the mosasaurs of the late ,mesozoic were the only real sea serpents of the geologic ages4 ,they were something like the ichthyosaurs1 but with smaller heads and much longer1 more slender1 serpentlike bodies4 ,some grew to be thirty or forty feet long4 ,plesiosaurs were perhaps the strangest of all the ,mesozoic marine reptiles4 ,they grew to be forty to fifty feet long1 with stout body1 very long1 slender neck1 small head1 short tail1 and four long1 powerful swimming paddles which were distinctly leglike4 ,these and the mosasaurs were both flesh eaters1 as shown by the sharp teeth4 ,illustration3 ,plate #ae4--7;a7 ,restoration of a ,late ,paleozoic 7,coal ,age7 ,landscape4 ,showing the main kinds of plants which have entered into the making of most of our coal4 ,giant club mosses both with and without branches1 in the left background2 giant horsetail 7or scouring rush7 plants on the right2 and seed ferns in the left foreground4 ,a primitive reptile in the water2 two large amphibians or giant salamanders1 called stegocephalians1 on the land2 and a great dragon fly1 two feet wide1 in the air4 7,from a drawing by ,prof4 ,williston4 ,courtesy of ,d4 ,van ,nostrand ,co47 ,illustration3 ,plate #ae4--7;b7 ,photograph of a ,fossil ,fern or ,seed ,fern ,frond on a ,piece of ,shale ,millions of ,years ,old4 ,the specimen is of the ,pennsylvanian ,age and was taken from the coal fields of ,pennsylvania4 7,after ,white1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #af4--,restoration ,showing the ,general ,appearance of ,some of the ,largest ,animals ,which ,ever ,trod the ,earth4 ,a mounted skeleton in the ,american ,museum of ,natural ,history is sixty-seven feet long1 and the skeleton of a similar creature in the ,carnegie ,museum1 ,pittsburgh1 is eighty-seven feet long4 ,they lived millions of years ago during the middle and late ,mesozoic era4 7,after ,c4 ,r4 ,knight4 ,courtesy of the ,american ,museum of ,natural ,history1 ,new ,york47 ,the most remarkable walking reptiles of all time were the dinosaurs or terrible lizards4 ,we shall describe enough types of these unique creatures to give the reader a fair idea of their appearance and habits4 ,most astonishing of all were the sauropods including the largest animals which ever trod the earth4 ,they grew to be as much as sixty to ninety feet or more in length4 ,remarkably well preserved skeletons have been found1 one from ,utah1 eighty-seven feet long1 being mounted in the ,carnegie ,museum of ,pittsburgh4 ,the largest of these brutes stood fifteen to twenty feet high and they must have weighed thirty to fifty tons4 ,the very long1 serpentlike neck and tail1 and very small head were grotesque features4 ,considering the structure of the dinosaurs1 the kind of strata in which they are embedded1 and the associated fossil remains1 it seems clear that they mostly lived in and near fresh water and on near-by lowlands4 ,the character of their teeth shows that they fed entirely on soft plants which they must have habitually bolted because their teeth were not well adapted to grinding food4 ,it is difficult to believe that a single huge beast could have consumed less than a few hundred pounds of vegetable matter per day1 and1 on account of the very small size of the head1 he must have spent most of his time eating4 ,also the comparatively very small size of the brain1 and its simplicity of structure1 render it certain that they were extremely stupid creatures4 ,to make up for this they had an enormous enlargement of the spinal cord in the sacral region 7i4e41 over the hind legs74 ,this sacral brain--if we may so call it--was ten to twenty times bigger than the cranial brain4 ,it was necessary in order to work the powerful hind legs and tail4 7,le ,conte47 ,illustration3 ,fig4 #fj4--,skeleton of a great four-legged 7sauropod7 dinosaur4 ,a mounted skeleton in the ,american ,museum of ,natural ,history1 ,new ,york1 is sixty-seven feet long4 ,this creature lived millions of years ago during the ,jurassic period4 7,after ,marsh47 ,another dinosaur1 in some respects like the sauropod1 was the stegosaur which grew to be twenty to thirty feet long1 and heavier than the elephant4 ,unlike the sauropod1 it had a short neck and was armored with a double row of great plates over its back1 and sharp spines 7one to three feet long7 toward the end of the tail4 ,the excessive stupidity of the creature is proved by the fact that its very simple brain weighed less than three ounces6 ,stegosaurs were plant eaters as indicated by the tooth structure1 and1 though they looked ferocious1 they were probably not fighters1 certainly at least nothing like the carnivorous types of dinosaurs we shall soon describe4 ,illustration3 ,fig4 #fa4--,skeleton of the curious kind of dinosaur 7stegosaur7 of ,mesozoic ,age with great bony plates over the back4 ,length about thirty feet4 7,after ,marsh47 ,the ferocious dinosaurs of ,mesozoic time were carnivorous1 or flesh eaters1 as shown by their numerous sharp teeth in relatively large heads4 ,the largest known type is the tyrannosaur1 an almost perfect skeleton of which1 #dj feet long and #af feet high1 is mounted in the ,american ,museum of ,natural ,history in ,new ,york 7,plate #ag74 ,so far as known1 this was the largest carnivorous animal which ever walked on the earth4 ,it is evident from the structure that it walked on its hind legs1 the front ones having been much shorter and used something like arms4 ,there were also various other smaller forms of two-legged flesh-eating dinosaurs1 many of the wonderfully preserved tracks in the ,triassic sandstones of the ,connecticut ,river ,valley having been made by such creatures when they walked around over soft1 sandy mud flats at least eight or ten million years ago4 ,the sandy mud with its tracks became somewhat hardened and then deeply buried under much more sediment which1 through the ages1 has been eroded off1 thus exposing to view certain of the layers covered with tracks4 ,some bones of dinosaurs have also been found in the ,connecticut ,valley4 ,illustration3 ,fig4 #fb4--,skeleton 7restored7 of a great two-legged dinosaur of the ,mesozoic era4 ,this type of plant eater grew to be fully twenty-five feet long4 7,after ,marsh47 ,another remarkable type of two-legged dinosaur was much like the flesh eaters just described1 but they were plant eaters4 ,the largest of these grew to be #cj feet long and #ae to #bj feet high1 comparable1 therefore1 to the tyrannosaur in size4 ,a wonderful collection of almost perfect skeletons may be seen in the museum in ,brussels1 ,belgium4 ,in mining coal #a1jjj feet below the surface in ,belgium1 twenty-two complete skeletons and several partial skeletons were found in an ancient river deposit of ,cretaceous ,age4 ,a marvelously preserved specimen of one of these two-legged plant eaters found in ,wyoming1 has been called a dinosaur mummy because the skin and much of the flesh of the creature had shriveled down upon its bones4 ,the minutest details of the texture of its skin are almost perfectly preserved4 ,illustration3 ,fig4 #fc4--,skeleton of a dinosaur 7triceratops7 with a large remarkable head4 ,this creature grew to be twenty-five feet long during ,cretaceous time4 7,after ,marsh47 ,another type of dinosaur1 so different from the others1 should be briefly described4 ,this was triceratops1 or the three-horned face beast1 so named because of the three powerful horns which projected forward from the top of the very large1 flattened skull4 ,it grew to be twenty to twenty-five feet long4 ,skulls six to eight feet long have been unearthed4 ,just where the brain might have developed1 the skull dished downward1 and so one authority considers triceratops to have had the largest head and smallest brain of all the great reptiles4 ,it is well known that dinosaurs of many types lived during the great ,age of ,reptiles1 though by no means all types ranged through the whole era4 ,no dinosaurs are definitely known to have crossed the line into the ,cenozoic era4 ,one of the most astonishing facts in the history of animal life is the extinction of the mighty dinosaurs1 but no very satisfactory explanation has yet been offered4 ,probably their great size was a contributing factor1 for it is well known that while very large animals spend nearly all their time in eating1 small animals spend a small proportion of theirs1 and most of it in other activities4 ,now1 as long as food is abundant1 the larger animals of a race have the better chances1 but if a scarcity of food ensues1 the larger animals may all be suddenly swept out of existence4 7,matthew47 ,whatever may have been the real reason for dinosaur extinction we can at least be sure that with the extensive changes in the elevation of land areas 7,rocky ,mountain ,revolution7 which mark the close of the ,mesozoic1 came the withdrawing of the great inland ,cretaceous seas along the low-lying shores of which the dinosaurs had their home1 and with the consequent restriction of old haunts1 came the blotting out of a heroic race4 ,their career was not a brief one1 for the duration of their recorded evolution was twice that of the subsequent mammalian 7,cenozoic7 age4 ,they do not represent a futile attempt on the part of nature to people the world with creatures of insignificant moment1 but are comparable in majestic rise1 slow culmination1 and dramatic fall to the greatest nations of antiquity4 7,schuchert47 ,among the most extraordinary animals not only of the ,mesozoic1 but also of all time1 were the flying reptiles or literal dragons of the air4 ,some were very small1 while others were the largest creatures which ever flew1 with a spread of wing of twenty to twenty-five feet--twice that of any modern bird4 ,unlike birds they had no feathers1 but the two wings consisted of large membranes 7batlike7 supported by one enormously elongated finger of each front limb4 ,the other fingers were armed with sharp claws4 ,the early ,mesozoic flying reptiles had sharp teeth1 while the later ones were mostly entirely toothless1 but all were carnivorous4 ,their short bodies were supplied with tails of varying lengths1 one long-tailed species having a rudder at the end4 ,their heads were fairly large1 but of light build4 ,the creature called pteranodon was not only the largest of the flying reptiles1 but also probably the most highly specialized creature which ever lived1 everything possible apparently having been sacrificed to facilitate flight 7,plate #ah74 ,the hollow bones were so wonderfully light and strong that it has been estimated that the living animal1 with twenty-five foot spread of wing1 and head four feet long1 could not have weighed more than twenty-five pounds6 ,the rear portions of the body and hind limbs were very weak4 ,illustration3 ,fig4 #fd4--;,a small carnivorous flying reptile of ,mesozoic time4 ,spread of wings about two feet4 7,restored by ,marsh47 ,it should not be thought that the above-described groups of reptiles were the only ones which existed during ,mesozoic time4 ,there were also certain groups still living1 like turtles1 lizards1 and crocodiles1 but they were doubtless mostly completely under the dominance of certain of the now long-extinct types above described4 ,the oldest-known fossil snakes are from very late ,mesozoic rocks1 where they are small and comparatively rare4 ,more than likely they evolved from lizards by deterioration of the legs4 ,poisonous snakes were not evolved until early in the next 7,cenozoic7 era4 ,we shall now turn our attention to next to the highest class of ,vertebrate animals--the birds4 ,they and the mammals are the only warm-blooded animals4 ,what is their ancestry8 ,from what original stock did they branch off8 ,the oldest-known bird lived during the ,jurassic period1 and it was so decidedly reptilian in character as to render it practically certain that birds are specialized descendants of certain ,mesozoic reptiles1 though not1 as might be supposed1 of the flying reptiles4 ,the few known specimens of the ,jurassic birds were found in the famous lithographic limestone quarries of ,bavaria4 ,at least two of the specimens are in a marvelous state of preservation1 with practically the whole skeleton intact and almost perfect impressions of the feathers on the rock4 ,that the creature was really a bird is proved not only by its feathers1 but also its beak1 brain1 limb bones1 and feet4 ,among the reptilian characters are its long1 vertebrated tail1 teeth set in sockets1 and long claws on the wings4 ,this reptilian bird was about the size of a small crow4 ,by late ,cretaceous time the birds made notable evolutionary progress and they became diversified1 more than thirty species being known from ,cretaceous rocks4 ,these were distinctly more modern in structure and appearance than the ,jurassic bird4 ,the only important reptilian characteristic still retained was the possession of teeth4 ,the tail had become much shortened and the brain was still relatively smaller than in modern birds4 ,one type1 about nine inches high1 was a powerful flier1 as shown by the strong keel and wing bones4 ,another important ,cretaceous type was almost wholly a water dweller1 with powerfully developed legs used in swimming4 ,its teeth were set in grooves instead of in sockets1 thus indicating degeneration of tooth structure4 ,this type was notable for its size--five to six feet in length4 ,illustration3 ,fig4 #fe4--;,an early type of bird with teeth4 ,this bird grew to a height of about nine inches in ,cretaceous time1 millions of years ago4 7,restored by ,marsh47 ,during the early part of the ,cenozoic era birds became still more advanced and numerous1 with many modern groups represented4 ,some of the more primitive types were1 however1 still left over during the ,tertiary1 as1 for example1 a toothed bird1 in which the teeth were merely dentations of the bill1 thus being the most degenerate of all types of tooth structure4 ,mammals comprise the highest class of all animals4 ,they are1 of course1 all warm blooded and characterized by suckling their young4 ,so far as known1 mammal life began in the early ,mesozoic era as a branch of primitive reptiles1 but they made little progress throughout the era when they occupied a very subordinate position in the animal world4 ,they were few in number1 small1 and primitive in structure4 ,there is no evidence for the ,mesozoic existence of any of the higher forms of mammals1 that is1 those which give birth to well-formed young which are prenatally attached to the mother by the so-called placentum4 ,during the eons of the ,mesozoic1 from late ,triassic time until its close1 the mammals 7including the remote progenitors of humanity7 were in existence1 but held in such effective check 7by reptiles7 that their evolutionary progress was practically insignificant4 ,this curb is strikingly illustrated by the wonderful series of tiny jaws and teeth of these diminutive creatures found in the ,comanchian 7early ,cretaceous7 of ,wyoming1 in actual association with the single tooth of a carnivorous dinosaur1 many times the bulk of the largest mammalian jaw4 ,the removal of this check resulted 7in the ,tertiary period7 in the speedy evolution of the archaic mammals4 7,schuchert47 ,the phenomenal development of mammals during the ,tertiary period forms one of the most wonderful chapters in the whole evolution of organisms4 ,even very early in the ,tertiary1 many important higher 7placental7 types of mammals had evolved1 and the simpler1 more primitive ,mesozoic forms became very subordinate4 ,by the close of the ,tertiary the higher types of mammals had become marvelously differentiated into most of the present-day groups or types4 ,a very significant feature of the evolution was the steady increase in relative size of brain4 ,the vast numbers of fossil skeletons and bones of mammals found in ,tertiary strata is scarcely believable4 ,in our brief discussion we can do no more than describe a few representative examples of the ,cenozoic evolution of mammals4 ,the great diversity of modern placental animals may be suggested by a few examples1 as the tiger1 dog1 horse1 camel1 elephant1 squirrel1 hedgehog1 whale1 monkey1 and man4 ,forms like these1 traced back through their ancestors to the very early part of the ,tertiary period1 gradually become less and less distinct until they cannot be at all distinguished as separate groups1 but rather there are ancestral generalized forms which show combinations of features of the later groups4 ,those early ,tertiary generalized placental mammals had four feet of primitive character1 with five toes on each foot2 the whole foot1 which from toe to heel touched the ground1 was not adapted to swift running2 the teeth were simple 7primitive7 in type and of full original number 7forty-four72 the toes were supplied with nails which were about intermediate between real claws and hoofs in structure2 and the brain was relatively much smaller and simpler in structure than in most modern mammals4 ,illustration3 ,fig4 #ff4--;,chart showing the main features of interest in the evolution of the horse family through several million years of the present 7,cenozoic7 era of geologic time4 7,after ,matthew1 ,american ,museum of ,natural ,history47 ,the history of the horse family furnishes an excellent illustration of certain evolutionary changes among mammals4 ,skeletons of many species1 ranging from the early ,tertiary to the present1 have been found in remarkable state of preservation representing every important change in the history of the horse family4 ,a study of the chart will make clear some of the most striking changes which have taken place4 ,the oldest member of the horse family represented on the chart was about the size of a small fox1 with four toes and a degenerated fifth toe 7splint7 on the front foot1 and three toes and splint on the hind foot4 ,since the chart was made a still more primitive form1 even more closely resembling the original five-toed ancestor1 has been found4 ,gradually the middle toe enlarged1 while the others disappeared except the two splints or very degenerate toes still left in the modern horse4 ,increase in size of the animal and brain capacity accompanied these changes4 ,also the teeth underwent notable change1 and two originally separate bones 7radius and ulna7 of the foreleg became consolidated into a single stronger bone4 ,the even-toed hoofed mammals of to-day1 like the deer1 pig1 and camel1 are also the product of evolution much like that of the horse1 except that two of the original five toes have been equally developed1 while the others have either greatly degenerated1 as in the pig1 or disappeared entirely1 as in the camel4 ,the elephants1 or trunk-bearing animals1 illustrate a very different kind of evolution4 ,they seem to have reached their climax of development in the late ,tertiary when they grew to be as much as #ad feet high1 and were more abundant and widespread over the earth than at any other time4 ,the modern elephant1 like the horse1 has been traced back through many intermediate forms to its primitive early ,tertiary ancestry4 ,some of the most important evolutionary changes took place in the head portion4 ,the trunk is a highly developed form of snout1 the earliest form of which was much like that of the modern tapir4 ,the tusks are highly specialized and elongated teeth4 ,during the earlier history the chin was very long and supported short tusks1 so that there were then four tusks4 ,carnivorous mammals1 like tigers and wolves1 and gnawers1 like rats and squirrels1 may also be traced back to generalized early ,tertiary types4 ,another kind of evolution is well illustrated by certain mammals which1 even in early ,tertiary time1 so thoroughly adapted themselves to a water environment as to become whales1 porpoises1 etc4 ,illustration3 ,fig4 #fg4--;,comparison of feet of monkeys and man4 ,the primates include the highest group of all ,vertebrates1 and therefore of all animals4 ,monkeys1 apes1 and man belong to the primates4 ,there is no evidence whatever for the appearance of even the simplest and most primitive forms before the opening of the ,cenozoic era1 but even very early in ,tertiary time1 lemurs and primitive types of monkeys existed4 ,later in the ,tertiary true monkeys and apes were common1 and by the close of the period some apes were highly enough developed to strongly resemble certain of the oldest and most primitive types of man4 ,we have1 however1 no positive knowledge of the existence of man in even the latest ,tertiary4 ,in the light of much evidence in regard to the antiquity of man1 it seems improbable that true human fossils will ever be found in rocks older than the ,quaternary1 though if we are willing to descend 7far enough in the human scale toward apes7 it is not unlikely that man-apes may be discovered in very late ,tertiary rocks4 ,the difficulty comes in the classification4 ,where are we to draw the line between the higher apes and the lowest forms of man8 ,but this very difficulty is one of the strongest arguments in favor of the organic evolution of man because practically all intermediate forms between true man and certain other high-grade primates are known from the strata4 ,the following tabular summary of the geological history of man is based upon the work of most of the ablest students of the subject4 #c4 ,homo sapiens ,historic 7bronze and iron7 age4,modern 7e4g41 modern man7 ,neolithic 7recent stone7 age ,postglacial 7carefully shaped and but polished stone implements7 prehistoric ---------------------------------------- ------------------------------- #b4 ,homo primigenius ,upper ,paleolithic 7ancient ,late ,glacial 7e4g41 ,neanderthal stone7 age 7rough bone man7 and stone implements1 cave frescoes1 bone carvings1 etc47 ,lower ,paleolithic 7ancient ,middle ,glacial stone7 age 7rude stone implements of so-called river man7 ---------------------------------------- ------------------------------- #a4 ,early ancestral forms ,possibly some very crude ,early ,glacial 7e4g41 ,pithecanthropus stone implements and possibly erectus7 late ,tertiary ---------------------------------------- ------------------------------- ,of the early ancestral forms1 that is1 those which were rather distinctly man-apes1 two will be very briefly referred to4 ,one of these1 known as ,pithecanthropus erectus1 was a remarkable creature whose partial skeleton1 consisting of the upper part of a skull1 lower jaw1 several teeth1 and a thigh bone1 was found in early ,quaternary deposits in ,java in #ahia4 ,it was certainly a man-ape or possibly ape-man of low order1 about #e-a_/b feet high4 ,the skull has a low crown1 very receding forehead1 and prominent brow ridges1 but the brain capacity is #hej cubic centimeters1 as compared to #ejj cubic centimeters in ordinary higher apes1 and nearly #a1ejj cubic centimeters in the average modern man4 ,the very recently extinct very low-type aborigines of ,tasmania had a skull capacity of #a1aii cubic centimeters4 ,in #aijg the lower jaw of an anthropoid or manlike ape set with rather human teeth was found associated with very crude stone implements seventy-five feet below the surface in river-deposited sand in ,germany4 ,it is of either early or middle ,glacial time and quite certainly represents a lower order creature than the oldest ,paleolithic man as described below4 ,many bones and implements of ,paleolithic man 7see above table7 have been found mainly in river gravels and caves4 ,the relative ages of ,paleolithic human bones and implements are best determined by the associated fossil animals4 ,thus the most ancient truly human fossils are found directly associated with bones of very old types of elephants1 rhinoceroses1 and hippopotamuses which are definitely known to have lived during middle or early middle ,glacial 7,quaternary7 time corresponding to early ,paleolithic time4 ,a very conservative estimate would make the age of such very old human remains at least #aej1jjj to #bej1jjj years because the ,ice ,age was at least #ejj1jjj years long4 ,in a later human stage there are many associations with extinct animals like an older type of mammoth1 cave bear1 cave hyena1 and others of later ,glacial time estimated at #ej1jjj to #aej1jjj years ago4 ,last of all was the latest ,paleolithic stage corresponding to the close of the ,ice ,age1 the human remains of which are found associated with reindeer and the latest mammoths which roamed in great numbers across ,europe4 ,this was probably not more than #cj1jjj to #ej1jjj years ago4 ,paleolithic man is so called because he fashioned stone weapons and implements4 ,the structure of skull and skeleton shows him to have been a low-type savage1 something over five feet high on the average1 with a forward stooping carriage4 ,the average ,paleolithic brain was not greatly inferior in size to that of modern civilized man1 but it was not so highly organized and occupied a thick skull with much lower forehead and heavy brow ridges4 ,the bushmen of ,australia and the recently extinct ,tasmanians are the nearest modern resemblances4 ,many fine specimens of ,paleolithic man have been found1 especially in cave deposits4 ,that he was an expert hunter is proved by the great accumulation of bones of now extinct animals found in and about his haunts or camps1 bones representing at least #ajj1jjj horses having been found around a single camp site6 ,illustration3 ,fig4 #fh4--;,comparison of skulls3 a1 ,paleolithic 7,neanderthal7 man2 b1 modern man4 7,after ,woodward1 ,british ,museum47 ,only two among the many known ,paleolithic man localities will be briefly described4 ,in the ,perigord district of southwestern ,france a number of caves contain human relics ranging in age from early to late ,paleolithic4 ,of special interest among these relics are fishhooks made of bone1 and crude sketches of animals such as the mammoth and reindeer now extinct in that region4 ,the ,aurignac cave1 also in ,france1 was no doubt a family or tribal burial place4 ,seventeen ,paleolithic human skeletons1 associated with bones of extinct animals and crude art works1 were found in the cave4 ,near the entrance there were ashes and charcoal mixed with burned and split bones of extinct animals4 ,certain of the caves occupied by late ,paleolithic man have their walls decorated with sketches and even colored pictures4 ,these are1 therefore1 the oldest known art galleries4 ,an excellent example is the cave at ,altamira in northern ,spain4 ,as we glance at the pictures one of the first things to impress us is the excellence of the drawing1 the proportions and postures being unusually good4444 ,the next observation may be that1 in spite of this perfection of technique1 there is no perspective composition--that is1 no attempt to combine or group the figures4444 ,it is also clear that the work of many different artists is represented1 covering a considerable period of time4 ,the walls show traces of many other paintings that were erased to make way for new work4 7,wissler47 ,illustration3 ,fig4 #fi4--,sketch of a painting by ,paleolithic man found in a cave in west-central ,france4 ,various animals1 including the extinct mammoth elephant1 are represented4 7,courtesy of ,american ,museum of ,natural ,history47 ,the ,neolithic1 or recent stone age was a gradual development from the late ,paleolithic1 and man was then more highly developed and more similar in structure to modern man4 ,his stone implements were more perfectly made1 and often more or less polished and ground at the edges4 ,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 beds of lakes near shore where they sometimes built their dwellings upon piles4444 ,neolithic man in ,europe had learned to make pottery1 to spin and weave linen1 to hew timber1 and build boats1 and to grow wheat and barley4 ,the dog1 horse1 ox1 sheep1 goat1 and hog had been domesticated4 7,norton47 ,man is linked to the past through the system of life1 of which he is the last1 the completing creation4 ,but1 unlike other species of that closing system of the past1 he1 through his spiritual nature1 is more intimately connected with the opening future4 7,j4 ,d4 ,dana47 ,,chapter ,,xx ,,mineralogy ,we are more or less familiar with the division of all materials of nature into the animal1 vegetable1 and mineral kingdoms4 ,with slight exceptions minerals are the materials which make up the known part of the earth4 ,in a very real sense1 then1 mineralogy is the most fundamental of the various branches of the great science of geology because the events of earth history1 as interpreted by the geologist1 are recorded in the mineral matter 7including most rocks7 of the earth4 ,when we examine the rocky material or mineral matter of the earth in any region we find that it consists of various kinds of substances each of which may be recognized by certain characteristics4 ,each definite substance 7barring those of organic origin7 is called a mineral4 ,or1 more specifically1 a mineral is a natural1 inorganic1 homogeneous substance of definite chemical composition4 ,according to this definition a mineral must be found ready made in nature1 must not be a product of life1 must be of the same nature throughout1 and its composition must be so definite that it can be expressed by a chemical formula4 ,all artificial substances1 such as laboratory and furnace products1 are excluded from the category of minerals4 ,coal is not a mineral because it is both organic and of indefinite composition4 ,a few examples of very common substances which perfectly satisfy the definition of a mineral are quartz1 feldspar1 mica1 calcite1 and magnetite4 ,only two substances--water and mercury--are ordinarily liquid minerals4 ,there are nearly a thousand distinct mineral species1 and to them and their varieties several thousand names have been applied4 ,it is a surprising fact that of the eighty or more chemical elements1 that is substances which cannot be subdivided into simpler ones1 only eight make up more than #ih per cent of the weight of the crust of the earth1 though1 with one very slight exception1 none of the eight exist as such in mineral form4 ,the eight elements are oxygen 7nearly #ej per cent71 silicon 7over #be per cent71 aluminum 7over #g per cent71 iron 7over #e per cent71 calcium 7or lime71 magnesium 7or magnesia71 sodium 7or soda71 and potassium 7or potash74 ,certain rock formations are made up essentially of but one mineral in the form of numerous grains as1 for example1 limestone1 which consists of calcite 7carbonate of lime74 ,most of the ordinary rocks are1 however1 made up of two or more minerals mechanically bound together4 ,thus1 in a specimen of granite on the author's desk several distinct mineral substances are distinguishable by the naked eye4 ,these mineral grains are from one to five millimeters across4 ,most common among them are hard1 clear1 glassy grains called quartz2 nearly white1 hard grains1 with smooth faces1 called feldspar2 small1 silvery white plates1 easily separable into very thin flakes1 called mica2 and small1 hard1 black grains1 called magnetite4 ,it is the business of the mineralogist to learn the characters of each mineral1 how they may be distinguished from each other1 how they may be classified1 how they are found in nature1 and what economic value they may have4 ,it is an important part of the business of the geologist to learn what individual minerals combine to form the many kinds of rocks1 how such rocks originate1 what changes they have undergone1 and what geological history they record4 ,it is thus clear that the great science of geology is much broader in its scope than mineralogy4 ,one of the most remarkable facts about minerals is that most of them by far have a crystalline structure1 that is they are built up of tiny particles known as molecules4 ,such crystalline minerals are often more or less regular solid forms bounded by plane faces and sharp angles1 such forms being known as crystals4 ,how do crystals develop such regularity of form8 ,any solid is considered to be made up of many very tiny 7submicroscopic7 molecules held together by an attractive force called cohesion4 ,in liquids the molecules may more or less freely roll over each other1 thus altering the shape of the mass without disrupting it4 ,in gases the molecules are considered to be relatively long distances apart and moving rapidly4 ,during the process of change of a substance from the condition of a liquid or gas to that of a solid1 due to lowering of temperature or evaporation1 the cohesive force pulls the particles 7molecules7 together into a rigid mass4 ,under favorable conditions such a solid has a regular polyhedral form4 ,this results from the fact that the particles or molecules of the substance which1 while it was liquid or gaseous1 rolled about on one another1 have been in some way arranged1 grouped and built up4 ,to illustrate this1 suppose a quantity of small shot to be poured into a glass3 the shot will represent the molecules of a substance in the liquid state1 as for example a solution of alum4 ,if1 now1 we suppose these same shot to be coated with varnish or glue so that they will adhere to each other1 and imagine them grouped as shown in ,figure #gj;a1 they will represent the arrangement of the molecules of the alum after it has become solid or crystallized4 ,this arranging1 grouping1 and piling up of molecules is called crystallization1 and the solid formed in this way is called a crystal4 ,figures #gj;b and #gj;c show the shot arranged to reproduce two common forms of crystals 7e4g41 fluorite and calcite74 7,whitlock47 ,illustration3 ,fig4 #gj4--,piles of shot arranged to give some idea of the manner in which molecules are bound together in various crystal forms4 7,after ,whitlock1 ,new ,york ,museum47 ,a combination of certain facts regarding crystals furnish all but absolute proof of some sort of regularity of arrangement of particles within them4 ,among such facts are the following3 7#a7 the wonderful regularity of arrangement of faces upon crystals is practically impossible to account for except as the outward manifestation of regularity of structure or systematic network arrangement of the interior2 7#b7 most crystals split or cleave more or less perfectly in one or more directions presumably in accordance with certain layered structure of the constituent particles2 7#c7 all of the many known forms of crystals can be accurately grouped in regard to their effects upon the passage of light 7especially polarized light7 through them1 each kind or type of network structure presumably producing a different effect upon light2 and 7#d7 ,x-ray photographs have proved that particles1 or at least groups of particles1 are very systematically arranged within crystals4 ,it will be instructive for us to make a comparison between the growth of crystals and organisms4 ,both really grow1 but each species of organism is rather definitely limited in size while there is no known limit to the size which may be attained by a crystal so long as material is supplied to it under proper conditions4 ,as a matter of fact crystals vary in size from microscopic to several feet in length1 those less than an inch in length being most abundant by far4 ,organisms mostly grow from within1 while crystals grow from material externally added4 ,it is an astonishing fact that in crystals as well as organisms growth takes most rapidly on a wound or broken place4 ,thus if a crystal is removed from the solution in which it is growing and put back after a corner has been broken off1 the fractured surface will build up more rapidly than the rest4 ,finally1 crystals are not necessarily limited in age like organisms4 ,under certain natural conditions1 as1 for example1 weathering1 crystals may decay or be broken up2 but where they are protected as constituent parts of rock formations well below the earth's surface they may remain unchanged for indefinite millions of years4 ,thus in a ledge of the most ancient known or ,archeozoic rock only recently laid bare by erosion one may see crystals which are precisely as they were when they crystallized many millions of years ago4 ,one of the most remarkable properties of a crystal is its symmetry1 by which is meant the greater or less degree of regularity in the arrangement of its faces1 edges1 and vertices4 ,a given substance may1 according to circumstances1 crystallize in a variety of forms or combinations of forms1 but1 with very few exceptions1 all crystals of a given substance exhibit the same kind or grade of symmetry4 ,there are three kinds of crystal symmetry1 namely1 in respect to a plane1 a line or axis1 and a point or center4 ,a plane of symmetry divides a crystal into halves in such a way that for every point on one side of the plane there is a corresponding point directly opposite on the other side4 ,crystals may be cut into halves along various surfaces which are not symmetry planes4 ,an axis of symmetry is a line about which a complete rotation 7or in a few cases rotation combined with reflection7 brings the crystal into the same relative position two1 three1 four or six times1 these being called two1 three1 four1 and sixfold axes of symmetry--no others being possible4 ,a crystal has a center of symmetry when any line passing through it encounters corresponding points at equal distances from it on opposite sides4 ,there are just #cb classes or combinations of the symmetry elements among crystals and just #bcb definite crystal forms4 ,not only is it demonstrable that no more can exist1 but actual experience with crystals of hundreds of species of minerals has never revealed any more4 ,obviously1 then1 symmetry furnishes us with a very scientific basis of classification of crystals1 all of the #bcb crystal forms constituting the #cb symmetry classes being in turn referable to seven fundamental crystal systems4 ,to bring out the relations of the faces of a crystal and further aid in classification1 prominent1 straight lines or directions passing through the center of a crystal are chosen as crystallographic axes4 ,such axes may or may not coincide with symmetry axes4 ,basing our definitions upon both symmetry axes and crystallographic axes1 the seven systems are as follows3 #a4 ,isometric4 ,there must be at least four threefold axes of symmetry1 while the highest grade symmetry class of the five in the system includes three fourfold1 four threefold1 and six twofold axes of symmetry2 nine planes of symmetry2 and a center of symmetry4 ,there are three interchangeable crystallographic axes at right angles to each other4 ,illustration3 ,fig4 #ga4--;,figures showing1 a1 crystal axes of ,isometric system2 b1 points of emergence of the nine axes of symmetry in a cube of the ,isometric system2 c1 nine planes of symmetry in a cubic crystal4 7,after ,whitlock1 ,new ,york ,state ,museum47 #b4 ,tetragonal4 ,there must be one and only one fourfold symmetry axis1 while the highest of its seven symmetry classes contains also four twofold axes of symmetry2 five planes2 and a center4 ,characterized by three crystallographic axes at right angles to each other1 only two of them interchangeable4 #c4 ,trigonal4 ,characterized by one and only one threefold symmetry axis1 the highest of the five classes having also three twofold axes2 four planes2 and a center4 ,crystallographic axes as for hexagonal4 #d4 ,hexagonal4 ,one and only one sixfold axis of symmetry must be present1 but the highest of the seven classes also has six twofold axes2 seven planes2 and a center4 ,characterized by four crystallographic axes1 one vertical and three interchangeable horizontal axes making angles of #fj degrees with each other4 #e4 ,orthorhombic4 ,there must be no axis of symmetry higher than a twofold and three prominent directions 7i4e41 parallel to important faces7 at right angles to each other1 the highest grade of the three classes having three twofold axes2 three planes2 and a center4 ,there are three noninterchangeable crystallographic axes at right angles4 #f4 ,monoclinic4 ,there is no axis of symmetry higher than a twofold and only two prominent directions at right angles to each other1 the highest of the three classes having one twofold axis2 one plane2 and a center4 ,there are three noninterchangeable crystallographic axes1 only two of which are at right angles4 #g4 ,triclinic4 ,there is no axis of symmetry of any kind1 and there are no prominent directions at right angles4 ,one of the two classes has a center of symmetry only1 and the other no symmetry at all4 ,characterized by three noninterchangeable crystallographic axes1 none at right angles4 ,a fact which should be strongly emphasized is that crystals only1 of all the objects of nature1 can be definitely referred to the above seven systems comprising the #cb classes of symmetry1 and #bcb crystal forms4 ,since there are about #a1jjj mineral species and only #bcb fundamental forms1 it necessarily follows that two or more species may crystallize in the same form within a class1 so that it is not always possible to tell the species of mineral merely by its crystal form4 ,it is1 however1 a remarkable fact that1 where two or more substances crystallize in the same class 7i4e41 show the same grade of symmetry7 each substance almost invariably exhibits crystal habit which is a pronounced tendency to crystallize in certain relatively few forms or combinations of forms out of the many possibilities4 ,it is clear1 then1 that grade of symmetry combined with habit are of great practical value in determining crystallized minerals1 because1 on the basis of symmetry1 a crystal is referred to a certain definite symmetry class in which only a limited number of substances crystallize1 and then1 by its characteristic habit1 the particular substance can be told4 ,illustration3 ,fig4 #gb4--;,figures illustrating three crystal forms with exactly the same symmetry elements2 a and b are separate forms1 and c is a combination of the two4 ,the mineral garnet nearly always crystallizes in one of these forms4 ,from the above discussion it should not be presumed that crystals always develop with perfect geometric symmetry4 ,as a matter of fact such is seldom the case because1 due to variations of conditions or interference of surrounding crystals in liquids 7ordinary or molten71 a crystal usually grows more rapidly 7by building out faces7 in certain directions than in others4 ,under such conditions actual crystals are said to become distorted because they are not geometrically perfect4 ,whether geometrically perfect or not1 all crystals respond to the law of constancy of interfacial angles which means that on all crystals of the same substances the angles between similar 7corresponding7 faces are always equal4 ,this is one of the most fundamental and remarkable laws of minerals4 ,that it must be true follows from the fact that the crystal faces merely outwardly express in definite form the definite internal structure or arrangement of particles which have built up the crystal4 ,in other words1 the real structural symmetry of a crystal never varies no matter how much its geometric symmetry may vary4 ,the practical application of the law of constancy of interfacial angles lies in the fact that in many cases a mineral may actually be identified merely by measuring the interfacial angles of its crystal form4 ,the relative lengths of the crystallographic axes is a very important feature of all crystals except those of the isometric system in which the axes are always of equal length so that the ratio is #a3a3a4 ,in all the other systems1 however1 at least one axis differs in length from the others and1 since the amount of difference is absolutely characteristic of each substance1 the axial ratio of a crystal1 when carefully determined by measurement of the angles between the different faces1 affords a never-failing method of determining the mineral for all systems except the isometric4 ,by way of illustration1 the tetragonal crystal of the mineral zircon1 with only one axis different in length1 shows the very definite axial ratio #a3a3j4fd1 while the orthorhombic crystal of sulphur1 with all three axes of different lengths1 has an axial ratio #j4hac3a3a4ijc4 ,these ratios of course always hold true no matter what the size or particular outward form of the crystal4 ,as might be expected from the above discussion of the remarkable structure of crystals1 experience has proved that the relative lengths of all intercepts 7or distances from the center7 of all faces upon any crystal can be expressed by whole numbers1 definite fractions1 or infinity4 ,it necessarily follows that the ratios between the intercepts of the faces of any face on a crystal to those of any other face on the same crystal may always be expressed by rational numbers1 and this is known as the law of definite mathematical ratio4 ,it is a remarkable fact that very small whole numbers or fractions1 or infinity or zero1 will always express the intercepts of any crystal face4 ,thus far our discussion has centered about crystals as individuals1 but1 in most cases by far1 they form groups or aggregates4 ,most commonly crystal grouping is very irregular1 but by no means rare is parallel grouping where whole crystals1 or more usually parts of crystals1 have all corresponding parts exactly parallel4 ,but most remarkable of all are the twin crystals in which two or more crystals intergrown or in contact have all corresponding parts in exactly reverse order4 ,the conditioning circumstances under which twin crystals develop are unknown4 ,in the light of the facts and principles above explained1 the reader will more than likely agree with the author that crystals rank very high among nature's most wonderful objects4 ,but there are still other characteristic features of crystals naturally resulting from their marvelous structure4 ,some of these will now be briefly referred to4 ,illustration3 ,fig4 #gc4--;,figures illustrating twin crystals3 a1 gypsum 7,monoclinic system72 b1 fluorite 7,isometric system72 c1 cassiterite 7,tetragonal system74 7,after ,new ,york ,state ,museum ,bulletin47 ,many crystals and crystalline substances exhibit the important property known as cleavage which is the marked tendency to break easily in certain directions yielding more or less smooth plane surfaces4 ,as would be expected1 a cleavage surface is always parallel to an actual1 or at least a possible1 crystal face1 and it takes place along the surfaces of weaker molecular cohesion4 ,the degree of cleavage varies from almost perfect1 as in mica1 to very poor or none at all1 as in quartz4 ,the number of cleavage directions exhibited by common minerals is illustrated as follows3 mica1 one2 feldspar1 two2 calcite1 three2 and fluorite1 four4 ,it is a striking fact that when a crystal or cleavage piece is placed in a solvent1 the action proceeds with different velocities in crystallographically different directions and little pits or cavities1 called etching figures1 are developed on some or all of the faces4 ,since the symmetry of these etching figures and their arrangement upon the faces are directly related to1 and natural effects of the crystal symmetry1 the figures often furnish an important method of placing a doubtful crystal or even merely a cleavage fragment in its proper symmetry class4 ,another marvelous property of crystals and crystalline substances is their effect upon light4 ,since the study of the passage of light through crystals has really become a large separate branch of mineralogical study1 we can no more than state a few fundamental facts and principles in the short space at our disposal4 ,light is caused by vibrations of the so-called ether1 and always travels in straight lines4 ,the vibration directions are at right angles to the direction of transmission of the light4 ,when a ray of light enters a crystal or crystalline mineral representing any crystal system except the isometric it is doubly refracted 7i4e41 broken into two rays71 each of the two rays is polarized 7i4e41 made to vibrate in a single plane only71 and one ray vibrates almost at right angles to the other4 ,double refraction is strikingly shown by placing a piece of clear calcite 7,iceland spar7 over a dot on paper when two dots instead of one are visible4 ,the amount of double refraction varies with the substance1 and in some degree according to the direction of passage of light through a crystal4 ,isometric crystals only are singly refracting and hence a ray of light is not affected in passing through them4 ,crystals of all the other six systems doubly refract and polarize light and in three systems--tetragonal1 hexagonal1 and trigonal--one direction 7coincident with the main axis of symmetry7 produces single refraction only1 while in the remaining three systems--orthorhombic1 monoclinic1 and triclinic--there are always two directions of single refraction whose positions vary with the substance4 ,many crystals outside the isometric system also exhibit a remarkable tendency to absorb light differently in different crystallographic directions1 thus producing two or three color tints1 which vary according to the substance4 ,after gaining a practical knowledge of the above and many other optical properties of crystals1 it is possible by the aid of a specially constructed 7polarizing7 microscope1 to recognize 7with few exceptions7 each one of the many mineral species4 ,this method is of great value in determining the various minerals which are aggregated in the form of a rock1 in which case a very thin slice of the rock is studied with the microscope4 ,an important criterion for the recognition of minerals is hardness1 by which is meant the resistance of a smooth surface to abrasion or scratching4 ,the generally adopted scale of hardness follows3 #a4--,soft1 greasy feel1 and easily scratched by the finger nail 7e4g41 talc74 #b4--;,just scratched by the finger nail 7e4g41 gypsum74 #c4--;,just scratched by a copper coin 7e4g41 calcite74 #d4--;,easily cut by a knife1 but does not cut glass 7e4g41 fluorite74 #e4--;,just scratches soft glass1 and is cut by a knife 7e4g41 apatite74 #f4--;,harder than steel1 and scratches glass easily 7e4g41 orthoclase74 #g1 #h1 #i1 and #aj4--;,harder than any ordinary substance and represented in order by quartz1 topaz1 corundum1 and diamond4 ,illustration3 ,plate #ag4--,skeleton of the ,great ,two-,legged1 ,carnivorous ,dinosaur ,reptile1 ,called ,tyrannosaurus1 ,which ,lived ,during ,cretaceous ,time4 7,courtesy ,american ,museum of ,natural ,history47 ,small ,picture4--,restoration of the ,earliest ,known ,bird of ,which ,several ,nearly ,perfect ,skeletons ,have ,been ,found4 ,this feathered creature with reptilian characteristics lived at least #e1jjj1jjj years ago4 ,it had a long vertebrated tail1 claws on the ends of the wings1 and teeth4 7,by ,e4 ,w4 ,berry47 ,plate #ah4--7;a7 ,skeleton of the ,largest ,known ,creature ,that ,ever ,flew4 ,it was a flying reptile with spread wings of nearly twenty-five feet1 and lived during the ,cretaceous period several million years ago4 7,courtesy of the ,american ,museum of ,natural ,history47 ,plate #ah4--7;b7 ,skeleton of a ,remarkable ,swimming ,reptile of the ,mesozoic ,era4 ,length about twelve feet4 ,parts of skeletons of unborn young are seen4 7,courtesy of the ,american ,museum of ,natural ,history47 ,minerals also show a great variety of colors4 ,many of them like quartz and calcite are colorless or white1 others like galena 7steel-gray7 and pyrite 7brass-yellow7 show inherently characteristic colors1 while still others like amethyst 7purple7 and sapphire 7blue7 are colored by impurities4 ,there is also a great range in relative weights or density of minerals1 commonly called the specific gravity1 which range from less than one for ice to #ba4e for platinum1 and even somewhat higher4 ,the average specific gravity of all minerals of the earth is about #b4f4 ,in the light of the above discussion of the general properties of minerals1 we shall now proceed to name and briefly describe some of the minerals which are either very common1 or of special interest1 or of special economic importance4 ,only those features are listed by which the mineral species may be recognized at sight1 or by the aid of very simple nonchemical tests4 ,illustration3 ,fig4 #gd4--;,drawings showing forms of crystals of common minerals3 a and b1 garnet 7,isometric72 c and d1 feldspars 7,monoclinic72 e1 f1 and g1 quartz 7,trigonal72 h1 i1 and j1 calcite 7,hexagonal72 k1 augite 7,monoclinic72 l4 hornblende 7,monoclinic72 m1 pyrite 7,isometric74 ,amphibole4 ,a number of species closely related in composition1 crystal form1 and properties are here included4 ,they are silicates of lime and magnesia usually with aluminum and iron4 ,most common by far are those which crystallize in the monoclinic system with prismatic faces and two good prismatic cleavages meeting at about #bd degrees4 ,color1 commonly brown to black1 but sometimes green or white4 ,hardness varies from #e to #f1 and specific gravity from #c to #c4d4 ,hornblende1 the most common species1 is a dark colored silicate of lime1 magnesia1 aluminum1 and iron4 ,it is one of the few most common of all mineral species1 especially in igneous and metamorphic rocks4 ,tremolite is a white to light gray silicate of lime and magnesia found especially in metamorphic limestones4 ,actinolite is a green silicate of lime1 magnesia1 and iron especially common in certain metamorphic rocks4 ,one kind of jade is an amphibole similar to tremolite and actinolite in composition1 while the other kind is a pyroxene 7see below74 ,jade is and has been highly prized in the east 7especially in ,china7 where it has been carved into many objects of exceptional variety and beauty4 ,jade is probably the toughest 7not hardest7 of all minerals because of its wonderful microscopically fibrous structure4 ,in color it is white1 gray1 and green4 ,apatite4 ,crystallizes in the hexagonal system with a six-sided prism usually capped at each end by a six-sided pyramid 7see ,figure #ge;g74 ,composition1 a phosphate of lime4 ,color variable1 but mostly white1 green1 or brown4 ,hardness of #e1 or just enough to scratch soft glass4 ,specific gravity1 #c4b4 ,no good cleavage4 ,tiny crystals are widely disseminated through many common rocks--igneous1 metamorphic1 and sedimentary4 ,in certain metamorphic limestones excellent crystals a foot or more in length have been found4 ,apatite1 mostly in uncrystallized form1 is the source of most of our phosphate fertilizers4 ,azurite4 ,an azure-blue hydrous carbonate of copper which crystallizes commonly in small monoclinic crystals4 ,hardness1 nearly #d1 and specific gravity1 nearly #d4 ,commonly occurs in veins deposited by underground water4 ,one of the great ores of copper1 especially in ,arizona1 ,chile1 and ,australia4 ,barite4 ,a sulphate of barium crystallizing in orthorhombic prisms usually of tabular habit4 ,white to light color shades4 ,hardness1 #c4e2 specific gravity1 #d4e1 which is notably higher than the average of light-colored minerals4 ,three good cleavages parallel to principal crystal faces4 ,a common and widely distributed mineral1 especially in many vein deposits associated with certain ores4 ,used in ground form to give weight to certain kinds of paper and cloth1 and a barium compound used for refining sugar is made from it4 ,illustration3 ,fig4 #ge4--;,crystal forms cf common minerals3 a1 galena 7,isometric72 b1 sphalerite 7,isometric72 c1 beryl 7,hexagonal72 d1 hematite 7,hexagonal72 e1 magnetite 7,isometric72 f1 barite 7,orthorhombic72 g1 apatite 7,hexagonal72 h4 sulphur 7,monoclinic72 i1 gypsum 7,monoclinic72 j1 chalcopyrite 7,tetragonal72 k1 fluorite 7,isometric72 l1 zircon 7,tetragonal72 m1 tourmaline 7,trigonal72 n1 corundum 7,hexagonal74 ,beryl4 ,a silicate of aluminum and the rare chemical element beryllium4 ,hexagonal crystals usually of very simple six-sided prismatic habit 7see ,figure #ge;c74 ,color white1 green1 blue1 or yellow4 ,specific gravity1 #b4h4 ,cleavage practically absent4 ,it is a very exceptionally hard mineral1 being #h in the scale4 ,very large crystals have been found1 as1 for example1 in ,new ,hampshire1 where single crystals several feet long weigh a ton or more4 ,beryl is also of special interest because two of its varieties--emerald 7green7 and aquamarine 7blue7--are well-known gem stones1 the emerald being one of the most highly prized gems4 ,the colors are due to slight impurities4 ,beryl most commonly occurs in dikes of coarse granite called pegmatite1 but also in certain metamorphic and sedimentary rocks4 ,calcite4 ,commonly called calc spar4 ,a carbonate of lime4 ,hexagonal crystals in a great variety of forms1 but all with crystal faces arranged in sixes around the principal or vertical axis forming rhombohedrons1 prisms1 or double-pointed pyramids4 ,the principal axis of symmetry is sixfold by a combination of rotation and reflection4 ,very perfect cleavages in three directions yielding fragments whose faces make angles of #ge and #aje degrees4 ,color1 white when pure1 but variously colored when impure4 ,hardness1 #c 7very easily scratched by a knife72 specific gravity1 #b4g4 ,calcite is a very common mineral1 especially in limestone 7including chalk7 and marble which are usually largely made up of it4 ,also commonly found in veins1 and as spring and cave deposits 7stalactites74 ,a porous1 stringy variety1 called travertine1 is deposited by certain hot springs1 as at ,mammoth ,hot ,springs in ,yellowstone ,park4 ,a very transparent crystalline variety is called ,iceland spar4 ,calcite is a very useful mineral4 ,limestone and marble are widely used as a building stone1 and for decorative purposes1 statuary1 etc4 ,limestone is burned for quicklime1 used as a flux in smelting certain ores1 in glass making1 etc4 ,cassiterite4 ,the one great ore of tin whose composition is oxide of tin4 ,tetragonal crystallization 7,figure #gc;c74 ,hardness greater than steel1 being over #f in the scale4 ,specific gravity #g1 which is notably high4 ,color1 brown to nearly black4 ,cleavage1 practically absent4 ,fairly widespread in small amounts1 and in commercial quantities in only a few localities1 usually in veins in granite or metamorphic rocks near granite1 as at ,cornwall1 ,england1 also in the form of rounded masses in gravel deposition as in the ,malay region4 ,chalcocite4 ,crystallizes in the orthorhombic system1 usually in tabular form1 but crystals not common4 ,a black sulphide of copper with metallic luster4 ,hardness1 nearly #c2 specific gravity1 nearly #f4 ,no cleavage4 ,chalcocite occurs in vein deposits as one of the important copper ores1 especially at ,butte1 ,montana4 ,chalcopyrite4 ,known as copper pyrites1 7,figure #ge;j74 ,a deep brass-yellow sulphide of iron and copper4 ,seldom crystallized in tetragonal forms4 ,hardness1 #c4e2 specific gravity1 over #d4 ,no cleavage4 ,metallic luster4 ,widely distributed in vein deposits associated with other metal-bearing minerals4 ,a very important ore of copper1 especially at ,rio ,tinto1 ,spain4 ,chlorite4 ,a soft1 green mineral1 usually in small tabular crystals1 in general appearance much like mica 7see below71 but unlike mica1 the almost perfect cleavage leaves are not elastic1 though they are flexible4 ,composition1 a silicate of aluminum and magnesia4 ,always of secondary origin as a result of chemical alteration of certain other minerals1 such as biotite-mica1 pyroxene or amphibole4 ,cinnabar4 ,a vermilion-red sulphide of mercury4 ,an extra soft metallic mineral1 only #b4e in the scale4 ,specific gravity over #h1 which is notably high4 ,completely vaporizes on being heated4 ,small trigonal crystals rare4 ,cinnabar is the one great ore of mercury1 occurring in veins1 especially in ,california and ,spain4 ,copper4 ,copper as such 7so-called native copper7 is widely distributed in veins1 usually in small amounts with other copper minerals1 but in the great mines of northern ,michigan it occurs in immense quantities as the only important ore4 ,it is readily recognized by its color1 softness 7less than #c71 and notable weight 7specific gravity1 nearly #i74 ,isometric crystals uncommon4 ,corundum4 ,an oxide of aluminum of hexagonal crystallization1 usually in six-sided prisms1 capped by very steep pyramidal faces 7see ,figure #gen74 ,it is next to the hardest of all known minerals 7#i in the scale71 the diamond only exceeding it4 ,specific gravity about #d4 ,three good cleavages making angles of nearly #ij degrees with each other4 ,the color of corundum is usually brown1 but it varies greatly4 ,two of the most highly prized of all precious stones--ruby 7red7 and sapphire 7blue7--are nearly transparent varieties of corundum1 colored by certain impurities4 ,oriental topaz 7yellow71 oriental emerald 7green71 and oriental amethyst 7purple7 are also clear varieties of corundum4 ,it occurs in various igneous and metamorphic rocks1 and in some stream gravels4 ,the finest rubies1 associated with some sapphires1 occur in gravels in ,burma1 ,siam1 and ,ceylon4 ,emery is a fine-grained mixture of corundum and other minerals1 especially magnetite4 ,diamond4 ,this mineral is remarkable not only because it is the king of precious stones1 but also because it is easily the hardest known substance 7#aj in the scale74 ,specific gravity1 #c4e4 ,very brilliant luster4 ,crystals of usually octahedral habit in the isometric system4 ,usually colorless1 but often variously tinted4 ,composition1 pure carbon4 ,burns completely away at high temperature4 ,the greatest mines in the world are in ,south ,africa1 where the diamonds occur in masses of rather soft 7decomposed7 igneous rock1 evidently having crystallized during the cooling of the molten masses4 ,in ,brazil and ,india diamonds are found in stream gravels4 ,feldspar ,group4 ,the feldspars are by far the most abundant of all minerals in the crust of the earth4 7,figures #gd;c1 #gd;d47 ,there are several important species or varieties of feldspar with certain features in common as follows3 crystal forms1 either monoclinic or triclinic 7closely resembling monoclinic71 in prismatic forms whose faces usually meet at or near #ij or #abj degrees2 two good cleavages at or near #ij degrees1 hardness at or near #f2 specific gravity1 a little over #b4e2 color1 usually white1 gray1 or pink2 and composition1 silicate of aluminum with potash1 soda1 or lime4 ,the two potash feldspars are orthoclase and microcline1 the former being monoclinic1 with cleavages at exactly #ij degrees1 and the latter triclinic1 with cleavages a little less than #ij degrees4 ,a kind of green microcline is known as ,amazon stone4 ,the soda-lime feldspars go by the general name plagioclase4 ,they are triclinic1 with cleavages meeting at approximately #hf degrees4 ,very commonly one of the cleavage faces exhibits characteristic1 well defined striations or fine parallel lines caused by multiple twinning during crystal growth4 ,some of the common plagioclases are albite1 a white soda feldspar1 including most so-called moonstone2 oligoclase1 a usually greenish-white to reddish-gray soda-lime feldspar including sunstone2 and labradorite1 a lime-soda feldspar1 usually gray to greenish-gray with a beautiful play of colors4 ,the feldspars occur in all three great groups of rocks1 but they have most commonly crystallized during the cooling of molten masses of igneous rocks4 ,where many sedimentary rocks have undergone great change 7metamorphism7 under conditions of heat1 pressure1 and moisture1 feldspars have very commonly formed4 ,orthoclase and microcline feldspar are used in the manufacture of porcelain and chinaware4 ,some special varieties of feldspar are cut or polished for semiprecious stones or decorative purposes4 ,fluorite4 ,a common mineral whose composition is fluoride of lime4 7,figure #gc;b47 ,isometric crystals1 usually cubes with edges modified1 are common4 ,twinned cubes are also common4 ,easily scratched by a knife 7hardness1 #d71 and specific gravity a little over #c4 ,clear and colorless when pure1 but variously colored1 especially green1 blue1 yellow1 and brown1 due to impurities in solution during crystallization4 ,remarkable because of its four good cleavages meeting at such angles as to permit good cleavage octahedrons to be broken out of crystals4 ,fluorite is widely distributed1 most commonly in vein deposits1 often associated with metallic ores4 ,occurs also as crystals in some limestones and igneous rocks4 ,some fissure veins of fluorite in limestone in southern ,illinois are twenty to forty feet wide4 ,used mostly as a flux in the manufacture of certain steel1 in glass making1 and in making enamel ware4 ,galena4 ,commonly as isometric crystals either as cubes or combinations of cubes and octahedrons4 ,composition1 sulphide of lead4 7,figure #ge;a47 ,color1 lead-gray with metallic luster4 ,hardness1 #b4e2 specific gravity high1 #g4e4 ,very brittle4 ,three excellent cleavages at right angles and parallel to the crystal faces of the cube4 ,nearly all of the lead of commerce comes from the smelting of galena4 ,it is mined in many parts of the world where it nearly always occurs in typical vein deposits often associated with sphalerite 7see below74 ,garnet ,group4 ,the members of this very interesting mineral group very commonly occur in isometric crystallized forms1 mostly twelve and twenty-four faced figures or both combined1 as shown by ,figure #gb4 ,all the six species of garnets are silicates1 mostly of aluminum usually with either lime1 magnesia1 or iron4 ,cleavage1 very imperfect or absent4 ,hardness great1 #f4e to #g4e1 and specific gravity #c4a to #d4c1 varying according to species4 ,color also varies with composition1 but most commonly red1 brown1 and more rarely yellow1 black1 and green4 ,garnets are most common as crystals embedded in metamorphic rocks1 especially highly altered strata4 ,also occurs in many igneous rocks and in some sands4 ,commonly used as a semiprecious stone1 and also ground for use as an abrasive1 especially in making a kind of sand 7or garnet7 paper4 ,gold4 ,gold as such 7native gold7 is1 in small amounts1 really a very widely distributed mineral4 ,it is characterized by its yellow color1 softness 7less than #c in the scale71 great weight 7specific gravity1 over #ai71 and extreme malleability4 ,most of the commercial gold occurs in river gravels 7so-called placer deposits71 and in veins associated with the very common mineral quartz4 ,graphite4 ,commonly called black lead1 but it is not lead at all4 ,its composition is pure carbon--the same as that of the diamond4 ,we here have a very remarkable example of a single substance 7carbon7 which1 according to circumstances1 crystallizes in two distinctly different systems 7diamond in isometric1 and graphite in hexagonal7 yielding very thin1 flexible flakes2 greasy in feel2 and easily rubs off on paper4 ,it weighs less than the average mineral 7specific gravity1 a little over #b74 ,good crystals of hexagonal tabular form are rare4 ,the most natural home of graphite is in the metamorphic rocks1 especially certain of the highly altered strata1 where it occurs in the form of more or less abundant flakes1 having originated from organic matter4 ,some also occurs in igneous rocks and in veins4 ,large quantities are made at ,niagara ,falls from anthracite by electricity4 ,gypsum4 ,monoclinic crystals common1 usually of simple forms1 as shown by ,figure #ge;i4 ,sometimes twin crystals4 ,composition1 sulphate of lime4 ,colorless or white when pure4 ,can be scratched by the finger nail 7hardness1 #b74 ,specific gravity1 #b4c4 ,three good cleavages1 especially the prismatic1 yielding cleavage plates with angles of #ff and #aad degrees4 ,thin cleavage layers1 moderately flexible4 ,there are several varieties3 7#a7 selenite1 which is clear1 crystalline2 7#b7 satin spar1 fibrous with silky luster2 7#c7 alabaster1 fine-grained and compact crystalline2 and 7#d7 rock gypsum1 massive granular or earthy4 ,gypsum is common and widespread especially among stratified rocks often as thick beds which have mostly resulted from evaporation of bodies of water containing it in solution1 and often associated with salt beds4 ,also occurs as scattering crystals in shales and clays1 and in some veins4 ,in greatest quantities it is burned to make plaster of ,paris4 ,satin spar and alabaster are often cut and polished for ornaments1 etc4 7,see ,figure #gc;a47 ,halite4 ,common salt4 ,composition1 chloride of soda4 ,isometric crystals1 nearly always in cubes with three good cleavages at right angles1 and parallel to the faces of the cube4 ,hardness1 #b4e2 specific gravity1 #b4e4 ,colorless to white when pure4 ,characteristic salty taste4 ,abundant and widespread1 often as extensive strata in rocks of nearly all ages1 having resulted from evaporation of inland bodies of salt water4 ,also in vast quantities in solution in salt lakes and the sea4 ,halite has many uses1 as for example1 cooking and preservative purposes1 indirectly in glass making and soap making1 glazing pottery1 and in many ore-smelting and chemical processes4 ,hematite4--,one of the common and important iron oxides with less iron than magnetite and no water as has limonite4 ,crystallizes in hexagonal forms4 ,color1 black1 with metallic luster1 when crystalline1 otherwise usually dull red4 ,hardness1 about #f2 specific gravity1 about #e4 ,no cleavage4 ,red streak when rubbed on rough porcelain4 ,hematite is extremely widespread in rocks of all ages1 especially in metamorphic and sedimentary rocks4 ,some occurs as crystals in igneous rocks1 and some in vein deposits4 ,it is the greatest ore of iron in the ,united ,states1 especially in ,minnesota1 ,michigan1 ,wisconsin1 and ,alabama4 ,kaolin4 ,commonly called ,china clay4 ,composition1 a hydrous silicate of aluminum4 ,crystallizes in scalelike monoclinic forms1 but usually forms compact claylike masses4 ,hardness1 a little over #b2 specific gravity1 #b4f4 ,color when pure1 white4 ,usually feels smooth and plastic4 ,very abundant and widespread1 especially forming the main body of clay and of much shale4 ,always of secondary origin1 generally resulting from the decomposition of feldspar4 ,it is the main constituent of chinaware1 pottery1 porcelain1 tiles1 bricks1 etc4 ,limonite4 ,an important oxide of iron in composition like hematite except for its variable water content4 ,never crystallized4 ,hardness1 about #e2 specific gravity1 nearly #d4 ,color1 light to dark brown to nearly black4 ,leaves a characteristic yellowish-brown streak when rubbed on rough porcelain4 ,exceedingly common and widely distributed1 always as a mineral of secondary origin as a product of weathering of various iron-bearing minerals4 ,where accumulated in considerable deposits it is an iron ore of some importance4 ,magnetite4 ,one of the three important oxides of iron containing no water1 and richer in iron than hematite4 7,see ,figure #ge;e47 ,commonly crystallizes in isometric octahedral forms alone or combined with twelve-faced forms4 ,hardness1 #f2 specific gravity1 #e4 ,color1 black with metallic luster4 ,leaves black streaks on rough porcelain4 ,characteristically highly magnetic4 ,wide-spread as crystals in nearly all kinds of igneous rocks1 and as large segregation masses in certain igneous rocks4 ,also very common in metamorphic rocks1 in many cases forming lenses and beds as ore deposits4 ,occurs in some strata and sands4 ,it is an important ore of iron4 ,malachite4 ,a light-green hydrous carbonate of copper4 ,in almost every way1 except difference in color and slight difference in composition1 it is very much like azurite 7see above74 ,mica ,group4 ,the micas rank high in abundance among the most common minerals of the earth4 ,all of the several species are silicates of aluminum combined with other chemical elements according to the species4 ,all crystallize in monoclinic six-sided prisms whose angles are nearly #abj degrees4 ,these prisms closely approach true hexagonal forms4 ,all are characterized by one exceedingly good cleavage at right angles to the prismatic faces1 yielding very thin elastic cleavage sheets4 ,hardness1 #b to #b4e2 specific gravity1 #b4g to #c4 ,the various species or varieties are not always sharply separated from each other4 ,most common are3 muscovite1 or so-called isinglass1 a potash mica which is colorless and transparent in thin sheets when pure2 biotite1 an iron-magnesia mica1 black to dark green2 and phlogopite1 a brown magnesia mica4 ,olivine4 ,often called chrysolite4 ,a silicate of iron and magnesia4 ,orthorhombic crystals1 usually in stout prismatic form4 ,color1 usually yellowish green4 ,hardness1 nearly #g2 specific gravity1 #c4c4 ,transparent to translucent4 ,no real cleavage4 ,its hardness1 color1 and crystal form generally characterize it4 ,it is a fairly common mineral found mainly as crystalline grains in certain dark-colored igneous rocks4 ,a clear green variety1 called peridot1 is used as a gem stone4 ,opal4 ,an oxide of silicon1 like quartz in composition except that it is combined with a varying amount of water4 ,it never crystallizes1 probably because of its rather indefinite composition4 ,hardness #e4e to #f4e 7softer than quartz72 specific gravity1 about #b4 ,varieties variously colored4 ,common opal1 usually translucent with greasy luster4 ,precious opal1 translucent with beautiful play of colors1 used as a gem4 ,fire opal1 with bright red to orange internal reflections4 ,hyalite1 colorless and transparent in small rounded masses4 ,wood opal1 wood petrified by opal4 ,geyserite1 a white1 porous1 stringy variety deposited by certain hot springs like the ,yellowstone geysers4 ,tripolite1 fine-grained1 chalklike in appearance1 consisting of tiny siliceous shells of very simple plants called diatoms4 ,platinum4 ,this mineral occurs as an impure native metal1 usually alloyed with certain other metals4 ,native platinum1 hardness1 #d4e 7exceptionally high for a metal72 specific gravity as usually alloyed1 #ad to #ai4 ,pure platinum1 specific gravity1 over #ba1 or one of the very heaviest known substances4 ,color1 light steel-gray1 with metallic luster4 ,very malleable and ductile4 ,a rare metal found commercially mostly in gravel or placer deposits mostly in the ,ural ,mountains1 also as grains in certain dark igneous rocks4 ,used for many scientific instruments1 in the electrical industry1 as jewelry1 etc4 ,pyrite4 ,commonly called iron pyrites4 ,sometimes called fool's gold4 7,see ,figure #gdm47 ,a sulphide of iron which commonly crystallizes in the isometric system mostly as cubes1 twelve-faced pyritohedrons1 octahedrons1 or combinations of these4 ,color1 light brass-yellow1 with metallic luster4 ,cleavage1 practically absent4 ,hardness1 greater than that of steel 7over #f in the scale72 specific gravity1 about #e4 ,leaves greenish black streak when rubbed on rough porcelain4 ,differs from chalcopyrite by paler color and much greater hardness4 ,it is a common and very widely disseminated mineral in rocks of all kinds and ages1 but especially in metamorphic rocks as veins1 and banded or lenslike deposits4 ,most igneous rocks contain small scattering grains of pyrite4 ,many deposits of commercial value are known4 ,great quantities are burned for the manufacture of sulphuric acid 7oil of vitriol7 which is one of the most important of all chemicals4 ,pyroxene ,group4 ,along with quartz and feldspars1 the pyroxenes rank among the most common of all minerals4 7,see ,figure #gdk47 ,composition1 very similar to amphibole 7see above74 ,pyroxenes crystallizing in the monoclinic system are the most important4 ,these crystals are prismatic in habit1 with prism faces making angles of nearly #de or #ij degrees instead of about #abd degrees as in the monoclinic amphiboles which the monoclinic pyroxenes greatly resemble4 ,two fairly good prismatic cleavages cross at an angle of nearly #ij degrees1 instead of at about #abd degrees as in the monoclinic amphiboles4 ,hardness1 #e to #f2 specific gravity1 #c4b to #c4f4 ,color1 variable according to species4 ,the most common variety of pyroxene is augite1 a dark-green to black silicate of aluminum1 iron1 lime1 and magnesia4 ,certain pyroxenes also crystallize in the orthorhombic system4 ,pyroxene is most abundantly represented as crystals in many kinds of igneous and metamorphic rocks4 ,it is practically useless except as one kind of jade4 ,quartz4 ,next to the feldspars1 quartz is probably the most common of all minerals1 especially at and near the earth's surface4 7,see ,figures #gd;e1 #gd;f1 and #gd;g47 ,composition1 oxide of silicon4 ,often crystallizes in the trigonal system almost always as six-sided prisms capped by six-sided pyramids1 which are really combined three-sided forms1 often with alternate corners modified by small faces4 ,these small modifying faces1 etching figures1 and microscopic tests show that quartz is really trigonal in spite of the common occurrence of simple six-sided outward forms4 ,the pyramidal faces make different angles than those of either apatite or beryl1 both of which are somewhat like quartz in crystal form4 ,hardness1 #g 7distinctly high1 cannot be scratched by the knife72 specific gravity1 #b4f 7about average for all minerals74 ,cleavage1 practically absent1 and breaks like glass4 ,colorless when pure1 but varieties exhibit many colors4 ,a few only of the many varieties will be briefly described4 ,among the distinctly crystalline varieties are3 rock crystal1 pure colorless2 amethyst1 purple2 rose quartz1 pink2 milky quartz1 white2 and smoky quartz1 dark--due to tiny inclusions of carbon4 ,among the fine-grained1 compact more or less indistinctly crystalline or noncrystalline varieties1 usually translucent with a waxy luster1 are3 chalcedony1 bluish gray1 waxy looking1 usually in small rounded masses2 carnelian1 red2 prase1 green2 agate1 with parallel bands1 usually variously colored2 flint and jasper1 opaque to translucent1 dark to red4 ,quartz is exceedingly abundant in all the great groups of rocks4 ,it constitutes the main bulk of sandstones1 is common in shales1 and occurs in certain other strata4 ,in many igneous rocks1 like granite1 it is a very prominent constituent4 ,most of the metamorphic rocks contain its crystalline forms in greater or less amounts4 ,quartz is the most common of all vein minerals1 in many cases associated with valuable ores4 ,various varieties are widely used for ornamental purposes4 ,used in making sandpaper1 glass1 porcelain1 mortar1 concrete1 and in certain ore-smelting processes4 ,sandstone is widely used as a building stone4 ,serpentine4 ,a hydrous silicate of magnesia never in distinct crystals as such1 but shown to be monoclinic under the microscope4 ,hardness variable1 #b4e to #e2 specific gravity1 about #b4f4 ,mostly of variegated green or yellowish green color with waxy luster1 except a fibrous variety 7asbestos7 which is light green to white4 ,the fibrous variety of serpentine is the principal source of asbestos1 an amphibole asbestos being less common4 ,ordinary serpentine 7sometimes miscalled green marble7 is widely used as a building and decorative stone4 ,serpentine is common and widespread1 especially in igneous and metamorphic rocks1 but never as a really original mineral4 ,it always results from alteration of certain other magnesia-bearing silicate minerals1 such as pyroxene1 amphibole1 olivine1 etc4 ,silver4 ,native silver is not a very rare mineral and it is mined in certain parts of the world1 but most of the metal is obtained from certain silver-bearing minerals1 especially sulphides and a chloride4 ,silver crystallizes rather rarely in the isometric system4 ,more commonly it occurs as irregular masses1 plates1 and wirelike forms4 ,characterized by its color1 metallic luster1 softness 7less than #c in the scale71 and exceptional weight 7specific gravity1 #aj4e74 ,usually occurs in vein deposits1 commonly associated with other metals or metal-bearing minerals1 especially copper4 ,sphalerite4 ,a sulphide of zinc commonly in crystalline form belonging in the isometric system1 especially in tetrahedral combination forms 7see ,figure #ge;b74 ,color1 usually brown1 yellow or nearly black with resinous luster4 ,hardness1 nearly #d2 specific gravity1 #d4 ,several good cleavages1 yielding fragments whose faces meet at #ij and #abj degrees4 ,sphalerite is a fairly common and widespread mineral1 occurring nearly always in veins in most kinds of rocks4 ,it is very often associated with other ores1 particularly the great ore of lead 7galena74 ,sphalerite is by far the greatest ore of zinc4 ,sulphur4 ,native sulphur4 ,crystallization1 orthorhombic1 usually in combination pyramidal forms4 7,see ,figure #ge;h47 ,characterized by yellow color1 resinous luster1 softness 7about #b in the scale71 low specific gravity 7about #b71 and very poor cleavages4 ,it has most commonly resulted from alteration of certain sulphur-bearing minerals1 especially gypsum1 the decomposition of which has yielded vast deposits4 ,some also of volcanic origin4 ,great quantities are used in making sulphuric acid1 matches1 gunpowder1 fireworks1 and for vulcanizing and bleaching rubber goods4 ,talc4 ,often called steatite4 ,monoclinic crystals rare4 ,one perfect cleavage1 yielding very thin1 flexible leaves4 ,very soft 7hardness1 #a74 ,feels greasy1 and looks waxy to pearly4 ,color1 white1 gray1 to light green4 ,specific gravity1 #b4h4 ,composition1 a hydrous silicate of magnesia1 much like that of serpentine4 ,talc is always of secondary origin1 generally derived by chemical alteration of various common minerals rich in silicate of magnesia4 ,soapstone is a common variety resulting from alteration of whole rock masses4 ,soapstone has many practical uses as for washtubs1 table tops1 electrical switchboards1 hearthstones1 stove and furnace linings1 blackboards1 gas tips1 etc4 ,talc proper is used as a lubricant1 to weight paper1 in soap1 as dustless crayon1 talcum powder1 etc4 ,topaz4 ,a silicate of aluminum and fluorine4 ,orthorhombic crystals common1 usually prisms capped at one end by pyramided faces and abruptly terminated at the other4 ,colorless when pure1 but often variously colored due to impurities4 ,very exceptionally hard 7#h in the scale72 specific gravity1 #c4e4 ,one good cleavage across the prism zone2 usually found as crystals in1 and in cavities in1 igneous rocks4 ,appears always to have formed from highly heated vapors or liquids given off by cooling molten rock masses4 ,topaz is one of the more highly prized of the gem stones4 ,tourmaline4 ,composition1 very complex1 but chiefly a silicate of boron and several metals and semimetals4 ,commonly as crystals in the trigonal system in both long and short prismatic forms1 as shown by ,figure #gem1 with opposite ends not unlike4 ,extra hard 7#g in the scale72 specific gravity1 about #c4 ,color1 widely various1 but brown and black are most common4 ,practically no cleavage4 ,tourmaline probably always originated as a high temperature mineral1 especially as crystals in granites and related rocks and in certain metamorphic rocks which have been subjected to high temperature and pressure4 ,certain transparent colored varieties of tourmaline rank high among the semiprecious stones4 ,turquoise4 ,a hydrous phosphate of aluminum4 ,massive noncrystalline1 blue to green1 waxy luster1 mostly opaque1 hardness of #f1 and specific gravity of about #b4g4 ,turquoise is a high temperature mineral found in veins and cavities in certain igneous rocks4 ,it is a rare mineral used as a gem stone4 ,zircon4 ,a silicate of zirconium usually crystallized in the tetragonal system as simple four-sided prisms capped by four-sided pyramids4 7,see ,figure #gel47 ,very poor cleavages4 ,color usually brown4 ,hardness1 #g4e 7extra high72 specific gravity1 nearly #d4g4 ,brilliant luster4 ,zircon is very commonly present as scattering crystals of varying size in most igneous rocks4 ,also common as crystals in various metamorphosed stratified rocks1 and less common in some sand and gravel deposits4 ,certain transparent varieties1 especially the brown and pink ones called hyacinth1 are used as gem stones4 ,zircon is also the source of oxide of zirconium used in making mantles for certain incandescent lights4 ,,chapter ,,xxi ,,economic ,,geology ,in this chapter it is our purpose to briefly consider geology in its direct relations to the arts and industries4 ,when we realize that the value of strictly geologic products taken from the earth each year in the ,united ,states alone amounts to billions of dollars1 we can better appreciate the practical application of geological science4 ,such products include coal1 petroleum1 natural gas1 many valuable metal-bearing minerals1 and many nonmetalliferous minerals and rocks4 ,in most cases these valuable products of nature have been slowly accumulated or concentrated at many times and under widely varying conditions throughout the millions of years of known geological time4 ,to trace the extent of1 and most advantageously remove1 such deposits for the use of man is always invariably impossible unless geological knowledge is brought to bear4 ,in many cases the problems involved are intricate1 and only the trained geologist is able to at all successfully cope with them4 ,in such cases it is necessary not only to have a thorough knowledge of minerals and rocks as such1 but also of their origin and structure4 ,much of the practical application of geology is carried out by the mining engineer who should have1 above all1 a thorough knowledge of the great principles of geology4 ,our plan of discussion is to consider1 first1 coal1 petroleum1 and natural gas2 then the most important metalliferous deposits of ores2 and finally nonmetalliferous minerals and rocks of exceptional commercial importance4 ,underground waters have already been discussed from the practical standpoint in the chapter on ,waters ,within the ,earth4 ,certain minerals have already been sufficiently considered from the economic standpoint in the chapter on ,mineralogy4 ,,coal1 ,,petroleum1 ,,and ,,natural ,,gas ,coal4 ,most valuable of all geological products is coal4 ,although it is not1 strictly speaking1 a mineral1 both because of its organic origin and lack of definite chemical composition1 coal is generally classed among our mineral resources4 ,some idea of the national importance of coal in the ,united ,states may be gained when we realize that the energy derived from a single year's output is equivalent to that of hundreds of millions of men working full time through the year4 ,the uses of coal are too well known to need mention here4 ,coal is1 beyond question1 of organic 7plant7 origin as shown by its very composition2 perfect gradations between plant deposits like peat and true coal2 and the presence of microscopic plant remains and spores in the coal4 ,an excellent summary of just what happens during the transition of ordinary vegetable matter into coal has been given by ,d4 ,white as follows3 ,all coal was laid down in beds analogous to the peat beds of to-day4 ,all kinds of plants1 especially such species as were adapted to the particular region where the deposit was located1 in whole or in part went into the deposit4 ,plants are composed chiefly of cellulose and proteins4 ,the former1 comprising by far the larger bulk1 constitute the framework1 whereas the latter are concerned in the vital functions4 ,with these are associated many other substances1 among which are chiefly starch1 sugars1 and fats and oils1 constituting reserve foodstuffs2 waxes1 resin waxes1 resins1 and higher fats1 performing mainly protective functions4444 ,these components differ widely in their resistance to various agencies4 ,those substances involved in the life function and the support of the plant are relatively very stable under the conditions imposed upon them4 ,at the death of the plants1 governed by conditions imposed in the bog1 a partial decomposition1 maceration1 elimination1 and chemical reduction begins1 brought about by various agencies1 chiefly organic1 mainly fungi at first and bacteria later4 ,the most labile are removed first1 the more resistant next1 and so on1 as the conditions require1 leaving the most resistant behind in a residue called peat4 ,the process of decomposition1 elimination1 and chemical reduction begun in peat1 chiefly by biochemical means1 is taken up and continued by dynamochemical means into and through the various successive later stages1 and results in the various grades of coal1 as lignite1 sub-bituminous1 and cannel coal1 and anthracite4 ,the principal chemical elements involved in the changes which take place are carbon1 oxygen1 and hydrogen1 as shown by the following analyses of about average samples of each member of the so-called coal series4 ,the coal series ,carbon ,oxygen ,hydrogen ,nitrogen ---------------------------------------- -------------------- ,wood 7cellulose7 #ej #dc #f #a ,peat #ei #cc #f #b ,lignite #fi #be #e4e #j4h ,bituminous coal #hb #ac #e #j4h ,anthracite coal #ie #b4e #b4e trace ,graphite #ajj 44 44 44 ---------------------------------------- -------------------- ,from this table it is seen that the oxygen relatively diminishes while the carbon relatively increases1 though1 of course1 all three elements actually decrease during the chemical change from cellulose to coal4 ,these three elements disappear mainly in the form of gases1 such as water vapor1 marsh gas1 and carbonic acid gas4 ,the final or graphite stage is almost reached by the graphitic anthracite of ,rhode ,island1 which is so nearly pure carbon as to be really useless as coal4 ,the conditions under which successive layers of vegetable matter 7later turned into coal7 become embedded in the earth's crust have been outlined in the chapter on the ,evolution of ,plants4 ,the most perfect conditions for prolific plant growth1 and accumulation as great beds in the earth's crust1 were during the ,pennsylvanian period of the late ,paleozoic era in many parts of the world1 but especially in the ,united ,states1 ,china1 ,great ,britain1 and ,germany4 ,most of the world's great supply of coal comes from rocks of ,pennsylvanian ,age1 while next in importance are ,cretaceous rocks1 and some comes from strata of other ages later than the ,pennsylvanian1 even as late as the ,tertiary4 ,the ,united ,states not only has the greatest known coal fields1 but it also produces far more coal than any other country4 ,in #aiah the production was #fgh1jjj1jjj tons1 the greatest in our history1 or enough1 if loaded into cars of forty tons capacity1 to fill a train which would reach around the earth at the equator about six times6 ,equally amazing is the fact that this coal was nearly all consumed by this one nation6 ,in #aiai the production fell to #edd1jjj1jjj tons4 ,is there real danger that our supply of coal will soon run out8 ,hardly so when we consider1 first1 the fact that probably not more than #a per cent of the readily available coal has thus far been removed1 and1 second1 the high probability that rate of increase in coal production for the last twenty years will not continue4 ,in fact1 during the last two or three years the production has fallen off considerably4 ,but even so1 coal1 which is our greatest natural resource1 and which can never be replaced1 should be scientifically conserved4 ,in the case of the very restricted anthracite coal fields what might be called a crisis has already been reached1 because a very considerable part of the available supply has been taken out4 ,something like #cej1jjj square miles of the ,united ,states are underlain with one or more beds of workable coal 7not including lignite7--in some areas five to twenty or more beds one above the other4 ,there are also about #aej1jjj square miles of country underlain with the more or less imperfect coal called lignite4 ,it has been estimated that there are more than a trillion tons of easily accessible coal1 and another trillion tons accessible with some difficulty in the principal coal fields of the ,united ,states4 ,the greatest production of coal by far is from the ,appalachian ,mountain and ,allegheny ,plateau districts1 from the western half of ,pennsylvania to ,alabama1 where all the coal is bituminous of ,pennsylvanian ,age4 ,here as well as elsewhere the coal beds are interstratified with various kinds of sedimentary rocks1 most commonly with shales and sandstones4 ,in the ,appalachian field the strata including coal beds are more or less folded toward the east1 while they are nearly horizontal toward the west4 ,the famous ,pittsburgh coal bed is probably the most extensive important single coal bed known4 ,it covers an area of over #ab1jjj square miles and is workable1 with a thickness of five to fifteen feet1 over an area of #f1jjj square miles of parts of western ,pennsylvania1 ,ohio1 and ,west ,virginia4 ,illustration3 ,fig4 #gf4--,map of the ,united ,states1 showing the principal coal fields4 ,cross-lined areas represent lignitic coals4 7,after ,u4 ,s4 ,geological ,survey47 ,the greatest production of anthracite coal by far is from central-eastern ,pennsylvania1 where strata of ,pennsylvania ,age1 including a number of anthracite beds1 are mostly highly folded4 ,most remarkable of all in this district is the so-called mammoth bed of anthracite1 nearly everywhere present1 with a thickness up to as much as fifty or sixty feet4 ,less than #ejj square miles are there underlain by workable anthracite coal4 ,next to the greatest production of coal in the ,united ,states is from the two large areas in the middle of the ,mississippi ,valley4 ,it is all bituminous coal1 associated with nearly horizontal strata of ,pennsylvanian ,age4 ,the scattering areas through the ,rocky ,mountains yield all types of coal--anthracite1 bituminous1 and lignite4 ,in some of these areas the coal beds have been but little disturbed from their original horizontal position1 but usually they are more or less folded along with the inclosing strata1 the crustal disturbances affecting the coal beds having taken place late in the ,mesozoic era and early in the ,cenozoic era4 ,practically all of these coals are of ,cretaceous and ,tertiary ,ages1 the best being ,cretaceous4 ,very little of the ,rocky ,mountain coal is anthracite4 ,on the ,pacific ,coast coal production is relatively very small4 ,the coals are there bituminous to lignitic of ,tertiary ,age1 usually folded in with the strata4 ,in ,alaska there are widely distributed1 relatively small coal fields1 but they have been little developed4 ,alaskan coals range in age from ,pennsylvanian to ,tertiary1 and in kind from anthracite to lignite4 ,petroleum4 ,crude oil or petroleum is an organic substance consisting of a mixture of hydrocarbons1 that is1 it is made up very largely of the two chemical elements carbon and hydrogen1 in rather complex and variable combinations4 ,it is practically certain that petroleum has been derived by a sort of slow process of distillation from organic matter--animal or vegetable or both--in stratified rocks within the earth4 ,many strata1 as for example carbonaceous shales1 are more or less charged with dark-colored decomposing organic matter4 ,the chemical composition itself1 the kinds of rocks with which it is associated1 and certain optical 7microscopic7 tests all point to the organic origin of petroleum4 ,in southern ,california at least1 certain of the oils have quite certainly been derived from the very tiny oily plants called diatoms which fill many of the strata4 ,illustration3 ,fig4 #gg4--,profile and structure section showing folding of strata1 with included coal beds1 across one of the anthracite coal fields of eastern ,pennsylvania4 ,length of section1 a little over #b miles4 7,after ,u4 ,s4 ,geological ,survey47 ,during the last twenty years petroleum has come to be one of the most important and useful natural products4 ,among the many substances artificially derived from petroleum are kerosene1 gasoline1 naphtha1 benzine1 vaseline1 and paraffine4 ,the ,united ,states leads in the production of petroleum1 while southern ,russia and ,mexico are very important producers4 ,in the ,united ,states the principal areas underlain with petroleum-bearing strata are the northern ,appalachian field 7through western ,pennsylvania to central ,west ,virginia72 the ,ohio-,indiana field 7central ,indiana to northwestern ,ohio72 the mid-continental field 7southeastern ,kansas and northeastern ,oklahoma72 the southeastern ,texas-,louisiana field2 and the southwestern ,california field4 ,the total areas underlain with oil total about #aj1jjj square miles4 ,in the ,appalachian1 ,ohio-,indiana1 and mid-continental fields the strata carrying oil range in age from ,ordovician to ,pennsylvanian1 and they are mostly but little disturbed from their original horizontal position4 ,the ,texas-,louisiana oils come mainly from ,cretaceous and ,tertiary strata which gently downtilt under the ,coastal ,plain toward the ,gulf4 ,in ,california the oil-bearing strata are of ,tertiary ,age and generally considerably disturbed and folded4 ,under proper conditions below the earth's surface the derived oil accumulates in porous or fractured rocks4 ,there must1 of course1 be a source from which the petroleum is derived or distilled2 a porous or fractured rock formation to take it up2 a cap rock or impervious layer to hold it in2 and a proper geologic structure to favor accumulation4 ,the most common porous 7containing7 rock is sandstone1 and the most common cap rock is shale4 ,oil is rarely found without gas1 and saline water is likewise often present4 ,if the containing strata are horizontal1 the oil and gas are usually irregularly scattered1 but if tilted or folded1 and the beds porous throughout1 they appear to collect at the highest point possible4 ,it was the result of observations along this line that led ,i4 ,c4 ,white to develop what is known as the anticlinal theory4 ,according to this theory1 in folded areas the gas collects at the summit of the fold 7anticline71 with the oil immediately below1 on either side1 followed by the water4 ,it is1 of course1 necessary that the oil-bearing stratum shall be capped by a practically impervious one4 ,illustration3 ,fig4 #gh4--,map showing the principal petroleum and natural gas fields of the ,united ,states4 7,after ,u4 ,s4 ,geological ,survey47 ,illustration3 ,fig4 #gi4--;,a vertical 7structure7 section showing a very common type of oil-bearing structure4 ,in this anticline1 water1 oil1 and gas arranged in order of their specific gravities4 ,removal of the gas would allow the oil and water to rise higher toward the apex of the porous layer4 7,after ,indiana ,geological ,survey47 ,illustration3 ,plate #ai4--;,general ,view in the ,appalachian ,mountains ,along ,new ,river1 ,virginia4 ,this is a typical portion of the great area which1 during ,mesozoic time1 was reduced by erosion to the condition of a low-lying plain 7peneplain74 ,since early ,cenozoic time the peneplain has been upraised and ,new ,river has carved out its ,v-shaped valley to its present depth1 while tributary streams have carved out a series of valleys along belts of weak rocks nearly at right angles to the main valley4 ,the remarkably even sky line marks approximately the old peneplain surface4 7,photo by ,hillers1 ,u4 ,s4 ,geological ,survey47 ,illustration3 ,plate #bj4--7;a7 ,a ,big ,glacial ,bowlder of ,plutonic ,igneous ,rock ,carried ,miles from ,its ,parent ,ledge by the ,ice ,sheet ,which ,passed ,over the ,adirondack ,mountains ,during the ,ice ,age4 7,photo by the author47 ,illustration3 ,plate #bj4--7;b7 ,a ,long1 ,winding ,ridge of ,sand and ,gravel 7,called an ,esker7 ,deposited by a ,stream in a ,channel in the ,ice ,near the ,margin of the ,great ,glacier ,during ,its ,retreat from the ,adirondack ,mountains4 7,photo by the author47 ,if the rocks are dry1 then the chief points of accumulation of the oil will be at or near the bottom of the syncline 7downfold71 or lowest portion of the porous bed4 ,if the rocks are partially saturated with water1 then the oil accumulates at the upper level of saturation4 ,in a tilted bed1 which is locally porous1 and not so throughout1 the oil1 gas1 and water may arrange themselves according to their gravity in this porous part4 7,ries47 ,although the term oil pool is commonly used1 there is really no actual pool or underground lake of oil1 but rather porous rock saturated with oil4 ,it has been estimated that in an oil field of average productiveness a cubic foot of the porous rock contains from six to twelve pints of oil4 ,the life of a well drilled into an oil pool varies from a few months to twenty or thirty years1 or sometimes even more1 but a heavy producer 7especially a gusher7 almost invariably falls off very notably in production in a few months1 or at most a few years4 ,the typical ,pennsylvanian oil well is said to last about seven years4 ,the fact that the ,united ,states is still able to increase oil output is because new fields are found and developed1 the most recent being in the interior and northern parts of ,texas4 ,it is practically certain1 however1 that the climax of oil production in the ,united ,states will be reached before many years--long before that of bituminous coal4 ,it is a well-known fact that oil1 as well as natural gas1 is usually under more or less pressure within the earth4 ,the pressure is so great in some cases that where1 in the course of drilling1 oil or gas accumulated under proper conditions1 as for example those shown by ,figure #gi1 are encountered the pressure may be hundreds of pounds per square inch1 or enough to blow to pieces much of the drilling outfit4 ,it is under such conditions that great gushers are struck4 ,a wonderful case in point was the famous ,lakeview gusher1 struck in ,california in #aiaj4 ,within a few days the well was far beyond control4 ,it continued to flow 7for a time shooting high into the air7 for eighteen months1 finally stopping after it had produced over #h1jjj1jjj barrels of oil1 about #f1jjj1jjj of which had been saved4 ,the daily production of the well varied greatly1 reaching a maximum of #fe1jjj barrels4 7,pack47 ,one very common cause of oil pressure is the expansive force of the associated imprisoned gas which steadily increases as the gas is generated4 ,another cause which is seemingly applicable in many cases is hydrostatic pressure1 where under certain structural conditions the pressure of water in a long-tilted layer is exerted against oil accumulated toward the top of an anticline 7or upbend7 in the strata4 ,the world's output of petroleum for #aiag was nearly #eei1jjj1jjj barrels1 of which the ,united ,states produced nearly #cch1jjj1jjj barrels1 ,mexico #hg1jjj1jjj barrels1 and ,russia #cd1jjj1jjj barrels of #db gallons4 ,natural ,gas4 ,the most perfect fuel with which nature has provided us is natural gas4 ,not only is it easily transported even long distances through pipes1 but also as a fuel it is easily regulated1 leaves no refuse1 and is less damaging to boilers than coal4 ,it is a colorless1 odorless1 free-burning gas1 consisting very largely of the simple hydrocarbon called marsh gas or fire damp4 ,petroleum nearly always has more or less natural gas associated with it1 but in some cases considerable quantities of gas may exist alone4 ,natural gas1 like petroleum1 is of organic origin--a product of slow natural distillation of vegetable or animal matter1 or both1 within the earth's crust4 ,one of the most common modes of occurrence of gas is at the top of an anticline 7upfold7 in porous rock 7like sandstone7 between impervious layers 7like shale74 ,figure #gi well illustrates the principle1 the gas lying above the oil1 and the oil above the water2 that is1 the three substances are arranged according to specific gravity4 ,gas may also exist in considerable quantities in irregular bodies of porous or fractured rocks4 ,natural gas is nearly always under pressure within the earth1 hundreds of pounds per square inch being common1 while more exceptionally1 as in certain ,west ,virginia wells1 pressures of over #a1jjj pounds have been registered4 ,the ,united ,states is by far the greatest world producer of natural gas1 the output for #aiah having been #gbj1jjj1jjj1jjj cubic feet4 ,west ,virginia easily headed the list1 with ,oklahoma and ,pennsylvania next in order4 ,areas underlain with natural gas are1 in the main1 the same as for petroleum1 and they total more than #aj1jjj square miles4 ,during the last forty years the waste of natural gas in the ,united ,states has been appalling4 ,in many cases wells in quest of oil have encountered gas and often such abandoned wells have been allowed to play millions of cubic feet of gas daily into the air for years4 ,a striking example was the ,murraysville well of western ,pennsylvania1 which shot #bj1jjj1jjj cubic feet of gas per day into the air for six years6 ,,metal-,,bearing 7,,ore7 ,,deposits ,iron4 ,without question the most useful of all metals is iron4 ,as such it is rare in nature1 but in chemical combination with other substances it is extremely widespread and very common4 ,iron makes up about #e per cent of the weight of the earth's crust1 but in the form of ore 7i4e41 a metal-bearing mineral or rock of sufficient value to be mined7 it is notably restricted in occurrence4 ,the three great ores of iron are the minerals hematite1 magnetite1 and limonite1 whose composition and characteristic properties the reader will find stated in the preceding chapter on ,mineralogy4 ,one of the worst impurities in iron ore is phosphorus1 which makes iron cold short1 i4e41 brittle when cold4 ,ore for the manufacture of ,bessemer steel must contain very little phosphorus 7less than #a_/ajjj of the metallic iron content of the ore74 ,sulphur as an impurity in the ore tends to make the iron hot short4 ,silica 7quartz7 is bad because it necessitates the use of more lime for flux in the furnace4 ,iron ores occur in rocks of most of the great geologic ages1 but in the ,united ,states principally in the pre-,paleozoic and ,paleozoic4 ,the ,united ,states is by far the greatest producer of iron ore in the world1 the output for #aiag having been about #ge1bhh1jjj tons1 the greatest in the history of this or any other country4 ,this one year's output loaded into cars of #dj tons capacity would have made a train about #ae1jjj miles long6 ,all but about #e1jjj1jjj tons of this tremendous production was hematite ore4 ,in #aiai the output of iron ore dropped to about #fj1jjj1jjj tons4 ,illustration3 ,fig4 #hj4--;,drawing showing details of part of an ore-bearing vein at ,pinos ,altos1 ,new ,mexico4 ,the chalcopyrite and sphalerite are the ores4 ,somewhat reduced in size4 7,after ,paige1 ,u4 ,s4 ,geological ,survey47 ,we shall now very briefly consider several of the greatest iron-mining districts of the ,united ,states1 giving some idea of the modes of occurrence and origin of the ores4 ,greatest of all is the ,lake ,superior region1 not far west and south of the lake in ,minnesota1 ,michigan1 and ,wisconsin4 ,considerably more than one-half the iron ore mined in the ,united ,states comes from the single ,state of ,minnesota1 and about one-fourth of it from ,michigan4 ,most of the ,minnesota ore by far is obtained from the so-called ,mesaba ,range1 which in #aiag produced #da1jjj1jjj tons of hematite ore4 ,the ore deposits are there of irregular shape1 lying at or near the surface 7usually covered only by glacial deposits74 ,none of them extend downward more than a few hundred feet4 ,the soft1 high-grade ore is removed by steam shovels in great open pits4 ,in the several districts of northern ,michigan and ,wisconsin the ores 7nearly all hematite7 are associated with more or less highly folded rocks at considerable depths4 ,the ,lake ,superior iron ores all occur in rocks of ,archeozoic and ,proterozoic ,ages4 ,according to the best explanation of their origin the iron of the ores was once part of a sedimentary series of rocks in the form of iron carbonate and silicate1 interstratified with layers of a flintlike rock associated with slate1 quartzite1 etc4 ,after these rocks were raised into land and subjected to weathering the old iron compounds were altered to oxides1 mainly hematite1 and somewhat concentrated4 ,further concentration of the ore was caused by dissolving out the flintlike layers of the old rocks4 ,the ,birmingham1 ,ala41 region is the second most important iron ore producer in the ,united ,states1 with an output of nearly #f1jjj1jjj tons in #aiah4 ,the ore is hematite1 forming part of the famous ,clinton iron ore deposits of ,silurian ,age4 ,this deposit1 named from ,clinton1 ,n4 ,y41 extends through central ,new ,york and in more or less interrupted parallel bands through the ,appalachian ,mountains to near ,birmingham where the richest deposits occur4 ,this ore appears to be an original bed 7or locally several beds7 of fairly rich iron ore deposited on the shallow ,silurian sea bottom and then covered by other strata4 ,at the time of the ,appalachian ,mountain revolution the iron ore was more or less highly folded in with other strata throughout the ,appalachians4 ,a remarkable fact regarding the ,birmingham district is that in the near vicinity of the ore there are both coal for fuel and limestone flux for smelting the ores4 ,the next most important mining region of the ,united ,states is the ,adirondack ,mountain region of northern ,new ,york1 where about #a1jjj1jjj tons of ore are obtained yearly4 ,magnetite is the ore1 and it occurs in more or less irregular lenses and bands in granite and closely associated rocks of pre-,paleozoic ,age4 ,one view regarding the origin of this ore is that it segregated during the process of cooling of the molten granite1 and another view 7recently advocated by the author7 is that it was derived from an older iron-rich igneous formation by either the molten granite or very hot solutions from it and concentrated into the ores4 ,about #b1ejj1jjj tons of magnetite were mined in the ,united ,states in #aiaf1 nearly one-half of it in the ,adirondacks4 ,the third important iron ore is limonite1 nearly #b1jjj1jjj tons of which were produced in the ,united ,states in #aiaf4 ,most of it came from the ,appalachian ,mountains4 ,all of this limonite is of secondary origin2 that is1 it has been derived from certain early ,paleozoic iron-bearing limestones either by weathering or solution1 and concentrated into ore deposits4 ,copper4 ,this is one of the most useful of all metals4 ,several of its very important uses are as a conductor of electricity in the form of wire2 in making alloys such as brass and bronze2 in copperplate engraving2 and in roofing and plumbing4 ,various minerals containing copper are found in many parts of the world1 but only about six of them are really important as ores4 ,these are native copper1 chalcopyrite1 chalcocite1 azurite1 malachite1 and cuprite1 most of which are described in the chapter on ,mineralogy4 ,the number of places where they may be profitably mined as ore is distinctly limited4 ,fifteen or twenty countries produce more or less copper1 but the ,united ,states is by far the greatest producer1 with an output of nearly #b1jjj1jjj1jjj pounds of copper in #aiaf1 the output having fallen off some in #aiah4 ,this was two-thirds of the world's output and ten times as much as the nearest competitor4 ,the other leading countries are ,japan1 ,chile1 ,mexico1 ,spain1 and ,canada4 ,in #aiah the four leading ,states in order were ,arizona1 ,montana1 ,michigan1 and ,utah1 with production ranging from nearly #gfe1jjj1jjj pounds to about #bcj1jjj1jjj pounds4 ,in ,arizona several great copper-mining districts lie in the southeastern one-fourth of the ,state4 ,almost invariably the ores are directly associated with limestone and an igneous rock 7granite71 both of late ,paleozoic ,age4 ,the ores are almost always near the border between the two rocks1 mostly as great irregular deposits within the limestone1 and less commonly as veins within the granite4 ,the original ores were carried in solution and deposited by hot liquids 7or vapors7 from the cooling granite4 ,at lower levels the ores are mainly sulphides of copper 7e4g41 chalcopyrite and chalcocite71 while at higher levels they are mostly carbonates 7malachite and azurite7 and oxides 7e4g4 cuprite74 ,the difference is due to the fact that the ores nearer the surface have been subjected to weathering and altered from their original condition4 ,the region around ,butte1 ,mont41 is next to the greatest copper producer4 ,nearly all the ores are sulphides of copper 7mainly chalcocite7 which occur with quartz in a great system of nearly parallel veins in granite of ,tertiary ,age4 ,it is supposed that in the copper veins the hot ore-bearing solutions ascended the fractures in the granite1 replacing the rock by ore1 and resulting in an intense alteration of the walls4 7,ries47 ,third in rank among the copper-producing ,states is ,michigan1 the mines being located on ,keweenaw ,peninsula1 which extends into ,lake ,superior4 ,for fully fifty years this district has been one of the most famous and important copper producers in the world4 ,a unique feature is that the ore is native copper1 associated with some native silver4 ,the rocks containing the ore are steeply tilted lava sheets and conglomerate 7cemented gravel7 strata of ,proterozoic ,age4 ,openings in porous lava and spaces between the conglomerate pebbles have been filled by metallic copper1 which was carried off in hot solutions from the cooling lavas4 ,certain of the mining shafts have been sunk more than #e1jjj feet below the surface1 these being next to the deepest in the world4 ,utah ranks fourth among the copper producers1 the greatest mining district being at ,bingham ,canyon1 southwest of ,salt ,lake ,city4 ,the rocks are late ,paleozoic strata1 pierced by a large body of igneous rock4 ,some of the sulphide ores 7mainly chalcopyrite7 occur in veins in the igneous rock and some in large tabular masses in the adjacent limestone4 ,hot solutions from lower portions of the uncooled igneous rock carried the ore in solution into the limestone and into cracks in the upper cooled igneous rock4 ,lead4 ,lead must surely be counted among the five or six most useful metals4 ,as in the case of nearly all the other most important natural resources1 the ,united ,states is the world's greatest producer of lead1 the output of metallic lead having been #eeb1jjj tons in #aiaf and somewhat less in #aiah4 ,most of this came from ,missouri1 ,idaho1 ,utah1 and ,colorado4 ,the leading other countries are in order--,spain1 ,germany1 ,mexico1 and ,australia4 ,nearly all the lead comes from the mineral galena 7a sulphide of lead71 which is described in the chapter on ,mineralogy4 ,among the many uses of lead are the following3 manufacture of certain high-grade paints from lead compounds2 making alloys such as pewter1 type metal1 solder1 babbit metal2 in plumbing2 in glass making2 and in the manufacture of shot4 ,the greatest lead-mining district is in the vicinity of ,joplin1 ,mo41 where the ore 7galena71 associated with much zinc ore1 occurs as veins and great irregular deposits in limestone of early ,paleozoic ,age4 ,it is generally agreed that underground waters dissolved the ores out of the limestone in which they were disseminated as tiny particles and deposited them in concentrated form at lower levels4 ,in the famous ,coeur d',alene district of northern ,idaho the great output of lead is really obtained from a lead-silver ore2 that is1 galena rich in silver4 ,this ore is in composition a lead-silver sulphide4 ,it occurs in great fissure veins1 mostly following fault fractures in highly folded strata of ,proterozoic ,age4 ,igneous rocks cut through the strata1 and it is believed that hot ore-bearing solutions given off from the highly heated igneous rocks rose in the fissures and deposited the ores4 ,the ,park ,city and ,tintic districts of ,utah are great producers of lead4 ,the lead ore 7galena7 is usually rich in silver4 ,it occurs mainly in veins and irregular deposits in limestone of ,paleozoic ,age closely associated with certain igneous rocks4 ,one of the most famous mining districts in the world is that around ,leadville1 ,col41 where ores of four metals--gold1 silver1 lead1 and zinc--have been extensively mined4 ,the salient points in the rather complex geology are the following3 ,paleozoic strata1 including much limestone1 rest upon a foundation of pre-,paleozoic granite4 ,sheets of igneous rock are interbedded with the strata and many dikes of igneous rocks cut through the whole combination4 ,after the last igneous activity all the rocks were somewhat folded and notably faulted in many places4 ,the ores were dissolved out of the igneous rock and deposited in large masses mostly in the limestone and in fissure veins1 especially along and near the fault zones4 ,zinc4 ,another of the few most useful metals is zinc4 ,it never occurs in metallic form in nature1 but most of it by far is obtained from the ore mineral sphalerite 7sulphide of zinc7 described in the chapter on ,mineralogy4 ,a red oxide of zinc ore1 called zincite1 assumes great economic importance in ,new ,jersey4 ,in #aiag the ,united ,states produced #fhf1jjj tons of metallic zinc and was easily the world's leader4 ,since #aiag the production has fallen notably4 ,the four greatest producing ,states are ,missouri1 ,montana1 ,new ,jersey1 and ,colorado4 ,germany and ,belgium are the greatest foreign producers4 ,most important of all in the ,united ,states is the district around ,joplin1 ,mo41 where the ore is closely associated with lead ore4 ,the mode of occurrence and origin of these ores are above referred to in the discussion of lead4 ,in ,montana some of the great east-west fissure veins in granite are rich in silver ores in the upper levels1 and in zinc ores 7mainly sphalerite7 at depths of from some hundreds of feet to nearly #b1jjj feet1 that is as far down as they have been mined4 ,they1 like the great copper veins of the same general district1 were carried by hot solutions which rose from the lower still very hot granite and deposited the ores in fissures of the same cooler rock higher up4 ,two great ore bodies in the general vicinity of ,franklin1 ,n4 ,j41 are of unique interest1 because they are mostly the red oxide of zinc called zincite4 ,the ore deposits occur in white limestone along or close to its contact with metamorphosed 7altered7 strata and granite of early ,paleozoic ,age4 ,it is not definitely known how the ore originated1 but it was probably derived in solution from the hot granite and deposited in the limestone by replacement of the latter4 ,in ,colorado the principal zinc mines are around ,leadville1 where lead ore is nearly always directly associated with the zinc ore4 ,this district is above described in the discussion of lead4 ,among many uses of zinc are for galvanizing2 for making certain high-grade paints2 brass and white metal2 and for roofing and plumbing4 ,gold4 ,this precious metal has been used and highly prized by man for thousands of years4 ,the discovery of gold in ,california in #ahdh was one of the most important events in the history of the mining world4 ,as early as #aheb that ,state reached its climax of production with an output of at least #ha1jjj1jjj worth of the metal4 ,the ,transvaal region of ,south ,africa has for two decades been the world's greatest gold producer4 ,though long known1 the metal has there been worked only since #ahhf4 ,in #aiae the peak of gold production in the world 7#dfh1gjj1jjj7 was reached and nearly maintained in #aiaf1 but since that time there has been a great falling off4 ,in #aiaf ,south ,africa produced gold to the value of about #bjj1jjj1jjj2 the ,united ,states over #ij1jjj1jjj2 ,australia over #dj1jjj1jjj2 ,russia over #bf1jjj1jjj2 and ,canada over #ai1jjj1jjj4 ,in tiny amounts gold is really very widespread4 ,it occurs in many stream gravels where so-called color may be obtained by washing gravel1 and it is even dissolved in sea water4 ,gold-mining localities are also numerous in many parts of the world1 but relatively few of them only have ever paid4 ,the total amount of money spent in actual gold-mining operations2 in hopeless but honest operations2 and for stock in fake gold mines has no doubt exceeded the actual value of gold produced4 ,in many a case acceptance of a report based upon a very brief examination of the ground by a competent geologist would have saved the cost of hopeless expenditure of money4 ,some one in nearly every community has a so-called gold mine4 ,most of the commercially valuable gold occurs in nature as native gold1 either mixed with gravel and sand 7i4e41 placer deposits7 along existing or ancient stream beds1 or in veins mechanically held in the mineral pyrite 7described in the preceding chapter7 in submicroscopic form1 or visibly mixed with quartz in vein deposits4 ,another kind of ore which assumes considerable importance1 as in parts of ,colorado1 is in the form of telluride of gold always found in veins4 ,in deep vein deposits it is quite the rule to find free or native gold mechanically and visibly mixed with quartz in the upper levels1 while deeper down the gold is mechanically1 but invisibly1 held in combination usually in pyrite1 which latter is associated with quartz4 ,this difference is due to the fact that the lower level ores are now just as they were formed1 while in the upper levels the ores have been weathered1 and the gold set free and often more or less further concentrated by solutions4 ,vein deposits1 including also telluride ores1 are found in many kinds of rocks--igneous1 sedimentary1 and metamorphic--of nearly all ages generally directly associated with igneous rocks4 ,in nearly all cases the best evidence indicates that the vein fillings were formed by hot ore-bearing solutions from the igneous rock1 which solutions deposited the ore plus quartz in fissures in either the igneous or adjacent rocks4 ,among the many localities where fissure veins of the kind just described are of great economic importance are the ,mother ,lode belt of the ,sierra ,nevada ,mountains of ,california2 ,cripple ,creek 7telluride ore71 ,georgetown and the ,san ,juan region of ,colorado2 ,goldfield1 ,tonopah1 and ,comstock ,lode of ,nevada2 and near ,juneau1 ,alaska4 ,placer deposits1 that is1 free gold mixed with gravel and sand1 also yield much gold4 ,they are most prominently developed in ,california and ,alaska4 ,these gold-bearing gravels represent the more resistant products of weathering1 such as quartz and native gold1 which have been washed down from the hills on whose slopes the gold-bearing quartz veins outcrop1 and were too heavy to be carried any distance1 unless the grade was steep4 ,they have consequently settled down in the stream channels1 the gold1 on account of its higher specific gravity1 collecting usually in the lower part of the gravel 7placer7 deposit4 7,ries47 ,such gold occurs as grains1 flakes1 or nuggets4 ,when a chunk of gold-bearing vein quartz1 with crevices filled by thin plates of the metal1 is carried downstream pieces are gradually broken away1 and the tough1 very malleable gold bends or welds together into a single mass called a nugget4 ,nuggets varying in weight up to over #b1jjj ounces have been found4 ,many placer deposits are along existing drainage channels1 while others occur in abandoned and even buried former channels4 ,most of the gold of ,south ,africa comes from ,witwatersrand district where the native metal occurs in a unique manner in beds or layers of conglomerate associated with other strata1 all the rocks being considerably folded and somewhat faulted4 ,some of the mines are more than a mile deep 7vertically71 the deepest in the world4 ,the gold either accumulated in placer form with gravel which later consolidated into conglomerate1 or it was introduced into spaces between the pebbles subsequently by ore-bearing solutions4 ,silver4 ,for many years the ,united ,states and ,mexico have been the world's greatest silver producers1 sometimes one and sometimes the other leading1 with ,canada third1 and ,australasia fourth4 ,in #aiah the ,united ,states produced nearly #fh1jjj1jjj ounces of silver and ,mexico over #fb1jjj1jjj ounces4 ,in the ,united ,states in #aiah the four leading ,states were ,montana1 ,utah1 ,idaho and ,nevada with outputs ranging from over #aj1jjj1jjj to over #ae1jjj1jjj ounces each4 ,in ,montana most of the silver is in the native form1 more especially in the upper portions of the great veins rich in copper and zinc ores near ,butte4 ,these ores and their origin are described above under the captions ,copper and ,zinc4 ,the two greatest silver districts of ,nevada are ,tonopah and ,comstock ,lode where silver and gold minerals are associated as ores in ,tertiary igneous rocks1 the ores having been deposited in veins by hot ore-bearing solutions from the igneous rocks4 ,in ,idaho the ,coeur d',alene district produces most of the silver1 the ore there being a silver-bearing lead ore 7galena74 ,the nature and origin of these deposits are described above under the caption ,lead4 ,in ,utah the silver is also obtained from silver-bearing galena especially in the ,tintic1 ,cottonwood ,canyon1 and ,bingham ,canyon districts where the ores occur mainly as irregular deposits and in fissure veins in ,paleozoic strata 7chiefly limestone7 directly associated with igneous rocks1 hot ore-bearing solutions from the igneous rocks having furnished the ores4 ,tin4 ,production of tin in the ,united ,states has never amounted to much1 a little mining having been carried on from time to time in ,south ,carolina1 ,black ,hills of ,south ,dakota1 and southern ,california4 ,about one-half of the world's supply of tin 7#aba1jjj long tons #aiah7 comes from the ,malay ,peninsula and two small islands near by4 ,the only other great producer is ,bolivia1 though a number of other countries produce from #a1jjj to #i1jjj tons each4 ,the only important ore of tin is the mineral cassiterite 7oxide of tin7 described above in the chapter on ,mineralogy4 ,in the ,malay region the ore all occurs in placer deposits and is1 therefore1 of secondary origin1 the source of the ore not being known4 ,in ,bolivia the tin ore occurs in veins in and close to granite1 the ore having been carried by very hot vapors or liquids which were derived from the still highly heated granite4 ,tin is used chiefly in the making of tin plate1 bronze1 pewter1 gun metal1 and bell metal4 ,aluminum4 ,the mineral called bauxite 7a hydrous oxide of aluminum7 is the great ore from which aluminum is obtained by an electrical process4 ,bauxite is noncrystalline1 relatively light in weight1 white to yellowish in color1 and in the form of rounded grains1 or earthy or claylike masses4 ,the ,united ,states and ,france are the only two great producers of bauxite1 most of which is treated for metallic aluminum4 ,in #aiah the ,united ,states produced more than #ajj1jjj tons of aluminum4 ,in the ,united ,states the principal deposits are in ,georgia1 ,alabama1 and ,arkansas4 ,bauxite is probably always a secondary mineral formed by decomposition of igneous rocks rich in certain aluminum silicate minerals4 ,in some cases1 as in the ,georgia-,alabama region1 the bauxite appears to have been formed and concentrated in deposits by hot solutions from uncooled igneous rocks4 ,aluminum is most used in the manufacture of wire for electric current transmission4 ,it is also mixed with certain other metals like copper1 zinc1 magnesium1 and tungsten to form special types of alloys1 some of which possess remarkable tensile strength up to nearly #ej1jjj pounds per square inch4 ,aluminum is used in powdered form to generate very high temperatures in certain welding processes4 ,it is also made into many kinds of utensils and instruments4 ,mercury4 ,this metal1 commonly known as quicksilver1 is of special interest because it is the only one which exists in liquid form at ordinary temperatures4 ,the metal occurs in only small quantities in nature1 most of it by far being obtained from the red mineral cinnabar described in the chapter on ,mineralogy4 ,in order of importance the greatest quicksilver producing countries in #aiaf were ,italy1 ,united ,states1 ,austria1 and ,spain4 ,in the ,united ,states1 ,california is by far the leading ,state1 while ,texas and ,nevada are the only other important producers4 ,in ,california most of the ore occurs in veins and irregular deposits in metamorphosed strata of ,mesozoic and ,cenozoic ages usually closely associated with igneous rocks4 ,there1 as well as in other parts of the world1 hot vapors from igneous rocks carried the volatile ore upward and deposited it in fissures4 ,among the many uses of mercury are in making fulminate for explosives2 making certain drugs and chemicals1 pigments1 electrical and physical apparatus2 silvering mirrors2 and in the amalgamation process of extracting gold and silver4 ,,other ,,economic ,,products ,building ,stones4 ,some of the principal features which should be considered in building stones are power to resist weathering1 power to withstand heat1 color1 hardness1 and density1 and crushing strength4 ,building stones representing rocks of nearly all important geologic ages are widely distributed throughout the world4 ,granite1 including certain other closely related rocks1 is one of the oldest and most useful building stones4 ,the ,new ,england ,states are the greatest producers1 while the ,piedmont ,plateau district 7east of the ,appalachians7 from ,philadelphia to ,alabama also contains important granite quarries4 ,in the ,adirondack ,mountains1 in ,wisconsin and ,minnesota1 through the ,rocky ,mountains1 and the ,sierra ,nevada ,mountains there are extensive areas of granite which are relatively little quarried4 ,the granite occurs only in regions of highly disturbed rocks1 usually in mountains or hills1 where great volumes of the molten rock were forced into the earth's crust1 cooled1 and later laid bare by erosion4 ,marble1 according to geological definition1 is a metamorphosed limestone1 that is a limestone which has been crystallized under conditions of heat1 pressure1 and moisture within the earth4 ,more loosely in trade any limestone which takes a polish may be called marble4 ,the greatest marble-producing districts of the ,united ,states are western ,new ,england 7especially ,vermont7 and the ,piedmont ,plateau and ,appalachian ,mountains in rocks of ,paleozoic age4 ,in northern ,new ,york and the mountains of the west there are relatively few marble quarries4 ,ordinary limestones are widely distributed in many ,states where they range in age from early ,paleozoic to ,tertiary4 ,most of the quarries supply stone for near-by markets4 ,the so-called ,bedford limestone of ,indiana has1 for many years1 been perhaps the most widely used limestone for building purposes in the ,united ,states4 ,sandstones1 which are stratified rocks consisting mainly of rounded quartz grains cemented together1 are widely used in building operations4 ,like limestones1 they are very widespread in formations of all ages except the very old4 ,there are many sandstone quarries supplying more or less local markets throughout the country4 ,two of the best known and most widely used sandstones are the so-called brown-stone of ,triassic ,age extending interruptedly from the ,connecticut ,valley of ,massachusetts to ,north ,carolina1 and the ,berea1 ,ohio1 sandstone of light gray color and uniform texture4 ,slate is mostly a metamorphosed shale1 that is a shale which has been subjected to great pressure within the earth so that the stratification has been obliterated and a well defined cleavage has been developed at right angles to the direction of application of the pressure4 ,good slate is fine-grained1 dense1 and splits readily into wide thin plates4 ,it occurs only where mountain making pressure and metamorphism have been brought to bear upon the strata4 ,most of our great slate quarries are located in early ,paleozoic rocks from ,new ,england through the ,piedmont ,plateau4 ,some quarries are also located in ,arkansas1 ,minnesota1 and westward to ,california4 ,clay4 ,clay1 which is one of the most widely distributed materials and one of the most valuable1 commercially1 may be defined as a fine-grained mixture of the mineral kaolinite with fragments of other minerals1 such as silicates1 oxides1 and hydrates1 and also often organic compounds1 the mass possessing plasticity when wet and becoming rock-hard when burned to at least a temperature of redness4 7,ries47 ,most clays originate by the weathering of rocks1 particularly igneous and metamorphic rocks rich in the mineral feldspar4 ,as a result of the decomposition of the feldspar1 much clay is formed1 the main substance of which is kaolin4 ,both feldspar and kaolin are described in the preceding chapter4 ,when the resulting clay rests upon the rock from which it has been derived it is called residual clay4 ,much of the clay is1 however1 carried away1 mainly by streams1 and deposited in lakes or the sea1 or on river flood plains4 ,some clay deposits are of wind-blown origin1 and still others are formed by the grinding action of glaciers4 ,clays are very widespread1 and they are directly associated with rocks of all geologic ages4 ,among the many important uses of clay are the following3 manufacture of common brick1 fire brick1 pottery1 chinaware1 porcelain ware1 tiles1 terra cotta1 and ,portland cement4 ,lime and ,cement4 ,limestone1 which is one of the most common and widespread of all stratified rocks1 forms the basis for the manufacture of the important substances lime 7or quicklime7 and ,portland cement4 ,lime results when pure limestone 7carbonate of lime7 is burned or heated to a temperature high enough to drive off the carbonic acid gas4 ,the greatest use of lime is for mixing with water and sand to make mortar4 ,a few of its other numerous uses are in plastering2 whitewashing2 purifying certain steel2 in making gas1 paper1 and soap2 and as a fertilizer4 ,certain limestones containing clay of the right kind and proportion are called natural cement rocks because1 after being burned1 they develop the property of setting1 like cement when mixed with water4 ,the setting of a cement is due to the fact that certain chemical compounds formed during the heating crystallize when mixed with water1 and the hard1 tiny interlocking crystals of the newly formed silicate minerals give rigidity to the mass4 ,of recent years ,portland cement has largely superseded the natural rock cements4 ,portland cement is the product obtained by burning a finely ground artificial mixture consisting essentially of lime1 silica1 alumina1 and some iron oxide1 these substances being present in certain definite proportions4 7,ries47 ,the necessary ingredients are generally obtained by grinding and burning carefully selected mixtures of limestone in some form1 and clay or shale4 ,the great and growing uses of cement need not be detailed here4 ,salt4 ,most of the common salt 7the mineral halite7 of commercial value occurs in nature in sea or salt lake water2 or in beds or strata of rock salt associated with other strata2 or as natural brine in openings or pores in certain rocks4 ,considerable salt is obtained by evaporation of tidewater1 as around ,san ,francisco ,bay1 and of salt lake water1 as at ,great ,salt ,lake1 ,utah4 ,it has been estimated that the ,great ,salt ,lake1 whose area is about #b1jjj square miles and greatest depth #ej feet1 contains several hundred million tons of common salt4 ,this salt has been washed out of the rocks of the surrounding country and gradually accumulated in the lake because it has no outlet4 ,most important of all sources of salt is the rock salt which occurs in the form of strata within the earth's crust4 ,such strata are found in rocks of nearly all ages from the early ,paleozoic to the present4 ,they resulted from the evaporation of salt lakes or salty more or less cut-off arms of the sea1 after which other strata accumulated on top of them4 ,thus in the ,silurian system of strata underlying all of southwestern ,new ,york ,state there occur almost universally from one to seven beds of salt4 ,the strata including the salt dip gently southward so that at ,ithaca1 ,new ,york1 seven salt beds were struck in a well at a depth of about #b1bjj feet4 ,northward the salt comes nearer and nearer the surface4 ,one well penetrated a layer of solid salt #cbe feet thick4 ,some of this salt is being mined much like coal1 but most of it is obtained by running water into deep wells to dissolve the salt1 the resulting brine being pumped out and evaporated4 ,under portions of southern ,michigan there are both salt beds and natural brines charging certain porous rock layers4 ,both the salt beds 7of ,silurian ,age7 and the brines 7of ,mississippian ,age7 supply great quantities of salt from brines pumped out and evaporated4 ,in #aiah the ,united ,states produced #ea1jjj1jjj barrels 7#bhj lbs4 each7 of salt4 ,michigan 7#ag1jjj1jjj barrels7 and ,new ,york were the leading ,states1 followed by ,kansas1 ,ohio1 ,west ,virginia1 and ,california4 ,some of the uses of common salt are given in the description of halite in the preceding chapter4 ,gypsum4 ,the composition and properties of this common and useful mineral are given in the chapter on ,mineralogy4 ,rock gypsum is the variety of great commercial importance4 ,it is widespread1 being quarried in many ,states1 and occurs interstratified with rocks of many ages where it has originated by evaporation or partial evaporation of salt water lakes or more or less cut-off arms of the sea4 ,salt beds are often associated with gypsum4 ,for about ten years the average yearly production of gypsum in the ,united ,states has been approximately #b1ejj1jjj tons1 or about ten times that of the nearest foreign competitor 7,canada74 ,new ,york1 ,iowa1 ,michigan1 and ,ohio are the chief producers4 ,in ,new ,york the rock gypsum 7usually four to ten feet thick7 lies between shale and limestone strata of ,silurian age1 and it is quarried from the central to the western part of the ,state4 ,in ,michigan the rock gypsum beds1 commonly five to twenty feet thick1 lie in ,mississippian strata in the southern portion of the ,state4 ,a great bed of exceptionally pure rock gypsum underlies about twenty-five square miles of ,webster ,county1 ,iowa1 in rocks of late ,paleozoic ,age4 ,the ,kansas gypsum deposits extend across the central part of the ,state in rocks of ,permian ,age4 ,rock gypsum is mainly used in making plaster of ,paris1 as a retarder in cement1 and as a fertilizer 7so-called land plaster74 ,,glossary ,,of ,,common ,,geological ,,terms ,names of subkingdoms and important classes of fossil plants and animals1 and mineral species1 are not included2 these being briefly and systematically discussed in chapters #ag1 #ah1 #ai1 and #bj1 respectively4 ,by using the index the reader can quickly locate the page where any one of these names is discussed4 ,some definitions in this glossary are taken from ,u4 ,s4 ,survey ,bulletin ,no4 #fac4 ,anticline4--,a kind of folded structure in which strata have been bent upward or arched4 ,archeozoic4--,the earliest known era of geologic time4 ,basalt4--,a common lava of dark color and of great fluidity when molten4 ,basalt is less siliceous than granite and rhyolite1 and contains much more iron1 calcium1 and magnesium4 ,base-level4--,the lowest level to which a stream can cut 7erode7 its channel4 ,a whole region may be base-leveled by erosion4 ,cambrian4--,the first or earliest period of the ,paleozoic era of geologic time4 ,cenozoic4--,the present era of geologic time4 ,it began at least several million years ago4 ,chalk4--,a soft1 fine-grained1 white limestone consisting mainly of tiny shells4 ,conglomerate4--,a sedimentary rock consisting of consolidated or cemented gravel4 ,often sandy4 ,cretaceous4--,the last period of the ,mesozoic era of geologic time4 ,crystal4--,a regular polyhedral form1 possessing a definite internal molecular structure1 which is assumed by a substance in passing from the state of a liquid or gas to that of a solid4 ,nearly every mineral1 under proper conditions1 will crystallize4 ,crystalline ,rock4--,a rock composed of closely fitting mineral crystals that have formed in the rock substance1 as contrasted with one made up of cemented grains of sand or other materials1 or with a volcanic glass4 ,crystallography4--,the study of crystals4 ,devonian4--,the middle one of the seven periods of the ,paleozoic era of geologic time4 ,dike4--,a mass of igneous rock that has solidified in a fissure or crack in the earth's crust4 ,drift4--,commonly called glacial drift4 ,the rock fragments--soil1 gravel1 and silt--carried by a glacier4 ,drift includes the unassorted material known as till 7ground moraine7 and deposits made by streams flowing from a glacier4 ,drowned ,river ,valley4--,when a land surface sinks enough to permit tidewater to enter the lower ends of its valleys to form estuaries1 a good example being the lower ,hudson ,valley4 ,era4--,a name applied to one of the broadest subdivisions of geologic time 7e4g4 ,paleozoic era74 ,erosion4--,the wearing away and transportation of materials at and near the earth's surface by weathering and solution1 and the mechanical action of running water1 waves1 moving ice1 or winds which use rock fragments as tools or abrasives4 ,exfoliation4--,the splitting off of sheets of rock of various sizes and shapes due to changes of temperature4 ,it is a process of weathering4 ,fault4--,a fracture in the earth's crust accompanied by movement of the rock on one side of the break past that on the other4 ,if the fracture is inclined and the rock on one side appears to have slid down the slope of the fracture the fault is termed a normal fault4 ,if1 on the other hand1 the rock on one side appears to have been shoved up the inclined plane of the break1 the fault is termed a reverse or thrust fault4 ,fault-block4--,a part of the earth's crust bounded wholly or in part by faults4 ,fault-scarp4--,the cliff formed by a fault4 ,most fault scarps have been modified by erosion since the faulting4 ,fissure4--,a crack1 break1 or fracture in the earth's crust or in a mass of rock4 ,flood-plain4--,the nearly level land that borders a stream and is subject to occasional overflow4 ,flood-plains are built up by sediment left by such overflows4 ,fold4--,a bend in rock layers or beds4 ,anticlines and synclines are the common types of folds4 ,formation4--,a rock layer1 or a series of continuously deposited layers grouped together1 regarded by the geologist as a unit for purposes of description and mapping4 ,a formation is usually named from some place where it is exposed in its typical character4 ,fossil4--,the whole or any part of an animal or plant that has been preserved in the rocks or the impression left on rock by a plant or animal4 ,preservation is invariably accompanied by some change in substance1 and from some fossils the original substance has all been removed4 ,geography4--,the study of the distribution of the earth's physical features1 in their relation to each other to the life of sea and land1 and human life and culture4 ,geology4--,the science which deals with the history of the earth and its inhabitants as revealed in the rocks4 ,glacier4--,a body of ice which slowly spreads or moves over the land from its place of accumulation4 ,gneiss 7pronounced nice74--,a metamorphic1 crystalline rock with mineral grains arranged with long axes more or less parallel1 giving the rock a banded appearance4 ,derived from either igneous or stratified rocks well within the earth under conditions of pressure1 and usually also heat and moisture4 ,igneous ,rocks4--,rocks formed by the cooling and solidification of a hot liquid material1 known as magma1 that has originated at unknown depths within the earth4 ,those that have solidified beneath the surface are known as intrusive rocks1 or if the cooling has taken place slowly at great depth1 as plutonic rocks1 e4g4 granite4 ,those that have flowed out over the surface are known as effusive rocks1 extrusive rocks1 or lavas1 e4g41 basalt4 ,volcanic rocks include not only lavas1 but bombs1 pumice1 tuff1 volcanic ash1 and other fragmental materials or ejecta thrown out from volcanoes4 ,joints4--,nearly all rocks1 except very loose surface materials1 are separated into blocks of varying size and shape by a system of cracks called joints4 ,they may be caused by earth-crust movements1 contraction during solidification of molten rocks1 or contraction during drying out of sediments4 ,jurassic4--,the middle one of the three periods of the ,mesozoic era of geologic time4 ,lava4--,an igneous rock which in molten condition has poured out upon or close to the earth's surface1 e4g4 basalt4 ,limestone4--,a sedimentary rock consisting essentially of carbonate of lime which generally represents accumulation of shells of organisms1 but in some cases precipitates from solution4 ,often impure4 ,loess 7pronounced lurse with the r obscure74--,a fine homogeneous silt or loam showing usually no division into layers and forming thick and extensive deposits in the ,mississippi ,valley and in ,china4 ,it is generally regarded as in part at least a deposit of wind-blown dust4 ,marble4--,a crystalline limestone1 usually a metamorphic rock1 the limestone having been altered by heat1 pressure1 and moisture within the earth4 ,meander4--,to flow in serpentine curves4 ,a loop in a stream4 ,most streams in flowing across plains develop meanders4 ,mesa4--,a flat-topped hill or mountain left isolated during the general erosion or cutting down of a region4 ,mesozoic4--,next to the present era of geologic time4 ,metamorphic ,rock4--,any igneous or sedimentary rock which has undergone metamorphism1 that is notable alteration from its original condition4 7,see ,metamorphism47 ,metamorphism4--,any change in rocks effected in the earth by heat1 pressure1 solutions1 or gases4 ,a common cause of the metamorphism of rocks is the intrusion into them of igneous rocks4 ,rocks that have been so changed are termed metamorphic4 ,marble1 for example1 is metamorphosed limestone4 ,mineral4--,an inorganic substance of definite chemical composition found ready made in nature1 e4g4 calcite1 quartz4 ,mississippian4--,a period of the ,paleozoic era of geologic time--in order of age1 the third from the last of the era4 ,moraine4--,glacial drift carried on1 within1 or under a glacier and deposited at the end1 along the sides1 or under the glacier4 ,oil-pool4--,an accumulation or body of oil in sedimentary rock that yields petroleum on drilling4 ,the oil occurs in the pores of the rock and is not a pool or pond in the ordinary sense of these words4 ,ordovician4--,next to the earliest period of the ,paleozoic era of geologic time4 ,ore4--,a metal-bearing mineral or rock of sufficient value to be mined4 ,outcrop4--,that part of a rock formation which appears at the surface4 ,the appearance of a rock at the surface or its projection above the soil4 ,often called an exposure4 ,paleontology4--,the study of the world's 7geologically7 ancient life1 either plant or animal1 by means of fossils4 ,paleozoic4--,an old era of geologic time--third back from the present4 ,peneplain4--,a region reduced almost to a plain by the long-continued normal erosion of a land surface4 ,it should be distinguished from a plain produced by the attack of waves along a coast or the built-up flood plain of a river4 ,pennsylvanian4--,next to the last period of the ,paleozoic era of geologic time4 ,period4--,a name applied to one of the subdivisions of an era of geologic time1 e4g4 ,cambrian period4 ,permian4--,the last period of the ,paleozoic era of geologic time4 ,petrology4--,the study of rocks1 including igneous1 sedimentary1 and metamorphic rocks4 ,physiography4--,the study of the relief features of the earth and how they were produced4 ,placer ,deposit4--,a mass of gravel1 sand1 or similar material resulting from the crumbling and erosion of solid rocks and containing particles or nuggets of gold1 platinum1 tin1 or other valuable minerals1 which have been derived from rocks or veins4 ,plutonic ,rock4--,an igneous rock solidified from a molten condition well within the earth4 7,see ,igneous ,rocks47 ,proterozoic4--,next to the earliest known era of geologic time4 ,quartzite4--,a metamorphic rock composed of sand grains cemented by silica into an extremely hard mass4 ,quaternary4--,the later of the two periods of the ,cenozoic era of geologic time4 ,rejuvenated4--,any region which has been subjected to erosion for a greater or less length of time and then reelevated so that the streams are renewed in activity4 ,rock4--,any extensive constituent of the crust of the earth1 usually consisting of a mechanical mixture of two or more minerals1 e4g4 granite1 shale4 ,less commonly a rock consists of a single mineral 7e4g4 pure marble71 or of organic matter 7e4g4 coal74 ,sandstone4--,a sedimentary rock consisting of consolidated or cemented sand4 ,often shaly or limy4 ,schist4--,a rock that by subjection to heat and pressure and usually moisture within the earth has undergone a change in the character of the particles or minerals that compose it and has these minerals arranged in such a way that the rock splits more easily in certain directions than in others4 ,it is a metamorphic rock derived from either sedimentary or igneous rock1 more commonly the former4 ,sedimentary ,rocks4--,rocks formed by the accumulation of sediment in water 7aqueous deposits7 or from air 7eolian deposits74 ,the sediment may consist of rock fragments or particles of various sizes 7conglomerate1 sandstone1 shale72 of the remains or products of animals or plants 7certain limestones and coal72 of the product of chemical action or of evaporation 7salt1 gypsum1 etc472 or of mixtures of these materials4 ,some sedimentary deposits 7tuffs7 are composed of fragments blown from volcanoes and deposited on land or in water4 ,a characteristic feature of sedimentary deposits is a layered structure known as bedding or stratification4 ,each layer is a bed or stratum4 ,sedimentary beds as deposited lie flat or nearly flat1 but subsequently they have often been deformed by folding and faulting4 ,shale4--,a sedimentary rock consisting of hardened thin layers of fine mud4 ,silurian4--,a period of the ,paleozoic era of geologic time--in order of age1 the third from the beginning of the era4 ,slate4--,a rock that by subjection to pressure within the earth has acquired the property of splitting smoothly into thin plates4 ,the cleavage is smoother and more regular than the splitting of schist along its grain4 ,it is a metamorphic rock nearly always derived from shale4 ,soil4--,the mantle of loose material resting upon bedrock1 either in its place of origin or transported by water1 wind1 or ice4 ,strata 7or stratified rocks74--,sedimentary rocks which1 by the sorting power of water 7less often by wind71 are arranged in more or less definite layers or beds separated by stratification surfaces4 ,stratification4--,the separation of sedimentary rocks into more or less parallel layers or beds4 ,stratigraphy4--,the branch of geologic science that deals with the order and relations of the strata of the earth's crust4 ,structure4--,in geology1 the forms assumed by sedimentary beds and igneous rocks that have been moved from their original position by forces within the earth1 or the forms taken by intrusive masses of igneous rock in connection with effects produced mechanically on neighboring rocks by the intrusion4 ,folds 7anticlines and synclines7 and faults are the principal mechanical effects considered under structure4 ,schistosity and cleavage are also structural features4 ,syncline4--,a kind of folded structure in which strata have been bent downward4 ,it is an inverted arch--the opposite of an anticline4 ,talus 7pronounced tay'lus74--,the mass of loose rock fragments that accumulates at the base of a cliff or steep slope4 ,terrace4--,a steplike bench on a hillside4 ,most terraces along rivers are remnants of valley bottoms formed when the stream flowed at higher levels4 ,other terraces have been formed by waves4 ,some terraces have been cut in solid rock1 others have been built up of sand and gravel1 and still others have been partly cut and partly built up4 ,tertiary4--,the earlier of the two periods of the ,cenozoic era of geologic time4 ,triassic4--,the earliest period of the ,mesozoic ,era of geologic time4 ,unconformity4--,a break in the regular succession of sedimentary rocks1 indicated by the fact that one bed rests on the eroded surface of one or more beds which may have a distinctly different dip from the bed above4 ,an unconformity may indicate that the beds below it have at some time been raised above the sea and have been eroded4 ,in some places beds thousands of feet thick have been washed away before the land again became submerged and the first bed above the surface of unconformity was deposited4 ,if beds of rock may be regarded as leaves in the volume of geologic history1 an unconformity marks a gap in the record4 ,vein4--,a mass of mineral material that has been deposited in or along a fissure in the rocks4 ,a vein differs from a dike in that the vein material was introduced gradually by deposition from solution1 whereas a dike was intruded in a molten condition4 ,quartz and calcite are very common vein minerals4 ,volcanic ,rocks4--,igneous rocks erupted at or near the earth's surface1 including lavas1 tuffs1 volcanic ashes1 and like material4 ,weathering4--,the group of processes1 such as the chemical action of air and rain water1 and of plants and bacteria1 and the mechanical action of changes of temperature1 whereby rocks on exposure to the weather change in character1 decay1 and finally crumble into soil4 ,transcriber ,note ,all illustrations splitting paragraphs were moved before or after the paragraph4 ,all simple typos were corrected 7i4e41 ,reudemann to ,ruedemann1 pryoxene to pyroxene74