,transcriber's ,notes3 ,small capitals have been converted to ,,solid capitals4 ,illustrations have been moved so they do not break up paragraphs4 ,typographical and punctuation errors have been silently corrected4 ,,how ,,old ,,are ,,fossils8 ,,by ,,sharat ,k4 ,,roy ,,assistant ,,curator ,,of ,,invertebrate ,,paleontology ,illustration ,,geology ,,leaflet #i ,,field ,,museum ,,of ,,natural ,,history ,,chicago #aibg ,,list ,,of ,,geological ,,leaflets ,,issued ,,to ,,date ,no4 #a4 ,model of an ,arizona ,gold ,mine 4#aj ,no4 #b4 ,models of ,blast ,furnaces for ,smelting ,iron 4#aj ,no4 #c4 ,amber--,its ,physical ,properties and ,geological ,occurrence 4#aj ,no4 #d4 ,meteorites 4#aj ,no4 #e4 ,soils 4#aj ,no4 #f4 ,the ,moon 4#aj ,no4 #g4 ,early ,geological ,history of ,chicago 4#be ,no4 #h4 ,agate 4#ej ,no4 #i4 ,how old are ,fossils8 4#be ,d4 ,c4 ,,davies1 ,,director ,,field ,,museum ,,of ,,natural ,,history ,,chicago1 ,u4,s4,a4 ,illustration3 ,,leaflet #i4 ,,plate ,i4 ,a ,,view ,,of ,,the ,,grand ,,canyon ,,of ,,the ,,colorado ,,river1 ,,arizona4 ,,zoroaster ,,in ,,background4 ,photograph by ,o4 ,c4 ,farrington4 ,,field ,,museum ,,of ,,natural ,,history ,,department ,,of ,,geology ,,chicago1 #aibg ,,leaflet ,,number #i ,how old are ,fossils8 8,how old is that fossil and how do you know it80 is a question frequently asked by visitors going through the hall of fossils in the ,museum4 ,a precise answer to such a question is impossible and an adequate one demands a longer time than can usually be afforded4 ,the consequences of inadequate explanation have often proved to be unsatisfactory4 ,the visitor becomes skeptical and instead of taking interest in the subject1 he seems to be confirmed in his doubts4 ,in this leaflet is given a condensed1 general statement of methods of determining the age of ancient life4 ,the information is drawn from the works of various authors1 especially ,barrell's 8,rhythms and ,measurements of ,geologic ,time40 ,it is intended for those who are interested in the age of past life and yet do not intend an exhaustive study of the subject1 nor have free and easy access to its literature4 ,the birth of life was the most momentous occasion in the history of the earth4 ,when one considers the myriads of evidences unearthed by paleontologists and paleobotanists1 they seem to leave no room to doubt the great conception that the life of the land has emerged from the sea4 ,it is1 therefore1 only a natural impulse to look for the remains of this life in the rocks laid down by the ancient seas and to wonder at the vastness of time behind them4 ,since traces of the lowest forms of life have been found in practically the oldest known sedimentary strata1 the problem of determining the age of life necessarily involves the determination of the age of those strata4 ,but unravelling the dead past is not an easy task4 ,one trying to unlock the 8secrets of the cemetery of ,nature's dead10 walks on a shadowy road4 ,his difficulties are many4 ,it is like crossing a deep moat1 climbing a steep wall4 ,various methods have been applied to estimate the age of different periods of the earth's history and much progress has been made toward a successful issue4 ,broadly1 the procedure of different methods is the same4 ,they do not differ in principle4 8,the rates of certain changes at the present day are determined as accurately as possible1 and in imagination1 the respective processes are traced backward in time until limiting conditions are arrived at40 ,until the epoch-making discovery of radium1 the two most outstanding methods used in calculating geologic time were 7#a7 the rate of land erosion and deposition and 7#b7 the rate of derivation of salt 7sodium chloride7 from the land and its accumulation in the oceans4 ,theoretically1 it is simple to use the rate at which sediments are being deposited or solutions gathered into the ocean1 as 8geologic clocks0 for estimating the length of past time4 ,but in practice each method encounters its own difficulties and the results deduced give us at best only a rough idea of the immensity of time involved4 ,i shall not deal with these individual estimates1 but give the mean of several1 which is #ajj1jjj1jjj years1 speaking roundly4 ,as stated before1 it is only a rough estimate4 ,nevertheless1 it confirms the fact that the earth is very old--indeed much older than is commonly believed4 ,in #afej1 ,bishop ,ussher1 in his interpretation of the 8,in the beginning0 of ,genesis1 estimated that the earth was created #djjd years before the birth of ,christ4 ,according to this view the earth is #eica years old today4 ,many cosmogonists and even some geologists of the #aith century held this ,biblical interpretation to be the age of the earth4 ,other ancient religions held that the earth was created much earlier then #djjd ,b4,c4 ,hutton1 one of the founders of ,geology1 in his studies1 found 8no vestige of a beginning--no prospect of an end40 ,one cannot help sympathizing with ,hutton4 ,whoever has made a trip to the ,grand ,canyon of the ,colorado ,river1 ,arizona1 must remember the awe-inspiring depth of the ,paleozoic strata and thousands of feet of ,proterozoic sediments beneath them 7,pl4 ,i74 ,if he has traveled farther north to the ,cabinet ,range1 ,montana1 he must have carried with him an undying impression of the #ce1jjj ft4 of the rocky monument1 there built up by the ,proterozoic seas4 ,illustration3 ,,leaflet #i4 ,,plate ,,ii4 ,,comparison ,,of ,,an ,,ancient ,,and ,a ,,modern ,,marine ,,animal ,,of ,,allied ,,groups4 #a1 ,a ,,cambrian ,,trilobite 7,,after ,,walcott74 #b1 ,a ,,modern ,,horseshoe ,,crab4 ,the present rate of denudation in the ,hudson ,bay region is one foot in about #dg1jjj years4 ,how long then1 has it taken the seas to lay down these miles of sediments1 which are but a small fraction of the whole geologic column8 ,having considered all this1 can we estimate1 even approximately1 the vast length of time that has elapsed between these periods of sedimentation1 the so-called 8gaps0 or 8breaks0--the torn1 illegibly written pages of the history of the earth8 ,indeed1 there are moments when all may feel that it is much beyond their comprehension4 ,but1 man1 by nature1 is at once humble and exalted4 ,he is willing to admit his defeat1 yet his thirst to conquer new knowledge1 to know the truth1 is never satiated4 ,the reason for the failure to arrive at an absolute result is not very far to seek4 ,in computing geologic time1 one has to calculate that which has elapsed1 by some process in nature that takes place in one direction only and that does not change its rate when conditions alter4 ,whatever the method applied1 be it the deposition of sediments1 or the gathering of solutions1 or the losing of heat by the sun and the earth1 its rate of action should be uniform and uninterrupted4 ,it should be independent of the changing conditions of the earth4 ,uniformity of the rate of action is the criterion for precise calculation4 ,but we know that the past was quite different from the present4 ,different conditions have existed at different times during the earth's history4 ,the configuration of the earth1 its climate1 humidity1 temperature and many other factors have varied from time to time and with them the rate of erosion1 deposition and solution has either accelerated1 diminished or ceased4 ,the doctrine of uniformitarianism cannot be assumed in a changing world1 even though our knowledge of the earth of the past can only be gained from a fuller study of the earth of the present4 ,when we consider the rate of sedimentation as a method for estimating geologic time1 we take the total observed thickness of the geologic column1 7estimated to be #gj miles7 and divide it by the rate at which the sediments are now being laid down4 ,but do we know this rate8 ,by taking the average rate of sedimentation of nine large rivers#a now in existence and assuming it to be the rate at which sediments were deposited in the past1 an approximate conclusion can be arrived at4 ,but it will be noticed from the figures of the rate of sedimentation of different rivers that they are widely variable1 the highest being many times greater than the lowest4 ,can we then use an average of nine figures so out of proportion and yet expect a reliable quantitative value8 ,furthermore1 the rate of deposition along the coast1 near the mouth of a large and active river1 is much higher than on a coastline where no rivers empty their sediments into the sea4 ,the rate is also largely controlled by the character of the sediments deposited4 ,sandstone and shale are more rapidly deposited than limestone4 ,moreover1 just as there is no knowledge of the duration of time of erosion between periods of sedimentation1 there is also no record of the amount of detritus that has fallen off the edge of the continental shelf during widespread emergence of the continents4 #a ---------------------------------------- ------------------------ ,river ,drainage ,total tons ,ratio of ,height in feet of areas in annually sediment column of sediment square to water with a base of miles in weight #a square mile ---------------------------------------- ------------------------ ,potomac #aa1jdc #e1eeg1bej #a3 #c1ege #d4j ,mississippi #a1bdd1jjj #djf1bej1jjj #a3 #a1ejj #bda4d ,rio ,grande #cj1jjj #c1hcj1jjj #a3 #bia #b4h ,uruguay #aej1jjj #ad1ghb1ejj #a3aj1jjj #aj4f ,rhone #cd1hjj #cf1jjj1jjj #a3 #a1gge #ca4a ,po #bg1ajj #fg1jjj1jjj #a3 #ijj #ei4j ,danube #cbj1cjj #ajh1jjj1jjj #a3 #b1hjj #ic4b ,nile #a1ajj1jjj #ed1jjj1jjj #a3 #b1jej #ch4h ,irrawaddy #abe1jjj #bia1dcj1jjj #a3 #a1faj #bji4j ---------------------------------------- ------------------------ ,babb4 ,science1 ,vol4 ,,xxi1 p4 #cdc1 #ahic4 ,we have also no record of the vast quantity of shallow water sediments that were stirred up by the penetration of storm waves1 and carried to abyssal depths by the currents and tides4 ,similar uncertainties beset us when we consider the rate of chemical denudation1 that is1 the rate at which salts have been dissolved from the lands and accumulated in the oceans1 as a measure of geological time4 ,here1 again1 we take the total amount of salts that is in the oceans today and divide it by the present rate of annual supply4 ,we know with reasonable accuracy the quantity of salts in the oceans and if it were possible to assume the present rate of supply to be a true mean for all geological time1 a satisfactory age of the oceans might be obtained4 ,but it cannot be assumed as such4 ,an assumption of this nature will only lead us from the domain of exactness to that of uncertainty4 ,aside from various other factors1 neither the area of the continents1 nor their relief was in the past the same as today4 ,consequently1 the stream gradient and its power of dissolving salts from the land surfaces have not been the same4 ,it is also not known how much salt the ocean derived from the shore line and from beds beneath the ocean1 nor how much of the rock-salt beds on the earth that has been precipitated out of ocean water4 ,it is plain1 therefore1 that the rate of any process that is controlled by so many conditions cannot be used 7even making generous allowances for irregularities and inaccessible data7 as a reliable guide to evaluate geologic time4 8,it is a clock10 says ,harker1 8which now hurries and now creeps or stands still1 and it cannot be trusted as a timekeeper40 ,any estimate based on the temperature of the earth1 or of the sun1 encounters similar practical difficulties1 for the temperature of a body may not be constant4 ,it may rise or it may fall4 ,further1 the rate of change of temperature is controlled by a variety of conditions1 such as the amount of energy radiated1 the supply of energy and so forth4 ,nor is there any record of the immense quantity of heat produced by igneous agencies and radio-activity4 ,another estimate1 one of the earliest1 was based on the rate of life transformation in successive periods4 ,the geological series were divided into twelve periods and it was believed that #bj1jjj1jjj years were required for an entire change in the species of each period1 or #bdj1jjj1jjj years in all4 ,this does not include the time in which we have no record of plant or animal life4 ,there is no reasonable debate as to the passage of one species to another4 ,it is clearly manifested in the succession of fauna found today all over the world in the sedimentary rocks4 ,even the most casual student of paleontology is convinced of this glaring truth4 8,the brutal cogency of a slab of fossils could be hated and fought1 but could not be gainsaid40 ,but when we are confronted with the question of setting a standard of measuring geologic time by means of this paleontological record1 more precisely1 through this biological process1 we cannot help pondering over the grave uncertainty of the result4 ,when we fix our gaze upon a trilobite1 a three-lobed1 crab-like creature 7,pl4 ,,ii fig4 #a7 that ruled the seas in the dim days of the ,cambrian period 7p4 #aa7 and see that it was equipped with gills and swimming organs1 with powers of digestion and excretion1 with specific organs of circulation and reproduction and with motor and sensory nerves1 and compare it with one of its tribe1 a present day horseshoe crab1 7,pl4 ,,ii fig4 #b7 we do not find any noticeable progress in structure1 in intricacy or in the degree of specialization4 ,yet the time that has elapsed since the ,cambrian is1 according to a moderate estimate1 nearly #fjj1jjj1jjj years6 7p4 #aa74 ,geologic record testifies that evolution awaits environmental change1 that animals in some way adjust themselves to their environment1 either by discarding or modifying old characters or by acquiring new ones4 ,yet1 what are known as 8immortal0 types1 such as the brachiopods1 ,lingula1 ,crania and ,terebratula 7,pl4 ,,iii figs4 #a-c7 or the pelecypods1 such as ,pecten1 ,pinna and ,arca 7,pl4 ,,iii figs4 #d-f7 or the gastropods1 such as ,pleurotomaria1 ,natica and ,trochus 7,pl4 ,,iii figs4 #g-i71 have withstood all possible environmental changes and have steadfastly held their own ever since we have records of their very early appearance on earth4 ,on the contrary1 we have records of types that have yielded so rapidly to change that their evolution is almost explosive4 ,it is almost incomprehensible how1 within such a limited period of time1 fishes have changed into amphibians1 amphibians into reptiles1 and reptiles into birds and mammals 7,pl4 ,,iv figs4 #a-d74 ,with these conflicting evidences staring us in the face1 with the knowledge that the entire organic world has been subject to earth-wide periods of long stagnation and rapid intensive change1 one may well ponder whether it is within our power to establish a standard for measuring geologic time on the evidence of life transformation4 ,the study of the succession of faunas--the change of one species to another1 can only indicate the magnitude of time involved4 ,it cannot afford any basis1 whatsoever1 for a concrete expression of geologic time4 ,during the last three decades1 a number of radio-active changes of one chemical element into another have been discovered and studies of certain minerals and rocks containing various radio-active elements have created means to calculate their age with remarkable accuracy4 8,a study of the various radio-active elements contained in minerals and rocks10 says ,harker1 8has shown that it is possible1 in certain favorable cases1 to calculate directly their age in years40 ,the radio-active minerals are commonly found in igneous rocks4 ,they are widely distributed all over the world4 ,the parents of the whole series of radio-active elements are uranium and thorium4 ,they possess the highest atomic weights of all known elements4 ,each of these parental elements transforms itself through a succession of changes4 ,the final product of uranium is the formation of the metal lead and the gas helium4 ,these transformations take place in one direction only1 that is1 from an element of higher atomic weight to an element of lower atomic weight4 ,it has also been demonstrated beyond question that these transformations are unalterable by any process whatsoever and that they are independent of temperature1 pressure or any other physical or chemical state4 ,temperatures up to #b1ejj ,c4 and pressures up to #fjj tons per square inch have not been found to influence the rate of transformation4 ,time estimated on the basis of these processes1 therefore1 offers a more reliable result than that obtained by any other method hitherto known4 ,detailed descriptions of how the metal uranium slowly and regularly breaks down in a descending series into the metal lead and the gas helium1 will be found in the literature on radio-activity4 ,for our purpose1 it suffices to say that according to ,barrell1 an atom of uranium which breaks up will ultimately give rise as a stable product to eight atoms of helium and one atom of lead4 ,since the rate of transformation is known1 data for calculating the age of the mineral and with it the rock formation of which it is a part1 can be obtained by measuring the quantity of helium and lead in the rock and comparing it with the quantity of uranium in the same volume of material4 ,but1 as helium is a gas1 it is likely that a certain portion of it leaks out and consequently the estimate of age on the basis of how long helium had been in contact with uranium and lead is to be regarded as a minimum estimate4 ,for example1 the age of the mineral thorianite that occurs abundantly in the sands and gravels of ,ceylon has been estimated to be #bhj1jjj1jjj years1 but the mineral is doubtless much older1 as1 ever since it was broken away from its original home in the pegmatite dikes of ,ceylon1 it lay exposed to the action of weathering and it was1 therefore1 very likely that during all these years a certain percentage of its helium contents had leaked away4 ,but estimates based on the lead ratios of radio-active minerals offer results consistent among themselves4 ,that is1 whenever fresh1 primary1 uranium-bearing minerals of the same geological age have been examined1 the lead ratios are always found to remain constant4 ,the value of the ratios increases or decreases as the geological age of the respective mineral increases or decreases4 ,in other words1 the lead ratios are in keeping with the geological age4 ,the procedure of applying the lead ratio in calculating geological time can be briefly stated thus3 ,the rate of production of lead from uranium can be readily calculated4 ,the rate at which helium is generated is accurately known and the quantity of lead liberated in the same time is approximately #f4e times that of helium4 ,in a year one gram of uranium produces #a4be #aj grams of lead1 and at this rate #h1jjjb million years will be required for the production of one gram of lead4 #b ,a more recent and accurate computation reduces this #h1jjj million years to #g1ejj million years4 ,there is no serious difficulty in applying this method for measuring geologic time1 except that it is necessary to determine whether the lead is of radio-active origin or original lead4 ,the presence of original lead is likely to mar the constancy of the lead ratio essential for accurate results4 ,but ordinary lead need not be confused with uranium lead1 as the atomic weight of ordinary lead is #bjg4a and that of uranium lead is #bjf4b4 ,values between these two figures imply a mixture of two types of lead4 ,for reliable calculations1 a series of fresh1 primary minerals of the same geological age showing a constant lead ratio of atomic weight #bjf4b needs to be examined4 ,the following table shows the geologic time that has elapsed between the first evidences of life and the present1 as calculated by ,barrell from radio-active data4 ,the figures are his minimum and maximum estimates4 ,it will be noticed from the figures in the table that the earliest life of which we have fossil records is about #a1ejj1jjj1jjj years old4 ,from this1 it could be safely concluded that the inception of life on earth must have taken place much earlier4 ,it is quite significant that each geological era1 occasionally a geological period1 has its characteristic grouping of life developed from the life of preceding periods4 ,as we climb higher in the geological column1 life becomes more and more complex and specialized4 ,from the one-celled life of the ,archeozoic it passes through the invertebrates--fishes--amphibians--repti les--birds and mammals to man of the ,recent time4 ,since ,barrell's publication of the estimates of geologic time as measured by means of radio-activity1 some further studies have been made along the same line1 but1 as no generally accepted results have shown any marked differences from ,barrell's results1 it has been deemed advisable to use his age data as perhaps our present most adequate guide as to the length of geologic periods4 ,although the measurable forces of radio-activity give on the whole a remarkably satisfactory time gauge and are doubtless more accurate than any method here discussed1 it must not be considered that the ages given 7p4 #aa7 are absolute4 ,the knowledge of geological time is of more importance for the comparative than for the absolute magnitude of the results obtained4 ,just as the study of astronomy gives us the conception of the vastness of space1 so does the study of geology reveal to us that of the immensity of time4 ,,sharat ,k4 ,,roy4 ,illustration3 ,,leaflet #i4 ,,plate ,,iii4 8,,immortal0 ,,types4 #a1 ,,lingula4 #b1 ,,crania4 #c1 ,,terebratula4 #d1 ,,pecten4 #e1 ,,pinna4 #f1 ,,arca4 #g1 ,,pleurotomaria4 #h1 ,,natica4 #i1 ,,trochus4 7,figs4 #a1 #b1 #g1 after ,hall1 #c1 #i1 after ,zittel1 #d1 #f1 after ,dall1 #e1 ,pal4 ,n4 ,j4 ,i1 #h1 after ,cragin74 ,drawings by ,carl ,f4 ,gronemann4 ,,the ,,geological ,,time ,,table #a4j ,one million years ---------------------------------------- ------------------------ ,,time ,,scale ,,eras ,,periods 7,after ,barrell7 ,,characteristic -------------- ,,life ,minimum,maximum ---------------------------------------- ------------------------ ,,psychozoic ,recent ----------------------------- #a #a4e ,,age ,,of ,,man ,pleistocene ---------------------------------------- ----------- ,pliocene #g #i ------------------------------- ,,cenozoic ,miocene #ai #bc ,,age ,,of ,,mammals ------------------------------- ,,and ,,modern ,oligocene #ce #ci ,,flowering ,,plants ------------------------------- ,eocene #ee #fe ---------------------------------------- ----------------------- ,cretaceous #ie #aae ------------------------------- ,comanchian #abj #aej ,,mesozoic ------------------------------- ,,age ,,of ,,reptiles ,jurassic #aee #aie ------------------------------- ,triassic #aij #bdj ---------------------------------------- ----------------------- ,permian #bae #bhj ,,age ,,of ,,amphibians ------------------------------- ,,and ,pennsylvanian #bej #ccj ,,ancient ,,floras ---------------------------------------- ----------- ,mississippian #cjj #cgj ------------------------------- ,,age ,,of ,,fishes ,,paleozoic ,devonian #cej #dbj ---------------------------------------- ----------- ,silurian #cij #dfj ------------------------------- ,,age ,,of ,,higher ,ordovician #dhj #eij ,,shelled ------------------------------- ,,invertebrates ,cambrian #eej #gjj ---------------------------------------- ----------------------- ,,proterozoic #ibe ,,age ,,of ,,primitive ,,invertebrates ------------ ,systematic ----------------------------------- classification ,,long ,,erosional ,,interval ------------ variable ----------------------------------- ,,dawn ,,of ,,unicellular ,,archeozoic #aejj ,,life1 ,,algal ,,forms ,,reported ---------------------------------------- ------------------------ ,,bibliography ,,barrell1 ,j4--,rhythms and ,measurements of ,geological ,time4 ,bull4 ,geol4 ,soc4 ,am41 ,vol4 #bh1 #aiag1 pp4 #gde-ijd4 ,,becker1 ,,geo4 ,e4--,the ,age of the ,earth4 ,smithsonian ,misc4 ,coll41 ,,lvi41 ,no4 #f1 #aiaj4 ,,chamberlain1 ,t4 ,c4--,diastrophism and the ,formative ,processes1 ,,xiii4 ,the ,time over which the ,ingathering of the ,planetesimals was ,spread4 ,jour4 ,geol41 ,vol4 ,,xxviii1 #aibj1 pp4 #fge-ha4 ,,harker1 ,a4--,geology in relation to the exact sciences1 with an excursus on geological time4 ,nature1 ,vol4 #ie1 #aiae1 pp4 #aje-aji4 ,,holmes1 ,a4--,the ,age of the ,earth4 ,harper and ,bros41 ,london and ,new ,york1 #aiac4 ,,joly1 ,j4--,radioactivity and ,geology4 ,van ,nostrand1 ,n4 ,y41 #aiji2 ,an ,estimate of the ,geological ,age of the ,earth4 ,trans4 ,roy4 ,soc4 ,dublin1 ,,vii4 #ahii1 pp4 #bc-ff2 ,the ,age of the ,earth4 ,phil4 ,mag41 #fth ser41 ,vol4 ,,xxii1 #aiaa1 pp4 #cei-hj4 ,,sollas1 ,w4 ,j4--,presidential ,address4 ,quart4 ,journ4 ,geol4 ,soc41 ,vol4 #fe1 #aiji1 pp4 cxii2 ,proc4 ,geol4 ,soc4 of ,london1 ,sess4 #aijh-i1 pp4 i-cxxii4 ,,walcott1 ,c4 ,d4--,geologic ,time1 as indicated by the sedimentary rocks of ,north ,america4 ,smithsonian ,rep4 #ahic1 pp4 #cja-ccd2 ,jour4 ,geol4 ,vol4 ,,iii1 #ahic1 pp4 #fci-fgd4 ,illustration3 ,,leaflet #i4 ,,plate ,,iv4 ,,evolution ,,of ,,fish ,,to ,,mammal-,,like ,,reptile4 #a1 ,,fish4 #b1 ,,amphibian4 #c1 ,,reptile4 #d1 ,,mammal-,,like ,,reptile4 7,figs4 #a1 #b1 after ,klaatsch1 #c1 based on ,williston1 #d1 based on ,gregory74 ,drawings by ,sharat ,k4 ,roy4 ,,printed ,,in ,,the ,,united ,,states ,,of ,,america ,,by ,,field ,,museum ,,press