, # • .. • ' • • •
HOW OLD IS THE ' • 't EARTH? + s. o eo(, '-J,1 • "' • • I BY CHESTER A. REEDS • .. , .. - • • •• • • ~ ••
• •• • - ~ -. •
• • GUIDE LEAFLET SERIES, No. 75 THE AMERICAN MUSEUM OF NATURAL _HISTORY • • NEW YORK, N. Y. • .. Reprint~ from Natural History, March-April, 1931
., THE INNER GORGE OF THE GRAND CANON OF THE COLORADO RIVER
From a Painting by Gunar Wildforss, 1930
LINCOLN ELI.SWORTH COLLECTION
IEW looking west-northwest across the mouth of Bright Angel Canon V from near the Kaibab Suspension Bridge. The Colorado River is in the left center, and flows here at an elevation of 2450 feet above sea level. The varied rocks of the north wall of the inner gorge appear in the foreground and in the mid-distance, with the isolated Tower of Set, 5997 feet, appearing in the background, left center. In the right center the towering mass of the Cheops Pyramid, 5350 feet, crowns the slope to the inner gorge. The t win peaks of Isis T emple form the highest elevation 7028 feet, in the right background. T o the south of the river in the left margin appears a portion of the rocks in the south wall of the inner gorge with the isolated peak of Dana Butte, 5025 feet, prominently in the background. The geological section, which is of special interest, is explained more fully in the article " How Old Is the Earth ?" HOW OLD IS THE EARTH?
BY CHESTER A. REEDS
GUIDE LEAFLET SERIES No. 75
THE AMERICAN MUSEUM OF NATURAL HISTORY NEW YORK, N. Y .
R ep rinted from Natural History, March-April, 1931 • @ E. H . N ewman, Pub /;shers Photo Service, N . Y.
THE COL ORADO R rvER AT T H E BoTTOM OF T H E G R AND CANON OF Am- ZONA HOW OLD IS THE EARTH? The Earth Reveals Its Age By Hour-glass Depo ition of Sodium and Sediments, and the Atomic Disintegration of Radioactive Element
BY CHESTER A. REEDS Curator of Geology and Invertebrate Palreontology, American Museum
T may be stated at thP outset that primitive forms of life in rocks of very nobody knows just how old the earth ancient origin, all point to physical condi Iis. There are certain criteria available, tions on the surface of the earth that are however, which indicate that the oldest similar in every respect to those enduring rocks are of the order of 2000 million today. Various folded gneisses and years. There are data which imply that schists, without vestiges of life, much the upper limit of the age of the minerals distorted and frequently impregnated with is about 3000 million years. This may be volcanic injections, constitute the oldest considered the lower limit of the age of the rocks exposed on the earth' surface. earth's material. Iron meteorites have The earth, although very old, has a re been analyzed which yield a maximum markable history. The various steps in age of 2600 million years. These are its development are in some instances still stupendous figures. The lower figure of obscure, but they are becoming more two billion years as a minimum age for apparent with the growth of knowledge the earth implies that it has encircled the concerning the earth. sun as many times, and that during this Spectroscopic analyses reveal that 49 period it has turned on its axis 730,500,- of the 90 chemical elements found on the 000,000 times to afford as many days earth have been recognized in the sun. of light and darkness. In fact, astronomy teaches that the 1091 The presence of ripple marks, sun-crack members of the solar system have orig impressions in muds, water-worn pebbles, inated from the same material. Various rounded sand grains, seasonally banded theories as to the origin of the earth clay , lime tone depo it , and vestiges of postulate that the earth and the other 4 NATURAL HISTORY planetary bodies in our olar system were planetary velocities varying from 9 to born of our sun when it was in a giant-star 4 7 miles per second. Due to the great stage. This transformation of the sun i resistance offered to their passage by the supposed to have been induced by the earth's atmosphere, which is estimated to close approach of a passing star several be 90 to 100 miles in thickness, the solid times more massive than the sun itself . portions of most meteors burn up before The resulting effect of such a close ap reaching the earth. In addition to the proach was the setting up of great tidal ash of burnt-out meteors a minimum of stre ses in the sun and the drawing out of one meteorite per day reaches the earth's two long filaments of gaseous matter from surface. opposite sides of the sun's surface. After The portions of 700 meteoritic falls ex the large star pa sed on, the filament on hibited in various museums are composed the far side of the sun as well as a portion primarily of either nickel-iron, or of stone of that on the near side may have been specimens, or, of combinations of these drawn back into the sun; however, a two kinds of matter. The stony meteorites considerable portion of the filament re re emble the light colored felsitic lavas mained in space subject to the influence of of the earth. There are differences in the sun. In the course of time t}:ie matter texture in each, however, which the in this filament was gathered together skilled observer readily detects. The about certain nuclei to form the nine iron meteorites with nickel, troilite, planets and their satellites. The ma carbon, and other inclusions are not found terial was original- duplicated on the ly in a gaseous state. earth. Some 29 Later it passed to a elements found on liquid state through the earth have been loss of heat by radi detected in meteor ation from its sur ites. On the other face, and fin ally, as hand, six mineral in the case of the compounds have earth, into a solid been noted in mete state, at least for orites, which have the outer crustal not been found on portion which may the earth. be 40 miles in thick It may be Rtated ness or about 1/200 thus that the earth, of the radius of the the meteorites, the American Museum of Natural History earth. sun, the moon, and A TONE METEORITE, JOH STOW I CO LORADO, The meteors, METEORITIC H OWER the stars are dis which enter t h e This stony meteorite weighing 42 lbs. oz., was seen to fall tantly related. The following four explo ions, at 4:20 P .M ., July 6, 1924. It is upper levels of the coated with a thin black crust. The gray stony matrix of earth and its moon the interior is shown by the white spots where the crust has earth's atmosphere been peeled off with diameter of in great numbers, 7918 and 2162 estimated to be 20 million per day, may miles, re pectively, are intimately re be remnants of the original filaments, or lated to the sun, which is 866,400 miles of like matter from outer space. l\fost of in diameter. Although their densitirs, the e meteors are small, one to two-tenths as compared with equal volumes of water, of an inch in diameter. Upon entering vary, the density of the earth being 5.52, the earth's atmosphere they travel at the moon 3.40, the sun 1.39, the e differ- HOTT OLD I THE EARTH?
THE MY TIC POWER OF THE COLORADO RIVE R I N ARIZO'.\:A A view from the upwarped plateau rim, 7000- 000 feet above the sea, into the outer and inner gorge of the Grand Cn f1on where the river flows at a level of 2400 feet. This canon is 217 miles long, from 8 to 20 miles wide, and more than a mile deep. It was eroded by the river during the last one million years ence are explained by the different states earth to pass from a gaseous to a olid of like matter, the earth and moon bring state is of course problematical. solid and the sun gaseous The fact that According to the Planetesimal Hypo the earth rotates on its axis at a rate of thesis proposed by the late T. C. Chamber 1 .5 mile per second, and about the sun lain and F. A. Moulton of the Univer ity at a rate of 66,000 miles pc>r hour, also of Chicago in 1905, all but the central implies that the mass of the earth, which core of the earth, which is 4346 miles in ,Yeigh 6590 million million million tons, diameter, has been built up by the infall of is controlled by the larger mass of the sun, planetesimal matter. Since but a mall which revolves once on its axis in 25 amount of such planetesimal, or meteoric days, and weighs 1. 983 X 1031 grams. matter, is now ad Jo• SECTOR' Of LOCITl(S OF phic rock , there are EARTH SHOY..tN THOUAKE \\..4\£"S In addition to the outer LAYEPS FROM T \ARIOUS DEPTHS SURFACE TO TH V v uccessive zones of C E NTER l (1<"'1 .'stil(P\ "1 /Su) ed im entar y layer, ROCK INl[R PR ETA 55 32(11 AFTER R.A.0ALY,19 0 57 Co3J material and a central VELOCITIES OF DISCOUTl ~ UITY which varies in thick EARTHQ,UAK E WAVE 30 C.a025 Co37 Af'"fER 8 , U UTEN8E PCi 't S OJ Co J7 core which differ from 0ISC0N TlfliUI T'( ness from 0-5 km. with Q.'t 5 7 9 't '+ ·t,O- 70 80 't5 one another in density, OISCO NTI NUITV density 2. 7, the crust i b0·70 7 9 ,.'t 1200 122 5 675 in chemical compo i 1700 125 7 2 !5 compo ed of three 2~50 13 25 7 5 2900 130 725 tion, and in elasticity. DISCO NT l l'I UI TY zones of rock each 2 900 8 5 The earth as a whole is 39.59M T£R)OJ70 II 0 separated by planes of more rigid than steel. A 30° ECTOR OF THE EARTH discontinuity as follow : SHOWING LAYER FROM THE SU RFACE TO THE Earthquake waves are CENTER. Tars DIFFERENTIATION OF THE IN- (1) granitic layer 0-30 TERIOR OF THE EARTH INTO ZONES IS BASED transmitted through it. U PON VARIATIONS NOTED IN THE TRANSMIS- km. in thickness, den SION OF E ARTHQU AKE WAVES THROUGH THE Each earthquake re EARTH . THE INNER CORE DOES NOT TRANSMIT ity 2.7 THE SECON DARY OR TRANSVERSE SEI MIC cords three principal WAVES, A KIND DEVELOPED ONLY IN SOLID , (2) granodiorite layer BENCE, IT l BELIEVED TO BE IN A ''LIQU ID" kind of waves on a OR ''GASEOU S'' STATE 30-45 km. in thickne , ei mograph, namely: density 2.7+ primary, secondary, and main waves. (3) gabbro layer 45 to 60-70 km. , The primary or longitudinal waves pass density 2.9. through all portions of the earth. The Below the crust lie a hot, vitreous, econdary or transver e wave , a kind basaltic layer 60-1200 km. in thicknes , developed only in solids, pass through only density 3.3. This is followed by a layer the outer portion of the earth; they do 1200- 2450 km. in thickness, which Daly not pass below a depth of 2900 km. It is believes may partake of the nature of at this depth, 0.45 of the radius of the peridotite, while H . Jeffries (1929) refers earth, that the inner core begins. Since to it as the dunite layer, den ity 5.0. At this type of wave is not tran mitted a depth of 2450- 2900 km. there occurs a through the inner core, thi portion of the zone composed perhaps of pla tic iron, HOW OLD I S THE EARTH? 7 density 9, where seismographic waves successive geologic stages involved in the low down. This would indicate that it is upbuilding of the earth. If one could transitional in character from the more or visualize, even in the crudest fashion the le s silicate layers above to the great inner changes in geography that have taken core of the earth below. The inner core place at regular intervals, say 100,000 with great pressures and temperatures years, the equential history of the earth resulting from its superimposed load is would be in large measure solved. In believed by H . Jeffries (1929) to be liquid accordance with the normal sequence of iron, by Daly (1930) to be in a "fluid" events such a series of pictures should or "gaseous" state. Its average density is begin with the birth of the earth, from 11.5. Tt i probable that the inner core of the parent body, the sun. One hundred the earth was originally composed of ma thousand years later a sufficient change terial resembling that found in iron mete would have taken place in the earth to orites. Iron meteorites have a specific depict the second scene. A large number gravity of 7 or higher. The idea of a of pictures would have to be sketched, liquid inner core is supported by present 30,000 in fact, if the earth is three billion day seismology, for the secondary or years old, before the present day is transverse wave of an earthquake, a kind reached, with its magnificent panorama of appearing only in solids, is not trans continents, oceans, irregular coast lines, mitted through the inner core. mountains, plateaus, plains, river , lakes, The methods of palreogeography afford seas, snow fields, glaciers, deserts, and theoretically a splendid insight into the various forms of plant and animal li fe, © E. H . Ne:wman, Publishers Photo Service, N . Y. HERMIT CAMP AT THE EN'D OF THF, HERMIT TRAIL, GRAND CA:filON This t_ourist ca!Ilp, 3700 feet belo,!· the south.rim, is a half mile east of Columbus Point, the imposing central rock tower. This towering spur 1s composed of horizontal sediments that are green, mauve, red, and gray in color. The camp oYerlooks the inner gorge of the Colorado River, 700 feet deep NATURAL HISTORY FOOTPRl:\'TS OF A LABYRINTHODO T, COCO;\"l NO SAi\" D. TONE, GHA:s;D CA -o:-; Footprints made by an amphibian of Permian age as the sands of the Coconino formation were being deposited 210 million of years ago. The ands were moist when the impressions were made. The weil:(ht of the animal compacted them and the footprints were covered and preserved not to mention the cities and other works taken and analyzed. From more than of man. 8000 analyses F. v\T. Clarke in hi Data To fault i to be found with the idea, on Georhem1·stry, 1924, observe$ that for geologic processes are continuou, and taking the continents as a whole they are they have been so throughout the im lowered by solvent denudation one foot in mensity of geologic time. The difficultv 30,000 years. From measurements of the in preparing such a series of pictures suspended matter collected in the analyzed arises from the fact that the records of ample he concludes that the chemical past events, which are preserved in the denudation represents but 30 per cent of earth itself, are omewhat fragmentary the total denudation. This gives a mean and, furthermore, they are not dated in rate of total denudation at the pre cnt terms of yf'ars, as man dates his present time of one foot in 600 year . chronology. The average height of all lands above The data most frequently used in sea level has been computed to be approxi estimating the age of the earth are tho e mately 2300 feet. The average depth of ba ed on g<"ologic proces e such as ero the oceans i8 about 13,000 feet. If the ion, sedimentation1 and deformation. land surface is lowered one foot in 8600 The e processes are in evidence on the years and the average height of land urface of the lands. For epochs; other above the sea is 2300 feet then it would than the present, these data are to be take 19,780,000 years to erode the lands found in the stratigraphic record as to sea level, assuming that the rate con preserved in the crust of the earth. tinued uniform to the end, which i not The rate of erosion of the lands is of likely. Granting that the oldest rocks on value as a criterion. Sample of water from the surface of the earth are approximately representative streams for various 2,000,000,000 years old, that the rate of climates and topographic relief have been erosion continued to be onr foot in 8500 HOW OLD IS THE EARTH? 9 years throughout all this time, and that cerned; at such times shallow seas have the lands were uplifted at the close of spread far and wide over base-leveled each complete erosion period, then the lands; new areas of deposition have thus lands would have to have been uplifted arisen; sedimentation accompanied by 101 time to afford continuous erosion. slow subsidence in well defined trough The American geologists, Powell, Dut followed; then folding, crumpling and ton, and Davis, have shown that the overthrusting of the horizontal strata ap lands have been base-leveled frequently peared as the result of lateral com pre sion; during geologic time. To this level this was followed by a general uplift of the surface Davis applied the term peneplain. folded rocks into high mountains by forces Each peneplain was developed as the acting from beneath the crust. Such up result of a cycle of erosion. Many ancient lifts were frequently accompanied by the peneplains lie buried and preserved as intrusion of igneous and volcanic rocks unconformities between different beds of into the distorted mass. With the uplift sedimentary rock; others have been of the region a new cycle of erosion was elevated and more or less destroyed by inaugurated, the agents of erosion again later cycles of erosion. These later cycles renewed their f'fforts to reduce the new are uncompleted, since before any one of landscape to a peneplain. This in brief them could be finished the lands were up is the history of various regions of the lifted and a new cycle inaugurated. In earth's crust, particularly where numerous fact no extensive peneplains, not up old and young mountains exist. lifted or dissected, are known to exist at While the rate of denudation in the the present time. various cycles of erosion has not been evertheless, it is apparent to geologists preserved, the sediments that were de that the earth has been in repose re posited in the shallow seas lying upon and peatedly, as far as denudation is con- about the margins of the continents and FO, IL ALG./E IN A ROCK WALL, PHANTOM RAKCH ME . HOUSE, BRIGHT A'.' Each era is divided into period 1 which r r f 1 THE RELATION OF THE ROCK URF ACE are ba ed upon a definite series of rock Z.'1'11 OF THE EARTH TO EA LEVEL zsooo A diagrammatic representation of the relative representing one or more cycle of sedi areas of the rock surface (lithosphere) at z.0000 various levels above and below sea-level, mentation, developed during an undefined expre sed in feet and in percentages. (:.Iodi- 15000 fied from R . D . alisbury) interval of time. Locally f'ach period 10000 E.p1conicinenta.l Zone M~~r,__l,.AfJ4 __1,..:e:'~1:._=---- 5ea. Level ------_t'.1_e_a._~ _l:,,;_v_ej -~--E;~i:_t_h)__ ~~.rf ~_c~------~ ~;~• 7500 11) 00 15000 lOOOO lSOOO )0000 35000 10 ,. MILLIONS LEflD M. GEOL. GEOLOGICAL M. GEOLOGI CAL AGES CENOZOIC EPO CHS M. OF Y EARS RAT I OS YE ARS ERAS PERJODS, SYSTEMS Y. PREDOMINANT LIFE RADIOACTIVE ORE S Y. RECENT IOOOO YRS . PLEISTOC NE PLIOCENE b 7 MIOCENE 12 19 100 OLIGOCENE lb 35 EOCE NE 2 0 55 PALEOCENE 5 6 0 RADIOACTIVE ORES : 200 ERM \AN . ;25 cc·::·: URANINITE S , PENNSYLVANIAN 35 : .,:c.;=J AfrE_OF/ NORTH CARO LINA 239 MISSISSlPPIAN= 50 . ·\, f' MPr\BlANS 100 URANINITE S , 300 0 GLAS TON BUR Y, COIJ N. 2 83 DEVONIAN 50 . 'Acfr ·_OF . o.o5 _ 360 _N 374 URANINITES , 0 SI LU RIAN ./. i+O ... ,:. FJSHES. 400 w BRANCHVILLE CON N.3 74 44-3 0.06 _ 4- 30 . J _ OR VIC IAN {j):€lf5 CORACITE <( ~'a /'./\tt ~'eQf )";: -~ LAKE SUPERIOR 4-4- 3 500 0.07 _ 498 .0.. . ·\7-o -}~VE'.R--TE BRAT!:S 573 0.08 - PITCH BLE NOES, 590 KATA NG-, AFRICA 573 600 HO RIAN ITES, 64-0 0.09 - 63 CE Y LO N 587 700 0.10 _ 700 0. 11 _ 800 8't0 0.12_ 831 ft.-,..r : :. ~ 890 0. 13 _ 89 ·;[./· BR OC.GER \TES, 900 1,~-: MO S S.NORWAY 89 C ·-~~? 0. 14- _ 96 I ;:.t, RADIOACTIVE ORE, ~7 0 0 LA RY, AUSTRALIA 9 B 7 IODO URANINITE S , 1024- 0. 15_ 1026 r.,!,-~'::-,';±-l:,H,..:,,,,,-:,F.-"'-:-,-1, >'·~ ONTARIO,CAN A OA l0 2 ~ 1056 ,:)• CLf: V EITES, 1087 0. 16_ 108 /\-;_; ARE NOAL, NOrtWAY I 05 6 1100 3\":_::: SAMA RS KIT ES, ;\ •J..,• DOUGLAS CO ., COL . 1087 0. I 7 _ I I 5 ::r-, 1200 1200 0. 18_ 1212 b.\;,""*-l,:,,:;t,"'*-",;,:' 't~; 1257 HACKINTOSHITE, 0.19_ 12 7 WODINGAAU STRALIA 1257 1300 14- 0 URANINITE , 14-65 K E Y STONE S. D. 1469 1500 0.24 _ 1600 0.25 _ 1700 1800 URANINITE, 0 .28_ 183 SINYAYA PALA, 18521------f'.:..S.:.:""'-"""-"'='"""""' CARE LIA U 5.5 R. I 852 POSITION ONE ATOM OF CAMBRIAN AND LATERR0C! Ewing Gallou:ay, N . Y. IXNER GORGE OF THE GRAXD CANO:-.', AIHZOXA View as in fronti piece. Looking down the Colorado River from the Kaibab suspen ion bridge. Rock ection from river bed to top of Isi T emple: Archeozoic: V. Vi hnu schist; Proterozoic: B. Bass li mestone; H . H akatai hale, h. hinumo quartzite; Palaeozoic: (Cambrian) T. T apeats sand tone, BA. Bright Angel shale: (M ississippian) R . Red wall lime tone, (Permian) s. upai sandstone and shale, C. Coconino and tone and system of rocks is further divided into mentary rocks ; lime tone , andstone , epochs and formations of rocks. These and shale representing the Permian, local de ignations, which are numerous Mississippian, and Cambrian period of and variable from place to place, have not the Palaeozoic era. Below the e level been included in this general chart. strata the river has cut its inner gorge To illustrate the meaning of portions of through tilted sedimentary rock ; quartz this chart the wonderful section of rock ite, limestone, and shale, ome two mile exposecl in the Grand Canon of the thick, which are of Middle Proterozoic age. Colorado River in Arizona may be cited. Below this series the river ha cut it way Across a plateau, the upper surface of into a cry talline basement rock, without which ri es from 7000 to 8000 feet above tratification,which belongs to the earlie t sea level, the Colorado River has eroded a era, the Archreozoic. While thi great trench about 217 miles long and a mile section is wonderfully impres ive to tho e deep at the we tern end. This trench who vi it the Canon, the tory ha been consists of two conspicuous feature , one, but partly told. an outer cafion, which i 4600 feet deep The ancient basement rock are epa from the north rim and from 8 to 20 miles rated from the overlying Proterozoic eries across, the other, an inner o-orge which is by a great erosion interval. This interval another 1000 feet deep, narrow, and V- is represented in the ection by an uneven haped in cross section. The buttressed surface known as an unconformity. Prior wall of the outer canon are composed of a to occupying their present position, these succes ion of horizontally stratified sedi- ba ement rocks in the bed of the river HOW OLD IS THE EARTH? 13 which arc 3000 feet above sea level, were worth have collected primitive form of deeply buried, crushed, smashed, and re plants known a fossil algre, from the crystallized by the processes of diastro Bass limestone, which appears just above phism as they lay at a lower level beneath the base of the Unkar series. The e are a thick cover of rock. This cover was the oldest fossils in the Grand Canon. removed slowly by surface weathering, Following the long period of deposition, wind, and running water, acting through diastrophism renewed its activities. The out a complete cycle of erosion. The Proterozoic series was faulted and tilted, preF-ent erosion of the Grand Cafion is but in fact, great block mountains thou ands a small beginning as compared with the of feet in height were uplifted. grrat erosion period under consideration This epoch of mountain uplift wa fo l which was completed perhaps 1,200,000,- lowed by a econcl long period of erosion, 000 year ago. during which time the region wa again On t.op of the gently subsiding pene worn to a smooth plain, except for a few phlin thus established, sediments of the low lying hills. In many places the thick Proterozoic Chuar and Unkar series were Proterozoic series was entirely removed and slowly deposited. It was a long enduring the basement rocks again exposed except period, for the deposits are more than where a few downfaulted blocks of the 11,000 fept in thickness. The sediments Proterozoic rocks were presnved. These were at least partly of marine origin, for downfaulted blocks of sediments are all Dr. David \Yhite and Mr. Lincoln Ells- that remain of the great Proterozoic era in SILICIFIED FORM OF FOSSIL ALGA'J I N THE BAS, LIMESTONE Collected by i\Ir. Lincoln E llsworth from the middle Proterozoic rocks, near the mouth of Bright Angel Canon, Arizona. This specimen is of exceptional interest to students of the early remains of life. The Radioactive Chart of Geological Time indicates that it is about 940 million years old AT[:RAL HI TORY this region. A second great line of uncon tended over the whole di trict of the fo rmity separates the Proterozoic rock Grand Canon. The distant strata repre- from those of Palreozoic age. ent depo it of Permian, Mesozoic, and As the land again ank, the ea of Tertiary age. They are about a mile in Cambrian time rolled in over the smoothed thickness. Two major cycle of erosion plain to inaugurate ano ther great era of are preserved in these rock , one at the depo ition in thi region, the Palffiozoic. end of the Permian, the other at the clo e Horizontally disposed sandstones, shales, of the Me ozoic era. Each denote pro and lime tones were deposited. Amongst longed ero ion and a great interval of time. them may be found the fo il shells of Thi Grand Canon section, although ex Yarious invertebrates, the tracks and re tremely intere ting and impre sive, repre main of trilobite , et cetera. In the sent but portion of the geological time wonderfully impressive Palreozoic series scale. of bed , the Orodovician, Silurian, and We have now suggested briefly the part Devonian periods are missing. They are played by the great geologic processe, repre ented by an unconformity. We during the upbuilding of the earth. The hall know more about what happened to question arises how long have the e force them when t he Grand Canon is more fully been acting? While various criteria have explored. been used in the investigation of thi As one views the Grand Canon from the problem, the data mo t frequently con rim at El Tovar, it is difficult to realize sulted are the sodium alts of the ocean , that the rim rock, the Kaibab limestone, the thickness of the sedimentary rocks, of Permian age, is not the top of the and the radioactivity of the igneou rock . serie of ediments. The great cliff The sodium in the oceans has been on the north and east that overlook the derived from the land by the weathering region are the higher strata that once ex- of igncou rocks. It has been transported GLACI AL BOWLDER OF VARVED CLAY OF MIDDLF: PROTEROZOIC AGE The seasonal layers of this compact rock were depo ited in a glacial lake of Cobaltian time, 1100 million years ago, in Ontario Province, Canada. This specimen of the oldest known glacial period was carried by the ice of the last Plei tocene glaciation to Battle Creek, :\lichigan. E . :\I. Brigham collector HOW OLD IS THE EARTH? 15 from the land to the sea by river carry mentary deposits of various ages as ing it in solution. As noted by J. Joly of 529,000 feet or 100 miles. J . H. Bretz, Dublin in 1899, the mass of the ore.an 1926, on the basis of several estimates waters is 1,180,000 million million ton . obtains an average rate of accumulation The percentage of sodium in the oceans was of one foot in 880 years. These figures calculated by him give a total of to be 1.08 per cent 465,520,000 years as by weight, so that the a.mount of time there are 12,600 reqµired for the de million million tons position of the sedi of sodium in the me.qtary record. oceans. The This estimate amount of odium does not include, contributed by the however, the beds rivers to the sea w h i c h w e re d e annually has been p o sited in e pi variously estimat con tin en tal seas, ed. After applying uplifted and subse certain corrections, quently removed A. Holmes, 1927, by erosion, leaving calculated that the only an erosion yearly increment V ARVED CLAY OF LATE PLEISTOCENE AGE plane as a record amounts to 35 mil HAVERSTRAW, N. Y. of the event . This partial section of Haverstraw brick clay, 30,000 years l i o n to n s. The old, was deposited seasonally in fresh water as the ice of the Neither does it take last glaciation retreated northward. The space between pin~ amount of sodium represents a year. The lighter colored layers of fine sand are into consideration the summer deposits; the dark bands of clay are the winter in the sea divided layers those great gaps by this annual rate separating the five gives 360,000,000 years as the age of eras of geologic time when sedimenta the oceans. tion was · presumably confined to the This calculation is based upon the pres margins of the continental platforms. ent rate of denudation and delivery. It is Ocean waters now cover the margins of most probable that the rate is much the continental platforms to a depth of higher now than during many of the 600 feet and embrace continental areas former geologic ages when the lands were totaling 10,000,000 square miles. J. less highi less extensive, and the seas more Barrell, 1917, notes that geologic proces widespread. To account for these differ ses, embracing erosion, sedimentation, and ences, J. W. Gregory (1921) recommends deformation recur in composite rhythms that the present estimates based upon in which landscapes alternate with sea sodium should be multiplied by five giving scapes and geosynclinal areas of sedi a total of 1,800,000,000 years as the age of mentation with high mountains. The the oceans. processes of sedimentation are complex The age of the earth based upon the and variable, defying rates of deposition. thickness of the stratified formations is Areas of sedimentation alternate with more difficult to apply since the average scour and fill, the resulting product annual rate of deposition of sediments is represents merely the balance between not definitely known for the present or for these two processes. In some areas sedi past epochs of geologic time. A. Holmes, ments may not always have reached so 1927, estimates the thickness of the sedi- far, in others they may have been carried 16 ATURAL HISTORY away to more distant spots, leaving mall Hence, whether the mass be 10, 100, or or large gaps in the horizontally disposed 1000 times larger, it disintegrates at the ediments known as di conformities. On same rate. the basis of these numerous defici encies in Uranium disintegration is thus a trictly the stratigraphic record it would seem uniform process whose velocity has re that the above e timate of 465,520,000 mained unchanged throughout geological years should be multiplied by a small time. Von Hevesy says that it is the figure such as 4, to account for the total nucleus which is involved in the decay, time involved since sedimentation began, and nuclear processes proceed inde namely, 1,862,080,000 years ago. pendently of temperature, pre sure, and Another lin e of evidence, which has other external conditions. Hence, he yielded remarkable resul ts as to the age asserts there is absolutely no reason to of the earth i the radio-active method. believe that the process has gone forward It was first used in this connection by at any different rate than at pre ent at Boltwood of Yale in 1907. It is based any period in the earth's history. upon the invariable rate of disintegration To students of this subject it is a well of the radioactive substances1 such as known fact that the disruption of a uranium, thorium, radium and actinium, uranium atom is always accompanied by which possess high atomic weights and the radiation of an alpha-particle, which disintegrate with the continuous emana is a charged helium atom, or by the loss of tion of helium into substances of lower and a beta-particle, which is a free electron. lower atomic weights, terminating in The alpha-particles leave the atom with a lead. While chemists and physicists have velocity of 8800 miles per second and analyzed but a comparatively small travel a distance of about 2.8 cm. in air number of rocks of different ages contain and about 0.013 mm. in mica before they ing radioactive minerals, the determina become powerless. The beautifully color tions so far made yield results which are ed "pleochroic halos" seen in mica (biotite) in accord with the sequence of roch as under polarized light are produced by determined by geologists. The radio these alpha particles as they are emitted active method affords ao-e determinations by the contained uranium and the which are more accurate than that pro decomposed products of uranium. The duced by any other known method. fact that the halos, corresponding to the According to G. von Hevesy in Sc,£ence, various radioactive substances, have the ov. 21, 1930, single atoms of uranium same diameter, indicates that the rate of and other radioactive substances explode. decay ha remained the same throughout The number of particles exploding and the ages. To apply the rate of uranium decaying in unit time is strictly propor decay as a measure of time it is necessary tional to the number present. Thus as von Hevesy says to obtain (1) the total where one atom of uranium out of 1,000,- quantity of uranium that has decayed in 000,000,000 atoms, (or a mas weighing some mineral ince the solidification of 1/ 40,000,000,000 of a gram) explodes and the earth, and (2) the rate of that decay. disintegrates every 5 days, 73.05 atoms Accompanying the radiation of alpha di integrate in like manner in the course particles from uranium it is known that of a year. If the ma s and the number of one atom of helium, an inert gas, rises atoms be 10 times as large, 10 atoms will from the decay of each atom of uranium. decompose in fi ve days. If the mass be Although a mall portion of this helium 100 or 1000 times as large, 100 or 1000 e capes, mo t of it collects in the uranium atom will decompose in the ame time. bearing rock, where it volume give~ n HOW OLD IS THE EARTH? 17 P hotograph by B arnum B rou,-n STRA'l'IFIED CRETACEOUS D1 1 0 AUR BEDS, ALBERTA PROV! CE, CANADA The man near the center of the picture sta nds on the contact between the marine Pierre beds below and the fresh-water Belly Ri,·er dinosaur beds above. The contact denotes not onl y a change in the character of the ~ediments, but a lost interval of time occurring about 65 million years ago measure of the age of the rock. Lord from the known rate of the production of Rayleigh noted that one cubic centimeter helium from uranium, A. Holmes, 1927, of helium may be produced from one calculates that a million grams of uranium gram of uranium in 9,000,000 years. give rise to 1/ 7400 of a gram of lead every Since a small portion of the helium year. Holmes also presents formulre for gradually escapes, this method gives but a making age determinations from the minimum age. On this basis, age determi various radioactive minerals. Thus after nations of ancient rocks have been made determinating the lead content of the to the amount of 570,000,000 years. uranium minerals it is possible to calcu Uranium has an atomic weight of 238, late what proportion of the uranium has helium 4. Hence, as the decay proceeds decomposed since the mineral was formed. and helium is liberated, the products of In the American Journal of Science for the decay have atomic weights, 234, 230, March, 1927, A. Holmes and R. W. Law 226, 222, 218, 214, 210 and 206. The son reviewed the methods of determining atomic weight of a beta-particle is 1/ 1800, the radioactive disintegration of 18 hence, when it is lost, the atomic weight is samples and presented 22 determinations, decreased by an insignificant amount. the results of which have been incor The atomic weight 206, which is lead porated in the left margin of the geologi derived from uranium, is of special interest cal time scale on page 137. In the in radioactive determinations, since it is a same journal, Aug., 1930, A. F. Kovarik solid product and does not disintegrate. described two additional analyses of It may be observed that 238 parts of ancient rocks, one for 1,465,000,000 years, uranium produce 206 parts of lead as the other for 1,852, 000,000 years. These 32 parts of helium are developed. Hence, have also been added to the chart. 1 NATURAL HISTORY The radioactive method, which is based method is not only of value in deter upon the natural disintegration of uran mining the lower limit of the age of the ium to lead, is of great importance for earth's materials, but of the chemical it enables us to determine the following elements. A a chemist he considers that interesting things about the earth: the transformation of u'ranium into lead 1. The age of the had already pro oldest igneous rocks gressed to a certain containing radioactive point while the minerals, that is, the earth's material minimum age of the earth. was still ga eous. 2. The date of vari He as erts that thi ous events in the later lead with atomic history of the earth. weight 206 did not 3. The nature per remain isolated, but haps of various trans fo rmations in the gas mixed with lead eous and liquid stages (208) formed by the of the earth's history. decay of thorium 4. The maximum and as a result com age of the eart h. mon lead (207) was As to these vari produced. He goes ous points it may on to say that ap be said that the , ECTIO1 OF FORDHAM GNEI, ,, ARCHIEOZOIC AGE proximately half of oldest radioactive EW YORK, N. Y. our common lead The folded and contorted bands of light and dark colored mineral so far de minerals represent lines of segregat ion of the mineral matter, was formed from and folding when in a plastic state. It is typical of many termined is a speci Archre ozoic rocks. Specimen from excavation, eastern abut uranium before the ment of Fort Wasbington bridge over Harlem River. Age men of Uraninite problematical, perhaps 1800 million years earth's materials from Sinya Pala, had solidified. He Carelia, innorthwestern U.S.S.R., and that cites F. W. A ton a having proved re its age is 1,852,000,000 years as determined cently that ordinary lead is a mixture of by A. F. Kovarik, Sloane Laboratory, uranium-lead and thorium lead. He Yale University, August, 1930. Another considers that lead formed in uranium specimen of the same mineral from Key minerals has had no opportunity to mix stone, South Dakota, as determined by with thorium lead and consequently it has Prof. Kovarik, gave 1,465,000,000 years. remained fixed as uranium-lead. Thus It is probable that other specimens yield the ratio of all the uranium to about half ing an even greater age may be found the common lead (plus the uranium-lead) and that the minimum age of the earth, present in the whole earth must give the that is, the formation of the crust, may age of the earth's material. His con idera be considered to have begun approxi tions give about 3000 million years as the mately 2,000,000,000 years ago. upper limit of the age of the minerals ; The age determinations of various it is also the lower limit of the age of the events in the later history of the earth earth's materi al. He draws a distinction have been entered in t he geological time between the few radioactive element , cale on page 137. which have altered according to accurately The third and fourth points are of known laws during this long time, and the special intere t, ince the early history of other elements which built up the earth' the earth is still obscure. According to constituents and have undergone no von Hevesy, 1930, the uranium-lead change. HOW OLD IS THE EARTH? 19 MUD FILLING OF "SU N CRACK S, " UPAI FORMATION A specimen of lower P ermian age that is some 215 millio n years old. It exhibits the same physical phenomenon as is found in the Hakatai shale speci men 935 million mars old of middle Proterozoic age. F rom the Grand Canon of Arizona. Lincoln Ellsworth Collection, 1930 The foregoing determi nations have had to do with the crust of the earth. Since the earth's interior is inaccessible, the geochemist turns to the meteorites and as sumes that the iron mete orites correspond to the core of the earth, and the stony meteorites to the more or less silicate- like material lying between the core and materials of the earth and of meteorites the crust. may have come from a common celestial F. Paneth of Berlin developed in 1926 source. It also implies that the youth the methods for determining the helium ful earth, which grew presumably from the content of meteorites. He notes that the inner core outward by the addition of iron meteorites when heated to a red heat layers of planetoid and planetesimal loos0 no trace of helium. According to matter, began its development 2,600,- von Hevesy, 1930, Paneth has found for 000,000 years ago. The oldest surface the iron meteorites a maximum age of rock so far analyzed yields an age of 2600 million years. 1,852,000,000 years. The difference in age These data are significant. It lends between the oldest rock and the oldest support to the theory that the original meteorite is 748,000,000 years. May not this difference, or some 600,000,000 years, repre sent the time consumed in the upbuilding of the primrval earth? In conclusion it may be stated that these radi oactive determinations are not only astounding, but rem arkable. Al- PROT EROZOI C RIPPLE M ARKS AN D " UN CR ACK " IM- PRES IONS T his slab of red H akatai shale of middle P roterozoic age is some 935 million years old. It shows t hat the ~ame physical phPnomena were in fo rce during t he early eras of the rarth 's history as are in evidence today. Specimen from t he Grand Canon of Arizona. Lincoln E lls- worth Collection, 193 1 20 ATURAL HISTORY though the method is still young the we may consider the crust of the earth to results are dependable. The method is be 1,852,000,000 or about 2,000,000,000 based upon the natural rate of disinte years old; the inner core, 2,600,000,000 o-ration of the atoms of the few radio years old; and the upper limit of the active elements. This rate cannot be minerals, or materials of the earth, as changed by any known human or phy ical 3,000,000,000 years old, as noted below agency. It is thus a reliable and in the radioactive clock of geological thoroughly scientific method. When its time. The radioactive determination application has been extended to numer and the oldest fo ils indicate that prim ous samples of radioactive rocks and itive life was present on the earth one minerals from all parts of the world, and one-half billion years ago; stone im ple embracing rocks of all ages, then we shall ments and human remains in Pliocene know, in all probability, how old the deposits imply that the human race was on earth is. the earth about one and one-half million Upon the basis of knowledge for 1931, years ago. IX ~ P ROTERO ZOI C ERA f\J Prim1t1 ve closses or Life 650 m,11,on y ea rs A R C H EOZO IC ERA r a rl, es , ~ Li f e Boo million yeo.rs T his cl m· k [ace or 12 hours hows how ~000 milli on y a r · lll!LY he a ll ot te • PRECAMBRIAN ROCKS GENERAL- RA DI0LAfflA,SPONGES, Tl/BES OLDES T GL AC I AL DEPOSIT, ACTIVE ENO PRODUCT LY METAM0RPM0SED;JrzNE0US PRE AND TRAILS, ( GffAPH I TE)JN COBALT/A N, II D0M.Y. 2000 ORES I S L EAD. 0 0MINANT;SEDIMENTAffY PRESENT H U RON/AN, 900-1200 M. Y. ; C.AREEDS FEB. /9 JI RAD I O ACT I V E CH ART OF GEO LO G I CAL TI M E Note: For every J,000,000,000,000 uranium atoms (or a mass weighing J/40,000,000,000 of a gram) one atom ex plodes every five days. Five eras are shown on this chart; sec page 146 for radioactive clock of geological time, showing seven eras. 12 ATURAL HISTORY