A History of Geologic Time: "It Is Perhaps a Little Indelicate to Ask Of

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A History of Geologic Time: "It is perhaps a little indelicate to ask of our Mother Earth her age..." (Arthur Holmes 1913) Author(s): Henry Faul Source: American Scientist, Vol. 66, No. 2 (March-April 1978), pp. 159-165 Published by: Sigma Xi, The Scientific Research Society Stable URL: http://www.jstor.org/stable/27848511 . Accessed: 10/09/2013 15:34

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This content downloaded from 134.173.140.64 on Tue, 10 Sep 2013 15:34:22 PM All use subject to JSTOR Terms and Conditions Henry Faul A History of Geologic Time

"It isperhaps a little indelicate to ask of our Mother Earth her age . . ." (Arthur Holmes 1913)

hundred and so, less than a Time was conceived in geology. I Physical infinity is always relative, students, was to the after Hutton's Edin mean the absolute, continuous, end and Hutton responding decade death, the fountain ofWer less time in which the history of convention of the day, but compared burgh became lives of the Hut nerian (J. 1973). planets and stars is reckoned. It to the Patriarchs, ideology Eyles time was infinite indeed. happened inEdinburgh, in the 1770s, ton's The Industrial Revolution in a small-group of thinkers which generated from biblical new demand for and called itself the Oyster Club and in Hutton's departure coal, metals, was rooted in his unifor facilities. The of cluded Joseph Black (1728-99), the chronology transport sinking to the of mines and the of roads and chemist; Adam Smith (1723-90), the mitarian approach history building was that information economist; and James Hutton the Earth. He convinced canals required geologic where little or had been (1726-97), the physician-farmer geologic processes in ages long past nothing known before. New studies turned geologist. In more conven were no different from the processes produced new became tional circles, the largest unit of time now active and thus open to obser interest, geology popular, of the coun was still the human life span, and the vation. Geology could be interpreted and geologic knowledge ca As age of the world was accepted as es without recourse to postulated try rapidly expanded. geologic as evidence theWernerian tablished by Bishop Ussher's artful tastrophes such the Noachian accumulated, lost and the biblical arithmetic with biblical chronology, Flood. The Earth was forever chang viewpoint ground the same. of time faded. The but the Oyster Club discussions were ing, but its nature remained concept slowly its William Smith not bound by the limits of such pre The concept was radical for discovery by (1769 on. that no matter how dif conceptions. Hutton had taken his time-and slow to catch 1839) strata, ferent are of the friends to see the rocks along the they may appear, at same when contain the same Scottish coast, and they observed that A rival school arose in Germany, age they of fossils the every rock formation, no matter how the new Mining Academy inFreiburg, assemblage provided Werner means of the strati old, appeared to be derived from where Abraham Gottlob establishing and other rocks, older still. (1749-1817) developed an impressive graphic sequence thus, indirectly, a time base. following by teaching an orderly, newly expanded simplistic kind of geology inwhich all Uniformitarianism the of rocks, including granite and basalt, Largely through writings a Charles the uni Years went by before Hutton saw fit were said to have precipitated from Lyell (1797-1875), formitarian view to put the idea in print. He finally put primeval ocean in a regular, universal eventually displaced and the it this way in 1788: "The result, sequence. Without mentioning Noah Wernerian ideas, conception of no tied to Mosaic therefore, of our present enquiry is, orMoses, Werner presented a picture time, longer that we find no vestige of a begin of the Earth's past that anyone could chronology, expanded inordinately. That Under the oldest lavas ofMt. Etna, ning,-no prospect of an end." reconcile with religious opinion. was the root of his success. Lyell found fossils that looked to him like shells now living in theMediter One of Werner's disciples, Robert ranean, and he wrote of "an indefinite at the Jameson became Pro lapse of ages having been comprised Henry Faul is professor of geophysics (1774-1854), at the Uni within each of the more modern University of Pennsylvania in Philadelphia. fessor ofNatural History pe He was trained at M.LT. (S.B. 1941; Ph.D. versityof Edinburgh in 1803and held riods of the earth's history" (1833). served in the Manhattan Project same Charles 1949), the post for fifty years. About Imbued with the spirit, (1942-46) and in the U.S. Geological Survey 1800-20, the Freiburg Academy was Darwin (1809-82), who had studied (1947-63), and was professor of geophysics at trade but with Jameson but came to follow the Southwest Center for Advanced Studies, a small provincial school, of made an un now the University of Texas in Dallas the University of Edinburgh had Lyell's view time, (1963-66). The author is indebted to A. 0. C. more than fifteen hundred students characteristically hasty calculation of Nier candid reminiscences his at for of days and was one of the world leaders in the time required for erosion to ex Harvard. Address: Department of Geology, medicine and science. Jameson's cavate theWealden Valley in south University of Pennsylvania, Philadelphia, PA a His 19104. classes were large, usually fifty to eastern England (1859, p. 285).

1978 March-April 159

This content downloaded from 134.173.140.64 on Tue, 10 Sep 2013 15:34:22 PM All use subject to JSTOR Terms and Conditions Figure 1. Calculations of the "age of the world" were made long before Bishop Ussher. (From Cooper's Chronicle, London, 1560.)

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C&fefegr Co ?times larger than present esti ty. Knowing the temperatures at various mates (Holmes 1965). Darwin needed depths and making assumptions lots of time for his natural selection The of the Earth about the thermal conductivity of the process, but the uniformitarians had cooling intervening rock masses, Kelvin was overreached themselves. The Industrial Revolution also had its able to estimate outward heat flow. impact on physics-physicists be He concluded that the Earth also had As the sequence of sedimentary rocks came concerned with dynamics and to be cooling and that the time in Britain and western Europe was energy. The origin of the sun's heat elapsed since a solid crust first formed studied and gradually worked out, was an obvious problem, and by the on a presumably molten globe could thinking about time became tied to middle of the nineteenth century it be calculated (Thomson 1862b). As he the new stratigraphy. Lyell had re was clear that the sun could not re saw it, the Earth and the sun were peatedly mentioned "indefinite pe main hot forever. Its thermal energy both about 100million yearsold, but riods of time" required for various was very large but clearly finite. considering all the assumptions that geologic processes, but it was clear William Thomson, later Baron Kelvin had to be made in those calculations, that strata and groups of strata were (1824-1907) was not the first to con he feltobliged to ascribewide limits individually finite. There were gaps sider the problem, but he was cer of error to his result: a neat factor of in the record as it was exposed in tainly the most vigorous proponent of four. The Earth could be anywhere England, but some of the gaps could the new idea. He added up the energy from 25 to 400 million years old. be filled by observing rocks of the emitted by the sun, assumed physical same age in other countries. There conditions and heat capacities that Geologists paid no attention to Kel was no evidence that any of the gaps seemed plausible at the time, and vin's attacks on uniformitarianism could be of "indefinite" duration. The calculated a rate of cooling that untilhe decided to take the troubleto mid-nineteenth-century geologist had turned out to be surprisingly rapid advertise them in an address to the only a crude idea of the rates of de (Thomson 1862a). All movement of Glasgow Geological Society in 1868 position of sedimentary rocks, but it air and water on the surface of the (Thomson 1871).When he finallygot was enough to make clear that the Earth is produced by energy received his reaction, itwas swift, elegant, and whole magnificent layer cake of from the sun. Weathering of rocks, ineffective. Thomas Huxley rose to British strata represented an enor transport of sediment by wind, water, defend uniformitarianism against

160 American Scientist,Volume 66

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Figure 2. Surrounded by a chalk escarpment, underestimated the erosive power of rivers and northof the edge of thechalk, and he chose the theWealden valley isa breachedanticline with both Lyell and Darwin thoughtthat the cliffs valley in a vain effort to illustrate the vastness the soft rocks under the chalk removed by must have been cut by an invading sea. Dar of geologic time. (FromLyell 1833.) stream erosion. The uniformitarians greatly win's home was south of Farnborough, just

Kelvin's attacks in 1869, but came up lantic and Archibald Geikie on the formore than a generation and also a with littleor nothingthat Kelvin had other both made their objections on great popular hero, thanks to the on not already considered. Then as now, geologic grounds, and a few others publicity that surrounded his work itwas difficult for geologists to argue joined in, but itwas a matter more of the transatlantic telegraph. His lec with the results of physics. faith than of hard facts. (For a de ture was a ceremonious revival of the tailed and perceptive view of the old theoretical conclusions, now pre Kelvin's approach was direct and his controversy read Burchfield 1975.) sumably vindicated by elegant ex a premises made sense. The uncer Darwin had dropped the Weald cal periments. It was fancy "I told you as a tainty he quoted was large enough to culation and softened his early uni so," hardly intended challenge for allow most geologists to make an or formitarian stand in successive edi further debate. derly retreat from their concept of tionsof theOrigin of Species, but had a very long time; but as the years went been unable to bring himself to accept But challenge it turned out to be. T. by, calculations tightened and Kel the short time scale. Kelvin's num C. Chamberlin (1843-1928), professor vin's time became shorter and short bers were incompatible with the time at the new University of Chicago, had er. In 1880, Clarence King (1842 required for natural selection. been workingon thehypothesis that 1901) and Carl Barus (1856-1935) set Something had to be wrong with the Earth and the other planets ac up a laboratory in the U.S. Geological them, and Darwin vainly sought what cumulated in a cold state from small Survey formeasuring physical prop it could be. He remained convinced of pieces, which he called "planetesi erties of rocks, including thermal the validity of his theoretical con mals." In his view, the Earth had conductivity and heat capacity clusions and simply refused to follow never been completely molten. The (Barus 1893).With the new results, his contemporaries onto the Kelvin concept was not mature in his mind King further reduced the uncertainty bandwagon. and was published only much later ofKelvin's cooling rate for the Earth. (Chamberlinand Moulton 1909),but By the time most geologists had cap Meanwhile, Kelvin's time became too it was clearly incompatible with itulated to the overwhelming power short even for some of his supporters. Kelvin's model of an originally in of Kelvin's physics, King had ob Perhaps it was their picking that candescent Earth. Not being bashful, tained an age of 24 million years made him return to the subject of Chamberlin attacked immediately (1893). time in a grand speech to the Victoria (1899). "The fascinating impressive Institute in 1897 (Kelvin 1898). By ness of rigorous mathematical anal Not all geologists gave up, of course. that time he had been Britain's lead ysis, with its atmosphere of precision C. D. Walcott on this side of theAt ingphysicist (in contemporaryeyes) and elegance, should not blind us to

1978 March-April 161

This content downloaded from 134.173.140.64 on Tue, 10 Sep 2013 15:34:22 PM All use subject to JSTOR Terms and Conditions the defects of the premises that con rivers over geologic time. From the rors tended to compensate. Whatever dition the whole process," he wrote, present salt concentration in the sea their inadequacy, they clearly re and he went on to take Kelvin's and a weighted average of the flow established the very long time scale. one one. arguments apart, by Much of rates and#salt contents ofmajor rivers, Boltwood himself was painfully aware what he said had been said before Joly obtained an age between 80 and of the wide uncertainty in the urani without great damage to the cooling 90 million years for the time when the um decay-rate figure available at the concept, but then he reached beyond surface temperature of the Earth time, the hazy understanding of the the limits of previous discussion: dropped below the boiling point of uranium decay scheme, and the se water (Joly 1901). vere limits of existing chemical tech What the internal constitution of the niques. Moreover, his health was atoms may be is yet an open question. It Joly's papers bristled with numbers, failing and his interest in geology was is not improbable that they are complex in fair antidote to Kelvin's, and pro marginal. He dropped the age work organizations and the seats of enormous duced lively discussion for a few and preferred to follow his calling as no careful chemist energies. Certainly, years. His approach was boldly Rutherford's chemist, studying ra would affirm either that the atoms are quantitative but was beset with seri dioactive-decay systematics as a pri elementary or that theremay not be really ous uncertainties. Radio mary problem. locked in them of the first geological up energies was in ... activity the wind and Joly order of magnitude. Nor would he himself soon abandoned the salt of The debacle of Kelvin's war on uni probably feel prepared to affirmor deny the sea in favor of radium formitarianism had had little effect that the extraordinary conditions which (Joly on reside in the center of the sunmay not set 1903). his public image. His Popular free a portion of this energy_A geolo Lectures (1894) had remained stan gist begins to grow dizzy contemplating Radioactivity dard fare among the reading public. such thermal possibilities. Why should Thus it happened that, about 1905, a not and whatever else It was a from the "radiations atoms, atomecules, long way physics teacher in Gateshead High lies one after have their emitted bodies" to below, another, by phosphorescent School inNewcastle-on-Tyne handed energies squeezed out of them; and the the of radioactive discovery decay, a copy to a very bright pupil named outer the be heated and regions [of sun] but the subsequent development of Arthur Holmes a an (1890-1965) (for lighted for unknowable period at their nuclear was The physics explosive. good biography see Dunham 1966). expense? first of radioactive trans glimmer Thus the torch was passed, though mutation came from the emanation one could hardly think of two char That was written in the of of Elster and Geitel spring experiments acters more divergent than the over 1899. Henri at the Becquerel's experiment (1902), working Herzogliches powering, self-quoting, deadly-seri with those fateful flakes of a common in Wolfenb and Gymnasium ttel, ous Kelvin and the mild, humorous, uranium salt sul Rutherford and at (potassium uranyl Soddy (1902), open-minded Holmes. The only on a McGill Ernest Rutherford fate) photographic plate University. thread between them was a lifelong in black had been re wrote the basic wrapped paper (1871-1937) equa passion for the study of geologic ported on 24 February 1896. Kelvin tions of radioactivity in 1904, calcu time. had been aware of Becquerel's ex lated the amount of heat released by periments when he spoke to the Vic radium present in ordinary rocks, and Holmes began his long career in the toria Institute (Kelvin, Carruthers concluded that "the time during laboratory of R. J. Strutt in Imperial Beattie, and Smoluchowski de which the Earth has been at a tem College in 1910, and his first paper, Smolan 1896,1897). Chamberlin, two perature capable of supporting the "The association of lead with urani years later, may or may not have presence of animal and vegetable life um in rock-minerals, and its appli been, but even if he had followed may be very much longer than the cation to the measurement of geo them closely, he would have found estimate made by Lord Kelvin from logical time" (1911) fairly presaged little in them to lead him toward other data." R. J. Strutt (1875-1947) his life's work. There was, however, postulating a nuclear energy source showed in 1905 that the amount of another side of him, and periodically for the sun. Early nuclear physicists helium in uranium minerals was he would go on long geological surveys were concerned with the excitation of larger than what could have accu inAfrica and Asia, usually employed the puzzling "Becquerel rays" and mulated from alpha decay inKelvin's by commercial interests. In that way had said practically nothing about the geologic time. Before long, the evi he developed, in addition to his ex source of their energy by the time dence became overwhelming, but ceptional mastery of the new physics, Chamberlin wrote his prophetic guess Kelvin remained unimpressed. He an impressive command of general about the energy of the sun. died convinced itwas all wrong. geology (see Holmes 1965). In 1927 he proposed his first time scale in a Also in 1899, John Joly of Dublin At Yale University, Bertram Bolt popular booklet, The Age of the presented his first version of an wood (1870-1927)collected published Earth, perhaps to show that he knew new of elaborate calculation the age analyses of pure uranium minerals itwas based more on his geologic in on of the Earth based the amount of (mostlyfrom Hillebrand 1890) and tuition than on hard physical data. in an salt the sea, idea originally pro calculated ages from the lead content, His subsequent revisions (1937,1947) a posed byEdmond Halley in 1693 (V. assuming uranium half-life of IO10 continued in the light vein, stead more Eyles, 1973). Joly assumed that the years, than twice the present fastly undaunted by the shortage of oceans condensed from a primeval value (Boltwood 1907). His ages reliable measurements. atmosphere as pure water and that ranged up to 2,000 million years, and was some are the salt derived from the weath of them not far from pres Isotopic age determinations on rocks, ering of rocks and brought down by ently accepted values because his er securely tied to the fossil record, fi

162 American Scientist, Volume 66

This content downloaded from 134.173.140.64 on Tue, 10 Sep 2013 15:34:22 PM All use subject to JSTOR Terms and Conditions nally became available in the late his studentshad studied dozens of fifties (including a few from this common lead samples, but always writer), and Holmes snapped them up obtained about the same value, una with glee. "My 1947 attempt to con ware of the coincidental fact that ... struct a time-scale has now out isotopic differences in common leads lived its usefulness. Of the two alter tend to cancel out in the gravimetric natives then proposed, the 'B scale' in determination (Nier, pers. com.). particular enjoyed an unexpected success for ten now I years-But The come to bury the B scale, not to praise breakthrough it," he began (1960), and he went on The systematics of the uranium and to develop his "1959 scale" with the thorium decay chains were then rea new data. That time scale is still with sonably well known, but analytical us and is not likely to change very difficultieshad not diminishedmuch much in the future. It is amusing to since Boltwood's time. Standard contemplate that Holmes's old esti chemical techniques were still inad no mates, based on what now look like equate, and the decay constant of the data at all, were rarely off from cur rarer isotope of natural uranium, values more than was its content in rently accepted by - 235U, hazy because, 240 riJ 2S : ^m^ 234 *S2 20%. ATOMIC (MASS UNITS normal uranium was known only roughly. The breakthrough came mass Figure 3. Nier's determination of the from a Harvard postdoc named Al The proper of uranium in 1939made possible the samples spectrum fred O. Nier (1911-), who had come calculation of the decay rate of^HJ and thus On thisside of theAtlantic, the from Minnesota in a study became the key to the Holmes-Houtermans 1936, bringing on the form of of geologic time took (1946) computationof the age of the Earth. new ingredient, the magic touch of a structured organization. In 1923, the Each point in the spectrum represents a swing born instrumenter. Committee on Measurement of of theballistic galvanometer and theheight of each is to the amount of the Time Atomic Disinte peak proportional Mass had been known Geologic by The value of the spectrometers of the National Research corresponding isotope. gration 238TJ/236TJrati0 stai standswithin his limitsof for some time, but a new electrometer was with C. Council appointed, A. uncertainty. vacuum tube had just been perfected, Lane (1863-1948) ofTufts College as withhigh gain and extremelylow grid chairman. The Committee on the Age current, which made it possible to also of the National of the Earth, was something else again. Decades amplify and measure very small Research Council, was set up in 1926, went by beforethe effortfinally bore electric currents. Nier used it to withYale's AdolfKnopf (1882-1966) fruit. measure the beam of uranium and in charge. Knopfs committee pro lead ions in his mass spectrometer. 80 was duced the much read Bulletin One of Lane's good customers The new instrument was accurate Research Council a to make (National 1931), Gregory P. Baxter (1876-1953), pro enough and stable enough volume five short hefty containing fessor of chemistry at Harvard and a the first isotopic analyses suitable for articles without much residual value of time. specialist in the gravimetric deter calculations geologic Using a and masterfully organized 336-page mination of atomic weights. He and uranium halides and lead iodide me review, ranging from Becquerel's ex ticulously prepared by Baxter from periments to the state of the isotopic Lane's judiciously selected mineral ' art as itstood in 1931,by theubiqui specimens (Nier, pers. com.), Nier com . tous Arthur Holmes. Lane's % 1 ISOTOPESOF determined the isotopic composition mittee issued encouraging annual I 0>jDiNARY j*6*0 -; of natural uranium (Nier 1939) and reports until 1955, but its main thus was able to calculate the decay function, while the chipper chairman rate of the rarer isotope, 235U, from was still alive, was to serve as a base available measurements of the com forhis worldwide search fordatable bined alpha activity of natural ura mineral specimens. Highly pure ura nium. He also measured the isotopic are rare nium minerals and lack of composition of the lead extracted was a suitable samples standard from 29 samples of radioactive min complaint among the early geochro erals and 25 lead ores (Nier 1938, nologists. 1939;Nier, Thompson, andMurphey 1941). Then the war came, Nier went In 1936,at age 73,Lane had refused on to other things, and everything to sign the Massachusetts teachers' pertaining to uranium became SE 212 210 208 206 . 204 202 oath and formally retired,but he Atomicmaw units. CRET. continued to use his boundless energy to collect radioactive SI and re specimens Figure 4. Nier's mass spectrometer cleanly The of theEarth lead ores from every corner of the solved the lead spectrum (1938).^Pb ispro age world and distribute them to every duced in the decay of 232Th (thorium),207Pb Nier's data remained available in li from235U, aoepb from 238U. ^Pb, which isnot respectable analyst who showed in braries, of course, for anyone who radiogenic, is often used as the reference to accumulat terest. Analytical results express the variation in the radiogenic iso would find them.Two independent ed, but their geological interpretation topes. thinkers went to work on them and

1978 March-April 163

This content downloaded from 134.173.140.64 on Tue, 10 Sep 2013 15:34:22 PM All use subject to JSTOR Terms and Conditions both finally showed that such mea products in radioactive minerals surements permit calculating the age could be handled. Accurate mass of the Earth as a planet. In Germany spectrometry, isotope dilution, flame it was F. G. Houtermans (1903-66); photometry, and neutron activation and at the University of Edinburgh, developed into reliable analytical where he had held a chair since 1943, tools. Besides lead from uranium, it who else but Arthur Holmes? The was now possible to measure the idea may seem obscure at first, but it argon accumulated from the decay of ismarvelously simple. We have in the potassium and the strontium from Earth two different parent isotopes, the decay of rubidium in geologic 235U and 238U, each decaying into a time. In the early fifties, laboratories different daughter isotope of lead, for isotopic age-determination were 207Pb and 206Pb respectively, at a established across the country, different rate. Different isotopes of a mainly by students ofHarold C. Urey heavy element are, of course, chemi (1893-) and Nier. At last, Hutton's cally identical and thus geochemically time was open to measurement. inseparable. Uranium and lead are chemically very different, however, The wealth of new data placed the and pure lead, cleanly separated from history of the Earth and the moon on uranium by natural processes, is a quantitative basis for the first time. common in nature. We assume that More than that, it provided the point each lead deposit was concentrated of departure for a major conceptual a from large rock system (containing breakthrough in geology, perhaps the some uranium) at some time. In greatest of all time: age determina principle, all that is needed is the tions on volcanic rocks found on land, isotopic composition of three natural Combined with measurements of the lead deposits of different and inde r manent magnetization of those pendently known geologic ages, and same rocks, showed that the Earth's we can extrapolate back in time to the magnetic polarity has reversed itself origin of the Earth. frequently in the geologic past. The resulting paleomagnetic time scale Houtermans (1946) wrote the equa was found to match the magnetic tion this way: profiles measured by ships over mid ocean ridges, thus proving conclu - that the sea floor is ff ffo= Ac D _ 1 ex'w-ex'P sively spreading - - and that the continents are a a0 Ra G 139# eXw exP really drifting, as had long been suspected by some. Itwas the beginning of a new His a symbols and stand for the era in man's understanding of the ratios 206Pb/204Pb and 207Pb/204Pb, Earth. respectively, in a lead deposit of age p. The subscript 0 refers to these same ratios in "primordial" lead ("nat rliches Blei" of Houtermans), the lead at the time of the originof References the w, in other uncon Earth, words, Barns, Carl. 1893. The fusion-constants of ig taminated by the decay products of neous rock. Part III. The thermal capacity uranium. Ac D (actinium-D) and Ra of igneous rock, considered in its bearing on the relation of to G (radium-G) are now obsolete sym melting-point pressure. bols for 207Pb and A is the Phil. Mag. 35:296-307. 206Pb, 5. In Holmes's final time scale Figure (1960) Becquerel, Henri. 1896. Sur les radiations decay constant of 238U, X' the the of Earth correlated decay covering part history by mises par phosphorescence. Comptes constant of 235U, and 139 is the fossils, Holmes used known thicknesses of Rendus 122:420-21; Sur les radiations in sediments to between present ratio of these uranium iso interpolate isotopic age visibles mises par les corps phosphores measurements made on rocks tied topes (see Fig. 3). geologically cents, 501-03. to well-defined points in the fossil record. Boltwood, Bertram. 1907. On the ultimate of the radio-active Our present of the age of disintegration products knowledge elements. Part II. The the Earth disintegration prod derives directly from these ucts of uranium. Am. Jour. Sci. 23:77-88.

interpretations (Holmes 1946, 1950; In thewords ofCharles Lyell (1830,p. Burchfield, J. D. 1975. Lord Kelvin and the Houtermans 1946,1947). The values 71), "we have now arrived at the era Age of the Earth. Science History Publica they originally reported were about of living authors, and shall bring to a tions. 3,000 million years. Later refinements conclusion our sketch." The super Chamberlin, T. C. 1899. On Lord Kelvin's ad with more dress on the age of the Earth as an abode many lead analyses by charged endeavors of the Manhattan fitted for life. Science, n.s. 9:889-901 and other workers produced the not built the presently Project (1942-46) only 10:11-18. Reprinted in Smithsonian Ann. accepted date for the accumulation of bombs, but also boosted nuclear an Rep. for 1899 (1900):223-46. the about planets: 4,600 million years alytical technology to a point where Chamberlin, T. C., and F. R. Moulton. 1909. ago. the minute quantities of daughter Science 30:642-45.

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This content downloaded from 134.173.140.64 on Tue, 10 Sep 2013 15:34:22 PM All use subject to JSTOR Terms and Conditions Am. phey. 1941. The constitution of ra Darwin, Charles. 1859. On the Origin of King, Clarence. 1893. The age of the Earth. isotopic diogenic leads and the measurement of Species. Murray. Jour. Sci. 45:1-20. time. III. Rev. 60:112-16. vol. geological Phys. Dunham, K. C. 1966. Arthur Holmes. Bio Lyell, Charles. 1830. Principles of Geology, Rutherford, Ernest. 1904. graphical Memoirs of Fellows of the Royal 1.Murray. Radio-Activity. 12:291-310. Cambridge Univ. Press. Society -. 1832. Principles of Geology, vol. 2. Elster, J. P. L. J., and F. K. H. Geitel. 1902. Murray. Rutherford, Ernest, and Frederick Soddy. in dem elek 1902. The cause and nature of ber eine fernere Analogie -. 1833. Principles of Geology, vol. 3. radioactivity, trischen Verhalten der nat rlichen und der 2. Phil. 4:569-85. Murray. part Mag. durch abnorm leitend Becquerelstrahlen National Research Council. 1931. The Age of Strutt, R. J. 1905. On the radio-active minerals. Luft. Zeit. 2:590-93. gemachten Physikal. the Earth. National Research Council Bull. Proc. Royal Soc. A 76:88-101. J. M. 1973. Robert. Die. Sci. Eyles, Jameson, 80. Thompson, S. P. 1910. The Life of William 7:69-71. Biogr. Thomson, Baron Kelvin of Largs. Macmil Nier, A. 0. 1938. Variations in the relative V. A. 1973. Joly, John. Die. Sci. Biogr. lan. Eyles, abundances of the isotopes of common lead 7:160-61. from various sources. Jour. Am. Chem. Soc. Thomson, William. 1862a. On the age of the Hillebrand, W. F. 1890. On the occurrence of 60:1571-676. sun's heat. Macmillan's Magazine 5:288 in uraninite and on the 93. nitrogen composition -. 1939a. The isotopic constitution of uraninite in Condensed from a of general: uranium and the half-lives of the uranium --. 1862b. On the secular cooling of the bulletin of the U.S. Geological Earth. Soc. Edin. Trans. forthcoming isotopes. Phys. Rev. 55:150-53. Roy. 23(l):157-69. Sci. 40:384-94. Survey. Am. Jour. Reprinted in Phil. Mag. 25:1-14. -. 1939b. The isotopic constitution of Holmes, A. 1911. The association of lead with -. leads and the measurement of 1871. On Geological Time. Glasgow and its radiogenic uranium in rock minerals, application Geol. Soc. Trans. 3:1-28. in geological time. II. Phys. Rev. 55:153-63. Reprinted to the measurement of time. Proc. geological Popular Lectures 2:10-64. Nier, A. O., R. W. Thompson, and B. F. Mur Roy. Soc. A 85:248-56.

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