THE AGE OF THE EAKTH AND THE AGE OF THE OCEAN 



By Adolph Knopf 



I. THE AGE OF THE EARTH : SUMMARY OF PRINCIPAL RESULTS 



At the beginning of the present century the problem of the age of 

 the earth was envisaged as requiring the reconciliation of three inde- 

 pendent estimates, all of the same order of magnitude. These esti- 

 mates were G. H. Darwin's, of 57,000,000 years, based on the separa- 

 tion of the moon from the earth: Lord Kelvin's, of 20,000,000 to 

 40,000,000, based on the secular cooling of the globe; and Joly's, of 

 80,000,000 to 90,000,000, based on the rate of accumulation of sodium 

 in the world ocean. To these should be added Helmholtz's estimate 

 of 22,000,000 years, based on the source of the sun's heat and its 

 probable duration. 



Shortly after the opening of the century the discoveries of radio- 

 activity destroyed the foundations on which the principal physical 

 methods of estimating geologic time had previously rested. These 

 discoveries, however, gave us methods based on atomic disintegration 

 which soon indicated that geologic time is ten to twenty times as long 

 as had been deemed probable from the estimates previously con- 

 sidered most trustworthy. These methods appear to involve far 

 fewer assumptions than the geologic methods for measuring time, 

 and the problem as we now see it is to reconcile estimates differing by 

 a whole order of magnitude. In short, the radioactive evidence indi- 

 cates that post-Cambrian time, i. e., from Ordovician onward, is 

 450,000,000 years, a span that is easily reconcilable with the geologic 

 evidence, and that the age of the earth is at least 2,000,000,000 years, 

 an estimate which, although not incompatible with the geologic 

 evidence, is less readily reconcilable. 



The oldest method for determining the length of geologic time is 

 based on the thickness of the strata that accumulated during that 

 time. Estimates of this kind have been made many times, but as 

 our knowledge of the earth increases the known thickness of the 

 strata has steadily increased. Schuchert now finds that the maximum 

 thickness of strata on the North American continent deposited since 



^ Reprinted by permission from Bulletin of the National Research Council, No. 80, June, 

 1931. 



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