EADIOACTIVITY AND THE EARTH's AGE 129 



Mellard Eeade/* taking a low rate for erosion, computed a period of 95 

 million years since the base of the Cambrian. All of these estimates may 

 have been more or less unconsciously influenced by Kelvin's great paper 

 on the secular cooling of the earth, which appeared in 1862. In 1860, 

 however, John Phillips ^^ estimated that the time required for the deposi- 

 tion of the stratified rocks lay between 38 and 96 million years. 



As the figures themselves show, the uncertainty is great, because the 

 data are imperfect; but the premises have been derived from different 

 regions in different ways, and the conclusion geems fairly well established 

 that a few tens of millions of years will cover the period of sedimentation. 



A radically different and very important method is due to Mr Joly, 

 who sought to determine the age of the ocean from its sodium content on 

 the hypothesis that this is derived at a uniform rate from the decomposi- 

 tion of the rocks of the land surface. In 1899 *° Mr Joly reached 80 to 

 90 million years by his method, and in 1900 *^ he increased this estimate 

 by 10 million in the course of an answer to a criticism by Mr Sollas,*' 

 who still thinks Joly's estimate too large by 30 or 4.0 million years. In 

 1902 Mr Mackie *^ discussed the same method in much detail. He ap- 

 pears to me to succeed in showing that Mr Joly's estimate is too large, 

 and himself reaches 25 million without as, I think, justifying so great a 

 reduction. Data are now accumulating on the composition of waters, 

 and it will soon be practicable to apply Mr Joly's method more satisfac- 

 torily than was possible in 1900.** 



It is noteworthy that the estimates from sedimentation (with a single 

 exception) and those from the oceanic sodium content vary between sub- 

 stantially the same limits, 25 to 100 million years — wide limits truly, 

 but better than unlimited guesses. 



Years ago biologists were inclined to claim vast periods of time in 

 which to accomplish the evolution of species, but recent investigations 

 seem to have convinced them that the rate of evolution even today is far 



=8 Geological Magazine, vol. 10, 1893, p. 99. 



'^ "Life on the eartti," etcetera, 1860, p. 119. I tiave checlced the computation. Mr 

 Reade, in his "Evolution of the earth's structure," 190.3, p. 2.56, is somewhat severe on 

 Phillips for stating in his "Treatise on geology," 1839, that the average wastage in the 

 drainage basin of the Ganges is 1/40,000 yard, which he is quoted as saying is about 

 1/111 of an inch. In the U. S. Geological Survey's copy of the treatise, 12mo edition, 

 undated, but catalogued as 1837-39, the fraction given is 1/4,000 yard, and this is 

 about 1/111 inch. The 40,000 in Mr Reade's copy is merely a misprint for 4,000. 



« Transactions of the Royal Society of Dublin, vol. 7, 1899, p. 23. 



" British Association for the Advancement of Science, 1900, p. 369. 



*^ Mr Sollas repeats his criticism in his volume, "The age of the earth," etcetera, 

 1905. 



^^ Transactions of the Edinburgh Geological Society, vol. 8, 1902, p. 240. 



" Cf. Mr F. W. Clarke's "Data of geochemistry." U. S. Geological Survey Bulletin 

 no. 330, 1908, p. 110. 



XIII— Bull. Geol. Soc. Am., Vol. 19, 1907 



