THE AGE OF THE EARTH. 1 



By J. Joly, F.R.S. 



The recent contributions to the data bearing on the subject of the 

 age of the earth have strengthened the evidence derived by two very 

 different methods of computation ; that based on the study of solvent 

 denudation and that based on the accumulation of radioactive waste 

 products in minerals. While the indications of both lines of inquiry 

 seem individually rendered more definite by these advances, the diver- 

 gence in their final results have, if anything, become intensified. I 

 propose in the following pages to review the opposing methods, as 

 briefly as the many details permit, and to discuss the possibility of 

 reconciliation. 



THE AGE OF THE OCEAN DERIVED FROM SOLVENT DENUDATION. 



Three recent contributions to this subject have appeared: Prof. 

 Sollas's Presidential Address to the Geological Society of London, 

 1909; a paper on "A preliminary study of chemical denudation," by 

 F. W. Clarke (Smithsonian Miscellaneous Collections, vol. 56, June, 

 1910) ; and a paper by G. F. Becker on "The age of the earth" (Smith- 

 sonian Miscellaneous Collections, vol. 56, June, 1910). 



These recent discussions chiefly center round the ascertainment of 

 the true present rate of supply of sodium to the ocean. The limita- 

 tions of the method are also discussed. 



My own original estimate of the age of the ocean 2 was based on the 

 only data then available — the estimates made by Sir John Murray of 

 the average chemical composition of river water and the probable 

 total annual discharge of the rivers into the ocean. Calculating from 

 its estimated volume and mean chemical composition the mass of 

 sodium now in the ocean, and dividing this by the calculated amount 

 of sodium entering annually from the rivers, the uncorrected age of 

 99.4 million years was obtained. To this I applied certain corrections, 

 to some of which I shall refer later. The final result of these correc- 

 tions left the age as from 80 to 90 million years. 



1 Reprinted by permission, after revision by the author, from the Philosophical Magazine, London, 

 S. 6, vol. 22, No. 122, September, 1911, pp. 358-370. 

 a Trans. Roy. Dublin Soc, vol. 7, 1899. 



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