THE BIRTH-TIME OF THE WORLD 45 



but if we could extract all the rock-salt (the chloride of sodium) 

 from the ocean we would have enough to cover the entire dry 

 land of the earth to a depth of 400 feet. It is this gigantic 

 quantity which is going to enter into our estimate of the earth's 

 age. The calculated mass of sodium contained in this rock-salt 

 is 14,130 million million tonnes. 



If now we can determine the rate at which the rivers supply 

 sodium to the ocean, we can determine the age. 1 As the result 

 of many thousands of river analyses, the total amount of sodium 

 annually discharged to the ocean by all the rivers of the world 

 is found to be probably not far from 175 million tonnes. 2 

 Dividing this into the mass of oceanic sodium we get the age 

 as 807 millions of years. Certain corrections have to be applied 

 to this figure which result in raising it to a little over 90 millions 

 of years. By this method Sollas gets the age as between 

 80 and 150 millions of years. My own result 3 was between 

 80 and 90 millions of years ; but I subsequently found that upon 

 certain extreme assumptions a maximum age might be arrived 

 at of 105 millions of years. 4 Clarke regards the 807 millions of 

 years as certainly a maximum in the light of certain calculations 

 by Becker. 5 



The order of magnitude of these results cannot be shaken 



1 Trims. R.D.S. 1899. A paper by Edmund Halley, the astronomer, in the 

 Philosophical Transactions of the Royal Society for 171 5, contains a suggestion 

 for finding the age of the world on somewhat similar lines. He proposes to make 

 observations on the saltness of the seas and ocean at intervals of one or more 

 centuries, and from the increment of saltness arrive at their age. The measure- 

 ments, as a matter of fact, are impracticable. The salinity would only gain (if all 

 remained in solution) one millionth part in 100 years ; and, of course, the con- 

 tinuous rejection of salts by the ocean would invalidate the method. The last 

 objection also invalidates the calculation by T. Mellard Reade (Proc. Liverpool 

 Geol. Soc. 1876) of a minor limit to the age by the calcium sulphate in the ocean. 

 Both papers were quite unknown to me when working out my method. Halley's 

 paper was, I think, only brought to light in 1908. 



* J. W. Clarke, A Preliminary Study of Chemical Denudation (Smithsonian 

 Miscellaneous Collections, 1910). 



3 Loc. cit. 



* "The Circulation of Salt and Geological Time" (Geol. Mag. 1901, p. 350). 



5 Becker (loc. cit.), assuming that the exposed igneous and archcean rocks alone 

 are responsible for the supply of sodium to the ocean, arrives at 74 millions of 

 years as the geological age. This matter was discussed by me formerly (Trans. 

 R. D. S. 1899, pp. 54 et seq.). The assumption made is, I believe, quite inad- 

 missible. It is not supported by river analyses, or by the chemical character of 

 residual soils from sedimentary rocks. There may be some convergence in the rate 

 of solvent denudation, but -as I think on the evidence— in our time unimportant. 



