AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 263 



of the primeval crust exposed to denudation. The hitter assumption 

 appears justified, not only for the reason that any other is gratuitous 

 and prima facie unwarranted, but also from the fact that the sodium 

 contents of the sedimentaries, as now existing, if increased by what is 

 now the ocean, reverts very nearly to that of Clarke's primary crust. 



The assumption of this acidic denudation of the primeval rocks leaves 

 the ocean charged principally with chlorides at the dawn of geological 

 history. Carbonates nmst also have entered into the composition of 

 the primeval ocean, probably as minor constituents. Sulphates pos- 

 si})ly also existed in relatively small quantities. 



Sterry Hunt believed that the waters imprisoned in the pores of the 

 older stratified rocks, and which are "vastly richer in salts of lime and 

 magnesia than those of the present sea," might be regarded as the 

 fossil sea-waters of the ancient ocean. He gives a theory of the subse- 

 quent changes in ocean chemistry, suggesting that the carbonates of 

 the alkalies and the alkaline earths in subsequent geological history 

 carried to the sea by the rivers, would first precipitate the dissolved 

 alumina and the heavy metals, "after which would result a decompo- 

 sition of the chloride of calcium of the sea water, resulting- in the pro- 

 duction of carbonate of lime or limestone and chloride of sodium or 

 common salt."^ 



III. — THE SUPPLY OF SODIUM BY THE RIVERS. 



Before turning to other considerations we musi attend to a correction 

 which we have already touched upon and which is not negligible. In 

 deducting from the river supply of sodium a quantity equal to 10 per 

 cent of the sodium chloride as being derived directl}' from the sea, we 

 evidently reduce our divisor and so increase our estimate of geological 

 time. The deduction of 10 per cent can, of course, be accepted as no 

 more than a rough allowance — possibly a little excessive. 



The quantity of sodium chloride thus assumed as derived from the 

 sea is 1,057 tons per cubic mile of river water, or 108 X 10^ tons for the 

 entire annual river discharge. Calculating the sodium only, this 

 becomes 42x10^ tons per annum. We have already calculated the 

 quantity of sodium in the ocean of to-day, and found it to amount to 

 15,627x10^" tons. But of this we have reason to believe 1,1)72x10'' 

 tons are to be ascribed to the rapid denudation of the original rocks, 

 leaving 13,655 X 10'" tons to l)e accounted for l)y sul)sequent supply from 

 the rivers. This river suppl}" amounts to a total of 15,727x10^ tons 

 per annum, to which must be applied the correction for the observed 

 supply to rivers of sodium abstracted from the sea and precipitated 

 upon coastal countries by rain water. This, as we have just seen, is 



1 Chemical and Geological Essays, 1897, p. 41. See also Bischof's Chemical and 

 Physical Geology, London, 1855, Vol. I, p. 7, and Sir A. Geikie's Text-Book of 

 Geology, third edition, p. 412, Deposits in Salt and Bitter Lakes. 



