JoLY — An Estimate of the Geological Age of the Earth. 39 



subsequent geological history carried to the sea by the rivers, would first 

 I^recipitate the dissolved alumina and the heavy metals, "after which would 

 result a decomposition of the cldoride of calcium of the sea-water, resulting in the 

 production 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 must 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 directly 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 

 1657 tons per cubic mile of river water, or 108 x 10' tons for the entire annual 

 river discharge. Calculating the sodium only, this becomes 42 x 10^ tons per 

 annum. We have already calculated the quantity of sodium in the ocean of 

 to-day, and found it to amount to 15,627 x 10^^ tons. But of this we have reason 

 to believe 1972 x 10'- tons are to be ascribed to the rapid denudation of 

 the original rocks, leaving 13,655 x 10'^ tons to be accounted for by subsequent 

 supply from the rivers. This river supply amounts to a total of 15,727 x 10^ tons 

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

 rivers of sodium abstracted from the sea and precipitated upon coastal countries 

 by rain-water. This, as we have just seen, is estimated at 42 x 10' tons per annum. 

 Hence the river supply is now reduced to 15,307 x 10^ tons. The quotient of 

 13,655 X 10'^ by 15,307 x 10* is 89-2 x 10^ To this number of years may be 

 added the decimal 0*1 x 10'^ years as the period approximately required to effect 

 the denudation of the primitive rocks to the extent of fixing the free hydrochloric 

 acid, giving, finally, as the estimate of the duration of denudation, 89*3 x 10'' 

 years. 



It must not be understood from the foregoing that we claim a degree of 

 accuracy for our estimate approximating to so small a time interval as 100,000 

 years. The period is only taken into account as arising from our figures. It 

 will be seen later on that a far larger margin of error is of necessity assumed. 



*•" 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," 3rd ed., p. -J12, 

 "Deposits in Salt and Bitter Lakes." 



TEAMS. ROy. DUB. SOC, N.S. VOL. VII., PAET III. H 



