JoLty—An Estimate of the Geological Age of the Earth. 39 
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 decomposition of the chloride of calcium of the sea-water, resulting in the 
production of carbonate of lime or limestone, and chloride of sodium or common 
salt.” * 
II].—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 supply 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. 1., p. 7; and Sir A. Geikie’s ‘‘ Text-Book of Geology,” 8rded., p. 412, 
“Deposits in Salt and Bitter Lakes.’ 
TRANS, ROY. DUB. SOC., N.S. VOL, VII., PART III. H 
