26 Joty—An Estimate of the Geological Age of the Earth. 
Although the uncertainty attending the estimates of the volume of sedi- 
mentary rocks involved in the recognized geological method of prosecuting 
inquiry into the Age of the Harth must be admitted, it will be seen further 
on that the sodium contents of the sea considered in connexion with the know- 
ledge we possess of the chemical composition of the sedimentary and igneous 
rocks lends support to estimates that have been made of the total bulk of the 
sedimentaries. Here the two methods of inquiry into the Geological Age of the 
Harth appear to mutually support one another. 
This is involved in the following consideration which for the sake of clearness 
may be outlined here. The mean composition of the siliceous sedimentary rocks 
can be arrived at approximately by taking the average of the many reliable 
analyses available of various classes of such rocks as are most abundant. For 
present purposes, it is only necessary to consider the alkali content of these rocks. 
Furthermore, the mean alkali content of the principal igneous rocks, can, in a 
similar manner, be investigated. The mean composition of these rocks has been 
estimated with more especial care by I’. W, Clarke of the United States Geological 
Survey, and the composition of the older crust of the Earth in this way approxi- 
mately determined. It is in the first place found that the alkali content of the 
latter 1s considerably in excess of that of the former. Accepting now an approxi- 
mate estimate of the bulk of siliceous sedimentary rocks, and restoring to this the 
sodium now contained in the ocean, the sodium content of the original crust, 
or of the average of-the eruptives, is obtained with a fair degree of approxi- 
mation. 
Here we observe in the sodium-deficit of the detrital siliceous sediments the 
results of its gradual abstraction by the influences of denudation. There can surely 
be but this one legitimate explanation of the fact that the great bulk of the detrital 
sedimentaries is deficient in sodium by just that amount of this body as is 
contained in the ocean, plus a relatively small allowance for the deposits of 
Rock Salt. It is to be observed that we can effect such a restoration in the case 
of no other elemental body dissolved in the sea. The amount present of the 
chemically related substance potassium will not fit the detrital sediments. It 
exhibits a deficiency. For obvious reasons the calcium and magnesium salts will 
also be deficient.* 
An interesting fact, however, is in the case of the potassium revealed as the 
result of very simple calculation. The present potassium discharge of the rivers, 
if prolonged into the past, as the duration of this is determined by the sodium 
constituent, would have fed into the ocean just about the missing quantity of 
* The reasons referred to are principally the continual abstraction and precipitation of these bodies 
from the sea, giving rise to Limestones and Dolomites, and the presence of calcium and magnesium in the 
ocean sediments, 
