42 Joty—An Estimate of the Geological Age of the Earth. 
saline deposits. They furnish a striking support to the Uniformitarian views 
here advocated. 
At the best the stratified salt deposits of the Earth must form only a very 
small fraction of what has accumulated in the waters of the ocean, The Rock Salt 
of the latter would cover the entire dry land of the Earth to a depth of 400 feet. 
The other deposits are entirely local, and but rarely attain this thickness. We 
see from what has been said that the fractional part of some of these deposits, 
which actually go to throw error into our calculations, makes so small a part of a 
small correction, that we are not concerned with its estimation. 
V.—The Alkalies of the Rocks. 
It is a fact of great interest in connexion with our present consideration that 
the igneous and eruptive rocks, as a whole, possess an amount of soda alkali which 
preponderates over potash alkali, while, in the case of the sedimentary rocks, this 
is in the very large number of cases reversed, the potash alkali exceeding the 
soda alkali. 
This becomes clear in the light of what we have already considered as regards 
the gradual derivation of the salts of the ocean in the process of formation of the 
sedimentaries, coupled with the fact that, under or during conditions of weathering, 
potash aluminium silicates are more resistant than soda aluminium silicates. 
This and another cause for the retention of the latter salts will be reverted to 
again. ‘The fact we wish to dwell on here is the ultimate one that this chemical 
distinction, broadly speaking, exists between the igneous and sedimentary rocks. 
We shall also find that the restoration of the known amount of sodium in the ocean 
to the sedimentary rocks will bring them up to the sodium percentage of the 
igneous rocks. A like restoration cannot be effected for the potash alkali owing 
to reasons we have briefly to point out further on.* 
The average igneous and eruptive primitive crust-rock arrived at by Mr. Clarke 
(ante) possesses the alkali percentage. 
KAO), Ss ae ee ee ee 3 
WaO,  « o o o o il 
This we may compare with the results of averaging the rock analyses selected by 
H. Rosenbusch in his ‘‘ Elemente der Gesteinslehre.” f 
* In dealing with this question in this and the ensuing section the sodium and potassium of the sea 
will be calculated as the oxides for the convenience of frequent references to rock analyses. 
+ Stuttgart, 1898. 
