42 JoLY — Ati Estimate oj 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. 



K2O, 2-83 



Na^O, 3-61 



This we may compare with the results of averaging the rock analyses selected by 

 H. Roseubusch in his " Elemente der Gesteinslehre."t 



* In dealing with this question in this and the ensuing section the sodium and potassium of the sea 

 will he calculated as the oxides for the convenience of frequent references to rock analyses, 

 t Stuttgart, 1898. 



