48 



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



one constituent to pass over without loss from the one rock to the other, and use 



its percentage as a standard of reference by which to compare the loss of alkalies. 



We take the alumina for this purpose. 



The alumina of the original rock in Clarke's average of analysis is 15'38. 



This is for the parent rock. For the derived rocks we may refer to Rosenbusch's 



tables (loc. cit.), Avhere we find a mean of 16 "06 per cent, of AI2O3, in the several 



groups of Slates, Sandstones, Phyllites, Schists, and Gneisses already referred 



to.* We now have the data required for our calculation. On these we find, 



closely : — 



Na20 lost = 60 per cent. 



,, saved = 40 ,, 



Referring now to the mean loss of soda and of entire rock given in soil- 

 formations, we find that on the same ratio of sodium loss to gi-oss loss, Clarke's 

 original crust-rock should have lost 33 per cent, in affording sediments of the 

 present soda percentage. 



If, following this estimate exactly, we assume that the loss by solution in the 

 process of denudation and formation of the detrital siliceous sediments had been 

 33 per cent, about, and still assuming 1*1 mile as the thickness of the detrital 

 sedimentary mass spread over the land, the mass of the parent rock would calculate 

 out as 95 X 10'" tons, and the restored soda would amount to 3-21 per cent. 



Hence it ajjpcars that, if a thickness of 1"1 viilc of rock S2)read over the land area 

 represents the bulk of the entire detrital siliceous sedimentary rocks, inclusive of submarine 

 detritus, and this constitutes (SI per cent, of the entire sedimentaries of the Earth, including 

 matter in solution in the sea, the sodium contained in the sea, added to what is left over in 

 the detrital sediments, tvould suffice to restore to the entire mass a soda percentage almost 

 equal to that in the eruptive, igneous, and crystalline rocks ; the deficiency, about 

 0*4 per cent., exists partly in Rock Salt deposits. Some of the calcareous rocks, 



* As follows : — 



