48 Joty—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 (doc. cit.), where we find a mean of 16:06 per cent. of Al,O;, 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 :— 
Na,O lost = 60 per cent. 
», saved = 40 re 
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 gross 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. 
Fence wt appears that, if a thickness of 1:1 mile of rock spread over the land area 
represents the bulk of the entire detrital siliceous sedimentary rocks, mcelusive of submarine 
detritus, and this constitutes 67 per cent. of the entire sedimentaries of the Earth, including 
matter mm solution im the sea, the sodium contained in the sea, added to what is left over in 
the detrital sediments, would 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 :— 
Mean of 15 Sandstones, &e. (p. 391), . : 5 SPaK 
0 13 Clays, &c. (p. 420), . . 6 2) L946 
9» 18 Clay-slates, &c. (p. 425), . : o Weil 
. 6 Calcareous Clay-slates (p. 428), . 14:95 
,» 20 Phyllites(p. 483), . |. . . 20:85 
% 10 Sericite-schists (p. 486), . : 5 Diloey 
99 4 doubtful Phyllites (p. 486), . . 15:42 
99 13 ‘‘ Pelit-gneisse’ (p. 470) . 6 o Soe 
55 8 ‘¢ Psammit-gneisse’”’ (p. 471), . > LiOs 
> 3 Amphibole-gneisses (p. 484), . . 18°54 
0 6 Pyroxen-geneisses (p. 486), . 14:93 
o 4 Mica-schists (p. 497), : 6 . 13°82 
Mean of all, . 16:06 
