46 Joty—An Estimate of the Geological Age of the Earth. 
The restoration of this to the rocks therefore raises their amount of Na,O to 
34:8 x 10” tons. 
The total bulk of sedimentary rock, on Mr. Reade’s estimate, is however equal 
to a layer two miles deep over the dry land.* This amounts to 116 x 10” tons: 
hence, with the sodium of the ocean restored to them, we find the soda percentage 
from the fraction 3?;48, which is 3:00 per cent. This is about the soda percentage 
of many Granites, Gneisses, and Diorites, &c., but falls somewhat short of the 
average of the eruptive and igneous rocks. The stratified salt deposits would 
somewhat raise the figure to over 3 per cent. Clarke’s average original crust has 
3°61 per cent. 
It appears very probable that we may in part trace the deficiency to the estimate 
of sedimentary rock beneath the ocean. This must be mainly precipitated material ; 
the detrital deposits can only be a fraction of that upon the land. We can easily 
see how an estimate on somewhat different and, it is submitted, more satisfactory 
bases may be effected, bringing almost exact agreement between the restored 
sediments and the primal rock. 
We can use the broad fact—to be presently shown—that the comparison of 
disintegrated and decomposed rock material of the present day, constituting soils 
of various rock-formation, reveals a loss of constituents of parent rock amounting 
on the average to 38 per cent. When it is remembered that such soils represent 
in many cases, extreme stages of weathering never attained to by many sediments, 
but that these latter are often the result of little more than disintegration and 
transportation, it appears probable that 30 per cent. may be assumed as the loss by 
solution of the entire detrital sediments. We accept one mile deep of these on 
the land, and, confining ourselves to purely detrital siliceous sediments, assume that 
as much as 10 per cent. of what is on the land is in the sea, or, say, a total of 
1-1 mile deep over the land area. We include in this the Pre-Cambrian detrital 
sediments. 
To recover from this the original mass of parent rock, we assume that a loss of 
30 per cent by solution occurred in the process of denudation, or in other words, the 
1:1 mile of detrital sediments is 70 per cent. of the original mass of parent rock. 
The mass of 1:1 mile deep of sedimentary rock of specific gravity of 2:5 will be 
64 x 10% tons. This being assumed as 70 per cent. of the original mass, the latter 
is 91 x 10" tons. 
* In stating that there is as much sedimentary rock under the sea as upon the land, Mr. Reade possibly 
implies that the submarine sediments are to be estimated as possessing a thickness of one mile. Mr. Reade’s 
calculation of the geological age of the Earth on the rate of denudation of the sediments appears, however, 
to involve that the dul of sedimentary material beneath the ocean is, in his opinion, to be taken as about 
equal to what is upon the land, or the total bulk is equal to the land-area covered to a thickness of two 
miles. Geol. Mag., Dec. 3, vol. x., pp. 97-100. 
