Joty—An Estimate of the Geological Age of the Earth. 45 
in its formation, be adequate to yield to the ocean the sodium that is in it, assuming 
these sedimentaries to be derived from rocks having the mean composition of the 
important eruptive masses now known. Iiven more, the result of the calculation 
indicates that what is in the ocean is not quite a full measure of the sodium washed 
from these rocks. Recollecting that the stratified Rock Salt—the former inland-sea 
deposits——should enter the estimate on the side of the amount credited to the ocean, 
the result must be regarded as satisfactorily favouring the hypothesis. The restora- 
tion of the potash is attended with difficulties to be referred to later which render 
such a satisfactory result impossible. 
For an estimate of the amount of sedimentary rocks on the Earth’s surface we 
are indebted to Mr. T. Mellard Reade.* For the average thickness of sedimentary 
rocks down to the base of the Cambrian, Mr. Reade takes a volume equal to the 
land area covered to the depth of one mile: this being based on the results of 
borings, sections, &c. This commends itself as a good approximation. He 
further, however, assumes that a similar volume of sediment exists under the sea. 
The latter assumption is probably excessive, even if it includes the relatively 
small additional amount of dissolved matter in the ocean. Pre-Cambrian sed1- 
mentary rocks are so comparatively limited in amount that the inclusion even of 
these, as defined by our present knowledge, can hardly justify the total of the 
estimate. However, we will provisionally accept it, and carry out our calculation 
applied to the mass so defined. 
Mr. Readet estimates 10 per cent. of the land sediments to consist of calcareous 
rocks, and also that the total mass of calcareous rocks of the Earth would suffice 
to cover its surface to a depth of one-tenth of a mile. ‘To arrive at the amount of 
siliceous detrital sediments from these estimates, we must deduct from his estimate 
of the total sedimentaries such a mass of calcareous rocks as would cover the Earth 
to a depth of one-tenth of a mile, and further make an allowance for precipitated 
materials other than calcareous. Neglecting the last deduction as being a compara- 
tively small one, we find that the deduction of the calcareous rocks leaves his estimate 
of rocks other than calcareous to amount to a layer 1°6 of a mile in thickness over the 
land-area of the Earth. Hence the mass in tons is equal to 558 x 10° x 1:6 x 2°5 x 
42 x 10°, or 94 x 10 tons nearly. The value 558 x 10° is the area of the layer in 
square miles, 1-6 its thickness in miles, 2°5 the assumed specific gravity of the rock, 
and 42 x 10% the mass in tons of a cubic mile of water. The mean soda of the 
more abundant sedimentaries amounts, as we have seen, to 1:47 per cent. Hence 
13°8 x 10” tons of soda exist in this mass of detrital sedimentary rocks. To this 
must be added the known amount of soda in the sea, which is obtained by con- 
verting 15,627 x 10” tons of the chloride to the oxide; giving 21 x 10* tons. 
* Geol. Mag., vol. x., 1898, p. 97. 
+ Geol. Mag., vol. vi., 1879, and Proc. Roy. Soc., vol. xxymt., 1879, p. 281. 
