JoLY — Ah 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 novs^ known. Even 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 sedi- 

 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. Readef 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 658 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., 1893, p. 97, 



t Geol. Mag., vol. vi., 1879, and Proc. Eoy. Soc, vol. xxvm., 1879, p. 281. 



