AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 269 



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 

 favoring the hypothesis. The restoration of the potash is attended 

 with difficulties to be referred to later, Avhich 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, etc. 

 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 probablj^ excessive, even if it includes the relatively 

 small additional amount of dissolved matter in the ocean. Pre-Cam- 

 brian sedimentary 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. Reade ' 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 woidd 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 sedimen- 

 taries 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 pre- 

 cipitated materials other than calcareous. Neglecting the last deduc- 

 tion as being a comparatively 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 -1:2 X 10^ 

 or 94X10^'* tons nearly. The value 558X10'^ is the area of the 

 layer in square miles, 1.0 its thickness in miles, 2.5 the assumed specific 

 gravity of the rock, and 42x10* 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 converting 

 15,627x10^' tons of the chloride to the oxide, giving 21x10^^ tons. 

 The restoration of this to the rocks therefore raises their amount of 

 NagO to 84. 8 X 10'' tons. 



'Geol. Mag., Vol. X, 1893, p. 97. 



^Geol. Mag., Vol. VI, 1879, and Proc. Roy. Soc, Vol. XXVIII, 1879, p. 281. 



