270 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



The total bulk of sedimentary rock on Mr. Reade's estimate is, how- 

 ever, equal to a layer 2 miles deep over the dry land.^ This amounts 

 to 116x10" tons; hence, with the sodium of the ocean restored to 

 them, we find the soda percentage from the fraction ff|, which is 3 per 

 cent. This is about the soda percentage of many granites, gneisses, 

 and diorites, etc., but falls somewhat short of the average of the erup- 

 tive 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 frac- 

 tion 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 com- 

 parison 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 lepresent 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 3(» per cent ma}^ be assumed as 

 the loss by solution of the entire detrital sediments. We accept 1 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 denu- 

 dation, 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 eiXlO'*^ tons. 

 This being assumed as 70 per cent of the original mass, the latter is 

 91X10" tons. 



The mass of 64 X 10" tons contains 940 X 1( »^-' tons of Na^O. Adding 

 the amount in the ocean (21x10" tons), we obtain 30.4X10" tons. 



* In stating that there is as much sedimentary rock under the sea as upon the land 

 Mr. Reade possibly implies that the sul)marine sediments are to be estimated as 

 possessing a thickness of 1 mile. Mr. Reade' s calculation of the geological age of the 

 earth on tlie rate of denudation of the sediments appears, however, to involve that 

 the bulk 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 2 miles. (Geol. Mag., Dec. 3, Vol. X, pp. 97-100.) 



