Reviews — J. Johjs Age of the Earth. 131 



we read : " Water from a greater depth [not specified] holds dissolved 

 chlorides as well. Chloride of sodium is by far the most common, 

 but chlorides of magnesium and calcium are often added. Such 

 waters are usually called saline. The presence of chlorides is 

 seldom shown in waters at less than 100 feet in depth. . . ." ** It 

 is very rare that the drill descends to 300 feet without encountering 

 saline water." These waters are from Silurian beds. 



The conclusion to be drawn from these remarks is, that some 

 of the sodium found in river waters may probably be derived from 

 the " fossil waters " of old sea muds, and if that is the case, it is in 

 circulation from the ocean to the stratified rocks and back again. 

 No doubt much sodium may find its way into rivers immediately 

 from the felspar of rocks, but we are not justified in crediting all the 

 sodium conveyed to the ocean as having been supplied de novo in 

 that manner. This consideration would lengthen Professor Joly's 

 estimate of the world's age by an unknown period. 



An enquiry not without interest may be made as follows : — 

 Supposing the whole of the land area to consist of granite ; for how 

 many years would a foot thickness of the land supply the sodium as 

 carried down by the rivers at the present rate ? Mr, Merrill's book 

 supplies the data to answer this question.^ He describes a section 

 where solid granite occurs at the bottom, and, after passing through 

 an intermediate stage of weathering, is disintegrated into sand at 

 the surface. It appears legitimate to assume that this process of 

 disintegration keeps pace with the denudation of the surface. Mr. 

 Merrill says that the solid granite, in the process of disintegration 

 into the residual sand, loses 28-62 per cent, of its soda. Now the 

 weight of the sodium is the weight of the soda mhms the weight of 

 the combined oxygen, so that it will be 23/31 the weight of the 

 soda. Also he tells us that the weight of the soda is 2-68 per cent, 

 of the solid granite ; and according to Mallet a cubic foot of granite 

 weighs 198-34: pounds. 



With these data it appears that the weight of the sodium in 

 a cubic foot of granite is 0-0004:7029 of a ton. 



Accepting Professor Joly's figures, the land area in square feet 

 will be 55,814,000 x (5,280)^, which gives us the volume of a layer 

 a foot thick covering the land area ; also the weight of the sodium 

 annually discharged by the rivers is 157,267,544: tons. Dividing this 

 by the volume of the one foot layer multiplied by the weight of 

 sodium in a cubic foot, we get 1/5000 of a foot of granite as 

 competent to supply the sodium of the rivers in the annual process 

 of disintegration. 



We may hence conclude that, if the land area consisted of 

 granite, the rate of denudation would be one foot in 5,000 years. 

 Sir A. Geikie, in his " Text Book of Geology " (p. 444), accepts 

 one foot in 6,000 years as a probable mean for the rate of denudation. 

 It is certainly remarkable that tlie result we have obtained comes 

 near this. But when we remember that but a small portion of the 

 land surface consists of crystalline rocks, and that the mean 

 i " On Rocks and Rock Weathering," p. 209. 



