Revieics — J. Jol//'s Age of the Earth. 127 



sodium by rivers to the ocean ai'e considered. The estimate of 

 89'3 millions of years is finally arrived at as " based on the most 

 complete estimates of probabilities." 



IV. We next have a short geological discussion on the origin of 

 beds of rock salt ; but the author concludes that these have scarcely 

 any bearing upon his theory. 



V. In this section the rates of percentages of soda and potash in 

 igneous and sedimentary rocks respectively are considered. Seeing 

 that the sedimentaries have been ultimately derived from the igneous, 

 the difference needs to be accounted for. The difference is con- 

 siderable ; for while in the igneous the percentages are of potash 

 2'83 and of soda 3'6], in the sedimentaries they are of potash 249 

 and of soda 1'47. "If now the infei*ence is right that the missing 

 alkalies [i.e. the deficiency of soda in the sedimentaries] were 

 supplied to the ocean, we should expect to find, on a rough approxi- 

 mation of the bulk of sedimentaries, and hence of the original rocks 

 giving ri&e to them, that such a mass of parent rock would be 

 adequate to supply the sodium in the ocean." This the author claims 

 to be the case, allowing for the sodium retained in beds of rock salt. 

 The weight of this argument clearly depends upon what reliance we 

 can place upon the estimate of the bulk of the sedimentaries, which 

 is taken from Mr. Mellard Eeade.^ Professor Joly accepts his 

 estimate of a layer 2 miles thick over the land area, which on 

 deducting the calcareous rocks is reduced to 1'6 miles. This, how- 

 ever, is reduced still further by a course of somewhat complicated 

 reasoning to 1*1 miles, and the conclusion, accentuated by italics, is 

 thus stated : — 



^^ Hence it appears that, if a thichness of Vl mile of rock spread 

 over the land area represent the bulk of the entire detrital siliceous 

 sedimentary rocks, inclusive of submarine detritus, and this constitutes 

 €7 per cent, of the entire sedimentaries of the earth, including matter in 

 solution in the sea, the sodium contained in the sea, added to lohat is left 

 over in the detrital sediments, would suffice to restore to the entire mass 

 a soda percentage almost equal to that in the eruptive, igneous, and 

 crystalline rocks ; the deficiency, about 0*4 per cent., exists partly in 

 rock-salt deposits." 



The bearing of this section upon the general argument is, that 

 the sodium in the ocean may be regarded as having been all of it 

 derived from the original rock magma. 



VI. The ratio of potash to soda in the ocean is 1 to 31 nearly, 

 whereas in the river discharge it is 1 to 2*8, that is, there is about 

 eleven times as much potash compared to soda in the rivers as there 

 is in the ocean. It is therefore clear that at this rate the present ratio 

 of these alkalies in the ocean could not have been contributed by the 

 rivers. " We must then suppose that the rivers are now supplying 

 more potash relatively to soda than formerly ; or that some process 

 of abstraction of the potash from the ocean is in continual progress." 

 The author comes to the conclusion that there is no reason to 

 suppose that the ratio of the alkalies in the river supply differed in 



1 Geol. Mag., Dec. Ill, Vol. X (1893), p. 97. 



