Reviews — J. Joly's Age of the Earth. 125 



•' Of the sodium cbloride 39'32 per cent, is sodium. In the sea, 

 there is therefore ainass of sodium amounting to 14,151 x 10'^ tons." 



The materials in tons per cubic mile of nineteen of the prhicipal 

 rivers of the world (after Sir J. Murray) are next given, and from 

 these it is calculated that they contain 21,106 tons of sodium per 

 eubic mile. " The total volume [of water] discharged by the rivers 

 into the ocean is 6,521 cubic miles per annum." 



The mass of sodium in the ocean divided by the mass annually 

 brought down by the rivers gives the length of time in which, at 

 the present rate, the mass in the ocean can have accumulated. The 

 result is 89,565,000 years. But Professor Joly prefers to use^ 

 a later estimate of the volume of the ocean, which would lengthen 

 the " period of geological denudation to 91,800,000 years nearly." 



The foregoing gives the pith of Professor Joly's very ingenious 

 theory of the age of the earth. The remaining sections are subsidiary 

 to it, and ai'e apparently intended to meet possible objections, and 

 to render the first estimate more accurate. 



II. " On the Original Condition of the Ocean." This is a necessary 

 inquiry ; for unless it can be premised that there was no sodium in 

 it, the argument clearly fails, and the earth's age will be pro- 

 portionately shorter by an unknown amount. The author assumes 

 that the earth was once molten, and considers that, upon cooling, 

 " the upper part of what is now the earth's crust must have contained 

 as silicates, in the form of slag, lava, or rock, the alkaline earths 

 now appearing chiefly as carbonates, the alkalies now distributed 

 between the salts of the sea and the alkali silicates of the rocks, 

 along with iron and alumina. The early hydrosphere must for 

 want of other known alternative be supposed to have contained 

 a quantity of hydrochloric acid, roughly represented by the chlorine 

 now in the ocean." 



We next find speculations as to the sequence of events preceding 

 and following the first condensation of water on the surface ; and 

 it is considered improbable that a uniform ocean ever covered the 

 entire globe ; and the author inclines to the opinion that the sub- 

 oceanic crust is more dense than the continental, and that ocean 

 basins have been permanent. This part of the essay is of mucli 

 interest, but does not very closely concern the main question of the 

 age of the earth. 



We have here quoted an important analysis by Mr. F. W. Clarke 

 (Bull. U.S.A. Geol. Surv., 1897), " which may fairly represent the 

 composition of the older crust of the earth." 



"SiOj 59-77 NaoO 3-61 



AI2O3 15-38 H,0 1-51 



FeoOj 2-65 Ti02 0-53 



FeO 3-44 Pa O5 0-'21 



CaO 4-81 



MgO 4-40 99-14 



K2O 2-83 



" Such a rock, or lava, attacked by a heated solution of hydro- 

 chloric acid, must ultimately yield its iron, calcium, magnesium. 



