JoLY — An Estimate of the Geological Age of the Earth. 33 



lithological differences in the subaqueous and subaerial portions of the lithosphere. 

 It is to be anticipated that beneath the ocean the effects of the primordial 

 conditions of fusion in presence of volatile matter at high pressure would be more 

 perfectly preserved than over the early land areas, where the reduction of pressure 

 and still shallow crust would tend to the expansion and extrusion of the original 

 magma. A diminished mean density of the sub-oceanic crust does not, however, 

 necessarily follow. On the contrary, the conditions of greater pressure under 

 which it was formed must be supposed to have conferred greater density upon 

 it, and to have favoured the differentiation and crystallization of the denser 

 silicates. If sufficient time elajised for these differences to become deeply 

 established in the crust of the Earth, a subsequent reversal of the distribution 

 of 2)ressure must be improbable. It is difficult to conceive that the limited range 

 of transportation attending denudation can have led to any extensive subsequent 

 redistribution of equilibrium. Tidal convulsions would appear to be the only 

 refuge of those who object to the permanence of the continents.* 



The upper part of what is now the Earth's solid crust must, as we have 

 urged, 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. Carbonic anhydride also entered into its composition, and the 

 atmosphere, enveloping all, must have still been largely in excess of our present 

 atmosphere, principally owing to the presence of carbonic anhydride and hydro- 

 chloric acid. The waters of the early ocean, and the rain which then fell upon the 

 lavas and rocks of the land, possessed solvent powers greatly in excess of what we at 

 present observe. Those who have maintained that the sea was " salt " from the 

 first, if they paused here, would doubtless find considerable support to their views ; 

 and, of course, the right or wrong of the matter turns upon what one means by 

 " salt." We are only concerned now with one element of the ocean, the sodium, 

 and it will be easy to show that complete neutralisation of the acid hydrosphere 

 would have been attended, by only a relatively small introduction of sodium into 

 the ocean. 



We can make a rough estimate of the results of this primeval chemical denu- 

 dation — and hence of the correction on the estimate of geological time involved 

 in the primitive saltness of the ocean — by allocating the action of the acid among 

 the constituents of the early crust ; but we have first to inquire into the percen- 

 tage relations of these constituents. 



* See " Physios of the Earth's Crust " (by the Eev. Osmond Fisher : Macmillan & Co., 1889, pp. 297, 

 298), where increased density of the lithosphere beneath the ocean is for other reasons inferred. 



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