AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 257 



unloading- must have been eti'ected in a eoniparatively short period — 

 "instantaneously," if contrasted with the slow unloading- effected 

 by denudation,' Such a redistribution of pressures must have inau- 

 gurated remarkable 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 earl}' land areas, where the reduction of pres- 

 sure and still shallow crust would tend to the expansion and extrusion 

 of the original magma. A diminished mean density of the suboceanic 

 crust does not, however, necessarily follow. On the contrary, the con- 

 ditions of greater pressure under which it was formed must be sup- 

 posed to have conferred greater density upon it, and to have favored 

 the differentiation and crystallization of the denser silicates. If suffi- 

 cient time elapsed for these differences to become deeply established 

 in the crust of the earth, a subsequent reversal of the distribution of 

 pressui'e 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 con- 

 vulsions 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 chieffy 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 earh' hydro- 

 sphere must, for want of other known alternative, be supposed to 

 have contained a quantit}" 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 hydrochloric 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 for their views; and, of course, the right or 

 wrong of the matter turns upon what one means by "salt." We are 



Ut is not to be supposed that tidal disturbances permitted this allocation of the 

 surface to take place quietly, and without swaying at each vibration of our satellite, 

 then possibly much closer to the terrestrial surface. ^ 



'''See Physics of the Earth's Crust (by the Rev. Osmond Fisher: Macmillan ct Co., 

 1889, pp. 297, 298), where increased density of the lithosphere beneath the ocean is 

 for other reasons inferred. 



SM 99 17 



