322 CONDITION OF THE FLOOR OF THE OCEAN. 



Lord Kelvin has pointed out that, when the solid crust began to form, 

 it would rapidl.y cool over its whole surface, the precipitation of water 

 would accelerate this process, and there would soon be an approxima- 

 tion to present conditions. As time went on the plastic or critical 

 layer — the tektosphere — immediatel}^ beneath the crust would gradu- 

 ally sink deeper and deeper, while ruptures and readjustments would 

 become less and less frequent than in earlier stages. With the tirst 

 fall of rain the silicates of the crust would be attacked by water and 

 carbon dioxide, which can at low temperatures displace silicon dioxide 

 from its combinations. The silicates, in consequence, have been con- 

 tinuously robbed of a part or the whole of their bases. The silica 

 thus set free goes ultimately to form quartz veins and quartz sand on 

 or about the emerged land, while the bases leached out of the disin 

 tegrating rocks are carried out into the ocean and ocean basins. A 

 continuous disintegration and differentiation of materials of the litho- 

 sphere accompanied by a sort of migration and selection among min- 

 eral substances, is thus alwaj's in progress. Through the agency of 

 life, carbonate of lime accumulates in one place; through the agency 

 of winds, quartz sand is heaped up in another; through the agency of 

 water, beds of clay, of oxides of iron and of manganese are spread 

 out in other directions. 



The contraction of the centrosphere supplies the force which folds 

 and crumples the lithosphere. The combined effect of hydrosphere, 

 atmosphere, and biosphere on the lithosphere gives direction and a 

 determinate mode of action to that force. From the earliest geological 

 times the most resistant dust of the continents has been strewn along 

 the marginal belt of the sea floor skirting the land. At the present 

 time the deposits over this area contain on the average about TO per 

 cent of free and combined silica, mostly in the form of quartz sand. 

 In the abysmal deposits far from land there is an average of only about 

 30 per cent of silica, and hardly any of this in the form of quartz sand. 

 Lime, iron, and the other bases largely predominate in these abysmal 

 regions. The continuous loading on the margins of the emerged land 

 by deposits tends by increased pressure to keep the materials of the 

 tektosphere in a solid condition immediately beneath the loaded area. 

 The unloading of emerged land tends by relief of pressure to pro- 

 duce a viscous condition of the tektosphere immediately beneath the 

 denuded surfaces. Under the influence of the continuous shakings, 

 tremors, and tremblings always taking place in the lithosphere the 

 materials of the tektosphere yield to the stresses acting on them, and 

 the deep-seated portions of the terrigenous deposits are slowly carried 

 toward, over, or underneath the emerged land. The rocks subse- 

 quently reformed beneath continental areas out of these terrigenous 

 materials, under great pressure and in hydrothermal conditions, Avould 

 be more acid than the rocks from which they were originally derived, 



