108 GEOLOGY. 



a selective action of this kind began. The surface material of the 

 areas that remained exposed lost more of its basic than of its acidic 

 constituents, while the submerged material lost less, and perhaps gained 

 something by the redeposition of the matter borne in from the land. 

 As the plane tesimals were being gathered in on land and water alike, 

 those that fell on the land suffered some atmospheric change, while 

 those that fell into the water were mainly protected from it. As this 

 differential action affected each successive layer of growth after the 

 accumulation of surface waters began, the specific gravity of the land 

 areas came to be less than that of the submerged areas. 



Permanent versus temporary effects. — It is not the temporary 

 specific gravity that resulted from the change of physical state involved 

 in disintegration that is to be considered here, but rather what may 

 be termed the inherent or permanent specific gravity, i.e., the specific 

 gravity that would be retained after any metamorphism which the 

 material might probably suffer in the future had taken place. So 

 likewise it is not the temporary chemical combinations arising imme- 

 diately from the weathering, but the potential future combinations 

 that are significant. For example, any rock likely to arise from the 

 residual sands, earths, and clays by any probable metamorphism, or 

 even by remelting, would have a lower specific gravity than the original 

 average rock, or than any rock likely to be developed from the alkalies 

 and alkaline earths removed by the leaching process in connection 

 with the original rock. The leaching of the land material had, there- 

 fore, a permanent effect on its specific gravity, an effect not eliminated 

 by any probable change resulting from its burial under later accumula- 

 tions. The earth segments built up by accretion on the land were 

 hence lighter than the segments built up under the waters, and the 

 difference increased as the segments grew in thickness. 



The progressive depression of water-covered areas. — It follows from 

 the greater weight of the water-covered segments that the compression 

 beneath them, as they became more and more weighted with incoming 

 material, was greater than the compression beneath the land-segments, 

 and hence the water-covered areas were depressed relatively more 

 than the land-areas. The waters drawn in upon the depressed seg- 

 ments augmented the depressing effects due to difference in specific 

 gravity. 



It is not necessary to suppose that there was, at the outset, a 



