354 GEIKTE 



now possesses, and that the crumpling of its outer layers, 

 whether due to mere contraction or, as has been suggested, 

 to the escape also of subterranean vapours, affords evidence 

 of this diminution. A little reflection suffices to show us 

 that, even without any knowledge of the actual history of 

 the contraction, we might anticipate that the effects would 

 neither be continuous nor everywhere uniform. The solid 

 crust would not, we may be sure, subside as fast as the 

 mass inside. It would, for a time at least, cohere and sup- 

 port itself, until at last, gravitation proving too much for 

 its strength, it would sink down. And the areas and 

 amount of descent would be greatly regulated by the vary- 

 ing thickness and structure of the crust. Subsidence would 

 not take place everywhere; for, as a consequence of the 

 narrower space into which the crust sank, some regions 

 would necessarily be pushed up. These conditions appear 

 to have been fulfilled in the past history of the earth. There 

 is evidence that the terrestrial disturbance has been renewed 

 again and again, after long pauses, and that, while the 

 ocean basins have on the whole been the great areas of 

 depression, the continents have been the lines of uprise or 

 relief, where the rocks were crumpled and pushed out of 

 the way. Paradoxical, therefore, as the statement may 

 appear, it is nevertheless strictly true, that the solid land, 

 considered with reference to the earth's surface as a whole, 

 is the consequence of subsidence rather than of upheaval. 



Grasping, then, this conception of the real character of 

 the movements to which the earth owes its present surface 

 configuration, we are furnished with fresh light for explor- 

 ing the ancient history and growth of the solid land. The 

 great continental ridges seem to lie nearly on the site of 

 the earliest lines of relief from the strain of contraction. 

 They were forced up between the subsiding oceanic basins 

 at a very early period of geological history. In each suc- 

 ceeding epoch of movement they were naturally used over 

 again, and received an additional push upward. Hence 

 we see the meaning of the evidence supplied by the sedi- 

 mentary rocks as to shallow seas and proximity of land. 

 These rocks could not have been otherwise produced. They 

 were derived from the waste of the land, and were depos- 



