THE HISTORY OF THE NORTH ATLANTIC 75 



been a permanence of the position of the continents and oceans 

 throughout geological time, but with many oscillations of these 

 areas, producing submergences and emergences of the land. 

 In this way we can reconcile the vast vicissitudes of the conti- 

 nental areas in different geological periods with that continuity 

 of development from north to south, and from the interiors 

 to the margins, which is so marked a feature. We have, for 

 this reason, to formulate another apparent geological paradox, 

 namely, that while, in one sense, the continental and oceanic 

 areas are permanent, in another, they have been in continual 

 movement. Nor does this view exclude extension of the con- 

 tinental borders or of chains of islands beyond their present 

 limits, at certain periods; and indeed, the general principle 

 already stated, that subsidence of the ocean bed has produced 

 elevation of the land, implies in earlier periods a shallower 

 ocean and many possibilities as to volcanic islands, and low 

 continental margins creeping out into the sea ; while it is also 

 to be noted that there are, as already stated, bordering shelves, 

 constituting shallows in the ocean, which at certain periods 

 have emerged as land. 



We are thus compelled, as already stated, to believe in the 

 contemporaneous existence in all geological periods, except 

 perhaps the earliest of them, of the three distinct conditions of 

 areas on the surface of the earth, defined in chapter second — 

 oceanic areas of deep sea, continental plateaus and marginal 

 shelves, and lines of plication and folding. 



In the successive geological periods the continental pla- 

 teaus, when submerged, owing to their vast extent of warm and 

 shallow sea, have been the great theatres of the development of 

 marine life and of the deposition of organic limestones, and 

 when elevated, they have furnished the abodes of the noblest 

 land faunas and floras. The mountain belts, especially in 

 the north, have been the refuge and stronghold of land life 

 in periods of submergence ; and the deep ocean basins have 



