5 o TEE POPULAR SCIENCE MONTHLY. 



us to the further consideration that, while the ocean-beds have been 

 sinking, other areas have been better supported, and constitute the 

 continental plateaus ; and that it has been at or near the junctions of 

 these sinking and rising areas that the thickest deposits of detritus, 

 the most extensive foldings, and the greatest ejections of volcanic 

 matter have occurred. 



There has thus been a permanence of the position of the conti- 

 nents and oceans throughout geological time, but with many oscilla- 

 tions of these areas, producing submergences and emergences of the 

 land. In this way we can reconcile the vast vicissitudes of the con- 

 tinental areas in different geological periods with that continuity of 

 development from north to south and from the interiors to the mar- 

 gins, which is so marked a feature. We have for this reason to for- 

 mulate 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 continental borders or of chains of islands beyond 

 their present limits at certain periods ; and, indeed, the general prin- 

 ciple 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 to believe in the contemporaneous exist- 

 ence in all geological periods, except perhaps the earliest of them, of 

 three distinct conditions of areas on the surface of the earth : 



1. Oceanic areas of deep sea, which always continued to occupy in 

 whole or in part the bed of the present ocean. 



2. Continental plateaus and marginal shelves, existing as low flats 

 or higher table-lands, liable to periodical submergence and emergence. 



3. Lines of plication and folding, more especially along the bor- 

 ders of the oceans, forming elevated portions of land, rarely altogether 

 submerged, and constantly affording the material of sedimentary accu- 

 mulations, while they were also the seats of powerful volcanic ejec- 

 tions. 



In the successive geological periods the continental plateaus 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 fur- 

 nished the abodes of the noblest land faunas and floras. The mount- 

 ain-belts, especially in the north, have been the refuge and stronghold 

 of land life in periods of submergence, and the deep ocean-basins have 

 been the perennial abodes of pelagic and abyssal creatures, and the 

 refuge of multitudes of other marine animals and plants in times of 

 continental elevation. 



