18 REPoni— 1886. 



of to-day is much smaller than it was in those times «7hen it spread 

 widely over the continental plains and slopr ., and much larger than it 

 has been in times of continental elevation. This leads us to the further 

 conaideration that, while the ocean beds have been sinking, other areas have 

 teen better snpp jrted, 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 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 continental areas in difiFerent geological 

 periods with that continuity of development from north to south, and 

 from the interiors to the margins, which is so marked a feature. Wo 

 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. !N'or does 

 this view exclude extension of the continental borders or oi' 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 emorged as land. 



We are thus compelled to believe in the contemporaneous existence 

 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 borders of the oceans, forming elevated portions of 

 land, rarely altogether submerged and constantly affording the material 

 of sedimentary accumulations, while they were also the seats of powerful 

 volcanic ejections. 



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 de • 

 position 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 been the 

 perennial abodes of pelagic and abyssal creatures, and the refuge of mul- 

 titudes of other marine animals and plants in times of continental eleva- 

 tion. These general facts are full of importance with reference to the 



