Presidential Address. 225 



already stated, that subsidence of the ocean-bed has pro- 

 duced 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 

 existence in all geological periods, except perhaps the earl- 

 iest 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 pli- 

 cation and folding, more especially along the borders of the 

 oceans, forming elevated portions of land, rarely altogether 

 submerged and constantly affording the material of sedi- 

 mentary accumulations, while they were also the seats of 

 powerful volcanic ejections, 



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 de- 

 velopment 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 becm the perennial abodes of pelagic 

 and abyssal creatures, and the refuge of multitudes of other 

 marine animals and plants in times of continental elevation. 

 These general facts are full of importance with reference to 

 the question of the succession of formations and of life in 

 the geological history of the earth. 



So much time has been occupied with these general views, 

 that it would be impossible to trace the history of the Atlantic 

 in detail through the ages of the Palaeozoic, Mesozoic, and 

 Tertiary. "We may, however, shortly glance at the changes 

 of the three kinds of surface already referred to. The bed 



