78 THE HISTORY OF THE NORTH ATLANTIC 



vations were not always alike on the two sides of the Atlantic. 

 The Salina period of the Silurian, for example, and the Jurassic, 

 show continental elevation in America not shared by Europe. 

 The great subsidences of the Cretaceous and the Eocene were 

 proportionally deeper and wider on 'the eastern continent, and 

 . this and the direction of the land being from north to south, 

 cause more ancient forms of life to survive in America. These 

 elevations and submergences of the plateaus alternated with 

 the periods of mountain-making plication, which was going on 

 at intervals, at the close of the Eozoic, at the beginning of the 

 Cambrian, at the close of the Siluro-Cambrian, in the Permian, 

 and in Europe and Western America in the Tertiary. The 

 series of changes, however, affecting all these areas was of a 

 highly complex character in detail.^ 



We may also note a fact which I have long ago insisted on,^ 

 the regular pulsation of the continental areas, giving us alter- 

 nations in each great system of deep-sea and shallow-water 

 beds, so that the successive groups of formations may be di- 

 vided into triplets of shallow-water, deep-water, and shallow- 

 water strata, alternating in each period. This law of succession 

 applies more particularly to the formations of the continental 

 plateaus, rather than to those of the ocean margins, and it 

 shows that, intervening between the great movements of plica- 

 tion there were subsidences of those plateaus, or elevations of 

 the sea bottom, which allowed the waters to spread themselves 

 over all the inland spaces between the great folded mountain 

 ranges of the Atlantic borders. 



In referring to the ocean basins we should bear in mind 

 that there are three of these in the northern hemisphere — the 

 Arctic, the Pacific, and the Atlantic. De Ranee has ably 



thousand years ago, and was itself not of very great duration. Thus in 

 Pleistocene times the land must have been submerged and re-elevated in a 

 very rapid manner. 



* " Acadian Geology." 



