72 THE HISTORY OF THE NORTH ATLANTIC 



sea the products of the waste of land, sorting them into fine 

 clays and coarser sands ; and the cold currents which cling to 

 the ocean bottom, now determined in their courses, not merely 

 by the earth's rotation, but also by the lines of folding on both 

 sides of the Atlantic, would carry south-westward, and pile up 

 in marginal banks of great thickness the debris produced from 

 the rapid waste of the land already existing in the Arctic 

 regions. The Atlantic, opening widely to the north, and 

 having large rivers pouring into it, was, especially, the ocean 

 characterised, as time advanced, by the prevalence of these 

 phenomena. Thus, throughout the geological history it has 

 happened that, while the middle of the Atlantic has received 

 merely organic deposits of shells of foraminifera and similar 

 organisms, and this probably only to a small amount, its 

 margins have had piled upon them beds of detritus of im- 

 mense thickness. Professor Hall, of Albany, was the first 

 geologist who pointed out the vast cosmic importance of these 

 deposits, and that the mountains of both sides of the Atlantic 

 owe their origin to these great lines of deposition, along with 

 the fact, afterwards more fully insisted on by Rogers, that the 

 portions of the crust which received these masses of debris 

 became thereby weighted down and softened, and were more 

 liable than other parts to lateral crushing. 



Thus, in the later Eozoic and early Palaeozoic times, which 

 succeeded the first foldings of the oldest Laurentian, great 

 ridges were thrown up, along the edges of which were beds of 

 limestone, and on their summits and sides, thick masses of 

 ejected igneous rocks. In the bed of the central Atlantic 

 there are no such accumulations. It must have been a flat, or 

 slightly ridged, plate of the ancient gneiss, hard and resisting, 

 though perhaps with a few cracks, through which igneous mat- 

 ter welled up, as in Iceland and the Azores in more modern 

 times. In this condition of things we have causes tending to 

 perpetuate and extend the distinctions of ocean and continent, 



