474 THE DEPTHS OF THE SEA. [CHAP. x. 



Erom a glance at the map (PI. VIII.), and remem 

 bering that nearly the same arrangement exists in 

 regard to the newer rocks of North America, it would 

 seem that the sum of these minor elevations and 

 subsidences has produced a general elevation of the 

 edges, and a general contraction, of a basin the long 

 axis of which coincides roughly with the long axis of 

 the Atlantic. The Jurassic beds crop out along the 

 outer edge of the basin, the cretaceous beds form a 

 middle band, while the tertiaries occupy the troughs 

 and valleys. All of these, however, maintain a cer 

 tain parallelism determined by the contour of the 

 earlier land and the direction of the older mountain 

 ridges, to one another, and to the shores of the 

 present sea. 



From the parallel of 55 north latitude, at all 

 events to the equator, we have on either side of 

 the Atlantic a depression 600 or 700 miles in width, 

 averaging 15,000 feet in depth. These two valleys 

 are separated by the modern volcanic plateau of the 

 A9ores. It does not seem to us to be at all probable 

 that any general oscillations have taken place in the 

 northern hemisphere sufficient either to form these 

 immense abysses, or, once formed, to convert them 

 into dry land. 



Reasoning partly upon physical and partly upon 

 palseontological grounds, Mr. Prestwich thinks it 

 probable that the ancient chalk ocean which formed 

 a great transverse belt entirely across southern and 

 eastern Europe and central Asia on the one hand, and 

 across the Isthmus of Panama and southern North 

 America on the other, was cut off by a land barrier 

 from the Arctic Sea, and on that account possessed a 



