£7 THE DEPTHS OF THE SEA. [crrap. x. 
From a glance at the map (PI. VIIT.), 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 
Acores. 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 
paleeontological 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 
