OCEANS AND THEIR HISTORY 7 



bottom water from the Antarctic and preventing its 

 entering from the south into the eastern Atlantic Valley. 

 Since there is a corresponding submarine barrier, the 

 Rio Grande Ridge, running westward from the Mid- 

 Atlantic Ridge but broken through by a wide submarine 

 channel, the Antarctic bottom current is free to enter 

 from the south into the western Atlantic Valley, where 

 its cooling effect on the bottom temperature is apparent 

 as far north as the vicinity of the Bermudas. South of 

 the equator a narrow branch from this westerly Antarc- 

 tic bottom current runs toward the northeast, entering 

 the eastern Atlantic Valley through the Romanche 

 Channel. (Figure 3 is a rough representation of the 

 Antarctic bottom current.) 



Figure 4 shows what the Pacific and Indian Oceans 

 would look like after the same fall of ocean surface by 

 13,000 feet. In the Pacific a vast submarine ridge or 

 plateau, laid bare by the retreating ocean water, would 

 stretch northeast and north in a mighty curve from the 

 Antarctic continent, reaching the west coast of Central 

 America. This ridge now supports the very few East 

 Pacific islands. 



In contrast to these few in the east there is an abun- 

 dance of islands in the central and western parts of the 

 ocean, especially south of the equator. They are mainly 

 of magmatic origin — volcanic cones rising steeply from 

 great depths, a few crowned by lofty peaks towering 

 thousands of feet above the ocean surface. The great 



