THE OCEANS 



105 



Features of the Sub-Oceanic Elevations and Depressions. 



Among the great sub-oceanic elevations the most extensive is the 

 Mid-Atlantic Rise (Supan-38, plate 12), which extends from Ice- 

 land, over the Azores, southward to Tristan da Cunha, a distance of 

 14,000 kilometers and an area of 10 million square kilometers. It 

 is bounded by the 4,000-meter line, and its width in the South At- 

 lantic is approximately indicated by the longitudes of Ascension and 

 St. Helena islands. From the temperatures found in the depres- 

 sions on either side, and especially from the high temperature of the 

 south African trough, it is thought that the axis of this rise nowhere 



Guam 



McdLnilla 



Fig. 17. Two cross-sections of the Marian Trench, a fore-deep in the 

 western Pacific. The upper is east from Guam, and passes 

 through the Nero Deep, 9,636 meters. The lower is east from 

 Medinilla, and passes through lesser deeps, north of the preced- 

 ing, and also shows a "ridge" east of the trench. (After Kriim- 

 mel.) 



falls below 3,000 meters. North of the equator it is less continuous 

 than south of it. With its branches it divides the Atlantic Ocean 

 bottom into a number of great depressions: a North American 

 basin of 13 million square kilometers area, a Brazilian and an Ar- 

 gentine basin with a combined area of 16 million square kilometers, 

 a North African basin of 9 million square kilometers area, and a 

 West African trough of 1 1 million square kilometers area. This 

 great Mid-Atlantic swell does not join the Antarctic continent, but 

 between 30° and 40° S. lat. it has two branches — a northwestern 

 one, the Rio Grande rise, nearly separating the Argentine and Brazil 

 basins, and a western one, the IJlialcs rise, which extends from 

 Tristan da Cunha island to the South African continent, and ef- 



