THE OCEANS 



lOI 



ordinarily penetrates. The littoral belt is, therefore, the submerged 

 edge of the continental area, and most of it forms the margins of 

 the great oceans. The remainder of the oceans constitutes the 

 abysmal or abyssal area, in the discussion of which we may include 

 the steep transitional slopes between — 200 meters and — 2,400 me- 

 ters (see ante). The upper zone of the ocean as a whole, irrespec- 

 tive of the position of the sea floor, forms the pelagic district, of 

 especial importance in marine bionomy, under which heading it and 

 the abyssopelagic zone will be more fully considered. (See Chap- 

 ter XXVI.) 



Conformation of the Ocean Floor. The ocean floor is char- 

 acterized by elevations and depressions, often of great extent and 



fosse 

 des Tonga 



Ande; 



Fig. 16. (a) Diagrammatic cross-section of the Pacific Ocean near lat. 20° S., 

 showing two fore-deeps, the Tonga and the Atacama. 

 (b) A similar section across the Sonth Atlantic and Indian oceans 

 near lat. 24° S. The Mid-Atlantic Rise is shown in section in the 

 Atlantic. (After Hang.) 



diversity. (Fig. 16.) The following terminology has been de- 

 veloped for these irregularities by the International Commission for 

 submarine nomenclature (39:131): 



I. Grand or Major Features, or Those of Wide Extent. 



Continental Shelf (G. Schelf, Fr. socle or plateau continental). 



The submerged border of the continents to the lOO-fathom or 



200-meter line, where the descent is abrupt. 

 Sub-oceanic Depressions. 



a. Basin (G. Beckett, Fr. bassin). Approaching a circular 



form. 



b. Trough (G. lUulde, Fr. vallee). Elongated and broad de- 



pressions with gradually sloping sides. 



c. Trench (G. Graben. Fr. raz'in). Long l)ut narrow depres- 



sions along the continental border, with steep sides, of 

 which the continental is higher than the marine. 



