io6 PRINCIPLES OF STRATIGRAPHY 



fectively cuts off the South African trough from the Antarctic cold 

 waters. In the Pacific, west of South America, is the great Easter 

 Island rise, with an area exceeding that of Africa. The South 

 Indian Ocean shows two great swells or rises : the Kerguelen rise, 

 from Antarctica and Australia, and the Crozct sivell, or rise, ex- 

 tending southward from Africa. As examples of trenches, or 

 Graben, may be mentioned the Marian trench with the Nero deep, 

 and the Japan trench with the Tuscarora deep. They are regarded 

 as submarine fault or rift valleys, just as are those upon the land 

 {e. g., Rhine Graben). The two cross-sections of the Marian trench 

 here reproduced show their general characteristics (Fig. 17). 

 Ridges are illustrated by the Wyville-Thomson ridge, between 

 North Britain and the Faroe Islands, and the Faroe Island ridge, 

 which separates the deep Arctic waters of the East Greenland Sea 

 from the North Atlantic. The Wyville-Thomson ridge has its 

 lowest point within 576 meters of the sea-level, and shows a num- 

 ber of notches of the wind-gap type, which suggest that it is a part 

 of an old land ridge. The floor of the Faroe-Shetland gully, which 

 terminates in this ridge, has a fairly uniform depression of 1,170 

 to 1,189 riieters. As an example of a plateau may be mentioned the 

 Blake Plateau, a nearly flat submerged tableland averaging 700 to 

 800 meters below sea-level, and bounded on the west by the steep 

 slope rising- to the Florida-Carolina shelf, and on the east by the 

 equally steep slope descending to the North American basin. From 

 this basin rises the small isolated Bermuda plateau, while the Pacific 

 has in its central part the equally steep-sided Hawaiian plateau, 

 both of which are well shown on Supan's map, above cited. The 

 Pourtales plateau, south of Florida, with a depth of 90 to 300 

 fathoms (165 to 549 meters), is an example of a small plateau bor- 

 dering the continent. 



II. Intracontinental Seas. 



The continental blocks are divided into continents by arms of 

 the sea penetrating deep into the land, or indented by shallow or 

 deep marginal seas more or less land-locked or enclosed by islands. 

 These are the intracontinental seas, among which two types can be 

 distinguished : the independent,* and dependent. Independent 

 seas are more or less distinct from the oceans, lying in depres- 

 sions which are independent of the main ocean basins, and sepa- 

 rated from them by a rim, which may be largely submerged or 

 may rise above sea-level, leaving only a slight superficial connection 



* Not in the sense of Kriimmel. See ante. 



