28 PHYSIOGRAPHIC GEOLOGY. 



also a mountain-border, and higher than the western. By these 

 border-ranges the interior of Africa is mostly shut off from the sea^ 

 — it is a shut-up continent, as Guyot calls it. The loftiest mountains 

 are in Abyssinia and Zanguebar, facing the Indian Ocean. Abys- 

 sinia is, to a great extent, an elevated plateau, 6000 to 7000 feet in 

 height, with ridges reaching to 11,000 and 13,500 feet ; and farther 

 south, in 3° 40', stands the snowy Kilimanjaro, which, according 

 to report, is 20,000 feet high, and probably the source of the Nile. 

 The interior of the northern or east-and-west half consists of (1) 

 the Great Sahara region, a plateau of about 1500 feet elevation, 

 with its undulations and ridges ; (2) an east-and-west depression 

 on the north, between Sahara and the border-mountains, nearly to 

 the ocean's level in some parts, and being the region of the oases ; 

 (3) a partial east-and-west depression about the parallels 10° to 

 15° N., separating the Sahara plateau from the southern, and con- 

 taining Lake Tchad, at an elevation of 800 feet. The interior of 

 the southern half is a plateau 2000 to 2500 feet in average height : 

 the great lake Uniamesi, south of the equator, between the meri- 

 dians 25° and 35°, is stated by Livingstone to have its surface 2000 

 feet above the sea. 



Fig. 21. 



The sections figs. 21 and 22 give a general idea of these features 

 Fig. 21 is a section from south to north (the heights necessarily 

 much exaggerated in proportion to the length); a, the southern 

 mountains ; b, the southern plateau ; c, Lake Tchad depression ; 

 d, Sahara plateau ; e, oases depression ; /, mountains on the Medi- 

 terranean, of which there are two or three parallel ranges. Fig. 22 



Tie. 22. 



W E 



represents the surface-outline from west to east through the 

 southern half of the continent. In all these sections all minor 

 details are omitted, in order to bring out clearly the system, or 

 continental model. 



Africa has, therefore, the basin-form, but is a double basin ; and 

 its highest mountains are on the side of the largest ocean, the 

 Indian. The height of the mountains adjoining the Mediter- 

 ranean is the only exception to the relation to the oceans ; and this 

 is small. Moreover, the position of the head of the continent 



