EIVERS. 5 



tableland, intersected by mountain ranges resting on foundations of from 3,000 to 

 4,500 feet. The Congo and Nile basins confine on tbe north and west this region of 

 plateaux, which comprises about a third of the whole continent. On the other 

 sides the border ranges are considerably less elevated and much more divided than 

 those of South and East Africa. They are nowhere continuous, but rise in scattered 

 fragments between the Congo and Niger, between the Nile and Lake Tsad, in the heart 

 of the Sahara, which is broken by the two isolated masses of Tibesti and Ahaggar, in the 

 extreme west, where the scarps of the plateaux run parallel with the coasts of Upper 

 Guinea and Senegambia ; lastly in Mauritania, where the Atlas range constitutes a 

 distinct orographic system, formerly connected with those of South Europe. South 



Fig. 2. — Hydrography of Africa according to Medieval Geographers. 



of this system the continent may be roughly described as a vast plane inclined in a 

 north-westerly direction. 



Rivers. 



The rudimentary character of its general relief is also reflected in its hydrographie 

 system. The African rivers, still to a great part entangled in the intricacies of the 

 plateau, have a somewhat irregular and unfinished course, often forcing their way 

 through narrow rocky gorges, and obstructed by numerous falls and rapids. Even 

 the more copious streams are relatively less accessible to navigation than those of 

 the other continents. In this respect the contrast is specially striking between 

 Africa and South America, the two divisions of the globe which are more frequently 

 compared with each other. The *' Dark Continent " is entirely destitute of the 

 great estuaries and broad arteries giving access in the New World from the Atlantic 

 seaboard almost to the foot of the Andes. The comparative absence of navigable 

 waters, of islands and good harbours, combined with the great extent of desert 

 wastes, has mainly contributed to exclude Africa from the general life of the com- 

 mercial world. 



All the great rivers — Nile, Congo, and Niger — are interrupted by cataracts and 



