tableland, intersected by mountain ranges resting on foundations of from 3,000 to 4,500 feet. The Congo and Nile basins confine on the 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 or Africa according to Medieval Geographers.
of this system the continent may be roughly described as a vast plane inclined in a north-westerly direction.
The rudimentary character of its general relief is also reflected in its hydrographic 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 commercial world.
All the great rivers—Nile, Congo, and Niger—are interrupted by cataracts and