Within this border, or fringe, of maritime heights thus traced, we may consider all southern Africa as a vast plateau having a general elevation of about three thousand feet above the sea.
The most prominent interior ranges which rise from this section of the plateau are those called the Mushinga Mountains. These "seem to have an east and west direction, separating the wide basins of the Congo and Zambesi Rivers, and the mountains to the westward of Lakes Albert and Victoria Nyanza and Tanganyika." The latter "form the western edge of the great plateau of eastern equatorial Africa, the center of which is occupied by Unyamwezi, and slope down towards the broad vale of the central Congo."
If we look at the map, "Northern Africa, between the higher southern plateau and the mountains of Barbary on the Mediterranean coast, appears to be generally lower, or at an average elevation of from one thousand to fifteen hundred feet above the sea, though the plateau formation remains the same.
"The prominent lines of height known within it are those which extend from the Marrah Mountains of Dafur, between the Nile basin and that of Lake Tchad, northwestward through the mountain land of Tibesti, in the center of North Africa, to the series of plateaus occupied by the Tuareg tribes south of the plateau of Barbary.
CROSSING THE DESERT.
"A remarkable volcanic belt is traced through the Bight of Biafra in the line of the islands of Annobon, St. Thomas, Princes, and Fernando Po, ten thousand one hundred and ninety feet high, to the high Cameroon Mountains, thirteen thousand seven hundred and sixty feet high, on the coast of the mainland, and thence inland on the same abrupt line to Mounts Alantika and Mendif, midway to Lake Tchad in the interior."
If we could follow the course taken by the early settlers and the missionaries in Africa, we should, in going from the southern coast northward, have to cross three mountain systems and two table lands, before we could reach the great central plateau.
The mountains would present "steep, wall-like faces on the coast side, as if they were raised to prevent all access to the interior; on the other side they descend gradually to the broad, elevated tracts, which retain the name karroos, barren plains, given them by the natives on account of their general appearance and character."
In such a section as Cape Colony, we might expect that the karroos, enclosed by mountains, would become converted into lakes, did we not remember that little rain falls there, and then for only a brief season. "The karroos appear to have been lakes at some very remote period in the history of the earth's surface; for much of the soil is impregnated with salt; shallow salt lakes or marshes are numerous, and beds of sea shells are to be found here and there."