Page:Africa by Élisée Reclus, Volume 2.djvu/521

 CLDCATE— FLORA— FAUNA. 427 whoso highest peaks attain an altitude of 2,320 feet, and, lastly, those of Tummo, standing at about the same elevation. The last-named, however, form rather a hani4da intcrsectetl by watercourses, than a group of mountains in the strict eenae of tlie term. Tummo, which merges in the southern plateau of Fezzan, constitutes a limestom- tableland, overlaid by a layer of blackish sandstone. It is furrowed in all directions, and cut up into separate blocks, which assume the appearance of towers. The tabular surface of the plateau, covered here and there with a layer of cby and shingle, is almost pt^rfectly level, with a slight general incline in the direction from north-east to south-west. Through the Biban, or " Gates," one of the depressions in the Tummo uplands, runs the most frequented trade route across the desert, leading from Murzuk in Fezzan to Kuka on the west shore of Lake Tsad. Caravans coming from the south usually sjx^nd several days in this delightful sjwt, where all find abundance of pure fresh water, springing in five streams from the foot of a sandstone cliff. The steep sides of the rocks are here covered with names and inscriptions, and round about the camping-grounds have been accumulated vast quantities of camel droppings, yielding an inexhaustible supply of fuel for the passing convoys. Towards the west the Tibesti hills fall gradually down to the plains. But in the south-west the surface is broken by sandstone heights of fantastic form, leading to the deep valleys of Borku, whose main axis runs panillel with that of the Tilx»8ti highlands. The lowest parts of these longitudinal depressions stand at an absolute elevation of scarcely 6G0 feet. They are separated by intervening white, red, or violet limestone rocks from the Bahr-el-Ghazal, or " Sea of Gazelles," till recently a vast lacustrine basin, but now dried up. Even some parts of Borku belong to this depression, which communicated through a narrow channel with Lake Tsad, forming between the river basins draining towards the sea, the Nile, Niger, and Congo, the true centre of the African continent. Sweet or brackish waters bubble up in the hollows of the Borku district; but some of the valleys have no springs visible on the surface, which is often covered with alum. The oases are also threatened, by some ranges of sandhills, formed by the disintegration of the sur- rounding sandstone rocks. Climate — Flora — Fauna. Most of these rocks are bare and arid, without scrub or mosses. But for the little rain that yearly falls the whole country would be absolutely uninhabitable. The clouds gather chiefly in the month of August : consequently Tibesti Ix'longsin respect of its climate to the Sudanese zone, the slight rainfall occurring in summer when the sun is at the zenith. Rains of a few hours' duration suffice completely to flood the narrow rocky valleys, where nothing is lost by infiltration. The torrent born of a shower rushes wildly down the slopes, in its impetuous course sweeping away the domestic animals, goats, sheep, at times even the camel. After it has passed, the valley again becomes dry, but the pure water is retained in the fissures of the rocks and in deep caverns, around which arc grouinxi the habitations of man