Page:Africa by Élisée Reclus, Volume 2.djvu/89

 rUYSICAL FEATURES. 6» which has been followed by no subsequent western explorer. Twenty years later Lyon surveyed the chief trade route connecting Tripoli through Jofra with Murzuk, and determined a few astronomical jKniits, which were afterwards extended by the researches of Oudnoy, Denham, and Clapporton. The expedition of the year 18f50, associated with the names of Barth, Overweg, and Richardson, followed the direct highway across the Retl llamada wilderness. Then came the important explorations of Vogel, Duveyrier, Beurraann, Rohlfs, Von Bary, and Nachtigal, who have not only laid down the network of their own itineraries, but have also supplemented them with many others, on the authority of numerous Arab informers. Thus, to mention one instance, Rohlfs has published an account of the discovery of one of the Wan oases by Mohammed-el-Tarhoni, an Arab of Zella. In its general outlines, Fezzan presents the form of an amphitheatre gradually inclined towards the east, and on the other three sides encircled by plateaux. Its mean altitude is about l,OoO feet, the lowest levels of the oases nowhere probably falling below G50 feet. According to Barth, the deepest depression occurs at the Sharaba wells, east of Murzuk, where a lacustrine basin receives the drainage of an extensive area, and remains flooded for months together. Physical Features. The vast region enclosed b}' the escarpments of the plateau is itself a somewhat broken country, the general relief of which, as well as its mean elevation, shows that it has not certainly formed a marine basin during recent geological times, notwithstanding the theories lately advanced to the contrary by some eminent geographers, not only for Fezzan, but for the whole of the Sahara. Nevertheless in many places traces are visible of the former presence of salt water, and the submergence of the land at some very remote period is attested both by the undulating lines of shifting sands driving before the winds on the western plateau, and by the polished pebbles of diverse colours strewn like mosaics over the surface of the eastern serirs. The space encircled by the surrounding plateau consists in great part of secondary terraces, whose main axis runs in the direction from west to east, and which are separated from each other by crevasses with a mean depth of about 150 feet. These narrow, tortuous intermediate depressions take the name of ** wadies," like the beds of temporary watercourses in the northern parts of Trijx)litana, but as they are never flushed by any freshets, a more appropriate designation would be that of Lofra or " ditch" which in fact is applied to one of these depressions in the Murzuk district. Some are mere ravines of sand or hard clay, while others present the aspect of verdant glens shaded by overhanging palm-trees. Although not forming a fluvial system p»operly so called, they generally converge one towards another, without, however, always reaching the common bed towards the east of Fezzan. In this direction the unfinished channels are obstructed by sands and reefs. The southern slopes of the Jebel-es-Soda and of the Black Ilaruj present a very gradual incline. They are prolonged southwards by the spurs and terraces of