Page:Africa by Élisée Reclus, Volume 2.djvu/265

 RIVEB8 OF ALGERIA. S16 120 miles in the central port of the plateau. It is divided by the Kheider isthmus into two basins, of whirh the western has an extreme breadth of 15 miles. East of the Shelif the plateau region presents nothing but small basins, such as the Dhaya Dakhla, north of the Ukait range, and south of that range the eastern and western Zahrez, which according to one estimate contain some six hundred million tons of suit. North-east of Bu-S&da stretches the extensive Shott-el-Ilodna, which at a former geological epoch was certainly an Alpine lake. Farther east are some smaller scbkhas, the most important of which is the Tarf, whose waters attain the highest possible degree of saturation, or twenty-seven per cent. Most of the streams flowing from the southern border chains towards the Sahara are absorbed by irrigation works soon after leaving the mountain gorges. Some, however, flow from oasis to oasis for a long distance from the hills. In the west these wadies take a southerly course ; but near the Tunisian frontier the vast basin of the now-dried-up Igharghar is inclined in the opposite direction towards the Shott Mclghigh depression. Lofty uplands lying in the Sahara far to the south of Algeria give to the whole of the intervening region a northerly tilt, and this is a point of primary imix)rtance in the physical geography of the desert. While the running waters formerly flowed in the east, either towards an " inland sea," or towards the Gulf of Cabes, they drained in the west in a southerly direction either to the Niger, or even directly to the Atlantic by trending round to the west. Although the problem is not j-et solved, the reports of recent explorers render the former hyix)thesi8 the most probable. "Within the present limits of Algeria, all the other streams rising on the escarpments of the plateau run dry in the sandy dunes which lie some 60 miles forther south. Such are the Wed Nemus, which rises in the neighbourhood of Tiut ; the "Wcd-el-Gharbi ; the "NVed-cs-Segguer, flowing from Brezina, south of Geryville ; the Wed Zergoun, fed by the torrents of the Jebel Amur ; the Wed Lua, skirting the east side of the Mzab plateau. The other streams of this region flow to the Wed Mzi, the chief branch of the Wed Jeddi, which forms a geological limit between the cretaceous plateaux and the sands of the Quaternary plains. After a course of about 300 miles, the Jeddi merges in the vast depression of the Shott Melghigh. Like other rivers of the Sahara flowing over rocky beds, it is subject to sudden and formidable freshets, the dry channel at the confluence of the Wed Biskra being sometimes flooded to an extent of 6 or 7 miles from bank to bank in a few hours. The Wed Msif, also in the Ilodna district, suddenly assumes the proportions of a river nearly 2 miles wide, sweeping away escarpments and whole flocks of sheep in its impetuous course. Other wadies coming from the gorges in the Aures and Shcshar mountains, or rising in the desert itself at the foot of the rocky escarpments, converge towards the depression of the shotts, without always reaching it. By far the largest of these dried-up watercourses is the Igharghar, which has its farthest headstreams in the Jebel Ahaggar, and which develops a vast chonnel 1 to 6 miles wide, and large enough to contain the waters of a Nile or a Mississippi. In some places it is completely obliterated and choked with shifting dunes to such an extent that the