The region of arid waste lands called the Sahara lies between the Sudan on the south and the Atlas Mountains and the Egyptian coast on the north. It is a part of an arid belt extending eastward to Baluchistan, the entire area meas uring about 4,000,000 square miles. Of this area at least two-thirds lies west of Suez, and is known in general as the Sahara. It is all an elevated plain, into which many valleys have been eroded by the ancient drainage systems which are now the only marked topographical features of the region. The whole area may, therefore, be divided into certain regions, limited by natural features. First, the so-called Ara bian or Nubian Desert; the area between the Nile, the only living river, that crosses the arid zone, and the Red Sea. This is marked in its southern portion by the continuation of the vol canic; uplands of Abyssinia, which lessen in height toward the north, but border the Red Sea in a line of jagged mountains, many of which exceed 4000 feet, and one, Soturba, reaches 6900 feet. In the south is the great rift of the Wady Mahal', probably an ancient Nile channel ; and in Lower Egypt are tlie rifts occupied by the Khar geh, Dak and others, forming a line of notable oases. \Vest of the Nile rises the desolate pla teau of the Libyan Desert, which covers the whole region from central Darfur to the Mediter ranean (long. 1S° to 30° E.), excepting the few oases above mentioned. Its general altitude varies from about 1500 feet in the south to 500 on the Mediterranean, where it breaks down in hills. A line of elevations extending northwest ward from the Marra Mountains in Darfur to the Algerian Atlas forms a sort of -boundary to the Libyan Desert, and makes possible the thinly inhabited oasis regions of Tibesti and Murzuk. Further west there are wadies, or dried-up river valleys, of which one, with numerous branches, is traceable from the Tropic of Cancer north to the "shotts" or swampy lakes which occupy the ]urge, low plain (in places below sea-level) west of the Gulf of Cabes. It is believed that within
2500 years this valley was occupied by a flowing river, but now only a few pools and springs exist through the dry season. West of this more broken region between Algeria and Lake Chad there stretches an enormous space of waterless waste land, with shifting sand dunes, broken by lines of rugged and naked elevations having a general northeast and southwest direction. This waste extends to the Atlantic coast all the way from about lat. 1S° to 28° N., that is, from the hills of Senegal to the western extremity of the Atlas. The elevation of the Sahara throughout the greater part of its extent exceeds 1000 feet, diminishing gradually from the south toward the north in the Libyan Desert. and from its cen tre in the western half of the desert toward the Lake Chad Basin and the Niger, and toward the coast of Tunis and Tripoli. Only very small and irregular areas along the northern border are below the level of the Mediterranean.
The elevated district called the Atlas Region, with its littoral margin along the Atlantic Ocean and Mediterranean Sea, is a part of the great Alpine system of Europe, to which it is linked by the mountains of Spain and the Pyrenees. Unlike other African mountains, the Atlas have a folded structure and an Alpine character, and present many parallel zones. These ranges ex tend in a nearly straight line from Cape Nun, on the Atlantic, northeast to the headlands of Tunis, where they are broken through by the narrows of the Mediterranean. Along the Medi terranean coast the elevations are volcanic, and descend very abruptly. Toward the interior, irregular ranges form a long line of heights of Paleozoic rocks, which is sometimes called the Tell Atlas; but this is more prominent in Algeria than in Morocco, where the seaward side is a rough plateau. The Atlas stretches over a dis tance of about 1400 miles, and attains its great est elevation in the western portion, where it rises to a height of nearly 15,000 feet.