SAHARA (Arabic Sah'ra), the vast desert region of North Africa, stretching from the Atlantic to the Nile, and from the S. confines of Morocco, Algeria, Tunis, and Tripoli S. to the vicinity of the Niger and Lake Tchad. It is usual to regard the Libyan Desert, lying between Egypt, the Central Sudan, and Tripoli, as a sep arate division. Both are, however, links in the chain of great deserts that girdle the Old World from the Atlantic coast across Africa, Arabia, Persia, Turkestan, and Mongolia to the Pacific. It was long customary to assert that the Sahara was the bed of an ancient inland sea, and that it consisted of a vast, uniform ex panse of sand, swept up here and there into ridges by the wind. But this idea is utterly erroneous. Since the French became masters of Algeria, they have completely revolutionized our knowledge of the Sahara, at all events of the coun try immediately to the S. of Algeria and Tunis. The surface, instead of being uni form and depressed below sea-level, is highly diversified, and attains in one place an altitude of fully 8,000 feet.
From the neighborhood of Cape Blanco in the W. a vast bow or semicircle of sand-dunes stretches right round the N. side of the Sahara to Fezzan, skirting the Atlas Mountains and the mountains of Algeria. This long belt of sand hills varies in width from 50 to 300 miles, and is known by the names Igidi and Erg, both meaning "sand hills." The hills rise to 300 feet (in one place, it is said, to more than 1,000 feet), though the aver age elevation is about 70 feet. Water is nearly always to be found below the sur face in the hollows between the different chains of these sand hills, and there a few dry plants struggle to maintain a miserable existence. S. of Algeria, on the other side of the Erg, the country rises into the lofty plateau of Ahaggar (4,000 feet), which fills all the middle parts of the Sahara. Its surface runs up into veritable mountains 6,500 feet high, which, incredible as it may seem, are covered with snow for three months in the year. On the S. it apparently falls again toward the basins of the Niger and Lake Tehad; nevertheless there are moun tain ranges along the E. side reaching 8,000 feet in Mount Tusidde in the Tibbu country, and a mountain knot in the oasis of Air (or Asben) which reaches up to 6,500 feet. Mountainous tracts occur
also in the W., between Morocco and Tim buctoo, but of inferior elevation (2,000 feet). These mountainous parts embrace many deep valleys, most of them seamed with the dry beds of ancient rivers, as the Igharghar and the Mya, both going some hundreds of miles N. toward the "shotts" of Algeria and Tunis. These valleys always yield an abundance of wa ter, if not on the surface in the water courses, then a short distance below it, and are mostly inhabited, and grazed by the cattle and sheep and camels of the natives.
Another characteristic type of Saharan landscape is a low plateau strewn with rough blocks of granite and other rocks, and perfectly barren. In very many parts of the Sahara, especially in the valleys of the mountainous parts, in the recesses or bays at the foot of the hills, alongside the watercourses, and in the hollows of the sand-dunes, in all which localities water is wont to exist, there are oases— habitable, cultivable spots, islands of ver dure in the midst of the ocean of desert. These oases occur in greatest number along the S. face of the Atlas and the Algerian mountains, on the N. side of the Ahaggar plateau. These lines of oases mark the great caravan routes between the Central Sudan States and the Medi terranean.
A large portion of the Sahara, though not the whole, was undoubtedly under water at one time, probably in the Cre taceous period and earlier. Then the surface seems to have been in great part elevated, so that the waters remained only in some lakes and in gulfs near the Mediterranean coast. The Romans had colonies or military posts a long way S., in what are now desert regions; and both Herodotus and Pliny tell us that the ele phant, the rhinoceros, and the crocodile, all animals that only live near abundant supplies of water, were common through out North Africa in their day. None of the Egyptian inscriptions or animal sculp tures represent the camel, nor do the Greek and Roman historians mention it either as being a denizen of North Africa. The camel is now the principal carrier across the Sahara, and must have been introduced since the beginning of the Christian era. The inference from these and other facts is that the process of desiccation has gone on more rapidly dur ing the last 2,000 years.