South of the Saharan Atlas lies the desert, comprising about 2,000,000 square miles, the topography of which is extremely varied. For present purposes it is sufficient to say that in the extreme southern portion of the Algerian Sahara, and crossed by the Tropic, there is an extensive highland, the plateau of Idghagh, where an elevation exceeding 5,000 feet is reached. All of the Sahara to the west of this plateau, or to the west of a line drawn north from it, appears to be above sea-level (much of it having an altitude of 1,000 feet), and of greater geological age than that portion of the desert lying to the east. North from the plateau of Idghagh the country gradually descends to the depression of which the great Chott Melghir is a part, a region below the level of the sea. Here extends also one of the longest oueds of the Sahara, the Oued Igharghar, which takes its rise in the plateau of Idghagh and empties in the Chott Melghir, an entire length exceeding 700 miles. In the western part of the Sahara the surface descends to the Atlantic, but in the eastern part it falls away to the Mediterranean. Turning now to consider the part of southern Algeria with which this study especially deals—lying between Laghouat and Ghardaia, between Ghardaia and Ouargla, and between the latter place and Biskra, all to the south of the Saharan Atlas—we find topographical details which are probably representative of much of the rest of the Great Desert. Laghouat has an elevation of 2,400 feet. It lies on the northern edge of the region of the dayas. This region is characterized by the poor development of its drainage and has a slightly undulating surface with frequent depressions, each the center of an area of rather small extent, from which it receives flood-waters. The dayas differ from the other undrained areas, the chotts, in that they do not contain an excess of salt, owing probably to efficient subdrainage. In the region of the dayas the surface falls away to the south or the southeast until the region of the Chebka is reached, which extends to the territory of the Beni M’Zab. In the Chebka low and flat-topped mountains are so irregularly disposed as to give rise to the name, meaning a net; they are separated by valleys, narrow toward the northern portion of the region, but expanding into small plains as one proceeds towards Ghardaia. Ghardaia, the country of the Beni M’Zab, marks the southern limit of the Chebka. At Ghardaia the altitude is 1,600 feet. Between Ghardaia and Ouargla are undulating stony plains, the Gantara (_hamada_), large salt spots, the chotts, and a prominent range of sand mountains, _areg_ desert, possibly 1,000 feet high. At the eastern edge of the Gantara the general level of the country drops suddenly about 200 feet to the Ouargla plain (_reg_ desert), with an altitude less than 500 feet; this is an eroded flood-plain of the Oued Igharghar or its tributaries.