The Central Sahara with these three groups of mountains differs materially from the Eastern Sahara. Although our data for the latter are more limited by lack of knowledge, the structure of the surface immediately west of the Nile Valley appears characteristically to be a series of closed basins. The area is covered with depressions into which insignificant channels flow, and from which there appear to be no outlets. Compared with the river systems of the west, the stream beds are small and ill-defined. One valley of some magnitude, the Bahr Bela Ma which Rohlfs tried to find on his famous journeys in the Libyan desert, has been identified either as a dry channel of the Nile running roughly parallel to it, or alternatively as a valley which starts from N.E. Tibesti and terminates near or in the Wadi Natrun depression just west of the Nile and level with the apex of the Delta. The upper part which drains Tibesti has been called the W. Fardi; elsewhere it is the W. Fareg; the shallow depression crossed by Hassanein Bey on his journey from Jalo to Kufra seems to be part of this system. Examples of closed basins separated from one another by steppe or desert are the oases of Kufra, the Jaghbub-Siwa, Jalo and Lake Chad depressions. In these areas cultivation is frequently intense; salt and fresh water are abundant; and the vegetation sometimes develops luxuriantly into veritable forests of date palms such as exist at Kufra. Between these hollows the intervening Libyan desert is probably the largest and most sterile area of its sort in the world. The Western Sahara, on the other hand, is essentially an area of well-defined river systems with watersheds and dry beds fashioned on a vast scale. The valleys which extend from the mountains of Ahaggar and the Fezzan to the present River Niger have corresponding channels on the other side of the water-parting running through Southern Algeria or Tunisia towards the Mediterranean. There are good reasons for believing that the original course of the Niger terminated in a swamp or marsh north of Timbuctoo, probably the same collecting basin as that west of Ahaggar into which certain rivers from the Atlas also used to flow. The lower Niger from the eastern side of the great bend where the river now turns south-east and south drained the Central Sahara by a great channel which had its head-waters in Ahaggar and the Fezzan, and ran west of Air.