158 THE DESICCATION OF AFRICA. 



Nigeria ; this is continued into the Sahara, through the high- 

 lands of Air, or Asben, up to the Tasili, or Plateau, of Azdjer. 

 From this side only two affluents now reach the lake, the 

 Kamerduga Yobe, from the direction of Kano and the 

 Jedseram, to the south. The Benue thrusts itself boldly into 

 the south-western drainage area, and on the north-west the 

 Saharan highlands have now no moisture to contribute to 

 waters of the lake. There is, however, an oasis in the desert, 

 east of Air, called Agram, which owes its existence to under- 

 ground moisture. These oases, as one learns from the 

 Igharghar, are on the courses of dried-up streams, and the 

 underground rivers are relics of former streams that flowed 

 above ground. On the south, the Lagone affluent is being 

 filched by the Benue, through the Tuburi Swamps, and there 

 remains only the Shari River, with its many tributaries, as a 

 constant and settled contributor. 



On the east, there is the Bahr-el-Ghazal, which has 

 nothing to do with the Nilotic river of that name. The eastern 

 boundary of the Chad basin is formed by the Tibesti High- 

 lands and the Ennedi Hills; one would expect these to drain 

 towards the lake. One finds, however, that at the foot of the 

 Tibesti Highlands there are places, visited by Nachtigal, which 

 are 350 feet below the level of Lake Chad. This is evidence 

 that the waters of Lake Chad are held up by silt and sand 

 above the normal height, and explains the expansion of the 

 river into a lake; it also shows that this river system once 

 flowed from the lake towards the Tibesti Highlands in an 

 easterly direction. The whole system of wadys on the 

 western side of the Tibesti Highlands is collected by larger 

 wadys directed towards the south-east ; that is to say, when 

 there was sufficient water to fill these wadys, it did not flow 

 towards Lake Chad, but turned round southwards and made 

 for a gap in the Ennedi Hills. The inclination of the Bahr-el- 

 Ghazal is in the same sense eastwards. The Bodele and the 

 Djourab are districts lying under the Ennedi Hills, at the 

 eastern end of the Bahr-el-Ghazal, with a general level of some 

 200 to 300 feet below that of Lake Chad. According to the 

 geological map of this region, by Captain Arnaud, the depres- 

 sion of the Bahr-el-Ghazal lies within a semi-circle of high 

 granite hills, Tibesti on the north and a series of heights to 

 the south, about Wadai ; these connect with the granite of the 

 coastal rampart. The Shari and Logone Rivers pass through 

 a gap in the granite hills. Between the Tibesti massif and the 

 southern one, there is the gap of the Ennedi Hills, which are 

 composed of horizontal sandstones, such as Foureau has made 

 us familiar with in the Tasili, or Plateau, of Azdjer, which 

 lies to the north-west. Foureau found this latter breached by 

 a vast canyon, in which the Igharghar once flowed from the 



