THE DESICCATION OF AFRICA. 151 



plains around them were moist with an irrigation brought 

 there from more favoured regions. 



On the north of the Ahaggar massif, which is of granite 

 (Mt. Hamane, 7,150 feet), the great Igharghar waddy cuts 

 first a deep canyon through the Tasili, or plateau, of the 

 Azdjer, consisting of Devonian and Carboniferous rocks, and 

 then becomes lost under the great Erg, or region of shifting 

 dunes. On the north of the Erg, the wady reappears and its 

 course is marked by wells and irrigation works, showing the 

 presence of abundant underground water. It seemed possible 

 at one time, that the Lake Chad might have discharged its 

 waters northwards and, through a rift in the Ahaggar high- 

 lands, have led down into the Igharghar, but the documents 

 of the Foureau-Lamy mission are so definite as to the great 

 escarpment facing south and barring all drainage from south 

 to north, that one had to abandon the idea. 



To come back to the Niger, the river, after leaving the 

 neighbourhood of Timbuktoo, enters the Burrum Gorge, 

 flowing in it due east for 250 miles in a trench scarcely 300 

 feet wide. Thence it follows along the junction of granite on 

 the south-west and soft Cretaceous and Eocene rocks on the 

 north-east, until it reaches the Falls of Bussa, where Mungo 

 Park perished. Here the granite stretches across the river 

 in a comparativeh' narrow bar and. forms, on the east, the 

 continuation of the coastal rampart, which rises near Bauchi 

 to 6,400 feet. The Niger in this region consists of two 

 branches, the Niger proper and the Kaduna River- Both 

 originally rose in the coastal rampart. The Kaduna has 

 eaten back by headstream erosion, but, its upper tributaries 

 draining a compact granite area and having only hard rock 

 to work on, has not progressed far into the interior; the 

 Niger, on the other hand, having only a narrow bar of hard 

 rock to eat back through, and, behind that, the soft rocks 

 of the Cretaceous and Eocene formations, has tapped the 

 whole of the drainage area behind the coastal rampart. The 

 course of the Niger between Timbuktoo and the Bussa Falls, 

 was once a river rising on the north of the coastal rampart 

 and flowing north-east. The region of dead water in the 

 Debo Swamp indicates where the two northwards-flowing 

 rivers originally met. 



The stages in the erosion of the rivers of the Gold Coast 

 are so complete in their varying degree of reaching the 

 coastal rampart, and the action is so similar, on a large 

 scale, to what is found going on in so many of the rivers of 

 Africa, like in the Kowie, that one is justified in drawing the 

 conclusion that the Niger once flowed into the Sahara and 

 made it a habitable region. The fact that has enabled the 

 Niger to work this act of spoliation is the sudden thinning 



