152 THE DESICCATION OF AFRICA. 



of the bar of hard rock at Bussa, which elsewhere guards the 

 drainage of the interior, and the presence, behind it, of soft 

 uncompacted strata. 



The future development of the Niger region is full of 

 disaster for North Africa. The Volta has completed its 

 spoliation; the Bandama is busy stealing the Bagoe and 

 Baoule tributaries; the vigorous headstreams in Sierra Leone 

 and Liberia are eating into the area of the upper Niger, and 

 some time in the future this will drain to the sea by the St. 

 Paul or the Moa River; the Senegal River is stealing into the 

 basin of the Tankisso River. Thus, the ultimate evolution 

 of the south-western Sahara will result in an absolutely 

 waterless desert. Not one of its present streams, whose 

 waters reach the sea by such a long route, can compete with 

 the boring headstreams of the coast rivers. The only thing 

 that would prevent the fulfilment of this would be a bodily 

 tilting of the whole continent, and such readjustments, due 

 to the lightening of the earth's crust by the mass of rock 

 weathered away and carried to the sea by the rivers, and 

 also by the absence of millions of tons of ground-water 

 that now permeate the soil, have taken place in the past, and 

 may do so again in the future- 



For the geology of this portion of Africa I have used the 

 work of P. Lemoine in Steinmann and Wilken's " Handbuch 

 der Regionalen Geologie," and for the. topography, geology 

 and archiology of the central Sahara the two volumes of the 

 "Mission Foureau-Lamy," Paris, 1905. 



THE BENUE. 



This great tributary of the Niger is, at its confluence, a 

 larger river than the mam stream; it comes fresh from the 

 Adamawa Highlands and the coastal rampart hills, whereas 

 the waters of the Niger have come a very long way, and have 

 visited the outskirts of the Sahara. The Benue, in the 

 original plan, began in the coastal rampart, or escarpment, 

 through which it has now cut back. The physiography 

 of the country immediately adjoining the stream is very 

 characteristic. Above Yola, where it occupies a river-bed in 

 which the waters formerly flowed towards Lake Chad, instead 

 of away from it, there is the same deep, narrow channel that 

 we saw in the Niger, below Timbuktoo, and which we shall 

 see again below Stanley Pool, on the Congo, and m the 

 Kebrabasa Falls, on the Zambesi. Below Yola, where head- 

 stream erosion has been in play, the coastal rampart appears 

 as two great ranges bounding the valley, at some places 

 approaching, at others retiring some distance from the river. 

 It is very instructive to compare this river gorge with that 



