1 50 THE DESICCATION OF AFRICA. 



Sultan of Segu ; Park estimated his present of 5,000 as being 

 worth about twenty shillings. The cheapness and profusion 

 of these shells would lead to the improvident natives dropping 

 quantities of them, and hence their presence round the city 

 can be explained. 



The great salt desert of El Juf, with the salt mines at 

 Taodeni on the eastern margin, is the relic of a dried-up lake, 

 rather than that of a sea, and further, the levels of Timbuktoo 

 (787 feet) and Taodeni (722 feet) rule out this theory, unless 

 we bring in movements of the earth's crust, which can only 

 be purely speculative. To the north of El Juf there are the 

 Ergs, regions of shifting sand-dunes, which are, according to 

 Foureau, the broad basins of a quaternary river system. It 

 has now vanished, but it once led north into the great 

 Igharghar and thence to the sea by the shotts, or lagoons, of 

 Algeria. Just south of Taodeni, Lenz marks a depression 

 only 400-500 feet (120-150 m.) above sea-level, inviting the 

 waters of the Niger to flow into it and to form a great central 

 Saharan lake.* Further north, there are the oases of Twat, 

 Gourara and Ouargla in the course of such a supposed river. 

 Comparatively high ground lies between Gourara and 

 Ouargla, but the whole of this area is covered with enormous 

 thicknesses of recent debris, washed down from the Atlas 

 Mountains; this would obscure any pre-existing river valley 

 and would also have aided in the obstruction and final 

 diversion of the stream. 



There is another possible outlet for this river system, 

 namely, through the Adrar Highlands, bordering the west 

 coast. At Cape Blanco there is a great estuary, between four 

 and five miles broad, now filled in with light, friable, 

 sandstones, that could have been the mouth of this river. 

 This, however, would not have given the requisite moisture 

 to the Ahaggar Mountains and allowed them to pour plentiful 

 rivulets out into the present deserts, which, joined together, 

 scoured the great river beds we find m these regions; the 

 larger ones, such as the Tanegrouf, In Azoua and Tafassasset, 

 are now, indeed, dry, but at one time they were filled with 

 water and communicated with the sea. In the heart of the 

 Ahaggar, in the pools, there still live the river fish, Barbus 

 deserti and B. biscarensis, which must have originally swum 

 up a river that flowed far from their present haunts. These 

 central Saharan highlands are so hemmed in on all sides by 

 vast plains, separating them from the coastal regions, that no 

 climatic changes would suffice to bring rain-laden clouds in 

 such quantities as to allow of their giving rise to great rivers 

 that could flow the 1,200-1,500 miles to the coast, unless the 



* See Lieut. Cortier's " Route March from Timbucktoo to 

 Taodini." La Geographie, 1906, pp. 317-341. 



