6 NORTH-EAST AFRICA. 



rapids, whicli cut off from outward intercourse populous regions whose fluvial 

 systems ramify over many hundred millions of acres. The Nile and Congo rising 

 amid the higher plateaux, where the slope is still undecided, traverse in their upper 

 courses many great lakes, which according to a vague tradition once constituted a 

 single lacustrine basin of enormous extent. In the sixteenth century the Portuguese 

 explorers had some idea of this hydrographie system. But in tracing the outlines 

 of the great equatorial lakes they seem to have rather copied older maps than relied 

 on positive information. But, however this be, they appear to have believed in the 

 existence of a single source for the Nile, Congo, and even the Zambesi. But the 

 streams were also supposed to traverse extensive underground regions, and an 

 Italian map engraved in the middle of the fifteenth century represents a Nile with 

 three heads, separated by a vast space from the emissaries of the chief fountain. 

 This Nile is moreover made to flow in the direction from north to south, a small 

 Egyptian delta corresponding to a much larger delta in South Africa. 



The first modern explorers of the same region were also influenced by these 

 traditional ideas. Even Speke traced the course of four rivers issuing from various 

 parts of Lake Nj^inza to form the Nile, while Stanley made Tanganyka the source 

 of two effluents, one flowing northwards to the Nile, the other westwards to the 

 Conffo. But althousrh these great arteries do not rise in a common source, the 

 water-parting between them is in some places so low and imdecided that a slight 

 disturbance of the surface would suffice to change the direction of many affluents. 

 It is even possible that on the dividing line of some basins there may exist lakes or 

 swamps draining in both directions. 



The unfinished aspect of the central rivers, the cataracts interrupting their 

 course, the lacustrine reservoirs scattered over the plateaux, produce a certain resem- 

 blance between equatorial Africa and the Scandinavian peninsula. But in the 

 northern region, still under ice within a comparatively recent geological epoch, the 

 rivers have scarcely commenced their work of erosion. The climatic conditions are 

 of course entirely different, and although the existence of an old glacial period may 

 be suspected even in the torrid zone, the long ages that have elapsed since that 

 remote epoch must have effaced nearly all trace of glaciers and moraines. Hence 

 the rudimentary character of these fluvial basins is probably due to a different cause. 

 The climate, which was formerly much more humid in the Sahara, may possibly 

 have been correspondingly drier in the south-eastern region of the Nyanza plateau. 

 In the absence of a copious rainfall the rocks would remain uneroded, and the now 

 flooded cavities unfilled by the alluvia of running waters. During its long geolo- 

 gical life the earth has witnessed many shif tings of the climatic zones. If the rains 

 are more abundant in some places than formerly, in others they are more rare, and 

 the Tgharghar basin, for instance, in North-west Africa, belongs to one of these 

 dried-up regions. 



East of the Nile and of the great lakes there is no space between the plateaux 

 and the coast for the development of large streams. From the Egyptian uplands 

 the Red Sea receives nothing but intermittent wadies, and along a seaboard of about 

 2,400 miles southwards to Mozambique the Indian Ocean is fed only by such 



