THE DESICCATION OF AFRICA. 165 



or when two rivers meet in such an unnatural manner, that 

 changes have taken place recently, and that the river has 

 not yet adapted itself to the new conditions. Here, however, 

 we come up against the Rift-system of East Africa, which 

 is a region of block uplift as well as of trough faults, and the 

 whole original plan of drainage is obscured. One is tempted 

 to suggest that the original river-bed extended from the Marra 

 Hills as the Bahr-el-Arab, then down the Bahr-el-Ghazal, down 

 the Sobat and out to sea by the Juba River; that the 

 mountains of block uplift rose across this and turned the 

 Sobat from a south-easterly flowing river to a north-westerly 

 one. Given sufficient time and no further shifting of the 

 earth's crust, the Juba River will eat back through the high- 

 lands of Abyssinia, tap the Sobat and actually cause such 

 a river to come into existence. 



At the time when such a condition of things obtained, 

 the Rift Valleys of the Red Sea and Gulf of Aden had not 

 yet been formed, and Arabia was joined on to the continent 

 of Africa. Africa then was a symmetrical block, with a 

 central extension north and south, together with a northern 

 cross-piece, the West African shoulder being balanced by the 

 Arabian shoulder on the east. The central portion was drained 

 and fertilised by the Proto-Congo, flowing from the Congo- 

 Zambesi divide to the Great Syrtis Gulf. On the west the 

 Niger and the Igharghar drained the country from south to 

 north and on the east; the Arabian shoulder was drained by 

 the twin Niles. The whole of the continent was then as well 

 watered as the North American Continent. With the Rift 

 system we cannot deal here, but for the rest, the drying up 

 of North Africa is due to the capture and diversion of its 

 natural river system whereby the waters which once tra- 

 versed the continent are now hurried in short, precipitous 

 courses to the sea- 



SOUTH AFRICA. 



In South Africa we have the central depression of the 

 Kalahari, surrounded on all sides by high ground. In the 

 centre lies Lake Ngami, corresponding, in many respects, 

 with Lake Chad. East of Lake Ngami there is another great 

 depression, occupied by the Soa Salt Pan and the Ntwetwe 

 Pan, and constituting the Makarikari depression. The Soa 

 Pan. covers an area of about 2,000 square miles and is 56 

 miles from north to south; the Ntwetwe Pan is 86 miles long, 

 by 5-8 miles broad. If the whole of the Makarikari depres- 

 sion, which lies some 150 feet below the level of Lake Ngami, 

 were to be filled by the overflow from that lake through the 

 Botletle River, a lake of 15,000 square miles in area would 



