GEOGRAPHICAL ASPECTS OF THE NILE LYONS. 493 



some parts considerable crops are raised in the rainy season ; but tlie 

 rainfall rapidly diminishes as we move northward, and at Khartoum 

 it only amounts to about 4 or 5 inches annually, which falls in the 

 four months June to September, the remainder of the year being hot 

 and dry. We have now left the thorn forest and the savannah type 

 of country in which the gum acacia predominates, for north of 

 latitude 16° even these become rare, and we enter the rainless desert 

 of northern Africa. 



At Khartoum very different conditions are encountered, not only 

 does the rainless region begin immediately north of it, but the heavy 

 rainfall of Abyssinia furnishes the flood of the Blue Nile which both 

 waters the lower reaches of the river and carries down to them the 

 red-brown silt which forms the flood plains of Egypt. Tlie great 

 volume of the Blue Nile flood, as compared with that of the IVliite 

 Nile, has already been alluded to, as well as its occurrence during a 

 short season of four months in the summer.** After leaving the hills 

 of Abyssinia the Blue Nile flows in the channel which it has eroded 

 in the alluvial deposits which overlie the crystalline rocks of the 

 region, and the banks are of sufficient height to prevent the flooding 

 of the lands which border it. The slope of the river at low stage is 

 about 1 in 3,000 between Fazogli and Roseires, but decreases to 1 

 in 8,000 below the latter place. It is highly probable that on the 

 whole the river is slowly eroding its bed as the cataracts below Ber- 

 ber are being worn away, but the change of slope between Singa and 

 Sennar is very remarkable. Here the slope is only 1 in 10,900, while 

 upstream of this reach it is 1 in 8,300 and below it 1 in 9,100. Since, 

 so far as is known, there is no ridge of hard rock at this point to 

 account for the change of slope, a gradual warping of this part of 

 the country may be the cause, and the extreme meandering of the par- 

 allel streams, the Rahad and the Dinder, in the same region gives 

 some support to the hypothesis. 



Below Khartoum the river enters the region of the cataracts, which 

 are generally described as being six in number; but this is not strictly 

 accurate, for one portion, which is but a deep and narrow gorge, is 

 included as the sixth cataract, while, on the other hand, the impor- 

 tant series of rapids which occur immediately downstream of Abu 

 Hamed are ignored. At no point is there any considerable vertical 

 fall, but each so-called " cataract " consists of one or more series of 

 rapids, in which the water slope is from about 1 in 2,000 to 1 in 800. 

 In every case the rock channel is formed of granite, gneiss, or crys- 

 talline schists, which have usually been greatly crushed by earth 

 movements, in consequence of which lines of weakness have been 



"Cf. Geographical Journal. September, 1906; and "Physiography of the Nile 

 Basin," Chap. VI. Cairo, lUOG. 



