GEOGRAPHICAL ASPECTS OF THE NILE LYONS. 495 



The next cataract, the fifth, begins about 30 miles below Berber, 

 and consists of three or four separate groups of rapids. Together 

 with the intervening reaches of low slope, they occupy nearly 100 

 miles of valley, in which the river descends 82 feet; beyond this a 

 reach of low slope for 30 miles extends to Abu Hamed. This reach 

 upstream of Abu Hamed seemed at first to ofi'er many advantages for 

 a reservoir, but a detailed survey showed that the valley opened out 

 widely at the northern end until any economical storage of water in it 

 became impracticable. 



Here the Nile makes its great bend to the westward, flowing at times 

 even southward ; but the causes which determined this in the past have 

 yet to be found, and careful examination of this portion of the basin 

 is needed to lead to a solution of the problem. From Abu Hamed to 

 Merowe, a distance of 150 miles, the river flows principally over 

 crj^stalline rocks, and rapids occur frequently at short intervals. In 

 the first 17 miles the river falls 60 feet, and afterwards follows an 

 irregular course through these rocks for another 125 miles before 

 reaching the head of the fourth cataract at Shirri Island, 10 miles be- 

 low which is Merowe. Shirri Island was specially examined as pos- 

 sibly providing a site for a dam, but the valley between this point 

 and the cataract at Abu Hamed was not large enough to contain the 

 necessary volume of water. (See pi. 1.) 



A long reach then follows, in which, for IGO miles, sandstone rock 

 alone occurs on either side of the alluvial plain, and it is not until Abu 

 Fatma, 40 miles north of Dongola, that the third cataract commences. 



From this point to Wadi Haifa the rapids, which are collectively 

 grouped as the third and second cataracts, occupy a great part of the 

 river's course. In general character they have much in common with 

 each other and with those which have been already mentioned, but 

 each has peculiarities of its own, which, however, exceed the scope of 

 the present paper. The erosive action of the mass of silt-laden water 

 which pours down them in flood, and which amounts to nearly half a 

 cubic mile of water per diem when the river is in high flood, has for 

 many thousand years been wearing down the rocks of these rapids, 

 and carrying the material to the delta of Egypt and to the sea. The 

 character of these rapids may be well seen in various parts of the 

 third cataract, and among those which occur between it and the second 

 or Wadi Haifa cataract. At Hannek, in the third cataract (pi. 2), 

 bands of granite traversing the gneiss, which forms the fundamental 

 rock, give rise to obstructions to the river's flow. A point at the up- 

 per end of the Dal cataract seemed to offer certain advantages as a 

 dam site. Here the rock is a granite, which has been highly crushed 

 along certain lines, leaving uncrushed masses as kernels in an envelope 

 of gneiss; the result has been to form a number of small elongated or 

 rounded islands of granite, between which the water has eroded its 



