494 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1908. 



developed, thus determining the direction of the various water 

 channels, which often follow the line of intrusive dikes. 



As the river has cut its way down through the overlying sandstone 

 of Cretaceous age, it has met with portions of the uneven floor of 

 crystalline rocks on which the sandstone was originally deposited, 

 and the position of these rock}' ridges has determined the position 

 of the cataracts; but the directions and positions of the water chan- 

 nels, and of the different rapids, are due to the structure of the rock 

 masses themselves as they now exist, after the crushing and disloca- 

 tion to which they have been subjected during the past history of the 

 continent. 



Upstream of each of these outcrops of harder rock, which form a 

 series of steps down which the Nile flows, there is a reach of low slope, 

 about 5 or 6 inches per mile, in which the river flows placidly through 

 the sandstone regions in the narrow alluvial plain which it has de- 

 posited during past centuries. These barriers of hard rock seemed 

 to promise suitable sites on which masonry dams might be erected 

 for the purpose of storing the additional water which was needed for 

 the irrigation of the cultivable lands in Egypt; but a survey of the 

 whole length of the river from Khartoum to Wadi Haifa, and the de- 

 tailed examination of the three reaches which offered most prospect 

 of storing the necessary quantity of water, showed that nowhere was 

 there a site offering the advantages of that at the Aswan cataract, 

 while in more than one of the sites examined the rocks had been so 

 crushed and fissured as to afford but an unsafe foundation for large 

 works. 



Fifty miles below Khartoum the Nile, which has been flowing 

 through a sandstone plain, enters the Shabluka gorge, which is 7^ 

 miles long and cuts directly through the hill mass of this name. This 

 is commonly called the " sixth cataract," but the total fall of the water 

 level at low stage is only 28 inches in this length, and it is the rush 

 of the whole river through a deep and narrow channel which causes 

 the troubled water and suggests a steep slope; the depthis very great, 

 being as much as 100 feet at one point in the middle of the gorge in 

 January, and 80 feet at another point near the downstream end. It 

 is very remarkable that the river should have taken its course through 

 this isolated hill mass of crystalline rocks instead of excavating a 

 channel through the softer gneisses which now form the low ground 

 around. This point demands detailed investigation, but there is evi- 

 dence that the gorge existed in some form at a very remote period; 

 that it was filled with sandstone in Cretaceous times, and that in the 

 modern erosion of the country the river has reoccupied the ancient 

 gorge by excavating the sandstone which filled it; further examina- 

 tion, however, is needed before this question can be considered as 

 finally settled. 



