496 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



channels in the softer gneiss. (See pi. 3.) These rocks vary greatly 

 in the resistance which they ofi'er to erosion, and while depths of 15 

 and 20 feet were found at low stage in certain channels, at other 

 points not far distant depths of 60 and 70 feet were not uncommon, 

 and at one place a depth of as much as 130 feet was recorded. Again, 

 at the Atiri rapids, south of Semna, an intermixture of granite and 

 schists is the cause of the unequal erosion which has left the rocks and 

 islands of the harder material to obstruct the fairway of the river. 

 (See pi. 4.) At Semna, about 12 miles above the second cataract, 

 inscriptions on the rocks appear to show that since 2000 B. C. the river 

 has lowered its bed by about 27 feet, and recent excavations in Nubia 

 also indicate that, since the time when predynastic man inhabited the 

 valley, the river level has fallen, doubtless in consequence of the ero- 

 sion of the Aswan cataract, while the existence of terraces of water- 

 rolled detritus at several levels in the side valleys furnishes confirm- 

 atory evidence of this. 



Throughout this region a narrow valley in a rainless climate 

 offered little opportunity or encouragement for a population to de- 

 velop and thrive. In the southern portion sufficient rain falls in the 

 form of summer storms to enable the nomad Arab to find water and 

 forage in the valleys sufficient for himself and his flocks, but in the 

 northern part the country is too inhospitable for him to do more 

 than move through it between the fertile valley of Egypt and the 

 Sudan region of the monsoon rains, excej)t among the hills which 

 border the Red Sea, where water is more plentiful. Here and there 

 in the desert are wells and in the El Kab depression, to the west of 

 Dongola, water is found at a short distance from the surface. The 

 origin of this water is of much interest ; but as yet verified facts are 

 so few that we are left to choose between several more or less probable 

 hypotheses. It is, however, certain, from such measurements as have 

 been made, that there is a considerable loss of water between Khartoum 

 and Wadi Haifa in flood, which is greater than can be accounted for 

 by evaporation. Much of this water makes its way into the allu- 

 vial flood plain, and it seems more than probable that it also perco- 

 lates into the porous Nubian sandstone. In the rainless Nubian cli- 

 mate the underground water table must slope away from the river, 

 and this water must find its way into the lower layers. In default 

 of any accurate levels or borings in the desert, no more can be said at 

 present, but the observations on discharge made at the Aswan dam 

 should, when published, show what loss takes place in this manner 

 in the Wadi Haifa-Aswan reach. It has, moreover, been shown that 

 when the river level falls below that of the water table in the flood 

 plain, water drains back into the river at low stage, and Mr. Craig " 



« " The Nile flood and rains of the Nile Basin, 1906," Survey Dept., Carlo. 



