74 DE. J. BALL OW THE [Feb. I903,. 



of 7*9 metres in 4200 years, or nearly 2 millimetres per year, to 

 account for. Such erosion corresponds to .. ' — or 200 cubic 



metres of rock, weighing approximately 500 tons, per year. 



The amount of rock removed by simple solution in the stream 

 during 4200 years must in itself be considerable ; but it is im- 

 possible of accurate estimation, and is probably so insignificant in 

 comparison with that removed by mechanical action, that we may 

 neglect it in the following consideration. 



The yearly discharge of the Nile past Semna is very nearly 

 100,000 million tons of water, at a mean velocity of 4| kilometres 

 per hour at high Nile and 2| kilometres per hour at low Nile ; and 

 it should be remembered that these velocities, which in themselves 

 are capable of accounting for the sweeping along of large pebbles, 

 are locally so much increased by the numerous rocky obstructions 

 as to permit of the movement of boulders and rock-fragments bigger 

 than a mans head. Further, quite apart from the pebbles and 

 boulders which are swept over the barrier and dropped in the deeper 

 water beyond, the quantity of fine silt or rock-flour carried by the 

 stream is certainly not less than 60 million tons per year. 1 And, 

 finally, one must not lose sight of the circumstance, that the strike 

 of the foliation-planes of the gneiss is parallel to the direction of the 

 current, and thus most favourable to the creation of channels across 

 the barrier. 



When the foregoing facts are borne in mind, the removal of 

 500 tons of rock per year under the existing conditions is not only 

 not impossible, but highly probable. Thus such an amount of 



, . .. i * • i 500x1000,000 - .„. 



erosion corresponds to the removal of only i qq qou 000 000 0r mllll ~ 



grammes of rock, per ton of silt-laden water. Or, looking at the 

 matter another way, the potholing action certainly accounts for at 

 least two-thirds of the action, leaving, let us say, 170 tons to be 

 accounted for by the action of the fine silt : this only corresponds to 



aiinnnnkn or about 3 grammes of rock per ton of fine silt swept 

 00,000,000 & i i- 



over the barrier'. 



The rate of erosion demanded by this simple explanation of the 



facts observed is, moreover, not greater than that which would be 



suggested by a comparison of measurements of river-erosion made in 



1 The average flow past Assuan is given by Sir W. Willcocks (' Egyptian 

 Irrigation ') as 2990 cubic metres per second, or 94.292 million tons per year. 

 At Semna it will, of course, be in excess of* this, as some water is taken up by 

 evaporation and irrigation between Semna and Assuan. 



The most reliable data as to the amount of silt carried in suspension by the 

 Nile are those obtained by Dr. Mackenzie, Principal of the School of Agriculture 

 at Giza, and published in the Public Works Administration Eeports for 1896 

 (p. 43) & 1898 (p. 101). Prom the mean of monthly determinations extend- 

 ing over three vears, the amount of silt carried past Cairo is 57 parts in 100,000 



, , rp' 57x100,000,000,000 _ .... 



of water. This gives ' — — or 57 million tons per year. 



IUU,UU , 

 Bearing in mind the thick deposits which are thrown down on the lands of 

 Upper Egypt, the quantity of silt passing Semna must be considerably over 

 00 million tons per year. 



