Vol. 59.] SEMNA CATAEACT OF THE NILE. 75 



other districts. To quote a single well-known case, the Biver 

 Simeto, flowing through the Etna lava-stream of 1603, had by 1828 

 cut for itself a passage from 50 to several hundred feet wide, and 

 in some parts from 40 to 50 feet deep. 1 There is doubtless a 

 considerable difference between such lava and the Semna gneiss in 

 point of hardness ; but even when due allowance has been made for 

 this fact, the comparison is not unfavourable for the truth of the 

 explanation suggested. Nor can it be objected that the evidence at 

 other points of the Nile Yalley is in any way contradictory. It must 

 be borne in mind that, except at the cataracts, the processes of' 

 erosion and deposition of silt in the Egyptian portion of the valley 

 of the Nile nearly balance each other ; while at the cataracts the 

 action is purely erosive, and in fact corresponds more to that of a 

 mountain-stream than to that of a peacefully-flowing river. 



A study of the Semna rapid, which may be taken as the simplest 

 possible case of a Nile cataract, throws considerable light on the 

 study of more complex cases. All the Nile cataracts exist at points 

 where igneous and metamorphic masses crop out through the softer 

 sedimentary rocks, and are the consequence of the greater difficulty 

 which the river finds in eroding a course through such masses. We 

 are still far from having sufficient information, concerning these 

 interesting features of the great Egyptian river, to be able completely 

 to trace its past history as a geological agent ; but my own researches 

 at the Assuan Cataract, and those of my colleague Dr. W. F. Hume 

 at the higher ones, have brought to light a number of very striking 

 facts, more especially concerning the influence of dykes and faults 

 in determining the rivers course, and there is ground for hope, that 

 when all the observations have been worked out and co-ordinated, a 

 very considerable advance in the study of the problem will have been 

 made. At Assuan and Silsila the river has entirely changed its 

 channel and suffered a considerable lowering 2 within geologically 

 recent times, though before the historical period ; and it is probable 

 that such changes were largely brought about by the removal of 

 long pre-existent hard barriers such as the one at Semna. There 

 can be no doubt that at all these cataracts erosion is very rapidly 

 going on, though masked from observation by the absence of records 

 of sufficient antiquity and by the great variations in the floods of 

 different years. At Silsila and at Kalabsha in Lower Nubia, the 

 river passes rapidly through narrow gorges in hard rocks ; and when 

 soundings and borings were taken there a few years ago for the 

 proposed reservoir-dams, it was found that the bottom of the stream, 

 far from being, as was expected, solid rock, was in fact in each case 

 composed of sand to a depth of over 20 metres. 3 It is impossible to 

 explain the existence of this thick sand-stratum, unless we assume 



1 Lyell, ' Principles of Geology ' 11th ed. (1872) vol. i, pp. 352-53. 



2 My own observations give the fall at Assuan as at least 20 metres (65 feet), 

 while Prof. Schweinfurth gives that at Silsila as 20 to 22 metres, Petermann's 

 Mitth. vol. xlvii (1901) p. 9. If, in these cases, the lowering of the stream has 

 proceeded at a rate similar to that recorded at Semna, the period correspond- 

 ing to the changes is about 10,000 years. 



3 For this information I am indebted to Mr. Marshall Hewat, who conducted 

 the borings in question for the Egyptian Government. 



