Vol. 59.] SEMNA CATARACT OF THE NILE. 73 



•origin, as the rock shows signs of a certain amount of crushing. 

 Magnetite is entirely absent. The order of crystallization would 

 appear to be (1) sphene, (2) augite, and (3) hornblende, as the last- 

 named mineral frequently occupies interspaces around the augites. 

 Unfortunately, as the exceptional nature of the rock was not 

 recognized in the field, its relations to the associated gneiss and 

 schist were not made out ; it appeared, however, to form part of the 

 basic intrusion which has been suggested as the origin of the schist, 

 -and thus to be younger than the gneiss. 



Physical Geology. 



To account for the deep central channel across the bar, I was at 

 first inclined to believe in the existence of a soft basic dyke, such 

 as accounts for most of the channels between the islands of the 

 Assuan Cataract. But I failed to find any evidence of such a dyke, 

 which would in fact have a strike perpendicular to that of the syenite- 

 porphyry veins already mentioned ; and I believe this channel, like 

 the parallel smaller channels which are only filled at high Nile, to 

 be due to the simple erosive action of the river, the regularity being 

 due to the coincidence of the direction of flow with the strike of the 

 foliation-planes of the gneiss. It is obvious that at an early stage 

 of the erosion some one channel would be slightly deeper than the 

 rest, owing to accidental circumstances ; and this deepest channel, 

 -conveying more silt-laden water than the others, would be more 

 rapidly deepened. 



The sketch-map published by Lepsius in his 4 Denkmiiler ' (pt. i, 

 sheet 3) was drawn in the latter half of the month of July, when the 

 river would have risen considerably above its lowest level : the true 

 nature of the rocky barrier is thus imperfectly exhibited, and at the 

 same time the extent of the valley shown above and below the 

 barrier was insufficient for a true judgment to be formed as to the 

 manner in which the fall of the river-level had been brought about. 

 From the survey just accomplished at low Nile, it will, I think, be 

 easy to show that the explanation is to be sought in the wearing away 

 of the barrier itself. Horner indeed put this forward as an hypo- 

 thesis, but dismissed it as requiring too long a period of time. In 

 thus dismissing it he was almost certainly wrong, for the barrier is 

 of small extent, and the yearly removal of a comparatively small 

 amount of it will explain the facts observed. It should be premised 

 that relatively deep water exists both above and below the barrier, 

 which is only about 200 metres (650 feet) in average width ; and this 

 condition of things, owing to the difference in hardness of the rocks, 

 must have existed for long ages. Further, the enormous amount of 

 potholing shows that by this means the river is rapidly breaking up 

 the barrier, and that ordinary direct erosion plays a subordinate part. 



The area of the barrier is approximately 500 x 200 or 100,000 

 square metres. Taking the lowest group of ancient inscriptions of 

 the time of Amenemhat III above mentioned, as marking the average 

 high Nile of the period about 2300 e.c, we have a vertical erosion 



