76 DK. J. BALL ON THE [Feb. I903, 



that there formerly existed barriers at these places, the fall of water 

 over which would cause a great deepening of the channel on the 

 down-stream side, and as the barrier was gradually worn back the 

 local deepening of the channel would become filled with silt. 



Suprisingly deepsoundingshave been taken quite recently below the 

 new reservoir-dam at the Assuan Cataract, and these again can only 

 be explained on a like assumption. Thus it would appear that, 

 after allowing for the well-known tendency to gross exaggeration in 

 early ' travellers' tales,' there may have been some foundation for 

 the statements of numerous classical writers, 1 who describe the 

 Assuan Cataract as a distinct waterfall, tumbling over a precipice 

 with a loud noise ; more especially if they relied on a tradition 

 handed down from earlier times, rather than on the evidence of an 

 eye-witness of their own day. Finally, the fact that the whole of 

 the cultivated lands of Lower Nubia, and the large alluvial tract 

 which forms the Kom Ombo plain, lie at so great an elevation as to 

 be impossible of irrigation at the present day without the aid of 

 water-raising appliances, is easily explained by the hypothesis that, 

 at Silsila and Assuan at least, the river was formerly dammed back 

 by natural obstructions which have since been eroded away. 



A very good site for a quantitative determination of the rate of 

 erosion in granite by the silt-laden waters of the Nile will be 

 furnished by the sluices of the new dam at Assuan. In this case all 

 large stones will be kept out by grids, so that only the smaller 

 pebbles and sand will be carried through, and some years will 

 doubtless havo to elapse before the erosion has proceeded sufficiently 

 for a reliable measurement to be obtained. And with regard to 

 the rate at which potholing action is removing the barrier at Semna, 

 it would appear not difficult to make a fairly accurate experimental 

 determination within a few years, for the action is certainly a 

 relatively rapid one, and the holes are laid dry regularly every year 

 at low Nile.- It would be easy to mark a few average holes, 

 and after temporarily removing their contents, to measure their 

 transverse dimensions and their depths below artificial datum-marks 

 deeply sculptured in the "sides of the holes ; if then the stones were 

 replaced within the holes exactly as found, and the action allowed 

 to go on for a few years, a remeasurement would indicate the average 



1 Diodorus Siculus. Hist. 1. i, c. 3 ; Strabo, Geog. 1. xvii, c. 1, § 49 ; 

 Pomponius Mela, De situ orbis lib. 1, c. 9 ; Seneca, Nat. Quasi. 1. iv, c. 2. 



" That potholing may, under certain favourable conditions, proceed at such 

 a rate as to permit of easy measurement after a few years' action, is proved by an 

 observation in Sweden, recorded by Axel Erdmann (' Bidrag till Kannedomen 

 om Sveriges qvartara Bildningar ' Sver. geol. Undersokn. ser. C, No. 1, 1868, 

 p. 82). About 1858, when the Oena paper-mills were built on the western bank 

 of the Gota Elf, near the falls of Troll hattan, the required water for driving 

 the mills was conducted from the river through a channel, which was at the 

 time blasted entirely out of the rock. When an extension of the mills took 

 place some 8 or 9 years later, it became necessary to widen this channel, for 

 which purpose the water was drained off. It was then found that three small 

 potholes had been formed in the bottom of the channel, of a maximum depth 

 of 1 J feet, and of diameters varying from 6 to 12 inches, with the rounded 

 pebbles (of trap) still remaining within them. Thus 8 or 9 years had been a 

 sufficient time for the formation of these potholes, through the rotating motion 

 given to the stones by the water flowing through the channel. 



