Vol. 52.] PEOF. E. HULL ON THE GEOLOGF. OF THE NILE VALLEr. 317 



by granite ridges, instead of having merely to find its way 

 through a wide plain as at the latter place. Near the northern 

 end of the railway-cutting above Assuan we find a deposit of the 

 old Nile mud with bands of pebbles resting on the upturned edges 

 of the ancient schists. 



Fig. 5. — Section from the river-banh opposite the Island of 

 Sehel at the First Cataract. 1 



S = Nubian Sandstone and Conglomerate. 



G = Granite forming the sides of the present and ancient channels of the Nile. 

 [Distance = oyer 2 miles.] 



The valley now described was recognized by Leith Adams as an 

 ancient river-channel, and in the alluvial deposits in the ravines 

 north of it he was fortunate in discovering numerous shells, such as 

 JEtheria semilunata, Iridina nilotica, and Bulimus pullns. 2 Time 

 did not permit of a careful search on my own part. 



From the foregoing facts and considerations it will be observed 

 that the evidence of a former higher Nile surface afforded by the old 

 terraces is confirmed by that of the old channels. The waters 

 which formerly extended over the floor of the Nile Valley with a 

 breadth of several miles are now confined by banks which are 

 seldom | mile apart, and this has resulted, not in consequence 

 of the deepening of the channel, but by reason of the diminution 

 in volume of the waters themselves. This is shown by the fact 

 that there is no scouring of the Nile channel, the current being 

 insufficient for this purpose ; and besides this, the borings which 

 have been made show that the bed of the river is composed of 

 alluvial mud of considerable depth. 3 The bed is, in fact, rising by 

 accessions of deposit comparable with that which is annually spread 

 over the cultivated lands on either side during the high floods, and 

 which has been estimated by Mr. Willcocks, Engineer to the 

 Public Works Department, to amount to 0-12 metre (4*7 inches) 

 in 100 years. 4 We are therefore obliged to have recourse to 

 another explanation — namely, the decrease, and in part cessation, 

 of the rainfall over the entire hydrographical basin of the river — 

 in order to account for the decrease in volume. 



The hypothesis of a former greater volume of the river has the 

 support of Leith Adams, Zittel, Lyons, and others, and may there- 

 fore be considered as an accepted hypothesis : though it is not, as it 

 seems to me, brought forward with sufficient prominence by writers 



1 It is near this place that the great Nile embankment is to be made. 



2 Quart. Journ. Geol. Soc. vol. xx. (1864) p. 15. The species were determined 

 by the late S. P. Woodward, of the Natural History Museum. 



3 At Silsileh, where the Nubian Sandstone crosses the river, the solid rock 

 was still not met with after boring to a depth of 20 to 25 metres (<>5 to 73 feet) 

 below low Nile, ' Kep. Technical Commission,' p. 21 (1804). 



4 Eeport on ' Perennial Irrigation of Egypt,' 1894, p. 12. 



