Vol. 52.] PROF. E. HULL ON THE GEOLOGY OF THE NILE VALLEY. 309 



period, upon the rising of the land, there can be little doubt ; as it 

 is becoming more clear that this was the special epoch of elevation, 

 disturbance of strata, and denudation over all the Egyptian and 

 Syrian region. 1 But there was a second period of groat fluviatile 

 inundation in what may be designated 'the Pluvial period/ ex- 

 tending from later Pliocene times into, and including, the Pleisto- 

 cene ; and to this latter stage I shall have occasion to recur later on. 

 At the present day erosion has almost ceased, and the fact that 

 the cultivated terraces bordering the river, as well as the plain 

 of Lower Egypt, are gradually rising with regard to the sea-level 

 by reason of deposition of sediment shows that the bed of the river 

 is also rising. 2 There is, therefore, no scouring action going on. 



The fall of the river from Assuan to Cairo is a little over 6 inches 

 per mile, a fall just sufficient to keep the fertilizing sediment in 

 suspension when the river is not at its lowest. The magnificent 

 Nile Valley, of an average breadth of 10 miles, cut down through a 

 table-land of an average elevation of 800 to 1000 feet, capped by 

 Eocene limestone, and extending above Cairo for a distance of 300 

 miles, is a physical feature which must impress the observer with 

 the conviction of the enormous length of time during which the 

 Miocene erosion was in progress. 3 



The next point to which I wish to refer is the fact that in the Nile 

 we have a conspicuous example of a river running across escarpments. 

 Taking these in succession from the north up stream, they are : — 

 1, the escarpment of the Eocene limestone ; 2, that of the Cre- 

 taceous limestone ; 3, that of the Nubian Sandstone ; and 4, in a 

 less degree, that of the granitic and schistose rocks of Assuan. The 

 dip of the strata towards the north, although generally almost 

 imperceptible, and sometimes reversed, is greater than the slope of 

 the river-channel. In consequence of this the transverse course 

 of the river was a physical necessity, if it was to enter the Medi- 

 terranean ; but owing to the low angle of dip of the beds, and their 

 wide range, the fact is not so evident as it would be if the dip were 

 greater, and the range more contracted. Zittel has suggested that 

 the river once entered the Red Sea. This I cannot conceive to 

 have been possible at any point above Cairo, except during early 

 Pliocene times when Lower Egypt was submerged to a depth of 

 about 220 feet, and the waters of the Red Sea, united to those of 

 the Mediterranean, overflowed the entire region below this level. 

 In this sense the statement is true, but not otherwise. As in the case 

 of the streams traversing the Chalk and Greensand escarpments in 



1 It is now becoming generally recognized that Miocene strata are absent 

 from the Egyptian area, which bears out the view above stated. See ' The 

 Geology of Egypt,' by Philip Lake, 'Science Progress,' vol. iv. (1896) p. 395; 

 also Prof. Mayer-Eymar, ' Zur Geologie Aegyptens,' Vierteljahrschr. Naturf. 

 Gesellsch. Zurich, 1886. 



2 Otherwise the river would have ceased to reach the cultivated plain on its 

 banks. 



3 From Zittel's section across the Nile Valley it will be seen that the plateau 

 on the eastern side of the valley rises over 1500 feet in some places, and I am 

 inclined to think that the average height may be 1000 feet. 



t2 



