318 PK0E. E. HULL Off THE GEOLOGY OP THE NILE VALLEY. [May 1 896, 



on Egyptian geography, who have had generally other objects of 

 investigation before their minds, but I should like to quote in this 

 connexion the language of Prof. Zittel as follows : — ■ 



'Alle diese Thatsachen beweisen, dass der Nil einst ein weit 

 machtigerer und reissenderer Strom als heutzutage war, und dass 

 die Gattung JEtheria, welche jetzt erst siidlich von Assuan beginnt, 

 friiher weiter nach Norden verbreitet war.' l 



Pakt IV. 



8. The Pluvial Period. 



When traversing, in 1883-84, the fine valleys of the Sinaitic 

 Peninsula and Arabia Petraea, the bottoms of which, now dry, are 

 •composed of alluvial deposits, I came to the conclusion that at a 

 former period, and under different climatic conditions, they con- 

 stituted the channels of an extensive river-system draining into the 

 Red Sea. 2 It is only at rare intervals that rain falls over this 

 region, in the form of spasmodic thunderstorms of short duration, 

 and they are quite in sufficient to account for the formation of valleys, 

 sometimes a mile or more in breadth and hundreds of feet in depth. 

 The dry river-valleys which open into the Nile, chiefly along the 

 -eastern side, tell a similar tale. 3 The streams which flowed along 

 them, and by which they were excavated, have dried up and 

 disappeared. The period during which this process of valley- 

 erosion, of terrace-formation, and of high floods went on may well be 

 designated ' Pluvial ' — extending from the Pliocene down through 

 the post-Pliocene, and terminating with recent times. It is a term 

 indicative of meteorological rather than geological conditions, though 

 not unconnected with these. 



Conclusion. 



The conclusion to which we are driven from a consideration of 

 the above phenomena is that the Mle has decreased in volume to 

 a large extent, as compared with that of primaeval times. It only 

 remains to consider how and when this decrease has arisen. 



(1) As regards the manner in which this change took place, 

 there can be only one answer : by the drying up of its sources and 

 tributaries owing to decrease in the rainfall. Throughout 1200 miles 

 of its course, the river runs through a region well-nigh rainless, 

 where its waters are subject to a constant drain through evaporation ; 

 in consequence of which its volume at Khartum is considerably 

 larger than it is at Cairo ; and the only wonder is, when one con- 

 templates the extent of this evaporation, especially during the 



1 ' Palasontographica,' vol. xxx. (1883) p. 137. Zittel founds this opinion 

 •mainly on the discoveries by Leith Adams of fluviatile shells in the terraces 

 beyond the reach of the highest floods. 



2 ' Geology of Arabia Petrsea, etc.,' Mem. Palest. Expl. Fund, passim. 



3 Such as the Wadi Sonnur, opposite Beni Suef ; W. Tarfeh, north of 

 Minieh ; W. Siut, opposite the town of the same name ; W. Gassab, opposite 

 Girgeh ; W. Keneh, a valley with numerous branches, opposite Keneh ; and 

 W. Abu Wassel, below Luxor. 



