Vol. 5 2.] PROF. £. HULL OH THE GEOLOGY OF THE NILE VALLEY. 319 



hotter months, that its waters ever reach the Mediterranean at all. 

 That they do so is owing to the fact that it is during these months 

 that the waters rise by reason of the Abyssinian floods. The 

 numerous dry valleys which enter the Nile Valley in Middle and 

 Upper Egypt and Nubia show that this region was once abundantly 

 watered. 



When we consider the enormous area of the hydrographical basin 

 of the Nile, estimated at 1,100,000 square miles, we can under- 

 stand how a slight climatic change in the direction of increased 

 humidity and decreased temperature would cause an enormous 

 expansion of volume of Egypt's great river. This is really what I 

 believe took place ; and it now remains to consider the period to 

 which we should refer these altered conditions. 



(2) During the Miocene period, when the primseval river was 

 channelling out its bed, and when the land was relatively higher 

 than at present as regards the surface of the Mediterranean, the 

 climatic conditions may have been altogether different from those 

 of the present day over the Nile Basin ; but whether this was the 

 case or not, we have good grounds for believing that during the 

 subsequent Pliocene period, when Lower Egypt was submerged to a 

 depth of over 200 feet, and the sea stretched up the Nile Valley for 

 several hundred miles, the conditions were different from those of 

 the present time. The increase of water-surface must have been 

 accompanied by increased humidity, and a lowering of tempera- 

 ture, compared with that of Miocene times. And when these 

 Pliocene conditions gave place to those of the Pleistocene period 

 the climatic conditions in the same direction must have been still 

 further advanced. 1 The lowering of temperature over all the 

 Europasian regions to the north must have greatly affected those 

 we are now discussing. To what extent the annual mean tempe- 

 rature of the subtropical regions of Africa was lowered during the 

 Glacial period of the temperate zone can scarcely be estimated with 

 precision, but that the general effect was brought about cannot, as 

 it seems to me, be contested. My own view is that in these 

 regions the climatic conditions were similar to those of Europe at 

 the present day, both as regards temperature and rainfall ; and if 

 such were even approximately the case, it is easy to account for the 

 vastly greater volume of the Nile waters as compared with those 

 which render Egypt not only a habitable but a fertile country. 

 For this epoch I adopt the term first, I believe, suggested by 

 Mr. Jameson for this part of the world — namely, the Pluvial 

 period. 2 



1 Lyons considers that at this period (post-Pliocene) there was 'a con- 

 siderable rainfall ' over the area of the Libyan Desert, though not excessive. 

 Quart. Journ. Geol. Soc. vol. 1. (1894) p. 542. See also E. A. Floyer, ibid. 

 vol. xlviii. (1892) p. 580. 



2 Capt. Lyons has unintentionally misrepresented my meaning when he 

 suggests that I have restricted the term ' Pluvial period ' to that represented 

 by the Glacial of Europe. On the contrary, I expressly regard it as extending 

 from the Pliocene down to the close of the post-Pliocene, as will be seen on 

 referring to my memoir on the ' Physical Geology of Arabia Petraea and 

 Palestine,' Mem. Palest. Explor. Fund (1886), pp. 69, 113. 



