Vol. 50.] PHYSIOGRAPHY OF THE LIBYAN DESERT. 543 



time, but I have no doubt that they also represent streams which 

 flowed when the calcareous springs were forming the tufa above 

 mentioned, and when the evergreen oak and other similar plants 

 were growing near. 



I would refer to this same period the large gravel-sheets which in 

 Nubia extend for a length of 7 or 8 miles along the Nile bank at 

 Debera, 8 miles north of "Wadi Haifa, a period when a more generous 

 rainfall furnished streams which eroded the beds of the neighbouring 

 plateau and fed the Nile, which deposited its beds of alluvium 100 

 feet above its present level, and maintained the beds of JEtheria 

 semilunata, Cyrena fluminalis, Unio, Paludina, and other shells 1 

 which are found at this level at Wadi Haifa, Debera, Derr, etc. 

 Deposits of impure kaolin in the gullies of the Second Cataract 

 point to the same condition of things, when the rain was weathering 

 the felspars of the crystalline rocks. 



Shortly to recapitulate the conditions under which this area has 

 attained its present state, we have firstly an elevation of the Eocene 

 rocks and a certain amount of erosion of them, followed by a 

 depression, at all events in the northern area, where the marine 

 Miocene beds were deposited. A gradual elevation seems to have 

 taken place, and the Jebel Ahmar Sandstone was deposited. An 

 elevation 2 of the area in Pliocene times caused the Nile to erode its 

 bed deeply, and the main work of plateau erosion and the formation 

 of the southern oases was commenced. A depression of the area 

 at the time of the deposition of the late Pliocene sea-beaches, 3 

 near Cairo, checked this eroding action and probably caused the 

 deposit of the high-level Nile mud with beds of JEtheria, etc. A 

 later elevation with a climate of moderate rainfall enabled the 

 river to cut out its present bed below the earlier Nile-mud deposits, 

 and one is almost tempted to go further and attribute the present 

 silting-up of the Nile bed below the First Cataract to a recent 

 depression of the area, at all events to the north, as is shown by 

 Roman tombs near Alexandria, which are now below sea-level. 



Much has been made of the fact that the rainfall of to-day has 

 formed deep valleys on the eastern bank of the Nile, but I am 

 inclined to think that, while the rainfall of post- Pliocene times 

 eroded both sides of the river, the physical conditions were widely 

 different. On the west was a gently rising plateau which was not, 

 perhaps, very deeply eroded, and since then thousands of years of 

 wind- and sand-action under desert conditions have ground away 

 and obliterated most of the traces of this rainfall. On the east a 

 high ridge of crystalline rocks gave the streams a more rapid fall, 

 and so vastly increased their eroding power, while then as now they 

 caused precipitation of moisture carried by currents of air from the 

 eastward ; thus the gorges and valleys once formed have been kept 



1 Leith Adams and S. P. Woodward, Quart. Journ. Geol. Soc. vol. xx. (1864) 

 pp. 14 and 19. 



2 Jukes-Browne. 'Physical Geology,' pt. iii. ch. ii. 2nd ed. 1892. 



3 These are small patches, too minute to show on any but a large-scale map. 



