GEOLOGICAL HISTORY OF EGYPT AND THE NILE VALLEY. 323 
Nile Valley, by which the strata of Nummulite limestone 
have been let down about 250 feet on the west side of the 
valley, as. shown by Schweinfurth and Dawson, as com- 
pared with their position in Mokattam Hill, on the east bank* 
(Fig. 3). This le of fault, trending in a N.—S, direction 
along the base of the eastern cliff, corresponds with the line 
of the valley for a distance of nearly 100 miles, and doubt- 
less caused the river to select its course in this particular 
district. With the rising of the sea-bed into land it became 
necessary for the various affluents of the Upper Nile to select 
a channel seawards, and it is easy to show that no other 
course than that actually selected was possible. It has been 
suggested more.than once that the Nile at one time flowed 
through some abandoned channel into the Red Sea; but no 
such channel exists or was possible. The long range of the 
Abyssinian and Arabian Highlands, formed of ancient rocks, 
effectually barred any outlet in an easterly direction. And 
for a similar reason a westerly course, which would have 
brought the river into contact with the Tibesti and Ahagegai 
Highlands, was impossible. A river in seeking an outlet 
towards the sea necessarily flows along the lowest accessible 
ground, and such a tract appears to have existed generally 
along the present Nile Valley, formed of Cretaceous and 
Tertiary limestone strata, which was probably slightly 
depressed as compared with the bordering tracts. The fault 
in the valley of Lower Egypt above described, which was 
an effectual guide to its course there, probably gave place 
further south to a slight depression or channel in the old sea- 
bed, and along this the river seems to have had a line of flow 
of least general resistance to its course towards the outer sea. 
We may therefore suppose that, as the sea-bed over the 
Egyptian and Libyan areas gradually rose, and became land, 
the river waters followed the line of retreat of the sea waters 
northwards, the several streams converging into one central 
channel; and when the whole tract had been reclaimed 
from the sea, this channel would become deeper by reason 
of the increased eroding action of the waters, tending to 
deepen the channel back from the outlet.t 
* Proc. German Geol. Survey, 1883, quoted by Dawson, Joc. cit., p. 538. 
The disturbances and dislocations in the rocks near the Second Cataract, 
described by Leith Adams, are probably referable to the same period, 
Quart. Jour. Geol. Soc., vol. xx, p. 11. 
+ These streams are well shown in Ramsay’s Orographical Map of 
Africa, published by E. Stanford, and in the “ Carte du bassin du Nil” 
in Reclus’ Nouv. Geog., vol. x. 
