GEOLOGICAL HISTORY OF EGYPT AND THE NILE VALLEY. 315 
distance of 100 miles, and opens out on the Nile at the 
village of the same name. ‘The Wady el Arabah, which 
opens on the Gulf of Suez, is remarkable for its width and 
the loftiness of the cliffs by which it is bounded, and has 
been graphically described by Schweinfurth, who has dis- 
covered Palaeozoic strata therein.* 
These valleys are now waterless, but from their ramifying 
form, and the physical features of the sides, indicating exten- 
sive erosion of the strata, together with the alluvial gravel 
of their floors, they enable us to infer, without any hesitation, 
that they were originally formed by water-action, and that 
they were once the channels of no inconsiderable streams 
and rivers. They have their counterparts in Arabia Petreea 
and Southern Palestine, and other districts of the African 
Continent. 
ParT II].—TuHEeE NILE. 
The region thus described is traversed from south to north 
by the one river of Egypt, the Nile, which, rising in the great 
central lake, embosomed amidst lofty mountains, the Victoria 
Nyanza, at a level of 3,740 feet, flows for a distance of 
2,000 miles, till it enters the Mediterranean by its two great 
branches, those of Rosetta and Damietta; the only repre- 
sentatives of seven original outlets known to ancient geo- 
graphers. . 
In its upper reaches, south of lat. 10° N., the Nile receives 
many tributaries, but north of this, there are but two, the 
Bahr el Azraq (or the Blue River) and the Atbara or Bahr el 
_Aswad (or the Black River), which descend from the highlands 
of Abyssinia. It is now known, from the account of Sir 
Samuel Baker and others, that the periodic inundations of 
the Nile in Egypt are due to the thunderstorms which burst 
upon the Abyssinian mountains about the summer solstice, 
and which, pouring down in torrents by its great rivers, 
chiefly the Atbara, carry along the muddy sediment derived 
from the breaking down of their banks. At the island of 
Phila, about five miles south of Assouan, the Nile may be con- 
sidered to make its entrance into Egypt. At Assouan, the 
level of the river is, according to the barometric measure- 
ments of Russegger about 300 feet above Cairo, and 365 feet, 
* Bull. Institut Egyptien, No. 6 (1885). There is another and greater 
valley of the same name in Arabia Petra, extending northward from 
the Gulf of Akabah. 
