Assuan (First Cataract), the course of the river is interrupted by
outcrops of granites and other crystalline rocks, which have been
uncovered by the erosion of the overlying sandstone, and to-day form
the mass of islands, with numerous small rapids, which are described
not very accurately as cataracts; no good evidence exists in support
of the view that they are the remains of a massive barrier, broken
down and carried away by some sudden convulsion. From 25 deg. N.
northwards for 518 m. the valley is of the "rift-valley" type, a level
depression in a limestone plateau, enclosed usually by steep cliffs,
except where the tributary valleys drained into the main valley in
early times, when there was a larger rainfall, and now carry off the
occasional rainstorms that burst on the desert. The cliffs are highest
between Esna and Kena, where they reach 1800 ft. above sea-level. The
average width of the cultivated land is about 10 m., of which the
greater part lies on the left (western) bank of the river; and outside
this is a belt, varying from a few hundred yards to 3 or 4 m., of
stony and sandy ground, reaching up to the foot of the limestone
cliffs, which rise in places to as much as 1000 ft. above the valley.
This continues as far as 29 deg. N., after which the hills that close
in the valley become lower, and the higher plateaus lie at a distance
of 10 or 15 m. back in the desert.

_The Fayum._--The fertile province of the Fayum, west of the Nile and
separated from it by some 6 m. of desert, seems to owe its existence
to movements similar to those which determined the valley itself.
Lying in a basin sloping in a series of terraces from an altitude of
65 ft. above sea-level in the east to about 140 ft. _below_ sea-level
on the north-west, at the margin of the Birket-el-Kerun, this province
is wholly irrigated by a canalized channel, the Bahr Yusuf, which,
leaving the Nile at Derut esh Sherif in Upper Egypt, follows the
western margin of the cultivation in the Nile valley, and at length
enters the Fayum through a gap in the desert hills by the XIIth
Dynasty pyramids of Lahun and Hawara (see FAYUM).

_The Delta._--About 30 deg. N., where the city of Cairo stands, the
hills which have hitherto run parallel with the Nile turn W.N.W. and
E.N.E., and the triangular area between them is wholly deltaic. The
Delta measures 100 m. from S. to N., having a width of