Appendix Il: Phytogeography and Geology. 1093 
IV. The Desert region. 
The relief of the deserts lying on either side of the Nile 
is very unequal’). On the east (D. a.) the divide between the 
Nile and the Red Sea is formed by a range of ancient crystalline 
rocks running parallel to the coast at a short distance from it, and 
which rises to a considerable height since many of its peaks reach 
1200 metres while some few attain or even exceed 2000 metres. 
From the foot of this range the plateau, formed of cretaceous and 
tertiary sandstones and limestones, slopes gradually westwards to- 
wards the valley of the Nile but even here the edge of the plateau 
rises to 200 and 250 metres above the valley floor in many places. 
On the west of the valley conditions are very different (D. 1.); the 
desert plateau rises rapidly from the valley, often as steep cliffs, 
and more gradually for some 10—20 kilometres beyond this. To 
this succeeds an almost horizontal plateau without any well defined 
drainage lines, rising here and there to low flatt-topped ridges, but 
on the whole falling very gently to the westward. ‘Thus there is 
no catchment basin of any extent on the west of the Nile, and the 
feebly marked drainage lines extend but a few kilometres back on 
to the plateau; only such rainstorms as fall near the plateau edge 
are drained towards the river and but rarely does the water reach 
the margin of the cultivation. What falls on the plateau drains 
into shallow wind-worn depressions and there soaks into the rock 
or is soon evaporated. The area of the basin west of the Nile 
may in this part of its course be limited to the 5 to kilometres 
beyond the limites of the cultivation and of this area none of it 
can be said to be effective seemg how small a quantity of rain 
falls. A few rainstorm occur every winter but they are usually 
very local in their effect. On the eastern side the much larger 
area, and the steeper slopes, together with a greater frequency of 
rain near the Red Sea Hills make the winter rainfall a more im- 
portant factor; in about every second year one or other of the larger 
wadies comes down in flood, sometimes so suddenly as to carry 
away camels and sheep which may be grazing in the valleys and 
pours a large volume of water into the Nile. As a contribution to 
the river supply these “Seils’” or rainfloods are unimportant but 
their effect in eroding the desert is immense. ‘The rain falling 
heavily for a few hours on the bare rock surface of the desert where 
no vegetation exists to delay it, pours rapidly down the gentle slope 
of the plateau into the nearest valley carrying with it the material 
which the rapid variation of temperature in the summer months has 
loosened from the rock surface. 
1) Lyons: The Physiography of the River Nile and its basin. — Cairo 1906. 
