1086 Appendix II: Phytogeography and Geology. 
Assiut) relatively to the eastern. The former took place as the 
result of changed geographical conditions on the continent to the 
south at the time in question, with which however we need not 
deal here. On a homogenous mass of rock weathering has little 
power to form depressions of any magnitude, and this is the cause 
of the continuous unbroken plateau which stretches southwards 
from the Faytim, the under-lying rocks being one continuous thick 
mass of hard limestone. Wherever softer intercalations are present 
differential weathering takes place, and all the great depressions of 
the Libyan desert owe their origin to the presence oft soft easily 
denuded strata; if the great homogenous mass of the Nile Valley 
limestone had stretched unchanged westwards, the Little Oasis and 
Farafra would never have existed. 
Not one endemic species exists in this district and only a few 
species not found again in other districts of Egypt, these are the 
following: 
Medicago granatensis. | Atriplex tataricum. 
Astragalus brachyceras. Najas pectinata. 
Vicia gracilis. Panicum eruciforme. 
Myriophyllum spicatum. Alopecurus agrestis. 
c) The narrower Nile-Valley (N. v. N. v. mer.). 
North of Aswan the Nile flows through a fertile and highly 
cultivated valley which opens out into the Delta 25 kilometres north 
of Cairo, and in this part of its basin the river occupies the western 
margin, all drainage lines of any importance coming in from the 
east on the right bank. This is due to the very unequal relief of 
the country lying on either side of the river. On the east 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. On the west of the valley conditions are very different; 
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 flat-topped ridges, but 
on the whole falling very gently to the westward. Only such rain 
storms 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-worm 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 
