xlvi INTRODUCTION. 
The physical features of both sides of the Nile indicate the existence of a period long 
antecedent to the present, in which a considerable rainfall prevailed, as in the eroded 
valleys of the desert there may be observed rocky ravines which have been carved 
out by the action of water, which has left behind it dry channels over which waterfalls 
had once precipitated themselves, and others down which cataracts had once raced. 
The rainfall of the present is not sufficient to account for such a degree of erosion, 
unless it be supposed that it has been prolonged over an almost incalculable period of 
time, a supposition which does not meet with support when other considerations are 
brought under review. On the other hand, the entire evidence seems to sanction the 
conclusion that a material change in the character of the climate of North-eastern 
Africa, so far as its rainfall is concerned, has taken place since the days when the 
ancient Egyptians represented on the monuments of Sakkarah the various animals 
with which they were familiar. The camel was seemingly unknown to that ancient 
people : at least no figure of it has been bequeathed by them to us, whereas, for example, 
representations of the elephant 1 are present, and on ivory carvings of an almost pre- 
historic age figures of elephants are found associated with others of the ibex of the 
Arabian desert. The elephant, rhinoceros, hippopotamus, lion, and hysena, along with 
some other large mammalia, existed in Central Europe in Miocene times, but were 
then unknown in Africa, which they reached only at a comparatively modern period. 
From the nature and habits of some of these animals it is self-evident that the climate 
then must have been a moist one and with a rainfall which ensured an abundant supply 
of vegetation and water. The gradual disappearance of this rain and consequent slow 
conversion of North-east Africa into a semi-desert led to the retirement southwards of 
the larger of these mammals to the region of periodical rains. 
Besides the slight and rare rainfall along the course of the Nile Valley, a certain 
amount of clew falls at night even on the desert. In winter the dew on the plants on 
the cultivated lands has now and again been noticed in early morning sparkling with 
icicles, whilst the surface of the desert may in places be white with hoar frost. 
The intense heat of the desert is the cause of a great indraught of cold air from the 
north during the greater part of the year, so that winds from that quarter are six times 
more prevalent than those from the south. In March to May the hot khamsin charged 
with dust blows sometimes for days at a time, while the sun is above the horizon, 
drying up vegetation and exhausting animal life. 
The great triangular level alluvial expanse of the Delta of the Nile is enclosed on its 
eastern and western sides respectively by the Arabian and Libyan deserts. Nearly the 
whole of the seaward face of the Delta is occupied by a series of great lakes and swamps 
of brackish and even saline waters, surrounded more or less by sedgy banks. These 
extensive lakes are separated from the sea by sand-dunes, generally resting on a solid 
reef, made up of sea and freshwater shells, which forms a barrier against the encroach- 
1 The elephant is unmistakably represented in Roman mosaics, Ac, iD Tunisia. Johnston, P. Z. S. 189S, 
p. 353. 
