PHYSICAL FEATURES. xxxi 
Qurun, Wadi Eaian, and Wadi Natrun are further examples of depressions. The 
first, which occupies the deepest part of the Fayum, had its surface-waters, in 1892, 
43"30 metres below the level of the sea, but the bed of the lake lies from 5 to 17 
metres lower. The second is a bare desert, 40 metres below sea-level, whilst the third 
is slightly under the level of the Nile at Teraneh. The springs which give rise to the 
fertility of the more southerly oases derive their supplies of water, according to Zittel, 
from a water-bearing bed fed by the water that finds its way down to it from 
Dar-Fur, a region which has a heavy rainfall. Numerous wells, some of them of great 
depth, once existed, more or less, throughout all the oases, but many have been choked 
up with sand. The Libyan plateau is studded over, in places, with flat-topped or 
conical hills, 60 to 80 metres high, the remains of a tableland which formerly had its 
general surface at the level of their summits, and now and again a low isolated 
mountain rises from it. This tableland of limestone is covered with sand that either 
accumulates in the form of drifts in the hollows, or covers large areas with shifting 
sand-dunes 3 to 5 metres in height, directed from south-east to north-west by the 
polar winds. This sand, which is so characteristic a feature of this desert and more 
especially of the great area to the west of the oases, and designated by the members of 
the Eohlfs Expedition "Das grosse libysche Sandmeer," is derived from the denudation 
of the Nubian sandstones, chiefly brought about by the action of the wind aided by the 
extremes of heat and cold, and by rare falls of rain. The sand-charged wind has 
played a powerful role in modifying the Libyan desert, but the nature and extent of 
its action are questions to be decided by physical geographers. However, from 
a biological standpoint there can be no doubt that it materially influences the 
distribution of plants, and thus also affects animal life. The vast plains swept by 
the winds are practically devoid of plants and animals, and this is markedly the case 
on the plateau of Nubian sandstone, and amid the huge sand-billows 100 to 150 metres 
in height, and the intervening wavelets of the great sand-sea which defied the passage 
of the Eohlfs Expedition. 
The surface of the desert at midday is, as a rule, much hotter than the air which 
becomes heated by the amount of caloric reflected from off the soil, be it sand, stones, 
or locks. The most uniform temperature is met with in such localities as the 
" Sandheim," to the north-west of Dakhel, in the uninterrupted surface of the great 
sand-sea. If a thermometer be sunk into the sand for a depth of from 1 to 2 centi- 
metres the temperature of the surface can be ascertained and compared with that of the 
air ; and from the observations made by Jordan in the Dakhel oasis, in the Eegenfeld, 
and in the " Sandheim " it appears that the temperature of the surface sand is, on 
an average, from l°-3 to l D -8 Cent, higher than that of the atmosphere. In the 
latter locality, on the 1 2th February, the temperature of the air at 8 o'clock a.m. rose 
above that of the sand, but by noon the latter had become heated 2°-4 Cent, in excess 
of the air. On the stony desert the variations of temperature during the day are 
