DESERTS AND THEIR INHABITANTS 127 
eye can reach, and even much farther; but sooner or 
later ridges and bands of pebbles, or of solid rock, will 
be met with cropping up among the sand, while fre- 
quently, as in the Libyan Desert, there are mountain 
ranges rising to a height of several thousand feet above 
the level of the plain. And it is these exposed rocks 
which form the source whence the sand was, and still is, 
derived. These mountains naturally attract what moisture 
may remain in the air, and in their valleys are found a 
more or less luxuriant vegetation. Oases, too, where the 
soil is more or less clayey, occur in most deserts; and it 
is in such spots that animal and vegetable life attains 
the maximum development possible in the heart of the 
desert. 
In the most arid and typical part of the Libyan Desert 
the sand is blown into large dunes, which are frequently 
flat-topped, and show horizontal bands of imperfectly con- 
solidated rock ; and between these are open valleys, partly 
covered with sand and partly strewn with blocks of rock 
polished and scored by the sand-blast. In such sand- 
wastes the traveller may journey for days without seeing 
signs of vegetation or hearing the call of a bird or the 
hum of an insect’s wing. But even in many of such dis- 
tricts it is a mistake to suppose that vegetable and animal 
life is entirely absent throughout the year. In the western 
Sahara, for instance, showers generally moisten the ground 
two or three times a year; and after each of these a 
short-lived vegetation springs suddenly up, and if no other 
form of animal life is observable, at least a few passing 
birds may be noticed. 
Among the most important and extensive deserts of the 
world we have first the great Sahara, with an approximate 
area of sixteen thousand square miles, nearly connected 
