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 



