94 THE TROnCAL WORLD. 



ful districts penetrate far into its bosom, like large peninsulas 

 or promontories jutting into the sea. 



Until within the last few years, it was supposed to be a low 

 plain, partly situated even below the level of the ocean ; but 

 the journeys of Barth, Overweg, and Vogel have proved it, on 

 the contrary, to be a high table-land, rising 1,000 or 2,000 feet 

 above the sea. Nor is it the uniform sand-plain which former 

 descriptions led one to imagine ; for it is frequently traversed 

 by chains of hills, as desolate and wild as the expanse from 

 which they emerge. But the plains also have a different 

 character in various parts : sometimes over a vast extent of 

 country the ground is strewed with blocks of stone or small 

 boulders, no less fatiguing to the traveller than the loose drift 

 sand, which, particularly in its western part (most likely in 

 consequence of the prevailing east winds), covers the dreary 

 waste of the Sahara. Often also the plain is rent by deep 

 chasms, or hollowed into vast basins. In the former, par- 

 ticularly on the northern limits of the desert, the rain de- 

 scending from the gulleys of the Atlas, sometimes forms streams, 

 which are soon swallowed up by the thirsty sands, or dried by 

 the burning sunbeams. In spite of this short duration, the 

 sudden appearance of these streams is not unfrequently the 

 cause of serious distress to the oases which border the northern 

 limits of the desert. 



For this reason, as soon as the Atlas veils itself with clouds, 

 horsemen from the oases of the Beni-Mzab are sent at full 

 speed into the mountains. They form a chain as they proceed, 

 and announce, by the firing of their rifles, the approach of the 

 waters. The inhabitants of the oases instantly hurry to their 

 gardens to convey their agricultural implements to a place of 

 safety. A rushing sound is heard ; in a short time the ground 

 is inundated; and the little village seems suddenly as if by 

 magic transported to the banks of a lake, from which the 

 green tufts of the palm-trees emerge like islands. But this 

 singular spectacle soon passes away like the fantastic visions of 

 the mirage. 



The deeper basins of the Sahara are frequently of great 

 extent, and sometimes contain large deposits of salt. Wherever 

 perennial springs rise from the earth, or wherever it has been 

 possible to collect water in artificial wells, green oases, often 



