96 ORIGIN OF THE DESERTS. 



bands, characterised as " Rainless Districts." Of these the larger 

 occupies all the northern region of Africa, and the greater portion of 

 Arabia, Syria, Persia, and Beloochistan, embracing an area of 80 of 

 longitude over 17 of latitude. The other extends over the table- 

 lands of Thibet and the Gobi. It is in form an irregular ellipsis, 

 obliquely inclined from south-west to north-east. Its length is about 

 1100 leagues ; its width, 450. From the former it is only separated 

 by a narrow belt. In the region marked by these two species rain is 

 an extraordinary phenomenon ; several years will pass without the 

 clouds shedding a single drop of water. This permanent, and nearly 

 absolute, aridity, establishes a very marked difference between the 

 Deserts properly so called, and the Landes, Steppes, and Prairies, con- 

 demned as these are during the hot season to a deadly dryness, but 

 in winter inundated with rain or covered with snow ; and in spring 

 converted into immense marshes, where an exuberant vegetation 

 makes its appearance, frequently capable of resisting the* action of 

 the summer sun and the withering winds. 



In the Rainless Districts vegetation is a nullity ; it becomes 

 reduced to a very small number of saline plants and dwarf bushes, 

 nourished by the brackish waters which, the soil conceals. Finally, 

 the desert region may not only be compared to a sea in its aspect and 

 immensity, but it is a true sea, or at least the bed of an ancient sea, 

 which formerly communicated, and, perhaps, was confounded with 

 the Mediterranean, and whose drying up, though still incomplete, 

 took place at a recent geological epoch. We may reasonably conclude 

 that, owing to a series of gradual upheavals, this sea was at first 

 broken up into vast lagoons; that most of these successively disap- 

 peared, but not without leaving some certain evidences of the 

 primitive submersion of the continent. " If we might hazard a 

 conjecture," says a recent writer,* "it would be that the same 

 convulsions and upheavals which at the close of the tertiary epoch 

 indented the southern coasts of Europe, at the same time drained the 

 ocean which hitherto had rolled over the plains of the Sahara, and 

 * Rev. H. B. Tristram, " The Great Sahara," p. 360. 



