They become, therefore, vast wastes of dry and barren sands in which no root can find nourishment, and of dreary rocks to which not even a lichen can cling.
The most extensive and remarkable rainless region on the earth is a vast tract extending through the interior and northern part of Africa, and the southwestern part of Asia.

The Red Sea penetrates into this tract from the south, and thus breaks the outline and continuity of its form, without, however, altering, or essentially modifying its character.

It divides it, however, and to the different portions which this division forms, different names have been given.

The Asiatic portion is called Arabia Deserta; the African tract has received the name of Sahara; while between these two, in the neighborhood of Egypt, the barren region is called simply _the desert_.

The whole tract is marked, however, throughout, with one all-pervading character: the absence of vegetable, and, consequently, of animal life, on account of the absence of rain.
The rising of a range of lofty mountains in the center of it, to produce a precipitation of moisture from the air, would probably transform the whole of the vast waste into as verdant, and fertile, and populous a region as any on the globe.
[Illustration: VALLEY OF THE NILE] As it is, there are no such mountains.