The soil is generally covered with a thick stratum of sand; but in several places it also exhibits great walls of rock, and in others masses of rolled or shattered pebbles. The subsoil is nearly always of a gypseous or calcareous nature, rarely clayey; wherever it is porous and permeable, it is impregnated with salt, which rises to the surface, or is held in solution in the subterranean basins of water, the thermal springs, the ponds, and the lakes. The saline efflorescences of the deserts of Persia and Oriental Asia not only suffice for the wants of the inhabitants, but supply the great Asiatic caravans with their principal article of exportation. The atmosphere of the Deserts is not less dry than their sands and rocks. The sky wears a perennial azure, more or less veiled in haze, or rather spotted with a few clouds. Johnstone represents them, in his admirable "Physical Atlas," by two white unequal 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, condemned 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.