"If the Desert of the Plateaux," he says,[57] "be the image of a sea suddenly fixed during a level calm, the Desert of Sand represents to us a sea which may have been solidified during a violent tempest. The Dunes, or sand-hills, like waves, rise one behind another even to the limits of the horizon, separated by narrow valleys which represent the depressions of the great billows of the ocean, all whose various aspects they simulate. Sometimes they narrow themselves into keen-edged crests, or shoot upwards in pyramids, or swell into cylindrical domes. Seen from a distance, these Dunes also remind us at times of the appearances of the _névé_ (or granulated snow) in the amphitheatres and on the ridges which lie contiguous to the loftiest Alpine summits. Their colours still further enhance the illusion. Moulded by the winds, the burning sands of the desert assume the same forms as the _névés_ of the glaciers." Whoever has seen the Dunes on the coast of Norfolk, or more particularly in Gascony, may gain a very accurate conception of the Desert. The only notable differences are in the extent, which here seems infinite, like that of ocean; the purity of the heaven, which is seldom sullied by a cloud; and the colour, which is of a soft, intense blue. The nature of the soil is the same; it is a very fine, shifting, silicious sand, white sometimes, like that of Fontainebleau, and sometimes reddened by the presence of oxide of iron. In the Sahara this sand gathers in veritable Dunes, hillocks which the wind upheaves, displaces, and transforms from one day to another. Only the _lettes_, or valleys, which in our Dunes receive the pluvial waters and preserve a sufficient amount of fertility, are here just moistened by rare saline infiltrations, and almost always remain in a condition of absolute sterility. Nevertheless, in some localities, the presence of gypsum gives the sand a certain fixity, which permits a small number of plants to germinate and develop themselves. This gypsum is never found but in the valleys, and never in tabular masses, as on the plateaux, but only in crystals of various forms, penetrated by silica. "You pick up a pebble," says M. Martins, "and find it to be a crystal." The villages are surrounded by crenellated ramparts built of crystals; the houses which compose these villages are constructed of the same materials; and very weird and splendid is the scene presented by these edifices with their sun-illuminated walls. Notwithstanding their small dimensions and mean architecture, when thus lit up in glorious radiance, they seem to realize the wonders told in fairy tales of the enchanted palaces of the genii! CHAPTER IV. PHENOMENA OF THE DESERT.