Here, horizontal strata of mud and gypsum, or sulphate of lime, are deposited upon the shores, as it were, of the great Sandy Sea. The gypsum reposing on the mud is composed of plates in such close juxtaposition as to resemble an artificial pavement. "It covers the surface of vast plateaux which have not been encroached upon by the waters; whether those waters were marine currents at the epoch when the Sahara was a vast sea, or diluvian torrents which descended from the mountains after their elevation, little matters; the gypsum, produced by the violent evaporation of the Saharan sea, has withstood their operation, and composes the plateaux of which we are speaking. Their surface is so smooth, that vehicles might roll for leagues upon this natural pavement, which echoes like a vault under the horses' hoofs. A plateau of this kind, the small Desert of Mourad, extends from Biskra to the banks of the great salt lake called _Chott Mebrir_ by the Arabs. The gypseous surface is not everywhere exposed: most frequently it is covered by a layer of small rounded pebbles, nearly all quartzose, exhibiting the greatest variety of tints, from the purest white to the most vivid red; they are mixed with black calcareous stones split on the surface. Whence came these pebbles, which have evidently been 'rolled' by the waters? We know not. They are the mysterious witnesses of those grand diluvian torrents which have left the traces of their passage over the surface of the whole earth, though the geologist cannot always discover the mountains or rocks that furnished the materials of this diluvium."[54]