"At fifteen parasangs from Teheran,[48] commences the _Salt Desert_, which, from east to west, extends to the very frontiers of India. This immense basin, eastward, has no other limits than the horizon; to the west, to the north, to the south, it is bounded by hills of sand which completely represent the Dunes of France. The soil, of a fawn-coloured yellow, is composed of clay and sand, exactly resembling the mud which occupies the bottom of a dried-up basin. It is said that at many points a man on horseback will disappear without his body being ever again discovered.[49] I have seen one of these places, near Sivas; the soil is everywhere impregnated with salt mingled with nitre, which crystallizes on the surface. For the rest, if you dig two or three inches deep, you find water, though very brackish in quality. The general opinion is that the desert was once occupied by a sea, which suddenly disappeared on the night that Mohammed was born. And it seems to me that there is no reason to doubt this sudden disappearance, since even in our own days, and only a few years ago, the salt lake of Ourmiah (Urumiyeh), in the province of Azerbaïdjan, vanished completely for twenty-four hours; it is true that the waters emerged again from their subterranean basin. I think it almost absolutely demonstrated, from inspection of these localities, that at a remote epoch this sea communicated with the Caspian, and formed one united basin of water. I am not sure but that in the south it also communicated with the Indian Sea, for I have not travelled in that direction. The apparition of the Elburz chain has cloven the two basins, and the sea, receiving only inconsiderable streams, insensibly receded, until the day when it was wholly dried up, leaving only two lakes: one, the lake of Sivas, which disappeared in the seventh century; the other, the lake of Seistan, which is still extant, and receives several of the important rivers of Afghanistan. At all events, the great sea itself had disappeared some generations prior to the epoch of Alexander. "The great humidity of the soil," adds Doctor Cloquet, "struck me vividly. Does not this humidity appear to indicate the presence of vast subterranean sheets of water, which sweat, so to speak (_transsuderaient_), through the porosities of the earth?"