The riches of the desert in rock-salt, and the fact of rock-salt having been used in building, have been known since the time of Herodotus. The salt zone of the Sahara (zone salifère du désert), is the southernmost of three zones, stretching across Northern Africa from south-west to north-east, and believed to be connected with the beds or deposits of rock-salt of Sicily and Palestine, described by Friedrich Hoffman and by Robinson. (Fournel, sur les Gisements de Muriate de Soude en Algérie, p. 28-41; Karsten über das Vorkommen des Kochsalzes auf der Oberfläche der Erde, 1846, S. 497, 648, and 741.) The trade in salt with Soudan, and the possibility of cultivating dates in the Oases, formed by depressions caused probably by falls or subsidences of the earth in the gypsum beds of the tertiary cretaceous or keuper promotions, have alike contributed to enliven the Desert, at least to some extent, by human intercourse. The high temperature of the air, which makes the day’s march so oppressive, renders the coldness of the nights, (of which Denham complained so often in the African Desert, and Sir Alexander Burnes in the Asiatic), so much the more striking. Melloni, (Memoria sull’ abassamento di temperatura durante le notti placide e serene, 1847, p. 55), ascribes this cold, produced doubtless by the radiation from the ground, less to the great purity and serenity of the sky, (irrigiamento calorifico per la grande serenità di cielo nell’ immensa e deserta pianura dell’ Africa centrale), than to the profound calm, the nightly absence of all movement in the atmosphere. (Consult also, respecting African meteorology, Aimé in the Exploration de l’Algérie, Physique génerale, T. ii., 1846, p. 147.)