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Show ANNOTATIONS AND ADDITIONS. 105 ferent plane-a. north-south one. Ascending uninterruptedly from Constantine, at an elevation of 332 toises (2122 Eng. feet), the culminating point is found between Batnah and Tizur, at an elevation of only 560 toises (3580 Eng. feet). In the part of the Desert situated between Biscara and Tuggurt, Fournel has had a series of Artesian wells dug with succes (Comptes Rendus de l' Acad. des Sciences, t. xx. 1 45, pp. 170, 882, and 1305). We learn from the old accounts of Shaw, that the inhabitants of the country knew of a subterranean supply of water, and relate fabulous tales of a "sea under the earth (bahr toht el-erd)." Fresh waters flowing between clay and marl strata of the old cretaceous and other sedimentary deposits, under the action of hydrostatic pressure form gushing fountains when the strata are pierced (Shaw, Voyages dans plusieurs parties de la Berberie, t. i. p. 169; Rennell, Africa, Append. p. lxxxv). That fresh water in this part of the world should often be found .near beds of rock salt, need not surprise geologists acquainted with mines, since Europe offers many analogous phenomena. 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 salifere du desert) 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 Gisemens de Muriate de Soude en Algerie, pp. 28-41; Karsten iiber das Vorkommen des Kochsalzes auf der Oberftache 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 t.he more striking. Melloni (Memoria sull' abassamento di temperatura durante le notti placide e serene, 184 7, p. 55) ascribes this cold, produced doubtless by tho |