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 eleva- 

 tion 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 success (Comptes Rendus de TAcad. des 

 Sciences, t. xx. 1845, 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 Berbe>ie, t. i. p. 169 j Rennell, Africa, Append, p. 

 Ixxxv). 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 

 Yorkommen des Kochsalzes auf der Oberflache der Erde, 1846, s. 

 497, 648, and 741.) The trade in salt with Soudan, and the possi- 

 bility 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 sulF abassamento di temperatura durante le notti placide 

 e serene, 1847, p. 55) ascribes this cold > produced doubtless by the. 



