ANNOTATIONS AND ADDITIONS. 117 



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. 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 Gisements de 

 Muriate de Soude en Algerie, p. 28-41 ; Karsten iiber das 

 Vorkommen des Kochsalzes auf der Oberflache 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 subsi- 

 dences 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), 



