ILLUSTRATIONS (16). THE SAHARA. 91 



the radiant heat of a stratum of chalk, inclined at an angle 

 of 65 towards the south, and which is full of the shells of 

 Inoceramus*. "Arrived at Biscara (Biskra)," says Fournel, 

 "an indefinite horizon, like that of the sea, lay spread 

 before us." Between Biscara and Sidi Ocba the land is 

 only 243 feet above the sea's level. The inclination increases 

 considerably towards the south. In another workf, where I 

 have brought together all the points that refer to the depres- 

 sion of some portions of continents below the level of the sea, I 

 have already noticed that, according toLe Pere, the bitter lakes 

 (lacs amers) on the isthmus of Suez, when they have but little 

 water, and, according to General Andreossy, the Natron lakes 

 of Fayoum, are also lower than the level of the Mediterranean. 



Among other manuscript notices of M. Fournel, I possess a 

 geognostic vertical profile, with all the inflexions and inclina- 

 tions of the strata, representing the surface the whole way 

 from the coast near Philippeville to a spot near the Oasis of 

 Biscara in the Desert of Sahara. The direction of the line 

 on which the barometric measurements were taken is south 

 20 west; but the points of elevation determined are pro- 

 jected, as in my Mexican profiles, on a different plane, one 

 from N. to S. Ascending uninterruptedly from Constantine, 

 whose elevation is 2123 feet, the highest point is found be- 

 tween Batnah and Tizur, at only 3581 feet. In the part of 

 the desert which lies between Biscara and Tuggurt, Fournel 

 has succeeded in digging a series of artesian wells J. We 

 learn from the old accounts of Shaw, that the inhabitants of 

 the country were acquainted with a subterranean supply of 

 water, and related fabulous tales of a " sea under the earth 

 (bahr tohl el-erd)." Fresh waters, which flow between clay 

 and marl strata of the old chalk and other sedimentary for- 

 mations, under the action of hydrostatic pressure, form gushing 

 fountains when the strata are pierced. The phenomenon oi 

 fresh water being often found near beds of rock salt, need not 

 surprise the geognosist, acquainted with mining operations, 

 since Europe offers many analogous phenomena. 



* Fournel, Sur les Gisemens de Muriate de Soude en Algerie, p. 6, 

 in the Annales des Mines, 4me serie, t. ix. 1846, p. 546. 



t Asie centrale, t. ii. p. 320. 



t Comptes rendus de I'Academie des Sciences, t. xx. 1845, pp. 170, 

 882, 1305. 



See Shaw, Voyages dans plusieur* parties de la Berberie, t. i. p. 

 169, and Rennel, Africa, Append, p. Ixxxv. 



