378 Messrs. Playfair and Letouraeux on the 



along an impermeable bed. The same may be said of the 

 Oued Gghaghar, whose source, never yet visited by Europeans, 

 is in the Touarey country. 



These dry watercourses have all enormous beds with deeply 

 worn banks, and they join the central depression by immense 

 estuaries, wliich prove how great a volume of water they had 

 once discharged as their tribute to the great Lake Tritonis, of 

 which the Chott Melghir and the salt lakes of the Tunisian 

 Sahara are tlie insignificant remains. 



What has drained this great river, and transformed into a 

 series of salt marshes the Lake Tritonis, which, if we can be- 

 lieve Lucan, communicated with the sea ? 



It is probable that this is mainly ownng to a gentle and 

 progressive upheaval of a great part of the Sahara, and partly 

 perhaps to the disappearance of those great forests, once the 

 home of the African elephant. 



AVhatever the cause, the eastern depression has now no 

 running water except on its northern border. But sheets of 

 water, driven from the surface, still exist, in the bowels of the 

 earth, as a vast subterranean sea, the waters of which are 

 strongly impregnated with saline matter. 



From time immemorial artesian wells have existed here, 



and have everywhere spread with their waters life and wealth. 



The water, which in the lowest part of the depression is 



found at a dc]ith of 20 metres, is, at the edges of the basin, 



50, 60, or 100 metres from the smi'ace of the soil. 



Its existence, however, is not only indicated by artesian 

 wells : throughout the whole extent of the Oued Ghir, and 

 even to the south of it, depressions are found full of water, 

 which appear to be as it were the spiracles of the subterranean 

 lake ; they are styled by the natives hahr (sea) ; the French 

 call them goujfres. 



\n the 8ouf the water circulates close to the surface of the 

 soil, enclosed in a sandy substratum which is concealed by a 

 bed, more or less thick, of sulphate of lime, crystallized on the 

 upper surface and anuirphous in the lower part. One has only 

 to penetrate this layer of gy]isum to create a avcU. When it 

 is intended to ])lant a date-grove, the industrious Souafa re- 

 move the entire crust of gypsum and jilant their palms in the 

 aquiferous sand beneath. Their green summits rise above the 

 plain around, thus forming orchards excavated like ants' nests, 

 sometimes 8 metres below the level of the ground. 



This complicated distribution of water in the lower Sahara 

 gives rise to the dilfcrcnt kinds of oases. 



Running streams, dannned bv barrages and distributed in 

 canals, make the river oases (Ziban). 



