llydrographlcal St/sUm of Aljeria. 377 



far as ( Joloa, after wliicli it turns towards the north along the 

 plateau of the lienl M'Zab. 



Tlie Uued (ihirj the Souf, N'gou^a, and the greater ])art of 

 the Ziban have a less elevation than 100 metres; Biskra and 

 Ouargla are hardly higher, while the Chott Melghir and part 

 of the Oued ( Jiiir are below the level of the sea. 



The Chott ^lelgiiir, whieii occupies the bottom of the de- 

 pression, is sunk in the gypseous soil, and forms a sheet of 

 water Salter than the sea. It is of no great depth, and in 

 summer, owing to evaporation, it is partly covered with a 

 thick and brilliant coatnig of crystals; so that the eye can 

 scarcely distinguish where the salt terminates and the water 

 begins. The bottom is an abyss of black and viscous mud, 

 emitting an odour of garlic, due possibly to the presence of 

 bromides. Nevertheless it is not without veins of more solid 

 ground, forming natural causeways, on which the people of 

 the country do not hesitate to trust themselves. 



The rivers of the Aurasic system, essentially torrential in 

 the mountains when confined within steep and narrow gorges, 

 serve to irrigate the oases, where their waters are retained and 

 absorbed by means of dams. That which percolates through these 

 and forms streams lower down their courses is again absorbed 

 by the Sakias or canals of irrigation. It is only after the co- 

 pious rains of winter, and the melting of the snow in the moun- 

 tains, that their beds are tilled and their waters reach the Chott. 



The smaller springs and streams which have their origin at 

 the foot of the mountains are always absorbed by the oases 

 or by the cereals which the inhabitants of the Ziban cultivate 

 wherever a thread of the })recious li(juid is found. 



On the west the Cued Djedi joins the Chott ; it rises on the 

 southern slopes of Jebel Amour, fertilizes the oasis of El- 

 Aghouat, and, skirting the plateaux of the higher Sahara, tra- 

 verses the lower Sahara from west to east. It is ordy in the 

 upper part of its course that this Oued is a permanent stream; 

 lower down its Avater is to a great extent dried up by the solar 

 rays or absorbed by barrages ; the rest disappears m the per- 

 meable strata, or filters through the sand and flows along the 

 clayey bottom which underlies it. Like the rivers of the 

 Aurl's, but even more rarely than these, its course is only 

 filled by the melting of the snows, or dm-ing the heavy rains 

 on the High Plateaux. 



The foregoing remarks apply equally to the other rivers 

 which, rising in the eastern part of the higher Sahara, flow 

 towards the region of N'gou(ja. 



In the south the Oued 3Iia presents always the appearance 

 of a dry watercourse, below the sand of which water flows 



