Ilifdiogrujihical System of Algeria. ."379 



Water abstirboil by j)enncable strata con.stitutes (I) the 

 oases with onHiiary \\v\\a (( )ulad DjcUal cV:c.), (2) oases with 

 artesian wells (Tii^';^art, N'gouya, Ouarghi, &C.), (3) the ex- 

 cavated oases (Soutj. 



Sometimes two systems are tomul united in the same phioe;. 



Tlie hii^her Sahara extends iVuin the western Uniits ot" the 

 lower one to within the t'rontiers ot" ^loroeeo ; to the south it 

 reaches beyond Cxoleah, and on the north it is bounded by the 

 last chains of the IIiy:h Plateaux. 



It is principally composed ot" rocky steppes, only the depres- 

 sions between which are tilled with sand. 



Towards the east deset luls almost ])erpendieuhirly from north 

 to south a large promontory which rises below El-A,u'houat to 

 nearly i)UOO metres, and sinks gradually towards Guleali, sc- 

 j)arated from the plateau of Tademait by a sort of isthmus 

 4(,)0 metres high. It is in this plateau that the Oued Mia and 

 its affluents arise, which, in French territory at least, contain 

 only slight intiltrations of water under a sandy bed. 



In the centre the rocky plateaux fall rather abruptly as far 

 as the zone of the Areg^ or country of sand-hills, occupying 

 a depression the bottom of which is about 400 metres above 

 the sea. 



Finally, towards the extreme west, where the chains of the 

 High Plateaux descend lower, the Saharan plateaux also de- 

 scend further south, leaving between them numerous valleys. 



In each of these three divisions the water-system is ditlerent. 

 The eastern promontory, the crests of which are directed to- 

 wards the west, sends out no spurs towards the zone of the 

 Areg ; but it is fuiTOwed towards the east by immense ravines, 

 of which the principal bear the names of Oued Ensa and 

 Oued M'Zab. llaiu seldom falls in the lower part ; and the 

 southern crevasses are almost all deep ravines, without Avater 

 or vegetation. Even in the upper part it is only during severe 

 storms, and when more than usually abundant snow has 

 melted on the High Plateaux, that the waters pouring on the 

 Sahara unite in the deep defiles, forming a mighty wave, 

 which during twenty-four or forty-eight hours precipitates 

 itself into the estuaries of the lower Sahara. When this tor- 

 rent has passed, nothing remains in its dry bed save a few 

 pools where the gazelle drinks, and a slight subterranean 

 percolation which serves to supply the few wells at which the 

 caravans draw Avater. 



These periodical inundations are quite inadequate to supply 

 the Beni M'Zab, who have established gardens in the very 

 beds of the great ravines Avhich dominate their seven cities. 

 In vain they treasure up a store in their reservoirs ; they are 



