90 THE DATE PALM. 



alkaline soil and the high salt content of the irrigation water preclude other profitable 

 cultures. 



The artesian water at Ourlana, as elsewhere in the Oued Rirh, is confined below a 

 compact stratum of pudding stone which lies some 175 to 250 feet below the surface. 

 Below this pudding stone is a layer of loose quartz sand, more or less mixed with 

 pebbles, which contains an abundant supply of water under sufficient pressure to 

 give a ready flow, frequently to the tops of the lower hillocks in the plain. 



The French engineers Jus and Holland, who have studied exhaustively the ques- 

 tion of the origin of the water supply of the Oued Rirh, agree in believing that the 

 original source is in the Atlas Mountains to the north, where the heavy rainfall and 

 snowfall (some 5^ feet annually) is absorbed by the upturned cretaceous strata and 

 conducted in these strata to the south, where it first reappears in the great springs 

 of the Zab region along the northern border of the Sahara. The water of these 

 springs and of many others which are believed to exist, though the water never 

 reaches the surface, soaks into the pervious strata of the Saharan formation and flows 

 southward toward the Oued Rirh country, o becoming imprisoned beneath an imper- 

 vious pudding-stone layer, except where natural openings exist and allow the water 

 to reach the surface & or where artesian wells have been put down. 



On the 1st of October, 1885, Oued Rirh contained 114 flowing wells put down by 

 the French and tubed with iron, 492 flowing wells constructed by the natives, and 22 

 natural springs, which were used for irrigating. The total supply of water furnished 

 by these wells and springs was 253,698 liters per minute, or 4 cubic meters (over 

 1,050 gallons) per second, having an average temperature of 25.1 C. The largest 

 flowing well is No. 4, at Sidi Amran, which was put down in 1884. It flows 6,000 

 liters per minute. 



The beneficial effect of French occupation has been very marked in the Oued 

 Rirh, where in 1856 there were 33 oases, all in a state of decay. They were nourished 

 by 58,000 liters of water per minute and contained only 136,000 date palms, for the 

 most part old and yielding but little fruit. Thirty years later, thanks to the artesian 

 wells put down by the French, the total yield of water had been raised to more than 

 253,000 liters per minute; all the old oases had been put in a flourishing condition 

 and new ones had been created, so that in 1885 there were 43 oases containing 

 509,375 date palms in full bearing, and about 138,000 young palms from 1 to 7 years 

 old. The native population had more than doubled during this time and the value 

 of the oases had increased more than fivefold. 



The oasis of Ourlana, of which a special study was made, is located nearly 100 

 miles south of Biskra, at an altitude of 113 feet above sea level, and is in the most 

 fertile part of the Oued Rirh. Within a radius of 10 miles of Ourlana there are no 

 fewer than 15 oases irrigated from 32 artesian wells (30 of which are modern tubed 

 wells of French construction) and from 16 springs " behour." These 15 oases con- 

 tained in 1882 over 182,000 date palms, and nearly half of these oases have been 

 much enlarged since then, so that they now, doubtless, contain over 200,000 date 

 palms. 



The water of these springs of the western Zab contains on the average 0.203 per 

 cent of dissolved salts. Those springs which reach the surface indirectly after filter- 

 ing a distance through the superficial strata yield water showing a larger per cent of 

 alkali about 0.268 per cent on the average and by the time the water has^soaked its 

 way through the Saharan strata and flowed to the Oued Rirh country, the alkali con- 

 tent has risen to an average of 0.487 per cent. 



& Forming the springs and small lagoons called " behour" and "chria" by the 

 Arabs. 



