380 Messrs. Playfair and Lctouineux on the 



obliged to have recourse to deep wells cut in the rock, which 

 collect the infiltration of water in the calcareous strata. 



Above the promontory it is only El-Aghouat and Ain 

 Madhi, situated in a depression at the foot of the mountains, 

 that can utilize almost at all seasons of the year, by means of 

 barrages, the upper waters of the Oued Djedi,- which flow from 

 east to west. 



In the middle, Brezina and several oases placed at the very 

 foot of the mountain-range can also irrigate their date-groves 

 with running water ; but further south the water flowing 

 along the rocky plateaux encounters the mo\nng sands of the 

 Areg, which arrest its course and cause pools or marshes 

 (Dliaya), neither usually very large or very deep. These little 

 Chotts present the same phenomena as the greater depressions 

 in the lower Sahara, their ancient banks, now quite dry, 

 attesting a very marked decrease in the volume of their 

 waters. 



Towards the east, on the other hand, where the mountains 

 in the plateaux rise to a greater height than 900 metres, and 

 present a vast surface, the ravines are the bed of veritable 

 rivers, which render abundant imgation possible, and, uniting 

 in two principal streams, form the Oued Mersaoud, Avhich 

 descends southwards to an unknown distance. 



8uch is the upper Algerian Sahara, of which the greatest 

 de])rfSsion does not descend to within 400 metres of the sea, 

 while in the lower one there is not a single point attaining 

 that altitude. In the one the plateau is the prevailing featiu"e, 

 in the other the depression ; here rocks abomid, there they are 

 entirely absent. As to moving sand, which the Arabs com- 

 pare to a net, it occupies a sufficiently extensive zone in both 

 regions ; but still it does not cover one third part of the Alge- 

 rian Sahara. 



II. Distribution of Species. 



The ichthyology of Algeria is yet imperfectly known ; and 

 future discoveries will probably augment the comparatively 

 small number of twenty-one species, which our researches 

 have established in the fresh and brackish waters (yi the 

 colony. 



The Tell, as might be imagined, is the richest region : 

 there sixteen species, or three fourths of the total number, are 

 found. Of this number only two are common to the three 

 regions, Barhua callensh and^l ngulUo vulgaris. The Lcuciscua 

 mllcnsls is common to the Tell and the lligli Phitcaux ; ami 

 the ('//jJiinoJon raldrifdnus inhabits iMiiially the brackish 



