368 REPORT— 1882. 



the Sabara are really from the south ; for if not, the winds, instead of 

 carrying the aqueous vapours to the north, would drive them in the 

 opposite direction — to the Sahara — without submitting them to the 

 condensation produced by the Aures Mountains, so that the creation 

 of the large mass of vapours would be of no benefit at all to the 

 country. Unfortunately this most impoi'tant meteorological question is 

 still very far from being decided ; indeed, many scientific men in France 

 admit that at Biskra and Tuggurt the north-west, north-east, and north 

 winds are prevalent. 



As for the statement that the Gulf of Gabes communicated formerly 

 with the Tritonis lacus (El-Fedjedj) and penetrated deeply into the interior 

 of the Sahara, this statement of Commander Roudaire, which has been 

 strongly attacked both in France and Germany, is, as I think to have 

 proved,' perfectly founded and abundantly supported not only by historical 

 but also by geological facts. 



Let us now resume, in a few words, the most prominent features of 

 the geological history of the Sahara- Lybian desert. 



1. The records of this history are very old, for the southern region of 

 the present Sahara was represented during the Devonian period by a 

 certain number of isolated masses of limestone, gneiss, and mica-schist, 

 the limestone containing Devonian fossils. Those masses preserved 

 their insular position through all the succeeding ages, and never sank 

 again tinder the sea. 



2. It was during the cretaceous epoch that a large portion of the 

 present Sahara was upheaved in the shape of variously ramified masses, 

 so that the sea of the following geological periods could penetrate into 

 their interior, forming numerous gulfs and bays. 



3. The Sahara was represented down to the quaternary epoch chiefly 

 by those cretaceous masses which since their upheaval have never been 

 covered by the sea. During the quaternary period, among the gulfs 

 which washed the shores of the cretaceous land, the largest occupied the 

 present country of Igharghar ; the northern extremity of that gulf reached 

 the place of Biskra, and the southern the cretaceous plateaux of Tademayt 

 and Tinghert ; the town Wargla occupying almost the central part of the 

 gulf. As the last was entirely secluded from any communication with 

 the sea from the north, the littoral part of Algeria having been upraised 

 long before, and consisting then, as now, of more or less high mountains, 

 this large quaternary gulf could not find any other way into the interior 

 of the Saharan cretaceous continent than through the present Gulf of 

 Gabes ; and the narrow strip of diluvial deposits which, surrounded by 

 cretaceous rocks, extends from Gabes to the salt lake of Bl-Fedjedj 

 (Tritonis lacus) proves that here was really the entrance of the quaternary 

 gulf. This geological fact is important with reference to the question, 

 so long discussed, of the ancient communication between the lake and 

 the sea ; it confirms the hypothesis of Commander Roudaire, and I am 

 not aware that this argument, which I consider as the strongest of 

 all, has ever been urged in his favour. The upheaval of the quaternary 

 large gulf (and of many other smaller ones) was the last marine phase 

 which the Sahara underwent. 



4. Once entirely raised up in all its parts, the Sahara had still to 

 undergo a subaerial operation which consisted of the formation and 



' Loe. eit. p. 641. 



