THE DESERTS OF AFRICA AND ASIA. 357 



that the ideas which were formerly entertained of the Saharan Desert 

 have at present undergone an entire change. It has been ascertained 

 that those sand-deposits, which completely hide the solid framework of 

 the country, are comparatively local phenomena, and that in the greatest 

 part of the Sahara-Lybian Desert the subjacent strata are perfectly con- 

 spicuous, either by cropping out through the superficial deposits, or by 

 rising as mountains and hills, which almost all belong to the cretaceous 

 formation, and cover an immense tract of this part of Africa. M. Rolland, 

 who has particularly studied the cretaceous formation of the Sahara, 

 speaks ' with astonishment of its extraordinary development, not only in 

 the French Sahara, where it occupies an area equal to that of all France, 

 but also in the whole African Desert, touching the Red Sea on the east 

 and the Atlantic on the west. ' For in all those regions,' says M. Rolland, 

 ' cretaceous strata, containing the same fauna and having the same mine- 

 ralogical composition, are developed on a line of 60 degrees in length, 

 with 3 to 6 degrees in width. No later sediments repose on those rocks, 

 with the exception only of some quaternary deposits, filling up, in the 

 Lybian Desert, the depressions between the cretaceous mountains.' 



The western extremity of this immense mountain range being situated 

 in Marocco, cannot have been as well examined as the part crossing the 

 French Sahara and the Lybian Desert, where the cretaceous formation is no 

 less completely developed than in the northern regions of Algeria — regions 

 which may be considered in this respect as truly classic ground ; for the 

 energetic explorations of M. Ooquand have ascertained there all the known 

 subdivisions of this formation, characterised by fossils more numerous 

 and more various than perhaps in any country of a similar extent, the 

 amount of new ci'etaceous species in northern Algeria being, according to 

 him, no less than 227. 



The regions of the Sahara not occupied by cretaceous mountains and 

 hills consist of large surfaces, more or less horizontal, composed either of 

 loose sands or diluvial (quaternary) deposits. These last seem to have 

 formed gulfs which, after the emergence of the cretaceous masses, re- 

 mained covered by the sea, and were filled up in a compai-atively recent 

 epoch, for they contain shells of mollusks belonging to still living species. 



As for the rocks which underlie the sandy deposits, what we know 

 of them is due to the numerous wells sunk by the French along the 

 northern boundaries of the Sahara, particularly in the province of Con- 

 stantino. The learned engineer, M. Jus, who during twenty years has 

 directed those admirable works, ranges in the pliocene formation the dif- 

 ferent rocks (limestone, sandstone, marls, gypsum, &c.), pierced by the 

 soundings, as well as the impermeable water-bearing clay which forms 

 the bottom of the wells. This clay presents the most astonishing dis- 

 crepancies in its level, being sometimes many hundred feet under the 

 surface of the soil, and sometimes approaching it very closely. So, for in- 

 stance, in the region of Wadi-Rir, two wells named Ain-Kerma and 

 Unel-Thiur, are distant one from the other about forty miles, yet the 

 depth of the first is only 44 feet, and that of the second 321 feet. In 

 the country of Honda, the well named Nemechdib is 10 feet deep, whereas 

 the well Barika, almost close to it, is 117 feet. Again, at Batna and at 

 Biskra, the soundings have been pushed through more than 540 feet, 

 without reaching any subterranean water, so that the works were aban- 



' Com^ptei Rendiis des Seances de VAcad. det Sc, 1879, vol. Ixxviii. p. 778. 



