306 



THE GEOLOGIST. 



then rests on the impermeable rock, draining through a very thin stratnrn of 

 gravel or sand into any depressions, whence it is raised by artesian wells, and 

 creates an oasis. 



Prom the Sebaa Eons to Laghonat, all these langes appear to belong to the 

 lower chalk formation. Limestone predominates, and forms the ridges of the 

 Sahari, SenallDa, and Djellal mountains. It is of saccharoid structure, and of 

 a variable colour, generally greyish white. In many of the plains there is 

 sandstone, sometimes hard, and at otlier times so soft as to yield to the pressure 

 of the fingers. This sandstone encloses nodules of flint of various colours 

 and semi-transparent. By dis-aggregation they become detached from the softer 

 medium in which they were embedded. As the wind sweeps the sand they 

 form shingly beaches of pebbles, many of them of a pretty chalcedony, which is 

 exported in some quantity to Paris. 



The upper deposit of Innestone is marked by regular beds of gypsum of vast 

 extent, which are found in every district of the Sahara, but never in the 

 secondary formation of the Atlas region. 



South of Laghonat, the furthest Erench outport, we came upon a shallow 

 alluvial deposit of the very latest tertiary and diluvian formation. Near the 

 mountains this is often composed of rolled pebbles in a limestone matrix. On 

 the plains it is a white calcareous rock, a sort of crust very hard at the sur- 

 face, but soft and friable below, where it is mixed with green or grey clay, and 

 encloses many crystals of gypsum. 



The diluvian formation may be traced more or less distinctly, I believe, 

 between all the ranges, even as far north as the Zahrez, near Djelfa. 



I was particularly struck by the fact that several of my fossil shells from these 

 superficial deposits proved specifically identical with fresh-water tertiary fossils 

 from tlie region of the Black Sea. May not further research perhaps reveal 

 that at no very distant geologic epoch a vast chain of fresh-water lakes, similar 

 to those of ISorth America at the present day, extended from the plateaux of 

 the western Sahara as far as the neighbourhood of the Caspian ? 



The basin of the M'zab country further still to the south supplied me only 

 witli a few fossils, apparently miocene. 



In turning from the M'zab southwards to Waregla, and thence north-east 

 towards Tuggurt and the Gulf of Cabes, the geological system appears to be 

 the same, but with fewer distinct little basins, and with more extensive dilu- 

 vian deposits. 



As far as we could trace them, the basins are generally horizontal up to 

 Biskra in tlie north, and Gufza in the east, or very slightly inclined, consisting 

 of alternating beds of greensand (?), gypsum, aiid clay. These beds extend 

 alinost witliout interruption, or with very slight depressions, from latitude 

 lliirty -one degrees north to thirty-five degrees north, and from longitude five 

 degrees cast to nine degrees east. 



Tli(! most interesting portion of this district is the Wed H'hir, a long line of 

 (U>prcssion slo})iiig from the Tonareg desert, latitude thirty degrees north, and 

 h)iigiliidc live degrees cast (circitu), with its surface occasionally moistened by 

 sail lakes, hnl without any springs of fresh water, yet affording at intervals 

 throughoui, its whole extent a never-failing supply of sweet water, through 

 arU>si;m wrlls ixnu l rat iiig the upper limestone. An immense population is 

 suppoiiiMl hy this \\ ctl K'hir, which is for many days' journey one continuous 

 line of oasrs, sueli as Ivl Maricr, Tamerna, Tuggurt," Temafin, and after a 

 further inlcrval, in which its traces arc lost, it reappears in the oases of 

 N'Gonssa ami ^Vangla, and gradually is lost in the highlands of the south. 

 But it is |)rnhai)U' lhal c^vcu hcvc the subterranean course of the water can be 

 1 raced, and t hat the Tonareg owe ihcir means of subsistence to theii* knowledge 

 ol wells on this line. 



