306 
TlIK (GEOLOGIST. 
then rests on the impcnncablc rock, draining tlirough a very thin straium of 
gravel or sand into any depressions, whence it is raised by artesian wells, and 
creates an oasis. 
From the Scbaa Hons to Laglioaat, all tlicse langcs appear to belong to the 
lower chalk toniiation. Limestone predominates, and forms tlie ridges of the 
Sahari, Senalba, and Djellal mountains. It is of saeciiaroid structure, and of 
a variable colour, gencn-ally greyish white. In many of the plains tliere is 
sandstone, sometimes iiard, and at other times so soft as to yield to the pressure 
of the fingers. This sandstone encloses nodules of tlint of various colours 
and semi-t rauspareut. By dis-aggregation they become detached from the softer 
medium in which tiicy were enibeddcd. As tlie wind sweeps the sand they 
form shiii^'ly beaches of pebbles, many of them of a pretty chalcedony, wliich is 
exported in some quantity to Paris. 
The upper deposit of hmestone 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 furtlicst French 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. 
1 was particularly struck by the fact that several of my fossil shells from these 
sujierficial deposits proved spccilicaUy identical with fresh-water tertiary fossils 
from the 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 Korth America at the present day, extended from the plateaux of 
the western Saliara as far as tlie neighbourhood of the Caspian ? 
The basin of the M'zab country further still to tlie 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 w'ith fewer distinct little basins, and with more extensive dilu- 
vian deposits. 
As far as wc could trace them, the basins are generally horizontal up to 
Biskra in the nortii, and Gufza in the east, or very slightly inclined, consisting 
of alternating beds of greensand (?), gypsum, and clay. These beds extend 
almost without interniption, or with very slight depressions, from latitude 
thirty-one degrees north to thirty-live degrees north, and from longitude five 
degrees cast to nine degrees cast. 
The most interesting portion of this district is the Wed R'hir, a long line of 
depression sloping from the Touareg desert, latitude thirty degrees north, and 
longitude live degrees east (eircitu), with its surface occasionally moistened by 
salt lakes, but without any springs of fresh water, yet affording at intervals 
throughout its whole extent a never-failing supply of sweet water, through 
artesian wells penetrating the upper limestone. An immense population is 
supported by this Wed ll'liir, which is for many days' journey one continuous 
Ihie of oases, such as El JMarier, Tamerna, Tuggurt, Temafin, and after a 
further interval, in which its traces are lost, it reappears in the oases of 
N'Gonssa and Wangla, and gradually is lost in the highlands of the south. 
But it is ))robable that even here the subterranean course of the water can be 
traced, and that the Tonareg owe their means of subsistence to their knowledge 
of wells on this line. 
