rowed by an infinite number of little ravines, of which the principal are the Wed Itel and the Wed Retem.

It is difficult to trace the precise limits of the Chott Melr❜hir, but a depression of the average breadth of thirty miles extends from El Marier in the north of the Wed R'hir as far as the Gulf of Cabes.

APPENDIX II.

ON THE GEOLOGICAL SYSTEM OF THE CENTRAL SAHARA OF ALGERIA.

THOUGH the Atlas generally is of the Tertiary epoch, the whole country between it and Laghouat is the Secondary, rocky, and with mountain ranges running from N.E. to S.W. There are, however, some striking exceptions, which may be illustrated by the Guern el Meila, a day's ride to the N.W. of Laghouat. To understand this formation we must conceive several elliptical basins of diminishing size piled one upon another. The lowest and largest rests upon the flat surface of secondary rock, which is the base of the whole system. Several great fissures, which pervade all these superimposed basins, allow the water to percolate.

Again, on the other side of Laghouat, if we examine the chains of the Dj. Moudloua, Dj. Dakla, Dj. Zebecha, and Ras El Aïoum, we shall perceive that all four chains belong to the same system of basins, which, instead of being simply elliptical, as those of Guern el Meila, have experienced in two portions of their extent a contraction from upheavals, which has given them the form of the elevated ribs of a gourd with two depressed valleys. One of these contractions forms the dry bed of the Wed Mzi.

The Guern el Haouatha forms a third basin, incomplete towards the S. W.

The Dj. Djelouaj forms a fourth basin without any definite limits southwards, so far as I could trace it.

At Laghouat there is a fifth basin, formed by the Dj. Trisgrarine to the west and the Dj. Seridjab to the east.

From the Sebaa-Rous to Laghouat all these ranges of the

secondary period seem to belong to but one geologic epoch, that of the lower chalk formation. Limestone predominates in this formation, and it is this which constitutes the ridges of the Sahari, Senalba, Djellal, and the system of basin deposits so extraordinary about Laghouat. It is generally of a saccharoïd structure, and of a variable colour, greyish white predominating. By atmospheric action the surface of this limestone has become furrowed and polished, and presents a peculiar wavy appearance. It affords good lime for building purposes. It contains considerable deposits of freestone with much comminuted quartz, which varies in colour and hardness. At Guelt Estel and at Djelfa it supplies good building-stone; elsewhere, as at Recheg, it is so soft as to yield to the pressure of the fingers. Its general colour is yellow and red. This sandstone encloses nodules of flint of various colours, and semi-transparent. By disaggregation they become detached from the softer medium of sandstone in which they are embedded. As the wind removes the sand, they form a sort of shingly beach of pebbles, many of them a very pretty chalcedony, which can be cut and polished, and which are exported in some quantity to Paris for the manufacture of cane and knife handles.

There are also, as near the Rochers de Sel, a few deposits of marl, green and red, and of remarkable brilliancy.

The upper deposit of limestone is marked by regular beds of gypsum of vast extent. Gypsum has been found everywhere at Djelfa, Aïn el Ibel, &c. The regularity and extent of these beds of gypsum is a peculiar characteristic of these secondary rocks, and does not present itself in the secondary formation of the Atlas district.

But to the south and east of Laghouat, in the district of the dayats, we come upon a superincumbent and shallow alluvial soil of the very latest tertiary or of diluvial formation. At the foot of the mountains this is composed of rolled pebbles embedded in a limestone matrix. Near Laghouat the surface is covered by a débris of chalk, with much limestone intermixed.

The further we proceed from the mountains the nodules diminish in size, and the soil is often composed only of a yellowish white calcareous rock, which is raised here and there in ridges more or less thick. It is a kind of crust which covers the soil as with a cloak. Very hard at the surface, it is, on the contrary, extremely friable below, and there it is mixed with green or grey clay. This lastmentioned rock presents very considerable deposits of diluvial soil, and encloses crystals of gypsum of greater or less size, which are often sufficiently numerous to deserve the name of regular deposits.

The diluvial formation exhibits superficial deposits more or less extensive between all the mountain ranges. Even as far north as the two Zahrez* it forms the soil of the great basin which encloses these lakes. It appears again on the banks of the Wed Melah, where I found several fossils, between the Rochers de Sel and Djelfa; and again may be detected between the Dj. Senalba and Djellal.

The only diluvial traces between Djellal and Laghouat consist of very limited deposits of rolled pebbles upon the plateaux, and beyond the reach of any existing or extinct streams. I was particularly struck by the fact that several of my fossil shells from this district, in the superficial deposit, proved specifically identical with fresh-water tertiary fossils given me by my friend Capt. Spratt, and obtained by him in the fresh-water deposits of the region of the Black Sea. May not further research, perhaps, show us that, at some no very distant geologic epoch, a vast chain of fresh-water lakes, similar to those of North America at the present day, extended from the plateaux of the Western Sahara as far as the neighbourhood of the Caspian?

The geological system of the M'zab country, where it differs from that of the Central Sahara, may perhaps be explained by the supposition of partial cataclysms occasioned by earthquakes. The basin appears to be composed

* Vide page 71.

of a triple bed, the lower of which consists of a dark limestone of great hardness, the second of white sandstone, and the upper of a white limestone of very fine texture, well adapted for building purposes. The water drainage has created between the two upper beds an accidental and very thin bed of sandstone, porous, friable, and mixed with silex. It is solely to the supply of water in this narrow stratum that the oases of the M'zab owe their existence. In the two lower beds, which crop out in some of the ridges round, a few fossils (meiocene?) may be obtained. I gathered here some good specimens of Cyprina Islandica. This country, like the Northern Sahara, appears to have experienced a double aqueous invasion: the first at the more distant period of salt water: the second at the transitionary period of fresh water alluded to above. This is marked by the character of the fossils, but the whole basin may be looked upon as of the upper chalk period.

In such a formation but little mineral wealth can be expected. The most interesting mineral products are the salt hills and lakes. The Rochers de Sel have already been described. It has been suggested to me that these should be considered as an eruption of argillaceous, calcareous mud, gypsum, and rock-salt, upheaved across the secondary and tertiary deposits.

The tertian epoch appears to be marked by some scanty remains in the basin of the Zahrez, under the shallow diluvial deposits. It appears again in the banks of the Wed Melah, composed of regular beds of puddingstone and of yellowish sandstone, analogous in appearance to the tertiary formation at Boghar. The fossils which I collected near the Rochers de Sel have been assigned to the meiocene period. From this we may conjecture that, while the Dj. Senalba was above the ocean, a deep basin formed its deposits in all the country to the northwards. Above the salt-rock, which is about 130 feet in height, the deposit consists of fragments of limestone of every colour, mingled with crystals of white and red gypsum, and united by a marly cement easily acted upon by atmospheric causes.