Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/106

 floods arising suddenly is ever present to the Arabs, who uniformly refuse to camp in the water-way of a dry river-bed, but always pitch their tents on one of the lower alluvial terraces. This winding valley, known as Wady Omongraf, leads into the broad expansion of Wady Sidreh, called Syeh Sidreh, or, by Lottin de Laval, " Le ras des quatre Wadys," just above the junction of Wady Mokuttub.

The New Red Sandstone appears again as soon as the open country is reached from Wady Chamile, occupying the low ground at the foot of the granite rocks, which form a magnificent line of escarpment, about 1500 or 2000 feet high, running generally east and west towards Wady Ghenneh, below which point the valley again closes, being cut through hard rocks. The lithological character of the sandstone, though generally similar to that observed on the northern side of the granite, has one marked difference, namely, the absence of the thin limestone forming the middle member of the series, so that it becomes difficult to correlate the sections exactly ; and this difficulty is increased by the fact that the beds are not in their normal position, but are faulted down against the granite on the north, and in like manner brought against higher beds on the southern side of Wady Sidreh.

Ghenneh. — The valley known as Wady Ghenneh is formed by the junction of three or four tributaries, the most important being that on the western side, called Wady Maghara. The lower part of the gorge below the junction is narrow, between steep cliffs of brownish- yellow sandstone, probably representing the upper part of the series ; but above this point the country is open, with low hills and broad gravel-terraces, until the granite is reached. This last rises as a bare cliff, without the slightest vestige of alluvial soil or loose covering of any kind, to about 1500 feet above the plain or downland below. That the line of demarcation between the two kinds of rock is a fault is evidenced by a small outlying patch of the lower red sandstones, resembling a low obelisk on a broad base, crowning the highest point of the hill, and forming the peak known as Dehemi, at the head of the Ghenneh valley.

A large dyke breaks through the sandstones near their northern boundary, and appears to have flowed over the higher beds, leaving a thick mass capping a great portion of the hills to the south. Lithologically this lava is generally similar to that seen near the head of the Chamile pass.

Mines of Wady Maghara. — Turquoise-mining has been carried on in the sandstones in Wady Sidreh, about 250 feet above the valley-level, and more extensively on the right-hand side of Wady Maghara, where the workings are at about the same height, and extend for about 300 yards along the face of the cliff. They are in two beds, about 15 or 20 feet apart, in different parts of the district, the most considerable being in the upper one. The rock is a soft coarse-grained quartzose sandstone, of a light-yellow colour, mottled with red and brown patches where more ferruginous. The turquoises are found lining the small open joints which cross the rock in a general north and south direction, and also in the solid sandstone a