32 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [DeC. 9, 



floods arising suddenly is ever present to the Arabs, who uniformly 

 refase 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 ISTew 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 to- 

 wards 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 sand- 

 stones, 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 x}ortion of the hills to the south. Li- 

 thologically 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 ge- 

 neral north and south direction, and also in the solid sandstone a 



