Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/110

 The size of these mollusks, which are far too small to be eaten, as well as the position in which the shells were found, is enough to show that they have not been brought in by human agency ; so that we are compelled to admit that, within modern times, or at any rate since the formation of the present drainage- system, the Sinaitic climate has been so far different that freshwater mollusks have lived in places that now form part of a dry stony desert.

Cretaceous Rocks. — The sandstone series of Ghenneh is overlain to the westward, in Wady Sidreh and Nagb el Bedra, by Cretaceous rocks. These are principally soft bright-green sands, alternating with thin clayey limestones, — the lowest beds containing numerous Echinoderms. In the long line of escarpment which extends in a south-westerly direction towards Wady Mokuttub the beds are not in their natural order, as the upper part of the sandstones, the brown beds, are first faulted against granite, and then, by a parallel dislocation, with a downthrow in the opposite direction, are brought, at a point immediately south of Wady Ghenneh, against the flint- conglomerate, which is very strongly developed in the form of alternations of coarse flint-shingle with thin coral-limestones and beds with a large coarsely ribbed Pecten. The total thickness of this group must be very considerable, as it rises in the hill called Abooalagha to a height of 2424 feet above the sea-level, or more than 1600 feet above Ghenneh ; and even this position is due to its being on the downthrow side of a considerable fault. In the direction of Mokuttub the throw of the fault lessens, so that the brown sandstones are brought against a small exposure of Cretaceous rocks (calcareous sandstones and shales with green sands, which are succeeded by a soft grey limestone, covered by a thick bed of blue shaly clay with a little salt and gypsum, and about 500 feet of a soft chalky limestone with bands and nodules of flint, forming an inaccessible cliff, strongly recalling the aspect of an ordinary English chalk-cliff). The total height of the summit above the bottom of Wady Mokuttub cannot be less than 1000 feet. This escarpment goes by the general name of Gharabi.

From Wady Mokuttub the Triassic sandstones take a general south-easterly direction, and are last seen in Wady Ferran, forming a chain of small outliers on the crystalline schists, about twenty miles below the old town. Lower down the latter valley they are again covered by the green sands and thin sandy limestones of the Cretaceous period, having the same south-westerly dip of from 17° to 20°. These, after an outcrop of about 1-1/2 mile along the valley, are covered by the white limestone, beyond which point towards the sea I have not followed the section.

Age of the White Limestone. — Hitherto no opinion has been expressed as to the age of the white limestone, and it now becomes needful to consider this point. On the ground, I was inclined to take the limestone as representing the chalk- with-flints, from the strong physical resemblance to that formation ; but we have seen that a like resemblance holds good in the bituminous chalk-with-flints of the Gharandel and Wady Husseid, which are proved by their fossils to be Nummulitic. Another piece of evidence bearing against this