36 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 9, 



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 hare 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 dislo- 

 cation, 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 alter- 

 nations 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 Abooa- 

 lagha 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 sand- 

 stones are brought against a small exposure of Cretaceous rocks (cal- 

 careous 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 gene- 

 ral name of Gharabi. 



From Wady Mokuttub the Triassic sandstones take a general 

 south-easterly direction, and are last seen in Wady Eerran, 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| 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 ex- 

 pressed 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 hke 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 



