5. _Thick-bedded White Chalk and grey crystalline limestone._—This series conformably overlies the upper member of the last. On the west side the chalk forms a well-marked, snow-white, tortuous escarpment, lying at a considerable distance (5 to 10 kilometres) from the oasis-wall. Towards the south, however, it approaches the depression, its escarpment following immediately above and behind that of the underlying series. South of the extreme end of the depression proper, the chalk escarpment forms a long narrow bay, the furthest point of which is 20 kilometres distant. Thence it runs northwards, so that to the south-east of the depression the beds of the series are seen at some little distance from the oasis-edge, but further north they appear to come to the edge and to overlie the brown limestone as part of the main scarp in the neighbourhood of the large hill already mentioned in latitude 28° 5′ N., where the strata dip so steeply into the plateau. There is some difficulty in correlating the beds of this group on the two sides of the oasis, for while on the west they are chalky and in places fossiliferous, being only in part altered to a crystalline limestone, on the east, doubtless owing to the folding which produced the strong dipping already referred to, they are almost entirely composed of hard crystalline and horny greyish-white limestone, apparently devoid of fossils. (_Vide_ p. 60). Within the depression it is probable that the White Chalk is represented in part by some of the hard gray crystalline limestone in the synclinal fold of Jebel Hefhuf. On the west and south-west sides of the depression the White Chalk covers a considerable area, which is characterized by its rough surface and by numerous depressions, some of considerable size, eroded by wind-borne sand. To the south it extends to Farafra, being continuous with the chalk of that oasis. Lithologically, on the west side of the oasis, the series is represented by a thick-bedded, snow-white, pure foraminiferal chalk, 30 to 45 metres thick, partly altered in the upper part into hard gray crystalline limestone. It was probably deposited in water of considerable depth. On the west side of the depression the White Chalk yielded a fair assemblage of fossil-remains, the chief among which were great numbers of corals. In the upper part Nautili are occasionally met with, while _Spirorbis_, _Pecten_, _Gryphæa_, fragments of _Inoceramus_ and other shells are fairly common. In addition echinids occur sparingly and sharks’ teeth are seen here and there. One of the best fossil localities is 24 kilometres north of Ain el Haiss. The following is a list of those obtained:—