7.—_Sandstones, Clays and Marls_—These, the lowest and oldest beds, form the floor and parts of the walls of the depression. They are best developed and exposed in the north end, where their maximum thickness is about 170 metres; the base of the series is not seen, and thus their total thickness cannot be estimated. In the north part of the oasis they are capped, with apparent conformability, by Eocene limestones, although an intervening band of limestone-grit may occasionally occur. In the south, the series is followed by the higher Cretaceous divisions, while in the isolated hills within the depression these beds are capped either by Eocene limestones, by basalt or dolerite, or by Post-Eocene ferruginous sandstones and quartzites. They consist of friable false-bedded variegated sands and sandstones, with harder dark-brown ferruginous bands, alternating with sandy shales and clays, passing through every gradation. Some of the sandstones are micaceous. The clays are frequently saliferous, and bands of fibrous gypsum are occasionally seen. Large well-formed groups of barytes crystals are occasionally found in the sandstones of the oasis-floor, and hard thin vein-like masses of a sandstone, in which the cementing material is barytes, are frequently met with in the eastern part. The weathering of these veins, which are evidently produced by the action of infiltrated solutions in cracks, gives rise to the peculiar spheroidal belted nodules which often strew the ground, and of which the existence has been already mentioned by Ascherson and Zittel. Although these strata are, as a rule, unfossiliferous, organic remains are common in certain beds, and especially in the dark-brown ferruginous concretionary bands of the sandstones. The commonest form is _Exogyra_, individuals of which are found crowded together in some bands; in most cases they are pseudomorphs in ironstone. In the hills between Mandisha and El Bawitti this series consists of soft false-bedded variegated sands and sandstones with ferruginous bands, alternating with sandy shales, the whole locally showing a slight westerly dip of between 3° and 7°. Some of the sandstones contain flakes of colourless mica. The whole series is capped and protected by a hard bed of dolerite, which has been intruded along a bedding plane into the series, and now forms the summit of these hills. The following section was measured about 4 kilometres west of Mandisha— _Top._ Metres. More or less columnar dolerite, much broken up by weathering. 10 Sandstones, sandy clays; unfossiliferous 65 Ferruginous sandstone with casts of _Exogyra_; sandrock and sandstone, with ferruginous unfossiliferous bands 10