Lithologically the beds in question present a considerable similarity to the quartzites and hard ferruginous sandstones of Jebel el Ghudda and Gar el Hamra on the road from Feshn to the oasis (ante, p. 18, 20), and also to the beds of Jebel Ahmar, near Cairo, and in parts of the Fayum. In the absence of evidence of their precise age, they may be provisionally classed as Oligocene. [Sidenote: Jebel Horabi.] A short description of these beds in a few special localities will now be given. This well-marked hill, situated at the extreme northerly end of the depression, consists of a mass of ferruginous material, including limonite, pisolitic iron-ore, red and yellow ochre, etc., lying on a series of sandy shales, clays and sandstones belonging to the Cenomanian (Series No. 7). Thin bands of limonite, etc., occur in the clays and sandstones, but the great mass of mineral appears to form a distinct deposit capping, and in part replacing, the former. The iron-ore occurs in every stage of purity. Samples from Jebel Horabi, analysed by Mr. A. Lucas, gave the following results:— Pisolitic ironstone Ferric oxide 58·68% ≡ 41·07% iron. Limonite „ 81·06% ≡ 58·84% „ [Sidenote: Isolated hills in centre of Oasis.] These hills all show the same characters, with the exception of one or two capped by limestone or basalt. They consist of sandstones, with occasional shales and clayey bands, of the Cenomanian series (No. 7) capped by a hard brown or black silicified ferruginous grit, which frequently passes into a typical quartzite. This cap may be of any thickness up to some seven or eight metres. Its junction with the sandstones below is generally obscured by the mass of talus lying on the slopes, but where visible it is difficult to draw any line of demarcation between the two, there being in some cases a more or less gradual increase of hardness and ferruginous material from the upper part of the sandstones upwards. In one hill an ordinary yellow sandstone was observed, when followed up, to contain an increasing number of ferruginous concretions, at first in isolated lumps, or strings, but higher in such quantity as to present the appearance of a breccia, which latter gradually passed up into a hard dense mass of ferruginous quartzite. This at first sight suggested a similar age for the whole rock-section, but would be quite explicable on the supposition of infiltration, as suggested above.