2.—_Basalt and Dolerite._—Three large hills in the north-west of the oasis, notably Jebel Mayesra, Jebel Mandisha, and the northern half of Jebel Hefhuf, are capped with a basic volcanic rock, the existence of which was noted by Cailliaud[54] so long ago as 1820. Ascherson[55] further studied the distribution of this rock in 1876, and collected specimens which were carefully examined later by Prof. Zirkel.[56] The latter diagnosed the rock as a typical plagioclase-basalt, strongly resembling that of the Giant’s Causeway; it is finely holocrystalline, containing augite, plagioclase and olivine, with some magnetite and ilmenite, and very sparing flakes of biotite; the resemblance of the rock to basaltic intrusions in Tripoli and at Abu Zabel (between Cairo and Bilbeis) is remarked on by Zittel and Ascherson, and the opinion that it was intruded into the sandstones in later Tertiary times is put forward. The basaltic intrusions occur at four separate points in the oasis, representing two more or less inclusive areas; the total area covered by the rock is about 14 square kilometres. The intrusion appears to have been in the form of laccolites; in Jebel Hefhuf the basalt is intrusively interbedded in the Cenomanian, between the lower sandstones and the overlying limestone, both of which rocks show distinct signs of contact-metamorphism. The sandstone shows generally very little alteration, though in some places much hardened; while the limestone is highly crystalline and of a beautiful red colour near the igneous rock. The crystalline nature is, however, more due to folding than to contact-metamorphism, as it occurs in all the limestones of the neighbourhood. In several places the vertical pipes, up which the molten mass was thrust, with disturbance of the sandstone, can be traced; and dykes are occasionally seen near the edge of the deposits. The section seen in Jebel Mandisha is as follows:— _Top._ Metres. Columnar basalt and dolerite, much broken up by weathering 9·2 Sandstones, clays and sandy shales 64·0 Ferruginous sandstones with casts of _Exogyra_ } (thin bed) } 9·2 } Sand-rock and sandstone with ferruginous bands } ---- 82·4