| Pisolitic ironstone | Ferric oxide | 58·68% ≡ 41·07% | iron. |
| Limonite | „ | 81·06% ≡ 58·84% | „ |
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.
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 brokenup by weathering | 9·2 | ||
| Sandstones, clays and sandy shales | 64·0 | ||
| Ferruginous sandstones with casts ofExogyra (thin bed) | ⎱ ⎰ | 9·2 | |
| Sand-rock and sandstone with ferruginousbands | |||
| 82·4 |
In a small ⅂-shaped hill in lat. 28° 46½′ N., long. 28° 48′ E. of G. a mass of basalt occurs which appears to be mainly the remains of a large pipe or “neck.” The igneous rock here shows flow-structure at the sides, being hard and crystalline in the centre. It has altered the sandstones at the contact very considerably, converting them into a greenish rock full of chalcedony. Here, as in the other deposits, the basalt shows distinct columnar jointing and the resulting blocks weather into spheroidal masses where exposed.
The igneous rocks of Baharia are therefore Post-Cretaceous in age and it seems reasonable to assume, as suggested by Mayer-Eymar,[57] that they are of Lower Oligocene age, contemporaneous with the basalt-sheets of the Fayum, of Abu Roash and the desert to the west, and of Abu Zabel. The andesite neck passed on the road between Maghagha and the oasis ([p. 22]) and the larger masses of Bahnessa and other places in the Western Desert were likewise probably erupted at the same time.
RECENT.
1.—Sand-dunes, Salines and Superficial Deposits.