The third vein is indicated by Specimen No. 15, which is a dark gray laminated rock, weathering to reddish brown, very fine in texture, with small clear crystals of phenocrysts sparsely disseminated through the ground-mass. The rock-slide shows, under the microscope, a great similarity to Specimen No. 11 as described above. The felspar here, however, forms a much finer crypto-crystalline ground-mass than in the former, and the ægirine microlites are smaller, tapering, and not so well formed. This rock may also be tentatively called a fine ægirine-felsite.
The Ouenat Hills are mainly composed of rocks represented by Specimens 17 and 21, the chief constituent of which is a gray alkali-felspar (possibly orthoclase with some microcline). Quartz is well represented in idiomorphic forms; no mica is noticeable in the hand specimens, but many well-developed prismoidal crystals of very dark or dark green hornblende are thickly distributed throughout the mass.
No slide was made of these specimens owing to their fragile condition on account of weathering, but the rock may be determined as a coarsely crystalline gray hornblende granite.
Specimen No. 18 is another representative rock from, and constitutes a considerable bulk of, the Ouenat Hills. It may be termed a red granite, approximating to an aplite with very little mica, which decomposes and forms oxides of iron which have stained the rock a brownish red; quartz and felspar form the main bulk of the rock.
As in the case of the Arkenu syenites, so here in the Ouenat granites we find other examples of endogenous veins running through the parent rock, represented by Specimens 16, 19, and 22.
Specimen No. 16 represents a vein of purplish felsite, in the felsitic ground-mass of which occur phenocrysts of idiomorphic felspar.
Specimen No. 19 represents a vein of pure white granular quartz rock which occurs in and may have been the cause (through denudation) of the cave found in the foot-hills of the Ouenat range.
Specimen No. 22, found at Garet Shezzu, is a typical quartzite which may also occur as a vein in the granites.
Two specimens found inside the cave in the Ouenat Oasis are of particular interest. These specimens are Nos. 20 and 21. The former, a laminated travertine, could only have been deposited from running water, as the formation of ripple-markings confirms; and from notes made by the explorer at the time of his inspection, we learn that there was quite a lot of it lying about on the floor of the cave. Under the microscope spheroidal structure is displayed, representing the ripple-markings, and in the matrix of calcite many fragments of quartz, felspar, etc., are conspicuous, these having been derived from the denuding granites. No organic remains were observed.
The second specimen (No. 21) is a fragment of the hornblendic granite of which the Ouenat Hills are chiefly composed, and which forms the roof of the cave; this is coated on one side with a thin black iron-manganese film, similar to the well-known deposit on the rocks in the Nile at or near the Asswan Dam.