The Arkenu Hills are mostly composed of rocks represented by Specimens 12 and 14, which are really somewhat similar in composition.
No. 12 consists of a holocrystalline aggregate of large crystals of a light gray (possibly a decomposed or kaolinized orthoclase) alkali-felspar, which constitutes the main bulk of the rock. No quartz is visible in the hand specimen, which is greatly weathered, and only gives a specific gravity of a little over 2.5. Small crystals of dark greenish hornblende are well formed, and occur in fewer numbers than in Specimens 17 and 21, which are representatives of the rockmass of the Ouenat Hills to be described presently.
Specimen 14 is an unweathered gray rock chiefly composed of a mottled gray alkali-felspar, with hornblende crystals in similar numbers to those in Specimen 12.
The microscopic examination of a rock-section made from Specimen 14 corroborates the above description, but introduces the possibility of the presence of nephelin in granular-like patches in the slide, which correspond to darker slightly lustrous areas in the hard specimen; however, no nephelin has been actually identified.
Specimens Nos. 12 and 14 may therefore be called syenites.
Running through the syenites of the Arkenu Hills are veins of intrusive rocks represented by Specimens Nos. 11, 13, and 15, and no doubt many others occur.
Specimen No. 11 represents a vein of a hemicrystalline, hard dark green rock weathering brown on the outer surface, with innumerable small dark specks which are scarcely discernible in the unweathered portion of the specimen.
Under the microscope this rock is found to be of considerable interest. It consists of an aggregate of small phenocrysts of idiomorphic felspar, which in places assumes the appearance of a crypto- or microcrystalline felspathic matrix crowded with acicular crystals of a green mineral resembling ægirine. The latter are in places irregularly distributed, but in areas where the felspar occurs in roughly rectangular or lozenge form, the ægirine microlites are crowded round the edges of the latter.
No quartz is noticeable in the rock-slide, and the rock may be tentatively determined as an ægirine-felsite, apparently similar to a rock described and figured in Harker’s “Petrology for Students.”
A second vein in the Arkenu Hills is represented by Specimen No. 13, which is a brownish quartzite.