Fig. 25.—Kersantite, from a dyke at Gebel Fereyid [11,504], × 40. f, felspar (mainly oligoclase); b, biotite; a, augite, with a border of hornblende (h); ap, apatite.
Only one occurrence of a rock which can be with certainty classed as a lamprophyre has been noted in South-Eastern Egypt. The single occurrence referred to is that of a kersantite [11,504] which forms a dyke cutting east-and-west through the granite of Gebel Fereyid. In the hand specimen (see [Plate XXIII]) it is a fine grained dark-grey rock, marbled with veinlets of brownish-white granular (felspathic?) material, and containing here and there porphyritic dark-brown platey crystals up to three millimetres in diameter. The sp. gr. is 2·81. Under the microscope, the rock presents a very fresh appearance, and is seen to be composed mainly of felspars and brown biotite, with a little accessory augite, green hornblende, apatite and magnetite. The biotite, the most conspicuous constituent, is seen partly in basal sections of perfectly idiomorphic forms, and partly as long lath-shaped sections; it is strongly pleochroic, the colour varying from pale yellow-brown to a very dark reddish-brown. The felspar, which has undergone some alteration, forms a sort of matrix round the biotite; here and there large crystals show a tendency to idiomorphism, but the felspar is essentially allotriomorphic. The felspar appears to be mainly oligoclase; but there are also some crystals which show only simple twinning, and these are doubtless orthoclase. The augite and hornblende are very sparingly present, the former in nearly colourless to purplish crystals, the latter in tiny forms of a deep emerald or bluish-green colour, with very high double refraction. Magnetite is liberally scattered in small grains through the rock, seldom included in the biotite. Apatite occurs in very fine long needles among the felspars.
Andesites.
Andesites, the volcanic representatives of the diorites, are much more scarce in the south portion of the Eastern Desert than they are further north. In the district here treated of, only a single deposit, that of Gebel Sufra, has been noted as belonging certainly to the class of andesitic lavas. Some other volcanic rocks consisting chiefly of hornblende and plagioclase have been met with, as for instance at the hill of Ti Keferiai and in the Wadi Huluz; but these are of so basic a nature that they are more properly classed as hornblende-basalts. It is also practically certain that some of the rocks which must be classed as schists on account of their structure are metamorphosed andesites; among the schists of the Wadi Muelih, for example, are rocks which in thin section present a ground mass still distinctly andesitic in character, but the hornblendic constituent, instead of forming well-defined porphyritic crystals as it doubtless originally did, is dragged out into fibrous forms, and the same action can be traced in the hornblendes of the ground mass.
The andesite of Gebel Sufra [10,597] occurs as a columnar deposit overlying syenites and diorites at the top of the mountain, which rises to 690 metres above sea-level in latitude 24° 39′. The rock, which is a fine-grained greenish-grey one weathering to a brown colour on exposed surfaces, often shows a banded structure (see [Plate XXIII]). The sp. gr. is 2·67. The microscopic section shows the rock to be highly altered, but sufficient traces of its original nature can be made out to leave little doubt of its being an andesitic lava. The slides show porphyritic felspars in a cryptocrystalline ground mass composed of felspar with a little hornblende and biotite. The porphyritic felspars are too much kaolinised for twinning to be made out, but a little calcite is visible in their decomposition products, and the crystals are in rather elongated forms which suggest plagioclase rather than orthoclase. The minerals of the ground mass are likewise much decomposed, but tiny grains of green hornblende and wisps of brown biotite, both altering to chlorite, can be seen. There is an almost complete absence of primary iron oxides, but a single large porphyritic crystal in the slide shows separated hæmatite in flakes and in strings down its cleavage planes. The nature of this single crystal is not very clear, and it is doubtful if any of its original substance remains; its form and cleavage are suggestive of augite, but the clear spaces unoccupied by the iron oxide have the appearance of quartz or clear felspar under crossed nicols.
Fig. 26.—View near the top of Gebel Sufra, showing the columnar structure of the andesite.
Fig. 27.—Andesite of Gebel Sufra [10,597], as seen between crossed nicols, × 40. Porphyritic felspar crystals in a cryptocrystalline ground mass.