QUARTZ-FELSITE.
Near Gebel Kolaiqo.
NATURAL SIZE.
Normal pink granite is the form of granite most commonly met with in South-Eastern Egypt. It forms the principal rock of many of the mountains, such as Gebels Nugrus, Selaia, Faraid, Um Reit, Shigigat, Niqrub, Abu Brush, and Hamra Dom, and also covers large expanses of low hill country, and occurs as knolls scattered over sandy plains such as that of Selaia and the tract west of Um Reit. The rock [11,505], a full size representation of which is shown on [Plate XXII,] is composed of pink orthoclase, white oligoclase, quartz, and biotite, with hornblende, sphene, apatite, and magnetite as accessory minerals. The pink orthoclase is the dominant constituent, and gives the colour to the mass; it is sometimes in grains of about the same size as those of the quartz, viz., two to four millimetres in diameter, but frequently tends to assume a porphyritic habit, the crystals then measuring a centimetre or more across, and inclining to idiomorphic forms. Microcline is occasionally sparingly present. The oligoclase is white, and is far less abundant and less conspicuous than the orthoclase; in the porphyritic varieties of the rock, the oligoclase is mixed with the quartz and mica of the ground mass. In some specimens the felspars are fresh, in others [12,133] they show clouding due to decomposition, with formation of kaolin and occasionally of sericite. Quartz is present to about half the amount of the felspars, in grains ranging up to three millimetres in diameter. The biotite is frequently the only ferro-magnesian mineral present, occurring in wisps and little nests among the other minerals; under the microscope it is strongly pleochroic, the colour varying from usually a pale olive brown to nearly black when a crystal is rotated over a nicol prism, though in a specimen [11,531] from the little hill called Sikeit, about five kilometres west of Berenice, colour range is from pale yellow to deep green. Hornblende is typically either absent or present only in a very subordinate amount to the biotite. It is sometimes altered to chlorite, with separation of granules of iron oxide and formation of epidote. Sphene, apatite, and magnetite are usually only very sparingly present, and are only visible on microscopic examination. In a slide cut from the rock of Gebel Fereyid, however, sphene forms a crystal measuring 1·6 millimetres across, of the characteristic wedgelike form (see [Fig. 4]).
Fig. 4.—Granite, Gebel Fereyid [11,505], × 10. f, felspar (mostly orthoclase), somewhat clouded by decomposition; q, quartz; b, biotite; s, sphene.
The principal variation in mineral composition of the rock is the greater or less abundance of the biotite, and the occasional presence of appreciable amounts of hornblende. In a specimen from Wadi Kreiga [12,133], the biotite is almost absent, its place being taken by green hornblende, now largely altered to chlorite and epidote. The ferro-magnesian minerals are as a rule present in smaller amounts than in average granites, and in some places, especially near the periphery of the great intrusions, they vanish almost entirely, and the rock passes gradually into a pegmatite.
Variations in appearance of the rock are also conditioned by the size and habit assumed by the felspar crystals, by the variations in general coarseness or fineness of grain, and by the pressures and weathering influences to which the masses have been subjected. Where the rock has been crushed, or subjected to unequal pressures in different directions during consolidation, the porphyritic crystals tend to lie along definite planes, and the rock may approximate in appearance to a gneiss. The microscopic sections from these places show further evidence of crushing in the cracking of crystals and undulose extinction between crossed nicols. Where the rock is much weathered it usually takes on a whiter aspect due to the bleaching of the orthoclase, and sometimes exhibits green spots due to the formation of chlorite from the decomposition of biotite and hornblende.
It is the pink granite which by its disintegration gives rise to much of the granitic sand which is met with in wadis and on plains. The rock disintegrates very readily under changes of temperature, the cleavage planes of the orthoclase aiding in the process. Change of the felspar to kaolin goes on under weathering influences, and the felspars become bleached, but the bulk of the disintegration is a mechanical process, a large part of the sand consisting of unaltered felspar.
Red pegmatitic granite is closely associated with the normal pink granite, occurring in great masses peripherally to the latter, as for instance at Gebel Hamrat Mukbud and in the low hills about the lower parts of Wadi Gemal and Wadi Kreiga. The red pegmatitic granite is in fact simply an extreme variation of the normal pink granite in which the ferro-magnesian minerals (biotite and hornblende) are either absent or present in very small amounts. That the rock often forms dykes is probably a consequence of its peripheral position rather than of any special manner of formation. The great masses such as Gebel Hamrat Mukbud appear, like the normal granites, to have solidified under plutonic conditions, and the dykes are off-shoots from the main mass. There is, therefore, no reason in this locality for classifying the pegmatites separately from the granites into a special division of dyke-rocks, as is done by Prof. Rosenbusch, and we shall consider the red pegmatitic granite as simply a coarse grained mica-free granite rich in orthoclase.