INTERMEDIATE IGNEOUS ROCKS.

Ball.—Geography & Geology of South-Eastern Egypt.PLATE XXII.ACID IGNEOUS ROCKS.PORPHYRITIC GRANITE.Gebel Fereyid.PEGMATITIC RED GRANITE.Wadi Gemal.BIOTITE-GRANITE.Gebel Abu Hegilig.HORNBLENDE-GRANITE.Gebel Elba.QUARTZ-FELSITE.Near Gebel Kolaiqo.QUARTZ-FELSITE.Near Gebel Kolaiqo.NATURAL SIZE.Normal pink graniteis 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 onPlate 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 oligoclaseis 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 (seeFig. 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 graniteis 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.The pegmatitic granite is usually of a far more pronounced red colour than the normal granite. This is doubtless in part due to thegreater abundance of orthoclase, but it also arises in some cases from the orthoclase having itself a deeper colour. The rocks about the lower parts of Wadi Gemal and Wadi Kreiga have a strong brick-red aspect. Their proximity to the coast suggests that there may possibly be some connexion between the intensity of their coloration and their position near the sea; this idea is supported in some measure by the strong red colouration of the dioritic rocks of Ras Benas, which occupy a similar position, but the actual manner of production of the oxide of iron which gives the colour is not clear.Fig. 5.—Red pegmatitic granite, from low hills near the mouth of Wadi Gemal [12,106], × 10.q, quartz;f, felspar (orthoclase) turbid and full of finely disseminated red oxide of iron.In the hand specimen (seePlate XXII), the red pegmatitic granite [10,389] is a coarse-grained aggregate of red orthoclase and quartz. The quartz is generally of grey aspect in the mass, running in strings and networks through the felspar. The rock usually breaks easily into irregular pieces under the hammer, owing to the facility with which the large felspar crystals can be cleaved, but the very red variety from near the mouth of Wadi Gemal [12,106] is extremely hard and tough, and strikes fire very easily under the hammer. Under the microscope (seeFig. 5) the red colour of the orthoclase often persists even in the thinnest sections, being evidently due to finely disseminated iron oxide which clouds the felspars. The quartz shows sharply defined angular forms, and appears to have got somewhat the start of the felspar in crystallisation. Owing to its cloudy nature, the felspar looks like a ground mass, and between crossed nicols shows a peculiar patchy appearance, so that a first glance at the slide suggests a quartz porphyry; but the whole of the patches forming a crystal extinguish at once, and moreover the characteristic twinning of the orthoclase can be seen in some of the crystals. A little oligoclase is also present in addition to the orthoclase. Crushing of the rock is frequently shown by cracked and brecciated crystals, more especially in the hard form of the rock from Wadi Gemal.Dykes of pegmatitic granite are very frequent in the neighbourhood of Gebel Hamrat Mukbud; they are typically rather paler in colour than the larger masses, and occasionally show graphic structure.In ascending Gebel Migif, the gneiss slopes of the mountain were found to be strewn here and there with crystals of green microcline [10,366], similar to the well-known Pike’s Peak mineral. The crystals, which are of imperfect idiomorphic form, measuring about one to two centimetres in length, sometimes show cross-striations on certain faces. Though the rock was not tracedin situ, the crystals are doubtless derived from pegmatite dykes traversing the gneiss.Aplite, or fine-grained binary granite consisting essentially of quartz and felspar, with very little or no mica or hornblende, differs from pegmatite not only in its greater fineness of grain, but in its structure, which is essentially granitic rather than pegmatitic, the two main minerals being both in allotriomorphic grains. In colour, different aplites vary with that of their felspars from red to white, and most of them have tiny dark spots in them, sometimes only visible with a lens, due to the presence of small quantities of hornblende or biotite. As already mentioned onp. 268,most of the granites of South-Eastern Egypt incline to the acid or aplitic type, and one is frequently in doubt whether to classify an acid granite, such as that of Gebel Elba for instance, as an aplite with a little hornblende, or as a hornblende granite. Under aplites will be here considered only those granites in which the proportion of ferro-magnesian minerals is extremely small, not forming more than about one or two per cent of the rock. When the definition of an aplite is thus limited, aplites are comparatively scarce in the area.Gebel Abu Hireiq consists of a red granitic rock; I did not visit the range, but the guide sent to erect the triangulation beacon brought back as the typical rock a pink aplite [12,134] of rather fine grain, consisting essentially of quartz and pink felspar, with sparsely scattered specks of hornblende.The dykes [11,536] which seam the granite and diorite near where the Wadi el Kreim joins Wadi Garara may be classed as siliceous altered aplites. They are greyish-white to purplish-pink rocks of rather fine grain, with a few blackish specks, the latter visible only with a lens. The microscopic slide shows granitic quartz and red-stained decomposedmatter, with a rather plentiful sprinkling of grains and powder of iron oxide; the decomposed matter is almost certainly the result of alteration of felspar and hornblende, the latter in very small proportion.The hill called Marwot Rod el Ligaia, near the head of Wadi Muelih, is a boss of pink aplite, and is of interest as lying in close connexion with certain quartz veins which are believed to be of igneous origin (seep. 266).Fig. 6.—Biotite-granite near Gebel Abu Hegilig [10,390], × 17.q, quartz;f, felspar (orthoclase and oligoclase), somewhat turbid by decomposition;b, biotite;s, sphene.Biotite-graniteof a well-defined type [10,390] occurs somewhat largely in the mountains round the heads of Wadi Abu Ghusun and Wadi el Abiad. It is a whitish rock with dark-brown to black spots (seePlate XXII). The white portion of the rock consists of quartz and felspar; the dark spots are biotite. Generally the rock is of medium grain, the biotite being in nests two to four millimetres diameter scattered through the mass, giving it a speckled appearance. Sometimes the biotite is more uniformly scattered, giving the rock a grey aspect. At some places the rock shows signs of crushing, this being indicated in hand specimens by a tendency to laminar arrangement of the biotite patches. Under the microscope (seeFig. 6), the felspars, which are somewhat more abundantly present than the quartz, are seen to consist of orthoclase and oligoclase, with here and there crystals showing the characteristic cross hatching of microcline. The biotite is mostly in nests and clusters of ragged-looking crystals, but it also occurs in tiny flakes included in the felspars, sometimes showing a distinct arrangement along the cleavage planes of the latter. The biotite is usually fairly fresh; it is highly pleochroic, with colour ranging from pale yellow to deep olive-green in different positions over the nicol. Associated with the biotite are a few wisps of colourless mica, probably muscovite, and irregular granules of opaque iron oxides. Granules of epidoteare to be seen in the altering felspars, and there are a few small irregular granules of a brownish highly refracting mineral, probably sphene.A much more acid type of biotite granite [11,507] occurs in close proximity to serpentine in low hills near the wells of Abraq. In this the proportion of biotite is so small that the rock might almost be classed as an aplite. It is a fine-grained white rock, very fresh-looking, with dark spots; in hand specimens it almost exactly resembles the hornblende granite of Gebel Elba (seePlate XXII), but examination with a lens shows the dark spots to consist of biotite instead of hornblende. The microscopic slide shows quartz, orthoclase, and oligoclase with granitic structure, with here and there small straggling crystals of brown biotite; the brown biotite is sometimes altering to chlorite with change of colour from brown to green.Muscovite-graniteis rarely met with in South-Eastern Egypt. A specimen from Gebel Adar Qaqa[128][12,138] is a fine-grained hard pinkish rock of sp. gr. 2·62, composed of quartz, orthoclase, oligoclase, muscovite, and garnet.A very coarse-grained variety of muscovite-granite, in which the individual crystals measure several centimetres across, forms white hills to the east of Wadi Nugrus.Hornblende-graniteoccurs in several forms, giving rise to rocks of different aspects in different parts of the district.The most important and widely distributed form of hornblende-granite is a fine-grained and very acid rock of sp. gr. 2·59, which but for the manner in which it occurs might almost be spoken of as an aplite. It is a white rock, sometimes with a faintly pink or greenish cast, of medium to fine grain, with black specks of hornblende about a millimetre in diameter scattered through it (seePlate XXII). This rock forms Gebels Muelih and Elba; it also occurs in the low country round Gebel Hamrat Mukbud, and a gneissose variation of it forms the great boss of Um Rasein. Dykes of the same rock traverse the more basic rocks of Um Bisilla.Fig. 7.—Hornblende-granite, Gebel Elba [12,118], × 17.q, quartz;f, felspar (mostly orthoclase), clouded by kaolinisation;h, hornblende;b, biotite, altering with separation of flakes of limonite.Under the microscope the rock from Gebel Elba [12,118] is seen to consist of quartz, orthoclase, a little oligoclase, hornblende, andsmall quantities of biotite (seeFig. 7). The orthoclase is somewhat clouded in strips parallel to the clino-axis of the crystals. The hornblende is in irregular grains of sharply marked curved outlines; it is of a very deep bottle-green colour. The accessory biotite is brown, occurring as small wisps associated with the hornblende.The rock of Gebel Muelih [10,355] is similar to that of Gebel Elba, but the hornblende here is of an extremely pale greenish brown colour, only slightly pleochroic, with well-marked vertical cleavage and a very small extinction angle, so that it might be mistaken for biotite, which latter mineral appears to be absent.Fig. 8.—Hornblende-granite, Gebel Hamata [10,405], × 10.q, quartz (the rock contains a larger percentage of quartz than appears in the drawn portion of the slide);f, felspar (orthoclase), clouded;h, hornblende, altered.Another type of hornblende-granite, differing from that last described in being of coarser grain and containing much more abundant hornblende, and thus having much more the appearance of an ordinary grey granite, occurs in the lower part of Gebel Hamata and the surrounding mountains. This rock [10,405] is slightly heavier than the foregoing type, having a sp. gr. of 2·66. Under the microscope (seeFig. 8) the hornblende is seen to be of a dark green colour, very much altered and clouded by separated iron oxides and epidote.A third type of hornblende-granite is formed by the variation of the normal pink granite already referred to, in which the place of biotite is largely taken by hornblende. The mass of Gebel Mishbih appears[129]to consist of a rock of this type, which passes by insensible gradations into a syenite.Granite-porphyry.Fig. 9.—Granite-porphyry, Kreishim Hill [12,150], viewed between crossed nicols, × 10.q, quartz (the dark triangular area to the left of figure is also a quartz crystal in the position of extinction);f, felspar (oligoclase);g, micro-granitic ground mass.Under this heading are comprised rocks of granitic composition in which porphyritic crystals of quartz, felspar, and mica are surrounded by a fine-grained ground mass of entirely granitic (holocrystalline) character. They differ from porphyritic granites in the much finer grain of the general body of the rock, and from the quartz felsites in the entire absence of glassy matter from the ground mass. Rocks of this type are scarce in South-Eastern Egypt. An example [12,150] occurs at the hill called Kreishim, sixteen kilometres west of Halaib. Here the rock, which appears to form a small boss, consists of rounded white porphyritic crystals set in a finely crystalline grey ground mass. The sp. gr. is 2·69. Under the microscope (Fig. 9) the porphyritic crystals are seen to consist of quartz and oligoclase, the latter in approximately idiomorphic crystals, often beautifully zoned; the ground mass, of micro-granitic structure, is made up of smaller allotriomorphic crystals of quartz, felspar, and biotite.Dykes of granite-porphyry also occur traversing the granite of Gebel Kahfa [11,537 B] and in the basic rocks of Gebel Um Bisilla [11,518]. At both these places the rocks are far less fresh than that above described. In the hand specimen they are of a greyish-white or greenish-white colour, strongly resembling fine-grained aplites in appearance. With the lens, crystals of quartz and felspar can be made out, and specks of hornblende and chlorite. The microscopic slides show the characteristic structure of granite porphyry, but the felspars are all clouded by decomposition products, and the hornblende, which is rather sparingly present in the ground mass, is mostly changed to chlorite and epidote. The specimen from Gebel Kahfa contains a small amount of muscovite.Quartz-felsite.Though covering only small areas, quartz-felsite is of very wide distribution in South-Eastern Egypt. It typically forms dykes and intrusive sheets traversing the plutonic and metamorphic rocks, but likewise occurs occasionally in larger masses. Felsite dykes are specially abundant in the neighbourhood of Gebels Muelih, Zergat Naam, and Um Reit. Larger masses occur in the Wadi Huluz, and form the summits of Gebels Nigrub el Foqani and Hamata.Felsites are frequently met with in a highly altered condition. The commonest alteration, especially in dykes, is kaolinisation of the felspars and a clouding of the whole rock with finely disseminated iron oxides. In some dykes traversing the granite of Um Reit, a bleaching action has reduced the felsite to the appearance of a limestone. In other cases the rock has been devitrified and indurated to a high degree; this is well seen in Gebel Igli el Iswid (latitude 25°) and at Gebel Hadarba (latitude 22°), where extensive hill-tracts consist of felsitic rocks of almost flinty hardness.The quartz-felsites are in general among the youngest of the igneous rocks of the district, since they commonly form dykes and intrusions, not only in the schists, but also in the granites and other eruptive rocks.Fig. 10.—Quartz-felsite, Wadi Huluz [10,394], × 10.q, quartz;f, felspar (orthoclase and oligoclase);g, ground mass, showing flow structure round the porphyritic quartz and felspar.The quartz-felsite of Wadi Huluz [10,394] occurs in considerable masses about a kilometre below the water holes of Um Gerifat. In the hand specimen, it is of granitoid appearance, with opalescent quartz crystals two millimetres or more in diameter plentifully scattered in a light greyish ground mass. The sp. gr. is 2·71. Under the microscope the crystals of quartz are seen to have rounded forms and to be accompanied by other porphyritic crystals of orthoclase and oligoclase, often approximating to idiomorphic shape (seeFig. 10).The porphyritic constituents are embedded in a cryptocrystalline ground mass, in which augite and minute grains of quartz, felspar, and biotite can be made out, and there are some strings and granules of epidote and fairly large specks of iron oxides. The ground mass shows a fluidal arrangement, the little biotite flakes in it being often arranged in lines which sweep round the porphyritic crystals. The rock has undergone considerable alteration, the felspar crystals being clouded and full of tiny micaceous flakes of high double refraction; the epidote is also doubtless due to the alteration of augite and other minerals in the ground mass.The quartz-felsite which forms the upper part of Gebel Hamata [10,906] is a dark brown coarse-textured rock, with remarkably glassy porphyritic quartz in granules about two millimetres diameter scattered plentifully through it. It is a very hard rock which rings under the hammer and weathers into rusty brown blocks. The sp. gr. is 2·71. Under the microscope the quartz crystals are seen to be much cracked. There are also porphyritic crystals of orthoclase and oligoclase, mostly of irregular shape, and a few crystals, of still more irregular form, of dark green hornblende. The cryptocrystalline ground mass consists chiefly of quartz and felspar, through which are scattered tiny granules of dark green hornblende. The ground mass shows a matted texture between crossed nicols; there is no trace of fluidal movement. Like the foregoing, this rock is somewhat altered, the hornblende in particular being very much attacked; the felspars are fairly fresh, but the crystals, like those of the quartz, are often cracked, and thus show the rock to have undergone considerable crushing.Fig. 11.—Quartz-felsite from dyke at Gebel Kolaiqo [12,145], × 10.q, quartz (a group of idiomorphic crystals);f, felspar (mostly orthoclase);g, microgranitic ground mass of quartz and felspar, with wisps of hornblende.One of the quartz-felsite dykes traversing the schists at Gebel Kolaiqo [12,145] is a very similar rock to that of Gebel Hamata, except that here the ground mass, instead of being dark brown in the hand specimen, is of a reddish brown colour (seePlate XXII), and theporphyritic crystals are partly pink felspar and partly glassy quartz. The sp. gr. is 2·62. Under the microscope the quartz and felspar crystals are seen to be less crushed than in the Hamata rock, while the ground mass, instead of being of a matted structure, is microgranitic, with tiny elongated grains of green hornblende scattered through it (seeFig. 11).Another quartz-felsite dyke at Gebel Kolaiqo [12,135] has a rather different composition, and in the hand specimen has more the appearance of a felspar porphyry, porphyritic white felspars being scattered with quartz grains through a dark grey to black ground mass (seePlate XXII). Its quartz crystals frequently approximate to idiomorphic forms; they are much corroded by the ground mass. The felspar is chiefly oligoclase, in more or less idiomorphic crystals, a little clouded by decomposition products. The ferro-magnesian mineral here is chiefly brown biotite, in crystals of smaller size than those of the quartz and felspar, scattered porphyritically through the ground mass. One of the quartz crystals includes a crystal of biotite. The ground mass is crystalline, and appears to consist of quartz and felspar with a little biotite and some glassy matter.A peculiar type of quartz-felsite [10,377] occurs associated with serpentine and ophicalcites in the upper part of Gebel Ghadir. The rock, which has a sp. gr. of 2·66, is nearly white, with a faint greyish tinge and scattered reddish-brown specks. At first the rock was taken for a granulite, which it very much resembles in appearance, and the reddish-brown specks for garnet. But examination with a lens shows the specks to be ferric oxide, and here and there little crystals of glassy quartz can be seen. Under the microscope the rock is found to consist mainly of an extremely fine-grained semi-granulitic colourless ground mass in which are sparsely scattered clear crystals of quartz, sometimes showing corrosion, and straggling irregular patches of opaque iron oxide, often mixed with nearly colourless wisps of mica. A few of the clear crystals are in four-sided forms, and may possibly be felspar; but they are perfectly free from alteration or twinning. The iron oxide is doubtless the product of alteration of a ferro-magnesian mineral, probably biotite. The minerals of the ground mass are difficult of determination owing to the minuteness of the grains and the absence of any colour, but apparently consist of quartz, felspar, a colourless hornblende, and sericite.The quartz-felsite dykes [10,356] which cut through the granite and schists of Gebel Muelih appear as grooves in the granite, and as back-bone ridges in the surrounding schists. They are pinkish-brown close-textured rocks, with a dark marbling in places. The microscopic slides show quartz and felspar in a confused ground mass, with little flakes of a white micaceous mineral (sericite?) aggregated round the porphyritic crystals and distributed through the felspars and the ground mass.The highly altered quartz-felsite [12,158] which forms Gebel Butitelib, near Gebel Niqrub el Tahtani, appears to have formed a boss or neck in the surrounding schists. The rock is a very fine-grained grey to purplish one, in which no crystals can usually be made out with the unaided eye. The sp. gr. is very low, being only 2·32. Under the microscope one sees small highly corroded crystals of quartz scattered through a clouded and glassy ground mass. No traces of felspars can be seen, the crystals, if they existed, having been decomposed and become undistinguishable from the ground mass; nor can any ferro-magnesian minerals be made out, though there are tiny granules and wisps of iron oxides which in places show an arrangement suggestive of their having been derived from the decomposition of such minerals.The specimen [12,121] brought back from the summit of Gebel Shendib, by the guide who was sent to erect the triangulation beacon on it, is a compact brown rock with reddish patches, which under the microscope appears to be an altered and brecciated felsite.[130]The slide shows quartz and altered felspars in a spherulitic ground mass, the whole of the mineral being very much clouded by finely disseminated ferric oxide.The felsite dykes [11,538] which seam the granite of Gebel Um Reit are a still more highly altered rock. The dykes are about two metres wide, of a white colour, and resemble limestone in appearance; being soft, they have weathered more rapidly than the granite around them, leaving vertical-sided chasms. But for the manner of its occurrence, the rock would have been easily mistaken in the field for a sedimentary one. The microscope, however, confirms its eruptive origin. The slides cut from the rock show a confused and cloudedsemi-crystalline aggregate, containing clearer small areas of quartz. Even the quartz crystals are full of specks, and only the faintest traces can be seen of the felspars, which are so decomposed as to be hardly distinguishable from the ground mass.Fig. 12.—Altered quartz-felsite, Gebel Igli el Iswid [10,372], as seen between crossed nicols, × 40.f, felspar (mostly orthoclase in the position of extinction);h, a crystal of hornblende;g, hemi-crystalline ground mass;m, micropegmatitic intergrowth of quartz and felspar, separating the porphyritic crystals.In the rocks of Gebels Igli el Iswid, Mahali, and Hadarba, we have quartz-felsites which have been altered in quite a different way. The rocks are extremely hard, and almost flinty, breaking with a sub-conchoidal fracture. They are typically of a brown to nearly black colour, with white spots measuring a millimetre or two across. Under the microscope these white spots are seen in the rock from Gebel Igli el Iswid [10,372] to be chiefly porphyritic felspar and quartz crystals, sometimes corroded by the ground mass; while the black ground mass is largely composed of extremely fine micro-pegmatitic intergrowths of quartz and felspar, with some glassy matter, and here and there irregular small clouded and altered crystals of hornblende. The micro-pegmatitic material has possibly originated by devitrification of an originally glassy ground mass. Variations of the rock occur in which the porphyritic felspars are infrequent or even altogether absent, the rock passing gradually into a hornfels [10,371]. The slide from Gebel Mahali [10,402] exhibits clear porphyritic crystals of quartz and orthoclase, both minerals in more or less idiomorphic forms, embedded in, and occasionally corroded by, a semi-crystalline ground mass. The ground mass contains some calcite and iron oxides, probably arising from the decomposition of a hornblendic or micaceous mineral.Fig. 13.—Microperthitic structure in felspar of quartz-felsite, Gebel Hadarba [12,147], as seen between crossed nicols, × 40.The rocks of Gebel Hadarba [12,146-12,149] are essentially similar to those of Gebel Igli el Iswid, but in some cases they show bright red veining; where they have been exposed to the polishing agency of the sand blast, these veined varieties look as though they had beenstreaked with melted sealing-wax. In some of the slides the felspars show a well marked microperthitic structure, while in others they are so decomposed that they are barely distinguishable from the ground mass. The ground mass sometimes contains granules and strings of iron-oxide, possibly referable to alteration of a hornblendic constituent, but the rock is too much altered for one to be sure.Fig. 14.—Crushed oligoclase crystal in quartz-felsite, Wadi Huluz [10,404], as seen between crossed nicols, × 40.An altered and crushed quartz-felsite [10,404] which occurs in the Wadi Huluz, near where the Wadi el Abiad joins it about eight kilometres to the north-west of Gebel Hamata, somewhat resembles the rock of Gebel Hadarba in appearance. It is a jaspery looking rock of dark colour with red and greenish patches, in which with a lens one can see scattered grains of glassy quartz. The slide cut from the rock shows clear porphyritic crystals of quartz, orthoclase and oligoclase in a fine grained ground mass. The porphyritic crystals are mostly in rounded forms, but some of the felspars show a tendency to idiomorphism. Many of the crystals are smashed, and some show undulose extinction. The ground mass consists of quartz, felspar, and some glassy matter, with abundant epidote in nests and strings, and a small amount of green hornblende in straggling forms.The east-and-west dykes which traverse the syenite of Gebel Zergat Naam, and form the actual summit of that mountain, consist of a very hard compact flesh-coloured rock which weathers brown on the surface. The microscopic section [11,525] shows the rock to bea highly indurated felsite. The original felspar crystals can be made out by their shapes, but between crossed nicols they are seen to consist of a very fine-grained mosaic polarising in low greys, probably the result of alteration by siliceous solutions. These altered felspars are scattered with some quartz in a cryptocrystalline ground mass.No volcanic rocks of acid type have been with certainty identified in South-Eastern Egypt. It is, however, possible that some of the rocks above described are in reality of volcanic origin, though the manner of their occurrence is more suggestive of intruded masses and sheets. It is also quite likely that some of the hornfels found associated with the schists, as for instance at Gebel Um Semiuki, near Abu Hamamid, are altered forms of glassy acid lavas; but their vast age and the intense metamorphism to which they and the surrounding rocks have been subjected render it impossible to be certain of the manner of their origin.INTERMEDIATE IGNEOUS ROCKS.(a)SUB-ACID ROCKS.Syenite and Syenite-porphyry.Though hornblendic varieties of granite are fairly common, true syenite (i.e., orthoclase-hornblende rocks with little or no quartz) is amongst the rarest of rocks in South-Eastern Egypt. It occurs near Gebel Nazla (between Bir Um Gubur and Bir Masur), in the two remarkable conical hills called El Nahud, which rise from the plain near the head of Wadi Natash, and at Gebel Zergat Naam. In all these three localities the syenite appears to form intrusive bosses rising through the surrounding rocks.The specimens from near Gebel Nazla [10,625] and from El Nahud [10,857] are very fine-grained reddish-brown rocks, which can be seen with a lens to be largely made up of red orthoclase crystals with dark specks of hornblende.Fig. 15.—Syenite of Gebel Zergat Naam [11,515], × 10.o, orthoclase felspar, showing striations parallel to the basal planes;h, hornblende deep green in colour;g, interstitial quartz.Fig. 16.—Syenite-porphyry, Gebel Zergat Naam [11,512], as seen between crossed nicols, × 10.f, porphyritic felspar (orthoclase);h, hornblende;g, microgranitic ground mass containing felspar, hornblende, and some quartz.The syenite of Gebel Zergat Naam [11,515] rises as a great boss from among the surrounding dark schistose rocks. It is typically a pinkish-brown rock of rather fine grain (seePlate XXIII), in which can be seen shining crystals, three to four millimetres long, ofpink orthoclase, and specks of dark hornblende, with here and there a little glassy-looking quartz. The sp. gr. is 2·62. Under the microscope the rock is found to be mainly composed of orthoclase, with a much smaller amount of hornblende and a little interstitial quartz. The felspar is fairly clear, in forms approximating to idiomorphic, and showing the characteristic simple twinning of orthoclase. The crystals are slightly clouded in streaks parallel to the basal planes, so that even in ordinary light a faint herring-bone structure is visible, the streaks on either side of the trace of the plane of composition being inclined to each other at a large angle. Between crossed nicols this herring-bone structure is very strongly marked, and is evidently due to a perthitic intergrowth (of albite?). The intergrown lamellæ are slightly irregular; though following generally the direction of the basal cleavage planes they are not perfectly straight, nor always continuous across the half of the crystal. The lamellæ extinguish and attain their maximum of brightness simultaneously with the respective halves of the crystal in which they occur, so that they become invisible in certain positions of the nicols; but as the nicols are turned they appear as well marked dark bands, clearly visible even under low powers. The hornblende is of exceptionally dark green colour, in irregular straggling masses, often considerably decomposed and clouded with iron oxide. Accessory minerals, other thanthe clear interstitial quartz, appear to be almost entirely absent in the slide examined.Ball.—Geography & Geology of South-Eastern Egypt.PLATE XXIII.INTERMEDIATE IGNEOUS ROCKS.PINK SYENITE.Gebel Zergat Naam.TRACHYTE.Gebel Kahfa.DIORITE.Gebel Allawi.DIORITE WITH PRISMATIC HORNBLENDE.Wadi Muqur.BANDED ANDESITE.Gebel Sufra.VEINED KERSANTITE.Gebel Fereyid.NATURAL SIZE.In places, the rock of Gebel Zergat Naam takes on a finer grain and a greyer aspect in the mass. A slide cut from this variety [11,512] shows essentially the same composition, but the felspars are here porphyritic in a crystalline ground mass of felspar and hornblende. The rock thus passes into syenite-porphyry. The porphyritic felspar crystals show the same lamellar structure as those in the more coarsely crystalline rock.Trachyte.Fig. 17.—Trachyte, from a dyke at Gebel Kahfa [11,537 A] × 10. A clear crystal of orthoclase felspar is seen in the centre of the field, surrounded by a finely crystalline ground mass of hornblende and felspar.Trachyte, the volcanic representative of syenite, occurs in dykes [11,537 A] seaming the granite of Gebel Kahfa. It is a light grey rock, of very fine grain, breaking with a rough surface, containing pores and small stumpy white pearly-looking porphyritic crystals of orthoclase (seePlate XXIII). The sp. gr. is 2·56. The microscope shows the ground mass surrounding the porphyritic idiomorphic orthoclase crystals to be a holocrystalline mixture of hornblende and felspar. The felspars of the ground mass are usually in more elongated forms than the porphyritic crystals, and appear to be partly plagioclase. The hornblende, in small and very irregular crystals of dark olive-green colour, often clouded and dirty looking, is scattered plentifully among the feslpars of the ground mass. Accessory primary minerals appear to be entirely absent.A rock which occurs in a hill at the head of Wadi Amba-ut [10,375] appears to be essentially similar to the above, but the porphyritic orthoclases are more numerous and the ground mass contains small quantities of augite and magnetite. This rock is in a highly altered condition, the felspars being full of kaolin and epidote, while the hornblende, which is here of a paler colour than in the trachyte of Gebel Kahfa, is highly chloritised.(b)SUB-BASIC ROCKS.Diorite.Though not forming such conspicuous features as the granites, rocks of dioritic composition are very widely distributed over the country, and are specially abundant in the districts to the south of Ras Benas. They are almost always sharply marked-off from the granites, being generally closely connected with the more basic igneous rocks and schists. The coarser-grained varieties usually form parts of irregular intrusive masses, shading off gradually into the more basic forms of diabase, gabbro, and hornblende rock; the finer-grained types occur as dykes and irregular bands traversing other igneous rocks and schists, and are themselves frequently so crushed as to resemble schists. Owing to their intimate association with other dark rocks, it is no easy matter to trace the limits of the diorites in the field. The same difficulty is found when they are microscopically examined, for one finds in their mineralogical composition every gradation from true diorites, through the intermediate stage of augite-diorite to diabase and gabbro, while many of the harder dark schists and hornblende gneisses turn out to be merely highly crushed diorites, so that classification must be more or less arbitrary.Diorite typically forms low hill country of dark aspect. The rock weathers as a rule far more easily than granite, and in some cases the debris of rounded grains set free by disintegration cover the surface and render the climbing of the hills somewhat dangerous, the effect being like one would imagine to result from walking over slopes covered with hard peas.

ACID IGNEOUS ROCKS.

PORPHYRITIC GRANITE.Gebel Fereyid.PEGMATITIC RED GRANITE.Wadi Gemal.

PORPHYRITIC GRANITE.Gebel Fereyid.

PEGMATITIC RED GRANITE.Wadi Gemal.

BIOTITE-GRANITE.Gebel Abu Hegilig.HORNBLENDE-GRANITE.Gebel Elba.

BIOTITE-GRANITE.Gebel Abu Hegilig.

HORNBLENDE-GRANITE.Gebel Elba.

QUARTZ-FELSITE.Near Gebel Kolaiqo.QUARTZ-FELSITE.Near Gebel Kolaiqo.

QUARTZ-FELSITE.Near Gebel Kolaiqo.

NATURAL SIZE.

Normal pink graniteis 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 onPlate 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 oligoclaseis 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 (seeFig. 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 graniteis 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.

The pegmatitic granite is usually of a far more pronounced red colour than the normal granite. This is doubtless in part due to thegreater abundance of orthoclase, but it also arises in some cases from the orthoclase having itself a deeper colour. The rocks about the lower parts of Wadi Gemal and Wadi Kreiga have a strong brick-red aspect. Their proximity to the coast suggests that there may possibly be some connexion between the intensity of their coloration and their position near the sea; this idea is supported in some measure by the strong red colouration of the dioritic rocks of Ras Benas, which occupy a similar position, but the actual manner of production of the oxide of iron which gives the colour is not clear.

Fig. 5.—Red pegmatitic granite, from low hills near the mouth of Wadi Gemal [12,106], × 10.q, quartz;f, felspar (orthoclase) turbid and full of finely disseminated red oxide of iron.

In the hand specimen (seePlate XXII), the red pegmatitic granite [10,389] is a coarse-grained aggregate of red orthoclase and quartz. The quartz is generally of grey aspect in the mass, running in strings and networks through the felspar. The rock usually breaks easily into irregular pieces under the hammer, owing to the facility with which the large felspar crystals can be cleaved, but the very red variety from near the mouth of Wadi Gemal [12,106] is extremely hard and tough, and strikes fire very easily under the hammer. Under the microscope (seeFig. 5) the red colour of the orthoclase often persists even in the thinnest sections, being evidently due to finely disseminated iron oxide which clouds the felspars. The quartz shows sharply defined angular forms, and appears to have got somewhat the start of the felspar in crystallisation. Owing to its cloudy nature, the felspar looks like a ground mass, and between crossed nicols shows a peculiar patchy appearance, so that a first glance at the slide suggests a quartz porphyry; but the whole of the patches forming a crystal extinguish at once, and moreover the characteristic twinning of the orthoclase can be seen in some of the crystals. A little oligoclase is also present in addition to the orthoclase. Crushing of the rock is frequently shown by cracked and brecciated crystals, more especially in the hard form of the rock from Wadi Gemal.

Dykes of pegmatitic granite are very frequent in the neighbourhood of Gebel Hamrat Mukbud; they are typically rather paler in colour than the larger masses, and occasionally show graphic structure.

In ascending Gebel Migif, the gneiss slopes of the mountain were found to be strewn here and there with crystals of green microcline [10,366], similar to the well-known Pike’s Peak mineral. The crystals, which are of imperfect idiomorphic form, measuring about one to two centimetres in length, sometimes show cross-striations on certain faces. Though the rock was not tracedin situ, the crystals are doubtless derived from pegmatite dykes traversing the gneiss.

Aplite, or fine-grained binary granite consisting essentially of quartz and felspar, with very little or no mica or hornblende, differs from pegmatite not only in its greater fineness of grain, but in its structure, which is essentially granitic rather than pegmatitic, the two main minerals being both in allotriomorphic grains. In colour, different aplites vary with that of their felspars from red to white, and most of them have tiny dark spots in them, sometimes only visible with a lens, due to the presence of small quantities of hornblende or biotite. As already mentioned onp. 268,most of the granites of South-Eastern Egypt incline to the acid or aplitic type, and one is frequently in doubt whether to classify an acid granite, such as that of Gebel Elba for instance, as an aplite with a little hornblende, or as a hornblende granite. Under aplites will be here considered only those granites in which the proportion of ferro-magnesian minerals is extremely small, not forming more than about one or two per cent of the rock. When the definition of an aplite is thus limited, aplites are comparatively scarce in the area.

Gebel Abu Hireiq consists of a red granitic rock; I did not visit the range, but the guide sent to erect the triangulation beacon brought back as the typical rock a pink aplite [12,134] of rather fine grain, consisting essentially of quartz and pink felspar, with sparsely scattered specks of hornblende.

The dykes [11,536] which seam the granite and diorite near where the Wadi el Kreim joins Wadi Garara may be classed as siliceous altered aplites. They are greyish-white to purplish-pink rocks of rather fine grain, with a few blackish specks, the latter visible only with a lens. The microscopic slide shows granitic quartz and red-stained decomposedmatter, with a rather plentiful sprinkling of grains and powder of iron oxide; the decomposed matter is almost certainly the result of alteration of felspar and hornblende, the latter in very small proportion.

The hill called Marwot Rod el Ligaia, near the head of Wadi Muelih, is a boss of pink aplite, and is of interest as lying in close connexion with certain quartz veins which are believed to be of igneous origin (seep. 266).

Fig. 6.—Biotite-granite near Gebel Abu Hegilig [10,390], × 17.q, quartz;f, felspar (orthoclase and oligoclase), somewhat turbid by decomposition;b, biotite;s, sphene.

Biotite-graniteof a well-defined type [10,390] occurs somewhat largely in the mountains round the heads of Wadi Abu Ghusun and Wadi el Abiad. It is a whitish rock with dark-brown to black spots (seePlate XXII). The white portion of the rock consists of quartz and felspar; the dark spots are biotite. Generally the rock is of medium grain, the biotite being in nests two to four millimetres diameter scattered through the mass, giving it a speckled appearance. Sometimes the biotite is more uniformly scattered, giving the rock a grey aspect. At some places the rock shows signs of crushing, this being indicated in hand specimens by a tendency to laminar arrangement of the biotite patches. Under the microscope (seeFig. 6), the felspars, which are somewhat more abundantly present than the quartz, are seen to consist of orthoclase and oligoclase, with here and there crystals showing the characteristic cross hatching of microcline. The biotite is mostly in nests and clusters of ragged-looking crystals, but it also occurs in tiny flakes included in the felspars, sometimes showing a distinct arrangement along the cleavage planes of the latter. The biotite is usually fairly fresh; it is highly pleochroic, with colour ranging from pale yellow to deep olive-green in different positions over the nicol. Associated with the biotite are a few wisps of colourless mica, probably muscovite, and irregular granules of opaque iron oxides. Granules of epidoteare to be seen in the altering felspars, and there are a few small irregular granules of a brownish highly refracting mineral, probably sphene.

A much more acid type of biotite granite [11,507] occurs in close proximity to serpentine in low hills near the wells of Abraq. In this the proportion of biotite is so small that the rock might almost be classed as an aplite. It is a fine-grained white rock, very fresh-looking, with dark spots; in hand specimens it almost exactly resembles the hornblende granite of Gebel Elba (seePlate XXII), but examination with a lens shows the dark spots to consist of biotite instead of hornblende. The microscopic slide shows quartz, orthoclase, and oligoclase with granitic structure, with here and there small straggling crystals of brown biotite; the brown biotite is sometimes altering to chlorite with change of colour from brown to green.

Muscovite-graniteis rarely met with in South-Eastern Egypt. A specimen from Gebel Adar Qaqa[128][12,138] is a fine-grained hard pinkish rock of sp. gr. 2·62, composed of quartz, orthoclase, oligoclase, muscovite, and garnet.

A very coarse-grained variety of muscovite-granite, in which the individual crystals measure several centimetres across, forms white hills to the east of Wadi Nugrus.

Hornblende-graniteoccurs in several forms, giving rise to rocks of different aspects in different parts of the district.

The most important and widely distributed form of hornblende-granite is a fine-grained and very acid rock of sp. gr. 2·59, which but for the manner in which it occurs might almost be spoken of as an aplite. It is a white rock, sometimes with a faintly pink or greenish cast, of medium to fine grain, with black specks of hornblende about a millimetre in diameter scattered through it (seePlate XXII). This rock forms Gebels Muelih and Elba; it also occurs in the low country round Gebel Hamrat Mukbud, and a gneissose variation of it forms the great boss of Um Rasein. Dykes of the same rock traverse the more basic rocks of Um Bisilla.

Fig. 7.—Hornblende-granite, Gebel Elba [12,118], × 17.q, quartz;f, felspar (mostly orthoclase), clouded by kaolinisation;h, hornblende;b, biotite, altering with separation of flakes of limonite.

Under the microscope the rock from Gebel Elba [12,118] is seen to consist of quartz, orthoclase, a little oligoclase, hornblende, andsmall quantities of biotite (seeFig. 7). The orthoclase is somewhat clouded in strips parallel to the clino-axis of the crystals. The hornblende is in irregular grains of sharply marked curved outlines; it is of a very deep bottle-green colour. The accessory biotite is brown, occurring as small wisps associated with the hornblende.

The rock of Gebel Muelih [10,355] is similar to that of Gebel Elba, but the hornblende here is of an extremely pale greenish brown colour, only slightly pleochroic, with well-marked vertical cleavage and a very small extinction angle, so that it might be mistaken for biotite, which latter mineral appears to be absent.

Fig. 8.—Hornblende-granite, Gebel Hamata [10,405], × 10.q, quartz (the rock contains a larger percentage of quartz than appears in the drawn portion of the slide);f, felspar (orthoclase), clouded;h, hornblende, altered.

Another type of hornblende-granite, differing from that last described in being of coarser grain and containing much more abundant hornblende, and thus having much more the appearance of an ordinary grey granite, occurs in the lower part of Gebel Hamata and the surrounding mountains. This rock [10,405] is slightly heavier than the foregoing type, having a sp. gr. of 2·66. Under the microscope (seeFig. 8) the hornblende is seen to be of a dark green colour, very much altered and clouded by separated iron oxides and epidote.

A third type of hornblende-granite is formed by the variation of the normal pink granite already referred to, in which the place of biotite is largely taken by hornblende. The mass of Gebel Mishbih appears[129]to consist of a rock of this type, which passes by insensible gradations into a syenite.

Fig. 9.—Granite-porphyry, Kreishim Hill [12,150], viewed between crossed nicols, × 10.q, quartz (the dark triangular area to the left of figure is also a quartz crystal in the position of extinction);f, felspar (oligoclase);g, micro-granitic ground mass.

Under this heading are comprised rocks of granitic composition in which porphyritic crystals of quartz, felspar, and mica are surrounded by a fine-grained ground mass of entirely granitic (holocrystalline) character. They differ from porphyritic granites in the much finer grain of the general body of the rock, and from the quartz felsites in the entire absence of glassy matter from the ground mass. Rocks of this type are scarce in South-Eastern Egypt. An example [12,150] occurs at the hill called Kreishim, sixteen kilometres west of Halaib. Here the rock, which appears to form a small boss, consists of rounded white porphyritic crystals set in a finely crystalline grey ground mass. The sp. gr. is 2·69. Under the microscope (Fig. 9) the porphyritic crystals are seen to consist of quartz and oligoclase, the latter in approximately idiomorphic crystals, often beautifully zoned; the ground mass, of micro-granitic structure, is made up of smaller allotriomorphic crystals of quartz, felspar, and biotite.

Dykes of granite-porphyry also occur traversing the granite of Gebel Kahfa [11,537 B] and in the basic rocks of Gebel Um Bisilla [11,518]. At both these places the rocks are far less fresh than that above described. In the hand specimen they are of a greyish-white or greenish-white colour, strongly resembling fine-grained aplites in appearance. With the lens, crystals of quartz and felspar can be made out, and specks of hornblende and chlorite. The microscopic slides show the characteristic structure of granite porphyry, but the felspars are all clouded by decomposition products, and the hornblende, which is rather sparingly present in the ground mass, is mostly changed to chlorite and epidote. The specimen from Gebel Kahfa contains a small amount of muscovite.

Though covering only small areas, quartz-felsite is of very wide distribution in South-Eastern Egypt. It typically forms dykes and intrusive sheets traversing the plutonic and metamorphic rocks, but likewise occurs occasionally in larger masses. Felsite dykes are specially abundant in the neighbourhood of Gebels Muelih, Zergat Naam, and Um Reit. Larger masses occur in the Wadi Huluz, and form the summits of Gebels Nigrub el Foqani and Hamata.

Felsites are frequently met with in a highly altered condition. The commonest alteration, especially in dykes, is kaolinisation of the felspars and a clouding of the whole rock with finely disseminated iron oxides. In some dykes traversing the granite of Um Reit, a bleaching action has reduced the felsite to the appearance of a limestone. In other cases the rock has been devitrified and indurated to a high degree; this is well seen in Gebel Igli el Iswid (latitude 25°) and at Gebel Hadarba (latitude 22°), where extensive hill-tracts consist of felsitic rocks of almost flinty hardness.

The quartz-felsites are in general among the youngest of the igneous rocks of the district, since they commonly form dykes and intrusions, not only in the schists, but also in the granites and other eruptive rocks.

Fig. 10.—Quartz-felsite, Wadi Huluz [10,394], × 10.q, quartz;f, felspar (orthoclase and oligoclase);g, ground mass, showing flow structure round the porphyritic quartz and felspar.

The quartz-felsite of Wadi Huluz [10,394] occurs in considerable masses about a kilometre below the water holes of Um Gerifat. In the hand specimen, it is of granitoid appearance, with opalescent quartz crystals two millimetres or more in diameter plentifully scattered in a light greyish ground mass. The sp. gr. is 2·71. Under the microscope the crystals of quartz are seen to have rounded forms and to be accompanied by other porphyritic crystals of orthoclase and oligoclase, often approximating to idiomorphic shape (seeFig. 10).The porphyritic constituents are embedded in a cryptocrystalline ground mass, in which augite and minute grains of quartz, felspar, and biotite can be made out, and there are some strings and granules of epidote and fairly large specks of iron oxides. The ground mass shows a fluidal arrangement, the little biotite flakes in it being often arranged in lines which sweep round the porphyritic crystals. The rock has undergone considerable alteration, the felspar crystals being clouded and full of tiny micaceous flakes of high double refraction; the epidote is also doubtless due to the alteration of augite and other minerals in the ground mass.

The quartz-felsite which forms the upper part of Gebel Hamata [10,906] is a dark brown coarse-textured rock, with remarkably glassy porphyritic quartz in granules about two millimetres diameter scattered plentifully through it. It is a very hard rock which rings under the hammer and weathers into rusty brown blocks. The sp. gr. is 2·71. Under the microscope the quartz crystals are seen to be much cracked. There are also porphyritic crystals of orthoclase and oligoclase, mostly of irregular shape, and a few crystals, of still more irregular form, of dark green hornblende. The cryptocrystalline ground mass consists chiefly of quartz and felspar, through which are scattered tiny granules of dark green hornblende. The ground mass shows a matted texture between crossed nicols; there is no trace of fluidal movement. Like the foregoing, this rock is somewhat altered, the hornblende in particular being very much attacked; the felspars are fairly fresh, but the crystals, like those of the quartz, are often cracked, and thus show the rock to have undergone considerable crushing.

Fig. 11.—Quartz-felsite from dyke at Gebel Kolaiqo [12,145], × 10.q, quartz (a group of idiomorphic crystals);f, felspar (mostly orthoclase);g, microgranitic ground mass of quartz and felspar, with wisps of hornblende.

One of the quartz-felsite dykes traversing the schists at Gebel Kolaiqo [12,145] is a very similar rock to that of Gebel Hamata, except that here the ground mass, instead of being dark brown in the hand specimen, is of a reddish brown colour (seePlate XXII), and theporphyritic crystals are partly pink felspar and partly glassy quartz. The sp. gr. is 2·62. Under the microscope the quartz and felspar crystals are seen to be less crushed than in the Hamata rock, while the ground mass, instead of being of a matted structure, is microgranitic, with tiny elongated grains of green hornblende scattered through it (seeFig. 11).

Another quartz-felsite dyke at Gebel Kolaiqo [12,135] has a rather different composition, and in the hand specimen has more the appearance of a felspar porphyry, porphyritic white felspars being scattered with quartz grains through a dark grey to black ground mass (seePlate XXII). Its quartz crystals frequently approximate to idiomorphic forms; they are much corroded by the ground mass. The felspar is chiefly oligoclase, in more or less idiomorphic crystals, a little clouded by decomposition products. The ferro-magnesian mineral here is chiefly brown biotite, in crystals of smaller size than those of the quartz and felspar, scattered porphyritically through the ground mass. One of the quartz crystals includes a crystal of biotite. The ground mass is crystalline, and appears to consist of quartz and felspar with a little biotite and some glassy matter.

A peculiar type of quartz-felsite [10,377] occurs associated with serpentine and ophicalcites in the upper part of Gebel Ghadir. The rock, which has a sp. gr. of 2·66, is nearly white, with a faint greyish tinge and scattered reddish-brown specks. At first the rock was taken for a granulite, which it very much resembles in appearance, and the reddish-brown specks for garnet. But examination with a lens shows the specks to be ferric oxide, and here and there little crystals of glassy quartz can be seen. Under the microscope the rock is found to consist mainly of an extremely fine-grained semi-granulitic colourless ground mass in which are sparsely scattered clear crystals of quartz, sometimes showing corrosion, and straggling irregular patches of opaque iron oxide, often mixed with nearly colourless wisps of mica. A few of the clear crystals are in four-sided forms, and may possibly be felspar; but they are perfectly free from alteration or twinning. The iron oxide is doubtless the product of alteration of a ferro-magnesian mineral, probably biotite. The minerals of the ground mass are difficult of determination owing to the minuteness of the grains and the absence of any colour, but apparently consist of quartz, felspar, a colourless hornblende, and sericite.

The quartz-felsite dykes [10,356] which cut through the granite and schists of Gebel Muelih appear as grooves in the granite, and as back-bone ridges in the surrounding schists. They are pinkish-brown close-textured rocks, with a dark marbling in places. The microscopic slides show quartz and felspar in a confused ground mass, with little flakes of a white micaceous mineral (sericite?) aggregated round the porphyritic crystals and distributed through the felspars and the ground mass.

The highly altered quartz-felsite [12,158] which forms Gebel Butitelib, near Gebel Niqrub el Tahtani, appears to have formed a boss or neck in the surrounding schists. The rock is a very fine-grained grey to purplish one, in which no crystals can usually be made out with the unaided eye. The sp. gr. is very low, being only 2·32. Under the microscope one sees small highly corroded crystals of quartz scattered through a clouded and glassy ground mass. No traces of felspars can be seen, the crystals, if they existed, having been decomposed and become undistinguishable from the ground mass; nor can any ferro-magnesian minerals be made out, though there are tiny granules and wisps of iron oxides which in places show an arrangement suggestive of their having been derived from the decomposition of such minerals.

The specimen [12,121] brought back from the summit of Gebel Shendib, by the guide who was sent to erect the triangulation beacon on it, is a compact brown rock with reddish patches, which under the microscope appears to be an altered and brecciated felsite.[130]The slide shows quartz and altered felspars in a spherulitic ground mass, the whole of the mineral being very much clouded by finely disseminated ferric oxide.

The felsite dykes [11,538] which seam the granite of Gebel Um Reit are a still more highly altered rock. The dykes are about two metres wide, of a white colour, and resemble limestone in appearance; being soft, they have weathered more rapidly than the granite around them, leaving vertical-sided chasms. But for the manner of its occurrence, the rock would have been easily mistaken in the field for a sedimentary one. The microscope, however, confirms its eruptive origin. The slides cut from the rock show a confused and cloudedsemi-crystalline aggregate, containing clearer small areas of quartz. Even the quartz crystals are full of specks, and only the faintest traces can be seen of the felspars, which are so decomposed as to be hardly distinguishable from the ground mass.

Fig. 12.—Altered quartz-felsite, Gebel Igli el Iswid [10,372], as seen between crossed nicols, × 40.f, felspar (mostly orthoclase in the position of extinction);h, a crystal of hornblende;g, hemi-crystalline ground mass;m, micropegmatitic intergrowth of quartz and felspar, separating the porphyritic crystals.

In the rocks of Gebels Igli el Iswid, Mahali, and Hadarba, we have quartz-felsites which have been altered in quite a different way. The rocks are extremely hard, and almost flinty, breaking with a sub-conchoidal fracture. They are typically of a brown to nearly black colour, with white spots measuring a millimetre or two across. Under the microscope these white spots are seen in the rock from Gebel Igli el Iswid [10,372] to be chiefly porphyritic felspar and quartz crystals, sometimes corroded by the ground mass; while the black ground mass is largely composed of extremely fine micro-pegmatitic intergrowths of quartz and felspar, with some glassy matter, and here and there irregular small clouded and altered crystals of hornblende. The micro-pegmatitic material has possibly originated by devitrification of an originally glassy ground mass. Variations of the rock occur in which the porphyritic felspars are infrequent or even altogether absent, the rock passing gradually into a hornfels [10,371]. The slide from Gebel Mahali [10,402] exhibits clear porphyritic crystals of quartz and orthoclase, both minerals in more or less idiomorphic forms, embedded in, and occasionally corroded by, a semi-crystalline ground mass. The ground mass contains some calcite and iron oxides, probably arising from the decomposition of a hornblendic or micaceous mineral.

Fig. 13.—Microperthitic structure in felspar of quartz-felsite, Gebel Hadarba [12,147], as seen between crossed nicols, × 40.

The rocks of Gebel Hadarba [12,146-12,149] are essentially similar to those of Gebel Igli el Iswid, but in some cases they show bright red veining; where they have been exposed to the polishing agency of the sand blast, these veined varieties look as though they had beenstreaked with melted sealing-wax. In some of the slides the felspars show a well marked microperthitic structure, while in others they are so decomposed that they are barely distinguishable from the ground mass. The ground mass sometimes contains granules and strings of iron-oxide, possibly referable to alteration of a hornblendic constituent, but the rock is too much altered for one to be sure.

Fig. 14.—Crushed oligoclase crystal in quartz-felsite, Wadi Huluz [10,404], as seen between crossed nicols, × 40.

An altered and crushed quartz-felsite [10,404] which occurs in the Wadi Huluz, near where the Wadi el Abiad joins it about eight kilometres to the north-west of Gebel Hamata, somewhat resembles the rock of Gebel Hadarba in appearance. It is a jaspery looking rock of dark colour with red and greenish patches, in which with a lens one can see scattered grains of glassy quartz. The slide cut from the rock shows clear porphyritic crystals of quartz, orthoclase and oligoclase in a fine grained ground mass. The porphyritic crystals are mostly in rounded forms, but some of the felspars show a tendency to idiomorphism. Many of the crystals are smashed, and some show undulose extinction. The ground mass consists of quartz, felspar, and some glassy matter, with abundant epidote in nests and strings, and a small amount of green hornblende in straggling forms.

The east-and-west dykes which traverse the syenite of Gebel Zergat Naam, and form the actual summit of that mountain, consist of a very hard compact flesh-coloured rock which weathers brown on the surface. The microscopic section [11,525] shows the rock to bea highly indurated felsite. The original felspar crystals can be made out by their shapes, but between crossed nicols they are seen to consist of a very fine-grained mosaic polarising in low greys, probably the result of alteration by siliceous solutions. These altered felspars are scattered with some quartz in a cryptocrystalline ground mass.

No volcanic rocks of acid type have been with certainty identified in South-Eastern Egypt. It is, however, possible that some of the rocks above described are in reality of volcanic origin, though the manner of their occurrence is more suggestive of intruded masses and sheets. It is also quite likely that some of the hornfels found associated with the schists, as for instance at Gebel Um Semiuki, near Abu Hamamid, are altered forms of glassy acid lavas; but their vast age and the intense metamorphism to which they and the surrounding rocks have been subjected render it impossible to be certain of the manner of their origin.

Though hornblendic varieties of granite are fairly common, true syenite (i.e., orthoclase-hornblende rocks with little or no quartz) is amongst the rarest of rocks in South-Eastern Egypt. It occurs near Gebel Nazla (between Bir Um Gubur and Bir Masur), in the two remarkable conical hills called El Nahud, which rise from the plain near the head of Wadi Natash, and at Gebel Zergat Naam. In all these three localities the syenite appears to form intrusive bosses rising through the surrounding rocks.

The specimens from near Gebel Nazla [10,625] and from El Nahud [10,857] are very fine-grained reddish-brown rocks, which can be seen with a lens to be largely made up of red orthoclase crystals with dark specks of hornblende.

Fig. 15.—Syenite of Gebel Zergat Naam [11,515], × 10.o, orthoclase felspar, showing striations parallel to the basal planes;h, hornblende deep green in colour;g, interstitial quartz.Fig. 16.—Syenite-porphyry, Gebel Zergat Naam [11,512], as seen between crossed nicols, × 10.f, porphyritic felspar (orthoclase);h, hornblende;g, microgranitic ground mass containing felspar, hornblende, and some quartz.

Fig. 15.—Syenite of Gebel Zergat Naam [11,515], × 10.o, orthoclase felspar, showing striations parallel to the basal planes;h, hornblende deep green in colour;g, interstitial quartz.

Fig. 16.—Syenite-porphyry, Gebel Zergat Naam [11,512], as seen between crossed nicols, × 10.f, porphyritic felspar (orthoclase);h, hornblende;g, microgranitic ground mass containing felspar, hornblende, and some quartz.

The syenite of Gebel Zergat Naam [11,515] rises as a great boss from among the surrounding dark schistose rocks. It is typically a pinkish-brown rock of rather fine grain (seePlate XXIII), in which can be seen shining crystals, three to four millimetres long, ofpink orthoclase, and specks of dark hornblende, with here and there a little glassy-looking quartz. The sp. gr. is 2·62. Under the microscope the rock is found to be mainly composed of orthoclase, with a much smaller amount of hornblende and a little interstitial quartz. The felspar is fairly clear, in forms approximating to idiomorphic, and showing the characteristic simple twinning of orthoclase. The crystals are slightly clouded in streaks parallel to the basal planes, so that even in ordinary light a faint herring-bone structure is visible, the streaks on either side of the trace of the plane of composition being inclined to each other at a large angle. Between crossed nicols this herring-bone structure is very strongly marked, and is evidently due to a perthitic intergrowth (of albite?). The intergrown lamellæ are slightly irregular; though following generally the direction of the basal cleavage planes they are not perfectly straight, nor always continuous across the half of the crystal. The lamellæ extinguish and attain their maximum of brightness simultaneously with the respective halves of the crystal in which they occur, so that they become invisible in certain positions of the nicols; but as the nicols are turned they appear as well marked dark bands, clearly visible even under low powers. The hornblende is of exceptionally dark green colour, in irregular straggling masses, often considerably decomposed and clouded with iron oxide. Accessory minerals, other thanthe clear interstitial quartz, appear to be almost entirely absent in the slide examined.

Ball.—Geography & Geology of South-Eastern Egypt.PLATE XXIII.INTERMEDIATE IGNEOUS ROCKS.PINK SYENITE.Gebel Zergat Naam.TRACHYTE.Gebel Kahfa.DIORITE.Gebel Allawi.DIORITE WITH PRISMATIC HORNBLENDE.Wadi Muqur.BANDED ANDESITE.Gebel Sufra.VEINED KERSANTITE.Gebel Fereyid.NATURAL SIZE.

INTERMEDIATE IGNEOUS ROCKS.

PINK SYENITE.Gebel Zergat Naam.TRACHYTE.Gebel Kahfa.

PINK SYENITE.Gebel Zergat Naam.

TRACHYTE.Gebel Kahfa.

DIORITE.Gebel Allawi.DIORITE WITH PRISMATIC HORNBLENDE.Wadi Muqur.

DIORITE.Gebel Allawi.

DIORITE WITH PRISMATIC HORNBLENDE.Wadi Muqur.

BANDED ANDESITE.Gebel Sufra.VEINED KERSANTITE.Gebel Fereyid.

BANDED ANDESITE.Gebel Sufra.

VEINED KERSANTITE.Gebel Fereyid.

NATURAL SIZE.

In places, the rock of Gebel Zergat Naam takes on a finer grain and a greyer aspect in the mass. A slide cut from this variety [11,512] shows essentially the same composition, but the felspars are here porphyritic in a crystalline ground mass of felspar and hornblende. The rock thus passes into syenite-porphyry. The porphyritic felspar crystals show the same lamellar structure as those in the more coarsely crystalline rock.

Fig. 17.—Trachyte, from a dyke at Gebel Kahfa [11,537 A] × 10. A clear crystal of orthoclase felspar is seen in the centre of the field, surrounded by a finely crystalline ground mass of hornblende and felspar.

Trachyte, the volcanic representative of syenite, occurs in dykes [11,537 A] seaming the granite of Gebel Kahfa. It is a light grey rock, of very fine grain, breaking with a rough surface, containing pores and small stumpy white pearly-looking porphyritic crystals of orthoclase (seePlate XXIII). The sp. gr. is 2·56. The microscope shows the ground mass surrounding the porphyritic idiomorphic orthoclase crystals to be a holocrystalline mixture of hornblende and felspar. The felspars of the ground mass are usually in more elongated forms than the porphyritic crystals, and appear to be partly plagioclase. The hornblende, in small and very irregular crystals of dark olive-green colour, often clouded and dirty looking, is scattered plentifully among the feslpars of the ground mass. Accessory primary minerals appear to be entirely absent.

A rock which occurs in a hill at the head of Wadi Amba-ut [10,375] appears to be essentially similar to the above, but the porphyritic orthoclases are more numerous and the ground mass contains small quantities of augite and magnetite. This rock is in a highly altered condition, the felspars being full of kaolin and epidote, while the hornblende, which is here of a paler colour than in the trachyte of Gebel Kahfa, is highly chloritised.

Though not forming such conspicuous features as the granites, rocks of dioritic composition are very widely distributed over the country, and are specially abundant in the districts to the south of Ras Benas. They are almost always sharply marked-off from the granites, being generally closely connected with the more basic igneous rocks and schists. The coarser-grained varieties usually form parts of irregular intrusive masses, shading off gradually into the more basic forms of diabase, gabbro, and hornblende rock; the finer-grained types occur as dykes and irregular bands traversing other igneous rocks and schists, and are themselves frequently so crushed as to resemble schists. Owing to their intimate association with other dark rocks, it is no easy matter to trace the limits of the diorites in the field. The same difficulty is found when they are microscopically examined, for one finds in their mineralogical composition every gradation from true diorites, through the intermediate stage of augite-diorite to diabase and gabbro, while many of the harder dark schists and hornblende gneisses turn out to be merely highly crushed diorites, so that classification must be more or less arbitrary.

Diorite typically forms low hill country of dark aspect. The rock weathers as a rule far more easily than granite, and in some cases the debris of rounded grains set free by disintegration cover the surface and render the climbing of the hills somewhat dangerous, the effect being like one would imagine to result from walking over slopes covered with hard peas.

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