CHAPTER X.

Fig. 47.—Serpentine derived from an almost pure olivine-rock, from a low hill east of Bir Abraq [11,506], × 17. The outlines of the original olivine crystals (o) and the cracks in them are marked by granules and strings of magnetite. A serpentinised crystal of bronzite is seen atb.Fig. 48.—A small portion of the serpentine derived from olivine in the same slide, viewed between crossed nicols under a higher power, showing the cellular structure.A serpentine rock found associated with fine-grained diorite at Gebel Um Tenedba [11,517 B] may have been derived from an olivine diabase poor in felspar. The rocks in this locality are highly altered and crushed, and frequently more or less schistose. In a slide cut from the serpentine of the lower part of the mountain, which has a sp. gr. of 2·71, all the original minerals of the rock have been replaced by alteration products. The main portion of the slide is probably altered augite; it consists of serpentine with fairly distinct mesh structure in which iron oxides are very abundant in clots and strings. Some patches, which show a more uniform grey aspect under crossed nicols, are doubtfully referred to altered felspar. Smaller areas, usually free from iron oxide grains and mostly consisting of almost isotropic serpentine with a substance polarising in low yellow tints running along irregular cracks in all directions, are believed to represent altered olivine; one of these areas is nearly filled with little brushes of the substance just mentioned, giving it almost a spherulitic appearance as the nicols are rotated.Fig. 49.—Serpentine, derived from wehrlite, Gebel Gerf [12,128], × 17.s, serpentine (mainly altered olivine, with strings of magnetite);o, unaltered olivine, mixed with granules of augite;d, diallage.The predominant rock of the mountain mass of Gebel Gerf [12,128] is a serpentine apparently derived from a wehrlite or olivine-diallage rock. In the hand specimen, it is a rather heavy black rock (sp. gr. 2·75) with small pea-green spots and a few small bronzy and glassy looking crystals here and there. The microscopic slide (seeFig. 49) shows the olivine to be largely altered to serpentine in the ordinary manner with separation of magnetite, but some granules of the original mineral remain, especially aggregated, together with augite granules, round the diallage crystals. The diallage is tolerably fresh, and easily recognisable by its fine striation and oblique extinction. Besides the diallage, there is some ordinary augite, not always very easily distinguishable from the unaltered olivine owing to its cleavage not being very marked. Some of the diallage crystals show undulose extinction as the result of strain.Fig. 50.—Serpentinisation of bronzite, as seen in a serpentine derived from bronzite-rock, Gebel Gerf [12,119], under a high power between crossed nicols. The clearer portions of the figure are unaltered portions of a single large crystal of bronzite; the serpentine is seen forming mainly along the cleavages of the original mineral, with subsidiary cross-fibres.At many points the serpentine of Gebel Gerf contains a great deal of bronzite, crystals of this mineral of typical aspect and measuring up to five millimetres diameter being visible in the hand specimen scattered liberally through the duller serpentine. The sp. gr. of this portion of the mass is 2·81. The slide [12,119] shows practically nothing but bronzite altering to serpentine, so that locally the parent rock has been a pyroxenite rather than a peridotite. The serpentinisation of the bronzite has proceeded mainly along the fibres of the original mineral, but there are numerous cross-fibres (seeFig. 50), and here, as in the hornblende of the rock of Gebel Abu Dahr, the change to serpentine appears to have been accompanied by the production oftremolite. In another slide [12,127] the main constituent appears to be still bronzite, but here it is of a pronounced olive-brown colour even in thin section, and is accompanied by some nearly colourless diallage; some of the olive-brown fibrous crystals show extinction-angles of a few degrees, and it is possible that these are hornblende.Though the greater part of the Gerf serpentine has been derived from rocks free, or practically free, from felspar, it is possible that these were mixed with other rocks, such as gabbros and diabases, in which basic plagioclase formed an essential constituent. One of the specimens brought back from the mountain [12,115], having a sp. gr. of 2·67, is a somewhat wedge-shaped block, about ten centimetres square and four centimetres thick, covered entirely with the shining black to greenish glaze which is characteristic of shattered serpentine, except where chipped with the hammer to test its coherence and to look for remains of crystals in it. The fractured surface is mostly dull and nearly black, with little strings and spots of greenish matter and scattered shining specks. But although the block so thoroughly resembles a serpentine, the slide cut from it proves the rock to be a fine-grained diabase, consisting essentially of augite and plagioclase, with a good deal of iron oxide. The plagioclase is tolerably fresh, in lath-shaped forms still showing twinning very clearly, while the augite is very clouded and is changing to hornblende and chlorite or serpentine. In the field nothing was noted which would separate this rock from the rest of the mass, which it resembles almost perfectly in appearance, and whether the diabase forms a dyke or sheet, or the specimen represents a fragment of diabase caught up by the magma, is not certain. A dyke of this material, shattered and serpentinised on all the fractured surfaces, would be indistinguishable from the surrounding serpentine. It is even possible, though not likely, that the diabase is not a separate intrusion, but merely a variation of the same plutonic mass which has given origin to the main bulk of the serpentine.Other variations in the serpentine which point to parts of the Gerf mass having originally been of a gabbroid type, occur in the mountains round Bir Korbiai and Bir Meneiga. A specimen from near Bir Korbiai [12,125], is a greyish-black fine-grained rock full of little white and greenish specks and strings, with pale citron-green serpentinous matter covering the slickensided surfaces of the fragmentsinto which the mass readily separates. The sp. gr. is 2·67. The microscopic slide shows serpentine with pronounced knitted structure in places, as though derived from augite, and considerable patches of calcite which may represent an original lime-felspar. The calcite has, however, been largely redistributed along cracks, where it is often mixed with extremely coarse fibres or plates of serpentine.The specimen from the mountains round Bir Meneiga [12,104] is a hard dull slatey-grey rock with black streaks and spots; it is strongly magnetic and shows a tendency to schistose structure. The sp. gr. is 2·54. On examination with a lens the grey matter has in places something of a resemblance to saussuritised felspar. The slide cut from this specimen contains no original minerals. The bulk of it seems to be composed of very minute fibres or plates, generally showing a distinctly parallel arrangement along the planes of schistosity, but exhibiting knitted structure in places, and thus probably consisting in part of kaolinic matter derived from altered felspar, and in part of serpentinous matter derived from alteration of augite; while scattered through the slide are abundant patches and strings of calcite and magnetite. The rock is too intensely crushed and altered for one to be certain of its origin, but it appears to have been originally a diabase.Fig. 51.—Serpentine derived from alteration of harzburgite, Gebel Korabkansi [12,140], × 17.s, serpentine, mostly formed from olivine;o, unaltered olivine;b, altering bronzite;c, chromite or magnetite, enveloping olivine.The serpentine of Gebel Korabkansi, to the west of Gebel Gerf, is an altered olivine-bronzite rock (harzburgite) of moderately coarse grain. A slide [12,140] cut from the least altered portion, where the sp. gr. of the rock is 2·72, shows olivine to have been the most abundant constituent; it is largely serpentinised in the typical manner, but abundant kernels remain unaltered. Mixed with the olivine crystals are others of a fibrous mineral, altered partly to serpentine and partly to a confused mass of tremolite fibres; in the few cases where any of the original material remains, this fibrous mineral shows straight extinction, and is probably bronzite. The alterationof the pyroxene has here proceeded more rapidly than that of the olivine. The pyroxenic mineral is easily picked out in the slide by ordinary light, owing to the fact that the olivine has altered to greenish-yellow serpentine, while the mixed material produced from the pyroxene is of a dirty-white aspect. Another slide from Gebel Korabkansi [12,112] exhibits in places mesh-structure with clear kernels which extinguish very obliquely to the meshes, showing augite or diallage also to have been a constituent of the parent rock; and there is a single ragged-looking crystal of dark green fibrous hornblende, altering to nearly colourless serpentine. The sp. gr. of the specimen from which this latter slide was cut is 2·63. Both the slides from Gebel Korabkansi are rather poor in iron oxides, but there are a few irregular crystals of chromite, generally enveloping olivine.Alteration Products of Serpentine. Magnesite and Asbestos.In many of the serpentine-masses the effects of solution and pressure can be traced in the formation of veins and pockets of magnesite, asbestos, and occasionally talc in the rock. Magnesite veinlets are frequently seen, for instance, round about Bir Meneiga [12,142] and Bir Muqur, while asbestos has been found in pieces of several kilogrammes weight in the Wadi Sherefa el Sherqi [12,122] and near Bir Muqur [12,153]. At the last-named locality it occurs along the crush-planes of the serpentine, and appears to be preceded by the formation of long fibres of a green substance.[133]Where thickest, the glaze on the slickensided surfaces of the serpentine fragments seems to consist of this same green substance, which splits into hard fibres often reaching ten centimetres or more in length and only a few millimetres in diameter [12,153 A] and these hard fibres, which have a sp. gr. of 2·64, break up gradually, apparently under weathering influences, into the much finer and more flexible fibres of white asbestos. The quality of the asbestos is, however, inferior, for a specimen of the softest kind sent to London for an expert opinion, was reported to be of no commercial value; even had it been otherwise, the quantity seen was too small to justify working, though of course a thorough search among the mountains might reveal the presence of bigger deposits than were encountered during the survey.[125]FromMarwa, the Arabic term for quartz.[126]The numbers in square brackets in this and the following chapter are the registration numbers of the specimens in the Geological Museum, Cairo.[127]I may here recall that the quartz mass which constitutes the so-called “alabaster” quarry north of Aswân, and for which I have inferred an igneous origin, is likewise closely associated with aplitic or pegmatitic intrusions. SeeA Description of the First or Aswân Cataract. Cairo, 1907, p. 84.[128]This specimen was brought back by a guide sent for the purpose to Gebel Adar Qaqa. It is stated to be the main rock of the mountain, but as I did not visit the mountain I am unable to vouch personally for this being the case.[129]From a specimen brought back by the guide who was sent to erect the triangulation beacon on Gebel Mishbih.[130]I did not visit Gebel Shendib, but as seen from Gebel Elba the mountain appeared to be granite; the felsite probably forms a dyke passing through the summit.[131]There is another Gebel el Anbat near the Wadi Hodein, but this latter consists of schists.[132]Preliminary Report on the Geology of the Eastern Desert of Egypt, between latitude 22° N. and 25° N., by Dr. W. F. Hume. Cairo, 1907. p. 57.[133]This fibrous green substance is probably the columnar variety of serpentine known asPicrolite; it strongly resembles the specimens thus labelled in the London Museum of Practical Geology.CHAPTER X.METAMORPHIC ROCKS.Metamorphic rocks, comprising a great variety of gneisses and schists, are widely and abundantly distributed in South-Eastern Egypt, forming about half the entire area treated of in this volume. The irregular way in which the metamorphic rocks alternate with those of the igneous and sedimentary groups will be better appreciated by a reference to the geological map onPlate XXthan by any verbal description. As a whole, the metamorphic rocks represent the remains of a complex of igneous and sedimentary deposits which were laid down in remote geological periods; subsequently these deposits were crushed, folded, and elevated into mountains, their original structures being largely obliterated in the dynamical process; in succeeding ages the metamorphosed complex was penetrated by the great granitic and other intrusions which have been described in the preceding chapter, and then planed down by denudation as an old land surface before being wholly or in part submerged beneath the waters of the Cretaceous sea in which the Nubian sandstone was deposited. The elevation of the land after the Cretaceous period was accompanied by further folding which must have increased the degree of metamorphism of the rocks; but it is most probable that the forces which produced these post-Cretaceous movements were of less intensity and duration than those of more ancient periods, and we may regard the metamorphic character of the older rocks as having been for the most part impressed on them in Archæan, or at latest in Palæozoic times. It is not impossible that some of the more highly foliated gneisses may even have been parts of the primitive crust of consolidation of the earth, while the clay schists may be the remains of the very earliest sediments.Besides the evidences of dynamo-metamorphism above-mentioned, we find occasionally traces of contact-metamorphism, where igneous dykes have locally altered the rocks into which they haveintruded; but, in general, the subsequent compression of the masses has obliterated these traces, and the effects of contact-metamorphism are negligible in comparison with those produced by dynamic action.The metamorphic rocks occurring in this part of Egypt may be classified as follows:—Gneisses.Granite-gneiss(crushed granite).Diorite-gneiss(crushed diorite).Highly foliated gneisses(in which all traces of primitive structure have been obliterated by foliation and recrystallization).Schists.Quartz-schists(in some cases probably metamorphosed sandstone, in others crushed aplitic rocks).Clay-schists(metamorphosed clays).Hornblende-schists and crushed volcanic rocks.Hornfels(metamorphosed acid lavas).Mica-schists(sometimes containing beryl and tourmaline).Chlorite-schists.Talc-schists.Calcareous schists and marble.Graphite-schists.Breccias.Fluidal breccias, in which broken fragments of solid rock have been included in an intrusive magma.Breccias proper, formed by fracture and recementing together of fragments without fusion.Mineral Veins.Quartz veins(sometimes containing gold and traces of copper).Calcite veins(with more or less chalybite).Magnesite veins(in serpentine).Asbestos veins(in serpentine).Gneisses.The most conspicuous occurrence of gneiss in South-Eastern Egypt is the great mass which extends from Gebel Mudergeg and Gebel Migif in the north, down the range of Gebel Hafafit, and along the greater part of the Wadi Gemal. With this mass is closely connected another stretch of gneiss to the south of Gebel Abu Khrug; the hills which rise from the sandy plain round the head of Wadi Natash are mostly gneiss, so that the gneiss is probably continuous across the plain except where it is penetrated by the syenitic cones called El Nahud. The total area covered by gneiss in this locality alone is some two thousand square kilometres, and includes some remarkable mountain masses, such as Gebels Migif and Hafafit.Other gneissose areas of less extent occur round Bir Abu Beid and to the south-west of Gebel Hamrat el Feg; while still smaller areas are found in the hills to the north-west of Berenice, in the remarkable triple-headed mountain called Gebel Um Rasein, and to the north of Bir Um Bishtit.Gneiss typically forms bold scenery of precipitous hills, the colour of which varies with the composition of the rock, from pink to grey. At Gebel Migif one ascends the mountain most easily by the northern slopes, which are formed by the foliation-planes of the rock, while to the south these are cut across by sheer precipices; the same is the case at Gebel Um Rasein.Fig. 52.—Granite-gneiss, Gebel Um Rasein [12,108], viewed between crossed nicols, × 30.q, quartz;ol, oligoclase;or, orthoclase;h, hornblende;m, “mortar” of crushed quartz and felspar between the larger crystals. The black areas are quartz in the position of extinction.Of gneisses evidently produced by the deformation of granite, we may take the rock of Gebel Um Rasein [12,108][134]as a type. The bulk of the mountain is formed of a nearly white acid rock, of sp. gr. 2·67, with dark spots, resembling the granite of Gebel Elba in appearance, except that here the dark minerals have a marked arrangement along definite planes. (Compare the coloured figures of the two rocks on PlatesXXIIandXXV). In places the rock contains thin bands of almost pure biotite, and these are often thrown into sharp contortions by the strong compression which the mass has undergone. The strike of the foliation-planes in Gebel Um Rasein itself is about south-south-east, with a dip of about 20° to the east-north-east, but thestrike swings round to about south at the south end of the mass, and the inclination of the foliation-planes become much steeper. The gneiss shows very marked pitting due to weathering, the holes ranging from a few centimetres to a metre or more in diameter and depth. A microscopic section of the rock shows it to be essentially an acid hornblende-granite similar to that of Gebel Elba, but crushed so that there is a “mortar” of broken up quartz and felspar between the larger crystals, and the larger where unbroken frequently show undulose extinction as the result of strain.Fig. 53.—Diorite-gneiss, Wadi Nugrus [10,386], × 17.h, hornblende, with small enclosures of felspar and sphene;pl, plagioclase felspar;b, biotite;q, quartz;ap, apatite;sp, sphene.In the Wadis Nugrus and Gemal there is a great deal of strongly foliated grey gneiss [10,386-7], derived from the intense crushing of diorites. A figure from a typical hand specimen of this gneiss, looking at the edges of the foliation-planes, is given onPlate XXV.The sp. gr. of the rock is 2·88. The microscopic slides show plagioclase and green hornblende in about equal proportions, with some quartz and small amounts of biotite, sphene, iron oxides, and apatite. The hornblende is very fresh-looking, in strongly pleochroic (yellowish to deep blue-green) straggling crystals, aggregated together along the foliation-planes; the crystals, though variously oriented, show a tendency to lie with their cleavages in the direction of foliation. A verystriking feature of the slides is the abundance of small grains of enclosed felspar in the hornblende, giving the green crystals a perforated appearance in ordinary light. The felspars, which are likewise fairly fresh, are much smashed up, forming with the quartz a mosaic between the irregular bands of hornblende; the grains often show undulose extinction. Biotite, of a warm brown colour where not bleached, is sparingly mixed with the hornblende. The sphene is in small rounded grains, mixed with, and sometimes containing, granules of ilmenite; the grains show a tendency to form little aggregates, sometimes in the hornblende, and sometimes in the felspathic portions of the slide. One or two stumpy grains of apatite are enclosed in the felspars, but this mineral is extremely scarce in the slides examined, and does not show its usual long prismatic habit.Ball.—Geography & Geology of South-Eastern Egypt.PLATE XXV.METAMORPHIC ROCKS.GRANITE-GNEISS.Gebel Um Rasein.RED GNEISS.Gebel Abu Beid.DIORITE-GNEISS.Wadi Nugrus.GREEN BRECCIA.Gebel Hamata.EMERALD AND QUARTZ IN MICA-SCHIST.Gebel Sikait.NATURAL SIZE.As an example of gneisses in which the foliation is so intense that all original structures have been obliterated by recrystallization, we may take the red gneiss of the Abu Beid Hills [10,659], of which a coloured natural size representation is given onPlate XXV.In this rock, which has a sp. gr. of 2·65, the main constituent is a pink felspar, mixed with a little quartz and arranged in fine lenticles separated by greenish laminæ which are doubtless altering biotite. This rock bears a strong resemblance to the well-known red gneiss of the Erzgebirge, which, from its chemical composition, is usually believed to be a metamorphosed granite poor in mica.Grey gneisses, equally intensely foliated with the Abu Beid rock, occur in the range of Gebel Hafafit; these contain a much larger proportion of biotite, and remind one of the grey gneisses of the Freiberg district. But as no analyses have yet been made of the Egyptian rocks, the absolute similarity to those of the Erzgebirge is not quite certain.Quartzites and Quartz-schists.About ten kilometres south of Gebel Abu Gurdi is a tract of low hills and ridges composed mainly of quartz-schists. The limits of these rocks have not been mapped, but they appear to cover a somewhat elliptical area with its long axis, measuring some eight or ten kilometres, pointing east and west. Where first encountered, at a point about twelve kilometres east of Gebel Selaia, on the march from thathill towards the pass into the head of Wadi Lahami, the rocks strongly resemble rather friable white to brown fine-grained sandstones, with bedding planes dipping about 35° to the north. The rock [10,413] consists almost entirely of white or slightly iron-stained quartz grains, with, in places, scattered minute dull black rods which are apparently hornblende. Traced further east, the rocks have more and more the character of quartz schists, with harder bands of quartzite, and the planes of separation, which gradually become nearly horizontal, have more the appearance of foliation-planes. The harder quartzite bands [10,414] are usually brownish to greenish-white in colour, but sometimes take on a purple tint, and show micaceous and chloritic scales on cleaved surfaces; in some places the rock contains “eyes” of minette-like composition. To the south the bedding or cleavage planes seem to reverse their dip, this being now to the south as though there were an anticlinical fold in the mass. There is considerable mixture of other schists with the rocks, and dykes of diorite and pegmatite, as well as veins of quartz, are frequent. A dioritic dyke which cuts through the rocks at a point nine kilometres south-south-west of Gebel Abu Gurdi, and which is itself crushed almost into a hornblende-schist, has hardened and altered the quartzose rock on either side of it into a close-grained horny looking rock [10,415] apparently containing a great deal of finely granular andalusite.These quartz-schists may have arisen from the metamorphism either of sandstones, in which case the parts to the west of the tract are the least altered, or of fine-grained aplitic rocks, on which view the sandstone-like aspect of the western portion of the tract is due to weathering; the rock is certainly more highly weathered where it most resembles an unaltered sandstone, and the presence of hornblende and the “eyes” of micaceous matter are suggestive of igneous rock. From a consideration of all the evidence I was able to gather in a rapid march over the sea, however, I incline to the view that the rocks are altered sandstone older than the Nubian series, and probably of about the same age as the ancient clays which gave rise to the clay-schists of Zabara and elsewhere. The rocks are completely detached from the nearest undoubted Nubian sandstone beds (which lie some forty kilometres to the south-west, beyond Gebel Zergat Naam), and they are mixed with schists and veined and altered to a degree which is nowhere approached by beds known to be of the Nubian series.Still, it is just possible that the quartz-schists represent the remains of Nubian sandstone strata, for the nearest beds of that series show-considerable folding and faulting near Gebel Zergat Naam, and we should expect still greater disturbance and alteration here near the main watershed and close to a principal axis of mountain elevation.A quartz-schist which is certainly derived from the metamorphism of aplitic dykes or sheets, occurs as great inclined bands cutting across the Wadi Shenshef close to some old ruins. The main rock of the district, in which the bands occur, is a medium-grained diorite. The quartz-schist is a greyish-white fissile rock, which splits easily into great slabs; it has supplied an excellent building material for the houses of the ruined town or encampment, being used not only for the walls but also for lintels over doors and windows.Clay-schists.Clay-schists cover extensive areas in the South-Eastern Desert. They occur abundantly in the high hills to the east of Gebel Nugrus and form a large portion of Gebel Zabara. They abound also to the south of Wadi Antar about the longitude of Gebel Um Goraf, in the hills on the east side of Gebel Abu Dahr, in the Wadi Hodein round about where Wadi Um Tenedba joins it, in the hills flanking the sandstone plateau of Gebels Dif and Anfeib, at Gebels Kolaiqo and Eqrun, on the western flanks of the serpentine mass of Gebel Meneiga, and in the low hills on the plain between Gebels Gerf and Korabkansi.Clay-schists are typically of a grey colour, though greenish, purple, and reddish varieties occur. They are soft rocks, easily scratched with a knife. In some places, as for instance at Gebel Zabara, they are practically slates, cleaving into slabs often of considerable size, while in other localities, as for instance near Gebel Abu Dahr, they are crushed into matchwood-like splinters, which can be used as slate-pencils. They are usually associated with other metamorphic rocks such as crushed diorites and hornblende and mica-schists. The direction of foliation of the slaty and slabby forms is often persistent—at Gebel Zabara, for instance, a general south-easterly dip of about 28° is maintained over a considerable area; but there arenumerous local variations, and the number of observations thus far made is too small for any general conclusions to be drawn as to the regional distribution of dip. On the whole, the clay-schists are remarkably free from knots or other irregularities, though knotted varieties, doubtless the result of contact alteration, have been observed on the south side of Wadi Seiga and at a few other places.The clay-schists have probably for the most part been produced by the metamorphism of ancient argillaceous sediments; in some places, as for instance in the Wadi Beida, a passage can be observed into what appears to be a crushed conglomerate, while in other places they pass into quartzites which doubtless represent altered intercalated sandstones. The relation between the direction of the foliation-planes and that of the original bedding is unknown; it would appear likely that in most cases the traces of bedding have been entirely obliterated by pressure. Nor have any fossils been found in the rocks, though a sharp look-out was kept in the more likely-looking localities, so that the age of the beds is unknown. At present I am inclined to regard them as most likely of Archæan age, antedating the great granitic intrusions; but further evidence, either in the finding of fossils or from a more careful study of the field relations of these rocks to the others, is required before one can be certain that they are of so great an antiquity.Hornblende-schists and Crushed Volcanic Rocks.Under the heading of hornblende-schists and crushed volcanic rocks are classified a great variety of rocks, ranging in colour from green to grey and almost black, in fissility from the finest lamellation to almost massive forms in which the schistosity is barely evident in the hand specimen, but all definable as fine-grained dark-coloured schistose rocks consisting largely either of hornblende or of its alteration products. Their origin is not always clear, but many of them appear on microscopic examination to be altered fine-grained syenites, diorites, and lavas, and it is probable that practically the whole are altered forms of various intermediate and basic igneous rocks. The reason for including the hornblende-schists and crushed volcanic rocks in one great group is the difficulty of separating the different classes in the field. They pass one into the other, and many rockswhich in the field would be put down as hornblendic turn out on detailed examination to contain little or none of that mineral, its place being taken by decomposition products.Schists of the types above defined occur probably more abundantly than any other class of metamorphic rocks in the South-Eastern Desert. They enter largely into the composition of some of the main mountain masses, such as those of Gebels Abu Hamamid and Abu Gurdi, besides covering great expanses of lower hill country. In the mountain ranges they rise in steep-sided ridges with knife edge summits, a good idea of which is given by the photograph of Gebel Abu Hamamid onPlate VII.In the lower hill country we have typically a dreary confused looking waste of thousands of dark hills and ridges separated by small winding wadis; sometimes there is a semblance of system in the distribution of the hills, owing to the occurrence of parallel dykes harder than the schists which they penetrate, forming thus back-bones for long lines of schist-ridges.Fig. 54.—Schist produced by crushing of syenite, near Gebel el Anbat (Wadi Hodein). [11,532 B], × 30.f, crushed and clouded felspar, probably orthoclase;lc, mixture of calcite and limonite, produced by alteration of hornblende, no trace of which remains.Schist derived from Syenite.—Among the schists occurring near Gebel el Anbat, near the Wadi Hodein, is one which appears to be a crushed and altered syenite. It is a speckled grey and reddish-brown rock with a dull and rather granular fracture, of sp. gr. 2·92. The slide [11,532 B] shows a rather coarse granitic mixture of clouded felspar and heavily iron-stained calcite. The felspar is considerably altered, showing no twinning, but between crossed nicols it extinguishes in irregular bands and is seen to be much crushed. The ferruginous and calcareous matter is almost certainly altered hornblende; it forms irregular masses, in which the iron oxide is mostly arranged in parallel lines as though along the cleavage planes of the parent mineral, while a clouded calcite fills up the spaces between the lines. It would seem that in addition to pressure, carbonic acid has been the main agent of metamorphism here, the ferro-magnesiansilicate being attacked and its silica entirely removed, while the aluminous silicate of the felspar has remained to some extent unchanged.Fig. 55.—Hornblende-schist (probably a metamorphosed fine-grained diorite), Wadi Muelih [10,357], × 40.h, pale green hornblende;f, mass of decomposed felspar, containing abundance of finely acicular hornblende and some little patches of quartz mosaic;m, magnetite.Schists derived from Diorites.—Hornblendic schists which appear to have been produced by the metamorphism of fine-grained diorites occur largely in the low hill country round the upper parts of the Wadi Muelih, and in the mountain mass of Gebel Abu Gurdi. A specimen of the less altered rock from the Wadi Muelih [10,357] is a fine-grained hard grey basaltic-looking rock, of sp. gr. 3·04, in which the schistosity in not very evident, though it is well seen in the mass. The microscopic slide shows pale green hornblende in ragged fibrous forms in a matted-looking clouded ground mass made up of finely acicular green hornblende and plagioclase, with a little quartz and orthoclase and a few grains of magnetite. Hornblende fibres are often enclosed in the decomposing felspars, which rarely show definite outlines and appear much shattered. The quartz is probably of secondary formation, occurring as little patches of mosaic.The schist which forms the summit of Gebel Abu Gurdi [10,416] is probably also a crushed and altered diorite; it is a rather fine-grained hard greenish-grey rock, which with a lens can be seen to be a mixture of greenish-white felspathic material and dark hornblende; the felspar is mostly dull, while the hornblende, on the other hand, is frequently in shining crystals. The sp. gr. of the rock is 3·02. The microscopic slide shows the hornblende to be of a very pale green colour, in large irregular crystals, often including felspars. The felspar appears to be plagioclase; it is typically in smaller crystals than the hornblende, with a strongly marked tendency to idiomorphism, highly cracked and almost entirely changed to kaolin. Some rather large straggling crystals of a dark brown clouded and semi-opaque highly refractingmineral, white by reflected light, are perhaps altered sphene. Round the larger crystals is more finely crystalline matter, much clouded, apparently composed of altered felspar and hornblende.The crushed dioritic dykes already mentioned as cutting the quartz-schist to the south-west of Gebel Abu Gurdi are possibly offshoots from the same magma which formed the main mass of the mountain. Specimen [10,415], taken from one of these dykes, is a fine-grained grey rock, of sp. gr. 2·93, with an even more decided schistosity than that of the main mountain. The microscopic slide shows clear hornblende of a green colour, strongly pleochroic (greenish-yellow to blue-green) in irregular grains which are frequently aggregated into nests and strings running in the direction of foliation of the rock; the remaining material is a mosaic of clear quartz, clouding kaolin, and sericite, presumably representing altered felspar.Fig. 56.—Schist composed of fragments of various volcanic rocks, summit of Gebel Abu Hamamid [10,397], × 30.Schists formed by crushing of Volcanic Rocks.—Schists derived from the crushing of volcanic rocks are very abundant in the mountains round Gebel Abu Hamamid and in the Wadi Beida. Typical specimens from the summit of Gebel Abu Hamamid [10,397] are hard green to grey rocks, of sp. gr. 2·7, breaking with a rough dull fracture. With a lens, spots and strings of dull white matter, with ill-defined outlines, are seen in a green to nearly black ground mass. The microscopic slides show the rock to be a breccia rather than a simple crushed rock, for in the same slide very various structures can be seen. Some portions, evidently andesitic, consist of perfectly idiomorphic lath-shaped felspars scattered with a little decomposed hornblende in a glassy brown ground mass. Others, more abundant, seem to be altered quartz-diorite-porphyrite; in these parts, porphyritic quartz and orthoclase crystals, in forms strongly inclined to idiomorphism, clouded and strained, are scattered in a cryptocrystalline ground mass containing a good deal of green chlorite and epidote. In other parts of the slide, again, the chief porphyritic constituent is formedby large green grains, which have evidently once been biotite or hornblende, but which now consist of chlorite. In yet other parts of the rock we have fragments of devitrified glassy lava. The different parts, which are not always well outlined, are separated by schistose bands composed mainly of strings of chlorite and epidote. Whether the rock is a tuff, or a crushed conglomerate of igneous boulders and pebbles, or due to complicated crushing of a series of contiguous igneous rocksin situ, is not quite clear. In some places, especially about the Wadi el Sheikh, the schists look like crushed conglomerates, but these may possibly be rocks crushedin siturather than accumulations of rolled fragments transported by streams.The hornblende-schists which surround Gebel Zergat Naam are of a peculiar type of which the origin is not evident, but are possibly altered andesites. The typical rock [11,526] is a hard grey basaltic one of sp. gr. 2·95, with a rusty-brown skin on exposed surfaces. The microscopic slide shows little plates and brushes of nearly colourless hornblende, liberally scattered in a clouded ground mass consisting mainly of hornblende fibres in radiating and dendritic groups, with a small amount of kaolinic matter and a few grains of magnetite. The slide contains no distinct quartz, but possibly a small amount of this mineral may be present in very minute grains with the kaolinic matter.To the metamorphism of andesitic lavas, too, are somewhat doubtfully ascribed the grey and green schists of the Wadi Beida, which have specific gravity of about 2·75. The slides from the grey variety of the schists [12,116 and 12,159] show a very fine-grained clouded rock, apparently a mosaic of quartz, kaolin, chlorite and sericite, with larger scattered irregular plates of dark green chlorite. In a pale green variety from the head of the wadi [12,111], there is less chlorite and a considerable amount of calcite.Hornblende Schists of doubtful origin.—In the more highly foliated and harder varieties of hornblende schist, which are typically of a darker colour than most of those already described, we have rocks in which the process of re-crystallization has been so complete that no trace of the original rock remains. These rocks, which are true hornblende-schists in the narrowest sense of the term, are less abundant than the foregoing types, and generally occur as comparatively narrow bands associated with gneisses and mica and talc schists. All theminerals in them being of secondary origin, they are usually in a quite fresh state, and fractured surfaces, examined with a lens, exhibit a mass of glistening small crystals of hornblende, quartz, and felspar. The density is usually about 2·9. Microscopic slides show elongated crystals of clear green hornblende, strongly pleochroic (pale yellowish-green to deep blue-green) arranged along the planes of schistosity, separated by clear granules of quartz and a little felspar, with scattered magnetite grains.

Fig. 48.—A small portion of the serpentine derived from olivine in the same slide, viewed between crossed nicols under a higher power, showing the cellular structure.

A serpentine rock found associated with fine-grained diorite at Gebel Um Tenedba [11,517 B] may have been derived from an olivine diabase poor in felspar. The rocks in this locality are highly altered and crushed, and frequently more or less schistose. In a slide cut from the serpentine of the lower part of the mountain, which has a sp. gr. of 2·71, all the original minerals of the rock have been replaced by alteration products. The main portion of the slide is probably altered augite; it consists of serpentine with fairly distinct mesh structure in which iron oxides are very abundant in clots and strings. Some patches, which show a more uniform grey aspect under crossed nicols, are doubtfully referred to altered felspar. Smaller areas, usually free from iron oxide grains and mostly consisting of almost isotropic serpentine with a substance polarising in low yellow tints running along irregular cracks in all directions, are believed to represent altered olivine; one of these areas is nearly filled with little brushes of the substance just mentioned, giving it almost a spherulitic appearance as the nicols are rotated.

Fig. 49.—Serpentine, derived from wehrlite, Gebel Gerf [12,128], × 17.s, serpentine (mainly altered olivine, with strings of magnetite);o, unaltered olivine, mixed with granules of augite;d, diallage.

The predominant rock of the mountain mass of Gebel Gerf [12,128] is a serpentine apparently derived from a wehrlite or olivine-diallage rock. In the hand specimen, it is a rather heavy black rock (sp. gr. 2·75) with small pea-green spots and a few small bronzy and glassy looking crystals here and there. The microscopic slide (seeFig. 49) shows the olivine to be largely altered to serpentine in the ordinary manner with separation of magnetite, but some granules of the original mineral remain, especially aggregated, together with augite granules, round the diallage crystals. The diallage is tolerably fresh, and easily recognisable by its fine striation and oblique extinction. Besides the diallage, there is some ordinary augite, not always very easily distinguishable from the unaltered olivine owing to its cleavage not being very marked. Some of the diallage crystals show undulose extinction as the result of strain.

Fig. 50.—Serpentinisation of bronzite, as seen in a serpentine derived from bronzite-rock, Gebel Gerf [12,119], under a high power between crossed nicols. The clearer portions of the figure are unaltered portions of a single large crystal of bronzite; the serpentine is seen forming mainly along the cleavages of the original mineral, with subsidiary cross-fibres.

At many points the serpentine of Gebel Gerf contains a great deal of bronzite, crystals of this mineral of typical aspect and measuring up to five millimetres diameter being visible in the hand specimen scattered liberally through the duller serpentine. The sp. gr. of this portion of the mass is 2·81. The slide [12,119] shows practically nothing but bronzite altering to serpentine, so that locally the parent rock has been a pyroxenite rather than a peridotite. The serpentinisation of the bronzite has proceeded mainly along the fibres of the original mineral, but there are numerous cross-fibres (seeFig. 50), and here, as in the hornblende of the rock of Gebel Abu Dahr, the change to serpentine appears to have been accompanied by the production oftremolite. In another slide [12,127] the main constituent appears to be still bronzite, but here it is of a pronounced olive-brown colour even in thin section, and is accompanied by some nearly colourless diallage; some of the olive-brown fibrous crystals show extinction-angles of a few degrees, and it is possible that these are hornblende.

Though the greater part of the Gerf serpentine has been derived from rocks free, or practically free, from felspar, it is possible that these were mixed with other rocks, such as gabbros and diabases, in which basic plagioclase formed an essential constituent. One of the specimens brought back from the mountain [12,115], having a sp. gr. of 2·67, is a somewhat wedge-shaped block, about ten centimetres square and four centimetres thick, covered entirely with the shining black to greenish glaze which is characteristic of shattered serpentine, except where chipped with the hammer to test its coherence and to look for remains of crystals in it. The fractured surface is mostly dull and nearly black, with little strings and spots of greenish matter and scattered shining specks. But although the block so thoroughly resembles a serpentine, the slide cut from it proves the rock to be a fine-grained diabase, consisting essentially of augite and plagioclase, with a good deal of iron oxide. The plagioclase is tolerably fresh, in lath-shaped forms still showing twinning very clearly, while the augite is very clouded and is changing to hornblende and chlorite or serpentine. In the field nothing was noted which would separate this rock from the rest of the mass, which it resembles almost perfectly in appearance, and whether the diabase forms a dyke or sheet, or the specimen represents a fragment of diabase caught up by the magma, is not certain. A dyke of this material, shattered and serpentinised on all the fractured surfaces, would be indistinguishable from the surrounding serpentine. It is even possible, though not likely, that the diabase is not a separate intrusion, but merely a variation of the same plutonic mass which has given origin to the main bulk of the serpentine.

Other variations in the serpentine which point to parts of the Gerf mass having originally been of a gabbroid type, occur in the mountains round Bir Korbiai and Bir Meneiga. A specimen from near Bir Korbiai [12,125], is a greyish-black fine-grained rock full of little white and greenish specks and strings, with pale citron-green serpentinous matter covering the slickensided surfaces of the fragmentsinto which the mass readily separates. The sp. gr. is 2·67. The microscopic slide shows serpentine with pronounced knitted structure in places, as though derived from augite, and considerable patches of calcite which may represent an original lime-felspar. The calcite has, however, been largely redistributed along cracks, where it is often mixed with extremely coarse fibres or plates of serpentine.

The specimen from the mountains round Bir Meneiga [12,104] is a hard dull slatey-grey rock with black streaks and spots; it is strongly magnetic and shows a tendency to schistose structure. The sp. gr. is 2·54. On examination with a lens the grey matter has in places something of a resemblance to saussuritised felspar. The slide cut from this specimen contains no original minerals. The bulk of it seems to be composed of very minute fibres or plates, generally showing a distinctly parallel arrangement along the planes of schistosity, but exhibiting knitted structure in places, and thus probably consisting in part of kaolinic matter derived from altered felspar, and in part of serpentinous matter derived from alteration of augite; while scattered through the slide are abundant patches and strings of calcite and magnetite. The rock is too intensely crushed and altered for one to be certain of its origin, but it appears to have been originally a diabase.

Fig. 51.—Serpentine derived from alteration of harzburgite, Gebel Korabkansi [12,140], × 17.s, serpentine, mostly formed from olivine;o, unaltered olivine;b, altering bronzite;c, chromite or magnetite, enveloping olivine.

The serpentine of Gebel Korabkansi, to the west of Gebel Gerf, is an altered olivine-bronzite rock (harzburgite) of moderately coarse grain. A slide [12,140] cut from the least altered portion, where the sp. gr. of the rock is 2·72, shows olivine to have been the most abundant constituent; it is largely serpentinised in the typical manner, but abundant kernels remain unaltered. Mixed with the olivine crystals are others of a fibrous mineral, altered partly to serpentine and partly to a confused mass of tremolite fibres; in the few cases where any of the original material remains, this fibrous mineral shows straight extinction, and is probably bronzite. The alterationof the pyroxene has here proceeded more rapidly than that of the olivine. The pyroxenic mineral is easily picked out in the slide by ordinary light, owing to the fact that the olivine has altered to greenish-yellow serpentine, while the mixed material produced from the pyroxene is of a dirty-white aspect. Another slide from Gebel Korabkansi [12,112] exhibits in places mesh-structure with clear kernels which extinguish very obliquely to the meshes, showing augite or diallage also to have been a constituent of the parent rock; and there is a single ragged-looking crystal of dark green fibrous hornblende, altering to nearly colourless serpentine. The sp. gr. of the specimen from which this latter slide was cut is 2·63. Both the slides from Gebel Korabkansi are rather poor in iron oxides, but there are a few irregular crystals of chromite, generally enveloping olivine.

In many of the serpentine-masses the effects of solution and pressure can be traced in the formation of veins and pockets of magnesite, asbestos, and occasionally talc in the rock. Magnesite veinlets are frequently seen, for instance, round about Bir Meneiga [12,142] and Bir Muqur, while asbestos has been found in pieces of several kilogrammes weight in the Wadi Sherefa el Sherqi [12,122] and near Bir Muqur [12,153]. At the last-named locality it occurs along the crush-planes of the serpentine, and appears to be preceded by the formation of long fibres of a green substance.[133]Where thickest, the glaze on the slickensided surfaces of the serpentine fragments seems to consist of this same green substance, which splits into hard fibres often reaching ten centimetres or more in length and only a few millimetres in diameter [12,153 A] and these hard fibres, which have a sp. gr. of 2·64, break up gradually, apparently under weathering influences, into the much finer and more flexible fibres of white asbestos. The quality of the asbestos is, however, inferior, for a specimen of the softest kind sent to London for an expert opinion, was reported to be of no commercial value; even had it been otherwise, the quantity seen was too small to justify working, though of course a thorough search among the mountains might reveal the presence of bigger deposits than were encountered during the survey.

[125]FromMarwa, the Arabic term for quartz.[126]The numbers in square brackets in this and the following chapter are the registration numbers of the specimens in the Geological Museum, Cairo.[127]I may here recall that the quartz mass which constitutes the so-called “alabaster” quarry north of Aswân, and for which I have inferred an igneous origin, is likewise closely associated with aplitic or pegmatitic intrusions. SeeA Description of the First or Aswân Cataract. Cairo, 1907, p. 84.[128]This specimen was brought back by a guide sent for the purpose to Gebel Adar Qaqa. It is stated to be the main rock of the mountain, but as I did not visit the mountain I am unable to vouch personally for this being the case.[129]From a specimen brought back by the guide who was sent to erect the triangulation beacon on Gebel Mishbih.[130]I did not visit Gebel Shendib, but as seen from Gebel Elba the mountain appeared to be granite; the felsite probably forms a dyke passing through the summit.[131]There is another Gebel el Anbat near the Wadi Hodein, but this latter consists of schists.[132]Preliminary Report on the Geology of the Eastern Desert of Egypt, between latitude 22° N. and 25° N., by Dr. W. F. Hume. Cairo, 1907. p. 57.[133]This fibrous green substance is probably the columnar variety of serpentine known asPicrolite; it strongly resembles the specimens thus labelled in the London Museum of Practical Geology.

[125]FromMarwa, the Arabic term for quartz.

[126]The numbers in square brackets in this and the following chapter are the registration numbers of the specimens in the Geological Museum, Cairo.

[127]I may here recall that the quartz mass which constitutes the so-called “alabaster” quarry north of Aswân, and for which I have inferred an igneous origin, is likewise closely associated with aplitic or pegmatitic intrusions. SeeA Description of the First or Aswân Cataract. Cairo, 1907, p. 84.

[128]This specimen was brought back by a guide sent for the purpose to Gebel Adar Qaqa. It is stated to be the main rock of the mountain, but as I did not visit the mountain I am unable to vouch personally for this being the case.

[129]From a specimen brought back by the guide who was sent to erect the triangulation beacon on Gebel Mishbih.

[130]I did not visit Gebel Shendib, but as seen from Gebel Elba the mountain appeared to be granite; the felsite probably forms a dyke passing through the summit.

[131]There is another Gebel el Anbat near the Wadi Hodein, but this latter consists of schists.

[132]Preliminary Report on the Geology of the Eastern Desert of Egypt, between latitude 22° N. and 25° N., by Dr. W. F. Hume. Cairo, 1907. p. 57.

[133]This fibrous green substance is probably the columnar variety of serpentine known asPicrolite; it strongly resembles the specimens thus labelled in the London Museum of Practical Geology.

METAMORPHIC ROCKS.

Metamorphic rocks, comprising a great variety of gneisses and schists, are widely and abundantly distributed in South-Eastern Egypt, forming about half the entire area treated of in this volume. The irregular way in which the metamorphic rocks alternate with those of the igneous and sedimentary groups will be better appreciated by a reference to the geological map onPlate XXthan by any verbal description. As a whole, the metamorphic rocks represent the remains of a complex of igneous and sedimentary deposits which were laid down in remote geological periods; subsequently these deposits were crushed, folded, and elevated into mountains, their original structures being largely obliterated in the dynamical process; in succeeding ages the metamorphosed complex was penetrated by the great granitic and other intrusions which have been described in the preceding chapter, and then planed down by denudation as an old land surface before being wholly or in part submerged beneath the waters of the Cretaceous sea in which the Nubian sandstone was deposited. The elevation of the land after the Cretaceous period was accompanied by further folding which must have increased the degree of metamorphism of the rocks; but it is most probable that the forces which produced these post-Cretaceous movements were of less intensity and duration than those of more ancient periods, and we may regard the metamorphic character of the older rocks as having been for the most part impressed on them in Archæan, or at latest in Palæozoic times. It is not impossible that some of the more highly foliated gneisses may even have been parts of the primitive crust of consolidation of the earth, while the clay schists may be the remains of the very earliest sediments.

Besides the evidences of dynamo-metamorphism above-mentioned, we find occasionally traces of contact-metamorphism, where igneous dykes have locally altered the rocks into which they haveintruded; but, in general, the subsequent compression of the masses has obliterated these traces, and the effects of contact-metamorphism are negligible in comparison with those produced by dynamic action.

The metamorphic rocks occurring in this part of Egypt may be classified as follows:—

Gneisses.

Granite-gneiss(crushed granite).

Diorite-gneiss(crushed diorite).

Highly foliated gneisses(in which all traces of primitive structure have been obliterated by foliation and recrystallization).

Schists.

Quartz-schists(in some cases probably metamorphosed sandstone, in others crushed aplitic rocks).

Clay-schists(metamorphosed clays).

Hornblende-schists and crushed volcanic rocks.

Hornfels(metamorphosed acid lavas).

Mica-schists(sometimes containing beryl and tourmaline).

Chlorite-schists.

Talc-schists.

Calcareous schists and marble.

Graphite-schists.

Breccias.

Fluidal breccias, in which broken fragments of solid rock have been included in an intrusive magma.

Breccias proper, formed by fracture and recementing together of fragments without fusion.

Mineral Veins.

Quartz veins(sometimes containing gold and traces of copper).

Calcite veins(with more or less chalybite).

Magnesite veins(in serpentine).

Asbestos veins(in serpentine).

The most conspicuous occurrence of gneiss in South-Eastern Egypt is the great mass which extends from Gebel Mudergeg and Gebel Migif in the north, down the range of Gebel Hafafit, and along the greater part of the Wadi Gemal. With this mass is closely connected another stretch of gneiss to the south of Gebel Abu Khrug; the hills which rise from the sandy plain round the head of Wadi Natash are mostly gneiss, so that the gneiss is probably continuous across the plain except where it is penetrated by the syenitic cones called El Nahud. The total area covered by gneiss in this locality alone is some two thousand square kilometres, and includes some remarkable mountain masses, such as Gebels Migif and Hafafit.

Other gneissose areas of less extent occur round Bir Abu Beid and to the south-west of Gebel Hamrat el Feg; while still smaller areas are found in the hills to the north-west of Berenice, in the remarkable triple-headed mountain called Gebel Um Rasein, and to the north of Bir Um Bishtit.

Gneiss typically forms bold scenery of precipitous hills, the colour of which varies with the composition of the rock, from pink to grey. At Gebel Migif one ascends the mountain most easily by the northern slopes, which are formed by the foliation-planes of the rock, while to the south these are cut across by sheer precipices; the same is the case at Gebel Um Rasein.

Fig. 52.—Granite-gneiss, Gebel Um Rasein [12,108], viewed between crossed nicols, × 30.q, quartz;ol, oligoclase;or, orthoclase;h, hornblende;m, “mortar” of crushed quartz and felspar between the larger crystals. The black areas are quartz in the position of extinction.

Of gneisses evidently produced by the deformation of granite, we may take the rock of Gebel Um Rasein [12,108][134]as a type. The bulk of the mountain is formed of a nearly white acid rock, of sp. gr. 2·67, with dark spots, resembling the granite of Gebel Elba in appearance, except that here the dark minerals have a marked arrangement along definite planes. (Compare the coloured figures of the two rocks on PlatesXXIIandXXV). In places the rock contains thin bands of almost pure biotite, and these are often thrown into sharp contortions by the strong compression which the mass has undergone. The strike of the foliation-planes in Gebel Um Rasein itself is about south-south-east, with a dip of about 20° to the east-north-east, but thestrike swings round to about south at the south end of the mass, and the inclination of the foliation-planes become much steeper. The gneiss shows very marked pitting due to weathering, the holes ranging from a few centimetres to a metre or more in diameter and depth. A microscopic section of the rock shows it to be essentially an acid hornblende-granite similar to that of Gebel Elba, but crushed so that there is a “mortar” of broken up quartz and felspar between the larger crystals, and the larger where unbroken frequently show undulose extinction as the result of strain.

Fig. 53.—Diorite-gneiss, Wadi Nugrus [10,386], × 17.h, hornblende, with small enclosures of felspar and sphene;pl, plagioclase felspar;b, biotite;q, quartz;ap, apatite;sp, sphene.

In the Wadis Nugrus and Gemal there is a great deal of strongly foliated grey gneiss [10,386-7], derived from the intense crushing of diorites. A figure from a typical hand specimen of this gneiss, looking at the edges of the foliation-planes, is given onPlate XXV.The sp. gr. of the rock is 2·88. The microscopic slides show plagioclase and green hornblende in about equal proportions, with some quartz and small amounts of biotite, sphene, iron oxides, and apatite. The hornblende is very fresh-looking, in strongly pleochroic (yellowish to deep blue-green) straggling crystals, aggregated together along the foliation-planes; the crystals, though variously oriented, show a tendency to lie with their cleavages in the direction of foliation. A verystriking feature of the slides is the abundance of small grains of enclosed felspar in the hornblende, giving the green crystals a perforated appearance in ordinary light. The felspars, which are likewise fairly fresh, are much smashed up, forming with the quartz a mosaic between the irregular bands of hornblende; the grains often show undulose extinction. Biotite, of a warm brown colour where not bleached, is sparingly mixed with the hornblende. The sphene is in small rounded grains, mixed with, and sometimes containing, granules of ilmenite; the grains show a tendency to form little aggregates, sometimes in the hornblende, and sometimes in the felspathic portions of the slide. One or two stumpy grains of apatite are enclosed in the felspars, but this mineral is extremely scarce in the slides examined, and does not show its usual long prismatic habit.

Ball.—Geography & Geology of South-Eastern Egypt.PLATE XXV.METAMORPHIC ROCKS.GRANITE-GNEISS.Gebel Um Rasein.RED GNEISS.Gebel Abu Beid.DIORITE-GNEISS.Wadi Nugrus.GREEN BRECCIA.Gebel Hamata.EMERALD AND QUARTZ IN MICA-SCHIST.Gebel Sikait.NATURAL SIZE.

METAMORPHIC ROCKS.

GRANITE-GNEISS.Gebel Um Rasein.RED GNEISS.Gebel Abu Beid.

GRANITE-GNEISS.Gebel Um Rasein.

RED GNEISS.Gebel Abu Beid.

DIORITE-GNEISS.Wadi Nugrus.GREEN BRECCIA.Gebel Hamata.

DIORITE-GNEISS.Wadi Nugrus.

GREEN BRECCIA.Gebel Hamata.

EMERALD AND QUARTZ IN MICA-SCHIST.Gebel Sikait.

NATURAL SIZE.

As an example of gneisses in which the foliation is so intense that all original structures have been obliterated by recrystallization, we may take the red gneiss of the Abu Beid Hills [10,659], of which a coloured natural size representation is given onPlate XXV.In this rock, which has a sp. gr. of 2·65, the main constituent is a pink felspar, mixed with a little quartz and arranged in fine lenticles separated by greenish laminæ which are doubtless altering biotite. This rock bears a strong resemblance to the well-known red gneiss of the Erzgebirge, which, from its chemical composition, is usually believed to be a metamorphosed granite poor in mica.

Grey gneisses, equally intensely foliated with the Abu Beid rock, occur in the range of Gebel Hafafit; these contain a much larger proportion of biotite, and remind one of the grey gneisses of the Freiberg district. But as no analyses have yet been made of the Egyptian rocks, the absolute similarity to those of the Erzgebirge is not quite certain.

About ten kilometres south of Gebel Abu Gurdi is a tract of low hills and ridges composed mainly of quartz-schists. The limits of these rocks have not been mapped, but they appear to cover a somewhat elliptical area with its long axis, measuring some eight or ten kilometres, pointing east and west. Where first encountered, at a point about twelve kilometres east of Gebel Selaia, on the march from thathill towards the pass into the head of Wadi Lahami, the rocks strongly resemble rather friable white to brown fine-grained sandstones, with bedding planes dipping about 35° to the north. The rock [10,413] consists almost entirely of white or slightly iron-stained quartz grains, with, in places, scattered minute dull black rods which are apparently hornblende. Traced further east, the rocks have more and more the character of quartz schists, with harder bands of quartzite, and the planes of separation, which gradually become nearly horizontal, have more the appearance of foliation-planes. The harder quartzite bands [10,414] are usually brownish to greenish-white in colour, but sometimes take on a purple tint, and show micaceous and chloritic scales on cleaved surfaces; in some places the rock contains “eyes” of minette-like composition. To the south the bedding or cleavage planes seem to reverse their dip, this being now to the south as though there were an anticlinical fold in the mass. There is considerable mixture of other schists with the rocks, and dykes of diorite and pegmatite, as well as veins of quartz, are frequent. A dioritic dyke which cuts through the rocks at a point nine kilometres south-south-west of Gebel Abu Gurdi, and which is itself crushed almost into a hornblende-schist, has hardened and altered the quartzose rock on either side of it into a close-grained horny looking rock [10,415] apparently containing a great deal of finely granular andalusite.

These quartz-schists may have arisen from the metamorphism either of sandstones, in which case the parts to the west of the tract are the least altered, or of fine-grained aplitic rocks, on which view the sandstone-like aspect of the western portion of the tract is due to weathering; the rock is certainly more highly weathered where it most resembles an unaltered sandstone, and the presence of hornblende and the “eyes” of micaceous matter are suggestive of igneous rock. From a consideration of all the evidence I was able to gather in a rapid march over the sea, however, I incline to the view that the rocks are altered sandstone older than the Nubian series, and probably of about the same age as the ancient clays which gave rise to the clay-schists of Zabara and elsewhere. The rocks are completely detached from the nearest undoubted Nubian sandstone beds (which lie some forty kilometres to the south-west, beyond Gebel Zergat Naam), and they are mixed with schists and veined and altered to a degree which is nowhere approached by beds known to be of the Nubian series.Still, it is just possible that the quartz-schists represent the remains of Nubian sandstone strata, for the nearest beds of that series show-considerable folding and faulting near Gebel Zergat Naam, and we should expect still greater disturbance and alteration here near the main watershed and close to a principal axis of mountain elevation.

A quartz-schist which is certainly derived from the metamorphism of aplitic dykes or sheets, occurs as great inclined bands cutting across the Wadi Shenshef close to some old ruins. The main rock of the district, in which the bands occur, is a medium-grained diorite. The quartz-schist is a greyish-white fissile rock, which splits easily into great slabs; it has supplied an excellent building material for the houses of the ruined town or encampment, being used not only for the walls but also for lintels over doors and windows.

Clay-schists cover extensive areas in the South-Eastern Desert. They occur abundantly in the high hills to the east of Gebel Nugrus and form a large portion of Gebel Zabara. They abound also to the south of Wadi Antar about the longitude of Gebel Um Goraf, in the hills on the east side of Gebel Abu Dahr, in the Wadi Hodein round about where Wadi Um Tenedba joins it, in the hills flanking the sandstone plateau of Gebels Dif and Anfeib, at Gebels Kolaiqo and Eqrun, on the western flanks of the serpentine mass of Gebel Meneiga, and in the low hills on the plain between Gebels Gerf and Korabkansi.

Clay-schists are typically of a grey colour, though greenish, purple, and reddish varieties occur. They are soft rocks, easily scratched with a knife. In some places, as for instance at Gebel Zabara, they are practically slates, cleaving into slabs often of considerable size, while in other localities, as for instance near Gebel Abu Dahr, they are crushed into matchwood-like splinters, which can be used as slate-pencils. They are usually associated with other metamorphic rocks such as crushed diorites and hornblende and mica-schists. The direction of foliation of the slaty and slabby forms is often persistent—at Gebel Zabara, for instance, a general south-easterly dip of about 28° is maintained over a considerable area; but there arenumerous local variations, and the number of observations thus far made is too small for any general conclusions to be drawn as to the regional distribution of dip. On the whole, the clay-schists are remarkably free from knots or other irregularities, though knotted varieties, doubtless the result of contact alteration, have been observed on the south side of Wadi Seiga and at a few other places.

The clay-schists have probably for the most part been produced by the metamorphism of ancient argillaceous sediments; in some places, as for instance in the Wadi Beida, a passage can be observed into what appears to be a crushed conglomerate, while in other places they pass into quartzites which doubtless represent altered intercalated sandstones. The relation between the direction of the foliation-planes and that of the original bedding is unknown; it would appear likely that in most cases the traces of bedding have been entirely obliterated by pressure. Nor have any fossils been found in the rocks, though a sharp look-out was kept in the more likely-looking localities, so that the age of the beds is unknown. At present I am inclined to regard them as most likely of Archæan age, antedating the great granitic intrusions; but further evidence, either in the finding of fossils or from a more careful study of the field relations of these rocks to the others, is required before one can be certain that they are of so great an antiquity.

Under the heading of hornblende-schists and crushed volcanic rocks are classified a great variety of rocks, ranging in colour from green to grey and almost black, in fissility from the finest lamellation to almost massive forms in which the schistosity is barely evident in the hand specimen, but all definable as fine-grained dark-coloured schistose rocks consisting largely either of hornblende or of its alteration products. Their origin is not always clear, but many of them appear on microscopic examination to be altered fine-grained syenites, diorites, and lavas, and it is probable that practically the whole are altered forms of various intermediate and basic igneous rocks. The reason for including the hornblende-schists and crushed volcanic rocks in one great group is the difficulty of separating the different classes in the field. They pass one into the other, and many rockswhich in the field would be put down as hornblendic turn out on detailed examination to contain little or none of that mineral, its place being taken by decomposition products.

Schists of the types above defined occur probably more abundantly than any other class of metamorphic rocks in the South-Eastern Desert. They enter largely into the composition of some of the main mountain masses, such as those of Gebels Abu Hamamid and Abu Gurdi, besides covering great expanses of lower hill country. In the mountain ranges they rise in steep-sided ridges with knife edge summits, a good idea of which is given by the photograph of Gebel Abu Hamamid onPlate VII.In the lower hill country we have typically a dreary confused looking waste of thousands of dark hills and ridges separated by small winding wadis; sometimes there is a semblance of system in the distribution of the hills, owing to the occurrence of parallel dykes harder than the schists which they penetrate, forming thus back-bones for long lines of schist-ridges.

Fig. 54.—Schist produced by crushing of syenite, near Gebel el Anbat (Wadi Hodein). [11,532 B], × 30.f, crushed and clouded felspar, probably orthoclase;lc, mixture of calcite and limonite, produced by alteration of hornblende, no trace of which remains.

Schist derived from Syenite.—Among the schists occurring near Gebel el Anbat, near the Wadi Hodein, is one which appears to be a crushed and altered syenite. It is a speckled grey and reddish-brown rock with a dull and rather granular fracture, of sp. gr. 2·92. The slide [11,532 B] shows a rather coarse granitic mixture of clouded felspar and heavily iron-stained calcite. The felspar is considerably altered, showing no twinning, but between crossed nicols it extinguishes in irregular bands and is seen to be much crushed. The ferruginous and calcareous matter is almost certainly altered hornblende; it forms irregular masses, in which the iron oxide is mostly arranged in parallel lines as though along the cleavage planes of the parent mineral, while a clouded calcite fills up the spaces between the lines. It would seem that in addition to pressure, carbonic acid has been the main agent of metamorphism here, the ferro-magnesiansilicate being attacked and its silica entirely removed, while the aluminous silicate of the felspar has remained to some extent unchanged.

Fig. 55.—Hornblende-schist (probably a metamorphosed fine-grained diorite), Wadi Muelih [10,357], × 40.h, pale green hornblende;f, mass of decomposed felspar, containing abundance of finely acicular hornblende and some little patches of quartz mosaic;m, magnetite.

Schists derived from Diorites.—Hornblendic schists which appear to have been produced by the metamorphism of fine-grained diorites occur largely in the low hill country round the upper parts of the Wadi Muelih, and in the mountain mass of Gebel Abu Gurdi. A specimen of the less altered rock from the Wadi Muelih [10,357] is a fine-grained hard grey basaltic-looking rock, of sp. gr. 3·04, in which the schistosity in not very evident, though it is well seen in the mass. The microscopic slide shows pale green hornblende in ragged fibrous forms in a matted-looking clouded ground mass made up of finely acicular green hornblende and plagioclase, with a little quartz and orthoclase and a few grains of magnetite. Hornblende fibres are often enclosed in the decomposing felspars, which rarely show definite outlines and appear much shattered. The quartz is probably of secondary formation, occurring as little patches of mosaic.

The schist which forms the summit of Gebel Abu Gurdi [10,416] is probably also a crushed and altered diorite; it is a rather fine-grained hard greenish-grey rock, which with a lens can be seen to be a mixture of greenish-white felspathic material and dark hornblende; the felspar is mostly dull, while the hornblende, on the other hand, is frequently in shining crystals. The sp. gr. of the rock is 3·02. The microscopic slide shows the hornblende to be of a very pale green colour, in large irregular crystals, often including felspars. The felspar appears to be plagioclase; it is typically in smaller crystals than the hornblende, with a strongly marked tendency to idiomorphism, highly cracked and almost entirely changed to kaolin. Some rather large straggling crystals of a dark brown clouded and semi-opaque highly refractingmineral, white by reflected light, are perhaps altered sphene. Round the larger crystals is more finely crystalline matter, much clouded, apparently composed of altered felspar and hornblende.

The crushed dioritic dykes already mentioned as cutting the quartz-schist to the south-west of Gebel Abu Gurdi are possibly offshoots from the same magma which formed the main mass of the mountain. Specimen [10,415], taken from one of these dykes, is a fine-grained grey rock, of sp. gr. 2·93, with an even more decided schistosity than that of the main mountain. The microscopic slide shows clear hornblende of a green colour, strongly pleochroic (greenish-yellow to blue-green) in irregular grains which are frequently aggregated into nests and strings running in the direction of foliation of the rock; the remaining material is a mosaic of clear quartz, clouding kaolin, and sericite, presumably representing altered felspar.

Fig. 56.—Schist composed of fragments of various volcanic rocks, summit of Gebel Abu Hamamid [10,397], × 30.

Schists formed by crushing of Volcanic Rocks.—Schists derived from the crushing of volcanic rocks are very abundant in the mountains round Gebel Abu Hamamid and in the Wadi Beida. Typical specimens from the summit of Gebel Abu Hamamid [10,397] are hard green to grey rocks, of sp. gr. 2·7, breaking with a rough dull fracture. With a lens, spots and strings of dull white matter, with ill-defined outlines, are seen in a green to nearly black ground mass. The microscopic slides show the rock to be a breccia rather than a simple crushed rock, for in the same slide very various structures can be seen. Some portions, evidently andesitic, consist of perfectly idiomorphic lath-shaped felspars scattered with a little decomposed hornblende in a glassy brown ground mass. Others, more abundant, seem to be altered quartz-diorite-porphyrite; in these parts, porphyritic quartz and orthoclase crystals, in forms strongly inclined to idiomorphism, clouded and strained, are scattered in a cryptocrystalline ground mass containing a good deal of green chlorite and epidote. In other parts of the slide, again, the chief porphyritic constituent is formedby large green grains, which have evidently once been biotite or hornblende, but which now consist of chlorite. In yet other parts of the rock we have fragments of devitrified glassy lava. The different parts, which are not always well outlined, are separated by schistose bands composed mainly of strings of chlorite and epidote. Whether the rock is a tuff, or a crushed conglomerate of igneous boulders and pebbles, or due to complicated crushing of a series of contiguous igneous rocksin situ, is not quite clear. In some places, especially about the Wadi el Sheikh, the schists look like crushed conglomerates, but these may possibly be rocks crushedin siturather than accumulations of rolled fragments transported by streams.

The hornblende-schists which surround Gebel Zergat Naam are of a peculiar type of which the origin is not evident, but are possibly altered andesites. The typical rock [11,526] is a hard grey basaltic one of sp. gr. 2·95, with a rusty-brown skin on exposed surfaces. The microscopic slide shows little plates and brushes of nearly colourless hornblende, liberally scattered in a clouded ground mass consisting mainly of hornblende fibres in radiating and dendritic groups, with a small amount of kaolinic matter and a few grains of magnetite. The slide contains no distinct quartz, but possibly a small amount of this mineral may be present in very minute grains with the kaolinic matter.

To the metamorphism of andesitic lavas, too, are somewhat doubtfully ascribed the grey and green schists of the Wadi Beida, which have specific gravity of about 2·75. The slides from the grey variety of the schists [12,116 and 12,159] show a very fine-grained clouded rock, apparently a mosaic of quartz, kaolin, chlorite and sericite, with larger scattered irregular plates of dark green chlorite. In a pale green variety from the head of the wadi [12,111], there is less chlorite and a considerable amount of calcite.

Hornblende Schists of doubtful origin.—In the more highly foliated and harder varieties of hornblende schist, which are typically of a darker colour than most of those already described, we have rocks in which the process of re-crystallization has been so complete that no trace of the original rock remains. These rocks, which are true hornblende-schists in the narrowest sense of the term, are less abundant than the foregoing types, and generally occur as comparatively narrow bands associated with gneisses and mica and talc schists. All theminerals in them being of secondary origin, they are usually in a quite fresh state, and fractured surfaces, examined with a lens, exhibit a mass of glistening small crystals of hornblende, quartz, and felspar. The density is usually about 2·9. Microscopic slides show elongated crystals of clear green hornblende, strongly pleochroic (pale yellowish-green to deep blue-green) arranged along the planes of schistosity, separated by clear granules of quartz and a little felspar, with scattered magnetite grains.

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