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 on Plate XX than 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 have intruded; 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.

Of gneisses evidently produced by the deformation of granite, we may take the rock of Gebel Um Rasein [12,108] 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 Plates XXII and XXV). 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 the strike 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.

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 on Plate 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 very striking 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.

=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 on Plate 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 that hill 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 are numerous 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 rocks which 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 on Plate 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.

=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-magnesian silicate being attacked and its silica entirely removed, while the aluminous silicate of the felspar has remained to some extent unchanged.

=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 refracting mineral, 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.

=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 formed by 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 rocks in 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 crushed in situ rather 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 the minerals 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.

Near Gebel Eqrun are found hornblende-schists which exhibit a curious banding in planes at right angles to the main foliation, in the form of darker lenticular stripes a few millimetres wide and about the same distance apart. A slide [12,117] cut from this variety shows little trace of the banding, the lighter spaces between the dark bands merely showing a clouding of the quartz and felspar by tiny granules of epidote. The stripes are most probably the consequence of a secondary compression in a direction perpendicular to the original one rather than relics of a banded structure in the parent rock.

=Actinolite-schists.=—Very beautiful bright green schists, in which the hornblende is in the fibrous to silky form called actinolite, occur in small quantity associated with mica and talc schists at Sikait [10,380] and elsewhere. In these rocks the actinolite fibres, which often reach two centimetres in length, are generally aggregated into bundles, with radiating structure. In the microscopic slide the rock presents even a more beautiful appearance than in the mass, the long fibres of actinolite polarising in the most brilliant tints; associated with the actinolite, there is nearly always more or less chlorite and talc.

=Hornfels.=

Associated with the schists of Gebel Abu Hamamid and the neighbouring mountains there are great masses of very hard horny-looking rock [10,401] of green to grey colour, breaking with a sub-conchoidal fracture, and of such close texture as to appear homogeneous even with a strong lens. The pyramidal peak called Gebel Um Semiuki, which rises to 1,282 metres above sea, three kilometres to the north-east of Gebel Abu Hamamid, is almost entirely composed of rocks of this type; in the mountain faces the rock looks red, but this is only due to a film covering weathered surfaces, the interior being of a green to grey colour. The rock, which has a sp. gr. of 2·71, is frequently beautifully banded, light and dark layers alternating with each other, and often contains tiny cubes of pyrites [10,399]. The microscopic slide from Gebel Um Semiuki shows a very fine-grained clouded compact rock, apparently consisting of glassy matter with minute granules of quartz and altered felspar, together with a little sericite, the latter especially along certain bands. The slide from Gebel Abu Hamamid is similar, but here the granules of quartz and felspar are a little larger, though they are still too small to be seen with a lens in the hand specimen; the appearance is that of a quartz felsite on a small scale. It has already been mentioned (p. 281) that the quartz felsites of Gebels Igli and Hadarba pass gradually into hornfels, and when we remember that the schists of the Abu Hamamid district are mostly crushed volcanic rocks, it becomes almost certain that the hornfels associated with them is a crushed and devitrified glassy lava of acid composition.

A yellowish horny rock with grey streaks [10,379], which occurs near Gebel Sabahia, is conspicuous in the field owing to its weathered surfaces being covered with a bright red ferruginous skin, resembling cinnabar in colour. The sp. gr. is 2·52. Examination with a lens shows the grey streaks to be filled with myriads of brilliant yellow specks of pyrites. The microscopic slide shows these to be aggregates of little cubes, while the bulk of the rock is a schistose felsitic mass of quartz and felspar, with scattered larger felspar crystals, much broken up and bent. In this rock too we have therefore a rolled up and altered felsite.

=Mica-schists.=

Mica-schists, composed mainly of golden-brown lustrous laminæ of biotite with more or less quartz, occur near the base of Gebel Zabara and at Gebel Sikait [10,626], as well as in small quantity at one or two other points. They are always associated with gneiss, and appear to form irregular bands, alternating and mixed with talc and other schists. The laminæ of mica can seldom be separated in any large size, breaking up at a touch into small scales; they are often highly contorted.

=Emeralds= (Beryl).—At Zabara and Sikait the mica-schists contain crystals of beryl (silicate of beryllium and aluminium, Be3Al2Si6O18), the clear variety of which forms the gem emerald. The beryls are mostly found in lenticular bands of quartz which occur in the mica-schist, but sometimes they can be seen in the schist itself. The crystals are mostly well developed hexagonal prisms of a pale emerald-green colour, with characteristic vertical striation. The coloured figure on Plate XXV will give a good idea of the usual appearance of the mineral. In microscopic slides (see Fig. 58) the beryls are conspicuous only by their clear cut hexagonal outlines; they are quite colourless, with low polarisation colours about the same as those of quartz. Both at Zabara and Sikait there are numerous ruins and ancient mines where emeralds have been sought; most of them are irregular shafts and tunnels, twisting about as the old miners followed the varying directions of the bands of schists. It is commonly believed that gem emeralds were at one time extracted from these mines, and it seems incredible that the mining should have been carried on to so great an extent as is shown by the ruins and old workings, unless stones of considerable value were obtained. The Zabara mines were re-opened by Cailliaud in the time of Mohammad Ali Pasha (1817), but the stones extracted were of little value, being clouded and full of flaws. A similar result followed a more recent (1904-5) vigorous attempt by Mr. James, acting on behalf of Mr. Edwin Streeter, of London, to work the emerald mines of Sikait; plenty of beryls were found, but none clear enough to be of any great value, and the enterprise was abandoned, Mr. James concluding that either the ancient miners had worked out all the bands containing stones of any value, or else, what is perhaps more likely, the ancients were satisfied with a duller stone for a gem than our modern jewellers. The dull forms of beryl are in our own day of very little value, being principally used as a source for beryllium salts in chemical laboratories.

=Tourmaline.=—Besides beryls, the mica and talc schists of Sikait contain in places abundance of black tourmaline in well-developed crystals. At some spots this mineral is so plentiful as to form practically small patches of tourmaline-rock [10,395]. In thin section [9,874 and 9,908] the tourmaline crystals, which are much clouded and irregularly cracked, show beautiful pleochroism (colourless to deep orange), and very high double refraction colours in prismatic sections. Like the beryl, however, tourmaline is only of value as a gem when it is clear and transparent, and all the crystals so far obtained are dull and opaque.

Calcite, in rhomb-shaped crystals of a brown colour due to presence of included iron oxides [10,382] likewise occurs in places in the mica-schists of Sikait.

=Chlorite-schists.=

Though many of the decomposed hornblende-schists contain more or less chlorite, I have only in two localities come across rocks in situ which contain so large a proportion of this mineral as to deserve the name of chlorite-schists. The first is in the hills of Um el Huetat (latitude 25°), where typical chlorite-schists are mixed with mica, talc, and hornblende schists. The second locality is between Gebels Ras Shait and Nugrus, where the rock [10,388] is remarkable not only in its peculiar appearance but also by its strongly magnetic character. It is a thoroughly schistose rock of a rather pale greyish-green colour and rather silky appearance, with rusty looking spots. The sp. gr. is 2·77. The microscopic slide shows the stone to consist essentially of an aggregate of elongated plates and fibres of low double refraction, which from the hand specimen seem to be chlorite, but in the slide look more like antigorite. Magnetite grains are liberally scattered through the chloritic mass. The rusty spots visible in the hand specimen are translucent foxy red in the slide, in irregular broken forms with well-marked cleavage and nearly straight extinction. They are somewhat doubtfully regarded as deeply iron-stained hornblende. Mixed with the foxy red material are aggregates of granules of a highly refracting but isotropic mineral of deep bottle-green colour (? spinel), and chloritic wisps. There are also some clear colourless grains, resembling apatite in appearance except that they sometimes show well-marked vertical cleavage and high extinction angles; these are possibly a colourless augite, but the double-refraction colours are far lower than is usual with this mineral.

Typical chlorite-schists occur in the Wadi Salib Abiad, and near Gebels Ribdab and Muqsim, in the extreme south-west portion of the region. Hearing, from some wandering Arabs while at Gebel Abu Dahr in February 1907, that prospectors were at work in the Wadi Salib Abiad, I sent a guide to find out who they were and what they were doing; the guide reported that on his arrival they had gone away, but there were some old workings in a green rock of which he brought a sample. I did not get an opportunity of visiting the locality personally, but the specimen brought back by the guide [11,523] is a beautiful apple-green chlorite-schist, with some brownish calcareous-looking bands. The microscopic slide shows some little quartz and talc besides the chlorite, and there are scattered minute highly-refracting rounded grains of a feebly translucent mineral of a reddish-brown colour, probably rutile. A similar rock from the eastern side of Gebel Muqsim has been reported by Mr. Charteris Stewart, who also records a normal chlorite-schist as occurring on the north side of Gebel Ribdab.

=Talc-schists.=

Talc-schists, though not widely distributed, are abundant in certain localities. They form the main rock in many of the hills in the district called Um el Huetat, to the west of Gebel Atut in latitude 25°, where there are numerous old mines. In this district there are immense masses of talc-schist of remarkable purity. The rock [10,364] is of a grey to green or brown colour, distinctly schistose, cleaving easily with a soapy feel, and easily scratched by the finger nail; more massive forms occur which can be easily carved into pipes, and these are frequently smoked by the Bedouin. I descended one of the old shafts at Um el Huetat, and found no evidence of other minerals having been worked than the talc itself. A somewhat more earthy variety of talc-schist [10,396] was met with near the Rod el Ligaia. Talc-schists also occur at Gebel Sikait [10,383] and Gebel Zabara, where, as at Um el Huetat, they alternate with mica and hornblende schists. But at Sikait and Zabara the talc-schists are decidedly subordinate to the mica-schist in which the beryls are found.

To the class of talc-schists is also probably best referred a fissile and very rotten purple-brown schist, containing cubes of limonite, which was found near the triangulation station on Gebel Hamida, and again near the old mines of Romit. The microscopic slide of the Romit specimen [12,139] shows the bulk of the rock to consist of talc and chlorite, with blotchy stains and floculent-looking masses of brown iron oxide and occasional clear irregular granules of quartz. In this are plentifully embedded perfect little cubes of limonite. The limonite cubes [12,129 and 12,136] weather out easily from the rotten matrix, and can be gathered from the ground at the foot of the exposed surfaces.

=Calcareous Schists and Marble.=

Some very curious calcareous schists are found in Gebel el Anbat and its neighbourhood, near the Wadi Hodein. In Gebel el Anbat itself a mass of these rocks rises to a height of over 250 metres above the wadi level. They are earthy-looking rocks of varying colour, chiefly brown and reddish, often purplish on the rough weathered surfaces, and sometimes have a talcose feel. A typical specimen [11,532 A] has a sp. gr. of 2·92, and the slide cut from it shows it to be almost entirely composed of grains of calcite, with here and there larger irregular granules of quartz and of what looks like altered felspar, and a liberal sprinkling of iron oxide. It is difficult to assign an origin to this rock, but a variation of it [11,532 B] has been already described (p. 339) as probably an altered syenite, and the main rock may therefore represent the extreme form of alteration of an igneous rock rich in lime felspars.

A hard close-grained reddish-brown rock of sp. gr. 2·91, with dark streaks [12,107], which forms a high ridge, swathed in blown sand, rising to 350 metres above sea-level near the Wadi Kreiga, eleven and a half kilometres to the south-east of Gebel Beida, resembles a quartzite in appearance, but turns out on careful examination to be a calcareous schist. The microscopic section shows a very fine-grained mosaic of calcite with a little quartz, and scattered grains and strings of iron oxide. The origin of this rock is uncertain; it may be a metamorphosed limestone.

The summit of a high hill rising to 686 metres above sea, on the east side of the Wadi Um Khariga in latitude 24° 56′ 30″, is a gozzany mass resembling the outcrop of a mineral vein traversing the schists. On a fresh fracture, the interior of the rock [10,369] is seen to consist mainly of dark crystalline calcite, with some cubical crystals of pyrites, numerous rusty looking spots and patches of limonite, and veinlets of white calcite. The microscopic slide shows a mixture of calcite with kaolinic and serpentinous matter, with a very pronounced schistose structure, containing “eyes” of mixed calcite and iron oxides; the rock is therefore in reality a ferruginous calcareous schist. The mass is too highly metamorphosed for more than a guess as to its origin; but the slide contains some granules resembling picotite and one or two small patches of what looks like altering felspar, and the suggestion is that the schist is a metamorphosed basic igneous vein.

=White crystalline marble= has been found only at one point, namely, about three kilometres south of the jagged peak called Qash Amir, west of Gebel Elba, where it forms a small patch in crystalline rocks.

=Graphite-schists.=

A considerable mass of graphite-schist occurs associated with diorite rocks near some ancient gold workings at Gebel Allawi. The extent of the graphite-schist has not been mapped, but it occurs on the north slope of the mountain, and as seen from the summit it appears to run out westward for miles as a broad black band. The rock [10,378], which has a sp. gr. of 2·70, consists of a mixture of graphite with calcareous and earthy matter. The parts richest in graphite are readily cut with a knife, and mark paper easily. Analysis shows that the graphite, though sufficiently abundant to appear in the hand specimen the dominant mineral of the rock, is nevertheless present in surprisingly small proportion; a sample tested by Mr. Pollard was found to contain only a trifle over one per cent. of carbon.

A similar graphite-schist occurs associated with mica-schist in one of the emerald mines of Gebel Sikait, and it may be remarked that graphitic schists have also been observed by Dr. Hume near the lodes in the gold mines of Baramia and Um Garaiart, both of which localities, however, lie outside the limits of the area here described.

The origin of the graphite-schists is uncertain. Perhaps the hypothesis presenting the least difficulty is that which supposes them to have been formed from ancient sedimentary rocks containing the remains of plants; on this view the close association of dioritic rocks suggests that the great of igneous intrusion may have played a part in the distillation of the more volatile matters of the plant remains, while the schistose structure shows that pressure has also been active in the formation of the rock. But the absence of any associated rocks which bear distinct signs of having originally been ancient sediments makes one hesitate to refer the graphite to an organic origin, and possibly the carbonaceous matter may have been produced from quite another source than that of plant life.

=Breccias.=

A very curious rock [10,385] which from a distance looks like a giant diorite, occurs as a band in gneiss on the west side of the lower part of Wadi Nugrus, is doubtless a fluidal breccia. It contains ovoid masses of white aplite, sometimes measuring five centimetres in diameter, embedded in a dark fine-grained dioritic matrix with a marked tendency to schistose structure. Most likely the rock has been formed by the catching up of the fragments of a crushed aplite in an intrusive diorite, and then the whole mass has been subjected to the same pressure which foliated the surrounding gneisses.

More normal breccias are found at various points. One which occurs between schists and hornfels near the summit of Gebel Abu Hamamid [10,398] is made up of fragments of various altered volcanic rocks, with large black flint-like lumps of hornfels, all cemented into a very hard rock which breaks across the fragments composing it. This breccia was doubtless formed by the same movements which produced the schists of the summit of the mountain; these latter (see p. 341) are themselves almost as much fine breccias as schists.

To the west of Gebel Zergat Naam the stones in the wadis are sometimes cemented into hard breccias by calcareous matter, doubtless deposited by drainage waters which have dissolved out the lime from felspathic rocks.

In the hill called Ti Keferiai, a little below the triangulation point which marks the summit, a highly altered fine-grained dioritic rock, containing much epidote, has been crushed into a coarse breccia [12,123] cemented by rose-coloured quartz.

A remarkable green breccia is found in Gebel Hamata, where it appears to form a large mass in the mountain-side to the east of the main peak. This rock [10,407], which has a sp. gr. of 2·92, is darker in colour and somewhat softer than the ornamental “breccia verde antico” of the Wadi Hammamat district further north, but it also is a very beautiful rock. In the hand specimen, it consists of black angular fragments, up to two centimetres in diameter, embedded in a dark green ground mass, the whole of very fine grain and barely scratchable with a knife. Under the microscope, the black fragments seem to be of basaltic nature, while the green matrix is probably a highly crushed and brecciated diorite-porphyrite; the whole of the slide is clouded by decomposition products.

=Fault-breccias=, produced by differential movement of the two sides of faults, occur in the neighbourhood of the Wadi Saalek, where the sandstones and schists are much faulted (see p. 359). The breccias here [11,539] are narrow bands which stand up like dykes; they are very calcareous and highly ferruginous, with occasional green stains, perhaps due to traces of copper.

=Mineral Veins.=

Quartz veins deposited by solutions in cracks and fissures of the igneous and metamorphic rocks are very numerous and widely distributed, especially in the Sukari district. They vary immensely in size, from mere strings to veins two metres or more in thickness; they cut the rocks in every direction and at every angle of dip. Steeply inclined veins are by far the most numerous, but some have a flat inclination and are then styled “reefs” by the miners.

The principal interest attaching to the quartz veins is the fact that they frequently contain gold, though seldom in particles visible to the unaided eye. The quartz veins were worked for their gold by the ancients, the remains of whose dwellings and stone grinding pans are found at numerous places, as for instance in the Wadi Hangalia, at Kurdeman, near Gebel Sabahia, at Gebels Sukari and Allawi, in the Wadi Lewewi, and at Romit and Darahib. Our modern prospectors have found these ruins of ancient mining camps and grinding pans to be the best guide to auriferous veins, gold being seldom found except in and near the old workings. As mentioned on p. 27, the ancients worked the mines by convict labour, and they could for that reason afford to work ores which are too poor to pay under modern conditions. But in certain cases the veins have been found rich enough to give possibilities of a commercial return to modern mining enterprise; a list of the prospecting licences and mining leases now in force is given on p. 28. As to the source of the gold, it is not known whether it came up in solutions from below, or has been secreted laterally from the country rock.

Besides gold, some of the quartz veins contain traces of copper, but none of the occurrences of copper ore within the area specially treated of in this book appear to be capable of yielding any considerable quantity of the metal, most of them in fact being mere stains due to oxidation and carbonatisation of traces of sulphides.

Calcite veins are much more rarely met with than those of quartz. In only one of the veins I have examined is calcite present in any considerable quantity, namely in the vein of the old gold mines of Romit. In this vein, white to brown crystalline calcite is found mixed with chalybite, limonite, and smoky quartz [12,105 and 12,141], the last-named only being apparently auriferous. There did not appear to be enough chalybite and limonite in the vein to make it worth following up for iron ore, especially in view of the expense of transport from the place.

Magnesite and asbestos veins occur in the serpentines of the Gebel Gerf district. These occurrences, which appear not to be large enough to be worth working, have been described on p. 330.

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