Fig. 57.—Hornblende-schist, near Gebel Eqrun [12,117], × 30.h, hornblende;q, quartz;m, magnetite. A little felspar is present mixed with the quartz, from which it is distinguishable only in polarised light.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.Fig. 58.—Beryl and quartz, from a lenticle in mica-schist near Sikait [10,580], × 30.b, beryl in idiomorphic hexagonal crystals;q, quartz, allotriomorphic.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 onPlate XXVwill give a good idea of the usual appearance of the mineral. In microscopic slides (seeFig. 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.Fig. 59.—Tourmaline crystals in graphitic talc-schist, Sikait mines [9,908], × 17.t, tourmaline crystals, irregularly cracked and clouded;g, talc-schist, heavily clouded by graphite.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 rocksin situwhich contain so large a proportion of this mineral as to deserve thename 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 weatheredsurfaces, 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 marblehas 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 centimetresin 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 (seep. 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,[135]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 (seep. 359). Thebreccias 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[136]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 onp. 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 onp. 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 considerablequantity of the metal, most of them in fact being mere stains due to oxidation and carbonatisation of traces of sulphides.[137]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 onp. 330.[134]The figures in square brackets in this and the preceding chapter are the numbers under which the specimens are registered in the Geological Museum, Cairo.[135]SeeBarron and Hume’s “Eastern Desert.” Cairo, 1902. p. 263.[136]Other quartz veins which probably originated in quite a different manner are treated of under the heading of Igneous Rockssee(p. 266).[137]The copper smelted in ancient times at Kubban, on the east bank of the Nile opposite Dakka, in latitude 23° 10′, was possibly obtained from mines at Abu Seyal (sometimes misspelt Absciel), north of the Wadi Alaqi in latitude 22° 47′, longitude 33° 44′, where there are extensive old workings, Abu Seyal lies outside the region described in this volume; a reference to the mines will be found in theReport of the Egyptian Department of Mines for 1906, p. 34.CHAPTER XI.TECTONICS AND GENERAL GEOLOGY.Evidence of a former Pluvial Period.Given a sufficiently long period for their activity, the denuding and transporting agencies at work at the present day are capable of accounting for most of the superficial sculpturing of South-Eastern Egypt. The country is not absolutely rainless, and within a decade most of the dry valleys have been for a few hours the beds of streams, the result of rain storms. There is practically no frost in this part of the world, so that disintegration by the freezing of water in crevices of the rock does not occur on any large scale; the diurnal variations of temperature, are, however, so great that this cause alone is very potent in breaking up rock material. The disintegrated matter accumulates as heaps of debris and sand, ready to be transported towards the Nile or the sea by the streams which follow the next rainfall. Both in erosion and in the transport of sand, wind is a very active agent, and accounts for the formation and distribution of immense quantities of sand. Thus the mountains are slowly being lowered, and the rocky valleys between them are being widened and deepened, even at the present day, and the accumulations of sand on the coast-plain and elsewhere are being slowly increased in thickness.But when we look at the great wadis, often hundreds of kilometres in length, cut to a depth of fifty metres with a width of half a kilometre through the sandstone plateaux which separate the mountain ranges from the Nile, it is difficult to conceive that rainfall and denudation have not in the past been greater than at present. In our own day, it is but seldom that the great wadis convey streams as far as the Nile or the sea, their waters being usually absorbed by the sandy bed before the end is reached; erosion nowadays is practically confined to the upper reaches of the wadis, and unless we postulate greater rainfall in the past, inconceivable ages must have been occupied inthe erosion of these great channels. We are thus driven to believe that what is now a very dry area was formerly one of considerable rainfall. This belief is supported by the traces of glaciation in Europe, for it is natural to infer that when temperate Europe had an arctic climate, northern Africa had a temperate one; the effect, whatever its cause, being practically equivalent to an increase of latitude. This change of climate is equally evidenced by geological observation in other parts of Egypt. It is even likely that the climate of Egypt may be slowly changing at present; but the change within the historical period has been so small as to be practically negligible.Origin of the Red Sea.If the 200-metre contour of the bottom of the Red Sea, shown onPlate I,be examined, it will be found to exhibit great indentations towards the great mountain masses, while there is a curious projection including the Island of Zeberged which mimics the present Ras Benas. Some of the indentations of the contour line lie in the direct prolongation of existing great wadis, such as those of Lahami, Khoda, Hodein, Di-ib, and Serimtai. The obvious suggestion from this coincidence is that the sea has encroached on the land since the drainage-system had substantially its present form, and we infer a sinking of the region at no very remote geological epoch. The central parts of the Red Sea attain depths of over 2,000 metres; thus this sea was a great and deep one even when the level of its waters, relative to the land, was 200 metres lower than now. We have no information which would give us a clue to the origin of this primitive sea, but the inference from the contours is that thepresentextent of the Red Sea has been caused by a great general subsidence of the land, and not by trough-faulting as has hitherto been usually stated.[138]The subsidence just referred to was even greater than would be gathered from a consideration of the present coast-line. At intervals along the entire eastern coast of Egypt are hills of gypsum; these are never found except close to the present sea-borders, and the natural deduction is that the gypsum beds were deposited when the sea was at a higher level than at present. At Ras Benas, the gypseous stratareach altitudes of nearly 200 metres, so that at the time when the gypsum was formed the Red Sea must have covered a much greater area than now, extending in fact approximately to the contour of 200 metres above present sea-level. As to the epoch when this greater extent of the sea existed, we should have a clue if we knew the age of the gypsum beds, which unfortunately is not the case; but they are almost certainly younger Tertiary beds, possibly Miocene or even Pliocene, so that in any case the Red Sea is a depression of considerable antiquity.Possible Former Extent of the Eocene Rocks.Eocene rocks are entirely absent from the district, and the same is the case with Cretaceous rocks younger than the Nubian sandstone. But if we go westward along the parallel of Berenice, across the Nile into longitude 23° 30′, we come to the plateau face of Gebel Garra, where there are exposed thicknesses of about ninety metres of Eocene limestone and 240 metres of Cretaceous marls. These beds cover great expanses further west and north; they evidently once extended beyond their present limits, and we may ask whether they ever reached over the Red Sea mountains here, as is the case in North-Eastern Egypt. To this question no answer can be given; from Berenice to Gebel Garra is a distance of over 300 kilometres, and even a very gradual thinning of the beds eastward would account for their absence from the main mountains; at the same time the denudation which has removed every trace of the hard Eocene limestone from the plain between Gebel Garra and the Nile may well have done the same further east.Original Extent of the Nubian Sandstone.The Nubian sandstone is found on both sides of the watershed ranges, and the question raised by its distribution is in regard to its possible original continuity. Did the Nubian sandstone once extend over the present igneous mountains, or was it laid down on either side of a great island ridge? In other words, did the Red Sea mountains exist as such in Cretaceous times, or have they been subsequently elevated, and the Nubian sandstones which covered them denuded away? To this question it may be stated at once that no certain answer can be given; but a good deal of study has been devoted to the facts bearing upon it, and these facts will be briefly enumerated and discussed below.Evidence from Volcanic Intrusions.—That the Nubian sandstone is younger than the igneous rocks in general is proved by the almost complete absence of intrusions into the sandstone. Apart from an interbedded diabase sheet in the north part of the region and a small basic dyke near Gebel Awamtib, the igneous rocks appear nowhere to penetrate the Nubian beds. The sandstone isa fortioriyounger than the metamorphic rocks into which the igneous masses have been intruded.Evidence from Present Distribution.—Coming now to the distribution of the sandstone, though we find it on both sides of the main ranges, there is not a trace of sandstone among the higher mountains; this of course proves nothing, for denudation would be most active among the peaks and in districts of maximum up-thrust, and sandstone is an easily erodible rock. The long tongue of sandstone plateau which terminates in Gebel Anfeib extends indeed right across the main watershed, but a reference to the orographical map (Plate I) will show that this extension is along a north-west to south-east general depression, where the Wadis Hodein and Garara form a cut across the map separating the main mountain masses into north and south groups. The presence of the sandstone tongue here would be equally well accounted for whether the mountains were elevated before or after the deposition of the Nubian beds.Not only is the sandstone absent from the main mountains, but it is never found in such close proximity to them as to render a decisive answer to our question. Thus the eastern scarp of the sandstone plateau in the north part of the area overlooks a broad tract of low country between it and the mountains; and similarly from Gebel Anfeib one overlooks low country to the north, east, and south. On the eastern side of the mountains the sandstone deposits are restricted to small patches considerably removed from the main summits, and even in most cases from their foot-hills.Evidences from Structural Features.—With regard to the structural evidences to be gathered from the sandstone itself, it was hoped that the observations of the dip of the beds at different places would throw light on the question as to whether their deposition antedated the mountain formation or no.To the north of latitude 24°, the eastern edge of the sandstone scarp showed beds differing but little from the horizontal, though very gentle folding in various directions is probable.Further south, near Gebel Zergat Naam, much more decided evidences of folding, and even of dislocation, were met with. The head of the Wadi el Kreim, south-west of Zergat Naam (seethe geological map onPlate XX) is probably a line of fault, for here on the south-west of the wadi we have sandstones coming right down to the wadi floor, with a dip of 30° to the south-west near the edge, becoming flatter the further we go from the wadi, while on the other side are crushed and brecciated schists. Going further west, in the sandstone hill-mass which lies twelve kilometres west of Zergat Naam, the dip of the beds is in the opposite direction, being 10° to 15° north-east, and granite appears at the foot of the steep west-south-west slope; this, with the preceding observation, seems to indicate a synclinal fold terminated near Zergat Naam by a fault up-thrusting the schists and syenite.Where the Wadi Garara cuts through the sandstone hills to receive the Wadi el Kreim, the beds dip markedly to the south, the observed inclinations being 60° or more at the north edges, rapidly falling to 20° or less further south; the north faces show granite and schists at their base.Further west, on the way to Gebel Um Harba, the sandstone of the hills showed dips to the east of 15°. At Gebel Um Harba itself there are thick beds of sandstone dipping 13° east-north-east, while all around the mountain one looks out over beds having approximately the same inclination.At Gebel Um Khafur, the dip is 13° to 14° to the north-north-east, and is very constant over a large area. From the north side, where the plain is 380 metres above sea-level, the hills rise with a succession of dip slopes and basset-edges over a horizontal distance of 2·9 kilometres (measured perpendicular to the strike) to the triangulation beacon at 560 metres above sea, the beds all along dipping at 13° or 14°. Unless there is step-faulting here along the strike-wadis(seeFig. 60) the total thickness of Nubian sandstone here is over 450 metres. This is a much greater thickness than has been noted anywhere else in Egypt, and I am inclined to think that there is step-faulting along at least two of the strike wadis which separate the hill-mass into ridges.Fig. 60.—Section of Sandstone at Gebel Um Khafur.In approaching Gebel Awamtib from the north-west, I crossed over a small patch of diorite in the sandstone at the pass from Wadi Um Terbi into Wadi Awamtib, and a basic dyke was found cutting the sandstone of a spur of Gebel Awamtib. The beds of Awamtib itself dip pretty uniformly a little north of west.Fig. 61.—Sketch section of junction of sandstone with granite, west of Gebel Um Keit.About nine kilometres to the south of Gebel Awamtib, a station was taken on a sandstone headland with schists and quartz veins at its floor, and afterwards I skirted the limit between sandstone and granitic rocks on the way to Gebel Um Reit. All along this route there was no suggestion of sharp folding or faulting; the beds were nearly horizontal right up to the limit, where the granite hills rise suddenly (seeFig. 61).From the top of Gebel Um Reit, which itself is granite, the sandstone limit could be seen to the north and east; the beds dip 30° in places; in those on the east the direction of dip is about south. The region round Um Reit is evidently one of considerable disturbance, but the sandstone is too far off for one to get any precise idea of its nature from the summit.Fig. 62.—Sketch of faulting near Wadi Saalek.Going northwards from Um Reit across the Wadi Saalek and up one of its branches into the head of Wadi Muegil, there is a good exposure of faulted sandstone overlying schists just before reaching the pass. Here (seeFig. 62) one of the faults is a distinct overthrust, with a north-easterly strike, and there are other faults in a parallel direction.From the Wadi Muegil northwards past the Galt el Aguz to Gebel Um Harba there are gentle dips and curvings of the sandstone beds, with a predominant dip about north-east. At the Galt el Aguz the sandstone rocks are much tumbled about, but this appears to be due to fall of over-hanging beds.At the pass from Wadi Um Arta into the head of Wadi Silsila, the dip of the sandstone is north-north-east. In its lower part, Wadi Silsila passes between sandstone ridges the dip of which is constantly east-north-east.Round Bir Abraq and the triangulation station on Gebel Abraq (seelarge scale map onPlate XV) the main direction of dip is east-south-east; the pool of Bir Abraq itself is under projecting slabs of sandstone dipping in this direction. Bir el Sunta, a little further north, appears to lie in a syncline, the beds of the hills to the north dipping south-east, while those to the south dip north-west.Approaching the sandstone from the east by Wadi Hodein, one traverses granite country with felsite dykes and enters suddenly into high sandstone plateaux. The beds at the Abu Saafa Springs dip about 5° northwards.The beds of Gebel Dif and Gebel Anfeib have a prevalent dip on the west side to the east and north-east, while on the east face the dip appears to be in the reverse direction. This great mass of sandstone plateau may thus be a syncline with a north-westerly strike (i.e., a strike roughly parallel to the main direction of the watershed mountain ranges), but the eastern face was not examined in detail, and it may be that the whole mass dips to the north-east; some outlying small sandstone masses near the head of Wadi Edunqul show no reversal, the dip being still east-north-east. At Bir Dif the sandstones are variously tilted, and probably faulted.The sandstones on the east side of the watershed ranges, near Gebel Ranga, were observed by Dr. Hume to dip towards the sea. In the south part of the region there are some low hills forming two series of ridges between the sea and Gebel Kolaiqo, separated by hills ofred aplitic granite; these hills were not actually visited, but were mapped from some little distance, and the dip of the beds was not measured, but the nature of the rocks was confirmed by specimens of sandstone brought back by guides sent to the place.Summing up the dips and disturbances of the sandstone beds at the different places, it is clear that the strata are the more disturbed, the more closely they approach the main mountain ranges; this and the seaward dip of the beds near El Ranga are so far in favour of the view that the sandstone may once have extended right over the present mountains. But it will be seen that the disturbances of bedding are of an irregular character, especially between Gebels Zergat Naam and Um Reit, where the folding and faulting are often in directions quite distinct from that of the main mountain axes, and it may be that these disturbances are due to later movements rather than to the elevation of the main mountain ranges. So far as observations on the sandstone itself go, therefore, the question as to whether the Nubian sandstones of the Red Sea border ever directly joined those on the west of the mountains remains open; the observed disturbances of the beds give us clear proof that considerable earth movements have taken place since the sandstone was deposited, but they are inconclusive as to whether these same movements caused the elevation of the great mountain ranges.The Igneous and Metamorphic Rocks.Turning now to the main mountain-forming rocks, we find them composed of the two great classes, igneous and metamorphic. Though in places we have transition members, such as granites passing into gneiss, yet on the whole the two groups are quite distinct, and we find typical igneous masses rising in the midst of equally typical schists. Under these circumstances there can be no hesitation in considering the two groups to be of different ages, and that the igneous rocks must be the younger.The igneous masses are divisible geologically into the three classes of (a) lavas, (b) dykes, and (c) plutonic rocks.Lavas.—Amongst the igneous rocks, the lavas are those most poorly represented. Apart from the andesite of Gebel Sufra, and the diabase sheets in or under the Nubian sandstone, there are only a fewoccurrences of volcanic rocks in all the area, a circumstance which is easily explained by the enormous denudation which has gone on and the fact that lavas, being superficial out-pourings, will have been most exposed to denuding forces.Dykes.—Dykes seam the schists and igneous rocks, frequently in such numbers and with such parallelism as to give to the land the form of a succession of ridges separated by long narrow depressions, forming “dyke country.” In nature, the dykes vary from extremely acid rocks like aplites and quartz-felsites, to very basic forms such as diabase and basalt. There is on the whole a preponderance of basic over acid types. As already remarked, only one instance of a dyke cutting the Nubian sandstone has been observed in the district. Another significant feature in the distribution of dykes is that while dykes of all kinds are found cutting schists and acid plutonic igneous rocks, the basic plutonic masses are as a rule free from dykes; this suggests that the basic plutonic rocks may be on the whole younger than the acid forms.Plutonic Rocks.—The plutonic rocks include granites, syenites, diorites, gabbros, pyroxenites, amphibolites, and peridotites. But these are not all of equal importance, and a natural division from the field observations is a two-fold one into acid and basic groups, which are tolerably well marked off from each other and are most likely of different ages. The acid class on this view consists of the granites and syenites, while the basic class contains all the other rocks above-mentioned. The granites are on the whole of a very acid type; syenite is very scarce and nearly always occurs in close association with granite. Of the basic group, probably the most abundant and most typical rock is gabbro; diorites and the various ultra-basic rocks are almost always closely associated with gabbro. Where acid and basic rocks occur in proximity (as for instance near Bir Abraq, where an acid granite occurs side by side with a serpentine) there is a sharp change, without transitional forms.The much greater abundance of dykes in the plutonic rocks of the acid group as compared with those of basic composition, as mentioned above, inclines us to regard the basic group as the younger—a view which is further supported by the greater frequency with which a tendency to gneissose structure is noticeable in the acid rocks. The fact that the basic members are frequently in a higher state of decompositionthan the acid ones is not contrary to this view, being due to the greater ease with which their constituents undergo weathering; and as explained onp. 315,the fissured state of the altered peridotes is probably not due to the same earth forces which have sheared the granites, but to internal stresses set up by the expansion of the rocks on serpentinisation.Though we may be fairly sure that the basic plutonic rocks are on the whole younger than the acid ones, we have no certain guide from which to estimate the geological age of either. We know from their relations to the sandstone that they are both older than the Upper Cretaceous, but we cannot say how much older; they may be anything from Archæan to Jurassic; the circumstance that similar rocks underlie Carboniferous strata in Sinai inclines us to place them at least as far back as Palæozoic times.Gneisses and Schists.—The gneisses and schists which cover so much of the country are obviously older than the plutonic masses which are intruded in them, and for these metamorphic masses we need have small hesitation in speculating on an Archæan age. They exhibit a wonderful variety of composition. Most of the gneisses are doubtless sheared ancient igneous rocks, such as granite and diorite, and a similar origin may be assigned to many of the schists. Other rocks, as for instance the clay-schists, graphite-schists, and marble, have almost certainly originated from the metamorphism of ancient sedimentary rocks. But in a large number of cases we have at present no clue to the parent rock from which the schists have been formed.Summary of Geological History of South-Eastern Egypt.Having in the foregoing pages discussed the evidences for the relative ages of the different classes of rocks and their mutual relations, we may now endeavour to reconstruct the past geological history of this part of Egypt from the information gathered. In this process we shall reverse the order of consideration taken above, and begin with the oldest rocks.The schists and gneisses probably represent, not the original crust of consolidation of the earth, but a complex of ancient sedimentary and igneous rocks, laid down in pre-Palæozoic times and subsequently crushed, folded, and faulted into mountains which were subsequentlydenuded and worn down. In parts of the main mountain-masses we may possibly still have the cores of some of these ancient elevations, but most of the present mountain peaks are formed of a later series of igneous rocks.The first igneous intrusions into the schists and gneisses were the granites which form such peaks as Gebels Nugrus, Faraid, and Elba. Then followed crushing and folding of these rocks, giving them often a gneissose structure and opening fissures, up which came later intrusions, mostly of basic rocks, in the form of dykes. With the dykes or later came great basic intrusions of gabbroid rocks, forming such masses as Gebels Dahanib, Gerf, and Meisah.A long interval now followed of which the rocks contain no record. We do not know whether the area was submerged or not in Palæozoic and early Mesozoic times; but if it was, all trace of the deposits of these ages has vanished in the great denudation which surely took place before the Upper Cretaceous sea swept over the country and deposited the Nubian sandstone. We do not know whether the Red Sea mountains then stood up as islands, or whether they were subsequently elevated. Nor do we know whether the area remained wholly or partly submerged during the Eocene period. But we are sure that at some time between the Upper Cretaceous and Oligocene epochs there was a great elevation of the land, with folding and faulting, especially in the areas now occupied by the great mountain masses, and possibly the mountains had their origin in this movement of elevation. The Red Sea depression may well have originated in a complementary sinking at this same period. The depth of the sea is of the same order (2,000 metres) as the height of the mountains on the adjacent land.In the succeeding Oligocene period the land was being sculptured into something like its present form, and probably the main drainage lines of to-day were then formed.About the Miocene epoch there was a sinking of the crust in this region. The Red Sea increased in area, and then, probably as the results of evaporation in a closed sea, deposits of gypsum were laid down. A subsequent elevation in Pliocene or post-Pliocene times raised these deposits along the present shores.In geologically recent times a further gentle elevation has gone on, giving rise to slightly raised coral-reefs and sea-beaches. During the glacial period of Europe, the rainfall in Egypt was probably greaterthan at present, and during this period the great wadis received almost their final sculpturing. After the change from this rainy climate to the dry one of to-day, erosion still went on, though more slowly, in the hills; but on the plains and along the coast accumulation of sands took place, partly owing to wind transport, and partly owing to the insufficiency of the drainage waters to carry their load as far as the Nile or sea. The abundance of coral-reefs in the Red Sea is largely conditioned by the lack of in-flowing streams of silt-laden water; for the coral animal flourishes only where the water is clear.
Fig. 57.—Hornblende-schist, near Gebel Eqrun [12,117], × 30.h, hornblende;q, quartz;m, magnetite. A little felspar is present mixed with the quartz, from which it is distinguishable only in polarised light.
Fig. 57.—Hornblende-schist, near Gebel Eqrun [12,117], × 30.h, hornblende;q, quartz;m, magnetite. A little felspar is present mixed with the quartz, from which it is distinguishable only in polarised light.
Fig. 57.—Hornblende-schist, near Gebel Eqrun [12,117], × 30.h, hornblende;q, quartz;m, magnetite. A little felspar is present mixed with the quartz, from which it is distinguishable only in polarised light.
Fig. 57.—Hornblende-schist, near Gebel Eqrun [12,117], × 30.h, hornblende;q, quartz;m, magnetite. A little felspar is present mixed with the quartz, from which it is distinguishable only in polarised light.
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.
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, 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.
Fig. 58.—Beryl and quartz, from a lenticle in mica-schist near Sikait [10,580], × 30.b, beryl in idiomorphic hexagonal crystals;q, quartz, allotriomorphic.
Fig. 58.—Beryl and quartz, from a lenticle in mica-schist near Sikait [10,580], × 30.b, beryl in idiomorphic hexagonal crystals;q, quartz, allotriomorphic.
Fig. 58.—Beryl and quartz, from a lenticle in mica-schist near Sikait [10,580], × 30.b, beryl in idiomorphic hexagonal crystals;q, quartz, allotriomorphic.
Fig. 58.—Beryl and quartz, from a lenticle in mica-schist near Sikait [10,580], × 30.b, beryl in idiomorphic hexagonal crystals;q, quartz, allotriomorphic.
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 onPlate XXVwill give a good idea of the usual appearance of the mineral. In microscopic slides (seeFig. 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.
Fig. 59.—Tourmaline crystals in graphitic talc-schist, Sikait mines [9,908], × 17.t, tourmaline crystals, irregularly cracked and clouded;g, talc-schist, heavily clouded by graphite.
Fig. 59.—Tourmaline crystals in graphitic talc-schist, Sikait mines [9,908], × 17.t, tourmaline crystals, irregularly cracked and clouded;g, talc-schist, heavily clouded by graphite.
Fig. 59.—Tourmaline crystals in graphitic talc-schist, Sikait mines [9,908], × 17.t, tourmaline crystals, irregularly cracked and clouded;g, talc-schist, heavily clouded by graphite.
Fig. 59.—Tourmaline crystals in graphitic talc-schist, Sikait mines [9,908], × 17.t, tourmaline crystals, irregularly cracked and clouded;g, talc-schist, heavily clouded by graphite.
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.
Though many of the decomposed hornblende-schists contain more or less chlorite, I have only in two localities come across rocksin situwhich contain so large a proportion of this mineral as to deserve thename 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, 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.
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 weatheredsurfaces, 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 marblehas 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.
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.
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 centimetresin 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 (seep. 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,[135]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 (seep. 359). Thebreccias 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.
Quartz veins deposited by solutions[136]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 onp. 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 onp. 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 considerablequantity of the metal, most of them in fact being mere stains due to oxidation and carbonatisation of traces of sulphides.[137]
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 onp. 330.
[134]The figures in square brackets in this and the preceding chapter are the numbers under which the specimens are registered in the Geological Museum, Cairo.[135]SeeBarron and Hume’s “Eastern Desert.” Cairo, 1902. p. 263.[136]Other quartz veins which probably originated in quite a different manner are treated of under the heading of Igneous Rockssee(p. 266).[137]The copper smelted in ancient times at Kubban, on the east bank of the Nile opposite Dakka, in latitude 23° 10′, was possibly obtained from mines at Abu Seyal (sometimes misspelt Absciel), north of the Wadi Alaqi in latitude 22° 47′, longitude 33° 44′, where there are extensive old workings, Abu Seyal lies outside the region described in this volume; a reference to the mines will be found in theReport of the Egyptian Department of Mines for 1906, p. 34.
[134]The figures in square brackets in this and the preceding chapter are the numbers under which the specimens are registered in the Geological Museum, Cairo.
[134]The figures in square brackets in this and the preceding chapter are the numbers under which the specimens are registered in the Geological Museum, Cairo.
[135]SeeBarron and Hume’s “Eastern Desert.” Cairo, 1902. p. 263.
[135]SeeBarron and Hume’s “Eastern Desert.” Cairo, 1902. p. 263.
[136]Other quartz veins which probably originated in quite a different manner are treated of under the heading of Igneous Rockssee(p. 266).
[136]Other quartz veins which probably originated in quite a different manner are treated of under the heading of Igneous Rockssee(p. 266).
[137]The copper smelted in ancient times at Kubban, on the east bank of the Nile opposite Dakka, in latitude 23° 10′, was possibly obtained from mines at Abu Seyal (sometimes misspelt Absciel), north of the Wadi Alaqi in latitude 22° 47′, longitude 33° 44′, where there are extensive old workings, Abu Seyal lies outside the region described in this volume; a reference to the mines will be found in theReport of the Egyptian Department of Mines for 1906, p. 34.
[137]The copper smelted in ancient times at Kubban, on the east bank of the Nile opposite Dakka, in latitude 23° 10′, was possibly obtained from mines at Abu Seyal (sometimes misspelt Absciel), north of the Wadi Alaqi in latitude 22° 47′, longitude 33° 44′, where there are extensive old workings, Abu Seyal lies outside the region described in this volume; a reference to the mines will be found in theReport of the Egyptian Department of Mines for 1906, p. 34.
TECTONICS AND GENERAL GEOLOGY.
Given a sufficiently long period for their activity, the denuding and transporting agencies at work at the present day are capable of accounting for most of the superficial sculpturing of South-Eastern Egypt. The country is not absolutely rainless, and within a decade most of the dry valleys have been for a few hours the beds of streams, the result of rain storms. There is practically no frost in this part of the world, so that disintegration by the freezing of water in crevices of the rock does not occur on any large scale; the diurnal variations of temperature, are, however, so great that this cause alone is very potent in breaking up rock material. The disintegrated matter accumulates as heaps of debris and sand, ready to be transported towards the Nile or the sea by the streams which follow the next rainfall. Both in erosion and in the transport of sand, wind is a very active agent, and accounts for the formation and distribution of immense quantities of sand. Thus the mountains are slowly being lowered, and the rocky valleys between them are being widened and deepened, even at the present day, and the accumulations of sand on the coast-plain and elsewhere are being slowly increased in thickness.
But when we look at the great wadis, often hundreds of kilometres in length, cut to a depth of fifty metres with a width of half a kilometre through the sandstone plateaux which separate the mountain ranges from the Nile, it is difficult to conceive that rainfall and denudation have not in the past been greater than at present. In our own day, it is but seldom that the great wadis convey streams as far as the Nile or the sea, their waters being usually absorbed by the sandy bed before the end is reached; erosion nowadays is practically confined to the upper reaches of the wadis, and unless we postulate greater rainfall in the past, inconceivable ages must have been occupied inthe erosion of these great channels. We are thus driven to believe that what is now a very dry area was formerly one of considerable rainfall. This belief is supported by the traces of glaciation in Europe, for it is natural to infer that when temperate Europe had an arctic climate, northern Africa had a temperate one; the effect, whatever its cause, being practically equivalent to an increase of latitude. This change of climate is equally evidenced by geological observation in other parts of Egypt. It is even likely that the climate of Egypt may be slowly changing at present; but the change within the historical period has been so small as to be practically negligible.
If the 200-metre contour of the bottom of the Red Sea, shown onPlate I,be examined, it will be found to exhibit great indentations towards the great mountain masses, while there is a curious projection including the Island of Zeberged which mimics the present Ras Benas. Some of the indentations of the contour line lie in the direct prolongation of existing great wadis, such as those of Lahami, Khoda, Hodein, Di-ib, and Serimtai. The obvious suggestion from this coincidence is that the sea has encroached on the land since the drainage-system had substantially its present form, and we infer a sinking of the region at no very remote geological epoch. The central parts of the Red Sea attain depths of over 2,000 metres; thus this sea was a great and deep one even when the level of its waters, relative to the land, was 200 metres lower than now. We have no information which would give us a clue to the origin of this primitive sea, but the inference from the contours is that thepresentextent of the Red Sea has been caused by a great general subsidence of the land, and not by trough-faulting as has hitherto been usually stated.[138]
The subsidence just referred to was even greater than would be gathered from a consideration of the present coast-line. At intervals along the entire eastern coast of Egypt are hills of gypsum; these are never found except close to the present sea-borders, and the natural deduction is that the gypsum beds were deposited when the sea was at a higher level than at present. At Ras Benas, the gypseous stratareach altitudes of nearly 200 metres, so that at the time when the gypsum was formed the Red Sea must have covered a much greater area than now, extending in fact approximately to the contour of 200 metres above present sea-level. As to the epoch when this greater extent of the sea existed, we should have a clue if we knew the age of the gypsum beds, which unfortunately is not the case; but they are almost certainly younger Tertiary beds, possibly Miocene or even Pliocene, so that in any case the Red Sea is a depression of considerable antiquity.
Eocene rocks are entirely absent from the district, and the same is the case with Cretaceous rocks younger than the Nubian sandstone. But if we go westward along the parallel of Berenice, across the Nile into longitude 23° 30′, we come to the plateau face of Gebel Garra, where there are exposed thicknesses of about ninety metres of Eocene limestone and 240 metres of Cretaceous marls. These beds cover great expanses further west and north; they evidently once extended beyond their present limits, and we may ask whether they ever reached over the Red Sea mountains here, as is the case in North-Eastern Egypt. To this question no answer can be given; from Berenice to Gebel Garra is a distance of over 300 kilometres, and even a very gradual thinning of the beds eastward would account for their absence from the main mountains; at the same time the denudation which has removed every trace of the hard Eocene limestone from the plain between Gebel Garra and the Nile may well have done the same further east.
The Nubian sandstone is found on both sides of the watershed ranges, and the question raised by its distribution is in regard to its possible original continuity. Did the Nubian sandstone once extend over the present igneous mountains, or was it laid down on either side of a great island ridge? In other words, did the Red Sea mountains exist as such in Cretaceous times, or have they been subsequently elevated, and the Nubian sandstones which covered them denuded away? To this question it may be stated at once that no certain answer can be given; but a good deal of study has been devoted to the facts bearing upon it, and these facts will be briefly enumerated and discussed below.
Evidence from Volcanic Intrusions.—That the Nubian sandstone is younger than the igneous rocks in general is proved by the almost complete absence of intrusions into the sandstone. Apart from an interbedded diabase sheet in the north part of the region and a small basic dyke near Gebel Awamtib, the igneous rocks appear nowhere to penetrate the Nubian beds. The sandstone isa fortioriyounger than the metamorphic rocks into which the igneous masses have been intruded.
Evidence from Present Distribution.—Coming now to the distribution of the sandstone, though we find it on both sides of the main ranges, there is not a trace of sandstone among the higher mountains; this of course proves nothing, for denudation would be most active among the peaks and in districts of maximum up-thrust, and sandstone is an easily erodible rock. The long tongue of sandstone plateau which terminates in Gebel Anfeib extends indeed right across the main watershed, but a reference to the orographical map (Plate I) will show that this extension is along a north-west to south-east general depression, where the Wadis Hodein and Garara form a cut across the map separating the main mountain masses into north and south groups. The presence of the sandstone tongue here would be equally well accounted for whether the mountains were elevated before or after the deposition of the Nubian beds.
Not only is the sandstone absent from the main mountains, but it is never found in such close proximity to them as to render a decisive answer to our question. Thus the eastern scarp of the sandstone plateau in the north part of the area overlooks a broad tract of low country between it and the mountains; and similarly from Gebel Anfeib one overlooks low country to the north, east, and south. On the eastern side of the mountains the sandstone deposits are restricted to small patches considerably removed from the main summits, and even in most cases from their foot-hills.
Evidences from Structural Features.—With regard to the structural evidences to be gathered from the sandstone itself, it was hoped that the observations of the dip of the beds at different places would throw light on the question as to whether their deposition antedated the mountain formation or no.
To the north of latitude 24°, the eastern edge of the sandstone scarp showed beds differing but little from the horizontal, though very gentle folding in various directions is probable.
Further south, near Gebel Zergat Naam, much more decided evidences of folding, and even of dislocation, were met with. The head of the Wadi el Kreim, south-west of Zergat Naam (seethe geological map onPlate XX) is probably a line of fault, for here on the south-west of the wadi we have sandstones coming right down to the wadi floor, with a dip of 30° to the south-west near the edge, becoming flatter the further we go from the wadi, while on the other side are crushed and brecciated schists. Going further west, in the sandstone hill-mass which lies twelve kilometres west of Zergat Naam, the dip of the beds is in the opposite direction, being 10° to 15° north-east, and granite appears at the foot of the steep west-south-west slope; this, with the preceding observation, seems to indicate a synclinal fold terminated near Zergat Naam by a fault up-thrusting the schists and syenite.
Where the Wadi Garara cuts through the sandstone hills to receive the Wadi el Kreim, the beds dip markedly to the south, the observed inclinations being 60° or more at the north edges, rapidly falling to 20° or less further south; the north faces show granite and schists at their base.
Further west, on the way to Gebel Um Harba, the sandstone of the hills showed dips to the east of 15°. At Gebel Um Harba itself there are thick beds of sandstone dipping 13° east-north-east, while all around the mountain one looks out over beds having approximately the same inclination.
At Gebel Um Khafur, the dip is 13° to 14° to the north-north-east, and is very constant over a large area. From the north side, where the plain is 380 metres above sea-level, the hills rise with a succession of dip slopes and basset-edges over a horizontal distance of 2·9 kilometres (measured perpendicular to the strike) to the triangulation beacon at 560 metres above sea, the beds all along dipping at 13° or 14°. Unless there is step-faulting here along the strike-wadis(seeFig. 60) the total thickness of Nubian sandstone here is over 450 metres. This is a much greater thickness than has been noted anywhere else in Egypt, and I am inclined to think that there is step-faulting along at least two of the strike wadis which separate the hill-mass into ridges.
Fig. 60.—Section of Sandstone at Gebel Um Khafur.
Fig. 60.—Section of Sandstone at Gebel Um Khafur.
Fig. 60.—Section of Sandstone at Gebel Um Khafur.
In approaching Gebel Awamtib from the north-west, I crossed over a small patch of diorite in the sandstone at the pass from Wadi Um Terbi into Wadi Awamtib, and a basic dyke was found cutting the sandstone of a spur of Gebel Awamtib. The beds of Awamtib itself dip pretty uniformly a little north of west.
Fig. 61.—Sketch section of junction of sandstone with granite, west of Gebel Um Keit.
Fig. 61.—Sketch section of junction of sandstone with granite, west of Gebel Um Keit.
Fig. 61.—Sketch section of junction of sandstone with granite, west of Gebel Um Keit.
Fig. 61.—Sketch section of junction of sandstone with granite, west of Gebel Um Keit.
About nine kilometres to the south of Gebel Awamtib, a station was taken on a sandstone headland with schists and quartz veins at its floor, and afterwards I skirted the limit between sandstone and granitic rocks on the way to Gebel Um Reit. All along this route there was no suggestion of sharp folding or faulting; the beds were nearly horizontal right up to the limit, where the granite hills rise suddenly (seeFig. 61).
From the top of Gebel Um Reit, which itself is granite, the sandstone limit could be seen to the north and east; the beds dip 30° in places; in those on the east the direction of dip is about south. The region round Um Reit is evidently one of considerable disturbance, but the sandstone is too far off for one to get any precise idea of its nature from the summit.
Fig. 62.—Sketch of faulting near Wadi Saalek.
Fig. 62.—Sketch of faulting near Wadi Saalek.
Fig. 62.—Sketch of faulting near Wadi Saalek.
Going northwards from Um Reit across the Wadi Saalek and up one of its branches into the head of Wadi Muegil, there is a good exposure of faulted sandstone overlying schists just before reaching the pass. Here (seeFig. 62) one of the faults is a distinct overthrust, with a north-easterly strike, and there are other faults in a parallel direction.
From the Wadi Muegil northwards past the Galt el Aguz to Gebel Um Harba there are gentle dips and curvings of the sandstone beds, with a predominant dip about north-east. At the Galt el Aguz the sandstone rocks are much tumbled about, but this appears to be due to fall of over-hanging beds.
At the pass from Wadi Um Arta into the head of Wadi Silsila, the dip of the sandstone is north-north-east. In its lower part, Wadi Silsila passes between sandstone ridges the dip of which is constantly east-north-east.
Round Bir Abraq and the triangulation station on Gebel Abraq (seelarge scale map onPlate XV) the main direction of dip is east-south-east; the pool of Bir Abraq itself is under projecting slabs of sandstone dipping in this direction. Bir el Sunta, a little further north, appears to lie in a syncline, the beds of the hills to the north dipping south-east, while those to the south dip north-west.
Approaching the sandstone from the east by Wadi Hodein, one traverses granite country with felsite dykes and enters suddenly into high sandstone plateaux. The beds at the Abu Saafa Springs dip about 5° northwards.
The beds of Gebel Dif and Gebel Anfeib have a prevalent dip on the west side to the east and north-east, while on the east face the dip appears to be in the reverse direction. This great mass of sandstone plateau may thus be a syncline with a north-westerly strike (i.e., a strike roughly parallel to the main direction of the watershed mountain ranges), but the eastern face was not examined in detail, and it may be that the whole mass dips to the north-east; some outlying small sandstone masses near the head of Wadi Edunqul show no reversal, the dip being still east-north-east. At Bir Dif the sandstones are variously tilted, and probably faulted.
The sandstones on the east side of the watershed ranges, near Gebel Ranga, were observed by Dr. Hume to dip towards the sea. In the south part of the region there are some low hills forming two series of ridges between the sea and Gebel Kolaiqo, separated by hills ofred aplitic granite; these hills were not actually visited, but were mapped from some little distance, and the dip of the beds was not measured, but the nature of the rocks was confirmed by specimens of sandstone brought back by guides sent to the place.
Summing up the dips and disturbances of the sandstone beds at the different places, it is clear that the strata are the more disturbed, the more closely they approach the main mountain ranges; this and the seaward dip of the beds near El Ranga are so far in favour of the view that the sandstone may once have extended right over the present mountains. But it will be seen that the disturbances of bedding are of an irregular character, especially between Gebels Zergat Naam and Um Reit, where the folding and faulting are often in directions quite distinct from that of the main mountain axes, and it may be that these disturbances are due to later movements rather than to the elevation of the main mountain ranges. So far as observations on the sandstone itself go, therefore, the question as to whether the Nubian sandstones of the Red Sea border ever directly joined those on the west of the mountains remains open; the observed disturbances of the beds give us clear proof that considerable earth movements have taken place since the sandstone was deposited, but they are inconclusive as to whether these same movements caused the elevation of the great mountain ranges.
Turning now to the main mountain-forming rocks, we find them composed of the two great classes, igneous and metamorphic. Though in places we have transition members, such as granites passing into gneiss, yet on the whole the two groups are quite distinct, and we find typical igneous masses rising in the midst of equally typical schists. Under these circumstances there can be no hesitation in considering the two groups to be of different ages, and that the igneous rocks must be the younger.
The igneous masses are divisible geologically into the three classes of (a) lavas, (b) dykes, and (c) plutonic rocks.
Lavas.—Amongst the igneous rocks, the lavas are those most poorly represented. Apart from the andesite of Gebel Sufra, and the diabase sheets in or under the Nubian sandstone, there are only a fewoccurrences of volcanic rocks in all the area, a circumstance which is easily explained by the enormous denudation which has gone on and the fact that lavas, being superficial out-pourings, will have been most exposed to denuding forces.
Dykes.—Dykes seam the schists and igneous rocks, frequently in such numbers and with such parallelism as to give to the land the form of a succession of ridges separated by long narrow depressions, forming “dyke country.” In nature, the dykes vary from extremely acid rocks like aplites and quartz-felsites, to very basic forms such as diabase and basalt. There is on the whole a preponderance of basic over acid types. As already remarked, only one instance of a dyke cutting the Nubian sandstone has been observed in the district. Another significant feature in the distribution of dykes is that while dykes of all kinds are found cutting schists and acid plutonic igneous rocks, the basic plutonic masses are as a rule free from dykes; this suggests that the basic plutonic rocks may be on the whole younger than the acid forms.
Plutonic Rocks.—The plutonic rocks include granites, syenites, diorites, gabbros, pyroxenites, amphibolites, and peridotites. But these are not all of equal importance, and a natural division from the field observations is a two-fold one into acid and basic groups, which are tolerably well marked off from each other and are most likely of different ages. The acid class on this view consists of the granites and syenites, while the basic class contains all the other rocks above-mentioned. The granites are on the whole of a very acid type; syenite is very scarce and nearly always occurs in close association with granite. Of the basic group, probably the most abundant and most typical rock is gabbro; diorites and the various ultra-basic rocks are almost always closely associated with gabbro. Where acid and basic rocks occur in proximity (as for instance near Bir Abraq, where an acid granite occurs side by side with a serpentine) there is a sharp change, without transitional forms.
The much greater abundance of dykes in the plutonic rocks of the acid group as compared with those of basic composition, as mentioned above, inclines us to regard the basic group as the younger—a view which is further supported by the greater frequency with which a tendency to gneissose structure is noticeable in the acid rocks. The fact that the basic members are frequently in a higher state of decompositionthan the acid ones is not contrary to this view, being due to the greater ease with which their constituents undergo weathering; and as explained onp. 315,the fissured state of the altered peridotes is probably not due to the same earth forces which have sheared the granites, but to internal stresses set up by the expansion of the rocks on serpentinisation.
Though we may be fairly sure that the basic plutonic rocks are on the whole younger than the acid ones, we have no certain guide from which to estimate the geological age of either. We know from their relations to the sandstone that they are both older than the Upper Cretaceous, but we cannot say how much older; they may be anything from Archæan to Jurassic; the circumstance that similar rocks underlie Carboniferous strata in Sinai inclines us to place them at least as far back as Palæozoic times.
Gneisses and Schists.—The gneisses and schists which cover so much of the country are obviously older than the plutonic masses which are intruded in them, and for these metamorphic masses we need have small hesitation in speculating on an Archæan age. They exhibit a wonderful variety of composition. Most of the gneisses are doubtless sheared ancient igneous rocks, such as granite and diorite, and a similar origin may be assigned to many of the schists. Other rocks, as for instance the clay-schists, graphite-schists, and marble, have almost certainly originated from the metamorphism of ancient sedimentary rocks. But in a large number of cases we have at present no clue to the parent rock from which the schists have been formed.
Having in the foregoing pages discussed the evidences for the relative ages of the different classes of rocks and their mutual relations, we may now endeavour to reconstruct the past geological history of this part of Egypt from the information gathered. In this process we shall reverse the order of consideration taken above, and begin with the oldest rocks.
The schists and gneisses probably represent, not the original crust of consolidation of the earth, but a complex of ancient sedimentary and igneous rocks, laid down in pre-Palæozoic times and subsequently crushed, folded, and faulted into mountains which were subsequentlydenuded and worn down. In parts of the main mountain-masses we may possibly still have the cores of some of these ancient elevations, but most of the present mountain peaks are formed of a later series of igneous rocks.
The first igneous intrusions into the schists and gneisses were the granites which form such peaks as Gebels Nugrus, Faraid, and Elba. Then followed crushing and folding of these rocks, giving them often a gneissose structure and opening fissures, up which came later intrusions, mostly of basic rocks, in the form of dykes. With the dykes or later came great basic intrusions of gabbroid rocks, forming such masses as Gebels Dahanib, Gerf, and Meisah.
A long interval now followed of which the rocks contain no record. We do not know whether the area was submerged or not in Palæozoic and early Mesozoic times; but if it was, all trace of the deposits of these ages has vanished in the great denudation which surely took place before the Upper Cretaceous sea swept over the country and deposited the Nubian sandstone. We do not know whether the Red Sea mountains then stood up as islands, or whether they were subsequently elevated. Nor do we know whether the area remained wholly or partly submerged during the Eocene period. But we are sure that at some time between the Upper Cretaceous and Oligocene epochs there was a great elevation of the land, with folding and faulting, especially in the areas now occupied by the great mountain masses, and possibly the mountains had their origin in this movement of elevation. The Red Sea depression may well have originated in a complementary sinking at this same period. The depth of the sea is of the same order (2,000 metres) as the height of the mountains on the adjacent land.
In the succeeding Oligocene period the land was being sculptured into something like its present form, and probably the main drainage lines of to-day were then formed.
About the Miocene epoch there was a sinking of the crust in this region. The Red Sea increased in area, and then, probably as the results of evaporation in a closed sea, deposits of gypsum were laid down. A subsequent elevation in Pliocene or post-Pliocene times raised these deposits along the present shores.
In geologically recent times a further gentle elevation has gone on, giving rise to slightly raised coral-reefs and sea-beaches. During the glacial period of Europe, the rainfall in Egypt was probably greaterthan at present, and during this period the great wadis received almost their final sculpturing. After the change from this rainy climate to the dry one of to-day, erosion still went on, though more slowly, in the hills; but on the plains and along the coast accumulation of sands took place, partly owing to wind transport, and partly owing to the insufficiency of the drainage waters to carry their load as far as the Nile or sea. The abundance of coral-reefs in the Red Sea is largely conditioned by the lack of in-flowing streams of silt-laden water; for the coral animal flourishes only where the water is clear.