The geography and geology of south-eastern Egypt

by John Ball

INTERMEDIATE IGNEOUS ROCKS.

(a) SUB-ACID ROCKS.

Syenite and Syenite-porphyry.

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

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

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

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

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

Ball.—Geography & Geology of South-Eastern Egypt.

PLATE XXIII.

INTERMEDIATE IGNEOUS ROCKS.

PINK SYENITE.
Gebel Zergat Naam.

TRACHYTE.
Gebel Kahfa.

DIORITE.
Gebel Allawi.

DIORITE WITH PRISMATIC HORNBLENDE.
Wadi Muqur.

BANDED ANDESITE.
Gebel Sufra.

VEINED KERSANTITE.
Gebel Fereyid.

NATURAL SIZE.

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

Trachyte.

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

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

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

(b) SUB-BASIC ROCKS.

Diorite.

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

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

Fig. 18.—Diorite of Gebel Allawi [10,313], × 10. pl, plagioclase felspar, clouded by decomposition products; h, hornblende, olive-brown in colour; ha, hornblende altering to pale-green chlorite; m, magnetite.

Diorite in the narrowest sense of the term (plagioclase-hornblende rock) is not by any means abundant. Curiously enough, it is generally found in the neighbourhood of old gold mines, as for instance in the Kurdeman and Allawi districts, and it was employed by the ancient miners for their crushing pans. The rock from Gebel Allawi [10,313] is a medium-grained one composed of black and milk-white minerals in about equal proportion (see [Plate XXIII]). Its sp. gr. is 2·95. Under the microscope the milk-white material is seen to be plagioclastic felspar, generally much decomposed, while the dark grains are of hornblende, pale green to olive-brown in thin section, with somewhat feeble pleochroism. Like the felspar, the hornblende is considerably altered; in places it has lost nearly all its colour and is converted into pale green chlorite. The accessory minerals are iron-oxides and a little sphene. Rocks of somewhat finer grain [10,358] occur in the Rod el Ligaia. In a slide from the last-named locality, granules of ilmenite are surrounded by sphene, suggesting the formation of sphene by the alteration of ilmenite. A variety of diorite [10,403] occurs in the Wadi Huluz in which the hornblende is nearly colourless, showing only a trace of green colour in thin section, and is accompanied by a small amount of augite.

Fig. 19.—Diorite, Wadi Baaneit [12,151], × 10. h, hornblende; pl, plagioclase; b, accessory biotite; q, accessory quartz.

A form of diorite which occurs in and about the Wadi Baaneit [12,151] is very similar to the rock just described, but differs from it in the more irregular manner in which the two principal mineral constituents are distributed. The hornblende is in patches varying from mere specks to eight millimetres in diameter; the felspars have a sugary appearance in the hand specimen. The sp. gr. is 2·81.

A more highly specialised type of diorite [12,103] occurs in the Wadi Muqur. This is a very beautiful heavy rock (sp. gr. 2·87), with shining prisms of hornblende, often three centimetres or more in length, running through a mass of white felspars (see [Plate XXIII]). Microscopic examination reveals the presence of some quartz mixed with the plagioclastic felspars.

Where the alteration of diorite has been very intense, as in the country rock [12,124] of the old Romit mine, the hand specimen differs from that of the unaltered rock in that not only are the white constituents of duller aspect, but the dark mineral, instead of being black and shining, has a dull greenish-grey appearance. Under the microscope one sees no trace of the original felspar, its place being taken by calcite and a confused aggregate of small plates of kaolin and sericite, while the hornblende is all replaced by chlorite, with little strings of limonite. Granules of quartz, often of relatively large size, are scattered through the mass; some of these are clouded by minute enclosures, and evidently represent the accessory quartz of the original rock, while others are clearer and are probably of secondary formation.

Fine-grained varieties of diorite are found much more abundantly than the normal plutonic type, occurring as irregular masses mixed with schists and as dykes in schists and other rocks. These fine-grained diorites are essentially similar to the coarser-grained rocks, but are generally even more altered. The fine-grained diorite [11,517 A], which forms the top of Gebel Um Tenedba, for instance, consists of a mixture of clouded and altered plagioclase and augite altering to chlorite, with separated iron oxide along the cleavage planes of the hornblende. Another specimen of fine-grained diorite, from a dyke in Wadi Kreiga [12,154], is even more highly altered, the hornblende being almost entirely chloritised, with formation of abundant little granules of epidote.

The main rock of Gebel Beida [12,160] may also be classed as an altered fine-grained diorite. It is a dark fine-grained greenish-grey rock with white and greenish-yellow spots, usually about two millimetres diameter, scattered through it, and abundant strings of a greenish yellow mineral. The sp. gr. is 2·96. Under the microscope it is seen to be of similar nature to the rocks last described, the hornblende being largely chloritised. There is a considerable amount of accessory augite, and this, though clouded, has resisted alteration better than the hornblende. The whitish spots seen in the hand specimen are made up of a fine mosaic of quartz grains, with tiny flakes of a micaceous mineral (sericite?) probably all of secondary origin, while the greenish-yellow strings are other alteration products in the shape of epidote and calcite.

Augite-diorite.

Under the head of augite-diorite are classed holocrystalline rocks containing, in addition to the plagioclase and hornblende of ordinary diorites, notable quantities of augite. The augite-diorites thus form a link between the diorites proper and the diabases or plagioclase-augite rocks.

The presence of augite along with the hornblende is difficult to ascertain in the field or in hand specimens, and can as a rule only be detected by the microscopic examination of thin sections. But augite diorites are generally of somewhat darker aspect than normal diorites in the mass, owing to a less abundance of felspar, and are generally tougher under the hammer.

Fig. 20.—Augite-diorite, Wadi Um Hargal [11,535], × 47. h, hornblende; a, augite, with celephytic border c, of hornblende and iron oxide; f, felspar (labradorite); ap, apatite.

An augite-diorite [11,535] which occurs on the pass at the head of Wadi Um Hargal, near Gebel Kahfa, is a heavy (sp. gr. 2·87) grey rock of medium grain, very fresh and hard, in which can be seen lustrous black crystals mixed with a rather small quantity of white felspars. In thin section, hornblende, the most abundant constituent, is in fairly large allotriomorphic crystals, with well marked cleavage and strong pleochroism (bluish-green to pale yellow), containing abundance of irregular granules of iron oxides. The augite, which is present to about one-third the amount of the hornblende, is in crystals of similar size, and likewise showing well-marked cleavage, easily distinguished by their pale brown colour, absence of pleochroism, and higher extinction angles (about 40°); some of the crystals show irregular cracking and clouding by decomposition products, and are surrounded by celephytic zones of greenish matter of rather lower double refraction containing flakes and strings of iron oxide, probably representing a marginal alteration to hornblende. The felspar, though considerably decomposed, still shows plagioclastic twinning clearly, and appears from the extinction angles to be an acid type of labradorite. The rock contains considerable amount of magnetite scattered through it, often in fairly large irregular grains, also a few small grains of apatite, and one or two small granules of quartz.

Fig. 21.—Augite-diorite, Gebel el Anbat [10,411], × 40. pl, plagioclase felspar; a, augite; h, hornblende, arising from alteration of augite.

The rock [10,411] which forms the dark hills called Gebel el Anbat, near the head of Wadi Kharit,[131] is likewise an augite-diorite. In the field it is seen weathered into rounded masses often resembling boulders, of great hardness, and covered with a blackish-brown skin. The sp. gr. is 2·97. Microscopic examination shows the rock, which is very fresh, to be essentially of the same type as that last described, but the augite is more abundant and so intimately mixed with the hornblende as to suggest even more strongly an alteration of augite to hornblende (see [Fig. 21]).

Mica-diorite.

Fig. 22.—Mica diorite, from a dyke at Gebel Abu Hegilig [10,391], × 17. f, felspar (mainly plagioclase); b, biotite altering with formation of limonite; h, hornblende; ap, apatite; m, magnetite.

Most of the diorites of South-Eastern Egypt contain little or no biotite as an accessory constituent. An exception occurs, however, in a great dyke [10,391] of very fine grained diorite of sp. gr. 2·87, which traverses the granite of Gebel Abu Hegilig, and which contains more biotite than hornblende. The dyke is so decomposed that it is difficult to get a coherent hand specimen; but a slide cut from one of the less altered portions shows the rock to be a very fine grained holocrystalline one, made up of plagioclase, biotite, hornblende, apatite and magnetite, with abundant alteration products such as epidote, kaolin, and chlorite. All the minerals, except the apatite and some of the iron oxides, are allotriomorphic. The felspars are very much altered, but appear to be mainly plagioclase. The biotite is in little brown ragged-looking plates, strongly pleochroic, frequently altered with separation of flakes of limonite. The hornblende is green, in small and very irregular-shaped crystals, which show very little trace of cleavage and are frequently chloritised. The apatite is in long hexagonal clear prisms. Iron oxides, sometimes showing square or hexagonal outlines, and epidote in granules, are liberally scattered through the rock. From the abundance of biotite and the fine grain and manner of occurrence of this rock it was taken in the field for a decomposed lamprophyre; but the entire absence of idiomorphism in the ferro-magnesian minerals show that it should rather be placed with the diorites.

Diorite-porphyrite.

Rocks which may be somewhat doubtfully classed as altered diorite-porphyrites occur at Gebel Abu Hodeid as well as near the ruins of Um Eleiga and at Gebel Um Heshenib.

Fig. 23.—Diorite-porphyrite, Gebel Abu Hodeid [12,143], × 40. f, porphyritic felspar (plagioclase); h, hornblende; b, biotite; g, ground mass, consisting chiefly of plagioclase and hornblende.

The triangulation point on Gebel Abu Hodeid not being an occupied station, I have not visited the mountain, but the guide sent to erect the beacon on the summit brought back a specimen [12,143] of the rock. It is a very fine-grained dark grey rock with tiny glistening specks. The sp. gr. is 2·88. The microscopic slide shows it to consist of porphyritic plagioclase in a very fine-grained holocrystalline ground mass composed principally of brown hornblende and plagioclase, with a little biotite and a plentiful sprinkling of tiny granules of iron oxides. The porphyritic plagioclases are inclined to idiomorphic forms, forming crystals about half a millimetre in length, and hence are not very conspicuous in the hand specimen; they are considerably clouded by decomposition, but still show repeated twinning very clearly. The hornblende, which forms the main constituent of the ground mass, is of a pale to dark yellow-brown colour, mostly in rounded granules in which cleavage is not very strongly marked. The plagioclase of the ground mass is likewise in tiny granules, mixed with the hornblende. Biotite is only sparingly present in the slide; it is in tiny brown flakes. All the minerals of the ground mass show more or less decomposition and clouding, and contain a fairly plentiful sprinkling of minute grains of iron oxides; flakes of this latter substance are specially evident in the decomposing biotite.

The diorite-porphyrite of Um Eleiga [11,527 B] occurs associated with fine grained gabbro round the old mines. It is a grey rock, breaking with a rough surface, in which porphyritic lath-shaped felspar crystals, up to three millimetres long, are somewhat sparsely scattered in a fine-grained ground mass. The sp. gr. is 2·82. The microscopic slide reveals the ground mass to be finely holocrystalline, composed of plagioclase and pale green to brown hornblende, with some magnetite. The plagioclase of the ground mass is partly in little laths, and the hornblende frequently shows a tendency to prismatic and fibrous forms. The whole rock is in a rather advanced state of alteration, all the crystals in the slide being strongly clouded by kaolin and other decomposition products.

The summit rock of Gebel Um Heshenib [10,392] appears to be a highly altered basic diorite-porphyrite forming a dyke in the surrounding schists. It is a dark-grey heavy rock (sp. gr. 3·04), of basaltic appearance, with white porphyritic patches, more or less rounded in form, scattered through it. The microscopic slide shows the white patches now to consist mainly of kaolinic matter, with which is mixed a clear mineral (sericite?), of very low refractive index, but showing rather high double-refraction colours; while the ground mass is a very fine-grained mixture of rather fibrous pale green hornblende with kaolinic matter. Hardly a trace of unaltered felspar remains, but it seems natural to ascribe the kaolin both of the porphyritic areas and of the ground mass to the decomposition of original felspars.

A dyke of fine-grained brown rock which occurs in the granite of the lower part of Wadi Kreiga [12,102] likewise appears to be an altered diorite-porphyrite. It is much less dense than the rock last described, its sp. gr. being only 2·68. The microscopic slide shows the main constituent to be plagioclase felspar in idiomorphic forms, much altered to kaolin and calcite and stained red by iron oxide. Between the felspars are irregular patches of chloritic and serpentinous matter, with calcite and flakes of limonite, the alteration products of a ferro-magnesian mineral which was probably originally hornblende.

Augite-porphyrite.

Fig. 24.—Augite-porphyrite, Wadi Muelih [10,359], × 40. a, augite; pl, plagioclase; h, hornblende, much clouded with iron oxides. The rock contains large porphyritic plagioclase and hornblende crystals not shown in the figure.

A dyke of augite porphyrite [10,353] occurs in the Wadi Muelih about half-way between Gebel Muelih and Erf el Fahid. It is a fine-grained reddish-brown rock of sp. gr. 2·79, with porphyritic white plagioclase crystals of considerable size (up to two centimetres in length) and other smaller porphyritic crystals of a dark schillerized-looking mineral. The ground mass, when examined with a hand lens, is seen to be a very fine-grained mixture of red and dark minerals, like a syenite in miniature. The microscopic slide reveals the dark porphyritic crystals as green hornblende, while the ground mass is a holocrystalline mixture of plagioclase and augite, with a little green to brown hornblende and abundance of magnetite granules. The felspars of the ground mass are mostly lath-shaped; they are much altered, and stained red by iron oxide. The augite, of a very pale purple colour, is abundant in the ground mass, sometimes in prismatic forms, but more often in rounded grains; it is altered in places to chlorite, becoming then green in colour. The extinction angles measured in the less-altered crystals range to over 40°.

Kersantite.

Fig. 25.—Kersantite, from a dyke at Gebel Fereyid [11,504], × 40. f, felspar (mainly oligoclase); b, biotite; a, augite, with a border of hornblende (h); ap, apatite.

Only one occurrence of a rock which can be with certainty classed as a lamprophyre has been noted in South-Eastern Egypt. The single occurrence referred to is that of a kersantite [11,504] which forms a dyke cutting east-and-west through the granite of Gebel Fereyid. In the hand specimen (see [Plate XXIII]) it is a fine grained dark-grey rock, marbled with veinlets of brownish-white granular (felspathic?) material, and containing here and there porphyritic dark-brown platey crystals up to three millimetres in diameter. The sp. gr. is 2·81. Under the microscope, the rock presents a very fresh appearance, and is seen to be composed mainly of felspars and brown biotite, with a little accessory augite, green hornblende, apatite and magnetite. The biotite, the most conspicuous constituent, is seen partly in basal sections of perfectly idiomorphic forms, and partly as long lath-shaped sections; it is strongly pleochroic, the colour varying from pale yellow-brown to a very dark reddish-brown. The felspar, which has undergone some alteration, forms a sort of matrix round the biotite; here and there large crystals show a tendency to idiomorphism, but the felspar is essentially allotriomorphic. The felspar appears to be mainly oligoclase; but there are also some crystals which show only simple twinning, and these are doubtless orthoclase. The augite and hornblende are very sparingly present, the former in nearly colourless to purplish crystals, the latter in tiny forms of a deep emerald or bluish-green colour, with very high double refraction. Magnetite is liberally scattered in small grains through the rock, seldom included in the biotite. Apatite occurs in very fine long needles among the felspars.

Andesites.

Andesites, the volcanic representatives of the diorites, are much more scarce in the south portion of the Eastern Desert than they are further north. In the district here treated of, only a single deposit, that of Gebel Sufra, has been noted as belonging certainly to the class of andesitic lavas. Some other volcanic rocks consisting chiefly of hornblende and plagioclase have been met with, as for instance at the hill of Ti Keferiai and in the Wadi Huluz; but these are of so basic a nature that they are more properly classed as hornblende-basalts. It is also practically certain that some of the rocks which must be classed as schists on account of their structure are metamorphosed andesites; among the schists of the Wadi Muelih, for example, are rocks which in thin section present a ground mass still distinctly andesitic in character, but the hornblendic constituent, instead of forming well-defined porphyritic crystals as it doubtless originally did, is dragged out into fibrous forms, and the same action can be traced in the hornblendes of the ground mass.

The andesite of Gebel Sufra [10,597] occurs as a columnar deposit overlying syenites and diorites at the top of the mountain, which rises to 690 metres above sea-level in latitude 24° 39′. The rock, which is a fine-grained greenish-grey one weathering to a brown colour on exposed surfaces, often shows a banded structure (see [Plate XXIII]). The sp. gr. is 2·67. The microscopic section shows the rock to be highly altered, but sufficient traces of its original nature can be made out to leave little doubt of its being an andesitic lava. The slides show porphyritic felspars in a cryptocrystalline ground mass composed of felspar with a little hornblende and biotite. The porphyritic felspars are too much kaolinised for twinning to be made out, but a little calcite is visible in their decomposition products, and the crystals are in rather elongated forms which suggest plagioclase rather than orthoclase. The minerals of the ground mass are likewise much decomposed, but tiny grains of green hornblende and wisps of brown biotite, both altering to chlorite, can be seen. There is an almost complete absence of primary iron oxides, but a single large porphyritic crystal in the slide shows separated hæmatite in flakes and in strings down its cleavage planes. The nature of this single crystal is not very clear, and it is doubtful if any of its original substance remains; its form and cleavage are suggestive of augite, but the clear spaces unoccupied by the iron oxide have the appearance of quartz or clear felspar under crossed nicols.

Fig. 26.—View near the top of Gebel Sufra, showing the columnar structure of the andesite.

Fig. 27.—Andesite of Gebel Sufra [10,597], as seen between crossed nicols, × 40. Porphyritic felspar crystals in a cryptocrystalline ground mass.

BASIC IGNEOUS ROCKS.

Gabbro.

Gabbros (or plutonic plagioclase-pyroxene rocks with or without olivine) are widely distributed in South-Eastern Egypt, entering largely into the composition of some conspicuous mountains such as Gebels Atut, Madaret Um Gamil, Um Gunud, Um Bisilla, Dahanib, Gerf, and Hadal Aweib Meisah, and also occurring in smaller patches at various other points.

Though they are all dark-coloured, tough and heavy rocks (specific gravity from 2·8 to 3·2), the gabbros vary very much in appearance at different places owing to variations in size of grain and in mineral composition. Thus we have every gradation from the coarse-grained gabbros such as those of Um Bisilla and Gerf, where the individual crystals measure sometimes two or three centimetres in length, through the medium grained rocks of Atut, Dahanib and Hadal Aweib Meisah, to the granulitic gabbro of Kolmanab hill, of which the grain is so fine that the rock looks almost like a basalt. In mineral composition, the gabbros show likewise great variety. Some, like the rocks of Um Bisilla, are relatively rich in felspar, and are lighter both in colour and in weight than others in which the pyroxenes predominate; in some cases the proportion of felspar almost vanishes and the rock passes into a pyroxenite. Some of the gabbros, such as those of Atut, contain olivine, while others, such as the rocks of Gebel Dahanib, do not. The nature of the pyroxene varies, being sometimes almost entirely diallage, while in others it is mainly ordinary augite, and in others, again, rhombic pyroxenes such as bronzite and hypersthene occur. A further variation is the presence of hornblende in some gabbros, either as an accessory primary constituent or as an alteration product of a pyroxene. In the uppermost rock of Gebel Um Bisilla we have an example of troctolite, a form of gabbro in which there is no pyroxene but only felspar and olivine.

In the field, mountains and hills formed of gabbro are typically of dark aspect, though frequently less dark than a freshly broken surface of the rock, owing to a film of iron-oxide which forms on weathered faces. This film is most strongly marked in the olivine-bearing varieties of the rock; it is very thin, good sound rock being usually found at a depth of a millimetre or so below the exposed surfaces. In form, hills of gabbro are usually in the form of flattish cones and ridges, whose surfaces and summits are covered with a debris of rusty-looking weathered blocks of the rock. This blocky type of summit is well seen at Gebel Atut (see the view on [Plate X,] p. 172).

Though sometimes sharply marked-off from the adjacent rocks, gabbros, when traced laterally in the field, are most frequently found to pass gradually into more basic forms such as pyroxenites, amphibolites, and serpentines. It is not always easy in the field to distinguish between augite or diallage and hornblende, and one or two rocks which were taken for gabbros turn out on microscopic study to be really basic diorites or hornblende-rocks; while a rock at Um Eleiga, which strongly resembles a rather fine-grained diorite in appearance, turns out to be a gabbro. The limit between gabbros and peridotites is exceptionally difficult to map, the proportions of olivine, augite, bronzite, and felspar changing very frequently in the same rock mass, as for instance at Gebel Gerf.

Almost all the gabbros contain a considerable amount of magnetite as an accessory constituent, and in some cases, as at Gebel Hadal Aweib Meisah, magnetite is present in such quantity as to render the rock strongly magnetic. Compass readings in the neighbourhood of large masses of gabbro are almost always subject to more or less error from this cause. In the case of a gabbro discovered by Dr. Hume to the west of Gebel Ranga, near the coast about latitude 24° 24′, concentration of the ferruginous matter has gone on to such a degree as to give rise to deposits of hæmatite containing 39 per cent of iron.[132]

Perhaps the most striking feature evident in the microscopic slides cut from the gabbros is the remarkable freshness of the felspars in most of the specimens, which, taken in conjunction with the basic nature of the rocks, inclines one to consider the basic rocks as probably on the whole amongst the youngest of the plutonic masses. Another characteristic feature is “celephytic” structure, in which a shell of green hornblende is found to surround the iron oxides and pyroxenes when they are embedded in, or in contact with, the surrounding felspar.

Fig. 28.—Gabbro, Gebel Dahanib [11,509], × 17. f, felspar (labradorite); a, augite; d, diallage; b, bronzite; h, hornblende, probably produced by alteration of augite.

A typical olivine-free gabbro [11,509] forms the main rock of Gebel Dahanib. A specimen taken from the summit, where the rock is rather finer in grain than that of the rest of the mountain, is very hard and heavy (sp. gr. 3·15), and is formed of a mixture of dark dull-looking mineral with shining white to colourless felspars. Under the microscope it is seen to be a holocrystalline aggregate of fairly fresh labradorite and pyroxene, both in allotriomorphic forms, in about equal proportions. About half the pyroxene is in the form of diallage, the remainder being mostly ordinary augite; it is almost colourless in thin section, but some of the crystals show a slight pleochroism, colourless to pale pinkish-brown. The augite crystals are much cracked, and frequently show signs of alteration with formation of calcite, epidote and serpentinous matter. There are a few small irregular areas of very pale green hornblende mixed with the augite, of which they may possibly be products of alteration. One or two elongated crystals in the slide, barely distinguishable from the augite under ordinary light, show a fibrous structure and straight extinction with low double-refraction colours; these are probably bronzite. There are only a few very tiny grains of iron oxide, and olivine appears to be absent from the slide examined.

Fig. 29.—Gabbro, Um Eleiga [11,527 A], × 17. pl, plagioclase; pc, clouded plagioclase; a, augite; m, magnetite; s, serpentinous matter probably from alteration of augite and hornblende.

A fine grained gabbro free from olivine occurs associated with diorite round the old mines of Um Eleiga [11,527 A]. It is a speckled black-and-white rock which would at first sight be taken for a fine-grained diorite rather than a gabbro. Its sp. gr. is 2·93. On microscopic examination the rock is found to be a holocrystalline aggregate of plagioclase, augite, altered hornblende, and magnetite, with granitic structure. The plagioclase (labradorite) is considerably altered and clouded by kaolinic matter, especially near the centres of the crystals, but still shows its characteristic twinning clearly. The augite, which is sometimes in the form of diallage, is fairly abundant, mostly in irregular grains, though occasionally inclining to prismatic forms; it is nearly colourless, but much cracked and slightly clouded. A clouded pale green to brown mineral, which sends off long tongues into cracks in the surrounding felspars, is also fairly abundant. This mineral polarises in yellows and greys as a confused serpentine-like aggregate of minute fibres, but contains clear and nearly colourless areas representing the original mineral from which it is derived; these clear areas, in which prismatic cleavage is usually well marked, are sometimes augite, but in many cases they polarise in lower colours (greys and yellows) than the augite, with low extinction angles, and are probably hornblende. Magnetite is very abundant in quite large irregular grains, often surrounded by a thin shell of hornblende or of the clouded alteration product just mentioned. There are a few small six-sided prisms of apatite, mostly included in the felspars.

Fig. 30.—Hypersthene-gabbro, Hadal Aweib Meisah [12,126], × 4. f, felspar (labradorite); a, augite; hy, hypersthene (the augite and hypersthene have the same appearance in ordinary light); m, magnetite; hb, hornblende, forming celyphitic borders round the augite and magnetite.

Another variety of fine-grained gabbro, likewise free from olivine, but containing hypersthene and some hornblende [12,126], forms the upper portion of Hadal Aweib Meisah, and a very similar rock [11,521] occurs in the hills five kilometres south-south-west of Marwot Elemikan. The rock is very hard, of a grey colour on fracture, weathering to blocks which have a rusty-brown skin. Its sp. gr. is 2·98. It is highly magnetic, and causes great disturbance of the compass needle in its neighbourhood. The hand specimen shows a mixture of shining white felspars with duller black minerals (see [Plate XXIV]). On microscopic study, the rock is found to be a holocrystalline aggregate of plagioclase, augite, hypersthene, hornblende and magnetite. The plagioclase, which forms about two-thirds of the rock, is a very clear and fresh labradorite of a rather acid type. The augite is usually in more or less rounded grains, often aggregated into irregular strings and mixed with hypersthene and magnetite. Only in a few cases does the augite show diallagic lamellation. It is nearly colourless, with a slight greenish or pinkish-brown tinge and faint pleochroism. The prismatic cleavages are usually distinct, and in addition the crystals are irregularly cracked. Twinning is fairly frequent, as also are inclusions of magnetite in the augite. The hypersthene occurs in grains similar to those of the augite, with which it is mixed, and from which at first sight it is not easily discriminated; but it can be picked out by its more marked pleochroism, straight extinction, and lower double refraction. Magnetite is abundant in large irregular grains, and in smaller granules included in the augite. Hornblende occurs in subordinate amount to the other minerals, and is principally seen as a celyphitic zone round the magnetite and augite crystals, especially between these crystals and the felspars. It is usually fairly clear, strongly pleochroic (deep greenish-brown to very pale yellowish-brown), and where surrounding two or three grains of other minerals the whole zone extinguishes at once, showing it to be a single crystal.

Ball.—Geography & Geology of South-Eastern Egypt.

PLATE XXIV.

BASIC AND ULTRA-BASIC IGNEOUS ROCKS.

OLIVINE-GABBRO.
Gebel Um Bisella.

FINE-GRAINED GABBRO.
Hadal Aweib Meisah.

TROCTOLITE.
Gebel Um Bisella.

DIABASE.
Rod el Nagi.

OLIVINE-BASALT.
Einiwai Hill.

SERPENTINE.
Gebel Korabkansi.

NATURAL SIZE.

Fig. 31.—Olivine-gabbro, Gebel Um Bisilla [11,514], × 10. pl, plagioclase felspar (labradorite); d, altered diallage; o, olivine, altered in places to serpentine (s), with separation of granules of iron oxide.

The main portion of Gebel Um Bisilla is formed of a gabbro [11,514] containing a relatively small proportion of pyroxene and a considerable amount of olivine. In the mass, it is a hard tough rock, consisting of a mixture of white to greenish felspars, showing plagioclastic twinning with the lens, with dull dark minerals, some of a greenish colour and others of a rusty-brown appearance (see [Plate XXIV]). The sp. gr. of the rock is 2·8. The labradorite, which forms about two-thirds of the whole, is very fresh, but is here and there decomposed with formation of calcite along cracks. The pyroxenic constituent is now mostly represented by dusty looking very pale greenish-brown straggling crystals interstitial to the felspars; it appears to have been originally diallage, but is in an advanced stage of alteration, polarising as a confused fibrous mass of hornblende and chlorite, with irregular banding in a direction inclined at about 30° to the general direction of the fibres. The olivine is in large rounded grains, nearly colourless where unaltered, showing the usual irregular cracks; some of the grains are altered to yellowish-green serpentine, with separation of granules of iron oxide.

Fig. 32.—Olivine-gabbro, Gebel Atut [10,365], × 17. o, olivine; a, augite; h, hornblende; f, felspar (labradorite).

Of the fine-grained olivine-gabbros, one of the principal types is the rock of Gebel Atut and Madaret Um Gamil [10,365]. It is a dark heavy rock, weathering into angular blocks with a thin rusty skin, of great hardness and ringing under the hammer. On a fresh fracture, it is seen to be made up of white glassy felspars and dark brownish minerals, some of which have a platey structure with cleavage surfaces which flash as the specimen is turned about in the sunlight. The sp. gr. is 3·01. Microscopic examination shows the rock to be a holocrystalline aggregate of plagioclase (labradorite), augite, hornblende and olivine, with a very little magnetite. The labradorite, which forms about half the rock, is very clear and fresh, in large crystals which frequently show a tendency to idiomorphism. Smaller crystals of labradorite are frequently included in the augite and hornblende. Augite, the next most abundant constituent after the felspar, is present in irregular almost colourless grains with well marked cleavage and numerous irregular cracks, and sometimes slightly clouded. The extinction angles measure up to 36°. The hornblende is in some crystals of a rather pale greenish-brown colour not showing very strong pleochroism, while in others it is more strongly coloured, varying from a rather deep reddish-brown to very pale yellowish-brown when turned over the nicol. The hornblende and augite are frequently associated in such a manner as to suggest that much of the hornblende in the rock originated from alteration of augite. The olivine is fairly abundant in large rounded grains, with the usual blackened irregular cracks; it is mostly fresh, but here and there are patches converted into nearly colourless serpentine with separation of numerous granules of iron oxide.

Fig. 33.—Olivine-gabbro, from a hill eleven kilometres east of Gebel Selaia [10,412], × 17. pl, plagioclase felspar (labradorite); a, augite; hy, hypersthene; o, olivine, with irregular cracks marked by separated magnetite; h, hornblende, enclosing the other minerals and forming a pseudocelephytic border round the olivine and augite. The rock typically contains a somewhat greater proportion of hornblende than appears in the figure.

Another olivine-gabbro [10,412], which forms a hill rising from the plain about eleven kilometres east of Gebel Selaia, resembles that of Gebel Atut, but is rather coarser in grain and contains less plagioclase, a larger proportion of hornblende, and probably a little accessory hypersthene. It is a dense dark and tough rock of sp. gr. 3·17, containing large black schillerized-looking crystals sometimes measuring one centimetre across, and a liberal sprinkling of white plagioclase with a few tiny grains of pyrite. Under the microscope, very clear and fresh labradorite is seen to form less than one-third of the rock; the crystals, which are mostly of irregular outline, are frequently enclosed in the hornblende. The hornblende occurs very abundantly as larger irregular green and brown crystals, with well-marked prismatic cleavage; it often encloses crystals of all the other minerals, forming a pseudo-celephytic border round the enclosed augite and olivine. Augite is somewhat less abundant than the hornblende, with which it is intergrown, in pale pinkish-brown crystals, slightly pleochroic, with very distinct vertical cleavage and irregular cracks. Twinning is not very frequent. The extinction angles are usually under 30°. A few crystals which show slightly stronger pleochroism (pinkish-brown to very pale green) than the ordinary augite, and straight extinction, are probably hypersthene. Olivine is present in about equal quantity with the augite, in large irregular grains, much cracked, but otherwise fairly fresh, polarising in brilliant colours; the cracks are blackened with separated iron oxide, but there is very little serpentinisation. There are a few small grains of pyrite and magnetite, but iron ores are not nearly so conspicuous as in some of the other rocks of this class.

Troctolite.

Fig. 34.—Troctolite of Gebel Um Bisilla [11,522], × 17. The crystals with the dark irregular cracks are olivine (o), altering to serpentine (s); the resulting expansion has crushed the surrounding clear plagioclase crystals (pl), forming large numbers of curved cracks into which little tongues of serpentine project.

If the gabbro of Gebel Um Bisilla is followed towards the summit, a diminution in the pyroxenic content, with a corresponding increase in the proportion of olivine, is noted, and at the top of the mountain we have a troctolite, or rock composed essentially of plagioclase and olivine [11,522]. It is a heavy speckled black-and-white rock, made up of colourless to milky and greyish-black grains about three millimetres diameter (see [Plate XXIV]). Its sp. gr. is 2·84. The rock looks like a diorite in the hand specimen; it weathers to rusty looking blocks of remarkable hardness. Under the microscope the mineral which looks greyish-black in the hand specimen is found to be olivine, colourless where unaltered in thin section; but the usual alteration to serpentine has gone on along irregular cracks, and the separated granules or iron oxide give the black colour to the mass. The felspar, which forms considerably more than half the rock, is a very fresh, though occasionally much cracked, labradorite. Both the constituent minerals are present in allotriomorphic grains.

Pyroxene-granulite.

Fig. 35.—Pyroxene-granulite, Kolmanab Hill [12,132], × 40. a, augite; h, hornblende; pl, plagioclase; m, magnetite.

The rock [12,132] forming the hill called Kolmanab, which rises from the coast-plain in latitude 22° 32′, resembles the fine-grained olivine free gabbros very closely in composition, but on account of its marked granulitic structure, is termed a pyroxene-granulite. The rounded form of the grains has probably been conditioned by movement of the magma during consolidation, but the rock contains no garnet or other typically metamorphic mineral, and there appears to be no reason for regarding this particular granulite as other than an igneous rock. It will have been noted that the augite grains in some of the fine grained gabbros show a marked tendency to granulitic forms, and the rock of Kolmanab appears to be merely an example of this tendency extending to the other constituents. The field relations are such as to suggest an intrusive boss. The rock is very hard and heavy, greyish-black and basaltic-looking; fresh fractures show tiny glistening grains when turned about in the hand. Its sp. gr. is 3·13. Microscopic examination shows it to be essentially a granulitic mixture of augite and plagioclase with a considerable amount of magnetite, and a little hornblende. All the minerals are of remarkable freshness, in rounded grains about a tenth of a millimetre in diameter. The augite, which forms about half the rock, is of a very pale green colour, sometimes showing faint traces of pleochroism with the same pinkish tints as hypersthene, from which, however, it is easily distinguished by its high extinction angles. It encloses abundant rounded colourless granules, which appear to be felspar. The felspar is an acid labradorite, and forms a mosaic among the augite grains; besides the twinned crystals, there are others which show no trace of this feature, and some of these may possibly be quartz. Hornblende occurs, not very abundantly, in larger crystals than the other constituents, grains of which it frequently encloses; it is of an olive-green colour. Magnetite is scattered through the entire rock in rounded grains, and is specially frequent enclosed in the hornblende.

Diabases.

Under the heading of diabase are included plagioclase-augite rocks, with or without olivine, of a character intermediate between gabbro and basalt. They differ from the gabbros in their finer grain, in the general absence of diallagic structures in the augite, and in the more or less porphyritic nature of their felspars, which are often ophitically intergrown with the augite. They differ from basalts, on the other hand, in being of coarser grain, and typically containing no glassy matter. The diabases of South-Eastern Egypt are more closely allied to the volcanic division of the basic rocks (basalts) than to the plutonic (gabbros), and many of the rocks here classed as diabases would be called dolerites by some English writers.

Fig. 36.—Diabase from under the Nubian sandstone, Rod el Nagi [10,417], × 40. pl, plagioclase felspar, ophitically intergrown with a, augite; l, limonite. The rock also contains large porphyritic felspars, not shown in the figure.

Perhaps the most interesting occurrence of diabase in this part of Egypt is a thick bed underlying the Nubian sandstone and exposed along the feet of its eastern scarps between latitudes 24° 30′ and 25°. Owing to the north-westerly trend of the scarp, it is cut obliquely by the meridian of 34°, and consequently only the southern portion of the deposit is shown on the geological map of [Plate XX.] The bed is well seen on the west of the road leading from Baramia to Dungash mines; this road follows the foot of the scarp for a long distance. The thickness of the diabase sheet as seen on this road is at least ten metres, and is very uniform for several kilometres. The rock is crushed and decomposed to such a degree that it is difficult to get a fair-sized specimen. Below the diabase, at a few points along the road, there are exposures of an ancient conglomerate of dark colour and extreme hardness. The relations of the bed to the sandstone are not absolutely certain, but the impression I got, when I passed the exposure on the two occasions of my outward and return marches, was that the diabase was an extensive outflow over the old conglomerate (thus indurating the latter), the sandstone being subsequently laid down on the undenuded diabase. The diabase is thus probably Cretaceous in age. A hand specimen [10,417] from one of the least decomposed portions of the sheet, in the Rod el Nagi, about fifteen kilometres west of Gebel Muelih, shows white to glassy porphyritic felspar crystals, frequently in lath-shaped forms measuring up to eight millimetres in length, in a fine-grained grey ground mass (see [Plate XXIV]). The rock contains many rusty specks, and occasionally as one turns the specimen about in the hand one can catch a rather dull flash from a crystal of some dark mineral. The sp. gr. of the rock is 2·85. Under the microscope the porphyritic felspars, which are very clear, are seen to be mostly plagioclase, though some of the crystals show simple twinning and may be orthoclase. The ground mass is holocrystalline, composed of felspar, augite, and limonite. The felspars of the ground mass are in the form of thin laths, with repeated twinning. The augite is nearly colourless, with a very pale brownish or greenish tinge, generally clouded by brown dusty matter and small irregular cracks. It forms irregular grains which are cut up in all directions by the ophitically intergrown felspars. The limonite is extremely abundant, scattered through the ground mass in rather large irregular grains which frequently show a tendency to square or hexagonal outlines; it is mostly opaque, but in some parts it is translucent, with a deep brown colour. The limonite is doubtless an alteration product of other minerals, probably magnetite and biotite, of which, however, no distinct traces now remain.

Besides forming the sheet under the Nubian sandstone, diabase occurs fairly abundantly in schists at various points, where it appears to form intrusions. A characteristic of these occurrences is its weathering into rusty looking brown “cannon balls.” Altered forms of diabase are associated with peridotites and other ultra basic rocks in the serpentine mass of Gebel Gerf (see [p. 328]). Diabase also forms veins, frequently magnetic, penetrating granites and schists, and at one point, near the foot of Gebel Awamtib, a dyke of diabase is even found penetrating the Nubian sandstone.

It is probable that these diabases are of very different ages at different points. While the occurrences associated with the sandstone are certainly of Cretaceous age, it seems difficult to imagine that those associated with the schists and serpentines are not vastly older; for though they are obviously younger than the schists, and possibly younger than the peridotites, they frequently show metamorphism to a degree which it is difficult to reconcile with a Cretaceous age, and which suggests that they antedated the folding which formed the ancient mountain chain on the flanks of which the Cretaceous strata were laid down.

Fig. 37.—Diabase, Gebel Abu Hamamid [10,400], × 40. a, augite; aa, augite, altering with separation of iron oxide; pl, plagioclase, often ophitically intergrown with the augite; s, green serpentinous mineral; m, magnetite.

As an example of a diabasic intrusion with “cannon ball” weathering, we may take the rock [10,400], which occurs in schists on the flanks of Gebel Abu Hamamid. When one of the rusty looking “cannon balls” is broken, the interior is seen to be a dark grey rock of very fine grain. The sp. gr. is 2·93. The microscopic slide shows the rock to be holocrystalline, consisting mainly of augite and plagioclase, ophitically intergrown, with some rather large grains and strings of a nearly isotropic clear yellow-green mineral, and scattered grains of iron oxides. The augite is of a pale brown colour; some of the crystals are fairly clear, while others show strong clouding and separation of iron oxide. The nature of the yellow-green mineral is somewhat uncertain; it polarises generally in very low colours, as an aggregate of fibres and tiny plates, sometimes showing a spherulitic structure between crossed nicols. Occasionally it includes granules of a highly refracting colourless mineral, possibly olivine; but there is a remarkable absence of the separated iron oxide which is usual with altering olivine, and the granules exhibit only very fine irregular hairlike cracks; if it is serpentine resulting from alteration of olivine, the olivine must have been a variety poor in iron.

Fig. 38.—Olivine-diabase, from a dyke at the junction of Wadis Huluz and Gemal [10,393], × 17. o, olivine; pl, plagioclase.

Turning now to the occurrence of diabase in dykes, a large dyke in the gneiss at the junction of Wadis Huluz and Gemal [10,393] consists of a very hard and heavy, strongly magnetic, greyish black rock of rather fine grain, in which a dark-brown platey mineral is mixed with grey and white matter. The sp. gr. is 2·95. Under the microscope the constituents are seen to be plagioclase, olivine, augite and magnetite, with small amounts of biotite and apatite. The plagioclase appears to have formed in two generations, for while the bulk of it is in small lath-shaped crystals (frequently with radial grouping), there is a very large porphyritic zoned crystal in the slide. The olivine is in large crystals, usually rounded, but occasionally tending to hexagonal outline with the usual strongly marked black irregular cracks. A little serpentinisation has gone on at the edges and along cracks of a few of the crystals, but, as a rule, the olivine is very fresh; it is never intergrown with felspars. The augite is slightly pleochroic, of a purple to brown tint, in irregular forms, partly in moderate sized crystals and partly in tiny grains in the ground mass. The crystals are much cracked. Ophitic structures are not conspicuous. Brown biotite is very sparingly present in small flakes. Magnetite is liberally scattered in small grains in the augite of the ground mass. Apatite occurs in minute prisms included in the felspars.

Fig. 39.—Diabase, from a dyke in Wadi Kreiga [12,110], × 17. pl, plagioclase felspar; a, augite; h, hornblende; l, limonite strings.

The diabase dykes which penetrate the granite in Wadi Kreiga [12,110] differ from the rock last described in their freedom from olivine and in showing marked ophitic structure. They are dense brown to black rocks of very fine grain, with porphyritic felspar crystals here and there. The sp. gr. is 2·98. Microscopic study shows them to be holocrystalline rocks, composed of an ophitic mixture of rod-shaped plagioclase with altering augite and hornblende. Both the ferro-magnesian minerals are very much clouded, and contain plentiful strings of iron oxide. The hornblende is dark green, often forming celephytic shells round the augite, and is probably largely an alteration from augite. There is very little of the nature of a ground mass, the augite and hornblende practically filling all the spaces between the felspars.

Mica-diabase.

Fig. 40.—Mica-diabase, Gebel Um Khariga [10,373] × 17. pl, plagioclase felspar; b, biotite, with separated opaque flakes of magnetite, often in geometric forms; a, clouded mineral, probably altered augite, with which the felspars are ophitically intergrown.

The rock which forms the top of Gebel Um Khariga [10,373] appears to be an altered mica-diabase. It is highly magnetic; the compass was found to point 20° out of its normal position at the station on the hill, while hand specimens broken off the rock showed strong polarity, some parts attracting and other parts repelling the needle; a fragment of the size of a pea deflected the compass needle several degrees when placed near it. It is a dark brown rock, of sp. gr. 2·83, very rotten, which in the mass looks like an altered dolerite. Microscopic study of a slide reveals the presence of altered plagioclase, in rather large lath-shaped crystals; biotite, largely altered to opaque iron oxide, the flakes of which show marked geometric forms; scattered grains of magnetite, and some secondary calcite. The brown clouded mineral polarises as a confused fine-grained and fibrous aggregate in low colours; it is probably altered augite, with which the plagioclases were ophitically intergrown, but is in too highly altered a state for certain identification. The strongly magnetic character of the rock would appear to indicate that the iron oxides produced by the alteration of the biotite are, like the primary grains, in the form of magnetite.

Basalt.

Basalt, the volcanic representative of the gabbros and diabases, is quite a scarce rock in South-Eastern Egypt, having been noted at only four or five points. Fairly fresh olivine-basalts, probably comparatively late intrusions, form the two conspicuous low hills near the coast called Gimeida and Einiwai; more altered rocks of basaltic type occur at the head of Wadi Um Deheisi (north of Gebel Kahfa), and in the hills on either side of the Wadi Huluz some ten kilometres north-west of Gebel Hamata; while an amygdaloidal rock which forms a large part of the hill-mass of Ti Keferiai has been classed as an altered hornblende-basalt.

Fig. 41.—Basalt, Gimeida Hill [12,156], × 40. Porphyritic crystals of plagioclase (pl) and olivine (o) in a hemicrystalline ground mass containing tiny crystals of plagioclase and granules of augite (a) and magnetite (m).

The basalt of Gimeida Hill [12,156], is a hard heavy block rock of sp. gr. 2·88, of dull aspect, with glassy white to colourless plagioclase crystals up to three millimetres diameter scattered through it, and here and there a dark diallagic crystal and some greenish glassy-looking grains of olivine. The microscopic slide shows the rock to be remarkably fresh; the porphyritic plagioclase and olivine crystals are seen to be embedded in a crystalline ground mass containing little lath-shaped plagioclases, with granules of pale brown augite, and abundant grains of magnetite.

Fig. 42.—Basalt, Einiwai Hill [12,144], × 17. o, olivine; a, augite; p, picotite, with a border of opaque chromite; gr, hemicrystalline ground mass.

The basalt of Einiwai [12,144] differs from that of Gimeida in the absence of porphyritic felspars. It occurs capping the red granite which forms the lower part of Einiwai Hill and the surrounding plain. It is a hard dull black rock with little glassy colourless to pale green crystals plentifully scattered through it (see [Plate XXIV]). The sp. gr. is 3·10. The microscopic slide shows porphyritic crystals of olivine, augite, and picotite, in a very fine-grained semi-glassy ground mass containing tiny laths of felspar and grains of augite and magnetite. The porphyritic crystals all show a remarkable absence of colour in the slide, which has been cut exceptionally thin on account of the darkness of the ground mass; this thinness of the slide doubtless accounts for the minerals all showing relatively low polarisation colours. The porphyritic crystals are mostly in six-sided and prismatic forms, but sometimes show as rounded grains; cleavage is usually indistinct, but irregular cracks are common, and some of the crystals are broken in two and the halves separated. Most of the six-sided crystals show straight extinction, and are probably a non-ferruginous olivine (forsterite); they show very little alteration, there being a general absence of serpentinisation or magnetite-separation along the cracks. The prismatic porphyritic crystals are in forms resembling those of augite, and some of the crystals show extinction sufficiently oblique to justify their identification as augite; others, however, showing straight extinction and very low polarisation colours, are probably enstatite. The picotite is in rather large isotropic rounded grains, of a pale green colour with a well marked opaque border of chromite.

A peculiar basaltic rock [10,408] occurs at the top of Wadi Um Deheisi, at the pass into Wadi Um Retba, on the road which passes from Bir Shadli to the north-east of Gebel Kahfa. It is a black rock of sp. gr. 2·98, and of such fine grain that practically nothing can be made out with a lens. It is remarkably magnetic, the compass being deflected by 13° from its normal direction at a plane-table station on the pass. The microscopic slide shows rounded and angular clear areas in a cryptocrystalline ground mass of greenish colour, full of specks of magnetite. The rounded clear areas are occupied by a very pale brownish mineral, probably augite, forming nests of crystals in different orientations, with fairly well-marked cleavages, high extinction-angle and fairly high double refraction. The angular clear areas are mostly colourless; they seem to consist chiefly of altered felspar, but some of them are formed of a fine mosaic of quartz granules. The ground mass is largely of chloritic nature, with small fibres of hornblende and some decomposed felspar. Its large content of rounded magnetite granules accounts for the magnetic character of the rock.

The basalt of the Wadi Huluz [10,410] is a close grained greenish-black rock containing white spots (amygdules) up to three millimetres diameter. Its sp. gr. is 2·93. The microscope shows the main bulk of the rock to be formed of lath-shaped plagioclase crystals, around and between which is green matter, now mostly chlorite. Here and there are small clear areas within the green patches, which show bright polarisation colours, and are probably augite, being the remains of the original mineral which has been largely chloritised. There is a fair amount of magnetite in scattered grains. The amygdules are partly filled with clear quartz, and partly with zeolites.

The hill mass of Ti Keferiai is largely made up of a fine-grained black rock [12,120], of sp. gr. 2·99, in which are greenish-white and pink amygdules up to two millimetres diameter. It is frequently much crushed, and in one place it has been broken into a coarse breccia and cemented with rose quartz [12,123]. The microscopic slide shows the main part of the rock to be a fine-grained mixture of pale hornblende with altered plagioclase, while the amygdules are filled with radiating zeolites. The amygdules seem to indicate that the rock is a volcanic one, and it has been classed as a basalt rather than as an andesite on account of its basic nature and high specific gravity. Its texture is microgranitic rather than basaltic, a circumstance perhaps in part due to the crushing it has undergone; the rock is passing into a schist.

Ultra-basic Igneous Rocks.

The ultra-basic igneous rocks (i.e., rocks practically free from felspar and composed entirely of ferro-magnesian silicates such as pyroxenes, amphiboles, and olivines), though forming but a small part of the earth’s crust in general, occur in very large proportion in the igneous masses of South-Eastern Egypt, where they cover several hundred square kilometres and form prominent mountain-masses such as those of Gebels Dahanib, Korabkansi, and Gerf. They may be classified into:—

(a) Pyroxenites (rocks composed essentially of pyroxenes);

(b) Amphibolites (rocks composed essentially of hornblende);

(c) Peridotites (rocks composed essentially of olivine, with or without pyroxenes and amphiboles);

All these ultra-basic rocks are easily altered to

(d) Serpentines, in which the original minerals may or may not be traceable.

A characteristic of the ultra-basic rocks here, as in other parts of the world, is their gradual transition into one another, showing that the various forms have arisen from consolidation of parts of one and the same magma owing to slight differences in composition or in the physical conditions under which consolidation has taken place. A further noteworthy circumstance is their gradual passage into basic rocks; there is no hard-and-fast line to be drawn, for instance, between basic diorites and amphibolites, nor between basic gabbros and pyroxenites, nor between olivine gabbros, poor in felspar and peridotites, these various classes being found to pass by insensible gradations one into another as they are followed up in the field. Moreover, being typically coarse-grained rocks, and pyroxenes and amphiboles being often indistinguishable in the hand specimen, great caution has to be exercised in naming a rock mass from a few microscopic slides which of necessity each embrace at most but a few square centimetres of section.

In the field, the appearance and cohesive strength of the ultra-basic rocks varies primarily with the extent to which they have been altered towards their final stage of serpentinisation. Where they are least altered, they form black masses of hard heavy crystalline rock of such toughness that they are only broken with difficulty with a sledge hammer; while in the cases where serpentinisation has proceeded to the greatest extent, they frequently form foxy red or even pink-looking hills which might almost be taken for granite from a distance, and they are so shattered that the rock comes off literally in tons at a mere touch; in these cases, long search is necessary to find a coherent piece large enough for a museum specimen. The brown or pink colour just referred to is of course only superficial, but in the untrodden and rainless wilderness surface films remain unbroken and give characteristic colours to the scenery. Freshly fractured surfaces are always dark green, dark brown, or black, with more or less crystal structure visible according as the rock is less or more altered; pyroxenites or amphibolites, when but little altered, are a mass of lustrous platey or fibrous dark crystals, while serpentines are typically of dull aspect. The specific gravity is high, ranging from as much as 3·1 in the less altered forms down to about 2·6 in those which are more completely serpentinised.

The process of serpentinisation is of course a chemical change, consisting largely in the combination of water with ferro-magnesian silicates free from alumina; but it is remarkable how frequently this chemical change has been accompanied by a parallel physical deformation. Serpentines are almost always shattered rocks, full of slickensided surfaces; when we compare the low sp. gr. of serpentine (2·6) with that of augite, hornblende, or olivine (about 3·2), we naturally conclude that the shattering of the rock is in all probability due to the expansion on hydration causing internal stress, and the slickensiding is due to the rock yielding along certain surfaces. The cracking of felspars and the forcing of serpentine into them, which are frequently seen in thin sections of olivine rocks, such as the troctolite shown in [Fig. 34] on p. 304, shows on a small scale the physical effect of expansion on serpentinisation, and should lead us to expect a corresponding effect in rock masses. It is thus not necessary to infer great tectonic movements to explain the shattering of the rock, and in fact the disposition of the serpentines in broad mountain tracts like Gebel Gerf is opposed to the idea of there being here any local accentuation of folding or crushing by general crust-crumpling. I have calculated that a horizontal sheet of pyroxenite of sp. gr. 3·1, ten kilometres wide, confined between fixed abutments and prevented from increasing its thickness, would rise into an arch having a height of about two and a quarter kilometres at its centre if converted into serpentine of sp. gr. 2·6; this is, of course, not given as a precise example of what may actually have taken place, but it will serve to show that expansion on hydration may produce dynamical effects not inferior to those of contraction of the earth’s crust, such as are believed to be the main cause of mountain formation, and to explain why we may find serpentines shattered to fragments and full of slickensided surfaces in areas where the surrounding rocks show comparatively little evidence of dynamo-metamorphism.

Pyroxenites.

Heavy dark green rocks, of medium to coarse grain, and consisting almost entirely of a schillerized-looking mineral, form the hill-masses of Gebels Um Ein and Qrein Salama, and similar rocks are found in connexion with gabbro at Gebel Um Gunud and elsewhere. From the diallagic appearance of these rocks in the hand specimen, they were classed in the field as pyroxenites or diallage rocks. But an examination of the slides cut from the specimens reveals the main constituent to be hornblende, and the rocks must therefore be placed in the division of amphibolites. It is, however, highly probable that these rocks were originally pyroxenites, the hornblende having originated mainly from the alteration of augite; the change from augite to hornblende in the gabbros has already ([p. 302]) been remarked, and in these ultra-basic forms the same process appears to have gone on.

Some parts of the great ultra-basic mass of Gebel Gerf consist of bronzite-rock. Specimens from the least altered portions consist almost entirely of bronzite, sometimes with a little olivine; the rocks are, however, generally found passing into serpentine, such as forms the main portion of the mountain, and on microscopic examination even the freshest portions always exhibit more or less serpentinisation. It will accordingly be preferable to treat of these rocks under the heading of serpentines.

Amphibolites.

Rocks consisting almost entirely of hornblende occur in considerable masses in the neighbourhood of Gebel Um Gunud, and form the conspicuous hills Gebel Um Ein and Qrein Salama; they also occur in a small patch about three kilometres east of Erf el Fahid. In the field, especially with the coarser-grained varieties, there is often considerable difficulty in identifying the main constituent of the rock as hornblende, owing to the schillerized appearance of the mineral being more suggestive of diallage; the mass east of Erf el Fahid, for instance, was classed in the field as a basic gabbro or diallage-rock, instead of a very basic diorite or hornblende-rock, such as the microscopic examination proves it to be. These rocks are always very heavy and extremely tough; they weather commonly into great rusty-looking rounded blocks, and a sledge hammer is required to get a specimen.

Fig. 43.—Amphibolite, from hills near Gebel Um Gunud [11,511], × 30. Almost the whole of the figure is occupied by hornblende, which is seen clouded and altering to granular epidote at e. Part of a large grain of calcite, bordered and streaked with iron oxide, is seen at c, while q is a small crystal of interstitial quartz. The slide also contains a small proportion of plagioclase felspar, not shown in the figure.

The amphibolite of the hills about Gebel Um Gunud [11,511] is a very hard and heavy coarse grained greenish-black rock composed almost entirely of shining platey-looking crystals of hornblende, which often reach two centimetres in length and breadth. The sp. gr. is 3·08. The microscopic slide shows the rock to be granitic in structure and to contain, in addition to hornblende, small amounts of plagioclase, quartz, calcite, and iron oxides. The irregular-shaped hornblende crystals show strong pleochroism, from pale olive-brown to moderately deep green. The extinction angles are large, being frequently over 20°. Twinning of the crystals is fairly common. Many of the crystals are clouded, and the alteration of the mineral has resulted in the formation of much epidote. The plagioclase is only present in very small quantity, strongly clouded by decomposition, but still showing twinning clearly. The quartz is clear, and is likewise present in very small proportion, mostly interstitial. Calcite is fairly plentiful in large rounded and irregular grains, rather turbid-looking, of a brownish or greenish tinge, outlined and streaked with strong lines of iron oxide, in a way which at times suggests that the calcite may possibly be the result of alteration of a lime olivine.

A specimen of amphibolite [11,528] obtained from a hill near the junction of Wadi Abu Marwa with Wadi Naait, about seven kilometres north-east of Gebel Um Gunud, is a dark green rock of rather fine grain, made up of shining plates and small fibrous crystals of hornblende and chlorite. Its sp. gr. is 2·97. Under the microscope the hornblende is in rather ragged-looking irregular crystals of various sizes, all interlocked with each other, rarely showing any approximation to idiomorphism except in the smaller crystals, which are commonly limited by prismatic faces; these smaller crystals are frequently included in the larger ones. The pleochroism is fairly strong, a pale olive brown, c moderately deep green. Prismatic cleavage is well marked. Extinction angles are often large, ranging up to 24°. A few of the crystals are slightly bent, and many show more or less clouding owing to partial alteration to chlorite, but on the whole the rock is fairly fresh. Except for a few specks of magnetite, and the alteration product chlorite, the rock contains nothing but hornblende.

The hornblende-rock or amphibolite of Gebel Um Ein [12,130] is of medium grain, almost entirely made up of shining crystals of hornblende with a little chloritic matter. The sp. gr. is 3·03. The microscopic slide shows large individuals of hornblende inclining to idiomorphism, set in a sort of ground mass of smaller crystals. The hornblende shows the same strong pleochroism and high extinction angles as in the two rocks just described; it is frequently clouded and speckled with iron oxides, and often full of lighter-coloured patches which extinguish with the rest of the crystal; in many cases these lighter patches contain a central granule of iron oxide, and they are doubtless due to a bleaching by segregation of the iron. Between crossed nicols many of the crystals are full of small fibres extinguishing differently from the rest of the crystal; some of these fibres polarise in low colours and are probably chlorite or serpentine, while others show brilliant tints and are possibly actinolite.

The amphibolite which occurs three kilometres east of Gebel Erf el Fahid [10,361] is of extremely coarse grain, looking like a very basic gabbro owing to the schillerized appearance of its large hornblende crystals and the presence of a little interstitial felspar. The sp. gr. of the rock is 2·98. The section shows the hornblende to be of a very pale green colour, with an almost entire absence of iron oxide grains and other alteration products. Between crossed nicols it has a fibrous woody appearance. The interstitial plagioclase (probably labradorite) is likewise very fresh, showing its repeated twinning very clearly; the crystals are full of tiny fibres of hornblende, and are traversed by broad cracks filled with a mosaic of smaller crystals of plagioclase and quartz.

Fig. 44.—Amphibolite of Qrein Salama [12,157], × 17. h, hornblende, strongly striated, altering to chlorite; o, olivine, and b, bronzite, both passing into serpentine.

The rock [12,157], of the hill called Qrein Salama, to the east of Gebel Gerf, is interesting as containing olivine and bronzite in addition to the more abundant hornblende, and thus forming a link between the amphibolites proper and the peridotites; but as about three-quarters of the rock is hornblende it is still classed as an amphibolite. The sp. gr. is 3·05. In the slide, the hornblende is seen in irregular crystals, colourless to very pale green, with a fibrous structure which is strongly marked by patches of shading of extremely fine black prismatic striations. With crossed nicols the fibrous structure is still more apparent, the mineral polarising as brilliant fibres separated by chloritic alteration products. The pronounced striation and faint colour of the crystals are more suggestive of diallage than of hornblende, but the extinction angles measured in the slide are all less than 22°. Many of the hornblende-crystals contain large numbers of small rounded grains and strings of iron oxide. The olivine is mostly in rounded crystals, frequently included in the hornblende; it is largely serpentinised and full of small grains of opaque iron oxides, but kernels of the original mineral remain. The bronzite, which is present in about equal proportion with the olivine, is likewise extensively serpentinised and full of iron oxide grains; it is distinguishable from the olivine by a more fibrous appearance (the serpentinisation having gone on mainly along the direction of the vertical axis instead of along irregular cracks), and by its generally lower polarisation colours in the unaltered portions.

Peridotites.

The peridotites, or felspar-free rocks consisting largely of olivine, are usually classified into:—

(a) Dunites, consisting entirely of olivine.

(b) Harzburgites, consisting of olivine and enstatite or bronzite.

(c) Wehrlites, containing olivine and diallage.

(d) Lherzolites, containing olivine, diallage, enstatite or bronzite, and picotite or chromite.

(e) Hornblende-picrites, containing olivine and hornblende.

(f) Mica-peridotites, containing olivine and biotite.

Most if not all of these classes are represented in South-Eastern Egypt, but in the altered form of serpentine. The change to serpentine has been so complete that it is now hardly possible to extract even a small specimen of the primitive rock; indeed, careful search is often required to obtain even specimens containing any unaltered mineral whatever, and one has frequently to rely on the structure of the serpentine for the identification of the rocks from which it originated.

Serpentine.

Serpentines cover about 400 square kilometres of South-Eastern Egypt, forming the principal rock of several remarkable mountain groups and also occurring in lower hill country.

The largest occurrence is that of the great mountain mass of Gebel Gerf, where serpentine with alternations of gabbroid and dioritic rocks can be followed from Bir Meneiga southward for some thirty kilometres to beyond the Sherefa pass, while the breadth of the tract from east to west is some fifteen kilometres or more at its widest part; this tract includes a vast assemblage of high peaks and ridges, towering up in many cases to more than a kilometre above sea-level. To the north, south, and east, the serpentine is bounded by gabbros and diorites, while on the west a tract of schistose rocks separates it from another great serpentine mass which forms Gebel Korabkansi.

Other remarkable serpentine masses are Gebel Abu Dahr and the upper part of Gebel Sikait; serpentines also enter into the composition of Gebels Ghadir and Um Tenedba, while lower hills formed of similar rocks occur near Bir Murra in Wadi Shait, in Wadi Um Khariga, near Gebel Kalalat, and on the plain east of Abraq Springs.

The foxy red colour of exposed faces of serpentine hills, and the generally shattered nature of the rock, have already been referred to. When one approaches the masses closely, the red colour often becomes less marked, because the fresh debris flanking the hills is of a darker aspect. The slopes of serpentine hills are usually steep, and this combined with the rotten nature of the rock renders their climbing not always quite free from danger.

Any attempt to map out in the field the precise limits of the different peridotites which were the parents of the serpentine ends in failure, partly because the constituent minerals can generally only be identified on microscopic examination, and partly because the different peridotites pass gradually one into another, and are evidently only produced by slight variations in the composition or conditions of consolidation of a single magma. Even the limits of the serpentine itself are not always very clear; for where the associated rock is gabbro, as for instance to the north of Gebel Gerf, there is a gradual passage through more or less serpentinised olivine gabbro to the true serpentine. Golden yellow veinlets of fibrous chrysotile can be seen running through the rock at many places, and occasionally veins and pockets of magnesite and an inferior kind of asbestos occur (see [p. 330]).

The serpentines are nearly always more or less magnetic, and sometimes show strong polarity. The compass was disturbed by 40° at the triangulation station on Gebel Sikait; the amount of disturbance changes greatly when one moves even a short distance, and all estimations of direction from compass readings in serpentine country are therefore liable to enormous error.

In a strictly petrographical sense it would be most systematic to describe the serpentines in classes according to the nature of the parent rock; grouping together, for instance, all those derived from pyroxenites into one class, those from amphibolites into another, those from dunites into a third, and so on. But besides the fact that the nature of the parent rock cannot always be determined with certainty, such a course would possess the objection of separating the different parts of one and the same mass; at Gebel Gerf, for example, we have serpentines derived from rocks of various of these classes, though probably all formed part of the same igneous intrusion and are thus genetically connected. In the descriptions which follow, therefore, the serpentines of each locality will be grouped together, irrespective of origin. The localities are taken in order of latitude from north to south.

Fig. 45.—Serpentine, probably derived from a mica-peridotite, east of Erf el Fahid [10,360], × 40. cm, chlorite and magnetite from alteration of biotite; s, clear serpentine, probably altered olivine.

A serpentine which occurs associated with amphibolite and various schists about three kilometres east of Gebel Erf el Fahid [10,360] may possibly have been derived from a mica-peridotite. In the hand specimen it is a dark brown rock of dull aspect with greenish patches, and shows marked magnetic polarity. The sp. gr. is 2·63. The microscopic slide shows mainly colourless serpentine, but there are strings and bundles of pale greenish-yellow chlorite, and specks and strings of magnetite. The arrangement of the magnetite (see [Fig. 45]) at once attracts attention, being frequently aggregated in parallel strings reminding one of the cleavage of mica. Between crossed nicols the chlorite is easily distinguished from the serpentine by its slightly higher polarisation-colours (low yellows as compared with greys); and its arrangement in long leaves and bundles, with distinct traces of the original mica, leaves no doubt as to its derivation from biotite. The parallel strings of magnetite are also most likely due to alteration of biotite, for it is difficult to account for their peculiar arrangement otherwise; they are mostly separated by material of lower double refraction than the chlorite above-mentioned, perhaps a variety of chlorite poorer in iron, owing to the previous separation of the oxide. The clear serpentine which forms the bulk of the slide does not contain much iron oxide, and does not include any trace of the original mineral from which it has been formed; but between crossed nicols it shows an irregular mesh structure which makes one almost certain of its derivation from olivine.

Fig. 46.—Serpentine, probably derived from lherzolite, Wadi Um Khariga [10,368], × 17. so, serpentine derived from olivine; sb and sd serpentine probably derived from bronzite and diallage; c, calcite; p, picotite; m, magnetite.

The serpentine of the hills on the west side of the Wadi Um Khariga [10,368] is remarkable in the field by its foxy red colour on all exposed surfaces. The rock, which has a sp. gr. of 2·63, is nearly black on fractured surfaces, and no crystals can be detected in it. The slide shows colourless to yellow serpentine, with a fair amount of calcite often arranged along cracks, a liberal sprinkling of magnetite, and one or two fairly large grains of picotite, or chromite. In some parts of the slide the serpentine shows an irregular mesh structure between crossed nicols; these portions, in which the magnetite-granules show an irregular honeycomb-like arrangement, are doubtless altered olivine. In other places the serpentine polarises in clear greys with a fibrous aspect, and the magnetite is arranged in parallel lines; calcite is typically developed in these fibrous portions, sometimes along the cleavages and sometimes in irregular patches. The fibrous-looking serpentine probably represents the alteration-products of both rhombic and monoclinic pyroxenes, and the parent rock was thus a medium-grained lherzolite, containing olivine, diallage, bronzite and picotite.

In the hills on the east side of the Wadi Um Khariga, about in latitude 24° 55′, there is some serpentine which has apparently resulted from the alteration of very basic dykes. The sp. gr. is 2·65. A slide cut from this rock [10,367] shows the same clear fibrous-looking patches as the specimen last described, and picotite, in a confused and nearly isotropic mass of fine fibres of serpentine, with but little magnetite and no calcite. No original mineral remains except the picotite. The clearer fibrous patches may represent diallage or bronzite, while the rest of the serpentine is somewhat doubtfully referred to olivine. There is some trace of banding in the rock, perhaps due to movement during consolidation.

A serpentine approaching an ophicalcite [10,376] forms the main rock at Gebel Ghadir, where it is associated with a peculiar quartz felsite resembling granulite. In the mass the rock, which has a sp. gr. of 2·67, is black to green, veined with calcite; surfaces of the debris are often covered with a brilliant green glaze. The microscopic slide reveals an irregular mixture of nearly colourless serpentine and cloudy-looking calcite, with abundant specks and a few larger granules of slightly translucent deep-brown chromite or picotite. Between crossed nicols, the serpentine is a mass of fibres and little plates, in which a strong tendency to linear arrangement can be seen, and here and there a lattice-structure. Though none of the original mineral remains there is not much doubt that the rock is an altered amphibolite or basic diorite; the calcite is possibly derived from the alteration of an original lime felspar, but it has been largely redistributed in the crushed rock by solution and redeposition.

The serpentine forming the hills on the north side of the mouth of Wadi Kalalat is a dark reddish-brown rock, of sp. gr. 2·76, in which shining crystalline specks of olivine can be seen. It has doubtless resulted from the alteration of a dunite, or rock consisting almost entirely of olivine. The slide [11,510] shows the rock still to contain abundant clear colourless kernels of the original olivine, in a mesh-work of pale olive-brown serpentine. The serpentinisation has taken place along irregular cracks in the olivine, each crack being generally marked by a thin streak of opaque iron oxide running longitudinally down its centre, with serpentine fibres running crosswise and filling the rest of the crack. Where the kernels first left have themselves become changed to serpentine, they are nearly isotropic, while the cracks polarise in clear greys, so that even where the whole of a crystal has been serpentinised the structure is still clear, both in ordinary light by the magnetite strings, and in polarised light by the way in which the serpentine of the cracks stands out from the more isotropic patches within the meshes. There is a little accessory diallage, easily recognisable by its fibrous appearance and the oblique extinction of its unaltered portions. The diallage, like the olivine, is passing into serpentine, but here the serpentine goes on along cleavage planes as well as along irregular cross cracks, giving the partially altered crystals a striped appearance between crossed nicols. Where the diallage has become entirely serpentinised, it can still be differentiated by its clearer appearance from the olivine-serpentine in ordinary light, and by its striated structure between crossed nicols; but of course unless kernels of the original mineral are left one cannot be sure whether it was diallage or bronzite. There are a few patches of calcite or magnesite, and veinlets of the same secondary minerals; these may have arisen from the alteration of a little original felspar, or from the diallage.

Near the head of Wadi Arais, to the south of Gebel Um Bisilla, there are some dykes of dark magnetic rock of sp. gr. 2·56. A specimen from one of these dykes [11,519], turns out on microscopic examination to be a schistose serpentine. It is traversed by parallel strings of opaque and brown translucent iron oxides, and granules of magnetite are also scattered over the interspaces. The interspaces between the iron oxide strings are filled with serpentine showing no recognisable structures, but containing here and there little nests of clouded green hornblende, suggesting the possibility of the rock being an altered basic diorite or amphibolite.

The highly crushed and rotten serpentine which forms the high and steep-sided mountain mass of Gebel Abu Dahr, differs considerably in composition in different parts. The most typical form of the rock, which has a sp. gr. of 2·77, is a dark brown serpentine with little strings of olive-green matter, and occasionally large dulled black crystals. A slide [11,516 A], cut from this portion of the rock shows mostly olivine, altering to serpentine in the usual manner with abundant clear kernels of the original mineral. Associated with the olivine is another mineral of a somewhat fibrous aspect, altered partly to serpentine and partly to another substance which polarises as a confused aggregate of fibres and flakes in brilliant colours, the flakes extinguishing with slight obliquity; the serpentine is formed mainly along the vertical cleavages of the mineral (though it also fills transverse irregular cracks), while the other substance (tremolite?) fills up interspaces. In some parts of the mass the rock has a blacker colour and a slightly higher density (2·87), somewhat resembling a basalt with large greenish fibrous-looking crystals scattered sparsely through it. The slide cut from this form of the rock [11,516 B] shows no olivine whatever, practically the whole slide being composed of the fibrous mineral just described, together with a little nearly colourless fibrous hornblende, in which the change above-mentioned appears to be going on. Thus it would seem likely that the rock of Gebel Abu Dahr is essentially an altered hornblende-picrite (or olivine-hornblende rock) with variations towards dunite (olivine-rock) on the one hand, and towards an amphibolite or hornblende-rock on the other. The alteration of hornblende here is not a purely serpentinous one, but results in the formation first of serpentine along cracks and cleavage planes, and then of a tremolitic mineral in the interspaces. It is worth remark, moreover, that in the slide free from olivine the change to tremolite preponderates, and there is much less serpentine in the altered hornblende than in the slide containing olivine; this suggests that the presence of the neighbouring olivine has in some way brought about more serpentine in the hornblende, perhaps by actual forcing of serpentine from the expanding olivine into cracks in the hornblende, or by the influence of pressure set up by the same expansion.

In the serpentine from some low hills on the plain a little to the east of Bir Abraq [11,506] we have a rock evidently derived from a dunite, though not a trace of unaltered olivine remains. The sp. gr. is 2·61. The microscopic slide consists of nearly colourless serpentine with strings of magnetite marking the cracks along which its formation began (see [Fig. 47]). Between crossed nicols (see [Fig. 48]) the main portion of the serpentine, doubtless derived from olivine, presents a very remarkable appearance; most of the polygonal spaces between the magnetite-meshwork are lined with plates of clear serpentine, while the centre is occupied by nearly isotropic material. Thus the slide in polarised light presents somewhat the aspect of an aggregate of cells with dark nuclei. Besides olivine, the slide shows a small amount of a fibrous mineral, probably bronzite, which is likewise almost entirely altered to serpentine, though a few original fibres remain; the magnetite grains in the serpentine derived from this mineral is typically aggregated in strings parallel to the fibres.

Fig. 47.—Serpentine derived from an almost pure olivine-rock, from a low hill east of Bir Abraq [11,506], × 17. The outlines of the original olivine crystals (o) and the cracks in them are marked by granules and strings of magnetite. A serpentinised crystal of bronzite is seen at b.

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

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

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

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

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

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

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

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

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

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

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

Alteration Products of Serpentine. Magnesite and Asbestos.

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

[125]From Marwa, the Arabic term for quartz.

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

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

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

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

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

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

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

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

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CHAPTER X.

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METAMORPHIC ROCKS.

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

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

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

Gneisses.

Granite-gneiss (crushed granite).

Diorite-gneiss (crushed diorite).

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

Schists.

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

Clay-schists (metamorphosed clays).

Hornblende-schists and crushed volcanic rocks.

Hornfels (metamorphosed acid lavas).

Mica-schists (sometimes containing beryl and tourmaline).

Chlorite-schists.

Talc-schists.

Calcareous schists and marble.

Graphite-schists.

Breccias.

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

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

Mineral Veins.

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

Calcite veins (with more or less chalybite).

Magnesite veins (in serpentine).

Asbestos veins (in serpentine).