Chapter IX (2)

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.

, × 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.

, × 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.=

, × 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.

, × 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.=

, × 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.=

, × 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.

, 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.

, × 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.

, × 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.

, × 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.

=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.]

, × 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.

, × 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.

, × 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.=

, × 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.=

, × 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.

, × 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.

, × 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.

, × 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.

, × 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.=

× 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.

, × 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.

, × 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.