CHAPTER IX.. * * * * *

* * * * * =IGNEOUS ROCKS.= * * * * *

Igneous rocks cover about one-third of the entire area of South-Eastern Egypt, forming irregularly distributed tracts, alternating with others occupied by metamorphic rocks, entering largely into the composition of the principal mountain masses and also underlying a large portion of the areas covered by sand on the coast-plain and elsewhere.

Regarded broadly, the igneous rocks of this portion of the Eastern Desert form two main divisions, namely, an acid division typified by granite rich in felspar, and a basic division typified by gabbro. Intermediate types occur, as well as ultra-acid and ultra-basic rocks, but these are less abundant, and are generally found in close association with one or other of the two main types. Rocks of the acid type predominate to the north of latitude 24°, while further south basic rocks form large portions of the main mountain tracts, with acid rocks at intervals on either side down to the Sudan frontier. Most of the igneous rocks are plutonic. There is a striking paucity of volcanic rocks, a circumstance doubtless to be ascribed largely to the enormous denudation which has taken place in the district. The plutonic masses, with the dykes seaming them, have been elevated and laid bare, while most of the volcanic outpourings have been denuded away and others have been altered or devitrified till their original volcanic nature has become less apparent. Metamorphism has also affected many of the plutonic masses to such a degree that it is often difficult to decide whether the rocks should now be placed in the igneous group, or classed as metamorphic rocks.

For purposes of classification, the igneous rocks of South-Eastern Egypt may be divided into five main groups, based on silica-percentage:—

1. Ultra-acid rocks, containing over 80 per cent of silica.

2. Acid rocks, containing from 65 to 80 per cent of silica.

3. Intermediate rocks:—

(a) Sub-acid rocks, with from 60 to 65 per cent of silica.

(b) Sub-basic rocks, with from 55 to 60 per cent of silica.

4. Basic rocks, containing from 45 to 55 per cent of silica.

5. Ultra-basic rocks, with less than 45 per cent of silica.

The classification is based on silica percentage, for although up to the present it has not been found possible to undertake the chemical analysis of the rocks, the microscopic examination of thin sections itself gives a very fair guide to the chemical composition, and the specific gravities of the different rocks, which increase progressively from the acid to basic groups, afford a further guide in this direction. The table on the following page shows the different rocks comprised within the five main classes.

The five classes are not all of equal importance. As already mentioned, the acid and basic types are predominant. The ultra-acid and sub-acid rocks are present in relatively small quantity, and are in close relationship with those of the acid series which occur in great abundance; the rocks of the sub-basic and ultra-basic groups, on the other hand, though by no means insignificant in their distribution (diorites and serpentines cover very large areas), appear to be closely linked in the field with those of the basic group. The district is so large, and the investigation of it has been so limited, that it would be unwise to draw from the above remarks the deduction that the rocks have originated from two main magmas; the statements are merely those of the facts of observation, so far as observation has proceeded.

The scheme tabulated above differs from established classifications only in the inclusion of an ultra-acid class of rocks. This inclusion is necessitated by the presence in the district of huge masses of quartz-rocks which are almost certainly igneous in origin. These rocks are almost pure silica, and though they are properly regarded as an extreme form of pegmatite, their silica-percentage (over 95) is too high for them to be placed with their near relations the granites; and on grounds of symmetry of classification it appears justifiable to have an “ultra-acid” class corresponding with that of ultra-basic rocks.

CLASSIFICATION OF IGNEOUS ROCKS.

--------+------------+-----------------------+-----------+-------------- | | Intermediate. | | Ultra- | Acid. | | Basic. | Ultra-basic. acid. | +----------+------------+ | | | Sub-acid.| Sub-basic. | | --------+------------+----------+------------+-----------+-------------- Quartz-|Granite |Syenite |Diorite |Gabbro |Pyroxenite rock | | | | | | | | | | | Normal pink| | Augite- | Troctolite|Amphibolite | granite | | diorite | | | | | | | | Red | | Mica- | Pyroxene- |Peridotite | pegmatitic | | diorite | granulite | | granite | | | | | | | | | | Aplite | | | |(Serpentine) | | | | | | Biotite- | | | | | granite | | | | | | | | | | Muscovite- | | | | | granite | | | | | | | | | | Hornblende-| | | | | granite | | | | | | | | | | | | | | |Granite- |Syenite- |Diorite- |Diabase | |porphyry |porphyry |porphyrite | | | | | | | | | | Augite- | Mica- | | | | porphyrite | diabase | | | | | | | | | Kersantite | | | | | | | | | | | | |Quartz- |Trachyte|Andesite |Basalt | |felsite | | | | --------+------------+----------+------------+-----------+--------------

ULTRA-ACID ROCKS.

=Quartz-rock= of igneous origin occurs in the form of intrusive bosses and veins penetrating older igneous and metamorphic rocks at Marwot Elemikan and elsewhere. The rock is practically pure vitreous quartz. Its dazzling white aspect in the sun renders it very conspicuous in the field. The shape of many of the masses (bosses), their close association with pegmatite and aplite in at least one locality, the absence from the veins of other minerals (calcite and various ores) such as usually accompany vein-quartz deposited from solution, and the occasional presence in the rock of mica flakes, all point to an igneous origin, and justify us in regarding this quartz-rock as the final product of differentiation of a granitic magma, representing its ultra-acid portion.

The most conspicuous occurrence of quartz-rock is in the three nearly conical hills called Marwot Elemikan, which rise from the granitic plain at the head of Wadi Elemikan in latitude 23° 53′, and which from their snow-white colour form landmarks visible from great distances. The largest and most northerly hill of the three is a boss about 300 metres in diameter, rising sixty-eight metres above the plain, which itself has here an altitude of 580 metres above sea. The other two hills are somewhat smaller; they lie respectively two kilometres south-east, and two and half kilometres south, of the largest one; a black hill of about equal size rises from the plain mid-way between them. The highest of the three white hills, which was occupied as a triangulation station, is composed almost entirely of glassy quartz, the only other mineral seen in it being a white mica, of which a few flakes were visible in the outer portions. The mass is cracked in all directions, and the faces of the separated blocks frequently show a slight yellowish staining.

Another remarkable boss of quartz, similar to that just described and likewise rising through granite, occurs on the south side of the Wadi Khoda (latitude 23° 42′) about seven kilometres above the point where the wadi emerges from the hills on to the coast-plain. A third occurrence, of much less size, in the floor of Wadi Abu Marwa, on the track from Wadi Lahami to Berenice via the Wadi Naait; the first-mentioned wadi derives its name from the quartz occurrence in it.

Of igneous quartz veins, the best example is the huge dyke which forms the back-bone of the ridge called Erf el Fahid, in latitude 25° 0′. Here the vein, which strikes east and west, is in schistose country. It is at least ten metres wide, rising to a height of ninety-five metres above the wadi, and can be traced for a length of nearly two kilometres. Like the rock of Marwot Elemikan, the quartz of Erf el Fahid shows faint iron-staining in places, but appears to contain no other minerals. It is weathered to a sintery appearance in places [10,362], and contains some cavities, from one of which I extracted a handful of loose quartz crystals [10,363] with pyramids developed at both ends; these last are evidently deposited from solution, but it is difficult to assign other than an igneous source for the vein, and the cavities and loose crystals are probably the product of solution subsequent to the igneous intrusion.

Going south-west from Erf el Fahid, down the Wadi Muelih, is another line of ridges, of which the back-bone is a great quartz vein traceable for some two and a half kilometres along its strike. Further down the wadi are networks of quartz veins seaming diorite in all directions on either side of a horse-shoe-shaped hill called Marwot Rod el Ligaia; this hill is of aplite, and probably represents a less acid part of the same magma which formed the quartz veins.

It is significant that there are no traces of mine workings at any of the places where quartz veins and bosses of the igneous type occur, notwithstanding the fact that the masses are so conspicuous as to have surely attracted the eye of every gold-seeking prospector. Though the loose crystals found at Erf el Fahid show that there at least aqueous solutions have acted on the rock to a slight extent, these great igneous quartz masses appear never to have been impregnated with gold or other ores.

ACID IGNEOUS ROCKS.

=Granites.=

Granites are the most abundant and most widely distributed of the igneous rocks of South-Eastern Egypt. They form a large proportion of the most prominent mountain masses, such as Gebels Hamrat Wogud, Nugrus, Hamrat Mukbud, Hamata, Faraid, Um Reit, Mishbih, Adar Qaqa, Adar Aweib, and Elba. They also occur in some great plain tracts, such as those round Gebel Selaia and to the west of Gebel Um Reit, where low hills of the rock rise through the coarse granitic sand which covers most of the plain.

In point of geological age, the granites appear to be the youngest of the plutonic rocks of the country, forming great intrusions in the more ancient schists and diorites.

Though sometimes occurring as rounded bosses, as at Gebels Muelih, Selaia, and Faraid, granite typically forms rather jagged mountains, more especially in the south parts of the area; the mountains of Mishbih and Elba, for instance, abound in spiky peaks, while Qash Amir, the “Scragged Hill” of the Admiralty Chart, is an extreme example of this mode of weathering. The spikes are often made up of more or less rounded blocks piled one on another, the separation and rounding of the blocks being brought about by jointing and the more rapid weathering of the corners of the separated masses. Granite mountains, though sometimes white, are usually of a pink or red colour and are often named accordingly by the Arabs. In Ababda country, all mountains named Hamrat, as for instance Hamrat Wogud and Hamrat Mukbud, are composed of red granite or granitoid gneiss; while in Bisharin country the corresponding name Adar, as used for example in Adar Qaqa and Adar Aweib, has the same significance, both the names meaning red. The beds of wadis draining from granitic mountains, and plains adjacent to them, are invariably covered with a thick accumulation of coarse felspathic and quartzose sand derived from the weathering of the rock. This sand is quite firm to walk on, and in this respect forms a pleasing contrast to the finer wind-borne sand derived from the disintegration of sandstone. The granitic sand being generally white in colour (the redness of the felspars having largely disappeared in the process of weathering), the wadis in whose floors it is largely displayed are frequently named “Wadi el Abiad” (abiad = white); there are numerous wadis called by this name, all possessing the common characteristic of draining from granitic mountains and consequently having a floor of granitic sand.

Besides the great mountain-forming masses, granites also occur frequently in the form of dykes or veins, penetrating the gneisses, schists, and other rocks.

Viewed as a whole, the granites of South-Eastern Egypt are characterised by their strongly acid composition. Though perfectly normal granites are found in many places, and a gradual passage into quartz-syenite may be occasionally traced, yet on the whole the rocks approximate to the aplitic and pegmatitic types in which quartz and felspar are associated with very small quantities of ferro-magnesian minerals.

We may consider the granites as falling mainly into the following classes:—

1. Normal pink granite.

2. Red pegmatitic granite.

3. Aplite.

4. Biotite-granite.

5. Muscovite-granite.

6. Hornblende-granite.

=PLATE XXII.=

=ACID IGNEOUS ROCKS.=

=PORPHYRITIC GRANITE.= Gebel Fereyid.

=PEGMATITIC RED GRANITE.= Wadi Gemal.

=BIOTITE-GRANITE.= Gebel Abu Hegilig.

=HORNBLENDE-GRANITE.= Gebel Elba.

=QUARTZ-FELSITE.= Near Gebel Kolaiqo.

=QUARTZ-FELSITE.= Near Gebel Kolaiqo.

NATURAL SIZE.]

=Normal pink granite= is the form of granite most commonly met with in South-Eastern Egypt. It forms the principal rock of many of the mountains, such as Gebels Nugrus, Selaia, Faraid, Um Reit, Shigigat, Niqrub, Abu Brush, and Hamra Dom, and also covers large expanses of low hill country, and occurs as knolls scattered over sandy plains such as that of Selaia and the tract west of Um Reit. The rock [11,505], a full size representation of which is shown on Plate XXII, is composed of pink orthoclase, white oligoclase, quartz, and biotite, with hornblende, sphene, apatite, and magnetite as accessory minerals. The pink orthoclase is the dominant constituent, and gives the colour to the mass; it is sometimes in grains of about the same size as those of the quartz, viz., two to four millimetres in diameter, but frequently tends to assume a porphyritic habit, the crystals then measuring a centimetre or more across, and inclining to idiomorphic forms. Microcline is occasionally sparingly present. The oligoclase is white, and is far less abundant and less conspicuous than the orthoclase; in the porphyritic varieties of the rock, the oligoclase is mixed with the quartz and mica of the ground mass. In some specimens the felspars are fresh, in others [12,133] they show clouding due to decomposition, with formation of kaolin and occasionally of sericite. Quartz is present to about half the amount of the felspars, in grains ranging up to three millimetres in diameter. The biotite is frequently the only ferro-magnesian mineral present, occurring in wisps and little nests among the other minerals; under the microscope it is strongly pleochroic, the colour varying from usually a pale olive brown to nearly black when a crystal is rotated over a nicol prism, though in a specimen [11,531] from the little hill called Sikeit, about five kilometres west of Berenice, colour range is from pale yellow to deep green. Hornblende is typically either absent or present only in a very subordinate amount to the biotite. It is sometimes altered to chlorite, with separation of granules of iron oxide and formation of epidote. Sphene, apatite, and magnetite are usually only very sparingly present, and are only visible on microscopic examination. In a slide cut from the rock of Gebel Fereyid, however, sphene forms a crystal measuring 1·6 millimetres across, of the characteristic wedgelike form (see Fig. 4).

The principal variation in mineral composition of the rock is the greater or less abundance of the biotite, and the occasional presence of appreciable amounts of hornblende. In a specimen from Wadi Kreiga [12,133], the biotite is almost absent, its place being taken by green hornblende, now largely altered to chlorite and epidote. The ferro-magnesian minerals are as a rule present in smaller amounts than in average granites, and in some places, especially near the periphery of the great intrusions, they vanish almost entirely, and the rock passes gradually into a pegmatite.

Variations in appearance of the rock are also conditioned by the size and habit assumed by the felspar crystals, by the variations in general coarseness or fineness of grain, and by the pressures and weathering influences to which the masses have been subjected. Where the rock has been crushed, or subjected to unequal pressures in different directions during consolidation, the porphyritic crystals tend to lie along definite planes, and the rock may approximate in appearance to a gneiss. The microscopic sections from these places show further evidence of crushing in the cracking of crystals and undulose extinction between crossed nicols. Where the rock is much weathered it usually takes on a whiter aspect due to the bleaching of the orthoclase, and sometimes exhibits green spots due to the formation of chlorite from the decomposition of biotite and hornblende.

It is the pink granite which by its disintegration gives rise to much of the granitic sand which is met with in wadis and on plains. The rock disintegrates very readily under changes of temperature, the cleavage planes of the orthoclase aiding in the process. Change of the felspar to kaolin goes on under weathering influences, and the felspars become bleached, but the bulk of the disintegration is a mechanical process, a large part of the sand consisting of unaltered felspar.

=Red pegmatitic granite= is closely associated with the normal pink granite, occurring in great masses peripherally to the latter, as for instance at Gebel Hamrat Mukbud and in the low hills about the lower parts of Wadi Gemal and Wadi Kreiga. The red pegmatitic granite is in fact simply an extreme variation of the normal pink granite in which the ferro-magnesian minerals (biotite and hornblende) are either absent or present in very small amounts. That the rock often forms dykes is probably a consequence of its peripheral position rather than of any special manner of formation. The great masses such as Gebel Hamrat Mukbud appear, like the normal granites, to have solidified under plutonic conditions, and the dykes are off-shoots from the main mass. There is, therefore, no reason in this locality for classifying the pegmatites separately from the granites into a special division of dyke-rocks, as is done by Prof. Rosenbusch, and we shall consider the red pegmatitic granite as simply a coarse grained mica-free granite rich in orthoclase.

The pegmatitic granite is usually of a far more pronounced red colour than the normal granite. This is doubtless in part due to the greater abundance of orthoclase, but it also arises in some cases from the orthoclase having itself a deeper colour. The rocks about the lower parts of Wadi Gemal and Wadi Kreiga have a strong brick-red aspect. Their proximity to the coast suggests that there may possibly be some connexion between the intensity of their coloration and their position near the sea; this idea is supported in some measure by the strong red colouration of the dioritic rocks of Ras Benas, which occupy a similar position, but the actual manner of production of the oxide of iron which gives the colour is not clear.

In the hand specimen (see Plate XXII), the red pegmatitic granite [10,389] is a coarse-grained aggregate of red orthoclase and quartz. The quartz is generally of grey aspect in the mass, running in strings and networks through the felspar. The rock usually breaks easily into irregular pieces under the hammer, owing to the facility with which the large felspar crystals can be cleaved, but the very red variety from near the mouth of Wadi Gemal [12,106] is extremely hard and tough, and strikes fire very easily under the hammer. Under the microscope (see Fig. 5) the red colour of the orthoclase often persists even in the thinnest sections, being evidently due to finely disseminated iron oxide which clouds the felspars. The quartz shows sharply defined angular forms, and appears to have got somewhat the start of the felspar in crystallisation. Owing to its cloudy nature, the felspar looks like a ground mass, and between crossed nicols shows a peculiar patchy appearance, so that a first glance at the slide suggests a quartz porphyry; but the whole of the patches forming a crystal extinguish at once, and moreover the characteristic twinning of the orthoclase can be seen in some of the crystals. A little oligoclase is also present in addition to the orthoclase. Crushing of the rock is frequently shown by cracked and brecciated crystals, more especially in the hard form of the rock from Wadi Gemal.

Dykes of pegmatitic granite are very frequent in the neighbourhood of Gebel Hamrat Mukbud; they are typically rather paler in colour than the larger masses, and occasionally show graphic structure.

In ascending Gebel Migif, the gneiss slopes of the mountain were found to be strewn here and there with crystals of green microcline [10,366], similar to the well-known Pike’s Peak mineral. The crystals, which are of imperfect idiomorphic form, measuring about one to two centimetres in length, sometimes show cross-striations on certain faces. Though the rock was not traced in situ, the crystals are doubtless derived from pegmatite dykes traversing the gneiss.

=Aplite=, or fine-grained binary granite consisting essentially of quartz and felspar, with very little or no mica or hornblende, differs from pegmatite not only in its greater fineness of grain, but in its structure, which is essentially granitic rather than pegmatitic, the two main minerals being both in allotriomorphic grains. In colour, different aplites vary with that of their felspars from red to white, and most of them have tiny dark spots in them, sometimes only visible with a lens, due to the presence of small quantities of hornblende or biotite. As already mentioned on p. 268, most of the granites of South-Eastern Egypt incline to the acid or aplitic type, and one is frequently in doubt whether to classify an acid granite, such as that of Gebel Elba for instance, as an aplite with a little hornblende, or as a hornblende granite. Under aplites will be here considered only those granites in which the proportion of ferro-magnesian minerals is extremely small, not forming more than about one or two per cent of the rock. When the definition of an aplite is thus limited, aplites are comparatively scarce in the area.

Gebel Abu Hireiq consists of a red granitic rock; I did not visit the range, but the guide sent to erect the triangulation beacon brought back as the typical rock a pink aplite [12,134] of rather fine grain, consisting essentially of quartz and pink felspar, with sparsely scattered specks of hornblende.

The dykes [11,536] which seam the granite and diorite near where the Wadi el Kreim joins Wadi Garara may be classed as siliceous altered aplites. They are greyish-white to purplish-pink rocks of rather fine grain, with a few blackish specks, the latter visible only with a lens. The microscopic slide shows granitic quartz and red-stained decomposed matter, with a rather plentiful sprinkling of grains and powder of iron oxide; the decomposed matter is almost certainly the result of alteration of felspar and hornblende, the latter in very small proportion.

The hill called Marwot Rod el Ligaia, near the head of Wadi Muelih, is a boss of pink aplite, and is of interest as lying in close connexion with certain quartz veins which are believed to be of igneous origin (see p. 266).

=Biotite-granite= of a well-defined type [10,390] occurs somewhat largely in the mountains round the heads of Wadi Abu Ghusun and Wadi el Abiad. It is a whitish rock with dark-brown to black spots (see Plate XXII). The white portion of the rock consists of quartz and felspar; the dark spots are biotite. Generally the rock is of medium grain, the biotite being in nests two to four millimetres diameter scattered through the mass, giving it a speckled appearance. Sometimes the biotite is more uniformly scattered, giving the rock a grey aspect. At some places the rock shows signs of crushing, this being indicated in hand specimens by a tendency to laminar arrangement of the biotite patches. Under the microscope (see Fig. 6), the felspars, which are somewhat more abundantly present than the quartz, are seen to consist of orthoclase and oligoclase, with here and there crystals showing the characteristic cross hatching of microcline. The biotite is mostly in nests and clusters of ragged-looking crystals, but it also occurs in tiny flakes included in the felspars, sometimes showing a distinct arrangement along the cleavage planes of the latter. The biotite is usually fairly fresh; it is highly pleochroic, with colour ranging from pale yellow to deep olive-green in different positions over the nicol. Associated with the biotite are a few wisps of colourless mica, probably muscovite, and irregular granules of opaque iron oxides. Granules of epidote are to be seen in the altering felspars, and there are a few small irregular granules of a brownish highly refracting mineral, probably sphene.

A much more acid type of biotite granite [11,507] occurs in close proximity to serpentine in low hills near the wells of Abraq. In this the proportion of biotite is so small that the rock might almost be classed as an aplite. It is a fine-grained white rock, very fresh-looking, with dark spots; in hand specimens it almost exactly resembles the hornblende granite of Gebel Elba (see Plate XXII), but examination with a lens shows the dark spots to consist of biotite instead of hornblende. The microscopic slide shows quartz, orthoclase, and oligoclase with granitic structure, with here and there small straggling crystals of brown biotite; the brown biotite is sometimes altering to chlorite with change of colour from brown to green.

=Muscovite-granite= is rarely met with in South-Eastern Egypt. A specimen from Gebel Adar Qaqa [12,138] is a fine-grained hard pinkish rock of sp. gr. 2·62, composed of quartz, orthoclase, oligoclase, muscovite, and garnet.

A very coarse-grained variety of muscovite-granite, in which the individual crystals measure several centimetres across, forms white hills to the east of Wadi Nugrus.

=Hornblende-granite= occurs in several forms, giving rise to rocks of different aspects in different parts of the district.

The most important and widely distributed form of hornblende-granite is a fine-grained and very acid rock of sp. gr. 2·59, which but for the manner in which it occurs might almost be spoken of as an aplite. It is a white rock, sometimes with a faintly pink or greenish cast, of medium to fine grain, with black specks of hornblende about a millimetre in diameter scattered through it (see Plate XXII). This rock forms Gebels Muelih and Elba; it also occurs in the low country round Gebel Hamrat Mukbud, and a gneissose variation of it forms the great boss of Um Rasein. Dykes of the same rock traverse the more basic rocks of Um Bisilla.

Under the microscope the rock from Gebel Elba [12,118] is seen to consist of quartz, orthoclase, a little oligoclase, hornblende, and small quantities of biotite (see Fig. 7). The orthoclase is somewhat clouded in strips parallel to the clino-axis of the crystals. The hornblende is in irregular grains of sharply marked curved outlines; it is of a very deep bottle-green colour. The accessory biotite is brown, occurring as small wisps associated with the hornblende.

The rock of Gebel Muelih [10,355] is similar to that of Gebel Elba, but the hornblende here is of an extremely pale greenish brown colour, only slightly pleochroic, with well-marked vertical cleavage and a very small extinction angle, so that it might be mistaken for biotite, which latter mineral appears to be absent.

Another type of hornblende-granite, differing from that last described in being of coarser grain and containing much more abundant hornblende, and thus having much more the appearance of an ordinary grey granite, occurs in the lower part of Gebel Hamata and the surrounding mountains. This rock [10,405] is slightly heavier than the foregoing type, having a sp. gr. of 2·66. Under the microscope (see Fig. 8) the hornblende is seen to be of a dark green colour, very much altered and clouded by separated iron oxides and epidote.

A third type of hornblende-granite is formed by the variation of the normal pink granite already referred to, in which the place of biotite is largely taken by hornblende. The mass of Gebel Mishbih appears to consist of a rock of this type, which passes by insensible gradations into a syenite.

=Granite-porphyry.=

Under this heading are comprised rocks of granitic composition in which porphyritic crystals of quartz, felspar, and mica are surrounded by a fine-grained ground mass of entirely granitic (holocrystalline) character. They differ from porphyritic granites in the much finer grain of the general body of the rock, and from the quartz felsites in the entire absence of glassy matter from the ground mass. Rocks of this type are scarce in South-Eastern Egypt. An example [12,150] occurs at the hill called Kreishim, sixteen kilometres west of Halaib. Here the rock, which appears to form a small boss, consists of rounded white porphyritic crystals set in a finely crystalline grey ground mass. The sp. gr. is 2·69. Under the microscope (Fig. 9) the porphyritic crystals are seen to consist of quartz and oligoclase, the latter in approximately idiomorphic crystals, often beautifully zoned; the ground mass, of micro-granitic structure, is made up of smaller allotriomorphic crystals of quartz, felspar, and biotite.

Dykes of granite-porphyry also occur traversing the granite of Gebel Kahfa [11,537 B] and in the basic rocks of Gebel Um Bisilla [11,518]. At both these places the rocks are far less fresh than that above described. In the hand specimen they are of a greyish-white or greenish-white colour, strongly resembling fine-grained aplites in appearance. With the lens, crystals of quartz and felspar can be made out, and specks of hornblende and chlorite. The microscopic slides show the characteristic structure of granite porphyry, but the felspars are all clouded by decomposition products, and the hornblende, which is rather sparingly present in the ground mass, is mostly changed to chlorite and epidote. The specimen from Gebel Kahfa contains a small amount of muscovite.

=Quartz-felsite.=

Though covering only small areas, quartz-felsite is of very wide distribution in South-Eastern Egypt. It typically forms dykes and intrusive sheets traversing the plutonic and metamorphic rocks, but likewise occurs occasionally in larger masses. Felsite dykes are specially abundant in the neighbourhood of Gebels Muelih, Zergat Naam, and Um Reit. Larger masses occur in the Wadi Huluz, and form the summits of Gebels Nigrub el Foqani and Hamata.

Felsites are frequently met with in a highly altered condition. The commonest alteration, especially in dykes, is kaolinisation of the felspars and a clouding of the whole rock with finely disseminated iron oxides. In some dykes traversing the granite of Um Reit, a bleaching action has reduced the felsite to the appearance of a limestone. In other cases the rock has been devitrified and indurated to a high degree; this is well seen in Gebel Igli el Iswid (latitude 25°) and at Gebel Hadarba (latitude 22°), where extensive hill-tracts consist of felsitic rocks of almost flinty hardness.

The quartz-felsites are in general among the youngest of the igneous rocks of the district, since they commonly form dykes and intrusions, not only in the schists, but also in the granites and other eruptive rocks.

The quartz-felsite of Wadi Huluz [10,394] occurs in considerable masses about a kilometre below the water holes of Um Gerifat. In the hand specimen, it is of granitoid appearance, with opalescent quartz crystals two millimetres or more in diameter plentifully scattered in a light greyish ground mass. The sp. gr. is 2·71. Under the microscope the crystals of quartz are seen to have rounded forms and to be accompanied by other porphyritic crystals of orthoclase and oligoclase, often approximating to idiomorphic shape (see Fig. 10). The porphyritic constituents are embedded in a cryptocrystalline ground mass, in which augite and minute grains of quartz, felspar, and biotite can be made out, and there are some strings and granules of epidote and fairly large specks of iron oxides. The ground mass shows a fluidal arrangement, the little biotite flakes in it being often arranged in lines which sweep round the porphyritic crystals. The rock has undergone considerable alteration, the felspar crystals being clouded and full of tiny micaceous flakes of high double refraction; the epidote is also doubtless due to the alteration of augite and other minerals in the ground mass.

The quartz-felsite which forms the upper part of Gebel Hamata [10,906] is a dark brown coarse-textured rock, with remarkably glassy porphyritic quartz in granules about two millimetres diameter scattered plentifully through it. It is a very hard rock which rings under the hammer and weathers into rusty brown blocks. The sp. gr. is 2·71. Under the microscope the quartz crystals are seen to be much cracked. There are also porphyritic crystals of orthoclase and oligoclase, mostly of irregular shape, and a few crystals, of still more irregular form, of dark green hornblende. The cryptocrystalline ground mass consists chiefly of quartz and felspar, through which are scattered tiny granules of dark green hornblende. The ground mass shows a matted texture between crossed nicols; there is no trace of fluidal movement. Like the foregoing, this rock is somewhat altered, the hornblende in particular being very much attacked; the felspars are fairly fresh, but the crystals, like those of the quartz, are often cracked, and thus show the rock to have undergone considerable crushing.

One of the quartz-felsite dykes traversing the schists at Gebel Kolaiqo [12,145] is a very similar rock to that of Gebel Hamata, except that here the ground mass, instead of being dark brown in the hand specimen, is of a reddish brown colour (see Plate XXII), and the porphyritic crystals are partly pink felspar and partly glassy quartz. The sp. gr. is 2·62. Under the microscope the quartz and felspar crystals are seen to be less crushed than in the Hamata rock, while the ground mass, instead of being of a matted structure, is microgranitic, with tiny elongated grains of green hornblende scattered through it (see Fig. 11).

Another quartz-felsite dyke at Gebel Kolaiqo [12,135] has a rather different composition, and in the hand specimen has more the appearance of a felspar porphyry, porphyritic white felspars being scattered with quartz grains through a dark grey to black ground mass (see Plate XXII). Its quartz crystals frequently approximate to idiomorphic forms; they are much corroded by the ground mass. The felspar is chiefly oligoclase, in more or less idiomorphic crystals, a little clouded by decomposition products. The ferro-magnesian mineral here is chiefly brown biotite, in crystals of smaller size than those of the quartz and felspar, scattered porphyritically through the ground mass. One of the quartz crystals includes a crystal of biotite. The ground mass is crystalline, and appears to consist of quartz and felspar with a little biotite and some glassy matter.

A peculiar type of quartz-felsite [10,377] occurs associated with serpentine and ophicalcites in the upper part of Gebel Ghadir. The rock, which has a sp. gr. of 2·66, is nearly white, with a faint greyish tinge and scattered reddish-brown specks. At first the rock was taken for a granulite, which it very much resembles in appearance, and the reddish-brown specks for garnet. But examination with a lens shows the specks to be ferric oxide, and here and there little crystals of glassy quartz can be seen. Under the microscope the rock is found to consist mainly of an extremely fine-grained semi-granulitic colourless ground mass in which are sparsely scattered clear crystals of quartz, sometimes showing corrosion, and straggling irregular patches of opaque iron oxide, often mixed with nearly colourless wisps of mica. A few of the clear crystals are in four-sided forms, and may possibly be felspar; but they are perfectly free from alteration or twinning. The iron oxide is doubtless the product of alteration of a ferro-magnesian mineral, probably biotite. The minerals of the ground mass are difficult of determination owing to the minuteness of the grains and the absence of any colour, but apparently consist of quartz, felspar, a colourless hornblende, and sericite.

The quartz-felsite dykes [10,356] which cut through the granite and schists of Gebel Muelih appear as grooves in the granite, and as back-bone ridges in the surrounding schists. They are pinkish-brown close-textured rocks, with a dark marbling in places. The microscopic slides show quartz and felspar in a confused ground mass, with little flakes of a white micaceous mineral (sericite?) aggregated round the porphyritic crystals and distributed through the felspars and the ground mass.

The highly altered quartz-felsite [12,158] which forms Gebel Butitelib, near Gebel Niqrub el Tahtani, appears to have formed a boss or neck in the surrounding schists. The rock is a very fine-grained grey to purplish one, in which no crystals can usually be made out with the unaided eye. The sp. gr. is very low, being only 2·32. Under the microscope one sees small highly corroded crystals of quartz scattered through a clouded and glassy ground mass. No traces of felspars can be seen, the crystals, if they existed, having been decomposed and become undistinguishable from the ground mass; nor can any ferro-magnesian minerals be made out, though there are tiny granules and wisps of iron oxides which in places show an arrangement suggestive of their having been derived from the decomposition of such minerals.

The specimen [12,121] brought back from the summit of Gebel Shendib, by the guide who was sent to erect the triangulation beacon on it, is a compact brown rock with reddish patches, which under the microscope appears to be an altered and brecciated felsite. The slide shows quartz and altered felspars in a spherulitic ground mass, the whole of the mineral being very much clouded by finely disseminated ferric oxide.

The felsite dykes [11,538] which seam the granite of Gebel Um Reit are a still more highly altered rock. The dykes are about two metres wide, of a white colour, and resemble limestone in appearance; being soft, they have weathered more rapidly than the granite around them, leaving vertical-sided chasms. But for the manner of its occurrence, the rock would have been easily mistaken in the field for a sedimentary one. The microscope, however, confirms its eruptive origin. The slides cut from the rock show a confused and clouded semi-crystalline aggregate, containing clearer small areas of quartz. Even the quartz crystals are full of specks, and only the faintest traces can be seen of the felspars, which are so decomposed as to be hardly distinguishable from the ground mass.

In the rocks of Gebels Igli el Iswid, Mahali, and Hadarba, we have quartz-felsites which have been altered in quite a different way. The rocks are extremely hard, and almost flinty, breaking with a sub-conchoidal fracture. They are typically of a brown to nearly black colour, with white spots measuring a millimetre or two across. Under the microscope these white spots are seen in the rock from Gebel Igli el Iswid [10,372] to be chiefly porphyritic felspar and quartz crystals, sometimes corroded by the ground mass; while the black ground mass is largely composed of extremely fine micro-pegmatitic intergrowths of quartz and felspar, with some glassy matter, and here and there irregular small clouded and altered crystals of hornblende. The micro-pegmatitic material has possibly originated by devitrification of an originally glassy ground mass. Variations of the rock occur in which the porphyritic felspars are infrequent or even altogether absent, the rock passing gradually into a hornfels [10,371]. The slide from Gebel Mahali [10,402] exhibits clear porphyritic crystals of quartz and orthoclase, both minerals in more or less idiomorphic forms, embedded in, and occasionally corroded by, a semi-crystalline ground mass. The ground mass contains some calcite and iron oxides, probably arising from the decomposition of a hornblendic or micaceous mineral.

The rocks of Gebel Hadarba [12,146-12,149] are essentially similar to those of Gebel Igli el Iswid, but in some cases they show bright red veining; where they have been exposed to the polishing agency of the sand blast, these veined varieties look as though they had been streaked with melted sealing-wax. In some of the slides the felspars show a well marked microperthitic structure, while in others they are so decomposed that they are barely distinguishable from the ground mass. The ground mass sometimes contains granules and strings of iron-oxide, possibly referable to alteration of a hornblendic constituent, but the rock is too much altered for one to be sure.

An altered and crushed quartz-felsite [10,404] which occurs in the Wadi Huluz, near where the Wadi el Abiad joins it about eight kilometres to the north-west of Gebel Hamata, somewhat resembles the rock of Gebel Hadarba in appearance. It is a jaspery looking rock of dark colour with red and greenish patches, in which with a lens one can see scattered grains of glassy quartz. The slide cut from the rock shows clear porphyritic crystals of quartz, orthoclase and oligoclase in a fine grained ground mass. The porphyritic crystals are mostly in rounded forms, but some of the felspars show a tendency to idiomorphism. Many of the crystals are smashed, and some show undulose extinction. The ground mass consists of quartz, felspar, and some glassy matter, with abundant epidote in nests and strings, and a small amount of green hornblende in straggling forms.

The east-and-west dykes which traverse the syenite of Gebel Zergat Naam, and form the actual summit of that mountain, consist of a very hard compact flesh-coloured rock which weathers brown on the surface. The microscopic section [11,525] shows the rock to be a highly indurated felsite. The original felspar crystals can be made out by their shapes, but between crossed nicols they are seen to consist of a very fine-grained mosaic polarising in low greys, probably the result of alteration by siliceous solutions. These altered felspars are scattered with some quartz in a cryptocrystalline ground mass.

No volcanic rocks of acid type have been with certainty identified in South-Eastern Egypt. It is, however, possible that some of the rocks above described are in reality of volcanic origin, though the manner of their occurrence is more suggestive of intruded masses and sheets. It is also quite likely that some of the hornfels found associated with the schists, as for instance at Gebel Um Semiuki, near Abu Hamamid, are altered forms of glassy acid lavas; but their vast age and the intense metamorphism to which they and the surrounding rocks have been subjected render it impossible to be certain of the manner of their origin.

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.

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.

=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.=

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.

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.

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.

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.

The rock [10,411] which forms the dark hills called Gebel el Anbat, near the head of Wadi Kharit, 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.=

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.

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.=

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.=

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.

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.

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.

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.

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.

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.]

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.

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.

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.=

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.=

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.

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.

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.

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.

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.=

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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. 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.