* * * * *
=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[125] 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],[126] 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.[127]
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.=
[Illustration: Ball.—Geography & Geology of South-Eastern Egypt.
=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).
[Illustration: Fig. 4.—Granite, Gebel Fereyid [11,505], × 10. _f_,
felspar (mostly orthoclase), somewhat clouded by decomposition; _q_,
quartz; _b_, biotite; _s_, sphene.]
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.
[Illustration: Fig. 5.—Red pegmatitic granite, from low hills
near the mouth of Wadi Gemal [12,106], × 10. _q_, quartz; _f_,
felspar (orthoclase) turbid and full of finely disseminated red
oxide of iron.]
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).
[Illustration: Fig. 6.—Biotite-granite near Gebel Abu Hegilig
[10,390], × 17. _q_, quartz; _f_, felspar (orthoclase and
oligoclase), somewhat turbid by decomposition; _b_, biotite; _s_,
sphene.]
=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[128] [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.
[Illustration: Fig. 7.—Hornblende-granite, Gebel Elba [12,118],
× 17. _q_, quartz; _f_, felspar (mostly orthoclase), clouded by
kaolinisation; _h_, hornblende; _b_, biotite, altering with separation
of flakes of limonite.]
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.
[Illustration: Fig. 8.—Hornblende-granite, Gebel Hamata [10,405],
× 10. _q_, quartz (the rock contains a larger percentage of quartz
than appears in the drawn portion of the slide); _f_, felspar
(orthoclase), clouded; _h_, hornblende, altered.]
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[129] to consist of a rock of this type, which passes by
insensible gradations into a syenite.
=Granite-porphyry.=
[Illustration: Fig. 9.—Granite-porphyry, Kreishim Hill [12,150],
viewed between crossed nicols, × 10. _q_, quartz (the dark triangular
area to the left of figure is also a quartz crystal in the position
of extinction); _f_, felspar (oligoclase); _g_, micro-granitic
ground mass.]
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.
[Illustration: Fig. 10.—Quartz-felsite, Wadi Huluz [10,394],
× 10. _q_, quartz; _f_, felspar (orthoclase and oligoclase); _g_,
ground mass, showing flow structure round the porphyritic quartz
and felspar.]
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.
[Illustration: Fig. 11.—Quartz-felsite from dyke at Gebel Kolaiqo
[12,145], × 10. _q_, quartz (a group of idiomorphic crystals); _f_,
felspar (mostly orthoclase); _g_, microgranitic ground mass of quartz
and felspar, with wisps of hornblende.]
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.[130]
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.
[Illustration: Fig. 12.—Altered quartz-felsite, Gebel Igli
el Iswid [10,372], as seen between crossed nicols, × 40. _f_,
felspar (mostly orthoclase in the position of extinction); _h_,
a crystal of hornblende; _g_, hemi-crystalline ground mass; _m_,
micropegmatitic intergrowth of quartz and felspar, separating the
porphyritic crystals.]
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.
[Illustration: Fig. 13.—Microperthitic structure in felspar of
quartz-felsite, Gebel Hadarba [12,147], as seen between crossed
nicols, × 40.]
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.
[Illustration: Fig. 14.—Crushed oligoclase crystal in
quartz-felsite, Wadi Huluz [10,404], as seen between crossed nicols,
× 40.]
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.
[Illustration: Fig. 15.—Syenite of Gebel Zergat Naam [11,515],
× 10. _o_, orthoclase felspar, showing striations parallel to the
basal planes; _h_, hornblende deep green in colour; _g_, interstitial
quartz.]
[Illustration: Fig. 16.—Syenite-porphyry, Gebel Zergat Naam
[11,512], as seen between crossed nicols, × 10. _f_, porphyritic
felspar (orthoclase); _h_, hornblende; _g_, microgranitic ground
mass containing felspar, hornblende, and some quartz.]
The syenite of Gebel Zergat Naam [11,515] rises as a great boss
from among the surrounding dark schistose rocks. It is typically
a pinkish-brown rock of rather fine grain (_see_ Plate XXIII), in
which can be seen shining crystals, three to four millimetres long,
of pink orthoclase, and specks of dark hornblende, with here and
there a little glassy-looking quartz. The sp. gr. is 2·62. Under the
microscope the rock is found to be mainly composed of orthoclase,
with a much smaller amount of hornblende and a little interstitial
quartz. The felspar is fairly clear, in forms approximating to
idiomorphic, and showing the characteristic simple twinning of
orthoclase. The crystals are slightly clouded in streaks parallel to
the basal planes, so that even in ordinary light a faint herring-bone
structure is visible, the streaks on either side of the trace of
the plane of composition being inclined to each other at a large
angle. Between crossed nicols this herring-bone structure is very
strongly marked, and is evidently due to a perthitic intergrowth
(of albite?). The intergrown lamellæ are slightly irregular; though
following generally the direction of the basal cleavage planes they
are not perfectly straight, nor always continuous across the half
of the crystal. The lamellæ extinguish and attain their maximum of
brightness simultaneously with the respective halves of the crystal in
which they occur, so that they become invisible in certain positions
of the nicols; but as the nicols are turned they appear as well marked
dark bands, clearly visible even under low powers. The hornblende is
of exceptionally dark green colour, in irregular straggling masses,
often considerably decomposed and clouded with iron oxide. Accessory
minerals, other than the clear interstitial quartz, appear to be
almost entirely absent in the slide examined.
=PLATE XXIII.=
[Illustration: Ball.—Geography & Geology of South-Eastern Egypt.
=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.=
[Illustration: Fig. 17.—Trachyte, from a dyke at Gebel Kahfa
[11,537 A] × 10. A clear crystal of orthoclase felspar is seen in
the centre of the field, surrounded by a finely crystalline ground
mass of hornblende and felspar.]
Trachyte, the volcanic representative of syenite, occurs in dykes
[11,537 A] seaming the granite of Gebel Kahfa. It is a light grey
rock, of very fine grain, breaking with a rough surface, containing
pores and small stumpy white pearly-looking porphyritic crystals
of orthoclase (_see_ Plate XXIII). The sp. gr. is 2·56. The
microscope shows the ground mass surrounding the porphyritic
idiomorphic orthoclase crystals to be a holocrystalline mixture
of hornblende and felspar. The felspars of the ground mass are
usually in more elongated forms than the porphyritic crystals, and
appear to be partly plagioclase. The hornblende, in small and very
irregular crystals of dark olive-green colour, often clouded and
dirty looking, is scattered plentifully among the feslpars of the
ground mass. Accessory primary minerals appear to be entirely absent.
A rock which occurs in a hill at the head of Wadi Amba-ut [10,375]
appears to be essentially similar to the above, but the porphyritic
orthoclases are more numerous and the ground mass contains small
quantities of augite and magnetite. This rock is in a highly altered
condition, the felspars being full of kaolin and epidote, while the
hornblende, which is here of a paler colour than in the trachyte of
Gebel Kahfa, is highly chloritised.
(_b_) =SUB-BASIC ROCKS.=
=Diorite.=
Though not forming such conspicuous features as the granites, rocks of
dioritic composition are very widely distributed over the country,
and are specially abundant in the districts to the south of Ras
Benas. They are almost always sharply marked-off from the granites,
being generally closely connected with the more basic igneous rocks
and schists. The coarser-grained varieties usually form parts of
irregular intrusive masses, shading off gradually into the more basic
forms of diabase, gabbro, and hornblende rock; the finer-grained types
occur as dykes and irregular bands traversing other igneous rocks
and schists, and are themselves frequently so crushed as to resemble
schists. Owing to their intimate association with other dark rocks,
it is no easy matter to trace the limits of the diorites in the
field. The same difficulty is found when they are microscopically
examined, for one finds in their mineralogical composition every
gradation from true diorites, through the intermediate stage of
augite-diorite to diabase and gabbro, while many of the harder dark
schists and hornblende gneisses turn out to be merely highly crushed
diorites, so that classification must be more or less arbitrary.
Diorite typically forms low hill country of dark aspect. The rock
weathers as a rule far more easily than granite, and in some cases
the debris of rounded grains set free by disintegration cover the
surface and render the climbing of the hills somewhat dangerous,
the effect being like one would imagine to result from walking over
slopes covered with hard peas.
[Illustration: Fig. 18.—Diorite of Gebel Allawi [10,313], ×
10. _pl_, plagioclase felspar, clouded by decomposition products;
_h_, hornblende, olive-brown in colour; _ha_, hornblende altering
to pale-green chlorite; _m_, magnetite.]
Diorite in the narrowest sense of the term (plagioclase-hornblende
rock) is not by any means abundant. Curiously enough, it is generally
found in the neighbourhood of old gold mines, as for instance in the
Kurdeman and Allawi districts, and it was employed by the ancient
miners for their crushing pans. The rock from Gebel Allawi [10,313]
is a medium-grained one composed of black and milk-white minerals
in about equal proportion (_see_ Plate XXIII). Its sp. gr. is
2·95. Under the microscope the milk-white material is seen to be
plagioclastic felspar, generally much decomposed, while the dark
grains are of hornblende, pale green to olive-brown in thin section,
with somewhat feeble pleochroism. Like the felspar, the hornblende
is considerably altered; in places it has lost nearly all its colour
and is converted into pale green chlorite. The accessory minerals
are iron-oxides and a little sphene. Rocks of somewhat finer grain
[10,358] occur in the Rod el Ligaia. In a slide from the last-named
locality, granules of ilmenite are surrounded by sphene, suggesting
the formation of sphene by the alteration of ilmenite. A variety of
diorite [10,403] occurs in the Wadi Huluz in which the hornblende
is nearly colourless, showing only a trace of green colour in thin
section, and is accompanied by a small amount of augite.
[Illustration: Fig. 19.—Diorite, Wadi Baaneit [12,151], × 10. _h_,
hornblende; _pl_, plagioclase; _b_, accessory biotite; _q_, accessory
quartz.]
A form of diorite which occurs in and about the Wadi Baaneit [12,151]
is very similar to the rock just described, but differs from it
in the more irregular manner in which the two principal mineral
constituents are distributed. The hornblende is in patches varying
from mere specks to eight millimetres in diameter; the felspars have
a sugary appearance in the hand specimen. The sp. gr. is 2·81.
A more highly specialised type of diorite [12,103] occurs in the
Wadi Muqur. This is a very beautiful heavy rock (sp. gr. 2·87),
with shining prisms of hornblende, often three centimetres or more
in length, running through a mass of white felspars (_see_ Plate
XXIII). Microscopic examination reveals the presence of some quartz
mixed with the plagioclastic felspars.
Where the alteration of diorite has been very intense, as in the
country rock [12,124] of the old Romit mine, the hand specimen
differs from that of the unaltered rock in that not only are the white
constituents of duller aspect, but the dark mineral, instead of being
black and shining, has a dull greenish-grey appearance. Under the
microscope one sees no trace of the original felspar, its place being
taken by calcite and a confused aggregate of small plates of kaolin
and sericite, while the hornblende is all replaced by chlorite, with
little strings of limonite. Granules of quartz, often of relatively
large size, are scattered through the mass; some of these are clouded
by minute enclosures, and evidently represent the accessory quartz
of the original rock, while others are clearer and are probably of
secondary formation.
Fine-grained varieties of diorite are found much more abundantly than
the normal plutonic type, occurring as irregular masses mixed with
schists and as dykes in schists and other rocks. These fine-grained
diorites are essentially similar to the coarser-grained rocks,
but are generally even more altered. The fine-grained diorite
[11,517 A], which forms the top of Gebel Um Tenedba, for instance,
consists of a mixture of clouded and altered plagioclase and augite
altering to chlorite, with separated iron oxide along the cleavage
planes of the hornblende. Another specimen of fine-grained diorite,
from a dyke in Wadi Kreiga [12,154], is even more highly altered,
the hornblende being almost entirely chloritised, with formation of
abundant little granules of epidote.
The main rock of Gebel Beida [12,160] may also be classed as an
altered fine-grained diorite. It is a dark fine-grained greenish-grey
rock with white and greenish-yellow spots, usually about two
millimetres diameter, scattered through it, and abundant strings of a
greenish yellow mineral. The sp. gr. is 2·96. Under the microscope
it is seen to be of similar nature to the rocks last described,
the hornblende being largely chloritised. There is a considerable
amount of accessory augite, and this, though clouded, has resisted
alteration better than the hornblende. The whitish spots seen in
the hand specimen are made up of a fine mosaic of quartz grains,
with tiny flakes of a micaceous mineral (sericite?) probably all
of secondary origin, while the greenish-yellow strings are other
alteration products in the shape of epidote and calcite.
=Augite-diorite.=
Under the head of augite-diorite are classed holocrystalline
rocks containing, in addition to the plagioclase and hornblende of
ordinary diorites, notable quantities of augite. The augite-diorites
thus form a link between the diorites proper and the diabases or
plagioclase-augite rocks.
The presence of augite along with the hornblende is difficult to
ascertain in the field or in hand specimens, and can as a rule only
be detected by the microscopic examination of thin sections. But
augite diorites are generally of somewhat darker aspect than normal
diorites in the mass, owing to a less abundance of felspar, and are
generally tougher under the hammer.
[Illustration: Fig. 20.—Augite-diorite, Wadi Um Hargal [11,535],
× 47. _h_, hornblende; _a_, augite, with celephytic border _c_, of
hornblende and iron oxide; _f_, felspar (labradorite); _ap_, apatite.]
An augite-diorite [11,535] which occurs on the pass at the head
of Wadi Um Hargal, near Gebel Kahfa, is a heavy (sp. gr. 2·87)
grey rock of medium grain, very fresh and hard, in which can be
seen lustrous black crystals mixed with a rather small quantity
of white felspars. In thin section, hornblende, the most abundant
constituent, is in fairly large allotriomorphic crystals, with well
marked cleavage and strong pleochroism (bluish-green to pale yellow),
containing abundance of irregular granules of iron oxides. The augite,
which is present to about one-third the amount of the hornblende,
is in crystals of similar size, and likewise showing well-marked
cleavage, easily distinguished by their pale brown colour, absence
of pleochroism, and higher extinction angles (about 40°); some of
the crystals show irregular cracking and clouding by decomposition
products, and are surrounded by celephytic zones of greenish matter of
rather lower double refraction containing flakes and strings of iron
oxide, probably representing a marginal alteration to hornblende. The
felspar, though considerably decomposed, still shows plagioclastic
twinning clearly, and appears from the extinction angles to be an
acid type of labradorite. The rock contains considerable amount of
magnetite scattered through it, often in fairly large irregular
grains, also a few small grains of apatite, and one or two small
granules of quartz.
[Illustration: Fig. 21.—Augite-diorite, Gebel el Anbat [10,411],
× 40. _pl_, plagioclase felspar; _a_, augite; _h_, hornblende,
arising from alteration of augite.]
The rock [10,411] which forms the dark hills called Gebel el Anbat,
near the head of Wadi Kharit,[131] is likewise an augite-diorite. In
the field it is seen weathered into rounded masses often resembling
boulders, of great hardness, and covered with a blackish-brown
skin. The sp. gr. is 2·97. Microscopic examination shows the rock,
which is very fresh, to be essentially of the same type as that last
described, but the augite is more abundant and so intimately mixed
with the hornblende as to suggest even more strongly an alteration
of augite to hornblende (_see_ Fig. 21).
=Mica-diorite.=
[Illustration: Fig. 22.—Mica diorite, from a dyke at Gebel Abu
Hegilig [10,391], × 17. _f_, felspar (mainly plagioclase); _b_,
biotite altering with formation of limonite; _h_, hornblende; _ap_,
apatite; _m_, magnetite.]
Most of the diorites of South-Eastern Egypt contain little or no
biotite as an accessory constituent. An exception occurs, however, in
a great dyke [10,391] of very fine grained diorite of sp. gr. 2·87,
which traverses the granite of Gebel Abu Hegilig, and which contains
more biotite than hornblende. The dyke is so decomposed that it is
difficult to get a coherent hand specimen; but a slide cut from one
of the less altered portions shows the rock to be a very fine grained
holocrystalline one, made up of plagioclase, biotite, hornblende,
apatite and magnetite, with abundant alteration products such as
epidote, kaolin, and chlorite. All the minerals, except the apatite
and some of the iron oxides, are allotriomorphic. The felspars are
very much altered, but appear to be mainly plagioclase. The biotite is
in little brown ragged-looking plates, strongly pleochroic, frequently
altered with separation of flakes of limonite. The hornblende is
green, in small and very irregular-shaped crystals, which show very
little trace of cleavage and are frequently chloritised. The apatite
is in long hexagonal clear prisms. Iron oxides, sometimes showing
square or hexagonal outlines, and epidote in granules, are liberally
scattered through the rock. From the abundance of biotite and the
fine grain and manner of occurrence of this rock it was taken in
the field for a decomposed lamprophyre; but the entire absence of
idiomorphism in the ferro-magnesian minerals show that it should
rather be placed with the diorites.
=Diorite-porphyrite.=
Rocks which may be somewhat doubtfully classed as altered
diorite-porphyrites occur at Gebel Abu Hodeid as well as near the
ruins of Um Eleiga and at Gebel Um Heshenib.
[Illustration: Fig. 23.—Diorite-porphyrite, Gebel Abu Hodeid
[12,143], × 40. _f_, porphyritic felspar (plagioclase); _h_,
hornblende; _b_, biotite; _g_, ground mass, consisting chiefly of
plagioclase and hornblende.]
The triangulation point on Gebel Abu Hodeid not being an occupied
station, I have not visited the mountain, but the guide sent to
erect the beacon on the summit brought back a specimen [12,143]
of the rock. It is a very fine-grained dark grey rock with tiny
glistening specks. The sp. gr. is 2·88. The microscopic slide shows
it to consist of porphyritic plagioclase in a very fine-grained
holocrystalline ground mass composed principally of brown hornblende
and plagioclase, with a little biotite and a plentiful sprinkling
of tiny granules of iron oxides. The porphyritic plagioclases
are inclined to idiomorphic forms, forming crystals about half a
millimetre in length, and hence are not very conspicuous in the hand
specimen; they are considerably clouded by decomposition, but still
show repeated twinning very clearly. The hornblende, which forms the
main constituent of the ground mass, is of a pale to dark yellow-brown
colour, mostly in rounded granules in which cleavage is not very
strongly marked. The plagioclase of the ground mass is likewise in
tiny granules, mixed with the hornblende. Biotite is only sparingly
present in the slide; it is in tiny brown flakes. All the minerals
of the ground mass show more or less decomposition and clouding,
and contain a fairly plentiful sprinkling of minute grains of iron
oxides; flakes of this latter substance are specially evident in
the decomposing biotite.
The diorite-porphyrite of Um Eleiga [11,527 B] occurs associated
with fine grained gabbro round the old mines. It is a grey rock,
breaking with a rough surface, in which porphyritic lath-shaped
felspar crystals, up to three millimetres long, are somewhat
sparsely scattered in a fine-grained ground mass. The sp. gr. is
2·82. The microscopic slide reveals the ground mass to be finely
holocrystalline, composed of plagioclase and pale green to brown
hornblende, with some magnetite. The plagioclase of the ground mass
is partly in little laths, and the hornblende frequently shows a
tendency to prismatic and fibrous forms. The whole rock is in a
rather advanced state of alteration, all the crystals in the slide
being strongly clouded by kaolin and other decomposition products.
The summit rock of Gebel Um Heshenib [10,392] appears to be a highly
altered basic diorite-porphyrite forming a dyke in the surrounding
schists. It is a dark-grey heavy rock (sp. gr. 3·04), of basaltic
appearance, with white porphyritic patches, more or less rounded in
form, scattered through it. The microscopic slide shows the white
patches now to consist mainly of kaolinic matter, with which is
mixed a clear mineral (sericite?), of very low refractive index, but
showing rather high double-refraction colours; while the ground mass
is a very fine-grained mixture of rather fibrous pale green hornblende
with kaolinic matter. Hardly a trace of unaltered felspar remains, but
it seems natural to ascribe the kaolin both of the porphyritic areas
and of the ground mass to the decomposition of original felspars.
A dyke of fine-grained brown rock which occurs in the granite of
the lower part of Wadi Kreiga [12,102] likewise appears to be an
altered diorite-porphyrite. It is much less dense than the rock
last described, its sp. gr. being only 2·68. The microscopic slide
shows the main constituent to be plagioclase felspar in idiomorphic
forms, much altered to kaolin and calcite and stained red by iron
oxide. Between the felspars are irregular patches of chloritic
and serpentinous matter, with calcite and flakes of limonite, the
alteration products of a ferro-magnesian mineral which was probably
originally hornblende.
=Augite-porphyrite.=
[Illustration: Fig. 24.—Augite-porphyrite, Wadi Muelih [10,359],
× 40. _a_, augite; _pl_, plagioclase; _h_, hornblende, much clouded
with iron oxides. The rock contains large porphyritic plagioclase
and hornblende crystals not shown in the figure.]
A dyke of augite porphyrite [10,353] occurs in the Wadi Muelih
about half-way between Gebel Muelih and Erf el Fahid. It is a
fine-grained reddish-brown rock of sp. gr. 2·79, with porphyritic
white plagioclase crystals of considerable size (up to two
centimetres in length) and other smaller porphyritic crystals of a
dark schillerized-looking mineral. The ground mass, when examined
with a hand lens, is seen to be a very fine-grained mixture of red
and dark minerals, like a syenite in miniature. The microscopic slide
reveals the dark porphyritic crystals as green hornblende, while the
ground mass is a holocrystalline mixture of plagioclase and augite,
with a little green to brown hornblende and abundance of magnetite
granules. The felspars of the ground mass are mostly lath-shaped;
they are much altered, and stained red by iron oxide. The augite, of
a very pale purple colour, is abundant in the ground mass, sometimes
in prismatic forms, but more often in rounded grains; it is altered
in places to chlorite, becoming then green in colour. The extinction
angles measured in the less-altered crystals range to over 40°.
=Kersantite.=
[Illustration: Fig. 25.—Kersantite, from a dyke at Gebel Fereyid
[11,504], × 40. _f_, felspar (mainly oligoclase); _b_, biotite;
_a_, augite, with a border of hornblende (_h_); _ap_, apatite.]
Only one occurrence of a rock which can be with certainty classed
as a lamprophyre has been noted in South-Eastern Egypt. The single
occurrence referred to is that of a kersantite [11,504] which
forms a dyke cutting east-and-west through the granite of Gebel
Fereyid. In the hand specimen (_see_ Plate XXIII) it is a fine grained
dark-grey rock, marbled with veinlets of brownish-white granular
(felspathic?) material, and containing here and there porphyritic
dark-brown platey crystals up to three millimetres in diameter. The
sp. gr. is 2·81. Under the microscope, the rock presents a very fresh
appearance, and is seen to be composed mainly of felspars and brown
biotite, with a little accessory augite, green hornblende, apatite
and magnetite. The biotite, the most conspicuous constituent, is seen
partly in basal sections of perfectly idiomorphic forms, and partly
as long lath-shaped sections; it is strongly pleochroic, the colour
varying from pale yellow-brown to a very dark reddish-brown. The
felspar, which has undergone some alteration, forms a sort of matrix
round the biotite; here and there large crystals show a tendency to
idiomorphism, but the felspar is essentially allotriomorphic. The
felspar appears to be mainly oligoclase; but there are also some
crystals which show only simple twinning, and these are doubtless
orthoclase. The augite and hornblende are very sparingly present,
the former in nearly colourless to purplish crystals, the latter in
tiny forms of a deep emerald or bluish-green colour, with very high
double refraction. Magnetite is liberally scattered in small grains
through the rock, seldom included in the biotite. Apatite occurs in
very fine long needles among the felspars.
=Andesites.=
Andesites, the volcanic representatives of the diorites, are much
more scarce in the south portion of the Eastern Desert than they
are further north. In the district here treated of, only a single
deposit, that of Gebel Sufra, has been noted as belonging certainly
to the class of andesitic lavas. Some other volcanic rocks consisting
chiefly of hornblende and plagioclase have been met with, as for
instance at the hill of Ti Keferiai and in the Wadi Huluz; but these
are of so basic a nature that they are more properly classed as
hornblende-basalts. It is also practically certain that some of the
rocks which must be classed as schists on account of their structure
are metamorphosed andesites; among the schists of the Wadi Muelih, for
example, are rocks which in thin section present a ground mass still
distinctly andesitic in character, but the hornblendic constituent,
instead of forming well-defined porphyritic crystals as it doubtless
originally did, is dragged out into fibrous forms, and the same
action can be traced in the hornblendes of the ground mass.
The andesite of Gebel Sufra [10,597] occurs as a columnar deposit
overlying syenites and diorites at the top of the mountain, which
rises to 690 metres above sea-level in latitude 24° 39′. The rock,
which is a fine-grained greenish-grey one weathering to a brown colour
on exposed surfaces, often shows a banded structure (_see_ Plate
XXIII). The sp. gr. is 2·67. The microscopic section shows the rock
to be highly altered, but sufficient traces of its original nature can
be made out to leave little doubt of its being an andesitic lava. The
slides show porphyritic felspars in a cryptocrystalline ground
mass composed of felspar with a little hornblende and biotite. The
porphyritic felspars are too much kaolinised for twinning to be
made out, but a little calcite is visible in their decomposition
products, and the crystals are in rather elongated forms which suggest
plagioclase rather than orthoclase. The minerals of the ground mass
are likewise much decomposed, but tiny grains of green hornblende and
wisps of brown biotite, both altering to chlorite, can be seen. There
is an almost complete absence of primary iron oxides, but a single
large porphyritic crystal in the slide shows separated hæmatite in
flakes and in strings down its cleavage planes. The nature of this
single crystal is not very clear, and it is doubtful if any of its
original substance remains; its form and cleavage are suggestive of
augite, but the clear spaces unoccupied by the iron oxide have the
appearance of quartz or clear felspar under crossed nicols.
[Illustration: Fig. 26.—View near the top of Gebel Sufra, showing
the columnar structure of the andesite.]
[Illustration: Fig. 27.—Andesite of Gebel Sufra [10,597], as seen
between crossed nicols, × 40. Porphyritic felspar crystals in a
cryptocrystalline ground mass.]
BASIC IGNEOUS ROCKS.
=Gabbro.=