* * * * *
=METAMORPHIC ROCKS.=
* * * * *
Metamorphic rocks, comprising a great variety of gneisses and schists,
are widely and abundantly distributed in South-Eastern Egypt,
forming about half the entire area treated of in this volume. The
irregular way in which the metamorphic rocks alternate with those
of the igneous and sedimentary groups will be better appreciated
by a reference to the geological map on Plate XX than by any verbal
description. As a whole, the metamorphic rocks represent the remains
of a complex of igneous and sedimentary deposits which were laid
down in remote geological periods; subsequently these deposits
were crushed, folded, and elevated into mountains, their original
structures being largely obliterated in the dynamical process;
in succeeding ages the metamorphosed complex was penetrated by the
great granitic and other intrusions which have been described in
the preceding chapter, and then planed down by denudation as an
old land surface before being wholly or in part submerged beneath
the waters of the Cretaceous sea in which the Nubian sandstone was
deposited. The elevation of the land after the Cretaceous period
was accompanied by further folding which must have increased the
degree of metamorphism of the rocks; but it is most probable that
the forces which produced these post-Cretaceous movements were of
less intensity and duration than those of more ancient periods,
and we may regard the metamorphic character of the older rocks
as having been for the most part impressed on them in Archæan,
or at latest in Palæozoic times. It is not impossible that some
of the more highly foliated gneisses may even have been parts of
the primitive crust of consolidation of the earth, while the clay
schists may be the remains of the very earliest sediments.
Besides the evidences of dynamo-metamorphism above-mentioned, we find
occasionally traces of contact-metamorphism, where igneous dykes
have locally altered the rocks into which they have intruded; but,
in general, the subsequent compression of the masses has obliterated
these traces, and the effects of contact-metamorphism are negligible
in comparison with those produced by dynamic action.
The metamorphic rocks occurring in this part of Egypt may be
classified as follows:—
=Gneisses.=
_Granite-gneiss_ (crushed granite).
_Diorite-gneiss_ (crushed diorite).
_Highly foliated gneisses_ (in which all traces of primitive
structure have been obliterated by foliation and recrystallization).
=Schists.=
_Quartz-schists_ (in some cases probably metamorphosed sandstone,
in others crushed aplitic rocks).
_Clay-schists_ (metamorphosed clays).
_Hornblende-schists and crushed volcanic rocks._
_Hornfels_ (metamorphosed acid lavas).
_Mica-schists_ (sometimes containing beryl and tourmaline).
_Chlorite-schists._
_Talc-schists._
_Calcareous schists and marble._
_Graphite-schists._
=Breccias.=
_Fluidal breccias_, in which broken fragments of solid rock have
been included in an intrusive magma.
_Breccias proper_, formed by fracture and recementing together
of fragments without fusion.
=Mineral Veins.=
_Quartz veins_ (sometimes containing gold and traces of copper).
_Calcite veins_ (with more or less chalybite).
_Magnesite veins_ (in serpentine).
_Asbestos veins_ (in serpentine).
=Gneisses.=
The most conspicuous occurrence of gneiss in South-Eastern Egypt is
the great mass which extends from Gebel Mudergeg and Gebel Migif in
the north, down the range of Gebel Hafafit, and along the greater part
of the Wadi Gemal. With this mass is closely connected another stretch
of gneiss to the south of Gebel Abu Khrug; the hills which rise from
the sandy plain round the head of Wadi Natash are mostly gneiss, so
that the gneiss is probably continuous across the plain except where
it is penetrated by the syenitic cones called El Nahud. The total
area covered by gneiss in this locality alone is some two thousand
square kilometres, and includes some remarkable mountain masses,
such as Gebels Migif and Hafafit.
Other gneissose areas of less extent occur round Bir Abu Beid and to
the south-west of Gebel Hamrat el Feg; while still smaller areas are
found in the hills to the north-west of Berenice, in the remarkable
triple-headed mountain called Gebel Um Rasein, and to the north of
Bir Um Bishtit.
Gneiss typically forms bold scenery of precipitous hills, the
colour of which varies with the composition of the rock, from pink
to grey. At Gebel Migif one ascends the mountain most easily by the
northern slopes, which are formed by the foliation-planes of the
rock, while to the south these are cut across by sheer precipices;
the same is the case at Gebel Um Rasein.
[Illustration: Fig. 52.—Granite-gneiss, Gebel Um Rasein [12,108],
viewed between crossed nicols, × 30. _q_, quartz; _ol_, oligoclase;
_or_, orthoclase; _h_, hornblende; _m_, “mortar” of crushed
quartz and felspar between the larger crystals. The black areas are
quartz in the position of extinction.]
Of gneisses evidently produced by the deformation of granite, we
may take the rock of Gebel Um Rasein [12,108][134] as a type. The
bulk of the mountain is formed of a nearly white acid rock, of
sp. gr. 2·67, with dark spots, resembling the granite of Gebel
Elba in appearance, except that here the dark minerals have a marked
arrangement along definite planes. (Compare the coloured figures of
the two rocks on Plates XXII and XXV). In places the rock contains
thin bands of almost pure biotite, and these are often thrown into
sharp contortions by the strong compression which the mass has
undergone. The strike of the foliation-planes in Gebel Um Rasein
itself is about south-south-east, with a dip of about 20° to the
east-north-east, but the strike swings round to about south at the
south end of the mass, and the inclination of the foliation-planes
become much steeper. The gneiss shows very marked pitting due to
weathering, the holes ranging from a few centimetres to a metre or
more in diameter and depth. A microscopic section of the rock shows
it to be essentially an acid hornblende-granite similar to that of
Gebel Elba, but crushed so that there is a “mortar” of broken up
quartz and felspar between the larger crystals, and the larger where
unbroken frequently show undulose extinction as the result of strain.
[Illustration: Fig. 53.—Diorite-gneiss, Wadi Nugrus [10,386], ×
17. _h_, hornblende, with small enclosures of felspar and sphene;
_pl_, plagioclase felspar; _b_, biotite; _q_, quartz; _ap_, apatite;
_sp_, sphene.]
In the Wadis Nugrus and Gemal there is a great deal of strongly
foliated grey gneiss [10,386-7], derived from the intense crushing
of diorites. A figure from a typical hand specimen of this gneiss,
looking at the edges of the foliation-planes, is given on Plate
XXV. The sp. gr. of the rock is 2·88. The microscopic slides show
plagioclase and green hornblende in about equal proportions, with
some quartz and small amounts of biotite, sphene, iron oxides, and
apatite. The hornblende is very fresh-looking, in strongly pleochroic
(yellowish to deep blue-green) straggling crystals, aggregated
together along the foliation-planes; the crystals, though variously
oriented, show a tendency to lie with their cleavages in the direction
of foliation. A very striking feature of the slides is the abundance
of small grains of enclosed felspar in the hornblende, giving
the green crystals a perforated appearance in ordinary light. The
felspars, which are likewise fairly fresh, are much smashed up,
forming with the quartz a mosaic between the irregular bands of
hornblende; the grains often show undulose extinction. Biotite,
of a warm brown colour where not bleached, is sparingly mixed with
the hornblende. The sphene is in small rounded grains, mixed with,
and sometimes containing, granules of ilmenite; the grains show a
tendency to form little aggregates, sometimes in the hornblende,
and sometimes in the felspathic portions of the slide. One or two
stumpy grains of apatite are enclosed in the felspars, but this
mineral is extremely scarce in the slides examined, and does not
show its usual long prismatic habit.
=PLATE XXV.=
[Illustration: Ball.—Geography & Geology of South-Eastern Egypt.
=METAMORPHIC ROCKS.=
=GRANITE-GNEISS.= Gebel Um Rasein.
=RED GNEISS.= Gebel Abu Beid.
=DIORITE-GNEISS.= Wadi Nugrus.
=GREEN BRECCIA.= Gebel Hamata.
=EMERALD AND QUARTZ IN MICA-SCHIST.= Gebel Sikait.
NATURAL SIZE.]
As an example of gneisses in which the foliation is so intense that
all original structures have been obliterated by recrystallization,
we may take the red gneiss of the Abu Beid Hills [10,659], of which
a coloured natural size representation is given on Plate XXV. In
this rock, which has a sp. gr. of 2·65, the main constituent is
a pink felspar, mixed with a little quartz and arranged in fine
lenticles separated by greenish laminæ which are doubtless altering
biotite. This rock bears a strong resemblance to the well-known
red gneiss of the Erzgebirge, which, from its chemical composition,
is usually believed to be a metamorphosed granite poor in mica.
Grey gneisses, equally intensely foliated with the Abu Beid rock,
occur in the range of Gebel Hafafit; these contain a much larger
proportion of biotite, and remind one of the grey gneisses of the
Freiberg district. But as no analyses have yet been made of the
Egyptian rocks, the absolute similarity to those of the Erzgebirge
is not quite certain.
=Quartzites and Quartz-schists.=
About ten kilometres south of Gebel Abu Gurdi is a tract of low
hills and ridges composed mainly of quartz-schists. The limits
of these rocks have not been mapped, but they appear to cover a
somewhat elliptical area with its long axis, measuring some eight or
ten kilometres, pointing east and west. Where first encountered,
at a point about twelve kilometres east of Gebel Selaia, on
the march from that hill towards the pass into the head of Wadi
Lahami, the rocks strongly resemble rather friable white to brown
fine-grained sandstones, with bedding planes dipping about 35° to
the north. The rock [10,413] consists almost entirely of white or
slightly iron-stained quartz grains, with, in places, scattered
minute dull black rods which are apparently hornblende. Traced
further east, the rocks have more and more the character of
quartz schists, with harder bands of quartzite, and the planes of
separation, which gradually become nearly horizontal, have more the
appearance of foliation-planes. The harder quartzite bands [10,414]
are usually brownish to greenish-white in colour, but sometimes take
on a purple tint, and show micaceous and chloritic scales on cleaved
surfaces; in some places the rock contains “eyes” of minette-like
composition. To the south the bedding or cleavage planes seem to
reverse their dip, this being now to the south as though there were
an anticlinical fold in the mass. There is considerable mixture of
other schists with the rocks, and dykes of diorite and pegmatite,
as well as veins of quartz, are frequent. A dioritic dyke which
cuts through the rocks at a point nine kilometres south-south-west
of Gebel Abu Gurdi, and which is itself crushed almost into a
hornblende-schist, has hardened and altered the quartzose rock on
either side of it into a close-grained horny looking rock [10,415]
apparently containing a great deal of finely granular andalusite.
These quartz-schists may have arisen from the metamorphism either
of sandstones, in which case the parts to the west of the tract
are the least altered, or of fine-grained aplitic rocks, on which
view the sandstone-like aspect of the western portion of the tract
is due to weathering; the rock is certainly more highly weathered
where it most resembles an unaltered sandstone, and the presence of
hornblende and the “eyes” of micaceous matter are suggestive of
igneous rock. From a consideration of all the evidence I was able
to gather in a rapid march over the sea, however, I incline to the
view that the rocks are altered sandstone older than the Nubian
series, and probably of about the same age as the ancient clays
which gave rise to the clay-schists of Zabara and elsewhere. The
rocks are completely detached from the nearest undoubted Nubian
sandstone beds (which lie some forty kilometres to the south-west,
beyond Gebel Zergat Naam), and they are mixed with schists and veined
and altered to a degree which is nowhere approached by beds known
to be of the Nubian series. Still, it is just possible that the
quartz-schists represent the remains of Nubian sandstone strata,
for the nearest beds of that series show-considerable folding and
faulting near Gebel Zergat Naam, and we should expect still greater
disturbance and alteration here near the main watershed and close
to a principal axis of mountain elevation.
A quartz-schist which is certainly derived from the metamorphism
of aplitic dykes or sheets, occurs as great inclined bands cutting
across the Wadi Shenshef close to some old ruins. The main rock of the
district, in which the bands occur, is a medium-grained diorite. The
quartz-schist is a greyish-white fissile rock, which splits easily
into great slabs; it has supplied an excellent building material
for the houses of the ruined town or encampment, being used not only
for the walls but also for lintels over doors and windows.
=Clay-schists.=
Clay-schists cover extensive areas in the South-Eastern Desert. They
occur abundantly in the high hills to the east of Gebel Nugrus and
form a large portion of Gebel Zabara. They abound also to the south
of Wadi Antar about the longitude of Gebel Um Goraf, in the hills
on the east side of Gebel Abu Dahr, in the Wadi Hodein round about
where Wadi Um Tenedba joins it, in the hills flanking the sandstone
plateau of Gebels Dif and Anfeib, at Gebels Kolaiqo and Eqrun,
on the western flanks of the serpentine mass of Gebel Meneiga,
and in the low hills on the plain between Gebels Gerf and Korabkansi.
Clay-schists are typically of a grey colour, though greenish, purple,
and reddish varieties occur. They are soft rocks, easily scratched
with a knife. In some places, as for instance at Gebel Zabara, they
are practically slates, cleaving into slabs often of considerable
size, while in other localities, as for instance near Gebel Abu Dahr,
they are crushed into matchwood-like splinters, which can be used
as slate-pencils. They are usually associated with other metamorphic
rocks such as crushed diorites and hornblende and mica-schists. The
direction of foliation of the slaty and slabby forms is often
persistent—at Gebel Zabara, for instance, a general south-easterly
dip of about 28° is maintained over a considerable area; but there
are numerous local variations, and the number of observations thus
far made is too small for any general conclusions to be drawn as to
the regional distribution of dip. On the whole, the clay-schists are
remarkably free from knots or other irregularities, though knotted
varieties, doubtless the result of contact alteration, have been
observed on the south side of Wadi Seiga and at a few other places.
The clay-schists have probably for the most part been produced by
the metamorphism of ancient argillaceous sediments; in some places,
as for instance in the Wadi Beida, a passage can be observed
into what appears to be a crushed conglomerate, while in other
places they pass into quartzites which doubtless represent altered
intercalated sandstones. The relation between the direction of the
foliation-planes and that of the original bedding is unknown; it
would appear likely that in most cases the traces of bedding have
been entirely obliterated by pressure. Nor have any fossils been
found in the rocks, though a sharp look-out was kept in the more
likely-looking localities, so that the age of the beds is unknown. At
present I am inclined to regard them as most likely of Archæan age,
antedating the great granitic intrusions; but further evidence,
either in the finding of fossils or from a more careful study of
the field relations of these rocks to the others, is required before
one can be certain that they are of so great an antiquity.
=Hornblende-schists and Crushed Volcanic Rocks.=
Under the heading of hornblende-schists and crushed volcanic rocks are
classified a great variety of rocks, ranging in colour from green to
grey and almost black, in fissility from the finest lamellation to
almost massive forms in which the schistosity is barely evident in
the hand specimen, but all definable as fine-grained dark-coloured
schistose rocks consisting largely either of hornblende or of its
alteration products. Their origin is not always clear, but many of
them appear on microscopic examination to be altered fine-grained
syenites, diorites, and lavas, and it is probable that practically
the whole are altered forms of various intermediate and basic igneous
rocks. The reason for including the hornblende-schists and crushed
volcanic rocks in one great group is the difficulty of separating
the different classes in the field. They pass one into the other,
and many rocks which in the field would be put down as hornblendic
turn out on detailed examination to contain little or none of that
mineral, its place being taken by decomposition products.
Schists of the types above defined occur probably more abundantly
than any other class of metamorphic rocks in the South-Eastern
Desert. They enter largely into the composition of some of the
main mountain masses, such as those of Gebels Abu Hamamid and Abu
Gurdi, besides covering great expanses of lower hill country. In
the mountain ranges they rise in steep-sided ridges with knife edge
summits, a good idea of which is given by the photograph of Gebel Abu
Hamamid on Plate VII. In the lower hill country we have typically a
dreary confused looking waste of thousands of dark hills and ridges
separated by small winding wadis; sometimes there is a semblance of
system in the distribution of the hills, owing to the occurrence
of parallel dykes harder than the schists which they penetrate,
forming thus back-bones for long lines of schist-ridges.
[Illustration: Fig. 54.—Schist produced by crushing of syenite,
near Gebel el Anbat (Wadi Hodein). [11,532 B], × 30. _f_, crushed
and clouded felspar, probably orthoclase; _lc_, mixture of calcite
and limonite, produced by alteration of hornblende, no trace of
which remains.]
=Schist derived from Syenite.=—Among the schists occurring
near Gebel el Anbat, near the Wadi Hodein, is one which appears
to be a crushed and altered syenite. It is a speckled grey and
reddish-brown rock with a dull and rather granular fracture, of
sp. gr. 2·92. The slide [11,532 B] shows a rather coarse granitic
mixture of clouded felspar and heavily iron-stained calcite. The
felspar is considerably altered, showing no twinning, but between
crossed nicols it extinguishes in irregular bands and is seen to
be much crushed. The ferruginous and calcareous matter is almost
certainly altered hornblende; it forms irregular masses, in which the
iron oxide is mostly arranged in parallel lines as though along the
cleavage planes of the parent mineral, while a clouded calcite fills
up the spaces between the lines. It would seem that in addition to
pressure, carbonic acid has been the main agent of metamorphism here,
the ferro-magnesian silicate being attacked and its silica entirely
removed, while the aluminous silicate of the felspar has remained
to some extent unchanged.
[Illustration: Fig. 55.—Hornblende-schist (probably a metamorphosed
fine-grained diorite), Wadi Muelih [10,357], × 40. _h_, pale green
hornblende; _f_, mass of decomposed felspar, containing abundance of
finely acicular hornblende and some little patches of quartz mosaic;
_m_, magnetite.]
=Schists derived from Diorites.=—Hornblendic schists which appear
to have been produced by the metamorphism of fine-grained diorites
occur largely in the low hill country round the upper parts of
the Wadi Muelih, and in the mountain mass of Gebel Abu Gurdi. A
specimen of the less altered rock from the Wadi Muelih [10,357] is
a fine-grained hard grey basaltic-looking rock, of sp. gr. 3·04,
in which the schistosity in not very evident, though it is well seen
in the mass. The microscopic slide shows pale green hornblende in
ragged fibrous forms in a matted-looking clouded ground mass made up
of finely acicular green hornblende and plagioclase, with a little
quartz and orthoclase and a few grains of magnetite. Hornblende fibres
are often enclosed in the decomposing felspars, which rarely show
definite outlines and appear much shattered. The quartz is probably
of secondary formation, occurring as little patches of mosaic.
The schist which forms the summit of Gebel Abu Gurdi [10,416]
is probably also a crushed and altered diorite; it is a rather
fine-grained hard greenish-grey rock, which with a lens can be seen
to be a mixture of greenish-white felspathic material and dark
hornblende; the felspar is mostly dull, while the hornblende, on
the other hand, is frequently in shining crystals. The sp. gr. of
the rock is 3·02. The microscopic slide shows the hornblende
to be of a very pale green colour, in large irregular crystals,
often including felspars. The felspar appears to be plagioclase;
it is typically in smaller crystals than the hornblende, with a
strongly marked tendency to idiomorphism, highly cracked and almost
entirely changed to kaolin. Some rather large straggling crystals
of a dark brown clouded and semi-opaque highly refracting mineral,
white by reflected light, are perhaps altered sphene. Round the
larger crystals is more finely crystalline matter, much clouded,
apparently composed of altered felspar and hornblende.
The crushed dioritic dykes already mentioned as cutting the
quartz-schist to the south-west of Gebel Abu Gurdi are possibly
offshoots from the same magma which formed the main mass of the
mountain. Specimen [10,415], taken from one of these dykes, is a
fine-grained grey rock, of sp. gr. 2·93, with an even more decided
schistosity than that of the main mountain. The microscopic slide
shows clear hornblende of a green colour, strongly pleochroic
(greenish-yellow to blue-green) in irregular grains which are
frequently aggregated into nests and strings running in the direction
of foliation of the rock; the remaining material is a mosaic of
clear quartz, clouding kaolin, and sericite, presumably representing
altered felspar.
[Illustration: Fig. 56.—Schist composed of fragments of various
volcanic rocks, summit of Gebel Abu Hamamid [10,397], × 30.]
=Schists formed by crushing of Volcanic Rocks.=—Schists derived
from the crushing of volcanic rocks are very abundant in the mountains
round Gebel Abu Hamamid and in the Wadi Beida. Typical specimens from
the summit of Gebel Abu Hamamid [10,397] are hard green to grey rocks,
of sp. gr. 2·7, breaking with a rough dull fracture. With a lens,
spots and strings of dull white matter, with ill-defined outlines, are
seen in a green to nearly black ground mass. The microscopic slides
show the rock to be a breccia rather than a simple crushed rock,
for in the same slide very various structures can be seen. Some
portions, evidently andesitic, consist of perfectly idiomorphic
lath-shaped felspars scattered with a little decomposed hornblende
in a glassy brown ground mass. Others, more abundant, seem to be
altered quartz-diorite-porphyrite; in these parts, porphyritic quartz
and orthoclase crystals, in forms strongly inclined to idiomorphism,
clouded and strained, are scattered in a cryptocrystalline ground mass
containing a good deal of green chlorite and epidote. In other parts
of the slide, again, the chief porphyritic constituent is formed
by large green grains, which have evidently once been biotite or
hornblende, but which now consist of chlorite. In yet other parts of
the rock we have fragments of devitrified glassy lava. The different
parts, which are not always well outlined, are separated by schistose
bands composed mainly of strings of chlorite and epidote. Whether the
rock is a tuff, or a crushed conglomerate of igneous boulders and
pebbles, or due to complicated crushing of a series of contiguous
igneous rocks _in situ_, is not quite clear. In some places,
especially about the Wadi el Sheikh, the schists look like crushed
conglomerates, but these may possibly be rocks crushed _in situ_
rather than accumulations of rolled fragments transported by streams.
The hornblende-schists which surround Gebel Zergat Naam are of a
peculiar type of which the origin is not evident, but are possibly
altered andesites. The typical rock [11,526] is a hard grey basaltic
one of sp. gr. 2·95, with a rusty-brown skin on exposed surfaces. The
microscopic slide shows little plates and brushes of nearly colourless
hornblende, liberally scattered in a clouded ground mass consisting
mainly of hornblende fibres in radiating and dendritic groups, with
a small amount of kaolinic matter and a few grains of magnetite. The
slide contains no distinct quartz, but possibly a small amount of this
mineral may be present in very minute grains with the kaolinic matter.
To the metamorphism of andesitic lavas, too, are somewhat doubtfully
ascribed the grey and green schists of the Wadi Beida, which have
specific gravity of about 2·75. The slides from the grey variety
of the schists [12,116 and 12,159] show a very fine-grained clouded
rock, apparently a mosaic of quartz, kaolin, chlorite and sericite,
with larger scattered irregular plates of dark green chlorite. In
a pale green variety from the head of the wadi [12,111], there is
less chlorite and a considerable amount of calcite.
=Hornblende Schists of doubtful origin.=—In the more highly
foliated and harder varieties of hornblende schist, which are
typically of a darker colour than most of those already described,
we have rocks in which the process of re-crystallization has been
so complete that no trace of the original rock remains. These rocks,
which are true hornblende-schists in the narrowest sense of the term,
are less abundant than the foregoing types, and generally occur as
comparatively narrow bands associated with gneisses and mica and talc
schists. All the minerals in them being of secondary origin, they are
usually in a quite fresh state, and fractured surfaces, examined with
a lens, exhibit a mass of glistening small crystals of hornblende,
quartz, and felspar. The density is usually about 2·9. Microscopic
slides show elongated crystals of clear green hornblende, strongly
pleochroic (pale yellowish-green to deep blue-green) arranged along
the planes of schistosity, separated by clear granules of quartz
and a little felspar, with scattered magnetite grains.
[Illustration: Fig. 57.—Hornblende-schist, near Gebel Eqrun
[12,117], × 30. _h_, hornblende; _q_, quartz; _m_, magnetite. A
little felspar is present mixed with the quartz, from which it is
distinguishable only in polarised light.]
Near Gebel Eqrun are found hornblende-schists which exhibit a curious
banding in planes at right angles to the main foliation, in the form
of darker lenticular stripes a few millimetres wide and about the
same distance apart. A slide [12,117] cut from this variety shows
little trace of the banding, the lighter spaces between the dark
bands merely showing a clouding of the quartz and felspar by tiny
granules of epidote. The stripes are most probably the consequence of
a secondary compression in a direction perpendicular to the original
one rather than relics of a banded structure in the parent rock.
=Actinolite-schists.=—Very beautiful bright green schists, in which
the hornblende is in the fibrous to silky form called actinolite,
occur in small quantity associated with mica and talc schists at
Sikait [10,380] and elsewhere. In these rocks the actinolite fibres,
which often reach two centimetres in length, are generally aggregated
into bundles, with radiating structure. In the microscopic slide
the rock presents even a more beautiful appearance than in the mass,
the long fibres of actinolite polarising in the most brilliant tints;
associated with the actinolite, there is nearly always more or less
chlorite and talc.
=Hornfels.=
Associated with the schists of Gebel Abu Hamamid and the neighbouring
mountains there are great masses of very hard horny-looking rock
[10,401] of green to grey colour, breaking with a sub-conchoidal
fracture, and of such close texture as to appear homogeneous even
with a strong lens. The pyramidal peak called Gebel Um Semiuki, which
rises to 1,282 metres above sea, three kilometres to the north-east
of Gebel Abu Hamamid, is almost entirely composed of rocks of this
type; in the mountain faces the rock looks red, but this is only
due to a film covering weathered surfaces, the interior being of
a green to grey colour. The rock, which has a sp. gr. of 2·71, is
frequently beautifully banded, light and dark layers alternating with
each other, and often contains tiny cubes of pyrites [10,399]. The
microscopic slide from Gebel Um Semiuki shows a very fine-grained
clouded compact rock, apparently consisting of glassy matter with
minute granules of quartz and altered felspar, together with a little
sericite, the latter especially along certain bands. The slide from
Gebel Abu Hamamid is similar, but here the granules of quartz and
felspar are a little larger, though they are still too small to be
seen with a lens in the hand specimen; the appearance is that of
a quartz felsite on a small scale. It has already been mentioned
(p. 281) that the quartz felsites of Gebels Igli and Hadarba pass
gradually into hornfels, and when we remember that the schists of the
Abu Hamamid district are mostly crushed volcanic rocks, it becomes
almost certain that the hornfels associated with them is a crushed
and devitrified glassy lava of acid composition.
A yellowish horny rock with grey streaks [10,379], which occurs near
Gebel Sabahia, is conspicuous in the field owing to its weathered
surfaces being covered with a bright red ferruginous skin, resembling
cinnabar in colour. The sp. gr. is 2·52. Examination with a lens
shows the grey streaks to be filled with myriads of brilliant yellow
specks of pyrites. The microscopic slide shows these to be aggregates
of little cubes, while the bulk of the rock is a schistose felsitic
mass of quartz and felspar, with scattered larger felspar crystals,
much broken up and bent. In this rock too we have therefore a rolled
up and altered felsite.
=Mica-schists.=
Mica-schists, composed mainly of golden-brown lustrous laminæ
of biotite with more or less quartz, occur near the base of Gebel
Zabara and at Gebel Sikait [10,626], as well as in small quantity
at one or two other points. They are always associated with gneiss,
and appear to form irregular bands, alternating and mixed with talc
and other schists. The laminæ of mica can seldom be separated in
any large size, breaking up at a touch into small scales; they are
often highly contorted.
[Illustration: Fig. 58.—Beryl and quartz, from a lenticle in
mica-schist near Sikait [10,580], × 30. _b_, beryl in idiomorphic
hexagonal crystals; _q_, quartz, allotriomorphic.]
=Emeralds= (Beryl).—At Zabara and Sikait the mica-schists
contain crystals of beryl (silicate of beryllium and aluminium,
Be3Al2Si6O18), the clear variety of which forms the gem emerald. The
beryls are mostly found in lenticular bands of quartz which occur
in the mica-schist, but sometimes they can be seen in the schist
itself. The crystals are mostly well developed hexagonal prisms of a
pale emerald-green colour, with characteristic vertical striation. The
coloured figure on Plate XXV will give a good idea of the usual
appearance of the mineral. In microscopic slides (_see_ Fig. 58) the
beryls are conspicuous only by their clear cut hexagonal outlines;
they are quite colourless, with low polarisation colours about
the same as those of quartz. Both at Zabara and Sikait there are
numerous ruins and ancient mines where emeralds have been sought;
most of them are irregular shafts and tunnels, twisting about as
the old miners followed the varying directions of the bands of
schists. It is commonly believed that gem emeralds were at one time
extracted from these mines, and it seems incredible that the mining
should have been carried on to so great an extent as is shown by
the ruins and old workings, unless stones of considerable value were
obtained. The Zabara mines were re-opened by Cailliaud in the time of
Mohammad Ali Pasha (1817), but the stones extracted were of little
value, being clouded and full of flaws. A similar result followed
a more recent (1904-5) vigorous attempt by Mr. James, acting on
behalf of Mr. Edwin Streeter, of London, to work the emerald mines
of Sikait; plenty of beryls were found, but none clear enough to
be of any great value, and the enterprise was abandoned, Mr. James
concluding that either the ancient miners had worked out all the
bands containing stones of any value, or else, what is perhaps more
likely, the ancients were satisfied with a duller stone for a gem
than our modern jewellers. The dull forms of beryl are in our own
day of very little value, being principally used as a source for
beryllium salts in chemical laboratories.
[Illustration: Fig. 59.—Tourmaline crystals in graphitic
talc-schist, Sikait mines [9,908], × 17. _t_, tourmaline crystals,
irregularly cracked and clouded; _g_, talc-schist, heavily clouded
by graphite.]
=Tourmaline.=—Besides beryls, the mica and talc schists of Sikait
contain in places abundance of black tourmaline in well-developed
crystals. At some spots this mineral is so plentiful as to form
practically small patches of tourmaline-rock [10,395]. In thin section
[9,874 and 9,908] the tourmaline crystals, which are much clouded
and irregularly cracked, show beautiful pleochroism (colourless to
deep orange), and very high double refraction colours in prismatic
sections. Like the beryl, however, tourmaline is only of value as
a gem when it is clear and transparent, and all the crystals so far
obtained are dull and opaque.
Calcite, in rhomb-shaped crystals of a brown colour due to presence
of included iron oxides [10,382] likewise occurs in places in the
mica-schists of Sikait.
=Chlorite-schists.=
Though many of the decomposed hornblende-schists contain more or
less chlorite, I have only in two localities come across rocks
_in situ_ which contain so large a proportion of this mineral as
to deserve the name of chlorite-schists. The first is in the hills
of Um el Huetat (latitude 25°), where typical chlorite-schists are
mixed with mica, talc, and hornblende schists. The second locality
is between Gebels Ras Shait and Nugrus, where the rock [10,388]
is remarkable not only in its peculiar appearance but also by its
strongly magnetic character. It is a thoroughly schistose rock of
a rather pale greyish-green colour and rather silky appearance,
with rusty looking spots. The sp. gr. is 2·77. The microscopic
slide shows the stone to consist essentially of an aggregate of
elongated plates and fibres of low double refraction, which from the
hand specimen seem to be chlorite, but in the slide look more like
antigorite. Magnetite grains are liberally scattered through the
chloritic mass. The rusty spots visible in the hand specimen are
translucent foxy red in the slide, in irregular broken forms with
well-marked cleavage and nearly straight extinction. They are somewhat
doubtfully regarded as deeply iron-stained hornblende. Mixed with the
foxy red material are aggregates of granules of a highly refracting
but isotropic mineral of deep bottle-green colour (? spinel),
and chloritic wisps. There are also some clear colourless grains,
resembling apatite in appearance except that they sometimes show
well-marked vertical cleavage and high extinction angles; these are
possibly a colourless augite, but the double-refraction colours are
far lower than is usual with this mineral.
Typical chlorite-schists occur in the Wadi Salib Abiad, and near
Gebels Ribdab and Muqsim, in the extreme south-west portion of the
region. Hearing, from some wandering Arabs while at Gebel Abu Dahr in
February 1907, that prospectors were at work in the Wadi Salib Abiad,
I sent a guide to find out who they were and what they were doing;
the guide reported that on his arrival they had gone away, but there
were some old workings in a green rock of which he brought a sample. I
did not get an opportunity of visiting the locality personally,
but the specimen brought back by the guide [11,523] is a beautiful
apple-green chlorite-schist, with some brownish calcareous-looking
bands. The microscopic slide shows some little quartz and talc besides
the chlorite, and there are scattered minute highly-refracting rounded
grains of a feebly translucent mineral of a reddish-brown colour,
probably rutile. A similar rock from the eastern side of Gebel Muqsim
has been reported by Mr. Charteris Stewart, who also records a normal
chlorite-schist as occurring on the north side of Gebel Ribdab.
=Talc-schists.=
Talc-schists, though not widely distributed, are abundant in certain
localities. They form the main rock in many of the hills in the
district called Um el Huetat, to the west of Gebel Atut in latitude
25°, where there are numerous old mines. In this district there are
immense masses of talc-schist of remarkable purity. The rock [10,364]
is of a grey to green or brown colour, distinctly schistose, cleaving
easily with a soapy feel, and easily scratched by the finger nail;
more massive forms occur which can be easily carved into pipes, and
these are frequently smoked by the Bedouin. I descended one of the old
shafts at Um el Huetat, and found no evidence of other minerals having
been worked than the talc itself. A somewhat more earthy variety of
talc-schist [10,396] was met with near the Rod el Ligaia. Talc-schists
also occur at Gebel Sikait [10,383] and Gebel Zabara, where, as at
Um el Huetat, they alternate with mica and hornblende schists. But
at Sikait and Zabara the talc-schists are decidedly subordinate to
the mica-schist in which the beryls are found.
To the class of talc-schists is also probably best referred a fissile
and very rotten purple-brown schist, containing cubes of limonite,
which was found near the triangulation station on Gebel Hamida,
and again near the old mines of Romit. The microscopic slide of
the Romit specimen [12,139] shows the bulk of the rock to consist
of talc and chlorite, with blotchy stains and floculent-looking
masses of brown iron oxide and occasional clear irregular granules
of quartz. In this are plentifully embedded perfect little cubes of
limonite. The limonite cubes [12,129 and 12,136] weather out easily
from the rotten matrix, and can be gathered from the ground at the
foot of the exposed surfaces.
=Calcareous Schists and Marble.=
Some very curious calcareous schists are found in Gebel el Anbat and
its neighbourhood, near the Wadi Hodein. In Gebel el Anbat itself a
mass of these rocks rises to a height of over 250 metres above the
wadi level. They are earthy-looking rocks of varying colour, chiefly
brown and reddish, often purplish on the rough weathered surfaces,
and sometimes have a talcose feel. A typical specimen [11,532 A]
has a sp. gr. of 2·92, and the slide cut from it shows it to be
almost entirely composed of grains of calcite, with here and there
larger irregular granules of quartz and of what looks like altered
felspar, and a liberal sprinkling of iron oxide. It is difficult to
assign an origin to this rock, but a variation of it [11,532 B] has
been already described (p. 339) as probably an altered syenite, and
the main rock may therefore represent the extreme form of alteration
of an igneous rock rich in lime felspars.
A hard close-grained reddish-brown rock of sp. gr. 2·91, with dark
streaks [12,107], which forms a high ridge, swathed in blown sand,
rising to 350 metres above sea-level near the Wadi Kreiga, eleven
and a half kilometres to the south-east of Gebel Beida, resembles a
quartzite in appearance, but turns out on careful examination to be a
calcareous schist. The microscopic section shows a very fine-grained
mosaic of calcite with a little quartz, and scattered grains and
strings of iron oxide. The origin of this rock is uncertain; it may
be a metamorphosed limestone.
The summit of a high hill rising to 686 metres above sea, on the
east side of the Wadi Um Khariga in latitude 24° 56′ 30″, is
a gozzany mass resembling the outcrop of a mineral vein traversing
the schists. On a fresh fracture, the interior of the rock [10,369]
is seen to consist mainly of dark crystalline calcite, with some
cubical crystals of pyrites, numerous rusty looking spots and patches
of limonite, and veinlets of white calcite. The microscopic slide
shows a mixture of calcite with kaolinic and serpentinous matter,
with a very pronounced schistose structure, containing “eyes”
of mixed calcite and iron oxides; the rock is therefore in reality a
ferruginous calcareous schist. The mass is too highly metamorphosed
for more than a guess as to its origin; but the slide contains some
granules resembling picotite and one or two small patches of what
looks like altering felspar, and the suggestion is that the schist
is a metamorphosed basic igneous vein.
=White crystalline marble= has been found only at one point, namely,
about three kilometres south of the jagged peak called Qash Amir,
west of Gebel Elba, where it forms a small patch in crystalline rocks.
=Graphite-schists.=
A considerable mass of graphite-schist occurs associated with diorite
rocks near some ancient gold workings at Gebel Allawi. The extent of
the graphite-schist has not been mapped, but it occurs on the north
slope of the mountain, and as seen from the summit it appears to run
out westward for miles as a broad black band. The rock [10,378],
which has a sp. gr. of 2·70, consists of a mixture of graphite
with calcareous and earthy matter. The parts richest in graphite are
readily cut with a knife, and mark paper easily. Analysis shows that
the graphite, though sufficiently abundant to appear in the hand
specimen the dominant mineral of the rock, is nevertheless present
in surprisingly small proportion; a sample tested by Mr. Pollard
was found to contain only a trifle over one per cent. of carbon.
A similar graphite-schist occurs associated with mica-schist in
one of the emerald mines of Gebel Sikait, and it may be remarked
that graphitic schists have also been observed by Dr. Hume near
the lodes in the gold mines of Baramia and Um Garaiart, both of
which localities, however, lie outside the limits of the area here
described.
The origin of the graphite-schists is uncertain. Perhaps the
hypothesis presenting the least difficulty is that which supposes
them to have been formed from ancient sedimentary rocks containing
the remains of plants; on this view the close association of dioritic
rocks suggests that the great of igneous intrusion may have played a
part in the distillation of the more volatile matters of the plant
remains, while the schistose structure shows that pressure has
also been active in the formation of the rock. But the absence of
any associated rocks which bear distinct signs of having originally
been ancient sediments makes one hesitate to refer the graphite to
an organic origin, and possibly the carbonaceous matter may have
been produced from quite another source than that of plant life.
=Breccias.=
A very curious rock [10,385] which from a distance looks like a giant
diorite, occurs as a band in gneiss on the west side of the lower
part of Wadi Nugrus, is doubtless a fluidal breccia. It contains
ovoid masses of white aplite, sometimes measuring five centimetres
in diameter, embedded in a dark fine-grained dioritic matrix with
a marked tendency to schistose structure. Most likely the rock has
been formed by the catching up of the fragments of a crushed aplite
in an intrusive diorite, and then the whole mass has been subjected
to the same pressure which foliated the surrounding gneisses.
More normal breccias are found at various points. One which occurs
between schists and hornfels near the summit of Gebel Abu Hamamid
[10,398] is made up of fragments of various altered volcanic rocks,
with large black flint-like lumps of hornfels, all cemented into a
very hard rock which breaks across the fragments composing it. This
breccia was doubtless formed by the same movements which produced the
schists of the summit of the mountain; these latter (_see_ p. 341)
are themselves almost as much fine breccias as schists.
To the west of Gebel Zergat Naam the stones in the wadis are sometimes
cemented into hard breccias by calcareous matter, doubtless deposited
by drainage waters which have dissolved out the lime from felspathic
rocks.
In the hill called Ti Keferiai, a little below the triangulation
point which marks the summit, a highly altered fine-grained dioritic
rock, containing much epidote, has been crushed into a coarse breccia
[12,123] cemented by rose-coloured quartz.
A remarkable green breccia is found in Gebel Hamata, where it appears
to form a large mass in the mountain-side to the east of the main
peak. This rock [10,407], which has a sp. gr. of 2·92, is darker
in colour and somewhat softer than the ornamental “breccia verde
antico” of the Wadi Hammamat district further north,[135] but it
also is a very beautiful rock. In the hand specimen, it consists of
black angular fragments, up to two centimetres in diameter, embedded
in a dark green ground mass, the whole of very fine grain and barely
scratchable with a knife. Under the microscope, the black fragments
seem to be of basaltic nature, while the green matrix is probably a
highly crushed and brecciated diorite-porphyrite; the whole of the
slide is clouded by decomposition products.
=Fault-breccias=, produced by differential movement of the two sides
of faults, occur in the neighbourhood of the Wadi Saalek, where the
sandstones and schists are much faulted (_see_ p. 359). The breccias
here [11,539] are narrow bands which stand up like dykes; they are
very calcareous and highly ferruginous, with occasional green stains,
perhaps due to traces of copper.
=Mineral Veins.=
Quartz veins deposited by solutions[136] in cracks and fissures
of the igneous and metamorphic rocks are very numerous and widely
distributed, especially in the Sukari district. They vary immensely in
size, from mere strings to veins two metres or more in thickness; they
cut the rocks in every direction and at every angle of dip. Steeply
inclined veins are by far the most numerous, but some have a flat
inclination and are then styled “reefs” by the miners.
The principal interest attaching to the quartz veins is the fact that
they frequently contain gold, though seldom in particles visible
to the unaided eye. The quartz veins were worked for their gold by
the ancients, the remains of whose dwellings and stone grinding pans
are found at numerous places, as for instance in the Wadi Hangalia,
at Kurdeman, near Gebel Sabahia, at Gebels Sukari and Allawi, in
the Wadi Lewewi, and at Romit and Darahib. Our modern prospectors
have found these ruins of ancient mining camps and grinding pans
to be the best guide to auriferous veins, gold being seldom found
except in and near the old workings. As mentioned on p. 27, the
ancients worked the mines by convict labour, and they could for
that reason afford to work ores which are too poor to pay under
modern conditions. But in certain cases the veins have been found
rich enough to give possibilities of a commercial return to modern
mining enterprise; a list of the prospecting licences and mining
leases now in force is given on p. 28. As to the source of the gold,
it is not known whether it came up in solutions from below, or has
been secreted laterally from the country rock.
Besides gold, some of the quartz veins contain traces of copper,
but none of the occurrences of copper ore within the area specially
treated of in this book appear to be capable of yielding any
considerable quantity of the metal, most of them in fact being
mere stains due to oxidation and carbonatisation of traces of
sulphides.[137]
Calcite veins are much more rarely met with than those of quartz. In
only one of the veins I have examined is calcite present in any
considerable quantity, namely in the vein of the old gold mines of
Romit. In this vein, white to brown crystalline calcite is found
mixed with chalybite, limonite, and smoky quartz [12,105 and 12,141],
the last-named only being apparently auriferous. There did not
appear to be enough chalybite and limonite in the vein to make it
worth following up for iron ore, especially in view of the expense
of transport from the place.
Magnesite and asbestos veins occur in the serpentines of the Gebel
Gerf district. These occurrences, which appear not to be large enough
to be worth working, have been described on p. 330.
* * * * *
[Footnote 134: The figures in square brackets in this and the
preceding chapter are the numbers under which the specimens are
registered in the Geological Museum, Cairo.]
[Footnote 135: _See_ Barron and Hume’s “Eastern Desert.” Cairo,
1902. p. 263.]
[Footnote 136: Other quartz veins which probably originated in quite
a different manner are treated of under the heading of Igneous Rocks
_see_ (p. 266).]
[Footnote 137: The copper smelted in ancient times at Kubban, on
the east bank of the Nile opposite Dakka, in latitude 23° 10′,
was possibly obtained from mines at Abu Seyal (sometimes misspelt
Absciel), north of the Wadi Alaqi in latitude 22° 47′, longitude
33° 44′, where there are extensive old workings, Abu Seyal lies
outside the region described in this volume; a reference to the
mines will be found in the _Report of the Egyptian Department of
Mines for 1906_, p. 34.]