olivine) are widely distributed in South-Eastern Egypt, entering
largely into the composition of some conspicuous mountains such as
Gebels Atut, Madaret Um Gamil, Um Gunud, Um Bisilla, Dahanib, Gerf,
and Hadal Aweib Meisah, and also occurring in smaller patches at
various other points.
Though they are all dark-coloured, tough and heavy rocks (specific
gravity from 2·8 to 3·2), the gabbros vary very much in appearance
at different places owing to variations in size of grain and
in mineral composition. Thus we have every gradation from the
coarse-grained gabbros such as those of Um Bisilla and Gerf, where
the individual crystals measure sometimes two or three centimetres in
length, through the medium grained rocks of Atut, Dahanib and Hadal
Aweib Meisah, to the granulitic gabbro of Kolmanab hill, of which
the grain is so fine that the rock looks almost like a basalt. In
mineral composition, the gabbros show likewise great variety. Some,
like the rocks of Um Bisilla, are relatively rich in felspar, and
are lighter both in colour and in weight than others in which the
pyroxenes predominate; in some cases the proportion of felspar almost
vanishes and the rock passes into a pyroxenite. Some of the gabbros,
such as those of Atut, contain olivine, while others, such as the
rocks of Gebel Dahanib, do not. The nature of the pyroxene varies,
being sometimes almost entirely diallage, while in others it is mainly
ordinary augite, and in others, again, rhombic pyroxenes such as
bronzite and hypersthene occur. A further variation is the presence of
hornblende in some gabbros, either as an accessory primary constituent
or as an alteration product of a pyroxene. In the uppermost rock of
Gebel Um Bisilla we have an example of troctolite, a form of gabbro
in which there is no pyroxene but only felspar and olivine.
In the field, mountains and hills formed of gabbro are typically of
dark aspect, though frequently less dark than a freshly broken surface
of the rock, owing to a film of iron-oxide which forms on weathered
faces. This film is most strongly marked in the olivine-bearing
varieties of the rock; it is very thin, good sound rock being usually
found at a depth of a millimetre or so below the exposed surfaces. In
form, hills of gabbro are usually in the form of flattish cones
and ridges, whose surfaces and summits are covered with a debris
of rusty-looking weathered blocks of the rock. This blocky type of
summit is well seen at Gebel Atut (_see_ the view on Plate X, p. 172).
Though sometimes sharply marked-off from the adjacent rocks, gabbros,
when traced laterally in the field, are most frequently found to pass
gradually into more basic forms such as pyroxenites, amphibolites,
and serpentines. It is not always easy in the field to distinguish
between augite or diallage and hornblende, and one or two rocks
which were taken for gabbros turn out on microscopic study to be
really basic diorites or hornblende-rocks; while a rock at Um Eleiga,
which strongly resembles a rather fine-grained diorite in appearance,
turns out to be a gabbro. The limit between gabbros and peridotites
is exceptionally difficult to map, the proportions of olivine, augite,
bronzite, and felspar changing very frequently in the same rock mass,
as for instance at Gebel Gerf.
Almost all the gabbros contain a considerable amount of magnetite as
an accessory constituent, and in some cases, as at Gebel Hadal Aweib
Meisah, magnetite is present in such quantity as to render the rock
strongly magnetic. Compass readings in the neighbourhood of large
masses of gabbro are almost always subject to more or less error
from this cause. In the case of a gabbro discovered by Dr. Hume to
the west of Gebel Ranga, near the coast about latitude 24° 24′,
concentration of the ferruginous matter has gone on to such a degree
as to give rise to deposits of hæmatite containing 39 per cent
of iron.[132]
Perhaps the most striking feature evident in the microscopic slides
cut from the gabbros is the remarkable freshness of the felspars
in most of the specimens, which, taken in conjunction with the
basic nature of the rocks, inclines one to consider the basic
rocks as probably on the whole amongst the youngest of the plutonic
masses. Another characteristic feature is “celephytic” structure,
in which a shell of green hornblende is found to surround the iron
oxides and pyroxenes when they are embedded in, or in contact with,
the surrounding felspar.
[Illustration: Fig. 28.—Gabbro, Gebel Dahanib [11,509], × 17. _f_,
felspar (labradorite); _a_, augite; _d_, diallage; _b_, bronzite;
_h_, hornblende, probably produced by alteration of augite.]
A typical olivine-free gabbro [11,509] forms the main rock of
Gebel Dahanib. A specimen taken from the summit, where the rock is
rather finer in grain than that of the rest of the mountain, is very
hard and heavy (sp. gr. 3·15), and is formed of a mixture of dark
dull-looking mineral with shining white to colourless felspars. Under
the microscope it is seen to be a holocrystalline aggregate of fairly
fresh labradorite and pyroxene, both in allotriomorphic forms, in
about equal proportions. About half the pyroxene is in the form of
diallage, the remainder being mostly ordinary augite; it is almost
colourless in thin section, but some of the crystals show a slight
pleochroism, colourless to pale pinkish-brown. The augite crystals are
much cracked, and frequently show signs of alteration with formation
of calcite, epidote and serpentinous matter. There are a few small
irregular areas of very pale green hornblende mixed with the augite,
of which they may possibly be products of alteration. One or two
elongated crystals in the slide, barely distinguishable from the
augite under ordinary light, show a fibrous structure and straight
extinction with low double-refraction colours; these are probably
bronzite. There are only a few very tiny grains of iron oxide,
and olivine appears to be absent from the slide examined.
[Illustration: Fig. 29.—Gabbro, Um Eleiga [11,527 A], × 17. _pl_,
plagioclase; _pc_, clouded plagioclase; _a_, augite; _m_, magnetite;
_s_, serpentinous matter probably from alteration of augite and
hornblende.]
A fine grained gabbro free from olivine occurs associated
with diorite round the old mines of Um Eleiga [11,527 A]. It
is a speckled black-and-white rock which would at first sight
be taken for a fine-grained diorite rather than a gabbro. Its
sp. gr. is 2·93. On microscopic examination the rock is found
to be a holocrystalline aggregate of plagioclase, augite, altered
hornblende, and magnetite, with granitic structure. The plagioclase
(labradorite) is considerably altered and clouded by kaolinic
matter, especially near the centres of the crystals, but still
shows its characteristic twinning clearly. The augite, which is
sometimes in the form of diallage, is fairly abundant, mostly in
irregular grains, though occasionally inclining to prismatic forms;
it is nearly colourless, but much cracked and slightly clouded. A
clouded pale green to brown mineral, which sends off long tongues
into cracks in the surrounding felspars, is also fairly abundant. This
mineral polarises in yellows and greys as a confused serpentine-like
aggregate of minute fibres, but contains clear and nearly colourless
areas representing the original mineral from which it is derived;
these clear areas, in which prismatic cleavage is usually well marked,
are sometimes augite, but in many cases they polarise in lower colours
(greys and yellows) than the augite, with low extinction angles, and
are probably hornblende. Magnetite is very abundant in quite large
irregular grains, often surrounded by a thin shell of hornblende or
of the clouded alteration product just mentioned. There are a few
small six-sided prisms of apatite, mostly included in the felspars.
[Illustration: Fig. 30.—Hypersthene-gabbro, Hadal Aweib Meisah
[12,126], × 4. _f_, felspar (labradorite); _a_, augite; _hy_,
hypersthene (the augite and hypersthene have the same appearance in
ordinary light); _m_, magnetite; _hb_, hornblende, forming celyphitic
borders round the augite and magnetite.]
Another variety of fine-grained gabbro, likewise free from olivine,
but containing hypersthene and some hornblende [12,126], forms
the upper portion of Hadal Aweib Meisah, and a very similar rock
[11,521] occurs in the hills five kilometres south-south-west of
Marwot Elemikan. The rock is very hard, of a grey colour on fracture,
weathering to blocks which have a rusty-brown skin. Its sp. gr. is
2·98. It is highly magnetic, and causes great disturbance of
the compass needle in its neighbourhood. The hand specimen shows
a mixture of shining white felspars with duller black minerals
(_see_ Plate XXIV). On microscopic study, the rock is found to be
a holocrystalline aggregate of plagioclase, augite, hypersthene,
hornblende and magnetite. The plagioclase, which forms about
two-thirds of the rock, is a very clear and fresh labradorite of
a rather acid type. The augite is usually in more or less rounded
grains, often aggregated into irregular strings and mixed with
hypersthene and magnetite. Only in a few cases does the augite show
diallagic lamellation. It is nearly colourless, with a slight greenish
or pinkish-brown tinge and faint pleochroism. The prismatic cleavages
are usually distinct, and in addition the crystals are irregularly
cracked. Twinning is fairly frequent, as also are inclusions of
magnetite in the augite. The hypersthene occurs in grains similar
to those of the augite, with which it is mixed, and from which at
first sight it is not easily discriminated; but it can be picked
out by its more marked pleochroism, straight extinction, and lower
double refraction. Magnetite is abundant in large irregular grains,
and in smaller granules included in the augite. Hornblende occurs
in subordinate amount to the other minerals, and is principally
seen as a celyphitic zone round the magnetite and augite crystals,
especially between these crystals and the felspars. It is usually
fairly clear, strongly pleochroic (deep greenish-brown to very pale
yellowish-brown), and where surrounding two or three grains of other
minerals the whole zone extinguishes at once, showing it to be a
single crystal.
=PLATE XXIV.=
[Illustration: Ball.—Geography & Geology of South-Eastern Egypt.
=BASIC AND ULTRA-BASIC IGNEOUS ROCKS.=
=OLIVINE-GABBRO.= Gebel Um Bisella.
=FINE-GRAINED GABBRO.= Hadal Aweib Meisah.
=TROCTOLITE.= Gebel Um Bisella.
=DIABASE.= Rod el Nagi.
=OLIVINE-BASALT.= Einiwai Hill.
=SERPENTINE.= Gebel Korabkansi.
NATURAL SIZE.]
[Illustration: Fig. 31.—Olivine-gabbro, Gebel Um Bisilla [11,514],
× 10. _pl_, plagioclase felspar (labradorite); _d_, altered diallage;
_o_, olivine, altered in places to serpentine (_s_), with separation
of granules of iron oxide.]
The main portion of Gebel Um Bisilla is formed of a gabbro
[11,514] containing a relatively small proportion of pyroxene and
a considerable amount of olivine. In the mass, it is a hard tough
rock, consisting of a mixture of white to greenish felspars, showing
plagioclastic twinning with the lens, with dull dark minerals, some
of a greenish colour and others of a rusty-brown appearance (_see_
Plate XXIV). The sp. gr. of the rock is 2·8. The labradorite, which
forms about two-thirds of the whole, is very fresh, but is here
and there decomposed with formation of calcite along cracks. The
pyroxenic constituent is now mostly represented by dusty looking
very pale greenish-brown straggling crystals interstitial to the
felspars; it appears to have been originally diallage, but is in an
advanced stage of alteration, polarising as a confused fibrous mass
of hornblende and chlorite, with irregular banding in a direction
inclined at about 30° to the general direction of the fibres. The
olivine is in large rounded grains, nearly colourless where unaltered,
showing the usual irregular cracks; some of the grains are altered to
yellowish-green serpentine, with separation of granules of iron oxide.
[Illustration: Fig. 32.—Olivine-gabbro, Gebel Atut [10,365],
× 17. _o_, olivine; _a_, augite; _h_, hornblende; _f_, felspar
(labradorite).]
Of the fine-grained olivine-gabbros, one of the principal types is
the rock of Gebel Atut and Madaret Um Gamil [10,365]. It is a dark
heavy rock, weathering into angular blocks with a thin rusty skin,
of great hardness and ringing under the hammer. On a fresh fracture,
it is seen to be made up of white glassy felspars and dark brownish
minerals, some of which have a platey structure with cleavage surfaces
which flash as the specimen is turned about in the sunlight. The
sp. gr. is 3·01. Microscopic examination shows the rock to be
a holocrystalline aggregate of plagioclase (labradorite), augite,
hornblende and olivine, with a very little magnetite. The labradorite,
which forms about half the rock, is very clear and fresh, in large
crystals which frequently show a tendency to idiomorphism. Smaller
crystals of labradorite are frequently included in the augite and
hornblende. Augite, the next most abundant constituent after the
felspar, is present in irregular almost colourless grains with
well marked cleavage and numerous irregular cracks, and sometimes
slightly clouded. The extinction angles measure up to 36°. The
hornblende is in some crystals of a rather pale greenish-brown
colour not showing very strong pleochroism, while in others it is
more strongly coloured, varying from a rather deep reddish-brown to
very pale yellowish-brown when turned over the nicol. The hornblende
and augite are frequently associated in such a manner as to suggest
that much of the hornblende in the rock originated from alteration
of augite. The olivine is fairly abundant in large rounded grains,
with the usual blackened irregular cracks; it is mostly fresh,
but here and there are patches converted into nearly colourless
serpentine with separation of numerous granules of iron oxide.
[Illustration: Fig. 33.—Olivine-gabbro, from a hill eleven
kilometres east of Gebel Selaia [10,412], × 17. _pl_, plagioclase
felspar (labradorite); _a_, augite; _hy_, hypersthene; _o_, olivine,
with irregular cracks marked by separated magnetite; _h_, hornblende,
enclosing the other minerals and forming a pseudocelephytic border
round the olivine and augite. The rock typically contains a somewhat
greater proportion of hornblende than appears in the figure.]
Another olivine-gabbro [10,412], which forms a hill rising from
the plain about eleven kilometres east of Gebel Selaia, resembles
that of Gebel Atut, but is rather coarser in grain and contains
less plagioclase, a larger proportion of hornblende, and probably a
little accessory hypersthene. It is a dense dark and tough rock of
sp. gr. 3·17, containing large black schillerized-looking crystals
sometimes measuring one centimetre across, and a liberal sprinkling
of white plagioclase with a few tiny grains of pyrite. Under the
microscope, very clear and fresh labradorite is seen to form less than
one-third of the rock; the crystals, which are mostly of irregular
outline, are frequently enclosed in the hornblende. The hornblende
occurs very abundantly as larger irregular green and brown crystals,
with well-marked prismatic cleavage; it often encloses crystals of
all the other minerals, forming a pseudo-celephytic border round the
enclosed augite and olivine. Augite is somewhat less abundant than
the hornblende, with which it is intergrown, in pale pinkish-brown
crystals, slightly pleochroic, with very distinct vertical
cleavage and irregular cracks. Twinning is not very frequent. The
extinction angles are usually under 30°. A few crystals which show
slightly stronger pleochroism (pinkish-brown to very pale green)
than the ordinary augite, and straight extinction, are probably
hypersthene. Olivine is present in about equal quantity with the
augite, in large irregular grains, much cracked, but otherwise fairly
fresh, polarising in brilliant colours; the cracks are blackened with
separated iron oxide, but there is very little serpentinisation. There
are a few small grains of pyrite and magnetite, but iron ores are
not nearly so conspicuous as in some of the other rocks of this class.
=Troctolite.=
[Illustration: Fig. 34.—Troctolite of Gebel Um Bisilla [11,522],
× 17. The crystals with the dark irregular cracks are olivine (_o_),
altering to serpentine (_s_); the resulting expansion has crushed the
surrounding clear plagioclase crystals (_pl_), forming large numbers
of curved cracks into which little tongues of serpentine project.]
If the gabbro of Gebel Um Bisilla is followed towards the summit, a
diminution in the pyroxenic content, with a corresponding increase in
the proportion of olivine, is noted, and at the top of the mountain
we have a troctolite, or rock composed essentially of plagioclase
and olivine [11,522]. It is a heavy speckled black-and-white rock,
made up of colourless to milky and greyish-black grains about three
millimetres diameter (_see_ Plate XXIV). Its sp. gr. is 2·84. The
rock looks like a diorite in the hand specimen; it weathers to
rusty looking blocks of remarkable hardness. Under the microscope
the mineral which looks greyish-black in the hand specimen is found
to be olivine, colourless where unaltered in thin section; but the
usual alteration to serpentine has gone on along irregular cracks,
and the separated granules or iron oxide give the black colour to the
mass. The felspar, which forms considerably more than half the rock,
is a very fresh, though occasionally much cracked, labradorite. Both
the constituent minerals are present in allotriomorphic grains.
=Pyroxene-granulite.=
[Illustration: Fig. 35.—Pyroxene-granulite, Kolmanab Hill
[12,132], × 40. _a_, augite; _h_, hornblende; _pl_, plagioclase;
_m_, magnetite.]
The rock [12,132] forming the hill called Kolmanab, which rises from
the coast-plain in latitude 22° 32′, resembles the fine-grained
olivine free gabbros very closely in composition, but on account of
its marked granulitic structure, is termed a pyroxene-granulite. The
rounded form of the grains has probably been conditioned by movement
of the magma during consolidation, but the rock contains no garnet
or other typically metamorphic mineral, and there appears to be
no reason for regarding this particular granulite as other than
an igneous rock. It will have been noted that the augite grains
in some of the fine grained gabbros show a marked tendency to
granulitic forms, and the rock of Kolmanab appears to be merely an
example of this tendency extending to the other constituents. The
field relations are such as to suggest an intrusive boss. The rock
is very hard and heavy, greyish-black and basaltic-looking; fresh
fractures show tiny glistening grains when turned about in the
hand. Its sp. gr. is 3·13. Microscopic examination shows it to be
essentially a granulitic mixture of augite and plagioclase with a
considerable amount of magnetite, and a little hornblende. All the
minerals are of remarkable freshness, in rounded grains about a tenth
of a millimetre in diameter. The augite, which forms about half the
rock, is of a very pale green colour, sometimes showing faint traces
of pleochroism with the same pinkish tints as hypersthene, from which,
however, it is easily distinguished by its high extinction angles. It
encloses abundant rounded colourless granules, which appear to be
felspar. The felspar is an acid labradorite, and forms a mosaic among
the augite grains; besides the twinned crystals, there are others
which show no trace of this feature, and some of these may possibly
be quartz. Hornblende occurs, not very abundantly, in larger crystals
than the other constituents, grains of which it frequently encloses;
it is of an olive-green colour. Magnetite is scattered through the
entire rock in rounded grains, and is specially frequent enclosed
in the hornblende.
=Diabases.=
Under the heading of diabase are included plagioclase-augite rocks,
with or without olivine, of a character intermediate between gabbro
and basalt. They differ from the gabbros in their finer grain,
in the general absence of diallagic structures in the augite, and
in the more or less porphyritic nature of their felspars, which
are often ophitically intergrown with the augite. They differ from
basalts, on the other hand, in being of coarser grain, and typically
containing no glassy matter. The diabases of South-Eastern Egypt
are more closely allied to the volcanic division of the basic rocks
(basalts) than to the plutonic (gabbros), and many of the rocks here
classed as diabases would be called dolerites by some English writers.
[Illustration: Fig. 36.—Diabase from under the Nubian sandstone,
Rod el Nagi [10,417], × 40. _pl_, plagioclase felspar, ophitically
intergrown with _a_, augite; _l_, limonite. The rock also contains
large porphyritic felspars, not shown in the figure.]
Perhaps the most interesting occurrence of diabase in this part of
Egypt is a thick bed underlying the Nubian sandstone and exposed
along the feet of its eastern scarps between latitudes 24° 30′
and 25°. Owing to the north-westerly trend of the scarp, it is
cut obliquely by the meridian of 34°, and consequently only the
southern portion of the deposit is shown on the geological map of
Plate XX. The bed is well seen on the west of the road leading from
Baramia to Dungash mines; this road follows the foot of the scarp
for a long distance. The thickness of the diabase sheet as seen on
this road is at least ten metres, and is very uniform for several
kilometres. The rock is crushed and decomposed to such a degree that
it is difficult to get a fair-sized specimen. Below the diabase,
at a few points along the road, there are exposures of an ancient
conglomerate of dark colour and extreme hardness. The relations
of the bed to the sandstone are not absolutely certain, but the
impression I got, when I passed the exposure on the two occasions of
my outward and return marches, was that the diabase was an extensive
outflow over the old conglomerate (thus indurating the latter), the
sandstone being subsequently laid down on the undenuded diabase. The
diabase is thus probably Cretaceous in age. A hand specimen [10,417]
from one of the least decomposed portions of the sheet, in the Rod
el Nagi, about fifteen kilometres west of Gebel Muelih, shows white
to glassy porphyritic felspar crystals, frequently in lath-shaped
forms measuring up to eight millimetres in length, in a fine-grained
grey ground mass (_see_ Plate XXIV). The rock contains many rusty
specks, and occasionally as one turns the specimen about in the
hand one can catch a rather dull flash from a crystal of some dark
mineral. The sp. gr. of the rock is 2·85. Under the microscope the
porphyritic felspars, which are very clear, are seen to be mostly
plagioclase, though some of the crystals show simple twinning and
may be orthoclase. The ground mass is holocrystalline, composed
of felspar, augite, and limonite. The felspars of the ground mass
are in the form of thin laths, with repeated twinning. The augite
is nearly colourless, with a very pale brownish or greenish tinge,
generally clouded by brown dusty matter and small irregular cracks. It
forms irregular grains which are cut up in all directions by the
ophitically intergrown felspars. The limonite is extremely abundant,
scattered through the ground mass in rather large irregular grains
which frequently show a tendency to square or hexagonal outlines;
it is mostly opaque, but in some parts it is translucent, with a
deep brown colour. The limonite is doubtless an alteration product
of other minerals, probably magnetite and biotite, of which, however,
no distinct traces now remain.
Besides forming the sheet under the Nubian sandstone, diabase occurs
fairly abundantly in schists at various points, where it appears
to form intrusions. A characteristic of these occurrences is its
weathering into rusty looking brown “cannon balls.” Altered forms
of diabase are associated with peridotites and other ultra basic rocks
in the serpentine mass of Gebel Gerf (_see_ p. 328). Diabase also
forms veins, frequently magnetic, penetrating granites and schists,
and at one point, near the foot of Gebel Awamtib, a dyke of diabase
is even found penetrating the Nubian sandstone.
It is probable that these diabases are of very different ages at
different points. While the occurrences associated with the sandstone
are certainly of Cretaceous age, it seems difficult to imagine that
those associated with the schists and serpentines are not vastly
older; for though they are obviously younger than the schists,
and possibly younger than the peridotites, they frequently show
metamorphism to a degree which it is difficult to reconcile with a
Cretaceous age, and which suggests that they antedated the folding
which formed the ancient mountain chain on the flanks of which the
Cretaceous strata were laid down.
[Illustration: Fig. 37.—Diabase, Gebel Abu Hamamid [10,400], ×
40. _a_, augite; _aa_, augite, altering with separation of iron oxide;
_pl_, plagioclase, often ophitically intergrown with the augite;
_s_, green serpentinous mineral; _m_, magnetite.]
As an example of a diabasic intrusion with “cannon ball”
weathering, we may take the rock [10,400], which occurs in schists
on the flanks of Gebel Abu Hamamid. When one of the rusty looking
“cannon balls” is broken, the interior is seen to be a dark
grey rock of very fine grain. The sp. gr. is 2·93. The microscopic
slide shows the rock to be holocrystalline, consisting mainly of
augite and plagioclase, ophitically intergrown, with some rather
large grains and strings of a nearly isotropic clear yellow-green
mineral, and scattered grains of iron oxides. The augite is of
a pale brown colour; some of the crystals are fairly clear, while
others show strong clouding and separation of iron oxide. The nature
of the yellow-green mineral is somewhat uncertain; it polarises
generally in very low colours, as an aggregate of fibres and tiny
plates, sometimes showing a spherulitic structure between crossed
nicols. Occasionally it includes granules of a highly refracting
colourless mineral, possibly olivine; but there is a remarkable
absence of the separated iron oxide which is usual with altering
olivine, and the granules exhibit only very fine irregular hairlike
cracks; if it is serpentine resulting from alteration of olivine,
the olivine must have been a variety poor in iron.
[Illustration: Fig. 38.—Olivine-diabase, from a dyke at the
junction of Wadis Huluz and Gemal [10,393], × 17. _o_, olivine;
_pl_, plagioclase.]
Turning now to the occurrence of diabase in dykes, a large dyke
in the gneiss at the junction of Wadis Huluz and Gemal [10,393]
consists of a very hard and heavy, strongly magnetic, greyish black
rock of rather fine grain, in which a dark-brown platey mineral is
mixed with grey and white matter. The sp. gr. is 2·95. Under the
microscope the constituents are seen to be plagioclase, olivine,
augite and magnetite, with small amounts of biotite and apatite. The
plagioclase appears to have formed in two generations, for while
the bulk of it is in small lath-shaped crystals (frequently with
radial grouping), there is a very large porphyritic zoned crystal
in the slide. The olivine is in large crystals, usually rounded,
but occasionally tending to hexagonal outline with the usual
strongly marked black irregular cracks. A little serpentinisation
has gone on at the edges and along cracks of a few of the crystals,
but, as a rule, the olivine is very fresh; it is never intergrown
with felspars. The augite is slightly pleochroic, of a purple to
brown tint, in irregular forms, partly in moderate sized crystals
and partly in tiny grains in the ground mass. The crystals are much
cracked. Ophitic structures are not conspicuous. Brown biotite is very
sparingly present in small flakes. Magnetite is liberally scattered
in small grains in the augite of the ground mass. Apatite occurs in
minute prisms included in the felspars.
[Illustration: Fig. 39.—Diabase, from a dyke in Wadi Kreiga
[12,110], × 17. _pl_, plagioclase felspar; _a_, augite; _h_,
hornblende; _l_, limonite strings.]
The diabase dykes which penetrate the granite in Wadi Kreiga [12,110]
differ from the rock last described in their freedom from olivine
and in showing marked ophitic structure. They are dense brown to
black rocks of very fine grain, with porphyritic felspar crystals
here and there. The sp. gr. is 2·98. Microscopic study shows them
to be holocrystalline rocks, composed of an ophitic mixture of
rod-shaped plagioclase with altering augite and hornblende. Both
the ferro-magnesian minerals are very much clouded, and contain
plentiful strings of iron oxide. The hornblende is dark green, often
forming celephytic shells round the augite, and is probably largely
an alteration from augite. There is very little of the nature of a
ground mass, the augite and hornblende practically filling all the
spaces between the felspars.
=Mica-diabase.=
[Illustration: Fig. 40.—Mica-diabase, Gebel Um Khariga [10,373]
× 17. _pl_, plagioclase felspar; _b_, biotite, with separated opaque
flakes of magnetite, often in geometric forms; _a_, clouded mineral,
probably altered augite, with which the felspars are ophitically
intergrown.]
The rock which forms the top of Gebel Um Khariga [10,373] appears to
be an altered mica-diabase. It is highly magnetic; the compass was
found to point 20° out of its normal position at the station on the
hill, while hand specimens broken off the rock showed strong polarity,
some parts attracting and other parts repelling the needle; a fragment
of the size of a pea deflected the compass needle several degrees
when placed near it. It is a dark brown rock, of sp. gr. 2·83, very
rotten, which in the mass looks like an altered dolerite. Microscopic
study of a slide reveals the presence of altered plagioclase,
in rather large lath-shaped crystals; biotite, largely altered to
opaque iron oxide, the flakes of which show marked geometric forms;
scattered grains of magnetite, and some secondary calcite. The
brown clouded mineral polarises as a confused fine-grained and
fibrous aggregate in low colours; it is probably altered augite,
with which the plagioclases were ophitically intergrown, but is in
too highly altered a state for certain identification. The strongly
magnetic character of the rock would appear to indicate that the
iron oxides produced by the alteration of the biotite are, like the
primary grains, in the form of magnetite.
=Basalt.=
Basalt, the volcanic representative of the gabbros and diabases,
is quite a scarce rock in South-Eastern Egypt, having been noted at
only four or five points. Fairly fresh olivine-basalts, probably
comparatively late intrusions, form the two conspicuous low hills
near the coast called Gimeida and Einiwai; more altered rocks of
basaltic type occur at the head of Wadi Um Deheisi (north of Gebel
Kahfa), and in the hills on either side of the Wadi Huluz some ten
kilometres north-west of Gebel Hamata; while an amygdaloidal rock
which forms a large part of the hill-mass of Ti Keferiai has been
classed as an altered hornblende-basalt.
[Illustration: Fig. 41.—Basalt, Gimeida Hill [12,156], ×
40. Porphyritic crystals of plagioclase (_pl_) and olivine (_o_) in
a hemicrystalline ground mass containing tiny crystals of plagioclase
and granules of augite (_a_) and magnetite (_m_).]
The basalt of Gimeida Hill [12,156], is a hard heavy block rock
of sp. gr. 2·88, of dull aspect, with glassy white to colourless
plagioclase crystals up to three millimetres diameter scattered
through it, and here and there a dark diallagic crystal and some
greenish glassy-looking grains of olivine. The microscopic slide
shows the rock to be remarkably fresh; the porphyritic plagioclase
and olivine crystals are seen to be embedded in a crystalline ground
mass containing little lath-shaped plagioclases, with granules of
pale brown augite, and abundant grains of magnetite.
[Illustration: Fig. 42.—Basalt, Einiwai Hill [12,144], × 17. _o_,
olivine; _a_, augite; _p_, picotite, with a border of opaque chromite;
_gr_, hemicrystalline ground mass.]
The basalt of Einiwai [12,144] differs from that of Gimeida in the
absence of porphyritic felspars. It occurs capping the red granite
which forms the lower part of Einiwai Hill and the surrounding
plain. It is a hard dull black rock with little glassy colourless
to pale green crystals plentifully scattered through it (_see_
Plate XXIV). The sp. gr. is 3·10. The microscopic slide shows
porphyritic crystals of olivine, augite, and picotite, in a very
fine-grained semi-glassy ground mass containing tiny laths of felspar
and grains of augite and magnetite. The porphyritic crystals all
show a remarkable absence of colour in the slide, which has been cut
exceptionally thin on account of the darkness of the ground mass;
this thinness of the slide doubtless accounts for the minerals all
showing relatively low polarisation colours. The porphyritic crystals
are mostly in six-sided and prismatic forms, but sometimes show
as rounded grains; cleavage is usually indistinct, but irregular
cracks are common, and some of the crystals are broken in two and
the halves separated. Most of the six-sided crystals show straight
extinction, and are probably a non-ferruginous olivine (forsterite);
they show very little alteration, there being a general absence
of serpentinisation or magnetite-separation along the cracks. The
prismatic porphyritic crystals are in forms resembling those of
augite, and some of the crystals show extinction sufficiently oblique
to justify their identification as augite; others, however, showing
straight extinction and very low polarisation colours, are probably
enstatite. The picotite is in rather large isotropic rounded grains,
of a pale green colour with a well marked opaque border of chromite.
A peculiar basaltic rock [10,408] occurs at the top of Wadi Um
Deheisi, at the pass into Wadi Um Retba, on the road which passes
from Bir Shadli to the north-east of Gebel Kahfa. It is a black rock
of sp. gr. 2·98, and of such fine grain that practically nothing
can be made out with a lens. It is remarkably magnetic, the compass
being deflected by 13° from its normal direction at a plane-table
station on the pass. The microscopic slide shows rounded and angular
clear areas in a cryptocrystalline ground mass of greenish colour,
full of specks of magnetite. The rounded clear areas are occupied
by a very pale brownish mineral, probably augite, forming nests of
crystals in different orientations, with fairly well-marked cleavages,
high extinction-angle and fairly high double refraction. The angular
clear areas are mostly colourless; they seem to consist chiefly of
altered felspar, but some of them are formed of a fine mosaic of
quartz granules. The ground mass is largely of chloritic nature,
with small fibres of hornblende and some decomposed felspar. Its
large content of rounded magnetite granules accounts for the magnetic
character of the rock.
The basalt of the Wadi Huluz [10,410] is a close grained
greenish-black rock containing white spots (amygdules) up to three
millimetres diameter. Its sp. gr. is 2·93. The microscope shows
the main bulk of the rock to be formed of lath-shaped plagioclase
crystals, around and between which is green matter, now mostly
chlorite. Here and there are small clear areas within the green
patches, which show bright polarisation colours, and are probably
augite, being the remains of the original mineral which has been
largely chloritised. There is a fair amount of magnetite in scattered
grains. The amygdules are partly filled with clear quartz, and partly
with zeolites.
The hill mass of Ti Keferiai is largely made up of a fine-grained
black rock [12,120], of sp. gr. 2·99, in which are greenish-white
and pink amygdules up to two millimetres diameter. It is frequently
much crushed, and in one place it has been broken into a coarse
breccia and cemented with rose quartz [12,123]. The microscopic
slide shows the main part of the rock to be a fine-grained mixture
of pale hornblende with altered plagioclase, while the amygdules
are filled with radiating zeolites. The amygdules seem to indicate
that the rock is a volcanic one, and it has been classed as a basalt
rather than as an andesite on account of its basic nature and high
specific gravity. Its texture is microgranitic rather than basaltic,
a circumstance perhaps in part due to the crushing it has undergone;
the rock is passing into a schist.
=Ultra-basic Igneous Rocks.=
The ultra-basic igneous rocks (_i.e._, rocks practically free from
felspar and composed entirely of ferro-magnesian silicates such as
pyroxenes, amphiboles, and olivines), though forming but a small part
of the earth’s crust in general, occur in very large proportion in
the igneous masses of South-Eastern Egypt, where they cover several
hundred square kilometres and form prominent mountain-masses such
as those of Gebels Dahanib, Korabkansi, and Gerf. They may be
classified into:—
(_a_) _Pyroxenites_ (rocks composed essentially of pyroxenes);
(_b_) _Amphibolites_ (rocks composed essentially of hornblende);
(_c_) _Peridotites_ (rocks composed essentially of olivine,
with or without pyroxenes and amphiboles);
All these ultra-basic rocks are easily altered to
(_d_) _Serpentines_, in which the original minerals may or may
not be traceable.
A characteristic of the ultra-basic rocks here, as in other parts
of the world, is their gradual transition into one another, showing
that the various forms have arisen from consolidation of parts of
one and the same magma owing to slight differences in composition
or in the physical conditions under which consolidation has taken
place. A further noteworthy circumstance is their gradual passage
into basic rocks; there is no hard-and-fast line to be drawn,
for instance, between basic diorites and amphibolites, nor between
basic gabbros and pyroxenites, nor between olivine gabbros, poor
in felspar and peridotites, these various classes being found to
pass by insensible gradations one into another as they are followed
up in the field. Moreover, being typically coarse-grained rocks,
and pyroxenes and amphiboles being often indistinguishable in the
hand specimen, great caution has to be exercised in naming a rock
mass from a few microscopic slides which of necessity each embrace
at most but a few square centimetres of section.
In the field, the appearance and cohesive strength of the ultra-basic
rocks varies primarily with the extent to which they have been
altered towards their final stage of serpentinisation. Where they
are least altered, they form black masses of hard heavy crystalline
rock of such toughness that they are only broken with difficulty
with a sledge hammer; while in the cases where serpentinisation has
proceeded to the greatest extent, they frequently form foxy red or
even pink-looking hills which might almost be taken for granite from a
distance, and they are so shattered that the rock comes off literally
in tons at a mere touch; in these cases, long search is necessary
to find a coherent piece large enough for a museum specimen. The
brown or pink colour just referred to is of course only superficial,
but in the untrodden and rainless wilderness surface films remain
unbroken and give characteristic colours to the scenery. Freshly
fractured surfaces are always dark green, dark brown, or black,
with more or less crystal structure visible according as the rock is
less or more altered; pyroxenites or amphibolites, when but little
altered, are a mass of lustrous platey or fibrous dark crystals,
while serpentines are typically of dull aspect. The specific gravity
is high, ranging from as much as 3·1 in the less altered forms down
to about 2·6 in those which are more completely serpentinised.
The process of serpentinisation is of course a chemical change,
consisting largely in the combination of water with ferro-magnesian
silicates free from alumina; but it is remarkable how frequently
this chemical change has been accompanied by a parallel physical
deformation. Serpentines are almost always shattered rocks, full of
slickensided surfaces; when we compare the low sp. gr. of serpentine
(2·6) with that of augite, hornblende, or olivine (about 3·2),
we naturally conclude that the shattering of the rock is in all
probability due to the expansion on hydration causing internal stress,
and the slickensiding is due to the rock yielding along certain
surfaces. The cracking of felspars and the forcing of serpentine into
them, which are frequently seen in thin sections of olivine rocks,
such as the troctolite shown in Fig. 34 on p. 304, shows on a small
scale the physical effect of expansion on serpentinisation, and
should lead us to expect a corresponding effect in rock masses. It
is thus not necessary to infer great tectonic movements to explain
the shattering of the rock, and in fact the disposition of the
serpentines in broad mountain tracts like Gebel Gerf is opposed
to the idea of there being here any local accentuation of folding
or crushing by general crust-crumpling. I have calculated that a
horizontal sheet of pyroxenite of sp. gr. 3·1, ten kilometres wide,
confined between fixed abutments and prevented from increasing its
thickness, would rise into an arch having a height of about two and
a quarter kilometres at its centre if converted into serpentine of
sp. gr. 2·6; this is, of course, not given as a precise example of
what may actually have taken place, but it will serve to show that
expansion on hydration may produce dynamical effects not inferior to
those of contraction of the earth’s crust, such as are believed to
be the main cause of mountain formation, and to explain why we may
find serpentines shattered to fragments and full of slickensided
surfaces in areas where the surrounding rocks show comparatively
little evidence of dynamo-metamorphism.
=Pyroxenites.=
Heavy dark green rocks, of medium to coarse grain, and consisting
almost entirely of a schillerized-looking mineral, form the
hill-masses of Gebels Um Ein and Qrein Salama, and similar rocks are
found in connexion with gabbro at Gebel Um Gunud and elsewhere. From
the diallagic appearance of these rocks in the hand specimen, they
were classed in the field as pyroxenites or diallage rocks. But
an examination of the slides cut from the specimens reveals the
main constituent to be hornblende, and the rocks must therefore
be placed in the division of amphibolites. It is, however, highly
probable that these rocks were originally pyroxenites, the hornblende
having originated mainly from the alteration of augite; the change
from augite to hornblende in the gabbros has already (p. 302) been
remarked, and in these ultra-basic forms the same process appears
to have gone on.
Some parts of the great ultra-basic mass of Gebel Gerf consist of
bronzite-rock. Specimens from the least altered portions consist
almost entirely of bronzite, sometimes with a little olivine;
the rocks are, however, generally found passing into serpentine,
such as forms the main portion of the mountain, and on microscopic
examination even the freshest portions always exhibit more or less
serpentinisation. It will accordingly be preferable to treat of
these rocks under the heading of serpentines.
=Amphibolites.=
Rocks consisting almost entirely of hornblende occur in considerable
masses in the neighbourhood of Gebel Um Gunud, and form the
conspicuous hills Gebel Um Ein and Qrein Salama; they also occur
in a small patch about three kilometres east of Erf el Fahid. In
the field, especially with the coarser-grained varieties, there is
often considerable difficulty in identifying the main constituent of
the rock as hornblende, owing to the schillerized appearance of the
mineral being more suggestive of diallage; the mass east of Erf el
Fahid, for instance, was classed in the field as a basic gabbro or
diallage-rock, instead of a very basic diorite or hornblende-rock,
such as the microscopic examination proves it to be. These rocks are
always very heavy and extremely tough; they weather commonly into
great rusty-looking rounded blocks, and a sledge hammer is required
to get a specimen.
[Illustration: Fig. 43.—Amphibolite, from hills near Gebel Um
Gunud [11,511], × 30. Almost the whole of the figure is occupied by
hornblende, which is seen clouded and altering to granular epidote
at _e_. Part of a large grain of calcite, bordered and streaked
with iron oxide, is seen at _c_, while _q_ is a small crystal of
interstitial quartz. The slide also contains a small proportion of
plagioclase felspar, not shown in the figure.]
The amphibolite of the hills about Gebel Um Gunud [11,511] is a very
hard and heavy coarse grained greenish-black rock composed almost
entirely of shining platey-looking crystals of hornblende, which often
reach two centimetres in length and breadth. The sp. gr. is 3·08. The
microscopic slide shows the rock to be granitic in structure and to
contain, in addition to hornblende, small amounts of plagioclase,
quartz, calcite, and iron oxides. The irregular-shaped hornblende
crystals show strong pleochroism, from pale olive-brown to moderately
deep green. The extinction angles are large, being frequently over
20°. Twinning of the crystals is fairly common. Many of the crystals
are clouded, and the alteration of the mineral has resulted in the
formation of much epidote. The plagioclase is only present in very
small quantity, strongly clouded by decomposition, but still showing
twinning clearly. The quartz is clear, and is likewise present in very
small proportion, mostly interstitial. Calcite is fairly plentiful
in large rounded and irregular grains, rather turbid-looking, of a
brownish or greenish tinge, outlined and streaked with strong lines
of iron oxide, in a way which at times suggests that the calcite
may possibly be the result of alteration of a lime olivine.
A specimen of amphibolite [11,528] obtained from a hill near the
junction of Wadi Abu Marwa with Wadi Naait, about seven kilometres
north-east of Gebel Um Gunud, is a dark green rock of rather fine
grain, made up of shining plates and small fibrous crystals of
hornblende and chlorite. Its sp. gr. is 2·97. Under the microscope
the hornblende is in rather ragged-looking irregular crystals of
various sizes, all interlocked with each other, rarely showing any
approximation to idiomorphism except in the smaller crystals, which
are commonly limited by prismatic faces; these smaller crystals are
frequently included in the larger ones. The pleochroism is fairly
strong, a pale olive brown, c moderately deep green. Prismatic
cleavage is well marked. Extinction angles are often large, ranging
up to 24°. A few of the crystals are slightly bent, and many show
more or less clouding owing to partial alteration to chlorite, but
on the whole the rock is fairly fresh. Except for a few specks of
magnetite, and the alteration product chlorite, the rock contains
nothing but hornblende.
The hornblende-rock or amphibolite of Gebel Um Ein [12,130] is
of medium grain, almost entirely made up of shining crystals of
hornblende with a little chloritic matter. The sp. gr. is 3·03. The
microscopic slide shows large individuals of hornblende inclining to
idiomorphism, set in a sort of ground mass of smaller crystals. The
hornblende shows the same strong pleochroism and high extinction
angles as in the two rocks just described; it is frequently clouded
and speckled with iron oxides, and often full of lighter-coloured
patches which extinguish with the rest of the crystal; in many cases
these lighter patches contain a central granule of iron oxide,
and they are doubtless due to a bleaching by segregation of the
iron. Between crossed nicols many of the crystals are full of small
fibres extinguishing differently from the rest of the crystal; some
of these fibres polarise in low colours and are probably chlorite
or serpentine, while others show brilliant tints and are possibly
actinolite.
The amphibolite which occurs three kilometres east of Gebel Erf el
Fahid [10,361] is of extremely coarse grain, looking like a very
basic gabbro owing to the schillerized appearance of its large
hornblende crystals and the presence of a little interstitial
felspar. The sp. gr. of the rock is 2·98. The section shows the
hornblende to be of a very pale green colour, with an almost entire
absence of iron oxide grains and other alteration products. Between
crossed nicols it has a fibrous woody appearance. The interstitial
plagioclase (probably labradorite) is likewise very fresh, showing
its repeated twinning very clearly; the crystals are full of tiny
fibres of hornblende, and are traversed by broad cracks filled with
a mosaic of smaller crystals of plagioclase and quartz.
[Illustration: Fig. 44.—Amphibolite of Qrein Salama [12,157],
× 17. _h_, hornblende, strongly striated, altering to chlorite;
_o_, olivine, and _b_, bronzite, both passing into serpentine.]
The rock [12,157], of the hill called Qrein Salama, to the east of
Gebel Gerf, is interesting as containing olivine and bronzite in
addition to the more abundant hornblende, and thus forming a link
between the amphibolites proper and the peridotites; but as about
three-quarters of the rock is hornblende it is still classed as an
amphibolite. The sp. gr. is 3·05. In the slide, the hornblende
is seen in irregular crystals, colourless to very pale green,
with a fibrous structure which is strongly marked by patches
of shading of extremely fine black prismatic striations. With
crossed nicols the fibrous structure is still more apparent,
the mineral polarising as brilliant fibres separated by chloritic
alteration products. The pronounced striation and faint colour of
the crystals are more suggestive of diallage than of hornblende,
but the extinction angles measured in the slide are all less than
22°. Many of the hornblende-crystals contain large numbers of
small rounded grains and strings of iron oxide. The olivine is
mostly in rounded crystals, frequently included in the hornblende;
it is largely serpentinised and full of small grains of opaque iron
oxides, but kernels of the original mineral remain. The bronzite,
which is present in about equal proportion with the olivine, is
likewise extensively serpentinised and full of iron oxide grains;
it is distinguishable from the olivine by a more fibrous appearance
(the serpentinisation having gone on mainly along the direction of
the vertical axis instead of along irregular cracks), and by its
generally lower polarisation colours in the unaltered portions.
=Peridotites.=
The peridotites, or felspar-free rocks consisting largely of olivine,
are usually classified into:—
(_a_) _Dunites_, consisting entirely of olivine.
(_b_) _Harzburgites_, consisting of olivine and enstatite or
bronzite.
(_c_) _Wehrlites_, containing olivine and diallage.
(_d_) _Lherzolites_, containing olivine, diallage, enstatite or
bronzite, and picotite or chromite.
(_e_) _Hornblende-picrites_, containing olivine and hornblende.
(_f_) _Mica-peridotites_, containing olivine and biotite.
Most if not all of these classes are represented in South-Eastern
Egypt, but in the altered form of serpentine. The change to serpentine
has been so complete that it is now hardly possible to extract even
a small specimen of the primitive rock; indeed, careful search is
often required to obtain even specimens containing any unaltered
mineral whatever, and one has frequently to rely on the structure
of the serpentine for the identification of the rocks from which
it originated.
=Serpentine.=
Serpentines cover about 400 square kilometres of South-Eastern Egypt,
forming the principal rock of several remarkable mountain groups
and also occurring in lower hill country.
The largest occurrence is that of the great mountain mass of Gebel
Gerf, where serpentine with alternations of gabbroid and dioritic
rocks can be followed from Bir Meneiga southward for some thirty
kilometres to beyond the Sherefa pass, while the breadth of the tract
from east to west is some fifteen kilometres or more at its widest
part; this tract includes a vast assemblage of high peaks and ridges,
towering up in many cases to more than a kilometre above sea-level. To
the north, south, and east, the serpentine is bounded by gabbros and
diorites, while on the west a tract of schistose rocks separates it
from another great serpentine mass which forms Gebel Korabkansi.
Other remarkable serpentine masses are Gebel Abu Dahr and the upper
part of Gebel Sikait; serpentines also enter into the composition of
Gebels Ghadir and Um Tenedba, while lower hills formed of similar
rocks occur near Bir Murra in Wadi Shait, in Wadi Um Khariga, near
Gebel Kalalat, and on the plain east of Abraq Springs.
The foxy red colour of exposed faces of serpentine hills, and the
generally shattered nature of the rock, have already been referred
to. When one approaches the masses closely, the red colour often
becomes less marked, because the fresh debris flanking the hills is
of a darker aspect. The slopes of serpentine hills are usually steep,
and this combined with the rotten nature of the rock renders their
climbing not always quite free from danger.
Any attempt to map out in the field the precise limits of the
different peridotites which were the parents of the serpentine ends
in failure, partly because the constituent minerals can generally
only be identified on microscopic examination, and partly because
the different peridotites pass gradually one into another, and are
evidently only produced by slight variations in the composition or
conditions of consolidation of a single magma. Even the limits of the
serpentine itself are not always very clear; for where the associated
rock is gabbro, as for instance to the north of Gebel Gerf, there is
a gradual passage through more or less serpentinised olivine gabbro
to the true serpentine. Golden yellow veinlets of fibrous chrysotile
can be seen running through the rock at many places, and occasionally
veins and pockets of magnesite and an inferior kind of asbestos occur
(_see_ p. 330).
The serpentines are nearly always more or less magnetic, and
sometimes show strong polarity. The compass was disturbed by
40° at the triangulation station on Gebel Sikait; the amount of
disturbance changes greatly when one moves even a short distance,
and all estimations of direction from compass readings in serpentine
country are therefore liable to enormous error.
In a strictly petrographical sense it would be most systematic to
describe the serpentines in classes according to the nature of the
parent rock; grouping together, for instance, all those derived from
pyroxenites into one class, those from amphibolites into another,
those from dunites into a third, and so on. But besides the fact
that the nature of the parent rock cannot always be determined with
certainty, such a course would possess the objection of separating the
different parts of one and the same mass; at Gebel Gerf, for example,
we have serpentines derived from rocks of various of these classes,
though probably all formed part of the same igneous intrusion and
are thus genetically connected. In the descriptions which follow,
therefore, the serpentines of each locality will be grouped together,
irrespective of origin. The localities are taken in order of latitude
from north to south.
[Illustration: Fig. 45.—Serpentine, probably derived from a
mica-peridotite, east of Erf el Fahid [10,360], × 40. _cm_, chlorite
and magnetite from alteration of biotite; _s_, clear serpentine,
probably altered olivine.]
A serpentine which occurs associated with amphibolite and various
schists about three kilometres east of Gebel Erf el Fahid [10,360]
may possibly have been derived from a mica-peridotite. In the
hand specimen it is a dark brown rock of dull aspect with greenish
patches, and shows marked magnetic polarity. The sp. gr. is 2·63. The
microscopic slide shows mainly colourless serpentine, but there are
strings and bundles of pale greenish-yellow chlorite, and specks and
strings of magnetite. The arrangement of the magnetite (_see_ Fig. 45)
at once attracts attention, being frequently aggregated in parallel
strings reminding one of the cleavage of mica. Between crossed nicols
the chlorite is easily distinguished from the serpentine by its
slightly higher polarisation-colours (low yellows as compared with
greys); and its arrangement in long leaves and bundles, with distinct
traces of the original mica, leaves no doubt as to its derivation
from biotite. The parallel strings of magnetite are also most likely
due to alteration of biotite, for it is difficult to account for
their peculiar arrangement otherwise; they are mostly separated by
material of lower double refraction than the chlorite above-mentioned,
perhaps a variety of chlorite poorer in iron, owing to the previous
separation of the oxide. The clear serpentine which forms the bulk
of the slide does not contain much iron oxide, and does not include
any trace of the original mineral from which it has been formed;
but between crossed nicols it shows an irregular mesh structure
which makes one almost certain of its derivation from olivine.
[Illustration: Fig. 46.—Serpentine, probably derived from
lherzolite, Wadi Um Khariga [10,368], × 17. _so_, serpentine derived
from olivine; _sb_ and _sd_ serpentine probably derived from bronzite
and diallage; _c_, calcite; _p_, picotite; _m_, magnetite.]
The serpentine of the hills on the west side of the Wadi Um Khariga
[10,368] is remarkable in the field by its foxy red colour on all
exposed surfaces. The rock, which has a sp. gr. of 2·63, is nearly
black on fractured surfaces, and no crystals can be detected in
it. The slide shows colourless to yellow serpentine, with a fair
amount of calcite often arranged along cracks, a liberal sprinkling
of magnetite, and one or two fairly large grains of picotite,
or chromite. In some parts of the slide the serpentine shows an
irregular mesh structure between crossed nicols; these portions,
in which the magnetite-granules show an irregular honeycomb-like
arrangement, are doubtless altered olivine. In other places the
serpentine polarises in clear greys with a fibrous aspect, and
the magnetite is arranged in parallel lines; calcite is typically
developed in these fibrous portions, sometimes along the cleavages
and sometimes in irregular patches. The fibrous-looking serpentine
probably represents the alteration-products of both rhombic and
monoclinic pyroxenes, and the parent rock was thus a medium-grained
lherzolite, containing olivine, diallage, bronzite and picotite.
In the hills on the east side of the Wadi Um Khariga, about in
latitude 24° 55′, there is some serpentine which has apparently
resulted from the alteration of very basic dykes. The sp. gr. is
2·65. A slide cut from this rock [10,367] shows the same clear
fibrous-looking patches as the specimen last described, and picotite,
in a confused and nearly isotropic mass of fine fibres of serpentine,
with but little magnetite and no calcite. No original mineral remains
except the picotite. The clearer fibrous patches may represent
diallage or bronzite, while the rest of the serpentine is somewhat
doubtfully referred to olivine. There is some trace of banding in
the rock, perhaps due to movement during consolidation.
A serpentine approaching an ophicalcite [10,376] forms the main
rock at Gebel Ghadir, where it is associated with a peculiar quartz
felsite resembling granulite. In the mass the rock, which has a
sp. gr. of 2·67, is black to green, veined with calcite; surfaces
of the debris are often covered with a brilliant green glaze. The
microscopic slide reveals an irregular mixture of nearly colourless
serpentine and cloudy-looking calcite, with abundant specks and a
few larger granules of slightly translucent deep-brown chromite or
picotite. Between crossed nicols, the serpentine is a mass of fibres
and little plates, in which a strong tendency to linear arrangement
can be seen, and here and there a lattice-structure. Though none
of the original mineral remains there is not much doubt that the
rock is an altered amphibolite or basic diorite; the calcite is
possibly derived from the alteration of an original lime felspar,
but it has been largely redistributed in the crushed rock by solution
and redeposition.
The serpentine forming the hills on the north side of the mouth of
Wadi Kalalat is a dark reddish-brown rock, of sp. gr. 2·76, in which
shining crystalline specks of olivine can be seen. It has doubtless
resulted from the alteration of a dunite, or rock consisting almost
entirely of olivine. The slide [11,510] shows the rock still to
contain abundant clear colourless kernels of the original olivine,
in a mesh-work of pale olive-brown serpentine. The serpentinisation
has taken place along irregular cracks in the olivine, each crack
being generally marked by a thin streak of opaque iron oxide
running longitudinally down its centre, with serpentine fibres
running crosswise and filling the rest of the crack. Where the
kernels first left have themselves become changed to serpentine,
they are nearly isotropic, while the cracks polarise in clear greys,
so that even where the whole of a crystal has been serpentinised the
structure is still clear, both in ordinary light by the magnetite
strings, and in polarised light by the way in which the serpentine
of the cracks stands out from the more isotropic patches within the
meshes. There is a little accessory diallage, easily recognisable by
its fibrous appearance and the oblique extinction of its unaltered
portions. The diallage, like the olivine, is passing into serpentine,
but here the serpentine goes on along cleavage planes as well as
along irregular cross cracks, giving the partially altered crystals
a striped appearance between crossed nicols. Where the diallage has
become entirely serpentinised, it can still be differentiated by its
clearer appearance from the olivine-serpentine in ordinary light,
and by its striated structure between crossed nicols; but of course
unless kernels of the original mineral are left one cannot be sure
whether it was diallage or bronzite. There are a few patches of
calcite or magnesite, and veinlets of the same secondary minerals;
these may have arisen from the alteration of a little original
felspar, or from the diallage.
Near the head of Wadi Arais, to the south of Gebel Um Bisilla,
there are some dykes of dark magnetic rock of sp. gr. 2·56. A
specimen from one of these dykes [11,519], turns out on microscopic
examination to be a schistose serpentine. It is traversed by parallel
strings of opaque and brown translucent iron oxides, and granules of
magnetite are also scattered over the interspaces. The interspaces
between the iron oxide strings are filled with serpentine showing no
recognisable structures, but containing here and there little nests
of clouded green hornblende, suggesting the possibility of the rock
being an altered basic diorite or amphibolite.
The highly crushed and rotten serpentine which forms the high and
steep-sided mountain mass of Gebel Abu Dahr, differs considerably in
composition in different parts. The most typical form of the rock,
which has a sp. gr. of 2·77, is a dark brown serpentine with little
strings of olive-green matter, and occasionally large dulled black
crystals. A slide [11,516 A], cut from this portion of the rock
shows mostly olivine, altering to serpentine in the usual manner
with abundant clear kernels of the original mineral. Associated with
the olivine is another mineral of a somewhat fibrous aspect, altered
partly to serpentine and partly to another substance which polarises
as a confused aggregate of fibres and flakes in brilliant colours,
the flakes extinguishing with slight obliquity; the serpentine is
formed mainly along the vertical cleavages of the mineral (though it
also fills transverse irregular cracks), while the other substance
(tremolite?) fills up interspaces. In some parts of the mass the
rock has a blacker colour and a slightly higher density (2·87),
somewhat resembling a basalt with large greenish fibrous-looking
crystals scattered sparsely through it. The slide cut from this form
of the rock [11,516 B] shows no olivine whatever, practically the
whole slide being composed of the fibrous mineral just described,
together with a little nearly colourless fibrous hornblende, in
which the change above-mentioned appears to be going on. Thus it
would seem likely that the rock of Gebel Abu Dahr is essentially
an altered hornblende-picrite (or olivine-hornblende rock) with
variations towards dunite (olivine-rock) on the one hand, and towards
an amphibolite or hornblende-rock on the other. The alteration of
hornblende here is not a purely serpentinous one, but results in
the formation first of serpentine along cracks and cleavage planes,
and then of a tremolitic mineral in the interspaces. It is worth
remark, moreover, that in the slide free from olivine the change to
tremolite preponderates, and there is much less serpentine in the
altered hornblende than in the slide containing olivine; this suggests
that the presence of the neighbouring olivine has in some way brought
about more serpentine in the hornblende, perhaps by actual forcing of
serpentine from the expanding olivine into cracks in the hornblende,
or by the influence of pressure set up by the same expansion.
In the serpentine from some low hills on the plain a little to
the east of Bir Abraq [11,506] we have a rock evidently derived
from a dunite, though not a trace of unaltered olivine remains. The
sp. gr. is 2·61. The microscopic slide consists of nearly colourless
serpentine with strings of magnetite marking the cracks along which
its formation began (_see_ Fig. 47). Between crossed nicols (_see_
Fig. 48) the main portion of the serpentine, doubtless derived
from olivine, presents a very remarkable appearance; most of the
polygonal spaces between the magnetite-meshwork are lined with plates
of clear serpentine, while the centre is occupied by nearly isotropic
material. Thus the slide in polarised light presents somewhat the
aspect of an aggregate of cells with dark nuclei. Besides olivine, the
slide shows a small amount of a fibrous mineral, probably bronzite,
which is likewise almost entirely altered to serpentine, though a
few original fibres remain; the magnetite grains in the serpentine
derived from this mineral is typically aggregated in strings parallel
to the fibres.
[Illustration: Fig. 47.—Serpentine derived from an almost pure
olivine-rock, from a low hill east of Bir Abraq [11,506], × 17. The
outlines of the original olivine crystals (_o_) and the cracks in
them are marked by granules and strings of magnetite. A serpentinised
crystal of bronzite is seen at _b_.]
[Illustration: Fig. 48.—A small portion of the serpentine derived
from olivine in the same slide, viewed between crossed nicols under
a higher power, showing the cellular structure.]
A serpentine rock found associated with fine-grained diorite at
Gebel Um Tenedba [11,517 B] may have been derived from an olivine
diabase poor in felspar. The rocks in this locality are highly
altered and crushed, and frequently more or less schistose. In a
slide cut from the serpentine of the lower part of the mountain,
which has a sp. gr. of 2·71, all the original minerals of the
rock have been replaced by alteration products. The main portion
of the slide is probably altered augite; it consists of serpentine
with fairly distinct mesh structure in which iron oxides are very
abundant in clots and strings. Some patches, which show a more uniform
grey aspect under crossed nicols, are doubtfully referred to altered
felspar. Smaller areas, usually free from iron oxide grains and mostly
consisting of almost isotropic serpentine with a substance polarising
in low yellow tints running along irregular cracks in all directions,
are believed to represent altered olivine; one of these areas is
nearly filled with little brushes of the substance just mentioned,
giving it almost a spherulitic appearance as the nicols are rotated.
[Illustration: Fig. 49.—Serpentine, derived from wehrlite, Gebel
Gerf [12,128], × 17. _s_, serpentine (mainly altered olivine, with
strings of magnetite); _o_, unaltered olivine, mixed with granules
of augite; _d_, diallage.]
The predominant rock of the mountain mass of Gebel Gerf [12,128] is
a serpentine apparently derived from a wehrlite or olivine-diallage
rock. In the hand specimen, it is a rather heavy black rock
(sp. gr. 2·75) with small pea-green spots and a few small bronzy and
glassy looking crystals here and there. The microscopic slide (_see_
Fig. 49) shows the olivine to be largely altered to serpentine in
the ordinary manner with separation of magnetite, but some granules
of the original mineral remain, especially aggregated, together
with augite granules, round the diallage crystals. The diallage is
tolerably fresh, and easily recognisable by its fine striation and
oblique extinction. Besides the diallage, there is some ordinary
augite, not always very easily distinguishable from the unaltered
olivine owing to its cleavage not being very marked. Some of the
diallage crystals show undulose extinction as the result of strain.
[Illustration: Fig. 50.—Serpentinisation of bronzite, as seen
in a serpentine derived from bronzite-rock, Gebel Gerf [12,119],
under a high power between crossed nicols. The clearer portions
of the figure are unaltered portions of a single large crystal of
bronzite; the serpentine is seen forming mainly along the cleavages
of the original mineral, with subsidiary cross-fibres.]
At many points the serpentine of Gebel Gerf contains a great
deal of bronzite, crystals of this mineral of typical aspect and
measuring up to five millimetres diameter being visible in the hand
specimen scattered liberally through the duller serpentine. The
sp. gr. of this portion of the mass is 2·81. The slide [12,119]
shows practically nothing but bronzite altering to serpentine,
so that locally the parent rock has been a pyroxenite rather than a
peridotite. The serpentinisation of the bronzite has proceeded mainly
along the fibres of the original mineral, but there are numerous
cross-fibres (_see_ Fig. 50), and here, as in the hornblende of the
rock of Gebel Abu Dahr, the change to serpentine appears to have
been accompanied by the production of tremolite. In another slide
[12,127] the main constituent appears to be still bronzite, but
here it is of a pronounced olive-brown colour even in thin section,
and is accompanied by some nearly colourless diallage; some of the
olive-brown fibrous crystals show extinction-angles of a few degrees,
and it is possible that these are hornblende.
Though the greater part of the Gerf serpentine has been derived from
rocks free, or practically free, from felspar, it is possible that
these were mixed with other rocks, such as gabbros and diabases, in
which basic plagioclase formed an essential constituent. One of the
specimens brought back from the mountain [12,115], having a sp. gr. of
2·67, is a somewhat wedge-shaped block, about ten centimetres square
and four centimetres thick, covered entirely with the shining black
to greenish glaze which is characteristic of shattered serpentine,
except where chipped with the hammer to test its coherence and to
look for remains of crystals in it. The fractured surface is mostly
dull and nearly black, with little strings and spots of greenish
matter and scattered shining specks. But although the block so
thoroughly resembles a serpentine, the slide cut from it proves the
rock to be a fine-grained diabase, consisting essentially of augite
and plagioclase, with a good deal of iron oxide. The plagioclase
is tolerably fresh, in lath-shaped forms still showing twinning
very clearly, while the augite is very clouded and is changing to
hornblende and chlorite or serpentine. In the field nothing was
noted which would separate this rock from the rest of the mass,
which it resembles almost perfectly in appearance, and whether the
diabase forms a dyke or sheet, or the specimen represents a fragment
of diabase caught up by the magma, is not certain. A dyke of this
material, shattered and serpentinised on all the fractured surfaces,
would be indistinguishable from the surrounding serpentine. It is
even possible, though not likely, that the diabase is not a separate
intrusion, but merely a variation of the same plutonic mass which
has given origin to the main bulk of the serpentine.
Other variations in the serpentine which point to parts of the Gerf
mass having originally been of a gabbroid type, occur in the mountains
round Bir Korbiai and Bir Meneiga. A specimen from near Bir Korbiai
[12,125], is a greyish-black fine-grained rock full of little white
and greenish specks and strings, with pale citron-green serpentinous
matter covering the slickensided surfaces of the fragments into which
the mass readily separates. The sp. gr. is 2·67. The microscopic
slide shows serpentine with pronounced knitted structure in places,
as though derived from augite, and considerable patches of calcite
which may represent an original lime-felspar. The calcite has,
however, been largely redistributed along cracks, where it is often
mixed with extremely coarse fibres or plates of serpentine.
The specimen from the mountains round Bir Meneiga [12,104] is a
hard dull slatey-grey rock with black streaks and spots; it is
strongly magnetic and shows a tendency to schistose structure. The
sp. gr. is 2·54. On examination with a lens the grey matter has
in places something of a resemblance to saussuritised felspar. The
slide cut from this specimen contains no original minerals. The
bulk of it seems to be composed of very minute fibres or plates,
generally showing a distinctly parallel arrangement along the planes
of schistosity, but exhibiting knitted structure in places, and thus
probably consisting in part of kaolinic matter derived from altered
felspar, and in part of serpentinous matter derived from alteration
of augite; while scattered through the slide are abundant patches and
strings of calcite and magnetite. The rock is too intensely crushed
and altered for one to be certain of its origin, but it appears to
have been originally a diabase.
[Illustration: Fig. 51.—Serpentine derived from alteration of
harzburgite, Gebel Korabkansi [12,140], × 17. _s_, serpentine,
mostly formed from olivine; _o_, unaltered olivine; _b_, altering
bronzite; _c_, chromite or magnetite, enveloping olivine.]
The serpentine of Gebel Korabkansi, to the west of Gebel Gerf,
is an altered olivine-bronzite rock (harzburgite) of moderately
coarse grain. A slide [12,140] cut from the least altered portion,
where the sp. gr. of the rock is 2·72, shows olivine to have been the
most abundant constituent; it is largely serpentinised in the typical
manner, but abundant kernels remain unaltered. Mixed with the olivine
crystals are others of a fibrous mineral, altered partly to serpentine
and partly to a confused mass of tremolite fibres; in the few cases
where any of the original material remains, this fibrous mineral shows
straight extinction, and is probably bronzite. The alteration of the
pyroxene has here proceeded more rapidly than that of the olivine. The
pyroxenic mineral is easily picked out in the slide by ordinary light,
owing to the fact that the olivine has altered to greenish-yellow
serpentine, while the mixed material produced from the pyroxene
is of a dirty-white aspect. Another slide from Gebel Korabkansi
[12,112] exhibits in places mesh-structure with clear kernels which
extinguish very obliquely to the meshes, showing augite or diallage
also to have been a constituent of the parent rock; and there is
a single ragged-looking crystal of dark green fibrous hornblende,
altering to nearly colourless serpentine. The sp. gr. of the specimen
from which this latter slide was cut is 2·63. Both the slides from
Gebel Korabkansi are rather poor in iron oxides, but there are a
few irregular crystals of chromite, generally enveloping olivine.
=Alteration Products of Serpentine. Magnesite and Asbestos.=
In many of the serpentine-masses the effects of solution and pressure
can be traced in the formation of veins and pockets of magnesite,
asbestos, and occasionally talc in the rock. Magnesite veinlets
are frequently seen, for instance, round about Bir Meneiga [12,142]
and Bir Muqur, while asbestos has been found in pieces of several
kilogrammes weight in the Wadi Sherefa el Sherqi [12,122] and near
Bir Muqur [12,153]. At the last-named locality it occurs along the
crush-planes of the serpentine, and appears to be preceded by the
formation of long fibres of a green substance.[133] Where thickest,
the glaze on the slickensided surfaces of the serpentine fragments
seems to consist of this same green substance, which splits into hard
fibres often reaching ten centimetres or more in length and only a few
millimetres in diameter [12,153 A] and these hard fibres, which have
a sp. gr. of 2·64, break up gradually, apparently under weathering
influences, into the much finer and more flexible fibres of white
asbestos. The quality of the asbestos is, however, inferior, for a
specimen of the softest kind sent to London for an expert opinion,
was reported to be of no commercial value; even had it been otherwise,
the quantity seen was too small to justify working, though of course
a thorough search among the mountains might reveal the presence of
bigger deposits than were encountered during the survey.
[Footnote 125: From _Marwa_, the Arabic term for quartz.]
[Footnote 126: The numbers in square brackets in this and the
following chapter are the registration numbers of the specimens in
the Geological Museum, Cairo.]
[Footnote 127: I may here recall that the quartz mass which
constitutes the so-called “alabaster” quarry north of Aswân,
and for which I have inferred an igneous origin, is likewise closely
associated with aplitic or pegmatitic intrusions. See _A Description
of the First or Aswân Cataract_. Cairo, 1907, p. 84.]
[Footnote 128: This specimen was brought back by a guide sent for
the purpose to Gebel Adar Qaqa. It is stated to be the main rock
of the mountain, but as I did not visit the mountain I am unable to
vouch personally for this being the case.]
[Footnote 129: From a specimen brought back by the guide who was
sent to erect the triangulation beacon on Gebel Mishbih.]
[Footnote 130: I did not visit Gebel Shendib, but as seen from Gebel
Elba the mountain appeared to be granite; the felsite probably forms
a dyke passing through the summit.]
[Footnote 131: There is another Gebel el Anbat near the Wadi Hodein,
but this latter consists of schists.]
[Footnote 132: Preliminary Report on the Geology of the Eastern
Desert of Egypt, between latitude 22° N. and 25° N., by
Dr. W. F. Hume. Cairo, 1907. p. 57.]
[Footnote 133: This fibrous green substance is probably the columnar
variety of serpentine known as _Picrolite_; it strongly resembles the
specimens thus labelled in the London Museum of Practical Geology.]