All materials, both sedimentary and igneous, composing the earth’s
crust, are liable to be involved in movements of deformation, becoming
subject to pressures and chemical agencies, which are often intense
in character and productive of far-reaching internal changes. The
result is the formation of a series of rocks, which may be of divers
origins, but which having lost their original characteristics,
may be grouped as metamorphic. By the action of compression
and dislocation not only are the major masses folded or broken,
but their individual constituents are brought into new relations
both as regards their external conditions and internal molecular
arrangement. Where lateral pressure has been active as regards the
finer-grained rocks, such as clays, the individual particles arrange
themselves with their longer axes at right angles to the pressure,
the result being the production of a fissile structure or cleavage,
totally distinct from the original bedding-planes of the strata.
Clays under such circumstances pass into slates, in which no very
definite mineral banding is observed, or into schists, in which there
is a tendency for the separate minerals, such as mica and quartz, to
have a pseudo-stratified appearance, though the individual layers,
if closely examined, are found to be lenticular, not parallel in
shape. These schists and slates are developed on a large scale in
the Red Sea hills and Sinai.
The extent to which compression and tension has been carried in
these ancient sedimentary strata is well indicated in Sinai, where
rounded pebbles of quartz have been fractured, the two broken parts
somewhat separated, and the whole re-cemented in a schistose matrix.
In many instances, besides mechanical stress, another factor of
metamorphism of rocks has to be invoked to explain the phenomena
observed. Mineral changes take place in these rocks which are due
to the superadding of thermal action, consequent on their direct
contact with molten masses of granitic and other plutonic rocks. In
Sinai, for instance, new minerals are called into existence by
these contact effects, members of the _andalusite_ group, which
are formed at high temperatures, being produced near the junctions,
garnets being present in the mica-schists in a zone somewhat further
removed from the contact area, while beyond these is a belt where the
slaty strata show only traces of the formation of the new minerals
in the presence of ill-defined knots, or segregations forming the
so-called “Knotenschiefer” of the Germans.
It is possible to trace the influence of these contact and dynamic
effects, not only in the sedimentary rocks, but also in those of
plutonic origin. An excellent example of such changes, known as
foliation, can be observed in the Dal Cataract, where a granite is
present containing large crystals of red felspar. It is possible
to trace the change of this rock into a variety which instead
of being massive, is banded, the bands containing the larger red
felspars all tending to lie in one plane. These are not parallel,
but lenticular, certain of the minerals, especially the felspars,
remaining comparatively unaffected as “eyes,” round which the
other minerals appear to sweep. The quartz in many instances is
found to be crushed into a mosaic of small grains, and the plates
of mica have been dragged out and separated from one another by
the shearing. These rocks, having a general composition resembling
the granites, have been termed gneisses, and in some cases, similar
effects have been produced by the intrusion of veins of granite along
the horizontal planes of a more basic rock already banded. This result
is well seen in the Third Cataract, where a foliated rock composed of
a white felspar and mica has been pierced by veins of highly quartzose
granite, which has then spread along the laminæ of the older member.
Similarly in Sinai, the granite, where in contact with the schistose
sedimentary beds, has in certain favourable localities penetrated
between the schistose layers, producing a banded rock which approaches
a gneiss in character.
In Egypt, we are not at present acquainted with such gigantic
overfolding, fracture, and pushing of whole masses of strata one
over the other, as have been studied, for example, in the North-West
Highlands of Scotland, but there is in Egypt itself, a vast series
of gneisses, crystalline schists, marbles and dolomites whose
origin is but dimly understood, and which may represent changes
long anterior those exhibited by the ancient rocks of which we have
been speaking. Developed in the untrodden wastes of the Sudan, in
the wilder stretches of the Cataracts, in the desolate regions of
the Etbai Desert of Egypt, their study is attended with difficulty
and all our hypotheses must depend on the comparison of isolated
specimens with similar rocks from more favoured regions.