The principles and objects of geology, with special reference to the geology of Egypt — David Hume — John Shaqi
The principles and objects of geology, with special reference to the geology of Egypt
David Hume · en
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.