VII.—THE FORMATION OF METAMORPHIC ROCKS.
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.