[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.
CHAPTER X.
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.