II.—ORIGIN OR FORMATION OF ROCKS.
The simplest illustration of the formation of a rock is one which may be observed by the dweller in Egypt every year during the Nile flood. The “red water” of the Nile, if collected during this period, and evaporated to dryness, leaves behind it the fine-grained sediment, or Nile mud, forming the soil to which this land owes its fertility; while in the central portions of the river, the coarser material, consisting largely of sand-grains, is being transported by the stronger current. As a result, the river flows for the main part over a sandy bed, the clays being restricted to the sides where the water is moving less swiftly, or to the fields on which the finer sediment is deposited. This “red water” has been traced step by step to its parent source, and has been proved to be derived from the wearing away of the widely-spread volcanic rocks of Abyssinia, disintegrated by differences of temperature, etc., and denuded by the destructive rainstorms which break over that region in the early summer. It is equally a matter of experience that on drying, this sediment passes from the condition of a soft and sticky mud to a hard and resistant clay, which, drying during the heat of the summer, cracks in every direction. The fluviatile character of these clay deposits is often revealed by the presence of the river shells enclosed in them, and in each succeeding year slight differences in composition in the material brought down are indicated by the layers being sharply marked off from one another, and so presenting the familiar stratified appearance. Again, much sand and clay is being carried seaward and deposited, the former, in general, nearer the land on account of its greater specific gravity and less finely divided character.
These clays and sands are forming both on the land and in the sea, a point which does not need elaboration, but when considering the origin of the limestones, and how it is that they seem at times to be built up of fossil shells, as a rule the general student would be at a loss for an answer. The researches in the great oceans, which have been carried on with such assiduity during recent years, have shown that the upper layers of their waters are crowded with a vast number of living organisms, apparently simple in structure, but having the power of extracting the carbonate of lime in solution and constructing shells of complicated and beautiful form. As the animals die, these tiny shells rain down to the bed of the ocean, slowly forming a muddy white calcareous paste which encloses the sea-urchins and other marine animals living in the depths of the sea. Thus, step by step, muds (which on drying are as genuine limestones as any now forming the cliffs and scarps of Egypt) are laid down on one another, separated into strata whenever some external change, such as the addition of some clayey matter transported from a river in flood, slightly alters the composition. The alteration is subsequently indicated either by variation in tint or by differing resistance to the wearing influences of the meteorological agencies. But how are these argillaceous muds transformed into the solid clays, the sands into sandstones, and the calcareous muds into fossiliferous limestones? How have these loose materials become consolidated so as to form the compact rock-masses with which we become acquainted in the most casual study of the physical structure of Egypt?
III.—DEFORMATION OF ROCKS.
Examination of the rock exposures at many localities in the neighbourhood of Cairo bears witness to the fact that the strata of limestone which were laid down horizontally upon the bed of the sea are no longer in the position they once occupied, but are now inclined to the horizontal plane at angles which are easily perceptible. This is especially noticeable in the two shallow cuttings under the Great Pyramid, where the strata are inclined 5 degrees to the south, revealing the reason why the whole Pyramid slope descends so rapidly in the southward direction. Again, if standing at the base of the same mighty structure, the gaze be allowed to wander over the broad expanse of the Nile Valley to the scarp of the Moqattam hills behind the Citadel, it will be seen that the white limestone of their lower slopes forms, not a long horizontal wall, but an arch, sloping strongly both north and south of the Citadel. There has been obviously change of form, but how has it taken place?
We learn with surprise that an earthquake shock in distant San Francisco or in the inhospitable regions of Turkestan has recorded itself through the delicately-poised recorder at Helwan, long before the dire news of destruction has flashed along the telegraph wire to the same destination. There is a realization of the instability of the earth’s crust, in spite of the solidarity of its component parts, but though the sudden shocks bring home this truth, it is not so readily grasped that day by day and hour by hour parts of the earth’s crust are slowly rising and others sinking, offering stern problems to the dwellers on the shores where these changes are most markedly taking place. On the eastern shores of England, towns and forests are being submerged beneath the relentless advance of the sea; on the coasts of the Scandinavian peninsula, etc., on the other hand, beaches formerly beneath the sea now stand high above the influence of its waves, and in Egypt coral-reefs which once grew beneath the waters of the Red Sea rise in places to over two hundred metres above it in bold hills or steep-sided terraces. Though possibly of far greater importance and significance than the sudden convulsions which have left so deep an impression on the mind of man, these movements are nevertheless so imperceptible that they arouse little attention.
A second type of deformation is noted where heavy masses of one rock rest on other and softer materials. The underlying beds are then often compressed and contorted; clays, for instance, are drawn out into thinner laminæ, giving rise to shales, while the massive rocks above either slip on the surface of the lower ones, if these be impermeable, and so permit of a water-layer forming along the junction, or else descend by sheer weight, producing a confused area of mixed materials in front of the still unshattered cliffs. Examples of this nature abound in Egypt wherever the Eocene limestones rest on the Cretaceous clays; on the railway from Armant to Matana the resulting effects are well observed near Shagab. Here in the main cliff the massive limestones are seen resting on the soft bluish clays, while in front is a wilderness of low hills in which limestones and clays are mixed, broken and contorted. But while these deformations may be irregular and local, there remain greater pressure-effects which have been regional in character, and by whose agency the solid rocks have been folded in the most remarkable manner, strata once horizontal being thrown into arches, or anticlines, and basin-like curves, or synclines. The importance of these changes cannot be over-estimated, and some of the marked features of Egyptian scenery depend directly on these effects. Reference has already been made to the slope of the Pyramid plateau, but the character of Egypt in far broader outline depends on the results of these pressures and the foldings so produced. A glance at the map suffices to show that many of the salient features in this country present a remarkable similarity and parallelism. The eastern cliff-wall of Kharga Oasis preserves a notable parallelism to a portion of the Nile Valley, though the latter be separated by many kilometres of wild desert plateau from the oasis; another portion of the Nile Valley also agrees with the Gulf of Suez in the broad outlines of its trend.
Fig. 1.—Two Anticlines and a Syncline.[3]
A section taken from Baharia or Kharga Oasis to the shores of the Red Sea reveals the fact that Egypt proper is bounded on the west by a low flat arch which has brought the underlying sandstones nearer to the surface, giving rise to the great oases.[4] These are mainly present in the sandstone areas, and are in part bounded by cliff-walls composed in many instances of clays at the base and limestones at the summit. To the east, on the other hand, rise the Red Sea hills, the central core of a steeply inclined arch in which the resistant granites and schists now rise high above the low-lying sandstone country which flanks them. Between these two arches is the flat-bedded syncline in which the nummulitic limestone is the conspicuous member, the strata in central Egypt having in consequence a half cup-shaped form, of which one half, the northern, may have disappeared by fracture beneath the waters of the Mediterranean Sea. This cup-like structure may be due to Egypt having not only been folded in a north-west and south-east direction but also almost at right angles, this latter folding giving rise to such remarkable features as the Wadi Araba, the Qena bend, and possibly affording the fundamental explanation for the great S-shaped bends of the Nile. In other and more mountainous regions, such as the Alps and Himalayas, these solid earth-waves may be under such immense pressure that their crests begin first to turn over like those of an advancing wave of the sea, and then may be broken in such a way that mighty masses of strata are rent asunder, those portions which are uppermost being thrust for great distances over the underlying beds. These extreme folds ending in fracture, or overthrusts, are as yet but little known in Egypt, though Dr. Ball has reported an interesting case from the neighbourhood of Abu Harba, and some of the phenomena of dislocation observed near the borders of the Gulf of Suez may possibly be explained as resulting from movements of this nature.