William Fraser Hume

“ Nothing is more striking than the presence of the dark films on the desert limestones in regions which are liable to a certain amount of rainfall, and nothing more convincing as to their origin than their absence in those portions of the south-western desert of Egypt where rain is of great rarity. Near the Nile, the Red Sea and the Mediterranean, dew may take the part of rain in action, and in a sense the results of its activity may appear more intense, as rain is liable to wash away the products of its own handiwork. ”

“ 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. ”

“ Egypt clearly illustrates this remarkable succession, and thus gives additional grounds for believing that it is of fundamental importance. Volcanic activity was developed on a gigantic scale when the most ancient sediments of Egypt, now forming the folded and altered slates and schists of the Red Sea hills, were being laid down. To this part of the cycle belong some of the most interesting rocks of the country, the imperial porphyry of Dokhan, the dark andesites that crown the highest summits in Sinai, and the country-rock in which some of the most ancient of Egypt’s gold-mines are situated. ”

“ Any beds of soft sands and clays are rapidly dissected by the torrent waters, a feature which readily explains the absence of conspicuous hills in the Eastern Desert east of Esna, where the Cretaceous clays form the dominant constituent in the geological structure of the country. Whatever the effects of sand-erosion in the Western Desert or rain-erosion in the hills and on the plateaus of the Eastern Desert of Egypt, they come relatively but little under the notice of the dweller on the Nile, to whom the river-erosion and the reformation of new materials become of primary importance. ”

“ If fracture by overthrust be unknown in Egypt, another type of fracture has produced effects of a far-reaching character. In certain [8] regions of the earth, folding is no longer the conspicuous method by which the rock-components of its crust are displaced with regard to one another. In many instances small fissures have been observed in which the beds on one side have been thrown down to a lower level than they are on the other. ”

“ The geological features of Egypt as presented to-day are the results of the formation through varying conditions and the subsequent deformation of the rocks composing its solid crust; we may next consider the varied agencies through whose action these are now undergoing transformation. A comparatively small portion of this country is undergoing erosion by the sea, and if elevation be still taking place, there is rather gain from the sea than destruction by it. ”

“ All opinions agree that the mineral oil is derived by some form of chemical action, though whether it arises from the decomposition of organic remains or whether it be of inorganic origin is still matter of dispute. Geological students have on the whole ranged themselves on the side of the first-named view, pointing out that the petroleum fields are all associated with sedimentary strata, whether sands or limestones. ”

“ A molten magma varies greatly in its behaviour according to its position, the solidification of that portion which remained deep-seated in the hidden reservoir beneath the earth’s crust taking place slowly and imperceptibly. A rock such as granite is not formed suddenly by instantaneous cooling, but step by step individual minerals are crystallized from the complex solution, and tend to develop as regular and often most beautiful crystals. ”

“ The study of coal-producing regions tends to show that the change to coal of high commercial value requires not only conditions favourable to the loss of the more volatile gases, but also that the beds must have been involved in great earth-movements, which have hastened the tendency to their being enriched in carbon, both favourable conditions of deposition and marked disturbance of the strata being thus required to obtain the much-desired result. ”

“ In broad outline, it may be stated that in southern Egypt, limestone rests on clays alternating with limestone, these on sandstones which in their upper part alternate with clay, and the sandstone on granite and metamorphic rocks (slates, schists, etc.) . The inclination or dip of the various beds is such that should a boring be made through the limestone near Cairo, one might expect to reach first a succession of beds where clay was predominant, followed by beds of sandstone, and finally the series of which the granite of Aswan is a conspicuous member. ”

“ Finally, [22] the last-formed minerals have to occupy the interspaces left by their consolidated neighbours, and in the more acid rocks this unenviable position is usually reserved for the free silica, forming quartz. These deep-seated masses, slowly solidified and entirely crystalline in their structure, have been called the Plutonic rocks, the example familiar to all dwellers in Egypt being the red Aswan, or monumental granite. In it, the felspars and micas or hornblendes present tend towards their true crystal form, while the quartz occupies the interspaces. ”

“ 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. ”

“ As every material expands and contracts according as it is heated or cooled, so the different component parts of the rocks composing the earth’s crust are in constant movement with regard to one another, and the less homogeneous they are the greater the effect in breaking them up into small masses or particles. ”

“ In geology, the term rock is applied alike to the soft and hard materials composing the earth’s crust, as sands may pass into solid sandstones, soft muds become the most tenacious of clays, and hard basalt originate as a molten lava.
[2] This statement may not absolutely apply to Cairo itself, seeing that in the disturbed region of Abu Roash sandstone is actually at the surface, and the clays are less conspicuously developed than is the case to the south. ”

“ In Egypt and the Sudan the Nile passes from areas where it flows peacefully and quietly, usually of considerable breadth and bounded by fertile lands, to others in which it is restricted, dashing down steep slopes in rapids and cataracts. A geological examination has shown that in the first case the river is flowing over and between sedimentary homogeneous rocks, such as the limestones and sandstones, while in the second instance it has entered regions composed of heterogeneous igneous and metamorphic rocks, such as the granites, gneisses and schists. ”

“ It is a source of surprise to most to learn that the greater number of mineral substances, if free to develop from solution without external hindrances, tend to form solid bodies which are fashioned on geometrical principles, constituting one of the most interesting and mysterious problems with which scientific thought has to deal. ”

“ Considering the earth-movements of Egypt as a whole, the evidence shows folding to have been the more important type of displacement on its western side, and intense faulting to have been most conspicuous in the east, while between these two extremes, the relative importance of folding and faulting will remain a contested point. ”

“ 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 [25] those exhibited by the ancient rocks of which we have been speaking. ”

“ 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. ”

“ 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. ”

“ Of greater importance to the world at large are the gradual changes which vegetable matter (collected under specially favourable circumstances free of all sandy and clayey admixture) has undergone through vast periods of time, causing the slow evolution of the oxygen, hydrogen and nitrogen, originally present, with a gradual predominance of the carbon. ”

“ A river is, in fact, the main agent combining the effects of transformation and reformation, new strata being produced in its plain tract as the result of the eroding activities in its upper reaches. Much of the detrital material is also carried seaward to form deposits of marine sands and muds along the shore-lines of the continents, these themselves becoming, should subsequent differential movement of land and sea take place, the sandstones and clays of future continental areas. ”

“ Other internal chemical activities are at work, producing changes which are still the cause of debate and earnest study. The origin of petroleum must undoubtedly be traced to chemical transformations of a complicated character, if we may judge by the number of experimental methods which yield petroleum as a product. ”

“ The inorganic view has been held as tenaciously by a number of men experienced in the search for oil, and it is capable of argument that sulphur dioxide and sulphuretted hydrogen, if being produced simultaneously, may result in the alteration of limestone to gypsum, free sulphur and petroleum being also obtained in the reaction. ”

“ With the close of the Eocene epoch a reverse movement set in, by which the pure white marine limestones of the Moqattam hills were capped by the sandy limestones and clays forming the brown-tinted beds of their upper portion, the whole being, in its turn, covered by the coarse flint and quartz gravel which containing silicified trees [5] is the chief desert-former [10] in the immediate neighbourhood of Cairo, as also of a large area in north-west Egypt. ”

“ Similarly, going westward and crossing the great limestone desert, on reaching the edge of the scarp which bounds the oases, clays appear from under the limestones, and in their turn overlie sandstones forming the floors of the oasis depressions. Closer examination reveals the fact that in the upper part of the sandstone series seams of clay alternate with the sandy layers, and in that part of the cliff where clay bands predominate, beds of limestone alternate with them. ”

“ Joints may also be called into being by the effects of internal pressures and movements within the earth’s crust, such structures having been experimentally reproduced by Daubrée in materials under stress by torsion and by simple pressure. ”

“ The granite of Aswan displays such jointing to a marked degree, giving rise to remarkable hills composed of huge boulders of granite piled on one another.
V.—CHEMICAL TRANSFORMATION OF ROCKS.
Besides the mechanical effects of river, rain, and wind, other changes whose wide-reaching significance cannot be over-estimated, [15] are taking place on and below the earth’s surface. Chemical action is slowly at work producing effects of the first importance to man. Rain-water has the power of absorbing important quantities of carbonic acid gas and oxygen from the atmosphere. ”

“ Closer study has shown that these wind-swept expanses afford no protection or resting-place for the finer sands and that consequently their floor must be formed of the more solid materials which wind cannot carry before it. If this be realized, no astonishment will be felt that the Libyan Desert surface is composed of limestone, or of coarse gravels from between whose larger fragments all the finer sand has been swept away.
Fig. 5.—Sand-erosion of Sandstone Cliff at Gebel el Tunb. Wadi Qena, Eastern Desert. ”

“ A vein may sometimes grow by the accretion of successive layers, which, owing to local causes, such as the relative content of iron oxide, etc., may display slightly different colours, one of the results being the production of so interesting a rock as the Egyptian alabaster, which is a carbonate of lime. ”

“ Intermediate between the extruded volcanic rocks, and the highly crystalline varieties of the internal reservoirs of molten material, are the dyke or hypabyssal rocks, which as narrow vertical or nearly vertical intrusions pass in all probability from the deep-seated source towards the surface of the earth, when they reach it giving rise to volcanic effects. ”