The principles and objects of geology, with special reference to the geology of Egypt

by W. F. Hume

The nature of a river system need not be dealt with in much detail here, as I have already discussed this subject in “Survey Notes” for April, 1907, under the title of “River Characteristics as illustrated by the Nile.” It may be well, however, to recall that a normal river passes through three distinct phases of activity. In its mountain tract (for most large rivers arise in the higher altitudes) there is maximum erosion and backward growth of the river system. In its central portion, or valley tract, the stream is acting as a transporter of eroded material, and such erosion as there is, is downward rather than sideward. Finally, the plain tract is the region of deposition of the materials so carried, erosion being lateral, and the growth of the stream bed forward in the form of a fan-shaped delta where the transported sands and clays enter the sea.

But this general succession may be further complicated by circumstances depending upon the geological conditions. 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. The production of these rapids is due to the combination of steep slope and the difference between harder and softer materials, the rapidly-moving waters wearing away those more easily denuded, while the compact members remain as obstacles to their advance, and are only slowly worn away along joint-planes and other lines of weakness. In the Third Cataract, hard bars of granite rising through softer gneiss at right angles to the river course have produced the main rapids; elsewhere, as at the Bab el Kebir, near Wadi Halfa, the river has taken advantage of a thin dyke of soft rock traversing an extremely hard diorite, so that the stream has worn a narrow gully between steep rock-walls, where the intensity of the rush of water is greatly exaggerated owing to its being restrained and fettered by the narrowness of the passage. In some cases the same result has been produced owing to the existence of a line of fracture, or fault, across the stream, the waters taking advantage of this line of least resistance. The general erosion in these rapids is accompanied by great local effects where eddies and whirlpools are produced, and the sand and rocky fragments act as abrading agents. Pot-holes are formed in the solid rock, and rapidly deepened by the intense effects of this nature produced during times of flood, the result being splendidly illustrated in some of the smaller islands of the First Cataract at Aswan.

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.

But there are other agencies at work as transformers on and within the earth’s crust. There are in most rocks a series of divisional planes, which may be either vertical or inclined, and to which the name of joints has been given. These may arise from various causes. Both in sedimentary and igneous rocks they are in part due to contraction during consolidation—in the former when they lose their contained water, in the latter when they solidify from a molten condition. 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, 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. On the average, rain-water contains 1·77 per cent by volume of dissolved carbonic acid gas, and 33·76 per cent of dissolved oxygen. In passing through the soil, rain-water also absorbs the organic acids formed by the decomposition of plant remains. These dissolved gases and organic acids render rain an active chemical agent in the alteration of rocks, its effects being conveniently classified under the headings: (1) Oxidation; (2) Solution; (3) Formation of Carbonates; and (4) Hydration.

(1) Oxidation results in the formation of thin crusts on the surface of rocks, the compounds of manganese and iron so frequently present in them being also rusted or hydrated by the action of the rain-water. 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.

(2) The effects of Solution are of the greatest importance, limestone being soluble to the extent of about 1 part in 1,000 in water saturated with carbonic acid. In many limestone countries of the world the solution effects are marked by the production of underground caves and channels and in some parts of the north-eastern desert of Egypt, where chalky limestones are the main constituent, this action has produced remarkable results—large caves, cylindrical channels, and natural bridges being of not uncommon occurrence.

(3) Formation of Carbonates. Owing to the rains in Egypt being of very brief duration, but nevertheless extremely active while they last, the soluble material in the condition of the unstable bicarbonate of lime is carried only a short distance, and losing its loosely combined carbonic acid is redeposited in the cracks of the rocks, as veins of carbonate of lime, or as the cementing material by which broken fragments are consolidated into compact breccias. This action may be seen in the cliff face south of the Pyramids, near the Sphinx, where the sandy limestones forming the top of the hill have been attached by the rain containing carbonic acid. The calcareous tests of the shells in the sandy limestones have been dissolved away, leaving only the sandy internal casts of the shells behind, and the material so removed has been redeposited in intricate interlacing veins in a clayey band immediately below. 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. As a rule, the term alabaster is applied to the sulphate rather than to the carbonate of lime. Probably much carbonate of lime is also carried in solution to the sea, and there forms the source of the material which hundreds of living animals seize upon for the production of the shells in which they dwell. I was much struck last year, during a journey from the Pyramids to Wasta, to note how the oyster-beds of one age (the Pliocene) formed themselves upon oyster-beds of a long preceding period (the Eocene), probably on account of the greater amount of carbonate of lime at those localities, present owing to solution of the earlier shell-structures.

That veins of carbonate of lime should be present in limestone districts is, in view of the above statements, not surprising, but it does appear somewhat startling at first sight, to find marked deposits of carbonate of lime lining the floors and sides of torrent-beds in districts entirely composed of igneous or volcanic rocks of complicated mineral structure. Experience has shown, however, that the lime silicates, so abundant in the more basic members of the igneous series, such as diabases and diorites, are liable to the attack of the rain-waters containing carbonic acid, carbonate of lime being produced by the reaction.