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.
(4) Of the results of Hydration, the most striking examples in Egypt are the formation of kaolin near Aswan, due to the absorption of water by the felspars of the granitic and gneissose rocks, and the thick zone of decomposition (kaolinic) products, which was cut through in excavating the navigation canal in the syenite which forms the main rock at that locality.
The total effect of all the above-mentioned meteorological influences results in the weathering of the rock-surface, involving the softening and crumbling of the harder materials, but sometimes leading to the solidification of materials previously loosely aggregated by substances left as cementing agents when the water containing them in solution has evaporated.
In addition to the various direct results of the meteorological activities upon the earth’s surface, there are others which indicate more subtle changes. Perhaps amongst the most interesting of these is the formation of concretions—bodies composed of one material aggregated in more or less rounded or irregular form in a rock of another composition. Among the most interesting and abundant of these are the layers of flint, which form bands of strikingly parallel character in the limestones of Upper Egypt. These have not yet been submitted to the detailed study which similar concretions have received in Europe, but there is little doubt that they, in large measure, represent the aggregation of gelatinous silica round decomposing organic materials, the shells of organisms and the framework of siliceous sponges often forming their centre. In some cases, as in the fossil trees, the replacement appears to have taken place molecule by molecule, as the outlines of every cell of the once woody fibre are now replaced in silica. By a well-known transition, this once gelatinous material has now become one of the most solid of substances.
Ferruginous concretions, composed of oxide of iron, are present in many of the Egyptian sandy clays, some of the beautifully-tinted purple, yellow and red ochres being found in this form; and the natives collect them for the use of the women as ornamental coloration.
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. This passage from vegetable matter to coal has been noted in Egypt in connection with the Nubian sandstone, beds of carbonaceous material deserving the name of lignite or even bituminous coal having been found at various localities. The deposits found up to the present time are of such tenuity that it is not possible on the evidence available to express optimistic opinions as to the probable occurrence of workable coal in Egypt, but still they are of sufficient interest to be kept constantly in mind while the Geological Survey is prosecuting its researches. From time to time the finding of coal-seams has been reported at Edfu, in Kharga, at Saqiet el Teir and Abu Radham[7] in the Eastern Desert, but the efforts hitherto made have resulted in failure.
The evidence thus far available shows that great rivers were entering the sea in Nubia during an early geological period (the Cretaceous), typical fresh-water shells having been found south of Aswan covered with marine worm-tubes; leaf-imprints are abundant in some of the sandy layers, and in isolated instances they have collected in sufficient quantity to give rise to lignite and bituminous coal-layers of extreme thinness, showing that this interesting and important change has taken place, at least to some extent, in Egypt itself. 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.
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. 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. 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.
In this connection it is interesting to note that gypsum, sulphur and petroleum are associated at Jemsa, on the Gulf of Suez.
One of the most interesting features in connection with petroleum is the phenomenon presented in most oil-fields of oil-wells separated perhaps by only thirty metres emitting oil under pressure at the same time; also the great pressures indicated by the remarkable fountain flows which are of constant occurrence in the principal petroleum fields.[8]