VI.—THE FORMATION OF IGNEOUS ROCKS.
The external and internal transforming agents are therefore of the deepest interest and of the highest economic importance, and the further study of the deep-seated changes leads directly to a consideration of the formation and sustained activity of the molten materials which find their main present-day expression in the phenomena of volcanoes. Their extension in the past is also revealed by the wearing hand of time in the wide distribution of coarsely crystalline granites and other igneous rocks, once deep-seated, but now exposed in regions which are either the cores of ancient continents or centres of exceptional deformation.
Igneous action and movements of the earth’s crust stand in intimate relation to one another, a point which has been clearly stated by Dr. Harker[9] as follows:—“Setting aside operations conducted in hypothetical intercrustal magma-basins, or generally in the unknown depths of the earth’s crust, we recognize the actual manifestations of igneous action chiefly in the forcing outward of molten magmas from a lower to a higher level within the crust or through the crust to the surface.” This study has led directly to the recognition of three phases of igneous action, of which two are intrusive, being phases in which the molten materials are raised from a lower to a higher level within the earth’s crust, and an extrusive phase, in which these materials are raised to the surface and poured out there as lavas.
Study in all parts of the world has further shown that these events follow a definite sequence or cycle of igneous activity, volcanic phenomena marking its commencement, followed by the movement of deep-seated igneous molten magmas (representing the plutonic phase), and closing with a number of minor intrusions, which may seam both the volcanic, plutonic, and sedimentary strata. 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. Here, too, belong the banded and columnar lavas of the Sixth Cataract, and the fragments of volcanic rocks which play an important part in the characteristic conglomerates of the Eastern Desert.
Still more conspicuous is the phase of plutonic activity, vast masses of granite, diorite, and other highly crystalline rocks as molten magmas having been in contact with or intruded into the overlying sediments and volcanic materials. From Sinai to the Sudan there is a geographical complex due to the intermingling of these deep-seated igneous rocks with the older and metamorphosed volcanic and sedimentary members. Desolate volcanic hills of dull-green shade, whose sides are covered with weathered debris of irregular outline, alternate with broad plain, out of which rise rounded masses of granite, or with mountain ranges, whose precipitous sides and serrated outlines constitute some of the most striking features of the Red Sea hill scenery.
The final phase, that of the minor intrusions, is admirably illustrated in Egypt, large areas of the desert consisting of low ridges formed of parallel bands or dykes of hard rocks, which have seamed the softer granite, the latter having subsequently been worn down by erosion. In other cases the dyke is of more basic composition and more easily denuded than the adjacent rocks.
When these evidences of past igneous activity are examined, they reveal the interplay of such varied and complicated chemical and physical effects that a large volume would be required to set forth the great body of facts observed. Of these only a bare statement of the most incomplete order must suffice. Igneous rocks are of very variable chemical composition, and for the purposes of discussion of their complex relationships have been subdivided into three series according to the amount of silica they contain, viz.:—The Acid Rocks with 50 to 70 per cent silica; The Intermediate Rocks with 50 to 60 per cent silica; The Basic Rocks under 50 per cent silica. As a whole, the igneous rocks are compounds of the silicates of alumina, magnesia, lime, potash and soda, with oxide of iron; the silicates of alumina, soda and potash are most abundant at the acid end of the scale, and those of lime, magnesia, with the oxides of iron and phosphates at the basic extreme. In consequence, the basic rocks are usually darker in colour and heavier than the more acid varieties. Not only have the igneous rocks a varying composition, but also a varying structure depending on the differences of origin and position. We may note their history as involving several stages, viz.:—(1) Solidification from the initial molten magma; (2) Deformation when under the influence of great earth pressures; (3) Transformation under the meteorological influences, at or near the earth’s surface. In addition there are the contact alterations produced near their junctions with the overlying strata, both within themselves and in the beds affected by the molten material.
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. 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. The minerals, as a rule, tend to crystallize out directly in the order of their basicity, the oxides of iron with their eight-sided crystals being among the first, so that their octahedral outlines are usually well preserved. The other minerals of the rocks follow in succession; sometimes complicated intergrowths occur owing to two crystals of different composition starting their formation at the same time in the same portion of the solution. Finally, 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.
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. The dykes have a tendency to be parallel to one another, and in some regions are the most conspicuous features, if harder than the surrounding rock giving rise to marked ridges separated in many cases by comparatively shallow valleys. Not a few of the main summits in the Red Sea hills and Sinai have assumed their present form and outline owing to the unequal denudation of one of these hard bands, while many an ascent, which otherwise would have been by no means free from danger, is simplified by the formation of gullies due to the wearing away of the softer and more basic members of this series.
In the hypabyssal varieties, as well as in surface flows, an examination of hand-specimens often reveals the presence of larger crystals (usually termed porphyritic crystals) scattered in a ground-mass in which no definite structure is visible except under the microscope. These may in part have been formed before the molten magma of the rock had begun its upward ascent, but in general they and the finer-grained base, have crystallized under similar conditions. Examinations of the ground-mass microscopically also shows it to be entirely crystalline in its nature, but the individual components are of very small size.
In the extruded volcanic rocks, on the other hand, the porphyritic crystals are probably of much earlier date than the ground-mass in which they are present, the latter being solidified above-ground, where cooling was rapid, pressure suddenly reduced, and many of the gaseous constituents were free to escape readily. As crystals require a certain temperature and a slow liberation of the heat for their formation, in many volcanic rocks the rapid cooling results in the production of a glass instead of a crystalline aggregate, so that the presence of a glassy matrix is in itself evidence of a quick loss of heat.