Popular Science Monthly/Volume 9/August 1876/Rock-Structure
By Rev. J. MAGENS MELLO, M. A., F. G. S.
THE study of rock-structure is one of great interest to the geologist, and not only does it teach him the various materials of which any particular rock is built up, but it will often lead him to the knowledge of wonderful facts relating to its origin and past history, and will enable him to trace some of the many changes to which it may have been subjected during the lapse of time.
I propose to illustrate this by taking some familiar specimen and showing the ways in which we may investigate its nature and history.
Suppose we take a piece of granite and see what we may learn about it. There are few persons but are acquainted with this rock in some one or more of the forms in which it is found. Our public buildings often present us with splendid illustrations of granite, sometimes roughly hewed, as it has come from the quarry; in other cases highly polished. We have seen the fine gray stones from Aberdeen, or the beautiful red ones from Peterhead and elsewhere. Now, when we begin to examine a piece of one of these granites, we see at once that it is not an homogeneous stone—such, for instance, as is a bit of flint but that it is built up of various dissimilar-looking materials; and we may notice, moreover, that one or more of those materials is crystalline, that it is shaped in some regular geometrical form. We shall probably be struck with certain whitish or flesh-colored crystals, more conspicuously prominent than the other substances of which the specimen is composed. With some care we may be able to make out in part the form of these crystals, and perhaps to measure one or more of their angles; then, too, we shall notice that these crystals are apparently imbedded in a more glassy-looking substance of a clear and grayish color, and here and there we shall observe some bright spangles of a thin flaky mineral. We shall thus have seen the three principal minerals of which typical granite rock is composed; the larger opaque crystals, whether white or pink, are feldspar, the glassy mineral is quartz, and the little glittering spangles are mica. We may next proceed to a more detailed examination of each of these in turn. We will first ask the chemist what he can tell us of their composition. The chemist is not satisfied with merely knowing that a certain mineral occurring in certain definite crystal-line or other forms is quartz, another feldspar, and so on; but he asks further: "What is this quartz? Is it a simple body, or is it, simple as it may appear to sight, a compound of two or more elements?" He takes various specimens of quartz, some perhaps from the granite, others from some other rocks, and subjects them to the analytical processes of the laboratory: the result is, that he finds all quartz, no matter what its color may be, whether white or pink or black, or pure and colorless as glass, to be a compound of the metalloid silicon and the gas oxygen; in other words, that it is an oxide of silicon, to which he assigns the name silica. By a series of analyses he is able to correlate the quartz of the granite with all other forms, and they are many in which this mineral occurs. The flint of the chalk, the white
Fig. 1.—Section of Granite from Cornwall (polarized), magnified 26 diameters.
veins so often met with in the older slaty rocks, the agates picked up on the sea-shore and elsewhere, the beautiful crystals known as cairngorms, amethysts, and others, are all found to be but varying forms of the same substance, colored sometimes by adventitious matter, as iron, etc.; and he finds, too, that the exquisite skeletons of some of the sponges, the delicate valves of the Diatomaceœ and other minute specimens of organic life, consist of this very same silica, which is indeed one of the most important compounds entering into the structure of the earth's crust. Suppose the student next picks out one of the feldspar-crystals: this on analysis will be, as was the quartz, found to be also a combination; in it he will also find silica, but the silica in this instance is found to be combined with the metals aluminium and potassium—in fact, is a double silicate of alumina and potash. There are many varieties of feldspar: some of them differ from that most common in granite, which is called plagioclase, in containing lime or soda instead of potash; these are also distinguished from the orthoclastic series by their crystalline structure, which will afford, as we shall see, a ready method for their recognition, when they are microscopically examined. When the granite rocks become decomposed, as they often do in Cornwall and elsewhere, through the wear and tear of the weather, we frequently find the disintegrated materials so separated that the silicate of alumina of the feldspar forms thick deposits of the beautiful white clay known as kaolin, and which is so valuable to the china-manufacturer.
The mica of granite is usually a variety called Muscovite, or potash mica; this again on chemical analysis is found to contain, as did the feldspar, silica, alumina, and potash, and also often some iron and manganese. There are several different sorts of mica, also, sometimes found in granite, especially Biotite, the composition of which varies from the above; but all the micas may be known by their being found in flatfish crystals, which may be split up into an infinity of thin leaflets. Thus far our unaided eyesight and the help of the chemist have
Fig. 2. Orthoclase Feldspar.Fig. 3.—Plagioclase Feldspar.
shown us what granite is made of; but we are now beginning to learn that, would we know something of the real history of a rock, a far minuter examination is needful, and geologists are rapidly learning that they must turn to the microscope if they would receive answers to many important questions, both as to the history and also as to the composition of rocks. A marvelous light has been shed during the past few years on rock-structure through this minute investigation, especially with the aid of polarized light. The intricacies of the closest-grained rocks have been disentangled, their component parts distinguished from each other, and the very order and history of their combination in the mass revealed. Now, when we examine our granite beneath the microscope, which can be done by having thin slices prepared, we shall learn something about it which we could hardly hope to have discovered without this aid. There has been much speculation as to the origin of granite, whether it is a plutonic—that is, an old volcanic rock—or whether it is only a deposit from water consolidated and altered during the lapse of long ages by heat and pressure: the microscope will help us to the truth. When magnified and examined with the polariscope, a thin section of granite is a very beautiful object, and its different constituent parts stand revealed with the greatest distinctness: we at once learn to see the crystals of feldspar, somewhat opaque and cloudy as they usually are in granite, but now and then clear and beautifully striped, and also the crystals of mica, imbedded in the clear quartz, which will be at once known by its bright clear colors and by the margin of rainbow-like tints which border its patches. Ordinary orthoclase feldspar is usually some-what opaque and dirty-looking under the microscope, and by this it may be distinguished from the clear, glassy sanidine which is frequently found in igneous rocks, and presents under the microscope, when polarized, pure rich colors as well as sharply-defined crystals similar in form to those of the common orthoclase. The orthoclastic feldspars may be very readily distinguished from the plagioclastic by their structure, as revealed by the polariscope; the latter invariably are seen to be striped with variously-colored bands, showing what is called twin crystallization; and the orthoclase, though often forming twins on a larger scale, does not present the minutely-banded
Fig. 4.—Mica (Biotite).
appearance of the plagioclastic feldspars. The mica in the granite section will not be difficult to recognize, especially if Biotite; often we shall observe it as forming fairly-shaped hexagonal crystals, and the polariscope will also help us to know it by its thinly-laminated structure, giving rise to fine parallel striæ on the surface of its crystals. Its colors, also, when polarized will be duller than those of the quartz, for which it might sometimes be mistaken at first sight, should it be a light-colored mica; and then, again, it will frequently be found that when the prisms of the polariscope are crossed the mica becomes perfectly opaque, its sections having been formed across the optical axis. But let us now look at the quartz. We shall observe that this quartz is generally not crystallized in definite forms, as are the feldspar and the mica; it appears as a matrix which has been at some time or other soft and so is penetrated by the other crystals, the interspaces of which it fills up: this shows us at once that it must have been solidified after them, and so was unable to assume its regular forms. This is a very remarkable fact, and helps us toward the secret of the formation of the granite. We know that quartz requires a higher temperature to melt it than does either the feldspar or the mica, and so, had the granite been formed as are regular volcanic rocks in the ordinary way of igneous fusion, we should certainly have found that the quartz would have crystallized before either the feldspar or the mica, and it would have been seen in definite crystalline form, and its crystals would have interfered with and penetrated those of the other mineral constituents of the rock. Again, if we look carefully at the quartz with a moderately high power, we shall see in it certain small cavities, and some of these will be seen to contain a certain amount of liquid, and also an air-bubble, which will move as the specimen is moved. This liquid has been proved to be water, and from the fact of its not entirely filling the cavity we learn that a reduction of temperature has taken place since the water was first caught up by the quartz, causing the contents of the cavities to contract. Sometimes we shall find other cavities, which, instead of containing water, contain small crystals, or even air only. Now, from all these facts it appears tolerably certain that the granite was formed under peculiar circumstances; it has never been such a purely molten rock as is the lava of a volcano, which is poured out from its crater to the light of day. We gather that it was rather formed at great depths in the earth, where it may have been partially melted, partially subjected to the action both of water and of steam, charged with various mineral substances, and subjected to enormous pressure. What the original condition of granite was we cannot tell; some have gone so far as to think that it may have been that of a sedimentary rock, which has been metamorphosed by the forces just alluded to. But, whatever the primary state of granite may have been, its present condition shows it to belong undoubtedly to the igneous class of rocks, but to have been formed under conditions differing from those which have given rise to lavas reaching the surface. As far as can be gathered, the granite rocks, as such, have never seen the light of day until exposed by denudation, etc.; their origin was deep in the central portions of ancient volcanoes, where, by partial melting and slow cooling, under intense pressure, and in the presence of some water, the various minerals came together and crystallized into granite.—Science-Gossip.