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Part 10

Common Minerals and Rocks · William O. Crosby — chapter 10 of 22 · ~2,451 words · public domain

Textures of Rocks.

Texture is a general name for those smaller structural features of rocks which can be studied in hand specimens, and which depend upon the forms and sizes of the constituent particles of the rocks, and the ways in which these are united.

By “constituent particles” we mean, not the atoms or molecules of matter composing the rocks, but the pebbles in conglomerate, grains of sand in sandstone, crystals of quartz, feldspar, and mica in granite, etc. The four most important textures are:—

(1) Fragmental texture.—The rock is composed of mere irregular, angular, or rounded, but visible, fragments. Examples: sand, sandstone, gravel, conglomerate, etc. Specimens 30, 31, 28, 29.

(2) Crystalline texture.—The constituent particles are chiefly, at least, distinctly crystalline, as shown either by external form, or cleavage, or both. Examples: granite, diabase, gneiss, etc. Specimens 45, 1, 41.

(3) Compact texture.—The constituent particles are indistinguishable by the naked eye, but become visible under the microscope, appearing as separate crystalline grains or as irregular fragments. In other words, if, in the case of either the granular or crystalline textures, we conceive the particles to become microscopically small, then we have the compact texture. Examples: clay, slate, many limestones, basalt, etc. Specimens 34, 35, 39.

(4) Vitreous texture.—The texture of glass, in which the constituent particles are absolutely invisible even with the highest powers of the microscope, and may be nothing more than the molecules of the substance, which thus, so far as our powers of observation are concerned, presents a perfectly continuous surface. Examples: obsidian, glassy quartz, and some kinds of coal. Specimens 47, 15.

These four textures, which, it will be observed, are determined by the forms and sizes of the constituent particles, may be called the primary textures, because every rock must possess one of them. We cannot conceive of a rock which is neither fragmental, crystalline, compact, nor vitreous. But in addition to one of the primary textures, a rock may or may not have one or more of what may be called secondary textures. These are determined by the way in which the particles are united, the mode or pattern of the arrangement, etc. Following are definitions of the principal secondary textures:—

(1) Laminated texture.—This exists where the particles are arranged in thin, parallel layers, which may be marked simply by planes of division, or the alternate layers may be composed of particles differing in composition, form, size, or color, etc. Among the laminated textures we thus distinguish: (a) the banded texture, where the layers are contrasted in color, texture, or composition, but cohere, so that there is no cleavage or easy splitting parallel with the stratification; and (b) the schistose or shaly texture, where such fissility or stratification-cleavage exists. If a fragmental, compact, or vitreous rock is fissile, we use the term shaly; but a fissile, crystalline rock is described as schistose. The banded texture may occur with the fragmental,—banded sandstones, etc.; with the crystalline,—many gneisses, etc. (specimen 41); with the compact,—many slates, limestones, felsites, etc. (specimens 34, 42); with the vitreous,—banded obsidian, furnace slags, and some coal. The schistose texture may occur with the crystalline,—mica schist, etc. (specimen 43); and the shaly texture with the compact and fragmental, but rarely with the vitreous.

(2) Porphyritic texture.—We have this texture when separate and distinct crystals of any mineral, but most commonly of feldspar, are enclosed in a relatively fine-grained base or matrix, which may be either crystalline, compact, or vitreous, but rarely fragmental. Specimens 5, 6, 7 are examples of the porphyritic compact texture.

(3) Concretionary texture.—When one or more constituents of a rock have the form, in whole or in part, not of distinct angular crystals, but of rounded concretions, the texture is described as concretionary, the concretions taking the place in this texture of the isolated crystals in the porphyritic texture. This texture occurs in connection with all the primary textures, but the most familiar example is oölitic limestone.

(4) Vesicular texture.—A rock has this texture when it contains numerous small cavities or vesicles. These are most commonly produced by the expansion of steam and other vapors when the rock is in a plastic state; and hence the vesicular texture is found chiefly in volcanic rocks. Except rarely, it is associated only with the compact texture,—ordinary stony lavas (specimen 49); and with the vitreous texture,—pumice (specimen 48).

(5) Amygdaloidal texture.—In the course of time the vesicles of common lava are often filled with various minerals deposited by infiltrating waters, giving rise to the amygdaloidal texture, from the Latin amygdalum, an almond, in allusion to a common form of the vesicles, or amygdules, as they are called, after being filled. The amygdaloidal texture is thus necessarily preceded by the vesicular, and is limited to the same classes of rocks. Specimen 50.

Besides the foregoing, there are many minor secondary textures. The rocks known as tufas have what may be called the tufaceous texture. Then we have kinds of texture depending on the strength of the union of the particles, as strong, weak, friable, earthy, etc.

Classification of Rocks.

Having finished our preliminary observations on the characteristics of rocks, we are now about ready to begin a systematic study of the rocks themselves; but it is needful first to say a few words about the classification of rocks, since upon this depends not only the order in which we shall take the rocks up, but also the ideas that will be imparted concerning their relations and affinities. The classifications which have been proposed at different times are almost as numerous as the rocks themselves. Some of these are confessedly, and even designedly, artificial, as when we classify stones according to their uses in the arts, etc. But we want something more scientific, a natural classification; that is, one based upon the natural and permanent characteristics of rocks. Rocks have been classified according to chemical composition, mineralogical composition, texture, color, density, hardness, etc.; but these arrangements, taken singly or all combined, are inadequate.

A natural classification may be defined as a concise and systematic statement of the natural relations existing among the objects classified. Now the most important relations existing among rocks are those due to their different origins. We must not forget that lithology is a branch of geology, and that geology is first of all a dynamical science. The most important question that can be asked about any rock is, not What is it made of? but How was it made? What were the general forces or agencies concerned in its formation? Rocks are the material in which the earth’s history is written, and what we want to know first concerning any rock is what it can tell us of the condition of that part of the earth at the time it was made and subsequently.

Classification of Rocks.

The geological agencies, as we have already learned, may be arranged in two great classes: first, the aqueous or superficial agencies originating in the solar heat, and producing the sedimentary or stratified rocks; and, second, the igneous or subterranean agencies originating in the central or interior heat, and producing the eruptive or unstratified rocks. Hence, we want to know first of any rock whether it is of aqueous or igneous origin. Then, if it is a sedimentary rock, whether it has been formed by the action chiefly of mechanical forces, or of chemical and organic forces. And, if it is an eruptive rock, whether it has cooled and solidified below the earth’s surface in a fissure, and is a dike or trappean rock, or has flowed out on the surface and cooled in contact with the air, and thus become an ordinary lava or volcanic rock.

Here we have the outlines of our classification, and it will be observed that we have simply reached the conclusion, in a somewhat roundabout manner, that there should always be a general correspondence between the classification of rocks and the classification of the forces that produce them. The general plan of the preceding scheme of the classification must now be clear, and the details will be explained as we go along.

Descriptions of Rocks.

1.—Sedimentary or Stratified Rocks.

1. MECHANICALLY FORMED OR FRAGMENTAL ROCKS.—These consist of materials deposited from suspension in water, and the process of their formation is throughout chiefly mechanical. The materials deposited are mere fragments of older rocks; and, if the fragments are large, we call the newly deposited sediment gravel; if finer, sand; and, if impalpably fine, clay. These fragmental rocks cannot be classified chemically, since the same handful of gravel, for instance, may contain pebbles of many different kinds of rocks, and thus be of almost any and very variable composition. Such chemical distinctions as can be established are only partial, and the classification, like the origin, must be mechanical. Accordingly, as just shown, we recognize three principal groups based upon the size of the fragments; viz.:—

(1) Conglomerate group. (2) Arenaceous group. (3) Argillaceous group.

This mode of division is possible and natural, simply because, as we observed in an early experiment, materials arranged by the mechanical action of water are always assorted according to size. When first deposited, the gravel, sand, and clay are, of course, perfectly loose and unconsolidated; but in the course of time they may, under the influence of pressure, heat, and chemical action, attain almost any degree of consolidation, becoming conglomerate, sandstone, and slate, respectively. The pressure may be vertical where it is due to the weight of newer deposits, or horizontal where it results from the cooling and shrinking of the earth’s interior. The heat may result from mechanical movements, or contact with eruptive rocks; or it may be due simply to the burial of the sediments, which, it will be seen, must virtually bring them nearer the great source of heat in the earth’s interior, on the same principle that the temperature of a man’s coat, on a cold day, is raised by putting on an overcoat. The effect of the heat, ordinarily, is simply drying, coöperating with the pressure to expel the water from the sediments; but, if the temperature is high, it may bake or vitrify them, just as in brick-making. Sediments are consolidated by chemical action when mineral substances, especially calcium carbonate, the iron oxides, and silica are deposited between the particles by infiltrating waters, cementing the particles together. This principle is easily demonstrated experimentally by taking some loose sand and wetting it repeatedly with a saturated solution of some soluble mineral, like salt or alum, allowing the water to evaporate each time before making a fresh application. The interstices between the grains are gradually filled up, and the sand soon becomes a firm rock. But the student should clearly understand that, in geology, gravel, sand, and clay are just as truly rocks before their consolidation as after. It is plain then that in each of the principal groups of fragmental rocks we must recognize an unconsolidated division and a consolidated division.

(1) Conglomerate group.—The rocks belonging in this group we know before consolidation as gravel, and after consolidation as conglomerate.

Gravel.—The pebbles, as we have already seen, are usually, though not always, well rounded or water-worn; and they may be of any size from coarse grains of sand to boulders. As a rule, however, the larger pebbles, especially, are of approximately uniform size in the same bed or layer of gravel, with, of course, sufficient fine material to fill the interstices. Although the same limited mass of gravel may show the widest possible range in chemical and mineralogical composition, yet hard rocks are evidently more likely than soft rocks to form pebbles; and hence quartz and quartz-bearing rocks usually predominate in gravels. Specimen 28.

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