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.