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.