(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.