Common Minerals and Rocks

by William O. Crosby

(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.