dictated by common-sense, with the object of establishing and maintaining, in the words of the old maxim of the sanitarian, mens sana in corpore sano.
THE study of rock-structure is one of great interest to the geologist, and not only does it teach him the various materials of which any particular rock is built up, but it will often lead him to the knowledge of wonderful facts relating to its origin and past history, and will enable him to trace some of the many changes to which it may have been subjected during the lapse of time.
I propose to illustrate this by taking some familiar specimen and showing the ways in which we may investigate its nature and history.
Suppose we take a piece of granite and see what we may learn about it. There are few persons but are acquainted with this rock in some one or more of the forms in which it is found. Our public buildings often present us with splendid illustrations of granite, sometimes roughly hewed, as it has come from the quarry; in other cases highly polished. We have seen the fine gray stones from Aberdeen, or the beautiful red ones from Peterhead and elsewhere. Now, when we begin to examine a piece of one of these granites, we see at once that it is not an homogeneous stone—such, for instance, as is a bit of flint but that it is built up of various dissimilar-looking materials; and we may notice, moreover, that one or more of those materials is crystalline, that it is shaped in some regular geometrical form. We shall probably be struck with certain whitish or flesh-colored crystals, more conspicuously prominent than the other substances of which the specimen is composed. With some care we may be able to make out in part the form of these crystals, and perhaps to measure one or more of their angles; then, too, we shall notice that these crystals are apparently imbedded in a more glassy-looking substance of a clear and grayish color, and here and there we shall observe some bright spangles of a thin flaky mineral. We shall thus have seen the three principal minerals of which typical granite rock is composed; the larger opaque crystals, whether white or pink, are feldspar, the glassy mineral is quartz, and the little glittering spangles are mica. We may next proceed to a more detailed examination of each of these in turn. We will first ask the chemist what he can tell us of their composition. The chemist is not satisfied with merely knowing that a certain mineral occurring in certain definite crystal-line or other forms is quartz, another feldspar, and so on; but he asks further: "What is this quartz? Is it a simple body, or is it, simple as