Common Minerals and Rocks ------------------------------------------------------------------------ Boston Society of Natural History GUIDES FOR SCIENCE-TEACHING NO. XII COMMON MINERALS AND ROCKS BY WILLIAM O. CROSBY D. C. HEATH & CO., PUBLISHERS BOSTON NEW YORK CHICAGO ------------------------------------------------------------------------ _Copyright_ BY THE BOSTON SOCIETY OF NATURAL HISTORY 1881 I D 2 ------------------------------------------------------------------------ INTRODUCTION. [Illustration] Minerals and rocks, or the inorganic portions of the earth, constitute the proper field or subject-matter of the science of Geology. Now the inorganic earth, like an animal or plant, may be and is studied in three quite distinct ways, giving rise to three great divisions of geology, which, as will be seen, correspond closely to the main divisions of Biology. First, we may study the forces now operating upon and in the earth—the geological agencies—such as the ocean and atmosphere, rivers, rain and frosts, earthquakes, volcanoes, hot springs, etc., and observe the various effects which they produce. We are concerned here with the dynamics of the earth; and this is the great division of _dynamical geology_, corresponding to physiology among the biological sciences. Or, second, instead of geological causes, we may study more particularly geological effects, observing the different kinds of rocks and of rock-structure produced by the geological agencies, not only at the present time, but also during past ages. This method of study gives us the important division of _structural geology_, corresponding to anatomy and morphology. All phenomena present two distinct and opposite aspects or phases which we call _cause_ and _effect_; and so in dynamical and structural geology we are really studying the opposite sides of essentially the same classes of phenomena. In the first division we study the causes now in operation and observe their effects; and then, guided by the light of the experience thus gained, we turn to the effects produced in the past and seek to refer them to their causes. These two divisions together constitute what is properly known as physiography; and they are both subordinate to the third great division of geology,—_historical geology_,—which corresponds to embryology.