Minerals and rocks, or the inorganic portions of theearth, constitute the proper field or subject-matter of thescience of Geology. Now the inorganic earth, like an animalor plant, may be and is studied in three quite distinct ways,giving rise to three great divisions of geology, which, aswill be seen, correspond closely to the main divisions of Biology.First, we may study the forces now operating upon andin the earthmdash;the geological agenciesmdash;such as the oceanand 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.
