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.

The great object of the geologist is, by studying the geological formations in regular order, from the oldest up to the newest, to work out, in their proper sequence, the events which constitute the earth’s history; and dynamical and structural geology are merely introductory chapters, the alphabet, as it were, which must be learned before we are prepared to read understandingly the grand story of the geological record.

Our work in this short course will be limited to the first two divisions,—i.e., to dynamical and structural geology. We will attempt, first, a general sketch of the forces now concerned in the formation of rocks and rock-structures; and after that we will study the composition and other characteristics of the common minerals and rocks.

The scope of this work, and its relations to the whole field of geology, are more clearly indicated by the following classification of the geological sciences:—

Many teachers will desire to fill in some of the details of the outline sketch presented in this Guide, and for this purpose the following works are especially recommended:—

Elements of Geology. By Prof. Joseph Le Conte. 1882. D. Appleton & Co., New York. Nearly 600 pages.

Manual of Geology. By Prof. J. D. Dana. Third edition. 1880. 800 pages.

Text-Book of Geology. By Prof. A. Geikie. 1882. Macmillan & Co., London. Nearly 1000 pages.

As a reference-book for mineralogy, the following treatise is unsurpassed:—

Text-Book of Mineralogy. By Edward S. Dana. 1883. John Wiley & Sons, New York.

And, as an introduction to the study of minerals, and, through these, to the study of rocks,—

First Lessons in Minerals. Science Guide No. XIII. By Mrs. E. H. Richards.

cannot be too highly recommended. Teachers will find this little primer of 46 pages invaluable with young children, and with all who have had no previous training in chemistry.

As an admirable continuation of the work begun in these pages, teachers are referred to Professor Shaler’s “First Book in Geology.” In this our brief sketch of the geological agencies is amplified and beautifully illustrated; and rarely have the wonderful stories of the river, ocean-beach, glacier, and volcano been told so effectively. In the chapter on the history of life on the globe the main outlines of historical geology are skillfully brought within the comprehension of beginners. The directions to teachers are fully in accord with the modern methods and ideas, and are a very valuable feature of the book.