Geology is a unit and, though for the purpose of orderly treatment, it is necessary to divide the subject into various provinces, it should be clearly understood that these provinces are rather the various aspects and phases of the same science than actual divisions. Every part of the subject is so intimately related to every other part, that any possible arrangement involves the more or less violent separation of things, that belong together and requires much anticipation and repetition. The past is meaningless unless we understand the present, and a full understanding of the present can only be gained through a knowledge of the past, yet it is obvious that we cannot deal with both past and present simultaneously. Although it is an undoubted evil, some classification is necessary, if we would avoid losing ourselves in bewildering labyrinths of detail.

The departments into which geology is usually divided are as follows: -

1. Dynamical Geology

Dynamical Geology, or the study of the forces which are now at work in modifying the surface of the earth, and of the chemical and mechanical changes which they effect. This is the key by which we may interpret past changes.

2. Structural Geology

Structural Geology, or the study of the materials of which the earth is composed and of the manner in which they are arranged; together with such explanations of the modes in which the arrangement was produced as may be inferred from the structure.

3. Geomorphology

Geomorphology (also called Physiographical Geology, or Physiography) is an examination of the topographical features of the earth and of the manner in which they were produced. Primarily, this subject is a province of physical geography, but it is a valuable adjunct to geology.

The three foregoing divisions together constitute a larger division, which is called Physical Geology, and which is contrasted with:

4. Historical Geology

This is the study of the earth's history, the changes of level between land and sea, of topography, of climate, and of the successive groups of animals and plants which have lived upon the globe. The historical is the dominant standpoint in geology, the main problem of which is to interpret the records of the earth's history. The other departments are the means, to this great end.

While the geologist needs the help of almost all the other physical and natural sciences, he has his peculiar province in the rocks which make up the accessible crust of the earth. These rocks are aggregates of a comparatively few common minerals, called, for that reason, the rock-forming minerals. A study of the processes now going on shows that rocks are formed in various ways and, in accordance with these modes of formation, they may be grouped in three great classes: I. Igneous Rocks, or those which have solidified by cooling from a state of fusion and are therefore not divided into layers or beds, are either glassy or crystalline, and are composed of complex minerals. The igneous rocks have forced their way upward from the earth's interior, thus penetrating the overlying rocks in various ways. A familiar example of this group is lava. II. Sedimentary, or Stratified Rocks, those which were accumulated under water or on land in a series of successive beds, or strata, from material derived from the disintegration of older rocks, and are generally fragmentary, or non-crystalline, and composed of simpler minerals than those which make up the igneous rocks.

Speaking broadly, the beds of the sedimentary rocks were originally laid down in a horizontal position, and hence when they are found to be tilted, inclined, or folded, it follows that they have been disturbed from their original attitudes. III. Metamorphic Rocks, or those igneous or sedimentary rocks which have been more or less profoundly reconstructed in place, often with the generation of entirely new minerals.

In the accessible part of the earth's crust, rocks of all kinds (other than loose materials, such as sand) are divided into pieces, by vertical and horizontal partings, which are called joints (see p. 369). In addition, the rocks are divided into still larger masses, or blocks, by a profounder system of fissures, and planes of dislocation, or faults (see p. 353). The blocks are of all sizes, up to thousands of square miles and down to areas of a few square feet, and thus the surface of the earth has been well compared to a vast mosaic of rock-pieces.

The crust of our planet is called the lithosphere, a, shell of rocks of unknown thickness. Within the lithosphere is the great mass of the earth, or centrosphere, concerning which we know only that it is highly heated, of great density, and under enormous pressure. The surface of the globe is very irregular .and covered with elevations and depressions. The deeper depressions are filled with water and constitute the ocean basins which in area bear to the land the proportion of 2.54:1, and this incomplete envelope of water is the hydrosphere. If the surface of the earth were smooth, the ocean would cover it entirely to a depth of nearly two miles. Finally, the atmosphere is a gaseous envelope, which encloses the earth completely.