we can properly understand the appearances presented by the rocks which compose the crust of the earth, we must have some knowledge of the agencies by which rocks are now being formed, and by which the form of the earth's surface is being modified. Again, before we can attempt to construct any history of the formation of the earth's crust we must be acquainted with the leading facts and conclusions of Paleontology, as that branch of the science is termed which deals with the structure and classification of former existences.

Descriptions of the various natural operations which are now in progress upon the earth, and of the changes which they bring about, were aptly termed by Sir Charles Lyell, "the Principles of Geology"; since it is only by a careful study of the changes which are taking place in the present that we can hope to interpret the many changes which have taken place in the past.

A knowledge of these first principles ought, therefore, to form the groundwork or introduction to the study of those subjects which are, perhaps, the more special province of geological science.

It seems, then, the most natural and logical treatment of the subject in the first place to describe the geological processes which can be seen at work in various parts of the world, and to show the student how these operations will explain the facts observable in the crust of the earth, and how they will account for the diversified features which its surface presents. By these steps we can proceed to the history of the earth since it became a habitable globe, as interpreted by means of the observations and deductions previously arrived at. This treatise, therefore, will be divided into five parts, which may be designated and defined as follow.

1. Dynamical Geology.-An account of the actual results. produced by the geological agencies now in operation.

2. Geognosy, or Structural Geology.-Being the study of rocks and rock-masses, without reference to the geological time of their production. This branch may be divided into two sections: a. Petrology-the study of the mineral components of rocks as visible in detached specimens. b. Tectonic Geology-the study of rock-masses; their struc

tural characters, and mutual relations, as visible in the earth.

3. Physiography.-An explanation of the origin of hills. and valleys, lakes, escarpments, and mountains.

4. Paleontology.-Under this head will be given an outline of the laws which govern the distribution of life, both in space and time; but to deal fully with this part of the subject a separate volume would be required.

5. Stratigraphy, or Historical Geology.-Being a chronological account of the successive groups, or series of strata, which are found in the earth's crust. This will deal chiefly with the rocks of the British Islands, and will necessarily be only a condensed account, but prominence will be given to those facts which enable us to realize the relative positions of land and sea, and the general physical geography of each geological epoch.

PART I.

DYNAMICAL GEOLOGY.

SECTION I. Changes produced by the Influence of Internal or Subterranean Causes.

Fig. 1. Diagram of a Segment of the Terrestrial Globe, to show its probable internal structure.

THE

CHAPTER I.

THE GENERAL STRUCTURE OF THE EARTH.

HE Shape of the Earth, and its Consequences. -Everyone knows, or should know, that the earth has been both measured and weighed; and that the form of the globe is not that of a perfect sphere, but of an oblate spheroid. According to Mr. Clark, the equatorial diameter of the earth is 7925 604 miles in length, while the length of the polar diameter is only about 7899-114 miles; the difference between them is, therefore, a little less than 26.5 miles. That is to say, that the polar axis is almost exactly 26 miles shorter than an equatorial axis or line passing through the centre of the earth from one side of the equator to the other: or, again, as calculated along radii from the centre, an equatorial radius is about 13 miles, or 70,000 feet longer than a polar radius.

If, therefore, a true sphere were to be described within the earth, with a radius equal to that of the polar radius, the surface of this sphere would lie at the depth of 70,000 feet below the actual surface at the equator.

Now the deepest soundings in the ocean are less than 28,000 feet, and the highest mountain in the world, viz., Mount Everest in the Himalayas, is only 29,000 feet above the sea. Compared, therefore, with the bulging protube rance of the earth's mass at the equator, the irregularities of its surface, which we know as ocean-depths and mountain heights, are obviously small.

It is also seen that this equatorial bulge of 70,000 feet, though only about part of an equatorial radius, represents the bulk or weight of an enormous mass of matter, and its existence involves very important consequences.