PART I.

PHYSICAL GEOLOGY.

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

CHAPTER I.

THE MASS AND GENERAL STRUCTURE OF THE EARTH.

THE 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 M. Bessel, the equatorial diameter of the earth is 7925-6 miles in length, while the length of the polar diameter is only about 7899-11 miles; the dif ference 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 70,000feet 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 30,000 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, repre

sents the bulk or weight of an enormous mass of matter, and its existence involves very important consequences.

The first of these consequences is the stability of the earth's axis. The earth is a body which revolves upon its own axis, that is, upon an imaginary line connecting its two poles, and this line is its shortest diameter. The actual circumference of the earth's equator is about 83 miles greater than that of the true sphere enclosed by it, consequently its linear velocity is more rapid than that of the inner sphere, because any point on it is carried round that greater space in the same period of time. Any cause, therefore, which tended to make the earth revolve upon any other axis than that of its shortest diameter, would have to overcome the resistance of the greater momental force residing in the equatorial protuberance; and ever since the earth first acquired its present form, it is very difficult to imagine any cause sufficient to have produced this result.

Hence we may conclude that the position of the earth's axis has remained unchanged throughout all geological time, and that the present north and south poles have always been its poles.

The second conclusion deducible from the peculiar shape of the earth is the fact that it is almost exactly that of a spheroid of rotation; in other words, it is the form which the globe would have assumed had it been originally a pasty or fluid mass revolving with its present velocity. If a ball of molten matter were set to spin round one of its diameters as an axis, it would naturally tend to bulge about the parts farthest from that axis, the bulging being great in proportion to the velocity of its rotation, until for any given velocity a balance was ultimately produced between the centrifugal force of the rotation and the centripetal force due to the attraction of gravity in the

mass.

The fact that the earth has this form raises a strong presumption that its condition was at one time sufficiently plastic to allow it to adjust its shape to the impulse of its motion; in short, that it was once in a semi-fluid or pasty condition. Although this original plasticity is by no means a certainty, yet it is generally assumed as a probability,

agreeing very well with some other known facts which are in favour of the idea, that the earth has been continually parting with heat, and that its own heat was once sufficient to keep its substance in a molten or semi-molten condition.

Internal Temperature of the Earth.-Without, then, further considering the question of the possible temperature and condition of the earth in the earliest times, we will pass to a subject with which geology is more directly concerned; viz., the probable state and temperature of its interior at the present time.

This is, of course, a very difficult matter upon which to obtain any evidence, but the following facts tend to show that it consists of a cool envelope surrounding a heated interior, and that the latter possesses a high temperature of its own, quite independently of any heat derived from the sun or other external source.

1. Specific Gravity of the Earth.-When we come to consider the mass of the earth, we shall find that its density is not so great as it would be if nothing impeded or counteracted the force of gravitation. This force would probably compress the rocks into a much closer and denser condition, and so cause the mass of the earth to have a greater density, unless there were some counteracting expansive force, and we know of no such force except that of heat.

2. Volcanoes.-The numerous volcanic orifices, which occur here and there all over the earth, and from which streams of lava or molten rock are so often poured forth, prove that some parts of the interior at any rate are so heated that solid rocks are rendered perfectly fluid. But the lavas derived from existing volcanoes form only a small portion of the bulk of igneous rocks in the crust of the earth, all of which have certainly come from the interior.

3. Temperature of Deep Mines.-The influence of the summer's heat or winter's cold does not penetrate far into the interior of the earth, and it is estimated that at a depth of about fifty feet beneath any given place on the earth's surface a thermometer would mark the same temperature all the year round.