The marginal regions of oceans, where most deposition takes place, seem to undergo slow subsidence, while the continents seem in most places to be as slowly rising. Modern geologists are inclined to think that as denudation wears down a continental surface, removing from it a great quantity of solid rocky matter (see chap. v., pp. [161]-[163]), the pressure below is somewhat lessened, or in other words, so much weight is taken off; but that, on the other hand, as this extra amount of material accumulates on the bed of a neighbouring ocean the pressure is increased by a corresponding amount, and so the balance between internal and external forces is upset, and movements consequently take place. We have already seen that the external parts of the earth are much more subject to movements than might have been expected; and for our part, we are willing to believe that in this simple way upheaving forces might be called into play sufficient to account for even the elevation of mountain-chains. For suppose a great mass of strata to continue sinking as they were formed, for long periods of time; what seems to follow? The downward movement would go on until a time would come when the strata, in endeavouring to settle down at a lower level, would (as by the contraction theory above explained) be forced to fold themselves into ridges, and in this way long strips of them might even be elevated into mountain-ranges.
Another ingenious idea was suggested by the late Mr. Scrope, whose work on volcanoes is well known. His idea was that when a large amount of sedimentary material has accumulated on any large area of the bed of the ocean, it somewhat checks the flow of heat from within, and therefore the temperature of the rocks forming part of the earth's crust below will be increased, much in the same manner as a glove checks the escape of heat from the hand and keeps it warm. The consequence of this would be expansion; and as such expansion would be chiefly in a horizontal direction, the area would bulge upwards and cause elevation of the strata resting on it. But there are several difficulties which this theory fails to explain.
And lastly, Professor Le Conte, holding that the contraction theory is unsatisfactory, accounts for earth-movements of all kinds by supposing that some internal parts of the earth cool and contract faster than others. Those parts that cool fastest, according to this theory, are those that underlie the oceanic basins or troughs; while the continental areas, not cooling so rapidly, are left standing up in relief. This theory, which does not seem very satisfactory, is based upon the idea that some parts of the earth's interior may be capable of conducting heat faster than others. We know that some substances, like iron, are good conductors of heat, while others are bad conductors; and it is therefore conceivable that heat may be flowing faster along some parts of the earth than along others; and if so, there would be differences in the rate of contraction.
There are various theories with regard to the nature of the earth's interior. One of these already referred to, but now antiquated, supposes our planet to consist of a thin, solid crust lying on a molten interior, so that the world would be something like an egg with its thin shell and liquid, or semi-liquid, interior. Now, there are grave reasons for refusing to accept this idea. In the first place, a certain slow movement of the earth known as "precession," because it causes the precession of the equinoctial points on the earth's orbit, could not possibly take place as it does if the earth's interior were in this loose and molten condition. That is a matter decided by mathematical calculation, on which we will not dwell further. Secondly, we obtain some very valuable evidence on this abstruse subject from the well-known daily phenomenon of the tides, caused, as the reader is probably aware, by the attractions of the sun and moon; but much more by the moon, because she is nearer, and so exerts a greater pull on the ocean as each part of the world is brought directly under her by the earth's daily rotation on its axis. The waters of our oceans rise up twice each day as they get in a line with the moon, and then begin to fall again. Thus we get that daily ebb and flow seen on our shores. Now, it has been clearly proved by Sir William Thomson, and others, that if any considerable portion of the interior of the earth were in a fluid condition, it too would rise and fall every day as the ocean does. So we should in that case have a tide below the earth as well as on its surface, and the one would tend to neutralise the other, and the ocean tide ought to appear less than it actually is. Even if the earth's crust were made of solid steel, and several hundreds of miles thick, it would yield so much to the enormous pulls exerted by both the sun and moon that it would simply carry the waters of the ocean up and down with it, and we should therefore see no appreciable rise and fall of the water relatively to the land. As a matter of fact, there is a very slight tide in the solid earth below our feet, but so slight that it does not practically affect the tide which we see every day in the ocean. But we wish to show that were the interior of the earth in anything approaching, to a fluid or molten condition, the phenomena of the tides would be very different from what they actually are.
All geologists are therefore agreed that we must consider our earth as a more or less solid body, and not as being something like an india-rubber ball filled with water.
The only question is whether it is entirely solid throughout. Some authorities consider this to be the case. But others venture to think that while the great mass of the globe is solid, there may be a thin liquid layer lying somewhere below the surface. Sir William Thomson calculates that there must be a solid crust at least two thousand or twenty-five hundred miles thick (the diameter of the earth is about eight thousand miles) and that the mass of the earth "is on the whole more rigid certainly than a continuous solid globe of glass of the same diameter."
One other question with regard to the earth's interior may be mentioned in conclusion. Astronomers have calculated the weight of our planet, and the result is curious; for it turns out to be at least twice as heavy as the heaviest rocks that are found on or near the surface. It is about five and a half times as heavy as a globe of water of the same size would be, whereas most rocks with which we are acquainted are about two and a half, or at most three times heavier than water. This fact seems to open out curious consequences; for instance, it is quite possible that metals (which are of course much heavier than water) may exist in the earth's interior in considerable quantities. The imagination at once conjures up vast quantities of gold and silver. What is the source of the gold and silver, and other metals found in mineral veins? This question cannot as yet be fully answered. Very small quantities of various metals have been detected in sea-water; and so some geologists look upon the sea as the source from which metals came. But it is possible that they were introduced from below,—perhaps by the action of steam and highly heated water during periods of volcanic activity,—and that their source is far down below in the depths of the earth.
But perhaps we have already wandered too far into the regions of speculation.