But as ages go on and the process of cooling progresses, the crust reaches a density when it supports itself, like a couple of great arches; it no longer wrinkles; it no longer follows downward the receding molten mass within; mountains cease to be formed; and at length we have a red-hot ball revolving in a shell or crust, with a space between the two, like the space between the dried and shrunken kernel of the nut and the nut itself. Volcanoes are always found on sea-shores or on islands. Why? Through breaks in the earth the sea-water finds its way occasionally down upon the breast of the molten mass; it is at once converted into gas, steam; and as it expands it blows itself out through the escape-pipe of the volcano; precisely as the gas formed by the gunpowder coming in contact with the fire of the percussion-cap, drives the ball out before it through the same passage by which it had entered. Hence, some one has said, "No water, no volcano." While the amount of water which so enters is small because of the smallness of the cavity between the shell of the earth and the molten globe within, this process is carried on upon a comparatively small scale, and is a safe one for the earth. But suppose the process of cooling to go on uninterruptedly until a vast space exists between the {p. 72} crust and the core of the earth, and that some day a convulsion of the surface creates a great chasm in the crust, and the ocean rushes in and fills up part of the cavity; a tremendous quantity of steam is formed, too great to escape by the aperture through which it entered, an explosion takes place, and the crust of the earth is blown into a million fragments. The great molten ball within remains intact, though sorely torn; in its center is still the force we call gravity; the fragments of the crust can not fly off into space; they are constrained to follow the master-power lodged in the ball, which now becomes the nucleus of a comet, still blazing and burning, and vomiting flames, and wearing itself away. The catastrophe has disarranged its course, but it still revolves in a prolonged orbit around the sun, carrying its broken _débris_ in a long trail behind it.