There are mighty fire-forces below; molten materials pent up, waiting for the slightest yielding of the solid crust to fight their way out; heated and imprisoned gases struggling for freedom. Sometimes these captive giants escape through what may be termed the orthodox safety-valves, old volcanic vents. Sometimes they break open new craters for themselves. Sometimes they fail to get out, and only shake the ground, or force it gently upward. Though earthquakes are more usually ascribed to the effects of a steady “crust-creep,” they are sometimes caused by heated gases seeking liberty. These heavings and twistings, these crumplings and swayings, of the earth-crust do not belong only to land, but go on in the same manner under the ocean. Many an earthquake felt on land has its origin under the sea; and others are so far out as to be felt only by passing ships. In the Pacific alone it is said that about two thousand five hundred shocks occur in the course of a year. Of the eight or ten thousand shocks, believed to be about the average number throughout the world annually, fully half have their origin beneath ocean-waters. CHAPTER XIII. OLD OCEAN AS A BUILDER “Grain by grain His Hand Numbers the unmeasurable sand.”--C. G. ROSSETTI. “... The ever changing strand Of shifting and unstable sand Which wastes beneath the steady chime And beating of the waves.”--WHITTIER. As a vast Cathedral is made of separate blocks of stone, laid one upon another and cemented together, so a stratified rock is formed of tiny particles of substance, placed one over another and pressed into solidity. Let us scoop up a little sand from the sea-shore, and look at it under a magnifying-glass. We shall find a number of loose yellow grains, not all alike in shape, but all somewhat rounded. Most of them are characterised by a glittering hardness, and probably some are transparent. For the most part these grains are of a substance called _quartz_, which may be almost any colour from white to black. Some beautiful forms of it are known to us, such as amethyst, agate, and jasper. But we have now to do with the humbler relatives of those aristocratic gems; with the more commonplace kinds of quartz, such as sandstone. Sandstone rock is chiefly made of grains of sand, firmly compacted together. An especial interest belongs to the rounded shape of these little grains. Like many voiceless things in the world of inanimate Nature, it tells its own tale, if only we will pause to hear. Why should grains of sand be rounded? Why not square, or pointed, or angular? That is just what they were, not very long ago. Each sand-speck, when first detached from rock or stone, had its angles and corners. But these have been gradually rubbed away; and by the very same process which, in the course of years, rubs away angularities and eccentricities from the characters of human beings.