That the earth-crust is restless, heaving, disposed to rise and sink, to crumple into furrows, to fold into ravines and mountain ranges, we now know. Yet this may not be the whole explanation of sea-shells upon high summits. It may be also that the surface of the sea itself—the general level of the ocean—has not been always the same. It too may have risen and sunk; not once only, but many times.
Between one and two hundred years ago there was a theory that the sea had as a whole sunk lower, on account of the enormous drying up of its surface. Of course the surface does dry up very much and very rapidly. Yet this deficit in ocean-waters being met by equally enormous down-pourings of rain and gifts of river-water, evaporation will not serve to account for a lowered ocean-level all over the earth. So the notion had to be given up; and in later years it was looked upon as certain that the land must have risen, and that the ocean-level must have been changeless.
Until recently this was regarded as established; but in the curious see-saw of scientific theories the pendulum has begun to swing back.
Not all the way. That the land has risen in parts, shaking its skirts free from ocean-tides, is not doubted. Scientists are, however, maintaining that while this is true, the other need not be untrue. They hold that, while land in many places has undoubtedly risen and sunk, the ocean-surface may have risen and sunk also.
The ocean could not, indeed, like the land have risen or sunk only in parts. Except where it is forcibly held at a higher level by some strong force of local attraction, this would be impossible. Yet causes might exist which should affect the whole ocean-surface.
One such may be mentioned. The rising or sinking of great tracts of land could not fail to affect the ocean-level.
That it must do so may be easily understood. Let us fill a basin with water, to within an inch of the brim, and then slowly lower into the water a ball, or any other solid body. As the ball enters, the level of the water rises—must rise. Then let us slowly lift the ball out again; and as it leaves the water, the level of the latter sinks—must sink.
Precisely the same result would follow upon the rising or sinking of some great continental mass. When it rises, the ocean contains less land, and the water-level must go down. When it sinks, the ocean contains more land, and the water-level must go up.
This question has been long under discussion. For many years, as with other questions, each side looked upon its own view for the time being as the only right one. But—as again with many difficult problems—the fairer and wider view recognises the possibility of truth in both explanations, and probably in the end both will prove to be in a measure right.
Many other kinds of rock, as well as sandstone, have been built under ocean-waters in past ages, constructed of minute particles, piled together, and hardened. Some sorts, like sandstone, are only or chiefly mineral; while others are only or chiefly “organic”—that is, are mainly composed of the remains of “organised” or living beings. About the latter much has to be said in future chapters.