From the rocks we learn that, once upon a time, long ago, they were put together, scrap by scrap, under water, not upon dry land. And from the fossil remains in those rocks we learn with equal certainty that, once upon a time, they lay under ocean-waters. Fossil remains, buried in rocks, are of many kinds. Not only relics of animals, or of parts of animals, such as teeth and bones and shells. Not only relics of sea-weeds and other ocean vegetation. Not merely the actual remains, hardened into stone; but also the casts or shapes of all such remains, found impressed in the substance of the rocks. The whole of these various records of the past must be included under the term “fossils.” Firmer substances, like shells and bones, have become often transformed into actual solid stone. But softer living substances have more frequently passed out of existence, leaving only an impress or _cast_, which is filled up later by some other substance. When upon high mountain-tops such fossils are found--the remains of sea-weeds and sea-creatures embedded in rock--the question must naturally arise, How could these ocean-inhabitants have found their way to such a height? That the sea must once have flowed over them cannot be doubted. But how did it come about? Were those water-built heights under the sea, because in those times the land lay lower, or because the sea-surface stood higher? 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.