3. _The Subsidence of Land._--Elevations can be shown to have taken place by fossils, and by other evidences of former sea levels which are left on the surface; but depressions leave but few indications of change of level. It is yet doubtful whether the depression is equal to the elevation; that is, whether the amount of land remains nearly constant, or whether there has been an augmentation of the dry land within the tertiary and recent periods. We are certain that the augmentation, if any, has not been equal to the elevation, for subsidences to a great amount are known to have taken place. There are occasional instances of submerged forests seen at low tide, at some distance from the shore. There are several near the coast of England and Scotland, and near the coast of Massachusetts. They are but a few feet below low water, and do not indicate a subsidence of more than about twenty feet. Numerous instances are on record of the sinking down of wharfs and buildings near the sea during earthquakes. Almost every violent earthquake is accompanied by a change of level. The changes of this kind which have been noticed are in seaport towns, because greater facilities are there afforded for detecting them, and because loss of property awakens attention to them; but there is every reason to suppose that these changes of level extend to great distances both into the country and into the sea. [Illustration: Fig. 60.] An immense area in the Indian and Pacific Oceans, probably ten millions of square miles, is undergoing change of level. The lines A B and D G (Fig. 60) represent nearly the axes of depression; while an intermediate and two exterior parallel lines would represent axes of elevation. The evidence of these changes is found principally in the peculiarities of the wall of coral rock encircling the islands. The following figures represent, in sections, modifications of form of the same island. The coral wall built up around the island by the polyps, from the depth of fifty, or at most of a hundred feet, is shown at _c c_ (Fig. 61). If the island is elevated, this wall becomes a _fringing reef_ (Fig. 62), _b'_ becoming the level of the sea, and the animal begins a new wall at the same depth as before. But if the island is gradually sinking, the wall is kept built up to the surface, and becomes a _barrier reef_ (Fig. 63). A channel is thus left between the island and the reef, which, though gradually filling up with broken coral or other sediment, is generally deep enough for a ship-channel. If the island continue to subside till it disappears, and the coral wall is still kept at the surface, it then becomes an _atoll_, a circular coral island (Fig. 64), often of many leagues in diameter, beaten by the surf on the outer edge, but enclosing a quiet lake, which communicates only by occasional channels with the ocean. [Illustration: Fig. 61.] [Illustration: Fig. 62.] [Illustration: Fig. 63.] [Illustration: Fig. 64.]