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.
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.
