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272 Guinea in the Polynesian seas. In this area are 204 islands, very few of which are high or with land still above the sea. Southward of this area are many mountainous islands, or with highlands, surrounded with reefs, evidently beyond the line of greatest subsidence.

The evidence is satisfactory that the depressed area has gone down, in comparatively recent geologic time, many thousands of feet, and yet the subsidence may have been less than the elevation of lands elsewhere; for we have the elevation of the Rocky Mountains, Andes, Alps, and Himalayas, modern events in geologic history. It is more than probable that great subsidence in one section is correlated by elevations elsewhere. And the depression of the Pacific area may correspond to the elevation of northern lands which probably caused the cold and glaciers of the glacial age. The movement was one of those great secular changes of the earth's crust which dates far back in its history.

There is evidence that the Pacific subsidence has ceased, or nearly so, and that local elevations have long since commenced. In about 40 instances, Pacific coral-islands have been elevated since reefs were formed upon them. Many of these elevations are a few feet only, others, a few hundred feet; as many as 600 feet in one or two instances. These elevated masses of coral-rock have the perpendicular walls and configuration before described. Metia, or Aurora Island (Fig. 15), is one of the Panmotu group; its walls of coral-limestone are 250 feet high, and resemble the Palisades on the Hudson.



Along the outer margins of the elevated islands are deep caverns, showing, by their contour, the wearing and wasting action of waves. The Bermudas are remarkable for their caverns; the coral-made land being in places 260 feet above the level of the sea. On the island of Oahu, the Rev. John "Williams entered one by a descent of 20 feet, and wandered a mile in one of its branches. Innumerable openings presented themselves on all sides. The roof, a superb stratum of coral-rock, 15 feet thick, was supported by stalactitic columns, and thickly hung with stalactites.