The islands contiguous to the lines A B and C D ([Fig. 60]) are uniformly atolls, or are surrounded by barrier reefs, and are therefore subsiding; while the islands at a distance from these lines are surrounded by fringing reefs, which indicate that they are rising.
A well-authenticated instance of gradual subsidence is that of Greenland. The entire western coast, from its southern extremity to Disco Island, a distance of six hundred miles, has for the last two centuries been slowly subsiding. The dwelling-houses and places of worship built by the early European settlers are now in part or entirely submerged. The natives are said to be aware of the subsidence, and never build their huts near the sea.
4. We have thus seen that both elevation and depression may take place. There is reason to believe that these changes of level have, in some cases, been several times repeated. In one of the eastern ranges of the Andes, opposite to Chili, there is a mass of marine strata of five thousand feet in thickness. About the middle of the series there occurs a silicified forest. In one place a clump of coniferous trees was found of more than fifty in number, and a foot or more in diameter. The base of the strata must have been twenty-five hundred feet below the surface of the sea, in order to admit of the deposition of the first half of it. It was then elevated, so that a forest grew upon its surface. It was then depressed at least twenty-five hundred feet, more, to admit of the deposition of the subsequent strata, and the whole is now uplifted to form a mountain range of eight thousand feet in height.
The temple of Jupiter Serapis, near Naples, in Italy, was built near the sea, about eighteen hundred years ago. It was gradually submerged, and finally lost by the deposition of sediment nearly to the top of the columns. It was afterwards elevated, so as to be entirely above the level of the sea. The remains of the temple ([Fig. 65]) were afterwards discovered by the columns projecting a little above the ground. The sediment was removed to the depth of forty-six feet, when the workmen came to the base of the columns, and to a pavement seventy feet in diameter. In 1807 an artist was employed to take drawings of the ruin. The pavement was then above the level of the sea. Sixteen years afterwards the same artist found the pavement covered with water, and the depth has continued to increase since that time. It is considered that for the last forty years the depression has been three-fourths of an inch a year.