II. SUBSIDENCE INDICATED BY ATOLLS AND BARRIER REEFS.

Looking at atolls as covering buried islands, we observe that through the equatorial latitudes such marks of subsidence abound from the Eastern Paumotus to the Western Carolines, a distance of about six thousand geographical miles. In the Paumotu Archipelago there are about eighty of these atolls. Going westward, a little to the north of west they are found to dot the ocean at irregular intervals; and the Kingsmill or Gilbert Group, the Marshall Group, and the Carolines comprise seventy-five or eighty atolls.

If a line be drawn from, Pitcairn's Island, the southernmost of the Paumotus, by the Gambier Group, the north of the Society Group, the Navigators, and the Solomon Islands to the Pelews, it will form nearly a straight boundary, trending N. 70° W., running between the atolls on one side and the high islands of the Pacific on the other, the former lying to the north of the line, and the latter to the south.

Between this boundary line and the Hawaiian Islands, an area nearly two thousand miles wide and six thousand long, there are two hundred and four islands, of which only three, exclusive of the eight Marquesas, and the Ladrones with Yap, Hunter's, and Los Matelotas in the line of the Ladrones and Pelews, are not of coral. These three are Kusaie or Ualan, Ponape, and Truk or Hogoleu, all in the Caroline Archipelago. South of the same line, within three degrees of it, there is an occasional atoll; but beyond this distance, there are none excepting the few in the Friendly Group, and one or two in the Feejees.

If each coral island scattered over this wide area indicates the subsidence of an island, we may believe that subsidence

was general throughout the area. Moreover, each atoll, could we measure the thickness of the coral constituting it, would inform us nearly how much subsidence took place where it stands; for they are actually so many registers placed over the ocean, marking out, not only the site of a buried island, but also the depth at which it lies covered. We have not the means of applying the evidence; but there are facts at hand, which may give at least comparative results.

a. We observe, first, that the barrier reefs are, in general, evidence of less subsidence than atoll reefs (p. 266). Consequently, the great preponderance of the former just below the southern boundary line of the coral island area, and, farther south, the entire absence of atolls, while atolls prevail so universally north of this line, are evidence of some depression just below the line; of less, farther south; and of the greatest amount, north of the line or over the coral area.

b. The subsidence producing an atoll, when continued, gradually reduces its size until it finally becomes so small that the lagoon is obliterated; and, consequently, a prevalence of these small islands is presumptive evidence of the greater subsidence. We observe, in application of this principle, that the coral islands about the equator, five or ten degrees south, between the Paumotus and the Gilbert Islands, are the smallest of the ocean; several of them are without lagoons, and some not a mile in diameter. At the same time, in the Paumotus, and among the Gilbert and Marshall Islands, there are atolls twenty to fifty miles in length, and rarely one less than three miles. It is probable, therefore, that the subsidence indicated was greatest at some distance north of the boundary line, over the region of small equatorial islands, between the meridians of 150° and 180° W.

c. When, after thus reducing the size of the atoll, the

subsidence continues its progress, or when it is too rapid for the growing reef, it finally sinks the coral island, which, therefore, disappears from the ocean. Now, it is a remarkable fact that while the islands about the equator above alluded to indicate greater subsidence than those farther south, there is over a region north of these islands, that is, between them and the Hawaiian Group, a wide blank of ocean without an island, which is nearly twenty degrees in breadth. This area lies between the Hawaiian, the Fanning and the Marshall Islands, and stretches off, between the first and last of these groups, far to the northwest.

Is it not, then, a legitimate conclusion that the subsidence, which was least to the south beyond the boundary line and increased northward, was still greater or more rapid over this open area; that the subsidence which reduced the size of the islands about the equator to mere patches of reef, was further continued, and caused the total disappearance of islands that once existed over this part of the ocean?

d. That the subsidence gradually diminished southwestwardly from some point of greatest depression situated to the northward and eastward, is apparent from the Feejee Group alone. Its northeast portion (see chart) consists of immense barriers, with only a few points of rock remaining of the submerged land; while in the west and southwest there are mountain islands of great magnitude. Again, along the north side of the Vanikoro Group, Solomon Islands, and New Ireland, there are coral atolls, but scarcely one to the south.

In view of this combination of evidence, we are led to believe that the subsidence increased from the south to the northward or northeastward, and was greatest between the Navigator and Hawaiian Islands, near the centre of the area

destitute of islands, about longitude 170° to 175° W., and latitude 8° to 10° N.

But we may derive some additional knowledge respecting this area of subsidence from other facts.

Hawaiian Range. - We observe that the western islands. in the Hawaiian Range, beyond Bird Island, are atolls, and all indicate a large participation in this subsidence. To the eastward in the range, Kauai and Oahu have only fringing reefs, yet in some places these reefs are half a mile to three fourths in width. They indicate a long period since they began to grow, which is borne out by the features of Kauai showing a long respite from volcanic action. We detect proof of subsidence, but not of a large amount. Moreover, there are no deep bays; and, besides, Kauai has a gently-sloping coast plain of great extent, with a steep shore acclivity of one to three hundred feet. The facts favor the idea of much less subsidence since the time when the corals began to grow in the region of Kauai and Oahu than along the range to the westward. The rather small width of the reefs about these two islands may be owing to the former action of their volcanoes, which may have been burning during the earlier part of the coral-reef era. But deep-sea soundings must be made along the whole chain, including the line to the westward, before we can speak positively about the change of level.

The western islands of the range bear some evidence of having, in recent times, commenced a new subsidence after a temporary cessation. They all have little dry land and vegetation about the reefs. Brooks's Island, in latitude 28° 15′ N., and longitude 177° 20′ W., eighteen miles in circumference, has on its north and east sides a compact coral wall of about five feet elevation, which continues for four and a quarter miles, and then becomes a line of detached rocks at tide level.

This bare wall, thus described by Capt. Wm. Reynolds, U. S. N., appears to be an indication that the land was once finished off under a cessation of subsidence, but that a sinking of small amount has since taken place, amounting perhaps to four or five feet.

Ocean Island, in 28° 25′ N., 178° 25′ W., another of this range, is very similar to Brook's in its wall of coral rock on the east; and so also is Pearl and Hermes' reef, in 27° 50′ N., 176° W., though the wall of the latter is more a series of detached rocks than a continuous parapet.

Marquesas.-The Marquesas are remarkable for their abrupt shores, often inaccessible cliffs, and deep bays. The absence of gentle slopes along the shores, their angular features, abrupt soundings close alongside the island, and deep indentations, all bear evidence of subsidence to some extent; for their features are very similar to those which Kauai or Tahiti would present, if buried half its height in the sea, leaving only the sharper ridges and peaks out of water. They are situated but five degrees north of the Paumotus, where eighty islands or more have disappeared, including one at least fifty miles in length. There is sufficient evidence that they participated in the subsidence of the latter, but not to the same extent. They are nearly destitute of coral, and apparently because of the depth of water about the islands.

Gambier Group.-In the southern limits of the Paumotu Archipelago, where, in accordance with the foregoing views, the least depression in that region should have taken place, there are actually, as we have stated, two high islands, Pitcairn's and Gambier's. There is evidence, however, in the extensive barrier about the Gambier's (see cut on page 266), that this subsidence, although less than farther north, was by no means of small amount. On page 157, we have estimated;