[13] The section has been made from the chart given in the “Atlas of the Voyage of the _Coquille_.” The scale is .57 of an inch to a mile. The height of the island, according to M. Lesson, is 4,026 feet. The deepest part of the lagoon-channel is 162 feet; its depth is exaggerated in the woodcut for the sake of clearness. [Illustration: Vertical section of an island of Bolabola.] AA—Outer edge of the reef at the level of the sea. BB—Shores of the island. A′A′—Outer edge of the reef, after its upward growth during a period of subsidence. CC—The lagoon-channel between the reef and the shores of the now encircled land. B′B′—The shores of the encircled island. N.B.—In this, and the following woodcut, the subsidence of the land could only be represented by an apparent rise in the level of the sea. It is evident in this section, that a line drawn perpendicularly down from the outer edge of the new reef to the foundation of solid rock, exceeds by as many feet as there have been feet of subsidence, that small limit of depth at which the effective polypifers can live—the corals having grown up, as the whole sank down, from a basis formed of other corals and their consolidated fragments. Thus the difficulty on this head, which before seemed so great, disappears. As the space between the reef and the subsiding shore continued to increase in breadth and depth, and as the injurious effects of the sediment and fresh water borne down from the land were consequently lessened, the greater number of the channels, with which the reef in its fringing state must have been breached, especially those which fronted the smaller streams, will have become choked up with the growth of coral: on the windward side of the reef, where the coral grows most vigorously, the breaches will probably have first been closed. In barrier-reefs, therefore, the breaches kept open by draining the tidal waters of the lagoon-channel, will generally be placed on the leeward side, and they will still face the mouths of the larger streams, although removed beyond the influence of their sediment and fresh water;—and this, it has been shown, is commonly the case. [Illustration: Vertical section of an island of Bolabola.] A′A′—Outer edges of the barrier-reef at the level of the sea. The cocoa-nut trees represent coral-islets formed on the reef. CC—The lagoon-channel. B′B′—The shores of the island, generally formed of low alluvial land and of coral detritus from the lagoon-channel. A″A″—The outer edges of the reef now forming an atoll. C′—The lagoon of the newly formed atoll. According to the scale, the depth of the lagoon and of the lagoon-channel is exaggerated.