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thus been led, solve the curious problem,—what has givento each class of reef its peculiar form ?
Let us in imagination place within one of the subsidingareas, an island surrounded by a “ fringing-reef,”—thatkind, which alone offers no difficulty in the explanationof its origin. Let the ^unbroken lines and the obliqueshading in the woodcut 4) represent a vertical sectionthrough such an island; and the horizontal shading will
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Ltrel nf Sett.
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 ofsubsidence.
CC—The lagoon-channel between the reef and the shores of the now encircledland.
B'B'—The shores of the encircled island.
N.B. — In this, and the following woodcut, the subsidence of the land couldonly be represented by an apparent rise in the level of the sea.
represent the section of the reef. Now, as the island sinksdown, either a few feet at a time or quite insensibly, wemay safely infer from what we know of the conditionsfavourable to the growth of coral, that the living massesbathed by the surf on the margin of the reef, will soonregain the surface. The water, however, will encroach,little by little, on the shore, the island becoming lowerand smaller, and the space between the edge of the reefand the beach proportionally broader. A section of thereef and island in this state, after a subsidence of severalhundred feet, is given by the dotted lines: coral-islets are