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Updated: June 20, 2025
In this instance the whole line of reef has been converted into land; but usually a snow-white line of great breakers, with only here and there a single low islet crowned with cocoa-nut trees, divides the dark heaving waters of the ocean from the light green expanse of the lagoon-channel.
BB, The shores of the fringed island. A'A', Outer edges of the reef, after its upward growth during a period of subsidence, now converted into a barrier, with islets on it. B'B', The shores of the now encircled islands. CC, Lagoon-channel. No theory on the formation of coral-reefs can be considered satisfactory which does not include the three great classes.
Coral islets are supposed to have been formed on the reef; and a ship is anchored in the lagoon-channel. This channel will be more or less deep, according to the rate of subsidence, to the amount of sediment accumulated in it, and to the growth of the delicately branched corals which can live there. We can now at once see why encircling barrier-reefs stand so far from the shores which they front.
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.
It cannot be that the corals will not grow close to the land; for the shores within the lagoon-channel, when not surrounded by alluvial soil, are often fringed by living reefs; and we shall presently see that there is a whole class, which I have called Fringing Reefs from their close attachment to the shores both of continents and of islands.
A section of the reef and island in this state, after a subsidence of several hundred feet, is given by the dotted lines: coral-islets are supposed to have been formed on the new reef, and a ship is anchored in the lagoon-channel. The height of the island, according to M. Lesson, is 4,026 feet.
This fringe consists of coral-sand and detritus thrown up from the lagoon-channel, and of soil washed down from the hills; it is an encroachment on the channel, analogous to that low and inner part of the islets in many atolls which is formed by the accumulation of matter from the lagoon. The lagoon channels may be compared in every respect with true lagoons.
Internally the reef either slopes gently into the lagoon-channel, or ends in a perpendicular wall sometimes between two and three hundred feet under water in height: externally the reef rises, like an atoll, with extreme abruptness out of the profound depths of the ocean. What can be more singular than these structures?
It will, perhaps, occur to some, that the actual reefs formed of coral are not of great thickness, but that before their first growth, the coasts of these encircled islands were deeply eaten into, and a broad but shallow submarine ledge thus left, on the edge of which the coral grew; but if this had been the case, the shore would have been invariably bounded by lofty cliffs, and not have sloped down to the lagoon-channel, as it does in many instances.
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