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Updated: June 20, 2025
The horizontal line is the level of the sea, from which on the right hand a plummet descends, representing a depth of 200 fathoms, or 1,200 feet. The vertical shading shows the section of the land, and the horizontal shading that of the encircling barrier-reef: from the smallness of the scale, the lagoon-channel could not be represented. AA. Outer edge of the coral-reefs, where the sea breaks.
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.
The principal small detached reefs within the lagoon-channel have in this instance been represented. HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the "Atlas of the Voyage of the 'Astrolabe," compiled from the surveys of Captains Duperrey and D'Urville; the depth of the immense lagoon-like space within the reef is not known.
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. 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.
I will give only a few instances. In barrier-reefs it has long been remarked with surprise, that the passages through the reef exactly face valleys in the included land, even in cases where the reef is separated from the land by a lagoon-channel so wide and so much deeper than the actual passage itself, that it seems hardly possible that the very small quantity of water or sediment brought down could injure the corals on the reef.
The depth within the lagoon-channel also varies much; from 10 to 30 fathoms may be taken as an average; but at Vanikoro there are spaces no less than 56 fathoms or 336 feet deep.
As a current is formed by the water thrown over the edge of atoll-formed reefs, which carries sediment with it through the deep-water breaches, the same thing probably takes place in barrier-reefs, and this would greatly aid in preventing the lagoon-channel from being filled up.
In all reefs, owing to the sediment being washed out of the lagoon-channel to leeward, that side is least favourable to the long-continued vigorous growth of the corals; hence dead portions of reef not unfrequently occur on the leeward side; and these, though still retaining their proper wall-like form, are now in several instances sunk several fathoms beneath the surface.
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. Let us in imagination place within one of the subsiding areas, an island surrounded by a "fringing-reef," that kind, which alone offers no difficulty in the explanation of its origin.
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? We see
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