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Updated: June 28, 2025
Closely resemble in general form and structure atoll-reefs. Width and depth of the lagoon-channels. Breaches through the reef in front of valleys, and generally on the leeward side. Checks to the filling up of the lagoon-channels. Size and constitution of the encircled islands. Number of islands within the same reef. Barrier-reefs of New Caledonia and Australia.
I have consulted, as far as I was able, every original voyage and map; and the colours were first laid down on charts on a larger scale. The same blue colour, with merely a difference in the depth of tint, is used for atolls or lagoon-islands, and barrier-reefs, for we have seen, that as far as the actual coral-formation is concerned, they have no distinguishing character.
Barrier-reefs, when encircling small islands, have been comparatively little noticed by voyagers; but they well deserve attention. In their structure they are little less marvellous than atolls, and they give a singular and most picturesque character to the scenery of the islands they surround.
The fragments torn up during gales from the outer margin are thrown over the reef on the shores of the island. Fringing-reefs, like barrier-reefs, both surround islands, and front the shores of continents. The external margin of this reef is described, as formed of projecting points, within which there is a space, from six to twelve feet deep, with patches of living coral on it.
Encircling barrier-reefs are of all sizes, from three miles to no less than forty-four miles in diameter; and that which fronts one side, and encircles both ends, of New Caledonia, is 400 miles long. Each reef includes one, two, or several rocky islands of various heights; and in one instance, even as many as twelve separate islands.
Description of the coloured map. Proximity of atolls and barrier-reefs. Relation in form and position of atolls with ordinary islands. Direct evidence of subsidence difficult to be detected. Proofs of recent elevation where fringing-reefs occur. Oscillations of level. Absence of active volcanoes in the areas of subsidence. Immensity of the areas which have been elevated and have subsided.
Between barrier-reefs, encircling either one lofty island or several small low ones, and atolls including a mere expanse of water, a striking series can be shown: in proof of this, I need only refer to the first plate in this volume, which speaks more plainly to the eye, than any description could to the ear.
I have not thought it worth while to introduce all those small and very numerous reefs, which occur within the lagoons of most atolls and within the lagoon-channels of most barrier-reefs, and which stand either isolated, or are attached to the shores of the reef or land.
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.
In the second section of the first chapter, I have shown that there are in the neighbourhood of atolls, some deeply submerged banks, with level surfaces; that there are others, less deeply but yet wholly submerged, having all the characters of perfect atolls, but consisting merely of dead coral-rock; that there are barrier-reefs and atolls with merely a portion of their reef, generally on the leeward side, submerged; and that such portions either retain their perfect outline, or they appear to be quite effaced, their former place being marked only by a bank, conforming in outline with that part of the reef which remains perfect.
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