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Updated: May 21, 2025
In the three southern large atolls, the separate portions of reef between these channels have the ordinary structure, and are linear; but in the other atolls, especially the more northern ones, these portions are ring- formed, like miniature atolls. Other ring-formed reefs rise out of the lagoons, in the place of those irregular ones which ordinarily occur there.
The ring-formed reefs on the sides of this bifurcating channel are elongated, so that the northern and southern portions of Mahlos Mahdoo may claim, as far as their external outline is concerned, to be considered as distinct and perfect atolls.
The ring-formed reefs of the Maldiva atolls. The submerged condition of parts or of the whole of some annular reefs. The disseverment of large atolls. The union of atolls by linear reefs. The Great Chagos Bank. Objections from the area and amount of subsidence required by the theory, considered. The probable composition of the lower parts of atolls.
We may also infer that the central ring-formed reefs are modifications of those irregular ones, which are found in the lagoons of all common atolls. It appears from the charts on a large scale, that the ring-like structure is contingent on the marginal channels or breaches being wide; and, consequently, on the whole interior of the atoll being freely exposed to the waters of the open sea.
Even a huge land-crab is furnished by nature with the means to open and feed on this most useful production. The ring-formed reef of the lagoon-island is surmounted in the greater part of its length by linear islets. On the northern or leeward side there is an opening through which vessels can pass to the anchorage within.
In most lagoons the depth is considerably greater in the centre than in the channels; but in Tilla-dou-Matte, where the marginal ring-formed reefs stand far apart, the same depth is carried across the entire atoll, from the deep-water line on one side to that on the other.
Considerable doubts should be entertained regarding the nature of a reef of this kind, with a very shallow lagoon, and standing far from any other atoll, on account of the possibility of a crater or flat bank of rock lying at the proper depth beneath the surface of the water, thus affording a foundation for a ring-formed coral-reef.
Ring-formed reefs, marginal and central. Great depths in the lagoons of the southern atolls. Reefs in the lagoons all rising to the surface. Position of islets and breaches in the reefs, with respect to the prevalent winds and action of the waves. Destruction of islets. Connection in the position and submarine foundation of distinct atolls. The apparent disseverment of large atolls.
It is conformable with this view, that the ring-formed reefs on the margin, even where most perfect and standing furthest apart, generally have their longest axes directed in the line which the reef would have held, if the atoll had been bounded by an ordinary wall.
Hence the building up of those enormous ring-formed craters which are seen in such vast numbers on the moon's surface some of them being no less than a hundred miles in diameter, with which those of Etna and Vesuvius are the merest molehills in comparison.
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