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Updated: June 1, 2025
A perfect series such as we have here traced, impresses the mind with an idea of actual change; and it will hereafter be seen, that the theory of subsidence, with the upward growth of the coral, modified by accidents of probable occurrence, will account for the occasional disseverment of large atolls. The Great Chagos bank alone remains to be described.
To add to my doubts, immediately on the outside of this barrier-like reef, TURNEFFE, LIGHTHOUSE, and GLOVER reefs are situated, and these reefs have so completely the form of atolls, that if they had occurred in the Pacific, I should not have hesitated about colouring them blue.
Creeping steadily northward, we passed the Cosmoledo group of atolls without paying them a visit, which was strange, as, from their appearance, no better fishing-ground would be likely to come in our way.
The shores of this great land were doubtless fringed by coral reefs; and, as it slowly underwent depression, the hilly regions, converted into islands, became, at first, surrounded by fringing reefs, and then, as depression went on, these became converted into encircling reefs, and these, finally, into atolls, until a maze of reefs and coral-girdled islets took the place of the original land masses.
The lagoon seemed alive with large and small fish none of which, Niâbon said, were poisonous, like two thirds of those of the Marshall Island atolls, and the beaches and sand-flats were covered with small golden-winged plover, who displayed not the slightest fear of our presence, letting us approach them within a few yards, then rising and settling down again.
I may here remark that the outlying reefs on the west coast, between latitude 19 deg and 24 deg, are the only ones in the Red Sea, which approach in structure to the true atolls of the Indian and Pacific Oceans, but they present only imperfect miniature likenesses of them.
THEORIES OF CORAL REEFS. Fringing reefs require no explanation, since the depth of water about them is not greater than that at which coral can grow; but barrier reefs and atolls, which may rise from depths too great for coral growth demand a theory of their origin.
There are also Barrier-Reefs, usually enclosing an island in the deep sea, and Lagoon Islands or Atolls, which enclose a lagoon, or lake, such as the one where Sammy now was.
In some cases, the less inclination of a certain part of the external slope, for instance of the northern extremities of the two Keeling atolls, is caused by a prevailing current which there accumulates a bed of sand.
And as by our theory the areas including atolls and barrier-reefs are subsiding, we ought occasionally to find reefs both dead and submerged.
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