Coral Reefs

SECTION I.—DESCRIPTION OF KEELING ATOLL. Corals on the outer margin.—Zone of Nulliporæ.—Exterior reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari and Holuthuriæ subsisting on corals.—Changes in the condition of the reefs and islets.—Probable subsidence of the atoll.—Future state of the lagoon.
SECTION II.—GENERAL DESCRIPTION OF ATOLLS. General form and size of atolls, their reefs and islets.—External slope.— Zone of Nulliporæ.—Conglomerate.—Depth of lagoons.—Sediment.—Reefs submerged wholly or in part.—Breaches in the reef.—Ledge-formed shores round certain lagoons.—Conversion of lagoons into land.
SECTION III.—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK. Maldiva Archipelago.—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.—The Great Chagos Bank.—Its submerged condition and extraordinary structure.
SECTION I.—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS FAVOURABLE TO THEIR INCREASE.
SECTION II.—ON THE RATE OF GROWTH OF CORAL-REEFS.
SECTION III.—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
PLATE I.

PLATE I.—MAP SHOWING THE RESEMBLANCE IN FORM BETWEEN BARRIER CORAL-REEFS SURROUNDING MOUNTAINOUS ISLANDS, AND ATOLLS OR LAGOON ISLANDS.
In the several original surveys, from which the small plans on this plate have been reduced, the coral-reefs are engraved in very different styles. For the sake of uniformity, I have adopted the style used in the charts of the Chagos Archipelago, published by the East Indian Company, from the survey by Captain Moresby and Lieutenant Powell. The surface of the reef, which dries at low water, is represented by a surface with small crosses: the coral-islets on the reef are marked by small linear spaces, on which a few cocoa-nut trees, out of all proportion too large, have been introduced for the sake of clearness. The entire annular reef , which when surrounding an open expanse of water, forms an “atoll,” and when surrounding one or more high islands, forms an encircling “barrier-reef,” has a nearly uniform structure. The reefs in some of the original surveys are represented merely by a single line with crosses, so that their breadth is not given; I have had such reefs engraved of the width usually attained by coral-reefs. 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. At Peros Banhos none of the lagoon-reefs rise to the surface of the water; a few of them have been introduced, and are marked by plain dotted circles. A few of the deepest soundings are laid down within each reef; they are in fathoms, of six English feet.