Coral Reefs

Page 09

Figure 4.--BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago, from the survey by Captain Beechey, R.N.; the lagoon is choked up with reefs, but the average greatest depth of about twenty fathoms, is given from the published account of the voyage.

Figure 5.--BOLABOLA, in the Society Archipelago, from the survey of Captain Duperrey in the "Coquille:" the soundings in this and the following figures have been altered from French feet to English fathoms; height of highest point of the island 4,026 feet.

Figure 6.--MAURUA, in the Society Archipelago; from the survey by Captain Duperrey in the "Coquille:" height of land about eight hundred feet.

Figure 7.--POUYNIPETE, or SENIAVINE, in the Caroline Archipelago; from the survey by Admiral Lutke.

Figure 8.--GAMBIER ISLANDS, in the southern part of the Low Archipelago; from the survey by Captain Beechey; height of highest island, 1,246 feet; the islands are surrounded by extensive and irregular reefs; the reef on the southern side is submerged.

Figure 9.--PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in the Indian Ocean; from the survey by Captain Moresby and Lieutenant Powell; not nearly all the small submerged reefs in the lagoon are represented; the annular reef on the southern side is submerged.

Figure 10.--KEELING, or COCOS ATOLL (or lagoon-island), in the Indian Ocean; from the survey by Captain Fitzroy; the lagoon south of the dotted line is very shallow, and is left almost bare at low water; the part north of the line is choked up with irregular reefs. The annular reef on the north-west side is broken, and blends into a shoal sandbank, on which the sea breaks.



Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.-- Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae subsisting on corals.--Changes in the condition of the reefs and islets.-- Probable subsidence of the atoll.--Future state of the lagoon.


A.--Level of the sea at low water: where the letter A is placed, the depth is twenty-five fathoms, and the distance rather more than one hundred and fifty yards from the edge of the reef.

B.--Outer edge of that flat part of the reef, which dries at low water: the edge either consists of a convex mound, as represented, or of rugged points, like those a little farther seaward, beneath the water.

C.--A flat of coral-rock, covered at high water.

D.--A low projecting ledge of brecciated coral-rock washed by the waves at high water.

E.--A slope of loose fragments, reached by the sea only during gales: the upper part, which is from six to twelve feet high, is clothed with vegetation. The surface of the islet gently slopes to the lagoon.

F.--Level of the lagoon at low water.

KEELING or COCOS atoll is situated in the Indian Ocean, in 12 deg 5' S., and longitude 90 deg 55' E.: a reduced chart of it was made from the survey of Captain Fitzroy and the Officers of H.M.S. "Beagle," is given in Plate I., Figure 10. The greatest width of this atoll is nine miles and a half. Its structure is in most respects characteristic of the class to which it belongs, with the exception of the shallowness of the lagoon. The accompanying woodcut represents a vertical section, supposed to be drawn at low water from the outer coast across one of the low islets (one being taken of average dimensions) to within the lagoon.

The section is true to the scale in a horizontal line, but it could not be made so in a vertical one, as the average greatest height of the land is only between six and twelve feet above high-water mark.

I will describe the section, commencing with the outer margin. I must first observe that the reef-building polypifers, not being tidal animals, require to be constantly submerged or washed by the breakers. I was assured by Mr. Liesk, a very intelligent resident on these islands, as well as by some chiefs at Tahiti (Otaheite), that an exposure to the rays of the sun for a very short time invariably causes their destruction.

Charles Darwin

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