Chapter 1

ATOLLS OR LAGOON-ISLANDS.

SECTION 1.I.--KEELING ATOLL.

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.

(PLATE: UNTITLED WOODCUT, VERTICAL SECTION THROUGH KEELING ATOLL.)

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. Hence it is possible only under the most favourable circumstances, afforded by an unusually low tide and smooth water, to reach the outer margin, where the coral is alive. I succeeded only twice in gaining this part, and found it almost entirely composed of a living Porites, which forms great irregularly rounded masses (like those of an Astraea, but larger) from four to eight feet broad, and little