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 less in thickness. These mounds are separated from
each other by narrow crooked channels, about six feet deep, most of which
intersect the line of reef at right angles. On the furthest mound, which I