We did not return on board till late in the evening, for we stayed a
long time in the lagoon, examining the fields of coral and the gigantic
shells of the chama, into which, if a man were to put his hand, he
would not, as long as the animal lived, be able to withdraw it. Near
the head of the lagoon I was much surprised to find a wide area,
considerably more than a mile square, covered with a forest of
delicately branching corals, which, though standing upright, were all
dead and rotten. At first I was quite at a loss to understand the
cause afterwards it occurred to me that it was owing to the following
rather curious combination of circumstances. It should, however, first
be stated, that corals are not able to survive even a short exposure in
the air to the sun's rays, so that their upward limit of growth is
determined by that of lowest water at spring tides. It appears, from
some old charts, that the long island to windward was formerly
separated by wide channels into several islets; this fact is likewise
indicated by the trees being younger on these portions. Under the
former condition of the reef, a strong breeze, by throwing more water
over the barrier, would tend to raise the level of the lagoon. Now it
acts in a directly contrary manner; for the water within the lagoon not
only is not increased by currents from the outside, but is itself blown
outwards by the force of the wind. Hence it is observed, that the tide
near the head of the lagoon does not rise so high during a strong
breeze as it does when it is calm. This difference of level, although
no doubt very small, has, I believe, caused the death of those
coral-groves, which under the former and more open condition of the
outer reef has attained the utmost possible limit of upward growth.
A few miles north of Keeling there is another small atoll, the lagoon
of which is nearly filled up with coral-mud. Captain Ross found
embedded in the conglomerate on the outer coast, a well-rounded
fragment of greenstone, rather larger than a man's head: he and the men
with him were so much surprised at this, that they brought it away and
preserved it as a curiosity. The occurrence of this one stone, where
every other particle of matter is calcareous, certainly is very
puzzling. The island has scarcely ever been visited, nor is it
probable that a ship had been wrecked there. From the absence of any
better explanation, I came to the conclusion that it must have come
entangled in the roots of some large tree: when, however, I considered
the great distance from the nearest land, the combination of chances
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