Considering that Keeling atoll, like other coral formations, has been
entirely formed by the growth of organic beings, and the accumulation of
their detritus, one is naturally led to inquire how long it has continued,
and how long it is likely to continue, in its present state. Mr. Liesk
informed me that he had seen an old chart in which the present long island
on the S.E. side was divided by several channels into as many islets; and
he assures me that the channels can still be distinguished by the smaller
size of the trees on them. On several islets, also, I observed that only
young cocoa-nut trees were growing on the extremities; and that older and
taller trees rose in regular succession behind them; which shows that these
islets have very lately increased in length. In the upper and south-eastern
part of the lagoon, I was much surprised by finding an irregular
field of at least a mile square of branching corals, still upright, but
entirely dead. They consisted of the species already mentioned; they were
of a brown colour, and so rotten, that in trying to stand on them I sank
halfway up the leg, as if through decayed brushwood. The tops of the
branches were barely covered by water at the time of lowest tide. Several
facts having led me to disbelieve in any elevation of the whole atoll, I
was at first unable to imagine what cause could have killed so large a
field of coral. Upon reflection, however, it appeared to me that the
closing up of the above-mentioned channels would be a sufficient cause; for
before this, a strong breeze by forcing water through them into the head of
the lagoon, would tend to raise its level. But now this cannot happen, and
the inhabitants observe that the tide rises to a less height, during a high
S.E. wind, at the head than at the mouth of the lagoon. The corals, which,
under the former condition of things, had attained the utmost possible
limit of upward growth, would thus occasionally be exposed for a short time
to the sun, and be killed.