As The Barrier-Reef Slowly Sinks Down, The
Corals Will Go On Vigorously Growing Upwards; But As The
Island Sinks, The Water Will Gain Inch By Inch On The Shore -
The Separate Mountains First Forming Separate Islands Within
[Picture]
one great reef - and finally, the last and highest pinnacle
disappearing.
The instant this takes place, a perfect atoll
is formed: I have said, remove the high land from within an
encircling barrier-reef, and an atoll is left, and the land has
been removed. We can now perceive how it comes that
atolls, having sprung from encircling barrier-reefs, resemble
them in general size, form, in the manner in which they are
grouped together, and in their arrangement in single or
double lines; for they may be called rude outline charts of
the sunken islands over which they stand. We can further
see how it arises that the atolls in the Pacific and Indian
Oceans extend in lines parallel to the generally prevailing
strike of the high islands and great coast-lines of those
oceans. I venture, therefore, to affirm, that on the theory of
the upward growth of the corals during the sinking of the
land, [13] all the leading features in those wonderful
structures, the lagoon-islands or atolls, which have so long
excited the attention of voyagers, as well as in the no less
wonderful barrier-reefs, whether encircling small islands or
stretching for hundreds of miles along the shores of a
continent, are simply explained.
It may be asked, whether I can offer any direct evidence
of the subsidence of barrier-reefs or atolls; but it must be
borne in mind how difficult it must ever be to detect a
movement, the tendency of which is to hide under water the part
affected. Nevertheless, at Keeling atoll I observed on all
sides of the lagoon old cocoa-nut trees undermined and falling;
and in one place the foundation-posts of a shed, which
the inhabitants asserted had stood seven years before just
above high-water mark, but now was daily washed by every
tide: on inquiry I found that three earthquakes, one of them
severe, had been felt here during the last ten years. At
Vanikoro, the lagoon-channel is remarkably deep, scarcely
any alluvial soil has accumulated at the foot of the lofty
included mountains, and remarkably few islets have been
formed by the heaping of fragments and sand on the wall-like
barrier reef; these facts, and some analogous ones, led
me to believe that this island must lately have subsided and
the reef grown upwards: here again earthquakes are frequent
and very severe. In the Society archipelago, on the
other hand, where the lagoon-channels are almost choked up,
where much low alluvial land has accumulated, and where in
some cases long islets have been formed on the barrier-reefs
- facts all showing that the islands have not very lately
subsided - only feeble shocks are most rarely felt. In these
coral formations, where the land and water seem struggling
for mastery, it must be ever difficult to decide between the
effects of a change in the set of the tides and of a slight
subsidence:
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