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Feb.
1826.
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*\’in^' second voyage and visited several of the coral islands, and
/ was founded, no doubt, upon the experience wliich he had derived
upon that voyage. But considering tlie extent of some of these islands,
it is evident that if this be their form, the lithophytes, tlic aiiiinals which
construct them, must commence their operations at very great deptlis,
a fact which is doubted by naturalists. The general opinion now is,
that they have their foundations upon submarine iiioiiiitaiiis, or upon
extinguished volcanoes, whicli are not more than four or five hundred
feet immersed in the ocean ; and that their shape depends upon the figure
of the base whence they spring. It would be immaterial which of these
theories were correct, were it not that iu the latter instance the lagoon
that is formed in all the islands of this description might be o icasioned
by the shape of the crater alone, whereas, in the formei. it must
result from the propensity of the coral animals, and this, if true, forms
a remarkable and interesting feature in their natural history. Mr.
Forster* thought this peculiarity might arise from the instinct of the
animalcules forming the reefs, which from a desire to shelter their
habitation from the impetuosity of the winds, and the power and rage
of the ocean, endeavoured to construct a ledge, within which was a
lagoon entirely screened against the power of the elements, and where
a calm and sheltered place was by these means afforded to the animals
in the centre of the island.
Another reason why the consideration of the nature of their foundation
is not immaterial is, that if the form of the islands arose from the
peculiar shape of the craters, and it be admitted that the lithophytes are
unable to exist at greater depths than those above-mentioned, we shall
have examples of craters of considerably larger dimensions, and more
complete in their outline, than any that are known upon the land, which,
if true, is a curious fact. Until the voyage of the Blossom, it was not
generally known that the lagoons in these islands were of such depths, or
that the wall of coral which encircles them was so narrow and perfect, as iu
almost every instance it has been found; nor that the islands were of such
dimensions, as they were designated groups, or chains of islands, iu coiise-
* F o rs te r’s Observations, 4to, pag e 150.
quence of the wall being broken by channels into the lagoon; but on
examination, the chain is found continuous under water; and as in all
probability it will in time reach the surface and become dry, the whole
group may be considered as one island.
In the plans which I have delivered into the Admiralty, the figure
and extent of thirty coral islands, out of the many which exist in the
Pacific, are carefully delineated, and a reference to them will more fully
explain the nature of them than any description I can give here. One
of these plans* being of particular interest, I have inserted it in the
present work, as it exhibits, not only tlie coral chain enclosing the
lagoon, which is the common character of the coral islands ; but, also,
an example of several volcanic islands rising within i t ; and likewise
the peculiarity of the inequality in the sides of the chain mentioned iu
page 189.
The subject of the formation of these islands is one of great interest,
and will require a numerous and careful collection of facts before
any entirely satisfactory couclusioii can be arrived at. I regret that
my time did not permit me to inquire more particularly into this
curious matter; but iiaviug to survey about fifty islands, some of
which W'ere of great extent, iu the space of about four months, I
could not accomplish more than w'as absolutely necessary to the purposes
of a safe navigation of the Archipelago. We w'ere, liowever, not
inattentive to the subject, and when opportunity offered, soundings
were tried for at great depths, and the descent of the islands w’as repeatedly
ascertained as far as the common lines would extend. Some
of these experiments are given iu the annexed plate, representing
a section of a coral island from actual measiireiiient.
In considering the subject of these coral formations, ray attention
W'as drawn to the singularity of the occurrence of openings iu them,
either opposite to, or iu the direction of some stream of fresh tvater
from the mouiitaiiis; and on searching several charts, I find so many
corroborations of the fact, that I have uo doubt of the trutli of i t : as
far as my own observations extended, it W'as always so. The aversion
* See the pUui o f Gambier group.
C H A P .
V H i .
Mardi,
I82G.