the form of a truncated cone, and then, their progress being arrested,
they work laterally, so that if several of them were near each other they
would unite and form a shelf similar to that which has been described
round the margins of some of the lagoons.
The depth of these lagoons is various: in those which w-e entered
it was from twenty to thirty-eight fathoms, but in others, to whicli w-e
had no access, by the light-blue colour of the water it appeared to
be very small. It is, however, tolerably certain that the coral forms
the basis of them, and consequently, unless depositions of sand or other
substances, obnoxious to the coral insects, take place, their depth must
depend upon their age.
Very little offered itself to our notice, by which we could judge
of the rapidity of the growth of the coral, as the islands which we
examined had never been described with the accuracy necessary for
this purpose; and there were, consequently, no means of comparing
the state in wliich they were found by us, with that ivhich was presented
to our predecessors ; but from the report of the natives, the coral
bordering the volcanic islands does not increase very fast, as we never
heard of any channels being filled u p ; but, on the contrary, that the
passages through the reefs were apparently always iu the same condition.
The only direct evidence, however, which 1 could obtain of this fact was
that of the Dolphin reef off Point Venus in Otaheite. This reef, when first
examined by Captain Wallis in 1769, had “ two fathoms water upon
it.” Cook sounded upon it a few years afterwards, and gave its depth
fifteen feet. In our visit to this place, we found, upon the shallowest
part of it, thirteen feet and a half. These measurements, though at
variance, from the irregularity of the surface of the reef, are sufficiently
exact to warrant the conclusion that it has undergone no very material
alteration during an interval, it should be recollected, of fifty-six years.
But the Dolphin, as w-ell as the above-mentioned reefs and channels,
are within the influence of rivers, which, iu my opinion, materially
retard their increase, and their growth must not be taken as a criterion
of that of the islands of which 1 have been speaking. YYith regard to
them, there is one fact worthy of consideration, and upon which every
person must form his own judgment. I allude to the remains of the CH AR
Matilda, a ship which a few pages back is stated to have been cast away ,
upon one of these coral islands. In my description of Matilda Island,
Feb.
I82(i.
it is stated, that one of the anchors of this ship, a ton iu weight, a four-
pounder gun, her boilers and iron-work, are lying upon the top of the
reef, two hundred yards from the present break of the sea, and are dry
at low water*. The nature of these articles and the quantity oi iron
bolts and other materials lying with them renders it probable that the
vessel went to pieces in that spot, for had the sea been heavy enough
to wash the anchor from deeper water, the boiler must have been
carried much beyond i t ; and the question is, whether the hull of a
vessel of the Matilda’s tonnage could be washed upon a reef dry at
low water, and be deposited two hundred yards within the usual break
of the sea. The circumstance of the hatches, staves of casks, and part
of the vessel, being deposited in parts of the dry land not far distant,
and scarcely more than four feet from the present level of the sea,
offers a presumption that the sea did not rise more than that height
above its ordinary level, or it would have washed the articles further
and left them in the lagoon, whence they would have been carried to
sea by the current.
The materials were not in the least overgrown with coral, nor had
they any basin left round them by w’hich the progress of the coral
could be traced; and yet, in other parts of this reef, we noticed the
chama gigas of seven or eight inches in diameter so overgrown by it,
that there was only a small aperture of two inches left for the extremity
of the shell to open and shut.
When the attention of men of science was called to these singular
formations by the voyages of Captain Cook, one opinion, among others,
respecting their formation was. that they sprung from a small base and
extended themselves laterally as they grew perpendicularly towards the
surface of the sea ; and that they represented upon a large scale the
form which is assumed by some of the corallines. In particular this
theory was entertained by Mr. John R. Forster, wlio accompanied Captain
* The rise of the tide is about two feet.