190 
VOYAGE TO THE 
CHAP, 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 
Feb. 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 we entered 
it was from twenty to thirty-eight fathoms, but in others, to which we 
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 httle 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 which they were found by us, with that which was pre- 
sented 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 up ; but, on the contrary, that the 
passages through the reefs were apparently always in the same condition. 
The only direct evidence, however, which I 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 well as the above-mentioned reefs and channels, 
are within the influence of rivers, which, in my opinion, materially 
retard their increase, and their growth must not be taken as a criterion 
of that of the islands of which I have been speaking. With regard to 
them, there is one fact worthy of consideration, and upon which every 
