66 CORAL FORMATIONS. 
may be remarked that the tides in the Paumotus are two to three feet, 
and about E:nderby’s Island five to six feet in height. 
Maldives.—Chagos Bank.—The Maldives have been often appealed 
to in illustration of coral structures. ‘They are particularly described 
by Mr. Darwin, from information communicated to him by Captain 
Moresby, and from the charts of this officer and Lieutenant Powell.* 
The point of special interest in their structure is the occurrence of 
atolls or rings within the larger atolls. The islets of the lagoon, and 
those of the encircling reef, are in many instances annular reefs, 
each with its own little lake. Gems within gems are here clustered 
together. 
The annular islets of the main encircling reef are oblong, and lie 
with the longest diameter, which is sometimes three miles long, in 
the line of the reef. ‘Those of the lagoon are generally less than two 
miles across. ‘The lagoons they contain vary from five fathoms or 
less to twelve fathoms in depth. 
The Maldives are among the largest atoll-reefs known; and they 
are intersected by many large open channels; and Mr. Darwin ob- 
serves, that the interior atolls occur only near these channels, where 
the sea has free access. We may view each large island in the 
archipelago as a sub-archipelago of itself. Although thus singular in 
their features, they illustrate no new principles with regard to reef- 
formations. t 
* Darwin on Coral Reefs, p. 32. See also Journal of the Royal Geographical Society, 
on the Geography of the Maldives, by J. J. Horsburgh, ii. p.72 ; and by Captain W. F. W. 
Owen, ibid, p. 81; also vol. v., p. 398, on the Northern Atolls of the Maldives, by Cap- 
tain Moresby. 
+ Mr. Darwin thus remarks, (Op. cit. pp. 33, 34,)—“TI can in fact point out no 
essential difference between these little ring-formed reefs, (which, however, are larger, 
and contain deeper lagoons than many true atolls that stand in the open sea,) and the 
most perfectly characterized atolls, excepting that the ring-formed reefs are based on a 
shallow foundation instead of the floor of the open sea, and that instead of being scattered 
irregularly, they are grouped closely together.”—“ It appears from the charts on a large 
scale, that the ring-like structure is contingent on the marginal channels or branches 
being wide, and consequently on the whole interior of the atoll being freely exposed to the 
waters of the open sea. When the channels are narrow, or few in number, although the 
lagoon be of great size and depth, (as in Suadiva,) there are no ring-formed reefs ; where 
the channels are somewhat broader, the marginal portions of reef, and especially those 
close to the larger channels, are ring-formed, but the central ones are not so: where they 
are broadest, almost every reef throughout the atoll is more or less perfectly ring-formed. 
Although their presence is thus contingent on the openness of the marginal channels, the 
