solid stone is stated to consist of shell
and coral, united by sand; but masses of very compact limestone are also
found even in the uppermost and newest parts of the reef, such as could
only have been produced by chemical precipitation. Professor Agassiz
also informs me that his observations on the Florida reefs (which
confirm Darwin's theory of atolls to be mentioned in the sequel) have
convinced him, that large blocks are loosened, not by shrinkage in the
sun's heat, as Chamisso imagined, but by innumerable perforations of
lithodomi and other boring testacea.

The carbonate of lime may have been principally derived from the
decomposition of corals and testacea; for when the animal matter
undergoes putrefaction, the calcareous residuum must be set free under
circumstances very favorable to precipitation, especially when there are
other calcareous substances, such as shells and corals, on which it may
be deposited. Thus organic bodies may be inclosed in a solid cement, and
become portions of rocky masses.[1120]

The width of the circular strip of dead coral forming the islands
explored by Captain Beechey, exceeded in no instance half a mile from
the usual wash of the sea to the edge of the lagoon, and, in general,
was only about three or four hundred yards.[1121] The depth of the
lagoons is various; in some, entered by Captain Beechey, it was from
twenty to thirty-eight fathoms.

The two other peculiarities which are most characteristic of the annular
reef or atoll are first, that the strip of dead coral is invariably
highest on the windward side, and secondly, that there is very generally
an opening at some point in the reef affording a narrow passage, often
of considerable depth, from the sea into the lagoon.

[Illustration: Fig. 117.]

_Maldive and Laccadive Isles._--The chain of reefs and islets called the
Maldives (see fig. 117.), situated in the Indian Ocean, to the
south-west of Malabar, forms a chain 470 geographical miles in length,
running due north and south, with an average breadth of about 50 miles.
It is composed throughout of a series of circular assemblages of islets,
all formed of coral, the larger groups being from forty to ninety miles
in their longest diameter. Captain Horsburgh, whose chart of these
islands is subjoined, states, that outside of each circle or _atoll_, as
it is termed, there are coral reefs sometimes extending to the distance
of two or three miles, beyond which there are no sou