rock; and others are scattered quite irregularly where the sea is very
shallow; these in most respects are allied to those of the fringing class,
but they are of comparatively little interest.

I have given a separate chapter to each of the above classes, and have
described some one reef or island, on which I possessed most information,
as typical; and have afterwards compared it with others of a like kind.
Although this classification is useful from being obvious, and from
including most of the coral-reefs existing in the open sea, it admits of a
more fundamental division into barrier and atoll-formed reefs on the one
hand, where there is a great apparent difficulty with respect to the
foundation on which they must first have grown; and into fringing-reefs on
the other, where, owing to the nature of the slope of the adjoining land,
there is no such difficulty. The two blue tints and the red colour
(replaced by numbers in this edition.) on the map (Plate III.), represent
this main division, as explained in the beginning of the last chapter. In
the Appendix, every existing coral-reef, except some on the coast of Brazil
not included in the map, is briefly described in geographical order, as far
as I possessed information; and any particular spot may be found by
consulting the Index.

Several theories have been advanced to explain the origin of atolls or
lagoon-islands, but scarcely one to account for barrier-reefs. From the
limited depths at which reef-building polypifers can flourish, taken into
consideration with certain other circumstances, we are compelled to
conclude, as it will be seen, that both in atolls and barrier-reefs, the
foundation on which the coral was primarily attached, has subsided; and
that during this downward movement, the reefs have grown upwards. This
conclusion, it will be further seen, explains most satisfactorily the
outline and general form of atolls and barrier-reefs, and likewise certain