The Structure and Distribution of Coral Reefs | Page 9

breakers, with here
and there an islet crowned by cocoa-nut trees, separates the smooth
waters of the lagoon-like channel from the waves of the open sea. The
barrier-reefs of Australia and of New Caledonia, owing to their
enormous dimensions, have excited much attention: in structure and
form they resemble those encircling many of the smaller islands in the
Pacific Ocean.
With respect to fringing, or shore-reefs, there is little in their structure
which needs explanation; and their name expresses their comparatively
small extension. They differ from barrier-reefs in not lying so far from
the shore, and in not having within a broad channel of deep water.
Reefs also occur around submerged banks of sediment and of
worn-down 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 peculiarities in their structure. The distribution,
also, of the different kinds of coral-reefs, and their position with
relation to the areas of recent elevation, and to the points subject to
volcanic eruptions, fully accord with this theory of their origin. (A brief
account of my views on coral formations, now published in my Journal
of Researches, was read May 31st, 1837, before the Geological Society,
and an abstract has appeared in the Proceedings.)
(DESCRIPTION OF THE PLATES.
PLATE I.--MAP SHOWING THE RESEMBLANCE IN FORM
BETWEEN BARRIER CORAL-REEFS SURROUNDING
MOUNTAINOUS ISLANDS, AND ATOLLS OR LAGOON
ISLANDS.)
In the several original surveys, from which the small plans on this plate
have been reduced, the coral-reefs are engraved in very different styles.
For the sake of uniformity, I have adopted the style used in the charts
of the Chagos Archipelago, published by the East Indian Company,
from the survey by Captain Moresby and Lieutenant Powell. The

surface of the reef, which dries at low water, is represented by a surface
with small crosses: the coral-islets on the reef are marked by small
linear spaces, on which a few cocoa-nut trees, out of all proportion too
large, have been introduced for the sake of clearness. The entire
ANNULAR REEF, which when surrounding an open expanse of water,
forms an "atoll," and when surrounding one or more high islands, forms
an encircling "barrier-reef," has a nearly uniform structure. The reefs in
some of the original surveys are represented merely by a single line
with crosses, so that their breadth is not given; I have had such reefs
engraved of the width usually attained by coral-reefs. I have not
thought it worth while to introduce all those small and very numerous
reefs, which occur within the lagoons of most atolls and within the
lagoon-channels of most barrier-reefs, and which stand either isolated,
or are attached to the shores of the reef or land. At Peros Banhos none
of the lagoon-reefs rise to the surface of the water; a few of them have
been introduced, and are marked by plain dotted circles. A few of the
deepest soundings are laid down within each reef; they are in fathoms,
of six English feet.
Figure