Coral Reefs - Introduction - Description of Plates

CORAL-REEFS.

INTRODUCTION.

The object of this volume is to describe from my own observation and the
works of others, the principal kinds of coral-reefs, more especially those
occurring in the open ocean, and to explain the origin of their peculiar
forms.  I do not here treat of the polypifers, which construct these vast
works, except so far as relates to their distribution, and to the
conditions favourable to their vigorous growth.  Without any distinct
intention to classify coral-reefs, most voyagers have spoken of them under
the following heads: "lagoon-islands," or "atolls," "barrier" or
"encircling reefs," and "fringing" or "shore-reefs."  The lagoon-islands
have received much the most attention; and it is not surprising, for every
one must be struck with astonishment, when he first beholds one of these
vast rings of coral-rock, often many leagues in diameter, here and there
surmounted by a low verdant island with dazzling white shores, bathed on
the outside by the foaming breakers of the ocean, and on the inside
surrounding a calm expanse of water, which from reflection, is of a bright
but pale green colour.  The naturalist will feel this astonishment more
deeply after having examined the soft and almost gelatinous bodies of these
apparently insignificant creatures, and when he knows that the solid reef
increases only on the outer edge, which day and night is lashed by the
breakers of an ocean never at rest.  Well did Francois Pyrard de Laval, in
the year 1605, exclaim, "C'est une merueille de voir chacun de ces
atollons, enuironne d'un grand banc de pierre tout autour, n'y ayant point
d'artifice humain."  The accompanying sketch of Whitsunday island, in the
South Pacific, taken from Captain Beechey's admirable "Voyage," although
excellent of its kind, gives but a faint idea of the singular aspect of one
of these lagoon-islands.

(PLATE: UNTITLED WOODCUT, WHITSUNDAY ATOLL.)

Whitsunday Island is of small size, and the whole circle has been converted
into land, which is a comparatively rare circumstance.  As the reef of a
lagoon-island generally supports many separate small islands, the word
"island," applied to the whole, is often the cause of confusion; hence I
have invariably used in this volume the term "atoll," which is the name
given to these circular groups of coral-islets by their inhabitants in the
Indian Ocean, and is synonymous with "lagoon-island."

(PLATE: UNTITLED WOODCUT, REEF AT BOLABOLA ISLAND.)

Barrier-reefs, when encircling small islands, have been comparatively
little noticed by voyagers; but they well deserve attention.  In their
structure they are little less marvellous than atolls, and they give a
singular and most picturesque character to the scenery of the islands they
surround.  In the accompanying sketch, taken from the "Voyage of the
'Coquille'," the reef is seen from within, from one of the high peaks of
the island of Bolabola.  (I have taken the liberty of simplifying the
foreground, and leaving out a mountainous island in the far distance.)
Here, as in Whitsunday Island, the whole of that part of the reef which is
visible is converted into land.  This is a circumstance of rare occurrence;
more usually a snow-white line of great 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 1.--VANIKORO, situated in the western part of the South Pacific;
taken from the survey by Captain D'Urville in the "Astrolabe;" the
soundings on the southern side of the island, namely, from thirty to forty
fathoms, are given from the voyage of the Chev. Dillon; the other soundings
are laid down from the survey by D'Urville; height of the summit of the
island is 3,032 feet.  The principal small detached reefs within the
lagoon-channel have in this instance been represented.  The southern shore
of the island is narrowly fringed by a reef: if the engraver had carried
this reef entirely round both islands, this figure would have served (by
leaving out in imagination the barrier-reef) as a good specimen of an
abruptly-sided island, surrounded by a reef of the fringing class.

Figure 2.--HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the
"Atlas of the Voyage of the 'Astrolabe,'" compiled from the surveys of
Captains Duperrey and D'Urville; the depth of the immense lagoon-like space
within the reef is not known.

Figure 3.--RAIATEA, in the Society Archipelago; from the map given in the
quarto edition of "Cook's First Voyage;" it is probably not accurate.

Figure 4.--BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago,
from the survey by Captain Beechey, R.N.; the lagoon is choked up with
reefs, but the average greatest depth of about twenty fathoms, is given
from the published account of the voyage.

Figure 5.--BOLABOLA, in the Society Archipelago, from the survey of Captain
Duperrey in the "Coquille:" the soundings in this and the following figures
have been altered from French feet to English fathoms; height of highest
point of the island 4,026 feet.

Figure 6.--MAURUA, in the Society Archipelago; from the survey by Captain
Duperrey in the "Coquille:" height of land about eight hundred feet.

Figure 7.--POUYNIPETE, or SENIAVINE, in the Caroline Archipelago; from the
survey by Admiral Lutke.

Figure 8.--GAMBIER ISLANDS, in the southern part of the Low Archipelago;
from the survey by Captain Beechey; height of highest island, 1,246 feet;
the islands are surrounded by extensive and irregular reefs; the reef on
the southern side is submerged.

Figure 9.--PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in
the Indian Ocean; from the survey by Captain Moresby and Lieutenant Powell;
not nearly all the small submerged reefs in the lagoon are represented; the
annular reef on the southern side is submerged.

Figure 10.--KEELING, or COCOS ATOLL (or lagoon-island), in the Indian
Ocean; from the survey by Captain Fitzroy; the lagoon south of the dotted
line is very shallow, and is left almost bare at low water; the part north
of the line is choked up with irregular reefs.  The annular reef on the
north-west side is broken, and blends into a shoal sandbank, on which the
sea breaks.