Chapter 2 - Barrier Reefs

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CHAPTER II.--BARRIER REEFS.














Closely resemble in general form and structure atoll-reefs.--Width and
depth of the lagoon-channels.--Breaches through the reef in front of
valleys, and generally on the leeward side.--Checks to the filling up of
the lagoon-channels.--Size and constitution of the encircled islands.--
Number of islands within the same reef.--Barrier-reefs of New Caledonia and
Australia.--Position of the reef relative to the slope of the adjoining
land.--Probable great thickness of barrier-reefs.

The term "barrier" has been generally applied to that vast reef which
fronts the N.E. shore of Australia, and by most voyagers likewise to that
on the western coast of New Caledonia.  At one time I thought it convenient
thus to restrict the term, but as these reefs are similar in structure, and
in position relatively to the land, to those, which, like a wall with a
deep moat within, encircle many smaller islands, I have classed them
together.  The reef, also, on the west coast of New Caledonia, circling
round the extremities of the island, is an intermediate form between a
small encircling reef and the Australian barrier, which stretches for a
thousand miles in nearly a straight line.

The geographer Balbi has in effect described those barrier-reefs, which
encircle moderately sized islands, by calling them atolls with high land
rising from within their central expanse.  The general resemblance between
the reefs of the barrier and atoll classes may be seen in the small, but
accurately reduced charts on Plate I. (The authorities from which these
charts have been reduced, together with some remarks on them and
descriptive of the Plates, are given separately.), and this resemblance can
be further shown to extend to every part of the structure.  Beginning with
the outside of the reef; many scattered soundings off Gambier, Oualan, and
some other encircled islands, show that close to the breakers there exists
a narrow shelving margin, beyond which the ocean becomes suddenly
unfathomable; but off the west coast of New Caledonia, Captain Kent
(Dalrymple, "Hydrog. Mem." volume iii.) found no bottom with 150 fathoms,
at two ships' length from the reef; so that the slope here must be nearly
as precipitous as off the Maldiva atolls.

I can give little information regarding the kinds of corals which live on
the outer margin.  When I visited the reef at Tahiti, although it was low
water, the surf was too violent for me to see the living masses; but,
according to what I heard from some intelligent native chiefs, they
resemble in their rounded and branchless forms, those on the margin of
Keeling atoll.  The extreme verge of the reef, which was visible between
the breaking waves at low water, consisted of a rounded, convex,
artificial-like breakwater, entirely coated with Nulliporae, and absolutely
similar to that which I have described at Keeling atoll.  From what I heard
when at Tahiti, and from the writings of the Revs. W. Ellis and J.
Williams, I conclude that this peculiar structure is common to most of the
encircled islands of the Society Archipelago.  The reef within this mound
or breakwater, has an extremely irregular surface, even more so than
between the islets on the reef of Keeling atoll, with which alone (as there
are no islets on the reef of Tahiti) it can properly be compared.  At
Tahiti, the reef is very irregular in width; but round many other encircled
islands, for instance, Vanikoro or Gambier Islands (Figures 1 and 8, Plate
I.), it is quite as regular, and of the same average width, as in true
atolls.  Most barrier-reefs on the inner side slope irregularly into the
lagoon-channel (as the space of deep water separating the reef from the
included land may be called), but at Vanikoro the reef slopes only for a
short distance, and then terminates abruptly in a submarine wall, forty
feet high,--a structure absolutely similar to that described by Chamisso in
the Marshall atolls.

In the Society Archipelago, Ellis (Consult, on this and other points, the
"Polynesian Researches," by the Rev. W. Ellis, an admirable work, full of
curious information.) states, that the reefs generally lie at the distance
of from one to one and a half miles, and, occasionally, even at more than
three miles, from the shore.  The central mountains are generally bordered
by a fringe of flat, and often marshy, alluvial land, from one to four
miles in width.  This fringe consists of coral-sand and detritus thrown up
from the lagoon-channel, and of soil washed down from the hills; it is an
encroachment on the channel, analogous to that low and inner part of the
islets in many atolls which is formed by the accumulation of matter from
the lagoon.  At Hogoleu (Figure 2, Plate I.), in the Caroline Archipelago
(See "Hydrographical Mem." and the "Atlas of the Voyage of the
'Astrolabe'," by Captain Dumont D'Urville, page 428.), the reef on the
south side is no less than twenty miles; on the east side, five; and on the
north side, fourteen miles from the encircled high islands.

The lagoon channels may be compared in every respect with true lagoons.  In
some cases they are open, with a level bottom of fine sand; in others they
are choked up with reefs of delicately branched corals, which have the same
general character as those within the Keeling atoll.  These internal reefs
either stand separately, or more commonly skirt the shores of the included
high islands.  The depth of the lagoon-channel round the Society Islands
varies from two or three to thirty fathoms; in Cook's (See the chart in
volume i. of Hawkesworth's 4to edition of "Cook's First Voyage.") chart of
Ulieta, however, there is one sounding laid down of forty-eight fathoms; at
Vanikoro there are several of fifty-four and one of fifty-six and a half
fathoms (English), a depth which even exceeds by a little that of the

interior of the great Maldiva atolls.  Some barrier-reefs have very few
islets on them; whilst others are surmounted by numerous ones; and those
round part of Bolabola (Plate I., Figure 5) form a single linear strip.
The islets first appear either on the angles of the reef, or on the sides
of the breaches through it, and are generally most numerous on the windward
side.  The reef to leeward retaining its usual width, sometimes lies
submerged several fathoms beneath the surface; I have already mentioned
Gambier Island as an instance of this structure.  Submerged reefs, having a
less defined outline, dead, and covered with sand, have been observed (see
Appendix) off some parts of Huaheine and Tahiti.  The reef is more
frequently breached to leeward than to windward; thus I find in
Krusenstern's "Memoir on the Pacific," that there are passages through the
encircling reef on the leeward side of each of the seven Society Islands,
which possess ship-harbours; but that there are openings to windward
through the reef of only three of them.  The breaches in the reef are
seldom as deep as the interior lagoon-like channel; they generally occur in
front of the main valleys, a circumstance which can be accounted for, as
will be seen in the fourth chapter, without much difficulty.  The breaches
being situated in front of the valleys, which descend indifferently on all
sides, explains their more frequent occurrence through the windward side of
barrier-reefs than through the windward side of atolls,--for in atolls
there is no included land to influence the position of the breaches.

It is remarkable, that the lagoon-channels round mountainous islands have
not in every instance been long ago filled up with coral and sediment; but
it is more easily accounted for than appears at first sight.  In cases like
that of Hogoleu and the Gambier Islands, where a few small peaks rise out
of a great lagoon, the conditions scarcely differ from those of an atoll,
and I have already shown, at some length, that the filling up of a true
lagoon must be an extremely slow process.  Where the channel is narrow, the
agency, which on unprotected coasts is most productive of sediment, namely
the force of the breakers, is here entirely excluded, and the reef being
breached in the front of the main valleys, much of the finer mud from the
rivers must be transported into the open sea.  As a current is formed by
the water thrown over the edge of atoll-formed reefs, which carries
sediment with it through the deep-water breaches, the same thing probably
takes place in barrier-reefs, and this would greatly aid in preventing the
lagoon-channel from being filled up.  The low alluvial border, however, at
the foot of the encircled mountains, shows that the work of filling up is
in progress; and at Maura (Plate I., Figure 6), in the Society group, it
has been almost effected, so that there remains only one harbour for small
craft.

If we look at a set of charts of barrier-reefs, and leave out in
imagination the encircled land, we shall find that, besides the many points
already noticed of resemblance, or rather of identity in structure with
atolls, there is a close general agreement in form, average dimensions, and
grouping.  Encircling barrier-reefs, like atolls, are generally elongated,
with an irregularly rounded, though sometimes angular outline.  There are
atolls of all sizes, from less than two miles in diameter to sixty miles
(excluding Tilla-dou-Matte, as it consists of a number of almost
independent atoll-formed reefs); and there are encircling barrier-reefs
from three miles and a half to forty-six miles in diameter,--Turtle Island
being an instance of the former, and Hogoleu of the latter.  At Tahiti the
encircled island is thirty-six miles in its longest axis, whilst at Maurua
it is only a little more than two miles.  It will be shown, in the last
chapter in this volume, that there is the strictest resemblance in the
grouping of atolls and of common islands, and consequently there must be
the same resemblance in the grouping of atolls and of encircling
barrier-reefs.

The islands lying within reefs of this class, are of very various heights.
Tahiti is 7,000 feet (The height of Tahiti is given from Captain Beechey;
Maurua from Mr. F.D. Bennett ("Geograph. Journ." volume viii., page 220);
Aitutaki from measurements made on board the "Beagle"; and Manouai or
Harvey Island, from an estimate by the Rev. J. Williams.  The two latter
islands, however, are not in some respects well characterised examples of
the encircled class.); Maurua about 800; Aitutaki 360, and Manouai only 50.
The geological nature of the included land varies: in most cases it is of
ancient volcanic origin, owing apparently to the fact that islands of this
nature are most frequent within all great seas; some, however, are of
madreporitic limestone, and others of primary formation, of which latter
kind New Caledonia offers the best example.  The central land consists
either of one island, or of several: thus, in the Society group, Eimeo
stands by itself; while Taha and Raiatea (Figure 3, Plate I.), both
moderately large islands of nearly equal size, are included in one reef.
Within the reef of the Gambier group there are four large and some smaller
islands (Figure 8, Plate I.); within that of Hogoleu (Figure 2, Plate I.)
nearly a dozen small islands are scattered over the expanse of one vast
lagoon.

After the details now given, it may be asserted that there is not one point
of essential difference between encircling barrier-reefs and atolls: the
latter enclose a simple sheet of water, the former encircle an expanse with
one or more islands rising from it.  I was much struck with this fact, when
viewing, from the heights of Tahiti, the distant island of Eimeo standing
within smooth water, and encircled by a ring of snow-white breakers.
Remove the central land, and an annular reef like that of an atoll in an
early stage of its formation is left; remove it from Bolabola, and there
remains a circle of linear coral-islets, crowned with tall cocoa-nut trees,
like one of the many atolls scattered over the Pacific and Indian Oceans.

The barrier-reefs of Australia and of New Caledonia deserve a separate
notice from their great dimensions.  The reef on the west coast of New
Caledonia (Figure 5, Plate II.) is 400 miles in length; and for a length of
many leagues it seldom approaches within eight miles of the shore; and near
the southern end of the island, the space between the reef and the land is
sixteen miles in width.  The Australian barrier extends, with a few
interruptions, for nearly a thousand miles; its average distance from the
land is between twenty and thirty miles; and in some parts from fifty to
seventy.  The great arm of the sea thus included, is from ten to twenty-five
fathoms deep, with a sandy bottom; but towards the southern end, where
the reef is further from the shore, the depth gradually increases to forty,
and in some parts to more than sixty fathoms.  Flinders (Flinders' "Voyage
to Terra Australis," volume ii., page 88.) has described the surface of
this reef as consisting of a hard white agglomerate of different kinds of
coral, with rough projecting points.  The outer edge is the highest part;
it is traversed by narrow gullies, and at rare intervals is breached by
ship-channels.  The sea close outside is profoundly deep; but, in front of
the main breaches, soundings can sometimes be obtained.  Some low islets
have been formed on the reef.

(PLATE: UNNAMED, THREE VERTICAL SECTIONS (WOODCUT DIAGRAMS):

1. VANIKORO, from the "Atlas of the Voyage of the 'Astrolabe'," by D.
D'Urville.

2. GAMBIER ISLAND, from Beechey.

3. MAURUA, from the "Atlas of the Voyage of the 'Coquille'," by Duperrey.

The horizontal line is the level of the sea, from which on the right hand a
plummet descends, representing a depth of 200 fathoms, or 1,200 feet.  The
vertical shading shows the section of the land, and the horizontal shading
that of the encircling barrier-reef: from the smallness of the scale, the
lagoon-channel could not be represented.

AA.--Outer edge of the coral-reefs, where the sea breaks.

BB.--The shore of the encircled islands.)

There is one important point in the structure of barrier-reefs which must
here be considered.  The accompanying diagrams represent north and south
vertical sections, taken through the highest points of Vanikoro, Gambier,
and Maurua Islands, and through their encircling reefs.  The scale both in
the horizontal and vertical direction is the same, namely, a quarter of an
inch to a nautical mile.  The height and width of these islands is known;
and I have attempted to represent the form of the land from the shading of
the hills in the large published charts.  It has long been remarked, even
from the time of Dampier, that considerable degree of relation subsists
between the inclination of that part of the land which is beneath water and
that above it; hence the dotted line in the three sections, probably, does
not widely differ in inclination from the actual submarine prolongation of
the land.  If we now look at the outer edge of the reef (AA), and bear in
mind that the plummet on the right hand represents a depth of 1,200 feet,
we must conclude that the vertical thickness of these barrier coral-reefs
is very great.

I must observe that if the sections had been taken in any other direction
across these islands, or across other encircled islands (In the fifth
chapter an east and west section across the Island of Bolabola and its
barrier-reefs is given, for the sake of illustrating another point.  The
unbroken line in it (woodcut No. 5) is the section referred to.  The scale
is .57 of an inch to a mile; it is taken from the "Atlas of the Voyage of
the 'Coquille'," by Duperrey.  The depth of the lagoon-channel is
exaggerated.), the result would have been the same.  In the succeeding
chapter it will be shown that reef-building polypifers cannot flourish at
great depths,--for instance, it is highly improbable that they could exist
at a quarter of the depth represented by the plummet on the right hand of
the woodcut.  Here there is a great APPARENT difficulty--how were the basal
parts of these barrier-reef formed?  It will, perhaps, occur to some, that
the actual reefs formed of coral are not of great thickness, but that
before their first growth, the coasts of these encircled islands were
deeply eaten into, and a broad but shallow submarine ledge thus left, on
the edge of which the coral grew; but if this had been the case, the shore
would have been invariably bounded by lofty cliffs, and not have sloped
down to the lagoon-channel, as it does in many instances.  On this view
(The Rev. D. Tyerman and Mr. Bennett ("Journal of Voyage and Travels,"
volume i., page 215) have briefly suggested this explanation of the origin
of the encircling reefs of the Society Islands.), moreover, the cause of
the reef springing up at such a great distance from the land, leaving a
deep and broad moat within, remains altogether unexplained.  A supposition
of the same nature, and appearing at first more probable is, that the reefs
sprung up from banks of sediment, which had accumulated round the shore
previously to the growth of the coral; but the extension of a bank to the
same distance round an unbroken coast, and in front of those deep arms of
the sea (as in Raiatea, see Plate II., Figure 3) which penetrate nearly to
the heart of some encircled islands, is exceedingly improbable.  And why,
again, should the reef spring up, in some cases steep on both sides like a
wall, at a distance of two, three or more miles from the shore, leaving a
channel often between two hundred and three hundred feet deep, and rising
from a depth which we have reason to believe is destructive to the growth
of coral?  An admission of this nature cannot possibly be made.  The
existence, also, of the deep channel, utterly precludes the idea of the
reef having grown outwards, on a foundation slowly formed on its outside,
by the accumulation of sediment and coral detritus.  Nor, again, can it be
asserted, that the reef-building corals will not grow, excepting at a great
distance from the land; for, as we shall soon see, there is a whole class
of reefs, which take their name from growing closely attached (especially
where the sea is deep) to the beach.  At New Caledonia (see Plate II.,
Figure 5) the reefs which run in front of the west coast are prolonged in
the same line 150 miles beyond the northern extremity of the island, and
this shows that some explanation, quite different from any of those just
suggested, is required.  The continuation of the reefs on each side of the
submarine prolongation of New Caledonia, is an exceedingly interesting
fact, if this part formerly existed as the northern extremity of the
island, and before the attachment of the coral had been worn down by the
action of the sea, or if it originally existed at its present height, with
or without beds of sediment on each flank, how can we possibly account for
the reefs, not growing on the crest of this submarine portion, but fronting
its sides, in the same line with the reefs which front the shores of the
lofty island?  We shall hereafter see, that there is one, and I believe
only one, solution of this difficulty.

One other supposition to account for the position of encircling barrier-reefs
remains, but it is almost too preposterous to be mentioned; namely,
that they rest on enormous submarine craters, surrounding the included
islands.  When the size, height, and form of the islands in the Society
group are considered, together with the fact that all are thus encircled,
such a notion will be rejected by almost every one.  New Caledonia,
moreover, besides its size, is composed of primitive formations, as are
some of the Comoro Islands (I have been informed that this is the case by
Dr. Allan of Forres, who has visited this group.); and Aitutaki consists of
calcareous rock.  We must, therefore, reject these several explanations,
and conclude that the vertical thickness of barrier-reefs, from their outer
edges to the foundation on which they rest (from AA in the section to the
dotted lines) is really great; but in this, there is no difficulty, for it
is not necessary to suppose that the coral has sprung up from an immense
depth, as will be evident when the theory of the upward growth of
coral-reefs, during the slow subsidence of their foundation, is discussed.