The Reef Runs At A
Greater Or Less Distance From The Included Land; In The
Society Archipelago Generally From One To Three Or Four
Miles; But At Hogoleu The Reef Is 20 Miles On The Southern
Side, And 14 Miles On The Opposite Or Northern Side, From The
Included Islands.
The depth within the lagoon-channel also
varies much; from 10 to 30 fathoms may be taken as an
average; but at Vanikoro there are spaces no less than 56
fathoms or 363 feet deep.
Internally the reef either slopes
gently into the lagoon-channel, or ends in a perpendicular
wall sometimes between two and three hundred feet under
water in height: externally the reef rises, like an atoll, with
extreme abruptness out of the profound depths of the ocean.
What can be more singular than these structures? We see
[picture]
an island, which may be compared to a castle situated on the
summit of a lofty submarine mountain, protected by a great
wall of coral-rock, always steep externally and sometimes
internally, with a broad level summit, here and there breached
by a narrow gateway, through which the largest ships can
enter the wide and deep encircling moat.
As far as the actual reef of coral is concerned, there is not
the smallest difference, in general size, outline, grouping,
and even in quite trifling details of structure, between a
barrier and an atoll. The geographer Balbi has well remarked,
that an encircled island is an atoll with high land rising out
of its lagoon; remove the land from within, and a perfect
atoll is left.
But what has caused these reefs to spring up at such
great distances from the shores of the included islands? It
cannot be that the corals will not grow close to the land;
for the shores within the lagoon-channel, when not surrounded
by alluvial soil, are often fringed by living reefs;
and we shall presently see that there is a whole class, which
I have called Fringing Reefs from their close attachment
to the shores both of continents and of islands. Again, on
what have the reef-building corals, which cannot live at
great depths, based their encircling structures? This is a
great apparent difficulty, analogous to that in the case of
atolls, which has generally been overlooked. It will be
perceived more clearly by inspecting the following sections
which are real ones, taken in north and south lines, through
the islands with their barrier-reefs, of Vanikoro, Gambier,
and Maurua; and they are laid down, both vertically and
horizontally, on the same scale of a quarter of an inch to
a mile.
It should be observed that the sections might have been
taken in any direction through these islands, or through
[picture]
many other encircled islands, and the general features would
have been the same. Now, bearing in mind that reef-building
coral cannot live at a greater depth than from 20 to 30
fathoms, and that the scale is so small that the plummets on
the right hand show a depth of 200 fathoms, on what are
these barrier-reefs based? Are we to suppose that each
island is surrounded by a collar-like submarine ledge of rock,
or by a great bank of sediment, ending abruptly where the
reef ends?
If the sea had formerly eaten deeply into the islands,
before they were protected by the reefs, thus having
left a shallow ledge round them under water, the present
shores would have been invariably bounded by great precipices,
but this is most rarely the case. Moreover, on this
notion, it is not possible to explain why the corals should
have sprung up, like a wall, from the extreme outer margin
of the ledge, often leaving a broad space of water within,
too deep for the growth of corals. The accumulation of a
wide bank of sediment all round these islands, and generally
widest where the included islands are smallest, is highly
improbable, considering their exposed positions in the central
and deepest parts of the ocean. In the case of the barrier-reef
of New Caledonia, which extends for 150 miles beyond
the northern point of the islands, in the same straight line
with which it fronts the west coast, it is hardly possible to
believe that a bank of sediment could thus have been
straightly deposited in front of a lofty island, and so far
beyond its termination in the open sea. Finally, if we look
to other oceanic islands of about the same height and of similar
geological constitution, but not encircled by coral-reefs,
we may in vain search for so trifling a circumambient
depth as 30 fathoms, except quite near to their shores; for
usually land that rises abruptly out of water, as do most of
the encircled and non-encircled oceanic islands, plunges
abruptly under it. On what then, I repeat, are these barrier
reefs based? Why, with their wide and deep moat-like channels,
do they stand so far from the included land? We shall
soon see how easily these difficulties disappear.
We come now to our third class of Fringing-reefs, which
will require a very short notice. Where the land slopes abruptly
under water, these reefs are only a few yards in width,
forming a mere ribbon or fringe round the shores: where
the land slopes gently under the water the reef extends
further, sometimes even as much as a mile from the land;
but in such cases the soundings outside the reef always show
that the submarine prolongation of the land is gently inclined.
In fact, the reefs extend only to that distance from the shore,
at which a foundation within the requisite depth from 20 to
30 fathoms is found. As far as the actual reef is concerned,
there is no essential difference between it and that forming
a barrier or an atoll: it is, however, generally of less width,
and consequently few islets have been formed on it. From
the corals growing more vigorously on the outside, and from
the noxious effect of the sediment washed inwards, the outer
edge of the reef is the highest part, and between it and the
land there is generally a shallow sandy channel a few feet in
depth.
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