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of ocean, of more than a thousand miles in one direction and
several hundreds in another, scattered over with islands,
none of which rise to a greater height than that to which
waves can throw fragments, or the wind heap up sand.
Now if we leave out of the question subsidence, the
foundation on which these reefs are built, must in every
case come to the surface within that small limit (we may
say twenty fathoms) at which corals can live. This conclusion
is so extremely improbable that it may at once be
rejected: for in what country can there be found a broad and
grand range of mountains of the same height within a hundred
and twenty feet ? But on the idea of subsidence, the
case is at once clear: as each point, one after the other
according to its altitude, was submerged, the coral grew
upwards, and formed the many islets now standing at one
level.
Having endeavoured on general grounds not only to remove
any extreme degree of improbability in the belief of
a general subsidence, but likewise to show that it is almost
necessary to account for the existence of a vast number of
reefs on one level, we will now see how far the same idea
will apply to the peculiar configuration in the several classes.
Let us imagine an island merely fringed by reefs extending
to a short distance from the shore; in which case, as we have
before remarked, there is no difficulty in understanding their
structure. Now let this island subside by a series of movements
of extreme slowness, the coral at each interval growing
up to the surface. Without the aid of sections it is not
very easy to follow out the result, but a little reflection will
show that a reef encircling the shore at a greater or less distance,
according to the amount of subsidence, would be
produced. If we suppose the sinking to continue, the
encircled island must, by the submergence of the central
land but upward growth of the ring of coral, be converted
into a lagoon island. If we take a section of some encircled
island on a true scale, as for instance Gambler, which has
been so well described by Captain Beechey, we shall not find
the amount of movement very great, which would be necessary
to change a well-characterized encircling reef, into as
characteristic a lagoon island.
It will at once be evident that a coral reef, closely skirting
the shore of a continent, would, in like manner after
each subsidence, rise to the surface; the water, however,
always encroaching on the land. Would not a barrier reef
necessarily be produced, similar to the one extending parallel
to the coast of Australia ? It is indeed but uncoiling one
of those reefs which encircle at a distance so many islands.
Thus the three great classes of reef, lagoon, encircling,
and barrier, are connected by one theory. It will perhaps
be remarked, if this be true, there ought to exist every
intermediate form between a closely-encircled and a lagoon
island. Such forms actually occur in various parts of the
ocean : we have one, two, or more islands encircled in one
reef ; and of these some are of small proportional size to the
area enclosed by the coral formation ; so that a series of
charts might be given, showing a gradation of character
between the two classes. In New Caledonia, where the
double line of reef projects 140 miles beyond the island, we
may imagine we see this change in progress. At the
northern extremity, reefs occur, some of which are of the
encircling kind, and others almost with the character of true
lagoon islands. The line of reef which fronts the whole west
coast of this great island, has by some been called a barrier.
It is four hundred miles long ; and may be said thus to form
a link between an ordinary encircling reef and the great
Australian barrier.
I should perhaps have entered before into the consideration
of one apparent difficulty in the origin of lagoon
islands. It may be said, granting the theory of subsidence,
a mere circular disc of coral would be formed, and not a
cup-shaped mass. In the first place, even in reefs closely
fringing the land (as before remarked), the corals
do not grow on the shore itself, but leave a shallow
channel. Secondly, the strong and vigorous species which
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