Thus you have the reef forming a bed thicker upon the
flanks of the island; but the edge of the reef will be very much further
out from the land, and the lagoon will be many times deeper; in short,
your fringing reef will be converted into an encircling reef. And if,
instead of this being an island, it were a great continent like
Australia, then you will have the phenomenon of a barrier reef which I
have described. The barrier reef of Australia was originally a
fringing reef; the land has gone slowly down; the consequence is the
lagoon has deepened until its depth is now 25 fathoms and the corals
have grown up at the outer edge until you have that prodigious
accumulation which forms the barrier reef at present. Now let this
process go on further still; let us take the land a further step down,
so as to submerge even the peak. The coral, still growing up, will
cover the surface of the land, and you will have an atoll reef; that is
to say, a more or less circular or oval ring of coral rock with a
lagoon in the middle. Thus you see that every peculiarity and
phenomenon of these different forms of coral reef was explained at once
by the simplest of all possible suppositions, namely, by supposing that
the land has gone down at a rate not greater than that at which the
coral polypes have grown up.
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