That where you find, as you often do in the Pacific, a mere atoll, or
circular reef of coral, with a shallow pond of smooth water in the
centre, and deep sea round, that is a pretty sure sign that the
mountain-top has sunk completely into the sea, and that the corals
are going on building where its peak once was.
And more. On working out the geography of the South Sea Islands by
the light of this theory of Mr. Darwin's, the following extraordinary
fact has been discovered:
That over a great part of the Pacific Ocean sinking is going on, and
has been going on for ages; and that the greater number of the
beautiful and precious South Sea Islands are only the remnants of a
vast continent or archipelago, which once stretched for thousands of
miles between Australia and South America.
Now, applying the same theory to limestone beds, which are, as you
know, only fossil coral reefs, we have a right to say, when we see in
England, Scotland, Ireland, limestones several thousand feet thick,
that while they were being laid down as coral reef, the sea-bottom,
and probably the neighbouring land, must have been sinking to the
amount of their thickness--to several thousand feet--before that
later sinking which enabled several hundred feet of millstone grit to
be laid down on the top of the limestone.
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