Now it is known that, as a general rule, the level of the land is either
stationary, or is undergoing a slow upheaval, in the neighborhood of
active volcanoes; and, therefore, neither atolls nor encircling reefs
ought to be found in regions in which volcanoes are numerous and active.
And this turns out to be the case. Appended to Mr. Darwin's great work
on coral reefs, there is a map on which atolls and encircling reefs are
indicated by one colour, fringing reefs by another, and active volcanoes
by a third. And it is at once obvious that the lines of active volcanoes
lie around the margins of the areas occupied by the atolls and the
encircling reefs. It is exactly as if the upheaving volcanic agencies
had lifted up the edges of these great areas, while their centres had
undergone a corresponding depression. An atoll area may, in short, be
pictured as a kind of basin, the margins of which have been pushed up by
the subterranean forces, to which the craters of the volcanoes have, at
intervals, given vent.
Thus we must imagine the area of the Pacific now covered by the
Polynesian Archipelago, as having been, at some former time, occupied
by large islands, or, may be, by a great continent, with the ordinarily
diversified surface of plain, and hill, and mountain chain. The shores
of this great land were doubtless fringed by coral reefs; and, as it
slowly underwent depression, the hilly regions, converted into islands,
became, at first, surrounded by fringing reefs, and then, as depression
went on, these became converted into encircling reefs, and these,
finally, into atolls, until a maze of reefs and coral-girdled islets
took the place of the original land masses.
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