e
found scattered, have undergone a depression of some hundreds, or,
it may be, thousands of feet, Mr. Darwin made a supposition which had
nothing forced or improbable, but was entirely in accordance with what
we know to have taken place over similarly extensive areas, in other
periods of the world's history. But Mr. Darwin subjected his hypothesis
to an ingenious indirect test. If his view be correct, it is clear that
neither atolls, nor encircling reefs, should be found in those portions
of the ocean in which we have reason to believe, on independent grounds,
that the sea-bottom has long been either stationary, or slowly rising.
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.

Thus the atolls and the encircling reefs furnish us with clear, though
indirect, evidence of changes in the physical geography of lar