these structures exist.

It seems, at first, a startling paradox, to suppose that the land
is less fixed than the sea; but that such is the case is the uniform
testimony of geology. Beds of sandstone or limestone, thousands of
feet thick, and all full of marine remains, occur in various parts of
the earth's surface, and prove, beyond a doubt, that when these beds
were formed, that portion of the sea-bottom which they then occupied
underwent a slow and gradual depression to a distance which cannot
have been less than the thickness of those beds, and may have been
very much greater. In supposing, therefore, that the great areas of
the Pacific and of the Indian Ocean, over which atolls and encircling
reefs are 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 neighbourhood 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