the western shores of South America, between the
summit of the Andes and the Pacific (a region of earthquakes and active
volcanoes), we find signs of recent elevation, not attested indeed by
coral formations, which are wanting there, but by upraised banks of
marine shells. Then proceeding westward, we traverse a deep ocean
without islands, until we come to a band of _atolls_ and encircled
islands, including the Dangerous and Society archipelagoes, and
constituting an area of subsidence more than 4000 miles long and 600
broad. Still farther, in the same direction, we reach the chain of
islands to which the New Hebrides, Salomon, and New Ireland belong,
where fringing reefs and masses of elevated coral indicate another area
of upheaval. Again, to the westward of the New Hebrides we meet with the
encircling reef of New Caledonia and the great Australian barrier,
implying a second area of subsidence.

The only objection deserving attention which has hitherto been advanced
against the theory of atolls, as before explained (p. 759.), is that
proposed by Mr. Maclaren.[1132] "On the outside," he observes, "of coral
reefs very highly inclined, no bottom is sometimes found with a line of
2000 or 3000 feet, and this is by no means a rare case. It follows that
the reef ought to have this thickness; and Mr. Darwin's diagrams show
that he understood it so. Now, if such masses of coral exist under the
sea, they ought somewhere to be found on _terra firma_; for there is
evidence that all the lands yet visited by geologists, have been at one
time submerged. But neither in the great volcanic chain, extending from
Sumatra to Japan, nor in the West Indies, nor in any other region yet
explored, has a bed or formation of coral even 500 feet thick been
discovered, so far as we know."

When considering this objection, it is evident that the first question
we have to deal with is, whether geologists have not already discovered
calcareous masses of the required thickness and structure, or precisely
such as the upheaval of atolls might be expected to expose to view? We
are called upon, in short, to make up our minds both as to the internal
composition of the rocks that must result from the growth of corals,
whether in lagoon islands or barrier reefs, and the external shape which
the reefs would retain when upraised gradually to a vast height,--a task
by no means so easy as some may imagine. If the reader has pictured to
himself large masses of e