proceeded to examine in South America many fine illustrations of the older
igneous rock-masses, and especially of the most highly crystalline types of
the same, and then on his way home had opportunities of reviving the
impression made upon him by the fresh and unaltered volcanic rocks. Some of
the general considerations suggested by these observations were discussed
in a paper read by him before the Geological Society, on March 7th, 1838,
under the title "On the Connection of Certain Volcanic Phenomena, and On
the Formation of Mountain-chains, and the Effect of Continental
Elevations." The exact bearing of these two classes of facts upon one
another are more fully discussed in his book on South American geology.

The proofs of recent elevation around many of the volcanic islands led
Darwin to conclude that volcanic areas were, as a rule, regions in which
upward movements were taking place, and he was naturally led to contrast
them with the areas in which, as he showed, the occurrence of atolls,
encircling reefs, and barrier-reefs afford indication of subsidence. In
this way he was able to map out the oceanic areas in different zones, along
which opposite kinds of movement were taking place. His conclusions on this
subject were full of novelty and suggestiveness.

Very clearly did Darwin recognise the importance of the fact that most of
the oceanic islands appear to be of volcanic origin, though he was careful
to point out the remarkable exceptions which somewhat invalidate the
generalisation. In his "Origin of Species" he has elaborated the idea and
suggested the theory of the permanence of ocean-basins, a suggestion which
has been adopted and pushed farther by subsequent authors, than we think
its originator would have approved. His caution and fairness of mind on
this and similar speculative questions was well-known to all who were in
the habit of discussing them with him.