Round the World," Darwin writes that he does so "with grateful pleasure, as
an acknowledgment that the chief part of whatever scientific merit this
journal and the other works of the author may possess, has been derived
from studying the well-known and admirable 'Principles of Geology.'"

The second volume of Lyell's "Principles" appeared after Darwin had left
England; but it was doubtless sent on to him without delay by his faithful
friend and correspondent, Professor Henslow. It appears to have reached
Darwin at a most opportune moment, while, in fact, he was studying the
striking evidences of slow and long-continued, but often interrupted
movement on the west coast of South America. Darwin's acute mind could not
fail to detect the weakness of the then prevalent theory concerning the
origin of the ring-shaped atolls--and the difficulty which he found in
accepting the volcanic theory, as an explanation of the phenomena of
coral-reefs, is well set forth in his book.

In an interesting fragment of autobiography, Darwin has given us a very
clear account of the way in which the leading idea of the theory of
coral-reefs originated in his mind; he writes, "No other work of mine was
begun in so deductive a spirit as this, for the whole theory was thought
out on the west coast of South America, before I had seen a true
coral-reef. I had therefore only to verify and extend my views by a
careful examination of living reefs. But it should be observed that I had
during the two previous years been incessantly attending to the effects on
the shores of South America of the intermittent elevation of the land,
together with the denudation and deposition of sediment. This necessarily
led me to reflect much on the effects of subsidence, and it was easy to
replace in imagination the continued deposition of sediment by the upward
growth of corals. To do this was to form my theory of the formation of
barrier-reefs and atolls."