subject. He admitted to me that had he known, when he wrote his work, of
the abundant deposition of the remains of calcareous organisms on the sea
floor, he might have regarded this cause as sufficient in a few cases to
raise the summits of submerged volcanoes or other mountains to a level at
which reef-forming corals can commence to flourish. But he did not think
that the admission that under certain favourable conditions, atolls might
be thus formed without subsidence, necessitated an abandonment of his
theory in the case of the innumerable examples of the kind which stud the
Indian and Pacific Oceans.

A letter written by Darwin to Professor Alexander Agassiz in May 1881 shows
exactly the attitude which careful consideration of the subject led him to
maintain towards the theory propounded by Mr. Murray:--"You will have
seen," he writes, "Mr. Murray's views on the formation of atolls and
barrier-reefs. Before publishing my book, I thought long over the same
view, but only as far as ordinary marine organisms are concerned, for at
that time little was known of the multitude of minute oceanic organisms. I
rejected this view, as from the few dredgings made in the "Beagle", in the
south temperate regions, I concluded that shells, the smaller corals, etc.,
decayed and were dissolved when not protected by the deposition of
sediment, and sediment could not accumulate in the open ocean. Certainly,
shells, etc., were in several cases completely rotten, and crumbled into
mud between my fingers; but you will know whether this is in any degree
common. I have expressly said that a bank at the proper depth would give
rise to an atoll, which could not be distinguished from one formed during
subsidence. I can, however, hardly believe in the existence of as many
banks (there having been no subsidence) as there are atolls in the great
oceans, within a reasonable depth, on which minute oceanic organisms could
have accumulated to the depth of many hundred feet."