the
reef level with or within a few fathoms of the surface, so that, as
subsidence proceeded, the distance between the outer rim of the reef and
the sinking land would continually increase, with the result that a
barrier-reef would be formed separated by a wide channel from the
central peak. As corals and other organisms with calcareous skeletons
live in the channel, their remains, as well as the accumulation of coral
and other debris thrown over the outer edge of the reef, would maintain
the channel at a shallower depth than that of the ocean outside.
Finally, if the subsidence continued, the central peak would disappear
beneath the surface, and an atoll would be left consisting of a raised
margin of reef surrounding a central lagoon, and any pause during the
movement of subsidence would result in the formation of raised islets or
a strip of dry land along the margin of the reef. Darwin's theory was
published in 1842, and found almost universal acceptance, both because
of its simplicity and its applicability to every known type of
coral-reef formation, including such difficult cases as the Great Chagos
Bank, a huge submerged atoll in the Indian Ocean.

Darwin's theory was adopted and strengthened by J. D. Dana, who had made
extensive observations among the Pacific coral reefs between 1838 and
1842, but it was not long before it was attacked by other observers. In
1851 Louis Agassiz produced evidence to show that the reefs off the
south coast of Florida were not formed during subsidence, and in 1863
Karl Semper showed that in the Pelew islands there is abundant evidence
of recent upheaval in a region where both atolls and barrier-reefs
exist. Latterly, many instances of recently upraised coral formations
have been described by H. B. Guppy, J. S. Gardiner and others, and
Alexander Agassiz and Sir J. Murray have brought forward a mass of
evidence tending to shake the subsidence theory to its foundations.
Murray has pointed out that the deep-sea soundings of the "Tuscarora"
and "Challenger" have proved the existence of a large number of
submarine elevations rising out of a depth of 2000 fathoms or more to
within a few hundred fathoms of the surface. The existence of such banks
was unknown to Darwin, and removes his objections to Chamisso's theory.
For although they may at first be too far below the surface for
reef-building corals, they afford a habitat for numerous echinoderms,
molluscs, crustacea and deep-sea corals, who