on these small, barren, and rocky
islands, and still more so at its diverse, yet analogous, action on
points so near each other.

_V.--The Coral Islands of the Indian Ocean_

Having completed the survey of the coasts and islands of the South
American continent, the Beagle sailed across the wide Pacific to Tahiti,
New Zealand, and Australia, in order to carry out the chain of
chronometrical measurements round the world. From Australasia a run was
then made for Keeling or Cocos Island in the Indian Ocean. This lonely
island, 600 miles from the coast of Sumatra, is an atoll, or lagoon
island. The land is entirely composed of fragments of coral.

There is, to my mind, much grandeur in the view of the outer shores of
these lagoon islands. The ocean, throwing its waters over the broad
barrier-like reef, appears an invincible, all-powerful enemy. Yet these
low, insignificant coral islets stand and are victorious; for here
another power, as an antagonist, takes part in the contest. Organic
forces separate the atoms of carbonate of lime, one by one, from the
foaming breakers, and unite them in a symmetrical structure. Let the
hurricane tear up its thousand huge fragments, yet what will that tell
against the accumulated labour of myriads of architects at work night
and day, month after month?

There are three great classes of coral reefs--atoll, barrier, and
fringing. Now, the utmost depth at which corals can construct reefs is
between twenty and thirty fathoms, so that wherever there is an atoll a
foundation must have originally existed within a depth of from twenty to
thirty fathoms from the surface. The coral formation is raised only to
that height to which the waves can throw up fragments and the winds pile
up sand. The foundation, such as a mountain peak, therefore, must have
sunk to the required level, and not have been raised, as has hitherto
been generally supposed.

I venture, therefore, to affirm that, on the theory of the upward growth
of the corals during the sinking of the land, all the leading features
of those wonderful structures, the lagoon-islands or atolls, as well as
the no less wonderful barrier-reefs, whether encircling small islands,
or stretching for hundreds of miles along the shores of a continent, are
simply explained. On the other hand, coasts merely fringed by reefs
cannot have subsided to any perceptible amount, and therefore they must,
since the growth of their corals, either have remained s