ber of craters, all so nearly of the same altitude.

In shallow tracts of sea, coral reefs no doubt tend to assume the
well-known circular form, but the difficulty was to account for the
numerous atolls which rise to the surface from the abysses of the ocean,
while the coral-forming zoophytes can only live near the surface.

Darwin showed that so far from the ring of corals resting on a
corresponding ridge of rocks, the lagoons, on the contrary, now occupy
the place which was once the highest land. He pointed out that some
lagoons, as for instance that of Vanikoro, contain an island in the
middle; while other islands, such as Tahiti, are surrounded by a margin
of smooth water separated from the ocean by a coral reef. Now if we
suppose that Tahiti were to sink slowly it would gradually approximate
to the condition of Vanikoro; and if Vanikoro gradually sank, the
central island would disappear, while on the contrary the growth of the
coral might neutralise the subsidence of the reef, so that we should
have simply an atoll with its lagoon. The same considerations explain
the origin of the "barrier reefs," such as that which runs for nearly a
thousand miles, along the north-east coast of Australia. Thus Darwin's
theory explains the form and the approximate identity of altitude of
these coral islands. But it does more than this, because it shows that
there are great areas in process of subsidence, which though slow, is of
great importance in physical geography.

The lagoon islands have received much attention; which "is not
surprising, for every one must be struck with astonishment, when he
first beholds one of these vast rings of coral-rock, often many leagues
in diameter, here and there surmounted by a low verdant island with
dazzling white shores, bathed on the outside by the foaming breakers of
the ocean, and on the inside surrounding a calm expanse of water, which,
from reflection is generally of a bright but pale green colour. The
naturalist will feel this astonishment more deeply after having examined
the soft and almost gelatinous bodies of these apparently insignificant
coral-polypifers, and when he knows that the solid reef increases only
on the outer edge, which day and night is lashed by the breakers of an
ocean never at rest. Well did Francois Pyrard de Laval, in the year 1605
exclaim, 'C'est une merveille de voir chacun de ces atollons, environne
d'un grand banc de pierre tout autour, n'y ayant point d'artific