CORAL. 3385

Dr. Carpenter gives the following interesting account of
the formation of coral islands :—‘ A large proportion of the
coral islands of the Polynesian Archipelago are shaped like
a crescent, sometimes like a complete ring; and these islands
never rise many feet above the surface of the ocean. The
highest part is always on the windward (easterly) side, against
which the waves are almost constantly dashing. Within the
crescent or ring, is a basin termed a lagoon, and this usually
communicates with the open sea by a channel, sometimes
of considerable width, on the leeward side of the island.
Occasionally this channel is completely filled up by the
growth of the coral; and the lake, thus enclosed, only com-
municates with the sea by filtration through the coral rock.
The coral polypes never build above low-water mark; and
they are not, therefore, immediately concerned in the eleva-
tion of the surface from beneath the waves. This is prin-.
cipally accomplished by the action of the sea itself. Large
masses are often detached by the violence of the waves, from
the lower part of the structure, and these are washed up on
the windward side of the reef. Shells, coral-sand, and
various other dééris, accumulate upon it in like manner,
until at last it is changed into an island, upon which there
is a calcareous soil, capable of supporting various kinds of