Corals and coral islands.

STRUCTURE OF CORAL ISLANDS.

179

blocks of coral, with their usual rough angular features and
blackened surface. There is but little depth of coral soil,
although the land may appear buried in the richest foliage.
In fact, the soil is scarcely any thing but coral sand. It is
seldom discolored beyond four or five inches, and but little of
it to this extent; there is no proper vegetable mould, but only
a mixture of darker particles with the white grains of coral
sand. It is often rather a coral gravel, and below a foot or
two, it is usually cemented together into a more or less compact coral sand-rock.
One singular feature of the shore platform, occasionally
observed, remains to be mentioned. Huge masses of reefrock are sometimes found upon it, some of which lie loose
upon the reef, while others are firmly imbedded in it below,
and so cemented to it as to appear to be actually a part of the
platform rock. Sketches of two of these masses are here given.
-......, -
i 2
BLOCKS OF OORAL ROCK ON THE SHORE PLATFORM.
Figure 1 represents a mass on the island of Waterland
(one of the Paumotus), six feet high and about five in diameter; it was solid with the reef-rock below, as though a part
of it, and, about two feet above its base, it had been so nearly
worn off by the waters as to have become irregularly topshaped. Another mass, similarly attached to the reef at base,
observed on Kawehe (Vincennes Island), was six feet high
above low-water level, and seven feet in its longest diameter