Corals and coral islands.

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CORALS AND CORAL ISLANDS.

finally cemented and so made solid. At Tongatabu and
among the Feejee Islands, reefs thus formed of corals standing
in their growing positions are common. Though now mere
dead rock, and exceedingly firm and compact, the limits of
the several constituent coral masses may be distinctly made
out. Some individual specimens of Porites in the rock of the
inner reef of Tongatabu are twenty-five feet in diameter; and
Astraeas and Maeandrinas, both there and in the Feejees, measure twelve to fifteen feet. These corals, when growing beneath the water, form, as has been stated, solid hemispheres,
or rounded hillocks; but on reaching the surface, the top dies,
and enlargement takes place only on the sides; and in this
manner the hemisphere is finally changed to a broad cylinder
with a flat top. This was the condition of the Astr.eas and
Porites in the reef-rock referred to. Such a platform looks
like a Cyclopean pavement, except that the calcareous cementing material, filling in between the huge masses, is more
solid than in any work of art: it even exceeds in compactness
the corals themselves. Other portions of reefs consist of
branching corals, with the intervals filled in by sand and small
fragments; for even in the stiller waters fragments are to some
extent produced. A rock of this kind is often used for buildings and for walls on the island of Oahu. It consists mainly
of Porites, and in many parts is still cavernous, or but imperfectly cemented.
There is also to be found about inner reefs, over large
areas, the solid white limestone already described, showing
internally no evidence of its coral origin, and containing rarely
a shell or other imbedded fossil. It is a result of the consolidation of the fine coral sand or mud that is made and accumulated through the action of the light waves that work over
the inner reefs. It has been said that large regions of barren