146 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 
Astras and Meeandrinas, both there and in the Feejees, meas- 
ure twelve to fifteen feet. These corals, when growing be- 
neath 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 Astraas and 
Porites in the reef-rock referred to. Such a platform looks 
like a Cyclopean pavement, except that the calcareous ce- 
menting material, fillmg im 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 build- 
ings and for walls on the island of Oahu. It consists mainly 
of Porites, and in many parts is still cavernous, or but imper- 
fectly 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 consoli- 
dation of the fine coral sand or mud that is made and accu- 
mulated through the action of the light waves that work over 
the inner reefs. It has been said that large regions of barren 
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