364 On Coral Reefs and Islands. 
side of them a depth of many fathoms. They sometimes seem 
to grow up from a narrow base, like a mushroom; and a ship 
striking one with her keel may crush it and glide on. More fre- 
quently, they are below like the solid reef above described, and | 
the contest is more likely to be fatal to the vessel than to the coral 
tch. Corals grow over them, as in the shallow waters about 
other reefs; and, as elsewhere, there are deep cavities among the 
congregated corals, in which a lead will sometimes sink to a depth 
of several feet, or even fathoms. These holes about, growing 
reefs often give much annoyance to the boat which may venture 
to anchor upon them; and in many an instance in the course of 
the surveys, diving was the only resource left for frecing the foul 
anchor. 
The rock of the inner reefs is peculiar in being but sparingly 
fragmentary. The corals composing it stand to a great extent as 
they grew; yet it is not less compact or firm in its texture than 
the rock of the outer reefs. The cavities among the branches 
and growing masses gradually become filled with coral sand, and 
the whole is finally cemented and thoroughly compacted. At 
Tongatabu and among the Feejee Islands, reefs thus made of 
corals standing in their growing positions are common. Though 
now mere dead rock, 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 were twen- 
ty-five feet in diameter; and Astras and Meandrinas, both there 
and in the Feejees, measured twelve to fifteen feet. These corals, 
when growing beneath the water, form solid hemispheres, oF 
rounded hillocks; but on reaching the surface, the top dies, and 
enlargement takes place ouly on the sides. In this manner the 
hemisphere is finally changed to a broad cylinder with a flat top. 
This was the condition of the Astraeas and Porites in the reef-rock 
referred to. _'The platform looks like a Cyclopean pavement, €X- 
cept that the cementing material, filling in between the huge 
masses, is more solid than any work of art: it even exceeds In 
compactness the corals themselves. Other portions of these reefs 
consist of branching corals, with the intervals filled in by sand 
and small fragments; for even in the more still waters fragments 
are to some extent produced.* There is also to be found here, 
and frequently over large areas, the solid white limestone already 
escribed, showing internally no evidence of its coral origin, and 
containipg rarely a few shells or imbedded fossils. 
The formation of the inner reefs goes on at a less rapid rate than 
that of the guter, as the process depends on growth unaided, eX- 
cept in a comparatively small degree, by the action of the waves. 
Moreover, as we shall explain more particularly in another place, : 
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pe cameos ae Porit taxi ip-moeny pants in xl canstiost ; me 
cem is the material of the large church at Honolua. 
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