FRINGING AND BARRIER REEFS. 39 
edge of lifeless coral rock, sometimes of great extent, there are other 
patches, still submerged, which are covered with growing corals 
throughout. They are of different elevations; and though at times 
but a few yards in breadth, there is often alongside 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 frequently, 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 patch. 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 freeing the foul anchor. 
The rock of the inner reefs is peculiar in being but sparingly frag- 
mentary. ‘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 posi- 
tionsarecommon. ‘Though now mere dead rock, the limits of the seve- 
ral constituent coral masses may be distinctly made out. Some indi- 
vidual specimens of Porites in the rock of the inner reef of Tongatabu 
were twenty-five feet in diameter; and Astreas and Meandrinas, both 
there and in the Feejees, measured twelve to fifteen feet. "These corals, 
when growing beneath the water, form solid hemispheres, or rounded 
hillocks; but on reaching the surface, the top dies, and enlargement 
takes place only 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 Astreeas and Porites in the reef-rock referred to. The 
platform looks like a Cyclopean pavement, except that the cement- 
ing material, fillmg 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 
* 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, It is the material of the large church at Honolulu. 
