STRUCTURE OF CORAL REEFS. 145 
a few yards in breadth, there is often alongside of them a 
depth of many fathoms. The mushroom shape described 
above is common among them; and a ship striking one with 
her keel may crush it and glide on. More frequently, they 
are at bottom like the solid reef above described, and the con- 
test is more likely to be fatal to the vessel than to the coral 
patch. Ina passage between two reefs near Tongatabu, called 
the Astrolabe channel, the sloop-of-war Vincennes ran on a 
coral patch, which had been laid down as a reef. It stopped 
the ship for a moment, but broke away under her; and in the 
survey of the passage afterward, says Captain Wilkes, “no shoal 
was found in the place where the ship had struck, and we had the 
satisfaction of knowing that we had destroyed it without injury 
to the vessel.” Corals grow over these patches, as in the shal- 
low waters about other reefs ; and, as elsewhere, there are deep 
cavities among the congregated corals, in which a lead will some- 
times sink to a depth of many 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 diving is found to be the only resource left for free 
ing the foul anchor. 
The margins of the reefs in and about the inner channels 
are often luxuriant with magnificent corals quite to the edge, 
so that while the reef is elsewhere solid rock to its very top, 
here at the margin it is alive and may be said literally to be 
growing. 
The rock of the inner reefs seldom consists of rolled or 
broken fragments of coral like a large part of that of the 
outer reef. It is often made of dead corals, standing to a 
great extent as they grew; yet it is generally compact and 
firm in texture. The cavities among the branches and masses 
gradually become filled with coral sand, and the whole is 
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