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. In a 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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