STRUCTURE OF CORAL REEFS. 115 



there are seldom any large accumulations of coral debris. The 

 margin is generally less abrupt ; yet there is every variety of 

 slope, from the gradually inclined bed of corals to the bluff de- 

 clivity with its clinging clumps. In different parts, there are 

 many portions still under water at the lowest tides ; and here 

 (as well as upon the outer banks) fine fishing sport is afforded 

 the natives, who w^de out at ebb tide with spears, pronged 

 sticks, and nets, to supply themselves with food. The lover of 

 the marvellous may find abundant gratification by joining in 

 such a ramble ; for, besides living corals, there are myriads of 

 other beings which science alone has named, of various beau- 

 tiful forms and colours, as becomes the inhabitants of a coral 

 world. 



Between the large reefs, which spread a broad surface, at the 

 water's edge, of lifeless coral rock, sometimes of great extent, 

 there are other patches, still submerged, that are covered 

 with growing corals throughout. They are of different eleva- 

 tions under the water's surface ; and though at times but 

 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 Vmcemtes 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 



