ON CORAL AND CORAL REEFS 1(27 



urchins ; and even sometimes encloses the eggs of tur- 

 tles in a state of petrification. The active and vigorous 

 growth of the reef goes on only at the seaward margins, 

 where the polypes are exposed to tlie wash of the surf, 

 and are thereby provided with an abundant supply of 

 air and of food. The interior portion of the reef may 

 be regarded as almost wholly an accumulation of dead 

 skeletons. Where a river comes down from the land 

 there is a break in the reef, for the reasons which have 

 been already mentioned. 



The origin and mode of formation of a fringing reef, 

 such as that just described, are plain enough. The 

 embryos of the coral polypes have fixed themselves 

 upon the submerged shore of the island, as far out as 

 they could live, namely, to a depth of twenty or twenty- 

 five fathoms. One generation has succeeded another, 

 building itself up upon the dead skeletons of its prede- 

 cessor. The mass has been consolidated by the in- 

 filtration of coral mud, and hardened by partial solu- 

 tion and redeposition, until a great rampart of coral 

 rock one hundred or one hundred and fifty feet high 

 on its seaward face has been formed all round the is- 

 land, with only such gaps as result from the outflow of 

 rivers, in the place of sally-ports. 



The structure of the rocky accumulation in the en- 

 circling reefs and in the atolls is essentially the same 

 as in the fringing reef. But, in addition to the differ- 

 ences of depth inside and out, they present some other 

 peculiarities. These reefs, and especially the atolls, 

 are usually interrupted at one part of their circumfer- 

 ence, and this part is always situated on the leeward 

 side of the reef, or that which is the more sheltered side. 

 Now, as all these reefs are situated within the region 

 in which the tradewinds prevail, it follows that, on the 



