GOO 



POEMATION OF COEAL EEEFS. 



[Ch. XLIX. 



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shore, which is kept open by the efflux of the sea as the 

 tide goes down. 



must 



of subsidence before explained, be traced back to causes 

 Avhich were in action during the existence of the encirclino- 

 reef, and when an island or mountain top rose within it, for 

 such a reef precedes the atoll in the order of formation. 

 Now in those islands in the Pacific, which are large enough 

 to feed small rivers, there is generally an opening or channel 

 in the surrounding coral reef at the point where the stream 

 of fresh water enters the sea. The depth of these channels 

 rarely exceeds 25 feet; and they may be attributed, says 

 Captain Beechey, to the aversion of the lithophytes to' fresh 

 water, and to the probable absence of the mineral matter of 

 which they construct their habitations.^ 



Mr. Darwin, however, has shown, that mud at the bottom 



mor 



the water in preventing the growth of the polypi, for the 

 walls which enclose the openings are perpendicular, and do 

 not slant off gradually, as would be the case, if the nature 

 of the element presented the only obstacle to the increase of 

 the coral-building animals. 



When a breach has thus been made in the reef, it wiU be 

 prevented from closing up by the efflux of the sea at low 

 tides ; for it is sufficient that a reef should rise a few feet 

 above low-water mark to cause the waters to collect in the 

 lagoon at high tide, and when the sea falls, to rush out at 

 one or more points where the reef happens to be lowest or 

 Aveakest. This event is strictly analogous to that witnessed 



estuaries where a body of salt water accumulated 

 during the flow issues with great velocity at the ebb of the 

 tide, and scours out or keeps open a deep passage through 

 the bar, which is almost always formed at the mouth of a 



At first there are probably many openings, but the 

 growth of the coral tends to obstruct all those which do not 

 serve as the principal channels of discharge ; so that their 

 number is gradually reduced to a few, and often finally to 



m our 



river. 



* Voyage to the Pacific, &c., p. 191. 



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