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Sect. VI.] 



GEOLOGY. 



191 



cov in difierent zones of depth ; so that in collecting 

 sp'^^imens, the depth at wliich each kind is found, and at 

 tvhich it is most abundant, should be carefully noted. 

 It ought always to be recorded whether the specimen came 

 from the tranquil waters of a lagoon or protected channel, 

 or fiom the exposed outside of the reef. The small reefs 



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atolls not a single reef rises within several fathoms of the 



level. It would be a curious point to ascertain 



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whether the corals in these cases consisted of the same 

 species ; and if so, on what possible circumstance this sin- 

 gular difference in the amount of their upward growth 

 has depended. 



Any facts which can elucidate the rate at w^hich corals 

 «an grow under favourable circumstances, will ever be 

 interesting : nor should negative facts, showing that within 



iven period reefs have not increased either lateral! 

 or vertically upwards, be neglected. In a full-grow 

 forest, to judge of its rate of growth, a part must be first 

 cut down ; so is it probably with reefs of corals. The 

 aborigines of some of the many coral islands in the great 



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n 



oceans might perhaps adduce positive facts on this head ; 

 for instance, the date might be known when a channel 

 had been cut to float out a large canoe^ and which had 

 since grown up. 



For the classification of coral reefs, the most important 



point to be attended to, is the inclination of the bed of the 

 adjoining sea; and secondly, the depth of the interior 

 lagoon in the case of atolls, and of the channel between 

 the land and the reef, in Encircling or Barrier, and in 



