tc. 



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d 



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^^'lailai 





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inai- 



roond its 



■wted from tie 

 *uich a line of 



m^ 





w) 



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partly of 



nrrounded br s 



M rut nnlr tlie 



. th: ''yri, 



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will 



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 ;AMt^ of wli^^'^ 





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strip 



of w^' 



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* 



Ch. XLIX.] 



ORIGIN OF THEIR FORM. 



593 



circling or a barrier reef. 



Two classes of reefs, tlierefore;^ liave now been considered ; 

 first, tlie atoll, and, secondly, tbe encircling and barrier reef, 

 botli agreeing perfectly in structure, and tlie sole diiference 

 lying in tlie absence in tlie case of tlie atoll of all land, and 

 in tlie others tlie presence of land bounded either by an en- 



But there is still a third class of 

 reefs, called by Mr. Darwin "^fringing reefs,' which approach 

 much nearer the land than those of the encircling and barrier 

 class, and which indeed so nearly touched the coast as to 

 leaA^e nothing in the intervening space resembling a lagoon. 

 ' That these reefs are not attached quite close to the shore 

 appears to be the result of two causes ; first, that the water 

 immediately adjoining the beach is rendered turbid by the 

 surf, and therefore injurious to all zoophytes ; and, secondly, 

 that the larger and efficient kinds only flourish on the outer 

 edge amidst the breakers of the open sea.'"^ 



It will at once be conceded that there is so much analogy 

 between the form and position of the strip of coral in the 



Fig. lo6. 



/ 



\ 



Supposed section of an island with an encircling reef of coral, 

 A. The island. 



A 



h, c. Highest points of the encircling reef between ^vhich and the coast is seen a space 



occupied by still water, 



atoll, and in the encircling and barrier reef, that no explana- 

 tion can be satisfactory which does not include the whole. 

 If we turn, in the first place, to the encircling and barrier 

 reefs, and endeavour to explain how the zoophytes could 

 have found a bottom on which to begin to build, we are met 

 at once with a great difficulty. It is a general fact, long since 

 remarked by Dampier, that high land and deep seas go 



together. 



In other words, steep mountains coming down 



^ Darwin's Journ., p. 557, 2nd edit. chap. 20, and Coral Islands, chapters 



1 *> R 



A^ ^j O. 



TOL. II. 



Q Q 



