I 50 MANUAL OF ZOOLOGY. 



ing force is found in the vital activity of the living corals which 

 form the seaward margin of the reef, and which, by their 

 growth, prevent the sea from always destroying the masses of 

 sediment which it may have thrown up. 



B. The coral-producing polypes cannot exist at depths 

 exceeding some 15 to 30 fathoms. It follows from this that no 

 coral-reef can be commenced upon a sea-bottom deeper than 

 about 30 fathoms. The question now arises In what way 

 have reefs been produced, which, as we have seen, rise out of 

 depths of 300 fathoms or more? This question has been 

 answered by Darwin, who showed that the production of 

 barrier-reefs and atolls was really to be ascribed to a gradual 

 subsidence of the foundations upon which they rest. Thus, if 

 a fringing-reef which surrounds an island is supposed gradually 

 to sink beneath the sea, the upward growth of the corals will 

 neutralise the downward movement of the land, so far, at any 

 rate, that the reef will appear to be stationary, whilst it is 

 really growing upwards. The island, however, as subsidence 

 goes on, will gradually diminish in size, and a channel will be 

 formed between it and the reef. If the depression should be 

 still continued, the island will be reduced to a mere peak in 

 the centre of a lagoon ; and the reef, from a " fringing-reef," 

 will have become converted into an "encircling barrier-reef." 

 As the growth of the reef is chiefly vertical, the continued 

 depression will, of course, have produced deep water all round 

 the reef. If the subsidence be continued still further, the 

 central peak will disappear altogether, and the reef will become 

 a more or less complete ring surrounding a central expanse 

 of water; thus becoming converted into an "atoll." The 

 production, therefore, of encircling barrier-reefs and atolls is 

 thus seen to be due to a process of subsidence of the sea- 

 bottom. The existence, however, of fringing-reefs is only 

 possible when the land is either slowly rising, or is stationary ; 

 and, as a matter of fact, fringing-reefs are often found to be 

 conjoined with upraised strata of post-tertiary age. Atolls and 

 encircling barrier- reefs, on the other hand, are not found in the 

 vicinity of active volcanoes regions where geology teaches us 

 that the land is either stationary, or is undergoing slow upheaval. 



C. Different portions of a coral reef are occupied by differ- 

 ent kinds of corals. According to Agassiz, the basement of a 

 coral-reef is formed by a zone of massive Astraans. These 

 cannot flourish at depths of less than six fathoms of water, 

 and consequently when the surface of the reef has reached this 

 level, the Astrczans cease to grow. Their place is now taken 

 by Meandrinas (Brain-corals) and Forties; but these, too, 



