118 MANUAL OP ZOOLOGY. 



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

 depths of 300 fathoms or more ? This question has been an- 

 swered by Darwin, who showed that the production of barrier- 

 reefs arid atolls was really to be ascribed to a gradual subsi- 

 dence 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 de- 

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

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

 tral 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 produc- 

 tion, 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 mat- 

 ter of fact, fringing-reefs are often found to be conjoined with 

 up-raised strata of post-tertiary age. 



C. Different portions of a coral reef are occupied by different 

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

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

 not flourish at depths of less than six fathoms of water, and 

 consequently when the surface of the reef has reached this 

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

 by Meandrinas (Brain- corals) and Porites, but these, too, can- 

 not extend above a certain level. Finally the summit of the 

 reef is formed by an aggregation of less massive corals, such as 

 Madreporidce, Milleporidce, and Gorgonidce. 



DISTRIBUTION OF ACTINOZOA IN TIME. With the single ex- 

 ception of the Mollusca, no division of the animal kingdom 

 contributes such important and numerous indications of its 

 past existence, as the Actinozoa. 



In the Palaeozoic Rocks, the majority of corals belong to the 

 division Rugosa, these seeming to have filled the place now 

 taken by the sclerodermic Zoantharia. The order Rugosa is 

 entirely Palaeozoic, with the single exception of the genus Holo- 

 cystis, which is represented in the Secondary Bocks by a single 



