1 84 MANUAL OF ZOOLOGY. 



The question now arises In what way have reefs been pro- 

 duced, 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 be supposed gradually to sink be- 

 neath 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 be- 

 tween it and the reef. If the depression should be still con- 

 tinued, 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 up- 

 raised 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 Astrceans cease to grow. Their place is now taken 

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

 cannot extend above a certain level. Finally, the summit of 

 the reef is formed by an aggregation of less massive corals, 

 such as Madreporidce, Milleporidcz, and Gorgonida. 



DISTRIBUTION OF ACTINOZOA IN TIME. With the single exception of 

 the Mollusca, no division of the animal kingdom contributes such import- 

 ant and numerous indications of its past existence as the Actinozoa. 



In the Palaeozoic rocks the majority of corals belong to the division 



