CORALS 89 



A barrier reef may extend along a thousand or more 

 miles of a continent's shore, as the Great Barrier Reef 

 off Queensland, or, again, it may surround an island 

 or a mere speck of land. Were the latter removed we 

 would have an atoll, and the obviousness of this fact 

 was, pending the detailed examination of the reefs 

 themselves, the cause of the prevalence for several 

 decades of the view that the atoll ring-reefs were 

 formed on the subsidence of the land which they 

 once surrounded, first as fringing and later as barrier 

 reefs. 



We may now refer to the chief theories put forward 

 to account for the formation and origin of coral reefs, 

 much information bearing on which may be obtained 

 by any traveller of average ability. These theories 

 centre on the atoll, since it is easy to see that any 

 theory must be capable of explaining the formation 

 of both barrier and atoll reefs. A caution may be at 

 once issued to the effect that it is unlikely that any one 

 theory will explain the formation and origin of all 

 coral reefs ; doubtless as great a diversity exists in 

 the composition of the earth's structure beneath the 

 ocean's bed as in the land. As to the formation 

 of fringing reefs and isolated reef masses in shallow 

 waters there is no dispute, but to explain their building 

 we require briefly to consider the bionomics of the coral 

 animals. 



The structure we know by the name of " coral " is 

 the skeleton of animals common round the coasts of 

 temperate lands, closely allied to the well-known sea- 

 flowers or anemones. It is formed of carbonate of 

 lime, which is deposited by the animal in some way 

 beneath itself, producing a firm but rough seat, from 

 which the animal is itself incapable of stirring. Some 

 of these anemones are solitary beasts, but most form 



