234 CORAL AND ATOLLS 



Agassiz and Semper both showed that frequently the base was 

 a rising one, but Semper admitted that at times it was not 

 impossible for atolls to be formed on sinking bases. He says 

 (" Animal Life," English Trans., International Scientific Series, 

 vol. xxi. 1906, p. 456): "I regard it as quite possible that 

 under certain circumstances a subsidence may be combined 

 with the formation of atolls, and even that it may once have 

 been the sole cause of their formation ; but I cannot admit that 

 subsidence is alone sufficient to explain all the conditions and 

 relations of coral reefs, or even of predominant importance." 



The theory of subsidence was accepted, and strongly ad- 

 vocated, by Dana, and he even went a step farther than its 

 originator, for he was unable to conceive that, even if banks 

 did exist at the proper depth, an atoll structure could be 

 developed upon them without the intervention of a movement 

 of subsidence. This possibility Darwin had always allowed. 

 He said : " If such a bank lay a few fathoms submerged, the 

 simple growth of the coral without the aid of subsidence would 

 produce a structure scarcely to be distinguished from a true 

 atoll ; for in all cases the corals on the outer margin of the 

 reef, from having space and being freely exposed to the open 

 sea, will grow vigorously and tend to form a continuous ring, 

 whilst the growth of the less massive kinds on the central 

 expanse will be checked by the sediment formed there, and 

 by that washed inward by the breakers ; and as the space 

 becomes shallower, their growth will also be checked by the 

 impurities of the water, and probably by the small amount of 

 food brought by the enfeebled currents, in proportion to the 

 surface of living reefs studded with innumerable craving 

 mouths." This I consider to be one of the truest pictures 

 ever drawn of the development of an atoll reef, but Da.rwin 

 did not imagine it to be the mode of formation of the majority 

 of atolls ; and Dana observed : " If such patches of submerged 

 bank existed, the lagoon structure would still be as inexplicable 

 as ever " {o'p. cit. p. 220). 



The newest epoch in the investigation of coral structures 

 is opened by the detailed study of the atoll of Funafuti carried 



