236 DALY. 



great lagoons of atolls and the barrier-bounded channels of the Feejees 

 and other archipelagoes and those of the outer waters about islands 

 or their barriers, show no tendency to grow with large depressed 

 centres, but rather with flat tops, as vegetation might grow, or else 

 with elevated centres .... It is only through continued subsidence 

 under such conditions that the margins can be made to grow so much 

 faster than the interior as to produce thereby a basin-like interior 

 50 to 300 feet deep." 



It is strange that his recognition of only 300 feet (91 m.) as approxi- 

 mately the maximum depth of lagoons inside both atoll and barrier 

 reefs did not lead Dana to question the subsidence theory more seri- 

 ously. Darwin did anticipate this objection and tried to meet it, as 

 shown in the following passages : " Another and less obvious objection 

 to the theory may perhaps be advanced, namely, that, although atolls 

 and barrier-reefs are supposed to have gone on subsiding for a long 

 period, yet that their lagoons and lagoon-channels have only rarely 

 come to exceed 40 and never 60 fathoms in depth. But if our theory 

 is worth consideration, we already admit that the rate of subsidence 

 has not ordinarily exceeded that of the upward growth of the massive 

 corals which live on the margins of the reefs, so that we have only 

 further to suppose that the rate has never exceeded that at which 

 lagoons and lagoon-channels are filled up by the growth of the delicate 

 corals which live there, and by the accumulation of sediment. As the 

 filling-up process, in the case of barrier-reefs lying far from the land, 

 and of the larger atolls, must be an extremely slow one, we are led to 

 conclude that the subsiding movement has always been equally slow\ 

 And this conclusion accords well with what is known of the rate of 

 recent movements of elevation 



" And with respect to the whole amount of subsidence necessary to 

 have produced the many atolls widely scattered over immense spaces, 

 the movement, as alread}^ shown, must either have been uniform and 

 exceedingly slow, or effected by small steps separated from each other 

 by long intervals of time, so as to have allowed the reef-constructing 

 polypifers to bring up their solid framework to the surface; and this 

 is one of the most interesting conclusions to which we are led by the 

 study of coral-formations." ^^ Statements of similar import have not 

 been discovered in Dana's book. 



The multitude of new charts published since Darwin and Dana 

 wrote their books, have essentially confirmed their generalization as 



68 C. Darwin, Coral Reefs, London, 3rd ed., pp. 153-4 and 192-3 (1889). 



