Theories Regarding Reef Structure. 119 



elevation, and of your belief that the islands have 

 not since subsided, I have no doubt that I should 

 have considered them as formed during subsidence. 

 But I should have been much troubled in my mind 

 by the sea not being so deep as it usually is round 

 atolls, and by the reef on one side sloping so grad- 

 ually beneath the sea; for this latter fact, as far 

 as my memory serves me, is a very unusual and al- 

 most unparalleled case. I always foresaw that a bank 

 at the proper depth beneath the surface would give 

 rise to a reef which could not be distinguished from 

 an atoll formed during subsidence. I must still ad- 

 here to my opinion that the atolls and barrier- reefs 

 in the middle of the Pacific and Indian Oceans indi- 

 cate subsidence ; but I fully agree with you that 

 such cases as that of the Pelew Islands, if of at all 

 frequent occurrence, would make my general conclu- 

 sions of very little value. Future observers must de- 

 cide between us. It will be a strange fact if there 

 has not been subsidence of the bed of the great 

 oceans, and if this has not affected the forms of the 

 coral reefs. 



" Yours very sincerely, 



" Charles Darwin." 



Darwin's descriptions of the three classes of coral 

 reefs are too well known to require especial mention, 

 and have been presented by him to the world in a 

 work entitled " Structure and Distribution of Coral 

 Reefs," which if read in connection with Semper's 

 "Animal Life" and other works on corals of to-day, 

 will show the remarkable care and reasoning which 



