132 BULLETIN OF THE 



islets which dot the great lagoon-like waters of the Gambier group, 

 "leaving scarcely any doubt in the mind that the islets were the 

 emerged points of sunken lands ; and if this is evidence of subsidence, 

 then the atoll [of Keeling] which he [Darwin] examined was proof of 

 further subsidence, that is, one that had continued to the disappearance 

 of the sinking peaks." This is the proof which Darwin believed to be 

 almost certain evidence of subsidence. 



Dana adds, as an argument in favor of subsidence, the existence of 

 deep fiord-like indentations in the rocky coasts of islands, both those in- 

 side of barriers and those not bordered by reefs. Certainly this is a 

 most unsafe method of reasoning, unless accompanied by sounding in the 

 fiords to show the continuation of the slope of erosion. As to the non- 

 existence in the ocean now, and the extreme improbability of the exist- 

 ence at any time, of submarine volcanoes or chains of mountains having 

 their numerous summits within a hundred feet of the surface, which has 

 been a favorite argument against the possibility of a volcanic base for 

 reefs, the recent deep-sea soundings of the Atlantic in volcanic districts, 

 like that off the west coast of Africa by the Talisman, have shown the 

 existence of numerous peaks and submarine banks, which in the track of 

 oceanic currents would soon be built up to the level at which corals can 

 thrive, and produce the very conditions denied by Darwin. A similar 

 state of things has been developed by the soundings of the Blake in the 

 West Indies. 



Dana mentions the great width of a reef as an indication of subsid- 

 ence. I am unable to see the force of that argument. It seems merely 

 to indicate the great length of time which has elapsed since it began to 

 build. We might take for granted the evidence of subsidence as de- 

 duced by Dana for the Tahitian group and for the Samoan group, for 

 instance, and yet we should not have the proof that this subsidence 

 was coexistent with the formation of the different kinds of reefs. 



If, as is supposed, we can have submarine banks of limestone formed 

 upon volcanic mountains or other steep slopes, the steepness of the slope 

 off the coral reef, does not argue anything in favor of subsidence. I do 

 not see that the large debris offer positive proof of subsidence, if they 

 have, as Murray supposes, gradually rolled down the steep talus of the 

 sea face of the reef, and have, as is certainly the case in the Sandwich 

 Islands, formed the surface, which may be of great thickness. Dana 

 infers, from the statement I made in regard to the former connection of 

 the Windward Islands 1 with South America, that there has been a sub- 

 1 Bull. Mus. Comp. Zool., 1879 ; Am. Joum. of Science, XVIII. 230, 1880. 



