138 BULLETIN OF THE 



mass of coral sand some twenty-five feet thick, progressing inland. He 

 also describes the mode in which the free coral sand is rapidly cemented 

 into limestone by the action of rain-water containing carbonic acid, 

 which takes up a little of the lime and on evaporating forms the succes- 

 sive crust lines of demarcation between various layers of sand, forming 

 the stratification and lamination of the ^Eolian rocks. The section given 

 by Thomson, as exposed by the cutting made for the floating dock in 

 1870, seems to prove a slight subsidence, as there was found a bed of a 

 kind of peat at a depth of forty-seven feet, containing stumps of cedar in 

 a vertical position lying upon the hard bare rock. But it does not prove 

 that this subsidence, or a greater one, which cannot be proved, has been 

 the cause of the atoll shape of the Bermudas, any more than the slight 

 elevations of from twenty to fifty feet, such as we so often meet with in 

 volcanic districts, prove that the special type of coral reefs existing there 

 have been due to their influence. 



Lieutenant Nelson 1 has given an account of the geological details of 

 the appearance of the different islands composing the Bermudas, and of 

 the encroachments by the sea and sands, and it did not escape him that 

 the whole of the Bermudas " may be called organic formations, as they 

 present but one mass of animal remains in various stages of comminution 

 and disintegration," and he also called attention to the organic com- 

 position of what he calls Bermuda chalk, which corresponds evidently to 

 what has more recently been called coral ooze. He was among the first 

 to notice the important action of Serpulse in cementing together pieces 

 of coral, and in certain localities forming even small independent reef 

 patches. This has been fully confirmed by other observers in other 

 districts. 



Nelson has also suggested the possibility of the formation of submarine 

 mountains by the growth of marine invertebrates round any base they 

 may meet, the decay of their calcareous remains adding stability and 

 bulk to the colony, while around their summits coral reefs would grow. 

 He also says, very truly, " Zoophytes affect a vertical growth, and in 

 this attitude have a tendency to add to the accumulations of the exterior 

 fence, to the prejudice of the space circumscribed." 



When we pass to the very regions explored by Darwin, Mr. Henry 

 0. Forbes, 2 who in 1879 examined the Keeling Atoll, forty-three years 

 after Darwin's visit, — the very one which Darwin first examined, and 

 which suggested to him his whole theory, — could not satisfy himself 



1 Trans. Geol. Soc. of London, V., Part I., 1840, p. 103. 



2 A Naturalist's Wanderings in the Eastern Archipelago, London, 1885. 



