APPENDIX. 263 



they do not front more than a sixth part of the circumference 

 of the island), they would necessarily from their position 

 have been coloured as barrier-reefs ; as the case stands they 

 are left uncoloured. I suspect that after a small amount of 

 subsidence, the corals were killed by sand and mud being 

 deposited on them, and the reefs being thus prevented from 

 growing upwards, the banks of madreporitic rock were left 

 in their present submerged condition. 



The Bermuda Islands have been carefully described 

 by Lieut. Nelson, in an excellent Memoir in the Geol. 

 Transactions (vol. v. part i. p. 103). In the form of the 

 bank or reef, on one side of which the islands stand, there 

 is a close general resemblance to an atoll; but in the 

 following respects there is a considerable difference, — first, 

 in the margin of the reef not forming (as I have been 

 informed by Mr. Chaffers, R.N.) a flat, solid surface, laid 

 bare at low water, and regularly bounding the internal 

 space of shallow water or lagoon ; secondly, in the border 

 of gradually shoaling water, nearly a mile and a half in 

 width, which surrounds the entire outside of the reef (as is 

 laid down in Capt. Hurd's chart) ; and thirdly, in the size, 

 height, and extraordinary form of the islands, which present 

 little resemblance to the long, narrow, simple islets, seldom 

 exceeding half a mile in breadth, which surmount the 

 annular reefs of almost all the atolls in the Indian and 

 Pacific Oceans. Moreover, there are evident proofs 

 (Nelson, ibid., p. 118), that islands similar to the existing 

 ones, formerly extended over other parts of the reef. It 

 would, I believe, be difficult to find a true atoll with land 

 exceeding thirty feet in height; whereas, Mr. Nelson 

 estimates the highest point of the Bermuda Islands to be 

 260 feet; if, however, Mr. Nelson's view, that the whole 

 of the land consists of sand drifted by the winds, and 



