THE GEOLOGY OF BERMUDA. 9 



inclines to the belief that it is not an atoll, and calls attention to the 

 following points in which Bermuda differs from atolls in general : " First, 

 in the margin of the reef not forming a flat, solid surface, which is laid 

 bare at I6w water; secondly, in the water gradually shoaling for nearly 

 a mile and a half in width round the entire reef; 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."* These differences are cer- 

 tainly of great importance; yet I believe that they are all capable of 

 explanation on the theory that Bermuda is an atoll. I believe we may 

 recognize the causes of these peculiarities in the peculiar history of the 

 islands. Bermuda has a special interest in view of its position. It is 

 perhaps the only atoll in the Atlantic Ocean, the atoll character of the 

 Bahamas being at least doubtful.t Bermuda is also remarkable as being 

 more remote from the equator than any other coral formation, the growth 

 of corals there being rendered possible by the influence of the Gulf 

 Stream. 



THE CORAL LIMESTONE AND ITS VARIETIES. 



As already stated, the only rock found in situ in Bermuda, if we ex- 

 cept the peat or muck of the bogs, and the "red earth," which will be 

 specially treated of hereafter, is limestone. The rock varies, however, 

 exceedingly in texture. The extremes are an unconsolidated, calcareous 

 sand, a subcrystalline rock of almost flinty compactness, and a coarsely 

 crystalline stalagmite. The hard, compact rock is locally called " base 

 rock," and the name is employed in that sense by Nelson ; | but it does 

 not uniformly underlie the softer rocks, nor is there any evidence that 

 it is older than they. 



Dana distinguishes in coral reefs and islands three kinds of rock in 

 respect of the mode of formation: viz., reef- rock, beach sand-rock, and 

 drift sand-rock. § The reef-rock is that which constitutes the reefs 

 proper. It is formed by the accumulation of the more or less finely 

 comminuted material of the corals, shells, and other skeletons of marine 

 animals, on the grounds where the corals are growing. It can there- 

 fore never be formed at an elevation much above low-tide. The beach 

 sand-rock is formed by the action of the waves sweeping up against the 



• Coral Reefs, p. 264. 



t Darwin, Coral Reefs, pp. 256-259; Dana, Corals and Coral Islands, pp. 213-218. 



tOp. cit., p. 109. 



$ Corals and Coral Islands, Ch. II. 



