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 low 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 suberystalline 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. 
+ Darwin, Coral Reefs, pp. 256-259; Dana, Corals and Coral Islands, pp. 213-218. 
t Op. cit., p. 109. 
§ Corals and Coral Islands, Ch. II. 
