8 THE GEOLOGY OF BERMUDA. 
low-tide level. The crest of these circular ridges is formed in large part 
of the calcareous tubes of tubicolous worms. They are appropriately 
called by Nelson “serpuline reefs.”* The elevation of these serpu- 
line reefs above low-tide level is due to the fact that these worms, un- 
like the coral-forming anthozoa and hydrozoa, can survive an exposure 
for some hours out of water. There are circular ridges of coral reef 
similar to these serpuline reefs, except that they are less elevated, their 
upward growth being limited by the inability of the coral animals to 
survive an exposure above the water. These circular reefs are called, 
commonly, “boilers.” The form of both varieties of these ‘ boilers” 
illustrates well Chamisso’s theory of atolls—a theory which, though in- 
adequate for the explanation of atolls in general, recognized a principle 
which has played an important part in the history of coral formations.t 
The depth of water in the elliptical lagoon inclosed by the outer reef © 
is generally 6 or 8 fathoms, though there are many patches of reef scat- 
tered through the lagoon. Outside of the reef the water deepens gradu- 
ally for a mile or more, the average depth at the distance of a mile being 
only about 12 fathoms. A little further from the shore a more abrupt 
descent commences, the depth at a distance of 10 miles in every direc- 
tion except the south-west being from 1,500 to 2,250 fathoms. “Twenty 
miles to the southwest-by-west from the Bermudas there are two sub- 
merged banks, 20 to 47 fathoms under water, showing that the Bermudas 
are not completely alone, and demonstrating that they cover a summit 
in a range of heights.”t The Challenger expedition obtained a sound- 
ing of 2,950 fathoms about 300 miles further on in the same direction, 
indicating apparently that the range is not of great extent in that direc- 
tion.§ 
IS BERMUDA AN ATOLL? 
The general form of the Bermuda Archipelago, as represented on & 
map, is strikingly suggestive of the belief that it is a compound atoll, 
similar to Mahlos Mahdoo and some other atolls of the Maldiva Archi- 
pelago. The great depth of water within a few miles of the islands, 
and the exclusively calcareous character of the rocks of which the 
islands are composed, tend to confirm this impression. Danaj|| and 
Thomson] regard Bermuda as truly an atoll. Darwin apparently 
SR ea eee *Op. cit., pp. 105, 116. 
t Darwin, Coral Reefs, pp. 78, 118. 
t Dana, Corals and Coral Islands, p. 370. 
§ Thomson, op. cit., Vol. L., p. 286. 
|| Corals and Coral Islands, p. 218. 
4 Op. cit., Vol. I., p. 280. 
