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 i^elson "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 exj)osure 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 iniioortant part in the history of coral formations.f 

 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 dei^th 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."| 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 a 

 map, is strikingly suggestive of the belief that it is a comj)ound atoll, 

 similar to Mahlos Mahdoo and some other atolls of the Maldiva Archi- 

 pelago. The great depth of water witbin a few miles of the island^, 

 and the exclusively calcareous character of the rocks of which the 

 islands are composed, tend to confirm this imiiressiou. Dana|| and 

 Thomson fl regard Bermuda as truly an atoll. Darwin apparently 



*0/). cit, pp. 105, 116. 



t Darwin, Coral Reefs, pp. 78, 118. 



t Dana, Corals and Coral Islands, p. 370. 



$ Thomson, op. cit., Vol. I., p. 286. 



II Corals and Coral Islands, p. 218. 



If Op. cit., Vol. I., p. 280. 



