30. DARWIN ON CORAL ISLANDS. 631 



30. Formation of Coral Islands. — From the grand 

 scale on which these operations have been carried on 

 in the Pacific, and the powerful volcanic action of which 

 those latitudes have been the theatre, as shown by the 

 elevatory movements to which the neighbouring conti- 

 nents have been subjected, the extremely small extent of 

 dry land in that world of waters is a very striking pheno- 

 menon. This remarkable fact was supposed by Mr. Lyell* 

 to admit of explanation, on the supposition that a gradual 

 subsidence had been going on for ages over a vast area of 

 the bed of the Pacific, which occasioned the solid materials 

 produced by the coral zoophytes, to sink down beneath the 

 waters, and therefore no considerable additions were made 

 to the dry land above the level of the sea : the Polynesian 

 Archipelago, and the submerged coral reefs, alone indicating 

 the stupendous changes effected by zoophytal agency. This 

 opinion has been confirmed by the observations of Mr. 

 Darwin, w r hose explanation of the mode in which the for- 

 mation of Coral Islands takes place, is a beautiful example 

 of philosophical induction. 



The coral reefs are described by Mr. Darwin as of three 

 distinct kinds, arising from the different circumstances at- 

 tending their production. First, Atolls ;f secondly, barrier 

 reefs, which are ridges of coral either extending in straight 

 lines in front of the shores of a continent or large island, or 

 encircling a group of small islands, and separated from the 

 land by a deep channel of water; thirdly, fringing reefs, or 

 banks of coral, superimposed on the slopes of the adjacent 

 land, and at no great distance from the shore. 



By the gradual subsidence of the land, and the coincident 

 upward growth of the corals, fringing reefs are gradually 

 converted into barrier reefs, and the latter into Atolls. 

 Hence it is inferred, that coasts merely fringed by reefs 



* Principles of Geology, 1st edition. 



-} An Indian name for the circular or lagoon islands. 



