50 James D. Dana, Esq., on the Structure, 



shewing that all the ingredients of coral are actually con- 

 tained in the waters of the ocean.* 



It has been common to attribute the origin of the lime of 

 corals to the existence of carbonic acid springs in the vicinity 

 of coral islands. But it is an objection to such a hypothesis, 

 that, in the first place, the facts do not require it ; and in 

 the second, there is no foundation for it. The islands have 

 been supposed to rest on volcanic summits, thus making one 

 hypothesis the basis of another. Carbonic acid springs are 

 by no means a universal attendant on volcanic action. The 

 Pacific afi'ords no one fact in support of such an opinion. 

 There are none on Hawaii, where are the most active fires 

 in Polynesia ; and the many explorations of the Society and 

 Navigator Islands have brought none to light, Some of the 

 largest reefs of the Pacific, those of New Holland and New 

 Caledonia, occur where there is no evidence of former vol- 

 canic action. t 



The currents of the Pacific are constantly bearing new 

 supplies of water over the growing coral beds, and the whole 

 ocean is thus engaged in contributing to their nutriment. 

 Fish, molluscs, and zoophytes are thus provided with earthy 

 ingredients for their calcareous secretions, if their food fails 

 of giving the necessary amount ; and by means of the powers 

 of animal life, bones, shells, and corals alike are formed. 



The origin of the lime in solution throughout the ocean is 

 an inquiry foreign to our present subject. It is sufficient here 

 to shew that this lime, whatever its source, is adequate to 

 explain all the results under consideration. 



3. Causes influencing the Growth of Coral Zoophytes. 

 Marine zoophytes generally require pure ocean water, and 

 they abound especially in the broad inner channels among 

 the reefs, or the large lagoons, and in the shallow waters 

 outside of the breakers. In these channels, at the Feejee 

 group, there are species of every genus, and they grow in 

 the greatest luxuriance, exceeding in profusion and display 

 all that was elsewhere seen in the Pacific. Here are found the 



* Trans. Hoy. Soc. of Edin., xvi., 145, 1846; Amer. Jour. Sci., 2d Ser., ii., 

 114, 1846. 



t See also Darwin, Op eit., p. 60. 



