7© 



reefs. Mr. Darwin's attention was attracted to the subject in 

 the course of his voyage in the Eeagle, in the years 1 831-6, and 

 his theory is based upon the facts — firstly, " that, reef-building 

 corals only flourish at a very limited depth ; and secondly, that 

 throughout areas of vast dimensions none of the coral reefs or 

 coral islets rise to a greater height above the level of the sea 

 than that attained by matter thrown up by the winds and 

 waves." These reefs, consequently, must have foundations ; 

 and what is their nature ? Are they formed of great banks 

 of sediment, or do they consist of solid rock ? Mr. Darwin 

 contended that the form or disposition of the groups of atolls 

 afford' no countenance for the first suggestion, and that the 

 latter alternative must accordingly be adopted. Fringing reefs 

 present no difficulties for solution, as they simply occur in those 

 limited depths where coralligenous Actinozoa are capable of 

 flourishing, and barrier reefs and atolls represent successive 

 stages of subsidence of the floor on which fringing reefs were 

 erected. As the foundation sinks the coral-formers are obliged 

 to build upwards to maintain the level at which they can exist. 

 The upper parts of such reefs are, then, the living portions, 

 being built upon the skeletons of their predecessors. The 

 above view of the formation of the varied forms of coral-reef, 

 although very generally adopted, has not, however, satisfied all 

 those naturalists who have bestowed attention on the subject ; 

 and Mr. Murray has recently stated, as the result of his obser- 

 vations on board the •' Challenger," that he has arrived at the 

 conclusion " that it is not necessary to call in subsidence to ex- 

 plain any of the characteristic features of barrier reefs or atolls, 

 and that all these features would exist alike iu areas of slow 

 elevation, of rest, or of slow subsidence." He thinks that he 

 has succeeded in proving that foundations have been prepared 

 for these two classes of reefs by the disintegration of volcanic 

 islands, and by the building up of submarine volcanoes by the 

 deposition on their summits of organic and other sediments, 

 maintaining that when " coral plantations build up from sub- 

 marine banks they assume an atoll form, owing to the more 

 abundant supply of food to their outer margins, and the re- 



