368 



MENARD AND I>ADD 



[chap. 15 



the temperature duriiiii the year falls below about 18 C for ap])reciable intervals 

 (Wells. 1957). The importance of temperature in large part explains the absence 

 of reefs around islands such as Ascension and St. Helena in the eastern Atlantic 

 and the (Jalapagos Islands in the eastern Pacific. In the eastern parts of both 

 oceans the u])welling of cold waters prevents reef development. 



Many oceanic islands, for example Eua in the Tonga Group (Hoffmeister, 

 1{)32) and Oahu in Hawaii, are partly or completely encircled hy fringing reefs 



Fig. 1. Seamovints, low islands and submarine banks, volcanic islands and active volcanoes 

 in the Pacific Basin. Broken line marks the structural boundary of the basin on the 

 southwest (andesite line). 



— platforms that may extend outward from the shore for distances of .several 

 hundred feet. Islands such as Ongea and many others in Fiji are protected by 

 barrier reefs, structures comparable in size and constitution to fringing reefs 

 but separated from the shore by a lagoon that may be several miles in width 

 (Ladd and Hoffmeister, 1945). On some islands, such as Lakemba, Fiji, both 

 fringing and barrier reefs are developed. Some reef-encircled islands are com- 

 posed entirely of volcanic rock, others entirely of limestone ; on some islands 

 both types of rock are exposed, the limestones resting unconformably on the 

 volcanic foundation. 



