LIFE : ITS MECHANICAL WOBK AND KOCK COISTKIBuTlOlsS. 145 



foundation to build upon. (3) The presence of volcanic action, which, 

 through occasional submarine action, destroys the life of a coast. (4) The 

 depth of water on precipitous shores ; for the reef -making corals do not 

 grow where the depth exceeds 150 feet. 



For the last-mentioned reason, reefs are prevented from commencing 

 to form in the deep ocean. But if by other accumulations, or in any other 

 way, the bottom is brought up to the limiting distance from the surface, 

 Corals may commence the making of reefs. 



Coral formations are most abundant in the tropical Pacific, where there are 290 coral 

 islands, besides extensive reefs around other islands. The Paumotu Archipelago, east of 

 Tahiti, contains between 70 and 80 coral islands ; the Carolines, including the Radack, 

 Ealick, and Gilbert groups, as many more ; and others are distributed over the interme- 

 diate region. The Tahitian, Samoan, and Fiji Islands are famous for their reefs ; also 

 New Caledonia and the islands to the northwest. There are reefs also about some of the 

 Hawaiian Islands. The Laccadives and Maldives, in the Indian Ocean, are among the 

 largest coral islands in the world. The East Indies, the eastern coast of Africa, the West 

 Indies, and southern Florida abound in reefs ; and Bermuda, in latitude 32° N., is a coral 

 group. Reef-forming Corals are absent from western America, except along the coast of 

 Central America as far north as the Gulf of California, and they are mostly absent from 

 western Africa, on account of the cold extratropical currents that flow toward the equator : 

 for the same reason, there are no reefs on the coast of China. (See the Physiographic 

 Chart.) 



1. Coral Islands. 



1. Forms. — Atolls. — A coral island commonly consists of a narrow rim of 

 reef, surrounding a lagoon, as illustrated in the annexed sketch (Fig. 143). 



143. 



Coral island, or atoll. 



Such islands are called atolls, — a name of Maldive origin. Maps of two 

 atolls are given in Figs. 144, 145, showing the rim of coral reef, the salt- 

 water lake or lagoon, and the variations of form. They are never circular. 



144. 



145. 



^^^u,^^. 



0'' 





^^^^0^m,, 





I f 

 i>.. - -m 



^.sjsss*^, 



^psiasiSMiS^i 



Atolls.— Fig. 144, Apia, one of the Gilbert Islands ; 145, Menchikoff, one of the Carolines. 



The size varies from a length of fifty miles to two or three ; and, when quite 

 small, the lagoon is wanting, or is represented only by a dry depression. 

 Dana's manual — 10 



