CORAL FORMATIONS. 



615 



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

 mencing to form in the deep ocean. The notion that coral islands 

 are rising from its depths has no support in facts : they must have 

 the land within a few fathoms of the surface to begin upon. 



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

 coral islands, besides extensive reefs around other islands. The Paumotu Archi- 

 pelago, east of Tahiti, contains between seventy and eighty coral islands ; the 

 Carolines, including the Radack, Ralick, and Kingsmill groups, as many 

 more; and others are distributed over the intermediate region. The Tahitiam 

 Sanioan, and Feejee Islands are famous for their reefs; also New Caledonia and 

 islands northwest. There are reefs also about some of the Hawaian 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. Eeefs are absent from western America, except along by Pa- 

 nama, and mostly from western Africa, on account of the cold extratropical 

 currents that flow towards the equator : for the same reason, there are no reefs 

 on the coast of China. (See the Physiographic Chart.) 



1. Coral Islands. 



Forms. — Atolls. — The larger part of coral islands consist of a nar- 

 row rim of reef surrounding a lagoon, as illustrated in the annexed 

 sketch (fig. 848). Such islands are called atolls, — a name of Mal- 

 dive origin. Maps of two atolls are shown in figs. 849, 850, showing 

 the rim of coral reef, the salt-water lake or lagoon, and the varia- 



848. 



Coral island, or atoll. 



tions of form in these islands: they are -never 'circular. 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 de- 

 pression. 



The reef is usually to a large extent bare coral rock, swept by the 

 waves at high tide. In some, the dry land is confined to a few iso- 

 lated points, as Menchikoff Island, of the Caroline group (fig. 850) ; 

 in others, one side is wooded continuously, or nearly so, while the 

 other is mostly bare, or is a string of green islets, as in fig. 849, 

 representing Apia, one of the Kingsmill Islands. The higher or 



