618 



DYNAMICAL GEOLOGY. 



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 two hundred 

 and ninety coral islands, besides extensive reefs around other islands. The Paumotu Ar- 

 chipelago, east of Tahiti, contains between seventy and eighty coral islands ; the Caro- 

 lines, including the Radack, Ralick, and Gilbert groups, as many more ; and others are 

 distributed over the intermediate region. The Tahitian, Samoan, and Feejee Islands an 

 famous for their reefs; also New Caledonia and the islands to the northwest. There ar' 

 reefs also about some of the Hawaian Islands. The Laccadives and Maldives, in the In 

 dian Ocean, are among the largest coral islands in the world. The East Indies, the east- 

 ern 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 Amer- 

 ica, except along the coasts of Central America, and as far north as the Gulf of Califor- 

 nia, and mostly 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. 

 (Seethe Physiographic Chart.) 



1. Coral Islands. 



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

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

 (Fig. 960). Such islands are called atolls, — a name of Maldive origin. 



Fig. 960. 



Coral island, or atoll. 



Maps of two atolls are given in Figs. 961, 962, showing the rim of 

 coral reef, the salt-water lake, or lagoon, and the variations 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 depression. 



Fig. 962. 



Atolls. — Fig. 961, Apia, one of the Gilbert Islands ; 962, Menchikoff, one of the Carolines. 



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 isolated 

 points, as in Menchikoff Island, of the Caroline group (Fig. 962) ; in 



