ACTINOZOA. 



89 



temperature of the sea shall not be less during winter than 66 ; and coral- 

 reefs are, therefore, not found in temperate seas. Keefs, however, abound 

 in all the seas not far removed from the equator, being found chiefly on the 

 east coast of Africa and the shores of Madagascar, in the Red Sea and 

 Persian Gulf, throughout the Indian Ocean and the whole of the Pacific 

 Archipelago, around the West-Indian Islands, and on the coast of Florida. 

 The headquarters, however, of the reef-building corals may be 'said to be 

 around the islands and continents of the Pacific Ocean, where they often 

 form masses of coral many hundreds of miles in length. According to 

 Darwin, coal-reefs may be divided into three principal forms viz., Fringing- 

 reefs, Barrier-reefs, and Atolls, distinguished by the following charac- 

 ters: 



1. Fringing-reefs (Fig. 27, 1). These are reefs, usually of a moderate 

 size, which may either surround islands or skirt the shores of continents. 

 These shore-reefs are not separated from the land by any very deep channel, 

 and the sea on their outward margins is not of any great depth. 



2. Barrier-reefs (Fig. 27, 2). These, like the preceding, may either 

 encircle islands or skirt continents. They are distinguished from fring- 

 ing- reefs by the fact that they usually occur at much greater distances 

 from the land, that there intervenes a channel of deep water between them 

 and the shore, and soundings taken close to their seaward margin indicate 

 great depths. 



FIG. 27. Structure of Coral-reefs. 1. Fringing-reef; 2. Barrier-reef; 8. Atoll; a Sea- 

 level ; & Coral-reef; c Primitive land ; d Portion of sea within the reef, forming a chan- 

 nel or lagoon. 



As an example of this class of reefs may be taken the great barrier-reef 

 on the northeast coast of Australia, the structure of which is on a gigantic scale. 

 This reef runs, with a few trifling interruptions, for a distance of more than 

 a thousand miles, with an average breadth of thirty miles, and an area of 



