128 CORALS AND CORAL ISLANDS. 



CHAPTER II. 



STRUCTURE OF CORAL. REEFS AND 

 ISLANDS. 



Coeal reefs and coral islands are structures of the same 

 kind under somewhat different conditions. They are made 

 in the same seas, by the same means ; in fact, a coral island 

 has in all cases been a coral reef through a large part of its 

 history, and is so still over much of its area. The terms how- 

 ever are not synonymous. Coral islands are reefs that stand 

 isolated in the ocean, away from other lands, whether now 

 raised only to the water's edge and half submerged, or covered 

 with vegetation ; while the term coral reef, although used for 

 reefs of coral in general, is more especially applied to those 

 which occur along the shores of high islands and continents. 

 There are peculiarities in each making it convenient to describe 

 them separately. 



I. COEAL REEFS. 



I. GENERAL FEATURES. 



Coral reefs are ban 1 , s of coral rock built upon the sea-bot- 

 tom about the shores of tropical lands. In the Paciiic, these 

 lands, with the exception of New Caledonia and others of 

 large size to the westward, are islands of volcanic or igneous 

 rocks, and they often rise to mountain heights. The coral 

 reefs which skirt their shores are ordinarily wholly submerged 

 at high tide ; but, at the ebb, they commonly present to view a 

 broad, flat, bare surface of rock, just above the water level, 



