CHAPTER 11. 



STRUCTURE OF CORAL REEFS AND ISLANDS. 



'Coral 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, however, 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 vege- 

 tation ; 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. CORAL REEFS. 

 GENERAL FEATURES. 



Coral reefs are banks of coral rock built upon the sea- 

 bottom about the shores of tropical lands. In the Pacific, 

 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 



