128 CORALS AND CORAL ISLANDS. 
CHAPTER II. 
STRUCTURE OF CORAL REEFS AND ISLANDS. 
Corat 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. CORAL REEFS. 
IL GENERAL FEATURES. 
Coral reefs are bans of coral rock built upon the sea-bot- 
tom about the shores of tropical lands. In the Pacitic, 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, 
os 
