6 9 



Corals of one kind or another are more or less plentiful 

 throughout the seas of the globe, but the reef-forming species 

 are restricted to certain regions, and the areas which they oc- 

 cupy are determined by the temperature of the ocean, depth, 

 nature of the shore of the adjoining land, and other circum- 

 stances. All appear to require an equable and elevated tem- 

 perature in order to flourish — this temperature, according to 

 Dana's observations, being not below 68° F. Further, no reef- 

 form seems to be capable of living and thriving, except at 

 depths exceeding 20 to 40 fathoms ; and in some cases the 

 growth of reefs appears to be prevented by the nature of the 

 coast, though additional researches are required on this point. 



Navigators have been long accustomed to divide coral reefs 

 into three groups, namely, fringing or shore reefs ; barrier reefs, 

 and atolls, or lagoon islets. Of these, the first are found skirt- 

 ing the coasts of islands and continents at distances varying from 

 less than half a mile to one or two miles. The outer portion of 

 such reefs is higher than the inner, in consequence of the 

 stronger corals growing more vigorously where they are exposed 

 to the wash of the waves, and the channel between the reef and 

 the land has in most cases a sandy bottom. The second class 

 (barrier reefs) either extend in straight lines in front of the 

 shores of a continent or great island, or encircle smaller islands, 

 being separated from the land by a broad and rather deep 

 channel, the calm water of which usually presents a striking 

 contrast to the violent surf which breaks on the outer margin 

 of the reef. In some cases the entire line of reef is converted 

 into land, but usually only limited portions of it are so, assum- 

 ing the form of low islands more or less clothed with vegetation, 

 of which the cocoa-nut palm is a prominent feature. The last 

 kind of reefs — atolls, or lagoon islands — are annular in form. 

 They vary much in dimensions, and surround calm expanses of 

 water destitute of islands. A considerable portion of their 

 surface is raised above the sea-level, usually presenting the 

 appearance of a chain of small islets. 



A considerable number of theories have been advanced to 

 account for the different peculiarities of these three kinds of 



