154 FORMATION OF CORAL REEFS. 



salt, it is true, in its lower part, and the fact that 

 it is affected by the rise and fall of the tides 

 shows that it is not entirely secluded from com- 

 munication with the ocean outside ; but the salt 

 water, being heavier, sinks, while the lighter 

 rain-water remains above, and it is to all appear- 

 ance actually changed into a fresh-water lake. 



I need not dwell here on the further history 

 of such a Coral island, or follow it through the 

 changes by which the summit of its circular wall 

 becomes covered with a fertile soil, a tropical 

 vegetation springs up upon it, and it is at last 

 perhaps inhabited by man. There is something 

 very attractive in the idea of these green rings 

 enclosing sheltered harbors and quiet lakes in 

 mid-ocean, and the subject has lost none of its 

 fascination since the mystery of their existence 

 has been solved by the investigations of several 

 contemporary naturalists, who have enabled us to 

 trace the whole story of their structure. I would 

 refer all who wish for a more detailed account 

 of them to Charles Darwin's charming little vol- 

 ume on " Coral Reefs," where their mode of 

 formation is fully described, and also to James 

 D. Dana's " Geological Report of the United 

 States Exploring Expedition." 



Coral Reefs are found only in tropical regions : 

 although Polyps, animals of the same class as 

 those chiefly instrumental in their formation, are 



