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 fertiite 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 Expediticn.” 
Coral Reefs are found only in tropical regions: 
although Polyps, animals of the same class as 
those chiefly instrumental in their formation, are 
