188 AGE OF CORAL REEFS. 



This brings us to the shore-bluffs, consisting 

 simply of another Reef exactly like those already 

 described, except that in course of time it has been 

 united to the main-land by the complete filling up 

 and consolidation of the channel which once di- 

 vided it from the extremity of the peninsula, as 

 a channel now separates the Keys from the shore- 

 bluffs, and the outer Reef, again, from the Keys. 

 These three concentric Reefs, then, the outer 

 Reef, the Keys, and the shore-bluffs, if we meas- 

 ure the growth of the two latter on the same 

 low estimate by which I have calculated the rate 

 of progress of the former, cannot have reached 

 their present condition in less than twenty thou- 

 sand years. Their growth must have been suc- 

 cessive, since, as we have seen, all Corals need 

 the fresh action of the open sea upon them, and 

 if either of the outer Reefs had begun to grow 

 before the completion of the inner one, it would 

 have effectually checked the growth of the latter. 

 The absence of an incipient Reef outside of the 

 outer Reef shows these conclusions to be well 

 founded. The islands capping these three reefs 

 do not exceed in height the level to which the 

 fragments accumulated upon their summits may 

 have been thrown by the heaviest storms. The 

 highest hills of this part of Florida are not over 

 ten or twelve feet above the level of the sea, 

 and yet the luxuriant vegetation with which they 



