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 
