190 AGE OF CORAL REEFS. 
of successive concentric Reefs, we must believe 
that hundreds of thousands of years have elapsed 
since its formation began. Leaving aside, how- 
ever, all that part of its history which is not 
susceptible of positive demonstration in the pres- 
ent state of our knowledge, I will limit my re- 
sults to the evidence of facts already within our 
possession ; and these give us as the lowest pos- 
sible estimate a period of seventy thousand years 
for the formation of that part of the peninsula 
which extends south of Lake Okee-cho-bee to the 
present outer Reef. 
So much for the duration of the Reefs them- 
selves. What, now, do they tell us of the per- 
manence of the Species by which they were 
formed? In these seventy thousand years has 
there been any change in the Corals living in the 
Gulf of Mexico? I answer most emphatically, 
No. Astrzans, Porites, Mzandrinas, and Mad- 
repores were represented by exactly the same 
Species seventy thousand years ago as they are 
now. Were we to classify the Florida Corals 
from the Reefs of the interior, the result would 
correspond exactly to a classification founded 
upon the living Corals of the outer Reef to-day. 
There would be among the Astreans the differ- 
ent Species of Astrea proper, forming the close 
round heads,—the Mussa, growing in smaller 
stocks, where the mouths coalesce and run into 
