AGE OF CORAL REEFS. 175 
CHAPTER XII. 
AGE OF CORAL REEFS AS SHOWING PERMANENCE OF 
SPECIES. 
A Few miles from the southern extremity of 
Florida, separated from it by a channel, narrow 
at the eastern engy but widening gradually to- 
ward the west, and rendered every year more 
and more shallow by the accumulation of mate- 
rials constantly collecting within it, there lies a 
line of islands called the Florida Keys. They 
are at different distances from the shore, stretch- 
ing gradually seaward in the form of an open 
crescent, from Virginia Key and Key Biscayne, 
almost adjoining the main-land, to Key West, at 
a distance of twelve miles from the coast, which 
does not, however, close the series, for sixty miles 
farther west stands the group of the Tortugas, 
isolated in the Gulf of Mexico. Though they 
seem disconnected, these islands are parts of a 
submerged Coral Reef, parallel with the shore of 
the peninsula and continuous underneath the 
water, but visible above the surface at such points 
of the summit as have fully completed their 
growth. 
