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 end, 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. 



