109 



"Several of the Florida Keys, such as Key West and Indian Key, are 

 already large, inhabited islands, several miles in extent. The interval 

 between them and the mainland is gradually filling up, by a process similar 

 to that by which the islands themselves were formed. 



' ' The gentle landward slope of the reef, and the channel between it and 

 the shore, are covered with a growth of the more branching lighter Corals, 

 such as Sea-Fans, Corallines, &c., answeiing the same purpose as the 

 intricate roots of the Mangrove tree. All the debris of the reef, as well as 

 the sand and mud washed from the shore, collect in this net-work of Coral 

 growth within the channel, and soon transform it into a continuous mass, 

 with a certain degree of consistence and solidity. This forms the foundation 

 of the mud-flats which are now rapidly filling the channel, and must 

 eventually connect the Keys of Florida with the present shore of the 

 peninsula. 



"Outside the Keys, but not separated from them by so great a distance 

 as that which intervenes between them and the mainland, there stretches 

 beneath - the water another reef, abrupt, like the first, on its seaward side, 

 but sloping gently toward the inner reef, and divided from it by a channel. 

 This outer reef and channel are, however, in a much less advanced state 

 than the preceding ones. Only here and there a sand-flat large enough 

 to afford a foundation for a beacon, or a light-house, shows that this 

 reef also is gradually coming to the surface, and that a series of islands 

 corresponding to the Keys must eventually be formed upon its summit. 



"What is now the rate of growth of these Coral reefs ? We cannot, perhaps, 

 estimate it with absolute accuracy, since they are now so nearly completed ; 

 but Coral growth is constantly springing up wherever it can flnd a foot-hold, 

 and it is not difficult to ascertain approximately the rate of growth of the 

 different kinds. Even this, however, would g^ve us far too high a standard; 

 for the rise of the Coral reef is not in proportion to the height of the living 

 Corals but to their sohd parts, which never decompose. Add to this, that 

 there are many brittle, delicate kinds that have a considerable height when 

 alive, but contribute to the increase of the reef only so much additional 

 thickness as their branches would have if broken "and crushed down upon 

 its surface. A forest in its decay does not add to the soil of the earth a 

 thickness corresponding to the height of its trees, but only such a thin 

 layer as would be left by the decomposition of its whole vegetation. In the 

 Coral reef also we must allow not only for the deduction of the soft parts, 

 but also for the comminution of all these little branches, which would be 

 broken and crushed by the action of the storms and tides, and add, 

 therefore, but little to the reef, in proportion to their size when aUve. 



"The foundations of Fort Jefferson, which is built entirely of Coral rock, 

 were laid on the Tortugas Islands in the year 1846. A very intelligent 

 head-workman watched the growth of certain Corals that established 



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