ORGANIC AGENCIES. 113 



added is probably not less than 12,000 to 15,000 square 

 miles, and the area which will be added at least half as 

 much more. 



Cooperation of Other Agencies. 



We have seen that the reefs of Florida are unique. It 

 seems certain, therefore, that they were formed under 

 unique conditions. The things to be accounted for are 



1. The constant growth of land ; and, 2. The formation 

 of barriers where there was no subsidence. 



1. The constant growth of land southward shows 

 that there was a continual extension southward of the 

 conditions of coral-growth, i. e., of moderate depth. In 

 other words, there must have been a gradual extension 

 southward of the submarine bank, on the edge of which 

 the corals grew. If there had been a preexisting bank, 

 obviously the corals would have grown as only one reef 

 on its outer edge ; the formation of successive reefs, one 

 beyond the other, proves that the shallow bank on which 

 they grew must have extended successively in that direc- 

 tion. 



Thus much seems certain, but the cause of the exten- 

 sion is more uncertain. It is probable, however, that the 

 bank was formed and extended by sedimentary deposit by 

 the Gulf Stream.* 



Thus, then, the extension of the Peninsula of Florida 

 in recent times has been the result of the cooperation of 

 several agencies : 1. The Gulf Stream built up from deep- 

 sea bottom a bank, and extended it by the same process. 



2. The corals took up the work by forming successive bar- 

 * At one time the sediments were supposed to be mechanical sedi- 

 ments from the Gulf rivers, especially the Mississippi. But now they 

 are believed to be organic sediments, partly brought by the Gulf 

 Stream from other coral banks, e. g., the Yucatan bank, but mainly 

 formed in place by the growth of successive generations of deep-sea 

 shells ; the Gulf Stream bringing only the conditions of heat and 

 food necesary for rapid growth. 



LE CONTE, GEOL. 8 



