160 ORGANIC AGENCIES 



At one time * the writer thought that the bank was formed and ex- 

 tended by mechanical sediments brought by the Gulf Stream, and de- 

 posited on the inner side of its curve ; but Alexander Agassiz has 

 shown f that the sediments are more probably organic, and the bank 

 was formed partly by such sediments brought by the Gulf Stream 

 from other coral banks in the Caribbean Sea, but mostly built up in 

 situ by the accumulation of shells of successive generations of deep- 

 sea animals, the Gulf Stream bringing only the conditions of rapid 

 growth in the form of warmth and abundant food. 



It is probable, therefore, that the southern portion of the peninsula 

 of Florida is due to the co-operation of four or five different agencies, 

 viz. : 1. The Gulf Stream building up a submarine bank to the dotted 

 line n' n\ Fig. 128, within 100 feet of the surface; 2. Then corals 

 building up to the surface ; 3. Then waves raising it twelve to fifteen 

 feet above the surface ; 4. And, finally, debris from the peninsula, on 

 the one side, and the reef and keys on the other, filling up the inter- 

 vening channels, and afterward raising the level of the swamps or 

 Everglades thus formed; 5. In this last process the mangrove-trees 

 have assisted. 



2. The reefs of Florida are barrier reefs. Barriers are usually sup- 

 posed to indicate subsidence. The Pacific barriers, according to Dar- 

 win, commenced as fringes and became barriers by subsidence. But 

 in Florida there has been no subsidence. They did not commence as 

 fringes. The probable explanation is this : Corals will not grow in 

 muddy water. On a gently-sloping shore with mud bottom, such as 

 probably always existed on the southern shore of Florida, a fringing 

 reef could not form, because the bottom would be always chafed by the 

 waves and the water rendered turbid. But at a distance from shore, 

 on the edge of the bank, where such a depth was attained that the 

 waves no longer chafed the bottom, a barrier would form, limited on 

 the one side by the muddiness, and on the other by the depth, of the 

 water. Also the proximity of the Gulf Stream, carrying warmth and 

 food, would contribute to the same result. 



It will be observed that this view of the formation of barriers differs 

 from both that of Darwin and that of Murray. It has been adopted 

 by Captain Guppy for some of the barriers of the Pacific. \ 



Shell-Deposits. 



Eivers carry carbonate of lime in solution to the sea (p. 82). In 

 some bays, where large quantities of this material are carried by rivers 



* American Journal of Science, Second Series, vol. xxiii, p. 46, 185*7. 

 f Memoirs of the American Academy of Science, vol. xi, p. 107. 

 X Nature, vol. xxxv, p. 77, 1886. 



