40 FLORIDA REEFS. 



the keys and the main-land. In still more remote ages the present mud 

 flats may become swamps, elevated above the reach of the tide-waters, 

 like the Everglades ; and this process may perhaps be extended to the 

 present ship-channel. But nnless some great revolution in nature modifies 

 the present relative level between land and sea, it may safely be maintained 

 that the present outer reef is the final southern boundary of the North 

 American continent, and that the sooner a system of lighthouses and 

 signals is established along the whole reef, the better; for this is, after all, 

 the shore which is to be lighted, and not the range of keys which is within 

 the reef In relation to the western range of keys, and the western ex- 

 tremity of the reef, we may expect, in course of time, to see the depression 

 between the Marquesas and Tortugas gradually lessened by the increase of 

 the reef, so that the westernmost group of islands may finally stand in as 

 close connection with the keys more to the Avest as they now bear to each 

 other, the passage between them being reduced to as narrow a channel as 

 Boca Grande, between the Marquesas and the Mangroves. 



The shoals west of Cape Sable may, undoubtedly, also increase in extent 

 westward ; but how far the currents from the northwest may limit this 

 accumulation, in connection with the changes which the currents themselves 

 may undergo by the increase of the kej^s to the west, it is beyond the 

 power of human foresight to determine. 



These practical results, — for so we venture to call the general conclusions 

 last presented, — although they are purely scientific deductions from general 

 principles, may satisfy the most obstinate supporters of the matter-of-fiict 

 side of all questions, of the advantages of scientific illustrations in the daily 

 walks of life, and also justify the course which has been followed with so 

 much success by the Coast Survey, in combining the strictest scientific 

 methods with its practical operation. 



Kespectfully submitted, 



L. AGASSIZ. 

 Professor A. D. Bache, 



Superintendent of the Coast Survey. 



