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 unless 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 tlie 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 depi^ession 
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 west 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 keys 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 genei'al 
principles, may satisfy the most obstinate supporters of the matter-of-fact 
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 pi'actical operation. 
Respectfully submitted, 
L. AGASSIZ. 
Professor A. D. Bache, 
Superintendent of the Coast Surve>/. 
