60 THREE CRUISES OF THE “ BLAKE." 
such as we now find to the west of the Tortugas. Continuing 
the parallel, it is easy to imagine how, little by little, since east- 
erly winds and currents prevailed, жаңа coming from the 
small outside reefs and held in suspension were абай driven 
to the westward, and accumulated finally upon what was then 
the extremity of the great Florida Bank. There they gave rise 
to knolls similar to those upon which the Tortugas have been 
built. From the moment these knolls attained a sufficiently 
favorable elevation for the growth of corals, a western reef was 
formed, holding to the small Florida Reef then existing very 
much the same relations as the Tortugas at the present day hold 
to the great Florida Reef. Again, by the same agencies, the, 
chansi which once undoubtedly ran back into the mud flats 
to the north of the oldest keys were gradually closed, since such 
channels as are still open, in part or wholly, in the more recent 
and westerly portions of the reef, as, for instance, the entrance 
to Key West Harbor, run from the south to the north across 
from the reef to the mud flats. In like manner these channels, 
and those which form an extensive strait in the most recent part 
of the reef between Rebecca Shoal and the Tortugas, will in time 
disappear, and become, in consequence of the extension of the 
mud flats beyond Rebecca Shoal, narrow channels, like those of 
Key West, of the Pine Keys, and of the Marquesas. By this time 
there will also have been formed an extension of the outer reefs 
along the twenty-fathom line, connecting the Tortugas with the 
present reef, and only broken here and there by passages simi- 
lar to those already existing along the reef, by which vessels 
find free access to the middle passage between the reef and 
keys. These channels are kept open by the same tidal agen- 
cies as are now more powerfully at work at the Tortugas, but 
which have gradually diminished in force from the Pme Key 
Channel to the northward. The depth of the passage between 
the Tortugas and Rebecca Shoal allows a larger and stronger 
body of water to pass through that channel than through any 
other. 
It has been clearly proved by Hunt that the extensive flats to 
the northward of the keys have been formed by the agency of 
the tides, the whole triangular space between the Rebecca Shoal 
