60 THREE CRUISES OF THE " BLAKE." 



such as we now lind to the west of the Tortugas. Continuing 

 the parallel, it is easy to imagine how, Httle by little, since east- 

 erly winds and currents prevailed, materials coming from the 

 small outside reefs and held in susjDension were gradually 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 

 channels 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, comiecting 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 Pine 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 



