110 THE TORTUGAS AND FLORIDA REEFS. 



the time when the Florida Reef formed but an insignificant point south of the line 

 extending from Cape Sable to Key Biscayne, until it reached the extremity of the 

 present continuous mud flat, about ten miles to the west of the Marquesas. As is 

 well shown on the Coast Survey maps, the mud flats, keys, and reef dip as a whole 

 to the southwest, as does also the Florida plateau to the westward of the Marquesas. 

 It is only upon such parts of this plateau as from some cause or other have attained 

 a sufficient elevation to allow corals to flourish, that the reef may be expected to 

 extend. The knoll rising above the general level upon which the Tortugas have little 

 by little been built up, is such, an area, and such also is the patch to the westward of 

 the Tortugas, upon which, as I shall show hereafter, an incipient coral reef is already 

 forming, at a depth of a little less than twenty fathoms (Plate I.). It is not difficult 

 to go back to a time when the great mud flats of Florida did not exist. In their 

 place was a steep slope, such as we now find to the west of the Tortugas. Follow- 

 ing the comparison backward, it is easy to imagine how, little by little, from the 

 existence of the prevailing easterly winds and currents, the materials coming from 

 the small outside reefs and held in suspense were little by little driven to the west- 

 ward, accumulating 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 at once 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 ; 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, running 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 on 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 

 similar 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 agencies as are now more powerfully at work at the Tortugas, 



