THE FLOKIDA REEFS. 59 



ing uninterruptedly, a few feet below the surface of the water, 

 to the north Weird of the line of keys. (Fig. 34.) The northern 

 part of this flat, from Cape Sable to Key Biscayne, is fringed on 

 the southeast face by the line of narrow keys reaching from 

 Cape Florida to Bahia Honda. (Fig. 34.) In the oldest part of 

 the reef, the bay to the north of the keys, the waters of which 

 once undoubtedly covered the whole space between Pine Keys 

 and Cape Sable, has little by little been filled up and transformed 

 for the greater part into the wide, shallow mud flats now ex- 

 tending over that area. Next comes, from the Pine Keys to 

 Rebecca Shoal, a comparatively more recent portion of the reef, 

 in which the noiihern extremities of the keys rise somewhat 

 higher above the general level of the mud flats. 



These two adjoining regions of flats and keys run paraUel to 

 the main reef, at a distance of from one to nine miles from the 

 outer line of reefs, — the reef, that is, 2^ar excellence, over the 

 whole surface of which the living corals still prosper. Farther 

 yet to the westward, at a distance of fifteen miles from the west- 

 ern extremity of the outer parts of the reef, rise the Tortugas. 

 In this group a condition of things prevails at the present day 

 which must have been repeated over and over again, from 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 which 

 extends to the westward of the Marquesas. It is only upon 

 such parts of this plateau as from some cause or other have at- 

 tained a sufficient elevation to allow corals to flourish that the 

 reef may be expected to extend. Such an area is the knoU ris- 

 ing above the general level upon which the Tortugas have little 

 by little been buflt up ; and such also is the patch to the west- 

 ward 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. (Fig. 38.) 



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. 



