THE FLORIDA REEFS. 59 
ing uninterruptedly, a few feet below the surface of the water, 
to the northward 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 northern extremities of the keys rise somewhat 
higher above the general level of the mud flats. 
These two adjoining regions of flats and keys run parallel to 
the main reef, at a distance of from one to nine miles from the 
outer line of reefs, — the reef, that is, par 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 knoll ris- 
ing above the general level upon which the Tortugas have little 
by little been built 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, 
