20 FLORIDA REEFS. 
rated from the layers below. There could hardly be a more conclusive 
evidence that the keys have been formed above high-water mark. These 
islands take their name from the pine woods with which they are partially 
covered. A beautiful palmetto flourishes there also, considerably smaller 
than the cabbage-palmetto of the Southern States, and, as far as I can ascer- 
tain, an undescribed species. It is remarkable for its smooth stem, and for 
the silvery lustre of the lower surface of its leaves. The great Avidth of the 
mud flats surrounding the Pine Islands, and the peculiar direction of that 
group of keys, suggest the idea that they were formed when the Gulf dis- 
charged its waters more freely from the north into the Gulf Stream, and 
when the range of keys extended only abreast of Cape Sable. This supposi- 
tion is sustained by the fact that the whole reef dips to the west. The 
outer reef rises to the same level as the Marquesas ; but west of Key West, 
as far as the Tortugas, the main range of keys has not yet reached the same 
height above the sea level as the eastern parts of the reef We may there- 
fore suppose, that the group of Key West, which is again as high as Key 
Largo, stood to the Key Vaccas in the same relation as the Tortugas now 
stand to the Marquesas, leaving a broad, deep depression between them. This 
has gradually been filled by drifting sand, just as the Marquesas, by like accu- 
mulations, are spreading toward the Tortugas. The groups of Boca Chica, 
of which Key West and Saddle Bunch Key are the largest, have again 
a more westerly direction, corresponding with the general curve of the reef 
These islands share the character of the eastern reef, but have also some 
features of the Pine Islands. Their lower strata consist of coral boulders 
and coral breccia, as was ascertained during the excavation preparatory to 
the foundation of the fort of Key West. Their upper layers, however, 
namely, all that portion which rises above low-water mark, rest upon 
a coarse oolite, above which follow layers of finer grain, overlaid by very 
thin layers of muddy limestone. These surface layers resemble those of 
the Pine Keys, especially on their northern and northeastern shores, which 
are very level, spreading out into submarine mud flats, while the southern 
shore is more abrupt, and worn by the action of the tides. The stratification 
of Key West is not, however, so regular as that of the Pine Islands, nor do 
the strata dip so uniformly to the north as in tlie latter. They incline, 
indeed, in all directions on the western shore, showing that the trend of the 
tidal deposits has been constantly shifting. The cross-stratification is 
nowhere better seen, in the main range of keys, than on the westernmost 
