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 width of the 

 mud flats surrounding the Phie 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 the 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 



