FLORIDA REEFS. 21 



promontory of Key West. Upon this island, also, the intervening seams of 

 compact limestone between layers of oolite are especially numerous ; and 

 here the superficial crust is more continuous than elsewhere, frequently con- 

 sisting, beside the compact limestone, of small fragments of oolite rock, with 

 patches here and there of oolitic sand. Occasionally such a mixture 

 entirely fills large excavations, — excavations worn by the tides in former 

 ages, and filled again by later deposits. Along the northern shores, where 

 flat beds, similar to those of the Pine Islands, extend under the mud flats, 

 their dip is but from two to three or four degrees. Near the fort, where the 

 oolitic beds have been quarried for building purposes, they have a dip of 

 seven or eight degrees, the stratification being clearly indicated by suc- 

 cessive seams of compact limestone, and also by the projecting edges of the 

 seams of coarser oolite rising from the vertical walls of the artificial excava- 

 tions. The so-called Mangrove Islands, west of Key West, constitute 

 a group of low keys connected by mud flats, similar to all the other man- 

 grove islands north of the Pine Islands and of Key West. They may be 

 considered a prolongation of the extensive flats encircling the whole group 

 of keys from the upper Bahia Hondas to Boca Grande. They are separated 

 from Key West by a navigable ship-channel, running north-northwest like 

 those intersecting the Pine Islands, showing that, notwithstanding the more 

 westerly course of Key West and the whole Boca Chica group, all this part 

 of the reef has a common character. The westernmost groups, the Marque- 

 sas and Tortugas, although they lie in the direction of the main range of 

 keys, have again another character. 



The islands forming this group are among the most interesting of the 

 whole reef, because, without the phenomena of subsidence to which the 

 Atoll or Lagoon Islands of the Pacific are due, they nevertheless closely 

 resemble them in character. The Atolls in the Pacific are formed by the 

 sinking of some island around which corals have established themselves. 

 By the growth of the corals the reef rises as the foundation subsides, and 

 finally reaches the surface as a ring often broken by channels and enclosing 

 a harbor. In such cases the wall of the reef is precipitous and often sinks 

 to a depth at which the reef-building corals cannot live. But, as the process 

 is very gradual, the dead base of the reef affords a foundation for the living 

 portion until the latter finally reaches the water level. 



It is my belief that the Tortugas (though no change has taken place in 

 the sea-bottom on which they rest) were built up by the coral growth 



