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 woi'n 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 tlie reef-building corals cannot live. But, as the process 
is very gi-adual, 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 
