FLORIDA REKFS. 15 
with concentric layers, like true oolite, but of rounded, minute fragments of 
corals cemented together. These rocks are generally of a pure white color, 
though occasionally tinged with gray or brown, — the coloring matter derived, 
no doubt, from decomposed organic matter. At higher levels a fine-grained 
oolite usually comes in with more distinct traces of stratification. Even 
here, however, the oolitic grains are also mostly comminuted fragments of 
coral, as the microscope readily shows. Only here and there the most 
minute oolites seem to be ibruied by several concentric coatings of amor- 
phous limestone. Alternating with these different layers of oolite are 
found small seams, varying from a line to half an inch in thickness, of com- 
pact, homogeneous limestone, so uniform as to ring under the hammer. An 
important fact — inasmuch as it may help to explain the formation of this 
compact limestone — is that a layer of it constantly occurs upon the surface of 
all other rocks forming the keys. It forms a last surface, as it were, even 
upon the highest points of the most elevated keys, a crust following all the 
external sinuosities and irregularities. Evidently, therefore, it cannot have 
been formed under the level of the water, but merely by the action of the 
spray ; and the successive seams of similar compact limestone, intervening 
between the layers of oolite, would indicate the successive surfaces of the 
keys during the progress of their formation. 
A careful survey of the character of the rocks in the keys affords satis- 
factory evidence that they have been formed, at whatever height they 
may rise, by the same action which is now going on upon the reef, — that 
is, by the accumulation of loose materials above the water-level. That part 
of the keys which rises above the level of the water is, therefore, a sub- 
aerial and not a submarine accumulation of floating matter, thrown above 
high-water mark by the tempestuous action of the water. We insist 
upon the fact that the keys furnish in themselves, by the internal struc- 
ture of their rock, the fullest evidence that they have been formed al>ove 
high-water mark by the action of gales and hurricanes, instead of having 
grown as a reef up to the water-level, and been subsequently raised 
to their present height. The evidence of this statement rests upon 
certain facts obtained from observation of the reef itself, at Sand Key 
and the Sambos. 
These facts are as follows : First, that their stratification has all the 
character of a tidal shore stratification ; secondly, that their different layers 
are separated by crusts of compact limestone similar to that now found 
