FLORIDA EEEFS. 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 formed 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 tlie 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 above 

 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 



