GEOLOGY. 273 



When a growing reef has attained its maximum height, or reached 

 the level of low water, a new process begins, consisting chiefly in the 

 accumulation of loose materials upon its summit. Large coral bowl- 

 ders are thrown up, and gradually ground into fragments, coral gravel 

 and sand, and finally deposited in more or less regular beds, presenting 

 all the complications of torrential stratification, which are finally ce- 

 mented, by the infiltration of amorphous limestone, into compact coral 

 rock. When the materials are combined in a coarse state of decom- 

 position, they form a kind of coral breccia ; but when cemented after 

 they have been reduced to small globular fragments, they constitute a 

 sort of oolite, and even compact limestone, when the deposit is formed 

 by precipitation. Thin layers of such compact limestone occur fre- 

 quently as dividing seams in the larger masses of oolite ; and there is 

 everywhere such a layer of compact limestone upon the surface of all 

 coral rocks rising above the level of the sea ; a circumstance which 

 seems to indicate that such layers are not formed under a permanent 

 sheet of water, but must be the result of action of gales and the spray. 

 This is the more probable since this superficial crust is nowhere hori- 

 zontal, but follows all the irregularities of the soil. 



If it were asked how corals which, during their growth, have with- 

 stood so effectually the violence of the sea, become such an easy prey 

 to the waves after the reef has reached the surface of the water, it would 

 require only to point at the innumerable boring shells and worms which 

 establish themselves in the dying part of their stems, and at the brit- 

 tleness arising from these perforations, to satisfy every careful observer 

 that the peculiar mode of life of these boring animals is a provision of 

 nature subservient of the secondary purpose of the corals, to furnish 

 materials for the increase of the solid parts of our globe. 



Along the outer reef of Florida, and in the main range of keys, many 

 islands may be selected and described, in such an order as to form a nat- 

 ural series, from a living reef, without a dead fragment upon its edge, to 

 an extensive island, apparently formed entirely of coral rock or of oolite, 

 or even of compact limestone ; but, in reality, presenting only a cap 

 of such dead materials, overlying a true reef, once living, and now 

 buried under its own fragments. The circumstance that the main 

 keys and the shore blufls, which have been formed successively, rise to 

 the same height above the level of the ocean, is an unquestionable evi- 

 dence that the ground, over which the general reef of Florida extends, 

 has undergone no change of level ; that it has neither been raised nor 

 subsided. This evidence may be carried further, by comparing, also, 

 the Everglades with their intervening ridges and hammocks, which 

 are, in reality, inland keys and islands, similar to the main keys and 

 Mangrove islands, formed in the same manner as those now surrounded 

 by the sea, and which, by the uniformity of their level, furnish addi- 

 tional evidence that the whole region has been stationary ever since 

 corals began to grow in those latitudes. 



_ The question of most interest to the Coast Survey, in the examina- 

 tion of these coral formations, was that of the probability of another 

 reef being formed at some future time beyond the present outer reef. 

 Until some conclusion could be arrived at in regard to this question, 



