14 FLORIDA EEEFS. 



lying each other in such a manner as to present the same irregularity which 

 is found in all drift stratification. Layers upon layers are seen resting 

 unconformably, dipping in different directions so as to present all the modi- 

 fications which may be observed in torrential stratification, each layer fol- 

 lowing, with more or less regularity, the course of the flood under which it 

 has been accumulated. 



This dip varies for several degrees, but rarely exceeds 7 to 8 degrees 

 upon any extensive surface where the sea has had full play. In enclosed 

 bays or narrow channels, however, the materials accumulate at much steeper 

 angles. In sheltered places, slopes of coral sand are formed at angles of 

 20 or even 30 degrees. At the Tortugas, Lieutenant Wright pointed out to 

 us accumulations of coral sand sloping at an angle of more than 33 degrees, 

 exceeding decidedly, therefore, the steepest slopes of silicious sand-banks. 

 This circumstance corroborates the assertion of Mr. Darwin, that coral sand 

 possesses an adhesive property which enables it to accumulate at higher 

 angles than other loose materials, owing, perhaps, to the viscosity of the 

 animal matter still pervading the coral fragments. 



By a process not yet fully understood, but to which we shall return here- 

 after, these loose collections are gradually cemented into solid rock, 

 presenting the most diversified appearance, according to the substances 

 of which it is composed. Then we find a coarse breccia, consisting of 

 larger fragments of corals and shells, enclosing sometimes coral boulders; 

 and this is the sort of rock which generally overlies the immediate surface 

 of that portion of the keys which has been formed by the progress of the 

 reef, growing in situ. Such rock was seen among the foundations of 

 the new lighthouse at Sand Key, where the large boidders are very numer- 

 ous, and seem almost as fresh as if they had been lying on the spot but 

 for a few years. It may be, indeed, that during the hurricane of 1846, the 

 whole cap of the reef was renewed at that spot. 



Similar rock, but of more breccia-form character, containing remains of 

 shells in greater quantity than I have seen elsewhere in the coral rock, was 

 brought up from excavations made in twelve feet water, when the founda- 

 tions of Fort Taylor were laid in Key West. The same coarse-grained rock 

 is observed, at low water, in many places where the tidal action has worn 

 away parts of the primitive rock of the keys. Above these materials are 

 generally found layers of what seems, at first sight, a coarse-grained oolite, 

 but upon closer examination it is seen to consist, not of grains of limestone 



