276 On the Geology of the Key of Sombrero, W. I. 



an uplift of the bottom from a considerable depth to above or 

 near the sea-level), suggests that the peak was thus brought 

 within the reach of surface action, or that a passage was thus 

 in some way opened for the introduction of its finest detritus in 

 suspension. The obstruction of this passage, or the investment 

 of the peak with a fringing reef, may have prevented the for- 

 mation of the clay in other shallow-water periods and its inter- 

 mixture with the overlying limestone. The distribution of the 

 detritus would also depend upon the relative position of the pre- 

 sent surface and the peak, with reference to the prevailing 

 winds, the character of the inlets, the entrance of the ground- 

 swells, and the shape of the lagoon. 



We have now arrived at a period at which there is a blank 

 in the record, closing the submarine movements. We have 

 little clue by which to determine the original thickness of bed 

 F, or the number and thickness of the subsequent beds. It is 

 probable that the later beds, if any, were not attended by uphea- 

 vals of great range and violence, since they produced no system 

 of fissures. 



§ 23. Sub-aerial Movements. — The following is a summary of 

 the subsequent and sub-aerial movements. 



Sixth elevation, to a point far above the sea-level. The for- 

 mation shattered into a system of joints. 



Seventh subsidence. Erosion of joints. Formation of a 

 superficial guano-deposit,* and its transference into the widened 

 joints. Partial pseudomorphic change in the limestone beds. 

 Subsidence to sea-level and denudation of surface. 



Seventh elevation, to a less height than by the sixth, but to 

 at least fifty feet. Formation shattered anew. 



Eighth subsidence. Erosion of joints. Formation of a 

 second guano deposit and its transference into the joints. For- 



* In one vein of roek-gnano of the northern level, numerous perfect impressions 

 or remains were found of a species of Choanopoma, identical with that now living, 

 and the only existing land-shell on the Key. The impression of an extinct Pupa 

 (Slrophia) also occurred. 



