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



waters continually aerated and to keep the corals washed 

 clean from fine sand, the shifting nature of the sandy bottom, 

 occasionally s'irred up by the ground-swells, the occasional 

 slight freshening of the lagoon by rains, etc. The presence 

 of Calc-Spar in this bed, as well as in A, also suggests the 

 more frequent entrance in these two periods of the ocean- 

 waters, which must have had a greater content of carbonic acid 

 and solvent power than the waters of a closed lagoon. During 

 the whole of this subsidence the water remained shallow and 

 thus especially favorable to an abundance of shells and corals. 

 The elevation which next occurred, the fifth in the series, there- 

 fore reached a small height, as it was confined below the sea- 

 level. 



Bed F. 



§ 21. Character and History. — Only a remnant of the lower 

 portion of this bed remains, resting in place upon E and vary- 

 ing from one to three feet in thickness. This seems to consist 

 chiefly of reefs of Madrepore, like the lower portion of E or 

 of A. 



This abundance of corals, and the absence of lagoon mud, and 

 a system of fissures in connexion with the surface of E, prove 

 that the range of the fifth uplift was small and did not much 

 interrupt the open communication with the sea ; though suffi- 

 ciently so, perhaps, to occasion a more quiet lagoon and the 

 substitution of Madrepore for the massive kinds of coral. 



CONCLUSION. 



§ 22. General Remarks. — It has been assumed in the fore- 

 going statements : 



First, That every upheaval consisted of a single thrust. It is 

 less probable but possible, however, that one or more were 

 caused by a succession of shocks. 



Secondly, That every subsidence was an imperceptibly slow 



