On the Geology of the Key of Sombrero, W. I. 271 



panying an uplift of so great a range are denoted by the mul- 

 titude and length of the fissures, though, as in A, the superficial 

 network has doubtless been produced by contraction on atmo- 

 spheric exposure. 



Bed E. 



§ 19. Character. — The upper limit of this bed has escaped 

 denudation in only one locality, viz. beneath and in the vicinity 

 of the remnant of bed F (Fig. 1). Its thickness is there from 

 5£ to 9 feet, but elsewhere it varies from a few inches to 13£ 

 feet, averaging 10 feet. It seems to thicken considerably 

 towards the west or northwest, and must probably have had a 

 thickness of 20 or more feet on the west cliff before denudation. 

 The bed wholly consists of Porites limestone, perhaps most fre- 

 quently capped with a few inches of Sand limestone. In the 

 lower portion of the bed the color is snow-white and the struc- 

 ture loose and crumbling, while the higher is more compact, 

 hard, and crystalline, with a bluish tinge. 



The shells belong to many species, and the spines of Strom- 

 bus are often rounded or water-worn. A study of this bed 

 might throw light upon the molluscs of the present West Indian 

 coral banks, as the solitary dispersion of the Strombits, the con- 

 trary gregarious habits of the Bullidoe, the frequent insertion 

 of the Solenidm in masses of Astraa, etc., are visible at a 

 glance. A tiny cylinder, connecting the casts of Tellina, fre- 

 quently indicates the hole perforated by some borer through 

 the original shell. Occasionally the thickest portions of the 

 shell of Strombus are perfectly preserved, but generally with its 

 superficies converted into a soft snow-white mass. One spe- 

 cimen of Echinus has been found. 



The corals predominate in quantity over the shells. The 

 reefs of Madrepore are the bulkiest, but the spheres of the mas- 

 sive corals sometimes reach 3 to 3j feet in horizontal diameter. 

 Usually, however, though numerous, they are of small size, 3 



