On the Geology of the Key of Sombrero, W. I. 265 

 Bed C. 



§ 14. Character. — This has been entirely removed, like the 

 succeeding beds, north of the slope between the two levels. 

 The variation of its thickness in the southern level is from three 

 to six feet, and its average five feet; but it increases rapidly to 

 ten feet towards the south. This is the only increase in the 

 present remnant. The bed consists of Sand limestone, of Bulla 

 limestone, or of two bands, the lower being one or two feet in 

 thickness and composed of Bulla limestone or a mixture of the 

 two. The presence of Madrepore limestone also in some sec- 

 tions near the northern and southern extremities of the remnant 

 suggests the former predominance of coral towards those points 

 (in the portions now carried away), as in B. 



Its upper limit is as difficult to distinguish as its lower, par- 

 ticularly when the materials of the tangent surfaces of C and D 

 are identical so that the two beds coalesce. In many sections 

 a great many seams, running horizontally, or with a slight obli- 

 quity, divide the rock, within a foot of this probable junction, 

 into layers or discs a few inches in thickness. The fourth mode 

 of fossilization prevails. 



§ 15. History. — It is probable that the bedding-line material, 

 which caps the northern part of B, does not signify a closed 

 lagoon and the reproduction of calcareous mud, but that an 

 empty seam intervened between the two beds, which has been 

 filled with the lagoon-mud from the top of T> through the 

 fissures of the second system. 



Deposits of shells and of sand of increasing fineness were 

 successively formed, in this as in the former period, and pro- 

 bably with coral-reefs in the shallower waters near the shores. 



From the absence of a system of fissures betokening violence 

 and tension, and from the marked scarcity of coral-reefs in the 

 beginning of the next period, it may be inferred that the ele- 

 vation by which this subsidence was closed was of compara- 

 tively small range, even less than that of the last, since it was 



