On the Geology of the Key of Sombrero, W. I. 263 



to be represented by a few isolated patches of Madrepore or soli- 

 tary spheres (in one case three feet in horizontal diameter), of 

 Mceandrina, Bulla limestone taking its place. Within from six 

 inches to three feet of the upper limit the material is either 

 Sand or Bulla Limestone. The following series will thus repre- 

 sent the composition of this bed. 



1 — Sand limestone, 



2 — Sand and Bulla limestones intermixed, 



3 — Bulla limestone, 



4 — Ditto, slightly intermixed with Coral, and 



5 — Coral limestone : 

 of which JSTos. 4 and 5 are wanting in the centre of its present 

 remnant, but are successively introduced towards the N., E., 

 and S., and of which Nos. 1, 2, 3, and 4, seem to decrease suc- 

 cessively in the same directions. In parts of the northern level, 

 and there alone, the upper portion of this bed, about two feet 

 in thickness, is separated from the main body by a horizontal 

 seam. The upper limit of the bed is marked by a bedding- 

 line, three-eighths of an inch in depth, only in this level ; while 

 in the southern level it is possible to distinguish this limit, on a 

 section, only where the lower part of the next bed above 

 abounds in fossils which contrast with its own unfossiliferous 

 Sand limestone, or vice-versa. The fourth method of fossiliza- 

 tion prevails, but there are rare instances of the second. 



§ 13. History. — After the elevation last mentioned a new 

 period of subsidence began, the sea-water permeated the bar- 

 rier (probably through fissures), a shallow but deepening lagoon 

 was produced within, and, perhaps on account of the gentle 

 attrition of the ripples of the quiet waters, a calcareous mud was 

 deposited as an impalpable sediment, filled the fissures and over- 

 spread the whole bottom, i. e. the upper surface of Bed A. 

 This must have been forming during a considerable period and 

 at the same time consolidated, since there is little adhesion 

 between the material of this " bedding-line" and that of B. 



Subsequently the green clay was formed, which will be par- 



