On the Geology of the Key of Somhrero, IF. I. 257 



Bulla limestone is a congeries of small shells, chiefly Bulla, 

 with sometimes a few corals interspersed. Color white. Struc- 

 ture compact. Fracture uneven. Specific gravity =2.20 — 2.28. 



Sand limestone mainly consists of a compact limestone, inter- 

 mixed with a few individuals of Bulla, etc. Corals rarely 

 or never occur. Color white, but generally not so pure as in 

 the former varieties. Fracture uneven to even. Specific gra- 

 vity= 2.36— 2.55. 



Lagoon limestone consists of a greyish (sometimes pinkish), 

 hard, fine-grained, tough limestone, susceptible of polish. Frac- 

 ture even to sub-conchoidal, and smooth. Detached fragments 

 or protuberances of all the limestones clink musically beneath 

 the hammer ; but this sonorous property, extreme compactness, 

 and excellent preservation of the fossils, especially characterize 

 this variety. The fossils are rare, (a few shells, chiefly Bulla, 

 but no corals,) and often wanting. Fracture even to sub-con- 

 choidal, and smooth. Specific gravity =2.55 — 2.71. 



The limestones increase in compactness from the first to the 

 fifth. The only trace of crystalline character is the Calc Spar, 

 which occasionally fills the cavities of the fossils. An oolitic 

 structure has never been observed. 



The upper surface of certain beds consists of a thin layer or 

 scale of limestone differing from the preceding. Its color is 

 usually light to dark brown. Fracture even. Specific gravity 

 =2.67. It is always very fine-grained and compact, sometimes 

 massive, but often displays on its vertical section, a series of 

 fine horizontal lines of lamination. It is evidently a deposit of 

 an impure carbonate of lime from at least partial solution in 

 water; and as it occurs in layers intervening between the beds, 

 thus marking their stratification, its section (which is always 

 quite distinct in contrast with the white limestones) may be 

 styled the bedding-line. It always adheres to the bed on 

 which it lies, and is partially incorporated with it. 



A few series of specific gravity determinations have been 

 made upon specimens of the limestone of each bed. selected on 



