On the Geology of the Key of Sombrero, W. J. 267 



exhibits a great number of pebbles and blocks, all more or less 

 angular and tabular, up to one foot in diameter, always con- 

 sisting of Sand or Lagoon limestone, and often having portions 

 of the bedding-line attached. Two rarer varieties of these peb- 

 bles are worthy of mention. The first possess a compactness 

 excelling that of any of the rock in situ. They have a con- 

 choidal fracture, are translucent on their edges, and have a spe- 

 cific gravity =2.56 — 2.69. The second are peculiar for their 

 color, which varies from greyish-white to deep brownish-black, 

 through shades of brown. When these transitions occur in a 

 single pebble it sometimes resembles a half burned piece of 

 Anthracite. In one pebble faint traces of lamination were 

 detected. The color is due to a number of black points scat- 

 tered through the material (Lagoon limestone), which remain in 

 an insoluble carbonaceous sediment, on the solution of a pebble 

 in Hydrochloric acid, while Sulphuretted Hydrogen is strongly 

 perceptible in the gases evolved. Their fracture is even to sub- 

 conchoidal, they are sometimes translucent on their edges, and 

 their specific gravity ranges up to 2.73. These varieties of 

 course show the greater fineness of the deposits, in some por- 

 tions of the area, and the intermixture of organic matter. 



§ 17. Veins (of the Second System). The superficial portion 

 of the Lagoon limestone is very commonly intersected by reti- 

 dilations of fine veins (Fig. 2) as in Bed A. Other fine veins, 

 from one-eighth to half an inch in width, mostly vertical, run 

 down to all distances from the bedding*line, commonly stop at 

 or before the bedding-line of A, and sometimes reach the sea- 

 level where observation is cut off. They gradually thin out as 

 they approach their terminations, and are very abundant in 

 some sections. Some of these veins, however, of various 

 lengths, range in width from several inches to two feet, while 

 others possess the form of vast basins or receptacles in the sur- 

 face of the bed. The depth of the latter varies from two or 

 three feet down to an unknown distance beneath the sea-level. 

 Their areas in the southern level, wherever they could be mea- 



