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



of more delicate structure. The upper limit of the band is gene- 

 rally well defined, with slight undulations, though occasionally 

 (especially in the northern part ot the Key) it swells up until it 

 nearly reaches the top of the bed. Next above comes a layer 

 abounding in shells, which finally graduates into an almost 

 unfossiliferous rock. A bedding-line, about half an inch in 

 thickness, marks the upper limit of this bed with a narrow 

 brown line. The fourth method of fossilization is most common 

 in some sections of the upper two bands, but the fifth (with 

 Calc Spar) generally prevails in the lower two. In the latter 

 case fossils are often found projecting from a water-worn sur- 

 face, identical with the original shell in form and markings. 



§ 10. Veins. — The band of Sand Limestone is more or less 

 intersected by fine vertical veins, usually from two to twelve 

 inches in depth ; sometimes also a layer of the bed, reaching a 

 few inches below the bedding-line, is completely cut up into 

 small fragments or angular pebbles by a network of fine and 

 short veins running irregularly in every plane. These veins 

 are generally covered by the bedding-line, and filled with its 

 material, but never with that of the next bed, B. Towards the 

 northern end of the bed, however, a light green clay often occu- 

 pies them. Sometimes many shallow depressions occur in the 

 surface of the bed, and the bedding-line is abruptly interrupted, 

 while angular pebbles, from two to six inches in length (often 

 having a fragment of the bedding-line material attached), are 

 scattered over the top of the bed. In one locality of the north- 

 ern level, a layer of conglomerate, one or two feet in thickness, 

 is intercalated between the beds A and B, its cement consisting 

 of indurated green clay and its pebbles of the material of the 

 Sand Limestone. 



§ 11. History. — The total thickness of this bed is indefinite, 

 since its lower limit lies everywhere beneath the sea-level. At 

 the point at which observation commences, the whole subsiding 

 area was covered with a close and uninterrupted madrepore 

 reef. From the few species and individuals of the shells in the 



