268 On the Geology of the Key of Sombrero, W. 1. 



sured, vary from eight to sixty square yards, and in the northern 

 level, where they especially abound, from three to fifty. Their 

 sides are usually either vertical or slope at various angles down- 

 wards and inwards, but sometimes they burrow a few feet 

 underneath the Lagoon limestone. They may therefore in 

 general be described as funnel-, wedge-, trough-, pan-, or basin- 

 shaped cavities sunk in the surface of this bed, of which those 

 of the northern level have of course been decapitated by denu- 

 dation. 



The finer veins are mostly filled with the bedding-line mate- 

 rial, but the larger with what has already been styled the Green 

 Conglomerate. The smaller pebbles of this rock are more usu- 

 ally rounded than the larger, but they have already been 

 described. The cementing material is of three kinds. The 

 first is the ordinary bedding-line material. The second is 

 a soft light green marl of so fine a grain that when rubbed 

 between hard surfaces no siliceous grittiness is observed, 

 but only brittle and easily crushed particles of Carbonate of 

 Lime. Often on weathered surfaces either it is mottled with 

 rusty-red spots, or its color is more or less changed to a light- 

 brown. The third kind is more common than either of these 

 two, being their mixture in varying proportions. It is harder 

 than the second, and its colors equally various. Its specific gra- 

 vity = 2.46. Two analyses are here appended. 



I. II. 



Water 4.25 4.11 



Silica 22.56 



Protoxide of Iron.. .56 

 Alumina 12.19 



Insoluble silicate 35.89 35.31 



Soluble silica 57 .18 



Carbonic acid 20.91 20.93 



Sulphuric acid trace trace 



Lime 27.58 28.46 



