On the 'Geology of the Key of Sombrero, W. L 259 



Fourthly, The inner and outer casts alone remain, the cavity 

 being empty. This is the most common of all the modes, with 

 both shells and corals. 



Fifthly, The cavity between the casts has been partially or 

 entirely tilled by infiltrating solutions, sometimes with Calc 

 Spar and sometimes with Phosphate of Lime. A collection has 

 been made of most of the fossil species of this formation, all of 

 which are supposed to be recent. Its material is probably 

 similar to, if not identical with, whatever deposits may be now 

 forming in the "West India seas. 



The characters and histories of the beds may now be succes- 

 sively considered. 



Bed A. 



§ 9. Character. — Bed A underlies the whole Key and varies 

 in thickness (from the sea-level to the upper limit) from seven 

 to twenty-four feet, averaging about ten feet in the southern 

 part and seventeen in the central and northern parts. It not 

 only thickens gradually thus towards the north, but rather sud- 

 denly at various points from west to east. This irregularity is 

 due to the varying thickness of the lowest of the three bands 

 of which it is composed, whose respective thicknesses are as 

 follows: 



The casts of the coral branches in the lowest band, especially 

 near the bottom and sometimes throughout, have been tilled 

 with sand or with Calc Spar, and consequently present a blurred 

 appearance. Its upper portion very often, if not generally, 

 consists, to the depth of about a foot, of another species of coral 



