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



disposed, vertically, according to the submarine depth reached 

 and the vertical range of species, as well as to the nature of the 

 barrier reef. In relation to these movements, the position of the 

 Key on the prolongation of the volcanic axis marked by the 

 leeward islands of the Lesser Antilles (one of which. Saba, is in 

 sight to the S. by E.), and the coincidence with this axis of the 

 trend of the last three systems of joints, are facts worthy of 

 note. In all the oscillations no irregular movements ever 

 occurred to cause the plane of the rim of the basin to deviate 

 from its coincidence with that of the horizon. 



The atoll-character of this Key seems to be an anomaly in the 

 West Indies, but these islands have hitherto been very imper- 

 fectl} 7 explored, and it may be fairly suspected that many of 

 the isolated keys and banks with which its archipelagoes 

 abound, may reveal to future examination the possession, (in 

 their former history if not at present,) of a true atoll con- 

 struction. 



§ 7. Varieties of Limestone. — The limestone, of which the 

 Key is mainly composed, is similar to that of modern coral 

 islands. It comprises five varieties, which may be designated 

 as follows, for the sake of convenient reference : Madrepore 

 limestone, Porites limestone. Bulla limestone, Sand limestone, 

 and Lagoon limestone. 



Madrepore limestone is of pure white color, and consists 

 almost entirely of the tubular casts of this coral, being usually 

 rendered, therefore, exceedingly vesicular. The coral evidently 

 grew as one uninterrupted reef, with its interstices filled with 

 sand and a few small shells, Bulla, Cerithium, etc. 



Porites limestone is characterized by the heterogeneousnets 

 of its organic constituents, the abundance of the massive 

 kinds of coral, especially Porites and Mceandrina, and its vari- 

 able but generally very vesicular and coarse struc:nre. Its 

 corals formed, not an unbroken reef, but a " coral-plantation." 

 Color usually white, but sometimes slightly bluish. Specific 

 Gravity = 2.09 — 2.23, in hand-specimens. 



