hill: geology of JAMAICA. 209 



Oligocene, which was of far reaching importance in the Gulf, Caribbean, 

 and Antilloan regions, although they but slightly affected our Coastal 

 Plain. This profound subsidence is manifested by the nature and occur- 

 rence in the present upland structure of parts of the Antilles, Barbados, 

 and Trinidad, of oceanic deposits composed largely of Globigerinae and 

 Radiolarians, such as are now known to occur at oceanic depths varying 

 from 1,200 to 3,000 fathoms. 



These beds are synchronous with the so called Jackson and Vicksburg 

 formations of our southern coast, and their equivalents in Yucatan and 

 Costa Kica. The exact geologic age and correlation of these beds have 

 been ascertained by paleontologio data, and the extent and differential 

 variations of this downward movement may be indicated in the com- 

 position of the synchronous deposits in different localities. 



The effect of this subsidence upon our own Gulf coast east of the Mis- 

 sissippi was a slight northward transgression of the Gulf, as shown by 

 the more mixed character of the formation around the American littoral 

 in Mississippi and Alabama. Subsidence in that direction is distinctly 

 recorded in the change of the Tertiary sediments from the impure land- 

 derived character of the Lignitic and Claiborne beds into the 500 feet of 

 limestones of the Jackson and Vicksburg epochs. The littoral nature of 

 synchronous deposits in Costa Eica also indicates a shallower condition 

 towards the continental shore line in that direction. In Cuba, near 

 Havana and Santa Clara, the Vicksburg limestones contain small peb- 

 bles, probably indicative of persisting inequalities of bottom, as the land 

 was being submerged there. Elsewhere in the Antilles and Barbados, 

 the formation is of the deep oceanic nature described. 



From the geographic occurrence of these beds I am inclined to believe 

 that they were deposited in the troughs or deeper slopes of two great 

 basins or depressions, separated by a ridge along the present site of the 

 Caril:)bee Islands. One of these basins, the Barbadian, lay in the Atlantic 

 proper, while the other was in the heart of the Caribbean, having an 

 elliptical outline, whose longer and deeper axis extended through the 

 Windward passage from Mississippi to Trinidad. This depression so 

 largely drowned the Antilles that only the higher summits of Cuba, 

 Haiti, and Jamaica remained above sea level as small islands. This 

 basin shallowed rapidly towards its periphery, the Coastal Plain of the 

 United States, the old lands of Costa Rica and Yucatan. The northern 

 and eastern periphery of this basin during this subsidence could have 

 been only the Bahaman, Floridian, and Windward banks, which must 

 have been largely submerged. 



VOL. xxxiv. 14 



