HILL: GEOLOGY OF JAMAICA. 209 
Oligocene, which was of far reaching importance in the Gulf, Caribbean, 
and Antillean 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 Globigerinz 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 Rica. The exact geologic age and correlation of these beds have 
been ascertained by paleontologic 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 Lignitie and Claiborne beds into the 500 feet of 
limestones of the Jackson and Vicksburg epochs. The littoral nature of 
synchronous deposits in Costa Rica also indicates a shallower condition 
towards the continental shore line in that direction. In Cuba, near 
Havana and Santa Clara, the Vieksburg limestones contain small peb- 
bles, probably indicativo 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 1 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 
Caribbee 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 
