hill: geology of Jamaica. 159 



This Bubsidonce was one of the most important and far-reaching 

 events in all Antilleau history as we shall show in our next Part. 

 For present purposes, however, it is best to consider how far it in- 

 volved the pre-existing land topography of Jamaica. Canyon cuttings 

 through the collar of the limestone plateau which now encircles the 

 peaks of Blue Mountain structure show that the latter extends down 

 to sea level and nearly everywhere out to the present margin of the 

 island. The limestone deposits of this subsidence encrust this moun- 

 tainous core to a height of 3,000 feet, and hence only the portion of 

 the summits above that altitude could have been dry land when this 

 subsidence was at its maximum. This land, then, was restricted to an 

 area of what is now the upper slopes and summit region of the Blue 

 Mountain ridge proper. The remaining parts of the island, including 

 the limestone plateau and the Clarendon and Jerusalem Mountains, 

 were completely submerged. The culmination of this subsidence can 

 be fixed by the paleontologic evidence at the close of the Eocene period, 

 Vicksburg epoch. 



The next event in Jamaican history was the re-elevation of the sea 

 bottom and the restoration of the land area to proportions far beyond 

 its present outline, connecting it with the adjacent island of Haiti on 

 the east and possibly the Central American region to the south of west. 

 This elevation is attested, first, by the shallowing nature of the upper 

 limestone Moneague formations of the Oceanic Series, in which simple 

 corals and MoUusca appear; and, secondly, by the emergence of those 

 strata into land during another period of mountainous folding, ac- 

 companied by great laccolithic intrusions of igneous granitoid rocks. 



This emergence of the island to beyond its present outline is evi- 

 dent. This emergence brought up with it that portion of the old 

 presubmerged mountainous topography now encrusted by a coating 

 1,000 feet or more of oceanic chalks. That the land extended still 

 farther beyond its present margins than at present is shown by the 

 truncated termination of the formations of the back coast topography, 

 and the unconformity of the later formations against their eroded 

 bluffs, the submerged benches of the island, and the biologic proof 

 that dissemination of the land Mollusca between the islands took place 

 at this time.^ 



This emergence was dominated by a low east and west fold through 

 the geographic centre of the island, which caused its present elongate 



1 Distribution of the Land and Fresh Water MoUusks of the West Indian 

 Region. Proc. U. S. Nat. Mus., Vol. XVII. pp. 423-450, 1894. 



