HILL: GEOLOGY OF JAMAICA. 159 
This subsidence was one of the most important and far-reaching 
events in all Antillean history as we shall show in our next Part. 
Por 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 coro 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, 
Vere completely submerged. The culmination of this subsidence can 
e fixed by the paleontologic evidence at the close of the Eocene period, 
ieksburg epoch. 
The next event in Jamaican history was the re-elevation of the sea 
Jtom and the restoration of the land area to proportions far beyond 
'ts present outline, connecting it with the adjacent island of Haiti on 
1 east and possibly the Central American region to the south of west. 
‘his elevation is attested, first, by the shallowing nature of the upper 
'Mestone Moneague formations of the Oceanic Series, in which simple 
“orals and Mollusca appear; and, secondly, by the emergence of these 
Strata into land during another period of mountainous folding, ac- 
“ompanied by great laccolithic intrusions of igneous granitoid rocks. 
"his emergence of the island to beyond its present outline is evi- 
Mt. This emergence brought up with it that portion of the old 
Presubmerged mountainous topography now encrusted by a coating 
a feet or more of oceanic chalks. That the land extended still 
Ather beyond its present margins than at present is shown by the 
'incated termination of the formations of the back coast topography, 
‘nd the unconformity of the later formations against their eroded 
ufs, the submerged benches of the island, and the biologic proof 
at dissemination of the land Mollusca between the islands took place 
18 time.! 
This emergence was dominated by a low east and west fold through 
: geographic centre of the island, which caused its present elongate 
En, ; 
Reg tribution of the Land and Fresh Water Mollusks of the West Indian 
lon. Proc. U, S, Nat. Mus., Vol, XVII. pp. 423-460, 1894. 
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