I 



hill: geology of Jamaica. 1G5 



To the physicist who estimates the amplitude of these oscillations 

 there is apparently only one available datum plane, — the position of 

 sea level relative to that portion of the island now protruding above the 

 water. This is an imperfect bench mark, however, for a large portion 

 of the former island surface is now submerged, and sea level may have 

 changed. Fortunately, we have in the geomorphology, stratigraphy, and 

 paleontology of the present land area, and the knowledge of the subma- 

 rine topography and bathymetric distribution of living marine organisms, 

 important collateral data which give valuable facts for estimates. 



The first or great primary oscillation was that embraced in the cycle 

 of the Cambridge-Montpelier descending, and the early Oligocene ascend- 

 ing movement. This subsidence was certainly 3,000 feet, as can still be 

 measured by the height to which the bases of the old pre-existing moun- 

 tain summits are plastered with its deposits. When we remember that 

 the present land surface of Jamaica is only the crest of a larger Jamaica 

 still submerged, it would be no stretch of the imagination to believe that 

 the movement was from twice to thrice the amount given. In fact, there 

 is very direct evidence which leads us to the latter conclusion. 



The purity of the Montpelier chalks and their dominant composition 

 of Globigerinse would indicate that they were deposited like the deep sea 

 oozes of to-day at depths of 1,200 and 2,300 fathoms. The fact that in the 

 eastern end of the adjacent island of Cuba the stratigraphic and time 

 position of these beds is occupied by Radiolarian earths which are now 

 known to abound from 2,000 to 4,000 fathoms (12,000 to 24,000 feet) 

 justifies us in premising that the Globigerina beds of Jamaica were laid 

 down in very deep waters. It would not be at all unfair to assume the 

 average between the minimum and maximum of the Globigerina and 

 Radiolarian depths as 1,750 fathoms, or over 10,000 feet, as the mean of 

 this subsidence at Jamaica. The assumption of a minimum depth has 

 additional support in the fact that the Jamaican Globigerina deposits 

 were close to the slope of the nucleal islands which persisted as tips of 

 land during this epoch. 



The succeeding Post-Montpelier upward movement brought up the 

 bottom not only to sea level ( + 10,000 feet), but raised it at least 3,000 

 feet above it, wdiich would equal an elevation of 13,000 feet. As the 

 highest mountain peaks of Jamaica now stand 4,325 feet above the ma- 

 terial deposited in the Montpelier subsidence, they must have reached at 

 the culmination of this elevation altitudes of over 17,000 feet above the 

 sea. As great as these heights may seem, they do not greatly exceed 

 that of the summit of the Sierra de Santa Marta, which now stands 



