HILL: GEOLOGY OF JAMAICA. 165 
To the physicist who estimates the amplitudo 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 
9f 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 arca, and the knowledge of the subma- 
ine 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- 
Mg 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 
vf Globigerinze would indicate that they were deposited like the deep sea 
0zes of to-day at depths of 1,200 and 2,300 fathoms. The fact that in the 
®astern end of the adjacent island of Cuba the stratigraphic and time 
Position of these beds is oceupied 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 
Own in very deep waters. It would not be at all unfair to assume the 
rage between the minimum and maximum of the Globigerina and 
“diolarian 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 
Vero close to tho slope of the nucleal islands which persisted as tips of 
nd 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 
ect above it, which would equal an elevation of 13,000 feet, As the 
"ghost mountain penks of Jamaica now stand 4,325 feet above the ma- 
torial deposited in the Montpelier subsidence, they must have reached at 
12 culmination of this elevation altitudes of over 17,000 feet above the 
Sea 
ü * As great as these heights may seem, they do not greatly exceed 
l 
at of tho summit of the Sierra de Santa Marta, which now stands 
