HILL: GEOLOGY OF JAMAICA. 21 
representing the western end of the Blue Mountain ridges, terminate on 
the eastern side of the great basin of St. Thomas-in-the-Vale. 
The mountains are composed of the friable or loosely consolidated 
Shales, clays, and conglomerates of the Blue Mountain Series, with here 
and there an exceptional local bed of limestone or an occasional dike or 
mass of igneous rock usually decomposed, all of which aro intensely pli- 
cated and folded. Their present configuration is due to the readiness 
with which they yield to erosion. When one considers how rapidly 
degradation is going on and has gone on, he can only conclude that the 
mountains were once of much greater altitude and extent. ‘There is no 
reason why its summits in times past may not have extended as high as 
their kindred in the Sierra Maestra of Cuba, over 8,000 feet, or in San 
Domingo, over 10,000 feet. 
These Blue Mountains are the highest of an extensive system of cor- 
rugations which were partially buried, especially west of the centre of 
the island, during a former period of subsidence, beneath the veneering 
of white limestone, and which has since: been re-elevated to a height of 
3,000 feet. Only the Blue Mountain ridges persisted as land during 
this epoch of subsidenco, while the Clarendon and other westward groups 
Were covered by the ocean's waters. 
The old Blue Mountain structure and material reappear in many places 
in the great central valleys of St. Thomas-in-the-Vale, Clarendon Parish, 
Great River, and elsewhere to the west, where the later crust of the 
White Limestone Plateau has been worn away. It is also seen in the 
face of the back coast bluffs along the western half of the north side of 
the island below the limestone and above the narrow coastal benches. 
These were originally & part of the same grand system as the Blue 
Mountains, which were buried beneath the white limestones, and are 
now re-exposed by erosion of the latter. Let us examine some of these 
localities moro closely. 
The Clarendon Mountains. — The exact geographic centre of Jamaica 
is marked by a most interesting topographie feature, an anticlinal valley 
worn out of the crest of the low arch of the White Limestone Plateau. 
This is an elongated oval area, fifty miles in length, lying mostly in 
Clarendon Parish, but extending on the southeast into St. Catherine 
and on the northwest into Trelawney. This great amphitheatre is com- 
pletely surrounded by the inward facing breaks of the Limestone Plateau, 
which rise 2,558 feet on the south and 3,000 feet on the west side. 
Most of the area of the valley is occupied by two parallel mountainous 
ridges with laterals and disconnected outliers, "The most southern ridge 
