hill: geology of Jamaica. 21 



representing tho western end of the Blue Monntain ridgos, 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 are 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 subsidence, 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-Yale, Clarendon Parish, 

 Great Eiver, 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 a 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 more closely. 



The Clarendon Mountains. — The exact geographic centre of Jamaica 

 is marked by a most interesting topographic 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 x)utliers. The most southern ridge 



