22 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



— the drainage divide of the Minho and St. Thomas — is known as the 

 "Main Ridge." The other, which lies between the drainage of the 

 Minho and Cave and Pindars Eivers, may be called the Santa Maria 

 Ridge. It culminates eastward in Bull Head, an elongated summit. A 

 view of the latter mountain is shown on Plate VII. These ridges con- 

 sist of the same material as the Blue Mountain Pddge, present similar 

 slopes with cuchillate salients, and differ only in their crests, which are 

 not so serrated. They have a general north of west trend, parallel to 

 that of the Blue Mountain Ridge. They are not continuous with the 

 latter, whose west end terminates e?i echelon many miles northeast of 

 where the former begin. 



The present relief of these ridges, while greatly resembling that of the 

 Blue Mountains, has been produced during late epochs by the deep erosion 

 of the river valleys which are parallel to them. The heights of these 

 mountains (Bull Head, 2,885 feet, and the main ridge, 2,542 feet) do 

 not anywhere exceed that of the circumscribing White Limestone Plateau. 

 It is our opinion that they are merely modern drainage divides, their 

 summits representing an older floor of a former central basin valley like 

 those described on later pages. Furthermore, the data indicate that this 

 older and higher floor was once completely covered by the formations 

 of the White Limestone Plateau. 



In the southeast corner of Planover Parish, in the valley of Great 

 River, at Jerusalem Mountain in the north central portion of Westmore- 

 land and along the northwest coast of Hanover, the Central Mountain 

 rocks and structure are again exposed by denudation of the once overly- 

 ing wliite limestone sheet as in the Clarendon district. Jerusalem Moun- 

 tain is a hill 600 feet above the plain, surrounded by an amphitheatre 

 of white limestone hills. The other exposures are usually shown in the 

 valleys of stream ways. Probably the Great River and Jerusalem Moun- 

 tain localities represent exposures of a third line of old mountain folds 

 lying south of the Clarendon trend. 



In all the localities mentioned, the Central Mountain structure is 

 intensely folded. In the cast it is a crumpled anticline, and has evi- 

 dently been subjected to the additional disturbance of a later intru- 

 sion of a great mass of granitoid porphyry, with many auxiliary dikes, 

 which is now exposed by erosion on the north side of the west end 

 of the ridges in St. Mary. Westward the structure is that of over- 

 thrown closed folds, as shown in Plate XXII., which is a view of the 

 structure on the north coast near Lucea. The geologic section further 

 exhibits the deformation which has produced the Blue Mountains of 

 Jamaica. 



