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 Rivers, 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 Ridge, 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 en échelon 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 valloys which are parallel to them. The heights of these 
mountains (Bull Head, 2,885 feet, and the main ridge, 2,542 feet) do 
not any where 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 Hanover 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 white 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 streamways. 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 east 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. 
