156 . Mr. De la Beche on the Geology of Jamaica. 



towards the Blue Mountain Peaks by Sheldon, Abbey Green, and Portland 

 Gap, I observed syenite, composed of white and flesh-coloured felspar, 

 colourless quartz, and greenish black hornblende, in considerable abundance 

 between Sheldon and Abbey Green, near Farm Hill : this syenite seemed to 

 be associated with grauwacke; in what manner, however, it would be difficult 

 to say, as their relative position was very obscure. 



Greenstone (white felspar, and blackish green hornblende, the grains 

 being of a moderate size) is seen singularly mixed up with slate and limestone 

 immediately above Abbey Green House ; and between this place and Port- 

 land Gap, an association of porphyry, slate resembling grauwacke slate, green- 

 stone and limestone will be met with. The porphyry is principally of a dark 

 claystone or trappean base with white crystals of felspar varying in size in 

 different places. 



The ridge which extends from Portland Gap to the base of that from which 

 the Blue Mountain Peaks rise here and there, presents, where the tangled 

 and moss-covered roots will permit a portion of rock to appear, that variety of 

 trap which is not unfrequently observable in districts composed of this class 

 of rocks; it is not precisely a cornean, but in some instances approaches it; 

 it occurs of a dark colour on this ridge, and gives out an earthy smell when 

 breathed upon. 



On the ridge from whence the highest Blue Mountain rises, I found a dark- 

 coloured trappean rock, containing a very few small crystals of felspar in the 

 mass ; it gives out an earthy smell when breathed on, and occasionally has a 

 sandstone appearance : the Peaks seemed in a great measure composed of a 

 similar substance. I did not observe any greenstone, syenite, or porphyries 

 upon the higher parts of the Cold Ridge or Blue Mountain Peaks, but merely 

 the above rocks ; their connexion with well characterized trap is therefore 

 not clear, and they may be mere varieties of grauwacke or some other subme- 

 dial rock, though from their general appearance, and their resemblance to 

 rocks in some trappean districts elsewhere, I am inclined to refer them to the 

 trappean class. 



Trap will be observed mixed with argillaceous slate and grauwacke at the 

 base of the Blue Mountain Range, between Whitehall and Bath (St. Thomas- 

 in-the-East); it is exposed in the bed of the Plantain Garden River, and is 

 principally composed of a dark claystone porphyry, with crystals of white fel- 

 spar, and of a rather earthy rock, of a gray, greenish, and occasionally reddish 

 colour, traversed by numerous veins of a substance having a zeolitic character. 

 This rock forms some picturesque cliffs upon the river, and presents no ap- 

 pearance of stratification. 



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