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



The road over the hills from Stony Hill to Green Castle (St, Mary's) pre- 

 sents trap rocks closely resembling that afforded by the Agua Alta, being not 

 far distant from it. Between Belmont (St. Andrew's) and Green Castle I 

 observed a porphyritic rock having a light brown earthy base containing cry- 

 stals of whitish felspar, a few pieces of green hornblende^ and a few specks of 

 mica. Between the latter places I also found some masses of a steatitic rock 

 mixed with trap, which latter was much decomposed to some depth throughout 

 the greater part of this district. 



In several places the trap had a sandy appearance arising from the decom- 

 position of clinkstone, which. Dr. MacCulloch observes, " frequently acquires 

 an arenaceous aspect on the surface after exposure to the weather ; so as 

 even to have been confounded with sandstone by incautious observers*." 



At Pimento Grove, a coffee settlement between Rio Magno Pen (St. Tho- 

 mas-in-the-Vale) and Tremolesworth (St. Mary's), trap rocks are met with, 

 which apparently are continued from those above mentioned. They here 

 constitute mountains that rise among others of the white limestone formation, 

 which latter rests upon and covers the trap on the North-east and South- 

 west. The trap in the vicinity of Pimento Grove principally consists of 

 greenstone and porphyry. One singular variety of the latter composed of a 

 dark claystone base studded with crystals of white felspar, will be observed 

 on the left hand side of the road, having the porphyritic pieces or concretions 

 rounded and separated from each other by a soft argillaceous substance, among 

 which strings of chalcedony are sometimes found. The above is the only 

 instance of this kind of structure that came under my observation while in 

 Jamaica, with the exception of part of an extinct volcano to be mentioned 

 hereafter. Here the porphyry is of a very common kind, and is associated 

 with greenstone porphyries : one of these contains brownish red crystals of 

 felspar, and crystals of augite ; and another, grayish white crystals of felspar 

 with crystals of augite ; the last mineral being more abundant in the latter 

 than in the former of these varieties. 



In the neighbourhood of Williamsfield and Sandy Gully (St. Thomas-in- 

 the-Vale) greenstone and porphyry, very much decomposed where exposed 

 to the weather, form the lower parts of the valley, which has all the characters 

 of a valley of denudation, the higher parts of the mountains being capped by 

 white limestones, which seem once to have formed parts of the same mass. 

 This trappean country appears to be connected with that range of trap, above 

 described, which extends from St. David's north westerly into St. Mary's; its 



* Synopsis of the Overlying Rocks, in his Geological Classification of Rocks, p. 495. 



