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Mr. De la Beche on the Geology of Jamaica. 167 



junction with it being- concealed by the white limestone formation on the 

 East of Williamsfieldj &c. Another portion of decomposed trap appears on 

 the road between Williamsfield and Hampshire * estates. 



A small portion of porphyritic conglomerate occurs near Mount Sinai 

 estate^ on the Yallahs River^ being a continuation of the Hope River por- 

 phyritic conglomerates on the South-east ; it is however greatly covered up on 

 the Yallahs River by the white limestone formation. 



Another extensive district of trap rocks, constituting the principal part of 

 the St. John's Mountains, and extending westerly into Clarendon, occurs at a 

 considerable distance from that which has been above described, and is sur- 

 rounded on all the sides which I have examined by the white limestone for- 

 mation : it however seems referrible to the same epoch of formation as that 

 above noticed. Ascending the St. John's Mountains by the road from St. 

 Dorothy's Rectory to Luidas Vale, trap appears on the northern side of the 

 river near Mountain River estate, forming the greater part of the mountain 

 on which the Barracks are placed. Greenstone and porphyry are here seen 

 in general very much decomposed, as is very commonly the case in the St. 

 John's Mountains. Among these rocks I found a very fine-grained green- 

 stone, as also a purplish brown rock, apparently with a decomposed felspathic 

 base, in which were a few light greenish spots of decomposed felspar. The 

 porphyries are in such a state of decomposition to a considerable depth, that 

 their various bases cannot be distinguished without considerable difficulty : 

 I however met with one fragment having a blackish green claystone base, 

 with crystals of greenish white felspar. This trap mountain is covered to- 

 wards Luidas Vale by the white limestone formation, which also occurs in 

 patches upon it, as may be seen at the Barracks, &c. 



A decomposed trap rock occurs at Pleasant Hill among these mountains ; 

 and this spot is interesting, as the trap can be observed almost in contact with 

 the white limestone that bounds it on the northward, and forms the slope of 

 the mountain towards Luidas Vale. 



That part of the St. John's Mountains which is represented on the map as 

 composed of the rocks under consideration, may be described as formed of 

 various claystone porphyries, greenstones, and rocks approaching the basaltic 

 character, with various modifications of that rock '^^ of which," says Dr. Mac- 

 Culloch, " indurated clay or wacke may be placed at one extreme, and com- 

 pact felspar at the other ; the intermediate member being claystone and clink- 



* I am not quite certain of the name of this estate ; it is however a little to the left of the 

 road hetween Williamsfield and Rio Magno Pen. 

 VOL. II. SECOND SERIES. Z 



