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



stone *." These rocks pass into each other in such a manner that it is impos- 

 sible to say what extent of country is occupied by any one. They are well 

 exposed on the Juan de Bolas Great River, forming picturesque cliffs of vari- 

 ous heights on both banks. Considerable variety will be found in the vicinity 

 of Lemon Hall estate on this river. The porphyries are there principally 

 composed of a dark claystone base with white crystals of felspar ; the base of 

 some have the appearance of a fine-grained greenstone, and I observed one 

 with a brownish red claystone base and reddish white crystals of felspar. 

 Among the other trap rocks of this place I remarked a fine-grained green- 

 stone-looking rock containing small brilliant facets, which effervesce and ap- 

 pear to be carbonate of lime. 



The trap mountains of St. John's attain considerable elevation, and are 

 formed into very sharp ridges winding considerably, the course of the main 

 ridges being about East and West. 



The St. John's trap extends into that part of Clarendon which is principally 

 drained by the mountain portion of the Rio Minho, and the streams descend- 

 ing from the elevated land of which the Bull Head forms the highest part. 

 Porphyritic conglomerate is seen stratified near Savoy estate (Clarendon), 

 with a dip of a few degrees to the Eastward : the same rock, which corre- 

 sponds with the porphyritic conglomerate of the Hope River, is also found at 

 Morgan's Valley. Porphyry appears to be associated with the conglomerate 

 near Savoy estate. 



A porphyry occurs near Port Antonio (Portland), separating the white 

 limestone from the submedial rocks ; but the manner in which it is exposed 

 makes it difficult to refer it to any particular epoch of formation. This rock 

 will be observed not far from the town on the road to Moore Town (a Maroon 

 town) ; it consists of a reddish gray earthy claystone base, with crystals of 

 white felspar, and crystals of augite. Not far from this there is another por- 

 phyry having a reddish brown claystone base, containing small reddish cry- 

 stals of felspar intermingled with moderately sized crystals of augite. 



To this list may be added the earthy claystone rocks which are found in 

 one part of the descent from Monte Diablo into St. Thomas-in-the-Vale, and 

 form some part of the southern face of that mountain ; among them may be 

 seen a porphyritic rock composed of a light brown earthy base thinly sprinkled 

 with white crystals of felspar. From the information I received it would 



* Geological Classification of Rocks, p. 480. 



