88 Dr. KiDD on the Mineralogy of St. David's. 



porphyry of a light drab colour, containing small crystals of felspar 

 nearly of the same colour, together with completely tarnished cu- 

 bical crystals of iron pyrites : there is also in this vein an occasional 

 appearance of decaying hornblende or chlorite. The base of this 

 porphyritic vein wears away by the action of the weather, and 

 leaves the crystals of the felspar projecting from the surface of the 

 weathered part. 



This vein of porphyry is inclosed in a stratum of friable schist, 

 neither the character nor position of which are at all altered by the 

 immediate contact of the vein. The adhesion between the schist 

 and the vein is so very slight that it is extremely difficult, if at all 

 possible, to separate a specimen which shall unequivocally shew the 

 junction of the two. 



The schist, which is traversed by filamentous veins of quartz, 

 appears to the eye of a very delicately laminated structure, yet does 

 not readily separate in the direction of the planes of the laminae. 

 The surfaces of many of the natural rifts have a brownish black 

 tarnish. 



Carvay. 



The cliffs in the neighbourhood of this spot, the precise situation 

 of which is not recollected, but it is not far distant from the fore- 

 going, consist of highly inclined strata of indurated greenish-grey 

 freestone ; of red and coarsely laminated slaty freestone, which 

 Is used in building ; and of a soft argillaceous freestone with nume- 

 rous veins of sparry quartz. Traces of chlorite are very frequent 

 in the rocks of this neighbourhood, and the slaty freestone is 

 often interspersed with particles of a substance intermediate in 

 its character to mica and chlorite. 



