Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/98

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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 cubical 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 laminæ. The surfaces of many of the natural rifts have a brownish black tarnish.


The cliffs in the neighbourhood of this spot, the precise situation of which is not recollected, but it is not far distant from the foregoing, 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 numerous 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.