PORPHYRY

is a compound mineral or rock, composed e entially of a base of hornstone, interspersed with crystals of felspar. It frequently contains also quartz, mica, and hornblende. That most esteemed is the antient porphyry of Egypt, with a ground of a fine red colour pa ing into purple, having snow-white crystals of felspar imbedded in it. Most beautiful specimens of it are to be seen in the antique colo al statues in the British Museum. Porphyry occurs in Arran, and in Perth shire between Dalnacardoch and Tummel bridge. It is much used for making slabs, mullers, and mortars.