PRIMITIVE ROCKS. fpar, which on clofe examination appears granu lar foliated ; * further, alfo, pitchftone, pearlftone, and obfidian. The imbedded parts are moft com monly felfpar and quartz, which are ufually cry- ftallized, more or lefs perfectly, and hence they appear fometimes granular. The quartz is ufual ly cryftallized, and in double fix-fided pyramids. The felfpar cryftals are broad fix-fided prifms, but ufually very indiftin£t. The felfpar is more or lefs freffi, fometimes even glaffy, fometimes com pletely difintegra'ted and earthy, and fometimes appears only like white fpecks. It is difficult to determine, whether or not this is original, or the confequence of weathering. The frequency and magnitude of thefe mixed parts, modify the ap pearance of the different kinds of porphyry very much. Sometimes one, fometimes the other, but more frequently both occur together, and along with thefe feveral others which are lefs frequent, as cryftals of hornblende and mica. The bafis and the mixed parts of the porphyry alfo differ in colour and feveral other properties. It fometimes contains chalcedony and agate, fometimes in mal- five pieces, fometimes in fmall layers or plates. Further, there fometimes occur balls of a greater or lefs fue ; in clay-porphyry the centre of thefe balls is chalcedony, but their exterior is hornftone porphyry. In Pitchftone-Porphyry, thefe balls are compofed of a particular kind of conchoidal horn ftone, but the centre is quartz. Thefe balls are nearly