Porphyry. 335 neweft period, where it reaches its maximum, and beyond this period it fcarcely appears. 5. It is very nearly allied to the rocks of the trap formation, and therefore it is defcribed im mediately after them. 6. The three immediately preceding primitive rocks, namely, Primitive Limeftone, Primitive Trap and Serpentine,are not fo abundant, nor fo general as the four older formations. Porphyry, whofe geo- gnoftic relations we are now to defcribe, is one of , the moft widely extended formations. 8. Porphyry. Po r ph i r.—Werner. Porphyre, Brochant. 1. The Grecian word from which the name Porphyry is derived, fignifxes redj hence the name of the formation is borrowed from that kind, which is denominated Antique Red Porphyry. It is wor thy of remark, that red, or colours bordering on it, or palling into it, prevail in rocks belonging to the Porphyry Formation. 2. It is a compound rock, having a balis, in which the other cotemporaneous conftituent parts are imbedded, either in tl\e form of grains or crys tals. Neither the bafe nor the imbedded parts are always of the fame kind. On the differences of the firft, depends the diftindion of the different kinds of porphyry. The bafe is fometimes clay- ftone, fometimes hornftone, fometimes compad fel- fpar,