dine, it becomes trachyte porphyry, of which there are examples in Cos. Antrim and Down, in Ireland. In the trachyte of the Drachenfels, oligoclase accom panies sanidine as a constituent, with magnesia-mica, and hornblende. 1 The varieties of this rock, conse quent on changes in chemical composition, or the presence of accessory minerals, and different rates of cooling from a molten state, are endless ; for while on the one extreme, we have a crystalline granular rock, MAGNIFIED SECTION OF TRACHYTE FROM PUT CAPUCIN (a. V. LASACLX). resembling granite, on the other we find the same constituents passing into obsidian (or volcanic glass), or pumice-stone so porous as to float on water. In this condition it passes beyond the category of build ing stones, and requires no further consideration here. Generally speaking, trachytes are the representa tives of the acidic (a highly silicated) class of volcanic rocks, as basalts are of the basic class. 1 Zirkel, Petrog. ii. 180.
