trachysoma

[Pref. trachy-, and Gr. soma IMG:content-1291.png:[blocks in formation] dicated the predominant mineral constituent, hence sanidine-trachyte, oligoclase-trachyte, . With the exception of the rocks of a few localities, this word is now used as the name of a group of rocks having certain physical and chemical resem- blances in common, but differing considerably in their mineralogical composition. For their miner- alogical composition, structure and cla ification, see Rosenbusch, Mikroskopische Physiographie d. ma igen Gesteine (Stuttgart, 1877), and other petro- logical works. trachyte-porphyry, s. Petrol.: The same as QUARTZ-FELSITE (q. v.). trachyte-tuff, s. Petrol.: A tuff (q. v.), consisting of either fragmentary or loosely-compacted earthy, volcanic materials, having the composition and structure of trachyte (q. v.).