Steatite—see soapstone.
Syenite—a granitoid rock composed of orthoclase and hornblende, or other ferromagnesian mineral; the name was formerly applied to a granitoid aggregate of quartz, feldspar, and hornblende.
Tachylite (hyalomelane, basaltic glass)—a black glass of basaltic nature corresponding to the acidic glasses, obsidian and pitchstone.
Till (bowlder clay)—a stony or bowldery clay or rock rubbish formed by glaciers.
Trachyte—a name formerly applied to a rock possessing a peculiar roughness due to its cellular structure; but at present mainly confined to a compact, usually porphyritic igneous rock, consisting mainly of sanidine associated with varying amounts of triclinic feldspar, augite, hornblende, and biotite.
Trap—a general term for igneous rocks of the darker basaltic types.
Travertine—a limestone deposited from calcareous waters, chiefly springs; usually soft and cellular, and hence also called calcareous tufa, calc sinter.
Tuff (tufa)—a term including certain porous granular or cellular rocks of diverse origins; the volcanic tuffs embrace the finer kinds of pyroclastic detritus, as ashes, cinders, etc.; the calcareous tufa embrace the granular and cellular deposits of springs; the better usage limits the term to volcanic clastics.
Water-lime—an impure argillaceous limestone possessing hydraulic properties.
Wacke—a dark earthy or granular deposit formed from basic tuffs or from the disaggregation of basaltic and similar rocks; a term which may well come into more general use to distinguish the silicate sands that arise from the disaggregation, but only partial decomposition, of basic rocks, as arkose does, the like products of the acidic or granitoid rocks, and as sandstone does, the granular products of complete chemical decomposition.