THE ORIGIN AND DESCENT OF ROCKS. 473 



easily into slabs or flakes, less uniform than slate; they are mainly composed 

 of the silicate minerals. 



Scoriae — ^light, cellular fragments of volcanic rock, coarser than pumice; cinders. 



Septaria — concretions the interior of which have parted, and the gaping cracks 

 become filled with calcite or other mineral deposited from solution (Figs. 375-77). 



Serpentine — a rock consisting largely of serpentine; derived in most cases 

 by alteration from magnesian silicate rocks. 



Shale — a more or less laminated rock, consisting of indurated muds, silts, 

 or clays. 



Slate — an argillaceous rock which is finely laminated and fissile, either due to 

 very uniform sedimentation or (more properly) to compression at right angles 

 to the cleavage planes; e.g., common roofing-slate (Fig. 362). 



Soapstone (steatite) — a soft unctuous rock, composed mainly of talc. 



Stalactites — pendant icicle-like forms of calcium carbonate deposited from 

 dripping water. 



Stalagmite — the complement of stalactites formed by calcareous waters drip- 

 ping upon the floors of caverns. 



Steatite — see soapstone. 



Syenite — a granitoid rock composed of orthoclase and hornblende, or other 

 ferromagnesian mineral; the name was formerly ajDplied to a granitoid aggregate 

 of quartz, feldspar, and hornblende. 



Tachylite (hyalomelane, basaltic glass) — a black glass of basaltic nature corre- 

 sponding 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 pyroclast'c detritus, 

 as ashes, cinders, etc.; the calcareous tufa embrace the granular and cellular 

 deposits of springs; the better usage limits the term to volcanic elastics. 



Waterlime — 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 disaggrega- 

 tion, but only partial decomposition, of basic rocks, as arkose does, the like prod- 

 ucts of the acidic or granitoid rocks, and as sandstone does, the granular products 

 of complete chemical decomposition. 



