MATERIALS OF THE EARTH 87 



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. 30). 



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



Stalactites. Pendant icicle-like forms of calcium carbonate deposited 

 from dripping water. 



Stalagmites. The complement of stalactites formed by calcareous waters 

 dripping upon the floors of caverns. 



Steatite. (See soapstone.) 



Syenite. A granitoid rock composed of orthoclase and hornblende, or 

 other f erromagnesian mineral ; the name was formerly applied to a granitoid 

 aggregate of quartz, feldspar, and hornblende. 



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 rough- 

 ness 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 calcarous tufas 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. 



Laboratory work. Laboratory study of the common rock-making miner- 

 als and of the common types of rocks should be undertaken in connection 

 with this chapter. The groups of minerals mentioned on page 26 should be 

 made familiar first. The identification of these minerals in the rocks should 

 follow. The arrangement of the minerals is important in some cases, especi- 

 ally in metamorphic rocks. It is as important to stop these studies at the 

 right point, in this connection, as to take them up. The technicalities of 

 mineralogy, and the refinements of rock classification should not be touched 

 , at this point. Appropriate limitations for the study of igneous rocks, at this 

 point, are suggested on pages 27-30, and 59-62. 



