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Silex. One of the simple earths, nearly pure in'rock crystal. 
Silt. The finer particles of the earths suspended and transported by water. 
Simple^ when applied to minerals and rocks, has reference to their ho- 
mogeneousness, and not to the number of elements which enter into 
their composition. 
Stalactite. A pendant cylindrical mass of carbonate of lime, attached to 
the roof of a cave. 
Stalagmite^ is a mass of concreted limestone formed beneath the stalac- 
tite, by water dropping from it charged with carbonate of lime. 
Stratum^ strata^ stratification. The arrangement of the layers of a rock 
in parallel position. 
Strike. The direction in which the edge of a stratum appears at the sur- 
face. 
Syenite^ sienite. A variety of granite, in which hornblende replaces the 
mica. 
Synclinal line, or axis. Where the strata dip downwards like the sides 
of a gutter. 
Talus. A collection of rocks and stones beneath a cliff. 
Tertiary. The fourth grand division of the rocky strata, composed of 
clays, gravels, and sometimes of consolidated beds of limestone and 
siliceous matter; all of which are more recent than those of the three 
preceding divisions, as primary, transition and secondary. 
Testacea. Molluscous animals having a shelly covering. 
Tepid. Warm. 
Thermal. Hot. 
Thin out. The diminution in the thickness of strata, until they disappear. 
Trap^ trappean rocks. Ancient volcanic rocks composed of feldspar, 
hornblende, and hornblende and augite. Basalt, greenstone, amygda- 
loid and dolerite, are the most common of this class of rocks. They 
are sometimes called superincumbent rocks, because they rest or repose 
irregularly or indifferently on any of the other rocks. The pure horn- 
blende rocks were formerly termed primitive trap. 
Traveaiin. A deposit of carbonate of lime, more or less porous and 
earthy, from the water of springs; sometimes it is hard and semi- 
crystalline. 
Tufa, calcareous and siliceous. Porous or earthy deposits from springs 
of water, containing lime or silex. 
Tuff, or Tufa, Italian. A name for a volcanic rock of an earthy texture. 
Unconformable. See Conformable. 
Veins. Openings or fissures in rocks, filled with stony or metallic mat- 
ter. 
Zoophiles. Animals of the lowest order, as sponges, corals, &c. 
