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479 
Sacchoroidal. Resembling loaf sugar. 
Schist, schistose. Slate and slaty. 
Septaria. Flattened balls of stone traversed by seams of calcareous 
spar or some other mineral. The outer surface has the appearance 
of a turtle's back ; and hence they have been considered, by persons 
of little observation, as petrified turtles. 
Shale. Generally used as synonimous v^rith slate. 
Shell Marl. The name is applied to recent fresh-water deposits, which 
consist of marl and numerous shells of Helices, Planorbis, &c. in- 
termixed. 
Shingle. Beaches and shores covered with waterworn pebbles and 
gravel. 
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 
homogeneousness, and not to the number of elements which enter in- 
to 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 sta- 
lactite, 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 
surface. 
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 
silicious matter ; all of which are more recent than those of the three 
preceding divisions, as primary, transition and secondary. 
Testacea. Moluscous animals having a shelly covering. 
Tepid. Warm. 
Thermal. Hot. 
Thin out. The diminution in the thickness of strata, until they disap- 
pear. * 
Trap, trappean rocks. Ancient volcanic rocks composed of feldspar, 
hornblende, and hornblende and augite. Basalt, greenstone, amyg- 
daloid 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 hornblende rocks were formerly termed primitive trap. 
Travertin. A deposit of carbonate of lime, more or less porous and 
earthy, from the water of springs ; sometimes it is hard and semi- 
crystalline. 
