e are many of different
external forms.

SILICA. One of the pure earths. _Etym._, _silex_, flint, because
found in that mineral.

SILICATE. A chemical compound of silica and another substance, such
as silicate of iron. Consult elementary works on chemistry.

SILICEOUS. Of or belonging to the earth of flint. _Etym._, _silex_,
which see. A siliceous rock is one mainly composed of silex.

SILICIFIED. Any substance that is petrified or mineralized by
_siliceous_ earth.

SILT. The more comminuted sand, clay, and earth, which is
transported by running water. It is often accumulated by currents in
banks. Thus the mouth of a river is silted up when its entrance into
the sea is impeded by such accumulation of loose materials.

SIMPLE MINERAL. Individual mineral substances, as distinguished from
rocks, which last are usually an aggregation of simple minerals.
They are not simple in regard to their nature; for when subjected to
chemical analysis, they are found to consist of a variety of
different substances. Pyrites is a simple mineral in the sense we
use the term, but it is a chemical compound of sulphur and iron.

SINTER, CALCAREOUS OR SILICEOUS. A German name for a rock
precipitated from mineral waters. _Etym._, _sintern_, to drop.

SLATE. See "Cleavage" and "Schist."

SOLFATARA. A volcanic vent from which sulphur, sulphureous, watery,
and acid vapors and gases are emitted.

SPORULES. The reproductory corpuscula (minute bodies) of cryptogamic
plants. _Etym._, [Greek: spora], _spora_, a seed.

STALACTITE. When water holding lime in solution deposits it as it
drops from the roof of a cavern, long rods of stone hang down like
icicles, and these are called _stalactites_. _Etym._, [Greek:
stalazo], _stalazo_, to drop.

STALAGMITE. When water holding lime in solution drops on the floor
of a cavern, the water evaporating leaves a crust composed of layers
of limestone: such a crust is called _stalagmite_, from [Greek:
stalagma], _stalagma_, a drop, in opposition to _stalactite_, which
see.

STATICAL FIGURE. The figure which results from the equilibrium of
forces. From [Greek: statos], _statos_, stable, or standing still.

STERNUM. The breast-bone, or the flat bone occupying the front of
the chest.

STILBITE. A crystallized simple mineral, usually white, one of the
Zeolite family, freq