GLOSSARY.
297
Salient Angle.—The outward projecting angle formed by the meeting of two planes.
Shak.—A cavity in a limestone rock.
Silica.—The dioxide of silicon ; often used as a term to include all the naturally occurring forms of this oxide.
Solfataua. —A small vent in the neighbourhood of a volcano, emitting sulphurous vapours.
Solid Angle.—The corner formed by the meeting in a point of three or more planes.
Specific Gravity.—The ratio of the weight of a substance in air to the weight of an equal volume of water at its maximum density.
Strass.—The glass used for the production of artificial gems.
Surface Charge of Electricity.—A
charge of electricity on a surface, the potential or electrical tension
varying at different points <5n the surface, as at the two ends of a
Tourmaline crystal.
Surface Tension.—A
force acting between a surface and a medium with which it is in
contact. The total force in a given case is proportional to the area of
the surface.
Thermal Springs.—Springs of uprising heated water.
Thermo-metamorphism.—A change induced in rocks as the result of the action of heat, and probably of heated water.
Trace (of a plane).—The line along which the plane cuts another plane.
Translucent.—Allowing light to pass ; distinguished from transparent in not allowing objects to be definitely seen.
Transparent.—Allowing light to pass so freely that definite images of the objects reflecting or emitting the light are formed on the retina.
Trisoctahedron.—A
generally octohedral form of the cubic svstem in which each face of the
octahedron is replaced by three planes ; when each of these planes has
three angles the form is a trigonal trisoctahedron; when each of the
planes has four angles the form is a tetragonal trisoctahedron (Fig.
24).
Unconformity.—The relation between two rocks when the
