|
GLOSSARY.
293
Dihedral Angle.—The angle formed by the meeting of two
planes in an edge. Divided Arcs.—Portions of a circle, usually made in brass,
graduated into units and fractions of angular measurement. Dodecahedron.—A form of the cubic system having twelve
faces each the shape of a rhomb. This is the rhombic
dodecahedron (Fig. 23). Other twelve-faced forms also
occur. Druse.—A cavity lined with crystals. Dynamo-metamorphism. — Change induced in a rock as a result
of mechanical movement. Since this movement usually
occurs under great pressure it is usually accompanied by
great heat. Electroscope.—An instrument for detecting an electrical
charge. Epicene Minerals.—Minerals formed from others by the
slow downward percolation of water at a relatively low
temperature. Eruptive Bocks.—Volcanic rocks which have been poured out
on the surface of the land or in the sea. Facet.—An artificially produced plane surface on a gem stone. Fault.—A surface or fissure in a rock mass where a differential
gliding movement has occurred. Fault Breccia.— A mass of angular fragments of rocks lying
in a fault fissure. Ferroviagxesian Silicate.—A mineral consisting largely of
silicate of iron and magnesium ; usually found in rocks
containing a low percentage of silica. Fritting.—The caking of the ingredients in the making of
glass. Fumaroles.—Small vents around a volcano from which gases,
and sometimes mud, escape : often found on volcanoes of
declining activity. Geode.—The same as Druse (q. v.). Gurgulho.—A deposit of angular rock fragments in which
Diamonds are found. Habit (of a crystal).—A term used to indicate the general
|
||
|
|
||