Gneiss is a hard, granular rock which exhibits a coarsely banded
structure (resulting from metamorphism). The bands are evident because
of color differences due to different mineral content; those dark in
color are commonly rich in dark mica (biotite) or hornblende (a dark
green to black, hard mineral), whereas the light bands contain feldspar
and quartz. Many gneisses have about the same mineral composition as
granite; hence, for our nontechnical purposes, a banded rock, otherwise
granite-like, is a gneiss.
[Illustration: Gneiss typically banded.]
Gneiss is a metamorphic rock, a _changed_ rock. The banded structure was
developed by a combination of very high pressure, high temperature, and
solutions acting on a previously existing rock in essentially a solid
condition. The original rock may have been an igneous or sedimentary
rock which has been crushed or made to flow into bands, or has been
re-crystallized. The tremendous pressure which operated during the
banding of most gneisses also crumpled square miles of rock thousands of
feet thick into folded and broken (faulted) mountains. True slates,
marbles, and some quartzites are formed from soft shales, limestones,
and sandstones, respectively, in the metamorphic process.
Almost no metamorphic rock of this regional type crops out in Missouri,
but the boulders of gneiss which are found in the glacial deposits were
picked up in Canada or the northern United States and carried to
Missouri by a continental glacier thousands of years ago.
Except for use as bulk stone or possible structural purposes the gneiss
in Missouri has no value. The glistening yellowish mica sometimes seen
in gneiss is not gold, of course, and is likewise valueless.