Gneiss
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.
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.
Hematite
Hematite (“keel”) is a heavy, red to purplish red, dull to glistening mineral which leaves a red mark or streak when rubbed on a hard white rock (like chert) or on unglazed porcelain. This red color of hematite coating or stain is responsible for our red clays, red soils, red iron rust, reddish creek-water, and almost every bit of natural, red mineral matter in Missouri. Hematite is iron oxide, Fe₂O₃, and has a close associate, limonite, which is yellow to brown in color, and has the chemical composition Fe₂O₃·nH₂O. The two are mentioned together here because they are commonly associated in nature, where they can be recognized in mixture by the yellowish red or reddish brown colors on rocks or soils. Individual discussion is given limonite under its heading, but its relationship to hematite is repeated here for obvious reasons.
