GNEISS is a metamorphic rock composed of roughly parallel bands of
minerals. It is medium grained to coarse grained and is generally light
in color. The names given to gneiss emphasize a distinctive texture or
mineral or indicate composition. For example, biotite gneiss emphasizes
a mineral, and granite gneiss indicates the composition of the rock.
SCHIST is much like gneiss but is fine grained and has a thinly layered
structure that makes the rock break with a wavy surface. Some common
types of schistose rocks are talc schist, chlorite schist, and
hornblende schist. As the names indicate, they are characterized by
their dominant mineral. Mica schist may be formed by the metamorphism of
either sedimentary or igneous rocks.
Gneiss and schist are not native to Illinois but are found in the
glacial drift.