PERIDOTITE is the only igneous rock native to Illinois that crops out at
the surface. It is found as dikes (irregular veins) or sills (thin
sheets) that were formed when molten rock from deeper in the earth
intruded into cracks and fissures in the bedrock of southeastern
Illinois (Hardin, Pope, Gallatin, and Saline Counties). Peridotite
pebbles and boulders also may be found in the glacial drift.
Peridotite ranges from very fine grained to medium grained and has an
even texture. It is dark gray to greenish gray, depending on the
minerals present. In general it is composed of olivine, hornblende,
pyroxene, and mica, with little or no feldspar or quartz.