PERIDOTITE (5)
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.