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. GNEISS AND SCHIST (6) 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. CONGLOMERATE (7) CONGLOMERATE is a sedimentary rock made up of pebbles or other rock fragments cemented in a background or matrix of finer material, generally silica, calcium carbonate, clay, iron oxide, or a mixture of these substances. The rounded rock fragments have been worn by being rolled in streams and along beaches. [Illustration: uncaptioned] If the pebbles embedded in the matrix are sharp and angular, freshly broken and not worn, the rock is called breccia and is generally found near the place where the fragments originated. Conglomerate or breccia may be made up of any type of rock or mineral, most commonly durable material such as chert, quartz, quartzite, granite, and gneiss. In Illinois, conglomerates commonly are found at the base of sandstone formations and as beds in the lower “Coal Measures.” They also are found in some gravel deposits. SANDSTONE (8) SANDSTONE is a clastic sedimentary rock consisting of sand-sized grains (one sixty-fourth to one-sixteenth inch in diameter) held together by a cementing material. As sandstones become finer grained they grade into siltstones; as they become coarser grained they grade into conglomerate. The shape of sand grains in sandstones ranges from rounded to angular.