The Pennsylvanian is one of our most important groups of rock strata because it contains all of our minable coal beds, as well as important deposits of limestone, shale, clay, sandstone, and some oil and gas. The Pennsylvanian rocks are very widespread in Illinois, occurring under the glacial drift from depths of a few feet to several hundred feet throughout about two-thirds of the glaciated area (fig. 1). Next below the Pennsylvanian are the Mississippian rocks. We in Illinois are particularly interested in this division of rocks because they take their name from the excellent exposures along the Mississippi River valley in western Illinois, southeastern Iowa, and eastern Missouri. They are composed of extensive beds of limestone and cherty limestone, sandstone, and shale. Mississippian rocks are of great economic importance in the structural area known as the Illinois Basin, where they are the most important oil producing rocks. They also contain our fluorspar deposits and along the valley bluffs are an excellent source of limestone for quarrying. Rocks older than the Mississippian—except for small areas along the Mississippi and Illinois River valleys—are found at the surface only in the northern quarter of the state and locally in Hardin County near the southern tip of the state. They are nonetheless economically important because from these older rocks are produced lead and zinc, some oil and gas, silica sand, limestone, dolomite, and shale. On the generalized rock column (fig. 2) these older rocks are grouped into two units. The uppermost contains the Devonian and Silurian and the lower contains the Ordovician and Cambrian. In general they include dolomite, limestone, and shale, with sandstone at several places, especially in the lower unit. DESCRIPTION OF ROCKS AND MINERALS The terms mineral and rock are often confused. They are frequently used together and the materials they describe are closely related. In general, a mineral is a naturally occurring chemical element or compound formed by inorganic processes, whereas a rock is a mixture of particles or grains of several minerals. However, when we refer to mineral resources or industrial minerals, we generally include materials that are technically rocks—such as limestone, dolomite, shale—and also coal and oil that are in fact organic substances. On the other hand, we include in the rock category high purity sandstone that is composed almost entirely of one mineral (quartz) and high purity limestone that is composed largely of the mineral calcite. MINERALS A few minerals are composed of only one element, such as diamond (carbon) and native copper, but most minerals are chemical compounds that contain several elements.