Next below the Pennsylvanian are the rocks of the Mississippian System, shown in blue on the map (M¹ and M²). The lower and middle Mississippian rocks (M¹) are largely limestone in and near the areas mapped, but in the central and eastern part of the state where they are buried under Pennsylvanian rocks they contain much siltstone and cherty limestone. The upper Mississippian rocks (M²) consist of a succession of sandstone, shale, and limestone formations. This system of rocks takes its name from the Mississippi River because there are excellent exposures of these strata along the Mississippi Valley in western Illinois, southeastern Iowa, and eastern Missouri. The Mississippian rocks are a source of limestone, fluorspar, zinc, and ganister, and are of greatest economic significance in southeastern Illinois where they are the most important of our oil-producing rocks. The Devonian (D, dark gray on the map), Silurian, (S, violet), Ordovician (O, light pink), and Cambrian (C, deep pink) rocks, in the order named, are older than the Mississippian strata. In general, they include dolomite, limestone, shale, and sandstone. Except for small areas along the Mississippi and Illinois River Valleys, these older rocks are found at the surface only in the northern quarter of the state and locally in Alexander, Hardin, Jackson, Monroe, Pike, and Union Counties. They are nevertheless economically important because they yield limestone, dolomite, silica sand, oil, zinc and lead, tripoli, novaculite, and novaculite gravel. The rocks of the Cambrian through Pennsylvanian Systems belong to the Paleozoic Era. The Paleozoic rocks overlie crystalline rocks, such as granites, that extend to unknown depths in the crust of the earth. The crystalline rocks are not exposed in Illinois but are encountered in the drilling of some deep wells and may be seen in the nearby Missouri Ozarks and in central Wisconsin. STRUCTURAL GEOLOGY The rock formations appear to lie flat in most of Illinois, but they are slightly inclined in most places. In some areas they are down-warped into basins and troughs (synclines), upfolded into domes and arches (anticlines), or broken by faults. The largest structural feature in Illinois is a great spoon-shaped basin—the Illinois Basin—that extends southeastward into Indiana and Kentucky. The deepest part of the basin is in southeastern Illinois. [Illustration: Figure 3—When rocks are down-warped and the surface is leveled by erosion, the youngest rocks are preserved in the center and the older rocks are exposed at the margins.]