Records of wells (well logs) and rock cores or samples collected when test holes are drilled into the bedrock also reveal much about the sequence of rock strata beneath the surface. The kinds of rocks encountered tell, in addition, something about ancient geography, for some were formed on land and others were deposited in long-departed seas. Such knowledge is extremely important in finding and developing mineral resources such as coal and oil. ECONOMIC GEOLOGY Minerals produced commercially in Illinois include crude oil, coal, limestone, dolomite, clay, sand, gravel, fluorspar, tripoli, ganister, novaculite gravel, silica sand, and the metals zinc and lead. The distribution of mineral industries, shown in the several maps of figures 8 through 12, is, of course, related to the distribution of the rocks (plate 1). For example, the coal mines are scattered along the margin of the area of Pennsylvanian rocks where the coals are at relatively shallow depths. The mineral fuels, coal and petroleum, are the leading mineral products of Illinois, making up about 70 percent of the annual value of all minerals produced in the state. For many years coal was at the top of the list, but petroleum now holds first place. Petroleum In recent years production of petroleum has averaged about 78 million barrels each year, making Illinois the eighth largest petroleum producing state. The major oil area is the deep part of the Illinois Basin in the southcentral and southeastern parts of the state, but significant discoveries have been made recently in central and western Illinois. There are about 490 oil fields in Illinois, ranging from a few to several thousand acres in size. Oil has been discovered in rocks of the Pennsylvanian, Mississippian, Devonian, Silurian, and Ordovician Systems, but the Mississippian are the most productive and account for about 76 percent of our total oil production. [Illustration: Figure 8—Oil pools in Illinois as of January 1, 1961. The pools are concentrated mainly in the Illinois Basin and along the LaSalle Anticlinal Belt.] [Illustration: Figure 9—Active coal mines are scattered around the margin of the basin where the coals are at relatively shallow depths.] KEY SHIPPING MINES Underground Strip Slope LOCAL MINES OVER 25,000 TONS Underground Strip Coal Illinois ranks fourth in the nation among states that produce bituminous coal, the type of coal most widely mined in the United States. Coal is used by many manufacturing industries, by public utilities companies that burn coal to generate electricity, and by individuals for heating homes.