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.