Lead and Zinc Lead and zinc are the only metallic minerals mined in Illinois. Deposits are found in Jo Daviess County in northwestern Illinois and in the fluorspar area of Hardin and Pope Counties in extreme southern Illinois. In northwestern Illinois the ore occurs in rocks of Ordovician age, but in southeastern Illinois it is associated with fluorspar in rocks of Mississippian age. Between 1820 and 1865, the mining area of which northwestern Illinois is a part was the nation’s principal producer of lead ore. In recent years production of zinc has increased and that of lead has decreased. Water Supplies Water, too, is a mineral resource, and our most necessary one. The source of nearly all water is rain and snow, which collect in lakes and streams or seep into the ground to form our two types of water supplies—surface water and ground water. Ground water in Illinois collects underground in deposits of sand and gravel or in the porous sandstone or limestone layers of bedrock. These rock materials are called “aquifers” because of their ability to hold water and to allow it to flow into wells that are drilled into them. The aquifers are not equally distributed throughout Illinois, so ground-water supplies are abundant in some parts of the state but are scarce in others. Some Illinois cities depend wholly or partly on surface supplies from lakes and rivers. The sand and gravel aquifers yield more ground water than any others in Illinois. The major sand and gravel aquifers were deposited by streams and lie along river valleys. Some of these river valleys are no longer visible to us because they were buried by glacial drift. They are recognized, however, from well records or drilling records. The northern third of Illinois has the most abundant supplies of ground water. Bedrock formations of Devonian through Cambrian age are favorable aquifers and supply drinkable (potable) water at depths of from 200 to 2,000 feet. These formations, however, lie much deeper in the Illinois Basin, where the water they furnish is too salty for most uses. West of the Illinois River and at the southern tip of the state, the Mississippian, Devonian, and Silurian limestones are the chief aquifers. Their yield, however, is small. In the Illinois Basin all these formations are overlain by Pennsylvanian rocks which offer only small, scattered supplies of ground water. ILLINOIS STATE GEOLOGICAL SURVEY