Although the lead deposits of extreme northwestern Illinois (now Jo Daviess County) were reported by the French explorers in 1658 and are said to have been worked by the Indians, the influx of white settlers in the early 1800’s marked the beginning of an extensive mining industry, which was an important factor in the early development of that part of the state. The town of Galena takes its name from the mineral galena, which was the principal ore mined. [Illustration: Figure 18—Galena cubes.] In 1845, the Upper Mississippi Valley produced 60 percent of all lead mined in the United States, which then ranked first in world lead production. Until about 1860, the zinc ore was considered useless, but today its total value is several times that of lead. Lead mining began in southern Illinois in 1842. Lead and zinc production in this area is a by-product of fluorspar mining. WHAT IS THE VALUE OF THESE METALS PRODUCED IN ILLINOIS? In 1963, Illinois produced 20,377 tons of zinc valued at about $4,677,500 and 2,901 tons of lead worth about $626,600. GROUND WATER WHAT IS GROUND WATER? Ground water is water that fills all openings in earth materials in the zone of permanent saturation. The top of the zone of saturation is called the water table. The source of ground water is precipitation (rain and snow) that seeps into the soil and percolates downward. Below the water table, ground water moves slowly toward places of discharge such as springs, lakes, rivers, marshes, and wells. Water falls to the ground, moves through the rocks, returns to the surface, and finally gets back to the atmosphere by evaporation and from plants (fig. 19). This cycle is continuously repeated. WHAT KINDS OF ROCKS YIELD GROUND WATER? Ground water is most readily obtained from saturated rocks that have fairly large openings between grains (such as sand, gravel, and sandstone) or have interconnected cracks or channels (such as limestones). Rocks that contain ground water and that will yield it to wells are called aquifers. Sand and gravel beds are widely used aquifers in Illinois. [Illustration: Figure 19—Source, movement, and occurrence of ground water. Arrows in aquifers show direction of flow of water.] _EVAPORATION_ SAND AND GRAVEL WELL Water Table River sand and gravel ARTESIAN WELLS _EVAPORATION_ River WELL COLLECTING AREA FOR ARTESIAN WATER PRECIPITATION Crater WATER TABLE WELL _Lake_ Glacial pebbly clay Glacial sand and aquifer Shale Sandstone aquifer Limestone aquifer Shale GROUND-WATER RESERVOIR IS WATER FOUND IN UNDERGROUND LAKES AND RIVERS IN ILLINOIS? No. There are a few large springs that flow from rocks along river bluffs in Illinois, but most ground water occurs in tiny openings within the rocks. Ground water reservoirs may be regarded as similar to sponges rather than as underground lakes or rivers. WHY DO WELLS SOMETIMES GO DRY?