The mineral is fluorite (fig. 9), commonly called fluorspar. It is composed of calcium fluoride (CaF₂), a compound of calcium and fluorine, and is a glassy mineral that is generally white or gray but may be purple, rose, yellow, blue, or green. In rare instances it is colorless. Fluorspar mining in Hardin and Pope Counties began with lead mining. Galena was first discovered there in 1839 in a well being dug at the town of Rosiclare. Mining of galena began in the early 1840’s, and somewhat later ore was being smelted by three furnaces, all of which have long since disappeared. The veins that were worked for galena also contained fluorspar, but as there was little or no use for fluorspar in the 1840’s it was considered waste. In time, uses developed, however, and about 1870 it was mined and shipped in commercial quantities. Since then the tonnage and value of the fluorspar produced from the Rosiclare area have increased until fluorspar is the major product. [Illustration: Figure 9—Group of cubic fluorite crystals.] The fluorspar mining district north of the town of Cave in Rock in eastern Hardin County also was an early producer of galena. In that area the fluorspar-galena deposits are elongate and approximately flat. The first miners followed the ore bodies from outcrops by tunneling into the hillsides. In the late 1930’s and early 1940’s, many holes were drilled into the bedrock in search of new deposits. Ore was found that contained not only galena and fluorspar but also important amounts of sphalerite. _Vein Deposits._—In the Rosiclare district, the fluorspar and its accompanying minerals occur as steeply inclined veins a few inches to 25 feet or more wide (fig. 10), usually in limestone strata. The veins are not uniformly thick but widen or narrow from place to place both vertically and horizontally. They occur along faults—planes along which the rocks of the earth’s crust have broken and slipped. A fault may be a single plane of slippage but more often is a zone of broken and displaced rocks. In most of the faults that contain fluorspar, the slippage is vertical, or nearly so. Along one of the faults in the Rosiclare district, the rocks on one side of the fault have moved downward as much as 650 feet in relation to the rocks on the other side. Some faults are more than 10 miles long, and the depth to which they extend into the earth is unknown. Fluorspar has been mined from one of them at depths of 800 feet. Not all faults, nor all parts of any one fault, contain fluorspar. [Illustration: Figure 10—Diagrammatic cross section of fluorspar vein along a fault. The strata on the left side of the fault have moved downward with reference to those on the right side.] SOIL FAULT FLUORSPAR VEIN ALONG FAULT down SANDSTONE A LIMESTONE B SHALE C SANDSTONE D LIMESTONE E up LIMESTONE B SHALE C SANDSTONE D LIMESTONE E