About $53,900,000 worth of clay products were manufactured in Illinois in 1963. SAND AND GRAVEL WHERE DO SAND AND GRAVEL COME FROM? Glacial deposits of sand and gravel are the chief source of these materials in Illinois, but chert or flint gravels that are older than the glacial deposits are known and utilized locally in western and extreme southern Illinois. Sands of recent origin are dredged from deposits in the larger rivers and streams (fig. 17), and commercial silica sand is produced from sandstone bedrock. [Illustration: Figure 17—Dredge equipment used in producing sand and gravel.] WHAT IS THE VALUE OF COMMON ILLINOIS SAND AND GRAVEL PRODUCTION? Production of common sand and gravel totaled 27,115,000 tons at a value of $24,800,000 in 1963. SILICA SAND WHAT IS SILICA SAND? Although natural sands are aggregations of many kinds of small rock and mineral fragments, silica sand consists almost entirely of fine grains of a single mineral, quartz. The principal sources of Illinois silica sand are in LaSalle (Ottawa, Utica, Wedron, and Troy Grove areas) and Ogle (Oregon area) Counties. HOW IS SILICA SAND USED? Silica sand, produced in northern Illinois, is famous for its high purity, and is widely used in making glass. More than two-thirds of the raw material in common glass is silica sand. Quantities of silica sand are used as molding sand, because it can withstand the high temperatures produced in casting steel and other metals. It also is used for grinding and smoothing plate glass, for sand blasting, and for fracturing sand used to increase the production of oil wells. Some silica sand is ground to a fine powder and utilized as an ingredient in scouring compounds, paint fillers, pottery, glazes, and enamels. A specially sized sand is produced from the St. Peter Sandstone Formation (Ordovician in age) that is used in testing the strength of cements and as a laboratory standard in various kinds of tests. HOW VALUABLE IS SILICA SAND TO THE ECONOMY OF THE STATE? Illinois is a major producer of silica sand, having produced 2,900,000 tons in 1963. The silica sand and ground silica produced in the same year was valued at $11,400,000. TRIPOLI WHAT IS TRIPOLI AND WHERE IS IT FOUND? Tripoli, also known as amorphous silica, is mined in Alexander County in southern Illinois. It is prepared for market by being ground to a fine powder. It consists of tiny particles of quartz. WHAT ARE SOME OF THE USES FOR TRIPOLI? Tripoli is used as “white rouge” in optical lens polishing, as a paint filler, as a fine abrasive, in the ceramic industry, and for many other purposes. FLUORSPAR WHAT IS FLUORSPAR?