The Illinois silica sand industry is based on the St. Peter Sandstone. It centers around Ottawa, Wedron, Troy Grove, and Utica in LaSalle County and in Oregon in Ogle County. Two principal grades of silica sand are produced—washed and crude. The value of the silica sand produced in Illinois in 1963 was about $9,000,000. _Washed Sand._—Although the St. Peter Sandstone is composed almost entirely of quartz grains, a small amount of clay is present. For some uses it is not necessary to remove the clay, for others its elimination is important and is achieved by washing the sand. In the mining of silica sand that is to be washed, the sandstone is first blasted loose from the parent deposit to break it into sand or pieces of various sizes. Some of the larger pieces may require a second blasting to disintegrate them. At some pits the material is loaded mechanically and transported to the washing plant. At others a powerful stream of water is directed against the broken sandstone (fig. 14) and the resulting mixture of sand and water flows to a collecting basin from which it is pumped through large pipes to the processing mill. In both types of operation the sand is thoroughly washed at the plants. After it is washed, the sand is further processed to suit the needs of its users. Much of it is screened into different size grades. _Uses of Washed Silica Sand._—The washed silica sand produced in Illinois has many uses, some of which are briefly mentioned below. The suitability of the sand for some purposes depends in part on its having been screened to specified sizes. The high purity of Illinois washed silica sand makes it suitable for making glass, which is more than half silica sand. Each year over a million tons is used for this purpose. The purity of the sand also is of importance for chemical and metallurgical uses such as the manufacture of sodium silicate and silicon carbide and in alloying. The hardness of the sand makes it useful for grinding large sheets of plate glass to prepare them for polishing and also makes it an effective abrasive agent for sandblasting. Metal castings in foundries and the exteriors of buildings are cleaned by this process. Illinois produces thousands of tons of sand yearly for such abrasive purposes. Because the coarser grains of the washed silica sand are rounded, strong, and available in uniform sizes, oil operators use thousands of tons of it annually in the hydraulic fracturing of oil-bearing strata. The sand is mixed with oil, other petroleum products, or water and is forced by powerful pumps into sandstone or limestone formations that contain oil. The great force thus exerted opens fractures in the rock strata and pushes the liquid and sand into them. When the pressure is relieved, the sand grains serve as props to hold the fractures open. The oil can then flow more easily into the wells and oil production is thus increased.