The middle or lower slopes of bluffs are common sites for spring-deposited tufa or travertine. Deposits have been seen in various parts of Illinois, but are not known to have been worked, except in Calhoun County where small quantities of tufa were produced for use as agricultural limestone. It is thought that the tufa and travertine deposits of Illinois are relatively small, but they may be of local importance. [Illustration: Figure 23—Calcareous tufa from Pike County. In addition to the large visible pores there are numerous tiny ones.] Pyrite and Marcasite The mineral pyrite consists of iron and sulfur as the compound iron sulfide (FeS₂) and, because of its shiny, brassy yellow color, is sometimes called fool’s gold. Marcasite has the same composition as pyrite, but its crystals have a different shape and it is often lighter colored. Both minerals occur in various parts of the state; they are particularly prevalent in some coal seams, and when so occurring are in some cases called coal brasses or “sulfur.” Pyrite and marcasite are used commercially as raw materials for making sulfuric acid, although sulfur itself is more extensively used for that purpose. At one time coal brasses recovered during coal-cleaning operations at a northwestern Illinois coal mine were sold for acid making. A large quantity of coal brasses probably could be recovered from such operations at Illinois coal mines. Uranium Uranium has been sought in Illinois by many people in recent years. The Geological Survey also carried out a wide search for uranium and particular attention was paid to certain clays and other rocks in Hardin County and certain black shales in other parts of the state. About 200 samples from Hardin County and 175 samples of the shales were tested by the Survey. No deposits were found that are known to be of the required richness and quantity. Iron Ore About the middle of the 19th century two furnaces in Hardin County in extreme southern Illinois for a time produced iron from local limonite ores. The ore is said to have occurred as pellets, chunks, and masses scattered through soil and clay and apparently was of irregular distribution. Little can be seen of the deposits today. Their extent is not known, but they are believed to be of limited size. The Illinois State Geological Survey carries on a continuous program of research on the industrial minerals and metals of Illinois and their uses. In addition to the investigations mentioned in this booklet, many others have been made or are in progress. Such studies are necessarily a continuing activity if the full potentialities of Illinois mineral resources are to be realized, for industry continually demands new raw materials and changes its requirements for those now used.