The early settlers of Illinois recognized the value of limestone and quarried stone blocks and slabs for making foundations, chimneys, and even houses. For mortar they used a mixture of sand and lime to hold the blocks together. The lime was made by heating limestone red hot in simple furnaces or kilns, the ruins of a few of which may still be seen. Kinds of Limestone Illinois has two principal varieties of limestone, referred to technically as limestone and dolomite. “Limestone” may be used as a general name for both varieties. Limestone consists principally of crystalline particles of the mineral calcite (fig. 1). This mineral is glassy in appearance and is composed of calcium, carbon, and oxygen combined to form calcium carbonate—CaCO₃. Dolomite is largely made of crystalline particles of the mineral dolomite, which also has a glassy appearance and consists of calcium, magnesium, carbon, and oxygen—CaMg(CO₃)₂. The crystalline particles of limestone and dolomite vary in size. Some are coarse enough to be seen easily, others are so small that they can be distinguished only with a microscope. Formation of Limestone and Dolomite Almost all Illinois limestones were formed in seas that covered Illinois millions of years ago. The many different limestone formations in Illinois suggest that oceans covered all or part of the area several times. Numerous kinds of shell fish, corals, and other marine animals lived in these oceans and had shells and other hard parts made of calcium carbonate. Through countless generations, these animal remains accumulated on the ocean floor and gradually were compacted and cemented into limestone (fig. 2). Other Illinois limestones, however, were formed by the hardening of muds composed mainly of calcium carbonate that accumulated on the floors of the ancient seas. Still other limestones were formed of a combination of animal remains and lime mud. [Illustration: Figure 1—Calcite crystals. Limestone is made up mainly of calcite crystals, but they are less perfectly formed and are crowded together.] The coral reefs of the South Pacific Ocean have their counterparts in Illinois. The ancient Illinois oceans contained extensive reefs that were built up just as the modern reefs have been. In northern Illinois, around Chicago for instance, a number of the ancient reefs are now the site of stone quarries. In southwestern Illinois such reefs are a source of petroleum.