The New Stone Age

sical state admitting of fine grinding with as low power consumption as possible. The finished product must contain silica, lime, and alumina in the right proportions, and must be free from excessive amounts of ferric oxide, magnesium, potash, and soda. The raw materials therefore must be so chosen as to provide the proper ingredients. The silica and alumina are customarily derived from clay or shale. The lime is obtained as limestone or marl, if it contains not less than eighteen per cent. of lime, and clay or chalk. In compounding the raw material mix, due allowance is made for such clay as may be present in the marl. By-products, such as slag and caustic soda waste, are also used. In calcareous materials lime predominates, and this class is represented by limestone, marl, chalk, and alkali waste. Silica and alumina predominate in the agrillaceous materials, such as clay, shale, slate waste, and slag. Combinations having the right proportion of the three essential elements may be madeup from cement rock and limestone, or from limestone with shale or clay, sometimes from slag and limestone, and frequently from marl or shale with clay. Less frequently, alkali waste is proportioned with clay. A typical cement rock found near Copely, Pennsylvania, will analyze as follows: calcium carbonate, 67.14; magnesium carbonate, 2.90; silica, 18.34; iron oxide and alumina, 7.49; sodium and potassium oxides, 0.19; water and undetermined, 3.94. Cement rock from other local...