CHAPTER VIII. THE ENERGY RESOURCES--COAL, OIL, GAS (AND ASPHALT) 115 COAL 115 ECONOMIC FEATURES 115 World production and trade 115 Production in the United States 117 Coke 118 Classification of coals 119 GEOLOGIC FEATURES 123 PETROLEUM 127 ECONOMIC FEATURES 127 Production and reserves 128 Methods of estimating reserves 134 Classes of oils 136 Conservation of oil 137 GEOLOGIC FEATURES 140 Organic theory of origin 140 Effect of differential pressures and folding on oil genesis and migration 142 Inorganic theory of origin 143 Oil exploration 144 OIL SHALES 150 NATURAL GAS 151 Economic features 151 Geologic features 151 ASPHALT AND BITUMEN 151 Economic features 151 Geologic features 153 CHAPTER IX. MINERALS USED IN THE PRODUCTION OF IRON AND STEEL (THE FERRO-ALLOY GROUP) 154 GENERAL FEATURES 154 IRON ORES 158 ECONOMIC FEATURES 158 Technical and commercial factors determining use of iron ore materials 158 Geographic distribution of iron ore production 160 World reserves and future production of iron ore 162 GEOLOGIC FEATURES 166 Sedimentary iron ores 166 Iron ores associated with igneous rocks 171 Iron ores due to weathering of igneous rocks 171 Iron ores due to weathering of sulphide ores 173