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 o