GEOLOGIC FEATURES OF THE COMMON ROCKS 88
Building stone 88
Crushed stone 90
Stone for metallurgical purposes 91
Clay 91
Limitations of geologic field in commercial investigation
of common rocks 92

SOILS AS A MINERAL RESOURCE 94
Origin of soils 94
Composition of soils and plant growth 96
Use of geology in soil study 97

CHAPTER VII. THE FERTILIZER GROUP OF MINERALS 99

GENERAL COMMENTS 99

NITRATES 101
Economic features 101
Geologic features 102

PHOSPHATES 104
Economic features 104
Geologic features 105

PYRITE 107
Economic features 107
Geologic features 108

SULPHUR 109
Economic features 109
Geologic features 110

POTASH 111
Economic features 111
Geologic features 112

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
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