CHAPTER VI. THE COMMON ROCKS AND SOILS AS MINERAL RESOURCES 80 ECONOMIC FEATURES OF THE COMMON ROCKS 80 Granite 82 Basalt and related types 82 Limestone, marl, chalk 82 Marble 83 Sand, sandstone, quartzite (and quartz) 84 "Sand and gravel" 84 Clay, shale, slate 85 The feldspars 86 Hydraulic cement (including Portland, natural, and Puzzolan cements) 86 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