Manganese minerals (braunite, hollandite, pyrolusite) 73 Marble 75 Mica 76 Obsidian and vitrophyre 77 Opal 78 Pegmatite 79 Pyrite 80 Quartz 81 Quartzite 84 Rhyolite 85 Sand and sandstone 85 Schist 87 Serpentine 87 Shale 88 Silver minerals (argentite, cerargyrite, native silver) 89 Sulfur 90 Talc and soapstone 93 Topaz 94 Tourmaline 94 Uranium minerals (carnotite, uranophane, pitchblende) 95 Volcanic ash (pumicite) 97 Composition, hardness, and specific gravity of some Texas minerals 99 Books about rocks and minerals 100 Nontechnical books for beginners 100 Textbooks and other reference books 100 Selected references on Texas rocks and minerals 100 Glossary 102 Index 104 Illustrations Page Guadalupe Peak and El Capitan in the Guadalupe Mountains, Culberson County, Texas 1 Earth’s outer crust 2 Geologic time scale 3 Generalized geologic map of Texas 4-5 A mineral is made up of chemical elements 7 A rock is made up of minerals 8 Extrusive igneous rocks form at the earth’s surface 9 Intrusive igneous rocks form beneath the earth’s surface 10 Soils develop from weathered rock and associated organic material 11 Conglomerate from Webb County, Texas 11 Precipitated sediments lining a teakettle 12 Contact metamorphism 13 A scalenohedron 14 Barite specimen showing radial form 15 Chalcedony showing botryoidal form 16 Transparent mineral 16 Streak plate 17 Conchoidal fracture 18 Stalactites and stalagmites in the Caverns of Sonora, Sutton County, Texas 19 Calcite geode from Travis County, Texas 20 Petrified wood from Texas Gulf Coastal Plain 20 Prospector’s hammer 22 Hand lens 22 Physiographic outline map of Texas 42 Massive anhydrite 43 Amphibole asbestos from Gillespie County, Texas 44 Barite cleavage fragment from west Texas 44 Basalt from Brewster County, Texas 45 Calcite has perfect rhombohedral cleavage 46 Calcite crystals (dog-tooth spar) from the Terlingua area of Brewster County, Texas 47 Celestite cleavage fragment from Lampasas County, Texas 48 Cinnabar and calcite crystals from the Terlingua area of Brewster County, Texas 50 Bentonite is used as a drilling-fluid additive 51 Hazel copper-silver mine, Culberson County, Texas 53 Dolomite rock from Burnet County, Texas 54 Feldspar cleavage fragment from Llano County, Texas 55 Microcline feldspar crystals from Llano County, Texas 56 Fluorite has octahedral cleavage 57 Galena has perfect cubic cleavage 57 Garnet crystal forms 58 Gneiss from Blanco County, Texas 59 Placer gold in stream gravels 60 Granite from Gillespie County, Texas 61 Texas State Capitol building at Austin is made of Burnet County granite 62