S salt (_see also_ halite): 40, 65, 66 domes: 43, 58, 64, 66, 91, 97 lakes: 66 rock: 40, 65 Salt Fork of the Brazos River: 43 sand: 41, 58, 72, 85, 91 blast: 86 building: 85 filtering: 86 glass: 85-86 sandstone: 41, 52, 55, 58, 59, 72, 76, 81, 84, 86 Sandy Creek: 60 San Saba County: 70, 88 Santa Anna: 86 satin spar: 64 Satorsky, Cyril: vii scale, geologic time: 2, 3 scintillation counter: 96 schist: 41, 56, 58, 72, 73, 76, 81, 87, 93 actinolite: 87 graphite: 63, 87 hornblende: 87 mica: 76, 87 talc: 87, 93 schistosity: 87 schorl: 95 Scurry County: 98 sedimentary rocks, definition of: 10 sediments: 8, 11 chemical: 12 selenite gypsum: 63, 64, 76 series: 2 serpentine: 34, 40, 44, 75, 87, 93 serpentinite: 87 Seven Heart Gap: 45 Shafter: 57, 58, 60, 67, 90 Shafter Lake: 83 shale: 12, 13, 19, 40, 45, 70, 87, 88 oil: 88 Shelby, Cader A.: vii Sheridan prospect: 53 siderite: 70 Sierra Blanca: 58, 67, 82 silicified wood (_see also_ petrified wood): 20 silt: 12, 85 siltstones: 12 silver: 26, 52, 57, 59, 89 glance: 90 horn: 90 minerals: 89 native: 26, 89 Silver Creek: 58 sinter, calcareous: 46 slate: 13 Smith County: 70 smoky quartz: 82, 83 soapstone: 40, 93 soils: 10 Solitario uplift: 42, 88 Sonora: 18 Caverns of: 19 specific gravity: 18, 99 specular hematite: 67 sphalerite: 57 Spiller mine: 74 Spindletop salt dome: 91 Spring Creek: 56 stalactites: 18, 19 stalagmites: 18, 19 Starr County: 84, 98 static metamorphism: 13 Stonewall County: 52 streak or powder of minerals: 17 streak plate: 17, 22 Streeter: 48, 94 striations: 81, 95 strontianite: 49 strontium: 49 sulfur: 29, 30, 31, 32, 34, 36, 81, 90 Sutton County: 18, 19 system: 2 T talc: 28, 29, 34, 87, 93 schist: 87, 93 Talc Rock: 93 Taylor County: 45, 49, 60 Terlingua area: 46, 47, 49, 50, 65 Tertiary: 2, 3, 9, 21, 45, 49, 51, 52, 56, 57, 60, 70, 77, 78, 79, 83, 84, 85, 86, 90, 97, 98 terrazzo— chips: 55, 76 floors: 88 tests, chemical (_see_ chemical tests) thorium: 80 Thrall oil field: 88 time, geologic: 2, 3 tin: 47 topaz: 36, 38, 94 tourmaline: 33, 82, 94 Town Mountain: 95 Trans-Pecos area: 9, 10, 45, 60, 62, 69, 78, 82, 83, 84, 85, 87, 89, 90, 98 trap rock: 45 travertine: 46 Travis County: 20, 45, 46, 49, 68, 69, 70, 88 tremolite: 44, 93 Triassic: 3, 45, 87 Trinity County: 98 tufa, calcareous: 46 twinned crystals: 46, 63 tyuyamunite: 97 U Upshur County: 70 uranium: 80, 95, 96 minerals: 95 uraninite: 97 uranophane: 31, 95, 97 Uvalde County: 45, 60, 84, 88