Gneiss Interlocking grains are in straight or wavy bands Schist Splits in thin layers; some schist is soft enough to be scratched by a knife 4. FRAGMENTAL appearance (rocks are made up of fragments that are either loose or cemented together) Breccia Angular, gravel-size fragments that are cemented together Conglomerate Rounded, gravel-size fragments that are cemented together Coquina Shells and shell fragments that are cemented together Gravel Loose fragments Pulverulent Loose, powdery fragments; dilute limestone hydrochloric acid fizzes on them Sand Loose fragments no larger than a pinhead Sandstone Sand fragments that are cemented together Volcanic ash Loose, fine, gritty particles [Illustration: Physiographic outline map of Texas.] DESCRIPTIONS OF SOME TEXAS ROCKS AND MINERALS The pages that follow contain descriptions of Texas rocks and minerals. The descriptions are given in alphabetical order, except that related varieties are described together. For example, agate, amethyst, chert, flint, jasper, onyx, and chalcedony are discussed under quartz, because they are varieties of quartz. The descriptions include the properties of the rock or mineral that will help you identify it and also include information on where the rock or mineral can be found in Texas, some of its uses, and how it may have formed. The chart on page 99 lists chemical composition, specific gravity, and hardness of various Texas minerals. Agate. _See_ Quartz. Agatized Wood. _See_ Quartz. Alabaster. _See_ Gypsum. Albite. _See_ Feldspar. Almandite. _See_ Garnet. Amethyst. _See_ Quartz. Amphibole Asbestos. _See_ Asbestos. Anhydrite Anhydrite, calcium sulfate, is a rather soft mineral that you can scratch with a pocket knife, although not with a fingernail. It has a glassy or a pearly luster and is transparent or translucent. Most anhydrite is white, but impurities cause it to be grayish, bluish, or reddish. When rubbed across a streak plate, anhydrite gives a white streak. This mineral has an uneven fracture, and it cleaves in three directions that are at right angles to each other. It commonly occurs as rectangular cleavage fragments or as sugary crystalline masses.