Physiographic outline map—shows location of natural regions (p. 42). Playa lake—a temporary shallow lake in a nearly level, closed basin, which has no drainage outlet. Pleistocene—the first epoch of the Quaternary Period (_see_ p. 3). Pliocene—the last epoch of the Tertiary Period (_see_ p. 3). Precambrian—comprises the Early and the Late Precambrian Eras, the earliest great divisions of geologic time. Rocks that formed more than 600 million years ago are known as Precambrian rocks. Pyritohedron—a solid that has twelve 5-sided faces. Quaternary—the present period of geologic time; the second period of the Cenozoic Era (_see_ p. 3). Recent—the present epoch of geologic time; the second epoch of the Quaternary Period (_see_ p. 3). Sectile—describes material, such as soap, that can be cut smoothly with a knife. Sediments—material deposited by water, wind, or ice on the earth’s surface. Sedimentary rocks—rocks made up of sediments. Series—a subdivision of a system that includes all rocks formed during an epoch. Specific gravity—the ratio of the weight of a substance to the weight of an equal volume of water. Streak—the color of the powder of a mineral. System—all rocks formed during a period. Tertiary—the first period of the Cenozoic Era (_see_ p. 3). Translucent—light will pass through, but objects cannot be seen. Transparent—light will pass through, and objects can be seen. Trans-Pecos—area of Texas located west of the Pecos River (_see_ p. 42). Volcanic rocks—igneous rocks that have formed on the earth’s surface; extrusive rocks. Index A acid: 18, 22 acid tests (_see also_ chemical tests): 18 actinolite: 82 schist: 87 agate: 21, 83 agatized wood: 20, 84 age, earth’s crust: 2 alabaster: 65 albite: 55 alkali lakes: 66, 83 Allamoore: 87, 93 alloys: 52 almandite: 58 Alpine: 57, 78, 82, 90 Altuda Mountain: 57, 90 amalgam: 51 amazonstone: 56 amethyst: 37, 82 Amethyst Hill: 82 amorphous minerals, definition of: 15 amphibole asbestos: 29, 33, 34, 44 Anderson County: 70 Andrews County: 83 Angelina County: 52 anhydrite: 28, 29, 30, 35, 36, 43, 65, 66, 91 anthophyllite: 93 antigorite: 87 Apache Mountains: 65 apatite: 72 Archer County: 52 argentite: 26, 27, 89, 90 argentiferous galena: 57, 90 Armstrong County: 79 asbestos: 43 amphibole: 29, 33, 34, 44 chrysotile: 44, 87 ash, volcanic: 41, 97 augite: 45 Austin: 46, 62 autunite: 97 azurite: 35, 52, 53, 54