*The Geology of Texas, Vol. II, Structural and Economic Geology, by E. H. Sellards, C. L. Baker, and others. Univ. Texas Bull. 3401 (1935). *Texas Mineral Resources, by E. H. Sellards and others. Univ. Texas Pub. 4301 (1946). *Geological Resources of the Trinity River Tributary Area in Texas and Oklahoma, by H. B. Stenzel, A. E. Weissenborn, and others. Univ. Texas Pub. 4824 (1948). Uranium at Palangana Salt Dome, Duval County, Texas, by A. D. Weeks and D. H. Eargle. _In_ U. S. Geol. Survey Prof. Paper 400-B (1960). Geology of the Quicksilver Deposits of the Terlingua District, Texas, by R. G. Yates and G. A. Thompson. U. S. Geol. Survey Prof. Paper 312 (1959). GLOSSARY Amorphous—without crystalline structure and therefore without regular form. Balcones fault zone—a system of faults extending from north of Waco in McLennan County, through Travis and Bexar counties, to near Del Rio in Val Verde County (_see_ p. 42). Boulder—a large rock or mineral fragment that has a diameter greater than 256 millimeters (about 10 inches). Breccia—a rock made up of sharp-cornered, cemented fragments with diameters greater than 2 millimeters (about ⁸/₁₀₀ of an inch). Cambrian—the earliest period of the Paleozoic Era (_see_ p. 3). Cenozoic—the present era, one of the great divisions of geologic time (_see_ p. 3). This era began about 63 million years ago. Clastic—made up of broken fragments of rocks or minerals. Cleavage—occurs when minerals split along smooth flat surfaces that are parallel to possible crystal faces. These planes as well as crystal faces are controlled by the crystal lattice or atomic structures of the minerals. Cleavage fragment—a mineral specimen that has been broken along its planes of cleavage. Cobble—a rock or mineral fragment that has a diameter between 64 and 256 millimeters (about 2½ and 10 inches). Conchoidal—a curved fracture surface shaped like the inside of a shell or spoon. Conglomerate—a rock composed of cemented, rounded rock or mineral fragments, most of which are of gravel size. Cretaceous—the third and latest period of the Mesozoic Era (_see_ p. 3). Cryptocrystalline—made up of tiny crystalline particles that are too small to be distinguished even under high magnification. Crystalline—having a definite, orderly internal structure. Cube—a solid that has six equal, square sides. Dodecahedron—a solid that has twelve plane, four-sided faces. Element—a basic building block of all matter, which cannot be separated into different substances by ordinary chemical means. Eocene—the second epoch of the Tertiary Period (_see_ p. 3). Epoch—a unit of geologic time that is a subdivision of a period. Era—a major division of geologic time, which consists of several periods. Extrusive rocks—igneous rocks formed from magma that was extruded on the earth’s surface. Fault—a break in the rocks or strata of the earth’s crust along which movement or slippage has taken place.