Part 28

Fault—a break in the rocks or strata of the earth’s crust along which movement or slippage has taken place.

Fluid—a substance made up of particles that can move freely about; it can be a liquid or a gas.

Formation—rocks or strata that are recognized and mapped as a unit.

Fracture—the kind of surface obtained if a mineral is broken in a different direction from that of the cleavage or parting. Commonly, fracture surfaces are rough, uneven, or curved, whereas cleavage surfaces are smooth.

Geologic map (areal)—shows the extent and distribution of formations exposed at the earth’s surface.

Granular—the texture of a rock or mineral that is made up of visible grains. If all the grains are about the same size, the term equigranular is used.

Granule—a rock or mineral fragment that has a diameter of from 2 to 4 millimeters (about ⁸/₁₀₀ to ¹⁵/₁₀₀ of an inch).

Gravel—uncemented rock or mineral fragments that have diameters greater than 2 millimeters (about ⁸/₁₀₀ of an inch).

Gulf Coastal Plain—an area that extends, in Texas, from the Gulf of Mexico to the Balcones fault zone and in which Quaternary, Tertiary, and Upper Cretaceous strata crop out at the surface (see p. 42).

High Plains—an area in northwest Texas extending from the Pecos River valley north to the Oklahoma-Texas boundary (see p. 42).

Igneous rocks—rocks formed by the cooling and hardening of hot, molten rock material.

Intrusive rocks—igneous rocks that have formed below the surface of the earth.

Lava—molten rock material that has poured out onto the earth’s surface from volcanoes; also the rock that is formed after the molten material has cooled and hardened.

Llano uplift—an area in central Texas where Precambrian and early Paleozoic rocks occur at the earth’s surface (see p. 42).

Magma—hot, molten rock material from which igneous rocks form.

Massive—in a mass, without a regular or complete form.

Mesozoic—an era, one of the great divisions of geologic time (see p. 3). This era began about 230 million years ago and lasted until about 63 million years ago.

Metamorphic rock—rock formed from igneous or sedimentary rocks that are altered by heat, pressure, and fluids below the earth’s surface.

Miocene—the fourth epoch of the Tertiary Period (see p. 3).

Mississippian—the fifth period of the Paleozoic Era (see p. 3).

Nodule—a small, rounded mass or lump.

Octahedron—a solid that has eight triangular faces.

Opaque—no light can pass through.

Ordovician—the second period of the Paleozoic Era (see p. 3).

Paleozoic—an era, one of the great divisions of geologic time (see p. 3). This era began at the end of Precambrian time and lasted until about 230 million years ago.

Parting—occurs when a mineral breaks along a flat surface that is not a true cleavage plane.

Pebble—a rock or mineral fragment that has a diameter between 4 and 64 millimeters (about ¹⁵/₁₀₀ and 2½ inches).

Pennsylvanian—the sixth period of the Paleozoic Era (see p. 3).