Texas Rocks and Minerals: An Amateur's Guide

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.

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).

Period—a unit of geologic time, a subdivision of an era.

Permian—the last period of the Paleozoic Era (see p. 3).

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.

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