Chapter VII: Introduction (5)

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.

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

B
Babyhead: 72
Balcones fault zone: 10, 42, 45, 88
Bandera County: 45
Baringer Hill pegmatite: 80
barite: 15, 28, 31, 32, 35, 36, 38, 44, 48
barium: 45
Barrilla Mountains: 85
Baylor County: 45, 52, 98
basal cleavage: 76, 93, 94
basalt: 40, 41, 45
basement: 2, 9
Bastrop County: 64
Bell County: 69
bentonite: 51, 52
beryllium: 80
Big Bend area: 9, 42, 51, 75, 77
Big Branch Gneiss: 59
biotite: 32, 33, 34, 76
Bird mine: 90
Blanco County: 44, 58, 59, 61, 70, 83, 87, 94
black mica (_see_ biotite)
blast furnace: 70, 71
blast sand: 86
bleaching clay: 51
blue copper: 53
Boerne: 18
Boling salt dome: 91
books about rocks and minerals: 24, 39, 100
brass: 52
braunite: 26, 27, 73
Brazoria County: 91
Brazos County: 98
Brazos River, Double Mountain Fork and Salt Fork of: 43
breccia: 12, 41, 81
Brewster County: 45, 46, 47, 49, 50, 57, 65, 70, 76, 77, 78, 82,
84, 85, 90
brimstone: 90
bronze: 52
Brooks County: 64
Brown, Thomas E.: vii
Brown County: 45, 49
brown iron ore (_see also_ limonite): 70
Bryan Mound salt dome: 91
building sand: 85
Burnet: 56, 80, 97
Burnet County: 18, 53, 54, 55, 56, 58, 59, 60, 61, 62, 63, 70, 76,
79, 80, 81, 82, 83, 97