Part 17

Fault. A break in the rocks in which there has been displacement of the two sides relative to each other.

Fault block range. A mountain range bounded on two or more sides by faults.

Feldspar. A group of light-colored aluminum silicate minerals that are major constituents of igneous rocks. They contain potassium, sodium, and calcium in differing proportions.

Fold. A bend in rock layers.

Foraminiferida. One-celled marine animals with microscopic, perforated, many-chambered calcium carbonate shells, often called forams.

Fossil. The remains or traces of an animal or plant which has been preserved in the rock.

Fusulinid. One-celled marine animals (forams) with shells which look like a grain of wheat in shape and size, frequently abundant in Colorado Pennsylvanian rocks.

Galena. A heavy gray metallic mineral (PbS), often cubic in form, that is the most important ore of lead.

Gangue. Nonvaluable minerals occurring in veins with ore minerals.

Glaciation. Alteration of the earth’s surface by erosion and deposition by glacier ice.

Glacier. A body of ice originating on land by recrystallization of snow, and showing evidence of movement by flowing.

Gneiss. A coarse-grained metamorphic rock usually banded with streaks of darker, finer-grained rock.

Granite. An intrusive igneous rock consisting essentially of sodium or potassium feldspar and quartz, often speckled with dark-colored minerals.

Graptolite. Extinct marine organisms without known close living relatives, with small black sawblade-like chitinous hard parts preserved as fossils.

Hematite. A steel gray or metallic grayish black or reddish gray mineral (Fe₂O₃) that is an important ore of iron.

Hogback. A sharp-crested ridge formed by a resistant layer of steeply dipping rock.

Huebnerite. A heavy reddish brown mineral (MnWO₄) that is a major ore of tungsten.

Igneous rocks. Rocks formed by solidification from a molten state, either at the surface (extrusive) or below the surface (intrusive).

Intrusive rocks. Igneous rocks formed when molten rock material solidifies without reaching the surface.

Joint. A fracture in the rock, along which no discernible movement has taken place.

Kerogen. Solid bituminous material in oil shales.

Laccolith. A lens-shaped mass of igneous rock intruded into layered rocks.

Lava. Fluid or molten rock such as that which issues from a volcano.

Lode. A rock mass, often a vein, containing valuable minerals.

Massif. A mountainous mass that has relatively uniform geologic characteristics and which may embrace a number of peaks.

Mesa. A flat-topped mountain bounded on at least one side by a steep cliff.

Metamorphic rock. Rock formed by alteration of pre-existing rock, especially by great temperatures and pressures.

Mollusk. Any one of the large group of invertebrate animals which includes the snails, clams, octopuses, squids, and their extinct relatives.