Alluvial fan. A cone-shaped mass of sediment built by rivers or streams
as they issue from mountains onto more level ground.
Alluvium. Stream deposits formed in recent geologic time, composed of
sand, gravel, and stones.
Ammonite. One of a large group of extinct mollusks related to the living
chambered _Nautilus_. Ammonite shells, usually cone-shaped or
coiled, are divided into many chambers by crenellated septa.
Angular unconformity. A surface separating tilted or folded layers of
rock from overlying less disturbed layers.
Anticline. An upward fold or elongated arch in rock layers.
Aquifer. A rock layer that is water-bearing.
Artesian water. Groundwater that is under sufficient pressure to rise
above the level at which it is encountered in a well. It does not
necessarily rise completely to the surface.
Basalt. An extrusive igneous rock, fine-grained and dark colored,
composed mainly of calcium-rich feldspar and the black mineral
pyroxene.
Basement. A name commonly applied to metamorphic or igneous rocks
underlying the sedimentary rock layers.
Batholith. A large body of intrusive igneous rock, 40 square miles or
more in outcrop area, which extends downward to an unknown depth.
Bedrock. The solid rock which underlies soil, sand, clay, or other loose
surface material.
Belemnite. The cigar-shaped internal shell of an extinct marine mollusk
similar to a squid.
Brachiopod. One of a large group of marine shelled animals having two
unequal, bilaterally symmetrical shells.
Bryozoa. A large group of tiny colonial marine animals that secrete
calcareous or horny coverings in a great variety of shapes.
Caldera. A large basin-shaped depression caused by explosion or collapse
around a volcanic center.
Cassiterite. A heavy, brown to brownish black mineral composed of tin
and oxygen (SnO₂) that is an ore of tin.
Cephalopod. A marine mollusk with a head surrounded by tentacles. Squids
and octupuses belong to this group, as do fossil forms having
straight or coiled shells divided into numerous interior chambers.
Chalcopyrite. A reddish-gold colored ore of copper (CuFeS₂).
Cirque. A deep, steep-walled recess in a mountain, caused by glacial
erosion at the head of a valley.
Concretion. A nodular or irregular concentration of minerals such as
calcite or limonite, formed by precipitation of the mineral from
groundwater around a nucleus.
Conglomerate. A rock containing coarse fragments of an older rock,
usually as rounded water-worn stones or pebbles.
Conodont. One of a group of tiny dark brown tooth-like fossils thought
to be dermal or dental parts of some extinct group of fish.
Diatreme. A volcanic vent or pipe drilled through rocks by the explosive
energy of gas-charged molten rock, now containing igneous rock and
often altered or unaltered fragments of the surrounding rock.
Dike. A vertical or nearly vertical sheet of igneous rock which cuts
across the structure of adjacent rocks.
Diorite. An intrusive igneous rock composed of sodium-rich feldspar and
dark minerals, with only small amounts of quartz.
Dip. The angle at which a layer of rock is inclined below the
horizontal.
Dome. A roughly circular upfold in which the rock layers dip outward in
all directions from the center.
Dowsing. Searching for underground water or ore with a divining rod,
usually a forked stick supposed to locate spots where the desired
substance may be found under the surface.
Echinoderm. One of a large group of marine invertebrate animals, most of
which have pentagonal symmetry and a skeleton of many calcite
plates. Many forms are spiny. The group includes starfish and sea
urchins.
Evaporite. Chemical sediments precipitated when water (usually sea
water) evaporates.
Extrusive rocks. Igneous rocks formed when molten rock material is
ejected onto the surface. Synonymous with volcanic rocks.
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.
Molybdenite. A soft bluish gray, metallic mineral (MoS₂) that is a major
ore of molybdenum.
Monocline. A steplike fold in otherwise horizontal or gently dipping
rock layers.
Moraine. An accumulation of unsorted rock material built up by the
action of glacier ice.
Native gold. Gold occurring in nature uncombined with other elements.
Peneplain. A land surface worn down by erosion to a nearly flat or
broadly undulating plain.
Petzite. A heavy black or steel gray metallic telluride ore of gold and
silver (Ag₃AuTe₂).
Placer. A sand or gravel deposit containing particles or nuggets of gold
or other heavy valuable minerals.
Plateau. An elevated, comparatively flat surface of land, usually larger
than a mesa, sometimes composed of many mesas, and often dissected
by deep stream valleys.
Porphyry. An igneous rock, usually intrusive, which contains conspicuous
large crystals in a fine-grained matrix.
Pyrite. A brass-yellow metallic mineral (FeS₂) that is an important
source of sulfur. It is commonly known as fool’s gold.
Reef. A moundlike limestone structure built in the sea by sedentary
organisms such as corals.
Rhyolite. A light-colored volcanic rock with quartz and feldspar as the
principal constituents.
Schist. A metamorphic rock characterized by parallel orientation of
flat-grained minerals like mica.
Sedimentary rocks. Rocks formed of fragments of other rock transported
by wind or water, or formed by precipitation from solution.
Sphalerite. An amber-yellow to black mineral (ZnS) that is an important
ore of zinc.
Stalactite. A cylindrical or conical deposit of calcite hanging from the
roof of a cavern, formed by evaporation of water droplets containing
calcium carbonate.
Stalagmite. Columns or ridges of calcite rising from the floor of a
cavern, formed by water containing calcium carbonate dripping from a
stalactite.
Stock. A mass of igneous intrusive rock that covers less than 40 square
miles, has steep sides, and extends to an unknown depth.
Tennantite. A metallic gray mineral that contains copper, iron, and
arsenic, and is an ore of copper.
Tetrahedrite. A brittle, dark gray to black, metallic mineral containing
copper, iron, zinc, and silver.
Trilobite. One of a primitive group of extinct marine crustaceans,
related to crabs and lobsters, having segmented bodies divided by
longitudinal grooves into three lobes.
Unconformity. A surface separating layers of rock, formed by a period of
nondeposition or erosion.
Vein. A crack or fissure filled with mineral material, often with
valuable ore minerals.