Part 16

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.