GeoWord of the Day

The GeoWord of the Day is a free service of the American Geosciences Institute. All of the terms and definitions are from the Glossary of Geology, 5th Edition Revised.


labradorescence (lab''-ra-dor-es'-cence). An optical phenomenon consisting of flashes of a laminated iridescence of a single bright hue that changes gradually as a mineral or gemstone is moved about in reflected light, caused by internal structures that selectively reflect only certain colors; specif. the light-interference effect exhibited by labradorite and set up in thin plates of feldspar (produced by repeated twinning or by exsolution), resulting in a series of vivid colors (usually brilliant blue or green) spread over large areas. Syn: change of color.

cystiphragm (cys'-ti-phragm). A skeletal partition in stenolaemate bryozoans, extending from zooecial wall into zooecial cavity and recurved inwardly to form a compartment that is generally closed by abutting zooecial wall or diaphragm, or by another cystiphragm. Cystiphragms encircle the living chamber with a hollow collar or ring.

weathering shot . In seismic exploration, the use of a small seismic source to determine velocity characteristics and thickness of the weathering or low-velocity layer. Syn: short shot; poop shot.

potassic-chloropargasite . A vitreous black monoclinic mineral of the amphibole group: (K,Na)Ca2(Mg,Fe2+)4Al(Si6Al2)O22(Cl,OH)2. It represents pargasite with Cl>(OH), and with K as the dominant alkali cation.

gullet [paleont] (gul'-let). (a) A variably tubular invagination of the cytoplasm of various protists (such as tintinnids) that sometimes functions in the intake of food. (b) A longitudinal groove present in certain algae (such as some Cryptophyceae and Euglenophyceae). (c) In vertebrates, the esophagus or other, often distensible, modifications of the anterior alimentary canal for taking food into the body.

axial skeleton . In the vertebrates, the vertebral column, ribs, braincase, and median fins when present. In common usage the axial skeleton also includes the entire skull and visceral skeleton.

nuclear log . (a) neutron log. (b) radioactivity log.

oral (o'-ral). adj. (a) Said of the surface on which the mouth of an invertebrate is situated, such as the upward-directed actinal surface of the theca of an edrioasteroid. Also, relating to or located on an oral surface, or situated at, near, or toward the mouth or peristome (such as of an echinoderm); e.g. an "oral pole" representing the end theca containing the mouth in a cystoid. Ant: aboral. (b) Pertaining to the orifice (not the mouth) of a bryozoan zooid. (c) Toward the upper side of a conodont element. (d) In vertebrates, a general term related to any aspect of the mouth. n. An oral plate of an echinoderm.

caulescent (cau-les'-cent). In plants, producing or having a stem above ground (Lawrence, 1951, p.743). Etymol: Latin "caulis," stem, stalk.

heterotactic [struc petrol] . Said of the symmetry of a fabric in which not all the subfabrics agree in symmetry. Cf: homotactic [struc petrol].

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