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.

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fold [paleont] . (a) A major rounded elevation on the surface of a brachiopod valve (generally the brachial valve ), externally convex in transverse profile and radial from the umbo, and usually median in position. It is typically associated with the sulcus. Cf: carina. (b) A spirally wound ridge on the interior of the wall of a gastropod shell; e.g. columellar fold and parietal fold.

McKelvey box . A diagram that portrays the formal classification of mineral reserves and resources proposed by McKelvey (1973). Reserves and resources are classified on one axis according to degree of certainty and along the other according to feasibility of economic recovery.

heyite (hey'-ite). A yellow-orange monoclinic mineral: Pb5Fe2+2O4(VO4)2 .

vitrinertite (vit-ri-ner'-tite). A coal microlithotype that contains a combination of vitrinite and inertinite totalling at least 95%, and containing more of each than of liptinite. It generally occurs in high-ranking bituminous coals.

fission tracks . The paths of radiation damage made by nuclear particles in a mineral or glass by the spontaneous fission of uranium-238. They are similar in occurrence and formation to alpha-particle recoil tracks but are larger and less numerous. Fission-track density is established by etching and subsequent microscopic examination and counting.

species postlap zone . "A zone designated by the part of the range of the marker species occurring after the cutoff of range of another marker species" (McLean, 1968, p.10).

prismatic structure . (a) A microstructure of mutually parallel, adjacent, polygonal columns that do not strongly interdigitate along their mutual boundaries and may be separated by organic matrix (Boggild, 1930, p.246; Taylor et al., 1969, p.4; Carter, 1980a). (b) columnar joints.

focal length . A general term for the distance from the principal point or center of a lens to the principal focus. In photogrammetry, the term "equivalent focal length" is the distance measured along the lens axis from the rear nodal point of the lens to the position of the focal plane that provides the best average definition in the aerial negative, and the term "calibrated focal length" is an adjusted value of the equivalent focal length computed to distribute the effect of lens distortion over the entire field of the negative. Symbol: f.

camera axis . A line perpendicular to the focal plane of the camera and passing through the interior perspective center or emergent (rear) nodal point of the lens system.

alamosite (al-a-mos'-ite). A colorless, white, cream or light gray monoclinic mineral: PbSiO3 . It has a 12-repeat pyroxene-like chain.

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