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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monument [surv] . A natural or artificial physical structure that marks the location on the ground of a corner or other survey point; e.g. a pile of stones indicating the boundary of a mining claim, or a road or fence marking the boundary of real property. See also: boundary monument.

spilitization (spi''-lit-i-za'-tion). Albitization of a basalt to form a spilite.

Saurischia (Saur-is'-chi-a). One of the two orders of archosaurian reptiles commonly treated as dinosaurs, characterized by a triradiate pelvis. It includes bipedal predators (Carnosauria, Coelurosauria), quadrupedal long-necked herbivores (Sauropods), and presumptive ancestors of the latter (Prosauropoda). Range, Middle Triassic to Upper Cretaceous. Cf: Ornithischia.

principal point . (a) The foot of the perpendicular from the interior perspective center of a lens to the plane of the photograph; the geometric center of an aerial photograph, or the point where the optical axis of the lens meets the film plane in an aerial camera. Symbol: p. See also: fiducial mark; photograph center. Syn: center point. (b) Either of two points on the axis of a lens such that a ray from any point of the object directed toward one principal point will emerge from the lens in a parallel direction but directed through the other principal point. (c) The point at which a principal visual ray intersects a perspective plane.

bedding void . An open space between successive lava flows, formed where an overlying flow does not completely conform to the solidified crust of lava beneath it.

palimpsest [meta] . adj. Said of a structure or texture in a metamorphic rock in which remnants of some pre-existing structure or texture, perhaps even the original one, are preserved, and sometimes even megascopically visible. The term was first used by Sederholm (1891). Cf: relict [meta].

Albian (Al'-bi-an). Stage of the Standard Global Chronostratigraphic Scale: uppermost Lower Cretaceous, (above Aptian, below Cenomanian).

disconformity (dis-con-form'-i-ty). An unconformity in which the bedding planes above and below the break are essentially parallel, indicating a significant interruption in the orderly sequence of sedimentary rocks, generally by a considerable interval of erosion (or sometimes of nondeposition), and usually marked by a visible and irregular or uneven erosion surface of appreciable relief; e.g. an unconformity in which the older rocks remained essentially horizontal during erosion or during simple vertical rising and sinking of the crust (without tilting or faulting). The tendency is to apply the term to breaks represented elsewhere by rock units of at least formation rank (Stokes and Varnes, 1955, p.157). The term formerly included what is now known as paraconformity. Syn: parallel unconformity; erosional unconformity; nonangular unconformity.

lithothamnion (lith-o-tham'-ni-on). A plant of the genus Lithothamnion, an encrusting or nodular red calcareous alga of the family Corallinaceae, abundant in post-Jurassic rocks, and reported as a living form from considerable depths and very cold waters. It is most abundant on the seaward edge of reef flats, where it acts as a cementing medium of some coral reefs.

inert component (in-ert'). A component whose amount in a rock after a metasomatic process depends on its initial concentration rather than on its chemical potential as externally fixed by the environment. Cf: mobile component; perfectly mobile component.


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