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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GeoWord of the Daydepositional dip . primary dip . See also: regional dip .

GeoWord of the Dayfrustule (frus'-tule). The siliceous cell wall of a diatom, consisting of two halves, the epivalve and the hypovalve. It is ornate, microscopic, and boxlike.

GeoWord of the Daybeta quartz . The polymorph of quartz that is stable from 573°C to 870°C, that has a vertical axis of six-fold symmetry and six horizontal axes of two-fold symmetry, and that has a lower refractive index and birefringence than those of alpha quartz. It occurs as phenocrysts in quartz porphyries, graphic granite, and granite pegmatites. Also spelled: β-quartz. Syn: high quartz.

GeoWord of the Daymean high water . Average of high water values. Used in England, Wales, and some states to determine ownership of coastal property. Abbrev: MHW.

GeoWord of the Dayoccludent margin (oc-clud'-ent). The margin of scutum and tergum forming the aperture in a cirripede crustacean and occluding it with comparable margins of opposed scutum and tergum.

GeoWord of the Dayduns . A term used in SW England for a shale or massive clay associated with coal.

GeoWord of the Daylithothamnion ridge . An algal ridge built by Lithothamnion and other crustose coralline algae, rising about 1 m above the surrounding reef and extending to depths of 6-7 m below sea level. See also: algal ridge, cup reef; algal reef; boiler; breaker [reef].

GeoWord of the Dayaltimeter (al-tim'-e-ter). An instrument, usually an aneroid barometer, for determining height above ground or above mean sea level, based on the fall of atmospheric pressure accompanying an increase in altitude.

GeoWord of the Dayatoll structure . In a metamorphic rock, porphyroblasts with hollow centers resembling atolls. The ring may be almost complete or consist of a chain of granules (Joplin, 1968).

GeoWord of the Daywash [geomorph] . (a) Erosion effected by wave action. (b) The wearing away of soil by runoff water, as in gullying or sheet erosion; rainwash.