arithmetic mean diameter . An expression of the average particle size of a sediment or rock, obtained by summing the products of the size-grade midpoints and the frequency of particles in each class, and dividing by the total frequency. Syn: equivalent grade.
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.
undation theory (un-da'-tion). A theory proposed by Van Bemmelen (1933) that explains the structural and tectonic features of the Earth's crust by vertical upward and downward movements caused by waves that are generated by deep-seated magma. The concept is no longer accepted. Cf: blister hypothesis.
intake (in'-take). (a) recharge. (b) The openings in water-bearing materials through which water passes into a well.
cave beard . A variety of cave cotton where parallel, fibrous strands of gypsum or other sulfate minerals are free-hanging, usually from a cave ceiling.
pollenkitt (pol'-len-kitt). A sticky material, produced by the tapetum, that may hold pollen grains together during dispersal. Etymol: German "Kitt," putty, cement.
sea reach . The reach of the lower course of a stream where it approaches the sea.
roho system . A system of major, listric, basinward-dipping growth faults that gradually flatten with depth into conformity with an allochthonous salt sheet or salt weld . Sediment wedges in the fault blocks dip and thicken landward but become younger seaward (Schuster, 1995). Cf: stepped counter-regional system.
veil [cryst] . An aggregate of minute bubbles creating a whitish or cloudlike appearance in quartz.
disequilibrium system . A geomorphic system that tends toward equilibrium but has not attained a steady state (Renwick, 1992). Cf: nonequilibrium system.
geogram (ge'-o-gram). A term used by Marr (1905, p. lxii) for a hypothetical geologic column, connoting principally lateral variations in lithology and in organic assemblages that could be traced in a deposit over an area of any width up to that of the circumference of the Earth.
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