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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mound [arch] . A small, human-made hill, composed either of debris accumulated during successive occupations of the site (syn: tell; teppe ) or of earth heaped up to mark a burial site.

palisade tissue . Tissue composed of long cylindrical chlorophyllous cells oriented normal to the lamina beneath the upper epidermis of angiospermous leaves. Syn: palisade mesophyll.

iridium (i-rid'-i-um). (a) An element of the platinum group, symbol Ir. (b) A metallic white cubic mineral consisting of more than 80 percent Ir, the remainder being osmium, palladium, or related elements.

granular chert . A compact, homogeneous, hard to soft chert, common in insoluble residues, composed of distinguishable and relatively uniform-sized grains, characterized by an uneven or rough fracture surface and by a dull to glimmering luster (Ireland et al., 1947, p.1486); it may appear saccharoidal. See also: granulated chert. Cf: smooth chert; chalky chert. Syn: crystalline chert.

melaphyre (mel'-a-phyre). A term originally applied to any dark-colored porphyritic igneous rock but later restricted to altered basalt, esp. of Carboniferous and Permian age. Obsolete.

directional drilling (di-rec'-tion-al). The intentional drilling of a well at controlled departures from the vertical and at controlled azimuths, often utilizing a whipstock or a special downhole assembly with a bent (angled) subassembly and a turbine motor located close to the drill bit. It is done to establish multiple wells from a single location such as an offshore platform, and for other purposes. Cf: deviation [drill]; sidetracking.

convergent margin . A continental edge that is also a plate edge and is being deformed by collision with another plate; active margin.

soil forming processes . Processes that form soil horizons and depend on the relative influence of the soil formation factors.

helicospore (hel'-i-co-spore). A cylindric, spiral, or convolute, usually septate, asexual fungal spore. Cf: Saccardoan spore groups.

irarsite (ir-ar'-site). A metallic black cubic mineral of the gersdorffite group: (Ir,Ru,Rh,Pt)AsS . It forms a series with hollingworthite.