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aboriginal— Referring to the native inhabitants of any region.

accretion— Outbuilding of continental crust.

actinolite— A mineral, monoclinic amphibole similar to the mineral tremolite but iron-bearing; a typical formula is Ca₂ (MgFe)₅ Si₈ O₂₂ (OH₂ ).

acuminate— Tapering to the apex, the sides more or less pinched in before reaching the tip. Compare acute .

acute— Tapering to the apex, with the sides straight or nearly so; usually less tapering than acuminate.

adiabatic— An atmospheric process in which descending air is compressed and warmed and ascending air expands and is cooled.

aeolian— Pertaining to the wind; especially said of such deposits as loess (windblown dust of Pleistocene age) and dune sand, of sedimentary structures such as windformed ripple marks, or of erosion and deposition accomplished by the wind.

aggregation— The collection of units or parts into a mass or whole.

akene— A small, dry, hard, one-seeded fruit that remains closed at maturity.

albite— Triclinic mineral of feldspar group, NaAlSi₃ O₈ , a variety of the mineral plagioclase.

alkali dust— Fine particles (salts) in desert soils that are concentrated on dry lake beds.

alkaline— Pertaining to substances with a pH greater than 7; basic.

allopatric— Occurring in different geographic regions.

alluvial deposits— Poorly consolidated sand, gravel, silt, and clay.

alluvial fan— An outspread, gently sloping mass of alluvium deposited by a stream, especially in an arid or semiarid region where a stream issues from a narrow canyon onto a plain or valley floor. Viewed from above, it has the shape of an open fan, with the apex at the valley mouth.

alluvial soil— A soil formed from materials deposited by moving water, as along a river bed.

alluvium— A general term for clay, silt, sand, gravel, or similar unconsolidated detrital material deposited by running water.

Alpine Fell-fields— The plant community at elevations above treeline, composed of cushion plants and other low perennials interspersed between large rock surfaces and presenting little exposure to the elements.

Alpine Steppe— Arid plant community above treeline.

alpine Tundra— See Alpine Fell-fields .

altocumulus— A principal cloud type that appears in many forms but is always considered a cloud of median height above the terrain, as opposed to a low (cumulus or stratus) or high (cirrus) cloud.

Amphisbaenia— An order of limbless, burrowing reptiles.

anaerobic— Lacking available free oxygen for processes such as respiration and metabolism.

anal fin— The median ventral fin located just posterior to the anus.

andalusite— Varicolored orthorhombic mineral (i.e., crystal system with three axes of symmetry that are mutually perpendicular), Al₂ SiO₅ . Metamorphic mineral formed at medium pressures and temperatures.

annealing— Recrystallization involving grain coarsening due to the long-term attendance of relatively high metamorphic temperatures.

anomaly— A deviation from the normal value for the same region (usually) in temperature or precipitation over a specified period.

antenna— Flexible appendage on head of arthropods, used for touching and smelling.

anther— The pollen-bearing part of the stamen. (See Fig. 6.1).

anthesis— The action or period of opening of a flower.

anticline, anticlinal— A fold, generally convex upward, whose core contains stratigraphically older rocks.

anticlinorium— A composite anticlinal structure of trough- and arch-shaped folds.

anticyclone— A large-scale atmospheric circulation in which the airflow is clockwise as viewed from above — the opposite direction to that of a cyclone.

apposite— Proceeding from opposite sides of a stem, as opposite leaves. (See Fig. 6.1).

appressed— Pressed flat against another organ or surface.

arachnid— A member of the Class Arachnida, characterized by the presence of one pair of preoral appendages with two or three joints (e.g., spiders, scorpions, mites, and ticks).

arc— Islands or mountains arranged in a great curve.

archaeology— The study of human lifeways by means of material remains.

arête— A rocky, sharp-edged ridge or spur, commonly present above the snowline in rugged mountains sculptured by glaciers, and resulting from the continued backward growth of the walls of adjoining cirques.

arthropods— Segmented invertebrates having jointed legs, such as insects and spiders.

artifact— Portable object modified by human agency.

assemblage— Relatively homogeneous group of fossils that recur at the same stratigraphic level.

atlatl— Spear thrower; an implement used to propel an arrowlike projectile or "dart."

attenuation— A reduction in the amplitude of a signal, seismic wave, or rock fold.

auditory— Pertaining to hearing.

aureole— A zone surrounding an igneous intrusion in which the country rock shows the effects of contact metamorphism.

avifauna— The species of breeding birds that inhabit a region; all species of birds generally present in a region at any season.

awl— A sharp, pointed bone or wood tool used in basket making.

axil— Upper angle formed by a leaf branch with the stem. (See Fig. 6.1.)