Any of various old Spanish units of length used in Latin America and the
Southwestern United States, equal in different localities to between 31 in
and 34 in (78.7 cm and 86.4 cm); e.g., a unit equal to 33.3333 in (84.666
cm) in Texas, to 33.372 in (84.764 cm) in California, to 33.00 in (83.82
cm) in Arizona and New Mexico, and to 32.9931 in and 32.9682 in (83.802 cm
and 83.739 cm) (among others) in Mexico. For other values, see ASCE
(1954). AGI
Rock failure above a tunnel due to ring stresses. These cause rock to
crack across a weakness plane and fall. The final shape is a reentrant V
rather than a rounded arching. Syn:arching to a weakness
See also:arching
This magnetometer is available in two models, one for airborne surveys and
the other for use on the ground. Both measure the total magnetic field of
the Earth rather than its components. Dobrin
The angle by which the compass needle deviates from the true north;
subject to annual, diurnal, and secular changes. Called more properly
declination of the needle. See also:declination
Standard, 2; Fay
A compass of delicate construction for observing the variation of the
magnetic needle. Webster 2nd
Said of a sediment or sedimentary rock, such as red beds or sandstone,
showing variations of color in irregular spots, streaks, blotches,
stripes, or reticulate patterns. AGI
Bornite, erubescite. Pryor, 3
See:bornite
A mineral that is either present in considerable amounts in a rock or
characteristic of the rock; a mineral that distinguishes one variety of
rock from another. Syn:distinctive mineral
In mineralogy, a mineral showing differences in color, other physical
properties, or minor variations in composition from the material
considered typical of the species. An example is emerald, the
green-colored gem beryl. Hess
A plot of the variance (one-half the mean squared difference) of paired
sample measurements as a function of the distance (and optionally of the
direction) between samples. Typically, all possible sample pairs are
examined, and grouped into classes (lags) of approx. equal distance and
direction. Variograms provide a means of quantifying the commonly observed
relationship that samples close together will tend to have more similar
values than samples far apart.
A pea-size spherule, usually composed of radiating crystals of plagioclase
or pyroxene. This term is generally applied only to such spherical bodies
in basic igneous rock, e.g., variolite. CF:spherulite
A fine-grained igneous rock of basic composition containing varioles. Not
recommended usage. AGI
Said of the texture of a rock, esp. a basic igneous rock, composed of
pea-size spherical bodies (varioles) in a finer-grained groundmass.
CF:spherulitic
A geophysical device for measuring or recording variations in terrestrial
magnetism; a variable inductance provided with a scale. AGI
a. An orthorhombic mineral, AlPO4 .2H2 O ; dimorphous with
metavariscite; waxy; forms nodular masses in cavities where phosphate
water acts on aluminous rock. Variscite is a popular gem material for
cabochons and various carved objects, commonly substituting for turquoise.
Syn:lucinite; utahlite.
b. The mineral group mansfieldite, scorodite, strengite, and variscite.
CF:amatrice
A suspended multiple deck screen with increased slopes in the second and
third deck. It is used principally for coal and other large feed materials
and combines scalping and sizing operations. The oversize lump material is
removed on the top deck, egg or range size on the second, and nut size on
the bottom deck. See also:trislope screen; vibratory screen.
Nelson
A mineral that is perhaps a variety of cassiterite.
A monoclinic mineral, (Na,Ca)2 (Mn,Fe)3 (PO4 ) (sub
3) ; alluaudite group; forms dull olive-green granular masses; at
Varutrask, Sweden.
a. A sedimentary bed or lamina or sequence of laminae deposited in a body
of still water within 1 yr's time; specif. a thin pair of graded
glaciolacustrine layers seasonally deposited, usually by meltwater
streams, in a glacial lake or other body of stillwater in front of a
glacier. A glacial varve normally includes a lower summer layer consisting
of relatively coarse-grained, light-colored sediment (usually sand or
silt) produced by rapid melting of ice in the warmer months, which grades
upward into a thinner winter layer, consisting of very fine-grained
(clayey), often organic, dark sediment slowly deposited from suspension in
quiet water while the streams were ice bound. Counting and correlation of
varves have been used to measure the ages of Pleistocene glacial deposits.
AGI
b. Any cyclic sedimentary couplet, as in certain shales and
evaporites.--Etymol: Swedish. AGI
See:varved clay