The process of one lithospheric plate descending beneath another.
See also:obduction
A long, narrow belt in which subduction takes place; e.g., along the
Peru-Chile trench or in the volcanic arc belts of the western Pacific
Ocean. AGI
The part of identified resources that does not meet the economic criteria
of reserves and marginal reserves. USGS, 2
A subvariety of provitrain in which the corky origin of the cellular
structure is microscopically visible. CF:periblain; xylain. AGI
a. A variety of provitrinite characteristic of suberain and consisting of
corky tissue. AGI
b. A maceral of brown coal and lignite derived from the suberin layer in
corkified cell walls of some Mesozoic and younger plants.
CF:periblinite; xylinite; telinite. AGI
a. Said of a mature lithic wacke (or lithic graywacke) in which quartz
grains and fragments of siliceous and argillaceous rocks predominate, and
feldspars make up less than 10% of the rock and may be altogether lacking.
Such rocks have also been called subgraywackes. AGI
b. Said of a mature lithic arenite containing abundant quartz grains and
fragments of the more stable rocks (such as cherts), and less than 10%
feldspar grains. AGI
A layer, stratum, or material immediately beneath some principal surface;
specif. a layer of earth or rock that is graded to receive the foundation
of an engineering structure. Often it is the soil or natural ground that
is prepared and compacted to support, and that lies directly below, a
road, pavement, building, airfield, or railway. CF:subbase
The surface of the earth or rock prepared to support a structure or a
pavement system. ASCE
The most common type of sandstone, intermediate in composition between
orthoquartzite and graywacke (Pettijohn, 1957). Composition is typically
30% to 65% quartz and chert, less than 15% clay matrix, more than 25%
unstable materials such as feldspar grains and rock fragments, and voids
or mineral cement exceeding the amount of clay matrix. The rock is lighter
colored and better sorted, and has less matrix, than graywacke.
CF:graywacke
a. Said of a mineral grain that is bounded partly by its own rational
faces and partly by surfaces formed against preexisting grains as a result
of either crystallization or recrystallization. AGI
b. Said of the shape of such a crystal, intermediate between euhedral and
anhedral. AGI
Macerals having a low hydrogen content, such as fusinite.
Tomkeieff
An inclined shaft along the footwall of a reef on the Rand. It develops
and extracts ore from areas below the main haulage level.
See also:staple shaft
One of the small crystals that often unite in parallel growths to build up
larger crystals of the same general habit. Standard, 2
Said of a stratum situated immediately under a particular higher stratum
or below an unconformity. Ant: superjacent. Syn:underlying
The subjective brightness of a surface is determined by two factors, the
light flux radiated from the surface and the sensitivity of the eye under
the conditions in which the surface is seen. The sensitivity of the eye is
partly controlled by the contrasts presented over the visual field, but is
mainly dependent on its adaptation brightness level. Roberts, 2
a. A secondary level for working ore in top slicing and sublevel caving; a
companion heading. Nelson
b. An intermediate level opened a short distance below the main level; or,
in the caving system of mining, 15 to 20 ft (4.6 to 6.1 m) below the top
of the orebody, preliminary to caving the ore between it and the level
above. See also:sublevel stoping; caving system.
See:sublevel stoping
a. A stoping method in which relatively thin blocks of ore are caused to
cave by successively undermining small panels. The ore deposit is
developed by a series of sublevels spaced at vertical intervals of 18 to
25 ft or 30 ft (5.5 to 7.6 m or 9.1 m) and occasionally more. Usually only
one or two sublevels are developed at a time, beginning at the top of the
orebody. The sublevels are developed by connecting the raises with a
longitudinal subdrift from which timbered slice drifts are driven right
and left opposite the raises to the ore boundaries or to the limits of the
block. Usually alternate drifts are driven first, and caving back from
them is begun and continued while the intermediate slices are being
driven. The caving is begun at the ends of the slices by blasting out cuts
and retreating in the same manner toward the raises. The broken and caved
ore formerly was shoveled into cars and trammed to the raises, but in
recent years it is dragged to the raises by power scrapers. Successively
lower sublevels are developed and caved back until the entire block has
been mined. This method is intermediate between block caving and top
slicing, since part of the ore is mined as in top slicing and part is
caved. See also:top slicing combined with ore caving
b. Similar to top slicing from which it is thought to have been developed.
The general plan of operations is to mine every other slice by driving
crosscuts (slice drifts) from 18 to 36 ft (5.5 to 11.0 m) apart. The ore
between the crosscuts as well as that in the slice above is then mined,
thus causing the overlying material to cave. The method is applicable to
irregular and steeply dipping orebodies that cannot be worked by top
slicing. The present tendency is to sink vertical shafts in the footwall
rather than inclined shafts as formerly done. Also called subdrift caving.
Lewis
A drive often made in a section, esp. in gently inclined deposits, that
divides the deposit into narrower panels and zones. They are narrower, and
the support and equipment for them is more simple than that required in
level drives. Stoces
See:sublevel stoping
See:top slicing combined with ore caving; sublevel stoping.