a. The formal geochronologic unit of highest rank, next above era. The
Phanerozoic Eon encompasses the Paleozoic, Mesozoic, and Cenozoic Eras.
AGI
b. One billion years. Also spelled aeon. AGI
A monoclinic mineral, MnAl(PO4 )(OH)2 .H2 O ; forms a
series with childrenite; pink to rose red; in granite pegmatites
associated with manganese phosphates.
A sensitive torsion balance for measuring variations in the density of the
underlying rocks; it records the horizontal gradient of gravity.
Webster 3rd
See:torsion balance
See:torsion balance
A five-sided step-cut gem resembling a shoulder ornament (epaulet) in
outline.
See:epeirogeny
As defined by Gilbert (1890), a form of diastrophism that has produced the
larger features of the continents and oceans, for example, plateaus and
basins, in contrast to the more localized process of orogeny, which has
produced mountain chains. Epeirogenic movements are primarily vertical,
either upward or downward, and have affected large parts of the
continents, not only in the cratons but also in stabilized former orogenic
belts, where they have produced most of the present mountainous
topography. Some epeirogenic and orogenic structures grade into each other
in detail, but most of them contrast strongly. Adj. epeirogenic.
Syn:epeirogenesis
A prefix signifying on or upon. CF:cata-
Asterism seen by reflected light, as in star ruby or sapphire cut en
cabochon to reveal asteria. The effect is created when light is reflected
from suitably oriented inclusions within the stone.
A bottom sampler consisting of a pair of sheet-metal skis attached to a
light framework for a silk or nylon net. Removable rakers in front of the
net stir up the bottom as the dredge advances, permitting the net to
capture the benthic fauna and flora contained in the sediment. A
bottom-walking wheel connected to a small counter indicates the distance
over the bottom the device travels during a haul. Hunt
In oceanography, animals and plants found living below low tidemark and
above the 100-fathom (183-m) line. CTD
Situated upon a continental plateau or platform, as an epicontinental sea.
Fay
A name proposed as a replacement for epidiorite. AGI
An orthorhombic mineral, NaBeSi3 O7 (OH) ; dimorphous with
eudidymite; forms colorless tabular crystals in nepheline-syenite
pegmatites with albite, elpidite, and analcime at Mont St. Hilare, PQ,
Canada; Narsarsuk, Greenland; and Langesundfjord, Norway.
A metamorphosed gabbro or diabase in which generally fibrous amphibole
(uralite) has replaced the original clinopyroxene (commonly augite). It is
usually massive but may have some schistosity. See also:epidiabase
AGI
A metamorphic rock consisting of epidote and quartz, and generally
containing other secondary minerals such as uralite and chlorite.
AGI
a. A basic silicate of aluminum, calcium, and iron. One form is CA2
(Fe3+ ,Al)3 (SiO4 )3 (OH) ; monoclinic; Mohs
hardness, 6 to 7; sp gr, 3.25 to 3.5; and a common secondary constituent
of igneous rocks. Pryor, 3
b. A monoclinic mineral, 2[Ca2 FeAl2 O(OH)(Si2 O (sub
7) )(SiO4 )] ; green; forms a series with clinozoisite; a common
rock-forming mineral with albite and chlorite in low-grade metamorphic
rocks and an accessory in some igneous rocks; may be used as a minor
gemstone; formerly called pistacite, arendalite, delphinite, thalalite.
c. The mineral group allanite, allanite-(Y), clinozoisite, epidote,
hancockite, mukhinite, piemontite, and zoisite.
The hydrothermal introduction of epidote into rocks or the alteration of
rocks in which plagioclase is albitized, freeing the anorthite molecule
for the formation of epidote and zoisite, often accompanied by
chloritization. These processes are characteristically associated with
metamorphism. AGI
a. Said of a geologic process, or of its resultant features, occurring at
or near the Earth's surface. CF:hypogene
b. Pertaining to a crystal that is not natural to its enclosing material;
e.g., a pseudomorph. AGI
a. The change in the mineral character of a rock as a result of external
influences operating near the Earth's surface, e.g., mineral replacement
during metamorphism. AGI
b. The changes, transformations, or processes, occurring at low
temperatures and pressures, that affect sedimentary rocks subsequent to
their compaction, exclusive of surficial alteration (weathering) and
metamorphism; e.g., postdepositional dolomitization. AGI