Glossary - Geology of Oregon - page 246

that produced discrete plump blobs of cooled
basalt.
Placer -An ore deposit where the host rock is stream
or beach gravel or sand.
Plunge -The dip of a fold axis. Anticlines and syncline
may plunge.
Plutonic rocks -Igneous rocks formed at depth. The
term includes rocks formed in a batholith.
Porosity -The ratio of the aggregate volume of void
space or interstices in a rock or soil to its total
volume. It is usually stated as a percentage.
Porphyry -Igneous rocks containing conspicuous pheno-
crysts (large crystals) in a fine-grained or
aphanitic groundmass.
Pressure ridge -Large wave-like forms on the surface of
solidified lava flows caused by movement and
wrinkling of the cooling surface lava above the
molten rock below.
Pumice -A sort of volcanic froth made up of excessively
cellular, glassy lava, generally of the compo-
sition of rhyolite.
Pyroclastic rock -A general term applied to fragmental
volcanic material that has been explosively or
aerially ejected from a volcanic vent.
Radiolaria -A marine planktonic protozoan. A single-
celled microfossil with a skeleton of opaline
silica that is used to age date oceanic rocks.
Reverse fauIt -A fault whose hanging wall (that part
above one standing on the fault plane) moved
up relative to the footwall.
Reverse or inverted topography -A common occurrence
in volcanic terrains where streambeds become
filled with lava then stand out in relief
after erosion has removed tha adjacent
softer soil and rock.
Rhyolite -An extrusive rock with a high-silica content
which is the fine-grained equivalent of a
granite. When the lava cools quickly it may
form obsidian and when gassy it may form a
froth called pumice.
Rock -Any naturally formed aggregate of mineral matter
constituting an essential and appreciable part
of the earth's crust.
Scarp (also escarpment) -A steep slope or cliff. It is
called a fault scarp if caused by faulting.
Schist -A medium to coarse-grained metamorphic rock
that is composed largely of tabular minerals
such as mica and hornblende.
Scoria -Vesicular basaltic lava, dark gray to dark red in
color. The vesicles make up a significant
proportion of the rock.
Sedimenl -Any particles settling from suspension in
water or air. Sedimentary rocks are those made
up of sediment, loosely applied to all stratified
rocks.
Series -A time-stratigraphic unit ranked next below a
system, such as the Eocene Series.
Serpentinite -A rock consisting almost entirely of the
mineral serpentine derived from the alteration
of previously existing olivine and pyroxene.
Shale -A compacted mud or clay, generally possessing a
thinly bedded or laminated structure.
Silicate -Minerals or rocks containing crystalline
compounds of silicon and oxygen.
Sill -An intrusive body of igneous rocks of approxi-

mately uniform thickness. A sill is relatively
thin compared with its lateral extent, which has
been emplaced parallel to the bedding or
schistosity of the intruded rocks.
Siltstone -Aconsolidated sedimentary rock consisting of
fragments between the size of sand and clay
grains.
Slate -A fine-grained metamorphic rock which tends to
split in slabs parallel to the alignment of mica
rather than along bedding planes.
Slickenside -Polished and grooved rock surface where
rock masses have rubbed together under
pressure along a fault plane.
Spit -A long peninsula of unconsolidated sand in a
bar projecting from a headland.
Slack -A steep-sided off-shore pinnacle, isolated by
wave action from a sea cliff of which it was
once a part.
Stratification -Parallel structure produced by deposi-
tion of sediments in beds or layers.
Strike -The course or direction of the intersection
of an inclined bed or fault with the
horizontal plane surface.
Strike-sIip -Term applies to faults that have a prom-
inent horizontal movement parallel to the plane
of the fault.
Subduction zone -A curved planar surface where one
major plate, usually the denser oceanic plate,
is under-thrusting the less dense continental
plate.
Superposition -The laying down of beds in order one on
top of the other with the younger bed being on
top.
Superimposed stream -A stream that was established on
a new surface, usually a sedimentary cover,
which maintained its course despite different
preexisting lithologies and structures encoun-
tered as it eroded downward into the under-
lying rocks.
Syncline -A trough-shaped downfold with limbs rising
from the axis, in which the youngest beds make
up the axial part of the fold.
System -A world-wide time-rock division containing
rocks formed during a period. Rock laid down
during the Permian Period would be the
Permian System.
Talus -Rock debris that accumulated in a slope at the
foot of a cliff.
Tectonics -Study of the broader structural features of
the earth and their causes. Plate tectonics
relates to movements of large segments
(plates) of the earth's crust and their
interrelationship.
Tephra -A collective term for all clastic volcanic debris
which during an eruption is ejected from a
crater or vent and transported through the air,
including bombs, blocks, dust, ash, cinders,
lapilli, scoria, and pumice.
Tephrachronology -Relative ages of strata based on
layers of volcanic ash.
Terrane -A suite of rocks bounded by fault surfaces that
has been displaced from its point of origin.
Tethyan -A term for rocks and environments of the
Mesozoic Tethys Sea which was a tropical
ocean extending from the Mediterranean