BUREAU OF GEOLOGY


pyrolysis Decomposition of organic substances by heat in the
absence of air.

quartz (mineral) Crystalline silica, an important rock-forming mineral:
SiO2. It is, next to feldspar, the commonest mineral. Quartz forms the
major proportion of most sands.

radiocarbon dating See carbon-14 dating.

radiometric dating Calculating an age in years for geologic materials
by measuring the presence of a short-life radioactive element, e.g.
carbon-14; or by measuring the presence of a long-life radioactive ele-
ment plus its decay product, e.g., potassium-40/argon-40. The term
applies to all methods of age determination based on nuclear decay of
natural elements.

reduced To change a chemical compound by removing oxygen or
adding hydrogen so that the valence of the positive element is lower.

reed-sedge peat (American Society of Testing and Materials (ASTM)
classification) Peat containing at least 33.33 percent plant fibers, half of
which are reed-sedge and other nonmosses. NOTE: ASTM is presently in
the process of revising this classification. The above term will no longer
be used.

salt-water encroachment Displacement of fresh surface or ground-
water by the advance of saltwater due to its greater density, usually in
coastal and estuarine areas, but also by movement of brine from beneath
a playa lake toward wells discharging freshwater. Encroachment occurs
when the total head of the saltwater exceeds that of adjacent fresh-
water. Syn: encroachment; saltwater intrusion; seawater encroachment.

sapric peat (U.S. Department of Agriculture classification) Peat con-
taining less than 33.33 percent recognizable plant fragments of any
type; consists of the most extensively decomposed plant material.

sapropel An unconsolidated, jelly-like ooze or sludge composed of
plant remains, most often algae, macerating and putrifying in an anaero-
bic environment on the shallow bottoms of lakes and seas. It may be a
source material for petroleum and natural gas.

sheet flow An overland flow or downslope movement of water tak-
ing the form of a thin, continuous film over relatively smooth soil or rock
surfaces and not concentrated into channels larger than rills.


silviculture The cultivation of forest trees.