Post-Tertiary or Quaternary: the youngest group of systems. See Table. Pre-Cambrian or Archæan: the oldest system of rocks.

Pumice any froth-like, foam-like, spongy, porous, or cellular lava. Pyroxene (Gr. pur, fire; xenos, a guest): a family of minerals, common constituents of many crystalline igneous, and of some schistose rocks. Quadersandstein: name given in Saxony, Bohemia, and Silesia to the Cretaceous system; so called because the sandstone of which it is chiefly composed is traversed by abundant well-marked vertical joints, that cause the rock to weather into square, tabular, and pyramidal hills, and pillarlike masses.

Quaquaversal (L. quaqua, wheresoever; versus, turned): applied to strata which dip outwards in all directions from a common centre; dome-shaped

strata.

Quartz (Ger.): common form of native silica; the most common of all rockforming minerals.

Quaternary: alternative name for Post-Tertiary.

Raised beaches: see Beaches.

Recent period: the latest of the geological systems; passes gradually into the present or existing condition of the earth.

Reversed faults: in these the hade or inclination of the fault is in the direction of upthrow-lower rocks having been pushed over higher rocks. See Overthrust and Thrust-plane.

Revived rivers: when the rivers of a region have succeeded in cutting their channels down to the base-level, they have a slight fall and flow sluggishly. Should the whole region then be elevated, while the direction of its slopes remains unchanged, the erosive energy of the rivers is renewed, and they are said, therefore, to be revived.

Rhætic (from the Rhætian Alps): one of the subdivisions of the Triassic system. Rhyolite (Gr. rheo, to flow; lithos, stone): an acid volcanic rock.

Roches moutonnées: rocks rounded like the back of a sheep; name given to rocks which have been abraded, rounded, and smoothed by glacial action. Rothliegendes (Ger.): one of the subdivisions of the Permian system. Rumpfgebirge (Ger.): same as Horste (q. v.).

Salses (Fr.): another name for mud-volcanoes or Macalubas (q. v.).

Schist (Gr. schistos, easily split): a crystalline rock in which the constituent minerals are arranged in rudely alternate parallel layers or folia; a foliated rock.

Scoriæ (Gr. skoria, dross): loose fragments of slaggy, cindery lava.

Screes (Icel. skritha, fallen rocks on a hillside): a Westmoreland term for the sheets of loose angular stones which gather upon hillsides and at the base of cliffs, etc.

Shearing the yielding of a rock to compression, strain, and tension during crustal movements, whereby the solid mass is compelled to flow, so that a kind of fluxion-structure is developed in it; frequently under such conditions dislocation takes place-the rock gives way and one mass is pushed over another.

Sheet: molten matter intruded between bedded rocks.

Stalactites (Gr. stalaktos, dropping): the icicle-like pendants hanging from the roofs of limestone caves, formed by the drip of water holding carbonate of lime in solution.

Stalagmites (Gr. stalagmos, a dropping): the calcareous deposit formed upon the floor of a cavern by the drip of water from the roof.

Stoss-seite: see Lee-seite.

Striæ, glacial: scratches, furrows, etc., engraved upon rock-surfaces by glacial

action.

Strike: the general direction or run of the outcrops of strata.

Swallow-holes: see Dolina.

Syenite (from Syene, Egypt): a holocrystalline igneous rock of deep-seated origin.

Syncline (Gr. syn, together; klino, I lean): a basin or trough-shaped arrangement of strata; the strata dip from opposite directions inwards to one common axis. When the axis is vertical the syncline is symmetrical; when inclined, unsymmetrical.

Systems: the larger divisions of strata included under the Paleozoic, Mesozoic, Cainozoic, and Quaternary groups.

Terrigenous: applied to marine accumulations the materials of which have been derived from land; opposed to abysmal, applied to marine deposits the constituents of which have not been so derived.

Thrust-plane: a Reversed fault (q. v.), the hade or inclination of which approaches horizontality; a common structure in regions of highly flexed rocks.

Till: another name (Scottish) for Boulder-clay (q. v.).

Tors: the peculiar and often fantastic prominences met with in regions of granite which have been long exposed to weathering, as on Dartmoor. The kopjes of Mashonaland are an example of the same phenomenon.

Trachyte (Gr. trachys, rough): a hemicrystalline volcanic rock.

Travertine: another name for Calc-sinter (q. v.).

Triassic (Gr. trias, three): one of the Mesozoic systems.

Tufa, or calcareous tufa: same as Calc-sinter, Travertine (q. v.). Tuff: a volcanic fragmental rock; usually applied to the finer-grained ejecta of volcanic eruptions; may consist almost entirely of lapilli (q. v.) or of the finest sand and dust, or of a mixture of coarse and fine ingredients.

Unconformable: not conforming in position, or not having the same inclination or dip with underlying rocks; applied to strata which rest upon an eroded surface of older rocks; unconformity or unconformability, the condition of not being conformable.

Underclay the bed upon which a coal-seam rests.

Uniclinal (L. unus, one; Gr. klino, to lean): applied to a series of strata dipping in one and the same direction.

Upthrow, upcast: that side of a fault on which the strata lie at a higher level than their continuations on the other side of the fault. Normal faults are usually described as downthrows; reversed faults as upthrows.

Wâdy (Ar.): a ravine or watercourse, dry except in the rainy season. Some wâdies are perennially dry.

Weathering: applied to the decomposition, disintegration, and breaking up of the superficial parts of rocks under the general action of changes of temperature, and of wind, rain, frost, etc.

Zeolites (Gr. zeo, I boil; lithos, stone): a group of minerals, so called because they bubble up in the blowpipe flame; often met with filling up vesicular cavities, etc., in igneous rocks.