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.

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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. wâdies are perennially dry.

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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.