_Structure._--In geology, the forms assumed by sedimentary beds and igneous rocks that have been moved from their original position by forces within the earth, or the forms taken by intrusive masses of igneous rock in connection with effects produced mechanically on neighboring rocks by the intrusion. Folds (anticlines and synclines) and faults are the principal mechanical effects considered under structure. Schistosity and cleavage are also structural features. _Syncline._--A kind of folded structure in which strata have been bent downward. It is an inverted arch--the opposite of an anticline. _Talus_ (pronounced t[=a]y'lus).--The mass of loose rock fragments that accumulates at the base of a cliff or steep slope. _Terrace._--A steplike bench on a hillside. Most terraces along rivers are remnants of valley bottoms formed when the stream flowed at higher levels. Other terraces have been formed by waves. Some terraces have been cut in solid rock, others have been built up of sand and gravel, and still others have been partly cut and partly built up. _Tertiary._--The earlier of the two periods of the Cenozoic era of geologic time. _Triassic._--The earliest period of the Mesozoic Era of geologic time. _Unconformity._--A break in the regular succession of sedimentary rocks, indicated by the fact that one bed rests on the eroded surface of one or more beds which may have a distinctly different dip from the bed above. An unconformity may indicate that the beds below it have at some time been raised above the sea and have been eroded. In some places beds thousands of feet thick have been washed away before the land again became submerged and the first bed above the surface of unconformity was deposited. If beds of rock may be regarded as leaves in the volume of geologic history, an unconformity marks a gap in the record. _Vein._--A mass of mineral material that has been deposited in or along a fissure in the rocks. A vein differs from a dike in that the vein material was introduced gradually by deposition from solution, whereas a dike was intruded in a molten condition. Quartz and calcite are very common vein minerals. _Volcanic Rocks._--Igneous rocks erupted at or near the earth's surface, including lavas, tuffs, volcanic ashes, and like material. _Weathering._--The group of processes, such as the chemical action of air and rain water, and of plants and bacteria, and the mechanical action of changes of temperature, whereby rocks on exposure to the weather change in character, decay, and finally crumble into soil. Transcriber Note All illustrations splitting paragraphs were moved before or after the paragraph. All simple typos were corrected (i.e., Reudemann to Ruedemann, pryoxene to pyroxene).