136 GUIDEBOOK OF THE WESTERN UNITED STATES, 



Stratigraphy. The branoh of geologic science that deals with, the order and rela- 

 tions of the strata of the earth's crust. 



Strike. The direction along which an inclined rock layer would meet the earth's 

 surface if that surface were level. The outcrop (which see) of a bed on a plain is 

 coincident with its strike. 



Structure. In sreolosv the forms assumed hv snfHmr'nlMrv "hprla ^nA 



ismeous 



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 pro- 

 duced mechanically on neighboring rocks by the intrusion. Folds (anticlines 

 and synclines) and faults are the principal mechanical effects considered under 

 structure, Schistopity and cleavage are also structural features. 



Syncline, An inverted arch of bedded or layered rock suggestive in form of a canoe. 



Talus (pronounced tay^his). 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 arc remnants of 

 valley bottoms formed when the land was lower or 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. (See PI. XXIX, A, p. 119.) 



Till. The deposit of mingled bowlders, rock fragments, and soil left behind by a 

 melting glacier or deposited about its margin. 



Tuff. A rock consisting of a layer or layers of lava particles blown from a volcano. 



Typ 



A fine tuff is often called volcanic ash and a coarse tuS breccia. 

 e locality. The place at which a formation is typically displayed and from 

 which it is named; also the place at which a fos?il or other geologic feature is dis- 

 played in typical form . 



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 

 show 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 sur- 

 face of unconformity was deposited. If beds of rock may })e considered as leaves 

 in the volume of geologic histofy, an unconformity marks a gap in the record. 



Vein. A maes 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. 



Volcanic bomb. A rounded mass of lava thrown out while in a hot and pasty con- 

 dition fiom a volcano. A bomb, like a raindrop, is rounded in its passage through 

 the air and may be covered with a cracked crust due to quick cooUng. 



Volcanic cone. A mountain or hill usually of characteristic conical fom, built up 

 around a volcanic vent. The more nearly perfect cones are composed principally 

 of lava fragments and volcanic ashes. 



Volcanic glass. Lava that has cooled and solidified before it has had time to 

 crystallize. 



Volcanic neck. A plug of lava that formerly congealed in the pipe of a volcano. 

 W hen the tuffs and lava flows that make up most of a volcano have been washed 

 away by erosion the neck may remain as an isolated liill 



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 

 wa.er and of plants and bacteria and the mechanical action of changes of tem- 

 perature whereby rocks on exposure to the weather change in character, decav, 

 and finally crumble into soil. (See PI. XXVII, p. 109.) 



