GLOSSAEY OF GEOLOGIC TEEMS. 235 



Slate. A rock that by subjection to pressure within the earth has acquired the 

 property of splitting smoothly into thin plates. The cleavage is smoother and 

 more regular than the splitting of schist along its grain. 



Stratigraphy. The branch 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 geology, the forma assumed by sedimentary beds and igneous rocks 

 thathavebeen 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. (See figs. 12 and 13, p. 100.) Schistoaity and cleavage are also struc- 

 tural features. 



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



Talus (pronounced tay'^us). 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 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. 



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

 melting glacier or deposited about its margin. 



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

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



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

 which it is named; also the place at which a fossil or other geologic feature is 

 displayed in typical form. 



trnconformity, 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. (See p, 42.) 



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. 



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

 dition from 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 cooling. 



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

 around a volcanic vent. The more nearly perfect cones are compo^d princi- 

 pally of lava fragments and volcanic ashes. 



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

 tallize. 



