176 



STRATIFIED OR SEDIMENTARY ROCKS. 



In such cases the folded structure is not visible at one view, but only 

 brought out by extended survey. In cases of strong folding the strata 



Fig. 144.— Contorted Strata (from Logan). 



are often broken and dislocated (Figs. 196, 197, etc.). In all cases of 

 elevated strata, whether level or tilted and folded, large portions of the 



7 C 



Fig. 145.— Section of Appalachian Chain. 



upper parts are carried away by erosion and the remainder is left in 

 isolated patches and basins, or else standing at all angles, with their 



Fig. 146.— Section of the Jura Mountains. 



edges exposed (Figs. 144, 145, and in all the other figures). Such ex- 

 posure on the surface of the edges of eroded strata is called an outcrop. 



K&m 



X 



m^&r- 



\ d 



Fig. 147.— Upturned and Eroded Strata, Elk Mountains, Colorado (after Hayden). 



Definition of Terms. — There are certain terms in frequent use by 

 geologists which must now be defined. These are dip and strike, anti- 

 cline and syncline and conformity and unconformity. 



Dip and Strike. — The dip of strata is their inclination to a hori- 

 zontal plane. Thus in Fig. 148 the strata dip southward about 30°. 

 The angle of dip is measured by a clinometer (Fig. 149), and the direc- 



