186 



STRUCTURAL GEOLOGY. 



FIG. 97. Contorted strata. (From Logan.) 



earth, several thousand feet thick, are folded all together 

 so that their foldings form great mountain-ridges, and 

 can only be made out by extensive surveys (Fig 98). As 



FIG. 98. Section of Appalachian chain. 



might be expected, the strata by such violent movements 

 are usually broken and dislocated, and always, as seen in 

 Eigs. 97 and 99, large portions of their upper parts have 



FIG. 99. 



been carried away by erosion, leaving their edges exposed 

 on the surface. Such exposure of strata on the surface is 

 called outcrop. 



^ s 



FIG. 100. 



