248 



STRUCTURAL GEOLOGY. 



duced by erosion of the dried-up lake-deposits of this 

 region (Fig. 152). 



2. Gently Undulating Strata. These, also, by ero- 

 sion give rise to table-topped mountains ; but, if carefully 



FIG. 152. -Mauvaises Terres, Bad Lands. (After Hayden.) 



examined, the ridges are seen to be synclinal, and the val- 

 leys anticlinal. Fine examples of this form are found on 

 the western slopes of the Appalachian chain, where the 

 folds of the strata are dying away in gentle undulations 

 (Fig. 149). The reason of this form is that the hollows 



Ki<;. 15:!. Section of coal-field of Pennsylvania. (After Lesley.) 



become hardened by compression, while the original 

 saddles are loosened or even broken by tension, and ero- 

 sion therefore takes effect mainly on these latter. 



