STRUCTURES COMMON TO ALL ROCKS. 49 



3. Highly Inclined Outcropping Strata. These 

 give rise to sharp ridges, determined each by the outcrop 

 of a hard stratum, with intervening valleys determined by 

 the outcrop of softer strata (Fig. 154). This structure is 



FIG. 154. Parallel ridges. 



finely displayed on the flanks of Western mountains and 

 the mountains of Tennessee, and especially in the moun- 

 tains of Virginia. Standing on the top of Warm Springs 

 Kidge, twelve or more mountain-waves may be counted, 

 each crest determined by the outcrop of a hard sandstone. 



4. Very Gently Inclined Outcropping Strata. 

 These, in the Plateau region, give rise to a remarkable 

 series of nearly level tables, terminated by cliffs, a hard 

 stratum forming the surface of the table. In Fig. 156, 

 taken from Powell, the successive tables are fifteen to 

 twenty miles wide, and the cliffs 1,500 to 2,000 feet high. 

 The manner in which these are formed is illustrated in 

 the diagram, Fig. 155, in which a, b, c, d are hard strata. 

 The dotted space shows the general erosion. 



5. Highly Metamorphic and Granitic Rocks. 

 These reveal intornal Suruoture much less perfectly than 



PlO. 155. Dotted lines show material carried away by erosion. 



unchanged stratified rocks. Usually the inequalities are 

 very irregular, the peaks bein<( determined by harder, and 

 the valleys by softer, spots. In some cases, however, the 

 peculiar forms may be easily explained. Thus, in the 



