DISLOCATIONS OF STRATA. 



93 



Such a dislocation along a fracture is called a fault. Faults vary from 

 an inch or less of displacement to thousands of feet. Along the line 

 bb, there is not only a fault, but also at the junction a bending of the 

 layers, arising from the friction of one side against the other when the 

 dislocation took place. In Fig. 97, the fracture is an opened one filled 

 with rock. In 97 A, the fracture was a crooked one, and consequently 

 the sliding of one side on the other left a series of open spaces to be- 

 come subsequently filled. On p. Ill, other faults are represented. 



Folds or flexures are most common in the rocks of mountainous 

 regions. They show that, in mountain-making, rock-formations have 

 been bent as sheets of wax may be bent. Single folds are often miles 

 in span, scores of miles in length, and thousands of feet in height. A 

 ridge sometimes corresponds to one such fold. On the other hand, a 

 stratum may have its bed bent into small zigzags or contortions, like 

 those of Fig. 99 e. The large folds are not now entire because most 

 upturned rocks are easily worn away by flowing waters ; but their orig- 

 inal forms are indicated by the inclinations of the strata that remain. 



Fig. 98, by Rogers, is from an actual section in the Appalachians, 



Fig. 98. 



?i vviYiv" m i" 



six miles in length, and shows the foldings of the strata. These are 

 numbered so that the bendings of a given stratum may be followed. 

 Thus in bends over n, to the left of the middle of the figure, and the 

 right portion .descends to come up again in in at the right end of the 

 figure ; again, iv, to the left, rises and bends over in and n, though 

 disjoined about the top of the fold by denudation. 



Flexures, as illustrated in the transverse sections, Fig. 99 a-d, may 



A Fis:. 99. B 



x\ 



be gentle or bold ; and they generally have the opposite slopes unlike, 

 and, consequently, the axial plane, a x, inclined. In Fig. a, a a; is 

 nearly vertical ; but in b, c, d, it is much inclined. In d, the top of 



