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 

 b b, there is not only a fault hut also at the junction a hending 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. 

 Fig. 98 is an actual section, by Rogers, of a part of the Appala- 



Fie. 98. 



VVITIV 



chians, six miles in length, showing the foldings and contortions of the 

 strata in those mountains. The different strata are numbered, and by 

 these numbers 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. 



Some of the kinds of flexures and curvatures are shown in the an- 



Fig. 99 



nexed figures a-e, to appreciate which it must be understood that pli- 

 cations vary in extent from an inch and less to scores of miles ; that 

 they stretch over vast regions, and sometimes make lofty mountains. 



The plaitings and smaller foldings, but a few feet or yards in 

 breadth, are local and superficial, confined often to single layers or 

 thin beds ; and this is usually true of those that are many scores of 

 yards broad. It is always of the highest importance to distinguish 

 these local flexures from the profound bendings of great formations, 

 in the course of which they occur. 



The foldings of a region are generally in ranges nearly parallel to 

 one another ; and, where one fold dies out along the range, another 



