522 GEOLOGY. 



on the crests of folds where local stretching took place, and that reversed 

 faults might occur even in regions of tension. But such cases must 

 usually be local, and capable of detection and elimination by a study 

 of the phenomena of the surrounding region. These exceptional cases 

 aside, the general inference from prevailing normal faults is that the 

 regions where they occur have undergone stretching, while the infer- 

 ence from the less widely distributed reversed faults is that the surface 

 where they occur has undergone compression. 



In view of the current opinion that the crust of the earth has been 

 subjected to great lateral thrust a3 a result of coohng, it is well to make 

 especial note of the fact that the faults which imply stretching are called 

 normal because tHey are the more abundant ; and that the faults which 

 imply thrust are less common, and are styled reversed. The numerical 

 ratio of normal to reversed faults has never been closely determined, 

 but normal faults very greatly preponderate, and are estimated by some 

 writers to embrace 90-odd per cent, of the whole. The testimony of 

 normal faults is supported by the prevalence of gaping crevices, and of 

 veins which are but crevices that stood open until they were filled by 

 deposition. All these phenomena seem to testify to a stretched con- 

 dition of the larger part of the surface of the continents. This will 

 again claim attention in the study of Earth Movements. 



Effect of faulting on outcrops. — Faulting may bring about numerous 

 complications in the outcrop of rock formations. In a series of forma- 

 tions having a monoclinal structure (Fig. 

 436), many changes may be introduced. 

 Let it be supposed in the following cases 

 that, after faulting, the surface has been 

 reduced to planeness by erosion. If the 

 fault-plane be parallel to the strike of the 

 beds {ah, Fig. 436), and hence a strike fault, 



Fig. 436. — Diagram showing an ^, ^ c • i i j t 



area of rock with monocUnal the outcrop of a given layer may be dupli- 

 structure. One layer notably cated {H, Fig. 437), or it may be elimi- 

 iinlike the others. ^^^^^ altogether (Fig. 438). If the fault- 



plane be parallel to the direction of dip {cd, Fig. 436), a dip fault, the layer 

 H will outcrop, as in Fig. 439, if the downthrow was on the far side, or as 

 in Fig. 440 if the downthrow was on the opposite side. In both cases 

 the outcrop H is offset, the amount of the offset decreasing with increasing 

 angle of dip and increasing with increasing throw of the fault. If the fault 



