PHYSIOGRAPHIC EVIDENCE OF FAULTING 199 



The incision of the initial scarp by ravines, which increase in depth as 

 the scarp is increased in altitude by repeated faulting, will in time de- 

 stroy its continuity, and when the total uplift has reached a thousand 

 feet or more nearly all the face of the scarp will have been consumed 

 between the slanting ravine walls ; but so long as recent renewal of fault- 

 ing holds the base of the scarp close to the fault line, the spurs into 

 which the greater part of the scarp has been transformed must terminate 

 in triangular facets. The slanting surface of the facets, not much worn 

 back from the fault surface, must truncate the structure of the uplifted 

 mass in such a way that the bases of all the facets stand along a single 

 simple line. 



As long as such facets are well defined, there need be no doubt about 

 their marking a fault, even if no rock outcrops are seen on its lower side. 

 The chain of evidence in such a case is indeed of the essentially same 

 nature as that on which geologists usually rely for the demonstration of 

 faulting, although composed of different links. The usual geological evi- 

 dence of faulting is the occurrence of corresponding structures in dis- 

 cordant attitudes on the two sides of a line; the fault fracture is seldom 

 seen, unless revealed by mining, and the movement implied in the term 

 fault is never seen. In view of such an occurrence, a geologist would say : 

 "Those two corresponding structures can not have been made independ- 

 ently ; they must be two parts of an originally single mass, now fractured 

 and displaced by a fault." And this inference of fracturing and dis- 

 placement in the unobservable past is commonly spoken of in geological 

 circles as if it possessed the same order of verity as the directly observ- 

 able occurrence of the two discontinuous structures — that is, it is com- 

 monly treated as a "fact." 



The physiographic evidence of faulting in the case here considered is 

 the occurrence of a highly specialized group of forms, namely, the ter- 

 minal facets of a series of spurs, peculiarly related to each other and to 

 the structures that they truncate. In view of such an occurrence, a 

 physiographer would say: "Those facets can not be the original termina- 

 tion of the structures that they now truncate; the action of the sea being 

 excluded, no process of erosion can have worn back a former greater ex- 

 tension of the structural mass to so systematic a termination in trian- 

 gular facets along a simple, non-structural line; hence the visible mass 

 must be part of an originally greater mass, the other part of which has 

 been relatively faulted down out of sight." This inference of fracturing 

 and displacement is not today universally spoken of in geological circles 

 as if it possessed a high degree of verity; but there was once a time 



