FAULTS IN GENERAL. 519 



consider the general problem of faulting, briefly here, more fully in our 

 own stuclie"^ ; we, second, examine carefully the facts and arguments on 

 which our belief in the occurrence of faults is based ; we, third, recognize 

 a marked difference in the completeness of argument by which the 

 various faults are defined, some examples reaching complete geologic 

 demonstration, while other examples are hardly more than provisional 

 geologic inferences. 



Under the term, fault, we include a fracture of greater or less extent on 

 which differential movement of the adjacent masses has taken place. 

 Faults may be minute and clean cut, as often seen on joint planes in 

 quarries ; they may be of great dimensions and complicated structure, 

 as known in our Appalachians and elsewhere. They are manifestly 

 associated with different classes of strains ; sometimes indicating an ex- 

 tension of the region involved, sometimes a comj^ression, sometimes a 

 lateral or tangential shearing. They are on the one hand associated 

 with monoclinal folds, and on the other hand with overthrust folds. The 

 plane of the faults may have any inclination to the horizon, and the move- 

 ment of the adjacent masses may have any directions on the fault-j^lanes. 

 After the faulting, the fault plane itself may be deformed or faulted, and 

 the dislocated masses may be deeply eroded, even to baselevelling. 



Faults are ordinarily recognized by some lapse of continuity in masses 

 that were once continuous. This is variously determined. In the un- 

 derground work of mining, the fault-fracture itself may be encountered; 

 the dissimilarity of the adjacent masses is then clearh^ demonstrated, 

 and the occurrence of mechanical disturbance on the fault-plane is in- 

 dicated by ])reccias and slickensides ; but not once in a hundred times 

 is a fault thus determined ; and when thus determined, the visible por- 

 tion of the fault-plane is perliaps less than a hundred-thousandth of its 

 entire area. Faults are sometimes visible on sea-cliffs and in railroad 

 cuts ; they are occasionally seen on barren canyon walls, and still more 

 rarely and intermittently on valley sides and in stream beds, l:)ut the 

 actual visibility of a fault plane is a rare and local matter. 



Most of the faults that have been represented on geological maps and 

 sections are not known ]>y the visible occurrence of their fractures. They 

 are inferred from the lack of continuity of rock masses whose former 

 continuity can be reasonably postulated. The safety of such inferences 

 depends chiefly on the distinctness of the rock masses concerned, on the 

 certainty of their former- continuity across the fault-line, on the impos- 

 sibility of explaining their })resent discontinuity by other means than 

 faulting, on the closeness with which the fault-line may be traced, and 

 on the association with it of various minor indications of mechanical 

 disturbance. 



