ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 61 



ment either one or both of the walls have " dragged," and so are 

 curled back in the immediate neighborhood of the fault plane 

 (Fig. 44). 



When, as is quite generally the case, the actual plane of dis- 

 placement of a fault is not open to inspection, the movement may 

 be proven by the observation of 

 abrupt, as contrasted with grad- 

 ual, changes in the strikes and dips 

 of neighboring exposures (Fig. 45) ; 

 or by noting that some easily rec- 

 ognized formation has been 

 sharply offset in its outcrops (Fig. 

 46). 



There are in addition many in- 

 dications rather than proofs of the 

 presence of faults, which must be 

 taken account of in every general 

 study of the geology of a district. 

 Thus the outcrops of all neighbor- 

 ing formations may terminate 

 abruptly upon a straight line which 

 intersects all alike. Deep-seated 

 fissure springs may be aligned in 

 a striking manner, and so indicate 



the course of a prominent fracture, 

 though not necessarily of a fault. 

 Much the same may be said of the 

 dikes of cooled magma which have 

 been injected along preexisting frac- 

 tures. 



The base of the geological map. 

 Modern topographic maps form an im- 

 portant part of the library of the serious 

 student of physiography; they are the 

 gazetteer of this branch of science. 

 Every civilized nation has to-day either completed a topographic 

 atlas of its territory, or it is vigorously prosecuting a survey to 

 furnish maps which represent the relief with some detail, and pub- 

 lishing the results in the form of an atlas of quadrangles. Thus 



FIG. 45. Map to show how a fault 

 may be indicated in abrupt changes 

 of the strike and dip of neighboring 

 exposures. 



FIG. 46. A series of parallel 

 faults indicated by successive 

 offsets in the course of an 

 easily recognizable rock for- 

 mation. 



