870 STUDIES FOR STUDENTS 



area are observed at the individual exposures. In areas of 

 folded rocks minor curvings of bedding planes are usually exhib- 

 ited, and generally in great perfection. Changes in the strike 

 and the dip of the beds are, when large or numerous exposures 

 are found, observed to be, in general, gradual rather than abrupt; 

 and, if abrupt, search will reveal intermediate values for the dip, 

 or the strike, or both, as the case may be. Foliation planes are 

 characteristic of, and connected with, folding, but not with 

 faulting. 



CRITERIA FOR RECOGNIZING A SYSTEM OF FAULTS. 



The characteristics of a jointed and normally faulted area of 

 the earth's crust have been touched upon in the last section. 

 With abundant outcrops this structure may be disclosed upon 

 the areal map by the generally rectilinear or zigzag boundaries 

 of formations. If not, some indication may be afforded by the 

 prevalence of straight lines and sharp bends in the topography, 

 particularly if the lines fall into parallel and intersecting series. 

 Such a relief may not be apparent without careful examination 

 of the map, for the reason that the particular combination of 

 directions is not known, and it may yet be quite striking when 

 once discovered. Again, the drainage lines may compose a net- 

 work which in direction conforms both to that of the topographic 

 lines, and that of the formation boundaries. The prominent 

 joints observed at the individual exposures, it may be supposed, 

 will also be in conformity with the directions of the geologic and 

 topographic lines above enumerated. This has never been better 

 expressed than in the classic work of Professor Th. Kjerulf upon 

 the geology of southern and central Norway, which concludes 

 with these words: 



Thus the great systems of fissures which cut up the surface produce the 

 primary lines in the aspect of the surface of Norway. The mysterious network 

 of these lines is stamped in indelible characters ; this may indeed remain a 

 long time unnoticed; if, however, it has once been seen, it will never again 

 escape observation. Like a moss-grown inscription upon a plate of marble, it 

 is there and may be recognized. Here all embodied representations of 

 plateaus, of tilted plains, and of every sort of erosion have not prevailed to 

 hide the writing or to withdraw it from observation. Push these all aside and 



