STRUCTURAL GEOLOGY 413 



ment along joint planes, but faulting is not limited to such displace- 

 ments. Horizontal displacements may take place along a joint 

 plane, with no vertical displacement, and this also is faulting. 

 Horizontal displacement often accompanies vertical displacement, 

 and the former is as much a part of the faulting as the latter is. 

 The tendency of recent study, whether based on theory or on field 

 observation, is to emphasize the importance of the horizontal 

 movement in faulting. In numerous mines, for example, where 

 the walls of the shafts and tunnels afford excellent opportunity for 

 determinations, horizontal movement is much more in evidence 

 than vertical. 



There are various displacements of rock bodies not mentioned 

 above which are akin to faulting, if not to be regarded as such. 

 Thus when strata are folded there is some slipping of layer on layer. 

 Occasionally there is displacement of layer on layer, even when 

 the beds are not folded. Such a case with a well developed " slicken- 

 side" surface is known in Ohio, between beds which are nearly 

 horizontal. The recognition of such movements as faults opens 

 a wide door. The great variety of displacements along joints or 

 other partings in the rock, shows the difficulty of defining faults 

 sharply. Many movements of displacement, which can hardly 

 be separated from faults logically, are not usually called faults. 



Map work. The sections of the Structure Section Sheets of the folios of 

 the United States Geological Survey furnish abundant illustrations of a va- 

 riety of structural features, such as folding and faulting, and the relations of 

 sedimentary, metamorphic, and igneous locks. The sections of various Bul- 

 letins, Professional Papers, etc., of the same Survey afford other illustrations. 



