284 MATERIALS AND THEIR ARRANGEMENT 



their interpretation. Most gravity faults indicate an extension 

 of the surface sufficient to permit the fault-blocks to settle down 

 unequally. Thrust faults, as a rule, signify a compression of the 

 surface which required the blocks to overlap one another. In other 

 words, normal faulting usually implies tensional stress, and reversed 

 faulting compressional stress. Exceptional cases aside, the infer- 

 ence from gravity faults is that the regions where they occur have 

 undergone stretching, while the inference from thrust-faults is that 

 the surface when they occur has undergone compression. 



In view of the current opinion that the crust of the earth has 

 been subjected to great lateral thrust as a result of shrinking, it is 

 well to make especial note of the fact that the faults which imply 

 stretching are called normal because they are the more numerous; 

 and that the faults which imply thrust are the less common. The 

 testimony of normal faults in favor of tension is supported by the 

 prevalence of gaping crevices, and veins. All these phenomena 

 seem to testify to a stretched condition of the larger part of the 

 surface of the continents. 



Faulting may bring about numerous complications in the out- 

 crops of rock formations. These are difficult of detection in some 

 cases, especially after erosion has destroyed the fault-scarps l 



Faults of horizontal displacement. Horizontal displacement 

 may take place along a joint-plane, with no vertical displacement. 

 This also is faulting. Horizontal displacement accompanies verti- 

 cal displacement, in many cases, and the former is as much a 

 part of the faulting as the latter. 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 many 

 mines, for example, where the walls of shafts and tunnels afford 

 excellent opportunity for observation, horizontal movement is 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. 

 In places there is displacement of layer on layer, even when the beds 

 are not folded. Such a case with a well developed "slickenside" 

 contact 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 



1 See authors' Geologic Processes, pp. 522-524.- 



