622 STUDIES FOR STUDENTS 



the zone of flowage, the necessary deformation is accomplished 

 by folding. In regions of close folds it is probable that 

 before the superincumbent bods were removed by erosion many 

 of the latter were faulted instead of folded, for they were in the 

 zone of fracture and flowage, and in the zone of fracture 

 rather than the zone of flowage. 



Possibly the depth at which important faults disappear is 

 in many cases not more than 5000 meters, although the dis- 

 cussion of the depth of earthquake shocks due to faults leads 

 to the conclusion that some faults extend to the depth of a 

 number of miles. 



If there are inclined planes of weakness in the deep-seated 

 zone of flowage, the deformation may largely occur by faulting 

 along these planes."^ Such inclined planes of weakness may be 

 in sedimentary rocks or in igneous rocks. Since masses of 

 intrusive rocks, either in the form of dikes or of bosses, usually 

 have vertical or steeply inclined exteriors, faulting is particu- 

 larly likely to occur at the contacts of igneous rocks with one 

 another, or at the contacts of igneous with sedimentary rocks. 

 It has already been explained that such deep-seated faults 

 would differ in no way from the differential movements result- 

 ing in cleavage or fissility, except that the movements are 

 mainly confined to narrow zones. This results in great dis- 

 placement at the fault zones and little displacement in the areas 

 between the faults. In such supposed deep-seated faulting it 

 is to be remembered that the displacement takes place without 

 crevice or joint. At any given movement the rock is to be 

 regarded as welded together. The different parts simply shear 

 over one another along the plane of greatest weakness. 



RELATIONS OF FAULTS TO STRATIGRAPHY. 



Faults may be used to discriminate between series in pre- 

 cisely the same way as joints, and the criterion has far 

 greater weight. If an inferior set of formations has a more 



' The Mechanics of Appalachian Structure, by Bailey Willis. Tliirteenth Ann 

 Rept., U. S. Geol. Surv., pp. 217-274, 1893. Especially Pis. XCV and XCVI. 



