MATERIALS OF THE EARTH 



65 



sometimes without connection with folding. Faults with great 

 throw probably pass into folds below, in many cases at least (Fig. 39) . 



The rock on either side of a fault- 

 plane is often smoothed as the result 

 of the friction of movement. Such 

 surfaces are slickensides. A slicken- 

 sided surface has a slight resemblance 



to a glaciated surface, but generally Fig. 39. The fault above 

 . , ,. T, i grades into a fold below, 



gives evidence of greater rigidity be- 



Thickening and thinning 

 of layers next the fault- 

 plane is evident. Based 

 on experiments of Willis. 

 (13th Ann. Rept., U. S. 

 Geol. Surv.) 



tween the moving surfaces. The rock 



along a fault-plane is sometimes 



crushed, making a crushed zone a few 



inches or feet wide. Such a zone 



becomes a passageway for ground-water, and so the site of 



ground-water work. Not a few such zones are "mineralized," and 



have become the sites of important mines. 



Effect of faulting on outcrops. Faulting may bring about 



numerous complications in the outcrops of rock formations. In 



a series of formations having a 

 monoclinal structure (Fig. 40), 

 many changes may be introduced. 

 Let it be supposed in the following 

 cases that, after faulting, the 

 surface has been reduced to plane- 

 ness by erosion. If the fault-plane 



is parallel to the strike of the 

 Fig. 40. Diagram of an area ,,,,.. , . 



of rock with monoclinal beds (ab, Fig. 40), and hence a 

 structure. One layer notably strike fault, the outcrop of a given 



layer (H) may be duplicated (Fig. 



41), or it may be eliminated altogether (Fig. 42). If the fault- 

 plane is parallel to the direction of dip (cd, Fig. 40), a dip fault, 

 the layer H will outcrop as in Fig. 43, if the downthrow was on the 

 far side. The outcrops of H are offset, the amount of the offset 

 decreasing with increasing angle of dip, and increasing with increas- 

 ing throw of the fault. If the fault is oblique to the direction of 

 dip and strike (ef, Fig. 40), an oblique fault, the outcrop of such 

 a layer as H will have the relations shown in Fig. 44 if the down- 



