NOKMAL STEP FAULTS. 



237 



The two surfaces along the line of movement which have been rubbed 

 together are commonly very dense, hard and highly polished, form- 

 ing what is usually termed " slickensides." The common appearance of 

 these faults as they are encoun- 

 tered in mining is represented in 

 figure 1, w^hich is taken from the 

 Bloomfield shaft near Des Moines. 

 It is typical of quite a large num- 

 ber occurring in this and other 

 mines. Sometimes the slips are 

 quite small (figure 2 ) and die out 

 in the coal vein itself, as shown 

 in the Thistle mine in Appanoose 

 county. An especially interesting 

 fault has been observed in the 

 American mine in ^I ah a ska 



county, where the line of movement has been directed through a 

 narrow nodular band or elongated mass of clay ironstone (figure 3). 

 The line of breakage in passing from the softer to the harder layer is 



Figure I.— Fault in Bloomfield Shaft, 

 Des Moines, Iowa. 





Figure 2. — Deformation in Roof of Thistle Mine, Appanoose County, Iowa. 



changed in direction, being bent toward the normal, while after passing 

 into the coal seam again it assumes its original direction. This phe- 

 nomenon is essentially the same which has been recognized in slates 

 where hard grit bands are intercalated. 



Step Faults. — Where the ordinary normal fault is repeated a number 

 of times within a short distance the step fault is formed. Exposures of. 

 this kind are well shown in the Davison mine in Jasper county. Figure 

 4 represents a section through part of one of the entries, although the 

 entire series is not here reproduced. The fall of the displacements is 



