238 C. R. KEYES — CRUSTAL ADJUSTMENT IN MISSISSIPPI VALLEY. 



from six inches to several feet, the inclinations being about forty degrees. 

 Another similar series occurs in the Deep Vein mine in Monroe county. 

 In all cases the faults are practically parallel to one another. In some 

 places the effect of the strain instead of appearing in faults is relieved 



^:?^vF^J^y'^y; 





Figure 3.— Fault in American Mine, Mahaska County, Iowa. 



partly by small sharp folds without direct fractures. From an examina- 

 tion of numbers of these small faults and of others of similar character 

 in mountainous regions it would seem that the adjustment of tension 

 in the earth's crust here, as in districts of greater deformation, is accom- 

 plished by means of many small slips rather than by a few large ones. 



Figure 4.— Step Fault in Davison Mine, Jasper County, Iowa. 



Similar faulting has been worked out in the mountain districts of Cali- 

 fornia, where recently Becker has made some extended and extremely 

 interesting observations in this direction. 



Trough Faults. — A special case of the ordinary normal fault is when 

 two slips with hades in opposite directions occur close together, allowing 



