ooi B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



of all angular shapes varying from a few inches to 25 feet on a side. 

 The surfaces are slickened over wide areas, and where they preserve 

 their orientation in the cliffs the slickens demonstrate much relative 

 horizontal displacement of adjacent fragments. Certain fracture planes 

 are in fact steep fault surfaces along which displacement has occurred 

 in the direction of the strike rather than in that of the dip. Such faults 

 are, however, without apparent system. In other places, as north of 

 Altyn, the cliffs present mural faces traversed by remarkably regular 

 lines of bedding which are crossed by nearly vertical joints (see figure 2, 

 plate 47). 



Viewed in the large, the structure of the Altyn limestone sometimes is 

 that of major and minor thrust faults. Yellow mountain, as seen from 



Upper member, Altyn formation. 



Lower member, minor thrusts 



Lewis thrust 



Figure 5. — Chief Mountain, looking north. 



Showing the zone of minor thrusts in massive limestone between the Lewis thrust at the base 

 and the undisturbed upper member of the Altyn formation. 



Chief Mountain ridge, exhibits these relations very clearly (see figure 6). 

 The basal major thrust lies at the foot of the cliffs, somewhat obscured 

 by talus, but sloping about 8 degrees in a curve which on the left is less 

 inclined and descends more rapidly to the right. Springing from it are 

 several minor thrusts, which dip more steeply and which upward pass 

 out either into the air or into an upper major thrust. The upper major 

 thrust is at the base of argillites which dip gently and without appreci- 

 able disturbance to the southwest. It simulates an unconformity. 



In Chief mountain a similar structure is more strikingly exhibited (see 

 figure 5). The base of massive Altyn limestone is traversed by minor 

 thrusts which are often subparallel Ito the bedding, so far as it can be 

 made out. These thrusts dip 30 degrees and occupy a zone about 1,000 

 feet thick above the Lewis major thrust. They are limited above by an 



