THE LOMBARD OVERTHRUST AND RELATED FEATURES 285 



forms the upper part of the Carboniferous system. The Quadrant 

 formation consists of two members, as noted by Dr. Peale. 1 

 The lower is a red arenaceous limestone overlain by bands of 

 shale and limestone. The upper member is thinly bedded cherty 

 limestones alternating with quartzite layers. The top of the 

 formation is somewhat arbitrarily placed by Dr. Peale at the 

 base of a very massive and persistent quartzite layer which is 



Fig. 11. — Double horseshoe canyon of the Missouri River. View east showing 

 Lombard Station, and mouth of Sixteenmile Creek canyon. 



considered to be the basal member of the overlying Ellis formation 

 of Mesozoic age. 



The writer obtained a thickness of about 400 feet for the Quad- 

 rant north of Logan and 674 feet near Lombard. The exposure 

 of the Quadrant formation in the canyon near Lombard is excellent, 

 and a section was measured straight up the side of the canyon from 

 the top of the massive cliff of the gray Madison limestone to the 



1 A. C. Peale, op. cit., p. 39. 



