FOLDING OF UPPER HUEOlSriAN SERIES. 161 



31 W., and swings east north of Lake Mary into sec. 32, T. 43 N., R. 31 W. 

 Here ' ferruginous slates are exposed, bordering the Michigamme River 

 at the s6-caJled Grhdden exploration. The extension of these lowermost 

 Upper Hurouian beds east from this point soon passes under the sand plains 

 and drift hills and is lost. The higher beds of the series are, however, 

 exposed in the lower course of the Michigamme, Paint, and Brule rivers, 

 which give good sections across them. In this portion of the area discussed 

 the extension of even these higher parts of the formation can not, however, 

 be followed farther east than the Michigamme River. 



That the Crystal Falls synclinal basin is not simple, but has minor 

 rolls, is shown by the way in which the Upper Huronian series indents the 

 Lower Huronian at the eastern end. Also the close and complicated folding- 

 is shown by mining work, and can be nicely seen in the open pits of the 

 Columbia and Crystal Falls mines, in the exposures in the railroad cut near 

 the Crystal Falls mine, and also along both banks of the Paint River near 

 the town of Crystal Falls. PI. XIV shows the general character of this 

 syncline. The folding has produced extensive " reibungs-breccias." Near 

 Crystal Falls, along the river bank, about one-fourth mile south of the rail- 

 road bridge, may be seen such a breccia, which has been formed at the 

 junction of a chei't with the slates. 



TIME OF FOLDING OF THE UPPER HURONIAN. 



The latest folding to which the rock of the Crystal Falls district has 

 been subjected is that which affected the Upper Huronian and likewise 

 involved the underlying Archean and Lower Huronian rocks. Therefore 

 the determination of this period of folding is of especial interest, as mark- 

 ing the close of orogenic movements in this district. 



Overlying the Upper Huronian is the Potsdam Cambrian, or Lake 

 Superior sandstone. The beds of this formation are horizontal, or else 

 show a very slight tilting, following the general inclination of the district, 

 which perhaps to a great extent may be explained by the initial dips of the 

 beds. They overlie with strong unconformity the upturned and strongly 

 plicated beds of the Upper Huronian. This unconformity marks a lapse of 

 time represented in other districts by the following events: (1) A period of 

 upheaval and denudation of the Upper Huronian; (2) the subsidence and 

 deposition upon the truncated Uj^per Huronian sediments of the hetero- 

 MON xxxvi 11 



