CHAPTER V. 

 THE UPPER HURONJAN SERIES. 



The upper series of this district is connected in the northeastern part 

 of the area with the Upper Marquette series of the Marquette district 

 ah-eady described in the Fifteenth Annual Report and Monograph XXVIII. 

 In these reports the Upper Marquette series is regarded as part of the Upper 

 Huronian. As has been stated, the Crystal Falls district is the southwestern 

 extension of the Marquette district, and consequently we should expect the 

 chief formations of the two districts to be continuous, as they are. Because of 

 the drift and because of a change in the character of the rocks, in mapping 

 the Avestern part of the Crystal Falls district it has not been practicable to 

 divide the Upper Huronian into several formations, corresponding to those 

 in the Marquette district. No independent name will be given to it, but 

 it will simply be called Upper Huronian, with the understanding that it 

 corresponds stratigraphically to the Upper Marquette series. 



DISTRIBUTION, BXPOSTJEES, A]Srr) TOPOGRAPHY. 



Beginning in the northeastern part of the area discussed by me (see 

 PI. Ill), this series covers the southern parts of T. 46 N., Rs. 31 and 32, 

 where it is only 4 miles in width. It is here a northwest-southeast syncline. 

 From this place it stretches beyond the northern limit of the map. With 

 slight interruptions where intrusives occur, it extends in a broad area to the 

 west and soiith about the Hemlock volcanics to a point lying beyond the 

 limit of the map. On the eastern side of the district it abuts against and is 

 folded in synclines in the Archean granite. 



Exposures are scanty for the greater part of the area in the Crystal 

 Falls district underlain by the Upper Huronian series. This is due to two 

 conditions, iirst, to the soft character of the rocks constituting the series, and, 

 second, to the presence in places of the Cambrian sandstone, and more 

 especially to the deep covering of glacial drift which is found spread over 

 the entire district. The Upper Huronian is composed in great measure of 



