CHAPTEE II 



GEOGRAPHICAL LIMITS, STRUCTURE AND STRATIGRAPHY, 

 AND PHYSIOGRAPHY. 



GEOGRAPHICAL LIMITS. 



The portion of the district here described extends from the north Hne 

 of T. 47 N. to the south Hne of T. 42 N., and from the center of R. 31 W". 

 to the west hne of R. 33 W., and contains approximately 540 square miles. 



Upon the small sketch map at bottom of PI. HI is outhned the por- 

 tions of the district which have been studied and described by the different 

 authors. 



The detail character of the formations is unknown for parts of the 

 area under discussion. This is especially true of the north, west, and 

 southwest parts, where, owing- to the readily decomposable nature of the 

 rocks, as determined by the few ledges observed, and to the drift mantle, 

 very few outcrops are to be found. 



STRUCTURE AND STRATIGRAPHY. 



The Crystal Falls district is not sharply defined petrographically, but 

 is continuous with the Marquette district on the northeast and the Menomi- 

 nee district on the southeast (PI. I). It is, however, remarkable for the 

 vast accumulation of volcanic rocks, which, while by no means absent from 

 the adjoining districts, do not there play so conspicuous a role. 



Structurally this district can hardly be better separated from the 

 Menominee and Marquette districts than it can be petrographically. The- 

 important sedimentary troughs of the two adjacent districts are separated 

 by an average width of 40 miles. The area between the districts on a, 

 direct course is occupied chiefly by Archean rocks, with narrow infolded 

 trouo-hs of Huronian rocks playing a very subordinate role. At the east 



