48 REPORT OF THE 



points it abuts upon the coast, reappearing again in the Huron 

 islands on the west, and Granite Island on the east. Another 

 granitic boss rises up in the district south of the Iron Region, 

 and covers about twelve townships, and still others, on a small 

 scale, are found east of the mouth of the Machigamig river. 



The rock throughout these exposures is seldom a true granite, 

 being composed mostly of feldspar and quartz, with occasional 

 intermixtures of mica in small quantity. Hornblende sometimes 

 replaces the mica, and the rock becomes syenite. The plutonic 

 rocks on the south shore of Lake Superior, appear to have been 

 upheaved after or towards the close of the Azoic period. 

 II.— VOLCANIC GROUP. 



A range of volcanic rocks extends from the extremity of 

 Keweenaw Point to Montreal river, running nearly parallel 

 with the lake coast, and having a width varying from two to 

 eight miles. About twelve miles east of Montreal river the belt 

 suddenly widens to about fifteen miles, sending a spur off on 

 the south side toward the southern extremity of Agogebic 

 lake. Another spur sets off north to the Porcupine Mountains. 

 To the east of Portage lake this belt is in reality two belts — 

 the "northern" one consisting of interstratified masses of 

 amygdaloid, conglomerate and coarse sandstone ; the " south- 

 ern," or "Bohemian" range being a mass of crystalline trap. 

 About a mile north of the northern range, another narrow belt 

 curves round parallel with the coast from a point opposite 

 Manitou Island, to the eastern point of Sand Bay. The belt 

 called the Northern Range contains the larger number of copper 

 locations. These rocks were erupted during the period of the 

 Lake Superior Sandstone. A contemporaneous range forms the 

 basis of Isle Royal.* 



III.— THE AZOIC SYSTEM. 



An immense thickness of unfossiliferous strata is interposed 

 between the crystalline rocks just referred to, and the Lake Su- 

 perior sandstone. These, in the Upper Peninsula, commence at 



• For information concerning the rocks of Lake Superior, see Foster and Whitney's Report. 



