Some Points in the Geology of Northern Wisconsin. Ill 



great group — 6 ve hundred feet thick — of magnetic and specular 

 schists, carrying a larger portion of the oxjds than the general 

 run. The northernmost, highest, or newest portion of the groups con- 

 sists, so far as known, almost entirely of diorites, diorite slates 

 and diorite schists, one locality, however, of quartz slate is 

 known, and no doubt more occurs, since this portion of the 

 group is much concealed by drift, and, indeed, has not been 

 examined with the thoroughness devoted to the lower members. 

 Between the exposures of this uppermost member and the 

 next one beneath, there is always a gap without exposures, at 

 least a thousand feet in width, on the southern side of which 

 rises the Penokie Ridge, whose mass is chiefly made up, at least 

 on its western portion, of the middle member of the group. 

 It is evident that some general cause gives rise to the lack 

 of exposures in this interval, which cause I take to be greater 

 softness of material ; and, inasmuch as in the Huronian of the 

 Marquette region, the soft portions are chiefly the soft hema- 

 tites, which are never found outcropping, for this reason, and 

 for others, I have reco .amended in my report, that this part of 

 the county be thoroughly searched by test-pitting. 



The total thickness of the Huronian series at Penokie Gap, 

 as calculated from actual measurement, is about 4,000 feet, as 



follows : 



Feet. 



-. , , i not exp., 900 



Lowest member j in sight, 700 



Central member 600 



Space witliout e iposure 1400 



Upper member 400 



4000 



About fifty miles east of Penokie Gap, Messrs. Purapelly 

 and Brooks estimate the thickness of the same belt at -ijOOO 

 feet, a remarkable uniformity of its structure is thus shown. 



Copper Bearing Series. — Next north of, and immediately 

 overlying the Huronian, are the enormously thick beds of the 

 copper-bearing series, which have, in all, an apparent thick- 



