80 THE PENOKEE IKON-BEAEING SERIES. 



accept the six groups of the first cohimn even as a mere fjeneral strati- 

 graphic succession. Group i, called " Granite and Syenite with Gabbro," 

 should rather read gabbro with some reddish acidic rocks. Here belong 

 the enormous masses of gabbro which appear in tlie Bad river country of 

 Wisconsin, and over a large area in northern Minnesota at tlie base of the 

 Keweenaw series. That these rocks belong rather to the Keweenaw series 

 itself than a separate group, is indicated both by their lithological similarity 

 to that series, and the very striking unconformity which they present with 

 regard to the lower slaty i-ocks, both in northern Minnesota and the Penokee 

 district. 



The second, or mica-schist group, is hardly a valid one unless the term 

 mica-schist be made to include an enormous mass of rocks which is not 

 ordinarily covered by such a name. It is true that mica-bearing schistose 

 rocks, quite different, liowever, from those completely crystalline mica- 

 schists which belong at a much lower horizon in the lake Superior region, 

 ai-o met with in certain places in the lake Superior region at the summit of 

 what has ordinarily been called the Hurouian series ; but these micaceous 

 schistose rocks are after all a mere phase and by no means the predominant 

 one of a great thickness of genuine fragmental rocks, which mei'ge, both 

 horizontally and vertically, into those occurring farther below, called the 

 carbonaceous and arenaceous black slate group. 



The fourth group, the hydromica and magnesian slate group, has, so 

 far as we are aware, no distinct existence; certainly none at any such 

 stratigraphical horizon as here indicated. Hydromica-slates and magnesian 

 slates are liere and there stratified among the Iron-bearing rocks of different 

 parts of the lake Sujierior region, but they never constituted, so far as we 

 are aware, any continuous or well marked hoi'izon. 



The fifth group, which, however, has certainly no separate and dis- 

 tinct existence, is fanly well represented in the Marijuette, Menominee, and 

 Penokee districts, though Prof. Winchell's reference to this one horizon of 

 the Huronian series of Canada has, we think, no basis. 



The sixth grouj) correctly enough includes those granites and gneisses 

 which, with certain schists, also belong throughout the lake Superior 

 country beneath all the stratiform rocks. 



