GENERAL GEOLOGY OF THE DISTRICT, 465 



the iron formation, Ijut in most localities it took place within a compara- 

 tively short distance. Trom this time onward the accumulating beds 

 of the Penokee series were very uniform in character. The material depos- 

 ited constitutes the Upper slate member. This member, in places more than 

 10,000 feet in thickness and making up at least six-sevenths of the whole 

 series when at its maximum thickness, is in turn graywacke and gray wacke- 

 slate, mica-schist and mica-slate, with less frequent clay-slates. The 

 original material for all of these phases of rocks was very largely quartz 

 and feldspar, the only differences being the relative proportions of the two 

 mmerals and their fineness of comminution. Thick beds of fine grained 

 black clay-slates and gray wacke-slates show that through a large proportion 

 of the area the material was very finely pulverized. From Penokee gap 

 and westward the original rock has been metamorphosed into a mica-slate 

 or mica-schist. Here the material furnished by the underlying gneiss and 

 granite was almost wholly feldspar. This peculiarity is explained by the 

 fact that the area of granite and granitoid gneiss stretching to the south 

 and west is very strongly feldspathic. After the deposition of at least 

 12,000 feet of these materials, and perhaps other kinds of sediments which 

 have been subsequently swept away, came the end of Penokee time. 



The district was then elevated above the sea, gently folded, and suf- 

 fered a long period of atmospheric denudation. The ei'osion was .sufli- 

 cient at the west and east ends of the district to entirely remove the series. 

 What fraction of it has been removed at its place of present maxinuun thick- 

 ness is unknown. Following the Penokee series came the great succession 

 of eruptive and fragmental layers which built up the Keweenaw series. This 

 series has been already treated in a monograph of the Geological Survey,' 

 and nothing will be here said as to the conditions which prevailed or as to 

 the succession. But the probable connection between the eruptives of this 

 series and the dikes of the Penokee series is to be noted. The little altered 

 diabases of the Southern Complex and those of the Penokee series have 

 been described in detail and thek relations to the containing fonnation 

 given (chapter vii). It has been seen that these same dikes have been pro- 

 foundly altered in the iron formation, where they have been subjected to 



' Copper-Beariug Rocks of Lake Superior ; by R, D, Irving, U. S. Geol. Survey, Mouograph v, 

 MON XIX 30 



