420 THE PE^rOKEE IROX-BEAEING SERIES. 



series is derived from g-ranites, gneisses, mica-schists, hornblende-schists, 

 etc.; in other words, from rocks wliich are always strongly quartzose and 

 which contain a large quantity of- acid feldspar. From materials of this 

 sort quartzites, graywackes, and slates are naturally formed. That the 

 quartz and acid feldspars are often in a very fresli condition is not 

 strange, but it has ^een seen that even these acid feldspars have exten- 

 ■ sively, in the Upper slate member, altered to the more basic minerals, 

 mica and chlorite, with the simultaneous separation, of quartz and the 

 resultant formation of a mica-schist or chlorite-schist. These mica-schists 

 are unmistakably j^arts of the Penokee series, and are as much changed 

 from their original condition as is the basic eruptive material of the 

 greenstone-conglomerates, notwithstanding the fact that the latter are 

 composed of materials very readily alterable. Basic eiiiptive rocks are 

 well known to have undergone extensive changes, even when occurring in 

 solid masses \\liich l)elong to geologic periods long subsequent to that of 

 the Penokee series. It thus appears that the widespread alteration of the 

 basic detritus ill these' greenstone-conglomerates is no proof that they are 

 not of the saiue age as the rocks of the typical succession. Also in the 

 greenstone-conglomerates themselves is found a small quantitv of well worn 

 fragmental (piartz and feldspar, which in every respect resembles like 

 material in the graywackes and quartzites. It is thus plain that the litho- 

 logical evidence points toward a classification of the greenstone-conglomer- 

 ate with a series subseqiieiit to the Southern Complex, a series of clastic 

 rather than crystalline rocks. 



Finally, the association and relations of the greenstone-conglomerates 

 to the other classes of rock adjoining, whicli muiuestionably are the equiv- 

 alent of the main Pent)kee succession, are such as to make it scarcely 

 conceivable that they could be a part of the eruptives of the overlying 

 Keweenaw series. 



Sfratif/raphical euidcnce as to rquivalencc with the main Penokee area. — 

 The stratigra])hical cvidcuu-e that the eastern area rocks belong to a series 

 separated botli from the schists, gneisses, and gnuiites to the south of them 

 and trom tlu^ great range of greenstones to the nortli is precisely like 

 th;it wliich proves tluit the clastic formations to the west belong to a dis- 



