THE ERUPTIVES. 349 



clayey rocks, penetrable with great difficulty if at all Vjy percolating water, 

 contains greenstones which are relatively little altered. 



Comparison of Pcnokec f/reeti stones ivifh greenstones of the Southern Com- 

 plex and Keweenaw series.— V\\v rocks nnderlying the Penokee series — the 

 Southern Complex — contain liere and there exposures whicli in all essential 

 respects are like the diabases nf that series. However, parts of the basic 

 eruptives of the older succession are massive, profoundly altered green- 

 stones and schistose rocks which are believed to have been greenstones, 

 but if so they have undergone extreme metamorphism. That there should 

 be eruptives here which do not differ in degree of alteration from those 

 contained in the Penokee series is what woidd be expected, for throufh 

 these underlying rocks must have passed the dikes which traversed the 

 Penokee series itself, and doubtless the fresher greenstones represent i)arts 

 of the .same dikes which are found in tlie higher series. 



Tlie Keweenawan eniptives dilfer from those contained in tlie Peno- 

 kee series in their great variety and also in tlieii- manner of alteration. 

 The immediately overlying rock is in turn a coarse grained gal^bro, a 

 dialiase, a diorite which at times is schistose or fpiailzose, a melaphyre, an 

 augite-porphyrite, a porjihyrite, and Jin amygdaloid. Not onlv do all 

 phases of these basic rocks here occur, l)ut in the same locality several 

 may be found closely associated. In short, nearly all of the phases of the 

 basic eruptives of the Keweenaw series occur, and in the sudden alterations 

 from one phase to another and their intricate mingling thev are typical of 

 the series to which they belong.' 



The Keweenawan eruptives above the Penokee series are tlie volcanic 

 equivalents of the Penokee dikes and sheets. Along tlie contMct line of the 

 two series the eruptives are sometimes intricately related. These facts 

 strongly suggest that the Penokee dikes are the pi])es through- which flie 

 volcanics passed. The identical lithological character of tlie dikes and 

 sheets suggests that the fonner fed the latter. We are thus led to the 

 hypothesis that all of these eruptives belong to a single period, the Ke- 

 weenawan. 



'Roland D. Irving: Jlouograph U. S, GeoJ. Survey, vol. v, Copper-bearing Rocka of L»k6 

 Superior, 



