466 THE penokep: iron-bearing series. 



long continued leaching-. In that jiart of the Penokee series which has 

 been the seat of mining operations, the large number of these dikes and 

 the fact tliat they cut the containing formations perpendicularly are 

 shown by the descriptions, pp. "271-275 and Pis. xxx and xxxi. That 

 these diabases are the pipes through which has passed, from deep within 

 the earth, the vast amount cff material which formed tlie basic volcanic Hows 

 of the Keweenaw series can hardly be doubted. The trap range north of 

 the Penokee series is a set of rocks varying, it is true, greatly iii structural 

 character, being here amygdaloids and there diabases and gabbros, but ni 

 chemical composiou these rocks are practically the same as those that are 

 found in the form of dikes within the Penokee succession. 



« 



After or during Keweenaw time began the orographic movement, 

 accompanied and followed by erosion, which made the synclinal trough of 

 lake Superior, and which upturned and truncated the whole great thickness 

 of formations constituting the Keweenaw and Penokee series. We have no 

 measure by which to estimate the time required for this work, but it was 

 sufficient to bring to the surface a continuous succession of beds more than 

 50.000 feet thick. ' 



Prior to this time, during it, and subsequent to it went on the alter- 

 ations which clianged the rocks of the Penokee series from their original 

 condition to their present somewhat metamorphosed one. There is no means 

 of placing the time at which the change from the feldspathic fragmental rocks 

 to the mica-schists occurred. There seems to be tolerably clear evidence 

 that the transformation of tlie cherty iron carbonates to the many ])hases of 

 rock now found in the formation took place during tlie ujjlifting and erosion 

 or subsequent to it. 



After the Penokee and Keweenaw series had assumed their jiresent 

 inclined position and had sutlered vast erosion, there was deposited upon 

 their uptm-ned edges the Eastern sandstone, which now cuts them l)oth off 

 just west of Gogebic lake. 



Whi/ the district is given a separate memoir. — The reason is now more 

 apparent than at any time before for giving this Penokee district a separate 

 memoii-. It stands out in the lake Superior country unique in its simplicity 

 and isolation. It is a great series of water deposited sediments, the origin 



