460 THE PENOKEE IRON-BEARING SERIES. 



slate and Iron bearing members in nearly uniform thicknesses shows that 

 at the time of their deposition there was for this distance an approximately 

 level basin. If no part of the npper slates and iron formation have been 

 swept away by erosion, we must believe that there was inaug-urated before 

 the end of the latter period an orographic movement which formed a syn- 

 cHnal trough, the center of which was Tylers fork, and thus a great thick- 

 ness of up))er slates there accumulated, while to the east and west the 

 thickness became less and less until it finally disappeared, as well as a part 

 or the wliole of the lower members of the series. That a great earth move- 

 ment of tliis sort could occur without disturbing the perfect conformity of 

 the formation of the series is improbable. The discordance in the strikes of 

 the layers would indicate the change, but as all the members are in appar- 

 ent jierfect accordance, one layer has followed upon another without dis- 

 turbance. This being the case, the Penokee series was originally probably 

 of rather uniform thickness, and doubtless tliis thickness continued l)oth 

 east and west of the points at which tlie series can now be traced; for tkere 

 are many reasons for believing that this district was a part of a great basin 

 which extended to the Marquette district on the east and to the quartzites of 

 the Chipjjewa valley on the west. 



The relations just sketched can, then, have but one meaning — that a 

 great unconformity separates tlie Penokee and Keweenaw series. This 

 is the only ^^ossible explanation of the facts that the Keweenaw series is 

 above an Upper slate member 13,000 feet thick at Tylers fork, and east and 

 west not a great distance is above a belt much thinner, while a little far- 

 ther east and west this slate disappears altogether, and yet a little farther 

 to tlie west tlie lower members are hidden, and west f>f Numakagon lake 

 the Keweenaw series apparently rests directly upon the Southern Complex. 

 During this intervening period the rocks of the Penokee series were raised 

 above the sea, and then suffered long continued denudation until in places 

 the entire succession was carried away, exposing the underlying rocks. 

 How thick the series as a whole was at the beginning of this erosion there 

 is no available evidence, but the amount of material swept away at Numa- 

 kagon lake was 14,000 feet more than at Tylers loi-k, while the difference 

 in amount of erosion between the last place and Sunday lake is scarcely 



