HISTORICAL SKETCH— LAKE SUPERIOR REGION. I2I 



carbonaceous and ferruginous shales and slates. The upper 

 slate formation includes the great mass of the Upper Huronian 

 series. Its maximum thickness is more than ten thousand feet. 



In certain areas, during Upper Huronian time, there was 

 great volcanic activity, as a result of which, peculiar formations 

 were piled up, wholly different from any of th"e ordinary mem- 

 bers of the series. Also this volcanic activity greatly disturbed 

 the regular succession, so that for each of the volcanic districts 

 an independent succession exists, the sedimentary and volcanic 

 formations being intimately interlaminated. The two areas 

 which are best known are the Michigamme iron district north 

 of Crystal Falls and the east end of the Penokee district. 

 Similar volcanics also occur in the Marquette district. In 

 the Michigamme iron district is an extensive area of green- 

 stones, greenstone -conglomerates, agglomerates and surface 

 lava flows, many of which are amygdaloidal. In the Penokee 

 district the materials are almost identical. The typical suc- 

 cession for this district extends in unbroken order for fifty 

 miles or more, but east of Sunday Lake this is suddenly dis- 

 turbed by the appearance of the volcanics. The character of the 

 rocks and their order soon becomes so different that if one were 

 not able to trace the change from one into the other, there would 

 be a great temptation to regard the part of the series bearing 

 volcanics earlier than or later than the Penokee series proper. 

 But the continuity of the two cannot be doubted. Thus this 

 occurrence well illustrates that lithological character in pre- 

 Cambrian, as in post-Cambrian time is no certain guide as to 

 relative age. Finally, associated with the Lake Superior Upper 

 Huronian rocks are many later intrusive dikes and interbedded 

 sills, chiefly diabases, gabbros and diorites, but local granitic 

 intrusives also occur, particularly in the Felch Mountain and 

 Crystal Falls districts, and possiblv also in the Menominee 

 district. 



The typical districts in which the Upper Huronian series can 

 be best studied are the Penokee, Marquette, Mesabi and Animikie. 

 Remote from the Lake Superior region proper, the rock series 



