5 l6 C. K. LEITH 



i. East of the western Archean oval, to the Hemlock volcanics 

 found there and the overlying Groveland formation. 



2. At Michigamme Mountain, to the Mansfield slates and the Grove- 

 land formation. 



3. In the Mansfield area, to the Mansfield slates and the Hemlock 

 volcanics occurring there. 



4. In the southeastern part of the district, to the Mansfield and 

 Groveland formations. 



The replacement of an iron-bearing formation by the great vol- 

 canic formation just described is exactly paralleled in the Upper 

 Huronian rocks of the Penokee iron-bearing series, where the pure 

 iron-bearing formation is replaced at the east end of the district by a 

 great volume of volcanic rocks intercalated with slates and containing 

 bunches of iron-formation material. 



Following the deposition of the Lower Huronian series, the region 

 was raised above the sea and eroded to different depths in different 

 places. In the Felch Mountain range the only formations above the 

 Randville dolomite are a thin bed of slate and the Groveland iron 

 formation. In the northeastern part of the district only a thin belt of 

 iron-formation rocks remains. In the central and western parts of the 

 district there is a great thickness of volcanics. This, however, does not 

 imply a difference of erosion equal to the difference in thickness of 

 these rocks, for doubtless when the volcanics were built up there 

 was contemporaneous subsidence, so that at the end of Lower Huro- 

 nian time there may have been little variation in the elevation of 

 the upper surface of the series, but very great differences in its 

 thickness. 



The Upfte)' Huronian. — After the Lower Huronian series was 

 deposited the district was raised above the sea, may have been greatly 

 folded, and was eroded to different depths in different parts of the 

 district. 



Following the earth movements and erosion, the waters for some 

 reason advanced over the district, and the Upper Huronian series was 

 deposited. The basal horizon was a conglomerate, which has, however, 

 very different characters in different parts of the district. 



In the eastern half were Archean rocks, the Sturgeon quartzite, the 

 Mansfield slate, and the Groveland iron formation. Upon these was 

 deposited a sandstone which locally was. very ferruginous. This has 



