INTEODUCTION. XXI 



is found above the Ajibik formation. The occupation, in the western part 

 of the district, by the Hemlock volcanics of the same part of the geological 

 column as the Hemlock volcanics east of the western Archean oval, the 

 Mansfield slate, and the Groveland formation, is explained by the fact 

 that in the western part of the district the volcanoes first broke out and 

 there continued their activity longest. While north of Crystal Falls the 

 volcanic rocks were being laid down, the Mansfield formation was being 

 deposited in the southeastern part of the district. This activity continued 

 there through the time that the Grroveland formation was being deposited 

 in other parts of the district. 



From the foregoing it appears that the Hemlock formation in the 

 western part of the district is equivalent: 



(1) 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 Groveland 

 formation ; 



(3) In the Mansfield area, to the Mansfield slates and the Hemlock 

 volcanics occurring there; and 



(4) In the southeastern part of the district, to the Mansfield and 

 Groveland formations. 



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

 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 north- 

 eastern 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 Penokee iron-bearing district of MioWgan and Wisconsin, by R. D. Irving and C. R. Van 

 Hise: Mon. U. S. Geol. Survey, Vol. XIX, 1892, pp. 428-4,33. 



