C URRENT PRE- CA MBRIA N LITER A TURE 5 I 5 



of the Lake Superior region. In all important respects these rocks 

 are similar to those of the Negaunee formation of the Marquette dis- 

 trict, with the exception that in the southeastern part of the Crystal 

 Falls district, associated with the nonclastic material^ there is a con- 

 siderable proportion of clastic deposits. The Groveland formation 

 contains iron carbonate and possibly glauconite, from which its other 

 characteristic rocks were derived. 



The variability in the character of the deposits overlying the 

 Randville formation is probably caused by the great volcanic outbreaks 

 in the western part of the district. In the southern and southeastern 

 parts of the area the deposit overlying the Randville formation is the 

 Mansfield slate and schist. North of Michigamme Mountain and of 

 the Mansfield area the Mansfield formation is replaced along the 

 strike by the Hemlock volcanic formation, which directly overlies the 

 limestone for most of the way about the western Archean oval. The 

 effect of the volcanic outbreak apparently did not reach so far as the 

 northeastern part of the district. 



Overlying the Mansfield formation in the southeastern part of the 

 district and the Randville formation in the central part of the district 

 is the Groveland iron-bearing formation. In the Mansfield slate area 

 the iron-bearing rocks appear near the top of the Mansfield formation 

 intercalated with the slates. The Groveland formation cannot be cer- 

 tainly traced farther north than the northeastern portion of the western 

 Archean oval. It is apparently replaced along the strike by the Hem- 

 lock volcanics. 



In the northeastern part of the district the Groveland formation, 

 equivalent to the Negaunee formation of the Marquette district of Michi- 

 gan, is found above the Ajibik formation. The occupation, in the west- 

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

 the geological column as occupied by the Hemlock volcanics east of the 

 western Archean oval, the Mansfield slate, and the Groveland forma- 

 tion, is explained by the fact that in the western part of the district the 

 volcanoes broke out and there continued their activity longest. While 

 north of Crystal Falls the volcanic rocks were laid down, the Mansfield 

 formation was being deposited in the southeastern part of the district. 

 This activity continued there through the time in which the Groveland 

 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 : 



