XX INTRODUCTION. 



are: (1) The Mansfield formation, (2) tlie Hemlock volcanic formation, and 

 (3) tlie Groveland formation. 



The Mansfield formation was a mudstone, which has subsequently been 

 transformed into a slate or schist. The Hemlock formation is mainly a 

 great volcanic mass, including' both basic and acid rocks, lavas, and tuffs, 

 but it contains also subordinate interbedded sedimentary rocks. This for- 

 mation occupies a larger area than any other of the Lower Huronian forma- 

 tions and is perhaps the most characteristic feature of the Crystal Falls 

 district. The Groveland is the iron-bearing formation. It includes sideritic 

 rocks, cherts, jaspilites, iron ores, and other varieties characteristic of the 

 iron-bearing formations of the Lake Superior region. In all important 

 respects these rocks are similar to those of the Negaunee formation of the 

 Marquette district, with the exception that in the southeastern part of the 

 Crj^stal Falls district, associated with the nonclastic material, there is a 

 considerable 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 dis- 

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

 Groveland iron-bearing fonnation. In the Mansfield slate area the iron- 

 bearing rocks appear near the top of the Mansfield formation intercalated 

 with the slates. The Groveland formation can not be certainly traced 

 farther north than the northeastern portion of the western Archean oval. 

 It is apparently replaced along the strike by the Hemlock volcanics. 



In the northeastern part of the district the Groveland formation, 

 equivalent to the Negaunee formation of the Marquette district of Michigan, 



