514 C. K. LEITH 



therefore, that the Sturgeon sea gradually overrode the district, and that 

 at the time the Sturgeon quartzite was deposited in the southeastern part 

 of the area the Archean was not yet submerged in the central and north- 

 eastern parts of the district. However, since the quartzite resting on 

 the Archean in the latter area cannot be separated lithologically from 

 the Sturgeon quartzite, both are given the same formation color, but 

 the later quartzite is given a separate letter symbol. The quartzite 

 color therefore represents the transgression deposit of the same gen- 

 eral lithological character, rather than a formation all parts of which 

 have exactly the same age. While nowhere in the district is there any 

 marked discordance between the schistosity of the Archean and the 

 Sturgeon quartzite, the conglomerates at the base of the latter forma- 

 tion in the Sturgeon River tongue are believed to indicate a great 

 unconformity between the Archean and the Lower Huronian series. 

 The change from the Sturgeon deposits to those of the Randville was a 

 transition. 



The Randville dolomite is a nonclastic sediment, and is believed 

 to mark a period of subsidence and transgression of the sea to the 

 northeast, resulting in deeper water for much of the district. Since 

 the Randville dolomite has its full thickness on the Fence River just 

 east of the western Archean oval, and does not appear at all about the 

 Archean oval a short distance to the northeast, it is probable that the 

 shore line, during Randville time, was between these two areas and 

 that the land arose somewhat abruptly toward the northeast. As the 

 Randville formation has a thickness of 1500 feet, it probably represents 

 a considerable part of Lower Huronian time. 



Following the deposition of the Randville dolomite, deposits of 

 very different character occur in different parts of the district. These 

 deposits are: (1) The Mansfield formation, (2) the Hemlock volcanic 

 formation, and (3) the 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 sedi- 

 mentary rocks. This formation occupies a larger area than other of 

 Lower Huronian formation, and is perhaps the most characteristic 

 features 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 



