INTRODUCTION. XIX 



thickness iii the Sturgeon River tongue to less than 100 feet in thickness at 

 places in the Felch Mountain range, and is altogether absent in the north- 

 eastern part of the district. 



In the southeastern part of the district the Sturgeon quartzite is over- 

 lain by the Randville dolomite. In the central part of the district the 

 quartzite between the Archean and the Randville is so thin that it can not 

 be represented on the maps as a separate formation. In the northeastern 

 part of the district a quartzite resting- on the Archean, but occupying a 

 higher position stratigraphically than the Randville dolomite, is overlain by 

 an iron-bearing formation. It appears, 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 northeastern parts of the district. However, 

 since the quartzite resting on the Archean in the latter area can not be sepa- 

 rated 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 a transgression deposit of the 

 same general lithological character, rather than a formation, all parts of 

 which have exactly the same age. Wliile 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 formation 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 waters 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 Rand- 

 ville time, was between these two areas and that the land arose somewhat 

 abruptly toward the northeast. As the Randville formation has a thick- 

 ness of 1,500 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 difiFerent parts of the district. These dejDOsits 



