XXn INTRODUCTION. 



the difference in thickness of tliese rocks, for doubtless when the volcanics 

 were built itp there was contemporaneous subsidence, so that at the end 

 of Lower Huronian time there may have been little variation in the 

 elevation of the upper surface of the series, but very great difference in its 

 thickness. 



THE UPPER HUROlSriAN. 



After the Lower Huronian series was deposited the district was raised 

 above the sea, may have been gently folded, and was eroded to different 

 depths in different parts of the district. 



Following the earth inovemeuts and erosion the waters for some reason 

 advanced over the district and the Upper Huronian series was deposited. 

 The basal horizon was a conglomerate, which has, however, very different 

 characters in different parts of the district. 



In the eastern half were Archean rocks, the Sturgeon quartzite, the 

 Mansfield slate, and the Groveland iron formation. Upon these was depos- 

 ited a sandstone which locally was very ferruginous. This has subsequently 

 been changed into a ferruginous quartzite. The typical occurrence of this 

 quartzite is at the east end of the Felch Mountain range. It also appears 

 between the Archean ovals in the northeastern part of the district. If 

 distinct conglomerates were foriiied at the bottom of this quartzite, they are 

 buried under glacial deposits or have disappeared as the result of meta- 

 niorphism. 



In the western part of the district the rocks of the Lower Huronian at the 

 surface are the great Hemlock formation, and here the basal horizon of the 

 Upper Huronian is a slate or slate conglomerate, the fragments of which are 

 derived mainly from the underlying Hemlock formation. The sandstones 

 and conglomerates varied upward into shales and grits, which have been 

 subsequently altered into mica-slates and mica-schists. After a considerable 

 thickness of mudstone and grit was deposited, there followed a layer of 

 combined clastic and nonclastic sediments, the latter including u-on-bearing 

 carbonates. These appear to be at a somewhat persistent horizon, and in 

 this belt are found the iron-formation rocks, and iron ores in the Upper 

 Huronian in the vicinity of Crystal Falls. Above these ferruginous rocks 

 there was deposited a great thickness of shales and grits, which have been 

 transformed into mica-slates and mica- schists. 



