C URRENT PRE- CAM PR /A N LITER A TURE 5 I 7 



subsequently been changed iuto a ferruginous quartzite. The typical 

 occurrence of this quartzite is at the end of the Felch Mountain range. 

 It also appears between the Archean ovals in the northeastern part of 

 the district. If distinct conglomerates were formed at the bottom of 

 this quartzite, they are buried under glacial deposits or have disap- 

 peared as the result of metamorphism. 



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 slaty 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 non- 

 clastic sediments, the latter-including iron-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 trans- 

 formed into mica-slates and mica-schists. 



Since the deposition of the Upper Huronian the rocks of the dis- 

 trict have been folded. The more complex folds vary from a north- 

 south to an east-west direction. The closer folds in the northeastern 

 part of the area are nearly north-south. In the central part of the 

 area the closer folds strike northwest-southeast. In the eastern and 

 southeastern parts of the district the closer folds are nearly east-west. 

 All of these folds have steep pitches. 



Subsequent to, or during the late stage of, this time of folding there 

 was a period of great igneous activity, probably contemporaneous with 

 the Keweenawan. At this time there were introduced into both 

 the Lower and Upper Huronian vast bosses and numerous dikes. 

 The intrusives vary from those of an ultrabasic character, such as 

 peridotites, through those of a basic character, such as gabbros and 

 dolorites, to those of an acid character, such as granites. These 

 intrusives, while altered metasomatically, do not show marked evi- 

 dence of dynamic metamorphism ; therefore the conclusion that they 

 were introduced later than the period of intense folding already 

 described. 



Cambrian rocks overlie unconformably the rocks above described. 



