OUTLIjSTE of this monograph. XXXV 



stained mica-schists, with thin, iuterbanded beds of ferruginous and micaceous quartzite. Neither 

 kind shows traces of clastic origin. From their strnctures and general relations they are believed to 

 have been derived from sedimentary rocks by metamorphism. 



Section IX. The Algonkiau rocks are cut by intrusives, among which both acid and basic rocks 

 are represented. The acid rocks are granites occurring in narrow dikes. No dikes of granite are 

 known to cut the Raudville or Mansiield formations. 



Chapter IV treats of the Michigamme Mountain and Fence River areas. These areas occur in 

 the centra] part of the district. In the Fence River area the structure is very simple. In the Michi- 

 gamme Mountain area the structure is complex. 



Section I treats of the Arohean. The prevalent rock is granite, cut by acid and basic dikes. 



Section II treats of the Sturgeon formation. This is scarcely known as a distinct Algonkiau 

 member in this area apart from the Randville formation. In one section purely clastic sediments 

 were observed, for which it is convenient to retain the name. These exposures consist of slates and 

 graywackes, with some layers of a coarser texture. 



Section III treats of the Randville dolomite. In the Fence River area the dolomite lies on the 

 east side of the Archean and occupies a belt about one-half mile in width, and extending from the 

 mouth of the Fence River about 10 miles to the north and west, where it leaves the portion of 

 the district studied. In the Michigamme Mountain area the dolomite tops the low arch in a broad 

 crumpled sheet. The formation in the Fence River area occurs in an eastward-dipping monocline 

 with a number of minor plications. An average thickness of about 1,.500 feet is estimated for the 

 formation in this area. In the scattered outcrops of the Michigamme Mountain area the dolomite 

 strikes and dips toward all points of the compass, caused by the gentle arching from the general 

 northwest-southeast axis, combined with sharp local folds which run nearly east and west. Petro- 

 graphically the formation ranges from coarse saccharoidal marbles, sometimes very pure but usually 

 filled with secondary silicates, to fine-grained, little-altered limestones, which are occasionally so 

 impure as to be calcareous or dolomitic sandstones and shales. The prevalent colors are white, but 

 v.arious shades of pink, light and deep blue, and pale green occur. Some of the varieties are oolitic. 

 This structure does not seem to have been noted previous to this in limestones of pre-Cambrian age. 



Section IV treats of the Mansfield formation. The typical locality of this formation is in the 

 vicinity of the Mansfield mine, which lies to the west of the district studied. Where it occurs in the 

 Michigamme Mountain area, the formation consists of phyllites or mica-slates of various colors. The 

 structure of this area is so complex and the outcrops so few as to forbid any but an approximate 

 outlining of the general boundaries of the formation. The geological position of the formation is 

 free from doubt. It overlies and passes downward into the Randville dolomite. The formation does 

 not seem to influence the topography. Like the preceding one, it has been extensively folded. 

 The average thickness is probably not less than 400 feet. The mica-slates or phyllites possess no 

 especial petrographical interest. 



Section V treats of the Hemlock formation. The Mansfield formation of the Michigamme 

 Mountain area changes along the strike into rocks of a ditt'erent character, to which the above name 

 is given. In the Fence River area it occupies a belt between 2,000 and 3,000 feet in width, between 

 the Randville dolomite on the west and the Groveland formation on the east. The best exposures 

 occur on the sections made by the Fence River. No folds have been observed within this formation. 

 The thickness probably varies from to 2,300 feet as a maximum. The rocks of the formation are 

 chiefly chloritic and ophitio schists, with which are associated schists bearing biotite, ilmenite, and 

 ottrelite; greenstone, conglomerates or agglomerates, and amygdaloids. The general characters of the 

 schists are (1) a groundmass composed of chlorite, quartz, magnetite, epidote, and in some cases 

 plagioclase microlites, and (2) the presence in this groundmass of much larger porphyritic individuals 

 of several secondary minerals. As evidence of the origin of these schists, first, there is the absence 

 of rocks possessing any sedimentary characters; next, lavas and also greenstone-conglomerates or 

 agglomerates are undoubtedly present in the series ; furthermore, the minerals which compose the 

 schist are those which would result from the alteration in connection with dynamic metamorphism of 



