392 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



(3) The mica-schists are not widely distributed in the portion of the 

 Archean areas included in the Felch Mountain map. They are well rep- 

 resented in the northern Archean area beyond the limit of the area mapped, 

 but within this limit they are known only in sees. 34 and 35, T. 42 N., R. 

 29 W., where an overthrust fault bring-s them into successive contact with 

 the Randville dolomite and Sturgeon quartzite for a distance of three- 

 fourths of a mile. An excellent section, which includes the faulted con- 

 tact with the dolomite, is exposed along the Sturgeon River below the 

 dam in the northern portion of section 35. Though so feebly represented, 

 they possess an unusual interest both in their tield relations and in their 

 microscopic characters. 



The mica-schists when fresh are dark gray, rather soft rocks, of fine to 

 medium grain, with a generally well-developed schistose structure. The 

 most noticeable constituent, in spite of the dark color, is muscovite, which 

 occurs in pearly flakes of large size plentifully sprinkled along the cleav- 

 age surfaces, and is especially characteristic of thin seams, which are much 

 more fissile than the rest of the rock and part it into parallel bands of much 

 regularity. Biotite, however, is the more abundant mica, although in 

 smaller and less conspicuous plates, and to it the dark color of the rock is 

 due. Quartz and sometimes feldspar may also be recognized. 



These rocks offer little resistance to the weather. The biotite gives up its 

 iron with great ease, staining the outcrop a dull red. The final product is a 

 slightly coherent ferruginous mixture in which the large muscovite plates 

 alone are recognizable. At a less advanced stage of weathering the alterna- 

 tion of layers more rich in biotite produces color banding in reds and grays. 



The mica-schists contain many intruded dikes and sheets of flesh-colored 

 pegmatite and also of amphibolite, both of which are generally parallel to 

 the foliation. The pegmatites are typical " schrift-granits," the feldspar being 

 microcline. Both pegmatites and amphibolites show ragged and intrusive con- 

 tacts with the schists when these are examined in detail. Both also are foliated. 



Under the microscope the mica-schists are thoroughly crystalline aggre- 

 gates of quartz, biotite, and muscovite, always with more or less microcline. 

 Magnetite is always present as a primary mineral, and hematite or some hydrous 

 oxide of iron between hematite and limonite is very abundant in the zone of 

 weathering. Besides these, tourmaline is an abundant accessory in some 

 slides, and apatite, zircon, titanite, pyrite, and chlorite also commonly occur. 



