19G TUE MARQUETTE IROX-BEAEIiSrG DISTRICT. 



of a inntted mass of tiiiv sericitc or kaolin fibers may represent an original 

 feldsj)ar, but if so, no otlier evidence of its former existence remains. 



HIOTirK-SCinsTS. 



The biotite-schists are much more abun(hint than tlie muscovitic vari- 

 leties. As seen in the liand specimen, they vary from cora^iact or slightly 

 schistose, dark-gray rocks (No. 1G922, analysis, p. 202), reseml)ling fine- 

 grained, dark qnartzites, to sandy, slaty, liglit-gray ones (No. 16913, analysis, 

 p. 202), resendjling friable sandstones. 



Under the microscope thev sometiines appear almost massive. Usually, 

 lio^vever, their mica flakes are arranged with tlieir long axes approximatelj' 

 parallel, and their other components are more or less elongated in the same 

 •direction. Quartz is the principal component. Its grains are elongated, 

 and where thev are in C(intact they interlock by irregular sutures. Thev 

 frequently contain, as inclusions, spicules of gi'een hornblende and small 

 flakes of biotite. Feldspar is also abundant. A few irregular grains of 

 clear plagioclase and kaolinized gi-ains of an untwiimed feldspar, probaljlv 

 orthoclase, lie between the cpiartzes, but they are found only occasionallv. 

 The greater portion of the feldspar is altered into kaolin, chlorite, etc. 

 Biotite is the characteristic component. It is found in large and small 

 reddish-brown flakes Iving- between the quartz grains, and often including 

 several of them. Magnetite, zircon, epidote, limonite, and hematite are 

 found in all sections, l)ut in verv minute quantities. Muscovite is a little 

 more ])lentiful, 1)ut this also is rare. It is present in the kaolinized feldspar, 

 Ijut not 43lsewhere in the slides. In some few cases the biotite has been 

 - changed to chlorite, when it loses its brown color. Otherwise the rocks 

 ai"e very monotonous in their features (see fig. 8, on the opposite page). 



FKLDSPATIHC UK ITITE-SCHIST.S. 



The feldspathic scliists are more A^aried in character, mainly l^ecaiise 

 of the larg-e quantities of feldspar present in them. This, by its alteration, 

 gives rise to various secon<lar\' products. The rocks ai"e very much like 

 gray wackes in their macroscopic appearance. They are fine-grained, frag- 

 mental-looking, grav rocks, with l>ands of lighter and darker shades (No. 

 16765, analysis, p. 202). In thin section they are seen to be composed mainly 



