324 THE PENOKEE IRON BEARING SERIES. 



most cases the decomposition of the felcLspars h;is been (somplete, and that all of the 

 biotite and much of the <iuartz are secondary products. Scattered uniformly through 

 the section are rather plentiful grains and crystals of pyrite. 



41. Chloritic sericite-schist, from an uppermost horizon, mingled with Keweenaw 

 greenstones. Specimens 9029 (slide M30); 9630 (slide 3345), 120 N., 150 W., Sec. 17, 

 T. 45 N., R. 1 W., Wisconsin. 



The rocli is light gray, moderately fine grained, banded, and brealis with a sub- 

 conchoidal fracture. 



The thin section has a groundmass of small blades and fibers of pale green chlo- 

 rite and colorless sericite, with a little biotite, which contains numerous small frag- 

 montal grains of qiuirtz and fewer of feldspar. Tlie individuals of the latter mineral 

 are usually badly decomposed, the alteration products being chlorite, sericite, and bio- 

 tite. In some cases cores of feldspar surrounded by these secondary products are 

 comparatively fresh. 



Section between Tylers fork and Potato river. 



42. Chloritic graywacke-slate, from a lower middle horizon. Specimens 12777 

 (slide 5467); 12778 (slide 5468), the SW. i Sec. 24, T. 45 N., R. 1 W., Wisconsin, on 

 the Wisconsin Central Railway. 



Tliese rocks are almost precisely like 32, the only differences being that 12778 is 

 sligiitly schistose, while 12773 is massive and wliite mica Hakes are plentiful. 



In thin section these rocks are almost precisely like those of 32. The only dif- 

 ferences are that fragmental white mica (sericite) is more, biotite less, and pyrite more 

 plentiful than in that number. 



43. Chloritic graywacke-slate, from a lower middle horizon. Specimen 12779 

 (slide 5469), NE. i of the SW. i Sec. 24, T. 45 N., R. 1 W., Wisconsin, on the Wis- 

 cousin Central Railway. 



Tills rock is like 12773 in 32, except that it is of a darker color. 

 Tlie thin section is like 5463 in 32, except tliat the amount of black material, 

 either impure iron oxide or iron oxide, pyrite, and carbonaceous nuiterial is greater. 



44. Biotite-graywacke, from an upper horizon. Specimen 9598 (slide 3334), N., 

 600 W., Sec. 15, T. 45 N., R. 1 W., Wisconsin. 



The rock is of a dark gray color, medium grained, uniform texture, and breaks 

 witli conchoidal fracture. 



This section is coarser grained tlian tliat of any of the previously described hio 

 titic grjiywackes, but resembles them in essential points. Clastic particles of »pnirtz 

 and feldspar, well rounded, compose two-thirds or more of the mass of the rock. A 

 few of the quartz grains are finely complex, and nearly all of them are distinctly 

 enlargcil. The alteration of feldspar to biotite is very nicely shown. The freshest of 

 the feldspar grains are surrounded by and more or less deeply intersected with sec- 



