314 THE PENOKEE lEON-BEARING SERIES. 



spar, biotite, and black opaque material, which is inostly somewhat altered pyrite or 

 pyrrhotite, but which may also contain carbonaceous material. A portion of the 

 matrix is ceutainly fVagmcntal, as is shown by secondary enlargements of the quartz 

 grains. The biotite is all believed to be due to the alteration of feldspar; much of it 

 is certainly of this nature. 



11. Black chiastolitic! biotitic slates, from west side of fault at lower middle hori- 

 zons. Specimen 0570 (slide 3381), Gi5(» N., 1350 W.; specimen 9574 (slide 3380), 070 

 N., 1350 W.; specimen 0572 (slide 3379), 080 N., 1350 W., Sec. 11, T. 44 N., R. 3 W., 

 Wisconsin. 



The rooks are black, and exceedingly fine grained to aphanitic. Specimen 9572 

 breaks with a conchoidal fracture and 0574 and 9570 are finely laminar. In 9576 

 are large cleavage areas like those described in 10. In the specimens numerous crys- 

 tals of pyrite are contained. 



The thin section 3381 is almost precisely like the thin section 3322 in 10, above 

 described, the only difference being that the decomposition of the large fragmental 

 feldspars has proceeded somewhat further. Thin section 3380 diflers from 3381 in 

 that the large feldspars have been less numerous apparently and the alteration of 

 those present has not gone so far. The api)earance of the rocks as seen in hand 

 specimens corresponds with their appearance under the microscope, the extent of 

 the alteration of the ft'I(lsj)ars as determined by t lie thin sections corresponding exactly 

 with the condition of tlu; large feldsjiars as seen macroscopically. In specimen 9550, 

 in which the areas of feldspar are generally well developed and show distinct cleavage 

 surfaces, the biotitic alterations characteristic of these rocks have taken place to but 

 a comparatively small extent. In specimen 9574 the obscurely outlined feldspars and 

 irregular cleavage surface are accompanied by extensive biotitic alteration. The 

 section of 9572 is much like the finer parts of 0550 (in 10) and 0570. Scattered sparsely 

 through the finer mnterial of 0572 and 0574 are (piite large crystals and intersecting 

 clusters of crystals of cliiastolitc. These chiastolites often include such a quantity of 

 the other minerals of the section as to be almost indistinguishable in ordinary light. 

 The clouds of in(;lusions are arranged in parallel lines, which correspond to the some- 

 what obscure slaty cleavage of the rock. Pyrite is an abundant accessory. 



12. Biotite-slate, from west side of fault at a lower middle horizon. Specimen 

 1440 Wis., (slide 201), 700 N., W., Sec. 10, T. 44 N., E. 3 W., Wisconsin. 



The thin section is vei'y fine grained, and consists of intimately mingled quartz, 

 feldsi)ar, biotite, and minute particles of dark opaque material a i)ortion of whicli is 

 (loul)fless pyrite. The biotite composes about one-third of the rock. A portion of 

 this mineral is certainly secMJudary to feldspar, and all of it is probably of this origin. 



l.'>. Biotitic and nuiscovitic slates, from west side of fault, at a lower middle hori- 

 zon. Specimens 120 Wr. and 130 Wr., 8,50 N., 2(»0 W., Sec. 10, T. 44 N., R. 3 W., 

 Wisconsin. 



