THE UPPER SLATE MEMBER. 321 



in tilt' IViruiatiiiii of ;i siiijfli' individual oi' (jiiartz, in which (;ase the more or less 

 detached areas pohirize tofjetlier. In otlicr cases the dec^oinposition of a feldspar has 

 resulted in the forniatiou of many fjrains of quartz as well as numerous folia of mica. 

 Tlie proof of these alterations in the single section is not complete, but taken in con- 

 nection with the other mica schists of the formation it is conclusive. A reticulating 

 area of saturating i|uart/. ottcn contains detached cores of feldspar, which polarize 

 as a unit. Sometimes these feldspars have not altered U) such an extent as described 

 above. In almost every case their rounded exteriors are lost, but irregular areas of 

 considerable size <iuitc frequently remain, which iuclude few folia of biotite and little 

 saturating quartz. It is possible that a portion of the silica composing the saturating 

 quartz coincs from extraneous sources. (PI. xxxiv, Fig. 1.) 



29. Chloritic and nniscovitic graywacke, from a high horizon, higher than the 

 preceding, although not heie an up])ermost horizon. Specimen .'il64 (slide 3213), Wis. 

 N., 1,()0» W., Sec. 27, T. 45 N., R. 2 W., VViscoTisin. 



The thin section was originally composed of rather small clastic particles of 

 quartz and feldspar. The (puirtz grains ai'e mostly enlarged. Secondary interstitial 

 quartz has appeared. The feldspars Jiave largely decomposed, their places now being 

 occupied by chlorite, uuiscovite, saturating quartz, and to a small extent biotite. 



Section at and west of Tylers fork. 



:H). Black biotite-slate, from a low horizon. Specimen 12770 (slide .5461); .short 

 distance east of north (piarter post, Sec. 1, T. 44 N., R. 2 W., Wi.sconsin. 



The rock is black, aphunitic, finely laminated, very soft, and apparently carbo- 

 naceous. 



The thin section shows an exceedingly fine grained background, composed of 

 flinty and i>erhaps amorphous (juartz, of kaolin, and of black material which may be 

 carbonaceous. Contained in this background aie numerous small fragments of (piartz 

 and feldspar and dark brown folia of l>iotite. 



.SI. Biotitic graywackeslate, from a low horizon. Si)ecimen 12771 (slide .'5462), 

 SE. [ of the SW. J^ of Sec. M, T. 45 N., R. 1 \V.. Wisconsin, on the Wisconsin Cen- 

 tral Railroad. 



The rock is dark gray, very line grained, clcavable, and shows here anil there 

 lustrous flakes of white mica. 



The thin section consists of very small i)articles of (juartz, feldspar, biotite, 

 chlorite, brown iron oxide, and api>arently some kiudin. Much of the quartz is frag- 

 mental. The feldspar fragments are much altered, the resultant products being 

 chlorite, biotite, and quartz. 



.■?2. Chlorite graywacike-slates, from a middle horizon. Specimens 12772 (slidfi 

 540.3), 1277:5 (slide 5464), NE. ^ of the ]SIW. 4 Sec. 32, T. 45 N., R. 1 W., Wisconsin. 

 Jti'JJ XIX— 21 



