THE UPPEK SLATE ]MEMP>ER. 319 



The rocks an^ dark sray, fine jjraiiitMl, finely laniiniited. In tliem are TimneroMs 

 roumlisli spots of a darker color than tlie remainder of the rock. Pyrite is a pleiitifnl 

 accessory. 



The thin sections show both fine sniiiicd and coarse grained materials. Kather 

 small well rounded grains feldspar compose more than one-ludf of tliecoarsei- parts, 

 while the particles oi this mineral, although of very small size, arc^ abundant in the 

 finer jiortions. The grains of quartz are frequently enlarged. The feldspar compri.ses 

 orthoclase, microcli e, and plagioclase. Many of its grains are much altered (ddorite, 

 biotite, and quartz, resulting from their decomposition. In the finer parts of the sec 

 tious are plentiful chlorite, biotite, and finely crystalline quartz, which may in part 

 be fragmeutal, and many tine particles of a dark colored sidjstance, which is ]irobal)ly 

 carbonaceous material. In the coarser portions of the rock much of the chlorite and 

 biotite are in well defiiuMl flakes. The dark colored roundish spots, seen niacroscop 

 ically when examined in ordinary light, appear to be but little different from the finer 

 grained p.arts of the remainder of the rocks, but in iioiarized liglit they almost com 

 pletely extinguish the light. 



'M. Biotitic graywacke, from a middle horizon. Specimen 1)582 (slide .'{,326), 22(1 

 N., W., Sec. 5, T. 44 N., K. 2 W., Wisconsin. 



The rock is light gray, fine grained, quite massive, and breaks witli a conchoidid 

 fracture. 



The thin section is composed largely of fragmeutal particles of (|uartz iind feld- 

 spar of varying sizes, which are set in a finely crystalline fnatrix, consisting mostly of 

 quartz. Biotite in minute folia is scattered between the fragments and through the 

 fine matrix. A portion of it is certaiidy secondary to feldspar, and probably all of it 

 is of this nature. The grains of quartz are frequeutly enlarged. INIuch of the finer 

 grained interstitial quartz is j)robably secondary. 



25. Black biotite-slate from a middle horizon. Specimen !).'>.S4 (slide .'{;>27) from 

 600 N.,0 W., Sec. 5, T. 44 N., 11. 2 W., Wisconsin. 



The roitk is almost black, aphanitic, cleavable, and contains nuuu!rous black, 

 obscure areas which give it a faint mottled appearance. 



The thin section consists of excessively minute particles nC quartz, biotite, 

 chlorite, kiudin, and pyrite. A few grains of (juartz somewhat larger than the 

 remainder have a fragmental ai)pearance. Biotite is very abundant. Contained in 

 this fine material are routuled areas which appear to be almost conii)letcIy ([('(•(im- 

 posed feldspar, the minerals formed by this alteration being the same as those in 

 the remainder of the section. 



20. Bliiek biotite-slate from an uppci- horizon. Si)ecimen !W!)2 (slide .'{.'{.'it*), 

 1950 N., 1750 W., Sec. 4, T. 44 N., K. 2 W., Wisconsin. 



The .specimen is like 9550 in Ul and 057(1 in 11, excc])t that the cleavage areas 

 which give the rock a mottled appearance are less distinct than in them. Comi)osi- 



