THE UPPER SLATE MEMBER. 315 



Si)eciineii 129 Wr. is like 0570 in 11, above described. Spi'cimeu 130 Wr. differs 

 from 120 in bciiii^- of a dark gray color, and in that itsliows but few cleavage surfaces. 



The tliin section of 120 Wr. resembles very closely .•5;]22 in 10 and ;i;5.Sl in 11; the 

 only point of dilVercnci' heinij that ])yrite and black material in minute specks are 

 particularly abundant. In I.iO Wr. the alteration of large fragniental feldsi)ar t<) 

 muscovite and biotite is nicely shown. The flakes of muscovite, which are of a 

 greater magnitude than those of biotite, are clustered about and i>euetrate the parti- 

 cles of feldspar. The section is unusually free from black material. In other respects 

 it does not differ from the other mica-slates of the vicinity. (PI. xxxiv, Fig. 3.) 



14. Black biotite-slates, from west side of fault, at a middle horizon. Specimens 

 9549 (slide 3;521), 10.50 N., IGOO W.; 9548 (slidf 3320), 1070 N., 1(500 W.; 9547 (slide 

 3094), 1090 N., lOOO W., Sec. 11, T. 44 N., R. 3 W., Wisconsin. 



The rocks are black, exceedingly fine grained, finely laminar, and cleave readily 

 along the plane of lamination. As in 9550 in 10, and 9570 in 11, which they are almost 

 precisely like, nnmerons roiinded cleavage ai'eas are contained in the fine material. 

 Pyrite in small crystals is an accessory. 



The thin sections are almost precisely like 3322 in 10, the only difference of 

 importance being that in them the biotitic alteration of the fragmental feldspars has 

 taken iilace more largely than in .3322. The corresj)ondcnce between (he ai)]tearance 

 of hand specimens and thin sections described as applying to 10 and 11, due to feld- 

 spar decomposition, is here ecpially well seen. 



15. Muscovitic graywacke, from west side of fault, at a middle horizon. Speci- 

 men 0540 (sli.le 3093), 1100 N., 1000 W., Sec. 11, T. 44 N., R. 3 W., Wisconsin. 



The rock is a light gray, medium grained one, with uniform texture, which 

 cleaves irregularly along a schistose jdane. 



The thin section is composed of coarse grains of quartz, having generally oval or 

 roundish forms set in a fine matrix, which also appears to be chiefly quart/.ose, 

 although with it are feldspar, chlorite, and muscovite. While there is a wide grada- 

 tion in the size of the quartz grains, there is an approximate separation of the coarse 

 and fine (piartz. At first sight the section appears to have but a small amount of 

 coarse fragmental fehls|)ar. However, upon closer examination it is seen that the 

 remnants of many such feldspars are pi'esent. In some cases these grains arc honey- 

 combed by saturating quartz, many detached areas often being in a single individual. 

 Again a feldspar area contains many small individuals of quartz mingled witli chlorite 

 and muscovite, and occasionally a feldsjiar particle is found which is tolerably fresh. 

 Evidently this rock was originally mostly composed of nmndt'd fragmental partieles 

 of quartz and feldsi)ar. The fragmental quartz grains are enlarged, and thus are now 

 sharply angular. The ]>articles of feldspar are largely rejilaceil oi- decompo.sed, as 

 above described, and consequently tlu^ rock has now become at first, sight an almost 

 comi>letely (-rystalline one. Had the i)rocess continued until all the feldspars had 



