THE UPPER SLATE MEMBEK. 327 



In tliiu section siiiiiU i)artii'l('S of iiuartz, geiieially enlarged and sometimes 

 finely complex, with fragments of fcldsi)ar. both of greatly varying magnitudes, com- 

 pose from ouebalf to two-thirds of the rock. The feldsj)ar includes orthoclase, 

 microcliue, and plagiodase. Many of the feldspars are nuich altered, while some of 

 them are almost or (juitc decomitosed, the resulting products being mainly chlorite 

 and (piartz, although with these minerals are found sericite, or kaolin and ferrite. 

 Tin- matrix consists chiefly of finely crystalline (juartz, with sonic amorphous silica, 

 mingled with which are a considerable (juantity of chlorite and a little kaolin or seri- 

 cite and ferrite. In the matrix are also numerous irregtilar patches of opaque 

 material, which is in part gray and in part black. These patches are probably mix- 

 tures of ierrite and partly altered jtyrite. The section is that of a fine grained 

 typical graywackc. 



53. Chloritic graywackc, from a middle horizon. Specimen 9124 (slide 2908), 

 1200 N., W., Sec. 8, T. -15 N., K. 1 E., Wisconsin. 



The rock is like 52. 



The thin section is like that of 52. 



54. Chloritic graywackc, ti'om an upper horizon. Specimen 9126 (slide 2909), 

 1000 N., 55 W., Sec. 5, T. 45 N., R. 1 E., Wisconsin. 



The rock is coarser grained than 52 and 53 and contains crystals of pjTite, but 

 otherwis(^ is like them. 



The tliin section diflers from those of 52 and 53 only in being coarser grained. 



Section near east range line of R. 1 £., Wisconsin. 



55. Magnetitic day-slate, from base of formation. Specimen 9148 (slide 2924), 

 1100 N., 40 W., Sec. 1, T. 45 N., R. 1 E., Wisconsin. 



The rock is gray, fine grained, finely laminated, and easily deavable along the 

 lamination. Composition: Silica, 52'58; alumina, 20'76; iron sesquio.xide, 12'17; iron 

 protoxide, 4-08; manganous oxide, "21; calcium iijxide, -30; magnesium oxide, 1*33; 

 potassa, 4-87; .soda, .37; lithia, trace; water, 3-43: =100-10. 



In thin section the groundmass is very finely divided. It appears to consist of 

 quartz, feldspar, kaolin, with perhaps chlorite and biotite. In this groundmass is 

 contained very luimerous sniall crystals of magnetite. The analysis well bears out 

 the appearance of the section. 



50. Magnetitic clay-slate, from near base of formation. Specimen 9139 (slide 

 2911), 1200 N., 1975 W., Sec. G, T. 45 N., R. 2 E., Wisconsin. 



The specimen differs from 55 only in being of a darker color. Composition: 

 Silica, 53'44 ; alumina, 19-G2; iron ses((uioxide, 11-38; iron protoxide, 5-35; manganous 

 oxide, trace; calcium oxide, -42; magnesium oxide, 1-58; potassa, 1'73; soda, 2-fil; 

 lithia, trace; water, 4-07; phosphoric acid, trace: ^ 100-20. 



