THE UPPEli SLATE MEMBER. 323 



The rocks are black, fine grained to apliauitic, and break readily across the 

 slaty cleavage with a subconcboidal fracture. 



Tlie thin sections consist of intimately mingled minute particles of quartz, feld- 

 spar, biotite, chlorite, and dark opaque patches of ferrite or altered pyrite. In these, 

 as in the previously described sections, the biotite and chlorite are in part at least 

 plainly secondary to feldspar. These slates are somewhat coarse grained, and there- 

 fore show more plainly their fragmental character than do most of the slates of the 

 district. 



37. Chloritic biotite-slate, from an upper middle horizon. Specimen 9608 (slide 

 3;34(»), 475 N., 910 W., Sec. 21, T, 45 N., R. 1 W., Wisconsin. 



The rock is Uke 36. 



The thin section is finer grained than those in 36, but is otherwise like them. 



38. Biotite graywacke, from an upper middle horizon, iuterstratitied with 9608. 

 Specimen 9607 (slide 3339), 550 N., 910 W., Sec. 21, T. 45 N., R. 1 W., Wisconsin. 



The rock is dark gray, of a uniform medium grain, massive, and breaks with a 

 subconchoidal fracture. 



In thin section this rock difters fi'om 387 in 34, in that the chlorite is less and 

 biotite more abundant. They were originally of the same composition. The difler 

 euce at the present time is due to the fact that the feldspaj's underwent difterent 

 alterations in the two cases. Pyrite is present in abundant small grains and crystals. 



39. Chloritic graywacke, from an upper middle horizon, iuterstratified with 9007 

 and 9608. Specimen 9606 (slide 4427), 715 N., 885 W., Sec. 21, T. 45 N., R. 1 W., 

 Wisconsin. 



The rock is like 38. 



The thin section resembles closely 3341 aud 3342 iu 34. It differs from them 

 chiefly in containing a smaller proportion of finely crystalline interstitial material, 

 and in that the fragmental (juartz and feldspar are in larger gTaiiis. The almost 

 conii)lete decomposition of the feldspar aud the resultant formation of chlorit*, 

 biotite, muscovite or sericite, and finely crystalline quartz are nicely shown. 



40. Biotite-slate, from au upper middle horizon. Specimen 9605 (sUde 3338), 

 740 K, 885 W., Sec. 21, T. 45 K, R. 1 W., Wisconsin. 



The rock is dark gray, exceedingly fine grained; contains numerous miiuite par 

 tides of pyrite, and is given a mottled api)earaiice by cleavage surfaces of feldspar. 



The thin section consists chiefly of biotite, quartz, and feldspar, the latter min- 

 eral now being much less abundaut than the other two. The little remaining feldspar 

 is in the last stages of alteration, its resultant products being mainly biotite and 

 (piartz. The decomposition of each grain of feldspar produced many particles of quartz 

 and folia of biotite. In many cases polarized light is necessary to discover the remain- 

 ing feldspar of a partly decomposed individual, so closely do its alteration i)roducts 

 resemble in appeiu'ance the remainder of the section. It is quite probable also that in 



