THE UPPER SLATE MEMBER. 325 



oiidary biotitc. These grains yet retain their well rounded form, but in many cases 

 the orifi'lnal exteriors of the feldspar jjrains are lost. Often the entire surfaces of the 

 feldspars include very numerous particles of the biotite, there remaining throughout 

 such areas here and there little spots of feldspar, which together polarize as units. 

 The rather sparse matrix of the rock consists of finely crystalline quartz, of feldspar, 

 and of biotite. Much of this fine quartz is doubtless a secondary infiltration material 

 and the biotite is believed to be secondary to feldspar. 



This section is one of the finest yet described to show the alteration of feldspathic 

 areas into numerous small folia of b'Otite. (PL xxxii, Fig 3.) 



45. Biotitic graywacke, fi'om near the top of the series. Specimen 9595 (slide 

 3332), 700 N., 1940 W., Sec. 14, T. 45 N., R. 1 W., Wisconsin. 



The rock is dark gray, rather fine grained, massive, and breaks with a subcou- 

 choidal fracture. 



In thin section medium sized clastic particles of quartz and feldspar are set in a 

 finer matrix, which consists of particles of these minerals mingled with much biotite 

 in small folia antl with less plentiful white mica (sericite). As in the mica-schists 

 previously described, many of the feldspar iiarticles are altered to biotite to a greater 

 or less degree. 



Section at Potato river and vicinity. 



46. Chloritic graywacke-slate, from a lower middle horizon. Specimen 9107 

 (slide 2789), 1925 N., 1990 W., Sec. 19, T. 45 N., R. 1 E., Wisconsin. 



The rock is dark gray, exceedingly fine grained, finely schistose, and shows upon 

 the cleavage surface numerous minute particles of pyrite. 



In thin section small fragmental particles of quartz and feldspar are set in an 

 exceedingly fine matrix, consisting chiefly of quartz and chlorite. Mingled with the 

 chlorite are flakes of sericite or kaolin. Throughout the section are many small areas 

 of dirty somewhat altered pyrite. 



47. Biotitic and chloritic graywacke, from an upper middle horizon. Specimen 

 9109 (slide 4418^, 1800 N., W., Sec. 13, T. 45 N., R. 1 W., Wisconsin. 



The rock is dark gray, medium grained, massive, and breaks with conchoidal 

 fracture. 



The thin section is almost precisely like that of 38. The particles of quartz are 

 distinctly enlarged; many of them are notable for the very numerous partly liquid 

 filled ca\aties which they contain. (PI. xxxii. Fig. 4.) 



48. Biotitic and chloritic graywacke, from an upi)er horizon. Specimen 9110 

 (slide 2902), 390 X., 1980 W., Sec. 7, T. 45 N., R. 1 B., Wisconsin. 



The rock is like 47. 



In thin section the rock is almost precisely like that of 47. 



