404 THE PENOKEE IKON BEARING SERIES. 



36. Chloritic iuirt graphitic schist, from ii low horizon, north of 35. Specimen 

 12595 (slide 5350), 1770 X., 232 W., Sec. 24, T. 47 N., R. 43 W., Michigan. 



The rock is strongly schistose, and shows alternate bands of gray quartzose and 

 dark green to black material, the latter being the more pleutifnl. The cleaved sur- 

 face shows a black lustrous appearance due to grapliite. . 



The thin section has a finely crystalline background, consisting of quartz and 

 ferro-dolomite in about equal quantity. This background contains much chlorite, seri- 

 cite, and black graphitic material, arranged in parallel, wavy lines. A portion of the 

 quartz is plain'y fragmental, and a part of the ferro-dolomite is in small perfect rhom- 

 bohedi'a. 



37. Sericitic slate, from a low horizon, north of 36. Specimen 12596 (slide 5351), 

 1824 N., 232 W., Sec. 24, T. 47 N., E. 43 W., Michigan. 



The rock is dark green, of a uniform texture and finely foliated. 



The section has a background consisting of finely crystalline quartz, chlorite, 

 sericite, and pyrite. Set in this background, and making up about one-half of the sec- 

 tion, are rather small fragments of quartz and feldspar. These fragments are with 

 great uniformity scattered through the rock with their longer axes in a common direc- 

 tion. The appearance which has resulted from this arrangement of tine grained 

 material is that of a net which has been drawn out diagonally, the grouudmass repre- 

 senting the meshes, and the fragments of quartz and feldspar the open spaces. 



38. Graywacke-slate, from a low horizon, north of 37. Specimen 12597 (slide 

 5352), 1852 N., 232 W., Sec. 24, T. 47 N., E. 43 W., Michigan. 



The rock is dark green, foliated, of a uniform grain, coarse enough to show par- 

 ticles of quartz, feldspar, and pyrite. It closely resembles 30. 



The section differs but little from that of 30, except that it is coarser grained. 



39. Sericiteslate, from a low horizon, north of 38. Specimen 12598 (slide 5353), 

 1876 N., 232 W., Sec. 24, T. 47 N., R. 43 W., Michigan. 



The rock is somewhat finer grained than, but otherwise precisely like 37. 



Tlie thin section diti'ers from that of 37, in that the background is more abun- 

 dant, and in that the sericite is very plentiful, while chlorite is subordinate. in 

 quantity. This supposed sericite is in minute particles, with the fibers in a common 

 direction, and often so concentrated as to make up solid sheets, all of the constituent 

 fibers of which extinguish simultaneously. 



40. (Juartzite, from a middle horizon. Specimen 12599 (slide 5354), N., 1973 

 W., Sec. 18, T. 47 N., R. 42 W., Michigan. 



The rock is a coarse grained, vitreous (piartzite. The contained fragments of 

 quartz have a somewhat different coh>r from the matrix in which they are contained, 

 so that their roundeil outlines are distinctly seen. They vary greatly in magnitude, 

 the largest ones being one-foui'th of an inch in diameter, while from this they run to 

 those so small that they can not be distinguished from the matrix in which they are 



