330 THE PENOKEE IKON-BE A KING SERIES. 



The thiu sectious closely resemble that of 54. Very plainly fragmental quartz 

 and feldspar iji grains of medium size compose two-thii'ds of the sections. The quartz 

 grains are often slightly enlarged. The feldspars are frequently fresh and often also 

 much altered, the alterations resulting in the formation of sericite or nuiscovite, chlo- 

 rite, and perhaps a little biotite. The matrix is like those of the previously described 

 chloritic graywackes, except that sericite or muscovite is here present. 



68. Chloritic and sericitic graywacke, from near top of series. Specimen 9032 

 (slide 2770), 720 N., 1280 W., Sec. 14, T. 4G N., K. 2 E., Wisconsin. 



The rock is like 67, except that it is finer grained. 



The thin section is liner grained than that of 67. It also differs greatly from it 

 in that it has a much more crystalline appearance, etiualiug in this respect many of 

 the chloriteschists west of tlie south end of Gogebic lake. The crystalline appear- 

 ance is made still sti'onger by the fact that while the quartz particles are exceedingly 

 angular, it is with the greatest difficulty that enlargements can be seen to cause this 

 angularity — in most grains ijuite impossible. The mineral constituents are identical 

 with those of 67, but sericite is much more i>leiitiful than in it, and both the sericite 

 and chlorite are mostly in rather small, well defined leaflets. That all of these two 

 minerals are secondary to feldspar can not be shown, although a large jjortion of 

 them certainly is of this nature. However, from the association with and likeness to 

 67, it can not be doubted that both are of like origin — that is, altered mechanical 

 sediments. It is also certain that the crystalline appearance of 67 is due to the prev- 

 alence of the sericitic and chloritic alteration of feldspar, combined with the separa- 

 tion of quartz in the interstices and the enlargement of the quartz particles. 



Section at and west of Black river. 



69. Chloritic clay-slate, from a middle horizon. Specimen 9194 (slide 2932), 1200 

 N., 1140 W., Sec. 13, T. 47 N., R. 47 W., Michigan. 



The rock is gray, aphanitic, and shows a slaty cleavage which cuts across the 

 bedding plane. 



The section is exceedingly fine grained. It appears to consist of a confused mix- 

 ture of chlorite, kaolin, quiirtz, ferrite, mingled with which are here and there a few 

 larger grains of quartz which appear to be clastic. 



70. Pyritic clay-slate, from a lower horizon. Specimen 12530 (slide 5334), 1000 

 N., 1200 W., Sec. 17, T. 47 N., K. 46 W., Michigan. 



The rock is light gray, very fine grained, and contains a large amount of i>yrite, 

 both disseminated through the rock and in large irregular masses. 



The thin section consists of a confused mass of minute particles of quartz, feld- 

 spar, chlorite, kaolin, ferrite, and perhaps other clay-formmg minerals. Contained in 

 this material are abundant crystals and areas of pyrite. 



71. Chloritic graywacke, from an upper horizon. Specimen 10417 (slide 4021), 

 800 N., 1340 W., Sec. 10, T. 47 N., R. 46 W., Michigan. 



