168 THE PENOKEE IRON-BE ARINd SERIES. 



1935 N., 1940 W.; 7510 (slide 1966), from 1925 N., lS!t(» \V., Sec. 14, T. 47 N., R. 45 W., 

 Michigan. 



These are fine grained slaty rocks, of a uniform texture, and greenish to pinkish 

 gray colors iijion the cleavage surfaces. Except in 04,'!2 tlie slieen of mica is seen. 



The thin sections of tliese rocks, with the exception of .'Jl.SO, which is peculiar in 

 containing a relatively small quantity of sericite, are very close to 2771 and 2774 in IS. 



45. Quartz-slate and chh)ritic slate. Specimens 945.3 (slide .'^074), 9452 (slide 

 307.3). From 200 N., 800 W., Sec. 10, T. 47 N., R. 45 W., Michigan. 



These slates consist of alternating bands of (!oars(; quartzitic and aphanitic 

 greenish material. 



The aphanitic portions of these rocks, as seen inacroscojiically, show in the thin 

 se(^tion an intimate mixture of minute particles of (|U:irtz, feldspar, chlorite, kaolin, 

 and biotite. The coarser seams show a gronndmass of the s;nne nature, in which are 

 carried an abundance of rounded fragnu^nts of quartz, witli which are associated 

 numerous fragments composed of an exceedingly minutely crystalline silica, mingled 

 with tlakes of kaolin, or of a micaceous material. It seems not imjjrobable that each 

 of these fragments may be due t(j the decomposition of a single feldspar. 



46. Qnartzite and slate, from an upper middle horizon. Si»ecimens 7495 (slide 

 1940), from .JOO N., W.; 749G (slide 1941), from 140 N., W., Sec. 10, T. 47 N., R. 45 

 W., Michigan. 



' These specimens represent an interstratiflcation of grc^eiiish gray, semivitreous, 

 medium grained quartzite, and argillaceous slate. 



The thin secticnis of these rocks are from the moi-e quartzitic portions of the 

 specimens. They closely resemble those of 37. 



47. Ohloritic and sericitic quartzite, from a liigh horizon. Specimen 94.54 (slide 

 3075), from 370 N., 800 W., Sec. 10, T. 47 N., R. 45 W., Michigan. 



A fine grained, greenish gray, ma.ssive (luaitzite, with a couehoidal fracture. 



The thin section shows large fragnu'uts of (piartz and feldspar buried in a 

 gronndmass of minutely crystalline silica, mingled with chlorite and sericite tiakes. 

 The feldspar fragments are much altered, the lesnlling i)rodn<-ts being clilorite, seri- 

 cite, and (piai'tz. The manifest production of these three secondary minerals from 

 the larger fragnuMits of feldspar makes exceedingly ]n((bal)le a similar origin for the 

 same minerals in the matrix. 



48. Quartzite, from immediately beneath tlic Iron Ix'aring member at the top of 

 the Quartz-slate imMnber. Specimen 7513 (slide 19.54), from 42(» N., 873 VV., Sec. 10, 

 T. 47 N., R. 45 W., Michigan. 



A medium grained, massive, semivitreons (|ua.rtzite. 

 The thin section of tliis rock closely resembles 3069 in 37. 



