160 THE I'ENOKEE lEOISr-BEARING SERIES. 



W.; !»1}84 (slide -782), from !t70 N., 201) W.; itltSIJ (sliilc 2783), from !)(»(} N., 18.5 W.; 

 9088 (.slide 2784), from 060 N., 18.'i W.; 0080 (slide 2785), from 010 N., 105 W.; 9099 

 (slide 2788), from 888 X., M W.; 9100 (slide 289(1), from 889 N., 37 W.; 9101 (slide 

 2897), from 890 N., .38 W., Sec. 19, T. 15 N., K. 1 E., Wisconsin. 



Except at the extreme north, where the qiiartzite 11 is found, the exposures of 

 the <i)u;irtz-sliite member on Potato river consist of interstratiflcations of thinly 

 bedded pale pinkisli (piartzitic layers, and greenish (chloritic) gray and brown argil- 

 laceous slates. These grade into one another and are all described here together. 



The thin sections from the more ijuartzitic ])ortions of the Potato river exposures 

 show fragments of quartz as the predominant ingredient, but mingled with the (puirtz 

 fragments are plentiful ones of feldspar, including orthoclase, microcline, and i)lagio- 

 clase. The quartz fragments have nearly always received enlargenjcnts which 

 have produced an interlocking of the quartz areas. The feld.spar fragments on the 

 whole are singularly fresh. Chlorite, ferrite, ami calcite are present as accessories in 

 some of the sections, occurring as narrow films and areas between the fragmental 

 particles. Prom the (prartzitic i)hases there is tbund a gradation to those varieties 

 which are entirely aphaiiitic in the hand specimens, and are so exceedingly line 

 grained and clay(\y that in the thin section little is to l.)e made out save that minute 

 particles of (luaitz and feldspar are scattered through the clayey background, which 

 is taken to Ije composed of pulvei-ized and kaolinized felds])ar, mingled with clilorite, 

 sericite, etc. Sliile 2788 is at the extreme as to fineness of grain and small number of 

 recognizable quartz and feldspar ])articles. In the intermediate varieties half or more 

 than half of each section is coniiiosed of niiuute but still distinctly recognizable frag- 

 ments of quartz and feldspar, the quartz grains being almost always provided with 

 plainly visible enlargements, even when widely separated from one another by the 

 intermediate clayey matiix. This matrix in many sections, in addition to the minerals 

 menlioiied above (namely, chlorite, sericite, kaolin, and ferrite), contains calcite and 

 occasionally crystals of magm^tite, which last mineral is particularly met with in .sec- 

 tions from specimens which come from not very far from the underlying green s<'hists. 



14. (^uartzite, from the uppermost horizon of the Quartz-slate member, imme- 

 diately beneath the Iron-bearing member. Si)eciiiien 9082 (slide 2780). Prom 1000 

 N., 210 W., vSec. 19, T. 45 X., K. 1 E., Wisconsin. 



A coarse grained, vitreous ([uartzite, of a \>n\i' jdnk color. 



The section is almost wholly comi)oscd of clear (juartz in large intricately inter- 

 locking areas. Each one of these areas, howcvci', is furnished with a very distinctly 

 outlined fragmental core, and occasionally tlu' secoinlary enlargement has received 

 crystal outlines. The cores and enlargements are separated from one another by films 

 of ferritic material. This section liiriiishes one of the very finest illustrations that we 

 have met with of the prodnclion of a vitreous (|uart/.ife from a completely fragmental 

 sandstone by the enlargcnicnl piiMcss. (PI. XX, Pigs. 1 and 2.) 



