THE QUARTZ SLATE MEMBER. 159 



10. Chloritic and bidlitic shiU'. Specimen 4.-).{2 (.slide UO.i). From NE. J Sec. 

 17, T. 44 N., R. J \V.. Wisconsin. 



Tlie thin section is cDmiJoscd of cxcei'din^ly line paitifles of ((iiartz, feldspar, 

 biotite, clilorite, and .sericite. The f;eneral apiieaiance is a fraj^nienta! one, but the 

 j;iain is so minute that it \vi)uld be dithcult to prove a fragmental nature from thi.s 

 section alone. 



From the Tijlets J'urk xcrtion. 



11. Chloritic and biotitic slate. Specimen '.MJi."> (slide ;;o:i!t). From 113.3 N., 1980 

 W., See. 34, T. 4.5 N., R. 1 W.. Wi.seoiisin. 



A tine grained, slaty rociv, breaking with eonehoidal fractuic across the lamina- 

 tion. Upon cleaved surfaces are seen numerous flakes of white mica of a light gray 

 color. 



The chief constituents of the thin section aie quarly, and feldspar in phunly frag 

 mental particles, the former being very plentiful, and having the fragments ordinarily 

 enlarged. Between the grains of quartz and feldspar, and al.scj included in the feld- 

 spar fragments as secondary products, are numerous small folia ot clijoi iie, biotite, 

 and sericite. 



Front the I'oUito riper xitHoh. 



12. Magnetitic (juartz-slate and conglomerate, from the base of the Qnartz-slate 

 member. Specimens 0094 (slide 32tl7), OO'.Ki (slide L'.Stt.j), !ll7.-> (slides 2f)!»4, 2!)!).">). All 

 from SSS N., 35 W., Sec 1!), T. l.") N., R. 1 E., \\'isc,ousin. 



The lowest portion of the Quartz-slate nu'inber on I'otalu river, at and near the 

 junction with the underlying gre<'n schists, differs from the body of the nicTnlier above 

 described only in tlie presence of much magnetite, alon.i; with ininu'rons |)el)bles and 

 bowlders derived fi'oin the lower schists, also fewer small liagnu'iits of jasper and chert. 



The ba<'kground of the siM'tions dilfers fioni the slates before described onlv in that 

 they contain a \ery considerable (piantity of iinijnietite in well defined crystals, and 

 aggregates of crystals, these being at tinu's so large and nuineioiis as to make n|i most 

 of the sections. As indicated in the macroscopic description, these rocks contain |)eb- 

 bles and bowlders <leri\cd from the gieen schist iMiinediately Ijclow, and also fewer of 

 quartz, chert, and Jasper. In stM'tions 2!t!t4 and 2!I'J."') is seen a large cherty area in 

 which are numerous magnetite particles. The occiuren<'e in these rocks of mnnerons 

 fragments of cherty and .jasi)eiy nmterial, assoi'iated with an abundance of magnetite, 

 calls to miinl the peculiar magm-titic l)rei'cia which lies at this same horizon on Bad 

 river, wheic, however, there lies l)etween it and the lower schists a thickness of some 

 '.to feet of limestone and white chert. The ('herry liinestone mendier is entirely lack- 

 ing at Potato river, but the occurrence Justdescribed suggests that it and a jasper 

 formation may have formerly been here. (T'l. xvii, Fi;;. I.) 



13. Feldspathic<|uartzites, oigraywackes. chloritic quartz-slates, and gray wacke- 

 slates, from middle ami lower horizons. Specimens 90«3 (slide 2781), from 950 i^., 180 



