156 THE PENOKEE IRON-BEARING SERIES. 



3157), from 1575 N., W., 9648; (slide 3158) from 1635 N., W., Ser. 24, T. 44 N., 

 R. 4 W., VViscoiisiii. 



A medimii jiriiincd, massive, semi-vitreous qnartzite, mottled light gray and dark 

 green. 



Fragments of a very limjiid quartz compose tbree- fourths of the thin sections. 

 These fragments are often partly or wholly separated from each other by an interstitial 

 material, conijiosed mainly of clilorite in aggregates of pale green flakes, and horn- 

 blende in small gieenisli needles and blades. However, over the greater part of the 

 sections tlie quartz grains tit closely, or even interlock, and at times the original out- 

 lines of the fragments are quite obliterated. Here and there a little interstitial car- 

 bonate is seen. 



From the section in Sec. 17, T. 44 N., R. 3 W., Wisconsin. 



3. Chloritic qnartzite. Specimen 4521 (slide 1402). From SW. i, Sec. 17, T. 44 

 N., R. 3 W., Wisconsin. 



Rather large interlocking quartz grains C(nnpose the greater part of the section. 

 These ai'e believed to be enlarged fragments of quartz, mainly because in most of the 

 similar rocks (if this belt they are manifestly so; but taking this section alone, the 

 proof ol' the fragmcntal origin is not conclusive. In masses and films between the 

 grains are aggregates of chlorite, the section as a whole being closely allied to those of 2. 



4. liiotitic slate. Specimen 9044 (slide 3154). From lu'ar the center of Sec. 17, 

 T. 44 N., R. 3 W., Wisconsin. 



This is for the most part a linely laminated slate, but occasionally bands of greater 

 width and coarser grain occur. It is of a greenish gray color and shows strongly the 

 sheen of mica. 



The thin section is composed mainly of very finegrained quartz and mica, tlie two 

 uunerals being about ecpially abundant and evenly nnngle<l. Tlie fiagniental char- 

 acter of this line matrix is not <Mitirely phiin, hut a coarser band iniining across the 

 section shows the usual large fragments of quartz, each with a wide secondary enlarge- 

 ment, wliile occasional quartz grains of the same chaiacter are scattered through the 

 line graine<l groundmass. The line of dcnnirkation between these quartz fragments 

 and the enlargements is conimoidy niarkeil by minute Hakes of mica. Tlieie are also 

 occasionally ju'esent large flakes of a rectangularly extinguishing chlorite, and soine 

 fragments ol' feldspar. It is not imjjrobable that much of the line graineil (|uartz has 

 developed from the alteration of the feldspar. (IM. xix. Figs. 1 aiul 2.) 



r,. Chloritic (pnirtzite. S]iecimen 1114 Wr. From 1000 N., 1000 W., Sec. 17, T. 

 44 N., li.3 \V., Wisconsin. 



This is a medium grained, gray, vitreous (piartzite, which resenddes closely 2, 

 but dilfers in lacking the peculiar mottling shown by those rocks. 



Intricately interlocking <'leai' ([uartz grains compose nearl.\' the whole section, 

 the only other minerals preseid l)eing chloille and ferrite, unless some of the greenish 



