310 THE PENOKEE IRON-BEARING SERIES 



The feldspar jireas are rounded and are decomposed to a greater or less extent, tlie 

 secondary products being biotite and quartz. This alteration is more extensive u]iiin 

 the outside than in the interiors of the particles. Tliese feldspars also include ferrite. 



From the section at east end of EngUsli lake. 



2. Biotite-slates from a middle horizon. Specimen 12G Wr., from 1,300 N., 200 

 W., and specimen 128 Wr., from 1,000 N., (JOO W., Sec. 9, T. 44 N., R. 3 W., Wisconsin. 



These rocks are dark gray to black, tine grained, quite massive, mottled, the 

 mottling being due to numerous small cleavage surfaces. 



In the thin section the cleavage areas are seen to Ije well rounded, altered grains 

 of feldspar. They are set in the groundmass, which consists of intimately mingled 

 small grains of quartz* and small, brown particles of biotite, with a considerable 

 (puintity of ferrite. The partial defiompositiou of the clastic feldspars has resulted 

 in the formation of very numerous small folia of biotite and a few larger ones of 

 nuiscovite, the transition to these minerals being beautifully shown. In places also 

 the feldspar is replaced by saturating (juartz. In each of the feldspar areas the 

 .secondary biotite and muscovite are found most plentifully nt or near the exterior ot 

 the grain, although in almost every case the alteration has proceeded in a greater or 

 less degree quite to the center. In the matrix it is impossible to determine whii^h 

 part, if any, of the quartz is fragmental. The biotite of the matrix is precisely 

 hke that found in the feldspar; it is all deep brown, very strongly <lichroic, and 

 therefore probably bears a large percentage of iron. Tliis biotite has doubtless been 

 furnished its iron by the abundant oxide present. The i)eculiar spotted appearance 

 of the section when held up to the light, taken in connection with its a])pearauce 

 under the microscope, gives a clear idea of the manner in which the rock reached 

 its present condition. (PI. xxxiii. Figs. 2, 3, and 4.) 



3. Muscovitic biotite-schist from an upper middle horizon. Specimen 73, Wis. 

 (slide 24), fro7n south shore of English lake, Sec. 0, T. 44 N., R. 3 W., Wisconsin. 



The thin section is a rather line grained, apparently completely crystalline, 

 typical mica-schist. The groundmass consists chietly of ipiartz, mingled with which 

 is feldspar, both orthoclase and plagioclase. Biotite in rather small fine folia of 

 uniform size is very plentiful; muscovite is much less abundant. That all the mica 

 is a seconilary alteration of i'cldsjKir can not be proven, but a i)ort.ion of it is certainly 

 of this nature. Many grains of feldspar arc partly surrounded and cut by folia of 

 mica, while many of the larger particles of feldspar contain numerous Hakes of mica, 

 which in magnitude and ap])earance are precisely like tlie great mass of the mica in 

 the section, (^uite nunu'rous black grains ami crystals of a mineral which is taken 

 to he pyrite are inchuled alike in the <inartz, feldsi)ai', and mica. 



4. Muscovitic biotite-schists from an u])i)er horizon. Si)ecimen 78 Wis. (slide 

 20), specimen 7!t Wis. (slide 27^, from SW. \ of Sec. 4, T. 44 N., R. 3 W., Wisconsin. 



