398 THE PENOKEE IRON BEARING SERIES. 



8. Ohloritic, gi-aywafkc-slate, uortli of 7. Sptwimcii !»L'{r>yV (slide 4475), L'(K) N., 

 375 W., Sec. 17, T. 47 N., E. 43 W., Michigan. 



The rock is dark green and fine graine^l, but contnins jnuuerons sniidl grains of 

 (|uartz and altered fragments of feldspar of suflicient size to be perceptible to the 

 naked eye. 



The thin section shows a rather abundant grounduiass, consisting of linely 

 crystalline quartz, chlorite, kaoliu, and iron oxide. Contiiined in this matrix are 

 abundant grains of both (juartz and fehlspar of widely varying sizes, the foiiner 

 often being slightly enlarged, and in such cases merging gradually into the clayey 

 matrix. 



9. Clay-slate, north of 8, at top of belt. Specimen !tL'o5 (slide 3003), 400 N., 

 800 W., Sec. 17, T. 47 N., R. 43 W., Michigan. 



The rock is dark gray to black, ai)hanitic, and has a well developed slaty 

 cleavage. 



The thin section is exceedingly fine grained, but appears to consist of an inter- 

 mingled mass of chlorite, (puirtz, kaolin,' and ferrite, witli a little ferrodolomite. 



tiection mostly in the loest half of Sees. 16 and 21, T. 47 N., R. 43 W., Mich if/an. 



10. Ohloritic graywackeslate, from south part of belt. Specimen 7416 (slide 

 1875), 1S30 N., 100 W., See. liO, T. 47 N., R. 43 W., Michigan. 



The rock closely I'esembles 7. 



The thin section differs from those of 7 only in containing in the interstices 

 nuich more chlorite and apparently a little magnetite. 



11. Ferrodolomitic graywackes, north of 10, at south part of belt. Specimens 

 7307 (slide 1860), 9234 (slide 2947), 1900 N., 210 W., Sec. 20, T. 47 K, R, 43 W., Mich- 

 igan. 



The rocks are dark gray, rather line grained, and contain very lumu'rous dark 

 brown areas of mingled ferro-tlolomite and iron oxide, and white areas of terro- 

 doloniite. 



The sections have an exceedingly line grained matrix, consisting of crystalline 

 ;tnd amorphous silica, of chlorite, and of black opaque iron oxide, a little of which is 

 magnetite. Scattered plentifully through this matrix are fragmental i)aiticles of 

 quartz and feldspar. The areas which give the rocks their mottled appearance are 

 the ferrodolomite or limouite or the two combined. All of these areas were plainly 

 once ferro-dolonute, and the alteration of this mineral has ]>roduced limouite. All 

 stages of the change may be seen, from those areas which are pure ferrodolomite to 

 those which are wholly limouite. 



12. Ohloritic graywacke-slate, from lower middle i)art of the belt. SpecinuMi 

 7387 (slide 1852), 230 N., 1850 VV., Sec. 16, T. 47 N., R. 43 W,, Michigan. 



Tlie lock is dark gray, tine grained, schistose, but weathers to a dull pale yellow. 

 The thin section diliers in no important rt^sjicct from that of 8. 



