220 THE PKNOKEE IROIf-BEARlKG SERIES. 



This reseuiblaiicf holds in the thin sections, save that the garnet is less abund- 

 ant and that the aetiuolite has been much nioie laigely altered. The alteration 

 produets are biotite and chlorite, but more or less brown iron oxide accompanies the 

 biotite. 



From the Punokee raiif/e, Sec. !>, T. 14 N., R. ~^'ir., Wificonsin. 



15. Magnetitic and aetinolitie quartz-schist, from a low horizon. Specimen 2004 

 Wis. (slide 3(10), from ^'., 1000 W., See. !», T. 44 N., R. t> W., Wisconsin. 



The thin section of this rock does not ditt'er in any essential respect from those 

 of the Penokee gap ac-tinolitic schists, 11 and 12. The actinolite is arranged in the 

 usual radiating bundles and is more or less stained with brown and red iron oxides. 

 The quartz, as usual in these rocks, is very minutely crystalline, and wholly without 

 any appearance of a fragmeutal origin. 



From the (ior<ie of Tylers fori-, Wisconsin. 



16. Ferruginous quartz schists, from a low horizon. Specimens 9620 (slide 3147), 

 from 1170 N., 160 W.; 9624 (slide 3149), from 1235 N., 193 W., See. 33, T. 45 N., R. 1 

 W., Wisconsin. 



The exjiosures of the Iron-bearing member at the gorge of Tylers fork i)resent 

 rocks having a general resemblance to those of the I'enokee gap section, including 

 darker colored, miue magnetitic, and lighter colored more ([uartzose pba.ses; but one 

 is impressed at once with the relatively small i)roportion of magnetite contained in 

 the Tylers fork section where the light colored, more (piartzose kinds A-er>' greatly 

 predominate, and where the thin handing which characterizes the I'enokee section is 

 only developed in one or two jdaces. Specimens 9620 and it624 are mainly made up 

 of the lighter colored phase, which, it slumhl be said, is still lighter colored than any 

 of the I'enokee gap rocks. No actinolite is i)erceptible with the magnifying glass, 

 while the ai)|)earancc of the (|uartz is (piite i)eculiar, suggesting some sort of a con- 

 cretionary' arrangement. The lean ferruginous seams are exceedingly irregular in 

 thickness and In course, blanching antl running in all sorts of iiregular fashions 

 insteail of lying i)arallel to the general bedding of the rock. Sp. gr. of the quartzose 

 jiortioii of 0020, 2-03; of nmgnetitic Ijaiiding scams, 5-01. 



The thin sectiims are composed of (piartz, actinolite, and magnetite, and so far 

 are analogous to the aetinolitie and magnetitic schists of Peuokee gap above described, 

 but there are some im|)ortaut dilferences. In the first place, the actinolite in the Tylers 

 folk rocks is very niucli less plentilul, and as a rule is in much more minute blades. 

 A much mjore pronounced difference, however, consists in the striking tendencry toward 

 a concretionary development that these Tylers fork rocks .show. This is brought out 

 pai ticulaily in the arrangement of the individuals of the greatly predominating 

 (|uartz ol' the groundmass and comes out with siiecial prominence in the polarized 

 light, when there is seen a tendency of the coarser individuals of the quartz to 



