326 THE PENOKEE IRON-BEARING SERIES. 



49. Biotitic graywackeslate, from au upper horizon. Specimeus 9111 (slide 

 2790); 9112 (slide 2923), 35 N., 940 W., Sec. 12, T. 45 N., R. 1 W., Wiscousiu. 



The rock is dark gray, fine grained, and varies from massive to finely laminar. 

 In all essential points the thin section is like 3329 in 27. As in it, there is much 

 biotite, which is certainly secondary to feldspar. 



50. Biotite-slate, from near top of series. Specimen 9113 (slide 2903), 300 N., 

 940 W., Sec. 12, T. 45 N., R. 1 W. Wisconsin. 



The rock is dark gray to black, aphanitic, finely laminated, and mottled by 

 very numerous black cleavage areas. 



The thin section, with the exception that black opaque material and pyrite are 

 sparsely present, is exactly like 3322 in 10. (PI. xxxiv, Fig. 4.) 



51. Feldspathic quartzites and conglomerates from top of series, mingled with 

 Keweenawan greenstones. Specimens 9115 (slide 4419), 911G (slide 2905), 470 N., 30 

 W.; 9118 (slide 2906), 9119 (shde 3298), 470 N., 05 W., Sec. 11, T. 45 N., R. 1 W., 

 Wisconsin. 



The rocks vary from coarse grained gray vitreous quartzite to a conglomerate 

 containing numerous pebbles several inches in diameter. 



In thin sections large fragmeutal grains of (juartz compose a good portion of 

 these rocks. Mingled with the quartz is a considerable quantity of fragmeutal feld- 

 spar, also in large individuals. The quartzes are usually enlarged, this fact generally 

 being easy to discover. Between the fragmeutal i^articles is finely crystalline quartz 

 and other accessories in large quantity. In slide 2905 the finely crystalline quartz is 

 mingled with actinolite, being cut through and through in every direction by it. The 

 actiuolite needles are also always included in the enlargements of the clastic quartz 

 grains, but never in the cores. It is, then, plainly a secondai'y mineral, and about it 

 the iufiltrated quartz has crystallized. Some of the feldspaj-s have decomposed, and 

 in them is found, as secondary and replacing product, actinolite and quartz. In 

 slides 2906 and 3298 the feldspars are badly decomposed, the secondaiy products 

 being kaolin, large brilliantly polarized flakes of muscovite, and many smaller ones 

 of biotite. These minerals are also included in the interstitial quartz and in slide 

 2900, and also in the enlargements of the old quartz grains. This section nicely 

 illustrates the micaceous alteration of feldspar, by the decomposition of which, accom- 

 panying the micas, saturating quartz has been formed. Many of the pebbles in the 

 conglomerate are large, interlocking, complex fragments of quartz. (I'l. xxxii, Fig. 1 .) 



Section in the north part of T. 45 N., li. 1 E., Vi^isconsin. 



52. Chloritic graywacke, from a lower middle horizon. Specimen 9123 (slide 

 2794), 300 N., W., Sec. 8, T. 45 N., R. 1 E., Wisconsin. 



The rock is dark gray, fine grained, massive, and breaks with conchoidal 

 fracture. 



