THE IRON-BEARING MEMBER. 225 



From the Penohr range in Sec. 10, T. 15 X., R. 1 E., Wisconsin. 



23. Eerniyii 10118 and uctiiiolitic chert, or quartz ami chert. Si>eciineii OIS.T 

 (slide 41'11); fioiii N., liiOO W., Sec. 10, T. 45 N., R. 1 E., Wisconsin. 



Th(' rock is a fine grained chert-schist, much stained with hrown iron oxide. 

 On dose examination it is seen to be minutely i)orous. The Iresh tractiire is studded 

 with little clusters of bright lustered magnetite, of which mineral the weight shows 

 that there is a considerable jiortion present. Sp. gr., 3'26. 



The section is a rather confused one, on account of the large amount of brown 

 iron oxide, but careful examiuatiou shows that it consists of a number of more or less 

 thoroughly detached areas, between whi(th is a background of miiuitely crystalline 

 silica. The areas referred to are of an angular t« roundish outline, and in them tlie 

 oxides of iron, including the lirowii oxide and magnetite, are mainly aggregated. 

 When the iion oxide is least thick they show sheaf-like aggregations of actinolite 

 blades in a background of minutely crystalline sUica. Actinolite needles also occur 

 in the grouudmass. 



From the Penokee range in Sec. 13, T. 45 If., R. 1 E., Wisconsin. 



24. Ferrugiuous chert-schist. Specimen 9 189 (slide 2930) ; from 1920 N., 1825 W., 

 Sec. 12, T. 45 N., R. 1 E., Wisconsin. 



A very flue grained chert, analogous to 9185, deeply stained with brown and 

 red iron oxides, which minerals occur also in little bands of some thickness. 



The thin section is made from some of the less ferruginous portions. It presents 

 essentially the same appearance as that of 26, below described. The silica individuals 

 are, however, more generally of somewhat larger size than in the latter section, 

 although still (luite small, and the concreti<mary structure is not so strongly devel- 

 oped. Neither is there so much iron oxide present. The concretionary areas and 

 api)areut ttagineuts seen in 2G are less evident in the i)resent case because tliey have 

 less iron remaining in them, but many are readily seen in ordinary light, minute films 

 of iron oxide still outlining them, wliile their interiors are commonly composed of a 

 somewhat more minutely divided silica than that of the nmtrix. 



25. Hematite iron ore in thin seains directly interstratified witli 24. Specimen 

 9187 (slide 310.-)); from I'J.IO N., 1825 W., Sec. 12, T. 45 N., R. 1 E., Wisconsin. 



Tile rock is a dark purplish red slaty hematite iron ore. 



The thin section of this rock dillers from that of 24 only in that it is mainly com- 

 posed of a nearly opaque mass of particles of the red and brown oxides of iron. 

 Small areas of minutely crystalline silica are seien, and a single vein of quartz of later 

 origin than the iron oxide traverses the section. The quartz of this vein is in quite 

 large individuals compared with those of the grouudmass proper. 

 3ION XIX 15 



