184 THE PENOKEE IRON-REARING SERIES. 



represent tlie riclicr i)()rtious of the belt, the layers from wliicli they come 

 alternating with others which run down to only a small percentage of iron. 



In the more eastern portion of the belt the total amount of metallic iron 

 may perhaps be nearly as great as at Penokee ga]), but it tends to grciiter 

 concentration, the result of which is the production of genuine ore bodies 

 and of considerable thicknesses of rock, in which relatively little iron is 

 present, though in these cases iron oxide is usually contained in bands, 

 seams, and finely disseminated particles. 



Ahyuptness of transition from the underh/im/ Quartz-slate member. — As in- 

 dicated already in Chapter II, the fundamental distinction betweei^ tlie 

 Quartz-.slate and Iron-bearing members lies in the fact that the former is 

 wholh' of clastic origin, being still made up mainly of fragmental material 

 readily recognizable as such, while the latter not only contains no frag- 

 mental material whatever, but presents us with no evidence at all of ever 

 having accumulated in a detrital condition. Upon the other hand, the slaty 

 rocks which overlie the Iron-bearing member, though occasionally so nuicli 

 changed by metasomatic processes as to have lost their fragmental character, 

 have in the main j)re.served it thoroughly, and are plainly of a detrital 

 origin. The transition, then, from the Quartz-slate member to the Iron- 

 bearing member is one from a detrital to a nondetrital formation, l^he 

 contact between these two formations is to be seen iit numerous points, and 

 at all of them the change is al)rupt. This conclusion has been reached, 

 not merely by jui examination in the field, l)ut as a result of a careful study 

 of thin sections of specimens collected on each side of the contact with 

 the very object of ol)taining light upon this ])oint. In the more eastern 

 portion of the district, and so far west as the vicinit\- of the passage of 

 Tylers fork, in T. 45 N., R. 1 W., Wisconsin, this contact has been 

 brought to view ])\ mining operations at man\" points, the rule being, as 

 further explained hereafter, that the principal deposits of ore lie at the base 



siu; (14) trench NE. J Sec. 15, T. 44 N., R. 3 W., Wisconsin; (15) exposure SW. J Sec. 17, T. 44 N., R. 

 3 W., Wisconsin; (16) px])Osurc NW. } Sec. 16, T. 44 N., 1{. 3 W., Wi8con.sin; (17) exposure NE. i .Sec. 

 14, T. 44 N., R. 3 W., Wisconsin. 'I'lie s:ini|ilps from which these analyses were made were all selected 

 with reference to deterniininf; the value of these schists as iron ores, and therefore, while fairlv 

 representiuj; the percentages of iron in the thicknesses named, were of course selected from the 

 richer portions of the esposiu'es. 



