THE quartz-slatp: member. 175 



Change to the Iron-bear iiuj member. — The contact between the Quartz- 

 slate and the Iron-bearing member which immediately overlie.s it is exposed 

 in scores of places, as a result of mining operations. Nowliere wliere 

 it has l)een seen, however, does this contact suggest anything but the most 

 abrupt change from one formation to the other. In places the ujjper part 

 of the quartzite appears to be no more tlian a citarse sand, and occasionally 

 blocks of it are contained in the 1)asement layers of the iron formation. 

 This would seem to indicate that here and there the quartzite was some- 

 what broken before the begiiming of tlie deposition of the overlying mem- 

 ber or else by dynamic movements. Also, the upper part of tlie quartzite 

 is often heavily stained with iron oxide which has been carried down along 

 the cracks Ijy leaching action. Nevertheless, the change from one forma- 

 tion to tlie other is astonishingly abrupt, it often being possible to locate to 

 a fraction of an inch the plane between the two tbrmations. Upon one side 

 of this plane is the coarse fragmental quartzite; upon the other the noufrag- 

 mental varied rocks of the Iron-bearing member. 



Prominent exposures. ^K.'6 stated at the begiiming of this chapter, the 

 Quartz-slate is one of the best and most continuously exposed members of 

 the entire Penokee series. This will be realized from an inspection of tlie 

 detailed maps herewith (Pis. v to xiii), upon which, howevei', are placed 

 only the more accurately located exposures. Further detailed work and 

 measurements would undoubtedly enable us to locate about as many more. 

 In the following notes we refer oidy to those exposures which are particu- 

 larly prominent, either on account of their size or because they show some 

 noteworthy peculiarity. Beginning at the west, the first exposures worthy 

 of mention are the larg'e ones running along the north side of the Marengo 

 river in the extreme southwestern portion of Sec. 14, and northwestern 

 ]iortion of Sec. "23, T. 44 N., R. 5 W., Wisconsin. The river runs nearly 

 along the strike of the slate, and also apparently along its contact mth the 

 more southern granite, which shows in bold exposures all along the south 

 side of the river.- The slate is one of the biotitic and chloritic varieties. In 

 the extreme northeastern })art of Sec. 24, T. 44 N., R. 4 W., Wisconsin, 

 several exposures of the quartzite, which forms tlie uppermost horizon of 

 the f]uartz-slate, are to be seen. One nf these lies on the eastern line of 



