THE UPPER SLATE MEMBER. 297 



This Upper slate is a belt of fraginental rocks, or rocks which were 

 originally fraginental. It has the same basis for separation from the sedi- 

 ments of the Iron-bearing member that the fragmental Quartz-slate below 

 it has. In oreneral the fragrmental character of the thin sections of these rocks 

 is recognizable at a glance, althoi;gh, as will be seen later, certain pttrtions 

 of the belt have l)y subsequent alterations taken a crystalline character. 

 These rocks both in hand specimen and thin section are separated with cer- 

 tainty and ease from those of the underlying Iron-beaiing member; for they 

 ai'e fundamentally i;nlike in texture and mineral content. 



Transition from Iron-hcariny to the Upper slate memher. — The change 

 from the Quartz-slate to the Iron-bearing member has been shown to be 

 abrupt, taking place usually without any transition belt. In the few locali- 

 ties in which the traitsition from the Iron-bearing to the Upper slate 

 member is exposed the change is quite gradual. Beginning at the west, 

 the first known exposure of this sort is in the extreme southern jiart of 

 the SW. \ of Sec. 11, T. 44 N., R. 3 W., Wisconsin. Here, at the foot of 

 the northern slope of a large bluff belonging to the iron-bearing formation, 

 is found a rock (described later) which in its characteristics is intermediate 

 between the black slates found in the upper belt at this locality and the 

 biotitie actinolite-.slates of the Iron-bearing member. Its location in either 

 formation is somewhat arbitrary, l)ut it is placed in the tabulations of the 

 Upper slate member. In the west part of Sec. 6, T. 45 N., R. 2 E., Wis- 

 consin, and near by, exposures in the Iron-bearing and in the Upper slate 

 members are found quite close to each other. That in the latter is a ti-ansi- 

 tion rock, as it contains iron oxide as a chief constituent, a large part of which 

 is magnetite, although it is nearer the slates than the iron-belt rocks. At 

 the Black river, in the southern part of Sec. 12, T. 47 N., R. 46 W., Miclii- 

 gan, there is a continuous exposure of 200 feet of rock at the horizon in whicli 

 the change from the Iron-bearing to the Upper slate member occurs. Here 

 is a transition, and the location of the line between the two belts is more or 

 less arbitrary. Just south of this line, although the specimens contain frag- 

 mental material in considerable quantity, the nonfragmental material is pre- 

 ponderant; and at the southern end of the exposure, 150 feet south of this 

 line, the fragmental material is still tolerably aliundant North <if tlic line 



