THE ARCELEAN ROCKS. 



493 



BLACK RIVER YALLEY. 



The first exposures of crystalline rocks met with in ascending Black river are found a 

 short distance below the town of Black River Falls, T. 21, R. 4 W., in Jackson county. 

 From here they occur in the bed and on the sides of the stream, with only occasional inter- 

 ruptions, as far north as town 28, in Clark county. For the greater part of this distance, 

 they are concealed, away from the river, by overlying horizontal sandstone, through 

 which, however, they occasionally rise in knobby projections. In some of the branch 

 streams, also, the sandstone is cut through and the crystalline rocks exposed. Along 

 the river the rock ledges, in few places only, rise to any considerable height above the 

 water. 



In the vicinity of Black River Falls the exposures are large and interesting. The 

 map of Plate XVII shows the relative positions of the various outcropping beds, their bed- 

 ding and dip directions being shown by the accompanying section. From these it will 

 lie seen that at this place a central, nearly structureless, granitic mass is bounded on the 

 southwest by layers of gneiss, dipping southwestward, into which it appears to grade; 

 and on the northeast, by a succession of scliistose beds, dipping northeastward, but not hav- 

 ing exactly the same strike directions as the gneiss on the other side. In the following 

 detailed descriptions, the various rock masses are numbered as on the map and section, 

 beginning with the gneiss on the southwest: 



FIG. 20. 



UNCOHFORMABILITT, BLACK RIYER FALLS. 



I. Gniess (1,015): rather fine-grained, very plainly laminated, pink and gray 

 banded; dipping S. W. 60, striking N. 32 W. The constituent minerals 

 are: pinkish orthoclase, predominating; pinkish and colorless translucent 

 quartz; mica in very fine black scales, but quite abundant, and much more 

 plenty in certain layers than in others; and also a whitish, partly altered, 

 felspar, in fine facets. In some places, especially micaceous portions have 

 a much darker color than usual; in others large nests of coarse, cleavable 



