510 



GEOLOGY OF CENTRAL WISCONSIN. 



which have had one of these slaty layers removed from above them, show a peculiar 



ridgy appearance, evidently due to the passage into them of the slaty cleavage planes. 



At the summit of the east bluff, near its southern end, indications of a somewhat 



lower dip than elsewhere are seen, whilst at the Devil's Nose, surfaces occur slanting as 



FIG. 25. 



much as 29 northward. At the latter place, many 

 cross-joints obscure the bedding, nearly all of the 

 planes, however, sloping northward. Some very 

 large ones were noted, with as high an angle as 82, 

 covered with a shining, soft, greasy film. In seams 

 and nests in the quartzite, in this vicinity, occurs 

 a compact, but soft, clay-like substance (1254) of a 

 lilac color, which is penetrated by fine white strings, 

 and con tains: silica, 62.16; alumina, 29.67; iron ox- 

 ide, 4.17; lime, 0.16; water, 2.50=99.36. This sub- 

 stance appear to be the same as that which per- 

 vades and gives character to the quartz-schists of 

 the region, and is closely allied to the red " pipe- 

 stone," that occurs with the quartzites of Barron 

 county, and again in southwest Minnesota. 

 On the summits and sides of all the cliffs about the lake and valley, two sets of very 

 marked vertical cross-joints are to be seen, the more prominent and persistent set trend- 

 ing N. 45* W. These joints have produced, on the upper portions of the cliffs, a striking 

 columnar appearance, the separate columns of quartzite, 20 to 40 feet in height, often 

 standing entirely detached by the joint cracks from the main cliff. In some cases, in- 

 tervening masses of quartzite have fallen, and left entirely isolated needles at a distance 

 from the cliff face. 



As in the ravine at the southwest corner of the lake, so also in many other places on 

 the north flank of the ridge, horizontal ledges of sandstone and very coarse conglomer- 

 ate occur, abutting against, and unconformably overlying the quartzite. At the north- 

 ern point of the east bluff, the contact of the two formations, is beautifully exposed, and 

 the very instructive section represented in Fig. 26 occurs. Here the ends of columnar, 



FIG. 26. 



SLATY CLEAVAGE IN QUABTZ SLATE AT 

 DEVIL. 'SLAKE. 



POTSDAM BOWLDER-CONGLOMERATE AND SANDSTONE ON AROH.BAN Q.UAKTZITE, DEV-IL'S LAKE 



Scale, ten feet to the inch. 

 A. B , Quartzite. B. C., Sandstone. C. D., Qnartzite. D. E., Sandstone. 



