510 



GEOLOGY OF CENTRAL WISCONSIN. 



which have had one of these slaty layera removed froin above them, show a peeuHar 



ridgy appearance, evidently due to the passage into tlaem 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 



much as 29° northward. At the latter place, many 

 cross-joints obsom-e the beddmg, nearly all of the 

 planes, however, sloping northward. Some very 

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

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

 and nests m the quartzite, in tliis vicinity, occurs 

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

 hlao color, which is penetrated by fine white strings, 

 and contains: silica, 62.16; alumina, 29.67; iron ox- 

 ide, 4.17; Ume, 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 aUied to the red "pipe- 

 stone," that occurs with the quaitzites of Barron 

 county, and again in southwest Minnesota. 

 On the summits and sides of all the cHffs 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 chffs, a striking 

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

 standmg 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 clifl' face. 



As in the ravine at the southwest comer 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 columnai-. 



Fig. 26. 



Slaty Olbavase in Quartz Slatb at 

 Dbvil'sLake. 



ToTSDAM BoWLDBB-OONaLOMEBATK AND SANDSTONE ON Ar.nil.EAN (iUAllTZlTE, DevIL'B LaKJJ 



Scale, ten feet to the incli. 

 A. B , Quartzite. B. C, Saudstone. 0. D., Qnartzite. D. E., SaiulBtcne. 



