478 



GEOLOGY OF CENTRAL WISCONSIN. 



FIG. 12. 



At Conant's Rapids, sections 18 and 17, T. 23, R. 

 8 E., Portage county, are large rock exposures in the 

 "bed and on the sides of the Wisconsin river. The 

 sketch-map, Fig. 12, shows the localities of the occur- 

 rences at this place, as also at the Stevens Point rap- 

 ids, above. 



On the west side of the river the rock exposures and 

 rapids are found continuing further down stream than 

 on the east, on account of the northeast strike of the 

 rocks. Beginning at the foot of the rapids on the 

 west side, we note first on the N. W. qr. of the N. E. 

 qr. of Sec. 17, a low exposure some 500 feet in length, 

 under the river bank, No. I of map. The rock here 

 (775) is a fine-grained, pinkish-gray gneiss, showing 

 fine-granular, translucent quartz, predominating; fine- 

 faceted white felspar, abundant; black mica in very 

 fine separate scales, arranged in lines. The lamina- 

 tion is quite close and distinct. The weathered sur- 

 face of the exposure is brownish in color, smooth, ana 

 highly polished by the running action of the river 

 From this smoothed and brown- tinted surface numer- 

 ous reddish granite veins stand out in bold relief, hav- 

 ing resisted the eroding action more successfully than 

 the suiTOunding rock. The bedding is not very dis- 

 tinct, being obscured by many cross-joints; the strike 

 is N. 81 E., and the dip N. W. 60 to 67. Other 

 quite prominent joints occur, bearing N. 55 E., and 

 standing vertical. Numerous veins are to be seen 

 here, both of white quartz, and of reddish felspathic 

 granite, varying in thickness up to several inches. 

 One granite vein 5 inches wide trends N. 26 E., for 

 a distance of 50 feet. The vein matter (775) is a very 

 fine-grained felspathic granite, in which all the ingredients can, however, be distin- 

 guished. Numerous thin feeders extend from the vein into the rock around. The next 

 exposure above, No. II of the map, is a large one, stretching across a side channel of the 

 river, just about on the line beetween sections 8 and 17. The rock here (778) is a very 

 coarse-grained, micaceous, granite, consisting of very large-flaked brilliant black mica; 

 white, very distinctly striated felspar, in facets up to % inch by % inch in size; limpid 

 quartz; some brownish- stained mica; and some little hornblende. In some places a 

 very distinct tendency to crumble is perceptible, and then the mica is much iron-stained, 

 and the rock is blotched with large patches of white, kaolinized felspar. Even where 

 exposed to the running water this rock presents nowhere the peculiar smoothed and 

 glistening appearance of the exposure below, but on the contrary shows everywhere a 

 rough, coarsely pitted surface. This is rendered the more striking by the innumerable 

 reddish granite and white quartz veins which intersect the rock in every direction, mak- 

 ing up nearly half of the exposure, for these having resisted better the weathering in- 

 fluences, show the smoothed appearance alluded to, and stand out in relief from the 

 lighter colored, jagged surface of the surrounding rock. The veins are from 1 inch to 18 

 inches in width. The reddish ones are of granite, having a large predominance of red- 

 dish felspar, which, in some of them, occurs along the sides of the vein free from ad- 

 mixture, and in large crystalline surfaces. One vein 2^ inches wide showed alternating 

 bands of white quartz, pink, coarsely crystalline felspar, and felspathic granite (777). 



SKETCH-MAP SHOWING LOCALITIES 

 OF HOCK EXPOSURES AT CONANT'S 

 RAPIDS AND STEVENS POINT RAP- 

 IDS. 



Scale, one mile to the inch. 



