GEOLOGY OF CENTRAL WISCONSIN. 



pink orthoclase occur. The laminae are for the most part not over 

 l-32d inch in width, remarkably regular and parallel, and without 

 contortion. Two sets of veins traverse the rock, both of reddish felspar, 

 those of one set being but mere strings and faulting the others, which are 

 one-fourth to one-half inch in width. This gneiss is exposed for several 

 hundred feet along the river opposite Ledyard's old mill; and, as shown on 

 the map and section, is, at the lower end of the exposure, overlaid by 25 to 

 30 feet or more of horizontal sandstone, which fills in the depression in the 

 very irregular upper surface of the gneiss. The exact junction of the two 

 formations is distinctly to be seen for a long distance. In some places the 

 gneiss shows no alteration at its contact with the sandstone; in others 

 again, as it is traced upward from the water's edge to the line of contact, 

 a rapidly increasing decomposition is observed, until immediately below 

 the sandstone the change to a soft bluish-white clay or kaolin (1,018) is 

 complete. The kaolin retains still very plainly the fine lamination of the 

 unaltered gneiss, it being even possible in some cases to trace individual 

 laminae from the unchanged into the kaolinized rock. Immediately below 

 the sandstone the laminae of the softened rock are seen to be bent over as 

 though by the weight of the superincumbent sandstone. This is a fact of 

 some interest, since it would confirm the view already presented, that the 

 kaolinization was subsequent to the deposition of the sandstone; having 

 been caused possibly by the currents of carbonated water which found pas- 

 sage along the junction line of the two formations. A section through the 

 sandstone, kaolin, and gneiss, is presented in Fig. 20. Up the river 

 gneiss continues to show, losing, gradually, its distinct lamination, to with- 

 in a short distance of the wagon bridge, above which, after an interval 

 without exposures, granite appears. 



la. Granite (1,008): medium-grained, pinkish, consisting of a nearly uniform 

 admixture of pinkish orthoclase, in facets up to l-16th inch, and fine- 

 grained translucent quartz. Some mica is present, in fine scales, showing 

 sometimes a slightly stringy arrangement. This granite is exposed from a 

 short distance above the wagon bridge, as far north as the north line of 

 Sec. 14, the river in this distance passing through a gorge whose walls 

 sometimes reach a height of 80 feet. In the large exposures at the falls, 

 the parallel grain of the gneiss below is almost entirely lost, being only 

 occasionally indicated in an obscure arrangement of the mica. The rock 

 here is traversed by several sets of joints mostly somewhat irregular, those 

 showing the greatest irregularity trending N. 80 E. and dipping 72 S. E. 

 but having no corresponding structure in the rock. Above, the granite 

 shows the same general characters as at the falls, occasionally as in the 

 railroad cut on the west side of the river, just above the falls showing a 

 darker kind than usual from a greater quantity of fine dark mica. In this 

 cut there are to be seen two sets of planes equally marked, one set trend- 

 ing N. W. and dipping N. E., the other trending N. E. and dipping N. 

 W. A distinct stringy arrangement of the mica was noted parallel to the 

 former set. Near the north line of Sec. 15, the granite exposures cease 

 suddenly on the east side of the river, whilst they continue some distance 

 farther on the west side a fact to be explained by the northwest strike of 

 the succeeding slaty rocks. 



II. Hornblende rock or schist (501): fine-grained, crystalline-textured; dark- 

 colored to black; breaking with conchoidal fracture; weathering out; into 

 rough prismatic fragments, with dirty brown color; striking, as a whole, 



