Geological Excursion into Central Wisconsin — Hall. 261 



coarse as are those at Rhinelander, and are much more weathered. 

 Towards the lower end of the rapids the gneissic rock becomes more 

 granitic and carries as an inclusion a dark colored rock of rather 

 fine and uniform texture. As a rule this dark colored rock is amjg- 

 daloidal. The amygdules are very irregular in shape and diverse in 

 size. The structural relations of the gneiss and the inclusions are 

 set forth in the accompanying figure (Fig. 4 A^ J?, C). The gradual 

 disappearance of the gneiss, with the constant increase of the 

 darker colored rock; the rounded character of the inclusions and 

 the inclosing situation of the gneiss, and finally, the finer texture of 

 the darker colored material in proximity to the gneissic all point to 

 the irruption of a dike with a shattering of the penetrated rock 

 along the plane of contact. Exposures showing the same general 

 characters seen here are not infrequent among the old crystalline 

 rocks of the Northwest. 



In lithologic characters the gneissic rock is granitoid and of 

 the biotite type. Hornblende occurs in small proportion. Quartz 

 is very abundant, and frequently occupies large areas with firmly 

 interlocking grains. A portion of the feldspar is triclinic, with a 

 low extinction; as a rule it is so badly altered that twinning striae 

 can be seen only around its borders, while within are nests of kao- 

 lin (?) and epidote. While some orthoclase was seen, more than 

 half of the feldspathic constituent is microcline. This is .appar- 

 ently the least altered of all the feldspars. In the mosaic which 

 usually lies between the larger and more or less altered indi- 

 viduals, small, brightly polarizing grains can be seen, which repre- 

 sent at least two species of feldspar, which are certainly secondary, 

 orthoclase and microcline. 



The darker colored rock, the inclusion as it is here designated, 

 is granitic in habit. It carries feldspars, quartz, hornblende, bio- 

 tite and epidote. At its contact with the gneiss the two are firmly 

 knitted together and the typical characters of each are carried up 

 to the very plane of contact, save perhaps in a finer texure and a 

 larger sprinkling of epidote. The hornblende in its general 

 anatomy bears strong evidence of secondary origin; the quartz is 

 infiltrated into certain portions of the mass in larger proportion 

 than elsewhere and the feldspars seem to no little extent to have 

 the general outline of the lathlike or diabasic type. Occasionally 

 an individual shows the alteration phases into epidote and kaolin, 

 but as a rule the feldspars are very fresh and clear. 



