Geological Excursion into Central Wisconsin — Hall. 265 



the gneisses of the valley. A rounding o£ the feldspar individuals 

 is quickly seen. These individuals are partly orthoclase and partly 

 albite and oligoclase, as determined from the relics of a twinning 

 striation. 



Scarcely more than mention need be made of the gneisses of 

 Stevens Point, for details have been published by the Wisconsin 

 geological survey.* The strike of these rocks in one place was N. 

 45° E., and in another, where the direction was determined rather 

 by the parallel position of the feldspar crystals than by any dis- 

 tinct lamination, N. 65*^ E., with a northwesterly dip of 75°. 

 There is considerable variation in the texture, some areas being 

 quite fine and others normally coarse, and almost everywhere 

 granitic veins of greater or less extent appear. In lithologic com- 

 position the gneiss is a hornblende biotite one, the last-named 

 mineral predominating. Muscovite can be seen in a few scattered 

 folia, but for a Wisconsin valley gneiss, epidote is singularly want- 

 ing. The feldspars, both orthoclase and the plagioclases, are con- 

 siderably altered internally and corroded externally, some of them, 

 without doubt, to entire obliteration, as the areas of quartz mosaic 

 extending from the circumference to the center of the eyes seem 

 to show. That the silica which now abounds in these rocks must 

 have come in part from some external source seems clear. These 

 rocks have until recently been covered with the Cambrian sand- 

 stone (see page 251), whose outlyers still stand in the vicinity, be- 

 coming a continuous sheet towards the south. f One of these 

 knobs still stands only a few hundred paces away from the gneissic 

 exposures sampled. The suggestion comes with no little force that 

 the acidic character of the gneiss may be due in part to the silica 

 which has percolated from the overlying sandrock into the rock 

 beneath, to take the place of those constituents which have been 

 gradually dissolved and removed. The changed feldspars and the- 

 surrounding mosaic of secondary quartz grains are represented in 

 figure 6, plate iii. 



The Granites. — Around Trapp city and Kickbusch and just 

 above Wausau are several exposures of granite which have assumed 

 considerable economic importance. The fresh condition of the rock, 

 its bright, cheerful color, its texture and freedom from fracture 

 make it a most valuable quarry stone. The color is usually reddish^ 



*Geology of Wisconsin, Vol. ii, p. 478, et seq. 

 IGeology of Wisconsin, Vol. iv, p. 627. 



