Geological Excursion into Central Wisconsin — Hall. 263 



particles of epidote and quartz. It may be mentioned here as the 

 only occurrence of the kind noted by the writer in the Wisconsin 

 valley, a few very well formed crystals of epidote lie in the matrix 

 of essential constituents. 



A half mile further down the river at Grandfather falls, a 

 long rapid barely mentioned in the Wisconsin reports,* the rock is 

 almost identical both in the strongly marked features of the bold 

 rock masses over which the river plunges and in its more detailed 

 microscopic characters, with that at Morin's farm. In color 

 this rock is a beautiful gray of varying dark and light tone, and 

 persistent banding. Weathering bleaches its color very noticeably. 

 Its texture is medium with a general tendency towards the eye 

 structure called by the Germans "augengneiss.'" The strike at 

 the falls is N. 25° E. magnetic and the S. S. W. dip is 65o. 



Quartz is the prevailing mineral. In the eyes the grains are 

 of good size, possessing the wavy extinction so common in this 

 structure and induced, as is generally supposed, by stretching or 

 pressure. Elsewhere the grains are very minute and exhibit a 

 striking mosaic when polarized. The feldspars are small and 

 fresh-looking, probably all of secondary origin. Small size and 

 great elongation parallel with the lamination of the rock 

 characterize the hornblende and biotite individuals. The speci- 

 mens taken show a great excess of the latter mineral over the 

 former. Epidote is plentifully present. Veins and segregations 

 of quartz course through the rock. 



Not until Merrill was reached were further samples of gneiss 

 taken, although exposures lie in the river and along the carriage 

 road at several intermediate points. At this city, on the island 

 formed between the mill-race and the river, occurs a considerable 

 exposure and a bountiful contact of gneiss and an eruptive rock 

 which has already been noted as a quartz diorite of gabbroid an- 

 cestry (ante, p. 258). A darker colored, coarser textured and more 

 shattered gneiss is seen here than at Grandfather falls and Morin's. 

 A concentric Aveathering is very strikingly displayed; the rock 

 seems to be rotted away, save the few hard, spherical nodules left 

 on the surface. Where one of these nodules lies up against the 

 contact zone of the diorite the cohesion of the two is quite firm. 



To detail the microscopic characters of this Merrill hornblende 

 biotite gneiss would be to repeat the description of that at Grand- 



*Vol. IV. p. 702 and accompanying sketch, Map viii. 



