260 Geological Excursion into Central Wisconsin — Hall. 



doubtedly the source of the hornblende. Epidote abounds in these 

 inclusions, as well as in the normal gneiss. While the normal 

 gneiss carries biotite in large proportion, here only occasional areas 

 are to be seen. 



If one asks for the proofs of the gabbroid nature of the lentic- 

 ular hornblendic masses, his attention must be directed to several 

 considerations; in the first place, the proximity of large eruptive 

 masses of gabbro and the undoubtedly shattered condition of the 

 penetrated gneissic rocks render such a nature quite possible; 

 again, they are more altered than the large masses of gabbro, 

 as, indeed, they should be, buried as 

 they are in a rock of diverse chemical 

 coDiposition and molecular constitu- 

 tion; the feldspars of the gabbro and 

 of the inclusions seem to be of the 

 labradorite-anorthite type and iden- 

 tical, while together they differ from 

 those of the gneisses; biotite plays 

 an important role in the gneisses, 

 while here it takes an insignificant 

 place; the texture of the two is di- 

 verse; and those metamorphic 

 changes represented by corroded 

 constituents, kaolinized feldspars, 

 conversion of hornblende into bio- 

 tite and the general sprinkling 

 throughout the mass of epidote 

 granules similar to those so conspic- 

 uous in the matrix gneiss are largely 

 wanting in the inclusions. 



Going south from Rhinelander 

 the next exposures of rocks of any 

 kind are the gneissic rocks at Grand- 

 mother falls. Section 10, T. 33, R. 6 

 E. Here the rocks are partly gran- 

 itic, although the gneissic condition 

 prevails ; they frequently become epi- 

 dotic through the occurrence of bands 

 or veins of the mineral epidote. 

 In texture, the rocks are not so 



Fia. 4.— Sketched from the 

 gneissic rocks at Grandmother 

 falls. A represents the largest 

 proportion of the amygdaloidal 

 basic rock; i? represents a med- 

 ium proportion of the basic and 

 gneissic rocks; while G shows 

 the matrix gneiss carrying only 

 a few small masses of the in- 

 cluded material. 



