Geological Excursion into Central Wisconsin — Hall. 257 



many granitic veins of a lighter color than the gabbro itself, a 

 more or less shattered condition, and well-worn and rounded sur- 

 faces. But the penetrated rock in that case was a hornblende 

 gneiss; in this, a silicious rock, more or less schistose, and of sed- 

 imentary origin. A greenish brown color prevails, which is not 

 cheerful nor neat to the eye as it appears in the quarry and in the 

 walls of many buildings in the city constructed of this stone. 



The further structural features of this rock are well set forth 

 in the report of Irving and Van Hise* and in the earlier published 

 •field report of Professor Irving.f 



In situation and extent the exposures visited lie on the west 

 side of the Wisconsin river, and on the islands, natural and arti- 

 ficial, within the stream. The rapids at Wausau, the Big Bull 

 falls of the early settlers, are formed by these rocks opposing a 

 barrier to the stream for a distance of a mile or more. For the 

 most part the rock is quite massive, although here and there gneissic 

 areas appear, having a northeast and southwest trend. Hornblendic 

 veinsj and segregations were frequently noted. 



Microscopically, quartz is everywhere present in minute, clear 

 grains, often apparently taking the place of corroded feldspars. 

 Possibly some of it occurred as one of the primary constituents of 

 the rock. The feldspar is plagioclastic, partly of the labradorite 

 type and partly albitic, and is present in two generations, as is the 

 quartz. Frequently large areas are singularly flecked with inter- 

 growths of another species, which, according to the writer's obser- 

 vations, extinguishes but few degrees from the extinction angle of 

 the host. Again, considerable areas are burrowed and filled with 

 vermicular quartz. In other places minute crystals of hornblende 

 and biotite stud the feldspar field thickly, although, as a rule, these 

 minerals lie in proximity to the pyroxenic constituents. 



Diallage is better developed here than in the gabbros of Rhine- 

 lander. The peculiar markings, the rod-like structure, of this min- 

 eral in two series, parallel or coincident with the cleavages, are very 

 common. It alters into hornblende, and numerous crystal plates of 

 hornblende are strewn thoughout even the freshest diallages, while 

 the borders of the grains are entirely changed into the secondary 

 substance. Less biotite is to be seen here than at Rhinelander. 



*Geology of Wisconsin, Vol. iv, pp. 661, 662. 



IGeology of Wisconsin, Vol. ii, pp. 486-488, and elsewhere. 



^Compare Geology of Wisconsin, Vol. ii, p. 487. 



