GEOLOGY OF CENTRAL WISCONSIN, 



partially dissolved, or replaced in part by crystalized silica; the latter presenting in the 

 polarized light colored patches which have frequently their crystallographic axes all 

 lying in the same direction. On the other hand, many of the quartz grains enclose crys- 

 tals of felspar, but these may be instances where the felspathic material has been 

 almost entirely replaced by the silica. The quartz resembles vein quartz and contains a 

 few fluid inclusions. 



1430. Quartz-Porphyry. HTTROXIAN? From head of Lake Buffalo, N. E. qr. 

 of Sec. 8, T. 14, R. 9. E., Marquette county. Dark reddish-brown; porphyritic texture. 

 The cleavage facets of the felspar are the only mineral ingredients that can be recog- 

 nized by the naked eye. With the microscope, the base presents a semi-granular moss- 

 like texture, in which are strewn a few crystals of orthoclase and quartz, also numerous 

 small crystal grains of magnetite. To the latter is due the dark color of the rock. 



