Name:  Porphyritic Biotite Granite

Geologic Name:  "Rockville" Granite

Reference:  Thiel, G.A. and Dutton, C.E., The architectural, structural and monumental
stones of Minnesota.  Minnesota Geol. Survey Bull. 25, p. 65-69, 1935.

Microscopic characters:  Coarse hypautomorphic granular texture.  Microcline,
oligoclase, biotite, quartz, hornblende, and orthoclase are the essential constituents.
The biotite contains numerous pleochroic halos.  Some of the plagioclase is zoned.  
Sphene, apatite, magnetite and chlorite are the accessories.  The feldspar is partly
clouded with sericite and kaolin.