U. S. GEOLOGICAL SURVEY 
BULLETIN NO. 229 PL. V 
THIN SECTIONS OF ALTERED PORPHYRITIC DIKE NEAR TIN CREEK. 
A. Magnified 80 diameters: a, feldspar; b, fluorite; C, fine-grained sericitic minerals; d, groundmass of 
fine-grained quartz, fluorite, sericite, and calcite. 
B. Magnified 54 diameters: a, quartz phenocryst; b, groundmass, consisting mainly of fluorite and second- 
ary quartz; c, zinnwaldite mica. 
