394 GEOLOGY OP THE EUEEKA DISTRICT. 



more of the other minerals, pass gradually into the surrounding groundmass; they 

 have numerous gas cavities, some containing a fluid, besides very few glass inclu- 

 sions. The augite is in part so clouded with dust-like particles as to be almost 

 opaque. Hypersthene occurs among the larger phenocrysts, having a very irregular 

 form, and the same pleochroism that it exhibits in the andesite of this region. Asso- 

 ciated with the phenocrysts is light reddish brown mica, which has a small angle 

 between the optic axes; it is apparently of primary origin. There are, besides, large 

 grains of iron oxide and considerable calcite. 



