Vol. 4] Lawson. — The Robinson Mining District. 



305 



Another sample of garnet-ehal copy rite ore from the shaft 

 on the Taylor claim consists mostly of garnet with rough jagged 

 fracture shot through with chalcopyrite. The rock has a rudely 

 sheeted structure. In thin section the field is murky and con- 

 sists of nothing but impure doubly refracting garnet, more or 

 less distinctly idiomorphic, with interstitial chalcopyrite carry- 

 ing small irregular grains of quartz. 



Petrographical Character. — The normal facies of this intru- 

 sive mass is a medium textured gray rock composed of whitish 

 feldspar and black hornblende as a matrix in which are im- 

 bedded large flesh tinted fresh feldspars up to 15 mm. in size. 

 A very sparing amount of quartz may be detected with a lens 

 on weathered surfaces. The feldspar was assumed in the field 

 to be orthoclase and the rock was designated a syenite. A micro- 

 scopic study of the rock, however, shows that there is a large 

 proportion of plagioclase giving extinction angles in sections 

 normal to (010) up to 20°, and having a refractive power greater 

 than that of balsam. This feldspar, having the characters of 

 andesine, is not less in amount than the orthoclase, a fact which 

 clearly indicates that the rock should be classed with the mon- 

 zonites rather than with the syenites. The orthoclase is distin- 

 guished in general from the andesine by the absence of twinning 

 and by the fact that it shows a lower refractive power than the 

 balsam. The hornblende occurs in fairly idiomorphic crystals 

 of small size. Quartz is very sparingly represented in the sec- 

 tion. The accessory minerals are apatite, titanite, and mag- 

 netite. The structure is hypidiomorphic granular as regards 

 the general body of the rock ; but this serves as a matrix for 

 large phenocrysts of orthoclase. The phase of this monzonite 

 cutting the limestone and shale at the Taylor Mine to the west 

 of Pilot Knob shows some minor variations from the prevailing 

 character of the mass underlying Weary Flat. In the specimens 

 taken from this locality the ground mass is finer grained and 

 the phenocrysts are more numerous. The hornblende, where 

 fresh, occurs in longer prisms, but much of it is represented only 

 by pseudomorphs of limonite. Quartz is a little more abundant. 

 Apatite and titanite are more prominently developed. The large 

 phenocrysts of orthoclase attain a length of 25 mm. and they 



