thp: eocks of the washoe district. 47 



isotropic substance. The iron ore is in part magnetite, while in f>tlier cases 

 it appears to be ilnienite. Ajjatites of the usual colorless variety are fre- 

 quent. Zircons are not unconnnon, and there are occasional small i)atches 

 of titanite. 



Field habit. — The croppings of the quartz-porphyry are usually exceed- 

 ingly rough, and the nearest approach to a structure is indicated in some 

 localities by the separation of the rock into uneven sherdy fragments. Its 

 appearance is almost identical all over the district, except in the small area 

 where the quartz is macroscopically suppressed. Here it shows various 

 brown and green colors, and sometimes a smooth fracture like a fine-grained 

 hornblende-andesite. In this area the color and texture vary every few feet. 

 This macroscopical difference appears to correspond to no microscopical 

 peculiarity beyond a finer grain. As quartz-})orphyry is the only quartzose 

 rock in the district, it is readily distinguishable. 



Various determinations. — As lias becu sceu iu the rc'sumc of former memoirs, 

 the quartz-porphyry has been variously determined by the eminent geolo- 

 gists who have discussed the Washoe District. Baron v. Richthofen very 

 positively asserted that the circumstances of its occurrence rendered it cer- 

 tain that this porjihyry was intermediate in age between the granitic and the 

 volcanic rocks, and 1 entirely agree Avith him. The absolute unifonuity of 

 the rock from the Overman mine to the southern extremity of the mass, with 

 the exception of a small felsitic area, utterly precludes the supposition that 

 it is separable into different species of Tertiary and pre-Tertiary origin. 

 The felsitic modification comes in contact only with granite and basalt, but 

 its microscopical character is identical with that of the coarser porphyry; it 

 strongly resembles well-known varieties of quartz-porphyry, and I can see 

 no evidence on the ground sufficient to sej^arate it from that species. That 

 in the Overman and Caledonia mines and the Forman shaft the porphyry 

 vertically underlies horidilende-andesite is beyond question; both optical 

 tests and specific gravity determinations show that it is an orthoclase rock; 

 and the character and association of the inclusions in the quartzes are pre- 

 cisely those which are so very common in old quartz-porphyry. Professor 

 Zirkel determined the lai'ger proportion of this rock as a dacite, but on 

 reexamining his slides I found that they corresponded in every respect to 



