THE EOCKS OF THE WASHOE DISTRICT. 43 



ravine, constitutes tlie principal area of the granitic diorite at tlie surface; 

 wliile botli to the nortli and the south porphyritic varieties prevail, and 

 nearly all the diorite to the east of the Lodk is of the same character. 



METAMORPHIC DIORITE. 



Origin and association. — Tlic soutliem portiou of the district contains a large 

 area of this rather puzzling rock. It was mentioned by Mr. King as a "com- 

 pact, black, crystalline rock, which in hand specimens would unquestionably 

 be classed as a basalt," but which can be shown to be of nietamorphic origin. 

 Professor Zirkel determined it as a peculiar basalt. The most ordinary 

 variety is of a black or iron-gray color, and shows an irregular crystalline 

 fracture; but certain varieties (the more feldspathic ones) are light in color, 

 and considerably resemble Mount Davidson diorite. There is no little diffi- 

 culty in determining whether this rock shall be regarded as of metamorphic 

 origin, or as eruptive. To the west of the Florida mine the contact between 

 it and the underlying metamorphic rocks appears as sharp as possible. At 

 the Wales Consolidated, a mine opened on a deposit lying between this rock 

 and the granite, there is no evidence whatever of bedding. Near the Amazon 

 mine it is weathered in round boulders, precisely like those produced by the 

 action of frost on basalt; and close to the Volcano mine it forms a distinct 

 breccia. On the other hand, in some of the railroad cuts, there appear to 

 be ti'ansitions into rocks of evidently sedimentary origin. But such appear- 

 ances need very cautious treatment, for between metamorphism and decom- 

 position, a contact might readily assume the appearance of a transition. The 

 microscopical character of the rock offers nothing decisive as to its origin, 

 and the point wliicli has mainly determined -me to regard it as metamorphic 

 is its relation to the quartz-poi-phjry. An inspection of the map will show 

 tliat it is invariably associated with the quartz-porphyry, and that if it had 

 resulted from the metamorphism of the sedimentar}- strata by porphyry 

 eruptions, subsequent erosion nnist have exposed it in relations almost iden- 

 tical with those observed. Its composition also indicates a metamorphic 

 rather than an eruptive origin. 



