THE EOCKS OF THE WASHOE DISTRICT. 69 



clase, on tlie supposition of a mere mixture of species. It appears to me 

 more probable, however, from the character of the zonal plagioclases, that 

 many of the feldspars are not chemically refera1)le to either species. 



The results of the application of Thoulet's method agree excellently 

 with those of the microscopical examination, and together render it impossi- 

 ble to classify this rock otherwise than as a hornblende-audesite, in spite of 

 a maci'oscopical appearance exceedingly like ordinary varieties of trachyte, 

 and very dissimilar to common andesite. 



Remarkable glass inclusion in feldspar. Tlie fcldsparS COUtaiu glaSS inclusioilS iu 



all the slides of this rock, but these are most abundant to the north of the 

 Utali. In the quarry close to the hoisting works of that mine some of these 

 inclusions are of a peculiar character, forming negative feldspar crystals of 

 a more or less perfect shape. These were mentioned by Professor Zirkel 

 with admiration. No such fine example occurs in my slides as in that de- 

 scribed by him, and in his slide number 284 there is but one which can 

 have furnished his description. This is illustrated in Fig. 14, Plate III. 

 It is not a sanidiu, however, but probably a labradorite crystal. 



Groundmass. — The grouudmass of the more trachytic varieties is entirely 

 crystalline, though never granular like some of the older hornblende-ande- 

 sites; its texture is also very loose and open, a fact which often influences 

 the course of decomposition. To the north of the Utah patches of glass 

 similar to that which is included in the feldspars of the same locality are 

 distributed through the groundmass, and on the ridge running east by south 

 from the Geiger Grade toll-house, D. 1, as well as at the point where the 

 Grade cuts the younger hornblende-andesite area, the glass prevails to such 

 an extent that the rock approaches an obsidian in character. Its pitchy 

 black color is due merely to the bisilicates and magnetite, the glass and 

 feldspar being transparent. 



Field character. — The more trachytic varieties near Shaft III. of the Sidro 

 Tunnel, and on the southwesterly spur of Mount Rose, are red or purple, 

 and highly porphyritic, very soft and rough rocks, quite incapable of being 

 confounded with any other occurrence in the district. They do not exhibit 

 regular partings or columnar structure. Mount Rose and Mount Emma are 

 largely composed of tufa and tufaceous breccia. The tufa is not macro- 



