262 GEOLOGY OF THE EUREKA DISTRICT. 



Fortieth Parallel Exploration were led into error in sup})osing that all the 

 rocks classed as pvroxeue-andesite in the Great Basin belong to the same 

 time jieriod and were identical as regards their geological position in the 

 oi-der of succession, whereas there are two distinct periods, the earlier of 

 which is represented by the pyroxene-andesites of Washoe and preceded 

 the hornblende-mica-andesites, dacites, and rhyolites, and the latter by the 

 pyroxene-andesites which followed the rhyolites, as developed on so grand 

 a scale at Richmond Mountain. 



Accessory Minerals.— Disseminated through the lavas at Eureka four 

 minerals have been recognized, which in all cases occur simply as accessory 

 constituents, us in no single instance do they enter largely into the compo- 

 sition of the rocks. These n'linerals are apatite, zircon, garnet, and allanite. 

 Apatite and zircon iu a perfectly unaltered condition have been determined 

 in every t}'pe rock of both feldspathic and pyroxenic magmas. The apa- 

 tites are much like those described in volcanic rocks elsewhere, with well 

 developed terminations and a characteristic basal cleavage. Zircons in 

 both long, slender prisms and short, stout, colorless crystals are by no 

 means uncommon, and, judging from their distribution, occur apparently 

 uninfluenced by the nature of the lava, notwithstanding their high specific 

 gravity. They are found especially well developed in the andesitic pearl- 

 ites, the crystalline forms, as drawn by Mr. Iddings, having already been 

 employed as illustrations of microscopic zircons in recent text-books. 



The presence of apatite is indicated by analyses in the determination 

 of ])hosphoric acid, but the amount of zirconia present has not yet been 

 estimated in any of these lavas. Judging- from the analyses, thephosphoric 

 acid increases with the basicity of the lava, starting in with only VG per 

 cent in the rhyolite from Rescue Canyon and reaching '29 per cent in the 

 basalt from the summit of Richmond Mountain. The two silicates, garnet 

 and allanite, have been detected only in the acidic magmas, but both of 

 them have apparently been developed in the same type of rocks. The 

 garnets, although minute, inay be easily recognized by the naked eye, 

 standing out as brilliant dark red crystals in contrast with the light colored 

 pumices, tuffs, and pearlites which carry them. They occur in both the 

 Rescue Canyon and Pinto Peak rhyolites. They are well developed at 



