232 GEOLOGY OF THE EUEEKA DISTRICT. 



movements were followed by a long period of time before the outbursts of 

 lavas is evident by the amount of erosion wliic-h took place before the ex- 

 tnision of the latter rocks. This is shown by the position of the lavas in 

 the bottoms of deeply eroded canyons, by the blocking up of old drainage 

 channels and the cutting of new ones, all of which must have required 

 considerable time to be accomplished by slow geological processes. On 

 the other hand, based upon much the same kind of e'sadenee, the amount 

 of erosion since the volcanic period seems relatively slight. Although evi- 

 dence of geological age of these lavas may be difficult to obtain over the 

 greater part of the Great Basin, wherever such proof is observed it always 

 points to the conclusion that the eruptions took place since the coming in 

 of Tertiary time and for the most part during the Pliocene period. Nearly 

 all geologists who have examined the volcanic areas of Nevada and Utah 

 are in accord with the opinion that they belong to Tertiary time. In the 

 region of the Montezuma and Kawsoh i-anges, in western Nevada, the geol- 

 ogists of the Fortieth Parallel Ex})loration^ have described the cutting by 

 intnisive acid lavas of upturned Miocene strata carrying fresh-water mol- 

 luscan shells and the overlying of the latter beds by basaltic flows. Evi- 

 dence has also accumulated to show that several of the great rhyolite flows 

 that preceded the basalt belong- to the Pliocene epoch. From unques- 

 tioned evidence in other parts of the Great Basin as to the geological posi- 

 tion of identical lavas, it is assumed that those of the Eureka District were 

 also poured out during the Tertiary age. As regards the dui'ation of vol- 

 canic acti\'ity there is scarcely any evidence. From the earliest outbursts, 

 consisting of horublende-andesites, to the latest eruptions of highly glassy 

 basalts, volcanic activity may have extended o^'er the greater jjart of late 

 Tertiary time. That this acti\nty continued at varying intervals through- 

 out a long period seems clear from the amount of erosion, which, though 

 relatively not excessive, must have recpiired considerable time. There is 

 no evidence to show that volcanic energy with varying intensity may not 

 have extended through Pliocene w^ell on into Quaternary time, although 

 there is no reason to suppose that outbursts of })asalt have taken place 

 within what may be called historic periods. 



' U. S. Geol. Explor., 40th Par. Descriptive Geology, p. 771. 



