AGE OF ERUl'TION. 231 



Distribution of Extrusive Lavas.— In regard tO the distribution of VolciUlic 



rocks in the Eureka District it will be noticed by reference to the map tliat 

 there lire none in that part of tlie Diamond Range which conies witiiin tlic 

 area of the map. Althougli the southern end is coTupletely cut otf by recent 

 lavas from the mouutaiu Wock of Comity Peak, nowhere do thev seem to 

 penetrate into tiu- range itself In the southern end of the Pifion Kange on 

 the opposite side of Diamond Valley, there is the same ab.sence of hivas, if 

 weexce})t a small outburst northwest of The Gate, which has but little to do 

 with the main Itody of the Eureka j\b;)untains. The Mahogany Hills west 

 and north of Dry Lake; furnish one or two small exjtosures of rhyolite along 

 lines of faulting in limestone, notably at the head of Brown's Canyon, 1)ut 

 they are of no special geological significance. In the Fish Creek Mountains 

 no outbreaks of extrusive rocks are known and the same holds true of the 

 mountains bordering the Spring Valley fault. In the extreme southeast 

 corner of the map the Cliff Hills come in, formed of basic andesites similar 

 to those of Richmond Mountain, but they lie wholly lieyond the borders of 

 the Eureka Mountains. This confines the area of the principal volcanic 

 extravasations either to the region east of the sununit of Prospect Ridge or 

 to the country encircling the southern end of that ridge, where it sinks below 

 Fish Creek Valley. Nearly every type and many of the varieties of vol- 

 canic rocks found upon the Nevada plateau have been eru})ted within these 

 restricted limits of the Eureka District. Indeed, the region furnishes many 

 rocks which may be taken as typical of a broad area of country lying 

 between the Wasatch and the Sierra Nevada ranges. 



Age of Eruptions.— It sluHild 1)6 clcarlv Understood that the P^ureka Dis- 

 trict, like many other regions of central Nevada, offers no direct e^^dence 

 as to the age of volcanic eru})tions, as there occur no sedimentary forma- 

 tions between Upper Coal-measure limestones and recent Pleistocene de- 

 posits. While })ositive evidence may be wanting as to their precise age, 

 there can be no doubt that the eruptions took place subsequent to the 

 dynamic movements which l)rouglit about the flexing, folding, and mountain- 

 building, and these, while it may not have lieen demonstrated, have been 

 assigned upon excellent grounds to a ])ost-Jurassic upheaval, already dis- 

 cussed in Chapter 11. of this work. Moreover, that these orographic 



