AGE OF ERUPTION. 231 



Distribution of Extrusive Lavas. In regard to the distribution of volcanic 

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

 there are none in that part of the Diamond Range which comes within the 

 area of the map. Although the southern end is completely cut off by recent 

 lavas from the mountain block of County Peak, nowhere do they seem to 

 penetrate into the range itself. In the southern end of the Pinon Range on 

 the opposite side of Diamond Valley, there is the same absence of lavas, if 

 we except a small outburst northwest of The Gate, which has but little to do 

 with the main body of the Eureka Mountains. The Mahogany Hills west 

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

 lines of faulting 111 limestone, notably at the head of Brown's Canyon, but 

 they are of 110 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 

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

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

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

 extravasations either to the region east of the summit 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 erupted 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 should be clearly understood that the Eureka Dis- 

 trict, like many other regions of central Nevada, offers no direct evidence 

 as to the age of volcanic eruptions, as there occur no sedimentary forma- 

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

 posits. While positive 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 brought about the flexing, folding, and mountain- 

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

 assigned upon excellent grounds to a post-Jurassic upheaval, already dis- 

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



