RELATIVE AGE OF LAVAS. 249 



tain is a vast accumulation of pyroxene-andesite similar in its geological 

 occurrence to the smaller hills of basalt which have broken out at numer- 

 ous points along the fractures caused by the elevation of the County Peak 

 and Silverado block. 







Mr. Clarence King, in summing up the observations of the geologists 

 connected with the Geological Exploration of the Fortieth Parallel upon 

 the mode of occurrence of the rhyolites between the Sierra Nevada and 

 Wasatch ranges, makes the following concise generalization: 



Where a great mountain block has been detached from its direct connections 

 and dropped below the surrounding levels, there the rhyolites have overflowed it and 

 built up great accumulations of ejecta. Wherever the rhyolites, on the other hand, 

 accompany the relatively elevated mountain blocks, they are present merely as bor- 

 dering bauds skirtiug the foothills of the mountain mass. There are few instances 

 in which hill masses were riven by dikes from which there was a limited outflow over 

 the high summits; but the general law was, that the great ejections took place in 

 subsided regions. 1 



Nowhere within the Great Basin does this description hold true with 

 greater force than in the Eureka District. It holds true, however, for the 

 entire hornblende-andesite and dacite groups, as in their mode of occurrence 

 they can not be separated from the more acidic lavas. It holds equally 

 well for the pyroxene-andesites, since such broad masses as make up Rich- 

 mond Mountain are simply relatively large accumulations of lavas at cen- 

 ters of great dislocation in highly disturbed regions in every way similar to 

 those of other lavas. In the case of the hornblende-andesites and rhyolites 

 they have poured over and nearly submerged a depressed sedimentary 

 region, whereas the rhyolites, pyroxene-andesites, and basalts, which have 

 broken out in proximity to the Silverado and County Peak region, appear 

 more as an encircling belt to a relatively elevated country. 



Relative age of Volcanic Rocks. In the Eureka District the hornblende- 

 andesite and the closely related hornblende-mica-andesite are the earliest 

 of the Tertiary lavas, all others with which they are associated being found 

 either to break through or overlie them. Hornblende-andesite, wherever it 

 occurs in the district, is a crystalline rock and forms a central body, which, 

 by insensible transitions, passes into a rock with a more and more glassy 



'U. S. Geol. Explor. 40th Par., 1878, vol. I, Systematic Geology, p. 6W. 



