RELATIVE A(JI>: OF LAVAS. 249 



tain is a vast accuiiuilatioii 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 iip 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 f^reat inoiiiitain block has been detached from its direct connections 

 and (liopj)ed below the surronnding levels, there the rhyolites have overflowed it and 

 built n\) great accumulations of cjecta. Wherever the rhyolites, on the other hand, 

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

 dering bands skirting 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 liigh summits: but the general law was, that the great ejections took place in 

 subsided regions.' 



Nowhere within the Great Basin does this description hold true with 

 g"reater force than in the Eureka District. It holds true, however, for the 

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

 they can not be separated from the more acidic lavas. It holds ('([ually 

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

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

 ters of great dislocation in highh" disturbed regions in every way similar to 

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

 thev have i)oured over and nearly submerged a dej)ressed sedimentary 

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

 broken out in proximity to the Silverado and C*ountv Peak region, appear 

 more as an encircling ])elt to a relatively elevated country. 



Relative age of Volcanic Rocks.— Ill the Eurekil District thl' lloi'nl)len(le- 



andesite and the closely related homblende-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. Ilornblende-andesite, wherever it 

 occurs in the district, is a crystalline rock and forms a central l)0(h', which, 

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



'U. S. Geol. Explor. 40tli Par., 1878, vol. i, Systematic Geology, p. 694. 



