CHAPTEE VII I. 



TERTIARY AND POST-TERTIARY VOLCANIC ROCKS. 



Eureka a Volcanic Center.— lu the Eureka District the veceiit volcanic erup- 

 tions play a far more important part than the granites and porphyries just 

 described. They occur in much larger masses, cover more extensive areas, 

 and are more widely distributed over the distinct. While the older crystal- 

 line rocks have exerted little influence upon the surface features of the 

 country, the volcanic rocks have greatly modified its topographical outlines, 

 have built up isolated mountains, broad table-lands, and numeroTis small 

 hills, and in coming to the surface have disturbed and broken up sedi- 

 mentary formations, greatly complicating geological structure. Moreover, 

 the volcanic rocks are of special interest from an economic point of view, 

 owing to their intimate geological connection with the argentiferous lead 

 deposits occm'ring in the adjoining Paleozoic rocks. As the Eureka Moun- 

 tains are surrounded on nearly all sides by the characteristic broad valleys 

 of the Nevada plateau, this volcanic region occupies a somewhat isolated 

 position with reference to the neighboring ranges, constituting a region 

 quite apart from all other centers of similar eruptions, yet at the same time 

 bearing the closest resemblance in the nature of its extravasated material 

 to many other localities in the Great Basin. Nowhere in the district have 

 the accumulated lavas attained any great elevation above the surround- 

 ing mountains, but as regards mode of occurrence, peculiarities of distri- 

 bution, and varieties of modification they offer a wide field for investigation. 

 There is no such piling up of enormous masses of erupted material as in 

 the Washoe District, and no such opportunity for an investigation of tlie 

 more coarsely crystalline rocks, but, on the other hand, the relationship 

 between the extrusive lavas and the uplifted blocks of Paleozoic strata is 

 better shown than in any other carefully studied area in the Great Basin. 



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