280 GEOLOGY OF THE EUREKA DISTRICT. 



The same writers have demonstrated the nonexistence of trachyte as 

 one of the natural divisions of volcanic lavas in the Great Basin, the 

 occurrence of orthoclase rocks free from quartz secretions being almost 

 unknown in that region. These recent advances in our knowledge of vol- 

 canic rocks tends to simplify the law of sequence so far as their occurrence 

 in the Great Basin is concerned, since two of the groups, the propylite and 

 the andesite, as laid down by Richthofen, have been merged into one, 

 and the trachytes either relegated to some variety of andesitic lavas or 

 placed among quartz-bearing rocks, either dacite or rhyolite. The impor- 

 tance of these observations lies in the fact that there is no interpolation of a 

 strongly alkaline magma in the series of lavas, and that andesitic lavas pass 

 over directly into rhyolite. Not only in the Great Basin but in many other 

 regions as well, rhyolite is far more closely related to andesites derived 

 from a feldspathic magma than to trachytes. 



Having thus briefly reviewed the literature bearing upon the genesis 

 of lavas and their order of succession, it becomes a matter of much interest 

 to see how far the facts observed in a carefully studied and surveyed region 

 like Eureka are in accord with the Adews expressed by the eminent writers 

 quoted, since it is only by the accumulation of vast amount of evidence 

 from many widely separated fields that we can hope to attain anything like 

 definite laws governing the mutual relations of igneous rocks. 



In only one other i-egion of the Great Basin have volcanic phenomena 

 been investigated in a manner at all comparable with Eureka and that one 

 the much discussed area of the Washoe District. At Washoe the conditions 

 are in some respects very different, volcanic activity having extended 

 through a longer period of time. The coarse crystalline rocks which form 

 the long slopes of Mount Davidson do not make their appearance at Eureka, 

 and for that matter are wanting over the greater part of the Nevada plateau. 

 They belong to an earlier period, forming a distinct chapter in the Tertiary 

 history of volcanic action. 



The earliest eruptions at Eureka may be correlated with the horn- 

 blende-mica-andesite of Washoe (trachytes of Richthofen and King and 

 later hornblende-andesites of Becker) in mineral composition and structural 

 features. They may be regarded from the point of view of this chapter as 



