270 GEOLOGY OF THE EUREKA DISTRICT. 



composition of magmas, this series of lavas may be taken as representative 

 of many others in widely separated regions throughout the world. To the 

 lavas of Hungary they show very close resemblance. 



Beginning with the hornblende-andesite the feldspathic magma became 

 gradually more siliceous until the close of the rhyolitic eruptions without 

 any abrupt break in the outpourings or the intervention of any percepti- 

 ble change in geological conditions. It seems impossible, therefore, to 

 consider these lavas in any other light than as a continuous succession of 

 flows, interrupted only by time intervals of longer or shorter duration. 

 Notwithstanding these gradual transitions, certain type rocks prevail to a 

 far greater degree than others, both as regards bulk and distribution, nota- 

 bly the homblende-mica-andesite and the Pinto Peak variety of the rhyo- 

 lite, the two standing out prominently as the principal eruptions of the 

 feldspathic series. The dacites are greatly limited in their bulk, and the 

 same is true of all rocks of intermediate composition, the greater part of 

 them being easily classed under one or the other of the natural groups. 



The earliest outbursts along diflPerent profound fissure planes have 

 not necessarily been identical in composition or syuchi'onous in time. Along 

 some of these the first overflows observed are hornblende-mica-andesite, 

 in others highly siliceous andesitic pearlites, in still others dacites, and in 

 several of them rhyolites, but in no single instance, whatever may have 

 been the nature of the earliest lava poured out, has a more basic member 

 of the feldspathic series been recognized as breaking out along the same 

 fissure. It is as if certain of these fissures were opened by the forcing- 

 upward of the lavas at different periods of eruptive energy and the vents 

 filled by a magma of definite composition at that time coming to the surface 

 simultaneously through all the fissures. It is also worthy of note that 

 along the meridional faults the andesitic material for the most part broke 

 out at the northern ends, the lavas in general growing more acidic toward 

 the south. Furthermore, certain fissures becoming filled and choked by 

 cooling and ciystallization have prevented the more acidic lavas from find- 

 ing an outlet at the surface along the same line where the earlier portions 

 of the molten mass broke out. 



