290 GEOLOGY OF THE EUREKA DISTRICT. 



Silverado and County Peak block and the depressed Carboniferous block 

 between the Hoosac and Pinto faults ; third, they occur in numerous dikes 

 penetrating the limestones ; fourth, they occur in one or two relatively 

 large bodies, notably Richmond Mountain and Pinto Peak, along lines of 

 displacement already mentioned. 



All the lavas may be classed under the heads : hornblende-andesite, 

 hornblende-mica-andesite, dacite, rhyolite, pyroxene-andesite, and basalt. 

 They pass by insensible gradations from one to the other. All division lines 

 are more or less arbitrary ; they are necessary for the purposes of classifica- 

 tion, although they may not exist in nature. 



Field observations clearly show that the order of succession of these 

 natural groups into which the lavas have been divided was as follows : First, 

 that the homblende-audesite was the earliest of all the erupted material ; 

 second, that the hornblende-mica-andesite followed the hornblende-ande- 

 site ; third, that the dacite followed the hornblende-mica-andesite ; fourth, 

 that the rhyolite closely followed the dacite; fifth, that the pyroxene-ande- 

 site succeeded the rhyolite ; sixth, that the basalt was the most recent of all 

 these volcanic products. 



In chemical composition this entire series of lavas shows a range in 

 silica amounting to about 25 per cent, a range which is quite as wide as is 

 usually found in most centers of eruption even where the volume of lavas 

 thrown out is vastly greater and the duration of volcanic energy far longer. 

 Analyses show endless transition products between the extreme basic and 

 acidic lavas, with a tendency of the alkalies and silica to accumulate at the 

 acidic end and the material forming the ferro-magnesian minerals at the 

 basic end. 



It is maintained in this work that all the varied products of eruption 

 are derived from a common source, a homogeneous molten mass. Under a 

 process of differentiation this earlier mass split up into two magmas, desig- 

 nated as a feldspathic and a pyroxenic magma. The lavas at Eureka are the 

 result of the same process of differentiation derived from one or the other 

 of these magmas. Beginning with hornbende-andesite, the earliest lava, the 

 feldspathic magma became more siliceous until the close of rhyolitie erup- 

 tions. The rhyolite was followed by pyroxene-andesite and the eruptions 



