162 Scientific Intelligence. 



The region oilers no direct proof of the age of volcanic energy ; 

 yet all evidence points to the conclusion that the eruptions belong 

 to the Tertiary era and for the most part to the Pliocene period. 

 They may have extended well on into Quaternary time, although 

 there is no reason to suppose that eruptions took place within 

 historic time. 



As regards their mode of occurrence the principal eruptions may 

 be classed under four heads : First, they broke out through 

 profound fissures along the three great meridional lines of dis- 

 placement, the Hoosac, Pinto, and Rescue faults, and to some 

 extent along the lesser parallel faults; second, following the lines 

 of orographic fracture, they border and almost completely encircle 

 the large uplifted masses of sedimentary strata like the Silverado 

 and County Peak block and the depressed Carboniferous block be- 

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

 dikes penetrating the limestone ; 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 classification, 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 hornblende-andesite was the 

 earliest of all the erupted material; second, that the hornblende- 

 mica-andesite followed the hornblende-andesite; third, that the 

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

 rhyolile 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, designated 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 witb hornblende-andesite, the earliest 

 lava, the feldspathic magma became more siliceous until the close 



