SEQUENCE OF LAVAS. 283 



Analyses Nos. ii to vii, inclusive, represent Tertiaay rocks older tluiu 

 any found at Eureka, but from Nos. viii to xii, inclusive, together with No. 

 I, they coiTespoud fairly well to similar lavas at the latter localit}'. In this 

 table, however, pyroxeue-andesites similar to those of Richmond Mountain 

 and of the same geological position, associated with basalts and later than 

 the rhyolites, were not shown, for the reason alread}' stated: that they lie 

 beyond the limits of the mining districts. 



Nowhere else between the Wasatch and Sierra have the lavas been so 

 carefully mapped, and only in a few places do they appear so varied and 

 complete. In many centers of eruption, even where the amount of lava 

 poured out is large, certain types of rock are wanting, and in others their 

 relative position can not well be detei-mined owing to fretjuent breaks in the 

 continuity of exposures. 



The history of volcanic action may be fragmeutal and oidy partially 

 recorded in any one locality, but throughout the Great Basin, where the 

 physical and geological conditions were much the same during the volcanic 

 period, it is probable that the sequence of lava will be found to be in accord 

 in many places with the observed facts at Eureka. As a center of eru>|itive 

 energy in Teitiary time the Great Basin stands out as a geological unit. 



The earliest lavas erupted at Eureka can-y fi-om G5 to 67 per cent of 

 silica and are of intermediate composition, in accordance with the broad 

 generalization of Richthofeu and the facts observed by others else- 

 where. From this middle ground, however, the lavas increase in acidity 

 tmtil they attain the composition of the extreme acid types. The latter are 

 in turn followed by lavas that are also intermediate in composition, but 

 which inci'ease in l)asicit>- until tlicy attain the extreme basic type found in 

 the later basalt. 



Starting from a magma closeh" related in composition, they ditl'er- 

 entiate in opposite directions from tliis common ground until they reach tlie 

 exti'eme type. It will be borne in mind that the existence of both an acid 

 and a basic magma at Eureka have been clearly established, and t^) this 

 extent conform to the views held by Bunsen, Nowhere are the two 

 magmas better exhibited, as shown in their distribution, mode of occuiTence, 

 and even in the outlines of the lava masses, both types of rock being sharply 



