254 G KG LOGY OF THE EUREKA DISTRICT. 



to one or the other of two well defined groups, in each of which the lavas, 

 although possessing a wide range in chemical composition, are so intimately 

 related and so interdependent as to suggest that they must necessarily have 

 been derived from some common source. In other words, all lavas at Eureka 

 may be divided into two sharply contrasted groups, the one acid, and the 

 other basic. 



A microscopical examination in the laboratory of a large amount 

 of material collected in the field in the opinion of the writer 

 lends support to this view of two magmas. It is brought about by 

 a study of the gradual transition in mineral composition and by cer- 

 tain peculiarities of structure and crystallization characteristic of each 

 magma. The acid magma was the earlier in age, the eruptions begin- 

 ning with hornblende-andesite and closing with the extreme acidic forms 

 of rhyolite. In general the lavas of this acid series are light in color, 

 the microcrystalline groundmass being composed for the most part of an 

 aggregation of feldspar and quartz grains without the accompaniment of 

 ferro-magnesian silicates. The hornblendes play no part in the composi- 

 tion of the groundmass, being present as porphyritic secretions, whereas the 

 pyroxenes, in the few instances where they have been recognized, at the 

 basic end of the series do not occur as porphyritic minerals, but only in 

 minute microlitic forms developed in the groundmass. The glass is always 

 highly acidic. 



Sharply contrasted with these acidic lavas the basic lavas are char- 

 acterized by a predominance of the pyroxenic minerals, the prevalence of 

 lime-soda feldspars and the structural features of a groundmass peculiar either 

 to pyroxene-andesite or to basalt. The basic magma came in with pyroxene- 

 andesite and closed with numerous outflows of basalts. The two magmas 

 so sharply defined by mineralogical and structural distinctions may be 

 designated respectively as the feldspathic and pyroxenic magmas. A dis- 

 cussion as to their nature will bring out still more clearly their diagnostic 

 points of difference and the importance of this division in its bearing upon 

 the origin of the sequence of lavas. Further on in this chapter it will be 

 maintained that both these magmas are simply differentiated products of 

 an earlier homogeneous molten mass. 



