284 GEOLOGY OF THE EUREKA DISTEICT. 



contrasted in their surface features. In the opinion of the writer, however, 

 there are too many insurmountable physical obstacles and too few estab- 

 lished facts to warrant the acceptance of any theory which attempts to 

 account for the varied products of eruption by supposing them to be admix- 

 tm-es from wholly distinct reservoirs. The observed geological phenomena 

 at Eureka tend to controvert such a theory where the two magmas, although 

 in close proximity, fail to show any mingling of products from separate 

 reservoirs. 



Furthermore, there are no evidences of any alternating flows of 

 feldspathic and pyroxenic magmas, nor of oscillations in relative acidity 

 within any acid magma, which would certainly be the case had there been 

 any basic material injected into the feldspathic lava. Within limited range 

 any large outburst of lava doubtless may disjjlay slight variations in com- 

 position, but this also holds true for diiferent parts of the same flow, and is 

 still more noticeable in pyroxenic magmas owing to the greater liquidity of 

 basic lava streams and the consequent tendency of the basic mineral secre- 

 tions to lag behind. The first vi<ilent explosions after cessations of activity 

 might readily thi-ow out a lava slightly different in composition from the 

 regular even flow of the mass, and again the last portions might vary some- 

 what in character from the great bulk of molten material. 



E\adence is wanting at Eureka that tlie lavas were thrown out, geolog- 

 ically speaking, from great distances below the surface or from very vary- 

 ing depths; at least the lavas themselves do not indicate that there were 

 any profound orgraphic movements during the eruptions. Nor is there any 

 evidence of oscillation in depth from which the material was derived, even 

 if we accept differences in specific gravity as evidence of increase of dis- 

 tance from the surface. There was one, and only one, great break in the 

 mineralogical character of the lava. Changes in specific gi'avity were 

 gradual, but at the same time they covered nearly the entire range of varia- 

 tion ordinarily found in volcanic lavas. Such heavy minerals as zircon, 

 allanite, and garnet occur in the rocks of the lowest specific gravity, and in 

 the case of zircons they are nowhere found better developed than in the 

 glassy rocks which must have cooled near the sm-face. As these heavy 

 infusible minerals were the first to crystallize out, they should have sunk 



