272 GEOLOGY OF THE EUEEKA DISTEICT. 



stand alone, having broken tln-ough rhyolite. A large pcirtion of the rock 

 masses designated as pyroxene-andesite would hardly be classed as typical 

 rock of that natural group, and the same may be said of many of the 

 basaltic flows which are far too rich in silica and wanting in olivine to be 

 regarded as normal basalt. It is probable that many modern volcanoes 

 would show the same wide range in basic lavas as is developed in the region 

 of Richmond Mountain. 



Throughout a wide range in composition and over an extended geo- 

 graphical area the pyroxenic magma fails to show the tendency, so strongly 

 marked in the feldspathic magma, to separate into well defined natural 

 groups, nor is the evidence by any means clear that during the period of 

 extravasation a steady increase in the basicity of the lava took place with- 

 out occasional oscillations in composition. Nevertheless, it is evident that 

 whatever oscillations there were must have been confined within very narrow 

 limits and restricted to lavas of intermediate composition between pyroxene- 

 andesite and basalt. No pyroxene-andesite dikes have been observed pen- 

 etrating either the basalts or the intermediate lavas. 



It seems evident from field observations that there were no abrapt 

 alterations of feldspathic and pyroxenic lavas after the appearance of the 

 earliest pyroxene-andesite. 



Speculative Theories.— It does not come within the scope of this chapter, 

 which is mainly devoted to a presentation of observed facts, to enter upon a 

 full discussion of the speculative theories advanced by geologists to account 

 for the condition of the molten masses beneath the surface, nor the physical 

 causes leading to their separation into the varied products found either as 

 interbedded sheets and laccolites within the superficial crust of the globe, 

 or poured out upon the surface as extrusive lavas. Yet, at the same time, 

 after having devoted so much study to the constitution of the different 

 lavas and their order of succession, this chapter would be incomplete with- 

 out calling attention to the importance of the phenomena presented at 

 Eureka and pointing out the bearing of tlie observed facts upon the prob- 

 lems offered in volcanic regions elsewhere. Without entering npon a review 

 in detail or a critical discussion of the opinions held by others who have 

 considered these speculative matters, it is necessary to recall, bi'iefly, the 



