268 GEOLOGY OF THE EUREKA DISTRICT. 



from an earlier primordial molten mass is no more difficult than to conceive 

 of the breaking up of the feldspathic magma into a homblende-mica- 

 andesite and a rhyolite group, and the latter has been shown to take 

 place, so far as it is possible to demonstrate it from surface evidences, 

 along fissure planes through which the lavas issued. The original magma 

 separated into a heavier and a lighter portion, the groundmass structure 

 of the two being fundamentally different. It will be borne in mind 

 that the earlier magma consisted of a groundmass made up of an aggre- 

 gation of feldspar and quartz grains, through which were disseminated 

 porphyritic secretions of hornblende and mica, but no pyroxene, except 

 in a few instances of pyroxene microlites in the groundmass of some 

 varieties of audesite. The later magma consisted of a groundmass com- 

 posed of lath-shaped lime-soda feldspars and pyroxene microlites, so intri- 

 cately interwoven as to form the so-called felt-like structure characteristic 

 of pyroxene-andesite, through which were scattered the heavier ferro- 

 magnesian minerals already described. 



History of Volcanic Action. The geological history of volcanic action at 

 Eureka during Tertiary time is in many respects simple and, after a careful 

 study of its details, easily deciphered. There are among the lavas no masses 

 of coarsely crystalline rocks slowly cooled beneath the surface under 

 physical conditions different from those usually found accompanying extru- 

 sive flows. No powerful displacements have brought into juxtaposition 

 igneous rocks of different ages, crystalline structure and mineral composi- 

 tion, and although faulting attending extravasation doubtless did occur it 

 was not of a kind to obscure geological structure. Again, the sequence of 

 events was not complicated or broken by long intervals of activity and 

 rest through successive geological epochs during which an older and a 

 younger series of eruptions took place; but on the contrary the lavas were 

 apparently poured out under very similar physical conditions from the 

 beginning to the end of volcanic action. In coming to the surface these 

 lavas were not forced upward as one continuous eruption or rapid series of 

 eruptions, but were the result of a succession of overflows accumulating 

 slowly, although at times spasmodically, along lines of volcanic activity 

 coincident with lines of orographic displacement. The material thus poured 



