266 GEOLOGY OF THE EUREKA DISTRICT. 



rocks to the andesites, while the presence of quartz as an essential con- 

 stituent .places it more correctly among the dacites. For the erupted 

 material of Eureka it stands as one of the most basic rocks of the feld- 

 spathic magma, rich in porphyritic quartz secretions. 



The most basic of the feldspathic lavas analyzed is an andesitic pearl- 

 ite, very limited in extent, containing 65'13 per cent of silica, the complete 

 analysis of which will be found in column 6 of the table. It carries well 

 developed feldspars, with some hornblende and biotite, but is especially 

 noticeable for the numerous pyroxene microlites which enter into the 

 structure of the very glassy groundmass. The rock, although belonging 

 to the acidic lavas, is allied to the basic magma by the coming in of these 

 microlites of pyroxene, which more or less modify the nature of the glassy 

 groundmass and relate it in structural habit to the rocks of Richmond 

 Mountain. It is doubtful if any fresh rock of the feldspathic magma would 

 fall much below 65 per cent in silica. An analysis of a typical rock from 

 Richmond Mountain, given in column 7 of the table, yielded 6T58 per cent 

 of silica. The most acidic rocks derived from the pyroxenic magma, as 

 shown by a series of silica determinations in partial analyses, is 62'41 per 

 cent. As these analyses are only partial, they are not published. They 

 show variations from 49 to 62 per cent of silica, with a gradual falling off 

 in soda and potash as the rocks develop more and more magnetite and 

 olivine. The most basic basalt examined yielded about 49 per cent 

 of silica. 



By reference to the table of complete analyses it will be seen that the 

 lime, magnesia, and oxides of iron increase from the acidic to the basic end 

 of the series. Of these bases, lime is the most regular in its behavior and 

 presents the widest range, starting with less than 1 per cent in the rhyolite 

 of Pinto Peak and reaching over ] per cent in the dike of intrusive basalt 

 which cuts the pyroxene-andesite near the summit of Richmond Mountain. 

 It should be borne in mind that the Pinto Peak rock carries no ferro- 

 magnesian minerals and the feldspars are for the most part sanidine. Mag- 

 nesia stands second in this uniform increase, but is wholly wanting in the 

 rhyolites, coining in with the first appearance of the ferro-magnesian- 

 silicates and increasing rapidly with the development of pyroxene and 



