SUMMAEY. 291 



became more and more basic until the close of the volcanic period. The 

 feldspathic and pyroxenic lavas do not approach each other in their tenure 

 of silica within 2 - 25 per cent. In chemical composition the earliest erup- 

 tions of both magmas resemble each other, but from this common ground 

 they differentiate steadily until the feldspathic reaches the extreme acidic, 

 and the pyroxenic the extreme basic end of their respective series. The 

 extreme products of differentiation in any volcanic center in the Great 

 Basin are rhyolite and basalt. 



