SUMMAKY. 291 



beciiiiu^ iiKii'c and more hasic until the close of the \olcanic period. Tlu^ 

 fel(Isj)atlii(' and px'roxenie la\as do not approach each other in tlieii' tenure 

 of silica within 2'25 per cent. In chemical composition tlie earliest ei-u|i- 

 tions of both magmas resemble each other, but from this common L;round 

 they differentiate steadily until the feldspathic reaches the extreme acidic, 

 and the pyroxenic the extreme basic end of their respective sei'ies. The 

 extreme products of differentiation in any volcanic center in the Grreat 

 Basin are rhyolit(i and l)asalt. 



