DIFFERENTIATIOX OF LAVAS. 2l-(7 



tively large quantity of pyroxenie magma, but less basic. If the lava 

 which crystallizefl out from this latter magma upon cooling is forced 

 upward to the surface, it may consist of both j)yro.\ene-an(lesite and basalt, 

 as at Eureka. It may be wholly a noniial l)asalt, as shown in a Huiiil)er 

 of localities in the Great Basin, or it niay be largely made up of magnetite 

 and other iron minerals, forming a basic rock not yet recognized in the 

 Great Basin, but known elsewhere at several widely separated places in the 

 world. It is a matter of observation in many localities that where the bulk 

 of rhyolite is excessive the basalt outflows frequently occur in small l)odies, 

 and it will probably be found that where there are relatively large basic 

 flows a portion of them will at least show an andesitic habit. 



Differentiation of Lavas.— The cxisteuce at Eureka of two groups of lavas, 

 difteriug primarily in structure and the chemical nature of their transitif)n 

 products, has been clearly demonstrated and evidence has been advanced 

 to show' that they were derived from a still earlier molten mass. Processes 

 of differentiation similar to those by which the molten material lieneatli the 

 surface is sujjposed to be capable of breaking up into rhyolite and basalt, 

 are sufficient not only to account for the breaking up of a primordial 

 mass into a feldspathic and pyro.xenic magma, l)ut also to account for the 

 existence of partial magmas and an entire series of transition lavas such as 

 found at Eureka. The first products of .such a molten mass would naturallv, 

 but not necessarily, be a lava of intermediate composition, such as are 

 often seen as the earlie.st eruptions in volcanic centers. The first ei'uptions 

 at Washoe being earlier than those at Eureka were consetjuently more 

 uniform in composition. Differentiation in the magma had taken place 

 only to a limited degree, and it is by no means easy to distinguish 

 hornblende-aiidesite from pyroxeue-andesite. Tiie splitting up of both 

 the feldspathic and pyroxenie magmas, the former into hiiniblende- 

 mica-andesite, dacite, and rhyolite, and the lattei- into pyroxeue-andesite 

 and basalt, has already been described. It is difficult tt) conceive a con- 

 trolling physical force acting u])on one magma which could not under 

 similar conditions of heat and pressure exert the same influences uj)on 

 fractional magmas, the difl'erentiated products of a primordial molten mass. 



