DIFFERENTIATION OF LAVAS. 287 



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

 which crystallized out from this latter magma upon cooling is forced 

 upward to the surface, it may consist of both pyroxene-andesite and basalt, 

 as at Eureka. It may be wholly a normal basalt, as shown in a number 

 of localities in the Great Basin, or it may 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 bodies, 

 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 existence at Eureka of two groups of lavas, 

 differing primarily in structure and the chemical nature of their transition 

 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 beneath the 

 surface is supposed 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 pyroxenic magma, but 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 naturally, 

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

 often seen as the earliest eruptions in volcanic centers. The first eruptions 

 at Washoe being earlier than those at Eureka were consequently 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 

 homblende-audesite from pyroxene-andesite. The splitting up of both 

 the feldspathic and pyroxenic magmas, the former into hornblende- 

 mica-andesite, dacite, and rhyolite, and the latter into pyroxene-andesite 

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

 trolling physical force acting upon one magma which could not under 

 similar conditions of heat and pressure exert the same influences upon 

 fractional magmas, the differentiated products of a primordial molten ma. 



