286 GEOLOGY OF THE EUKEKA DISTEICT. 



fissures, under precisely similar geological conditions, in too many localities 

 not to realize their mutual relations. Such occurrences appear far too 

 common the world over to permit us to suppose them to be derived from 

 wholly independent reservoirs, yet everywhere occupying the same relative 

 positions with the basalt superimposed upon the rhyolite. Basalt and 

 rhyolite may be the final products from the same common source, but not 

 necessarily differentiated by a simple process of specific gravity separation 

 as demanded by Mr. King. 



Within the area of the Great Basin there does not appear to be any 

 rock whose composition is due to a mingling of minerals characteristic of both 

 basalt and rhyolite. Both rocks, while they exhibit considerable range in 

 chemical composition, always remain sharply contrasted as regards mineral 

 constituents. Variations from normal rhyolite carrying orthoclase and 

 quartz in most instances show a transition toward hornblende-mica- 

 andesite through dacite, and never toward a pyroxenic magma, which 

 could hardly be the case if the process was due wholly to the dropping 

 out of the heavier minerals. Plagioclastic feldspars may be developed in 

 large numbers, but they belong to less basic species than those which char- 

 acterize normal basalt. In like manner variations from normal basalt tend 

 toward pyroxene-andesite and do not carry orthoclase. The process by 

 which the two magmas are formed is more in the nature of a differentiation 

 by molecular change and changes of density in the molten mass under 

 varying conditions of pressure and temperature than by a separation of 

 minerals during crystallization based upon differences of specific gravity. 

 In the Great Basin and probably all through the northern Cordillera con- 

 ditions were favorable in many localities for a complete differentiation of a 

 normal magma to its final products, rhyolite and basalt. 



Now, if we suppose a magma of intermediate composition, from 

 which the necessary material to form rhyolite has been withdrawn, the 

 chemical constitution of the residue will depend largely upon the quantity 

 of rhyolite produced. If the quantity of rhyolitic magma thus formed is 

 relatively large, the remaining basaltic magma may be correspondingly 

 small and necessarily basic in composition. Again, if the bulk of acid or 

 feldspathic magma which separated out is small, there will remain a rela- 



