DIFFERENTIATION OF LAVAS FROM A UNIFORM TYPE. H3 



but is occasionally met, more often than hornblende. In all the dacites and 

 rhyolites,, the dark mineral is almost exclusively biotite. The earlier andesite, on 

 the other hand, contains abundantly both hornblende and biotite, with some augite, 

 while the later andesite contains much augite and biotite, with some hornblende. 

 The basalt, again, contains abundantly both augite and hornblende, the latter often 

 partly or wholly resorbed by magmatic action and pseudomorphosed into aggre- 

 gates of iron-oxide crystals. No biotite is present. The presence, or evidence of 

 the former presence, of hornblende is thus shown in nearly all the Tonopah 

 volcanics, from the very siliceous to the very basic, and emphasizes their consan- 

 guinity. But the number of hornblende crystals (it is possible that some of these 

 pseudomorphs were also after augite) indicated by the pseudomorphs above 

 described as having been originally present in the unconsolidated rhyolitic magma 

 is large, being equaled only in the earlier andesites and the basalts. 



DERIVATION OF KHYOLITE AND HASA l,T FROM INTERMEDIATE MAKMA. 



Statement of theory. The Oddie rhyolite is considerably separated from the 

 earlier andesites in age, while it was nearly contemporaneous with the basalt of 

 Siebert Mountain. In this basalt the partly corroded and pseudomorphosed 

 hornblende crystals indicate that the hornblende was an earlier crystallization, not 

 entirety stable under the later conditions of the magma, which produced naturally 

 iittgite. That is to say, both the highly siliceous rhyolitic magma and the basic 

 basaltic magma developed, as first mineral, hornblende, which in each case was 

 unsuited to later conditions; the magma of the rhyolite became more siliceous and 

 alkaline, so that biotite was formed as the dark mineral, and that only sparingly; 

 the magma of the basalt became more basic and calcareous, so that abundant 

 augite was formed. If this is so, then these two magmas at the time of the Hrst 

 hornblende crystallization must have been more nearly intermediate in nature and 

 approached each other more closely; and as they were erupted at nearly the 

 same locality they may possibly have been nearly or quite the same. Such a 

 common intermediate magma might have a composition like that, for example, 

 of an andesite. These considerations would harmonize with the hypothesis that 

 the writer adopted several years ago, that the contemporaneous "complemen- 

 tary" rhyolites and basalts of the Great Basin region were the results of the 

 splitting up of a magma of intermediate composition. a 



jRhy elite-bandit differentiation theory tested by analyses. Complete analyses of the 

 basalt and of the Oddie rhyolite were, unfortunately, not made; one partial analysis 

 of each shows the relative amounts of silica, lime, and the alkalies. These analyses 

 may be compared in considering the theory that the basalt and the rhyolite are the 



"Succession and relation of the lavas of the Great Basin: Jour. Geol., vol. s, pp. 621-616. 



