MAGMATIC ALTERATION OF CERTAIN MINERALS 281 
involved in an eruption and the conditions under which the 
magma reaches the surface are probably so complex and cer- 
tainly to so large an extent unknown to us that any explanation 
of such phenomena must be, after all, largely mere speculation at 
the present time. 
Still it must be granted that the explanation of the above 
facts based on the process and consequences of alteration are 
within the bounds of reason, and our brief review points rather 
to than away from the conclusion that alteration of hornblende 
or biotite is responsible for a great part of the acid augite-ande- 
sites, as well as some of the more basic members of the group. 
The foregoing remarks lead us directly to the consideration 
of another application of the ideas advanced in these pages to 
theoretical petrology, which may be briefly touched upon before 
closing this paper. I refer to the succession of erupted rocks at 
volcanic centers. In many places we find pyroxene-andesites 
among the later rocks following hornblende or biotite-andesites. 
Thus in the Eureka district the order of succession as given by 
Hague* is: hornblende-andesite, hornblende-biotite-andesite, 
dacite, rhyolite, pyroxene-andesite, and lastly basalt. Again at 
Sepulchre Mountain? the lower breccias are hornblende or biotite- 
andesites, while the upper and consequently later breccias are 
almost wholly of pyroxene-andesite. While recognizing the 
fact that the causes and circumstances involved are extremely 
various and complex, it may be suggested that such successions 
may be due in part to causes similar to those already spoken of 
in connection with the variation of mineralogical composition 
with geological occurrence; namely, that the hornblende or 
biotite of the original magma may have been replaced by augite 
during a period of liquidity under diminished pressure. 
SUMMARY. 
After a brief discussion of the current theories referring the 
alteration of hornblende and biotite to a resorptive action of the 
* HAGUE, Geol. Eureka Distr., Mon. XX, U.S. G. S., 290, 1892. 
2 IDDINGS, op. cit., 634, 635. 
