234 Hague and Iddings— Volcanoes of California, ete. 
tion with chemical determinations has shown that hypersthene 
is the substitute for olivine in rocks differing from basalt by a 
somewhat higher silica percentage, the former mineral being, as it 
is, simply the bi-silicate of the same Fe, Mg, elements. It is not 
strange then to find rocks of similar structure and appearance 
differing solely in the relative proportion of olivine and hypers- 
thene, and passing by easy gradations from basalt to hypers- 
thene-andesite. 
Between hornblende-andesite and hypersthene-andesite occur 
varieties in which only a small amount of hornblende is found, 
and which with equal propriety might as hand specimens be 
considered as modifications of oné or the other types. 
n the same way forms are found intermediate between horn- 
of many colors, some bearing more or less hornblende and 
presenting the transition forms to hornblende-andesite, which 
occurs in abundance. The variety resembling domite is from 
anne Cone and Black Butte, Strawberry Valley near Mt. 
asta, 
accompanied by a variety of hornblende-andesite of most 
curious structure. It is a very porous evengrained mass com- 
posed of tabular plagioclases, hornblende and colorless glass, 
