IGNEOUS ROCKS. 57 
almost the entire thickness. In the Harmony Mountains the for- 
mation consists of rhyolite, trachyte, and conglomerate, while in the 
lava area southwest of Iron Mountain only the hornblende andesite 
breccia is present, with a little limestone and conglomerate. 
The biotite-hornblende-pyroxene andesite is a light pinkish-gray 
porphyritic rock, with feldspar as the main phenocryst. Biotite, 
hornblende, and diOpside are of equal importance. Quartz and 
magnetite are present. The feldspars are mainly labradorite, but a 
little orthoclase is present. They show zonal growth and have inclu- 
sions of apatite. A few fragments of quartz are present. The diop- 
side is light green in color, while the hornblende and biotite are dark 
brown and show alteration to ferrite. The groundmass is amor- 
phous with finely crystalline feldspar and specks of ferrite. 
The hornblende andesite breccia is a dark purplish-gray porphy- 
ritic rock, with phenocrysts of brown hornblende and a groundmass 
of small lath-shaped crystals of labradorite, orthoclase, monoclinic 
and rhombic pyroxene, separated by irregular cavities. The rhyo- 
lite and trachyte layers have the same texture as the underlying 
tufl'aceous rhyolite and trachyte formations. The limestones and 
conglomerate are like those of the Tertiary, but thinner and less con- 
solidated. Their presence in the uppermost lava formation shows 
that all the lavas were submerged after its deposition. 
CHEMICAL and mineral composition of the 
IGNEOUS ROCKS. 
Analyses have been made of the intrusive andesite and of the four 
principal flows, namely, the early trachyte (1), early rhyolite (2), lat- 
est trachyte (5), and dacite (7). Of the rest of the flows, some, while 
they have considerable horizontal distribution, are very thin, while 
the others are local in their distribution. 
The following table gives the chemical composition, the approxi- 
mate mineral composition of the same specimens determined from 
thin sections, and the mineral composition of the same specimens 
calculated as modes from the chemical composition for each of these 
formations, in order of age: 
