52 IRON ORES OF IRON SPRINGS DISTR T CT, UTAH. 
so than orthoclase. Quartz shows strong resorption, but is otherwise 
fresh and limpid. Under the high power, it shows inclusions. The 
biotite is black and shiny and has inclusions of apatite. Magnetite 
and ferrite are present in small quantities, the latter as an alteration 
product of the ferrous silicates. Zircon is rare. The groundmass 
is partly crystalline and partly eutaxitic and amorphous. The 
crystalline part appears to be mostly quartz and feldspar. There 
are numerous chalcedony-filled cavities throughout the rock. 
The band of early rhyolite along the northwestern front of the 
Swett Hills is bright pink with phenocrysts of quartz and sanidine, 
the latter with Carlsbad twinning. Both quartz and feldspar are 
fresh and glassy. The former frequently shows resorption. The 
groundmass is pink and dense, but with numerous little cavities. 
It appears to be amorphous with lenses of lighter colored crypto- 
crystalline material. 
The areas of lower rhyolite in the Eightmile Hills and the Har- 
mony Mountains, like those first described, are more or less tuffaceous 
in character, but as the groundmass becomes denser they grade into 
a fragmental rhyolite. The little neck of the early rhyolite bed west 
of Stoddard Canyon has a number of areas of trachyte and dacite 
very much like those in the purple trachyte east of Birch Canyon. 
These are very irregular in extent and distribution and could not be 
structurally so separated as to correlate them with the overlying 
trachyte and dacite. 
Later trachyte. — Wherever the later trachyte is present it is com- 
posed of 2 distinct layers — at the base a black trachytic pitchstone 
averaging 5 to 10 feet in thickness and above this a dense red trachyte. 
The pitchstone contains abundant phenocrysts of feldspar and 
biotite, and fewer of diopside, hornblende, magnetite, and apatite. 
The feldspars are largely sanidine, showing Carlsbad twinning. A 
little plagioclase is present. The feldspars are all fresh and unaltered, 
and have numerous inclusions of apatite and zircon. The biotite is 
dark brown and strongly pleochroic. It has few inclusions which 
are mainly zircon. Magnetite fragments are abundant, some of them 
showing alteration to hematite. Hornblende is rare. 
The groundmass is black and glassy and under the high power 
appears to be made up of innumerable crystallites with simple and 
branching hairlike forms grouped along lines of flow. In it are red 
spherulites, some of which in addition to the spherulitic structure 
show flow structure like the rest of the groundmass. 
The trachyte above the pitchstone is a reddish-brown porphyritic 
rock with phenocrysts of feldspar and biotite, about equally abundant, 
and together making up perhaps one-tenth of the mass of the rock. 
Both orthoclase and plagioclase are present, the latter being probably 
