56 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
the successive zones of growth being very thin, owing to rapidly 
alternating conditions in the formation of the crystal. There is con- 
siderable alteration along fracture lines, the most important decom- 
position product being calcite. The feldspars have inclusions of 
apatite and biotite. The biotite is dark brown and greenish brown, 
highly pleochroic, and has inclusions of feldspar. The diopside is 
light green in color and has numerous inclusions of magnetite. It 
is often very much decomposed, the products being calcite and fer- 
rite. Fragments of quartz, which frequently show resorption, are 
present. The hornblende is of the brown basaltic variety. Frag- 
ments of magnetite are abundant. 
The groundmass is amorphous and frequently shows flow struc- 
ture. Through it are scattered tiny specks of the same minerals as 
compose the phenocrysts. 
Late tuffaceous rhyolite. — The late tuffaceous rhyolite has the same 
character throughout, except for an agglomerate at the base. The 
latter consists of big bowlders and smaller fragments of dark igneous 
material, both acidic and basic. About halfway up in the tuff forma- 
tion there is, in a few places, a thin layer of coarse sandy material, 
which appears to be water deposited. Frequently the upper part of 
the formation contains numerous concretion-like spheres, ranging 
from 1 inch to 5 inches in diameter, which are very much harder and 
darker than the rest of the rock. These have the same minerals as 
the rest of the formation, but the groundmass is denser and the 
cavities have all been filled with chalcedony. 
The late tuffaceous rhyolite has a porphyritic texture with pheno- 
crysts of feldspar, quartz, biotite, and hornblende, and a porous 
glassy groundmass. The main phenocryst is quartz, which is very 
fresh and generally shows crystal outlines. Orthoclase and plagio- 
clase are present, but not abundant. Most of them have suffered 
surface alteration. The biotite is frequently twinned and has inclu- 
sions of apatite. Fragments of hornblende and pyroxene are present, 
the latter being in included rock fragments. Magnetite particles 
are scattered through the rock. 
The groundmass is amorphous with numerous cavities filled or 
lined with calcite. For this reason calcite forms quite a large per- 
centage of the rock. 
Biotite-JiornUende-pyroxene andesite. — In the Swett Hills the bio- 
tite-hornblende-pyroxene andesite bed consists of several types of 
rock. In the western part there is at the base a dark hornblende 
andesite breccia and above this several layers of rhyolite and tra- 
chyte interlayered with limestone and conglomerate. In the eastern 
part all but the breccia disappear, and a biotite-hornblende- pyroxene 
andesite, which to the west only caps the lower series, makes up 
