48 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
is slightly above the mean density of any combination of the over- 
lying sediments. The density of the Iron Springs laccoliths, now 
2.65, was considerably lower before solidification, presumably lower 
than 2.54, the mean density of the overlying sediments, suggesting 
that the Iron Springs laccoliths came to rest not in places deter- 
mined by the law of hydrostatic equilibrium but in places determined 
by the competency of the restraining limestone and sandstone strata; 
in other words, the Iron Springs laccoliths, if their density at the 
time of the intrusion was that common to acidic intrusives of this type, 
would have been intruded more nearly at the surface had it not 
been for the strength of the restraining limestone. 
PETROGRAPHY. 
The intrusive andesite is the rock with which the iron-ore deposits 
are associated. Deposits are found on the borders of all the andesite 
areas except Stoddard Mountain, where its absence is probably due 
to the absence of limestone. 
The rock is a light-gray biotite andesite with porphyritic texture. 
The phenocrysts, consisting of feldspar, biotite, hornblende, and 
diopside, are numerous, occupying more than half of the rock mass, 
and varying in size up to an eighth of an inch in diameter. The 
most abundant phenocryst is plagioclase of the variety labradorite, 
but orthoclase is occasionally present. A large number of feldspars 
show zonal growth. They are comparatively fresh, showing altera- 
tion only along cracks, along lines of zonal growth, and on the sur- 
face. The alteration products are calcite, kaolin, quartz, and seri- 
cite, typical katamorphic products. The next most abundant 
phenocryst is biotite, in shiny black hexagonal plates, often altered 
to phlogopite with a golden luster and frequently having reaction 
rims of magnetite. In the Stoddard Mountain area and parts of the 
other areas, the biotite is almost entirely decomposed to ferrite. In 
a few cases it has altered to green chlorite. The biotite has abun- 
dant inclusions, mainly apatite but sometimes quartz, magnetite 
and zircon. The hornblende is generally in dark-green prismatic 
crystals, with inclusions of magnetite, biotite, and quartz. Like 
the biotite, the hornblende in the andesite of the Stoddard Mountain 
area is almost entirely decomposed to ferrite. The diopside is of a 
light-green color, and is noticeably associated with magnetite, which 
is present as inclusions and around the borders. It is generally 
more or less altered to uralite along cracks and around the border. 
Fragments of magnetite are abundant throughout the rock. Ferrite 
is present as an alteration product of the ferrous silicates. 
The groundmass is cloudy from alteration, but seems to be com- 
posed mainly of fine crystalline quartz and feldspar, both orthoclase 
and plagioclase. Biotite, hornblende, pyroxine, and magnetite are 
also represented, but less abundantly. 
