16 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
GENERAL GEOLOGY. 
The dominating geological features of the district are 3 large 
andesite laccoliths, constituting The Three Peaks (PL VII, A), 
Granite Mountain, and Iron Mountain, lying in a northeast-south- 
west line across the area mapped. Three unconformable sedimentary 
series, aggregating 4,000 feet in thickness, outcrop in successive 
rings around these laccoliths (PL VII, B), dipping outward asymmet- 
rically, very steeply at the contact, less steeply farther away. Still 
farther from the laccoliths lava flows 2,000 feet thick rest in nearly 
horizontal attitude on the tilted sedimentary rocks. These general 
relations are modified by faulting. All of the rock formations of the 
district are more or less covered on the middle and lower slopes 
by unconsolidated and partly consolidated erosion debris, both 
aqueous and subaerial, which spreads out on the lower ground to 
make the deserts. The detailed succession is shown in fig. 2. 
The laccoliths are exposed only in their upper parts — at no place 
has erosion shown their bottoms in section. The rock is an andesite 
of remarkably uniform texture and composition. Within the area 
of the laccoliths are a few veins of iron ore and fault blocks of ore 
and Carboniferous and Cretaceous sediments. 
In contact with the laccoliths for the most part is Carboniferous 
limestone, a pure, dense, blue limestone, with a few feet of sandy 
material appearing locally at the base. The contacts are at most 
localities nearly vertical, this being due partly to faulting and 
partly to the fact that erosion has cut down to the sides of the lac- 
coliths, exposing the vertical part of their contact. Locally, where 
erosion has not gone down so far, the limestone dips distinctly away 
from the andesite at an angle as low as 10°. The limestone is altered 
at the contact with the laccolith for a maximum of 1,000 feet, as 
measured on the erosion slopes, by loss of carbonates, development 
of anhydrous silicates, and replacement by ore. 
Cretaceous sediments outcrop in a zone outside of the Carbonifer- 
ous limestone, except where locally they are faulted down against 
the laccolith, or where the laccolith has penetrated the Cretaceous 
and erosion has not yet cut down to the Carboniferous limestone, as 
on the west side of Iron Mountain. The Cretaceous rocks are prin- 
cipally sandstone, with layers or lenses of shale, conglomerate, and 
limestone breccia. At the contact with the laccoliths the sandstone 
has been indurated and amphibole has developed. 
The relations of the Cretaceous sandstone to the underlying Car- 
boniferous limestone are those of apparent conformity, but the con- 
tact is rendered somewhat obscure by the presence of shale at the base 
of the Cretaceous sandstone. The lower portion may be in part 
Jurassic, but separation could not be made. The greatly varying 
