CHAPTER IV. 
IGNEOUS ROCKS. 
The igneous rocks of the district are both intrusive and effusive. 
The intrusives are biotite andesite laccoliths intruded into the Paleo- 
zoic and to a less extent into the Mesozoic rocks after the deposition 
of the Tertiary sediments. The effusives are later and form a bedded 
series varying in thickness from 1,000 to 2,000 feet and consisting 
of rhyolitic, trachytic, and andesitic flows, tuffs, and breccias. 
The lavas rest on the eroded and upturned edges of the Eocene 
and Cretaceous sediments, indicating a considerable period of erosion 
between the intrusion of the biotite andesite and the outpouring of 
the effusives. The latter have been correlated with the Miocene 
lavas of the Wasatch Mountains, hence the andesite intrusion is 
post-Eocene and probably early Miocene. 
The Quaternary basalts, present on all sides of the area, are not 
present within the district. 
LACCOLITHS (EARLY MIOCENE). 
DISTRIBUTION AND STRUCTURE. 
The laccoliths are exposed in three main areas forming the cores 
of the principal mountains of the district — The Three Peaks and 
Granite Mountain in the northeastern and Iron Mountain in the 
southwestern part. A fourth, Stoddard Mountain, lies mainly beyond 
the southwest side of the area mapped. The laccolith areas are cir- 
cular, with local irregularities due to faulting and other causes. 
Northwest of the The Three Peaks a small area of biotite andesite 
is brought up in contact with the Claron limestone by a great fault. 
Northwest of Iron Mountain a wide dike of andesite breaks through 
the Pinto sandstone a short distance from the main laccolithic mass, 
while to the southwest, surrounded by sediments, are several small 
andesite areas, parts of the large andesitic mass exposed through 
erosion of the low-dipping overlying limestone. In The Three Peaks 
and Iron Mountain laccolithic areas there are small patches of sedi- 
ments faulted down into the andesite or left as erosion remnants. 
In general the sediments dip away from the andesite, although 
locally, as east of Iron Mountain, the dip is steeply toward it, sug- 
gesting overturned strata. The strike of the beds is always parallel, 
or nearly so, to the andesite contact, except where faulting has 
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