40 IEON ORES OF IRON SPRINGS DISTRICT, UTAH. 
the Homestake formation. In the northeast quarter of the district 
this conglomerate also has been mapped. 
Above the conglomerate beds throughout the district is a great 
thickness of gray, yellowish-brown, or red sandstones (7), forming by 
far the largest part of the Pinto formation. These are normally 
coarse and friable, but where parts of the Pinto formation come in 
contact with the laccolithic andesite they are fine grained and 
quartzitic. The sandstone becomes coarser in its upper part, and 
near the top contains a layer of coarse conglomerate. During the 
erosion interval at the end of the Cretaceous this conglomerate was 
so largely removed that at present it is found only locally near the 
overlying Eocene deposits. 
Limestone lenses appear in the Pinto sandstone along the south- 
western edge of The Three Peaks, immediately southwest of Iron 
Springs, and in the middle of the valley halfway between these areas. 
Other lenses are exposed in Oak Springs Flat, east of Iron Mountain, 
where they are partly obscured by late unconsolidated deposits. 
These lenses are in the lower part of the Pinto formation, but their 
exact position in the preceding succession is not known. 
The limestones are much fractured and breccia ted. Their color 
varies, in different parts of the district, from dark gray to light gray 
and even pink. The latter is very much like the Claron limestone 
and is distinguished from it only by its brecciated character. 
CONTACT METAMORPHISM BY LACCOLITHS. 
At or near the contact of the Pinto sandstone with the laccolithic 
andesite the sandstone is quartzitic, and the yellow, brown, and 
red colors are replaced by gray and white. In places the rocks are 
mottled or have concentric dark-colored rings (PL XXI, A, p. 74), 
which, under the microscope, appear to be irregular aggregates of 
small elongated crystals of colorless amphibole. The metamorphism 
is commonly noticeable for a few hundred yards from the contact, and 
west of Iron Mountain it is seen as far as a quarter of a mile. 
FOSSILS. 
No fossils were found in the Pinto sandstone. This absence of 
fossils and the association of red sandstones and shales suggest ter- 
restrial deposition. 
STRUCTURAL RELATIONS AND THICKNESS. 
The relations of the Pinto sandstone to the underlying Homestake 
limestone have already been discussed. The relations with the 
overlying Claron formation are unconformable, as evidenced by a 
basal conglomerate and by the partial erosion of certain well- 
recognizable beds near the top of the formation. There seems to be 
