SEDIMENTARY FORMATIONS. 37 
the large-scale detail maps of the ore deposits (Pis. Ill- VI, in pocket) 
the lower sandy phase and the metamorphosed phases of the blue 
limestone have been separated from the typical blue limestone but 
not from each other. 
Unless the structural relations are clear, both the sandy and sili- 
cated contact phases of the limestone are likely to be confused with 
the Cretaceous sandstone which occurs in isolated patches in the 
andesite. 
FOSSILS. 
The fossils found in the Homestake limestone are few and poorly 
preserved. Only one genus, Aviculopecten, ranging from Silurian 
to Triassic, was identified by Prof. Eliot Blackwelder, of the Univer- 
sity of Wisconsin. There are fragments, however, which appear to 
be referable to Ed?nondia, a Carboniferous genus. 
STRUCTURAL RELATIONS AND THICKNESS. 
The Homestake limestone is intruded by the laccoliths. The 
contacts are usually nearly vertical, sometimes because of later fault- 
ing. In a few localities, as at the Desert Mound, in the southwestern 
part of the Iron Mountain laccolith, and south of The Three Peaks, 
the limestone rests against and upon the laccolith with a gently dip- 
ping plane of contact, though locally even here the contacts are steep 
because of faulting. In the circular ridge east and southeast of the 
Iron Mountain laccolith the dip of the Pinto sandstone and the Claron 
formation is steeply toward the andesite, suggesting an overturned 
fold in which the Homestake limestone probably participates. 
The Homestake limestone is overlain conformably by the Pinto 
sandstone, with a shale at the base. 
The thickness of the Homestake limestone, as shown by exposures, 
ranges from 500 to 50 feet. The average thickness has been taken to 
be about 200 feet. The variation is probably due to intrusion of 
laccoliths at different horizons. If the intrusion has followed the 
base the variation in thickness may indicate unconformity with the 
overlying Pinto sandstone. 
CRETACEOUS SYSTEM.** 
PINTO SANDSTONE. 
DISTRIBUTION. 
The Pinto sandstone for the most part borders the Homestake 
limestone on the side away from the laccoliths. On the west side of 
The Three Peaks it extends several miles westward from the band of 
Homestake limestone and disappears under the desert clays and 
a Jurassic in part, perhaps. 
