SEDIMENTARY FORMATIONS. 39 
as east of the Homestake mine, south of Crystal Springs, and around 
the southwest corner of the Iron Mountain laccolith. They are com- 
paratively soft and friable and are generally of a deep maroon color; 
at the base are beds of white sandstone having deep red spots. The 
grains are mostly quartz and the different colors are due to iron oxide 
in the cement. 
The cherty limestone breccia (3), although thin, is another char- 
acteristic bed in the lower part of the Pinto formation. It is present 
around The Three Peaks laccolith, north of the Granite Mountain 
laccolith, and east of the Desert Mound, at all of which places it occu- 
pies the same part of the series. It is a dark grayish-blue brecciated 
limestone. The fragments vary in dimensions from a fraction of an 
inch to 6 inches, and are separated by narrow bands or veinlets of 
chert, which on weathered surfaces may project as much as a quarter 
of an inch above the rest of the rock. Under the microscope the 
fragments appear to be made up of exceedingly fine grains of calcite, 
with here and there veins of coarser calcite. 
The lower conglomerate (4) is associated, almost without excep- 
tion, with the brecciated limestone, but it is also found in parts of 
the district where the latter is not present. Wherever identified it 
has been mapped with the conglomerate symbol. It is exceedingly 
well cemented, so that on breaking, the fractures occur for the most 
part through the pebbles instead of around them. This is true 
especially in the northeast quarter of the area. The pebbles are 
chiefly quartz, black chert, quartzite, and dark limestone, the latter 
probably being obtained from the Homestake formation. The 
matrix is composed of rather fine-grained sand, the whole being well 
cemented by silica. 
Between the lower and upper conglomerates there are a number 
of sandstone and shale beds (5), none of which are continuous over 
any considerable area. West of The Three Peaks the lower part 
is characterized by light-green shale about 20 feet in thickness, 
overlain by a purple shale about half as thick. Above this are 
fine-grained flaggy sandstones, white, gray, and mottled in color. 
West of Granite Mountain and south of The Three Peaks the shales 
are entirely absent, being replaced by coarse- and fine-grained 
sandstone. 
The upper conglomerate (6) appears throughout most of the 
northeast quarter of the area. It consists of heavy conglomerate 
layers alternating with thin layers of sandstone, the pebbles com- 
posing the former being of the same kind as those of the lower 
conglomerate but as a ride larger, especially where the formation 
is thick, and less firmly cemented. Around the Iron Mountain 
laccolith only one conglomerate is present, presumably the lower, 
since at several places a maroon sandstone is found between it and 
