16 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
These fossils, according to Girty, indicate a Pennsylvanian or u Coal 
Measures" age. 
Lower in the section, near Nelson, Ariz., Mississippian forms were 
obtained as follows: 
Spirifer striatus var. madisonensis. 
Straparollus sp.* 
Menophyllum excavatum. 
Schuchertella inaequalis. 
Spirifer centronatus. 
Girty states that this is the Eomississippian fauna, which has a wide 
range over the West, correlating with the lower " Wasatch " a lime- 
stone of Utah, the Madison limestone of Yellowstone Park, and the 
Chouteau limestone of Missouri. 
The Redwall limestone has been described by Gilbert b as consisting 
of " Upper Carboniferous" above and " Lower Carboniferous" below, 
and King c makes a similar division of the " Wasatch" limestone in 
Utah. It is probable that in the area here described the line of 
separation between the Pennsylvanian and the Mississippian is to be 
drawn between the upper massive and the lower laminated divisions 
of the Redwall. 
TERTIARY. 
Eruptive rocks. — If sediments of Triassic, Jurassic, or Cretaceous 
age were deposited in western Arizona, they were removed previous 
to the opening of the Tertiary period. The Tertiary rocks are prin- 
cipally effusive andesites and rhyolites, which occur in isolated moun- 
tain masses and in broad sheets having a maximum thickness of 3,000 
feet or more. The oldest effusive rock is the andesite of the northern 
end of Black Mesa. (See p. 27.) It is a dark-colored porphyritic 
rock lying above the granite and beneath the rhyolites and later 
andesites. 
The older andesite is overlain by extensive beds of rhyolitic ash, 
tuff, and flows, which extend over a large part of western Arizona, and 
which were in turn followed by extrusions of andesite. The rhyolites 
and younger andesites are closely associated and must at present be 
described together. (See pp. 83-87.) They are conspicuously exposed 
in the Black Mesa, White Hills, Kingman Mesa, and Aquarius Moun- 
tains, but extend eastward beyond the area mapped, and farther south 
occur in extensive masses in the Chocolate and Dome Rock ranges. 
Some of the older basalt sheets of the region may also belong to the 
Tertiary, but this is not definitely known. They cut the rhyolites and 
andesites in dikes and overlie them in sheets. East of Chloride they 
a Present usage of the Geological Survey restricts the name Wasatch to the Eocene formation. As 
soon as sufficient detailed work can be done, another name will be assigned to the Carboniferous forma- 
tion. 
?' Gilbert, G. K., Final Rept. U. S. Geog. Surv. W. 100th Mer., vol. 3, 1875, pt. 1, p. 178. 
c King, Clarence, Final Rept. U. S. Geol. Explor. 40th Par., vol. 1, p. 155. 
