58 KECONNAISSANCE OF PART OF WESTERN ARIZONA. 
Eruption of andesite. — The oldest effusive rock thus far found in 
western Arizona is the andesite in the vicinity of Gold Roads, a min- 
ing camp on Black Mesa, west of Kingman. The andesite rests upon 
granite and underlies an extensive series of rhyolites and younger 
andesites, but no further evidence of its age was obtained. It may 
be a time equivalent of the great masses of andesite comprising the 
Pine Valley Mountains and. resting upon the Eocene sediments in the 
southwestern part of Utah. These masses, according to Huntington 
and Goldthwaite, were probably extruded at the close of the Eocene. 
The provisional reference of the Arizona andesite to the close of the 
Eocene is based on the fact that, in the general sequence of events 
here described, its extrusion occurred between the period of early 
Tertiary erosion and the long period of base-leveling, during which 
part of the Mohave peneplain was formed. (See p. 59.) 
Crusted movements and erosion.— Either during or shortly after the 
extrusion of the older andesites further uplift apparently occurred in 
western Arizona, followed by renewed erosion, which carried away 
much of the andesite and again exposed the granite. The Paleozoic 
sediments that formerly extended much farther to the west were 
eroded away at about the same time and the region reduced to a 
peneplain, parts of which are still preserved in the Truxton Plateau 
and in the plateau lying south of Iceberg Canyon, in which soft and 
hard layers alike are truncated. This may prove to be a part of the 
Mohave peneplain in southwestern Utah, which in the opinion of 
Huntington and Goldthwaite 6 was formed after the period of folding 
of the early Miocene. 
Certain phenomena described by Spun*/ from Meadow Valley Can- 
yon, a tributary of Virgin River in southern Nevada, strengthen this 
supposition. He describes a succession of rocks very similar to those 
found in northwestern Arizona. Extensive beds of rhyolite rest on 
an eroded surface of older rocks and are overlain by extensive depos- 
its of conglomerate and soft sandstone which he refers with a query 
to the Pliocene, and these in turn are overlain by basalt and Pleisto- 
cene gravel. His description of the Pliocene (?) conglomerate and 
its position in the sections given indicate that it is probably equivalent 
to the Temple Bar conglomerate, which is here regarded as Pleisto- 
cene. Fig. 16 of Spurr's paper shows faulted Paleozoic sediments 
peneplaned and overlain by the conglomerate. This faulting may 
prove to be part of the early faulting of the Plateau region, and the 
peneplain beneath the conglomerate to be part of the Mohave pene- 
plain, in which case the relations in Meadow Valley Wash would 
a Huntington, Ellsworth, and Goldthwaite, J. W., The Hurricane fault in the Toquerville district, 
Utah: Bull. Mus. Comp. Zool. Harvard Coll., vol. 42, 1904, p. 217. 
b Idem, p. 226. 
fldem, p. 222. 
dSpurr, J. E., Descriptive geology of Nevada south of the fortieth parallel and adjacent portions of 
California: Bull. IT. S. Geol. Survey No. 208, 1903, pp. 139-148. 
