GEOLOGIC HISTORY. 59 
correspond with those found in the Plateau region and would form 
another link in the chain of evidence that the pre-Quaternaiy course 
of Colorado River was through southern Nevada, to the north of the 
present canyon. 
Eruption of rhyolite and younger andesite. — After the formation of 
the peneplain, extensive masses of rhyolite and andesite, in the form 
of tuff, breccia, and flow, were spread over its surface, filling the 
depressions and covering all but the higher parts of the land surface. 
Although largely removed by later erosion, these lavas still cover 
wide areas, such as Black Mesa, a large part of the Truxton Plateau, 
and the mesa in the vicinity of Kingman. These lavas occur also to 
the east beyond the area described, and far to the south, where they 
form Castle Dome and the Chocolate Mountains in southwestern 
Arizona. 
In places where the rhyolites and younger andesites occur in more 
or less regular horizontal beds, as near Kingman and in Black Mesa, 
they have a maximum thickness of about 3,000 feet. In more moun- 
tainous regions, as at the southern end of the Aquarius Mountains, 
they are apparently thicker. In other places, notably in the Truxton 
Plateau, they thin and disappear. The thickness of the rhyolites 
and younger andesites, and their areal distribution, indicate that 
they originally covered a large part of northwestern Arizona. 
The time at which these great masses of igneous rock were extruded 
is not definitely known. It was after the Mohave peneplain had been 
formed, at least in large part, for the lavas rest upon the eroded sur- 
face. Furthermore, the amount of erosion which followed the extru- 
sion of these lavas places this event far back in the Tertiary. For 
these reasons, and because of the evidence that the close of the Mio- 
cene was a time of mountain forming and of great volcanic disturb- 
ance in the Great Basin and the mountains west of it, the extrusions 
of rhyolites and younger andesites may be provisionally referred to 
the close of the Miocene. 
Formation of the Grand Wash Trough. — Sometime during the mid- 
dle Tertiary, and apparently before the completion of the Mohave 
peneplain, the Grand Wash fault was formed, with a displacement of 
several thousand feet, giving rise to the Grand Wash Trough. This 
faulting probably corresponds to the early displacements of the Hur- 
ricane and other faults described by Huntington and Goldthwaite, 
and should not be confused with the faulting which accompanied the 
later uplift of the plateau. 
Deposition of the Greggs breccia. — At the north end of the region 
described local deposition followed the extrusion of the rhyolites and 
younger andesites. These sediments in the vicinity of Temple Bar 
lie unconformably beneath the Temple Bar conglomerate and consisl 
a Huntington, Ellsworth, and Goldthwaite, J. W., op. cit., p. 
