SOUTHERN PACIFIC LINES 221 
on a prominent serrated ridge which extends to July 4 Butte and 
probably marks a great igneous dike. The old mail road from Phoenix 
to Yuma crossed these mountains in a pass between Cimmerian and 
Woolsey Peaks and, descending Woolsey Arroyo, reached the bank 
of the Gila River at Agua Caliente (ca-liane’ tay). It passed Woolsey 
Well, formerly a favorite camping place, about 3 miles west of Woolsey 
eak, where some interesting geologic features are exposed. The 
basement lavas are overlain by conglomerate and sandstone of Ter- 
tiary age, tilted gently to the west. These rocks are capped by a 
sheet of basalt and intersected by several dikes and sills of basalt. 
Some of the boulders in the conglomerate are from 1 to 3 feet in 
diameter. 
About 2 miles northwest of Gillespie is the Old Dixie mine, where 
shale and andesitic agglomerate are cut by quartz monzonite por- 
phyry that was probably intruded in Tertiary time. 
ix~ =~ 
eae RS are ota 1 =s 
tr FP FueX, ete fe Se ie =~ 
Fagg oy iy) ‘Cagev!s AS ANSINITE 
ER Pe ee if FI eS 
a 
ae 
‘ 
ges 
SIN eS TCO 
FIGURE 60.—Section in Yellow Medicine Butte and adjoining mountai 
Arizona. Tb, basalt; Te, conglomerate; Tt, light tuff; Ta, andesite, etc.; 
Agr, granite 
On approaching Harqua siding the railroad passes through cuts of 
granite in a small exposure in the midst of the volcanic succession. 
From the divide at Harqua siding there is a rapid 
Harqua. descent into the wide alluvial flat of Quail Spring 
Po gana ag ~~ Wash. To the north from a point near Saddle siding 
miles. "there is an excellent view of Yellow Medicine Butte, 
which consists of a high southward-sloping cuesta of 
basalt capping a thick mass of tuffs. A fault traverses this cuesta, 
breaking it into two portions. This succession and the basement of 
granite on which it lies is general throughout the region, notably in 
the Montezuma Cuesta and its companion to the south; in Columbus 
Peak, where the dip is 20°; and in the Agua Caliente Mountains. 
The relations are shown in Figure 60. 
Beyond Papago siding a gap leads between basalt-capped mesas. 
Passing Montezuma and Camel sidings the lowlands on the north 
side of the Gila River are entered. To the north is a fine view of a 
feature known as Montezuma Face, which, as shown in Plate 32, A, 
presents a remarkably natural face profile looking upward, 
