140 
VOLCANIC ROCKS—CANONS OF THE GILA. 
Plate X, fig. 4, displays the geological character at the place where the river leaves the moun¬ 
tains ; the stream taking the advantage of the chasm between the plutonic axis and the 
elevated strata on its southern side. The upheaving mass on the north side of the river is 
variously composed—thus, commencing at the water’s edge and ascending the right bank of the 
river: 
1st. Hornblende porphyry, (felspar and hornblende, in crystals.) 
2d. Basalt. 
3d. Hornblende porphyry ; crystals of hornblende, large. 
4th. Amygdaloidal trachyte; a brownish felspar rock. 
5th. Felspar in dykes, walled by 
6th. Trachyte. 
The basaltic dyke has a thickness varying from 15 to 20 feet, and rapidly merging into the 
hornblende porphyry on either side. The felspar dyke has a greater breadth and does not 
merge into the trachytic amygdaloid, with which it is in contact. 
This intrusive mass of varied plutonic character is itself a foreign body, thrust through the 
granitic basis which here and there (though not appearing in the immediate locality) shows 
itself. In its elevation it has carried up with it two beds of stratified sandstone and a capping 
of basalt and purplish trachytic amygdaloidal felspar. The latter is a layer about six feet in 
thickness, and underlies the basalt, which is about 15 feet thick. The great mass of stratified 
rock is a yellow sandstone conglomerate, which dips away from the central mass at an angle 
of 15°, and generally toward the east; hut at this point there is no uniform dip over any 
extent of ground. South of this point a few hundred yards is a canon in the bed of a creek, 
emptying itself lower down into the Gila, where the sandstone is better displayed and slopes 
gently, with a dip 12° east; it is here yellow conglomerate, about 60 feet thick; the pebbles 
being granitic and volcanic. It was up this creek the trail led to reach the Gila, as it is 
scarcely safe to travel through the canon along the immediate hank. Cotton-wood and good 
grass were on the narrow river bottom at the entrance of the canon. 
The following is the list of plutonic and igneous rocks met with at the canon and a few miles 
up the river on each bank. Plate XI, fig. 1, shows the structure of the canons. 
A. Felspar rock with quartz crystal disseminated, resembling u eurite.” 
B. Felspathic granitoid rock, with epidote. 
C. Felspathic rock, lamellar, (in dykes mostly.) 
D. Syenite. 
E. Amygdaloidal trachyte, felspathic rock with glassy crystals. 
F. Basalt, compact and amygdaloidal. 
All these varieties may he found congregated within the space of a few miles. Plate X, fig. 3, 
represents an exposure afforded by the river in travelling up, on July 6th; the section embraces 
ten miles. 
A few ranges in the lower part of the canon, though of an elevation from 500 to 800 or even 
1,000 feet, are yet comparatively narrow in their base or in cross section, and appear to he made 
up wholly or in great part of variety O. Such are the “ Spire” hills, which present an appear¬ 
ance alike fantastic and grand ; from the warm tone of the flesh-colored felspar, the absence of 
vegetation, its extreme roughness in ascent from the huge masses of rock on its side and base, 
and the pointed pinnacles and turrets which its outline, sharply defined on a clear sky, presents, 
the observer is forcibly struck with the singularity of the landscape. 
