140 VOLCANIC EOCKS — CANONS OF THE GILA, 



Plate X, fig. 4, displays the geological cliaracter at the place where the river leaves the moun- 

 tains ; the stream taking the advantage of the chasm hetween 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 : 



Ist. Hornblende porphyry, (felspar and hornblende, in crystals.) 



2d. Basalt. 



3d. Hornblende porphyry ; crystals of hornblende, large. 



4th. Amygdaloidal trachyte ; a brownish felsjjar 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 ; but at this point there is no uniform dip over any 

 extent of ground. South of this point a few hundred yards is a caiion 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 caiion along the immediate bank. Cotton-wood and good 

 grass were on the narrow river bottom at the entrance of the caiion. 



The following is the list of plutonic and igneous rocks met with at the caiion and a few miles 

 up the river on each bank. Plate XI, fig. 1, sliows the structure of the caiions. 



A. Felspar rock with quartz crystal disseminated, resembling " 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 be 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 caiion, 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 be made 

 up wholly or in great part of variety C. 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. 



