PORPHYRITIC RANGES OF THE DESERT. 131 



disseminated in bundles through the mass. Epidote is in places substituted for felspar, and 

 sometimes increases in amount until the whole mass becomes of an uniform greenish tint from 

 its predominance. This variety of granite is a very common rock at the base of the isolated 

 hills immediately east of the fort, and lying between it and Big Horn mountains. It forms the 

 base of the dome hills which lie northeast of the fort^ and cross the river higher up. 



GEOLOGY OF THE GILA RIVER. 



This river in its course westward has formed its channel by wearing its way through the 

 softer strata and more recent deposits. Like some of the large rivers of the west of this conti- 

 nent, its course is across the strike of the mountain ranges, and seems to occupy the line of a 

 great depression or fault lying at the southern termination of the Sierra Blanca and MogoUon 

 ranges. In this respect it resembles the Humboldt and Columbia rivers further north, and 

 differs from the streams of California, which, almost without exception, roll in the valleys 

 corresponding with the trends of the mountain axis. 



For the last 60 miles of its course the Gila has escaped from this fault, and traverses the 

 great sloping plain constituting the northeastern boundary of the desert land — the Colorado 

 and Sonora desert. In its passage toward the Colorado river it crosses the line of several 

 isolated hills, whose general trend is northwest and southeast. Round and between these hills 

 it winds its way, having worn itself a channel or bottom several feet below the general level, 

 producing well marked terraces or mesas upon the sides of the hills upon which it impinges. 

 The river bottom within ten miles of its junction varies from ^ to ^ mile in width ; and by the 

 height of the drift shows that the overflow is occasionally 12 feet in depth. The growth is 

 obione, willow, and acacia, and upon the terraces an occasional cereus, which commences to 

 appear about 14 miles east of the fort. The soil of the bottom land the first 12 miles up the 

 Gila is brownish clay, mixed with pebbles of quartz, felspar and jaspery silicious rock. The 

 impalpable soil is calcareous, effervescing very strongly. Below this alluvium, which in many 

 places is not more than three feet deep, is a bed of sand, blackened by a layer of mica crystals? 

 which are deposited in horizontal strata. 



Dome hills, the first series of hills east of Camp Yuma met with ascending the Gila, are of 

 a blackish and red brown color, and rugged outline, destitute of vegetation, and composed of 

 jaspery metamorphic quartz, having very little trace of sedimentary origin, cut up by veins of 

 translucent quartz and dolerite, and passing gradually into amygdaloid, with imbedded glassy 

 quartz crystals, a felspathic trachytic rock. 



The strike of this range is north 65° west, and can be traced by the eye fifty miles (or 

 further) to the north, where it reaches the Colorado, and crossing that river forms the turretted 

 hills on the Colorado desert. The summits scarcely attain a thousand feet of elevation. At 

 the base, forming the terrace over which the trail runs, is granitic rock, very friable, contain- 

 ing small hexagonal mica and albite. Between these porphyritic hills, which lie 14 miles 

 east of Fort Yuma, and the next series of hills, (Big Horn or Goat mountains,) a distance of 

 45 miles, lies an extended level plain, slightly sloping to the southwest, unbroken in its level, 

 except by the river, where it has worn its way through it ; this is the northern limit of the 

 Sonora desert. The material of the surface is granitic or felspatic, modified by the addi- 

 tion of a few minerals, derived from the occasional elevations or low hills which rise abruptly 

 out of the plain to the height of 300 or 500 feet, composed of quartz, conglomerate and mica- 

 ceous sandstone of a dark red or brown color, and without any traces of lamination or deposit, 



