CHAPTER XX. 
CANONS OF THE GILA RIVER AND PINALENO MOUNTAINS. 
Basalt overflow.—East of the pimas villages.—Physical appearance and topography of the canons.—Trend of the 
HILL RANGES.-SADDLE MOUNTAINS.-BREADTH OF THE PRIMARY ROCK.-ELEVATION OF THE SEDIMENTARY BEDS, EASTWARD. 
Subsequent alterations of level.—Plutonic forces.—Aggregate of igneous rocks.—Spire hills, their structure.— 
Basalt dykes.—Metamorphic sandstone.—Its lithological character and geological age.—San pedro river.—Its 
course.—Fertile valley, gypseous beds of.—Denudation, thickness, position, variety and probable age of.—Con¬ 
glomerate OF THE UPPER PART OF THE VALLEY.—THICKNESS AND POSITION.—AURIFEROUS GRAVEL. 
Thirty-two miles east of the Pimas villages, along the river course, a region of basaltic 
overflow is again met with. The soil of the bed of the river is light, sandy and granitic, and 
the pebbles volcanic greenstone—basalt, jasper, greenish and reddish porphyry, with brown 
fine conglomerate. The river is terraced on each bank, composed of fine gravel drift overlying 
basalt, which appears to flow in a sloping mesa or plain from the north, where a pyramid 
shaped hill appears to present the outline of an ancient crater. Three miles further up the 
river the country becomes greatly disturbed, and the elevated crests, which run north and 
south, crossing the river, narrow its bed, and form what is known as the canons of the Gila. 
This is the commencement of a region very much disturbed in its geological relations and 
presenting a great deal of confusion in the appearance and stratification. 
At the entrance of the canon, and ascending an elevated point over the river, (camp July 6,) 
the topography of the vicinity appears simple. So many as six ranges of hills can be seen, 
coming from the north and running southward, (S. 40° E.) These hills, generally speaking, 
increase in altitude to the eastward, the last of which in view has the well known form of the 
Saddle mountains in its range. Through this last range, and north of Saddle mountains, the 
Gila canons, and in its downward course it cuts its way across those smaller ranges to the west, 
until finally it emerges on the open plain leading to the Pimas lands. 
This belt of mountain country, travelled in this route, is 34 miles from the entrance of the 
canon to the mouth of the San (Jose) Pedro river. This breadth of country is wholly occupied 
by igneous and erupted rock. The western or lowest hills are of plutonic and primary rock, 
while the more eastern are sedimentary and capped with basalt and amygdaloidal trachyte. 
Upon many of these flat capped summits trees and vegetation grow, and they appear to have 
been at one time the level surface of the country. Several hundred feet below these is at present 
the river bed ; and its size and volume appear totally incapable of having produced such results 
of denudation and removal as must have occurred. Two forces have been at work : 1st. The 
primary upheaval of the granitic basis, carrying the sedimentary beds with it; and, 2d. The 
plutonic force, as evidenced by the amygdaloid and basaltic lavas. These are best displayed at 
the western end of the canon, where the river leaves the mountain region. At the very point 
where it finally escapes the river bed exposes a section of an anticlinal axis with plutonic 
intrusions, and the small valley around has much the appearance of a centre of plutonic action, 
judging from the high dip of the strata and the semicircular form of the igneous rocks around. 
