CHAPTER XX. 



CANONS OF THE GILA RIVER AND PINALENO MOUNTAINS. 



Basalt oveuflow.— East of the purAS villages. — Physical appeahanob and topographt op the oaSons. — Trend or thb 



HILL RANGES. — SADDLE MOUNTAINS. BrEADTH OF THE PRIMABT HOCK. ElEV.^TION OF THE SEDIMENTARY BEDS, EASTWARD. 



SCBSEQUENT ALTERATIONS OF LEVEL, — PlUTONIO FORCES. — AoOREQ ATE 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 porjjhyry, 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 caiions 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 caiion, 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 caiions, 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 caiion, 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 ar nind. 



