CHAPTER XIX. 



FORT YUMA TO THE PIMAS VILLAGES. 



Fort tuma. — Colorado. — Ancient tebrace and rivee bed.— Aspect of gila river near its modth. — Occasional overflows. — 

 Vegetation of river bottom. — Soil, limits, and constitution of the sonora desert. — Bir. horn mountains. — Geology of. — 

 Native copper ore — Remarks on. — Basalt overflow, its great extent. — Pimas jornada, geological structure of. — 

 Granitic axis. — Quartz veins. — Titanium ore.— Plain at the Maricopas wells. — Water of. — Influence op subterranean 



SPRINGS UPON vegetation. PiMAS VILLAGES. — CULTIVATED LANDS. — CHARACTER OF THE BOIL. — NECESSITY OP IRRIGATION. — 



Recapitulation. 



Fort Yuma is situated on a granitoid porphyry hill, 75 feet above low water, on the Colorado 

 river, which, running south to this point, turns abruptly westward, rounding the eminence on 

 ■which the camp is placed, the southern base of which is denuded and partly removed by the 

 force of the current that here, meeting with the Grlla, flows in an united stream to the Gulf of 

 California. 



The Colorado, north of the fort, spreads out into a wide stream, with low swampy banks, 

 for some miles, when it is narrowed again by the proximity of volcanic rocks through which it 

 caiions. 



In its course through the lowlands below the fort it is constantly changing its banks and 

 altering the course of the stream ; indeed, its present channel, and the confluence of the two 

 rivers at the fort, is at such an unusual angle, that it is certain its present course is but a 

 recent and a temporary one. 



The terrace which extends from Cook's well to Pilot Knob, and thence to the fort, is about 

 35 feet high, and is capped by rounded drift pebbles of granitoid and amygdaloid rock, 

 cemented by a tufaceous deposit. This bank, which is remarkably uniform in its height, 

 would appear to indicate the former course of the Colorado when it flowed more southwesterly 

 into an open sea, (the present Colorado desert,) and when the Gila, instead of turning 

 abruptly north to flow into the Colorado, took a course west and south of the granitoid mass 

 on which the fort is now placed, converting it either into an island, or else leaving it on the 

 east side of the two rivers. 



The waters of the Gila mingle with those of the Colorado coming from an opposite point of 

 the compass. South of the present course of the former river is a low terrace facing the south 

 and touching the low erupted range opposite to that on which the fort lies. Between this and 

 some blufi's lower down on the Colorado is a deep alluvial bottom, in which, it is highly 

 probable, lay the ancient course of the Gila; in other words, from the appearance of the district 

 it is evident that these two rivers united about one-fourth of a mile west of their present junc- 

 tion at some not very remote period, and that the fort hill lay east of the t^vo rivers and not 

 west, as its situation now is. 



Plate V, fig. 5 illustrates this alteration. The porphyry of Fort Yuma may be looked upon 

 as an amphibolic granite, made up of brownish felspar, with small crystals of hornblende 



