CHAPTER XIX. 
TORT YUMA TO THE PIMAS VILLAGES. 
Port yuma.—Colorado.—Ancient terrace and river bed.—Aspect of gila river near its month.—Occasional overflows.— 
Vegetation of river bottom.—Soil, limits, and constitdtion of the sonora desert.—Big 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 of subterranean 
springs upon vegetation.' — Pimas villages.—Cultivated lands.—Character of the soil.—Necessity of 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 Gila, 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 hanks, 
for some miles, when it is narrowed again by the proximity of volcanic rocks through which it 
canons. 
In its course through the lowlands below the fort it is constantly changing its hanks 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 hut 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 hank, 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 Grila, 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 bluffs 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 two rivers and not 
west, as its situation now is. 
Plate Y, fig. 5 illustrates this alteration. The porphyry of Fort Yuma may he looked upon 
as an amphibolic granite, made up of brownish felspar, with small crystals of hornblende 
