| CHAPTER III. 
GEOLOGY OF COUNTRY BORDERING THE LOWER COLORADO, 
GENERAL FEATURES OF COLORADO BASIN.—DESERT PLAINS AND NET WORK OF MOUNTAINS.—VOLCANIC CHARACTER OF THE DISTRICT.— 
Loca, GEOLOGY—MovTH oF THE COLORADO TO FORT YUMA.—TRANSPORT AND DEPOSITION OF SEDIMENT BY THE RIVER.—Fort 
YUMA.—PurpLe HILLS.—HIGHLY COLORED PORPHYRIES, TRACHYTES, AND TUFAS—ExXpPLorer’s pass.—GRANITE—GOLD, COPPER, 
IRON, AND LEAD OF PURPLE HILLS.—CHIMNEY PEAK.—TURRETED SUMMITS OF VOLCANIC MOUNTAINS IN THIS REGION —THEORY 
OF THEIR FORMATION.—BARRIER ISLANDS —QUATERNARY AND TERTIARY CONGLOMERATES.—SPIRE RANGE.—LIGHT-HOUSE ROCK.— 
METAMORPHOSED RED SANDSTONES —GNEISSOID ROCKS OF CHOCOLATE MOUNTAINS—-GRAVEL BEDS BORDERING RIVER.—GREAT 
CoLoRADO VALLEY.—STRATIFICATION OF SEDIMENTS.—ORIGIN OF CROSS AND A 
SAND AND CLAY.—SUN-CRACKED CLAY SEAMS.—HALF-WAY AND RIVERSIDE MOUNTAINS—MBsTAMORPHIC LIMESTONE.—TALCOSE 
SLATE.—METALLIFEROUS ROCKS.—MONUMENT MOUNTAINS AND CANON.— TERTIARY CONGLOMERATES.— GRANITE WITH METALLIFEROUS 
8.—M SILVER, COPPER, LEAD, IRON.—GEOLOGICAL SCENERY. 
VRIN' 
ELNDS. 
GENERAL FEATURES. 
The geological structure of the banks of the Colorado, from the Black cafion, near the mouth 
of the Rio Virgen, to the head of the Gulf of California, though exhibiting considerable local 
diversity, has a common character throughout; a character shared by all parts of a wide area 
bordering the Colorado through the lowest five hundred miles of its course. This area is 
bounded on the west by the peninsular extension of the Sierra Nevada, and on the east by 
several ranges belonging to the same system, of which the Mojave and Black mountains, the 
Cerbat, and, perhaps, even the Aquarius ranges, may be considered as examples; the moun- 
tains of Sonora and those crossing the Gila east of Fort Yuma holding the same relation to the 
southern portion of this area that the former ranges do to the northern, some of them being 
but the southeastern prolongation of the same lines of upheaval. 
As a whole, this area is one of depression; a synclinal trough lying between the principal 
mountain ranges I have mentioned, the lowest positions of which are scarcely elevated above 
the ocean level. It is a continuation northward of the great valley, if such it may be called, 
now occupied by the Gulf of California. It is only, however, when viewed as a whole, and 
taken in connexion with the great physical features of the region which surrounds the trough 
of the Colorado, that it deserves the term I have applied to it; and the traveller who passes 
through it, unless carrying from side to side a line of barometric observations, will be very 
likely to be deceived by the local variety which its surface presents, and fail to receive a just 
Impression of its general configuration. The greater part of the surface of the Colorado basin 
is composed of arid sand or gravel plains, exhibiting but little variation of level, of which the 
Colorado desert, and that lying east of the Colorado and south of the Gila, (‘‘the great Sonora 
desert,’’) afford the most striking examples. These plains are enclosed in a net work of bare 
and rugged mountain chains, not of great altitude, but exceedingly picturesque in their out- 
lines. Of these chains, the most important and most continuous have the northwest and south- 
east trend of the Sierra Nevada system; the others are but interlocking spurs; the whole forming 
a series of isolated basins which usually are longest in a northwest and southeast direction, and 
of which the surfaces, composed of gravel or sand, form the desert plains before described. 
Into this labyrinth the Colorado pours, as it issues from its trough cut in the great table-lands 
which skirt the western base of the Rocky mountains, where it takes its rise. Doubtless in 
earlier tmies it filled these basins to the brim, thus irrigating and enriching all its course. 
In the lapse of ages, however, its accumulated waters, pouring over the lowest points'inthe . 
