PURPLE HILLS—GOLD, SILVER, COPPER, ETC. 21 
southwesterly of several neighboring and parallel ranges, to which, from the general color 
they exhibit, the name of Purple Hills was given. They are composed of granite and mica 
slates, associated with which are purple porphyries and trachytes, in sufficient quantity to 
impart to them their prevailing color. Where we passed through the first of these ranges, it 
has nearly a northwest and southeast trend, and in the immediate vicinity of the river is com- 
posed of a gray, massive granite, which, yielding somewhat readily to the action of the ele- 
ments, as most of the granites of the Sierra Nevada system do, has formed slopes receding from 
the river, giving the pass an outline strikingly in contrast with that of most of the cafions cut 
in the porphyritic rocks higher up. Boot Mountain, a prominent peak in this range on the 
east side of the river, is composed of trap. 
Traversing Explorer’s Pass we entered an oval valley cut in various channels by the river. 
Of this valley the higher portion is occupied by beds of gravel deeply cut by washes and cov- 
ered with the giant cactus, (Cereus giganteus,) and other characteristic forms of the vegetation 
of the desert. The gravel beds are made up of material of two different classes: first, angular 
fragments of granite, trap, porphyry, &c., brought down from the neighboring hills, and form- 
ing long slopes about their bases; second, rolled and rounded pebbles of various sizes, com- 
posed of all forms of erupted rock, with silicified wood, jasper, chalcedony, and occasionally 
chert containing corals and crinoidal stems; the latter derived, as I subsequently ascertained, 
from the Carboniferous limestone many hundred miles above. The channel of the river, 
where it cuts through the second range of the Purple Hills is bordered on either side by walls 
of trap, trachyte, and porphyry, exhibiting a great variety both of color and consistence, 
though purple trachytes and porphyries predominate. Toward the upper end of this passage 
the scenery becomes bolder; the hills higher and more craggy, showing considerable variety 
and contrast of color. The materials which compose them are porphyries, trachytes, and 
tufas, pink, purple, white, blue, yellow, brown, &c. The colors are all vivid, and obscured 
Y no vegetation form a landscape very different from any before presented to our eyes. 
Proceeding up the river we traversed another valley similar to the one just described, bor- 
dered on the east by the desert, with its characteristic geology and botany. We then entered the 
- pass through the third range of the Purple Hills, which are here, however, not purple, but gray; 
being composed of mica slate, with conspicuous veins of quartz and hornblende. Many parts 
of the Purple Hills are rich in metallic minerals. In the second range a vein of argentiferous 
galena has been opened by Mr. Halsted, of Fort Yuma, and promises well. In the immediate 
Vicinity of the river, the third range of the Purple Hills affords the most unmistakable evi- 
ence of the existence of valuable minerals. Gold, copper, iron, and lead are found there; 
the gold in small quantities, and probably not in such abundance as to compensate the miner. 
Some of the veins of copper are, however, rich, and have already attracted the attention of the 
residents at the fort. From the claim of Messrs. Hooper, Halsted, and Johnson, I received 
Specimens of erubesite and black copper, not inferior in quality to the ores from the copper 
mines of the Gila. 
Above the mica slate hills, the red, white, green, pink, and blue tufas, porphyries, and tra- 
chytes, described as occurring below, reappear, giving the same fantastic appearance to the 
Scenery. These rocks, with trap and scoria, extend from the river to Chimney Peak. 
Chimney Peak is a remarkably picturesque double pinnacle which crowns a mountain chain, 
probably the northwestern prolongation of the middle range of the Purple Hills. Like the 
other peaks of the range it is composed of trap, and affords a striking example of the tendency 
to form columnar summits exhibited by all the mountains of this vicinity. Dome mountain on 
the east of the Colorado, present the same features in nearly an equal degree. The mountains 
which have this form are all trappean in character, and doubtless owe their peculiar outlines 
to the manner in which this material yields to the action of the elements. The trap is usually 
ore orless columnar in structure, the cleavage planes which bound the columns being perpen- 
