40 GEOLOGY. 
the cation holds the same general character. Nearly perpendicular walls rise on either side to — 
the height of a thousand feet or more, with few interruptions and no open valleys or alluvial 
land. The structure of the Black mountains is here scarcely different from what it has been 
described to be below. Masses of granite are exposed at several points in its section, but in 
magnitude they are far surpassed by the towering heaps of trap, porphyry, trachyte, &c., &c., 
which make up the bulk of the chain. 
Probably nowhere in the world is there a finer display of rocks of volcanic origin than may 
be seen about the southern entrance to the cafion. The beetling crags which form its massive 
portals are composed of dark-brown porphyry of hardest and most resistant character. Just 
within the cafion, on the west side of the river, this porphyry is mingled with huge convoluted 
masses of light-brown trachyte ; tufa, pure white or white veined with crimson, and pale blue 
obsidian, (pearl-stone) ; amygdaloids of various kinds, their cavities filled with different zeolites ; 
black and gray basalts, sometimes columnar ; scoria, red, orange, green, or black, and of every 
grade of texture ; porphyries in great variety, including some of unequalled beauty; trachytes 
and tufas of all colors ; obsidian in its various forms; all these are abundantly exposed in the 
immediate vicinity. 
Five miles west of the cafion are some hills, composed of soft amygdaloid, everywhere 
traversed by crevices lined with beautiful specimens of opalescent chalcedony, and rosettes of 
crystals of quartz which have a stellar arrangement. The soft trap rock having been 
weathered away, these silicious minerals are left covering the surface and sparkling in the 
sunshine like so many diamonds. 
The view of the western slope of the Black mountains which we obtained from the summits 
bordering the cafion is scarcely equalled, in its wild Savage grandeur, by any I have elsewhere 
seen. A thousand subordinate pinnacles spring from the mountain side, all displaying the 
ragged outlines which the materials composing them are so prone to assume, while their colors 
are as striking and varied as their forms. Nota particle of vegetation is visible in the land- 
scape. Here and there a spiny cactus clings to the rocks, but its color blends with theirs, as 
its thorny and repulsive nature harmonizes with the forbidding features of the surrounding 
scenery. 
As the eye of the traveller sweeps over this wilderness of sunburnt summits, which stand 
so stark and still, glittering in the burning sunlight and yet so desolate, he shrinks from the 
unearthly scene with a feeling of depression which must be felt to be imagined. 
Rock salt—On the banks of a tributary of the Rio Virgen, a few miles from the Colorado, 
the Indians obtain large quantities of rock salt. Much of it is very pure and beautifully 
crystallized, and it is said to exist in immense quantities. I regret that I was unable to visit 
the locality and determine its geological position. 
Gypsum also occurs in the vicinity in 
abundance. 
I suspect the rocks which contain these minerals are not older than the Tertiary, but it is 
possible that the Secondary strata, which are so largely developed east of the Little Colorado, 
and which contain so much salt and gypsum, recur on the Rio Virgen. 
