32 GEOLOGY. 
Wahsatch mountains as to that of the Sierra Nevada system. Its geological relations are also 
interesting, from the fact that it forms the first of the series of steps by which the ascent is 
made from the trough of the Colorado to the surface of the high table-lands occupying the 
central portion of the continent. 
The intimate structure of the Black mountains does not differ essentially from that of some of 
the ranges which have already been described; particularly that upon which Chimney Peak 
is set. As a whole, it is characterized by a predominance of rocks of a distinctly eruptive 
character, such as massive granite, trap, porphyry, and trachyte, and the rarity of gneiss, mica 
slate, clay slates, &c., which are probably metamorphic. 
That portion of the range which forms the eastern boundary of the Mojave valley is composed 
principally of trap, trachyte, and porphyry, of which the prevailing dark colors have suggested 
the name given to the entire chain. Further north its composition is more varied, embracing 
granite, porphyries, trachytes, and tufas in great variety, whose colors are scarcely less striking 
than those of the Purple Hills. 
DEAD MOUNTAIN. 
On the west side of the river, near the north end of the Mojave valley, is a high and pic- 
turesque mountain, which forms a prominent landmark for all this region. This mountain is 
supposed by the Mojaves to be the dwelling-place of their disembodied spirits, and hence it has 
received the name it bears. From this great mass, as a starting point, a continuous mountain 
chain stretches to the northwest, in harmony with the other ranges of the Sierra Nevada system 
which we have already described. A spur from Dead mountain extends across to the Black 
mountains, forming the northern limit of the Mojave valley. This valley is bounded on the 
west by a range of granitic mountains, which, at first sight, seems to be a spur running off 
towards the northwest from Dead mountain, but which is, in fact, formed by several distinct lines 
of upheaval parallel with that of the Dead mountain range, and of which the southeastern termini 
are en echelon. Dead mountain, like all those on the west side of the Mojave valley, is composed 
mainly of granite. This is generally white and massive, highly felspathic, and frequently 
friable. In some localities it is gneissoid, and is associated with clay slate, mica slate, &c. In 
several places low hills of trap and scoria rise near the base of the range. 
PYRAMID CANON. 
As has been stated, a spur from the Dead mountains crosses the Colorado, forming the 
northern boundary of the Mojave valley. The river cutting through this range has produced 
Pyramid cafion, a name suggested by a remarkable monument-like pinnacle of porphyritic rock 
which crowns the left bank near the northern entrance. This range has nowhere a great alti- 
tude, nor does the caiion cut through it bear comparison, in point of magnitude, with several 
others which we traversed. Yet, in this locality, I was furnished with the most ‘satisfactory 
proof of the truth of the theory I have advanced to explain the mode of formation of the cafions 
of the Colorado. The walls of Pyramid cafion are, for the most part, formed of massive granite, 
which exhibit perpendicular faces two hundred feet or more in height. On either side of the 
stream, among the summits of the granite hills, full two hundred and fifty feet above its bed, 
are masses of stratified gravel, identical in composition, and continuous with the gravel terraces 
which border the river both above and below the cafion. Here, then, we have conclusive 
evidence that where now is Pyramid cafion was once an unbroken barrier, stretching across the 
course of the Colorado, and raising its waters to an elevation of at least two hundred and fifty 
feet above their present level. This barrier has been cut through by the action of the river, 
but these beds of gravel and sand remain as monuments of the time when the valley above 
Pyramid cafion, as well as that of the Mojaves below, were in great part covered by sheets of 
