22 GEOLOGY. 
dicular to the cooling surface. . When, as most frequently occurs, these planes are vertical, by 
the erosion of rains and flowing streams, perpendicular walls are formed, and large masses 
usually exhibit mural faces. This will account for the peculiar outline which many of the trap- 
pean summits of the mountains of the Colorado basin present. Their great altitude, as com- 
pared with the mass of the ranges which they crown, is doubtless due to the resistance offered 
by their material to the atmospheric influences which have removed perhaps several hundred 
feet of the more yielding tufas and trachytes once surrounding them. 
About the bases of some of the isolated mountains of the Colorado basin the material 
washed from the sides and summits has accumulated, and. where these detrital slopes have 
been opened in the beds of the ‘‘washes’’ I have described, I was able roughly to measure 
the amount of denudation the mountains had suffered. From these data it would appear that 
many of them have once been fully twice their present size. 
At Precipice Bend and Barrier islands the river impinges against huge masses of red tra- 
chyte, west of which the highly colored rocks before described are very conspicuous, and 
extend for many miles along the base of Chimney Peak range. 
_o 
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Fig. 1.—cHIMNEY PEAK FROM THE NORTH. 
A mile or two above Barrier islands are high perpendicular bluffs of stratified and consoli- 
dated gravel, forming a conglomerate of great thickness. The upper part of these hills is com- 
posed of uncemented gravel, and is continuous with the desert surface, which here forms a 
mesa about one hundred and fifty feet above the level of the river. 
of these bluffs is horizontally stratified and not at all metamorphosed. Its constituents do not 
materially differ from those of the gravel above. They are rolled and angular fragments of 
porphyry, trachyte, trap, scoria, jasper, &c., in great variety, cemented by carbonate of lime. 
It may not be possible to determine the exact age of these beds of conglomerate; but Iam 
disposed to regard them as entirely Quaternary: 5% 
The consolidated portion 
