24 GEOLOGY 
structure, its trend, and position to Chimney Peak range, marking, however, a distinct line of 
upheaval, perhaps but a continuation of the more northern ranges of the Purple Hills. 
The river here bends abruptly to the east and skirts the base of another prominent and con- 
tinuous range of mountains, which it bursts through at Porphyry Gate. That portion of this 
chain which lies west of the river is composed of porphyries, tufas, and trachytes, of which 
the prevailing color is a dark brownish purple, and hence they have received the name of 
Chocolate mountains. 
The cliffs which border the river at the point where it passes through this range are of 
similar composition with the western portion of the chain, that on the right bank consisting of 
a deep purple porphyritic trachyte. In the middle of the stream stands a picturesque conical 
rock, composed of purple trachyte, and called, from its form, Light-house rock. The adjacent 
hills upon the left bank are formed of trachyte, porphyry, tufa and volcanic conglomerate, of 
varied colors and tonsistence, closely resembling the colored rocks of the Purple Hills. In 
these hills I discovered a series of highly metamorphosed red sandstones, very distinctly 
stratified, the strata standing nearly vertical. Where most changed these beds seemed to be 
converted into purplish trachytes and porphyries, similar to those which I have described as 
characteristic of this range where we had before observed it. At the time of making these 
observations I was entirely unable to determine the age of these strata ; but to whatever epoch 
they might belong, I was disposed to consider them as the source from which had been derived, 
by fusion, a portion, at least, of the purplish trachytes and porphyries so characteristic of the 
region I had recently traversed. Long subsequently, when we had left the basin of the Colo- 
rado, and had descended from the surface of the high table-land lying east of it to the bottom 
of the Great Cafion of the upper Colorado, in the red sandstones and shales which there form 
the base of the sedimentary series, I thought I saw the equivalents of the sandstones of the 
Chocolate mountains. Some of the tufas about Porphyry Gate are highly charged with alkaline 
salts, and their decomposed portions frequently contain plates of selenite. 
Eastward from the point at which the Colorado cuts through the Chocolate mountains they 
assume a new aspect. They are no longer purple, but gray, and are composed of gneiss 
traversed by veins of granite and quartz. The gneissoid rocks are frequently foliated in 
structure and much convoluted ; even a hand specimen including curved layers of hornblende 
and felspar with veins of quartz, well representing the structure of the chain. The aspect of 
these rocks is such as to lead an observer more readily to refer them to a metamorphic origin 
than any other granitic rocks seen on our route. 
At Camp 18, located near the base of the granitic portions of the Chocolate mountains, the 
Quaternary strata are very fully exposed. They consist of beds of gravel and sand with brown 
‘and white clays, which form low table-hills, extending as far as the eye could reach toward 
the east in an accurately horizontal line along the mountain bases. The sides and summits of 
these terraces are covered with rolled pebbles of various sizes, packed so closely as to occupy 
the entire surface. The harder of these pebbles are curiously polished and blackened, as 
though with plumbago, while the softer ones, particularly those composed of limestone, are 
etched and eroded much in the same manner as those found on the surface of the Colorado 
desert. This effect may be due to blown-sand, but at present no sand is here visible, and the 
erosion seems rather attributable to chemical action. 
GREAT COLORADO VALLEY. 
For several days after leaving the Chocolate mountains we were occupied in crossing a broad 
valle ley, of which the geological structure exhibited a perfect monotony throughout. On either 
side of the river are broad, gravel, desert plains; that on the southeast occupying all the 
interval between the parallel ranges of the Chocolate and Monument mountains. The surface 
