108 
GEOLOGY. 
found on each side for several miles from the camp, as before observed. It is through these 
that the creek has cut its channel, and, where the stratification remains horizontal, the tops of 
the hills are flat and covered with water-worn boulders and thick layers of pebbles, which are, 
apparently, the continuation or source of the accumulations of drift already noticed as covering 
the slope of the desert near the mountains. One of the isolated hills a short distance north of 
the trail was found to be capped by a thick layer of fossils, closely impacted together. Several 
specimens were detached very quickly, and further observations were deferred until our return, 
as I was already far behind the party. These fossils were chiefly oysters and pectens. Just 
before reaching the hill which leads up to the long slope of the desert, we cached a sack of 
barley, to be fed to the mules on our return. The fossils I had procured were also hid, and the 
whole was covered with sand. 
The ascent of the hill leading up from the valley of the creek brought us to a full view of the 
broad expanse of the Great Desert. It lay stretched out before us, a vast plain reaching to the 
horizon, the only mountains visible being those on the northeast side, rising dim and faint in the 
distance, and a single isolated ridge, much nearer and almost south of us. This was Signal 
mountain , a solitary peak, serving as a guide to the traveller when on the desert. The road 
could be traced down the slope extending off to the southeast, this being the direction of the 
mouth of the Gila in the Colorado, about 90 miles distant. The phenomena of mirage added 
greatly to the grandeur of the scene, supplying the forms of mountains or buildings upon the 
distant edge of the desert; tall blue columns loomed up on the horizon, and castles, towers, 
and the spires of cathedrals, seemed to stand beyond the limits of the great plain. Their out¬ 
lines were distinctly projected against the morning sky, but they were undergoing constant 
change, becoming lower or higher, and occasionally spreading out like a cross, or merging to¬ 
gether into one grand pile. 
Some of the outlines were sketched and are represented in the figures; but they convey no 
idea of the beauty of the scene. See, also, View XII. 
OUTLINES OF THE MIRAGE, COLORADO DESERT. 
The slope of the desert from the hill on which we stood was very gradual, and the soil firm, 
but gravelly and barren. After traversing it a short distance, we passed the trail made by the 
wagons a few days previously; then, reaching the foot of the slope, found a hard, smooth surface 
of bluish clay, containing shells, the same species as before observed on the portions of the 
desert further north. Before we passed upon the clay, however, several accumulations of 
drift-pebbles and rocks, from three to eight inches in diameter, were found along the trail. 
The surfaces of nearly all of these were smooth and polished. This is due to the continued 
action of the blown sand and dust in its passage over them. The greater part of these 
masses are of volcanic origin, and many specimens of scoriaceous and vesicular lava were found. 
Fine specimens of silicified wood were also found to be abundant and of various sizes, from one 
or two inches in length to as many feet. They are generally of a brown color, and retain all 
the appearance of wood ; the grain and knots show distinctly, and resemble the wood of the 
mezquit. The surfaces of these specimens were also curiously polished, and some of them ap¬ 
pear to have been deeply cut and grooved by the moving sand. 
