112 GEOLOGY. 
‘Colorado River.—At the time of my visit the river was nearly at its lowest stage, and was 
flowing rapidly, about twenty feet below the edge of the bank The water was highly charged 
with fine, red mud, which gave it a decided red color and opacity. This red clay receives its 
color from a large amount of peroxide of iron; considerable quantities of it were deposited in 
all the shallows and quiet places along the banks. 
The amount of silt thus annually transported to the Gult of California by this river must be 
very large, and very considerable additions to and alterations of its delta must result. 
From the Algodones, (or the Indian village near the Colorado,) the trail followed up the 
right bank of the river, winding about in a thick growth of willows. The terrace and sand- 
hills still bounded the view on the left, and were about half a mile distant from the stream. 
We soon reached Pilot Knob, which had so long been visible at a distance, and, in fact, is a 
well known landmark or guide to the traveller of the desert. It rises, solitarily, above the level 
plain, about three miles distant from the point of the range that borders the desert on the north- 
east. The whole surface of the rocks is of a jet black color, and they have a peculiar polished 
exterior which glistens in the sun as if it had been varnished or highly polished. Linesof struc- 
ture or lamination were, however, distinctly visible in the granite, and were much bent and 
contorted. Their trend is about 5° W. of N. (magnetic.) The Knob also appears to be tra- 
versed by dykes or ridges of a dark volcanic rock, which resembles basalt, or some dark 
varieties of crystalline trap. I had not time to follow out and determine the line of junction 
between the granite and this rock. 
The granite was traversed by narrow seams and fissures, which were filled up by a cellular 
and porous substance, apparently a calcareous sinter or travertin. 
On the rocks of this Knob, about forty or fifty feet above the level of the river, I found 
numerous rounded agates and carnelians, and a mass of rolled and water-worn flint, which 
contained small fossil shells, apparently Carboniferous or Cretaceous. All these pebbles and 
the loose fragments of the rock were highly polished, and presented a dark brown or black 
color, glistening in the sun's rays like polished ebony or japanned ware. 
The sand-hills terminate near the western side of Pilot Knob, but the bank or terrace upon 
which they are formed appears to be continuous beyond the Knob to the vicinity of Camp Yuma. 
Its upper stratum, at Pilot Knob, is a bed of conglomerate about five feet thick, similar to that 
occurring back of the Indian village. "The pebbles are closely and firmly cemented together or 
imbedded in a paste of carbonate of lime, and they are principally fragments of volcanic rocks of 
various colors, some of them being dark red and green porphyries of great beauty. This con- 
glomerate is underlaid by strata of sand, in an unconsolidated state, and they much resembled 
post tertiary deposits. 
The river makes a great bend near this ridge, and almost washes its base, leaving a space 8° 
narrow between the rocks and the bank that the road leads over a low point of the Knob. A 
fine view of the Colorado and the timber along its banks is presented from the elevation ; it 18 
seen to curve around towards another but much smaller butte—the site of Fort Yuma—about 
eight miles distant. The bank or terrace, which was found to border the trail after leaving 
Cook’s Well, is seen to be the margin of an elevated and level plain extending between the 
mountains, and perfectly devoid of trees or, vegetation ; while the lower ground or bottom- 
land along the river is covered with groves of mezquite, willows and cotton-wood. The accom- 
panying sketch was taken from the Knob, looking up the river, and a little north of east, and 
shows the plain on the left, and the peculiarly sharp and pointed outlines of the distant moun- 
