118 
GEOLOGY. 
like chalk, and appeared to have been produced by the long exposure of the specimen to the 
weather. This difference between the color of the interior and the surface showed, in a manner 
very striking, the peculiar surface discoloration which is found on all the rocks of that region, 
and which it is yet impossible to explain. 1 
Having at length arrived at the base of the range, it was found to he composed of naked 
rocks, there being no soil or vegetation. The whole surface had that peculiar blackened and 
glistening appearance observed at Pilot Knob. The outlines of the range were very rugged 
and sharp, and long re-entering valleys or angles were numerous. We climbed to an elevation 
of about two hundred and fifty feet on the extreme point, and thus gained a full view of the 
Desert towards the north, west, and south. The ridge upon which we stood appeared to he one 
of many others extending similarly out into the Desert, and overlapping, thus bounding long, 
narrow valleys. It was, however, evident that it extended further south than the others, and 
Pilot Knob was in its line of prolongation, and apparently three or four miles distant. The 
butte upon which Camp Yuma was located was in full view, and the windings of the Colorado, 
with its green border of timber, could be traced in the plain far to the south. The space 
between the mountain and Pilot Knob was nearly level, and paved for the whole distance with 
the brilliant pebbles. There were, however, slight channels, or depressions of the surface, 
which indicated that’ the direction of the drainage was southwest. 
The rocks were found to be compact gneiss, or gneissose granite, the layers being nearly ver¬ 
tical. Several white quartz veins, from three inches to a foot or two in thickness, were seen ; 
one could be traced by the eye for a long distance on the side of the ridge. The sand-hills were 
in full view for their entire extent, and were seen to form a narrow belt only, and did not 
appear to extend far back from the bank or terrace commencing at Pilot Knob. The surface 
intervening between the hills and the ridge appeared to be hard and gravelly, and was free 
from sand. After taking many bearings to distant mountains, we returned to Camp Yuma. 
% 
1 For a description f the structure of specimens of the fossil wood of the Desert, see Appendix, Article YI. 
