116 
GEOLOGY. 
northeast from Salt creek, and probably near the lowest point of the bed of^the ancient lake. 
Clouds of dust were thrown up from various parts of the Desert at the time of the principal 
shock, and the probability is, that there were many small openings and rents for subterranean 
gasses, that have never been found, or that have since been entirely obliterated. 
The position of these, the most important and active vents, and the fact that they were opened 
in the lowest parts of the surface, and through the recent clay formations subject to overflow, 
indicates a connexion of the phenomena of disturbance with the infiltration of water. No 
overflow of the Colorado, however, was known to have taken place that season; hut rain had 
fallen in the months of July, August, and September, preceding the earthquake. 1 
December 9 —Fort Yuma to the mountains north of Pilot Knob .—In company with Lieutenant 
Hendershot I made a trip to the mountains northwest of the fort, and nearly north of Pilot 
Knob. The mountains are near the end, or at that point form the end, of the apparently con¬ 
tinuous range extending southeasterly from San Bernardino mountain, and hounding the 
valley of the Desert-on the north and east. We estimated the distance to the nearest point of 
these rocks at thirteen miles; they, however, seemed to he much nearer, and to he separated 
from us by only a narrow belt of river-bottom and of the plain beyond. To an observer from 
the Atlantic States, whose eye had not become educated in estimating distances in that region 
of lofty peaks and pure air, the distance would have appeared to he less than half its reality. 
Our course, at first, lay over the bottom-lands of the Colorado, among cottonwoods, willows, 
and clumps of mezquite trees. We soon passed from these to a wide area covered with gigantic 
Chenopodiaceous weeds, through which it was not easy to force a passage with our mules. We 
saw beyond us, the borders of the upper plain, marked by a horizontal line, apparently about 
thirty or forty feet above the level of the bottom-land. This line was broken at short intervals 
by gullies worn out from the top to the bottom of the hank by drainage water during the 
showers, leaving in some places long projecting points of the plain between two gullies. These 
little valleys, thus formed by erosion, were from twenty to sixty or one hundred feet in width., 
and were hounded by steep banks of the horizontal strata on each side. In some places the 
thick bed of conglomerate, which overlies the sand, was visible, and in others, it had been 
gradually undermined and washed away until the height of the hank was much reduced. The 
bottom or floor of these little valleys was sandy, and strewn in places with pebbles from the con¬ 
glomerate. A large quantity of black sand was also found covering the surface of ripple- 
marks, and in some places it was so abundant as to form black patches of several square feet 
in area and from one -quarter to half an inch in depth. A sample of this was collected, and is 
found to contain many small, hut beautifully formed, garnets. In the solitude of these narrow 
valleys, excavated below the level expanse of the desert plain above, I was agreeably surprised 
to find several forms of vegetation of unique and peculiar appearance, many of them, doubtless, 
yet undescrihed. One of the most conspicuous was a large shrub, or tree, probably the green- 
harked Acacia of Major Emory and others. It forms a low tree which spreads out over a large 
surface, hut at that locality did not rise higher than the level of the adjoining hanks. 
The foliage is light and airy, and not unlike that of the mezquite or screw-bean in its general 
aspect, hut is quite different in color, being a whitish or ashy-green. The limbs and trunks 
are round and very smooth, and the bark is greenish-white. The plant is so peculiar and 
striking in its appearance that it cannot fail to arrest the attention of the most casual observer. 2 
1 The author is indebted to Major Heintzelman, U. S. A., for these facts. 
2 Specimens of this plant and others, were subsequently collected by Major G. H. Thomas, United States army, and are 
described and figured by Dr. Torrey in the Appendix. 
