Reports of explorations and surveys, to ascertain the most practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean: Vol. 5, Pt. 2

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; but rain ad
fallen in the months of July, August, and September, preceding the earthquake.'
D)ecember 9-Fort Yuma to the mountains north of Pilot Knob.-In company with Lieutenant
IHendershot 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 bounding 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 be much nearer, and to be 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 be 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 plaint, 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 bank 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 bounded 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 heig,ht of the bank was much reduced. The
bottom or floor of these little valleys was sandy, and strewn in places with pebbles from the conglomerate. A large quantity of black sand was also found covering the surface of ripplemarks, 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, but beautifully formed, garnets. In the solitude of these narrow
valleys, excavated below the level expanse of the desert plain above, I was agreeably surprised
yet undescribed. One of the most conspicuous was a large shrub, or tree, probably the greenbarked Acacia of Major Emory and others. It forms a low tree which spreads out over a large
surface, but at that locality did not rise higher than the level of the adjoining banks.
The foliage is light and airy, and not unlike that of the mezquite or screw-bean in its general
aspect, but 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.
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.
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