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.
be intrusive, and probably the cause of the disturbance of the beds. Additional evidence of
violent disturbances of the strata was found in the presence of many faults or fractures of the
beds. The preceding figure represents two faults, of about four feet each, seen on the face of
one of the vertical exposures of strata. They were, in all probability, produced during
earthquakes.
Seams and layers of gypsum were abundant in the banks on the side of the hill as we descended to the dry bed of the creek; and by the decomposition and wearing down of the beds,
this mineral has become detached, and lies distributed on the surface in thin, transparent plates
and fibrous masses. Other banks, higher up the creek, also contained thin seams of gypsum,
associated with layers of sand, containing a large amount of peroxide of iron.
The cache of barley and the fossils, which we had made when on our way to the Gila, was
found undisturbed, and I left the trail to give the locality of the fossils a further examination.
The hill is on the north side of the valley, and stands alone, having a flat top, and the sides
partly covered with great blocks, which have fallen from the upper stratum. This stratum is
over fifteen feet thick,' and is formed entirely of the fossil shells and their fragments, mingled
with a little clay. These fossils were so firmly impacted together that it was difficult to detach
a single shell.
They all had a dark-brown color, and appeared to be changed in composition or silicified,
but consisted of carbonate of lime.2
The top of this hill presents a level surface not over one hundred feet in breadth. A thin
stratum of drift-stones was lying upon it, all of them being very smooth and glistening, looking as if they had lain there for ages. Among them I found several masses of silicified wood,
one of them so large that I was unable to bring it away; smaller specimens were, however, obtained. In these the grain of the wood was well preserved; and all the small knots,
and the rings of annual growth, were remarkably distinct. One of the specimens, in which the
rings were perfectly shown on the ends, appeared to have been flattened by pressure, so as to
produce ellipticity in all the rings. The specimens picked up on the desert slope were of similar formation and color. Wherever these specimens have lain out upon the surface exposed to
the continued action of the loose sand, they have become worn and polished, so that the grain of
the former wood is more distinctly displayed, by which their beauty is greatly increased.
CARRIZO CREEK TO SAN DIEGO.
December 15.-Carrizo Creek to Vallecito, 21 miles.-The water of Carrizo creek does not
appear at the surface for many miles west of the entrance to its valley from the Desert. The
greater part of the valley is entirely dry and sandy, and almost as forbidding as the Desert.
The monotony is broken by a clump of palm trees on the north of the trail, and a green bank
from which springs issue, known as the "Palm Springs."
These were situated under a bank of argillaceous, sedimentary beds. The water rises at
various places, and seems to saturate the ground so thoroughly for a space two or three hundred feet in diameter, that a hole dug in any part of it soon becomes filled. The water was
sulphurous, and gave off a slight quantity of sulphuretted hydrogen gas. A slight efflorescence
of nitre was seen on the surface of the ground around the pools. The water, however, was not
1 I find this thickness recorded in my notes; but five feet more nearly agrees with my rec(llection of the locality.1855.
For a description of these shells, see Appendix, Article II.
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