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

UPRAISED STRATA-AURIFEROUS SLATES.
These strata consist of alternations of conglomerates, sandstone, and shales, in beds of
variable thickness and order of succession. The color varies from gray to buff-yellow, and
further south to red. The strata are in a semi-consolidated state, and wear away rapidly by
the action of the weather. Beds of hard sandstone were, however, seen, and may, perhaps, be
found in all of the outcrops. These outcrops of sandstone were found on both sides of the trail
for about two miles, and the formation appears to extend northeasterly in a narrow valley between
the granite or metamorphic rocks and a ridge of talcose and micaceous slates rising south of
them.
The extent of this valley, and its position relatively to the pass, will be seen on the small
map of the region, (Map II.) The outcrops are also represented on the general section of the
Bernardino Sierra, which extends through this pass.
No intruded rocks in the form of dykes or otherwise were found among these upraised strata,
although careful search along the line of exploration was made. As the strata rest immediately
upon the adjacent granite, it at first appears that their disturbance and flexed condition was
produced by its intrusion. I looked in vain for an exposure of the line of junction between the
two formations, neither could I detect any great metamorphism of the sediments, which would
probably have resulted if they had been upraised by the inruption of the contiguous granite.
It is, therefore, probable that the disturbance was produced by some igneous intrusion at a distance, and not visible along the line of observation. This intrusion may have displaced a
portion of the granite together with the strata. Further explorations must determine to what
agency these strata owe their present position, whether to a granitic intrusion between them
and the Great Basin, or to the inruption of dykes of porphyry, or the approximation of the
ridges of granitic and talcose rocks, between which the flexures are the most bold.
Lower down the creek, in the vicinity of the ridge of talcose slates, (see section,) the sandstone formations on the right bank are not so much tilted and bent as higher up in the pass;
consequently they do not form such abrupt and precipitous hills. The valleys become more
broad, and in some places the strata are nearly horizontal. A large quantity of oxide of iron
also enters into their composition, and imparts a red color to some of the beds.
Age of the sand3tone strata.-With regard to the age of these strata, I am unable to affirm
positively, but am disposed to regard them as Tertiary. Not a single fossil was found in all the
outcrops, and therefore there is no such evidence of their age. Their resemblance to the strata
at the southern end of the Tulare valley was noted; yet they do not show such a great thickness, nor such thick and firmly consolidated beds.
Ridge of talcose and auriferous states.-The first distinct outcrop of the talcose and gold-bearing rocks met with south of the Mariposa district is exposed in this pass about nine miles from
the summit. At this point the brook issues from among the hills of sandstone, and impinges
upon a slate ridge, and afterwards follows the dividing line between it and the sandstones for a
long distance.
These slates form a distinct ridge of several hundred feet in elevation above the bed of the
canon, trending nearly due east and west, (magnetic,) and they can be seen to extend in an
unbroken ridge for several miles eastward. A narrow valley is thus formed between them and
the granite, which is partly occupied by the outcrops of tilted sandstone just described. The
dip of the slate is at an angle of 70~ northwards, and is very regular and uniform. The rock
differs somewhat from those generally described as talcose, found in the auriferous region of the
Atlantic States, and also from those seen in Mariposa county. The color is dark gray, passing
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