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 difficult, if not impossible, to trace the connexion of the formations without the aid of fossils.
Even the fossils have very different aspects, and present a different grouping of species at each
locality. The difference in mineral characters, which apparently is so great, is, no doubt, in
part due to the limited examinations which have been made; and it is probable that more
extended explorations will show that each well-marked group of strata has a wide extent along
the coast. The distances between the localities described, and the different positions of the strata
relatively to the great lines of elevation about which they were deposited, are also sufficient to
lead us to expect the strata, even of the same age, to be very different in their appearance.
The Miocene strata of Ocoya Creek are about eighty miles removed from those of San Per nado-supposed to be also Miocene-and a range of mountains, with the lowest passes over
three thousand feet in elevation, extends between them. One series of strata is at the eastern
base of the Sierra Nevada, the other at the southern base of the Bernardino Sierra. The out crops at San Fernando are one hundred and forty miles distant from those of San Diego, and
those of San Diego fifty miles or more from the fossiliferous strata of Carrizo Creek, and on the
opposite side of the high Peninsula mountains. So, also, the strata of the Great Basin are
separated from the other localities by mountains on all sides.
Although we fail to trace a resemblance between the strata of the same age over the whole
extent of the region which has been considered, it is believed that the strata which are developed
at distant points along the same base of either of the great mountain ranges have a general
similarity of mineral contents; in other words, the great lithological differences of the strata
are coincident with the great variations in the relief of the surface, and are also found on the
opposite sides of the same chain of mountains.
There is little doubt that formations of the age of those of Ocoya Creek, and bearing a general
resemblance to them in mineral composition, extend along the whole western base of the Sierra
Nevada, although interrupted at many points, or removed by denudation and the action of
rivers. The rounded hills passed on the 21st of July, and described in the Itinerary, are believed to correspond in age with those of Ocoya Creek. So, also, thick horizontal strata at
Mokelumne hill, composed, in great part, of volcanic ashes and pumice; and strata found at
several points further north are probably of the same period. The deposits of Chico Creek,
represented by Miocene fossils, are, perhaps, a continuation of the same series, but may be, and
probably are, older, or lower in the group than those of Ocoya Creek. Dr. Trask has noted
the occurrence of marine fossils at Willow Springs and other points in the Sacramento Valley.
The deposit at Volcano Ridge, so far as it is represented by the specimens in hand, does not
appear to be of the same series, but is, apparently, older.
The Miocene deposits are not confined to the base of the Sierra Nevada, but are found on the
opposite side of the valley at the base, or forming the foot-hills, of the Coast Mountains.
They were, also, found at the extreme southern end of the Tulare Valley, resting upon the
upturned edges of the older sandstone (probably Eocene) near San Amedio.
That these are Miocene, or more recent, is not only indicated by their unconformity with the
supposed Eocene strata, but by fossils of the genera /eretrix and Stramonita, found near the
foot-hills. Mr. Conrad regards these as Miocene, and describes them as Meretrix Tbtlaranza and
Stramonita petrosa. A fine specimen of the genus Arca, described us Arca microdonta, (Plate
III., fig. 29,) was obtained from the hills of the Coast Mountains near the Tulares. This is
regarded as Miocene by Mr. Conrad, who states that it has some resemblance to A. arata, Say,
of the Maryland Miocene.
The relation of these Miocene deposits to the recent drft or the aurtfero~t drift, of the slope
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