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

OBSERVATIONS ON THE TERTIARY DEPOSITS.
of the Sierra Nevada is obscure for want of sufficient explorations and examination. These more
modern accumulations attain such a great development, and are frequently of such fine materials, including, also, volcanic sand and ash, fragments of pumice, and the like, that they
might easily be mistaken as the equivalents of the Ocoya Creek series. Whether the horizontal
strata, seen near the Tuolumne, at Fort Miller under the basalt, at Bear Creek, (see Section 2,
Sheet I,) and at the crossing of- the Chowchiilas, can be referred-to the Ocoya Creek series, or
are much more recent, can only be determined by fossils or further exploration, so as to trace
the continuity. The strata in the vicinity of the Tuolumne and the Merced rivers, forming the
flat-topped hills, were, at the time of their examination, supposed to be the equivalents of the
sandstone strata of the Coast Mountains on the opposite side of the valley.
The Miocene strata of San Diego are, without much doubt, extended northwards along the
whole slope of the Peninsula Sierra, and connect with those which appear in outcrops at San
Fernando and other places. The strata of all this slope may be regarded as one group, and are
believed to have a general similarity of lithological characters and fossils.
A great difference is, however, presented between the strata of San Diego and those on the
opposite side of the mountains, although they are probably synchronous in origin The high
Peninsula Mountains were, doubtless, a barrier between the seas at the time of the deposition of
the strata, as, at the present day, they are between the waters of the Gulf and the Pacific.
There is also a remarkable difference in the appearance of the fossils from the east and west sides
of the chain. While on the Pacific side we find a variety of genera and species, both of univalves and bivalves, the deposit on the eastern, or Gulf side, consists of an enormous bed almost
wholly formed of Os8tracea and cPectens. Although an entire separation of the Miocene seas is
indicated, it is probable that they were connected, during a part, at least, of the Tertiary
period, through the pass of San Bernardino; Lower, or Old California, being thus left as an
island, which it was formerly supposed to be. A depression of only 2,808 feet would be suffi cient to again unite the head of the Gulf with the Pacific, and thus form an island of the
Peninsula.
The San Francisco sandstone is probably co-extensive with the Coast Mountains north and
south-south as far as the Bernardino Sierra, and north even into Oregon and Washington
Territories. There is much reason to believe that the coal beds of Bellingham Bay and Puget's
Sound occur in this series of strata. A block of sandstone from the coal strata of the former
place is nearly identical in its mineral constitution and appearance with that of the vicinity of
San Francisco. It contains two large Pectens and masses of coal.1
The southern extension of the great deposits of microscopic, silicious organisms at Monterey
is shown by the specimen from the vicinity of San Luis Obispo, and by the rolled silicious masses
charged with bitumen, and of undoubted infusorial origin, which are cast up by the surf on the
shores of San Pedro.
Although the observations which have been made are comparatively limited —if we compare
them with the great extent of surface of California-they are sufficient to show that the Tertiary
attains an enormous development on the Pacific coast; and that, in fact, a large part of the area
of the State is either occupied by, or underlaid by, strata of that period. They probably constitute the principal part of the ranges of the Coast Mountains, where they attain their greatest
development and thickness. It is not yet possible to form a reliable estimate of the thickness of
the whole series of strata, but exposures at several points are not less than;2,500 or 3,000 feet.
1 A descriptive section of the coal-bearing strata is given in Chapter XIX.
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