OBSERVATIONS ON THE TERTIARY DEPOSITS. 
189 
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 mate¬ 
rials, including, also, volcanic sand and ash, fragments of pumice, and the like, that they 
might easily he 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 Chowchillas, can he referred to the Ocoya Creek series, or 
are much more recent, can only he determined hy 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 he 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 douht, 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 he 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 uni¬ 
valves and bivalves, the deposit on the eastern, or Gulf side, consists of an enormous bed almost 
wholly formed of Ostracea and Pectens. 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 con¬ 
stitute 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. 
A descriptive section of the coal-hearing strata is given in Chapter XIX. 
