26 
CONDITIONS DURING THE MIOCENE PERIOD. 
It is remarkable of the tertiary beds of the Coast Range that they all belong to the middle 
group, or to what is known as Miocene beds in Europe, and that no Eocene beds have been 
discovered in this survey. The fossils found in these beds approach, in some respect, the 
Atlantic Miocene beds in Virginia, and bear a remote resemblance, indeed, to the European 
beds of analogous position. Like the European strata of the Miocene period, these beds are 
destitute of vegetation, there being but feeble remains of sea-weed in the sandstones of Santa 
Barbara, and but one evidence of land vegetation in the remains of fossil exogenous wood, found 
in a boulder derived from the blue calcareous bed of the Sierra Monica group, the uppermost 
series of the whole. Such slight evidence of the existence of dry land, to any extent, appears 
to warrant the statement already advanced, namely, that during this period of geological history 
this portion of California was separated from the main continent by a wide ocean, occupying 
the region between the Sierra Nevada and the mountains near the Rio Grande. They must 
have existed as mere oceanic islands, whose vegetation, if any, was drifted away from where it 
grew. The Miocene group, wherever observed, appears to prove that there was less land above 
water then than at the period preceding (Eocene) or succeeding (Pliocene,) and the chief bulk 
of the vegetation is of that character which indicates a moist and warm climate. 
Such a climate must have reacted sensibly upon the fauna of the period, and to this, perhaps, 
more than to any other cause, would be due the different forms of mollusc life which distinguish 
the Californian beds from any others yet known. The Virginian beds are its nearest represen¬ 
tatives, but the latter were deposited in waters under latitude 37°, a difference of four degrees 
on the western side of the continent, not involving, certainly, much variation of parallel, but a 
vast diversity of climate, for, in the Miocene period, the Panama isthmus was under water, and 
the equatorial warm belt of water, flowing across the continent, warmed the western side more 
than it is at present, and by its not forming the Gulf stream, cooled the Virginian Miocene 
waters in a corresponding degree. Thus the difference between the two coasts was not merely 
in latitude, but in temperature. Much greater was the difference between the Californian and 
the European beds; besides a cooler ocean, the latter also was placed near latitude 47°, or fifteen 
degrees further to the north. 
Deposited in waters removed from a continent by a large interval, the sediments subsided 
slowly and in order, and not having suffered much from subsequent denudation, excepting in 
the present river valleys, these beds have attained a thickness and importance which cause 
them to overshadow the tertiaries' elsewhere on this continent, and to be regarded as one of 
the most interesting geological features of our western shores. 
Partly from their lithological character, and partly from the arrangement of their fauna, the 
Miocenes of the Coast Range may be conveniently divided into three subdivisions, the upper, 
middle, and lower beds, whose total thickness exceeds 2,000 feet. 
The absorption of the trappean and trachytic ranges of the coast into the Sierra Nevada, in 
Los Angeles county, previously alluded to, is only apparent, and arises from a false view taken 
of the course of elevation of the ranges. It is usual to trace a mountain range from north to 
south, because the map happens to have its north at top ; it would be erroneous to consider that 
this was the order of upheaval of these chains ; two circumstances would lead to the belief that 
the elevation occurred first in the south, and thence extended northward. The first of these is, 
that these ranges in the south are all loftier than towards the north or San Francisco, and not 
only the mountains but the valleys are on elevated table-land, from 900 to 2,000 feet in altitude. 
The second is derived from a study of the valley at the foot of the Sierra Nevada, known a 
