TERTIARY FAUNAS OF THE PACIFIC COAST R27 
first-mentioned areas from the beginning of the Eocene, but the final 
approach toward base level was probably not attained until the close 
of the upper Miocene. Volcanic activity had ceased on the Coast 
Ranges south of San Francisco during the inauguration of the upper 
Miocene, and had become subdued if not suppressed in the coastal 
belt to.the north. In Oregon" and possibly also in the vicinity 
of Mount Diablo, east of San Francisco, in northeastern California, 
and in Washington volcanoes still persisted. 
FAUNAS AND CLIMATE OF THE UPPER MIOCENE 
The upper Miocene as here mapped and described embraces 
several formations, each carrying a more or less well-defined fauna. 
The most characteristic of these, in the order of age, are the Santa 
Margarita, typically developed in San Luis Obispo and Monterey 
counties, Cal., the Empire of Oregon, and the San Pablo of the San 
Joaquin Valley. All three of these indicate conditions approaching 
those of the present day, though leaning toward warmer climates. 
Toward the end of the Miocene and the beginning of the Pliocene, 
the forerunners of the upper Pliocene sub-boreal invasion which was 
to come, began to be felt. A cool-water fauna is found in the upper- 
most Etchegoin (upper Miocene) formation in the Coalinga district, 
this being followed by a freshwater fauna. In the lower Pliocene 
faunas of southern California are the last representatives of certain 
unique species of Pecten which were abundant in the upper Miocene 
of central California, but which migrated southward during the 
late Miocene, and became extinct before the Pliocene in the territory 
where they formerly had been so abundant. The abundance of 
huge oysters, pectens, and certain subtropical echinoid types in the 
Santa Margarita implies shallow, rather warm, water—these con- 
ditions being due in part, at least, to the local sheltered bodies of water 
which occupied the southern Coast Ranges during that period. The 
Empire fauna, best developed along the edge of the open upper 
Miocene ocean, extended from at least as far north as the Straits 
of Fuca to the region of the Santa Cruz Mountains and possibly 
farther south. 
The strong resemblance between the Etchegoin fauna of the 
t J.C. Merriam, Bull. Dept. Geol. Univ. Cal., Vol. V, p. 173. 
