244 RALPH ARNOLD 



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 



I J. C. Merriam, Bull. Dept. Geol. Univ. Cal., Vol. V, p. 173. 



