TERTIARY FAUNAS OF THE PACIFIC COAST 241 



VOLCANIC ACTIVITY IN THE LOWER MIOCENE 



The most important display of volcanic phenomena on the Pacific 

 Coast took place during the early and middle Miocene, and probably 

 reached its climax at the time of the widespread post-early middle 

 Miocene (post-Monterey) disturbances. Great volcanoes were active 

 throughout eastern Washington and Oregon and in the Coast Ranges 

 of California from the Santa Cruz Mountains at least as far south 

 as the Santa Ana Mountains in Orange County. The lavas and tuffs 

 emitted by these volcanoes, and the associated intrusions, were basic 

 in character. Certain facies of the Monterey are believed by Lawson 

 and Posada' to consist of fine volcanic ash ejected from distant 

 volcanoes of the period. 



FAUNAS AND CLIMATE OF THE LOWER MIOCENE 



The marine faunas of the lower Miocene or Vaqueros are well 

 known and of widespread occurrence in the Coast Ranges of Cali- 

 fornia; those of the Monterey, owing to the peculiar character of its 

 sediments, are meager and little understood. A general survey of 

 the fauna, however, indicates conditions approximate to those now 

 existing in the coastal provinces, although certain forms of southern 

 extraction, such as large cone shells, numerous areas, and other types, 

 indicate possible warmer environment. The evidence of the mol- 

 lusks is supported by that of the plant remains, at least in so far as it 

 relates to the region of Puget Sound, for there, according to Knowlton,^ 

 the presence of sumacs, chestnuts, birches, and sycamores in the 

 upper Puget group [probable lower Miocene] would seem to indicate 

 an approach from the subtropical conditions of the Eocene to the 

 conditions prevailing at the present day. 



PERIOD OF DIASTROPHISM IN THE MTODLE MIOCENE 



One of the most widespread and important periods of diastro- 

 phism in the Tertiary history of the Pacific Coast was that immediately 

 following the deposition of the Monterey or lower middle Miocene. 

 Its effects are visible from Puget Sound to southern California. It is 

 marked as much by readjustment, by local faulting and folding as by 

 general movements of elevation and subsidence. In some regions the 



1 Bull. Dept. Geol. Univ. Cal., Vol. I, pp. 24 ff. 



2 Tacoma Folio, p. 3. 



