524 RALPH ARNOLD 
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 Cal- 
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 coastai provinces, although certain forms of southern 
extraction, such as large cone shells, numerous arcas, 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 MIDDLE 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. 1, pp. 24 ff. 
2 Tacoma Folio, p. 3. 
