232 RALPH ARNOLD 



the deposition of the Chico were accompanied by basic igneous 

 intrusions. No profound movements and no volcanic activity 

 accompanied the post-Chico (post-Cretaceous) movements in Cali- 

 fornia as they did in Washington. 



Steep mountains bordered the youthful Eocene sea in southern 

 Oregon, northeastern California, and north of San Diego, and 

 occupied portions of one or more large islands in the region of Mon- 

 terey and Santa Barbara counties south of San Francisco. Elsewhere 

 the relief of the land appears to have been comparatively low and the 

 shore-lines with few bays or estuaries. 



DISTRIBUTION AND CHARACTER OF SEDIMENTS 



Rocks of marine origin and Eocene age are found at many local- 

 ities throughout Washington and Oregon west of the Cascade Range, 

 and over considerable areas of the Coast Ranges in central and 

 southern California. Although Eocene rocks probably once fringed 

 the greater part of the western base of the Sierra Nevada, they are 

 now all removed by erosion or covered by later formations except at 

 one locality near Merced Falls. For the most part the Eocene rocks 

 of the Pacific Coast are either sandstone or shale. Conglomerate is 

 found at the base of the formation throughout southeastern Oregon, 

 north of San Diego, and at a few localities along the northeastern 

 flanks of the Coast Range; and at Port Crescent, Washington, 

 Eocene fossils are associated with tufif; but these occurrences are 

 exceptional. Also, diatomaceous shales occur at the top of the 

 Eocene series in the vicinity of Coalinga, Cal., where they are believed 

 to be the source of important deposits of petroleum. Coal and 

 other indications of shallow- and brackish-water conditions are 

 found over much of Washington and Oregon and California, usually 

 overlying marine Eocene beds. The maximum thickness of the 

 Eocene sediments varies from 8,500 feet east of the Cascades,' 

 10,000 to 12,000 feet in western Oregon^ to 9000 ± feet in southern 

 California. 3 



CONDITIONS PREVAILING DURING THE EOCENE 



During the early part of the Eocene, marine conditions prevailed 

 over a considerable territory that later was covered by brackish- or 



1 G. O. Smith, Mt. Stewart Folio. 



2 J. S. Diller, Rosebtirg, Coos Bay, and Port Orford Folios. 



3 Ralph Arnold, U. S. Geol. Stirv. Bull. 321, p. 21. 



