In the western Santa Monica Mountains the Miocene volcanics consist of olivine basalt, basalt, andesite and dacite flows, breccias, pyroclastic rocks, volcanic-derived epiclastic rocks, and associated intrusive bodies of related compositions. Pillow breccias are abundant in some areas, almost everywhere near the base of the volcanic sequence. True pillow lavas are scarce. Chemical analysis of these rocks show them to be low in FeO, Fe₂O₃, and MgO, and very low in K₂O. Silica content ranges from 45.31 to 66.12 percent.

The more basic rocks generally are present in the lower half of the volcanic sequence and the less basic are present in the upper third of the sequence. However, there is no consistent variation trend and the tendencies are of limited usefulness for purposes of stratigraphic correlation. Some tentative statement about a rock's probable stratigraphic position can be made if it contains olivine or is largely a pillow breccia (low in sequence), or if it contains primary quartz with or without oxyhornblende (high in sequence).

A comparison of the volcanic rocks near the eastern end of their continuous outcrop area with those near the western end shows the latter to be of similar petrographic types but much more extensively altered. Toward the western end, fresh ferromagnesian rock types are scarce, the minerals having been replaced in part or completely by bowlingite, iddingsite, etc. The plagioclase is commonly affected and partly replaced by kaolinite or carbonate and commonly partly albitized. The glass has devitrified or has been replaced.

The dominant structure of the volcanics in the mountains is that of a north-dipping homocline broken by steep east-west or east-northeast to west-northwest faults. Broad east-west folding also is recognized at several places.

End_of_Article - Last_Page 460------------