1916] 



Dickerson: Tejo-n Eocene of California 



405 



is identical with this rhyolite and some of the boulders in the 

 conglomerate are likewise composed of this rock. The rhyolite areas 

 mapped are in close association with the Sutter formation as a rule. 



One of the best exposures for study is seen on a ridge one mile 

 southwest of West Butte. The beds at this place cap the underly- 

 ing Cretaceous. The overlying beds dip 10° W, while the underlying 

 Chico strata dip 40° AV. The strata are composed chiefly of what 

 appears to be a much altered rhyolitic tuff-breccia which looks at 

 first sight to be a sandstone. This rock is light gray when 

 weathered and a dark gray on unweathered surfaces. It is com- 

 posed of very angular quartz grains, altered biotite and feldspar. 

 Gypsum commonly fills the joint planes and in places occurs be- 

 tween the bedding planes. Lenses of conglomerate occur interbedded 

 with this rock and two thin flows of igneous rock four to eight 

 inches in thickness were also noted. 



Farther west on this same ridge, this formation is found in fault 

 contact with the Tejon at the top of a small divide crossed by the 

 county road. This divide is located one and a half miles S 50° W 

 of West Butte. The Sutter formation at this point has a dip of 22° W 

 and a strike of N 20° W. The beds vary locally, cross-bedding being 

 common. Besides the rocks described above, a light-gray tuff com- 

 posed of fine ash particles occurs here. The andesite is sharply 

 separated from the Sutter formation at this locality and it has a 

 dip of only 6° W. This same relation is seen in the 547-foot hill one 

 mile north. Similar relations between the Sutter and the underlying 

 and overlying formations Avere found south of South Butte. This 

 formation is named for its occurrence in Sutter County, California, 

 in what is now known as the Marysville Buttes (formerly Sutter 

 Buttes) . 



Historical Geology 



The sequence of events which gave us the Marysville Buttes in 

 their present form appears to be as follows : 



(1) The lowering of the continental margin and the deposition 

 of Chico Cretaceous strata by a transgressing sea from the west ; 

 (2) a great time-interval during which this site was land; (3) sub- 

 mergence at the end of Tejon time which resulted in the deposition 

 upon the outer edge of the continental shelf of Eocene strata com- 

 posed of foraminiferal shales, the deep-water equivalents of the in- 

 shore lone of the Sierran foothills; (4) uplift; (5) an intrusion of 



