Geol— Vol. II.] ANDERSON— CRETACEOUS DEPOSITS. 53 



portion of this section much resembles the Cretaceous at 

 Spanish Gulch, while the lower part, consisting of soft dark 

 shales with an occasional thin, hard stratum, is an exact 

 duplicate of the Knoxville, as it is usually developed in 

 California and Southern Oregon. The total thickness of 

 the section is hardly less than 3,500 to 4,000 feet, of which 

 the shales probably make up more than one-half. At the 

 lower end of the Mitchell Knoxville section the shales dip 

 westerly for a short distance, but the west side of the anti- 

 cline is covered by Tertiary formations." The fossils of 

 the upper portion of the section show it to be of Lower 

 Chico age. 



3. The Peridotite Intrusions. 



The relations of the serpentines of the Coast Ranges to 

 both the Knoxville (Paskenta) and the Chico strata form 

 another convincing proof of the unconformity of the Chico 

 upon the former. It is well known that the peridotites 

 from which the serpentines have been derived have been 

 intruded into the Knoxville beds at many places in the 

 Coast Ranges, and that this has happened especially, also, 

 throughout the very region from which the Horsetown 

 strata are entirely missing. A few of these cases may be 

 given, though an extended and complete list of them, that 

 have from time to time been noticed, would be superfluous 

 for the purpose of this paper. 



On the map of the Great Western Quicksilver Mine, 

 Napa County, published by Becker (1888, p. 358), tongues 

 of serpentine are shown penetrating the " Neocomian" 

 shales. Such occurrences are said to be abundant, and so 

 closely and generally are serpentine and Knoxville shales 

 associated in that region as to suggest to Becker the deri- 

 vation of the serpentine from sedimentary rocks. He 

 (Becker, 1888, p. 121) says: "Highly inclined strata 

 strike into serpentine areas in such a manner as to wholly 

 preclude the supposition that the serpentine represents an 

 earlier mass." At Mount Diablo, also, Mr. Turner (1891) 

 has shown similar dikes of serpentine cutting the Knoxville 



(5) November 24, 1902 



