1918] Davis: The Radiolarian Cherts of the Franciscan Group 313 



The Radiolarian Cherts 



Although the various types of radiolarian rocks, previously men- 

 tioned, may reach local importance, the type which is most common 

 among rocks containing radiolaria, is that exemplified by the Fran- 

 ciscan cherts. They might be referred to as bedded radiolarian cherts, 

 for the reason that they occur in thin beds, generally separated by 

 shale partings. It is a rather peculiar fact that when the radiolarian 

 rocks are dense cherty rocks, they generally show this peculiar bed- 

 ding. The more massive varieties of radiolarian rocks are almost 

 always soft and poorly consolidated. Since the bedding is so charac- 

 teristic the simple term "radiolarian cherts" may be used to desig- 

 nate them. Radiolarian rocks of this bedded type are called "kiesel- 

 schiefer" by the Germans. They are apparently included under the 

 term "phthanites" by the French and Belgian geologists. 



In addition to the radiolarian cherts of the Franciscan group, 

 radiolarian cherts are found in some of the older rocks of the Pacific 

 Coast region. 



KLAMATH MOUNTAINS 



In the Klamath Mountains, there are red cherts associated with 

 fossiliferous limestones of Devonian age. 51 These show the usual 

 round, clear spots and appear to be radiolarian cherts, though no defi- 

 nite lattice structures are now discernible. 



SIERRA NEVADA 



Radiolarian cherts are found in some of the members of the bed 

 rock complex of the Sierra Nevada. Specimens of red chert from 

 the Sierra Nevada can not be distinguished from Franciscan cherts. 



Professor Lawson has found radiolaria in cherts of the Calaveras 

 near Colfax. 



Waldemar Lindgren 52 described radiolarian rocks from the Fed- 

 eral Loan area near Nevada City. These rocks are a part of the 

 Calaveras and are described by Lindgren as "siliceous argillite." 

 The rock has been considerably metamorphosed and while the de- 

 scriptions indicate that it somewhat resembles the cherts of the Fran- 

 ciscan formation, it is almost entirely massive and shows no distinct 

 bedding with argillaceous material. 



In the Nevada City Special Folio, 53 Lindgren described bedded 



51 Diller, J. S., Am. Jour. Sci., vol. 165, p. 345, 1903. 



52 The Gold Quartz Veins of Nevada City and Grass Valley Districts, 17th 

 Ann. Eep. U. S. Geol. Surv., part II, p. 1, 1896. 



53 U. S. Geol. Surv., Folio no. 29, 1896. 



