1918] Davis: The Radiolarian Cherts of the Franciscan Group 247 



Bedding of the Cherts 

 two types of chert 



The Franciscan cherts vary somewhat in their nature, but they 

 may be separated into two main types according to the relative pro- 

 portion of shaly material which they contain. 



Cherts of the well bedded type consist of layers of chert alternat- 

 ing with shale. The layers of chert usually range in thickness from 

 half an inch to three inches. They are rarely less than a quarter of 

 an inch thick and sometimes reach a thickness of a foot or more. 

 The average thickness is about two inches. The shale partings vary 

 from mere films up to beds three or four inches thick. The usual 

 range of thickness of shale in well bedded cherts is between one- 

 eighth inch and one inch. The average thickness of the shale part- 

 ings in the well bedded varieties is between one-quarter and half an 

 inch. Occasionally much thicker bedding is seen; in rare cases the 

 chert beds may be eighteen to twenty-four inches thick while the 

 shale partings are six to eighteen inches thick. 



The separation between the chert and shale is clean cut ; there is 

 no gradation between them. In some sections of well bedded cherts, 

 hundreds of separate beds are visible in rhythmic alternation 

 (plate 25a and plate 26a). Generally the well bedded varieties are 

 red, or brownish red, in color. Less commonly they are green, black, 

 white, yellow-gray, or rarely purple. Almost invariably the color 

 of the shale is the same as that of the associated cherts. 



The massive type of chert differs from the well bedded type in 

 that the shale partings are very thin. Usually the massive cherts 

 are white or gray, and the shale which is present in the partings is 

 pale gray in color. Occasionally a red chert with thin films of red 

 shale shows this massive character, but such occurrences are not com- 

 mon; the red cherts are usually well bedded. The thickness of sep- 

 arate layers of chert in the massive type is about the same as in the 

 well bedded type. 



The massive cherts stand up in bold outcrops, and in the areas 

 occupied by them, large isolated blocks of rock appear ranging up 

 to ten feet in diameter, while locally, where they have not been 

 greatly fractured, the outcrops present high, steep cliffs. The well 

 bedded cherts with thick shale partings, on the other hand, seldom 

 outcrop boldly. One may go over considerable areas of chert with- 

 out finding any well developed beds of shale exposed in natural out- 

 crops. While the slopes underlain by this type of chert may be steep, 



