1918] Davis: The Radiolarian Cherts of the Franciscan Group 283 



interrupt the ordinary alternation of chert and shale. The cherts 

 may dip at steep angles and may occasionally show intense plication 

 and contortion, but such severe disturbances are not as common as in 

 the Franciscan cherts. There is also less disturbance by minor faults. 



As in Franciscan cherts, if one views a section of Monterey cherts 

 from a distance, he gets the impression that they are in definite and 

 persistent beds. If, however, the section be examined carefully at 

 close range, certain remarkable irregularities will be found. 



Since the beds are usually less disturbed, it is possible to see these 

 minor irregularities of bedding to much better advantage than in the 

 Franciscan cherts. The irregularities consist of a thickening and 

 thinning of beds of shale and chert and in lenslike terminations of 

 both chert and shale beds of exactly the same nature as those observed 

 in the Franciscan cherts, and described above. 



Fig. 10. Occurrence of minor lenses of chert in a depression of the surface 

 between a sandstone and a chert formation in the Monterey group. 



The lenslike development of shale beds is much more common in 

 the Monterey cherts than in the Franciscan cherts. The lenslike char- 

 acter of the beds of chert is about equally developed in both forma- 

 tions. In the photographs (plates 33 and 34a), numerous examples 

 of the wedging-out of beds of chert and shale may be seen, together 

 with the resultant branching of neighboring beds of chert or shale, 

 as well as examples of thickening and thinning. 



When the contact between the cherts and sandstones is observ- 

 able, it is seen that the shale partings in the cherts run parallel to the 

 contact between the two rocks. There are, however, some local varia- 

 tions which might be represented diagrammatically. In the little 

 irregularities of the contact plane, one often sees short lenses covered 

 by overlying layers of chert in the manner indicated in figure 10. 



There are numerous lithologic variations in the bedded cherts. 

 They range from rather soft cherts, which can be easily scratched 

 with a knife, to varieties which are quite hard, though hone of them 

 scratch quartz. In color they may be white, cream color, pink, yel- 

 low, brown or black, depending upon the nature of the impurities 



